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.