molecules Article Tuning Photophysical Properties by p-Functional Groups in Zn(II) and Cd(II) Complexes with Piperonylic Acid Francisco Sánchez-Férez 1 , Joaquim Mª Rius-Bartra 1 , José A. Ayllón 1 , Teresa Calvet 2 , Mercè Font-Bardia 3 and Josefina Pons 1, * 1 Departament de Química, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain; francisco.sanchez.ferez@uab.cat (F.S.-F.); joaquim.rius.bartra@gmail.com (J.M.R.-B.); joseantonio.ayllon@uab.cat (J.A.A.) 2 Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain; mtcalvet@ub.edu 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; mercef@ccit.ub.edu * Correspondence: josefina.pons@uab.cat; Tel.: +34-935812895 Abstract: Aggregation between discrete molecules is an essential factor to prevent aggregation- caused quenching (ACQ). Indeed, functional groups capable of generating strong hydrogen bonds are likely to assemble and cause ACQ and photoinduced electron transfer processes. Thus, it is possible to compare absorption and emission properties by incorporating two ligands with a different bias toward intra- and intermolecular interactions that can induce a specific structural arrangement. In parallel, the π electron-donor or electron-withdrawing character of the functional groups could  modify the Highest Ocuppied Molecular Orbital (HOMO)–Lowest Unocuppied Molecular Orbital  (LUMO) energy gap. Reactions of M(OAc)2 ·2H2 O (M = Zn(II) and Cd(II); OAc = acetate) with Citation: Sánchez-Férez, F.; 1,3-benzodioxole-5-carboxylic acid (Piperonylic acid, HPip) and 4-acetylpyridine (4-Acpy) or isoni- Rius-Bartra, J.M.; Ayllón, J.A.; Calvet, T.; Font-Bardia, M.; Pons, J. Tuning cotinamide (Isn) resulted in the formation of four complexes. The elucidation of their crystal structure Photophysical Properties by showed the formation of one paddle-wheel [Zn(µ-Pip)2 (4-Acpy)]2 (1); a mixture of one dimer and p-Functional Groups in Zn(II) and two monomers [Zn(µ-Pip)(Pip)(Isn)2 ]2 ·2[Zn(Pip)2 (HPip)(Isn)]·2MeOH (2); and two dimers [Cd(µ- Cd(II) Complexes with Piperonylic Pip)(Pip)(4-Acpy)2 ]2 (3) and [Cd(µ-Pip)(Pip)(Isn)2 ]2 ·MeOH (4). They exhibit bridged (1, µ2 -η1 :η1 ), Acid. Molecules 2022, 27, 1365. bridged, chelated and monodentated (2, µ2 -η1 :η1 , µ1 -η1 :η1 and µ1 -η1 ), or simultaneously bridged and https://doi.org/10.3390/ chelated (3 and 4, µ2 -η2 :η1 ) coordination modes. Zn(II) centers accommodate coordination numbers molecules27041365 5 and 6, whereas Cd(II) presents coordination number 7. We have related their photophysical proper- Academic Editors: Mário J. F. Calvete ties and fluorescence quantum yields with their geometric variations and interactions supported by and Mariette M. Pereira TD-DFT calculations. Received: 19 January 2022 Keywords: Zn(II) and Cd(II); 4-acetylpyridine; isonicotinamide; piperonylic acid; photoluminescence Accepted: 15 February 2022 Published: 17 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- The correlation between composition, structure and properties has always been the iations. foundation of materials design. Understanding this interplay allows to comprehend the structure–properties relationship and make significant progress in fields inter alia luminescence [1], sensing [2] or photoelectrical conductivity [3,4], allowing to design Copyright: © 2022 by the authors. prominent materials. In the field of optical materials, the conception of fluorescent light Licensee MDPI, Basel, Switzerland. emitting materials based on discrete molecular complexes provides a crucial advantage This article is an open access article compared to their polymeric analogues as they have better solvent processability [5]. distributed under the terms and Pyridine based fluorophores have been developed as a fitting family of ligands with conditions of the Creative Commons which to design fluorescent complexes. They excel at being sensitive to electronic per- Attribution (CC BY) license (https:// turbation and are capable of coordinating both soft and hard metal ions [6]. However, creativecommons.org/licenses/by/ they commonly present fluorescence quenching associated with intramolecular charge 4.0/). transfer (ICT) and intra-ligand charge transfer (ILCT) transitions, photoinduced electron Molecules 2022, 27, 1365. https://doi.org/10.3390/molecules27041365 https://www.mdpi.com/journal/molecules Molecules 2022, 27, 1365 2 of 29 transfer (PET) processes, or aggregation-caused quenching (ACQ) by exciplex formation that promote non-radiative decays. In this scenario, a well-established strategy is to incor- porate transition metal ions that can stabilize the excited state as well as minimize PET processes by fixing non-bonding electrons on the heteroatoms of the ligand and lowering the energy of the lone pair orbitals. Diamagnetic metal ions as Zn(II) and Cd(II) are able to coordinate with the fluorophore and present outstanding performance by minimizing relaxation via non-radiative decays [7,8]. Their fully populated d orbitals do not partake in electronic transitions and, therefore, ligand centered π. . . π* transitions are favored, which are desirable in the seeking of high emissive complexes [9]. In particular, being coordinated with N- and O-donor ligands, they tend to construct highly blue emissive materials [10]. In terms of their metal preferences, Zn(II) presents coordination numbers from 3 to 6, whereas Cd(II) can accommodate up to 8. Their geometries range from tetrahedral to octahedral or, in the case of Cd(II), the less common square-antiprism [11]. In complexes containing carboxylic acids and pyridine derivative ligands, coordination numbers 4–6 are preferred by Zn(II), while Cd(II) is likely to present 6 and 7. The Zn(II) center tends to form dimeric motifs as [Zn2 (CO2 )2 ] with two carboxylates coordinated in a bridging mode or square pyramidal [Zn2 (CO2 )4 ] paddle-wheels with two apical N-donors. Conversely, Cd(II) is likely to construct double bridged [Cd2 (CO2 )2 ] cores with four N-donor ligands, being the heptacoordinated environment is completed by chelate carboxylates and by designing a [Cd2 (CO2 )4 ]−2+2 motif. This core is commonly hindered for Zn(II) complexes [12]. Therefore, they provide a versatile tool to evaluate how the structure of the resulting complexes affects their photophysical properties. In recent studies, our group has examined the structure–properties relationship of several Zn(II) and Cd(II) metal complexes with benzoic acid or piperonylic acid (HPip) and 3-phenylpyridine, 4-phenypyridine and 4-acetylpyridine (4-Acpy) [13,14]. From these studies, three dimeric [Zn2 (CO2 )4 ] paddle-wheel and three dimeric [Cd2 (CO2 )4 ]−2+2 double bridged complexes were synthesized and their crystal structures were elucidated. The UV-Vis absorption measurements identified 4-Acpy as a promising ligand to avoid ACQ by minimizing