Hydrogenation by cyanocobaltate. Part IV. Hydrogenation of conjugated dienes by 2,2′-bipyridinetricyano-, tricyano(ethylenediamine)-, and tricyano(1,10-phenanthroline)-cobaltate(II) complexes

Abstract

Hydrogenation of butadiene and isoprene catalysed by aminetricyanocobaltate(II) ions [amine = ethylenediamine (en), 2,2′-bipyridine (bipy), and 1,10-phenanthroline (phen)] has been studied. The effect of the CN : Co ratio on the selectivity and the optical, e.s.r., and n.m.r. spectra suggests that the active species is [Co(CN)₃(am)]^–. Intermediate complexes have been detected by n.m.r. spectroscopy, and a high-field shift by the ‘ring current’ observed with the bipy and phen complexes indicated that the am ligands co-ordinate to the cobalt atom in the cis-configuration with respect to an alkenyl ligand. Pseudo-contact shifts have been observed with the intermediate complexes in the en system, and broadening without shift in the bipy and phen systems. These behaviours are explained in terms of the formation of cyanide-bridged paramagnetic dimeric complexes. Catalytic activity is controlled by the lability of the Co–H bond of hydrido-complexes [Co(CN)₃(am)H]^– rather than the stability of the intermediate complexes.