New palladium and platinum polyfluorophenyl complexes with pyrazolyl N-donor ligands. Analysis of the restricted rotation of the polyfluorophenyl rings

Article abstract of DOI:10.1039/b108139j

The new N-donor chelate ligands bis(pyrazol-1-yl)(anisol-2-yl)methane, bpzmArOMe, bis(3,5-dimethylpyrazol-1-yl)hydrocarbylmethane (hydrocarbyl = anisol-2-yl, bpz*mArOMe; cyclohexyl, bpz*mCy) and bis(3,5-dimethylpyrazol-1-yl)ferrocenylmethane, bpz*mFec, have been prepared and characterized. New palladium and platinum polyfluorophenyl complexes were prepared with these ligands and those previously described: bis(pyrazol-1-yl)(phenol-2-yl)methane, bpzmArOH; bis(3,5-dimethylpyrazol-1-yl)(phenol-2-yl)methane, bpz*mArOH; 2-(pyrazol-1-yl)-pyridine, pzpy and 2-(pyrazol-1-yl)-pyrimidine, pzpm. For the bpzmR or bpz*mR complexes only one isomer has been found with the R group in an axial orientation. The fluxional behaviour of the new complexes, mainly the polyfluorophenyl rotation, has been analyzed. For the complexes with the nonplanar ligands, derived from bis(pyrazol-1-yl)methane, rotation of the pentafluorophenyl groups has been found to occur with different energy barriers. The presence of methyl groups in position 3 of the pyrazole rings reduces this barrier. The process is not affected by the presence of coordinating solvents or anions (C1^-) or by a change in the complex concentration. For the derivative Pd(2,3,4,6-C₆HF₄)₂(pzpm), containing a planar ligand, two atropisomers are observed even at high temperature, which excludes the existence of polyfluorophenyl rotation. Processes of Pd-N bond rupture or cis,trans-isomerization have not been observed for this complex on the NMR time scale. The molecular structure of complex Pd(C₆F₅)₂(bpz*mFec) has been determined by an X-ray diffraction study.

Full text of DOI:10.1039/b108139j

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New palladium and platinum polyfluorophenyl complexes with
pyrazolyl N-donor ligands. Analysis of the restricted rotation
of the polyfluorophenyl rings
M. Carmen Carrion,a Alberto Dıaz,a Ana Guerrero,a Felix A. Jalon,*a Blanca R. Manzanoa
and Ana Rodrıguezb
a
´
Facultad de Quımicas, Universidad de Castilla-La Mancha, Avda. Camilo J. Cela, 10,
E-13071 Ciudad Real, Spain. E-mail: fjalon@qino-cr.uclm.es
b
´
Escuela Tecnica Superior de Ingenierıa Industrial, Universidad de Castilla La Mancha,
Avda. Camilo J. Cela, 3, E-13071 Ciudad Real, Spain
´
Received (in Strasbourg, France) 3rd September 2001, Accepted 19th November 2001
First published as an Advance Article on the web
The new N-donor chelate ligands bis(pyrazol-1-yl)(anisol-2-yl)methane, bpzmArOMe, bis(3,5-dimethylpyrazol-
1-yl)hydrocarbylmethane (hydrocarbyl ¼ anisol-2-yl, bpz*mArOMe; cyclohexyl, bpz*mCy) and bis(3,5-
dimethylpyrazol-1-yl)ferrocenylmethane, bpz*mFec, have been prepared and characterized. New palladium
and platinum polyfluorophenyl complexes were prepared with these ligands and those previously described:
bis(pyrazol-1-yl)(phenol-2-yl)methane, bpzmArOH; bis(3,5-dimethylpyrazol-1-yl)(phenol-2-yl)methane,
bpz*mArOH; 2-(pyrazol-1-yl)-pyridine, pzpy and 2-(pyrazol-1-yl)-pyrimidine, pzpm. For the bpzmR or
bpz*mR complexes only one isomer has been found with the R group in an axial orientation. The fluxional
behaviour of the new complexes, mainly the polyfluorophenyl rotation, has been analyzed. For the complexes
with the nonplanar ligands, derived from bis(pyrazol-1-yl)methane, rotation of the pentafluorophenyl groups
has been found to occur with different energy barriers. The presence of methyl groups in position 3of the
pyrazole rings reduces this barrier. The process is not affected by the presence of coordinating solvents or anions
(ClÀ) or by a change in the complex concentration. For the derivative Pd(2,3,4,6-C6HF4)2(pzpm), containing a
planar ligand, two atropisomers are observed even at high temperature, which excludes the existence of
polyfluorophenyl rotation. Processes of Pd–N bond rupture or cis,trans-isomerization have not been observed
for this complex on the NMR time scale. The molecular structure of complex Pd(C6F5)2(bpz*mFec) has been
determined by an X-ray diffraction study.
In recent years Pd(II) complexes bearing N-donor ligands1
have received increasing attention. The reasons for this interest
include the catalytic activity of these systems in the poly-
merization of alkenes,2 alkene/CO copolymerization,3 alkene/
alkyl-acrylate copolymerization4 and other catalytic processes5
and also the study of important insertion reactions6 or
dynamic processes7–9 carried out on complexes of this nature.
