[C6F5Xe]+ and [C6F 5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C 6F5)

Article abstract of DOI:10.1021/ic702259c

New examples of [C₆F₅Xe]⁺ salts of the weakly coordinating [BY₄]^- (Y = CN, CF₃, or C₆F₅) anions were synthesized by metathesis of [C ₆F₅Xe][BF₄] with M^I[BY₄] (M^I = K or Cs; Y = CN, CF₃, or C₆F₅) in CH₃CN at -40°C, and were crystallized from CH ₂Cl₂ or from a CH₂Cl₂/CH ₃CN solvent mixture. The low-temperature (-173°C) X-ray crystal structures of the [C₆F₅Xe]⁺ cation and of the [C₆F₅XeNCCH₃]⁺ adduct-cation are reported for [C₆F₅Xe][B(CF₃)₄], [C₆F₅XeNCCH₃][B(CF₃)₄], [C₆F₅Xe][B(CN)₄], and [C₆F ₅XeNCCH₃][B(C₆F₅)₄]. The [C₆F₅Xe]⁺ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF₃)₄]^- anion. The solid-state Raman spectra of the [C₆F₅Xe]⁺ and [C₆F₅XeNCCH₃]⁺ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C₆F₅XeNCCH₃]⁺ is approximately half that of [FXeNCCH₃]⁺. Coordination of CH₃CN to [C₆F₅Xe]⁺ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.

Full text of DOI:10.1021/ic702259c

Inorg. Chem. 2008, 47, 3205-3217
[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of the Weakly Coordinating Borate
Anions, [BY4]- (Y ) CN, CF3, or C6F5)
Karsten Koppe,†,‡ Hermann-J. Frohn,*,† Hélène P. A. Mercier,‡ and Gary J. Schrobilgen*,‡
Anorganische Chemie, UniVersität Duisburg-Essen, Lotharstrasse 1, D-47048 Duisburg, Germany,
and Department of Chemistry, McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada
Received November 16, 2007
New examples of [C6F5Xe]+ salts of the weakly coordinating [BY4]- (Y ) CN, CF3, or C6F5) anions were synthesized
by metathesis of [C6F5Xe][BF4] with MI[BY4] (MI ) K or Cs; Y ) CN, CF3, or C6F5) in CH3CN at -40 °C, and were
crystallized from CH2Cl2 or from a CH2Cl2/CH3CN solvent mixture. The low-temperature (-173 °C) X-ray crystal
structures of the [C6F5Xe]+ cation and of the [C6F5XeNCCH3]+ adduct-cation are reported for [C6F5Xe][B(CF3)4],
[C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4]. The [C6F5Xe]+ cation, in each structure,
interacts with either the anion or the solvent, with the weakest cation–anion interactions occurring for the [B(CF3)4]-
anion. The solid-state Raman spectra of the [C6F5Xe]+ and [C6F5XeNCCH3]+ salts have been assigned with the
aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor–acceptor bond
dissociation energy of [C6F5XeNCCH3]+ is approximately half that of [FXeNCCH3]+. Coordination of CH3CN to
[C6F5Xe]+ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial
charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a
2c–2e to a 3c–4e bond.
Introduction
The syntheses of several new salts of the [C6F5Xe]+ cation,
namely, those of the [B(CF3)4]-, [B(CN)4]-, and [B(OTeF5)4]-
anions, as well as the [B(CF3)4]- and [B(C6F5)4]- salts of
the [C6F5XeNCCH3]+ cation, have recently been described
along with their solution characterizations by multi-NMR
spectroscopy, stabilities, and reactivities.8 The ability of the
[C6F5Xe]+ cations of the [B(CF3)4]- and [B(C6F5)4]- salts
to coordinate CH3CN to xenon is primarily a consequence
of the weakly coordinating natures of these anions. The use
of very weakly coordinating anions suggested that it may
be possible to isolate and characterize the [C6F5Xe]+ cation
in a solid-state environment that more closely resembles the
gas-phase cation, provided a suitable weakly coordinating
solvent is used for synthesis and crystallization. The present
paper reports the solid-state structural characterizations of
[C6F5Xe][BY4] (Y ) CF3 or CN) and [C6F5XeNCCH3][BY4]
(Y ) CF3 or C6F5) by low-temperature X-ray crystallography
and Raman spectroscopy and electronic structure calculations
of the gas-phase [C6F5Xe]+ and [C6F5XeNCCH3]+ cations,
The [C6F5Xe]+ cation has been a subject of interest since
its discovery in 1989.1–3 Although the prior characterization
of [C6F5Xe]+ has included the X-ray crystal structure
determinations of several of its salts, none of the structures
have provided a well-isolated [C6F5Xe]+ cation. In
[C6F5Xe][AsF6],4 there is a close contact to one fluorine
(2.672(5) Å) of the anion. The [C6F5XeNCCH3][(C6F5)2BF2]
(2.681(8) Å)3 and [C6F5XeNC5H3F2][AsF6] (2.694(5) Å)5
salts contain [C6F5Xe]+ cations that are weakly coordinated
to nitrogen bases, whereas in C6F5XeOC(O)C6F5, the Xe---O
interaction is significantly stronger (2.367(3) Å).6,7
* To whom correspondence should be addressed. E-mail: h-j.frohn@uni-
due.de (H.-J.F.); schrobil@mcmaster.ca (G.J.S).
†
Universität Duisburg-Essen.
McMaster University.
‡
(1) Naumann, D.; Tyrra, W. J. Chem. Soc., Chem. Commun. 1989, 47–
50.
(2) Frohn, H.-J.; Jakobs, S. J. Chem. Soc., Chem. Commun. 1989, 625–
627.
(3) Frohn, H.-J.; Jakobs, S.; Henkel, G. Angew. Chem., Int. Ed. Engl.
1989, 28, 1506–1507. ; Angew. Chem. 1989, 101, 1534–1536.
(4) Frohn, H.-J.; Klose, A.; Schroer, T.; Henkel, G.; Buss, V.; Opitz, D.;
Vahrenhorst, R. Inorg. Chem. 1998, 37, 4884–4890.
(5) Frohn, H.-J.; Schroer, T.; Henkel, G. Z. Naturforsch. 1995, 50b, 1799–
1810.
(6) Frohn, H.-J.; Klose, A.; Henkel, G. GIT Fachz. Lab. 1993, 752–755.
(7) Frohn, H.-J.; Klose, A.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1993,
32, 99–100. ; Angew. Chem. 1993, 105, 114–115.
(8) Koppe, K.; Bilir, V.; Frohn, H-J.; Mercier, H. P. A.; Schrobilgen, G. J.
Inorg. Chem. 2007, 46, 9425–9437.
10.1021/ic702259c CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 8, 2008 3205
Published on Web 03/26/2008

Koppe et al.
