Steric control over the formation of cis and trans bis-chelated palladium(ii) complexes using a new series of flexible N,P pyridyl-phosphine ligands

Article abstract of DOI:10.1039/b709032c

The deprotection of phosphonium chloride salts [PR₂(CH ₂OH)₂]⁺Cl^- and subsequent condensation reaction with N-methyl-2-aminopyridine has been carried out to give a series of ligands of the form PR

Full text of DOI:10.1039/b709032c

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Steric control over the formation of cis and trans bis-chelated palladium(II)
complexes using a new series of flexible N,P pyridyl–phosphine ligands†‡
Deborah A. Clarke, Philip W. Miller, Nicholas J. Long* and Andrew J. P. White
Received 14th June 2007, Accepted 17th July 2007
First published as an Advance Article on the web 13th August 2007
DOI: 10.1039/b709032c
The deprotection of phosphonium chloride salts [PR2(CH2OH)2]+Cl− and subsequent condensation
reaction with N-methyl-2-aminopyridine has been carried out to give a series of ligands of the form
PR2CH2N(CH3)C5H4N (R = Ph 6, Cy 7, t-Bu 8) which have been fully characterised either as the pure
ligand (6) or the air stable borane adducts (R = Cy 7a, t-Bu 8a). The 1 : 1 reactions of 6, 7 and 8 with
PdCl2(COD) gave the N,P chelate complexes [Pd{PR2CH2N(CH3)C5H4N}Cl2]; the Cy (10) and t-Bu
(11) complexes were characterised by X-ray crystallography. The bisligated species
[Pd{PCy2CH2N(CH3)C5H4N}2Cl2] (12) was obtained when the reaction was carried out at higher
temperatures and the ligands were found to be coordinated to the metal in a trans configuration
through the phosphorus donors. Abstraction of the chlorides from the bis-ligated species 12, using
silver salts, resulted in the coordination of the pyridine ring forming the bis-chelate complex
[Pd{PCy2CH2N(CH3)C5H4N}2]2+ 13. In comparison, the palladium bis-chelate complex of ligand 5
[Pd{PPh2CH2N(CH3)C5H4N}2]2+ (14) was shown to form in a cis configuration and was fully
characterised by X-ray crystallography.
documented.21,22 Most notably, the use of these ligands in pal-
ladium catalysed cross-coupling reactions has been demonstrated
Introduction
to activate aryl chloride substrates even under mild conditions.23–31
The ability of bulky alkyl phosphines to provide highly coor-
dinatively unsaturated nucleophilic catalytic precursors is key
to their effectiveness.24 The combination of bulky electron rich
phosphines with quaternised amine groups has been investigated
most notably for aqueous phase cross-coupling reactions,23,32 and
ring opening33 ring closing34 metathesis polymerisation using
the aminophosphine ligands 1–4 (Scheme 1). The effects of a
hemilabile nitrogen donor using the aminophosphine ligand 5 for
the promotion of aryl amination reactions has been demonstrated
by Hii et al.24
The chemistry of pyridylphosphines has been much developed
in recent years owing to the versatility of these ligands in
coordination chemistry and catalysis.1–5 The most frequently used
pyridylphosphine, 2-pyridyldiphenylphosphine (dppy), displays a
variety of coordination modes to metal centres; most commonly
P-coordination and N,P bridging are observed.6 The formation
of N,P metal chelate complexes are obviously difficult to achieve
using dppy owing to ring strain in the resultant four-membered
chelate; recently however a number of ligands with organic spacer
groups between the nitrogen and phosphorus atoms have been
synthesised and shown to display N,P-coordination to a variety of
metals.7–19 These bidentate ligands show a much greater potential
for application to a range of catalytic reactions.
The interest in using bidentate P, N ligands for catalysis stems
from their hemilability, an effect arising from having both hard and
soft donor atoms in the same molecule.20 The numerous modes
of chelation, along with the strong coordinate bonding of the
phosphorus to late transition metals results in the stabilisation of
the metal centre in a variety of oxidation states and geometries.
In addition, the chelate effect may impart stability on the active
catalyst in the absence of a substrate, whilst in the presence of
a substrate the harder nitrogen atom can dissociate to provide a
vacant site for binding in a catalytic reaction.
Scheme 1 Some known examples of aminophosphine ligands.
We thought it would be interesting to extend the range of bulky
electron rich aminophosphines that have the ability to act as
hemilabile ligands to metal centres owing to their potential in
catalytic applications. Herein we report the synthesis of a series
of new N,P ligands (Scheme 2), based on a pyridine ring with a
methylene amino spacer between the phosphorus donor, and their
coordination chemistry to palladium(II).
The recent use and development of electron rich, bulky
phosphine ligands for catalytic applications has been well
Department of Chemistry, Imperial College London, South Kensington,
London, UK SW7 2AZ. E-mail: n.long@imperial.ac.uk; Tel: +44 (0)20
7594 5781
Results and discussion
† CCDC reference numbers 650212–650216. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b709032c
‡ Electronic supplementary information (ESI) available: Additional crystal
structure data. See DOI: 10.1039/b709032c
The new N,P ligands 6, 7 and 8 were prepared by the direct
condensation of the appropriate PR2CH2OH precursor with
N-methyl-2-aminopyridine (Scheme 2). Phosphonium chloride
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Scheme 2 Conditions: (i) NEt3, methanol, reflux 2 h; (ii) N-methyl-2-
aminopyridine, reflux 24 h; (iii) BH3S(CH3)2·THF, diethyl ether, stirred at
room temperature overnight.
precursor salts were prepared from their corresponding diphenyl
or dialkyl phosphine, in high yield, according to established
literature procedures.35 The chloride salt was deprotected in situ
using triethylamine, forming hydroxymethyl-diphenylphosphine
or hydroxymethyl-dialkylphosphine prior to reaction with N-
methyl-2-aminopyridine. Subsequent precipitation with diethyl
ether and removal of the base salt gave the required ligand.
Ligand 6 was obtained in high yield as a white air stable solid,
1
the 31P{ H} NMR spectrum showed a single peak at −21.3 ppm.
A similar ligand synthesised by Durran et al.6 shows a peak
at −17.4 ppm. Ligands 7 and 8 were obtained as viscous, air
sensitive oils in crude yields of ca. 70%—the bulky alkyl Cy and tBu
phosphine groups appeared to hinder this condensation reaction
compared to the Ph derivative. NMR spectroscopic data for these
ligands and their phosphine precursors are summarised in Tables 1
and 2.
