Structure Guiding Supramolecular Assemblies in Metal-Organic Multi-Component Compounds of Mn(II): Experimental and Theoretical Studies

: Two multi-component coordination compounds of Mn(II), viz. [Mn(H 2 O) 6 ](2-Mepy) 2 (4-NO 2 bz) 2 · 2H 2 O ( 1 ) and [Mn(H 2 O) 6 ][Mn(2,3-PDCH) 3 ] 2 ( 2 ) (where, 2-Mepy = 2-methylpyridine, 4-NO 2 bz = 4-nitrobenzoate, 2,3-PDC = 2,3-pyridinedicarboxylate), have been synthesized and characterized using elemental, spectroscopic (FT-IR and electronic), TGA and single-crystal X-ray diffraction analyses. Complex 1 is a co-crystal hydrate of Mn(II) involving uncoordinated 2-Mepy, 4-NO 2 bz and water molecules; while compound 2 is a multi-component molecular complex salt of Mn(II) comprising cationic [Mn(H 2 O) 6 ] 2+ and anionic [Mn(2,3-PDCH) 3 ] − complex moieties. The uncoordinated 2-Mepy and 4-NO 2 bz moieties of 1 are involved in lone-pair (l.p)- π and C–H · · · π interactions which stabilize the layered assembly of the compound. The crystal structure of compound 2 has been previously reported. However, we have explored the unusual enclathration of complex cationic moieties within the supramolecular host cavities formed by the molecular assembly of complex anionic moieties. The supramolecular assemblies obtained in the crystal structure have been further studied theoretically using DFT calculations, quantum theory of atoms-in-molecules (QTAIM) and non-covalent interaction plot (NCI plot) computational tools. Theoretical studies reveal that the combination of π -staking interactions (l.p-π , π - π and C–H ··· π ) have more structure-guiding roles compared to the H-bonds. The large binding energy of π -stacking interactions in 2 is due to the antiparallel orientation of aromatic rings and their coordination to the metal centers, thereby increasing the contribution of the dipole–dipole interactions.


Introduction
Metal-organic compounds of transition metals have attracted the researchers not only because of their intriguing structural topologies but also due to their myriad applications in various fields [1][2][3][4].Using organic moieties as ligands, various research groups have reported coordination compounds with fascinating topologies and network architectures [5].Multi-component compounds (composed of more than two components), mainly classified as co-crystals, molecular salts and polymorphs, have also received remarkable emphasis because of their significant physicochemical and pharmaceutical applications [6,7].The co-crystallization technique has been effectively employed in the literature to develop potential co-crystals with significant pharmaceutical properties [8].Metal-organic multicomponent co-crystals have also held a unique place in crystal engineering because of their applications in pharmaceuticals, electronic devices and in synthetic chemistry [9,10].
The uses of ancillary N-donors with aromatic carboxylate ligands have drawn considerable emphasis in the design and synthesis of metal-organic coordination compounds [19,20].Researchers often employed aromatic carboxylates as ligands for the synthesis of compounds due to their multi-coordination modes for transition metals [21,22].Pyridine dicarboxylate (PDC) derivatives have also been successfully employed as building blocks to construct metal-organic compounds of fascinating structural topologies [23,24].A 2methylpyridine (2-Mepy) moiety has also been used to construct metal-organic compounds with interesting structural topologies [25,26].

Materials and Methods
Manganese(II) chloride tetrahydrate, 2-methylpyridine, 4-nitrobenzoic acid, 2,3-pyridine dicarboxylic acid were bought from Sigma Aldrich (Darmstadt, Germany) and Merck (Darmstadt, Germany) and used without further purification.Elemental analyses of the compounds were recorded in a Perkin Elmer 2400 Series II CHNS/O analyzer (Waltham, MA, USA).A Bruker ALPHA II Infrared spectrophotometer (West Perth, WA, Australia) was used to record the FTIR spectra of the compounds.The electronic spectra of the compounds were recorded in a Shimadzu UV-2600 spectrophotometer (Duisburg, Germany).To record the UV-Vis-NIR spectra, BaSO 4 was used as reference.A Sherwood Mark 1 Magnetic Susceptibility balance(Cambridge, UK) was used to calculate the magnetic susceptibility of the compounds.Thermogravimetric analyses of the compounds were performed in a Mettler Toledo TGA/DSC1 STAR e system (Columbus, OH, USA) under the flow of N 2 gas at a heating rate of 10 • C min −1 .

