Convenient access to the tricyanoborate dianion B(CN)32- and selected reactions as a boron-centred nucleophile

Article abstract of DOI:10.1039/c5cc00555h

Alkali metal tricyanoborates M₂B(CN)₃ (M = Na, K) are accessible by the reaction of tricyanofluoroborates with alkali metals (i) in liquid NH₃ or (ii) in THF-naphthalene. The M₂B(CN)₃ are versatile starting materials for the synthesis of K[RB(CN)₃] (R = Et, C₆F₅, CH₂CHCH₂).

Full text of DOI:10.1039/c5cc00555h

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Convenient access to the tricyanoborate
2À
dianion B(CN)3 and selected reactions
Cite this:DOI: 10.1039/c5cc00555h
as a boron-centred nucleophile†
Received 20th January 2015,
Accepted 17th February 2015
Johannes Landmann,a Jan A. P. Sprenger,a Rudiger Bertermann,a Nikolai Ignat’ev,b
¨
Vera Bernhardt-Pitchougina,ac Eduard Bernhardt,c Helge Willnerc and Maik Finze*a
DOI: 10.1039/c5cc00555h
www.rsc.org/chemcomm
Alkali metal tricyanoborates M2B(CN)3 (M = Na, K) are accessible by
the reaction of tricyanofluoroborates with alkali metals (i) in liquid
NH3 or (ii) in THF–naphthalene. The M2B(CN)3 are versatile starting
Q
materials for the synthesis of K[RB(CN)3] (R = Et, C6F5, CH2 CHCH2).
So far, only a few well-defined boron-centred nucleophiles have
been isolated and characterized in detail. Their nucleophilic
character has been demonstrated by reactions with selected
electrophiles. Relevant examples for such boron species are
summarized in Scheme 1. The unusual lithiated boryl anion A
was reported in 2006.1 Since then a number of related anions
have been described and their chemistry has been studied.2
Furthermore, related transition metal boryl complexes are highly
valuable starting materials for transition metal catalysed trans-
formations and they exhibit a certain degree of nucleophilic
character at the boron atom, as well.3 The anionic dimanganese
borylene complex B reveals a nucleophilic behavior at the linear
coordinated boron centre.4 The carbene-stabilized p-boryl anion
Scheme 1 Selected nucleophilic boron species.
In addition to the reactions found for the nucleophilic boron
C was found to react with methyl iodide to yield the corre- derivatives depicted in Scheme 1, the high synthetic value of
sponding methyl substituted donor-stabilized borole5 and with related nucleophilic boron species was demonstrated by reactions
[Et3NH]+ to give the respective protonated derivative.6 Both of a few other boryl anions that have been described but that could
reactions provided some evidence for a nucleophilic reactivity not be isolated. Trapping reactions with various electrophiles have
of C. However, recently an alternative non-nucleophilic pathway been conducted especially with the carbene-stabilized boryl anion
À
for reactions of C via a radical intermediate was discussed.7 Further NHC–BH2 (NHC = 1,3-bis-(2,6-diisopropylphenyl)imidazol-
examples for nucleophilic boron derivatives are the molecule D,8 2-ylidene)11 and the phosphane substituted boryl anion (cyclo-
À 12
the dicyanoborate monoanion E9 and the tricyanoborate dianion C6H11)3P–BH2
.
2À
2À
B(CN)3 (1).10
The homoleptic cyanoborate dianion B(CN)3 (1),10 which
is isoelectronic to the tricyanmethanide anion C(CN)3À, is the
only doubly negatively charged boron-centred base described in
the literature.10 The negative charge of all other related boron-
centred bases is fully (D) or in part (A–C and E) compensated by
further substituents, e.g. a N-heterocyclic carbene. So far, an alkali
metal tetracyanoborate M[B(CN)4] (M = Li, Na, K)13 is required as
starting material for the preparation of M21 (M = alkali metal).10
The [B(CN)4]À anion is either reduced with an alkali metal in
liquid ammonia or one of its four cyano groups is removed by
the action of a strong base, for example n-butyl lithium.10a Here
we report on (i) alternative syntheses for alkali metal salts of the
a
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg,
¨
Am Hubland, 97074 Wurzburg, Germany. E-mail: maik.finze@uni-wuerzburg.de
b Merck KGaA, PM-ATI, Frankfurter Straße 250, 64293 Darmstadt, Germany
c
¨
FB C Anorganische Chemie, Bergische Universitat Wuppertal, Gaußstraße 20,
42119 Wuppertal, Germany
† Electronic supplementary information (ESI) available: Experimental and spec-
troscopic details, 19F MAS NMR spectrum and IR as well as Raman spectrum of
K21ÁKF. CCDC 1039991 (K4Á0.5THF), 1039992 (K6), 1031561 and 1039993 (K5),
1039994 (K3Á0.5(CH3)3CO). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5cc00555h
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Scheme 2 Synthesis of M21ÁMF (M = K, Na).
2À
tricyanoborate anion B(CN)3 (M21, M = Na, K) and (ii) on
reactions of K21 with selected electrophiles that demonstrate
the nucleophilic nature of the boron atom of 1.
In the course of our studies on the chemical and electro-
chemical stabilities of tricyanofluoroborates, which are valuable
materials for electrochemical applications,14 we have developed
