Mono and tetranuclear gold(I) complexes of tris( 1-benzylimidazole-2-yl)- phosphine

Article abstract of DOI:10.1021/ic901562d

The reaction of tris(1-benzyllmldazole-2-yl)phosphine, (Bzim)₃P, 1, with Ph₃AsAuCl In 1:1 stoichiometric ratio produced (Bzim) ₃PAuCl, 2. The reaction of (Bzim)₃PAuCl with NaAuCl ₄ in 1:1 stoichiometry in dichloromethane gives an orange-yellow crystalline tetranuclear gold(I) cluster [{μ-N,N-(Bzim)₃PAuCl} ₂Au₂][AuCl₂][AuCl₄], 3. Complex 4, [{μ-N,N-(Bzim)₃PAuCl}₂Au₂][AuCl ₂]₂ is formed when the reaction stoichiometry of (Bzim)₃PAuCl and AuCl₄^-is 2:1. The crystal structure of 3 shows the formation of a 12-membered macrocycle with Au ... Au distances of ～3.0 A. The structures of (Bzim)₃PAuCl and 3 show Au ... H - C interactions ranging from 2.57 to 2.95 A. Complex 2 crystallizes in the monoclinic space group P2₁/n (Z=4), a=9.1927(5), b=13.528(2), c=22.995(2) A, and ss = 94.537(5)°. Complex 3 crystallizes in the monoclinic space group P2₁/c (Z=4), a = 13.785(4), b = 21.426(6), c = 25.203(8) A, and ss = 96.51(6)°.

Full text of DOI:10.1021/ic901562d

Inorg. Chem. 2010, 49, 513–518 513
DOI: 10.1021/ic901562d
Mono and Tetranuclear Gold(I) Complexes of Tris(1-benzylimidazole-2-yl)-
phosphine
Alfredo Burini,*,† Rossana Galassi,† Simone Ricci,† Fiorella Bachechi,‡ Ahmed A. Mohamed,§ and
John P. Fackler, Jr.*,§
†
ꢀ
Dipartimento di Scienze Chimiche, Universita di Camerino, via S. Agostino 1, I-62032 Camerino, Italy,
‡Istituto di Cristallografia, CNR, Area della Ricerca di Roma, C.P. 10, I-00016 Monterotondo St., Roma, Italy,
and §Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255
Received August 5, 2009
The reaction of tris(1-benzylimidazole-2-yl)phosphine, (Bzim)3P, 1, with Ph3AsAuCl in 1:1 stoichiometric ratio
produced (Bzim)3PAuCl, 2. The reaction of (Bzim)3PAuCl with NaAuCl4 in 1:1 stoichiometry in dichloromethane gives
an orange-yellow crystalline tetranuclear gold(I) cluster [{μ-N,N0-(Bzim)3PAuCl}2Au2][AuCl2][AuCl4], 3. Complex 4,
[{μ-N,N0-(Bzim)3PAuCl}2Au2][AuCl2]2 is formed when the reaction stoichiometry of (Bzim)3PAuCl and AuCl4- is 2:1.
˚
The crystal structure of 3 shows the formation of a 12-membered macrocycle with Au Au distances of ∼3.0 A. The
3 3 3
˚
structures of (Bzim)3PAuCl and 3 show Au H-C interactions ranging from 2.57 to 2.95 A. Complex 2 crystallizes in
3 3 3
o
ꢁ
˚
the monoclinic space group P21/n (Z = 4), a = 9.1927(5), b = 13.528(2), c = 22.995(2) A, and β = 94.537(5) . Complex
ꢁ
˚
3 crystallizes in the monoclinic space group P21/c (Z = 4), a = 13.785(4), b = 21.426(6), c = 25.203(8) A, and β =
96.51(6)o.
Introduction
Since aurophilic bonding is directional,4 the energetic and
structural similarities between aurophilic and hydrogen
bonding suggested the use of aurophilic attraction to control
the supramolecular frameworks of Au(I) complexes. Several
studies demonstrated that aurophilic interaction could be
used to build supramolecular architectures equivalent to
those created by hydrogen bonding.5 Exploring aurophilicity
in the formation of supramolecular structures and secondary
interactions in the solid state has received considerable
interest. Also non conventional hydrogen bonding such as
Weak gold-gold, aurophilic attractive interactions1 arise
from correlation effects strongly enhanced by relativistic
contributions.1 Aurophilic attractions are weaker than cova-
lent or ionic bonds but stronger than van der Waals contacts
and comparable to hydrogen bonds.1a,c In the case of mono-
nuclear Au(I) complexes, the Au Au contacts are 3.00-
3 3 3
˚
3.50 A and associated with a bond energy in the order of 21-
46 kJ mol-1 as calculated by theoretical and experimental
methods.2 Au(I) complexes of the type L-Au-X with L =
phosphine ligands in particular and X = halide or pseudo-
halide are able to aggregate into pairs, rings, and chains.3 In
the monomeric complexes the Au(I) coordination geometry
is very close to linear while the presence of gold-gold
interaction produces a deviation in which the gold atoms
are drawn together slightly.
