Copper(I) mediated oligomerisation of a phosphaalkyne

Article abstract of DOI:10.1039/b712161j

The oligomerisation of tert-butylphosphaalkyne, ^tBuC≡P, mediated by Cu(i) complexes yields an unprecedented C₄P₅ cage compound, which is stabilised in a matrix of copper(I) iodide. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712161j

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Copper(I) mediated oligomerisation of a phosphaalkyne{
Ulf Vogel,a John F. Nixonb and Manfred Scheer*a
Received (in Cambridge, UK) 7th August 2007, Accepted 14th September 2007
First published as an Advance Article on the web 28th September 2007
DOI: 10.1039/b712161j
The oligomerisation of tert-butylphosphaalkyne, tBuCMP,
mediated by Cu(I) complexes yields an unprecedented C4P5
cage compound, which is stabilised in a matrix of copper(I)
iodide.
amounts of red crystals as a side product which was identified as
[(tBuCP)4Cu2I2]2 (3).
Complexes 2 and 3 are insoluble in common organic solvents
which made it impossible to investigate their NMR spectroscopic
properties.
Following the successful isolation of tert-butylphosphaalkyne 1 by
Becker et al,1 the first compound featuring a carbon–phosphorus
triple bond which is stable at room temperature, its coordination
chemistry2,3 as well as its element organic chemistry3,4 has been
widely explored. One focus of these explorations was the
oligomerisation of 1, since this reaction yields a range of novel
carbon–phosphorus cage compounds, often stereoselectively.5 The
oligomerisation of 1 can be induced by a variety of methods, such
as thermal activation and reactions with Lewis acids. For the latter
strategy, which often proceeds in a more controlled fashion than
thermal oligomerisation, both main group and transition metal
Lewis acids have been used.6 The resulting carbon–phosphorus
cage compounds feature multiple phosphorus atoms that can act
as coordination sites for transition metals.7 This suggested to us
that the rigid cages could act as building blocks for supramolecular
aggregates. Our group has recently synthesised a variety of
supramolecular aggregates using group 10 metals and Pn-ligand
complexes as building blocks,8 so it was of interest to see if the
function of the latter could possibly also be fulfilled by carbon–
phosphorus cage compounds. Since we previously successfully
used the Lewis acidic copper(I) halides for the synthesis of
supramolecular assemblies, we decided to explore the reactivity of
1 with similar complexes. It can be expected that the metal halide
serve as both a Lewis acid to initiate the oligomerisation of 1 and
at the same time function as a supramolecular matrix9 to stabilise
any resulting unusual carbon–phosphorus cages.
The crystal structure of 2§ is depicted in Fig. 1 and 2. It shows a
t
novel Bu4C4HP5 cage that coordinates to three CuI units of a
ladder-like motif, in such
a way that a one-dimensional
coordination polymer is obtained. The polymeric strands are
oriented along the crystallographic c-axis. The tBu4C4HP5 cage in
2 (Fig. 1) has not been previously observed as an oligomer of 1.
Since the composition of the cage molecule is not a multiple of the
phosphaalkyne 1 it is obvious that the stoichiometry of the cage
cannot be described by a simple oligomerisation of 1. We assume
that the cage was formed by an initial pentamerisation reaction of
1 followed by the elimination of a tBuC moiety, which probably
was caused by the steric crowding in the CuI matrix, and a
subsequent protonation of the vacant coordination site. The cage
structure in 2 can be described as a P5C3 cubane where a CHtBu
unit is inserted into a P–P bond. All bonds in the cage are in the
