Synthesis and complexation of heptafluoroisopropyldiphenylphosphine

Article abstract of DOI:10.1039/b918348e

We report a one-step synthesis of the phosphine, PPh₂ ⁱC₃F₇ from commercially available precursors. The stereoelectronic properties of the phosphine were probed by coordination to transition metals. Mo(CO)₅PPh₂ⁱC ₃F₇ was synthesised and the synthesis and structure of trans-PtCl₂(PPh₂ⁱC₃F ₇)₂ are described. PPh₂ⁱC ₃F₇ was found to be a bulky electron-withdrawing ligand. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b918348e

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Synthesis and complexation of heptafluoroisopropyldiphenylphosphine†‡
Lesley C. Lewis-Alleyne,a Makeba B. Murphy-Jolly,a Xavier F. Le Goffb and Andrew J. M. Caffyn*a,c
Received 7th September 2009, Accepted 26th October 2009
First published as an Advance Article on the web 11th November 2009
DOI: 10.1039/b918348e
i
i
We report a one-step synthesis of the phosphine, PPh2 C3F7
gives PPh2 C3F7 1 in 53% yield (Scheme 1). Analytically pure
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PPh2 C3F7 was observed to crystallise directly after distillation.§
from commercially available precursors. The stereoelectronic
properties of the phosphine were probed by coordination to
transition metals. Mo(CO)5PPh2 C3F7 was synthesised and
the synthesis and structure of trans-PtCl2(PPh2 C3F7)2 are
described. PPh2 C3F7 was found to be a bulky electron-
i
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Scheme 1 Synthesis of heptafluoroisopropyldiphenylphosphine.
Thermolysis of 1 with Mo(CO)6 in boiling octane gave
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withdrawing ligand.
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Mo(CO)5PPh2 C3F7 2 in 28% yield, along with some decompo-
Conventional phosphines with branched alkyl groups constitute
an important group of bulky tertiary alkyl phosphines. Our group
is interested in developing the chemistry of bulky strong p-acceptor
phosphines. Ligands with this combination of properties have
been highlighted as having an interesting stereoelectronic profile.1
The coordination chemistry of such ligands is poorly developed.
The heptafluoroisopropyl group is conceptually the simplest
branched perfluoroalkyl substituent. Few perfluoroisopropyl-
substituted phosphorus(III) compounds have been synthesised2-5
and their coordination chemistry has not been explored. A general
lack of development of methodologies for the introduction of
electron-poor substituents, and specifically perfluoroalkyl sub-
stituents, at phosphorus(III) has been noted.6 We have recently
developed a convenient protocol for the general synthesis of
perfluoroalkylphosphines starting from commercially available
P(OPh)3 (or P(O-p-C6H4CN)3), RfSiMe3 (Rf = n-CnF2n+1) and CsF
or other fluorides.7 We initially attempted to extend this strategy to
secondary perfluoroalkyl groups by employing iC3F7SiMe3 under
the same conditions. However, after treatment of P(OAr)3 and
sition material (Scheme 2). The 31P NMR spectrum showed a
doublet resonance at d 57.5 ppm (2JPF = 80 Hz).§ In the IR
spectrum of 2 the A1 n(CO) band was observed at 2081 cm-1. This
(2)
value is essentially identical to that obtained for the analogous
complexes Mo(CO)5PPh2CF3 and Mo(CO)5PPh2C2F5 (Table 1).
Clearly, this suggests that the increase in bulk of the perfluoroalkyl
group maintains the relative p-acidity of the phosphine. Further-
more, 2 may be confidently regarded as being more electron-
withdrawing than PPh3, but significantly less electron-withdrawing
than tris(perfluoroalkyl)phosphines such as P(CF3)3.
Scheme 2 Reaction of heptafluoroisopropyldiphenylphosphine with
Mo(CO)6 and K2PtCl4.
