Femtosecond Infrared Studies of Chemical Bond Activation (original) (raw)
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Journal of the American Chemical Society, 1998
The Si-H bond activation reactions by group VIIB, d 6 organometallic compounds η 5-CpM(CO) 3 (M) Mn, Re; Cp) C 5 H 5) were studied in neat triethylsilane under ambient conditions. Utilizing femtosecond and nanosecond pump-probe spectroscopic methods, the spectral evolution of the CO stretching bands was monitored from 300 fs to tens of microseconds following UV photolysis. The reactive intermediates observed on the ultrafast time scale were also studied using ab initio quantum chemical modeling. It was found that photolysis of the manganese tricarbonyl resulted in dicarbonyls in their singlet or triplet electronic states, whereas photolysis of the rhenium complex led only to the singlet dicarbonyl. The branching ratio of the two manganese intermediates was measured and was related to different electronic excited states. For both the Mn and Re complexes, the reactions were found to be divided into two pathways of distinct time scales by the initial solvation of the dicarbonyls through the Si-H bond or an ethyl group of the solvent molecule. The time scale for the Si-H bond-breaking process was, for the first time, experimentally derived to be 4.4 ps, compared to 230 ns for breaking an alkane C-H bond. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the reaction mechanisms for these reactions.
Journal of Physical Chemistry A, 1999
The photochemical Si-H bond activation reaction by the group VB, d 4 organometallic compound η 5-CpV-(CO) 4 (Cp) C 5 H 5) has been studied in neat triethylsilane under ambient conditions. The spectral evolutions of the metal-bonded CO stretching bands were monitored from 300 fs to 800 ps following UV photolysis using femtosecond pump-probe spectroscopic methods. The reactive intermediates observed on the ultrafast time scale were also studied using density functional theory as well as ab initio quantum chemical modeling. It was found that photolysis of vanadium tetracarbonyl resulted in the formation of tricarbonyls in either the singlet or triplet electronic state following CO loss. The subsequent reaction was partitioned into two pathways by the initial solvation of the tricarbonyls through the Si-H bond or an ethyl group of the solvent molecule. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the chemical reactivity.
Photochemical mechanisms in intermolecular C-H bond activation reactions of organometallic complexes
Coordination Chemistry Reviews, 1994
This article describes observations obtained from detailed investigations of the photochemistry of (rlX&)Rh(CO), (R = H, Me) and (HBPz*&Rh(CO), (Pz* = 3,5dimethylpyrazolyl) complexes in hydrocarbon solutions. Photochemical quantum efficiency data have been recorded for ligand photosubstitution and intermolecular Si-H/C-H bond activation processes following excitation at various wavelengths in the visible and near-UV regions. The results obtained, taken in conjunction with prior photochemical measurements (viz. matrix isolation, flash photolysis and low temperature studies in liquefied noble gases) lead to a proposed mechanistic scheme that explains both the ligand photosubstitution and C-H/Si-H bond activation reactions.
The Journal of Physical Chemistry A, 1999
The photochemical Si-H bond activation reaction by the group VB, d 4 organometallic compound η 5 -CpV-(CO) 4 (Cp ) C 5 H 5 ) has been studied in neat triethylsilane under ambient conditions. The spectral evolutions of the metal-bonded CO stretching bands were monitored from 300 fs to 800 ps following UV photolysis using femtosecond pump-probe spectroscopic methods. The reactive intermediates observed on the ultrafast time scale were also studied using density functional theory as well as ab initio quantum chemical modeling. It was found that photolysis of vanadium tetracarbonyl resulted in the formation of tricarbonyls in either the singlet or triplet electronic state following CO loss. The subsequent reaction was partitioned into two pathways by the initial solvation of the tricarbonyls through the Si-H bond or an ethyl group of the solvent molecule. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the chemical reactivity.
