pubs.acs.org/IC Article Controlling the Formation of Two Concomitant Polymorphs in Hg(II) Coordination Polymers Francisco Sánchez-Férez, Xavier Solans-Monfort, Teresa Calvet, Merce ̀ Font-Bardia, and Josefina Pons* Cite This: https://doi.org/10.1021/acs.inorgchem.1c03762 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ABSTRACT: Controlling the formation of the desired product in Downloaded via UNIV AUTONOMA DE BARCELONA on March 24, 2022 at 19:50:17 (UTC). the appropriate crystalline form is the fundamental breakthrough of crystal engineering. On that basis, the preferential formation between polymorphic forms, which are referred to as different assemblies achieved by changing the disposition or arrangement of the forming units within the crystalline structure, is one of the most challenging topics still to be understood. Herein, we have observed the formation of two concomitant polymorphs with general formula {[Hg(Pip)2(4,4′-bipy)]·DMF}n (P1A, P1B; Pip = piperonylic acid; 4,4′-bipy = 4,4′-bipyridine). Besides, [Hg- (Pip)2(4,4′-bipy)]n (2) has been achieved during the attempts to isolate these polymorphs. The selective synthesis of P1A and P1B has been successfully achieved by changing the synthetic conditions. The formation of each polymorphic form has been ensured by unit cell measurements and decomposition temperature. The elucidation of their crystal structure revealed P1A and P1B as polymorphs, which originates from the Hg(II) cores and intermolecular associations, especially pinpointed by Hg···π and π···π interactions. Density functional theory (DFT) calculations suggest that P1B, which shows Hg(II) geometries that are further from ideality, is more stable than P1A by 13 kJ·mol−1 per [Hg(Pip)2(4,4′-bipy)]·DMF formula unit, and this larger stability of P1B arises mainly from metal···π and π···π interactions between chains. As a result, these structural modifications lead to significant variations of their solid-state photoluminescence. ■ INTRODUCTION The rise of crystal engineering, triggered by the identification coordinated to the metal center,14 temperature is a simple way to control the formation of polymorphic forms. Energetic data and understanding of the structural features in materials, of organic polymorphs15 show that usually energy differences enabled the prediction of crystal structures and, therefore, fall within the range of 0−10 kJ·mol−1, but scarce data about improved the ability to design products with desired chemical relative energy calculation of coordination polymers have been and physical properties by tuning their crystal structure.1,2 found.16 Interestingly, these mere differences in the order of These structural modifications required understanding of weak interactions are responsible for significant modification in molecular and supramolecular preferences and identifying the resulting properties. Recent photoluminescence studies of interaction patterns.3,4 However, since crystallization is a Au(I) coordination complexes have shown emission-depend- kinetic process, metastable intermediates can be achieved as ent properties caused by slight structure differences after the multiple local minima during the self-assembly of the ligands absorption of guest solvent molecules.13 By this token, with the metal ion.5 These structures are mainly stabilized by polymorphism in Cu(I) complexes evinced the impact of the steric requirements of the linkers and by hydrogen bonds such structural differences in the photophysical properties.17 and noncovalent interactions inter alia π···π, C−H···O, C− Although there is a large amount of polymorphism data H···π or the limited case of Hg···π.6,7 Therefore, providing about crystallization of organic salts18 and cocrystals19 of access to many potential structure accommodations. Not pharmaceutical interest, and despite the knowledge of surprisingly, concomitant polymorphs can be formed within polymorphs in discrete coordination complexes such as the this landscape.8 Within this frame, slight modifications, intermediate variations, or major structural changes (polymorphism) Received: December 3, 2021 throughout the entire crystal structure are promoted by factors such as time,9 temperature,10 solvent,11,12 or the introduction of templates,13 which influence both the nucleation rate and crystal growth. While the role of the solvent varies from filling voids and partaking in intermolecular interactions to being © XXXX The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.inorgchem.1c03762 A Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article archetypal [Pt(2,2′-bipyridine)Cl2],20 there is a scarce number diamond window in the range of 4000−500 cm−1. 1H, 13C{1H}, of structures bearing coordination polymers reported hitherto and distortionless enhancement by polarization transfer (DEPT)-135 comprising Cu(I),21−23 Ag(I),14,24,25 Co(II) and Ni(II),26−29 NMR spectra were recorded on an NMR-FT Bruker360 MHz Cd(II),16,30 or Pb(II).31 In this scenario, free rotation ligands spectrometer in DMSO-d6 solution at RT. All chemical shifts (δ) are given in ppm. Solid-state photoluminescence measurements were can drive the formation of conformational poly- recorded using a Varian Cary Eclipse Fluorescence spectrophotom- morphs.25,28,32,33 Besides, Hg(II) as softer metal compared to eter between 500 and 660 nm. CIE 1931 chromaticity diagram was Zn(II) and Cd(II)34 is capable of accommodating several generated using Origin Pro 2019b software. distorted geometries and partake in weak intermolecular Synthesis of the Polymorphs Mixture: {[Hg(Pip)2(μ-4,4′-bipy)]· interactions. Thus, a combination of them can trigger the DMF}n (P1A and P1B). DMF (2.5 mL) was placed into a 10 mL vial assembly of different arrangements.7,35 and heated in a furnace until 95 °C. Once the temperature was In pursuit of extending our knowledge on the structure− reached, 4,4′-bipy (44.1 mg, 0.282 mmol) and HPip (94.2 mg, 0.567 property relationship of Hg(II) compounds,36,37 we have mmol) were introduced. When the solution became transparent, combined Hg(OAc)2, 1,3-benzodioxole-5-carboxylic acid Hg(OAc)2 (90.1 mg, 0.283) was added and the suspension was (piperonylic acid, HPip) and the free rotational 4,4′-bipyridine sonicated for a minute until dissolution. The reaction was sealed and kept under autogenous pressure at 95 °C for 45 min and then allowed ligand (4,4′-bipy). The synthesis performed at 95 °C resulted to cool down out of the furnace for 1 h until 25 °C. After cooling, in two concomitant polymorphs bearing the same zigzag one- several prism-like colorless crystals were formed, filtered, and washed dimensional (1D) structure and accommodating guest N,N- twice with 5 mL of cold Et2O. Careful inspection of them revealed the dimethylformamide (DMF) molecules with general formula presence of two different crystal sizes but sharing the same crystalline {[Hg(Pip)2(4,4′-bipy)]·DMF}n (P1A and P1B). During the habit (the smaller ones were around 70%, while the large ones were attempts to isolate each polymorphic forms, we recognized the about 30%). These single crystals were mechanically sorted for their formation of an additional zigzag 1D coordination polymer X-ray crystal structure elucidation, revealing that the small crystals with formula {[Hg(Pip)2(4,4′-bipy)]}n (2). All of them were were P1A while the big crystals were P1B. characterized by analytical and spectroscopic techniques. In P1A and P1B: Yield: 141 mg (66%). dT = 192−196 °C. Anal. addition, periodic density functional theory (DFT) calculations Calcd for C29H25N3O9Hg (760.11 g·mol−1): C, 45.82; H, 3.31; N, have been performed to set the relative stabilities of both 5.53. Found: C, 45.56; H, 3.28; N, 5.32%. FTIR-ATR (wavenumber, cm−1): 3097(w)−3014(w) [ν(CH)]ar, 2985(w)−2784(w) [ν(CH)]al, polymorphic forms and rationalize the origin of the different 1659(m) [ν(CO)]DMF, 1628(w)−1576(m) [ν(CC/CN)], stability. Finally, these structural differences provoked dramatic 1542(m) [ν(COO)]as, 1502(m)−1482(m) [ν(CC/CN)], variations in their solid-state photoluminescence. 1429(m) [ν(COO)]s, 1418(sh.), 1368(s)−1229(s) [δ(CC/C ■ N)], 1163(m), 1128(w), 1101(m), 1069(m), 1034(s) [δ(C−H)]ip, EXPERIMENTAL SECTION 1003(m) [δ(C−H)]ip, 936(m), 920(m), 888(m), 866(w), 821(m)− 771(s) [δ(C−H)]oop, 721(m), 682(m), 660(m), 639(m), 630(m), Chemical Risks. Hg(II) complexes are toxic, and any manipu- 582(m), 551(m), 534(m), 503(w). 1H NMR (360 MHz; DMSO-d6; lation of the samples has to be carried out into the fume hood and 298 K): δ = 2.72 and 2.88 [6H, s, N−(CH3)2]DMF, 6.10 [4H, s, O− wearing gloves. CH2−O], 6.97 [2H, d, 3J = 8.1 Hz, O2C−C−CH−CH], 7.41 [2H, s, Materials and General Details. Hg(II) acetate (Hg(OAc)2), O2C−C−CH−CO], 7.58 [2H, d, 3J = 8.2 Hz, O2C−C−CH−CH], 1,3-benzodioxole-5-carboxylic acid (piperonylic acid, HPip), 4,4′- 7.92 [4H, d, 3J = 4.3 Hz, m-Hpy], 7.94 [1H, s, CHO]DMF, 8.77 [4H, d, bipyridine (4,4′-bipy) ligands, methanol (MeOH), N,N-dimethylfor- 3 J = 5.3 Hz, o-Hpy]. mamide (DMF), acetic acid (HOAc), and diethyl ether (Et2O) as solvents were purchased from Sigma-Aldrich. The water used in the Synthesis of Compound [Hg(Pip)2(μ-4,4′-bipy)]n (2). To a reactions was Milli-Q water. Deuterated dimethyl sulfoxide-d6 solution of Hg(OAc)2 (100 mg, 0.313 mmol) and HPip (105 mg, (DMSO-d6) was used for the NMR experiment and was purchased 0.631 mmol) in DMF (3 mL), a solution of 4,4′-bipy (49.0 mg, 0.313 from Eurisotop. All of them were used without further purification. mmol) in DMF (2 mL) was added dropwise under vigorous stirring. Reactions and manipulation were carried out in a Digitheat-TFT Immediately, a yellowish solid appeared. The reaction was stirred for furnace (JP Selecta) using sealed vials under an autogenous pressure 1 h. The solid obtained was filtered and washed with 10 mL of cold of DMF at 95 °C for the synthesis of the mixture (P1A and P1B) and MeOH. Suitable crystals were obtained by slow diffusion of 5 mL of the isolation of either P1A or P1B. Compound 2 was synthesized in MeOH into 1 mL of the mother liquors for 5 days. The phase purity DMF at room temperature (RT) or both in MeOH and Milli-Q water of the sample was confirmed by PXRD. at RT and 95 °C. Powder X-ray diffraction (PXRD) patterns were The synthesis of compound 2 was also achieved at RT and at 95 °C measured with a PANalytical X’Pert PRO MPD θ/θ powder using MeOH or Milli-Q water as a solvent. diffractometer of 240 mm radius, in a configuration convergent Yield: 165 mg (77%). dT = 201−202 °C. Anal. Calcd for beam with a focalizing mirror and a transmission geometry with flat C26H18N2O8Hg (687.02 g·mol−1): C, 45.45; H, 2.64; N, 4.08. Found: samples sandwiched between low-absorbing films. A Cu Kα radiation C, 45.38; H, 2.38; N, 3.84%. FTIR-ATR (wavenumber, cm−1): with λ = 1.5418 Å was used (45 kW and 40 mA). All of them were 3107(w)−3045(w) [ν(CH)]ar , 2983(w)−2891(w) [ν(CH)] al, recorded from 2θ = 5 to 30° with a step scan of 0.0263° and a 2792(w), 1631(w), 1606(m) [ν(CC/CN)], 1574(m), measuring time of 300 s per step. Thermal decomposition 1541(m) [ν(COO)]as, 1496(m) [ν(CC/CN)], 1484(m) temperature (dT) was measured on a Stuart Melting Point Apparatus [ν(CC/CN)], 1429(m) [ν(COO)]s, 1418(m), 1367(s)− SMP30 with a heating ramp of 2.0 °C·min−1 in a temperature range of 1230(s) [δ(CC/CN)], 1160(m), 1106(m), 1094(m), 20−210 °C. Elemental analyses (C, H, N) were carried out on a Euro 1067(m), 1034(s) [δ(C−H)]ip, 1008(m) [δ(C−H)]ip, 930(m), Vector 3100 instrument. Simultaneous thermogravimetric (TG)/ 919(m), 890(m), 818(m)−765(s) [δ(C−H)]oop, 721(m), 681(m), differential thermal analysis (DTA) determinations were performed 670(m), 633(m), 586(m), 549(m). 1H NMR (360 MHz; DMSO-d6; with a Netzsch STA 409 instrument, using an aluminum oxide 298 K): δ = 6.08 [4H, s, O−CH2−O], 6.95 [2H, d, 3J = 8.0 Hz, powder crucible and an oxide powder as a standard (Al2O3, O2C−C−CH−CH], 7.38 [2H, s, O2C−C−CH−CO], 7.56 [2H, d, 3J PerkinElmer 0419-0197) and heating at 5 °C·min−1 from 25 to 350 = 7.9 Hz, O2C−C−CH−CH], 7.89 [4H, d, 3J = 4.3 Hz, m-Hpy], 8.74 °C, under nitrogen atmosphere with a flow rate of 80 mL·min−1. The [4H, d, 3J = 4.3 Hz, o-Hpy]. 13C{1H} NMR (360 MHz; DMSO-d6; Fourier transform infrared-attenuated total reflection (FTIR-ATR) 298 K): δ = 169.07 [O2C−C], 150.63 [N−CH−CH], 150.13 [O2C− spectra were recorded on a PerkinElmer spectrometer, equipped with C−(CH)2−C], 147.13 [O2C−C−CH−C], 144.95 [N−(CH)2−C], a universal attenuated total reflectance (ATR) accessory with 127.64 [O2C−C], 125.17 [O2C−C−CH−CH], 121.97 [N−CH− B https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Table 1. Crystal Structure Refinement Data for Compounds P1A, P1B, and 2 P1A P1B 2 empirical formula C29H25HgN3O9 C116H100Hg4N12O36 C26H18HgN2O8 formula weight 760.11 3040.43 687.01 T (K) 100(2) 100(2) 100(2) wavelength (Å) 0.71073 0.71073 0.71073 system, space group triclinic, P1̅ triclinic, P1̅ monoclinic, C2/c unit cell dimensions a (Å) 6.0027(7) 13.6326(15) 17.803(6) b (Å) 13.5859(17) 20.267(2) 11.556(4) c (Å) 17.769(2) 21.899(2) 12.339(4) α (deg) 71.181(4) 105.796(4) 90 β (deg) 83.249(4) 105.114(4) 115.059(14) γ (deg) 80.217(4) 100.923(4) 90 V (Å3) 1348.6(3) 5395.3(10) 2299.5(13) Z 2 2 4 Dcalc (g·cm3) 1.872 1.872 1.984 μ (mm−1) 5.769 5.768 6.751 F(000) 744 2976 1328 crystal size (mm3) 0.250 × 0.105 × 0.045 0.303 × 0.177 × 0.060 0.701 × 0.198 × 0.148 hkl ranges −8 ≤ h ≤ 8 −19 ≤ h ≤ 19 −25 ≤ h ≤ 22 −18 ≤ k ≤ 19 −28 ≤ k ≤ 28 0 ≤ k ≤ 16 0 ≤ l ≤ 25 −31 ≤ l ≤ 31 0 ≤ l ≤ 17 2θ range (deg) 2.427−30.602 1.966−30.551 3.526−30.365 reflections collected/unique/[Rint] 8230/8230 216 059/33 014 3368/3368 [Rint] = 0.1828 [Rint] = 0.0574 [Rint] = 0.0405 completeness to θ (%) 99.9 99.9 97.8 absorption correction semiempirical semiempirical semiempirical max. and min. transmis. 