inorganics Article Synthesis of a Heterometallic [Zn2Ca] Pinwheel Array Stabilized by Amide-Amide Synthons Daniel Ejarque 1 , Teresa Calvet 2 , Mercè Font-Bardia 3 and Josefina Pons 1, * 1 Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain 2 Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain 3 Unitat de Difracció de Raig-X, Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), Universitat de Barcelona, Solé i Sabarís, 1-3, 08028 Barcelona, Spain * Correspondence: josefina.pons@uab.es; Tel.: +34-935-812-895 Abstract: The rational design of heterometallic compounds bearing s-block metal ions have been a difficult task for chemists owing to their lack of preferential geometries. However, some strategies, such as the design of coordinating pockets with different sizes and/or donor atoms, have offered great results. In this work, this strategy has been tested using Ca(II) as an s-block metal ion and a com- pound previously obtained by our group with the formula [Zn3 (µ-ACA)6 (4-phpy)2 ], which contains tetrahedral N,O- and octahedral O-coordinating pockets as a model structure. From this work, the corresponding heterometallic compound with the formula [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·EtOH (1) has been successfully synthesized, and fully characterized, and its crystal structure has been elucidated. Furthermore, we have compiled all the crystal structures containing [Zn2 M] pinwheel secondary building units (SBUs), where M stands for an s-block metal ion, and the observed tendencies, as well as the promising applications as template SBUs for the preparation of 1D–3D coordination polymers, Citation: Ejarque, D.; Calvet, T.; have been discussed. Finally, solid-state UV-Vis and photoluminescence have been recorded and Font-Bardia, M.; Pons, J. Synthesis of compared with the homometallic [Zn3 (µ-ACA)6 (4-phpy)2 ] compound. a Heterometallic [Zn2 Ca] Pinwheel Array Stabilized by Amide-Amide Keywords: heterometallic complexes; trinuclear complexes; Zn(II); Ca(II); secondary building unit; Synthons. Inorganics 2022, 10, 118. X-ray crystal structure; α-acetamidocinnamic acid; solid-state photoluminescence https://doi.org/10.3390/ inorganics10080118 Academic Editors: Marius Andruh, Wolfgang Linert and Daniel 1. Introduction García-Vivó Coordination compounds presenting diverse metal ions within the same molecule Received: 19 July 2022 denoted as heterometallic complexes have recently received special attention owing to Accepted: 12 August 2022 their superior structural versatility compared with their homometallic counterparts [1], as Published: 14 August 2022 well as their unique physical and chemical properties arising from the synergistic effect of their different metallic ions [2,3]. As a result, this subclass of coordination complexes have Publisher’s Note: MDPI stays neutral been used in the fields of photoluminescence [4], magnetism [5], or catalysis [6], among with regard to jurisdictional claims in others [7]. published maps and institutional affil- Researchers focused on heterometallic chemistry have been mostly devoted to the iations. rational design of d-d and d-f metal complexes [8,9], while those presenting s-block metal ions have remained less explored caused by their challenging obtention [10]. This difficulty mainly arises from the larger atomic radii of s-block compared with d-block metals, which Copyright: © 2022 by the authors. allows a broad range of coordination numbers and geometries for the s-block metals Licensee MDPI, Basel, Switzerland. lack any geometrical preference [11–13]. One of the strategies that have successfully This article is an open access article resulted in the obtention of d-s heterometallic compounds relies on the formation of distributed under the terms and different coordinating pockets within the same complex providing variable coordination conditions of the Creative Commons environments and sizes where different metal ions can selectively bind [14,15]. Following Attribution (CC BY) license (https:// this strategy and benefiting from the oxophilic character of s-block metal ions [16,17], the creativecommons.org/licenses/by/ formation of four- and six-coordinating pockets based on O-donor or mixed N,O-donor 4.0/). atoms is a feasible way to selectively bind different metal ions toward the formation of Inorganics 2022, 10, 118. https://doi.org/10.3390/inorganics10080118 https://www.mdpi.com/journal/inorganics Inorganics 2022, 10, 118 2 of 13 d-s heterometallic compounds [18–20]. Interestingly, the heterometallic arrays prepared by the aforementioned strategy can be utilized as secondary building units (SBUs) toward the formation of heterometallic 1D–3D coordination