Transition metal catalyzed D2/H2O exchange: Distinguishing between the single and double exchange pathways

Article abstract of DOI:10.1016/j.molcata.2006.11.037

Isotopic exchange between D₂ (g) and H₂O (l) is catalyzed by the water soluble complexes Rh(TPPTS)₃Cl and Rh(TPPMS)₃Cl (where TPPTS = tris(3-sulfonatophenyl)phosphine, trisodium salt and TPPMS = 3-sulfonatopheny

Full text of DOI:10.1016/j.molcata.2006.11.037

Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
Transition metal catalyzed D2/H2O exchange: Distinguishing
between the single and double exchange pathways
Jennifer L. Carrikera, Paul S. Wagenknechta,∗, Mitra A. Hosseinib, Patrick E. Flemingb
a Department of Chemistry, Furman University, Greenville, SC 29613, United States
b Department of Chemistry, San Jose State University, San Jose, CA 95192, United States
Received 21 September 2006; received in revised form 20 November 2006; accepted 21 November 2006
Available online 26 November 2006
Abstract
Isotopic exchange between D2 (g) and H2O (l) is catalyzed by the water soluble complexes Rh(TPPTS)3Cl and Rh(TPPMS)3Cl (where
TPPTS = tris(3-sulfonatophenyl)phosphine, trisodium salt and TPPMS = 3-sulfonatophenyldiphenylphosphine, sodium salt). This isotope exchange
has been monitored by gas chromatographic analysis of the gaseous headspace over the aqueous catalyst solution. Upon binding to the catalyst, the
hydrogen isotopologues can either exchange one or both atoms with solvent. Kinetic schemes have been developed for both situations. Analysis
of exchange data demonstrates that for both catalysts at pH ≥ 8 the D2 molecule only undergoes one exchange event per visit to the active site. At
lower pH values this changes. Rh(TPPMS)3Cl shows evidence of double exchange at pH 7 and below. Rh(TPPTS)3Cl shows evidence of double
exchange at pH 6 and below. Rate constants for both single and double exchange can be determined and have been analyzed in the pH range from
3 to 12. Mechanistic implications of the rate constants are discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: H/D exchange; Hydrogenase enzymes; Dihydrogen complex acidity; Kinetics; Water soluble phosphines; H2/D2O exchange
1. Introduction
Exchange studies involving the hydrogenase enzymes often
employ a mass spectroscopic measurement of the gaseous
products in the headspace over a solution of the enzyme
[1,3,4,18–26]. Such studies can distinguish between single
exchange (Eq. (1)) and double exchange (Eq. (2)).
Transition metal catalyzed H2/D+ and D2/H+ exchange reac-
tions, where the H+ and D+ originate from water or alcohols,
have been of significant interest because of the relevance of
these processes to the study of the hydrogenase enzymes [1–11],
transition metal dihydrogen complexes [7–13], and catalytic
hydrogenations [14–17]. For example, the D2/H2O exchange
catalyzed by the hydrogenase enzymes has been instrumen-
tal in monitoring the activity and studying the mechanism of
this important class of enzymes [1–4,18–26]. Consequently,
this exchange process has often been a primary screening tool
for functional models of the hydrogenase enzymes [5–11].
Such functional models usually invoke heterolytic cleavage
of dihydrogen through the intermediacy of a transition metal
dihydrogen complex [7–13]. Recent interest in performing
hydrogenations in aqueous solution has also kindled an inter-
est in the ability of water soluble hydrogenation catalysts to
catalyze this type of H/D exchange [15–17].
D2 + H2O → HD + HDO
D2 + 2H2O → H2 + 2HDO
(1)
(2)
ThedistinctionhereiswhetheronlyonenucleusofD2 exchanges
or both nuclei exchange during one visit to the active site. The
observation of more H2 than HD in the headspace at early
reaction times indicates that double exchange predominates.
