The vital role of ditopic N–N bridging ligands with different lengths in the formation of new binuclear dioxomolybdenum(vi) complexes: synthesis, crystal structures, supramolecular framework and protein binding studies

Abstract

A new series of binuclear dioxomolybdenum(VI) complexes 1–4 of general formula [(MoO₂L)₂(N–N)] with an ONS donor Schiff base ligand (H₂L = S-benzyl-β-N-(5-bromo-2-hydroxyphenyl)methylenedithiocarbazate) and bridging auxiliary ligands having different lengths and flexibilities [N–N donor 4,4′-bipyridine (4,4′-bipy), 1,2-bis(4-pyridyl)ethene (bpe), 1,3-bis(4-pyridyl)propane (tmp) and 1,4-bis[(1H-imidazole-1-yl)methyl]benzene (bix)] have been synthesized. The complexes have been thoroughly characterized by elemental analyses, IR, ¹H NMR and UV-Vis spectroscopy, cyclic voltammetry and thermal analyses. The molecular structures of all four complexes 1–4 have been determined by single-crystal X-ray diffraction techniques. Crystal structures of binuclear molybdenum complexes having bridging 1,3-bis(4-pyridyl)propane and 1,4-bis[(1H-imidazole-1-yl)methyl]benzene ligands are reported for the first time. Each molybdenum site in the binuclear complexes adopts a distorted octahedral geometry. The auxiliary spacers (N–N) act as bis-monodentate ligands occupying one axial position of each molybdenum centre thereby connecting two molybdenum(VI) atoms. The influence of auxiliary ligands on structural and supramolecular features of the complexes has been studied. Hirshfeld surface and fingerprint plots uncover the central role of different intermolecular interactions and their comparative dimension to build up the crystal architectures of the molybdenum complexes. Supportive DFT calculations regarding non-covalent interactions and molecular orbitals have been carried out to support the experimental data. Apart from these interesting structural features, the effects of binding of the ligand H₂L and complexes 1–4 with bovine serum albumin (BSA) have been explored using absorption and steady-state fluorescence titration measurements. Molecular docking studies have also been carried out to further deepen the understanding of interaction patterns and binding modes of the ligand and binuclear dioxomolybdenum(VI) complexes with BSA.