Molecular and electronic structure of square-planar gold complexes containing two 1,2-di(4-tert-butylphenyl)ethylene-1,2-dithiolato ligands: [Au(2L)2]1+/0/1-/2-. A combined experimental and computational study

Article abstract of DOI:10.1021/ic061181u

From the reaction of in situ generated 1,2-di(4-tert-butylphenyl)ethylene- 1,2-dithiol, ²LH₂, and Na[AuCl₄]·H ₂O in 1,4-dioxane, green brown crystals of diamagnetic [N(n-Bu) ₄][Au^III(²L)₂] (1) were obtained. As shown by cyclic voltammetry, 1 is a member of an electron-transfer series comprising the dianion [Au^II(²L)₂] ^2-, the monoanion [Au^III(²L)₂] ^-, the neutral species [Au^III(²L ^?)(²L)]⁰ ? [Au^III( ²L)(²L^?)]⁰, and the monocation [Au^III(²L^?)₂]⁺. (²L^?)^1- represents the π radical anion (S_rad = 1/2) of the one-electron oxidized closed-shell dianion ( ²L)^2-. Oxidation of 1 in CH₂Cl₂ with ferrocenium hexafluorophosphate affords green, paramagnetic microcrystals of [Au^III(²L^?)(²L)] ? [Au^III(²L)(²L^?)] (2) (S = 1/2). Complexes 1 and 2 have been characterized by X-ray crystallography. Both species possess square-planar monoanions and neutral molecules, respectively. From the oxidation reaction of 1 or [N(n-Bu)₄][Au^III( ³L)₂] with 2-3 equiv of [NO]BF₄ in CH ₂Cl₂, a green solution of [Au^III( ²L^?)₂]⁺ and green microcrystals of [Au^III(³L^?)₂]BF₄ (3) were obtained, respectively; (³L)^2- represents the dianion 1,2-di(4-diphenyl)ethylene-1,2-dithiolate, and (³L ^?)^1- is its π radical monoanion. The electronic structures of this series of gold species have been elucidated by UV-vis, EPR spectroscopies, and DFT calculations. It is shown computationally by density functional theoretical (DFT) methods that the electronic structure of [Au ^III(¹L^?)₂]⁺ is best described as a singlet diradical (S_t = 0); the ligand mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a Au^III ion and two closed-shell dianionic ligands; and the corresponding dianions [Au(L)₂]^2- are best described as an intermediate Au^II/Au^III species with a metal-ligand delocalized SOMO (25% Au 5d, 75% 3p of four S atoms). (¹L) ^2- is the dianion 1,2-di(phenyl)ethylene-1,2-dithiolate, and ( ¹L^?)^1- is the π radical monoanion. The neutral species [Pd^II(²L^?)₂] (4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as described for [Au^III( ¹L^?)₂]⁺ (singlet diradical), whereas that of the monoanion [Pd^II(²L^?) (²L)]^- ? [Pd(²L)(²L ^?)]^- corresponds to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [Au^III(mnt) ₂]^-, where mnt = maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt^?)] (E_1/2 = 0.91 V vs Fc ⁺/Fc). The optical and EPR spectroscopies of [Au(mnt)(mnt ^?)] were consistent with those observed for the corresponding di(tert-butylphenyl)-ethylenedithiolate complex 2.

