Synthesis and X-ray structures of dinuclear and trinuclear gold(I) and dinuclear gold(II) amidinate complexes

Article abstract of DOI:10.1021/ic048855q

The structures of the trinuclear gold(I), [Au₃(2,6-Me ₂-form)₂-(THT)Cl], the dinuclear [Au₂(2,6- Me₂-form)₂], and the oxidative-addition product [Au ₂(2,6-Me₂-form)

Full text of DOI:10.1021/ic048855q

Inorg. Chem. 2005, 44, 166−168
Synthesis and X-ray Structures of Dinuclear and Trinuclear Gold(I) and
Dinuclear Gold(II) Amidinate Complexes†
Hanan E. Abdou, Ahmed A. Mohamed, and John P. Fackler, Jr.*
Laboratory for Molecular Structure and Bonding, Department of Chemistry,
Texas A&M UniVersity, College Station, Texas 77843-3255
Received August 19, 2004
Scheme 1
The structures of the trinuclear gold(I), [Au3(2,6-Me2-form)2-
(THT)Cl], the dinuclear [Au2(2,6-Me2-form)2], and the oxidative-
addition product [Au2(2,6-Me2-form)2Cl2] formamidinate complexes
are reported. The trinuclear complex is stable with gold
−gold
distances 3.01 and 3.55 Å. The gold gold distance in the dinuclear
−
complex decreases upon oxidative-addition with halogens from 2.7
to 2.5 Å, similar to observations made with the dithiolates and
ylides.
Anionic, bridging, nitrogen bonded ligands display a rich
chemistry with gold(I), not unlike the extensive chemistry
developed with ylide ligands.1 Furthermore, small molecular
gold clusters which serve as catalyst precursors for CO and
olefin oxidation have prompted additional interest in gold
cluster syntheses, especially with cluster precursors not
containing elements that are potential catalyst poisons.2 The
amidinates contain only C, H, N, and sometimes O and/or
halogens. Gold(I) cluster compounds also have attracted
much attention due to their intriguing structural diversity and
short metal-metal aurophilic bonding interactions. This is
the case for tetranuclear gold(I) amidinate complexes where
short Au‚‚‚Au contacts are often observed.3-7 Visible pho-
toluminescence under UV excitation also is present.
Among the best known examples are the “paddlewheels”.9
Upon attempting to synthesize gold(I) complexes with the
ligand Hpp, 1,3,4,6,7,8-hexahydro-pyrimido[1,2-a]pyrimidi-
nate, used successfully by Cotton9 to form [M2]4+ paddle-
wheel complexes, an oxidized, dinuclear Au(II) product was
isolated.10 This compound formed spontaneously upon react-
ing the anionic ligand with the (THT)AuCl in CH2Cl2. It
has the shortest Au-Au (2.47 Å) distance recorded to
date. By removing the halogenated solvent from the reaction,
using THF, the same product was obtained in about 50%
yield, along with Au metal. Thus, dinuclear amidinate
complexes support the formation of metal-metal bonded
Au(II)-Au(II) products.
The tetranuclear gold(I) clusters [Au4(ArNC(H)NAr)4], Ar
) C6H4-4-OMe,3 C6H3-3,5-Cl2,3 C6H4-4-Me,3 C6H4-2-CF3,7
C6F5,7 and C10H7, 7 were synthesized in ∼55-80% yield by
reaction of sodium or potassium amidinate with gold(I)
(3) Mohamed, A. A.; Abdou, H. E.; Irwin, M. D.; Lo´pez-de-Luzuriaga,
J. M.; Fackler, J. P., Jr. J. Cluster Sci. 2003, 14, 253-266.
(4) Beck, J.; Strahle, J. Angew. Chem., Int. Ed. Engl. 1986, 25, 95.
(5) Mohamed, A. A.; Lo´pez-de-Luzuriaga, J. M.; Fackler, J. P., Jr. J.
Cluster Sci. 2003, 14, 61.
(6) Yang, G.; Raptis, R. G. Inorg. Chem. Acta 2003, 352, 98-104.
(7) Abdou, H. E.; Mohamed, A. A.; Lo´pez-de-Luzuriaga, J. M.; Fackler,
J. P., Jr. J. Cluster Sci. 2004, 15, 397-411.
Formamidines, ArNHC(H)NAr, are capable of forming
flexible coordination modes (see graphic with structures) with
metal ions, which lead to various molecular arrangements.8
* To whom correspondence should be addressed. E-mail: fackler@
mail.chem.tamu.edu. Fax: (979) 845-2373. Tel: (979) 845-0648.
† Dedicated to Professor F. A. Cotton on the occasion of his 75th birthday.
(1) Grohmann, A.; Schmidbaur, H. ComprehensiVe Organometallic
Chemistry, II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: New York, 1995; Vol 3.
(8) (a) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219-300
and references therein. (b) Patai, S. The Chemistry of Amidines and
Imidates; John Wiley and Sons: New York, 1975; Vol. 1.
(9) (a) Clerac, R.; Cotton, F. A.; Dunbar, K. R.; Murillo, C. A.; Wang,
X. Inorg. Chem. 2001, 40, 420-426. (b) Cotton; F. A.; Lin, C.;
Murillo, C. A. Inorg. Chem. 2000, 39, 4574-4578.
(2) Choudhary, V.; Sivadinarayana, C.; Chusuei, C. C.; Datye, A. A. k.;
Fackler, J. P., Jr.; Goodman, D. W. J. Catal. 2002, 297, 247.
(10) Irwin, M. D.; Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Chem.
Commun. 2003, 2882-2883.
166 Inorganic Chemistry, Vol. 44, No. 2, 2005
10.1021/ic048855q CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/21/2004

