Filling a Niche in “Ligand Space” with Bulky, Electron-Poor Phosphorus(III) Alkoxides

Article abstract of DOI:10.1002/chem.201804805

The chemistry of phosphorus(III) ligands, which are of key importance in coordination chemistry, organometallic chemistry and catalysis, is dominated by relatively electron-rich species. Many of the electron-poor P^III ligands that are readily available have relatively small steric profiles. As such, there is a significant gap in “ligand space” where more sterically bulky, electron-poor P^III ligands are needed. This contribution discusses the coordination chemistry, steric and electronic properties of P^III ligands bearing highly fluorinated alkoxide groups of the general form PR_n(OR^F)_3?n, where R=Ph, R^F=C(H)(CF₃)₂ and C(CF₃)₃; n=1–3. These ligands are simple to synthesize and a range of experimental and theoretical methods suggest that their steric and electronic properties can be “tuned” by modification of their substituents, making them excellent candidates for large, electron-poor ligands.

Full text of DOI:10.1002/chem.201804805

A Journal of
Accepted Article
Title: Filling a niche in “ligand space” with bulky, electron-poor
phosphorus (III) alkoxides
Authors: Sharifa Hussein, Denis Priester, Paul Beet, Jonathon Cottom,
Sam J. Hart, Tim James, Robert J. Thatcher, Adrian C.
Whitwood, and John Martin Slattery
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To be cited as: Chem. Eur. J. 10.1002/chem.201804805
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Filling a niche in “ligand space” with bulky, electron-poor
phosphorus (III) alkoxides
Sharifa Hussein,[a] Denis Priester,[a] Paul Beet,[a] Jonathon Cottom,[a] Sam J. Hart,[a] Tim James,[a]
Robert J. Thatcher,[a] Adrian C. Whitwood[a] and John M. Slattery,*[a]
2i, 3]
Abstract: The chemistry of phosphorus(III) ligands, which are of key
importance in coordination chemistry, organometallic chemistry and
catalysis, is dominated by relatively electron-rich species. Many of
the electron-poor P(III) ligands that are readily available have
relatively small steric profiles. As such, there is a significant gap in
“ligand space” where more sterically bulky, electron-poor P(III)
ligands are needed. This contribution discusses the coordination
chemistry, steric and electronic properties of P(III) ligands bearing
properties.[2f,
When combined, these data can provide a
stereo-electronic map of phosphine “ligand space” that can be
used to link structure to function and aid in the design of ligands
and complexes for particular applications. Such design
principles are exemplified in concepts such as Ligand
Knowledge Bases (LKBs).[2h, 2i, 4]
highly fluorinated alkoxide groups of the general form PRn(ORF)3-n
,
where R = Ph, RF = C(H)(CF3)2 and C(CF3)3; n = 1-3. These ligands
are simple to synthesize and a range of experimental and theoretical
methods suggest that their steric and electronic properties can be
“tuned” by modification of their substituents, making them excellent
candidates for large, electron-poor ligands.
Introduction
Phosphorus(III)-centered Lewis bases are amongst the most
commonly encountered ligands in organometallic chemistry and
catalysis. The ability to “tune” the steric and electronic properties
of these ligands by modification of the ligand substituents allows
the properties of a metal complex to be tailored to suit a
particular application. According to the Orpen-Connelly model,[1]
P(III) ligands are -donors through their lone pairs and -
acceptors through their P-R *-orbitals. The degree of -
donor/-acceptor character for a particular ligand can be
influenced by the substituents at phosphorus e.g. through
inductive effects or by changing the relative energies of the
frontier orbitals. Similarly, the size of the substituents has a
profound influence on the steric properties of these ligands. In
order to facilitate the application of P(III) ligands a number of
parameters may be used to describe these steric and electronic
properties. Ligand “cone angles”, S4′ and He8 parameters,
amongst others, are frequently used to describe steric
properties.[2] The CO stretching vibrations of metal-carbonyl
complexes e.g. [Ni(L)(CO)3], [W(L)(CO)5], [Rh(L)(CO)2Cl] and
[CpIr(L)(CO)], where L = the ligand of interest, or other
spectroscopic features such as metal-phosphorus coupling
constants are commonly used as indicators of electronic
Figure 1. Selected examples of electron-poor P(III) ligand systems that have
been explored. R1 and R2 are a range of alkyl, aryl or heteroatom-based
functional groups and X- is a suitable anion.
It has been noted, when considering a map of ligand space,
derived from computed steric and electronic parameters, that a
large proportion of available P(III) ligands are species that are
relatively electron rich.[2f] The electron-poor P(III) ligands
available are often relatively small, e.g. P(CF3)3 (cone angle =
o
137 ). Therefore, there exists a significant gap in ligand space
corresponding to electron-poor, sterically bulky P(III) ligands.
These species are expected to be relatively poor -donors and
stronger -acceptors than typical phosphines such as PPh3.
Thus, their ability to bind to, and stabilize, metals in low
oxidation states or to generate highly electrophilic metal centers
is enhanced. Likewise, their steric bulk may promote the
dissociation of other ligands (e.g. in catalysis) or stabilize low-
coordinate metal centers.
Several groups have attempted to fill this gap in ligand
space, e.g. with ligands such as those in Figure 1. One
approach has been to prepare P(III) ligands bearing
perfluorinated
tert-butyl,
iso-propyl
and
cyclo-hexyl
substituents.[2f, 5] However, studies involving these ligands are
still relatively rare and their syntheses are often not trivial.
Brisdon et al. and others have worked extensively on
perfluorovinyl-containing phosphines (PR3-n(vinylF)n {where R =
[a]
Dr S. Hussein, Mr D. Priester, Dr J. Cottom, Mr S. J. Hart, Mr T.
James, Dr R. J. Thatcher, Dr. A. C. Whitwood and Dr. J. M. Slattery.
