Synthesis and structure of a new tetrakis(pentafluorophenyl)borate salt of the silver(I) cation with novel trigonal planar tris(benzene) coordination

Article abstract of DOI:10.1021/om050388c

A new, analytically pure silver(I) salt, Ag(C₆H ₆)₃⁺B(C₆F₅) ₄^- (1), was prepared, and its structure determined by single-crystal X-ray crystallography. The geometry around the silver atom in salt 1 is trigonal planar, which represents a novel coordination mode for a silver(I) coordinated with three independent arene molecules. The three benzene molecules are bound to the silver atom with estimated hapticities of η^1.36, η^1.31, and η^1.44. No contact was found between the silver cation and the B(C₆F₅) ₄^- anion.

Full text of DOI:10.1021/om050388c

4842
Organometallics 2005, 24, 4842-4844
Synthesis and Structure of a New
Tetrakis(pentafluorophenyl)borate Salt of the Silver(I)
Cation with Novel Trigonal Planar Tris(benzene)
Coordination
Kohei Ogawa, Toshikazu Kitagawa,* Shintaro Ishida, and Koichi Komatsu*
Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan
Received May 14, 2005
Summary: A new, analytically pure silver(I) salt,
Ag(C6H6)3+B(C6F5)4- (1), was prepared, and its structure
determined by single-crystal X-ray crystallography. The
geometry around the silver atom in salt 1 is trigonal
planar, which represents a novel coordination mode for
a silver(I) coordinated with three independent arene
molecules. The three benzene molecules are bound to the
silver atom with estimated hapticities of η1.36, η1.31, and
TPFPB- (1), and its X-ray structure, clearly demon-
strating the presence of a silver ion coordinated with
three benzene molecules in a trigonal planar fashion.
Results and Discussion
Treatment of Li+TPFPB- in diethyl ether with aque-
ous AgNO3 at room temperature in the dark resulted
in the formation of Ag+TPFPB-, which was isolated
from the organic layer as a highly light-sensitive color-
less solid. This solid was recrystallized from benzene
to afford analytically pure Ag(C6H6)3+TPFPB- (1) in
83% yield as colorless crystals. The crystalline solid is
nonhygroscopic and thermally stable at temperatures
up to 138 °C and can be handled under air without any
special precautions. It is also stable in the presence of
ordinary light. The salt is moderately soluble in organic
solvents such as diethyl ether, tetrahydrofuran, toluene,
dichloromethane, and chloroform, while it is insoluble
in hexane. No benzene molecule is released from the
silver ion under reduced pressure (1 × 10-4 Torr, 24 h
η
1.44. No contact was found between the silver cation and
-
the B(C6F5)4 anion.
Introduction
Silver(I) and thallium(I) salts of a weakly coordinating
anion1 such as the tetrakis(pentafluorophenyl)borate
anion (TPFPB-),2 as well as the carborane anion
(CB11H12-) and its derivatives,3 are useful for generating
reactive cationic species via halide-ion abstraction.
While Tl+TPFPB- has recently been reported as a stable
salt and has been shown to be useful for metathesis
reactions,4 Ag+TPFPB- is more difficult to handle
without aromatic molecules coordinated to the silver ion.
Thus, except for patents,5 reports of Ag+TPFPB- are
quite limited. To the best of our knowledge, this Ag+
salt has been reported as a catalyst only in glycosyla-
tion6 or Friedel-Crafts reactions,7 and no X-ray struc-
tures of Ag+TPFPB- with aromatic ligands have been
at room temperature), unlike Ag+CB11H12-‚2C6H69 and
- 10
Ag(C6D6)3+BF4
.
