as efficient thermal precursors for {Tp
0 Rh[P(OMe) 3 ]}. Several hydrocarbon activation products were observed by exchange for methane in 8 (Scheme 4). In the
alkyne activations, no evidence was seen for the formation of alkyne π-complexes,
again pointing to the need for a strong σ-donor to be present to stabilize the η
2 -
ligation. In addition, the activation of pentane using 8 was unsuccessful, giving only
decomposition after several hours. Instead, Tp
0 Rh[P(OMe) 3 ](n-pentyl)H was prepared by irradiation of 7 in pentane at 10
C. Also, attempted activation of
cyclopentane, CH 3 CF 3 , and CH 2 F 2 was unsuccessful, giving only small quantities
of the desired products (not enough for use in kinetic studies).
-40
-30
-20
-10
0
10
20
30
40
50
80
90
100
110
120
130
Carbon-Hydrogen Bond Strengths (kcal/mol)
Calculated M-C Bond Strengths
(kcal/mol)
c-pentyl
methyl
n-pentyl
phenyl
t-butylvinyl
DFT: M062x
CHF 2
CH 2 F
CH 2 OMe
CH 2 O
t Bu
ArCH 2
-CaCR, R = Ph, SiMe 3 , hexyl, pCF 3 C 6 H 4 ,
t Bu, CF 3 , p-MeOC 6 H 4
CH 2 CF 3
slope = 1.76(11)
slope = 1.53(19)
CH 2 C(O)M
sp
2
sp
sp
3
Tp'Rh(PMe 3 )RH
Fig. 6 DFT-calculated plot of relative M–C bond strengths vs. C–H bond strengths for Tp
0 Rh
(PMe 3 )(R)H. The lower line is fit to the hydrocarbons (blue filled box, y ¼ 1.531x À 162.9), and the
upper line is fit to the –CH 2 X and CHF 2 substrates (red filled triangle, y ¼ 1.756x À 198.0). Data
for CH 3 CF 3 activation is also shown, but not included in the fits. M062X method and basis set 6–
31g** for first row atoms and pseudopotentials, additional functions optimized by Stuttgart group
for atoms beyond the second row. Experimental C–H bond strengths were used for all substrates
except the alkynes. Alkyne C–H bond strengths were calculated (B3LYP) since experimental
values are unavailable [13]. The vertical separation of the lines at D C–H ¼ 100 kcal mol
À1 is
12.6 kcal mol
À1
. Reproduced with permission of the ACS from Jiao et al. [18]
The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as. . .
83
0 Rh[P(OMe) 3 ]}. Several hydrocarbon activation products were observed by exchange for methane in 8 (Scheme 4). In the
alkyne activations, no evidence was seen for the formation of alkyne π-complexes,
again pointing to the need for a strong σ-donor to be present to stabilize the η
2 -
ligation. In addition, the activation of pentane using 8 was unsuccessful, giving only
decomposition after several hours. Instead, Tp
0 Rh[P(OMe) 3 ](n-pentyl)H was prepared by irradiation of 7 in pentane at 10
C. Also, attempted activation of
cyclopentane, CH 3 CF 3 , and CH 2 F 2 was unsuccessful, giving only small quantities
of the desired products (not enough for use in kinetic studies).
-40
-30
-20
-10
0
10
20
30
40
50
80
90
100
110
120
130
Carbon-Hydrogen Bond Strengths (kcal/mol)
Calculated M-C Bond Strengths
(kcal/mol)
c-pentyl
methyl
n-pentyl
phenyl
t-butylvinyl
DFT: M062x
CHF 2
CH 2 F
CH 2 OMe
CH 2 O
t Bu
ArCH 2
-CaCR, R = Ph, SiMe 3 , hexyl, pCF 3 C 6 H 4 ,
t Bu, CF 3 , p-MeOC 6 H 4
CH 2 CF 3
slope = 1.76(11)
slope = 1.53(19)
CH 2 C(O)M
sp
2
sp
sp
3
Tp'Rh(PMe 3 )RH
Fig. 6 DFT-calculated plot of relative M–C bond strengths vs. C–H bond strengths for Tp
0 Rh
(PMe 3 )(R)H. The lower line is fit to the hydrocarbons (blue filled box, y ¼ 1.531x À 162.9), and the
upper line is fit to the –CH 2 X and CHF 2 substrates (red filled triangle, y ¼ 1.756x À 198.0). Data
for CH 3 CF 3 activation is also shown, but not included in the fits. M062X method and basis set 6–
31g** for first row atoms and pseudopotentials, additional functions optimized by Stuttgart group
for atoms beyond the second row. Experimental C–H bond strengths were used for all substrates
except the alkynes. Alkyne C–H bond strengths were calculated (B3LYP) since experimental
values are unavailable [13]. The vertical separation of the lines at D C–H ¼ 100 kcal mol
À1 is
12.6 kcal mol
À1
. Reproduced with permission of the ACS from Jiao et al. [18]
The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as. . .
83