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.