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-kap pa P)ferrocenyl]-ethyl}-3,5-dimethyl-1H-pyrazole-kappa N}palladium; 2, dich loro{1-{(R)-1-[(S)-2-(diphenylphosphino-kappa P)ferrocenyl]ethyl}-3-phenyl- 5-methyl-1H-pyrazole-kappa N}palladium; 3, dichloro{1-{(R)-1-[(S)-2-(diphen ylphosphino-kappa P)ferrocenyl]ethyl}-3-(2,4,6-trimethylphenyl)-1H-pyrazole -kappa N}palladium), previously used as catalyst precursors in the asymmetr ic 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-r ay crystallography and revealed unusual structural properties. A severe dev iation from the ideal square-planar geometry was observed, as well as extre mely elongated Pd-P bonds (Pd-P = 2.4559(16) Angstrom for 5 and 2.504(2) An gstrom for 6). The nature of the uncommon structural features observed in t he bis(trichlorosilyl) derivatives has been explored with density functiona l theory calculations and the combined quantum mechanics/molecular mechanic s (QM/MM) method. Using the combined QM/MM approach a systematic series of model systems have been constructed in which the steric and electronic infl uences of the substituent groups were selectively removed or altered. The t echnique has allowed the exact nature of the geometric distortions to be pi npointed.