Reaction of [Pd2Cl2(μ-dppm)2] with arenesulfonyl azides. Kinetics and mechanism of formation of arenesulfonylimido-bridged A-frame adducts

Full text of DOI:10.1021/ic980937p

3944
Inorg. Chem. 1999, 38, 3944-3946
Reaction of [Pd2Cl2(µ-dppm)2] with Arenesulfonyl
Azides. Kinetics and Mechanism of Formation of
Arenesulfonylimido-Bridged A-Frame Adducts
Isabella Foch, Ga´bor Besenyei, and La´szlo´ I. Sima´ndi*
Institute of Chemistry, Chemical Research Center
Hungarian Academy of Sciences, P.O. Box 17
H-1525 Budapest, Hungary
ReceiVed August 7, 1998
Figure 1. Dependence of the pseudo-first-order rate constant ko on
the ArSO2N3 concentration. Ar ) 4-MeOC6H4 (a), 2-NO2C6H4 (e),
4-AcC6H4 (f), 4-NO2C6H4 (g). Solvent CH2Cl2, T ) 25 °C, [Pd2Cl2-
(µ-dppm)2] ) 4.75 × 10-4 M.
Introduction
Organic azides represent an important source of the RN:
moiety and are used in transition metal chemistry for the
preparation of metal imido complexes.1-3 The reaction of a
metal-metal bond with an organic azide usually affords
complexes with a bridging imido group.4 Recently, interest has
been increasing in the mechanism of reactions between aromatic
azides and metal complexes.5-8
The scarcity of mechanistic information on the reactions of
arenesulfonyl azides with metal complexes prompted us to carry
out a kinetic study of reaction 1, which takes place at measurable
rates at room temperature with all seven azides.
Experimental Section
In our earlier work aimed at the phosgene-free synthesis of
arenesulfonylureas, we have found that N-chloroarenesulfona-
midates can be catalytically carbonylated with CO to arene-
sulfonylisocyanatesinthepresenceofpalladium(II)complexes.9-11
These isocyanates are precursors of arenesulfonylureas, repre-
sentatives of a family of low dose herbicides widely used in
plant protection. The mechanism of the N-carbonylation reaction
is not known, but palladium sulfonylnitrene complexes may be
implicated as possible intermediates.
In a study to elucidate their chemistry, we reacted the dimeric
Pd(I) complex [Pd2Cl2(µ-dppm)2] (1), where dppm is bis-
(diphenylphosphino)methane, with arenesulfonyl azides (2). The
observed reaction 1 was accompanied with the evolution of N2,
affording a series of novel arenesulfonylnitrene-bridged di-
nuclear A-frame complexes, [Pd2Cl2(µ-dppm)2(µ-NSO2Ar)]
(3).12
[Pd2Cl2(µ-dppm)2] was synthesized by a known method.13 Arene-
sulfonyl azides were prepared by reacting the corresponding arene-
sulfonyl chlorides with sodium azide in aqueous ethanol.14 Imido-
bridged (nitrene) complexes [Pd2Cl2(µ-dppm)2(µ-NSO2Ar)] (Ar )
4-CH3OC6H4, 4-CH3C6H4, C6H5, 4-FC6H4, 2-NO2C6H4, 4-AcC6H4,
4-NO2C6H4) were prepared and analyzed as described previously.12
UV-vis spectra were recorded as a function of time on a Hewlett-
Packard 8452A diode-array spectrophotometer equipped with a ther-
mostated cell compartment. The kinetics were monitored at selected
wavelengths using the sets of spectra obtained.
Results and Discussion
The reaction of [Pd2Cl2(µ-dppm)2] (1) in CH2Cl2 with
arenesulfonyl azides 2a-g was monitored by UV-vis spec-
troscopy. The typical time evolution of the spectra is shown in
the Supporting Information (S2) for the reaction of 1 with a
40-fold excess of tosyl azide (2b) at 25 °C. The appearance of
sharp, well-defined isosbestic points indicates a single reaction
involving the absorbing species. The spectra of the starting Pd
dimer 1 and the product nitrene complexes 3a-g are available
from independent measurements. The reaction of 1 with the
other six arenesulfonyl azides gave analogous spectral changes,
with very close similarity between the spectra of the product
nitrene complexes. Tosyl and phenylsulfonyl azide (2b and 2c)
do not absorb above 300 nm, but the 2- and 4-nitro derivatives
(2e and 2g) contribute to the absorbance up to about 400 nm.
To avoid excessive absorbance and still have readily measurable
rates, certain limitations were imposed on the sulfonyl azide
concentrations used in the kinetic runs.
[Pd Cl (µ-dppm) ]
+ ArSO2N3 f
2
2
2
(1)
(2)
[Pd Cl (µ-dppm) (µ-NSO Ar)]
+ N2 (1)
2
2
2
2
(3)
Ar ) (a) 4-CH3OC6H4; (b) 4-CH3C6H4; (c) C6H5;
(d) 4-FC6H4; (e) 2-NO2C6H4; (f) 4-AcC6H4; (g) 4-NO2C6H4
* Corresponding author. Fax: +36 1 325 7554. E-mail: simandi@
cric.chemres.hu.
