Homoleptic Permethylpentalene Complexes: “Double Metallocenes” of the First-Row Transition Metals

The synthesis of the bimetallic permethylpentalene complexes Pn*2M2 (M = V, Cr, Mn, Co, Ni; Pn* = C8Me6) has been accomplished, and all of the complexes have been structurally characterized in the solid state by single-crystal X-ray diffraction. Pn*2V2 (1) and Pn*2Mn2 (3) show very short intermetallic distances that are consistent with metal−metal bonding, while the cobalt centers in Pn*2Co2 (4) exhibit differential bonding to each side of the Pn* ligand that is consistent with an η5:η3 formulation. The Pn* ligands in Pn*2Ni2 (5) are best described as η3:η3-bonded to the metal centers. 1H NMR studies indicate that all of the Pn*2M2 species exhibit D2h molecular symmetry in the solution phase; the temperature variation of the chemical shifts for the resonances of Pn*2Cr2 (2) indicates that the molecule has an S = 0 ground state and a thermally populated S = 1 excited state and can be successfully modeled using a Boltzmann distribution (ΔH° = 14.9 kJ mol−1 and ΔS° = 26.5 J K−1 mol−1). The solid-state molar magnetic susceptibility of 3 obeys the Curie−Weiss law with μeff = 2.78μB and θ = −1.0 K; the complex is best described as having an S = 1 electronic ground state over the temperature range 4−300 K. Paradoxically, attempts to isolate the “double ferrocene” equivalent, Pn*2Fe2, led only to the isolation of the permethylpentalene dimer Pn*2 (6). Solution electrochemical studies were performed on all of the organometallic compounds; 2−5 exhibit multiple quasi-reversible redox processes. Density functional theory calculations were performed on this series of complexes in order to rationalize the observed structural and spectroscopic data and provide estimates of the M−M bond orders.