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

摘要

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 co-ligands 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 dc magnetometry, ¹H NMR spectroscopy, and low-temperature 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]⁺. DFT calculations on the neutral (S = ½) and cationic (S = 0) Co₃ clusters reveal that oxidation occurs at an oribital 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.