Electronic effects on the acidity of phosphorus(III) triflates

Article abstract of DOI:10.1016/S0022-328X(01)01460-7

A range of chiral phosphorus(III) triflates has been prepared and their donor-acceptor behaviour towards the representative donor 4-phenylpyridine investigated. Correlations between ³¹P-NMR shifts, relative binding strengths (K_r) and

Full text of DOI:10.1016/S0022-328X(01)01460-7

Journal of Organometallic Chemistry 643–644 (2002) 516–521
www.elsevier.com/locate/jorganchem
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Electronic effects on the acidity of phosphorus(III) triflatesꢀ
Sylvie L. Renard, Terence P. Kee *
School of Chemistry, Uni6ersity of Leeds, Leeds LS2 9JT, UK
Received 10 August 2001; accepted 14 November 2001
Abstract
A range of chiral phosphorus(III) triflates has been prepared and their donor–acceptor behaviour towards the representative
donor 4-phenylpyridine investigated. Correlations between 31P-NMR shifts, relative binding strengths (Kr) and the electronic
nature of the phosphorus complex are observed. A scale of relative Lewis acidity (Ar) is proposed for this class of phosphorus
compound towards donor molecules using 4-phenylpyridine as a reference. © 2002 Published by Elsevier Science B.V.
Keywords: Phosphorus(III) triflates; Electronic effects; Chiral
tronic nature to their carbon cousins the Arduengo
class of carbenes.
1. Introduction
In this contribution, we wish to extend our earlier
work in the definition of a relative basicity scale (Br) for
various donor molecules towards a reference phospho-
rus(III) triflate [3b] towards defining the degree to
which phosphorus Lewis acidity may be influenced by
distal, non-steric substitution of the organic framework.
The method we have chosen is based on isothermal
There has been considerable interest over the last 20
years in phosphenium compounds [1] which are for-
mally examples of |2u3 phosphorus and recently, ef-
forts have intensified due to the range of applications
envisaged in organometallic and co-ordination chem-
istry championed, among others, by Nakazawa and
Baker, respectively, [2]. We [3] and others [4] have
measurements of relative binding strengths of phospho-
examined chiral phosphorus(III) halides and triflates,
rus(III) triflates against 4-phenylpyridine at fixed con-
the latter of which have been shown to possess signifi-
centrations. We recognise that this approach is less
cant phosphenium character both in the solid state and
rigorous than full NMR titration analysis but has been
solution, especially when in the presence of donor
used effectively nonetheless in a relative capacity.
molecules such as amines and phosphines [3]. Our
studies were inspired by earlier work of Bertrand et al.
on the heterolysis of the phosphorusꢀhalogen bond in
the presence of a strong nucleophile such as 1,8-diaz-
2. Results and discussion
abicyclo[5.4.0]undec-7-ene [5]. More recently, some
beautiful work by Gudat and colleagues [6] has re-
Full synthetic details of all halide compounds 4 and
triflate derivatives 5 are available as Section 3 but are
vealed that, on the basis of structural, computational
summarised in Scheme 1; representative general meth-
and donor–acceptor interactions with Lewis bases,
ods for the synthesis of one such derivative, 2a–5a have
been reported previously [3b]. Each X-substituent is in
1,3,2-diazaphospholene triflates are comparable in elec-
a position to cause minimal steric differentiation at the
phosphorus centre, a feature we have explored in some
detail on the ligand frameworks alone [7]. Indeed, the
substituents are some seven bonds distant from the
phosphorus centre and thus, might be expected to have
ꢀ C’est un tre`s grand plaisir pour nous de pre´senter cette petite
offrande en l’honneur du Professeur Franc¸ois Mathey; un chimiste et
une personne d’excellence.
* Corresponding author. Tel.: +44-113-233-6421; fax: +44-113-
a somewhat attenuated through-bond electronic influ-
ence, especially since any extended conjugation should
233-6565.
E-mail address: p.kee@chem.leeds.ac.uk (T.P. Kee).
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(01)01460-7

