Tp*Rh

The pentamethylcyclopentadienyl derivatives of rhodium Cp RhL (L = PMe3, C2H4) oxidatively add thiophene preferentially via the C—S activation route compared to that based on the C—H activation [880M1171,94JOM(472)311]. The Tp derivatives by contrast yield mainly the latter. Tp Rh(PEt3) acts almost selectively and forms exclusively 225 (R = Et), whereas Tp Rh(PMc3) forms a major amount of 225 (R = Me) and minor amount of 226 (960M2678). 

The vinyl ehloride derivative [(> -Tp )Rh(CNBu-t)(CH2=CH)Cl] when rea-eted with Cp2ZrH2 gives the vinyl hydride [(j -Tp )Rh(CNBu-t)(CH2=CH)H] (990M495). In CgDg at room temperature, the produet transforms into the ethylene eomplex [(>j -Tp )Rh(CNBu-t)(j7 -CH2=CH2)] by intramoleeular isomerization. A similar eomplex, [(>j -Tp )Rh(CN-2,6-xylyl)(>j -CH2=CH2)], is also known (980M4784,980M5148). Photolysis of [(j -Tp )Rh(CNBu-t)(PhN=C=NBu-t)]... 

Note that reaction of the vinyl chloride complex Tp Rh(CNCH2 Bu)(CH=CH2)Cl with [Cp2ZrH2]2 did not give the hydrozirconation reaction on the vinyl group but led quantitatively to the formation of the vinyl hydride complex Tp Rh(CNCH2 Bu) (CH=CH2)H [231]. 

Photolysis of a CO ligand from the parent compound initiates the reaction. The resultant >j3-Tp Rh(CO) complex has a coordinatively vacant site that is quickly occupied by a solvent molecule. The time scale for such a solvation process is generally on the order of 2 ps in room temperature liquids (28). The solvated 3-Tp Rh(CO)(S) (S = alkane) complex exhibits a single CO-stretching band (vCo) at 1972 cm 1 (the v = 0 1 band) as shown in Fig. 4a. For this particular system the large... 

Figure 4 Transient difference spectra in the CO stretching region for t73-Tp Rh(CO)2 in room-temperature alkane solution at various time delays following 295 nm photolysis. Panel (e) 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. 29.)...

The fact that the final product 3-Tp Rh(CO)(H)(R) does not appear on the ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater than 5.2 kcal/mol for the alkane C-H bond activation. Nanosecond step-scan FTIR experiments on the 3-Tp Rh(CO)2/cyclohexane system show that the remnant of the 2-Tp Rh(CO)(S) peak persists for 280 ns after photoexcitation, while the product CO stretch at 2032 cm-1 rises with a... 

The ability to follow the C-H bond activation by 3-Tp Rh(CO) from its initiation to completion allows one to assemble a comprehensive reaction profile delineated in Fig. 8. The proposed scheme provides a direct assessment of the time scale and energy barrier for the C-H bond-breaking step. A comparison of these results with those of the isoelectronic Si-H bond activation requires a better understanding of chemical bonds, to be discussed in the next section. 

Figure 8 A proposed reaction mechanism for the alkane C-H bond activation by j3-Tp Rh(CO)2 covering the ultrafast dynamics to nanosecond kinetics. 

The oxidative addition of alkanes to Rh1 and Ir1 species CpML is very facile. In the case of the tris(pyrazolyl)borate complex Tp Rh(CO)2 [Tp = HB(2,4-Me2pyr)3], the lifetimes of the intermediates were determined by ultrafast time-resolved infrared spectroscopy. In this case de-chelation of one of the pyrazolyl arms was found to precede the C—H oxidative addition step. The proposed intermediates for R—H addition and their lifetimes are shown in Fig. 21-1.127... 

Figure 21-1 Proposed mechanism and energy diagram for the C—H activation reaction of Tp Rh(CO)2 in alkane solution. These energy differences are estimates from separate ultrafast and nanosecond experiments. The stabilities of the intermediates are shown relative to each other and are not intended to be absolute (from Ref. 127).

Similar TRIR experiments were performed with Tp Rh(CO)2 and Bp Rh(CO)2 [Tp = hydridotris(3,5-dimethylpjn azolyl)borate Bp = dihydridobis(3,5-dimethyIpyrazolyl)borate] in liquid Xe solution at 223 K 48). From the positions of the observed transient infrared bands, it was found that the Tp species forms two xenon complexes upon photolysis, (7j -Tp )Rh(CO)Xe and (>j -Tp )Rh(CO)Xe, and that Bp Rh(CO)Xe is produced from Bp Rh(CO)2. 

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