The first complexes and cyclodimerisations of methylphosphaalkyne (P≡CMe)

Article abstract of DOI:10.1039/b607489h

The first complexes and cyclodimerisations of methylphosphaalkyne, P≡CMe, are reported to arise from its reactions with a range of platinum(O) complexes and [W(CO)₅(THF)]. A number of differences between the chemistry of this phosphaalkyne and that of its bulkier analogues have been highlighted and explained on steric grounds. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b607489h

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The first complexes and cyclodimerisations of methylphosphaalkyne
(P≡CMe)†
Cameron Jones,* Christian Schulten and Andreas Stasch
Received 26th May 2006, Accepted 15th June 2006
First published as an Advance Article on the web 22nd June 2006
DOI: 10.1039/b607489h
The first complexes and cyclodimerisations of methylphos-
phaalkyne, P≡CMe, are reported to arise from its reactions
with a range of platinum(0) complexes and [W(CO)5(THF)].
A number of differences between the chemistry of this
phosphaalkyne and that of its bulkier analogues have been
highlighted and explained on steric grounds.
The chemistry of sterically stabilised phosphaalkynes has been
extensively explored since the preparation of P≡CBut in 1981.1
Since that time, these compounds have been shown to behave
more like alkynes than nitriles in their coordination chemistry.
In addition, the special properties of phosphaalkynes have led to
their use as building blocks in the metal mediated syntheses of
an impressive array of organophosphorus heterocycles and cage
compounds.2 It has been a goal of low coordination phosphorus
chemists to extend this work to the unhindered phosphaalkynes,
P≡CR, R = H or Me, not least because these are the direct P-
analogues of acetylene and propyne which are feed-stocks for a
number of important industrial processes. The problem here has
Scheme 1 Reagents and conditions: i, [(P–P)Pt(C2H4)] (P–P = dppe
been that, until recently, the synthesis and handling of unhindered
phosphaalkynes has been technically difficult.3 In addition, they
were thought to be unstable at room temperature, though it is now
clear that solutions of P≡CMe, although pyrophoric, can be stored
for days at room temperature without decomposition. This fact,
combined with the high yield, multi-gram synthesis of P≡CMe
reported by Guillemin et al. in 2001,4 prompted us to begin
an examination of the coordination and cyclooligomerisation
chemistry of this phosphaalkyne.5 In order to draw comparisons,
this initial report details the treatment of P≡CMe with tungsten(0)
and platinium(0) precursors that have previously been utilised
in reactions with more bulky phosphaalkynes. This study does,
indeed, highlight significant reactivity differences (and similarities)
between P≡CMe and, for example, P≡CBut.
or (PEt3)2) or [Pt(PCy3)2], toluene–diethyl ether; ii, dry in vacuo,
−1/2 P≡CMe; iii, [W(CO)5(THF)], THF.
most note are the phosphaalkyne resonances (1: d 102.4 ppm; 2:
d 90.4 ppm) which shifted upfield by ca. 200 ppm, suggesting the
presence of saturated P-centres in the final products.‡ This was
confirmed by the X-ray crystal structures of these compounds, 3
and 4, (vide infra) which revealed each to contain a phosphaalkyne
molecule bridging two platinum fragments. The mechanism of
formation of 3 and 4 probably involves 1 and 2 being in equilibrium
with the free phosphaalkyne, thus allowing attack of 1 and 2
by a second “Pt0(P–P)” fragment upon removal of volatiles,
including P≡CMe, from the reaction mixtures. Similar secondary
reactions do not occur for complexes of bulkier phosphaalkyne-
Hindered phosphaalkynes are well known to form stable
2
complexes of the type [Pt(P–P)(g -P≡CR)], P–P = (PPh3)2, dppe
2
platinum(0) complexes, e.g. [Pt(dppe)(g -P≡CBut)],7 presumably
etc., R = But or mesityl, in their reaction with, for example,
because attack at the P–C multiple bond is not facile for steric
reasons. In line with this proposal, we have found that when
methylphosphaalkyne is coordinated to a very bulky platinum
2
[Pt(P–P)(g -C2H4)].6 In concert with these results, treatment of
related Pt(0) precursors with an excess of P≡CMe led to clean
solutions of the analogous complexes, 1 and 2 (Scheme 1), as
2
fragment in [Pt(PCy3)2(g -P≡CMe)] 5 (Scheme 1), the compound
1
determined by their 31P{ H} NMR spectra (ESI†). However, after
is completely stable to phosphaalkyne loss and formation of
volatiles were removed from the reaction solutions in vacuo and
2
[{Pt(PCy3)2}2(l-g -P≡CMe)], cf. 3 and 4.
