Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes Page: 19,134
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Journal of the American Chemical Society
1.4
1.2
S0.8
0.6
0.4
- 0.2
0.0
0 0.5 1 1.5
[1] (x 10-3 M)Figure 3. Plot of [ethylbenzene] (M) after 30 min as a function of [1]
(M) at 100 OC with 0.1 MPa of ethylene in neat benzene (R2 = 0.99).0.03 0.05 0.07 0.09
[Ethylene] (M)0.14
0.12
0.1
0.08
N
0.06
. 0.04
0.02 +
0-
0 0.5 1 1.5 2 2.5 3
[Benzene] (M)
Figure 4. Plot of [ethylbenzene] (M) after 2 h as a function of
[benzene] (M) at 100 OC in perfluoropyridine with [ethylene] =
0.16(5) M and [1] = 0.03 M.
of ethylene hydrophenylation with C6H6 and C6D6 were used to
determine the kinetic isotope effect (KIE) for catalytic reactions.
In separate experiments that incorporated either C6H6 or C6D6,
using the ratio of total TO after 4 h of reaction (taken from
multiple experiments under identical conditions) a KIE for the
catalytic reaction of 1.8(4) was determined.
To study stoichiometric benzene C-H activation by
[(tbpy)Pt(R)]+ systems, we used the reaction of [(tbpy)Pt-
(Ph-d,)(THF) ] [n = 5 (1-d5) or 0 (1)] with excess C6H6 or
C6D6. The reaction of 1-ds and C6H6 at 21 OC in CD2C12 leads to
the formation of 1 and C6DsH with an observed pseudo-first-
order rate constant of 7.1(4) x 10-5 s-1. A pseudo-first-order
rate constant of 5.0(3) x 10-5 s is observed for the same
reaction between 1 and C6D6 to form 1-ds and C6HsD. The
activation of a benzene C-H/D bond results in a KIE of 1.4(1)
(Scheme 1), which is statistically indistinguishable from the
catalytic KIE, and the magnitude of the KIE is similar to that
previously observed for arene and aliphatic C-H activation by
Pt(II) systems.32'34,46,47 The impact of the weakly coordinating
Lewis base THF on the rate of benzene C-D activation by 1 was
investigated. The reaction rate decreases with increasing THF
concentration (Figure 6). At 30 OC in CD2C12, the conversion of
1 to 1-ds with 0.18 M C6D6 proceeds with an observed rate ofFigure 5. Plot of [ethylbenzene] (M) after 2 h as a function of
[ethylene] (M) at 100 OC in C6D6 and [1] = 0.028 M.
4.6(4) x 10-s s-1. At low concentrations of C6D6, a first-order
dependence on concentration of C6D6 is observed, and as the
concentration of C6D6 is increased, saturation conditions are
reached (Figure 7).
Two pathways for stoichiometric benzene C-H activation by
1 are shown in Scheme 2, which are distinguished by associative
versus dissociative pathways for benzene/THF exchange. Rate
laws were derived assuming that benzene C-H/D coordination
is the rate-limiting step and by applying the steady-state approxi-
mation (reaction intermediates are not observed). The small
primary KIE of 1.4(1) is consistent with similarly small KIEs
(kH/kD 1.1-1.4) for C-H/D activation by d8 metal centers,
which have been interpreted as rate-determining C-H coordina-
tion.46,48,49 Rate-limiting C-H/D activation by d8 metals have
been reported to exhibit KIEs >_ 2.5.50o,51 Both pathways predict
an inverse dependence on the concentration of THF; however,
only the rate law for the dissociative pathway is consistent with
the observation of saturation kinetics for variation of benzene
concentration. Thus, we suggest that the dissociative pathway
with rate-determining C-H coordination is most likely. Studies
of C-H activation by other cationic Pt(II) systems with hydro-
carbyl ligands suggest similar pathways that likely involve three-
coordinate Pt complexes.41 It should be noted that rapid pre-
equilibrium conditions would be consistent with either
pathway.s2 The KIE (C6H6 vs C6D6) for the catalytic cycle is
larger [kH/kD = 1.8(4)], but the deviation is also larger. Thus, the
KIE for the catalytic cycle is consistent with either rate-limiting
C-H coordination or C-H activation. A change of the rate-
determining step for stoichiometric benzene C-H activation
versus the catalytic cycle might be explained by a similar
magnitude for the two steps (i.e., C-H coordination and
C-H bond breaking) and a change in relative barrier height
for C-H activation by a Pt-phenyl versus Pt-CH2CH2Ph.
Computational Study of Benzene C-H Activation by
[(tbpy)Pt(CH2CH2Ph)]+. The mechanisms by which Pt(II) sys-
tems activate the C-H bonds of hydrocarbons have been studied
extensively,41,S3-55 and the two mechanisms most commonly
invoked are electrophilic substitution and oxidative addition
(Scheme 3).32'41'4656-59 In addition, recent studies suggest
that exchange of phenyl groups in [(N-N)Pt(Ph)(C6H6)]+dx.doi.org/10.1021/ja206064v IJ. Am. Chem. Soc. 2011, 133, 19131-19152
1.6
1.2
N
0.8
0.42 2.5
0.11 0.13
19134
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McKeown, Bradley A.; Gonzalez, Hector Emanuel; Friedfeld, Max R.; Gunnoe, T. Brent; Cundari, Thomas R., 1964- & Sabat, Michal. Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes, article, November 8, 2011; [Washington, DC]. (https://digital.library.unt.edu/ark:/67531/metadc107788/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.