Synthesis, Structural, and Physicochemical Characterization of a Ti6and a Unique Type of Zr6Oxo 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 TiCl4with H3dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K2[TiIV6(μ3-O)2(μ-O)3(OCH3)4(CH3OH)2(μ-Hdihybo)6]·CH3OH (1), while the reaction of ZrCl4with H3dihybo and either nBu4NOH or KOH also gave the hexanuclear zirconium(IV) compounds 2and 3, respectively. Compounds 1–3have the same structural motif [MIV6(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 Zr6clusters reported thus far, and a unique Zr6core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters 1and 3retain 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 TiO2and ZrO2to 1.48 and 2.34 eV for the titanium and zirconium compounds 1and 3, respectively. The application of 1and 3in photocurrent responses was investigated. ESI-MS measurements of the clusters 1and 3revealed 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 NICSzzscan 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 NICSzz(1) values equal to −30 and −42 ppm for the Ti (compound 1) and Zr (compound 2) systems, respectively, comparable to the NICSzz(1) value of the benzene ring of −29.7 ppm calculated at the same level of theory.