New palladium complexes with rigid scorpion-type ligands. Crystal structure of complexes [Pd(η3-2-CH3-C3H 4)(bpz*mPh)](CF3SO3) and [Pd(bpz*mpy)2](BF4)2. bpz*mPh=phenyl-bis(3,5-dimethylpyrazol-1-yl)methane; Bpz*mpy=pyridin-2-yl-bis(3,5-dimethylpyrazol-1-yl)methane

Article abstract of DOI:10.1016/S0022-328X(00)00160-1

Two α-R substituted bis(3,5-dimethylpyrazol-1-yl)methane ligands (R=Ph, bpz*mPh and 2-py, bpz*mpy) have been used to synthesise new palladium(II) compounds with the general formulae PdXX′(bpz*mR) (R=Ph, X=X′=Cl; 1a, X=X′=Me; 2a, X=Cl, X′=Me; 3a, X=X′=C₆F₅; 4a or R=2-py, X=X′=Cl; 1b, X=Cl, X′=Me; 3b, X=X′=C₆F₅, 4b), [Pd(η³-2-CH₃-C₃H ₄)(bpz*mR)]X (X=PF₆^-, R=Ph, 5a; X=CF₃SO₃^-, 2-py, 5b) and [Pd(bpz*mR)₂](BF₄)₂ (R=Ph, 6a; 2-py, 6b). In all the complexes the bpz*mR ligands adopt a rigid conformation with the R group in an axial position and, as a consequence, no boat-to-boat interconversion is observed. In 1b and 3b-5b the pyridine group is uncoordinated. Complex 5b is present as a single isomer, while 5a exists as two isomers that are in equilibrium and differ in the orientation of the allyl group with respect to the NN ligand. For complex 6b the four possible anti isomers are observed in solution. The molecular structures of the major isomer of 5a and one of the isomers of 6b have been determined by X-ray structure analysis. In both structures the bpz*mR ligands are coordinated as bidentate. For 5a the ligand backbone confirms the stated rigidity. For 6b a pyridine-pyrazole coordination of the two bpz*mpy ligands is observed. The fluxional behaviour implying interconversion of the two pyrazole rings in 3b, of the two isomers in 5a and the rotation of the C₆F₅ groups in 4a and 4b has also been studied.

Full text of DOI:10.1016/S0022-328X(00)00160-1

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2000) 174–184
New palladium complexes with rigid scorpion-type ligands. Crystal
structure of complexes [Pd(h3-2-CH3ꢀC3H4)(bpz*mPh)](CF3SO3)
and [Pd(bpz*mpy)2](BF4)2.
bpz*mPh=phenyl-bis(3,5-dimethylpyrazol-1-yl)methane;
bpz*mpy=pyridin-2-yl-bis(3,5-dimethylpyrazol-1-yl)methane
Nieves Arroyo, Felipe Go´mez-de la Torre, Fe´lix A. Jalo´n *, Blanca R. Manzano,
B. Moreno-Lara, Ana M. Rodr´ıguez
Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Uni6ersidad de Castilla-La Mancha, Facultad de Qu´ımicas,
A6da. Camilo J. Cela, 10, 13071 Ciudad Real, Spain
Received 21 December 1999; received in revised form 28 February 2000
Dedicated to Professor Elguero on the occasion of his 60th birthday for his outstanding contribution to chemistry and as an expression of
personal gratitude to him
Abstract
Two a-R substituted bis(3,5-dimethylpyrazol-1-yl)methane ligands (R=Ph, bpz*mPh and 2-py, bpz*mpy) have been used to
synthesise new palladium(II) compounds with the general formulae PdXX%(bpz*mR) (R=Ph, X=X%=Cl; 1a, X=X%=Me; 2a,
X=Cl, X%=Me; 3a, X=X%=C6F5; 4a or R=2-py, X=X%=Cl; 1b, X=Cl, X%=Me; 3b, X=X%=C6F5, 4b), [Pd(h3-2-
CH3ꢀC3H4)(bpz*mR)]X (X=PF−6 , R=Ph, 5a; X=CF3SO3−, 2-py, 5b) and [Pd(bpz*mR)2](BF4)2 (R=Ph, 6a; 2-py, 6b). In all
the complexes the bpz*mR ligands adopt a rigid conformation with the R group in an axial position and, as a consequence, no
boat-to-boat interconversion is observed. In 1b and 3b–5b the pyridine group is uncoordinated. Complex 5b is present as a single
isomer, while 5a exists as two isomers that are in equilibrium and differ in the orientation of the allyl group with respect to the
NN ligand. For complex 6b the four possible anti isomers are observed in solution. The molecular structures of the major isomer
of 5a and one of the isomers of 6b have been determined by X-ray structure analysis. In both structures the bpz*mR ligands are
coordinated as bidentate. For 5a the ligand backbone confirms the stated rigidity. For 6b a pyridine–pyrazole coordination of the
two bpz*mpy ligands is observed. The fluxional behaviour implying interconversion of the two pyrazole rings in 3b, of the two
isomers in 5a and the rotation of the C6F5 groups in 4a and 4b has also been studied. © 2000 Elsevier Science S.A. All rights
reserved.
Keywords: Palladium; Bis-pyrazolylmethane; Rigidity; Fluxional behaviour; Scorpion
1. Introduction
configurational possibilities. A number of studies have
been performed on bispyrazolylmethane [1] or borate
[1–3] ligands coordinated to a metallic centre, and it
was found that when a methine substituent is present,
two conformations are expected (see Scheme 1) depend-
ing on the axial or equatorial disposition of the a-R
substituent. In the present work, we prepared
two a-substituted bis(3,5-dimethylpyrazol-1-yl)methane
derivatives, phenyl-bis(3,5-dimethylpyrazol-1-yl)meth-
ane (bpz*mPh) and pyridin-2-yl-bis(3,5-dimethyl-
Rigidity is one of the most desirable qualities of a
ligand intended for certain specific catalytic uses such
as, for example, polymerisation processes. This prop-
erty is associated with the selectivity of a system in
adopting only one of the different conformational or
* Corresponding author. Fax: +34-926-295318.
