Synthesis and structure of tritylium salts

Article abstract of DOI:10.1007/s11224-015-0638-0

Several tritylium compounds [Ph₃C][A] have been isolated by salt metathesis or halide abstraction reactions starting from Ph₃C-X (X = halogen) and Lewis acids in good yields (A = BF₄, BCl₄, AlCl₄, GaCl₄, PF₆, AsF₆, SbF₆, SbCl₆, CHB₁₁H₅Cl₆, CHB₁₁Cl₁₁, CHB₁₁H₅Br₆, CF₃COO, CF₃SO₃, N₃). The structures of 15 tritylium salts bearing different types of weakly coordinating anions (A^-) have been determined. The structures are discussed on the basis of ion pairing versus covalent bond formation and solid-state interactions in comparison with gas-phase data.

Full text of DOI:10.1007/s11224-015-0638-0

Struct Chem (2015) 26:1641–1650
DOI 10.1007/s11224-015-0638-0
ORIGINAL RESEARCH
Synthesis and structure of tritylium salts
1
Alexander Hinz1 Rene Labbow Fabian Reiß1,2 Axel Schulz1,2
•
•
•
•
´
Katharina Sievert1 Alexander Villinger1
•
Received: 6 July 2015 / Accepted: 8 July 2015 / Published online: 22 July 2015
Ó Springer Science+Business Media New York 2015
Abstract Several tritylium compounds [Ph3C][A] have
been isolated by salt metathesis or halide abstraction
reactions starting from Ph3C–X (X = halogen) and Lewis
acids in good yields (A = BF4, BCl4, AlCl4, GaCl4, PF6,
AsF6, SbF6, SbCl6, CHB11H5Cl6, CHB11Cl11, CHB11H5
Br6, CF3COO, CF3SO3, N3). The structures of 15 tritylium
salts bearing different types of weakly coordinating anions
(A-) have been determined. The structures are discussed
on the basis of ion pairing versus covalent bond formation
and solid-state interactions in comparison with gas-phase
data.
sodium bromide and the tritylium salt, trityl malonate
Trit[HC(COOEt)2] (trityl = triphenylmethyl = [Ph3C]?,
Scheme 1) [1]. This paper might be one of the first to deal
with tritylium salts. Many chemists have studied the chem-
istry of triphenylmethyl compounds and their reactivity [1–
22], among them famous persons like (1) Paul Walden [3, 18]
who discovered that, e.g., triphenylmethyl halides display
electric conductivity once dissolved in liquid SO2 due to
dissociation into tritylium cations and halides, (2) Moses
Gomberg [7, 13] who studied the reaction of triphenylmethyl
chloride with Zn yielding the first example of an organic free
radical, the triphenylmethyl radical or (3) Adolf Baeyer and
Victor Villiger who postulated the formation of a salt when
triphenylmethane was treated with concentrated sulfuric
acid. Baeyer und Villiger coined such trivalent tritylium
cations ‘‘carbonium salts’’ [5], while Gomberg preferred the
term ‘‘carbyl salts’’ [6]. Hence, these investigations regard-
ing the chemistry of triphenylmethyl compounds can be
regarded as the birth of carbocation chemistry. It should be
noted that today trivalent carbocations are referred to as
carbenium ions (R3C?) [16], while carbonium ions (also
non-classical cations) describe tetra- or pentacoordinated
carbocations such as [CR5]? or [C2R5]? [8, 9].
Keywords Tritylium Á Synthesis Á Structure Á Bonding
Introduction
As early as 1878, Henderson described the reaction of
triphenylmethyl bromide with ethylic sodiomalonate
(sodium salt of malonic acid diethyl ester) to give
Dedicated to Professor Dr. Magdolna Hargittai on the occasion of her
70th birthday.
Ever since the discovery of trityl salts, many examples
have been published mostly in combination with (new)
weakly coordinating anions [23] such as perfluorinated
alkoxy [24–28] or amido [24] aluminates, borates [BR4]-
(R = C6F5, [29] R = CF3, CN), [30] [(CF3)3BCN]-, [30]
[B12X12]2- (X = Cl, [31, 32] F [33]), carborates [34] (e.g.,
[HCB11H11]-), [35] cyanide Lewis base/Lewis acid com-
plexes (e.g., [(C6F5)3B–CN–B(C6F5)3]-), [36, 37]
[N(SO2F)2]-, [38] [C5(CN)5]-, [39] or classical metallates
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-015-0638-0) contains supplementary
material, which is available to authorized users.
