Synthesis and π-Tweezer Properties of tripodCobalt-Bisalkynyl Compounds [CH3C(CH2PPh2)3Co(C≡CR)2] − Application to the Oxidative Coupling of Alkynyl Groups

Abstract

tripodCoCl2 [CH3C(CH2PPh2)3CoCl2] (1) reacts with alkynyllithium reagents RC≡CLi to produce tripodCo(C≡CR)2 (2). The neutral paramagnetic CoII species 2 can be reduced to the anionic diamagnetic CoI compounds 2−, which can also be obtained by treating tripodCoCl (3) with two equivalents of LiC≡CR. Compounds 2 contain an M(C≡CR)2 group with the alkynyl substituents in cis-positions to each other. The expectation that they might therefore act as π-tweezer ligands is born out by the characterisation of tripodCo(C≡CR)2−NiCO (4). The cis-position of the alkynyl groups in 2 should also facilitate their oxidative coupling. This type of reaction is, in fact, observed when compounds 2 are treated with one-electron oxidants. Reaction of 2 with Cp2Fe+ leads to [tripodCo(η2-RC≡CC≡CR)]+ (5) in which one of the triple bonds of the diyne acts as a four-electron donor ligand while the other one remains uncoordinated. One-electron oxidation of 2 thus initiates a two-electron oxidative coupling of the alkynyl groups with concomitant one-electron reduction of CoII (2) to CoI (5). All compounds were characterised by the usual analytical and spectroscopic techniques including EPR spectroscopy. The structure of compounds 5 is exemplified by an X-ray analysis of [tripodCo(η2-tBuC≡CC≡CtBu)](PF6) (5b).