The air sensitive ligands 7 and 8 were isolated as their corre-
sponding borane salts following reaction with borane dimethyl
sulfide in THF (Scheme 2).36 The borane adducts 7a and 8a were
obtained as air stable white crystalline solids after flash column
1
chromatography. Their 31P{ H} NMR spectra showed expected
downfield shifts, relative to the unprotected ligands (Table 1), and
coupling to spin active 10B and 11B nuclei. There was no significant
change to the proton spectra of either ligand when coordinated to
boron.
Crystals of the borane adduct 7a suitable for single crystal
X-ray diffraction were obtained by recrystallisation from hot
ethanol. The crystal structure of 7a (Fig. 1) shows the expected
ligand structure with coordination of the borane occurring solely
to the phosphorus atom and not to any of the potentially
competing nitrogen atoms. The P–C(1)–N(2)–C(3) dihedral angle
is ca. 107◦.
The borane could be readily removed from 7a and 8a to give
the purified ligands (in overall 10% and 9% yields respectively) by
stirring overnight with fluoroboric acid dimethyl ether complex in
THF.37 31P NMR spectra showed single peaks for both ligands.
The purity of the ligands was significantly improved compared to
the crude products prior to the borane protection step.
1
Table 1 31P{ H} NMR shifts for the intermediates and products of the
reaction shown in Scheme 2
1
31P{ H} NMR (ppm) [CDCl3]
Ligand
PR2CH2OH
Ligand
Borane adduct
6: R = Ph
7: R = Cy
−10.1
1.4
20.3
−21.3
−10.1
17.2
—
41.9
25.0
t
8: R = Bu
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would result in decreased shielding and explain their downfield
chemical shift relative to the complex.
Single crystals suitable for X-ray diffraction analysis of complex
10 were obtained by layering a dichloromethane (DCM) solution
of 10 with diethyl ether. The crystal structure of 10 (Fig. 2,
Table 3) shows the expected chelating mode of the ligand
bonding to the palladium via the pyridyl nitrogen and phosphine
group.
Fig. 1 The molecular structure of borane adduct 7a.
Coordination to palladium(II)
To investigate the chelating ability of ligands 6, 7 and 8, they were
stoichiometrically reacted with PdCl2(COD) affording the Pd(II)
species as shown in Scheme 3. In each case the ligand chelated
to the metal centre to form the expected six-membered chelate
rings. This was evidenced by the characteristic downfield shifts
1
observed in the 31P{ H} NMR spectra of all three complexes 9, 10
and 11 upon coordination to the palladium centre. Coordination
of the pyridine nitrogen of the ligands to palladium was observed
in the H NMR spectra of the complexes. The proton ortho to
Fig. 2 The molecular structure of complex 10.
1
the pyridine nitrogen was significantly shifted downfield relative
to that of the free ligand owing to donation of electron density
In order to chelate to the palladium centre the ligand has
twisted about the C(1)–N(2) bond such that the P–C(1)–N(2)–C(3)
dihedral angle is ca. 83◦, cf. 107◦ in the free ligand 7a (vide supra).
The six-membered chelate ring has a twisted boat conformation,
1
from the aromatic ring to the metal centre. The 31P{ H} NMR
1
and H NMR chemical shifts of the free ligands and complexes
˚
are summarised in Table 2.
{C(1),P,Pd,N(4)} being coplanar to within ca. 0.04 A with N(2)
˚
and C(3) lying ca. 1.07 and 0.73 A respectively out of this plane on
the same side. Interestingly, the coordination of the metal to the
pyridyl nitrogen N(4) is noticeably distorted, the metal lying ca.
˚
0.83 A out of the pyridyl ring plane such that the N(4)–Pd vector is
inclined by ca. 22◦ to the plane of the ring. Both the conformation
of the six-membered ring and the distorted pyridyl coordination
are features also seen in the closely related structure cis-dichloro-
(N-(cyclohexyl)-N-((diphenylphosphino)methyl)pyridine-2-
amine-Nꢀ,P)-palladium(II).17
Scheme 3 Coordination of ligands to palladium(II).
Single crystals of complex 11 suitable for X-ray diffraction
were obtained by dissolution of the compound in hot DCM
and subsequent slow cooling. The crystal structure of the t-butyl
complex 11 (Fig. 3, Table 3) is similar to that of its cyclohexyl
counterpart 10 (Fig. 2), though notable differences can be seen
in the angles at the metal centre. In particular, the P–Pd–Cl(2)
angle differs by more than 5◦ between the two complexes, though
there is no obvious reason for this. The ligand is again twisted
Notably, there is a considerable upfield chemical shift, of
approximately 1 ppm, for the methylene protons of all three ligands
upon complexation to palladium. This may be explained by the
increase in shielding of these protons by the lone pair on the
nitrogen as a result of chelation to palladium. In the free ligand
there is greater rotational freedom about the N–CH2P bond which
◦
˚
Table 3 Comparative selected bond lengths (A) and angles ( ) for 10 and 11
10 11
10
2.4132(5)
2.0995(15)
85.759(19)
90.013(19)
177.91(4)
11
2.3945(4)
2.1024(11)
91.044(15)
87.817(15)
175.16(3)
Pd–P
Pd–Cl(2)
P–Pd–Cl(1)
P–Pd–N(4)
Cl(1)–Pd–N(4)
2.2092(5)
2.2935(5)
175.700(17)
92.55(4)
2.2460(3)
2.2869(4)
177.781(13)
91.80(3)
Pd–Cl(1)
Pd–N(4)
P–Pd–Cl(2)
Cl(1)–Pd–Cl(2)
Cl(2)–Pd–N(4)
91.70(4)
89.47(3)
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respectively. Single crystal X-ray analysis of 12 (Fig. 4) confirmed
the suspected trans bis-ligated complex.
Fig. 3 The molecular structure of complex 11.
about the C(1)–N(2) bond compared to 7a, the P–C(1)–N(2)–
C(3) dihedral angle being ca. 84◦, cf. 107◦ in 7a and 83◦ in 10.
As was seen in the structure of 10, the six-membered C2N2PPd
chelate ring has a twisted boat conformation, {C(1),P,Pd,N(4)}
˚
Fig. 4 The molecular struct◦ure of the C2-symmetric complex 12. Selected
89.649(18), P–Pd–Pꢀ 173.907(17), P–Pd–Clꢀ 90.870(18), Cl–Pd–Clꢀ
being coplanar to within ca. 0.08 A with N(2) and C(3) lying ca.
