Reductive Disproportionation of CO₂ with Bulky Divalent Samarium Complexes

Abstract

The base-free divalent samarium complex Cp tt 2 Sm ( 1; Cp tt = 1,3-( t Bu) 2 (C 5 H 3 )) has been synthesized in diethyl ether by salt metathesis of SmI 2 . Crystals of 1 suitable for X-ray study have been obtained by sublimation at 116 °C under reduced pressure. The dissolution of 1 in thf and pyridine solution leads to the solvent adducts Cp tt 2 Sm(thf) 2 ( 3 ) and Cp tt 2 Sm(py) ( 4 ), respectively, while drying 3 under reduced pressure yields Cp tt Sm(thf) ( 5 ). The reaction of CO 2 with the base-free divalent samarium complexes Cp tt 2 Sm ( 1 ) and Cp ttt 2 Sm ( 2; Cp ttt =1,2,4-( t Bu) 3 (C 5 H 2 )) leads to the clean formation of bridged carbonate samarium dimers [Cp ttt 2 Sm] 2 (μ-CO 3 ) ( 7 ) and [Cp tt 2 Sm] 2 (μ-CO 3 ) ( 8 ). This is indicative of the reductive disproportionation of CO 2 in both cases with release of CO. This contrasts with the formation of the oxalate-bridged samarium dimer reported from the reaction of CO 2 with the Cp* 2 Sm(thf) 2 complex. Otherwise, the reaction with CO does not proceed with the bulky complexes, while traces of O 2 have led to the formation of the original bridged peroxo samarium dimer [Cp ttt 2 Sm] 2 (μ-O 2 ) ( 6 ). The mechanism for these reactions is studied herein by experiments and also by theoretical computations. The key result is that the different pathways are rather close in energy, which also explains why the nature of the final product, if only one is present, is difficult to predict a priori in this chemistry.