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C. The Reaction Mechanism — Resolving a Convolved Chemical Reaction

C. The Reaction Mechanism — Resolving a Convolved Chemical Reaction

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100



Yang and Harris



Figure 16 A proposed reaction mechanism for the silane Si–H bond activation

by the Á5 -CpM(CO)3 (M D Mn, Re) covering the ultrafast dynamics to nanosecond

kinetics.



allows for a direct experimental assessment of the Si–H bond-breaking

step. The ¾4.4 ps time scale of that step is indicative of a very small

energy barrier, thus providing experimental support for a previous theoretical prediction put forth by Musaev and Morokuma (8). The marked

difference in the bond-breaking barriers for the isoelectronic C–H and

Si–H bond activation reflects their distinct bonding characteristics. The

d orbitals on the Si atom make it easier for the Si–H bond to interact

with the transition metal center both energetically and spatially.

Figure 16 also demonstrates the complexity of a chemical reaction

in the liquid phase. A photochemical reaction such as those discussed in

this section can easily span several orders of magnitude in time from its

initiation to completion, as results of the intricate dynamical processes in



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Bond Activation Reactions



101



solution. Following photoexcitation, the reaction may begin with a dynamical partitioning in the dissociative excited state that, on the ¾100 fs time

scale, leads to reactive intermediates such as Á5 -CpMn(CO)2 in either the

singlet or triplet spin states. The nascent singlet Á5 -CpM(CO)2 may interact

either with the chemically inert ethyl site of Et3 SiH to form the ethyl

solvated Á5 -CpM(CO)2 Et3 SiH or with the reactive Si–H bond to form

the final product. Under the dynamical influence of the solvent bath, the

weakly coupled ethyl solvate together with the surrounding solvent shell

may undergo various reorganization until the metal center encounters a

reactive Si–H bond to complete the reaction. The time scale of such a

procession, ranging from hundreds of picoseconds to a few microseconds, is

expected to depend upon the specific metal-alkane interactions, the number

of active sites in a solvent molecule, and steric interactions. Therefore,

the macroscopic reaction rate is determined by the rearrangement process,

which sets the time scales for the two solvation-partitioned product formation pathways three decades apart.

Another important aspect brought to light by this study is the

realization of the dynamics of a high-spin, 16-electron transition metal

center in a two-electron oxidative-addition reaction. As noted earlier,

transition metal–mediated reactions normally occur at an unsaturated metal

center that may potentially exist in more than one spin state. Conventional

thinking advises that if such a reaction begins with an S D 0 metal center,

for example, the system will follow a reaction coordinate in the same

electron-spin manifold. This thinking, together with the prevailing postulate

that most organometallic reactions can be understood by invoking 16- or

18-electron intermediates or transition states (14), has been influential in

describing a reaction mechanism. Not until recently was the importance

of spin-state changes in the reactivity of unsaturated transition metal

complexes recognized (53,54). For instance, a recent study by Bengali,

et al. shows that the photogenerated Á5 -CpCo(CO) and Á5 -CpŁ Co(CO) do

not form stable adducts with alkanes or rare gas atoms (Xe, Kr) but react

readily with CO molecules at a diffusion-limited rate (55). Utilizing both

DFT and ab initio computational methods, Siegbahn later attributed these

observations to an Á5 -CpCo(CO) species in the ground triplet manifold.

The triplet Á5 -CpCo(CO) then undergoes a rapid spin-flip to form the

singlet Á5 -CpCo(CO)2 under the influence of an incoming CO, but not

when the incoming ligand is a more weakly binding alkane molecule

(49,56,57). Ultrafast infrared studies by Dougherty and Heilweil show

that Á5 -CpCo(CO) reacts very quickly (
strongly-binding ligand 1-hexene to form presumably a singlet complex,



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Yang and Harris



in which a 1-hexene molecule complexes to the Co metal through its C C

double bond (58). A similar reactivity has also been observed in the reaction

of CO and N2 with triplet Á3 -Tpi Pr,Me Co(CO) (Tpi Pr,Me D HB-Pzi3 Pr,Me ,

Pzi Pr,Me D 3-iso-propyl-5-methylpyrazolyl) (59) or Á5 -CpŁ MoCl(PMe3

(60,61), in the oxidative addition of benzene or aldehydes C–H bonds to

unsaturated Á5 -CpŁ Co Á2 -H2 C CHSiMe3 (62–64) and most recently in

the silane Si–H bond activation by Á5 -CpV(CO)4 (65). In view of the above

examples, it would seem that a stronger metal-ligand interaction tends to

facilitate a high-spin to low-spin crossover in an organometallic compound.