˚
bond lengths (A) and angles ( ); Pd–P 2.3376(5), Pd–Cl 2.3048(5); P–Pd–Cl
˚
1.13 and 0.76 A respectively out of this plane on the same side.
The pyridyl coordination is likewise similar, the palladium atom
170.23(2).
˚
lying ca. 0.99 A out of the pyridyl ring plane (the N(4)–Pd vector
is inclined by ca. 27◦ to the plane of the ring).
Unlike the chelating coordination modes seen in the structures
of complexes 10 and 11, here ligand 7 acts in a monodentate
fashion coordinating only via the phosphorus donor (Fig. 4).
The two ligands are oriented trans to each other, presumably a
consequence of the steric bulk of the cyclohexyl groups, and it is
When the reaction of cyclohexyl ligand 7 (Scheme 4) with
PdCl2(COD), in a 1 : 1 ratio, was carried out at room temperature
1
two peaks were observed in the 31P{ H}NMR. One peak was
a minor product at 50.0 ppm, the chelated product 10, while
a second peak appeared at 17.5 ppm and was suspected to be
the non-chelated bis-ligated complex formed by the coordination
of two ligands via the softer phosphine groups. An approximate
bis/mono ligated ratio of 1 : 3 was obtained by integration of the
˚
notable that the Pd–P bond length [2.3376(5) A] is longer than
those seen in the mono-ligated complexes 10 and 11 where the
˚
ligand is bidentate [2.2092(5) and 2.2460(3) A respectively]. The
complex has crystallographic C2 symmetry about an axis that
passes through the metal centre perpendicular to the Cl2P2Pd
plane. This square plane is noticeably distorted, the {Pd,Cl,P}
and {Pd,Clꢀ,Pꢀ} planes being twisted with respect to each other by
ca. 12◦. Not being forced into a chelate ring, the P–C(1)–N(2)–
C(3) dihedral angle is enlarged to ca. 124◦. This is even larger than
that seen in the crystal structure of borane adduct 7a [107◦], and
may in part be associated with a short intermolecular C–H · · · p
approach to the pyridyl ring from a methyl proton on C(9) of an
adjacent molecule [see ESI‡ for more details].
When the bis-ligated complex 12, which has two free un-
coordinated pyridyl groups, was reacted with two equivalents
of silver perchlorate, the two chloride ions were abstracted
from the palladium centre resulting in the coordination of the
pyridyl groups to the metal centre (Scheme 5). Evidence for the
coordination of the pyridyl groups was given by the IR stretching
frequency of the pyridyl ring which displayed a characteristic
1
31P{ H} spectrum. Repeating this reaction at a lower temperature
of −10 ◦C gave only the chelate complex 10; no evidence of a
second peak at 17.5 ppm was seen in the 31P NMR spectrum.
This suggests that the kinetic product of the reaction is complex
10, formed at the lower temperature, while a second product,
the thermodynamic product, forms when the temperatures are
higher. This new complex (12) was isolated in its pure form from
the reaction mixture as a yellow crystalline solid by layering a
DCM solution with diethyl ether. The proton NMR spectrum of
12 showed no changes in chemical shift of the pyridyl protons
or of the methylene protons from the free ligand, this indicated
that the pyridyl ring remained uncoordinated to the Pd centre.
Further evidence for the bis-ligated complex was corroborated by
comparing the 31P NMR chemical shift of 12 with the related
trans complexes Pd(PCy3)2Cl2 and Pd(PCy2{allyl})2Cl2 which
1
have 31P{ H} NMR chemical shifts of 24.8 ppm38 and 19 ppm39
Scheme 4 Formation of the bis-ligated complex 12.
Scheme 5 Formation of the bis-chelate complex 13.
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Table 4 Chemical shifts of complexes 12, 13 and 14
1H NMR (ppm)
1
Complex
31P{ H} NMR
a
b
c
d
e
f
12
13
14
17.5
55.9
30.8
8.18
7.38
7.86
6.65–6.51
6.62
6.64
7.48
7.81
7.73
6.65–6.51
7.26
7.06
4.48
3.70
4.45
3.29
3.31
3.22
shift to higher wavenumbers40,41 (1605 cm−1) compared to the free
ligand (1594 cm−1). The 31P NMR spectrum (Table 4) showed a
distinct downfield chemical shift from 17.5 ppm (complex 12) to
55.9 ppm for the phosphine resonance upon ligation of the pyridyl
equation. Further cooling of the sample revealed the splitting of
this signal into two pseudo triplets (overlapping dd) at 3.8 ppm and
3.2 ppm which are the result of the ‘freezing-out’ of two distinct
chemical environments for these methylene protons. A marked
upfield chemical shift of the pyridyl ortho protons from 7.38 ppm
at 293 K to 7.07 at 193 K was also observed which further supports
a conformation change.
1
groups. The H NMR spectrum of the bis-chelate complex 13
displayed characteristic downfield chemical shifts for the pyridyl
protons except for a marked and unexpected upfield chemical
shift for the ortho protons (7.38 ppm) relative to the free ligand
(8.12 ppm). This upfield shift of the ortho pyridyl protons can be
explained by the increase in electron density on the palladium
centre resulting from the coordination of two electron rich
dicyclohexyl phosphine groups. The methylene protons resonance
of the bis-chelate complex 13 are noticeably broadened at ambient
temperature compared to the monochelate palladium dichloride
complex 10 which show a sharp doublet resulting from coupling to
the phosphorus atom. The broadening of the methylene protons
in 13 indicates a fluxional conformation change of these protons
occurring on the NMR timescale. Variable temperature 1H NMR
(Fig. 5) of complex 13 revealed an approximate coalescence
temperature of 273 K, which corresponds to a Gibbs free energy
of activation of 48.7 kJ mol−1 for this process from the Eyring
The attempted preparation of the analogous complex L2PdCl2
complex using the diphenylphosphino ligand 6 resulted in the
formation of a mixture of cis and trans isomers, as might be
expected,42 appearing at 25.6 ppm and 10.0 ppm respectively in
1
the 31P{ H} NMR (CDCl3) spectrum. An approximate cis : trans
1
ratio of 5 : 1 was obtained based on 31P{ H} NMR integration
which showed no change when heated (50 ◦C) in a sealed
NMR tube for 24 h; confirming a thermodymanic equilibrium.