C-H Cl6 or π-π interactions7 have been found to exist
3 3 3
along with the Au Au interaction in crystal packing.
3 3 3
Gold(I) is an electron rich, soft metal ion and forms stable
compounds with soft element ligands. Thus most gold(I)
chemistry has sulfur, phosphorus, or carbon containing
ligands.8 However, Au(I) also shows moderate affinity to-
ward nitrogen ligands although the structure of [Au(NH3)2]Br
*To whom correspondence should be addressed. E-mail: alfredo.burini@
unicam.it (A.B.), fackler@mail.chem.tamu.edu (J.P.F.).
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Chem. Soc. 2001, 123, 5376. (b) Hunks, W. J.; Jennings, M. C.; Puddephatt, R. J.
Inorg. Chem. 2000, 39, 2699. (c) Jones, P. G.; Ahrens, B. New J. Chem. 1998,
1041. (d) Ahrens, B.; Jones, P. G.; Fisher, A. K. Eur. J. Inorg. Chem. 1999, 1103.
(e) Hollatz, C.; Schier, A.; Schmidbaur, H. J. Am. Chem. Soc. 1997, 119, 8115.
(6) (a) Aakeroy, C. B.; Evans, T. A.; Seddon, K. R.; Palinko New J. Chem.
1999, 145. (b) Freytag, M.; Jones, P. G. Chem. Commun. 2000, 277.
(7) (a) Puddephatt, R. J. Chem. Commun. 1998, 1055. (b) Hao, L.;
Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. Inorg. Chem. 1999, 38, 4616.
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Vol. 5. (b) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and
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r
2009 American Chemical Society
Published on Web 12/17/2009
pubs.acs.org/IC

514 Inorganic Chemistry, Vol. 49, No. 2, 2010
Burini et al.
Chart 1. Structure of (1-Benzylimidazole-2-yl)phosphine, (Bzim)Ph2P,
and Tris(1-benzylimidazole-2-yl)phosphine, (Bzim)3P
C-H bond activation of one of the phenyl rings of the
(Bzim)Ph2P ligand was observed by adding [{Ir(μ-Cl)(cod)}2]
to [(Bzim)Ph2PIrCl(cod)] to give the hydride complex
[IrCl(cod){μ-PPh(C6H4)-BzIm}IrHCl(cod)].25 Here we report
the X-ray crystal structures of (Bzim)3PAuCl, 2, and [μ-N,
N0-{(Bzim)3PAuCl}2Au2][AuCl2][AuCl4], 3, which show that
(Bzim)3P ligand is unexpectedly capable of giving rise to
secondary interactions involving Au and H.
Experimental Section
Synthesis of [{(Bzim)Ph2PAuCl}2AuCl2]Cl and[{(Bzim)Ph2-
PAuCl}2AuCl2]BF4. To a solution of (Bzim)Ph2PAuCl20 (0.066 g;
0.155 mmol) in 6 mL of degassed CH2Cl2 was added solid
was reported about sixty years9 later than that of the ana-
logous silver complex.10 Several Au(I)-N complexes have
been described with amines,11 ketimines,12 nitriles,13 pyrazo-
lates,14 poly(pyrazolyl)borates,15 amidinates,16 benzylimida-
zolates, and carbeniates.17 A pentacoordinate nitrido Au(I)
cluster[μ5-N(AuPPh3)5]2þ has been structurally characterized.18
In this study we present the coordination chemistry of the
P,N-donor ligand tris(1-benzylimidazole-2-yl)phosphine,
(Bzim)3P, (Chart 1), with Au(I). The new tertiary phosphine
ligand is similar to poly(pyrazol-1-yl)borates.19 Previous
investigations with the (1-benzylimidazole-2-yl)phosphine
analogue, (Bzim)Ph2P, demonstrated that it can react at the
P atom forming complexes such as (Bzim)Ph2PAuCl20 and
[(Bzim)Ph2PMCl(cod)] (M = Rh(I), Ir(I)),21 or at P and N
centers, as a bidentate ligand, forming dinuclear cationic
complexes such as [μ-(Bzim)Ph2PM]2[X]2 (M=Au(I), Ag(I);
X=PFh, BFh, NOh),22 [μ-(Bzim)Ph2PHgClO4]2[ClO4]2,23 and
NaAuCl4 2H2O (0.023 g; 0.0575 mmol), and the reaction mixture
3
was stirred overnight. The colorless solution slowly turns to yellow,
and the formation of a white precipitate of NaCl was observed. The
solution was then filtered and pumped down to dryness. The crude
compound was washed with hexane and crystallized by CH2Cl2
