Diphosphinine derivatives of terpyridine: A new class of neutral π-accepting PNP-pincer ligands

Article abstract of DOI:10.1002/chem.200801337

A study was conducted to illustrate the development of a synthetic access to a novel neutral PNP-pincer ligand containing two phosphinine donors and a bridging pyridine moiety. In contrast to its terpyridine analogue facile coordination of this tridentate ligand towards a neutral Cu¹ center was observed. The corresponding Cu¹Br complex was characterized crystographically and revealed a distorted tetrahedral coordination geometry of the metal center as a result of an unusual coordination mode of the two phosphinine ligands. Due to the presence of electronically rather than inequivalent donor atoms, this novel ligand represents a novel class of PNP-pincer systems. This is expected to lead to transition-metal complexes with new properties and applications, especially in homogeneous catalysis and as optoelectronic devices in the near future.

Full text of DOI:10.1002/chem.200801337

COMMUNICATION
DOI: 10.1002/chem.200801337
Diphosphinine Derivatives of Terpyridine:
A NewClass of Neutral p-Accepting PNP-Pincer Ligands
Christian Müller,*[a] Evgeny A. Pidko,[a] Martin Lutz,[b] Anthony L. Spek,[b] and
Dieter Vogt[a]
Phosphinines (phosphabenzenes, phosphorins), the higher
homologues of pyridines are low-coordinated phosphorus
compounds that possess significantly different steric and
electronic properties compared to common ligands based on
trivalent phosphorus.[1,2]
We have recently started to investigate the preparation of
Figure 1. Tridentate neutral PNP-pincer ligand 1 in comparison with tpy.
functionalized phosphinines via the pyrylium salt route, orig-
inally described by Märkl for the synthesis of 2,4,6-triphe-
nylphosphinine.[1a] It turned out that this procedure allowed
us to introduce substituents into specific positions of the het-
erocyclic framework; a feature which is essential for ligand
design and their potential application in homogeneous cata-
lytic reactions. In this way a variety of donor-functionalized
and axially chiral monophosphinines, as well as a trans-coor-
dinating diphosphinine have been designed and prepared by
us lately.[3,4]
We have now extended this modular approach to the syn-
thesis of the tridentate pyridyl-bridged diphosphinine 1 (Fig-
ure 1).[2a] This compound resembles not only a diphosphi-
nine analogue of the well-known 2,2’:6’,2’’ terpyridine (tpy)
but represents at the same time a new class of neutral p-ac-
cepting PNP-pincer ligands.
We assume that the recrystallization procedure affords the
non-protonated compound 2 and not the analogous pyridini-
um salt as also observed before for the related 2-pyridyl-4,6-
diphenyl pyrylium tetrafluoroborate.[3d] In fact, the triplet at
d=8.1 ppm (3JH,H =8.0 Hz) in the 1H NMR spectrum of 2
(CD3CN) is indicative for the resonance of a proton in Hg
position of a pyridine-ring, rather than of a pyridinium ion.
The elemental analysis further confirms the presence of 2
rather than of 2·HBF4.
Reaction of the bis-pyrylium salt
2 with excess P-
[6]
AHCTRE(UNG CH2OH)3 only leads to traces of the desired product as
confirmed by 31P NMR spectroscopy. However, 2 was suc-
cessfully transformed into the corresponding pyridyl-bridged
[7]
diphosphinine 1 by reaction with PACHTRE(UNG SiMe3)3 in acetonitrile
Reaction of commercially available 2,6-diacetylpyridine
with four equivalents of benzylidene-acetophenone in the
presence of HBF4·Et2O at T=708C affords the bis-pyrylium
salt 2 (Scheme 1).[5] Compound 2 was obtained as a yellow
solid after recrystallization from hot methanol in 56% yield.
at T=808C according to Scheme 1. Compound 1 was ob-
tained as a yellow solid in 24% yield after column chroma-
[a]Dr. C. Müller, Dr. E. A. Pidko, Prof. Dr. D. Vogt
Department of Chemical Engineering and Chemistry
Schuit Institute of Catalysis, Eindhoven University of Technology
5600 MB Eindhoven (The Netherlands)
Fax : (+31)40-245-5054
E-mail: c.mueller@tue.nl
[b]Dr. M. Lutz, Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research
Crystal and Structural Chemistry, Utrecht University, Padualaan 8
3584 CH Utrecht (The Netherlands)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801337.
Scheme 1. Synthesis of PNP-pincer 1.
