Tuning the redox potentials of dinuclear tungsten oxo complexes [(Cp*W(4,4'-R,R-2,2'-bpy)(mu-O))2][PF6]2 toward photochemical water splitting.

A series of novel dinuclear tungsten(IV) oxo complexes with disubstituted 4,4'-R,R-2,2'-bipyridyl (R(2)bpy) ligands of the type [(CpW(R(2)bpy)(mu-O))(2)]PF(6) (R=NMe(2), tBu, Me, H, Cl) was prepared by hydrolysis of the tungsten(IV) trichloro complexes [CpW(R(2)bpy)Cl(3)]. Cyclic voltammetry measurements for the tungsten(IV) oxo compounds provided evidence for one reversible oxidation and two reversible reductions leading to the oxidation states W(V)W(IV), W(IV)W(III) and W(III)W(III). The corresponding complexes (Cp*W(R(2)bpy)(mu-O))(2) PF(6) (n=0 for R=Me, tBu, and 1, 3 for both R=Me) could be isolated after chemical oxidation/reduction of the tungsten(IV) oxo complexes. The crystal structures of the complexes [(CpW(R(2)bpy)(mu-O))(2)]BPh(4) (R=NMe(2), tBu) and (Cp*W(Me(2)bpy)(mu-O))(2)PF(6) (n=0, 1, 2, 3) show a cis geometry with a puckered W(2)O(2) four-membered ring for all compounds except [(CpW(Me(2)bpy)(mu-O))(2)] which displays a trans geometry with a planar W(2)O(2) ring. Examining the interaction of these novel tungsten oxo complexes with protons, we were able to show that the W(IV)W(IV) complexes [(CpW(R(2)bpy)(mu-O))(2)]PF(6) (-) (R=NMe(2), tBu) undergo reversible protonation, while the W(III)W(III) complexes [(CpW(R(2)bpy)(mu-O))(2)] transfer two electrons forming the W(IV)W(IV) complex and molecular hydrogen.