PGSE NMR diffusion overhauser studies on [Ru(Cp*)(η6- arene)][PF6], plus a variety of transition-metal, inorganic, and organic salts: An overview of ion pairing in dichloromethane

Article abstract of DOI:10.1002/chem.200800222

PGSE diffusion, ¹F, ¹H HOESY and ¹³CNMR studies for a series of [Ru(Cp*)(η⁶-arene)][PF₆] (1) salts are presented. The solid-state structure of [Ru(Cp*) (η⁶-fluorobenzene)][PF₆] (1c) is reported. The extent of the ion pairing and the relative positions of the ions are shown to depend on the arene. For the solvent dichloromethane, new and literature PGSE data for PF⁶_- salts of transition-metal, inorganic, and organic salts are compared. Taken together, these new results show that the charge distribution and the ability of the anion to approach the positively charged positions (steric effects due to molecular shape) are the determining factors in deciding the amount of ion pairing. DFT calculations of the charges in four salts of type 1, as well as in a variety of other salts, using a natural population analysis (NPA), support this view. This represents the first attempt, using experimental data, to understand, correlate, and partially explain the various degrees of ion pairing in a widely different collection of salts.

Full text of DOI:10.1002/chem.200800222

FULL PAPER
DOI: 10.1002/chem.200800222
6
PGSE NMR Diffusion Overhauser Studies on [Ru
A
H
R
U
G
A
H
R
N
(h -arene)]
A
H
R
U
G
6
Plus a Variety of Transition-Metal, Inorganic, andOrganic Salts:
An Overview of Ion Pairing in Dichloromethane
[
a]
[a]
[b]
[c]
Aitor Moreno, Paul S. Pregosin,* Luis F. Veiros, Alberto Albinati, and
[
c]
Silvia Rizzato
1
9
1
Abstract: PGSE diffusion,
F,
H
metal, inorganic, and organic salts are
compared. Taken together, these new
results show that the charge distribu-
tion and the ability of the anion to ap-
proach the positively charged positions
(steric effects due to molecular shape)
are the determining factors in deciding
the amount of ion pairing. DFT calcu-
lations of the charges in four salts of
type 1, as well as in a variety of other
salts, using a natural population analy-
sis (NPA), support this view. This rep-
resents the first attempt, using experi-
mental data, to understand, correlate,
and partially explain the various de-
grees of ion pairing in a widely differ-
ent collection of salts.
13
HOESY and C NMR studies for a
(h -arene)][PF ] (1)
salts are presented. The solid-state
[Ru(Cp*)(h -
[PF ] (1c) is reported.
The extent of the ion pairing and the
relative positions of the ions are shown
to depend on the arene. For the solvent
dichloromethane, new and literature
6
series of [Ru
A
C
H
T
R
E
U
N
G
(Cp*)
A
C
H
T
R
E
U
N
G
A
H
E
N
6
6
structure
of
A
H
R
U
G
fluorobenzene)]
ACHTREUNG
6
Keywords:
density functional calculations
anions
·
cations
·
·
ion pairs · NMR spectroscopy
ꢀ
PGSE data for PF6 salts of transition-
Introduction
“innocent”. However, a number of reports in the recent lit-
erature suggest that the choice of the anion can be impor-
tant, both in terms of the structure of the salt and its reactiv-
Transition-metal salts are synthesized or generated in an in-
creasing variety of reactions. Frequently, the nature of the
anion is ignored or the anion is considered to be relatively
[1–13]
ity.
1) Diels–Alder catalysis, 2) polymerization chemistry, 3)
Relevant examples include an anion dependence in
[1]
[2]
[3]
catalytic hydrogenation, 4) asymmetric ring opening reac-
[4]
tions of oxabicyclic alkenes, 5) asymmetric cyclopropana-
[
a] A. Moreno, Prof. P. S. Pregosin
Laboratory of Inorganic Chemistry
ETHZ, Hçnggerberg 8093 Zürich (Switzerland)
Fax : (+41)44-633-10-71
[
5]
[6]
tion, and 6) C,H-activation reactions, amongst others.
The anions may or may not coordinate to the metal
[7]
center. Normally, one does not find an explanation for the
E-mail: pregosin@inorg.chem.ethz.ch
effect of the anion; however, occasionally, ion pairing is
[
b] Prof. L. F. Veiros
[11,17]
mentioned as a possible contributor.
Centro de Química Estrutural
Complexo I, Instituto Superior TØcnico
Av. Rovisco Pais 1, 1049-001 Lisbon (Portugal)
Fax : (+351)21-846-44-55
It is not always evident how one should measure and rec-
ognize this ion pairing; however, recent NMR studies, which
combine both pulsed gradient spin-echo (PGSE) diffusion
E-mail: veiros@ist.utl.pt
[14,15]
and Overhauser NMR spectroscopy,
, have begun to
[
c] Prof. A. Albinati, Dr. S. Rizzato
shed light on how selected anions interact with their respec-
tive cations in solution. Usually, the literature studies involv-
ing NMR diffusion measurements emphasize the use of this
methodology in connection with recognizing changes in mo-
Department of Structural Chemistry and School of Pharmacy
University of Milan, 20133 Milan (Italy)
Fax : (+39)02-5031-4454
E-mail: alberto.albinati@unimi.it
[16–30]
Supporting information for this article is available on the WWW
under http://www.chemeurj.org or from the author. It contains .cif
files for the structure of 1c, plus Tables S1 and S2, with the NPA
charge distributions and optimized coordinates. In addition Figures
S1 and S2 show optimized geometries of the ion pairs and a represen-
tation of the molecular electrostatic potential for the cations.
lecular volumes.
Nevertheless, one finds an increasing
number of reports involving NMR data and ion pair-
[15,17,31–33]
ing.
Assuming that the two ions can be measured
separately, inspection of the magnitudes of the NMR diffu-
sion constants provides a direct estimate of the extent of the
Chem. Eur. J. 2008, 14, 5617 – 5629
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5617

ion pairing. For 100% ion pairing (and in the absence of,
e.g., hydrogen-bonding, or encapsulation effects), the diffu-
sion constants (D values) for the anion and cation will be
P(o-tolyl) one of the o-tolyl substituents coordinates to
A
H
T
E
U
G
3
6
[39]
afford [Ru
A
H
R
U
G
A
H
R
U
G
A
H
R
U
G
[1415]
13
identical within the experimental error.
Despite the
C NMR spectroscopy: In connection with understanding
recent progress, the relevant reports focus primarily on one
related set of salts and thus are rather specific. We know of
no study attempting to understand and correlate ion pairing
in very different materials.
the positive charge distribution in our complexes we have
1
3
measured and assigned the C spectra of our salts and show
these data in Table 1. Figure 1 shows how some of these as-
1
3
1
signments were made with the help of C, H correlations.
