Synthesis, structure, and bonding of weakly coordinating anions based on CN adducts

Article abstract of DOI:10.1021/ja902450b

The addition of alkali or silver salts of dicyanoamide (dca), tricyanomethanide (tcm) and tetracyanoborate (tcb) to a solution of B(C ₆F₅)₃ in diethyl ether affords salts containing very voluminous B(C₆F₅)₃ adduct anions of the type [E(CN)_n^-]·[B(C₆F₅) ₃]_n:E = N (dca-nb with n = 1, 2; b ) B(C₆F ₅)₃); E = C (tcm-nb with n = 1, 2, 3), and E = B (tcb-nb with n = 1, 2, 3, 4). Salts bearing these anions such as B[(CN)·B(C ₆F₅)₃]₄^- (= [B(CN) ₄^-]·[ B(C₆F₅) ₃]₄), C[(CN)·B(C₆F₅) ₃]₃^- (= [C(CN)₃^-] ·[ B(C₆F₅)₃]₃), and N[(CN)·B(C₆F₅)₃]₂ ^- (=[N(CN)₂^-]·[ B(C₆F ₅)₃]₂) can be prepared in good yields. They are thermally stable up to over 200°C and dissolve in polar organic solvents. Depending on the stoichiometry mono-, di-, tri-, or tetraadduct formation is observed. The solid state structures of dca-2b, tcm-3b and tcb-4b salts show only long cation...anion contacts and thereby weak interactions, large anion volumes and only small distortions of the dca, tcm or tcb core enwrapped between B(C₆F₅)₃ groups. That is why these anions can be regarded as weakly coordinating anions. On the basis of B3LYP/6-31+G(d) computations the energetics, structural trends and charge transfer of the adduct anion formation were studied. Since tcm-3b and tcb-4b are easily accessible and can also be prepared in large quantities, these anions may be utilized as a true alternative to other widely used weakly coordinating anions. Moreover, for both steric and electronic reasons it seems reasonable to expect that as counterions for cationic early transition metal catalysts such anions may show reduced ion pairing and hence increased catalytic activity.

Full text of DOI:10.1021/ja902450b

Published on Web 05/26/2009
Synthesis, Structure, and Bonding of Weakly Coordinating
Anions Based on CN Adducts
Arne Bernsdorf,† Harald Brand,† Robert Hellmann,† Martin Ko¨ckerling,†
Axel Schulz,*,†,‡ Alexander Villinger,† and Karsten Voss†
Abteilung Anorganische Chemie, Institut fu¨r Chemie, UniVersita¨t Rostock, Albert-Einstein-Strasse
3a, 18059 Rostock, Germany, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock,
Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
Received March 27, 2009; E-mail: axel.schulz@uni-rostock.de
Abstract: The addition of alkali or silver salts of dicyanoamide (dca), tricyanomethanide (tcm) and
tetracyanoborate (tcb) to a solution of B(C6F5)3 in diethyl ether affords salts containing very voluminous
B(C6F5)3 adduct anions of the type [E(CN)n-]·[ B(C6F5)3]n: E ) N (dca_nb with n ) 1, 2; b ) B(C6F5)3); E
) C (tcm_nb with n ) 1, 2, 3), and E ) B (tcb_nb with n ) 1, 2, 3, 4). Salts bearing these anions such as
-
-
B[(CN) · B(C6F5)3]4 () [B(CN)4-] · [ B(C6F5)3]4), C[(CN) · B(C6F5)3]3 () [C(CN)3-] · [ B(C6F5)3]3), and
-
N[(CN)·B(C6F5)3]2 ()[N(CN)2-]·[ B(C6F5)3]2) can be prepared in good yields. They are thermally stable
up to over 200 °C and dissolve in polar organic solvents. Depending on the stoichiometry mono-, di-, tri-,
or tetraadduct formation is observed. The solid state structures of dca_2b, tcm_3b and tcb_4b salts show
only long cation · · ·anion contacts and thereby weak interactions, large anion volumes and only small
distortions of the dca, tcm or tcb core enwrapped between B(C6F5)3 groups. That is why these anions can
be regarded as weakly coordinating anions. On the basis of B3LYP/6-31+G(d) computations the energetics,
structural trends and charge transfer of the adduct anion formation were studied. Since tcm_3b and tcb_4b
are easily accessible and can also be prepared in large quantities, these anions may be utilized as a true
alternative to other widely used weakly coordinating anions. Moreover, for both steric and electronic reasons
it seems reasonable to expect that as counterions for cationic early transition metal catalysts such anions
may show reduced ion pairing and hence increased catalytic activity.
[Al{OC(CF3)3}4]-),7 and teflate-based anions (e.g. [M(OTeF5)6]-
with M ) As, Sb, Bi, Nb)8-10 display exceptional characteristics
Introduction
Weakly coordinating anions (WCA) have achieved great
interest due to academic and commercial importance in olefin
polymerization, ionic liquids, stabilizing highly reactive inter-
mediates and battery technology.1 This interest has grown over
two decades of basic research on the physical and chemical
properties of anions containing bulky hydrocarbon and fluoro-
carbon functionality.1 WCAs such as borate-based anions (e.g.
[B(C6F5)4]-),2 carborane-based anions (e.g. [CB11H6X6]- ion
with X ) Cl, Br),1b,3-6 alkoxy- and aryloxymetallates (e.g.
such as low nucleophilicity, chemical inertness, solubility,
leaving group lability, and weak coordination resulting in
applications throughout organic and inorganic chemistry.1a They
are particularly useful for stabilizing highly reactive cations (e.g.
[CX3]+ ions with X ) Cl, Br, I),11,12 stabilizing weakly bound
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Anderson, O. P.; Grieco, P. A.; Strauss, S. H. Organometallics 1996,
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Trans. 2004, (16), 2496.
† Abteilung Anorganische Chemie, Institut fu¨r Chemie, Universita¨t
Rostock.
‡ Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock.
(1) Reviews: (a) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43,
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Chem. ReV. 1993, 93, 927, and references therein.
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Bardin, V. V.; Frohn, H. J. Main Group Met. Chem. 2002, 25, 589.
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9
8958 J. AM. CHEM. SOC. 2009, 131, 8958–8970
10.1021/ja902450b CCC: $40.75  2009 American Chemical Society

Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
Figure 1. Concept of composing weakly coordinating anions based on Lewis acid base adducts. (Example for n ) 3: tcm_3b ) tricyanomethanide attached
to three B(C6F5)3. Example for n ) 4: tcb_4b tetracyanoborate attached to four molecules of B(C6F5)3.)
complexes (e.g. [Ag(P4)2]+)13-15 and as counterions for cationic
Lewis acid catalysts in a wide variety of electrophilic chemis-
try.16
electrolytes, as reaction media, as components for solar and fuel
cells, or as lubricants,1,20-27 the BMIM (1-butyl-3-methylimi-
dazolium) and EMIM (1-ethyl-3-methylimidazolium) salts have
also been prepared and fully characterized.
Almost ten years ago Bochmann and co-workers introduced
an interesting simple approach to synthesize very weakly
nucleophilic anions by coordinating the strong Lewis acid
B(C6F5)3 to strongly basic bridging groups Z to give WCAs of
the type [Z{B(C6F5)3}n]x- (Z ) CN, Ni(CN)4, NH2; x ) 1, 2;
n ) 2, 4), a procedure also applied by workers at Dow.16c-e,17-19
This concept can be generalized as shown in Figure 1: A WCA
complex is formed (i) by attaching an element E to a Lewis
base linker, and (ii) by adding a strong bulky Lewis acid with
chemically robust functional groups. In such a Lewis acid/Lewis
base complex the negative charge is delocalized over a large
anion surface. Based on this synthetic concept, we want to report
here on novel donor-acceptor complexes (with CN linker)
containing anions, which can be regarded as very weakly
coordinating. The synthesis, structure and charge transfer are
discussed on the basis of experimentally observed and theoreti-
cally obtained data. Since ionic liquids are receiving ever-
increasing attention due to their wide potential to serve as
Discussion
According to Grimm’s hydride displacement law the hydrides
of the second row, BH4-, CH3 , NH2 and OH-, can be
-
-
,
regarded as formal pseudohalides.28-30 However, chemically
there is a significant difference in the basicity between these
pseudohalides and the halides, since halides represent only very
weak bases while such formal pseudohalides are strong bases.
-
Only the successive substitution of the hydrogen atoms in BH4 ,
-
-
CH3 and NH2 by electron-withdrawing groups such as CN
leads to the class of resonance-stabilized, nonlinear pseudoha-
lides.30 To further decrease the basicity into the range of the
halides especially in case of CH3 and NH2-, which possess
-
one lone pair localized in a p-type atomic orbital (AO), it is
also important that this electron-withdrawing group is capable
(20) Wasserscheid, P.; Welton, T. Ionic Liquids in synthesis; Wiley-VCH:
2003.
(21) Shobukawa, H.; Tokuda, H.; Susan, A. B. H.; Watanabe, M.
Electrochim. Acta 2005, 50, 3872.
(13) Krossing, I. J. Am. Chem. Soc. 2001, 123, 4603.
(14) Krossing, I.; van Wu¨llen, L. Chem. Eur. J. 2002, 8, 700.
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Eur. J. 2004, 10, 5041.
(22) Shobukawa, H.; Tokuda, H.; Tabata, S.-I.; Watanabe, M. Electrochim.
Acta 2004, 50, 1.
(23) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Usami, A.; Mita,
Y.; Watanabe, M.; Terada, N. Chem. Commun. 2006, 544.
(24) Wang, P.; Zakeeruddin, S. M.; Shaik, M.; Humphry-Baker, R.; Gra¨tzel,
M. Chem. Mater. 2004, 16, 2694.
(16) (a) Bochmann, M.; Jaggar, A. J.; Nicholls, J. C. Angew. Chem., Int.
Ed. Engl. 1990, 29, 780. (b) Yang, X.; Stern, C. L.; Marks, T. J.
Organometallics 1991, 10, 840. (c) Lancaster, S. J.; Walker, D. A.;
Thornton-Pett, M.; Bochmann, M. Chem. Commun. 1999, 1533. (d)
Zhou, J.; Lancaster, S.J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.;
Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223. (e) Hannant, M. H.;
Wright, J. A.; Lancaster, S. J.; Hughes, D. L.; Horton, P. N.;
Bochmann, M. Dalton Trans. 2006, 16.
(25) Wang, P.; Wenger, B.; Humphry-Baker, R.; Moser, J.-E.; Teuscher,
J.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Zakeeruddin, S. M.;
Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 6850.
(26) Mazille, F.; Fei, Z.; Kuang, D.; Zhao, D.; Zakeeruddin, S. M.; Gra¨tzel,
M.; Dyson, P. J. Inorg. Chem. 2006, 45, 1585.
(17) Lancaster, S. J.; Rodriguez Lara-Sanchez, A.; Hannant, M. D.; Walker,
D. A.; Hughes, D. L.; Bochmann, M. Organometallics 2002, 21, 451.
(18) LaPointe, R. E. WO99/42467, 1999.
(19) LaPointe, R. E.; Roof, G. R.; Abbboud, K. A.; Klosin, J. J. Am. Chem.
Soc. 2000, 122, 9560.
(27) Angell, C. A.; Xu, W. Science 2003, 302, 422.
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22.
