A Thiolate-Bridged, Fully Delocalized Mixed-Valence Dicopper(I,II) Complex that Models the CuA Biological Electron-Transfer Site.

Robert P. Houser, Victor G. Young, Jr.,& Willliam B. Tolman Journal of the American Chemical Society 1996, 118, 2101-2102.

SUMMARY: This paper describes the synthesis of a novel complex, [L(iPrdacoS)Cu]2(O3SCF3) that replicates essential elements of the structural and spectroscopic features of the unusual CuA metalloprotein electron transfer site. This site contains a novel dithiolate-bridged, delocalized mixed-valence (Cu1.5Cu1.5) core with 4-coordinate, distorted tetrahedral copper ions in close proximity (~2.6 � apart). A key spectroscopic signature that supports the delocalized nature of the site is an EPR signal with 7-line hyperfine coupling patterns in its low field components arising from spin interactions with both I = 3/2 copper ions. The model complex was assembled by mixing the sodium salt of 1-isopropyl-5-mercaptoethyl-1,5-diazacyclooctane [L(iPrdacoS)] with Cu(O3SCF3)2 in a 3:2 ratio in MeOH. An X-ray crystal structure determination revealed it to contain a planar {Cu2(�-SR)2}1+ core comprised of symmetry-related copper ions in distorted trigonal pyramidal coordination environments (avg. Cu���Cu = 2.92 �, Cu-S = 2.27 �, Cu-S-Cu = 80.0�). Importantly, the X-band EPR spectrum at 4.2 K contains a nearly axial signal with clearly recognizable 7-line hyperfine splitting in the lower field components (g1 = 2.010, g2 = 2.046, g3 = 2.204, A2Cu = 36.3 G, A2Cu =49.9 G, as determined by spectral simulation) that is strikingly similar to the signals reported for the CuA sites in nitrous oxide reductase, cytochrome c oxidase, and engineered proteins.