Department of Chemistry
University of York, Heslington, York YO10 5DD (UK)
Fax: (+44) 01904 322516
i
Ph, NMe2, NEt2, EtO, Pr, Cy, BuO; n =1,2; vinylF = CF=CF2,
E-mail: john.slattery@york.ac.uk
CCl=CF2, CF=CFH, CCCF3} and the P-stereogenic phosphine
nBuPhP(CF=CF2).[5d, 6] These perfluorovinyl groups were found
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201804805
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to be quite electron withdrawing, similar electronically to alkoxy
groups. However, the origin of their similar electronic properties
is different: perflourovinyl groups have an inductive effect that
withdraws electron density from the P lone pair (leading to
weaker -donation), while alkoxy groups reduce the energy of
the *-orbital (leading to better -acceptor character). Sterically,
perfluorovinyl groups are smaller than a perfluorinated phenyl
group.
An alternative approach to the use of fluorinated, electron-
withdrawing groups in the preparation of electron-poor
phosphines has been to include cationic groups at phosphorus.
The area of α-cationic phosphines has seen tremendous growth
in recent years and has uncovered some exciting ligands whose
strongly electron-withdrawing properties have unlocked novel
catalytic processes.[7] While these are undoubtedly exciting
ligands, they also have some disadvantages, which mean they
will not be appropriate for all situations. These include changes
in solubility compared to neutral species, weaker M-L bonding
and the potential for unwanted reactions at the cationic
component. In addition, the more highly charged ligands, which
are the most electron poor, have so far displayed only limited
coordination chemistry. As such, there is still significant scope
for the development bulky P(III) ligands that are electron poor
and simple to synthesize to complement existing species and
help to fill the gap in ligand space.
cycloaddition reactions, for example those involving substrates
that are electronically not well differentiated.[11] Recent work has
shown that 1 can play an important role in the development of
active catalysts for C-H functionalization reactions, primarily with
Rh-based systems, but also with Pd.[11f, 11g, 12] As part of this
work, Yanagisawa et al. demonstrated that the use of very bulky,
electron-poor ligands appears to be essential for successful
catalysis when [RhCl(CO)L2], where L = 1, 2 and 3, complexes
are used as catalysts for the direct C-H coupling of heteroarenes
with haloarenes.[12a] When L = 1, 94 % conversions can be
achieved, but when the number of fluorinated alkoxy groups at P
are reduced (i.e. when L = 2) conversions drop to 31 % and no
conversion is seen for PPh3 for the same substrates. It has also
been possible to utilize 1 for the Ir-catalysed hydrosilylation of
amides.[13]
Results and Discussion
Synthetic routes to compounds 1-3 have previously been
reported by reaction of PPh3-nCln (n
= 1-3) with either
LiOC(H)(CF3)2 or HOC(H)(CF3)2 and NEt3. However, in our
hands, reaction of the relevant P-chlorophosphine with
NaOC(H)(CF3)2 in dry, degassed CH2Cl2 under ultrasonic
activation has proved the most convenient approach. Compound
4 has previously only been synthesized by reaction of PCl3 with
ClOC(CF3)3.[14] The preparation of this hypochlorite, from ClF
and HOC(CF3)3 with the elimination of HF, makes this route less
accessible for standard synthetic labs and it was pleasing to find
that reaction of 3 equivalents of NaOC(CF3)3 with PCl3 also
gives 2. Compounds 5 and 6 have, to the best of our knowledge,
not been reported previously and can be synthesized in a similar
manner. Although all compounds reported here can be purified
by distillation (sublimation in the case of 4) it is often possible to
use the ligands as prepared in CH2Cl2 solution, after filtration to
remove NaCl, without further purification. The 31P{1H}, 19F and
1H NMR spectroscopic data for each compound can be found in
Table 1.
This paper discusses the coordination chemistry, steric
and electronic properties of a series of phosphite, phosphonite
and phosphinite ligands with the general formula PR3-n(ORF)n {R
= Ph, RF = C(CF3)3, C(H)(CF3)2; n = 1-3} (Figure 2) using a
combination of experimental and theoretical approaches. These
species are simple to synthesize and the fluorinated alkoxide
groups impart both steric bulk, and significant -acceptor
character to the ligands. The steric and electronic properties of
these species can be tuned by varying R and RF.
Table 1. Spectroscopic data and isolated yields for ligands 1-6.
Ligand
Isolated
Yield (%)

31P{1H}

19F (ppm)
 1H (ppm)[a]
Figure 2. Phosphorus ligands investigated in this work.
(ppm)
1
2
39[b]
61
140
190
-74.7
4.80
4.64
The chemistry of fluorinated phosphorus alkoxides has a long
history and compounds 1, 2, 3 and 4 have previously been
described.[8] However, their ligand chemistry and catalytic
applications are not well developed. To the best of our
knowledge 5 and 6 have not previously been reported. Of all the
ligands that are known, the catalytic applications of
P{OCH(CF3)2}3 1 have been investigated in the most detail. In
early work, van Leeuwen investigated the use of 1 as a ligand in
the Ni-catalysed cyclodimerisation of isoprene.[9] However, the
observed yields were quite low. The same group later found that
1, along with other sterically bulky, electron-poor ligands, formed
highly active hydroformylation catalysts with Rh.[10] Ligand 1 has
also been used successfully in catalytic systems for [4+2]
-74.3
and -74.1[c]
3
4
5
6
70
53
69
69
143
149
190
132
-73.8
-72.3
-71.4
-71.7
4.71
NA
NA
NA
[a] Selected 1H NMR chemical shifts, for OC(H)(CF3)2 groups only, are
reported. [b] Although 31P and 19F NMR spectroscopy suggests that the
formation of 1 is quantitative, the volatility of 1 means that loss of some
product under vacuum is difficult to avoid during isolation.[c] Two
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environments are observed for each C(H)(CF3)2 group in the 19F NMR of 2,
due to atropisomerism because of hindered rotation around either the P-O or
O-C bond.