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A R T I C L E S
Bernsdorf et al.
nor moisture sensitive. They dissolve in polar solvents such as
CH2Cl2, diethyl ether, or thf, but slowly decompose in water
under formation of the H2O·B(C6F5)3 adduct as shown by 19F
and 11B NMR studies.40-42 The astonishing stability against
water can be attributed mainly to kinetic hindrance by the
B(C6F5)3 groups. For comparison, it is known that for instance
in CH3CN·B(C6F5)3 the acetonitrile molecule can partially be
removed by water, which has been studied by means of
equilibrium titration.40 All mentioned tcb, tcm, and dca adduct
salts are easily prepared in bulk and are infinitely stable when
stored in a sealed tube in the dark (silver salts).
Salts of tcb_4b, tcm_3b, and dca_2b are thermally stable up
to over 200 °C (Table 1). The melting points of Ktcb_4b and
Ktcm_3b are significantly decreased upon adduct formation
although the formula weight increases considerably (∆Tmp(KX
- KX_nb, X ) tcm and tcb): 77.1 °C tcm, 131.1 °C tcb), while
for K[dca]/K[dca_2b] an increased melting point is found (∆Tmp
) -24.0°). Further decrease in the melting points is achieved
by exchanging the metal cations by EMIM and BMIM cations
with the smallest melting points always found for the BMIM
salts: 122.3° dca_2b, 161.6° tcm_3b and 147.9° tcb_4b (Table
1). Hence, neither of these salts can be regarded as a true ionic
liquid (mp < 100 °C).43
Figure 2. Delocalization of the p-type AO lone pair of the central atom
into π*(C-N) orbitals in dca and tcm. σ(B-C) f π*(C-N) hyperconju-
gation in tcb (only one interaction is shown for clarity).
of delocalizing the p-type AO lone pair e.g. into empty π*
orbitals which is the case in resonance-stabilized pseudohalides
such as dicyanoamide or tricyanomethanide (Figure 2). But also
the π*(CN) orbitals in B(CN)4- are slightly populated according
to NBO analysis31-34 due to a small σ(B-C) f π*(C-N)
hyperconjugative effect35 as displayed in Figure 2. Although
the basicity in B(CN)4- (tcb ) tetracyanoborate), C(CN)3- (tcm
-
) tricyanomethanide) and N(CN)2 (dca ) dicyanamide) is
strongly decreased by CN substitution, these anions cannot be
regarded to be weakly coordinating as shown on numerous
occasions due to the basic terminal nitrogen atom of the cyano
group.36-38 Hence, further delocalization of the anion charge
(besides the introduction of steric strain) is achieved by adding
a strong bulky Lewis acid such as B(C6F5)3 resulting in the
Spectroscopic Studies. The 11B and 13C data for a number of
known compounds and for compounds described in this work
are listed in Table 1. 11B and 13C NMR spectroscopy are
particularly well suited to distinguish between three-coordinate
borane and the four-coordinate boron found in the Lewis acid-
base adducts for which the 11B resonance (-11.8 dca_2b, -10.1
tcm_3b, and -8.5 ppm tcb_4b) is shifted to lower frequency
with respect to free B(C6F5)3 by more than 65 ppm (cf. B(C6F5)3
in CD2Cl2: 59.1 ppm).44-49 The data illustrate that when boron
is directly attached to nitrogen, the 11B{1H} resonance is sharp
and is observed in the range of -7 to -12 ppm. If the central
four-coordinate boron atom is attached to the carbon atoms of
CN group as in tcb_4b, the signal is found at -36.1 (cf. -38.6
in naked tcb) ppm. A shift of ca. 10 to 12 ppm for the CN
resonance is observed in the 13C spectra upon adduct formation
(120.6/108.0 dca/dca_2b; 122.1/111.6 tcm/tcm_3b; 121.9/111.2
ppm tcb/tcb_4b). The 19F NMR spectra of dca_2b, tcm_3b and
tcb_4b show resonances shifted to lower frequency compared
to B(C6F5)3 but similarly broad.
-
formation of the new WCAs such as B[(CN) ·B(C6F5)3]4
(tcb_4b) and C[(CN) ·B(C6F5)3]3- (tcm_3b). Salts of N[(CN) ·B
(C6F5)3]2 (dca_2b) have already been described.16e,39 This
-
approach can be generalized with respect to the Lewis base
linker as well as to the bulky Lewis acid (Figure 1).
Synthesis. Synthesis of M+B[(CN) ·B(C6F5)3]4-, M+C[(CN) ·B
(C6F5)3]3- and M+N[(CN)·B(C6F5)3]2- (M ) alkali metal, Ag,
NMe4, EMIM, BMIM) is easily achieved by adding the
corresponding MX salt (X ) tcb, tcm, and dca) to a solution of
B(C6F5)3 (slight excess) in diethyl ether. Within a short period
(ca. 30 min) the formation of a clear colorless solution can be
observed. Removal of the solvent followed by washing with
n-hexane (removal of excess B(C6F5)3) gives the corresponding
borane adduct salts in good yields. Recrystallization from diethyl
ether yields pure colorless crystals. Often solvent molecules are
embedded in the crystals which, however, can be removed
readily by thermal treatment (50-60 °C) of the crystalline
material over a period of 6-24 h. Furthermore, EMIM and
BMIM salts can also be prepared by salt metathesis reaction
starting from the silver salt.
The IR and Raman data of all considered B(C6F5)3 adduct
anions in Table 1 show sharp bands in the expected region
2290-2380 cm-1, which can be assigned to the νCN stretching
frequencies. As previously shown, the coordination of B(C6F5)3
to a NC-R species causes a significant band shift to higher
Properties. Alkali, ammonium, imidazolium, and silver salts
of M[X_nb] (X ) tcb, tcm, and dca; n ) 4, 3, 2) are neither air
(31) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO
Version 3.1.
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R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581.
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22, 1994.
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Soc., Dalton Trans. 1996, 3931.
(32) Carpenter, J. E.; Weinhold, F. J. Mol. Struct. (THEOCHEM) 1988,
169, 41.
(33) Weinhold, F.; Carpenter, J. E. The Structure of Small Molecules and
Ions; Plenum Press: 1988; p 227.
(34) Weinhold, F.; Landis, C. Valency and Bonding. A Natural Bond Orbital
Donor-Acceptor PerspectiVe; Cambridge University Press: 2005. and
references therein.
(43) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
(44) Vei, I. C.; Pascu, S. I.; Green, M. L. H.; Green, J. C.; Schilling, R. E.;
Anderson, G. D. W.; Rees, L. H. Dalton Trans. 2003, 2550.
(45) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.; Cafferkey,
S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun. 1998, 2529.
(46) Doerrer, L. H.; Galsworthy, J. R.; Green, M. L. H.; Leech, M. A.;
Mueller, M. J. Chem. Soc., Dalton Trans. 1998, 3191.
(47) Doerrer, L. H.; Galsworthy, J. R.; Green, M. L. H.; Leech, M. A.
J. Chem. Soc., Dalton Trans. 1998, 2483.
(35) Klapo¨tke, T. M.; Schulz, A. Quantum Chemical Methods in Main
Group Chemistry; with an invited chapter of R. D. Harcourt about
VB Theory; John Wiley & Sons: New York, 1998.
(36) Tcb salts; for example: Ku¨ppers, T; Bernhardt, E.; Willner, H.; Rohm,
H. W.; Ko¨ckerling, M. Inorg. Chem. 2005, 44, 1015.
(37) Tcm salts; for example: Zhang, L.-Y.; Shi, L.-X; Chern, Z.-N. Inorg.
Chem. 2003, 42, 633.
(38) Dca salts; for example: Costes, J.-P.; Novotchi, G.; Shova, S.; Dahan,
F.; Donnadiue, B.; Tuchagues, J.-P. Inorg. Chem. 2004, 43, 7792.
(39) The synthesis of the sodium and trityl salt has been reported. Crystal
data are only available of the trityl salt.
(48) Doerrer, L. H.; Graham, A. J.; Green, M. L. H. J. Chem. Soc., Dalton
Trans. 1998, 3941.
(49) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999,
4325.
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Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
Table 1. Thermal Analysisa and Spectroscopic Datab of dca_2b, tcm_3b and tcb_4b Based Salts along with the Data of the Adduct Free
Potassium Salts
K[dca]
K[dca_2b]
EMIM[dca_2b]
BMIM[dca_2b]
Ag[dca]
Ag[dca_2b]
formula wt in g/mol
mp in °C
105.14
1129.10
1201.17
1229.22
173.91
1197.87
onset
226.6
228.5
250.2
252.5
152.5
153.8
119.6
122.3
e
154.6
155.0
peak
dec point in °C
onset
280.5c
2212
252.5d
2365
2286
2369
210.0
2366
2290
2364
211.6
2367
2294
2374
251.7
2212
2183
2249
155.0d
2365
2291
2363
Raman νCN in cm-1
IR νCN in cm-1
NMR δ in ppm
11B-C6F5
-11.8
108.0
-11.8
108.0
-11.8
108.0
f
-12.0
107.5
13CN
120.6
K[tcm]
K[tcm_3b]
EMIM[tcm_3b]
BMIM[tcm_3b]
Ag[tcm]
Ag[tcm_3b]
formula wt in g/mol
mp in °C
129.16
1665.10
1737.17
1765.22
197.93
1733.87
onset
295.7
298.0
219.8
220.9
192.5
197.4
160.5
161.6
e
e
peak
dec point in °C
onset
387.9
2225
2173
2171
264.9
2353
2297
2292
275.9
2349
2297
2290
276.9
2346
2295
2289
394.8
2228
2170
2174
162.2
h
Raman ν CN in cm-1
IRνCN in cm-1
NMR δ in ppm
11B-C6F5
2290
-10.1
14.5
111.6
-10.3
14.5
111.6
-10.1
14.5
111.6
f
-10.1
14.5
111.6
13C-CN
6.4
122.1
13CN
K[tcb]
K[tcb_4b]
EMIM[tcb_4b]
BMIM[tcb_4b]
Ag[tcb]
Ag[tcb_4b]
formula wt in g/mol
mp in °C
153.98
2201.90
2273.96
2302.02
222.75
2270.67
onset
410.4
413.0
e
174.5
176.2
140.1
147.9
e
180.5
184.2
peak
dec point in °C
onset
554.3
2234
281.9d
2323
240.5
2330
230.0
2329
462.5
2232
243.0
h
Raman νCN in cm-1
NMR δ in ppm
g
11B-C6F5
-8.5
-36.1
111.2
-7.8
-36.1
110.8
-7.6
-36.1
110.7
f
-8.5
-36.1
111.2
11B-CN
13CN
-38.6
121.9
a Melting and decomposition points. b IR, Raman, and NMR. c Trimerization.51 d Decomposition begins with the melting. e Expected mp above
decomposition point. f Insoluble in standard organic solvents. g B(C6F5)3 11B NMR (CD2Cl2, 96 MHz, 25 °C): δ ) 59.1(s). 19F NMR (CD2Cl2, 282 MHz,
3
25 °C): δ ) -128.4 to -128.5 (m, o-F, 2F); -144.2 (t, p-F, 1F, JFF ) 20.5 Hz); -161.2 to -161.4 (m, m-F, 2F). 13C NMR (CD2Cl2, 75 MHz, 25 °C):
1
1
1
δ ) 148.7 (dm, o-C, 6C, JCF ) 249.7 Hz); 141.8 (dm, p-C, 3C, JCF ) 289.0 Hz); 138.0 (dm, m-C, 6C, JCF ) 256.2 Hz); 113.5 (br, C-B, 3C).
h Raman laser induced decomposition.
wave numbers (∆ν for potassium salts: 153 dca, 128 tcm, 89
tcb cm-1).50 Hence, both IR and Raman spectroscopy are also
particularly well suited to distinguish between B(C6F5)3 adduct
anions and the naked anions. The shift to higher wave numbers
upon adduct formation correlates nicely with a smaller C-N
distance (Table 6, see section on bonding and charge transfer)
and decreases along the series dca > tcm > tcb (Table 6).
