Angewandte
Chemie
DOI: 10.1002/anie.201412209
Cyanido-Phosphates
Tetracyanido(difluorido)phosphates M+[PF2(CN)4]ꢀ**
Jonas Bresien, Stefan Ellinger, Jçrg Harloff, Axel Schulz,* Katharina Sievert, Alrik Stoffers,
Christoph Tꢀschler, Alexander Villinger, and Cornelia Zur Tꢀschler
Abstract: The systematic study of the reaction of M[PF6] salts
and Me3SiCN led to a synthetic method for the synthesis and
isolation of a series of salts containing the unprecedented
[PF2(CN)4]ꢀ ion in good yields. The reaction temperature,
pressure, and stoichiometry were optimized. The crystal
structures of M[PF2(CN)4] (M = [nBu4N]+, Ag+, K+, Li+,
phosphorus(V) cyanides (e.g. P(CN)5) in the reaction of
P(CN)3 with (CN)2 by Gall and Schꢀppen.[29] They proposed
the in situ generation of P(CN)5, which, however, decom-
posed rapidly at standard conditions [Scheme 1, Eq. (1)]. In
+
H5O2 ) were determined. X-ray crystallography showed the
exclusive formation of the cis isomer in accord with 31P and 19
F
solution NMR spectroscopy data. Starting with the K-
[PF2(CN)4] the room temperature ionic liquid EMIm-
[PF2(CN)4] was prepared exhibiting a rather low viscosity.
Scheme 1. Synthesis of PV(CN)5 and cyanido(chlorido)phosphates.
a series of publications, the preparation of salts bearing
cyanido(chlorido)phosphate ions of the type [PCl6ꢀn(CN)n]ꢀ
(n = 1–3) were described, but none of these salts was fully
characterized.[30,31] The hitherto published syntheses started
from PCl6 salts (or from PCl6 salts in situ generated from
PCl5 and MCl; M = R4N+, R = nBu, Et) utilizing cyanido–
chlorido exchange agents, such as AgCN or KCN [Scheme 1,
Eq. (2)].
I
n the quest for new materials with improved properties, salts
containing robust weakly coordinating anions are of general
interest. Such anions can be used in ionic liquids (ILs),[1–6] in
electrolytes,[2,7–12] for the stabilization of unusual cat-
ions,[10,11,13] or transient species.[3] One major activity in
designing new robust anions focusses on the synthesis of
cyanide-based anions and their utilization in ILs.[4,14–26] Salts
bearing the tetracyanidoborate anion, [B(CN)4]ꢀ, were first
obtained by Willner et al. in the reaction of [Bu4N]X, BX3
(X = Br, Cl) and KCN in toluene.[27] A more efficient
synthesis for tetracyanidoborates on a larger scale starting
from the readily available reagents K[BF4], LiCl, and KCN
was published by the same group in 2003[23] along with studies
of the thermal behavior of the mixed cyanidofluoridoborates,
K[BFn(CN)4ꢀn] (n = 1–3). ILs containing the [B(CN)4]ꢀ ion
are used in electrochemical applications (e.g. solar cells and
batteries).[28] Owing to the laborious synthesis, making use of
huge amounts of lithium chloride in a sinter process
producing large amounts of waste, there was a need for
alternatives to cyanidoborate derivatives (e.g. cyanido-
(fluorido)phosphates) or more efficient routes to tetracyani-
doborates. As early as 1930, it was attempted to prepare
ꢀ
ꢀ
In 1980 Chevrier et al. reported solution 19F NMR spec-
troscopy data of the [PF5CN]ꢀ ion. Shortly thereafter, Dillon
et al. published 31P NMR data of reaction mixtures containing
[PF6ꢀn(CN)n]ꢀ (1 ꢁ n ꢁ 4) und [PF3Cl3ꢀn(CN)n]ꢀ (1 ꢁ n ꢁ
3).[32,33] These reaction mixtures had been generated by
treating PF5 with [Et4N]CN and [PCl4(CN)2]ꢀ with AgF,
respectively. Only recently, BMIm[PF3(CN)3] was obtained
by the reaction of BMIm[PCl3(CN)3] and Ag[BF4] after
4 days of reaction time (BMIm = 1-butyl-3-methylimidazo-
lium).[34] To our knowledge salts bearing simple CN-function-
alized phosphates of the type [PF2(CN)4]ꢀ have not been
isolated and fully characterized to date. Herein, we describe
facile large-scale syntheses of a series of salts bearing the
[PF2(CN)4]ꢀ ion, which can easily be transformed into room
temperature ILs by salt metathesis reaction.
[nBu4N][PF2(CN)4] (1) could easily be obtained in the
reaction of [nBu4N][PF6] with an excess of Me3SiCN under
autogenous pressure in a steel autoclave at high temperatures
[180–2108C, Equation (3)].[35] As summarized in Table S8
(see the Supporting Information), the best conditions to
isolated almost pure 1 in 40% yield were 10 equivalents of
Me3SiCN at 2008C for 16–18 h. When the excess of Me3SiCN
was decreased to 8 equivalents the yield of isolated product
dropped to 33% (and more impurities, such as [PF6ꢀn(CN)n]ꢀ
species, appeared), whereas raising the excess to above
10 equivalents did not lead to a higher yield. But it should
be noted that an increase of the excess of Me3SiCN up to
20 equivalents already gave small quantities of [PF(CN)5]ꢀ,
which was detected by 19F/31P NMR spectroscopy (Supporting
Information). Increasing the reaction temperature above
2008C led to the polymerization of Me3SiCN and decom-
position of the product as indicated by the formation of
[*] J. Bresien, Dr. J. Harloff, Prof. Dr. A. Schulz, K. Sievert, A. Stoffers,
Dr. A. Villinger
Institut fꢀr Chemie, Universitꢁt Rostock
Albert-Einstein-Strasse 3a, 18059 Rostock (Germany)
E-mail: axel.schulz@uni-rostock.de
Prof. Dr. A. Schulz
Abteilung Materialdesign, Leibniz-Institut fꢀr Katalyse e.V.
an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
S. Ellinger, C. Tꢁschler, C. Zur Tꢁschler
Lonza Ltd, Valais Works
Lonzastrasse, CH-3930 Visp (Switzerland)
[**] Financial support by the Lonza Group Ltd (K.S.) is gratefully
acknowledged. We also thank the DFG.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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