Synthesis, bonding properties and ether activation reactivity of cyclobutadienyl-ligated hybrid uranocenes

A series of hybrid uranocenes consisting of uranium(iv) sandwiched between cyclobutadienyl (Cb) and cyclo-octatetraenyl (COT) ligands has been synthesized, structurally characterized and studied computationally. The dimetallic species [(η(4)-Cb′′′′)(η(8)-COT)U(μ:η(2):η(8)-COT)U(THF)(η(4)-Cb′′′′)] (1) forms concomitantly with, and can be separated from, monometallic [(η(4)-Cb′′′′)U(THF)(η(8)-COT)] (2) (Cb′′′′ = 1,2,3,4-tetrakis(trimethylsilyl)cyclobutadienyl, COT = cyclo-octatetraenyl). In toluene solution at room temperature, 1 dissociates into 2 and the unsolvated uranocene [(η(4)-Cb′′′′)U(η(8)-COT)] (3). By applying a high vacuum, both 1 and 2 can be converted directly into 3. Using bulky silyl substituents on the COT ligand allowed isolation of base-free [(η(4)-Cb′′′′)U{η(8)-1,4-((i)Pr(3)Si)(2)C(8)H(6)}] (4), with compounds 3 and 4 being new members of the bis(annulene) family of actinocenes and the first to contain a cyclobutadienyl ligand. Computational studies show that the bonding in the hybrid uranocenes 3 and 4 has non-negligible covalency. New insight into actinocene bonding is provided by the complementary interactions of the different ligands with uranium, whereby the 6d orbitals interact most strongly with the cyclobutadienyl ligand and the 5f orbitals do so with the COT ligands. The redox-neutral activation of diethyl ether by [(η(4)-Cb′′′′)U(η(8)-C(8)H(8))] is also described and represents a uranium-cyclobutadienyl cooperative process, potentially forming the basis of further small-molecule activation chemistry.