Influence of Stoichiometry. Depending on the stoichiometry,
different adduct anions are formed with less than the maximum
number of B(C6F5)3 attached to the naked anion. In the case of
tcb salts we have studied the influence in more detail. When a
large excess of B(C6F5)3 is used, the final product is always
tcb_4b, which has been synthesized with a number of different
cations. However, when an excess of tcb salt was used, we were
able to observe the di- and triadducts. For instance, it was
possible to isolate the potassium salt with only two B(C6F5)3
groups attached to the tcb anion (K[tcb_2b], Figure 3, Table
S1 in the Supporting Information), and the 1-ethyl-3-methylimi-
dazolium salt with three B(C6F5)3 groups attached to the tcb
anion (EMIM[tcb_3b], Figure 4, Table S1 in the Supporting
Information). Both anions have been characterized by X-ray
crystallography, however, only poor data sets were obtained,
which do not allow a detailed discussion of structural parameters
but are good enough to prove unambiguously the existence of
both anions.
X-ray crystallography. The structures of M[dca_2b] (M )
Li, K, Ag), [K(18-crown-6)][tcm_3b] and M[tcb_4b] (M )
K, NEt4) have been determined. Tables 2 and 3 present the
X-ray crystallographic data. Selected molecular parameters
are listed in Table 4. The molecular structures of the three
adduct anions are depicted as superposition of ball-and-stick
and space-filling models in Figures 5, 6, and 7. ORTEP
(50) Jacobsen, H.; Berke, H.; Doering, S.; Kehr, G.; Erker, G.; Froehlich,
R.; Meyer, O. Organometallics 1999, 18, 1724.
(51) Irran, E.; Juergens, B.; Schnick, W Chem. Eur. J. 2001, 7, 5372.
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 8961

A R T I C L E S
Bernsdorf et al.
and 114.3(2)°, respectively, in CH3CN·B(C6F5)350). In the
crystal, the (NC)3B-C-N[B(C6F5)3] and the [(NC)3B]C-
N-B(C6F5)3 axes are close to linear, with average bond angles
of 176.3(2) and 176.4(2)°, respectively.
(2) [NEt4][tcb_4b]·1.5Et2O. [NEt4][tcb_4b]·1.5Et2O crystal-
j
lizes in the triclinic space group P1 with two formula units per
cell. The structure consists of separated ions with no significant
cation· · ·anion contacts. Only (ether)C-H· · ·F-C(aryl) interac-
tions are found. The crystallographic asymmetric unit contains
one ion pair [NEt4][tcb_4b] and 1.5 diethyl ether molecules.
Since there are no significant differences in the molecular
structure of the tcb_4b anion compared to that in the potassium
salt, we refer to the discussion above (see also Table 4).
(3) K[tcm_3b]. Since we did not succeed in obtaining single
crystals of K[tcm_3b], K[tcm_3b] was recrystallized in the
presence of 18-crown-6 ether in diethyl ether. [K(18-crown-
Figure 3. Molecular structure of the tcb_2b anion attached to two K+
centers in [K+][tcb_2b]. Color code: boron, red; carbon, dark gray; nitrogen,
blue; fluorine, light green; potassium, violet.
j
6)][tcm_3b] crystallizes in the triclinic space group P1 with four
formula units per cell. The asymmetric unit consists of two
independent [K(18-crown-6)]+ and tcm_3b units, as well as 0.46
diethyl ether molecules. No significant cation · · ·anion contacts
are found. A view along the a axis reveals stacked chains of
[K(18-crown-6)]+ and tcm_3b ions (Figure 8). Voids are
partially filled with diethyl ether molecules (0.92 ether molecule
per unit cell).
As depicted in Figure 6, the C(CN)3 core of the tcm_3b anion
is nearly trigonal planar (∑∠(Ccentral) ) 359.9(2)°). As already
discussed before, the (NC)3C-C-N[B(C6F5)3] and the
[(NC)3C]C-N-B(C6F5)3 axes are close to linear, with average
bond angles of 177.8(3) and 175.8(3)°, respectively. The boron
coordination geometry is slightly distorted tetrahedral with
average C-B-C bonding angles of 109.5°. The average length
of the B-CN and B-C(aryl) linkages amounts to 1.593(2) and
1.632 Å, respectively (cf. 1.595(1) in Ktcb,36 1.629 Å in
CH3CN·B(C6F5)350). The C-N bonds range between 1.130(3)
and 1.141(3) Å (average 1.134(4) Å), and an average C-C bond
length of 1.399(4) Å is found. Interestingly, the tricya-
nomethanide anion in K[tcm] has also approximate D3h sym-
metry with C-C bond distances averaging 1.39(2) Å and C-N
distances averaging 1.17(1) Å.52 The central C atom is 0.026
Å above the plane of the three N atoms.
Figure 4. Molecular structure of the tcb_3b anion in [EMIM][tcb_3b].
Color code: boron, red; carbon, dark gray; nitrogen, blue; fluorine, light
green.
representations of the asymmetric unit are shown in Figures
S1-S5 in the Supporting Information.
(1) K[tcb_4b]. K[tcb_4b] crystallizes in the triclinic space
j
group P1 with two formula units per cell. The structure consists
of separated K+(diethyl ether)4 and tcb_4b units with no
significant cation · · ·anion contacts. Only (ether)C-H· · ·F-C(aryl)
interactions are found. The crystallographic asymmetric unit
contains one ion pair [K+(diethyl ether)4][tcb_4b]. The K+ ion
is bonded to each oxygen atom of the four diethyl ether
molecules forming a tetrahedral coordination environment
around the cation with K-O distances between 2.659(2) and
2.708(2) Å. Two of the diethyl ether molecules are disordered.
The tcb anion (Figure 5) and the four B(C6F5)3 groups are
connected by means of strong B-N bonds (1.600(2), 1.601(2),
1.622(2) and 1.604(2) Å; cf. 1.616(3) Å in CH3CN·B(C6F5)3).50
The central boron atom of the B(CN)4 core and the boron atoms
of the four B(C6F5)3 are tetracoordinated. The average length
of the B-CN and B-C(aryl) linkages amounts to 1.593(2), and
1.632 Å, respectively (cf. 1.595(1) in Ktcb,36 1.629 Å in
CH3CN·B(C6F5)350). Compared to the naked tcb anion slightly
shorter C-N bonds ranging between 1.133(2) and 1.138(2) Å
(cf. 1.142(1) Å in Ktcb) are observed. The coordination
geometry around boron in the B(CN)4 core is slightly distorted
with the smallest angle of 107.9(1), and the largest 111.6(1)°.
The same holds true for the boron center of the B(C6F5)3 groups
with average N-B-C(aryl) bonding angles of 105.2(1)° and
average C(aryl)-B-C(aryl) angles of 113.2(1)° (cf. 104.0(2)
Strong B-N donor-acceptor bonds (1.582(4), 1.602(4),
1.563(4) Å; cf. 1.616(3) Å in CH3CN·B(C6F5)350) between the
tcm anion (Figure 6) and the three B(C6F5)3 groups are found
with an average distance of 1.582(4) Å being slightly shorter
than that in tcb_4b with 1.607(2) Å.
(4) K[dca_2b]. K[dca_2b] crystallizes in the monoclinic space
group P21/c with four formula units per cell. The asymmetric
unit consists of one K[dca_2b] unit with significant cation · · ·anion
contacts (Figure 9). A close inspection of the anion · · ·cation
interactions revealed ten K · · ·F(aryl) interactions (d(K-F) < 4
Å) with seven different C6F5 moieties (Figure 10). The shortest
K· · ·F contact is found at 2.670(3) Å (to F3). Among these ten
contacts are four bidentate and two monodentate coordination
modes.
(5) K[dca_2b]. K[dca_2b] contains the bent dca_2b anion of
approximate symmetry C2V (average bond lengths, C-Nbridge
1.294(6), C-Nterm 1.136(6) Å; average angles, N-C-N 174.9(5)°,
C-N-C 118.8(4)°; cf. [Ph3C]+ [dca_2b]-, C-Nbridge 1.282(13),
C-Nterm 1.145(12) Å; average angles N-C-N 172.1(10)°,
C-N-C 119.8(9)°;16e K[dca] average, C-Nbridge 1.33, C-Nterm
1.13, N-C-N 170°, C-N-C 120°).51 The average B-N
(52) Witt, J. R. Acta Crystallogr. 1971, B27, 1835.
9
8962 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
Table 2. Crystallographic Details of K[dca_2b], [Ag(Et2O)3][dca_2b] and [Li(thf)4][dca_2b]
K[dca_2b]
[Ag(Et2O)3][dca_2b]
[Li(thf)4][dca_2b]
chem formula
formula wt [g mol-1
color
cryst system
space group
a [Å]
C38B2F30KN3
1129.13
colorless
monoclinic
P21/c
9.579(2)
20.267(4)
19.729(4)
90.00
101.78(3)
90.00
3749(1)
4
C50H30AgB2F30N3O3
1420.26
colorless
C54H32B2F30LiN3O4
1385.39
colorless
]
triclinic
triclinic
j
j
P1
P1
9.313(2)
12.772(3)
23.118(5)
78.87(3)
83.65(3)
88.61(3)
2681.4(9)
2
1.759
0.527
0.71073
173(2)
61702
9.253(2)
13.331(3)
23.851(5)
83.84(3)
82.06(3)
79.71(3)
2856(1)
2
b [Å]
c [Å]
R [deg]
ꢀ [deg]
γ [deg]
V [Å3]
Z
Fcalc [g cm-3
]
2.000
0.331
0.71073
200(2)
11245
1.611
0.167
µ [mm-1
]
λMoKR [Å]
T [K]
0.71073
173(2)
65491
12967
8297
0.0448
1384
0.0440
0.1223
1.070
measd reflections
indep reflections
reflections with I > 2σ(I)
Rint
5944
3012
15577
13962
0.0771
2192
0.0513
0.1131
0.927
0.0214
1404
0.0310
0.0853
1.053
F(000)
R1 (R [F2 > 2σ(F2)])
wR2 (F2)
GOF
params
667
808
857
Table 3. Crystallographic Details of [K(18-crown-6)][tcm_3b] ·0.46Et2O, [K(Et2O)4][tcb_4b] and [N(Ete)4][tcb_4b]·1.5Et2O
[K(18-crown-6)][tcm_3b]·0.46Et2O
[K(Et2O)4][tcb_4b]
[N(Et)4][tcb_4b]·1.5Et2O
chem formula
formula wt [g mol-1
color
C71.82H28.38B3F45KN3O6.46
1963.11
colorless
C92H40B5F60KN4O4
2498.43
colorless
C90H35B5F60N5O1.50
2404.28
colorless
]
cryst system
space group
a [Å]
b [Å]
c [Å]
triclinic
P1
triclinic
P1
triclinic
P1
j
j
j
18.433(4)
19.776(4)
24.987(5)
68.15(3)
81.19(3)
71.50(3)
8011(3)
4
1.628
0.225
0.71073
173(2)
114777
28083
17.9158(5)
18.8215(6)
21.0662(6)
111.374(2)
111.849(2)
112.211(2)
4938.0(3)
2
1.680
0.221
0.71073
173(2)
129717
28210
14.916(8)
18.504(9)
20.187(9)
110.39(1)
105.51(1)
106.62(2)
4567(4)
2
1.748
0.189
0.71073
173(2)
34742
R [deg]
ꢀ [deg]
γ [deg]
V [Å3]
Z
Fcalc [g cm-3
]
µ [mm-1
]
λMoKR [Å]
T [K]
measd reflections
indep reflections
9855
reflections with I > 2σ(I)
18592
0.0252
3884
0.0573
0.1808
1.099
17868
0.0275
2472
0.0464
0.1292
1.051
7390
0.0394
2374
0.0334
0.0801
1.029
Rint
F(000)
R1 (R [F2 > 2σ(F2)])
wR2 (F2)
GOF
params
2629
1581
1550
donor-acceptor bond length is 1.560(6) Å, which is slightly
longer than that found in the Ph3C+ salt with 1.522 Å.16e
(6) [Li(thf)4][dca_2b]. [Li(thf)4][dca_2b] crystallizes in the
The silver cation is coordinated by three oxygen atoms of the
diethyl ether molecules at significantly different distances
(2.266(1), 2.274(1) and 2.341(1) Å, Figure 11). The local
coordination environment, however, is almost planar with
O-Ag-O angles of 105.30(4), 117.00(4) and 137.45(5)° (angle
sum 359.75°). These fairly different Ag-O structural parameters
can be attributed to steric repulsion between the ethyl groups
(Figures 11 and S4 in the Supporting Information). In agreement
with our experimental observations, theoretical results by Feller
and Dixon obtained for the gas phase species [Ag(DME)n]+ (n
) 1-4, DME ) dimethyl ether) show the same picture: For
Ag+, there is essentially no difference between the first and
j
triclinic space group P1 with two formula units per cell. The
asymmetric unit consists of one [Li(thf)4][dca_2b] unit with four
significant cation solvent interactions. The lithium cation has a
tetrahedral coordination environment formed by the four thf
molecules at 1.906 Å (average). Compared to the potassium
salt, similar structural features are found for the dca_2b anion
(Table 4).