S.L. Renard, T.P. Kee / Journal of Organometallic Chemistry 643–644 (2002) 516–521
517
be partially compromised by the presence of the meth-
ylene function. Nevertheless, in related electronic inves-
tigations, we have found that similar aromatic
substitution formally eight bonds distant from the ac-
tive site can influence reactivity at that site [8]. We
present here preliminary investigations into how such
distal electronic substitution may influence the elec-
tronic character at the phosphorus centre and degree of
interaction between that phosphorus centre in 5 and a
representative base, 4-phenylpyridine. This, we argue,
allows us to quantify phosphorus Lewis acidity in this
particular class of phosphorus triflates through defini-
tion of a relative acidity parameter (Ar, vide infra).
The 31P{1H} chemical shifts of compounds 4a–g and
5a–g at a concentration of 0.25 M (298 K; NMR probe
temperature) in CDCl3 and CD2Cl2 solvent respectively
are reproduced in Table 1. There is the suggestion,
from these data, especially when presented in graphical
format in Fig. 1 and Fig. 2 respectively, that there exists
a correlation between 31P chemical shifts in both 4 and
5 series and the electronic Hammett parameter of the
X-substituent. In both cases the trend is in the same
direction where higher resonance frequencies are fa-
voured by the more electron-donating substituents.
Whilst, a detailed explanation of this trend must be
treated with some caution due to the small changes
involved and rather lower than desirable R2 values, it is
worth commenting that there is an 13% variation in
triflate 31P-NMR resonances such that the nature of the
X-function, even when seven bonds distant from the
phosphorus centre has an influence over the 31P-NMR
chemical shift and hence the electromagnetic environ-
ment of the system. Given the considerable through-
bond distances involved our feeling is that field and
dipolar effects in solution may play a role.
In the presence of one equivalent (0.25M) of the base
4-phenylpyridine (Br=1.1 [3b]), triflates 5a–g return
far lower 31P-NMR chemical shifts due to rapid re-
versible interaction with the base (Scheme 2). Indeed, a
graphical representation of the change in 31P-NMR
shift (DlP) upon interaction with 4-phenylpyridine
against Hammett |p (Fig. 3) reveals a similar trend to
Scheme 1. Syntheses of compounds 4 and 5. (i) K2CO3, CHOC6H4X; (ii) LiAlH4; (iii) PCl3, N-methylmorpholine; (iv) Me3SiOSO2CF3 (TMSOTf).
See Section 3 for more details.
Table 1
Selected NMR and computed equilibrium data for compounds 4 and 5
lP (ppm) a System
lP (ppm) b
|
System
lP (ppm) c DlP (ppm) lC (ppm) d FI
Kr (mol−1 l)
Ar
e
System
p
4a
4b
4c
4d
4e
4f
181.9
180.0
179.9
182.4
177.4
178.0
183.0
5a
5b
5c
5d
5e
5f
260.7
264.9
262.7
256.7
223.7
238.3
272.1
0.0
0.24
0.26
−0.14
0.81
0.70
5a+base 163.6
5b+base 169.8
97.1
95.1
85.5
90.9
65.7
79.9
107.0
121.34
121.30
121.09
121.47
121.19
121.26
121.39
0.88
0.82
0.67
1.04
0.84
0.76
0.94
253
103
25
2100
125
54
7
10
27
–
19
13
3
5c+base
177.2
5d+base 165.8
5e+base
5f+base
158.0
158.4
4g
5g
−0.15
5g+base 165.1
1077
a In CDCl3; 0.25 M.
b In CD2Cl2; 0.25 M.
c Base=4-phenylpyridine; CD2Cl2; 0.25 M.
d CF3 signal, 0.25 M, CD2Cl2; lI=121.42 (nBu4NOTf) and lC=shift for base-free, precursor triflate 5a–g at same concentration and in same
solvent.
e Ar (relative acidity)=[lI−lS]/lI; ×104 [9].

