Reactions of the carboxylate A-frame complexes [Rh(μ-O 2CR)(CO) 2(μ-dppm) 2]ClO 4 with small molecules. Heterolytic activation of dihydrogen by the carbonyl adducts [Rh 2(μ-CO)(μ-O 2CR)(CO) 2(μ-dppm) 2]ClO 4 (R = CH 2Cl, CHCl 2, CCl 3, CF 3)

Abstract

The reactivity of the carboxylate A-frame complexes [Rh(μ-O 2CR)(CO) 2(μ-dppm) 2]ClO 4 (R = H, CH 3, CH 2Cl, CHCl 2, CCl 3, CF 3) towards CO, H 2/CO mixtures and halogens has been investigated. Unlike the acetate and formate complexes, whose reactions with CO have been reported elsewhere, all four halocarboxylates reacted with CO to give labile, isolable adducts, [Rh 2(μ-CO)(μ-O 2CR)(CO) 2(μ-dppm) 2]ClO 4 (R = CH 2Cl, CHCl 2, CCl 3, CF 3) at room temperature and pressure. In a most unusual reaction, the same halocarboxylate A-frame complexes promote the heterolytic activation of dihydrogen when reacted with an H 2/CO gas mixture. The carbonyl adducts have been shown to be intermediates in the formation of the product [Rh 2(μ-H)(μ-CO)(CO) 2(μ-dppm) 2]ClO 4. A rationale is proposed for this reaction in which only one metal centre is directly involved in the activation of H 2. The carboxylate A-frame complexes with R = H, CH 3 and CF 3 react with halogens to give the 1,2-transannular oxidative addition products [Rh 2(μ-O 2CR)X 2(CO) 2(μ-dppm) 2]ClO 4 (X = Cl, Br, I). With R = H, CH 3 these are the sole products but with R = CF 3, several unidentified minor products, apparently containing Rh(III) centres, were also formed.