A rare example of a krypton difluoride coordination compound: [BrOF 2][AsF6]-2KrF2

Article abstract of DOI:10.1021/ja9098559

The synthesis of [BrOF₂][AsF₆]-2KrF₂, its structural characterization, and bonding are described in this study. Although several KrF₂ adducts with transition metal centers have been previously reported, none have been crystallographically characterized. The solid-state Raman spectrum of [BrOF₂][AsF₆]?2KrF ₂ has been assigned with the aid of quantum-chemical calculations. The low-temperature (-173 °C) X-ray crystal structure of [BrOF ₂][AsF₆]?2KrF₂ consists of isolated molecular units and represents an example of KrF₂ coordinated to a main-group atom. The coordination geometry around the BrOF₂₊ cation renders the free valence electron lone pair more compact than in free BrOF ₂₊. The KrF₂ ligands are coordinated trans to the fluorine atoms of BrOF₂₊ with the AsF_6- anion coordinated trans to oxygen. The quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) analyses have been carried out in order to define the nature of the bonding in the complex. A significant amount of charge (0.25 e) is transferred to BrOF₂₊ from the two KrF₂ ligands (0.10e each) and from the AsF_6- anion (0.05 e). Significant polarization also occurs within the KrF₂ ligands, which enhances the anionic characters of the fluorine bridges. The interaction energy is mostly governed by the electrostatic interaction of the positively charged bromine atom with the surrounding fluorine atoms.

Full text of DOI:10.1021/ja9098559

Published on Web 02/22/2010
A Rare Example of a Krypton Difluoride Coordination
Compound: [BrOF2][AsF6]·2KrF2
David S. Brock,† Jonathan J. Casalis de Pury,† He´le`ne P. A. Mercier,†
Gary J. Schrobilgen,*,† and Bernard Silvi‡
Department of Chemistry, McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada, and
Laboratoire de Chimie The´orique (UMR-CNRS 7616), UniVersite´ Pierre et Marie Curie-Paris 06,
4 place Jussieu, 75252 Paris ce´dex, France
Received November 20, 2009; E-mail: schrobil@mcmaster.ca
Abstract: The synthesis of [BrOF2][AsF6]·2KrF2, its structural characterization, and bonding are described
in this study. Although several KrF2 adducts with transition metal centers have been previously reported,
none have been crystallographically characterized. The solid-state Raman spectrum of [BrOF2][AsF6]·2KrF2
has been assigned with the aid of quantum-chemical calculations. The low-temperature (-173 °C) X-ray
crystal structure of [BrOF2][AsF6]·2KrF2 consists of isolated molecular units and represents an example of
KrF2 coordinated to a main-group atom. The coordination geometry around the BrOF2+ cation renders the
free valence electron lone pair more compact than in free BrOF2+. The KrF2 ligands are coordinated trans
to the fluorine atoms of BrOF2+ with the AsF6- anion coordinated trans to oxygen. The quantum theory of
atoms in molecules (QTAIM) and electron localization function (ELF) analyses have been carried out in
order to define the nature of the bonding in the complex. A significant amount of charge (0.25 e) is transferred
to BrOF2+ from the two KrF2 ligands (0.10 e each) and from the AsF6- anion (0.05 e). Significant polarization
also occurs within the KrF2 ligands, which enhances the anionic characters of the fluorine bridges. The
interaction energy is mostly governed by the electrostatic interaction of the positively charged bromine
atom with the surrounding fluorine atoms.
n-C3F7);14,15 preliminary evidence but no structural characteriza-
Introduction
16
tion for KrF2 ·VF5 and KrF2 ·MnF4,17 and a series of KrF2
Lewis acid-base adducts with group 6 d0 transition metal
centers, namely, MOF4 ·KrF2 (M ) Cr,18 Mo,19 W19). The
structural characterizations of the latter KrF2 adducts were
limited to solution 19F NMR and solid-state Raman spectros-
copy. In all three cases, the Raman and 19F NMR spectra indicate
that the adducts result from weak coordination of KrF2 through
a fluorine bridge to the metal atom. In the absence of X-ray
crystal structures, an assessment of the degree of coordination,
based on the relative bond lengths of terminal and bridge Kr-F
bonds, could not be made.
To date, there is no X-ray crystal structure in which KrF2
serves as a ligand toward a metal atom, nor are there any
examples in which KrF2 coordinates to a main-group atom. Two
criteria are required for KrF2 coordination: (1) KrF2 must interact
with a Lewis acid center that is not sufficiently strong to
“completely” abstract a fluoride ion from KrF2, and (2) the
Lewis acid must be resistant to oxidation by the powerful
oxidative fluorinator, KrF2. These criteria are met in the
aforementioned low-temperature studies of the MOF4 ·KrF2
The precursor to all known krypton compounds, KrF2, has
been structurally well characterized1 and has been the subject
of several theoretical studies.2,3 The chemistry of krypton is
restricted to the +2 oxidation state, presently consisting of
several KrF+ and Kr2F3+ salts;4-12 Kr(OTeF5)2;13 a number of
nitrile adducts of KrF+, namely, FKrNt CR+ (R ) H, CF3, C2F5,
† McMaster University.
‡ Universite´ Pierre et Marie Curie.
(1) Lehmann, J. F.; Mercier, H. P. A.; Schrobilgen, G. J. Coord. Chem.
ReV. 2002, 233-234, 1–39.
(2) MacDougall, P. J.; Schrobilgen, G. J.; Bader, R. F. W. Inorg. Chem.
1989, 28, 763–769.
(3) Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2001,
40, 3002–3017.
(4) Selig, H.; Peacock, R. D. J. Am. Chem. Soc. 1964, 86, 3895.
(5) Frlec, B.; Holloway, J. H. J. Chem. Soc., Chem. Commun. 1973, 370–
371.
(6) Frlec, B.; Holloway, J. H. J. Chem. Soc., Chem. Commun. 1974, 89–
90.
(7) Gillespie, R. J.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.
1974, 90–92.
(8) Holloway, J. H.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.
1975, 623–624.
(14) Schrobilgen, G. J. J. Chem. Soc., Chem. Commun. 1988, 863–865.
(15) Schrobilgen, G. J. J. Chem. Soc., Chem. Commun. 1988, 1506–1508.
(9) Frlec, B.; Holloway, J. H. Inorg. Chem. 1976, 15, 1263–1270.
(10) Gillespie, R. J.; Schrobilgen, G. J. Inorg. Chem. 1976, 15, 22–31.
(11) Gillespie, R. J.; Martin, D.; Schrobilgen, G. J. J. Chem. Soc., Dalton
Trans. 1980, 1898–1903.
ˇ
ˇ
(16) Zemva, B.; Slivnik, J.; Smalc, A. J. Fluorine Chem. 1975, 6, 191–
193.
ˇ
(17) Lutar, K.; Jesih, A.; Zemva, B. Polyhedron 1988, 7, 1217–1219.
(12) Selig, H.; Holloway, J. H. In Topics in Current Chemistry; Boschke,
F. L., Ed.; Springer-Verlag: Berlin, 1984; Vol. 124, pp 33-90.
(13) Sanders, J. C. P.; Schrobilgen, G. J. J. Chem. Soc., Chem. Commun.
1989, 20, 1576–1578.
(18) Christe, K. O.; Wilson, W. W.; Bougon, R. A. Inorg. Chem. 1986,
25, 2163–2169.
(19) Holloway, J. H.; Schrobilgen, G. J. Inorg. Chem. 1981, 20, 3363–
3368.
9
10.1021/ja9098559  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 3533–3542 3533

A R T I C L E S
Brock et al.
adducts.18,19 Earlier studies have shown that the BrOF2+ cation
meets these criteria for the less strongly oxidizing XeF2 ligand
in [BrOF2][AsF6]·XeF2,20 which has been characterized in
solution by 19F and 129Xe NMR spectroscopy and in the solid
state by Raman spectroscopy, offering promise for the synthesis
of a KrF2 analogue.