The molecular structure of 1, determined by X-ray
crystallography, is shown in Figure 1. The silver atom
is bonded to three benzene molecules through one
carbon atom of each benzene ring. The geometry around
the silver atom is trigonal planar, as demonstrated by
the C-Ag-C angles [C1-Ag-C7 ) 119.7(2)°, C1-Ag-
C13 ) 122.4(2)°, and C7-Ag-C13 ) 117.9(2)°], which
are close to 120° and the sum of which is 360.0°. No
significant distortion in the complexed benzene rings
was observed [∆R(C-C) < 0.045 Å, the sum of the
internal angles ) 720.0°]. The slight deviation from an
equilateral hexagon can be attributed to packing effects,
which are shown by a weak C-H‚‚‚F contact (2.394 Å)
between one of the coordinating benzene molecules and
the anion. In contrast, the closest distance between the
silver and fluorine atoms in TPFPB- is 3.70 Å, indicat-
ing the absence of any bonding interactions; for refer-
ence, the sum of the van der Waals radii of Ag and F is
3.19 Å.11 The nearest Ag-C distances, 2.391(6) (Ag-
reported.8 Here we report on the preparation of the
TPFPB- salt of a new silver(I) complex, Ag(C6H6)3
-
+
* To whom correspondence should be addressed. E-mail: kitagawa@
scl.kyoto-u.ac.jp (T.K.); komatsu@scl.kyoto-u.ac.jp (K.K.).
(1) (a) Strauss, S. H. Chem. Rev. 1993, 93, 927. (b) Krossing, I.;
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WO 9627439, 1996; Chem. Abstr. 1996, 125, 276896. (b) Tsai, J. C.;
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141, 261198.
(8) The X-ray structure of the TPFPB- salt of an Ag+-macrocyclic
thioether complex has been reported: Blake, A. J.; Collison, D.; Gould,
R. O.; Reid, G.; Schro¨der, M. J. Chem. Soc., Dalton Trans. 1993, 521.
(9) Shelly, K.; Finster, D. C.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A.
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C. W.; Kilner, M. J. Organomet. Chem. 1998, 550, 59.
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10.1021/om050388c CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/27/2005

Notes
Organometallics, Vol. 24, No. 20, 2005 4843
to the angles (∼120°) observed in Ag-cyclic polyarene
complexes,14 in which the silver atom adopts a trigonal
pyramidal geometry. It is not usual for Ag+ to form a
trigonal planar complex.15 Polymeric Ag(CHPh3)+Me-
CB11F11- is the only known example of such a complex
that is coordinated with three aromatic rings,15 and to
our knowledge, 1 is the first reported trigonal planar
Ag complex with three independent arene ligands.16
This is in contrast to the reported structure of
Ag(C6D6)3+BF4-, in which Ag+ is coordinated in a
tetrahedral fashion with three C6D6 molecules and one
-
of the fluorine atoms of the BF4 anion.10
Theoretical calculations17 indicated that the differ-
ences in binding energy between the η1- and η2-
coordinations are quite small (<0.5 kcal mol-1) in silver-
benzene complexes and that the silver ion can migrate
on the entire π plane of benzene with virtually no
activation barrier. Experimentally, η1- and η2-coordina-
tions frequently coexist in the same crystal13 and are
sometimes observed even for the same silver atom.10,18
In the present case, however, the silver atom is located
above one of the carbon atoms of each benzene ring in
a nearly η1-fashion, where the electron density is the
highest.19 An η2-type coordination was not observed,
indicating the absence of π-back-donation from a filled
d orbital of the silver atom to the π* orbital of the arene.
These results can be interpreted to indicate that the
silver atom behaves simply as an electron acceptor, since
the counteranion (TPFPB-) has negligible electron-
donating ability.
Figure 1. (a) ORTEP drawing of Ag(C6H6)3+TPFPB- (1).
Hydrogen atoms are omitted for clarity. (b) Two additional
views of the cation moiety. Selected bond lengths (Å): Ag-
C1 ) 2.391(6), Ag-C2 ) 2.660(6), Ag-C7 ) 2.398(7), Ag-
C8 ) 2.737(8), Ag-C13 ) 2.400(6), Ag-C14 ) 2.646(6).
Selected angles (deg): C1-Ag-C7 ) 119.7(2), C1-Ag-C13
)
122.4(2), C7-Ag-C13 ) 117.9(2), C2-C1-Ag )
84.8(4), C6-C1-Ag ) 93.1(4), C12-C7-Ag ) 91.7(4), C8-
C7-Ag ) 88.2(4), C14-C13-Ag ) 83.9(4), C18-C13-Ag
) 92.2(4).
