Evidence for the Lability of a Bridging Phosphido Ligand under Hydrogen Atmosphere. Reactions of the Cluster Complex Ru3(μ3-P(C6H5)(C5H4N))(μ-P(C6H5)2(CO)8

Article abstract of DOI:10.1021/ja00224a023

In studies to determine free organic molecules that can be derived from the coordinated acyl group of the prototype complex Ru3(μ-C(O)(C6H5))(μ₃-P(C6H5)(C5H4N))(CO)9 (1) or its PR2H substituted derivatives Ru3(μ-C(O)(C6H5))(μ₃-P(C6H5)(C5H4N))(CO)8(PR2H) (2a, R = C6H5; 2b, R = C6H11), the species Ru3(μ₃-P(C6H5)(C5H4N))(μ-PR2)(μ-CO)2(CO)6 (4a, R = C6H5; 4b, R = C6H11) are respectively obtained in 80percent and 65percent yield through a mild thermolysis of 2 where coordinated PR2H is the source of the hydrogen atom releasing benzaldehyde.Benzene is the only organic product obtained from the reaction of 1 with H2, producing Ru3(μ-H)(μ₃-P(C6H5)(C5H4N))(CO)9 (3).By contrast, treatment of 1 with yields benzaldehyde and the salt 3-P(C6H5)(C5H4N))(CO)9 (<3*>).The high reactivity of 4a is first illustrated by a facile CO addition at 25 deg C involving metal-metal bond opening to give the 50-e cluster Ru3(μ₃-P(C6H5)(C5H4N))(μ-P(C6H5)2(CO)9 (5a) (yield, 95percent).The addition of CO is reversible under reduced pressure or under a stream of argon at 25 deg C.Facile ligand substitutions from 4a are exemplified by two reactions.With triphenylphosphine, Ru3(μ₃-P(C6H5)(C5H4N))(μ-P(C6H5)2(μ-CO)2(CO)5(P(C6H5)3) (6a) is produced in 80percent yield.Reaction of 4a with P(C6H5)2H, by contrast, gives Ru3(μ-H)(μ₃-P(C6H5)(C5H4N))(μ-P(C6H5)2)2(CO)6 (7a) (yield, 65percent), which exists in two isomeric forms 7a₁ (symmetric) and 7a₂ (asymmetric).The latter complex is formed by rapid oxidative addition of P-H bond of P(C6H5)2H to the metal, following its initial coordination.Addition of hydrogen to the title complex 4a (tetrahydrofuran; P(H2), 40 bar; 120 deg C; 2 h) promotes the conversion of the bridging phosphido group μ-P(C6H5)2 into a terminal P(C6H5)2H ligand to produce Ru3(μ-H)(μ₃-P(C6H5)(C5H4N))(CO)8(P(C6H5)2H) (8a) as the major product (yield, ca. 30percent), along with three additional species identified as 3, 7a₁ and 7a₂.Of these complexes, 8a is reconverted to 4a by heating under nitrogen.The formation of 3, 7a₁ and 7a₂ bears evidence of a hydrogen assisted intermolecular redistribution of phosphido groups.All complexes are characterized by elemental analysis, infrared spectroscopy, and 1H and 31P NMR.Details of the X-ray structures of 4b, 5a, and 6a are reported.The complex 4a is a catalyst precursor for the hydrogenation of cyclohexanone (typical experiment: tetrahydrofuran, 20 mL; 4a, 5E-2 mmol; cyclohexanone, 20 mmol; P(H2), 40 bar; 120 deg C; reaction time, 2h; yield, 77percent; turnover, 151).