The reaction of tert-butylphosphaalkyne 1 with CuI in CH3CN
leads to the formation of a red solution, which shows a singlet in
its 31P{1H} NMR spectrum at 258.1 ppm.{ This value is shifted
downfield in comparison to free 1 (d = 268.0 ppm in C6D6) which
indicates an initial complexation of 1 by the copper(I) halide. After
two days a solid which precipitated out of the solution, was
obtained as small needle shaped crystals if the reaction solution is
not agitated. This crystalline material was identified as
[H(tBuC)4P5(CuI)3(CH3CN)x]n (x = 1, 3) (2?xCH3CN) by single-
crystal X-ray structure determinations.§ During several different
attempts to repeat the reaction we occasionally obtained small
Fig. 1 C4P5 cage of 2 showing the coordination mode to the copper
atoms. Hydrogen atoms at the tert-butyl substituents have been omitted
˚
for clarity. Selected bond lengths (A) and angles (u): Cu(1)–P(1) 2.259(3),
aUniversita¨t Regensburg, Institut fu¨r Anorganische Chemie, 93040,
Regensburg, Germany. E-mail: manfred.scheer@chemie.uni-
regensburg.de; Fax: +49 941 943 4439; Tel: +49 941 943 4440
bChemistry Department, School of Life Sciences, University of Sussex,
Falmer, Brighton, UK BN19QJ
{ Electronic supplementary information (ESI) available: Experimental
details for the removal of CuI from 2 and the 31P NMR data of the
resulting products. See DOI: 10.1039/b712161j
Cu(2)–P(2) 2.230(3), Cu(3)–P(3) 2.256(3), P(1)–P(2) 2.233(4), P(1)–P(3)
2.193(5), P(1)–C(5) 1.854(10), P(2)–C(2) 1.897(10), P(2)–C(4) 1.852(9),
P(3)–C(2) 1.912(9), P(3)–C(3) 1.901(10), P(4)–C(2) 1.875(10), P(4)–C(3)
1.923(10), P(4)–C(4) 1.971(10), P(5)–C(3) 1.891(12), P(5)–C(4) 1.943(11),
P(5)–C(5) 1.830(10); Cu(1)–P(1)–P(2) 114.1(1), Cu(1)–P(1)–P(3) 114.3(2),
Cu(1)–P(1)–C(5) 136.2(4), P(2)–P(1)–P(3) 81.5(2), P(2)–P(1)–C(5) 99.9(3),
P(3)–P(1)–C(5) 96.8(4), Cu(2)–P(2)–P(1) 112.6(2).
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Fig. 2 Polymeric strand in 2 oriented along the crystallographic c-axis; tert-butyl groups and protons have been omitted for clarity.
range of normal single bonds. The high strain within the cage is
reflected in bond angles at the carbon atoms which can be as acute
as 87.7(5) for P(4)–C(4)–P(5). Coordination of the C4P5 cage to
three copper atoms is achieved via the P atoms P(1), P(2) and P(3).
The copper atoms Cu(1) and Cu(3) are included in ladder-like
Cu4I4 units, Cu(2) is part of a Cu2I2 ring. These subunits are
connected in such a way that a one-dimensional polymeric strand
is formed (Fig. 2). All copper atoms show a tetrahedral
coordination environment and the bond lengths in the CuI-
network are within normal ranges.
AlCl3.10 In 3 two of these cages are coordinated to a ladder-like
Cu4I4 unit, which is terminated by two molecules of CH3CN. All
bonds within the cage compound can be described as single bonds,
˚
except the bond between C(1) and P(1) (1.693(9) A) which is
formally a double bond.
To test if the method of Cu(I) assisted self assembly of 1 can be
used as a synthetic method for the preparation of free carbon
phosphorus cages, we tried to remove the metal atoms from the
cage. Cu(I) halides are known to form very stable cyano complexes
and the method to extract CuI from a phosphorus adduct was
recently imaginatively used to obtain a new allotrope of
phosphorus.11 Indeed, if 2 is suspended in a mixture of an
aqueous NaCN solution and Et2O, after 5 h all of the orange
powder has dissolved and the ether phase shows a pale yellow
colour (cf. ESI{). The 31P NMR spectrum of the Et2O phase is
complex and can be attributed to three major products, one of the
them being the expected free C4P5-cage 4 (Scheme 1) found in 2.