In the first section of this contribution we present NMR,
6
X-ray and DFT studies for a series of [Ru
A
H
R
U
G
A
H
R
U
G
A
C
H
T
R
E
U
N
G
[PF ] salts. After briefly describing the ion pairing and struc-
6
tures of these salts we move to the second, more significant
part in which we extend the discussion, using analogous data
ꢀ
from PF6 salts of various transition-metals, inorganic, and
organic salts, to form a more general picture of this phenom-
enon. This represents the first attempt to understand, corre-
late, and partially explain the differing degrees of ion pair-
ing in a very varied collection of salts by using experimental
NMR data.
Results andDiscussion
6
[Ru
A
C
H
T
R
E
U
N
G
(Cp*)
A
C
H
T
R
E
U
N
G
(h -arene)]
A
H
E
N
[PF ] salts (1): The ruthenium salts
6
were prepared by addition of a suitable excess of the arene
to a solution of [Ru(Cp*)(CH CN) ][PF ] in accordance
A
H
R
U
G
A
H
R
U
G
ACHTREUNG
3
3
6
[
34–36]
with the literature.
The solid-state structures of a
[37]
number of Ru–arene complexes are known and this litera-
[38]
ture includes examples involving Cp*. In some cases, the
complexation of the arene to the {Ru(Cp*)} fragment is pre-
ferred even when other good donor atoms are present, for
example, in the reaction of [Ru(Cp*)(CH CN) ][PF ] with
A
H
R
U
G
13
1
6
Figure 1. Section of the C, H correlation spectrum of 1b in [D ]acetone
1
13
1
showing the cross-peaks from the complexed arene due to J( C, H). The
A
H
R
U
G
A
H
R
U
G
A
H
R
U
G
proton assignment from left to right: ortho, meta, para.
3
3
6
1
3
6
Table 1. C Chemical shifts of the complexed arenes in 1–5 in [D ]acetone.
Salt
C1
C2
C3
C4
C7
C8
C9
C10
53.0
1
1
1
1
1
1
a X=H
b X=OMe
c X=F
d X=SiMe
e X=CO
f X=NO
87.4
97.6
133.6
135.6
89.5
96.7
97.3
96.9
98.2
98.0
100.3
9.8
90.4
87.1
79.0
87.3
84.4
88.6
76.7
87.1
88.2
89.2
88.6
86.0
87.2
89.0
91.2
11.1
10.7
10.0
9.3
57.2
3
2
Me
164.8
2
111.2
9.9
C1
110.9
110.7
C2
79.1
87.2
C3
110.9
91.0
C4
86.5
94.6
C5
88.6
89.4
C6
86.5
84.8
C7
102.6
101.4
C8
8.7
9.4
2
3
X=NO
X=Br
2
C2
135.4
C3
100.3
C3’
96.7
C4
79.3
C5
83.5
C6
83.6
C7
73.8
C7’
109.0
C8
92.9
C9
9.7
4
5
C1
130.6
C2
76.7
C3
71.5
C4
121.4
C7
96.6
C8
10.4
5618
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5617 – 5629

Ion Pairing
FULL PAPER
[
38]
As expected, arene complexation results in much lower
This equation is useful as it allows one to calculate a hy-
drodynamic radius from the viscosity (h) correction, and
thus compare the diffusion results from different solvents.
13
frequency C chemical shifts for all of the arene ligands.
Specifically, the coordination chemical shifts, Dd, for the
para-carbon atoms, fall in the range ꢀ35 to ꢀ44 ppm. Com-
pared to the changes in the para-carbon atoms within the
free arene compounds,
atoms in 1a–1 f, ꢁ3–4 ppm, are modest. The difference in
[14]
However, this relation has been criticized in that it is now
recognized that the value “6” is not correct for small-to-
medium sized molecules. An empirical correction has been
[40]
the changes in the para-carbon
[14a]
suggested
and the values in the table arise from using
both the value 6 and a semi-empirical correction. One can
compare these radii with those obtained from X-ray data
and we show such values in the footnotes to the tables.
In acetone, the measured D values for the cation and
anion are very different. The D values and calculated radii
for the anions are in good agreement with previous reports
[14a,15a]
in which little (but not zero) ion pairing was suggested.
The corrected rH values of approximately 4.3–4.4 Š seem a
little large for just the solvated anion; however, similar
ꢀ
values for PF6 have been obtained in methanol and it is not
a simple matter to estimate the size of the solvent shell. Al-
though acetone efficiently separates the ions, there is still a
ꢀ
10
2
ꢀ1
range of D values: 24.93(6)–27.78(6) (”10 m s ). The D
values for the cations range from 14.62(6) to 16.80(6) with
the smallest values associated with the larger cations, and
the largest with the smaller cations. The corrected rH values
fall in the range 5.2–5.7 Š and we will assume that these
values represent a reasonable estimate of the hydrodynamic
[41]
radius for the solvated cations in acetone.
1
3
the para-carbon C chemical shift between free nitroben-
zene and anisole is about 13.8 ppm, whereas for salts 1b and
1
In dichloromethane, the D values for the anions and cat-
ions are much closer. Indeed, for the dinitro and indole salts
2 and 4, respectively, the values for these two are almost
(but not quite) identical, suggesting, for 2, close to complete
f, this value is reduced to 2.6 ppm. For the F-substituted
ipso-carbon (para to the amino group) in the p-F-aniline salt
, the change is about 3 ppm,
5
relative to that for 1c. The
donor effect of an amino group
on the para position should be
much larger. This suggests that
resonance effects due to the
substituent R in salts 1 are not
as pronounced as in the free
organic arene compounds.
ꢀ
10
2
ꢀ1
[a]
Table 2. D [10 m s ] and rH [Š] values for the salts in CD Cl and [D ]acetone.
