Structures, reaction kinetics, and dioxygen chemistry of some pentacoordinate Schiff base copper complexes

[摘要] Crystal structures of the Cu(I)/Cu(II) complexes of the pentadentate nitrogen-bonding Schiff-base ligand, ${$bis-2,6- (1-(2-pyridin-2-ylethylimino)ethyl) pyridine$}$, (Cu$sp{m I/II}$(py)$sb2$DAP) $sp{m n+}$, 1, and of the similar ligand, ${$bis-2,6- (1-(2-imidazol-4yl-ethylimino)ethyl) pyridine$}$, (Cu$sp{m I}$(imidH)$sb2$DAP$sp+$, 2, have been solved as the tetrafluorobrorate salts. The complexes are all pentacoordinate about the copper center. The crystal structure of the tetracoordinate isomer of 1, the Cu(I) complex of ${$2- (1-(2-imidazol-4yl-ethylimino)ethyl) -6- ((1-tetrahydropyrido (3,4) imidazol)ethyl) pyridine$}$, (Cu$sp{m I}$(imidH)(imidH)$spprime$DAP) $sp{m n+}$, 4, has also been solved as the tetrafluoroborate salt.The kinetics of the electron self-exchange for the two couples (Cu$sp{m I/II}$(py)$sb2$DAP) $sp{m n+}$ and (Cu$sp{m I/II}$(imidH)$sb2$DAP) $sp{m n+}$ has been studied by dynamic NMR, and the kinetics opf electron cross-exchange has also been investigated by reducing (Cu$sp{m II}$(py)$sb2$DAP) $sp{2+}$ with the two similar complexes, 2, (Cu$sp{m I}$(imidH)$sb2$DAP) $sp+$, and 3, (Cu$sp{m I}$(imidR)$sb2$DAP) $sp+$, where R = p-xylyl, and monitoring the reactions with stopped-flow trechniques. These measured rates have been correlated by Marcus theory, and the good agreement suggests an outer-sphere electron transfer mechanism, although the possibility of an inner sphere electron-transfer mechanism has not been ruled out on the basis of ligand substitution kinetics studies and solution phase structural probes for complexes 1 and 2. These electron exchange results are apparently the first to be reported for synthetic Cu(I)/Cu(II) couples in which the coordination number is likely the same in the two oxidation states. (Cu$sp{m I}$(imidH)$sb2$DAP) $sp+$, 2, has been previously proposed to form a reversibly-bound dioxygen adduct, which, under removal of the dioxygen, regenerates (Cu$sp{m I}$(imidH)$sb2$ DAP) $sp+$. This regeneration step has now been determined to proceed by a route not related to reversible coordination of O$sb2$, but, instead, by a more complicated redox pathway.

[发布机构] Rice University

[效力级别] chemistry