and hexane. Yield 75%. Anal. Calcd for C44H38N4Au3Cl5P2: C,
36.37; H, 2.64; N, 3.86. Found: C, 36.49; H, 2.84; N, 3.64. 1H NMR
(CDCl3, δ, 295 K): 4.83 (s), 5.08 (s), 5.39 (s), 5.59 (s), 5.64, (s),
6.70-6.75 (m), 6.96-7.00 (m), 7.09-7.98 (m). 31P NMR (CDCl3,
δ, 295 K): 12.47 (s), 15.46 (s), 19.13 (s), 23.44 (s). IR (C-H
stretching region, cm-1): 3117.8, 3054.7, 3033.2, 2957.9, 2925.7,
2856.1. After treatment of [{(Bzim)Ph2PAuCl}2AuCl2]Cl with
a stoichiometric amount of AgBF4, the complex [{(Bzim)Ph2-
PAuCl}2AuCl2]BF4 was formed in a quantitative yield. The
elimination of the chloride counterion resulted in a stable complex
in solution as highlighted by its NMR. Anal. Calcd for
C44H38N4Au3Cl4P2BF4: C, 35.13; H, 2.55; N, 3.72. Found: C,
35.41; H, 2.72; N, 3.60. 1H NMR (CDCl3, δ, 295 K): 4.78 (s, 2H),
4.90 (s, 2H), 6.71-6.75 (m, 4H) 7.13-8.02 (m, 26H), 8.42 (s, 4H).
31P NMR (CDCl3, δ, 295 K): 23.38 (s).
6
4
3
[{μ-(Bzim)Ph2PM(cod)}2][BF4]2 (M=Rh(I), Ir(I)),21 or [(μ-
(Bzim)Ph2P)3M2][X]2 (M=Ag(I), Cu(I); X=CF3SOh, BFh).24
3
4
Synthesis of (Bzim)3P, 1. The synthesis can be carried out in
two alternative ways as reported in literature. Both methods are
successful and give similar yields.26 The following is a slightly
modified synthesis of the one reported for (py)3P by Schmid-
baur. To a stirred anhydrous diethyl ether solution (15 mL) of
1-benzylimidazole (0.4514 g, 2.85 ꢀ 10-3 mol) cooled to -70 °C,
under a nitrogen atmosphere, was added a 2.5 M hexane
solution of BuLi (1.14 mL, 2.85 ꢀ 10-3 mol). After 2 h the tem-
perature of the yellow solution was raised to -30 °C for 30 min
then cooled again to - 90 °C. At this point 2 mL of a diethyl
ether solution containing PCl3 (0.083 mL, 0.95 ꢀ 10-3 mol) was
added dropwise over 1 h. After 2 h the mixture was left to reach
room temperature, and stirred overnight. The white suspension
obtained was extracted with 11 mL of 2 M H2SO4, and then the
aqueous phase was neutralized by a saturated solution of
NaHCO3. The white waxy solid which formed was extracted
with dichloromethane. The yellow solution was dried over
anhydrous Na2SO4 and evaporated to dryness. A 0.319 g
portion of an oily yellow solid was obtained (yield ∼66%).
The (BzIm)3P ligand is easily oxidized, so no satisfactory
analysis was obtained. (BzIm)3P must be kept under a nitrogen
atmosphere. 31P {1H}NMR (acetone-d6; δ, 295 K): -61.76 (s).
A signal at 1.75 (s) is also present. This signal is less than 10% of
the ligand signal at -61.76 ppm, and it can be assigned to the
phosphine oxide.1H NMR (acetone-d6; δ, 295 K): 5.25 (s, 6H),
7.03-7.42 (m, 21H). IR (aromatic and aliphatic C-H stretching
region, cm-1): 3103, 3090 (shoulder), 3062.8, 3030, 3005.4, 2929.4.
Synthesis of (BzIm)3PAuCl, 2. (BzIm)3P, 0.159 g (3.15 ꢀ
10-4 mol), under a nitrogen atmosphere, was dissolved in 5.5 mL
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Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 515
of CH2Cl2 and cooled to 0 °C. To this pale yellow solution was
added solid Ph3AsAuCl (0.1698 g, 3.15 ꢀ 10-4 mol). The clear
solution was stirred for about 1 h. The completeness of the
reaction was followed by TLC (formation of free Ph3As). The
solution was then evaporated off under reduced pressure to
dryness, leaving a yellow oil. The product was treated several
times with hexane until a solid was obtained which dissolved in a
few milliliters of CH2Cl2. The solution was layered with hexane
to form colorless crystals suitable for an X-ray structure deter-
mination (0.101 g; yield 43%). Mp 216-219 °C.
