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A. The Reaction Intermediates — Solvation-Partitioned Pathways and Intersystem Crossing

A. The Reaction Intermediates — Solvation-Partitioned Pathways and Intersystem Crossing

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Bond Activation Reactions



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Figure 9 UV-Vis spectra of Á5 -CpMn(CO)3 and Á5 -CpRe(CO)3 in neat triethylsilane taken under experimental conditions. The excitation wavelengths are indicated

by arrows. (Adapted from Ref. 45.)



Figure 10 Solvation of the coordinatively unsaturated Á5 -CpRe(CO)2 intermediate by a Et3 SiH molecule partitions the silane Si–H bond activation reaction to

two pathways.



surrounding solvent. The ensuing reaction, however, may be further partitioned by solvation of a solvent molecule such as triethylsilane Et3 SiH,

Et D C2 H5 that has two chemically distinct Si H and C2 H5 sites. Using

the Re complex as an example, Fig. 10 illustrates such a partitioning via

solvation of the singlet Á5 -CpRe(CO)2 by a Et3 SiH molecule. The initial



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solvation results in the final product Á5 -CpRe(CO)2 (H)(SiEt3 or the ethylsolvate Á5 -CpRe(CO)2 Et3 SiH on time scales of ¾4.4 and ¾2.2 ps, respectively. The former 4.4 ps product appearance time is indicative of a very

small free energy barrier, if any, for activation of a Si–H bond. The Á5 CpM(CO)2 Et3 SiH ethyl solvate, for both the Mn and Re complexes,

remains stable on the ultrafast time scale.

Having discussed the likeness in the reaction pattern of the Mn and

Re complexes, we next turn to the differences in their photochemical properties. Figure 11 shows the fs-IR spectra of Á5 -CpMn(CO)2 in Et3 SiH

following 325 nm pump. The 1892 and 1960 cm 1 bands are assigned to

the ethyl-solvate Á5 -CpMn(CO)2 Et3 SiH in the singlet electronic ground

state, where the dicarbonyl intermediate is solvated through the ethyl moiety

of the Et3 SiH (47). The other two bands at 1883 and 2000 cm 1 that

appear in the early-time panels and decay away later are ascribed to the

unsolvated triplet Á5 -CpMn(CO)2 . These assignments are supported by

quantum chemical computations. Energetically, density-functional theory

using the B3LYP exchange correlation functional predicts a triplet Á5 CpMn(CO)2 that is 8.1 kcal/mol more stable than a singlet one. This result

is supported by a multiconfigurational SCF (MCSCF) calculation to the

second-order perturbation (PT2), which also predicts a more stable triplet

species by 9.9 kcal/mol. Furthermore, at the DFT/B3LYP level, the singlet

Á5 -CpMn(CO)2 assumes a different geometry than a triplet one, where the

singlet dicarbonyl exhibits a “bent” configuration (Fig. 12). In fact, the

qualitative correlation in the molecular configuration and spin states for

the 16-electron Á5 -CpML2 complexes has also been suggested by Hofmann

and Padmanabhan from the results of extended Hăuckel calculations (48).

How does the nascent triplet 5 -CpMn(CO)2 interact with a common

hydrocarbon solvent? In general, high-spin unsaturated organometallic

complexes do not interact very well with alkane solvents (49). In a dense

liquid environment, however, the triplet species may undergo a rapid,

concerted intersystem crossing and solvation to become a solvated complex

in the singlet state. The time scale for such a process can be on the order of

hundreds of picoseconds. Figure 13 shows the conversion of the unsolvated

triplet Á5 -CpMn(CO)2 to the singlet alkyl solvate Á5 -CpMn(CO)2 (alkane),

the latter of which remains stable in the ultrafast regime. This process can

be understood by a free-energy scheme depicted in Fig. 14, which mirrors

Marcus’s electron-transfer theory (50). The adiabaticity of the process

depends upon the magnitude of the spin-orbit coupling. An excess internal

energy may expedite the spin crossover/solvation process as demonstrated

by the faster decay of the 295 nm trace (¾90 ps) compared to the 325 nm



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Bond Activation Reactions



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Figure 11 Transient difference spectra in the CO-stretching region for

Á5 -CpMn(CO)3 in neat room-temperature triethylsilane at various time delays

following 325 nm photolysis. Under the experimental conditions, the large cross

section of the solvent Si–H band ¾2100 cm 1 and the parent CO bands (1947

and 2028 cm 1 ) make it difficult to access some regions of the spectrum. The

last panel is an FTIR difference spectrum before and after 308 nm photolysis. A

broad, wavelength-independent background signal from CaF2 windows has been

subtracted. (Adapted from Ref. 45.)



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Figure 12 Computed molecular geometries of Á5 -CpMn(CO)2 in its singlet and

triplet electronic manifolds at the DFT B3LYP/LANL2DZ level of theory. The left

column shows their energy differences calculated using DFT (8.1 kcal/mol) or ab

initio (9.9 kcal/mol) methods. The right column illustrates their interaction with an

alkane solvent molecule.



one (¾105 ps). A greater solvation energy also promotes the intersystem

crossing rate as long as the system configuration is in the Marcus normal

region.

B. The Reaction Barrier — Solvent Molecule Rearrangement



The above discussion demonstrates that the activation barrier for the silane

Si–H bond is relatively small compared to that for an alkane C–H bond.

This is surprising in that one might have expected comparable energy

barriers for activation of both the Si–H and C–H bonds based on the

similar enthalpy of activation H‡ from macroscopic kinetic measurements (41–44). Clearly other mechanisms are at work to make up the

energy barrier in the case of Si–H bond activation. To investigate if more

intermediates are involved, which may provide an explanation for the

reported apparent H‡ values, the reaction is followed extending into the

nano- and microsecond regime. The experiments show that only the previously discussed ethyl-solvate appears on these time-resolved IR spectra. Its

decay correlates very well with the product rise as displayed in Fig. 15.



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Bond Activation Reactions



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Figure 13 Ultrafast kinetics of Á5 -CpMn(CO)3 in neat room-temperature triethylsilane after (a) 295 nm; and (b) 325 nm photolysis. In each panel, the transients

are normalized against that of the singlet Á5 -CpMn(CO)2 Et3 SiH to demonstrate a

reduced singlet-to-triplet ratio when excited at a longer (325 nm) wavelength. The

time constants for the exponential fits are also shown in the plots. (Adapted from

Refs. 45 and 46.)



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