Synthesis and characterization of fluorophenylpalladium pincer complexes: Electronic properties of some pincer ligands evaluated by multinuclear NMR spectroscopy and electrochemical studies

Article abstract of DOI:10.1039/c1dt10446b

Palladium fluorophenyl complexes with different pincer ligands Pd(Ar)[2,6-(tBu₂PCH₂)₂C₆H ₃] (13), Pd(Ar)[2,6-(tBu₂PO)₂C ₆H₃] (14), Pd(Ar)[{2,5-(tBu₂PCH ₂)₂C₅H₂}Fe(C₅H ₅)] (15), and Pd(Ar)[{2,5-(tBu₂PCH₂) ₂C₅H₂}Ru(C₅H₅)] (16) were synthesized by the reaction of LiAr (Ar = C₆H₄F-4) with the respective trifluoroacetate palladium pincer complexes 9-12. The molecular structures of 14 and 16 were determined by an X-ray crystallographic method. Complexes 13-16 and {Pd(Ar)[{2,5-(tBu₂PCH₂) ₂C₅H₂}Fe(C₅H₅)]}PF ₆ (17) were studied by multinuclear NMR spectroscopy and cyclic voltammetry. On the basis of ¹⁹F NMR chemical shifts and ¹J(¹³C-¹⁹F) coupling constants, as well as Pd^II/Pd^IV oxidation potentials, electronic characteristics of the corresponding pincer ligands were elucidated.

Full text of DOI:10.1039/c1dt10446b

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PAPER
Synthesis and characterization of fluorophenylpalladium pincer complexes:
electronic properties of some pincer ligands evaluated by multinuclear NMR
spectroscopy and electrochemical studies†
Alexey V. Polukeev,a Sergey A. Kuklin,a Pavel V. Petrovskii,a Svetlana M. Peregudova,a
Alexander F. Smol’yakov,a Fedor M. Dolgushina and Avthandil A. Koridze*a,b
Received 16th March 2011, Accepted 27th April 2011
DOI: 10.1039/c1dt10446b
Palladium fluorophenyl complexes with different pincer ligands Pd(Ar)[2,6-(tBu2PCH2)2C6H3] (13),
Pd(Ar)[2,6-(tBu2PO)2C6H3] (14), Pd(Ar)[{2,5-(tBu2PCH2)2C5H2}Fe(C5H5)] (15), and
Pd(Ar)[{2,5-(tBu2PCH2)2C5H2}Ru(C5H5)] (16) were synthesized by the reaction of LiAr (Ar =
C6H4F-4) with the respective trifluoroacetate palladium pincer complexes 9–12. The molecular
structures of 14 and 16 were determined by an X-ray crystallographic method. Complexes 13–16 and
{Pd(Ar)[{2,5-(tBu2PCH2)2C5H2}Fe(C5H5)]}PF6 (17) were studied by multinuclear NMR spectroscopy
and cyclic voltammetry. On the basis of 19F NMR chemical shifts and 1J(13C–19F) coupling constants,
as well as PdII/PdIV oxidation potentials, electronic characteristics of the corresponding pincer ligands
were elucidated.
metric dehydrogenation of alkanes by transition metal complexes,
Introduction
Kaska, Jensen et al.,4 as well as Goldman et al.,5 showed that
Pincer complexes of transition metals have intensively been
iridium bis(phosphine) benzene-based pincer complexes 1 are
studied over the past several decades because of their ability to
effective catalysts for both hydrogen transfer and acceptorless
catalyze a wide range of chemical reactions.1 Especially impressive
results were achieved for iridium pincer complexes (Chart 1),
which catalyze selective alkane dehydrogenation, a challenging
transformation of saturated hydrocarbons and one of the most
significant tasks of organic chemistry and homogeneous catalysis.2
Following pioneer works of Crabtree3a–c and Felkin3d,e on stoichio-
alkane dehydrogenation. Later, Brookhart et al.6 established that
iridium bis(phosphinite) pincer complexes 2 are approximately
an order of magnitude more active than their bis(phosphine)
counterparts. Developed in our group7 iridium bis(phosphine)
pincer complexes with a metallocene backbone 3 and 4 have been
shown to be somewhat more active than type 2 complexes for
cyclooctane dehydrogenation in the presence of tert-butylethylene
as a hydrogen acceptor.
Taking into account not only the fundamental, but also
the practical importance of homogeneous catalytic alkane
dehydrogenation,2 considerable efforts have been undertaken to
investigate mechanistic details of catalytic alkane dehydrogenation
by iridium pincer complexes, using both experimental5d–g,6c,8a and
computational methods.5d–g,9 Apparently, the catalytic activity of
aA.N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, 28 Vavilov Street, 119991, Moscow, Russian
Federation. E-mail: koridze@ineos.ac.ru; Fax: (+)7(499)-135-50-85;
Tel: (+)7(499)-783-32-74
bInstitute of Organometallic Chemistry, I. Javakhishvili Tbilisi State Univer-
sity, 3 Chavchavadze Avenue, 0128, Tbilisi, Georgia; Tel: (+995 32)25-37-53
† CCDC reference numbers 817767 and 817768. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt10446b
Chart 1
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the systems considered should be affected by both a steric factor
(the accessibility of the catalytic center for a substrate), and an
electronic factor, i.e. the relative electron density at the iridium
atom. It is recognized that the steric accessibility of the iridium
atom is determined by the bulkiness of the organyl R groups of
the phosphorous donor atoms8 and by the value of the P–Ir–P
angle in the pincer complex.7 Evaluation of the electronic factor
appeared to be more complex. This ambiguity became apparent
during comparison of the electronic effects of the two benzene-
based ligands, bis(phosphine)8a and bis(phosphinite)6 pincers.
