Polycarbon Ligand Complexes:  Synthesis, Molecular Structures, and Selected EHMO Studies of Ru4, Ru5, and Ru6 Clusters with Carbon Ligands Derived from Phosphinodiynes

Thermolysis of the monosubstituted cluster Ru3(CO)11(Ph2PC⋮C−C⋮CR) (R = But, Ph, SiMe3), obtained via CO substitution in Ru3(CO)12, results in P−C and Ru−Ru bond cleavage. Recombination of the fragments yields a range of polyruthenium compounds containing −(C⋮C)− n ligands (n = 2−4). The complexes Ru4(CO)9(μ-PPh2)2{μ4-η1,η2,η2,η1-C⋮C−CC(But)−C⋮C−C⋮CBut} (2), Ru4(CO)10(μ-CO)(μ4-PPh){μ4-η1,η1,η2,η2-(ButC⋮C)C⋮CPh} (3), Ru4(CO)10(μ4-PPh)(μ4-η1,η1,η3,η3-PhC⋮C−C⋮CBut) (4), Ru5(CO)11(μ-CO)(μ-PPh2)2(μ3-η1,η1,η1-C⋮C−C⋮CBut)(μ4-C)(μ2-η1,η1-C−C⋮CBut) (5), Ru6(CO)13(μ-CO)2(μ-PPh2)(μ5-C)(μ3-η1,η1,η1-C−C⋮CR) (6a, R = But; 6b, R = Ph), and {Ru2(CO)6(μ-PPh2}2(μ-η1,η2:μ-η1,η2-C⋮C−C⋮C−) (7) have been fully characterized. The crystal structure of 2 revealed a polyunsaturated C8 chain formed by the head−tail coupling of two diynyl units bound to the metal core in μ4-η3 mode to produce a pendant penta-1-enediyne functionality. Clusters 3 and 4 feature square-planar arrangements of metal atoms capped on one side by a μ4-PPh ligand and on the other side by a −C⋮C−C⋮C− group. The hydrocarbyl units in 3 and 4 arise as a result of phenyl migration from a PPh2 group to a terminal carbon of the diynyl fragment. On heating, 3 readily decarbonylates to generate 4. The pentanuclear cluster 5 and hexanuclear cluster 6 both contain unusually coordinated C4 chains which may be considered as being comprised of carbido- and alkynyl-substituted alkylidyne ligands. The alkylidyne carbide ligand is coordinated across the basal face of a pentagonal pyramid in 6 and across a distorted spiked square in 5. The latter also features a second C4 group which forms a rare μ3-η1-butadiynyl ligand. An EHMO analysis of the model compound Ru6(CO)13(μ-CO)2(μ-PH2)(μ5-C)(μ3-η1,η1,η1-C−C⋮CH) (6a‘) suggests that the cluster is best described in terms of an Ru4C2 octahedron bicapped by a phosphido-bridged Ru2 unit, with the C−C−R (R = C⋮CH) radical contributing a total of seven electrons to the CVE count of the entire polyhedron.