Studies of intermediates in photoreactions of Cp2Fe2(CO)4 with CO and phosphorus ligands

Article abstract of DOI:10.1021/ja00058a024

A combination of flash photolysis with UV-visible and IR detection has been used to study the intermediates in the photochemical reactions of Cp₂Fe₂(CO)₄. The reaction kinetics of the intermediates have been investigated using conventional and laser flash photolysis at room temperature. Flash photolyses at low temperatures in hydrocarbon solutions under Ar or CO or in the presence of a phosphorus ligand were carried out for spectral characterizations of the unstable species. A CO-loss intermediate, assigned to the structure CpFe(μ-CO)₂(μ-η¹,η²-CO)FeCp, contains a semibridging CO group. This species is formed in relatively small amounts but is more stable than its structural isomer CpFe(μ-CO)₃FeCp, a well-established intermediate. A second new intermediate, assigned to (η⁵-Cp)(CO)₂Fe-Fe(CO)₃(η ³-Cp), decays (k = 5 × 10³ s^-1 at 22°C in hexane) to form a third species, assignable to (η⁵-Cp)(CO)Fe(μ-η¹η²-CO) Fe(CO)₂(η³-Cp), which in turn decays (k = 62.4 s^-1 at 25°C in hexane) to form Cp₂Fe₂(CO)₄. The second and third intermediates are formed more extensively under CO. These results provide new evidence for certain previously unexplained features observed on this system. Experiments involving the use of ¹³CO indicate that formation of the formally 19-electron radical Cp(CO)₃Fe from the 17-electron radical Cp(CO)₂Fe and CO and the subsequent reaction between the two radicals is a major pathway for formation of (η⁵-Cp)(CO)₂Fe-Fe(CO)₃(η ³-Cp) and thus (η⁵-Cp)(CO)Fe(μ-η¹,η ²-CO)Fe(CO)₂(η³-Cp). Evidence of formation of the two dimer species, even in photolysis under Ar, implies that photoinduced unsymmetrical cleavage of CO bridges in Cp(CO)Fe(μ-CO)₂Fe(CO)Cp to afford Cp(CO)₃Fe and Cp(CO)Fe also occurs, in addition to the well-established photoinduced symmetrical cleavage to give two Cp(CO)₂Fe. At low temperature, flash photolysis of Cp₂Fe₂(CO)₄ in the presence of a phosphorus ligand (L = P(i-Pr)₃, P(O-i-Pr)₃, P(OMe)₃, P(C₆F₅)₃) affords evidence of new transient species. The experimental results also support the existence of short-lived precursors to CpFe(μ-CO)₃FeCp; the latter has previously been considered to be a primary photoproduct via loss of a CO group from Cp₂Fe₂(CO)₄.