Phosphorus(III) ligands in fluorous biphase catalysis

Article abstract of DOI:10.1016/S0022-1139(99)00166-9

The synthesis, coordination chemistry and catalytic applications of a series of perfluoroalkyl-substituted phosphorus(III) ligands is illustrated.

Full text of DOI:10.1016/S0022-1139(99)00166-9

Journal of Fluorine Chemistry 101 (2000) 247±255
Phosphorus(III) ligands in ¯uorous biphase catalysis
Pravat Bhattacharyya, Ben Croxtall, Joanne Fawcett, John Fawcett, David Gudmunsen,
Eric G. Hope*, Ray D.W. Kemmitt, Danny R. Paige, David R. Russell,
Alison M. Stuart, Dan R.W. Wood
Department of Chemistry, University of Leicester, Leicester, LE1 7RH, UK
Received 9 May 1999; received in revised form 4 June 1999; accepted 4 June 1999
Abstract
The synthesis, coordination chemistry and catalytic applications of a series of per¯uoroalkyl-substituted phosphorus(III) ligands is
illustrated. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Fluorous biphase; Phosphorus; Homogeneous catalysis; Platinum group metals
1. Introduction
phorus(III) ligands have been used more widely than any
other single class of ligand, and here we outline the synth-
esis, properties and catalytic applications of phosphorus(III)
ligands prepared within our laboratory.
Following the disclosure of the ¯uorous biphase approach
to the heterogenisation of homogeneous catalysts in 1994
[1], the catalysis community has been polarised; they are
either staunchly in favour or strongly opposed to develop-
ments based upon the ¯uorous biphase concept. In essence,
this approach relies upon the unique physical and chemical
properties of per¯uorinated ¯uids which are immiscible with
many conventional organic solvents at ambient tempera-
tures, but which dissolve other highly ¯uorinated molecules.
Hence, by derivatising conventional homogeneous catalysts
with long per¯uoroalkyl sidechains (called ¯uorous pony-
tails, Fig. 1) it is possible to anchor the metal catalysts in the
per¯uorinated solvent. In addition, however, with an appro-
priate choice of ¯uorous and organic solvents (containing
the substrate), on warming or under pressure the ¯uorous
and organic solvents become miscible and catalysis occurs
homogeneously (Fig. 2). On cooling (or pressure release) the
biphase is re-established and, hence, catalyst/product
separation simpli®es to the separation of two, immiscible,
¯uids. Although research into the development of this
approach has been reported across a broad spectrum of
catalytic reactions (oxidation [2±6], C±C bond formation
[7,8], polymerisation [9], hydrogenation [10,11], hydrobora-
tion [12], hydroformylation [1,13,14,15,16],) using a wide
variety of per¯uoroalkyl-derivatised ligand systems, phos-
2. Ligand synthesis
The ®rst phosphorus(III) ligand prepared speci®cally for
its application in the ¯uorous biphase was P(C2H4C6F13)3
(Fig. 1). This molecule, prepared in 1994 [1] by the radical-
induced reaction of C6F13CH=CH2 with PH3, illustrates the
two key components for a ¯uorous biphase-compatible
ligand system: (i) long, linear, per¯uoroalkyl substituents
and (ii) a hydrocarbon spacer unit to insulate the donor atom
from the extremely electron-withdrawing ¯uorous ponytails.
Although there have been subsequent improvements in the
procedure and yield in this reaction [17,18], we [19] and
others [20] have found that reaction of the Grignard reagent
{C6F13C2H4MgI} with phosphorus chloride reagents (e.g.
PCl3, PPhCl2, PPh2Cl) offers a more general route into
ligands with per¯uoroalkyl substituents without the need
to use the highly toxic phosphine, PH3; a similar route via
organo-zinc reagents has also been described [21]. However,
in our hands, we were unable to extend this Grignard route to
derivatives with longer per¯uoroalkyl substituents [22].
Unfortunately, coordination chemistry studies (see below)
and theoretical calculations [14] indicate that the C2H4 unit
is not a particularly good electronic insulator, and although
these latter calculations suggest that a C3H6 group will be a
better spacer unit {P(C3H6C6F13)3 has subsequently been
*Corresponding author. Tel.: ‡44-116-252-2108 fax: ‡44-116-252-
3789.
E-mail address: eghl@le.ac.uk (E.G. Hope)
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 6 6 - 9

