Deactivation of Ruthenium Metathesis Catalysts via Facile Formation of Face-Bridged Dimers

Reaction of RuCl(dcypb)(μ-Cl)3Ru(dcypb)(N2) (3) with an excess of tert-butylacetylene at
ambient temperatures yields the dinuclear monovinylidene RuCl(dcypb)(μ-Cl)3Ru(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)Ph2). 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)(μ-Cl)2(μ-H)Ru(dcypb)(H2) (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 1H NMR. A minor product in this synthesis is proposed
to be RuCl(dcypb)(μ-Cl)2(μ2,η1-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 Ru2(μ-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 complexes, including systems
of the Grubbs type. Product identities were established by 1H, 2H, 13C, and 31P NMR and IR
spectroscopy and (for 4c and 5) by X-ray crystallography.