Experimental Evidence for a Temperature Dependent Transition between Normal and Inverse Equilibrium Isotope Effects for Oxidative Addition of H 2 to Ir(PMe2Ph)2(CO)Cl

Article abstract of DOI:10.1021/ja0362611

The equilibrium isotope effect (EIE) for oxidative addition of H ₂ and D₂ to Ir(PMe₂Ph)₂(CO)Cl has been measured over a large temperature range, thereby demonstrating that the inverse (<1) EIE previously observed at ambient temperature becomes normal (>1) at high temperature (>90 °C). The temperature dependence of the EIE for oxidative addition of H₂ and D₂ to Ir(PH ₃)₂(CO)Cl has been calculated using the geometry and vibrational frequencies obtained from DFT (B3LYP) calculations on Ir(PH ₃)₂(CO)ClH₂ and Ir(PH₃) ₂(CO)ClD₂, and is in accord with the experimentally observed transition from an inverse to normal EIE for oxidative addition of H₂ and D₂ to Ir(PMe₂Ph)₂(CO)Cl: the EIE is calculated to be inverse between 0 and 510 K, reach a maximum value of 1.15 at 867 K and then slowly decrease to unity as the temperature approaches infinity. This deviation from simple van't Hoff behavior, and the occurrence of a maximum in the EIE, is the result of the entropy term being temperature dependent. At low temperature, the enthalpy term dominates and the EIE is inverse, whereas at high temperatures the entropy term dominates and the EIE is normal. The observation of both normal and inverse EIEs for the same system indicate that inferences pertaining to the magnitude of an isotope effect at a single temperature may require more detailed consideration than previously realized.

Full text of DOI:10.1021/ja0362611

Published on Web 10/02/2003
Experimental Evidence for a Temperature Dependent
Transition between Normal and Inverse Equilibrium Isotope
Effects for Oxidative Addition of H to Ir(PMe Ph) (CO)Cl
2 2 2
Kevin E. Janak and Gerard Parkin*
Contribution from the Department of Chemistry, Columbia UniVersity,
New York, New York 10027
Received May 21, 2003; E-mail: parkin@chem.columbia.edu
2 2 2 2
Abstract: The equilibrium isotope effect (EIE) for oxidative addition of H and D to Ir(PMe Ph) (CO)Cl
has been measured over a large temperature range, thereby demonstrating that the inverse (<1) EIE
previously observed at ambient temperature becomes normal (>1) at high temperature (>90 °C). The
temperature dependence of the EIE for oxidative addition of H
using the geometry and vibrational frequencies obtained from DFT (B3LYP) calculations on Ir(PH
ClH and Ir(PH (CO)ClD , and is in accord with the experimentally observed transition from an inverse to
normal EIE for oxidative addition of H and D to Ir(PMe Ph) (CO)Cl: the EIE is calculated to be inverse
2
and D
2
3 2
to Ir(PH ) (CO)Cl has been calculated
3 2
) (CO)-
2
3
)
2
2
2
2
2
2
between 0 and 510 K, reach a maximum value of 1.15 at 867 K and then slowly decrease to unity as the
temperature approaches infinity. This deviation from simple van’t Hoff behavior, and the occurrence of a
maximum in the EIE, is the result of the entropy term being temperature dependent. At low temperature,
the enthalpy term dominates and the EIE is inverse, whereas at high temperatures the entropy term
dominates and the EIE is normal. The observation of both normal and inverse EIEs for the same system
indicate that inferences pertaining to the magnitude of an isotope effect at a single temperature may require
more detailed consideration than previously realized.
Introduction
The oxidative addition of dihydrogen to a metal center is one
of the most fundamental reactions in transition metal chemistry
and plays a role in many important processes, such as metal
catalyzed olefin hydrogenation. We have previously studied the
equilibrium isotope effect (EIE) for this transformation and in
1993 reported that the EIE for the oxidative addition of H2 and
D2 to W(PMe3)4I2 is inverse (i.e., KH/KD < 1), such that the
cleavage of the stronger D-D bond is thermodynamically more
favored than that of the weaker H-H bond.1 While this
observation may at first appear counterintuitive, and indeed
counter to the commonly held notion that deuterium prefers to
be located in the stronger bond to a greater extent than does
Figure 1. Vibrational modes associated with a C2-symmetric [MH2]
fragment and the vibrational, translational, and rotational modes of H2 from
which they are derived.
2
hydrogen, it is readily rationalized by the fact that the [MH2]
for other examples of oxidative addition of H2 to a metal center,3
fragment has a substantially greater number of isotopically
sensitive vibrations (i.e., two stretches and four bends) than that
of the single stretch in H2; the additional vibrations of the [MH2]
unit are derived from the rotational and translational degrees
of freedom of H2 molecule, as illustrated in Figure 1. Although
each of the vibrations of the [MH2] fragment are lower in energy
than that of the H2 stretch, the combination causes the total zero
point stabilization of [MD2] versus [MH2] to be greater than
that for D2 versus H2. Inverse EIEs have not only been reported
but have also been observed for coordination of H2 to give
2
4,5,6
dihydrogen complexes, [M(η -H2)].