In spite of using experimental and computational methods, it is
still not clear how the electronic properties of the bis(phosphine)
pincer ligand change when methylene CH2 groups in it are
replaced with oxygen atoms. Thus, based on the measurement
of the carbonyl stretching frequencies nCO of the respective
CO adducts Ir(CO)[2,6-(tBu2PO)2-4-X-C6H2] (Ir(CO)[PO,C,OP-
X], 2), Brookhart and co-workers6b supposed that bis(phosphinite)
pincer ligands are significantly less electron-donating than their
bis(phosphine) counterparts. On the other hand, computational
results obtained by Goldman and co-workers8a showed that the
thermodynamic and kinetic properties are affected in the same
directions, relative to the parent P,C,P ligand, by the presence of
the p-methoxy substituent and by the substitution of the ligand
methylene groups bound to phosphorus with the oxygen atoms
(to form a PO,C,OP ligand) even though these substitutions affect
the nCO values in opposite directions. Calculations also showed
that the iridium atom in bis(phosphinite) complexes bears a com-
parable or even more negative charge than in the bis(phosphine)
counterparts and the higher nCO value of Ir(CO)[PO,C,OP] is
most likely attributable to electrostatic effects.8a,10
Since a deep understanding of ligand electronic characteristics
is necessary for rational catalyst design, it is desirable to clarify
the electronic properties of different pincer ligands by the use of
experimental methods other than nCO values measurement.
19F NMR chemical shifts in aryl fluorides have been found to
be a sensitive tool for the examination of substituent electronic
effects. These chemical shifts correlate with the resonance effects
of para substituents in para-disubstituted fluorobenzenes and with
the inductive effects of meta substituents in meta-disubstituted
fluorobenzenes.11 Applications of this method to organometallic
systems are exemplified by the measurements of 19F NMR chem-
ical shifts of meta- and para- fluorophenyl derivatives of nickel,
palladium and platinum phosphine complexes12 as well as transi-
tion and non-transition metal fluorophenylcyclopentadienyls.13
Herein, we report the synthesis of p-fluorophenylpalladium
pincer complexes for 19F NMR and electrochemical studies
of the electronic properties of different pincer ligands. The
complexes studied include bis(phosphine) and bis(phosphinite)
benzene-based and bis(phosphine) ferrocene- and ruthenocene-
based pincers. The results obtained are compared with the existing
literature data on electronic properties of the pointed ligands.
cording to the literature procedures.14–16 Novel complex 6 was ob-
tained by cyclopalladation of 1,3-(tBu2PO)2C6H4.6a It was found
that palladium chlorides PdCl[RP,C,P] are not good candidates
for nucleophilic substitution reaction,17 therefore to facilitate the
reaction with the appropriate lithium reagent they were converted
to the corresponding palladium trifluoroacetates (Scheme 1).
Thallous trifluoroacetate was used instead of CF3COOAg in the
case of the metallocene complexes to avoid metallocene oxidation
which is likely to occur at least with the ferrocene-based complex.
Scheme 1
Fluorophenyl derivatives of palladium pincer complexes
were synthesized by methods similar to those reported by
Milstein18 and Wendt19 for the preparation of Pd(Ph)[iPrP,C,P] and
Pd(Ph)[tBuP,C,P], respectively. Treatment of 9–12 with LiC6H4F-4
gives the corresponding fluorophenyl derivatives 13–16 in yields
from low to moderate (Scheme 2).
The reaction is complicated by formation of the appropriate
PdBr[P,C,P] complexes (LiC6H4F-4 is generated from BrC6H4F-4
and this results in the presence of bromine-containing species in
the reaction mixture) and decomposition of the lithium reagent,
which impedes the purification. Compounds 13–16 were fully
characterized by means of multinuclear NMR spectroscopy and
elemental analysis; complexes 14 and 16 were also studied by
the X-ray diffraction method. Typical 1H NMR spectroscopic
features of 13 and 14 show a virtual triplet and overlapping
doublet of doublets for hydrogens meta and ortho with respect
to the fluorine atom in the case of benzene-based complexes; for
metallocene-based complexes all hydrogens within the Pd–C6H4F-
4 fragment became non-equivalent and appear as virtual triplets
1
or multiplets. The 31P{ H} and 19F NMR spectra of compounds
13–16 exhibit singlets; ipso carbon resonance of the Pd-C6H4F-
4 fragment undergoes splitting on both fluorine and phosphorus
1
nuclei thus appearing as triplet of doublets in the 13C{ H} NMR
spectra.
The molecular structures of 14 and 16 are illustrated in Fig.
1 and Fig. 2. In both compounds, the palladium atom has a
distorted square-planar geometry with the fluorobenzene ring
being virtually perpendicular to the plane of the benzene (94.8◦)
or the cyclopentadienyl (90.8◦) ring of the pincer backbones. The
Results and discussion
Synthesis and characterization of p-fluorophenyl derivatives of
palladium pincer complexes
˚
Initial palladium chloride complexes with benzene and metal-
locene backbones, compounds 5, 7, and 8, were synthesized ac-
Pd–C(1) distance in 16 (2.006(2) A) is slightly shorter then in
˚
14 (2.031(2) A); similar differences in the Ir–C(1) lengths were
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Scheme 2
Fig. 1 Molecular structure of 14 (ellipsoids at 50% probability level;
˚
hydrogen atoms are omitted for clarity). Selected bond lengths [A] and an-
gles [◦]: Pd(1)–C(1) 2.031(2), Pd(1)–C(11) 2.078(2), Pd(1)–P(1) 2.2794(6),
Pd(1)–P(2) 2.2835(6), C(1)–Pd(1)–C(11) 174.23(9), C(1)–Pd(1)–P(1)
79.79(7), C(1)–Pd(1)–P(2) 79.64(7), P(1)–Pd(1)–P(2) 158.63(2).
Fig. 2 Molecular structure of 16 (ellipsoids at 50% probability level;
˚
hydrogen atoms are omitted for clarity). Selected bond lengths [A] and an-
gles [◦]: Pd(1)–C(1) 2.006(2), Pd(1)–C(11) 2.104(2), Pd(1)–P(1) 2.3247(5),
Pd(1)–P(2) 2.3253(5), C(1)–Pd(1)–C(11) 171.41(7), C(1)–Pd(1)–P(1)
80.11(5), C(1)–Pd(1)–P(2) 80.16(5), P(1)–Pd(1)–P(2) 157.15(2).
reported for the iridium carbonyl derivatives of the corresponding
pincer complexes7 (2.025 A for Ir(CO)[tBuP,C,PFe] vs. 2.046 for
For the sake of comparison, we oxidized 15 using [Cp2Fe]PF6
to give the cationic compound 17 (Scheme 3). Although this
paramagnetic complex reveals broad signals in the region of
˚
Ir(CO)[tBuPO,C,OP]).
