Kinetics and Mechanism of the Formation of Palladium Bis(benzylamine) Complexes from Reaction of Benzylamine with Palladium Tri-o-tolylphosphine Mono(amine) Complexes

Article abstract of DOI:10.1021/ic961388v

The kinetics of the conversion of the palladium mono(benzylamine) complex Pd[P(o-tol)₃](p-C₆H₄CMe₃)[H ₂-NBn]Br (2) to the bis(benzylamine) complex Pd(p-C₆H₄CMe₃[H₂NBn]₂Br (3) established the second-order rate law: Rate = k₁[2][H₂NBn], where ΔH^? = 13.8 ± 0.3 kcal mol^-1 and ΔS^? = -29.7 ± 0.8 eu. Kinetics were consistent with a mechanism initiated by direct attack of benzylamine on 2 via an associative or interchange mechanism. Benzylamine exchange with both 2 and 3 was > 1.5 × 10³ times faster than conversion of 2 to 3 under comparable conditions. Complex 2 underwent phosphine exchange in the presence of P(1-naphthyl)₃ to form Pd[P(1-naphthyl)₃](p-C₆H₄CMe ₃)[H₂NBn]Br (9). Kinetics of the conversion of 9 to 2 were consistent with associative solvolysis of 9 to form Pd[P(1-naphthyL)₃](p-C₆H₄CMe ₃)[solvent]Br (V) followed by attack of P(o-tolyl)₃ to form the mixed bis(phosphine) intermediate Pd[P(o-tol)₃][P(1-naphthyl)₃](p-C₆H ₄CMe₃)Br (VI). Solvolytic displacement of P(o-tolyl)₃ from VI followed by reaction with amine would then form 2.

Full text of DOI:10.1021/ic961388v

2610
Inorg. Chem. 1997, 36, 2610-2616
Kinetics and Mechanism of the Formation of Palladium Bis(benzylamine) Complexes from
Reaction of Benzylamine with Palladium Tri-o-tolylphosphine Mono(amine) Complexes
H. Annita Zhong and Ross A. Widenhoefer*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed NoVember 20, 1996X
The kinetics of the conversion of the palladium mono(benzylamine) complex Pd[P(o-tol)3](p-C6H4CMe3)[H2-
NBn]Br (2) to the bis(benzylamine) complex Pd(p-C6H4CMe3)[H2NBn]2Br (3) established the second-order rate
law: Rate ) k1[2][H2NBn], where ∆Hq ) 13.8 ( 0.3 kcal mol-1 and ∆Sq ) -29.7 ( 0.8 eu. Kinetics were
consistent with a mechanism initiated by direct attack of benzylamine on 2 via an associative or interchange
mechanism. Benzylamine exchange with both 2 and 3 was > 1.5 × 103 times faster than conversion of 2 to 3
under comparable conditions. Complex 2 underwent phosphine exchange in the presence of P(1-naphthyl)3 to
form Pd[P(1-naphthyl)3](p-C6H4CMe3)[H2NBn]Br (9). Kinetics of the conversion of 9 to 2 were consistent with
associative solvolysis of 9 to form Pd[P(1-naphthyl)3](p-C6H4CMe3)[solvent]Br (V) followed by attack of P(o-
tolyl)3 to form the mixed bis(phosphine) intermediate Pd[P(o-tol)3][P(1-naphthyl)3](p-C6H4CMe3)Br (VI).
Solvolytic displacement of P(o-tolyl)3 from VI followed by reaction with amine would then form 2.
Introduction
Scheme 1
Palladium P(o-tol)3 (o-tol ) o-tolyl) complexes often exhibit
reactivity which differs markedly from the corresponding PPh3
complexes. For example, palladium P(o-tol)3 complexes are
effective catalysts for the cross-coupling of aryl bromides and
aminostannanes1-3 and for the cross-coupling of aryl halides
and amines in the presence of sodium tert-butoxide.4-7 In
contrast, attempts to effect intermolecular C-N bond formation
employing the corresponding palladium PPh3 complexes led
predominantly to reduction of the aryl halide. Palladium P(o-
tol)3 complexes have also been shown to produce higher reaction
rates than palladium PPh3 complexes in both the Heck arylation
reaction8 and Stille cross-coupling reaction.9 The enhanced
activity of palladium P(o-tol)3 complexes relative to PPh3
complexes has typically been attributed to the steric bulk of
the P(o-tol)3 ligand which promotes both phosphine lability and
coordinative unsaturation of the metal.9 In addition, P(o-tol)3
is more resistant toward quaternization than PPh3 which
promotes catalyst stability.10 Despite the high reactivity of
palladium P(o-tol)3 complexes and their relevance to syntheti-
cally significant C-C and C-N bond-forming protocols, ligand
substitution of palladium P(o-tol)3 and related complexes has
been scarcely investigated.
amination of aryl halides.11-13 For example, the palladium aryl
bromide dimer {Pd[P(o-tol)3](p-C6H4CMe3)(µ-Br)}2 (1) reacted
with 2 equiv of benzylamine (per dimer) to form the palladium
mono(amine) complex Pd[P(o-tol)3](p-C6H4CMe3)[H2NBn]Br
(2). Amine monomer 2 reacted with excess benzylamine in
C6D6 at 25 °C to form a 1:1 mixture of free P(o-tol)3 and the
palladium bis(benzylamine) complex Pd(p-C6H4CMe3)[H2-
NBn]2Br (3) (Scheme 1).13 Conversion of 2 to 3 was reversible;
addition of P(o-tol)3 to a solution of 3 regenerated 2 (Keq ≈ 1).
While the high rate of bridge cleavage (1 f 2) precluded kinetic
analysis, the conversion of 2 to 3 was conveniently monitored
by NMR spectroscopy, facilitating kinetic analysis. Further-
more, the conversion of 2 to 3 is relevant as a potential turnover-
limiting or chain-terminating step in the Pd-P(o-tol)3-catalyzed
arylation of primary amines.4-7 As a result, we initiated a study
of the kinetics and mechanism of the conversion of 2 to 3.
We have previously investigated the reactions of palladium
tri-o-tolylphosphine aryl halide dimers with amines in an effort
to gain insight into the corresponding palladium-catalyzed
Results and Discussion
X Abstract published in AdVance ACS Abstracts, May 15, 1997.