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8803

tography and shows the typical downfield resonance for
phosphinines at d=188.2 ppm in the 31P NMR spectrum
(C6D6). Any attempt to characterize 1 also crystallographi-
cally has failed so far due to the poor quality of the yellow
crystalline needles obtained after recrystallization of 1 from
hot acetonitrile.
Diphosphinine 1 resembles a terpyridine analogue in
which two of the three pyridine moieties have been substi-
tuted by the homologous aromatic phosphinine heterocycle.
In contrast to the well studied neutral PNP-pincer ligands of
the type I (Figure 2) containing predominantly s-donor
groups, the tridentate ligand 1 consists both of a s-donor
pyridine group as well as two p-acceptor phosphinine
donors due to the incorporation of the formally sp2-hybrid-
ized phosphorus atom into the aromatic ring system.[8]
Figure 3. Selected frontier orbitals of PNP-pincer ligand 1.
source.[13] On the other hand, structural informations on dis-
crete terpyridine–CuIX complexes (X=halogen) have, to
the best of our knowledge, not been reported to date in lit-
erature, although these species have been postulated in
atom-transfer radical polymerization reactions.[14] The only
structurally characterized CuI
ACHTRE(UNG tpy) complexes are cationic
Figure 2. PNP-pincer ligands with different electronic properties.
complexes containing either an extremely bulky terpyridine
derivative, which induces a strongly perturbed, almost
square-planer CuI coordination geometry or an additional
PPh3 ligand leading to a trigonal-bipyramidal CuI coordina-
tion geometry.[15] Also, a neutral one-dimensional coordina-
The electronic properties of 1 were evaluated by means of
DFT calculations and a selection of relevant frontier orbitals
is illustrated in Figure 3.[9,10] The LUMO+1 and LUMO,
which are close in energy enable the phosphinine moieties
to act as strong p-acceptors upon coordination to a metal
center. Furthermore, the HOMO and HOMOÀ1 can partici-
pate in both s-donation from the nitrogen atom and p-dona-
tion from the phosphorus atoms. The low-lying HOMOÀ3
and HOMOÀ4 represent the lone-pairs at the donor atoms
for participation mainly in s-bonding with a metal center.
As a matter of fact neutral PNP-pincers with such elec-
tronic properties are very rare. Examples are phosphinites
and phosphites based on bis(hydroxymethyl)pyridine
(Figure 1, II) although these systems tend to coordinate in
an intermolecular fashion towards a metal center due to the
more flexible side arms. Moreover, the nitrogen donor in
these complexes usually does not participate in any bonding
to the metal center as a result of the additional oxygen-
spacer. The most closely related systems to compound 1 are
the bis-iminopyridine analogue 2,6-bis(2-phosphaethenyl)-
pyridine (Figure 2, III), first reported by Geoffroy et al.[11]
and bis-phosphaalkene-pyridines of type IV (Figure 2) de-
scribed by Nieke and co-workers.[12]
tion polymer [Cu2(L)ACHTREU[NG m-I)2]n (L=4’pyridyl terpyridine;
pytpy) has been reported, containing hexacoordinated, dis-
torted square-pyramidal CuI centers and forming one-di-
mensional zigzag chains.[16] The preferred meridial coordina-
tion mode of this tridentate nitrogen-ligand results from the
sp2 hybridization of both carbon and nitrogen atoms causing
a directional orientation of the nitrogen lone pairs in the
plane of the heterocycles. Consequently, terpyridine prefer-
entially accommodates transition-metal centers with square-
planar or octahedral, rather than tetrahedral coordination
geometries, such as CuI.[17]
Reaction of 1 with equimolar amounts of CuBrACHTRE(UNG SMe2) in
CH2Cl2 does lead instantaneously and quantitatively to the
corresponding PNP-CuBr complex 3, as confirmed by
31P NMR spectroscopy (Scheme 2).
