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

Article abstract of DOI:10.1021/om0110888

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=CHBu^t, 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=CMe ₂)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.