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Poly[(tri-'butyl-phosphine)gold(I)]ammonhim Tetrafluoroborates and
Bis(tri-'butyI-phosphine)gold(I) Tetrafluoroborate
Alexander Sladek, H ubert Schmidbaur*
Anorganisch-Chemisches Institut der Technischen Universität München,
Lichtenbergstraße 4, D-85747 Garching
Z. Naturforsch. 50b, 859-863 (1995); received November 11, 1994
(Phosphine)gold(I) Complexes, [(Phosphine)gold(I)]ammonium Cations, Auriophilicity,
G o ld -G o ld Contacts
The reaction of tris[(tri-'butyl-phosphine)gold(I)]oxonium tetrafluoroborate with close to
stoichiometric quantities of ammonia in dichloromethane at -7 8 °C affords tetrakis[(tri'butyl-phosphine)gold(I)]ammonium tetrafluoroborate, the structure of which has been
determined by single crystal X-ray diffraction. The cation features a strongly distorted NAu4
tetrahedron with four short and two longer A u -A u edges. - With a large excess of ammonia,
only partially aurated ammonium salts are formed, as suggested by detailed NMR spectro­
scopic and mass spectrometric studies. These partially aurated intermediates have not been
isolated. Slow decomposition processes in these solutions lead to the precipitation of bis(tri'butyl-phosphine)gold(I) tetrafluoroborate. The structure of the bis(chloroform) solvate has
also been determined. The crystals contain two independent C3-symmetrical cations with
similar structures.
Gold clusters with interstitial atoms of the main
group metalloids have attracted considerable in­
terest in recent years owing to their unusual struc­
ture and bonding situation [1-3]. Surprisingly, the
non-classical structures appear to be caused by
significant A u -A u interactions between the seem­
ingly closed-shell metal centers. Pertinent phe­
nomena are particularly striking in clusters with
hypercoordinate interstitial atoms [4-8], but even
species which feature standard coordination num­
bers for the metalloid atoms often show struc­
tural anomalies.
A n example in case is the stereochemistry of
tetrakis[(phosphine)gold(I)]<7mA770/m/m cations of
the general formula [(R3P)Au]4N+. The first rep­
resentative of this series has been discovered by
Nesmeyanov et al. for R = Ph [9], For the BF4 salt
a crystal structure determ ination [10] has shown a
highly irregular structure with strong deviations of
the param eters from the tetrahedral standard.
Later work by Strähle et al. on the PF^ salt has
also confirmed a distorted structure, but with dif­
ferent details in the param eters [11], Finally, for
the fluoride salt an interesting C3-symmetrical
trigonal pyramidal structure of the cation was
* Reprint requests to Prof. Dr. H. Schmidbaur.
0932-0776/95/0600-0859 $06.00
found, with three gold atoms at the corners of a
small base triangle and the fourth gold atom set
apart in the axial position [12].
In the corresponding phosphonium system
no salt with a cation of the stoichiometry
[(Ph3P)Au]4P+ has been isolated, and only hyper­
coordinate species with more than four gold atoms
at the central phosphorus atom have been ob­
served for R = Ph [13]. It was only with the bulky
phosphine 'Bu3P that a salt [(fBu3P)Au]4P+BF4
became available, and for this a strongly distorted
tetrahedral structure was again detected [14]. This
finding was at variance with the results of theoreti­
cal studies [13-16], which have predicted cations
of this type to be square pyramidal.
Such a non-classical square-pyramidal structure
had indeed been confirmed [17] for the corre­
sponding arsonium salt with the cation
[(Ph3P)Au]4As+. These experimental data were
the starting point - and an im portant backing for the theoretical calculations. No analogous anti­
mony or bismuth compounds are known, but an
isoelectronic dication of the type [(R3P)Au]4S2+
has meanwhile been obtained and fully charac­
terized [3, 18],
At this state of affairs it was felt that the struc­
ture and properties of the ammonium system with
the bulky (rBu3P) ligand should also be investi-
© 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved.
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A. S ladek-H . Schmidbaur • Poly[(tri-'butyl-phosphine)gold(I)]ammonium Tetrafluoroborates
gated in order to further delineate the structural
preferences of tetra(aurated) nitrogen. In this
paper we report the results on metallo complexes
of ammonia with ('Bu3P)Au+ ligands.