polymers (CPs) [21–23]. Accordingly, previous examples of d-s heterometallic SBUs with pockets of different sizes based in N,O- and O-donor atoms have been found in the literature [24–26]. In this context, we have been working on the synthesis of Zn(II) trinuclear pinwheel SBUs with [Zn3 (µ-COO)6 ] array based on the α-acetamidocinnamate (ACA) ligand. This SBU presents two different types of metal ions, displaying two lateral tetrahedral Zn(II) ions coordinated in N,O-based pockets, where the N-atom comes from non-sterically hindered pyridines (4-phenylpyridine, 4-phpy; 4,40 -bipyridine, 4,40 -bipy), while the central metal ions display an octahedral environment surrounded by oxygen atoms from six ACA moieties [27,28]. In addition, it has been observed that pinwheel SBUs are good candidates for the formation of heterometallic complexes owing to their inequivalent metal ions showing different coordination environments [29,30]. Furthermore, we have observed in previous contributions that pinwheel SBUs presenting ACA promote the formation of a pattern of amide-amide intramolecular interactions, which stabilize the pinwheel disposition as in other examples found in the literature, where the patterns can be formed by either H-bonds [28,31,32], or planar interactions [33,34]. Bearing in mind all the requirements stated above, we envisioned that the introduction of an s-block metal ion, such as Ca(II), should be selectively coordinated into the octahedral O-based pocket, forming a heterometallic SBU. Therefore, we have synthesized one [Zn2 Ca] trinuclear compound with the formula [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·EtOH (1), which has been fully characterized. In addition, the crystal structure of [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·2EtOH (1C) has been elucidated, revealing the pinwheel formation, as well as the introduction of the Ca(II) ions in the octahedral and the Zn(II) ions in the tetrahedral pocket. Finally, we have analyzed its solid-state UV-Vis and photoluminescence, and we have compared it with its homometallic counterpart. 2. Experimental Section 2.1. Materials and General Methods Zinc(II) acetate dihydrate (Zn(OAc)2 ·2H2 O), calcium carbonate (CaCO3 ), nitric acid 69% (HNO3 ), α-acetamidocinnamic acid (HACA), 4-phenylpyridine (4-phpy) as reagents, and ethanol (EtOH) as solvent were purchased from Sigma-Aldrich. Calcium nitrate tetrahydrate Ca(NO3 )2 ·4H2 O was prepared using CaCO3 and HNO3 69% in a 1:2 molar ratio in Milli-Q water as solvent. Deuterated dimethylsulfoxide (DMSO-d6 ) was used for the NMR experiments and was purchased from Eurisotop. All of them were used without further purification. All the reactions and manipulations were carried out in air at room temperature (RT). The powder X-ray diffraction (PXRD) pattern was measured with a Siemens D5000 apparatus with 40 kW and 45 mA using CuKα radiation with λ = 1.5406 Å. The diffractogram was recorded from 2θ = 5◦ to 30◦ with a step scan of 0.02◦ , counting 1 s at each step. Elemental analyses (C, H, N) were carried out on a Euro Vector 3100 instrument. HR-ESI-MS measurements were recorded after dissolving the corresponding complex in MeOH in a MicroTOF-Q instrument equipped with an electrospray ionization source (ESI) in positive mode. Na+ ions come from the MeOH solvent which contains <50 ppb. Conditions were those used in routine experiments. The nebulizer pressure was 1.5 bar, the desolvation temperature was 180 ◦ C, dry gas was 6 L·min−1 , the capillary counter-electrode voltage was 5 kV, and the quadrupole ion energy, was 5.0 eV. FTIR-ATR spectra were recorded on a Tensor 27 (Bruker) spectrometer, equipped with an attenuated total reflectance (ATR) accessory model MKII Golden Gate with a diamond window in the range 4000–500 cm−1 . 1 H, 13 C{1 H} and DEPT-135 NMR spectra were recorded on a Bruker Ascend 300 MHz spectrometer in DMSO-d6 solutions at RT. All chemical shifts (δ) are given in ppm relative to Me4 Si as the internal standard. Solid-state UV-Vis spectra were carried out using a Cary 4000 spectrophotometer between 200–800 nm. Solid-state photoluminescence measurements were recorded using a Varian Cary Eclipse Fluorescence Inorganics 2022, 10, 118 3 of 13 spectrophotometer between 340 and 500 nm. CIE 1931 chromaticity diagram was generated using Origin Pro 2019b software. 