the residues redissolved, the spectra had substantially changed. Of
1
The 31P{ H} NMR spectroscopic data for 1, 2 and 5 are
2
similar to those for related complexes, e.g. [Pt(dppe)(g -P≡CBut)],7
Centre for Fundamental and Applied Main Group Chemistry, School
of Chemistry, Main Building, Cardiff University, Cardiff, UK CF10 3AT.
E-mail: jonesca6@cardiff.ac.uk
and in particular, their phosphaalkyne resonances display very
1
small JPtP coupling constants (1: 178.0, 2: 167.5, 5: 143.6 Hz).
Interestingly, these couplings for the bridged complexes, 3 and
4, are too small to be observable. The high field position of the
phosphaalkyne resonances (3: d −101.2 ppm, 4: d −115.5 ppm)
† Electronic supplementary information (ESI) available: Details of the
1
syntheses of 3–7, 31P{ H} NMR spectra of 1 and 2, and the molecular
structures of 4, 5 and 7. See DOI: 10.1039/b607489h
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in those complexes can, however, be compared to those in
other phosphaalkyne bridged complexes, e.g. [{CpMo(CO)2}2(l-
P≡CBut)], d −110 ppm.8 The X-ray crystal structures§ of 3–5
all displayed disorder of the coordinated phosphaalkyne over
two sites. Although successfully modeled, this disorder affects
the reliability of comment on the phosphaalkyne coordination
geometry to some extent. Saying that, the structure of 5 is
resultant 1,2- or 1,3-hetrocyclic complexes, a consequence of the
steric influence of the bulky tert-butyl groups.11
To test these differences in practice, we carried out the reaction
of P≡CMe with [W(CO)5(THF)]. In contrast to the related
reaction with P≡CBut, both head to head and head to tail coupled
complexes, 6 and 7, were observed in the reaction mixture in a
ca. 80 : 20 ratio (Scheme 1). The complexes were subsequently
purified by column chromatography and/or recrystallisation. In
accord with the aforementioned theoretical study, the preferential
formation of 6 does suggest that metal mediated head to head
couplings of unhindered phosphaalkynes are favoured over head
to tail couplings.
2
comparable with that of, for example, [Pt(PPh3)2(g -P≡CBut)],6a
˚
as both exhibit phosphaalkyne P–C distances (1.617 A (mean)
t
˚
and 1.672(17) A, respectively) longer than that in free P≡CBu
9
˚
˚
(1.548(1) A), but considerably shorter than those in 3 (1.744 A
˚
mean, Fig. 1) and 4 (1.728 A mean). Also of note are the Pt2PC
fragments of 3 and 4, which are non-planar and exhibit angles
Most informative of the characteristic data for 6 and 7 comes
between their three-membered rings of 96.4 and 96.8◦, respectively.
from their 31P{ H} NMR spectra which exhibit singlets with
1
183W satellites having characteristic one bond JPW couplings (6:
d −74.8 ppm, 1JWP = 148.2 Hz; 7: d −4.0 ppm, 1JWP = 252.2 Hz).
Crystal structure analyses of both complexes were carried out and
the molecular structure of 6 is depicted in Fig. 2. The intra-ring
distances in both are suggestive of significant delocalisation, as has
been previously observed in many 1,3-diphosphacyclobutadiene
complexes2 and the only structurally characterised complexes con-
4
taining a 1,2-isomer of this heterocycle type, viz. [Ti(COT)(g -1,2-
4
P2C2But2)], COT = cyclooctatetraarene,12 and [Fe(CO)3{g -1,2-
P2[W(CO)5]2C2But2}]13 (N.B. the latter complex was not formed
via a phosphaalkyne dimerisation).
