Substitution reactions between bis-chelate ligands in palladium(ii) alkenyl complexes: An unusual way to form unstable trans-P complexes. A study on the isomerization mechanism

Article abstract of DOI:10.1039/b910952h

The substitution reactions between asymmetric bis-chelate ligands and alkenyl chloro derivatives of palladium(ii) of the type [Pd(L-L′)(Rx)Cl] (L-L′ = 2-phenylsulfanylmethyl-pyridine (HN-SPh), 2-methyl-6- phenylsulfanylmethyl-pyridine (MeN-SPh), 2,2′-bipy

Full text of DOI:10.1039/b910952h

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Substitution reactions between bis-chelate ligands in palladium(II) alkenyl
complexes: an unusual way to form unstable trans-P complexes. A study on
the isomerization mechanism†
Luciano Canovese,*a Fabiano Visentin,a Gavino Chessa,a Claudio Santoa and Alessandro Dolmellab
Received 3rd June 2009, Accepted 19th August 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b910952h
The substitution reactions between asymmetric bis-chelate ligands and alkenyl chloro derivatives of
palladium(II) of the type [Pd(L–L¢)(Rx)Cl] (L–L¢ = 2-phenylsulfanylmethyl-pyridine (HN–SPh),
2-methyl-6-phenylsulfanylmethyl-pyridine (MeN–SPh), 2,2¢-bipyridinyl (BiPy), Rx =
=
=
=
–CCOOMe CMeCOOMe (Ra), –CCOOEt CMeCOOEt (Rb), –CCOOt-Bu CMeCOOt-Bu (Rc),
=
–(CCOOMe CCOOMe)2Me (Rd)) with phosphoquinoline moieties (8-diphenylphosphanyl-quinoline
(DPPQ), 8-diphenylphosphanyl-2-methyl-quinoline (DPPQ-Me)) usually leads to the formation of the
stable geometrical isomer bearing these groups in the cis position thanks to the mutual trans influence
of the alkenyl and phosphine groups. However, when the leaving group MeN–SPh and the entering
ligand DPPQ are involved, the fast and quantitative substitution reaction leads to the formation of a
couple of geometrical isomers [Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P (Rx = Ra, Rb,
Rc, Rd) in which the alkenyl and the phosphine groups are in mutual trans or cis position. The substrate
[Pd(DPPQ)(Rx)Cl]-trans P (Rx = Ra, Rb, Rc) slowly interconverts into its thermodynamically stable
-cis P counterpart while the bulky [Pd(DPPQ)(Rd)Cl]-trans P displays no tendency to isomerize, thereby
allowing separation of the two geometrical forms. Also, the ligand DPPQ-Me induces the formation of
the -trans P geometrical isomer which is only detectable at low temperature since it rapidly interconverts
into the -cis P derivative at RT. The kinetics of the interconversion process, a reasonable explanation of
the observed phenomenon based on theoretical calculations, and eventually an unequivocal structure
determination of the stable [Pd(DPPQ)(Rx)Cl]-cis P substrate are reported in the present paper.
complexes of mixed P–N ligands displayed a reduced catalytic
Introduction
activity towards olefin polymerization. Therefore such derivatives
did not get much attention4 although it was stated that differ-
ent isomers could impart different reactivity to the catalysts.5
Moreover, the exchange reactions in which bidentate ligands
act as entering groups substituting another chelating moiety
were not exhaustively investigated from the mechanistic point
of view.2 Notably, the interplay between the nucleophilic power
of the different entering atoms and the entropically modulated
chelating effect could play an unpredictable role. Quite recently
we have proposed a new approach to the syntheses of complexes
[Pd(L–L¢)(Rx)X] [L–L¢ = diphenylphosphinoethane (DPPE),
2,2¢-bipyridine (BiPy), 8-diphenylphosphanyl-2-methylquinoline
(DPPQ-Me); Rx = ZC CZMe, (ZC CZ)2Me (Z = COOMe,
COOEt, COOt-Bu); X = Cl, I] based on the metathetic ligand
exchange between the complex [Pd(MeN–SPh)(R)Cl] (MeN–
SPh = 2-methylthiophenyl-6-methylpyridine, R = methyl, vinyl,
butadienyl) and the appropriate ligand L–L¢.6 On the basis of
such a protocol we have now carried out the synthesis and studied
the reactivity related to the substitution reactions yielding the
derivatives reported in Scheme 1.
An impressive number of papers dealing with nucleophilic
substitution on planar tetracoordinate complexes in d8 metals
have hitherto been published and the inherent theoretical and
applicative aspects of such investigations have been exhaustively
reviewed.1 According to the well established mechanism, the
substitutionreactionproceedsviaretentionofconfigurationunless
isomerization takes place. Isomerization itself represents a well
developed field of interest which is still deeply investigated.2 In
this context, when Pd(II) or Ni(II) complexes are used as catalysts
the nature of the geometrical isomer could be very important. As a
matter of fact, it was noticed that in the olefin polymerization and
copolymerization the cis isomers derived from symmetric chelating
ligands (P–P or N–N) is the more productive catalyst.3
=
=
With planar tetracoordinate compounds derived from sym-
metric bidentate ligands, the complexes originate from mixed
chelate ligands (P–N, N–S), and two other unequal substituents
may exist as a couple of isomers. It was noticed that palladium
aDipartimento di Chimica, Universita` Ca¢ Foscari, Calle Larga S. Marta
2137, 30123 Venezia, Italy. E-mail: cano@unive.it
Apparently, addition of an asymmetric phosphorus–nitrogen
ligand (DPPQ, DPPQ-Me) (DPPQ = 8-diphenylphosphanylqui-
noline; DPPQ-Me = 8-diphenylphosphanyl-2-methyl-quinoline)
to the complexes [Pd(L–L¢)(Rx)Cl] should in principle gener-
ate a couple of isomers (namely [Pd(P–N)(Rx)Cl]-trans P and
bDipartimento di Scienze Farmaceutiche, Universita` di Padova, Via F.
Marzolo 5, 35131 Padova, Italy
† Electronic supplementary information (ESI) available: X-ray diffracto-
metric study. CCDC reference number 735035. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b910952h
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Scheme 1 Ligands (a) and complexes involved in the ligand substitution reaction (b).
[Pd(P–N)(Rx)Cl]-cis P in Scheme 1 bearing Rx in the trans or
Results
cis position with respect to the phosphorus atom, respectively)
independently of the starting complex. It was instead noticed
that only the ligand DPPQ when reacting with the complexes
[Pd(MeN–SPh)(Rx)Cl] (which are present in solution only as
[Pd(MeN–SPh)(Rx)Cl]-cis S isomer), yields a mixture of both
[Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P species
in the approximate ratio -trans P : -cis P = 7 : 1. It is noteworthy
that the complexes bearing P–N or S–N chelate ligands and
R groups were hitherto obtained mostly as cis isomers,7a-7g
and to the best of our knowledge only in one case as -trans
P species.7h Therefore, the substitution reactions between the
complexes [Pd(MeN–SPh)(Rx)Cl] and the ligand DPPQ yielding
both the isomeric species represent a remarkable opportunity
to be exploited. Moreover, in some cases the kinetics of iso-
merization can be studied by conventional techniques allowing
an in-depth insight into the intimate mechanism governing
such a geometrical transformation. The intimate mechanism
and the speculative analysis of the overall general rules gov-
erning this sort of reactions are the matter of the present
paper.
