Syntheses of mixed-ligand tetranuclear gold(I)-nitrogen clusters by ligand exchange reactions with the dinuclear gold(I) formamidinate complex Au 2(2,6-Me2Ph-form)2

Article abstract of DOI:10.1021/ic0612449

The reaction of the sterically crowded dinuclear gold(I) amidinate complex Au₂(2,6-Me₂Ph-form)₂, 1, with the less bulky bidentate nitrogen ligands results in the formation of tetranuclear gold(I) complexes. When the less bulky amidinate, K(4-MePh-form), A, was reacted with 1 in a 1:1 stoichiometric ratio, crystals containing equal amounts of the tetranuclear and dinuclear gold(I) aryl formamidinates, Au₄(4-MePh- form)₄ and Au₂(2,6-Me₂Ph-form)₂, where 2,6-Me₂Ph-form = B, were found in the same unit cell, 2·2THF: space group P1, a = 10.794(11) A, b = 14.392-(15) A, c = 25.75(3) A, α = 82.564(17)°, β = 85.443(18)°, γ = 82.614(19)°. The reaction of K(4-MePh-form), A, and 1 in a 1:2 ratio (excess) produced the tetranuclear complex only, 3. The potassium salt of the exchanged bulky ligand, K(2,6-Me₂Ph-form), formed as a byproduct. The reaction of the dinuclear gold(I) complex Au₂(2,6-Me ₂Ph-form)₂ with the 3,5-diphenylpyrazolate salt, K(3,5-Ph₂pz), resulted in the formation of two tetranuclear mixed-ligand complexes, Au₄(3,5-Ph₂pz)₂(2,6- Me₂Ph-form)₂·2THF, 4·2THF (space group P2₁/c, a = 11.5747(19) A, b = 25.497(4) A, c = 21.221(3) A, β = 96.979(3)°) and Au₄(3,5-Ph₂pz) ₃(2,6-Me₂Ph-form)·THF, 5·THF (space group P2₁/c, a = 23.058(5) A, b = 14.314(3) A, c = 18.528(4) A, β = 90.94(3)°. The block crystals from the tetranuclear complex, 4·2THF, contain mixed ligands with each pyrazolate ring facing an amidinate ring. The tetranuclear mixed ligand complex, 5·THF, was isolated as needles with ligands alternating above and below the Au₄ plane. The two tetranuclear mixed-ligand complexes emit at 490 and 530 nm, respectively, under UV excitation.

Full text of DOI:10.1021/ic0612449

Inorg. Chem. 2007, 46, 141−146
Syntheses of Mixed-Ligand Tetranuclear Gold(I)−Nitrogen Clusters by
Ligand Exchange Reactions with the Dinuclear Gold(I) Formamidinate
Complex Au2(2,6-Me2Ph-form)2
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 July 5, 2006
The reaction of the sterically crowded dinuclear gold(I) amidinate complex Au2(2,6-Me2Ph-form)2, 1, with the less
bulky bidentate nitrogen ligands results in the formation of tetranuclear gold(I) complexes. When the less bulky
amidinate, K(4-MePh-form), A, was reacted with 1 in a 1:1 stoichiometric ratio, crystals containing equal amounts
of the tetranuclear and dinuclear gold(I) aryl formamidinates, Au4(4-MePh-form)4 and Au2(2,6-Me2Ph-form)2, where
2,6-Me2Ph-form
)
B, were found in the same unit cell, 2
‚
2THF: space group P1h, a ) 10.794(11) Å, b ) 14.392-
(15) Å, c 25.75(3) Å, R ) 82.564(17)
)
°
,
â ) 85.443(18)
°
,
γ ) 82.614(19) . The reaction of K(4-MePh-form),
°
A, and 1 in a 1:2 ratio (excess) produced the tetranuclear complex only, 3. The potassium salt of the exchanged
bulky ligand, K(2,6-Me2Ph-form), formed as a byproduct. The reaction of the dinuclear gold(I) complex Au2(2,6-
Me2Ph-form)2 with the 3,5-diphenylpyrazolate salt, K(3,5-Ph2pz), resulted in the formation of two tetranuclear mixed-
ligand complexes, Au4(3,5-Ph2pz)2(2,6-Me2Ph-form)2
‚
2THF, 4
‚
2THF (space group P21/c, a
THF, 5
. The block crystals from the tetranuclear
)
11.5747(19) Å, b
)
25.497(4) Å, c
P21/c, a 23.058(5) Å, b
complex, 4
ligand complex, 5
tetranuclear mixed-ligand complexes emit at 490 and 530 nm, respectively, under UV excitation.