Table 1. Crystallographic Data for [C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(C6F5)4], and [C6F5XeNCCH3][B(CN)4]
[C6F5XeNCCH3][B(CF3)4]
[C6F5Xe][B(CF3)4]
[C6F5XeNCCH3][B(C6F5)4]
[C6F5Xe][B(CN)4]
chemical formula
space group
a (Å)
b (Å)
c (Å)
C12H3F17BNXe
Pca21
18.903(8)
7.472(4)
12.801(6)
90
90
90
1808(2)
4
2505.06
2.284
C10F17BXe
P21/c
6.751(2)
15.950(5)
14.715(4)
90
99.740(5)
90
1561.5(8)
4
C32F25H3BNXe
C10F5BN4Xe
Pbca
9.919(2)
16.269(4)
16.783(4)
90
90
90
2708(2)
8
3306.00
2.027
P21/c
10.606(2)
22.458(4)
13.835(2)
90
94.775(5)
90
3284(1)
4
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
molecules/unit cell
mol wt (g mol-1
)
2340.84
2.489
4073.86
2.060
calcd density (g cm-3
)
T (°C)
-173
-173
-173
-173
µ (mm-1
)
2.08
2.39
1.22
2.60
a
R1
R2
0.0356
0.0469
0.0439
0.0938
0.0569
0.1239
2
0.0364
0.0481
b
a R1 is defined as ∑|Fo| - |Fc|/∑|Fo| for I > 2σ(I). b R2 is defined as [∑[w(Fo - Fc )2]/∑w(Fo )2]1/2 for I > 2σ(I).
2
2
which are compared with the solid-state geometries and are
used to assist in the assignments of their vibrational frequen-
cies.
anions in this study are in good agreement with the published
parameters and require no further comment.
The geometrical parameters obtained for the [B(CN)4]-
anion in the present study are in good agreement with the
literature values; however, the closest contact distance
between a nitrogen atom and the xenon atom in
[C6F5Xe][B(CN)4], 2.716(3) Å, is noteworthy (Figure 4). The
latter contact is significantly less than the sum of the xenon
(2.16 Å) and nitrogen (1.55 Å) van der Waals radii14 and is
indicative of a significant cation–anion interaction. This
interaction, however, does not significantly manifest itself
in the geometry of the [B(CN)4]- anion, with the C-N bond
that is involved in the contact being equal in length, within
(3σ, to the other three C-N bonds. The Raman spectrum
in the CN stretching region, however, does show that the
anion symmetry is lowered by coordination to the [C6F5Xe]+
cation (see Raman Spectroscopy). The B-C bond lengths
and CBC bond angles are not significantly influenced by ion-
pair formation and are equal within (3σ. This is confirmed
by Raman spectroscopy and is attributable to the buffer effect
of the CN triple bond in the cation–anion interaction.
The Xe---N distance in [C6F5Xe][B(CN)4] (2.716(3) Å)
is comparable to the Xe---F bridge distances in
[C6F5Xe][AsF6] (2.714(5) and 2.672(5) Å).4 Aside from the
long Xe---N contact, the anion retains its Td symmetry and
gives no indication that the C-N bond of the coordinated
nitrogen atom is significantly elongated relative to the
remaining three C-N bonds. The ion-pair interaction is
apparently stronger with [AsF6]-, giving rise to a longer
As---F bridge bond having increased polarity, whereas the
stronger and more covalent C-N bond is little affected in
the [B(CN)4]- salt (vide supra). The interaction in
[C6F5Xe][B(CF3)4] is even weaker with an Xe---F distance
of 2.913(4) Å compared to the van der Waals sum of 3.63
Å for xenon and fluorine,14 thus providing the most weakly
coordinated [C6F5Xe]+ cation documented to date.
Results and Discussion
Preparation of [C6F5Xe]+ Salts. The syntheses of
[C6F5Xe][B(CF3)4], [C6F5Xe][B(CN)4], [C6F5XeNCCH3]-
[B(CF3)4], and [C6F5XeNCCH3][B(C6F5)4] were achieved in
nearly quantitative yields and in high purities by metatheses
of [C6F5Xe][BF4] with the corresponding MI[BY4] (Y ) CF3,
CN, or C6F5; MI ) K or Cs) salts in CH3CN at -40 °C (eq
1). Acetonitrile is the preferred solvent for the metatheses
CH3CN
[C6F5Xe][BF4] + MI[BY4]
8 [C6F5Xe][BY4] +
RT to -40 °C
MI[BF4]V (1)
because MI[BF4] salts have low room-temperature solubilities
(5 mmol L-1; negligible at -40 °C) and because
[C6F5Xe][BY4] (Y ) CF3, CN, or C6F5) and [C6F5Xe][BF4]
have high solubilities even at -40 °C. Minor amounts of
MI[BF4] contaminants were avoided at -40 °C when the
metatheses were carried out at high concentrations (1–1.5
mol L-1) which render MI[BF4] essentially insoluble. De-
tailed synthetic descriptions are provided in ref 8.
X-ray Crystal Structures of [C6F5Xe][B(CF3)4], [C6F5Xe]-
[B(CN)4], [C6F5XeNCCH3][B(CF3)4], and [C6F5XeNCCH3]-
[B(C6F5)4]. Summaries of the refinement results and other
crystallographic information are provided in Table 1. Im-
portant bond lengths and bond angles are listed in Table 2
along with calculated values.
The crystal structures of the [B(CF3)4]-
9
and
[B(C6F5)4]- 10 anions have been determined in their respec-
tive Cs+ and [H(OEt2)2]+ salts and that of the [B(CN)4]-
anion has been determined in M[B(CN)4] (M ) Li, Na, Cu,
Rb, Cs, [NH4], and Tl;11 Ag, K, [N(C4H9)4];12 and
[P(C6H5)4]).13 The structural parameters obtained for the
[B(CF3)4]- (Figures 1 and 2) and [B(C6F5)4]- (Figure 3)
(11) Küppers, T.; Bernhardt, E.; Willner, H.; Rohm, H. W.; Köckerling,
M. Inorg. Chem. 2005, 44, 1015–1022.
(9) Bernhardt, E.; Henkel, G.; Willner, H.; Pawelke, G.; Bürger, H.
Chem.sEur. J. 2001, 7, 4696–4705.
(12) Bernhardt, E.; Henkel, G.; Willner, H. Z. Anorg. Allg. Chem. 2000,
626, 560–568.
(10) Jutzi, P.; Müller, C.; Stammler, A.; Stammler, H.-G. Organometallics
2000, 19, 1442–1444.