Contrary to Pd–C(1), the distance between Pd and the ipso
carbon atom of the fluorophenyl group for 16 (2.104(2) A) is
longer than for 14 (2.078(2) A) which may indicate a stronger
trans influence of the ruthenocene-based pincer ligand.
1
33 to -118 ppm in the 1H NMR spectrum, in 31P{ H} and
˚
19F spectra sufficiently sharp signals are observed at d 94.2 (s)
from two equivalent phosphorus atoms, and -152.9 ppm (septet,
˚
Scheme 3
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Table 1 Carbonyl nCO stretching frequencies for pincer complexes
nCO (cm-1)
Chemical shifts were measured against an internal fluoroben-
zene standard in two solvents, C6D6 and CD2Cl2. 19F NMR
chemical shifts in Table 2 indicate an upfield resonance with
respect to C6H5F. Also, the values of the spin–spin coupling
constants 1J(13C–19F) are given in Table 2. It is known21 that
a satisfactory correlation exists between 19F NMR chemical
shifts and spin–spin coupling constants 1J(13C–19F), with coupling
constant decreasing while shielding of the fluorine nucleus rises.
This inverse dependence of fluorine nuclei shielding from 1J (13C–
19F) values was also demonstrated on several organometallic
derivatives of p-fluorophenylcyclopentadienyl compounds.22 As
can be seen from Table 2, this correlation exists for p-fluorophenyl
derivatives of the palladium pincer complexes studied.
Compound
Hydrocarbon solution CH2Cl2 solution
Ir(CO)[tBuP,C,PFe]
1926a
1926a
1904d
1905d
1951d
1913e
1938e
Ir(CO)[tBuP,C,PRu
]
{Ir(CO)[tBuP,C,PFe]}PF6
Ir(CO)[2,6-(tBu2PCH2)2C6H3] 1928b
Ir(CO)[2,6-(tBu2PO)2C6H3]
1949c
a Hexane solution, ref. 7. b Cyclooctane solution, ref. 5c. It should be noted
that in the very recent article, ref. 20, the value of the 1914 cm-1 is given for
a hexane solution of this complex. Such a marked difference between two
measurements in saturated hydrocarbon solvents is unexpected and needs
further investigation. c Pentane solution, ref. 6b. d Ref. 7. e Present work.
It is evident from the data presented in Table 2 that there is ap-
parent difference in 19F nuclei shielding between the benzene-based
bis(phosphinite) complex 14 and the bis(phosphine) complex 13
while changes in fluorine chemical shifts among bis(phosphine)
complexes 13, 15 and 16 are less pronounced. According to these
data, the lowest relative electron density should belong to the
palladium atom in the bis(phoshinite) complex 14. If we assume
that 19F NMR chemical shifts correctly reflect the relative electron
density at a metal atom in different type pincer complexes, it is
interesting to compare this data with nCO values given in Table 1
(see Fig. 3).
J = 711.4 Hz) from a PF6 anion, respectively. Previously it was
found that {PdCl[tBuP,C,PFe]}PF6 reveals a singlet at d 93.9 ppm
1
in the 31P{ H} NMR spectrum.15
Comparison of the 19F NMR chemical shifts. Electronic properties
of the pincer ligands
The existing approach for evaluation of electronic properties of
different pincer ligands is based on the measurement of the nCO
stretching frequencies of the respective iridium CO adducts. The
literature and our data is shown in Table 1.
This approach was criticised on the basis of the results obtained
by the computational methods as exemplified by comparison of
the bis(phosphine) and bis(phosphinite) benzene-based pincer
complexes.8a The calculations showed that the iridium atom in
bis(phosphinite) complexes bears a comparable or even more
negative charge than that in bis(phosphine) analogues and the
higher nCO value for Ir(CO)[PO,C,OP] is most likely attributable
to electrostatic effects.8a,10 In fact, substitution of the ligand
methylene groups bound to phosphorus with the oxygen atoms (to
form PO,C,OP) in the parent P,C,P ligand has two opposite effects:
oxygen atoms withdraw electron density from the phosphorous
atoms due to an inductive effect and donate electron density to the
benzene ring by resonance effect. As such, it is difficult to conclude
which effect will predominate in dictating electron density at the
metal atom.
Fig. 3 Linear correlation, with 17 excluded, of the nCO frequencies and
It is believed that 19F NMR chemical shifts of aryl fluorides
correlate with electronic properties of substituents in the benzene
ring. The lower the frequency of the 19F chemical shift is, the
19F NMR chemical shifts.
A linear least squares solution through the points corresponding
to 13–16 gives nCO = 2177.99 - 20.89*d with a correlation
coefficient of 0.999. Apparently, complex 17 deviates strongly
from this line. This fact should be preferably attributed to the
paramagnetic nature of the compound 17. In this case, it is likely
that a contribution from the paramagnetic center affects the 19F
shielding in spite of it being remote from the Fe atom, and the
19F NMR chemical shift fails to correctly reflect the electron
density at the Pd center. Indeed, both nCO and PdII/PdIV oxidation
potentials (see below) show a dramatic decrease in the donating
ability of the pincer ligand, while according to the 19F NMR
chemical shifts this changes only slightly and the Pd atom in 17
is almost as electron-rich as in the neutral complex 15, which is
pretty unrealistic.
more electron density substituent donates to the benzene ring. 19
F
chemical shifts of complexes 13–17 are listed in Table 2.
Table 2 19F NMR parameters for complexes 13–17
19F NMR chemical shiftsa (ppm)/
1J (13C–19F) coupling constants (Hz)
Compound (Ar = C6H4F-p)
C6D6 solution
CD2Cl2 solution
PdAr[tBuP,C,P] (13)
11.60
10.47
11.98
11.90
—
12.67/236.6
11.49/237.1
13.11/235.7
13.08
PdAr[tBuPO,C,OP] (14)
PdAr[tBuP,C,PFe ] (15)
PdAr[tBuP,C,PRu ] (16)
{PdAr[tBuP,C,PFe ]}PF6 (17)
12.95
Thus, it appears that the 19F NMR and nCO probes produce
comparable information for this series of compounds. Note that
a Upfield with respect to fluorobenzene.