In this paper, we report
(3) See, for example: (a) Abu-Hasanayn, F.; Krogh-Jespersen, K.; Goldman,
A. S. J. Am. Chem. Soc. 1993, 115, 8019-8023. (b) Shin, J. H.; Parkin, G.
J. Am. Chem. Soc. 2002, 124, 7652-7653. (c) Yan, S. G.; Brunschwig, B.
S.; Creutz, C.; Fujita, E.; Sutin, N. J. Am. Chem. Soc. 1998, 120, 10 553-
10 554.
(4) See, for example: (a) Bender, B. R.; Kubas, G. J.; Jones, L. H.; Swanson,
B. I.; Eckert, J.; Capps, K. B.; Hoff, C. D. J. Am. Chem. Soc. 1997, 119,
9
179-9190. (b) Gusev, D. G.; Bakhmutov, V. I.; Grushin, V. V.; Vol′pin,
M. E. Inorg. Chim. Acta 1990, 177, 115-120. (c) Hauger, B. E.; Gusev,
D.; Caulton, K. G. J. Am. Chem. Soc. 1994, 116, 208-214. (d) Bakhmutov,
V. I.; Bertr a´ n, J.; Esteruelas, M. A.; Lled o´ s, A.; Maseras, F.; Modrego, J.;
Oro, L. A.; Sola, E. Chem. Eur. J. 1996, 2, 815-825.
(
1) (a) Rabinovich, D.; Parkin, G. J. Am. Chem. Soc. 1993, 115, 353-354. (b)
Hascall, T.; Rabinovich, D.; Murphy, V. J.; Beachy, M. D.; Friesner, R.
A.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 11 402-11 417.
2) Wolfsberg, M. Acc. Chem. Res. 1972, 5, 225-233.
(5) For some isotope effects in binary metal hydrides, see: (a) Wiswall, R.
H., Jr.; Reilly, J. J. Inorg. Chem. 1972, 11, 1691-1696. (b) Luo, W.;
Clewley, J. D.; Flanagan, T. B. J. Chem. Phys. 1990, 93, 6710-6722.
(
10.1021/ja0362611 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 13219-13224
9
13219

A R T I C L E S
Janak and Parkin
Scheme 1
Scheme 2
experimental evidence for the interesting observation that the
EIE for oxidative addition of H2 to a metal center may undergo
a transition from an inverse to a normal value upon raising the
temperature.
Results and Discussion
Scheme 3
EIEs (and equilibrium constants in general) are typically
expected to vary in an exponential manner as the temperature
is raised. Recently, however, we reported calculations which
predict that the temperature dependence of the EIE for oxidative
addition of H2 to the 16-electron tungstenocene species {[H2-
Si(C5H4)2]W} would not exhibit such behavior. Thus, rather
than vary monotonically with temperature, the EIE for oxidative
addition of hydrogen to {[H2Si(C5H4)2]W} is predicted to exhibit
a maximum: the EIE is 0 at 0 K, increases to a maximum value
7
,8
of 1.57, and then decreases to unity at infinite temperature.
Although the equilibrium isotope effect for oxidative addition
of H2 is defined as the ratio KH/KD, it is evident that the EIE is
identical to the equilibrium constant for the isodesmic exchange
reaction (Scheme 2). An advantage of viewing the EIE in this
way is that its evaluation does not require any information
concerned with Ir(PH3)2(CO)Cl, but only requires thermody-
namic data on Ir(PH3)2(CO)ClH2, Ir(PH3)2(CO)ClD2, H2 and
D2. In this regard, it is conventional to determine the EIE by
the expression EIE ) KH/KD ) SYM ‚ MMI ‚ EXC ‚ ZPE,
Prompted by this result, we sought to obtain experimental
evidence that the EIE for oxidative addition of H2 to a transition
metal center could undergo a temperature-dependent transition
from an inverse to a normal value. Since we anticipated that
high temperatures would be required to observe this effect, we
elected to study oxidative addition of H2 in a system that is
thermally robust. For this reason, we chose to investigate the
Vaska system because previous studies have demonstrated that
the interconversion of Ir(PR3)2(CO)X and Ir(PR3)2(CO)XH2 is
well-characterized for a wide range of substituents (Scheme
12
where SYM is the symmetry factor, MMI is the mass-moment
3
,9,10,11
of inertia term, EXC is the excitation term, and ZPE is the zero
1
).
13,14
point energy term (Scheme 3).
The SYM term is determined
1. Computational Evaluation of the Temperature Depen-
by the symmetry number ratio of the species involved and is
unity for oxidative addition of H2/D2 to Ir(PH3)2(CO)Cl; the
MMI term is determined by their structures (i.e. their masses
dence of the EIE for Oxidative Addition of H2 and D2 to
Ir(PH3)2(CO)Cl. Previous experimental and computational
studies have demonstrated that the oxidative addition of H2 to
Ir(PR3)2(CO)Cl complexes is characterized by an inverse EIE
1
5
and moments of inertia); and the EXC and ZPE terms are
determined by their vibrational frequencies. The required
structural and vibrational data were obtained by DFT calcula-
tions using the B3LYP functional and the 6-31G**/LACVP**
basis sets. The geometry optimized structure of Ir(PH3)2(CO)-
Cl (Figure 2) corresponds closely to that of previous calculations3a
and the principal vibrations that are isotopically sensitive are
summarized in Table 1. It should be noted that the six normal
modes associated with the [IrH2] moiety mix with other
vibrational modes, as illustrated by the isotopic sensitivity of
at ambient temperature.3a,b Prior to experimentally determining
whether the EIE for oxidative addition of H2 to Ir(PR3)2(CO)-
Cl would exhibit a temperature-dependent transition from
7
inverse to normal, akin to that predicted for {[H2Si(C5H4)2]W},
we first performed calculations on the simplified Ir(PH3)2(CO)-
ClH2 system.