& Axel Schulz
axel.schulz@uni-rostock.de
1
Institut fu¨r Chemie, Abteilung Anorganische Chemie,
¨
Universitat Rostock, Albert-Einstein-Straße 3a,
18059 Rostock, Germany
(e.g., [AuCl4]-, [40] [Hf2Cl10]2-
,
[41] [M(O–
C(H)(CF3)2)6]-, [42] [M(OC6F5)6]- (M = Nb, Ta), [43, 44]
2
¨
Leibniz-Institut fu¨r Katalyse e.V. an der Universitat Rostock,
Albert-Einstein-Straße 29a, 18059 Rostock, Germany
chloridoplatinates, e.g., [Pt2Cl10]2- [45] and [SbCl6]-) [46].
123

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Struct Chem (2015) 26:1641–1650
Scheme 1 Synthesis of [Ph3C][HC(COOEt)2]
Due to a low-lying empty p-type orbital (Fig. 1, left),
strong Lewis acids such as the trityl cations were found to
be highly efficient catalysts, e.g., for the Diels–Alder
reaction for a range of substrates [55], intramolecular
cyclization of saturated alkoxysilyl hydrides [56], Michael
and aldol reactions [57] or the generation of frustrated
Lewis acid–base pairs [58]. Moreover, trityl salts of weakly
coordinating borate [59] and aluminate counter anions [60–
62] are highly effective stoichiometric activators for olefin
polymerization reactions [63–66]. In element organic
chemistry, trityl salts are frequently used in Bartlett–Con-
don–Schneider (BCS) type hydride transfer reaction to
generate, e.g., silylium ions (Scheme 2) [31, 47–54].
In this paper, we report on the synthesis, isolation and
full characterization of 15 different trityl salts bearing the
whole range of different weakly coordinating anions.
Scheme 2 Bartlett–Condon–Schneider type hydride transfer reaction
(E = Si, Ge) [31, 47–54]
salt metathesis reaction using silver (Eq. 1) or sodium salts
(Eq. 3). (2) In case of strong Lewis acids such as ECl3
(E = B, Al, Ga), a simple halide abstraction from the trityl
chloride was applied (Eq. 2). Most reactions were carried
out in polar solvents such as dichloromethane. In some
cases, mixtures of toluene and acetonitrile were utilized to
increase the solubility, e.g., in the synthesis of carborate
salts 9, 10 and 11 (their silver salts are not soluble in either
of these solvents), while a decrease in solubility by means
of n-pentane was necessary in case of tetrachloridogallate
salt 4. The synthesis of hexafluoridophosphate salt 5 was
carried out in acetonitrile, and for the azide metathesis
reaction, a mixture of CH3CN/toluene was used. All tri-
tylium salts can be prepared in bulk and obtained in good
yields (Table 1). Most tritylium salts display some mois-
ture sensitivity but are rather stable in air. They are usually
thermally stable up to over 100 °C, some even well over
200 °C (Table 1). Usually, tritylium salts are well soluble
in polar solvents such as CH2Cl2, CH3CN, toluene or
Results and discussion
Synthesis
To systematically study the structure of trityl salts, a series
of tritylium salts [Ph3C][A] [A = BF4 (1), BCl4 (2), AlCl4
(3), GaCl4 (4), PF6 (5), AsF6 (6), SbF6 (7), SbCl6 (8),
CHB11H5Cl6 (9), CHB11Cl11 (10), CHB11H5Br6 (11),
CF3COO (12), CF3SO3 (13), N3 (14)] have been prepared.
Three synthetic routes have been utilized (Scheme 3): (1)
Fig. 1 Left LUMO of the trityl
cation, right electrostatic
potential mapped onto the
electron density (colored MO
and ELF see ESI)
123

Struct Chem (2015) 26:1641–1650
1643
mixtures of them. It should be noted that most of the used
anions form ionic liquids with weakly coordinating cations,
in contrast to the strong Lewis acidic trityl cation.