Crystallographic Data Collection and Refinement
X-ray crystallographic data of the compounds were taken on a Bruker D8 Venture diffractometer (New York, NY, USA) containing a Photon III 14 detector, using an Incoatec high-brilliance IμS DIAMOND Cu tube.Data reduction, cell refinements as well as scaling and absorption corrections were performed using the Bruker APEX4 and SA-DABS program [27].Crystal structures were solved by the direct method and refined by full-matrix least-squares techniques with SHELXL-2018/3 [28] using WinGX [29].All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F 2 .Hydrogen atoms were inserted at calculated positions and refined as riders, except for those from water molecules, which were located using a Fourier difference map.Diamond 3.2 software was used to draw the structural diagrams [30].Table 1 contains the crystallographic data for the compounds.

Crystallographic Data Collection and Refinement
X-ray crystallographic data of the compounds were taken on a Bruker D8 Venture diffractometer (New York, NY, USA) containing a Photon III 14 detector, using an Incoatec high-brilliance IµS DIAMOND Cu tube.Data reduction, cell refinements as well as scaling and absorption corrections were performed using the Bruker APEX4 and SADABS program [27].Crystal structures were solved by the direct method and refined by full-matrix least-squares techniques with SHELXL-2018/3 [28] using WinGX [29].All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F 2 .Hydrogen atoms were inserted at calculated positions and refined as riders, except for those from water molecules, which were located using a Fourier difference map.Diamond 3.2 software was used to draw the structural diagrams [30].Table 1 contains the crystallographic data for the compounds.CCDC 2,123,024 and 2,123,025 contain the supplementary crystallographic data for the compounds 1 and 2, respectively.These data can be obtained free of charge at http://www.ccdc.cam.ac.uk (accessed on 13 July 2021) or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.

Computational Methods
The RI-BP86-D3/def2-TZVP [31,32] level of theory was used to compute the assemblies of compounds 1 and 2 using the X-ray coordinates and the Turbomole 7.2 program [33].The noncovalent interaction plot (NCIPlot) [34] and the quantum theory of "atoms-inmolecules" (QTAIM) [35] computational methods were computed using the MULTIWFN program [36] and represented by means of the VMD software [37]. 1)is obtained by reacting one equivalent of MnCl 2 •4H 2 O, two equivalents of 2-Mepy with two equivalents of Na-4-NO 2 bz at ambient conditions in water medium.Similarly, [Mn(H 2 O) 6 ][Mn(2,3-PDCH) 3 ] 2 (2) has been isolated by reacting MnCl 2 •4H 2 O and 2,3-pyridinedicarboxylic acid at a 1:2 molar ratio at room temperature in water medium.The compounds are soluble in water and in common organic solvents.Compounds 1 and 2 exhibit room temperature (298 K) magnetic moments (µ eff ) of 5.87 and 5.81 BM, respectively, which reveals the presence of five unpaired electrons per Mn(II) center [38].Although the crystal structure of the compound 2 has been previously reported [1], we have investigated the role of non-covalent interactions and the unusual enclathration of the complex cationic moieties during molecular association.

Crystal Structure Analysis
The molecular structure of compound 1 has been depicted in Figure 1.Table 2 represents the bond lengths and bond angles around the Mn(II) center.Compound 1 crystallizes in the triclinic P1 space group.The Mn(II) center is found to be present in a crystallographic center of inversion.The Mn(II) center has an octahedral geometry having six coordinated water molecules (O1W, O1W , O2W, O2W , O3W and O3W ).Moreover, two uncoordinated 2-Mepy, two uncoordinated 4-NO 2 bz and two uncoordinated water molecules are also present.The equatorial sites around the Mn(II) center are occupied by four oxygen atoms (O1W, O1W , O2W, O2W ) from four coordinated water molecules, whereas the axial positions are filled by O3W and O3W water molecules.The Mn-O axial bond lengths (2.178 Å) are slightly higher than that of Mn-O equitorial bond lengths (2.156 Å).The average Mn-O bond lengths are well consistent with those reported for Mn(II) compounds [39].uncoordinated 2-Mepy, two uncoordinated 4-NO2bz and two uncoordinated water molecules are also present.The equatorial sites around the Mn(II) center are occupied by four oxygen atoms (O1W, O1W′, O2W, O2W′) from four coordinated water molecules, whereas the axial positions are filled by O3W and O3W′ water molecules.The Mn-Oaxial bond lengths (2.178 Å) are slightly higher than that of Mn-Oequitorial bond lengths (2.156 Å).The average Mn-O bond lengths are well consistent with those reported for Mn(II) compounds [39].