As with the single-crystal X-ray structure of 4, all crystals of
7 are heavily disordered, even at 110 K. In the case of 7, this
was modelled with the entire phosphorus-oxygen core being
disordered over two positions (each with 50 % occupancy). As
the model is still relatively poor, an extensive structural
description is not appropriate. However, the data serve to
confirm structural connectivity and suggest that the phosphorus
centers exhibit distorted pyramidal geometries (with smaller O-
P-O angles than in an ideal tetrahedral geometry). In addition,
the phosphorus lone pairs point in opposite directions to each
other, presumably as a consequence of the steric bulk of the two
large -OC(CF3)3 groups at each phosphorus center. Structurally
characterized examples of free, uncoordinated pyrophosphites
are very rare. To the best of our knowledge only one previous
example is present in the Cambridge Structural Database, the
sterically congested pyrophosphite 6-[(2,4,8,10-tetrakis(1,1-
dimethylethyl)-dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl)oxy]-
2,4,8,10-tetrakis(1,1-dimethylethyl)-dibenzo[d,f][1,3,2]-
While 1, 2, 3, 5 and 6 are liquids under standard conditions,
4 is a solid that crystallizes readily from CH2Cl2 solution.
Although significant disorder (even at low temperature) appears
to be present in all crystals of 4 grown under a variety of
conditions, it was possible to obtain a single-crystal X-ray
structure of this compound of suitable quality to establish
structural connectivity (Figure 3). While a full discussion of the
structural parameters of 4 is not appropriate given the quality of
the model, the geometry around phosphorus appears to be
similar to related aryl phosphites (e.g. P(OPh)3, av. P-O 1.604
Å)[15]
A
space-fill representation of the structure of
4
(superimposed on Figure 3) suggests that 4 is a very sterically
bulky ligand. In addition, the perfluoro-t-butoxide substituents
appear to have little room for conformational flexibility, which
suggests that this phosphite should have a relatively rigid steric
profile compared to many phosphites..
dioxaphosphepin (CSD identifier MIKSUU) reported by DeBellis
et al.[16] This displays
a similar phosphorus-oxygen core
conformation to that shown by 7, with P-lone pairs pointing away
from each other. Although any comparison of structural
parameters is tentative, given the quality of the data for 7, it
appears that the P-O distances and O-P-O angles in 7 are
comparable to those reported by DeBellis (although any subtle
effects due to the inclusion of fluorinated alkoxides would not be
identifiable in these data).
Figure 3. Single-crystal X-ray structure of P{OC(CF3)3}3 (4). Monoclinic, C2/c,
110 K, R1 = 0.1565, wR2 = 0.4462. Whole-molecule disorder (modelled over
two positions) in addition to CF3 rotational disorder, is present in the structure.
Only one position is shown (and bond lengths and angles given for this) for
clarity. Selected bond lengths (Å) and angles (o): P(1)-O(1) 1.609(11), P(1)-
O(2) 1.615(11), P(1)-O(3) 1.610(12), O(1)-P(1)-O(2) 92.8(6), O(1)-P(1)-O(3)
93.0(6), O(2)-P(1)-O(3) 94.1(6).
The free ligands 1-6 are susceptible to hydrolysis, but
unlike many electron-rich P(III) ligands do not appear
susceptible to oxidation in air. In the case of 4, it was possible to
identify a product of partial hydrolysis, the pyrophosphite
(RFO)2P-O-P(ORF)2 {RF = C(CF3)3} (7) in NMR spectroscopic
(31P{1H} NMR  = 135 ppm; 19F NMR  = -71.4 ppm) studies
and crystals of this species were fortuitously obtained from a
reaction involving 4 where small amounts of water were
inadvertently introduced. This species gives some insight into
the mechanism of hydrolysis, but also suggests the possibility
that bidentate analogues of 4 may be accessible via a suitable
synthetic route. 7 has a characteristic multiplet at 135 ppm in the
31P{1H} NMR spectrum and crystals suitable for X-ray structural
analysis were grown from CH2Cl2 solution (Figure 4).
Figure 4. Single-crystal X-ray structure of (RFO)2P-O-P(ORF)2 {RF = C(CF3)3}
(7). Orthorhombic, Pbca, 110 K, R1 = 0.1170, wR2 = 0.3379. Extensive
disorder (modelled over two positions) is present in the structure. Only one
position is shown (and bond lengths and angles given for this) for clarity.
Selected bond lengths (Å) and angles (o): P(1)-O(1) 1.589(9), P(1)-O(2)
1.637(9), P(1)-O(5) 1.63(5), P(2)-O(3) 1.611(8), P(2)-O(4) 1.643(9), P(2)-O(5)
1.62(4), P(1)-O(5)-P(2) 136(3), O(1)-P(1)-O(2) 93.6(4), O(1)-P(1)-O(5) 96(2),
O(2)-P(1)-O(5) 96.2(9), O(3)-P(2)-O(4) 93.4(5), O(3)-P(2)-O(5) 101(2), O(4)-
P(2)-O(5) 100.2(8).
Steric properties
The steric properties of P(III) ligands are very important in
coordination chemistry and catalysis. Establishing the steric
parameters of ligands 1-6 is important for understanding their
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relationship to more commonly encountered ligands. Tolman’s
ligands 1-6 are somewhat larger than P(CF3)3 and PPh3 and
have similar steric properties to the bulky ligands P(But)3,
P(C6F5)3 and P(OBut)3. However, the conformational flexibility of
the –C6F5 and –OBut groups in the latter may allow these
ligands to change their steric profile quite dramatically
depending on the requirements of a particular metal fragment,
whereas significantly reduced conformational flexibility is
expected with ligands such as 4. As expected, P{OC(CF3)3}3 (4)
cone angles () and % buried volumes (%Vbur) have been
17]
chosen here to describe steric properties,[2a,
as they are
available for many other ligand systems, hence wider
comparison can be made, but also because some alternative
steric parameters (i.e. S4′) are known to fail for phosphites. A
ligand’s cone angle can vary quite considerably depending on
the metal fragment that it is attached to, as conformational
flexibility allows some ligands to change their steric profile to
respond to the steric requirements of other ligands at the metal
center. As such, we have calculated  (see ESI for details) for 1-
6 using structural data for a range of complexes (from both
experimental and DFT studies). The range of  values observed
for a particular ligand can be interpreted as an estimate of its
conformational flexibility. The results are shown in Table 2,
alongside data for some commonly encountered ligands for
comparison and %Vbur data for the same ligands.
o
shows a very large cone angle of 193 at Ni(CO)3, which is
considerably larger than P(But)3 and even approaches the size
o
of ortho-substituted aryl phosphines such as PMes3 ( = 212 ).
