STEREOSELECTIVITY IN REACTIONS OF METAL-COMPLEXES .14. USE OF CIRCULAR-DICHROISM TO DETERMINE THE STEREOSELECTIVE FORMATION OF [M(HIS)2] AND [M(PHET-SAL)2] (M = NI2+, CU2+, HIS = HISTIDINE, PHET-SAL = N-(1-PHENYLETHYL)SALICYLALDIMINE)

The stereoselective formation of the 1:2 complexes [M(his)2] and [M(PhEt-sal)2] (M = Ni2+, Cu2+, PhEt-sal = N-(1-phenylethyl)salicylaldimine) has been determined by circular dichroism (CD) measurements. Stereo-selectivity, defined as S = K(meso)/2K(rac), has been found to be 2.48 for [Ni(his)2], corresponding to a 21% excess of the mixed species relative to the statistical amount. This value is temperature-independent between 15 and 35-degrees. Whereas the absence of stereoselectivity in the formation of [Cu(prol)2] is confirmed, weak stereoselectivity is observed for [Cu(his)2] (2% excess of the mixed species). The CD intensity of the latter complex strongly depends on temperature and decreases by 12%, when the temperature is increased from 15 to 35-degrees. Small but significant stereoselectivity is found for the formation of the Schiff-base complexes [Ni(PhEt-sal)2] and Cu(PhEt-sal)2] in acetone with 1.0% and 2.4% excess, respectively, of the mixed species over the statistical value.