0.7461 and 0.5741 0.7461 and 0.5645 0.7461 and 0.3631 refinement method full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 full-matrix least-squares on |F|2 data/restrains/parameters 8230/1/330 33 014/0/1556 3368/0/168 goodness of fit (GOF) on |F|2 0.923 1.061 1.043 final R indices [I > 2σ(I)] R1 = 0.0458, wR2 = 0.0924 R1 = 0.0295, wR2 = 0.0579 R1 = 0.0218, wR2 = 0.0534 R indices (all data) R1 = 0.1549, wR2 = 0.1250 R1 = 0.0482, wR2 = 0.0680 R1 = 0.0229, wR2 = 0.0543 extinction coefficient 0.0027(3) n/a n/a largest. diff. peak and hole (e·Å−3) 1.807 and −1.841 2.418 and −1.894 2.128 and −1.765 CH], 109.63 [O2C−C−CH−C], 107.81 [O2C−C−CH−CH], 101.66 (COO)]as, 1504(m)−1453(m) [ν(CC/CN)], 1428(m) [ν- [O−CH2−O]. (COO)] s , 1419(m), 1367(s)−1230(s) [δ(CC/CN)], Synthesis of {[Hg(Pip)2(μ-4,4′-bipy)]·DMF}n (P1A). DMF (5 mL) 1163(m), 1101(m), 1070(m), 1036(s) [δ(C−H)]ip, 1006(sh), and HOAc (10.9 μL, 0.190 mmol) were introduced into a 10 mL vial, 935(m), 920(m), 887(m), 866(w), 819(m)−769(s) [δ(C−H)]oop, sealed, and heated in a furnace until 95 °C. Once the temperature was 720(m), 680(m), 661(m), 637(m), 630(m), 582(m), 554(m), reached, compound 2 (65.3 mg, 0.0950 mmol) was added and the 536(m). mixture was sonicated for 1 min until a yellowish transparent solution X-ray Crystallographic Data and Structural Analysis. Color- was obtained. Then, the vial was sealed, kept at 95 °C under an less prism-like P1A, P1B, and 2 specimens were used for the X-ray autogenous pressure for 45 min, and left to cool down slowly for 7 h crystallographic analysis. The X-ray intensity data were measured on a until 25 °C (Figure S1, Supporting Information). Suitable crystals of D8 Venture system equipped with a multilayer monochromator and a P1A were formed, collected, filtered off, and washed twice with 5 mL Mo microfocus (λ = 0.71073 Å). For P1A, P1B, and 2 the frames of cold Et2O. were integrated with the Bruker SAINT software package, using a Yield: 42.8 mg (59%). dT = 192−193 °C. FTIR-ATR (wave- narrow-frame algorithm. For P1A, the integration of the data using a number, cm−1): 3098(w)−3012(w) [ν(CH)]ar, 2895(w) [ν(CH)]al, triclinic unit cell yielded a total of 8231 reflections to a maximum θ 2780(w) [ν(CH)]al, 1658(s) [ν(CO)]DMF, 1628(w)−1576(m) value of 30.60° (0.70 Å resolution), of which 8231 were independent [ν(CC/CN)], 1542(m) [ν(COO)]as, 1503(m) [ν(CC/C (average redundancy 1.000, completeness = 99.0%, Rint = 18.28%, Rsig N)], 1483(m) [ν(CC/CN)], 1427(m) [ν(COO)]s, 1368(s)− = 17.88%) and 3761 (45.69%) were greater than 2σ(|F|2). For P1B, 1229(s) [δ(CC/CN)], 1163(m), 1101(m), 1069(m), 1035(s) the integration of the data using a triclinic unit cell yielded a total of [δ(C−H)]ip, 1003(sh), 935(m), 888(m), 861(w), 820(m)−771(s) 216 059 reflections to a maximum θ value of 30.55° (0.70 Å [δ(C−H)]oop, 719(m), 680(m), 660(m), 639(m), 629(m), 582(m), resolution), of which 33 014 were independent (average redundancy 550(m), 538(m), 511(m). 6.544, completeness = 99.9%, Rint = 5.74%, Rsig = 3.96%) and 26 560 Synthesis of {[Hg(Pip)2(μ-4,4′-bipy)]·DMF}n (P1B). The synthesis (80.45%) were greater than 2σ(|F|2). For 2, the integration of the data of P1B was performed following the same procedure as in the using a monoclinic unit cell yielded a total of 3368 reflections to a synthesis of P1A but without adding HOAc in the recrystallization of maximum θ value of 30.36° (0.70 Å resolution), of which 3368 were 2 (80.0 mg, 0.117 mmol). independent (average redundancy 1.000, completeness = 97.3%, Rint Yield: 64.9 mg (73%). dT = 198−199 °C. 3098(w)−3012(w) = 4.05, Rsig = 2.99%) and 3237 (96.11%) were greater than 2σ(|F|2). [ν(CH)]ar, 2894(w) [ν(CH)]al, 2786(w) [ν(CH)]al, 1658(s) [ν(C The structures were solved and refined using the Bruker SHELXTL O)]DMF, 1628(w)−1578(m) [ν(CC/CN)], 1540(m) [ν- Software Package (version-2018/3).38 For P1A, the final anisotropic C https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Table 2. Bond Lengths (Å), Bond and Torsion Angles (deg), and Intermolecular Interactions Present in P1Aa bond lengths Hg(1)−O(1) 2.192(5) Hg(1)−O(2) 2.901(9) Hg(1)−O(2) 2.901(9) Hg(1)−O(5) 2.228(9) Hg(1)−O(6) 2.647(9) Hg(1)−O(6) 2.647(9) Hg(1)−N(1) 2.215(6) Hg(1)−N(2) 2.341(6) Hg(1)−N(2) 2.341(6) bond angles N(2)−Hg(1)−O(1) 91.0(2) N(1)−Hg(1)−O(6) 85.3(3) N(1)−Hg(1)−O(6) 85.3(3) N(2)−Hg(1)−O(2) 101.7(2) O(1)−Hg(1)−O(2) 50.1(2) O(1)−Hg(1)−O(2) 50.1(2) N(2)−Hg(1)−O(5) 102.9(3) O(1)−Hg(1)−O(5) 95.5(3) O(1)−Hg(1)−O(5) 95.5(3) N(2)−Hg(1)−O(6) 84.1(3) O(1)−Hg(1)−O(6) 141.7(3) O(1)−Hg(1)−O(6) 141.7(3) N(2)−Hg(1)−N(1) 104.3(2) O(2)−Hg(1)−O(5) 137.6(3) O(2)−Hg(1)−O(5) 137.6(3) N(1)−Hg(1)−O(2) 82.5(2) O(2)−Hg(1)−O(6) 167.5(3) O(2)−Hg(1)−O(6) 167.5(3) N(1)−Hg(1)−O(1) 132.4(2) O(5)−Hg(1)−O(6) 49.5(4) O(5)−Hg(1)−O(6) 49.5(4) N(1)−Hg(1)−O(5) 123.3(3) Cg(I)−Cg(J) torsion angle, χ Cg(1)−Cg(1) 1.8(11) intermolecular interactions H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) C(15)−H(15)···O(9) 2.318 3.229(9) 0.950 160.4 C(17)−H(17)···O(9) 2.441 3.340(10) 0.950 158.3 π interactions Cg(I)···Hg(J) Cg···Hgb MeJ_Perp β Cg(1)···Hg(1) 3.953 3.491 27.91 Cg(I)···Cg(J) Cg···Cgc αd β, γe Cg(I)_Perp, Cg(J)_Perpf slippageg Cg(1)···Cg(1) 3.736(5) 0.0(4) 22.8, 22.8 3.443(3) 1.449 a Cg···Cg and Cg···Hg distances are given in Å. bCg···Hg = distance between ring centroid and Hg(II) center. cCg···Cg = distance between ring centroids (Å). dα = dihedral angle between planes I and J (deg). eOffset angles: β = angle Cg(I)−Cg(J) and normal to plane I (deg) and γ = angle Cg(I)−Cg(J) and normal to plane J (deg) (β = γ, when α = 0). fPerpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0). gSlippage = horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N1−C9−C10−C11−C12−C13. full-matrix least-squares refinement on |F|2 with 330 variables Table 3. Bond Lengths (Å) and Torsion Angles (deg) of converged at R1 = 4.58%, for the observed data and wR2 = 12.50% P1Ba for all data. For P1B, the final anisotropic full-matrix least-squares refinement on |F|2 with 1556 variables converged at R1 = 2.95%, for bond lengths the observed data and wR2 = 6.80% for all data. For 2, the final Hg(1) anisotropic