Preparation and X-ray structures of TiIV complexes of bis(carboxylato) ligands -: Formation of mono-, di-, tetra-, and hexanuclear complexes with or without OR and μ-O ligands

Treating the amine bis(carboxylato) ligand precursor pyridine-2,6-dicarboxylic acid (H2L1) with titanium tetra(isopropoxide) gave the mononuclear octacoordinate complex (L2TiIV)-Ti-1(dmf)(2) (1), in which each carboxylato ligand binds in an ill fashion and two solvent molecules are also coordinated to the metal. In contrast, employing the diamine bis(carboxylic acid) ligand precursor H2L2 gave a unique hexanuclear Ti-IV complex [Ti-6(IV)(OiPr)(16)(mu(2)-OiPr)(4)(mu-L-2)(2)] (2), where L-2 = (pyridin-2-ylmethyl)amino diacetate. The crystal structure reveals that all Ti-IV atoms exhibit octahedral geometry, where both carboxylate and isopropoxide groups serve as bridging ligands. Surprisingly, no oxido ligands appear in the structure, which indicates that the complex, obtained at room temp., was not exposed to any traces of water. The diamine bis(carboxylato) ligand coordinates in eta(2) modes of both carboxyl groups to give a total of six coordination sites. Employing the same ligand with a slight change in reaction conditions, which included heating to 35 degrees C, led to the formation of two O-bridged complexes: dinuclear [Ti-2(IV)(mu(2)-O)(OiPr)(2)- (L-2)(2)] (3) and tetranuclear [Ti-4(IV)(mu(2)-O)(4)(L-2)(4)] (4), where each bis(carboxylato) ligand binds to a single Ti center in an eta(1) fashion, and the only bridging ligands are oxide groups, with Ti-O-Ti angles of 180.0 degrees and 166.6 degrees for 3 and 4, respectively, indicating pi interactions in the dinuclear species. Employing a different diamine bis(carboxylato) ligand featuring a sequential connectivity of donor atoms allowed the identification of a mononuclear C-2-symmetrical bis(isopropoxido) complex, L3TiIV(OiPr)(2), (5) where L-3 = ethylenediamine diacetate. In 5, as opposed to 2 and similarly to 1, 4, and 5, the carboxylato ligand binds in an eta(1) fashion. Complexes 3, 4, and 5 are all inactive against ovarian OVCAR-1 and colon HT-29 cells. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).