)
21.221(3) Å, â ) 96.979(3)
°
) and Au4(3,5-Ph2pz)3(2,6-Me2Ph-form)
‚
‚
THF (space group
)
)
14.314(3) Å, c
)
18.528(4) Å, â ) 90.94(3)°
‚2THF, contain mixed ligands with each pyrazolate ring facing an amidinate ring. The tetranuclear mixed
‚THF, was isolated as needles with ligands alternating above and below the Au4 plane. The two
Introduction
short ligand N‚‚‚N bite distance suggested that the formation
of dinuclear gold complexes would require very short gold-
gold distances. Indeed this proved to be true, and as
communicated,4-6 dinuclear gold products have been ob-
tained with Au(I)‚‚‚Au(I) distances of about 2.7 Å and Au-
(II)-Au(II) distances of about 2.5 Å. The first compounds
obtained were new tetranuclear species,8 and later the
trinuclear and the dinuclear gold(I) complexes were ob-
tained.4 With sterically bulky groups in the ortho positions
of the aryl groups in ArNH(CH)NAr, such as Ar ) 2,6-
(CH3)2-C6H3 (2,6-Me2Ph) and 2,6-(C3H7)2-C6H3 (2,6-
It is often assumed that the “soft” metal ion gold(I) will
not effectively coordinate to the “hard” element nitrogen.1
The dearth of compounds of gold(I) coordinated to nitrogen
seems to bear this out.2 Nitrogen coordination to gold(I) has
produced interesting chemistry with pyrazolates, carbeniates,
benzylimidazolates,3 and recently, amidinates and related
ligands.4-8 Gold(I) compounds with amidinates and related
ligands had not been formed until recently. Furthermore, the
* To whom correspondence should be addressed. E-mail:
fackler@mail.chem.tamu.edu. Fax: (979) 845-2373. Tel: (979) 845-0548.
(1) Schmidbaur, H., Ed. Gold: Progress in Chemistry, Biochemistry and
Technology; John Wiley & Sons: Chichester, U.K., 1999.
(2) (a) Beck, J.; Strahle, J. Angew. Chem., Int. Ed. Engl. 1986, 25, 95.
(b) See also refs 3, 4, and 5.
(3) (a) Burini, A.; Mohamed, A. A.; Fackler, J. P., Jr. Comments Inorg.
Chem. 2003, 24, 253-280. (b) Mohamed, A. A.; Rawashdeh-Omary,
M. A.; Omary, M. A.; Fackler, J. P., Jr. Dalton 2005, 15, 2597-
2602. (c) Omary, M. A.; Mohamed, A. A.; Rawashdeh-Omary, M.
A.; Fackler, J. P., Jr. Coord. Chem. ReV. 2005, 249, 1372-1381.
(4) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Inorg. Chem. 2005,
44, 166-168.
(5) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Z. Naturforsch. B:
Chem. Sci. 2004, 59, 1480-1482.
(6) Irwin, M. D.; Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Chem.
Commun. 2003, 2882-2883.
(7) Mohamed, A. A.; Abdou, H. E.; Fackler, J. P., Jr. Inorg. Chem. 2006,
45, 11-13.
(8) 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. Abdou, H.
E.; Mohamed, A. A.; Lo´pez-de-Luzuriaga, J. M.; Fackler, J. P., Jr. J.
Cluster Sci. 2004, 15, 397-411.
10.1021/ic0612449 CCC: $37.00
Published on Web 12/10/2006
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 1, 2007 141

Abdou et al.
Shimadzu UV-2501 PC spectrometer. 1H spectra were recorded on
a Unity Plus 300 NMR spectrometer using the CHCl3 in the solvent
peak to reference the chemical shifts (δ). Emission and excitation
spectra were recorded on a SLM AMINCO Model 8100 spectrof-
luorometer equipped with a xenon lamp. Spectra were corrected
for instrumental response. Solid-state low-temperature measure-
ments were made using a cryogenic sample holder of local design.
Powder samples were attached to the holder with a mixture of
copper powder, Cryogen oil (used for mounting crystals for X-ray
structures), and collodion (an ether- and alcohol-soluble transparent
nitrocellulose). The glue was scanned for a baseline subtraction.
Liquid nitrogen was used to obtain the 77 K measurements using
a cell of local design. Mass spectrometry data (electrospray
ionization) were recorded at the Laboratory for Biological Mass
Spectrometry at Texas A&M University, using an MDS Series Qstar
Pulsar with a spray voltage of 5 keV.