(1) Wigley, D. E. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.;
John Wiley & Sons: New York, 1994; Vol. 42, p 239.
(2) Nugent, W. A.; Haymore, B. L. Coord. Chem. ReV. 1980, 31, 123.
(3) Cenini, S.; LaMonica, G. Inorg. Chim. Acta 1976, 18, 279.
(4) Herrmann, W. A.; Menjon, B.; Herdtweck, E. Organometallics 1991,
10, 2134.
(5) Dobbs, D. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 3836.
(6) Hanna, T. A.; Baranger, A. M.; Bergman, R. G. Angew. Chem., Int.
Ed. Engl. 1996, 35, 653.
The spectrophotometric measurements gave no indication of
any detectable intermediate in the reaction. Solutions of 3
prepared by dissolving the solid product in CH2Cl2 are stable.
Kinetic runs were carried out at 330, 360, and 418 nm in
CH2Cl2 under pseudo-first-order conditions, using a 10-fold or
greater excess of the arenesulfonyl azides over 1. The absor-
(7) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 6382.
(8) Fickes, M. G.; Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc.
1995, 117, 6384.
(9) Besenyei, G.; Ne´meth, S.; Sima´ndi, L. I. Angew. Chem., Int. Ed. Engl.
1990, 29, 1147.
(10) Besenyei, G.; Sima´ndi, L. I. Tetrahedron Lett. 1993, 34, 2839.
(11) Besenyei, G.; Ne´meth, S.; Sima´ndi, L. I. Tetrahedron Lett. 1994, 35,
9609.
(12) Besenyei, G.; Pa´rka´nyi, L.; Foch, I.; Sima´ndi, L. I.; Ka´lma´n, A. Chem.
Commun. 1997, 1143.
(13) Balch, A. L.; Benner, L. S. Inorg. Synth. 1982, 21, 47.
(14) Regitz, M.; Hocker, J.; Liedhegener, A. Org. Synth. 1968, 48, 36.
10.1021/ic980937p CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/05/1999

Notes
Inorganic Chemistry, Vol. 38, No. 17, 1999 3945
Table 1. Temperature Dependence of Rate Constant k (in M-1 s-1) for the Reaction of [Pd2Cl2(µ-dppm)2] with Various Arenesulfonyl Azides
a
ArSO2N3
Ar
t/°C
4-MeOC6H4
4-MeC6H4
C6H5
4-FC6H4
2-NO2C6H4
4-AcC6H4
4-NO2C6H4
35
30
25
20
15
10
5
0.0398
0.0366
0.0289
0.0183
0.0140
0.00948
0.033
0.0517
0.0370
0.118
0.085
0.061
0.044
0.032
0.123
0.096
0.199
0.176
0.135
0.104
0.0853
0.0283
0.0201
0.0161
0.0119
0.0102
0.0078
0.656
0.509
0.406
0.281
0.229
0.205
0.0258
0.058
0.0162
0.0111
0.034
0.0023
0
0.00702
0.021
0.0445
a Each value is the average of at least six runs reproducible to within (4%. Individual kinetic measurements were carried out at 3 wavelengths,
under pseudo-first-order conditions using various reactant concentrations.
Table 2. Activation Parameters from the Eyring Plot of Rate
Constant k (Eq 2) for Reaction 1 (Estimated Errors: ∆Hq, (2 kJ
Scheme 1
mol-1; ∆Sq, (4 J mol-1 K-1
)
∆Hq/kJ
∆Sq/J
∆Gq/kJ
2
mol-1
mol-1 K-1
mol-1 (25 °C)
4-CH3OC6H4SO2N3
4-CH3C6H4SO2N3
C6H5SO2N3
37.3
31.6
32.0
38.2
35.1
32.4
31.3
-150
-169
-164
-138
-147
-152
-143
82.1
82.1
81.0
79.0
78.9
77.5
74.0
4-FC6H4SO2N3
2-NO2C6H4SO2N3
4-AcC6H4SO2N3
4-NO2C6H4SO2N3
Figure 2. Hammett plot of the relative rate constant for reactions (1).
bance vs time curves showed excellent first-order behavior, as
demonstrated by the log(A - A∞) or log(A∞ - A) vs time plots
or Guggenheim plots. The latter were used in slower runs where
A∞ could not be determined with sufficient accuracy. After
preliminary tests by plotting all of the A vs time curves were
evaluated by fitting with a single-exponential function, and the
observed first-order rate constants (ko) from that source were
used as primary data. The value of ko was independent of the
concentration of the Pd dimer for all of the seven systems
studied.
The pseudo-first-order rate constant (ko) for each azide is
proportional to the arenesulfonyl azide concentration (Figure 1
and Supporting Information, S3). Consequently, the rate law is
of the form given by eq 2. The reproducibility of k was better
determined from the Eyring plots (Supporting Information, S4)
and are listed in Table 2 together with the rate constants at 25
°C.