Results and Discussion
Tris[(phosphine)gold(I)]oxonium salts are ex­
cellent aurating agents for ammonia and amines
[9-12, 19-21]. The recently prepared fully 'butylsubstituted compound has now been chosen for
the treatm ent of ammonia in varying stoichiomet­
ric ratios. With a large excess of ammonia present
(100:1), solutions in dichloromethane are ob­
tained, which exhibit a pro m in en t31P NMR singlet
signal at (3 = 86.7 ppm, accompanied by a minor
one at d = 87.7 ppm. Mass spectrometric studies
(FAB) of the residue remaining after evaporation
of the solvent give a parent peak at m/z = 814.5,
which can be assigned to the [('Bu3P)Au]2NH 2
cation. The compound could not be purified by
crystallization.
With a reduced excess of ammonia (70:1), the
solutions showed new 31P NM R signals at d = 85.4
and d = 84.1 ppm, the latter appearing as a 1:1:1
triplet with /(P N ) = 12.0 Hz. A signal at <3 = 86.7
ppm is present in low intensity. The mass spectrum
(FAB) of this mixture of products has m/z = 1214
as the parent peak corresponding to the cation
[('Bu3P)Au]3N H +, accompanied still by m/z =
814.5 (above).
With near-stoichiometric quantities of ammonia
and at very low tem perature (-7 8 °C) in dichloro­
m ethane the fully aurated ammonium cation is
generated. The tetrafluoroborate salt can be pre­
cipitated with pentane and recrystallized from dichlorom ethane/diethylether at -3 0 °C. The prod­
uct is obtained as a colorless, polycrystalline
material in 60% yield. The vacuum-dried samples
gave satisfactory elemental analyses. Single crys­
tals of the bissolvate can be grown from chloro­
form, decomp. temp. 115 °C. Solutions of this com­
pound in dichlorom ethane show the 31P{1H} NMR
singlet signal at d = 89.9 ppm, a ‘H doublet signal
at (3 = 1.52 ppm [./(PH) = 13.2 Hz], and two 13C
doublet signals at d = 32.4 and 39.7 ppm with J 2.7 and 22.1 Hz, respectively ('H -decoupled). In
the mass spectrum only the tris- and bis-aurated
cations have been detected (m/z = 1212.4 and
814.9, respectively).
Crystals of the bis-chloroform solvate are orthorhombic, space group Pbca (No. 61), with Z = 8
formula units in the unit cell. The lattice is com­
posed of independent tetrakis[(phosphine)gold]ammonium cations, tetrafluoroborate anions, and
solvent molecules. The cation has no crystallographically imposed symmetry. Its core is an irreg­
ular pyramid of four gold atoms with an interstitial
nitrogen atom and four peripheral phosphorus
atoms. All four N - A u - P axes are close to linear
[176.8(6)-178.7(5)°], and the four phosphine
“umbrellas” have standard dimensions (Fig. 1).
The most intriguing feature of the cation struc­
ture is the distortion of the NAu4 core which devi­
ates very strongly from the structure of a centered
regular tetrahedron. The A u - N - A u angles range
from the significantly subtetrahedral value of
104.2(9)° for A u l - N - A u 2 to a value of as much
as 125(1)° for A u 3 -N -A u 4 . Three other of the
six angles are found at 105.1(8), 105.7(7) and
105.8(9)°, and only one is 109.5(7)°. These angles
are associated with A u -A u distances ranging be­
tween 3.268(2) and 3.658(2) A for the six edges of
the “tetrahedron” . This virtually random struc­
tural irregularity is not caused by crystal packing,
since the approach of neither the counter ions, nor
of the solvent molecules is sufficiently close. It
should also be noted that intram olecular repulsion
Fig. 1. Structure of the cation in the crystal of
[ ('B u 3P ) A u ] 4N +B F j - 2 C H C 13 with atomic numbering
(SCHAKAL). The cation has no crystallographic
symmetry.
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A. S lad ek -H . Schmidbaur • Poly[(tri-'butyl-phosphine)gold(I)]ammonium Tetrafluoroborates
of the bulky phosphines should rather favour a
symmetrical ligand positioning at the surface of
the cation.
It therefore appears that the energy profile of
the distortion of the NAu4 tetrahedron is rather
flat owing to the efficient compensation of the en­
ergy loss associated with the bending of N -A u
bonds away from vertices of an ideal tetrahedron
by the gain in energy originating from peripheral
A u -A u approach. It has been pointed out pre­
viously [14, 17] that for the sum of standard cova­
lent radii for N and Au, the edges of a regular
NA u4 tetrahedron are obtained too long for effi­
cient A u -A u interactions as observed in non­
strained (unsupported) systems (3.01 ± 0.15 A).