(13) Finze, M.; Bernhardt, E.; Berkei, M.; Willner, H.; Hung, J.; Waymouth,
R. M. Organometallics 2005, 24, 5103–5109.
3206 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
Table 2. Experimental Geometrical Parameters for [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4], [C6F5XeNCCH3][B(C6F5)4], and [C6F5Xe][B(CN)4]
and Calculated Geometrical Parameters for [C6F5Xe]+ and [C6F5XeNCCH3]+
calcda
[B(CF3)4]- salt
[B(C6F5)4]- salt
[B(CN)4]- salt
[C6F5Xe]+
[C6F5Xe]+
[C6F5XeNCCH3]+
[C6F5Xe]+
[C6F5XeNCCH3]+
[C6F5XeNCCH3]+
SVWN
PBE1
SVWN
PBE1
bond lengths (Å)
2.100(10)
1.363(14)
1.353(11)
1.384(15)
1.328(12)
1.389(15)
1.351(12)
1.349(15)
1.346(13)
1.373(15)
1.336(11)
1.362(14)
2.610(11)
1.150(14)
1.445(18)
Xe(1)-C(1)
C(1)-C(2)
C(2)-F(1)
C(2)-C(3)
C(3)-F(2)
C(3)-C(4)
C(4)-F(3)
C(4)-C(5)
C(5)-F(4)
C(5)-C(6)
C(6)-F(5)
C(6)-C(1)
Xe(1)---N(1)
N(1)-C(7)
C(7)-C(8)
Xe(1)---N(4)
Xe(1)---F(10a)
2.104(5)
1.361(7)
1.331(6)
1.373(7)
1.334(6)
1.377(8)
1.330(7)
1.369(9)
1.335(6)
1.362(8)
1.324(6)
1.384(7)
2.100(6)
1.394(8)
1.315(8)
1.388(14)
1.372(10)
1.345(15)
1.335(8)
1.384(8)
1.308(7)
1.382(9)
1.341(6)
1.345(8)
2.640(6)
1.095(8)
1.478(9)
2.081(3)
1.380(4)
1.331(4)
1.379(4)
1.337(4)
1.377(5)
1.343(4)
1.378(5)
1.335(4)
1.380(4)
1.338(4)
1.377(4)
2.095
1.380
1.308
1.387
1.303
1.391
1.300
1.391
1.303
1.387
1.308
1.380
2.106
1.384
1.311
1.388
1.308
1.392
1.304
1.392
1.308
1.388
1.311
1.384
2.104
1.379
1.315
1.385
1.307
1.389
1.304
1.389
1.307
1.385
1.315
1.379
2.557
1.153
1.423
2.101
1.384
1.316
1.386
1.311
1.391
1.307
1.391
1.311
1.386
1.316
1.384
2.674
1.148
1.443
2.716(3)
2.913(4)
bond angles (deg)
176.9(3)
150.3(9)
177.6(13)
118.8(8)
C(1)-Xe(1)---N(1)
Xe(1)---N(1)-C(7)
N(1)-C(7)-C(8)
Xe(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(1)-C(2)-F(1)
F(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(2)-C(3)-F(2)
F(2)-C(3)-C(4)
C(3)-C(4)-C(5)
C(3)-C(4)-F(3)
F(3)-C(4)-C(5)
C(4)-C(5)-C(6)
C(4)-C(5)-F(4)
F(4)-C(5)-C(6)
C(5)-C(6)-C(1)
C(5)-C(6)-F(5)
F(5)-C(6)-C(1)
C(6)-C(1)-Xe(1)
C(6)-C(1)-C(2)
174.9(2)
155.0(7)
177.8(9)
117.8(5)
115.3(8)
121.7(7)
123.1(8)
123.2(9)
116.8(10)
120.1(10)
119.9(9)
120.3(8)
119.6(7)
118.6(7)
120.2(7)
121.1(6)
120.2(6)
120.1(6)
119.7(6)
119.3(5)
122.8(6)
179.9
180.0
180.0
118.6
118.6
120.9
120.5
119.5
120.3
120.2
121.0
119.5
119.5
119.5
120.2
120.3
118.6
120.5
120.9
118.6
122.8
179.98
179.98
179.97
118.7
118.5
120.9
120.6
119.6
120.1
120.2
121.0
119.5
119.5
119.6
120.2
120.1
118.5
120.6
120.9
118.7
122.7
117.8(4)
119.8(2)
119.1(3)
121.4(3)
119.5(3)
119.6(3)
120.4(3)
120.0(3)
121.4(3)
119.6(3)
119.0(3)
119.1(3)
120.4(3)
120.6(3)
119.6(3)
120.4(3)
120.0(2)
118.9(2)
121.3(3)
117.6
117.7
117.2
121.5
121.3
119.9
119.8
120.3
121.3
119.4
119.4
119.9
120.3
119.8
117.2
121.3
121.5
117.7
124.6
117.9(5)
120.8(5)
121.3(5)
119.9(5)
119.5(5)
120.6(5)
120.7(6)
118.9(6)
120.4(6)
120.9(5)
119.9(6)
119.2(6)
117.1(5)
122.1(5)
120.9(5)
118.4(4)
123.5(6)
119.6(10)
120.4(10)
119.9(10)
117.4(11)
119.5(11)
123.1(11)
122.2(11)
116.1(11)
121.7(10)
119.8(11)
117.8(11)
122.4(11)
118.7(11)
118.5(10)
122.8(11)
118.8(9)
117.1
121.6
121.3
119.9
119.8
120.3
121.2
119.4
119.4
119.9
120.3
119.8
117.1
121.3
121.6
117.6
124.8
122.1(10)
[C6F5Xe]+
[C6F5XeNCCH3]+
[B(CN)4]-
[B(CF3)4]-
[B(CF3)4]-
[B(C6F5)4]-
bond lengths (Å)
B-C
C-N
1.593(5)–1.609(5)
1.132(4)–1.144(4)
B-C
C-F
1.590(9)–1.617(8)
1.354(6)–1.372(7)
B-C
C-F
1.589(10)–1.629(11)
1.334(8)–1.347(9)
B-C
C-C
C-F
1.649(22)–1.682(19)
1.357(18)–1.407(18)
1.324(16)–1.389(16)
bond angles (deg)
C-B-C
B-C-N
106.8(3)–112.2(3)
174.6(4)–178.3(4)
C-B-C
B-C-F
F-C-F
107.5(5)–111.2(5)
113.0(5)–115.1(5)
103.5(4)–105.8(5)
C-B-C
B-C-F
F-C-F
108.4(7)–111.0(6)
111.4(6)–115.7(7)
103.0(6)–106.7(6)
C-B-C
B-C-C
C-C-C
C-C-F
100.7(10)–113.8(11)
119.5(11)–129.2(12)
119.0(12)–126.0(13)
113.3(12)–122.6(12)
a Calculated using SVWN/(SDB-)cc-pVTZ and PBE1PBE/(SDB-)cc-pVTZ levels.
The geometrical parameters of the unsolvated [C6F5Xe]+
cations and those of the nitrogen base adducts are equal,
within experimental error (Table 2), and are comparable to
those derived from the crystal structure of [C6F5Xe][AsF6].4
In particular, the Xe-C distances are in accord with the very
similar vibrational frequencies for modes containing signifi-
cant Xe-C stretching components (see Raman Spectros-
copy). As expected, the Xe-C bond lengths are compa-
rable to the I-C bond length in isoelectronic C6F5I (2.077(4)
Å.15 The only prior example showing a significant Xe-C
bond lengthening upon coordination of [C6F5Xe]+ is
C6F5XeOC(O)C6F5, with a Xe-C bond length of 2.122(4)
Å.6,7 It is noteworthy that [C6F5XeNCCH3]+ salts do not, in
all cases, crystallize from CH3CN/CH2Cl2 solutions. Crystal-
line salts containing the [AsF6]- anion that have been
(15) Frohn, H.-J.; Görg, S.; Henkel, G.; Läge, M. Z. Anorg. Allg. Chem.
1995, 621, 1251–1256.
(14) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3207

Koppe et al.
[C6F5Xe][BF4], and [C6F5XeNCCH3][B(C6F5)4] (Figure S1),
and their assignments are provided in Tables 3 and 4. The
frequency assignments are based on calculated gas-phase
frequencies of the [C6F5Xe]+ and [C6F5XeNCCH3]+ cations
(see Computational Results) and prior assignments of their
respective anions.
(a) [C6F5Xe]+, C6F5I, and [C6F5XeNCCH3]+. There
are no published vibrational frequency assignments for
the [C6F5Xe]+ or [C6F5XeNCCH3]+ cations, and only
unassigned infrared frequencies have been provided
6,7
for
C6F5XeOC(O)C6F5
and
[C6F5XeNCCH3][(C6F5)2BF2].3The frequency assignments for
[C6F5Xe]+ and [C6F5XeNCCH3]+ were made by comparison
with the calculated frequencies and Raman intensities (Tables
3 and 4) of the energy-minimized geometries (Figures 2 and
4) and, in the case of CH3CN modes, by comparison with
those of the free ligand and other CH3CN adducts.18 The
geometries and frequencies of C6F5H, C6F5Br, C6F5Cl, C6F6,
and isoelectronic C6F5I were also calculated using the same
methods and basis sets (Tables S1-S3), allowing compari-
sons to be made based on the new assignments.