General remarks on the substitution reactions between ligands
Addition under inert atmosphere (N2) of ligands DPPQ or DPPQ-
Me to a solution of the complexes [Pd(L–L¢)(Rx)Cl] (L–L¢ =
HN–SPh, BiPy; Rx = Ra, Rb, Rc, Rd) in CH2Cl2 or CHCl3
at RT yields immediately and quantitatively the corresponding
complexes [Pd(DPPQ)(Rx)Cl]-cis P and [Pd(DPPQ-Me)(Rx)Cl]-
cis P. However, when complexes [Pd(MeN–SPh)(Rx)Cl], (Rx= Ra,
Rb, Rc, Rd) react with ligand DPPQ it is apparent from 1H and
31P NMR experiments that the fast and quantitative MeN–SPh
ligand displacement yields a mixture of both [Pd(DPPQ)(Rx)Cl]-
trans P and [Pd(DPPQ)(Rx)Cl]-cis P isomers. In the case of
the butadienyl derivative complex (Rx = Rd) no isomerization
reaction is observed. Conversely, the [Pd(DPPQ)(Rx)Cl]-trans
P vinyl derivatives (Rx = Ra, Rb, Rc) undergo isomerization
even in the solid state.8 Taking advantage of this situation
and on the basis of the peculiar NMR features, we were
able to assign the geometrical configuration to each complex
under study, as can be deduced from the Scheme 2 and
Table 1.
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Table 1 Selected 1H NMR signals characterizing the isomers under study.
higher than the stoichiometric quantity (the exchange between
ligands is in any case quantitative irrespective of the vinyl
substituent Rx = Ra, Rc) the ensuing isomerization reaction
can be treated by non-linear regression of data to the mono-
exponential function (eqn (1)):
[Pd(DPPQ)(Rx)Cl)]-trans P
[Pd(DPPQ)(Rx)Cl)]-cis P
Cross peaks
Rd
COOMe (a) H2
COOMe (a) Ph, P
COOMe (b) Ph, P
COOMe (g) Ph, P
Me Ph, P
COOMe (g) H2, H4, H3
Me H2
[[Pd(DPPQ)(Rx)Cl]-cis P]t = [[Pd(DPPQ)(Rx)Cl]-trans
P]0(1-exp(-kobst) + [[Pd(DPPQ)(Rx)Cl]-cis P]0
(1)
1H NMR (ppm)
Rd
where [[Pd(DPPQ)(Rx)Cl]-cis P]0 and [[Pd(DPPQ)(Rx)Cl]-trans
P]0 represent the concentrations of the isomers in the initial
mixture at time t = 0, whereas [[Pd(DPPQ)(Rx)Cl]-cis P]t is the
concentration of the cis isomer at time t. [[Pd(DPPQ)(Rx)Cl]-trans
P]0 and kobs are the parameters to be optimized in the refinement
process. A typical 1H NMR experiment together with the related
numerical analysis is shown in Fig. 1.
Me 2.14
H2 9.78
Me 2.54
H2 9.73
Me 2.46
H2 9.73
Me 2.35
H2 9.69
Me 1.54
H2 10.25
Me 1.60
H2 10.20
Me 1.60
H2 10.24
Me 1.46
H2 10.21
Ra
Rb
Rc
The plot of the resulting kobs constants vs. the [DPPQ]0 (DPPQ
concentration in excess at t = 0) is described by the following
eqn (2):
31P NMR (ppm)
Rd
Ra
Rb
Rc
26.2
23.6
23.3
22.2
37.6
37.7
38.0
37.6
kobs = k1 + k2[DPPQ]0
(2)
in which k1 represents the monomolecular and k2 the bimolecular
path to the [Pd(DPPQ)(Rx)Cl]-cis P isomer.
The linear regression plots of rates vs. the concentration of free
DPPQ ligand, are summarized in Table 2, and shown in Fig. 2.
The ensuing k2 and k1 values are (5.1 0.6) ¥ 10-2, (1.39
0.06) ¥ 10-2 mol-1 dm3 s-1 and (1 8) ¥ 10-5, (2 2) ¥ 10-5 s-1
for the complexes [Pd(DPPQ)(Rx)Cl], Rx = Ra and Rx = Rc,
respectively.
The k1 values can also be measured from the isomerization
reactions triggered by the addition of the ligand DPPQ in
slightly stoichiometric defect relative to the complex [Pd(MeN–
SPh)(Rx)Cl]. The -trans P to -cis P isomerization reaction of the
immediately formed mixture of [Pd(DPPQ)(Rx)Cl]-trans P and
[Pd(DPPQ)(Rx)Cl]-cis P in the absence of free DPPQ ([DPPQ]0 =
0) was observed. The corresponding k1 values are reported in
Table 3.
Scheme 2 Geometrical configuration of the isomers.
Isomerization reactions
(A). Owing to the remarkable trans-influence exerted by the
phosphine and the vinyl group and the consequent induced mutual
labilization between the groups themselves, the transformation
of the complex [Pd(DPPQ)(Rx)Cl]-trans P into its more stable
isomer [Pd(DPPQ)(Rx)Cl]-cis P (Rx = Ra, Rb, Rc) was observed
(Scheme 3).
Table 2 Rate constants (s-1) for the isomerization reaction of the complex
[Pd(DPPQ)(Rx)Cl]-trans P to [Pd(DPPQ)(Rx)Cl]-cis P (Rx = Ra, Rc) in
CDCl3 at 25 ◦C determined by 1H NMR
a
b
[Pd(DPPQ)(Ra)Cl]0
[DPPQ]0
k
obs/s-1
2 ¥ 10-2
2 ¥ 10-2
2 ¥ 10-2
2 ¥ 10-2
0.27 ¥ 10-2
1 ¥ 10-2
(1.88 0.01) ¥ 10-4
(5.10 0.06) ¥ 10-4
(5.9 0.1) ¥ 10-4
1.31 ¥ 10-2
2 ¥ 10-2
(1.09 0.05) ¥ 10-3
[Pd(DPPQ)(Rc)Cl]0
2 ¥ 10-2
2 ¥ 10-2
2 ¥ 10-2
2 ¥ 10-2
1.2 ¥ 10-2
2 ¥ 10-2
3 ¥ 10-2
4 ¥ 10-2
(1.91 0.01) ¥ 10-4
(2.88 0.03) ¥ 10-4
(4.52 0.02) ¥ 10-4
(5.71 0.03) ¥ 10-4
Scheme 3 The isomerization reaction.
The kinetic studies by 1H NMR were carried out by
adding the appropriate amount of DPPQ ligand to a pre-
thermostated solution of the complexes [Pd(MeN–SPh)(Rx)Cl]
(see Scheme 1(b)). The immediate formation of a mixture of both
isomers [Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P
was observed followed by isomerization of [Pd(DPPQ)(Rx)Cl]-
trans P into its -cis P counterpart. When the amount of DPPQ was
a The concentrations reported in Table 2 (mol dm-3) represent the total con-
centration of the complex extant in solution at t = 0 ([Pd(DPPQ)(Rx)Cl]0 =
[[Pd(DPPQ)(Rx)Cl]-trans P]0 + [[Pd(DPPQ)(Rx)Cl]-cis P]0). b The concen-
tration of DPPQ ligand reported in Table 2 represents the concentration
of the free DPPQ in solution at t = 0 ([DPPQ]0 = [DPPQ]TOT
-
[Pd(DPPQ)(Rx)Cl]0; [DPPQ]TOT = total concentration of the ligand added
to the initial solution of the starting complex [Pd(MeN–SPh)(Rx)Cl].
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Fig. 1 Selected 1H NMR signals related to the isomerization reaction of the complex [Pd(DPPQ)(Ra)Cl]-trans P in CD3Cl at 298 K. Top insert:
non-linear regression analysis according to eqn (1)). [[Pd(DPPQ)(Ra)Cl]]0 = 2 ¥ 10-2 (mol dm-3); [DPPQ]0 = 1 ¥ 10-2 (mol dm-3).