2
2
6
CD
2
Cl
2
6
[D ]acetone
corr
H
corr
H
Compound
Fragment
D
D /D
c
a
rH
r
D
D /D
c
a
rH
r
1
1
1
a
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
12.50
13.33
12.42
13.02
12.39
12.95
11.67
12.65
12.10
13.14
11.75
12.28
11.30
11.57
11.43
12.02
12.07
12.28
12.23
12.55
0.94
4.2
4.0
4.3
4.1
4.3
4.1
4.5
4.2
4.4
4.0
4.5
4.3
4.7
4.6
4.6
4.4
4.4
4.3
4.3
4.2
5.1
4.9
5.2
5.0
5.2
5.0
5.4
5.1
5.2
5.0
5.3
5.2
5.5
5.4
5.4
5.3
5.2
5.2
5.2
5.1
16.43
25.87
16.35
24.93
16.80
26.14
15.61
26.31
15.54
27.00
15.64
26.07
14.62
25.10
15.00
27.23
15.88
27.78
15.88
25.95
0.64
4.4
2.8
4.4
2.9
4.3
2.7
4.6
2.7
4.6
2.7
4.6
2.8
4.9
2.9
4.8
2.6
4.5
2.6
4.5
2.8
5.3
4.4
5.3
4.4
5.2
4.4
5.4
4.3
5.5
4.3
5.4
4.4
5.7
4.4
5.6
4.3
5.4
4.3
5.4
4.3
[
b]
b
0.95
0.96
0.92
0.92
0.96
0.98
0.95
0.98
0.97
0.66
0.64
0.59
0.58
0.60
0.58
0.55
0.57
0.61
[
c]
c
[
d]
PGSE diffusion data: Diffusion
1d
1e
constants for the cations and
ꢀ
the PF6 ions were obtained by
using PGSE methods as de-
1
2
3
4
5
f
[14a,15]
scribed earlier.
Table 2
shows a compilation of these
results for 2 mm solutions of
the salts in dichloromethane
and acetone. In addition to the
experimental D values, hydro-
[e]
dynamic radii (r ) derived
H
from the Stokes-Einstein equa-
tion [Eq. (1)] are given.
H
[a] All at 2 mm. For the calculation of r , the viscosity of the nondeuterated solvent at 299 K was used.
ꢀ
3
ꢀ1
ꢀ1
ꢀ3
ꢀ1
ꢀ1
h(CD
2
Cl
2
)=0.41”10 kgs
m
; h([D
6
]acetone)=0.31”10 kgs
H
m . r was calculated using the Stokes–
corr
[14a]
Einstein equation, while r was calculated using a semiempirical estimation of c.
The measurements were
carried out using the H NMR resonance of Cp* for the cation and the F NMR resonance of PF for the
[c] r =4.7 Š. [d] For [Ru(Cp*)(h -phenyltri-
H
1
19
6
kT
phD
6
cryst
[38c]
cryst
6
anion. [b] For [Ru(Cp*)(h -anisole)][OTf], r =4.9 Š.
rH ¼ 6
ð1Þ
cryst
[38b]
6
cryst
[38c]
methylsilane)][OTf], r =5.1 Š.
[e] For [Ru(Cp*)(h -indole)][OTf], r =4.9 Š.
Chem. Eur. J. 2008, 14, 5617 – 5629
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5619

P. S. Pregosin et al.
[36b]
ion pairing. For the indole and aniline salts, 4 and 5,
re-
aniline salts, 4 and 5, respectively, the observed selective
spectively, hydrogen bonding between the NꢀH moieties
and the anion is likely to be a major contributor to the ob-
served D values. It is interesting that, despite the substantial
increase in ion pairing, relative to the acetone data, the cor-
rected rH values suggest that the cation is not larger than in
acetone.
HOESY contacts to the arene protons are consistent with
the presence of NꢀH···FꢀPF hydrogen bonds, thereby
5
bringing the anion close to the =CH protons proximate to
the N atoms.
19
1
[42]
F, H HOESY results: Although Branda et al.
have re-
cently summarized the combined use of diffusion and Over-
hauser methods, Macchioni and co-workers in a series of
[14,24,31]
19
1
papers
have made specific use of F, H HOESY
measurements on transition-metal salts, and have shown
that this is a viable method for placing the fluorine-contain-
ing anions relative to their cations, in three-dimensional
space.
Scheme 1. Selected HOESY results.
The HOESY results for 1 in dichloromethane (for which
we suspect significant ion pairing) can be summarized as fol-
lows: 1) one always finds a significant interaction of the
anion with the Cp* methyl groups as well as several contacts
An approach of the anion which brings it close to the
meta- and para-protons of the complexed arene might be ex-
pected based on steric grounds. However, it is not evident
why the PF6 ion would be attracted towards the region of
the electronegative O or F atoms, since this approach is ster-
ically less favorable.
ꢀ
to the complexed arene protons; 2) in all cases the PF6 ions
ꢀ
appear to be sitting close to the two planes of the ligands
near to the meta and para protons of the arene; and 3) in
two cases, 1b, R=MeO and 1c, R=F, the HOESY contacts
to the ortho-arene protons are somewhat stronger. This last
point is illustrated in Figure 2, which shows results for 1b
and 1 f and is summarized in Scheme 1. For the indole and
We will return to the solution D values and HOESY re-
sults for the salts 1 in connection with the second section.
Solid-state structure of 1c: The structure of 1c was deter-
mined by X-ray diffraction methods and Figure 3 gives an
ORTEP view of the cation, a space-filling model of the salt,
and selected interionic distances as well as selected bond
lengths and bond angles in the cation. The various RuꢀC
separations are all normal and in keeping with the literature
[37,38]
for such salts.
The presence of the F-substituent on the
arene does not seem to have induced any marked deviations
due to steric effects, as the six RuꢀC
A
H
R
U
G
are not significantly different.
From the space filling model (Figure 3, middle) it is clear
that the anion approaches both the Cp* and the coordinated
arene and that the observed closest contacts are under 3 Š
for both rings. These relatively short contacts are consistent
with the postulated strong ion pairing from the diffusion
ꢀ
data. The location of the PF6 ion close to the meta-proton
of the arene is also in agreement with the specificity ob-
served in the HOESY spectrum, in that the ortho-proton is
relatively close.
Calculations: To help in understanding the diffusion and
Overhauser data, we have calculated the charge distribution
in 1a*–c*, R=H, MeO, F and 1 f*, R=NO , using a natural
2
[43]
population analysis (NPA) performed on geometries opti-
[44]
mized by means of DFT calculations. The results are sum-
marized in Table 3 and in Tables S1 and S2 in the Support-
ing Information (the asterisk indicates that Cp rather than
Cp* was used in the calculations). The Ru atom carries only
a modest positive charge, about 0.20, in all four cases. A
more positive anisole in 1b* (0.51), with respect to nitroben-
1
9
1
Figure 2. The F, H HOESY NMR spectra of the salts 1b and 1 f at am-
bient temperature showing the selective interactions. In the methoxy-sub-
stituted-salt 1b, there are substantial contacts to both the ortho and meta
protons, whereas in the nitro-substituted salt 1 f, the contact to the ortho
protons is weak, but contacts to the meta and para protons stronger
(
2 6 6 2 2
CD Cl2/C D 5:1, for 1b and CD Cl for 1 f, 400 MHz, 10 mm).
5620
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5617 – 5629

Ion Pairing
FULL PAPER
Table 3. Selected positive charges from the NPA analysis of the cations.
