Manganese and titanium perfluorinated alkoxide complexes including HAA by {Mn(III)OH} and models for n-TiO₂
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Abstract
A new family of Mn(II) and Mn(III) complexes supported by fluorinated alkoxide ligands has been prepared and studied in detail. Complex 1 K[Mn(OH2)(pinF)2] can be deprotonated by KN(SiMe3)2 to form K2[Mn(OH)(pinF)2], 3, the first example of a mononuclear {Mn(III)OH} complex with all O-donors (pinF = perfluoropinacolate). Reactions of 1 with organic bases allowed the pKa to be estimated as 18.0 ± 0.3. Cyclic voltammetry reveals quasi-reversible redox behavior for both 1 and 3 with an unusually large ΔEp assigned to the Mn(III/II) couple. Using the Bordwell method, the bond dissociation free energy (BDFE) of the O-H bond in {Mn(II)-OH2}is estimated to be 67–70 kcal mol−1. Complex 3 abstracts H-atoms from 1,2-diphenylhydrazine, 2,4,6-TTBP, and TEMPOH, the latter of which supports a PCET mechanism. Under basic conditions in air, the synthesis of 1 results in K2[Mn(OAc)(pinF)2], 2, proposed to result from the oxidation of Et2O to EtOAc by a reactive Mn species, followed by ester hydrolysis. Complex 3 alone does not react with Et2O, but addition of O2 at low temperature effects the formation of a new chromophore proposed to be a Mn(IV) species. Several of these unusual mononuclear, all O-donor Mn complexes have been investigated by high-field electron paramagnetic spectroscopy (HFEPR) supported by DFT calculations. UV-vis spectroscopy reveals a trend in the absorption spectra for the five coordinate {Mn(III)(X/L)(pinF)2} complexes that depends on the identity of the non-pinF ligand. This trend was investigated using TD-DFT to calculate MO diagrams. TiO2 is a semiconducting metal oxide with Ti in all O-donor coordination. It is of interest for its chemical and photophysical properties which depend on the material’s structure and stoichiometry. Fluorinated alkoxides and aryloxides have been used to prepare Ti(IV) and Ti(III) complexes as models for Ti sites in pure and reduced TiO2. The unusual mononuclear titanyl complex (Me4N)2[Ti(O)(pinF)2], 12 reacts with H2O2 to form a peroxide complex (Me4N)2[Ti(O2)(pinF)2], 13, which displays a reversible O21-/2- redox feature in the cyclic voltammetry. The first hexacoordinate complexes of (OC6F5)1- have been prepared, namely dimeric {K2[Ti(IV)(OC6F5)6]}2, 14, and K3[Ti(III)(OC6F5)6], 17. Complex 14 is dimeric in both the solid and solution state, held together by non-covalent K...O/F interactions. Complex 17 displays Jahn-Teller distortion in the solid state that also manifests in the UV-vis spectroscopy. A second pair of Ti(IV/III) complexes K2[Ti(IV)(pinF)2], 15a, and K3[Ti(III)(pinF)3], 18, have been prepared, as well as a four coordinate Ti(III) complex K(crypt)[Ti(III)(OC4F9)4], 16. The electronic absorption and electrochemical properties of these Ti complexes is discussed in relation to other all O-donor Ti complexes found in the literature.
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2024