Enhanced Hydrogenation Activity and Recycling of Cationic Rhodium Diphosphine Complexes through the Use of Highly Fluorous and Weakly-Coordinating Tetraphenylborate Anions

Abstract

The effect of the nature of the anion on the performance of ionic rhodium catalysts has received little attention. Herein it is shown that the use of highly fluorous tetraphenylborate anions can enhance catalyst activity in both conventional and fluorous media. For hydrogenation catalysts of the type [Rh(COD)(dppb)][X] {COD=1,5-cis,cis-cyclooctadiene; dppb=1,4-bis(diphenylphosphino)butane; X=BF4- (1a), [BPh4]- (1b), [B{C6H4(SiMe3)-4}4]- (1c), [B{C6H3(CF3)2-3,5}4]- (1d), [B{C6H4(SiMe2CH2CH2C6F13)-4}4]- (1e), [B{C6H4(C6F13)-4}4]- (1f) and [B{C6H3(C6F13)2-3,5}4]- (1 g)} the activity towards the hydrogenation of 1-octene in acetone increased in the order 1c <1b <1e <1a <1d ~ 1f <1g with 1g being twice as active as the commonly applied 1a. Despite the fluorophilic character introduced by the substituted tetraarylborate anions, the presence of some perfluoroalkyl-substituents in the cation was still required for achieving high partition coefficients. Therefore, [Rh(COD)(Ar2PCH2CH2PAr2)][X] {Ar=C6H4(SiMe2CH2CH2C6F13)-4, X=[B{C6H3(C6F13)2-3,5}4]- (3f); Ar=C6H4(SiMe(CH2CH2C6F13)2)-4 and X=[B{C6H4(C6F13)-4}4]- (2g)} were prepared, which were active in the hydrogenation of 1-octene, 2g even more so than 3f. Both these highly fluorous catalysts could be recycled with 99% efficiency through fluorous biphasic separation, whereas the corresponding BF4- complex of 2g (2a) did not show any affinity for the fluorous phase.