(iPr)2Ph), the formation of the tetranuclear species is blocked
and isolation of di- and trinuclear gold(I) amidinates is
achieved.4
This new class of gold complexes show several features
such as reversible electrochemical oxidation, variable nucle-
arities,4,8 high catalytic activity as precursors for CO oxida-
tion,9 2-D supramolecular formation with Hg(CN)2,7 sus-
ceptibility to oxidative addition,4,5 luminescence at room
temperature, and formation of mixed gold-silver metal
complexes.8,10 A combination of electronic and steric factors
are probably involved in each of these properties.11 The
versatility of the system in that substituents on the phenyl
rings can be varied, which allows easy adjustment of the
electronic and steric properties of the complexes.
The yields of products obtained in the syntheses below are not
reported but are generally between 15 and 60%, based on recovery
of isolated, crystallographically characterized material. No attempt
was made to optimize the yields, which requires separation of the
partially soluble salts formed in the reaction from any residual
starting materials.
Synthesis of [Au2(2,6-Me2Ph-form)2][Au4(4-MePh-form)4], 2.
N,N′-di(4-Me)phenylformamidine (56 mg, 0.25 mmol) was stirred
with (14 mg, 0.25 mmol) of KOH in 15-20 mL of THF for 24 h.
Gradually, the colorless solution turned yellow. [Au2(2,6-Me2Ph-
form)2] (220 mg, 0.25 mmol) was added, and the mixture was stirred
for an additional 12 h. The solvent was removed under vacuum,
and the crude product was collected. The exchanged salt of the
ligand was extracted with water-ethanol after dissolution of the
crude product in CH2Cl2. The product 2‚2THF was recrystallized
from THF/hexanes.
Analysis. 1H NMR (298 K, CDCl3): δ 2.23 (s, 24H (CH3,
tetranuclear)), 6.84 (d, 16H (CH, phenyl tetranuclear)), 7.05 (br,
16H (CH, phenyl tetranuclear)), 8.25 (s, 4H (CH, amidinate
tetranuclear)), 2.47 (s, 24H (CH3, dinuclear)), 6.93 (br, 12H (CH,
phenyl dinuclear)), 7.43 (s, 2H (CH, amidinate dinuclear)).
Preparation of [Au4(4-MePh-form)4], 3. Although this com-
pound was reported previously by our group,8 the synthesis reported
here starts with a different gold complex. N,N′-di(4-Me)phenyl-
formamidine (226 mg, 1 mmol) was stirred with 56 mg (1 mmol)
of KOH in 15-20 mL of THF for 24 h. The colorless solution
turned yellow. [Au2(2,6-Me2Ph-form)2] (450 mg, 0.5 mmol) was
added, and the mixture was stirred for an additional 24 h. The
solvent was removed under vacuum, and the crude product was
collected. The exchanged salt of the ligand was extracted with
water-ethanol, 1:1, after dissolution of the crude product in CH2-
Cl2. Product 3 was recrystallized from THF/hexanes.
Analysis. 1H NMR (298 K, (CDCl3): δ 6.86 (d, 16H (CH, phenyl
tetranuclear)), 7.02 (br, 16H (CH, phenyl tetranuclear)), 8.25 (s,
4H (CH amidinate)), 2.23 (s, 24H (CH3)). Complex 3 was prepared
previously by the reaction of K(4-MePh-form), A, with Au(THT)-
Cl.8
Preparation of [Au4(3,5-Ph2pz)2(2,6-Me2Ph-form)2], 4. 3,5-
Diphenylpyrazole (110 mg, 0.5 mmol) was stirred with 20 mg (0.5
mmol) of KOH in 15-20 mL of THF for 24 h. Au2(2,6-Me2Ph-
form)2 (450 mg, 0.5 mmol) was added, and the mixture was stirred
for an additional 12 h. The solvent was removed under vacuum,
and the crude product was collected. The exchanged salt of the
ligand was extracted with water-ethanol after dissolution of the
crude product in CH2Cl2. The product was recrystallized from
THF-hexanes to give a mixture of complex 4‚2THF, Au4(Ph2pz)2-
(2,6-Me2Ph-form)2‚2THF, and 5‚THF, Au4(3,5-Ph2pz)3(2,6-Me2Ph-
In this paper, we describe new work which changes the
nuclearity of the sterically crowded dinuclear gold(I) amidi-
nate complex [Au2(2,6-Me2Ph-form)2], 1, by ligand exchange
with the anionic ligand 3,5-diphenylpyrazole and other less
bulky substituted amidinates, A, to form tetranuclear species.
Density functional theory calculations show that the tet-
rameric structure is more stable than the dimeric structure
for these gold(I) amidinates at both Gaussian 98 and ADF
levels, by about 20 kcal/mol. The reaction of less bulky
anionic ligands with the dinuclear gold(I) amidinate complex,
1, causes rearrangement to form more stable tetranuclear
gold(I) complexes. Previous studies in gold(I) chemistry,
wherein ligand exchange results in an increase in metal
nuclearity from two to four, have not been reported.