E-mail address: fjalon@qino-cr.uclm.es (F.A. Jalo´n)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00160-1

N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
175
(r.t.), except complex 2a, which decomposes slowly at
this temperature. This complex reacts with halogenated
solvents.
2.2. Characterisation of the new complexes
The IR spectra of the complexes are consistent with
the presence of the nitrogen donor, ancillary ligands
and the corresponding counteranions. Table 1 (1H-
NMR) and Table 2 (13C-NMR) show the chemical
shifts and coupling constants for the free ligands and all
the complexes, except for the mixture of isomers ob-
tained for complex 6b. The assignment of the signals
was made by considering the chemical shifts and cou-
pling constants expected for the different atoms and the
Scheme 1.
pyrazol-1-yl)methane (bpz*mpy) (see Scheme 1), and
explored their coordination abilities with different d8
Pd fragments in order to evaluate the contribution of
the a-R-substituent and the pyrazole methyl groups to
the molecular rigidity of the ligand. The two conform-
ers shown in Scheme 1 may be in a chemical exchange
through a boat-to-boat inversion, but the steric hin-
drance of R and the Me5 groups in conformer b could
give rise to an unequal distribution of the population
between the two conformations.
1
information obtained from H–1H COSY experiments.
The differentiation of Me3 and Me5 groups is based on
the NOE observed for the whole set of complexes
between Me5 and the Ha protons and between one of
the Me3 groups and the PdꢀMe group in 3a and 3b (see
Scheme 3). The NOE between the Hortho (Ph) or H3 (py)
signals and Ha was used to assign these resonances. A
noticeable effect is the systematic difference observed
(0.3–0.6 ppm) in the chemical shifts of the ortho pro-
tons of the Ph group when compared with the H3
protons of the py fragment in both series of com-
pounds. In the free ligands the chemical shifts of these
protons are nearly identical. This behaviour could be
due to the different population of the rotamers, which
is related to the free rotation around the Caꢀaryl bond.
In the compounds containing the pyridine group, a
weak interaction between the N atom and the anti-
bonding orbital perpendicular to the coordination plane
of the Pd centre could favour the corresponding ro-
tamer, whereas in the bpz*mPh series this rotamer
could be less probable for steric reasons.
2. Results and discussion
2.1. Synthesis of ligands and complexes
Bpz*mPh [1] and bpz*mpy [4] were prepared by
methods previously reported for related compounds.
Complexes 1a, 1b, 3a, 3b, 4a, 4b, 6a and 6b were
prepared by replacement of 1,5-cyclooctadiene (cod) or
CH3CN molecules in the appropriate Pd precursors by
the pyrazole derivatives. The removal of chloride an-
ions from the dimer [Pd(h3-2-CH3ꢀC3H4)Cl]2 with a
silver salt in a coordinating solvent, such as acetone,
and the addition of the ligands led to the allyl deriva-
tives. Complex 2a was obtained by reaction of 1a with
methyllithium (see Scheme 2).
Upon coordination, the proton and carbon reso-
nances of the pyrazole groups are shifted to lower field,
as is generally observed in coordinated heterocyclic
rings. A particular effect is observed for 4a and 4b,
The complexes are soluble in polar solvents such as
acetone, THF and dichloromethane and are stable un-
der an atmosphere of nitrogen at room temperature
Scheme 2.

176
N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
Table 1
1H-NMR data for ligands and complexes a
Compound
Pyrazole
Hh
Ph or py group
MeꢀPd or allylPd
H4
Me
bpz*mPh b
bpz*mpy b
5.86
5.88
2.19 (Me3)
2.21 (Me5)
2.18 (Me3)
2.21 (Me5)
7.63 6.91 (m, Ho)
7.30 (m, Hm,p
)
7.57 6.92 (dd, JHH=7.8, 1.0, H3)
7.69 (td, JHH=7.8, 1.7, H4)
7.26 (dd, JHH=7.8, 4.9, H5)
8.64 (ddd, JHH=4.9, 1.7, 1.0, H6)
7.38 7.05 (m, Ho)
PdCl2(bpz*mPh) b (1a)
PdCl2(bpz*mpy) b (1b)
6.05
6.04
2.52 (Me3)
2.59 (Me5)
2.54 (Me3)
2.58 (Me5)
7.61 (m, Hm,p
)
7.27 7.63 (d, JHH=7.8, H3)
7.98 (td, JHH=7.8, 1.7, H4)
7.47 (dd, JHH=7.8, 4.6, H5)
8.71 (m, H6)
PdMe2(bpz*mPh) c (2a)
PdClMe(bpz*mPh) d (3a)
5.45
2.35 (Me3, Me5) 6.72 6.69 (m, Ho)
7.18 (m, Hm,p
2.31 (Me3)
0.53
0.27
)
)
6.21 (H4)
7.82 6.80 (m, Ho)
7.38 (m, Hm,p
6.04 e (H4%) 2.66 (Me5)
2.40 e (Me3%)
2.57 e (Me5%)
