Versatile coordination chemistry of a bis(methyliminophosphoranyl)pyridine ligand on copper centres

Article abstract of DOI:10.1039/c4dt01794c

The coordination of a bis(methyliminophosphoranyl)pyridine ligand (L) to copper centres was studied. The use of copper(i) bromide precursors gave access to [LCuBr] (2) in which only one iminophosphorane arm is coordinated to the metal, as observed by X-ray crystallography and MAS ³¹P NMR. Its fluxional behaviour in solution was demonstrated by VT-³¹P NMR, and investigated by DFT calculations. On the other hand, coordination of L to [Cu(CH₃CN)₄]PF₆ gave a dimer [L ₂Cu₂](PF₆)₂ (3) in which the two copper centres do not have the same coordination sphere as shown by X-ray crystallography. Addition of a strong ligand such as PEt₃ allows the preparation of a cationic monomeric copper complex (4) in which L has a behaviour similar to that observed for 2. Synthesis of copper(ii) complexes was also achieved by chemical oxidation of 2, which shows an irreversible oxidation at -0.36 vs. Fc⁺/Fc, or directly via the coordination of L to CuBr₂. In [LCuBr₂] (5), L adopts a pincer coordination. Finally, the catalytic behaviour of copper(i) complexes 2 and 3 was investigated in cyclopropanation reactions and [3 + 2] cycloadditions.

Full text of DOI:10.1039/c4dt01794c

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Versatile coordination chemistry of a
bis(methyliminophosphoranyl)pyridine ligand
on copper centres†
Cite this: Dalton Trans., 2014, 43,
13399
Thibault Cheisson and Audrey Auffrant*
The coordination of a bis(methyliminophosphoranyl)pyridine ligand (L) to copper centres was studied.
The use of copper(I) bromide precursors gave access to [LCuBr] (2) in which only one iminophosphorane
arm is coordinated to the metal, as observed by X-ray crystallography and MAS 31P NMR. Its fluxional be-
haviour in solution was demonstrated by VT-31P NMR, and investigated by DFT calculations. On the other
hand, coordination of L to [Cu(CH3CN)4]PF6 gave a dimer [L2Cu2](PF6)2 (3) in which the two copper
centres do not have the same coordination sphere as shown by X-ray crystallography. Addition of a
strong ligand such as PEt3 allows the preparation of a cationic monomeric copper complex (4) in which L
has a behaviour similar to that observed for 2. Synthesis of copper(II) complexes was also achieved by
chemical oxidation of 2, which shows an irreversible oxidation at −0.36 vs. Fc+/Fc, or directly via the
coordination of L to CuBr2. In [LCuBr2] (5), L adopts a pincer coordination. Finally, the catalytic behaviour
of copper(I) complexes 2 and 3 was investigated in cyclopropanation reactions and [3 + 2] cycloadditions.
Received 16th June 2014,
Accepted 15th July 2014
DOI: 10.1039/c4dt01794c
www.rsc.org/dalton
Introduction
Over the past 3 decades, pincer ligands1,2 have gained an
important role in coordination chemistry with applications as
sensors,1,3 switches,4 and catalysts.5 This is related to their
modularity allowing fine tuning of both their steric and elec-
tronic properties.6 Various neutral pincer ligands incorporat-
ing three nitrogen donor functions with a central pyridine
have been reported, among which are the well-known terpyri-
dine7 and bis(iminopyridine)8 derivatives. Examples of such
pincers incorporating iminophosphoranes have also been
described (Fig. 1). Bochmann and coworkers used bis(imino-
phosphoranyl)pyridine (A, Fig. 1) complexes for ethylene poly-
merisation.9 More recently, Wang studied the coordination
chemistry of ligand B (Fig. 1) and the catalytic ability of some
of its complexes for ε-caprolactone polymerisation.10 Stephan
and coworkers have explored the coordination of ligand C
(Fig. 1) to palladium,11 and have shown that the coordination
by the nitrogen of the iminophosphorane functions allows a
shielding of the metal by the exocyclic phosphorus substitu-
ents. We were therefore interested in studying the coordination
Fig. 1 Iminophosphorane-pyridine or -phenyl ligands.
behaviour of ligand L, which combines the protective ability of
the peripheral triphenylphosphine groups, the electron donat-
ing capacity of iminophosphoranes, which act as strong σ and
π donors with poor accepting properties, and a central pyri-
dine. We thought that this core would balance the donation
from iminophosphoranes thanks to its accepting capacity. L
was described only recently12 but its bidentate version (D,
Fig. 1) has been employed for more than 10 years.13,14 As
Cadierno and coworkers recently highlighted the importance
of the hemilability15 of iminophosphorane ligands in their
catalytic applications,16 we were interested in studying such be-
haviour with L (Fig. 1), which should present a certain degree
of flexibility thanks to the methylene linkers. Therefore we
chose to investigate its coordination to copper centres. Indeed,
coordination of iminophosphorane ligands to this metal
was not much investigated and often restricted to anionic
Laboratoire de Chimie Moléculaire, Ecole Polytechnique, UMR CNRS 9168, F-91128
Palaiseau Cedex, France. E-mail: audrey.auffrant@polytechnique.edu
†Electronic supplementary information (ESI) available: VT-31P NMR Eyring plot
for 2 and 4, X-ray structure of [LCuBr(MeCN)][Ce(NO3)5] (6), cyclic voltammo-
gram of [LCuBr] (2), computational details. CCDC 996862–996866. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt01794c
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ligands.17 Actually, due to their electronic properties these
ligands are more suitable to stabilise electron-deficient
metals.18
The synthesis and characterisation of bis(methylimino-
phosphoranyl)pyridine copper(I) and copper(II) complexes are
reported. The hemilability of L on CuI centres was studied
using different techniques among which are solution and
solid state NMR studies, X-ray analysis, and DFT calculations.
