Stereodynamical interconversion of bis(N-aryl-α-isopropyl-β-aminovinylthionato)zinc(II) and -cadmium(II)

Abstract

Bis N-aryl(α-isopropyl-β-aminovinylthionato)zinc(II) and -cadmium(II) complexes [ I( a,b)] have been prepared in order to investigate their stereochemical non-rigidity and conformational interconversion. For the zinc(II) complexes [ I( a,b)], two types of energy barrier were observed by the use of dynamic 1H and 13C NMR spectroscopy. The lower barrier (Δ G‡ 298∼60 kJ mol −1) measured by averaging anisochronous o,o′-carbons (C-aryl ring A) is assigned to the hindered rotation of the β-phenyl ring. The higher energy barrier (Δ G‡ 298 = 77–78 kJ mol −1) was calculated for the interchange of the diastereotopic methyls of the α-isopropyl group for complexes I(a,b) and the anisochronous methyls of the N-aryl ring B in complexes I(a) during the ( R) ⇋ ( S) inversion. Whereas, the tetrahedral ( R) ⇋ ( S) inversion in the cadmium(II) complexes, I( a,b), is faster (Δ G‡ 298) = 51–53 kJ mol −1 compared with the zinc(II) complexes. In this case, the o,o′-carbons in the C-aryl ring manifest the ( R) ⇋ ( S) inversion process (Δ G‡ 298 = 47.5–49 kJ mol −1). The anisochronous o,o′-methyls of the N-aryl ring B may thus be used for studying the tetrahedral inversion at the metal centre along with the diastereotopic methyl groups in the type I complexes. The energy barrier for the intermolecular ligand degenerate exchange reaction (Δ G‡ 298 = 64.4 kJ mol −1) is higher than for the intramolecular ( R) ⇋ ( S) inversion at the metal centre.