248
P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
Fig. 1. Tris(1H,1H,2H,2H-perfluorooctyl)phosphine.
Fig. 2. Catalysis within a fluorous biphase system (FBS).
prepared [18]}, we turned our attention to the synthesis and
application of ligands containing aryl groups as electronic
insulators.
per¯uoroalkyl units which presumably ``protect'' the phos-
phorus atom from attack by incoming nucleophilic reagents.
In this structure, the per¯uoroalkyl groups radiate, linearly,
away from the aryl rings and line-up with other per¯uor-
oalkyl groups from adjacent molecules to generate ¯uorous
domains within the crystal lattice. This appears to be a
common observation in the structural determinations of
molecules with per¯uoroalkyl substituents to date. Unfor-
tunately, we have been unable to structurally characterise
any of our ligand systems, however, we have obtained single
crystals of O=P(3-C6H4C6F13)3 from a chloroform solution
of the free ligand left to stand for several weeks (Fig. 6).
Although the crystals were of relatively poor quality and the
ends of the per¯uoroalkyl groups showed considerable
disorder, the O=P(C6H4)3 core of the molecule is well
de®ned and offers an insight into the in¯uence of the
per¯uoroalkyl substituents by comparison with data for
triphenylphosphine oxide (Table 1) [23]. The slight length-
ening of the P=O and P±C bond lengths and compression of
the C±P±C bond angles indicate that the per¯uoroalkyl
groups have some in¯uence on the electron density at the
phosphorus atom. A crystal packing diagram for this struc-
ture illustrates how the per¯uoroalkyl units align within the
The copper-mediated cross-coupling reaction of a per-
¯uoroalkyl iodide with an aryl halide offers a convenient,
general, route to a comprehensive series of per¯uoroalky-
lated aryl intermediates (Figs. 3 and 4) [19]. In the reactions
with 2-, 3- and 4-C6H4BrI, the principle by-product is the
bis-substituted arene which is readily separated on distilla-
tion. Yields are generally good, except in the coupling
reaction with 2-C6H4BrI which is probably because of steric
interactions. For the per¯uoroalkylated aryl bromides,
lithiation at low temperature followed by reaction with a
range of phosphorus chloride reagents offers a widely
applicable route to a comprehensive series of ligands with
¯uorous ponytails. The steric congestion associated with the
ortho-C6F13 unit is re¯ected in the reaction of 2-
LiC6H4C6F13 with PCl3 which does not lead to complete
substitution but rather affords PCl(2-C6H4C6F13)2 as an
unusual air- and moisture-stable phosphorus(III) chloride
[22]. A structural determination (Fig. 5) con®rms the steric
congestion and indicates short intramolecular interactions
between phosphorus and various ¯uorine atoms on the

P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
249
Fig. 3. Synthetic routes to perfluoroalkyl-derivatised phosphine ligands.
crystal and the formation of the ¯uorous domains within the
structure (Fig. 7).
(Fig. 3) [24] whilst the synthesis of per¯uoroalkyl-substi-
tuted phenols (Fig. 4) allows the generation of a series of
phosphinite, phosphonite and phosphite ligands [19]. Here,
the additional oxygen atom alleviates any steric constraints
associated with the ortho-substituents.
Throughout this work, our primary goal has been the
application of these derivatised ligand systems in ¯uorous
biphase catalysis. Hence, preferential solubility of the
The residual electronic in¯uence of the per¯uoroalkyl
units on the phosphorus atom is re¯ected in our coordination
chemistry studies (see below), and consequently, we have
investigated the incorporation of additional hydrocarbon
spacer units. The reaction of F15C7CH2OTf with P(4-
C6H4OH)3 allows the insertion of an additional OCH2 unit

