Stereoselectivity in Reactions of Metal Complexes. Part XIV.. 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. Stereoselectivity, defined as S = Kmeso/2Krac' 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°. 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°. 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.