(
6) Bullock, R. M.; Bender, B. R. Isotope Methods in Homogeneous Catalysis.
In Encyclopedia of Catalysis; Horv a´ th, I. T., Ed.; 2002.
(
7) Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 6889-6891.
8) For other examples of situations in which there is a temperature-dependent
transition between a normal and inverse EIE, see ref 2.
(
(9) (a) Deeming, A. J.; Shaw, B. L. J. Chem. Soc. (A) 1969, 1128-1134. (b)
(12) The symmetry factor includes both external (σ) and internal (n) symmetry
numbers. See: Bailey, W. F.; Monahan, A. S. J. Chem. Educ. 1978, 55,
489-493.
(13) (a) Wolfsberg, M.; Stern, M. J. Pure Appl. Chem. 1964, 8, 225-242. (b)
Melander, L.; Saunders: W. H., Jr. Reaction Rates of Isotopic Molecules;
Wiley-Interscience: New York 1980. (c) Carpenter, B. K. Determination
of Organic Reaction Mechanisms, Wiley-Interscience: New York 1984.
(d) Ishida, T. J. Nucl. Sci. Technol. 2002, 39, 407-412. (e) Bigeleisen, J.;
Mayer, M. G. J. Chem. Phys. 1947, 15, 261-267.
(14) For some recent influential studies concerned with the computation of
isotope effects in organometallic systems, see: (a) Slaughter, L. M.;
Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R. J. Am. Chem. Soc.
2000, 122, 7953-7975. (b) Bender, B. R. J. Am. Chem. Soc. 1995, 117,
11 239-11 246. (c) ref 3a and 4a.
(15) Application of the Teller-Redlich product rule permits the MMI term to
be replaced by the vibrational product (VP) derived from the vibrational
frequencies. See ref 13.
Hyde, E. M.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1975, 765-767.
(
10) (a) Vaska, L.; Werneke, M. F. Ann. N. Y. Acad. Sci. 1971, 172, 546-562.
(
b) Zhou, P.; Vitale, A. A.; San Filippo, J., Jr.; Saunders: W. H., Jr. J.
Am. Chem. Soc. 1985, 107, 8049-8054. (c) Kunin, A. J.; Johnson, C. E.;
Maguire, J. A.; Jones, W. D.; Eisenberg, R. J. Am. Chem. Soc. 1987, 109,
2
963-2968. (d) Kunin, A. J.; Farid, R.; Johnson, C. E.; Eisenberg, R. J.
Am. Chem. Soc. 1985, 107, 5315-5317.
(
2 3 2
11) It should be noted that although oxidative addition of H to Ir(PR ) (CO)X
gives Ir(PR
3
)
2
(CO)XH
2
in which the H
2
has added parallel to the OC-
Ir-X axis, unstable isomers derived from addition parallel to the P-Ir-P
axis have also been detected by parahydrogen induced polarization. See:
(a) Hasnip, S. K.; Colebrook, S. A.; Sleigh, C. J.; Duckett, S. B.; Taylor,
D. R.; Barlow, G. K.; Taylor, M. J. J. Chem. Soc., Dalton Trans. 2002,
43-751. (b) Hasnip, S. K.; Duckett, S. B.; Sleigh, C. J.; Taylor, D. R.;
Barlow, G. K.; Taylor, M. J. J. Chem. Soc., Chem. Commun. 1999, 1717-
718.
7
1
13220 J. AM. CHEM. SOC.
9
VOL. 125, NO. 43, 2003

Oxidative Addition of H
2
to Ir(PMe
2
Ph)
2
(CO)Cl
A R T I C L E S
Figure 2. Geometry optimized structure of Ir(PH3)2(CO)ClH2. Selected
bond lengths (Å): Ir-P 2.317 and 2.318, Ir-H(trans to Cl) 1.588, Ir-H(trans to
CO) 1.632, Ir-Cl 2.529, Ir-CO 1.951.
Figure 4. Calculated EIE as a function of temperature for oxidative addition
of H and D to Ir(PH ) (CO)Cl, expressed in terms of exp(-∆∆H/RT)
2
2
3 2
and exp(∆∆S/R).
