A generic route to fluoroalkyl-containing phosphanes

Article abstract of DOI:10.1039/b909749j

The reaction of trimethylsilyl-containing phosphanes with perfluoroiodoalkanes provides a general and convenient route to perfluoroalkyl-containing phosphanes. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b909749j

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A generic route to fluoroalkyl-containing phosphanesw
Alan K. Brisdon* and Christopher J. Herbert
Received (in Cambridge, UK) 18th May 2009, Accepted 7th September 2009
First published as an Advance Article on the web 18th September 2009
DOI: 10.1039/b909749j
The reaction of trimethylsilyl-containing phosphanes with
perfluoroiodoalkanes provides a general and convenient route
to perfluoroalkyl-containing phosphanes.
Thus, CF3I was added to a solution of Ph2PSiMe3 in CDCl3
and the course of the reaction was followed using 19F and
31P{1H} NMR spectroscopy. The phosphorus resonance due
to Ph2PSiMe3, at ꢀ56.7 ppm, decreased in intensity and was
slowly replaced by a quartet (J = 73.8 Hz) centred at 2.5 ppm.
Similarly, the 19F NMR spectrum of the reaction mixture
showed replacement of the peak due to CF3I at ꢀ5.2 ppm16
with a doublet signal (J = 73.8 Hz) at ꢀ55.1 ppm.
These spectroscopic data are consistent with those previously
reported for Ph2PCF3.11 The principal by-product of the
reaction was identified as Me3SiI from 1H and 29Si NMR
studies (dH 0.8 ppm, dSi 9.7 ppm17).
Phosphorus(III) ligand systems of the type PR3 are amongst
the most widely studied and utilised in transition metal
chemistry, and they, or their metal complexes, find applications
in areas such as materials, drugs, organocatalysts and a variety
of C–C bond forming reactions.1 When one, or more, of the
substituents are perfluorinated (denoted by Rf) an unusual
combination of steric and electronic properties can result.2
Whilst a range of fluoroaryl-phosphanes are known,3 the only
perfluoroalkyl phosphanes to have been studied in detail are
When other perfluoroalkyliodides were investigated,
Table 1, we similarly observed a consumption of the starting
materials and new signals in the 31P{1H} and 19F NMR
spectra consistent with Rf-containing phosphines. For
Ph2P(CF3) and Ph2P(C2F5) these data are consistent with
those previously reported. Whilst for the new compounds
unequivocal assignment is possible based on the complex,
largely first order, NMR patterns observed in the mutually
coupled fluorine and phosphorus NMR spectra. Particularly
noteworthy is the reaction with t-C4F9I. Although this reaction
was not as clean as many of the others, some (CF3)3CH and
(CF3)2CQCF2 being observed by known decomposition
routes,18 it is to the best of our knowledge the first report of
a phosphine containing a tertiary perfluoroalkyl group being
prepared. Calculations performed by others suggest that such
ligands should possess unusual and interesting steric and
electronic properties.2
4
5
those containing the –CF3 and –CF2CF3 groups. This
limitation in phosphorus (and other element) organofluorine
chemistry largely arises because of a limited number of
suitable perfluoroalkyl-delivery reagents; perfluoroorganic
Grignard and lithium reagents are frequently unstable (e.g.
LiC2F5 is reported to decompose above ꢀ70 1C5) or inaccessible
(e.g. LiCF3). Thus fluoroalkyl phosphanes have been prepared
by a variety of specialised methods, including direct fluorination
of PR3 with F2 (followed by reduction of the resulting P(V)
species),6 electrochemical fluorination of PR3 in anhydrous
HF,7 the reaction of heavy metal transfer agents with PI3,8 or
reaction of CF3I with P4.9
In 2005, Caffyn and co-workers published a method for the
synthesis of P(Rf)3 compounds, based on the reaction of
Me3SiRf with P(OR)3 (R = Ph, p-C6H4CN).10 Subsequently
Togni et al. reported that CF3-containing phosphanes may be
prepared from Ph2PH or Ph2PSiMe3 and CF3-containing
hypervalent iodine compounds.11 Both of these methods benefit
from the commercial availability of Me3SiCF3, unfortunately
other Me3SiRf reagents are more difficult to obtain.
Replacement of Ph2PSiMe3 with i-Pr2PSiMe3 or the P-chiral
PhMePSiMe3 starting materials was also successful, which
suggests that this method may be applied to a number of
trimethylsilyl-containing phosphane starting materials.