(7) [Ag(Et2O)3][dca_2b]. [Ag(Et2O)3][dca_2b] also crystallizes
j
in the triclinic space group P1 with two formula units per cell.
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 8963

A R T I C L E S
Bernsdorf et al.
Table 4. Selected Structural Data of dca_2b, tcm_3b, and tcb_4b Saltsa
K[dca_2b]
[Ag(Et2O)3][dca_2b]
[Li(thf)4][dca_2b]
[K(18-crown-6)][tcm_3b]
[K(Et2O)x][tcb_4b]
[NEt4][tcb_4b]
d(C-N)
1.135(6)
1.136(6)
1.142(2)
1.145(2)
1.142(2)
1.144(2)
1.133(3)
1.139(4)
1.142(3)
1.133(2)
1.135(2)
1.137(2)
1.138(2)
1.590(2)
1.593(2)
1.594(2)
1.596(2)
1.600(2)
1.601(2)
1.604(2)
1.622(2)
173.8(1)
176.3(2)
177.0(2)
178.0(1)
173.1(1)
173.8(1)
179.0(1)
179.1(1)
1.076(3)
1.087(3)
1.103(3)
1.126(3)
1.591(5)
1.594(5)
1.602(5)
1.609(5)
1.604(4)
1.613(4)
1.627(4)
1.629(4)
169.1(3)
175.0(3)
175.1(3)
175.4(3)
173.0(3)
174.0(3)
175.6(3)
176.9(3)
d(E-CN)b
d(N-B)
1.290(6)
1.298(6)
1.285(2)
1.290(2)
1.286(2)
1.288(2)
1.389(4)
1.397(4)
1.400(4)
1.544(6)
1.575(7)
1.556(2)
1.564(2)
1.566(2)
1.571(2)
1.572(4)
1.591(4)
1.598(4)
∠(E-C-N)b
∠(C-N-B)
174.8(5)
175.0(5)
172.5(1)
174.3(1)
172.3(2)
173.1(2)
174.9(3)
176.9(3)
178.4(3)
164.4(5)
175.6(4)
169.6(1)
172.1(1)
171.1(2)
173.1(2)
174.9(3)
175.9(3)
175.9(3)
a Distances in Å, angles in deg. b E ) N dca, C tcm, and B tcb.
∆H0(MP2) ) 16.2 kcal/mol) is considerably smaller than that
of the first or second. Thus two small and one long Ag-O
distances (2.192 Vs 2.284 Å) have been calculated.53 It can be
assumed that the significant asymmetry in the coordination
sphere around the silver cation indicates already the transition
from tricoordination to dicoordination which is the usual
coordination mode of solvents found for silver salts such as
[Ag(NH3)2]+. The synthesis of [Ag(Et2O)3][BF4] has already
been reported but without any structural data,54 and in agreement
with our observation this salt loses the ether molecules at room
temperature. The WCA dca_2b seems to stabilize this unusual
tricoordination. Again, the structural parameters of the dca_2b
anion are very similar to those found in the potassium salt
(Table 4).
Anion Size and Lattice Enthalpies. The ion volumes of dca,
tcm and tcb and their adduct anions have been estimated from
the average atomic volumes simply by summing the volume
contributions from each atom in the molecular formula as
introduced by Hofmann (Table 5).55 It should be noted that
Hofmann’s approach is not always the best source of volumes.
For instance the anion and cation volumes are equal since no
charge considerations are made. Jenkins and Liebman’s isome-
gethic rule can be considered as the theoretical basis of
Hofmann’s approach. In addition, the isomegethic rule offers
another way to ion volumes.56
Figure 5. Molecular structure of the tcb_4b anion in [K(Et2O)4][tcb_4b]
as superposition of ball-and-stick and space-filling models. Color code:
boron, red; carbon, dark gray; nitrogen, blue; fluorine, light green.
As expected the anion volumes increase along the series dca
(63) < tcm (91) < tcb (116) and dca_2b (924) < tcm_3b (1382)
< tcb_4b (1838 Å3). Compared to the naked anions dca, tcm
Figure 6. Molecular structure of the tcm_3b anion in [K(18-crown-
6)][tcm_3b] as superposition of ball-and-stick and space-filling models.
Color code: boron, red; carbon, dark gray; nitrogen, blue; fluorine, light
green.
second binding energies ([Ag(DME)]+:53 ∆H0(MP2) ) 36.6
kcal/mol; [Ag(DME)2]+: ∆H0(MP2) ) 36.7 kcal/mol). The
binding energy of the third DME ligand ([Ag(DME)3]+:
Figure 7. Molecular structure of the dca_2b anion in K[dca_2b] as
superposition of ball-and-stick and space-filling models. Color code: boron,
red; carbon, dark gray; nitrogen, blue; fluorine, light green.
9
8964 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
Figure 10. K· · ·F contacts in K[dca_2b] (shown for d(K· · ·F) < 4.0 Å):
F3 2.670(3); F19 2.736(3); F20 2.835(3); F25 2.890(3); F10 2.955(3); F7
2.999(3); F16 3.097(3); F24 3.154(4); F6 3.290(3). F22 3.569(3). Carbon
atoms attached to the coordinating F atoms are also displayed but not
assigned for clarity reasons.
used to calculate the ion volume of the dca_2b according to eq
1 for an A+X- salt.57 With a cell volume of 3749.36 Å3, Z )
4 and V(K+) ) 14.4 Å3,58,59 an anion volume V(dca_2b) ) 922
Å3 is calculated (V(dca_2b) ≈ V([K(dca_2b)] - V(K+)), in
accord with the volume derived by Hofmann’s approach (cf.
924 Å3, Table 5).
Figure 8. Stacking diagram of [K(18-crown-6)][tcm_3b] ·0.46 Et2O (view
along 100). Color code: boron, red; carbon, dark gray; nitrogen, blue;
oxygen, brown; potassium, violet. Fluorine and hydrogen atoms are omitted
for clarity.
Vion(X-) ) [Vcell(A+X-)/Z]-Vion(A+)
(1)
Principles of volume-based thermodynamics (VBT), intro-
duced by Jenkins, Glasser and Passmore, have been used to
estimate the lattice potential energy (Upot), lattice enthalpy (∆HL)
and the standard entropy (S°298 in J mol-1 K-1 at 298.15 K and
101 kPa) of the new A+X- salts (A ) potassium, X ) dca_2b,
tcm_3b and tcb_4b) from its molecular volume (Table 5).58,59c
As expected the smallest lattice potential energy was found for
K[tcb_4b] with 299.6 kJ mol-1 followed by 318.3 for K[tcm_3b]
and 343.4 kJ mol-1 for K[dca_2b]. Compared to potassium salts
with the naked anions the lattice potential energy decreased by
311 (dca_2b), 287 (tcm_3b), and 272 kJ mol-1 (tcb_4b). The
high solubility of salts with these adduct anions in common
polar organic solvents such as CH2Cl2 or thf (in contrast to salts
with the naked anion) can directly be attributed to these
decreased lattice potential energies.1a For stability estimations
it might be of interest to know the standard entropy of B(C6F5)3
which can be derived from the volume 430.5 Å3 and amounts
Figure 9. ORTEP drawing of the molecular structure of K[dca_2b] in the
crystal. Thermal ellipsoids with 50% probability at 173 K. Color code:
boron, red; carbon, dark gray; nitrogen, blue; fluorine, light green; potassium,
violet.
to 601 J K-1 mol-1
.
Computations: Energetics of Adduct Formation, Structural
Trends, and Charge Transfer. Adduct anions such as dca_2b,
tcm_3b and tcb_4b are typical charge transfer complexes, and
the bond between the B(C6F5)3 and the naked anions can be
regarded as a donor-acceptor bond.62 The gas phase energies
for mono-, di-, tri- and tetraadduct formation according to eq 2
have been computed at the B3LYP/6-31+G(d) level of theory
(Table 6):
and tcb the volume of the B(C6F5)3 adduct anions dramatically
increases with the largest increase found for tcb_4b (cf. ∆Vion
) Vadduct-ion - Vion ) 861 dca_2b/dca, 1292 tcm_3b/tcm and
1722 tcb_4b/tcb). Each B(C6F5)3 group contributes to the ion
volume by about 430.5 Å3.
Since the potassium salt Kdca_2b crystallized without solvent
molecules, the experimentally determined cell volume could be
(53) Feller, D.; Dixon, D. J. Phys. Chem. A 2002, 106, 5136.
(54) Meerwein, H.; Hederich, V.; Wunderlich, K. Ber. Dtsch. Pharm. Ges.
1958, 291, 541.
E(CN) -] · [B(C6F5)3]m(g) + B(C6F5)3(g)
f
[
n
[E(CN)n-] · [B(C6F5)3]m+1(g) (2)
(55) Hofmann, D. W. M. Acta Crystallogr. 2002, B57, 489.
(56) (a) Jenkins, H. D. B.; Glasser, L; Klapo¨tke, T. M.; Crawford, M.-J.;
Bhasin, K. K.; Lee, J.; Schrobilgen, G. J.; Sunderlin, L. S.; Liebman,
J. F. Inorg. Chem. 2004, 43, 6238. (b) Jenkins, H. D. B.; Liebman,
J. F. Inorg. Chem. 2005, 44, 6359.
where E ) N, n ) 2, m ) 0, 1, 2; E ) C, n ) 3, m ) 0, 1, 2,
3; and E ) B, n ) 4, m ) 0, 1, 2, 3, 4.
(57) (a) Glasser, L.; Jenkins, H. D. B. Chem. Soc. ReV. 2005, 34, 866. (b)
Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg.