2.33 (Me3)
2.60 (Me5)
PdClMe(bpz*mpy) d (3b)
6.11
7.73 7.08 (d, JHH=7.8, H3)
7.80 (td, JHH=7.8, 1.6, H4)
7.40 (dd, JHH=7.8, 4.8, H5)
8.54 (d, JHH=4.8, H6)
7.92 6.85 (d, H3)
7.83 (td, H4)
7.49 (dd, H5)
8.55 (d, H6)
7.39 6.78 (m, Ho)
0.23
0.09
PdClMe(bpz*mpy) d (3b)
183 K
6.25 (H4)
2.20 (Me3)
2.65 (Me5)
6.15 e (H4%) 2.33 e (Me3%)
2.60 e (Me5%)
6.01
Pd(C6F5)2(bpz*mPh) b (4a)
Pd(C6F5)2(bpz*mpy) b (4b)
1.94 (Me3)
2.48 (Me5)
1.94 (Me3)
2.51 (Me5)
7.56 (m, Hm,p
7.32 7.11 (d, JHH=7.6, H3)
7.93 (t, JHH=7.6, H4)
)
6.01
7.47 (dd, JHH=7.6, 4.8, H5)
8.73 (d, JHH=4.8, H6)
8.01 6.47 (m, Ho)
[(h3-C4H7)Pd(bpz*mPh)]PF6 (5aM) 6.42
2.32 (Me3)
2.70 (Me5)
3.88 (Hsyn
3.14 (Hanti
1.14 (Me)
4.16 (Hsyn
1.71 (Hanti
2.15 (Me)
3.86 (Hsyn
2.71 (Hanti
1.52 (Me)
)
d
223 K
7.42 (m, Hm,p
)
)
)
)
5am d
6.39
6.18
2.23 (Me3)
2.74 (Me5)
8.27 6.95 (m, Ho)
7.46 (m, Hm,p
)
223 K
)
[(h3-C4H7)Pd(bpz*mpy)]Tf b (5b)
2.29 (Me3)
2.60 (Me5)
7.56 6.80 (dd, JHH=8.1, 1.0, H3)
7.83 (td, JHH=7.8, 2.0, H4)
7.31 (m, H5)
)
8.54 (dm, JHH=4.9, H6)
7.97 6.82 (dd, H3)
[(h3-C4H7)Pd(bpz*mpy)]Tf d (5b)
6.36
6.46
2.32 (Me3)
2.69 (Me5)
3.99 (Hsyn
2.93 (Hanti
1.52 (Me)
)
)
183 K
7.90 (td, H4)
7.44 (ddt, JHH=7.8, 4.9, 1.0, H5)
8.56 (dm, H6)
d
[Pd(bpz*mPh)2](BF4)2 (6a)
1.92 (Me3)
2.75 (Me5)
8.28 7.43 (m, Ho)
7.74 (m, Hm,p
)
a At room temperature if not specified. Chemical shifts in ppm and coupling constants in Hz. All resonances are singlets if not specified. o, m,
p: ortho, meta and para, respectively. d, doublet; t, triplet; m, multiplet.
b CDCl3.
c C6D6.
d (CD3)2CO.
e Signals corresponding to pyrazolyl groups trans to Me.
where the Me3 proton resonances are markedly shifted
to higher field, probably as a consequence of the elec-
tron current anisotropy [5] of the C6F5 rings. This effect
has also been observed in protons of coordinated hete-
rocyclic ligands situated close to metal aryls fragments
[6].

N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
177
Table 2
13C{1H}-NMR data for ligands and complexes a
Compound
Pyrazole
C4
Ca
Phenyl or 2-pyridyl
MeꢀPd or allylPd
Me3, Me5
C3, C5
Cortho
Cmeta
Cpara
Cipso
bpz*mPh b
1a b
107.32
108.36
106.60
12.36
14.27
11.47
15.09
14.23
14.38
10.99
11.58
14.02
14.95
11.44
13.91
11.26
14.88
11.88
15.14
137.33
141.54
134.05
141.91
139.30
150.98
139.77
140.96
152.47
152.73
144.71
74.40
69.27
68.45
68.49
127.29
126.62
129.01
128.85
130.66
148.83
155.04
136.93
135.42
129.77
2a c
125.60–129.24e
128.99
10.45
3a b
107.13
107.53
126.34
129.55
−8.36
4a d
108.79
69.82
69.57
70.84
126.74
126.68
127.65
129.95
129.89
130.75
129.76
130.18
131.75
135.82
153.83
5aM+5am d 108.28
59.79(CH2)
22.14 (Me)
e
e
6a d
110.50
149.83
155.89
e
C2
C3
C4
C5
C6
bpz*mpy b
1b b
106.85
108.44
107.71
108.32
107.66
11.39
13.78
11.60
15.15
11.29
14.45
11.61
13.88
11.55
15.04
140.70
74.57
69.73
70.34
69.12
69.02
155.58
123.12
125.05
124.36
124.37
124.32
136.8
137.93
137.52
138.25
137.96
122.22
123.61
124.06
122.03
122.19
149.43
150.48
149.83
150.50
149.95
148.52
e
e
3b d
143.80
152.71
142.03
153.39
144.28
152.69
155.71
159.35
153.22
−9.01
4b b
5b b
59.37(CH2)
22.64 (Me)
131.41(ꢀCꢁ)
ꢀ
a Room temperature. Chemical shifts in ppm. All signals are singlets.
b CDCl3.
c C6D6.
d (CD3)2CO.
e Non assigned or non-observed resonances.
The aforementioned NOEs between neighbouring
groups in these compounds (see Scheme 3) are in
accordance with a rigid boat conformation for the
metallacyclic ring with the R fragment in an axial
disposition on the methine Ca group. Accordingly,
given the rigidity of these ligands, the low temperature
1H-NMR spectra of all complexes showed unchanged
patterns. As a consequence, the boat-to-boat conforma-
tional exchange depicted in Scheme 1, which has been
observed in non-a-substituted bis(3,5-Me2-pyrazol-1-
yl)methane and borate Pd derivatives [1], can be ex-
cluded in our complexes. It is very probable that the
steric repulsion of the R group in the a position, with
the two Me5 fragments of the pyrazole rings, precludes
the formation of conformer (b) in Scheme 1. This
scorpion backbone of the ligand is, with some excep-
tions [7], the type normally observed in the solid state
for similarly a-substituted bis(pyrazol-1-yl)methane
derivatives and therefore it may be considered as the
most stable situation for the metallacyclic ring.