We have recently been working with two types of N-donor
ligands derived from pyrazole: (i) compounds containing the
bis(pyrazol-1-yl)methane backbone, which are consequently
non-planar9–11 [the groups of Canty,12 Trofimenko,13 Jordan14
and Minghetti15 have also prepared several types of palladiu-
m(II) bis(pyrazol-1-yl)methane derivatives] and (ii) systems
with an approximately planar structure composed of two16,17
or more18,19 distinct heterocyclic moieties. Different properties
of these compounds have been studied, with particular
emphasis on catalytic applications20 and fluxional behaviour
that involves Pd–N bond rupture processes.9,10,16–19 In the
work described here, we have focused our attention on the
synthesis of bis(polyfluorophenyl)palladium derivatives with
these two types of N-donor ligands. The main goal of our work
was to analyze the rotation of the polyfluorophenyl ring in the
new metal complexes. Cases in which free21,22 or restricted
rotation23,24 of polyfluorophenyl groups occurs in palladium
or platinum complexes have been described in the literature. It
is important to note that, in some cases, fluxional processes
involving the ancillary ligands can produce mirror planes on
the NMR time scale and give rise to an apparent aryl rota-
tion8,25,26 that could not actually occur. Clearly, when a real
mirror plane coincides with the coordination plane the rota-
tional process cannot be studied unless atropisomers are
formed due to the presence of two (or more) polyfluorophenyl
groups with an asymmetric substitution pattern.19,21 In our
complexes—with an appropriate choice of ligands—it would
be possible to evaluate the influence of different factors
that may be important for this polyfluorophenyl rotation:
(i) the geometry of the ligand, whether planar or non-planar,
(ii) the presence of methyl groups in the 3- and 5-positions of
the pyrazole rings and (iii) the type of group present on the
methynic carbon of the bis(pyrazolyl)methane backbone. For
the complexes bearing bis(pyrazol-1-yl)Rmethane ligands it is
also of interest to determine the orientation (axial or equa-
torial) of the R group at the methynic carbon and the factors
that influence on the boat-to-boat equilibrium that could
operate in these types of complexes.10,12,27 The ligands used are
indicated in Scheme 1.
Results and discussion
Synthesis of the ligands and complexes
The ligands bpzmArOH and bpz*mArOH have been reported
previously.28 The new ligands bpzmArOMe, 1, bpz*mArOMe,
DOI: 10.1039/b108139j
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Characterization of the new derivatives
The new derivatives were characterized by elemental analysis,
1H (Table 1), 13C{1H} (Table 2) and 19F NMR (Table 3) and
IR spectroscopy. The molecular structure of complex 10 was
determined by X-ray diffraction. The IR spectra in the X–C6F5
sensitive region30 (absorption at around 800 cmÀ1) show one
band for complexes 5–10, a situation in accordance with the
expected equivalence of the pentafluorophenyl groups. In
contrast, complexes 11–14 give rise to two bands, which is in
accordance with the asymmetry of the planar ligands.
The assignment of the NMR resonances was made by con-
sidering the chemical shifts and coupling constants31 expected
for the different atoms. In some cases, the information
obtained from 1H–1H and 1H–13C COSY and NOE spectra
was also taken into account. For complexes 5–10 (bpzmR or
bpz*mR derivatives) signals corresponding to one type of N-
donor ligand with equivalent pyrazolyl groups were found. In
general, the resonances are shifted to lower field after coordi-
nation to the metal (positive CIS, coordination induced shift,
CIS ¼ dcomplex À dligand). However, the protons situated near
the metal centre give rise to negative CIS values. This fact must
be due to the current anisotropy of the pentafluorophenyl
rings,32 an effect previously observed with other aromatic
rings33 as well as with polyfluorophenyl groups.19
Scheme 1
In principle, two types of isomers are possible for the bpzmR
or bpz*mR complexes, depending on whether the R group is in
an axial or equatorial orientation (see Scheme 4). A boat-to-
boat process should interconvert these two isomers. In our
complexes only one isomer is observed. For complex 5 we have
also confirmed that only one isomer exists at low temperature
(acetone-d6 , À90 ꢀC). The identity of the isomer present in
solution can be deduced from NOE spectra. The NOE between
Scheme 2
2, bpz*mCy, 3 and bpz*mFec, 4, were synthesized from the
corresponding bis(pyrazol-1-yl)ketone derivatives and the
aldehydes containing the R groups according to the procedure
described by Peterson et al.29 (see Scheme 2). The metal
complexes were prepared by replacement of cod or THF
ligands from the corresponding precursors (see Scheme 3). The
products are stable in the solid state and in solution under an
inert atmosphere. With the exception of complexes 6 and 8,
which are only sparingly soluble, the compounds are soluble in
common polar solvents. The bpzmR complexes (5 and 6) are
less soluble than those containing methylated bpz*mR ligands
(7–10).
0
0
Ha and Me5 or H5 is only possible when the R group is in an
axial orientation (see Scheme 4, conformation A). It is likely
that the isomer with the R group in the equatorial disposition
would lead to a high degree of steric hindrance between this
substituent and the pyrazolyl rings, particularly with Me5 .
This selectivity in the conformation of the N-donor ligand has
also been observed in other palladium complexes with similar
bpz*mR ligands.11 However, it should be noted that an
equatorial location for the methynic substituent has been
found in other types of complexes.34
In some palladium derivatives containing the pzpm ligand
coordinated to PdClMe, PdCl(COMe) and Pd(2-Me-allyl)
moieties16,17 we found that when a group of high trans influ-
ence is situated trans to the pyrimidine ring, the Pd–N(pm)
bond opens easily and produces an H4/H6 interchange. The
fact that these two protons give rise to narrow and separate
NMR signals in the polyfluorophenyl derivatives 12–14 (even
at 130 ꢀC in 1,10,2,20-tetrachloroethane-d2 for 14) indicates
that, as expected, the polyfluorophenyl groups have a smaller
trans influence than the methyl, acyl or Z3-2-Me-allyl groups
and that the Pd–N bond rupture must have a very high energy
barrier.
