Variable-temperature and -pressure kinetics and mechanism of the cyclopalladation reaction of imines in aprotic solvent

Article abstract of DOI:10.1021/om961099e

The cyclometalation reactions of benzylidenebenzylamines, -anilines, and -propylamine with palladium acetate have been studied in toluene solution. The cyclometalated compounds are formed via C-H electrophilic bond activation to produce different types of metallacycles, depending upon the polyfunctional nature of the ligand selected. If a five-membered endo metallacycle is possible, it is formed via aromatic C-H bond activation. Formation of a five-membered exo metallacycle only takes place when endo cyclometalation requires the formation of a six-membered compound via aliphatic C-H bond activation or when the steric hindrance affects the planarity of the imine in an important way. The activation of aliphatic C-H bonds has been achieved for endo six-membered metallacycles only when the alternative aromatic C-H bond activation would produce a four-membered exo compound; no activation of aliphatic C-H bonds to form exo five-membered complexes has been observed. The reactions have been monitored kinetically at different temperatures and pressures in order to establish the mechanism through which these spontaneous reactions occur. Three different sets of activation parameters are apparent from the results obtained: those related to the benzylamines and propylamine (ΔH^? = +4S/+67 kJ mol^-1, ΔS^? = -100/-180 J K^-1 mol^-1, ΔV^? = -11/-17 cm³ mol_-1), those related to the aniline derivatives (ΔH^? = +66/+75 kJ mol^-1, ΔS^? = -96/-123 J K^-1 mol^-1, ΔV^? = -21/-25 cm³ mol^-1), and finally those related to the imines producing aliphatic bond activation (ΔH^? = +487+49 kJ mol^-1, ΔS^? = -167/-177 J K^-1 mol^-1, ΔV^? = -20/-24 cm³ mol^-1). The results are interpreted as the formation of a highly ordered four-center transition state, involving the C-H and Pd-O(acetato) bonds, which is found to be very sensitive to the flexibility and steric hindrance of the imine ligands and very insensitive to electronic changes of the C-H bonds.

Full text of DOI:10.1021/om961099e

Organometallics 1997, 16, 2539-2546
2539
Va r ia ble-Tem p er a tu r e a n d -P r essu r e Kin etics a n d
Mech a n ism of th e Cyclop a lla d a tion Rea ction of Im in es
in Ap r otic Solven t
Montserrat Go´mez, J aume Granell, and Manuel Martinez*
Departament de Quı´mica Inorga`nica, Facultat de Quı´mica, Universitat de Barcelona,
Diagonal 647, E-08028 Barcelona, Spain
Received December 30, 1996X
The cyclometalation reactions of benzylidenebenzylamines, -anilines, and -propylamine
with palladium acetate have been studied in toluene solution. The cyclometalated compounds
are formed via C-H electrophilic bond activation to produce different types of metallacycles,
depending upon the polyfunctional nature of the ligand selected. If a five-membered endo
metallacycle is possible, it is formed via aromatic C-H bond activation. Formation of a
five-membered exo metallacycle only takes place when endo cyclometalation requires the
formation of a six-membered compound via aliphatic C-H bond activation or when the steric
hindrance affects the planarity of the imine in an important way. The activation of aliphatic
C-H bonds has been achieved for endo six-membered metallacycles only when the alternative
aromatic C-H bond activation would produce a four-membered exo compound; no activation
of aliphatic C-H bonds to form exo five-membered complexes has been observed. The
reactions have been monitored kinetically at different temperatures and pressures in order
to establish the mechanism through which these spontaneous reactions occur. Three different
sets of activation parameters are apparent from the results obtained: those related to the
benzylamines and propylamine (∆Hq ) +45/+67 kJ mol-1, ∆Sq ) -100/-180 J K-1 mol-1
∆Vq ) -11/-17 cm3 mol-1), those related to the aniline derivatives (∆Hq ) +66/+75 kJ mol-1
,
,
∆Sq ) -96/-123 J K-1 mol-1, ∆Vq ) -21/-25 cm3 mol-1), and finally those related to the
imines producing aliphatic bond activation (∆Hq ) +48/+49 kJ mol-1, ∆Sq ) -167/-177 J
K-1 mol-1, ∆Vq ) -20/-24 cm3 mol-1). The results are interpreted as the formation of a
highly ordered four-center transition state, involving the C-H and Pd-O(acetato) bonds,
which is found to be very sensitive to the flexibility and steric hindrance of the imine ligands
and very insensitive to electronic changes of the C-H bonds.
In tr od u ction
mechanistic information is limited.7 Very little infor-
mation is available about the nature of the species
existing in these reaction solutions; even the nature of
the starting material, palladium acetate, is rather
unclear.8 Although it is generally assumed that cyclo-
palladated compounds maintain their dimeric structure
in solution, it has been shown recently that acetone is
able to break the bridge of the cyclopalladated complex
[Pd(µ-Br)(C6H4CH(Me)NH2)]2, producing mononuclear
species.9
Cyclometalation reactions on Pd(II) complexes have
been throughly studied by a number of research groups.1
The synthesis of cyclopalladated complexes is a field of
great interest as a consequence of their applications, for
example, in organic synthesis2 or in the design of new
metallomesogenes3 or antitumor drugs.4 Optically ac-
tive cyclopalladated derivatives have also been used for
the enantiomeric excess determination5 and optical
resolution of amines and phosphines.6
Our interests have been centered in studies of this
type of reaction on iminic ligands10 as well as in the
mechanisms operating in organometallic reactions in-
volving the activation of C-H bonds on Pt(II) and
The number of these studies dealing with kinetic-
X Abstract published in Advance ACS Abstracts, May 1, 1997.