Ru
Cp
Arene
1
1
1
1
a*
b*
c*
f*
R=H
R=MeO
R=F
0.21
0.20
0.19
0.21
0.32
0.29
0.34
0.38
0.47
0.51
0.47
0.41
R=NO
2
ligand (0.38), which becomes more positive than in 1b*
0.29). This balance results in essentially the same metal
(
charge for the two complexes.
Continuing, the polarization of the CꢀH bonds is such
that the carbon atoms are negative by approximately ꢀ0.2
to ꢀ0.3 units and the hydrogen atoms positive by about the
same amount. However, two points are especially relevant:
1
1
) the substituted ipso-carbon atoms of the arene in 1b*,
c*, and 1 f* are positive for all three complexes, but with
considerably different charges, +0.37, +0.46, and +0.05, re-
spectively; and 2) the charge on the para-carbon atoms of
the arene ligands are all essentially the same (ca. ꢀ0.23 to
ꢀ
0.24). The second point supports the conclusions from the
C data with respect to the lack of strong resonance effects,
1
3
while the first suggests that, combined with the CꢀH bond
ꢀ
polarization, the PF ion will be attracted somewhat more
6
towards the ortho positions of the F- and MeO-substituted
arene (in agreement with the observed HOESY data), de-
spite the negative charges on the F and O atoms.
Additional DFT calculations were performed in order to
obtain the optimized geometry of the ion pairs formed by a
ꢀ
PF6 ion and each of the three model cation complexes,
1
b*
A
H
R
U
G
A
H
R
U
G
different structures were found, depending on the starting
ꢀ
geometry: A in which the cation and the PF6 ion are situat-
ꢀ
ed in a “side-by-side” arrangement, or B in which the PF6
is placed in a remote position relative to the R substituent
in the arene. For cations in 1b* and 1c*, the two arrange-
ments are clearly distinct. The ion pair resulting from geom-
Figure 3. On the structure of salt 1c. Top: Selected bond lengths [8] and
bond angles [8] in the cation: RuꢀC1 2.180(6), RuꢀC2 2.197(6), RuꢀC3
ꢀ
etry A has the PF6 ion placed in between the ortho- and
2
2
2
.187(6), RuꢀC4 2.183(6), RuꢀC5, 2.169(6), RuꢀC11 2.208(6), RuꢀC12
.207(7), RuꢀC13 2.198(7), RuꢀC14 2.213(8), RuꢀC15, 2.214(7), RuꢀC16
meta-H atoms of the arene ligand, while the ion pair result-
ing from geometry B has the anion approaching the meta-
and para-H atoms of that same ligand. In the case of cation
in 1 f*, the two geometries obtained are essentially equiva-
lent with the anion remote from the ortho-H atom and ap-
proaching the meta- and para-H atoms of the arene. In all
ꢀ
.227(7), C11 F 1.342(8), F-C11-C12 119.2(7), F-C11-C16 118.2(7).
Center: Space filling model of the salt, showing the placement of the
anion relative to one cation, in the unit cell. Bottom: Ball and stick
model indicating the calculated packing distances (only the first decimal
figure is significant).
ꢀ
cases the PF6 ion is placed in between the planes of the Cp
zene in 1 f* (0.41), reflects the better donor character of this
arene. In the nitro analogue (1 f*) the weaker electron dona-
tion from the arene to the metal is balanced by the Cp
and arene p ligands. The structures calculated for the ion
pairs are presented in the Supporting Information (Fig-
ure S1).
Chem. Eur. J. 2008, 14, 5617 – 5629
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www.chemeurj.org
5621

P. S. Pregosin et al.
The two ion-pair geometries (A and B) are practically iso-
might be extrapolated to other cations. Table 4 shows a se-
lection of literature PGSE diffusion data in dichloromethane
for a variety of monocationic transition-metal salts with the
PF6 ion. These include other ruthenium salts as well as iri-
dium, rhodium, and palladium examples, all in the concen-
tration range of about 1–2 mm.
From this table, one finds that the D values for the cations
are considerably smaller than those for the anions and that
the anion values fall in the range 9.87–14.70. Consequently
one can conclude that complete—or almost complete—ion
pairing is not the general case. The smaller D (and larger
rH) values for the anion are not what might be expected for
a “free” solvated anion (in methanol or acetone), so that
one can consider these data as representing examples of
varying intermediate ion pairing. To avoid the assumptions
associated with the Stokes–Einstein equation, and its modi-
fications, only the experimental D values will be used in the
energetic for the F and NO arene cations in 1c* and 1 f*
2
ꢀ
1
(
within 0.3 kcalmol ), but in the case of the methoxy-arene
ꢀ
cation in 1b* geometry
.7 kcalmol . The position of the PF ion relative to the
A
is clearly preferred by
ꢀ
1
ꢀ
5
6
arene ortho- and meta-H atoms (in geometry A) can be
evaluated through the hydrogen–phosphorus (HꢀP) separa-
tions in each calculated ion pair. The results obtained (see
ꢀ
Supporting Information) indicate that the PF6 approaches
the ortho-H atom of the arene in the case of cation in 1b*
(
HorthoꢀP=2.83 Š, H ꢀP=3.95 Š), while it moves closer
meta
to the meta-H atom in the case of complex cation in 1 f*
(
HorthoꢀP=4.97 Š, H ꢀP=2.86 Š). For 1c* an intermedi-
meta
ate situation occurs. This result agrees with the observed
HOESY data and with the conclusions drawn above, based
on the analysis of the calculated atomic NPA charges, there-
by validating this approach. The results above are further
confirmed by the calculated electrostatic potential for cat-
ions in 1b* and 1 f* (see Figure S2 in the Supporting Infor-
mation).
discussion that follows. Tables 2 and 4 show the ratios D /Da
c
(subscripts: c=cation, a=anion) immediately to the right of
the concentration values. We will assume that this ratio
qualitatively reflects differences in the amount of ion pair-
ꢀ
[45]
PF6 ion pairing in dichloromethane for a wide variety of
ing.
salts: For 1 in dichloromethane, a common solvent in organ-
ometallic chemistry, the data in Table 2 suggest a rather
large extent of ion pairing. It is not clear if this is a special
property of this anion and to what extent this observation
The largest ratio in Table 4 is found for the [RuCl-
6
A
H
R
U
G
(TMEDA=tetramethylethylenedi-
[31d]
A
T
E
N
amine) cation
(0.93) and the smallest value (0.57) for the
III
[46]
[Ir H {P
A
H
R
U
G
A
T
E
N
There are several fac-
2
ꢀ
10
2
ꢀ1
2 2
6ꢀ
[Š] values in CD Cl for PF salts.