Table 3 k1 values for the isomerization reaction obtained by addition of
DPPQ in stoichiometric defect in CDCl3, and determined by the initial
rate approximation9
Complex
k1/s-1
[Pd(DPPQ)(Ra)Cl]-trans P
[Pd(DPPQ)(Rb)Cl]-trans P
[Pd(DPPQ)(Rc)Cl]-trans P
(4.9 0.3) ¥ 10-6
(1.13 0.02 ¥ 10-5
(7.43 0.04) ¥ 10-6
mixtures produced in situ by addition of DPPQ in stoichiometric
defect with respect to the complexes [Pd(MeN–SPh)(Rx)Cl].
(B). Addition of DPPQ to a solution of the butadienyl
derivative [Pd(MeN–SPh)(Rd)Cl] in CDCl3 induces the immediate
formation of an isomeric mixture containing [Pd(DPPQ)(Rd)Cl]-
trans P and [Pd(DPPQ)(Rd)Cl]-cis P in the approximate ratio
of 1 : 1.6. However, the isomer [Pd(DPPQ)(Rd)Cl]-trans P does
not convert into its thermodynamically favoured cis counterpart.
This fact allows the separation of the two species by fractionated
crystallization.
Fig. 2 Linear regression of the rate constants of isomerization measured
at 25 ◦C in CDCl3 for the complexes [Pd(DPPQ)(Rx)Cl]-trans P (Rx =
Ra, Rc) vs. the concentration of free DPPQ.
The isomeric mixtures obtained by the substitution reactions
among the complexes [Pd(MeN–SPh)(Rx)Cl] and DPPQ can be
isolated by removing the solvent at low temperature (250 K).
These mixtures, when dissolved in CDCl3 at RT, display the same
isomerization rates (coincident k1 values) as those of the related
(C). As already stated, addition at RT of the ligand DPPQ-
Me in excess or in defect to the complexes [Pd(MeN–SPh)(Rx)Cl]
does not induce formation of both isomers. Only formation of
the thermodynamically favoured [Pd(DPPQ)(Rx)Cl]-cis P was
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Scheme 4 Reaction paths yielding the isomers.
observed. However, we were able to produce a highly reactive
isomeric mixture by working at low temperature (263 K), with
the ligand DPPQ-Me in defect and by taking advantage of
the inertness of the trans isomer induced by the bulky buta-
dienyl substituent Rd. In this case, at variance with the other
cases involving vinyl substituents, it was possible to detect the
fast substitution reaction between the ligands MeN–SPh and
DPPQ-Me and the slower subsequent isomerization process.
Unfortunately, in this case the reaction rates could not be
determined owing to the simultaneous presence of several dif-
ferent species ([Pd(MeN–SPh)(Rd)Cl], [Pd(DPPQ-Me)(Rd)Cl]-
cis P, [Pd(DPPQ-Me)(Rd)Cl]-trans P, uncoordinated DPPQ-Me
and MeN–SPh). Increasing the temperature (298 K) induces the
fast isomerization process yielding the thermodynamically stable
[Pd(DPPQ-Me)(Rd)Cl]-cis P species. It is however apparent that
the isomer ratio is kinetically controlled since the rate of substitu-
tion between ligands in this case and in those concerning the vinyl
derivatives is faster than that of the subsequent isomerisation.
No reactive [Pd(DPPQ)(Rx)Cl]-trans P isomers would ever have
been otherwise produced upon addition of DPPQ to the complex
[Pd(MeN–SPh)(Rx)Cl].
palladium center, thanks to its well known considerably high
nucleophilic capability which is associated with a high trans
effect.11 Under this reasonable hypothesis, the incoming transition
state and the resulting isomer would depend on the nature of the
leaving atom of the MeN–SPh ligand as described in Scheme 4.
It is worth noting that Scheme 4 is drawn on the basis of the well
established and accepted mechanism governing the nucleophilic
substitution reactions in planar tetracoordinate complexes which
proceeds with retention of configuration.1 In this respect, the
displacement of pyridine nitrogen or thioetheric sulfur represents
the key step in the formation of the -trans P or -cis P isomer,
respectively.
With the aim of clarifying the mechanism involving the two
different intermediates, we resorted to a theoretical calculation
based on the density functional theory (DFT).12-14 The energy
differences (DE) between the two transition states TN and TS
and between the final complexes [Pd(DPPQ)(Ra)Cl]-trans P and
[Pd(DPPQ)(Ra)Cl]-cis P were computed when the DPPQ displaces
MeN–SPh or the chelating HN–SPh from the complexes [Pd(N–
S)(Rx)Cl]-cis S. The schematic energy profiles are shown in
Fig. 3(a) and 3(b) respectively and the most relevant results are
summarized in Table 4.
(D). When the unreactive isomeric mixture of [Pd(DPPQ)-
(Rd)Cl]-trans P and [Pd(DPPQ)(Rd)Cl]-cis P undergoes the
transmetalation reaction with tributyl-phenylethynyl-stannane in
the presence of the stabilizing and accelerating olefin maleic
anhydride10 to give the coupling product (2Z,4Z)-tetramethyl-
7-phenylhepta-2,4-dien-6-yne-2,3,4,5-tetracarboxylate, it was no-
ticed that the more stable isomer [Pd(DPPQ)(Rd)Cl]-cis P is indeed
the more reactive species. As a matter of fact, the transmetalation
reaction undergone by the complex [Pd(DPPQ)(Rd)Cl]-cis P is
immediate whereas that undergone by [Pd(DPPQ)(Rd)Cl]-trans P
takes several hours.
X-Ray crystal structure†
The crystal structure for the complex [Pd(DPPQ)(Ra)Cl]-cis P
together with the relevant and selected bond lengths and angles
are reported in Fig. 4 and Table 5, respectively.
Further details are reported in the ESI.†
Discussion
The most intriguing experimental result emerging from the present
study is represented by the unequivocal formation of the ther-
modynamically unstable [Pd(DPPQ)(Rx)Cl]-trans P derivatives
(together with [Pd(DPPQ)(Rx)Cl]-cis P in a lesser amount).
This only occurs when the ligand MeN–SPh is displaced from
the complexes [Pd(MeN–SPh)(Rx)Cl] by the DPPQ moiety. We
Theoretical calculations
The substitution reaction of MeN–SPh with DPPQ yielding both
isomers [Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P
should probably proceed via attack of the phosphorus to the
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-1
˚
Table 4 Computed energies (Kcal mol ) and bond lengths (A) related to the species involved in the formation of the isomeric mixture [Pd(DPPQ)(Ra)Cl]-
trans P and [Pd(DPPQ)(Ra)Cl]-cis P from the reaction of [Pd(N–S)(Ra)Cl], (N–S = MeN–SPh, HN–SPh) with DPPQ (also see Fig. 3)
DE/Kcal mol-1
N–S = HN–SPh
Selected bond lengths/A
˚
N–S = MeN–SPh
N–S = HN–SPh
N–S = MeN–SPh
Ground state for complexes [Pd(N–S)(Ra)Cl]
0.0
0.0
Pd–N = 2.23
Pd–S = 2.42
Pd–N = 2.34
Pd–S = 2.42
TS (cis path)
TN (trans path)
IP–N (cis path)
9.8
8.7
-5.2
7.2
3.1
-8.3
Pd–N = 2.31
Pd–P = 2.39
Pd–S = 2.43
Pd–P = 2.50
Pd–N = 2.30
Pd–P = 2.41
Pd–S = 2.43
Pd–P = 2.51
IP–S (trans path)
1.7
-6.8
[Pd(DPPQ)(Ra)Cl]-cis P + N–S
[Pd(DPPQ)(Ra)Cl]-trans P + N–S
-17.9
-13.2
-23.1
-18.4
Fig. 3 Calculated energy profiles for the exchange reaction between ligands and the formation of the final isomeric mixture for the reactions (also
see Table 4): [Pd(N–S)(Ra)(Cl] + DPPQ → [Pd(DPPQ)(Ra)Cl]-trans P + N–S [Pd(N–S)(Ra)(Cl] + DPPQ → [Pd(DPPQ)(Ra)Cl]-cis P + N–S N–S =
MeN–SPh (Fig. 3(a)) N–S = HN–SPh (Fig. 3(b)).
