Abstract
Template condensation of 2,2‘-diaminobiphenyl, 1,4-bis(2-formylphenyl)-1,4-dithiabutane, and copper(II) tetrafluoroborate yields the new macrocyclic compound [CuI(bite)](BF4) (bite = biphenyldiimino dithioether). [CuI(bite)]BF4 crystallizes in the orthorhombic space group P212121 with a = 14.379(3) Å, b = 21.370(3) Å, c = 8.046(2) Å, V = 2534.7(7) Å3, Z = 4, R1 = 0.045, and R2 = 0.048. The X-ray structure of [CuI(bite)](BF4) reveals distorted tetrahedral N2S2 coordination about copper, with one unusually short Cu−S(thioether) bond of 2.194(2) Å. Oxidation of [CuI(bite)](BF4) with nitrosyl tetrafluoroborate gives [CuII(bite)](BF4)2. [CuII(bite)](BF4)2 crystallizes in the tetragonal space group I41/a with a = 11.640(2) Å, c = 39.527(3) Å, V = 5355.6(7) Å3, Z = 8, R1 = 0.061, and R2 = 0.063. X-ray crystallography of [CuII(bite)](BF4)2 reveals an approximately square planar CuN2S2 structure with two distant axial BF4- anions (Cu−F 2.546(4) Å) completing a “pseudo-octahedral” coordination sphere. Comparative EXAFS studies of solid samples and acetonitrile solutions of [CuI(bite)](BF4) and [CuII(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 1H NMR line broadening of [CuI(bite)]+ in the presence of [CuII(bite)]2+ reveal an overall first-order process with a rate constant of 21.7(1.9) s-1 at 295 K in acetone-d6. 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 1H NMR coalescence temperatures for methylene ligand protons of [CuI(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.
Published Version
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