Anal. Calcd for C30H27N6PAuCl: C, 49.03; N, 11.43; H, 3.70.
Found: C, 48.92; N, 11.43; H, 3.86. 31P {1H}NMR (acetone-d6; δ,
1
295 K): 19.75 (broad). H NMR (acetone-d6; δ, 295 K): 5.43 (s,
6H), 7.07-7.1 (m, 6H), 7.19-7.2 (t, 3H), 7.23-7.28 (m, 9H), 7.37-
7.38 (t, 3H). IR (aromatic and aliphatic C-H stretching region,
cm-1): 3114.3, 3094.2, 3063.3, 3034.1, 3008.6, 2069.5, 2931.8.
Synthesis of [{μ-N,N0-(Bzim)3PAuCl}2Au2][AuCl2][AuCl4],
3 and [{μ-N,N0-(Bzim)3PAuCl}2Au2][AuCl2]2, 4. To a stirred
CH2Cl2 solution (6 mL) of 2 (0.0355 g, 4.72 ꢀ 10-5 mol), under
a nitrogen atmosphere, was added the salt NaAuCl4 (0.01707 g,
4.72 ꢀ 10-5 mol). The mixture was stirred overnight at room
temperature. The yellow solution was filtered through Celite and
layered with hexane. After few days at -4 °C orange crystals grew.
Yield 65% (calculated based on complex 2). In a similar way but
using a 2:1 molar ratio of (Bzim)3PAuCl to NaAuCl4, the white
microcrystalline compound 4 was obtained (yield 80%).
Figure 1. Thermal ellipsoid plot of (Bzim)3PAuCl, 2. Ellipsoids are
shown at 40% probability.
software package27 and the SIR CAOS structure determination
package.28 Scattering factors and anomalous dispersion terms
were taken from the International Tables for X-ray Crystal-
lography.29
Anal. Calcd for 3, C60H54N12Cl8P2Au6: C, 29.17 ; H, 2.20
(2.35); N, 6.80 (6.63). Found: C, 29.03; H, 2.35; N, 6.63. Anal.
Calcd for 4, C60H54N12Cl6P2Au6: C, 30.03; H, 2.27; N, 7.00.
Found: C, 29.75; H, 2.29; N, 7.12. NMR spectra for 3 and 4.
31P{1H}NMR (CD2Cl2; δ, 293 K) 24.09 (s). 1H NMR (CD2Cl2;
δ, 293 K): 4.83 (s, broad, 2H), 5.40 (m, 8H), 5.6 (d, broad, 2H),
6.95-7.20 (m, 10H), 7.23-7.65 (m, 28H), 7.95 (s, 4H). IR
(aromatic and aliphatic C-H stretching region, cm-1): 3169,
3139, 3112.8, 3059.5, 3027.2, 2933.8, 2900, 2818, 2782, 2686,
2658.6, 2621.4, 2594.7.
Results and Discussion
The reaction of (Bzim)3P, 1, with Ph3AsAuCl in 1:1
stoichiometric ratio formed (Bzim)3PAuCl, 2, by displace-
ment of the Ph3As ligand. The crystal structure of 2 is shown
in Figure 1. Crystal data and selected bond lengths and angles
are listed in Tables 1 and 2. The structure of 2 consists of
monomeric units with Au(I) atoms in a nearly linear arrange-
ment with a P-Au-Cl angle of 176.65(8)°. The P-Au-Cl
angle of 176.65(8)o in 2 is close to the value of 175.1(3)°
reported for the analogous complex (Bzim)Ph2PAuCl20 in
Structure Determination. The X-ray structures of the pris-
matic crystals of 2 (colorless) and 3 (orange-yellow) were solved
in the monoclinic space groups P21/n and P21/c, respectively.
The unit-cell dimensions were determined by least-squares re-
finement of the setting angles of 24 high angle reflections care-
fully centered. X-ray diffraction effects were collected on a
Rigaku AFC5R diffractometer using graphite monochromated
Cu KR radiation and a 12 kV rotating anode generator.
Intensity data were collected by ω/2θ-technique to a maximum
2θ value of about 124°, 2, and 121°, 3. Three standard reflections
were measured every 100 reflections which were found to remain
constant. Corrections for Lorentz and polarization were applied
together with an empirical absorption correction that uses scan
data of three reflections at angles of about 90°. The minimum
normalized factors were in the range 0.55. Only the independent
reflections, which met the condition I > 3σ(I), were used in the
subsequent calculations.