It has been found that cone angles and % buried volumes are
strongly correlated for many metal-ligand combinations.[17a]
However, %Vbur places a greater emphasis on the steric
properties proximal to the metal, whereas  includes steric
effects at larger distances, which is important in some
systems.[21] Consideration of the %Vbur values in Table 2
confirms that 1-6 are large ligands, but highlights that most have
relatively remote steric hindrance rather than the significant
proximal steric profile presented by ligands such as PMes3.
When cone angles for 1-6 from all available crystal
structures/calculations are considered it is clear that 4 is
consistently larger than the other ligands across a range of
complexes. The bulky –OC(CF3)3 groups are locked into one
conformation in 4, rather than exhibiting the multiple
conformations often observed for phosphites, giving this ligand a
relatively stable steric profile across a range of coordination
environments. The phosphite 1 has a large cone angle for many
complexes, but in the case of [W(CO)5L]  is similar for 1 to
other ligands with the –OC(H)(CF3)2 group. This may be an
indication of conformational flexibility. Those ligands
with -OC(CF3)3 groups appear to be a little less flexible than
those with –OC(H)(CF3)2 groups (based on their generally
smaller range of  values). All ligands occupy the desired “bulky”
region of ligand space and are comparable, or larger than
recently reported -cationic phosphines.[7a]
Table 2. Ligand cone angles ( in o) and % buried volume data (%Vbur) for 1-6.
Where two ligands are present at the metal center,  for both ligands are
reported.
Ligand
[Ni(CO)3L][a]
171
[W(CO)5L][a]
153
[Ru 1] [b]
155
[Ru 2][c]
%Vbur
33.0[d]
32.9[d]
29.5[d]
42.0[d]
33.5[d]
33.9[d]
22.2[g]
26.6[h]
29.6[g]
28.4[h]
26.7[g]
37.3[g]
47.6[g]
1
-
2
176
161
143
142
144
-
3
154
151
145
4
193
177
182
5
177
163
171
-
6
166
161
162
143
PMe3
P(CF3)3
PPh3
P(OBut)3
PBut3
P(C6F5)3
118[f]
137[f]
145[f]
175[f]
182[f]
184[f]
212[f]
Electronic properties
The electronic properties of ligands have been assessed using a
variety of methods, all of which have positive and negative
aspects. A convenient approach, which we have used previously
to assess the donor properties of very electron-rich ligands, has
22]
been proposed by Gusev et al.[3d,
This involves geometry
[e]
PMes3
optimizations and vibrational frequency analyses of [(5-
C5H5)Ir(CO)L] complexes (where L is the desired ligand), using
DFT methods to obtain CO stretching frequencies and C-O bond
lengths that vary as a function of the donor/acceptor properties
of L. The use of this complex, as opposed to other commonly
used gauges of electronic properties {e.g. [Ni(L)(CO)3]}, allows
the comparison of a large range of different ligand classes using
the same scale. It has been noted, however, that methods to
assess ligand donor properties through (CO) in metal carbonyl
complexes can be affected by intramolecular interactions
between the ligands of interest and CO, which perturbs (CO).[7a,
[a] From optimised structures at the (RI-)BP86/SV(P) level. [b] From single-
crystal X-ray diffraction studies of [(5C5H5)Ru(NCMe)2L][PF6]. [c] From single-
crystal X-ray diffraction studies of [(5C5H5)Ru(NCMe)L2][PF6]. In the case of
ligands 3 and 6 two independent complexes are present in the asymmetric unit
and the average
structures of [Ni(CO)3L] with the M-P length set to 2.28Å using the SambVca
2.0 package.[19] [e] Mes = 2, 4, 6 trimethylphenyl. [f] Taken from references [20]
 values are reported. [d] Calculated from optimised
.
[g] Taken from reference [17a]. [h] Calculated from optimised structures of [(5-
C5H5)Ir(CO)L] with the M-P length set to 2.28Å using the SambVca 2.0
package.[19]
23]
As such, cautious analysis of the data is advisable for larger
ligands, where these interactions may be present. A plot of
The cone angles for [Ni(CO)3L] complexes of ligands 1-6
allow a comparison of their steric properties with a range of
examples from the literature (Table 2). These data suggest that
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(CO) against CO bond length for a range of ligands including 1-
6 is shown in Figure 5.
decomposed under these conditions. It appears that ligand
loss/exchange resulting in the formation of [W(CO)6] and
unidentified tungsten containing species is facile for these
ligands. EI-MS confirmed the presence of the tungsten
pentacarbonyl-ligand complexes ([M]+ observed) in all cases and
there is no evidence for the formation of [W(CO)4L2].
Strongly donating ligands, e.g. PMe3 appear at the top left
of Figure 5, as these increase the extent of back bonding from Ir
to the CO ligand. Electron-poor ligands, e.g. PF3, appear at the
bottom right of Figure 5, as they are more π-acidic and reduce
the amount back bonding from the metal to CO. PH3 has
electronic properties in-between these two extremes. Ligands 1
and 4 are found to have similar electronic properties to PF3.
Substituting fluorinated alkoxide groups for phenyl groups brings
the ligands’ electronic properties closer to PH3. An interesting
feature of these data is that although one would expect the
perfluoro-t-butyl phosphite 4 to have greater π-acidity than 1,
due to the extra CF3 groups, the data suggest that the electronic
properties of the two ligands are similar. This may be a
consequence of the steric bulk of 2, which prevents a close
approach of the ligand to the metal center. This would reduce
metal-phosphorus orbital overlap and prevent metal-ligand back
donation. There is a slight elongation of the Ir-P bond in the
optimized structure of [(5-C5H5)Ir(CO)(4)] (2.198 Å) compared
to [(5-C5H5)Ir(CO)(1)] (2.194 Å) that may be related to this.