Figure 1. Crystal structure of [C6F5Xe][B(CF3)4] where thermal ellipsoids
are shown at the 50% probability level.
obtained from CH3CN/CH2Cl2 solutions at ∼20 °C have no
CH3CN coordinated to [C6F5Xe]+.4 In this instance, the
[AsF6]- anion, which is generally regarded as a weakly
coordinating anion, is unexpectedly favored over CH3CN as
a coordinating base.
There is overall good agreement between the observed and
calculated frequencies for [C6F5Xe]+, [C6F5XeNCCH3]+, and
C6F5I, with slightly better agreement for the PBE1PBE
values, given in square brackets, below 900 cm-1. The bands
assigned to C6F5-ring modes are comparable to those
observed in other systems containing C6F5 groups, for
example, C6F5H,20 C6F6,21 C6F5Cl,22–24 and C6F5Br.22–24
The bands at 795 ([BF4]-), 794 ([B(CN)4]-), and 805
(C6F5I) cm-1 are assigned to ν(XeC) and ν(IC), which are
strongly coupled to C-F and C-C stretches. In contrast,
the frequencies corresponding to C-C stretching and C-C
stretching modes coupled to C-F modes occur at 1157–1665
cm-1 in C6F6.21 These bands were calculated at 772 [779]
([C6F5Xe]+) and 818 [829] (C6F5I) cm-1, respectively, and
were predicted to be intense in the infrared spectra, in
agreement with the very strong band observed at 808 cm-1
in the infrared spectrum of C6F5I.22–24 This frequency was
previously reported for the Cl (885 cm-1)22–24 and Br (836
cm-1)22–24 analogues, and is one of four bands that are
expected to display significant mass dependencies (Table S1).
In the prior studies, the 885 (Cl), 836 (Br), and 808 (I) cm-1
bands were assigned to pure Cl-C, Br-C, or I-C stretches
most likely because of their strong intensities in their infrared
spectra. The present study, however, reveals that the Xe-C
and I-C stretching modes also contribute to the most intense
Raman bands at 205 ([BF4]-), 201 ([B(CN)4]-), and 204
cm-1 (C6F5I) by coupling to an in-plane CCF bending mode
of the C6F5 ring. These coupled modes were calculated at
188 [193] and 204 [209] cm-1 for [C6F5Xe]+ and C6F5I,
respectively, and are predicted to be intense in their Raman
spectra. The calculated and experimental frequencies pro-
vided in the present work parallel the anticipated bond
The geometric parameters of the [C6F5XeNCCH3]+ cations
in the [B(CF3)4]- and [B(C6F5)4]- salts are essentially
identical to those published earlier for [C6F5XeNCCH3]-
[(C6F5)2BF2].3 In all three cases, the adduct-cation is
well separated from the anion. The experimental C-N
bond lengths for [C6F5XeNCCH3][(C6F5)2BF2] (1.140(8)
Å), [C6F5XeNCCH3][B(CF3)4] (1.096(9) Å), and
[C6F5XeNCCH3][B(C6F5)4] (1.157(19) Å) are equal, within
(3σ, to that observed in CH3CN (1.141(2) Å).16 This is in
agreement with the Raman spectra which show only a small
complexation shift for the C-N stretching frequency (vide
infra). This shift is much smaller for [C6F5XeNCCH3]-
[B(C6F5)4] (25 cm-1) and [C6F5XeNCCH3][B(CF3)4]) (30
cm-1) than for [FXeNCCH3]+ (59 cm-1)17 where the C-N
and Xe---N distances are 1.12(1) and 2.179(7) Å,18 respec-
tively, and are in accord with [XeF]+ being a significantly
stronger Lewis acid than [C6F5Xe]+ (see Computational
Results). The C-Xe---N bond angles are 174.8(2) and
177.1(4)° in the [B(CF3)4]- and the [B(CN)4]- salts,
respectively, andare comparable to that of the [(C6F5)2BF2]-
salt (174.5(3)°). The C-Xe---N bend is assigned at 161 and
164 cm-1 in the [B(CF3)4]- and [B(C6F5)4]- salts, respec-
tively, suggesting these angles are highly deformable and
that deviation from linearity in [C6F5XeNCCH3]+ arises from
crystal packing. A similar deviation from linearity has been
observed in [C6F5Xe][AsF6], where the C-Xe---F angles
were 170.5(3) and 174.2(3)°.4
Raman Spectroscopy. The vibrational frequencies ob-
tained from the low-temperature solid-state Raman spectra
of [C6F5Xe][B(CN)4], [C6F5XeNCCH3][B(CF3)4] (Figure 5),
(19) Bates, J. B.; Quist, A. S. Spectrochim. Acta 1975, 31A, 1317–1327.
(20) Steele, D.; Whiffen, D. H. Spectrochim. Acta 1960, 17, 368–375.
(21) Steele, D.; Whiffen, D. H. Trans. Faraday Soc. 1959, 55, 369–376.
(22) Long, D. A.; Steele, D. Spectrochim. Acta 1963, 19, 1947–1954.
(23) Long, D. A.; Steele, D. Spectrochim. Acta 1963, 19, 1955–1961.
(24) Hyams, I. J.; Lippincott, E. R. Spectrochim. Acta 1966, 22, 695–702.
(16) Enjalbert, R.; Galy, J. Acta Crystallogr. 2002, 58B, 1005–1010.
(17) Emara, A. A. A.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.
1987, 1644–1646.
(18) Fir, B. A. Master of Science Dissertation, McMaster University,
Hamilton, Ontario, Canada, 1999.
3208 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
Figure 2. (a) Crystal structure of [C6F5XeNCCH3][B(CF3)4] where thermal ellipsoids are shown at the 50% probability level; (b) calculated geometry for
[C6F5XeNCCH3]+ (SVWN/(SDB-)cc-pVTZ).
Figure 3. Crystal structure of [C6F5XeNCCH3][B(C6F5)4] where thermal
ellipsoids are shown at the 50% probability level.
strength trends for a series of xenon(II) cations in which
xenon is bonded to a second row element: ν(XeC) (201, 205
Figure 4. (a) Crystal structure of [C6F5Xe][B(CN)4] where thermal
ellipsoids are shown at the 50% probability level; (b) calculated geometry
for [C6F5Xe]+ (SVWN/(SDB-)cc-pVTZ).
cm-1, [C6F5Xe]+) < ν(XeN) (224 cm-1, [F5SN(H)Xe]+)25
<
ν(XeO) (491 cm-1, [XeOTeF5]+)26 < ν(XeF) (608, 610 cm-1;
[XeF]+).27 The bands at 106, 119 ([BF4]-); 85, 122, 126
([B(CN)4]-); and 110, 114 (C6F5I) cm-1 are assigned to the
δ(CCXe) and δ(CCI) out-of-plane and in-plane deformation
modes, respectively, coupled to out-of-plane and in-plane
δ(CCF) deformation modes, which are in good agreement
with the calculated values of 77 [81], 112 [122] ([C6F5Xe]+)
and 80 [82], 124 [133] (C6F5I) cm-1. The calculated trends
(cm-1) for the δ(CCX)oop and δ(CCX)ip modes (cm-1) are
as follows: H (804, 1142) [855, 1179] > F (135, 251) [138,
268] > Cl (107, 191) [109,135] > Br (90, 147) [92,134] >
I (80, 124) [82, 133] > Xe (77, 112) [81, 122].
