pubs.acs.org/IC Article Steric and Electronic Effects on the Structure and Photophysical Properties of Hg(II) Complexes Francisco Sánchez-Férez, Joaquim Ma Rius-Bartra, Teresa Calvet, Merce ̀ Font-Bardia, and Josefina Pons* Cite This: Inorg. Chem. 2021, 60, 3851−3870 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Since many factors influence the coordination around a metal center, steric and electronic effects of the ligands mainly determine the connectivity and, thus, the final arrangement. This is emphasized on Hg(II) centers, which have a zero point stabilization energy and, thus, a flexible coordination environment. Therefore, the unrestricted Hg(II) geometry facilitates the predominance of the ligands during the structural inception. Herein, we synthesized and characterized a series of six Hg(II) complexes with general formula (Hg(Pip)2(dPy)) (Pip = piperonylate, dPy = 3- phenylpyridine (3-phpy) (1), 4-phenylpyridine (4-phpy) (2), 2,2′- bipyridine (2,2′-bipy) (3), 1,10-phenanthroline (1,10-phen) (4), 2,2′:6′,2′-terpyridine (terpy) (5), or di(2-picolyl)amine (dpa) (6)). The elucidation of their crystal structures revealed the arrangement of three monomers (3, 5, and 6), one dimer (4), and two coordination polymers (1 and 2) depending on the steric requirements of the dPy and predominance of the ligands. Besides, the study of their photophysical properties in solution supported by TD-DFT calculations enabled us to understand their electronic effects and the influence of the structural arrangement on them. ■ INTRODUCTION Since the rise of crystal engineering,1 the development of compared to that of Zn(II) and Cd(II). In addition, Hg(II) complexes have potential applications in paper, paints, and structural design strategies has experienced a major break- cosmetics industry or in the production of manometers, mercury through. The understanding and use of either supramolecular batteries, and energy efficient fluorescent light bulbs. interactions2,3 or coordination bonds4,5 and their synergy in the In terms of the linkers, carboxylic acids are a recognized class crystal packing has led to the formation of solids with desired of versatile organic ligands for their potential coordination physical and chemical properties.6 modes, from monodentate to bidentate bridging, which can Within this frame, the selection of the appropriate metal ion as form a large variety of structures. Besides, the addition of well as the linkers will determine the molecular array, the pyridine derivative ligands (dPy) with different denticity, intermolecular interactions, and, thus, the final crystal packing. electronic donor properties, conjugation, and planarity or steric While metal ions act as nodes and mainly determine the requirements could result in diverse arrangements and dimensionality through its geometry, modulation of the organic linkers (flexibility, length, or symmetry) drives the assembly of functionalities.19−22 different architectures and therefore functionalities.7−10 In this scenario, the photophysical properties of these systems Among all of the potential metal nodes, those belonging to could be improved by adding conjugated π aromatic linkers by group 12 with a d10 electronic configuration, and therefore zero tuning their band gap and also driving the crystal packing crystal field stabilization energy, stand out for their flexible through C−H···π and π···π interactions.23 In particular, coordination environment and wide range of geometries,11,12 modulating the bite angle combined with a strong chelate effect usually presenting severe distortions. They enable metal−ligand using N^N-bidentate or N^N^N-tridentate chelate ligands has rearrangement during the structural inception, which gives rise proven to be a determining factor of the final molecular to highly ordered networks.13−15 Furthermore, complexes with geometry with a concomitant effect in the emission spectra.24 d10 metal nodes excel at presenting photoluminescence properties and, hence, have potential applications as fluores- cence-emitting materials.16 Received: December 13, 2020 It is essential to stress the lack of knowledge on the Published: February 25, 2021 coordination behavior and photophysics of Hg(II) nodes. Even though Hg(II) can adopt coordination numbers from 2 to 10,17 it tends to form low coordinated linear structures,18 with the formation of Hg(II) coordination polymers being scarce © 2021 American Chemical Society https://dx.doi.org/10.1021/acs.inorgchem.0c03640 3851 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Scheme 1. Outline of the Synthesis of Complexes 1−6 In a previous paper,25 our group studied and analyzed the (Pip)2(dpa)]·1/2H2O·1/2MeOH (6a) (Scheme 1), which were structure and photophysics from the reaction of M(OAc)2 (M = fully characterized and their crystal structures elucidated. We Zn(II), Cd(II)) with 1,3-benzodioxole-5-carboxylic acid further investigated their photophysical properties in MeOH (piperonylic acid, HPip) and two pyridine derivative ligands solution, and we performed TD-DFT calculations to identify the (dPy = 3-phenylpyridine (3-phpy) and 4-phenylpyridine (4- electronic transitions and comprehend how the structural and phpy)), which resulted in the formation of two Zn(II) dimeric electronic effect of the pyridine derivative influences the band paddle-wheels ([Zn(μ-Pip)2(dPy)]2) and two Cd(II) dimers gap, aggregation, and absorption spectrum. ■ ([Cd(μ-Pip)(Pip)(dPy)2]2). Recently, we have reported the reaction between Zn(II), EXPERIMENTAL SECTION Cd(II), Hg(II), and HPip, leading to the formation of one Chemical Risks. Hg(II) complexes are toxic, and any manipulation Zn(II) monomer ([Zn(Pip)2(H2O)2]) and three coordination of the samples has to be carried out in the fume hood and wearing polymers (two of Cd(II): [Cd(μ-Pip)2(H2O)]n and [Cd3(μ- gloves. Pip)6(MeOH)2]n and one of Hg(II): [Hg(μ-Pip)2]n]). All of Materials and General Details. Hg(II) acetate (Hg(OAc)2), 1,3- them showed different topologies, coordination numbers, and benzodioxole-5-carboxylic acid (piperonylic acid, HPip), 3-phenyl- modes, depending on the metal preferences.26 pyridine (3-phpy), 4-phenylpyridine (4-phpy), 2,2′-bipyridine (2,2′- Herein, we carried out the reaction of Hg(OAc)2 with HPip bipy), 1,10-phenanthroline (1,10-phen), 2,2′:6′,2′-terpyridine (terpy), and a comprehensive range of pyridine derivative ligands (dPy). and di(2-picolyl)amine (dpa) ligands and methanol (MeOH), ethanol The choice of the dPy was made considering that (a) the (EtOH), dichloromethane (CH2Cl2), diethyl ether (Et2O), and increasing denticity of dPy combined with the chelate effect fluorobenzene (C6H5F, Fbz) solvents were purchased from Sigma- Aldrich. Deuterated chloroform (CDCl3) and deuterated dimethyl minimizes ligand dissociation at lower concentrations; (b) sulfoxide (dmso-d6) were used for the NMR experiments and were bulkier ligands avoid solvent attack and, therefore, solvent purchased from Eurisotop. All of them were used without further quenching by complexation as well as reduce geometric changes; purification. Reactions and manipulation were carried out in air at room and (c) to minimize aggregation and, thus, self-quenching by temperature (RT) for compounds 1, 2, and 6 and under reflux avoiding the presence of potential hydrogen bond donors.27,28 conditions for 3−5. Thermal decomposition temperature (d.T.) was Therefore, the selected dPy were N-donor (3-phenylpyridine, 3- measured on a Stuart Melting Point Apparatus SMP30 (Cole-Parmer, phpy; 4-phenylpyridine, 4-phpy), N^N-donor (2,2′-bipyridine, U.K.) with a heating ramp of 2.0 °C/min in a temperature range from 2,2′-bipy; 1,10-phenanthroline, 1,10-phen), and N^N^N-donor 20 to 210 °C. Elemental analyses (EA, C, H, N) were carried out on a sites (2,2′:6′,2′-terpyridine, terpy; 1-(2-pyridinyl)-N-(2- Euro Vector 3100 instrument. HR-ESI+-MS measurements of complexes 1 and 2 in MeOH solution and 3−6 in MeOH/DMSO pyridinylmethyl)methanamine, di(2-picolyl)amine (dpa)). We (80/20) were recorded in a MicroTOF-Q (Bruker Daltonics GmbH, successfully isolated six Hg(II) complexes: [Hg(μ-Pip)2(3- Bremen, Germany) instrument equipped with an electrospray phpy)]n (1), [Hg(μ-Pip)2(4-phpy)]n (2), [Hg(Pip)2(2,2′- ionization source (ESI) in positive mode. Na+ ions come from the bipy)] (3), [Hg(μ-Pip)(Pip)(1,10-phen)]2·C6H5F (4a), [Hg- MeOH solvent which can contain <50 ppb. The conditions were those (Pip)2(terpy)]·EtOH (5), and [Hg- used in routine experiments. The nebulizer pressure was 1.5 bar, the 3852 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article desolvation temperature was 180 °C, the dry gas was at 6 L·min−1, the (calcd for {[Hg(Pip)2]2 + Na}+ = 1085.0046). FTIR-ATR (wave- capillary counter-electrode voltage was 5 kV, and the quadrupole ion number, cm−1): 3073(w) [ν(CH)]ar, 2907(w) [ν(CH)]al, 1614(w) energy was 5.0 eV. Simultaneous TG/DTA determination of [νas(COO)], 1560(m), 1550(m) [νas(COO)], 1502(m), 1485(m) compound 5 was carried out in a Netzsch STA 409 instrument, with [ν(CC), ν(CN)], 1433(s) [νs(COO)], 1373(s) [δ(CC), an aluminum oxide powder (Al2O3) crucible and heating at 5 °C·min−1 δ(CN)], 1336(s), 1256(s), 1227(s), 1168(m), 1109(m) [ν(C from 25 to 350 °C, under a nitrogen atmosphere with a flow rate of 80 OC)], 1073(m), 1031(s) [δip(CH)], 1012(w), 934 (w), 921(m), mL·min−1. Al2O3 (PerkinElmer 0419-0197) was used as a standard. 