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Table 3 PdII/PdIV oxidation potentials as measured by cyclic
the p-orbital belonging to the C-ipso of the C6H4F-4 unit is
virtually orthogonal to that of the benzene/Cp rings and they
cannot overlap with the same orbital of the Pd atom. Hence, p-
effects caused by the pincer ligand are transmitted to the C6H4F-
4 more indirectly and probably can be underestimated, while
in the case of the respective Ir–CO adducts direct conjugation
between the C-ipso and CO orbitals through the Ir atom is
possible. However, taking into consideration the sufficiently good
correlation mentioned above, it may be suggested that the p-effects
of substituents in pincer ligands do not play the major role in
dictating electron density at the metal atom. In fact, we suppose
that, although 19F NMR and nCO probes measure quite different
things, for the compounds presented they both mainly reflect
the ligand electronic effects being transmitted to the metal atom
through inductive/field mechanisms with resonance effects being
insufficient. That’s why the existence of some correlation between
19F chemical shifts and nCOs is possible, however it is dangerous to
ascribe much significance to such dependence. In particular, for the
PO,C,OP vs. P,C,P pair resonance effect of the oxygen atoms is not
competitive with their s electron withdrawing ability. For example,
replacement of a hydrogen in a para position with the methoxy
group in Ir(CO)[tBuP,C,P] and Ir(CO)[tBuPO,C,OP] only leads to a
2 cm-1 red shift in comparison with parent compounds (1947 cm-1
for Ir(CO)[PO,C,OP-4-OMe]6b and 1926 cm-1 for Ir(CO)[tBuP,C,P-
4-OMe]8a) due to the resonance effect of the –OMe group. At the
same time, the CO stretching frequency is much more sensitive
to inductive/field effects produced by substituents at P atoms:
substitution of the ligand methylene groups bound to phosphorus
with the oxygen atoms (to form PO,C,OP) leads to ca. 20 cm-1 blue
shift despite the resonance effect of the oxygens (about 4 cm-1 in
the opposite direction if one rather crudely assumes ortho and para
effects are comparable). Even more dramatic changes occur when
voltammetrya
Compound (Ar = C6H4F-p)
Eox (FeII/FeIII) (V)
Eox(PdII/PdIV) (V)
PdAr[tBuP,C,P] (13)
PdAr[tBuPO,C,OP] (14)
PdAr[tBuP,C,PFe] (15)
0.60
0.83
1.17
-0.10
a Conditions: glassy carbon electrode, scan rate 200 mV s-1, 0.1 M Bu4NPF6
in 9 : 1 CH3CN–CH2Cl2 mixture, Cp2Fe/Cp2Fe+ as internal reference.
the range obtained by van Koten et al. for related P,C,P, S,C,S and
N,C,N palladium pincer complexes.26 Note that for complex 15,
two redox processes were observed. The first redox process was
a reversible peak at -0.10 V which corresponds to one-electron
oxidation of the Fe atom and is close to that for PdCl[tBuP,C,PFe].15
The second redox process was observed at a more positive potential
of 1.17 V and corresponds to the irreversible PdII/PdIV oxidation.
In fact, we can consider this second process as an oxidation of the
in situ generated complex 17.
It is evident from Table 3 that electrochemical data sup-
ports the conclusions made from spectroscopic methods. Thus,
bis(phoshinite) complex 14 undergoes oxidation at a sufficiently
higher potential (DEox = 230 mV) than its bis(phosphine) counter-
part 13 which indicates that the Pd atom in 14 is less electron-rich.
Oxidation of the central atom of the metallocene makes a direct
comparison of complex 15 with 13 and 14 by electrochemistry
impossible; however, the strong electron-withdrawing effect of the
Fe-oxidized ligand in 17 could be observed. If we take Eox for 13 as
a rough measure for the hypothetical 15 Pd-based oxidation (nCO
and d 19F are rather close for these compounds), oxidation of the
ferrocene unit nearly doubles the Eox.
Remarkably, three experimental methods showed the same
trends in the estimation of the relative donor ability of the pincer
ligands. At the same time, the data obtained here support our
conclusion that the higher catalytic activity of complexes 2, 3, and
4 in alkane dehydrogenation versus that of complex 1 have mainly
steric reasons.7 Thus, compounds 2, 3, and 4 have a smaller P–Ir–P
angle than 1, which provides a higher accessibility of the Ir atom
for a substrate, while there is clear a similarity in the electronic
properties of the pincer ligands in complexes 1 and 3, 4. It should
be noted that recent DFT calculations gave further evidence to
the fact that the metal center of Ir(CO)[tBuPO,C,OP] is much less
sterically hindered than that of Ir(CO)[tBuP,C,P], which results in
improved catalytic activity.27
tBu groups are replaced with CF3. Thus, the nCO for the recently
23
published complex Ir(CO)[CF P,C,P] is 105 cm-1 greater than for
3
Ir(CO)[tBuP,C,P]. Such behavior can be explained by electrostatic
effects,8a,10 however we do think in these cases nCOs correctly
reflect electron density at the metal atom. Indeed, the presence of,
for example, a methoxy group obviously substantially changes the
nature of the p-system of an aryl ligand, but the extent to which this
effect is transmitted to the metal atom seems to be rather small.
For example, in terms of both trans influence and trans effect
phenyl and p-anisyl are quite similar.24 Besides, during a study
concerning metal-containing groups as substituents in benzene, a
number of m- and p-fluorophenyl transition metal complex sI and
sR constants were determined; in contrast to inductive parameters,
resonance parameters varied over a small range for rather different
complexes, and in related series of compounds (e.g., –Mn(CO)5 - x
,
Conclusions
Lx, or –Pt(PEt3)2X) they were almost invariant. It was concluded
that these minor changes in sR reflect the relative unimportance
of p-bonding to the aryl ring relative to other ligands.25
To summarize, we have prepared a series of new p-fluoro-
phenylpalladium complexes with four different pincer ligands.
The measurement of the 19F NMR parameters and PdII/PdIV
oxidation potentials allowed us to make some conclusions on
the electron density at the palladium atoms and the electronic
properties of the appropriate ligands, which correlate well with
previously reported data based on nCO stretching frequencies
of iridium carbonyl complexes with the same pincer ligands.
On the basis of 19F NMR parameters, PdII/PdIV oxidation
potentials of palladium p-fluorophenyl pincer complexes and nCO
frequencies of iridium carbonyl adducts, the donor ability of pincer
Cyclic voltammetry
In order to verify conclusions made from NMR spectroscopy
we undertook an electrochemical study of complexes 13, 14 and
15. Cyclic voltammograms were recorded in a CH3CN–CH2Cl2
9 : 1 mixture. For all pincer complexes an irreversible oxidation
wave was observed, which we attribute to PdII/PdIV oxidation.
