Titanium Oxo Cluster with Six Peripheral Ferrocene Units and Its Photocurrent Response Properties for Saccharides

A unique titanium oxo cluster with a ferrocene ligand was synthesized and characterized by single crystal X-ray analysis. Six ferrocene carboxylates coordinate to a D_3d Ti₆O₆ core to be a redox active cluster 1, [Ti₆O₆(OⁱPr)₆(O₂CFc)₆]. An analogue 2, [Ti₆O₆(OⁱPr)₆(O₂CⁱBu)₆], where the redox active ferrocene group is replaced by isobutyrate, is also reported as a contrast. The six ferrocene moieties in 1 are structurally identical to give a main redox wave at E_1/2 = 0.62 V in dichloromethane investigated by cyclic voltammetry. Photocurrent responses using electrodes of clusters 1 and 2 were studied, and the response properties of 1 are better than those of 2. The electronic spectra and theoretical calculations indicate that charge transfer occurs from ferrocene to Ti(IV) in 1, and the presence of the ferrocene moiety gives efficient electron excitation and charge separation. Cluster 1 is a cooperative system of TiO cluster and redox active ferrocene. Photocurrent response properties of an electrode of 1 for four saccharides, glucose, fructose, maltose, and sucrose, were tested, and only reducing sugars were responsive. The electrode of 2 is also photocurrent responsive to saccharides, but the current densities are lower than those of redox active 1.