Versatile {Cp₂Ti} Grafted Hetero-Polyoxotungstate Clusters: Synthesis, Crystal Structure, and Photocurrent Properties

Polyoxotungstate supported titanocene {Cp₂Ti}²⁺ clusters H₆{K₈(Cp₂Ti)₂P₄W₂₄O₈₈(PO₄)₂}·14H₂O (1), H₆[Na₂P₄W₁₄O₅₈(Cp₂Ti)₂]·12H₂O (2), and H₂[K₆{Cp₂Ti}{PW₉O₃₃(WO₂)}₂{NC₅H₃(COOK)₂}(NC₅H₃(CH₃)COOK)·22H₂O] (3) have been synthesized, and their single crystal X-ray structures have revealed unprecedented and intriguing structural features. The synthesized compounds have been characterized by various spectroscopic techniques including UV-vis, cyclic voltammogram, NMR, ESI-MS, and inductive coupled plasma spectroscopy (ICP) in solution and also by IR, TGA, and diffuse reflectance in the solid state. Clusters 1 and 2 are rare examples of lacunary POM supported titanocene clusters obtained by incorporating various phosphorus heteroatoms to form elusive phosphotungstate assemblies, whereas 3 is an unprecedented organometallic as well as heteroleptic pyridyl functionalized POM. Clusters 1-3 show transient photocurrent ON/OFF behavior upon UV-light irradiation and also exhibit characteristic Ti^IV/III intravalence electron transfer. This behavior is also established by their cyclic voltammograms in mixed phosphate buffers (Na₂HPO₄/NaH₂PO₄) which show the evidence of POM supported {Cp₂Ti}^2+/+ species in their redox solution. Furthermore, ESR line broadening is also observed in these clusters at room temperature, a fact which also confirms the formation of partially reduced/oxidized {Cp₂Ti}^2+/+ species leading to Ti^IV/III intravalence electron transfers within all three clusters. The {Cp₂Ti}²⁺ decorated polyoxometalate cluster 3 shows improved transient photocurrent behavior which may be due to the presence of pyridyl carboxyl ions which provide better surface contact for the cluster molecule through the carboxylate moiety to the ITO electrode.