However, no significant reaction was observed to occur over
a period of 5 days when either CF2QCFI or C6F5I was used.
On a preparative scale the reaction of i-C3F7I with
Ph2PSiMe3 at ꢀ20 1C results in an immediate reaction, after
addition of MeLi to facilitate removal of Me3SiI as volatile
Me4Si, and work-up a white, low-melting point solid was
isolated. NMR data (1H, 13C, 19F, 31P) and elemental analysis
confirmed the compound to be Ph2P(i-C3F7), 4, furthermore,
by slow cooling of the molten solid, we were able to obtain
single-crystals suitable for X-ray analysis.y
The molecular structure of 4 (Fig. 1), as expected, exhibits a
pyramidal geometry at the phosphorus centre with the P–C
bond length to the fluorinated fragment, 1.899(5) A, being
longer than the average distance to the non-fluorinated
groups, 1.830(5) A. These distances are consistent with the
other four X-ray structures of phosphanes containing one, or
more, perfluorinated groups that have been reported
Trimethylsilyl-containing phosphanes have been used,
often under transition metal catalysed conditions, to prepare
perprotio-phosphanes12 and to generate the fluorine-containing
phosphanes, Ph2P(CF2)nBr and Ph2P(CF2)nPPh2, from
Ph2PSiMe3 and Br(CF2)nBr (n = 113 or 214) and fluoroaryl-
phosphanes from polyfluoroarenes.14,15 It is surprising that a
similar strategy has not yet been successfully applied to the
synthesis of perfluoroalkyl-containing phosphanes.
The NMR-scale reactions of a series of primary, secondary
and tertiary RfI compounds with Ph2PSiMe3 were undertaken.z
School of Chemistry, The University of Manchester, Manchester,
UK M13 9PL. E-mail: alan.brisdon@manchester.ac.uk;
Fax: +44 (0)161 275 4598; Tel: +44 (0)161 306 4459
w Electronic supplementary information (ESI) available: Full
experimental details and copies of NMR spectra. CCDC 714473.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b909749j
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Table 1 Summary of reactions between RfI and R2PSiMe3
RfI
R2 in R2PSiMe3
Time for R2PSiMe3 consumptiona
31P{1H} NMR d (ppm)
Product, yieldb
CF3I
C2F5I
C8F17
i-C3F7I
s-C4F9I
c-C6F11
t-C4F9I
CF2QCFI
C6F5I
C2F5I
Ph2
Ph2
Ph2
Ph2
Ph2
Ph2
Ph2
Ph2
Ph2
i-Pr2
PhMe
6 days
5 days
6 days
30 min
30 min
30 min
30 min
—
2.5
ꢀ1.9
1.1
Ph2PCF3, 1, 75%
Ph2PC2F5, 2, 80%
Ph2P(C8F17), 3, 85%
Ph2P(i-C3F7), 4, 75%
Ph2P(s-C4F9), 5, 56%
Ph2P(c-C6F11), 6, 48%
Ph2P(t-C4F9), 7, 30%
—
I
ꢀ0.8
3.7
I
ꢀ3.4
15.2
—
—
24.5
—
2 h
30 min
—
i-Pr2P(C2F5), 8, 52%
PhMeP(i-C3F7), 9, 65%
i-C3F7I
ꢀ12
a
b
At room temperature. Typical NMR yields based on integration against an internal standard.
before—PPh2(C2F5),5 PPh(C2F3)2,19 PPh2(C6F5)20 and P(C6F5)3.21
The P–C distances observed for 4 are similar to those
found in the related fluoroalkyl phosphane PPh2(C2F5)
unable to observe any signals from in situ EPR studies which,
taken together, suggests that the rate determining step is not
radical-based. Whilst nucleophilic substitution would explain
the observed products, and the enhanced rate of reaction of
C2F5I with i-Pr2PSiMe3 compared with that of Ph2PSiMe3,
multinuclear (31P, 19F, 29Si and 1H) NMR studies, carried out on
a variety of reaction stoichiometries, suggest that the mechanism
is more complicated. In the slower reactions—those of the
straight chain perfluoroalkyl iodides—in situ monitoring via
NMR spectroscopy shows a significant peak due to Ph2PPPh2
[d(P–CF)
= 1.891(3) and d(P–C)av. = 1.834(3) A].
