Dinuclear versus mononuclear ruthenium(II) and osmium(II) complexes as potent mediators of glucose oxidase; crystal structure of [OsCl(4,4'-bpy)(bpy)2]BF4

New and known homo- and heterodinuclear Ru(II) and Os(II) complexes with 4,4'-bipyridine (4,4'-bpy), pyrazine, and 4-pyCH=CHpy-4' as bridging ligands (LL) of the type [Cl(bpy)(2)M(LL)MCl(bpy)(2)]X(2) (bpy=2,2'-bipyridine; X=PF(6) or BF(4)) have been studied in their capacity to exchange electrons with a reduced active site of glucose oxidase (GO) from Aspergillus niger. Cyclic voltammograms (CVs) of the dimers in the aqueous buffered solution, when compared with CVs of the parent monomeric species [MCl(LL)(bpy)(2)]BF(4) and [MCl(2)(bpy)(2)] which could be generated at pH approximately 7, if the dimers undergo monomerization, indicate that the dimers are the dominating species under such conditions. All electrochemically oxidized dinuclear complexes studied show high rates of oxidation of GO reduced by D-glucose and the corresponding observed second-order rate constants are in the range (5-64)x10(5) M(-1) s(-1) at 25 degrees C. However, these values are lower than that for the mononuclear complex [OsCl(4,4'-bpy)(bpy)(2)]BF(4) (1.1x10(7) M(-1) s(-1)), suggesting that potentially two-electron dimeric mediators have no advantage compared with corresponding monomeric complexes of Ru(II) and Os(II). The structure of [OsCl(4,4'-bpy)(bpy)(2)]BF(4) was confirmed by X-ray crystallography. The monodentate 4,4'-bpy ligand is coordinated cis to the chloride. Its higher reactivity toward reduced GO is accounted for in terms of the antenna effect of the monodentate 4,4'-bpy ligand. The antenna length equals 9.2 A and matches the depth of the enzyme active site pocket of ca. 10 A. The mechanism of the antenna effect is discussed.