Preliminary results of the luminescence studies of these
materials also are presented.
Experimental Section
General Procedures. All glassware were oven-dried prior to
use. Triethyl orthoformate (orthoester), p-toluidine, 2,6-dimethy-
laniline, o-anisidine (4-methoxyaniline), 3,5-diphenylpyrazole, KOH,
and NaOH were purchased from Aldrich. Tetrahydrothiophene was
purchased from TCI, Tokyo. The solvents, THF, CH2Cl2, hexanes,
toluene, ethanol, and ethyl ether, were purchased from Aldrich and
used as received. The dinuclear gold(I) amidinate complex, Au2(2,6-
Me2Ph-form)2, was prepared as described previously.4 Trinuclear
silver(I) pyrazolate, [Ag(µ-3,5-Ph2Pz)]3, was prepared following a
literature procedure.12 Elemental analyses were performed by
Guelph Chemical laboratories Ltd. and Chemisar Laboratories Inc.,
Guelph, Ontario, Canada. UV-vis spectra were recorded on a
(9) (a) Mohamed, A. A.; Burini, A.; Fackler, Jr. J. Am. Chem. Soc. 2005,
127, 5012-5013. (b) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler,
J. P., Jr.; Goodman, D. W. J. Am. Chem. Soc. 2005, 127, 1604-1605.
(10) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Unpublished results.
(11) Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219-300 and
references cited therein. (b) Patai, S. The Chemistry of Amidines and
Imidates; John Wiley and Sons: New York, 1975; Vol. 1.
(12) (a) Murray, H. H.; Raptis, R. G.; Fackler, J. P., Jr. Inorg. Chem. 1988,
27, 26-33. (b) Mohamed, A. A.; Perez, L. M.; Fackler, J. P., Jr. Inorg.
Chim. Acta. 2005, 358, 1657-1662.
142 Inorganic Chemistry, Vol. 46, No. 1, 2007

Mixed-Ligand Tetranuclear Au(I)-N Clusters
Table 1. Details of X-ray Data Collection and Structure Refinement
for Complexes 2‚2THF, 4‚2THF, and 5‚THF
form), in ∼25:75% ratio. The block crystals from 4‚2THF, which
give a pink emission at room temperature (RT) when irradiated
with UV light, were separated with a spatula from the needle
crystals, 5‚THF.
2‚2THF
4‚2THF
5‚THF
empirical
formula
fw
cryst syst
space group P1h
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
C102H114Au6N12O2 C72H76Au4N8O2 C66H60Au4N8O
Anal. Calcd for 4‚2THF (C72H76N8Au4O2): C, 46.15; H, 4.05.
1
2721.65
triclinic
1872
monoclinic
P21/c
1768
monoclinic
P21/c
Found: C, 48.13; H, 3.84. H NMR (298 K, CDCl3): δ 8.11 (br,
8H (Ph rings pyrazolate)), 7.84 (t, 12H (Ph ring pyrazolate)), 6.69
(s, 2H (CH pyrazolate)), 2.50 (s, 24H (CH3 amidinate)), 6.94-
7.02 (br, 12H (CH, phenyl dinuclear)), 7.45 (br, 2H (CH amidi-
nate)).
Preparation of [Au4(3,5-Ph2pz)3(2,6-Me2Ph-form)], 5. 3,5-
Diphenylpyrazole (80 mg, 0.38 mmol) was stirred with 22 mg (0.38
mmol) of KOH in 15-20 mL of THF for 24 h. Au2(2,6-Me2Ph-
form)2 (220 mg, 0.25 mmol) was added, and the mixture was stirred
for an additional 12 h. The solvent was removed under vacuum,
and the crude product was collected. The exchanged salt of the
ligand was extracted with water-ethanol after dissolution of the
crude product in CH2Cl2. The product 5‚THF, which emits light
blue/green, was recrystallized from THF-hexanes.
10.794(11)
11.5747(19)
25.497(4)
21.221(3)
90.00
96.979(3)
90.00
23.058(5)
14.314(3)
18.528(4)
90.00
90.94(3)
90.00
14.392(15)
25.75(3)
82.564(17)
85.443(18)
82.614(19)
3926(7)
2
6216.3(18)
4
6114(2)
4
dcalcd
1.922
1.950
1.922
(g/cm-3
)
µ (mm-1
R1,
)
9.363
9.462
9.616
0.0924,
0.1595
0.985
0.0566,
0.1227
0.770
0.0589,
0.1374
1.178
wR2
GOFc
Anal. Calcd. for 5‚THF (C66H60N8Au4O): C, 44.79; H, 3.39.
