Polynuclear Gold Complexes of the Carbodiimide Unit [N=C=N]2-

Article abstract of DOI:10.1515/znb-1996-0607

The reaction of N,N′-bis(trimethylsilyl)carbodiimide with equimolar quantities of tris[(triphenylphosphine)gold]oxonium tetrafluoroborate in dichloromethane at -40°C gives high yields of a trinuclear complex {[(Ph₃P)Au]₂NCN[Au(PPh₃)]}⁺ BF₄^-(1). According to variable temperature NMR data the cation is fluxional in solution with a rapid site exchange of the [(Ph₃P)Au] units, but has an unsymmetrical (N,N,N′) low temperature ground state configuration. A single crystal X-ray diffraction study has shown that in the crystal lattice these cations are associated further into cyclic, hexanuclear dimers through two short head-to-tail Au-Au contacts (auriophilicity). Treatment of complex 1 with one equivalent of [(Ph₃P)Au]⁺ BF₄^- in tetrahydrofuran affords a tetranuclear complex {[(Ph₃P)Au]₂NCN[Au(PPh₃)]₂} ²⁺ 2BF₄^- (2). The ambient temperature NMR spectra of 2 suggest a fluxional, pseudo-symmetrical structure, but at low temperature a set of three ³¹P signals (1:2:3 intensity) indicates an aggregation into higher nuclearity species with non-equivalent [Au(PPh₃)] units.

Full text of DOI:10.1515/znb-1996-0607

Polynuclear Gold Complexes of the Carbodiimide Unit [N=C=N]2-
W. Schneider, K. Angermaier, H. Schmidbaur*
Anorganisch-chemisches Institut der Technischen Universität München,
Lichtenbergstrasse 4, D-85747 Garching, Germany
Dedicated to Professor Marianne Bäudler on the occasion of her 75th birthday
Z. Naturforsch. 51 b, 801-805 (1996); received December 4, 1995
Carbodiimide, Gold Complexes, (Phosphine)gold Carbodiimides
The reaction of N,N'-bis(trimethylsilyl)carbodiimide with equimolar quantities of
tris[(triphenylphosphine)gold]oxonium tetrafluoroborate in dichloromethane at -40°C gives
high yields of a trinuclear complex {[(Ph3P)Au]2NCN[Au(PPh3)]}+ BF4~(1). According to
variable temperature NMR data the cation is fluxional in solution with a rapid site exchange of
the [(Ph3P)Au] units, but has an unsymmetrical (N,N,N') low temperature ground state configu-
ration. A single crystal X-ray diffraction study has shown that in the crystal lattice these cations
are associated further into cyclic, hexanuclear dimers through two short head-to-tail Au-Au
contacts (auriophilicity). Treatment of complex 1 with one equivalent of [(Ph3P)Au]+ BF4_ in
tetrahydrofuran affords a tetranuclear complex {[(Ph3P)Au]2NCN[Au(PPh3)]2}2+ 2BF4_ (2).
The ambient temperature NMR spectra of 2 suggest a fluxional, pseudo-symmetrical structure,
but at low temperature a set of three 11P signals (1:2:3 intensity) indicates an aggregation into
higher nuclearity species with non-equivalent [Au(PPh3)] units.
Introduction
as difunctional substrates. Although carbodiimines
have been employed as donors for a number of other
metals [33 -41], they have not yet been considered
as units with two nitrogen nucleation sites for gold.
In a series of recent studies it has been demon-
strated that ammonia [1 - 7], primary [8 - 20] and
secondary amines [21 - 27] can serve as aggre-
gation centers for high-nuclearity gold(I) clusters.
As many as five (and possibly six) [4] gold atoms
can be accommodated at the nitrogen nucleation
centers. While nitrogen is in the classical valence
configuration in the cases with lower coordination
numbers (C.N. 2, 3 and 4), the non-classical, hyper-
coordinate species (C.N. 5, 6) require a more so-
phisticated theoretical treatment for the description
of their structure and bonding. It appears that both
electron-deficient radial N-Au bonding and periph-
eral Au-Au interactions are responsible for the un-
expected stability of the clusters [28 - 31].
Results
A
number of aurating agents is available
for the introduction of [LAu]+ units at
a
given substrate, including (phosphine)gold fiu-
oroborates, trifiates and perchlorates, but also
tris[(phosphine)gold]oxonium salts and (phos-
phine)gold acetylacetonates. For the present pur-
pose we have chosen the oxonium route, because the
high affinity of the oxide function for silyl groups
(to form inert disiloxanes) was considered a use-
ful driving force for the synthetic reactions. And,
in fact, all preparative attempts with these reagents
were immediately successful. If the reaction is car-
ried out with equimolar quantities of the compo-
nents at -40°C in a mixture of dichloromethane
and tetrahydrofuran, the products are hexamethyl-
disiloxane and a trinuclear compound of formula 1
(eq. (1)).
