Redox-active tetrahydrosalen (salan) complexes of titanium

Abstract

A diarylamino-substituted N-methyl tetrahydrosalen (salan) ligand, ^An2NLH₂, is prepared in four steps and overall 53% yield from 5-bromosalicylaldehyde, with the key step a palladium-catalysed Hartwig–Buchwald amination of the tert-butyldimethylsilyl-protected 5-bromo-N-methylsalan ligand. Reaction of ^An2NLH₂ or its bromo analogue with Ti(OⁱPr)₄ or TiF₄ results in metalation of the ligand. The isopropoxide groups are readily exchanged with α- or β-hydroxyacids to form chelated complexes. X-ray crystallography and NMR spectroscopy indicate that the salan ligands are quite flexible, with ^An2NLTiF₂, for example, showing four stereoisomers in its ¹⁹F NMR spectrum. The major stereoisomer of (salan)Ti(X)(Y) depends principally on the trans influence of the X and Y groups. Complexes of ^An2NL show two reversible, closely spaced redox couples at approximately + 0.1 V vs.ferrocene/ferrocenium, and a second set of two closely spaced redox couples at ∼ + 0.8 V vs. Fc/Fc⁺.