Found: C, 44.61; H, 2.67. 1H NMR for 5 (298 K, CDCl3): δ 8.12-
8.15 (d, 12H (ph ring pyrazolate)), 7.82-7.87 (t, 18H (ph ring
pyrazolate)), 6.75 (s, 3H (CH pyrazolate)), 6.87-6.93 (br, 6H (CH,
phenyl dinuclear)), 7.47 (s, 1H (CH amidinate)), 2.50 (s, 12H (CH3
amidinate)).
Table 2. Selected Bond Distances (Å) and Angles (deg) for 2‚2THF,
4‚2THF, and 5‚THF
complex 2
Au(1)‚‚‚Au(2)
Au(2)‚‚‚Au(3)
Au(3)‚‚‚Au(4)
Au(1)‚‚‚Au(4)
Au(5)‚‚‚Au(5A)
Au(1)-N(1)
3.057(2)
2.926(3)
3.007(2)
3.006(3)
2.701(2)
2.044(13)
Au(1)‚‚‚Au(2)‚‚‚Au(3)
Au(4)‚‚‚Au(1)‚‚‚Au(2)
Au(2)‚‚‚Au(3)‚‚‚Au(4)
N(10)-Au(5)-N(9)
N(3)-Au(2)-N(2)
63.47(4)
114.30(5)
118.27(4)
169.0(5)
172.9(5)
174.4(6)
Structure Determinations. Suitable crystals for X-ray diffraction
of 2, 4, and 5 were obtained by slow diffusion of n-hexane into a
concentrated THF solution of the complexes. X-ray data were
collected using a Siemens (Bruker) SMART CCD-based (charge-
coupled device-based) diffractometer equipped with a LT-2 low-
temperature apparatus operating at 110 K. A suitable crystal was
chosen and mounted on a glass fiber using cryogenic grease. Data
were measured using ω scans of 0.3° per frame for 60 s, such that
a hemisphere was collected. The first 50 frames were recollected
at the end of data collection as a monitor for decay. No decay was
detected. Cell parameters were retrieved using SMART software
and refined using SAINT on all observed reflections.13 Data
reductions were performed using SAINT software.14 The structures
were solved by direct methods using SHELXS-97 and refined by
least-squares on F2, with SHELXL-97 incorporated into SHELXTL-
PC, version 5.03.15,16 Hydrogen atom positions were calculated by
geometrical methods and refined as a riding model. Cell parameters
and refinement results of 2‚2THF, 4‚2THF, and 5‚THF are
summarized in Table 1. Unfortunately, the refinement is not optimal
for any of the complexes because of the phenyl ring disorder and
THF solvent loss. The metal atoms and coordinated atoms are well
positioned, but a few H atoms on the THF molecules could not be
refined and a two-position disorder for the phenyl rings is observed
in 5. Selected bond lengths and angles are presented in Table 2.
The crystal structures are shown in Figures 1-3.
N(4)-Au(3)-N(5)
complex 4
Au(1)‚‚‚Au(2)
Au(3)‚‚‚Au(4)
Au(1)‚‚‚Au(4)
Au(2)‚‚‚Au(3)
N(8)-Au(1)-N(6)
3.1149(9)
3.1153(9)
3.1482(9)
3.1020(9)
174.2(5)
Au(2)‚‚‚Au(1)‚‚‚Au(4)
Au(3)‚‚‚Au(2)‚‚‚Au(1)
N(8)-Au(4)-N(6)
Au(3) Au(4) Au(1)
84.62(2)
94.93(2)
174.2(5)
94.00(2)
complex 5
Au(1)-N(1)
2.023(11)
3.0277(9)
3.2075(10) Au(3)‚‚‚Au(2)‚‚‚Au(1)
3.0541(9)
3.1870(10) N(1)-Au(1)-N(8)
N(7)-Au(4)-N(6)
Au(2)‚‚‚Au(3)‚‚‚Au(4)
93.05(3)
93.14(3)
86.13(3)
176.3(5)
174.6(5)
171.8(5)
Au(1)‚‚‚Au(4)
Au(1)‚‚‚Au(2)
Au(2)‚‚‚Au(3)
Au(3)‚‚‚Au(4)
Au(4)‚‚‚Au(1)‚‚‚Au(2)
N(2)-Au(2)-N(3)
Au(2)‚‚‚Au(3)‚‚‚Au(4) 93.05(3)
N(4)-Au(3)-N(5) 174.5(5)
complex Au2(2,6-Me2Ph-form)2, 1, in a 1:1 stoichiometry
ratio in THF forms the mixed crystal of the dinuclear and
tetranuclear complexes, [Au2(2,6-Me2Ph-form)2][Au4(4-
MePh-form)4]‚2THF, 2‚2THF. Each unit cell contains one
tetranuclear and one dinuclear molecule, Figure 1. The
potassium salt of the exchanged, bulky ligand, K(2,6-Me2-
1
Ph-form), forms as a byproduct. The H NMR spectrum of
the crude product shows a spectrum consistent with a mixture
of the two complexes. The addition of excess potassium salt,
A (Me), to an NMR tube charged with complex 2 in CDCl3
results in further exchange of the bulky anionic ligand [2,6-
Me2Ph-form]- in 2 with the formation of complex 3, as was
confirmed by the disappearance of the methine peak in the
1H NMR for 1 at 7.43 ppm. The reaction of A (Me) with 1
in a 1:2 ratio (excess) forms only 3, Chart 1. Similar results
were also obtained from the reaction of the salt K(4-OMePh-
form), A (OMe), with the dinuclear complex, 1.