250
P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
Fig. 4. Synthetic routes to perfluoroalkyl-derivatised phosphonite, phosphinite and phosphite ligands.
Fig. 5. Molecular structure of PCl(2-C6H4C6F13)2. Displacement ellip-
soids are shown at the 30% probability level.
ligand in a per¯uorocarbon/organic solvent biphase is essen-
tial and we have undertaken qualitative assessments of
¯uorous phase compatability using 31P NMR spectroscopy.
These experiments indicate that, for these ligand systems, at
Fig. 6. Molecular structure of O=P(3-C6H4C6F13)3. Displacement ellip-
soids are shown at the 30% probability level.

P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
251
Table 1
Selected bond length (A) and bond angle ( 8 ) data for O=PPh3 and O=P(3-
Table 2
1
Ê
The variation in JPtP (Hz) with L in cis-[PtCl2L2]
a
C6H4C6F13)3
L
1JPtP
b
O=PPh3
O=P(3-C6H4C6F13)3
PPh3
PPh2(4-C6H4C6F13
3672
3653
3635
3631
3633
P±O
1.46(1)
1.75(1)
1.49(2)
1.77(2)
)
P±C
PPh(4-C6H4C6F13)2
a
P±C
1.77(1)
1.84(2)
P(4-C6H4C6F13 3
PPh2(3-C6H4C6F13
PPh(3-C6H4C6F13 2
P(3-C6H4C6F13 3
P(2-C6H4C6F13)(4-C6H4C6F13 2
)
a
a
P±C
1.77(1)
1.84(2)
)
C±P±C
C±P±C
C±P±C
106.5(7)
107.2(7)
107.6(6)
105.0(10)
106.0(10)
106.7(10)
)
3602
b
)
b
)
PEtPh2
3640
3630
3617
3703
3680
aStandard deviations in parentheses.
bData taken from [23].
P(C2H4C6F13)Ph2
PEt(4-C6H4C6F13)2
P(4-C6H4OCH3)3
P(4-C6H4OCH2C7F15 3
)
aFormed as a mixture of cis- and trans-isomers.
bAll attempts to form cis-[PtCl2L2] failed, trans-[PtCl2L2] isolated
exclusively.
least three C6F13 units are required to confer preferential
¯uorous phase solubility, which is in line with the recently
reported >60% (w/w) ¯uorine requirement indicated by
quantitative solubility measurements [25]. Consequently,
only ligands which satisfy this criteria have been evaluated
in our preliminary catalytic studies (see below).
3. Coordination chemistry
At the outset of this work, the in¯uence of the per¯uor-
oalkyl substituents on the donor properties of phosphoru-
s(III) ligands, and consequently, the catalytic activity of
derivatised metal complexes was unknown. Since many
transition metal catalysts are generated in situ from the free
ligand and metal-containing precursors, the successful
development of the ¯uorous biphase approach to catalysis
depended upon the reactivities of these ligands mirroring
conventional phosphorus(III) ligands. We have investigated
the reactions of the ligands prepared in this study with a
broad spectrum of platinum-metal reagents (Figs. 8 and 9)
[26±28]. These investigations indicate that, for the most-
part, the per¯uoroalkyl substituents do not have a signi®cant
in¯uence on the donor properties of the phosphorus atoms
wherein the displacement of weakly coordinated ligands
and the cleavage of halide-bridged dimers occur in good
yields.
Spectroscopic investigations on these metal complexes
offer an insight into the electronic effects of the per¯uor-
1
oalkyl substituents. Table 2 compares the JPtP coupling
constants of the cis-[PtCl2L2] of the ¯uorous-derivatised
phosphines with the protio-parent ligands and shows how
the number and position of the per¯uoroalkyl groups affects
the 1JPtP values. These values can be correlated with the s-
donor strength or Hammett constant (sP) of the ligands [29]
whilst Table 3 {ꢀ(CO) for trans-[MCl(CO)L2] (M ˆ Rh,
Ir)} indicates the ꢁ-bonding characteristics of these ligands.
These data con®rm that:
Fig. 7. Crystal packing diagram for O=P(3-C6H4C6F13)3 illustrating short
intermolecular interactions and fluorous domains.