Table 1. Principal Isotopically Sensitive Vibrations in
Ir(PH3)2(CO)ClH2 and Ir(PH3)2(CO)ClD2
as the temperature is increased, whereas EXC decreases from
unity to a limiting value of 1/MMI. In view of the latter
relationship between EXC and MMI, it is convenient to analyze
the temperature dependence of the EIE in terms of the combined
Ir(PH
3
)
2
(CO)ClH
2
Ir(PH
3
)
2
(CO)ClD
)
2
13a
assignment
(cm-1)
(cm-1
ν1 stretch; trans to Cl
νCO
2240.6
2153.3
2042.7
903.7
814.1
845.1
750.2
1588.6
2121.6
1470.3
688.6
611.9
644.1
586.2
[SYM ‚ MMI ‚ EXC] term and the ZPE term. In this regard,
ν2 stretch; trans to CO
δ1 out of plane
δ2 out of plane
δ1 in plane
the [SYM ‚ MMI ‚ EXC] term may be viewed to correspond
closely to the entropy component, whereas the ZPE term
18
corresponds closely to the enthalpy component, as illustrated
δ2 in plane
by comparison of Figures 3 and 4.
In terms of enthalpy and entropy considerations, deviation
from the simple exponential relationship according to K ) exp-
(
-∆∆H/RT)exp(∆∆S/R) occurs if either ∆∆H or ∆∆S are not
constant over the temperature range studied (where ∆∆H )
HH - ∆HD and ∆∆S ) ∆SH - ∆SD). Examination of ∆∆H
∆
and ∆∆S as a function of temperature (Figure 5) indicates that
it is the variation of ∆∆S that is primarily responsible for the
deviation from van’t Hoff behavior and the occurrence of a
maximum in the EIE: thus, over the temperature range 0-1000
K, ∆∆S varies from 3.41 to 1.48 e.u. (i.e., a reduction of 56.6%),
1
whereas ∆∆H only varies from 1.22 to 1.20 kcal mol- (i.e.,
a reduction of 1.6%).
As illustrated in Figures 3 and 4, the [SYM ‚ MMI ‚ EXC]
(
entropy) component favors a normal EIE, varying from the
1
9
value of [SYM ‚ MMI] ) 5.55 at 0 K to unity at infinite
temperature, whereas the ZPE (enthalpy) component favors an
inverse EIE. The ZPE term is < 1 because the [MH2] fragment
has a greater number of isotopically sensitive vibrations than
that of the single stretch in H2 and the energies of these
vibrations are sufficient that they cause the total zero point
Figure 3. Calculated EIE as a function of temperature for oxidative addition
of H2 and D2 to Ir(PH3)2(CO)Cl expressed in terms of SYM, MMI, EXC,
and ZPE components.
the ν(CO) stretch in Ir(PH3)2(CO)ClH2 and Ir(PH3)2(CO)ClD2.
Therefore, the EIE calculation was performed using all frequen-
cies.
The temperature dependence of the individual terms and the
EIE for oxidative addition of H2/D2 to Ir(PH3)2(CO)Cl is
illustrated in Figure 3.16 Significantly, the EIE for oxidative
addition of H2 to Ir(PH3)2(CO)Cl exhibits a similar temperature
dependence to that for {[H2Si(C5H4)2]W}; that is, the EIE is
inverse between 0 and 510 K, reaches a maximum value of
(
17) The combined [SYM ‚ MMI] term is equivalent to the product of the
translational and rotational partition function ratios of the products and
P
P
R
R
reactants, i.e., (Qtr )(Qrot )/(Qtr )(Qrot ). This term is only strictly temperature
independent when T . Θrot, the rotational temperature. The EIE data
presented here are obtained by treating the rotational and nuclear partition
functions classically. Consideration of quantum effects serves to modify
the [SYM ‚ MMI ‚ EXC] (entropy) term at low temperature, but has no
effect on the low-temperature limit of the EIE which is dominated by the
ZPE (enthalpy) term.
(
18) It should be noted that the ZPE and enthalpy [exp(-∆∆H/RT)] terms are
not identical because the thermal population of vibrationally excited states
also provides a contribution to the enthalpy term. Correspondingly, the
1
.15 at 867 K and then slowly decreases to unity as the
temperature approaches infinity. Since the SYM and MMI terms
are temperature independent,17 the occurrence of a maximum
is a result of the ZPE and EXC terms opposing each other. Thus,
the ZPE term increases from zero to a limiting value of unity
[
SYM ‚ MMI ‚ EXC] and entropy [exp(∆∆S/R)] terms are not identical,
but differ by a similar component to that relating ZPE and enthalpy.
However, these differences are small such that ZPE ≈ [exp(-∆∆H/RT)]
and [SYM ‚ MMI ‚ EXC] ≈ [exp(∆∆S/R)] are good approximations. For
example, at 100 K, ZPE ) 0.002 and [exp(-∆∆H/RT)] ) 0.002, whereas
SYM ‚ MMI ‚ EXC ) 5.48, and exp(∆∆S/R) ) 5.48; at 1000 K, ZPE )
0
.542 and [exp(-∆∆H/RT)] ) 0.547, whereas SYM ‚ MMI ‚ EXC ) 2.12,
and exp(∆∆S/R) ) 2.11.
(19) EXC is unity at 0 K, and so [SYM ‚ MMI ‚ EXC] ) [SYM ‚ MMI]
(
16) The EIE at 300 K is 0.57 in good agreement to the value of 0.46 calculated
by Krogh-Jespersen and Goldman (ref 3a).
