Hafnium alkyl complexes of the anionic PNP pincer ligand and possible alkylidene formation

The diarylamido-based PNP pincer ligand can be used successfully for support of organometallic Hf complexes (PNP = [(4-Me-2-iPr2P–C6H3)2N]). (PNP)HfCl3 (3) was prepared via reaction of (PNP)Li (2) with HfCl4(OEt2). Reactions of (PNP)HfCl3 with alkyl Grignards led to triple alkylation to produce (PNP)HfMe3 (4) with a small methyl or only double alkylation to give (PNP)Hf(CH2SiMe3)2Cl (5) with a larger alkyl. Structures of 3, 4, and 5 in the solid state were established by X-ray diffraction studies. Structures of the alkyl complexes 4 and 5 display remarkably irregular coordination environments about Hf, while in 3 it is approximately octahedral. Compound 4 was found to be thermally stable at 75 °C. On the other hand, thermolysis of 5 at similar conditions led to a mixture of products, the major one of which is believed to be a Hf alkylidene on the basis of in situ NMR spectroscopic observations (e.g., δ 248.2 ppm in the 13C{1H} NMR spectrum).