Complexes with Tunable Intramolecular Ferrocene to TiIV Electronic Transitions: Models for Solid-State FeII to TiIV Charge Transfer
Turlington, M. D.; Pienkos, J. A.; Carlton, E. S.; Wroblewski, K. N.; Myers, A. R.; Trindle, C. O.; Altun, Z.; Rack, J. J.; Wagenknecht, P. S. Complexes with Tunable Intramolecular Ferrocene to TiIV Electronic Transitions: Models for Solid-State FeII to TiIV Charge Transfer, Inorg. Chem., 2016, 55(5), 2200-2211.
Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and TiIV has been prepared and characterized by UV-Visible spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, TMSCp, and comprise the complexes TMSCp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2C6H5)2 which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible TiIV/III reduction wave and an FeII/III oxidation that is irreversible for all complexes except TMSCp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M–1cm–1 < e < 8000 M–1cm–1) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the TiIV/III, and FeIII/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, l, have been determined using band shape analysis (2600 cm–1 < l < 5300 cm–1) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at TiIV and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)2 rapidly decomposes upon photolysis.