Chiral Palladium(II)−Bis(trichlorosilyl) Complexes. Synthesis, Structure, and Combined QM/MM Computational Studies

The PdCl2 complexes 1−3 (1, dichloro{1-{(R)-1-[(S)-2-(diphenylphosphino-κP)ferrocenyl]ethyl}-3,5-dimethyl-1H-pyrazole-κN}palladium; 2, dichloro{1-{(R)-1-[(S)-2-(diphenylphosphino-κP)ferrocenyl]ethyl}-3-phenyl-5-methyl-1H-pyrazole-κN}palladium; 3, dichloro{1-{(R)-1-[(S)-2-(diphenylphosphino-κP)ferrocenyl]ethyl}-3-(2,4,6-trimethylphenyl)-1H-pyrazole-κN}palladium), previously used as catalyst precursors in the asymmetric hydrosilylation of
olefins, were found to react with an excess of HSiCl3 in CH2Cl2 to afford the corresponding
bis(trichlorosilyl) derivatives 4−6, respectively. Two of the new complexes (5 and 6) were
characterized by X-ray crystallography and revealed unusual structural properties. A severe
deviation from the ideal square-planar geometry was observed, as well as extremely elongated
Pd−P bonds (Pd−P = 2.4559(16) Å for 5 and 2.504(2) Å for 6). The nature of the uncommon
structural features observed in the bis(trichlorosilyl) derivatives has been explored with
density functional theory calculations and the combined quantum mechanics/molecular
mechanics (QM/MM) method. Using the combined QM/MM approach a systematic series of
model systems have been constructed in which the steric and electronic influences of the
substituent groups were selectively removed or altered. The technique has allowed the exact
nature of the geometric distortions to be pinpointed.