Macrocyclic [Cu-I/II(bite)](+/2+) (bite = biphenyldiimino dithioether): An example of fully-gated electron transfer and its biological relevance

Template condensation of 2,2'-diaminobiphenyl, 1,4-bis(2-formylphenyl)-1,4-dithiabutane, and copper(II) tetrafluoroborate yields the new macrocyclic compound [Cu-I(bite)](BF4) (bite = biphenyldiimino dithioether). [Cu-I(bite)]BF4 crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 14.379(3) Angstrom, b = 21.370(3) Angstrom, c = 8.046(2) Angstrom, V = 2534.7(7) Angstrom(3), Z = 4, R-1 = 0.045, and R-2 = 0.048. The X-ray structure of [Cu-I(bite)](BF4) reveals distorted tetrahedral N2S2 coordination about copper, with one unusually short Cu-S(thioether) bond of 2.194(2) Angstrom. Oxidation of [Cu-I(bite)](BF4) with nitrosyl tetrafluoroborate gives [Cu-II(bite)](BF4)(2). [Cu-II(bite)](BF4)(2) crystallizes in the tetragonal space group I4(1)/a with a = 11.640(2) Angstrom, c = 39.527(3) Angstrom, V = 5355.6(7) Angstrom(3), Z = 8, R-1 = 0.061, and R-2 = 0.063. X-ray crystallography of [(Cu-II(bite)](BF4)(2) reveals an approximately square planar CuN2S2 structure with two distant axial BF4- anions (Cu-F 2.546(4) Angstrom) completing a ''pseudo-octahedral'' coordination sphere. Comparative EXAFS studies of solid samples and acetonitrile solutions of [Cu-I(bite)](BF4) and [Cu-II(bite)](BF4)(2) demonstrate that the primary coordination environments of both species are the same in solution as in the solid. Copper(I/II) electron self-exchange kinetics measured by H-1 NMR line broadening of [Cu-I(bite)](+) in the presence of [Cu-II(bite)](2+) reveal an overall first-order process with a rate constant of 21.7(1.9) s(-1) at 295 K in acetone-d(6). This result represents the first example of fully-gated electron transfer by small-molecule copper(I). The gating process likely involves inversion at sulfur and the tetrahedral-->square planar structural change coincident with electron transfer. Variable-temperature H-1 NMR coalescence temperatures for methylene ligand protons of [Cu-I(bite)](BF4) (287 K) demonstrate possible correlation of fast electron transfer with high ligand mobility for this and related small-molecule copper(I/II) couples. Comparison with other small-molecule copper systems also reveals that fast electron transfer is not always observed with coordination-number-invariance and conserved geometry during redox turnover, contrary to popular interpretations of the entatic state hypothesis for blue-copper protein active sites.