Some of the synthesised complexes were also used as catalysts
for two reactions: the alkene cyclopropanation and the [3 + 2]
cycloaddition of an organic azide on alkynes.
Scheme 2 Synthesis of complex 2.
1H NMR for the benzylic protons indicating a symmetric
species in solution.
However X-ray analysis performed on single crystals of 2,
obtained by the slow diffusion of benzene into a concentrated
THF solution, evidenced a non-symmetric structure (Fig. 2).
The copper center is only coordinated by the pyridine ring
and one of the iminophosphorane groups, with the second
one totally turned back without any interaction with the
copper or other molecules in the packing. Therefore, the
complex exhibits a trigonal geometry around the copper with a
constrained metallocycle (N1–Cu1–N3: 82.7(1)°). The metal is
almost coplanar with the pyridine ring (0.25 Å out of the
plane), whereas the bromine and the coordinated nitrogen are
slightly out of this plane with the N1–P1 bond almost coplanar
to the Cu1–Br1 bond (8.7° of deviation). The two P–N bonds
are not similar; indeed P1–N1 is longer than the non-co-
ordinated P2–N2 (1.588(3) vs. 1.565(3) Å).
In order to establish whether this unsymmetrical structure
is due to the crystallisation process or to insidious packing
inside the crystal, we recorded a solid-state MAS 31P NMR spec-
trum of a precipitated powder of 2 (Fig. 3, right), which shows
two independent peaks at 8.0 ppm and 22.7 ppm. These are in
agreement with the solid-state structure observed by X-ray
diffraction; the non-coordinated iminophosphorane appears at
8.0 ppm (δp = 5.9 ppm in THF for L) and the coordinated one
resonates at 22.7 ppm (vide infra). To gain further insight into
the behaviour of 2 in solution, we performed variable tempera-
ture 31P{1H} NMR experiments in THF. The characteristic
singlet of 2 starts to broaden around −70 °C and splits into
two broad singlets at 24.4 and 9.5 ppm at −95 °C (Fig. 3, left).
Results and discussion
The pincer ligand L was easily prepared by a bromination/
azidation sequence starting from 2,6-pyridinedimethanol
yielding the bis-azide 1, which was then reacted with triphenyl-
phosphine (Scheme 1).12 The ligand was isolated by precipi-
tation in Et2O in 78% overall yield, as no chromatographic
purification is required, the synthesis is easily scalable to mul-
tigrams (up to 10 g). For the key step (formation of the NvP
bond), we chose to rely on the Staudinger reaction rather than
the Kirsanov reaction11,19 because of the availability of the
starting diol and the stability of 1. Bis(methyliminophosphora-
nyl)pyridine L was characterised by multinuclear NMR and
elemental analysis.
It exhibits a singlet at 9.9 ppm in 31P{1H} NMR in CD2Cl2
and a doublet (3JP,H = 16.0 Hz) for the benzylic protons at
1
4.30 ppm in H NMR in agreement with literature data.12,13 As
most iminophosphoranes, L is sensitive to moisture and
decomposes in contact with air to the corresponding amine
and triphenylphosphine oxide within minutes (in solution)
and hours (in the solid state).
Synthesis of copper(I) complexes
Bis(methyliminophosphoranyl)pyridine ligand
L was first
reacted with various copper(I) bromide precursors (CuBr,
[CuBr(SEt2)] and [CuBr(PPh3)3]). In all cases, complex [LCuBr]
(2) was formed as the sole product (Scheme 2), this requires
few hours using [CuBr(SEt2)] or [CuBr(PPh3)3] and overnight
heating employing the poorly soluble polymeric CuBr. Interest-
ingly, L was able to displace three triphenylphosphines. 2 is
characterised in THF by a large singlet at 16.5 ppm in
31P{1H} NMR and a doublet at 4.24 ppm (3JP,H = 11.5 Hz) in
Fig. 2 ORTEP of the solid-state structure of 2 with 50% probability
thermal ellipsoids. Hydrogens and one benzene solvent molecule were
omitted for clarity. Selected bond lengths [Å] and angles (°): N1–Cu1
2.002(3), N3–Cu1 2.050(3), P1–N1 1.588(3), P2–N2 1.565(3), Cu1–Br1
2.2546(6); N1–Cu1–N3 82.7(1), N1–Cu1–Br1 141.17(8), N3–Cu1–Br1
135.37(8).
Scheme 1 Synthesis of the ligand L.
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Fig. 3 VT 31P{1H} NMR of 2 in THF (left), MAS 31P NMR spectrum of powdered 2 (right).
The exchange is too rapid to be completely blocked at this
This exchange process was also investigated by DFT calcu-
temperature; nevertheless, the coalescence was estimated lations, both associative and dissociative mechanisms were
around −90 2 °C. An Eyring plot (see ESI†) allows the evalu- computed (see Fig. 4). Starting from I (the optimised structure
ation of the activation barrier of the process at 7.3(1) kcal of 2), the dissociative pathway proceeds by the decoordination
mol−1
.
of the iminophosphorane leading to the linear intermediate II
Fig. 4 Energetic profile of the computed exchange.
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located at 9.7 kcal mol−1 above I. This value is slightly over the
experimental one, especially if a solvation continuum is used
(11.6 kcal mol−1, standard PCM in THF). The second pathway
proceeds through a concerted associative mechanism, a first
minimum (III), in which the second iminophosphorane
remains non-coordinated but has flipped on the copper side,
was located by performing a relaxed scan on the nitrogen–
copper distance. Despite the flatness of the energetic profile
near this point, a transition state labelled TS IV (Fig. 4) was
located at 7.8 kcal mol−1 above I. The latter value, in good
agreement with the experimental one, validates the hypothesis
of a concerted associative pathway.