Fig. 1 Molecular structure of 3 (25% thermal ellipsoids, non-methyl
˚
hydrogens omitted for sake of clarity). Selected bond lengths (A) and
angles (◦) relating to one of the two components of the disordered PCMe
ligand: Pt(1)–C(1) 2.075(13), Pt(1)–P(2) 2.2568(12), Pt(1)–P(3) 2.2618(12),
P(1)–C(1) 1.734(12), P(1)–Pt(2) 2.347(2), C(1)–C(2) 1.537(10), C(1)–Pt(2)
2.059(11), Pt(2)–P(4) 2.2498(10), Pt(2)–P(5) 2.2611(10); C(1)–Pt(1)–P(1)
45.4(3), P(2)–Pt(1)–P(3) 86.75(4), C(1)–Pt(2)–P(1) 45.8(3), P(4)–Pt(2)–P(5)
86.43(4), C(1)–P(1)–Pt(2) 58.3(4), C(1)–P(1)–Pt(1) 58.4(5), Pt(2)–P(1)–
Pt(1) 80.34(7), P(1)–C(1)–Pt(2) 75.9(4), P(1)–C(1)–Pt(1) 76.3(5),
Pt(2)–C(1)–Pt(1) 94.7(4).
Fig. 2 Molecular structu◦re of 6 (25% thermal ellipsoids). Selected bond
˚
lengths (A) and angles ( ): W(1)–P(1) 2.4772(15), P(1)–C(1) 1.799(6),
P(1)–P(2) 2.161(2), P(1)–W(3) 2.5314(15), C(1)–C(3) 1.527(8), C(1)–W(3)
2.353(5), W(2)–P(2) 2.4721(14), P(2)–C(2) 1.818(6), P(2)–W(3) 2.5099(14),
C(2)–C(4) 1.501(7), C(2)–W(3) 2.366(5); C(1)–P(1)–P(2) 78.04(18),
C(2)–C(1)–C(3) 127.2(5), C(2)–C(1)–P(1) 102.7(4), C(3)–C(1)–P(1)
129.3(4), C(2)–P(2)–P(1) 77.91(18), C(1)–C(2)–P(2) 101.4(4).
Bulky phosphaalkynes are well known to dimerise in the
coordination sphere of low valent transition metal fragments to
4
give g -diphosphacyclobutadiene complexes.2 Almost invariably,
this occurs in a head to tail fashion to give the 1,3-isomer of the
heterocycle. This is indeed the case in the reactions of P≡CR, R =
But or Mes, with [W(CO)5(THF)] which have yielded a variety
of 1,3-diphosphacyclobutadiene complexes.10 A theoretical study
on such dimerisations has suggested that head to head couplings,
to give 1,2-diphosphacyclobutadiene complexes, are significantly
more thermodynamically favourable for P≡CR, R = H or Me.
For P≡CBut, however, there is little energy difference between the
In conclusion, we have reported the first examples of
methylphosphaalkyne complex formations and cyclodimerisation
reactions. This study has highlighted both similarities and dif-
ferences between the chemistry of this phosphaalkyne and its
bulkier analogues. The differences appear to be derived from the
unhindered nature of the phosphaalkyne. A systematic study of
methylphosphaalkyne chemistry continues in our laboratory.
3734 | Dalton Trans., 2006, 3733–3735
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0.0268), R (on F) = 0.0484, wR (on F2) = 0.1112 (I > 2rI); 5·C6H5Me:
C45H77P3Pt, M = 906.07, monoclinic, space group P21/n, a = 11.466(2),
Acknowledgements
◦
3
˚
˚
b = 9.938(2), c = 41.029(8) A, b = 90.56(3) , V = 4675.0(16) A , Z = 4,
Dc = 1.287 g cm−3, F(000) = 1880, l(Mo-Ka) = 3.131 mm−1, 150(2) K,
8153 unique reflections (Rint = 0.0246), R (on F) = 0.0263, wR (on F2) =
0.0641 (I > 2rI); 6: C18H6O14P2W3, M = 1059.72, orthorhombic, space
We gratefully acknowledge financial support from the EPSRC
(partial studentship for C. S.). Thanks also go to the EPSRC Mass
Spectrometry Service.