This rigidity and the existence of the previously ex-
plained proximity between the a-R groups and the
palladium centre could imply blockage of one of the
perpendicular coordination positions of the palladium
centre. This aspect is crucial to avoid substitution pro-
Scheme 3. Main NOEs observed which establish the rigid conforma-
tion of the bpz*mR ligands.

178
N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
Scheme 4.
cesses such as alkene exchange in catalytic polymerisa-
tion reactions. The exchange is undesirable for an ex-
tended chain propagation [8].
shielding of the Ph or 2-py groups in the rigid confor-
mation of the bpz*mR ligand. In the isomer 5am the
chemical shift of the allylic methyl is in the usual region
but the Hanti protons are now especially shielded. This
shielding effect of the ligand bpz*mPh on an allylic
group has been previously observed in a molybdenum
complex [10]. In our case, this hypothesis was confi-
rmed after the determination of the molecular structure
of 5aM by an X-ray diffraction study.
Crystals of the cationic complex 5aM belong to the
orthorhombic space group Pnma and the molecular
structure consists of a mononuclear Pd cation and a
triflate counterion. The cation lies on a crystallographic
mirror plane containing Pd(1), C(2), C(3), C(10) and
C(13), and consequently only half of the molecule is
crystallographically independent. An ORTEP plot of the
cation is shown in Fig. 1 and the crystallographic data
and a selected list of bond distances and angles are
given in Tables 3 and 4, respectively. The geometry
around the Pd(1) atom is approximately square-planar
The equivalence of the two pyrazole rings in com-
plexes 1b, 4b and 5b above the melting points of the
corresponding solvents (see Table 1) implies the unco-
ordination of the pyridine group. This conclusion is in
contrast both with the more basic character of this
heterocycle when compared with pyrazole and with the
observations of other authors regarding non-substituted
bpzmpy ligands coordinated to PdXX% fragments. For
example, in PdIMe(bpzmpy) both isomers, pz2Pd and
pzpyPd, are observed in solution [4]. This implies a
more specific coordination of the bpz*mpy ligand,
which is an additional advantage for the rigidity of this
scorpion-type ligand.
The 1H- and 13C-NMR spectra of the allyl derivatives
5a and 5b show the symmetric environments expected
for a Cs symmetry (see Tables 1 and 2). While 5b is
present as only one isomer both in (CD3)2CO and
CDCl3 solutions even at low temperature, 5a exists, in
the temperature range allowed in the deuterated sol-
vents, in the form of two isomers in equilibrium
(5aM:5am) in a variable ratio (always \1, M=major,
m=minor isomer) depending on the solvent used.
These two isomers can be assigned to the two expected
different orientations of the 2-Me-allyl group in the
imposed symmetry of the Pdbpz*mR fragment, as de-
picted in Scheme 4. The existence of these types of
isomers has been previously observed in other bis(pyra-
zol-1-yl)methane and borate complexes when the boat-
to-boat interchange is frozen [1,9].
Complexes 5aM and 5b have identical structures,
with the allylic methyl group pointing towards the
a-substituent. This situation is deduced from the ob-
served NOEs between the Me3 group and the allylic
protons (syn and anti ), as well as the chemical shift of
the allylic methyl groups in the 1H-NMR spectrum.
The signal due to the allylic methyl is shifted to signifi-
cantly higher field (see Table 1) as a consequence of the
Fig. 1. Molecular structure of the cation of complex 5aM.

N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
179
with two coordinated pyrazole rings and the h3-bonded
allyl ligand. The Pd(1)ꢀN(1) and Pd(1)ꢀC(1) distances
Table 3
Crystal data and structure refinement for 5aM and 6b
,
are 2.115(4) and 2.111(5) A, respectively.
5aM
6b
The coordination of the bpz*mPh ligand gives rise to
a metallacycle, Pd(1)(NN)2C(9), with a boat conforma-
tion in which the phenyl group attached to the carbon
atom C(9) is located in an axial position, the situation
that has been established to occur in solution. The
methyl allylic group has a syn orientation with respect
to the phenyl ring, as emphasised in Fig. 2. This is the
same disposition found by NMR studies in solution
and produces the shielding effect previously stated.
If we define a plane containing the Pd and the two
nitrogen atoms N(1) and N(1A), then the terminal
Empirical formula
Formula weight
Temperature (K)
C21H27N4Pd·CF3SO3 C32H38N10Pd·B2F8
590.94
842.74
293(2)
293(2)
,
Wavelength (A)
0.71070
Orthorhombic
Pnma
0.71070
Monoclinic
I2/a
Crystal system
Space group
,
a (A)
17.058(3)
10.773(7)
13.487(1)
13.166(6)
14.107(1)
19.690(2)
100.14(2)
3600.0(17)
4
,
b (A)
,
c (A)
i (°)
3
,
Volume (A )
Z
2478.4(17)
4
,
allylic carbons are 0.105(4) A below this plane and the
Dcalc (g cm−3
)
1.584
1.555
,
central allylic carbon is 0.584(7) A above it. The plane
Absorption coefficient 8.86
(cm−1
F(000)
5.97
)
of the allyl group makes an angle of 112.9(5)° with the
coordination plane in such a way that the C(central) to
ꢀCH3 vector is pointing away from the metal.
1200
1712
Crystal size (mm)
Limiting indices
0.4×0.4×0.2
0.4×0.3×0.3
05h522, 05k514, 05h517,
1
The H- and 13C-NMR spectra of complex 6a both
05l517
05k518,
show a very simple pattern corresponding to a single,
symmetrical disposition of the two coordinated
bpz*mPh ligands. Considering the rigid character of
this ligand, two different stereoisomers are possible (syn
and anti, see Scheme 5) given the relative orientation of
the rigid boat conformation of the ligands on the
square plane of the palladium centre. Taking into ac-
count the resolved molecular structure for this type of
compound [11] and the steric requirements of the pyra-
zole substituents in the 3-position, the syn isomer can
be excluded.
