Access to catenated and branched polyphosphorus ligands and coordination complexes via a tri(pyrazolyl)phosphane

Article abstract of DOI:10.1039/c2cc18030h

We report a convenient and smooth protocol for the high-yielding synthesis of a triphosphane and an iso-tetraphosphane from a readily accessible tri(pyrazolyl)phosphane. The products are obtained from a one-pot reaction at room temperature and form comple

Full text of DOI:10.1039/c2cc18030h

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Chemical Communications
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Volume 48 | Number 36 | 7 May 2012 | Pages 4261–4376
ISSN 1359-7345
COMMUNICATION
Jan J. Weigand et al.
Access to catenated and branched polyphosphorus ligands and coordination
complexes via a tri(pyrazolyl)phosphane
1359-7345(2012)48:36;1-5

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Access to catenated and branched polyphosphorus ligands and
coordination complexes via a tri(pyrazolyl)phosphanewz
c
a
Kai-Oliver Feldmann,ab Roland Frohlich and Jan J. Weigand*
¨
Received 22nd December 2011, Accepted 27th January 2012
DOI: 10.1039/c2cc18030h
We report a convenient and smooth protocol for the high-
yielding synthesis of a triphosphane and an iso-tetraphosphane
from a readily accessible tri(pyrazolyl)phosphane. The products
are obtained from a one-pot reaction at room temperature and
form complexes with the Fe(CO)4-fragment.
Chemists have developed synthetic strategies to access essentially
any C–C bonded molecular framework. Many analogies have
been described between carbon and phosphorus that can be
justified by the diagonal relationship between these elements in
the periodic table.1 Although the P–P bond energy (200 kJ molꢀ1
)
Scheme 1 Examples of the preparation of tri- and iso-tetraphosphanes.
is relatively low compared to the C–C bond (350 kJ molꢀ1),2
a significant number of polyphosphanes,3 polyphosphide
anions4 and polyphosphorus cations5 has been isolated.
Nevertheless, the number of P–P bonded frameworks is small
compared to the vast array of C–C bonded compounds.6 The
preparation of organo-substituted polyphosphanes with more
than one P–P bond is challenging except for cyclo-polyphosphanes
(RnPn) (n = 3–6). Synthetic procedures for acyclic and
branched polyphosphorus compounds are limited. Some transition
metal7 and main group element mediated approaches have been
described. Scheme 1 shows selected examples. iso-Tetraphosphanes
can be prepared from P1 units by reacting halophosphanes with
either phosphides (formation of 1)8 or silylphosphanes (formation
of 2)9 in a metathesis reaction. Triphosphanes have also been
prepared by the reaction of a cyclo-tetrapnictane derivative 3
with MeI as an electrophile to yield triphosphane 4.10 Deri-
vative 5 is obtained via an aminolysis reaction involving a sec.
phosphane and an aminophosphane.11 While these examples12
are viable methods for the preparation of triphosphanes and
iso-tetraphosphanes they are either low yielding, require harsh
conditions such as elevated temperatures or highly basic media
or rely on preformed triphosphane fragments and use of
demanding reagents. These limitations mainly prevented the
establishment of polyphosphane chemistry to be used in
synthetic and coordination chemistry. This contribution
introduces an unprecedented and facile way for the prepara-
tion of a triphosphane (6) and an iso-tetraphosphane (1) and
demonstrates that the hitherto believed instability of 1 may
have been overestimated.8 Our approach is based on the known
phosphanylation reaction of primary and secondary phos-
phanes utilizing alkylaminophosphanes to generate P–P bonds
along with the elimination of the corresponding sec. amine.
These reactions typically require high reaction temperatures
and give the targeted phosphanes often in low to moderate
yields only.13,11 It has been shown that phosphanylation reac-
tions using alkylaminophosphanes in polymer supported synth-
eses of DNA are catalyzed by azoles. In these reactions azole
substituted phosphorus derivatives were identified as reactive
intermediates.14 During our investigation on the reactivity
of highly charged phosphorus cations with nucleophiles, we
realized that the P–N bond in pyrazolylphosphane derivatives
can be easily cleaved.15 In this context, we addressed the
question if this reactivity pattern can be applied for P–P bond
formation, using pyrazolyl-substituted phosphanes as easily
accessible P1 units in combination with secondary phosphanes.
Tris-(3,5-dimethyl-1-pyrazolyl)phosphane (8) was chosen as
a P1 source in the reaction with two eq. of Cy2PH (Cy =
cyclohexyl). We anticipated the generation of two new P–P
bonds via the elimination of two eq. of 3,5-dimethylpyrazole
