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

The synthesis of the bimetallic permethylpentalene complexes Pn*M-2(2) (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*V-2(2) (1) and Pn*Mn-2(2) (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 175:17 3 formulation. The Pn* ligands in Pn*Ni-2(2) (5) are best described as eta(3):eta(3)-bonded to the metal centers. H-1 NMR studies indicate that all of the Pn*M-2(2) species exhibit D-2h molecular symmetry in the solution phase; the temperature variation of the chemical shifts for the resonances of Pn*Cr-2(2) (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 (Delta H degrees = 14.9 kJ mol(-1) and Delta S degrees = 26.5 J K-1 mol(-1)). The solid-state molar magnetic susceptibility of 3 obeys the Curie-Weiss law with mu(eff) = 2.78 mu(B) and theta = -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*Fe-2(2), 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.