TY - JOUR
T1 - Photochemical substitution Reactions of [CpFe(CO)2]2 (Cp = η5-C5H5) in Hydrocarbon and Tetrahydrofuran Solution at Room Temperature
T2 - A Mechanistic Study with Time-Resolved Infrared Spectroscopy
AU - Dixon, Andrew J.
AU - George, Michael W.
AU - Hughes, Catherine
AU - Poliakoff, Martyn
AU - Turner, James J.
N1 - Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
PY - 1992/2/1
Y1 - 1992/2/1
N2 - 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 = η5-C5H5) with THF and P(OR)3 (R = Me, Et, and iPr) in cyclohexane and n-heptane solutions at 25 °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 v(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 v(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(μ-CO)3 and may well occur via a very short-lived and so far undetected precursor to Cp2Fe2(μ-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(μ-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(μ-CO)3, CpFe(CO)2, and CpFe(CO)P(OR)3; rate constants have been evaluated for most of the steps.
AB - 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 = η5-C5H5) with THF and P(OR)3 (R = Me, Et, and iPr) in cyclohexane and n-heptane solutions at 25 °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 v(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 v(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(μ-CO)3 and may well occur via a very short-lived and so far undetected precursor to Cp2Fe2(μ-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(μ-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(μ-CO)3, CpFe(CO)2, and CpFe(CO)P(OR)3; rate constants have been evaluated for most of the steps.
UR - http://www.scopus.com/inward/record.url?scp=0000419112&partnerID=8YFLogxK
U2 - 10.1021/ja00031a027
DO - 10.1021/ja00031a027
M3 - Article
AN - SCOPUS:0000419112
SN - 0002-7863
VL - 114
SP - 1719
EP - 1729
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 5
ER -