This behaviour markedly differs from the hemilability of
the pyridine moiety on copper(I) observed by van der Vlugt
et al. using phosphine analogues of L (i.e. R2PCH2(C5H3N)-
−
t
Fig. 5 ORTEP plot (50% of probability) of 3, hydrogens and two PF6
CH2PR2, R = Ph, Bu). Moreover, upon halide abstraction they
reported the isolation of a cationic T-shaped CuI complex.20
When abstracting the bromine from 2, a yellow solid poorly
soluble in THF is obtained. Its 31P{1H} NMR spectrum evi-
dences a sharp singlet at 28.8 ppm, which is in the range of
the copper coordinated iminophosphorane, and a heptuplet at
−144.1 ppm for the PF6 counter-anion. The same complex
3 was synthesised by reacting L with a stoichiometric amount
of [Cu(CH3CN)4]PF6 in THF (Scheme 3).
counter-anions have been omitted for clarity, and phenyl substituents
on P have been restricted to their Cipso for the same reason. Selected
bond lengths [Å] and angles (°): Cu1–Cu2 2.8863(3), N1–Cu1 1.881(2),
N4–Cu1 1.867(2), N2–Cu2 2.136(2), N5–Cu2 2.131(2), N3–Cu2 2.012(2),
N6–Cu2 2.011(2), P1–N1 1.587(2), P2–N2 1.582(2), N4–P3 1.597(2), N5–
P4 1.594(2); N1–Cu1–N4 174.09(7), N5–Cu2–N6 83.41(6), N2–Cu2–N3
82.58(6), N2–Cu2–N5 129.23(6).
values comparable to those obtained by X-ray crystallography.
This seems to indicate that the dimeric structure of 3 is main-
tained in solution; nevertheless, DFT calculations were con-
ducted in order to evaluate the stability of this structure in the
presence of the coordinating ligand. Enthalpy and Gibbs
energy of the coordination reaction are reported in Fig. 6. As
all the structures involved are cationic and the solvents used
are polar (THF, acetonitrile), solvation effects were modelled
with the polarisable continuum model (PCM).
These calculations show that the existence of the T-shaped
complex (VIII) is unlikely, since the coordination of an aceto-
nitrile ligand giving VII is favoured. Importantly, formation of
the tetracoordinated (or tricoordinated, not depicted) cationic
monomer with acetonitrile (VII) or an electron rich benzo-
nitrile such as 1,3,5-trimethoxy-benzonitrile23 (VI) is disfa-
voured compared to the dimer V by 18 and 10 kcal mol−1
respectively. This is in agreement with the results obtained
from the 1H DOSY NMR experiment. Experimentally, when
mixing 3 and 1,3,5-trimethoxy-benzonitrile in acetonitrile no
significant change was observed in 31P{1H} NMR. Moreover, X-
ray analysis of single crystals obtained by gas diffusion of
pentane evidenced the presence of 3.
The X-ray analysis of 3 (crystals were obtained by gas
diffusion of pentane into an acetonitrile solution) did not
show the expected T-shaped complex but the dimeric species
as depicted in Fig. 5.
The coordination spheres of the two copper atoms are quite
different; Cu1 is coordinated by two iminophosphorane
groups in a nearly linear mode (N2–Cu1–N4: 173.7(1)°),
whereas Cu2 exhibits a distorted tetrahedral geometry, with
the coordination of the two pyridine rings and two imino-
phosphoranes. Such divergent geometries for two coppers were
also observed by Piguet et al.21 Interestingly the NNvP–Cu bond
lengths are rather different with the N–Cu1 bonds (N1–Cu1:
1.881(2) Å, N4–Cu1: 1.864(2) Å) being shorter than the N–Cu2
(N2–Cu2: 2.136(2) Å, N5–Cu2: 2.131(2) Å). Moreover a small d10–
d10 interaction between the two copper atoms may exist (Cu1–
Cu2: 2.8863(3) Å),22 based on DFT calculations (see ESI†).
In order to determine whether 3 remains dimeric in solu-
1
tion, H DOSY experiments were performed on 2 and 3. Their
hydrodynamic radii were evaluated at 6.6 Å and 7.6 Å respecti-
vely. Approximating these complexes as a sphere gave volume
From these thermodynamic calculations, triethylphosphine
should be able to react with V to give IX located 4 kcal mol−1
below the dimer V. Indeed when a suspension of 3 in THF is
treated with an excess of PEt3, the mixture turned rapidly clear
to yield complex 4, which was characterised by multinuclear
NMR spectroscopy and X-ray analysis. Therefore, the formation
of a monomeric complex requires a strong ancillary ligand (Br,
PEt3) (Scheme 4).
It is noteworthy that we used an excess of PEt3 for practical
reasons but never observed the decoordination of L. In the
Scheme 3 Synthesis of [L2Cu2].
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Fig. 6 Enthalpy and Gibbs energy of the coordination of various ligands on V.
Scheme 4 Formation of 4 by addition of PEt3.
−
Fig. 7 ORTEP plot (50% of probability) of 4, hydrogens and a PF6
31P{1H} NMR spectrum both iminophosphorane moieties are
equivalent appearing as a singlet at 21.6 ppm and the co-
ordinated phosphine gave a singlet at −3.6 ppm. Therefore,
counter-anion have been omitted for clarity. Selected bond lengths [Å]
and angles (°): N1–Cu1 1.990(2), N3–Cu1 2.066(2), P1–N1 1.591(2), P2–N2
1.580(2), Cu1–P3 2.174(1); N1–Cu1–N3 83.1(1), N1–Cu1–P3 138.66(7).
