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 ?⁵:?³ formulation. The Pn* ligands in Pn*₂Ni₂ (5) are best described as ?³:?³-bonded to the metal centers. ¹H NMR studies indicate that all of the Pn*₂M₂ species exhibit D_2h 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 (?H° = 14.9 kJ mol^-1 and ?S° = 26.5 J K^-1 mol^-1). The solid-state molar magnetic susceptibility of 3 obeys the Curie-Weiss law with µ_eff = 2.78µ_B and ? = -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.