However, it is worth noting that the observation of initial H2/HD
ratios greater than one in the headspace could also result from
reaction of HD with a second enzyme molecule before escaping
from the aqueous phase into the gas phase. If this were the case,
increasing the concentration of the enzyme would increase the
observedH2/HDratio. Thistesthascommonlybeenemployedin
studies of hydrogenase enzymes in order to distinguish between
true double exchange and the more trivial concentration depen-
dent case [18–21]. Different hydrogenase enzymes differ in both
∗
Corresponding author. Tel.: +1 864 294 2905; fax: +1 864 294 3559.
E-mail address: paul.wagenknecht@furman.edu (P.S. Wagenknecht).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.11.037

J.L. Carriker et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
219
theircapacitytofacilitatedoubleexchange[3,18–25]andintheir
overall exchange activity as a function of pH [3,18–26].
Most studies with transition metal complexes, on the other
placed in a 5 mL gas-tight vial with a stir bar. The gas-tight
vial was a Kontes reaction vial fitted with a valved cap that
enables gas sampling by syringe. The vial was removed from
the glovebag, and D2 (g) was bubbled through the solution for
approximately one minute by inserting the needle through the
valve and slightly loosening the cap. The cap was tightened, the
needle was removed, and the solution was stirred at 25 ◦C on
a temperature-controlled stir plate. At regular intervals, using
a gas-tight syringe, 25 ␮L headspace samples were removed
through the valve and injected onto the GC column.
1
hand, have been performed by H NMR studies of H2/D2O
exchange. Here the total isotopic exchange is monitored by
measuring the increase in the HDO signal with time. Thus, no
distinction can be made between single exchange and double
exchange. Even if H2O were formed in a double exchange it
would result in HDO due to rapid isotope exchange. We have
´
been particularly intrigued with reports by Kovacs et al. con-
cerning H2/D2O exchange catalyzed by ruthenium and rhodium
complexes with water soluble phosphines such as TPPTS
(tris(3-sulfonatophenyl)phosphine, trisodium salt) and TPPMS
(3-sulfonatophenyldiphenylphosphine, sodium salt) [15–17].
These reports demonstrate that the pH profile of the turnover
frequency (TOF) is asymmetric. At high pH (∼11–12), no
exchange is observed, whereas the TOF approaches a constant
valueatlowpH. Wehavebeencurioustodetermineifthereisany
difference in single versus double exchange for these systems
as a function of pH and to use such information to gain a bet-
ter mechanistic understanding of these processes. Adding to our
interest in the Rh(TPPTS)3Cl and Rh(TPPMS)3Cl complexes is
that complexes of this type have been shown to be catalysts for
the reduction of NAD(P)+ to NAD(P)H using hydrogen gas [27].
Herein, we report the measurement of D2/H2O exchange with
Rh(TPPTS)3Cl and Rh(TPPMS)3Cl as a function of pH using
a gas chromatographic method that distinguishes between sin-
gle and double exchange. These studies demonstrate that under
basic conditions a single exchange mechanism predominates,
whereas under acidic conditions there is a significant component
of double exchange. Mechanistic implications of this discovery
will be discussed.
2.3. Kinetic modelling
D2, HD and H2 concentrations were expressed as percent-
ages of the total hydrogen concentration in the headspace of the
reaction vessel. Thus, the constraint
[D2] + [HD] + [H2] = 100%
(3)
holds at all times during the reaction, with each concentration
calculated as the percentage of total area under the curves for D2,
HD and H2 peaks in the gas chromatogram. It is assumed that the
concentration of each isotopologue in solution is proportional to
the partial pressure of the species in the head space, according
to what is predicted by Henry’s Law.
The measured concentrations were fit to the relationships in
Eqs. (8)–(10) as predicted by the effective mechanism shown
in Eqs. (5)–(7) (vide infra). Rate constants k1 and k2 were cho-
sen using a non-linear least squares fitting procedure so as to
minimize χ2 defined in Eq. (4).
ꢀ
χ2 =
([D2]t,obs. − [D2]t,calc.)2 + ([HD]t,obs. − [HD]t,calc.
)
2
t
2
2. Experimental
+ ([H2]t,obs. − [H2]t,calc.
)
(4)
It is estimated that the individual rate constants determined by
this procedure have an uncertainty of less than 5%. To study
variability among duplicate experiments, four replicate mea-
surements were performed for Rh(TPPMS)3Cl at pH 7. The
standard deviation for k1 was 19% of the mean and for k2 it was
11% of the mean.
