Degree Name

Master of Science (MS)

Semester of Degree Completion

1997

Thesis Director

Richard L. Keiter

Abstract

The kinetics and thermodynamics of isomerization of (OC)5CrPPh2CH2CH(PPh2)2 to its linkage isomer, (OC)5CrPPh2CH(PPh2)CH2PPh2, in chloroform-d have been studied with 31P{1H} NMR.

(OC)5Cr[η1-PPh2CH2CH(PPh2)2] ⇄ (OC)5Cr[η1-PPh2CH(PPh2)CH2PPh2]

The equilibrium constants for the reaction are 3.60, 2.61, 2.04, and 1.67 at 10 °C, 25 °C, 40 °C, and 53 °C, respectively. The forward reaction becomes more favorable as the temperature is decreased. The values of ΔH, ΔS, and ΔG(25 °C) were determined to be -13.6 kJ mol-1, -37.6 J mol-1K-1, and -2.4 kJ mol-1, respectively. The large decrease in entropy favors the reverse reaction while the decrease in enthalpy favors the forward reaction. Previous work has shown that for the analogous tungsten isomerization, values of ΔH, ΔS, and ΔG(25 °C) are -12.2 kJ mol-1, -28 J mol-1K-1, and -3.9 kJ mol-1, respectively.51 It can be concluded that the greater decrease in entropy for the chromium reaction accounts for its overall diminished favorableness as compared to the tungsten reaction.

Rate constants for the forward reaction in chloroform at 10 °C, 25 °C, 40 °C are 2.0 x 10-7 s-1, 2.1 x 10-6 s-1, and 1.7 x 10-5 s-1 with half-lives to equilibrium of 31 days, 3 days, and 8 hours, respectively. These reactions are about an order of magnitude slower than the analogous tungsten reaction, but about four orders of magnitude faster than isomerization of (OC)5CrPPh2CH2CH2P(tol)2. 55c The enthalpy of activation, ΔH≠, for the forward and reverse reactions are 105 kJ mol-1 and 120 kJ mol-1, respectively, larger by 12 kJ mol-1 and 15 kJ mol-1 than observed for tungsten. The entropy of activation, ΔS≠, for the forward and reverse reactions were found to be 1.4 J mol-1K-1 and 40 J mol-1K-1, respectively. These values are considerably more positive than those obtained previously for tungsten (-28 J mol-1K-1 and -1.0 J mol-1K-1).

It is concluded that abnormally fast isomerization rates for (OC)5MPPh2CH2CH(PPh2)2 (M = Cr, W) result because the short phosphine arm interacts with the equatorial carbonyl groups in the transition state, lowering the activation energy, and leading to labilization of the coordinated phosphorus atom which results in its replacement by the second phosphine arm. The concept of interaction between the short phosphine arm and the equatorial carbonyl groups is supported by long-range phosphorus-carbon coupling (4JPC), believed to augmented by a through-space mechanism. The entropies of activation suggest that phosphorus exchange in chromium has a much more significant dissociative component than for the analogous tungsten system. It would be expected that the smaller chromium atom would be less likely to form a stable 7-coordinate complex because of steric crowding.

Complexes, (OC)5WPPh2C(PPh2)=CH2 and [(OC)5WPPh2]2C=CH2 have been synthesized for the first time. The crystal structure of the former compound has been determined. Unlike the similar (OC)5WPPh2CH2PPh2 complex, the dangling phosphorus atom is not directed toward the equatorial carbonyl groups and no long-range phosphorus-carbon coupling (4JPC) is observed.

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