According to the theoretical considerations, it appears
to be possible that the silver atom shows a fluxional
behavior around the benzene ring in solution. In 1H and
13C NMR spectra of 1 in CD2Cl2, the benzene exhibited
a single peak at δ 7.43 and 128.3,20 respectively, at
ambient temperature, indicating the very rapid migra-
tion of the silver atom through an η1-η2-type exchange
or reversible dissociation. This interconversion is so fast
that no signal splitting or line broadening was observed
in spectra taken at -90 °C.
In conclusion, the facile synthesis and X-ray crystal
structure of a silver(I) salt of TPFPB-, 1, is described.
The silver ion has a trigonal planar geometry coordi-
nated with three benzene molecules in a nearly η1-
fashion, which has not been reported to date. This salt
is quite stable under air, and its distinct formula weight
permits its use in accurate molar quantities in reactions.
This suggests the utility of 1 as an excellent reagent
for metatheses and for one-electron oxidations to form
cationic species with a TPFPB- counteranion.21,22
Figure 2. General representation of the “grab angle” R,
which is defined as the angle between the normals to the
planes of the benzene rings.
C1), 2.398(7) (Ag-C7), and 2.400(6) Å (Ag-C13), are
compatible with that observed [2.400(7) Å] in the X-ray
crystal structure of Ag+CB11H12-‚2C6H6, which was
reported as the first example of the η1-coordination of
carbon to silver.9 The next closest Ag-C distances are
2.660(6) (Ag-C2), 2.737(8) (Ag-C8), and 2.646(6) Å
(Ag-C14). The C2-C1-Ag, C8-C7-Ag, and C14-
C13-Ag angles, 84.8(4)°, 88.2(4)°, and 83.9(4)°, respec-
tively, are somewhat less than 90°, which would be
expected for an ideal η1-coordination. If the hapticity of
coordination is estimated as ηx; x ) 1 + 2(d1 - D2)1/2
/
2
[(d12 - D2)1/2 + (d22 - D2)1/2],12 where d1 and d2 are the
closest and second closest Ag-C distances, respectively,
and D is the distance between Ag and the mean plane
of the benzene ring, the three coordinations are repre-
sented as η1.36, η1.31, and η1.44, respectively.
(14) (a) Kang, H. C.; Hanson, A. W.; Eaton, B.; Boekelheide, V. J.
Am. Chem. Soc. 1985, 107, 1979. (b) Heirtzler, F. R.; Hopf, H.; Jones,
P. G.; Bubenitschek, P. Tetrahedron Lett. 1995, 36, 1239. (c) Cohen-
Addad, P. C.; Baret, P.; Chautemps, P.; Pierre, J.-L. Acta Crystallogr.
1983, C39, 1346.
On the basis of structural data for 40 examples of Ag-
(arene)2+ or Ag(arene)3+ complexes that have no chemi-
cal linkage between the coordinating arene molecules,
Kochi et al. reported that the “grab angle” R (Figure 2)
falls into three classes, 95°, 130°, and 155° ((3°),
corresponding to the octahedral, tetrahedral, and linear
hybridization of silver, respectively.13 The R values
observed for 1, i.e., 119.2°, 121.4°, and 119.5°, belong
to none of these classifications and are rather similar
(15) Ivanov, S. V.; Miller, S. M.; Anderson, O. P.; Strauss, S. H.
Cryst. Growth Des. 2004, 2, 249, and references therein.
(16) Amma et al. have reported extensive structural studies on a
variety of silver-arene complexes. Griffith, E. A. H.; Amma, E. L. J.
Am. Chem. Soc. 1974, 96, 743, and references therein.
(17) Dargel, T. K.; Hertwig, R. H.; Koch, W. Mol. Phys. 1999, 96,
583.
(18) Gano, J. E.; Subramanian, G.; Birnbaum, R. J. Org. Chem.
1990, 55, 4760.
(12) Vasilyev, A. V.; Lindeman, S. V.; Kochi, J. K. Chem. Commun.