(7) to yield triphosphane 6 (Scheme 2a). The formation of 6
proceeds remarkably efficient at room temperature and the
product can be conveniently isolated by filtration and washing
a Department of Inorganic and Analytical Chemistry,
University of Munster, Corrensstr. 30, 48149 Munster, Germany.
¨
¨
E-mail: jweigand@uni-muenster.de; Fax: +49-215-8333126;
Tel: +49-215-8336074
b Graduate School of Chemistry, University of Munster,
¨
Corrensstr. 30, 48149 Munster, Germany
¨
c Department of Organic Chemistry, University of Munster,
Corrensstr. 40, 48149 Munster, Germany
¨
¨
w This article is part of the ChemComm ‘Frontiers in Molecular Main
Group Chemistry’ web themed issue.
z Electronic supplementary information (ESI) available. CCDC
860158 and 860159. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc18030h
c
4296 Chem. Commun., 2012, 48, 4296–4298
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Scheme 2 Preparation of triphosphane 6 and iso-tetraphosphane 1
from 8 and Cy2PH. (a) 2 Cy2PH, ꢀ2 3,5-dimethylpyrazole (7),
CH3CN, RT, 15 h, 91%; (b) 3 Cy2PH, ꢀ3 7, CH3CN, RT, 7 d, 95%.
Scheme 3 Preparation of complexes 9 and 10 from 6 and 1, respec-
tively. (a) Fe2(CO)9, ꢀFe(CO)5, THF, RT, 12 h, 56%; (b) Fe2(CO)9,
ꢀFe(CO)5, THF, RT, 12 h, 39%.
with CH3CN. Compound 6 was obtained as analytically pure
colourless material in high yield (91%). In a similar approach
iso-tetraphosphane 1 was obtained reacting 8 with three eq. of
Cy2PH (Scheme 2b).16 This reaction proceeds via the elimina-
tion of three eq. of 7 and formation of three P–P bonds. As is
in the case of 6 the isolation of 1 only involves filtration and
washing with CH3CN to yield an analytically pure colourless
product in excellent yield (95%). Compound 1 was previously
obtained from the metathesis reaction of K[P(SiMe3)2] with
three eq. of Cy2PCl in very low yield (10%, Scheme 1 top
reaction) and was only characterized by 31P NMR investiga-
tions and a melting point (86 1C). It was stated that 1 perishes
slowly in the solid state even under inert conditions and
decomposes in hot toluene (70 1C) quickly to Cy2P–PCy2
and Cy2PH via an unknown mechanism.8 In contrast, we
found that analytically pure 1 appears to be indefinitely stable
in the solid state (under Ar or N2) and has a defined melting
range (169–171 1C). Slight melting-associated decomposition
was only observed when 1 was heated at elevated temperatures
(>175 1C) for a longer period of time. Solutions of 1 in
toluene were stable for more than four weeks and heating to
70 1C (60 min) did not show any sign of decomposition.
The 31P{1H} NMR spectrum of 6 reveals an A2B spin system
(dA = 1.9 ppm, dB = 29.0 ppm; 1J(PAPB) = ꢀ271.8 Hz,
Dd/JAB E 19, 300 K, C6D6)16 with eight lines17 in an approxi-
mate intensity ratio of 2 : 1. For compound 1 a characteristic
coordinated intact to a transition metal.10 The isolation of
complex 10 shows that this is also valid for iso-tetraphosphanes.
The complexes 9 and 10 are characterized by distinct carbonyl
stretching vibrations (9: 2044, 1972, 1930, 1916(sh); 10: 2039,
1969, 1920(sh) cmꢀ1), which are reminiscent of the two a1 and
19
one e stretching vibrations of C3v symmetric [Fe(CO)]4
moieties but indicative of the overall lower symmetry of the
compounds.16 The 13C{1H} NMR spectrum displays charac-
teristic multiplets for the carbonyl ligands in 9 (d = 213.8 ppm)
and 10 (d = 215.8 ppm), respectively.20 The experimental and
fitted 31P{1H} NMR spectra along with the spin system and
chemical shifts are shown in Fig. 1. The low-field shift of the
coordinated P atom (9: d = 85.0 ppm; 10: d = 97.0 ppm) in
comparison to the respective polyphosphane is a characteristic
feature of this type of complexes and was previously observed
for related compounds.18b Both compounds were also crystallo-
graphically characterized. The molecular structures of complexes
9 and 10 along with selected bond lengths and angles are given
in Fig. 2. Compound 10 represents the first structurally
higher order AM3 spin system (dA = ꢀ108.9 ppm, dM
=
ꢀ5.2 ppm; 1J(PAPM) = ꢀ361.9 Hz, Dd/JAM E 49, 300 K, C6D6)16
is observed consistent with previously reported data.8 The
pronounced up-field shift of the resonance of the central
phosphorus atom in 6 in comparison to the respective nucleus
in 1 can be explained by the electron withdrawing effect of the
pyrazole substituent. Except for a very few examples,18,10 the
coordination chemistry of tri- and tetraphosphanes as ligands
is not developed mainly due to the belief that these ligands are
too unstable18b and require labour intensive and specialized
synthetic procedures. In contrast, we have now found that
polyphosphanes 6 and 1 react readily with Fe2(CO)9 to form
the corresponding iron-complexes 9 and 10 with an intact
ligand framework. The reaction is accompanied by the for-
