Substitution of CO Ligand by P(OPh)3 in Radical Cations of the Cymantrene Family: Relationships of Substitution Rates to E1/2 Values and Carbonyl IR Frequencies

Abstract

A series of piano-stool complexes of the cymantrene family (cymantrene = Mn(η5-C5H5)(CO)3, 1) undergoes facile replacement of a carbonyl ligand by P(OPh)3 when oxidized by one-electron in CH2Cl2/[NBu4][B(C6F5)4]. Data on the previously characterized complexes 1, Mn(η5-C5H4NH2)(CO)3 (3) and Mn(η5-C5Me5)(CO)3 (6) have been supplemented by cyclic voltammetry (CV) and IR spectroscopy on Mn(η5-C5H4Me)(CO)3 (2), Mn(η5-C5H4I)(CO)3 (4), and Mn(η5-C5H4C(O)H)(CO)3 (5). The substitution rates, determined by digital simulations of CV scans, ranged from 4 M−1 s−1 for 6+ to 3 × 105 M−1 s−1 for 5+. In general, a more strongly donating cyclopentadienyl substituent slows down the CO substitution rate. For mono-cyclopentadienyl substituted complexes, the logarithm of ksub is shown to increase linearly with either the weighted average of the CO stretching frequencies or the E1/2 value of the redox process. An exception to this generalization is the amine-substituted complex 3, for which the CO-substitution rate is higher than predicted by its E1/2 potential. The substitution rate of the pentamethylated Cp complex 6+ is slowed by about an order of magnitude owing to steric effects. The efficacy of this method to predict the CO-substitution rate of a cymantrene-tagged molecule was tested with a cymamtrene-derivatized diarylethene complex, 7. The measured P(OPh)3-for-CO substitution rate of 3.7 × 102 M−1 s−1 for 7+ was very close to that predicted by the E1/2 value of 7. A ligand electronic parameter, EL, of 0.62 was determined for the triphenylphosphite ligand. These studies build on the previous CO substitution-rate analyses by Sweigart and others.