Electronic Structure of Ru2(II,II) Oxypyridinates: Synthetic, Structural, and Theoretical Insights into Axial Ligand Binding

Reduction of (4,0)-Ru2(chp)4Cl (1) (chp = 6-chloro-2-oxypyridinate) with Zn or FeCl2 yields a series of axial ligand adducts of the Ru2(II,II) species Ru2(chp)4(L), with L = tetrahydrofuran (2), dimethyl sulfoxide (DMSO; 3), PPh3 (4), pyridine (5), or MeCN (6). Zn reduction in noncoordinating solvents such as toluene or CH2Cl2 leads to the dimeric species [Ru2(chp)4]2 (7) or [Ru2(chp)4]2­(ZnCl2) (8), whereas addition of strongly σ-donating ligands such as CO causes cleavage of the Ru–Ru bond. Density functional theory (DFT) models of these complexes, the axially free species, and the axial adducts of several other potential ligands (H2O, NH3, CH2Cl2, S-bound DMSO, N2, and CO) indicate that these compounds can be divided into three distinct categories, based on their Ru–Ru bond length and electronic structure. Compounds 2, 3, 5, 6, 7, and 8, the hypothetical axially free species, and adducts of H2O and NH3 fit in Category 1 with a (δ*)2(π*)2 ground state, as indicated by their electronic spectra, magnetic properties, and Ru–Ru bond distances. However, compound 4 and the CH2Cl2 adduct (Category 2) show a pseudo-Jahn-Teller distortion and spectroscopic signs of δ*/π* orbital mixing suggestive of a new electronic ground state intermediate between the (δ*)2(π*)2 and (δ*)1(π*)3 configurations. Category 3 consists of the hypothetical adducts of N2, S-bound DMSO, and CO, all of which are predicted to have a (δ*)1(π*)3 configuration. Electronic spectra were recorded and assigned using time-dependent DFT, allowing assignment of a band in the 10 000–13 000 cm–1 range as the δ → π* transition. The axial ligand’s π-acid character heavily influences the δ*−π* gap, and thereby the ground-state electronic configuration, but not the axial ligand binding strength, which is dictated more by the σ-donor character of the ligands. Thus, this work greatly expands the number of axial ligand adducts known for Ru2(II,II) complexes supported by N,O-donor ligands and provides a predictive theoretical framework for their stability and electronic structures.