28978-11-2Relevant academic research and scientific papers
Octahedral metal carbonyls. 63. Chelate ring displacement of DTHp from cis-(DTHp)W(CO)4 by Lewis bases (DTHp = 2,2,6,6-tetramethyl-3,5-dithiaheptane, Lewis base = L (phosphine, phosphite))
Dobson, Gerard R.,Cortés, José E.
, p. 3308 - 3314 (2008/10/08)
Reactions of cis-(DTHp)W(CO)4, in which DTHp forms a four-membered chelate ring, with L (DTHp = 2,2,6,6-tetramethyl3,5-dithiaheptane; L = P(OMe)3, P(O-i-Pr)3, P(OPh)3, P(OCH2)3CCH3, P(n-Bu)3) in chlorobenzene (CB) proceed via displacement of DTHp according to cis-(DTHp)W(CO)4 + 2L → cis- and tans-(L)2W(CO)4 + DTHp. Kinetics data in CB indicate the reaction to be biphasic, with cis-(L)(η1-DTHp)W(CO)4 present as a predominant species during the reaction's course; this intermediate has been characterized for L = P(OCH2)3CCH3. In contrast to results observed for related systems containing five-and six-membered chelating rings, evidence here suggests that significant L-W bond making takes place in the transition states leading to chelate ring opening and closing. This conclusion is supported by activation parameters both for chelate ring opening and for chelate ring closure after pulsed laser flash photolysis in 1,2-dichloroethane (DCE) and bromobenzene (BB) solvents (solv), which generates cis-[(solv)(η1-DTHp)W(CO)4]; evidence suggests that this intermediate is not produced thermally. The cis-(L)(η1-DTHp)W(CO)4 products produced via the first phase of the biphasic thermal process undergo unimolecular W-S bond fission. Rate constants for this process vary significantly and are largely influenced by the steric properties of coordinated L, increasing in the order of larger Tolman cone angles. This W-S bond dissociation is followed by solvation to afford cis-[(CB)(L)W(CO)4] intermediates, previously characterized, which then react via rapid reversible desolvation and attack at the five-coordinate, square-pyramidal [(L)W(CO)4] intermediates by L, to afford cis-(L)2W(CO)4. Subsequent nondissociative isomerization of this species forms the equilibrium mixture of cis- and trans-(L)2W(CO)4 products ultimately obtained. These several steps involved in the overall chelate displacement process are discussed in detail.
The Mechanism of Displacement of Diene from 4-diene)(CO)4> (M = Cr, Mo, or W, diene = norbornadiene; M = Mo, diene = cyclo-octa-1,5-diene) and the cis trans Isomerization of Complexes
Dixon, Denis T.,Kola, John C.,Howell, James A. S.
, p. 1307 - 1316 (2007/10/02)
Kinetic studies are reported of the displacement of the diene from 4-diene)(CO)4> by phosphorus ligands to give complexes .With the exception of , all reactions exhibit a bimolecular rate law of the type -d>/dt = Kobs.>, where A = which is interpreted mechanistically as a stepwise ligand-assisted dechelation of the diene; Kobs. may be seen to increase in the order Cr reflects the increasing order of ligand nucleophilicity. cis-trans Isomerization of (L = PBu3, P(OMe)3, or P(OPh)3; M = Cr, Mo, or W) has also been studied kinetically.With the exception of 2>, all isomerizations are intramolecular.Rates of isomerization increase in the order Mo eq (=/) increases in the order W ca.Mo Cr and in the order P(OPh)3 P(OMe)3 PBu3.These results are interpreted in terms of both steric and electronic factors.