252
P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
Fig. 8. Coordination chemistry of monodentate perfluoroalkyl-derivatised phosphorus(III) ligands.
Fig. 9. Coordination chemistry of bidentate perfluoroalkyl-derivatised phosphorus(III) ligands.
Table 3
Variation in ꢀ(CO) (cm 1) in trans-[MCl(CO)L2] (M ˆ Rh, Ir) with L
trans-[PtCl2L2] can be formed in preference to the cis-
isomer.
L
Rh
Ir
Further information on the donor properties of these
ligands can be obtained, in selected cases, from a compar-
ison of the data from single crystal structure determinations
with that from similar protio-ligand metal complexes;
Tables 4 and 5 illustrate two such comparisons. The most
important conclusion from these comparisons is that, for
both pairs of complexes, the per¯uoroalkyl substituents
have very little in¯uence on the ®rst coordination spheres
about the metal centres. For the trans-[RhCl(CO)L2] com-
plexes [26,30] the Rh±P, Rh±Cl and C±O distances are
unperturbed on substitution and only the variation in length
in Rh±C offers any suggestion of the electronic in¯uence of
the ¯uorous ponytails. This might have a slight knock-on
effect in the con®guration around the metal centre whereby,
although the rhodium, phosphorus, chlorine and carbonyl
carbon atoms are coplanar, the P±Rh±P axis is bent towards
the chlorine atom. Further structural studies, using Rh K-
edge EXAFS in the solid state and in per¯uorocarbon
solution indicate that, within the accuracy of this technique,
the metal ®rst coordination sphere is unperturbed on dis-
PPh3
1965
1982
1983
1993
1980
1984
1992
1958
1977
1953
1992
1953
1959
1972
1979
±
PPh2(4-C6H4C6F13
)
)
PPh(4-C6H4C6F13 2
)
P(4-C6H4C6F13)3
PPh2(3-C6H4C6F13
PPh(3-C6H4C6F13 2
P(3-C6H4C6F13 3
P(4-C6H4OCH3)3
P(4-C6H4OCH2C7F15 3
PEt3
P(C2H4C6F13 3
)
±
)
±
1961
1967
1929
1977
)
)
1. The aryl unit is a better electronic insulator than the
C2H4 unit, but that it is not perfect; even the C6H4OCH2
unit does not completely insulate the phosphorus atom.
2. The electronic influence through the aryl ring is purely
inductive whereby a meta-substituent has a greater
effect than a para-substituent.
3. Ortho- and meta-substitution can generate steric inter-
actions whereby the thermodynamically less favoured

