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 (H-3 dihybo) 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-1 and C-13 NMR, solid-state UV-vis, and ESI-MS spectroscopy. The reaction of TiCl4 with H-3 dihybo and KOH in methanol, at ambient temperature, yielded the hexanuclear titanium (IV) compound K-2[Ti-6(IV) (mu(3)-O)(2) (mu-O)(3)(OCH3)(4)(CH3OH)(2) (mu-Hdihybo)(6)]center dot CH3OH (1), while the reaction of ZrCl4 with H(3)dihybo and either (Bu4NOH)-Bu-n or KOH also gave the hexanuclear zirconium(IV) compounds 2 and 3, respectively. Compounds 1-3 have the same structural motif [M-6(IV)(mu(3)-O)(2) (mu-O)(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 Zr-6 clusters reported thus far, and a unique Zr-6 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 H(3)dihybo 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 M-3 (M = Ti, Zr) building blocks prior to their self-assembly into the hexanuclear M-6 (M = Ti, Zr) species. Density functional theory (DFT) calculations of the NICSzz scan curves of these systems revealed that the triangular M-3 (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.