J. AM. CHEM. SOC.
9
VOL. 125, NO. 43, 2003 13221

A R T I C L E S
Janak and Parkin
Table 2. Equilibrium Constant and Equilibrium Isotope Effect Data
for Oxidative Addition of H2 and D2 to Ir(PMe2Ph)2(CO)Cl in
Benzene (errors are reported at the 95% confidence level)
-
1
-1
T (°C)
K
H
(M )
K
D
(M )
H D
EIE ) K /K
4
4
4
3
3
3
2
2
2
2
2
1
1
4
4
4
4
3
3
2
2
2
2
1
1
2
3
3
4
5
7
8
5
0
5
5
5
0
5
2.5(2) × 10
2.03(9) × 10
1.5(2) × 10
6.3(3) × 10
3.4(3) × 10
1.97(8) × 10
8.9(3) × 10
3.8(3) × 10
3.12(4) × 10
2.0(1) × 10
1.36(3) × 10
1.8(1) × 10
1.21(7) × 10
7.4(3)
6.0(5) × 10
4.1(1) × 10
2.79(9) × 10
1.03(5) × 10
5.15(5) × 10
2.7(3) × 10
9.7(4) × 10
3.6(2) × 10
2.53(4) × 10
1.60(3) × 10
9.7(3) × 10
1.65(3) × 10
9.8(4)
0.41(4)
0.49(3)
0.52(6)
0.62(4)
0.65(5)
0.74(8)
0.92(5)
1.05(9)
1.23(3)
1.23(8)
1.41(6)
1.13(10)
1.24(9)
1.19(6)
1
1
1
1
1
00
10
20
30
40
Figure 5. Calculated temperature dependence of ∆∆H ) ∆HH - ∆HD
158
185
-
1
and ∆∆S ) ∆SH - ∆SD. ∆∆H varies from 1.22 to 1.20 kcal mol from
6.2(2)
0
to 1000 K, whereas ∆∆S varies from 3.41 to 1.48 e.u.
Table 3. Equilibrium Constant and Equilibrium Isotope Effect Data
for Oxidative Addition of H2 and D2 to Ir(PMe2Ph)2(CO)Cl in
Toluene (errors are reported at the 95% confidence level)
stabilization of [MD2] versus [MH2] to be greater than that for
D2 versus H2. Although EXC < 1, the combined [SYM ‚ MMI
‚
EXC] term is >1 because it is dominated by the [SYM ‚ MMI]
T (°C)
K
H
(M−1)
K
D
(M−1)
H D
EIE ) K /K
component (5.55). The combined [SYM ‚ MMI] term is
equivalent to the product of the translational and rotational
partition function ratios of the products and reactants, i.e.,
4
3
3
3
2
2
2
4
3
3
3
2
2
2
4
5
7
5
5
0
1.05(3) × 10
5.5(3) × 10
2.4(1) × 10
1.45(9) × 10
6.8(4) × 10
4.8(1) × 10
4.03(5) × 10
1.72(3) × 10
8.0(5) × 10
3.19(5) × 10
1.68(7) × 10
7.1(3) × 10
4.69(8) × 10
3.7(1) × 10
0.61(2)
0.69(6)
0.76(4)
0.87(7)
0.95(8)
1.02(3)
1.10(4)
1.36(10)
1.45(8)
1.29(14)
P
P
R
R 17
(Qtr )(Qrot )/(Qtr )(Qrot ) and the dominant component is
85
00
10
15
1
1
1
associated with the large difference in entropy between H2 and
D2 as compared to that between [MH2] and [MD2]. Specifically,
translational entropy of D2 is greater than that of H2 because of
2.51(6) × 102
1.9(1) × 102
120
130
2
0
2
2
a mass effect, whereas the rotational entropy of D2 is greater
than that of H2 because (i) the rotational levels of D2 are more
closely spaced than those of H2 due to a mass effect via the
moment of inertia,21 and (ii) the degeneracy of the rotational
levels for D2 are greater than those for H2.2
1.77(5) × 10
1.22(6) × 10
158
10.4(1)
8.1(3)
2
The calculations thus predict that the EIE for oxidative
addition of dihydrogen to Ir(PH3)2(CO)Cl would exhibit a
transition from an inverse to normal value due to competition
between the ZPE (enthalpy) and [SYM ‚ MMI ‚ EXC] (entropy)
terms. At low temperatures, the ZPE enthalpy component
dominates and the EIE is inverse, whereas at high temperatures
the [SYM ‚ MMI ‚ EXC] entropy component dominates and
causes a transition from an inverse to normal EIE. Experimental
evidence for this transition was, therefore, sought.
2
. Experimental Evidence for a Transition from Inverse
to Normal EIE for Oxidative Addition of H2 and D2 to Ir-
PMe2Ph)2(CO)Cl. For the experimental study, we chose to
investigate oxidative addition of H2 and D2 to Ir(PMe2Ph)2(CO)-
(
Figure 6. Temperature dependence of the EIE for oxidative addition of
H2 and D2 to Ir(PMe2Ph)2(CO)Cl in benzene.
2
3
1
Cl due to (i) the favorable H NMR spectroscopic properties
associated with the PMe2Ph versus PPh3 ligand, and (ii) the
fact that the reaction with H2 to give Ir(PMe2Ph)2(CO)ClH2 is
(
20) ∆Strans[D
2
- H
21) The energy of the rotational levels is given by ꢀ
I is the moment of inertia. Since I(D ) > I(H
more closely spaced for D
Rln[I(D )/I(H )] ) 1.36 e.u.
