Different Mechanisms of Photochemical Re-Me and Re-Et Bond Homolysis in [Re(R)(CO)3(4,4'-dimethyl-2,2'-bipyridine)]. A Time-Resolved IR Spectroscopic Study Ranging from Picoseconds to Micro
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- 作者：Anders Gabrielsson ; Ana Marí ; a Blanco-Rodrí ; guez ; Pavel Matousek ; Mike Towrie ; and Antoní ; n Vl
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- 刊名：Organometallics
- 出版年：2006
- 出版时间：April 24, 2006
- 年：2006
- 卷：25
- 期：9
- 页码：2148 - 2156
- 全文大小：323K
- 年卷期：v.25,no.9(April 24, 2006)
- ISSN：1520-6041
文摘
The photochemistry of two metal-alkyl complexes, [Re(R)(CO)3(dmb)] (R = methyl (Me), ethyl (Et)),was investigated by IR spectroscopy in the 
(CO) spectral region, time-resolved over an exceptionallybroad temporal range, from picoseconds to microseconds. Optical excitation of [Re(Et)(CO)3(dmb)] inMeCN produces within the first two picoseconds the radicals [Re(MeCN)(CO)3(dmb)]
and Et, togetherwith an excited state, which undergoes a slower (~90 ps) conversion to the same radicals. The reactiveexcited state was identified as 3MLCT (metal-to-ligand charge transfer) with an admixture of a 3SBLCT(sigma bond-to-ligand charge transfer) character. In CH2Cl2 solution, this excited state reacts with thesolvent molecules to produce the radical anion [Re(Cl)(CO)3(dmb)]- with ~130 ps kinetics. A series ofslower reactions follows, forming [Re(CH2Cl2)(CO)3(dmb)] and, ultimately, [Re(Cl)(CO)3(dmb)]. Thephotochemical mechanism of [Re(Me)(CO)3(dmb)] is different. Irradiation populates a 3MLCT excitedstate, which undergoes two parallel reactions: a thermally activated Re-Me bond homolysis and decayto the ground state. At room temperature, both reactions have the same time constant, ~34 ns, giving thephotochemical quantum yield of about 0.5 and 3MLCT excited-state lifetime of 17 ns. The same reactionmechanism operates in CH2Cl2 and MeCN. The photoreactivity and excited-state behavior of bothcomplexes are interpreted using qualitative potential energy surfaces. The striking difference betweenthe ethyl and methyl complex is caused by different relative energies of the optically excited 1MLCTstate, the dissociative 3SBLCT state, and the 3MLCT state along the Re-R coordinate.
(CO) spectral region, time-resolved over an exceptionallybroad temporal range, from picoseconds to microseconds. Optical excitation of [Re(Et)(CO)3(dmb)] inMeCN produces within the first two picoseconds the radicals [Re(MeCN)(CO)3(dmb)] and Et, togetherwith an excited state, which undergoes a slower (~90 ps) conversion to the same radicals. The reactiveexcited state was identified as 3MLCT (metal-to-ligand charge transfer) with an admixture of a 3SBLCT(sigma bond-to-ligand charge transfer) character. In CH2Cl2 solution, this excited state reacts with thesolvent molecules to produce the radical anion [Re(Cl)(CO)3(dmb)]- with ~130 ps kinetics. A series ofslower reactions follows, forming [Re(CH2Cl2)(CO)3(dmb)] and, ultimately, [Re(Cl)(CO)3(dmb)]. Thephotochemical mechanism of [Re(Me)(CO)3(dmb)] is different. Irradiation populates a 3MLCT excitedstate, which undergoes two parallel reactions: a thermally activated Re-Me bond homolysis and decayto the ground state. At room temperature, both reactions have the same time constant, ~34 ns, giving thephotochemical quantum yield of about 0.5 and 3MLCT excited-state lifetime of 17 ns. The same reactionmechanism operates in CH2Cl2 and MeCN. The photoreactivity and excited-state behavior of bothcomplexes are interpreted using qualitative potential energy surfaces. The striking difference betweenthe ethyl and methyl complex is caused by different relative energies of the optically excited 1MLCTstate, the dissociative 3SBLCT state, and the 3MLCT state along the Re-R coordinate.