Theoretical Cluster Model of a Zn-C Reformatsky Intermediate

Abstract

A large body of experimental data exists for the reactions of BrZnCH₂COOC(CH₃)₃ (I) and related organo-zinc compounds. Orsini et al. isolated a crystalline form of (I) from tetrahydrofuran; the X-ray structure determined by Boersma et al. showed a cyclic dimer of 68 atoms. In the present work, relativistic extended Huckel (REX) calculations using the method of Pyykko and Lohr have been carried out for the approach of CH₃ I and CH₃Br to the Zn-C bond of (I). Several mechanisms have been modeled, showing that the Br or I of an alkyl halide approaching Zn(II) plays a very important role in the cleavage of the Zn-C bond. The results of model cluster calculations on Zn(CH₃)₂, ZnCl₂, ZnH₂, and CH₃Cl using HONDO/5 and STO 6/4G basis have been compared to those obtained with REX and with the ab initio pseudopotential results of Ratner et al. with respect to Mulliken atomic charges and optimized geometries. The REX model agrees qualitatively with the experimental data and with the atomic charges of pseudopotential calculations, but the nonrelativistic ab initio HONDO/5 atomic charges in a minimum basis give a less clear description of the Zn-C bond when used with REX screening values. The REX model appears to be useful for clusters up to 100 atoms and 400 spin-orbitals and for large zinc-containing systems of biological interest.