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Ph2POAr (Ar = Ph, p-C6H4CN) with C3F7SiMe3/CsF in ether,
no evidence of P–iC3F7 bond formation was found by 31P or 19F
NMR spectroscopy.8
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PtCl2(PPh2 C3F7)2 3 was synthesised by treatment of aqueous
K2PtCl4 with an acetone solution of 1. The resulting red-orange
solution was left to stand at room temperature overnight, during
which time yellow crystals of 3, were obtained in 50% yield
(Scheme 2). The 31P NMR of the complex showed a multiplet
at d 25 ppm with associated 195Pt satellites (I = 12 , 33% abundant,
1JPtP = 3019 Hz).§ The magnitude of this coupling suggests a trans
stereochemistry for the complex (see Table 2). The synthesis of
PtCl2(PPh2CF3)2 has been reported and a trans geometry assigned
on the basis of no observed dipole moment.12 A trans geometry was
We therefore tried a different approach. Electron-rich P(NEt2)3
in combination with perfluoroalkyl bromides have been used
to form perfluoroalkyl-phosphorus bonds from P–Cl and
P–OPh linkages.9-11 We now report that treatment of a mixture of
perfluoroisopropyl iodide and Ph2PCl (in hexane) with P(NEt2)3
aDepartment of Chemistry, The University of the West Indies, St. Augustine,
Trinidad & Tobago
bLaboratorie “He´te´roe´le´ments et Coordination”, Ecole Polytechnique,
CNRS UMR 7653, 91128 Palaiseau, Ce´dex, France
cSchool of Biology, Chemistry & Health Science, Manchester Metropolitan
University, John Dalton Building, Chester St., Manchester, UK M1 5GD.
E-mail: a.caffyn@mmu.ac.uk; Tel: +44(0)161 2471443
(2)
Table 1 A1 n(CO) data for Mo(CO)5L complexes
Ligand/L
A1(2)/cm-1
Reference
† Electronic supplementary information (ESI) available: Experimental
details and details of cone angle calculations. CCDC reference numbers
746473 (1) and 746474 (3). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b918348e
i
PPh2 C3F7
2081
2080
2081
2077
2072
2104
2103
2070
This work
PPh2C2F5
PPh2CF3
PPh2CF CF2
19
20
21
22
23
24
22
‡ During preparation of this manuscript we became aware of a manuscript
=
i
detailing another synthesis of PPh2 C3F7 (A. K. Brisdon and C. J. Herbert,
PPh3
Chem. Commun., 2009, 6658.
P(CF3)3
PF3
PMe3
Part of this work was presented at 19th International Symposium on
Fluorine Chemistry, Abst. No. 78, August 23-28, 2009, Jackson Hole,
WY, USA.
1198 | Dalton Trans., 2010, 39, 1198–1200
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Table 2 Comparison of chemical shift and 1JPtP data for PtCl2L2
a pentafluoroethyl group to a heptafluoroisopropyl group results
in a modest increase in the steric demands of the phosphine. This
modest variation in size between ligands 1, PPh2C2F5 and PPh2CF3
would allow useful comparisons of complexes of interest for
catalytic screening. The increase in chain length and/or branching
of the perfluoroalkyl groups on phosphorus essentially maintains
the electronic characteristics of the phosphine. Therefore the use
of these perfluoroalkyl groups on phosphorus is indeed useful for
fine-tuning both the steric and electronic properties of phosphines.
We gratefully acknowledge Dr Alan Brisdon for helpful discus-
sions during the preparation of this manuscript.
Ligand/L
d (31P)/ppm
1JPtP/Hz
cis/trans
Reference
i
PPh2 C3F7
25
3019
2945
2637
3673
2400
3520
3698
trans
trans
trans
cis
trans
cis
cis
This work
6
25
25
26
26
21
PPh2C2F5
PPh3
18.5
19.8
14.3
12.3
9.6
PPh3
PEt3
PEt3
=
PPh2CF CF2
4.2
established for trans-[PtCl2(PPh2C2F5)2] by X-ray crystallography.6
Re-crystallisation of 3 from acetone yielded crystals suitable for an
X-ray diffraction study. The structure obtained (Fig. 1), confirmed
the formation of trans-3. 3 crystallises in the monoclinic space
group P21/c. The complex has a centre of inversion. The Pt–P and
Notes and references
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§ Synthesis of PPh2 C3F7: P(NEt2)3 (13.5 g, 55 mmol) dissolved in hexane
(30 ml) was added over 30 min to a solution of Ph2PCl (12.0 g, 55 mmol)
and iC3F7I (19.5 g, 66 mmol) in hexane (30 ml) with vigorous stirring, at
room temperature using a water bath. The mixture, which yielded a yellow
precipitate, was left stirring over 2 nights, after which time 31P NMR
showed that the reaction gave quantitative yield. The solution was filtered
via a cannula, the precipitate washed with 2 ¥ 10 ml hexane and the filtrates
were combined. Volatiles were removed in vacuo, yielding a solid residue.