intermolecular interactions. Fluorescence experiments confirmed the minimization of PET processes through complexation by enhanced fluorescence of the complexes with respect to the free ligands. Likewise, the work performed using Hg(II), HPip and a range of pyridine derivative ligands (3-phenylpyridine, 4-phenylpyridine, 2,20 -bipyridine, 1,10-phenanthroline, 2,20 :60 ,20 -terpyridine and di-(2-picolyl)amine) resulted in the formation of monomeric, dimeric and polymeric or oligomeric metal complexes [15]. Their UV-Vis measurements evinced that the aggregation effects can cause a significant shift on the absorption spectra, and the time-dependent density functional theory (TD- DFT) calculations displayed that the highest occupied molecular orbitals (HOMOs) were distributed along the Pip while the lowest occupied molecular orbitals (LUMOs) were over the pyridine ligands [15]. As a continuation of this work, in this paper, we have studied the reaction between M(OAc)2 (M = Zn(II) and Cd(II); OAc = acetate) with HPip and two N-donor ligands (dPy = 4-acetylpyridine (4-Acpy) and isonicotinamide (Isn)), considering that 4-Acpy has a good electron withdrawing group and Isn has a better electron donor group. From these reactions we have isolated one paddle-wheel complex [Zn(µ-Pip)2 (4-Acpy)]2 (1); a mixture of one dimer and two monomers [Zn(µ-Pip)(Pip)(Isn)2 ]2 ·2[Zn(Pip)2 (HPip)(Isn)]·2MeOH (2); and two dimeric complexes [Cd(µ-Pip)(Pip)(4-Acpy)2 ]2 (3) and [Cd(µ-Pip)(Pip)(Isn)2 ]2 ·MeOH (4). Besides, their molecular and supramolecular structures have been analyzed and their photophysical properties have been studied. These results have been supported by TD-DFT calculations. Molecules 2022, 27, 1365 3 of 29 2. Results and Discussion 2.1. Synthesis and General Characterization Complexes 1–4 were prepared via a combination of M(OAc)2 ·2H2 O (M = Zn(II) and Cd(II); OAc = acetate) salts and 1,3-benzodioxole-5-carboxylic acid (Piperonylic, HPip) with dPy = 4-acetylpyridine (4-Acpy) or isonicotinamide (Isn) in MeOH as solvent at room temperature (RT). The M:Pip:dPy molar ratio used in the synthesis was 1:2:4 for complexes 1 and 3; 1:2:1 for complex 2; and 1:2:2 for complex 4. The corresponding crystals suitable for X-ray crystallographic analysis were grown via slow evaporation of their mother liquors (1, 2 and 3) or via recrystallization in MeOH (4). All of the pure isolated metal complexes were characterized using elemental analysis, FTIR-ATR, 1 H, and 13 C{1 H} NMR spectroscopies, and single crystal X-ray diffraction method. In addition, their UV-Vis and fluorescence spectra were recorded and their molar absorptivity (ε) and quantum yield (φs ) calculated. The elemental analysis of all of the complexes agreed with the proposed formulas. FTIR-ATR spectroscopy. The FTIR-ATR spectra display the characteristic carboxylate bands of Pip ligand (S.I: Figure S1–S4). The carboxylate bands for υas (COO) appear at 1556 (1), 1534 and 1502 (2), 1539 and 1506 (3), or at 1539 and 1502 cm−1 (4), whereas bands for υs (COO) arise at 1385 (1), 1385 (2), 1384 (3) and at 1385 cm−1 (4).The calculated ∆ values (νas (COO)-νs (COO)) [16] were found to be 171 (1), 149 and 117 (2), 155 and 122 (3), 154 and 117 (4) cm−1 . These values suggest a bidentate bridged coordination mode of the Pip ligands (µ2 -η1 :η1 ) in 1, while in compounds 2–4 either bidentate bridged (µ2 - η1 :η1 ) or bridged and chelated (µ1 -η2 ) coordination modes are inferred. All of the spectra present bands attributable to the ν(C=O) belong either to the 4-Acpy at 1703 cm−1 (1) and 1693 cm−1 (3) or to the Isn ligand at 1701 cm−1 (2) and 1704 cm−1 (4). The ν(C=O) of 4-Acpy in complexes 1 and 3 is almost unvaried compared to the free 4-Acpy (1693 cm−1 ) due to small changes in the intermolecular interactions. In contrast, the ν(C=O) of Isn in complexes 2 and 4 is blue shifted compared to the free Isn (1655 cm−1 ), which is associated with a decrease in hydrogen bonding [17]. Besides, the ν(C=O) of the protonated HPip ligand in 2 appears at 1678 cm−1 , slightly blue shifted compared to the free HPip (1667 cm−1 ), suggesting that the hydrogen bonding is maintained [17]. Additional bands in all of the complexes attributable to the aromatic ring vibrations have also been identified [18,19]. NMR spectroscopy. The 1 H and 13 C{1 H} NMR spectra were recorded in DMSO-d6 . All of the spectra show the signals belonging to Pip and either to 4-Acpy or Isn. In the 1 H NMR spectra of all the compounds (S.I: Figures S5–S8), the signals attributable to the Pip ligand have been assigned to the aromatic protons between 7.55 and 6.80 ppm, and to the aliphatic protons of the dioxole unit between 6.07 and 5.95 ppm (HPip: 7.54–7.00 and 6.12 ppm). The signals from the aromatic and CH3 protons of the 4-Acpy ligand appear between 8.82 and 7.72 or between 2.63 and 2.39 ppm (4-Acpy: 8.68, 7.69 and 2.36 ppm). The aromatic signals from Isn appear between 8.71 and 7.72 ppm. The protons from the amide group appear between 8.23 and 7.66 ppm (Isn: 8.68, 8.21, 7.73, 7.69). In addition, the 1 H NMR spectra of 1–4 confirms the 2:1 (1 and 3), 5:4 (2) or 1:1 (4) molar ratio of the Pip with regards to the 4-Acpy or Isn ligands. In the 13 C{1 H} NMR spectra, the signals corresponding to the carboxylate group of the Pip ligands have been found at 171.4 (1), 171.2 (2), 172.3 (3), and 172.1 (4) ppm (S.I: Figures S9–S12); the aromatic carbons between 150.8 and 107.5 ppm; and the aliphatic carbon of the dioxole unit between 101.8 and 101.4 ppm. The signals attributable to the C=O4-Acpy at 198.1 (1) and 198.5 ppm (3), and those from the C=OIsn, are at 166.3 (2) and 166.6 ppm (4). Besides, those corresponding to the -CH3 group are found at 27.0 (1) and 27.3 ppm (3) [18]. The correct assignment of the C aromatic atoms required the use of DEPT-135 experiments for complexes 1 and 2. 