Chem. 1999, 38, 3609. (c) Jenkins, H. D. B.; Liebman, J. F. Inorg.
Chem. 2005, 44, 6359.
All adduct formation steps represent exothermic reactions.
The energy release upon adduct formation is always largest
when the first B(C6F5)3 group is attached and decreases along
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 8965

A R T I C L E S
Bernsdorf et al.
Table 5. Ionic Volumes, Atomic Distances for Ions Discussed in This Study and Thermodynamic Data Derived from Volume-Based
Thermodynamics (VBT)57,59c for the Potassium Salts
dca
tcm
tcb
dca_2b
tcm_3b
tcb_4b
Vion in Å3
63
91
116
924
1382
1838
(87)e
(113)e
(137)e
(901)e
(1359)e
16.007(6)
313.7
(1766)e
16.411(3)
294.8
dld(F· · ·F)a in Å
15.607(5)
343.4
1291.2
348.4
b
Upot(K+X-) in kJ mol-1
654.3
120.3
659.3
(656.3)
600.5
158.3
605.5
(602.8)
566.43
192.34
571.4
c
S°298(K+X-,s) in J K-1 mol-1
1914.1
318.7
(318.3)
2534.3
299.8
(299.6)
d
∆HL(K+X-) in kJ mol-1
(569.08)
(347.8)
b
a dld ) largest distance between two fluorine atoms in the adduct anion. Upot ) 2{[117.3/(Vm1/3)] + 51.9 kJ mol-1}. c S°298 ) 1360Vm + 15 J K-1
mol-1 d The lattice enthalpy can also be calculated by means of Bartlett’s relationship (value in parentheses):60 ∆HL ) [232.8/(Vm1/3)] + 110 kJ mol-1
. .
e Values in parentheses correspond to scaled Hoffmann volumes. Markedly, Hoffmann’s method does not take charge into account; however, cations of
the same summary formula are always smaller than anions. Krossing et al. found that VHoffmann correlates linearly with the experimental cation volumes
according to Vexp ) 0.964VHoffmann - 7 Å3 and for the anion according to 0.946VHoffmann + 27 Å3.61
the series -198.5 dca_1b, -150.3 tcm_1b and -110.6 tcb_1b
kJ/mol. For double (dca_2b, tcm_2b and tcb_2b) and triple
adduct formation (tcm_3b and tcb_3b) a significantly smaller
energy release is found (Table 6, values in parentheses), but
even when the fourth B(C6F5)3 group is added to the last free
cyano group in tcb_3b a significant energy gain of -36.2 kJ/
mol is estimated. It should be noted that B3LYP computations
give complexation energies which are intrinsically too small
due to an improper treatment of dispersion energy.63 This means
that these B3LYP values only represent lower limits. For the
anion tcb_4b, there is a chance that, upon introduction of this
anion, the strong Lewis acid B(C6F6)3 may concomitantly be
Figure 11. Coordination sphere around the Ag+ ion in [Ag(Et2O)3][dca_2b].
Distances in Å, angles in deg: Ag-O1 2.266(1), Ag-O2 2.341(1), Ag-O3
2.274(1); O1-Ag-O2 105.30(4), O1-Ag-O3 137.45(5), O2-Ag-O3
117.00(4).
introduced as a result of the weaker complexation enthalpy
tcb_4b f tcb_3b + B(C6F5)3 (36.2 kJ/mol). Hence in more basic
conditions, e.g. especially with bases that are preferable for
B(C6F5)3, tcb_4b may also serve as a source of B(C6F5)3.
(1) Structure. In agreement with the experimental data (X-
ray), the E-C-N moiety remains essentially linear (E ) N, C,
B in dca, tcm and tcb), and the local symmetry of the dca core
(C2V) in dca_2b, tcm core (D3h) in tcm_3b and tcb core (Td) in
tcb_4b is also nearly maintained. The donor-acceptor
CN-B(C6F5)3 bond increases along dca_2b < tcm_3b < tcb_4b
(1.558, 1.581, and 1.596 Å respectively), which is in accord
with experimental data (Table 4). With increasing number of
B(C6F5)3 molecules attached, the N-B bond lengths slightly
increase (for instance tcb_1b 1.576, tcb_2b 1.585, tcb_3b 1.591
and tcb_4b: 1.596 Å), while the C-N bond lengths slightly
decrease (for instance duncoordinated/dcoordinated: tcb 1.164/-, tcb_1b
1.163/1.153, tcb_2b 1.162/1.151, tcb_3b 1.161/1.150 and tcb_4b
-/1.149 Å). Thus successive coordination of the cyano groups
to the Lewis acid B(C6F5)3 always leads to a measurable
strengthening of the cyano triple bonds as displayed by short
C-N distance and a considerable shift of the νCN to higher wave
numbers (Table 1). Although the reduction of the C-N bond
length along the series dca > dca_1b > dca_2b, tcm > tcm_1b
> tcm_2b > tcm_3b, and tcb > tcb_1b > tcb_2b > tcb_3b >
tcb_4b, respectively, may seem small, it is clearly noticeable.
This bond contraction ∆dCN (anion - anion_adduct) is compat-
ible for experiment and theory and is about 0.01 Å. The origin
of this contraction has been studied by several groups indicating
that electrostatic interactions dominate the bonding in such
donor-acceptor complexes.50 However, orbital interactions
provide additional stabilization (σ donation and π back dona-
tion).50 The orbital mainly responsible for the σ donation is
assumed to possess some C-N antibonding character; hence
upon adduct formation electron density is removed from this
orbital and consequently strengthen and shorten the bond.
(2) Charge Transfer. According to NBO analysis (NBO )
natural orbital analysis),31 the charge transfer is between 0.33
and 0.42 e per B(C6F5)3 group attached to dca, tcm or tcb (Table
6) and therefore increases almost linearly with an increasing
number of B(C6F5)3 groups attached to the anions. Interestingly,
for tcm_3b (1.22 e), tcb_3b (1.18 e) and tcb_4b (1.51 e) more
than one electron (total charge transfer ∑QCT) is transferred to
the attached B(C6F5)3 groups, that is the tcm and the tcb cores
of these anions are now positively charged in contrast to the
singly negatively charged naked anions.
(58) Radii for K+ are found in the range 1.33 (Goldschmidt) to 1.51 Å
(Shannon) resulting in V(K+) ) 9.85 (Goldschmidt) and 14.4 Å3
(Shannon). Recently, Jenkins and Glasser published internally con-
sistent ion volumes, which is 27.7 Å3 for K+.
(59) (a) Goldschmidt, V. M. Chem. Ber. 1927, 60, 1263. (b) Shannon, R. D.
Acta Crystallogr. 1976, A32, 751. (c) Glasser, L.; Jenkins, H. D. B.
Inorg. Chem. 2008, 47, 6195.
(60) Mallouk, T. E.; Rosenthal, G. L.; Muller, G.; Busasco, R.; Barlett, N.
Inorg. Chem. 1984, 23, 3167.
(61) Preiss, U.; Slattery, J. M.; Krossing, I. Ind. Eng. Chem. Res. 2009,
48, 2290.
(62) (a) It is known that for adducts in the solid state and gas phase,
structural data and donor-acceptor energies can be quite different.
Leopold and co-workers have indicated that the donor-acceptor bond
is much shorter in the solid state than in the gas phase, and this change
has been associated with the substantial dipole moment of the
adduct. (b) Fiacco, D. L; Mo, Y.; Hunt, S. W.; Ott, M. E.; Roberts,
A.; Leopold, K. R. J. Phys. Chem. A 2001, 105, 484, and references
therein.
According to this fairly large charge transfer, the anion charge
is strongly delocalized over 73 (dca_2b), 109 (tcm_3b) and 145
atoms (tcb_4b) and hence such voluminous anion adducts can
be considered weakly coordinating anions.
Summary. In summary the formation of donor-acceptor
adducts of tris(pentafluorophenyl)borane with strongly coordi-
(63) Kristya´n, S.; Pulay, P. Chem. Phys. Lett. 1994, 229, 175.
9
8966 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
Table 6. Computed Structural Parameters, Reaction Energies and Total Charge Transfer
product/species
abbreviation
d(Ct N) in Å
d(N-B) in Å
∆E(eq1-4) in kJ/mola
∑Qct in eb
-
[N(CN)2
]
dca
1.181
1.174
1.164
1.173
1.167
1.160
1.156
1.164
1.153
1.151
1.150
1.149
[N(CN)2-]·[B(C6F5)3]1
[N(CN)2--]·[B(C6F5)3]2
dca_1b
dca_2b
tcm
tcm_1b
tcm_2b
tcm_3b
tcb
tcb_1b
tcb_2b
tcb_3b
tcb_4b
1.535
1.558
-198.5
-323.2 (-124.7)
0.50
0.92(0.42)
[C(CN)3
]
[C(CN)3-]·[B(C6F5)3]1
[C(CN)3-]·[B(C6F5)3]2
[C(CN)3--]·[B(C6F5)3]2
1.551
1.570
1.581
-150.3
-247.1 (-96.8)
-319.6 (-72.5)
0.47
0.86 (0.39)
1.22 (0.36)
[B(CN)4
]
[B(CN)4-]·[B(C6F5)3]1
[B(CN)4-]·[B(C6F5)3]2
[B(CN)4-]·[B(C6F5)3]3
[B(CN)4-]·[B(C6F5)3]4
1.576
1.585
1.591
1.596
-110.6
0.43
-191.2 (-80.6)
-252.6 (-61.4)
-288.8 (-36.2)
0.83 (0.39)
1.18 (0.36)
1.51(0.33)
a Values in parentheses correspond to the reaction eq(m) - eq(m - 1) of the mth step. b Values in parentheses correspond to ∑Qct(m) - ∑Qct(m - 1)
) qct of the mth step.
nating anions such as dicyanoamide, tricyanomethanide and
tetracyanoborate is a synthetically facile route to the bulky very
weakly coordinating anions [N(CN ·B(C6F5)3)2]- (dca_2b),
[C(CN·B(C6F5)3)3]- (tcm_3b), and [B(CN ·B(C6F5)3)4]- (tcb_4b),
which were isolated as stable alkali and silver salts. EMIM and
BMIM salts have also been prepared but with melting points
between 120 °C and 200 °C, such that these salts cannot be
considered as true ionic liquids.43 All studied salts with adduct
anions of the type dca_2b, tcm_3b and tcb_4b are thermally
stable up to over 200 °C.
As expected the volumes of the these adduct anions increase
along the series dca_2b (924 Å3) < tcm_3b (1382 Å3) < tcb_4b
(1838 Å3) with a contribution of each B(C6F5)3 group of about
430.5 Å3. Computations display an exothermic formation of
these adduct anions, relatively small lattice potential energies
and a fairly large charge transfer per B(C6F5)3 group slightly
decreasing with an increasing number of B(C6F5)3 groups
attached.
These new anions may be utilized as a true alternative to
other widely used weakly coordinating anions since they are
similarly easily accessible. Moreover, both for steric and
electronic reasons it seems reasonable to expect that if used as
counterions for cationic early transition metal catalysts such
anions may show reduced ion pairing and hence increased
catalytic activity.16
Computational Details. Our goal was to compare the struc-
tures and energetics of differently substituted adduct anions of
the type [E(CN)n-]·[B(C6F5)3]n: E ) N dca_nb with n ) 1, 2;
E ) C tcm_nb with n ) 1, 2, 3; E ) B tcb_nb with n ) 1, 2,
3, 4. Structural data of all considered species were calculated
by using the hybrid density functional theory (B3LYP) with
the program package Gaussian03.64 Three different types of
basis sets were used for the adduct free anions: (i) a 6-31+G(d),
(ii) an aug-cc-pVDZ, and (iii) a 6-311+G(2df) basis. Moreover,
trial MP2 calculations were carried out for methanide anions
only. Further details are presented in the Supporting Information.