19F NMR spectra and polyfluorophenyl rotation
In the bpzmR or bpz*mR complexes (5–10) one resonance due
to Fpara is observed in each case, which indicates the existence
of only one type of pentafluorophenyl ring. This is in accor-
dance with the Cs symmetry already deduced from the 1H and
13C NMR spectra. As far as the Fortho and the Fmeta signals are
concerned, one (7, 8 and 10) or two (5, 6, 9) resonances were
observed at room temperature for each type of fluorine. This
indicates that at this temperature a situation involving free or
restricted rotation, respectively, of the pentafluorophenyl ring
with respect to the C–Pd bond exists. In order to observe the
two signals separately and to detect the corresponding coa-
lescence temperatures, a number of variable temperature
Scheme 3
306
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Table 1 1H NMR data for derivatives 1–14. Coupling constants are given in Hz. d ¼ doublet, m ¼ multiplet, pt ¼ pseudotriplet, s ¼ singlet,
br ¼ broad
Pyrazole
0
0
0
0
0
H4
H3 =Me3
H5 =Me5
Ha
R
1a 6.31 t
2a 5.83s
3a 5.75 s
7.63 d
0
7.38 d
0
8.03s
6.94 d J6À5 ¼ 8.3(H 6); 6.95 td J4À5 ¼ 7.3, J4À6 ¼ 0.9 (H4);
7.38 td J3À5 ¼ 1.7 (H5); 6.89 dd J3À4 ¼ 7.9 (H3); 3.78 s (OMe)
6.89 d J6À5 ¼ 8.1 (H6); 6.91 t J4À5 ¼ 7.3(H 4); 7.31 td J3À5 ¼ 1.7 (H5);
6.82 dd J3À4 ¼ 7.6, J3À5 ¼ 1.7 (H3); 3.73 s(OMe)
0
0
J3 –4 ¼ 1.7 J5 –4 ¼ 2.4
2.20 s
2.19 s
2.10 s
2.36 s
7.68 s
5.83 d
JHaÀHipso ¼ 11.0
3.0 m (Hipso); 2.17–0.85 br m (CH2)
4a 5.76 s
5a 6.36 t
2.30 s
7.27 d
2.19 s
7.79 d
7.52 s
7.91 s
4.08 s (C5H5); 4.54 s (H2 þ H2 Cp.) 4.20 s (H3 þ H3 Cp.)
6.95 d J6À5 ¼ 8.4 (H6); 7.26 t J4À5 ¼ 7.6 (H4); 7.51 t (H5);
8.10 d J3À4 ¼ 7.9 (H3); 3.82 s (OMe)
0
0
0
0
0
0
J3 –4 ¼ 1.3 J5 –4 ¼ 2.7
6b 6.49 t
7.36 d 8.38 dd
0
8.60 s
6.95 dd J6À5 ¼ 8.06 (H6); 7.05 t J4À5 ¼ 7.7 (H4); 7.35 dt (H5);
7.87 dt J3À4 ¼ 7.06 (H3); 9.28 s (OH)
0
0
0
J3 –4 ¼ 2.2 J5 –4 ¼ 2.7,
0
0
J5 –3 ¼ 1.0
7a 5.92 s
8a 5.89 s
9a 5.84 s
10a 5.93s
1.92 s
1.90 s
1.91 s
1.88 s
2.45 s
7.47 s
7.50 s
7.01 d J6À5 ¼ 8.3(H 6); 7.21 t J4À5 ¼ 7.3(H 4);
7.54 ddd J5À3 ¼ 1.9 (H5); 7.28 d J3À4 ¼ 7.6 (H3); 3.73 s (OMe)
6.91 d J6À5 ¼ 7.8 (H6); 7.14 t J4À5 ¼ 7.6 (H4);
7.38 tm (H5); 7.24 d J3À4 ¼ 7.8 (H3); 6.58 br s (OH)
5.5 m (Hipso); 2.26–0.75 br m (CH2)
2.45 s
2.37 s
2.58 s
5.69 d
JHaÀHipso ¼ 11.2
0
0
7.85 s
4.22 s (C 5H5); 4.32 s (H2 þ H2 Cp); 4.44 s (H3 þ H3 Cp)
Pyridine or pyrimidine
Other groups
H3
H4
H5
H6
11b 6.87 pt
7.67 d
0
9.01 d
8.89 dd
8.93dd
8.86 m
8.31 d
J3À4 ¼ 8.5
8.39 pt
J4À6 ¼ 1.7
7.54 pt
7.98 d
0
0
0
J4 –5 ¼ 3.2 J3 –4 ¼ 1.9
12b 6.87 pt
7.76 d
J5À6 ¼ 5.4,
J5À4 ¼ 6.1
7.68 pt
J5À6 ¼ 5.5,
J5À4 ¼ 4.9
7.69 t
J5À6 ¼ 5.6,
J5À4 ¼ 4.7
7.70 t
–
9.17 pt
J4À6 ¼ 2.1
7.98 dd
8.67 dd
8.30 dd
0
0
0
0
J4 –5 ¼ 3.2 J3 –4 ¼ 2.0
13b 6.96 d
7.94 d
–
9.26 dd
J4À6 ¼ 2.1
0
0
0
0
J4 –5 ¼ 3.2 J3 –4 ¼ 1.9
14b 7.68 dc
6.87 m
6.66 m (C6HF4)
–
9.16 dd
J4À6 ¼ 2.1,
J4À5 ¼ 4.8
a
b
Chloroform-d. Acetone-d6. Overlapped signals.