In order to gain A u -A u bond energy the polyhe­
dron has to undergo al least some distortion which
allows intram olecular A u -A u approaches.
This idea is supported by a comparison of
the new data with literature values for hom olo­
gous compounds of the type [(R3P)Au]4N+X “,
and analogous organoammonium compounds
[(R3P)A u]3N R +X - [22]: While in the latter small
A u -N -A u angles and concomitantly short
A u -A u distances have been observed for the
NA u3 pyramids in all cases, the NAu4 tetrahedra
of the former are always distorted with both small
and large A u - N - A u angles and short and long
A u -A u distances. This distortion is even more
drastic in the phosphonium case with 'Bu3P
ligands [14], where the longer A u -P distances
allow for more space of the peripheral 'Bu groups
to move together following the A u -A u attraction.
On the basis of the new data we predict an even
more severe distortion of [(R3P)Au]4N + cations
for small groups R. Unfortunately, the structure of
the R = Me case has not yet been determined. The
species has been detected by mass spectrometry
and other spectroscopic and analytical techniques
[22], but no salts could be crystallized.
97.4 ppm (in CDC13). Its crystallization is not nec­
essarily accompanied by formation of elem ental
gold, which suggests that high-nuclearity gold clus­
ters containing interstitial nitrogen atoms are also
formed in the early stages of decomposition. The
31P NMR spectra of the solutions show several
peaks other than at d = 97.4 ppm, which have not
yet been assigned. The salt in question can also be
obtained in a metathesis reaction of the chloride
salt [23] with AgBF4 in ethanol (Exp. Part). The
product is identical with the decomposition prod­
uct of the poly(gold)ammonium salts.
Single crystals of a bis-chloroform solvate are
obtained from chloroform/diethylether-mixed sol­
vents. These crystals are trigonal, space group R3,
with six stoichiometric units in the unit cell. The
lattice is composed of two crystallographically in­
dependent cations (each with a crystallographic
threefold axis), two tetrafluoroborate anions and
two chloroform molecules (all with threefold sym­
metry), none of which shows subnormal intermolecular contacts.
The two independent cations have very similar
dimensions and conformations, which furtherm ore
are in excellent agreem ent with the data for the
same cation in the (solvent-free) chloride salt [23].
This result is suggesting that the structure shown
in Fig. 2 is representing a configuration which is
largely insensitive to the external field of anions
or solvent molecules in the crystal. The conform a­
tion as viewed down the P - A u - P axis is shown
in Fig. 3.
C H a ^
C21b
Au1
/ÄC21
P2
Bis(tri-'butyl-phosphine)gold(I) tetrafluoroborate
If any of the poly[(phosphine)gold]ammonium
salts are left in solution for a prolonged time
at ambient tem perature, colorless crystals of
['Bu3P]2Au+BF4 are formed, which appear to be
the primary product of decomposition. The com­
pound gives satisfactory elemental analyses and is
identified also by its 31P NM R singlet signal at d =
C21a
Fig. 2. Structure of the cation in the crystal of
[(fBu3P)2Au]+BF 4 -2CHC13 with atomic numbering
(ORTEP). The cation has a crystallographic threefold
axis.
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A. S ladek-H . Schmidbaur • Poly[(tri-fbutyl-phosphine)gold(I)]ammonium Tetrafluoroborates
Glassware was oven-dried and filled with nitrogen.
[('Bu3P)Au]30 +BF4 and ('Bu3P)2Au+Cl~ were pre­
pared according to literature m ethods [23, 24], NMR: Jeol GX 400. - MS: MAT 311 (FAB).
Tetrakisf (tri-'butylphosphine)gold (I) ] ammonium
tetrafluoroborate 1
Fig. 3. View down the threefold axis in the cation shown
in Fig. 2 (SCHAKAL).
Experim ental Part
All experiments were carried out under dry, puri­
fied nitrogen. Samples and solutions were pro­
tected against direct incandescent light. Solvents
were dried, distilled and saturated with nitrogen.
Empirical formula
Formula weight [g/mol]
Crystal system
Space group (No.)
«[A]
MA]
c[A]
a[°]
[g Cm 3]
Z
F(000) [e]
/<(M o-K „) [ c m '1]
T [°C]
h k l Range
Measured reflections
Unique reflections
Observed reflections
S e a le d
F>
Refined param eters
Weighting param eters 1/k
Weighting param eters a/b
H Atoms (found/calcd)
R/Rw
wR2/Rbased o n f (O M IT4)
{?fjn (max/min) [eA -3]
A solution of [('Bu3P)Au]30 +BF4 (0.50 g.