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Navarro-Ranninger, C.; Lo´pez-Solera, I.; Gonza´lez, V. M.; Pe´rez, J . M.;
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B.; Willis, A. C. J . Chem. Soc., Dalton Trans. 1995, 57.
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Chem. Soc., Dalton Trans. 1985, 2629. (b) Ryabov, A. D.; Yatsimirski,
A. K. Inorg. Chem. 1984, 23, 789. (c) Ryabov, A. D. Inorg. Chem. 1987,
26, 1252.
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G. H.; Sim, K. Y.; Tan, M. K. Tetrahedron: Asymmetry 1992, 3, 529.
(b) Kyba, E. P.; Rines, S. P. J . Org. Chem. 1982, 47, 4800. (c) Albert,
J .; Granell, J .; Muller, G.; Sainz, D.; Font-Bardia, M.; Solans, X.
Tetrahedron: Asymmetry 1995, 6, 325.
(8) Romm, I. P.; Kravtsova, S. V.; Perepelkova, T. I.; Petrov, E. S.;
Kalinovski, I. O.; Buslaeva, T. M. Russ. J . Coord. Chem. (Engl. Transl.)
1995, 21, 740.
(9) Vicente, J .; Saura-Llamas, I.; J ones, P. G. J . Chem. Soc., Dalton
Trans. 1993, 3619.
S0276-7333(96)01099-0 CCC: $14.00 © 1997 American Chemical Society

2540 Organometallics, Vol. 16, No. 12, 1997
Go´mez et al.
dinuclear Rh(II) complexes.11 The final goal of these
investigations is the study of the importance of the
relative influence of steric and electronic factors that
could tune the reactivity and thermodynamic prefer-
ences of the reactions involved in different
processes.12
in their electronic characteristics does not seem to affect
the process. In contrast, the influence of the steric
effects in the process is clearly observed. Thus, al-
though imine E could afford two different endo five-
membered metallacycles, under the experimental con-
ditions studied only the metalation of the less hindered
Caromatic-H bond was achieved.
Benzylidenebenzylamines I, J , and L can undergo the
metalation on different aromatic carbon atoms to afford
five-membered endo or exo metallacycles; in all cases
the endo metallacycles have been selectively obtained.
These results confirm the strong tendency of these
N-donor ligands to form endocyclic derivatives.10,14
Again, although imine L, containing a nitro substituent,
could afford two different five-membered endo metal-
lacycles (similarly to imine E), only metalation of the
less hindered bond has been observed. The cyclopalla-
dation reaction of imine K produced a reaction mixture
with a very complex 1H NMR spectrum. Reaction of the
solid thus obtained with PPh3 and LiBr affords the
mononuclear compounds [Pd(C-N)Br(PPh3)] which can
be separated and identified as the endo and exo palla-
dacycles 3k and 4k , respectively (Scheme 2). Formation
of the exocyclic compound with this imine can be
explained by the presence of a nitro group in the
2-position of the benzylidenic ring that hinders a planar
disposition of the iminic moiety and this aromatic ring.
Consequently, the formation of the four-center transi-
tion state (see below) leading to compound 1k is less
favored and the exo complex 2k is also obtained. Even
so, the endo compound, 1k , can be obtained in much
better yields when the reaction is performed under more
severe conditions or in protic solvent.
Imines M and N can produce both five-membered exo
metallacycles by activation of a Caromatic-H bond and
six-membered endo metallacycles by activation of a
Caliphatic-H bond. Although it is widely accepted that a
strong tendency exists to form five-membered metalla-
cycles via preferential activation of aromatic over
aliphatic C-H bonds,1 some exceptions are known.14b,15
Under the reaction conditions of this work, the five-
membered exo metallacycles 2m ,n are selectively ob-
tained (Scheme 3). The six-membered endo metallacy-
cles produced by activation of a Caliphatic-H bond were
only obtained from imines G and H (Scheme 4). Activa-
tion of a Caromatic-H bond would form, in these cases,
unusual four-membered metallacycles.1
RC H CHdNR′H
palladium acetate +
f
6
4
X
[Pd(µ-CH3CO2)(RC6H3CHdNR′H)]2
1x, endo
+
[Pd(µ-CH3CO2)(R′NdCHC6H4R)]2
2x, exo
In this paper we report the preparation and kinetic
study of the influence of temperature and pressure on
the reaction of cyclometalation of a wide variety of
imines, X (Chart 1), by palladium acetate. The imines
are suitable ligands to study cyclometalation reactions,
since they can undergo metalation on different carbon
atoms (polyfunctional ligands); consequently, they have
been selected to allow comparison of the metalation of
aromatic versus aliphatic carbon atoms. The electronic
and steric influence of the substituents in the process
and the importance of the size (five- or six-membered)
and nature of the metallacycle formed (endo, 1x, versus
exo, 2x; see above)13 have also been looked into. The
reactions have been carried out in toluene, a poor
coordinating solvent, to ensure the existence of the
dimeric forms of both starting material and cyclometa-
lated final complexes. The important differences in
kinetic, and thermal and pressure activation parameters
detected are associated with the flexibility of the iminic
ligands and to the formation of an initial species that
produces a lag time in the increase of the absorbance
accompanying the cyclometalation reaction.