If not otherwise stated, all discussed computational data refer
to the B3LYP/6-31+G(d) level of theory. Comparison of these
data sets using different basis sets exhibits differences in bond
lengths no larger than 0.01-0.02 Å. The bond angles in all these
molecules are also rather independent of the choice of basis
sets.65
to the large size of the adduct anions it was impossible to obtain
frequency data. Thus thermal corrections have not been carried
out.
NBO analyses31 were carried out to investigate the charge
transfer at the B3LYP level utilizing the optimized B3LYP
geometry. Details of the NBO analyses are summarized in
Table 6.
It should be emphasized that the computation was carried
out for single, isolated (gas-phase) anions. There may well be
significant differences among gas-phase, solution, and solid-
state results and derived conclusions.
Experimental Section
General Information. All manipulations were carried out under
oxygen- and moisture-free conditions under Argon using standard
Schlenk or drybox techniques.
Dichloromethane was purified according to a literature proce-
dure,66 dried over CaH2 and freshly distilled prior to use. Diethyl
ether was dried over Na/benzophenone and freshly distilled prior
to use. n-Hexane was dried over Na/benzophenone/tetraglyme and
freshly distilled prior to use. B(C6F5)3 was prepared by a modified
literature procedure originally developed by Massey et al.67 K[tcm]
and K[tcb] were prepared according to modified literature
procedures.68,69 K[dca] (99% Merck), [EMIM]Cl (99% Fluka) and
[BMIM]Cl (99.99% Merck) were used as received.
1
NMR. 19F{1H}, H and 13C{1H} NMR spectra were recorded
1
on Bruker spectrometers AVANCE 250, 300, or 500. The H and
13C NMR chemical shifts were referenced to the solvent signals
(13C δCD2Cl2 ) 54.0; 1H δCDHCl2 ) 5.31). The 19F and 11B chemical
shifts are referred (δ ) 0) to CFCl3 and B(OH)3 (δ ) 0),
respectively.
IR. Nicolet 6700 FT-IR spectrometer with a Smart Endurance
ATR device or Nicolet 380 FT-IR with Smart Orbit ATR module.
Raman. Bruker VERTEX 70 FT-IR with RAM II FT-Raman
module, equipped with a Nd:YAG laser (1064 nm).
MS. Finnigan MAT 95-XP from Thermo Electron.
CHN analyses. Analysator Flash EA 1112 from Thermo Quest.
DSC. Thermoanalytical measurements were performed with a
DSC 823e from Mettler-Toledo instrument. Two point calibrations
with In (mp 156.6 °C) and Zn (mp 419.6 °C) were carried out.
(65) For a more thorough discussion on the basis set dependence of
molecular geometries, see: Klapo¨tke, T. M.; Schulz, A. Ab initio
Methods in Main Group Chemistry; with an invited chapter by R. D.
Harcourt about VB Theor;, John Wiley & Sons: New York, 1998.
(66) Fischer, C. B.; Xu, S.; Zipse, H. Chem. Eur. J. 2006, 12, 5779.
(67) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc. 1963,
212.
(68) Trofimenko, S.; Little, E. L.; Mower, H. F. J. Org. Chem. 1962, 27,
433.
(69) Bernhardt, E.; Finze, M.; Willner, H. Z. Anorg. Allg. Chem. 2003,
629, 1229.
The reaction energies of the adduct formation were estimated
using the total energies without any thermal corrections. Due
(64) All calculations were performed using the Gaussian program package:
Frisch, M. J.; et al. Gaussian 03, ReVision E.01; Gaussian, Inc.:
Wallingford, CT, 2004.
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 8967

A R T I C L E S
Bernsdorf et al.
About 2-6 mg of the solid samples were weighed and contained
in sealed aluminum crucibles. They were studied in the temperature
range of 20 °C to 600 °C with a heating rate of 5 °C/min; throughout
this process the furnace was flushed with dry nitrogen. For the
evaluation of the output the Stare software was employed.
X-ray Structure Determination. X-ray quality crystals of all
samples were selected in Fomblin 1800 oil (Alfa Aesar) at ambient
temperatures. All measurements were carried out at 173(2) K
(except from K[dca_2b] at 200(2) K). The data were collected on
a Bruker-Nonius Apex X8 CCD diffractometer (Nonius Kappa CCD
diffractometer, K[dca_2b]), using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). The structures were solved by direct
methods (SHELXS-97)70 and refined by full-matrix least squares
procedures (SHELXL-97).71 Semiempirical absorption corrections
were applied except for K[dca_2b] (SADABS).72 All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were
included in the refinement at calculated positions using riding
models.
(w), 839 (w), 795 (s), 771 (s), 739 (w), 729 (w), 698 (w) 678 (s),
619 (w), 576 (w). Raman (1500 mW, 50 scans, 25 °C, cm-1): ν )
2365 (10) 1647 (2), 1520 (2), 1302 (1), 703 (1), 580 (4), 492 (4),
448 (4), 395 (4). MS m/z (%): 167 (12), 361 (40), 512 (68)
[B(C6F5)3]-, 578 (100) [NC-N-CN B(C6F5)3]-, 1059 (18), 1090
(5) [M - H]-. HRMS (ESI): calcd for C3811B2F30N3 ([M - H]-)
1089.98048, found 1089.98152.
[EMIM]N[(CN)B(C6F5)3]2, EMIM[dca_2b]. B(C6F5)3 (2.56 g,
5 mmol, 3 equiv) in 60 mL of diethyl ether and EMIM[dca] (0.177
g, 1 mmol, 1 equiv) give the product EMIM[dca_2b]. Yield of
EMIM[dca_2b]: 0.72 g (60%), mp 153.8 °C (DSC). C44H11B2F30N5
(1201.17): calcd N 5.83, C 44.00; found N 5.17, C 44.75. 1H NMR
(CD2Cl2, 300 MHz, 25 °C): δ ) 8.15 (s, N-CH-N, 1H);
7.35-7.34 (m, MeNCH, 1H); 7.32 - 7.30 (m, EtNCH, 1H); 4.24
(q, CH2, 2H, 3JCH3CH2 ) 7.4 Hz); 3.95 (s, NCH3, 3H); 1.59 (t, CH3,
3
3H, JCH3CH2 ) 7.3 Hz). 11B NMR (CDCl3, 96 MHz, 25 °C): δ )
-12.03 (s). 19F NMR (CDCl3, 282 MHz, 25 °C): δ ) -135.1 to
-135.3 (m, o-F, 2F); -159.4 (t, p-F, 1F, 3JFF ) 21.4 Hz); -165.4
to -165.6 (m, m-F, 2F). 13C NMR (CDCl3, 63 MHz, 25 °C): δ )
General Synthetic Procedure. For the synthesis of
M+B[(CN)·B(C6F5)3]4-, M+C[(CN)·B(C6F5)3]3- and M+N[(CN)·
148.7 (dm, o-C, 2C, 1JCF ) 243.6 Hz); 140.4 (dm, p-C, 1C, 1JCF
)
-
B(C6F5)3]2 (M ) alkali metal, Ag, NMe4, EMIM, BMIM) salts:
1
248.8 Hz); 137.6 (dm, m-C, 2C, JCF ) 246.4 Hz); 134.2 (br,
NCHN, 1C); 124.8 (s, MeNCH, 1C); 123.3 (s, EtNCH, 1C); 118.5
(br, C-B, 1C); 109.5 (br, CN, 1C); 46.6 (s, N-CH2, 1C); 37.3 (s,
N-CH3, 1C); 15.4 (s, NCH2-CH3, 1C). IR (ATR, cm-1): ν ) 2363
(w), 2290 (s), 1646 (s), 1600(w), 1569 (w), 1516 (s), 1453 (vs),1379
(s), 1282 (s), 1164 (w), 1096 (s), 970 (vs), 863 (w), 835 (w), 795
(w), 771 (s), 742 (w), 730 (w), 698 (w) 680 (s), 619 (w), 576 (w).
Raman (1500 mW, 50 scans, 25 °C, cm-1): ν ) 2368 (10) 1648
(3), 1384 (2), 580 (2), 485 (3), 446 (2), 390 (2).
To a solution of B(C6F5)3 (3-5-fold excess) in diethyl ether was
added the cyanide salt. After stirring for 30 min a colorless solution
was observed. Removal of the solvent led to a white solid, which
was washed three times with 20 mL of n-hexane (removal of the
B(C6F5)3 excess), and dried in vacuo. The products were obtained
as colorless powders. Recrystallization from diethyl ether yields
pure colorless crystals. Included solvents molecules can be removed
by thermal treatment at 50-60 °C in vacuo.
KN[(CN)B(C6F5)3]2, K[dca_2b]. B(C6F5)3 (1.530 g, 3 mmol,
3 equiv) in 60 mL of diethyl ether and K[dca] (0.105 g, 1 mmol,
1 equiv) give the product Kdca_2b. Yield of K[dca_2b]: 0.68 g
(60%), mp 252.5 °C (DSC). C38B2F30KN3 (1129.10): calcd N
3.72, C 40.42; found N 3.02, C 40.12. 13C NMR (CDCl3, 75
MHz, 25 °C): δ ) 147.7 (dm, o-C, 2C, 1JCF ) 240.7 Hz); 139.4
[BMIM]N[(CN)B(C6F5)3]2, BMIM[dca_2b]. B(C6F5)3 (2.56 g,
5 mmol, 3 equiv) in 60 mL of diethyl ether and BMIM[dca] (0.205
g, 1 mmol, 1 equiv) give the product BMIM[dca_2b]. Yield of
BMIM[dca_2b]: 0.74 g (60%), mp 122.3 °C (DSC). C46H15B2F30N5
(1229.22): calcd N 5.70, C 44.95; found N 5.25, C 44.94. 1H NMR
(CD2Cl2, 300 MHz, 25 °C): δ ) 8.14 (s, N-CH-N, 1H);
7.33-7.30 (m, CH-CH, 2H); 4.16 (t, N-CH2, 2H, 3JCH3CH2 ) 7.5
Hz); 3.95 (s, N-CH3, 3H); 1.92-1.82 (m, N-CH2-CH2, 2H);
1
1
(dm, p-C, 1C, JCF ) 249.6 Hz); 136.7 (dm, m-C, 2C, JCF
)
245.6 Hz); 117.5 (br, C-B, 1C); 107.5 (br, CN, 1C). 11B NMR
(CDCl3, 96 MHz, 25 °C): δ ) -11.99 (s). 19F NMR (CDCl3,
282 MHz, 25 °C): δ ) -135.1 to -135.2 (m, o-F, 2F); -159.5
3
1.43-1.31 (m, CH2-CH3, 2H); 0.97 (t, CH2-CH3, 3H, JCH3CH2
) 7.3 Hz). 11B NMR (CDCl3, 96 MHz, 25 °C): δ ) -12.07 (s).
3
(t, p-F, 1F, JFF ) 20.3 Hz); -165.5 to -165.6 (m, m-F, 2F).
