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

The chelating catechol/oxime ligand 2,3-dihydroxybenzaldehyde oxime (Hdihybo) has been used to synthesize one titanium(IV) and two zirconium(IV) compounds that have been characterized by single-crystal X-ray diffraction and H and C NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl with Hdihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium(IV) compound K[Ti(μ-O)(μ-O)(OCH)(CHOH)(μ-Hdihybo)]·CHOH (), while the reaction of ZrCl with Hdihybo and either BuNOH or KOH also gave the hexanuclear zirconium(IV) compounds and , respectively. Compounds - have the same structural motif [M(μ-Ο)(μ-Ο)] (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 Zr clusters reported thus far, and a unique Zr core structure. Multinuclear NMR solution measurements in methanol and water proved that the hexanuclear clusters and retain their integrity. The marriage of the catechol moiety with the oxime group in the ligand Hdihybo proved to be quite efficient in substantially reducing the band gaps of TiO and ZrO to 1.48 and 2.34 eV for the titanium and zirconium compounds and , respectively. The application of and in photocurrent responses was investigated. ESI-MS measurements of the clusters and revealed the existence of the hexanuclear metal core and also the initial formation of trinuclear M (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICS scan curves of these systems revealed that the triangular M (M = Ti, Zr) metallic ring cores exhibit pronounced metalloaromaticity. The latter depends upon the nature of the metallic center with NICS(1) values equal to -30 and -42 ppm for the Ti (compound ) and Zr (compound ) systems, respectively, comparable to the NICS(1) value of the benzene ring of -29.7 ppm calculated at the same level of theory.