new and easy syntheses for K2B(CN)3 (K21). These new syntheses
are especially attractive because the alkali metal tricyanofluoro-
borates K[BF(CN)3] and Na[BF(CN)3] (K2 and Na2)15 have become
easily accessible on large scale (4100 g).16 Reaction of K2 with
elemental potassium either in liquid ammonia or in THF in the
presence of catalytic amounts of naphthalene gives K21 in yields
of up to 89%, which contains one equivalent of KF as byproduct
(Scheme 2). So far, it was not possible to separate the KF.
However, it was not found to affect subsequent reactions with
electrophiles. Similar to the reaction of K[BF(CN)3] (K2) in NH3
and THF, Na2 is reduced with either sodium in liquid ammonia
or sodium naphthalide in THF. A comparative study on the
reaction of K2 with potassium, sodium and lithium naphthalide
in THF showed that the rate of reduction strongly decreases in
the order K 4 Na 4 Li. Attempted electrochemical reduction of
2 under the conditions of cyclic voltammetry and in the presence
of the weakly coordinating countercation [nBu4N]+ failed. Hence,
the electrochemical stability of [nBu4N]2 exceeds the one of THF.
The reduction of the [BF(CN)3]À anion (2) leads to a population
of its LUMO (Fig. 1) with electrons. Since this molecular orbital is
B–F antibonding the formation of the homoleptic cyanoborate
Fig. 2 11B MAS NMR spectrum and simulated NMR spectrum (red) as well
as 13C MAS NMR spectrum of K2B(CN)3ÁKF (K21ÁKF) derived from the
reaction of K[BF(CN)3] (K2) with elemental K in ammonia (* unknown
impurity, most probably as a result of the sample preparation).
2À
anion B(CN)3 (1) is rationalized. The similarity of the LUMO
of 2 to the HOMO of 1 is evident from the respective contour
plots in Fig. 1.
The mixed potassium salt K2B(CN)3ÁKF (K21ÁKF) was char-
acterized by elemental analysis, solid state NMR spectroscopy
and vibrational spectroscopy. The line shape of the 11B MAS
NMR spectrum reveals a second-order quadrupole MAS powder
pattern as expected for an anion with a trigonal-planar arrange-
ment at the quadrupolar nucleus. Simulation of the spectrum
as depicted in Fig. 2 resulted in an isotropic chemical shift
diso of À43.4 ppm, a quadrupolar coupling constant Cquad of
1.01 MHz and a quadrupolar asymmetry parameter Zquad of
0.19. This relatively small quadrupolar asymmetry parameter
also reflects the almost trigonal-planar geometry of the anion in
the solid state. The 13C MAS NMR spectrum shows a single
signal for the cyano groups at 169.4 ppm. The isotropic 11B and
13C chemical shifts observed for solid K21ÁKF are close to d(11B)
and d(13C) reported for K2B(CN)3 (K21) dissolved in liquid ND3
of À45.3 and 158.5 ppm, respectively.10a The signal of the
fluoride anion of the mixed salt K21ÁKF was observed at À133.7 ppm
(Fig. S1, ESI†), which is in good agreement to diso(19F) reported
for neat KF of À136 ppm.17 The IR and Raman spectrum of
K21ÁKF that are shown in Fig. S2 (ESI†) are almost identical to
the spectra reported for K21, earlier.10a
The nucleophilic character of the tricyanoborate dianion
2À
B(CN)3 (1) at its boron atom was demonstrated by reactions
with (a) water to yield K[BH(CN)3] (K3),10 (b) ethyl iodide to give
K[EtB(CN)3] (K4), (c) allyl chloride and allyl bromide to result in
K[CH2QCHCH2B(CN)3] (K5) and (d) hexafluorobenzene to give
K[C6F5B(CN)3] (K6). Salts of the anions 4–6 have been obtained
from the corresponding trifluoroborates earlier.18 However, the
synthetic method described in this publication does not rely on
the availability of the respective trifluoroborates. The reactions
Fig. 1 Different contour plots of the LUMO of the [BF(CN)3]À anion (2) and
of the HOMO of the B(CN)3 anion (1) [calculated at the PBE0/def2-
2À
TZVPP level of theory].
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reacts with allyl bromide to give an allyl substituted four-
coordinate boron derivative with a similar yield (36%).11 The
analogous reactivity unambiguously proves the nucleophilic
character at the boron centre of dianion 1.
The four potassium salts K3, K4, K5 and K6 were characterized
by multi-NMR and vibrational spectroscopy and single crystal
X-ray diffraction. Molecular structures of the four anions, which
show the almost tetrahedral arrangement at the boron atoms,
are depicted in Fig. 3 and experimental details on the crystal
structure analyses are given in the ESI.†
New and convenient syntheses for the potassium salt of the
unusual boron-centred nucleophile B(CN)3 (1)10 have been
2À
developed via reaction of the readily available K[BF(CN)3] (K2)16b
with potassium. K21 was found to be a valuable starting material
for the preparation of tricyanoborates of the type K[RB(CN)3]
(R = H (K3), Et (K4), CH2QCHCH2 (K5), C6F5 (K6)). Further detailed
studies on the chemistry of salts of dianion 1 with selected
compounds are in progress.
Scheme 3 Reactions of K21ÁKF with selected electrophiles.
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