2.1. Materials and methods
Rh(TPPMS)3Cl [28] and [RhCl(␩2-C2H4)2]2 [29] were pre-
pared according to the literature procedures. Rh(TPPTS)3Cl was
prepared by a slight modification of Method C by Herrmann and
Kohlpaintner [30] where [RhCl(␩2-C2H4)2]2 was used in place
of [RhCl(␩4-C8H12)]2. Buffer solutions (0.1 M) were prepared
and checked using a Fisher Scientific Accumet Model 25 pH
meter. The composition of each buffer solution was as follows:
pH 3, 4, 5 – phthalate buffer; pH 6, 7, 8, 12 – phosphate buffer;
pH 9 – borate buffer; and pH 10 – bicarbonate buffer [31]. Sep-
aration and quantification of H2, HD, and D2 was performed
by gas chromatography on a Hewlett Packard 5890 A gas chro-
matograph at 77 K using an Al2O3 column as described in the
literature [33].
3. Results and discussion
3.1. General exchange results
The progress of D2/H2O exchange reactions catalyzed by
Rh(TPPTS)3Cl and Rh(TPPMS)3Cl has been monitored in
buffered aqueous solutions between pH 3 and pH 12 at 25 ◦C
under 1 atm of D2. The headspace over the stirred aqueous cat-
alyst solution was sampled approximately every 20 min and
analyzed for %H2, HD, and D2. This is demonstrated in Fig. 1
for Rh(TPPTS)3Cl at pH 4 and in Fig. 2 for Rh(TPPTS)3Cl at pH
8. The turnover frequencies (TOF) as a function of pH for both
catalysts (calculated from the total moles of H appearing in the
headspace over the first 20 min: TOF = mol H/mol Rh·0.33 h)
are shown in Figs. 3 and 4.
2.2. Isotopic exchange methods
In a typical experiment, buffer solutions were degassed
on an Ar Schlenk line. Solutions of Rh(TPPTS)3Cl and
Rh(TPPMS)3Cl (2.2 mM) in 1 mL buffer solutions were pre-
pared in a glove bag under argon gas. The 1 mL solutions were

220
J.L. Carriker et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
Fig. 4. Turnover frequency for the Rh(TPPMS)3Cl catalyzed D2/H2O exchange
reaction as a function of pH. [Rh(TPPMS)3Cl] = 2.2 mM, initial pD2 = 1 atm.
Other than the method of analysis, there are four key differ-
ences in how we performed these experiments versus previously
published exchange reactions with these catalysts [15,16]. First,
the solutions in this report are buffered. Second, these stud-
ies were performed at 1 atm instead of 20 atm. Third, we have
performed D2/H2O exchange instead of H2/D2O exchange in
order to simplify buffer preparation. Fourth, we have not used
a three-fold excess of the phosphine ligands. With regard to
these differences: (1) Buffered solutions in this report are used
only as a precaution. (2) Lower pressures were necessitated by
Fig. 1. Percent composition as a function of time for the Rh(TPPTS)3Cl cat-
alyzed D2/H2O exchange reaction at pH 4. [Rh(TPPTS)3Cl] = 2.2 mM, initial
pD2 = 1 atm. Lines represent fits to the data. The observation of HD production
lagging behind the production of H2 is an indication of a significant contribution
due to double exchange.
´
our experimental set up. However, Kovacs et al. stated that for
one of the systems they fully analyzed, the rate was linear with
pressure at least down to atmospheric pressure of hydrogen.
Furthermore they described that higher than atmospheric pres-
sures were only necessary to maintain sufficient H2 in the small
´
headspace above the solution in the NMR tube. (3) Kovacs et
al. reported a negligible isotope effect (∼10%) for H2/D2O ver-
sus D2/H2O exchange catalyzed by Rh(TPPMS)3Cl [16], and
we also have not observed significant isotope effects for these
exchange reactions. (4) We tested the effect of excess ligand
on the Rh(TPPTS)3Cl catalyzed exchange and found no depen-
dence of the TOF on added ligand (0–10 equiv.). Thus, none
of these differences are expected to drastically affect the pH
profiles. Optimal turnover frequencies in our experiments are
´
in reasonable agreement with those reported by Kovacs et al.