518
S.L. Renard, T.P. Kee / Journal of Organometallic Chemistry 643–644 (2002) 516–521
Fig. 1. Correlation between 31P{1H}-NMR shifts for 4a–g vs. Hammett |p parameter of X (Scheme 1).
Fig. 2. Correlation between 31P{1H}-NMR shifts for 5a–g vs. Hammett |p parameter of X (Scheme 1).
those in Figs. 1 and 2. Using the same approach as
proposed earlier for quantifying the interactions be-
tween this class of phosphorus(III) triflates and donor
molecules [3b], it is possible to derive internally consis-
tent relative binding strengths for the interactions be-
tween 5a–g and 4-phenylpyridine. Thus, if we define
the equilibrium between Lewis acid and Lewis base as
in Scheme 2, the relative binding strength Kr may be
represented by the equation Kr=FI/{[Ao]. (1−FI)2}
where FI is the mole fraction of ionic form 6a–g and

S.L. Renard, T.P. Kee / Journal of Organometallic Chemistry 643–644 (2002) 516–521
519
[Ao] is the total concentration of phosphorus com-
pound in the system [3b]. We arrange that [Ao]=0.25
M and we calculate FI from the equation FI=[lS−
lC]/[lI−lC] where lS represents the NMR chemical
shift of the system under investigation (chosen to be
the 13C{1H} resonance of the triflate CF3 group) and
lC and lI are the corresponding chemical shifts of
base-free, precursor triflate 5a–g and ionic
(nBu4NOTf; lI 121.42) triflates, respectively, in the
same solvent and at same concentrations as the subject
samples. Representative 13C{1H}-NMR data and com-
puted FI and Kr values are given in Table 1 and
relationship between Kr and |p illustrated in Fig. 4.
The latter reveals a trend although, a distinctly non-
linear one such that the strongest degree of interac-
tions between phosphorus centre and 4-phenylpyridine
results from compounds 5 with the more electron-do-
ter-intuitive to that expected should through-bond
inductive effects play a large part. We envisage a full
explanation to involve components of dipole moment
and field effects.
In the same way as we proposed a scale of relative
basicity (Br) for donor molecules interacting with this
class of phosphorus(III) triflates, we can suggest a
complementary scale of relative acidity Ar=[lI−lS]/
lI, whereby, the smaller the Ar value, the greater the
relative acidity against 4-phenylpyridine (Br=1.1).
However, it is clear from Table 1 and graphical corre-
lation in Fig. 5, that the correlation between Ar and Kr
is far from regular, although, a trend is apparent. We
believe that this is due to the definition Ar=
[lI−lS]/lI, neglecting the vitally important feature
that each compound analysed has a different lC value
which is not accounted for explicitly in the aforemen-
tioned equation. Consequently, we advocate caution in
t
nating X substituents (e.g. Bu and Me), perhaps coun-
Scheme 2.
Fig. 3. Correlation between 31P{1H}-NMR shift differences for 5a–g in the presence and absence (DP) of 4-phenylpyridine (0.25 M) vs. Hammett
parameter of X (Scheme 1).
|
p

520
S.L. Renard, T.P. Kee / Journal of Organometallic Chemistry 643–644 (2002) 516–521
Fig. 4. Correlation between relative binding strength Kr for 5a–g in the presence of 4-phenylpyridine (K; 298 K; 0.25 M; CD2Cl2) vs. Hammett
parameter of X (Scheme 1).
|
p
Fig. 5. Correlation between relative binding strength Kr for 5a–g in the presence of 4-phenylpyridine (K; 298 K; 0.25 M; CD2Cl2) vs. relative
acidity parameter Ar (Scheme 1).
the assignment of such values and suspect, as in the
case of relative basicity parameters Br defined earlier
[3b] most effective correlations will result where the
number of system variables is limited.
3. Supplementary material
Full synthetic, characterising and NMR experimental
data for all compounds reported here are available as

S.L. Renard, T.P. Kee / Journal of Organometallic Chemistry 643–644 (2002) 516–521
521
Table 2
Triflate
m (g)
6/Otf
6p-Br/OTf
6p-tBu/OTf
6p-Cl/OTf
6p-CN/OTf
0.059
0.079
0.073
0.068
0.066
nBu4NCl
6o2F2/OTf
6p-Me/OTf
6o-NO2/OTf
6p-NO2/OTf
m (g)
0.049
0.068
0.063
0.070
0.070
Mass of triflate used.
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Acknowledgements
The authors extend their grateful thanks to the EP-
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that any traces of water were trapped. For each NMR tube the
total volume was 0.5 ml and the concentration of phosphorus(III)
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