Synthesis, Structural, and Physicochemical Characterization of a Ti₆ and a Unique Type of Zr₆ Oxo Clusters Bearing an Electron-Rich Unsymmetrical {OON} Catecholate/Oxime Ligand and Exhibiting Metalloaromaticity

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (H₃dihybo) has been used to synthesize one titanium­(IV) and two zirconium­(IV) compounds that have been characterized by single-crystal X-ray diffraction and ¹H and ¹³C NMR, solid-state UV–vis, and ESI-MS spectroscopy. The reaction of TiCl₄ with H₃dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium­(IV) compound K₂[Ti^IV₆(μ₃-O)₂(μ-O)₃(OCH₃)₄(CH₃OH)₂(μ-Hdihybo)₆]·CH₃OH (1), while the reaction of ZrCl₄ with H₃dihybo and either ⁿBu₄NOH or KOH also gave the hexanuclear zirconium­(IV) compounds 2 and 3, respectively. Compounds 1–3 have the same structural motif [M^IV₆(μ₃-Ο)₂(μ-Ο)₃] (M = Ti, Zr), which constitutes a unique example with a trigonal-prismatic arrangement of the six zirconium atoms, in marked contrast to the octahedral arrangement of the six zirconium atoms in all the Zr₆ clusters reported thus far, and a unique Zr₆ core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1 and 3 retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand H₃dihybo proved to be quite efficient in substantially reducing the band gaps of TiO₂ and ZrO₂ to 1.48 and 2.34 eV for the titanium and zirconium compounds 1 and 3, respectively. The application of 1 and 3 in photocurrent responses was investigated. ESI-MS measurements of the clusters 1 and 3 revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M₃ (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M₆ (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICS_zz scan curves of these systems revealed that the triangular M₃ (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICS_zz(1) values equal to −30 and −42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICS_zz(1) value of the benzene ring of −29.7 ppm calculated at the same level of theory.