amounts to ca. a = 33°. As expected and known for a long
time, the C1–C2 bond lengths are in the range of the C–C
˚
bond lengths of the phenyl rings (1.415 A) but rather short
˚
˚
with 1.444 A (cf. Rr(C–C) = 1.50, Rr(C = C) = 1.34 A)
[67] displaying some double-bond character. Both NBO
and MO data indicate that the central carbon atom is
strongly involved in delocalization of p electron density of
the phenyl rings (e.g., the pz orbital of C1 is occupied by
0.78 e). The computed NPA charges feature only a slightly
positively charged central C1 atom (Fig. 1) with ?0.22 e,
while all other C atoms are slightly negatively charged
(-0.12 to -0.24 e). Actually, the positive charge sits at the
hydrogen atoms bearing all ca. ?0.28 e. Hence, it can be
concluded that the overall positive charge is delocalized
over all hydrogen atoms and the central carbon atom;
however, the latter possesses much less than ?1 e. The
electrostatic potential mapped onto the electron density
(Fig. 1) supports also this view displaying a large area of
positive charge around the central C atom but also at the
edges of the phenyl rings while the phenyls are areas of
charge accumulation.
Spectroscopic characterization, structure
and bonding
Gas phase
According to computations (pbe1pbe/6-31G(d,p)), the tri-
tylium cation adopts a D3 symmetric structure (Fig. 2),
with the three phenyl groups attached to the central carbon
in a propeller type fashion. The deviation from planarity
Solution
Scheme 3 Synthesis of tritylium salts [E = Al, B, Ga; X = Cl, Br;
Y = BF4 (1), BCl4 (2), AlCl4 (3), GaCl4 (4), PF6 (5), AsF6 (6), SbF6
(7), SbCl6 (8), CHB11H5Cl6 (9), CHB11Cl11 (10), CHB11H5Br6 (11,
11ÁCH2Cl2, 11ÁCH3CN), CF3COO (12), CF3SO3 (13), N3 (14)]
Table 1 summarizes selected spectroscopic data of trity-
lium salts considered in this study. As can be seen from
Table 1 and in contrast to the IR data, 13C NMR studies are
Table 1 Physical properties and spectroscopic details of tritylium salts [Ph3C][A]
[A]
Method[a]
Yield/ %
Mp./°C
Color
13C (CPh3)
m
C–H/cm-1[j]
[BF4]
1
2
3
2
1
1
69
92
79
86
89
53
209[b]
188[b]
169[b]
174[b]
230[b]
222
213[b]
212[b]
230[b]
207[b]
Yellow
Yellow
Yellow
Yellow
Orange
Red–orange
Yellow
Yellow
Yellow
Yellow
Orange
Orange
Orange
Yellow
Yellow
Yellow
Colorless
213.08[d]
211.31[c]
211.31[c]
211.34[c]
213.1[d]
211.48[c]
211.52[c]
211.32[c]
213.02[d]
213.02[d]
211.32[c]
211.32[c]
213.08[d]
211.41[c]
95.47[c]
3095, 3070
[BCl4]
3085, 3066, 3058
3064
[AlCl4]
[GaCl4]
[PF6]
3065
3052
[AsF6]
3095
[SbF6]
1
[e]
58
[e]
not observed
3062
[SbCl6]
[CHB11H5Cl6]
1
92
91
99
98
81
81
65
3058
[CHB11Cl11
]
1
[f]
3043, 3022
3045
[CHB11H5Br6]
[CHB11H5Br6]ÁCH2Cl2
[CHB11H5Br6]ÁCH3CN
[CF3SO3]
240
[h]
1
[g]
3053
239[b]
118[b]
127
3047
1
1
3064
[CF3COO]
[N3]
3062, 3035
3
[i]
78
[i]
65
78.2[d]
57.5[c]
3085, 3062, 3033, 3022
3079, 3060, 3041, 3022
Trit-H
93
[a]
Preparation method according to Eqs. 1–3 in Scheme 3; [b] decomposition, [c] in CD2Cl2, [d] in CD3CN; [e] re-crystallization of commercially
[f]
[g]
(Alfa Aeser) available salt;
CN/toluene mixture; [h] forms 11 upon thermal treatment; [i] obtained by synthesis of [Me3Si]?—salts from [Ph3C][A]; [j] IR data, full data and
spectra are listed in ESI
obtained from 11ÁCH2Cl2 by thermal treatment;
obtained by re-crystallization of 11ÁCH2Cl2 from CH3
123

1644
Struct Chem (2015) 26:1641–1650
a valuable tool to study the amount of carbenium character
in the different tritylium salts in solution. Close inspection
of these data reveals that all salts with weakly coordinating
anions form separate solvated ions in polar solvents such as
CD2Cl2 or CD3CN as illustrated by the 13C NMR data with
a low-field shifted resonance d[13C] = 211.3 ppm in
CD2Cl2 and 213.1 ppm in CD3CN, respectively, for the
tricoordinate central carbon atom of the tritylium cation.