Significantly, although not surprisingly when compared with
structures of metal-bound n-fluoroalkyl and i-C3F7-containing
complexes,22 the sum of the angles around the phosphorus
centre is larger in 4, which contains a bulky perfluoroisopropyl
group, (309.61), than is observed in PPh2(C2F5) (304.21),
PPh(C2F3)2 (299.61) and PPh2(C6F5) (306.61) and similar to
that of P(C6F5)3 (309.91).
We have investigated a number of modifications to our
procedure; including the addition of fluoride catalysts, using
perfluoroalkyl bromides or chlorides, instead of iodides, and
replacement of Ph2PSiMe3 with Ph2PH. None of these were
found to be as successful. We have also investigated the
reaction between PhP(SiMe3)2 and P(SiMe3)3 with RfI, but
they were rapid and exothermic, resulting in a mixture of
products.
in the 31P{1H} NMR spectra (dP = ꢀ15 ppm). Interestingly, in
14
related work, such as the reactions of Ph2PSiMe3 with CBr4
(Ph2P)2PSiMe3 with BrCF2CF2Br23 and Ph2PH with CCl4
,
24
Ph2PPPh2 was also identified as a significant product or key
intermediate. We therefore propose that a similar process is
occurring here; initially R2PSiMe3 is converted to R2PPR2
which then reacts with RfI to produce R2PRf and R2PI. The
R2PI so formed can react with R2PSiMe3 to generate further
R2PPR2 and Me3SiI. Some support for the latter sequence of
reactions comes from the separate reaction of Ph2PPPh2 with
RfI, which produces both Ph2PRf and Ph2PI. Furthermore,
Ph2PI (dP = 38 ppm25) and RfSiMe3 (dSi = 4.2 ppm, 2J(SiF) =
38 Hz, Rf = CF3; 11.6 ppm, 2J(SiF) = 21.5 Hz, Rf = i-C3F7)
are also identified as intermediates in these reactions by NMR
spectroscopy. We are continuing to investigate the mechanism
of this reaction as well as its scope.
The observation of much faster reactions between RfI and
Ph2PSiMe3 for tertiary and secondary perfluoroalkyliodides
compared with primary analogues lead us to initially consider
a radical mechanism. However, the addition of radical traps
and radical initiators does not appear to significantly effect the
outcome, or rate, of these transformations; we were also
In conclusion, we have demonstrated that the reaction of
trimethylsilyl-substituted phosphanes with perfluoroalkyliodides
results in the facile generation of perfluoroalkyl-containing
phosphanes. The reaction is rapid for phosphanes containing a
secondary, tertiary or cycloperfluoroalkyl group, however, for
n-fluoroalkyl systems the reaction is slower. This method
provides, for the first time, a method by which a number of
new, sterically demanding, electron-poor fluorinated P(III)
ligands may be accessed from readily-available starting materials.
Notes and references
z NMR scale experiments were carried out on a 0.4 mmol scale by
dissolving the appropriate trimethylsilylphosphane in 0.75 cm3 of
CDCl3, and then adding a slight excess of RfI under anaerobic
conditions. Spectra were recorded [Bruker DPX 200 (1H, 19F and
31P at 200.131, 188.310 and 81.014 MHz) or Bruker AVANCE III 400
(1H, 19F and 31P at 400.131, 376.498 and 161.976 MHz) spectrometers
and referenced to external TMS, CFCl3 and 85% H3PO4, respectively]
immediately and then periodically until no further sign of reaction
occurred.
Fig. 1 An ORTEP representation of the molecular structure of
Ph2P(i-C3F7), 4, thermal ellipsoids are shown at 30% probability level.
Selected distances: P1–C1, 1.899(5); P1–C4, 1.828(5); P1–C10,
1.831(5); C1–F1, 1.400(5); C1–C2, 1.539(6), C1–C3, 1.531(7) A.
Selected angles: C1–P1–C4, 102.9(2)1; C1–P1–C10, 103.6(2)1;
C4–P1–C10, 103.1(2)1.
