Effects of Weak Intermolecular Interactions on the Molecular Isomerism of Tricobalt Metal Chains

Depending on the number of interstitial solvent molecules, n, crystals of the linear chain compound Co₃(dipyridylamide)₄Cl₂·nCH₂Cl₂ adopt either symmetrical or unsymmetrical metal chain structures. We explore here the possible reasons for such behavior using Hirshfeld surface analysis of intermolecular interactions as well as the charge density determined from 100(1) K X-ray diffraction data on the unsymmetrical complex Co₃(dipyridylamide)₄Cl₂·2.11CH₂Cl₂, u-1, and crystal structures of u-1 determined from single crystal synchrotron X-ray diffraction data at 20, 150, and 300 K. The new crystal structures are compared with previous structural results on a crystal with slightly different solvent content. This change in solvent content only affects the bond distances to atom Co(3), which are also strongly affected by temperature changes due to a spin crossover transition. Large differences in intermolecular interactions are revealed by the Hirshfeld surface analysis between symmetrical (s-1) and unsymmetrical (u-1) crystal solvates, suggesting that the molecular isomerism is strongly influenced by crystal packing effects. Topological analysis of the static electron density of u-1 suggests that there is direct metal−metal bonding for both the shorter Co(1)−Co(2) and the longer Co(2)−Co(3) contact. The approximate description of the system as a (Co₂)²⁺-dimer and an isolated Co²⁺-ion is reflected in the character of the metal−ligand interactions, which are more ionic for the isolated Co(3) atom, and the topological charges Co(1)^+0.50, Co(2)^+0.77, and Co(3)^+1.36. The two termini of u-1 are found to be very different, both in terms of structural surroundings as well as topology. The central Co(2) atom is similar to a cobalt atom in a tetragonally distorted octahedral environment resulting in preferred occupancy in the t_2g orbitals. The Co(1) atom has significant deformation in the xz and yz planes (z along the chain axis, x and y toward ligands) reflecting covalent interactions with the terminal chlorine atom Cl(1). The Co(3) atom has a relatively high occupancy of the d_x²−y² orbital and a relatively low occupancy of the d_xy orbital confirming that these orbitals are involved in the spin crossover process and predominantly responsible for the observed variation in bond lengths with temperature.