0.38 mmol) in C H 2C12 is cooled to -7 0 °C and
NH 3 (g) is very slowly condensed into this solution
with vigorous stirring. The reaction mixture is con­
centrated in a vacuum and pentane (50 ml) is
added. The white precipitate is crystallized from
CH2C12/E t20 at -3 0 °C, yield 0.29 g, 60%. 3IP{'H} NMR (CDC13, 20 °C): d = 89.9 [s, PPh3], 'H NMR (CDC13, 20 °C): (3 = 1.52 [d, 3/ Hp =
13.2 Hz, CH3], - 13C{1H} NM R (CDC13, 20 °C):
(3 = 32.4 [d, 2JCP = 2.8 Hz, C H 3]; Ö = 39.7 [d, 7 CP =
22.1 Hz, CP]. - MS (FAB pos.): m/z (% ) = 1213.4
(15.80) [(M -'B u 3PAu)+ + 1]; 814.9 (77.95)
[(M -2 rBu3PAu)+ + 1].
C48H W8A u4BF4NP4 (1697.97)
Calcd
Found
C 33.95
C 33.42
1
2
C4gHiosAu4BF4NP4 •2 CHC13
1936.64
orthorhom bic
Pbca (61)
19.120(2)
20.067(3)
38.361(6)
90
90
90
1.748
C24H S4AuBF4P2 •2 CHCI3
927.13
trigonal
R3 (146)
11.308(1)
11.308(1)
52.560(1)
90
90
120
1.587
8
7488
82.90
-6 2
+ 23/+20/+46
14832
10004
5422
0.1040
4cr(F)
311
1.00/0.001088
0/110
0.0913/0.0994
+ 2.103/-2.236
H 6.41% ,
H 6.28% .
Table I. Crystallographic
data for the compounds 1
and 2.
6
2784
43.23
-6 2
-1 4 /+ 1 4 /1 6 9
6656
6205
6205
0.0161
4cr(F)
223
0.0660/17.7190
0/56
0.1096 (0.1080)/
0.0348 (0.0352)*
+ 1.980/-1.780
* Refinement of the in­
verse model gave no sig­
nificant changes in the
values of w R 2/R hased on 1
(OM IT 4).
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A. S lad ek -H . Schmidbaur • Poly[(tri-'butyl-phosphine)gold(I)]ammonium Tetrafluoroborates
Bis(tri-'butylphosphine)gold(I) tetrafluoroborate 2
A solution of ('Bu 3 P) 2 A u+Cl~ (3.00 g, 4.71 mmol)
in ethanol ( 2 0 ml) is treated at room tem perature
with one equivalent of AgBF 4 (0.92 g, 4.71 mmol)
in ethanol (5 ml). A fter 30 min of stirring undis­
solved m aterial (AgCl) is filtered off. The solvent
is removed in vacuo and the residue crystallized
from C H 2 Cl2/ether at -3 0 °C, yield 3.24 g,
100%. - 3 1 P{rH} NM R (CDC13, 20 °C): Ö = 97.4
[s, PPh3],
C24H54A uBF4P2 (688.43)
Calcd
Found
D 41.87
C 41.22
H 7.91
H 7.75
Au 28.61%,
Au 29.20%.
Crystal structure determination
Colorless crystals of compound 1 were obtained
from CDCI 3 at room tem perature, and colorless
crystals of com pound 2 by carefully adding E t20
to a CDC13 solution at -3 0 °C. The samples were
m ounted in glass capillaries. Graphite-monochrom ated M o -K a radiation was used. The structures
were solved by D irect M ethods (SHELXTLPLUS [25]). Only gold and phosphorus atoms of
1
were refined with anisotropic displacement
param eters (SHELXTL-PLUS [25]), but all heavy
[1] H. Schmidbaur, Gold Bull. 23, 11-20 (1990).
[2] K. Angermaier, H. Schmidbaur, Inorg. Chem. 33,
2069-2070 (1994).
[3] K. Angermaier, H. Schmidbaur, Chem. Ber. 127,
2387-2391 (1994).
[4] F. Scherbaum, A. Grohm ann, B. Huber, C. Krüger,
H. Schmidbaur, Angew. Chem. 100, 1602-1604
(1988); Angew. Chem., Ind. Ed. Engl. 27, 15441546 (1988).
