Interconversion between Zwitterionic and Cationic Rhodium(I) Complexes of Demonstrated Value as Catalysts in Hydroformylation, Silylformylation, and Hydrogenation Reactions. Dynamic31P{1H} NMR Studies of (η6-PhBPh3)-Rh+(DPPB) and [Rh(DPPB)2]+BPh4-in Solution

Treatment of an orange solution of [Rh(COD)(DPPB)] + BF 4 - ( 2 ) in MeOH with 2 equiv of NaBPh 4 at room temperature (RT) afforded an orange precipitate, [Rh(COD)(DPPB)] + BPh 4 - ( 3 ), in 94% yield. Reaction of the cationic rhodium complex 3 with H 2 under ambient conditions in CH 2 Cl 2 for 1 h gave the zwitterionic complex (η 6 -PhBPh 3 ) - Rh + (DPPB) ( 4 ) in quantitative yield. Although 3 is stable in the solid state, it has the propensity in solution to convert to the zwitterionic complexes (η 6 -PhBPh 3 ) - Rh + (COD) ( 1 ) and (η 6 -PhBPh 3 ) - Rh + (DPPB) ( 4 ) along with a small amount of [Rh x (DPPB) 2 x ] x + [BPh 4 - ] x . Addition of 1, 2, and 4 equiv of DPPB to the CD 2 Cl 2 solution of (η 6 -PhBPh 3 ) - Rh + (NBD) ( 6 ) under N 2 resulted in the formation of [Rh(NBD)(DPPB)] + BPh 4 - ( 7 ) and [Rh(DPPB) 2 ] + BPh 4 - ( 5 ) in ratios of 90/10, 57/43, and 0/100, respectively, while addition of 2 equiv of DPPB to the CD 2 Cl 2 solution of 6, under an atmosphere of H 2 at RT, gave 4 and 5 in an ratio of 35/65. “Slowed” η 6 -PhBPh 3 - rotation about the (η 6 -PhBPh 3 ) - −Rh bond axis in (η 6 -PhBPh 3 ) - Rh + (DPPB) ( 4 ) was established by a variable-temperature 31 P{ 1 H} NMR study. Variable-temperature 31 P{ 1 H} NMR spectra of [Rh(DPPB) 2 ] + BPh 4 - ( 5 ) along with the low-temperature 31 P{ 1 H} COSY and EXSY NMR spectra demonstrated the presence of an equilibrium between [Rh(DPPB) 2 ] + ( 5 α) and [Rh(DPPB)(μ-DPPB)] 2 2 + ( 5 β).