As soon as strong interaction between the ions occurs such
as in acetate 12 or azide compound 14 with strong polar
covalent bonding, no dissociation is observed and a high-
field shifted resonance at d[13C] = 95.5 ppm (acetate 12)
and 78.2 ppm (azide 14, cf. 57.7 ppm in Ph3C–H 15) can
be detected.
Solid state: X-ray structure elucidation
The solid-state structures of [Ph3C][A] (A = BF4, N3,
SbCl6Ábenzene) have been known [46, 68, 69]; however,
we want to report here a new modification of Ph3C–N3. All
other X-ray data are reported here for the first time.
Crystals of tritylium salts suitable for single crystal
X-ray structure determination were usually obtained by
preparing saturated solutions of [Ph3C][A] (A = anion) in
CH2Cl2, CH3CN or toluene. [Ph3C][A] (A = AlCl4, GaCl4
and N3) crystallized with two independent formula units in
the unit cell, while only solvates were obtained for the
bromocarborates 11ÁCH2Cl2 and 11ÁCH3CN. Figures 3, 4,
Fig. 2 Computed gas-phase structure of [Ph3C]?. Selected bond
˚
lengths in [A] and angles in [°]: C1–C2 1.444, C2–C3 1.415; C2–C1–
C20 120.0, a = C20–C1–C2–C3 -33.1. NPA charges, q(X) in e:
X = C1: ?0.22, C2: -0.12, C3: -0.18, C4: -0.24, C5: -0.16, C6:
-0.24, C7: -0.18
Fig. 3 Molecular structures of 2 (left), 3 (middle) and 4 (right) in the
crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
C21 120.0(2), C33–C20–C21 121.0(2), C33–C20–C21 119.2(2), [
a(C1) = 32.3, [ a(C20) = 32.1. (4) C1–C2 1.438(3), C1–C8
1.448(3), C1–C14 1.445(3), C20–C21 1.448(2), C20–C27 1.441(3),
C20–C33 1.438(2), C2–C1–C8 119.47(2), C2–C1–C14 120.68(2),
C14–C1–C8 119.88(2), C27–C20–C21 119.95(2), C33–C20–C21
120.84(2), C33–C20–C21 119.23(2), [ a(C1) = 32.3, [ a(C20) =
32.0; colored version can be found in the ESI
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (2) C1–
C2 1.443(2), C1–C8 1.431(2), C1–C14 1.454(2), C2–C1–C8
119.3(2), C2–C1–C14 121.3(2), C8–C1–C14 119.6(2), [ a = 34.2.
(3) C1–C2 1.440(3), C1–C8 1.445(3), C1–C14 1.445(3), C20–C21
1.448(3), C20–C27 1.443(3), C20–C33 1.440(3), C2–C1–C8
119.5(2), C2–C1–C14 120.7(2), C14–C1–C8 119.9(2), C27–C20–
123

Struct Chem (2015) 26:1641–1650
1645
Fig. 4 Molecular structures of 5 (left), 6 (middle) and 7 (right) in the
crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
(6) C1–C2 1.442(2), C1–C8 1.447(2), C1–C14 1.445(2), C2–C1–C8
120.0(2), C2–C1–C14 118.5(2), C14–C1–C8 121.6(2), [ a = 28.6.
(7) C1–C2 1.452(2), C1–C8 1.446(3), C1–C14 1.437(2), C8–C1–C2
119.1(2), C14–C1–C2 120.2(2), C14–C1–C8 120.9(2), [ a = 35.7;
colored version can be found in the ESI
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (5) C1–
C2 1.452(2), C1–C8 1.439(3), C1–C14 1.450(2), C8–C1–C2
121.6(2), C8–C1–C14 120.5(2), C14–C1–C2 118.0(2), [ a = 31.9.
Fig. 5 Molecular structures of 9 (left) and 9ÁCH3CN (right) in the
120.9(2), C14–C1–C2 119.3(2), C14–C1–C8 119.8(2), [ a = 32.0.
