Aromatic vs Aliphatic C−H Cleavage of Alkyl-Substituted Pyridines by (PNP^iPr)Re Compounds

Both (PNP)Re(H)₄ and (PNP)ReH(cyclooctyne) (PNP^iPr = (ⁱPr₂PCH₂SiMe₂)₂N) react with alkylpyridines NC₅H₄R to give first (PNP)ReH₂(η²-pyridyl) and cyclooctene and then, when not sterically blocked, (PNP)Re(η²-pyridyl)₂ and cyclooctane. The latter are shown by NMR, X-ray diffraction, and DFT calculations to have several energetically competitive isomeric structures and pyridyl N donation in preference to PNP amide π-donation. DFT studies support NMR solution evidence that the most stable bis pyridyl structure is one that is doubly η²- with the pyridyl N donating to the metal center. When both ortho positions carry methyl substituents, cyclooctane and the carbyne complex (PNP)ReH(⋮<i>C</i>-pyridyl) are produced. Excess 2-vinyl pyridine reacts with (PNP)Re(H)₄ preferentially at the vinyl group, to give 2-ethyl pyridine and the σ-vinyl complex (PNP)ReH[η²-CHCH(2-py)]. The DFT and X-ray structures show, by various comparisons, the ability of the PNP amide nitrogen to π-donate to an otherwise unsaturated d⁴ Re^III center, showing short Re−N distances consistent with the presence of π-donation.