c
Table 2 13C{1H} NMR data for derivatives 1–14. Unless specified the signals are singlets. m ¼ meta, o ¼ ortho, p ¼ para
Pyrazole
0
0
0
0
0
C3
C4
C5
Me3 Me5 Ca
R
1a 140.6
.11051.956.6 (C 129.373
1); 124.1 (C2); 110.8 (C6); 120.7 (C4); 130.8 (C5); 127.8 (C3);
55.6 (–OMe)
6); 120.6 (C4); 129.9 (C5); 128.5 (C3); 55.8 (–OMe)
13.6 11.4 76.7 39.5 (Cipso); 26.2 (Cp); 29.4, 25.4 (Co, Cm)
2a 147.8
3a 147.4
4a 147.3
140.1
106.2
106.4
.180o6r.631911.38.3 110.7 (C
139.7
139.9
0
0
)
13.6 11.9 72.3 68.9 (Cp); 68.2 (Cp2,2 ); 69.5 (Cp3,3
72.8 157.2 (C 1); 124.0 (C2); 112.0 (C6); 121.7 (C4); 132.2 (C5); 129.0 (C3);
56.0 (–OMe)
5b 107.3144.5 5.9 13
6b 107.8
.73a1081.351341.8
8a 159.0
144.8
136.2
14.4 or 12.0 66.7 117.7 (C
73.3 155.6 (C1); 122.8 (C2); 116.7 (C6); 121.3(C 4); 132.4 (C5); 129.5 (C3)
2), 111.6 (C6); 122.7 (C4); 131.8 (C5); 130.1 (C3)
114.0
108.3
108.6
147.8
143.9
140.6
20.0 or 17.8 72.4 159.6 (C1); 125.6 (C2); 121.8 (C6); 128.1 (C4); 137.3 (C5); 135.8 (C3)
13.9 11.6 73.6 43.0 (Cipso); 26.5 (Cp); 29.0, 25.4 (Co, Cm)
9a 152.9
10a 153.7
14.4 12.2 69.2 69.3 (Cp); 68.3(C p2,2 ); 69.5 (Cp3,3
0
0
)
Pyridine or pyrimidine
C2
C3
C5
C4
C6
Other groups
11b 144.5
12b 145.8
13b 133.3c
111.3
111.6
112.3c
131.3
132.2
158.9c
150.3 113.4
155.3
124.8
121.7
150.0
162.3
162.0
143.2
159.3
122.5c
146.1c
JC-Pt ¼ 12.6 JC-Pt ¼ 22.4 JC-Pt ¼ 40.3
JC-Pt ¼ 22.8
JC-Pt ¼ 65.9
14b 145.8
112.0
122.1
122.2
162.5
159.9
100.3[m C 6HF4(CH)]
a
b
Chloroform-d. Acetone-d6. With platinum satellites.
c
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307

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Table 3 19F NMR data for complexes 5–14. Coupling constants are given in Hz. br s ¼ broad singlet, pd ¼ pseudodoublet, m ¼ multiplet,
Fm ¼ Fmeta , Fo ¼ Fortho , Fp ¼ Fpara
Fo
Fm
Fp
5b
6a
À111.13pd JFo-Fm ¼ 27.5
À111.56 pd JFo-Fm ¼ 30.5
À103.89 pd JFo-Fm ¼ 27.5
À104.44 pd JFo-Fm ¼ 27.5
À115 br s
À161.27 m À161.71 m
À154.16 m À154.58 m
À159.30 t JFp-Fm ¼ 19.8
À152.18 t JFp-Fm ¼ 19.8
7a
8a
À165.90 m
À163.09 t JFm-Fp ¼ 21.4
À115 br s
À165.96 m
À163.19 t JFm-Fp ¼ 19.8
9a
À117.06 m À119.18 m
À113.38 m
À167.93m
À165.41 pt JFm-Fp ¼ 19.8
10a
11a
12a
13b
À165.70 m
À162.88 t JFm-Fp ¼ 21.4
À117.20 m À117.41 m
À117.26 m À117.59 m
À120.69d m (4F)cJPt-F ¼ 457.8
À163.40 m À164.22 m
À163.40 m À164.08 m
À167.22 m À167.65 m
À160.79 t À160.18 t JFm-Fp ¼ 18.3
À160.27 t À160.83t JFm-Fp ¼ 21.4
À165.22d t JFm-Fp ¼ 21.3; JPt-F ¼ 18.2
À165.52d t JFm-Fp ¼ 21.3; JPt-F ¼ 18.2
F2
F3
F4
F6
14b
À111.63m (4F)
À170.90 m (2F)
À171.23m (2F)
À143.85 m (2F)
À92.05 m (4F)
À144.20 d ( J ¼ 21.3)
À144.25 d (J ¼ 18.3)
a
b
Chloroform-d. Acetone-d6 . Overlapped signals. With platinum satellites.
c
d
2-yl and show similar coalescence temperatures. The DGz
c
value is significantly higher (ca. 11.3kJ mol À1) for the complex
that contains the unmethylated ligand. The R groups that are
more remote from the coordination sphere seem to have only a
minor influence on the Pd–aryl rotation.