−255l525
4337/27/278
Data/restraints/
3134/21/207
parameters
Goodness-of-fit on F2 0.959
0.836
Final R indices
R1=0.0435,
wR2=0.0937
R1=0.0871,
wR2=0.1227
0.614 and −0.741
R1=0.0341,
wR2=0.0933
R1=0.0493,
wR2=0.1077
0.424 and
−0.338
[I\2|(I)] a
R indices
(all data)
Largest difference
peak and hole
−3
,
(e A
)
a R1=ꢁꢂFoꢀ−ꢀFcꢂ/ꢁꢀFoꢀ; wR2=[ꢁ[w(F2o−Fc2)2]/ꢁ[w(Fo2)2]]1/2
.
A relevant aspect in the anti isomer is the special
situation of the Me3 pyrazole substituents, which are
located at the face of the a-Ph group of the other
bpz*mPh ligand, a situation that leads to a remarkable
shielding of this methyl resonance in the 1H-NMR
spectrum (see Table 1).
1
In contrast with the very simple H- and 13C-NMR
spectra of complex 6a, those of complex 6b show a
Table 4
Selected bond lengths (A) and angles (°) for complexes 5aM and 6b
,
5aM
6b
Pd(1)ꢀC(1)
Pd(1)ꢀN(1)
Pd(1)ꢀC(2)
C(1)ꢀC(2)
C(2)ꢀC(3)
2.111(5) Pd(1)ꢀN(1)
2.115(4) Pd(1)ꢀN(5)
2.009(2)
2.035(2)
2.123(7) N(1A) bꢀPd(1)ꢀN(1) 95.54(12)
1.392(6) N(1)ꢀPd(1)ꢀN(5)
85.59(8)
1.523(11) N(5A) bꢀPd(1)ꢀN(5) 93.28(12)
C(1A) aꢀPd(1)ꢀC(1)
C(1)ꢀPd(1)ꢀN(1)
67.6(3)
N(1)ꢀN(2)ꢀC(11)
120.54(19)
120.47(17)
109.6(2)
102.30(18) C(12)ꢀN(5)ꢀPd(1)
N(1A) aꢀPd(1)ꢀN(1) 87.67(19) N(2)ꢀC(11)ꢀC(12)
N(2)ꢀN(1)ꢀPd(1)
N(1)ꢀN(2)ꢀC(9)
C(1A) aꢀC(2)ꢀC(1)
118.6(3)
119.1(4)
115.0(7)
N(5)ꢀC(12)ꢀC(11)
117.8(2)
N(2)ꢀC(9)ꢀN(2A) a 111.1(5)
N(2)ꢀC(9)ꢀC(10)
112.8(3)
a x, −y+1/2, z.
Fig. 2. Molecular projection showing the shielding effect of the
phenyl group over the allylic methyl in the cation of 5aM.
b −x+1/2, y, −z.

180
N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
Scheme 5.
Scheme 6. Possible anti coordination isomers of 6b.
complicated distribution of isomers. This is a conse-
quence of competition between the pyrazole and
pyridine heterocycles of the two coordinated ligands for
coordination to the palladium centre.
Assuming that the syn stereoisomers are less likely
than the anti, due to the aforementioned steric hin-
drance of the Me3 substituents, the number of possible
anti isomers is four. These possibilities are depicted in
Scheme 6.
In order to elucidate several aspects of the coordina-
tion of the bpz*mpy ligand to the Pd centre, monocrys-
tals of this isomeric mixture were obtained. The
molecular structure determined by an X-ray diffraction
study showed the presence of isomer II. The corre-
sponding ORTEP diagram is shown in Fig. 3 and the
crystallographic data as well as selected distances and
bond angles are given in Tables 3 and 4, respectively.
Crystals of the cationic complex 6b belong to the
monoclinic space group I2/a. The asymmetric unit con-
tains one half-molecule of the [Pd(bpz*mpy)2] cation
The number of different pyrazole and methyl groups
expected for each isomer is indicated below the struc-
tures in Scheme 6. In these anti isomers the uncoordi-
nated a-R substituent should shield the Me3 pyrazole
group situated at its face and, as such, six shielded
methyl resonances (indicated in parenthesis with the
symbol S for each isomer) are expected if the four
isomers I–IV are present. In practice six resonances,
three of which of the same intensity due to shielded
methyl groups (1.47–2.00 ppm), are observed in the
1
corresponding H-NMR spectrum. When the mixture is
allowed to stand for a long period of time (1 month) a
marked increase in the intensity of one shielded methyl
resonance together with three non-shielded methyl reso-
nances is observed. This corresponds to an enrichment
in the mixture of isomer I or II and, consequently, one
of these is the most thermodynamically stable. A de-
crease in the signals of equal intensity due to three
shielded methyl groups is also observed, which indicates
that isomer IV is now present to a lesser extent. A clear
change in the signals of the other two isomers is not
observed.
Fig. 3. Molecular structure of the cation of complex 6b.

N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
181
with the Pd centre sitting on a C2 axis. There is also
one BF−4 counteranion per half-molecule. The two neu-
tral ligands are coordinated in a bidentate fashion
through the nitrogen atoms of one pyrazole and the
pyridine rings. The two coordinated pyridine groups
are in a mutually cis disposition. The coordination
environment shows the Pd(1)ꢀN(1) distance to be
As previously stated, two isomers of 5a exist in
solution: 5aM and 5am. They are in a dynamic equi-
librium at r.t. both in (CD3)2CO and CDCl3 solution.