878(m), 845(m), 821(m), 804(m) [δoop(CH)], 767(s) [δoop(C The FTIR-ATR spectra were recorded on a PerkinElmer spectrometer, H)], 761(s) [δoop(CH)], 731(m), 719(m), 696(m), 677(s), equipped with a universal attenuated total reflectance (ATR) accessory 665(m), 621(m). 1H NMR (360 MHz, CDCl3, 298 K): δ = 6.02 with a diamond window in the range 4000−500 cm−1. 1H, 13C{1H}, and [4H, s, OCH2O], 6.83 [2H, d, 3J = 8.4 Hz, O2CCHCH], 7.51 DEPT-135 NMR spectra were recorded on an NMR-FT Bruker 360 [5H, m, O2CCHCO + (m+p)-Hph(4‑phpy)], 7.64 [2H, dd, 3J = 7.8 MHz spectrometer in CDCl3 or dmso-d6 solution at RT. All chemical Hz, 4J = 1.7 Hz, m-Hpy(4‑Phpy)], 7.68 [2H, m, o-Hph(4‑Phpy)], 7.72 [2H, dd, 3 shifts (δ) are given in ppm. The electronic spectra in solution of MeOH J = 8.4 Hz, 4J = 1.7 Hz, O2CCHCH], 8.79 [2H, d, 3J = 4.7 Hz, o- (1−6) were run on an Agilent HP 8453 UV−vis spectrophotometer Hpy(4‑Phpy)]. 13C{1H} NMR (360 MHz; dmso-d6; 298 K): δ = 168.97 with a quartz cell having a path length of 1 cm in the range 200−600 nm. [O2CC], 150.26 [NCH], 150.17 [O2CCCHCHC], Molar absorptivity values have been calculated as log(ε). Fluorescence 148.67 [NCHCHC], 147.14 [O2CCCHC], 136.49 measurements were carried out at 25 °C with a PerkinElmer LS 55 50 [N(CH)2CC], 129.83 [CC(CH)2CH or CC Hz fluorescence spectrometer using a 1 cm quartz cell, in MeOH CHCHCH], 129.34 [CC(CH)2CH or CCCH solution. The samples were excited at their absorption maxima, and the CHCH], 127.51 [O2CC], 127.08 [CCCH], 125.16 [O2C emission was recorded between 200 and 440 nm. Dilution effects on CCHCH], 122.12 [NCHCH], 109.59 [O2CCCH data were corrected by Origin Pro 8 software. C], 107.82 [O2CCCHCH], 101.66 [OCH2O]. UV−vis Synthesis of Compound [Hg(μ-Pip)2(3-phpy)]n (1). To a solution of (MeOH) λmax (ε) = 205 nm (4.78); 259 nm (4.50); 293 nm (4.22). Hg(OAc)2 (100 mg, 0.314 mmol) in MeOH (30 mL), a solution of Synthesis of Compound [Hg(Pip)2(2,2′-bipy)] (3). To a colorless HPip (104 mg, 0.627 mmol) and 3-phpy (197 mg, 1.27 mmol) in solution of Hg(OAc)2 (100 mg, 0.314 mmol) and HPip (104 mg, 0.628 MeOH (30 mL) was added dropwise under vigorous stirring at RT. mmol) in EtOH (25 mL), a solution of 2,2′-bipy (49.0 mg; 0.314 Immediately, a white solid appeared. The reaction remained under mmol) in MeOH (10 mL) was added dropwise under vigorous stirring stirring for 1 h. The solid obtained was filtered and washed with 10 mL and kept under reflux conditions for 4 h. A white powder precipitated. of cold diethyl ether. Suitable crystals were obtained by recrystallization The solid obtained was filtered and washed with 5 mL of cold MeOH in MeOH for 15 days. Yield: 127 mg (59%). d.T. = 172 °C. Anal. Calcd and 5 mL of cold diethyl ether. Suitable crystals were grown by for C27H19HgNO8 (686.04 g mol−1): C, 47.27; H, 2.79; N, 2.04. Found: recrystallization in MeOH for 5 days. Yield: 160 mg (75%). d.T. = 184 C, 46.99; H, 2.84; N, 1.97%. HR-MS (ESI+, MeOH): m/z (%) = °C. Anal. Calcd for C26H18HgN2O8 (687.02 g·mol−1): C, 45.45; H, 156.0813 (100%) (calcd for [3-phpy + H]+ = 156.0808); 554.9998 2.64; N, 4.08. Found: C, 45.30; H, 2.58; N, 3.86%. HR-MS (ESI+, (100%) (calcd for {[Hg(Pip)2] + Na}+ = 554.9977); 1085.0047 (77%) MeOH/DMSO): m/z (%) = 523.0570 (100%) (calcd for {[Hg(Pip)- (calcd for {[Hg(Pip)2]2 + Na}+ = 1085.0046). FTIR-ATR (wave- (2,2′-bipy)]}+ = 523.0579). FTIR-ATR (wavenumber, cm−1): 3107− number, cm−1): 3090(w) [ν(CH)]ar, 3035(w) [ν(CH)]ar, 2918(w) 3019 [ν(CH)]ar, 2999−2921(w) [ν(CH)]al, 1629(w), 1598(w), [ν(CH)]al, 1628(w), 1604(w) [νas(COO)], 1555(m) [νas(COO)], 1592(w), 1578(m), 1544(m) [νas(COO)], 1503(m), 1490(m) 1521(m), 1504(w), 1487(s) [ν(CC), ν(CN)], 1475(m), [ν(CC), ν(CN)], 1470(m), 1432(m) [νs(COO)], 1375(s) 1461(m), 1437(s) [νs(COO)], 1416(m), 1372(s) [δ(CC), δ(C [δ(CC), δ(CN)], 1337(m), 1314(m), 1256(s), 1239(s), N)], 1343(s), 1258(s), 1238(s), 1205(m), 1189(m), 1167(m), 1205(m), 1158(m), 1099(m) [ν(COC)], 1076(w), 1032(s) 1131(m), 1112(s) [ν(COC)], 1072(m), 1032(s) [δip(CH)], [δip(CH)], 1019(m), 981(w), 919 (m), 883(m), 819(m), 805(m) 934 (m), 921(m), 882(s), 839(w), 817(w), 804(m) [δoop(CH)], [δoop(CH)], 785(m) [δoop(CH)], 769(s) [δoop(CH)], 734(m) 772(s) [δoop(CH)], 753(s) [δoop(CH)], 721(m), 697(s), 680(s), [δoop(CH)], 720(m) [δoop(CH)], 679(m), 651(m), 627(m), 656(m), 625(m). 1H NMR (360 MHz, CDCl3, 298 K): δ = 6.06 [4H, s, 586(m), 540(m). 1H NMR (360 MHz, dmso-d6, 298 K): δ = 6.07 [4H, OCH2O], 6.89 [2H, d, 3J = 8.2 Hz, O2CCHCH], 7.52 [8H, s, OCH2O], 6.93 [2H, d, 3J = 8.2 Hz, O2CCHCH], 7.37 [2H, m, O2CCHCO + m-Hpy(3‑Phpy) + Hph(3‑Phpy)], 7.76 [2H, d, 3J = 8.2 d, 4J = 0.9 Hz, O2CCHCO], 7.54 [2H, dd, 3J = 8.0 Hz, 4J = 1.2 Hz, Hz, O2CCHCH], 8.01 [1H, ddd, 3J = 7.9 Hz, 4J = 2.3 Hz, 4J = 1.7 O2CCHCH], 7.76 [2H, dd, 3J = 7.6 Hz, 3J = 5.3 Hz, m- Hz, p-Hpy(3‑Phpy)], 8.64 [1H, dd, 3J = 5.4, 4J = 1.7 Hz, o-Hpy(3‑Phpy) Hpy(2,2′‑bipy)], 8.20 [2H, t, 3J = 7.7 Hz, p-Hpy(2,2′‑bipy)], 8.63 [2H, d, 3J = CH], 8.89 [1H, d, 4J = 2.1, o-Hpy(3‑Phpy)C]. 13C{1H} NMR (360 7.6 Hz, m-Hpy(2,2′‑bipy)C], 8.87 [2H, d, 3J = 4.9 Hz, o-Hpy(2,2′‑bipy)]. MHz; dmso-d6; 298 K): δ = 169.20 [O2CC], 150.57 [O2C 13 C{1H} NMR (360 MHz; dmso-d6; 298 K): δ = 169.31 [O2CC], C(CH)2C], 148.89 [NCHC], 148.05 [NCHCH], 150.84 [NCC], 150.06 [O2CC(CH)2C], 149.73 [N 147.49 [O2CCCHC], 137.05 [NCHCC], 136.42 CHCH], 147.11 [O2CCCHC], 139.43 [N(CH)2CH], [NCHC], 135.52 [N(CH)2CH], 129.59 [CCCH 128.20 [NCHCH], 125.87 [O2CC], 124.91 [O2CCCH CHCH or CC(CH)2CH]3‑phpy, 128.74 [CC(CH)2 CH], 122.11 [NCCH], 109.43 [O2CCCHC], 107.77 CH or CCCHCHCH]3‑phpy, 127.58 [O2CC], 127.26 [O2CCCHCH], 101.63 [OCH2O]. UV−vis (MeOH) [CCCH]3‑phpy, 125.54 [O2CCCHCH], 124.80 [N λmax (ε) = 203 nm (4.69); 215 nm (4.66); 244 nm (4.18); 260 nm CHCH], 109.91 [O2CCCHC], 108.18 [O2CCCH (4.25); 287 nm (4.29). CH], 102.02 [OCH2O]. UV−vis (MeOH) λmax (ε) = 212 nm Synthesis of Compound [Hg(μ-Pip)(Pip)(1,10-phen)]2 (4). To a (4.52); 258 nm (4.43); 278 nm (4.13); 293 nm (4.19). colorless solution of Hg(OAc)2 (101 mg, 0.316 mmol) and HPip (105 Synthesis of Compound [Hg(μ-Pip)2(4-phpy)]n (2). To a solution of mg, 0.633 mmol) in EtOH (25 mL), a solution of 1,10-phen (62.6 mg; Hg(OAc)2 (95.4 mg, 0.299 mmol) in MeOH (30 mL), a solution of 0.316 mmol) in MeOH (5 mL) was added dropwise under vigorous HPip (103 mg, 0.623 mmol) and 4-phpy (191 mg, 1.23 mmol) in stirring and kept under reflux conditions for 8 h. A white powder MeOH (30 mL) was added dropwise under vigorous stirring at RT. precipitated. The solid obtained was filtered and washed with 5 mL of Immediately, a white solid appeared. The reaction remained under cold MeOH. Suitable crystals were grown by recrystallization in stirring for 1 h. The solid obtained was filtered and washed with 10 mL fluorobenzene for 6 days, obtaining the compound [Hg(μ-Pip)(Pip)- of cold diethyl ether. Suitable crystals were obtained by recrystallization (1,10-phen)]2·C6H5F (4a). The characterization corresponds to 4. in MeOH for 18 days. Yield: 109 mg (53%). d.T. = 171 °C. Anal. Calcd Yield: 172 mg (76%). d.T. = 190 °C. Anal. Calcd for C56H36Hg2N4O16 for C27H19HgNO8 (686.04 g mol−1): C, 47.27; H, 2.79; N, 2.04%. (1422.09 g·mol−1): C, 47.28; H, 2.55; N, 3.94. Found: C, 47.22; H, Found: C, 46.99; H, 2.64; N, 1.98%. HR-MS (ESI+, MeOH): m/z (%) 2.43; N, 3.82%. HR-MS (ESI+, MeOH/DMSO): m/z (%) = 547.0576 = 156.0822 (100%) (calcd for [4-phpy·H]+ = 156.0761); 554.9990 (100%) (calcd for {[Hg(Pip)(1,10-phen)]}+ = 523.0580). FTIR-ATR (100%) (calcd for {[Hg(Pip)2] + Na}+ = 554.9977); 1085.0045 (71%) (wavenumber, cm−1): 3064−3014 [ν(CH)]ar, 2903−2782(w) [ν- 3853 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article (CH)]al, 1622(w) [νas(COO)], 1603(w), 1585(m), 1574(m), [Hg(Pip)2(dpa)]·1/2H2O·1/2MeOH (6a). The characterization corre- 1543(m) [νas(COO)], 1501(m), 1485(m) [ν(CC), ν(CN)], sponds to 6. Yield: 118 mg (54%). d.T. = 178 °C. Anal. Calcd for 1436(s) [νs(COO)], 1367(s) [δ(CC), δ(CN)], 1319(s), C28H23HgN3O8 (730.09 g·mol−1): C, 46.06; H, 3.18; N, 5.76. Found: 1260(s), 1240(s), 1163(m), 1142(w), 1107(m) [ν(COC)], C, 46.14; H, 3.22; N, 5.80%. HR-MS (ESI+, MeOH/DMSO): m/z (%) 1075(w), 1038(s) [δip(CH)], 937(m), 917(m), 892(w), 879(w), = 566.0994 (100%) (calcd for {[Hg(Pip)(dpa)]}+ = 566.1001). FTIR- 863(w), 853(m), 821(w), 802(m) [δoop(CH)], 768(s) [δoop(C ATR (wavenumber, cm−1): 3312(w) [ν(NH)], 3069 (w) [ν(CH)]ar, H)], 725(s) [δoop(CH)], 680(m) [δoop(CH)], 640(w), 580(m). 2902 (m) [ν(CH)]al, 1622(w), 1600(w), 1571(m) [νas(COO)], 1 H NMR (360 MHz, CDCl3, 298 K): δ = 5.97 [4H, s, OCH2O], 1545(m), 1502(w), 1485(m) [ν(CC), ν(CN)], 1433(s) 6.76 [2H, d, 3J = 8.1 Hz, O2CCHCH], 7.57 [2H, d, 4J = 1.5 Hz, [ν s (COO)], 1365−1321(s) [δ(CC), δ(CN)], 1285(m), O2CCHCO], 7.71 [2H, dd, 3J = 8.1 Hz, 4J = 1.6 Hz, O2CCH 1252(s), 1237(s), 1159(m), 1126(w), 1104(m) [ν(COC)], CH], 8.0 [2H, s, Hph(1,10‑phen)], 8.01 [2H, m, m-Hpy(1,10‑phen)], 8.53 [2H, 1072(w), 1036(s) [δip (CH)], 1012(m), 986(w), 935 (m), dd, 3J = 8.2 Hz, 4J = 1.5 Hz, p-Hpy(1,10‑phen)], 9.42 [2H, dd, 3J = 4.8 Hz, 4J 920(m), 885(w), 880(w), 804(m) [δoop(CH)], 769(s) [δoop(C = 1.7 Hz, o-Hpy(1,10‑phen)]. 13C{1H} NMR (360 MHz; dmso-d6; 298 K): H)], 719(m) [δoop(CH)], 678(m), 638(w), 583(m), 537(w), δ = 169.16 [O2CC], 150.37 [NCH], 149.49 [O2CCCH 524(w), 505(w). 