Compound 3 slowly precipitates as an orange solid from
the reaction mixture upon standing. It shows a broad reso-
nance at d=158.5 ppm in the 31P NMR spectrum (CD2Cl2),
which is in the typical range for transition metal complexes
of phosphinines. Orange crystals suitable for X-ray diffrac-
tion were obtained by slow diffusion of Et2O into a solution
of 3 in CH2Cl2. The copper complex 3 recrystallizes in the
We were further interested in the coordination chemistry
of 1 towards CuI as PNP-type pincer ligands of type I
(Figure 2) have recently been shown by our group to under-
go facile complexation upon reaction with a suitable CuI
¯
space group P1 (no. 2) with two independent metal com-
plexes and additional solvent molecules in the asymmetric
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Diphosphinine Derivatives of Terpyridine
COMMUNICATION
Figure 5. Molecular structure of 3 in the crystal (front view). Displace-
ment ellipsoids are shown at the 50% probability level. Only one inde-
pendent molecule is shown.
Scheme 2. Preparation of Cu-complex 3.
unit. The molecular structure of one independent molecule
is depicted in Figure 4 along with selected bond lengths and
[13]
À
(Figure 2, R=tBu) lacks in any Cu N bonding. This phe-
angles. Ligand 1 coordinates in a k3
A
nomenon can be attributed to the much stronger donor
character of phosphines in comparison to phosphinines,
which, along with the sterically demanding tBu groups obvi-
ously prevents coordination of the pyridine donor to the CuI
center in this particular complex.
metal center, while the fourth coordination site of CuI is oc-
cupied by the BrÀ anion. The tetrahedral environment of
CuI further enforces a non-coplanarity of the three heterocy-
cles leading to inter-planar angles for the P-heterocycles and
the N-heterocycle between 25.5(2) and 31.4(2)8.
The coordination behaviour of 1 can be rationalized by
the pronounced spherical character of the phosphorus lone-
pair in phosphinines and the particular shape of the LUMO,
HOMO and HOMOÀ1, which are responsible for p-back
donation and p-donor contributions, respectively (see
Figure 3). The resulting electronic situation apparently per-
mits coordination to the CuI center resulting in the observed
unusual coordination geometry.[18] Since the electronic prop-
erties of 1 are rather different compared to terpyridine it is
therefore not surprising that complementary CuI
ACHTRE(UGN tpy) com-
plexes have not been described in literature so far. The
facile complexation of the CuIBr fragment by ligand 1 can
qualitatively be explained by the fact that the “soft” CuI
center is much better stabilized by the two p-accepting
phosphinine ligands rather than by pyridine ligands in ter-
pyridine.[19,20] Consequently, these two ligands exhibit a very
different coordination behaviour.
Although phosphinines possess a rich and versatile coor-
dination chemistry this uncommon geometry has so far only
been observed in the CuI complex containing the silacalix-
Figure 4. Molecular structure of 3 in the crystal. Displacement ellipsoids
are shown at the 50% probability level. Only one independent molecule
is shown. Selected bond lengths [Š]and angles [ 8](second molecule in
À
À
square brackets): P11 C101 1.726(4) [1.728(4)], P11 C61 1.743(4)
À
À
[1.733(4)], P21 C271 1.734(4) [1.726(4)], P21 C231 1.738(4) [1.733(4)],
À
À
Cu1 P11 2.2522(13) [2.2757(12)], Cu1 P21 2.2641(12) [2.2719(12)],
À
À
Cu1 N1 2.101(3) [2.079(3)], Cu1 Br1 2.3344(7) [2.3222(7)], C101-P11-
C61 103.09(19) [103.19(19)], C271-P21-C231 102.63(19) [103.06(19)],
P11-Cu1-P21 137.20(5) [138.81(5)], N1-Cu1-Br1; 134.85(9) [135.07(9)].