have firstly assigned the geometrical structure to each isomer
and secondly we have undertaken the theoretical study whose
conclusions are reported further on. The [Pd(DPPQ)(Rd)Cl]-cis
P isomer is characterized by the cross-peaks in NOESY spectra
between the a, g, d COOCH3 and the protons of the phenyl groups
bound to phosphorus. The butadienyl terminal substituent CH3
also interacts with the phenyl protons indicating the proximity
of these groups with the phosphine moiety, no cross-peaks are
detectable between the COOCH3 and the CH3 with quinoline
protons. On the contrary, the [Pd(DPPQ)(Rd)Cl]-trans P isomer
displays cross-peaks among a and g COOCH3 and CH3 with
the quinoline protons (mainly H2). The characterization of the
stable isomers allows the subsequent identification of the reacting
ones. Such an identification was carried out by comparison of
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◦
˚
somehow confirmed by the calculated Pd–N length which is longer
in the case of [Pd(MeN–SPh)(Ra)Cl] than in the case of [Pd(HN–
Table 5 Selected bond lengths (A) and angles ( ) for [Pd(DPPQ)(Ra)Cl]-
cis P
˚
SPh)(Ra)Cl] (2.34 vs. 2.23 A).
Pd–Cl
Pd–P
Pd–N
2.334 (1)
2.201 (1)
2.135 (4)
2.006 (4)
N–C(6)
1.313 (6)
1.381 (6)
1.816 (4)
1.806 (4)
1.320 (6)
(iii) When the complex [Pd(MeN–SPh)(Ra)Cl] is taken into
consideration, the energy of the transition state TN is lower than
that of TS (3.1 vs. 7.2 Kcal mol-1; DE(TS-TN) = 4.1 Kcal mol-1).
(iv) The transition states TN and TS for the complex [Pd(HN–
SPh)(Ra)Cl], although displaying the same trend, are at con-
siderably higher energy (8.7 vs. 9.8 Kcal mol-1) with respect to
the transition states involved in the previously considered case.
N–C(14)
P–C(13)
P–C(15)
C(1)–C(3)
Pd–C(1)
Cl–Pd–P
177.0 (1)
93.4 (1)
90.9 (1)
84.7 (1)
91.0 (1)
174.5 (2)
Pd–N–C(6)
124.7 (3)
116.6 (3)
101.2 (1)
117.4 (1)
123.2 (3)
123.6 (4)
Cl–Pd–N
Cl–Pd–C(1)
P–Pd–N
P–Pd–C(1)
N–Pd–C(1)
Pd–N–C(14)
Pd–P–C(13)
Pd–P–C(15)
Pd–C(1)–C(3)
C(2)–C(1)–C(3)
Moreover, the corresponding values are very near (DE(TS-TN)
=
1.1 Kcal mol-1).
(v) The differences in energy between the IPS and IPN intermedi-
ates are very low in the case of the ligand MeN–SPh (DE(IPS-IPN)
=
1.5 Kcal mol-1) and markedly higher in the case of HN–SPh
(DE(IPS-IPN) = 6.9 Kcal mol-1).
(vi) The energy level of the intermediate IPS in the case of the
ligand HN–SPh is higher than that of the related starting complex
[Pd(HN–SPh)(Ra)Cl] (DE = 1.7 Kcal mol-1).
Point (i) confirms the experimental observation that the -cis
P isomer represents the final product of any exchange reac-
tion, whereas point (ii) indicates that the complex [Pd(MeN–
SPh)(Ra)Cl]—thanks to the distortion at the main coordination
plane—is energetically closer to the pentacoordinate transition
state and therefore more prone to nucleophilic substitution than
the complex [Pd(HN–SPh)(Ra)Cl].7f,7g,16,17 The high energy barrier
to TS and TN in the case of [Pd(HN–SPh)(Ra)Cl] and the energy
level referred to the intermediate IPS (points (iv) and (vi)) justify
in the authors’ opinion two experimental results. Firstly, the
aromatic nitrogen, irrespective of its reduced kinetic trans effect
and influence as compared with that of the thioetheric sulfur,18
displays the same remarkable coordinating capability and this fact
suggests that some sort of p back donation from palladium to the
quinoline (or pyridine) ring stabilizing the Pd–N bond would be
operative in the absence of distortion. Secondly, the similar energy
barriers for TN and TS (DE(TS-TN) = 1.1 Kcal mol-1) coupled with
Fig. 4 ORTEP15 view of the complex [Pd(DPPQ)(Ra)Cl]-cis P, together
with the numbering scheme. Thermal ellipsoids drawn at the 40%
probability level; hydrogen atoms have been omitted.
the proton and phosphorus chemical shifts. It is apparent that
the analogy among the chemical shifts related to 31P and 1H
(CH3, H2) signals belonging to different complexes is to be
traced back to an identical geometrical distribution. Thus, for
instance, the downfield shift (37.6 ppm) of the phosphorus of the
[Pd(DPPQ)(Rd)Cl]-cis P complex and the highfield shift (26.2)
of its -trans P counterpart allows the identification of all the
other parent compounds (cf. Table 1). Moreover, the nature of
the complex [Pd(DPPQ)(Ra)(Cl)]-cis P was also confirmed by the
31P and 1H NMR spectra of part of the crop subsequently used for
the X-ray structural characterization.
the high energy of the intermediate IPS (1.7 Kcal mol-1; DE(IPS-IPN)
=
6.9 Kcal mol-1) could eventually explain why no -trans P isomer is
experimentally observed in the case involving the starting complex
[Pd(HN–SPh)(Ra)Cl].
When the ligand MeN–SPh is involved, the intermediate IPN
is somewhat destabilized by the steric hindrance exerted by the
methyl substituent in position 2 of the coordinating pyridine (point
(v)). Therefore, at variance with the previous case, the path through
IPN does not represent a particularly advantageous way to the
final products so that the unstable -trans P species can be formed.
Moreover, the lowered energy barrier related to the transition state
TN (3.1 Kcal mol-1) kinetically favours the formation of the -trans
P isomer.
Theoretical calculations
From Fig. 3(a), (b) and Table 4 it is apparent that:
(i) The difference in energy between the final complexes
Notably, when the substituted chelate ligand is the symmetric
BiPy, the reaction proceeds with the formation of the more stable
-cis P isomer only. In this case, in the absence of alternative paths,
the substitution reaction of the symmetric chelate ligand yields
only the thermodynamically favored derivative.
[Pd(DPPQ)(Ra)Cl]-trans
P and [Pd(DPPQ)(Ra)Cl]-cis P is
DE(transP-cisP) = 4.7 Kcal mol-1); the [Pd(DPPQ)(Ra)Cl]-cis P deriva-
tive being the more stable, as expected.