Resu lts a n d Discu ssion
Com p ou n d s. The reaction of palladium acetate with
imines A-N (Chart 1) in toluene at 60 °C was studied.
When this reaction was carried out with imines A-F ,
the endo five-membered metallacycles 1a -f were ob-
tained, by metalation of the Caromatic-H bond (Scheme
1); these compounds have a dinuclear acetato-bridged
structure. The presence, on the aromatic ring to be
metalated, of substituents with important differences
The previously described compounds 1b,f,g,i,j and
1
2m ,n had been characterized by comparison of their H
(10) (a) Albert, J .; Ceder, R. M.; Go´mez, M.; Granell, J .; Sales, J .;
Solans, X. Organometallics 1990, 9, 1405. (b) Albert, J .; Ceder, R. M.;
Go´mez, M.; Granell, J .; Sales, J . Organometallics 1992, 11, 1536. (c)
Albert, J .; Granell, J .; Moragas, R.; Sales, J .; Font-Bardia, M.; Solans,
X. J . Organomet. Chem. 1995, 494, 95. (d) Albert, J .; Granell, J .; Sales,
J .; Font-Bardia, M.; Solans, X. Organometallics 1995, 14, 1393. (e)
Granell, J .; Sainz, D.; Sales, J .; Solans, X.; Font-Altaba, M. J . Chem.
Soc., Dalton Trans. 1986, 1785.
(11) (a) Crespo, M.; Martinez, M.; Sales, J . Organometallics 1992,
11, 1288. (b) Crespo, M.; Martinez, M.; Sales, J . Organometallics 1993,
12, 4297. (c) Gonza´lez, G.; Lahuerta, P.; Martinez, M.; Sanau, M.; Peris,
E. J . Chem. Soc., Dalton Trans. 1994, 545. (d) Estevan, F.; Lahuerta,
P.; Peris, E.; Ubeda, M. A.; Garc´ıa-Granda, S.; Go´mez-Beltra´n, F.; Pe´rez
Carren˜o, E.; Gonza´lez, G.; Martinez, M. Inorg. Chim. Acta 1994, 218,
189. (e) Estevan, F.; Gonza´lez, G.; Lahuerta, P.; Martinez, M.; Peris,
E.; van Eldik, R. J . Chem. Soc., Dalton Trans. 1996, 1045.
(12) (a) Crespo, M.; Martinez, M.; Sales, J . J . Chem. Soc., Chem.
Commun. 1992, 822. (b) Schmu¨lling, M.; Ryabov, A. D.; van Eldik, R.
J . Chem. Soc., Chem. Commun. 1992, 1609. (c) Gonza´lez, G.; Moullet,
B.; Martinez, M.; Merbach, A. E. Inorg. Chem. 1994, 33, 2330. (d)
Gonza´lez, G.; Martinez, M.; Rodriguez, E. J . Chem. Soc., Dalton Trans.
1995, 891.
NMR spectra with those published.10b,e,14 The metala-
tion compounds of imines G, M, and N have already
been described, but their dimeric acetato-bridged com-
pounds were not characterized.10b,14b
All new compounds were characterized by elemental
1
analysis, IR, and H NMR. The acetato bands appear
at ca. 1580 and 1420 cm-1 in the IR spectra of com-
pounds 1x and 2x, indicating that the ligand is in a
bridging position.16 In the 1H NMR spectra, the acetato
CH3 signal appears as a singlet, in accord with a trans
(14) Albert, J .; Granell, J .; Sales, J . J . Organomet. Chem. 1984, 273,
393. (b) Albert, J .; Granell, J .; Sales, J .; Solans, X.; Font, M. Organo-
metallics 1986, 5, 2567.
(15) (a) Alsters, P. L.; Engel, P. F.; Hogerheide, M. P.; Copjin, M.;
Spek, A. L.; van Koten, G. Organometallics 1993, 12, 1831. (b) De
Munno, G.; Ghedini, M.; Neve, F. Inorg. Chim. Acta 1995, 239, 155.
(16) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 3rd ed.; Wiley: New York, 1978; p 232.
(13) An imine cyclometalated compound is considered endocyclic
when the CdN bond is contained in the metallacycle.