(1) Kosugi, M.; Kameyama, M.; Migita, T. Chem. Lett. 1983, 927.
(2) Guram, A. S.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116, 7901.
(3) Paul, F.; Patt, J.; Hartwig, J. F. J. Am. Chem. Soc. 1994, 116, 5969.
(4) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1348.
(5) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 1996, 61, 1133.
(6) Wolfe, J. N.; Rennels, R. A.; Buchwald, S. L. Tetrahedron 1996, 52,
7525.
Kinetics of the Conversion of 2 to 3. In order to determine
the dependence of the rate of conversion of 2 to 3 on
benzylamine concentration, the pseudo-first-order rate constants
were measured as a function of benzylamine concentration from
0.20 to 1.64 M in C6D6 at 55 ( 0.5 °C by 1H NMR
(7) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609.
(8) (a) Mitsudo, T.; Fischetti, W.; Heck, R. F. J. Org. Chem. 1984, 49,
1640. (b) Fischetti, W.; Mak, K. T.; Stakem, F. G.; Kim, J. I.;
Rheingold, A. L.; Heck, R. F. J. Org. Chem. 1983, 48, 948.
(9) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585.
(10) Ziegler, C. B.; Heck, R. F. J. Org. Chem. 1978, 43, 2941.
(11) (a) Widenhoefer, R. A.; Zhong, H. A.; Buchwald, S. L. Organome-
tallics 1996, 15, 2745. (b) Widenhoefer, R. A.; Zhong, H. A.;
Buchwald, S. L. Unpublished results.
(12) Widenhoefer, R. A.; Buchwald, S. L. Organometallics 1996, 15, 2755.
(13) Widenhoefer, R. A.; Buchwald, S. L. Organometallics, 1996, 15, 3534.
S0020-1669(96)01388-2 CCC: $14.00 © 1997 American Chemical Society

Palladium Bis(benzylamine) Complexes
Inorganic Chemistry, Vol. 36, No. 12, 1997 2611
Table 1. Temperature and Solvent Dependence of the
Second-Order Rate Constants for the Conversion of 2 to 3 and 4 to
5
reacn
solvent
C6D6
C6D6
C6D6
C6D6
temp, °C
(104)k1, s-1 M-1
2 f 3
2 f 3
2 f 3
2 f 3
2 f 3
2 f 3
2 f 3
2 f 3
4 f 5
25
40
55
65
77
25
25
25
25
1.42 ( 0.09
4.41 ( 0.24
12.5 ( 0.6
25.2 ( 1.3
56.0 ( 1.8
1.46 ( 0.09
1.79 ( 0.10
0.71 ( 0.06
1.05 ( 0.24
C6D6
toluene-d8
THF-d8
dioxane-d8
C6D6
spectroscopy.14 Good pseudo-first-order plots for the disap-
pearance of 2 were obtained by plotting ln {[2]t/[2]0}versus
time (Figure S1, Table S1 (Supporting Information)), which
established the first-order dependence of the rate on the
concentration of 2. A plot of the pseudo-first-order rate
constants versus benzylamine concentration established the first-
order dependence of the rate on benzylamine concentration
(Figure S2).15 Under these conditions, the reaction obeyed the
second-order rate law shown in eq 1, where k1 ) (1.25 ( 0.06)
× 10-3 s-1 M-1 [∆Gq ) 22.7 ( 0.2 kcal mol-1] (Table 1).
Figure 1. Plot of kobs/[H2NBn] versus [P(o-tol)3]/[H2NBn] for the
conversion of 2 to 3 in C6D6 at 55 °C.
Scheme 2
d[2]
dt
rate ) -
) k1[2][NH2Bn]
(1)
independently (0.57),13 considering the narrow P(o-tol)3 con-
centration range employed in these kinetics.
The second-order rate constant (k1) for conversion of 2 to 3
was measured as a function of temperature from 25 to 77 °C
(Table 1, Figure S2). An Eyring plot of the second-order rate
constants provided the activation parameters: ∆Hq ) 13.8 (
0.3 kcal mol-1, ∆Sq ) -29.7 ( 0.8 eu (Figure S4). The second-
order rate constant (k1) for conversion of 2 to 3 was slightly
solvent dependent and ranged from (7.1 ( 0.6) × 10-5 s-1 M-1
in dioxane-d8 to (1.79 ( 0.10) × 10-4 s-1 M-1 in THF-d8 at
25 °C (Table 1). The second-order rate constant for conversion
of 2 to 3 at 25 °C was ∼25% greater than k1 for conversion of
the chloride mono(amine) complex Pd[P(o-tol)3](p-C6H4-
CMe3)[H2NBn]Cl (4) to Pd(p-C6H4CMe3)[H2NBn]2Cl (5) (Table
1). In addition, although the second-order rate constant for the
conversion of the iodide mono(amine) complex Pd[P(o-tol)3]-
(p-C6H4CMe3)[H2NBn]I (6) to Pd(p-C6H4CMe3)[H2NBn]2I (7)
was not determined, conversion of 6 to 7 at 25 °C in the presence
of 0.87 M benzylamine was ∼10 times faster than conversion
of 2 to 3 under comparable conditions ([H2NBn] ) 0.93 M).