Abstraction of the chlorides using silver perchlorate initially
gave a mixture of isomers; evaporation of the sample followed
by recrystallisation from hot DCM resulted in the isolation of
the cis isomer of the bis-chelate complex (Scheme 6). This was
indicated by a single resonance in the 31P NMR spectrum that
was markedly shifted downfield to 30.8 ppm, relative to the non-
1
chelated cis complex. The H NMR spectrum of the bis-chelate
complex 14 again showed distinct changes in the pyridyl proton
resonances, relative to the free ligand, indicating coordination of
the pyridyl ring. There was a general downfield shift of the pyridyl
resonances on coordination except for the ortho proton (a), which
again was unexpectedly shifted to an upfield position (7.86 ppm)
compared to the free ligand (8.21 ppm). The IR spectrum, again,
undoubtedly confirmed coordination of the pyridyl ring which
displayed a characteristic stretch at 1608 cm−1 compared to the
free ligand 1593 cm−1. Single crystals suitable for X-ray diffraction
were obtained by slow evaporation of a dichloromethane solution.
Single crystal X-ray diffraction of crystals of 14 showed the
expected bis-chelate dicationic complex having two N,P ligands
coordinating to a square planar palladium centre (Fig. 6, Table 5).
The square plane is distorted, the two {Pd,P,N} coordination
planes being twisted with respect to each other by ca. 11◦. The
complex has approximate C2 symmetry about an axis that passes
through the metal centre and bisects the P(1)–Pd–P(31) and
1
Fig. 5 Variable temperature (293 K to 193 K) H NMR of complex 13
run in CD2Cl2. * indicates that these protons behave fluxionally and that
the peaks change with temperature.
Scheme 6 Formation of the bis-chelate complex 14.
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◦
˚
In summary a series of new N,P ligands have been syn-
thesised and shown to chelate to a palladium(II) by both the
phosphorus and the nitrogen atoms. The sterically demand-
ing dicyclohexylphosphino ligand 7 was demonstrated to form
both mono-chelate and P-bound bis-ligated mondentate Pd(II)
complexes depending on the reaction temperature. A novel bis-
chelate complex 13 was formed when the chloride ions were
abstracted from the dicyclohexyl bis-ligated complex 12 resulting
in coordination of the pyridyl groups to the Pd(II) centre. A trans
geometry was assigned to complex 13 based on the geometry
of the precursor 12 and chemical shifts of comparable trans
cyclohexylphosphino Pd(II) complexes found in the literature. The
synthesis of related bis-chelate complex 14 using the diphenylphos-
phino ligand 6 gave exclusively the cis isomer, as characterised by
X-ray crystallography. The well documented steric interactions
of the much bulkier cyclohexylphosphino ligand are the likely
cause of the predominance of the trans isomer for complex 13.
We are currently investigating the role of these ligands for creating
heterogeneous mixed metal species and in catalytic C–C, C–N and
C–O bond forming reactions.
Table 5 Selected bond lengths (A) and angles ( ) for 14
Pd–P(1)
2.2319(7)
2.2411(7)
81.06(7)
Pd–N(4)
2.100(2)
2.112(2)
99.73(3)
172.93(7)
89.46(6)
Pd–P(31)
Pd–N(34)
P(1)–Pd–N(4)
P(1)–Pd–N(34)
N(4)–Pd–N(34)
P(1)–Pd–P(31)
N(4)–Pd–P(31)
P(31)–Pd–N(34)
168.09(7)
90.76(9)
Experimental
General considerations
All preparations were carried out using standard Schlenk line
techniques under an inert atmosphere of N2 unless otherwise
stated. Solvents were dried over standard drying agents and freshly
distilled under nitrogen before use. All starting materials were of
reagent grade, purchased from either Aldrich Chemical Company
or Acros Organics. Chromatographic separations were carried out
Fig. 6 The molecular structure of the dication in 14.
N(4)–Pd–N(34) angles. The main departure from C2 symmetry
is in the two six-membered C2N2PPd chelate rings. The P(1)/N(4)
ring has a twisted boat conformation, {P(1),C(1),N(2),C(3)} being
on Kieselgel 60 SiO2. H, 13C{ H} and 31P{ H} NMR spectra
were recorded on a Jeol JNM-Ex 270 MHz FT spectrometer
or on Bruker Av-400, DRX-400, Av-500 spectrometers. Variable
temperature 1H NMR spectra were run on a Bruker DRX-
400 MHz spectrometer. Chemical shifts are reported in ppm
using the residual proton impurities in the solvents. Pseudo-
triplets which occur as a result of identical J-value coupling
to two chemically inequivalent protons are assigned as dd and
are recognised by the inclusion of only one J-value. Positive-ion
FAB and electron ionisation mass spectra were recorded on a
Micromass Autospec Q spectrometer using a 3-nitrobenzyl alcohol
matrix. Electron ionisation was carried out at 70 eV. Infrared
spectra were recorded on a Perkin-Elmer 983G spectrophotometer
equipped with a Perkin-Elmer 3700 data station and recorded as
either KBr pressed discs or Nujol mulls. Elemental analyses were
carried out by Mr Stephen Boyer of the Department of Health
and Human Sciences, London Metropolitan University. X-Ray
diffraction analysis was carried out by Dr Andrew White of the
Department of Chemistry at Imperial College London.
1
1
1
˚
coplanar to within ca. 0.02 A with N(4) and Pd lying ca. 0.59
˚
and 1.79 A respectively out of this plane on the same side. The
conformation of the P(31)/N(34) chelate ring is much nearer to
an ideal boat; {Pd,N(32),C(33),N(34)} is coplanar to within ca.
˚
˚
0.08 A with P(31) and C(31) lying ca. 1.33 and 1.49 A respectively
out of this plane on the same side. In fact, these two atoms are
almost coplanar with the metal and N(32), the maximum deviation
˚
from planarity for {Pd,P(31),C(31),N(32)} being ca. 0.16 A. This
difference in conformation can be readily seen by comparing the P–
C–N–C dihedral angles; the P(1)–C(1)–N(2)–C(3) dihedral angle
is ca. 3◦ whilst that for P(31)–C(31)–N(32)–C(33) is ca. 79◦. Whilst
the latter is comparable to those seen in 10 and 11 [83 and 84◦
respectively] the former is vastly different to those seen in any of
the other structures.