Cyclopalladation of Imines in Aprotic Solvent
Organometallics, Vol. 16, No. 12, 1997 2541
Ch a r t 1a
a
Arrows indicate the activated position.
arrangement of the C-N ligands relative to the {Pd2(µ-
O2CCH3)2} fragment. The signal corresponding to the
CH2 protons of the benzylamine derivatives appears as
two doublets (spin system AB), as expected for a dimeric
acetato-bridged open-book structure (folded), already
found by X-ray diffraction studies for some cyclopalla-
dated acetato-bridged dimers.17 The aromatic and me-
compounds show that all imines, except K, afford
selectively the endo metallacyclic complexes. The pro-
ton NMR spectrum of the solid obtained after reaction
of this imine and palladium acetate is very complex, and
its reaction with PPh3 was studied in order to determine
its nature (Scheme 2). The aromatic region in the
proton NMR spectra of the derived compounds 3k and
4k (that could be separated as pure solids by their
different solubilities in acetone and dichloromethane),
provides conclusive evidence of the palladation position,
allowing the identification of these complexes as the
endo and exo metallacycles, respectively. The aromatic
protons of the palladated ring in both compounds appear
shifted to high field, indicating a cis arrangement of the
1
thinic proton signals for the H NMR spectra of these
(17) (a) Albinati, A.; Pregosin, P. S.; Ruedi, R. Helv. Chim. Acta 1985,
68, 2046. (b) Selbin, J .; Abboud, K.; Watkins, S. F.; Gutie´rrez, M. A.;
Fronczek, F. R. J . Organomet. Chem. 1983, 241, 259. (c) Balavoine,
G.; Clinet, J . C.; Zerbib, P.; Boubeukur, K. J . Organomet. Chem. 1990,
389, 259. (d) Garc´ıa-Ruano, J . L.; Lo´pez-Solera, I.; Massaguer, J . R.;
Navarro-Ranninger, C.; Rodr´ıguez, J . H.; Mart´ınez-Carrera, S. Orga-
nometallics 1992, 11, 3013.

2542 Organometallics, Vol. 16, No. 12, 1997
Go´mez et al.
Sch em e 1
Sch em e 2
phosphine and the metalated carbon atom.10b Further-
more, the methinic proton signal of 3k appears shifted
to high field (relative to that of the free imine) and
coupled with the phosphorus atom, 4J (PH) ) 8.2 Hz, in
good agreement with an endocyclic structure.10a In
contrast, the same proton signal for 4k appears shifted
to low field, in good agreement with an exocyclic
structure with the imine in the Z form relative to the
CdN bond.10a
tions were followed spectrophotometrically, and the
absorbance versus time traces were derived at the
wavelength where larger differences were detected.
After the appearance of a lag time in the increase of
the absorbance accompanying the reaction, strongly
dependent on the nature of the imine used, in all cases
a monophasic first-order behavior was observed. Moni-
toring of the overall reaction at the appropriate intervals
1
by H NMR spectroscopy agrees with the second reac-
Kin etics. The cyclometalation reaction of palladium
acetate with the different imines in toluene solution has
been studied under second-order conditions ([Pd]/[imine]
) 0.7-1.3) to avoid the formation of the insoluble bis-
(imine) complexes [Pd(CH3COO)2(imine)2]. The reac-
tion being the intramolecular cyclometalation (C-H
bond activation) and the initiation period being the
formation of a non-organometallic coordination com-
pound (see below).
Overall, the coordination of the imine to the dimeric

Cyclopalladation of Imines in Aprotic Solvent
Organometallics, Vol. 16, No. 12, 1997 2543
Ta ble 1. Kin etic a n d Activa tion P a r a m eter s for
th e Cyclom eta la tion Rea ction s of P a lla d iu m
Aceta te w ith th e Im in es In d ica ted in Ch a r t 1 in
Tolu en e Solu tion
Sch em e 3
∆Sq,
J K-1
mol-1
∆Vq,
cm3 mol-1
(T/K)
imine metalated 104k323
,
∆Hq, kJ
X
compd
s-1
mol-1
A
B
C
D
E
F
G
H
I
1a
1b
1c
1d
1e
1f
1g
1h
4.7
1.4
1.2
63 ( 5 -115 ( 16 -12 ( 3 (318)
71 ( 6 -102 ( 18 -23 ( 3 (343)
73 ( 10 -97 ( 30 -24 ( 5 (323)
66 ( 10 -123 ( 33 -23 ( 4 (343)
67 ( 12 -113 ( 36 -21 ( 2 (343)
75 ( 18 -96 ( 50 -25 ( 2 (343)
48 ( 3 -167 ( 9 -24 ( 3 (333)
49 ( 11 -177 ( 33 -20 ( 1 (343)
52 ( 3 -150 ( 10 -15 ( 2 (323)
65 ( 1 -110 ( 2 -12 ( 1 (333)
46 ( 4 -168 ( 13 -11 ( 1 (323)
45 ( 2 -180 ( 6 -15 ( 4 (323)
67 ( 5 -100 ( 15 -12 ( 1 (323)
62 ( 2 -120 ( 9 -17 ( 1 (323)
1.3a
1.6a
0.58
4.7
0.59
5.9
1i
1j
J
3.8
K
L
M
N
1k + 2k
1l
2m
2n
5.5b
3.3a
7.0
9.2a
a
b
A statistical factor of 2 has been applied. A statistical factor
of 1.1 has been applied according to the [1k ]/[2k ] ) 2.5 ratio
determined under these conditions.