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Ph2PCF3 (1). 31P NMR d = 2.5 [q, J(PF) = 73.8 Hz], 19F NMR
2
were solved by direct methods, with full-matrix least-squares refinement
on F2 using the SHELXL program.26 Non-hydrogen atoms were
refined with anisotropic thermal parameters; hydrogen atoms were
placed in idealised locations.
d = ꢀ55.1 [d, 2J(PF) = 73.8 Hz]. 1H NMR d = 7.7–7.4 (m), literature
(CDCl3), dP 2.8 [2J(PF) = 73.5 Hz], dF ꢀ55.0 [J = 73 Hz].11
Ph2PCF2CF3 (2). 31P NMR d = ꢀ1.9 [tq, 2J(PF) = 56.8, 3J(PF) =
3
3
16.7 Hz], 19F NMR d = ꢀ81.0 [3F, dt, J(PF) = 16.7, J(FF) = 3.0
Hz, CF3], ꢀ113.0 [2F, dq, 2J(PF) = 56.8, 3J(FF) 3.0, CF2], 1H NMR
d = 7.7–7.4 (m), literature (C6D6), dP ꢀ1.4 [J = 58, 17], dF ꢀ80.7
[J = 16.5, 3.1], ꢀ112.6 [J = 57.0, 3.1 Hz].5a
1 See for example: D. W. Allen, Organophosphorus Chem., 2006, 35, 1.
2 See for example: K. D. Cooney, T. R. Cundari, N. W. Hoffman,
K. A. Pittard, M. D. Temple and Y. Zhao, J. Am. Chem. Soc.,
2003, 125, 4318.
3 C. L. Pollock, G. C. Saunders, E. Smyth and V. I. Sorokin,
J. Fluorine Chem., 2008, 129, 142.
4 H. J. Emeleus and J. D. Smith, J. Chem. Soc., 1958, 527;
A. B. Burg and W. Mahler, J. Am. Chem. Soc., 1958, 80,
2334.
5 (a) R. G. Peters, B. L. Bennett, R. C. Schnabel and D. M. Roddick,
Inorg. Chem., 1997, 36, 5962; (b) J. D. Palcic, P. N. Kapoor,
D. M. Roddick and R. G. Peters, Dalton Trans., 2004, 1644.
6 J. J. Kampa, J. W. Nail and R. J. Lagow, Angew. Chem., Int. Ed.
Engl., 1995, 34, 1241.
7 V. Y. Semenii, V. A. Stepanov, N. V. Ignat’ev, G. G. Furin and
L. M. Yagupolskii, Zh. Obshch. Khim., 1985, 55, 2716; N. Ignat’ev
and P. Sartori, J. Fluorine Chem., 2000, 103, 57.
8 E. A. Ganja, C. D. Ontiveros and J. A. Morrison, Inorg. Chem.,
1988, 27, 4535.
9 F. W. Bennett, G. R. A. Brandt, H. J. Emeleus and
R. N. Haszeldine, Nature, 1950, 166, 225; F. W. Bennett,
H. J. Emeleus and R. N. Haszeldine, J. Chem. Soc., 1953, 1565.
10 M. B. Murphy-Jolly, L. C. Lewis and A. J. M. Caffyn, Chem.
Commun., 2005, 4479.
Ph2PC8F17 (3). dP 1.1 [m, 2J(PF) = 56.0 Hz], dF ꢀ81.7 [t, J(FF) =
7.8 Hz, 3F, CF3], ꢀ109.4 [dt, 2J(PF) = 56.0, J(FF) = 13.3, 2F, PCF2],
ꢀ122.1 [m, 2F], ꢀ122.6 [m, 6F], ꢀ123.5 [m, 2F], ꢀ127.0 [m, 2F].
dH 7.7–7.4 (m).
