Kinetics of Disproportionation of Tricarbonylbis(phosphine)iron(I) Cation Radicals Probed by Double Potential Step Chronocoulometry

Article abstract of DOI:10.1021/ja00274a031

Rates of substitution of carbon monoxide in a series of iron(I) radical complexes were measured by using double potential step chronocoulometry, a transient electrochemical technique.Carbon monoxide substitution in Fe(CO)3L2⁺ (L = a phosphine) radicals proceeds solely by a second-order process that is first order in both the metal radical and the entering pyridine nucleophile.The rate of substitution depends on the basicity and size of the Lewis base, as seen in the L = PPh3 system, where k₁ varies over 400-fold from 0.27 to 1.01 X 1E2 M^-1s^-1.Hammett analysis of the rate data shows that log k₁ correlates well with the ?-meta and ?-para values for nine 3- and 4-substituted pyridines.Nucleophilic attack of pyridine at Fe(CO)3(PCy3)2⁺ is 1E6 times slower than at Fe(CO)3(PMe3)2⁺, presumably because increased phosphine ligand size inhibits the associative pathway.Activation parameters further support the proposed associative mechanism: for L = PPh3, nucleophile = pyridine, ΔH^<*> = 9.8 +/- 0.3 kcal mol^-1 and ΔS^<*> = -21 +/- 1 cal mol^-1K^-1; for L = PCy3, nucleophile = 3,4-dimethylpyridine, ΔH^<*> = 14 +/- 1.5 kcal mol^-1 and ΔS^<*> = -27 +/- 5 cal mol^-1K^-1.After substitution of carbon monoxide by a nitrogen Lewis base, these complexes disproportionate via an outer-sphere electron-transfer process to yield Fe(II) and Fe(0) products.Comparison of the reactivities of Fe(CO)3(PPh3)2 and its 17-electron analogue, Fe(CO)3(PPh3)2⁺, shows that the cation radical is about 1E9 more reactive toward pyridine than its 18-electron precursor.