Ch a r t 2
Sch em e 4
constants, kobs, for the systems studied as a function of
imine, temperature, and pressure are available on
request. From these observed rate constants, the first-
order constants at 298 K, thermal activation param-
eters, and activation volumes collected in Table 1 are
derived. The values for both the enthalpy and entropy
of activation, as well as those for the activation volume,
are within the range of values observed for other C-H
bond activations via electrophilic substitution in the
absence of added acids.1e,11c-e The values determined
for ∆Vq are slightly less negative than those found for
these reactions, indicating a less crowded arrangement
of the transition state. The results agree with the
presence of a highly ordered transition state such as
that shown in Chart 2, where the neighboring terminal
acetato group acts as a proton acceptor to produce acetic
acid as a leaving group.11c-e Nevertheless, important
differences, strongly dependent on the nature of the
cyclometalating imine involved, are observed. Although
other interpretations are also possible, such as those
involving oxidative addition processes, for these types
of reactions the existence of the transition state depicted
in Chart 2 is fairly well-established.
As indicated above, the cyclometalation reaction of
palladium acetate with the imines X, depicted in Chart
1, produces in all cases the clean endo (1x) or exo (2x)
corresponding compounds; a mixture of both isomers is
detected in the final reaction mixture with imine K. As
for the changes in the UV-vis spectra, they are much
more complex; in all cases small initial changes in the
spectra are followed by further important and larger
variations. These initially small changes became much
more pronounced, with the imines having larger cone
angles around the donor N atom and less basic character
palladium center is comparatively fast in relation to the
cyclometalation reaction, as expected. The reaction
observed, and monitored in this work, corresponds solely
to the C-H bond activation on the already coordinated
imine. All the values found for the observed rate

2544 Organometallics, Vol. 16, No. 12, 1997
Go´mez et al.
(the aniline derivatives B-H). In these cases a lag time
in the increase of the absorbance versus time traces
becomes evident. Workup of the systems at these
reaction times produces the precipitation of the very
insoluble bis(imine) compounds;23 these complexes are
also detected in the UV-vis cells when concentrations
are on the high side. For the alkyl- and benzylamine
derivatives (A, I-N), these initial variations were of
very little importance.
Palladium acetate is a trinuclear compound in the
solid state, with all the acetato groups acting as bridging
ligands;18 nevertheless, the 1H NMR spectrum of a
saturated solution of palladium acetate in C6D6 shows
four singlets.19 The high-field signals can be assigned
to terminal acetato ligands and the low-field signals to
bridging acetato ligands.20 The relaxation time, T1,
values of these signals confirm this assignment; the
high-field signals have small T1 values, which is in
agreement with their higher mobility. When the spec-
tra were recorded at lower palladium acetate concentra-
tions, new signals were observed, their number increas-
ing with the dilution of the sample, suggesting that the
formation of solvato complexes with palladium acetate
occurs in solution. In this way trinuclear and dinuclear
species with terminal acetato ligands can be present in
the solution medium. It has been shown that the
addition of Lewis bases, as amines or phosphines, to
benzene solutions of palladium acetate affords, when
an excess of ligand is added, the mononuclear coordina-
tion compounds [Pd(CH3COO)2L2].8
F igu r e 1. Isokinetic plot for the cyclometalation reactions
in toluene solution of the imines indicated in Chart 1: (9)
aniline derivatives; (2) benzylamine derivatives; (O) propyl-
amine derivative; (4) imine K; (]) imines G and H. Empty
points have not been used for the regression.
equivalent to that for the less hindered benzylamine
derivatives, I-N.
With respect to thermal activation parameters, ∆Hq
and ∆Sq, the pattern in Figure 1 is rather significant.
Although no trends are clearly observed in Table 1,
Figure 1 indicates that three separate sets of thermal
activation parameters must be considered: those related
to the benzylamine derivatives (including the benzyli-
denepropylamine A), those related to the aniline deriva-
tives, and finally those related to the imines G and H,
which produce the activation of Caliphatic-H bonds.
As a consequence of all the aforementioned facts, the
cyclopalladation of imines in toluene can be proposed
as the following: once the formation of imine-palladium
coordination compounds, containing terminal and bridg-
ing acetato ligands, has taken place, the electrophilic
attack of the metal atom at the C-H bond with
elimination of acetic acid occurs, producing the final
dinuclear cyclopalladated derivatives.
In general it can be concluded that for the more
sterically hindered imines (the aniline derivatives)
enthalpies of activation are, as a whole, larger, due to
the fact that the important space reorganization on
going to the transition state cannot take place as easily
(less negative ∆Sq values). The values obtained for
imine K cannot be considered, because two different
compounds are produced; nevertheless, the fact that no
differences for imines M and N (producing exo deriva-
tives) have been detected made us assume that the
activation parameters could be considered in the same
lot. Much more dramatic differences are observed for
the formation of the endo six-membered metallacycles
via aliphatic C-H bond activation in imines G and H.
