Sulfur bridging interactions of cis-planar Ni^II-S₂N₂ coordination units with Nickel(II), Copper(I,II), Zinc(II), and Mercury(II): a library of bridging modes, including Ni^II(μ₂-SR)₂M^I,II rhombs

Abstract

Sulfur bridging interactions between three cis-planar Ni^II-S₂N₂ complexes and Ni^II, Cu^I,II, Zn^II, and Hg^II reactants were investigated by synthesis and X-ray crystal structures of some 24 complexes. This work was stimulated by recent crystallographic structures of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme A synthase. This bridged biological assembly has the minimal formulation [Fe₄S₄]-(μ₂-S_Cys)-[M((μ₂-S_Cys)₂Gly)Ni] with M = Ni^II, Cu^I, and Zn^II at sites distal and proximal, respectively, to the iron-sulfur cluster. Bridges supported by representations of the distal nickel site were sought by reactions of the complexes [Ni^II(LH-S₂N₂)]^2- and [Ni^II(LR-S₂N₂)], with 5-5-5 chelate ring patterns. Reaction products implicate the bridges Ni-(μ₂-S)_1,2-M in a variety of molecular structures, some with previously unknown connectivities of bridge atoms. The most frequently encountered bridge units are the nonplanar rhombs Ni(Mu₂-S)₂M involving both sulfur atoms of a given complex. Those with M = Ni^II are biologically relevant inasmuch as the catalytic metal at the proximal site is nickel. The complex [Ni(L-655)]^2-, containing the 6-5-5 ring pattern and coordination sphere of the distal nickel site, was prepared and structurally characterized. It was shown to sustain Ni₂(μ₂-S)₂ rhombic interactions in the form of trinuclear [{Ni(L-655)}₂Ni]^2- and [{Ni(L-655)}Ni(R₂PCH₂CH₂PR₂)] (R = Et, Ph) in which the second NiII simulates the proximal site. Bridging interactions of Ni^II-S₂N₂ complexes are summarized, and geometrical features of Ni₂(μ₂-S)₂ rhombs in these complexes, as dependent on ring patterns, are considered (L_H-S₂N₂ = N,N'-ethylenebis(2-mercaptoisobutyramide)(4-); L_R-S₂N₂ = trans-rac-N,N'-bis(2-mercapto-2-methylprop-1-yl)-1,2-cyclohexanediamine(2-); L-655 = N-(2-mercaptopropyl)-N'-(2'-mercaptoethyl)glycinamide(4-)).