2.2. Synthesis of [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·EtOH (1) An EtOH solution (5 mL) of Zn(OAc)2 ·2H2 O (50.0 mg, 0.228 mmol) and Ca(NO3 )2 ·4H2 O (26.9 mg, 0.114 mmol) was carefully added to an EtOH solution (5 mL) of HACA (140 mg, 0.682 mmol) and 4-phpy (70.7 mg, 0.456 mmol) at RT. The solution was gently stirred for 15 h until a white precipitate was obtained, and the resulting powder was kept on ice for 1 h. Afterward, the reaction was filtered, washed with two portions of cold Et2 O (2 × 10 mL), and dried under vacuum, yielding 1, which was characterized. Single crystals suitable for X-ray diffraction of [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·2EtOH (1C) were obtained after letting evaporate the mother liquors at RT for 15 days. 1. Isolated Yield: 80.3 mg (40.2% based on Zn). Elemental analysis calc(%) for C90 H84 N8 O19 Zn2 Ca (1752.53): C 61.68; H 4.83; N 6.39; found: C 61.55; H 4.54; N 6.36. HR- MS (ESI+ , MeOH): m/z (%) = 156.0822 (100%) (calc. for [4-phpy + H]+ = 156.0808); 228.0631 (100%) (calc. for [HACA + Na]+ = 228.0631); 716.0806 (100%) (calc. for [ZnCa(ACA)3 ]+ = 716.0894); 943.1456 (100%) (calc. for [ZnCa(ACA)4 + Na]+ = 943.1452). FTIR-ATR (wave number, cm−1 ): 3375(w) [ν(O-H)], 3225(w) [ν(N-H)], 3161–3003(br) [ν(C-H)ar + ν(C- H)alk ], 2980–2650(br) [ν(C-H)al ], 1667(m) [ν(C=O)], 1652(w), 1599(s) [νas (COO)], 1576(sh), 1523(m) [ν(C=C/C=N)], 1491(w), 1447(w), 1386(s) [νs (COO)], 1350(s) [δ(C=C/C=N)], 1279(m), 1225(w), 1210(w), 1183(w), 1161(w), 1120(w), 1076(w) [δip (C-H)], 1031(w) [δip (C- H)], 1015(w) [δip (C-H)], 971(w), 964(w), 932(w), 837(w), 786(w), 760(m) [δoop (C-H)], 730(w), 688(s) [δoop (C-H)], 624(w), 609(w), 590(w), 574(m), 522(w). 1 H NMR (300 MHz; DMSO-d6 ; Me4 Si; 298 K): δ = 9.14 [6H, s, NHACA ], 8.65 [4H, dd, 3 J = 4.6 Hz, 4 J = 1.6 Hz, o-Hpy,4-phpy ], 7.81 [4H, m, o-Hph,4-phpy ], 7.73 [4H, dd, 3 J = 4.6 Hz, 4 J = 1.6 Hz, m-Hpy,4-phpy ], 7.51 [18H, m, o-HACA + m-Hph,4-phpy + p-Hph,4-phpy ], 7.33 [12H, t, 3 J = 7.4 Hz, m-HACA ], 7.27 [6H, d, 3 J = 7.4 Hz, p-H 3 ACA ], 7.24 [6H, s, NH-C-CHACA ], 4.35 [1H, t, J = 5.0 Hz, OHEtOH ], 3.44 [2H, qd, J = 6.9 Hz, 4.9 Hz, CH2 ,EtOH ], 1.96 [18H, s, CO-CH3 ,ACA ], 1.05 [3H, t, 3 J = 7.0 Hz, 3 CH3 ,EtOH ]. 13 C{1 H} NMR (75 MHz; DMSO-d6 ; Me4 Si; 298 K): δ = 170.3 [NH-COACA ], 168.4 [CO2 ,ACA ], 150.2 [o-Cpy,4-phpy ], 147.7 [Ph-Cpy ,4-phpy ], 137.0 [Py-Cph ,4-phpy ], 135.3 [O2 C- CACA ], 129.9 [HN-C-CH-CACA ], 129.6 [o-CACA ], 129.4 [m-Cph ,4-phpy ], 129.3 [p-Cph ,4-phpy ], 128.6 [p-CACA ], 128.4 [m-CACA ], 128.1 [NH-C-CHACA ], 127.0 [o-Cph ,4-phpy ], 121.6 [m- Cpy ,4-phpy ], 56.2 [CH2 ,EtOH ], 23.2 [CO-CH3 ,ACA ], 18.6 [CH3 ,EtOH ]. DEPT-135 NMR (75 MHz; DMSO-d6 ; Me4 Si; 298 K): δ = 150.2 [o-Cpy,4-phpy ], 129.5 [o-CACA ], 129.3 [m-Cph ,4-phpy ], 129.3 [p-Cph ,4-phpy ], 128.5 [p-CACA ], 128.3 [m-CACA ], 128.1 [NH-C-CHACA ], 127.0 [o-Cph ,4-phpy ], 121.5 [m-Cpy ,4-phpy ], 56.1 [CH2 ,EtOH ], 23.2 [CO-CH3 ,ACA ], 18.6 [CH3 ,EtOH ]. 2.3. X-ray Crystallographic Data For compound 1C, a colorless prism-like specimen was used for the X-ray crystallo- graphic analysis. The X-ray intensity data were measured on a D8 Venture system equipped with a multilayer monochromator and a Mo microfocus (λ = 0.71073 Å). The frames of the compound were integrated with the Bruker SAINT Software package using a narrow-frame algorithm. All hydrogen atoms were refined using a riding model (AFIX) with an isotropic temperature factor equal to 1.2, the equivalent temperature factor of the atom to which are linked, and thus, the bond lengths of X-H were fixed. The structure was solved and refined using the SHELXTL Software Package (version 2018/3) [35]. The final cell constants and volume were based upon refinement of the XYZ- centroids of reflections above 20 σ(I). Data were corrected for absorption effects using the Multi-Scan method (SADABS). Crystal data and relevant details of structure refinement for 1C are reported in Table 1. Complete information about the crystal structure and molecular geometry is available in CIF format via CCDC 2190605 (1C). Molecular graphics were generated with Mercury 4.3.1 software [36], using the POV-Ray image package [37]. The color codes for all of the molecular graphics are as follows: dark blue (Zn), light green (Ca), red (O), light blue (N), gray (C), and white (H). The accessible void volumes have Inorganics 2022, 10, 118 4 of 13 been calculated with Mercury 4.3.1 software [36], using a probe radius of 1.2 Å [38]. The evaluation of the geometry distortion of Zn(II) and Ca(II) cores of 1C have been done using version 2.1 of SHAPE software from the corresponding cif files [39]. Hirshfeld surface analyses have been performed using CrystalExplorer 17.5 [40]. Table 1. Crystal data and structure refinement for 1C. 1C CCDC 2190605 Empirical formula C92 H90 CaN8 O20 Zn2 Formula weight 1798.53 T (K) 100(2) Wavelength (Å) 0.71073 System, space group Triclinic, P1 Unit cell dimensions a (Å) 11.6745(12) b (Å) 13.2064(13) c (Å) 15.5353(16) α (◦ ) 67.641(3) β (◦ ) 85.612(4) γ (◦ ) 73.680(4) V (Å3 ) 2124.6(4) Z 1 Dcalc (mg/m3 ) 1.406 µ (mm−1 ) 0.703 F (000) 938 Crystal size (mm3 ) 0.243 × 0.115 × 0.048 −16 <= h <= 16 hkl ranges −17 <= k <= 18 0 <= l <= 22 θ range (◦ ) 2.221 to 30.593 Reflections collected/unique/[Rint ] 13,031/13,031/[Rint = 0.0885] Completeness to θ (%) 99.9 Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.7461 and 0.6570 Refinement method Full-matrix least-squares on | F |2 Data/Restrains/Parameters 13,031/0/563 Goodness-on-fit on | F |2 1.046 Final R indices [I > 2σ(I)] R1 = 0.0429, wR2 = 0.1153 R indices (all data) R1 = 0.0900, wR2 = 0.1406 Extinction coefficient n/a Largest diff-peak and hole (e. Å−3 ) 1.516 and −0.421 3. Results and Discussion 3.1. Synthesis and Characterization of 1 Compound [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·EtOH (1) was prepared by combining Zn(OAc)2 ·2H2 O, Ca(NO3 )2 ·4H2 O, HACA, and 4-phpy in a 2:1:6:4 molar ratio using EtOH at room temperature (RT) (Scheme 1). The corresponding single crystals suitable for X-ray crystallographic analysis were obtained after allowing the mother liquors of the reaction to evaporate, yielding [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·2EtOH (1C). Additional details of the syn- thesis and procedure for obtaining the single crystals are given in the Experimental Section. Inorganics 2022, 10, x FOR PEER REVIEW 5 of 14 crystallographic analysis were obtained after allowing the mother liquors of the reaction to evaporate, yielding [Zn2Ca(μ-ACA)6(4-phpy)2]·2EtOH (1C). Additional details of the Inorganics 2022, 10, 118 5 of 13 synthesis and procedure for obtaining the single crystals are given in the Experimental Section. [Zn2Ca(μ-ACA)6(4-phpy)2]·EtOH (1) Scheme1. Scheme 1. Outline Outline of of the the synthesis synthesisof of1. 1. The The compound compound was was characterized characterized by by powder powder X-ray X-ray diffraction diffraction (PXRD), (PXRD), elemental elemental analysis 1 13 1 analysis (EA), (EA), HR-ESI-MS, HR-ESI-MS, FTIR-ATR, FTIR-ATR, H, 1H, C{ H}, and DEPT-135 NMR 13 1 NMR spectroscopies, spectroscopies, and and single single crystal crystalX-ray X-raydiffraction. diffraction. The The phase phase puritypurity of of the the bulk bulk sample sample of of 11 was was verified verified by by PXRD PXRD (SI:(SI: Figure Figure S1). S1). In In addition, addition, thethe EA EA of of compound compound 11 agreesagrees with with thethe proposed proposed formula. + formula. TheThe positive positiveionization ionizationmass massspectrum spectrum(ESI (ESI+-MS) -MS) ofof the the compound compound was was recorded recorded in MeOH as solvent. In these conditions, the complex displayed in MeOH as solvent. In these conditions, the complex displayed heterometallic fragments heterometallic fragments corresponding + + at m/z 716.0886 (100%) and corresponding to to[ZnCa(ACA) [ZnCa(ACA)33]]+ and and [ZnCa(ACA) [ZnCa(ACA)44 + + Na] Na]+ at m/z 716.0886 (100%) and 943.1456 943.1456 (100%), (100%), respectively respectively (SI: (SI: Figure Figure S2). S2). InIn thethe FTIR-ATR FTIR-ATR spectrum, spectrum, the the absence absence of of aa broad − 1 −1 broad band bandbetween between2704 2704andand24052405cm cm−1(ν(O-H) (ν(O-H)HACA HACA)) and and aa strong strong peak peakatat16371637cm cm−1 (ν(COOH) (ν(COOH)HACA ) suggests that the HACA was not protonated. In addition, the spectrum HACA) suggests that the HACA was not protonated. In addition, the spectrum presented −1 for ν (COO) and 1386 cm−1 presented the characteristic carboxylate the characteristic carboxylate bandsbands at 1599 cm at 1599 cm−1 for νas(COO) as and 1386 cm−1 for for ν (COO) (SI: Figure S3). The difference between these bands was 213 cmto−1 due to s νs(COO) (SI: Figure S3). The difference between these bands was 213 cm−1 due a highly aasymmetric highly asymmetric bridged coordination bridged coordination mode [41,42], mode [41,42], with a Δ value a ∆ value withlarger larger than its than its correspond- corresponding homometallic compound [27], in agreement ing homometallic compound [27], in agreement with the data obtained from the crystal with the data obtained from the crystal structure structure1[41,43]. [41,43]. The The 1HH NMR NMR spectrum spectrum was wasrecorded recordedto toascertain ascertainthe theratio ratiobetween betweenthe theACA ACA and and 4- 4-phpy ligands, resulting in a 6:2 (ACA:4-phpy) molar ratio phpy ligands, resulting in a 6:2 (ACA:4-phpy) molar ratio which agrees with the X-ray which agrees with the X- ray crystallographic data. It displayed a signal at 9.14 ppm corresponding to the NH crystallographic data. It displayed a signal at 9.14 ppm corresponding to the NH proton