Ph2PCF(CF3)(CF2CF3) (5). dP 3.7 ppm (dddqq, 2J(PFb) = 78.0
Hz, 3J(PFd) = 44.6 Hz, 3J(PFc) = 33.2 Hz, 3J(PFa) = 16.9 Hz,
4J(PFe) = 12.0 Hz). 19F NMR: dFa ꢀ67.8 ppm (3F, ddddq, 3J(PFa) =
16.9 Hz, 3J(FaFb) = 12.1 Hz, 4J(FaFc) = 12.3 Hz, 4J(FaFd) = 5.5 Hz,
5J(FaFe) = 8.6 Hz, PCF(CF3)(CF2CF3)), dFe ꢀ80.0 (3F, ddddq,
4J(PFe) = 12.0 Hz, 4J(FeFa) = 8.6 Hz, 4J(FeFb) = 12.0 Hz,
3J(FeFc)
=
0.1 Hz, 3J(FeFd)
=
0.6 Hz, PCF(CF3)(CF2CF3)),
1
3
dFc ꢀ110.6 ppm (1F, ddqdq, J(FcFd) = 295.6 Hz, J(PFc) = 33.2 Hz,
4J(FcFa) = 12.3 Hz, 3J(FcFb) = 12.0 Hz, 3J(FcFe) = 0.1 Hz,
PCF(CF3)(CFFCF3))
dFd ꢀ114.5 ppm (1F, dddqq, 1J(FdFc) = 295.6 Hz, 3J(FdP) = 44.6 Hz,
3J(FdFb) = 11.9 Hz, 4J(FdFa) = 5.5 Hz, 3J(FdFe) = 0.6 Hz,
2
PCF(CF3)(CFFCF3)), dFb ꢀ183.4 ppm (1F, dqdqd, J(FbP) = 78.0 Hz,
3J(FbFa) = 12.1 Hz, 3J(FbFc ) = 12.0 Hz, 4J(FbFe) = 12.0,
3J(FbFd) = 11.9 Hz, PCF(CF3)(CF2CF3)). dH 7.7–7.4 (m).
Ph2P(c-C6F11
)
(6) dP ꢀ3.4 ppm (td, 3J(PF2ax
) = 84.0 Hz,
2J(PF1) = 68.0 Hz). dF ꢀ110.5 (2F, 3J(PF) = 84.0 Hz, 2J(FF) =
298.3 Hz, F2a,6a), ꢀ122.6 (2F, d, 2J(FF) = 280.0 Hz, F3a,5a), ꢀ124.4
11 P. Eisenberger, I. Kieltsch, N. Armanino and A. Togni, Chem.
Commun., 2008, 1575.
2
2
ppm (1F, d, J(FF) = 288.8 Hz, F4a) ꢀ124.5 (2F, d, J(FF) = 298.3
Hz, F2e,6e), ꢀ138.2 (2F, d, 2J(FF) = 280.0 Hz, F3e,5e), ꢀ142.0 (1F, d,
2J(FF) = 285.5 Hz, F4e), and ꢀ185.8 (1F, m, 2J(PF) = 68.0 Hz, F1a).
dH 7.7–7.4 (m).
12 J. K. Stille, J. Org. Chem., 1987, 52, 748; V. S. Chan,
R. G. Bergman and F. D. Troste, J. Am. Chem. Soc., 2007, 129,
15122; S. E. Vaillard, C. Muck-Lichtenfeld, S. Grimme and
A. Studer, Angew. Chem., Int. Ed., 2007, 46, 6533.
13 M. Fild, P. G. Jones and K. Ruhnau, J. Fluorine Chem., 1991, 54,
387; M. L. Clarke, A. G. Orpen, P. G. Pringle and E. Turley,
Dalton Trans., 2003, 4393.
14 H.-G. Horn and H. J. Lindner, Chem.-Ztg., 1988, 112, 195.
15 L. I. Goryunov, J. Grobe, V. D. Shteingarts, B. Krebs,
A. Lindemann, E.-U. Wrthwein and C. Mck-Lichtenfeld,
Chem.–Eur. J., 2000, 6, 4612; L. I. Goryunov, V. D. Shteingarts,
J. Grobe, B. Krebbs and M. U. Triller, Z. Anorg. Allg. Chem.,
2002, 628, 1770.
16 D. Naumann and J. Kischkewitz, J. Fluorine Chem., 1990, 47,
283.
17 M. Sako, T. Kihara, K. Okada, Y. Ohtani and H. Kawamoto,
J. Org. Chem., 2001, 66, 3610.
18 A. Probst, K. Raab, K. Ulm and K. von Werner, J. Fluorine
Chem., 1987, 37, 223.
19 K. K. Banger, R. P. Banham, A. K. Brisdon, W. I. Cross,
G. Damant, S. Parsons, R. G. Pritchard and A. Sousa-Pedrares,
J. Chem. Soc., Dalton Trans., 1999, 427.
20 O. B. Shawkataly, M.-L. Chong, H.-K. Fun and K. Sivakumar,
Acta Crystallogr., Sect. C, 1996, 52, 1725.
21 A. Karipides and C. M. Cosio, Acta Crystallogr., Sect. C, 1989, 45,
1743.