1H NMR (360 MHz; dmso-d6; 298 K): δ = 4.16 CHC], 146.74 [O2CCCHC], 139.51 [N(CH)2CH], [4H, s, NHCH2], 5.10 [1H, br, NH], 6.03 [4H, s, OCH2O], 138.47 [NC], 128.89 [NCC], 128.66 [O2CC], 127.07 [N 6.87 [2H, d, 3J = 8.2 Hz, O2CCHCH], 7.37 [2H, br, m-Hpy], 7.51 CHCH], 125.47 [NCCCH], 124.60 [O2CCCHC], [2H, d, 3J = 8.2 Hz, O2CCHCH], 7.55 [4H, m, O2CCHCO 109.32 [O2 CCCHCH], 107.39 [O 2CCCHCH], + m-HpyCH2], 7.97 [2H, t, 3J = 7.7 Hz, p-Hpy], 8.75 [2H, d, 3J = 5.3 101.28 [OCH2O]. UV−vis (MeOH) λmax (ε) = 206 nm (4.78); Hz, o-Hpy]. 13C{1H} NMR (360 MHz; dmso-d6; 298 K): δ = 169.47 216 nm (5.03); 266 nm (5.05); 294 nm (4.93); 326 nm (3.68). [O2CC], 155.34 [NCCH2], 149.61 [O2CC(CH)2C], Synthesis of Compound [Hg(Pip)2(terpy)]·EtOH (5). To a colorless 14 9.42 [NCH ], 147.25 [O 2 C CC HC], 139.50 solution of Hg(OAc)2 (100 mg, 0.316 mmol) and HPip (105 mg, 0.632 [N(CH)2CH], 130.98 [O2CC], 124.96 [O2CCCH mmol) in EtOH (25 mL), a colorless solution of terpy (73.3 mg, 0.314 CH], 124.58 [NCCH + NCHCH], 110.04 [O2CC mmol) in EtOH (15 mL) was added dropwise under vigorous stirring, CHC], 107.85 [O2CCCHCH], 101.74 [OCH2O], and the solution turned yellow. The reaction was kept under reflux 50.77 [NCH2]. UV−vis (MeOH) λmax (ε) = 202 nm (4.17); 238 conditions for 3 h. The solution was concentrated until half of the nm (4.03); 281 nm (3.88); 307 nm (3.99). volume and cooled down in an ice bath until a yellowish powder X-ray Crystallography. Colorless needle-like (1 and 2), colorless precipitated. The solid was filtered and washed with 5 mL of cold prism-like (3, 4a, and 6a), and yellow needle-like (5) specimens were methanol. Suitable crystals were obtained by slow evaporation of used for the X-ray crystallographic analysis. The X-ray intensity data mother liquors on air for 12 days. Yield: 214 mg (89%). d.T. = 197 °C. were measured on a D8 Venture system equipped with a multilayer Anal. Calcd for C33H26HgN3O9 (810.17 g·mol−1): C, 48.92; H, 3.36; N, monochromator and a Mo microfocus (λ = 0.71073 Å). For 1−6a, the 5.19. Found: C, 48.74; H, 3.23; N, 5.04%. HR-MS (ESI+, MeOH/ frames were integrated with the Bruker SAINT Software package using DMSO): m/z (%) = 600.0837 (100%) (calcd for {[Hg(Pip)(terpy)]}+ a narrow-frame algorithm. For 1, the integration of the data using a = 600.0846). FTIR-ATR (wavenumber, cm−1): 3415(br) [ν(OH)], triclinic unit cell yielded a total of 17,255 reflections to a maximum θ 3099−3016(w) [ν(CH)]ar, 2964−2789(w) [ν(CH)]al, 1624(w), angle of 23.27° (0.90 Å resolution), of which 3230 were independent 1591(w), 1575(w), 1534(m) [νas(COO)], 1506(w), 1478(m) (average redundancy 5.342, completeness = 99.5%, Rint = 2.12%, Rsig = [ν(CC), ν(CN)], 1432(s) [νs(COO)], 1366(s) [δ(CC), 1.54%) and 3127 (96.81%) were greater than 2σ(|F|2). The calculated δ(CN)], 1347(s), 1312(s), 1253(s), 1196(w), 1160(m), 1114(w) minimum and maximum transmission coefficients (based on crystal [ν(COC)], 1071(m), 1053(m), 1038(s) [δip(CH)], 1010(m), size) are 0.5894 and 0.7449. For 2, the integration of the data using a 974 (w), 936(m), 916(m), 882(m), 852(w), 820(m), 805(m) triclinic unit cell yielded a total of 17,834 reflections to a maximum θ [δoop(CH)], 776(s) [δoop(CH)], 740(w), 721(w), 680(m), angle of 27.12° (0.78 Å resolution), of which 2591 were independent 650(m), 636(w), 608(w), 586(m), 516(w), 507(w). 1H NMR (360 (average redundancy 6.883, completeness = 99.8%, Rint = 2.91%, Rsig = MHz; CDCl3; 298 K): δ = 3.72 [2H, q, 3J = 7.0 Hz, HOCH2CH3], 1.73%) and 2437 (94.06%) were greater than 2σ(|F|2). The calculated 1.62 [1H, br, HOCH2CH3], 1.24 [3H, t, 3J = 7.0 Hz, HO minimum and maximum transmission coefficients (based on crystal CH2CH3], 5.94 [4H, s, OCH2O], 6.72 [2H, d, 3J = 8.2 Hz, size) are 0.5114 and 0.7461. O2CCHCH], 7.52 [2H, d, 4J = 1.5 Hz, O2CCHCO], 7.65 For 3, the integration of the data using a monoclinic unit cell yielded [2H, dd, 3J = 8.1 Hz, 4J = 1.6 Hz, O2CCHCH], 7.66 [2H, dd, 3J = a total of 22,978 reflections to a maximum θ angle of 30.56 (0.70 Å 8.2 Hz, 4J = 1.6 Hz, m-Hpy‑side(terpy)], 8.0 [2H, td, 3J = 7.7 Hz, 4J = 1.8 Hz, resolution), of which 3445 were independent (average redundancy p-Hpy‑side(terpy)], 8.23 [5H, m, m-Hpy‑side(terpy) + (m+p)-Hpy‑center(terpy)], 6.670, completeness = 98.6%, Rint = 4.64%, Rsig = 2.87%) and 3237 9.23 [2H, dd, 3J = 5.8 Hz, 4J = 1.6 Hz, o-Hpy(terpy)]. 13C{1H} NMR (360 (93.96%) were greater than 2σ(|F|2). The calculated minimum and MHz; dmso-d6; 298 K): δ = 169.38 [O2CC], 150.65 [NCH], maximum transmission coefficients (based on crystal size) are 0.3609 149.85 [O2CC(CH)2C], 147.27 [O2CCCHC], 140.30 and 0.7461. For 4a, the integration of the data using a monoclinic unit [NCCHCHside and NCCHCHcenter], 129.93 [N cell yielded a total of 58,489 reflections to a maximum θ angle of 30.46° CHCH and NCCHcenter], 126.90 [O2CC], 125.09 [O2C (0.70 Å resolution), of which 7936 were independent (average CCHCH], 123.78 [NCside and NCcenter], 123.22 [NC redundancy 7.370, completeness = 99.4%, Rint = 3.32%, Rsig = 2.02%) CHside], 109.95 [O2CCCHC], 107.89 [O2CCCHCH], and 7288 (96.83%) were greater than 2σ(|F|2). The calculated 101.80 [OCH2O], 56.49 [HOCH2], 19.02 [HOCH2 minimum and maximum transmission coefficients (based on crystal CH3]. UV−vis (MeOH) λmax (ε) = 205 nm (4.85); 253 nm (4.28); 282 size) are 0.5115 and 0.7461. For 5, the integration of the data using a nm (4.30); 298 nm (4.15); 322 nm (4.17). monoclinic unit cell yielded a total of 64,064 reflections to a maximum θ Synthesis of Compound [Hg(Pip)2(dpa)] (6). To a solution of angle of 30.56° (0.70 Å resolution), of which 4497 were independent Hg(OAc)2 (95.6 mg, 0.300 mmol) in MeOH (10 mL), a solution of (average redundancy 14.248, completeness = 99.8%, Rint = 2.96%, Rsig = HPip (103 mg, 0.618 mmol) in MeOH (20 mL) was added under 1.35%) and 4274 (95.04%) were greater than 2σ(|F|2). The calculated stirring. Then, a yellow solution of dpa (59.8 mg, 0.300 mmol) in minimum and maximum transmission coefficients (based on crystal MeOH (5 mL) was added and stirred for 4 h. The resulting yellow size) are 0.6261 and 0.7461. For 6a, the integration of the data using a solution was concentrated under vacuum until a dark yellow oil-like triclinic unit cell yielded a total of 159,036 reflections to a maximum θ reaction crude was formed, which was dissolved in 10 mL of CH2Cl2, angle of 36.35° (0.60 Å resolution), of which 25,925 were independent forced to precipitate with 15 mL of cold diethyl ether, and filtered (average redundancy 6.134, completeness = 99.3%, Rint = 3.86%, Rsig = (repeated twice). The final brownish powder was washed twice with 5 2.53%) and 23005 (88.74%) were greater than 2σ(|F|2). The calculated mL of cold diethyl ether. Recrystallization of the solid in MeOH and minimum and maximum transmission coefficients (based on crystal cooling down to 4 °C for 5 days resulted in suitable crystals of size) are 0.5108 and 0.7471. 3854 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 1. Crystal Structure Refinement Parameters for 1−3 1 2 3 empirical formula C27H19HgNO8 C27H19HgNO8 C26H18HgN2O8 formula weight 686.02 686.02 687.01 T (K) 100(2) 100(2) 100(2) wavelength (Å) 0.71073 0.71073 0.71073 system, space group triclinic, P̅ 1 monoclinic, C2/c monoclinic, P2/c unit cell dimensions a (Å) 8.8386(3) 19.3137(6) 11.2979(6) b (Å) 11.7440(5) 14.5623(4) 8.3924(5) c (Å) 12.0985(5) 8.3428(2) 12.9283(8) α (deg) 112.6490(10) 90 90 β (deg) 97.1300(10) 95.1020(10) 111.306(2) γ (deg) 97.3600(10) 90 90 V (Å3) 1128.60(8) 2337.13(11) 1142.04(12) Z 2 4 2 Dcalc (g cm3) 2.019 1.950 1.998 μ (mm−1) 6.876 6.641 6.797 F (000) 664 1328 664 crystal size (mm3) 0.198 × 0.081 × 0.072 0.402 × 0.048 × 0.045 0.315 × 0.189 × 0.135 hkl ranges −9 ≤ h ≤ 9 −24 ≤ h ≤ 24 −16 ≤ h ≤ 14 −13 ≤ k ≤ 13 −18 ≤ k ≤ 18 −11 ≤ k ≤ 11 −13 ≤ l ≤ 13 −10 ≤ l ≤ 9 −18 ≤ l ≤ 18 θ range (deg) 2.366−23.274 2.117−27.124 2.427−30.557 reflections collected/unique/[Rint] 17254/3229/[Rint] = 0.0212 17834/2591/[Rint] = 0.0291 22978/3445/[Rint] = 0.0464 completeness to θ (%) 99.5 99.8 99.1 absorption correction semiempirical semiempirical semiempirical max. and min. transmis. 0.7449 and 0.5894 0.7461 and 0.5114 0.7461 and 0.3609 refinement method full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 data/restraints/parameters 3229/0/334 2591/4/128 3445/0/168 goodness of fit (GOF) on |F|2 1.104 1.169 1.044 final R indices [I > 2σ(I)] R1 = 0.0114, R1 = 0.0459, R1 = 0.0196, wR2 = 0.0272 wR2 = 0.1238 wR2 = 0.0478 R indices (all data) R1 = 0.0134 R1 = 0.0504, R1 = 0.0218, wR2 = 0.0275 wR2 = 0.1346 wR2 = 0.0493 extinction coefficient n/a n/a n/a largest. diff. peak and hole (e Å−3) 0.318 and −0.587 2.485 and −1.987 0.635 and −1.564 The structures were solved using the Bruker SHELXTL software graphics are as follows: light gray (Hg), light blue (N), red (O), yellow package and refined using SHELX (version-2018/3).29 For 1, the final (F), gray (C), and white (H). anisotropic full-matrix least-squares refinement on |F|2 with 334 Methodology and Computational Details. All of the calcu- variables converged at R1 = 1.18% for the observed data and wR2 = lations have been performed using Gaussian 09 software, version 4.68% for all data. For 2, the final anisotropic full-matrix least-squares D.01.34 Since the chemistry of Hg(II) is still largely unexplored, there is refinement on |F|2 with 128 variables converged at R1 = 4.59% for the a lack of systematic computational research to better understand observed data and wR2 = 13.46% for all data. For 3, the final anisotropic