The present study extends the little studied coordination
chemistry of krypton to the synthesis and characterization of
the first main-group coordination compound of KrF2, namely,
[BrOF2][AsF6]·2KrF2. In addition to structural characterization
by solid-state Raman spectroscopy and single-crystal X-ray
diffraction, the nature of the adduct bonding is examined using
quantum-chemical calculations in conjunction with electron
localization function (ELF) analyses.
Table 1. Summary of Crystal Data and Refinement Results for
[BrOF2][AsF6]·2KrF2
chem formula
space group
a (Å)
b (Å)
c (Å)
AsBrOF12Kr2
P21/c (No. 14)
5.7166(6)
13.644(1)
15.105(2)
111.446(4)
1096.6(2)
4
2265.72
3.431
-173
14.91
ꢀ (deg)
V (Å3)
Z (molecules/unit cell)
mol wt (g mol-1
)
Fcalc (g cm-3
T (°C)
)
µ (mm-1
λ (Å)
)
0.71073
0.0693
a
R1
b
wR2
0.1715
Results and Discussion
b
a R1 ) ∑||Fo| - |Fc|/∑|Fo| for I > 2σ(I). wR2 is defined as {∑[w(Fo
2
1
2
- Fc2)2]/∑w(Fo )2} / for I > 2σ(I).
2
Synthesis and Properties of [BrOF2][AsF6]·2KrF2. Reaction
progress and the purities of all products were routinely
monitored by recording the low-temperature Raman spectra
(-150 °C) of the solids.
The [BrOF2][AsF6]·XeF2 adduct was synthesized as previ-
ously described.20 Rather than through the direct combination
of BrOF3 and AsF5, the salt, [BrOF2][AsF6], was synthesized
in high purity by removal of XeF2 from [BrOF2][AsF6]·XeF2
under dynamic vacuum at 0 °C (eq 1). This synthetic route to
[BrOF2][AsF6] circumvents difficulties associated with the
synthesis and isolation of BrOF3 from the reaction of K[BrOF4]
and [O2][AsF6] in HF21,22 and the possibility of explosion during
the hydrolysis of BrF5 to form BrOF3.23
[BrOF2][AsF6] upon removal of the solvent under dynamic
vacuum at -78 °C. Similar attempts to form a 3:1 adduct yielded
only [BrOF2][AsF6]·2KrF2 and unreacted KrF2.
X-ray Crystal Structure of [BrOF2][AsF6]·2KrF2. A summary
of the refinement results and other crystallographic information
are given in Table 1. Important bond lengths, bond angles, and
contacts are listed in Table 2.
The structure of [BrOF2][AsF6]·2KrF2 consists of a BrOF2
cation that interacts by means of short Br---F contacts with a
single fluorine atom of the AsF6 anion and a fluorine atom
from each of two KrF2 ligands (Figure 1a). The [BrOF2][AsF6]·
2KrF2 structural units are relatively isolated, with the shortest
intermolecular contacts (3.134-3.404 Å) occurring between the
fluorine and krypton atoms of neighboring KrF2 molecules,
which are near or slightly under the sum of the fluorine and
krypton van der Waals radii (3.49).24
+
-
[BrOF2][AsF6] · XeF2 dynamic vac8 [BrOF2][AsF6] + XeF2
(1)
0 °C
Addition of KrF2 to [BrOF2][AsF6] (2:1 molar ratio) that had
been precipitated and suspended in anhydrous HF (aHF) at -78
°C resulted in a significant volume increase with respect to the
original volume of suspended [BrOF2][AsF6]. The Raman
spectra of the solid product under frozen HF solvent and of the
product isolated by removal of HF at -78 °C were identical.
+
The primary coordination sphere of Br(V) in BrOF2 is
trigonal pyramidal. The secondary coordination sphere com-
-
prises a fluorine atom of the AsF6 anion coordinated trans to
the oxygen atom of BrOF2+ and the fluorine atoms of two KrF2
+
molecules coordinated trans to the fluorine atoms of BrOF2
so that the geometry of the F2OBrF3 moiety is pseudo-
octahedral.
+
Both spectra revealed that the bands corresponding to BrOF2
were shifted to lower frequencies relative to those of
[BrOF2][AsF6] and that the KrF2 stretching band, associated with
uncomplexed KrF2, was replaced by two pairs of Kr-F
stretching bands (see Raman Spectroscopy). The spectrum was
consistent with the formation of [BrOF2][AsF6]·2KrF2 according
to eq 2. The adduct is stable for at least 5 days at -78 °C as a
solid and under aHF solvent. The adduct is also stable in aHF
up to 25 °C for at least 1 h, with Raman spectroscopy showing
no discernible decomposition when the adduct was isolated by
removal of the solvent under dynamic vacuum at -78 °C.
The Kr-F bond lengths of both coordinated KrF2 molecules
are distorted relative to those of free KrF2 (1.894(5) Å),3 with
elongated bridge bonds (1.943(4), 1.933(4) Å) and terminal
bonds that are shortened by nearly equal amounts (1.840(5),
1.847(4) Å). The differences between the terminal and bridging
Kr-F bond lengths are significantly less than in KrF+ and Kr2F3+
salts: [KrF][AsF6] (Kr-Fb, 2.131(2) Å; Kr-Ft, 1.765(2) Å),3
[Kr2F3][AsF6]·[KrF][AsF6] (Kr-Fb, 2.061(6), 2.049(6), 2.106(6)
Å; Kr-Ft, 1.780(7), 1.803(6), 1.783(6) Å),3 [KrF][SbF6] (Kr-Fb,
2.140(3) Å; Kr-Ft, 1.765(3) Å),3 [KrF][BiF6] (Kr-Fb, 2.090(6)
Å; Kr-Ft, 1.774(6) Å),3 [Kr2F3]2[SbF6]2 ·KrF2 (Kr-Fb, 2.041(4),
2.065(4), 2.052(5), 2.056(4) Å; Kr-Ft, 1.805(5), 1.799(4),
1.797(5), 1.787(4) Å),3 and [Kr2F3][SbF6]·KrF2 (Kr-Fb, 2.027(5),
2.046(5) Å; Kr-Ft, 1.800(5), 1.790(5) Å).3 This indicates that
the Kr-Fb bonds in [BrOF2][AsF6]·2KrF2 have considerably
HF
[BrOF2][AsF6] + 2KrF2
8 [BrOF2][AsF6] · 2KrF2 (2)
12 h, -78 °C
Attempts to form the 1:1 adduct, [BrOF2][AsF6]·KrF2, by
reaction of a 1:1 molar ratio of [BrOF2][AsF6] and KrF2 in aHF
at -78 °C yielded only a mixture of [BrOF2][AsF6]·2KrF2 and
more covalent character relative to those of KrF+ and Kr2F3
+
salts and that the KrF2 molecules behave as coordinating ligands
rather than as fluoride ion donors.
The KrF2 molecules coordinate to the cation by means of
Br---F(3) and Br---F(5) contacts of 2.318(4) and 2.356(4) Å,
(20) Holloway, J. H.; Schrobilgen, G. J. Inorg. Chem. 1980, 19, 2632–
2640.
(21) Bougon, R.; Bui Huy, T. Compt. Rend. 1976, C283, 461–463.
(22) Gillespie, R. J.; Spekkens, P. J. Chem. Soc., Dalton Trans. 1977, 1539–
1546.
(23) Lehmann, J. F. Ph.D. Thesis, McMaster University, Hamilton, ON,
2004.