In these reactions the entropy values are much more
negative and are accompanied by very low activation
entropy values; it seems clear that, although thermo-
dynamic factors predict that these six-membered met-
allacycles with Caliphatic-palladium bonds are less stable,1
the kinetic information agrees with the fact that the
easier approach of the C-H bond to the Pd center in
the four-center transition state (Chart 2) results in a
much smaller enthalpy value. The flexibility of the
imine can also explain the values obtained for the
activation volumes; for all the benzylamine derivatives
∆Vq values are in the range -11 to -17 cm3 mol-1, while
those from aniline are in the range -20 to -25 cm3
mol-1. Again it is important to note that the benzyli-
denepropylamine is included in the first group, its
flexibility being very close to the benzylidenebenzyl-
amines.
From the data in Table 1, it can be seen that
important differences occur in kinetic and activation
parameters for all the series of imines used. With
respect to the bond activation, first-order rate constants
of benzylamine derivatives I-N are found to be higher
than those of the aniline derivatives B-H. No impor-
tant or meaningful differences are found between the
formation of endo or exo metallacycles (for example 1i
versus 2m ) for the group of benzylamine derivatives
used, I-N; the same applies for the substituents on the
aromatic ring.
Even more important is to note that, despite the fact
that for compounds 1g and 1h an aliphatic C-H
activation has taken place to produce a six-membered
endo metallacycle, no significant changes in the values
of k are detected in relation to the other aniline
derivatives. Finally, imine A seems to react at a rate
(18) (a) Skapski, A. C.; Smart, M. L. J . Chem. Soc., Chem. Commun.
1970, 658. (b) Cotton, F. A.; Han, S. Rev. Chim. Miner. 1983, 20, 496.
(c) Mawby, A.; Pringle, G. E. J . Inorg. Nucl. Chem. 1971, 33, 1989. (d)
Cotton, F. A.; Han, S. Rev. Chim. Miner. 1985, 22, 277.
(19) Values of δ (T1) for a saturated solution of palladium acetate
in C6D6: 1.60 ppm (1.31 s), 1.53 ppm (1.32 s), 1.49 ppm (1.22 s), and
1.48 ppm (1.24 s).
(20) Albert, J .; Granell, J .; Moragas, R.; Font-Bardia, M.; Solans,
X. J . Organomet. Chem. 1996, 522, 59.
(21) (a) Spitzer, M.; Gartig, F.; van Eldik, R. Rev. Sci. Instrum. 1988,
59, 2092. (b) Fleischman, F. K.; Conze, E. G.; Stranks, D. R.; Kelm, H.
Rev. Sci. Instrum. 1977, 99, 1427.
In summary, as found for similar systems, no impor-
tant mechanistic differences exist in cyclometalation

Cyclopalladation of Imines in Aprotic Solvent
Organometallics, Vol. 16, No. 12, 1997 2545
1d : yield 30%. Anal. Found (calcd) for C30H24N4O8Pd2: C,
46.0 (46.12); H, 3.0 (3.10); N, 7.0 (7.17). 1H NMR (δ, ppm):
8.55 (s, 2H, HCdN), 7.90 (d, J HH ) 7.6 Hz, 2H, aromatic),
7.30-7.15 (m, 6H, aromatic), 7.10 (d, J HH ) 7.6 Hz, 2H,
aromatic), 7.05 (t, J HH ) 7.6 Hz, 2H, aromatic), 6.90-6.80 (m,
4H, aromatic), 1.84 (s, 6H, CH3COO).
1e: yield 20%. Anal. Found (calcd) for C30H24N4O8Pd2: C,
46.3 (46.12); H, 3.2 (3.10); N, 6.9 (7.17). 1H NMR (δ, ppm):
8.10 (d, J HH ) 2.1 Hz, 2H, aromatic), 7.87 (s, 2H, HCdN), 7.75
(dd, J HH ) 7.5, 2.1 Hz, 2H, aromatic), 7.30 (d, J HH ) 7.5 Hz,
2H, aromatic), 7.20-7.10 (m, 6H, aromatic), 6.85-6.70 (m, 4H,
aromatic), 1.92 (s, 6H, CH3COO).
1h : yield 45%. Anal. Found (calcd) for C38H42N2O4Pd2: C,
56.9 (56.80); H, 5.1 (5.27); N, 3.2 (3.49). 1H NMR (δ, ppm;
CDCl3 + py-d5): 7.90 (s, 1H, HCdN), 7.20-6.50 (m, 6H,
aromatic), 2.90 (s, 2H, CH2-Pd), 2.70 (s, 3H, CH3), 2.25 (s,
3H, CH3), 1.80 (s, 3H, CH3), 1.70 (s, 3H, CH3).