1
The broad resonances of the H-NMR spectra and the
apparent existence of resonances for only one isomer in
the 13C-NMR spectrum suggest the existence of this
exchange equilibrium. At low temperature, the 1H-
NMR spectrum shows the expected narrow resonances
for the two isomers. Additional experiments indicate
clearly the existence of this exchange since the irradia-
tion of signals such as the allylic protons of the major
isomer produces magnetisation transfer over the ex-
changing protons of the minor component. The limit
imposed by the boiling point of the stated solvents has
prevented the observation of any coalescence and the
kinetic parameters of this exchange have been calcu-
lated in CDCl2ꢀCDCl2. The obtained DG‡c values for
the different reached coalescesces are in between 66.0/
305 and 66.6/325 (kJ mol−1/K). This type of exchange
has been observed both by ourselves and other authors
in allyl derivatives of Pd(II) and the corresponding
mechanism is thought to proceed via tricoordinate [15]
or pentacoordinate [16] intermediates.
,
2.009(2) A and the Pd(1)–N(5) distance to be 2.035(2)
,
A.
The metallacycle Pd(1)(NN)(NC)C(11) has a boat
conformation in which the two pyrazole groups at-
tached to the carbon atoms C(11) are located in a
mutually anti disposition. The dihedral angle between
each free pyrazole ring and the palladium coordination
plane is 123.6(1)°. In this situation the N(4) atom of the
pyrazole is at a non-bonded N(4)···Pd(1) separation of
,
2.876(3) A. The Me3 group of the coordinated pyra-
zoles (C(5) and C(5A) in Fig. 3) is situated at the face
of the free pyrazolyl ring of the other ligand. This is in
accordance with the shielding observed in solution for
these methyl groups.
2.3. Fluxional beha6iour of se6eral complexes
The 19F-NMR spectra of complexes 4a and 4b in
acetone solution at r.t. are quite comparable. Single
resonances for the ortho (broad), meta and para fluori-
nes are observed for 4a, whereas two ortho resonances
are observed for 4b. When the temperature is decreased
the single resonances of the ortho and meta fluoro
substituents split to give different signals and, at low
temperature, the pattern expected for two identical
C6F5 groups with restricted rotation is observed. This
behaviour is not unexpected considering the steric hin-
drance in the coordination environment of the Pd cen-
tre [17].
In both compounds, the two ortho fluorine reso-
nances observed at low temperature have a different
appearance. This should be due to the second order
spin system arising from the couplings with the meta
fluoro substituent, as demonstrated by homonuclear
irradiation, and no evidence for an inter-ring coupling
was observed [17a]. Considering the coalescences of the
Fo resonances, the free energies of activation at the
coalescence temperatures were estimated: (DG‡c (kJ
mol−1)/T (K): 53.9/283 for 4a and 57.1/311 for 4b). If
the pyridine group would participate in the rotational
process of 4b, the more plausible way would be by the
formation of a pentacoordinate intermediate through
the coordination of this heterocycle, a fact that is not
possible in 4a. This previsibly would lead to a decrease
in the DG‡c value. Considering that DGc‡ (4b)\DGc‡ (4a)
(a high influence of the temperature is not expected) we
propose that there is not a clear participation of the
pyridine fragment in the rotational process.
In contrast with the static behaviour observed in
complex 3a (from 183 to 323 K), which exhibits two
different pyrazole groups, 3b shows an apparent sym-
metric environment with two identical pyrazole rings at
r.t. both in the 1H- and 13C-NMR spectra in acetone-d6.
However, at low temperature (see Table 1) two clearly
1
different pyrazole units are observed in the H-NMR
spectrum in accordance with the C1 symmetry of this
compound. The free activation energy for the exchange
between the two pyrazole rings is 58.7 kJ mol−1 at 263
K (calculated from the coalescence of the H4 pyrazole
protons). This noticeable difference in the activation
barrier can be ascribed to the presence of the pyridine
ring in 3b but not in 3a. The easy exchange of free and
coordinated pyrazole fragments in tris(pyrazol-1-
yl)methane and borate derivatives of Pd(II) has been
previously observed [1,12,13] and such a tumbling pro-
cess is thought to occur through pentacoordinated in-
termediates with the pyrazole ligand being
tricoordinated to the metallic centre [14]. In complex 3b
the pyridine coordination could give rise to the penta-
coordinate intermediate. The energy barrier for the
tumbling process in tris(pyrazol-1-yl)methane and bo-
rate derivatives is especially low and in some cases the
process is beyond the NMR time scale. In the case of
complex 3b this barrier is considerably higher and this
should correspond to greater instability of the pentaco-
ordinate intermediate. The absence of an a-R sub-
stituent with coordination ability in the bpz*mPh
complexes, particularly in 3a, precludes this exchange
process.

182
N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
3. Experimental
was carried out by full-matrix least-squares analysis
SHELXL-97) [24]. Anisotropic temperature parameters
(
3.1. Starting materials and general conditions
were considered for all non-hydrogen atoms, while
hydrogen atoms were included in calculated positions
but not refined. For 5aM the triflate anion is disordered
over two positions with identical occupancies.
3.1.1. General comments
All manipulations were carried out under an atmo-
sphere of dry oxygen-free nitrogen using standard
Schlenk techniques. Solvents were distilled from the
appropriate drying agents and degassed before use.
[Pd(h3-2-Me-C3H4)(m-Cl)]2 [18], [Pd(C6F5)2(cod)] [19],
[PdCl2(cod)] [20], [PdClMe(cod)] [21] and [Pd-
(CH3CN)4](BF)2 [22] were prepared as described in the
literature. Elemental analyses were performed with a
Perkin–Elmer 2400 micro analyser. IR spectra were
recorded as KBr pellets or Nujol mulls with a Perkin–
Elmer PE 883 IR spectrometer. 1H- and 13C-NMR
spectra were recorded on a VARIAN UNITY 300
spectrometer. Chemical shifts (ppm) are given relative
3.2. Preparations
3.2.1.
