Synthesis, Electrochemical, and Computational Studies of Organocerium(III) Complexes with Ce-Aryl Sigma Bonds

A set of organolanthanide(III) complexes featuring Ce(III)-C_aryl σ-bonds have been synthesized and characterized. The syntheses follow salt metathesis reactions of the salts: ortho-lithiated oxazoline (Li-ortho-oxa) (L1) and Li₂-biphenyl (L2) with the corresponding metallocene precursors: 1-Ln ([(C₅Me₅)₂LnKCl₂(THF)]_n) Ln = La and Ce, 2-La, ([(C₅Me₄SiMe₃)₂LaKCl₂(THF)]_n) and 2-Ce ([((C₅Me₄SiMe₃)₂CeKCl₂)(Et₂O)_0.5]_n). Electrochemical studies were performed for the organolanthanide(III) complexes. Reversible Ce(III)/Ce(IV) redox couples were observed for (C₅Me₅)₂Ce(κ²-ortho-oxa) (3-Ce) and [Li(DME)₃][(C₅Me₅)₂Ce(biphen)] (5-Ce) complexes. However, complex (C₅Me₄(SiMe₃))₂Ce(κ²-ortho-oxa) (4-Ce) showed an irreversible Ce(III/IV) oxidation wave. A DFT-computed redox potential for 3-Ce showed good agreement with experiment. For 5-Ce, calculations indicated that the redox potential may be influenced strongly by speciation changes associated with the Li(DME)₃⁺ cation becoming outer-sphere. Bonding analysis suggests strong bond polarization in the title complexes (3-Ce and 5-Ce) with weights of 7-8% from Ce in the Ce−C_aryl donation-bond orbitals. Attempts to isolate Ce(IV) species from these complexes resulted into the decomposition of the products into unidentified Ce(III) species, indicating that the bulky and electron-donating cyclopentadienide derivatives, ortho-oxazolide ligand, and/or the biphenylide ligand do not effectively stabilize the organocerium(IV) complexes against decomposition under the conditions employed.