EPR/ENDOR detection of stable TiIII and TiIV radical products generated by single electron transfer from (PriO)3TiCH3

Abstract

Single electron transfer reactions between the stereoselective carbanion source methyltriisopropoxy-titanium(IV) as donor and α-diimines, ketones or α-dicarbonyls as π acceptor substrates exhibit a dichotomy with respect to the EPR-detectable paramagnetic ‘escape’ products. Substrates with very negative reduction potentials such as 2,2′-bipyridine yield Ti^III species, whereas more easily reducible molecules such as dinucleating 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine react to yield EPR-detectable Ti^IV complexes of the corresponding anion radicals. A very stable paramagnetic product has been isolated from the reaction between 3,5-di-tert-butyl-o-quinone (dtbq) and (PrⁱO)₃TiCH₃; this first thermally stable, but apparently configurationally labile, Ti^IV semiquinone complex [(dtbq^–1)Ti(OPrⁱ)₃]˙ was studied by UV–VIS–NIR absorption and EPR/ENDOR spectroscopy.