Transamination of divalent transition metal starting materials (M 2(N(SiMe 3) 2) 4, M = Mn, Co) with hexadentate ligand platforms RLH 6 ( RLH 6 = MeC(CHNPh-o- NR) 3 where R = H, Ph, Mes (Mes = Mesityl)) or H,CyLH 6 = 1,3,5-C 6H 9(NHPh-o-NH) 3 with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type (RL)Mn 3(py) 3 and ( RL)Co 3(PMe 2R′) 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 Mn 3 and Co 3 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 Mn 3 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 Fe 3 clusters. With the Co 3 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 Co 3 clusters affords diamagnetic cationic clusters of the type [( RL)Co 3(PMe 2R) 3] +. Density functional theory (DFT) calculations on the neutral (S = 1/2) and cationic (S = 0) Co 3 clusters reveal that oxidation occurs at an orbital with contributions from both the Co 3 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry