Complexes with Tunable Intramolecular Ferrocene to TiIV Electronic Transitions: Models for Solid State FeII to TiIV Charge Transfer

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• 作者：Michael D. Turlington ; Jared A. Pienkos ; Elizabeth S. Carlton ; Karlee N. Wroblewski ; Alexis R. Myers ; Carl O. Trindle ; Zikri Altun ; Jeffrey J. Rack ; Paul S. Wagenknecht
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：March 7, 2016
• 年：2016
• 卷：55
• 期：5
• 页码：2200-2211
• 全文大小：652K
• ISSN：1520-510X

文摘

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 Ti^IV has been prepared and characterized by UV–vis 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(C₂Ph)₂, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti^IV/III reduction wave and an Fe^II/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 < ε < 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 Ti^IV/III and Fe^III/II reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm^–1 < λ < 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 Ti^IV 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.