Effect of Temperature and Pressure on the Addition of 4- Tertiarybutylpyridine to Tricarbonyl (1-5-n-Dienyl) Iron (II) Complexes (Dienyl = C6H7, 2-MeoC6H6 or C7H9)

A detailed kinetic study of the addition of 4-tertiarybutylpyridine to the coordinated dienyl ligand in tricarbonyl (1-5-ɳ-dienyl) iron (II) complexes (1; dienyl = C6H7, 2-MeoC6H6 and C7H9) has been made as a function of temperatureand pressure in acetonitrile using stopped-flow techniques. The reactions proceed to completion under pseudo-first-order conditions, i.e. kobs = k1 [4-Me3CPy]. The observed rate sequence C6H7 > 2-Meo C6H6 > C7H9 (e.g. 18:3:1 at20oC and 0.1 MPa) and the low △ H≠ and large negative △ S≠ and △ V≠ values support direct addition (k1) to thedienyl fragments of complexes (1). The results contrast sharply with the reversible additions of 4-Cyanopyridine, 4-formyldypridine, 2-benzylpyridine and 3-iodopyridine to complexes (1) where kobs was not influenced by pressurechanges up to 150 MPa, indicating a zero, volume of activation and the consequent operation of the Odiaka’s “orderedtransition state” mechanism.