(19) Francl, M. M.; Hout, R. F., Jr.; Hehre, W. J. J. Am. Chem. Soc.
1984, 106, 563.
2001, 909.
(13) Lindeman, S. V.; Rathore, R.; Kochi, J. K. Inorg. Chem. 2000,
39, 5707.
(20) 1H and 13C chemical shifts for benzene, under the same
conditions, are δ 7.36 and 128.9, respectively.

4844 Organometallics, Vol. 24, No. 20, 2005
Notes
Anal. Calcd for C42H18AgBF20: C, 49.40; H, 1.78. Found: C,
49.38; H, 1.73.
Experimental Section
General Procedures. Commercially available reagents
X-ray Crystallography. Intensity data were collected at
100 K on a Bruker SMART APEX diffractometer with Mo KR
radiation and a graphite monochromator. The structure was
solved by direct methods (SHELXTL) and refined by full-
matrix least-squares on F2 (SHELXL-97). All non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms
were placed using AFIX instructions. Crystal data for 1: MF
) C42H18AgBF20: Mw ) 1021.24, 0.2 × 0.2 × 0.2 mm3,
monoclinic, C2/c, a ) 30.205(5) Å, b ) 11.0645(19) Å, c )
1
were of reagent-grade quality and were used as received. H
and 13C NMR spectra were recorded using CD2Cl2 as the
solvent at ambient temperature on a Varian Mercury-300
spectrometer and at -90 °C on a JEOL JNM-AL400 spec-
trometer.
Preparation of Ag(C6H6)3+TPFPB- (1). All procedures
were carried out in the dark. To a stirred solution of
Li+TPFPB-‚(Et2O)3.723 (0.170 g, 0.177 mmol) (Tokyo Chemical
Industry Co., Ltd.) in diethyl ether (3 mL) was added dropwise
a solution of AgNO3 (71.8 mg, 0.42 mmol) in H2O (5 mL)
dropwise. After stirring the reaction mixture at room temper-
ature for 15 min, the organic layer was separated and dried
(MgSO4), and the solvent was evaporated under vacuum. The
resulting colorless solid was recrystallized from hot benzene
(1.5 mL). The solution was let stand at room temperature for
24 h to give Ag(C6H6)3+TPFPB- (0.150 g, 0.147 mmol, 83%)
as colorless crystals: mp 138 °C (dec); 1H NMR (300 MHz, CD2-
23.910(4) Å, â ) 113.694(3)°, Z ) 8, V ) 7317(2) Å3, Dcalcd
)
1.854 g/cm3, 2θmax ) 50°, reflections: 18 648 collected, 6444
independent (Rint ) 0.0485), 6444 observed, 631 parameters,
R1 ) 0.0714, wR2 ) 0.1325, GOF ) 1.031, largest difference
peak and hole 2.695 and -3.926 e Å-3, respectively (around
Ag atom).
Acknowledgment. This work was supported by a
Grant-in-Aid for COE Research for Elements Science
(No. 12CE2005) from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan, and by a
JSPS fellowship (S.I.).
Cl2) δ 7.43 (s); 13C NMR (75.5 MHz, CD2Cl2) δ 148.8 (d, 1JCF
243 Hz, Cortho), 138.8 (d, 1JCF ) 246 Hz, Cpara), 136.9 (d, 1JCF
)
)
241 Hz, Cmeta), 128.3 (CH), 124.0 (Cipso); IR (KBr, cm-1) 1642,
1515, 1462, 1374, 1279, 1090, 979, 774, 770, 705, 684, 662.
Supporting Information Available: 1H and 13C NMR
spectra and complete details of the X-ray crystallographic
studies of 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
(21) Studies of the use of 1 in generating some unstable cations and
radical cations are currently underway in our laboratory.
(22) Ishida, S.; Nishinaga, T.; Komatsu, K. Chem. Lett. 2005, 34,
486.
(23) The number of molecules of diethyl ether was determined by
measuring the 1H NMR spectrum of the sample in the presence of a
known amount of an internal standard, hexamethylbenzene.
OM050388C