(24) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
9
3534 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Krypton Difluoride Coordination Compound
A R T I C L E S
Table 2. Experimental and Calculated (C1) Geometrical Parameters for [BrOF2][AsF6]·2KrF2
exptla
PBE1PBEa
B3LYPa
Bond Lengths (Å)
Br(1)-O(1)
Br(1)-F(1)
Br(1)-F(2)
Br(1)---F(3)
Br(1)---F(5)
Br(1)---F(7)
Kr(1)-F(3)
Kr(1)-F(4)
Kr(2)-F(5)
Kr(2)-F(6)
As(1)-F(7)
As(1)-F(8)
As(1)-F(9)
As(1)-F(10)
As(1)-F(11)
As(1)-F(12)
1.564(5)
1.727(4)
1.723(4)
2.318(4)
2.356(4)
2.576(4)
1.943(4)
1.840(5)
1.933(4)
1.847(4)
1.742(4)
1.711(4)
1.732(4)
1.732(4)
1.712(4)
1.709(4)
Br(1)-O(1)
Br(1)-F(1)
Br(1)-F(2)
Br(1)---F(3)
Br(1)---F(5)
Br(1)---F(7)
Kr(1)-F(3)
Kr(1)-F(4)
Kr(2)-F(5)
Kr(2)-F(6)
As(1)-F(7)
As(1)-F(8)
As(1)-F(9)
As(1)-F(10)
As(1)-F(11)
As(1)-F(12)
1.556
1.731
1.730
2.350
2.302
2.579
1.951
1.814
1.957
1.808
1.789
1.709
1.743
1.724
1.705
1.761
1.569
1.757
1.757
2.363
2.338
2.529
1.984
1.843
1.984
1.837
1.813
1.722
1.752
1.742
1.720
1.773
Bond Angles (deg)
F(1)-Br(1)-F(2)
F(1)-Br(1)-O(1)
F(1)-Br(1)---F(3)
F(1)-Br(1)---F(5)
F(1)-Br(1)---F(7)
F(2)-Br(1)-O(1)
F(2)-Br(1)---F(3)
F(2)-Br(1)---F(5)
F(2)-Br(1)---F(7)
O(1)-Br(1)---F(3)
O(1)-Br(1)---F(5)
O(1)-Br(1)---F(7)
F(3)-Kr(1)-F(4)
F(3)---Br(1)---F(5)
F(3)---Br(1)---F(7)
F(5)-Kr(2)-F(6)
F(5)---Br(1)---F(7)
Br(1)---F(3)-Kr(1)
Br(1)---F(5)-Kr(2)
Br(1)---F(7)-As(1)
F(7)-As(1)-F(8)
F(7)-As(1)-F(9)
F(7)-As(1)-F(10)
F(7)-As(1)-F(11)
F(7)-As(1)-F(12)
F(8)-As(1)-F(9)
F(8)-As(1)-F(10)
F(8)-As(1)-F(11)
F(8)-As(1)-F(12)
F(9)-As(1)-F(10)
F(9)-As(1)-F(11)
F(9)-As(1)-F(12)
F(10)-As(1)-F(11)
F(10)-As(1)-F(12)
F(11)-As(1)-F(12)
89.3(2)
103.3(3)
85.1(2)
162.4(2)
80.5(2)
102.8(3)
166.7(2)
84.9(2)
83.5(2)
90.2(2)
94.2(2)
172.6(2)
179.9(2)
96.9(2)
83.7(2)
178.7(2)
82.3(2)
132.1(2)
139.9(2)
131.1(2)
179.4(2)
89.1(2)
88.8(2)
88.4(4)
89.5(2)
90.4(2)
91.6(2)
91.2(2)
90.8(2)
177.9(2)
90.4(2)
89.1(2)
90.1(2)
90.4(2)
177.8(2)
F(1)-Br(1)-F(2)
89.1
102.0
83.9
166.9
89.9
100.0
172.5
82.4
89.8
102.0
85.7
166.5
86.1
100.1
173.7
83.4
F(1)-Br(1)-O(1)
F(1)-Br(1)---F(3)
F(1)-Br(1)---F(5)
F(1)-Br(1)---F(7)
F(2)-Br(1)-O(1)
F(2)-Br(1)---F(3)
F(2)-Br(1)---F(5)
F(2)-Br(1)---F(7)
O(1)-Br(1)---F(3)
O(1)-Br(1)---F(5)
O(1)-Br(1)---F(7)
F(3)-Kr(1)-F(4)
F(3)---Br(1)---F(5)
F(3)---Br(1)---F(7)
F(5)-Kr(2)-F(6)
F(5)---Br(1)---F(7)
Br(1)---F(3)-Kr(1)
Br(1)---F(5)-Kr(2)
Br(1)---F(7)-As(1)
F(7)-As(1)-F(8)
F(7)-As(1)-F(9)
F(7)-As(1)-F(10)
F(7)-As(1)-F(11)
F(7)-As(1)-F(12)
F(8)-As(1)-F(9)
F(8)-As(1)-F(10)
F(8)-As(1)-F(11)
F(8)-As(1)-F(12)
F(9)-As(1)-F(10)
F(9)-As(1)-F(11)
F(9)-As(1)-F(12)
F(10)-As(1)-F(11)
F(10)-As(1)-F(12)
F(11)-As(1)-F(12)
72.1
84.1
89.3
74.1
85.3
90.7
165.7
177.2
104.0
105.3
177.5
78.0
138.9
129.4
114.8
175.8
86.8
88.6
90.1
85.6
91.5
92.9
93.8
90.5
174.5
91.1
170.2
176.9
100.1
101.1
177.7
80.8
139.6
129.4
123.7
176.6
87.4
87.9
89.6
85.6
91.9
92.6
93.8
91.0
174.5
91.2
87.6
91.9
89.1
175.5
87.9
91.6
88.9
175.1
a The aug-cc-pVTZ(-PP) basis set was used. The symmetry of the energy-minimized geometry is C1. The labeling scheme corresponds to that used in
Figure 1a and b.
respectively, which are relatively short and significantly less
than the sum of the van der Waals radii of Br and F (3.32
Å). The fluorine bridges, Br(1)---F(3)-Kr(1) and Br(1)---
F(5)-Kr(2), are bent as a consequence of the AX2E2
VSEPR25 arrangement of bond pair and electron lone pair
domains of the bridging fluorine atom, resulting in angles of
132.1(2)° and 139.9(2)°, respectively (also see the Supporting
Information). The Br---Fb-Kr angles appear to be little
influenced by steric interactions, with Fb · · · Fb distances of
3.25-3.50 Å and Fb · · · FBr distances of 2.77-2.93 Å com-
pared to the van der Waals sum of 2.94 Å.24 The only other
short intramolecular contacts are Kr(1)---F(9) (3.22 Å) and
Kr(2)---F(10) (3.40 Å), which are near the van der Waals
sum for krypton and fluorine (3.49 Å). The KrF2 ligands
retain their linearity with F(3)-Kr(1)-F(4) and F(5)-Kr(2)-
F(6) angles of 179.9(2)° and 178.7(2)°, respectively. One
-
fluorine atom of the AsF6 anion forms a relatively short
+
Br---Fb bridge bond (2.576(4) Å) with the BrOF2 cation,
giving rise to a distorted octahedral arrangement around
arsenic in which the As-Fb bond (1.742(4) Å) is elongated
relative to the As-F bond trans to it (1.711(4) Å) and the
equatorial bonds, which range in length from 1.709(4) to
1.732(4) Å.
(25) Gillespie, R. J.; Hargittai, I. In The VSEPR Model of Molecular
Geometry; Allyn and Bacon: Boston, MA, 1991; pp 154-155.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3535

A R T I C L E S
Brock et al.
