N-Heterocyclic Carbene and Cyclic (Alkyl)(amino)carbene Complexes of Titanium(IV) and Titanium(III)

The reaction of one and two equivalents of the N-heterocyclic carbene IMes [IMes = 1,3-bis(2,4,6-trimethyl-phenyl)imidazolin-2-ylidene] or the cyclic (alkyl)(amino)carbene cAAC(Me) [cAAC(Me) = 1-(2,6-diisopropyl-phenyl)-3,3,5,5-tetra-methylpyrrolidin-2-ylidene] with [TiCl4] in n-hexane results in the formation of mono- and bis-carbene complexes [TiCl4(IMes)] 1, [TiCl4(IMes)(2)] 2, [TiCl4(cAAC(Me))] 3, and [TiCl4(cAAC(Me))(2)] 4, respectively. For comparison, the titanium(IV) NHC complex [TiCl4(IiPr(Me))] 5 (IiPr(Me) = 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene) has been synthesized and structurally characterized. The reaction of [TiCl4(IMes)] 1 with PMe3 affords the mixed substituted complex [TiCl4(IMes)(PMe3)] 6. The reactions of [TiCl3(THF)(3)] with two equivalents of the carbenes IMes and cAAC(Me) in n-hexane lead to the clean formation of the titanium(III) complexes [TiCl3(IMes)(2)] 7 and [TiCl3(cAAC(Me))(2)] 8. Compounds 1-8 have been completely characterized by elemental analysis, IR and multinuclear NMR spectroscopy and for 2-5, 7 and 8 by X-ray diffraction. Magnetometry in solution, EPR and UV/Vis spectroscopy and DFT calculations performed on 7 and 8 are indicative of a predominantly metal-centered d(1)-radical in both cases.