Femtosecond Infrared Study of the Dynamics of Solvation and Solvent Caging
Journal of the American Chemical Society, 2001
The ultrafast reaction dynamics following 295-nm photodissociation of Re 2 (CO) 10 were studied experimentally with 300-fs time resolution in the reactive, strongly coordinating CCl 4 solution and in the inert, weakly coordinating hexane solution. Density-functional theoretical (DFT) and ab initio calculations were used to further characterize the transient intermediates seen in the experiments. It was found that the quantum yield of the Re-Re bond dissociation is governed by geminate recombination on two time scales in CCl 4 , ∼50 and ∼500 ps. The recombination dynamics are discussed in terms of solvent caging in which the geminate Re(CO) 5 pair has a low probability to escape the first solvent shell in the first few picoseconds after femtosecond photolysis. The other photofragmentation channel resulted in the equatorially solvated dirhenium nonacarbonyl eq-Re 2 (CO) 9 (solvent). Theoretical calculations indicated that a structural reorganization energy cost on the order of 6-7 kcal/mol might be required for the unsolvated nonacarbonyl to coordinate to a solvent molecule. These results suggest that for Re(CO) 5 the solvent can be treated as a viscous continuum, whereas for the Re 2 (CO) 9 the solvent is best described in molecular terms.
Triplet Organometallic Reactivity under Ambient Conditions: An Ultrafast UV Pump/IR Probe Study
Journal of the American Chemical Society, 2001
The reactivity of triplet 16-electron organometallic species has been studied in room-temperature solution using femtosecond UV pump IR probe spectroscopy. Specifically, the Si-H bond-activation reaction of photogenerated triplet Fe(CO) 4 and triplet CpCo(CO) with triethylsilane has been characterized and compared to the known singlet species CpRh(CO). The intermediates observed were studied using density functional theory (DFT) as well as ab initio quantum chemical calculations. The triplet organometallics have a greater overall reactivity than singlet species due to a change in the Si-H activation mechanism, which is due to the fact that triplet intermediates coordinate weakly at best with the ethyl groups of triethylsilane. Consequently, the triplet species do not become trapped in alkyl-solvated intermediate states. The experimental results are compared to the theoretical calculations, which qualitatively reproduce the trends in the data.
Analysis and control of ultrafast photodissociation processes in organometallic molecules
European Physical Journal D, 2001
In this paper we characterize the ultrafast fragmentation in electronically excited Fe(CO)2(NO)2 and CpMn(CO)3 by means of femtosecond time-resolved spectroscopy combined with mass spectrometry. From the transient two-color multi-photon ionization data, it was possible to record the transients of the parent molecule ions and their photofragment ions. The experimentally observed decay times indicated an ultrafast loss of the first ligands (sub-100 fs decay times). Further we performed a feedback control experiment on the photofragmenting CpMn(CO)3 molecular system in order to maximize the yield of desired ionic products through pulse modulation. The shape of the pulses obtained from optimization reflect well the intrinsic molecular dynamics during photofragmentation and the change of the CpMn(CO)+/CpMn(CO)3+ ratio shows a clear evidence for the capability of the optimization method to find tailor-made system-specific pulses.
Journal of Organometallic Chemistry, 2000
The ultrafast dynamics of the Si H bond activation reaction by the Group 6 d 6 organometallic compounds M(CO) 5 (M=Cr, Mo, and W) have been studied in neat tri-substituted silanes under ambient conditions. The ultrafast spectral evolutions of the CO stretching bands were monitored following UV photolysis using femtosecond pump-probe spectroscopic methods. It was found that the coordinatively unsaturated species, which is formed following CO photolysis from the parent molecule, is quickly solvated (B 2 ps) via the C H bonds of the solvent. These species then rearranged to the silyl hydride product on a timescale of a few nanoseconds. These results were augmented by rearrangement studies in neat ethanol, propanol and hexanol solutions in which the initially formed metal C H complex rearranged to the metal hydroxyl complex. The mechanism of this rearrangement was discussed by comparison of the data with various models in the literature. It was found that a mechanism that is primarily dissociative in nature provided the best description of the experimental data.
Journal of the American Chemical Society, 1996
Using picosecond transient absorption spectroscopy we have examined the dynamics of Cp*Ir(CO) 2 (Cp*) C 5 Me 5) in cyclohexane and Cp*Rh(CO) 2 in n-pentane solution at room temperature following 295-nm UV excitation. A transient absorption with an instrument limited risetime was observed for Ir from 440 to 740 nm and for Rh from 500 to 650 nm. Each transient can be well fit to a biexponential decay consisting of a fast component of 2-3 ps and a slower component of 30-40 ps. These transients are attributed to excited state absorptions. Taking into account independent femtosecond IR studies of the ground state recovery and our data, we suggest that most excited state molecules relax through nondissociative excited states, decaying to the ground state without the loss of CO. These results offer an explanation for the low C-H bond activation quantum yields observed on preparative irradiation of Cp*Ir(CO) 2 and Cp*Rh(CO) 2 .