Results and Discussion
Synthesis. Reacting the potassium salt of the arylamidi-
nate, K(4-MePh-form), A (Me), with the dinuclear gold(I)
(13) SMART, version 4.043; Bruker Analytical X-Ray Systems: Madison,
WI, 1995.
(14) SAINT, version 4.035; Bruker Analytical X-Ray Systems: Madison,
WI, 1995.
(15) Scheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(16) SHELXTL, Program Library for Structure Solution and Molecular
Graphics, version 5.03 (PC version); Bruker Analytical X-ray
Systems: Madison, WI, 1995.
The reaction of the 3,5-diphenylpyrazolate salt, K(3,5-Ph2-
pz), with the dinuclear gold(I) complex 1 in a 1:1 stoichio-
metric ratio results in the formation of two tetranuclear
Inorganic Chemistry, Vol. 46, No. 1, 2007 143

Abdou et al.
Figure 1. X-ray structure of 2‚2THF, [Au2(2,6-Me2Ph-form)2][Au4(4-
MePh-form)4], with the THF removed.
Figure 3. Structure of the metal cluster of 5‚THF, [Au4(3,5-Ph2pz)3(2,6-
Me2Ph-form)]. All phenyl rings and protons of the pyrazolate and
formamidinate ligands are omitted for clarity.
Chart 1
OMePh-form)4 when the potassium amidinate salts react with
Au(THT)Cl.8 These intermediates have not been conclusively
identified (see Supporting Information) but may relate to the
formation of species of lower nuclearity.4
Figure 2. Structure of the metal cluster of 4‚2THF, Au4(3,5-Ph2pz)2(2,6-
Me2Ph-form)2. All phenyl rings and protons of the pyrazolate and
formamidinate ligands are omitted for clarity.
products which crystallize as blocks, [Au4(3,5-Ph2pz)2(2,6-
Me2Ph-form)2]‚2THF, 4‚2THF, and as needles, [Au4(3,5-
Ph2pz)3(2,6-Me2Ph-form)]‚THF, 5‚THF (Figures 2 and 3).
The block crystals from 4‚2THF, which give a red emission
at RT when irradiated with UV light, were separated with a
spatula from the needle crystals, 5‚THF. Attempts to isolate
the tetranuclear complex 4 only by carrying out the reaction
in 1:0.5 of 1/K(3,5-Ph2pz) still produced a mixture of
complexes 4 and 5. Adjusting the reaction ratio to 1:1.5
(excess) of the K(3,5-Ph2pz) resulted in the isolation of only
the tetranuclear mixed ligand complex 5‚THF.
Mass spectra (ESI positive field) of the tetranuclear
complex Au4(4-OMePh-form)4 in CH3CN showed peaks
from [Au2L2]+ (m/z ) 904), [[Au2L2]-OMe]+ (m/z ) 879),
and [[Au2L2]CH3CN]+ (m/z ) 950), in addition to [[Au3L2]-
+
OMe] (m/z ) 1064). The mass spectra of the dinuclear
complex Au2(2,6-Me2Ph-form)2 and the tetranuclear complex
Au4(4-MePh-form)4, 3, produced two similar peaks, although
in low intensity, at m/z ) 701 and 676, respectively, which
probably are related to structures such as [AuL2]+: fw )
701 for L ) (2,6-Me2Ph-form) and fw ) 680 for L ) (4-
MePh-form).
