Leveraging Chelating Amido Ligands to Support Metal–Metal Bonding in Dinuclear Cr(II) Complexes

Herein, we report a series of Cr(II) complexes containing triaryl, tetradentate ligands derived from o-phenylenediamide that illuminate how rigid, chelating amido ligands facilitate Cr–Cr bonding. The aminolysis reaction of Cr[N(SiMe3)2]2(thf)2 with H2(L1)a protonated N4 proligand containing flanking NMe2 groupsyielded square planar and mononuclear Cr(L1) (1), whereas the same reaction with N2S2 and N2O2 proligands with flanking SMe and OMe groups yielded dinuclear [Cr(L2)]2 (2) and [Cr(L3)]2 (3). The structures of 2 and 3 revealed Cr–Cr distances of 2.3356(6) and 2.3481(5) Å, consistent with metal–metal bonding, which was confirmed by the complete active space methods. The theoretical results suggest that Cr–Cr bonding is assisted by the chelating nature of the bridging amido ligands, which fold the dinuclear structure and orient the metals so that side-on overlap of Cr 3d orbitals can occur. Variable-temperature SQUID magnetometry and spectroscopic data (e.g., UV–vis-NIR, Raman, and IR) reported for 1–3 show differences indicative of the change in nuclearity and electronic structure. Collectively, these results reveal bridging amido ligand characteristics that support metal–metal bonding with Cr(II), and they help account for the wide range of metal–metal distances observed in dinuclear (or binuclear) Cr(II) complexes containing Cr2N2 cores.