1l: yield 50%. Anal. Found (calcd) for C32H28N4O8Pd2: C,
46.9 (47.48); H, 3.4 (3.48); N, 6.8 (6.92). 1H NMR (δ, ppm):
8.0-7.80 (m, 4H, HCdN, aromatic), 7.40 (dd, J HH ) 7.6, 2.2
Hz, 2H, aromatic), 7.20-7.10 (m, 8H, aromatic), 6.85-6.70 (m,
4H, aromatic), 4.65 (d, 2H, J HH ) 17.2 Hz, CH2N), 4.15 (d,
2H, J HH ) 17.2 Hz, CH2N), 2.25 (s, 6H, CH3COO).
Com p ou n d s 1k , 3k , a n d 4k . 1k . To a suspension of
palladium acetate (0.22 g, 1 × 10-3 mol) in 20 cm3 of acetic
acid was added a solution of imine K (0.24 g, 1 × 10-3 mol) in
10 cm3 of the same solvent. The mixture was brought to reflux
for 45 min and then concentrated under reduced pressure.
After addition of diethyl ether, a yellow solid was obtained,
which was purified by SiO2 column chromatography.
3k . A 0.25 × 10-3 mol (0.20 g) amount of 1k was treated
with PPh3 (0.13 g, 0.50 × 10-3 mol) and LiBr (43 × 10-3 g, 0.5
× 10-3 mol) in acetone solution at room temperature for 45
min, and the mixture was then filtered. The solution was
concentrated in vacuo, and the solid obtained after addition
of diethyl ether was recrystallized from CHCl3 to give 3k .
4k . A 0.25 × 10-3 mol amount of the solid obtained when
imine K was treated with Pd(AcO)2 in toluene at 60 °C for 1
h, which is a mixture of the endo and exo metallacycles 1k
and 2k , was treated with PPh3 (0.13 g, 0.50 × 10-3 mol) and
LiBr (43 × 10-3 g, 0.5 × 10-3 mol) in CH2Cl2 at room
temperature for 45 min. The solid formed was filtered, washed
with diethyl ether, and recrystallized from chloroform and
diethyl ether to afford 4k .
reactions via electrophilic substitution that can be
related to substituents on the phenyl rings to be
activated. Furthermore, our results in this work also
show that endo or exo activation does not seem to
produce differences in the mechanism or activation
parameters, as found for Pt(II) oxidative-addition
cyclometalations.11a,b,12a Only the steric backbone of the
imine systems studied seems to be important in the
values found for kinetic and activation parameters, once
the basicity of the imines has had its effect in the
previous coordination step; cyclometalation reactions for
dinuclear Rh(II) systems have also shown the same
effects.11c,e Finally, it is clear from our data that
formation of five- or six-membered metallacycles via
aromatic or aliphatic C-H activation produces some
differences that are not those traditionally established
from a thermodynamic point of view.
Exp er im en ta l Section
In str u m en ts a n d Ma ter ia ls. 1H and 31P{1H} NMR spec-
tra were recorded on Varian XL-200 (200 MHz) and Bruker
DRX-250 (101.26 MHz) spectrometers, respectively. Chemical
shifts (in ppm) were measured relative to SiMe4 for 1H and to
85% H3PO4 for 31P spectra; the solvent used was CDCl3. IR
spectra were recorded as KBr pellets on a Nicolet 520 FT-IR
spectrometer. Microanalyses were performed by the Institut
de Qu´ımica Bio-Orga`nica de Barcelona (CSIC) and by the
Serveis Cient´ıfico-Te`cnics de la Universitat de Barcelona. All
UV-vis spectra were recorded on a HP8452A diode array
instrument and on a Beckman UV5420 instrument equipped
with the high-pressure cell already described.11
Toluene and diethyl ether were dried over sodium-ben-
zophenone and distilled before use. The imines were prepared
according to procedures described in the literature.22
Com p ou n d s. The acetato-bridged cyclometalated endo
compounds 1b,f,g,i,j and the exo compounds 2m ,n have been
characterized previously.10b,e,14 Originally they were prepared
from palladium acetate and the corresponding imine in acetic
acid solution; in the present work, all have been prepared in
toluene solution at 60 °C over 1 h, in good yields.
Compounds 1a ,c-e,h ,l were all prepared in the same way.
To a suspension of palladium acetate (0.22 g, 1 × 10-3 mol) in
20 cm3 of toluene was added a solution of imine (1 × 10-3 mol:
for A, 0.15 g; for C, 0.21 g; for D, 0.23 g; for E, 0.23 g; for H,
0.24 g; for L, 0.24 g) in 10 cm3 of the same solvent; the mixture
was then kept at 60 °C for 1 h and concentrated under reduced
pressure to ca. 2 cm3. After addition of 10 cm3 of diethyl ether,
a yellow solid was obtained, which was purified by SiO2 column
chromatography using CHCl3/MeOH (90/10) as eluent.
Characterization data are as follows.
1a : yield 55%. Anal. Found (calcd) for C24H30N2O4Pd2: C,
46.1 (46.25); H, 4.9 (4.85); N, 4.6 (4.49). 1H NMR (δ, ppm):
7.95-7.15 (m, 10H, HCdN, aromatic), 3.20 (m, 4H, CH2N),
2.20-1.95 (m, 10H, CH2, CH3COO), 0.85 (m, 6H, CH3).