The exchange reaction of the bulky amidinate anionic
ligand, [2,6-Me2Ph-form]-, B, by the less bulky anionic
amidinate ligand A, R ) Me, OMe is irreversible. Further-
more, the tetranuclear clusters 4 and 5 do not react with
sodium pyrazolate to form the well-known trinuclear gold-
(I) pyrazolate complex, [(3,5-Ph2pz)Au]3, Chart 1.12
Although details of the density functional studies carried
out on these molecules are not reported here, both Gaussian
The exchange processes of 1 with bidentate nitrogen ligand
1
salts have been followed by H NMR in a CDCl3 solvent.
In the case of the exchange with K(4-MePh-form), new peaks
appear at 8.6 and 8.07 ppm, while K(3,5-Ph2pz) gives a new
resonance at 7.56 ppm (see Supporting Information). These
new peaks relate to the formation of an intermediate in the
exchange reaction and largely disappear when the reaction
is complete. Similar peaks are seen during the synthesis of
the tetranuclear complexes Au4(4-MePh-form)4 and Au4(4-
144 Inorganic Chemistry, Vol. 46, No. 1, 2007

Mixed-Ligand Tetranuclear Au(I)-N Clusters
98 and ADF level calculations done in collaboration with
Dr. Lisa Perez, suggest that the tetramer (sixteen-membered
ring) is more stable by about 20 kcal/mol than the dimer for
these gold(I) amidinates. Since the Au(I)‚‚‚Au(I) distance is
restricted to a shorter value in the dimer (eight-membered
ring) than the tetramer, the results of the calculations are
not surprising. Furthermore, by introduction of a Si atom in
place of the amidinate C atom, thereby increasing the bite
distance of the ligand, the energy differences between the
dimeric and tetrameric structures reduce to negligible values.
Crystallographic Results. The mixed crystal dinuclear/
tetranuclear complex [Au2(2,6-Me2Ph-form)2][Au4(4-MePh-
form)4]‚2THF, 2‚2THF, crystallizes with both the dinuclear
complex and the tetranuclear complex in the same unit cell,
Figure 1. The needle-like crystals of 2‚2THF are crystallized
in the triclinic space group, P1h, Table 1. The dinuclear
complex itself crystallizes in P1h. The tetranuclear complex
crystallizes in the C2/c space group (Table 1).8 The bond
distances and angles, Au‚‚‚Au ) ∼2.7 Å in the dinuclear
and ∼3.0 Å in the tetranuclear, are similar to those of the
parent complexes whose structures have been reported
previously (Table 2).4,8 The tetranuclear mixed-ligand gold-
(I) complexes, [Au4(Ph2pz)2(2,6-Me2Ph-form)2]‚2THF, 4‚
2THF, and [Au4(Ph2pz)3(2,6-Me2Ph-form)]‚THF, 5‚THF,
crystallize in the space group P21/c (Table 1). The 4‚2THF,
which crystallizes in blocks, contains pyrazolate rings facing
amidinate rings. This anti arrangement avoids steric crowd-
ing, Figure 2. The Au‚‚‚Au distances are ∼3.1 Å, slightly
longer than those found in the gold(I) amidinates [Au4(ArNC-
(H)NAr)4], 2.8-3.0 Å,8 Table 2. The tetranuclear mixed-
ligand complex 5‚THF was isolated as needles with ligands
alternating above and below the Au4 plane, Figure 3. In the
tetranuclear gold(I) pyrazolate complex, Au4[(3,5-t-Bu-pz)4],
reported by Raptis, the Au‚‚‚Au distances range from 3.11
to 3.18 Å,17 while in complexes 4 and 5, the Au‚‚‚Au
distances range from 3.10 to 3.15 Å, and from 3.03 to 3.21
Å, respectively. In these clusters, the Au‚‚‚Au distances for
atoms linked by the pyrazolate ligands are slightly longer,
as expected from the geometry of the ligand, than those
linked by the amidinate ligands.
Figure 4. Emission spectra of 1 (excitation at 343 nm), 2‚2THF (excitation
at 390 nm), and 3‚2THF (excitation at 350 nm) in the solid state at 77 K.
at 465 nm and a weak emission at 530 nm. The luminescence
of 2‚2THF in the solid state is dominated by the emission
from the tetranuclear species. The ligands themselves do not
show any visible emission under these experimental condi-
tions. Luminescence spectra, reported previously, of several
tetranuclear gold(I) amidinate complexes show them to emit
a bright blue-green light under UV excitation, with a strong
emission at ∼490 nm and a weak emission at ∼530 nm in
the solid state, at RT and 77 K. Lifetime measurements for
the emission of the tetranuclear gold(I) amidinates at 77 K
suggest that considerable metal contribution exists in the
lower energy, MMCT emission.8 Preliminary studies based
on the small Stokes shift and the symmetric band profiles
between the excitation and emission spectra suggest that the
high-energy emission at ∼490 nm is a fluorescence.