1
room temperature H, 13C and 31P NMR data seem to indicate
3
a symmetric species in solution; however, no JP,P is observed
on the 31P{1H} spectrum suggesting a dynamic behaviour as
observed for 2. VT-31P{1H} NMR spectroscopy (see ESI†) allows
copper presents a trigonal geometry, the metal–nitrogen bond
lengths and angles are comparable to those measured in 2.
As the pincer coordination of bis(methyliminophosphora-
nyl)pyridine should be favoured on a copper(II) centre, we syn-
thesised [LCuII] complexes either by a chemical oxidation of 2
or a direct reaction of L with CuBr2 (vide infra).
the estimation of the activation barrier at 8.8(1) kcal mol−1
.
Therefore, the hemilabile behaviour of L is maintained on a
cationic copper(I) fragment. It is noteworthy that, X-ray ana-
lysis of single crystals formed by slow diffusion of pentane into
its solution in THF gave a structure analogous to 2 (Fig. 7). The
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The cyclic voltammogram of 2 exhibits one irreversible oxi- Oxidation of copper(I) complex 2 was then realised with
dation wave at −0.36 V and one irreversible reduction wave at various oxidants (NOBF4, FcPF6, [Ce(NO3)6](NH4)2 (CAN),
−0.93 V vs. E1/2(Fc+/Fc) (see ESI†). We attributed the observed Scheme 5). In all cases, a rapid change to a green solution was
irreversibility to the rapid modification of the coordination observed, the product formed is characterised by a sharp
mode of the ligand when changing the oxidation state of the singlet around 40 ppm by 31P{1H} NMR spectroscopy. From
copper atom. After oxidation, the second iminophosphorane such a crude mixture obtained with CAN as the oxidant, single
coordinates with the metal and this pincer complex is then crystals were obtained. Their X-ray analysis evidenced a bi-
reduced inducing the rapid decoordination of one iminophos- metallic [LCuII(CH3CN)][CeIII(NO3)5] complex 6 (see ESI†). The
phorane group from the copper(I) centre. As anticipated the copper(II) complexes formed by oxidation were identified by
oxidation is slightly easier but the reduction is more difficult the addition of KBr to give the independently synthesised
compared to that of (2-pyridylmethyl)imine copper(I) com- complex 5 as ascertained by NMR spectroscopy and X-ray
plexes.24 In summary, both oxidation and reduction are irre- crystallography.
versible but the sequence redox change/reorganisation can be
repeated without loss of intensity.
Thus, as expected no hemilability occurs on the more elec-
tron-deficient copper(II) centre, in agreement with a stronger
interaction. Expecting that the versatile coordination of L may
generate under-coordinated copper(I) species, complexes 2 and
3 were tested as catalysts in two well-known copper catalysed
reactions (i.e. cyclopropanations and [3 + 2] cycloadditions).
Synthesis of copper(II) complexes
Prior to performing a chemical oxidation of 2, we coordinated
L to CuBr2 (see Scheme 5). The reaction was conducted in
THF, giving immediately a green solution. The green solid
obtained after workup exhibits a singlet at 42.7 ppm in CD2Cl2
in 31P{1H} NMR but its 1H NMR spectrum reveals broad and
slightly shifted peaks suggesting a paramagnetic complex.
This was further confirmed by the measurement of the mag-
netic moment of 5 by the Evans method25 (μeff = 1.6(1)μB), in
agreement with a S = 1/2 complex.
Crystals of 5 were obtained by slow evaporation of its THF
solution, the structure is presented in Fig. 8. The copper is pen-
tacoordinated and exhibits a nearly planar-square geometry
(RMS deviation of 0.183 Å out of planarity) with a second
bromine in the apical position, which is subjected to a Jahn–
Teller distortion (Cu1–Br1 2.3599(6) Å vs. Cu1–Br2 2.9871(6) Å).
The two iminophosphorane bonds are similar (N1–P1 1.597(3),
P2–N2 1.608(3)) and comparable to that observed in 2 for the
coordinated iminophosphorane but significantly longer than
the non-coordinated one in the same complex (1.565(3) in 2).
Catalytic tests
As a test reaction we have examined the cyclopropanation26 of
styrene with EDA (ethyldiazoacetate) in the presence of 1%
catalyst (Table 1). For comparable reactions, Reetz and co-
workers have reported good conversion with a promising
enantiomeric excess using a chiral iminophosphorane catalyst,
suggesting that the donor iminophosphorane ligand remains
bound to the metal during the catalysis.27 This prompted us to
evaluate complexes 2 and 3 in such reactions with the idea
that they should be able to stabilise the copper carbene
complex proposed as an intermediate in these cyclopropana-
tion reactions.28 First catalytic reactions with non-optimised
conditions showed that complex 2 mainly catalyses the dimeri-
sation of the diazo compound, while complex 3 is more
efficient for cyclopropanation. However, in the presence of
silver triflate, complex 2 becomes more selective. With a slower
addition of EDA, we were able to obtain 85% of cyclopropane
with 3 and above 90% with 2 after removal of the bromide.