proton atom of ACA, while the signals attributable to the aromatic protons of 4-phpy atom of ACA, while the signals attributable to the aromatic protons of 4-phpy were ob- were observed between 8.65 and 7.51 ppm, some of them being overlapped with the o-H served between 8.65 and 7.51 ppm, some of them being overlapped with the o-H atoms atoms from ACA in the signal of 7.51 ppm. In addition, the remaining aromatic protons from ACA in the signal of 7.51 ppm. In addition, the remaining aromatic protons from from ACA were observed at 7.33 and 7.24 ppm, followed by the hydrogen atom from the ACA were observed at 7.33 and 7.24 ppm, followed by the hydrogen atom from the alkene alkene group, which appeared at 7.24 ppm. Finally, the signals observed in the upfield group, which appeared at 7.24 ppm. Finally, the signals observed in the upfield region region corresponded to the EtOH hydrogen atoms at 4.35, 3.44, and 1.05 ppm, respectively, corresponded to the EtOH hydrogen atoms at 4.35, 3.44, and 1.05 ppm, respectively, to- together with the methyl proton atoms at 1.96 ppm (SI: Figure S4) [44]. gether with the methyl proton atoms at 1.96 ppm (SI: Figure S4) [44]. The 13 C{1 H} NMR displayed the characteristic bands of the carbonyl and carboxylate The 13C{1H} NMR displayed the characteristic bands of the carbonyl and carboxylate groups from ACA at 170.3 and 168.4 ppm, respectively (SI: Figure S5a). In addition, the groups from ACA at 170.3 and 168.4 ppm, respectively (SI: Figure S5a). In addition, the signals corresponding to the carbon atoms from the 4-phpy appeared between 150.2 and signals 129.3 corresponding ppm. In this zone,to thethe carbon carbon atoms atoms from from thethe 4-phpy alkene groupappeared of ACAbetween were also 150.2 and shown 129.3 ppm. In this zone, the carbon atoms from the alkene group at 135.3 and 128.1 ppm, in line with the absence of the first band in the DEPT-135 spectrum of ACA were also shown at 135.3 (SI: andS5b). Figure 128.1Furthermore, ppm, in line with the absence the aromatic carbonof theatoms first band frominACAthe DEPT-135 appeared spectrum between 129.9 and 128.4 ppm, while the methyl carbon atom was found at 23.2appeared (SI: Figure S5b). Furthermore, the aromatic carbon atoms from ACA ppm. Finally, between the 129.9 and 128.4 ppm, while the methyl carbon atom was found upfield region of the spectra also contained two signals attributed to the EtOH carbon at 23.2 ppm. Finally, the upfield region of the spectra atoms (56.2 and 18.6 ppm) (SI: Figure S5a).also contained two signals attributed to the EtOH carbon atoms (56.2 and 18.6 ppm) (SI: Figure S5a). 3.2. Structural Description and Hirshfeld Surface Analysis of 1C Compound 1C belongs to the triclinic P1 space group. It consisted of a heterometallic pinwheel array presenting two lateral Zn(II) and one central Ca(II) metal ions, as well as six ACA ligands with µ2 -η1 :η1 coordination modes that formed the trinuclear array, while two 4-phpy ligands occupied the apical positions (Figure 1a). The two Zn(II) lateral ions displayed [ZnO3 N] cores with tetrahedral geometries (S = 0.959, τ4 = 2.53) [45,46], while the central Ca(II) ion formed a [CaO6 ] core with an octahedral geometry (S = 0.100, Inorganics 2022, 10, 118 6 of 13 ata = 60◦ ) (Table 2; SI: Table S1) [45,47,48]. Furthermore, the supramolecular structure of 1C displayed voids with an accessible volume of 32.03 Å3 (1.5% of the unit cell volume) [38], where two EtOH molecules were situated. The bond lengths involving the Zn(II) metal center ranged between 1.9622(16) and 2.0504(19) Å, while those with Ca(II) as metal core presented bond lengths oscillating between 2.2055(17) and 2.2117(18) Å, giving rise to highly asymmetric bridged coordination modes, as the FTIR-ATR spectrum suggest. In addition, the bond angles ranged between 89.50(7) and 180◦ . All of these values are similar Inorganics 2022, 10, x FOR PEER REVIEW 7 of 14 to other heterometallic pinwheel arrays containing lateral Zn(II) and central Ca(II) metallic centers [49,50]. Figure 1. (a) Molecular structure of compound 1C. (b) Intramolecular interactions (c) Supramolec- Figure 1. (a) Molecular structure of compound 1C. (b) Intramolecular interactions (c) Supramolecular ular expansion of 1C along the (002) plane. (d) In-detail view of the intermolecular interactions in- expansion of 1C along the (002) plane. (d) In-detail view of the intermolecular interactions involved volved in the expansion along the (002) plane of 1C. (e) General view of the supramolecular expan- in the sion ofexpansion along 1C through the (002) the [001] plane of 1C. (e) General view of the supramolecular expansion of 1C direction. through the [001] direction. The