IR (ATR, cm-1): ν ) 2982 (w), 2516 (w), 2369 (w), 2285 (s),
1645 (s), 1515 (s), 1462 (vs), 1455 (vs), 1377 (w), 1282 (s),
1097 (s), 975 (vs), 874 (s), 865 (w), 797 (s), 772 (w), 762 (s),
750 (s), 737 (w), 728 (w), 705 (s), 680 (s), 672 (s), 632 (w),
619 (w), 607 (w), 576 (w). Raman (1500 mW, 50 scans, 25 °C,
cm-1): ν ) 2360 (10) 1650 (2), 1520 (2), 1302 (1), 700 (1), 580
(3), 490 (3), 449 (4), 396 (4). MS m/z (%): 167 (12), 361 (30),
512 (45) [B(C6F5)3]-, 578 (100) [NC-N-CN B(C6F5)3]-, 1090
(20) [M - H]-. HRMS (ESI): calcd for C3811B2F30N3 ([M -
H]-) 1089.98048, found 1089.9818.
19F NMR (CDCl3, 282 MHz, 25 °C): δ ) -135.1 to -135.2 (m,
3
o-F, 2F); -159.4 (t, p-F, 1F, JFF ) 20.0 Hz); -165.4 to -165.6
(m, m-F, 2F). 13C NMR (CDCl3, 63 MHz, 25 °C): δ ) 148.4 (dm,
o-C, 2C, 1JCF ) 242.5 Hz); 140.1 (dm, p-C, 1C, 1JCF ) 246.5 Hz);
1
137.4 (dm, m-C, 2C, JCF ) 242.4 Hz); 134.4 (br, NCHN, 1C);
124.7 (s, MeNCH, 1C); 123.6 (s, EtNCH, 1C); 118.5 (br, C-B,
1C); 109.0 (br, CN, 1C); 51.2 (s, N-CH2, 1C); 37.4 (s, N-CH3,
1C); 32.4 (s, N-CH2-CH2-CH2-CH3, 1C); 30.0 (s,
N-CH2-CH2-CH2-CH3, 1C), 13.4 (s, N-CH2-CH2-CH2-CH3,
1C). IR (ATR, cm-1): ν ) 2973 (w), 2374 (w), 2294 (s), 1646 (s),
1515 (s), 1452 (vs),1380 (s), 1280 (s), 1160 (w), 1099 (s), 973
(vs), 873 (w), 829 (w), 797 (w), 772 (s), 742 (w), 729 (w), 698
(w) 678 (s), 621 (w), 576 (w). Raman (1500 mW, 50 scans, 25 °C,
cm-1): ν ) 2364 (10) 1645 (2), 1025 (2), 580 (3), 490 (3), 447
(3), 390 (2).
AgN[(CN)B(C6F5)3]2, Ag[dca_2b]. B(C6F5)3 (1.530 g, 3 mmol,
3 equiv) in 60 mL of diethyl ether and Ag[dca] (0.173 g, 1 mmol,
1 equiv) give the product Ag[dca_2b]. Yield of Ag[dca_2b]: 0.86 g
(72%), mp 155.0 °C (DSC). C38AgB2F30N3 (1197.80): calcd N 3.51,
C 38.10; found N 3.48, C 37.07. 13C NMR (CD2Cl2, 75 MHz, 25
1
°C): δ ) 147.3 (dm, o-C, 6C, JCF ) 240.9 Hz); 139.0 (dm, p-C,
KC[(CN)B(C6F5)3]3, K[tcm_3b]. B(C6F5)3 (2.048 g, 4 mmol, 4
equiv) in 60 mL of diethyl ether and K[tcm] (0.129 g, 1 mmol, 1
equiv) give the product K[tcm_3b]. Yield of K[tcm_3b]: 1.17 g
(70%), mp 220.9 °C (DSC). C58B3F45KN3 (1165.10): calcd N 2.52,
C 41.84; found N 2.14, C 42.22. 13C NMR (CD2Cl2, 75 MHz, 25
°C): δ ) 148.5 (dm, o-C, 18C, 1JCF ) 242.0 Hz); 140.9 (dm, p-C,
3C, 1JCF ) 248.9 Hz); 136.3 (dm, m-C, 6C, 1JCF ) 248.7 Hz); 116.9
(br, C-B, 3C); 107.5 (br, CN, 1C). 11B NMR (CD2Cl2, 96 MHz,
25 °C): δ ) -12.0 (s). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ )
3
-135.1 to -135.2 (m, o-F, 2F); -159.5 (t, p-F, 1F, JFF ) 20.3
Hz); -165.5 to -165.6 (m, m-F, 2F). IR (ATR, cm-1): ν ) 2985
(w), 2890 (w), 2363 (w), 2291 (s), 1645 (s), 1515 (s), 1449
(vs),1382 (s), 1282 (s), 1086 (s), 1068 (s), 971 (vs), 931 (w), 867
1
1
9C, JCF ) 241.7 Hz); 137.6 (dm, m-C, 18C, JCF ) 243.1 Hz);
115.5 (br, C-B, 9C); 111.6 (s, CN, 3C); 14.5 (s, C(CN)3, 1C). 11
B
NMR (CD2Cl2, 96 MHz, 25 °C): δ ) -10.1 (s). 19F NMR (CD2Cl2,
282 MHz, 25 °C): δ ) -135.1 to -135.2 (m, o-F, 2F); -157.7 (t,
p-F, 1F, 3JFF ) 20.4 Hz); -164.5 to -164.8 (m, m-F, 2F). IR (ATR,
cm-1): ν ) 2980 (w), 2292 (s), 1647 (w), 1518 (s), 1456 (vs), 1383
(w), 1286 (w), 1101 (s), 970 (vs), 860 (w), 796 (w), 773 (s), 740
(w), 730 (w), 702 (w), 680 (s), 639 (w), 619 (w), 593 (w), 574
(70) Sheldrick, G. M. SHELXS-97: Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(71) Sheldrick, G. M. SHELXL-97: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(72) Sheldrick, G. M. SADABS. Version 2; University of Go¨ttingen:
Go¨ttingen, Germany, 2004.
9
8968 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Weakly Coordinating Anions Based on CN Adducts
A R T I C L E S
(w). Raman (1500 mW, 100 scans, 25 °C, cm-1): ν ) 2353 (8),
2297 (10), 1650 (5), 833 (2), 577 (3), 492 (4), 449 (3), 393 (3).
AgC[(CN)B(C6F5)3]3, Ag[tcm_3b]. B(C6F5)3 (2.048 g, 4 mmol,
4 equiv) in 60 mL of diethyl ether and Ag[tcm] (0.198 g, 1 mmol,
1 equiv) give the product Ag[tcm_3b]. Yield of Ag[tcm_3b]: 1.04 g
(60%), dec 162.2 °C (DSC). C58B3F45AgN3 (1733.87): calcd N 2.42,
C 40.18; found N 3.56, C 40.09. 11B NMR (CD2Cl2, 96 MHz, 25
°C): δ ) -9.7 (s). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ )
(75%), dec 281.9 °C (DSC). C76B5F60KN4 (2201.90): calcd N 2.54,
C 41.46; found N 2.56, C 41.03. 13C NMR (CD2Cl2, 75 MHz, 25
1
°C): δ ) 150.1 (dm, o-C, 6C, JCF ) 242.0 Hz); 142.9 (dm, p-C,
3C, 1JCF ) 242.7 Hz); 139.5 (dm, m-C, 6C, 1JCF ) 248.2 Hz); 113.8
1
11
(br, C-B, 3C); 111.2 (q, CN, 1C, JC ) 81.8 Hz). 11B NMR
B
(CD2Cl2, 96 MHz, 25 °C): δ ) -8.5 (br, CN B, 4B); -36.1 (s,
B(CN)4, 1B). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ ) -133.1 to
-133.2 (m, o-F, 2F); -154.5 (t, p-F, 1F, 3JFF ) 20.2 Hz); -162.2
to -162.4 (m, m-F, 2F). IR (ATR, cm-1): ν ) 2987 (w), 1646 (s),
1517 (s), 1456 (vs), 1383 (s), 1287 (s), 1099 (s), 970 (vs), 868
(w), 833 (w), 798 (w), 773 (s), 746 (w), 740 (w), 729 (w), 680 (s),
621 (w), 577 (w). Raman (1500 mW, 100 scans, 25 °C, cm-1): ν
) 2323 (10), 1648 (6), 1388 (3), 802 (1), 584 (6), 487 (6), 446
(7), 390 (5).
3
-135.1 to -135.2 (m, o-F, 2F); -157.7 (t, p-F, 1F, JFF ) 20.4
Hz); -164.6-164.8 (m, m-F, 2F). 13C NMR (CD2Cl2, 75 MHz, 25
°C): δ ) 148.6 (dm, o-C, 18C, 1JCF ) 242.6 Hz); 140.9 (dm, p-C,
1
1
9C, JCF ) 250.0 Hz); 137.8 (dm, m-C, 18C, JCF ) 239.1 Hz);
115.9 (br, C-B, 9C); 111.8 (s, CN, 3C); 14.0 (s, C(CN)3, 1C). IR
(ATR, cm-1): ν ) 2278 (s), 1647 (w), 1518 (s), 1456 (vs), 1383
(w), 1286 (w), 1101 (s), 970 (vs), 860 (w), 796 (w), 773 (s), 740
(w), 730 (w), 702 (w), 680 (s), 639 (w), 619 (w), 593 (w), 574
(w). Raman (dec).
AgB[(CN)B(C6F5)3]4, Ag[tcb_4b]. B(C6F5)3 (2.56 g, 5 mmol, 5
equiv) in 60 mL of diethyl ether and Ag[tcb] (0.223 g, 1 mmol, 1
equiv) give the product Ag[tcb_4b]. Yield of Agtcb_4b: 1.36 g
(60%), mp 184.2 °C (DSC). C76B5F60AgN4 (2270.67): calcd N 2.47,
C 40.20; found N 2.34, C 40.35. 11B NMR (CD2Cl2, 96 MHz, 25
°C): δ ) -7.5 (br, CN B, 4B); -36.2 (s, B(CN)4, 1B). 19F NMR
(CD2Cl2, 282 MHz, 25 °C): δ ) -135.1 to -135.2 (m, o-F, 2F);
[EMIM]C[(CN)B(C6F5)3]3, EMIM[tcm_3b]. B(C6F5)3 (2.048
g, 4 mmol, 4 equiv) in 60 mL of diethyl ether and EMIM[tcm]
(0.201 g, 1 mmol, 1 equiv) give the product EMIM[tcm_3b]. Yield
of EMIM[tcm_3b]: 1.18
g (68%), mp 197.4 °C (DSC).
3
C64H11B3F45N5 (1737.17): calcd N 4.03, C 44.25, H 0.64; found N
-156.7 (t, p-F, 1F, JFF ) 20.1 Hz); -164.3 to -164.5 (m, m-F,
1
3.75 %, C 44.46, H 0.68. H NMR (CD2Cl2, 300 MHz, 25 °C): δ
2F). 13C NMR (CD2Cl2, 63 MHz, 25 °C): δ ) 148.3 (dm, o-C, 6C,
1
) 8.13 (s, N-CH-N, 1H); 7.33-7.32 (m, CH-CH, 1H);
1JCF ) 246.5 Hz); 141.2 (dm, p-C, 3C, JCF ) 246.0 Hz); 137.6
3
1
7.30-7.29 (m, CH-CH, 1H); 4.22 (q, CH2, 2H, JCH3CH2 ) 7.3
(dm, m-C, 6C, JCF ) 248.0 Hz); 114.2 (br, C-B, 3C); 111.0 (q,
Hz); 3.93 (s, NCH3, 3H); 1.57 (t, CH3, 3H, 3JCH3CH2 ) 7.3 Hz). 13
C
)
CN, 1C, JC ) 81.6 Hz). IR (ATR, cm-1): ν ) 1646 (s), 1517
1
11B
NMR (CD2Cl2, 75 MHz, 25 °C): δ ) 148.4 (dm, o-C, 18C, 1JCF
(s), 1463 (vs), 1382 (s), 1288 (s), 1230 (w), 1179 (w), 1102 (s),
973 (vs), 886 (w), 861 (w), 797 (w), 774 (s), 740 (w), 682 (s), 626
(w), 577 (w). Raman (dec).