Pyridin-2-yl-bis(3,5-dimethylpyrazol-1-yl)methane
(bpz*mpy)
This ligand was prepared by a similar method [4] to
that described for pyridin-2-yl-bis(pyrazol-1-yl)methane
but using 3,5-dimethylpyrazole as starting reactive.
Yield: 58%. C16H19N5 (281.16): C, 68.34; H, 6.76; N.
24.89. Found: C, 68.32; H, 7.01; N, 24.72%.
1
to TMS (13C and H) or CF3Cl (19F). COSY spectra:
3.2.2. [PdCl2(bpz*mPh)] (1a)
standard pulse sequence with an acquisition time of
0.214 s, pulse width 10 ms, relaxation delay 1 s, number
of scans 16, number of increments 512. The NOE
difference spectra were recorded with the following
acquisition parameters: spectral width 5000 Hz, acquisi-
tion time 3.27 s, pulse width 18 ms, relaxation delay 4
s, irradiation power 5–10 dB, number of scans 240. For
variable temperature spectra the probe temperature (9
1 K) was controlled by a standard unit calibrated with
a methanol reference. Free energies of activation were
calculated [23] from the coalescence temperature (Tc)
and the frequency difference between the coalescing
signals (extrapolated at the coalescence temperature)
with the formula DG‡c =aT[9.972+log (T/lw)], a=
1.914×10−2. The estimated error in the calculated free
To a solution of PdCl2(cod) (0.80 g, 0.28 mmol) in 20
ml of dichloromethane, bpz*mPh was added (0.78 g,
0.28 mmol). The resulting yellow solution was stirred
for 16 h until the formation of an orange colour. The
resulting orange solid obtained after evaporation to
dryness was washed with diethyl ether (2×5 ml). The
solid was recrystallised from 1,2-dichloroethane–di-
ethylether and orange crystals were obtained. Yield
86%. C17H20Cl2N4Pd (457.49): Calc. C, 44.63; H, 4.37;
N, 12.24. Found: C, 44.61; H, 4.30; N, 12.34%. IR
(cm−1) (Nujol): 1556 (C=Npz); 346, 331 (PdꢀCl).
3.2.3. [PdCl2(bpz*mpy)] (1b)
A similar procedure to that used for 1a was followed
for 1b. The reaction was made in acetone for 4 h. Yield
91%. C16H19Cl2N5Pd (458.48): Calc. C, 41.90; H, 4.17;
N, 15.27. Found: C, 41.65; H, 3.44; N, 15.23%. IR
(cm−1) (Nujol): 1583 (C=Npy); 1553 (C=Npz); 346,
332 (PdꢀCl).
energies of activation is 90.5 kJ mol−1
.
3.1.2. X-ray structural determination of 5aM and 6b
Crystal, data collection, and refinement parameters
are collected in Table 3. Suitable prismatic and colour-
less crystals of both complexes were selected and
mounted on fine glass fibbers with epoxy cement. The
unit cell parameters were each determined from the
angular setting of least-squares fit of 25 strong high-an-
gle reflections. The asymmetric unit for 5aM and 6b
contained half-independent molecule. Reflections were
collected at 25°C on a NONIUS-MACH3 diffractome-
ter equipped with a graphite monochromated radiation
(u=0.7107 cm−1). Each sample showed no significant
intensity decay over the duration of data collection.
Data were corrected in the usual fashion for Lorentz
and polarisation effects and empirical absorption cor-
rection was not necessary (v=8.86 cm−1 (5aM) and
5.97 cm−1 (6b)). The space group was determined from
the systematic absences in the diffraction data. The
structures were solved by direct methods (SHELXS for
5aM and SIR92 for 6b) [24,25] and refinement on F2
3.2.4. [PdMe2(bpz*mPh)] (2a)
A suspension of 1a (0.82 g, 0.18 mmol) in 20 ml of
diethyl ether was kept cold (−30°C) and 220 ml of a
1.6 M MeLi solution was added. The temperature was
allowed to increase up to 0°C and then the solution was
kept at this temperature for 30 min. Degassed cold
water (1 ml) was added. The organic phase was sepa-
rated in a separatory funnel and dried with Na2SO4.
The colourless solution was filtrated and evaporated to
dryness. A pale yellow powdered solid was obtained.
Yield 30%. The presence of some impurities of palla-
dium metal makes this product unsuitable for a correct
microanalysis.
3.2.5. [PdClMe(bpz*mPh)] (3a)
To a solution of PdClMe(cod) (0.2 g, 0.75 mmol) in
30 ml of toluene, bpz*mPh was added (0.21 mg, 0.75

N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
183
mmol). The resulting solution was allowed to react for
6 h. A green pale solid was obtained after evaporation
to dryness. This solid was recrystallised from THF–di-
ethyl ether and pale yellow crystals were obtained.
Yield 82%. C18H23ClN4Pd (437.05): Calc. C, 49.46; H,
5.26; N, 12.81. Found: C, 49.24; H, 4.96; N, 12.79%.
IR (cm−1) (Nujol): 1557 (C=Npz), 308 (PdꢀCl).
from 1,2-dichoroethane–diethyl ether. Suitable crystals
for X-ray diffraction were obtained in a similar way
with CF3SO−3 as counteranion. Yield: 84%.
C21H27F6N4PPd (586.61): Calc. C, 42.98; H, 4.60; N,
9.50. Found: C, 43.20; H, 4.42; N, 9.77%. IR (cm−1
)
(KBr): 1571 (C=Npz); 748, 495 (PF6).