2.2. Crystal and Extended Structure of Zn(II) Complexes [Zn(µ-Pip)2 (4-Acpy])2 ]2 (1). It crystallizes in the triclinic P-1 space group and has a dimeric paddle-wheel structure (Figure 1a). Each Zn(II) center has a [ZnO4 N] core belonging Molecules 2022, 27, 1365 4 of 29 to the four Pip ligands and a 4-Acpy linker (Figure 1b) and exhibits a square pyramidal geometry (S.I.: Table S1) [20] with a value of τ = 0.004 [21], being pulled 0.340 Å out of the equatorial plane towards the axial position. The structure is held together by the four5 of Molecules 2022, 27, x FOR PEER REVIEW Pip 28 ligands exhibiting a bidentate bridged (µ2 -η1 :η1 ) coordination mode. The bond lengths and angles are within the range of analogous paddle-wheel complexes [13,14] (Table 1). Figure 1. (a) Molecular paddle-wheel like structure of complex 1 and (b) Zn(II) metal core. (c) Figure 1. (a) Molecular paddle-wheel like structure of complex 1 and (b) Zn(II) metal core. (c) Su- Supramolecular assemblyforming pramolecular assembly forming2D 2Dsheets sheetspromoted promotedby π· · · π byπ···π C-H· · ·and supportedbybyC-H···π supported π and C-H··in- C-H··O O interactions. In detail, π · · · π (light orange) and C-H · · · π/C-H · · · O (light green) associations teractions. In detail, π···π (light orange) and C-H···π/C-H···O (light green) associations between Pip between Pip andand 4-Acpy 4-Acpy ligands. ligands. [Zn(μ-Pip)(Pip)(Isn)2]2·2[Zn(Pip)2(HPip)(Isn)]·2MeOH (2). It crystallizes in the tri- clinic P-1 space group and comprises one dimeric and two monomeric structural units with two occluded MeOH molecules in the unit cell (Figure 2a). Such a structure is unu- sual and seems to be driven by strong interactions between the amide moieties, which also promoted the same behavior with Cu(II) [23]. Both the monomer and dimer have hexacoordinated Zn(II) atoms bearing [ZnO4N2] cores with a distorted octahedral geome- try (S.I: Table S1) presenting an ata (average twist angle) of 58.41° for Zn(1A) and 58.81° for Zn(1B) [24,25] (Table 2, Figure 2b). The dimeric unit is composed of four Pip and four Isn ligands. In contrast, the monomer contains two Pip, one HPip and two Isn ligands. The Zn(II) centers in the dimer are joined by two bidentate bridged (μ2-η1:η1) Pip ligands Molecules 2022, 27, 1365 5 of 29 Table 1. Bond lengths (Å), bond angles (◦ ) and intermolecular interactions in 1. Bond Lengths Zn(1)-O(5) 2.0372(12) Zn(1)-O(1)#1 2.0489(12) Zn(1)-O(2) 2.0388(11) Zn(1)-N(1) 2.0338(14) Bond angles N(1)-Zn(1)-O(5) 101.93(5) O(2)-Zn(1)-O(6)#1 87.85(5) N(1)-Zn(1)-O(2) 100.22(5) N(1)-Zn(1)-O(1)#1 98.73(5) O(5)-Zn(1)-O(2) 89.35(5) O(5)-Zn(1)-O(1)#1 88.98(5) N(1)-Zn(1)-O(6)#1 97.36(6) O(2)-Zn(1)-O(1)#1 160.91(5) O(5)-Zn(1)-O(6)#1 160.70(6) O(6)#1-Zn(1)-O(1)#1 87.47(5) Intermolecular H· · · A (Å) D· · · A (Å) D-H (Å) >D-H· · · A (◦ ) interactions C18-H18· · · Cg(1) 3.395 4.273(3) 0.950 154.63 C18-H18· · · O8 2.598(2) 3.467(3) 0.950 152.31 π· · · π interactions Cg· · · Cg Cg(I)· · · Cg(J) a αb β, γ c Cg(I)_Perp,Cg(J)_Perp d Slippage e Cg(2)· · · Cg(2)3.5075(10) 0 16.5, 16.5 3.3638(7), 3.3639(7) 0.994 Cg(2)· · · Cg(3)3.7861(10) 10.37(8) 31.1, 20.7 3.5410(7), 3.2430(7) - a #1 −x,−y+1,−z+1. Cg· · · Cg distances are given in Å. Cg· · · Cg = distance between ring centroids (Å); b α = dihedral angle between planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = C10-C11-C12-C14-C15-C16: Cg(2) = C2-C3-C4-C6-C7-C8; Cg(3) = N1-C17-C18-C19-C20-C21. These paddle-wheel units are assembled in groups of four by π· · · π interactions between Pip (Cg(2)) and 4-Acpy (Cg(3)) ligands in a 4-Acpy-Pip-Pip-4-Acpy sequence, stacking them through the [101] direction. Finally, the dimers are associated in chains along the [1] direction (Figure 1c) by weak C-H· · · π [22] and C-H· · · O interactions between the m-H aromatic H atom from the 4-Acpy ligand and both the aromatic ring (C18-H18· · · Cg(1), 3.395 Å) and O dioxole atom of Pip (C18-H18· · · O8, 2.598(2) Å) (Table 1, Figure 1c). All of these sets of interactions form 2D sheets of dimers along the ac plane. [Zn(µ-Pip)(Pip)(Isn)2 ]2 ·2[Zn(Pip)2 (HPip)(Isn)]·2MeOH (2). It crystallizes in the tri- clinic P-1 space group and comprises one dimeric and two monomeric structural units with two occluded MeOH molecules in the unit cell (Figure 2a). Such a structure is unusual and seems to be driven by strong interactions between the amide moieties, which also promoted the same behavior with Cu(II) [23]. Both the monomer and dimer have hexacoordinated Zn(II) atoms bearing [ZnO4 N2 ] cores with a distorted octahedral geometry (S.I: Table S1) presenting an ata (average twist angle) of 58.41◦ for Zn(1A) and 58.81◦ for Zn(1B) [24,25] (Table 2, Figure 2b). The dimeric unit is composed of four Pip and four Isn ligands. In contrast, the monomer contains two Pip, one HPip and two Isn ligands. The Zn(II) centers in the dimer are joined by two bidentate bridged (µ2 -η1 :η1 ) Pip ligands while the remaining two Pip units have a bidentate chelate coordination mode (µ1 -η2 ). Selected bond lengths and angles are displayed in Table 2. In addition, the Pip ligands in the monomer present µ1 -η2 and µ1 -η1 while HPip has a µ1 -η1 coordination mode. This µ1 -η1 coordination of the ligands is supported by an intramolecular O-H· · · O between the acidic proton of HPip and the uncoordinated carboxylate oxygen atom from the neighboring ligand. Selected bond lengths and angles are displayed in Table 2. In addition, the Pip ligands in the monomer present μ1-η2 and μ1-η1 while HPip has a μ1-η1 coordination mode. This μ1- η1 coordination of the ligands is supported by an intramolecular O-H···O between the Molecules 2022, 27, 1365 acidic proton of HPip and the uncoordinated carboxylate oxygen atom from the neigh- 6 of 29 boring ligand. Figure 2. (a) Molecular structure of the dimeric and monomeric units in complex 2. (b) Representa- Figure 2. (a) Molecular structure of the dimeric and monomeric units in complex 2. (b) Representation tion of the dimeric and monomeric Zn (II) cores. Inset of the intra- and intermolecular interactions of the dimeric between and monomeric HPip and MeOH withZn μ1(II) cores. -η1-Pip Inset of the intra- and intermolecular interactions between ligand. HPip and MeOH with µ1 -η1 -Pip ligand. Table 2. Bond lengths (Å), bond angles (°) and intramolecular interactions in 2. Table 2. Bond lengths (Å), bond angles (◦ ) and intramolecular interactions in 2. Monomer Bond lengths Monomer Zn(1A)-O(1A) Bond lengths 2.001(3) Zn(1A)-N(3A) 2.154(3) Zn(1A)-O(5A) Zn(1A)-O(1A) 2.052(3) 2.001(3) Zn(1A)-O(9A) Zn(1A)-N(3A) 2.184(3) 2.154(3) Zn(1A)-N(1A) Zn(1A)-O(5A) 2.140(3) 2.052(3) Zn(1A)-O(10A) Zn(1A)-O(9A) 2.257(3) 2.184(3) Bond angles Zn(1A)-N(1A) 2.140(3) Zn(1A)-O(10A) 2.257(3) O(1A)-Zn(1A)-O(5A) 117.34(11) N(1A)-Zn(1A)-O(9A) 91.03(11) Bond angles O(1A)-Zn(1A)-N(1A) 91.70(12) N(3A)-Zn(1A)-O(9A) 89.91(10) O(1A)-Zn(1A)-O(5A) 117.34(11) N(1A)-Zn(1A)-O(9A) 91.03(11) O(5A)-Zn(1A)-N(1A) 88.62(11) O(1A)-Zn(1A)-O(10A) 151.96(10) O(1A)-Zn(1A)-N(1A) O(1A)-Zn(1A)-N(3A) 91.70(12) 92.36(11) N(3A)-Zn(1A)-O(9A) O(5A)-Zn(1A)-O(10A) 89.91(10) 90.69(10) O(5A)-Zn(1A)-N(1A) O(5A)-Zn(1A)-N(3A) 88.62(11) 88.51(11) O(1A)-Zn(1A)-O(10A) N(1A)-Zn(1A)-O(10A) 151.96(10) 88.07(11) N(1A)-Zn(1A)-N(3A) O(1A)-Zn(1A)-N(3A) 175.79(12) 92.36(11) N(3A)-Zn(1A)-O(10A) O(5A)-Zn(1A)-O(10A) 88.89(10) 90.69(10) O(1A)-Zn(1A)-O(9A) O(5A)-Zn(1A)-N(3A) 92.89(10) 88.51(11) O(9A)-Zn(1A)-O(10A) N(1A)-Zn(1A)-O(10A) 59.09(9) 88.07(11) O(5A)-Zn(1A)-O(9A) N(1A)-Zn(1A)-N(3A) 149.77(11) 175.79(12) N(3A)-Zn(1A)-O(10A) 88.89(10) TA value (°) ATA value (°) O(1A)-Zn(1A)-O(9A) 92.89(10) O(9A)-Zn(1A)-O(10A) 59.09(9) O(10A)-Cg(3)-Cg(4)-N(1A) 46.72 O(5A)-Zn(1A)-O(9A) 149.77(11) N(3A)-Cg(3)-Cg(4)-O(5A) 64.48 58.41 TA value (◦ ) ATA value (◦ ) O(9A)-Cg(3)-Cg(4)-O(1A) 64.04 