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

Abstract

L-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+.