Catalytic Hydrolysis of Urea
.0 s, each scan required 6.35 s, and as many as 1000 scans were
Inorganic Chemistry, Vol. 36, No. 25, 1997 5919
5
solution became saturated with carbon dioxide, at the maximum
concentration of 0.10 M. The chemical shifts varied by less than (0.10
ppm, depending on the composition of the mixture and other conditions.
recorded. Spectra of either nucleus were recorded with and without
proton decoupling. In quantitative experiments, in which accurate
relative intensities were needed, decoupling was not used. The 13
C
resonances were integrated with an estimated error of (5%. Concen-
trations of the compounds were determined on the basis of these
integrals, the initial concentration of urea, and the known concentration
Results and Discussion
Use of 13C NMR Spectroscopy in Kinetics. On one hand,
quantitative analysis with C NMR spectroscopy is problematic,
mainly because of differential relaxation and the nuclear
Overhauser effect (NOE). Because these factors affect signal
intensity, concentrations are not accurately obtained from the
spectrum. On the other hand, C chemical shifts conveniently
1
3
6
of the C nuclei in the known volume of the solvent, acetone-d .
1
3
Equilibrium constants, rates, and rate constants were calculated from
the known concentrations of the reactants and products, with an
estimated error of 10-20%.
2
8
Binding of Urea to Palladium(II). These experiments were
1
3
13
performed by C NMR spectroscopy, in acetone-d
6
as the solvent, at
3
13 K. In one series, the solutions were made 0.30 M in the catalyst,
span a wide range. Differential relaxation was avoided in our
experiments when the delay time between the pulses was greater
than 5T1 of the slowest-relaxing nucleus, that in CO2. In the
2+
13
cis-[Pd(en)(H
2 2
O) ]
, and 0.10, 0.15, or 0.30 M in urea- C. In another
1
3
series, the solutions were made 0.30 M in urea- C and 0.075, 0.15, or
2
+
0
.30 M in cis-[Pd(en)(H
experiments was taken as the binding constant. The latter series of
experiments was done in acetone-d that was made 3.0 M in H O. An
2 2
O) ] . An average value from these six
presence of [Cr(acac)3], the delay time between the pulses
29
became sufficiently short for practical work.
The NOE
6
2
problem disappeared when all the spin-spin couplings were
preserved, i.e., when decoupling was not used. Initial rates were
determined in the first 2 h of the reaction course, while less
than 5% of the urea decomposed. In a series of quantitative
experiments with urea solutions of known concentrations, the
determinations based on the relative intensities of the 13C NMR
resonances of urea and acetone-d6 differed by only 2% from
average value from these three experiments was taken as the binding
constant of urea when it competes with water for coordination to the
catalyst.
6
Kinetics of Hydrolysis. The solvent was always acetone-d . The
temperature was 313 ( 0.5 K in experiments concerning the reaction
mechanism and 333 ( 0.5 K in those concerning catalytic turnover.
The reactions in eqs 1 and 2 were followed mostly by 13C NMR
spectroscopy. In a typical experiment, to a solution of a freshly
prepared complex shown in Chart 1 were added solid [Cr(acac)
sometimes another chemical, and finally solid urea, to start the reaction.
Acquisition of the spectra began within 1 min. The optional chemical
was an acid (DClO
dimethylformamide, or thiourea. The final concentrations of [Cr(acac)
and a base were 0.040 and 0.22 M, respectively. Other concentrations
were variously adjusted. Addition of either base to cis-[Pd(en)(H
did not cause precipitation. When urea was added, either before or
after the base, a yellow precipitate formed. It was dissolved in H
1
3
the actual concentrations. Evidently, C NMR spectroscopy
3
],
can be a reliable tool in quantitative analysis if precautions are
13
taken. We know only several other quantitative studies by
C
NMR spectroscopy and only one previous application of it in
4
or CF
3
3
COOH), a base (NaOH or Et N), N,N-
30-33
kinetics.
3
]
Relaxation Times T1 of the 13C Nuclei. A typical deter-
mination is shown in the Supporting Information, Figure S1.
The results are given in Table 1. The undetermined T1 times
were too long in the absence of the paramagnetic relaxation
agent, [Cr(acac)3], but they were conveniently determined in
its presence. Raising its concentration above 0.040 M did not
cause significant further shortening of the T1 times. The values
2+
O) ]
2 2
2
O
1
3
15
2
or D O and examined by C and N NMR spectroscopy.
The initial rates in Figures 1-4 and 7 were determined in the
experiments in which only the first 3-5% of a reaction was followed.
