Synthesis, Structural and Physicochemical Characterization of a Titanium(IV) Compound with the Hydroxamate Ligand...

The siderophore organic ligand N,2-dihydroxybenzamide (H₂dihybe) 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 H₂dihybe with TiCl₄ in methyl alcohol and KOH yielded the tetranuclear titanium oxo-cluster (TOC) [Ti^IV₄(μ-O)₂(HOCH₃)₄(μ-Hdihybe)₄(Hdihybe)₄]Cl₄∙10H₂O∙12CH₃OH (1). The titanium compound was characterized by single-crystal X-ray structure analysis, ESI-MS, ¹³C, and ¹H NMR spectroscopy, solid-state and solution UV-Vis, IR vibrational, and luminescence spectroscopies and molecular orbital calculations. The inorganic core Ti₄(μ-O)₂ of 1 constitutes a rare structural motif for discrete Ti^IV₄ 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 {Ti₄(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 H₂dihybe and was attributed to the high-energy ν(Ti₂-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 H₂dihybe. 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. ¹H and ¹³C NMR solution (CD₃OD) studies of 1 reveal that it retains its integrity in CD₃OD. The observed NMR changes upon addition of base to a CD₃OD 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.