Another difference between this structure and the structures of
10 and 11 is the coordination behaviour of the pyridyl rings. Here
in 14 the metal is much closer to being coplanar with the pyridyl
˚
rings, being ca. 0.32 A out of the N(4) ring plane [N–Pd vector
inclined by ca. 8◦ to the plane of the ring] and ca. 0.45 A out
˚
of the N(34) ring plane [N–Pd vector inclined by ca. 12◦ to the
plane of the ring]; in 10 and 11 the vectors were inclined by ca.
22 and 27◦ respectively. In the structure of the related literature
species bis((5-chloropyrid-2-ylaminomethyl)diphenylphosphine-
N,P)-platinum(II) bis(tetrafluoroborate) the two six-membered
chelate rings have very similar sofa conformations with
the phosphorus folded out of the C2N2Pt plane in each
case.14
Synthesis of [(diphenylphosphino)methyl]-
methyl-pyridin-2-ylamine, 6
To a solution of bis(hydroxymethyl)phosphonium chloride (0.5 g,
1.76 mmol) in methanol (10 ml) was added trimethylamine (234 ll,
1.76 mmol) and 2-(methylamino)pyridine (180 ll, 1.76 mmol) and
brought to reflux for 1 h. After this time, methanol was removed
in vacuo and diethyl ether added (20 ml) to extract the product.
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1
Cy CH2). 31P{ H} NMR (CDCl3, 270 MHz): d 41.9 (m). EI-MS:
The ether solution was filtered, via cannula, into a separate flask
and the ether removed in vacuo to yield a white solid (0.50 g, 93%).
Anal.: Calc. for C19H19N2P: C, 74.5; H, 6.3; N, 9.1. Found: C,
m/z 121 (100) [C7H9N2 ], 332 (9) [C19H34N2PB+].
+
1
74.6; H, 6.3; N, 9.1%. IR (v/cm−1) KBr: 1593 s (Py). H NMR
Synthesis of the borane adduct of di-tert-butylphosphinomethyl-
methyl-pyridin-2-yl-amine, 8a
(CDCl3, 270 MHz): d 8.21 (dd, 1H, 3JHH = 4.9 Hz, 4JHH = 2.0 Hz),
7.62–7.52 (m, 3H, Ar–H), 7.42 (ddd, 1H, 3JHH = 8.6 Hz, 3JHH
=
To a stirred solution of compound 8 (0.49 g, 1.70 mmol) in
EtOH (15 ml) was added borane–dimethyl sulfide complex in THF
(0.85 ml, 1.70 mmol) drop-wise. The mixture was stirred overnight
and the solvent removed in vacuo. A white solid formed (0.27 g,
53%) which was recrystallised from hot ethanol to give white
needle-like crystals (0.163 g, 32%). Anal.: Calc. for C19H34PBN2: C,
68.68; H, 10.31; N, 8.43. Found: C, 68.75; H, 10.38; N, 8.55%. IR
(v/cm−1) (Nujol): 2383, 2352 (mB-H). 1H NMR (CDCl3, 270 MHz) d
8.11 (d, 1H, 3JHH = 3.21 Hz, Ar–H), 7.47 (dd, 1H, 3JHH = 7.8 Hz,
7.2 Hz 4JHH = 1.9 Hz, Ar–H), 7.40–7.31 (m, 7 H, Ar–H), 6.56 (dd,
3
4
3
1H, JHH = 7.7 Hz, JHH = 4.9 Hz, Ar–H), 6.52 (d, 1H, JHH
=
8.7 Hz, Ar–H), 4.51 (d, 2 H, 2JPH = 3.94 Hz, CH2P), 2.93 (s, 3H,
CH3). 31P{ H} NMR (CDCl3, 109 MHz): d −21.3. 13C{ H} NMR
(CDCl3, 125 MHz): d 158.0 (s, Ar C), 147.7 (s, Ar CH), 137.6 (d,
1JPC = 15.2 Hz, Ar C), 136.9 (s, Ar CH), 133.1 (d, 2JPC = 17.7 Hz,
Ar CH), 128.6 (s, Ar CH), 128.3 (d, 3JPC = 7.0 Hz), 111.7 (s, Ar
1
1
1
CH), 106.2 (s, Ar CH), 52.4 (d, JPC = 10.2 Hz, CH2), 37.3 (d,
3JPC = 3.5 Hz, CH3). EI-MS: m/z (%): 121 (100) [C7H9N2 ], 322
+
2
Ar–H), 6.54–6.50 (m, 2H, Ar–H), 4.30 (d, 2 H, JH-P = 3.0 Hz,
(10) [C19H19N2PO+].
CH2P), 3.12 (s, 3H, CH3), 1.99–1.14 (m, 22H, Cy-H). 13C{ H}
1
NMR (CDCl3, 125 MHz): d 158.4 (s, Ar C), 147.2 (s, Ar CH),
137.6 (s, Ar CH), 112.3 (s, Ar CH), 106.5 (s, Ar CH), 39.8 (d,
1JPC = 35.7 Hz, CH2), 38.4 (s, CH3), 32.5 (d, 1JPC = 24.1 Hz, t-Bu
Synthesis of dicyclohexylphosphinomethyl-
methyl-pyridin-2-ylamine, 7
C), 28.2 (s, t-Bu CH3). 31P{ H} NMR (CDCl3, 270 MHz): d 25.0
1
To a Schlenk flask was added dicyclohexyl(hydroxymethyl)-
phosphonium chloride (0.25 g, 0.85 mmol) and dry, degassed
methanol (7 ml). Et3N was added (0.118 ml, 0.85 mmol) and the
mixture heated under reflux for 2 h. After cooling, N-methyl-2-
aminopyridine (0.087 ml, 0.85 mmol) was added and the mixture
heated under reflux overnight. Following this, the solvents were
removed in vacuo and dry, degassed diethyl ether added (50 ml)
to dissolve the ligand. Following cannula filtration under N2 into
another Schlenk flask the ether was removed in vacuo to give a
(m). EI-MS: m/z 279 (100) [C15H26N2PB+].
