Deactivation of ruthenium metathesis catalysts via facile formation of face-bridged dimers

Reaction of RuCl(dcypb)(mu-Cl)(3)Ru(dcypb)(N-2) (3) with an excess of tert-butylacetylene at ambient temperatures yields the dinuclear monovinylidene RuCl(dcypb)(mu-Cl)(3)Ru(dcypb)-(L) (4a; L=C=CHBut, dcypb = 1,4-bis(dicyclohexylphosphino)butane), rather than the expected mononuclear RuCl2(dcypb)(L). Attempted synthesis of an allenylidene derivative, via the corresponding reaction with 1,1-diphenyl-2-propyn-1-ol stops at the stage of hydroxyvinylidene 4b (L=C=CHC(OH)Ph-2). While formation of these dinuclear products may be an artifact of low solubility, the corresponding monoalkylidene species 4c (L=CHCH=CMe2) is obtained on treating soluble RuH(dcypb)(mu-Cl)(2)(mu-H)Ru(dcypb)(H-2) (5) with 3-methyl-3-chloro-1-butyne. Formation of the perchloro species 4c is consistent with facile homodimerization of the initially formed RuCl2(dcypb)(CHCH=CMe2) (2d), with expulsion of one alkylidene ligand as the free carbene. 2,7-Dimethylocta-2-,4,6-triene, the formal product of carbene coupling, is observed by H-1 NMR: A minor-product in this synthesis is proposed to be RuCl(dcypb)(mu-Cl)(2)(mu(2),eta(1)-CHCH=CMe2)RuCl(dcypb) (8). While the low activity of 4a/4b in ring-opening metathesis polymerization of norbornene is attributable to their low solubility, that of 4c points toward the stability of the Ru-2(mu-Cl)(3) entity. The low activity and facile formation of 4c reveals an important deactivation pathway for catalysts of type 2d, with additional relevance to other such chlororuthenium by H-1, H-2, C-13, and P-31 NMR and IR spectroscopy and (for 4c and 5) by X-ray crystallography.