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-à-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
[Cu3(py)(2-(3(5)-pz,6-Mepy)3]·0.5py (2) from
pyridine solutions. Species 2 rearranges in toluene to
yield a tetrameric
unit that precipitates as the molecular pair
[Cu4(2-(3(5)-pz,6-Mepy)4]2·3tol
(3). The two units are symmetry-related by virtue of a C2 axis and are coupled
via a unique Cu(I)···Cu(I) contact at
3.0045(13) Å. 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
[Cu2(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 [Cu4(2-(3(5)-pz,6-Mepy)6(CH3CN)2](PF6)2·CH3CN
(6). An analogous dinuclear species,
[Cu2(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
CuII3(μ3-O(H)) unit are
discussed.