Figure 2. Raman spectrum of [BrOF2][AsF6]·2KrF2 recorded at -150 °C
using 1064 nm excitation. Symbols denote FEP sample tube lines (/),
unreacted KrF2 (†), and instrumental artifact (‡).
bond domains, giving a tetrahedral AX2YE VSEPR25 arrange-
ment at Br(V). Thus, a stereochemically active valence electron
lone pair is expected to occupy a region at the center of the
triangular arrangement defined by the three long contacts that
comprise the more open face of the pseudo-octahedron. The
sum of the F(3)---Br(1)---F(5) (96.9(2)°), F(3)---Br(1)---F(7)
(83.7(2)°), and F(5)---Br(1)---F(7) (82.3(2)°) angles is 262.9(6)°,
whereas the face of the octahedron containing the primary
contacts is significantly more open (F(1)-Br(1)-O(1), 103.3(3)°;
F(1)-Br(1)-F(2), 89.3(2)°; F(2)-Br(1)-O(1), 102.8(3)°) with
an angle sum of 295.4(8)°. The angle sums are attributable
to the greater steric requirement of the Br-O double bond
domain. The short secondary contact distances observed in
[BrOF2][AsF6]·2KrF2 render the valence electron lone pair
domain of bromine more compact and localized around the
bromine atom relative to that of free BrOF2+. The steric
crowding of the Br(V) valence electron lone pair represents an
example of a “weakly active” electron lone pair32 (see Com-
putational Results).
Figure 1. (a) Structural unit in the X-ray crystal structure of
[BrOF2][AsF6]·2KrF2; thermal ellipsoids are shown at the 50% probability
level. (b) Calculated geometry (PBE1PBE/aug-cc-pVTZ(-PP)) of [BrOF2]-
[AsF6]·2KrF2 showing the pseudo-octahedral coordination around bro-
mine(V).
The Br-O bond length (1.564(5) Å) is equal, within (3σ,
to that of BrOF4- in [NO][BrOF4] (1.575(3) Å)26 and the neutral
parent molecule, BrOF3, in [NO2][BrF4]·2BrOF3 (1.569, 1.606
Å)26 but is significantly shorter than in the neutral species
O2Br-O-BrO2 (1.606 (12), 1.611(2), 1.613(2), 1.606(2) Å)27
and O2BrOTeF5 (1.595(4), 1.608(3) Å)28 and in the BrO2+ cation
of [BrO2][SbF6] (1.595(2) Å).29 The Br-O bond length is also
comparable to the Se-O bond length in isoelectronic SeOF2
(1.576 Å), which was measured in the gas phase by microwave
spectroscopy.30 The Br-F bond lengths in [BrOF2][AsF6]·2KrF2
(1.727(4), 1.723(4) Å) are equal, within (3σ, to the axial Br-F
Raman Spectroscopy. The low-temperature Raman spectrum
of [BrOF2][AsF6]·2KrF2 is shown in Figure 2. The observed
and calculated frequencies and their detailed assignments are
listed in Table 3. The spectral assignments for [BrOF2][AsF6]·
2KrF2 were made by comparison with the calculated vibrational
frequencies and Raman intensities (Table 3) of the energy-
minimized geometry (Figure 1b), as well as those of KrF2 (Table
S2 in the Supporting Information). In both the crystal structure
and the calculated geometry, the [BrOF2][AsF6]·2KrF2 structural
unit possesses C1 symmetry and the Raman spectrum has been
assigned under that symmetry. Vibrational frequencies calculated
at both the PBE1PBE and B3LYP (values in parentheses) levels
+
bond lengths of the BrF4 cation in [BrF4][Sb2F11] (Br-Fax,
1.728(3), 1.729(3) Å (Br-Feq, 1.664(3), 1.667(2) Å)),31 the
equatorial Br-F bond lengths in the neutral parent molecule,
BrOF3, in [NO2][BrF4]·2BrOF3 (Br-Fax, 1.820, 1.839, 1.822,
1.836 Å; Br-Feq, 1.725, 1.692 Å),26 and the Se-F bond lengths
of the isoelectronic SeOF2 molecule (1.7295 Å).30 They are,
however, significantly shorter than the Br-F bonds in [NO]-
[BrOF4] (1.846(2), 1.912(2) Å).26
The valence electron lone pair of bromine in [BrOF2][AsF6]·
2KrF2 is expected to occupy a region opposite the three primary
-
of theory reproduced the observed frequency trends. The AsF6
anion, under ideal octahedral symmetry (Oh), has three Raman-
active bands, ν1(A1 g), ν2(Eg), and ν5(T2g), two infrared-active
bands, ν3(T1u) and ν4(T1u), and one inactive band, ν6(T2u). In
the present instance, the fluorine-bridged AsF6 anion is
distorted, with local C1 symmetry which gives rise to 15 Raman-
(26) Ellern, A.; Boatz, J. A.; Christe, K. O.; Drews, T.; Seppelt, K. Z. Anorg.
Allg. Chem. 2002, 628, 1991–1999.
(27) Leopold, D.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1994, 33, 975–
976; Angew. Chem. 1994, 106, 1043-1044.
-
(28) Hwang, I.-C.; Kuschel, R.; Seppelt, K. Z. Anorg. Allg. Chem. 1997,
623, 379–383.
and infrared-active bands. Only eight bands were observed in
(29) Lehmann, J. F.; Riedel, S.; Schrobilgen, G. J. Inorg. Chem. 2008, 47,
8343–8356.
-
the Raman spectrum of AsF6 and their assignments were
(30) Bowater, I. C.; Brown, R. D.; Burden, F. R. J. Mol. Spectrosc. 1968,
28, 461–470.
(31) Vij, A.; Tham, F. S.; Vij, V.; Wilson, W. W.; Christe, K. O. Inorg.
Chem. 2002, 41, 6397–6403.
(32) Pilme´, J.; Robinson, E. A.; Gillespie, R. J. Inorg. Chem. 2006, 45,
6198–6204.
9
3536 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Krypton Difluoride Coordination Compound
A R T I C L E S
Table 3. Experimental and Calculated Vibrational Frequenciesa for [BrOF2][AsF6]·2KrF2
a Frequencies are given in cm-1 b The Raman spectrum was recorded in an FEP sample tube at -150 °C using 1064 nm excitation. Values in
.
parentheses denote relative Raman intensities. An additional band observed at 465(21) cm-1 was assigned to unreacted KrF2. The abbreviation
c
br denotes broad. The aug-cc-pVTZ(-PP) basis set was used. Values in parentheses denote Raman intensities (Å4 u-1). Values in square brackets
denote infrared intensities (km mol-1). d Vibrational assignments were based on modes calculated at the PBE1PBE level of theory. The abbreviations
denote stretch (ν), bend (δ), rock (Fr), twist (Ft), wag (Fw), in-plane bend (ip), and out-of-plane bend (oop). e This band is assigned to ν(BrF1) + ν(BrF2)
at the B3LYP level. f This band is assigned to ν(AsF13) + ν(AsF14) + ν(AsF10) + ν(AsF9) at the B3LYP level.
-
guided by comparison with other coordinated AsF6 anions
having local C1 or Cs symmetries.33,34
spectrum that cannot be accounted for by site symmetry
lowering alone because correlation of the gas-phase adduct
symmetry (C1) to the crystal site symmetry (C1) results in no
additional band splitting (Table S1 in the Supporting Informa-
tion). The additional bands are associated with vibrational
coupling within the crystallographic unit cell. Correlation of
the site symmetry to the centrosymmetric unit cell symmetry
(C2h with Z ) 4) results in equal apportioning of the 4(3N - 6)
vibrational modes among Ag, Au, Bg, and Bu symmetries. Thus,
of the 180 coupled vibrational modes for [BrOF2][AsF6]·2KrF2
All 45 vibrational modes of [BrOF2][AsF6]·2KrF2 belong to
A irreducible representations and are predicted to be Raman-
and infrared-active. Additional bands appear in the Raman
(33) Gerken, M.; Moran, M. D.; Mercier, H. P. A.; Pointner, B. E.;
Schrobilgen, G. J.; Hoge, B.; Christe, K. O.; Boatz, J. A. J. Am. Chem.
Soc. 2009, 131, 13474–13489.