experimental results, especially from an electronic perspective as in the full-matrix least-squares refinement on |F|2 with 168 variables converged field of photochemistry. It is worth mentioning that the reported at R1 = 1.96% for the observed data and wR2 = 4.93% for all data. For 4a, examples of TD-DFT calculations with Hg(II) complexes are scarce the final anisotropic full-matrix least-squares refinement on |F|2 with and no previously reported data have been found regarding electronic 334 variables converged at R1 = 2.68% for the observed data and wR2 = transition calculations. Geometry optimization of the ground state (GS) and vertical absorptions from the electronically excited state 6.10% for all data. For 5, the final anisotropic full-matrix least-squares (EES) for 1−5 have been done with density functional theory (DFT) refinement on |F|2 with 224 variables converged at R1 = 1.39% for the and time-dependent DFT (TD-DFT), respectively, using the ωB97X- observed data and wR2 = 3.27% for all data. For 6a, the final anisotropic D35,36 functional (Supporting Information: Tables S3−S7 and Figures full-matrix least-squares refinement on |F|2 with 755 variables converged S37−S41). A correlation consistent basis set was used for the Hg(II) at R1 = 1.87% for the observed data and wR2 = 4.33% for all data. atom, the effective core potential CrenbL,37 while the 6-311G- For 1−6a, the final cell constants and volume are based upon the (2d,p)38,39 basis set was used with C, H, N, and O atoms. MeOH refinement of the XYZ-centroids of reflections above 20 σ(I). Data were solvation effects were incorporated using the polarizable continuum corrected for absorption effects using the multiscan method model-linear response (PCM-LR).40,41 The frequencies were also (SADABS). Crystal data and relevant details of structure refinement computed for each optimized structure to ensure that the geometries for compounds 1−6a are reported in Table 1 and Table 2. Complete corresponded to an energy minimum. All of the optimized geometries information about the crystal structure and molecular geometry is in MeOH solution are similar to their corresponding ones determined available in CIF format via CCDC 2043891 (1), 2043892 (2), 2043890 by the single crystal X-ray diffraction method. The most significant (3), 2043893 (4a), 2043894 (5), and 2043895 (6a). Molecular conformational changes are observed in 5. For instance, this is reflected graphics were generated with Mercury 4.2.0 software30−32 using the in its pyridyl−pyridyl torsion angles. Unfortunately, all of the efforts to POV-Ray image package.33 The color codes for all of the molecular reach the geometry energy minima of complex 6 were unsuccessful. The 3855 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 2. Crystal Structure Refinement Parameters for 4a−6a 4a 5 6a empirical formula C62H41FHg2N4O16 C33H27HgN3O9 C57H52Hg2N6O18 formula weight 1518.17 810.16 1510.22 T (K) 100(2) 100(2) 100(2) wavelength (Å) 0.71073 0.71073 0.71073 system, space group monoclinic C2/c monoclinic, C2/c triclinic, P̅ 1 unit cell dimensions a (Å) 28.224(4) 22.9864(8) 9.5659(10) b (Å) 15.6228(19) 13.2575(5) 17.2009(19) c (Å) 12.2354(14) 10.7806(3) 17.2979(19) α (deg) 90 90 100.768(4) β (deg) 103.270(6) 116.3960(10) 100.641(4) γ (deg) 90 90 99.745(4) V (Å3) 5250.9(11) 2942.79(17) 2686.1(5) Z 4 4 2 Dcalc (g cm3) 1.920 1.829 1.867 μ (mm−1) 5.926 5.294 5.792 F (000) 2952 1592 1480 crystal size (mm3) 0.296 × 0.128 × 0.067 0.124 × 0.064 × 0.030 0.321 × 0.184 × 0.120 hkl ranges −40 ≤ h ≤ 40 −32 ≤ h ≤ 32 −15 ≤ h ≤ 15 −22 ≤ k ≤ 22 −18 ≤ k ≤ 18 −28 ≤ k ≤ 28 −17 ≤ l ≤ 17 −14 ≤ l ≤ 15 −28 ≤ l ≤ 28 θ range (deg) 2.578−30.463 3.073−30.556 2.466−36.354 reflections collected/unique/[Rint] 58489/7936/[Rint] = 0.0332 64074/4497/[Rint] = 0.0296 159036/25925/[Rint] = 0.0386 completeness to θ (%) 99.5 99.7 99.5 absorption correction semiempirical semiempirical semiempirical max. and min. transmis. 0.7461 and 0.5115 0.7461 and 0.6261 0.7471 and 0.5108 refinement method full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 data/restraints/parameters 7936/6/448 4497/0/2244 25925/3/755 goodness of fit (GOF) on |F|2 1.137 1.081 1.049 final R indices [I > 2σ(I)] R1 = 0.0268, R1 = 0.0139, R1 = 0.0187, wR2 = 0.0594 wR2 = 0.0320 wR2 = 0.0406 R indices (all data) R1 = 0.0304, R1 = 0.0160 R1 = 0.0242 wR2 = 0.0610 wR2 = 0.0327 wR2 = 0.0433 extinction coefficient n/a n/a n/a largest. diff. peak and hole (e Å−3) 2.118 and −2.068 0.486 and −0.910 1.266 and −1.758 highest occupied molecular orbital (HOMO) and lowest unoccupied graphically evaluated. The Δr index and TDM mapping have been molecular orbital (LUMO) were first examined and the band gap calculated using Multiwfn 3.7 software.44 ■ energies calculated. For 1−5, the first 60 vertical absorptions from the ground state to the excited states have been calculated and only the RESULTS AND DISCUSSION most probable transitions (higher f values) have been selected for the electronic analysis. General Materials and Methods. Complexes 1−6 were Electronic transitions are composed of multiple non-negligible prepared via combination of Hg(OAc)2, HPip, and the contributions from molecular orbitals (MOs), and their representation corresponding dPy. Reactions were performed in a 1:2:4 and evaluation as canonical orbitals could be intricated. Therefore, the (Hg:Pip:dPy) molar ratio for compounds 1 and 2, while 3−6 use of natural transition orbitals (NTOs)42 is a useful tool to identify were synthesized in a 1:2:1 proportion. The different molar ratio and represent the molecular orbitals implied on the transition. The between the syntheses of 1 and 2 and complexes 3−6 could lie in generation of NTOs is based on separately performing unitary the different coordination modes of the ligands (monodentate in transformation for occupied MOs and virtual MOs outstanding those 1 and 2 and chelate in 3−6). The synthesis of 1, 2, and 6 was pairs with predominant contributions, which makes the orbitals’ done in MeOH at room temperature (RT), while the synthesis inspection much easier. NTOs of the selected transitions for 1−5 have of 3−5 in EtOH was done under reflux conditions. Therefore, been generated using an isovalue of 0.02. Likewise, the identification of the synthesis of the complexes depends mainly on the molar each transition type(i) charge transfer (CT), in which the transition ratio and on the reaction conditions. The corresponding crystals leads to evident movement of charge density, or (ii) local excitation, where the electronic movement occurs in the same spatial region suitable for X-ray crystallographic analysis were grown via (LE)is not straightforward, and it has been afforded by calculating recrystallization in MeOH for 1−3 and 6, in fluorobenzene for 4, the Δr index43 (smaller Δr values (<2.0) define likely LE modes, while or by slow evaporation of mother liquors for 5. Complexes 4 and higher values (>2.0) pertain to charge transfers) and mapping the 6 crystallized occluding solvent molecules, yielding 4a and 6a. As transition density matrix (TDM) as a color filled 2D plot. The aforementioned, Hg(II) ions having a d10 electronic config- combination of both allows the electronic excitation mode to be uration entail a zero crystal field stabilization energy and enable established. The diagonal terms of the 2D plot highlight the primary an easier ligand rearrangement. Thus, the incorporation of the atoms involved in the transition, and the spatial extent can be dPy ligands has led to different arrangements, coordination 3856 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for 1 and 2a 1 Bond Lengths (Å) Hg(1)−N(1) 2.175(2) Hg(1)−O(1) 2.222(2) Hg(1)−O(5) 2.205(2) Hg(1)−O(2)#1 2.733(2) Hg(1)−O(6) 2.665(2) Hg(1)−O(2) 2.7779(2) Hg(1)−O(6)#2 2.855(2) Bond Angles (deg) N(1)−Hg(1)−O(5) 133.96(7) O(5)−Hg(1)−O(1) 89.57(6) N(1)−Hg(1)−O(1) 136.47(7) O(1)−Hg(1)−O(2) 51.40(6) O(1)−Hg(1)−O(2)#1 88.69(6) O(1)−Hg(1)−O(6)#2 128.13(6) O(1)−Hg(1)−O(6) 86.00(6) O(2)−Hg(1)−O(5) 136.39(6) O(2)−Hg(1)−O(2)#1 71.39(5) O(2)−Hg(1)−O(6)#2 168.97(5) O(2)−Hg(1)−O(6) 95.17(5) O(2)*−Hg(1)−O(6) 165.88(5) O(2)#1−Hg(1)−O(5) 91.77(6) O(5)−Hg(1)−O(6) 101.25(6) O(2)#1−Hg(1)−O(6)#2 119.19(5) O(6)−Hg(1)−O(6)#2 74.02(5) O(5)−Hg(1)−O(6)#2 50.38(6) O(2)#1−Hg(1)−N(1) 90.02(6) O(2)−Hg(1)−N(1) 87.24(6) O(6)#2−Hg(1)−N(1) 89.57(6) O(6)−Hg(1)−N(1) 84.88(6) 2 Bond Lengths (Å) Hg(01)−N(1) 2.204(7) Hg(01)−O(2)#2 2.656(9) Hg(01)−O(1) 2.258(6) Hg(01)−O(2) 2.728(9) Bond Angles (deg) N(1)−Hg(01)−O(1) 135.26(15) O(1)−Hg(01)−O(1)#1 89.5(2) N(1)−Hg(01)−O(2)#2 88.66(13) O(1)−Hg(01)−O(2)#2 86.7(2) O(1)−Hg(01)−O(2)#3 95.2(2) O(2)#2−Hg(01)−O(2)#3 177.32(18) O(1)−Hg(01)−O(2) 51.4(2) O(2)−Hg(01)−N(1) 86.37(12) O(2)−Hg(01)−O(1)#1 135.5(2) O(2)−Hg(01)−O(2)#2 78.2(3) O(2)−Hg(01)−O(2)#3 101.6(3) O(2)−Hg(01)−O(2)#1 172.74(17) a 1: #1 1 − x, 1 − y, 1 − z; #2 2 − x, 1 − y, 1 − z. 2: #1 x + 1, −y, −z + 1/2; #2 x, y + 1, z − 1/2; #3 −x + 1, −y + 1, −z + 1. numbers, and geometries and the concomitant variation of their The thermal decomposition temperature was recorded for photophysical properties. complexes 1−6, showing decomposition between 171 and 197 All of the six compounds were characterized by EA, FTIR- °C (see the Experimental Section). In addition, simultaneous ATR, 1H and 13C{1H}, and DEPT-135 NMR spectroscopies TG/DTA determination of 5 was carried out to confirm the and single crystal