Deprotection of borane salts
Procedure as described by Trost et al.37 Fluoroboric acid dimethyl
ether complex (0.7 ml, 5.83 mmol) was added to a stirred solution
of the borane salt 7a (0.25 g, 0.76 mmol) in dry, degassed DCM
(15 ml) at −15 ◦C. The reaction mixture was stirred overnight and
allowed to warm to ambient temperature. The reaction mixture
was diluted with dry, degassed DCM (15 ml), and degassed
saturated NaHCO3 solution (25 ml). Following a further 20 min
of vigorous stirring, the aqueous phase was extracted twice with
degassed DCM (2 × 20 ml), and the combined organic extracts
washed with brine and dried over MgSO4. The solvents were
evaporated to give ligand 7 as a viscous air sensitive colourless
1
viscous colourless oil. The 31P{ H} NMR spectrum of this crude
product contained a mixture of the desired compound 7 and
dicyclohexylphosphinomethanol starting material. The borane
protection and purification steps are described below.
Synthesis of di-tert-butylphosphinomethyl-
methyl-pyridin-2-yl-amine, 8
1
oil (0.13 g, 55%) H NMR (CDCl3, 270 MHz): d 8.12 (dd, 1H,
3JHH = 4.7 Hz, 4JHH = 1.8 Hz, Ar–H), 7.40 (td, 1H, 3JHH = 7.9 Hz,
4JHH = 1.9 Hz, Ar–H), 6.53–6.46 (m, 2H, Ar–H), 3.95 (d, 2 H,
2JPH = 2.70 Hz, CH2P), 3.05 (s, 3H, CH3), 1.80–1.20 (m, 22H,
Procedure was analogous to that described for compound 7.
Cy–H). 31P{ H} NMR (CDCl3, 270 MHz): d −10.1.
1
Synthesis of the borane adduct of dicyclohexylphosphinomethyl-
methyl-pyridin-2-yl-amine, 7a
The deprotection of borane salt 8a was carried out using the
same method to give ligand 8 as a viscous air sensitive colourless
The borane protection procedure described here is similar to that
used by Hii et al.36 To a solution of the ligand in dry, degassed
diethyl ether (60 ml) was added borane–dimethyl sulfide complex
(4.12 ml, 8.24 mmol) drop-wise and the mixture stirred overnight.
The ether was removed in vacuo to give a viscous oil. Following
flash column chromatography (2 : 1 petroleum ether 40–60 ◦C–
diethyl ether) and recrystallisation from hot ethanol, the product
was obtained as colourless crystals (0.50 g, 22%), Anal.: Calc.
for C15H30PBN2: C, 64.30; H, 10.79; N, 10.00. Found: C, 64.45;
1
oil (0.21 g, 38%). H NMR (CDCl3, 270 MHz): d 8.15 (dd, 1H,
3JHH = 4.9 Hz, 4JHH = 1.5 Hz, Ar–H), 7.40 (td, 1H, 3JHH = 7.9 Hz,
4JHH = 2.0 Hz, Ar–H), 6.57–6.43 (m, 2H, Ar–H), 4.10 (d, 2 H,
2JPH = 1.7 Hz, CH2P), 3.08 (s, 3H, CH3), 1.17 (d, 18 H, JPH
=
3
10.9 Hz, tBu-H). 31P{ H} NMR (CDCl3, 270 MHz): d 17.2.
1
Synthesis of [(diphenylphosphino)-methyl]-
methyl-pyridin-2-yl-amine palladium dichloride, 9
1
H, 10.69; N, 10.07%. IR (v/cm−1) (Nujol): 2364, 2339 (mB-H). H
To a solution of ligand 6 (0.4 g, 1.30 mmol) in dichloromethane
(5 ml) was added a solution of PdCl2(COD) (0.37 g, 1.30 mmol) in
dichloromethane (20 ml) and stirred for 1 h at room temperature.
Dichloromethane was removed under reduced pressure to yield a
yellow solid (0.61 g, 97%). Anal.: Calc. for C19H19Cl2N2PPd: C,
47.18; H, 3.96; N, 5.79. Found: C, 47.25; H, 3.95; N, 5.70%. IR
(v/cm−1) KBr: 1603 s (Py). H NMR (CDCl3, 270 MHz): d 9.32
(dd, 1H, JHH = 6.2 Hz, JHH = 1.7 Hz, Ar–H), 8.02–7.92 (m,
NMR (CDCl3, 270 MHz) d 8.15 (d, 1H, 2JHH = 2.97 Hz, Ar–H),
7.47 (dd, 1H, 2JHH = 6.89 Hz, Ar–H), 6.67–6.52 (m, 2H, Ar–H),
2
4.56 (d, 2 H, JPH = 3.21 Hz, CH2P), 3.12 (s, 3H, CH3), 1.30 (d,
1
18 H, 3JPH = 12.1 Hz, tBu-H). 13C{ H} NMR (CDCl3, 125 MHz):
d 158.2 (s, Ar C), 147.3 (s, Ar CH), 137.5 (s, Ar CH), 112.3 (s,
Ar CH), 105.8 (s, Ar CH), 40.0 (d, 1JPC = 37.7 Hz, CH2), 38.5 (s,
CH3), 31.2 (d, 1JPC = 29.7 Hz, Cy CH), 26.9 (m, Cy CH2), 26.0 (s,
1
3
4
4562 | Dalton Trans., 2007, 4556–4564
This journal is The Royal Society of Chemistry 2007
©

View Article Online
1
4H, Ar–H), 7.74 (ddd, 1H, 3JHH = 8.6 Hz, 3JHH = 7.2 Hz 4JHH
=
=
Found: C, 55.9; H, 7.7; N, 6.7%. H NMR (CDCl3, 270 MHz):
3
3
3
1.9 Hz, Ar–H), 7.62–7.45 (m, 6H, Ar–H), 6.95 (d, 1H, JHH
d 8.18 (d, 2 H, JHH = 4.9 Hz, Ar–H), 7.48 (dd, 2 H, JHH =
8.4 Hz, Ar–H), 6.89 (dd, 1H, 2JHH = 5.9 Hz, Ar–H), 3.53 (d, 2 H,
7.4 Hz, Ar–H), 6.58–6.51 (m, 4H, Ar–H), 4.48 (s, 4H, CH2), 3.29
2JPH = 3.7 Hz, CH2P), 2.94 (s, 3H, CH3). 31P{ H} NMR (CDCl3,
(s, 6H, CH3), 2.36, 0.85 (m, 44H, Cy-H). 31P{ H} NMR (CDCl3,
1
1
+
109 MHz): d 26.0. FAB-MS: m/z (%) : 121 (100) [C7H9N2 ], 447
270 MHz): d 17.5. FAB MS: m/z 779 (50) [C38H62N4P2ClPd+].
(30) [C19H19ClN2PPd+].
