Operationally unsaturated pincer/rhenium complexes form metal carbenes from cycloalkenes and metal carbynes from alkanes.
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abstract
Operationally unsaturated (i.e., 16/18-electron) (PNPR)Re(H)4, where PNPR is N(SiMe2CH2PR2)2, is reactive at 22 degrees C with cyclic olefins. The first observed products are generally (PNPR)Re(H)2(cycloalkylidene), with hydrogenated olefin as the product of hydrogen abstraction from the tetrahydride. The tetrahydride complex with R = tBu generally fails to react (too bulky), that with R = cyclohexyl suffers a (controllable) tendency to abstraction of 3H from one ring, forming an eta3-cyclohexenyl compound, and that with R = iPr generally gives the richest bimolecular reactivity. The cyclic monoolefins studied show distinct reactivity, C6 giving first the carbene and then coordinated cyclohexadiene, C5 giving carbene, then diene, and then eta5-C5H5, C8 giving carbene and then eta2-cyclooctyne, and C12 giving an eta3-allyl. Norbornene gives a pi-complex of the norbornene in thermal equilibrium with its carbene isomer; at 90 degrees C, hydrocarbon ligand Calpha-Cbeta bond cleavage occurs to give, for the first time, a carbyne complex from an internal olefin. Two compounds synthesized here have the formal composition "(PNPR)Re + olefin", and each of these is capable of dehydrogenating the methyl group of a variety of alkanes at 110 degrees C to form (PNP)ReH triple bond (CR).