˚
group Pccn, a = 14.323(3), b = 30.107(6), c = 12.311(3) A, V = 5308.7(19)
3
A , Z = 8, Dc = 2.652 g cm−3, F(000) = 3824, l(Mo-Ka) = 13.154 mm−1
,
˚
Notes and references
150(2) K, 5201 unique reflections (Rint = 0.0218), R (on F) = 0.0269, wR
(on F2) = 0.0500 (I > 2rI); 7: C18H6O14P2W3, M = 1059.72, monoclinic,
‡ Selected data for 3: (yield 34%). Mp 156–160 ◦C; 1H NMR (400 MHz,
˚
space group P21/n, a = 16.082(3), b = 12.237(2), c = 28.575(6) A, b =
◦
106.02(3) , V = 5405.1(19) A , Z = 8, Dc = 2.605 g cm−3, F(000) = 3824,
3
C6D6, 298 K): d 1.85 (m, 8H, PCH2), 4.12 (m, 3H, CCH3), 6.67–8.15
˚
1
(m, 40H, ArH); 31P{ H} NMR (121.6 MHz, C6D6, 298 K): d −101.2
l(Mo-Ka) = 12.919 mm−1, 150(2) K, 10511 unique reflections (Rint
=
1
0.0523), R (on F) = 0.0630, wR (on F2) = 0.1385 (I > 2rI). CCDC
reference numbers 609152–609156. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b607489h
(unresolv. m, PCMe), 36.4 (unresolv. m, JPtP = 3512 Hz, dppe), 45.8
1
(unresolv. m, JPtP = 3123 Hz, dppe); (MS/EI) m/z: 1244 [M+, 5%],
651 [(dppe)Pt(PCMe)+, 16%], 593 [(dppe)Pt+, 12%], 398 [dppe+, 100%];
acc. MS (EI) calc. for C54H51P5Pt2: 1244.1969, found 1244.1977; 4: (yield
57%). Mp 187–190 ◦C; 1H NMR (400 MHz, C6D6, 298 K): d 1.15 (m,
1 G. Becker, G. Gresser and W. Uhl, Z. Naturforsch., Teil B, 1981, 36, 16.
2 (a) K. B. Dillon, F. Mathey and J. F. Nixon, in Phosphorus: The Carbon
Copy, Wiley, Chichester, 1998; (b) Phosphorus–Carbon Heterocyclic
Chemistry: The Rise of a New Domain, ed. F. Mathey, Pergamon,
Amsterdam, 2001; (c) F. Mathey, Angew. Chem., Int. Ed., 2003, 42,
1578, and references therein.
3 See, for example: (a) S. Haber, P. Le Floch and F. Mathey, J. Chem.
Soc., Chem. Commun., 1992, 1799; (b) J.-C. Guillemin, T. Janati and
J.-M. Denis, J. Chem. Soc., Chem. Commun., 1992, 415.
4 J.-C. Guillemin, T. Janati and J.-M. Denis, J. Org. Chem., 2001, 66,
7864.
5 N. B. a formal methylphosphaalkyne complex, [{Co(CO)3}2(l-PCMe)],
has been prepared as an oil, not from free MeC≡P, but in the reaction
of [Co2(CO)8] with MeCCl2PCl2: D. Seyferth, J. S. Merola and R. S.
Henderson, Organometallics, 1982, 1, 859.
6 (a) J. C. T. R. Burckett-St. Laurent, P. B. Hitchcock, H. W. Kroto
and J. F. Nixon, J. Chem. Soc., Chem. Commun., 1981, 1141; (b) M.
Brym and C. Jones, Dalton Trans., 2003, 3665; (c) D. Himmel, M. Seitz
and M. Scheer, Z. Anorg. Allg. Chem., 2004, 630, 1220, and references
therein.
7 S. I. Al-Resayes, P. B. Hitchcock, M. F. Meidine and J. F. Nixon,
J. Chem. Soc., Chem. Commun., 1984, 1080.
8 M. F. Meidine, C. J. Meir, S. Morton and J. F. Nixon, J. Organomet.
Chem., 1985, 297, 255.
9 M. Y. Antipin, A. N. Chernega, K. A. Lysenko, Y. T. Struchkov and
J. F. Nixon, J. Chem. Soc., Chem. Commun., 1995, 505.
10 P. Kramkowski and M. Scheer, Eur. J. Inorg. Chem., 2000, 1869.
11 S. Creve, M. T. Nguyen and L. G. Vanquickenborne, Eur. J. Inorg.
Chem., 1999, 1281.
