Three Isomers in Equilibrium: Phosphine Exchange of Coordinated and Pendant Phosphines in a Unique Complex, (OC)₅WL (L = 1,2-Bis(diphenylphosphino)-1-di-p-tolylphosphinoethane), and Implications for Understanding the k₂ Term in the Rate Law for Phosphine Substitution in Group 6 Metal Carbonyl Complexes

Abstract

The linkage isomers [(OC) 5 W{κ 1 -PPh 2 CH 2 CH(PPh 2 )[P-(p-tol) 2 ]}] 3a, [(OC) 5 W{κ 1 -P(p-tol) 2 CH(PPh 2 )CH 2 PPh 2 }] 3b, and [(OC) 5 W{κ 1 -PPh 2 CH[P(p-tol) 2 ]CH 2 PPh 2 }] 3c isomerize at ambient temperatures to give a mixture of the three isomers in equilibrium.Rates of isomerization and equilibrium constants have been measured at temperatures from 10 to 55 °C, and from these, enthalpies and entropies have been determined. The isomerization reactions are initiated by a nucleophilic attack of a pendant phosphine on a bound carbonyl group followed by dissociation of a coordinated phosphine. These studies lead us to suggest that the k 2 term in the rate law for CO replacement by phosphines is based on a similar mechanism and offers a plausible clarification of the currently accepted understanding. The crystal structure of 3b, the most stable of the three isomers, has been determined.

Synthesis of P,P-bidentate caged phosphines via tungsten pentacarbonyl-promoted cycloaddition reactions of 1-alkyl-1,2-diphospholes

“…It is interesting to note that the isomerization pathway proposed in the literature 32–35 with participation of a carbon atom of one of the carbonyl groups is difficult and apparently unrealizable in our case due to the absence of points corresponding to a transition or some stable or metastable states on this potential energy surface (PES). Extremal points on the energy profile close to a W–C CO distance of 2.5 Å (Fig.…”

“…We decided to study this phenomenon in more detail, drawing upon existing literature concerning the migration of the W(CO) 5 group between the two phosphorus atoms. 30–33…”

“…We decided to study this phenomenon in more detail, drawing upon existing literature concerning the migration of the W(CO) 5 group between the two phosphorus atoms. [30][31][32][33] 1-Isopropyl-1,2-diphosphole (1b) reacts with in situ generated [W(CO) 5 (THF)] in THF at 0 1C to give complex 2b. Despite the steric hindrance posed by the isopropyl group and the W(CO) 5 fragment at the two phosphorus atoms, 2b displays significantly broadened signals in the 31 P{ 1 H} NMR spectrum at room temperature, indicating a dynamic process on the NMR time scale (Scheme 2).…”

“…According to the literature, [32][33][34][35] the 1,2-shift of a W(CO) 5 fragment can be initiated by a nucleophilic attack of the uncoordinated phosphorus atom on a nearby carbonyl carbon atom, thereby increasing the electron density at the tungsten atom. The molecular structure of complex 2b shows that the shortest distance of the s 3 l 3 phosphorus atom P 1 to the carbon atom of a carbonyl group is 3.721 Å, which exceeds the sum of the van der Waals radii (3.5 Å).…”

“…It is interesting to note that the isomerization pathway proposed in the literature [32][33][34][35] with participation of a carbon atom of one of the carbonyl groups is difficult and apparently Scheme 2 Migration of the W(CO) 5 group between the two phosphorus atoms in 1-alkyl-1,2-diphospholes.…”