Synthesis, chemistry, and spectroscopy of some tris(pentahaptocyclopentadienyl)uranium(IV) alkyl and aryl compounds

The systematic high yield syntheses of a number of (h⁵-C₅H₅)₃UR compounds from (A5-C5H5)3UC1 and the corresponding RLi or RMgX reagents are reported. The new compounds have been characterized by chemical and magnetochemical means and by vibrational and nmr spectroscopy. The allyl compound has a ground state monohaptoallyl geometry and is fluxional. In the isopropyl compound restricted rotation is observed about the U-R bond, indicating considerable crowding of ligands around the central metal. Analysis of the isotropic nmr chemical shifts indicates the contribution of large contact shifts; the mechanism of spin distribution involves negative spin density at the α-carbon atom of the alkyl or aryl group and is qualitatively in accord with SCF INDO molecular orbital calculations. The U-C bond is susceptible to attack by protonic solvents and, hence, is somewhat ionic. Most of the new compounds exhibit surprisingly high thermal stability, the general order of stability determined by kinetic studies in toluene solution being primary > secondary > tertiary. In the thermolysis, β elimination of olefin does not occur to any major extent, but intramolecular abstraction of hydrogen from a cyclopentadienyl ring occurs yielding RH; this takes place with essentially complete retention of stereochemical configuration at the α carbon atom of R. A homolytic U-C bond scission producing a highly constrained caged radical pair, or a concerted elimination of RH, explains these observations.