COMMUNICATION
Figure 2. Thermal ellipsoid drawing of 2. Selected distances (Å)
and angles (deg): Au(1)-N(1) 2.035(7), Au(1)‚‚‚Au(2) 2.711(3);
N(1)-Au(1)-N(3) 170.2(3).
Figure 1. Thermal ellipsoid drawing of 1. Selected distances (Å)
and angles (deg): Au(3)-Cl(1) 2.258(6), Au(3)‚‚‚Au(2) 3.0181(10),
Au(1)-N(1) 2.044(16), Au(1)-S(1) 2.245(6), Au(1)‚‚‚Au(2) 3.0132(12),
N(3)-Au(2)-N(2) 167.5(7).
chemistry, a trinuclear species forms when methyl groups
are in the ortho position. Similar results are obtained when
ortho isopropyl groups are used.12 In gold(I) carbeniate and
imidazolate chemistry, the trinuclear species are the only
neutral complexes isolated to date.13
chloride in THF (Scheme 1). The base-stabilized tetranuclear
gold(I) 3,5-diphenylpyrazolate cluster [(dppm)2Au4(Ph2pz)2]-
(NO3)2 also has been reported.5 The four gold atoms are
located at the corners of a rhomboid with the amidinate
ligands bridging above and below the near plane of the four
Au(I) atoms. The average Au‚‚‚Au distance is ∼3.0 Å,
typical of Au(I)‚‚‚Au(I) aurophilic interactions. Similar
structural arrangements have been found in the tetrameric
1,3-diphenyltriazenidogold(I) complex, [Au4(PhNNNPh)4]
(Au‚‚‚Au ) 2.85 Å),4 and the tetranuclear gold pyrazo-
late complex [Au4(µ-4-tBu-pz)4] (Au‚‚‚Au ) 3.1155(7)-
3.1886(7) Å).6
The dinuclear complex, 2, [Au2(2,6-Me2-form)2], is iso-
lated in quantitative yield by the reaction of (THT)AuCl and
the potassium salt of 2,6-Me2-form in a 1:1 stoichiometric
ratio (Supporting Information). The Au‚‚‚Au distance is
2.711(3) Å, and the N-Au-N angle is 170.2(3)° (Figure
2). To our knowledge, there is only one other example of a
symmetrically bridged dinuclear gold(I) nitrogen complex,
{Au2[(Me3SiN)2C(Ph)]2} with Au‚‚‚Au ) 2.646 Å.14
Few nitrogen ligand complexes of gold(II) are known
compared with the more common gold(I) and gold(III)
species. The gold(II) complexes generally are synthesized
by oxidative addition of halogens or pseudo-halogens to the
dinuclear gold(I) species, often forming darkly colored
products.15-17 The dihalide amidinate species (vida infra)
release the halogen upon gentle heating.
When 1 mmol of Cl2, Br2, or I2 is added to 1 mmol of
[Au2(2,6-Me2-form)2] in THF at room temperature, an
immediate color change from colorless to a very dark green
or brown occurs. Well-shaped dark orange or brown block
crystals grow out of the solution by slow evaporation.
Thermal gravimetric analysis and differential thermal analysis
show the release of solvent of crystallization followed by
the loss of the halogen. X-ray structures of the products (X
) Cl, Br, I) show slight differences in the Au‚‚‚Au distances,
from 2.71 Å in 2 to 2.51-2.57 Å in the gold(II) species.
The Au-X distances, Au-Cl ) 2.36 Å, Au-Br ) 2.47 Å,
Au-I ) 2.68 Å, are normal. The oxidized products also can
A trinuclear species [Au3(2,6-Me2-form)2(THT)Cl] is
isolated when a sterically bulky formamidinate ligand is used
such as the 2,6-dimethyl derivative (Supporting Information).
By the reaction of the potassium salt of the formamidinate
ligand and (THT)AuCl in a (1:1.5) stoichiometric ratio,
the trinuclear species forms and is isolated as monoclinic
(P21/n), prismatic crystals.11 Attempts to obtain a tetranuclear
gold species with this ligand, using different stoichiometric
ratios, were not successful, and instead, a dinuclear species
1
is isolated (vide infra). H NMR of the trinuclear species
shows the characteristic resonances of the tetrahydrothiophene
ligand. The complex is stable at room temperature and in
light for months. The structure contains two short Au‚‚‚Au
distances of ∼3.01 Å and a long Au‚‚‚Au distance of 3.55
Å (Figure 1). The Au-S distance is 2.245(6) Å, and
Au-Cl is 2.258(6) Å.
The chemistry of gold(I) pyrazolates includes various
species such as trinuclear, tetranuclear, and hexanuclear com-
pounds, with the trinuclear product being the most common.
A tetranuclear gold(I) pyrazolate compound forms when a
bulky group such as tert-butyl is used.6 In the amidinate
(12) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Unpublished results.
(13) Burini, A.; Mohamed, A. A.; Fackler, J. P., Jr. Comments Inorg. Chem.
2003, 24, 253-280.
(14) Fenske, D.; Baum, G.; Zinn, A.; Dehnicke, K. Z. Naturforsch., B:
Chem. Sci. 1990, 45, 1273-1278
(11) Crystal data for 1, C38H46Au3ClN4S: Mr ) 1217.22, monoclinic, space
group P21/n, a ) 10.8294(6) Å, b ) 21.4450(13) Å, c ) 16.7756(10)
Å, â ) 99.5270(10)°, V ) 3842.2(4) Å3, Z ) 4, R1 ) 0.0858, and
wR2 ) 0.1671. Crystal data for 2, C34H38Au2N4: Mr ) 896.62,
triclinic, space group P1h, a ) 7.354(6) Å, b ) 9.661(7) Å, c ) 11.421-
(10) Å, R ) 81.74(5)°, â ) 99.5270(10)°, γ ) 86.07(9)°, V ) 760.1-
(4) Å3, Z ) 1, R1 ) 0.0465, and wR2 ) 0.1243. Crystal data for 3,
C34H38Au2Cl2N4‚1.5C6H12: Mr ) 1093.76, monoclinic, space group
P21/c, a ) 11.012(2) Å, b ) 18.464(4) Å, c ) 19.467(4) Å, â )
94.86(3)°, V ) 3943.7(14) Å3, Z ) 4, R1 ) 0.0689, wR2 ) 0.1707.
(15) (a) Murray, H. H.; Raptis, R. G.; Fackler, J. P., Jr. Inorg. Chem. 1988,
27, 26-33. (b) Raptis R. G.; Fackler J. P., Jr. Inorg. Chem. 1988, 27,
4179-4182. (c) Laguna, A.; Laguna, M. Coord. Chem. ReV. 1999,
193-195, 837-856.
(16) (a) Mazany, A. M.; Fackler, J. P., Jr. J. Am. Chem. Soc. 1984, 106,
801. (b) Dudis, D. S. Ph.D. Thesis, Case Western Reserve University,
Cleveland, OH, 1984. (c) Raptis, R. G.; Porter, L. C.; Emrich, R. J.;
Murray, H. H.; Fackler, J. P., Jr. Inorg. Chem. 1990, 29, 4408 and
references therein.
(17) Fackler, J. P., Jr. Polyhedron 1997, 16, 1-17.
Inorganic Chemistry, Vol. 44, No. 2, 2005 167