Benzylamine Exchange with 2. Benzylamine exchange with
mono(amine) complex 2 was considerably faster than conversion
of 2 to 3. For example, addition of R,R-benzylamine-d2 (70
mM) to a solution of 2 (10 mM) in C6D6 at 25 °C led to
complete amine exchange as evidenced by a 1:7 ratio of
resonances corresponding to bound (δ 4.0, t, J ) 7.0 Hz) and
free benzylamine (δ 3.55, s) in the 1H NMR spectrum (Scheme
2). Assuming that g15% unscrambled 2 would have been
detected in this spectrum (δ 4.0:δ 3.55 g1:6), the observed 1:7
ratio of resonances indicates that amine exchange was g85%
complete (∼3 half-lives) after 2 min. Therefore, a lower limit
for the rate of amine exchange with 2 in the presence of 70
mM benzylamine of g1.7 × 10-2 s-1 (t1/2 e 40 s) was
estimated, which is >1.5 × 103 times faster than the conversion
of 2 to 3 under similar conditions. Analogous results were
Pseudo-first-order rate constants for approach to equilibrium
for the conversion of 2 to 3 were also measured as a function
of P(o-tol)3 concentration (0.048-0.21 M) in the presence of
excess H2NBn (∼0.33 M) in C6D6 at 55 °C.16 Good pseudo-
first-order plots for the approach to equilibrium were obtained
by plotting ln{([2]t - [2]eq)/([2]0 - [2]eq)} versus time (Figure
S3, Table S1). A plot of kobs/[H2NBn] versus [P(o-tol)3]/[H2-
NBn] was linear (Figure 1), in accord with the two-term rate
law shown in eq 2, where k1 ) (1.04 ( 0.05) × 10-3 s-1 M-1
,
kobs
[P(o-tol)3]
-1 [NH2Bn]
) k1 + k
(2)
[NH2Bn]
k-1 ) (2.7 ( 0.1) × 10-3 s-1 M-1, and Keq ) k1/k-1 ) 0.39 (
0.03.17 The value of k1 was in good agreement with the value
obtained under conditions of complete conversion, while the
value of Keq was in fair agreement with the value obtained
(14) Due to the slightly unfavorable equilibrium constant for the conversion
of 2 to 3 (Keq ) 0.57 at 55 °C), a 35-fold excess of benzylamine
relative to 2 was required to ensure g95% completion. As a result,
the dependence of the rate on benzylamine concentration below <0.20
M could not determined under these conditions.
(15) Plots of the pseudo-first-order rate constants versus amine concentration
also produced a small, positive intercept of the ordinate ((7 ( 6) ×
10-5 s-1 at 65 °C), approximately 5% of the magnitude of the second-
order term. Although a nonzero intercept of the ordinate suggests the
presence of a ligand-independent pathway, the large error associated
with the intercept, particularly considering the lack of data at low amine
concentrations, precludes this assignment. In addition, the temperature
and solvent dependence of the intercept was inconsistent with the
presence of an amine-independent pathway. Although an amine-
independent pathway for conversion of 2 to 3 cannot be rigorously
excluded, the mechanism of the conversion of 2 to 3 is clearly
dominated by an associative pathway at all but very low benzylamine
concentration.
(16) For related kinetics see: (a) Ohno, N.; Cusanovich, M. A. Biophys.
J. 1981, 36, 589. (b) Pelizzetti, E.; Mentasti, E. Inorg. Chem. 1979,
18, 583. (c) Bosnich, B.; Dwyer, F. P.; Sargeson, A. M. Aust. J. Chem.
1966, 19, 2213. (d) Bosnich, B.; Dwyer, F. P. Aust. J. Chem. 1966,
19, 2051. (e) Butler, J.; Davies, D. M.; Sykes, A. G. J. Inorg. Biochem.
1981, 15, 41. (f) Below, J. F.; Connick, R. E.; Coppel, C. P. J. Am.
Chem. Soc. 1958, 80, 2961.
(17) Howell, J. A. S.; Burkinshaw, P. M. Chem. ReV. 1983, 83, 557. (b)
Darensbourg, D. J. AdV. Organomet. Chem. 1982, 21, 113. (c) Cross,
R. J. Chem. Soc. ReV. 1985, 14, 197. (d) Collman, J. P.; Hegedus, L.
S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry; University Science Books: Mill
Valley, CA, 1987; Chapter 4.

2612 Inorganic Chemistry, Vol. 36, No. 12, 1997
Zhong and Widenhoefer
Scheme 3
Scheme 4
Scheme 5
obtained from reaction of benzylamine (∼70 mM) and Pd[P(o-
tol)3](p-C6H4CMe3)(H2NCD2Ph)Br (2-d2) (∼10 mM).
The mechanisms of ligand substitution at 16 electron square
planar platinum(II) complexes have been extensively studied
and are predominantly bimolecular in nature, initiated by attack
of solvent or excess ligand at the metal center.17-20 The first-
order dependence of the rate of conversion of 2 to 3 on
benzylamine concentration, the large negative entropy of
activation, and the lack of P(o-tol)3 rate inhibition were
consistent with a mechanism initiated via direct attack of
benzylamine on 2. Furthermore, the absence of a significant
intercept in the plot of pseudo-first-order rate constants versus
benzylamine concentration (Figure S2) indicated that a solvoly-
sis pathway via Pd(p-C6H4CMe3)[H2NBn](C6D6)Br (I) was not
significant.15 Similarly, because benzylamine exchange with
2 was much faster than solvolysis of 2 to form Pd[P(o-tol)3]-
(p-C6H4CMe3)(C6D6)Br (III) (see below), a bimolecular path-
way for amine exchange was inferred.
The bimolecular process which both exchanges benzylamine
and converts 2 to 3 could occur by either an associative or
interchange mechanism. In the associative mechanism, rapid
and reversible attack of benzylamine on 2 would generate the
trigonal bipyramidal five-coordinate bis(amine) mono(phos-
phine) intermediate Pd[P(o-tol)3](p-C6H4CMe3)[H2NBn]2Br (II),
which could undergo pseudorotation and loss of phosphine to
form 3. In the interchange mechanism, initial interaction of
benzylamine with 2 could form the outer-sphere 1:1 amine
adduct, 2‚H2NBn. Concerted exchange of outer- and inner-
sphere benzylamine ligands in 2‚H2NBn would result in amine
exchange while concerted exchange of the outer-sphere amine
ligand with the phosphine ligand of 2‚H2NBn would form 3
(Scheme 3).
Benzylamine Exchange with 3. The rate of benzylamine
exchange with bis(amine) complex 3 was also considerably
faster than the rate of conversion of 2 to 3. For example,
addition of R,R-benzylamine-d2 (70 mM) to a solution of 3‚H2-
NBn (10 mM) in C6D6 at 25 °C led to complete exchange within
2 min as evidenced by the 1:2.3 ratio of peaks corresponding
to bound (δ 3.8, br t, J ≈ 7 Hz) and free (δ 3.55, s) benzylamine
in the 1H NMR spectrum (Scheme 4).20 From this data, a lower
limit for the rate of amine exchange with 3 in the presence of
70 mM benzylamine of g1.5 × 10-3 s-1 (t1/2 e 40 s) was
estimated. Analogous results were obtained from the reaction
of benzylamine (∼70 mM) with Pd(p-C6H4CMe3)(H2NCD2-
Ph)2Br‚H2NCD2Ph (3-d4‚H2NCD2Ph) (∼10 mM).