and 15 [8] cm-1 upon coordination but, in practice, they are
observed at nearly the same frequencies in the [C6F5Xe]+
salts (794, 795 and 201, 205 cm-1) and [C6F5XeNCCH3]+
salts (795, 796 and 202, 203 cm-1). The insensitivity of these
modes to coordination is in accord with the insensitivity of
the [C6F5Xe]+ geometrical parameters to coordination (see
X-ray Crystal Structures). The C-N stretch is predicted to
shift by 10 [7] cm-1 upon coordination, which is less than
the observed shift (CH3CN, 2248 cm-1; [C6F5XeNCCH3]
[B(C6F5)4], 2273 cm-1; [C6F5XeNCCH3][B(CF3)4], 2278
cm-1), but is consistent with a weak donor–acceptor interac-
Both coupled modes involving Xe-C stretching compo-
nents are predicted to shift to higher frequency by 19 [23]
(26) Fir, B.; Whalen, J. M.; Mercier, H. P. A.; Sanders, J. C. P.; Dixon,
D. A.; Schrobilgen, G. J. J. Fluorine Chem 2001, 110, 89–107.
(27) Smith, G. L.; Mercier, H. P. A.; Schrobilgen, G. J. Inorg. Chem. 2007,
46, 1369–1378.
(25) Smith, G. L.; Mercier, H. P. A.; Schrobilgen, G. J. Inorg. Chem. 2008,
in press [DOI: ic702039f].
Inorganic Chemistry, Vol. 47, No. 8, 2008 3209

Koppe et al.
to A symmetries under the crystal site symmetry, C1,
resulting in removal of the E-degeneracy and the appearance
of three bands in the Raman spectrum. The fourth C-N
stretching band arises from the unique CN ligand coordinated
to the [C6F5Xe]+ cation. Additionally, a factor-group analysis
that treats only the C-N stretching modes was carried out
(Table S4). Although the analysis indicates that four ad-
ditional splittings should be observed on each Raman and
infrared band, these splittings and associated couplings within
the unit cell are too small to be resolved in the Raman
spectrum. Unlike the C-N stretches, the B-C stretches can
be assigned under Td symmetry, with only ν7(T2) showing a
splitting (2 cm-1; see Table 3, footnote b).
(ii) [B(C6F5)4]-. Although [B(C6F5)4]- has been widely
exploited as a weakly coordinating anion,29 no detailed
vibrational data and assignments appear to be available for
this anion. The experimental spectra of [B(C6F5)4]- in the
[C6F5XeNCCH3]+ (Figure S1) and Cs+ (Figure S2) salts were
therefore assigned by comparison with the frequencies
calculated from the optimized anion geometry using the
SVWN method (Tables 4 and S5 and Figure S3). There is
good overall agreement between the calculated and experi-
mental frequencies. The bands at 822 and 824 cm-1 are
assigned to the symmetric BC4 stretch, which is expected to
be very intense in the Raman spectrum and is calculated at
837 cm-1. The asymmetric BC3 stretches are expected to
occur at higher frequency, that is, 940, 986 cm-1, but only
one band is observed at 978 cm-1. Lower frequency modes
are calculated at 600 and 609 cm-1 which are best described
as coupled δ(CCC)oop and νas(BC3) modes, but they were
not observed in accord with their calculated very low
intensities. The out-of-phase [δ(BC2) - δ(BC2)] mode is
coupled to the δ(CCC)oop bending mode and is assigned to
the band observed at 684 cm-1 (calcd, 673 cm-1). The in-
phase [δ(BC2) + δ(BC2)] mode is coupled to the δ(CCC)oop
bending mode and is assigned to the band at 400 cm-1 (calcd,
409 cm–1). The BC2 rocking modes occur at 358 cm-1 and
are in good agreement with the calculated value of 364 cm-1.
The remaining medium intensity bands at 574, 758, 769, and
776 cm-1 are BC2 wagging modes that are strongly coupled
to ring bending modes, δ(CCC)oop or δ(CCC)ip. These modes
are calculated at 578, 749, 765, and 775 cm-1, respectively.
Computational Results. The electronic structures of the
[C6F5Xe]+ cation and the [C6F5XeNCCH3]+ adduct-cation
were calculated using pure density functional theory (DFT)
methods and (SDB-)cc-pVTZ basis sets starting from Cs and
C1 symmetries, respectively. All calculations resulted in
stationary points with all frequencies real. Although the
[C6F5Xe]+ cation4,30,31 and CH3CN31,32 molecule had previ-
ously been the subject of theoretical calculations, they were
recalculated under the same conditions as for
[C6F5XeNCCH3]+ to confirm the experimental changes that
Figure 5. Raman spectra (-150 °C, 1064-nm excitation) of (a)
[C6F5Xe][B(CN)4] and (b) [C6F5XeNCCH3][B(CF3)4].
tion (vide supra). The weak interaction is also supported by
the calculated low frequency for the Xe-N stretch (154 [141]
cm-1). This frequency is significantly lower than those
assigned for more covalent or stronger donor–acceptor
Xe---N bonds, for example, [FXeNCH]+, 330 cm-1;17,28
[FXeNCCH3]+, 268, 284, 288 cm-1;18 [FXeNSF3]+, 194
cm-1.27 The experimental frequency at 144 cm-1
([B(CF3)4]-) may be tentatively assigned to both ν(XeN) and/
or δ(CCF)ip + δ(CXeN)ip, because of the similarity of the
calculated frequencies for both modes (Table 4) using SVWN
and PBE1PBE (values in square brackets) methods. The in-
plane and out-of-plane δ(CXN) and δ(XNC) deformation
modes are calculated at 142 [134], 166 [168] cm-1 and 71
[74], 86 [87], 121 [113] cm-1, respectively, and are coupled
with in-plane and out-of-plane δ(CCF) deformation modes.
(b) [BY4]- Anions. The vibrational frequencies of
19
[B(CN)4]-,12 [BF4]-, and [B(CF3)4]– 9 are in agreement
with those reported earlier; their assignments have therefore
been made by comparison with the published assignments,
and they are not discussed further for the [BF4]- and
[B(CF3)4]- salts (Tables 3 and 4, footnotes c and b,
respectively).
(i) [B(CN)4]-. The [B(CN)4]- anion is weakly coordinated
through a single nitrogen atom in the crystal structure of
[C6F5Xe][B(CN)4] (see X-ray Crystal Structures). There are
four well-resolved bands in the C-N stretching region whose
frequencies are in close agreement with the previously
reported frequencies of the ν1(A1) and ν6(T2) C-N stretching
modes of [B(CN)4]-.12 The four bands are interpreted in
terms of a slightly distorted [B(CN)4]- anion having local
C3V symmetry in which the A1 and T2 symmetries under Td
symmetry correlate to A1 and A1 + E symmetries, respec-
tively, under C3V symmetry. The symmetries of the C-N
stretches of the three uncoordinated CN groups are reduced
(29) Piers, W. E.; Chivers, T. Chem. Soc. ReV. 1997, 26, 345–354.