The rate constants k
obtained by fitting intensities of the C NMR resonance of the
intermediate for 5 half-lives of urea decomposition to the appropriate
equation. The microscopic rate constants were obtained by fitting the
data in Figures 5 and 6 to the appropriate equations.
1
14
1
and k
2
in Figures 5 and 6, respectively, were
in Table 1 generally increase as the number of H and N nuclei
in the molecule decreases.
1
3
Binding of Urea to Palladium(II). Urea can variously
coordinate to metals; unidentate terminal modes are the most
common. Binding of hard metal ions, such as cobalt(III),
chromium(III), and rhodium(III), via the oxygen atom is both
All the experiments concerning the mechanism of the reactions were
2
+
done with cis-[Pd(en)(H
were usually made 0.30 M in it and in the enriched urea. The
concentration of adventitious water was ca. 1.5 M.
2
O)
2
]
as the catalyst. The initial solutions
7
-9
kinetically and thermodynamically favored.
Platinum(II),
which is a soft ion, initially loses an aqua ligand to yield the
oxygen-bound isomer, which then converts into the more stable
In experiments concerning the rate law, the concentrations of the
catalyst, the enriched urea, and water were varied, one at a time.
6
nitrogen-bound isomer. This process occurs over hours, and
the estimated equilibrium constant for it is ca. 10.
Usually the initial rate of formation of CO
0 spectra were taken during 1-2 h, depending on the reaction rate.
Acidity was adjusted with DClO (in determinations of initial rates)
or with CF COOH (in determinations of particular rate constants). The
net acid concentration was corrected for the contribution from the stock
2
was determined. At least
Because urea was enriched in both N and 13C, we applied
15
1
NMR spectroscopy of both nuclei to study reactions with cis-
4
2
+
3
2 2
[Pd(en)(H O) ] and, to a lesser extent, other palladium(II)
1
3
complexes in Chart 1. The C NMR spectra of mixtures
2
+
17
solution of cis-[Pd(en)(H
some experiments, ionic strength was varied with NaClO
was inhibited with different concentrations of NH NO
The reactions in eqs 1 and 2 were also followed by 15N NMR
spectroscopy, to determine the initial rate of NH formation. The
concentrations of urea and of the products were determined from the
known sum of their concentrations and the relative intensities of the
corresponding resonances. Possible substitution of ammonia for an
2
O)
2
]
, for which the first pK
a
is 5.6. In
2+
containing urea and cis-[Pd(en)(H2O)2] show the resonances
4
or the reaction
at 162.8, 165.5, and 158 ppm, respectively, of free urea and
the O-bound and N-bound ligands shown in Chart 2. These
4
3
.
6
values agree with those reported previously. Partial coalescence
3
of the first two resonances indicates that the exchange between
(
29) (a) Freeman, R.; Pachler, K. G. R.; LaMar, G. N. J. Chem. Phys. 1971,
5, 9. (b) Gurley, T. W.; Ritchey, W. M. Anal. Chem. 1976, 48, 1137.
5
2+
aqua ligand in cis-[Pd(en)(H
2
O)
8% enriched in N in both ions.
Composition of the Reaction Mixtures. The reactant, urea, an
intermediate, carbamic acid N-bound to palladium(II), and the products,
2
]
4 3
was studied with NH NO that was
(30) Johnson, H. L.; Thomas, D. W.; Elliss, M.; Gary, L.; DeGraw, J. I. J.
Pharm. Sci. 1977, 66, 1660.
1
5
9
(31) (a) Pfeffer, P. E.; Luddy, F. E.; Unruh, J.; Schoolery, J. N. J. Am. Oil
Chem. Soc. 1977, 54, 380. (b) Thiault, B.; Mersseman, M. Org. Magn.
Reson. 1975, 7, 575. (c) Thiault, B.; Mersseman, M. Org. Magn. Reson.
1
3
15
ammonia and carbon dioxide, were detected by C and N NMR
spectroscopy. Assignments of resonances were confirmed by spiking
the reaction mixtures with dry ice and NH
1
976, 8, 28. (d) Hajek, M.; Sklenar, V.; Sebor, G.; Lang, I.; Weisser,
O. Anal. Chem. 1978, 50, 773.
NO
3
containing 98% 15N in
(32) Blunt, J. W.; Munro, M. H. G. Aust. J. Chem. 1976, 29, 975.
(33) Chian, H. C.; Lin, L. J. Org. Magn. Reson. 1979, 29, 975.
4
both ions. The concentration of urea continued to decrease after the