P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
253
Table 4
lished homogeneous catalysts, which would facilitate such
separations, (e.g. ship-in-a-bottle, attachment to insoluble
supports, supercritical solvents) are under investigation [34]
and the ¯uorous biphase offers an interesting and potentially
valuable alternative strategy. However, a number of impor-
tant criteria would have to be satis®ed before a system using
per¯uorocarbon solvents could possibly be used on an
industrial scale:
Ê
Selected bond length (A) and bond angle ( 8 ) data for trans-
[RhCl(CO)L2]a
b
c
L ˆ P(CH3)3
L ˆ P(C2H4C6F13 3
)
Rh±P
2.307(1)
2.309(1)
1.770(4)
2.354(1)
1.146(4)
91.0(1)
2.300(2)
2.304(2)
1.807(5)
2.356(2)
1.152(6)
94.2(2)
Rh±P
Rh±C
Rh±Cl
C±O
C±Rh±P
C±Rh±P
Cl±Rh±P
Cl±Rh±P
P±Rh±P
1. The solvent and catalyst must be stable under process
conditions and both must be recoverable with 100%
efficiency.
2. The system should offer efficiency and/or selectivity
gains over established processes.
91.6(1)
89.3(1)
93.7(2)
85.5(1)
88.1(1)
86.5(1)
177.2(1)
172.1(1)
aStandard deviations in parentheses.
bData taken from Ref. [30].
cData taken from Ref. [26].
In our initial studies we have concentrated on two cat-
alytic processes; (a) the extensively studied hydrogenation
of styrene to allow a direct comparison between per¯uor-
oalkyl-derivatised catalysts and their protio-congeners and
(b) the hydroformylation of long chain alkenes, an important
industrial process where, currently, a relatively unreactive
and unselective cobalt-based catalyst has to be employed
because the more reactive and selective rhodium-based
catalysts cannot be separated from the long chain aldehyde
products without decomposition [34].
Table 5
a
Ê
Selected bond length (A) and bond angle ( 8 ) data for cis-[PtCl2L2]
b
c
L ˆ PPh3
L ˆ P(4-C6H4C6F13 3
)
Pt±P
2.251(2)
2.265(2)
2.356(2)
2.333(2)
97.8(1)
2.254(3)
2.271(3)
2.349(3)
2.328(2)
97.73(9)
89.97(9)
86.36(9)
86.30(9)
Pt±P
Pt±Cl
Pt±Cl
P±Pt±P
P±Pt±Cl
P±Pt±Cl
Cl±P±Cl
89.8(1)
In the hydrogenation of styrene using Wilkinson's cata-
lyst analogues (Table 6), although quantitative recovery of
87.1(1)
85.3(1)
aStandard deviations in parentheses.
bData taken from Ref. [33] .
cData taken from Ref. [27].
Table 6
Hydrogenation of styrene catalysed by rhodium complexesa
1
solution [26]. The packing diagram for the ¯uorous-deri-
vatised metal complex again shows short intermolecular
distances and ¯uorous domains as reported for the isostruc-
tural iridium complex [31,32]. For the cis-[PtCl2L2] com-
plexes [27,33] {when L ˆ P(4-C6H4C6F13)3 the cis-
complex co-crystallizes with its trans-isomer [27]} there
are slight asymmetries in the Pt±P and Pt±Cl bond lengths
and the coordination geometries around the metal centres in
the two complexes are virtually identical.
Our qualitative solubility assessments for these metal
complexes indicate that, in addition to requiring three
per¯uoroalkyl units per ligand, two monodentate ligands
per metal centre are essential to confer preferential solubi-
lity and that, even having satis®ed these criteria, the other
ligands at the metal centre can be important, i.e. although
trans-[RhCl(CO){P(4-C6H4C6F13)2] is preferentially ¯uor-
ous-phase soluble trans-[PdCl2{P(4-C6H4C6F13)2] is not.
Ligand
PPh3
Rate (mmol dm 3 h
)
155
211
160
276
177
139
100
218
201
79
b
PPh3
PPh3
c
d
PPh3
PEt3
P(4-C6H4CF3)3
P(3-C6H4CF3)3
P(4-C6H4OCH3)3
P(4-C6H4OCH2C7F15 3
P(C2H4C6F13 3
P(4-C6H4C6F13 3
P(3-C6H4C6F13 3
)
)
)
128
117
146
86
)
c
d
e
P(4-C6H4C6F13 3
P(4-C6H4C6F13 3
P(4-C6H4C6F13 3
)
)
)
115
aConditions: the ligand (1.257 Â 10 4 mol dm 3) (except where
stated) was dissolved in a degassed mixture of hexane (6 cm3), PP3
(10 cm3) and styrene (1.8 cm3; 15.7 mmol), pressurised to 1 bar with H2
and equilibriated with stirring at 63.58C (unless otherwise stated). A
solution of [RhCl(C2H4)2]2 (6.1 mg, 1.571 Â 10 5 mol) in toluene (2 cm3)
4. Catalytic studies
was added and the mixture was stirred for 1 h. Analyses by glc.
Rates Æ 4 mmol dm 3 h
1
.
bSolvent system toluene (18 cm3).
Although homogeneous catalysts can provide very high
activities and selectivities towards industrially desirable
products, their commercialisation can be thwarted by an
inability to separate them from the products. A number of
approaches directed towards the heterogenisation of estab-
cCatalytic runs at 758C and solvent system fluorobenzene (8 cm3)/PP3
(10 cm3).
dSolvent system toluene (8 cm3)/PP3 (10 cm3).
eSolvent system PP3 (18 cm3)(product extracted at the end of the
catalytic runs with toluene).