Table 3. Selected spectroscopic data for [W(CO)5L] complexes, where L = 1-
6.
Ligand

31P{1H}
[a]
(A1)1
νCO[b]
(A1)2
νCO[b]
(E)
νCO[b]
(B1)
νCO[b]
(1JPW
)
1
2
3
4
5
6
147 (452)
183 (357)
150 (291)
150 (292)
174 (374)
149 (304)
121 (496)
55 (300)
2097
2088
2080
2002
1970
1968
1975
2017
2002
1992
2007 [c]
1999 [c]
1996
-
1962
1953
1976 [c]
1967 [c]
1954
1975
1989
1952
1937
2097 [c]
1989 [c]
2089 [c]
2081
2101
2101
2081
2075
1980 [c]
1964
2005
2001
1952
1952
[25]
PF3
[26]
P(CF3)3
-
[27]
P(OMe)3
138 (386)
130 (381)
1980
-
P(OPri)3
[27a, 28]
[27b]
PPh3
21 (243)
-40 (230)
2075
2071
1942
1949
1942
1941
1980
-
[26]
PMe3
[a] Chemical shifts in ppm and coupling constants in Hz. [b] Stretching
frequencies in cm-1. Note that the antisymmetric {E and (A1)2} stretches are
greater in peak intensity, therefore are more easily identified in the IR
spectrum. As such, the reported vibrational frequencies for these stretches
may be more reliable. B1 symmetric stretches are observed for 1-6 as the
ligands result in complexes without perfect C4v symmetry. [c] IR data
extrapolated from (RI-)BP86/SV(P) calculations (see ESI for details).
1
Both the IR stretching frequencies and JWP coupling
constants in these complexes can be used as indicators of the
electronic properties of the ligands. The 1JWP coupling constants
appear to follow a trend where increasingly large values are
Figure 5. A plot of (CO)/cm-1 against CO bond length/Å for a range of ligands
including 1-6. NHC = 1,3-Dimethylimidazol-2-ylidene. Geometry optimisations
and vibrational frequency calculations performed at the (RI-)BP86/SV(P) level.
1
seen for more electron-poor, -acidic ligands. The JWP values
for 1-3 (452, 357 and 291 Hz respectively) follow a trend that fits
with the expectation that increasing the number of fluorinated
alkoxide groups at P increases the -acceptor character of the
In order to assess the electronic properties of these
ligands via experimental measurements, tungsten and rhodium
carbonyl complexes of 1-6 were prepared. Tungsten carbonyl
complexes of the form [W(CO)5L], where L = 1-6 were prepared
by reaction of [W(CO)5(THF)] with the relevant ligand in
tetrahydrofuran (THF) solution.[24] The 31P{1H} NMR
spectroscopic data for these complexes in THF, along with the
carbonyl IR stretching frequencies (solution phase in
THF/hexane) are summarized in Table 3. Four of these
complexes were amenable to purification by sublimation (to
remove excess [W(CO)6] present in the crude products).
However, complexes involving ligands 4 and 5 consistently
1
ligand. Steric effects appear to have an influence on the JWP
coupling constants for ligands involving the bulky perfluorinated
1
t-butoxy groups, as a simple relationship between JWP and the
number of alkoxy groups at P is not found for 4-6 (1JWP = 292,
374 and 304 Hz respectively). As observed with the [(5-
C5H5)Ir(CO)L] stretching frequencies, the perfluorinated t-butyl
phosphite 4 appears less -acidic than would be expected given
the number of fluorinated substituents. The steric bulk of 4
presumably prevents a close approach to the metal and this
effect appears to be more pronounced for [W(CO)5L] complexes,
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10.1002/chem.201804805
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due to the larger steric requirements of the W(CO)5 fragment.
When the JWP data for [W(CO)5L] complexes involving 1-6 are
compared to some common ligands in the literature the similarity
in electronic properties between PF3 and 1 are again evident
(496 vs. 452 Hz respectively). The other ligands span a range of
1JWP values that place their electronic properties between those
electron-poor ligand 1, support this suggestion. The [trans-
RhCl(CO)(L)2] complexes of 1, 2, 4, 5 and 6 from this work and
that of Itami et al. also allow a comparison of donor properties
across the series using (CO) and JRhP data in a similar way to
the Ir and W complexes described above. As this shows similar
1
1
features to the data discussed for the Ir and W species a
of P(CF3)3 and non-fluorinated phosphites at this metal fragment, summary of key data is included in the ESI.
consistent with their occupation of the electron-poor region of
ligand space.
The CO stretching frequencies for these tungsten
complexes also show a trend of increasing -acidity when more
fluorinated alkoxy groups are substituted on the P(III) ligands.
For example, the antisymmetric E stretch moves to lower
wavenumbers from 1-3 (1975, 1962, 1953 cm-1 respectively) and
from 4-6 (1976, 1967, 1954 cm-1 respectively). These data are
similar for the related –OC(H)(CF3)2 and –OC(CF3)3 substituted
ligands, presumably due to steric effects. When compared to
ligands in the literature, the CO stretching frequencies agree
with the trends seen in the tungsten-phosphorus (1JWP) coupling
constants in most cases, but there are some subtle differences.
For example, the similarity between 1 and PF3 is still evident {e.g.
(E) (CO) = 1975 cm-1 for both}. However, the relative ordering
of ligands 1-6 compared to P(CF3)3 and the non-fluorinated
1
phosphites is different to that suggested by the JWP data {with
P(CF3)3 appearing to be the best acceptor ligand and ligands 1-
6 showing better -acceptor properties (i.e. CO at higher
wavenumbers) than the non-fluorinated phosphites}. This
highlights the complexities involved in determining the relative
electronic properties of different ligands and is a reminder that
an electronic scale based on a single-property may not give a
complete picture. In fact, a ligand’s steric and electronic
properties are probably best assessed in a range of different
situations using several descriptors.