˚
which intermolecular Au Au contacts of 3.03(2) A are
present. The absence of Au Au interactions in 2 is not
3 3 3
3 3 3
reflected in the P-Au-Cl angle, which is not as close to
linear as in R3PAuCl complexes such as Et3PAuCl,
Pr3PAuCl, Ph3PAuCl, and (furil)3PAuCl (178.0°-178.9°).
˚
The Au-P bond length of 2.216(2) A found in 2 is shorter
than in(Bzim)Ph2PAuCl, 2.239(3) A.20 Few examples of such
˚
short Au-P bond length have been reported: Cl3PAuCl,
2.198(2) A; Me3PAuOSiMe3, 2.210(2) A;31 (furyl)3PAuCl,
30
˚
˚
32
˚
2.209(3) A.
The crystal structure of (Bzim)3PAuCl is characterized by
an interesting feature not seen in Au(I) complexes of benzy-
limidazolylphosphane. In (Bzim)Ph2PAuCl, the methylene
hydrogen atoms of the benzyl groups are placed close to
the gold atoms by rotation of the imidazolyl groups. One
benzylimidazolyl group assumes a conformation favorable
for the CH2 group to have one hydrogen atom at a close
Structures 2 and 3 were solved by Patterson and subsequent
Fourier syntheses and refined by full-matrix least-squares meth-
P
od with the minimized function w(|Fo| - |Fc|)2, where w =
1/σ2(Fo)2. The hydrogen atoms were included at calculated
positions and not refined. All the non-hydrogen atoms were
refined anisotropically while for complex 3 only the Au, Cl, and
P atoms were refined anisotropically. All the hydrogen atoms
were given the isotropic thermal factors of the parent carbon
atoms and not refined. The refinement converged at R1=0.048
and wR2 = 0.060, 2, and R1 = 0.068 and wR2 = 0.087, 3. All
calculations were performed using the TEXAN crystallographic
˚
distance from the Au center (Au H-C 2.62 A). In spite of
3 3 3
the Au H-C contact and the bulkiness of the 1-benzyl-2-
3 3 3
imidazolyl diphenylphosphane, the gold atom is still avail-
˚
able to strongly interact with another gold atom at 3.03 A. In
(Bzim)3PAuCl, two benzylimidazolyl groups can position
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(31) Bauer, A.; Schneider, W.; Angermaier, K.; Schier, A.; Schmidbaur,
H. Inorg. Chim. Acta 1996, 251, 249.
(32) Bachechi, F.; Burini, A.; Galassi, R.; Pietroni, B. R. J. Chem.
Crystallogr. 2004, 34, 743.
(27) EXSAN-TEXRAY Structure Analysis Package; Molecular Structure
Corporation: The Woodlands,TX , 1985.
(28) Camalli, M.; Capitani, D.; Cascarano, G.; Cerrini, S.; Giacovazzo,
C.; Spagna, R. et al. Patent No. 35403c/86; SIR CAOS User Guide; Istituto di
Strutturistica Chimica CNR: Rome, Italy.

516 Inorganic Chemistry, Vol. 49, No. 2, 2010
Burini et al.
Table 1. Crystal Data for Complexes 2 and 3
2
3
empirical formula
formula weight
C30H27AuClN6P
734.98
C60H54Au6Cl8N12P2
2470.57
crystal dimensions (mm) 0.10 ꢀ 0.25 ꢀ 0.40
0.07 ꢀ 0.07 ꢀ 0.50
monoclinic
P21/c (#14)
13.785(4)
21.426(6)
25.203(8)
96.51(6)
crystal system
space group
monoclinic
P21/n (#14)
9.1927(5)
13.528(2)
22.995(2)
94.537(5)
2850.5(6)
4
˚
a (A)
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
7396(1)
4
2.219
Z
Dc (g/cm3)
reflections collected
independent reflections
R1, wR2
1.712
5054
8345
4728 (Rint = 0.055) 7639 (Rint = 0.027)
0.048, 0.060
0.068, 0.087
˚
Table 2. Selected Bond Distances (A) and Angles (deg) for Complexes 2 and 3
Complex 2
Au(1)-Cl(1)
Cl(1)-Au(1)-P(1)
2.276(2)
176.65(8)
Au(1)-P(1)
2.216(2)
Figure 2. Crystal packing of complex (Bzim)3PAuCl, 2, showing the
secondary interactions.