An upper limit for the rate of benzylamine exchange with 3
was estimated from the absence of detectable excess line
broadening of the aryl methyl resonance of the closely related
bis(amine) complex Pd(p-C6H4CMe3)(H2NCH2-4-C6H4Me)2Br
(8) in the 1H NMR spectrum in the presence of 0.40 M
4-methylbenzylamine at 25 °C.20-23 Because the separation of
the aryl methyl resonances for bound and free 4-methylbenzyl-
amine (∆ν ) 50 Hz) was much larger than the excess
broadening of the aryl methyl resonance of 8 (∆ω1/2 e 2 Hz),
the slow-exchange approximation (∆ω1/2 ) kπ-1 24
was em-
)
ployed. Assuming that g2 Hz excess line broadening would
have been detected, an upper limit for the rate of amine exchange
with 8 in the presence of 0.40 M 4-methylbenzylamine of e6.2
s-1 was estimated.25 Therefore, amine exchange with 3 or 8
was between 1.5 × 103 and 1 × 105 times faster than the
conversion of 2 to 3 under comparable conditions.
A low-energy solvolysis pathway for amine exchange with
3 or 8 was confidently excluded as no low-energy pathway via
I was detected in the conversion of 2 to 3. Therefore,
bimolecular amine exchange with 3 was inferred and could occur
by either an associative mechanism via the five-coordinate tris-
(amine) intermediate Pd(p-C6H4CMe3)[H2NBn]3Br (IV) or by
an interchange mechanism via the 1:1 amine adduct Pd(p-C6H4-
CMe3)[H2NBn]2Br‚H2NBn (3‚H2NBn) (Scheme 5). The pos-
sibility of an interchange mechanism for amine exchange with
3 is suggested by the strong tendency of 3 and benzylamine to
(21) Wilkins, R. G. The Study of Kinetics and Mechanism of Reactions of
Transition Metal Complexes; Allyn and Bacon: Boston, MA, 1974.
(22) (a) Rabenstein, D. L.; Kula, R. J. J. Am. Chem. Soc. 1969, 91, 2492.
(b) Glass, G. E.; Schwabacher, W. B.; Tobias, R. S. Inorg. Chem.
1968, 7, 2471. (c) Taylor, P. W.; Feeney, J.; Burgen, A. S. V.
Biochemistry 1971, 10, 3866. (d) Pignolet, L. H.; Horrocks, W. D. J.
Am. Chem. Soc. 1968, 90, 922. (e) La Mar, G. N. J. Am. Chem. Soc.
1970, 92, 1806. (f) Rubini, P. R.; Poaty, Z.; Boubel, J. C.; Rodenhu¨ser,
L.; Delpuech, J. J. Inorg. Chem. 1983, 22, 1295.
(23) Merbach, A. E.; Moore, P.; Howarth, O. W.; McAteer, C. H. Inorg.
Chim. Acta 1980, 39, 129.
(24) Bovey, F. A. Nuclear Magnetic Resonance Spectroscopy, 2nd ed.;
Academic Press: San Diego, CA, 1988.
(18) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms;
Brooks/Cole: Monterey, CA, 1985; Chapter 4.
(19) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W. A.
Benjamin: New York, 1965.
(20) Complexes 3, 3-d4, and 8 were isolated as the corresponding hydrogen-
bonded mono(amine)solvate complexes 3‚H2NBn, 3-d4‚H2NCD2Ph,
and 8‚H2NCH2C6H4Me, respectively. As a result, solutions of these
complexes contatined 1 equiv of free amine.
(25) The rate of exchange of a single amine ligand is equivalent to the
rate of complete exchange. (a) Mønsted. L.; Mønsted, O. Acta Chem.
Scand. 1980, A34, 259. (b) Helm, L.; Elding, L. I.; Merbach, A. E.
Inorg. Chem. 1985, 24, 1719. (c) Swaddle, T. AdV. Inorg. Biochem.
Mech. 1983, 2, 95.

Palladium Bis(benzylamine) Complexes
Inorganic Chemistry, Vol. 36, No. 12, 1997 2613
Scheme 6
Figure 2. Benzylamine concentration dependence of the conversion
of 9 to 2 in the presence of ∼55 mM P(o-tol)3 at 45 °C in C6D6.
form the 1:1 hydrogen-bonded adduct 3‚H2NBn in both solution
and in the solid state.13
Scheme 7
Phosphine Exchange with 2. When a C6D6 solution of 2
(4 mM) was treated with excess P(1-nap)3 (1-nap ) 1-naphthyl)
at 25 °C, the P(o-tol)3 ligand was displaced with formation of
the corresponding P(1-nap)3 complex Pd[P(1-nap)3](p-C6H4-
CMe3)(H2NBn)Br (9) (Scheme 6). Phosphine exchange was
reversible and addition of P(o-tol)3 to a solution of 9 regenerated
2 {Keq ) [9][P(o-tol)3]/[2][P(1-nap)3] ) 0.52 ( 0.03 at 25 °C}.
In the presence of a ∼7 fold excess of P(1-nap)3 (55 mM), the
disappearance of 2 {[2]0 ) 8 mM} obeyed first-order kinetics
for approach to equilibrium over >3 half-lives with an observed
rate constant of kobs ) (4.7 ( 0.3) × 10-4 s-1 at 25 °C, which
corresponds to a half-life of ∼25 min (Figure S5). Under these
conditions, conversion of 2 to an 80:20 mixture of 9/2 was ∼3
times faster than the complete conversion of 2 to 3 in the
presence of ∼1 M benzylamine (k1 ) (1.42 ( 0.09) × 10-4
s-1 M-1 at 25 °C).
In a preparative-scale reaction, treatment of {Pd[P(1-nap)3]-
(p-C6H4CMe3)(µ-Br)}2 (10) with 2 equiv of benzylamine (per
dimer) led to the isolation of mono(amine) complex 9 in 83%
yield as a cream-colored solid (Scheme 6). The requisite
palladium aryl halide dimer 10 was isolated in 76% yield from
reaction of a 1:4:10 molar mixture of Pd2(DBA)3, P(1-nap)3,
and 4-tert-butylbromobenzene in benzene at 40 °C for 1 h.