Figure 6. Single-crystal X-ray structure of RhCl(CO)(PPh2ORF) (8), where RF
= C(CF3)3. Tetragonal, P42/n, 110 K, R1 = 0.0306, wR2 = 0.0645. Cl and CO
positions are disordered, disordered parts omitted for clarity. Thermal
ellipsoids are drawn at the 50 % probability level. Selected bond lengths (Å)
and angles (o): Rh(1)-P(1) = 2.2866(4), Rh(1)-P(2) = 2.3017(4), Rh(1)-Cl(1A)
= 2.381(1), Rh(1)-C(1A) = 1.779(8), C(1A)-O(1A) = 1.15(1), P(1)-Rh-P(2) =
177.96(2), C(1A)-Rh-Cl(1A) = 169.68(5), P(1)-Rh(1)-Cl(1A) = 92.34(3), P(2)-
Rh(1)-Cl(1A) = 87.22(3), C(1A)-Rh(1)-P(1) = 88.7(1), C(1A)-Rh(1)-P(2) =
91.62(14). No clathrate-type solvates, as seen in a recent related study, were
found for any complex.[30]
In addition to the W and Rh complexes described above,
Ru complexes of the form [(5-C5H5)Ru(NCMe)3-n(L)n][PF6],
where L = 1-6 and n = 1 or 2, have been synthesized by reaction
of the free ligand in the relevant stoichiometry with [(5-
C5H5)Ru(NCMe)3][PF6] in CH2Cl2. Ru complexes of this type are
of relevance in a range of catalytic transformations including
alkyne dimerisation, alkyne hydration etc.[31] The spectroscopic
data for these complexes are presented in Table 5. In all cases it
was possible to obtain crystals suitable for analysis by single-
crystal X-ray diffraction studies by slow diffusion of hexane into a
solution of the complex in CH2Cl2 at room temperature.
Representative structures of n = 1 and n = 2 complexes are
shown in Figure 7 and 8 respectively and a .cif file containing all
structures is included as supporting information.
Coordination chemistry
Unfortunately it was not possible to grow crystals of [W(CO)5L]
(L = 1-6) complexes that were suitable for X-ray structural
analysis. This is hampered in part by the slow decomposition of
all complexes, even at -20 oC under an inert atmosphere.
However, Itami et al. have reported Rh complexes of the form
[trans-RhCl(CO)(L)2] where L = 1 and 2 and explored their
application in catalytic C-C bond forming reactions.[18] In order to
make some comparisons across a range of ligand types we
have synthesized analogous complexes where L = 4, 5 and 6 by
reaction of [RhCl(CO)2]2 with two equivalents of the free ligands
in CD2Cl2. In the case of [trans-RhCl(CO)(6)2] (8), crystals
suitable for single-crystal X-ray diffraction studies were obtained
and the structure of this complex is shown in Figure 6. The
complex shows a slightly distorted square planar geometry
around Rh, with relatively short Rh-P distances (2.287 and 2.302,
av. 2.295 Å), which are significantly shorter than the average
Rh-P distance (2.328 Å) for [trans-RhCl(CO)(PR3)2] complexes
(with R1 < 0.1) in the Cambridge Structural Database (CSD).[29]
This may be a structural indication of increased -back donation
from Rh to this relatively electron-poor ligand. The observation
of even shorter Rh-P distances (2.2597(8) and 2.2550(7), av.
2.257 Å) in the trans-RhCl(CO)(L)2 complex of the more
In
all
structurally
characterized
[(η5-C5H5)Ru(NCMe)3-n(L)n][PF6] (L = 1-6, n = 1, 2) complexes
Ru is found to adopt a distorted tetrahedral geometry (if the
centroid of the C5H5 ring is taken as one vertex). The phosphite,
phosphonite and phosphinite ligands in these complexes show a
variety of conformations, due to rotation around the P-O bond, in
the different structures. In the majority of cases the oxygen lone
pairs on the ORF group(s) point either towards the metal or
broadly perpendicular to the M-P bond, which has the effect of
moving the RF group(s) away from the metal. This presumably
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10.1002/chem.201804805
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reduces steric repulsion between the RF groups and other
ligands. Interestingly, in all ruthenium complexes of ligands
containing the –C(H)(CF3)2 group the 19F NMR spectra show two
signals relating to inequivalent CF3 environments. This suggests
that coordination restricts rotation around the P-O or C-O bonds
at room temperature on the NMR timescale, resulting in
atropisomerism (atropisomers are only observed in the free
ligands for ligand 2, vide supra).
OC(CF3)3 and n = 1. However, it is interesting to note that the
Ru-P bond in the Ru-complex of phosphite 4 (2.233 Å) is
significantly longer than that seen in the hexafluoroisopropyl-
substituted analogue (complex 9, Ru-P = 2.206 Å), presumably
due to the much larger steric profile of 4. When comparing
complexes of the same ligand with n = 1 and n = 2 (only
complexes 12 and 13 allow this from the available data) it
appears that increasing the number of P-ligands at Ru leads to
an increase in the R-P bond lengths (from 2.264 to 2.278 Å in
the case of 12 and 13). This may be a result of competition
between the P-ligands for -backdonation from Ru or simply due
to steric repulsion between these relatively large ligands.
Table 5. Spectroscopic and structural data for [(5-C5H5)Ru(NCMe)3-n(L)n][PF6]
complexes, where L = 1-6 and n = 1 or 2.