The emissions observed from various coinage metal-
pyrazolate complexes are usually structured, typical of
ligand-based processes.18 For the gold(I) mixed-ligand
complexes we have studied, there is a vibronic structure in
the emission spectrum corresponding to the (CdN) or the
(NdN) stretching vibrational modes of the pyrazolate ligand.
On the basis of these observations, the emissions appear to
involve ligand-to-metal charge transfer, LMCT, from a
π-electronic ground state of the ligand. This is particularly
true of 1. The tetranuclear gold pyrazolates Au4[(3,5-t-Bu-
pz)4] emit at 541 nm in the solid state at ambient temperature.
The gold pyrazolates 4‚2THF and 5‚THF emit at 490 nm
with a weak emission at 530 nm. The emission at 550 nm is
more apparent in complex 5‚THF than it is in 4‚2THF
(Figure 5). While the dinuclear gold(I) amidinate complex
1 shows a vibronic emission at 77 K at high energy (430
nm), complexes 4‚2THF and 5‚THF give broad emissions
at lower energies. This suggests that the emissions from 4‚
2THF and 5‚THF may have an increased gold contribution
to the transition but further study is needed.
1
Luminescence Studies. Solution studies of 2 using H
NMR and UV-vis techniques show the spectra expected
for the presence of the independent dinuclear and tetranuclear
1
complexes. These results are as follows: H NMR (CDCl3),
methane 8.25 ppm, tetranuclear; 7.43 ppm, dinuclear. The
UV-vis spectrum (CH2Cl2) of 2 shows bands at 265, 310,
and 360 nm for the tetranuclear complex and 255 nm for
the dinuclear species. Complex 2 as a solid shows a blue-
green luminescence under UV radiation at RT and 77 K,
Figure 4. The dinuclear complex, 1, emits at ∼430 nm at 77
K in the solid state but shows no emission at ambient
temperature. The tetranuclear gold(I) complex 3 shows a
bright blue-green luminescence as a solid under UV light,
with a strong emission at ∼490 nm and a weak emission at
∼530 nm at RT and 77 K.8 The emission of solid 2 at RT
and 77 K shows two components: a broad, strong emission
Conclusions
This work describes the syntheses of mixed-ligand tetra-
nuclear gold(I) complexes. These complexes result from the
(18) (a) Omary, M. A.; Rawashdeh-Omary, M. A.; Diyabalanage, H. V.;
Dias, H. V. Inorg. Chem. 2003, 42, 8612-8614. (b) Dias, H. V. R.;
Diyabalanage, H. V. K.; Eldabaja, M. G.; Elbjeirami, O.; Rawashdeh-
Omary, M. A.; Omary, M. A. J. Am. Chem. Soc. 2005, 127, 7489-
7501.
(17) Yang, G.; Raptis, R. G. Inorg. Chim. Acta 2003, 352, 98-104.
Inorganic Chemistry, Vol. 46, No. 1, 2007 145

Abdou et al.
of a short Au(I)‚‚‚Au(I) distance. Ligand exchange with less
sterically bulky ligands provides a facile procedure for the
synthesis of tetranuclear mixed-ligand complexes. Recent
results from our laboratory have demonstrated the utilization
of this approach for the synthesis of tetranuclear mixed-metal
mixed-ligand gold-silver complexes containing pyrazolates
and amidinates.10
Acknowledgment. The Robert A. Welch Foundation of
Houston, Texas is acknowledged for financial support of this
work. Lisa Perez also is acknowledged with thanks for the
DFT calculations reported here, and Jose´ M. Lo´pez-
de-Luzuriaga for some of the emission spectra.
Figure 5. Emission spectra with excitation at 425 nm of 4‚2THF and
5‚THF in the solid state at 77 K, although crystal separation is
incomplete.
reaction of less bulky amidinates and pyrazolates with the
bulky ligand dinuclear Au2(2,6-Me2Ph-form)2, 1. The steric
effect of the bulky ligands can block the formation of the
tetranuclear species, allowing for isolation of the dinuclear
and trinuclear complexes. The DFT calculations suggest that
the tetranuclear complexes are energetically more stable than
the dinuclear complexes since the latter requires the presence
Supporting Information Available: X-ray crystallographic files
in CIF format for complexes 3‚2THF, 4‚2THF, and 5‚THF and
the NMR spectra related to the formation of intermediates in the
exchange. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0612449
146 Inorganic Chemistry, Vol. 46, No. 1, 2007