full-matrix least-squares refinement on |F|2 with 168 Hg(1)−O(1) 2.210(2) Hg(1)−O(2) 2.964(3) variables converged at R1 = 2.18%, for the observed data and wR2 = Hg(1)−O(5) 2.270(2) Hg(1)−O(6) 2.732(3) 5.43% for all data. For P1A, P1B, and 2, the final cell constants and Hg(1)−N(1) 2.350(3) Hg(1)−N(2) 2.192(3) volume are based upon the refinement of the XYZ-centroids of reflections above 20 σ(I). Data were corrected for absorption effects Hg(2) using the multiscan method (SADABS). Crystal data and relevant Hg(2)−O(13) 2.212(2) Hg(2)−O(14) 2.916(3) details of structure refinement for compounds P1A, P1B, and 2 are Hg(2)−O(9) 2.274(2) Hg(2)−O(10) 2.728(3) reported in Table 1. Selected bond lengths, angles, and intermolecular Hg(2)−N(3) 2.213(3) Hg(2)−N(4) 2.363(3) interactions of P1A are listed in Table 2. Bond lengths, angles, and Hg(3) intermolecular interactions of P1B are shown in Tables 3−5, Hg(3)−O(17) 2.216(2) Hg(3)−O(18) 2.885(3) respectively. Bond lengths, angles, and intermolecular interactions of Hg(3)−O(21) 2.260(2) Hg(3)−O(22) 2.740(3) 2 are listed in Table 6. Hg(3)−N(5) 2.376(3) Hg(3)−N(6) 2.212(3) The geometry evaluation of the three complexes has been Hg(4) performed using version 2.1 of SHAPE39 software, which is based Hg(4)−O(29) 2.245(2) Hg(4)−O(30) 2.794(3) on the low continuous-shape measure (CShM) value S.40 The corresponding atomic coordinates have been directly extracted from Hg(4)−O(25) 2.267(2) Hg(4)−O(26) 2.700(3) the .cif data. Hirshfeld surfaces with their 2D fingerprint plots have Hg(4)−N(7) 2.237(3) Hg(4)−N(8) 2.326(3) been generated using CrystalExplorer 17.5. 41 The essential Cg(I)−Cg(J) torsion angle, χ Cg(I)−Cg(J) torsion angle, χ interactions have been divided into O···H/H···O, C···C and those Cg(1)−Cg(3) 8.0(5) Cg(5)−Cg(6) 3.5(5) involving Hg(II). Cg(2)−Cg(4) 34.4(5) Cg(7)−Cg(7) 0.1(6) Complete information about the crystal structure and molecular a geometry is available in .cif format and deposited in the CCDC. Cg(1) = N2−C22−C23−C24−C25−C26; Cg(2) = N6−C74− CCDC numbers 2101056−2101058 contain the supporting data of C75C76−C77−C78; Cg(3) = N3−C27−C28−C29−C30−C31; this paper. Molecular graphics were generated using Mercury (version Cg(4) = N7−C79−C80−C81−C82−C83; Cg(5) = N4−C48− 4.3.1)42 with POV-Ray Package (version 3.7).43 Color codes for C49−C50−C51−C52; Cg(6) = N5−C53−C54−C55−C56−C57; molecular graphics: light slate blue (N), suva gray (C), white (H), Cg(7) = N8−C100−C101−C102−C103−C104. lavender gray (Hg), and red (O). Computational Details. DFT calculations of P1A and P1B were group and keeping the unit cell parameters to those determined by carried out using the VASP code.44,45 Geometry optimizations were single-crystal X-ray diffraction (SC-XRD). With the aim of analyzing performed at the PBE-D2 level of theory46,47 assuming a P1 space the suitability of the D2 Grimme’s empirical correction, we compared D https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Scheme 1. Outline of the Formation of the Mixture (P1A and P1B) and 2 and the Isolation of P1A and P1B Scheme 2. Optical Microphotographs of Single Crystals of P1A, P1B, and 2 from the Synthesis of the Mixture or Achieved by Recrystallization of 2a a Inset: P1A and P1B Mixture. the relative stability of P1B and P1A (in terms of potential energy per from the optimal geometry of a hypothetical isolated fragment (EDIS) [Hg(Pip)2(4,4′-bipy)]·DMF unit formula) with PBE-D2, PBE-D3 and (ii) the pure interaction energy (EINT) between the already and PBE-D*, a modification of the D2 Grimme’s empirical term distorted fragments (eq 1). suggested to be more suitable for the modeling of molecular crystals.48 Results are reported in Table S1 in the Supporting E = E DIS(1) + E DIS(2) + E INT (1) Information and show that all Grimme’s correction leads to a preference for the P1B polymorph and the differences between the These energy partition schemes are standard in computational three methods are small (5.5 kJ·mol−1). Moreover, according to the chemistry and examples where they have been applied with success literature, PBE-D3 is accurate enough for modeling molecular can be found in the literature.54,55 This analysis was made with crystals49,50 and, indeed, it has been even used to compute reference molecular calculations that were performed at the same PBE-D2 level values for the validation of less computationally demanding of theory and using Gaussian16 package.56 Main group elements and approaches.51 Ionic cores were described with the projector valence electrons of Hg(II) were represented with the Pople 6- augmented wave (PAW) pseudopotentials.52,53 The valence electrons 31+G(d,p)57,58 and Dunning’s aug-cc-pVDZ59 Gaussian-type basis were represented through a plane-wave basis set with a kinetic energy sets, respectively. The Hg(II) core was taken into account with the cutoff of 600 eV. Since cell size and the number of Hg(II) units in the pseudopotential associated with aug-cc-pVDZ basis set.60 ■ cell differs for P1A and P1B, the Brillouin zone was sampled with a different Monkhorst−Pack K-point mesh in each case, i.e., (4 × 4 × 4) RESULTS AND DISCUSSION and (1 × 1 × 1) for P1A and P1B, respectively. With the aim of getting further insights into the relative stability of Formation of P1A, P1B, and 2. From the reaction of the two polymorphs, we performed an energy partitioning scheme Hg(OAc)2, HPip and 4,4′-bipy in DMF as a solvent, three decomposing the association energy (E) between two metal dimers of different crystalline products, namely, {[Hg(Pip)2(μ-4,4′- vicinal chains in two terms: (i) the energy cost to distort the dimers bipy)]·DMF}n (P1A and P1B) and [Hg(Pip)2(μ-4,4′-bipy)]n E https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article polymorphs is only observed as of 75 °C and rapidly precipitates as temperature drops. Over 105 °C, complexes start to decompose. Their low solubility seems to facilitate the crystallization of different polymorphic intermediates, which suddenly nucleated and grew as temperature dropped.9 Therefore, temperature and thereby solubility are the key factors to the formation of the polymorphic mixture. The isolation of polymorphic forms being initially found to concomitantly crystallize as a mixture has been one of the pillars of crystal engineering. However, to attain the proper conditions to reach the target crystal form is often intricated. In this scenario, one of the forms usually tends to be less stable and, hence, more complicated to be achieved.16 The ability of the linkers to arrange into different polymorphic forms relies on directing factors inter alia time, temperature, concentration, solvent, or