Effect of crystal packing and coordinated solvent molecules on metal-ligand bond distances in linear trinuclear nickel compounds with bridging acetato and Schiff base ligands

A combined experimental and theoretical study on the molecular structure of linear trinuclear nickel compounds of general formula [Ni3(μ-Zsaltn)2(μ-AcO)2(S)2] (Z = H, 1S, and Br, 2S, where S identifies coordinated solvent molecules) is here presented. In these complexes the three metal ions are linked together by bridging acetato ions and tetradentate Schiff base ligands, these last derived from the condensation of 1,3-diaminopropane with two equivalents of 5-Z-salicylaldehyde. Two solvent molecules S can also interact with the terminal nickel ions to complete an octahedral coordination environment. We report the synthesis of [Ni3(μ-Brsaltn)2(μ-AcO)2] (2), and its crystal structure with two coordinated dimethylformamide molecules, [Ni3(μ-Brsaltn)2(μ-AcO)2(DMF)2] (2DMF). Its comparison with the crystal structure of 2 without solvent, together with the series of compounds 1, 1DMF, 1MeOH, 1DMSO, 1Py and 1H2O (MeOH = methanol, DMSO = dimethylsulfoxide, Py = pyridine) available in the literature, allows recognising modification in Ni–O and Ni–N coordination bond lengths and peculiar intermolecular contacts between neighbour molecules. In order to discern among the contribution of solvent molecules, crystal packing and substituent Z (H or Br) on bond distances (and angles), we performed a systematic theoretical study on four representative derivatives, namely 1, 1DFM, 2 and 2DMF, with the M06 functional and a careful choice of the best basis set. The results indicate that, while the major modulating factor is the presence or absence of the sixth solvent molecule to the peripheral nickel(II) ion, even if less strongly coordinated with Ni–O/N bond distances > 2.15 Å π-π stacking interactions and C–H⋯π and/or C–Br⋯π non-bonded interactions affect the coordination geometry in a significant way.