(9ÁCH3CN) C1–C2 1.441(3), C1–C8 1.447(3), C1–C14 1.444(3), C2–
C1–C8 120.2(2), C2–C1–C14 120.0(2), C14–C1–C8 119.5(2), [
a = 32.5; colored version can be found in the ESI
crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (9) C1–
C2 1.443(3), C1–C8 1.444(3), C1–C14 1.440(3), C2–C1–C8
Fig. 6 Molecular structures of 10 (left) and 10ÁC7H8 (right) in the
120.9(2), C2–C1–C14 119.7(2), C8–C1–C14 119.6(2), [ a = 32.8.
(10ÁC7H8) C1–C2 1.448(4), C1–C8 1.433(4), C1–C14 1.442(4), C8–
C1–C2 120.1(2), C8–C1–C14 120.0(2), C14–C1–C2 119.9(2), [
a = 32.7; colored version can be found in the ESI
crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (10) C1–
C2 1.434(2), C1–C8 1.441(3), C1–C14 1.450(3), C2–C1–C8
123

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Struct Chem (2015) 26:1641–1650
Fig. 7 Molecular structures of 11ÁCH2Cl2 (left) and 11ÁCH3CN
(right) in the crystal. Thermal ellipsoids drawn at 50 % probability
C2–C1–C8 121.5(4), C2–C1–C14 120.3(4), C8–C1–C14 118.2(2), [
a = 32.5. (11ÁCH3CN) C1–C2 1.437(6), C1–C8 1.439(7), C1–C14
1.442(7), C2–C1–C8 119.6(4), C2–C1–C14 119.8(4), C8–C1–C14
120.6(4), [ a = 32.6; colored version can be found in the ESI
˚
and 173 K. Selected bond lengths [A], angles [°] and torsion angles
[°]: (11ÁCH2Cl2) C1–C2 1.438(6), C1–C8 1.439(6), C1–C14 1.442(6),
Fig. 8 Molecular structures of 12 (left), 13 (middle) and 14 (right) in
the crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
119.11(9), C14–C1–C8A 123.2(4), [ a = 33.5. (14) C1–C2
1.535(2), C1–C8 1.533(2), C1–C14 1.544(2), C20–C21 1.535(2),
C20–C27 1.541(2), C20–C33 1.530(2), N1–C1 1.514(2), N1–N2
1.227(2), N2–N3 1.132(2), N4–C20 1.521(2), N4–N5 1.231(2), N5–
N6 1.132(2), C2–C1–C14 112.8(2), C8–C1–C2 111.02(9), C8–C1–
C14 111.6(2), C21–C20–C27 110.33(9), C33–C20–C21 112.4(2),
C33–C20–C27 111.9(2), [ a(C1) = 32.8, [ a(C20) = 43.7; colored
version can be found in the ESI
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (12) C1–
C2 1.529(2), C1–C8 1.531(2), C1–C14 1.524(2), C1–O1 1.510(2),
C20–O1 1.312(2), C20–O2 1.197(2), C2–C1–C8 112.33(11), C14–
C1–C2 116.0(2), C14–C1–C8 109.8(2), C20–O1–C1 122.6(2), O2–
C20–O1 130.8(2), [ a = 46.2. (13) C1–C2 1.441(2), C1–C8A
1.458(5), C1–C14 1.446(2), C2–C1–C8A 117.7(4), C2–C1–C14
˚
5, 6, 7, 8 and 9 display the molecular structures of [Ph3-
C][A], and a summary of selected structural data is listed in
Table 2. As can be seen from these data, except from the
azide and acetate compound respectively, which obviously
represent polar covalently bound species, all other tritylium
salts form ion pairs in the crystals with only small cation–
anion interactions. The molecular structure of azide 14
revealed the typical trans-bent structure of covalent azides
are rather long [Rrcov(C–O) = 1.38 A] [67] but still in a
range which can be considered as a polar covalent bonding
situation rather than an ion pair [Fig. 8 middle, cf. C–O
˚
1.453 A in CH3C(O)OCH3] [73]. On the contrary, the tri-
˚
flate species features a C1–O distance of 2.992(1) A, which
lies clearly outside the range of covalent bonding but is still
within the sum of van der Waals radii [RrvdW
˚
(CÁÁÁO) = 3.14 A] [74]. In contrast to the acetate, the tri-
˚
with a C1–N1 distance of 1.514(2) A which is slightly
flate clearly forms ion pairs with CÁÁÁO distances (Figs. 8
˚
and 10), ranging between 2.992 and 3.244 A, definitely
elongated compared to the sum of the covalent radii
˚
[Rrcov(C–N) = 1.45 A] [67] but well within the range of
typical molecular azides (Fig. 8 right) [70–72]. Also the
lying outside the range expected for a polar covalent bond.