intramolecular interactions of 1C were based on a hexagonal pattern of am- ide···amide homosynthons [51], presenting three different pairs of N-H···O interactions as their different contributions of H···O and O···H contacts indicated in their respective 2D fingerprint plots (SI: Figure S6). This pattern has been previously observed by our group, stabilizing the structure of similar complexes presenting pinwheel arrays (Figure 1b) [27,28]. On the other side, their intermolecular interactions propagated the structure along Inorganics 2022, 10, 118 7 of 13 Table 2. Selected bond lengths (Å), bond angles (◦ ), intramolecular and intermolecular interactions (Å) for compound 1C. Bond Lengths (Å) Zn(1)-O(1) 1.9622(16) Ca-O(2) 2.2115(17) Zn(1)-O(4) 1.9626(17) Ca-O(5) 2.2117(18) Zn(1)-O(7) 1.9375(16) Ca-O(8) 2.2055(17) Zn(1)-N(4) 2.0504(19) Bond Angles (◦ ) O(1)-Zn(1)-O(4) 110.43(7) O(2)-Ca-O(5)#1 93.98(7) O(1)-Zn(1)-N(4) 102.87(7) O(5)-Ca-O(5)#1 180.0 O(4)-Zn(1)-N(4) 100.42(7) O(8)-Ca-O(2) 88.09(6) O(7)-Zn(1)-O(1) 123.72(7) O(8)-Ca-O(2)#1 91.91(6) O(7)-Zn(1)-O(4) 117.39(8) O(8)-Ca-O(5) 89.50(7) O(7)-Zn(1)-N(4) 96.16(7) O(8)-Ca-O(5)#1 90.50(7) O(2)#1-Ca-O(2) 180.0 O(8)-Ca-O(8)#1 180.0 O(2)-Ca-O(5) 86.02(7) Twist Angles (◦ ) O(2)#1-Cg(1)-Cg(2)-O(5)#1 58.08 O(5)-Cg(1)-Cg(2)-O(8)#1 61.97 O(8)-Cg(1)-Cg(2)-O(2) 59.95 Intramolecular Interactions (Å) D-H···A D-H (Å) H···A (Å) D···A (Å) >D-H···A (◦ ) N(1)-H(1N)···O(6) 0.88 1.99 2.826(3) 158 N(2)-H(2N)···O(9) 0.88 2.13 2.840(2) 137 N(3)-H(3)···O(3) 0.88 2.17 2.892(3) 140 Intermolecular Interactions (Å) D-H···A D-H (Å) H···A (Å) D···A (Å) >D-H···A (◦ ) O(1W)-H(1WO)···O(4) 0.84 2.07 2.903(5) 173 O(2W)-H(2WO)···O(3) 0.84 2.24 3.053(5) 162 C(14)-H(14)···O(1W) 0.95 2.52 3.439(5) 162 C(30)-H(30)···O(1W) 0.95 2.48 3.390(6) 161 C(40)-H(40)···O(2W) 0.95 2.42 3.293(5) 152 C(18)-H(18)···Cg(3) 0.95 3.05 3.791(5) 136 C(44)-H(44)···Cg(4) 0.95 3.13 3.920(5) 142 #1: -x+1, -y+1, -z+1. Cg(1) = O(2)#1 O(5) O(8); Cg(2) = O(2) O(5)#1 O(8)#1; Cg(3) = C(4) C(5) C(6) C(7) C(8) C(9); Cg(4) = C(15) C(16) C(17) C(18) C(19) C(20) The intramolecular interactions of 1C were based on a hexagonal pattern of amide···amide homosynthons [51], presenting three different pairs of N-H···O interactions as their differ- ent contributions of H···O and O···H contacts indicated in their respective 2D fingerprint plots (SI: Figure S6). This pattern has been previously observed by our group, stabilizing the structure of similar complexes presenting pinwheel arrays (Figure 1b) [27,28]. On the other side, their intermolecular interactions propagated the structure along the (002) plane through their occluded EtOH molecules, which join together contiguous pinwheel arrays by H-bonds, involving the hydroxyl groups of the EtOH molecules and the carbonyl and carboxylate oxygen atoms from ACA, as well as three complementary C-H···O interactions formed by the oxygen atoms of the EtOH molecules and nearby hydrogen atoms of either ACA or 4-phpy ligands. All these interactions were clearly highlighted in the Hirshfeld surface of 1C as red spots, representing either the O···H or H···O contacts, with a 4.7% of the contact surface, where the O···H contacts show closer interactions compared with the H····O contacts (SI: Figure S7a,b). In addition, this propagation was also supported by C-H···π interactions between two ACA ligands of nearby trinuclear units, which were also observed in the curvedness representation of the Hirshfeld surface of 1C, representing a 17.1% contact surface in the 2D fingerprint plots, and displaying the characteristic wings shape (Figure 1c,d; SI: Figure S7c,d) [52]. Finally, the structure was also expanded through the [001] direction by reciprocal C-H···π interactions between o-H atoms from 4-phpy and vicinal ACA ligands, giving rise to a 3D net (Figure 1e). Inorganics 2022, 10, 118 8 of 13 3.3. CSD Study of Heterometallic [Zn2 M] Pinwheel SBUs (M = S-Block Metal) A search in the Cambridge Structural Database (CSD version 5.43 March 2022) of [Zn2 M] pinwheel SBUs, where M stands for an s-block metal ion, revealed a total of 35 hits. The synthetic methodologies followed mainly used nitrate salts as metal sources, while MCl2 , M(OH)2 , and Zn(OAc)2 ·2H2 O were used to a lesser extent. It also has been noticed that polar solvents, such as H2 O, EtOH, or DMF, have been used in all the syntheses. In addition, for the preparation of 27 complexes, high temperatures (over 80 ◦ C) and long reaction times (over 48 h) were required. Conversely, our compound was obtained in mild conditions, using EtOH as the solvent at RT. Of note, some examples were found where the preparation of these SBUs was performed starting from a