D-H•••A d(D-H) d(D-A) d(H•••A) <(DHA)
Compound 1 O2W-H2WA      The molecular structure of compound 2 has been depicted in Figure 4. Table 2 tains the bond lengths and bond angles around the metal centers.Compound 2 cry lizes in the trigonal P3 space group.The asymmetric unit of compound 2 compri dicationic complex moiety and two anionic complex moieties.Crystal structure ana reveals that the Mn1 center of compound 2 lies on three-fold symmetry axis, wherea Mn2 center lies not only on a three-fold axis but also on an inversion center.The ave Mn-O and Mn-N bond lengths are consistent with those reported for similar comp [43,44].The molecular structure of compound 2 has been depicted in Figure 4. Table 2 contains the bond lengths and bond angles around the metal centers.Compound 2 crystallizes in the trigonal P3 space group.The asymmetric unit of compound 2 comprises a dicationic complex moiety and two anionic complex moieties.Crystal structure analysis reveals that the Mn1 center of compound 2 lies on three-fold symmetry axis, whereas the Mn2 center lies not only on a three-fold axis but also on an inversion center.The average Mn-O and Mn-N bond lengths are consistent with those reported for similar complexes [43,44].3).These homo-and hetero-dimers have been further studied theoretically (vide infra).3).These homo-and hetero-dimers have been further studied theoretically (vide infra).Further analysis unfolds the enclathration of the cationic complex moiety viz.

Electronic Spectroscopy
Figures S3 and S4 represent the electronic spectra of the compounds 1 and 2, respectively.Absence of bands in the visible region is because of the Mn(II) center (d 5 system), for which all the electronic transitions from the 6 A 1g ground state are doubly forbidden [54,55].The absorption bands obtained at 230 and 269 nm in the solid state spectrum of 1 are due to the π→π* transition of the aromatic ligands [56,57].However, in the aqueous phase spectrum of 1, these absorption peaks are obtained at 228 and 271 nm, respectively.The bands at 223 and 268 nm in the UV-Vis-NIR spectrum of 2 can be ascribed due to π→π* transition of an aromatic ligand [56,57], while these peaks are obtained at 222 and 271 nm, respectively, in the aqueous phase UV-Vis spectrum of 2.

Theoretical Studies
The study of compound 1 was focused on the study of the interesting 2D assembly where the 2-Mepy and 4-NO 2 bz moieties are held together by C-H•••O, π-π and l.p.-π interactions.A tetrameric assembly (Figure 7) was used where the carboxylate group of 4-NO 2 bz has been protonated in order to use a neutral model and estimate the non-covalent contacts free from the influence of strong electrostatic forces.QTAIM and NCI plots, by means of the reduced density gradient (RDG) isosurfaces, were used to characterize the non-covalent contacts in the tetrameric assembly of compound 1, revealing an intricate combination of interactions; that is, four C-H•••O(nitro) interactions that are characterized by bond critical points (BCPs, represented as red spheres) and bond paths (BPs, represented as orange lines) connecting the O-atoms of the nitro groups to two adjacent aromatic Hatoms.The π-stacking interactions are characterized by two BCPs and BPs interconnecting two C-atoms of the 2-Mepy rings to two C-atoms of the 4-NO 2 bz moieties.
Interestingly, the QTAIM analysis ratifies the presence of the lp-π interaction involving one O-atom of the nitro group.It is characterized by a BCP and BP connecting the O-atom to one C-atom of the 2-Mepy ring.Finally, the QTAIM also confirms the importance of the C-H•••π interactions, characterized by a BCP and BP connecting the H and C-atoms.All these interactions are also revealed by green-colored (attractive) RDG isosurfaces.Those characterizing the C-H•••π and π-π interactions are quite extended, embracing most of the aromatic surfaces.We have evaluated the formation energy of the tetramer, starting from two different (2-Mepy)•••(4-NO 2 bz) dimeric assemblies, in order to investigate the relative importance of the H-bonds with respect to the π-based interactions.The dimerization energy starting from the pre-formed π-π dimer (represented in Figure 7a) is -6.2 kcal/mol, which corresponds to the contribution of the four H-bonds (1.55 kcal/mol each H-bond).If the formation of the tetramer is computed starting from the H-bonded dimer (Figure 7b), the binding energy is significantly larger (-14.4 kcal/mol), thus suggesting that the combination of π-interactions (l.p-π, π-π and C-H•••π) are energetically more relevant than the H-bonds.

