Molecular engineering of coordination pockets in chloro-tris-phenoxo complexes of titanium(IV)

The chloro-tris-phenoxo complexes [TiCl(OAr)3] (OAr = OC6H4CMe3-4 (1), OC6H3Me2-2,4 (2), OC6H2Me3-2,4,6 (4), OC6H3(CHMe2)2-2,6 (5), OC6H3(CMe3)2-2,4 (6) and OC6H4Ph-2 (8) are prepared by heating 3 equivalents of the phenol and [TiCl4] in toluene. X-ray crystal structure determinations show that 2 is a phenoxy-bridged dimer with the ortho-methyl groups making the beginning of a pocket about the terminal chloro ligand and 6 is a tetrahedral monomer in which the pocket is more well developed by the ortho-tert-butyl groups. Both 2 and 6 react with dmbipy to give [TiCl(OAr)3(dmbipy)] [OAr = OC6H3Me2-2,4 (3) and OC6H3(CMe3)2-2,4 (7)] in which the original pocket is destroyed. Reaction of TiCl4 with 3 equivalents of LiOC6H4Ph-2 in diethyl ether gives [TiCl(OC6H4Ph-2)3(diethyl ether)] (9) for which an X-ray crystal structure determination shows a trigonal bipyramidal coordination geometry with the diethyl ether lying trans to the chloro ligand. The three phenoxide ligands make up the equatorial plane which takes the 2-phenyl substituent on each phenoxo ligand away from the chloro ligand resulting in a partially collapsed cavity. The tied-back analogues of 2 and 6, [TiCl{(OC6H2Me2-2,4-CH2-6)3N}] (11) and [TiCl({OC6H2(CMe3)2-2,4-CH2-6}3N)] · diethyl ether (12), are prepared by adding (HOArCH2)3N [Ar = C6H2Me2-2,4 and C6H2(CMe3)2 2,4] to [TiCl(OCHMe2)3] in diethyl ether. An X-ray crystal structure of 12 showed a trigonal bipyramidal structure with a coordination environment about the terminal chloro ligand similar to that found in 6. Complex 12 reacts with pyridine to form the 6-coordinate complex [TiCl({OC6H2(CMe3)2-2,4-CH2-6}3N)(py)] (13).

The preparation and characterisation of chloro-tris-phenoxo complexes of titanium(IV) in which coordination pockets are produced by varying the substituent in the 2-position of the phenoxo ligand phenyl ring is described. Collapse of the pockets in these complexes and some nitrogen tied-back anologues is determined using bipyridyl and pyridine ligands.Figure optionsDownload as PowerPoint slide