(ii) The energies of the starting complexes [Pd(MeN–
SPh)(Ra)Cl] and [Pd(HN–SPh)(Ra)Cl] are different; the complex
[Pd(MeN–SPh)(Ra)Cl] being the less stable as can be deduced
from the superimposition of the energy levels determined in
both cases for the final complexes which are obviously the same
(E[Pd(MeN–SPh)(Ra)Cl] - E[Pd(HN–SPh)(Ra)Cl] ª 5 Kcal mol-1). This fact is
Isomerization reactions
(A). It is worthy to remember that the formation of the
-trans P species (and its -cis P counterpart) takes place in situ
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by adding an adequate amount of the ligand DPPQ to a solution
of [Pd(MeN–SPh)(Rx)Cl] complex. Under these circumstances,
formation of the thermodynamically unstable vinyl derivatives
[Pd(DPPQ)(Rx)Cl]-trans P (Rx = Ra, Rb, Rc) represents an
unusually favourable case which allows a detailed study of the
isomerization reactions of substrates bearing chelate ligands. In
this case it is apparent that the stepwise mechanism reported in
Scheme 5 is operative.
The inertness of such a species could be explained by the
bulkiness of the substituents and by some sort of stabilizing
intramolecular interaction between the butadienyl fragment and
the ligand DPPQ hindering the transition states of the dissociative
and associative paths. Unfortunately, several attempts at produc-
ing crystals suitable for diffractometric studies aimed at a better
understanding of this problem were unsuccessful.
(C). The introduction of the methyl group on position 2 of
the quinoline ring seems to remove the stabilization previously
discussed (B) and the presence of the thermodynamically unstable
[Pd(DPPQ-Me)(Rd)Cl]-trans P isomer can only be observed at
low temperature
The methyl group, while enhancing the basicity of the quinoline
nitrogen, induces an intrinsic instability on the ensuing complexes,
forcing them into a distorted geometry thanks to its steric
hindrance. This fact, which is not unprecedented, generally favours
the reactivity of the involved species as has been shown for allene
insertion on complexes of the type [Pd(R¢N–SPh)(Me)Cl] (R¢ =
H, Me, Cl) or transmetalation reactions.7f,7g,10,16,17
Notably, the general destabilization and the consequent en-
hanced reactivity of such a derivative if compared with that of
undistorted analogs was also apparent when the energies of the
complexes [Pd(HN–SPh)(Rx)Cl] and [Pd(MeN–SPh)(Rx)Cl] were
compared, as appears in the theoretical calculations section.
Scheme 5 Isomerization mechanism.
Path A is a bimolecular associative isomerization process,1e,1d,19
in which the free ligand phosphanylquinoline, present in stoi-
chiometric excess, attacks the complex [Pd(P–N)(Rx)Cl]-trans P
inducing the rearrangement to its -cis P counterpart. Coordination
of the phosphine of the free P–N ligand, the ring opening
at the quinoline nitrogen of the coordinate P–N moiety, the
subsequent ring closure of the incoming ligand and the consequent
displacement of the leaving P–N would describe the observed
isomerization in which P–N acts as a concentration-constant
promoter of the reaction.20 Such a step is clearly influenced by the
steric requirement of the Rx groups (Ra: k2 = (5.1 0.4) ¥ 10-2, Rc:
k2 = (1.34 0.06) ¥ 10-2 mol-1 dm3 s-1) as would be expected in case
of associative reactions.1d Path A represents an efficient process so
that a slight DPPQ excess warrants a quantitative isomerization
reaction. Independently of the DPPQ excess the reactions were
always described by a monoexponential relationship (see results).
Path B (k1) represents the monomolecular way to isomerization
which can also be observed in the absence of DPPQ in excess. In
this respect it is worth noting that the presence of the displaced
ligand MeN–SPh in the isomeric mixture does not alter the overall
rate of the isomerization process, and the calculated rate constants
(k1) were coincident with those determined from the isomerization
reaction of the isolated pure mixture of the -trans P and -cis P
complexes. Notably, path B is not affected by the steric nature of
the complexes involved (the measured k1 values are substantially
independent of the nature of Rx: Table 3). Moreover, such an
isomerization process is also observed in the solid state. The
isolated and purified isomeric mixture within a few days displays
the complete disappearance of the -trans P species. Apparently,
under the mutual trans influence exerted by the alkenyl and
the phosphine groups, isomerization probably takes place via a
T-shaped structure leading to a Y-shaped intermediate which
eventually converts into the thermodynamically stable -cis P
species.21
(D). In a former study dealing with transmetalation reactions
the mechanism proposed by some of us, suggested that the lability
of the nitrogen of the chelate P–N ligand would have enhanced the
overall reactivity of the complex involved in the transmetalation
processes.10 We therefore surmise that the lower reactivity of the
thermodynamically unstable [Pd(DPPQ)(Rd)Cl]-trans P can be
traced back to the scarce trans effect of the chlorine group which
does not induce a consistent labilization of the quinoline nitrogen.
On the contrary, in the [Pd(DPPQ)(Rd)Cl]-cis P complex the
quinoline nitrogen in trans position to the butadienyl moiety easily
undergoes destabilization. Thus, the labilization of the quinoline
nitrogen in trans position to the trans labilizing Rd group favours
the olefin pre-coordination which was supposed to be the key step
to the subsequent fast transmetalation process.10
Conclusion
The present work examines the results of the exchange reactions
between bidentate asymmetric ligands in planar tetracoordinate
Pd(II) complexes of general formula [Pd(L-L¢)(Rx)Cl]. The most
important issue is the possibility of determining the nature of
the geometrical isomers taking advantage of the features of the
leaving ligand only. Thus, the alkenyl palladium derivatives bearing
the pyridylthioether ligand MeN–SPh when reacting with the
phosphoquinoline DPPQ moiety, leads to the formation of both
the isomers [Pd(DPPQ)(Rx)Cl]-cis P and the quite rare and ther-
modynamically unstable [Pd(DPPQ)(Rx)Cl]-trans P, in significant
quantity. When the alkenyl group is the butadienyl derivative Rd,
the ligand exchange reaction produces a stable isomeric mixture
whose composition does not change with time allowing the
quantitative separation of the isomers. On the contrary when Rx =
Ra, Rb and Rc (vinyl residue) a slow isomerization is observed.
The detailed study of such isomerization in the case of Rx = Ra,
Rc was carried out and two mechanistic steps (associative and
(B). In our opinion the behaviour of the complex [Pd(DPPQ)-
(Rd)Cl]-trans P which does not undergo isomerization in the solid
state nor in solution is surprising.
9482 | Dalton Trans., 2009, 9475–9485
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dissociative) to the stable -cis P isomer were highlighted. When
the ligand DPPQ-Me is used the isomerization at RT turns out to
be immediate and in this case the unstable -trans P derivative can
only be observed at low temperature. The structure of the complex
[Pd(DPPQ)(Ra)Cl]-cis P which represents the thermodynamically
stable isomerization product was unequivocally determined by an
X-ray diffractometric study.
distilled dichloromethane. The reaction mixture was stirred for
6 h at room temperature. The resulting clear yellow solution
was concentrated at reduced pressure and the title complex was
precipitated by addition of diethyl ether. The clear yellow powder
was filtered off, washed with small aliquots of diethyl ether and
n-pentane, and vacuum dried (0.131 g, yield 92%). Crystals suitable
for X-ray analysis were obtained by slow diffusion of n-hexane into
a dichloromethane solution of the complex.