O(10A)-Cg(3)-Cg(4)-N(1A) 46.72 Dimer N(3A)-Cg(3)-Cg(4)-O(5A) Bond lengths 64.48 58.41 Zn(1B)-O(5B) O(9A)-Cg(3)-Cg(4)-O(1A) 2.025(3) 64.04 Zn(1B)-N(1B) 2.153(3) Molecules 2022, 27, 1365 7 of 29 Table 2. Cont. Monomer Dimer Bond lengths Zn(1B)-O(5B) 2.025(3) Zn(1B)-N(1B) 2.153(3) Zn(1B)-O(6B)#1 2.034(3) Zn(1B)-O(2B) 2.271(3) Zn(1B)-O(1B) 2.140(3) Zn(1B)· · · Zn(1B) 3.9893(6) Zn(1B)-N(3B) 2.151(3) Bond angles O(5B)-Zn(1B)-O(6B)#1 125.03(11) O(1B)-Zn(1B)-N(1B) 93.02(11) O(5B)-Zn(1B)-O(1B) 145.44(11) N(3B)-Zn(1B)-N(1B) 176.78(12) O(6B)#1-Zn(1B)-O(1B) 89.46(11) O(5B)-Zn(1B)-O(2B) 86.09(10) O(5B)-Zn(1B)-N(3B) 88.48(11) O(6B)#1-Zn(1B)-O(2B) 148.82(11) O(6B)#1-Zn(1B)-N(3B) 89.27(12) O(1B)-Zn(1B)-O(2B) 59.55(10) O(1B)-Zn(1B)-N(3B) 90.17(11) N(3B)-Zn(1B)-O(2B) 93.82(11) O(5B)-Zn(1B)-N(1B) 89.12(11) N(1B)-Zn(1B)-O(2B) 88.17(11) O(6B)#1-Zn(1B)-N(1B) 90.34(12) >D-H· · · A Intramolecular interactions H· · · A (Å) D· · · A (Å) D-H (Å) (◦ ) O(6A)-H(6A)· · · O(2A) 1.723 2.503(4) 0.840 153.7 π· · · π interactions Cg(I)· · · Cg(J) Cg· · · Cg a αb β, γ c Cg(I)_Perp, Cg(J)_Perp d Slippage e Cg(1)· · · Cg(2) 3.884(2) 1.7(2) 15.1, 16.1 3.7307(17), 3.7493(17) - TA value (◦ ) ATA value (◦ ) N(3B)-Cg(5)-Cg(6)-O(5B) 68.38 O(1B)-Cg(5)-Cg(6)-O(6B) 62.50 58.81 O(2B)-Cg(5)-Cg(6)-N(1B) 45.55 #1 −x+1,−y+1,−z. Cg· · · Cg distances are given in Å. a Cg· · · Cg = distance between ring centroids (Å); b α = dihedral angle between planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N1B-C17B-C18B- C19B-C20B-C21B; Cg(2) = N3B-C23B-C24B-C25B-C26B-C27B. Cg(3) = N3A-O9A-O10A; Cg(4) = N1A-O1A-O5A; Cg(5) = N3B-O2B-O1B; Cg(6) = N1B-O6B-O5B. Simultaneously, the carboxylate O atom interacts with the MeOH molecule through an intermolecular O-H· · · O hydrogen bond. Besides, the aromatic rings from Isn stacks in an intramolecular π· · · π interaction (Table 2). The monomers and dimers form a 3D supramolecular net via amide· · · amide, N- H/C-H· · · O and π· · · π interactions (Table 3). Amide· · · amide piles monomers and dimers between themselves while Isn, through its N-Hanti and m-H, form a dimer via association with an O atom from a chelate Pip ligand of a monomer and vice versa along [22] (Figure 3a). In addition, π· · · π interactions between Pip ligands stack monomers and dimers in an ordered sequence from Cg(3) to Cg(7) along the b axis (Figure 3b). Molecules 2022, 27, 1365 8 of 29 Table 3. Intermolecular interactions in 2. Intermolecular H· · ·A (Å) D· · ·A (Å) D-H (Å) >D-H· · ·A (◦ ) Interactions. N(2A)-H(2AA)· · · O(14A) 2.07 2.934(4) 0.88 168.4 N(2A)-H(2BA)· · · O(9A) 2.05 2.903(4) 0.88 164 N(4A)-H(4AA)· · · O(13A) 2.04 2.921(4) 0.88 176.1 N(4A)-H(4BA)· · · O(2B) 2.02 2.855(4) 0.88 157 N(2B)-H(2BA)· · · O(10B) 1.99 2.858(4) 0.88 166.8 N(2B)-H(2BB)· · · O(12A) 2.46 3.268(5) 0.88 153.6 N(4B)-H(4BA)· · · O(9B) 2.05 2.913(4) 0.88 165.9 N(4B)-H(4BB)· · · O(10A) 2.01 2.868(4) 0.88 164.7 O(1W)-H(1WO)· · · O(2A) 2.10 2.900(6) 0.84 160.1 π· · · π interactions a b β, γ c Slippage e Cg(I)· · · Cg(J) Cg· · · Cg α Cg(I)_Perp, Cg(J)_Perp d Cg(7)· · · Cg(6) 3.445(2) 1.24(18) 10.2, 9.0 3.4022(16), 3.3905(14) - Cg(7)· · · Cg(3) 3.468(2) 3.01(18) 13.6, 15.4 3.3432(16), 3.3710(14) - Cg(6)· · · Cg(5) 3.729(2) 4.64(17) 25.5, 22.5 3.4456(14), 3.3670(15) - Cg(5)· · · Cg(4) 3.561(2) 2.91(18) 16.2, 18.6 3.3740(15), 3.4202(16) - Cg(4)· · · Cg(3) 3.645(2) 2.60(18) 24.2, 21.7 3.3852(16), 3.3244(14) - Cg· · · Cg distances are given in Å. a Cg· · · Cg = distance between ring centroids (Å); b α= dihedral angle between Planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpen- dicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(3) = C18A-C19A-C20A-C21A-C23A-C24A; Cg(4) = C10A-C11A-C12A-C13A-C15A-C16A; Cg(5) = C2A-C3A-C4A-C5A-C7A-C8A; Cg(6) = C10B-C11B-C12B- C13B-C15B-C16B; Cg(7) = C2B-C3B-C4B-C5B-C7B-C8B. 2.3. Crystal and Extended Structure of Cd(II) Compounds [Cd(µ-Pip)(Pip)(4-Acpy)2 ]2 (3) and [Cd(µ-Pip)(Pip)(Isn)2 ]2 ·MeOH (4). They crystallize in the triclinic P-1 and monoclinic P2/c space groups, respectively, and contain two crystal- lographically independent molecules in the unit cell (molecules A and B). Both complexes have a dimeric structure with four Pip ligands and four 4-Acpy (3) (Figure 4a,b) or four Isn (4) (Figure 5a,b), in which the heptacoordinated Cd atoms present a [CdO5 N2 ] core with a distorted pentagonal-bipyramidal geometry (S.I.: Table S1). The bond lengths and angles are within the range of analogous dimeric Cd(II) complexes [13,14] (Tables 4 and 5). Molecules 2022, 27, x FOR PEER REVIEW 8 of 28 Molecules 2022, 27, 1365 9 of 29 Molecules 2022, 27, x FOR PEER REVIEW 9 of 28 2.3. Crystal and Extended Structure of Cd(II) Compounds [Cd(μ-Pip)(Pip)(4-Acpy)2]2 (3) and [Cd(μ-Pip)(Pip)(Isn)2]2·MeOH (4). They crystallize in the triclinic P-1 and monoclinic P2/c space groups, respectively, and contain two crys- tallographically independent molecules in the unit cell (molecules A and B). Both com- plexes have a dimeric structure with four Pip ligands and four 4-Acpy (3) (Figure 4a,b) or four Isn3.(4) Figure (Figure 5a,b), in which the Supramolecular heptacoordinated Cd atoms present N2detail, a [CdO5In ] core Figure 3. Supramolecular interactions interactions in in 22 (a) (a) promoted promoted by by Isn, Isn, Pip and Pip and HPip ligands. HPip ligands. In detail, with a distorted reciprocal N-H···Opentagonal-bipyramidal geometry and N-Hanti/C-H···O interactions. (b)(S.I.: π···πTable S1). The interactions bond lengths between and Pip aromatic reciprocal N-H· · · O and N-Hanti /C-H· · · O interactions. (b) π· · · π interactions between Pip aromatic rings highlighted angles are withininthe violet (Cg(4)), range pink (Cg(3)), of analogous black (Cg(7)), dimeric Cd(II) blue (Cg(6)), and complexes red (Tables [13,14] (Cg(5)). 4 and rings highlighted in violet (Cg(4)), pink (Cg(3)), black (Cg(7)), blue (Cg(6)), and red (Cg(5)). 