2
] ) (3/2)Rln[M(D )/M(H )] ) 2.04 e.u.
2
2
2
2
(
J
) (h /8π I)J(J + 1) where
2
2
), the rotational levels are
9
facile. The equilibrium constants for the oxidative addition of
2
2 2
. At high temperature, ∆Srot[D - H ] )
1
2
2
H2 and D2 in benzene and toluene were determined by H NMR
(
22) Homonuclear diatomic molecules with nuclei of integer spin require odd J
rotational levels to be coupled with I(2I + 1) antisymmetric nuclear spin
functions (i.e., para), whereas even J rotational levels are coupled with the
spectroscopy, from which the EIE at each temperature was
determined (Tables 2 and 3). Most interestingly, the data support
the theoretical prediction that the EIE undergoes a transition
from inverse to normal as the temperature is raised. Thus, while
KH is less than KD at 25 °C, KD decreases more rapidly than
does KH upon raising the temperature, such that they become
equal at ca. 90 °C; above this temperature, KH becomes greater
than KD (Figure 6). Hence, in accord with the calculations on
Ir(PH3)2(CO)ClH2 and Ir(PH3)2(CO)ClD2, the strongly inverse
EIE [0.41(4)] observed for oxidative addition of H2 and D2 to
Ir(PMe2Ph)2(CO)Cl at 25 °C in benzene becomes normal at
(
I + 1)(2I + 1) symmetric nuclear spin functions (i.e., ortho). For nuclei
with half-integer spins, the situation is reversed and odd J rotational levels
are coupled with (I + 1)(2I + 1) symmetric nuclear spin functions and
even J rotational levels are coupled with I(2I + 1) antisymmetric nuclear
spin functions. Thus, while both H
degenerate odd J levels, even J levels for D
degenerate, respectively. At high temperature, the entropy associated with
the different nuclear spin state degeneracies is ∆Snuc[D - H ] ) Rln-
+ 1) ] ) Rln(9/4) ) 1.61. See: (a) McQuarrie, D. A.
Statistical Mechanics”, Harper and Row: New York 1976. (b) Herzberg,
2
(I ) 1/2) and D
2
(I ) 1) have triply
2
and H are 6-fold and singly
2
2
2
2
2
{
“
D H
[(2I + 1) /(2I
G. Molecular Spectra and Molecular Structure. II Infrared and Raman
Spectra of Polyatomic Molecules, van Nostrand: New York 1959 pp 508-
5
09.
(
23) Deeming, A. J.; Shaw, B. L. J. Chem. Soc. (A) 1968, 1887-1889.
1
3222 J. AM. CHEM. SOC. VOL. 125, NO. 43, 2003
9

Oxidative Addition of H
2
to Ir(PMe
2
Ph)
2
(CO)Cl
A R T I C L E S
Figure 7. van’t Hoff plot for oxidative addition of H2 and D2 to Ir(PMe2-
Ph)2(CO)Cl in benzene over the range 25-130 °C.
Figure 8. van’t Hoff plot for oxidative addition of H2 and D2 to Ir(PMe2-
Ph)2(CO)Cl in toluene over the range 25-130 °C.
Table 4. Enthalpy and Entropy Data for Oxidative Addition of H2
and D2 to Ir(PMe2Ph)2(CO)Cl in Benzene and Toluene Determined
from van’t Hoff Plots (errors are one standard deviation)
temperatures greater than ca. 90 °C, and reaches a maximum
value of 1.41(6) at 130 °C (Figure 6). It should, however, be
noted that despite the qualitatively similar form of the experi-
mental temperature dependence of the EIE for oxidative addition
of H2 to Ir(PMe2Ph)2(CO)Cl to the computed value for Ir(PH3)2-
benzenea
tolueneb
∆
∆
HH (kcal mol-1)
-12.0(2)
-14.6(3)
2.6(4)
-20(1)
-27(1)
7(1)
-11.9(4)
-14.3(6)
2.4(7)
-19(1)
-25(2)
6(2)
-1
HD (kcal mol
)
(CO)Cl, there are quantitative differences. For example, the
∆∆H ) ∆HH - ∆H (kcal mol-1)
D
experimental transition from inverse to normal EIE for Ir(PMe2-
Ph)2(CO)Cl occurs at ca. 370 K (benzene), whereas that
computed for Ir(PH3)2(CO)Cl occurs at 510 K. Such discrepancy
is understandable on the basis that the experimental and
computational systems feature different PR3 ligands, i.e., PMe2-
Ph versus PH3, the latter being employed for computational
expediency. In addition, anharmonicity, which is not taken into
account in the computational study, will also influence the
temperature at which the transition from inverse to normal EIE
occurs. Specifically, introduction of anharmonicity to the
vibrational potential energy surface would result in the vibra-
tional energy levels becoming more closely spaced, thereby
affecting both the ZPE and EXC terms; however, the most
∆SH (e.u.)
∆
∆
SD (e.u.)