1
=
H NMR (CDCl3, 298 K): d 1.60 (s, 3H, CCH3), 3.50 (s, 3H,
Experimental
a-COOCH3), 3.56 (s, 3H, b-COOCH3), 7.39–7.48 (m, 5H, PPh2),
7.56–7.66 (m, 3H, PPh2), 7.70–7.76 (m, 2H, H3, H6), 7.81–7.88 (m,
2H, PPh2), 7.99 (ddd, 1H, J = 9.9 Hz; 7.2 Hz; 1.1 Hz, H7), 8.11
(dt, 1H, J = 8.1 Hz; 1.1 Hz, H5), 8.48 (dt, 1H, J =8.4 Hz; 1.6 Hz,
H4), 10.20 (dd, 1H, J =5.0 Hz; 1.6 Hz, H2).
Computational details
Calculations were performed with the ADF200712 package at the
BLYP13 level using DZP basis set in the frozen core approximation.
Scalar relativistic effects were taken into account by the zeroth-
order regular approximation (ZORA).14 The geometry optimiza-
tions were performed without any symmetry constraint, followed
by analytical frequency calculations to confirm that a minimum
or a transition state had been reached. Cartesian coordinates and
energies of stationary points are reported in the ESI.†
31P{ H} NMR (CDCl3, 298 K, ppm): d 37.7.
1
13
1
=
C{ H} NMR (CDCl3, 298 K, ppm): d 21.1 (CH3, CCH3),
51.1 (CH3, a-OCH3), 51.3 (CH3, b-OCH3), 123.1 (CH, C3), 128.0
(d, C, JCP = 7.2 Hz, a-C C), 129.9 (d, C, JCP = 48.6 Hz, C8), 131.4
=
(CH, C6), 132.0 (CH, C5), 137.1 (CH, C7), 139.1 (CH, C4), 150.4
10
9
2
=
(C, C ), 150.6 (C, C ), 154.2 (CH, C ), 155.8 (C, b-C C), 164.2
(CO, b-CO), 171.9 (d, CO, JCP = 4 Hz, a-CO).
IR (KBr pellet) n = 1696 cm-1 (C O); 1599 cm (C N).
Anal. Calcd for C28H25ClNO4PPd: C, 54.92; H, 4.12; N, 2.29.
Found: C, 54.82; H, 4.21; N, 2.36%.
-1
Materials
=
=
Unless otherwise stated, all manipulations were carried out under
an argon atmosphere using standard Schlenk techniques. All
solvents were purified by standard procedures and distilled under
argon immediately prior to use. 1D- and 2D-NMR spectra were
recorded using a Bruker 300 Avance spectrometer. Chemical shifts
(ppm) are given relative to TMS (1H and 13C NMR) and 85%
H3PO4 (31P NMR).
[Pd(DPPQ)(Rb)Cl]-cis P. 0.094 g (0.300 mmol) of DPPQ was
added in one portion to a solution of [Pd(MeN–SPh)(Rb)Cl]
(0.120 g, 0.222 mmol) in 10 ml of freshly distilled dichloromethane.
The reaction mixture was stirred for 24 h at room temperature. The
resulting clear yellow solution was concentrated under reduced
pressure and addition of diethyl ether gave the product as a
microcrystalline clear yellow solid. It was filtered through a
sintered glass-filter, washed with small aliquots of diethyl ether
and n-pentane, and dried under vacuum. (0.132 g, yield 93%).
1HNMR(CDCl3, 298K): d 1.09(t, 3H, J =7.1Hz, a-CH2CH3),
Peaks are labelled as singlet (s), doublet (d), triplet (t), quartet
(q), multiplet (m) and broad (br). The proton and carbon
1
1
assignment was performed by H–2D COSY, H–2D NOESY,
1H–13C HMQC and HMBC experiments.
IR spectra were recorded on a Perkin-Elmer Spectrum One
spectrophotometer.
DPPQ,22 DPPQ-Me,16 and complexes [Pd(MeN–SPh)(Rx)Cl]16
=
1.18 (t, 3H, J = 7.1 Hz, b-CH2CH3), 1.60 (s, 3H, CCH3), 3.77–
3.88 (m, 1H, a-CH2CH3), 4,21 (qd, 2H, J = 7.1 Hz; J = 2.6 Hz,
b-CH2CH3) 4.04–4.15 (m, 1H, a-CH2CH3), 7.37–7.49 (m, 5H,
PPh2), 7.55–7.76 (m, 5H, PPh2, H3,H6), 7.85–7.92 (m, 2H, PPh2),
8.00 (ddd, 1H, J = 9.9 Hz; 7.2 Hz; 1.1 Hz, H7), 8.10 (dt, 1H, J
=8.1 Hz; 1.1 Hz, H5), 8.46 (dt, 1H, J =8.4 Hz; 1.6 Hz, H4), 10.21
(dd, 1H, J = 5.0 Hz; 1.6 Hz, H2).
(Rx
=
Ra, Rb, Rc), [Pd(HN–SPh)(Ra)Cl],16 [Pd(DPPQ-
Me)(Ra)Cl],16 [Pd(BiPy)(Ra)Cl],16 [Pd(MeN–SPh)(Rd)Cl],16
[Pd(DPPQ-Me)(Rd)Cl]16 were prepared following literature
procedures. All other chemicals were commercial grade and were
used without further purification.
1
31P{ H} NMR (CDCl3, 298 K, ppm): d 38.0.
1
13C{ H} NMR (CDCl3, 298 K, ppm): d 13.9 (CH3, a-CH2CH3),
1H NMR kinetic measurements
=
14.1 (CH3, b-CH2CH3), 21.2 (CH3, CCH3), 59.8 (CH2, a-
In a typical NMR experiment 1.2 ¥ 10-2 mmol of the complex
under study ([Pd(MeN–SPh)(Rx)Cl)] Rx = Ra, Rc) was dissolved
in 0.6 ml of CD2Cl2 and the appropriate weighed amount of DPPQ
ligand was added in order to produce the molar ratio required
by the particular process The reaction progress was followed at
298 K by recording selected integrated signals of the reactants and
products. The ensuing rate constants were computed from non-
linear regression of the concentration profiles vs. time according
to the first order rate law (eqn (1)) (see results session).
CH2CH3), 59.9 (CH2, b-CH2CH3), 123.1 (CH, C3), 127.9 (d, C,
JCP = 7.2 Hz, a-C C), 130.4 (d, C, JCP = 55.3 Hz, C8), 131.4 (CH,
=
C6), 131.9 (CH, C5), 136.9 (CH, C7), 139.0 (CH, C4), 150.0 (C,
10
9
2
=
C ), 150.6 (C, C ), 154.2 (CH, C ), 155.6 (C, b-C C), 163.7 (CO,
b-CO), 171.5 (d, CO, JCP = 4 Hz, a CO).
IR (KBr pellet) n = 1697 cm-1 (C O); 1603 cm (C N).
Anal. Calcd for C30H29ClNO4PPd: C, 56.26; H, 4.56; N, 2.19.
Found: C, 56.12; H, 4.50; N, 2.30%.
-1
=
=
[Pd(DPPQ)(Rc)Cl]-cis P. 0.094 g (0.300 mmol) of DPPQ
was added to a solution of [Pd(MeN–SPh)(Rc)Cl] (0.120 g,
0.200 mmol) in 10 ml of freshly distilled dichloromethane. The
reaction mixture was stirred for 24 h at room temperature. The
resulting clear yellow solution was concentrated under reduced
pressure and the title complex was precipitated by addition of
Synthesis of the complexes
[Pd(DPPQ)(Ra)Cl]-cis P. 0.095 g (0.303 mmol) of the biden-
tate ligand DPPQ was added in one portion to a solution of
[Pd(MeN–SPh)(Ra)Cl] (0.120 g, 0.233 mmol) in 10 ml of freshly
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1
31P{ H} NMR (CDCl3, 298 K, ppm): d 37.3.
diethyl ether. The clear yellow powder was filtered off, washed
with small aliquots of diethyl ether and n-pentane, and vacuum
dried (0.134 g, yield 96%).