Oxidation potentials are listed in Table 3 and in general fall into
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ligand follows the order: [tBuP,C,PFe+]-, [2,6-(tBu2PO)2C6H3]-, [2,6-
(tBu2PCH2)2C6H3]-, [tBuP,C,PRu ]- and [tBuP,C,PFe]-.
should be protected from light. The column was washed by CH2Cl2
and the resulting solution was concentrated in vacuum followed
by addition of hexane and cooling to -20 C. The solution was
◦
decanted and the solid was washed with hexane and dried under
vacuum.
Experimental
General considerations
Pd(TFA)[2,6-(tBu2PCH2)2C6H3] (9). Starting from 0.265 g
(1.199 mmol) of CF3COOAg and 0.585 g (1.094 mmol) of 5 0.535
g (80%) of 9 was obtained. NMR spectra and elemental analysis
are in agreement with literature data.19,28
All manipulations on the synthesis of pincer ligands and
complexes were conducted under an argon atmosphere us-
ing standard Schlenk techniques. Resulting compounds 5–17
are air-stable and their purification doesn’t require an inert
atmosphere. All solvents were distilled under an argon at-
mosphere from the appropriate drying agents. Commercially
available reagents were used as received. Compounds PdCl[2,6-
(tBu2PCH2)2C6H3] (5),14 PdCl[{2,5-(tBu2PCH2)2C5H2}Fe(C5H5)]
(7),15 and PdCl[{2,5-(tBu2PCH2)2C5H2}Ru(C5H5)] (8)16 were pre-
pared according to the literature procedures. NMR spectra were
recorded on Bruker Avance 300 and 400 MHz spectrometers.
Pd(TFA)[2,6-(tBu2PO)2C6H3] (10). Starting from 0.259 g
(1.172 mmol) of CF3COOAg and 0.602 g (1.118 mmol) of 6 0.467
g (68%) of 10 was obtained. NMR spectra and elemental analysis
are in agreement with literature data.29
Pd(TFA)[tBuP,C,PFe ] (11). Starting from 0.390 g (1.230 mmol)
of CF3COOTl and 0.450 g (0.700 mmol) of 7 0.409 g of 11 was
obtained. According to NMR spectra, the sample contains ~25%
of the starting chloride 7 and was not subjected to elemental
1
1H and 13C{ H} NMR chemical shifts are reported in parts per
1
analysis. H NMR (300.13 MHz, CDCl3): d 4.24 (s, 2H, C5H2),
million downfield from tetramethylsilane; the residual signals of
deuterated solvents were used as references (7.26 ppm for CDCl3,
3.96 (s, 5H, C5H5), 2.92 (t, 1JHH = 15.2 Hz, 2H, CHACHBP), 2.51
(dt, 1JHH = 16.7 Hz, 3JPH = 4.6 Hz, 2H, CHACHBP), 1.57 (vt, J =
7.16 ppm for C6D6, 5.32 ppm for CD2Cl2). In 13C{ H} NMR
1
1
7.1 Hz, 2 tBu) 1.29 (vt, J = 6.6 Hz, 2 tBu). 31P{ H} NMR (121.49
measurements the signal of CD2Cl2 (53.7 ppm) was used as a
1
MHz, CDCl3): d 85.4. 19F{ H} NMR (282.40 MHz, CDCl3): d
1
reference. 19F{ H} chemical shifts are reported relative to internal
-75.33.
fluorobenzene for compounds 13–17 and to external CFCl3 for
1
others. 31P{ H} NMR chemical shifts are reported relative to an
Pd(TFA)[tBuP,C,PRu] (12). Starting from 0.480 g (1.514 mmol)
of CF3COOTl and 0.333 g (0.484 mmol) of 8 0.223 g (60%) of
12 was obtained. 1H NMR (400.13 MHz, CDCl3): d 4.62 (s, 2H,
C5H2), 4.40 (s, 5H, C5H5), 2.71 (dt, 1JHH = 16.7 Hz, 3JPH = 3.1 Hz,
external 85% solution of phosphoric acid in D2O. Assignments of
1
signals in the H NMR spectra of 15 and 16 were made using
NOESY spectrum. FTIR spectra were recorded on a Nicolet
Magna-IR 750 Fourier spectrometer. Elemental analyses were
performed at the A.N. Nesmeyanov Institute of Organoelement
Compounds of RAS. Despite several attempts, satisfactory ele-
mental analysis for the ruthenium-containing complexes 15 and
16 was not obtained. The products, however, appeared to be
analytically pure, as indicated by NMR.
1
3
2H, CHACHBP), 2.58 (dt, JHH = 16.7 Hz, JPH = 4.4 Hz, 2H,
CHACHBP), 1.34 (two overlapping vt, J1 = 6.7 Hz, J2 = 7.3 Hz, 4
1
1
tBu) 31P{ H} NMR (161.98 MHz, CDCl3): d 84.4. 19F{ H} NMR
(282.40 MHz, CDCl3): d -75.39.
General procedure for the syntheses of palladium p-fluorophenyls
13–16. A solution of LiC6H4F-4 in Et2O was added dropwise to
a suspension of complexes 9–12 in Et2O; the reaction mixture was
brought to room temperature and stirred overnight. After addition
of water, the layers were separated and the aqueous layer was twice
extracted with Et2O. The combined organic layers were dried over
MgSO4 and the volatiles were evaporated in vacuum. The residue
was purified by column chromatography and recrystallisation.
Synthesis of PdCl[2,6-(tBu2PO)2C6H3] (6). To a suspension of
NaH (0.700 g, 17.500 mmol, 60% dispersed in mineral oil) in 60 ml
of THF was added solution of resorcinol (0.870 g, 7.909 mmol)
in 15 ml of THF. The mixture was heated to reflux for 1 h, then
tBu2PCl (3.5 ml, 18.440 mmol) was added and the mixture was
refluxed for an additional 1 h. The volatiles were removed in
vacuum, the residue was dissolved in 50 ml of 2-methoxyethanol
and after addition of Pd(PhCN)2Cl2 (2.950 g, 7.702 mmol) refluxed
for 3 h. The pre-cooled reaction mixture was filtered through Celite
and the filtrate was evaporated in vacuum. The residue was purified
using column chromatography on silica gel (eluent CH2Cl2–hexane
1 : 1) and crystallised from a CH2Cl2–hexane mixture. Yield: 1.307
g (31%). Found: C, 49.04; H 7.19. Calc. for C22H39ClO2P2Pd: C,
LiC6H4F-430. It was prepared according to the literature pro-
cedure from BrC6H4F-4 and used immediately after preparation.
