Polynuclear complexes of copper(I) and the 2-(3(5)-pyrazolyl)-6-methylpyridine ligand: Structures and reactivity toward small molecules

The reactivity of trinuclear copper(I) centers vis-a-vis diagnostic diatomic and other small molecules, including carbon monoxide and dioxygen, is examined as part of an effort to unravel the pattern of dioxygen activation at multicopper sites of potential biological significance. The reaction of [Cu(CH3CN)(4)](PF6) with deprotonated 2-(3(5)-pyrazolyl),6-methylpyridine (2-(3(5)-pzH),6-Mepy, 1) affords the trinuclear copper(I) compound [Cu-3-(py)(2-(3(5)-pz,6-Mepy)(3)].0.5 py (2) from pyridine solutions. Species 2 rearranges in toluene to yield a tetrameric unit that precipitates as the molecular pair [Cu-4(2-(3(5)-pz,6-Mepy)(4)](2).3tol (3). The two units are symmetry-related by virtue of a C-2 axis and are coupled via a unique Cu(I)... Cu(I) contact at 3.0045(13) Angstrom. Compound 2 or 3 reacts with carbon monoxide to generate reversibly the trinuclear species [Cu(CO)(2-(3(5)-pz,6-Mepy)](3) (4), featuring coordination of one CO molecule per copper ion on the same face of a nine-membered Cu3N6 ring framework. Compound 2 reacts with dioxygen in tetrahydrofuran to yield the dinuclear species [Cu-2(2-(3(5)-pz,6-Mepy)(4)].thf (5) and an as yet uncharacterized green-brown crystalline material. In the presence of the hexafluorophosphate anion (PF6-) in CH3CN, the reaction with dioxygen affords the tetranuclear compound [Cu-4(2-(3(5)-pz,6-Mepy)(6)(CH3CN)(2)](PF6)(2) . CH3CN (6). An analogous dinuclear species, [Cu-2(2-(3(5)-pz,6-Mepy)(2)(MeOH)(4)](CF3SO3)(2) (7), is formed in wet methanol from Cu(CF3SO3)(2) and the deprotonated ligand 1. The nuclearity of the resulting species upon dioxygen activation and the fact that no compounds are observed containing the Cu-3(II)(mu(3)-O(H)) unit are discussed.