[a]
Table 4. D [”10 m s ] and r
H
[
31d]
[31g]
6
7
8a
(2.7 mm, 0.93)
8b
9
10
(
2.0 mm, 0.86)
(2.0 mm, 0.72)
(5.0 mm, 0.93)
(2.0 mm, 0.69)
(2.0 mm, 0.73)
(1.0 mm, 0.72)
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
cation
8.49
9.87
6.6
5.8
7.87
10.99
7.3
5.5
10.9
11.7
4.9
4.7
11.6
12.5
4.5
4.3
8.18
11.8
7.0
5.2
9.72
13.27
6.0
4.7
9.92
13.78
5.9
4.6
ꢀ
PF
6
[
b]
[b]
[c]
[d]
[51]
1
1
12
(1.0 mm, 0.57)
13a
(2.0 mm, 0.63)
13b
(2.0 mm, 0.62)
14
(2.0 mm, 0.71)
15a
(2.0 mm, 0.70)
15b
(2.0 mm, 0.67)
(
1.0 mm, 0.73)
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
D
r
H
cation
9.67
13.18
6.0
4.7
7.70
13.43
7.4
4.7
9.33
14.70
6.2
4.3
8.67
14.02
6.6
4.5
8.61
12.13
6.7
5.0
9.36
13.38
6.2
4.7
6.67
9.93
8.4
5.9
[
ꢀ
PF
6
14a]
[
a] Values in parenthesis after the concetration represent the quotient D
c
/D
a
, while r
H
was calculated by using a semiempirical estimation of c.
[
b] See reference [50]. [c] See reference [32c]. [d] See reference [67].
5622
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5617 – 5629

Ion Pairing
FULL PAPER
tors that are likely to contribute to the observed differences
in this parameter. Molecular weight (more properly molecu-
lar volume) will play a role, but this cannot be the only com-
for [N
A
H
T
E
U
G
A
H
R
U
G
strong HOESY contacts to all of the aliphatic resonances.
Extending this view, the binap salt 7 (binap=2,2’-bis(di-
phenylphosphino)-1,1’-binaphthyl),
donor set as in 6, shows a smaller D /D ratio (0.72), because
c a
[47]
[49]
ponent, since the ratio (0.86) for the Ru–bis(phosphine) 6
cation weight=618) is similar to that (0.80) for the ammo-
with almost the same
(
[48]
nium salt 17 (cation weight=366). Formal oxidation state
the anion has to find its way past the bulky binaphthyl
[29]
does not seem critical as indicated by a comparison of the
moiety in order to approach the P atoms. Salt 9, with its
relatively large phosphoramidite ligand, has the same prob-
lem. Experience has shown that despite the relatively small
size of the chloride ligand, the anion will not approach for-
mally negatively charged donors.
phine)–oxazoline complex (12),
I
III
[46]
Ir /Ir –phox salts 10 and 11, respectively, both of which
IV
have essentially identical D /D ratios. Moreover, the Ru –
allyl salt, [Ru
[PF ] (not shown in Table 4), has a D /D ratio of 0.90 (D
c
a
2
3
A
C
H
T
R
E
U
N
G
(Cp*)
A
H
R
N
(k -OAc)(h -CH ꢀCHꢀCH
A
H
R
U
G
2
[14,15]
A
C
H
T
R
E
U
N
G
The Ir–tris(phos-
6
c
a
[32e]
(
cation)=12.20 and D (anion)=13.51), not very far from
reveals the smallest ratio
the values for 1 in Table 2, again suggesting no major effect
of oxidation state.
(the least ion pairing), because 1) the positive charges are
likely to be distributed across four centers (three P atoms
and one N atom) and 2) the approach of the anion is hin-
dered by the nine substituted aromatic moieties.
We suggest that the positive charge distribution, together
with the ability of the anion to approach the positively
charged positions (steric effects due to molecular shape),
represent the determining factors. The structure of the
TMEDA salt 8a (D /D ratio=0.93), with its two partially
The dinuclear, hydroxide-bridged, binap–palladium salt
[51]
15b is informative in that it shows a relative small ratio of
0.67. One might have expected a larger value in that this
contains a dication (larger electrostatic interactions), plus
there is the possibility of a hydrogen bond between an F
c
a
positively charged ammonium-like N atoms concentrates the
positive charge near the metal, and the relatively small che-
lating TMEDA ligand will allow the anion to approach. The
atom of the anion and the OH donor(s). However, as
[47]
[14,15]
bis-phosphine 6 (D /D ratio=0.86), with its longer chains
noted,
the anion prefers to avoid the negatively charged
c
a
and modestly bulky PnBu3 ligands will keep the anion a
little further away from the positive P atoms. Indeed the
HOESY spectrum for 6, see Figure 4 (left), reveals only a
areas, and thus is forced to approach the positively charged
P atoms and consequently encounters significant steric inter-
actions from the various binap organic moieties. These steric
effects will be even more pronounced for the second anion,
if and when the first anion associates with the dication.
Continuing, NPA calculations have also been carried out
for the iridium(I)–1,5-cyclooctadiene (COD) complex 10*
weak contact to the PCH group, but shows strong contacts
2
to the remaining three aliphatic resonances. This is in con-
trast to what one observes in the corresponding spectrum
(
with methyl instead of tert-butyl and phenyl instead of o-
[50]
tolyl), and the rhodium(I)–carbene salt 13a* (with methyl
instead of cyclohexyl). Both of these salts show relatively
small D /D ratios. For 10*, the positive charges are located
c
a
primarily on the P atom (1.19), a second-row element, and
on the oxazoline ring carbon C10 (0.68), a close neighbor of
both the imine N and O atoms. The positive charge on the
Ir atom (0.24) is modest and similar to what was found in
the [Ru(Cp)
carbon atoms of the 1,5-COD carry modest negative charges
ꢀ0.25 to ꢀ0.30). Relatively large calculated positive charg-
es for the complexed P atoms of tertiary phosphines have
A
H
R
U
G
(
[51]
been reported earlier. Assuming that the anion will be at-
tracted towards C10 and the P atom, it is clear that steric ef-
fects will make a close approach of the anion more difficult.
The largest positive charge in the Rh–carbene salt 13a* is
located on the carbon of the CO ligand (0.58), reflecting a
1
9
1
Figure 4. Comparison of the F, H HOESY spectra for 6 and [N(nBu)
PF ]. Note that in 6 the contacts to the PCH group are very weak
CD Cl 400 MHz, 10 mm).