To a stirred solution of butyllithium (3.0 ml; 1.6 M in hexane) in
◦
diethyl ether (10 ml) at -30 C was added dropwise 1-bromo-4-
fluorobenzene (0.50 ml, 4.551 mmol) in diethyl ether (10 ml). Then
the reaction mixture was stirred for 45 min and brought to room
temperature.
48.99; H, 7.29. 1H NMR (400.13 MHz, CDCl3): d 6.95 (tt, 3JHH
=
8.0 Hz, 5JPH = 1.1 Hz, 1H, Ar–H), 6.53 (d, 3JHH = 8.0 Hz, 2H, Ar–
Pd(C6H4F-p)[2,6-(tBu2PCH2)2C6H3] (13). The reaction was
conducted with 0.535 g (0.874 mmol) of 9 and LiAr prepared
from 0.50 ml (4.551 mmol) of BrC6H4F-4 as described above. After
column chromatography on silica gel using hexane–CH2Cl2 5 : 1 as
an eluent and crystallisation from hexane–CH2Cl2 0.103 g (20%)
of 13 was obtained as a white powder. An additional 0.014 g (3%)
of pure compound 13 was yielded by repetitive chromatography
of the residue from the fractions left after first chromatography
containing trace amounts of 13. Found: C, 60.09; H, 8.25. Calc. for
H), 1.43 (m, 36H, 4 tBu). 31P{ H} NMR (161.98 MHz, CDCl3): d
1
192.1.
General procedure for the syntheses of palladium trifluoroacetates
9–12. Excess of CF3COOAg or CF3COOTl was added to a
solution of 5–8 in the appropriate quantity of THF. The reaction
mixture was stirred for 3 h and then filtered through a short silica
gel column (alumina was used instead in the case of metallocene-
based compounds 11 and 12). Until this stage, the reaction mixture
7206 | Dalton Trans., 2011, 40, 7201–7209
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C30H47FP2Pd: C, 60.55; H, 7.96. 1H NMR (400.13 MHz, CD2Cl2):
d 7.64 (vt, J = 8.0 Hz, 2H, 8- and 12-H), 7.05 (d, JHH = 7.4 Hz,
(s, C5H2), 64.88 (vt, J = 7.9 Hz, 3- and 4-C), 36.92 (vt, J = 5.8 Hz, 2
C(CH3)3), 35.06 (vt, J = 6.6 Hz, 2 C(CH3)3), 30.25 (vt, J = 2.9 Hz, 2
C(CH3)3), 28.93 (vt, J = 3.4 Hz, 2 C(CH3)3), 28.82 (vt, J = 11 Hz, 2
3
2H, 3- and 5-H), 6.91 (t, 3JHH = 7.4 Hz, 1H, 4-H), 6.75 (dd, 3JHH
=
3
1
1
8.0 Hz, JHF = 10.1 Hz, 2H, 9- and 11-H), 3.47 (m, 4H, 2 CH2),
CH2). 31P{ H} NMR (121.49 MHz, C6D6): d 71.7. 31P{ H} NMR
1
1
1.19 (vt, J = 6.8 Hz, 36H, 4 tBu). 13C{ H} NMR (100.61 MHz,
(121.49 MHz, C6D6): d 86.2. 19F{ H} NMR (282.40 MHz, C6D6):
CD2Cl2): 173.59 (t, 3JCP = 3.3 Hz, 1-C), 160.35 (d, 1JCF = 236.6 Hz,
10-C), 158.04 (td, J = 4.0 Hz, J = 12.3 Hz, 7-C), 150.86 (vt, J =
9.7 Hz, 2- and 6-C), 142.75 (m, 8- and 12-C), 124.60 (s, 4-C),
120.64 (vt, J = 9.2 Hz, 3- and 5-C), 112.00 (d, 2JCF = 16.1 Hz, 9-
and 11-C), 38.96 (vt, J = 10.8 Hz, 2 CH2), 35.74 (vt, J = 7.3 Hz, 4
d 11.98.
1
C(CH3)3), 29.68 (vt, J = 2.9 Hz, C(CH3)3). 31P{ H} NMR (121.49
1
MHz, C6D6): d 71.7. 19F{ H} NMR (282.40 MHz, C6D6): d
11.60.
Pd(C6H4F-p)[tBuP,C,PRu] (16). The reaction was conducted
with 0.407 g (0.532 mmol) of 12 and LiAr prepared from 0.22 ml
(2.003 mmol) of BrC6H4F-4 as described above. After column
chromatography on alumina using hexane–Et2O 5 : 1 as an eluent
and crystallisation from hexane 0.060 g (15%) of 16 was obtained
1
as a yellowish powder. H NMR (300.13 MHz, CD2Cl2): d 7.69
Pd(C6H4F-p)[2,6-(tBu2PO)2C6H3] (14). The reaction was con-
ducted with 0.607 g (0.985 mmol) of 10 and LiAr prepared from
0.43 ml (3.914 mmol) of BrC6H4F-4 as described above. After
column chromatography on silica gel using hexane–CH2Cl2 3 : 1
as an eluent and crystallisation from hexane–CH2Cl2 0.250 g (42%)
of 14 was obtained as a white powder. Found: C, 56.18; H, 7.23.
