SYNTHESIS AND CHARACTERIZATION OF HIGHLY FLUORINATED TUNGSTEN(II) METALLACYCLOPROPENE COMPLEXES - KINETICS AND MECHANISM OF AN UNPRECEDENTED ETA(2)-VINYL ISOMERIZATION

This report describes a novel migratory insertion reaction which leads to the formation of highly fluorinated eta(2)-vinyl complexes at a tu ngsten(II) metal center. Insight into the nature of this transformatio n was obtained by isolation and crystallographic characterization of a kinetic intermediate in the insertion process. Ambient-temperature ph otolysis of a toluene solution of the tungsten(II) tetrafluoroaryl met allacycle 2 and perfluoro-2-butyne accesses the kinetic eta(2)-vinyl c omplex 3 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. The solid-state structure of 3 has been determined. Upon heating, 3 is converted to the thermodynamic eta(2)-v inyl product 4 in which the fluoride ligands is now cis to the inserte d alkyne and trans to one CO and cis to the second CO ligand. Detailed kinetic studies are reported for this isomerization reaction using bo th infrared spectroscopy and UV-visible spectroscopy. In the absence o f added Ligand, this transformation displays first-order disappearance of 3 with Delta H-double dagger = 124(+/-2) kJ/mol, Delta S-double da gger = +48(+/-5) J/(mol . K) and Delta V-double dagger = +15.6(+/-0.4) cm(3)/mol. This is in accord with a limiting dissociative mechanism. Carbon monoxide enhances the rate of isomerization and shows first-ord er dependence in p(CO) with Delta H-double dagger = 98(+/-3) kJ/mol, D elta S-double dagger = -5(+/-9) J/(mol . K), and Delta V-double dagger = +3.9-(+/-0.2) cm(3)/mol. The P(OMe)(3)-promoted isomerization displ ays saturation behavior with Delta H-double dagger = 77(+/-2) kJ/mol, Delta S-double dagger = -46(+/-6) J(mol . K), and Delta V-double dagge r = +5.5(+/-0.4) cm(3)/mol at 2.2 M P(OMe)(3). The negative entropy an d positive volume of activation indicate that congestion is developed in the transition state. The kinetic data for the ligand-promoted tran sformations are in agreement with a mechanism involving a rapid pre-eq uilibrium (K) during which the adduct species [3 . P(OMe)(3)] is forme d, followed by a rate-determining step (h) to produce 5. For this proc ess, k(obs) = Kb[P(OMe)(3)]/(1 + k[P(OMe)(3)]). Visible spectroscopic evidence for the adduct species [3 . P(OMe)(3)] has been obtained. Non linear least-squares analysis of the k(obs) versus [P(OMe)(3)] plot de termined a K = 1.4 +/- 0.1 M(-1) and a k = (89 +/- 13) x 10(-5) s(-1), such that kK = 1.2 x 10(-3) M(-1) s(-1) for the adduct formation. It is unlikely that an actual 20-electron species is generated since the eta(2)-vinyl ligand can conceivably change its hapticity and function in an eta(1) mode.