Olefin and CO Competition Equilibria for Bis(cyclopentadienyl)vanadium(II)

• Published in: Organometallics 2004-03, Vol.23 (7), p.1519-1524
• Main Authors: Calderazzo, Fausto, Guelfi, Massimo, Pampaloni, Guido
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om034361+

The substitution reactions of VCp2(CO) with some selected olefins have been studied. Tetracyanoethylene (tcne) and fumaronitrile (fn), containing electron-withdrawing substituents, promptly react with VCp2(CO) with quantitative formation of VCp2(tcne) and VCp2(fn), respectively, while acrylonitrile and diethyl fumarate (defu) gave partial displacement of coordinated carbon monoxide under a CO atmosphere. No CO displacement from VCp2(CO) was observed with cyclooctene and norbornene. The carbonylation of VCp2(defu) (VCp2(defu) + CO ⇄ VCp2(CO) + defu) has been studied gas volumetrically from both sides, and the equilibrium constant for the displacement of coordinated defu by carbon monoxide was estimated to be 55 ± 8 at 298.5 K. Variable-temperature experiments between 298.5 and 327.7 K have shown the enthalpy change to be −2.4 kcal mol-1. The equilibrium constant for the addition of defu to VCp2 (VCp2 + defu ⇄ Cp2(defu)) has been evaluated spectroscopically to be 150 ± 11 at 298.5 K. From the available data, the equilibrium constant for the addition of carbon monoxide to VCp2 (VCp2 + CO ⇄ VCp2(CO)) has been calculated to be K a = (8.2 ± 0.4) × 103:  i.e., about 2 orders of magnitude higher than that for the complexation of defu. The present data for the 3d3 system of vanadium(II) (ν̃CO in toluene 1881 cm-1) are compared with those exhibited by the 5d10 system of AuCl(CO) (ν̃CO in toluene 2153 cm-1), whose behavior consistently shows an opposite dependence, as far as the olefin/CO competition is concerned, on the nature of the olefin.