Monoazadiene complexes of early transition metals. 2. Syntheses and structures of titanium 1-aza-1,3-diene complexes and their reactions with ketones

The novel dark green or violet and air-sensitive 1-aza-1,3-diene titanocene complexes Cp2Ti[N(R-1)CH=C(R-2)CH(Ph)] [R-1= t-Bu, R-2 = H (7a); R-1= C6H4-4-Me, R-2 = H (7b); R-1= c-C6H11, R-2 = Me (7c)] were prepared by the complexation of the 1-aza-1,3-dienes 1a-c to the titanocene "Cp2Ti" generated in situ by reduction of Cp2TiCl2 with magnesium. The solid-state structure of 7c shows a bent azatitanacyclic ring with a fold angle of 130.9(4)degrees. A series of electron-deficient 14e 1-aza-1,3-diene titanium complexes CpTi[N(R-1)CH=C(Me)CH(Ph)]Cl [R-1 = c-C6H11 (8a), t-Bu (8b), C6H4-2-Me (8c), C6H4-4-Me (8d)] has also been prepared by reduction of CpTiCl3 with magnesium in the presence of the 1-aza-1,3-dienes (RN)-N-1=CHC(Me)=CH(Ph) 1c-f. These new complexes were isolated as air-sensitive brown (8a,b) or dark red (8c,d) crystals in 50-65% yield. The X-ray crystal structure of 8c revealed that the coordination geometry for the 1-aza-1,3-diene ligands has substantial sigma(2),pi-eta(4)-metallacyclopent-4-ene character. The 1-aza-1,3-diene complexes 8a,c,d only exhibit supine geometry as confirmed by H-1 NMR spectroscopy, while 8b exists in both the conventional supine geometry and the prone geometry, which is demonstrated by quite different 1H NMR chemical shift values. Addition of 8c to 1 equiv of acetophenone gives the seven-membered metallacyclic ring system CpTi[N(C6H4-4-Me)CH=C(Me)CH(Ph)C(Me)PhO] (9), whose structure has also been characterized by NMR spectral data and by X-ray diffraction analysis. In contrast to 8c, the 1-aza-1,3-diene titanocene complex Cp2Ti[N(c-C6H11)CH=C(Me)CH(Ph)] (7c) does not react with acetophenone even at high temperatures.