[3]-phosphinine macrocyclic ligand and in CuACHTREUNG(tmbp)-
AHCTRE(UNG PPh3)2BF4 bearing the bidentate bipyridine analogue 4,4’-
5,5’-tetramethyl-2,2’-bisphosphinine (tmbp).[21] Due to the
À
rather long P Cu bond lengths in the latter compound
(2.3273(9); 2.3286(9) Š) the interaction between the copper
center and the bisphosphinine ligand has been described as
A remarkable feature of the “butterfly” structure of 3 is
the unusual non-directional coordination mode of the two
phosphinine ligands, which ultimately allows coordination of
the ligand to a metal with distorted tetrahedral geometry:
The metal atom is no longer located in the ideal axis of the
phosphorus lone pairs. Instead, the Cu–P vectors significant-
ly deviate from the plane defined by the heterocyclic rings
by 28.20(15) to 33.06(14)8 (Figure 5, front view).
The Cu–N vectors form angles of 8.28(18) and 9.8(2)8
with the corresponding pyridine planes and are thus approx-
imately coplanar. Additionally, the Cu atom is clearly shift-
ed towards the nitrogen atom resulting in Cu–N distances of
2.101(3) and 2.079(3) Š, while the Cu–P distances are in the
range of 2.2522(13)–2.2757(12) Š. Interestingly, and in con-
trast to our observations the corresponding complex
(I)CuIBr bearing the diphosphine-based PNP-pincer ligand
À
relatively weak. On the other hand, the P Cu bond lengths
in 3 are significantly shorter (2.2522(13)–2.2757(12) Š).
They are in the range of Cu P bond lengths observed in the
À
very few Cu–phosphinine complexes reported in literature
and in which the Cu atom is located in the ideal axis of the
phosphorus lone pair.[22] Moreover, the P–Cu and N–Cu dis-
tances in 3 are comparable with the ones observed in the
CuI complex [Cu
ACTHERNGU(MeCN)ACHTRE(UNG pnp)]PF6 (pnp=2,6-bis(2-phos-
phaethenyl)pyridine, III) reported by Hayashi et al.[23] The
values of the P=C bond lengths and C-P-C angles in 3 are
comparable to the values of uncomplexed 2,4,6-triaryl-sub-
stituted phosphinines.[3d,e]
Another interesting feature of 3 is the dimer formation by
intermolecular p–p stacking interactions between the pyri-
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In summary, we have developed a synthetic access to a
novel neutral PNP-pincer ligand containing two phosphinine
donors and a bridging pyridine moiety. In contrast to its ter-
pyridine analogue facile coordination of this tridentate
ligand towards a neutral CuI center was observed. The cor-
responding CuIBr complex was characterized crystallograph-
ically and revealed a distorted tetrahedral coordination ge-
ometry of the metal center as a result of an unusual coordi-
nation mode of the two phosphinine ligands. Due to the
presence of electronically rather inequivalent donor atoms,
we anticipate that this novel ligand represents a new class of
p-accepting PNP-pincer systems. This is expected to lead to
transition-metal complexes with new properties and applica-
tions, especially in homogeneous catalysis and as optoelec-
tronic devices in the near future. Further studies on this sub-
ject are currently carried out in our laboratories.
Acknowledgements
This work was supported in part (M.L., A.L.S.) by the Council for the
Chemical Sciences of the Netherlands Organization for Scientific Re-
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Keywords: coordination chemistry
·
copper
·
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Diphosphinine Derivatives of Terpyridine
COMMUNICATION
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Received: July 3, 2008
Published online: August 21, 2008
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8807