Mimicking of the histidine brace structural motif in molecular copper(I) compounds

Abstract

AbstractL‐Nτ‐methylhistidine methyl ester, MeHisOMe, has been employed as a potential ligand to mimic the histidine brace‐type coordination of copper ions in enzymes such as the particulate methane monoxygenase or lytic polysaccharide monooxygenases. MeHisOMe was prepared by double‐methylation of histidine methyl ester. Subsequently, its complexation by diphosphine copper(I) precursors [Cu(P^P)(MeCN)2]BF4 was tested, which led to the complexes [Cu(P^P)(MeHisOMe)]BF4 (P^P=dpePhos: 1, P^P=XantPhos: 2, P^P=dppf: 3). 1–3 were fully characterized, also by single crystal X‐ray analysis, thus providing first structural data for copper complexes with a synthetic, authentic histidine brace. The complexes proved inert in contact with dioxygen. To improve the biomimetic character attempts were made to formally replace the diphosphine ligands by bis(pyrazolyl)methanes, Bpm. Correspondingly, [BpmCu(NCMe)x]BF4 precursors were synthesized, with different substituents at the 3‐positions of the pyrazolyl (i. e. Bpm=di(3‐(phenyl)‐1H‐pyrazol‐1‐yl)diphenylmethane, di(3‐(mesityl)‐1H‐pyrazol‐1‐yl)methane and di(3‐(tert‐butyl)‐1H‐pyrazol‐1‐yl)diphenylmethane). Addition of MeHisOMe to these complexes led to products that were so sensitive towards oxidation by the environment that they eluded isolation. One experiment provided blue crystals as a product of such a reaction. They belonged to a salt with a complex cation consisting of a Cu(μ‐OH)2Cu core ligated by two MeHisOMe ligands, which dimerises in the solid state to give [Cu4(OH)4(MeHisOMe)4]4+.