Transcyclometalation : A Novel Route to (Chiral) Bis-Ortho-Chelated Bisphosphinoaryl Ruthenium(II) Complexes

Abstract

The RuII complexes [RuCl(R-PCP)(PPh3)] (R-PCP = [C6H3{CH(Me)PPh2}2-2,6]- (7a), [C6H3{CH(Et)PPh2}2-2,6]- (7b), or [C6H2{CH2PPh2}2-2,6-SiMe3-4]- (9)) were synthesized using distinct synthetic routes. One of these routes consists of the reaction of the parent arene compound R-PCHP (i.e., C6H4{CH(Me)PPh2}2-1,3 (6a), C6H4{CH(Et)PPh2}2-1,3 (6b), or C6H3(CH2PPh2)2-2,6-SiMe3-4 (8)) with [RuCl2(PPh3)3] in 1,2-dichloroethane. Following this procedure, complexes 7a and 7b were obtained. However, desilylation of 8 was observed with concomitant formation of complex [RuCl(C6H3{CH2PPh2}2-2,6)(PPh3)] (4). A second synthetic approach has been developed and applied in the high-yield synthesis of complexes 7a, 7b, and 9. This method, a transcyclometalation (TCM) reaction, involves the interconversion of one cyclometalated ligand metal complex, M(C,E), into another complex M(C,E') with the concomitant consumption and formation of the corresponding arenes HC,E' and HC,E, respectively. Using this method, previously observed problems such as nontolerance to the presence of a SiMe3 group, contamination of the final product by uncoordinated PPh3, or the formation of paramagnetic impurities were overcome. Investigations in solution of the formation of these square-pyramidal Ru complexes using two-dimensional NMR techniques (1H-1H COSY and 31P-1H COSY) have shown that both synthetic routes were stereoselective, yielding only three of the four expected stereoisomers of complexes 7a and 7b.