(30) Frohn, H-J.; Theißen, M. Angew. Chem., Int. Ed 2000, 39, 4591–
4593. ; Angew. Chem. 2000, 112, 4762–4764.
(31) Semenov, S. G. ; Sigolaev, Yu. F. Russ. J. Org. Chem. (Transl.) 2004,
40, 1835–1837; Zh. Org. Khim, 2004, 40, 1880–1881.
(32) Alia, J. M.; Edwards, H. G. M. J. Phys. Chem. A 2005, 109, 7977–
7987.
(28) Emara, A. A. A.; Schrobilgen, G. J. Inorg. Chem. 1992, 31, 1323–
1332.
3210 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
Table 3. Raman Frequencies and Intensities for [C6F5Xe]+ in [C6F5Xe][B(CN)4] and [C6F5Xe][BF4],a and Calculated Vibrational Frequencies,
Intensities, and Assignments for [C6F5Xe]+
a Raman spectra were recorded at -150 °C in 5-mm o.d. Pyrex precision glass NMR tubes using 1064-nm excitation. Frequencies are given in cm-1, and
relative Raman intensities are given in parentheses. b Bands at 2245(100) [νCN, ν1(A1)], 2240(54), 2234(41), 2227(44) [νCN, ν6(T2)], 951(6), 944(6) [νBC
,
ν7(T2)], 526(16), 524(13) [νBCN, ν3(E)], 486(10) [νBC, ν2(A1)], 329(5) [νBCN, ν5(T1)], and 143(14) [νCBC, ν9(T2)] cm-1 were assigned to [B(CN)4]- by
comparison with those given for K[B(CN)4] in ref 12. c Bands at 1107(11) [νBF, ν3(T2)], 760(32) [νBF, ν1(A1)], 524(10), 518(9) [ν4(T2)], and 363(14),
354(59) [ν2(E)] cm-1 were assigned to [BF4]- by comparison with previous assignments given in ref 19. d SVWN/(SDB-)cc-pVTZ and PBE1PBE/(SDB-)-
cc-pVTZ; calculated infrared intensities, in km mol-1, are given in square brackets and calculated Raman intensities, in Å4 amu-1, are given in
parentheses. The entry, <1, denotes an intensity that is 0.1–1.0. e The deformation modes of the C6F5 ring are denoted by δ and are relative to the plane
of the C6F5 ring; ip and oop denote in-plane and out-of-plane, respectively.
were observed upon coordination of CH3CN to [C6F5Xe]+.
Moreover, the previous calculations for [C6F5Xe]+ were at a
lower level [B3LYP/6–311G(3df,p) and B3LYP/6–311G(3d)]
and did not provide vibrational frequencies and intensities.
The present study takes into account relativistic effects by
employing semirelativistic effective core potentials (RLC
ECP), which was not the case in the earlier study. Key
optimized geometric parameters for [C6F5Xe]+ (Cs) and
[C6F5XeNCCH3]+ (C1) are listed in Table 2. The isoelectronic
systems, C6F5X (X ) Cl, Br, I), were also calculated.
There is good overall agreement between the observed
and the calculated bond lengths and bond angles. The
calculations also show that the Xe-C bond is expected
to elongate slightly upon coordination to nitrogen. The
Xe---N distance is slightly underestimated. The calculated
C-Xe---N angle (179.9°) differs from the observed
C-Xe---N angles (174.9(2)° and 176.9(3)°), suggesting
that the angle distortion at nitrogen arises from packing
effects. Such deviations from the ideal 180° angle about
Xe(II) have been encountered for RCN bases18 and F3SN27
coordinated to [FXe]+. The deformability of this angle is
also reflected in the low-frequencies calculated for the
C-Xe---N in-plane and out-of-plane bending modes
(Table 4). The Xe(1)-C(1) and N(1)-C(7) bond orders
Inorganic Chemistry, Vol. 47, No. 8, 2008 3211

Koppe et al.
Table 4. Raman Frequencies for [C6F5XeNCCH3][B(C6F5)4] and [C6F5XeNCCH3][B(CF3)4] and Calculated Frequencies for [C6F5XeNCCH3]+, CH3CN,
and [B(C6F5)4]-
3212 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
Table 4. Continued
Inorganic Chemistry, Vol. 47, No. 8, 2008 3213

Koppe et al.
Table 4. Continued
a Raman spectra were recorded at -150 °C in 5-mm o.d. Pyrex precision glass NMR tubes. Frequencies are given in cm-1, and relative Raman intensities
are given in parentheses. b SVWN/(SDB-)cc-pVTZ and PBE1PBE/(SDB-)cc-pVTZ; calculated infrared intensities, in km mol-1, are given in square brackets,
c
and calculated Raman intensities, in Å4 amu-1, are given in parentheses. Bands at 1293(9) [ν(10BCF3); ν1(A) and ν9(T)], 1279(15) [ν(11BCF3); ν1(A) and
ν9(T)], 1110(11) [ν(CF3), ν5(E)], 1034(8) [ν(CF3), ν11(T)], 724(65) [δ(CF3), ν2(A)], 696(8) [δ(CF3), ν13(T)], 546(8) [δ(CF3), ν6(E)], 525(15) [δ(CF3), ν14(T)],
519(11) [δ(CF3), ν15(T)], 318(21) [F(CF3), ν7(E)], 298(14) [F(CF3), ν16(T)], 226(9) [F(CF3), ν17(T)], and 52(9) [τ(CF3), ν19(T)] cm-1 were assigned to
[B(CF3)4]- by comparison with previous assignments given in ref 9. d Calculated geometrical parameters: (SVWN) N-C, 1.155 Å; C-C, 1.433 Å; C-H,
1.099 Å; ∠NCC, 180°; ∠CCH, 110.5°; ∠HCH, 108.4°; (PBE1PBE) N-C, 1.149 Å; C-C, 1.450 Å; C-H, 1.090 Å; ∠NCC, 180°; ∠CCH, 110.1°; ∠HCH,
108.9°. The entry, <1, denotes an intensity that is 0.1–1.0. e The C6F5 ring deformation modes are denoted by δ and are relative to the plane of the C6F5
ring; ip and oop denote in-plane and out-of-plane, respectively. f Anion and cation modes overlap. g SVWN/cc-pVTZ.
