Reductive Disproportionation of CO₂ with Bulky Divalent Samarium Complexes

The base-free divalent samarium complex Cp^tt₂Sm (<b>1</b>; Cp^tt = 1,3-(^<i>t</i>Bu)₂(C₅H₃)) has been synthesized in diethyl ether by salt metathesis of SmI₂. Crystals of <b>1</b> suitable for X-ray study have been obtained by sublimation at 116 °C under reduced pressure. The dissolution of <b>1</b> in thf and pyridine solution leads to the solvent adducts Cp^tt₂Sm­(thf)₂ (<b>3</b>) and Cp^tt₂Sm­(py) (<b>4</b>), respectively, while drying <b>3</b> under reduced pressure yields Cp^ttSm­(thf) (<b>5</b>). The reaction of CO₂ with the base-free divalent samarium complexes Cp^tt₂Sm (<b>1</b>) and Cp^ttt₂Sm (<b>2</b>; Cp^ttt =1,2,4-(^<i>t</i>Bu)₃(C₅H₂)) leads to the clean formation of bridged carbonate samarium dimers [Cp^ttt₂Sm]₂(μ-CO₃) (<b>7</b>) and [Cp^tt₂Sm]₂(μ-CO₃) (<b>8</b>). This is indicative of the reductive disproportionation of CO₂ in both cases with release of CO. This contrasts with the formation of the oxalate-bridged samarium dimer reported from the reaction of CO₂ with the Cp*₂Sm­(thf)₂ complex. Otherwise, the reaction with CO does not proceed with the bulky complexes, while traces of O₂ have led to the formation of the original bridged peroxo samarium dimer [Cp^ttt₂Sm]₂(μ-O₂) (<b>6</b>). 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 <i>predict</i> a priori in this chemistry