X-ray diffraction method. Compounds 1 and 2 presence of an occluded ethanol molecule in the powder sample were also characterized by HR-ESI+-MS in MeOH solution. In (Supporting Information: Figure S3). The measurement was addition, the thermal stability of complex 5 was studied via TG/ performed using 46.0 mg of sample. The complex starts to lose DTA determinations. Finally, the UV−vis and photolumines- the occluded ethanol molecule at 90 °C (weight loss: exptl 4.9%, cence spectra of 1−6 were recorded in MeOH solution and their calcd 5.6%) until 140 °C, followed up by decomposition at quantum yields calculated. The photophysical properties of 1−5 about 200 °C by the loss of the terpy ligand (weight loss: exptl were analyzed using TD-DFT calculations via 2D color mapping 29.1%, calcd 28.8%) and one Pip unit (weight loss: exptl 18.8%, of TDM supported by NTO analysis. calcd 20.3%). EA of all of the compounds agree with the proposal formula. In the FTIR-ATR spectra, the absence of bands between 2630 Besides, ESI+-MS of complexes 1−6 have been recorded. For and 2518 cm−1 attributable to hydrogen bonded ν(OH)HPip complexes 1 and 2, the weaker monodentate Hg−N and at 1667 cm−1 from ν(COOH) indicates that the HPip is coordination bond of 3-phpy (1) or 4-phpy (2) is broken deprotonated in the six complexes. The spectra of 1−6 display during the ESI+ fragmentation, resulting in the formation of the characteristic carboxylate bands in the range 1626−1530 fragments with m/z 156.0813 (100%) [3-phpy + H]+ and cm−1 for νas(COO) and 1436−1415 cm−1 for νs(COO) 156.0822 (100%) [4-phpy + H] + , respectively. As a (Supporting Information: Figures S4−S9). The difference consequence, the formation of {[Hg(Pip)2] + Na}+ species is between these bands (Δ = νas(COO) − νs(COO)) reveals the observed at m/z 554.9998 (100%, 1) and m/z 554.9990 (100%, coordination modes of the carboxylate linkers.45 Compounds 1, 2). Besides, {[Hg2(Pip)4] + Na}+ species are also identified at 2, and 4 display two different Δ values, 168 and 120, 179 and m/z 1087.0053 (77%, 1) and m/z 1087.0057 (71%, 2) 114, and 188 and 101 cm−1, suggesting bidentate bridged and (Supporting Information: Figure S1). On the other hand, for bidentate chelate (μ2-η2:η1) and chelate (μ1-η2) coordination complexes 3−6, the bidentate and tridentate dPy ligands are modes of the carboxylate groups. Compounds 3, 5, and 6 have a kept during ESI+ and a Pip linker is removed displaying values of Δ value of 112, 102, and 102 cm−1, indicating a bidentate chelate m/z 523.0570 (100%, 3), m/z 547.0576 (100%, 4), m/z (μ1-η2) coordination mode. All of these values are in line with 600.0837 (100%, 5), and m/z 566.0994 (100%, 6) correspond- the data from the X-ray analysis. Moreover, in 6, the peak ing to {[Hg(Pip)(dPy)]}+ fragments (Supporting Information: corresponding to the ν(NH) of the dpa is sharper compared Figure S2). to the free ligand, suggesting the N-amine coordination to the 3857 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Figure 1. 1D coordination polymers (a) 1 and (b) 2. In detail, the coordination environment around the Hg(II) node. Hydrogen atoms are omitted for clarity. Figure 2. Supramolecular associations between polymeric chains of (a) 1 and (b) 2 through π···π and C−H···O interactions. Hg···Hg distance is indicated as a dark solid double arrow line. Hydrogen atoms and negligible aromatic rings are omitted for clarity. Hg(II) center. Further bands assigned to ν(CC)/(CN), the carbon atom of the carboxylate group at 169.20 (1), 168.97 δ(CC)/(CN), δip(CH), and δoop(CH) from the (2), 169.01 (3), 169.16 (4), 169.38 (5), and 169.47 (6) ppm. aromatic rings have also been identified.46 The position of the pyridyl o-C atoms sorted in pairs of N- The 1H NMR spectra of complexes 1, 2, 4, and 5 were donors (148.89, 148.05 (1), and 150.26 (2)), N^N-donors recorded in CDCl3 solvent, while those of 3 and 6 were recorded (149.73 (3) and 150.37 (4)), and N^N^N-donors (150.65 (5) in dmso-d 6 due to their lower solubility (Supporting and 149.42 ppm (6)) enable us to better understand the electron Information: Figures S10−S15). The spectra of all of the donating ability of those ligands.47 These differences with the complexes show three signals assigned to the aromatic protons consequent upfield shift of those complexes containing 4-phpy of the Pip ligand between 7.77 and 6.71 ppm and to the −CH2− (2), 1,10-phen (4), and terpy (5) are essentially caused either by between 6.07 and 5.94 ppm. The bands of the corresponding resonance stabilization, inductive effects (−I), or both. In the dPy are located between 9.43 and 7.37 ppm, with the chemical case of 3-phpy (1) and 4-phpy (2), the better resonance shifts of o-Hpy being consistent with the presence of N- coordinated ligands (8.91 (1), 8.76 (2), 8.88 (3), 9.43 (4), 9.23 stabilization, the more −I effect (with the pKa value of the (5), and 8.75 (6) ppm).46 conjugate acid being 4.8 and 5.5,48 respectively), and the less The 13C{1H} NMR spectra of complexes 1−6 have been steric hindrance confer to 4-Phpy a better donor character. recorded in dmso-d6 solution as well as DEPT-135 experiments, Regarding the N^N-donors, the additional phenyl ring of 1,10- which were performed only for those complexes required to phen (4) with respect to 2,2′-bipy (3) results in a −I effect. ensure the correct assignation of the carbon atoms from the Finally, the pyridyl substituents have a −I effect in terpy (5) aromatic rings (Supporting Information: Figures S16−S21). compared to that of the −CH2− substituents in dpa (6), which is The spectra of all of the complexes display the band assignable to reflected in the upfield shift. 3858 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Structural Description. Crystal and Extended Structure prismatic geometry composed of three bidentate chelate ligands of Compounds 1 and 2. They belong to the triclinic P̅ 1 (1) and (μ1-η2), two Pip, and one 2,2′-bipy (Figure 3a). The Hg−N monoclinic C2/c (2) space groups, with both being linear bond lengths are slightly shorter compared to other similar coordination polymers which expand through the a or c axis, Hg(II) complexes with 2,2′-bipy.50,51 respectively. All of the distances of less than the sum of the van These monomeric units are held together forming chains der Waals radii have been considered to define the coordination along the b axis via double C−H···O interactions between the m- sphere49 (Table 3). The Hg(II) centers have a [HgO6N] core H of the 2,2′-bipy ligand and a carboxylate O atom of each Pip with a distorted capped octahedral geometry composed of two ligand (3.342(3) Å). These Pip ligands stack by π···π Pip and either one 3-phpy (1) or 4-phpy (2) ligand. The Pip interactions (Cg(1)···Cg(1)′ 3.530 Å) and a double C−H···O ligands define the formation of the edge-sharing polymeric array association between one hydrogen atom of each dioxole group acting at the same time as bidentate bridged and chelate (μ2- and the remaining carboxylate oxygen atom, which expands the η2:η1) linkers (Figure 1). Both have Hg−N bond lengths within crystal packing into 2D layers along the (1̅ 0 1) plane. Finally, a the range of other reported coordination polymers.23 π···π interaction between the 2,2′-bipy (Cg(2)···Cg(2)′ 3.886 With the 4-Phpy being less bulky than the 3-Phpy ligand, it Å) and a C−H···O interaction between the p-H of the 2,2′-bipy facilitates a closer packing of the structure, which is reflected in and the remaining carboxylate O atom (3.288(3) Å) define the the shortening of the Hg···Hg distance in 2 (4.1780(1) Å) with 3D supramolecular net (Figure 3b, Table 6). respect to 1 (4.4091(3) and 4.4763(3) Å). In 1, the polymeric Crystal and Extended Structure of Compound 4a. It chains are stacked throughout the b axis by weak π···π belongs to the monoclinic C2/c space group. It has a dimeric interactions (Figure 2a) between the Pip rings (Cg(1)··· array in which each Hg(II) has a [HgO5N2] core (Figure 4a), Cg(1)′ 3.761 Å), while, in 2, the Pip rings are too far composed of two Pip ligands and one 1,10-phen linker in a (Cg(2)···Cg(2)′ 4.679 Å) as well as the phenyl groups of the 4- distorted square antiprism geometry with bond angles ranging Phpy ligands (Cg(3)···Cg(3)′ 4.274 Å) to be considered as an between 54.76(8) and 140.16(10)° (Table 7). One Pip unit and effective π···π interaction (Figure 2b). Additional intermolecular one 1,10-phen unit have a bidentate chelate (μ1-η2) coordina- C−H···O interactions support the final crystal packing (Table tion mode, occupying four coordination positions, while the 4). remaining Pip linker joins the two Hg(II) centers through both bidentate chelate and bridged (μ2-η2:η1) coordination modes. In Table 4. Intermolecular Interactions of 1 and 2a addition, there is an occluded fluorobenzene (Fbz) molecule (Figure 4b). The Hg−N bond lengths are in range with similar 1 H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) reported complexes.50 C16−H16A···O5 2.631 3.581(3) 0.990 160.97 The 1,10-phen ligands act as hydrogen bond donors through C16−H16B···O8 2.565 3.263(3) 0.990 127.43 double and single C−H···O interactions with the Pip linkers C8−H8B···O1 2.549 3.485(3) 0.990 157.67 (Table 8). The double interaction occurs via both the p-H and C4−H4···O7 2.547 3.254(3) 0.951 131.29 the contiguous H from the phenyl ring with the same O atom C12−H12···O4 2.698 3.571(3) 0.950 153.03 from the carboxylate (Figure 5a) and arranges the dimeric units C24−H24···O8 2.619 3.521(3) 0.950 158.82 into 2D layers along the bc plane. C17−H17···O3 2.639 3.289(3) 0.950 126.02 The occluded Fbz molecules are π···π stacked (Cg(2)··· π···π Interactions Cg(2)′, 1.424 Å) in pairs and joined to the dimeric units by π···π Cg(1)···Cg(1) 3.761 Å 71.14° with the Pip rings (Cg(1)···Cg(2), 3.918 Å) and a C−H···F 2 H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) interaction (Table 8, Figure 5b). Besides, there is an additional C8−H8A···O1 2.324 3.31(2) 0.99 173.22 π···π (1,10-phen···Pip (Cg(3)···Cg(4), 3.523 Å)), cooperating π···π Interactions in the formation of these layers (Figure 5c). The remaining m-H Cg(2)···Cg(2) 4.679 Å 50.36° from the opposite pyridyl ring interacts with one dioxole O atom Cg(3)···Cg(3) 4.274 Å 77.41° along the [101̅] direction, supported by reciprocal π···π a 1: Cg(1) = C10−C11−C12−C13−C14−C15. 