We initially considered two plausible mechanisms for the
conversion of the P(o-tol)3 complex 2 to the P(1-nap)3 derivative
9. In the first mechanism, solvolysis of 2 would form the mono-
(amine) intermediate I, which could react with P(1-nap)3 to
generate 9. In the second mechanism, direct attack of P(1-nap)
on 2 via an associative or interchange mechanism could also
form 9. However, the rapid rate of conversion of 2 to 9 relative
to conversion of 2 to 3 clearly precluded exchange via phosphine
solvolysis, as no facile pathway via I was detected in the
conversion of 2 to 3. Likewise, it appeared unlikely that a
bimolecular reaction involving 2 and sterically bulky P(1-nap)3
would be faster than the bimolecular reaction involving 2 and
benzylamine. We therefore considered a more complex mech-
anism for the conversion of 2 to 9 initiated by solvolytic
displacement of the benzylamine ligand as outlined in Scheme
7.
The limited solubility of P(1-nap)3 in C6D6 precluded direct
kinetic analysis of the conversion of 2 to 9. As a result, the
kinetics of the conversion of 9 to 2 were investigated in an effort
to probe the mechanism of phosphine exchange. In order to
determine the rate dependence on P(o-tol)3 concentration, the
pseudo-first-order rate constants for the conversion of 9 to 2
were measured as a function of P(o-tol)3 concentration from
0.064 to 0.29 M in C6D6 at 25 °C. Each reaction proceeded to
g95% conversion and good pseudo-first-order plots for the
disappearance of 9 were obtained (Table S2, Figure S6). In
the absence of benzylamine, the rate was independent of P(o-
tol)3 concentration and the reaction obeyed the first-order rate
law shown in eq 3, where kobs ) (3.3 ( 0.2) × 10-3 s-1 (∆Gq
d[9]
dt
rate ) -
) kobs[9]
(3)
) 20.8 kcal mol-1). However, the rate was strongly inhibited
by excess benzylamine. In order to probe the benzylamine
concentration dependence of the rate of conversion of 9 to 2,
pseudo-first-order rate constants were measured as a function
of benzylamine concentration from 0.88 to 5.3 mM26 in the
presence of ∼55 mM P(o-tol)3 at 45 °C in C6D6 (Table S2,
Figure S7). A plot of the reciprocal of the observed rate
constants versus the [H2NBn]/[P(o-tol)3] ratio established the
inverse first-order dependence of the rate on the ratio of
[benzylamine]/[P(o-tol)3] for the conversion of 9 to 2 in the
presence of benzylamine (Figure 2).
Our kinetics for the conversion of 9 to 2 are consistent with
the mechanism shown in Scheme 7. In this mechanism,
solvolysis of 9 would form the mono(phosphine) intermediate
Pd[P(1-nap)3](p-C6H4CMe3)Br(solvent) (V), which could react
with P(o-tol)3 to form the four-coordinate mixed bis(phosphine)
intermediate Pd[P(1-nap)3][P(o-tol)3](p-C6H4CMe3)Br (VI). Sol-
volysis of VI would generate a second mono(phosphine)
intermediate Pd[P(o-tol)3](p-C6H4CMe3)Br(solvent) (III) which
could react with benzylamine to form 2. Because complete
(26) Kinetic analysis of the reaction at higher benzylamine concentration
was precluded by the competitive formation of palladium bis(amine)
complexes.

2614 Inorganic Chemistry, Vol. 36, No. 12, 1997
Zhong and Widenhoefer
(>95%) conversion of 9 to 2 was achieved under experimental
conditions, the reverse reaction (k-4) was ignored in the kinetic
analysis. Employing this assumption and assuming a steady-
state concentration of intermediates III, V, and VI generates
the rate law shown in eq 4a,b. This rate law is in accord with
d[9]
dt
rate ) -
)
k2k3[P(o-tol)3][9]
) kobs[9] (4a)
k
k3[P(o-tol)3] + -3 + 1 k-2[NH2Bn]
(
)
k4
k
-3 + 1 k-2
(
)
k4
[NH2Bn]
1
kobs
1
)
+
(4b)
k2
k2k3
[P(o-tol)3]
k
k3[P(o-tol)3] . -3 + 1 k-2[NH2Bn]
rate ) k2[9] (4c)
(
)
k4
our experimental rate data obtained in the presence of H2NBn,
where k2 ) (1.3 ( 0.1) × 10-2 s-1 and (k-2/k3)(k-3/k4 + 1) )
41 ( 5 at 45 °C. In the absence of benzylamine, the second
term of eq 4a approaches zero and eq 4a simplifies to eq 4c.
This rate law is the same form as the experimental rate law
Figure 3. Free energy-reaction coordinate diagram for the conversion
of 2 to 3 and for the conversion of 2 to 9. The free energy of mono-
(amine) complex 2 is arbitrarily set equal to zero.
obtained in the absence of benzylamine (eq 3), where kobs
k2.
)
drastic changes in solvent polarity, the low sensitivity of the
rate to solvent donicity could suggest the presence of a late,
productlike transition state.34
Although dissociative ligand substitution at 16 electron square
planar platinum complexes is rare,27-30 solvated intermediates
V and III could potentially be formed by an associative or
dissociative mechanism (Scheme 7). In order to distinguish
between these two possible pathways, the temperature and
solvent dependence of the first-order rate constant k2 for the
conversion of 9 to 2 was determined.31 An Eyring plot of k2
obtained at 10, 25, and 45 °C provided the activation param-
eters: ∆Hq ) 12.5 ( 0.8 kcal mol-1, ∆Sq ) -28 ( 3 eu (Figure
S8); the sign and magnitude of the entropy of activation are
clearly in accord with an associative pathway.
As noted above, saturation kinetics were observed for
conversion of 9 to 2 in the absence of benzylamine at P(o-tol)3
concentrations as low as 64 mM. It therefore appears likely
that saturation kinetics were also achieved for the conversion
of 2 to 9 in the presence of 55 mM P(1-nap)3 at 25 °C.
Therefore, from the observed rate constant for approach to
equilibrium (kobs 2 f 9 ) (4.7 ( 0.3) × 10-4 s-1), the first-order
rate constant for solvolysis of 2 in C6D6 at 25 °C to form III
was calculated to be k-5 ) (3.1 ( 0.2) × 10-4 s-1 (∆Gq )
22.0 ( 0.1 kcal mol-1).
