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Dynamical Intramolecular Metal-to-Metal Ligand Exchange of Phosphine and Thioether Ligands in Derivatives PtRu5(CO)166-C)

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posted on 05.04.2003 by Richard D. Adams, Burjor Captain, Wei Fu, Perry J. Pellechia
The complexes PtRu5(CO)15(PMe2Ph)(μ6-C) (2), PtRu5(CO)14(PMe2Ph)26-C) (3), PtRu5(CO)15(PMe3)(μ6-C) (4), PtRu5(CO)14(PMe3)26-C) (5), and PtRu5(CO)15(Me2S)(μ6-C) (6) were obtained from the reactions of PtRu5(CO)166-C) (1) with the appropriate ligand. As determined by NMR spectroscopy, all the new complexes exist in solution as a mixture of isomers. Compounds 2, 3, and 6 were characterized crystallographically. In all three compounds, the six metal atoms are arranged in an octahedral geometry, with a carbido carbon atom in the center. The PMe2Ph and Me2S ligands are coordinated to the Pt atom in 2 and 6, respectively. In 3, the two PMe2Ph ligands are coordinated to Ru atoms. In solution, all the new compounds undergo dynamical intramolecular isomerization by shifting the PMe2Ph or Me2S ligand back and forth between the Pt and Ru atoms. For compound 2, ΔH= 15.1(3) kcal/mol, ΔS = −7.7(9) cal/(mol·K), and ΔG298 = 17.4(6) kcal/mol for the transformation of the major isomer to the minor isomer; for compound 4, ΔH = 14.0(1) kcal/mol, ΔS = −10.7(4) cal/(mol·K), and ΔG298 = 17.2(2) kcal/mol for the transformation of the major isomer to the minor isomer; for compound 6, ΔH = 18(1) kcal/mol, ΔS = 21(5) cal/(mol·K) and ΔG298 = 12(2) kcal/mol. The shifts of the Me2S ligand in 6 are significantly more facile than the shifts for the phosphine ligand in compounds 25. This is attributed to a more stable ligand-bridged intermediate for the isomerizations of 6 than that for compounds 25. The intermediate for the isomerization of 6 involves a bridging Me2S ligand that can use two lone pairs of electrons for coordination to the metal atoms, whereas a tertiary phosphine ligand can use only one lone pair of electrons for bridging coordination.