5). Table 3. Intermolecular interactions in 2. Intermolecular Interactions. H···A (Å) D···A (Å) D-H (Å) >D-H···A (°) N(2A)-H(2AA)···O(14A) 2.07 2.934(4) 0.88 168.4 N(2A)-H(2BA)···O(9A) 2.05 2.903(4) 0.88 164 N(4A)-H(4AA)···O(13A) 2.04 2.921(4) 0.88 176.1 N(4A)-H(4BA)···O(2B) 2.02 2.855(4) 0.88 157 N(2B)-H(2BA)···O(10B) 1.99 2.858(4) 0.88 166.8 N(2B)-H(2BB)···O(12A) 2.46 3.268(5) 0.88 153.6 N(4B)-H(4BA)···O(9B) 2.05 2.913(4) 0.88 165.9 N(4B)-H(4BB)···O(10A) 2.01 2.868(4) 0.88 164.7 O(1W)-H(1WO)···O(2A) 2.10 2.900(6) 0.84 160.1 π···π interactions Cg(I)···Cg(J) Cg···Cg a αb β, γ c Cg(I)_Perp, Cg(J)_Perp d Slippage e Cg(7)···Cg(6) 3.445(2) 1.24(18) 10.2, 9.0 3.4022(16), 3.3905(14) - Cg(7)···Cg(3) 3.468(2) 3.01(18) 13.6, 15.4 3.3432(16), 3.3710(14) - Cg(6)···Cg(5) 3.729(2) 4.64(17) 25.5, 22.5 3.4456(14), 3.3670(15) - Cg(5)···Cg(4) 3.561(2) 2.91(18) 16.2, 18.6 3.3740(15), 3.4202(16) - Cg(4)···Cg(3) 3.645(2) 2.60(18) 24.2, 21.7 3.3852(16), 3.3244(14) - Cg···Cg distances are given in Å. a Cg···Cg = distance between ring centroids (Å); b α= dihedral angle between Planes I and J (°); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (°) and γ = angle Cg(I)-Cg(J) and normal to plane J (°) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Hori- Figure 4. Molecular structure zontal displacement ofbetween or slippage the crystallographically independent Cg(I) and Cg(J) (equal for bothdimeric units centroids whenin αcomplex 3. = 0). Cg(3) Figure 4. Molecular structure of the crystallographically independent dimeric units in complex 3. Units A and B (a) with their correspondingCg(4) = C18A-C19A-C20A-C21A-C23A-C24A; Cd(II)=cores (b). C10A-C11A-C12A-C13A-C15A-C16A; Cg(5) = Units A and B (a) with their corresponding Cd(II) cores (b). C2A-C3A-C4A-C5A-C7A-C8A; Cg(6) = C10B-C11B-C12B-C13B-C15B-C16B; Cg(7) = C2B-C3B-C4B- C5B-C7B-C8B. Molecules 2022, 27, x FOR PEER REVIEW 10 of 28 Molecules 2022, 27, 1365 10 of 29 Figure 5. Molecular structure of the crystallographically independent dimeric units in complex 4. Figure 5. Molecular structure of the crystallographically independent dimeric units in complex 4. Units A and B (a) with their corresponding Cd(II) cores (b). Units A and B (a) with their corresponding Cd(II) cores (b). Table 4. Bond lengths (Å), bond angles (°) and intramolecular π···π interactions in 3. Table 4. Bond lengths (Å), bond angles (◦ ) and intramolecular π· · · π interactions in 3. Molecule A Molecule Bond A lengths Cd(1A)-O(2A) Bond lengths 2.285(2) Cd(1A)-N(2A) 2.322(3) Cd(1A)-O(5A) Cd(1A)-O(2A) 2.330(2) 2.285(2) Cd(1A)-O(6A)#1 Cd(1A)-N(2A) 2.395(2) 2.322(3) Cd(1A)-O(5A)#1 Cd(1A)-O(5A) 2.482(2) 2.330(2) Cd(1A)-O(1A) Cd(1A)-O(6A)#1 2.485(2) 2.395(2) Cd(1A)-N(1A) Cd(1A)-O(5A)#1 2.332(3) 2.482(2) Cd(1A)···Cd(1A) Cd(1A)-O(1A) 3.7820(5) 2.485(2) Bond angles Cd(1A)-N(1A) 2.332(3) Cd(1A)· · · Cd(1A) 3.7820(5) O(2A)-Cd(1A)-N(2A) 93.46(9) N(2A)-Cd(1A)-O(5A)#1 92.29(8) Bond angles O(2A)-Cd(1A)-O(5A) 149.25(7) O(5A)-Cd(1A)-O(5A)#1 76.42(8) O(2A)-Cd(1A)-N(2A) 93.46(9) N(2A)-Cd(1A)-O(5A)#1 92.29(8) N(2A)-Cd(1A)-O(5A) 87.09(8) N(1A)-Cd(1A)-O(5A)#1 90.58(8) O(2A)-Cd(1A)-O(5A) O(2A)-Cd(1A)-N(1A) 149.25(7) 89.75(9) O(5A)-Cd(1A)-O(5A)#1 O(6A)#1-Cd(1A)-O(5A)#1 76.42(8) 53.41(7) N(2A)-Cd(1A)-O(5A) N(2A)-Cd(1A)-N(1A) 87.09(8) 172.17(9) N(1A)-Cd(1A)-O(5A)#1 O(2A)-Cd(1A)-O(1A) 90.58(8) 54.75(7) O(5A)-Cd(1A)-N(1A) O(2A)-Cd(1A)-N(1A) 86.51(8) 89.75(9) N(2A)-Cd(1A)-O(1A) O(6A)#1-Cd(1A)-O(5A)#1 87.86(8) 53.41(7) O(2A)-Cd(1A)-O(6A)#1 N(2A)-Cd(1A)-N(1A) 80.79(7) 172.17(9) O(5A)-Cd(1A)-O(1A) O(2A)-Cd(1A)-O(1A) 94.61(7) 54.75(7) N(2A)-Cd(1A)-O(6A)#1 O(5A)-Cd(1A)-N(1A) 95.65(8) 86.51(8) N(1A)-Cd(1A)-O(1A) N(2A)-Cd(1A)-O(1A) 88.17(8) 87.86(8) O(5A)-Cd(1A)-O(6A)#1 129.79(7) O(6A)#1-Cd(1A)-O(1A) 135.54(7) O(2A)-Cd(1A)-O(6A)#1 80.79(7) O(5A)-Cd(1A)-O(1A) 94.61(7) O(2A)-Cd(1A)-O(5A)#1 134.18(7) O(5A)#1-Cd(1A)-O(1A) 171.00(7) N(2A)-Cd(1A)-O(6A)#1 95.65(8) N(1A)-Cd(1A)-O(1A) 88.17(8) Molecule B O(5A)-Cd(1A)-O(6A)#1 129.79(7) Bond lengths O(6A)#1-Cd(1A)-O(1A) 135.54(7) O(2A)-Cd(1A)-O(5A)#1 Cd(1B)-O(5B) 134.18(7) 2.265(2) O(5A)#1-Cd(1A)-O(1A) Cd(1B)-N(1B) 171.00(7) 2.318(3) Cd(1B)-O(6B)#2 2.317(2) Cd(1B)-N(2B) 2.335(3) Cd(1B)-O(1B) 2.366(2) Cd(1B)-O(2B) 2.367(2) Cd(1B)-O(6B) 2.718(2) Cd(1B)···Cd(1B) 3.9452(6) Bond angles Molecules 2022, 27, 1365 11 of 29 Table 4. Cont. Molecule B Bond lengths Cd(1B)-O(5B) 2.265(2) Cd(1B)-N(1B) 2.318(3) Cd(1B)-O(6B)#2 2.317(2) Cd(1B)-N(2B) 2.335(3) Cd(1B)-O(1B) 2.366(2) Cd(1B)-O(2B) 2.367(2) Cd(1B)-O(6B) 2.718(2) Cd(1B)· · · Cd(1B) 3.9452(6) Bond angles O(5B)-Cd(1B)-O(6B)#2 127.84(8) N(1B)-Cd(1B)-O(1B) 93.92(8) O(5B)-Cd(1B)-N(1B) 96.03(9) N(2B)-Cd(1B)-O(1B) 86.04(8) O(6B)#2-Cd(1B)-N(1B) 85.79(8) O(5B)-Cd(1B)-O(2B) 85.45(8) O(5B)-Cd(1B)-N(2B) 88.90(8) O(6B)#2-Cd(1B)-O(2B) 145.20(8) O(6B)#2-Cd(1B)-N(2B) 86.31(8) N(1B)-Cd(1B)-O(2B) 102.18(9) N(1B)-Cd(1B)-N(2B) 172.09(9) N(2B)-Cd(1B)-O(2B) 84.33(9) O(5B)-Cd(1B)-O(1B) 141.05(7) O(1B)-Cd(1B)-O(2B) 55.62(7) O(6B)#2-Cd(1B)-O(1B) 90.36(8) π· · · π interactions a b β, γ c Slippage e Cg(I)· · · Cg(J) Cg· · · Cg α Cg(I)_Perp, Cg(J)_Perp d Cg(1)· · · Cg(2) 3.7430 16.0 25.0, 10.4 3.6813, 3.3912 - Cg(3)· · · Cg(4) 3.8415 13.0 21.2, 18.6 3.6418, 3.5810 - #1 −x,−y+2,−z, #2 −x+1,−y,−z+1. Cg· · · Cg distances are given in Å. a Cg· · · Cg = distance between ring centroids (Å); b α = dihedral angle between planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicu- lar distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N1A-C17A-C18A-C19A-C20A-C21A; Cg(2) = N2A-C24A-C25A-C26A-C27A-C28A; Cg(3) = N2B-C24B- C25B-C26B-C27B-C28B; Cg(4) = N1B-C17B-C18B-C19B-C20B-C21B. Table 5. Bond lengths (Å), bond angles (◦ ) and intramolecular π· · · π interactions of 4. Molecule A Bond lengths Cd(1A)-N(3A) 2.307(3) Cd(1A)-O(6A)#1 2.360(2) Cd(1A)-N(1A) 2.331(3) Cd(1A)-O(1A) 2.427(3) Cd(1A)-O(5A) 2.337(2) Cd(1A)-O(5A)#1 2.524(2) Cd(1A)-O(2A) 2.354(2) Cd(1A)· · · Cd(1A) 3.8026(6) Bond angles N(3A)-Cd(1A)-N(1A) 173.41(9) N(1A)-Cd(1A)-O(1A) 85.03(9) N(3A)-Cd(1A)-O(5A) 87.98(8) O(5A)-Cd(1A)-O(1A) 92.34(8) N(1A)-Cd(1A)-O(5A) 85.46(8) O(2A)-Cd(1A)-O(1A) 54.32(8) N(3A)-Cd(1A)-O(2A) 93.79(8) O(6A)#1-Cd(1A)-O(1A) 136.98(8) N(1A)-Cd(1A)-O(2A) 91.36(9) N(3A)-Cd(1A)-O(5A)#1 90.99(8) O(5A)-Cd(1A)-O(2A) 146.66(8) N(1A)-Cd(1A)-O(5A)#1 88.15(8) N(3A)-Cd(1A)-O(6A)#1 91.19(8) O(5A)-Cd(1A)-O(5A)#1 77.08(8) N(1A)-Cd(1A)-O(6A)#1 93.53(8) O(2A)-Cd(1A)-O(5A)#1 136.07(7) O(5A)-Cd(1A)-O(6A)#1 130.48(8) O(6A)#1-Cd(1A)-O(5A)#1 53.42(7) Molecules 2022, 27, 1365 12 of 29 Table 5. Cont. O(2A)-Cd(1A)-O(6A)#1 82.81(8) O(1A)-Cd(1A)-O(5A)#1 167.86(8) N(3A)-Cd(1A)-O(1A) 94.64(9) Molecule B Bond lengths Cd(1B)-O(2B)#2 2.306(2) Cd(1B)-O(5B) 2.362(2) Cd(1B)-N(3B) 2.321(2) Cd(1B)-O(6B) 2.411(2) Cd(1B)-O(1B) 2.328(2) Cd(1B)-O(2B) 2.572(2) Cd(1B)-N(1B) 2.329(3) Cd(1B)· · · Cd(1B) 3.7838(6) Bond angles O(2B)#2-Cd(1B)-N(3B) 86.45(8) N(3B)-Cd(1B)-O(6B) 86.89(8) O(2B)#2-Cd(1B)-O(1B) 131.21(8) O(1B)-Cd(1B)-O(6B) 137.68(8) N(3B)-Cd(1B)-O(1B) 92.87(9) N(1B)-Cd(1B)-O(6B) 91.31(8) O(2B)#2-Cd(1B)-N(1B) 86.99(8) O(5B)-Cd(1B)-O(6B) 54.62(7) N(3B)-Cd(1B)-N(1B) 173.17(9) O(2B)#2-Cd(1B)-O(2B) 78.31(8) O(1B)-Cd(1B)-N(1B) 92.89(9) N(3B)-Cd(1B)-O(2B) 91.92(8) O(2B)#2-Cd(1B)-O(5B) 145.66(8) O(1B)-Cd(1B)-O(2B) 52.93(8) N(3B)-Cd(1B)-O(5B) 91.89(8) N(1B)-Cd(1B)-O(2B) 88.63(8) O(1B)-Cd(1B)-O(5B) 83.13(8) O(5B)-Cd(1B)-O(2B) 136.03(7) N(1B)-Cd(1B)-O(5B) 92.44(8) O(6B)-Cd(1B)-O(2B) 169.35(7) O(2B)#2-Cd(1B)-O(6B) 91.05(8) π· · · π interactions a b β, γ c Slippage e Cg(I)· · · Cg(J) Cg· · · Cg α Cg(I)_Perp, Cg(J)_Perp d Cg(1)· · · Cg(1) 3.6090(19) 1 14.9 3.4870(14) - Cg(2)· · · Cg(2) 3.6435(18) 0 18.3 3.4597(13) 1.142 Cg(3)· · · Cg(3) 3.7740(19) 1 24.4 3.4372(14) - Cg(4)· · · Cg(4) 3.606(2) 3 2.5 3.6021(16), 3.6023(16) - a #1 −x + 1,y,−z + 1/2, #2 −x,y,−z + 1/2. Cg· · · Cg distances are given in Å. Cg· · · Cg = distance