EPR ENDOR DETECTION OF STABLE TI(III) AND TI(IV) RADICAL PRODUCTS GENERATED BY SINGLE ELECTRON-TRANSFER FROM (PR(I)O)3TICH3

Single electron transfer reactions between the stereoselective carbanion source methyltriisopropoxy-titanium(IV) as donor and alpha-diimines, ketones or alpha-dicarbonyls as pi-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(-l)Ti(OPri)3]. was studied by UV-VIS-NIR absorption and EPR/ENDOR spectroscopy.