Its 31P NMR spectrum shows five multiplets in a range from 190
to 233 ppm with a coupling pattern that can be assigned to the
free cage 4 (see ESI{). Unexpectedly, the spectrum also showed
signals attributable to two other carbon–phosphorus compounds,
which could be identified as 5 and 6 by their 31P NMR spectra
(Scheme 1). Binger et al. obtained the triphospha-Dewar-benzene 5
by a reaction of (COT9)Hf(tBu3C3P3) with C2Cl6 (COT9 = 1,4-
(Me3Si)2C8H6) and the reported NMR spectroscopic data matches
our observations.12 In addition to the spectroscopic identification
the structure of 6 was confirmed by X-ray crystallography (Fig. 4),
which has a crystallographically imposed mirror symmetry. The
formation of the novel cage compound 6, which is a valence
isomer of 4, gives rise to the assumption that the free cage 4
The molecular structure of 3, which has a crystallographically
imposed inversion symmetry, is depicted in Fig. 3 and consists of a
C4P4 cage which was already spectroscopically described as an
uncoordinated cage by Regitz and co-workers as the product of a
reaction sequence starting with the oligomerisation of 1 induced by
Fig. 3 The molecular structure of 3; hydrogen atoms and methyl
groups at the tert-butyl groups have been omitted for clarity. Symmetry
˚
code: 2 2 x, 2 2 y, 2z. Selected bond lengths (A) and angles (u): I(1)–
Cu(1) 2.599(2), I(1)–Cu(2) 2.587(1), I(2)–Cu(1) 2.739(2), I(2)–Cu(2)
2.725(2), I(2)–Cu(29) 2.704(2), Cu(1)–P(1) 2.233(3), Cu(1)–N(1) 2.017(8),
Cu(2)–P(29) 2.256(3), P(1)–C(1) 1.693(9), P(1)–P(2) 2.180(3), P(2)–C(2)
1.842(9), P(2)–C(3) 1.863(10), P(3)–C(2) 1.956(9), P(3)–C(3) 1.905(10),
P(3)–C(4) 1.897(10), P(4)–C(1) 1.862(9), P(4)–C(2) 1.840(9), P(4)–C(4)
1.889(10), C(3)–C(4) 1.578(10); Cu(1)–I(1)–Cu(2) 69.77(5), Cu(1)–I(2)–
Cu(2) 65.77(4), Cu(1)–I(2)–Cu(29) 93.90(4), Cu(2)–I(2)–Cu(29) 84.58(5),
I(1)–Cu(1)–I(2) 107.96(5), I(1)–Cu(1)–P(1) 119.44(8).
Scheme 1 Compounds 4–6.
5056 | Chem. Commun., 2007, 5055–5057
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lattice. For both structures a CH3CN molecule which coordinates to a Cu
atom is found, but in the second structure two additional molecules of
CH3CN are found in the crystal lattice in between two strands of the
coordination polymer. The structure solution of 2?3CH3CN was not
satisfactory due to the poor quality of the crystals obtained (unit cell: a =
˚
11.647(2), b = 13.348(3), c = 14.588(3) A, a = 97.47(3), b = 103.034(3), c =
114.73(3)u). All single crystals were mounted in an inert oil and transferred
to the cold stream of a Oxford Diffraction CCD area-detector
˚
diffractometer using Cu-Ka radiation (l = 1.54178 A). Crystal data for
¯
2?CH3CN: C22H40Cu3I3NP5, M = 1044.75, triclinic, space group P1, a =
˚
11.693(2), b = 12.892(2), c = 13.172(3) A, a = 89.99(1), b = 64.79(2),
3
˚
c = 77.48(1)u, V = 1744.2(6) A , T = 103(1) K, Z = 2, m(Cu-Ka) =
25.236 mm21, 11103 reflections measured, 3742 unique (Rint = 0.0487)
which were used in all calculations. The final R1 [I . 2s(I)] was 0.0368.
Crystal data for 3?2CH3CN: C44H78Cu4I4N2P8, M = 1644.64, monoclinic,
˚
space group P21/c, a = 15.505(3), b = 11.642(2), c = 18.001(3) A, b =
˚
3
21
,
111.43(2)u, V = 3025(1) A , T = 104(1) K, Z = 2, m(Cu-Ka) = 19.841 mm
13212 reflections measured, 3267 unique (Rint = 0.0466) which were used in
all calculations. The final R1 [I . 2s(I)] was 0.0391. Crystal data for
6?0.5C7H8: C23.5H41P5, M = 478.41, monoclinic, space group C2/m, a =
Fig. 4 Molecular structure of 6; hydrogen atoms have been omitted for
˚
clarity. Symmetry code: x, 1 2 y, z. Selected bond lengths (A) and angles
˚
23.7888(3), b = 11.2636(2), c = 10.3868(1) A, b = 108.202(1)u, V =
3
2643.85(6) A , T = 123(2) K, Z = 4, m(Cu-Ka) = 3.259 mm21, 25696
˚
(u): P(4)–P(5) 2.157(2), P(5)–C(3) 1.884(5), P(1)–C(1) 1.896(4), P(1)–C(2)
1.893(3), P(1)–C(3) 1.895(3), P(3)–C(1) 1.901(4), P(3)–C(2) 1.871(5), P(4)–
C(1) 1.891(4), C(4)–C(1) 1.539(5), C(2)–C(8) 1.536(7), C(3)–C(11) 1.562(7);
C(2)–P(1)–C(3) 87.74(16), P(1)–C(1)–P(3) 93.42(17), C(2)–P(1)–C(3)
87.74(16), C(2)–P(3)–C(1) 86.21(16), C(2)–P(3)–P(4) 101.90(16), C(1)–
P(3)–C(19) 86.21(16), P(1)–C(3)–P(19) 91.8(2), C(1)–P(4)–C(19) 86.21(16),
C(3)–P(5)–P(4) 92.88(16).