Fig. 2. Percent composition as a function of time for the Rh(TPPTS)3Cl cat-
alyzed D2/H2O exchange reaction at pH 8. [Rh(TPPTS)3Cl] = 2.2 mM, initial
pD2 = 1 atm. Lines represent fits to the data.
[16] when differences in pressure are accounted for. In addi-
tion, the pH profiles exhibit similar overall shapes to those
reported, even showing a slight dip in rate near neutral pH for
the Rh(TPPMS)3Cl catalyzed exchange.
Of particular interest is the shape of the plot of TOF versus
pH, that is, the rather steep descent from the maximum TOF
at pH > 7 but the flattening out of the TOF to a relatively
constant value under acidic conditions. Assuming the exchange
mechanism involves deprotonation and reprotonation of a
´
dihydrogen complex intermediate as Kovacs et al. suggest [16],
one would expect a maximum exchange rate at or near the
pKa of the dihydrogen complex intermediate with a relatively
symmetric decrease in exchange rate at higher and lower pH
values. Interestingly, the D2/H2O exchange processes catalyzed
by the hydrogenase enzymes often go through a maximum
near physiological pH and the rate typically falls off reasonably
Fig. 3. Turnover frequency for the Rh(TPPTS)3Cl catalyzed D2/H2O exchange
reaction as a function of pH. [Rh(TPPTS)3Cl] = 2.2 mM, initial pD2 = 1 atm.

J.L. Carriker et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
221
symmetrically at higher and lower pH values [3,18,19,21,22].
Thus, a closer examination of the details of the exchange rate
versus pH is necessary in order to understand the asymmetry of
the exchange activity for these catalysts.
3.2. Theoretical analysis of single versus double exchange
As mentioned above, hydrogenase enzymes from different
organisms differ in their capacity to effect single versus dou-
ble exchange [3,18–25]. For the case of D2/H2O exchange, if
H2 is observed before the appearance of HD, it is assumed
that double exchange is occurring. It is instructive, however, to
attempt to better quantify single versus double exchange. Eqs.
(5) through (7) are generalized equations intended only to dis-
tinguish between single exchange (Eqs. (5) and (6)) and double
exchange (Eq. (7)). Within the context of this article these pro-
cesses are assumed to be catalyzed by the rhodium catalysts and
thus require binding of dihydrogen to the rhodium complex.
Fig. 5. Predicted fractional composition of the headspace during D2/H2O
exchange in the presence of a large excess of H2O and assuming no isotope
effects and no double exchange. Note that if only single exchange is important,
H2 production necessarily lags behind HD production at early reaction times.
k
1
of single versus double exchange for a given set of conditions
should be possible.
D2 + H2O−→ HD + HDO
(5)
(6)
(1/2)k
1
HD + H2O −→ H2 + HDO
3.3. Analysis of exchange reactions catalyzed by
Rh(TPPTS)3Cl and Rh(TPPMS)3Cl
k
2
D2 + 2H2O−→ H2 + 2HDO
(7)
For the case of pure single exchange, the only way to obtain H2
is for HD that was formed in Eq. (5) to return to the active site
and undergo another exchange event (Eq. (6)). The rate constant
for this exchange event, ignoring any isotope effects, would be
(1/2)k1 due to a statistical factor that takes into account that
one-half of these exchange events are non-productive. These
equations also assume a large excess of H2O so that the reverse
processes are negligible. If both single and double exchange
are operable, then Eqs. (5)–(7) must be considered simultane-
ously. The resulting integrated rate expressions are given in Eqs.
(8)–(10), the derivations of which can be found in Appendix A.
For both catalysts in aqueous solutions at pH ≥ 8, the
D2/H2O exchange data shows no significant evidence of double
exchange, i.e., the data can be satisfactorily fit to a single rate
constant, k1, using Eqs. (11)–(13) as demonstrated in Fig. 2.
