Intramolecular exchange of coordinated and dangling phosphines in pentacarbonyl group 6 complexes of 1,1,2-tris(diphenylphosphino)ethane |
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Authors: | Richard L Keiter Ping Ye John William Benson Douglas E Brandt Arnold L Rheingold Kraig A Wheeler |
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Institution: | a Department of Chemistry, Eastern Illinois University, Charleston, IL 61920, United States b Department of Physics, Eastern Illinois University, Charleston, IL 61920, United States c Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, United States d Department of Chemistry, University of Nevada, Reno, Reno, NV 89557, United States |
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Abstract: | The linkage isomers, (OC)5Mκ1-PPh2 CH2CH(PPh2)2] 1 and (OC)5Mκ1-PPh2 CH(PPh2)CH2PPh2] 2 (M = Cr, Mo and W) exist in equilibrium at room temperature. Equilibrium constants for 1Cr ? 2Cr, 1Mo ? 2Mo and 1W ? 2W at 25 °C in CDCl3 are 2.61, 5.0 and 4.74, respectively. Enthalpy favors the forward reaction (ΔH = −13.5, −12 and −12.2 kJ mol−1, respectively) while entropy favors the reverse reaction (ΔS = −37.6, −28 and −28.2 J K−1 mol−1, respectively). Isomerization is much faster than chelation with 1Mo ? 2Mo ? 1W ? 2W > 1Cr ? 2Cr. Enthalpies of activation for 1Cr ? 2Cr and 1W ? 2W are 119.0 and 92.6 kJ mol−1, respectively, and entropies of activation are 1.4 and −28.2 J K−1 mol−1, respectively. Isomerization is 104 times faster for these complexes than for (OC)5Mκ1-PPh2CH2CH2P(p-tolyl)2]. A novel mechanism is proposed to account for the rate differences. The X-ray crystal structure of 2W shows that the phosphorus atom of the short phosphine arm lies very close to a carbon atom of the W(CO)4 equatorial plane (3.40 Å) which could allow “through-space” coupling, accounting in part for the observation of long-range JPC and JPW coupling. The X-ray structure of (OC)5Wκ1-PPh2 C( CH2)PPh2] 5W has been determined for comparison to 2W. |
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Keywords: | Group 6 pentacarbonyls Linkage isomerization Phosphine exchange Through-space coupling |
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