The effect on this dynamic process of factors such as con-
centration, solvent or addition of coordinating anions (ClÀ)
has also been studied. The results are the following: (i) the
previously reported11 DGz values and coalescence tempera-
Scheme 4
c
tures for complexes 15 and 16 in acetone-d6 fit within the
experimental error with those calculated herein in chloroform-
d; (ii) these parameters do not change with dilution of a
chloroform-d solution of 16, (iii) the addition of PPh4Cl to a
solution of 15 in chloroform-d does not modify the coalescence
19F NMR studies were carried out for 5 and 7 (chloroform-d)
and for 6, 9 and 10 (1,10,2,20-tetrachloroethane-d2). The cor-
responding values of the free energy of activation and the
respective coalescence temperatures obtained are collected in
Table 4 and represented in Fig. 1. Values for the previously
reported11 complexes Pd(C6F5)2(bpz*mR) (R ¼ phenyl, 15,
and pyridin-2-yl, 16; chloroform-d) have also been included for
the sake of comparison. Complexes bearing methylated pyr-
temperature or the DGz data.
c
When mechanistic considerations are taken into account,
rotation in the square planar complex25 or in three-coordinate
intermediates through dissociation of neutral35 or anionic
ligands23 has been proposed. In our case, the negligible effect
produced by a coordinating solvent or after the addition
of chloride allows us to exclude the formation of three- or
five-coordinate species. For positive effects of coordinating
solvents or anions in dynamic processes where three- or five-
coordinate intermediates are proposed see, for example, ref. 36
and 37. An intermolecular mechanism with the participation,
for example, of the coordinating atoms of the R group can also
be excluded because the process is not affected by dilution. We
propose that the rotation takes place in the four-coordinate
complexes. In principle, for rotation in a square planar geo-
metry one would expect a smaller barrier with the ligands
containing unmethylated pyrazole because these should
impose less steric hindrance and the pentafluorophenyl groups
would be able to rotate more freely. However, this hypothesis
is not in agreement with our experimental data. Crystal-
lographic Pd–N distances are not very sensitive to the sub-
stituent in position 3of pyrazole when bpzm ligands are
coordinated to the same PdX2 fragment.14 However, bulky
substituents in this position produce a significant distortion in
the boat metallacycle, reducing the PdNN/NNNN dihedral
angle and also the bite angle of the N-donor ligand14,38 (see
Scheme 5). This distortion pushes away the X ligands from the
aforementioned bulky substituent and opens the X–Pd–N
angle. In our case, this distortion could facilitate the penta-
fluorophenyl rotation. In any case, theoretical studies could
azolyl groups (bpz*mR) have DGz values that approximately
c
fit a straight line, giving an indication of a similar energy
barrier with a negligible influence of the R group. Unexpect-
edly the DGz data for the two complexes with unmethylated
c
pyrazole rings (5 and 6) are significantly higher. The effect of
the substituents in the pyrazole ring is more evident upon
comparing complexes 5 and 7, both of which have R ¼ anisol-
Table 4 Coalescence temperatures and free activation energies for
the Fortho–Fortho exchange (Pd–C6F5 rotation) in Pd(C6F5)2(bpz0mR)
complexes from the variable temperature 19F NMR spectraa
Tcb=K
DGz c=kJ molÀ1
c
5d
301
327
298
349
27352.1
28353
311
65.2
67.3
53.9
64.7
6e
7d
9e
10e
15d, f .9
16d, f
57.1
a
b
See Fig. 1 for representation. Coalescence temperature. Free
c
activation energy at the coalescence temperature calculated by
DGz ¼ aT [9.972 þ log(Tc=dn)] with a ¼ 1.914 Â 10À2
d
.
Chloroform-d.
c
e
f
1,10,2,20-tetrachloroethane-d2 . Ref. 11.
308
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Fig. 1 Linear plot of DGz (kJ molÀ1) versus Tc for the Fortho–Fortho
c
exchange in Pd(C6F5)2(bpz0mR) complexes (see Table 4 for values).
Numbers near the symbols refer to the corresponding complexes: (r)
&
complexes bearing the methylated bpz*mR ligands; ( ) complexes
bearing non-methylated bpzmR ligands.
Scheme 6 Schematic view of the possible atropisomers for complex
14 considering an averaged perpendicular orientation of the C6HF4
groups.
help in understanding this unexpected effect of the pyrazole
substituents.
temperature for the pentafluorophenyl complexes with pzpy
(11) or pzpm (12). We propose that in these complexes with
planar ligands the high difference in steric hindrance between
perpendicular or parallel disposition of the C6HnF5Àn groups
with respect to the coordination plane preclude the Pd–aryl
rotation.
19F NMR data for the complexes with planar ligands 11–13
show, as expected, two types of pentafluorophenyl ring (two
signals for Fpara). Two signals each were also observed for the
Fortho (isochronous for 13) and Fmeta atoms. This does not
constitute evidence of free rotation of the pentafluorophenyl
groups because the coordination plane is a mirror plane. Due
to this uncertainty we decided to synthesize a complex con-
taining 2,3,4,6-C6HF4 rings that consisted of two different
halves: Pd(2,3,4,6-C6HF4)2(pzpm) (14). Free rotation of the
polyfluorophenyl rings in this complex will give rise to two
different resonances for each type of fluorine atom. However,
in the case of restricted rotation one would expect two different
atropisomers19,21,35 (see Scheme 6), depending on the relative
orientation of the hydrogen atoms situated in the 5-position.
