Much attention has been paid to the oxidation of water to dioxygen by homogeneous catalysts. Of particular interest are di-and tetranuclear transition metal complexes, since extended X-ray absorption fine structure studies have indicated that the O 2 -evolving center (OEC) in photosystem II is composed of a tetranuclear Mn cluster with dimeric di-moxo Mn units. [1] A variety of di-and tetranuclear metal complexes have been prepared as structural models of the OEC, and four-electron oxidation of water has been attained with only a few dinuclear metal (Mn, Ru) complexes. [2±4] The key intermediates for O 2 evolution in the OEC is thought to be high-valent metal ± oxo complexes derived from aqua signals range from 0.1 ± 0.3 and increase as the temperature lowers. Finally, an empirical model based on PM3 conformational analysis permits a tentative assignment of configurations at the carbon atom a to the metal, relative ligand on the basis of both anisotropic effects and free energy criteria. Structural studies aimed at finding definitive proof for the assignment of the absolute configuration as well as studies of other organometallic species are underway.
Experimental Section
Organozinc bromides were obtained by reaction of active zinc with the corresponding bromides, previously distilled over P 2 O 5 , in dry THF at RT in an Ar atmosphere. [21] . (R)-1 and (S)-1 were purified by precipitation with ZnCl 2 (THF), extraction (0.2 m aqueous NaCN in CHCl 3 ), and vacuum distillation. (RS)-1 ([a] 25 D 0.0) was prepared by mixing equal amounts of the former reagents. The concentration in all the NMR experiments was 0.5 m in THF/C 6 D 6 (1/1), except for those performed at 150 8C in which it was 1.5 m in THF/[D 8 ]THF (4/1). TMS was added as an internal standard.