One implication of this good fit is that the assumption of a
negligible isotope effect holds for these systems, as had been
´
inferred by Kovacs et al. [16]. Both single and double exchange
must be considered for the TPPTS complex at pH ≤ 6 and for
the TPPMS complex at pH ≤ 7. These data sets can be satis-
factorily fit using Eqs. (8)–(10) as demonstrated in Fig. 1 and
this results in rate constants for both single exchange, k1, and
double exchange, k2. A graphical analysis of the rate constants
for single and double exchange for the TPPTS and TPPMS
complexes are shown in Figs. 6 and 7, respectively. Figs. 6 and 7
clearly demonstrate that double exchange predominates at lower
pH. The rate constants for single exchange proceed through
[D2]t = [D2]0e−(k +k )t
(8)
(9)
1
2
ꢁ
ꢂ
k1
1
1
2
[HD]t = [D2]0
(e−(1/2)k t − e−(k +k )t
)
k2 + (1/2)k1
[H2]t = [D2]0 − ([D2]t + [HD]t)
(10)
If only single exchange is occurring, i.e., k2 is negligibly small,
the isotopic compositions as a function of time simplify to Eqs.
(11)–(13).
[D2]t = [D2]0e−k t
(11)
(12)
(13)
1
[HD]t = [D2]0(2e−(1/2)k t − 2e−k t
)
1
1
[H2]t = [D2]0 − ([D2]t + [HD]t)
Theoretical isotopic composition curves for the single exchange
case are shown in Fig. 5. The shapes of the isotopic com-
position curves when both single and double exchange are
operable depend on the relative values of k1 and k2. By fit-
ting data obtained from catalyzed D2/H2O exchange reactions to
the above equations, a determination of the relative importance
Fig. 6. pH profile for D2/H2O exchange with Rh(TPPTS)3Cl catalyst.

222
J.L. Carriker et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
can thus be described in terms of the lifetime of species III
versus the “proton” exchange rate. If dihydrogen isotopologue
release from species III is more rapid than reprotonation of III
to form an isotopologue of II, single exchange should predom-
inate. If reprotonation of III is faster, then double exchange
predominates. For the catalysts cited herein, this competition is
controlled by pH (with double exchange predominating at low
pH).
Note that the mechanism in Fig. 8 involves protonation of I,
followed by deprotonation and reductive elimination. Another
mechanismthatseemedequallylikelytouswasdeprotonationof
I followed by protonation and reductive elimination. However,
the increase in the significance of double exchange at lower pH
values is entirely consistent with the proposed mechanism of
protonation occurring first. That is, double exchange requires
that protonation of III is fast relative to reductive elimination so
that multiple “proton” exchange reactions occur before release
of hydrogen. Since lower pH would accelerate the protonation
step, one would expect a higher likelihood of double exchange
at low pH if the mechanism in Fig. 8 is operative. On the other
hand, if deprotonation preceded protonation, one would expect
double exchange at higher pH. Thus, the pH profile supports the
proposed mechanism.
Finally, the pH at which each catalyst shows maximum sin-
gle exchange activity is noteworthy. Though the resolution of
the data cannot pinpoint the pH at which the maximum activ-
ity obtains, it appears that the maximum is between a pH of
8 and 9 for Rh(TPPMS)3Cl, and between a pH of 7 and 8 for
Rh(TPPTS)3Cl. As mentioned earlier, the exchange rate should
go through a maximum near the pKa of the “dihydrogen” com-
plex intermediate, II. Then, the pH profiles would suggest that
the dihydrogen intermediate for Rh(TPPTS)3Cl is more acidic
than the dihydrogen intermediate for Rh(TPPMS)3Cl. This is
intuitively satisfying as one would expect that the addition of
electron withdrawing sulfonates (in TPPTS, each phenyl ring
of triphenylphosphine is sulfonated, whereas for TPPMS, only
one of the phenyl rings is sulfonated) would increase the acid-
ity of the dihydrogen complex intermediate – which is what
the data appears to demonstrate. Thus, we believe that the pKa
of the dihydrogen complex intermediate, II, of Rh(TPPTS)3Cl
is between 7 and 8 and the pKa of the analogous dihydrogen
intermediate for Rh(TPPMS)3Cl is between 8 and 9.