For the tritylium salts with clearly separated ions
(all EX4-, EX6-, carborates and triflate), the closest
˚
C1–O bond lengths of C1–O1 1.510(2) A in the acetate 12
123

Struct Chem (2015) 26:1641–1650
1647
Fig. 9 Molecular structure of 14ÁC6H6 (left) and 14ÁC7H8 (right) in
-x ? y?1, -x ? 1, z; (4) -y ? 1, x-y, z. (14ÁC7H8) C1–C2
1.529(2), C2i–C1–C2ii 112.6(2), [ a = 30.3. Symmetry codes: (1)
-y ? 1, x-y ? 2, z; (2) -x ? y-1, -x ? 1, z; (3) -x ? y?1,
-x ? 1, z; (4) -y ? 1, x-y, z
the crystal. Thermal ellipsoids drawn at 50 % probability and 173 K.
˚
Selected bond lengths [A], angles [°] and torsion angles [°]: (14ÁC6H6)
C1A–C2A 1.527(2), C2Ai–C1A–C2A 112.90(7), [ a = 29.5. Sym-
metry codes: (1) -y ? 1, x-y ? 2, z; (2) -x ? y-1, -x ? 1, z; (3)
˚
Table 2 Selected structural data of tritylium salts [Ph3C][A] (distances in [A] and angles in [°])
sg (Z)[a]
[ Cq–C
R \ C1
a[b]
d
d(C1ÁÁÁXanion
)
d(c?a-)[e]
[c]
plane
[d]
[A]
[BF4]
F4132 (16)[f]
Pbca (8)
–
–
–
–
–
–
[BCl4]
[AlCl4]
1.443
1.443
1.443
1.444
1.442
1.447
1.445
1.445
1.444[g]
1.442
1.444
1.441
1.441
1.440
1.439
1.448
1.528
1.537
1.535
1.529
1.527
360.2
360.1
360.2
360.0
360.0
360.1
360.1
360.2
360.0[g]
360.0
359.7
360.2
360.0
360.0
360.0
360.0
338.1
335.4
334.6
337.8
338.7
34.2
32.3
32.1
32.3
32.0
31.9
35.7
28.6
32.3[g]
32.0
32.5
32.8
32.7
32.5
32.6
33.5
46.2
32.8
43.7
30.3
29.5
0.005
0.005
0.001
0.003
0.001
0.016
0.012
0.015
0.003[g]
0.014
0.003
0.004
0.003
0.009
0.015
0.011
0.422
0.451
0.457
0.426
0.415
3.440
3.755
3.658
3.975
3.642
3.080
3.070
3.058
4.05[g]
3.728
3.394
3.576
3.477
3.633
3.728
2.992
1.510[h]
1.514[h]
1.521[h]
–
2.842
2.786
P21/c (8)
[GaCl4]
P21/c (8)
2.786
[PF6]
P21/n (4)
P21/c (4)
P21/c (4)
P21/c[g]
2.499
2.483
2.437
2.796[g]
2.654
2.806
2.641
2.847
2.832
2.971
[AsF6]
[SbF6]
[SbCl6]
[CHB11H5Cl6]
[CHB11H5Cl6]ÁCH3CN
P21/c (4)
P21/c (4)
P21/c (4)
P21/c (4)
Pna21 (4)
Pna21 (4)
P21/c (4)
P21/c (4)
[CHB11Cl11
]
[CHB11Cl11]Á2 toluene
[CHB11H5Br6]ÁCH2Cl2
[CHB11H5Br6]ÁCH3CN
[CF3SO3]
2.513
[h]
[CF3COO]
[h]
ꢀ
P1 (4)
[N3]
ꢀ
R3:H (6)
H Á toluene
H Á benzene
[a]
–
–
ꢀ
R3:H (6)
–
sg space group and Z number of units per cell; [b] see definition in Fig. 2; [c] closest distance of central C1 atom to the plane spanned by C2–
[d]
[e]
[f]
C20–C200 see Fig. 2;
determination by A. H. Gomes des Mesquita et al. [68];
closest distance of central C1 atom to the anion;
[g]
closest distance between cation and anion;
[h]
only unit cell
no salts, but covalently bonded
[Ph3C][SbCl6]ÁC6H6 by J. Krauße et al. [46].;
˚
cation–anion distances are all within the range 2.4–3.0 A
(Table 2) displaying always weak XanionÁÁÁH–Ccation van
der Waals interactions (X = O, F, Cl or Br). For all these
salts, a large number of such weak XanionÁÁÁH–Ccation van
der Waals interactions are observed. For example, the
and all of these eight cations display FSbF6ÁÁÁH–Ccation
˚
interactions with FÁÁÁH distances smaller than 3.28 A
˚
[RrvdW(HÁÁÁF) = 3.28 A]. It is noteworthy to mention that a
broad array of hydrogen van der Waals radii ranging from
˚
0.47 to 1.81 A can be found in the literature [74]. Mostly,
-
˚
the value of 1.20 A tabulated by Bondi was used [75],
SbF6 ion is embedded in eight tritylium ions (Fig. 11),
123