2: Cg(2) = C2−C3− interactions between pairs of 1,10-phen aromatic rings C4−C5−C6−C7; Cg(3) = C12−C13−C14−C15−C3−C12. (Cg(5)···Cg(6), 3.603 Å) which drive the formation of the 3D net (Figure 6). Crystal and Extended Structure of Compound 3. It belongs Crystal and Extended Structure of Compound 5. It belongs to the monoclinic P2/c space group. It is a Hg(II) monomer to the monoclinic C2/c space group. It is a Hg(II) monomer with a [HgO4N2] core (Table 5) and a distorted trigonal with a [HgO4N3] core (Figure 7a) and a distorted capped Table 5. Selected Bond Lengths (Å) and Bond Angles (deg) for 3a bond lengths (Å) Hg(1)−O(1)#1 2.6281(16) Hg(1)−N(1) 2.3096(17) Hg(1)−O(2) 2.2527(16) bond angles (deg) O(2)#1−Hg(1)−O(2) 99.04(8) O(2)−Hg(1)−O(1)#1 91.50(5) O(2)#1−Hg(1)−N(1) 120.64(6) N(1)−Hg(1)−O(1)#1 144.76(6) O(2)−Hg(1)−N(1) 122.68(6) N(1)−Hg(1)−O(1) 83.61(5) N(1)−Hg(1)−N(1)#1 72.12(8) O(1)#1−Hg(1)−O(1) 128.26(7) O(2)#1−Hg(1)−O(1)#1 53.66(5) a 3: #1 −x, y, −z + 1/2. 3859 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Figure 3. (a) Molecular structure highlighting the coordination environment of the Hg(II) ion and (b) C−H···O and π···π intermolecular interactions in 3. Hydrogen atoms are omitted for clarity. Table 6. Intermolecular Interactions of 3a H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) C11−H11···O2 2.487 3.288(3) 0.950 141.95 C12−H12···O2 2.465 3.342(3) 0.950 153.63 C5−H5AB···O1 2.470 3.335(3) 0.990 145.62 π···π Interactions Cg(1)···Cg(1)′ 3.530 Å 92.13° Cg(2)···Cg(2)′ 3.886 Å 69.04° a Cg(1) = Cg(1)′: C2−C3−C4−C6−C7−C8. Cg(2) = Cg(2)′: N1−C9−C10−C11−C12−C13. Figure 4. (a) Coordination environment of the Hg(II) ions and (b) molecular structure of 4. Hydrogen atoms are omitted for clarity. octahedral geometry composed of two asymmetrically bidentate terpy···Pip) (Table 10). Both m-H of the lateral pyridyl rings chelate Pip (μ1-η2) and a tridentate chelate terpy (μ1-η3) ligand associate with one carboxylate O atom, while the Pip ligands (Figure 7b) with Hg−O and Hg−N bond lengths within the interact between them via the m-H and the remaining range of similar complexes52,53 (Table 9). carboxylate O atom. In addition, the EtOH molecule is hydrogen The terpy ligands arrange the monomeric units into 1D chains bonded to the same carboxylate O atom as the m-H (Figure 8). through reciprocal π···π stacking (Cg(1)···Cg(2), 3.601 Å) Crystal and Extended Structure of Compound 6a. It along the [1̅01] direction. These chains are joined through the b belongs to the triclinic P̅ 1 space group, having two crystallo- axis by two different C−H···O interactions (terpy···terpy and graphically independent monomeric units (A and B) and sharing 3860 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 7. Selected Bond Lengths (Å) and Bond Angles (deg) for 4aa bond lengths (Å) Hg(1)−O(1) 2.381(2) Hg(1)−N(1) 2.344(3) Hg(1)−O(2) 2.440(2) Hg(1)−N(2) 2.420(3) Hg(1)−O(2) 2.942(3) Hg(1)−O(5) 2.176(2) Hg(1)−O(6) 2.866(2) Hg(1)−O(6)′ 2.796(6) bond angles (deg) O(5)−Hg(1)−N(1) 127.28(10) O(6)′−Hg(1)−O(2) 116.64(17) O(5)−Hg(1)−O(1) 137.79(9) O(1)−Hg(1)−N(2) 90.58(8) N(1)−Hg(1)−O(1) 93.94(9) O(5)−Hg(1)−O(2) 126.41(9) O(5)−Hg(1)−N(2) 94.79(10) N(1)−Hg(1)−O(2) 87.35(9) N(1)−Hg(1)−N(2) 70.65(9) O(1)−Hg(1)−O(2) 54.76(8) O(1)−Hg(1)−O(2) 127.04(8) N(2)−Hg(1)−O(2) 138.04(8) N(1)−Hg(1)−O(2) 76.28(8) O(2)−Hg(1)−O(2) 76.68(7) O(5)−Hg(1)−O(2) 77.72(8) N(2)−Hg(1)−O(2) 131.20(8) O(6)−Hg(1)−O(2) 103.66(17) O(6)−Hg(1)−O(2) 95.95(14) O(6)−Hg(1)−O(1) 88.85(14) O(6)−Hg(1)−O(5) 49.64(15) O(6)−Hg(1)−N(2) 107.30(17) O(6)−Hg(1)−N(1) 176.52(14) O(2)−Hg(1)−O(2) 72.68(7) N(1)−Hg(1)−O(6)′ 161.07(17) N(2)−Hg(1)−O(6)′ 90.61(18) O(2)−Hg(1)−O(6)′ 109.24(15) O(6)′−Hg(1)−O(5) 49.50(16) O(1)−Hg(1)−O(6)′ 88.70(15) a O(6)′ defines a disordered carboxylate oxygen atom. Table 8. Intermolecular Interactions of 4aa H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) C21−H21···O6 2.495 3.278(7) 0.950 139.75 C19−H19···O6 2.445 3.233(7) 0.950 140.29 C25−H25···O8 2.47 3.34(2) 0.950 152.11 C26−H26···F1W 2.398 3.291(7) 0.950 156.48 π···π Interactions Cg(1)···Cg(1)′ 4.089 Å 95.30° Cg(3)···Cg(4) 3.523 Å 94.44° Cg(1)···Cg(2) 3.918 Å 104.68° Cg(5)···Cg(6) 3.603 Å 96.21° Cg(2)···Cg(2)′ 1.424 Å 30.62° a Cg(1) = Cg(1)′: C11−C12−C14−C15−C16−C17. Cg(2) = Cg(2)′: C1W−C2W−C3W−C4W−C5W−C6W. Cg(3): N2−C23−C24−C25− C26−C27. Cg(4): C2−C3−C4−C6−C7−C8. Cg(5): N2−C23−C24−C25−C26−C27. Cg(6): C20−C21−C22−C23−C27−C28. Figure 5. Representation of the double C−H···O interaction in 4 and highlighting of the π···π between Pip···Pip (a, light blue), Fbz and Pip (b, orange), and 1,10-phen···Pip (c, green) aromatic rings. Hydrogen atoms not involved in the intermolecular interactions are omitted for clarity. the same [HgO4N3] core (Table 11), both presenting distorted and one mer-tridentate chelate dpa (μ1-η3) ligand.54 The capped octahedral geometries (Figure 9a). The monomeric twisting of the pyridyl rings (angle between rings, 22.11°) is units are composed of two bidentate chelate Pip (μ1-η2) linkers favored by the free rotation of the −CH2− (Figure 9b). 3861 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Figure 6. Representation of the C−H···O interaction in 4 and the π···π interaction in pairs between 1,10-phen rings highlighted in light blue. Hydrogen atoms not involved in the intermolecular interactions are omitted for clarity. Figure 7. (a) Coordination environment of the Hg(II) ions and (b) molecular structure of 5. Hydrogen atoms are omitted for clarity. Table 9. Selected Bond Lengths (Å) and Bond Angles (deg) for 5a bond lengths (Å) Hg(1)−O(1)#1 2.2819(11) Hg(1)−N(1)#1 2.3995(12) Hg(1)−O(2) 2.7229(15) Hg(1)−N(2) 2.3620(16) bond angles (deg) O(1)−Hg(1)−O(1)#1 93.40(4) O(1)−Hg(1)−O(2) 51.83(4) O(1)−Hg(1)−N(2) 133.30(3) O(1)−Hg(1)−N(1)#1 84.34(4) N(2)−Hg(1)−N(1) 68.89(3) O(1)−Hg(1)−N(1) 126.34(4) O(1)#1−Hg(1)−O(2) 126.00(4) N(1)#1−Hg(1)−N(1) 137.79(4) N(1)#1−Hg(1)−O(2) 93.68(4) O(2)#1−Hg(1)−N(1)#1 87.23(4) N(2)−Hg(1)−O(2) 91.26(2) O(2)−Hg(1)−O(2)#1 177.47(3) a 5: #1 1 − x, y, 3/2 − z. Both monomers are associated by reciprocal N−H···O atom. B monomers are stacked in pairs between themselves interactions between the amino group and a carboxylate O through reciprocal π···π interactions promoted by the dpa 3862 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 10. Intermolecular Interactions of 5a define free rotation ligands. For this reason, we have defined a straightforward angle measurement (outer atoms angle) that was H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) done considering the planarity of the linkers and, therefore, O1W−H10···O2 2.025 2.775(3) 0.840 148.14 assuming that the steric effects are predominantly generated by C12−H12···O2 2.348 3.286(2) 0.950 169.36 two hydrogen outer atoms placed at the sides of the dPy ligands, C7−H7···O1 2.369 3.293(2) 0.950 163.99 especially remarkable for those having a chelate effect and free π···π Interactions rotation restrictions. All of the angles formed by two outer Cg(1)···Cg(2) 3.601 Å 74.99° hydrogen atoms of each ligand and its metal center have been a Cg(1): N1−C9−C10−C11−C12−C13. Cg(2): N2−C14−C15− examined from the crystallographic data using Mercury software C16−C15−C14. version 4.3.1,30−32 and only the bigger ones are listed in Table 13. ligands, while the corresponding aromatic rings in A are too far This angle permitted a reasonable comparison of the steric for an effective π···π stacking (B−B = Cg(3)···Cg(4), 3.751 Å; effects of the dPy ligands into the final complexes and explains A−A = Cg(1)···Cg(2), 4.203 Å). the different nuclearity of the complexes containing the The occluded water molecule acts simultaneously as an monodentate linkers 3-phpy and 4-phpy having smaller angles acceptor with a MeOH molecule and as a donor with the (50° less than chelate). Besides, the 1,10-phen linker being carboxylate O atom through O−H···O interactions (Figure 10). almost 5° less bulky than 2,2′-bipy promotes the formation of a Both combined with a π···π stacking (Cg(4)···Cg(5), 3.784 Å) dimeric structure instead of the monomeric array promoted by between one pyridyl ring, and the aromatic ring of a carboxylate the ligands having bigger angles (2,2′-bipy, terpy, and dpa). linker (Figure 11a) associate B monomers along the a axis. A and Photophysical Properties. UV−vis Spectroscopy. All of B monomers are joined through a C−H···O interaction between the samples were dissolved in MeOH, and their UV−vis spectra the −CH2− (A) and a carboxylate O atom (B) (Table 12, Figure were recorded at 298 K. Since π···π interactions are important 11b). noncovalent intermolecular forces and contribute to self- Structural Comparison and Steric Effects. Bidentate N^N′- assembly processes,59,60 additive UV−vis measurements were donor (2,2′-bipy and 1,10-phen) and tridentate N^N^N-donor performed within a concentration range from ∼1 × 10−9 to 1 × (terpy and dpa) ligands formed five-membered rings after 10−4 M to ensure the nonaggregation of the ligands and chelation with the Hg(II) centers. Since Hg(II) has a big van der complexes 1−6 at the selected concentration for the photo- Waals radius (1.7−2.0 Å)55 allowing high coordination numbers luminescence experiments and avoid the aggregation-caused (6 and 7), the steric effects of dPy ligands guide the formation of emission quenching (ACQ) effect61 (Supporting Information: complexes with diverse structural arrangements, varying from Figures S22 and S23). Complexes start to aggregate at 7.67 × monomers to coordination polymers. 