12 P. Binger, G. Glaser, S. Albus and C. Kru¨ger, Chem. Ber., 1995, 128,
1261.
36H, PCH3), 1.92 (m, 24H, PCH2), 3.92 (m, 3H, PCCH3); 31P{ H} NMR
1
(121.6 MHz, C6D6, 298 K): d −115.5 (unresolv. m, PCMe), 6.6 (unresolv.
m, 1JPtP = 3147 Hz, dppe), 7.7 (unresolv. m, 1JPtP = 3596 Hz); (MS/EI) m/z:
920 [M+, 11%], 802 [M+ − PEt3, 13%], 684 [M+ − 2PEt3, 20%], 489 [M+ −
Pt(PEt3)2, 18%], 431 [Pt(PEt3)2+, 100%]; acc. MS (EI) calc. for C26H63P5Pt2:
920.2908, found 920.2917; 5: (yield 47%). Mp 133–135 ◦C (decomp.); 1H
NMR (400 MHz, C6D6, 298 K): d 1.21–2.62 (m, 66H, CyH), 3.61 (m, 3H,
1
PCCH3); 31P{ H} NMR (121.6 MHz, C6D6, 298 K): d 32.7 (virt. t, 2JPP
=
1
2
1
24.0 Hz, JPtP = 3195 Hz, PCy3), 44.4 (virt. t, JPP = 24.0 Hz, JPtP
=
2
1
3195 Hz, PCy3), 86.6 (virt. t, JPP = 24.0 Hz, JPtP = 143.6 Hz, PCMe);
m/z (EI): 752 [Pt(PCy3)2+, 5%], 280 [PCy3+, 100%]; 6: (yield 23%). Mp
98–100 ◦C; 1H NMR (400 MHz, C6D6, 298 K): d 1.32 (virt. t, JPH
=
3
1
6.0 Hz, 6H, PCMe); 31P{ H} NMR (121.6 MHz, C6D6, 298 K): d −74.8 (s,
1JWP = 148.2 Hz, PCMe); IR (Nujol) m/cm−1: 2075(m), 2060(m), 1980(s),
1961(s), 1927(sh), 1914(sh); (MS/EI) m/z (on a crystalline mixture of
isomers 6 and 7): 1059 [M+, 64%], 1003 [M+ − 2CO, 13%], 919 [M+
−
5CO, 14%], 835 [M+ − 8CO, 62%], 664 [M+ − 14CO, 100%]; acc. MS
(EI) calc. for C18H6O14P2W3: 1059.7755, found 1059.7757; 7: (11%). Mp
100–103 ◦C; 1H NMR (400 MHz, C6D6, 298 K): d 1.20 (t, 3JPH = 16.0 Hz,
1
6H, PCMe); 31P{ H} NMR (121.6 MHz, C6D6, 298 K): d −4.0 (s, 1JWP
=
252.2 Hz, PCMe); IR (Nujol) m/cm−1: 2075(m), 2059(m), 1976(sh), 1963(br
s), 1929(br s), 1914(sh). Reproducible microanalyses could not be obtained
due to the presence of solvent of crystallisation and/or the air sensitivity
of the prepared complexes.
§ Crystal data for 3·Et2O: C58H61OP5Pt2, M = 1319.10, monoclinic, space
◦
˚
group P21/c, a = 13.450(3), b = 17.802(4), c = 22.023(4) A, b = 95.08(3) ,
3
V = 5252.7(18) A , Z = 4, Dc = 1.668 g cm−3, F(000) = 2592, l(Mo-
˚
Ka) = 5.512 mm−1, 150(2) K, 10300 unique reflections (Rint = 0.0300), R
(on F) = 0.0277, wR (on F2) = 0.0504 (I > 2rI); 4: C26H63P5Pt2, M =
920.79, orthorhombic, space group Pcab, a = 14.460(3), b = 20.302(4),
c = 24.218(5) A, V = 7110(2) A , Z = 8, Dc = 1.720 g cm−3, F(000) =
3
˚
˚
13 F. W. Heinemann, S. Kummer, U. Seiss-Brandl and U. Zenneck,
Organometallics, 1999, 18, 2021.
3600, l(Mo-Ka) = 8.100 mm−1, 150(2) K, 6945 unique reflections (Rint
=
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 3733–3735 | 3735
©