Ligand (L)

31P

19F [a]
Ru-P(1)
2.206
Ru-P(2)
-
Ru-N(1)
2.05
Ru-N(2)
2.05
1, n = 1 (9)[b]
162
-73.8,
-74.0
1, n = 2 [c]
-
-
-
-
-
2, n = 1 (10)
199
199
172
168
-73.4,
-73.6
2.224
2.061
2.065
2, n = 2 (11)
3, n = 1 (12)
3, n = 2 (13)
-73.2,
-73.4
2.262
2.264
2.278
2.265
-
2.059
2.059
2.046
-
-72.6,
-72.8
2.054
-
-72.4,
-72.7
2.277
4, n = 1 (14)
4, n = 2 [c]
117
-
-71.0
-
2.233
-
-
2.015
-
2.071
-
-
5, n = 1 (15)
5, n = 2 [c]
192
-
-70.5
-
2.254
-
-
2.047
-
2.061
-
-
Figure
7.
Single-crystal
X-ray
structure
of
[(η5-
6, n = 1 (16)
6, n = 2 (17)
167
185
-70.4
-69.1
2.263
2.289
-
2.069
2.048
2.065
-
C5H5)Ru(NCMe)2{PPh(OC{CF3}3)2}][PF6], 17. Monoclinic, P21/c, R1 = 0.0401,
wR2 = 0.0833. Hydrogen atoms and disordered parts (of [PF6]- anion) omitted
for clarity. Thermal ellipsoids are drawn at the 50 % probability level. Selected
distances (Å) and angles (˚): Ru(1)-P(1) = 2.2541(6), Ru(1)-N(1) = 2.047(2),
Ru(1)-N(2) = 2.061(2), N(1)-Ru(1)-P(1) = 94.77(6), N(2)-Ru(1)-P(1) = 97.49(6),
N(1)-Ru(1)-N(2) = 86.63(8), O(1)-P(1)-O(2) = 94.10(9).
2.299
[a] Two environments are observed for each C(H)(CF3)2 group in the 19F NMR
of complexes containing this functional group, due to atropisomerism. [b]
There is significant disorder present in this structure (for C5H5, [PF6]-), which
resulted in a relatively poor data set compared to other Ru complexes. [c] We
found no evidence of the formation of these species in solution NMR
spectroscopic studies. In 13, two molecules of CH2Cl2 and in 17 one molecule
of CH2Cl2 of crystallization were found in the asymmetric units. No clathrate-
type solvates, as seen in
complex.[30]
a recent related study, were found for any
The Ru-P distances in these complexes appear to follow a trend
based on the -acidity of the P-ligand(s) and the number of P-
ligands at Ru. In general the Ru-P distances are shortest for the
most -acidic ligands for each type of alkoxide substituent
{O(H)(CF3)2 and OC(CF3)3}. This can be interpreted as being
due to an increase in -backdonation from Ru to the more
electron-poor ligands, which strengthens the Ru-P bond. For
example, where RF = O(H)(CF3)2 and n = 1 a very short Ru-P
bond is seen for the phosphite 1 (2.206 Å) whereas a
significantly longer Ru-P bond is seen for the phosphinite 3
(2.264 Å). A similar trend of increasing Ru-P bond length with
decreasing -acidity of the P-ligand is also seen where RF
=
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10.1002/chem.201804805
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Figure
8.
Single-crystal
X-ray
structure
of
[(η5-
=
molecule of MeCN to
a
neutral sodium alkoxide cluster
C5H5)Ru(NCMe){PPh2(OC{CF3}3)}][PF6].0.5CH2Cl2, 17. Monoclinic, P21, R1
[Na4{OC(CF3)3}4], which was present as a low concentration
impurity in the sample of 4 used in this reaction. Cubic clusters
are common structural forms in alkoxides of this type.[33] To the
best of our knowledge compound 14 is the first structurally
characterized example of a metal complex of 4. The basic
structure of 14 is similar to the other complexes described above.
However, given the size of this ligand, the remarkably short Ru-
P bond length (2.233 Å) is particularly noteworthy and may be
an indication of relatively strong -backdonation from Ru to P.
Another interesting feature is the marked asymmetry in the Ru-N
bond lengths (2.015 and 2.071 Å), which appear in other
complexes described here to be relatively insensitive to the
nature of the ligand. Although the ligand is heavily disordered
(by rotation of CF3 groups in the –C(CF3)3 groups) it appears
that there is a closer approach of one C-F bond from 4 to the
CN bond of the coordinated MeCN that displays a particularly
short Ru-N distance (2.015 Å). It is possible that this asymmetry
in the weak intramolecular interactions in the solid-state
structure of 14 results in the asymmetry observed in the Ru-N
bond distances, rather than this being related to the nature of
the metal to P-ligand interaction.
0.0332, wR2 = 0.0745. The asymmetric unit contains two ion pairs, only one of
which is shown for clarity. Hydrogen atoms and solvent of crystallisaion
omitted for clarity. Thermal ellipsoids are drawn at the 50 % probability level.
Selected distances (Å) and angles (˚): Ru(1)-P(1) = 2.2881(9), Ru(1)-P(2) =
2.2988(7) Ru(1)-N(1) = 2.048(2), N(1)-Ru(1)-P(1) = 89.96(7), N(1)-Ru(1)-P(2)
= 96.01(6), P(1)-Ru(1)-P(2) = 99.07(3).
When compared to structural data in the literature, it was
found that complexes 9-17 have Ru-P bond lengths that are
significantly shorter than the average Ru-P distances for [(η5-
C5H5)Ru(NCMe)(L)2]+ (where L = any P(III) ligand) salts reported
in the CSD (av. Ru-P = 2.33 Å for > 180 structures with R1 < 0.1).
Indeed, complex 9 shows the shortest Ru-P distance for any
analogous complex with n = 1 or 2. The shortest Ru-P distance
previously reported was 2.231(2) Å.[32] The Ru-N distances are,
in most cases, relatively insensitive to the other ligands at Ru.
The solution chemistry in this system for the very bulky
phosphite 4 is more complex than for the other ligands.