Full text of DOI:10.1021/ic061181u

Inorg. Chem. 2007, 46, 1100−1111
Molecular and Electronic Structure of Square-Planar Gold Complexes
Containing Two 1,2-Di(4-tert-butylphenyl)ethylene-1,2-dithiolato Ligands:
+/0/1-/2-
[Au(2L)2]1
. A Combined Experimental and Computational Study
Swarnalatha Kokatam,† Kallol Ray,† Joseph Pap,† Eckhard Bill,† William E. Geiger,‡ Robert J. LeSuer,‡
Philip H. Rieger,§ Thomas Weyhermuller,† Frank Neese,† and Karl Wieghardt*,†
1
Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an der
Ruhr, Germany, Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405, and
Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912
Received June 28, 2006
From the reaction of in situ generated 1,2-di(4-tert-butylphenyl)ethylene-1,2-dithiol, 2LH2, and Na[AuCl4]
‚2H2O in
1,4-dioxane, green brown crystals of diamagnetic [N(n-Bu)4][AuIII(2L)2] (1) were obtained. As shown by cyclic
voltammetry, 1 is a member of an electron-transfer series comprising the dianion [AuII(2L)2]2-, the monoanion
-
[AuIII(2L)2]-, the neutral species [AuIII(2L•)(2L)]0 T [AuIII(2L)(2L•)]0, and the monocation [AuIII(2L•)2]+. (2L•)1 represents
the
with ferrocenium hexafluorophosphate affords green, paramagnetic microcrystals of [AuIII(2L•)(2L)]
(2) (S
1/2). Complexes 1 and 2 have been characterized by X-ray crystallography. Both species possess square-
planar monoanions and neutral molecules, respectively. From the oxidation reaction of 1 or [N(n-Bu)4][AuIII(3L)2]
with 2
3 equiv of [NO]BF4 in CH2Cl2, a green solution of [AuIII(2L•)2]+ and green microcrystals of [AuIII(3L•)2]BF4 (3)
were obtained, respectively; (3L)2 represents the dianion 1,2-di(4-diphenyl)ethylene-1,2-dithiolate, and (3L•)1 is
its radical monoanion. The electronic structures of this series of gold species have been elucidated by UV vis,
EPR spectroscopies, and DFT calculations. It is shown computationally by density functional theoretical (DFT)
π radical anion (Srad )
1/2) of the one-electron oxidized closed-shell dianion (2L)2-. Oxidation of 1 in CH2Cl2
T
[AuIII(2L)(2L•)]
)
−
-
-
π
−
methods that the electronic structure of [AuIII(1L•)2]+ is best described as a singlet diradical (St
) 0); the ligand
mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a AuIII ion and
-
two closed-shell dianionic ligands; and the corresponding dianions [Au(L)2]2 are best described as an intermediate
AuII/AuIII species with a metal
−
ligand delocalized SOMO (25% Au 5d, 75% 3p of four S atoms). (1L)2 is the
-
-
dianion 1,2-di(phenyl)ethylene-1,2-dithiolate, and (1L•)1 is the
π
radical monoanion. The neutral species [PdII(2L•)2]
(4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as
described for [AuIII(1L•)2]+ (singlet diradical), whereas that of the monoanion [PdII(2L•)(2L)]- [Pd(2L)(2L•)]- corresponds
to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [AuIII(mnt)2]-, where mnt
T
)
maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt•)] (E1
)
0.91 V vs Fc+/Fc). The optical and EPR
/
2
spectroscopies of [Au(mnt)(mnt•)] were consistent with those observed for the corresponding di(tert-butylphenyl)-
ethylenedithiolate complex 2.
Introduction
reported.1 (In this paper, we will designate specific dithiolene
ligands as (mL)2- or (mL•)1- with m ) 1, 2, or 3, whereas
simple (L) is a generic representation of dithiolene ligands
irrespective of their oxidation level.) The majority of these
species are diamagnetic, monoanionic [AuIII(L)2]- complexes
that possess a central diamagnetic Au(III) ion with a d8
Square-planar bis(dithiolene)gold complexes, [Au(L)2]n (n
) 0, 1- , 2-), have been investigated in depth in the past,
and their structural and spectroscopic properties have been
* To whom correespondence should be addressed. E-mail: wieghardt@
mpi-muelheim.mpg.de.
† Max-Planck-Institut fu¨r Bioanorganische Chemie.
‡ University of Vermont.
(1) Dithiolene Chemistry. Stiefel, E. I., Ed. Progr. Inorg. Chem. 2004,
52, 1-681.
§ Brown University.