Even if most cyclopropanation methodologies reported
focused on the stereoselectivity of this process it is interesting
to note that lower chemical yield of cyclopropane was obtained
with copper complexes featuring diamine ligands.29 In con-
trast to our expectation that the hemilability of the iminopho-
sphorane arm could disfavour the dimerisation of the diazo
compound, the addition of a poorly coordinating anion
remains essential to control this side reaction. Therefore we
decided not to further explore this process and turned our
attention to [3 + 2] cycloadditions, another emblematic cata-
lytic reaction with copper(I) catalysts.30
We first examined the cycloaddition of phenylacetylene and
4-methoxybenzylazide (Table 2). With 1% of both catalysts
and without solvent, the reaction was very rapid and exother-
mic inducing a brutal solidification of the medium (Table 1,
entry 1). Decreasing the amount of catalyst to 0.1% still allows
a rapid reaction with a slower solidification of the medium.
Full conversion is observed in this case within few minutes,
the triazole was isolated in excellent yield (entries 2 and 3).
Fig. 8 ORTEP plot (50% of probability) of 5, hydrogens have been
omitted for clarity. Selected bond lengths [Å] and angles (°): N1–Cu1
2.041(3), N2–Cu1 2.071(3), N3–Cu1 1.955(3), P1–N1 1.597(3), P2–N2
1.608(3), Cu1–Br1 2.3599(6), Cu1–Br2 2.9871(6); N1–Cu1–N2 151.0(1),
N3–Cu1–N1 79.6(1), N2–Cu1–N2 80.1(1), N3–Cu1–Br1 178.3(1), N3–
Cu1–Br2 81.0(1).
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Scheme 5 Synthesis of copper(II) complexes by salt metathesis or chemical oxidation.
6). As both catalysts display comparable activity, further exper-
iments were conducted with 2. When carrying out this reaction
in a biphasic toluene–water solvent mixture in air, we were
pleased to observe that the catalyst remains active. The yield is
lower compared to the anhydrous conditions, probably due to
partial decomposition. Moreover, when reacting sterically hin-
dered tert-butylacetylene with 4-methoxybenzylazide in the pres-
ence of 0.5% of 2, the triazole was isolated in 98% yield after 2 h.
These results are comparable to those described with bulky
NHC–Cu(I) complexes;31 however, 3-hexyne does not react in
the presence of 2. Therefore the cycloaddition proceeds
efficiently at a low catalyst loading even with a less reactive
alkyne, these performances outstrip those of [CuBr(PPh3)3]32
or of other iminophosphorane systems.33
Table 1 Cyclopropanation reactions
Catalyst
Additivea
Yield of Ab (%)
Yield of Bb (%)
3
2
2
None
AgOTf
AgNTf2
85
93 (84)
91
15
7
9
a 1 mol%. b GC yield (isolated yield) with respect to EDA.
Table 2 [3 + 2] cycloaddition
Conclusion
In conclusion, we evidenced the versatile coordination of bis-
(methyliminophosphoranyl)pyridine ligand
L
to copper
centres depending on both the oxidation state of the Cu and
the reaction conditions. Generally only one iminophosphorane
is bound to the metal in copper(I) complexes, and these mono-
meric complexes (2 and 4) exhibit a fluxional behaviour in
solution, which was studied by variable temperature NMR and/
or DFT calculations. When no strong coordinating ligand is
available a dimeric complex 3 is formed whose structure is pre-
served in solution. The oxidation of [LCuBr] is irreversible and
induces a change in the coordination mode of L. Indeed in
copper(II) complex 5, which has been synthesised by chemical
oxidation of 2 or from CuBr2, L acts as a pincer. Therefore the
hemilability of the iminophosphorane moieties can be redox
driven. In order to evaluate their catalytic ability copper(I) com-
plexes 2 and 3 were employed for cyclopropanation and [3 + 2]
cycloaddition of organic azides on alkynes. Current efforts
focus on exploring this switchable redox-induced hemilability
with other late transition metals.
Catalyst
Conditions
Time
Yielda
2 or 3
1 mol%, N2, neat
0.1 mol%, N2, neat
0.1 mol%, N2, neat
0.1 mol%, N2, dichloromethane
0.1 mol%, N2, dichloromethane
0.1 mol%, N2, toluene
0.1 mol%, N2, toluene
0.1 mol%, air, toluene/water
<1 min
3 min
5 min
20 min
30 min
45 min
2 h
n.d.b
94%
98%
96%
98%
96%
95%
77%
2
3
2
3
2
3
2
4 h
a Isolated yield. b Not determined due to the very rapid solidification of
the mixture.
Both catalysts allow also a rapid reaction in dichloromethane
(entries 4 and 5). In toluene, 2 proved more efficient than 3
which is probably explained by its better solubility (entries 5 and
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8.0 Hz, 2H, H2), 4.53 (s, 4H, H4). 13C NMR (CDCl3) δ 156.8 (C3),
138.3 (C1), 122.4 (C2), 33.6 (C4).
Experimental part
Synthesis
Synthesis of 1. NaN3 (0.813 g, 12.5 mmol) was dissolved in
All reactions were conducted under an atmosphere of dry DMSO (25 mL), dibromomethylpyridine (1.33 g, 5 mmol) was
nitrogen or argon, using standard Schlenk and glovebox tech- added and the mixture was stirred overnight at room tempera-
niques. Solvents and reagents were obtained from commercial ture. Water (25 mL) was added to quench the reaction, the
sources. Tetrahydrofuran, diethyl ether, toluene and petroleum mixture was then extracted with Et2O (3 × 50 mL), the com-
ether were dried with an MBraun MB-SPS 800 solvent purifi- bined organic layers were washed with water (2 × 100 mL) and
cation system. Pentane and acetonitrile were distilled from brine (100 mL), dried over MgSO4 and evaporated to dryness to
CaH2, under dry nitrogen. [CuBr(PPh3)3]34 and [CuBr(SEt2)]35 yield 1 as a yellow oil (0.940 g, 99%). 1H NMR (CDCl3) δ 7.76 (t,
were prepared following the literature procedure.