∆S ) ∆SH - ∆SD (e.u.)
a Temperature range 25-130 °C. b Temperature range 45-130 °C.
indicates that ∆∆H (ZPE) favors oxidative addition of D2 versus
H2 (i.e., an inverse EIE), whereas ∆∆S (SYM ‚ MMI ‚ EXC)
favors oxidative addition of H2 versus D2 (i.e., a normal EIE).
By comparison to the ∆∆H term, the ∆∆S component is small
[7(1) e.u.] so that a normal EIE is only obtained at high
temperatures when the T∆∆S term competes with ∆∆H.
The nature of an isotope effect (i.e., normal versus inverse)
is often invoked to infer details of the reaction coordinate and
structural changes of the molecules involved. However, since
both normal and inverse equilibrium isotope effects have now
been obtained in the same system, it is apparent that conclusions
derived from the magnitude of an isotope effect at a single
temperature may be more unreliable than previously considered.
For example, on the basis of the typical explanation for the
nature of an equilibrium isotope effect, i.e., deuterium prefers
2
4
significant effect has been reported to be on the ZPE term.
Anharmonic effects generate a less inverse ZPE term and
thereby result in both (i) a lower temperature for the transition
from inverse to normal and (ii) a larger maximum value for the
observed EIE.
Although the calculated temperature dependence of the EIE
does not follow van’t Hoff behavior over the full temperature
range (Figures 2 and 3), a linear dependence between lnK and
2
to be located in the highest frequency oscillator, the observation
of a normal EIE for oxidative addition of H2 at elevated
temperatures could have been interpreted as being due to a zero
point energy effect resulting from the H2 (D2) molecule having
a stronger bond than the combined Ir-H(D) bonds. However,
the zero point energy term is actually inverse and the origin of
the normal EIE at elevated temperature is purely due to entropy
effects dominating.
1
/T is observed for the experimental data over a significant
portion of the temperature range of the study for the individual
reactions of H2 and D2 (Figures 7 and 8). The enthalpies and
entropies determined from these plots (Table 4) indicate that
both ∆H and ∆S are more positive for addition of H2 than for
addition of D2: for benzene solution ∆∆H ) ∆HH - ∆HD )
1
2.6(4) kcal mol- and ∆∆S ) ∆SH - ∆SD ) 7(1) e.u. As noted
above, an important component of the ∆∆S term is the
Experimental Section
difference in entropies of H2 (34.0 e.u. at 300 K) and D2 (39.0
General Considerations. All manipulations were performed using
a combination of glovebox, high vacuum, and Schlenk techniques under
an argon or dinitrogen atmosphere. Solvents were purified and degassed
by standard procedures. Ir(PMe
method for related derivatives.27 Benzene-d
2
5,26
e.u. at 300 K).
The positive ∆∆H and ∆∆S values obtained from the
experimental study support the computational study which
2
Ph)
2
(CO)Cl was prepared by a literature
and toluene-d were
6
8
(
(
(
24) Torres, L.; Gelabert, R.; Moreno, M.; lluch, J. M. J. Phys. Chem. A 2000,
04, 7898.
25) Wooley, H. W.; Scott, R. B.; Brickwedde, F. G. J. Res. Natl. Bur. Standards
948, 41, 379-475.
26) Note that the entropy difference between H
obtained from Cambridge Isotopes and dried over 4Å molecular sieves
1
1
prior to use. H NMR spectra were measured on Bruker 300 DRX and
1
Bruker Avance 500 DMX spectrometers.
2
2
and D in the absence of
contributions from nuclear spin statistics is 3.4 e.u. See ref 18 and Handbook
of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; CRC Press: Boca
Raton 1991; p 5-35.
(27) Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Inorg. Chem.
2001, 40, 5626-5635.
J. AM. CHEM. SOC.
9
VOL. 125, NO. 43, 2003 13223

A R T I C L E S
Janak and Parkin
Determination of the EIE for Oxidative Addition of H
2
and D
2
Computational Details
All calculations were carried out using DFT as implemented in the
Jaguar 4.1 suite of ab initio quantum chemistry programs. Geometry
optimizations and frequency calculations were performed with the
B3LYP37 functional and the 6-31G** (C, H, O, P, and Cl) and
LACVP** (Ir) basis sets. The Cartesian coordinates for the derived
to Ir(PMe
Ph) (CO)Cl in C
saturated with H
2
Ph)
2
(CO)Cl. In a typical experiment, a solution of Ir(PMe
(0.5 mL of 15 mM) in a gastight NMR tube was
(1 atm at 24 °C). The sample was placed in a constant-
2
-
2
6 6
D
3
6
2
temperature bath ((1 °C), and removed periodically to monitor (by
H NMR spectroscopy) the conversion to the equilibrium mixture with
1
Ir(PMe
2
Ph)
2
(CO)ClH
2 2 2
. The molar ratio of Ir(PMe Ph) (CO)Cl to Ir-
geometries of Ir(PH
frequencies for Ir(PH
listed in the Supporting Information. The MMI term was determined
from the geometries of Ir(PH (CO)ClX and X (X ) H, D), whereas
3 2
) (CO)ClH
2
2
and H , together with the vibrational
1
(
PMe Ph)
2
2
(CO)ClH
2
was determined directly from the H NMR spectra,
)
3 2
(CO)ClH
2
, Ir(PH
3
)
2
(CO)ClD , H , and D are
2
2
2
whereas the molar concentration of dihydrogen in solution at equilib-
rium was determined by a method similar to that described previously,28
using a combination of Henry’s law with the mole fraction solubility
)
3 2
2
2
the EXC and ZPE terms at the temperature of interest were determined
from the vibrational frequencies (see Scheme 2). The EIE was
determined at 1 degree intervals over the range 0-3000 K.
