Kinetics and mechanism of addition of tertiary phosphines and phosphites to the dicarbonyl(η5-cyclopentadienyl)-(η-ethene)iron cation

Abstract

Kinetic studies of the addition of a range of tertiary phosphine and phosphite nucleophiles PR3 to the cation [Fe(cp)(CO)2(η-C2H4)]+1(cp =η5-C5H5)[equation (i)] revealed the general rate law, Rate =k1[Fe][PR3]. The second-order rate constants k1 decrease markedly down the order P(C6H4OMe-2)3 > PBun3 > P(C6H4OMe-4)3 > P(C6H4Me-4)3 > P(C6H4Me-4)Ph2 > PPh3 > P(C2H4CN-2)Ph2 > P(C2H4CN-2)3 > P(C6H4Cl-4)3 > P(OBun)3. This reactivity order parallels that of decreasing electron availability at the phosphorus centre, as shown quantitatively by the good correlation between log k1 and the Tolman Σχ values. An excellent fit to the Hammett and Bronsted equations is also observed for reaction (i) with the nucleophiles P(C6H4X-4)3. The moderate Bronsted slope α of [Fe(cp)(CO)2(η-C2H4)]++ PR3→[Fe(cp)(CO)2(C2H4PR3)]+(i) 0.46 establishes the importance of phosphine basicity in determining nucleophilicity towards the ethene ligand in cation 1. These results, together with the large negative entropy of activation with PPh3(ΔS‡1=–103 J K–1 mol–1), are interpreted in terms of direct addition (k1) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive charge on the phosphorus centre and considerable phosphorus–carbon bond formation.