1648
Struct Chem (2015) 26:1641–1650
Fig. 10 Electrostatic potential mapped onto the electron density (red areas of negative charge; blue areas of positive) of an ion pair of
[Ph3C][CF3SO3] (left), [Ph3C][SbF6] (middle) and [Ph3C][HCB11Cl11] (right); colored version can be found in the ESI (Color figure online)
rather than directly pointing to the central C1 atom. Hence,
the shortest cation–anion distances are always XanionÁÁÁH–
C
cation distances and not C1ÁÁÁX distances which are always
significantly longer (see Table 2). The smallest C1ÁÁÁX
distance [X = heavy atom of the anion with the closest
distance to the central carbon atom (C1) of the tritylium
-
ion] is between 2.99 A (CF3SO3 ) and 4.05 A [SbCl6-, cf.
˚
˚
˚
˚
˚
RrvdW(CÁÁÁF) = 3.17 A, CÁÁÁO 3.27 A, CÁÁÁCl 3.45 A,
˚
CÁÁÁBr 3.53 A].
As expected, the average C1–C bond lengths of the
˚
˚
tritylium salts (1.443–1.448 A, cf. 1.444 A for the com-
puted gas-phase species, vide supra) are significantly
shorter compared with the covalently bound compounds
˚
˚
(N3: 1.530–1.544 A, CF3COO: 1.528 A, cf. Ph3C–H:
˚
1.527–1.529 A) clearly indicating double-bond character,
while in case of the azide and acetate species a C1–C single
bond can be assumed. In accord with the short C1–C bond
length in case of the salts, the sum of the angles around C1
is almost 360°, displaying local planarity around the for-
mally sp2 hybridized C1, and the local symmetry of the
cation is close to D3 as observed for the gas-phase species
(vide supra). Thus, the agreement between the gas-phase
and the solid-state structural data is very good taking inter-
molecular interactions (solid-state effects) into account.
The deviation of the tritylium ion from planarity can also
be described by the distance (dplane in Table 2) of the C1
atom from the plane spanned by the three attached C atoms
of the phenyl groups, which is rather small, lying in the
Fig. 11 [SbF6]- ion embedded in eight tritylium ions in the crystal
([Ph3C]? ions drawn which lie within FÁÁÁH 3.3 A); colored version
˚
can be found in the ESI
although his approach was recently criticized. The choice
of the hydrogen van der Waals radius is not trivial and can
be discussed at length. To be on the safe side, we have used
˚
˚
˚
the largest value of 1.81 A for H atom and the standard
range 0.001 A (AlCl4)–0.015 A ([CHB11H5Br6]ÁCH3CN),
˚
˚
while the covalently bound species show large deviation
3
due to the formal sp type C1 atom ([0.4 A). The amount
value of 1.47 A for the F atom, summing up to 3.28 A.
Figure 10 shows the electrostatic potential mapped onto
the electron density of an ion pair of [Ph3C][CF3SO3] (left),
[Ph3C][SbF6] (middle) and [Ph3C][HCB11Cl11] showing
that always two or three (negatively charged) donor atoms
of the anion arrange in such a fashion that they can best
interact with the positively charged H atoms of the cation
˚
of propeller type character of the tritylium ion is best
described by the angle a (for definition see Fig. 2), which
is in the range between 28.6° (SbF6) and 34.2° (BCl4) and
thus comparable to the value of 33.1° for the gas-phase
species.
123

Struct Chem (2015) 26:1641–1650
1649
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