4
]
[
(
6
2
2
2
Chem. Eur. J. 2008, 14, 5617 – 5629
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5623

P. S. Pregosin et al.
neighboring electronegative O atom and, also electron do-
nation from the C atom to the metal. The three carbene–
Table 5. PGSE diffusion measurements for organic and inorganic salts in
ꢀ10
2
ꢀ1
[a]
CD
Compound
in CH Cl
N(nBu)
2
Cl
2
: D [”10 m s ] and r
H
[Š].
ACHTREUNG
corr
H
conc. [mM] fragment
D
D
c
/D
a
r
H
r
carbon donors are all positive (ca. 0.2). Somewhat surpris-
ingly, the rhodium atom is slightly negative (ꢀ0.16), perhaps
in keeping with the known good s-donor capability of such
heterocyclic carbene ligands. Assuming that the C atom of
the CO ligand cannot readily be approached due to the O
atom (which is strongly negative, ꢀ0.45 units), then the
anion will seek a path that will require it to pass the cyclo-
hexyl groups. Consequently, steric hindrance will play a role.
2
2
[
b]
[
4
][PF
6
]
2
1
1
2
2
2
2
2
2
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
cation
anion
11.66 0.88
13.22
4.5 5.3
4.0 5.0
5.3 6.0
3.6 4.7
4.9 5.7
3.9 4.9
3.7 4.8
3.6 4.7
4.7 5.5
4.4 5.2
5.4 6.0
3.6 4.7
4.5 5.3
3.7 4.7
4.5 5.3
3.8 4.8
4.6 5.3
3.6 4.7
[
c]
1
6
10.0
14.8
10.7
13.5
14.12 0.97
14.57
0.68
17
0.79
[
d]
1
1
8a
8b
[50]
The reported HOESY spectrum for 13a shows only signif-
icant contacts to the ring =CH protons in keeping with this
idea.
11.17 0.92
12.14
19
PCH
[P(nBu)Ph ][PF ]
9.80 0.66
14.90
11.72 0.81
14.48
11.76 0.84
13.94
11.73 0.79
14.82
Returning to the Cp*–arene complexes 1, all of which
show D /D ratios greater than 0.90, it seems clear that, due
[
e]
[
3
Ph
3
][PF
6
]
c
a
ꢀ
to the modest size of the cation, the PF6 ion can readily ap-
proach, so that there are no major steric problems which
might prevent tight ion pairing and this is also in accordance
with the solid-state results.
[f]
3
6
[
g]
[PPh
4
][PF
6
]
It is worth remembering that the values of the charges ob-
tained from the NPA calculations in, for example, 1a*–c*,
in CDCl
3
1
8a
2
2
cation
anion
cation
anion
9.63 0.99
9.71
8.46 0.99
8.50
4.2 5.2
4.2 5.2
4.8 5.7
4.8 5.7
1
f*, 10* and 13a*, show a strong dependence on the elec-
1
d
tronegativity differences between adjacent atoms and, also,
[52a]
on the size of the atoms.
[
2
a] For the calculation of r
99 K was used. h(CD Cl )=0.41”10 kgs
the Stokes–Einstein equation, while r
H
, the viscosity of the nondeuterated solvent at
Our increased understanding of the ion pairing in transi-
ꢀ
3
ꢀ1
ꢀ1
2
2
H
m . r was calculated using
tion-metal salts prompted us to prepare and measure D
corr
H
was calculated using a semiem-
ꢀ
values for several new PF6 salts of both inorganic and or-
cryst
[68]
cryst
pirical estimation of c. [ b]r =4.94 Š.
[c] For [BPh ] salt, r
4
=
[
69]
cryst
[70]
cryst
ganic cations and these results are shown in Table 5. The D
values for all three P-based cations are similar, suggesting
almost the same amount of ion pairing. Perhaps the amount
of ion pairing is slightly less for the larger tetraphenyl phos-
phonium salt. Figure 5 shows the HOESY spectrum for the
6.2 Š.
[d] For [BF ] salt, r =4.2 Š.
[e] For [BF ] salt, r
=
4
4
[
71]
cryst
[72]
cryst
[73]
4.8 Š. [f] For Br salt, r =4.8 Š. [g] r =5.0 Š.
[
PMePh3][PF ] analog and it is clear that the anion ap-
A
C
H
T
R
E
U
N
G
6
proaches the cation from the more accessible methyl group.
ꢀ
Using this pathway the PF6 will come close to the ortho-
phenyl protons, but remain remote from the meta- and para-
protons.
For the six nitrogen salts, [N
A
H
R
U
G
A
H
R
U
G
D /D ratios vary from 0.66 to 0.97 and the two extremes,
c
a
18a and 19, are the most interesting. For 18a, the positive
charge is localized and the anion has no problem approach-
ing from the side remote from the aromatic moiety. The
1
9
1
Figure 5. The F, H HOESY spectrum for [PMePh
contact is to the methyl group and a weaker contact is observed to the
ortho ring protons (CD Cl 400 MHz, 10 mm).
3 6
][PF ]. The strongest
2
2
5624
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5617 – 5629

Ion Pairing
FULL PAPER
HOESY spectrum, given in Figure 6, reveals contacts to the
=
CꢀMe group, the CH group, and a weak interaction with
2
the CH of the ethyl group. There are no contacts to the ar-
3
omatic protons. This modest-sized benzothiazonium cation
1
9
1
Figure 7. The F, H HOESY spectrum for 19. One finds the strongest
contacts to the vinyl protons, H . There are no contacts to either the aro-
a
matic protons or the central =CH vinyl proton, H
10 mm).
b 2 2
(CD Cl 400 MHz,
1
9
1
Figure 6. The F, H HOESY spectrum for 18a revealing the strong con-
tacts to the =CꢀCH
and the NCH groups (CD Cl 400 MHz, 10 mm).
3
2
2
2
tucks the anion in as indicated by the arrow. An NPA analy-
sis places a relatively large amount of positive charge, 0.6
Carbocation 16 is interesting in that the ratio of D values
is relatively small. The charges found from the NPA calcula-
tions suggest that C3, the carbonium ion carbon, is only
modestly positive (0.12), whereas C9 and C42 (directly at-
tached to the electronegative O atoms) carry most of the
charge (0.40). Perhaps the observed reduced amount of ion
pairing is due to the charge separation (in analogy to 19)
and the reluctance of the anion to approach the O atoms (in
analogy to the rhodium carbonyl salt, 13a). Unfortunately,
we have not been able to obtain HOESY data for 16, de-
spite several attempts.
[52a]
units, on the S atom, a second-row element,
supporting
the idea of localized charges. In an attempt to move the
[52b]
anion towards the S atom,
18b (with a relatively large
substituent on the nitrogen atom)
was prepared. The diffusion data
suggest slightly less ion pairing (Dc/
D =0.92); however, the HOESY
a
contacts (see structure below) indi-
cate that the anion remains close to
the N atom and has no strong inter-
est in the S atom.
