Applications of Cd N.M.R to Polycadmium Complexes. III. Effects of Zn/Cd and of Se/S Substitution in the Tetra-Adamantanoid Cage [S₄Cd₁₀(Sph)₁₆]^4- in Relation to Heterometal Metallothioneins

Abstract

Cadmium-113 n.m.r . spectra at 66.6 MHz of various mixtures of the six cage compounds [X₄M₁₀( SPh )₁₆]^4- (1; X = S, Se; M = Zn, Cd ) and [M₄( SPh )₁₀]^2- (M = Zn, Cd ), have revealed the exchange reactions of M and of X which occur within the different structural elements of (1), namely the triply bridging chalcogenide ligands X, the inner metal atoms Mⁱ, and the outer metal atoms M^o. At laboratory temperature the exchange time scales range from days (X) through hours (Mⁱ) to seconds (M^o and SPh ), according to the extent of atomic connection to each of these structural components of the aggregate. Metal exchange does not always follow random statistics, with biases towards equilibrium association of the same metal. The M- and X-substituted products, with lower symmetry than the high-symmetry precursors, reveal the mutual influences of the various structural elements on Cd chemical shift: S → Se substitution causes changes ( ΔδCd ) of -37 ppm at one bond, -3 ppm and -1.5 ppm at three bonds, and + 1 ppm five bonds distant; Cd → Zn substitution causes δCd to change by + 13 ppm and + 5 ppm at a connection of two bonds and by -3 ppm over four bonds. These effects of adjacent Cd → Zn substitution on δCd provide well defined models in support of the hypothesis of similar effects in the Cd n.m.r . spectra of mixed Zn, Cd metallothionein proteins.