Reaction of [Os₃(μ-H)₂(CO)₉(μ₃-CNC₅H₄CH=CH₂)] with phenylacetylene. a novel type of alkylidyne-alkyne coupling in a triosmium alkylidyne cluster

Reaction of the alkylidyne cluster [Os3(μ-H)2(CO)9(μ3-CNC5H4CH=CH2)] 1 with the unsymmetrical alkyne PhC≡CH occurred under thermal conditions affording two pairs of geometrical isomers [Os3(μ-H)(CO)9{μ3-η3-PhCCHC(CH=CHPh)}] 2a (30%), [Os3(CO)9(μ3-η1,η2,η2,η1-PhCHCH=CH-CH=CPh)] 2b (24%), [Os3(CO)7{μ-η2,η3-CH2=CHC(Ph)CHCPh}{μ3-η2-C(O)C(Ph)=CH}] 3a (8%), [Os3(CO)7{μ-η2,η3-PhCH=CHC(Ph)CHCH}{μ3-η2-C(O)C(Ph)=CH}] 3b (6%) and the known cluster [Os3(μ-H)(CO)9{μ-C(H)NC5H4(η2-CH=CH2)}] 4 (7%). The same reaction, carried out at room temperature in acetone, yielded another new cluster [Os3(CO)8(μ-η2-PhCCH2) (μ3-η2-CHCPhCH)] 5 as a major product (34%). All the compounds have been fully characterised by spectroscopic and X-ray diffraction methods. The structure of 2a reveals that the methylidyne carbon in the 'Os3C' core couples with two phenylacetylene molecules to form a novel C5hydrocarbyl co-ordinated in an allyl fashion on the trimetallic surface. Cluster 2b, an isomer of 2a, contains a cis,cis-1,5-diphenylpenta-1,3-diene ligand on a triangular array of Os atoms which, together with one of the osmium atoms forms an osmacyclohexa-2,4-diene ring with two localised double bonds. The structures of both 3a and 3b show the coupling of three alkyne molecules with 1, resulting in the formation of two discrete organic fragments. An interesting feature in both compounds is the formation of a keto ligand with a μ3-η2-co-ordination mode, which is derived from the coupling of one carbonyl ligand and a phenylacetylene molecule. The other fragment adopts a μ-η2,η3-co-ordination mode with a π and a η3-allyl bond to give a metallacyclic ring system. The two complexes are isomeric and differ only in the spatial positions of the phenyl substituents in the C5hydrocarbyl fragments. The structure of 5 contains a σ,π-vinyl group with a μ-η2-bonding mode and an allyl fragment with a μ3-η3-co-ordination mode. A proposed mechanism which involves the cleavage of the N-C(alkylidyne) bond in 1, resulting in the formation of a μ3-carbido species '[Os3(μ-H)2(CO)9(μ3-C)]' or a μ-alkylidene fragment '[Os3(μ-H)(CO)9(μ-CH)]' as the intermediate is put forward to explain the unusual coupling reaction sequence. The reaction of 1 with p-tolylacetylene in refluxing n-hexane was studied to support the suggested mechanism and another two pairs of geometrical isomers were fully characterised with the crystal structure of [Os3(CO)9(μ3-η1,η2,η2,η1-MeC6H4CHCH=CHCH=CC6H4Me)] 6b determined. The basic hydrocarbyl skeleton on the Os3triangle in 6b is essentially the same as in the analogous compound 2b except for the presence of two methyl groups on the phenyl rings. This precludes the existence of the original pyridine ring in the newly formed clusters.