Before continuing, two control experiments in CDCl (for
3
1d and 18a) are worth mentioning and the results for these
The cyanine salt, 19, has the posi-
are given at the bottom of Table 5. In cases in which small
tive charge distributed over two
remote N=C fragments and this is
also supported by the NPA results.
This delocalization weakens the ion
pairing and the HOESY spectrum
see Figure 7) shows that the anion sits almost exactly be-
cations are involved, one can ask whether the D
A
H
R
U
G
D
ACHTREUNG
(anion) values are coincidentally similar, that is, there is
no ion pairing but the two solvated species happen to have
about the same size. It is known that one finds almost com-
[14,15]
plete ion pairing in CDCl3.
Consequently, in this sol-
(
vent, one expects that 1) the ratio D /D should be close to
c
a
tween the two N atoms such that the two equivalent vinyl
protons at about 6.2 ppm show the strongest contacts. There
or equal to one and 2) the r values will be larger than in di-
H
chloromethane. The CDCl data reveal D /D ratios of 0.99
3
c
a
are also weaker contacts to the two NCH methylene pro-
for both salts and in both cases the rH values increase so
that our assumptions with respect to ion pairing in dichloro-
methane are supported.
2
tons. The single =CH vinyl proton, H , at about 8.5 ppm,
b
does not show any contact to the anion.
Chem. Eur. J. 2008, 14, 5617 – 5629
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5625

P. S. Pregosin et al.
Conclusion
sion the filtrate was slowly concentrated under vacuum. The resulting
crude was washed with Et
powder. Microanalytical data (C, H, N) for [PRPh
2
O and dried under vacuum affording a white
[PF ] [ R=Me (99%),
ꢀ
3
]
A
H
R
U
G
6
For all of the various PF6 salts in dichloromethane, the
nBu (92%), Ph (65%)], 18a (71%) and 19 (72%) are excellent (yield in
brackets).
PGSE diffusion results help us to understand ion pairing in
a more general sense and suggest, logically enough, that the
amount of ion pairing will depend on the nature of the salts.
NMR measurements: The PGSE measurements were carried out without
spinning and in the absence of external airflow. The sample was dissolved
in 0.55 mL of the deuterated solvent, with the concentration maintained
at 2 mm unless otherwise stated. The sample temperature was calibrated,
before the PGSE measurements, by introducing a thermocouple inside
the bore of the magnet.
II
Within any one class of salt, for example, the Ru –arene
complexes, there may be extensive or more modest ion pair-
ing. Combining the NMR data with calculations and crystal-
lography, one can now qualitatively rationalize the amount
of ion pairing. Further, the HOESY data contribute to our
understanding of the solution structure of the salt. Specifi-
cally, these measurements help to support the idea that
steric effects, due to the presence of large substituents, will
hinder the ability of the anion to approach the cation. The
predictive value associated with this combined approach is
limited in that it is not quantitative; nevertheless, this mix-
ture of NMR, DFT, and X-ray studies represents the first at-
tempt to understand and partially explain the various de-
grees of ion pairing in such a different collection of salts.
All the PGSE diffusion measurements were performed using the stan-
dard stimulated echo pulse sequence on a 400 MHz Bruker Avance spec-
trometer equipped with a microprocessor-controlled gradient unit and an
inverse multinuclear probe with an actively shielded Z-gradient coil. The
shape of the gradient pulse was rectangular, its duration d was 1.75 ms
and its strength varied automatically in the course of the experiments.
The calibration of the gradients was carried out by a diffusion measure-
ꢀ3
ment of HDO in D
2
O, which afforded a slope of 1.976”10 . The data
obtained were used to calculate the D values of the samples, according to
[
14,15,53]
the literature.
1
In the H-PGSE experiments, D was set to 167.75 ms. The number of
scans varied between 8 and 16 per increment with a recovery delay of 15
1
9
to 25 s. Typical experimental times were 1–2 h. For F, D was set to
1
2
17.75 or 167.75 ms; 8–16 scans were taken with a recovery delay of 12 to
0 s and with a total experimental time of about 1 ꢀ2 h.
All the spectra were acquired using 32 K points and a spectral width of
Experimental Section
1
19
2
796.4–4006.4 Hz ( H) and 1882.5 Hz ( F) and processed with a line
1
19
broadening of 1 Hz ( H) and 2 Hz ( F). The slopes of the lines, m, were
obtained by plotting their decrease in signal intensity versus G using a
All reactions and manipulations were performed under a N
2
atmosphere
2
using standard Schlenck techniques. Yields refer to purified compounds.
Solvents were dried and distilled under standard procedures and stored
under nitrogen. NMR spectra were recorded with Bruker Avance-400
and DPX-500 MHz at ambient temperature. Chemical shifts are given in
ppm and coupling constants (J) in Hz. Elemental analyses and mass spec-
troscopic studies were performed at ETHZ.
standard linear regression algorithm. Normally, 12–20 points were used
for regression analysis and all of the data leading to the reported D
values afforded lines whose correlation coefficients were >0.999. The
gradient strength was incremented in 3–5% steps from 3–5% to 42–
6
5%.
A measurement of H and F T
experiment, and the recovery delay set to five times T
1
19
1
was carried out before each diffusion
. We estimate the
We thank Prof J. Lacour (University of Geneva) for salts 16 and 17 and
1
2
3
Mr. S. Gruber (ETHZ 2007) for the gift of [Ru
CH CHCH{CH=CH })][PF ].
General procedure for the synthesis of [Ru
In a typical procedure, acetone (2–3 mL) was added to an oven-dried
Schlenk containing [RuCp*(CH CN) [PF ] (45–80 mg). After addition of
the arene (3 equiv) the brown reaction solution was stirred for 4 h at
08C. The solution was then slowly concentrated under vacuum and the
resulting crude product precipitated with acetone/pentane, affording a
brownish powder, which was washed with Et O. Variations on this ap-
A
H
R
U
G
A
H
R
U
experimental error in D values at ꢁ2%. The hydrodynamic radii, r
H
,
2
A
C
H
T
R
E
U
N
G
2
A
C
H
T
R
E
U
N
G
6
were estimated using the Stokes–Einstein equation (c=6) or by introduc-
ing the semiempirical estimation of the c factor that can be derived from
6
A
H
R
U
G
A
H
R
U
G
6
(Cp*)(h -arene)] [PF ] (1–4):
[54]
the micro friction theory proposed by Wirtz and co-workers, in which c
A
H
R
U
G
3
3
]
A
H
R
U
G
6
is expressed as a function of the solute to solvent ratio of radii.