(vt, J = 7.7 Hz, 1H, 7-H), 7.56 (vt, J = 7.5 Hz, 1H, 11-H), 6.80–6.65
(m, 2H, 8- and 10-H), 4.70 (s, 1H, C5H2), 4.35 (s, 5H, C5H5), 2.90
(d, JHH = 16.6 Hz, 2H, CHACHBP endo), 2.77 (dt, JHH = 16.5 Hz,
JHP = 4.1 Hz, 2H, CHACHBP exo), 1.28 (vt, J = 6.4 Hz, 18H, 2
tBu exo), 1.08 (vt, J = 6.9 Hz, 18H, 2 tBu endo). 13C{ H} NMR
1
(100.61 MHz, CD2Cl2): 162.38 (m, 6-C), 160.22 (d, 1JCF ª 235 Hz
(this value is very approximate due to inappropriate acquisition
of the spectrum and is not included in Table 2), 9-C), 143.12 (s)
and 141.40 (s, 7- and 11-C), 126.51 (t, JCP = 5.8 Hz, 1-C), 111.98
1
Calc. for C28H43FO2P2Pd: C, 56.14; H, 7.24. H NMR (300.13
MHz, C6D6): d 7.65 (vt, J = 7.7 Hz, 2H, 8- and 12-H), 7.09–6.98
(m, 3H, 4-, 9- and 11-H), 6.83 (d, 3JHH = 7.7 Hz, 2H, 3- and 5-H),
1
1.16 (vt, J = 7.3 Hz, 36H, 4 tBu). 13C{ H} NMR (100.61 MHz,
2
2
1
(d, JCF = 15.9 Hz) and 111.89 (d, JCF = 15.9 Hz, 8- and 10-C),
97.51 (vt, J = 13.7 Hz, 2- and 5-C), 71.77 (s, C5H2), 68.06 (vt, J =
7.9 Hz, 3- and 4-C), 36.81 (vt, J = 5.8 Hz, 2 C(CH3)3), 34.67 (vt,
J = 6.7 Hz, 2 C(CH3)3), 30.26 (vt, J = 2.9 Hz, 2 C(CH3)3), 28.89
CD2Cl2): 166.52 (vt, J = 6.2 Hz, 2- and 6-C), 160.52 (d, JCF
=
237.1 Hz, 10-C), 154.90 (td, J = 4.0 Hz, J = 11.8 Hz, 7-C), 141.65
(m, 8- and 12-C), 140.27 (m, 1-C), 127.68 (s, 4-C), 112.85 (d, 2JCF
=
16.9 Hz, 9- and 11-C), 104.66 (vt, J = 6.5 Hz, 3- and 5-C), 39.89 (vt,
(vt, J = 3.4 Hz, 2 C(CH3)3), 28.76 (vt, J = 11.3 Hz, 2 CH2).31P{ H}
1
1
J = 8.1 Hz, 4 C(CH3)3), 27.94 (vt, J = 3.7 Hz, C(CH3)3). 31P{ H}
NMR (161.98 MHz, C6D6): d 85.3. 19F{ H} NMR (282.40 MHz,
1
1
NMR (121.49 MHz, C6D6): d 191.4. 19F{ H} NMR (282.40 MHz,
C6D6): d 11.90.
C6D6): d 10.47.
Pd(C6H4F-p)[tBuP,C,PFe ] (15). The reaction was conducted
with 0.409 g (ca. 0.438 mmol of 11 and 0.146 mmol of 9) of 11
and LiAr prepared from 0.25 ml (2.276 mmol) of BrC6H4F-4 as
described above. After column chromatography on alumina using
hexane–Et2O 5 : 1 as an eluent and crystallisation from hexane
0.069 g of 15 was obtained as an orange powder in a 17% yield
with respect to the overall molar amount of the reagents. Found:
C, 57.91; H, 7.39. Calc. for C34H51FFeP2Pd: C, 58.09; H, 7.31. 1H
NMR (400.13 MHz, CD2Cl2): d 7.78 (vt, J = 7.6 Hz, 1H, 7-H),
7.57 (vt, J = 7.6 Hz, 1H, 11-H), 6.81 (vt, J = 8.2 Hz, 1H, 8-H),
6.71 (vt, J = 9.1 Hz, 1H, 10-H), 4.28 (s, 1H, C5H2), 3.94 (s, 5H,
C5H5), 3.08 (d, JHH = 16.6 Hz, 2H, CHACHBP endo), 2.66 (dt,
JHH = 16.6 Hz, JHP = 4.0 Hz, 2H, CHACHBP exo), 1.21 (vt, J =
6.3 Hz, 18H, 2 tBu exo), 1.16 (vt, J = 6.8 Hz, 18H, 2 tBu endo).
{Pd(C6H4F-p)[tBuP,C,PFe ]}PF6 (17). To a solution of 0.0060
g (0.0085 mmol) of 15 in dichloromethane (7 ml) was added
0.0027 g (0.0082 mmol) of [Cp2Fe]PF6. The reaction mixture was
stirred for 1 h which was accompanied by a color change from
orange to green. Volatiles were removed in vacuum and the residue
was twice washed with small portions of a hexane–benzene 1 : 1
mixture and dried in vacuum. Compound 17 was obtained as a
green powder (0.0069 g, 91%). Found: C, 46.66; H, 5.76. Calc. for
C34H51F7FeP3Pd ¥ 0.5CH2Cl2: C, 46.54; H, 5.89. 1H NMR (400.13
MHz, CD2Cl2): d 33.00 (br, 2H, C5H2), 22.97 (br, 5H, C5H5), 7.71
(s, 1H, Ar–H), 6.92 (s, 18 H, 2 tBu), 5.51 (s, 1H, Ar–H), 4.02 (s, 1H,
Ar–H), 0.89 (s, 1H, Ar–H), -9.89 (s, 18H, 2 tBu), -28.27 (s, 2H,
1
1
13C{ H} NMR (100.61 MHz, CD2Cl2): 162.91 (m, 6-C), 160.24
CHACHBP), -118.08 (s, 2H, CHACHBP). 31P{ H} NMR (121.49
(d, 1JCF = 235.7 Hz, 9-C), 143.08 (s) and 141.22 (s, 7-C and 11-C),
MHz, C6D6): d 94.2 (s, 2P, 2 tBu2P), -152.9 (sept, 1JPF = 711.4 Hz,
124.00 (m, 1-C), 112.02 (d, 2JCF = 16.2 Hz) and 111.95 (d, 2JCF
=
1P, PF6). 19F{ H} NMR (282.40 MHz, CD2Cl2): d -35.50 (d, 1JPF
=
1
15.8 Hz, 8-C and 10-C), 93.50 (vt, J = 13.3 Hz, 2- and 5-C), 70.36
711.4 Hz, 6F, PF6), 12.95 (s, 1F, ArF).
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Table 4 Crystal data, data collection and structure refinement parameters
for complexes 14 and 16
Kaska, R. E. Cramer and C. M. Jensen, J. Am. Chem. Soc., 1997, 119,
840–841; (c) C. M. Jensen, Chem. Commun., 1999, 2443–2449.