10−6 M (1), 8.41 × 10−6 M (2), 1.89 × 10−7 M (4), 4.44 × 10−7 Bite angle (∠N−M−N) measurements were an initial tool to M (5), and 1.63 × 10−5 M (6). Within this tendency, complex 3 evaluate steric effects of chelate ligands, but as only the donor which presented weaker interactions seems to avoid aggregation atoms are considered, the bulkiness of the ligand tends to be effects at higher concentrations. Those which exhibited stronger underestimated. Further steric parameters, such as the inter alia interactions in the solid state identified in the structural analysis tolman angle,56 exact ligand cone angle,57 and solid angle,58 better tend to easily aggregate in solution and promote a bathochromic Figure 8. Representation of the C−H···O interactions in 5 and the π···π interaction between terpy rings highlighted in light blue. Hydrogen atoms not involved in the intermolecular interactions are omitted for clarity. 3863 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 11. Selected Bond Lengths (Å) and Bond Angles (deg) for 6a bond lengths (Å) A Hg(1A)−O(1A) 2.2216(11) Hg(1A)−N(1A) 2.4579(13) Hg(1A)−O(5A) 2.3068(11) Hg(1A)−O(6A) 2.9420(10) Hg(1A)−N(3A) 2.3597(12) Hg(1A)−O(2A) 2.9530(10) Hg(1A)−N(2A) 2.3620(13) B Hg(1B)−O(1B) 2.2483(10) Hg(1B)−N(3B) 2.4243(12) Hg(1B)−O(5B) 2.2622(11) Hg(1B)−O(2B) 3.0310(10) Hg(1B)−N(1B) 2.3232(12) Hg(1B)−O(6B) 2.9070(10) Hg(1B)−N(2B) 2.4110(12) bond angles (deg) A O(6A)−Hg(1A)−O(5A) 48.40(4) O(1A)−Hg(1A)−N(1A) 80.33(4) O(6A)−Hg(1A)−N(1A) 70.93(4) O(1A)−Hg(1A)−N(2A) 139.75(4) O(6A)−Hg(1A)−N(2A) 86.12(4) O(1A)−Hg(1A)−N(3A) 125.63(4) O(6A)−Hg(1A)−N(3A) 116.98(4) O(1A)−Hg(1A)−O(2A) 48.51(4) O(6A)−Hg(1A)−O(1A) 109.80(4) O(2A)−Hg(1A)−N(1A) 120.15(4) O(6A)−Hg(1A)−O(2A) 145.44(4) O(2A)−Hg(1A)−N(2A) 128.21(4) O(5A)−Hg(1A)−N(1A) 114.71(4) O(2A)−Hg(1A)−N(3A) 77.12(4) O(5A)−Hg(1A)−N(2A) 119.31(4) N(1A)−Hg(1A)−N(2A) 70.14(4) O(5A)−Hg(1A)−N(3A) 93.72(4) N(1A)−Hg(1A)−N(3A) 139.70(4) O(5A)−Hg(1A)−O(1A) 97.42(4) N(2A)−Hg(1A)−N(3A) 71.14(4) O(5A)−Hg(1A)−O(2A) 102.30(4) B O(6B)−Hg(1B)−O(5B) 48.94(4) O(1B)−Hg(1B)−N(1B) 122.81(4) O(6B)−Hg(1B)−N(1B) 77.04(4) O(1B)−Hg(1B)−N(2B) 133.64(4) O(6B)−Hg(1B)−N(2B) 77.62(4) O(1B)−Hg(1B)−N(3B) 81.33(4) O(6B)−Hg(1B)−N(3B) 99.83(4) O(1B)−Hg(1B)−O(2B) 46.94(4) O(6B)−Hg(1B)−O(1B) 144.87(4) O(2B)−Hg(1B)−N(1B) 78.97(4) O(6B)−Hg(1B)−O(2B) 125.46(4) O(2B)−Hg(1B)−N(2B) 137.24(4) O(5B)−Hg(1B)−N(1B) 104.28(4) O(2B)−Hg(1B)−N(3B) 127.79(4) O(5B)−Hg(1B)−N(2B) 124.46(4) N(1B)−Hg(1B)−N(2B) 71.84(4) O(5B)−Hg(1B)−N(3B) 101.08(4) N(1B)−Hg(1B)−N(3B) 142.06(4) O(5B)−Hg(1B)−O(1B) 96.14(4) N(2B)−Hg(1B)−N(3B) 70.60(4) O(5B)−Hg(1B)−O(2B) 93.31(4) Figure 9. (a) Coordination environment of the Hg(II) ions on the two crystallographically independent monomers and (b) molecular structure of 6a. Hydrogen atoms except those from the −OH and H2O molecules are omitted for clarity. 3864 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Figure 10. Representation of the N−H···O interactions in 6a and the π···π interaction between dpa rings highlighted in light blue and light green. Hydrogen atoms not involved in the intermolecular interactions are omitted for clarity. Figure 11. Representation of (a) O−H···O and the π···π interaction between dpa rings in 6a highlighted in light blue. (b) C−H···O interactions between Pip ligands. Hydrogen atoms not involved in the intermolecular interactions are omitted for clarity. shift emphasized at higher wavelength absorption peaks.62 concentration extracted from the UV−vis data. The emission Especially, complexes 5 and 6 exhibited bathochromic spectra of complexes 1−6 show that the emission intensity aggregation effects reflected by the shift of absorption bands at increases in that order: 2 < 4 < 3 < 1 < 6 < 5 (Figure 12). The 333 and 302 nm, respectively. Likewise, after aggregation of 1, a spectrum of 3 has an unfolded emission band centered at 340 band at 293 nm emerged. Hereafter, the UV−vis measurements and 353 nm, while the rest of the complexes have only one of complexes 1−6 have been done using a concentration around predominant band centered between 316 and 356 nm. The ∼1.00 × 10−7 M (Supporting Information: Figure S24). The resultant emission color (λmax‑em) for complexes 1−6 at the UV−vis spectra of the ligands and L-tyrosine (L-tyr) have been selected λexc is blue, as displayed in CIE1931 chromaticity recorded using the same concentration (Supporting Informa- diagrams.63 The effect on the emission spectrum after tion: Table S1). The absorption maxima (λmax) of the complexes coordination of the ligands to the Hg(II) center has been and the ligands have been identified and their molar absorptivity analyzed by comparing the emission of the ligands with the values (ε) calculated (Table 14). corresponding complex being sorted in groups of three including Photoluminescence. All of the measurements were per- the pair of ligands (Pip + dPy) with their corresponding complex formed at 298 K, in MeOH as solvent, and using the (1−6), all being excited at the emission maxima of the 3865 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Table 12. Intermolecular Interactions of 6aa comparison with a reference (standard), the relative quantum yield (ϕs) of the selected product can be obtained.65 H···A D−H >D−H···A The quantum yield of 1−6 is calculated using eq 1 ij OD yzij I yzij n yz (Å) D···A (Å) (Å) (deg) ϕs = ϕref jjj ref zzzjjj s zzzjjj s zzz j ODs zj Iref zj nref z N2A−H2A···O2A 1.948 2.870(2) 1.000 152.02 2 k {k {k { N2B−H2B···O2B 2.333 3.126(2) 1.000 135.56 O2W−H2WA···O6B 1.92(2) 2.721(2) 0.81(2) 170.47 (1) O2W−H2WB···O2B 1.92(2) 2.725(2) 0.81(2) 170.03 O1W−H1W···O2W 1.979 2.797(2) 0.840 164.40 where ϕ is the quantum yield, OD is the optical density (or C20−H20C···O1A 2.250 3.200(2) 0.990 160.41 absorbance) at the excited wavelength, I is the area under the π···π Interactions curve of the emission spectra, and n is the refractive index of the A solvent. In this study, L-tyrosine has been used as the standard Cg(1)···Cg(2) 4.203 Å 69.10° (φref = 0.14)66 and ODref and Iref values have been obtained from B a solution at a concentration of 1.01 × 10−4 M in Milli-Q water Cg(3)···Cg(4) 3.751 Å 73.54° as solvent (nref = 1.3325)67 at 298 K. The values of As and Is of Cg(4)···Cg(5) 3.784 Å 65.44° HPip and dPy ligands as well as those of 1−6 have been a A = Cg(1): N1A−C1A−C2A−C3A−C4A−C5A; Cg(2): N3A− measured from solutions of 9.95 × 10−7 M (HPip) and ∼6.16 × C8A−C9A−C10A−C11A−C12A. B = Cg(3): N1B−C1B−C2B− 10−6 M (dPy and complexes 1−6) in MeOH as solvent (ns = C3B−C4B−C5B; Cg(4): N3B−C8B−C9B−C10B−C11B−C12B; 1.3314)68 at 298 K. The values of ϕs obtained for compounds Cg(5): C14B−C15B−C16B−C17B−C18B−C19B. 1−6 are 0.027 (1), 0.011 (2), 0.968 (3), 0.051 (4), 0.370 (5), and 0.047 (6) (Table 14). Table 13. Structural Parameters Regarding the Steric Effects Comparing the ϕs values sorted by the number of dPy N- of the Ligands donor sites, 1 has a slightly higher efficiency than 2 but is constant in the same order of magnitude. In the complexes with sample chelate angle (Pip) bite angle (dPy) outer atom anglea chelating ligands (3−6), even if chelation enhanced fluores- 1 50.38; 51.40 92.40° cence (CHEF) competes with steric crowding, the bigger size of 2 51.41 78.84° the Hg(II) center enables geometrical distortions without severe 3 53.66 72.12 146.25° alterations of the Hg−N bond lengths.69 In this sense, the non- 4a 54.76; 49.6 70.65 141.28° negligible better ϕs of complex 5 with respect to 6 is probably 5 51.83 68.89; 137.78 210.68° caused by the greater symmetry and rigidity of the terpy linker 6a 48.94; 46.94 70.60; 71.85; 142.06 221.68° compared to dpa, in which the aliphatic −(CH2)− groups a The angles are calculated between the outer atoms of the dPy ligand allowed severe bond length distortion of the [HgO4N3] core and, and the Hg(II) center using Mercury software.30−32 thus, facilitated a nonradiative decay. Finally, the largest difference in ϕs is reflected in complexes 3 and 4, with 3 having Table 14. UV−vis and Fluorescence Data of Complexes 1−6a a significantly larger value. Despite that the 1,10-phen ligand has sample λmax‑Abs (log(ε)) λex λmax‑em ϕS more rigidity than 2,2′-bipy, it is known that it has close-lying ππ* and nπ* singlet excited states, with nπ* excited states 1 212 (4.52); 258 (4.43); 230 346 0.03 278 (4.13); 293 (4.19)b promoting its