A series of U(IV) mixed-sandwich alkyls of the form [U(COTTIPS2)Cp*R] (R = Me, CH2Ph, CH2TMS, CH(TMS)2; COTTIPS2 = C8H6(Si i Pr3-1,4)2; Cp* = C5Me5; TMS = SiMe3) have been synthesized and structurally characterized, and their reactivity toward H2 and CO2 has been investigated. The alkyls R = Me, CH2Ph, CH2TMS react at room temperature with a stoichiometric amount of CO2 to form κ2-carboxylate complexes. Reaction of all four alkyls with H2 yields a monomeric, terminal hydride complex, [U(COTTIPS2)Cp*H], which is unstable with respect to hydrogen loss and reacts with CO2 to give the κ2-formate complex [U(COTTIPS2)Cp*(κ2-O2CH)]. Additionally, a common decomposition product of the alkyls and hydride complexactivation of a Cp* methyl group to give a “tucked-in” alkylhas been isolated and structurally characterized and its insertion chemistry toward CO2 has been examined.

The Th(iv) mixed-sandwich halide complexes Th(COT(TIPS2))Cp*X (where COT(TIPS2) = 1,4-{Si(i)Pr(3)}(2)C(8)H(6), X = Cl, I) have been synthesised, and structurally characterised. When Th(COT(TIPS2))Cp*I is reduced in situ in the presence of CO(2), a mixture of dimeric carboxylate and oxalate complexes {Th(COT(TIPS2))Cp*}(2)(μ-κ(1):κ(2)-CO(3)) and {Th(COT(TIPS2))Cp*}(2)(μ-κ(2):κ(2)-C(2)O(4)) are formed, possibly via a transient Th(iii) species. Th(COT(TIPS2))Cp*Cl is readily alkylated to yield the benzyl complex Th(COT(TIPS2))Cp*CH(2)Ph, which reacts with CO(2) to form a carboxylate and with H(2) to form a hydride; the latter inserts CO(2), giving the bridging formate complex {Th(COT(TIPS2))Cp*(μ-κ(1):κ(1)-O(2)CH)}(2).

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