between ring centroids (Å); b α= dihedral angle between Planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N1A-C17A- C18A-C19A-C20A-C21A; Cg(2) = N3A-C23A-C24A-C25A-C26A-C27A; Cg(3) = N1B-C17B-C18B-C19B-C20B-C21B; Cg(4) = N3B-C23B-C24B-C25B-C26B-C27B. These dimeric arrays are held together by the two Pip ligands, displaying both biden- tate bridge and chelate coordination modes (µ2 -η2 :η1 ), and the remaining two Pip linkers complete the coordination sphere by a bidentate chelate (µ1 -η2 ) coordination mode. Both dimers (3 and 4) present intramolecular π· · · π interactions between the aromatic rings of 4-Acpy and Isn, respectively (Tables 6 and 7). Molecules 2022, 27, 1365 13 of 29 Table 6. Intermolecular interactions in 3. Intermolecular H· · ·A (Å) D· · ·A (Å) D-H (Å) >D-H· · ·A (◦ ) Interactions C6B-H6BB· · · O10A 2.45 3.24(1) 0.99 135.9 C14B-H14B· · · O10A 2.497 3.06(1) 0.99 115.7 C25A-H25A· · · O2A 2.241 3.190(5) 0.95 177.2 C27B-H27B· · · O2B 2.365 3.228(5) 0.95 150.8 C30A-H30A· · · O6A 2.371 3.317(5) 0.98 161.9 C30B-H30E· · · O5B 2.480 3.423(6) 0.98 161.3 π· · · π interactions Cg(I)· · · Cg(J) Cg· · · Cga α b β, γ c Cg(I)_Perp, Cg(J)_Perp d Slippage e Cg(1)· · · Cg(7) 3.8236 9.0 25.6, 17.1 3.6555, 3.4478 - Cg(5)· · · Cg(6) 3.7876 1.0 29.0, 28.4 3.3325, 3.3129 - Cg(6)· · · Cg(6) 3.5058 0 18.3, 18.3 3.3287, 3.3287 1.100 Cg· · · Cg distances are given in Å. a Cg· · · Cg = distance between ring centroids (Å); b α = dihedral angle between Planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(1) = N1A-C17A-C18A-C19A-C20A-C21A; Cg(5) = C2A- C3A-C4A-C5A-C7A-C8A; Cg(6) = C10A-C11A-C12A-C14A-C15A-C16A; Cg(7) = C2B-C3B-C4B-C5B-C7B-C8B. Table 7. Intermolecular interactions in 4. Intermolecular >D-H· · ·A H· · ·A (Å) D· · ·A (Å) D-H (Å) Interactions. (◦ ) N2A-H2AA· · · O10A 1.997 2.873(3) 0.88 174.1 N2A-H2AB· · · O4A 2.161 2.909(4) 0.88 142.6 N4A-H4AA· · · O9A 2.046 2.899(3) 0.88 163.1 N4A-H4AB· · · O5B 2.015 2.849(3) 0.88 157.8 N4B-H4BA· · · O9B 2.050 2.928(4) 0.88 175.7 N4B-H4BB· · · O1W 2.172 2.980(8) 0.88 152.7 N2B-H2BA· · · O10B 2.011 2.874(4) 0.88 166.2 N2B-H2BB· · · O2A 2.022 2.832(3) 0.88 152.5 C26A-H26A· · · O5B 2.418 3.315(4) 0.95 157.3 π· · · π interactions a b β, γ c Slippage e Cg(I)· · · Cg(J) Cg· · · Cg α Cg(I)_Perp, Cg(J)_Perp d Cg(8)· · · Cg(7) 3.560(2) 6.19(18) 8.2, 13.4 3.4625(14), 3.5233(17) - Cg(5)· · · Cg(6) 3.824(2) 2.39(16) 26.2, 27.7 3.3857(14), 3.4303(13) - Cg(6)· · · Cg(7) 3.534(2) 1.95(17) 17.3, 15.6 3.4046(13), 3.3746(17) - Cg· · · Cg distances are given in Å. a Cg· · · Cg = distance between ring centroids (Å); b α = dihedral angle between Planes I and J (◦ ); c Offset angles: β = angle Cg(I)-Cg(J) and normal to plane I (◦ ) and γ = angle Cg(I)-Cg(J) and normal to plane J (◦ ) (β = γ, when α = 0); d Perpendicular distance (Å) of Cg(I) on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Horizontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(5) = C10A-C11A-C12A-C14A-C15A-C16A; Cg(6) = C2B-C3B-C4B-C5B-C7B-C8B; Cg(7) = C10B-C11B-C12B-C14B-C15B-C16B; Cg(8) = C2A-C3A-C4A-C5A- C7A-C8A. The intermolecular assembly of 3 is guided by π· · · π interactions supported by three pairs of weak C-H· · · O associations between the Pip and 4-Acpy ligands. The A dimers are stacked via planar interactions in trios, complemented by a reciprocal C-H· · · O between on plane J and perpendicular distance (Å) of Cg(J) on plane I (equal when α = 0); e Slippage = Hori- zontal displacement or slippage between Cg(I) and Cg(J) (equal for both centroids when α = 0). Cg(5) = C10A-C11A-C12A-C14A-C15A-C16A; Cg(6) = C2B-C3B-C4B-C5B-C7B-C8B; Cg(7) = C10B-C11B- C12B-C14B-C15B-C16B; Cg(8) = C2A-C3A-C4A-C5A-C7A-C8A;. Molecules 2022, 27, 1365 The intermolecular assembly of 3 is guided by π···π interactions supported by14 of 29 three pairs of weak C-H···O associations between the Pip and 4-Acpy ligands. The A dimers are stacked via planar interactions in trios, complemented by a reciprocal C-H···O between m- H m-Hand andHmethyl from Hmethyl 4-Acpy from 4-Acpyandandtwo twocarboxylate carboxylateOO atoms atomsforming formingchains chainsalong alongthe the [100] [100] direction. direction. In In contrast, contrast, B dimers only assemble assemble via via the theequivalent equivalentC-H C-H···O · · · Obetween betweenPip Pipand and 4-Acpy 4-Acpy in the same same axis. axis. Finally, · · · Bdimers Finally,AA···B dimers interact interact in pairs through · · · πinteractions through ππ···π interactions between between 4-Acpy and Pip ligands supportedsupported by by C-H · · · Obetween C-H···O betweenthe themethylene methylene H H atoms atoms from from Pip and thethe OO atoms atomsfrom from4-Acpy 4-Acpyalongalong[212]. [212].Both BothC-H · · · O and C-H···O · · · πinteractions and ππ···π interactions form 2D form 2D layers layers in in (102) (102) (Figure (Figure 6a,b). Figure 6. Intermolecular (a) π· · · π and (b) C-H· · · O interactions between A and B dimers present in 3. Color codes: Cg(1) black; Cg(2) orange, Cg(5) dark blue; Cg(6) dark green; Cg(7) violet. The dimeric units in 4 are held together by the amide· · · amide pattern engaged in a head-to-head disposition which orders the dimers in chains (A· · · A and B· · · B) parallel to the [10] direction. Besides, the N-Hanti also partake in a double N-H· · · O and m-C-H· · · O associating the chains between A· · · B dimers (Figure 7a–g). Planar π· · · π interactions support the assembly of dimers along [10] direction (Figure 7h). 2.4. Structure and Geometric Evaluation Detailed analysis of the geometric distortions present in Zn(II) and Cd(II) complexes have been performed using SHAPE [20,26] through the S parameter (S.I: Table S1). A recent structural search in the CCDC [27] has revealed that Zn(II) and Cd(II) mainly present coordination numbers ranging from 4 to 7, being scarce 3 and 8, the latter only affordable by Cd(II). The predominant structural motifs reported for Zn(II) are the dimeric µ-bridged [Zn2 (CO2 )2 ] units and the dimeric paddle-wheel [Zn2 (CO2 )4 ], bearing tetrahedral and octahedral geometries in [Zn2 (CO2 )2 ], or square pyramidal environments in [Zn2 (CO2 )4 ]. In turn, the most common motif in Cd(II) is the double bridged [Cd2 (CO2 )2 ], completed by two chelate ligands to form the [Cd2 (CO2 )2 ]−2+2 core and bearing coordination number 7 [12]. From these data it can be expected that complex 1 exhibits an almost ideal square pyramidal geometry (SPY-5, S = 0.240) [20] supported by the paddle-wheel structure that minimizes steric repulsion between the ligands. In the case of 2, both the monomer and dimer present uncommon structural motifs as the monomeric [Zn(CO2 )3 ] and the dimeric [Zn2 (CO2 )4 ]. The two display the same ligand disposition and distorted octahedral geometry, which seems to be stabilized by strong intermolecular N-H· · · O (dimer) and intramolecular O-H· · · O (monomer) interactions. In the monomeric unit, the µ1 -η2 coordination mode combined with the intramolecular interaction between the HPip and Pip ligands force geometric constraints that accommodate the metal octahedral geometry (OC-6, S = 2.769 (Zn1A)). By the same token, this deviation (OC-6, S = 3.801 (Zn1B)) [20] is emphasized in the dimer by both µ2 -η1 :η1 and µ1 -η1 - coordination modes of the Pip ligands. Cd(II) dimers Molecules 2022, 27, 1365 15 of 29 Molecules 2022, 27, x FOR PEER REVIEW 14 of 28 (3 and 4) present the common [Cd2 (CO2 )2 ]−2+2 core usually found bearing coordination number 7. Both dimers have similar S values (2.215 and 2.448 (3) or 1.995 and 2.015 (4)) [20] Figure 6. Intermolecular (a) π···π and (b) C-H···O interactions between A and B dimers present in 3. since the equatorial plane arranged by µ2 -η2 :η1 Pip linkers is almost equal. The strong Color codes: Cg(1) black; Cg(2) orange, Cg(5) dark blue; Cg(6) dark green; Cg(7) violet. double head-to-head amide· · · amide interaction between the dimers in 4 fix the Isn ligands and amend any distortion The dimeric units in that 4 arecannot be minimized held together by theinamide···amide complex 3, in which theengaged pattern interactions in a of 4-Acpy are weaker. Overall, the smaller radius of hexacoordinated Zn(II) (0.880 head-to-head disposition which orders the dimers in chains (A···A and B···B) parallel to Å), with respect to heptacoordinated Cd(II) (1.17 Å) [28], accentuate the geometric constraints that the [010] direction. Besides, the N-Hanti also partake in a double N-H···O and m-C-H···O occur during the formation of the dimeric arrays, which is reflected in the higher S value associating the chains between A···B dimers (Figure 7a–g). Planar π···π interactions sup- (3.801) of complex 2 compared to complexes 3 and 4 (S range between 1.995 and 2.448) [20]. port the assembly of dimers along [010] direction (Figure 7h). Figure 7. Intermolecular (a) N-H···O interactions of 4. In detail, (b) amide···amide interactions be- Figure 7. Intermolecular (a) N-H· · · O interactions of 4. In detail, (b) amide· · · amide interac- tween A dimers or (c) between B dimers; (d) MeOH···amide, (e) amide(A)···Pip(B) or (f) MeOH···Pip tions between association; andA(g) dimers or (c) between amide(B)···Pip(A) B dimers;(h) interactions. (d)π···π MeOH · · · amide,between interactions (e) amide(A) · · · Pip(B) or Pip rings. (f) MeOH· · · Pip association; and (g) amide(B)· · · Pip(A) interactions. (h) π· · · π interactions between Pip rings. 2.4. Structure and Geometric Evaluation Detailed analysis 2.5. Photophysical of the geometric distortions present in Zn(II) and Cd(II) complexes Properties have The beenabsorption performedand using SHAPEproperties emission [20,26] through of thethe S parameter complexes and(S.I: Tableas ligands, S1). A re- well as cent structural search in the CCDC [27] has revealed that Zn(II) and Cd(II) the references (L-tyrosine and phenanthrene), were recorded in a MeOH solution. The mainly present coordination absorption was numbers ranging measured from in the UV4region to 7, being of thescarce 3 and spectra 8, the from 200 latter to 345only nm,affordable while the by Cd(II). The predominant structural motifs reported emission was recorded between 270 and 450 nm at 298 K. for Zn(II) are the dimeric μ-bridged [Zn2(CO 2)2] units and the dimeric paddle-wheel [Zn2(CO2)4], bearing tetrahedral and oc- UV-Vis spectroscopy. To ensure the non-aggregation of the samples at the selected tahedral geometries concentrations influorescence for the [Zn2(CO2)2], experiments, or square pyramidal environments we performed additivein [Zn2(CO UV-Vis 2)4]. In measure- turn, mentsthe mosta common within motifrange concentration in Cd(II) fromis1the × 10double −9 to 1bridged × 10−5 [Cd 2(CO2)28). M (Figure ], completed by two chelate ligands to form the [Cd2(CO2)2]‒2+2 core and bearing coordination number 7 [12]. From these data it can be expected that complex 1 exhibits an almost ideal square pyramidal geometry (SPY-5, S = 0.240) [20] supported by the paddle-wheel structure that minimizes steric repulsion between the ligands. In the case of 2, both the monomer and dimer present uncommon structural motifs as the monomeric [Zn(CO2)3] and the dimeric references (L-tyrosine and phenanthrene), were recorded in a MeOH solution. The ab- sorption was measured in the UV region of the spectra from 200 to 345 nm, while the emission was recorded between 270 and 450 nm at 298 K. UV-Vis spectroscopy. To ensure the non-aggregation of the samples at the selected Molecules 2022, 27, 1365 concentrations for the fluorescence experiments, we performed additive UV-Vis measure- 16 of 29 ments within a concentration range from 1 × 10 to 1 × 10 M (Figure 8). −9 −5 a) 1.2 b) 1.6 1.0 1.4 1.2 Absorbance (a.u.) 0.8 Absorbance (a.u.) 1.0 0.6 0.8 0.4 0.6 0.4 0.2 0.2 0.0 0.0 195 220 245 270 295 320 345 195 220 245 270 295 320 345 Wavelength (nm) Wavelength (nm) c) 1.5 d) 1.4 1.2 1.0 Absorbance (a.u.) 1.0 Absorbance (a.u.) 0.8 0.6 0.5 0.4 0.2 0.0 0.0 195 220 245 270 295 320 345 195 220 245 270 295 320 345 Wavelength (nm) Wavelength (nm) Figure 8. Additive UV-Vis spectra of complexes (a) 1; (b) 2; (c) 3 and (d) 4 performed in a concentration range from 1 × 10−9 M to 1 × 10−4 M in MeOH as the solvent at 298 K. The arrows indicate the spectral changes upon increasing concentration. The samples do not seem to present aggregation in this range of concentration. The absorption spectrum of 2 displays a significant change from 4.49 × 10−8 M on, which has been ascribed to a change on the absorber rather than a change via aggregates formation. This is discussed below and supported by TD-DFT calculations. The absorption and emis- sion maxima of complexes 1–4 (λmax-Abs and λmax-Em , respectively), have been identified, and their molar absorptivity (ε) and relative quantum yield (φs ) calculated (Table 8). Table 8. UV-Vis and fluorescence data of complexes 1–4. Sample λex λmax-em ∆λ φs 1 263 354 9774 0.74 225 344 15,375 0.01 2 225 * 344 * 15,375 0.03 251 * 318 * 8,394 0.02 3 315 355 3,577 0.47 4 226 347 15,429 0.09 All of the wavelengths are given in nm. ∆λ are given in cm−1 . λex = excitation wavelength; λmax-em = maximum of emission; φS = quantum yield. * Bands arising from a change in the absorber (the dimer in 2). Photoluminescence. Heretofore, fluorescence measurements were performed using concentrations of 1.70 × 10−9 M (1); 1.08 × 10−8 M and 1.07 × 10−7 M (2); 1.04 × 10−7 M (3); and 1.01 × 10−7 M (4), extracted from the UV-Vis results after ensuring their non-aggregation to minimize ACQ [7]; these samples were excited at the wavelength of their emission max- ima. All of the relevant details have been summarized in Table 8. The spectra of 1 and 2 have single emission