˚
Pt–Cl bond distances of 2.3174(7) and 2.3055(7) A respectively, are
in the range of those seen in similar trans-platinum(II) dichloride
bis(phosphine) complexes.6,13,14 The P–Pt–Cl bond angle in 3 is
99.04(2)◦; this compares with P–Pt–Cl bond angles of 97.01(5)◦
in trans-PtCl2(PPh2C2F5)2 and 92.12(4)◦ in trans-PtCl2(PPh3)2.13
Although the ligands in 3 are close to planar, their arrangement
clearly deviates significantly from square. This suggests that the
increase in branching of the perfluoroalkyl group has an impact
by distorting the bond angles around the platinum metal centre.
Vacuum distillation (96 ◦C/0.5 mmHg) of the residue yielded PPh2 C3F7
i
(10.4 g, 53%), which crystallised on cooling. See ESI for characterisation.†
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[Mo(CO)5PPh2 C3F7]: Anal. Calc. for C20H10F7MoO5P: C, 40.70; H, 1.71.
Found: C, 40.75; H, 1.62%. IR (n-hexane) (cm-1) (CO region): 2081 (s),
2001 (w), 1965 (vs, br), 1962 (sh), 1956 (sh). 31P NMR (C6D6): d 57.5 (d,
2JPF = 80 Hz). 19F NMR (C6D6): d -65.5 (d, 3JFF = 9.1 Hz); -173.6 (dsep,
3
1
2JFP = 79.4 Hz, JFF = 9.3 Hz, CF). H NMR (C6D6): d 7.75 (m, 2H);
6.95–7.15 (m, 3H).
i
[PtCl2(PPh2 C3F7)2]: Anal. Calc. for C30H20Cl2F14P2Pt: C, 36.98; H, 2.07.
Found: C, 36.78; H, 2.33%. 31P NMR: (CDCl3): d 25.2 (m, 1JPPt = 3019 Hz).
19F NMR: (CDCl3): d -67.0 (d, JFF = 8.7 Hz, CF3); -172.5 (m, CF).
3
1H NMR: (CDCl3): d 7.95 (dd, 3JHP = 10.0 Hz, 3JHH = 6.0 Hz, 2H: ortho
C6H5); 7.55 (t, 3JHH = 4.0 Hz, 1H: para C6H5); 7.45 (m, 2H: meta C6H5).
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i
Fig. 1 An ORTEP drawing of trans-PtCl2(PPh2 C3F7)2. Thermal el-
8 E. J. Velazco, Ph.D. Thesis, University of the West Indies, St. Augustine,
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lipsoids shown at the 30% probability level and hydrog◦en atoms are
˚
omitted for clarity. Selected distances (A) and angles ( ): Pt(1)–P(1)
2.3174(7), Pt(1)–Cl(1) 2.3055(7), P(1)–C(1) 1.816(3), P(1)–C(7) 1.814(3),
P(1)–C(13) 1.929(3); Cl(1)–Pt(1)–P(1) 99.04(2), C(1)–P(1)–Pt(1) 113.58(8),
C(7)–P(1)–Pt(1) 110.28(9), C(13)–P(1)–Pt(1) 114.34(8), C(7)–P(1)–C(1)
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i
Commun., 2000, 56, e12–e15.
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for PPh2C2F5 using Peters and co-workers data.6 These estimates
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obtained from space-filling models. (It should be noted however,
that it has been suggested that this value is an overestimate.18)
In conclusion, the difference between the maximum cone angles
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20 M. Dhanoolal and L. C. Lewis-Alleyne, unpublished work.
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1200 | Dalton Trans., 2010, 39, 1198–1200
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©