Clusters built around the nitrogen atom of am-
monia or amines E-NFb with stepwise substitution
of hydrogen atoms by (phosphine)gold(I) moieties
have thus been investigated quite extensively, but
analogous studies with imines E=NH have been ini-
tiated only recently. Following our work on mono-
functional phosphine imines R3P=NH [32], we
subsequently turned to carbodiimines HN=C=NH
(Me3Si)2N2C + {[(Ph3P)Au]30 } +BF4~
(Me3Si)20 + {[(Ph3P)Au]3N2C}+BF4-
1
(1)
* Reprint requests to Prof. Dr. H. Schmidbaur.
0939-5075/96/0600-0801 $ 06.00
©
1996 Verlag der Zeitschrift für Naturforschung. A ll rights reserved.
K
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802
W. Schneider et al. ■Polynuclear Gold Com plexes o f the Carbodiimide Unit [N =C =N ]2
Table I. Crystallographic Data for
Tris[(triphenylphosphin)gold(I)]
carbodiimide(+l) tetrafluoroborate(-1)(1).
between the two nitrogen coordination sites (A). It
should be noted that a N,N,N structure (B) must
be considered as a likely alternative, which would
feature a common trigoldammonium unit attached
to a nitrile group. The latter is ruled out also on the
grounds of the IR spectrum, however, which has no
nitrile absorption band.
Empirical formula
Formula weight
Crystal system
Space group (No.)
a [A]
b [A]
c [A]
a[ °]
ß[°]
7 [°J3
V [A3]
C55H45Au3BF4N2P3*0.5 CH,C12
1547.01
triclinic
Pi (No.2)
13.994 (2)
14.079(2)
27.651 (3)
104.32(1)
95.81 (1)
99.64(1)
5145.7
.........
Ph3PAu.„„
Ph3PAu'
" N = (
.N—C=N
Ph3PAu^
Ph3PAu
\
/
AuPPh3
1.997
4
Pcalc [gem ' ]
Z
F(000) [e]
H (Mo-Kq) [cm-1]
T[°C]
2932
87.3
-62
/AuPPh3
VAuPPh3
Ph3PAu\
Ph3PAu.,.„
\uPPh3
■
N—C^N-
Diffractometer
Scan
Enraf Nonius CAD4
u>
N = G == N
Ph3PAu
/
Ph3PAu
Ph3PAu
hkl range
± 16/± 16/+32
Measured reflections 17912
Unique reflections
Observed reflections
15828
12581
0.000
4cr(F0)
647
Colourless single crystals of compound 1 are ob-
tained by layering dichloromethane solutions with
toluene and cooling to -25°C. The crystals are tri-
clinic, space group PI, with four formula units in
R \n t
F0 >
refined parameters
H-atoms
(found/calcd)
Absorption corr.:
- / 46
empirical
0.4327 / 0.9997
0.0423
Tmip/ Tmax
R*
0.0357
Rw
Weighting scheme***
Pfin(max/min) [eA-3] + 1.62/- 1.59
/= 1.0000/it = 0.000116
*/? = £(IIF oI-IFcN)/EIFoI;
**Rw = [£ (w (IF 0l-IFcl)2)/£ w (F 2)]l/2 ;
***w = l/[ a 2(F0) + k F 2].
The complex salt 1 is formed in about 70%
yield as a colourless stable material (m.p. 217°C),
which shows satisfactory elemental analysis data.
The FAB and FD mass spectra have the mass of the
complete cation as the parent ion in high abundance,
accompanied by fragment ions generated in the loss
of Ph^PAu and/or (Ph^P^Au units.
Fig. 1. Structure of a cation-dimer in the crystal of 1 with
atomic labeling. Hydrogen atoms are omitted for clarity.
Selected bond lengths [A] and angles [°]: Au2 - Au6
3.098( 1), Au3 - Au4 3.316(1), Aul - Au3 3.663( 1), Au5 -
Au6 3.473(1), Aul - N2 2.054(9), Au3 - N2 2.05(1), Au2
- NI 2.015(8), Au5 -N3 2.029(8), Au6 - N3 2.03(1), Au4
- N4 2.006(9),NI -C l 1.19(1),Cl - N2 1.25 (1),N3 - C2
1.28(2), C2 - N4 1.18(2), Aul - N2 - Au3 126.4(5). Aul
- N2 - Cl 116.7(8), Au3 - N2 - Cl 115.0(7), NI - Cl - N2
176(1), Au2-Nl - Cl 136.3(8), Au5-N3-Au6 117.6(5).