Complex 3
The reduction of the AuCl4- ion is well-established when
phosphorus or sulfur ligands are used as reducing agents.34
Also the reduction of Au(III) haloammine complexes by
iodide has been described.35 The reaction of Na[3,5-Ph2pz]
with Au(III)pyCl3 in tetrahydrofuran (THF) produced the
mixed valence Au(I)/Au(III) complex [Au3(3,5-Ph2pz)3-
Cl2].36 It is likely that compounds 3 and 4 result from the
Au(1)-Au(4)
2.989(2)
3.862(2)
3.766(2)
2.22(1)
2.04(3)
2.27(1)
2.26(2)
2.63(6)
173.5(4)
173.9(4)
90(1)
Au(1)-Au(2)
3.257(3)
3.962(2)
3.014(2)
2.29(1)
2.22(1)
1.98(3)
2.28(3)
2.54(6)
173(1)
175(1)
88(1)
Au(1)-Au(3)
Au(4)-Au(2)
Au(4)-Au(3)
Au(2)-Au(3)
Au(1)-P(1)
Au(1)-Cl(1)
Au(4)-N(3)
Au(2)-P(2)
Au(2)-Cl(2)
Au(3)-N(1)
Au(5)-Cl(4)
Au(5)-Cl(3)
Au(6)-Cl(6)
Au(6)-Cl(7)
Cl(1)-Au(1)-P(1)
Cl(2)-Au(2)-P(2)
Cl(3)-Au(5)-Cl(6)
Cl(4)-Au(5)-Cl(3)
Cl(7)-Au(6)-Cl(8)
N(7)-Au(4)-N(3)
N(9)-Au(3)-N(1)
Cl(4)-Au(5)-Cl(6)
Cl(5)-Au(5)-Cl(6)
-
reduction of AuCl4 with formation of the oxidized phos-
phine after coordination of the Au(III) to the nitrogen atoms
of the imidazole rings of the (Bzim)3PAuCl. When (Bzim)-
Ph2PAuCl was reacted37 with AuCl4-, the mixed valence
complex [{(Bzim)Ph2PAuCl}2AuCl2]Cl was isolated where
the Au(III) center is coordinated to two nitrogen atoms from
two (Bzim)Ph2PAuCl units.37
177(1)
163(1)
178(1)
one hydrogen atom of each CH2 group at close distance from
gold (2.83 and 2.95 A) (Figure 2). In a theoretical study on the
˚
stability of Au(I) phosphine complexes of the form
[Au(PH3)n]þ (n = 1-4) and [Au(PH3)nCl] (n = 1-3), struc-
Crystal data, selected bond lengths and angles for complex
3 are listed in Tables 1 and 3. Complex 3 (Figure 3) crystal-
lizes in the monoclinic space group P21/c (Z=4). It consists
of a 12-memberedmacrocycle containing two of the four gold
atoms coordinated to two (Bzim)3P ligands. The mean
˚
tures with conformations that allow Au H (2.691-2.980 A)
3 3 3
were found possible but with a high energy, ∼35 kJ mol-1
,
above the global minimum.33
In the crystal packing of (Bzim)3PAuCl, 2, the molecules
are connected by two kinds of weak interactions to form a
˚
Au-N distance is 2.01 A. It is in the range of the bond
lengths found in related tetranuclear nitrogen clusters com-
plexes such as [μ-3,5-tBu2pzAu]4,38 [(dppm)2Au4(3,5-Ph2-
Pz)2][NO3]2,39 [Au4(ArNC(H)NAr)4],16 and [Au4(μ3-L)-
(PPh3)4][BF4]2, L=8-aminoquinoline.40 The other two gold
atoms are coordinated to the phosphorus atom of (Bzim)3P
net: π-π interactions and C-H Cl hydrogen bonds. The
3 3 3
net is formed by chains along the b axis with lateral connec-
tions along the c axis. Along the chains the molecules are
connected by two weak π-π interactions between the cen-
troids of the imidazolyl and the benzyl rings and by two
˚
ligand with both P-Au bond lengths of 2.22(1) A. They
C-H Cl hydrogen bonds (Table 3, first four entries). The
3 3 3
complete their bicoordination with two chloride atoms with
chains are in turn connected by other very long C-H Cl
3 3 3
˚
Au-Cl=∼2.28 A. The strong deviation from linearity of the
hydrogen bonds (Table 3, entries 5-7).
In the attempt to obtain a mixed Au(I)/Au(III) complex,
the reaction of 2 with NaAuCl4 in a 1:1 stoichiometry was
perfomed. From the reaction mixture, the tetranuclear com-
plex 3 was isolated as an orange-yellow crystalline compound
(Scheme 1). The presence of the AuCl4- in 3 as the counterion
is indicative of an excess of the Au(III) salt in the reaction
mixture. In fact, when the reaction molar ratio of 2 with
NaAuCl4 is 2:1, the yield is higher, about 80%, and only
AuCl2- as a counterion is present, complex 4.
