1,2-CH Bond Activation of Pyridine across a Transient Titanium Alkylidene Radical and Re-Formation of the Ti=CHtBu Moiety

Reduction of the titanium alkylidene [(PNP)Ti= (CHBu)-Bu-t(OTf)] (PNP- = N[2-P(i)Pr2-4-methylphenyl](2)(-)) with KC8 in the presence of pyridine results in formation of the transient titanium(III) alkylidene radical [(PNP)Ti=-(CHBu)-Bu-t)] (A) or the adduct RPNP)Ti=cH(t)Bu)(NC5H8)] (B), which activates the C-H bond of pyridine to form the titanium(III) pyridyl alkyl complex [(PNP)Ti((CH2Bu)-Bu-t)(eta(2)-NC5H4)] (1) in 64% yield as brown microcrystals. Low temperature X-band EPR spectroscopy and single-crystal X-ray diffraction studies confirm the identity of 1 as a d(1) metal-centric radical with superhyperfine coupling to one nitrogen atom and having a side-on pyridyl moiety, which results in formation of the two isomeric forms la,b. Oxidation of 1 with [FeCp*2] [OTf] cleanly promotes alpha-hydrogen abstraction to re-form [(PNP)Ti=(CHBu)-Bu-t(OTf)] with concurrent elimination of pyridine and FeCp*(2). Re-formation of the alkylidene moiety most likely stems from an intermediate such as [(PNP)Ti((CH2Bu)-Bu-t)(eta(2)-NC5H4)(OTf)] (C)