Evidence
for
the
formation
of
[(5-
C5H5)Ru(NCMe)2(P{OC(CF3)3}3)][PF6] (14) was found in the
31P{1H} and 19F NMR spectra with the appearance of signals at
117 and -71.1 ppm respectively after addition of a CH2Cl2
solution of
4
to [(5-C5H5)Ru(NCMe)3][PF6]. These were
accompanied by the observation of free MeCN (at  = 2.10 ppm)
1
in the H NMR spectrum. However, even after 4 days of stirring
at room temperature ligand exchange is incomplete, with free 4
being seen in the 31P NMR spectrum and a significant quantity of
colorless crystalline material (undissolved 4, which is relatively
poorly soluble) seen in the reaction vessel. Heating the reaction
mixture at 43 oC for 24 hours in an attempt to solubilize ligand 4
and facilitate ligand exchange resulted in a decrease in the
intensity of signals for 14 in the NMR spectra and a new species
1
with (31P ) = 120.7 (doublet of multiplets, JPF = 1252 Hz). This
was
assigned
as
the
complex
[(5-
C5H5)Ru(NCMe)2(PF{OC(CF3)3}2)][PF5{OC(CF3)3}] (19), which is
supported by ESI-MS data which showed a strong signal at
768.9 m/z (for [M]+) in the positive mode and 576.9 m/z (for
[PF5{OC(CF3)3}]-) in the negative mode. 19F NMR data also
Figure 9. Single-crystal X-ray structure of the cation of [(η5-
C5H5)Ru(NCMe)2{P(OC{CF3}3)3}][Na4(ORF)4PF6.MeCN], 14, alongside the
Lewis structures of both ions for clarity. Monoclinic, P21/n, R1 = 0.0623, wR2 =
0.1677. Hydrogen atoms, anion and disordered parts (the C4F9 groups of both
the anion and cation are heavily disordered by CF3 group rotation) omitted for
clarity. Thermal ellipsoids are drawn at the 50 % probability level. Selected
distances (Å) and angles (˚): Ru(1)-P(1) = 2.233(1), Ru(1)-N(1) = 2.015(5),
Ru(1)-N(2) = 2.071(4), N(1)-Ru(1)-P(1) = 94.00(13), N(2)-Ru(1)-P(1) = 93.18
(10), N(1)-Ru(1)-N(2) = 86.70 (17), O(1)-P(1)-O(2) = 98.4(3), O(1)-P(1)-O(3) =
97.8(3), O(2)-P(1)-O(3) = 96.2(4).
suggest the presence of this anion (19F) = -62.3 (Fequatorial
,
1
2
doublet of doublet of multiplets, JPF = 746 Hz, JFF = 50
Hz) -73.86 (Faxial, doublet of quintets, JPF = 700 Hz, JFF = 50
Hz) and the P-F group on the substituted ligand (19F) = 1.25
1
2
1
4
(doublet of multiplets, JPF = 1252 Hz, JFF = 4 Hz) ppm. The
mechanism for this fluoride-alkoxide group exchange between
ligand 4 and the [PF6]- anion is unclear, but the driving force is
likely to be reduction of the steric bulk of the ligand and so
formation of a less sterically congested complex.
Conclusions
The steric profiles, electronic properties and coordination
chemistry of a range of phosphorus(III) ligands bearing highly
fluorinated alkoxide substituents have been investigated using a
range of experimental and computational approaches. These
ligands are relatively easy to synthesize and their steric and
electronic properties are tunable depending on the substituents
present at the phosphorus center. All ligands occupy the bulky,
electron-poor region of ligand space where there are currently
relatively few ligands available. Coordination of these ligands to
Although the reaction of 4 with [(η5-C5H5)Ru(NCMe)3][PF6]
does not proceed to completion at room temperature, it was
possible to identify crystals containing the desired cation (14),
after decanting to remove unreacted 4 and crystallisation from a
mixture of CH2Cl2 and Et2O. The molecular structure of 14 is
shown in Figure 9. While this contains the desired cation [(η5-
C5H5)Ru(NCMe)2{P(OC{CF3}3)3}]+, in 14 it crystallises as a salt
with the unusual counterion [Na4{OC(CF3)3}4PF6.MeCN]-. This
anion results from the coordination of a [PF6]- anion and a
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10.1002/chem.201804805
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techniques. Dichloromethane and hexane were purified with the aid of an
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Diethylether and tetrahydrofuran were dried over sodium and distilled
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NMR spectra were acquired on a Jeol ECX-400 (Operating frequencies
1H 399.78 MHz, 31P 161.83 MHz, 19F 376.17 MHz, 13C 100.53 MHz) or a
Bruker AVANCE 500 (Operating frequencies 1H 500.13 MHz, 31P 202.47
MHz, 13C 125.77 MHz). 31P and 13C spectra were recorded with proton
decoupling. Mass spectra were recorded on a Bruker micrOTOF or
Esquire 6000 using electrospray ionisation. Infrared (IR) spectra were
recorded on a Thermo-Nicolet Avator 370 FTIR spectrometer using CsCl
solution cells for sample insertion at ca. 200 mg/mL concentration. A
Unicam RS 10000E FTIR instrument, averaging 16 scans at resolution 1
cm-1 , was used with a SensIR ATR-IR accessory for solid samples.
Geometry optimisations and vibrational frequency analyses were
performed at the (RI-)BP86/SV(P) level with the full ligand substituents
used in the experimental study using TURBOMOLE.[34]
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EPSRC (DTA award to SH) for funding. We would also like to
thank Professor Robin Perutz and Dr Jason Lynam for valuable
discussions.
Keywords: Phosphorus ligands • coordination chemistry • ligand
design • catalysis • DFT
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Entry for the Table of Contents
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A comprehensive overview of the
synthesis, stereoelectronic properties
and coordination chemistry of large,
electron-poor P(III) ligands bearing
highly fluorinated alkoxides is
described.
Sharifa Hussein, Denis Priester, Paul
Beet, Jonathon Cottom, Sam J. Hart,
Tim James, Robert J. Thatcher, Adrian
C. Whitwood and John M. Slattery*
Page No. – Page No.
Filling a niche in “ligand space” with
bulky, electron-poor phosphorus (III)
alkoxides
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