At a given temperature rate constant k increases with
increasing electron-withdrawing power of the aromatic sub-
stituent (4-CH3O < 4-CH3 < 4-H < 4-F < 2-NO2 < 4-Ac <
4-NO2). At 25 °C a factor of about 25 is found upon going
from the 4-methoxy to the 4-nitro derivative. The 2-NO2
derivative is 6 times less reactive than the 4-NO2 compound,
which is apparently due to steric hindrance. The relative
reactivities of the 4-substituted benzenesulfonyl azides 2a-d,
f, g obey the Hammett relationship shown in Figure 2. The σ
values for the 4-substituents were taken from ref 19. From the
rate ) ko[Pd2Cl2(µ-dppm)2] )
k[Pd2Cl2(µ-dppm)2][ArSO2N3] (2)
than (3% for each azide, measured at three wavelengths and
various reactant concentrations. At 0-15 °C the scatter of k
was (4%.
The temperature dependence of the second-order rate constant
k was studied in the interval of 0-30 °C under pseudo-first-
order conditions. The second-order rate constants are listed in
Table 1 for the different azides. The activation parameters were
(15) Collman, J. P.; Kubota, M. F.; Vastine, D.; Sun, J. Y.; Kang, J. W. J.
Am. Chem. Soc. 1968, 90, 5430.

3946 Inorganic Chemistry, Vol. 38, No. 17, 1999
Notes
slope of the straight line a reaction constant of F ) 1.39 has
been obtained, suggesting the electrophilic attack of the arene-
sulfonyl azide on the Pd dimer. The observed kinetic behavior
and the activation parameters are consistent with an associative
bimolecular mechanism involving a transition state with a
relatively small extent of bond-breaking (small ∆Hq) but
extensive ordering of the reactants and solvent molecules (large
negative ∆Sq). Since the reaction takes place with extrusion of
a N2 molecule from the azide, complex formation between the
azide and Pd complex 1 would probably be required before the
rate-determining step. A concerted process seems to be unlikely
due to the low enthalpies of activation. The lack of detectable
complex formation between the reactants still permits preequi-
libria, producing reactive intermediate(s) in very low concentra-
tion. This would be consistent with the high degree of ordering
indicated by the large negative activation entropies.
Scheme 1 involves consecutive preequilibria producing
intermediates I1 and I2, followed by rate-determining expulsion
of the N2 molecule. Accordingly, the observed rate constant of
eq 2 can be given as k ) K1K2k3. In I1 the azide is terminally
bonded to one of the Pd atoms. Cyclic intermediate I2 is likely
to form, by analogy to [2+3] dipolar cycloadditions between
organic 1,3-dipolar reagents and dipolarophiles.18 The large
negative entropies of activation are probably associated with
the preequilibrium complexation (K1K2) in which the highly
ordered I2 is generated. Dinitrogen expulsion directly from I1
is unlikely since it requires the formation of the free nitrene,
which would then have to encounter 1 to produce imido complex
3. This kind of bond breaking is too energy-demanding to be
consistent with the observed low activation enthalpies. The high
selectivity of product formation also excludes the involvement
of free nitrene.
According to a recent report,6 PhN3 adds across a heterobi-
nuclear bond between Zr and Ir in a Cp complex through a
µ-phenyl azide intermediate, which can be thermally converted
to a bridging imido complex with N2 extrusion (between 75
and 105 °C, ∆Hq ) 122 kJ mol-1 ).
This suggests that intermediate I1 may in principle transform
to both an arenesulfonyl azide and a nitrene (imido)-bridged
complex. Apparently, there are special conditions that permit
the formation and detection of an arenesulfonyl azide-bridged
Pd dimer. Work is in progress to explore this possibility.
The observed reactivity pattern and activation parameters
resemble those reported by Collman et al. for the reaction of
organic azides with mononuclear rhodium and iridium halo-
carbonyl complexes, affording N2 complexes.15
Similarly large negative ∆Sq values have been reported by
James et al.16 for the reaction of 1 with CO, affording A-frame
CO adducts. A different picture emerged for the analogous
reactions of the corresponding Pt dimer,17 where positive ∆Sq
pointed to a dissociative process.
The reaction mechanism shown in Scheme 1 accommodates
both electrophilic attack by the arenesulfonyl azide and the large
negative ∆Sq.
Acknowledgment. This work was supported by the Hungar-
ian Science Foundation (OTKA Grant T16213).
(16) Lee, C. L.; James, B. R.; Nelson, D. A.; Hallen, R. T. Organometallics
1984, 3, 1360.
(17) Muralidharan, S.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 8104.
(18) Kosower, E. H. An Introduction to Physical Organic Chemistry; John
Wiley & Sons: New York, 1962; Chapter 18.
Supporting Information Available: Table of NMR data of
compounds 3a, 3d, and 3f and figures showing the time evolution of
the UV-vis spectra; dependence of the pseudo-first-order rate constant
on the arenesulfonyl azide concentration; typical Eyring plots.
(19) March, J. AdVanced Organic Chemistry, 4th ed.; John Wiley & Sons:
New York, 1992; p 280.
IC980937P