reflections measured, 2385 unique (Rint = 0.1366) which were used in all
calculations. The final R1 [I . 2s(I)] was 0.0810. The disordering of the
toluene molecule in the crystal lattice of 6 over two positions leads to a
somewhat elevated wR2 value. CCDC 656701–656703. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b712161j
1 G. Becker, G. Gresser and W. Uhl, Z. Naturforsch., Teil B, 1981, 36, 16.
2 J. F. Nixon, Chem. Soc. Rev., 1995, 319.
3 M. Regitz and P. Binger, Angew. Chem., Int. Ed. Engl., 1988, 27, 1484.
4 (a) Phosphorus: The Carbon Copy, ed. K. B. Dillon, F. Mathey and J. F.
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L. Weber, Adv. Organomet. Chem., 1997, 41, 1; (c) A. Mack and
M. Regitz, in Carbocyclic and Heterocyclic Cage Compounds and Their
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p. 199; (d) J. F. Nixon in Carbocyclic and Heterocyclic Cage Compounds
and Their Building Blocks, ed. K. K. Laali, J. A. I. Press, Stamford, CT,
1999, p. 257.
6 A. Mack and M. Regitz, Chem. Ber., 1997, 130, 823.
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1869; (c) D. Himmel, M. Seitz and M. Scheer, Z. Anorg. Allg. Chem.,
2004, 630, 1220.
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1737; J. Bai, E. Leiner and M. Scheer, Angew. Chem., Int. Ed., 2002, 41,
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undergoes rearrangement reactions, when the CuI matrix is
removed.
In summary we have shown that the oligomerisation of 1 in the
presence of CuI can be used for the synthesis of new carbon–
phosphorus cage compounds. Preliminary investigations on the
extraction of CuI from the insoluble supramolecular aggregates
have shown that it should be possible to remove CuI from the
reaction products and isolate the free cage compounds, which
partially undergo subsequent transformation, after removal of the
CuI matrix.
The authors thank the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial support. We are
grateful to Dr E. Peresypkina (Institute of Inorganic Chemistry,
Siberian Division of RAS, Novosibirsk, Russia) for the final
solution of the X-ray structure of 6. J. F. N. thanks the Alexander
von Humboldt foundation for the award of a Research Prize.
Notes and references
{ Typical experimental procedure: To a solution of CuI (460 mg, 2.59 mmol)
in 35 ml of CH3CN was added tert-butylphosphaalkyne 1 (472 mg,
4.72 mmol) under an atmosphere of dry N2. The mixture showed a red
colour immediately. 31P NMR spectrum of the solution: dP (161 MHz;
CD3CN; H3PO4) 258.1 (s). After a period of 4 days without agitation,
orange, microcrystalline 2?xCH3CN (x = 1, 3) (60 mg, 7%) precipitated
9 For examples of polyphosphorus cages stabilised in a CuI matrix, see:
(a) A. Pfitzner and E. Freundenthaler, Angew. Chem., Int. Ed. Engl.,
1995, 34, 1647; (b) A. Pfitzner and E. Freundenthaler, Z. Naturforsch.,
Teil B, 1997, 52, 199.
10 B. Breit, U. Bergstra¨ßer, G. Maas and M. Regitz, Angew. Chem., 1992,
104, 1043; B. Breit and M. Regitz, Chem. Ber., 1996, 129, 489.
11 A. Pfitzner, M. F. Bra¨u, J. Zweck, G. Brunklaus and H. Eckert, Angew.
Chem., Int. Ed., 2004, 43, 4228.
(Found: C, 24.1; H, 4.0. C20H37Cu3I3P5 requires C, 23.9; H, 3.7%); v˜/cm21
:
1957, 2898, 2861, 1629, 1522, 1473, 1391, 1362, 1260, 1210, 1195, 1087,
1042, 1021, 1002, 930, 899, 872, 795, 718, 699, 584, 529, 464. Occasionally a
few deep red crystals of 3 were also obtained.
§ Two different kinds of crystals for 2 were found. Both show the same
cage core, but differ in the number of solvent molecules in the crystal
12 P. Binger, S. Leininger, K. Gather and U. Bergstra¨ßer, Chem. Ber.,
1997, 130, 1491.
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Chem. Commun., 2007, 5055–5057 | 5057