J = 8.0 Hz, 1H, H1), 7.30 (d, J = 8.0 Hz, 2H, H2), 4.48 (s, 4H,
H4). 13C NMR (CDCl3) δ 155.9 (C3), 138.0 (C1), 121.1 (C2),
55.4 (C4).
Measurements
Nuclear magnetic resonance (NMR) spectra were recorded on a
Synthesis of L. In a Schlenk flask, 1 (1.40 g, 7.40 mmol) and
Bruker Av300 spectrometer operating at 300 MHz for 1H, PPh3 (3.88 g, 14.8 mmol) were mixed in dry Et2O (70 mL) indu-
75.5 MHz for 13C, and 121.5 MHz for 31P. Solvent peaks were cing nitrogen evolution. The mixture was stirred overnight at
used as internal references for 1H and 13C chemical shifts room temperature (if necessary the flask can be evacuated
(ppm). 31P peaks were referenced to an external 85% H3PO4. after few minutes to avoid over-pressure). The completeness of
The following abbreviations are used: br, broad; s, singlet; d, the reaction was verified by 31P{1H} NMR showing a singlet at
doublet; t, triplet; m, multiplet; hept, heptuplet. Labelling of 6.6 ppm for the bis(iminophosphorane). The reaction volume
atoms is indicated in Scheme 1. It should be mentioned that was reduced to about 15 mL resulting in the formation of a
solution 31P chemical shift of the compounds described white precipitate which was filtered under nitrogen, washed
herein can be remarkably affected by the nature of the solvent with pentane (2 × 30 mL) and dried under vacuum to yield 3
and, to a lesser extent, by dilution.36
(4.23 g, 87%). 31P{1H} NMR (CD2Cl2) δ 9.9 (s). 1H (CD2Cl2)
The 1H PGSE (DOSY) experiments were performed on the δ 7.79–7.59 (m, 15H, H1, H2, and H6), 7.57–7.30 (m, 18H, H7,
same spectrometer. The experiment was measured using the H8), 4.30 (d, JP,H = 16.0 Hz, 4H, H4). 13C NMR (CD2Cl2) δ 164.4
ledbpgp2s pulse program (Bruker) at a temperature of 298 K. A (d, JC,H = 23.5 Hz, C3), 136.7 (C1), 132.8 (d, JP,C = 9.0 Hz, C6),
relaxation delay of 10 s was employed along with a diffusion 132.4 (d, JP,C = 95.5 Hz, C5), 131.5 (d, JP,C = 3.0 Hz, C8), 128.7
time (Δ) of 50 ms and an eddy current delay of 5 ms. Bipolar (d, C, JP,C = 11.5 Hz, C7), 118.5 (C2), 51.4 (d, JP,C = 3.0 Hz, C4).
gradient pulses (δ/2) of 2.2 ms and homospoil gradient pulses Anal. Calcd for C43H37N3P2: C, 78.52; H, 5.67; N, 6.39. Found:
of 1.1 ms were used. The gradient strengths of the 2 homospoil 78.39; H, 5.74; N, 6.49.
pulses were −17.13% and −13.17%, respectively. Thirty-two
Synthesis of [LCuBr] (2). From CuBr: L (0.917 g, 1.39 mmol)
experiments of sixteen scans each were collected with the and CuBr (0.200 g, 1.39 mmol) were mixed in THF (10 mL)
bipolar gradient strength, initially at 2% (1st experiment), line- and heated overnight at 50 °C, the mixture turned yellow with
arly increased to 95% (32nd experiment). All gradient pulses the formation of a clear yellow precipitate. The completeness
were sine shaped and after each application a recovery delay of of the reaction was ascertained by 31P{1H} NMR, then the
200 µs was used. Further processing was achieved using the solvent volume was reduced to about 5 mL, and 10 mL of
MestReNova software.
pentane were added to achieve the precipitation. The resulting
MAS (magic angle spinning) 31P solid-state NMR experi- mixture was centrifuged, the supernatant was removed, the
ments were recorded at room temperature on a Tecmag solid was washed twice with pentane (10 mL) and finally dried
Apollo360 spectrometer using a CP/MAS Bruker probe. The 31
P
under vacuum overnight to yield the title compound as a clear
spectrum (Larmor frequency: 145.77 MHz, spinning rotation: yellow powder (1.05 g, 94%). 31P{1H} NMR (THF-d8) δ 16.5 (bs).
15 kHz) was externally referenced to a solution of H3PO4.
1H NMR (CD3CN) δ 7.77 (t, JH,H = 7.7 Hz, 1H, H1), 7.64–7.08
Elemental analyses were determined by Mr Stephen Boyer (m, 30H, H6, H7, H8), 7.04 (d, JH,H = 7.7 Hz, 2H, H2), 4.13 (d,
at London Metropolitan University.
Synthesis of dibromomethylpyridine. Adapted from
JP,H = 11.1 Hz, 4H, H4). 13C NMR (CD3CN) δ 161.6 (d, JP,C
19.0 Hz, C3), 138.6 (C1), 133.7 (C8), 133.6 (JP,C = 9.5 Hz, C6),
=
a
literature procedure:37 2,6-dihydroxymethylpyridine (3.48 g, 129.8 (JP,C = 12.0 Hz, C7), 128.3 (JP,C = 98.8 Hz, C5), 122.6 (C2),
25 mmol) was dissolved in 50 mL of DMF. The flask was 53.2 (C4). Anal. Calcd for C43H37BrCuN3P2: C, 64.46; H, 4.65;
cooled to 0 °C and PBr3 (57.5 mmol, 5.5 mL) was added drop- N, 5.24. Found: C, 64.31; H, 4.66; N, 5.17.
