Synthesis, Structural and Physicochemical Characterization of a Titanium(IV) Compound with the Hydroxamate Ligand N,2-Dihydroxybenzamide

The siderophore organic ligand N,2-dihydroxybenzamide (H(2)dihybe) incorporates the hydroxamate group, in addition to the phenoxy group in the ortho-position and reveals a very rich coordination chemistry with potential applications in medicine, materials, and physical sciences. The reaction of H(2)dihybe with TiCl(4) in methyl alcohol and KOH yielded the tetranuclear titanium oxo-cluster (TOC) [Ti(IV)(4)(μ-O)(2)(HOCH(3))(4)(μ-Hdihybe)(4)(Hdihybe)(4)]Cl(4)∙10H(2)O∙12CH(3)OH (1). The titanium compound was characterized by single-crystal X-ray structure analysis, ESI-MS, (13)C, and (1)H NMR spectroscopy, solid-state and solution UV–Vis, IR vibrational, and luminescence spectroscopies and molecular orbital calculations. The inorganic core Ti(4)(μ-O)(2) of 1 constitutes a rare structural motif for discrete Ti(IV)(4) oxo-clusters. High-resolution ESI-MS studies of 1 in methyl alcohol revealed the presence of isotopic distribution patterns which can be attributed to the tetranuclear clusters containing the inorganic core {Ti(4)(μ-O)(2)}. Solid-state IR spectroscopy of 1 showed the presence of an intense band at ~800 cm(−1) which is absent in the spectrum of the H(2)dihybe and was attributed to the high-energy ν(Ti(2)–μ-O) stretching mode. The ν(C=O) in 1 is red-shifted by ~10 cm(−1), while the ν(N-O) is blue-shifted by ~20 cm(−1) in comparison to H(2)dihybe. Density Functional Theory (DFT) calculations reveal that in the experimental and theoretically predicted IR absorbance spectra of the ligand and Ti-complex, the main bands observed in the experimental spectra are also present in the calculated spectra supporting the proposed structural model. (1)H and (13)C NMR solution (CD(3)OD) studies of 1 reveal that it retains its integrity in CD(3)OD. The observed NMR changes upon addition of base to a CD(3)OD solution of 1, are due to an acid–base equilibrium and not a change in the Ti(IV) coordination environment while the decrease in the complex’s lability is due to the improved electron-donating properties which arise from the ligand deprotonation. Luminescence spectroscopic studies of 1 in solution reveal a dual narrow luminescence at different excitation wavelengths. The TOC 1 exhibits a band-gap of 1.98 eV which renders it a promising candidate for photocatalytic investigations.