The First Hexanuclear Copper(I) Carboxylate: X-ray Crystal Structure and Reactivity in Solution and Gas-Phase Reactions

The carboxylate ligand-exchange reaction of copper(I) trifluoroacetate by 3,5-difluorobenzoate yielded a new product,
[Cu(O2C(3,5-F)2C6H3)] (1). Single crystals of 1 suitable for X-ray structural characterization were obtained by
sublimation-deposition procedures at 230 °C. An X-ray diffraction study revealed a remarkable planar hexanuclear
copper(I) core supported by bridging carboxylates, the first such structural type among other known copper(I)
carboxylates. The Cu···Cu distances within the core range from 2.7064(8) to 2.8259(8) Å and fall into the category
of cuprophillic interactions. The hexacopper unit remains intact upon gas-phase deposition with a planar polyarene,
coronene (C24H12), to give [Cu6(O2C(3,5-F)2C6H3)6](C24H12) (2). Density functional theory calculations suggest the
latter compound to be a cocrystallization product having electrostatic interactions between the hexacopper complex
and coronene. However, cocrystallization affects the photophysical properties of 2. While copper(I) 3,5-difluorobenzoate
(1) exhibits photoluminescence at ca. 554 nm (λex = 350 nm) in the solid state, compound 2 is nonluminescent
at room temperature in the visible region. Gas-phase and solution reactions of 1 with alkyne ligands, diphenylacetylene
(C14H10) and 1,4-bis(p-tolylethynyl)benzene (C24H18), result in the rupture of the [Cu6] core to afford dinuclear
organometallic copper(I) complexes. The latter have a dimetal core cis-bridged by two carboxylate groups with
acetylene ligands η2-coordinated to each copper(I) center.