mation of Fe(CO)5 (Scheme 3). The reactions proceed rather
cleanly (>80% of the desired product in solution by NMR),
however the isolated yields of the complexes 9 (56%) and 10
(39%) are moderate.16 Nevertheless, the formation and stabi-
lity of complexes 9 and 10 is remarkable, as it was reported
that transition metal polyphosphane complexes could not be
prepared via coordination of the free polyphosphane, but have
to be assembled at a transition metal.18b Wright and co-workers
were the first to prove that a triphosphane can indeed be
Fig. 1 31P{1H} NMR spectra (C6D6, 300 K) of the complexes 9 and
10. (a) AMX spin system for 9: dA = ꢀ1.4, dM = 33.3, dx = 85.0;
1J(PAPM) = ꢀ381.8 Hz, 1J(PMPX) = ꢀ325.6 Hz, 2J(PAPX) = 16.5
Hz; (b) AM2X spin system for 10: dA = ꢀ70.7, dM = 15.4, dx = 97.0;
1J(PAPM) = ꢀ464.9 Hz, 1J(PAPX) = ꢀ345.0 Hz, 2J(PMPX) = 45.8 Hz.
Expansions (inset) show the experimental (up) and fitted spectra (down).
c
This journal is The Royal Society of Chemistry 2012
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4 R. Pottgen, W. Honle and H. G. v. Schnering, Phosphides, Solid
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6 H. G. v. Schnering, Angew. Chem., Int. Ed. Engl., 1981, 20, 33.
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Allg. Chem., 1986, 544, 127.
Fig. 2 ORTEP plot of the molecular structures of compounds
(a) 9 and (b) 10. Thermal ellipsoids are drawn at 50% probability
(hydrogen atoms are omitted for clarity). Selected bond lengths [A]
and angles [1]: 9: P1–Fe 2.2765(7), P1–P2 2.2384(9), P2–P3 2.2133(9),
P2–N31 1.736(2), N31–N32 1.381(3), P2–P1–Fe 105.65(3),
N31–P2–P3 109.55(8); N31–P2–P1 100.51(8), P3–P2–P1 106.72(4);
10: P1–Fe 2.284(1), P1–P2 2.221(1), P2–P4 2.193(1), P2–P3 2.210(1),
N31–N32 1.381(3), P2–P1–Fe 108.76(4), P4–P2–P3 127.34(5);
P4–P2–P1 104.08(5), P3–P2–P1 106.88(5).
10 D. R. Armstrong, N. Feeder, A. D. Hopkins, M. J. Mays,
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11 L. Maier, Helv. Chim. Acta, 1966, 49, 1119.
characterized example of a metal complex of an iso-tetraphosphane.
In both complexes, the Fe(CO)4 moiety coordinates to a
terminal phosphorus atom. This mode of coordination was
reported to be thermodynamically favourable over the coordi-
nation of the central phosphorus atom for the related triphos-
phane complex [Ph2PP(Ph)P(Ph)2Fe(CO)4].18b Notably, the
polyphosphorus ligands occupy the axial position of the pseudo
trigonal bipyramidal coordination sphere of the iron atom.
Apparently the coordination site is determined by electronic
rather than steric factors which may be indicative of the low
p-acceptor properties of the polyphosphane ligands.21,18c
In summary, a highly efficient method affords symmetric
tri- and iso-tetraphosphanes in very high yields. This simple
protocol should allow for the systematic development of a
plethora of other tri- and iso-tetraphosphanes with a highly
diverse substitution pattern. We are currently investigating
this type of P–P bond formation using pyrazolyl-substituted
phosphanes as easily accessible P1 units in combination with a
series of primary and secondary phosphanes to access hitherto
unknown structural motives in polyphosphane chemistry. In
addition, this simple approach allows a systematic investiga-
tion of the coordination chemistry of polyphosphane ligands.
We are especially interested in using triphosphanes and
iso-tetraphosphanes as polydentate, chelating ligands and
explore their properties in the field of homogenous catalysis.
We gratefully acknowledge the FCI (fellowship for K.-O.F.),
12 Selected examples: (a) G. Fritz, E. Matern, H. Krautscheid,
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¨
16 Details of the synthesis, isolation and characterization of 1, 6, 9,
and 10 are available in the ESIz.
17 The unusually high multiplicity has been shown to originate
from the inapplicability of the secular approximation of the spin
hamiltonian rather than from magnetic inequivalence for a related
triphosphane by Crossley et al. (ref. 12f).
the Graduate School of Chemistry of the WWU in Munster, the
¨
DFG (WE 4621/2-1) and the European PhosSciNet (CM0802)
for funding. J.J.W. thanks Prof. F. E. Hahn for his generous
support and Prof. R. Wolf for helpful discussions.
18 (a) M. Scheer, C. Kuntz, M. Stubenhofer, M. Zabel and A. Y.
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2 Y. R. Luo, Comprehensive Handbook of Chemical Bond Energies,
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20 C. Sarmah, M. Borah and P. Das, Appl. Organomet. Chem., 2011,
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21 H. Haas and R. K. Sheline, J. Chem. Phys., 1967, 47, 2996.
c
4298 Chem. Commun., 2012, 48, 4296–4298
This journal is The Royal Society of Chemistry 2012