Homoleptic permethylpentalene complexes: “Double metallocenes” of the first-row transition metals

Andrew E. Ashley, Robert T. Cooper, Gregory Wildgoose, Jennifer C. Green, Dermot O’Hare

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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 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 (?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 µeff = 2.78µB 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.
Original languageEnglish
Pages (from-to)15662-15677
Number of pages16
JournalJournal of the American Chemical Society
Issue number46
Publication statusPublished - 21 Oct 2008

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