bands at 354 nm (1) and 344 or 318 nm (2) at being irradiated at 263 nm and 225 or 251 nm, respectively. The emission spectra of 3 and 4 present unfolded emission their molar absorptivity (ε) and relative quantum yield (ϕs) calculated (Table 8). Photoluminescence. Heretofore, fluorescence measurements were performed using concentrations of 1.70 × 10−9 M (1); 1.08 × 10−8 M and 1.07 × 10−7 M (2); 1.04 × 10−7 M (3); and 1.01 × 10−7 M (4), extracted from the UV-Vis results after ensuring their non-aggregation to minimize ACQ [7]; these samples were excited at the wavelength of their emission max- Molecules 2022, 27, 1365 ima. All of the relevant details have been summarized in Table 8. The spectra of 117and of 292 have single emission bands at 354 nm (1) and 344 or 318 nm (2) at being irradiated at 263 nm and 225 or 251 nm, respectively. The emission spectra of 3 and 4 present unfolded bands centered emission bandsat 355 andat347 centered 355nm, andat347 being nm,irradiated at 315 andat226 at being irradiated 315nm, andrespectively. As 226 nm, respec- displayed tively. As in the CIE 1931 displayed chromaticity in the diagram [29], CIE 1931 chromaticity the resultant diagram emission [29], the color resultant of 1–4 emission (318–355 nm)(318–355 color of 1–4 is violet nm) (Figure 9). (Figure 9). is violet Figure9.9. Representation Figure Representation of of the the emission emission color color of of complexes complexes 1–4 1–4 within within the the CIE CIE 1931 1931chromaticity chromaticity diagram. diagram. Emission intensities Emission intensities increase increase in inthe theorder order1 1< < 2 <2 3<<34 2σ(I)] wR2 = 0.1025 wR2 = 0.1442 R1 = 0.0469 R1 = 0.1404, R indices (all data) wR2 = 0.1102 wR2 = 0.1947 Extinction coefficient n/a n/a Largest. Diff. Peak and hole (e 1.540 and −0.544 2.970 and −1.091 Å−3 ) Table 10. Crystal structure refinement parameters for 3 and 4. 3 4 Empirical formula C60 H48 Cd2 N4 O20 C113 H92 Cd4 N16 O41 Formula weight 1369.82 2779.62 T (K) 100(2) 100(2) Wavelength (Å) 0.71073 0.71073 System, space group Triclinic, P-1 Monoclinic, P2/c Unit cell dimensions a (Å) 10.2552(6) 15.4163(10) b (Å) 11.9963(7) 17.3028(12) c (Å) 24.7728(15) 22.9004(17) α (◦ ) 82.798(2) 90 β (◦ ) 89.674(2) 105.565(2) γ (◦ ) 65.1870(10) 90 V (Å3 ) 2740.8(3) 5884.55 Z 2 2 Molecules 2022, 27, 1365 26 of 29 Table 10. Cont. 3 4 Dcalc (g cm3 ) 1.660 1.569 µ (mm−1 ) 0.862 0.806 F (000) 1384 2804 Crystal size (mm3 ) 0.131 × 0.092 × 0.042 0.400 × 0.120 × 0.040 −13 ≤ h ≤ 13 −22 ≤ h ≤ 21 hkl ranges −15 ≤ k ≤ 15 0 ≤ k ≤ 25 −32 ≤ l ≤ 32 0 ≤ l ≤ 33 θ range (◦ ) 2.191 to 27.526 2.190 to 31.067 Reflections collected/ 75664/12559/ 18818/18818/ unique/[Rint] [Rint ] = 0.0750 [R(int) = 0.0751] Completeness to θ (%) 99.9 99.8 Semi-empirical from Semi-empirical from Absorption Correction equivalents equivalents Max. and min. transmis. 0.7455 and 0.6770 0.7454 and 0.6631 Full matrix least-squares on Full-matrix least-squares on Refinement method |F|2 |F|2 Data/restrains/parameters 12559/0/788 18818/10/776 Goodness of fit (GOF) on |F|2 1.082 1.052 R1 = 0.0404, R1 = 0.0428, Final R indices [I > 2σ(I)] wR2 = 0.0814 wR2 = 0.1115 R1 = 0.0634, R1 = 0.0833, R indices (all data) wR2 = 0.0888 wR2 = 0.1439 Extinction coefficient n/a n/a Largest. Diff. Peak and hole (e 1.087 and −1.028 2.986 and −1.223 Å−3 ) Methodology and computational details. All of the calculations were performed using Gaussian09 software version D.01 [39]. The geometry optimization of the ground state and vertical absorptions from the electronically excited state for 1–4 were completed using density functional theory (DFT) and time-dependent DFT (TD-DFT), respectively, using ωB97X-D [40,41] functional (S.I: Tables S3–S7 and Figures S28–S32). A correlation consistent basis set was used for the Zn, Cd, C, H, N and O atoms, the effective core potential CrenbL[42]. The MeOH solvation effects were incorporated using the polarizable continuum model-Linear Response (PCM-LR) [43,44]. Since either monomeric or dimeric arrays seem to be involved in absorption and emission depending on the concentration, the geometry of the monomer and the dimer in 2 were optimized separately. The fre- quencies were also computed for each optimized structure to ensure that the geometries corresponded to an energy minimum. The HOMO and LUMO energetic levels were firstly examined and then the energy gaps calculated. For 1–4, the first 80 vertical absorptions from the ground state to the excited states were calculated and only the most probable transitions, those with higher oscillator strength (f ) values, were selected for the electronic analysis. The shift in the theoretical absorption spectra, with respect to the experimental profile, is within the range of typical TD-DFT calculations (~0.3 eV) and are caused by computing the absorptions as vertical transitions [35]. The methodology for the generation of the MOs was the same as has been previously reported [15] (S.I: Figures S18–S22). The analysis of the electronic transitions was sup- ported by the NTOs (S.I: Figures S23–S27) [45] to better identify and represent the main Molecules 2022, 27, 1365 27 of 29 contributor molecular orbitals of each transition. The NTOs were generated using Multiwfn software [46] version 3.7 with an isovalue of 0.02. Supplementary Materials: The following are available online, calculated S values from the crystal structures of 1–4, Table S1. Calculated S values from the optimized geometries of 1–4, Table S2. FTIR- ATR, 1 H NMR, 13 C{1 H} NMR and DEPT-135 spectra, Figures S1–S12. Solid state photoluminescence spectrum of 1, Figure S13. Comparison between experimental and theoretical UV-Vis spectra of complexes 1, 3 and 4, Figures S14–S16. HOMO–LUMO energy gaps, Figure S17. MOs representation of 1–4, Figures S18–S22. NTOs representation of 1–4, Figures S23–S27. Details about DFT optimization of 1–4, Tables S3–S7 and Figures S28–S32. Complete information about the crystal structure and molecular geometry is available in .cif format as Supporting Information. CCDC 2124892–2124895 contains the supplementary crystallographic data for this paper. These data can be retrieved free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 15 January 2022), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033. Author Contributions: Conceptualization, J.P.; data curation, F.S.-F., J.M.R.-B. and M.F.-B.; formal analysis, F.S.-F. and M.F.-B.; funding acquisition, J.P.; investigation, F.S.-F.; methodology, F.S.-F. and J.M.R.-B.; project administration, J.P.; resources, T.C. and J.P.; software, F.S.-F. and J.M.R.-B.; supervision, J.P.; validation, J.A.A., T.C. and J.P.; visualization, F.S.-F. and J.M.R.-B.; writing—original draft, F.S.-F.; writing—review and editing, J.A.A., T.C. and J.P. 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