4
2
T.V. Krasnyakova et al. / Journal of Catalysis 288 (2012) 33–43
(
1.274 Å) CACl (1.719 Å) and C@C (1.332 Å) [23] geometrically per-
reaction (9) could be neglected. Therefore, in order to provide a
thermodynamic driving force for the reaction (9), the heat of it
should be negative. In the course of reaction (9), the HACl, MACl,
mit to accomplish such a transition state.
The stoichiometric consequence of such a mode for chloromet-
allation step in the systems 2 and 3 is the formation of the appro-
priate metal complex with coordination vacancy on the same
2
2
and MA(
2 2
g -C H ) bonds are broken, the new HACl, MAC(sp )
2
and ClAC(sp ) bonds are formed, and the triple CAC bond is
transformed into double one. The heat of HACl bond rupture is
compensated by the formation of the new same bond. So, the heat
crystal plane in addition to
r-vinyl organometallic derivative.
Accordingly, the stepwise mechanism for acetylene catalytic
hydrochlorination in the systems 2 and 3 can be interpreted in
the following way (Scheme 4). Reaction begins with reversible
of reaction (9) is determined mainly by bonds dissociation energies
2
D
D
H = D(MACl) + D(MA(
g
-C
2
H
2
)) ꢂ D(MAC) ꢂ D(CACl)+
DE, where
(
M = Pd) or irreversible (M = Pt)
complex with coordination vacancy (step 1). Hydrochloric acid
molecule attack onto -acetylene complex with the assistance of
adjacent metal complex located on the (100) crystal plane (step
) leads to the intermediate formation of acetylene chlorometalla-
tion product and a new metal complex with coordination vacancy
on the same crystal plane. Fast protonolysis (step 3) of the -vinyl
p-coordination of acetylene to a
E is a heat of the triple bond transformation into double one.
The difference of reaction (9) heats for M = Pt ( ) and M = Pd
)) ꢂ D(MAC),
D(MAC) = D(PtAC) ꢂ
DH
2
2
p
(
D
H
3
) is
where
D(PdAC),
D
D
H
2
ꢂ
D
H
3
=
D
D(MACl) +
D
D(MA(
g
-C
2
H
2
D
D(MACl) = D(PtACl) ꢂ D(PdACl),
D
2
2
2
and
D
D(MA(
g
-C
H
2 2
)) = D(PtA(
g
-C
2
H
2
)) ꢂ D(PdA
2
(g
-C
2 2
H )).
r
Taking into consideration that
D
D(MACl) ꢆ 29 kJ/mol [24] and
organometallic derivative yields vinyl chloride. A local positive
charge of the crystal lattice defect in the form of a monoanion
D
D(MAC) ꢆ 34 kJ/mol [25] and the fact that platinum(II) chloro
complexes form more stable
(Sections 3.2.5 and 4.2), we have
)) > 0, that is, . Since the
this means that the value is closer to zero valuation than
3
DH one. Accordingly, degree of symmetry for transition state of
p-complexes than palladium(II) one
ꢃ ꢂ
2
[
PdCl
3
]
produced in the step 2 is favorable to a hydrogen atom
D
H
2
ꢂ
D
H
3
ꢆ
D
D(MA(
g -
of polar HCl molecule attack onto the intermediate organometallic
derivative.
C
2
H
2
D
H
2
>
D
H
3
DH values are negative,
DH
2
This hypothesis provides an explanation for two different HCl/
DCl kinetic isotope effects observed for the reaction (1). The first
one arises from H(D)ACl bond rupture with chlorine atom incorpo-
ration into the final product (step 2, Scheme 4), and the second
originates from H(D)ACl bond cleavage with hydrogen (deute-
rium) atom incorporation into vinyl chloride formed (step 3). The
chloropalladation step (step 2) is rate limiting; protonolysis (step
the reaction (9) in the system 2 is greater than in the system 3.
Probably, that is the reason for the less KIE value observed in the
system 3 in comparison with the KIE value determined [21] for
the system 2.
3
) proceeds faster. Therefore, it is difficult to say whether the pro-
5. Conclusions
tonolysis step occur according Eley–Rideal or Langmuir–Hinshel-
wood type mechanism.
Pre-grinding of K PdCl solid salt in a vibratory micromill under
2
4
Proposed mechanism for chloropalladation step (Scheme 3)
provides an explanation for high stereoselectivity of catalytic
hydrochlorination reaction as well as for a mode of active sites of
catalyst regeneration and excludes extremely unfavorable hetero-
acetylene or propylene atmosphere produces heterogeneous cata-
lyst for acetylene hydrochlorination. The active sites of catalyst
generated in the course of K PdCl salt mechanical pre-activation
2
4
are point defects of the crystalline structure in the form of palla-
+
ꢂ
lysis of HACl bond with H formation in the absence of solvation.
dium(II) complexes with a coordination vacancy, [PdCl ] , which
3
According to semi-classical theory, the highest value 6.9 for H/D
KIE at 25 °C is expected [18] for a reaction with linear and symmet-
ric transition state in the absence of tunneling contribution. The
symmetry of a transition state supposes that change of free energy
of such a reaction is equal to zero. Any deflection of a change of free
energy of a reaction from zero results in a decrease of KIE value as
well as any deviation of H-atom from the straight line connecting
H-donating and H-accepting groups in a transition state of a reac-
tion. Reasoning from the affinity between lattice parameters for
are able reversibly bind acetylene in a p-complex.
In the hydrochlorination of acetylene, the addition of HCl was
shown to occur in the anti-stereochemical configuration.
The two different HCl/DCl kinetic isotope effects for the cata-
lytic hydrochlorination of acetylene are observed proving that
HCl molecules participate in two separate steps of the catalytic
reaction. The KIE value equal to 2.8 was determined for the limiting
step of the overall catalytic reaction, and the KIE equal to 6.8 was
derived from the ratio of yields of vinyl chloride isotopomers
formed.
K
2
PtCl
structures of the transition state TS in the related systems 2 and
are very close. So, the difference in the KIE values for overall cat-
4 2 4
and K PdCl crystals, we may assume that geometrical
The first isotope effect is attributed to acetylene chloropallada-
tion step. This step involves donation of chlorine atom from HCl/
3
alytic reactions in these two systems is controlled not by geometric
but rather by thermodynamic factor.
DCl molecule to p-coordinated acetylene with simultaneous
removal of chloro ligand from adjacent palladium complex. In a
ð9Þ
The number of particles in the both sides of the reaction (9) is the
same, and so to the first approximation, the entropy factor of the
result, a new HCl (DCl) molecule and new palladium(II) complex
with a coordination vacancy are formed in addition to b-chlorovi-