13
1
=
C{ H} NMR (CDCl3, 298 K, ppm): d 22.2 (CH3, CCH3),
28.0 (CH3, a-OC(CH3)3, 28.6 (CH3, b-OC(CH3)3, 29.4 (CH3,
quinoline-CH3), 78.9 (C, a-OC(CH3)3), 79.7 (C, b-OC(CH3)3),
125.4 (CH, C3), 126.6 (CH, C6), 126.7 (d, C, JCP = 7.7 Hz, a-
1H NMR (CDCl3, 298 K): d 1.37 (s, 9H, a-t-Bu), 1.46 (s, 3H,
=
CCH3), 1.51 (s, 9H, b-t-Bu), 7.33–7.42 (m, 5H, PPh2), 7.54–7.60
(m, 1H, PPh2), 7.67–7.74 (m, 4H, PPh2, H3, H6), 7.83–7.90 (m,
2H, PPh2), 8.03 (ddd, 1H, J =9.9 Hz; 7.2 Hz; 1.1 Hz, H7), 8.06
(dt, 1H, J = 8.1 Hz; 1.1 Hz, H5), 8.44 (dt, 1H, J = 8.4 Hz; 1.6 Hz,
H4), 10.2 (dd, 1H, J =5.0 Hz; 1.6 Hz, H2).
C C), 130.4 (d, C, JCP = 52.2 Hz, C8), 131.2 (CH, C5), 134.0 (CH,
=
C7), 138.0 (CH, C4), 150.8 (C, C10), 151.0 (C, C9), 167.1 (C, C2),
2
=
156.3 (C, b-C C), 162.6 (CO, b-CO), 167.1 (C, C ), 170.4 (d, CO,
JCP = 5.5 Hz, a-CO).
31P{ H} NMR (CDCl3, 298 K, ppm): d 37.6.
1
IR (KBr pellet) n = 1698 cm-1 (C O); 1610 cm (C N).
Anal. Calcd for C35H39ClNO4PPd: C, 59.16; H, 5.53; N, 1.97.
Found: C, 59.27 H, 5.50; N, 1.94%.
-1
=
=
13
1
=
C{ H} NMR (CDCl3, 298 K, ppm): d 21.2 (CH3, CCH3),
28.1 (CH3, a-OC(CH3)3, 28.4 (CH3, b-OC(CH3)3, 78.8 (C, a-
OC(CH3)3, 79.6 (C, b-OC(CH3)3, 123.1 (CH, C3), 127.9 (d, C,
JCP = 7.5 Hz, a-C C), 130.0 (d, C, JCP = 55.9 Hz, C8), 130.9 (CH,
=
[Pd(DPPQ)(Rd)Cl]-cis
P and [Pd(DPPQ)(Rd)Cl]-trans P.
C6), 131.7 (CH, C5), 137.1 (CH, C7), 138.9 (CH, C4), 150.3 (C,
0.095 g (0.303 mmol) of the bidentate ligand DPPQ was added
to a dichloromethane solution (20 ml) of [Pd(MeN–SPh)(Rd)Cl]
(0.140 g, 0.213 mmol). The reaction mixture was stirred for
30 min at room temperature. The resulting clear yellow solution
was concentrated to a small volume (2 ml) and the product
was precipitated by addition of diethyl ether. The resulting solid
was filtered off, washed with small aliquots of diethyl ether and
n-pentane and finally dried under vacuum to give a clear yellow
powder (0.136 g, yield 85%) consisting of a mixture of the two
isomers cis (55%) and trans (45%). The crude solid was redissolved
in a 1 : 4 mixture of CH2Cl2 : n-hexane and cooled to 0 ◦C for
12 h. This allowed the complete precipitation of the less soluble
trans isomer as a white microcrystal solid (0.063 g). This product
was recovered by filtration through a sintered glass filter. The
filtered solution was evaporated to dryness on a rotary evaporator.
Addition of a 1 : 4 mixture of diethyl ether : n-hexane followed by
filtration of the pale-yellow solid afforded 0.064 g of virtually pure
trans isomer (0.064 g).
10
9
2
=
C ), 150.5 (C, C ), 154.1 (CH, C ), 156.4 (C, b-C C), 162.6 (CO,
b-CO), 171.0 (d, CO, JCP = 3.7 Hz, a-CO).
IR (KBr pellet) n = 1697 cm-1 (C O); 1601 cm (C N).
Anal. Calcd for C34H37ClNO4PPd: C, 58.63; H, 5.35; N, 2.01.
Found: C, 58.64 H, 5.39; N, 2.08%.
-1
=
=
[Pd(BiPy)(Rc)Cl]. 0.038 g (0.241 mmol) of BiPy was added
to a solution of [Pd(MeN–SPh)(Rc)Cl] (0.120 g, 0.200 mmol)
in 10 ml of dichloromethane. The reaction mixture was stirred
for 15 min at room temperature. Then, the clear yellow solution
was concentrated in vacuo and addition of diethyl ether gave the
product as a white powder. It was filtered through a sintered glass
filter, washed with small aliquots of diethyl ether and n-pentane,
and dried under vacuum (0.102 g, yield 93%).
1H NMR (CDCl3, 298 K): d 1.51 (s, 9H, a-t-Bu), 1.59 (s, 9H,
5
5¢
=
b-t-Bu), 2.39 (s, 3H, CCH3), 7.44–7.54 (m, 2H, H , H ), 8.00 (td,
1H, J = 7.9 Hz; J = 1.4 Hz, H3), 8.07 (td, 1H, J = 7.9 Hz; J =
1.4 Hz, H3¢), 8.08–8.16 (m, 2H, H4, H4¢), 8.91 (d, 1H, J =5.7 Hz,
H2), 9.14 (d, 1H, J =5.7 Hz, H2¢).
Trans-isomer. 1H NMR (CDCl3, 298 K): d 1.54 (s, 3H,
CCH3), 3.45 (s, 3H, g-COOCH3), 3.46 (s, 3H, a-COOCH3), 3.60
13
1
=
C{ H} NMR (CDCl3, 298 K, ppm): d 21.2 (CH3, CCH3),
=
28.2 (CH3, a-OC(CH3)3, 28.3 (CH3, b-OC(CH3)3, 79.8 (C, a-
(s, 3H, b-COOCH3), 3.69 (s, 3H, d-COOCH3), 7.37–7.57 (m, 6H,
PPh2), 7.63–7.71 (m, 2H, H3, H6), 7.82–7.94 (m, 5H, PPh2, H7),
8.04 (dt, 1H, J = 8.0 Hz; 1.1 Hz, H5), 8.41 (dt, 1H, J =8.4 Hz;
1.6 Hz, H4), 10.24 (dd, 1H, J = 5.2 Hz; 1.6 Hz, H2).
OC(CH3)3, 80.0 (C, b-OC(CH3)3, 121.5 (CH, C3¢), 122.1 (CH, C3),
126.2 (CH, C5¢), 126.9 (CH, C5), 127.6 (C, JCP = 7.5 Hz, a-C C),
=
138.7 (CH, C4¢), 139.5 (CH, C4), 149.3 (CH, C6¢), 153.1 (CH, C6),
2¢
2
=
153.5 (C, C ), 155.7 (C, C ), 158.8 (C, b-C C), 163.1 (CO, b-CO),
1
31P{ H} NMR (CDCl3, 298 K, ppm): d 37.6.
172.2 (d, CO, a-CO).