(34) Puddephatt, R. J. Chemistry of Gold.; Elsevier Scientific Publishing
Company: Amsterdam, The Netherlands, 1978.
(35) Elding, L. I.; Skibsted, L. H. Inorg. Chem. 1986, 25, 4084.
(36) Raptis, R. G.; Fackler, J. P., Jr. Inorg. Chem. 1990, 29, 5003.
(37) Galassi, R.; Bachechi, F.; Burini, A., private communication. A
paper describing the synthesis of these mixed valence, Au(I)/Au(III),
compounds and some related chemistry was prepared for publication and
submitted elsewhere but has been withdrawn.
(38) (a) Yang, G.; Raptis, R. G. Inorg. Chim. Acta 2003, 352, 98.
(b) Bonati, F.; Burini, A.; Pietroni, B. R.; Galassi, R. Gazz. Chim. Ital. 1993,
123, 691.
(39) Mohamed, A. A.; Lopez-de-Luzuriaga, J. M.; Fackler, J. P., Jr.
J. Cluster Sci. 2003, 14, 253.
(40) Schmidbaur, H.; Kolb, A.; Bissinger, P. Inorg. Chem. 1992, 31, 4370.
(33) Schwerdtfeger, P.; Hermann, H. L.; Schmidbaur, H. Inorg. Chem.
2003, 42, 1334.

Article
Inorganic Chemistry, Vol. 49, No. 2, 2010 517
˚
Table 3. Distances (A) between Centroids of Imidazolyl and Benzyl Rings and C-H Cl Hydrogen Bond Distances in the Crystal Packing of 2
3 3 3
˚
dist. (A)
C(1)-C(3) imidazolyl (x,y,z)
C(5A)-C(10A) benzyl (x,y,z)
Cl (x,y,z)
C(5A)-C(10A) benzyl (1/2-x,1/2þy,1/2-z)
C(1)-C(3) imidazolyl [(1/2-x,-1/2þy,1/2-z) þ (2,-1,0)]
H-C(4B) [(1/2-x,1/2þy,1/2-z) þ (2,-1,0)]
H-C(10B) [(1/2-x,1/2þy,1/2-z) þ (2,-1,0)]
H-C(7) [(-x,-y,-z) þ (1,0,0)]
3.96
3.94
2.78
2.90
2.89
3.00
3.14
H-C(9) [(-x,-y,-z) þ (2,1,0)]
H-C(10) [(-x,-y,-z) þ (2,1,0)]
Figure 3. Structural plots of [{μ-N,N0-(Bzim)3PAuCl}2Au2][AuCl2][AuCl4], 3. Au(1) Au(4) = 2.989(2), Au(2) Au(3) = 3.014(2), Au(1) Au(2)
3 3 3 3 3 3 3 3 3
˚
= 3.257(3), Au(3) Au(4) = 3.766(2), Au(2) H(2) = 2.48, Au(1) H(4) = 2.67, and Au(4)-Cl(7) = 3.33(5) A.
3 3 3 3 3 3 3 3 3
Scheme 1
angles: N(1)-Au(3)-N(9) 175(1)°, N(3)-Au(4)-N(7)
173(1)°, P(1)-Au(1)-Cl(1) 173.5(4)°, and P(2)-Au(2)-Cl(2)
173.9(4)° is consistent with the aurophilic interactions Au-
˚
(2) Au(3) 3.014(2) and Au(1) Au(4) 2.989(2) A. The
3 3 3
3 3 3
tetranuclear arrangement is composed of two dinuclear
moieties that rotate around each other by approximately
77° (Figure 3) and interact through Au(1) Au(2) 3.257(3)
3 3 3
˚
and Au(3) Au(4) 3.766(2) A). The latter distance is on the
3 3 3
Baukova et al.43 described the formation of Au H-C
long edge of the range considered for gold-gold interactions,
2.8-3.6 A, and it may originate from Coulombic repulsions
3 3 3
˚
interactions in diphenylmethane and diphenylethane orga-
nogold derivatives and classified them as agostic interac-
tions on the basis of the infrared spectroscopic evidence for
the shift of about 100 cm-1 to lower frequency of the C-H
stretches. On the basis of the Brookhart study,42 the intra-
molecular hydrogen-gold interactions present in complex 2
are compatible with an electrostatic interaction, analogous
between two formally positive Au(I) centers even though
short distances between Au(I) atoms supporting the same
charge (þ þ or - -) have been found.41
In complex 3 each methylene group has one hydrogen
˚
atom that interacts with a gold center: H(4) Au(1)=2.59 A,
3 3 3
˚
Au(1) H(4)-C(24) = 130°, H(2) Au(2) = 2.57 A, and
3 3 3
3 3 3
to the closed shell C-H Co interaction in [Et3NH]-
Au(2) H(2)-C(54)=139°. The nature of hydrogen-metal
3 3 3
3 3 3
[Co(CO)4]. Lippard42e has used the term anagostic for such
interactions. The ligand (Bzim)3P shows a peak at 2929 cm-1
in the infrared that can be assigned to the C-H stretch of the
methylene in the benzyl group. When the ligand is coordi-
nated to the gold atom in (Bzim)3PAuCl, 2, a new peak at
2969 cm-1 appears. The position of this signal is not
interactions has been extensively debated; they can involve tran-
sition metal atoms as hydrogen bond donors or acceptors.42
€
(41) (a) Conzelmann, W.; Hiller, W.; Strahle, J.; Sheldrick, G. M.