additional anions during the crystallization step.12 Therefore, we modified the synthetic conditions trying to isolate P1A and P1B. These polymorphs were synthesized as single crystals by changing concentration, time, and temper- Figure 1. Comparative diffractograms from top to bottom: single- ature or adding HOAc to identify the formation of the crystal (SC) XRD pattern of P1B, PXRD of bulk P1B, PXRD of P1B polymorphs. After the initially found concomitant formation of after heating at 60 °C for 8 h, PXRD of bulk P1B after heating at 60 P1A and P1B (Scheme 2), the isolation of P1B has been °C for 24 h, and SC-XRD of 2. accomplished by recrystallization of 2 in DMF at 95 °C and slow cooling down for 7 h. The recrystallization of 2 to achieve P1B has been performed in a concentration range of 1.1 × 10−3−2.5 × 10−2 M, from which the optimal conditions to crystallize P1B were found to be between 1.0 × 10−2 and 2.5 × 10−2 M. It is worth mentioning that P1B was formed regardless of the concentration while the formation of P1A was not observed. Since the initial reaction starting from Hg(OAc)2 resulted in the mixture of P1A and P1B, with P1A being the predominant form, we added equivalent amounts of HOAc to incorporate OAc− anions during the recrystallization of 2 in DMF at 95 °C. Interestingly, the addition of the OAc− anions drove the formation of P1A (Scheme 2). All of those syntheses were examined by several single-crystal X-ray diffraction analyses to determine their unit cell parameters, which are markedly different between P1A and P1B. In addition, the unit cell of crystals previously measured was redetermined after up to 3 weeks and no interconversion between P1A and P1B was observed. Some of the unit cell parameters measured are listed in Figure 2. Time-dependent FTIR-ATR spectra of P1B as of heating at Table S2, Supporting Information. The formation of each 60 °C. From top to bottom, t0 = as-synthesized P1B; t1 = after 1 h 30 min; t2 = after 12 h; t3 = after 19 h. The regions in which significant polymorphic form has been traced by single-crystal X-ray variations of the spectra occur have been highlighted in light green. diffraction (SC-XRD), and the phase purity of 2 was confirmed by powder X-ray diffraction (PXRD) (Figure S2, Supporting Information). We also provide the PXRD pattern of P1B, (2) have been isolated (Scheme 1) depending on the synthetic confirmed by unit cell measurements, to ensure the absence of conditions. 2 in the sample (Figure S3, Supporting Information). The The concomitant crystallization of P1A and P1B was interconversion between P1B and 2 was followed by PXRD observed when the reaction was performed in DMF at 95 °C. (Figure 1). Cooling down of the saturated solution for 1 h resulted in a General Characterization. The three compounds were crystals mixture of P1A and P1B. The main difference between characterized by decomposition temperature (dT), elemental both crystals at first sight is the size, being P1B larger than analysis (EA), FTIR-ATR and 1H NMR spectroscopies, TG/ P1A. Instead, the formation of 2 is achieved at RT in DMF, DTA, and single-crystal X-ray diffraction. Compound 2 was MeOH, and Milli-Q water or also at 95 °C in MeOH and characterized by 13C{1H} and DEPT-135 NMR spectros- Milli-Q water. The different formation of the mixture P1A and copies. In addition, the solid-state photoluminescence of the P1B or 2 could rely on the different solubility of the final three complexes has been recorded. complexes under the reaction conditions. Compound 2 rapidly We have recorded the FTIR-ATR spectra of P1A and P1B, precipitates as a yellow powder when the reaction is performed P1A, P1B, and 2. The FTIR-ATR spectrum of the mixture of at RT in DMF, while it is soluble at 95 °C. We have performed P1A and P1B is a combination of the spectra of the isolated the reaction in a temperature range of 25−115 °C, once every products. The absence of bands in all of the FTIR-ATR spectra 10 °C and the complete dissolution and formation of the between 2630 and 2518 cm−1, attributable to hydrogen- F https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Figure 3. One-dimensional polymeric chains of (a) P1A, (b) P1B, and (c) 2. Insets: sequence of the different Hg(II) cores for each compound. bonded ν(O−H)HPip and at 1667 cm−1 from ν(CO)HPip, ν(CO)DMF.6262 Additional bands from the aromatic rings indicates the deprotonation of the HPip ligand. The have also been identified.63,64 corresponding carboxylate bands appear at 1542 cm −1 The 1H NMR spectra of P1A and P1B mixture and 2 have (P1A), 1540 cm−1 (P1B), or 1560 cm−1 (2) for νas(COO) been recorded in DMSO-d6 (Figures S10 and S11, Supporting and at 1427 cm−1 (P1A), 1428 cm−1 (P1B), or 1430 cm−1 (2) Information). In the mixture of P1A and P1B, the presence of DMF is confirmed by signals at 7.94, 2.88, and 2.72 ppm. The for νs(COO) (Figures S4−S9, Supporting Information). The spectra show aromatic signals from the Pip linkers between difference between these bands (Δ = νas(COO) − νs(COO)) 7.58 and 6.95 ppm and the −CH2− from the dioxole group at reveals the coordination modes of the carboxylate linkers:61 6.10 ppm (P1A + P1B) and 6.08 ppm (2). The two signals 115 cm−1 (P1A), 112 cm−1 (P1B), and 130 cm−1 (2), from the 4,4′-bipy appear between 8.77 and 7.89 ppm. The suggesting bidentate chelate (μ1−η2) coordination mode in the 13 C{1H} and DEPT-135 NMR spectra of 2 display the three compounds. In addition, the spectra of P1A and P1B carboxylate band at 169.07 ppm and the −CH2− of the dioxole show a characteristic peak at 1658 cm−1 corresponding to the at 101.66 ppm. The remaining C signals appear between G https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Table 4. Bond Angles (deg) of P1B bond angles Hg(1) N(2)−Hg(1)−O(1) 133.44(9) N(1)−Hg(1)−O(6) 80.80(9) N(2)−Hg(1)−O(2) 84.86(9) O(1)−Hg(1)−O(2) 48.60(8) N(2)−Hg(1)−O(5) 122.70(9) O(1)−Hg(1)−O(5) 92.68(9) N(2)−Hg(1)−O(6) 89.53(9) O(1)−Hg(1)−O(6) 136.85(9) N(2)−Hg(1)−N(1) 104.53(9) O(2)−Hg(1)−O(5) 129.76(8) N(1)−Hg(1)−O(2) 103.18(9) O(2)−Hg(1)−O(6) 173.79(8) N(1)−Hg(1)−O(1) 89.83(9) O(5)−Hg(1)−O(6) 51.95(8) N(1)−Hg(1)−O(5) 107.84(9) Hg(2) N(3)−Hg(2)−O(13) 132.30(10) N(4)−Hg(2)−O(10) 82.42(9) N(3)−Hg(2)−O(14) 82.98(9) O(13)−Hg(2)−O(14) 49.39(8) N(3)−Hg(2)−O(9) 123.80(10) O(13)−Hg(2)−O(9) 94.7(1) N(3)−Hg(2)−O(10) 84.82(9) O(13)−Hg(2)−O(10) 142.50(9) N(3)−Hg(2)−N(4) 105.20(10) O(14)−Hg(2)−O(9) 130.69(9) N(4)−Hg(2)−O(14) 109.42(9) O(14)−Hg(2)−O(10) 164.88(8) N(4)−Hg(2)−O(13) 90.70(10) O(9)−Hg(2)−O(10) 51.72(9) N(4)−Hg(2)−O(9) 102.30(10) Hg(3) N(6)−Hg(3)−O(17) 129.80(10) N(5)−Hg(3)−O(22) 126.59(9) N(6)−Hg(3)−O(18) 