5 (a) F. Liu, E. B. Pak, B. Singh, C. M. Jensen and A. S. Goldman, J.
Am. Chem. Soc., 1999, 121, 4086–4087; (b) W.-W. Xu, G. P. Rosini,
M. Gupta, C. M. Jensen, W. C. Kaska, K. Krogh-Jespersen and A.
S. Goldman, Chem. Commun., 1997, 2273–2274; (c) F. Liu and A. S.
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(e) K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh, M. Kanzelberger,
N. Darji, P. D. Achord, K. B. Renkama and A. S. Goldman, J. Am.
Chem. Soc., 2002, 124, 10797–10809; (f) K. Krogh-Jespersen, M. Czerw,
N. Summa, K. B Renkema, P. D. Achord and A. S. Goldman, J.
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V. Kissin and A. S. Goldman, J. Am. Chem. Soc., 2003, 125, 7770–
7771.
6 (a) I. Go¨ttker-Schnetmann, P. S. White and M. Brookhart, J. Am.
Chem. Soc., 2004, 126, 1804–1811; (b) I. Go¨ttker-Schnetmann, P. S.
White and M. Brookhart, Organometallics, 2004, 23, 1766–1776; (c) I.
Go¨ttker-Schnetmann and M. Brookhart, J. Am. Chem. Soc., 2004, 126,
9330–9338.
7 S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernitskaya,
A. S. Peregudov, P. V. Petrovskii and A. A Koridze, Organometallics,
2006, 25, 5466–5476.
Complex
14
16
Formula
C28H43FO2P2Pd
598.96
C34H51FP2PdRu
748.16
Molecular weight
Crystal dimension/mm
Crystal system
Space group
0.30 ¥ 0.20 ¥ 0.15
0.24 ¥ 0.13 ¥ 0.07
Triclinic
Triclinic
¯
¯
P1
P1
˚
a/A
8.3067(5)
10.6772(6)
17.3068(10)
93.176(1)
99.251(1)
109.389(1)
1419.3(1)
2
10.5669(7)
10.6045(7)
15.8685(10)
100.877(1)
96.133(1)
107.634(1)
1638.4(2)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
d(calc.), g cm-3
qmax (◦).
1.402
28.0
7.96
0.775/0.890
6719 (0.0300)
5783
0.0330
0.0718
1.517
29.0
11.35
0.801/0.941
8639 (0.0268)
7533
0.0242
0.0570
m/cm-1
Transmission, Tmin/Tmax
No. unique refls (Rint
)
8 (a) K. Zhu, P. D. Achord, X. Zhang, K. Krogh-Jespersen and A. S.
Goldman, J. Am. Chem. Soc., 2004, 126, 13044–13053; (b) S. Kundu,
Y. Choliy, G. Zhuo, R. Ahuja, T. J. Emge, R. Warmuth, M. Brookhart,
K. Krogh-Jespersen and A. S. Goldman, Organometallics, 2009, 28,
5432–5444.
9 (a) S. Li and M. B. Hall, Organometallics, 2001, 20, 2153–2160; (b) H.-J.
Fan and M. B. Hall, J. Mol. Catal. A: Chem., 2002, 189, 111–118.
10 A. S. Goldman and K. Krogh-Jespersen, J. Am. Chem. Soc., 1996, 118,
12159–12166.
No. obs. refls (I > 2s(I))
R1 (on F for obs. refls)a
wR2 (on F2 for all refls)b
GOF
1.021
1.006
2
1/2
a R1 = RꢀFo| - |Fcꢀ/R|Fo| b wR2 = {R[w(Fo - Fc2)2]/Rw(Fo2)2}
11 (a) R. W. Taft, E. Price, I. R. Fox, I. C. Lewis, K. K. Andersen and G.
T. Davis, J. Am. Chem. Soc., 1963, 85, 3146–3156; (b) R. W. Taft, E.
Price, I. R. Fox, I. C. Lewis, K. K. Andersen and G. T. Davis, J. Am.
Chem. Soc., 1963, 85, 709–724; (c) C. Hansch, A. Leo and R. W. Taft,
Chem. Rev., 1991, 91, 165–195.
12 G. W. Parshall, J. Am. Chem. Soc., 1974, 96, 2360–2366.
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A. Ogorodnikova, J. Organomet. Chem., 1971, 32, 273–277; (b) S. P.
Gubin, A. A. Koridze, N. A. Ogorodnikova, A. A. Bezrukova and B.
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14 C. J. Moulton and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1976,
1020–1024.
15 A. A. Koridze, S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, V.
Yu. Lagunova, I. I. Petukhova, M. G. Ezernitskaya, A. S. Peregudov, P.
V. Petrovskii, E. V. Vorontsov, M. Baya and R. Poli, Organometallics,
2004, 23, 4585–4593.
16 S. A. Kuklin, F. M. Dolgushin, P. V. Petrovskii and A. A. Koridze,
Russ. Chem., Bull. Int. Ed., 2006, 55, 1950–1955.
X-ray diffraction study of 14 and 16
Single-crystal X-ray diffraction experiments were carried out with
a Bruker SMART APEX II diffractometer (graphite monochro-
mated Mo-Ka radiation, l = 0.71073 A, w-scan technique,
˚
T = 100(2)K). The APEX II software31 was used for collecting
frames of data, indexing reflections, determination of lattice
constants, integration of intensities of reflections, scaling and
absorption correction, and SHELXTL32 for space group and
structure determination, refinements, graphics, and structure
reporting. The structures were solved by direct methods and
refined by the full-matrix least-squares technique against F2 with
the anisotropic thermal parameters for all non-hydrogen atoms.
All hydrogen atoms were placed geometrically and included in the
structure factors calculation in the riding motion approximation.
The principal experimental and crystallographic parameters are
presented in Table 4.
17 J. Campora, P. Palma, D. del Rio and E. Alvarez, Organometallics,
2004, 23, 1652–1655.
18 M. Ohff, A. Ohff, M. E. van der Boom and D. Milstein, J. Am. Chem.
Soc., 1997, 119, 11687–11688.
19 R. Johansson, M. Jarenmark and O. F. Wendt, Organometallics, 2005,
24, 4500–4502.
Notes and references
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2702–2709.
1 The Chemistry of Pincer Compounds; D. Morales-Morales and C. M.
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