(34) Smith, G. L.; Mercier, H. P. A.; Schrobilgen, G. J. Inorg. Chem. 2007,
46, 1369–1378.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3537

A R T I C L E S
Brock et al.
in its unit cell, 45 Ag and 45 Bg Raman-active and 45 Au and
45 Bu infrared-active components are predicted. Of the predicted
90 Raman bands, only 25, including eight AsF6 bands, were
observed, implying vibrational coupling within the unit cell is,
except in a few instances, too weak to be observed.
CrOF4 ·KrF2 (486 cm-1
)
18 or MoOF4 ·KrF2 (462, 479 cm-1).19
Comparison of the frequency differences between the Kr-Ft
and Kr-Fb modes reveals an increase over the series
CrOF4 ·KrF2 (64 cm-1),18 MoOF4 ·KrF2 (102 cm-1),19 and
WOF4 ·KrF2 (116 cm-1),19 following the anticipated Lewis
acidity trend of the metal oxide tetrafluorides. This trend
suggests that the strengths of the Br---F adduct bonds in
[BrOF2][AsF6]·2KrF2, with a frequency difference between the
Kr-Ft and Kr-Fb modes of 84 cm-1, are intermediate with
respect to those of CrOF4 ·KrF2 and MoOF4 ·KrF2.
-
Upon coordination of KrF2, the cation stretching frequencies
shift to lower frequency relative to those of [BrOF2][AsF6].35
The highest frequency bands at 1047, 1053 cm-1 are assigned
to the factor-group split Br-O stretching mode. The in-phase
and out-of-phase BrF2 stretching bands occur at 644 and 625
cm-1, respectively, and show no factor-group splitting. The in-
phase band occurs at higher frequency and is more intense, in
agreement with the trends expected from the calculated values.
The two bands are also slightly shifted to lower frequency
The present vibrational assignments of coordinated KrF2 are
in accordance with those reported for XeF2 homoleptically
coordinated to a variety of metal cations.37,38 In these coordina-
tion complexes, the high-frequency Xe-F stretching bands are
assigned to Xe-Ft stretches and the low-frequency ones are
assigned to Xe-Fb stretches without invoking intramolecular
coupling in the vibrational mode descriptions of coordinated
XeF2. The calculated vibrational displacements of coordinated
KrF2 in [BrOF2][AsF6]·2KrF2 also do not show intramolecular
coupling. Instead, the vibrational coupling of the Kr-F stretches
is interligand in nature occurring between Kr-Fb stretching
modes that have near-equal displacement amplitudes and
between Kr-Ft stretching modes that have unequal displacement
amplitudes.
35
+
compared to those observed for free BrOF2
.
The trends in
the cation stretching frequencies can be accounted for by
donation of electron density from the KrF2 ligands to the
bromine atom, rendering bromine less electropositive (see
Natural Bond Orbital Analyses) and shifting the modes to lower
frequency. The cation bands at 314, 371/377, and 397 cm-1
+
are assigned to BrOF2 deformation modes and are in good
agreement with the calculated values.
The most intense modes in the spectrum of [BrOF2][AsF6]·
2KrF2 are those of the KrF2 ligand. Coordination of KrF2 to
BrOF2+ results in removal of the center of symmetry at krypton,
which is manifested in the Raman spectrum by the appearance
of bands to high and to low frequency of free KrF2 (ν(KrF),
465 cm-1),36 with the higher frequency and more intense band
assigned to the terminal Kr-Ft stretch and the lower frequency
band assigned to the bridging Kr-Fb stretch. These trends have
been observed in XeF2 adducts with metal cations.37,38 The
vibrational displacements calculated at the PBE1PBE and
B3LYP levels reveal that while there is no intraligand coupling
for the Kr-Ft and Kr-Fb stretching modes, interligand coupling
occurs giving rise to in-phase (KrFt) + ν(Kr′Ft) and out-of-
phase (KrFt) - ν(Kr′Ft) modes at 549 and 533 cm-1, respec-
tively, where the KrFt and Kr′Ft displacement amplitudes are
unequal in both coupled modes. These modes occur at similar
frequencies, in accordance with their calculated frequencies at
610 (577) and 587 (549) cm-1, respectively. The bands at 443
and 472 cm-1 are associated with analogous interligand coupling
of the Kr-Fb bridging stretching modes and are in good
agreement with the calculated values, 467 (450) and 489 (473)
cm-1. In contrast to the coupled Kr-Ft and Kr′Ft modes, the
coupled Kr-Fb and Kr′-Fb displacement amplitudes are nearly
equal. The Kr-Ft stretching frequencies are comparable to
The double degeneracy of the KrF2 bending mode of free
KrF2 (ν2, Πu) is removed when it is fluorine-bridged to bromine,
resulting in splitting into out-of-plane and in-plane Ft-Kr-Fb
bending modes with respect to the plane containing the two
KrF2 molecules and the bromine atom. The vibrational bands
are shifted to higher frequency relative to that of free KrF2 (236
cm-1 39 and occur at slightly different frequencies because one
)
KrF2 ligand is somewhat more strongly bound than the other in
the crystal structure and in the calculated gas-phase structure
(see X-ray Crystal Structure and Computational Results). The
δ(FKrF) modes are not coupled (Table 3), where δ(F5KrF6)ip
and δ(F5KrF6)oop are observed at 254 and 301 cm-1, respectively,
and δ(F3KrF4)ip is observed at 266 cm-1. The δ(F3KrF4)oop bend
was not observed but is calculated at 244 (234) cm-1 and is
expected to be weak. The calculated frequencies are also in
excellent agreement with the experimental frequencies. These
bands occur at much higher frequencies than those that are
assigned for the MOF4 adducts, i.e., 176 cm-1 (CrOF4 ·KrF2),18
170 cm-1 (MoOF4 ·KrF2),19 and 172 cm-1 (WOF4 ·KrF2),19
suggesting that the latter may have been erroneously assigned
in the earlier work and likely should be reassigned to the bands
reported at 256/283, 303/312, and 301/312, respectively.
Computational Results. The geometry of [BrOF2][AsF6] ·
2KrF2 was energy minimized starting from the crystallographic
coordinates and resulted in stationary points with all frequencies
real. The PBE1PBE/aug-cc-pVTZ(-PP) and B3LYP/aug-cc-
pVTZ(-PP) (B3LYP values are in parentheses) results are
reported in Tables 2 and 3 and Figure 1b.
ν(KrFt) of CrOF4 ·KrF2 (550 cm-1 18
than those of MoOF4 ·KrF2 (566, 579 cm-1
)
but are somewhat lower
19 and WOF4 ·KrF2
)
(571, 581 cm-1).19 The value is, however, much lower than that
of ν(KrF) in ꢀ-[KrF][AsF6] (615, 619 cm-1)3, [KrF][SbF6] (615
cm-1)3 and [KrF][BiF6] (604, 610 cm-1),3 indicating that the
coordinated KrF2 molecules in [BrOF2][AsF6]·2KrF2 are ad-
ducted and do not behave as fluoride ion donors toward the
Lewis acid, BrOF2+, as inferred from their relative crystal-
lographic bond lengths (see X-ray Crystallography).
(a) Geometries. The gas-phase geometry of [BrOF2][AsF6]·
2KrF2 optimized at C1 symmetry at both levels of theory and
did not deviate significantly from that observed in the X-ray
crystal structure (Figure 1a). The largest angle discrepancies
occur for F(3)---Br(1)---F(7) and Br(1)---F(7)-As(1), which are
over- and underestimated, respectively, with respect to the
experimental values (Figure 1b).
The Kr-Fb bridging frequencies are in better agreement with
19
those of WOF4 ·KrF2 (450, 469 cm-1
)
than with those of
(35) Bougon, R.; Bui Huy, T.; Charpin, P.; Gillespie, R. J.; Spekkens, P. H.
J. Chem. Soc., Dalton Trans. 1979, 6–12.
The calculated Br-O bond length was 1.556 (1.569) Å, and
the Br-F bond lengths were 1.731 (1.757) Å and 1.730 (1.757)
(36) Al-Mukhtar, M.; Holloway, J. H.; Hope, E. G.; Schrobilgen, G. J.