81.92(9) O(17)−Hg(3)−O(18) 49.83(9) N(6)−Hg(3)−O(21) 129.00(10) O(17)−Hg(3)−O(21) 96.70(10) N(6)−Hg(3)−O(22) 81.52(9) O(17)−Hg(3)−O(22) 124.63(9) N(6)−Hg(3)−N(5) 104.60(10) O(18)−Hg(3)−O(21) 146.43(9) N(5)−Hg(3)−O(18) 92.83(9) O(18)−Hg(3)−O(22) 139.98(8) N(5)−Hg(3)−O(17) 92.27(9) O(21)−Hg(3)−O(22) 51.70(9) N(5)−Hg(3)−O(21) 91.10(10) Hg(4) N(7)−Hg(4)−O(25) 120.80(10) N(8)−Hg(4)−O(30) 104.80(9) N(7)−Hg(4)−O(26) 82.95(9) O(25)−Hg(4)−O(26) 52.24(9) N(7)−Hg(4)−O(29) 133.5(10) O(25)−Hg(4)−O(29) 96.41(9) N(7)−Hg(4)−O(30) 82.74(9) O(25)−Hg(4)−O(30) 136.00(9) N(7)−Hg(4)−N(8) 104.2(10) O(26)−Hg(4)−O(29) 143.39(9) N(8)−Hg(4)−O(26) 80.87(9) O(26)−Hg(4)−O(30) 165.52(8) N(8)−Hg(4)−O(25) 104.10(10) O(29)−Hg(4)−O(30) 50.76(9) N(8)−Hg(4)−O(29) 91.2(10) 150.63 and 108.81 ppm. DEPT-135 experiment was required heating at 60 °C by losing the occluded DMF molecules to ensure the correct assignation of the carbon atoms from the (Figure 1). aromatic rings (Figure S12, Supporting Information). Likewise, FTIR-ATR spectra of P1B evinced the loss of TG−DTA determinations of the complexes were performed DMF molecules by the disappearance of the band at 1659 using 79.9 mg (P1A), 46.8 mg (P1B), or 81.2 mg (2). The TG cm−1 attributed to the υ(CO)DMF (Figure 2). Once the analysis of 2 exhibits a flat profile without any mass loss up to transformation into 2 is achieved, the sample can be 187 °C. Besides, its decomposition temperature is set to 199 recrystallized using the conditions mentioned in the Exper- °C from the DTA data (Figure S13, Supporting Information). imental Section to synthesize single crystals of P1A or P1B. The TG analysis of P1A and P1B evidences the loss of a DMF Therefore, polymorphs P1A and P1B seem to be monotropi- molecule (P1A, exp. 7.30%; calcd 9.60%; P1B exp. 8.60%; cally related, and interconversion between them is only feasible calcd 9.60%) between 50 and 121 °C (P1A) or between 90 via the formation of 2, which is promoted by the loss of the and 126 °C (P1B), and no more thermal events were observed DMF molecules and the consequent structural reorganization until decomposition, being stable up to 193 °C (P1A) or 199 ending in 2. °C (P1B). From these data, it seems that the release of DMF Crystal Structures of P1A, P1B, and 2. Both polymorphs by heating is favored in P1A with respect to P1B (Figures S14 (P1A and P1B) crystallize in the triclinic P1̅ space group, and S15, Supporting Information). whereas 2 crystallizes in the monoclinic C2/c space group. All Interconversion between P1A, P1B, and 2. All of the of them present the same connectivity forming zigzag 1D attempts to convert P1A into P1B and vice versa by applying polymeric structures, in which hexacoordinated Hg(II) centers temperature, unavoidably ended in their transformation into 2. bearing [HgO4N2] cores are assembled by bridging 4,4′-bipy Single crystals of P1A and P1B are stable under air exposure ligands along the [1̅01] (P1A) (Figure 3a), [21̅1] (P1B) and no interconversion over time was observed. PXRD (Figure 3b), or [101̅] (2) (Figure 3c) directions. The confirmed that in solid state, a single-crystal-to-single-crystal geometry of P1A and P1B is of distorted pentagonal pyramid transformation between P1B and 2 gradually occurs upon (PPY-6), a seldom reported geometry present in a few H https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Figure 4. (a) Stacking of chains in P1A through (b) π···π and Hg···π interactions or (c) between DMF and 4,4′-bipy by C−H···O interactions. Hg···C interactions are highlighted in dark green and correspond to Hg1···C12, 3.52(1) Å, and Hg1···C11, 3.801(9) Å. Hg···C contact: Hg1···C12, 3.52(1) Å; Hg1···C11, 3.801(9) Å. Cg(2) = N2−C14−C15−C16−C17−C18. complexes and reserved to Cd65 or Hg66 when they are P1B emulates the alternated sequence of interactions coordinated to linkers forcing geometric constrains. The present in P1A but splitting each of the two patterns into geometric distortions have been evaluated by the low three (Figure 5a). continuous-shape measure (CShM) value S39,40 for the three It exhibits cooperative π···π and Hg···π interactions potential geometries comprised in coordination number 6 involving Hg1−Hg4 (Figure 5b), Hg1 and Hg2 (Figure 5c), (Table S3, Supporting Information). The resulting S values or Hg3 and Hg4 (Figure 5d) and three different C−H··O agree with the better fitting of P1A (S = 5.748) and P1B (S: interactions with the occluded DMF molecules (Figure 5e and 7.550, Hg1; 6.450, Hg2; 7.466, Hg3; 5.863, Hg4) with a PPY-6 Table 5). The Hg−Cπ contact value of 3.52(1) Å in P1A and and the pairing of 2 with a trigonal prismatic geometry (TPY- between 3.315(4) and 3.641(3) Å in P1B fall within the range 6, S = 8.452). P1A is assembled by planar 4,4′-bipy linkers of previously reported examples.67 (Cg(1)−Cg(1); torsion angle, χ = 1.8(11)°) (Table 2), while By the same token, 2 assembles through π···π stacking but in P1B contains four Hg(II) centers connected by 4,4′-bipy a different manner. The piling of the chains is set in the ligands oriented with a χ up to 34.4(5)° (Cg(1)−Cg(3), χ = alternate sequence −Pip−Pip−4,4′-bipy− from perpendicular 8.0(5)°; Cg(5)−Cg(6), 3.5(5)°; Cg(2)−Cg(4), 34.4(5)°; polymeric chains (Figure 6a). The aromatic rings of the Pip Cg(7)−Cg(7), 0.1(6)°) (Table 3). Compound 2 and P1A ligands stack at a Cg(1)···Cg(1) distance of 3.906(2) Å, only have a single Cg(1)−Cg(1) χ value of 1.8(11)° (P1A) whereas the interaction between Pip and 4,4′-bipy is of and 0.1(3)° (2). Cg(1)···Cg(2), 4.000(2) Å (Table 6). This sequence is Such an unusual geometry of the Hg (II) centers in the repeated twice within the unit cell along [220] (Figure 6b) structure of P1A and P1B seems to be supported by the or [22̅0] (Figure 6c) directions. assembly of the polymeric chains in columns of stacked and These π···π interactions are supported by C−H···O coplanar 4,4′-bipy ligands. The cooperative π···π and Hg···π interactions, but, unlike P1A and P1B, only two m-H of interactions between the stacked 4,4′-bipy bring the chains 4,4′-bipy pointing in opposite directions act as donors and closer, thus promoting the displacement of the carboxylate interact with two coordinated carboxylate O atoms from oxygen atoms from the Pip ligands toward the basal plane. The parallel chains or with two dioxole O atoms from three complexes present similar bond lengths but markedly perpendicular chains. different bond angles. This is emphasized in the N−Hg−N Hirshfeld Surfaces and 2D Fingerprint Plots of P1A, bond angle being larger in P1A (104.3(2)°, Table 2) and P1B P1B, and 2. Intermolecular interactions have been analyzed (104.2(1)−105.2(1)°, Table 