Synthesis of bis-chelate palladium complex
[Pd{PCy2CH2N(CH3)C5H4N}2] [ClO4]2, 13
Synthesis of [dicyclohexylphosphinomethyl-methyl-
pyridin-2-yl-amine] palladium dichloride, 10
To
a solution of complex 12 (50 mg, 0.06 mmol) in
To a Schlenk flask was added PdCl2(COD) (0.12 g, 0.42 mmol) and
a magnetic stirrer bar. DCM (10 ml) was added and the mixture
stirred until the palladium complex had dissolved. This solution
was transferred by cannula into a second Schlenk flask containing
ligand 7 (0.12 g, 0.42 mmol) at −10 ◦C. The reaction mixture was
stirred at room temperature overnight. The solvent was removed
in vacuo yielding a yellow solid. Following recrystallisation from
DCM–diethyl ether, yellow block crystals were obtained (81.5 mg,
0.16 mmol, 42%). Anal.: Calc. for C19H31N2PPdCl2: C, 46.03; H,
6.30; N, 5.65. Found: C, 45.07; H, 6.50; N, 5.24%. IR (v/cm−1)
(Nujol): 1606 (mPy). 1H NMR (CDCl3, 270 MHz): d 9.20 (dd, 1H,
3JHH = 6.0 Hz, 4JHH = 1.5 Hz, Ar–H), 7.74 (td, 1H, 3JHH = 7.8 Hz,
4JHH = 1.9 Hz, Ar–H), 6.95–6.88 (m, 2H, Ar–H), 3.06 (s, 3H,
CH3), 2.99 (d, 2 H, 2JPH = 3.9 Hz, CH2P), 2.38–1.26 (m, 22H, Cy-
dichloromethane (3 ml) was added silver perchlorate (25 mg,
0.12 mmol) and stirred for 48 h at room temperature in darkness.
The solution was then filtered through a pad of Celite and
evaporated to dryness. Crystals were obtained after 2 d when a
solution of the complex (0.03 M) in dichloromethane was layered
with benzene. The crystals were collected, filtered off, washed
with diethyl ether and dried in vacuo (41 mg, 72%). Anal.: Calc.
for C38H62Cl2N4O8P2Pd: C, 48.44; H, 6.63; N, 5.95. Found: C,
48.34; H, 6.62; N, 5.91%. IR (cm−1) KBr: 1605 m (py). 1H NMR
3
3
(CD2Cl2, 400 MHz): d 7.81 (ddd, 2 H, JHH = 8.7 Hz, JHH
=
7.3 Hz 4JHH = 1.7 Hz, Ar–H), 7.38 (d, 2 H, 3JHH = 4.0 Hz, Ar–H),
7.26 (d, 2 H, 3JHH = 8.7 Hz, Ar–H), 6.23 (dd, 2 H, 3JHH = 6.5 Hz,
Ar–H), 3.70 (s, br, 4H, CH2P), 3.31 (s, 6H, CH3), 2.45–1.20 (m,
44H, Cy-H). 31P{ H} NMR (CD2Cl2 162 MHz): d 55.9. FAB-MS:
1
1
H). 13C { H} NMR (CDCl3, 400 MHz): d 162.2 (s, Ar C), 155.1
m/z (%): 121 (100) [C7H9N2 ], 743 (80) [C38H62N4P2Pd+], 843 (10)
+
(s, Ar CH), 140.4 (s, Ar CH), 117.0 (s, Ar CH), 112.7 (s, Ar CH),
41.8 (s with fine, CH2), 41.4 (s, CH3), 33.5 (d, 1JPC = 26.9 Hz, Cy
CH), 28.1 (d, JPC = 14.9 Hz, Cy CH2), 26.6 (t, JPC = 11.7 Hz,
+
[{C38H62N4P2Pd(ClO4)} ].
Synthesis of bis-chelate palladium complex
[Pd{PPh2CH2N(CH3)C5H4N}2] [ClO4]2, 14
1
Cy CH2), 25.8 (s, Cy CH2). 31P{ H} NMR (CDCl3, 270 MHz):
+
d 50.0. FAB MS: m/z 121 (100) [C7H9N2 ], 461 (87) [C19H31N2-
PPdCl+].
To a solution of the ligand 6 (200 mg, 0.64 mmol) in
dichloromethane (5 ml) was added PdCl2(COD) (92 mg,
0.32 mmol) in dichloromethane (5 ml) and stirred at room
temperature for 1 h. Silver perchlorate (133 mg, 0.64 mmol)
was then added and stirred for 48 h at room temperature in
darkness. The solution was then filtered through a pad of Celite
and evaporated to dryness (130 mg, 44%). Single crystals suitable
for X-ray diffraction were obtained by slow evaporation of a
dichloromethane solution. Anal.: Calc. for C38H38Cl2N4O8P2Pd:
C, 49.72; H, 4.17; N, 6.10. Found: C, 49.75; H, 4.12; N, 5.93%. IR
(v/cm−1) KBr: 1608 s (py). 1H NMR (CD3CN, 400 MHz): d 7.86
Synthesis of [di-tertbutylphosphinomethyl-methyl-
pyridin-2-yl-amine] palladium dichloride, 11
Procedure followed as for compound 10. After the reaction mix-
ture had been stirred overnight in DCM an insoluble precipitate
had formed. Solvents were removed in vacuo to give a crude
yellow solid (0.43 g). Recrystallisation from hot acetonitrile gave
yellow cubic crystals on cooling (0.216 g, 53%). Anal.: Calc. for
C15H27N2PCl2Pd: C, 40.61; H, 6.13; N, 6.31. Found: C, 40.59; H,
6.20; N, 6.39%. IR (v/cm−1): 1605.8. 1H NMR (CDCl3, 270 MHz):
3
3
(d, 2 H, JHH = 5.9 Hz, Ar–H), 7.73 (ddd, 2 H, JHH = 8.8 Hz,
3
4
d 9.17 (dd, 1H, JHH = 6.0 Hz, JHH = 1.7 Hz, Ar–H), 7.74 (td,
3JHH = 7.1 Hz, 4JHH = 1.7 Hz, Ar–H), 7.58–7.54 (m, 4H, Ar–H),
3
4
3
1H, JHH = 1.7 Hz, JHH = 7.9 Hz, Ar–H), 6.93 (d, 1H, JHH
=
3
7.42–7.34 (m, 16H, Ar–H), 7.06 (d, 2 H, JHH = 8.8 Hz, Ar–H),
8.5 Hz, Ar–H), 6.88 (td, 1H, 3JHH = 6.9 Hz 4JHH = 1.0 Hz, Ar–H),
6.64 (dd, 2 H, 3JHH = 6.4 Hz, Ar–H), 4.45 (d, 4H, 2JPH = 5.6 Hz,
2
3.22 (d, 2 H, JPH = 2.9 Hz, CH2P), 3.11 (s, 3H, CH3), 1.64 (d,
CH2P), 3.22 (s, 6H, CH3). 31P{ H} NMR (CD3CN 162 MHz)
1
9H, 3JPH = 14.4 Hz, tBu-H). 13C { H} NMR (CDCl3, 400 MHz):
1
d 30.8. FAB-MS: m/z (%): 323 (100) [C19H19N2PO+], 719 (65)
d 163.0 (s, Ar C), 155.3 (s, Ar CH), 140.3 (s, Ar CH), 116.2 (s,
[C38H38NP2Pd+], 918 (10) [{C38H38NP2Pd(ClO4)} ].
