An unusual solvent isotope effect in the reaction of W(CO)(5)(solv) (solv = cyclohexane or cyclohexane-d(12)) with THF

Citation
R. Paur-afshari et al., An unusual solvent isotope effect in the reaction of W(CO)(5)(solv) (solv = cyclohexane or cyclohexane-d(12)) with THF, ORGANOMETAL, 19(9), 2000, pp. 1682-1691
Citations number
74
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Organic Chemistry/Polymer Science
Journal title
ORGANOMETALLICS
ISSN journal
0276-7333 → ACNP
Volume
19
Issue
9
Year of publication
2000
Pages
1682 - 1691
Database
ISI
SICI code
0276-7333(20000501)19:9<1682:AUSIEI>2.0.ZU;2-O
Abstract
Time- and temperature-resolved infrared absorption spectroscopy is used to probe the reaction of THF with W(CO)(5)(solv) (solv cyclohexane, cyclohexan e-d(12)) to form W(CO)(5)-(THF). In cyclohexane-d(0), Delta H double dagger for the reaction is determined to be 3.6 +/- 0.2 kcal mol(-1) (15.2 +/- 0. 9 kJ mol(-1)) and Delta S double dagger = -13.7 +/- 2.5 eu (-57.3 +/- 10.5 J mol(-1) K-1). In cyclohexane-d(12), Delta H double dagger = 2.4 +/- 0.6 k cal mol(-1) (10.2 +/- 2.5 kJ mol(-1)) and Delta S double dagger = -18.3 +/- 3.5 eu (-76.6 +/- 14.6 J mol(-1) K-1). The low activation enthalpy in comp arison to the (CO)(5)W-cyclohexane bond dissociation energy, the negative e ntropy of activation, and comparison with other spectroscopic experiments i nvolving reactions of W(CO)(5)(cyclohexane) indicate that; the reaction pro ceeds by an associative interchange reaction mechanism. An unusual isotope effect, in which reaction proceeds more slowly in cyclohexane-d(12) even th ough Delta H double dagger is lower, is explained in terms of relative popu lations of low-frequency vibrations and the relative C-H and C-D bond stren gths in the solvent molecule released upon uptake of THF. These experimenta l results are supported by the results of an ab initio DFT calculation of t he structure and vibrational spectrum of W(CO)(5)(cyclohexane).