22 R. P. Hughes, J. S. Overby, A. Williamson, K.-C. Lam,
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5190.
23 I. Kovacs, E. Matern and G. Fritz, Z. Anorg. Allg. Chem., 1996,
622, 935.
Ph2PC(CF3)3 (7). dP 15.2 (dectet, 3J(PF) = 12.3 Hz), dF ꢀ60.0
3
(d, J(PF) = 12.2 Hz). dH 7.7–7.4 (m).
i-Pr2PC2F5 (8). dP 24.5 (tq, 2J(PF) = 41.7, 3J(PF) = 14.8 Hz),
dF ꢀ82.4 (dt, 3J(PF) = 14.8, 3J(PF) = 2.9 Hz ), ꢀ111.7 (dq, 2J(PF) = 41.7,
3J(PF) = 2.9 Hz ). dH 1.16 (6H, m, CH3), 2.2 (1H, dsept, J(PH) = 2.4,
2
3J(HH) = 7.1 Hz, CH).
PhMePCF(CF3)2 (9). dP ꢀ12.0 (dsept, 2J(PF) = 61.1, 3J(PF) =
16.4 Hz) dF ꢀ70.2 [ddq, 2J(PF) = 16.4, 3J(FF) = 11.2, 4J(FF) = 9.3 Hz,
CF3], ꢀ71.2 [ddqq, 2J(PF) = 16.4, 3J(FF) = 11.2, 4J(FF) = 9.3,
5J(FH) = 1.5 Hz, CF3], ꢀ190.3 [dsept, 2J(PF) = 61.1, 3J(FF) = 11.0,
2
CF] dH 7.28–7.40 (6H, m), 1.63 (3H, d, J(PH) = 6.0 Hz, CH3).
Synthesis of Ph2P(i-C3F7) (4): A dried Schlenk vessel was charged
with Ph2P(SiMe3) (2.2 cm3, 8.5 mmol) and dry hexane (20 cm3). The
solution was cooled to ꢀ30 1C and i-C3F7I (1.2 cm3, 8.5 mmol) was
added slowly over ca. 30 minutes. The solution was warmed to room
temperature overnight. MeLi (1.6 M in Et2O, 5.4 cm3, 8.6 mmol) was
added to the yellow solution and stirred for 30 minutes. The resulting
white precipitate was filtered off under an inert atmosphere (N2) and
the volatiles removed under vacuum to yield 4 as a white solid
(2.26 g, 75%). Elemental analysis C: 50.88, H: 2.71, P: 8.04%,
C15H10F7P requires: C: 50.84, H: 2.85, P: 8.75%. dP ꢀ0.8 [dsept,
2J(PF) = 74.0, 3J(PF) = 18.0 Hz] dF ꢀ69.6 [dd, 3J(PF) = 18.0,
3J(FF) = 11.9 Hz, CF3], ꢀ184.9 [dsept, 2J(PF) = 74.0, 3J(PF) =
11.9, CF] dH 7.24–7.45 (6H, m), 7.69–7.82 (4H, m)].
y Crystal data for 4: (C15H10F7P): Mr = 354.12; crystal size = 0.15 ꢂ
0.15 ꢂ 0.15 mm3; monoclinic, space group P21/c, a = 8.9091(2),
b = 26.2430(6), c = 6.3928(1) A, b = 99.406(1)1, V = 1474.55(5) A,
Z = 4, rcalcd = 1.595 g cmꢀ3, m = 0.257 mm, l = 0.71073 A, 150(2) K,
2ymax = 511, 2736 reflections collected, 208 parameters and 0 re-
24 P. Majewski, Phosphorus, Sulfur Silicon Relat. Elem., 1998, 134,
399.
straints, GOF on F2 = 1.082, final R indices for I 4 2s(I), wR2
=
25 E. Vincent, L. Verdonck and G. P. Van der Kelen, J. Mol. Struct.,
1980, 65, 239.
¨
26 G. M. Sheldrick, SHELXS97, University of Gottingen, Germany.
0.2181 R1 = 0.0796. Data were collected on a Nonius k-CCD 4-circle
diffractometer, and were corrected for Lorentz, polarisation and
absorption using the multi-scan method. The X-ray structural data
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This journal is The Royal Society of Chemistry 2009
6660 | Chem. Commun., 2009, 6658–6660