Fig. 7. pH profile for D2/H2O exchange with Rh(TPPMS)3Cl catalyst.
a reasonably sharp maximum as expected (vide supra). These
plots also demonstrate that the reason the turnover frequencies
shown in Figs. 3 and 4 level out at lower pH values is due to
the increase of a double exchange mechanism.
This double exchange also appears to be due to multiple
exchanges during one visit to the active site due to the fact that it
only appears at the lower pH values where total exchange rates
are slower. If the double exchange were of the type requiring
release from the transition metal followed by recoordination,
one would expect the double exchange to be more evident when
the net exchange rates were the highest. This is not observed. In
addition, for both catalysts at pH 6, where both single and dou-
ble exchange are significant, no increase in double exchange was
observed at higher catalyst concentrations.
3.4. Mechanistic implications
´
Kovacs et al. [16] have proposed a mechanism for
Rh(TPPMS)3Cl catalyzed D2/H2O exchange (Fig. 8). Here the
metal catalyst first undergoes oxidative addition of D2 to make
the dideuteride, I. The next step is protonation to form a bound
HD ligand from one of the deuteride ligands as shown in inter-
mediate II. Finally, loss of D+ to give hydride-deuteride, III, is
followed by reductive elimination of HD and oxidative addition
of D2 to return to I. The idea of single versus double exchange
4. Conclusions
Herein we have demonstrated that significantly more infor-
mation can be gained from D2/H2O exchange by measuring the
hydrogen isotopologues than by monitoring the HDO signal by
NMR. Namely, single and double exchange can be distinguished
and this distinction may have significant mechanistic implica-
tions. In this particular case, the data supports a mechanism
where protonation of the initial addition product, I, precedes
deprotonation. The results obtained are also consistent with the
contention that the maximum rate for single exchange should
occur at the pKa of the dihydrogen complex intermediate and
has allowed a bracketing of the pKa values of these impor-
tant intermediates. Such a method may be useful for obtaining
Fig. 8. Proposed mechanism for Rh(TPPMS)3Cl catalyzed D2/H2O exchange.

J.L. Carriker et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 218–223
223
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−d[D2]
= k1[D2] + k2[D2].
dt
H2O has been neglected in the differential rate equations because
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= k1[D2] − k1[HD].
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[27] (a)P.S. Wagenknecht, J.P. Penney, R.T. Hembre, Organometallics22(2003)
1180–1182;
d[HD]
1
2
= k1[D2]0e−(k +k )t
−
k1[HD],
1
2
dt
which is in the form
dx
= ae−bt − cx
dt
where a = k1[D2]0, b = k1 + k2, c = 1/2k1, and x = [HD]t.
The solution to the above differential equation is
a
xt =
e−bt + Ke−ct
c − b
where K is the constant of integration.
The constant of integration is determined by using the fact
that at t = 0, [HD] = 0, thus:
a
a
a
K = −
,
resulting in : xt =
e−bt
−
e−ct
.
(b) P.S. Wagenknecht, E.J. Sambriski, in: S.G. Pandalai (Ed.), Inorganic
Chemistry, Transworld Research Network, Kerala, India, 2003, pp. 33–50.
c − b
c − b
c − b
´
[28] F. Joo´, J. Kova´cs, A. Katho´, A.C. Be´nyei, T. Decuir, D.J. Darensbourg,
Back substitution for a, b, c, and x results in Eq. (9). Finally,
mass balance gives Eq. (10).
Inorg. Synth. 32 (1998) 1–8.
[29] R. Cramer, Inorg. Synth. 28 (1990) 86–87.
[30] W.A. Herrmann, C.W. Kohlpaintner, Inorg. Synth. 32 (1998) 8–25.
[31] Though Rh(TPPMS)3Cl catalyzes the hydrogenation of bicarbonate [32],
weobservenoevidenceofthatundertheconditionsemployedinthisarticle.
[32] F. Joo´, G. Laurenczy, L. Na´dasdi, J. Elek, Chem. Comm. (1999) 971–972.
[33] J.P. Collman, P.S. Wagenknecht, N.S. Lewis, J. Am. Chem. Soc. 114 (1992)
5665–5673.
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