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

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.