Effects of Formal Metal Oxidation State on the Preferred Structure Types in Binuclear Actinide Carbonyl Derivatives: Predicted Tetramerization of Carbon Monoxide to a Bridging Squarate Group in Uranium Chemistry

The structures and energetics of the binuclear cyclooctatetraene uranium carbonyls (C₈H₈)₂U₂(CO)n (n = 2, 3, 4, 5) have been studied by density functional theory. The most interesting observation from this work is the prediction of low-energy structures in the tetracarbonyl system of the type (C₈H₈)₂U₂(η⁴-μ-C₄O₄), in which the four CO groups couple to form a bridging C₄O₄ squarate unit. Such a tetramerization of carbon monoxide to give a squarate unit by organouranium compounds has been observed experimentally by Cloke and co-workers in sandwich compounds of the type (η⁵-Me₅C₅)U(η⁸-C₈H₆{SiR₃}₂) containing both five-membered and eight-membered rings. However, tetramerizations of CO groups to squarate were not predicted in theoretical studies of related (C₈H₈)₂Th₂(CO)₄ or (C₅H₅)₂M₂(CO)₄ systems (M = Th, U). These bridging squarate (C₈H₈)₂U₂(η⁴-μ-C₄O₄) structures found in this work are thermochemically favored to the extent that the lowest energy structure of the tricarbonyl (C₈H₈)₂U₂(CO)₃ is disfavored relative to disproportionation into such a bridging squarate tetracarbonyl structure and the lowest energy structure of the dicarbonyl (C₈H₈)₂U₂(CO)₂. In the remaining low-energy (C₈H₈)₂U₂(CO)n (n = 2, 3, 4, 5) structures, the carbonyl groups are all isolated, either as terminal CO groups similar to those bonding to d-block metals or as bridging η²-μ-CO groups bonded to uranium through both their carbon and oxygen atoms. The viability of formal uranium oxidation states from +3 to +6, as found experimentally in diverse stable molecules, leads to a variety of spin states and uranium-uranium bonding modes in the low-energy (C₈H₈)₂U₂(CO)n (n = 2, 3, 4, 5) structures. This contrasts with the previously studied thorium systems (C₈H₈)₂Th₂(CO)n (n = 2, 3, 4, 5)⁶, where the maximum viable formal thorium oxidation state of +4 limits the range of accessible structure types, metal-metal bonding modes, and spin states.