The blue copper site entatic state revisited: Electron self-exchange kinetics of four- and five-coordinate small molecule copper compounds

[摘要] Template condensation of 2,2$spprime$-diaminobiphenyl, 1,4-bis(2-formylphenyl)-1,4-dithiabutane, and copper(II) tetrafluoroborate yields the new macrocyclic compound (Cu$sp{m I}$(bite)) (BF$sb4).$ The X-ray structure of (Cu$sp{m I}$(bite)) (BF$sb4)$ reveals distorted tetrahedral N$sb2$S$sb2$ coordination about copper, with one unusually short Cu-S(thioether) bond of 2.194(2) A. Oxidation of (Cu$sp{m I}$(bite)) (BF$sb4)$ with nitrosyl tetrafluoroborate gives (Cu$sp{m II}$(bite)) (BF$sb4)sb2.$ X-ray crystallography of (Cu$sp{m II}$(bite)) (BF$sb4)sb2$ reveals an approximately square planar CuN$sb2$S$sb2$ structure with two distant axial BF$sb{4sp-}$ anions (Cu-F 2.546(4) A) completing a 'pseudo-octahedral' coordination sphere.Condensation of 2,6-diacetylpyridine, 1,10-diamino-4,7-dithiadecane, and silver(I) nitrate produces the five-coordinate macrocyclic N$sb3$S$sb2$ compound, (Ag$sp{m I}$(DAPS$sb2)brack(m BFsb4),$ upon crystallization with NaBF$sb4.$ Copper(II) transmetallation of (Ag$sp{m I}$(DAPS$sb2)brack(m BFsb4)$ produces $m lbrack Cusp{II}(DAPSsb2)brack(BFsb4)sb2$ which can be reduced with copper metal to yield $m lbrack Cusp{I}(DAPSsb2)brack(BFsb4). lbrack Cusp{I}(DAPSsb2)brack(BFsb4)$ reacts slowly with CDCl$sb3$ to produce $mlbrack Cusp{II}(DAPSsb2)Clbrack(BFsb4)$ which has a 'pseudo-octahedral' geometry (similar to $mlbrack Cusp{II}(bite)brack(BFsb4)sb2)$ with one distant BF$sb{4sp-}$ anion and one distant thioether sulfur atom bonded to Cu(II). The second thioether sulfur atom of the ligand remains uncoordinated.A comparative EXAFS study of solid state samples and acetonitrile solutions of $mlbrack Cusp{I}(bite)brack(BFsb4)$ and $mlbrack Cusp{II}(bite)brack(BFsb4)sb2$ demonstrates that the primary coordination environments of both compounds are the same in solution as in the solid. However, $mlbrack Cusp{I}(bite)brack(BFsb4)$ K-edge data indicates possible distant acetonitrile coordination or other structural change remote from the metal center.Copper(I/II) electron self-exchange kinetics measured by NMR line broadening reveals strikingly slow kinetics for $mlbrack Cusp{I/II}(DAPSsb2)brack(BFsb4)sb{1,2} ({$295 K, respectively) demonstrate possible correlation of fast electron transfer with high ligand mobility for these and related small-molecule copper(I/II) couples. Comparison with other copper systems also reveals that fast electron transfer is not always obtained in systems with coordination number invariance and conserved geometry during redox turnover, contrary to popular interpretation of the entatic state hypothesis for blue copper protein active sites.

[发布机构] Rice University

[效力级别] chemistry