PHOTOCHEMICAL SUBSTITUTION-REACTIONS OF [CPFE(CO)2]2 (CP = ETA-5-C5H5) IN HYDROCARBON AND TETRAHYDROFURAN SOLUTION AT ROOM-TEMPERATURE - A MECHANISTIC STUDY WITH TIME-RESOLVED INFRARED-SPECTROSCOPY

Microsecond and nanosecond time-resolved IR spectroscopy (TRIR) have been used to investigate both the kinetics and the nature of the intermediates in the photochemical substitution reactions of [CpFe(CO)2]2 (CP = eta-5-C5H5) with THF and P(OR)3 (R = Me, Et, and (i)Pr) in cyclohexane and n-heptane solutions at 25-degrees-C. An important feature of these experiments has been the use of both UV and visible photolysis wavelengths to distinguish between processes involving photoejection of CO, which principally occurs on UV irradiation, and homolysis of the Fe-Fe bond, which is promoted by both UV and visible light. TRIR signals from the depletion of [CpFe(CO)2]2 with 308-nm photolysis are used to determine the branching ratio (0.9:1) between homolysis of the Fe-Fe bond and photoejection of CO. These data then permit the evaluation of the IR extinction coefficient of the antisymmetric nu(C-O) band of the Fp radical, CpFe(CO)2, and hence the rate constant for dimerization of Fp. High-resolution microsecond TRIR based on a continuously tunable IR diode laser calibrated by FTIR is used to show that, contrary to previous work, there is no significant difference in wavenumber of the nu(C-O) band of the Fp radical between microsecond and picosecond TRIR experiments. UV photolysis of [CpFe(CO)2]2 in the presence of THF provides TRIR evidence for the formation of a previously unknown species, CP2Fe2(CO)3(THF), the formation of which involves neither Fp nor Cp2Fe2(mu-CO)3 and may well occur via a very short-lived and so far undetected precursor to Cp2Fe2(mu-CO)3. TRIR experiments on the reaction of [CpFe(CO)2]2 with P(OMe)3 reveal a very rapid substitution of one CO group in the Fp radical by P(OMe)3 followed by dimerization of CpFe(CO)P(OMe)3 to form [CpFe(CO)P(OMe)3]2. Similar results were obtained with P(OEt)3 and P(OiPr)3, although in these cases the [CpFe(CO)P(OR)3]2 compounds are labile and had not previously been detected. It is suggested that an IR band of an intermediate in these reactions, previously attributed to CpFe(CO)2(mu-CO)CpFe(CO)P(OiPr)3, is in fact due to [CpFe(CO)P(OiPr)3]2. An overall scheme is given for the reactions, in which all of steps can be explained on the basis of three intermediates, CP2Fe2(mu-CO)3, CpFe(CO)2, and CpFe(CO)P(OR)3; rate constants have been evaluated for most of the steps.