1100 Inorganic Chemistry, Vol. 46, No. 4, 2007
10.1021/ic061181u CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/19/2007

Molecular Structure of Square-Planar Gold Complexes
Scheme 1
has been concluded that the SOMO possesses ∼10% Au
character in agreement with relatively large 197Au hyperfine
coupling constants (AuII character).6,7
We present here a new electron-transfer series, which
involves the new, bulky dithiolene ligand 1,2-di(4-tert-
butylphenyl)ethylene-1,2-dithiolene (2LH2):8 [Au(2L)2]z (z )
1+, 0, 1-, 2-) (Scheme 1). For the first time, a mono-
cationic species has been isolated and characterized, [AuIII-
(3L•)2](BF4) (3), where (3L)2- represents the dianion 1,2-di-
(4-diphenyl)ethylene-1,2-dithiolate(2-), and (3L•)1- is its π
radical monoanion. In light of the recent previous work,5
we anticipated an electronic structure for this species that
involves a diamagnetic Au(III) central ion and two antifer-
romagnetically coupled π radical ligand monoanions yielding
an St ) 0 ground state: [Au(3L•)2]+. For comparison, we
have also prepared the isoelectronic, diamagnetic, but neutral
complex [PdII(2L•)2] (4) and structurally characterized it.
We report the experimentally established electronic struc-
tures of each member of the electron-transfer series [Au-
(L)2]z (z ) 1+, 0, 1-, 2-) and show that dithiolene ligands
exist as closed-shell diamagnetic dianions, (L)2-, or as their
one-electron oxidized π radical monoanion, (L•)1-, in such
complexes.
We have also performed DFT calculations on each
member of the series, which establish unequivocally that the
diamagnetic monocation [AuIII(1L•)2]+ is a singlet diradical.
(1L)2- represents the dianion 1,2-di(phenyl)ethylene-1,2-
dithiolate, and (1L•)1- is its π radical monoanion. Time-
dependent DFT calculations have allowed the assignment
of transitions in the electronic spectra in the near-infrared
region as an intervalence charge transfer (IVCT) band in
[AuIII(1L•)(1L)] T [AuIII(1L)(1L•)] and a ligand-to-ligand CT
band (LLCT) in [Au(1L•)2]+. The EPR spectral parameters
of the neutral and dianionic species have also been calculated.
electron configuration and two S,S-coordinated, closed-shell
benzene-1,2-dithiolate(2-) or, alternatively, two 1,2-disub-
stituted ethylene-1,2-dithiolate(2-) ligands (Scheme 1).
Recently, Garner et al.2 have reported a series of such
structurally characterized complexes containing two asym-
metric dithiolato ligands.
It has been discovered electrochemically that these mono-
anions represent the central member of an electron-transfer
series [Au(L)2]z (z ) 2-, 1-, 0).3 A neutral complex has
been structurally characterized, [AuIII(bdt•)(bdt)] T [AuIII-
(bdt)(bdt•)], where (bdt•)1- represents the π radical mono-
anion of benzene-1,2-dithiolate, (bdt)2-.4 For this and the
corresponding 3,4-di-tert-butylbenzene-1,2-dithiolate, (LBu•)1-,
containing neutral complex,5 an electronic structure has been
elucidated where a central, diamagnetic Au(III) ion is
coordinated to a π radical monoanion and a closed-shell
dianion. The ligand mixed valency had been clearly estab-
lished as being delocalized (class III). The latter species is
paramagnetic (St )1/2 ground state), and its EPR spectrum
has been reported.5 We report here the EPR spectrum of
[AuIII(mnt•)(mnt)]0 T [AuIII(mnt)(mnt•)]0, where (mnt)2-
represents maleonitriledithiolate(2-), and (mnt•)1- is its π
radical anion. For the neutral benzene-1,2-dithiolate species
[AuIII(LBu•)(LBu)], no resolved 197Au hyperfine coupling has
been reported,5 indicating a small Au character of the SOMO.
In a few instances, the paramagnetic dianion [AuII(L)2]2-
(St )1/2) has been investigated by EPR spectroscopy, and it
Experimental Section
The complex [N(n-Bu)4][AuIII(mnt)2] was prepared as described
in the literature.9a Reflux of 4,4′-di-tert-butylbenzoin with P4S10 in
1,4-dioxane affords the ligand 1,2-di-(4-tert-butylphenyl)-ethylene-
1,2-dithiolene, (2LH2), in situ according to refs 8 and 9b. The same
procedure has been used for the synthesis of 1,2-di(4-diphenyl)-
ethylene-1,2-dithiol, (3LH2).