31P NMR spectra of the compound in
dichloromethane show a broad singlet resonance
at 20°C, which is broadened further and finally split
into two resonances of the integral ratio 2 :1 as the
temperature is lowered progressively to -60°C. This
observation suggests a fluxionality process involv- Au5 - N3 - C2 118.6(7), Au6-N3-C2 123.2(7), N3 - C2
- N4 177( 1), C2 - N4 - Au4 143.8(9).
ing rapid positional exchange of the Ph^PAu groups
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W. Schneider et al. • Polynuclear Gold Com plexes of the Carbodiimide Unit [N = C = N ]2
803
the unit cell (Table I). The molecules are arranged
It can be isolated from the reaction mixture by
as crystallographically equivalent dimers, with no precipitation with diethyl ether as a colourless solid
crystallographically imposed internal symmetry.
The association follows a head-to tail pattern
(Fig. 1), with each mononuclear terminal unit
=N-AuPPh3 of one monomer approaching one of
the gold atoms of the bifurcated, dinuclear end
=N(AuPPh3)2 of a second monomer to form in-
termolecular “auriophilic” contacts. These contacts
are in the typical range for this type of aggregation
(decomposition temperature 212°C), but no single
crystals could be generated. The elemental analysis
data confirm the proposed composition, and in FD
mass spectra the dication is observed as the par-
ent peak, while FAB mass spectra show only the
cation of compound 1. Dichloromethane solutions
of compound 2 at room temperature show a sharp
singlet resonance in the 31P NMR spectrum, which
[Au3-Au4 3.316(1), Au2-Au6 3.098(1) A], Note broadens at -30°C and is finally split into a set of
that Aul and Au5 remain without any intra- or three signals of the intensity ratio 1:2:3. The chem-
intermolecular gold contacts. The dimers are thus ical shifts of these signals are different from those
macrocyclic systems.
of compound 1 in the same solvent and at the same
temperature. The process is fully reversible.
Regarding the NMR results, it is conceivable that
in solution the roles of free and engaged AuPPh3
units are changing rapidly for each dimer on the
NMR time scale, either in a dimer/monomer disso-
ciation equilibrium, in a ring opening at only one
end at a time, and/or through internal rearrange-
ments of the contacts of the dimer without ring
opening. No experiments have been carried out to
clarify the mechanism, which may be a combination
of all modes, with very similar energy characteris-
tics for the individual process.
Unfortunately there is no straightforward expla-
nation for this set of resonances, and only tenta-
tive structures can be advanced. It should be noted
that two isomers can be envisaged for the dication
of the tetranuclear complex. One is a symmetri-
cal, allene-type N,N,N',N'-structure (skeletal core
point group E>2d, C), and the other is a N,N,N,N'-
structure (point group C3V, D). None of these can
account alone for the pattern of the low temperature
31P NMR spectra, but a 2:1 mixture or adduct is in
The configuration of the two tri-coordinate nitro-
gen atoms (N2, N3) is virtually trigonal planar, with
sums of the angles for N2 at 358.1° and for N3 at
359.4°. The angles at the two two-coordinate nitro-
gen atoms are 136.3(8)° (for N l) and 143.8(9)° (for
N4), and thus far from the 120° or 180° standards,
indicating a mixed hybridization and a flat energy
profile for bending.
full agreement: [{[(Ph3P)Au]3NCN[Au(PPh3)]}2
{[(Ph3P)Au]2NCN[Au(PPh3)]2 }]6+. In the absence
of other evidence, we are reluctant to accept this
formula, and a definite answer will have to await
the results of future studies.
Experimental Part
General
The two crystallographically independent car-
bodiimide units N=C=N are quasi-linear, with the
angles N1-C1-N2 at 176(1)°, and N3-C2-N4 at
177(1)°. The planes Cl-Nl-Au2 and Cl-Aul-Au3
(C2-N4-Au4 and C2-Au5-Au6) are roughly perpen-
dicular, in agreement with the fundamental struc-
tural motif of allenes (and carbodiimides), which is
based on D2d (C2) symmetry.
Treatment of complex 1 with one equivalent of
(Ph3P)Au+BF4 _ (obtained in situ from the chlo-
ride and AgBF4 ~ in thf) gives high yields of the
tetranuclear complex {[(Ph3P)Au]4CN2 }2+ 2 BF4 ~
(2) (eq. (2)).