COMMUNICATION
CH2(S)PPh2; X ) Cl, Br, I) have short Au(II)-Au(II) bonds
with distances around 2.6 Å. The Au(I) precursors have
gold-gold distances close to 3.0 Å. This dramatic decrease
in the Au-Au distance is a result of the formation of two
Au-X bonds and a single Au-Au bond along the common
axis, forming stable, tetracoordinated d9-d9 Au(II) centers.17
While theoretical and experimental data exist relating the
aurophilic distance to “softness” of the ligands,19 recent
density functional theory calculations produce unexpected
results with these N ligands.20 The ground state for the
dinuclear gold(I) species is calculated to be a HOMO with
δ metal-metal antibonding, not σ metal-metal antibonding
as had been observed with the ylides.21
Figure 3. Thermal ellipsoid drawing of 3. Selected bond distances (Å)
and angles (deg): Au(1)-N(1) 2.020(8), Au(1)-Cl(1) 2.356(2),
Au(1)‚‚‚Au(2) 2.5176(7); N(1)-Au(1)-N(3) 174.9(3), N(1)-C(1)-N(2)
123.9(9).
Acknowledgment. The Robert A. Welch Foundation of
Houston, Texas, is acknowledged for financial support.
Supporting Information Available: X-ray crystallographic
files, in CIF format, for complexes 1, 2, and 3. Experimental details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
be isolated by using various halogenating agents such CH2X2,
CX4, or XCH2CH2X. Aqua regia also chlorinates 2 (Figure
3). An unsymmetrical product is obtained when methyl
iodide, CH3I, is added to 2 at a low temperature, in the
absence of light and oxygen.18 The dinuclear Au(II) com-
plexes [Au(CH2)PPh2X]2 and [Au(MTP)X]2 (MTP )
IC048855Q
(19) Pyykko, P. Chem. ReV. 1997, 97, 597.
(20) Perez, L. M. Private communication.
(21) Carlson, T. G.; Fackler, J. P., Jr.; Staples, R. J.; Winpenny, R. E. Inorg.
Chem. 1995, 34, 426.
(18) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Z. Naturforsch., B:
Chem. Sci. 2004, 59B, in proof.
168 Inorganic Chemistry, Vol. 44, No. 2, 2005