J. Chem. Soc., Dalton Trans. 1991, 2831–2834.
ˇ
(37) Tavcˇar, G.; Tramsˇek, M.; Bunicˇ, T.; Benkicˇ, P.; Zemva, B. J. Fluorine
Chem. 2004, 125, 1579–1584.
ˇ
(38) Tramsˇek, M.; Zemva, B. J. Fluorine Chem. 2006, 127, 1275–1284.
(39) Turner, J. J.; Pimentel, G. C. Science 1963, 140, 974–975.
9
3538 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Krypton Difluoride Coordination Compound
A R T I C L E S
Å, in very good agreement with the experimental Br-O
(1.564(5) Å) and Br-F (1.727(4), 1.723(4) Å) bond lengths.
The calculated O-Br-F1 (102.0 (102.0)°) and O-Br-F2 (100.0
(100.1)°) angles are more open than the F-Br-F angle (89.1
(89.8)°), and all three bond angles are in good agreement with
(Kr 0.64, 0.67; F 0.35, 0.39, 0.37, 0.40) of the coordinated
molecules are best represented by -1/2F---Kr+---F-1/2, the average
of Structures VI and VII, which are customarily used to describe
the bonding in KrF2.3 Upon adduct formation, the total of the
fluorine atom charges of KrF2 remains essentially unchanged;
however, the charge distributions of the KrF2 molecules are
polarized toward the positive bromine atom, with ∼0.07 e
transferred from Ft to Fb. A minor contribution from Structure VI
accounts for the charge drift and also accounts for the decreased
Kr-Fb bond order, increased Kr-Ft bond order, and increased Ft
valence. In addition, upon coordination, the krypton atoms become
somewhat more positively charged and there is an overall charge
transfer of 0.08 e from each KrF2 ligand to the [BrOF2][AsF6] ion
pair.
the experimental values. The three contact distances, Br---F3,5,7
,
2.350 (2.363) Å, 2.302 (2.338) Å, and 2.579 (2.529) Å,
respectively, reproduce the observed distances (2.318(4), 2.356(4),
2.576(4) Å), with the contact opposite the oxygen atom,
Br---F7, being the longest.
The KrF2 ligand geometries are also well modeled by the
calculations, i.e., the bridging krypton-fluorine bond lengths,
Kr-F3,5, 1.951 (1.984) Å and 1.957 (1.984) Å, respectively,
are longer than the terminal krypton-fluorine bond lengths,
Kr-F4,6, 1.814 (1.843) Å and 1.808 (1.837) Å, respectively.
The KrF2 ligands are also predicted to be near linear (177.2
(176.9)°, 177.5 (177.7)°), as observed experimentally.
The bond lengths, bond angles, and their trends for the
pseudo-octahedral fluorine-bridged AsF6- anion are also well
reproducedbutnotaswellasforthecation.Thearsenic-fluorine
bridge bond length, As-F7, 1.789 (1.813) Å, is elongated
relative to the other As-F bonds. The remaining calculated
As-F bond lengths range from 1.705 (1.720) Å to 1.761
(1.773) Å.
The valence bond description of BrOF2+ ·2KrF2, which takes
Structures IV and VI/VII into account, is represented by
Structures VIII and IX, where Structure VIII is the dominant
contributor and best accounts for the bonding in the adduct. A
minor contribution from Structure IX accounts for the small
increase in positive charge on the krypton atoms, the low Br-Fb
bond orders (0.09, 0.10), and a small degree of charge transfer
from the KrF2 ligands.
(b) Natural Bond Orbital (NBO) Analyses. The NBO40-43
analyses were carried out for the PBE1PBE- and B3LYP-
optimized gas-phase geometries of [BrOF2][AsF6]·2KrF2 and
KrF2 with the results given in Table S3 in the Supporting
Information. Both the PBE1PBE and B3LYP results are very
similar; only the PBE1PBE results are referred to in the ensuing
discussion.
The natural population analysis (NPA) charges given by the
NBO analysis for Br (+2.41), O (-0.72), and F (-0.45) in the
+
BrOF2 cation of [BrOF2][AsF6] · 2KrF2 total +0.79 and are
approximately half of the formal charges that are given by a
purely ionic model (+5, -2, and -1, respectively), indicating
that the cation bonds are polar covalent. The natural charges
are also consistent with a cation having a net charge of +1 where
the charge difference, -0.21, primarily arises from negative
charge transfer from the KrF2 ligands and, to a lesser extent
(c) QTAIM and ELF Analyses. The bonding was investigated
by complementary use of the quantum theory of atoms in
molecules (QTAIM)44 and the topological analysis45 of the
Becke and Edgecombe electron localization function (ELF).46
Both methods partition molecular space into adjacent nonover-
lapping regions with the help of the gradient dynamical system
theory, a technique very similar to that used in hydrology to
determine drainage basins and drainage divides. An outline of
QTAIM and ELF is provided in the Supporting Information.
For the ensuing discussion, the following abbreviations denote
-
from the AsF6 anion (-0.05), providing a total anion charge
of -0.95. Of the plausible valence bond contributions that can
be considered for the BrOF2+ cation (Structures I-IV), Structure
IV best represents the calculated charges, the Br-O/Br-F bond
order ratio (2.06), and Br/O/F/ valencies (2.27/0.95/0.46).
j
atomic populations, N(A); electron localization function, η(r);
core basins, C(A); valence basins, V(A, B, ...); monosynaptic
basins, V(A); disynaptic basins, V(A, B); and closed isosurfaces,
η(r) ) f, where f is defined as the isosurface contour. The
-
QTAIM and ELF analyses of KrF2, BrOF2+, and AsF6
fragments are provided in the Supporting Information.
Bonding in [BrOF2][AsF6]·2KrF2. The NBO, AIM, and ELF
bonding analyses provide a consistent picture of the reorganiza-
tion of electron density that results from the formation of the
KrF2 complex, i.e., the net electron density transfer toward
the BrOF2+ group and the polarization of the KrF2 ligands. The
Among the three plausible valence bond structures for KrF2, V,
VI, and VII, the calculated charges (Kr 1.08, 1.08; F -0.58, -0.43,
-0.58, -0.42), bond orders (0.24, 0.38, 0.25, 0.39), and valencies
(40) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735–746.
+
very large electronegativity differences between BrOF2 and
KrF2 (7.6 eV) and between BrOF2+ and AsF6- (14.2 eV), using
(41) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1998, 88, 899–
926.
(42) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO
Version 3.1; Gaussian Inc.: Pittsburgh, PA, 1990.
(43) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, C. M.; Morales, C. M.; Weinhold, F. NBO Version 5.0;
Theoretical Chemistry Institute, University of Wisconsin: Madison,
WI, 2001.
(44) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: Oxford, 1990.
(45) Silvi, B.; Savin, A. Nature 1994, 371, 683–686.
(46) Becke, A. D.; Edgecombe, K. E. J. Chem. Phys. 1990, 92, 5397–
5403.
9
J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3539

A R T I C L E S
Brock et al.
Scheme 1. Reduction of Localization Diagram for
[BrOF2][AsF6]·2KrF2 Showing the Ordering of Localization Nodes
and the Boundary Isosurface Value, η(r), at Which the Reducible
Domains Splita
a The labeling scheme corresponds to that used in Figure 3a.
In fact, the V(Br) basin accommodates its shape and volume
to the environment. In the complex it is confined within the
cage formed by the F1, F2, F3, F5, and F7 atoms, whereas there
are no constraints in the free cation. In contrast to the classical
AX6E arrangement predicted by VSEPR rules,25 the bond pair
and electron lone pair arrangement around Br appears to be a
hybrid of distorted octahedral, square pyramidal, and trigonal
pyramidal geometries that are predicted for AX6, AX5E, and
AX3E arrangements. The angles subtended at bromine have the
following values: O-Br-F1, 102.0°; O-Br-F2, 100.1°;
O-Br-F3, 90.7°; O-Br-F5, 85.3°; O-Br-V(Br), 133.2°; and
O-Br-F7, 170.3°.