254
P. Bhattacharyya et al. / Journal of Fluorine Chemistry 101 (2000) 247±255
Table 7
Hydroformylation of 1-hexene catalysed by rhodium complexesa
1
1
Ligand
Conversion (%)
Selectivity (%)
n/i
Rate constant (s
)
Initial rate (mol dm 3 s
)
b
c
c
3
PPh3
PPh3
99.6
99.0
98.3
99.6
99.7
99.7
96.1
98.2
89.2
92.0
82.3
89.3
2.5
3.1
3.8
2.9
6.4
6.6
2.3 Â 10
3
3.2 Â 10
3
2
P(4-C6H4C6F13)3
6.3 Â 10
1.3 Â 10
c
2
P(OPh)3
4.75 Â 10
3
2
P(OC6H4-4-C6F13)3
6.2 Â 10
1.2 Â 10
d
3
3
P(OC6H4-4-C6F13)3
3.2 Â 10
6.4 Â 10
aReaction conditions: [Rh(acac)(CO)2] ˆ 0.01 mol dm 3, ligand ˆ 0.03 mol dm 3, 1-hexene ˆ 1 cm3 in a mixture of toluene (2 cm3) and PP3 (2 cm3),
708C, 20 bar, 1 h, (unless otherwise stated), product analysis by GC.
bIn toluene (4 cm3).
cZero order throughout most of the reaction.
dPressure ˆ 8 bar.
the rhodium catalyst could be established by atomic absorp-
tion spectroscopy, the electronic in¯uence of the per¯uor-
oalkyl substituents (identi®ed in the spectroscopic studies)
is clearly evident; the best electronic insulator, as indicated
by the least reduction in rate between the derivatised and un-
derivatised ligands, is the C6H4OCH2 unit. Of greater
importance, however, is the signi®cant reduction in rate
solely on the introduction of the per¯uorinated solvent
which warrants further investigation. Similarly poor reac-
tivities have been reported in related work on the hydro-
genation of linear alkenes [10,11].
In contrast, in the hydroformylation of 1-hexene under
20 atm H2/CO 1 : 1 (Table 7), the introduction of per¯uoro-
1,3-dimethylcyclohexane solvent does not have a deleter-
ious effect upon the rate of reaction using a catalyst gen-
erated in situ using PPh3 and the introduction of the ¯uorous
ponytails is accompanied by a slight improvement in both
rate of reaction and selectivity. Further improvements in rate
and selectivity are obtained using a catalyst prepared from
P(O±4-C6H4C6F13)3 which is also active for the hydrofor-
mylation of internal alkenes and is the subject of further
detailed investigations [15,16].
Hamilton, Dr. D.F. Foster and Mr. G.P. Schwarz of the CATS
consortium for the hydroformylation studies and Dr. A.K.
Brisdon (UMIST) for assistance with the per¯uorovinyl-
lithium chemistry.
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