3.2.10. [Pd(p3-C4H7)(bpz*mpy)]Tf (5b)
3.2.6. [PdClMe(bpz*mpy)]·1/2C4H8O 3b·1/2C4H8O
A similar procedure to that used for 3a was fol-
lowed for the synthesis of 3b. The reaction was made
A similar procedure to 5a was followed for 5b. The
reaction was made in acetone for 4 h and the recrys-
tallisation from acetone–diethyl ether. Yield 86%.
C21H24F3N5O3SPd. (589.63): Calc. C, 42.61; H, 4.07;
N, 11.83. Found: C, 42.52; H, 3.74; N, 11.95%. IR
(cm−1) (KBr): 1590 (C=Npy), 1565 (C=Npz); 1265,
1147, 771, 638 (CF3SO3).
in tetrahydrofurane for
2
h. Yield 89%.
C17H22ClN5Pd·1/2C4H8O (474.06): Calc. C, 48.11; H,
5.52; N, 14.76. Found: C, 47.89; H, 4.94; N, 14.67%.
IR (cm−1) (Nujol): 1585 (C=Npy), 1555 (C=Npz),
318 (PdꢀCl).
3.2.11. [Pd(bpz*mPh)2](BF4)2 (6a)
3.2.7. [Pd(C6F5)2(bpz*mPh)] (4a)
To a solution of [Pd(CH3CN)4](BF4)2 (47.37 mg,
0.11 mmol) in 10 ml of acetone, bpz*mPh (60.00 mg,
0.21 mmol) was added. The solution was stirred at r.t.
for 5 min and the resulting solution was evaporated to
dryness. The resulting white solid was washed with
diethyl ether (2×10 ml). White crystals were obtained
after a recrystallisation from acetone–diethyl ether.
Yield: 89%. C34H40B2F8N8Pd (840.38): Calc. C, 48.59;
H, 4.76; N, 13.33. Found: C, 48.84; H, 5.25; N,
14.08%. IR (cm−1) (Nujol): 1558 (C=Npz); 1055
(BF4).
To a solution of Pd(C6F5)2(cod) (0.8 g, 0.15 mmol)
in 20 ml of dichloromethane, bpz*mPh (0.41 g, 0.15
mmol) was added. The resulting solution was stirred
for 16 h and a white solid was obtained after evapora-
tion to dryness. Colourless crystals could be obtained
from recrystallisation from 1,2-dichloroethane–pen-
tane. Yield 84%. C29H20F10N4Pd (720.71): Calc. C,
48.32; H, 2.78; N, 7.77. Found: C, 48.61; H, 2.60; N,
7.83%. 19F-NMR (CDCl3), r.t. l −114.47 (bs, Fo);
−162.91 (t, JFF=21.4 Hz, Fp); −165.68 (m, Fm); 213
K, l −114.38 (m, Fo), −115.11 (m, Fo); −162.91 (t,
J
FF=21.4 Hz, Fp); −165.10 (m, Fm), −164.82 (m,
3.2.12. [Pd(bpz*mpy)2](BF4)2 (6b)
Fm). IR (cm−1) (Nujol): 1564 (C=Npz); 1625, 1498,
A similar procedure to that used for 6a was fol-
lowed to obtain 6b. Yield 84%. C32H38B2F8N10Pd
(842.77): Calc. C, 45.61; H, 4.55; N, 16.62. Found: C,
45.99; H, 4.28; N, 17.10%. IR (cm−1) (Nujol): 1603
(C=Npy), 1560 (C=Npz); 1065 (BF4).
1059, 958 (C6F5).
3.2.8. [Pd(C6F5)2(bpz*mpy)]·CH2Cl2 4b·CH2Cl
A similar procedure to 4a was followed for 4b. The
reaction was carried out in acetone for 6 h. The
product was recrystallised from dichloromethane.
Yield 77%. C28H19F10N5Pd·CH2Cl2 (806.84): Calc. C,
43.17; H, 2.62; N, 8.68. Found: C, 43.38; H, 2.34; N,
8.93%. 19F-NMR, (CDCl3), r.t. l −113.93, −116.35
(bs, Fo); −162.81 (t, JFF=21.4 Hz, Fp); −165.56 (m,
Fm); 213 K: l −114.57 (m, Fo), −116.41 (m, Fo);
−161.08 (t, JFF=21.4 Hz, Fp); −164.68 (m, Fm),
−164.99 (m, Fm). IR (cm−1) (Nujol): 1595 (C=
Npy), 1567 (C=Npz); 1634, 1503, 1063, 960 (C6F5).
4. Supplementary material
Crystallografic data for the structural analysis have
been deposited in the Cambridge Crystallographic
Data Centre, CCDC no. 138315 for compound 5aM
and no. 138316 for compound 6b. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ (Fax: +44-1223-336033; or e-mail: de-
posit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk).
3.2.9. [Pd(p3-C4H7)(bpz*mPh)]PF6 (5a)
To a solution of [Pd(h3-C4H7)(m-Cl)]2 (200.00 mg,
0.5 mmol) in 20 ml of THF, AgPF6 (256.74 mg, 1.0
mmol) was added. The solution was stirred at r.t. for
4 h and the resulting suspension was filtered off. Then
bpz*mPh (284.70 mg, 1.00 mmol) was added to the
filtrate. After stirring for 1 h, the colourless solution
thus formed was evaporated to dryness. The resulting
white solid was washed with diethyl ether (2×20 ml).
White crystals were obtained after a recrystallisation
Acknowledgements
We gratefully acknowledge financial support from
the Direccio´n General de Investigacio´n Cient´ıfica y
Te´cnica (DGICYT) (Grant no. PB98-0315) of Spain
and Professor Trofimenko for instructive discussions.

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N. Arroyo et al. / Journal of Organometallic Chemistry 603 (2000) 174–184
8
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Ba¨ckvall, J. Am. Chem. Soc. 116 (1994) 3631.
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