The deviations from the octahedral (or square pyramidal)
value occur for those angles involving F1 and F2, which
correspond to the shortest F-O internuclear distances (largest
repulsions), and for F7, which is close to the V(Br) basin. It is
worth noting that the degenerate critical point mentioned above
lies on a line linking F9 to V(Br).
Figure 3. ELF localization domains for (a) [BrOF2][AsF6]·2KrF2 compared
with those of BrOF2+. The isosurface value is η(r) ) 0.75. Color code:
magenta ) core, brick-red ) monosynaptic basin.
Parr’s definition of electronegativity,47 accounts for the direction
of the charge transfer, whereas the hardness values47 of KrF2
-
(6.5 eV) and AsF6 (10.1 eV) account for the magnitudes of
the contributions. The NPA and QTAIM populations provide
global charge transfer values that are in agreement within a few
hundredths of an electron. About 0.1 e is transferred from each
KrF2 and 0.05 e from AsF6-. In the complex molecular graph,
the bromine atom is linked to the two bridging fluorines, F3
and F5, of the KrF groups and to F7 of AsF6-. The values of
the Laplacian of the electron density at the bond critical points
are positive and decrease with the Br-F internuclear distance,
i.e., 0.166 (Br-F5), 0.157 (Br-F3), and 0.122 (Br-F7). More-
over, there is a degenerate critical point between Br and F9.
The delocalization indexes between Br and the weakly bonded
fluorine atoms show almost the same trends: δ(Br, F3), 0.24;
δ(Br, F5), 0.22; δ(Br, F7), 0.14; and δ(Br, F9), 0.02. The
localization domains of the complex at η(r) ) 0.75 are shown
in Figure 3a, and the hierarchy of the ELF basins is given in
Scheme 1. Although the bromine coordination number has
increased to 6, the V(Br) (“valence electron lone pair on Br”)
basin population remains unchanged in the complex while its η(r)
) 0.75 localization domain appears to be contracted in the complex
(Figure 3a) compared to that of the free BrOF2+ cation (Figure
3b).
The interaction of the different groups also induces a
redistribution of their electronic densities among their basins.
+
The BrOF2 cation attracts 0.25 e, which is almost equally
distributed among the V(F) and V(O) basins, whereas the V(Br)
-
population remains unchanged. In the AsF6 anion, the V(As,
F) basins vanish in the complex and merge into the correspond-
ing V(F) basins. Within the KrF2 ligands, there is a density flow
from the terminal fluorine and the krypton atomic basins toward
the bridging fluorine whose net population is increased by 0.08 e.
Paradoxically, the populations of the V(F4) and V(F6) basins
increase with respect to the uncomplexed KrF2 molecule at the
expense of V(Kr). This polarization increases the covalent
character of the Kr-F4 and Kr-F6 interactions; the Kr atomic
basin contributions to V(F4) and V(F6) are 0.63 and 0.93 e,
whereas the V(F3) and V(F5) basins only belong to the F3 and
F5 atomic basins. This effect explains the contraction of the
(47) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, 1989; pp 90-98.
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3540 J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010

Krypton Difluoride Coordination Compound
A R T I C L E S
(c) [BrOF2][AsF6]·2KrF2. Approximately 0.3 mL of aHF was
condensed into an evacuated 1/4-in. o.d. FEP reaction tube containing
40.7 mg (0.126 mmol) of freshly prepared [BrOF2][AsF6] at -196
°C. The frozen HF was melted onto the [BrOF2][AsF6] sample at
-78 °C and was then refrozen at -196 °C. Krypton difluoride (45.0
mg, 0.369) was sublimed into the reactor at -196 °C, followed by
warming to -78 °C, whereupon KrF2 immediately reacted as
evidenced by the increased volume of the white solid that had
remained undissolved at -78 °C. The reactants were well mixed,
and the product was isolated after 2 h by removal of the HF solvent
under dynamic vacuum at -78 °C.
X-ray Crystallography. (a) Crystal Growth. Crystals of
[BrOF2][AsF6]·2KrF2 were grown by a previously described
procedure52 that entailed slow cooling of an HF solution of
[BrOF2][AsF6]·2KrF2, previously saturated at ca. -40 °C, from
-51 to -55 °C over the course of 5 h in a 1/4-in. o.d. FEP reactor
equipped with a side arm. When crystal growth had ceased, the
reactor was maintained at -55 °C, and the supernatant was decanted
into the side arm cooled to -78 °C. Once the majority of the
supernatant had been decanted, the contents of the side arm were
frozen at -196 °C, and the side arm was heat sealed off under
dynamic vacuum. The crystals were dried under dynamic vacuum
at -60 °C and stored at -78 °C until a suitable crystal could be
selected and mounted at low temperature on the diffractometer.
(b) Crystal Mounting. The dried crystals were dumped into an
aluminum trough cooled to -110 ( 5 °C by means of a cold stream
of dry N2 gas, allowing selection of individual crystals under a
stereomicroscope as previously described.53 A single crystal of
[BrOF2][AsF6]·2KrF2 was mounted at the tip of a glass fiber at
-110 ( 5 °C using a Fomblin oil as the adhesive. The crystal
used for data collection was a clear, transparent block measuring
0.20 × 0.20 × 0.18 mm.
Kr-F4 and Kr-F6 internuclear distances with respect to
uncomplexed KrF2.
Conclusion
+
The Lewis acid properties of the BrOF2 cation and its
resistance to oxidation have provided an avenue to the synthesis
of a KrF2 coordination complex with a main-group atom. The
synthesis and crystal structure of [BrOF2][AsF6]·2KrF2 provide
a rare example in which KrF2 functions as a ligand and represent
a significant extension of krypton chemistry, accounting for
much of what is presently known about the coordination
chemistry of KrF2. The vibrational assignments of the KrF2
ligands and their descriptions substantiate those of known
homoleptic XeF2 coordination complexes with metal cations.
The present findings may be expected to facilitate the extension
of KrF2 coordination chemistry to the syntheses of KrF2
complexes with other main-group and metal centers.
The NBO, AIM, and ELF bonding analyses indicate that
+
[BrOF2][AsF6]·2KrF2 is organized around BrOF2 and its
-
stabilization is due to its Coulomb interaction with the AsF6
anion and to electron delocalization and charge transfers
involving the KrF2 ligands. This charge transfer increases the
ionic character of the Br-O and Br-F1,2 bonds. The polarization
of the KrF2 ligands is a result of the electric field imposed by
+
the BrOF2 cation. Its main effect is to enhance the anionic
character of each bridging fluorine atom, thereby giving rise to
an electrostatic interaction with the positively charged bromine
atom of the [BrOF2][AsF6] ion pair. However, the stabilization
energy is not large enough to enable significant local rearrange-
ment of the ligands around the bromine atom, and consequently
the two KrF2 ligands are adjacent to one another. The study
has provided two structurally related examples that illustrate
strong (BrOF2+) and weak ([BrOF2][AsF6]·2KrF2) valence
electron lone pair behavior.32
(c) Collection and Reduction of X-ray Data. The single crystal
was centered on a SMART APEX II diffractometer, equipped with
an APEX II 4K charge-coupled device (CCD) and a 3-axis
goniometer, controlled by the APEX2 Graphical User Interface
(GUI) software,54 using graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). The diffraction data collection consisted of a
full φ-rotation (1010 frames collected at 0.36° intervals) at fixed ꢁ
) 54.74°, followed by a series of short ω scans (250 frames) at
various φ settings to fill the gaps. The crystal-to-detector distance
was 4.959 cm, and the data collection was carried out in a 512 ×
512 pixel mode using 2 × 2 pixel binning. Processing was carried
out by using the APEX2 GUI software,54 which applied Lorentz
and polarization corrections to three-dimensionally integrated
diffraction spots. The program SADABS55 was used for scaling
the diffraction data, the application of a decay correction, and an
empirical absorption correction based on redundant reflections.