19
1
The F, H HOESY measurements were acquired using the standard
four-pulse sequence on 400 MHz Bruker Avance spectrometer
equipped with a doubly tuned ( H, F) TXI probe. A mixing time of
00 ms was used. The number of scans was 8–16 and the number of incre-
5
a
1
19
2
8
proach, as well as yields (mg/%) and elemental analysis data (%) are
shown in Table 6.
ments in the F1 dimension 512. The delay between the increments was
set to 6 s. The concentration of the sample was 10 mm.
General procedure for the synthesis of the organic and inorganic salts: In
a typical procedure the halide salt (0.1–0.3 mmol) was dissolved in
Computational details: The calculations were performed using the Gaus-
[
55]
sian 03 software package, and the PBE1PBE functional, without sym-
metry constraints. That functional uses a hybrid generalized gradient ap-
CH
CH
2
Cl
2
(1.5–3 mL) and added to a solution of Ag
A
H
R
U
G
6
[PF ] (1 equiv) in
3
CN (1–2 mL). An immediate precipitate was formed and the reaction
[
56]
proximation (GGA), including 25% mixture of Hartree–Fock
ex-
solution stirred for 10 h at RT in the dark. After filtration of the suspen-
[
44]
change with DFT exchange-correlation, given by Perdew, Burke, and
[
57]
Ernzerhof functional (PBE).
The
Table 6. Reaction conditions and elemental analyses for compounds 1–4.
optimized geometries were obtained
[
58]
with the LanL2DZ basis set
aug-
Salt
t [h]
T [8C]
Yield [mg/%]
calcd/%
H
found/%
H
mented with an f-polarization func-
C
N
C
N
[
59]
tion, for Ru, and a standard 6-31G-
1
1
1
1
1
1
2
3
4
a
b
c
d
e
f
15
3
4
3
3
22
21
21
3
50
20
20
50
40
60
70
70
50
40/87
33/68
52/69
38/80
22/36
38/63
62/82
49/71
23/53
41.83
41.72
40.26
42.93
41.78
38.10
34.98
4.61
4.74
4.22
5.50
4.48
4.00
3.49
–
–
–
–
–
2.78
5.10
41.29
41.54
39.98
42.69
41.78
38.16
34.77
4.56
4.65
3.97
5.34
4.49
4.14
3.51
–
–
–
–
–
3.01
5.02
[60]
A
H
R
U
G
[43]
natural population analysis (NPA)
was used to obtain the charge distri-
bution.
Crystallography: Air stable, colorless
crystals of 1c, suitable for X-ray dif-
fraction, were obtained by crystalliza-
tion from dichloromethane. A crystal
was mounted on a Bruker APEX dif-
43.38
4.45
2.81
43.55
4.36
2.81
5626
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5617 – 5629

Ion Pairing
FULL PAPER
fractometer, equipped with a CCD detector, and cooled, using a cold ni-
trogen stream, to 120(2) K for the data collection. The space group was
determined from the systematic absences, while the cell constants were
refined, at the end of the data collection with the data reduction software
SAINT. The experimental conditions for the data collections, crystallo-
graphic and other relevant data are listed in Table 7 and in the Support-
ing Information.
Acknowledgement
P.S.P. thanks the Swiss National Science Foundation, and the ETH Zurich
for support, as well as the Johnson Matthey Company for the loan of pal-
ladium salts. The support and sponsorship by COST Action D24 “Sus-
tainable Chemical Processes: Stereoselective Transition Metal-Catalyzed
Reactions” is kindly acknowledged.
[
61]
Table 7. Experimental data for the X-ray diffraction study of compound
[
[
1] J. W. Faller, P. P. Fontaine, Organometallics 2005, 24, 4132.
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1
c.
formula
16 20 7
C H F PRu
M
T [K]
w
477.36
120 (2)
[3] S. P. Smidt, N. Zimmermann, M. Studer, A. Pfaltz, Chem. Eur. J.
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diffractometer
crystal system
space group (no.)
a [Š]
Bruker APEX CCD
monoclinic
P2 /n (no.14)
1
8.533(2)
14.581(3)
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b [Š]
c [Š]
14.234(3)
93.952(3)
1766.6(7)
4
1.795
10.43
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b [8]
V [Š ]
3
Z
ꢀ
3
1
calcd [gcm ]
ꢀ
1
m [cm
radiation
]
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MoKa (graphite monochrom.
l=0.71073 Š)
2.00–29.43
21026
4485
3633
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q range [8]
data collected
independent data
observed reflections (n
o
)
2
2
[
jF
o
j >2.0s(jFj )]
parameters (n
v
)
228
[
a]
R
int
0.0408
0.0756
0.2485
1.046
[
b]
R (obsd reflns)
2
[c]
wR (obsd reflns)
[
d]
GOF
2
o
2
o
2
o
2
2
[
{
a] Rint=ꢀjF ꢀ<F >j/ꢀF .
[
Rb ]= ꢀ(jF
o
ꢀ(1/k)F
c
j)/ꢀjF
o
2
j.
[
ꢀn
wc R] =
2
o
2
c
2
2
o
1/2
2
o
2
c
1/2
ꢀ[w(F ꢀ(1/k)F ) ]/ꢀwjF j ]} . [d]GOF=[ꢀ
w
(F ꢀ(1/k)F ) /(n
o
v
)] .
The collected intensities were corrected for Lorentz and polarization fac-
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[
61]
[62]
tors
and empirically for absorption using the SADABS program.
The structure was solved by direct and Fourier methods and refined by
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2
full-matrix least-squares methods, minimizing the function [ꢀw(F ꢀ(1/
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2
c
2
k)F ) ] and by using anisotropic displacement parameters for all atoms,
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The difference Fourier maps showed clearly the fluorine atoms in the
ꢀ
equatorial plane of one of the PF
6
octahedron, disordered over two po-
sitions, that were refined using isotropic displacement parameters. The
refinement yielded equal occupancy for both sites (0.52(2) and 0.48(2) re-
spectively). Upon convergence the final Fourier difference map showed
no chemically significant peaks.
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2
004, pp. 1–36.
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a riding model (B(H)=1.3”
2
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[
[
[
[
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H
(anion) values in either acetone
3
H
for 100% association; and 3) no aggregation, that is, the data were
to come from D values obtained at low concentrations, then it
would be possible to make a semi-quantitative estimate of the
[
amount of ion pairing from the r
are not always justified, the ratios D
H
values. Since these assumptions
/D provide an indication of
2
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would also be true for the N and proximate C atoms in 19*, that is,
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[
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Published online: May 26, 2008
Chem. Eur. J. 2008, 14, 5617 – 5629
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5629