Synthesis, Isolation and Structural Investigation of Schiff‐Base Alkoxytitanium Complexes: Steric Limitations of Ligand Coordination

Abstract

AbstractThis paper reports the reaction of Ti(OiPr)4 with a series of Schiff‐base ligands. The Schiff‐base proligands 1a–l are synthesised by the condensation of salicylaldehyde with a range of primary alkylamine and aniline derivatives. The treatment of 1a–f with Ti(OiPr)4 yields the octahedral bis(aryloxy–imine)Ti(OiPr)2 complexes 2a–f. X‐ray crystal structure analysis of 2c, 2d and 2e reveals complexes with a trans‐aldiminato oxygen atom and cis‐N,cis‐alkoxide ligand arrangement about the central metal atom. The reactions of Ti(OiPr)4 with the ligands 1g and 1h result in a sterically induced ligand rearrangement to form the octahedral complexes 2g and 2h, also characterised by X‐ray diffraction experiments, in which the nitrogen atoms of the O,N‐chelate are now trans‐orientated at the titanium centre. 1H NMR analysis reveals significant deshielding of the isopropoxide methine proton, induced by this coordination mode. In contrast, reactions of 1i and 1j with Ti(OiPr)4 result in the formation and isolation of the complexes 2i and 2j. X‐ray crystal structure analysis shows complex 2j to have a previously unobserved ligand orientation, in which both ligands are trans‐orientated, with respect to the aldiminato oxygen atoms, about the titanium centre, but steric bulk of the ligand inhibits the bidentate coordination of both O,N‐ligands. Further increase in thesteric bulk of the imine substituent results in a reduced reactivity for ligands 1k and 1l, such that Ti(OiPr)4 reacts with 1k to form the dimeric mono(Schiff‐base) complex 2k (characterised by X‐ray analysis). No reaction is observed for 1l. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)