In this case, each type of fluorine atom will give rise to four
resonances. In the 19F NMR spectrum of 14 at room tem-
perature in 1,10,2,20-tetrachloroethane-d2 , the resonances due
to the F4 atoms were the most informative. Four signals in a 1 :
1 : 1 : 1 ratio were observed for this fluorine. Such a pattern
indicates that the two atropisomers, equally populated, do not
interconvert at room temperature and, consequently, that the
rotation of the C6HF4 rings is restricted. At 130 ꢀC an identical
pattern of four different F4 signals was observed, which indi-
cates that both polyfluorophenyl rotation and cis-trans
isomerization are processes having very high energy barriers
in 14. Considering the result obtained with complex 14,
we believe that a restricted rotation also exists at room
X-Ray structure of 10
Crystals of 10 belong to the monoclinic space group P21/n. An
ORTEP plot of the molecule is shown in Fig. 2. The crystal-
lographic data and a list of selected bond distances and angles
are given in Tables 5 and 6, respectively. The geometry around
the Pd(1) atom is approximately square planar with two
coordinated pyrazole rings and two C6F5 groups. The bite
angle N(22)–Pd(1)–N(12) [84.22(13)ꢀ] and that formed by the
two C6F5 groups, C(61)–Pd(1)–C(51) [88.37(17)ꢀ], are smaller
than 90ꢀ. More open angles C(51)–Pd(1)–N(22) [92.26(15)ꢀ]
and C(61)–Pd(1)–N(12) [95.13(15)ꢀ] complete the square plane
geometry. The bite angle is intermediate between those found
in unsubstituted pyrazole or pyrazole substituted with bulky
groups in position 3.14 This fact may be in relation with the
stated fluxional behaviour of these types of compounds. The
respective Pd(1)–N and Pd(1)–C distances are comparable to
those found in other bis(pyrazolyl)methane11,14,15,39 and
polyfluorophenyl Pd(II) complexes.19,23,25 The coordination of
the bpz*mFec ligand gives rise to a boat-like metallacycle,
Scheme 5 (a) Distortion in the boat metallacycle due to the steric
hindrance of the substituent in position 3of the pyrazole ring (R). (b)
The distortion indicated in (a) reduces the NPdN angle and conse-
quently also the steric hindrance of the pyrazole N-donor atom with
the C6F5 groups.
Fig. 2 ORTEP view with atomic numbering of 10 (30% probability
ellipsoids).
New J. Chem., 2002, 26, 305–312
309

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Table 5 Crystal data and structure refinement for 10
pzpm) and contrast with the fluxional behaviour found in
complexes with the non-planar ligands.
Empirical formula
Formula weight
Temperature=K
Wavelength=A
Crystal system
C33H24F10FeN4Pd
828.81
293(2)
0.71070
Monoclinic
P21=n
12.8040(10)
16.86(3)
15.137(2)
105.41(4)
3150(6)
+
Experimental
Space group
General comments
+
a=A
+
b=A
+
All manipulations were carried out under an atmosphere of
dry, oxygen-free nitrogen using standard Schlenk techniques.
Solvents were distilled from the appropriate drying agents and
degassed before use. Elemental analyses were performed with a
Thermo Quest FlashEA 1112 microanalyzer. IR spectra were
recorded as KBr pellets or Nujol mulls with a Perkin–Elmer
PE 883IR spectrometer. 1H, 13C{1H} and 19F NMR spectra
were recorded on a Varian Unity 300 spectrometer. Chemical
shifts (ppm) are relative to TMS (1H, 13C NMR) and CFCl3
(19F). COSY spectra: standard pulse sequence, acquisition time
0.214 s, pulse width 10 ms, relaxation delay 1 s, 16 scans, 512
increments. The NOE difference spectra were recorded with
5000 Hz, acquisition time 3.27 s, pulse width 90ꢀ, relaxation
delay 4 s, irradiation power 5–10 dB. For variable temperature
spectra, the probe temperature ( 1 K) was controlled by a
standard unit calibrated with a methanol reference. Free
energies of activation were calculated40 from the coalescence
temperature (Tc) and the frequency difference between the
coalescing signals (extrapolated at the coalescence tempera-
c=A
b=ꢀ
+
Volume=A3
Z
4
11.22
7576
7576
Abs. coef.=cmÀ1
Total reflect.
Indep. reflect.
Rint
Final R indices [I > 2s(I)]
0.0328
R1 ¼ 0.0380
wR2 ¼ 0.0720
P
R1 ¼ kFoj À jFok =
P
P
P
jFoj; wR2 ¼ [ [w(Fo À Fc2)2] = [w(Fo2)2]]0.5
.
2
+
Table 6 Selected bond lengths (A) and angles (deg) for 10
Pd(1)–N(12)
Pd(1)–N(22)
Pd(1)–C(51)
Pd(1)–C(61)
C(1)–N(11)
C(1)–N(21)
C(1)–C(31)
2.131(5)
2.094(3)
2.025(6)
2.018(4)
1.452(5)
1.465(5)
1.506(6)
C(61)–Pd(1)–C(51)
C(61)–Pd(1)–N(22)
C(51)–Pd(1)–N(22)
C(61)–Pd(1)–N(12)
C(51)–Pd(1)–N(12)
N(22)–Pd(1)–N(12)
N(11)–C(1)–N(21)
N(11)–C(1)–C(31)
N(21)–C(1)–C(31)
88.37(17)
179.11(16)
92.26(15)
95.13(15)
176.15(15)
84.22(13)
110.3(3)
ture) (dg) with the formula DGz ¼ aT [9.972 þ log(T/dn)] with
c
a ¼ 1.914 Â 10À2. The estimated error in the calculated free
energies of activation is 1.0–1.1 kJ molÀ1. Pd(C6F5)2(cod),
Pt(C6F5)2(THF)2 and Pd(2,3,4,6-C6HF4) were prepared
similarly to other polyfluorophenyl complexes.41 Bis(pyrazol-
1-yl)ketone and bis(3,5-dimethylpyrazol-1-yl)ketone were
prepared according with reported methods.29 Equally,
bpzmArOH and bpz*mArOH were prepared according to the
literature.28 The aldehydes were purchased from Aldrich.