13
1
=
C{ H} NMR (CDCl3, 298 K, ppm): d 17.6 (CH3, CCH3),
-1
IR (KBr pellet) n = 1697 cm-1 (C O); 1602 cm (C N).
Anal. Calcd for C23H29ClN2O4Pd: C, 51.22; H, 5.42; N, 5.19.
Found: C, 51.34 H, 5.36; N, 5.12%.
=
=
51.2 (CH3, a-OCH3), 51.8 (CH3, g-OCH3), 51.9 (CH3, b-OCH3),
52.1 (CH3, d-OCH3), 123.1 (CH, C3 ), 127.8 (CH, C6), 129.5 (d, C,
JCP = 9.6 Hz, a-C C), 129.8 (d, C, JCP = 47 Hz, C8), 131.7 (CH,
=
5
7
=
=
[Pd(DPPQ-Me)(Rc)Cl]-cis P. 0.072 g (0.22 mmol) of DPPQ-
Me was added to a solution of [Pd(MeN–SPh)(Rc)Cl] (0.120 g,
0.200 mmol) in 10 ml of freshly distilled dichloromethane. The
reaction mixture was stirred for 15 min at room temperature. The
resulting clear yellow solution was concentrated under reduced
pressure and the title complex was precipitated by addition of
diethyl ether. The clear yellow powder was filtered off, washed
with small aliquots of diethyl ether and n-pentane, and vacuum
dried (0.134 g, yield 94%).
C ), 135.3 (g-C C,), 136.6 (CH, C ), 137.1 (d-C C,), 139.0 (CH,
4
10
9
2
=
C ), 150.0 (C, C ), 150.3 (C, C ), 154.4 (CH, C ), 162.1 (b-C C),
162.7 (CO, b-CO), 167.7 (CO, g-CO), 169.0 (C, d-CO), 171.9 (d,
CO, JCP = 4 Hz, a-CO).
IR (KBr pellet) n = 1709, 1683 cm-1 (C O); 1588 cm (C N).
Anal. Calcd for C34H31ClNO8PPd: C, 54.13; H, 4.14; N, 1.86.
Found: C, 54.22; H, 4.23; N, 1.82%.
-1
=
=
1
=
Cis-isomer. H NMR (CDCl3, 298 K): d 2.14 (s, 3H, CCH3),
1H NMR (CDCl3, 298 K): d 1.30 (s, 9H, a-t-Bu), 1.59 (s, 9H,
3.00 (s, 3H, g-COOCH3), 3.69 (s, 3H, d-COOCH3), 3.74 (s, 3H,
b-COOCH3), 3.90 (s, 3H, a-COOCH3), 7.38–7.64 (m, 9H, PPh2,
H3) 7.74 (t, 1H, J = 8.0 Hz; H6), 7.86–8.00 (m, 3H, PPh2, H7), 8.07
(dt, 1H, J = 8.0 Hz; 1.1 Hz, H5), 8.43 (dt, 1H, J = 8.2 Hz; 1.6 Hz,
H4), 9.78 (dd, 1H, J =5.2 Hz; 1.6 Hz, H2).
=
b-t-Bu), 1.67 (s, 3H, CCH3), 3.41 (CH3, 3H, quinoline-CH3),
7.33–7.46 (m, 5H, PPh2), 7.46 (d, CH, J =8.4 Hz, 1H, H3), 7.51–
7.62 (m, 2H, PPh2, H6), 7.90–7.97 (m, 3H, PPh2, H5), 8.04 (ddd,
1H, J =9.9 Hz; 7.2 Hz; 1.1 Hz, H7), 8.16 (dt, 1H, J =8.4 Hz;
1.6 Hz, H4).
1
31P{ H} NMR (CDCl3, 298 K, ppm):d 26.2.
9484 | Dalton Trans., 2009, 9475–9485
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13
1
=
6 L. Canovese, F. Visentin and G. Chessa, submitted to Inorganic
Syntheses, 2008.
C{ H} NMR (CDCl3, 298 K, ppm): d 19.4 (CH3, CCH3),
51.2 (CH3, g-OCH3), 51.8 (CH3, a-OCH3), 51.9 (CH3, d-OCH3),
7 (a) G. P. C. M. Dekker, A. Buijs, C. J. Elsevier, K. Vrieze, P. W. N. M.
van Leewuen, W. J. J. Smeets, A. L. Spek, Y. F. Wang and C. H. Stam,
Organometallics, 1992, 11, 1937–1948; (b) W. de Graaf, S. Harder, J.
Boersma, G. van Koten and J. A. Canters, J. Organomet. Chem., 1988,
358, 545–562; (c) T. G. Appleton, M. A. Bennett and I. B. Tomkins,
J. Chem. Soc., Dalton Trans., 1976, 439–44; (d) H. A. Ankersmit, B. H.
Løken, B. H. Kooijman, A. L. Spek, K. Eriksen, K. Vriese and G. van
Koten, Inorg. Chim. Acta, 1996, 252, 141–155; (e) H. A. Ankersmit,
N. Veldman, A. L. Spek, K. Eriksen, K. Goubitz, K. Vriese and G.
van Koten, Inorg. Chim. Acta, 1996, 252, 203–219; (f) L. Canovese, F.
Visentin, G. Chessa, P. Uguagliati and G. Bandoli, Organometallics,
2000, 19, 1461–1463; (g) L. Canovese, F. Visentin, G. Chessa, C. Santo,
P. Uguagliati and G. Bandoli, J. Organomet. Chem., 2002, 650, 43–56;
(h) E. Shirakawa, H. Yoshida and H. Takaya, Tetrahedron Lett., 1997,
38, 3759–3762.
3
=
52.0 (CH3, b-OCH3), 123.3 (CH, C ), 125.8 (C, g-C C), 128.0
(CH, C6), 132.2 (CH, C5), 132.3 (d, C, JCP = 55 Hz, C8), 132.8 (d,
7
4
=
C, JCP = 7.0 Hz, a-C C), 138.1 (CH, C ), 138.9 (CH, C ), 141.6
10
9
2
=
(C, d-C C), 152.6 (C, C ), 153.0 (C, C ), 159.6 (CH, C ), 162.6
=
(b-C C), 162.8 (CO, b-CO), 167.2 (CO, g-CO), 170.4 (C, d-CO),
175.2 (d, CO, JCP = 4 Hz, a-CO).
IR (KBr pellet) n = 1711 cm-1 (C O); 1591 cm (C N).
Anal. Calcd for C34H31ClNO8PPd: C, 54.13; H, 4.14; N, 1.86.
Found: C, 54.32; H, 4.20; N, 1.90%.
-1
=
=
Acknowledgements
8 It is noteworthy that the complexes [Pd(MeN–SPh)((Me)Cl] and
=
[Pd(MeN–SPh)(C( NR)Me)Cl] (R = 2,6-Me2C6H4) when reacting
The authors gratefully thank Dr F. Benetollo of the C.N.R.-
I.C.I.S. Institute (Padua, Italy) for allowing access to the PW1100
diffractometer, supervisioning the X-ray data collection process
and for help in the structure interpretation.
with DPPQ yield the corresponding -cis P isomer only.
9 J. H. Espenson, in Chemical Kinetics and Reaction Mechanisms,
McGraw Hill, Singapore, 2nd int. edn, 1995, ch. 2.
10 L. Canovese, F. Visentin, C. Levi and C. Santo, J. Organomet. Chem.,
2008, 693, 3324–3330.
11 P. D. Braddock, R. Romeo and M. L. Tobe, Inorg. Chem., 1974, 13,
1170–1175.
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Dalton Trans., 2009, 9475–9485 | 9485
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