The first-order rate constant k2 was solvent dependent and
increased by a factor of ∼5 in the order toluene-d8 (k2 ) 2.75
× 10-3 s-1) < C6D6 < THF-d8 < DMF-d7 (k2 ) 12.7 × 10-2
s-1) at 25 °C. Although the rate of conversion of 9 to 2 in
CD3OD/CDCl3 (5:1) at 25 °C was too fast to determine by
NMR, a lower limit for k2 of g4.6 × 10-2 s-1 was estimated
under these conditions.32 The dependence on k2 on solvent
donicity is also in accord with an associative pathway, although
the extent of rate acceleration is small considering the wide
range of solvent donicity.33 Although it is impossible to quantify
the effect of solvent donicity on the rate due to the
From our kinetic and related thermodynamic studies,12,13
a
free energy diagram was constructed for the ligand exchange
reactions involving 2, 3, and 9 (Figure 3). The overall
conversion of 2 to 3 and the conversion of 2 to 9 is nearly
ergoneutral (∆G° < 0.5 kcal mol-1).15 Benzylamine exchange
with 2 is facile (∆Gq e 18 kcal mol-1) with a barrier ∼5 kcal
mol-1 lower in energy than the barrier for conversion of 2 to 3
(∆Gq ) 22.7 ( 0.1 kcal mol-1). Bimolecular benzylamine
exchange with bis(amine) derivative 3 is also facile (15.5 kcal
mol-1 < ∆Gq < 18 kcal mol-1) and possesses a barrier ∼5-7
kcal mol-1 lower in energy than the barrier for conversion of 2
to 3. The barrier for solvolysis of 2 to form mono(phosphine)
intermediate III was slightly lower than the barrier for conver-
sion of 2 to 3 (∆∆Gq ≈ 0.7 kcal mol-1) and was ∼1.2 kcal
mol-1 higher in energy than the barrier for solvolysis of 9 to
form V. Conversion of 2 to the palladium aryl halide dimer 1
via loss of benzylamine is facile but endoergic (∆G° ) ∼3.4
kcal mol-1 Pd-1).12
(27) Romeo, R.; Minniti, D.; Lanza, S. Inorg. Chem. 1979, 18, 2362.
(28) Lanza, S.; Minniti, D.; Romeo, R.; Moore, P.; Sachinidis, J.; Tobe,
M. L. J. Chem. Soc., Chem. Commun. 1984, 542.
(29) Romeo, R.; Arena, G.; Scolaro, L. M.; Plutino, M. R.; Bruno, G.;
Nicolo, F. Inorg. Chem. 1994, 33, 4029.
(30) Romeo, R.; Grassi, A.; Scolaro, L. M. Inorg. Chem. 1992, 31, 4383.
(e) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem. Soc.
1989, 111, 8161.
(31) We thank several reviewers for suggesting these experiments.
(32) Conversion of 9 to 2 in CD3OD/CDCl3 (5:1) at 25 °C was complete
1
within 1 min by H NMR spectroscopy. Assuming g6% of 9 could
have been detected in the initial 1H NMR spectrum (∼4 half-lives), a
lower limit for the value of k2 for the conversion of 9 to 2 CD3OD/
CDCl3 (5:1) of g4.6 × 10-2 s-1 was estimated.
(34) (a) Belluco, A.; Oio, A.; Martelli, M. Inorg. Chem. 1966, 5, 1370. (b)
Belluco, U.; Graziani, M.; Nicolini, M.; Rigo, P. Inorg. Chem. 1967,
6, 721. (c) Belluco, U. Martelli, Orio, A. Inorg. Chem. 1966, 5, 582.
(d) Pearson, R. G.; Gray, H. B.; Basolo, F. J. Am. Chem. Soc. 1960,
82, 787.
(33) Cattalini, L. Inorganic Reaction Mechanisms; Edwards, J. O., Ed.;
Wiley-Interscience: New York, 1970.

Palladium Bis(benzylamine) Complexes
Inorganic Chemistry, Vol. 36, No. 12, 1997 2615
was >95% pure and was >98% deuterated at the benzylic positions.
1H NMR (C6D6, 25 °C): in addition to resonances corresponding to
free benzylamine-R,R-d2 (δ 7.15 and 0.70), resonances were observed
at δ 7.13-6.85 (m, 10 H), 2.43 (s, 4 H, H2NCD2Ph), and 1.27 (s, 9 H,
C6H4CMe3).
Conclusions
Kinetics of the conversion of the mono(phosphine) mono-
(amine) complex 2 to the corresponding bis(amine) complex 3
were consistent with an associative or interchange mechanism.
Benzylamine exchange with both 2 and 3 was considerably more
facile than conversion of 2 to 3. Kinetics of the conversion of
the 2 to the tri(1-naphthylphosphine) derivative 9 were in accord
with a mechanism initiated by associative solvolysis of the
benzylamine ligand followed by reaction with P(1-nap)3 to
generate the mixed bis(phosphine) intermediate Pd[P(1-nap)3]-
[P(o-tol)3](p-C6H4CMe3)Br (VI). Presumably, the small steric
size (cone angle ) 106°)35 and high nucleophilicity of benzyl-
amine promotes direct attack of the amine at palladium. In
contrast, the large steric bulk of P(1-nap)3 and P(o-tol)3 (cone
angle ) 195°)36 precludes direct interaction of the phosphine
and the palladium complex, leading to phosphine exchange via
initial solvolysis of the benzylamine ligand of 2.
{Pd[P(1-naphthyl)3](p-C6H4CMe3)(µ-Br)}2 (10). A purple solution
of Pd2(DBA)3 (500 mg, 0.55 mmol), P(1-naphthyl)3 (900 mg, 2.2 mmol)
and p-tert-butylbromobenzene (780 mg, 3.7 mmol) in 30 mL of benzene
was stirred at 40 °C for 1 h. The resulting brown solution was filtered
through Celite, and benzene was evaporated under vacuum. The
resulting oily residue was dissolved in Et2O (50 mL) and allowed to
stand at room temperature. The precipitate which formed over 3 h
was filtered out, washed with Et2O, and dried under vacuum to give
10 (599 mg, 76%) as a tan powder. 1H NMR (CDCl3, 55 °C): δ 8.30
(br), 7.79 (d, J ) 7.0 Hz), 7.68 (d, J ) 6.9 Hz), 7.10, 6.40, (br s, 2 H),
6.10 (d, J ) 7.6 Hz), 0.86 (s, C6H4CMe3). 31P {1H} NMR (CDCl3, 25
°C): δ 28.8 (br). Anal. Calcd (found) for C80H62Br2P2Pd2: C, 65.91
(65.67); H, 4.29 (4.38).