wise under vigorous stirring. The ice bath was removed and
From [CuBr(SEt2)]: 23.4 mg of [CuBr(SEt2)] (23.4 mg,
the mixture was stirred overnight. Water (125 mL) was slowly 0.1 mmol) and of L (65.8 mg, 0.1 mmol) were stirred in THF
added to quench the reaction, the mixture was extracted with (5 mL) for 2 hours. 2 was obtained after precipitation and
Et2O (3 × 125 mL), the combined organic layers were washed repeated washing with pentane (5 × 3 mL) (43.3 mg, 54%).
with water (2 × 150 mL) and brine (150 mL), dried over MgSO4
From [CuBr(PPh3)3]: L (0.131 g, 0.2 mmol) and [CuBr-
and evaporated to dryness giving an off-white solid (6.03 g, (PPh3)3] (0.186 g, 0.2 mmol) were stirred in THF (5 mL) for
91%). 1H NMR (CDCl3) δ 7.70 (t, J = 8.0 Hz, 1H, H1), 7.37 (d, J = 2 hours, the completeness of the reaction was verified by
13406 | Dalton Trans., 2014, 43, 13399–13409
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Paper
31P{1H} NMR showing 2, and the presence of free PPh3 (δp
=
General catalytic protocols for [3 + 2] cycloaddition
−5.2 ppm). The solvent was evaporated and the resulting solid
was washed with Et2O (6 × 5 mL), finally 2 was obtained as a
slightly yellow powder (64.2 mg, 40%).
In a glove-box, a Schlenk flask was charged with the catalyst
(1 mol% or 0.1 mol%), the flask was closed and removed from
the glove-box. Under a flux of nitrogen, the solvent or the
mixture of solvents (1 mL) was added, and then the azide
(1 mmol, 154 μL) and the alkyne (1 mmol, 110 μL) were intro-
duced. Depending on the reaction conditions, the flask was
either closed, evacuated and back-filled with nitrogen or let to
open air. The completion of the reaction was checked by TLC.
The reaction was quenched by the addition of water (5 mL),
extracted with EtOAc (3 × 5 mL), dried over MgSO4 and sub-
jected to rotary evaporation to yield the triazole.
1-(4′-Methoxybenzyl)-4-phenyl-1H-1,2,3-triazole (94%). Spectro-
scopic data were in agreement with the literature.38 The com-
pound was also univocally identified by X-ray analysis
of crystals obtained by slow evaporation of the NMR sample
(X-ray structure is available on request).
Complex [L2Cu2](PF6)2 (3). In a Schlenk flask, L (131.5 mg,
0.2 mmol) and [Cu(CH3CN)4]PF6 (74.4 mg, 0.2 mmol) were dis-
solved in THF (5 mL), the solution turned quickly to yellow/
green and was stirred for one additional hour upon which a
yellow precipitate formed. The completeness of the reaction
was checked by 31P{1H} NMR of the crude mixture. The solvent
was reduced to about 1 mL and 10 mL of petroleum ether were
added to complete the precipitation, the solid was separated
by filtration, washed with Et2O (5 mL) and petroleum ether
(10 mL). The resulting yellow solid was dried under vacuum
overnight to give 3 (147 mg, 85%). 31P{1H} NMR (CD3CN)
δ 30.8 (s), −142.2 (hept, JPF = 706 Hz). 1H NMR (CD3CN)
δ 7.78 (t, JH,H = 7.5 Hz, 1H, H1), 7.55–7.41 (m, 6H, H8), 7.38–7.25
(m, 12H, H6), 7.20–7.16 (m, 12H, H7), 6.98 (d, JH,H = 7.8 Hz, 2H,
1-(4′-Methoxybenzyl)-4-tbutyl-1H-1,2,3-triazole
1H NMR (CDCl3) δ 7.22 (d, J = 8.5 Hz, 2H, HAr), 7.11 (s, 1H,
triazole), 6.89 (d, J = 8.5 Hz, 2H, HAr), 5.41 (s, 2H, CH2), 3.81 (s,
(98%).
H2), 4.12 (bs, 4H, H4). 13C NMR (CDCl3) δ 161.7 (d, JC,P
17.6 Hz, C3), 138.9 (C1), 134.0 (JC,P = 2.9 Hz, C8), 133.8 (JC,P
=
=
H
9.5 Hz, C6), 130.0 (d, JC,P = 12.1 Hz, C7), 128.1 (JC,P = 99.1 Hz,
C5), 123.2 (C2), 53.7 (C4). Anal. Calcd for C86H74Cu2F12N6P6:
C, 59.62; H, 4.31; N, 4.85. Found: C, 59.51; H, 4.41; N, 4.82.
From 2: Using a vial, 2 (9.1 mg, 11.4 μmol) and AgPF6
(2.9 mg, 11.4 μmol) were mixed in THF-d8 (1 mL) inducing the
rapid precipitation of the formed AgBr salt. The mixture is fil-
tered and transferred to an NMR tube for spectroscopic analy-
sis which was in agreement with the data reported above. The
yield was not determined.
3H, OMe), 1.31 (s, 9H, tBu). 13C NMR (CDCl3) δ 159.96 (CIVAr),
158.21 (CIVAr), 129.74 (CHAr), 127.19 (CIVtriazole), 118.21
(CHtriazole), 114.55 (CHAr), 55.46 (OMe), 53.63 (CH2), 30.9
(CIV-tBu) 30.51 (tBu).
