Induced Acceleration of Phosphine Exchange in Metal Carbonyls by Pendant Groups of Coordinated Polyphosphines. Two Dangling Phosphine Arms Are Much Better Than One1

Abstract

The kinetics and thermodynamics of isomerization of the linkage isomers (OC)5W[η1-PPh2CH2CH(PPh2)2] (1) and (OC)5W[η1-PPh2CH(PPh2)CH2PPh2] (2) in chloroform have been studied by 31P{1H} NMR over the temperature range of 10−55 °C. Conversion of 1 to 2 is exothermic (ΔH = −12.25 ± 0.1 kJ mol-1) and is accompanied by a large decrease in entropy (ΔS = −28.2 ± 0.3 J mol-1 K-1). The reaction proceeds much faster than expected [k = (1.18 ± 0.01) × 10-5 s-1] at 25 °C, and its small activation enthalpy (ΔH⧧ = 92.6 ± 1.9 kJ mol-1) and large negative activation entropy (ΔS⧧ = −28.2 ± 6.2 J mol-1 K-1) suggest an associative mechanism. The reaction is 4 orders of magnitude faster than the isomerization of (OC)5W[η1-PPh2CH2CH2P(p-tol)2] (3) to (OC)5W[η1-P(p-tol)2CH2CH2PPh2] (4). Exchange of coordinated and dangling phosphines is faster than chelation for each of the four complexes, but chelation of 1 or 2 is much faster than chelation of 3 or 4. It appears that the second dangling phosphine arm present in 1 and 2 accel...