The Synthesis and Characterization of Binuclear Copper(I) Complexes as Models for Protein Active Sites

Citation

Dodge, John Austin (1983) The Synthesis and Characterization of Binuclear Copper(I) Complexes as Models for Protein Active Sites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/v7jq-zn64. https://resolver.caltech.edu/CaltechTHESIS:09032019-101145351

Abstract

A series of coordinatively unsaturated copper(I) complexes of binucleating nitrogenous ligands have been examined as potential models for protein active sites. The first complex discussed is derived from the ligand N,N,N',N'-tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN). The binuclear copper(I) complex Cu₂(TPEN)²⁺ reversibly binds two equivalents of carbon monoxide, giving an adduct which displays a pair of CO stretching absorptions at 2097 and 2107 cm^-1. Both Cu₂(TPEN)(BF₄)₂ and its carbonyl adduct have been examined by single-crystal X-ray diffraction. Each copper atom of Cu₂(TPEN)²⁺ is bound in a highly asymmetric environment, with an intramolecular copper-copper separation of 2.78Å. In the carbonyl adduct the metal-ligand bonds have rearranged, each copper has assumed a pseudotetrahedral geometry, and there is no longer any direct metal-metal interaction.

Another complex, Cu₂(CHXNpy)²⁺, has been prepared with a ligand analogous to TPEN, but with trans-1,2-cyclohexanediamine substituted for ethylenediamine. A second cyclohexanediamine-based ligand, CHXNbim, has been prepared which contains four benzimidazole groups rather than pyridine. Both Cu₂(CHXNpy)²⁺ and Cu₂(CHXNbim)²⁺ exhibit temperature dependent proton magnetic resonance spectra. The fact that fast exchange is observed only well above room temperature, in spite of the high lability of copper(I), is attributed to the sterically constrained nature of the ligands. Like Cu₂(TPEN)²⁺, Cu₂(CHXNpy)²⁺ forms a dicarbonyl adduct, but it is of lower stability. The benzimidazole complex Cu₂(CHXNbim)²⁺ does not react with carbon monoxide and reacts only slowly with oxygen. Furthermore, unlike Cu₂(TPEN)²⁺ and Cu₂(CHXNpy)²⁺, Cu₂(CHXNbim)²⁺ shows no tendency to disproportionate. This high stability is likely due to a combination of steric and electronic factors, which are discussed.

Finally, a series of complexes having different xylylene "backbones" have been synthesized and examined. Each ligand contains four biologically relevant imidazole groups. All of the complexes react rapidly with carbon monoxide and oxygen (irreversibly in the latter case). In contrast with another known xylylene-based complex containing pyridine groups, the reaction with oxygen occurs without ligand hydroxylation. Factors which could contribute to the reactivity difference are discussed.

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