decrease slightly upon complexation, and that of C(7)-C(8)
increases concomitantly. Coordination of CH3CN to
[C6F5Xe]+ has no significant effect on the geometric
parameters of the C6F5 ring. The weakness of the
donor–acceptor interaction is consistent with the small
Xe(1)---N(1) bond order (0.11). Upon adduct formation,
3214 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
Table 5. Calculateda Natural Atomic Charges, Mayer Bond Orders, and Mayer Natural Atomic Orbital Valencies for [C6F5Xe]+, [C6F5XeNCCH3]+,
and CH3CN
[C6F5Xe]+
[C6F5XeNCCH3]+
CH3CN
charge [valency]
Xe(1)
C(1)
F(1)
C(2)
F(2)
C(3)
F(3)
C(4)
F(4)
0.90 [0,71]
-0.25 [3.10]
-0.24 [0.83]
0.29 [3.13]
-0.22 [0.74]
0.30 [3.02]
-0.22 [0.74]
0.32 [3.03]
-0.22 [0.74]
0.30 [3.02]
0.94 [0.78]
-0.31 [3.05]
-0.26 [0.81]
0.30 [3.13]
-0.23 [0.73]
0.29 [3.02]
-0.23 [0.73]
0.31 [3.03]
-0.23 [0.73]
0.29 [3.02]
-0.26 [0.81]
0.30 [3.13]
-0.46 [2.04]
0.45 [3.01]
-0.80 [3.32]
0.31 × 3 [0.79 × 3]
C(5)
F(5)
-0.24 [0.83]
0.29 [3.13]
C(6)
N(1)
C(7)
C(8)
H(1–3)
-0.31 [1.96]
0.27 [3.01]
-0.79 [3.30]
0.28 × 3 [0.79 × 3]
bond order
Xe(1)-C(1)
C(1)-C(2)
C(2)-F(1)
C(2)-C(3)
C(3)-F(2)
C(3)-C(4)
C(4)-F(3)
C(4)-C(5)
C(5)-F(4)
C(5)-C(6)
C(6)-F(5)
C(6)-C(1)
Xe(1)---N(1)
N(1)-C(7)
C(7)-C(8)
C(8)-H
0.72
1.15
0.85
1.11
0.77
1.11
0.77
1.11
0.77
1.11
0.85
1.15
0.68
1.16
0.84
1.12
0.76
1.11
0.76
1.11
0.76
1.12
0.84
1.16
0.11
1.91
1.04
0.76 × 3
1.95
1.01
0.76 × 3
a SVWN/(SDB-)cc-pVTZ.
the Xe atom becomes more positive and the N atom
becomes more negative. There are no significant changes
in the C and F atom charges, except for the ipso-C atom,
which becomes more negative, and the C atom bonded to
N, which becomes more positive by similar absolute
amounts (-0.06 and +0.10, respectively). The smaller
change for the ipso-C is likely less because the C6F5 group
is able to compensate for additional electron density. The
Xe valency increases only slightly and that of nitrogen
decreases, in accord with the formation of a weak
donor–acceptor bond.
The gas-phase reaction energies corresponding to Xe---N
donor–acceptor bond dissociation were calculated at the
MP2/(SDB-)cc-pVTZ//SVWN/(SDB-)cc-pVTZ level of theory
for [C6F5XeNCCH3]+ (96.2 kJ mol-1) and [FXeNCCH3]+
(210.9 kJ mol-1). Under gas-phase conditions, it is clear that
[FXe]+ is a significantly stronger Lewis acid toward CH3CN
than [C6F5Xe]+, and this trend is paralleled by the gas-phase
fluoride ion dissociation energies for C6F5XeF (614.9 kJ
mol-1) and XeF2 (996.8 kJ mol-1).
the [C6F5Xe]+ cation in this series of weakly coordinating
anion salts. Raman spectroscopic and X-ray structure deter-
minations, complemented by electronic structure calculations,
reveal that the geometry of the [C6F5Xe]+ cation is rather
insensitive to CH3CN coordination. The [B(CF3)4]- salt
displays the weakest cation–anion interaction in its crystal
structure and provides the closest approximation to the
[C6F5Xe]+ cation in the gas-phase for the two unsolvated
[C6F5Xe]+ salts examined in this study. The formation of
the [C6F5XeNCCH3]+ salts of the [B(C6F5)4]- and [B(CF3)4]-
anions is consistent with their weakly coordinating natures,
allowing CH3CN to compete effectively with these anions
for the Lewis acidic xenon center of the cation. In contrast,
the interaction is apparently sufficiently strong to prevent
CH3CN coordination in the [B(CN)4]- salt. Although the
geometric parameters of the [B(CN)4]- anion are apparently
insensitive to coordination of a single CN group to xenon,
the interaction is detectable in the C-N stretching region of
[B(CN)4]-.
Experimental Section
[C6F5Xe][B(CF3)4] (1), [C6F5XeNCCH3][B(C6F5)4] (2),
[C6F5XeNCCH3][B(CF3)4] (3), and [C6F5Xe][B(CN)4] (4). The
metatheses of [C6F5Xe][BF4] with M[BY4] are described in detail
in ref 8. In a typical synthesis, equimolar amounts of MI[BY4] and
[C6F5Xe][BF4] (∼0.5 mmol) were dissolved in CH3CN (each 300
µL) and combined at room temperature. The suspension was cooled
Conclusions
The syntheses and solid-state structural characteri-
zations of [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4],
[C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4] have
provided several insights into the coordination behavior of
Inorganic Chemistry, Vol. 47, No. 8, 2008 3215

Koppe et al.
to -40 °C and centrifuged at this temperature. The mother liquor
was separated, and the solid residue was washed with cold CH3CN.
The solvent was then removed under dynamic vacuum (10-3 mbar)
at room temperature. The pale yellow solids were pumped under
vacuum (10-3 mbar) for more than 12 h at 20 °C. The
[C6F5Xe][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3]-
[B(C6F5)4] salts were obtained in essentially quantitative yields. It
is important to note that [C6F5Xe][B(CF3)4] has a tendency to
retain CH3CN even after pumping under vacuum. It is only after
repeated dissolutions (×5) in CH2Cl2 and evaporation under
dynamic vacuum that [C6F5Xe][B(CF3)4] was obtained free of
CH3CN (monitored by Raman and 1H NMR spectroscopies).
Applying this procedure to the salt containing the [B(C6F6)4]-
anion, [C6F5Xe---NCCH3][B(CF3)4] was obtained.8 The CH3CN-
free salts were stable indefinitely at ambient temperatures in the
inert atmosphere of a drybox.
Raman Spectroscopy. (a) Raman Sample Preparation. In the
drybox, freshly prepared [C6F5Xe]+ and [C6F5XeNCCH3]+ salts
were transferred into 5-mm o.d. Pyrex precision glass NMR tubes
(Wilmad 507) fused to ¼-in. o.d. lengths of glass tubing which
were attached to J. Young Teflon/glass stopcocks by means of ¼-in.
stainless steel Swagelok Ultra-Torr unions. The tubes had been
previously dried under dynamic vacuum (10-3 mbar) at ambient
temperature for at least 12 h and backfilled with dry argon prior to
use. To prevent dispersion of the solid over the walls of the NMR
tube, the solid material was loaded into the tube using a solids
syringe fabricated from 2-mm o.d. FEP tubing and a length of
1.5-mm o.d. a stainless steel rod that functioned as a piston.
After transfer of the solid, the NMR tube was connected to a
glass vacuum line, cooled to -196 °C, pumped under dynamic
vacuum (10-3 mbar), and heat sealed. The Raman spectra of
the resulting pale yellow powders were acquired at -150 °C.
The spectra of Cs[B(C6F5)4] (Table 4 and Figure S2), K[B(CF3)4],
CH3CN, and CH2Cl2 were also measured at -150 °C for
reference purposes.
main arm of the reactor. The arm containing the solution was placed
inside the glass Dewar of a low-temperature crystal growing
apparatus34 at a preset initial temperature and crystallized over
several hours: (1) After 8 h at -38 °C, clear, ∼10 mm long pale
yellow needles grew throughout the solution. The sample was
maintained at -40 °C for an additional 26 h. (2) Clear, colorless
needles grew at -25 °C throughout the solution over a period of
2 h. The temperature was maintained at -25 °C for a further 8 h.
(3) After the mixture was cooled to -8 °C clear, colorless square
plates grew throughout the solution. The temperature was main-
tained at -8 °C for ∼20 h and was then cooled to -20 °C over a
period of 2 h. (4) After 10 h at -63 °C, pale yellow needle-shaped
crystals grew throughout the solution. The sample was further
cooled to -66 °C over a 5 h period.