Pd[P(1-naphthyl)3](p-C6H4CMe3)[H2NBn]Br (9). Benzylamine
(20 mg, 0.19 mmol) was added to a brown solution of 10 (200 mg,
0.19 mmol) in CH2Cl2 (5 mL) and stirred at room temperature for 5
min. The resulting solution was concentrated to 2 mL under vacuum
and diluted with 25 mL of hexane. Cooling the solution via concentra-
tion to 10 mL under vacuum formed a precipitate which was filtered
out, washed with pentane, and dried under vacuum to give 9 (195 mg,
83%) as a cream-colored solid. 1H NMR (300 MHz, CDCl3, 55 °C):
δ 7.85, 7.68, 7.30, 6.97, 6.34, 3.72 (br, 2 H, H2NCH2Ph), 2.88 (br, 2
H, H2NCH2Ph), 0.94 (s, C6H4CMe3). 31P {1H} NMR (CDCl3, 25 °C):
δ 29.7. Anal. Calcd (found) for C47H40BrNPPd: C, 67.52 (67.27);
H, 4.82 (5.10).
Experimental Section
General Methods. Reactions were performed under an inert
atmosphere of nitrogen or argon in a glovebox or by standard Schlenk
techniques. Preparative-scale reactions were performed in flame- or
oven-dried Schlenk tubes equipped with a stir bar, side arm joint, and
a septum. 1H and 31P NMR spectra were obtained on a Varian XL-
300 spectrometer. Elemental analyses were performed by E+R
Microanalytical Laboratories (Corona, NY). Diethyl ether, hexane,
pentane, benzene, C6D6, toluene-d8, THF-d8, and dioxane-d8 were
distilled from sodium/benzophenone ketyl under argon or nitrogen.
Methylene chloride, methylene chloride-d2, and DMF-d7 were distilled
from CaH2; CDCl3 was distilled from P2O5. Benzylamine (Aldrich,
anhydrous) and CD3OD (Cambridge Isotopes Laboratories) were used
as received. Benzylamine-R,R-d237 was synthesized from the LiAlD4/
AlCl3 reduction of benzonitrile;38 1H NMR and GCMS analysis
indicated >95% chemical purity and g98% isotopic purity. Palladium
aryl halide dimers 1, 4, and 6 and palladium-amine complexes 2, 3,
and 8 were prepared by published procedures.11a,13
Kinetic Measurements. Samples for kinetic analysis were prepared
from stock solutions of the appropriate palladium aryl halide dimer
and/or benzylamine and were performed in oven-dried 5 mm thin-walled
NMR tubes capped with rubber septa. Solvent volume in the NMR
tubes was calculated from the solvent height measured at 25 °C
according to the relationship V (mL) ) H (mm) × 0.01384-0.006754
and from temperature dependence of the density of benzene.39 Kinetic
1
data was obtained by H NMR spectroscopy in the heated probe of a
Varian XL-300 spectrometer. Probe temperatures were measured with
either an ethylene glycol or methanol thermometer and were maintained
at (0.5 °C throughout data acquisition. Syringes employed in
measuring liquids for kinetic measurements were calibrated by mercury
displacement and were accurate to >95%. Error limits for rate
constants refer to the standard deviation of the slope and/or intercept
of the corresponding least-squares-fit line.
Pd[P(o-tol)3](p-C6H4CMe3)[H2NCD2Ph]Br (2-d2). Benzylamine-
R,R-d2 (1.2 µL, 1.1 × 10-2 mmol) was added via syringe to an NMR
tube which contained a solution of {Pd[P(o-tol)3](p-C6H4CMe3)(µ-Br)}2
(1) (∼7 mg, 5.5 × 10-3 mmol) in C6D6 (0.7 mL) to generate 2-d2 which
1
was > 95% pure by H NMR spectroscopy and was characterized by
1H NMR spectroscopy without isolation. 1H NMR (CDCl3, 50 °C): δ
7.80 (br, 3 H), 7.22 (m, J ) 6.4 Hz, 3 H), 7.07 (m, 4 H), 6.69 (s, 4 H),
3.01 (s, 2 H, H2NCH2Ph), 2.15 [br s, 9 H, P(o-tol)3], 1.17 (s, 3 H,
C6H4CMe3).
Benzylamine Dependence of the Rate of Conversion of 2 to 3.
Benzylamine (16 µL, 15.7 mg, 0.46 mmol, 0.20 M) was added via
syringe to an NMR tube containing a solution of 1 (2.0 mg, 1.6 ×
10-3 mmol, 2.2 mM) in C6D6 (total volume of 0.75 mL). Formation
of the mono(amine) complex 2 from 1 and benzylamine is both rapid
(t1/2 e 15 s at 25 °C) and exoergic (Keq ≈ 1 × 105).8 The tube was
shaken and placed in the probe of an NMR spectrometer preheated to
55 °C. The concentrations of 2 and 3 were determined by integrating
the tert-butyl resonances for 2 (δ 1.17) and 3 (δ 1.27) in the 1H NMR
spectrum and from the mass balance.40 The concentration of free
benzylamine was determined from the mass balance. The pseudo-first-
order rate constant for the conversion of 2 to 3 was determined from
a plot of ln([2]t/[2]0) versus time (Figure S1, Table S1). Pseudo-first-
order rate constants were also obtained at benzylamine concentrations
of 0.47, 1.06, and 1.64 M (Figure S1, Table S1). The second-order
rate constant for the conversion of 2 to 3 was obtained from a plot of
observed rate constants versus benzylamine concentration (Table 1,
Figure S2).
Pd(p-C6H4CMe3)[H2NCD2Ph]2Br‚H2NCD2Ph (3-d4‚H2NCD2Ph).