(d) Solution and Refinement of the Structures. The XPREP56
program was used to confirm the unit cell dimensions and the crystal
lattice. The structure was solved in the space group P21/c by use
of direct methods, and the solution yielded the positions of all the
atoms. The final refinement was obtained by introducing anisotropic
parameters for all the atoms, an extinction parameter, and the
recommended weight factor. The maximum electron densities in
the final difference Fourier maps were located around the bromine
and krypton atoms. The PLATON program57 could not suggest
additional or alternative symmetries.
Experimental Section
Apparatus and Materials. (a) General. All manipulations
involving air-sensitive materials were carried out under strictly
anhydrous conditions as previously described.48 Reaction vessels,
which also served as Raman sample tubes and NMR sample tubes,
were fabricated from 1/4-in. o.d. and 4-mm o.d. FEP tubing,
respectively, and were outfitted with Kel-F valves. All reaction
vessels and sample tubes were rigorously dried under dynamic
vacuum prior to passivation with 1 atm of F2 gas.
The starting material, [BrOF2][AsF6]·XeF2, was synthesized by
solvolysis of [XeOTeF5][AsF6] in BrF5 as previously described.49
Krypton difluoride was prepared and purified according to the
literature method.50 Anhydrous HF (Harshaw Chemicals Co.) was
purified by the standard literature method.51 High-purity Ar
(99.998%, Air Liquide) or N2 (obtained from liquid N2 boil-off
and dried by passage through a column of dry 3 Å molecular sieves)
gases were used for backfilling vessels.
(b) [BrOF2][AsF6]. In a typical synthesis, 62.1 mg (0.126 mmol)
of [BrOF2][AsF6]·XeF2 that had been prepared in a 1/4-in. o.d. FEP
reaction tube was pumped under dynamic vacuum for 12-18 h at
0 °C. The resulting white powder was shown to be [BrOF2][AsF6]
by low-temperature Raman spectroscopy22 and, in contrast with a
previous report,22 was stable for at least 12 h at room temperature
with no signs of decomposition.
(52) Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2001,
40, 3002–3017.
(53) Gerken, M.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2000, 39,
4244–4255.
(54) APEX2, release 2.0-2; Bruker AXS Inc.: Madison, WI, 1995.
(55) Sheldrick, G. M. SADABS (Siemens Area Detector Absorption
Corrections), version 2.10; Siemens Analytical X-ray Instruments, Inc.:
Madison, WI, 2004.
(56) Sheldrick, G. M. SHELXTL-Plus, release 6.14; Siemens Analytical
X-ray Instruments, Inc.: Madison, WI, 2000-2003.
(57) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(48) Casteel, W. J., Jr.; Dixon, D. A.; Mercier, H. P. A.; Schrobilgen, G. J.
Inorg. Chem. 1996, 35, 4310–4322.
(49) Keller, N.; Schrobilgen, G. J. Inorg. Chem. 1981, 20, 2118–2129.
(50) Chernick, C. L.; Malm, J. G. Inorg. Synth. 1966, 8, 254–258.
(51) Emara, A. A. A.; Schrobilgen, G. J. Inorg. Chem. 1992, 31, 1323–
1332.
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J. AM. CHEM. SOC. VOL. 132, NO. 10, 2010 3541

A R T I C L E S
Brock et al.
Raman Spectroscopy. Raman spectra were recorded on a Bruker
RFS 100 FT-Raman spectrometer at -150 °C using 1064 nm
excitation, 300 mW of laser power, and 1 cm-1 resolution as
previously described with a total of 1200 scans acquired.53
Computational Results. The optimized geometry and frequen-
cies of [BrOF2][AsF6]·2KrF2 were calculated at the B3LYP and
PBE1PBE levels of theory using aug-cc-pVTZ for all atoms.
Pseudopotentials (aug-cc-pVTZ-PP) for Kr, As, and Br were used.58
The combined use of aug-cc-pVTZ and aug-cc-pVTZ-PP is
indicated as aug-cc-pVTZ(-PP).58 The NBO analyses40-43 were
performed for the PBE1PBE and B3LYP optimized local minima.
Quantum-chemical calculations were carried out using the program
Gaussian 0359 for geometry optimizations, vibrational frequencies,
and their intensities. The program GaussView60 was used to
visualize the vibrational displacements that form the basis for the
vibrational mode descriptions given in Table 3.
The QTAIM analysis can only be performed with the total
density, which prevents the use of core pseudopotentials. The ELF
approach also requires, in principle, the all-electron wave function,
although small core pseudopotentials yield satisfactory results. For
all of the studied systems, the all-electron wave functions have been
calculated at the PBE1PBE/aug-cc-pVTZ optimized geometries with
the DGDZVP basis set.61 In order to validate the use of this basis
set in the present topological analyses, the ELF populations and
covariance matrix elements of KrF2 were calculated at the
PBE1PBE/aug-cc-pVTZ and PBE1PBE/DGDZVP levels and were
compared. The numerical values of all basins involving the fluorine
atoms agree within the numerical accuracy of the integration. There
are small differences in the Kr basins because the descriptions of
the inner shells are not identical (Table S4 in the Supporting
Information). ELF grids and basin integrations have been computed
with the TopMod package.62,63 The ELF isosurfaces have been
visualized with the Amira 3.0 software.64
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada for support in the form of a
Discovery Grant (G.J.S.), the Ontario Graduate Scholarship in
Science and Technology and the McMaster Internal Prestige
“Ontario Graduate Fellowships” Programs for support in the form
of scholarships (D.S.B.), and the computational resources provided
by SHARCNet (Shared Hierarchical Academic Research Computing
Network; www.sharcnet.ca).
Supporting Information Available: Complementary discus-
sion of the X-ray crystal structure of [BrOF2][AsF6]·2KrF2;
factor-group analysis for [BrOF2][AsF6]·2KrF2 (Table S1);
experimental and calculated Raman frequencies for KrF2 (Table
+
S2); NBO valencies, bond orders, and NPA charges for BrOF2 ,
[BrOF2][AsF6]·2KrF2, and KrF2 (Table S3); outline of QTAIM
and ELF; QTAIM and ELF analyses of KrF2, BrOF2+, and
AsF6- fragments; ELF localization domains of KrF2 (Figure S1);
ELF basin population and covariance matrix elements of KrF2
j
(Table S4); ELF basin population, N[Ω], covariance matrix
j
j
elements, 〈coV(N[Ω],N[Ω′])〉, and bromine atomic basin con-
tribution, (N[Ω|Br]), of BrOF2 (Table S5); ELF localization
domains for AsF6 (Figure S2); complete ref 59; and X-ray
+
j
-
crystallographic file in CIF format for the structure determination
of [BrOF2][AsF6]·2KrF2. This material is available free of
charge via the Internet at http://pubs.acs.org.
(58) Basis sets and pseudo-potentials were obtained from the Extensible
Computational Chemistry Environment Basis set Database, version
2/25/04, as developed and distributed by the Molecular Science
Computing Facility, Environmental and Molecular Science Laboratory,
which is part of the Pacific Northwest Laboratory, P.O. Box 999,
Richland, WA 99352.
(59) Frisch, M. J. et al. Gaussian 98, Revision A.11; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(60) GaussView, release 3.0; Gaussian Inc.: Pittsburgh, PA, 2003.
(61) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J. Chem.
1992, 70, 560–571.
JA9098559
(62) Noury, S.; Krokidis, X.; Fuster, F.; Silvi, B. Comput. in Chem. 1999,
23, 597–604.
(63) Matito, E.; Silvi, B.; Duran, M.; Sola`, M. J. Chem. Phys. 2006, 125,
024301.
(64) Amira 3.0; TGS, Template Graphics Software, Inc.: San Diego, CA,
2002.
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