113.7(4)
113.3(4)
Pd(1)–N(12)–N(11)–C(1)–N(21)–N(22), with the ferrocenyl
group attached to the carbon atom in an axial position. This is
in agreement with the data obtained for the complex in solu-
tion. The ferrocenyl unit adopts a orientation that minimizes
the possible steric repulsions with the Pd(C6F5)2 fragment,
having the unsubstituted Cp group pointing away from the
pentafluorophenyl groups.
Synthesis of the new derivatives
bpz*mArOMe, 1. A mixture of bis(pyrazol-1-yl)ketone
(500 mg, 3.08 mmol) and 420 mg (3.08 mmol) of ortho-
anisaldehyde and CoCl2 (ca. 14 mg) as catalyst is heated at
60 ꢀC for 7 h. The product is extracted with 20 mL of CH2Cl2 .
The solution is washed with water to eliminate the CoCl2 and
the solution is dried with anhydrous MgSO4 . The solution is
evaporated until dryness and the white solid of 1 is recrys-
tallized from diethyl ether–pentane (1 : 10). Yield: 57%. Anal.
calcd. for C14H14N4O (254.30): C, 66.13; H, 5.55; N, 22.03;
found: C, 65.83; H, 5.38; N, 21.60.
Conclusions
Square planar complexes of Pd(II) and Pt(II) have been pre-
pared through the coordination of planar and non-planar
chelate N-donor ligands to Pd(C6F5)2 , Pd(2,3,4,6-C6HF4)2 or
Pt(C6F5)2 fragments. For the complexes with a bis(pyrazol-1-
yl)methane (bpz*mR) backbone only one isomer is formed
with the R groups in an axial disposition and no boat-to-boat
inversion has been found. Different energetic barriers have
been measured on the NMR time scale for the Pd–poly-
fluorophenyl rotation. For the complexes with non-planar
ligands, the process is not affected by the presence of a co-
ordinating solvent or anion (ClÀ) or a change in the complex
concentration. These facts exclude the participation of three-
or five-coordinate intermediates and we propose that the
rotation occurs via four-coordinate species. Unexpectedly, the
presence of Me substituents in the position adjacent to the N-
donor atoms facilitates the rotation. The presence of more
remote groups, even those with coordinating ability, seems to
have less of an effect on the Pd–polyfluorophenyl rotation. The
rigidity observed, even at high temperature, for complex
Pd(2,3,4,6-C6HF4)2(pzpm), both in 1H and 19F NMR indicates
that Pd–N bond rupture, cis-trans isomerization and Pd–
polyfluorophenyl rotation are high-energy processes, falling
outside of the NMR time scale. This could be extrapolated to
the other Pd complexes containing planar ligands (pzpy and
bpz*mArOMe, 2. The method is similar to that used for
derivative 1. Amounts are as follows: 500 mg (2.29 mmol) of
bis(3,5-dimethylpyrazol-1-yl)ketone and 312 mg (2.29 mmol)
of ortho-anisaldehyde. A white solid is obtained. Yield: 60%.
Anal. calcd. for C18H22N4O (310.37): C, 69.65; H, 7.14; N,
18.05; found: C, 69.23; H, 6.93; N, 17.54.
bpz*mCy, 3. Bis(3,5-dimethylpyrazol-1-yl)ketone (1.00 g,
4.58 mmol) and 981 mg (4.58 mmol) of ferrocenecarbox-
aldehyde are heated under reflux in 25 mL of toluene during
5 h. The solution is evaporated until dryness and the residue is
recrystallized from diethyl ether–pentane (1 : 10). Brown-red
crystals of 3 are obtained. Yield: 78%. Anal. calcd. for
C17H26N4 (286.41): C, 71.29; H, 9.15; N, 19.56; found: C,
71.17; H, 9.08; N, 19.72.
bpz*mFec, 4. The method is similar to that used for 3.
Amounts are as follows: 500 mg (2.29 mmol) of bis(3,5-
dimethylpyrazol-1-yl)ketone and 0.276 mL (2.29 mmol) of
310
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cyclohexanecarboxaldehyde. The oily residue obtained after
the solvent evaporation becomes a white solid after 3days.
Yield: 60%. Anal. calcd. for C21H24N4Fe (388.29): C, 64.96;
H, 6.23; N, 14.43; found: C, 65.02; H, 6.42; N, 14.21.
solid is obtained. Yield: 85%. Anal. calcd. for C19H6F10N4Pd
(586.69): C, 40.38; H, 1.77; N, 8.97; found: C, 40.05; H, 1.37;
N, 8.77. IR (Nujol, cmÀ1): 786, 774 (C6F5).
Pt(C6F5)2(pzpm), 13. Pt(C6F5)2(THF)2 (60 mg, 0.09 mmol)
and 13mg (0.09 mmol) of pzpm are dissolved in 10 mL of
CH2Cl2 and the solution is stirred for 2 h at room temperature.
The solution is evaporated to dryness and a white solid of 13 is
obtained. Yield: 83%. Anal. calcd. for C19H6F10N4Pt (674.37):
C, 33.79; H, 0.89; N, 8.30; found: C, 34.06; H, 1.02; N, 8.10. IR
(Nujol, cmÀ1): 800, 772 (C6F5).