Theoretical Studies
The study of compound 1 was focused on the study of the interesting 2D assembly where the 2-Mepy and 4-NO2bz moieties are held together by C-H•••O, π-π and l.p.-π interactions.A tetrameric assembly (Figure 7) was used where the carboxylate group of 4-NO2bz has been protonated in order to use a neutral model and estimate the non-covalent contacts free from the influence of strong electrostatic forces.[61] which was specifically developed for H-bonds in X-ray structures.The values are given in Figure 8 using a red font.The H-bonds are strong, with a dissociation energy of 11.5 kcal/mol each HB, in line with the dark blue color of the RDG isosurfaces.The total dissociation energy of the dimer is 22.6 kcal/mol.Figure 8b shows the QTAIM/NCI Plot analysis of the heterodimer, where the binding energy is very large (-131.6 kcal/mol) due to the strong Coulombic attraction between the counterions.The strength of the H-bonds has been also assessed using the G r energy predictor to avoid the strong influence of electrostatic forces.In this case, the H-bonds are weaker than those of the homodimer, i.e., 2.2 and 5.1 kcal/mol, thus revealing that the H-bonds between the anionic units are stronger than those between the counterions.kcal/mol.Figure 8b shows the QTAIM/NCI Plot analysis of the heterodimer, where the binding energy is very large (-131.6 kcal/mol) due to the strong Coulombic attraction between the counterions.The strength of the H-bonds has been also assessed using the Gr energy predictor to avoid the strong influence of electrostatic forces.In this case, the H-bonds are weaker than those of the homodimer, i.e., 2.2 and 5.1 kcal/mol, thus revealing that the H-bonds between the anionic units are stronger than those between the counterions.Finally, we have also analyzed the π-stacking interaction between the anionic moieties.In this case, to avoid the electrostatic repulsion, two carboxylate groups have been protonated (see small arrows in Figure 9).The π-stacking interaction is characterized by a large and green RDG isosurface and two BCPs and BPs interconnecting two carbon atoms of the rings.Such a large isosurface reveals a strong complementarity between the aromatic surfaces.The assembly is further stabilized by two symmetric C-H•••O contacts established between the aromatic -CH groups and the O-atoms of the coordinated carboxylate groups.The strength of these H-bonds is modest (E dis = 1.2 kcal/mol each), thus revealing that the assembly is completely dominated by the π-stacking interactions, since the total binding energy is -18.0 kcal/mol).Such a large binding energy is related to the antiparallel orientation of the aromatic rings and their coordination of the metal centers, which increase the contribution of the dipole•••dipole interactions.Finally, we have also analyzed the π-stacking interaction between the anionic moieties.In this case, to avoid the electrostatic repulsion, two carboxylate groups have been protonated (see small arrows in Figure 9).The π-stacking interaction is characterized by a large and green RDG isosurface and two BCPs and BPs interconnecting two carbon atoms of the rings.Such a large isosurface reveals a strong complementarity between the aromatic surfaces.The assembly is further stabilized by two symmetric C-H•••O contacts established between the aromatic -CH groups and the O-atoms of the coordinated carboxylate groups.The strength of these H-bonds is modest (Edis = 1.2 kcal/mol each), thus revealing that the assembly is completely dominated by the π-stacking interactions, since the total binding energy is -18.0 kcal/mol).Such a large binding energy is related to the antiparallel orientation of the aromatic rings and their coordination of the metal centers, which increase the contribution of the dipole•••dipole interactions.

Figure 2 .
Figure 2. 2D network architectures of 1 along the crystallographic ac plane assisted by O-H C-H•••O, O-H•••N HB and non-covalent C-H•••C interactions.

Figure 6 .
Figure 6.(a) Enclathrated guest cationic complex moiety of 2 in the supramolecular hexameric cavity formed by the anionic complex moieties; (b) 2D architecture of 2 along the crystallographic ab plane with enclathration of guest cationic complex moieties.

Figure 6 .
Figure 6.(a) Enclathrated guest cationic complex moiety of 2 in the supramolecular hexameric cavity formed by the anionic complex moieties; (b) 2D architecture of 2 along the crystallographic ab plane with enclathration of guest cationic complex moieties.
QTAIM and NCI plots, by means of the reduced density gradient (RDG) isosurfaces, were used to characterize the non-covalent contacts in the tetrameric assembly of compound 1, revealing an intricate combination of interactions; that is, four C-H•••O(nitro) interactions that are characterized by bond critical points (BCPs, represented as red spheres) and bond paths (BPs, represented as orange lines) connecting the O-atoms of the nitro groups to two adjacent aromatic H-atoms.The π-stacking interactions are characterized by two BCPs and BPs interconnecting two C-atoms of the 2-Mepy rings to two C-atoms of the 4-NO2bz moieties.