of H
constant requires knowledge of the total amount of H
in the closed system, which is the sum of that in the gas phase above
the solution and that in solution. The number of moles of H in the gas
phase is determined using the ideal gas law and knowledge of the partial
2
obtained from the literature data. The calculation of equilibrium
2
initially present
Conclusions
2
In summary, an experimental investigation of the oxidative
addition of hydrogen to Ir(PMe2Ph)2(CO)Cl provides support
for the theoretical prediction that the EIE for this reaction
exhibits a temperature-dependent transition from an inverse
value to a normal value upon increasing the temperature. Since
the EIE for oxidative addition of hydrogen to {[H2Si(C5H4)2]W}
also exhibits this transition, it will be of interest to determine
how general is this phenomenon and whether it also applies to
the coordination of hydrogen to give dihydrogen complexes.
The observation of both normal and inverse EIEs for the same
system indicate that inferences pertaining to the magnitude of
an isotope effect at a single temperature may require more
detailed consideration than previously realized.
2
9
pressure of H
The number of moles of H
solubility expressions given by Clever. At equilibrium, the concentra-
tion of H is determined from the total number of moles of H present
at equilibrium. Specifically, the concentration of H in solution is
obtained by solving the two simultaneous equations that relate (i) the
number of moles of H in the gas phase to the partial pressure of H in
the gas phase via the ideal gas law, and (ii) the number of moles of H
2
and the volume of the headspace above the solution.
2
initially in solution is calculated from the
30
2
2
3
1
2
2
2
2
3
0
in solution to the partial pressure of H
The equilibrium constant (K ) was measured as a function of
temperature (Table 2) and ∆H and ∆S were determined from a van’t
Hoff plot over the temperature range 25-130 °C. An analogous
procedure was used to determine K as a function of temperature for
the corresponding reaction between Ir(PMe
which the EIE in benzene was determined. The equilibrium constants
for oxidative addition of H and D were measured in C CD using
2
in the gas phase.
H
3
2
D
33
2 2 2
Ph) (CO)Cl and D , from
Acknowledgment. We thank the U. S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-93ER14339) for
support of this research.
2
2
D
6 5
3
3
4,35
a similar protocol.
(
28) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M.
Supporting Information Available: Computational details (5
pages, print/PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1
987, 109, 203-219.
(29) In a system under 1 atm of pressure, the partial pressure of H
2
in the gas
phase is 1 atm minus the partial pressure of solvent: for benzene, log pbenzene
)
-1784.8/T + 7.9622, where T is the temperature at which the sample is
JA0362611
prepared; for toluene, log ptoluene ) -2047.3/T + 8.330. See: Handbook
of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; CRC Press: Boca
Raton 1991; p 6-84.
2
(34) Mole fraction solubility of H in toluene: ln x ) -6.0373-603.07/(T/K).
(
30) Mole fraction solubility of H
2
in benzene: ln x ) -5.6949-765.47/(T/K).
See: Clever, H. L. In Solubility Data Series; Young, C. L., Ed.; Pergamon
See: Clever, H. L. In Solubility Data Series; Young, C. L., Ed.; Pergamon
Press: Oxford, 1981; Volume 5/6: Hydrogen and Deuterium, p 169.
Press: Oxford, 1981; Volume 5/6: Hydrogen and Deuterium, p 164.
2
(35) Mole fraction solubility of D in toluene: ln x ) -6.0781-580.52/(T/K).
(
31) The total number of moles of H
2
(D
2
) present at equilibrium is determined
Ph) (CO)Cl and Ir(PMe Ph) (CO)-
(X ) H, D) which indicates how much of the originally present H
) has reacted.
See: Clever, H. L. In Solubility Data Series; Young, C. L., Ed.; Pergamon
Press: Oxford, 1981; Volume 5/6: Hydrogen and Deuterium, p 288.
(36) Jaguar 4.1 Schr o¨ dinger, Inc., Portland, Oregon, 2001.
by the relative integration of Ir(PMe
ClX
2
2
2
2
2
2
(D
2
(37) (a) Slater, J. C. Quantum Theory of Molecules and Solids, Vol. 4: The
Self-Consistent Field for Molecules and Solids; McGraw-Hill: New York,
1974. (b) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-
1211. (c) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37,
785-789. (d) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100. (e) Becke,
A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(
32) The van’t Hoff plot does not include the high-temperature data due to the
deviation from ideal behavior at high temperature (see text).
(
2
33) Mole fraction solubility of D in benzene: ln x ) -5.7399-743.44/(T/K).
See: Clever, H. L. In Solubility Data Series; Young, C. L., Ed.; Pergamon
Press: Oxford, 1981; Volume 5/6: Hydrogen and Deuterium, p 287.
13224 J. AM. CHEM. SOC.
9
VOL. 125, NO. 43, 2003