Abstract
Several new PN [1-(2-pyridyl)-2-(diphenylphosphino)ethane] complexes of rhodium and iridium have been synthesized. [Ir(PN)2]+ (1) was synthesized from [Ir(PN)(cod)]+ (cod = cyclooctadiene) and an excess of PN in acetone solution and characterized by IR, 1H NMR, and 31P NMR spectroscopy. 1 reacted with H2 in acetonitrile solution to form the cis-dihydride [Ir(PN)2(H)2]+ (2). Ir(PN)2(CO)Cl (3) and Ir(PN)(CO)Cl (4) were synthesized from the reaction of 2-methoxyethanol solutions of [Ir(CO)2Cl2]- with 2 and 1 equiv of PN, respectively. These four-coordinate square-planar complexes were characterized by IR and 31P NMR spectroscopy and by single-crystal X-ray analysis (3, monoclinic P21/a, a = 13.178 (2) Å, b = 13.579 (2) Å, c = 9.672 (2) Å, β = 93.84 (1)°, Z = 2, R = 0.025; 4-Et2O, triclinic P1, a = 11.079 (3) Å, b = 12.901 (3) Å, c = 8.377 (3) Å, α = 101.95 (3)°, β = 109.50 (3)°, γ = 73.01 (2)°, Z = 2, R = 0.025). In 3, both PN ligands are monodentate coordinated through the phosphorus atom. [Ir(PN)2(CO)]+ (5) was synthesized from 3 with KPF6 in acetone solution and characterized by IR and 31P NMR spectroscopy as a four-coordinate square-planar complex with one PN ligand bidentate and one monodentate bonded through the phosphorus atom. The reactivity of 5 with H2 and H+ was also examined. The reaction of PN with an acetone solution of [Rh(nbd)Cl]2 (nbd = norbornadiene) treated with AgPF6 produced a new chelated complex, [Rh(PN)2]PF6 (8(PF6)), which was characterized by IR and 31P NMR spectroscopy and by single-crystal X-ray diffraction (monoclinic P21/n, a = 10.714 (2) Å, 6 = 21.774 (2) Å, c = 15.915 (4) Å, β = 99.95 (2)°, Z = 4, R = 0.039), and was shown to have a distorted square-planar cis geometry. Complex 8 reacted with CO to form the carbonyl adduct, [Rh(PN)2(CO)]+ (9). The reaction of the tridentate ligand 2-[bis(diphenylphosphino)methyl]pyridine (PNP) with [Rh(CO)2Cl2]- under refluxing conditions of 2-methoxyethanol/toluene produced the 4-coordinate complex Rh(PN′)(CO)Cl (11). The ligand PN′ = 2-[(diphenylphosphino)methyl]pyridine resulted from P-C bond cleavage of PNP. The PN analogue of 11, Rh(PN)(CO)Cl (10), was also synthesized by reaction of [Rh(CO)2Cl2]-with PN in 2-methoxyethanol/toluene solution. Complexes 9-11 were characterized by IR and 31P NMR spectroscopy. The activities for the decarbonylation of benzaldehyde with use of 1(PF6) and 8(BF4) were found to be 9 and 10 turnovers/h, respectively, at 150°C in neat benzaldehyde, and their catalytic properties were studied. The results are discussed and compared to the catalytic activity and proposed mechanism of the decarbonylation reaction using [M(dppp)2]+ (M = Rh, Ir; dppp = 1,3-bis(diphenylphosphino)propane).