Figure 7 .
Figure 7. Combined QTAIM analysis (BCPs in red, BPs in orange) and NCI surfaces of the (a) π-stacked, (b) H-bonded dimers of compound 1 and (c) the tetramer.The gradient cut-off is ρ = 0.05 a.u., isosurface = 0.5, and the color scale is −0.04 a.u.< (signλ2)ρ < 0.04 a.u.Only intermolecular interactions are shown.The formation energies of the tetramer using the two dimers are also indicated.

Figure 7 .
Figure 7. Combined QTAIM analysis (BCPs in red, BPs in orange) and NCI surfaces of the (a) π-stacked, (b) H-bonded dimers of compound 1 and (c) the tetramer.The gradient cut-off is ρ = 0.05 a.u., isosurface = 0.5, and the color scale is −0.04 a.u.< (signλ 2 )ρ < 0.04 a.u.Only intermolecular interactions are shown.The formation energies of the tetramer using the two dimers are also indicated.For compound 2, the theoretical study is focused on the analysis of the O-H•••O H-bonds and π-stacking interactions, which have a significant structural directing role.The joint QTAIM and NCI plot analyses of two H-bonded dimers extracted from the solid state are provided in Figure 8.Each H-bond is characterized by a BCP, BP and dark-blue RDG isosurface connecting the H and O-atoms.The blue NCI Plot color discloses the strong nature of the H-bonds.However, the dimerization energy for the homodimer is positive (+11.3 kcal/mol) due to the electrostatic repulsion (anion•••anion interaction) and absence of counterions in the model.Consequently, the contribution of the H-bonds was also estimated free from the pure Coulombic repulsion by using the value of the Lagrangian Kinetic energy density (Gr) measured at the bond CP and the equation proposed by Vener et al. (E dis = 0.429 × G r )[61] which was specifically developed for H-bonds in X-ray structures.The values are given in Figure8using a red font.The H-bonds are strong, with a dissociation energy of 11.5 kcal/mol each HB, in line with the dark blue color of the RDG isosurfaces.The total dissociation energy of the dimer is 22.6 kcal/mol.Figure8bshows the QTAIM/NCI Plot analysis of the heterodimer, where the binding energy is very large (-131.6 kcal/mol) due to the strong Coulombic attraction between the counterions.The strength of the H-bonds has been also assessed using the G r energy predictor to avoid the strong influence of electrostatic forces.In this case, the H-bonds are weaker than those of the homodimer, i.e., 2.2 and 5.1 kcal/mol, thus revealing that the H-bonds between the anionic units are stronger than those between the counterions.

Figure 8 .
Figure 8. Combined QTAIM analysis (bond CPs in red, bond paths in orange) and NCI surfaces of two assemblies of compound 2, (a) one homodimer and (b) one heterodimer.The gradient cut-off is ρ = 0.05 a.u., isosurfaces = 0.5, and the color scale is −0.04 a.u.< (signλ2)ρ < 0.04 a.u.The dissociation energies of the H-bonds are given in red adjacent to the bond CPs in kcal/mol.The dimerization energies are also indicated.

Figure 8 .
Figure 8. Combined QTAIM analysis (bond CPs in red, bond paths in orange) and NCI surfaces of two assemblies of compound 2, (a) one homodimer and (b) one heterodimer.The gradient cut-off is ρ = 0.05 a.u., isosurfaces = 0.5, and the color scale is −0.04 a.u.< (signλ 2 )ρ < 0.04 a.u.The dissociation energies of the H-bonds are given in red adjacent to the bond CPs in kcal/mol.The dimerization energies are also indicated.

Figure 9 .
Figure 9. Combined QTAIM analysis (BCPs in red, BPs in orange) and NCI surfaces of the π-stacking assembly of compound 2. The gradient cut-off is ρ = 0.05 a.u., isosurfaces = 0.5, and the color scale is −0.04 a.u.< (signλ2)ρ < 0.04 a.u.The dissociation energies of the H-bonds are given in red adjacent to the bond CPs in kcal/mol.The dimerization energies are also indicated.

Figure 9 .
Figure 9. Combined QTAIM analysis (BCPs in red, BPs in orange) and NCI surfaces of the π-stacking assembly of compound 2. The gradient cut-off is ρ = 0.05 a.u., isosurfaces = 0.5, and the color scale is a.u.< (signλ 2 )ρ < 0.04 a.u.The dissociation energies of the H-bonds are given in red adjacent to the bond CPs in kcal/mol.The dimerization energies are also indicated.

Table 1 .
Crystallographic data and structure refinement details for the compounds 1 and 2.