Aromatic vs Aliphatic C−H Cleavage of Alkyl-Substituted Pyridines by (PNP<sup>iPr</sup>)Re Compounds

Both (PNP)Re(H)<sub>4</sub> and (PNP)ReH(cyclooctyne) (PNP<sup>iPr</sup> = (<sup>i</sup>Pr<sub>2</sub>PCH<sub>2</sub>SiMe<sub>2</sub>)<sub>2</sub>N) react with alkylpyridines NC<sub>5</sub>H<sub>4</sub>R to give first (PNP)ReH<sub>2</sub>(η<sup>2</sup>-pyridyl) and cyclooctene and then, when not sterically blocked, (PNP)Re(η<sup>2</sup>-pyridyl)<sub>2</sub> 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 η<sup>2</sup>- 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)<sub>4</sub> preferentially at the vinyl group, to give 2-ethyl pyridine and the σ-vinyl complex (PNP)ReH[η<sup>2</sup>-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 d<sup>4</sup> Re<sup>III</sup> center, showing short Re−N distances consistent with the presence of π-donation.