The supernatants were decanted from the crystals into the side-
arms of their respective reactors and were cooled to -196 °C,
whereupon the sidearms were heat sealed under dynamic vacuum
at -196 °C, and residual solvents were removed from the crystals
by pumping at (1) -67 °C for 30 min, (2) -75 °C for 15–20 min,
(3) -20 °C for 30 min, and (4) -62 °C for 30 min.
(b) Crystal Mounting and X-ray Data Collection. All crystals
were mounted at -110 ( 3 °C as previously described.33 The
crystals or crystal fragments used for the data acquisition had the
dimensions (1) 0.26 × 0.16 × 0.04 [C6F5Xe][B(CF3)4]; (2) 0.18 ×
0.08 × 0.06 [C6F5XeNCCH3][B(C6F5)4]; (3) 0.22 × 0.12 × 0.05
[C6F5Xe][B(CN)4]; (4) 0.04 × 0.04 × 0.08 [C6F5XeNCCH3]-
[B(CF3)4] mm3.
Crystals were centered on a P4 Siemens diffractometer, equipped
with a Siemens SMART 1K CCD area detector, controlled by
SMART,35 and a rotating anode emitting KR radiation monochro-
mated (λ ) 0.71073 Å) by a graphite crystal. Diffraction data
collection (-173 °C) consisted of a full Ψ-rotation at ꢁ ) 0° (using
1040 + 30) 0.3° frames, followed by a series of short (80 frames)
ω scans at various ψ and ꢁ settings to fill the gaps. The crystal-
to-detector distances were 4.970 cm for 1, 2, and 4, and 5.012 cm
for 3, and the data collections were carried out in a 512 × 512
pixel mode using 2 × 2 pixel binning. Processing of the raw data
was completed using SAINT+,36 which applied Lorentz and
polarization corrections to three-dimensionally integrated diffraction
spots. The program SADABS37 was used for the scaling of
diffraction data, the application of a decay correction, and an
empirical absorption correction on the basis of the intensity ratios
of redundant reflections.
(b) Raman Instrumentation and Spectral Acquisition. The
low-temperature (-150 °C) Raman spectra were recorded on a
Bruker RFS 100 FT Raman spectrometer using 1064-nm excitation
and a resolution of 1 cm-1 as previously described.33 The spectra
were recorded using laser powers of 100–300 mW and a total of
1500 scans.
X-ray Crystallography. (a) Crystal Growth. The following
quantities of [C6F5Xe]+ salts were weighed, in a drybox, into
previously vacuum-dried ¼-in. o.d. FEP T-shaped reactors, attached
to Kel-F valves, and anhydrous CH2Cl2 was condensed onto the
samples on a vacuum line: (1) [C6F5Xe][B(CF3)4] (19.12 mg, 32.7
µmol; 0.59 mL); (2) [C6F5XeNCCH3][B(C6F5)4] (43.87 mg; 44.9
µmol; 0.69 mL); (3) [C6F5Xe][B(CN)4] (40.56 mg, 98.2 µmol; 0.29
mL); (4) [C6F5XeNCCH3][B(CF3)4] (61.69 mg, 105.4 µmol; 0.59
mL). In salts 1, 3, and 4, suspensions resulted at room temperature
as well as in the case of salt 2, which was maintained at -20 °C.
Aliquots of CH3CN were condensed onto the samples at -196 °C
(2-4) until most (2) or all (3 and 4) of the solid material had
dissolved upon warming to room temperature. The amount of
CH3CN ranged from 20–200 µL depending on the solubility of the
[C6F5Xe]+ salt. The reactors were pressurized at ∼1 atm with dry
nitrogen at -80 °C and allowed to briefly warm to room
temperature to effect dissolution. Samples 1 and 2 were warmed
to ∼30 °C (e10 s) to provide a near-saturated solution which was
decanted into the sidearm of the reaction vessel. Upon sedimenta-
tion, the clear yellow mother liquor was decanted back into the
(c) Solution and Refinement of the Structure. The XPREP38
program was used to confirm the unit cell dimensions and the crystal
lattices. The solutions were obtained by direct methods, which
located the positions of the heavy atoms. The final refinements were
obtained by introducing anisotropic thermal parameters and the
recommended weights for all of the atoms. The maximum electron
densities in the final difference Fourier maps were located near the
heavy atoms. All calculations were performed using the SHELXTL-
plus package38 for the structure determination and solution refine-
ment and for the molecular graphics.
Calculations. Quantum chemical calculations were done using
the program Gaussian 03 (version C.02).39 Density functional
theory (DFT) calculations were performed with the SVWN and
(34) Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2001,
40, 3002–3017.
(35) SMART, version 5.6.11; Siemens Energy and Automation, Inc.:
Madison, WI, 1999.
(36) SAINT+, version 6.02; Siemens Energy and Automation, Inc.:
Madison, WI, 1999.
(33) Gerken, M.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2000, 39,
(37) Sheldrick, G. M. SADABS (Siemens Area Detector Absorption
Corrections), version 2.03; Bruker AXS, Inc.: Madison, WI, 1999.
4244–4255.
3216 Inorganic Chemistry, Vol. 47, No. 8, 2008

[C6F5Xe]+ and [C6F5XeNCCH3]+ Salts of [BY4]-
PBE1PBE exchange-correlation potentials and the standard all-
electron cc-pVTZ basis sets, except in the case of Xe for which
the semirelativistic large core (RLC) pseudopotential basis set
SDB-cc-pVTZ was used.40 The combined use of cc-pVTZ and
SDB-cc-pVTZ basis sets is indicated as (SDB-)cc-pVTZ. The geom-
etries were fully optimized at all levels of theory using analytical
gradient methods. The C-C and C-F bond lengths and frequencies
of C6F6 were calculated for use as benchmarks.
orbital analyses were performed using SVWN densities with the
NBO program.42–44
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (G.J.S.) and the
Deutsche Forschungsgemeinschaft and the Fonds der Che-
mischen Industrie (H.-J.F.) for support in the form of research
grants and SHARCNet (Shared Hierarchical Academic
Research Computing Network; www.sharcnet.ca) for the
computational resources. We also thank Prof. Helge Willner
for providing samples of K[B(CF3)4] and K[B(CN)4].
The program GaussView41 was used to visualize the vibra-
tional displacements that form the basis of the vibrational mode
descriptions given in Tables 3, 4, S1, and S2. Natural Bond
Supporting Information Available: Raman spectra of
[C6F5Xe][BF4] and [C6F5XeNCCH3][B(C6F5)4] (Figure S1), calcu-
lated vibrational frequencies for C6F5X (X ) H, F, Cl, Br, I, Xe+)
(Table S1), calculated and experimental vibrational frequencies
(Table S2) and calculated geometries (Table S3) for C6F5X (X )
H, F, Cl, Br, I), Raman spectrum of Cs[B(C6F5)4] (Figure S2),
discussion of the factor-group analysis for the distorted [B(CN)4]-
anion (also see Table S4), calculated geometries for the [B(C6F5)4]-
anion (Table S5 and Figure S3), and X-ray crystallographic files
in CIF format for the structure determinations of [C6F5Xe][B(CF3)4],
[C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3]-
[B(C6F5)4]. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Inorganic Chemistry, Vol. 47, No. 8, 2008 3217