A solution of 1 (50 mg, 0.08 mmol) and benzylamine-R,R-d2 (150 µL,
147 mg, 1.3 mmol) in THF (2 mL) was stirred overnight at room
temperature to give a colorless solution. Solvent was evaporated under
vacuum, and the residue was dissolved in THF (5 mL) and diluted
with pentane (5 mL). Cooling the resulting solution to -30 °C
overnight produced a precipitate which was filtered out, washed with
pentane, and dried under vacuum to give 3‚H2NBn-d6 (97 mg, 91%)
as a white fibrous solid. 1H NMR analysis indicated that 3‚H2NBn-d6
(35) Seligson, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1991, 113, 2520.
(36) Tolman, C. A. Chem. ReV. 1977, 77, 313.
(37) (a) Mukumoto, M.; Tsuzuki, H.; Tsukinoki, T.; Mataka, S.; Tashiro,
M.; Nagano, Y. J. Deuterium Sci. 1995, 4, 85. (b) Mure, M.; Klinman,
J. P. J. Am. Chem. Soc. 1995, 117, 8707. (c) Tsukinoki, T.; Ishimoto,
K.; Nakayama, K.; Kakinami, T.; Mataka, S.; Tashiro, M. J. Labelled
Compd. Radiopharm. 1994, 34, 839. (d) Kagabu, S.; Ando, C.; Ando,
J. J. Chem. Soc., Perkin Trans. 1 1994, 739. (e) Benincori, T.; Brenna,
E.; Sannicolo, F. J. Chem. Soc., Perkin Trans. 1 1993, 675. (f) Itoh,
S.; Mure, M.; Ogino, M.; Ohshiro, Y. J. Org. Chem. 1991, 56, 6857.
(g) Kim, J. M.; Cho, I. S.; Mariano, P. S. J. Org. Chem. 1991, 56,
4943. (h) Capdevielle, P.; Lavigne, A.; Sparfel, D.; Baranne-Lafont,
J.; Cuong, N. K.; Maumy, M. Tetrahedron Lett. 1990, 31, 3305.
(38) (a) Nystrom, R. F. J. Am. Chem. Soc. 1955, 77, 2544. (b) Frejd, T.;
Klingstedt, T. Synthesis 1987, 40.
The pseudo-first-order rate constants for the conversion of 2 to 3 in
C6D6 at 25, 40, 65, and 77 °C (Figures S9-S12, Table S3), for the
conversion of 2 to 3 in toluene-d8, THF-d8, and dioxane-d8 at 25 °C
(39) International Critical Tables of Numerical Data, Physics, Chemistry,
and Technology; Washburn, E. W., Ed.; McGraw-Hill: London, 1928;
Vol. III, pp 29, 39, 221.
(40) The conversion of 2 to 3, 4 to 5, and 6 to 7 was quantitative in all
cases.

2616 Inorganic Chemistry, Vol. 36, No. 12, 1997
Zhong and Widenhoefer
(Figures S13-S15, Table S4), and for the conversion of 4 to 5 in C6D6
at 25 °C (Figure S16, Table S5) were determined by procedures
analogous to that employed to determine kobs for the conversion of 2
to 3 at 55 °C. Second-order rate constants (k1) were determined from
plots of kobs versus [H2NBn] (Table 1, Figures S2, S17).
The concentrations of 2 and 9 were determined by integrating the tert-
butyl resonances for 2 (δ 1.17) and 9 (δ 1.03) in the 1H NMR spectrum
and from the mass balance. The pseudo-first-order rate constant for
the conversion of 9 to 2 was determined from a plot of ln([9]) versus
time (Figure S7, Table S2). Pseudo-first-order rate constants were also
obtained at benzylamine concentrations of 1.3, 1.8, 2.6, 3.5, and 5.3
mM in the presence of ∼55 mM P(o-tol)3 (Figure S7, Table S2). The
first-order rate constant k2 was obtained as the inverse of the intercept
of a plot of the observed rate constants versus [H2NBn]/[P(o-tol)3]
(Figure 2).
P(o-tol)3/H2NBn Dependence of the Rate of Conversion of 2 to
3. Benzylamine (25 µL, 24.5 mg, 0.23 mmol, 0.34 M) was added via
syringe to an NMR tube containing a solution of 1 (2.0 mg, 1.6 ×
10-3 mmol, 2.4 mM) and P(o-tol)3 (41.0 mg, 0.14 mmol, 0.21 M) in
C6D6 (total volume of 0.65 mL). The tube was shaken and placed in
the probe of an NMR spectrometer preheated at 55 °C. The concentra-
tions of 2 and 3 and free benzylamine were determined as described
above. The pseudo-first-order rate constant for approach to equilibrium
was determined from a plot of ln{([2]t - [2]eq)/([2]0 - [2]eq)} versus
time (Figure S3, Table S1). Pseudo-first-order rate constants for the
approach to equilibrium were also obtained at P(o-tol)3 concentrations
of 0.048, 0.091, and 0.14 M in the presence of ∼0.32 M benzylamine
(Figure S3, Table S1). The second-order rate constant k1 for the
Acknowledgment. We thank the National Science Founda-
tion, Dow Chemical, and Pfizer for their support of this work.
R.W. is an NCI Postdoctoral Trainee supported by NIH Cancer
Training Grant CI T32CA09112. We thank Drs. Michael
Palucki and Susan L. Hallenbeck for helpful discussions, and
we thank several thoughtful reviewers for suggesting the
solvolysis mechanism for the formation of intermediates III and
V. We thank Prof. Stephen L. Buchwald for helpful discussions
and for permission to publish this work.
conversion of 2 to 3 was obtained from the intercept of a plot of kobs
/
[H2NBn] versus [P(o-tol)3]/[H2NBn] (Figure 1); k-1 was obtained from
the slope of this plot.
P(o-tol)3 and Benzylamine Concentration Dependence of the Rate
of Conversion of 9 to 2. A solution of 9 (2.3 mg, 2.7 × 10-3 mmol,
4.1 mM) and benzylamine (0.06 mg, 5.7 × 10-4 mmol, 0.88 mM) in
C6D6 (0.65 mL) was added via syringe to an NMR tube containing
P(o-tol)3 (11.8 mg, 3.9 × 10-2 mmol, 0.055 M). The tube was shaken
and placed in the probe of an NMR spectrometer preheated to 45 °C.
Supporting Information Available: Tables and plots of kinetic
data (12 pages). Ordering information is given on any current masthead
page.
IC961388V