nonradiative decay and consequently vanishingly 2 205 (4.78); 259 (4.50); 293 (4.22) 250 348 0.01 low emission ϕs.70 On the other hand, the 2,2′-bipy ligand 3 203 (4.69); 215 (4.66); 233 340; 353 0.97 exhibited ππ* relaxation in polar solvents as MeOH.71 This is 244 (4.18); 260 (4.25); 287 (4.29) maintained after complexation with d10 metal ions, which do not 4 206 (4.78); 216 (5.03);b 230 345 0.05 have low-lying MLCT electronic levels and, thus, keep ligand 266 (5.05); 294 (4.93); 326 (3.68)b centered transitions. Only 1,10-phen functionalization, espe- 5 205 (4.85); 253 (4.28); 269 356 0.37 cially in 2,9-positions, led to significantly better ϕs values.70 282 (4.30); 298 (4.15);b 322 (4.17)b Electronic Calculations. All of the calculated UV−vis spectra 6 202 (4.17); 238 (4.03); 250 316 0.05 of complexes 1−5 agree reasonably well with the experimental 281 (3.88); 307 (3.99)b profiles. The shift in the theoretical absorption spectra with a All of the wavelengths are given in nm. ε values are given in M−1· respect to the experimental spectra lies within the range of cm−1. λmax‑Abs = maximum of absorption. λex = excitation wavelength; typical TD-DFT calculations (∼0.3 eV) and is caused by λmax‑em = maximum of emission; ϕS = quantum yield. bBands arising computing absorptions as vertical transitions.72 Only transitions from aggregation effects. with higher f values have been selected for the 2D color filled mapping of TDM and NTO analysis. The HOMO and LUMO complexes. These spectra are depicted in Figure S25 of the outline as well as energy band gaps can be found in Figure S26 in Supporting Information and show the quenching or enhance- the Supporting Information. Subsequently, the main contributor ment effect after complexation of the ligands. Complex 6 is the transitions, being either Hg, Pip, dPy, or a combination of them, only keeping the emission intensity (Supporting Information: have been analyzed for each absorption band to identify both the Figure S25d). On the other hand, complexes 3 and 5 exhibit an character and the regions involved in the electronic transition. emission enhancement effect (Supporting Information: Figures Each set of transitions has been represented as a 2D color filled S25c and e), while complexes 1, 2, and 4 display a quenching map of the TDM, and these results were confirmed by NTO effect (Supporting Information: Figures S25a, b, and d). analysis. The fluorescence quantum yield (ϕ) is defined as the ratio The HOMO and LUMO orbitals of 1−5 have π symmetry between the number of photons emitted and the number of with the HOMO being distributed among the Pip ligand and the photons absorbed and describes how efficient the sample is in LUMO being uniformly localized over the dPy linkers. It has converting the excitation light into photon emission.64 By been previously demonstrated for Pt complexes bearing N,N- 3866 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article Figure 12. Photoluminescence spectra of complexes 1−6 in MeOH solution excited at their corresponding emission maxima. Figure 13. (a) Molecular structure and atom labeling of compound 3. (b) Experimental and simulated UV−vis spectra of 3. 2D color filled mapping of TDM for transitions 3 (c) and 33 (d), in detail metal core region around the Hg(II) center (label 1). and N,N,N-donor ligands that this MO seclusion allowed the and Hg(II) contributions. Figure 13 shows the 2D color filled HOMO and LUMO energies to be separately modified by map of the TDM for transitions 3 (Figure 13c) and 33 (Figure functionalization, and thus, introduction of substituents on the 13d), which are representative of ligand centered (LC) and dPy or Pip ligands could selectively tune the band gap.73,74 Hg(II) involved transitions. Focusing on the diagonal of Analysis of complex 3 has been provided in Figure 13 to transition 3 centered at 257 nm (Figure13c), the region around exemplify the electronic analysis. The optimized molecular the atom labeled 43 stands out from the rest, indicating where geometry of 3 with the corresponding atom labeling scheme is the electronic transition occurs, which corresponds to the Pip displayed in Figure 13a. The calculated UV−vis spectrum ligand, in particular to the carboxylate and the subsequent matches with the experimental results (Figure 13b), and the carbons from the aromatic ring, congruent with the NTO primary significant transitions have been split into the ligands analysis results (Supporting Information: Figure S34). Both the 3867 https://dx.doi.org/10.1021/acs.inorgchem.0c03640 Inorg. Chem. 2021, 60, 3851−3870 Inorganic Chemistry pubs.acs.org/IC Article LMCT and MLCT transitions are predominantly responsible well as coordinates from the DFT geometry optimizations for the higher energetic transitions being located around 200 and TD-DFT calculation results (PDF) nm. At lower energies, the more red-shifted bands in the spectra arise from Pip in complexes 1, 2, and 3, while, in those Accession Codes containing more pi electron rich ligands (1,10-phen and terpy), CCDC 2043890−2043895 contain the supplementary crystallo- dPy are responsible for such transitions (4 and 5). Regarding graphic data for this paper. These data can be obtained free of metal centered transitions, these results display that MLCT charge via www.ccdc.cam.ac.uk/data_request/cif, or by email- transitions rise from Hg → Pip in complexes 1−4 (identified ing data_request@ccdc.cam.ac.uk, or by contacting The Cam- between 203 and 212 nm), while in 5 this contribution is not bridge Crystallographic Data Centre, 12 Union Road, Cam- bridge CB2 1EZ, UK; fax: +44 1223 336033. ■ observed. The role of dPy on MLCT transitions for 1−4 is neglectable, since dPy are not involved in them in complexes 1 AUTHOR INFORMATION and 2 and only minor contributions from 2,2′-bipy and 1,10- phen can be observed in 3 and 4. What should be noted is the Corresponding Author different behavior displayed in complex 5, in which the terpy Josefina Pons − Departament de Química, Universitat ligand facilitates the MLCT to be from Hg → terpy, causing a Autònoma de Barcelona, 08193 Barcelona, Spain; remarkable bathochromic shift on absorption (253 nm). This orcid.org/0000-0002-9546-6828; Email: Josefina.Pons@ difference is probably originated from the better π-acceptor uab.es character of the terpy ligand with respect to the remaining dPy. Authors The Supporting Information displays the complete electronic Francisco Sánchez-Férez − Departament de Química, analysis of the remaining complexes with additional 2D plots Universitat Autònoma de Barcelona, 08193 Barcelona, Spain (Supporting Information: Figures S27−S31 and Table S2) and Joaquim Ma Rius-Bartra − Departament de Química, NTO analysis (Supporting Information: Figures S32−S36). ■ Universitat Autònoma de Barcelona, 08193 Barcelona, Spain; orcid.org/0000-0003-2872-0539 CONCLUSIONS Teresa Calvet − Departament de Mineralogia, Petrologia i We synthesized and characterized a series of Hg(II) Geologia Aplicada, Universitat de Barcelona, 08028 coordination complexes bearing HPip and N-, N^N-, and Barcelona, Spain N^N^N-donor ligands, revealing that the use of the Hg(II) Mercè Font-Bardia − Unitat de Difracció de Raig-X, Centres metal node allowed the primacy of the geometric demands of the Científics i Tecnològics de la Universitat de Barcelona linkers with coordination numbers 6 and 7 to be accommodated. (CCiTUB), Universitat de Barcelona, 08028 Barcelona, Spain Despite that the Pip linker demonstrated its ability to form Complete contact information is available at: polymeric structures by μ2-η2:η1 coordination modes (1 and 2), https://pubs.acs.org/10.1021/acs.inorgchem.0c03640 both the increasing bulkiness of the dPy ligands and their chelate effect afforded the predominance of the dPy over the Pip ligand Notes (3−6) and drove the formation of discrete structures being two The authors declare no competing financial interest. monomers and a dimer. A comparison of the steric effects evinced that a slight increase in the bulkiness of dPy is sufficient to promote the formation of a monomer instead of a dimer, ■ ACKNOWLEDGMENTS J.P. acknowledges financial support from the CB615921 project, along with bulkier ligands inter alia terpy and dpa, which the CB616406 project from “Fundació La Caixa”, and the unavoidably formed monomers. These complexes were 2017SGR1687 project from the Generalitat de Catalunya. F.S.- associated by weak C−H···O and π···π interactions, which is F. acknowledges the PIF predoctoral fellowship from the reflected in the aggregation patterns, except for complex 3 that Universitat Autònoma de Barcelona. ■ avoided aggregation up to ∼1 × 10−4 M and presented less and weaker interactions. Furthermore, aggregation drove significant REFERENCES shifts on absorption spectra, especially remarkable for 5 and 6. (1) Schmidt, G. M. J. Photodimerization in the Solid State. Pure Appl. The experimental and theoretical photophysical study revealed Chem. 1971, 27, 647−678. that 1−5 have a localized HOMO on Pip while the LUMO is (2) Maginn, S. J. Crystal Engineering: The Design of Organic Solids over the dPy emerging as appropriate candidates for band gap by G. R. Desiraju. J. Appl. Crystallogr. 1991, 24, 265−265. modulation. The 2D and NTO analysis evidences that less (3) Liu, C. Y.; Chen, X. R.; Chen, H. 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