Trigonal Mn3 and Co3 Clusters Supported by Weak-Field Ligands: A Structural, Spectroscopic, Magnetic, and Computational Investigation into the Correlation of Molecular and Electronic Structure

Transamination of divalent transition metal starting materials (M2(N­(SiMe3)2)4, M = Mn, Co) with hexadentate ligand platforms RLH6 (RLH6 = MeC­(CH2NPh-o-NR)3 where R = H, Ph, Mes (Mes = Mesityl)) or H,CyLH6 = 1,3,5-C6H9(NHPh-o-NH2)3 with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type (RL)­Mn3(py)3 and (RL)­Co3(PMe2R′)3 (R′ = Me, Ph). While the sterically less encumbered ligand varieties, HL or PhL, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand (MesL) engenders local tetrahedral coordination. Both the neutral Mn3 and Co3 clusters feature S = 1/2 ground states, as determined by direct current (dc) magnetometry, 1H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn3 clusters, the long internuclear Mn–Mn separations suggest minimal direct metal–metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = −16.8 to −42 cm–1. Conversely, the short Co–Co interatomic distances suggest a significant degree of direct metal–metal orbital overlap, akin to the related Fe3 clusters. With the Co3 series, the S = 1/2 ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co3 clusters affords diamagnetic cationic clusters of the type [(RL)­Co3(PMe2R)3]+. Density functional theory (DFT) calculations on the neutral (S = 1/2) and cationic (S = 0) Co3 clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.