[5] F. Scherbaum, A. Grohm ann, G. Müller, H. Schmid­
baur, Angew. Chem. 101, 464-466 (1989); Angew.
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140-142 (1990).
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Angew. Chem. 103, 442-444 (1991); Angew. Chem.,
Int. Ed. Engl. 30, 433-435 (1991).
[8] E. Zeller, H. Schmidbaur, J. Chem. Soc. Chem.
Commun. 1992, 69-70.
[9] E. G. Perevalova, E. I. Smyslova, V. P. Dyadchenko,
K. I. Grandberg, A. N. Nesmeyanov, Izv. Akad.
Nauk SSSR, Ser. Khim. 1980, 1455.
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Chem. 277, 143-146 (1984).
[11] A. Brodbeck, J. Strähle, Acta Crystallogr. A 46,
C232 (1990).
[12] E. Zeller, H. Beruda, A. Kolb. P. Bissinger, J. Riede,
H. Schmidbaur, N ature 352, 141-143 (1991).
[13] H. Beruda, E. Zeller, H. Schmidbaur, Chem. Ber.
126, 2037-2040 (1993).
[14] E. Zeller, H. Beruda, H. Schmidbaur, Chem. Ber.
126, 2033-2036 (1993).
863
atoms of 2 (SHELXTL-93 [26]). The solution of
structure of 1 was complicated by disorder of the
BF 4 group. Its position could be accounted for by
using a split model of rigid groups with SOF = 0.5/
0.5. Rigid group refinem ent was also used for
three 'Bu groups at the structure of 1 and the BF 4
groups of 2. Hydrogen atoms, which could not be
refined isotropically, were placed in calculated
positions. A theoretical absorption correction was
carried out for both structures (DIFABS [27]).
The final cell param eters and specific data collec­
tion param eters are summarized in Table I. Details
of the X-ray structure determ inations have been
deposited at the Fachinformationszentrum Karls­
ruhe GmbH, D-76344 Eggenstein-Leopoldshafen,
Germany, and may be obtained on quoting the
names of the authors, the journal citation, and the
CSD num ber 58911.
Acknowledgements
This work was supported by the Deutsche For­
schungsgemeinschaft, Fonds der Chemischen In­
dustrie, and - through the donation of chemi­
cals - by Degussa AG and H eraeus GmbH. The
authors are grateful to Mr. J. Riede for carefully
establishing the X-ray data sets.
[15] J. Li. P. Pyykkö, Inorg. Chem. 32, 2630-2634 (1993).
[16] J. K. Burdett, O. Eisenstein, W. B. Schweizer, Inorg.
Chem. 33, 3261 -3268 (1994).
[17] E. Zeller, H. Beruda, A. Kolb. P. Bissinger, J. Riede,
H. Schmidbaur, N ature 352, 141-143 (1991).
[18] F. Canales, M. C. Gimeno, P. G. Jones, A. Laguna,
Angew. Chem. 106, 811-812 (1994); Angew. Chem.,
Int. Ed. Engl. 33, 769-770 (1994).
[19] a) A. G rohm ann, J. Riede, H. Schmidbaur, J. Chem.
Soc. Dalton Trans. 1991, 783-788;
b) Yi Yang, V. Ramamoorthy, P. R. Sharp, Inorg.
Chem. 32, 1946-1950 (1993).
[20] H. Schmidbaur, A. Kolb, P. Bissinger, Inorg. Chem.
31, 4370-4375 (1992).
[21] P. Lange, H. Beruda, W. Hiller, H. Schmidbaur, Z.
Naturforsch. 49b, 781-787 (1994).
[22] K. Angermaier, H. Schmidbaur, J. Chem. Soc.
Dalton Trans. 1995, 559-564.
[23] E. Zeller, A. Schier, H. Schmidbaur, Z. Naturforsch.
49b, 1243-1246 (1994).
[24] A. N. Nesmeyanov, E. G. Perevalova, Y. T. Struch­
kov, M. Y. Antipin, K. I. Grandberg, V. P. Dyad­
chenko, J. Organomet. Chem. 201, 343-349 (1980).
[25] SHELXTL-PLUS, Release 4.1, Siemens Analytical
X-Ray Instruments, Inc., Madison (Wisconsin)
(1990).
[26] G. M. Sheldrick, Program for the Refinement of
Structures, University of G öttingen (1993).
[27] a) N. Walker, D. Stuart, A cta Crystallogr. A 39, 158
(1983);
b) N. Walker, Program DIFABS, Version 9.0, BASF
AG (1993).
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