243.5 Hz); 140.9 (dm, p-C, 9C, 1JCF ) 237.4 Hz); 137.9 (dm, m-C,
1
18C, JCF ) 241.3 Hz), 133.9 (br, N-CH-N, 1C); 124.6 (s,
MeNCH, 1C), 123.1 (s, EtNCH, 1C); 115.6 (br, C-B, 9C); 111.6
(s, CN, 3C); 46.5 (s, N-CH2, 1C); 37.2 (s, N-CH3, 1C); 15.3 (s,
CH2-CH3, 1C); 14.5 (s, C(CN)3, 1C). 11B NMR (CD2Cl2, 96 MHz,
25 °C): δ ) -10.3 (s). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ )
[EMIM]B[(CN)B(C6F5)3]4, EMIM[tcb_4b]. B(C6F5)3 (2.56 g,
5 mmol, 5 equiv) in 60 mL of diethyl ether and EMIM[tcb] (0.226
g, 1 mmol, 1 equiv) give the product EMIM[tcb_4b]. Yield of
EMIM[tcb_4b]: 1.66 g (73%), mp 176.2 °C (DSC). C82H11B5F60N6
(2273.96): calcd N 3.70, C 43.31, H 0.49; found N 3.33 %, C 42.62,
H 0.40. 1H NMR (CD2Cl2, 300 MHz, 25 °C): δ ) 8.13 (s,
N-CH-N, 1H); 7.33-7.32 (m, MeNCH, 1H); 7.30-7.28 (m,
EtNCH, 1H); 4.21 (q, CH2, 2H, 3JCH3CH2 ) 7.4 Hz); 3.93 (s, NCH3,
3
-135.1 to -135.2 (m, o-F, 2F); -157.7 (t, p-F, 1F, JFF ) 20.0
Hz); -164.7 to -164.8 (m, m-F, 2F). IR (ATR, cm-1): ν ) 2290
(s), 1648 (w), 1519 (s), 1456 (vs), 1384 (w), 1286 (w), 1165 (w),
1103 (s), 972 (vs), 861 (w), 829 (w), 796 (w), 773 (s), 739 (w),
702 (w), 680 (s), 639 (w), 619 (w), 593 (w), 574 (w). Raman (1500
mW, 500 scans, 25 °C, cm-1): ν ) 2349 (8), 2297 (10), 1649 (5),
830 (2), 577 (5),510 (2), 491 (4), 448 (3).
[BMIM]C[(CN)B(C6F5)3]3, BMIM[tcm_3b]. B(C6F5)3 (2.048
g, 4 mmol, 4 equiv) in 60 mL of diethyl ether and BMIM[tcm]
(0.229 g, 1 mmol, 1 equiv) give the product BMIM[tcm_3b]. Yield
3
3H); 1.57 (t, CH3, 3H, JCH3CH2 ) 7.3 Hz). 13C NMR (CD2Cl2, 75
1
MHz, 25 °C): δ ) 148.3 (dm, o-C, 2C, JCF ) 242.9 Hz); 141.2
(dm, p-C, 1C, 1JCF ) 246.4 Hz); 137.8 (dm, m-C, 2C, 1JCF ) 246.4
Hz); 134.0 (br, NCHN, 1C); 124.6 (s, MeNCH, 1C); 123.1 (s,
1
11B
EtNCH, 1C); 113.7 (br, C-B, 1C); 110.8 (q, CN, 1C, JC
)
81.8 Hz); 46.4 (s, N-CH2, 1C); 37.2 (s, N-CH3, 1C); 15.2 (s,
NCH2-CH3, 1C). 11B NMR (CD2Cl2, 96 MHz, 25 °C): δ ) -7.8
(br, CN B, 4B). -36.1 (s, B(CN)4, 1B). 19F NMR (CD2Cl2, 282
MHz, 25 °C): δ ) -135.1 to -135.2 (m, o-F, 2F); -156.2 (t, p-F,
of BMIM[tcm_3b]: 1.06
g (60%), mp 161.6 °C (DSC).
C66H15B3F45N5 (1765.22): calcd N 3.97, C 44.91, H 0.86; found N
1
3.93, C 45.13, H 0.92. H NMR (CD2Cl2, 300 MHz, 25 °C): δ )
3
8.12 (s, N-CH-N, 1H); 7.30-7.27 (m, CH-CH, 2H); 4.13 (t,
N-CH2, 2H, 3JCH3CH2 ) 7.4 Hz); 3.92 (s, N-CH3, 3H); 1.90-1.80
(m, N-CH2-CH2, 2H); 1.40-1.28 (m, CH2-CH3, 2H); 0.93 (t,
1F, JFF ) 20.0 Hz); -164.1 to -164.3 (m, m-F, 2F). IR (ATR,
cm-1): ν ) 3148 (w), 1648 (s), 1519 (s), 1456 (vs), 1384 (s), 1288
(s), 1164 (s), 1101 (s), 974 (vs), 864 (w), 827 (w), 797 (w), 774
(s), 740 (w), 729 (w), 681 (s), 648 (w) 621 (w), 578 (w). Raman
(1500 mW, 700 scans, 25 °C, cm-1): ν ) 2330 (10), 1649 (3),
1393 (2), 807 (2), 585 (5), 488 (4), 447 (4), 391 (3).
3
CH2-CH3, 3H, JCH3CH2 ) 7.4 Hz). 13C NMR (CD2Cl2, 75 MHz,
1
25 °C): δ ) 148.5 (dm, o-C, 18C, JCF ) 244.6 Hz); 140.3 (dm,
1
1
p-C, 9C, JCF ) 242.5 Hz); 137.4 (dm, m-C, 18C, JCF ) 233.8
Hz); 134.2 (br, NCHN, 1C); 124.6 (s, MeNCH, 1C); 123.4 (s,
EtNCH, 1C); 115.5 (br, C-B, 9C); 111.6 (s, CN, 3C); 51.0 (s,
N-CH2, 1C); 37.2 (s, N-CH3, 1C); 32.1 (s, NCH2-
CH2-CH2-CH3, 1C); 19.8 (s, NCH2-CH2-CH2-CH3, 1C), 13.3
(s, NCH2-CH2-CH2-CH3, 1C); 14.5 (s, C(CN)3, 1C). 11B NMR
(CD2Cl2, 96 MHz, 25 °C): δ ) -10.1 (s). 19F NMR (CD2Cl2, 282
MHz, 25 °C): δ ) -135.1 to -135.2 (m, o-F, 2F); -157.7 (t, p-F,
[BMIM]B[(CN)B(C6F5)3]4, BMIM[tcb_4b]. B(C6F5)3 (2.56 g,
5 mmol, 5 equiv) in 60 mL of diethyl ether and BMIM[tcb] (0.254
g, 1 mmol, 1 equiv) give the product BMIM[tcb_4b]. Yield of
BMIM[tcb_4b]: 1.50 g (65%), mp 147.9 °C (DSC). C84H15B5F60N6
(2302.02): calcd N 3.65, C 43.83, H 0.66; found N 3.22, C 44.01,
H 0.72. 1H NMR (CD2Cl2, 300 MHz, 25 °C): δ ) 8.12 (s,
N-CH-N, 1H); 7.30-7.29 (m, CH-CH, 2H); 4.14 (t, N-CH2,
3
3
1F, JFF ) 20.3 Hz); -164.7 to -164.8 (m, m-F, 2F). IR (cm-1):
2H, JCH3CH2 ) 7.4 Hz); 3.92 (s, N-CH3, 3H); 1.91-1.81 (m,
ν ) 3174 (w), 2970 (w), 2289 (s), 1647 (w), 1596 (w), 1567 (w),
1518 (s), 1456 (vs), 1384 (s), 1285 (s), 1163 (w),1102 (s), 971
(vs), 861 (w), 829 (w), 796 (w), 773 (w), 739 (w), 701 (w), 680
(s), 639 (w), 621 (w), 593 (w), 574 (w). Raman (1500 mW, 300
scans, 25 °C, cm-1): ν ) 2346 (8), 2295 (10), 1649 (5), 830 (2),
577 (5),510 (2), 490 (4), 448 (3).
N-CH2-CH2, 2H); 1.41-1.29 (m, CH2-CH3, 2H); 0.94 (t,
3
CH2-CH3, 3H, JCH3CH2 ) 7.4 Hz). 13C NMR (CD2Cl2, 75 MHz,
1
25 °C): δ ) 148.3 (dm, o-C, 2C, JCF ) 242.3 Hz); 141.2 (dm,
p-C, 1C, 1JCF ) 240.0 Hz); 137.7 (dm, m-C, 2C, 1JCF ) 239.0 Hz);
134.2 (br, NCHN, 1C); 124.6 (s, MeNCH, 1C); 123.4 (s, EtNCH,
1
11B
1C); 113.8 (br, C-B, 1C); 110.7 (q, CN, 1C, JC ) 82.4 Hz);
KB[(CN)B(C6F5)3]4, K[tcb_4b]. B(C6F5)3 (2.56 g, 5 mmol, 5
equiv) in 60 mL of diethyl ether and K[tcb] (0.154 g, 1 mmol, 1
equiv) give the product K[tcb_4b]. Yield of K[tcb_4b]: 1.65 g
51.1 (s, N-CH2, 1C); 37.2 (s, N-CH3, 1C); 32.2 (s,
N-CH2-CH2-CH2-CH3, 1C); 19.8 (s, N-CH2-CH2-CH2-CH3,
1C); 13.2 (s, N-CH2-CH2-CH2-CH3, 1C). 11B NMR (CD2Cl2,
9
J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009 8969

A R T I C L E S
Bernsdorf et al.
96 MHz, 25 °C): δ ) -7.6 (br, CN B, 4B); -36.1 (s, B(CN)4,
1B). 19F NMR (CD2Cl2, 282 MHz, 25 °C): δ ) -135.1 to -135.2
(m, o-F, 2F); -156.2 (t, p-F, 1F, 3JFF ) 20.0 Hz); -164.1 to -164.3
(m, m-F, 2F). IR (ATR, cm-1): ν ) 3174 (w), 1647 (w), 1602 (w),
1568 (w), 1518 (s), 1456 (vs), 1384 (s), 1289 (s), 1163 (w),1101
(s), 971 (vs), 863 (w), 831 (w), 796 (w), 774 (w), 740 (w), 701
(w), 682 (s), 621 (w), 577 (w). Raman (1500 mW, 450 scans, 25
°C, cm-1): ν ) 2329 (10), 1648 (4), 1390 (2), 806 (2), 585 (5),
488 (4), 446 (4), 391 (3).
ment for compound K[dca_2b]. We are indebted to Prof. Dr.
Ingo Krossing (University of Freiburg) for most valuable
comments and suggestions.
Supporting Information Available: Additional material con-
cerning X-ray elucidation, VBT theory and computations,
utilized programs, derived ion volumes.64 Complete ref 64. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. Dedicated to Prof. I.-P. Lorenz (LMU
Munich) on the occasion of his 65th birthday. We thank Dr.
Peter Mayer (LMU Munich) for performing the X-ray measure-
JA902450B
9
8970 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009