Although such an intermolecular process can be qualitatively described

by Fig. 14, substantial efforts will be required, both in experiments and

in theoretical development, to reach the same level of understanding as

intramolecular intersystem crossing (66). The unique information provided

by ultrafast infrared spectroscopy, which includes the dynamics of IVR and

those of molecular morphology change, is expected to be crucial in future

developments.

V. C–Cl BOND ACTIVATION BY THE Re(CO)5 RADICAL



In a one-electron oxidative-addition reaction, only one chemical moiety

transfers to a transition metal center. Consequently, the formal charge of

the metal is changed by C1 (oxidation). Prototypical examples include

the abstraction of a halogen atom from a halogenated organic molecule

by transition-metal radicals that have formally 17 valence electrons at the

metal. Previous studies of such reactions have led to proposals that involve

either an intermediate that has 19 valence electrons at the metal center or a

charge-transfer intermediate (9–11). These two scenarios are illustrated in

Fig. 17 using the reaction of Cl atom abstraction by the CO5 Re radical

as an example. Due to the strong metal-carbonyl coupling, the CO ligands



Figure 17

Re(CO)5 .



Previously proposed reaction schemes for Cl atom abstraction by



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Bond Activation Reactions



103



may serve as a sensitive local probe for the charge density of the metal.

This offers an opportunity to clarify the reaction mechanism by comparing

the CO stretching frequencies in the reactive chlorinated methane solutions

with those in the chemically inert hexane solution. If the reaction proceeds

through a 19-electron intermediate, the increased metal electron density

will result in a red shift of the CO stretching frequency. On the other hand,

if the reaction proceeds through a charge-transfer intermediate of the form

[ CO 5 ReC C Cl–R], the positively charged rhenium pentacarbonyl will

exhibit a substantial blue shift in the CO-stretching frequency.

A. Clarification of the Reaction Pathway



In the current study, the reaction is initiated by photochemically splitting

the Re–Re bond of (CO)5 Re Re(CO)5 with UV pulses. The resulting

Re(CO)5 radical further reacts to abstract a Cl atom from a chlorinated

methane molecule CHn Cl4 n (n D 0, 1, 2) to form the final product

(CO)5 ReCl. In Fig. 18d, CO 5 ReCl shows two CO-stretching bands at

1982 and 2045 cm 1 in CCl4 solution (67). At shorter time delays

40 ns < t < 2.5 µs after photoexcitation, there appear five additional

bands marked by asterisks in Fig. 18c. These bands are assigned to the

equatorially solvated eq-Re2 CO 9 CCl4 , where a CCl4 solvent molecule

takes up the vacant site that is left behind by a leaving equatorial CO ligand,

in accordance with low-temperature studies (68). On the ultrafast time

scale, a broad feature comes into view at about 1990 cm 1 , as indicated

by the down-pointing arrow in Fig. 18b. To assign this broad feature, one

compares the spectrum in Fig. 18b with that taken in the chemically inert

hexane solution shown in Fig. 18a. In Fig. 18a, the most intense peak at

¾1992 cm 1 is assigned to the weakly solvated CO 5 Re radical in roomtemperature hexanes, in very good agreement with previously reported

Re(CO)5 band 1990 cm 1 in cyclohexane solution (69). It follows that

the ¾1990 cm 1 feature in Fig. 18b can be ascribed to CCl4 solvated

Re(CO)5 . The similarity in the CO-stretching frequencies of Re(CO)5 in

both hexanes and CCl4 solutions suggests that the electron density of

the Re center does not change appreciably in both solvents and that

the interaction of Re(CO)5 CCl4 is comparable in magnitude to that of

CO 5 Re (alkane). Both considerations are supported by DFT calculations.

The computed Mulliken population of Re only increases 8% (or 4%

using natural population) from Re(CO)5 /CH4 to Re(CO)5 CCl4 as shown

in Table 1 (70). The Re(CO)5 /CCl4 interaction energy is calculated to

¾ 0.6 kcal/mol at the DFT B3LYP level, comparable to ắ 0.6 kcal/mol



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