€
Z. Anorg. Allg. Chem. 1984, 512, 169. (b) Adam, H.-N.; Hiller, W.; Strahle, J.
Z. Anorg. Allg. Chem. 1982, 485, 81. (c) Bauer, A.; Schmidbaur, H. J. Am.
Chem. Soc. 1996, 118, 5324.
compatible with an agostic Au H-C interaction but
3 3 3
(42) (a) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Progress Inorganic
Chemistry; John Wiley & Sons, Inc.: New York, 1988; Vol. 36. Braga, D.;
Grepioni, F.; Desiraju, G. R. Chem. Rev. 1998, 98, 1375. (b) Desiraju, G. R.
J. Chem. Soc., Dalton Trans. 2000, 3745. (c) Crabtree, R. H. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 789. (d) Brookhart, M.; Green, M. L.; Parkin, G. Proc.
Natl. Acad. Sci. U.S.A. 2007, 104, 6908. (e) Sundquist, W. I.; Bancroft, D. P.;
Lippard, S. J. J. Am. Chem. Soc. 1990, 112, 1590–1596.
may represent an anagostic interaction. The crystallographic
data, however, is well matched for an agostic interaction
˚
wherein the M-H bond length is in the range of 2.3-2.9 A
and a M-H-C bond angle is 110-170°.42 Packing forces
positioning the methylene groups close to the gold atoms
may be involved. The IR spectrum of the tetranuclear
gold(I) complex, 3, in the region 4000-2400 cm-1 is
(43) (a) Baukova, T. V.; Kuz’mina, L. G.; Oleinikova, N. A.; Lemenovskii,
D. A.; Blumenfel’d, A. L. J. Organomet. Chem. 1997, 530, 27.

518 Inorganic Chemistry, Vol. 49, No. 2, 2010
Burini et al.
more complicated than that recorded for the mononuc-
lear compound. The signals of the C-H stretch of the
aromatic protons are shifted at higher energy with respect
to the free ligand. The aliphatic C-H bonds show several
signals below the 3000 cm-1 region. A large indented band
from 2820 cm-1 to 2590 cm-1 was observed while the new
oxidation,44 their structural features and the presence of
short Au Au interactions possibly influencing the optoe-
3 3 3
lectronic properties observed in various polynuclear Au(I)
compounds.45
Acknowledgment. This work was supported by grants
from University of Camerino and CNR (Roma) and The
Robert A. Welch Foundation, Houston, Texas.
band at 2900 cm-1 could be due to an anagostic Au H-C
3 3 3
interaction.
The 1H NMR spectrum of compound 3 shows
three groups of signals for the CH2 benzyl group, 2H at
higher field, 2H at lower field when compared with
the CH2 benzyl of the (Bzim)3PAuCl, and 8H due to the
four imidazoles involved in the N-Au-N bonds. The
diastereoscopic nature of the CH2 benzyl groups compli-
Supporting Information Available: The .cif files, the IR
spectra for the aromatic and the aliphatic C-H stretches, an
the 1H NMR of complexes 2 and 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
(44) (a) Choudhary, T. V.; Sivadinarayana, C.; Chusuei, C. C.; Datye, A.
K.; Fackler, J. P., Jr.; Goodmann, D. W. J. Catal. 2002, 297, 247. (b) Yan, Z.;
Sivadinarayana, C.; Mohamed, A. A.; Fackler, J. P., Jr.; Goodman, D. W. Catal.
Lett. 2006, 111, 15–18.
(45) Roundhill, D. M.; Fackler, J. P., Jr. Optoelectronic Properties of
Inorganic Compounds; Plenum Press: New York, 1999.
cate further analysis relative to any Au H-C interac-
3 3 3
tions.
The interest in tetranuclear gold clusters is due to their
potential application as precursors for CO and olefin