Modeling Biological Copper Clusters: Synthesis of a Tricopper Complex, and Its Chloride- and Sulfide-Bridged Congeners

The synthesis and characterization
of a family of tricopper clusters housed within a tris­(β-diketimine)
cyclophane ligand (H3L) that bear structural
similarities to biological copper clusters are reported. In all complexes,
each Cu atom is held within the N2-chelate of a single
β-diketiminate arm. Reaction of L3– with CuCl affords an anionic complex containing a μ3-chloride donor in the central cavity, whereas there is no evidence
for bromide incorporation in the product of the reaction of L3– with CuBr (Cu3L). Cu3L reacts with elemental sulfur to generate
the corresponding air-stable mixed-valent (μ3-sulfido)­tricopper
complex, Cu3­(μ3-S)L, which represents the first example of a sulfide-bridged copper
cluster in which each metal center is both coordinatively unsaturated
and held within a N-rich environment. The calculated LUMO is predominantly
Cu–S π* in character and delocalized over all three metal
centers, which is consistent with the isotropic ten-line absorption
(g ∼ 2.095, A ∼ 33
G) observed at room temperature in EPR spectra of the one-electron
chemically reduced complex, [Cu3­(μ3-S)L]−.