The first co-ordinatively unsaturated Group 8 allenylidene complexes: insights into Grubbs’ vs. Dixneuf–Fürstner olefin metathesis catalysts

Abstract

The reactions of [MCl₂(PPh₃)₃] with HCCCPh₂OH provided [MCl₂(CCCPh₂)(PPh₃)₂] (M = Ru 1a or Os 1b) the first examples of co-ordinatively unsaturated allenylidene complexes of Group 8 metals. The phosphine ligands of 1a are labile and readily replaced by PCy₃ to give [RuCl₂(CCCPh₂)(PCy₃)₂] 1c. Heating 1a with NaPF₆ in chloroform gave the known bimetallic salt [Ru₂(µ-Cl)₃(CCCPh₂)₂(PPh₃)₄]PF₆ 2·PF₆. The reaction of 1a with carbon monoxide provided [RuCl₂(CCPh₂)(CO)(PPh₃)₂] 3 which may also be prepared from [RuCl₂(CO)(dmf)(PPh₃)₂] and HCCCPh₂OH. The first macrocycle coligated allenylidene complex [RuCl(CCCPh₂)(PPh₃)([9]aneS₃)]Cl 4·Cl ([9]aneS₃ = 1,4,7-trithiacyclononane) was obtained from the reaction of 1a with [9]aneS₃. Alternatively, 4·PF₆ is also obtained by treating [RuCl₂(PPh₃)([9]aneS₃)] sequentially with NaPF₆ in acetonitrile followed by HCCCPh₂OH. The reaction of 1a with dppe and NaPF₆ yielded the known salt trans-[RuCl(CCCPh₂)(dppe)₂]PF₆ 5·PF₆. The complex [RuCl(CCCPh₂)(PCy₃){HB(pz)₃}] 6 (pz = pyrazol-1-yl) was obtained from the reaction of 1c with K[HB(pz)₃], whilst the related benzylidene complex [RuCl(CHPh)(PCy₃){HB(pz)₃}] 7 was obtained similarly from [RuCl₂(CHPh)(PCy₃)₂] and K[HB(pz)₃]. Heating [Ru₂(µ-Cl)₂Cl₂(η-cym)₂] (cym = ⁱPrC₆H₄Me-4) with PCy₃ and HCCCPh₂OH in refluxing benzene provided a mixture of 1c and the bimetallic complex [Ru₂(µ-Cl₂)Cl₂(CCCPh₂)(η-cym)] 8 and other unidentified products. The complex 8 may however be obtained quantitatively from the reaction of 1c with [Ru₂(µ-Cl)₂Cl₂(η-cym)₂]. These results suggest that the active species in ring-closure olefin metathesis processes mediated by the allenylidene pre-catalyst [RuCl(CCCPh₂)(PCy₃)(η-cym)]⁺ in non-polar arene solvents may be allenylidene analogues of the Grubbs’ alkene metathesis catalyst, viz. 1c and 8.