4) with respect to 2 (89.82(10)°, using CrystalExplorer 17.541 by Hirshfeld Surface and 2D Table 6). fingerprint plot analyses. The C−H···O interactions are P1A has one alternated cooperative interaction (Figure 4a), highlighted as red spots in the lateral region of the Hirshfeld in which planar aromatic rings of 4,4′-bipy stacks at 3.736 Å surfaces surrounding 4,4′-bipy ligands (Figure S16, Supporting together with a Hg···π interaction at 3.953 Å (Figure 4b). The Information). They are displayed in the 2D fingerprint plot as occluded DMF molecules are responsible for the coplanar broad wings representing a total of 27.3% (P1A), between 27.0 orientation of 4,4′-bipy. They act as double C−H···O and 28.0% (P1B) or 28.0% (2) of contact surface. The acceptors by associating with 4,4′-bipy through its four m-H complementary interactions from the DMF molecules are atoms and fixing the two pyridyl rings (Table 2). As a found in the Supporting Information (Figure S17). Besides, consequence, the 4,4′-bipy ligands are coplanar preventing the π···π interactions are pointed as a central region in the 2D formation of π···π interactions (Figure 4c). fingerprint plot that corresponds to the 4.1% (P1A), between I https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Figure 5. Stacking of chains in P1B. Inset of π···π and Hg···π interactions between (a) Hg3 and Hg4; (b) between Hg1 and Hg4; (c) Hg1 and Hg2, and (d) Hg3 and Hg4. (e) C−H···O interactions between pyridyl rings of 4,4′-bipy and DMF. Color codes: Hg1 in yellow, Hg2 in light green, Hg3 in pink, and Hg4 in light violet. Blue dashed lines indicate Hg···π interactions in (b)−(d) and C−H···O in (e). π···π interactions are shown as light green dashed lines. Hg···C contacts: Hg1···C75, 3.641(3) Å; Hg1···C76, 3.769(3) Å; Hg2···C23, 3.628(3) Å; Hg2···C24, 3.867(3) Å; Hg3··· C80, 3.396(3); Hg3···C81, 3.816(3); Hg4···C28, 3.315(4) Å; Hg4···C27, 3.737(3) Å. Cg(5) = N1−C17−C18−C19−C20−C21; Cg(6) = N4− C48−C49−C50−C51−C52; Cg(7) = N5−C53−C54−C55−C56−C57; Cg(8) = N8−C100−C101−C102−C103−C104. 2.7 and 4.1% (P1B) or to a 14.1% (2) of C−C contact surface. unchanged. This suggests that the present methodology It should be mentioned that despite the marked contribution accounts for the subtle van der Waals interactions. According from planar interactions in 2, these aromatic rings are on the to calculations, P1B is more stable than P1A by 13.4 kJ·mol−1 brink of effective π···π interactions. Finally, Hg(II) centers per [Hg(Pip)2(μ-4,4′-bipy)]·DMF formula unit. The energy display a 3.4% (P1A) or between 3.0 and 3.4% (P1B) of difference between the two polymorphs is small, and this is in contact surface toward the aromatic rings of 4,4′-bipy, while no agreement with the formation of the two polymorphs and the contribution is observed in 2 (Table S4, Supporting possibility of selectively obtaining one of the two species. A Information). partition energy scheme (Table 7) was performed to DFT Calculations. With the aim of getting further insights determine the origin for the preference for P1B. We into the relative stabilities of P1A and P1B polymorphs, we decomposed the association energy (E) between two metal performed periodic DFT(PBE-D2) calculations with VASP dimers of vicinal chains (see Figure S18 for the model systems code.44,45 The DFT optimized structures are close to those used in the partition scheme) in two terms: (i) distortion of determined by single-crystal X-ray diffraction and particularly the two dimers with respect to a hypothetical isolated dimer both the distorted pentagonal Hg(II) geometry and the and (ii) the interaction energy between the distorted fragments distance between 1D zigzag polymeric chains are essentially (further details can be found in the Experimental Section). J https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Table 5. Intermolecular Interactions Present in P1Ba P1B H···A (Å) D···A (Å) D−H (Å) >D−H···A (deg) C(49)−H(49)···O(33) 2.291 3.203(5) 0.950 160.8 C(54)−H(54)···O(33) 2.438 3.339(4) 0.950 158.3 C(51)−H(51)···O(35) 2.333 3.245(5) 0.950 160.8 C(57)−H(57)···O(35) 2.396 3.281(6) 0.950 154.9 C(101)−H(101)···O(34) 2.310 3.225(6) 0.950 161.6 C(103)−H(103)···O(34) 2.434 3.352(5) 0.950 162.4 C(18)−H(18)···O(36) 2.312 3.222(5) 0.950 160.2 C(20)−H(20)···O(36) 2.495 3.413(4) 0.950 162.5 π interactions Cg(I)···Hg(J) Cg···Hgb MeJ_Perp β Cg(1)···Hg(3) 4.004 Cg(2)···Hg(1) 3.850 3.619 19.93 Cg(3)···Hg(4) 3.867 3.228 33.42 Cg(4)···Hg(2) 4.121 Cg(I)···Cg(J) Cg···Cgc αd β, γe Cg(I)_Perp, Cg(J)_Perpf slippageg Cg(1)···Cg(4) 3.632(2) 6.1(2) 24.7, 18.6 3.442(1), 3.301(1) 1.516 Cg(2)···Cg(2) 3.741(5) 0.03(2) 5.0, 5.0 3.727(1), 3.727(1) 0.323 Cg(3)···Cg(3) 3.947(2) 0.02(16) 33.1, 33.1 3.306(1), 3.306(1) 2.156 a Cg···Cg and Cg···Hg distances are given in Å. bCg···Hg = distance between ring centroid and Hg(II) center. cCg···Cg = distance between ring centroids (Å). dα = dihedral angle between planes I and J (deg). eOffset angles: β = angle Cg(I)−Cg(J) and normal to plane I (deg) and γ = angle Cg(I)−Cg(J) and normal to plane J (deg) (β = γ, when α = 0). fPerpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0). gSlippage = horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N2−C22−C23−C24−C25−C26; Cg(2) = N6−C74−C75C76−C77−C78; Cg(3) = N3−C27−C28−C29−C30−C31; Cg(4) = N7− C79−C80−C81−C82−C83. Figure 6. (a) Crystal packing of 2 assembled by π···π interactions along [220] (highlighted in light orange) or [22̅0] direction (highlighted in light blue). Spacefill representation of the aromatic rings stacked along (b) [220] or (c) [22̅0]. Hydrogen atoms are omitted for clarity. K https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Table 6. Bond Lengths (Å), Bond and Torsion Angles (deg), and Intermolecular Interactions Present in 2a bond lengths Hg(1)−O(1)#1 2.1896(19) Hg(1)−N(1)#1 2.282(2) Hg(1)−O(2) 2.993(2) bond angles O(1)−Hg(1)−N(1) 132.82(7) O(1)−Hg(1)−O(1) 99.41(12) O(1)#1−Hg(1)−N(1) 103.67(8) O(1)−Hg(1)−O(2) 48.24(12) N(1)−Hg(1)−N(1)#1 89.82(10) O(1)−Hg(1)−O(2)#1 98.90(12) N(1)−Hg(1)−O(2) 127.06(7) O(2)−Hg(1)−O(2)#1 133.42(10) N(1)−Hg(1)−O(2)#1 87.56(8) Cg(I)−Cg(J) torsion angle, χ Cg(1)−Cg(1) 0.1(3) π interactions Cg(I)···Cg(J) Cg···Cgb αc β, γd Cg(I)_Perp, Cg(J)_Perpe slippagef Cg(1)···Cg(1) 3.906(2) 0 21.3, 21.3 3.640(1), 3.640(1) 1.418 Cg(1)···Cg(2) 4.000(2) 1.83(12) 30.2, 31.0 3.427(1), 3.457(1) a #1 −x + 1, y, −z + 1/2. Cg···Cg distances are given in Å. Cg···Cg and Cg···Hg distances are given in Å. bCg···Cg = distance between ring centroids (Å). cα = dihedral angle between planes I and J (deg). dOffset angles: β = angle Cg(I)−Cg(J) and normal to plane I (deg) and γ = angle Cg(I)− Cg(J) and normal to plane J (deg) (β = γ, when α = 0). ePerpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0). fSlippage = horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = C2−C3−C4−C6−C7−C8; Cg(2) = N1−C9−C10−C11−C12−C13. Table 7. Energy Partition Scheme Analysis of the Results indicate that the metal dimer distortion energies are all Association Energy between Vicinal Chains Following very similar (the largest variation is 4.9 kJ·mol−1). In contrast, Equation 1a the interaction energy between the distorted fragments differs significantly, and it is larger for the vicinal chains of P1B. fragment E EDIS(1) EDIS(2) EINT Consequently, the association between vicinal chains is 21.0 P1A and 36.1 kJ·mol−1 stronger for P1B than for P1A, and this Hg(1)−Hg(1)/Hg(1)−Hg(1) −144.8 −76.1 −76.1 −296.9 suggests that the thermodynamic preference for P1B originates P1B mainly from metal cation···π and π···π interactions, rather than Hg(2)−Hg(1)/Hg(1)−Hg(2) −170.1 76.9 76.9 −323.9 the distortion arising from crystal packing. Remarkably, Hg(3) Hg(3)−Hg(4)/Hg(4)−Hg(3) −180.9 72.0 72.0 −324.9 and Hg(4) centers, which present the less conventional Hg(2)−Hg(1)/Hg(4)−Hg(3) −165.8 76.9 72.0 −314.7 a coordination environment, show the smallest distortion E is the association energy, EDIS(1) and EDIS(2) are the energies energies and the highest interaction energies, thus suggesting required to distort the dimers to achieve the crystal structure, and EINT is the interaction energy between distorted fragments. All values that the final geometry around the metal center is tuned by are given in kJ·mol−1. several factors. Photoluminescence Studies. Photoluminescence proper- ties were recorded using single crystals of P1A, P1B, and 2 (Figure S19, Supporting Information). Under UV excitation of a pulse laser beam at λexc = 335 nm, the samples display the corresponding emission spectra with emission maxima (λmax‑em) located at 565 nm (P1A), 591 nm (P1B), and 562 nm (2). The three spectra are composed of unstructured bands suggesting charge transfer transitions in character (Figure 7). The formation of the coordination polymers caused a bathochromic shift in emission compared to the free 4,4′- bipy ligand (λmax‑em = 401 nm).68 The emission of P1B is considerably blue-shifted, whereas P1A and 2 present closer emission maxima. All of them present moderate Stokes shifts of 12 152 cm−1 (P1A), 12 930 cm−1 (P1B), and 12 057 cm−1 (2). As displayed in CIE 1931 chromaticity diagrams, both P1A and 2 display yellow emission colors (λmax‑em), while P1B is reddish-orange at being irradiated under the selected λexc (Figure S20, Supporting Information). A thorough study of the photophysical properties of biphenyl and related aromatic ligands69 evinced how molecular structure affects the shape and wavelength position of the emission spectrum. The case of Figure 7. Solid-state emission spectra of complexes 2 (λmax‑em = 562 biphenyl molecule remarks how the gain or loss of planarity nm, bright gold), P1A (λmax‑em = 565 nm, mikado yellow), and P1B modifies the absorption and emission properties. Since its (λmax‑em = 591 nm, bright orange). Inset of the samples under UV increase is rendered to sharper spectra and better quantum light of λ = 335 nm exposure. yields, the loss of planarity tends to provoke larger stokes shifts. L https://doi.org/10.1021/acs.inorgchem.1c03762 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article Noteworthily strong π···π stacking as well as Hg···π Authors interactions open relaxation pathways to nonradiative decay Francisco Sánchez-Férez − Departament de Química, processes and quench fluorescence.6 Therefore, the remarkable Universitat Autònoma de Barcelona, 08193 Bellaterra, quenching in the emission of P1A and P1B compared to 2 Barcelona, Spain; orcid.org/0000-0002-9546-6828 could be explained by both the combination of intermolecular Xavier Solans-Monfort − Departament de Química, Hg···π and π···π interactions, being the former only present in Universitat Autònoma de Barcelona, 08193 Bellaterra, P1A and P1B. Besides, the aromatic rings in 2 are on the brink Barcelona, Spain; orcid.org/0000-0002-2172-3895 of 4.0 Å for an effective π···π interaction (Table 6), whereas Teresa Calvet − Departament de Mineralogia, Petrologia i P1A and P1B have stacking of 4,4′-bipy at closer distances up Geologia Aplicada, Universitat de Barcelona, 08028 to 3.736 (P1A) or 3.632 Å (P1B). By the same token, the Barcelona, Spain; orcid.org/0000-0002-4058-7171 larger stokes shift of P1B could be understood by considering Mercè Font-Bardia − Unitat de Difracció de Raig-X, Centres the 4,4′-bipy torsion angles. Complexes P1A (χ = 1.8(11)°) Científics i Tecnològics de la Universitat de Barcelona and 2 (χ = 0.1(3)°) (Tables 2 and 6) present an adjacent (CCiTUB), Universitat de Barcelona, 08028 Barcelona, λmax‑em instead, P1B has χ between 0.1(6) and 34.4(5)° (Table Spain 3) with the consequent shift of emission up to 591 nm. ■ Complete contact information is available at: CONCLUSIONS https://pubs.acs.org/10.1021/acs.inorgchem.1c03762 We have successfully isolated two polymorphic forms (P1A Notes and P1B), initially found to be concomitantly formed, as well The authors declare no competing financial interest. ■ as their desolvated form 2. Interestingly, the separation between P1A and P1B was achieved by temperature or anion-template-dependent formation. Such control in poly- ACKNOWLEDGMENTS morphism is scarce, especially when both forms tend to J.P. acknowledges financial support from the CB615921 concomitantly crystallize. The crystal structures of the three project, the CB616406 project from “Fundació La Caixa”, compounds have been deeply analyzed revealing that Hg(II) is and the 2017SGR1687 project from the Generalitat de able to accommodate severe distortions and access to an Catalunya. X.S.-M. acknowledges financial support from uncommon distorted pentagonal pyramidal geometry. Those MICINN (PID2020-112715GB-I00) and the Generalitat de differences combined with the conformations of the 4,4′-bipy Catalunya (2017SGR1323). F.S.-F. acknowledges the PIF ligands resulted in significant variations of their photophysical predoctoral fellowship from the Universitat Autònoma de properties. Besides, distortions in P1B not only modify the Barcelona. ■ emission maxima, but, according to periodic-DFT calculations, they are counterbalanced leading to a more stable form as a REFERENCES consequence of stronger Hg(II)···π and π···π interactions. Therefore, this work contributes to the understanding of (1) Glusker, J. 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