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.