All experiments were routinely carried out under dry
and pure nitrogen. Solvents and glassware was dried
and saturated/filled with nitrogen. Conventional equip-
ment was used throughout. (MejSihNiC was purchased,
(Ph3P)AuCl and [(Ph3P)Au]30+BF4~ were prepared fol-
lowing literature methods [42, 43].
Tris[(triphenylphosphine)gold(I)]carbodiimide(+1)
tetrafluoroborate(-1) (1)
Bis(trimethylsilyl)carbodiimide (35 p\, 0.15 mmol) is
added at -40°C to a solution of
tris[(triphenylphosphine)gold(I)]oxonium tetrafluorobo-
rate (0.215 g, 0.15 mmol) in a dichloromethane /tetrahy-
drofuran mixed solvent (30 ml, 1:1 [vol]). The clear so-
lution is allowed to warm to 0°C in the course of 2 h. The
solvents are removed in a vacuum, the residue is taken
1 + [(Ph3P)Au]+BF4
-
{[(Ph,P)Au]4N2C}2+ 2BF4-
2
(2)
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804
W. Schneider et al. • Polynuclear Gold Com plexes o f the Carbodiimide Unit [N =C =N ]2
up twice in dichloromethane and precipitated by addition
of diethyl ether; yield 0.148 g (68%), m.p. 217°C. - 'H
NMR (CDCb): 6 = 7.35-7.45 ppm [m, Ph], ,3C{'H}
NMR (CDCb): b = 127.2 ppm [d, ‘./(CP) = 62.5 Hz, CPI;
are removed in a vacuum. The colourless residue is puri-
fied by precipitation from dichloromethane/diethyl ether
(0.266 g, 81% yield). - 'H NMR (CDC13): b = 7.30-7.60
[m, Ph], 31P{'H} NMR (CDC13): 20°C: b = 29.5 [br. s]:
128.4 [d, V(CP) = 18.2, C(meta)]; 131.3 [d, 47(CP) = -30°C: 28.8 [br. s]; -60°C: 29.5 [br. s, IP], 29.2 [br. s, 2P],
2.0, C(para)]: 133.0 [d, 27(CP) = 21.2, C(ortho)]: CN2 28.0 [br. s, 3P]. - MS (FD): m/e = 936.7 [100%, M2+],
was not observed. 3IP{'H} NMR (CD2C12): 20°C: b =
Analysis for C7iH6oAu4B-’FsN2P4 2 (2050.7)
30.6 [br. s]; 0°C: 30.5 and 30.3 [two br. s]; -30°C: 30.1
and 29.8 [two br. s]; -60°C: 29.6 [br. s, IP] and 29.3
[br. s, 2P], - MS (FAB): m/z = 1417.4 [43.0%, M+];
959.2 [7.5, M+-AuPPh3], 721.0 [35.5, (Ph3P)2Au+], 459.2
[100. Ph3PAu+], FD: 1418.5 [25.6, M++H], 721.7 [100,
(Ph3P)2Au+],
Calcd C 42.76 H 2.95 N 1.37 Au 38.41 F7.41%.
Found C 41.81 H 3.06 N 1.34 Au 35.7 F7.25%.
Crystal structure determination
Details of the X-ray structure determination of com-
pound 1 are summarized in Table I. Further informa-
tion on the structure determinations may be obtained
from Fachinformationszentrum Karlsruhe, Gesellschaft
für wissenschaftlich-technische Information GmbH.
D-76344 Eggenstein-Leopoldshafen, Germany, on quot-
ing the names of the authors, the journal citation and the
CSD number 59219.
Analysis for C55H43Au3BF4NtP3 1 (1504.6)
Calcd C 43.90 H 2.99 N 1.86 F5.05%,
Found C 43.45 H 3.07 N 1.95 F5.06%.
Tetrakis[(triphenylphosphine)gold(I)]carbodiimide(+2)
bis[tetrafluoroborate(-1)] (2)
A
solution of chloro(triphenylphosphine)gold(I)
(0.079 g, 0,16 mmol) in thf (20ml) is cooled to -40°C
and treated with AgBF4(0.031 g, 0.16 mmol). After 30
min of stirring the AgCl precipitate is removed by filtra-
tion and a solution of 1 (0.240 g, 0.16 mmol) in 20 ml
of CH2C12 is added. After stirring the mixture for 2 h
the solution is allowed to warm to 0°C, and the solvents
AcknowIedgements
J. Riede is thanked for establishing the X-ray data set
and F. R. Kreissl for mass spectrometric measurements.
Support by Deutsche Forschungsgemeinschaft and Fond
der Chemischen Industrie is gratefully acknowledged.
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