+
1
Ar CH), 111.4 (s, Ar CH), 43.7 (d, JPC = 33.5 Hz, CH2), 40.9
(s with fine, CH3), 38.1 (d, 1JPC = 16.9 Hz, t-Bu C), 30.4 (s, t-Bu
CH3). 31P{ H} NMR (CDCl3, 270 MHz): d 64.1. FAB MS: m/z
1
X-Ray crystallography
450 (100) [C15H27N2PCl2Pd+], 407 [C15H27N2PClPd+].
Table 6 provides a summary of the crystallographic data for
compounds 7a, 10, 11, 12 and 14. Data were collected using
Oxford Diffraction PX Ultra (7a and 14) and Xcalibur 3 (10,
11 and 12) diffractometers, and the structures were refined based
on F2 using the SHELXTL and SHELX-97 program systems.43
The B–H protons in the structure of 7a were found from DF maps
and refined with free isotropic thermal parameters subject to a
Synthesis of bis[cyclohexylphosphinomethyl-methyl-
pyridin-2-yl-amine] palladium dichloride, 12
Procedure followed as for compound 10 with stirring overnight
at 40 ◦C. The product was obtained as yellow needle-like crystals
by layering a DCM solution with diethyl ether (0.1 g, 0.12 mmol,
26%). Anal.: Calc. for C38H62Cl2N4P2Pd: C, 56.1; H, 7.7; N, 6.9.
˚
B–H distance constraint of 1.10 A.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 4556–4564 | 4563
©

View Article Online
Table 6 Crystal data, data collection and refinement parameters for compounds 7a, 10, 11, 12 and 14a
7a
10
11
12
14
Formula
Solvent
C19H34BN2P
—
C19H31Cl2N2PPd
CH2Cl2
C15H27Cl2N2PPd
—
C38H62Cl2N4P2Pd
—
[C38H38N4P2Pd](ClO4)2
CH2Cl2
Formula weight
Colour, habit
Crystal size/mm
Crystal system
Space group
332.26
580.65
443.66
Yellow prisms
814.16
Yellow needles
1002.89
Pale yellow blocks
Colourless prismatic needles Yellow blocks
0.26 × 0.15 × 0.14
Monoclinic
P21/n (no. 14)
13.6292(13)
9.5466(7)
15.4193(11)
90.817(7)
2006.0(3)
4
1.100
Cu-Ka
1.196
141
0.30 × 0.22 × 0.09 0.29 × 0.25 × 0.19 0.44 × 0.08 × 0.07 0.21 × 0.17 × 0.16
Monoclinic
P21/n (no. 14)
11.5230(13)
15.8628(19)
13.3819(12)
97.806(8)
2423.4(5)
4
Monoclinic
P21/c (no. 14)
8.5961(13)
13.265(2)
16.3961(3)
105.141(5)
1804.7(4)
4
Monoclinic
C2/c (no. 15)
21.171(4)
20.345(4)
9.4447(8)
104.668(12)
3935.4(11)
4b
Monoclinic
P21/n (no. 14)
9.8852(1)
19.9719(1)
21.56160(1)
98.528(1)
4209.76(5)
4
1.582
Cu-Ka
7.086
143
˚
a/A
˚
b/A
˚
c/A
b/◦
V/A
Z
3
˚
Dc/g cm−3
Radiation used
l/mm−1
1.592
Mo-Ka
1.283
1.633
Mo-Ka
1.408
1.374
Mo-Ka
0.721
2h max/◦
64
64
64
No. of unique refln measured 3706
7934
6141
254
0.034, 0.096
5755
5342
192
0.021, 0.052
6437
4982
214
0.029, 0.076
8162
6763
566
0.031, 0.084
obs, |Fo| > 4r(|Fo|)
3060
221
No. of variables
R1(obs), wR2(all)c
0.039, 0.117
a Details in common: 173 K, graphite monochromated radiation, refinement based on F2. b The molecule has crystallographic C2 symmetry. c R1
=
ꢀ
ꢀ
ꢀ
ꢀ
2
ꢁFo| − |Fcꢁ/ |Fo|; wR2 = { [w(Fo − Fc2)2]/ [w(Fo2)2]}1/2; w−1 = r2(Fo2) + (aP)2 + bP.
21 X. Y. Rueda and S. Castillo´n, J. Organomet. Chem., 2007, 692, 1628–
1632.
Acknowledgements
22 A. Ochida, H. Ito and M. Sawamura, J. Am. Chem. Soc., 2006, 128,
We would like to thank the Wellcome Trust for providing a VIP
award (PWM).
16486–16487.
23 R. B. DeVasher, L. R. Moore and K. H. Shaughnessy, J. Org. Chem.,
2004, 69, 7919–7927.
24 S. L. Parisel, L. A. Adrio, A. A. Pereira, M. M. Perez, J. M. Vila and
K. K. Hii, Tetrahedron, 2005, 61, 9822–9826.
25 J. H. Kirchhoff, C. Dai and G. C. Fu, Angew. Chem., Int. Ed., 2002, 41,
1945–1947.
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