General optimised catalytic protocols for cyclopropanation
In a glove-box, a Schlenk flask was charged with the catalyst
(1 mol%) and the silver salt (1 mol%). The flask was closed
and removed from the glove-box. Under a flux of nitrogen, dry
CH2Cl2 (3 mL) and styrene (460 μL, 4 mmol) were added using
a syringe, then the flask was cooled to about −5 °C and the
EDA (2 mmol, 210 μL) diluted in 5 mL of dry CH2Cl2 was
added dropwise over 1 h. The flask was then closed, evacuated
and back-filled with nitrogen and stirred overnight. The
mixture was analyzed by GC. The solvent was then evaporated
to yield the crude mixture which was purified by flash
Complex [LCu(PEt3)]PF6 (4). In
a vial, PEt3 (25 mg,
0.21 mmol) was added to [L2Cu2](PF6)2 (64.9 mg 0.038 mmol)
suspended in THF (2 mL), within few seconds the solution
turned clear. The solution was layered with pentane and
allowed to stand overnight at −35 °C to yield colorless crystals
which were collected, washed with pentane (5 mL) and dried
in a vacuum (29 mg, 39%). 31P{1H} NMR (THF-d8) δ 21.6 (s,
PV), −3.4 (s, PIII), −144.8 (hept, JPF = 710 Hz, PF6). 1H NMR
(THF-d8) δ 7.89–7.73 (m, 13H, H1, H6), 7.72–7.48 (m, 20H, H2,
H7, H8), 4.46 (d, JP,H = 10.0 Hz, 4H, H4), 0.92 (t, JH,H = 7.5 Hz,
6H, PCH2CH3), 0.69 (dt, JP,H = 15.5 Hz, JH,H = 7.5 Hz, 9H,
PCH2CH3). 13C NMR (THF-d8) δ 163.5 (d, JC,P = 24.0 Hz, C3),
139.0 (C1), 133.5 (d, JC,P = 9.0 Hz, C6), 133.1 (d, JC,P = 3.0 Hz,
C8), 130.5 (d, JC,P = 98.0 Hz, C5), 129.6 (d, JC,P = 11.5 Hz, C7),
chromatography (95/5 petroleum ether–EtOAc) to give
a
mixture of diastereoisomers with NMR data in agreement with
the literature.39
X-ray crystallography
Data were collected at 150 K on a Bruker Kappa APEX II
diffractometer using a Mo-Kα (λ = 0.71069 Å) X-ray source and
a graphite monochromator. The crystal structure was solved
using SIR 9740 and SHELXL-97 or SHELXL-2013.41 ORTEP
drawings were made using ORTEP III for Windows.42 X-ray
data are gathered in Table S1 (ESI†).
120.7 (C2), 53.3 (C4), 16.3 (d, JC,P = 23.7 Hz, CH2), 8.7 (d, JC,P
=
1.4 Hz, CH3). Anal. Calcd for C49H52CuF6N3P4: C, 59.79; H,
5.32; N, 4.27. Found: C, 59.65; H, 5.32; N, 4.36.
Complex [LCuBr2] (5). L (131.5 mg, 0.2 mmol) was sus-
pended in THF (10 mL) and anhydrous CuBr2 (44.7 mg,
0.2 mmol) was added. The mixture turned immediately green
and after 30 minutes of stirring a green precipitate formed.
Computational details
The solvent is evaporated under vacuum, the solid is washed All calculations were performed using the Gaussian 09 series
with Et2O (2 × 5 mL), dried under vacuum overnight to give the of programs (revision B.01).43 The ω-B97XD functional was
title compound as
a green powder (104.5 mg, 59%). used in combination with the 6-31G* basis set for carbons and
31P{1H} (CD2Cl2) δ 42.7 (s). μeff = 1.6μB (by the Evans method in hydrogens; the 6-311+G** basis set for bromine, phosphorus,
CD2Cl2). Anal. Calcd for C43H37Br2CuN3P2: C, 58.62; H, 4.23; nitrogen and oxygen; and the Def2-TZVP basis set for copper.44
N, 4.77. Found: C, 58.30; H, 4.38; N, 4.62.
The stationary points and transition states were characterised
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Dalton Trans., 2014, 43, 13399–13409 | 13407

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Dalton Transactions
by full vibration frequency calculations, with no imaginary fre-
quency for minima (stationary point), and one imaginary fre-
quency for the transition states. Solvent effects were
9 S. Al-Benna, M. J. Sarsfield, M. Thornton-Pett,
D. L. Ormsby, P. J. Maddox, P. Brès and M. Bochmann,
J. Chem. Soc., Dalton Trans., 2000, 4247–4257.
introduced through PCM single-point calculations,45 using 10 Z. Y. Chai, C. Zhang and Z. X. Wang, Organometallics, 2008,
standard Gaussian parameters, the solvent was THF. The PCM
Gibbs energies were calculated using the correction proposed
by Maseras and coworkers.46
27, 1626–1633; Z. Y. Chai and Z. X. Wang, Dalton Trans.,
2009, 8005–8012.
11 M. J. Sgro and S. W. Stephan, Dalton Trans., 2011, 40,
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12 Transition metal P-N complexes as polymerization catalysts
by LANXESS Deutschland GmbH and University of
Toronto, EP2641909, 2013.
Acknowledgements
Ecole Polytechnique and CNRS are thanked for financial 13 L. P. Spencer, R. Altwer, P. R. Wei, L. Gelmini, J. Gauld and
support. We are grateful to Dr S. Maron for recording MAS 31P
D. W. Stephan, Organometallics, 2003, 22, 3841–3854.
NMR, Dr E. Nicolas and Dr S. Labouille for fruitful discussions 14 R. Bielsa, R. Navarro, E. P. Urriolabeitia and T. Soler,
about DFT calculations and Dr G. Nocton for his help on VT
and DOSY NMR.
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