1c: yield 50%. Anal. Found (calcd) for C32H30N2O6Pd2: C,
51.6 (51.15); H, 3.9 (4.02); N, 3.8 (3.73). 1H NMR (δ, ppm):
7.60 (s, 2H, HCdN), 7.15-7.05 (m, 8H, aromatic), 6.85-6.75
(m, 4H, aromatic), 6.60 (dd, J HH ) 7.5, 2.2 Hz, 2H, aromatic),
6.05 (d, J HH ) 2.2 Hz, 2H, aromatic), 3.60 (s, 6H, CH3O), 2.10
(s, 6H, CH3COO).
Characterization data are as follows.
1k : yield 40%. Anal. Found (calcd) for C32H28N4O8Pd2: C,
47.8 (47.48); H, 3.6 (3.48); N, 6.8 (6.92). 1H NMR (δ, ppm):
8.10 (s, 2H, HCdN), 7.90 (dd, J HH ) 7.5, 2.1 Hz, 2H, aromatic),
7.55 (dd, J HH ) 7.5, 2.1 Hz, 2H, aromatic), 7.20-7.10 (m, 8H,
aromatic), 7.05-6.90 (m, 4H, aromatic), 4.55 (d, 2H, J HH
)
17.2 Hz, CH2N), 4.00 (d, 2H, J HH ) 17.2 Hz, CH2N), 2.20 (s,
6H, CH3COO).
3k : yield 50%. Anal. Found (calcd) for C32H26N2BrO2Pd:
C, 55.8 (55.88); H, 3.8 (3.81); N, 4.0 (4.07). 1H NMR (δ, ppm):
8.60 (d, J HP ) 8.2 Hz, 1H, HCdN), 7.70-7.50 (m, 6H,
aromatic), 7.40-7.20 (m, 14H, aromatic), 6.75 (t, J HH ) 7.2
Hz, 1H, aromatic), 6.70-6.50 (m, 2H, aromatic), 5.0 (s, 2H,
CH2N).
4k : yield 20%. Anal. Found (calcd) for C32H26N2BrO2Pd:
C, 55.5 (55.88); H, 3.6 (3.81); N, 4.2 (4.07). 1H NMR: (δ,
ppm): 9.15 (d, J HP ) 12.4 Hz, 1H, HCdN), 8.20 (m, 2H,
aromatic), 7.70-7.50 (m, 6H, aromatic), 7.40-7.20 (m, 11H,
aromatic), 7.10 (d, J HH ) 7.1 Hz, 1H, aromatic), 6.90 (t, J HH
)
7.1 Hz, 1H, aromatic), 6.45 (t, J HH ) 7.1 Hz, 1H, aromatic),
6.25 (t, J HH ) J HP ) 7.1 Hz, 1H, aromatic), 5.40 (s, 2H, CH2N).
Kin etic Mea su r em en ts. The reactions at atmospheric
pressure were followed by UV-vis spectroscopy in the full
750-300 nm range on a HP8542A instrument equipped with
a multicell transport, thermostated ((0.1 °C) with a circulation
bath. Observed rate constants were derived from the absor-
bance versus time traces at the wavelengths where a maximum
increase and/or decrease of absorbance was observed. No
(22) Bigelow, L. A.; Eatrough, H. In Organic Syntheses; Blatt, A.
H., Ed.; Wiley: New York, 1994; Vol. 1, p 80.
(23) For imine M the compound has been isolated and characterized.
To a suspension of 1 × 10-3 mol of palladium acetate in 20 cm3 of
toluene was added a solution of the imine (2.0 × 10-3 mol in 10 cm3 of
the same solvent) at room temperature. After it was stirred for 30 min,
the mixture was filtered, and the solid was washed with toluene and
diethyl ether and dried under reduced pressure. Yield: 0.68 g (94%).
Anal. Calcd for C40H48N2O4Pd: C, 66.06; H, 6.65; N, 3.85. Found: C,
66.14; H, 6.60; N, 3.92.

2546 Organometallics, Vol. 16, No. 12, 1997
Go´mez et al.
dependence of the values on the selected wavelengths was
detected, as expected for reactions where a good retention of
isosbestic points is observed. The general kinetic technique
is that previously described.11,12 Solutions for the kinetic runs
were prepared by dissolving the calculated amounts of the
compounds (palladium acetate and imine) in toluene. In all
cases no dependence on the concentration of palladium acetate
or imine was detected, and a [Pd]/[imine] ratio within the 0.7-
1.3 range was maintained to ensure the nonappearance of the
well-known insoluble [Pd(CH3COO)2(imine)2] species.8
on a Beckman UV5420 instrument at a fixed wavelength
chosen from the atmospheric pressure experiments. Rate
constants were derived from exponential least-squares fitting
by the standard routines. Least-squares errors for the rate
constants were always in the range of 10-15% of the calcu-
lated value. All post-run fittings to rate laws were done by
standard fitting programs commercially available.
Ack n ow led gm en t. We acknowledge financial sup-
port from the DGICYT and the CIRIT.
For runs at elevated pressure, a previously described
pressurizing system and high-pressure cell were used.21 In
these cases the absorbance versus time traces were recorded
OM961099E