Pd(C6F5)2(bpzmArOMe), 5. Pd(C6F5)2(cod) (100 mg, 0.18
mmol) is dissolved in 20 mL of acetone and 46 mg (0.18 mmol)
of bpzmArOMe are added. The solution is stirred at room
temperature for 2 h. After evaporating the solvent to dryness
and washing with pentane a white solid is obtained. White
crystals of 5 are obtained after recrystallization with 1,2-
dichloroethane and pentane (1 : 10). Yield: 49%. Anal. calcd.
for C26H14F10N4OPdꢁCH2Cl2 (694.82): C, 41.59; H, 2.07; N,
7.18; found: C, 41.03; H, 2.36; N, 6.94. IR (KBr, cmÀ1): 785
(C6F5).
Pd(C6HF4)2(pzpm), 14. Pd(C6HF4)2(cod) (100 mg, 0.19
mmol) and 28.6 mg (0.19 mmol) of pzpm are dissolved in 10
mL of tetrahydrofurane. The solution is stirred for 48 h. The
solution is evaporated to dryness, the residue is washed with
pentane and a white solid of 14 is obtained. Yield: 65%. Anal.
calcd. for C19H8F8N4Pd (550.71): C, 39.25; H, 1.46; N, 10.17;
found: C, 38.88; H, 1.50; N, 9.94. IR (Nujol, cmÀ1): 826, 782
(C6F5).
Pd(C6F5)2(bpzmArOH), 6. The method is similar to that
used for complex 5. Amounts are as follows: 100 mg (0.18
mmol) of Pd(C6F5)2(cod) and 43mg (0.18 mmol) of
bpzmArOH. 6 is obtained as a white solid. Yield: 66% . Anal.
calcd. for C25H12F10N4OPdꢁ0.5Et2O (730.78): C, 45.17;
H, 2.38; N, 7.80; found: C, 45.68; H, 2.29; N, 8.09. IR (KBr,
cmÀ1): 795 (C6F5).
Structure determination of 10
Suitable crystals of 10 were grown from toluene–pentane (1 : 10).
A white crystal of approximate dimensions 0.4 Â 0.3 Â 0.2 mm3
was mounted in a glass capillary. Intensity data were collected
Pd(C6F5)2(bpz*mArOMe), 7. The method is similar to that
used for complex 5. Amounts are as follows: 40 mg (0.07
mmol) of Pd(C6F5)2(cod) and 20 mg (0.07 mmol) of
bpz*mArOMe. White crystals of 7 are obtained. Yield: 53%.
Anal. calcd. for C30H22F10N4OPdꢁ0.5C2H4Cl2 (800.41): C,
46.52; H, 3.02; N, 6.99; found: C, 46.23; H, 2.55; N, 7.57. IR
(KBr, cmÀ1):790 (C6F5).
at 298 K on a Nonius-Mach3diffractometer equipped with
+
graphite monochromated Mo-Ka radiation (l ¼ 0.71070 A)
using the o/2y scan technique to a maximum value of 56ꢀ. The
intensities of two representative reflections measured every
hour did not change significantly during the course of the data
collection. The crystal is monoclinic, of space group P21/n, with
one molecule of 10 per asymmetric unit. Data were corrected in
the usual fashion for Lorentz and polarization effects and
empirical absorption correction was not necessary (m ¼ 11.22
cmÀ1). The structure was solved using direct methods
(SIR92).42 Refinement on F2 was carried out by full-matrix
least-squares techniques (SHELXL97).43 All non-hydrogen
atoms were refined with anisotropic thermal parameters. The
hydrogen atoms were included in calculated positions and were
refined isotropically.
Pd(C6F5)2(bpz*mArOH), 8. The method is similar to that
used for complex 5. Amounts are as follows: 60 mg (0.11
mmol) of Pd(C6F5)2(cod) and 30 mg (0.12 mmol) of
bpz*mArOH. A white solid of 8 is obtained. Yield: 78%. Anal.
calcd. for C29H20F10N4OPd (736.90): C, 47.27; H, 2.74; N,
7.60; found: C, 47.10; H, 2.97; N, 7.15. IR (KBr, cmÀ1): 780
(C6F5).
Pd(C6F5)2(bpz*mCy), 9. The method is similar to that used
for complex 5. Amounts are as follows: 52 mg (0.18 mmol) of 3
and 100 mg (0.18 mmol) of Pd(C6F5)2(cod) in 10 mL of acet-
one. After evaporating the solvent to dryness the residue is
recrystallized from 1,2-dichloroethane–pentane (1 : 10). White
crystals of 9 are obtained. Yield: 86%. Anal. calcd. for
C29H26F10N4Pd (726.94): C, 47.92; H, 3.61; N, 7.61; found: C,
48.01; H, 3.82; N, 7.63. IR (KBr, cmÀ1): 781 (C6F5).
CCDC reference number 168151. See http:==www.rsc.org=
suppdata=nj=b1=b108139j= for crystallographic data in CIF or
other electronic format.
Acknowledgements
This work was kindly supported by the Direccion General
de Ensenanza Superior e Investigaciones Cientıficas, Spain
(grant N. PB98-0315). We thank Prof. Espinet for helpful
discussions.
Pd(C6F5)2(bpz*mFec), 10. The method is similar to that
used for complex 9. Amounts are as follows: 80 mg (0.21
mmol) of 4 and 113mg (0.21 mmol) of Pd(C F5)2(cod) in 15
6
mL of acetone. Orange crystals of 10 are obtained. Yield:
72%. Anal. calcd. for C33H24F10FeN4Pd (828.82): C, 47.82; H,
2.92; N, 6.76; found: C, 47.77; H, 2.91; N, 6.63. IR (KBr, cmÀ1):
781 (C6F5).
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