pre-synthesized com- plex containing pivalate [53,54] or crotonate [55,56] ligands. Interestingly, compound {[Zn2 Ca(fda)4 ]·2Me2 NH2 }n (fda = 2,5-furandicarboxylate) was synthesized from a pre- formed 0D pinwheel SBU using pivalate and pyridine, which preserved its trinuclear array when their pivalate ligands were exchanged with 2,5-furandicarboxylates toward the formation of a 3D CP [54]. (SI: Table S2). The results of the CSD search are summarized in Figure 2 and the SI (Table S3). It was observed that 19 structures showed a central Ca(II) ion, followed by Mg(II) displaying 5 hits, Na(I) with 4 hits, Sr(II) and Ba(II) both presenting 3 hits, and K(I) containing 1 hit. Furthermore, complexes displaying 0D (11 hits) and 3D (14 hits) structures were more abundant than 1D or 2D CPs (both presenting 5 hits). Of note, the central s-block metal ions were located in octahedral O-based pockets in almost every structure, except for three examples containing Sr(II) or Ba(II), whose bigger ionic radii allowed the expansion of the coordination sphere reaching [SrO8 ]/[BaO8 ] cores [15,57,58]. Otherwise, the coordination sphere of the lateral Zn(II) ions showed diverse coordination pockets, displaying [ZnO4 ] (15 hits), [ZnO5 ] (1 hit), [ZnO3 N] (13 hits), [ZnO4 N] (4 hits), and [ZnO3 N2 ] (2 hits) cores. However, the five-coordinated environments were relatively scarce, being mainly orig- inated from µ2 -η2 : η1 carboxylate coordination modes [22,56,59,60], the coordination of water molecules [61,62], or by µ2 -η2 ditopic oxygen atoms from carboxylate moieties [63] (SI: Figure S8). Regarding the tetrahedral pockets, the two cores found differed in their lateral position, which could be either a N- or an O-atom. In 12 structures presenting [ZnO3 N] cores, the N-atoms arose from pyridine or bipyridine ligands displaying 0D or 1D complexes, such as in 1C. At the same time, there was a unique case where 2D sheets were formed by the expansion of a dicarboxylate ligand [64]. Conversely, all the structures with [ZnO4 ] cores displayed 2D or 3D arrays, showing mostly solvent molecules coordinated to their apical positions, while only in three examples the molecular expansion was done by both the carboxylates forming the SBU and those located in their apical positions [15,54,65]. Therefore, this overview showed that while the investigation regarding heterometallic com- plexes with [Zn2 M] (M = s-block metal ion) pinwheel SBUs has been scarce, it offers a great structural variability toward the formation of heterometallic arrays which are completely modulable, as it is possible to extend their structures through their apical positions or their central ligands either by bipyridines, carboxylates, or both. However, this compilation shows that further investigation about this SBU needs to be carried out to control the obtained structures. 3.4. Photophysical Properties The solid-state UV-Vis and photoluminescence of 1 and [Zn3 (ACA)6 (4-phpy)2 ] was recorded (Figure 3). The absorption spectra of both compounds showed similar profiles, presenting a broad unstructured signal with a maximum between 240 and 323 nm (1), and 240 and 330 nm ([Zn3 (ACA)6 (4-phpy)2 ]). When irradiated at an excitation wavelength of 320 nm, both compounds showed one unstructured signal with its maxima centered at 360 nm and a Stokes shift of 3472 cm−1 , suggesting local electronic transitions which have been tentatively attributed to intra-ligand charge transfers (ILCT) of either the ACA or the 4-phpy ligands [66,67]. The comparison between the two compounds showed that they only present minimal differences between their emission profiles. In addition, the resultant Inorganics 2022, 10, 118 9 of 13 emission color for both compounds was blue for 1 and electric violet for the homometallic compound, according to the CIE 1931 chromaticity diagram (SI: Figure S9) [68]. Therefore, it was observed that the exchange of a Zn(II) for a Ca(II) in the central position of the pinwheel SBU did not confer significant differences to the photoluminescence properties of Inorganics 2022, 10, x FOR PEER REVIEW 9 of 14 these complexes. 0D 1D 12 10 8 6 4 2D-3D 3D 2 ZnO4 0 ZnO5 0D ZnO3N 1D 2D ZnO4N 3D ZnO3N2 3D Inorganics 2022, 10, x FOR PEER REVIEW 10 of 14 Figure 2. Figure 2. Results Resultsofofthe theCSD CSDstudy of of study [Zn[Zn 2M] pinwheel 2 M] SBUs pinwheel (M =(M SBUs s-block metalmetal = s-block ion) with ion) awith repre- a sentation of the more abundant type of structures. representation of the more abundant type of structures. 3.4. Photophysical Properties The solid-state UV-Vis and photoluminescence of 1 and [Zn3(ACA)6(4-phpy)2] was recorded (Figure 3). The absorption spectra of both compounds showed similar profiles, presenting a broad unstructured signal with a maximum between 240 and 323 nm (1), and 240 and 330 nm ([Zn3(ACA)6(4-phpy)2]). When irradiated at an excitation wavelength of 320 nm, both compounds showed one unstructured signal with its maxima centered at 360 nm and a Stokes shift of 3472 cm−1, suggesting local electronic transitions which have been tentatively attributed to intra-ligand charge transfers (ILCT) of either the ACA or the 4-phpy ligands [66,67]. The comparison between the two compounds showed that they only present minimal differences between their emission profiles. In addition, the result- ant emission color for both compounds was blue for 1 and electric violet for the homome- tallic compound, according to the CIE 1931 chromaticity diagram (SI: Figure S9) [68]. Therefore, it was observed that the exchange of a Zn(II) for a Ca(II) in the central position of the pinwheel SBU did not confer significant differences to the photoluminescence prop- erties of these complexes. Figure 3. Figure 3. Normalized absorption absorption (dashed (dashed line) and emission (solid line) spectra of compound 1 (green) and [Zn3 (µ-ACA)6 (4-phpy)2]] (blue). (green) and [Zn3 (μ-ACA)6 (4-phpy) 2 (blue). 4. Conclusions In this contribution, we successfully synthesized and characterized one d-s hetero- metallic compound with a pinwheel array and formula [Zn2Ca(μ-ACA)6(4-phpy)2]·EtOH (1). This compound was obtained through the self-assembly of octahedral O- and tetrahe- dral N,O-based coordinating pockets that enable the selective coordination of Ca(II) and Zn(II) ions to the different coordination environments. The elucidation of the crystal struc- Inorganics 2022, 10, 118 10 of 13 4. Conclusions In this contribution, we successfully synthesized and characterized one d-s het- erometallic compound with a pinwheel array and formula [Zn2 Ca(µ-ACA)6 (4-phpy)2 ]·EtOH (1). This compound was obtained through the self-assembly of octahedral O- and tetrahe- dral N,O-based coordinating pockets that enable the selective coordination of Ca(II) and Zn(II) ions to the different coordination environments. The elucidation of the crystal struc- ture 1C confirmed the selective coordination between Zn(II)/Ca(II) through the formation of the aforementioned pockets. Furthermore, we compiled all the structures related to 1C presenting s-block metal ions in the central pocket of the pinwheel, observing the most com- mon synthetic conditions and the great versatility of this SBU. In addition, the solid-state UV-Vis and photoluminescence of 1 were recorded and compared with the corresponding homometallic compound and we observed that the substitution of a Zn(II) by a Ca(II) ion in the central octahedral pocket of the pinwheel SBU slightly altered both curve profiles. To conclude, the use of ACA presents a good option for the formation of heterometallic pinwheel SBUs using mild conditions compared with previously reported syntheses and presenting potential applications as a template for the preparation of extended networks using the pinwheel motif as a predetermined SBU. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/inorganics10080118/s1, Figure S1: PXRD patterns; Figure S2: ESI-MS fragments; Figure S3: FTIR-ATR spectrum; Figure S4: 1H NMR spectrum; Figure S5: 13C{1H} and DEPT-135 NMR spectra; Table S1: Geometric evaluation of the metal cores; Figures S6 and S7: Hirshfeld surface analysis; Table S2: Overview of the synthetic conditions utilized for synthesiz- ing the complexes found in the CSD search; Table S3: Overview of the main SBU features of the complexes found on the CSD search; Figure S8: Outline of the five-coordinated Zn(II) cores found on the CSD search; Figure S9: CIE 1931 chromaticity diagram. References [69–75] are cited in the Supplementary Materials. Author Contributions: Conceptualization, J.P.; methodology, D.E.; software, D.E.; validation, J.P. and T.C.; formal analysis, D.E. and M.F.-B.; investigation, D.E.; resources, J.P. and T.C.; data curation, D.E. and M.F.-B.; writing—original draft preparation, D.E.; writing—review and editing, J.P.; visualization, D.E.; supervision, J.P.; project administration, J.P.; funding acquisition, J.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the CB615921 project, the CB616406 project from “Fundació La Caixa”, and the 2017SGR1687 project from the Generalitat de Catalunya. D.E. acknowledges the PIF pre-doctoral fellowship from the Universitat Autònoma de Barcelona. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Accession code 2190605 (1) contains the supplementary crystallo- graphic data for this paper. 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