Macrocyclic [Cu^I/II(bite)]^+/2+ (bite = biphenyldiimino dithioether):  An Example of Fully-Gated Electron Transfer and Its Biological Relevance¹

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, R 1 = 0.045, and R 2 = 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, R 1 = 0.061, and R 2 = 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-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 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.

Synthesis, crystal structures and yeast RNA-binding properties of copper(II) and nickel(II) complexes with a bidentate Schiff base

“…1. The Cu-N 2.005(3) Å and Cu-S 2.289(2) Å distances are similar to reported values [11][12][13][14][15]. The angle between the furan and phenyl rings is 39.158.…”

ESR/UV–Vis–NIR cyclovoltammetry of macrocyclic complex [CuI(bite)]BF4 at different temperatures

“…The macrocyclic compound [Cu I (bite)]BF 4 (bite ¼ biphenyldiimino dithioether) is believed to represent a model compound of the type-I copper blue proteins [4] and has been recently intensively studied with respect to the electron transfer reactions of the [Cu II=I (bite)] 2þ=þ redox couple both using electrochemical and spectroscopic techniques [2,5]. Special attention is focused on the changes in the coordination sphere of this complex under electron transfer [5] as the environment around the metal centre and its coordination flexibility are the key factors for a metalloprotein to have a specific physiological function [6][7][8][9][10].…”

“…Special attention is focused on the changes in the coordination sphere of this complex under electron transfer [5] as the environment around the metal centre and its coordination flexibility are the key factors for a metalloprotein to have a specific physiological function [6][7][8][9][10].…”

“…The X-ray structure [5,11] of [Cu I (bite)](BF 4 ) reveals distorted tetrahedral N 2 S 2 coordination of the copper ion ( Fig. 1(a), Table 1).…”

“…[Cu II (bite)] 2þ has crystallographically imposed C 2 symmetry which implies both Cu-S bond lengths as well as N-C-C-C-C-S-Cu chelate rings to be equal. UV-Vis spectra indicate a four-coordinate, square-planar structure for this complex in the solution, thus the possibility of its coordination also by BF À 4 anions may be refused [5].…”