[N(n-Bu)4][Au(2L)2]‚2.25 CH2Cl2 (1). To a solution of 1,4-
dioxane (5 mL) and 4,4′-di-tert-butylbenzoin (0.41 g; 1.25 mmol)
was added P4S10 (0.4 g; 0.91 mmol). After the solution was heated
to reflux for 2.5 h in the presence of air, the solution was cooled
to 20 °C, and an excess of P4S10 was removed by filtration. To the
resulting yellow solution was added an aqueous solution (1 mL)
of Na[AuCl4]‚2H2O (0.22 g; 0.56 mmol). The mixture was heated
to reflux for 1 h, after which time a green-brown precipitate was
removed from the green solution by filtration. The solution was
(2) Tunney, J. M.; Blake, A. J.; Davies, E. S.; McMaster, J.; Wilson, C.;
Garner, C. D. Polyhedron 2006, 25, 591.
(6) Schlupp, R. L.; Maki, A. H. Inorg. Chem. 1974, 13, 44.
(7) (a) Van Rens, J. G. M.; Viegers, M. P. A.; de Boer, E. Chem. Phys.
Lett. 1974, 28, 104. (b) Ihlo, L.; Sto¨sser, R.; Hofbauer, W.; Bo¨ttcher,
R.; Kirmse, R. Z. Naturforsch. 1999, 54b, 597.
(8) Patra, A. K.; Bill, E.; Weyhermu¨ller, T.; Stobie, K.; Bell, Z.; Ward,
M. D.; McCleverty, J. A.; Wieghardt, K. Inorg. Chem. 2006, 45, 6541.
(9) (a) Waters, J. H.; Gray, H. B. J. Am. Chem. Soc. 1965, 87, 3534. (b)
Schrauzer, G. N.; Mayweg, V. P.; Heinrich, W. Inorg. Chem. 1965,
4, 1615. (c) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1965,
87, 1483.
(3) (a) Wang, K. Progr. Inorg. Chem. 2006, 52, 267 and references therein.
(b) Balch, A. L.; Dance, I. G.; Holm, R. M. J. Am. Chem. Soc. 1968,
90, 1139. (c) Schiødt, N. C.; Sommer-Larsen, P.; Bjørnholm, T.;
Folmer Nielsen, M.; Larsen, J.; Bechgaard, K. Inorg. Chem. 1995,
34, 3688. (d) Nakamoto, M. Kagaku to Kogyo (Osaka) 1994, 68, 463.
(4) Rindorf, G.; Thorup, N.; Bjørnholm, T.; Bechgaard, K. Acta Crys-
tallogr., Sect. C 1990, C46, 1437.
(5) Ray, K.; Wehyermu¨ller, T.; Goosens, A.; Craje´, M. W. J.; Wieghardt,
K. Inorg. Chem. 2003, 42, 4082.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1101

Kokatam et al.
Table 1. Crystallographic Data for 1‚2.25CH2Cl2, 2‚CH2Cl2,
then layered with 3 mL of an ethanolic solution of [N(n-Bu)4]Br
(0.19 g; 0.56 mmol). Within 24 h at 20 °C a microcrystalline, yellow
precipitate was obtained, which was filtered off and washed with
n-hexane. Recrystallization from a CH2Cl2/n-hexane (1:1 v/v)
mixture afforded yellow crystals suitable for X-ray crystallography.
Yield: 80 mg (13%). ESI mass spectrum: pos. ions m/z ) 242
(100%) [N(n-Bu)4]+; neg. ion m/z ) 905 (100%) [Au(2L)2]-. Anal.
Calcd for C60H88NS4Au: C, 63.52; H, 8.78; N, 1.23; S, 11.30; Au,
17.36. Found: C, 63.7; H, 8.5; N, 0.9; S, 10.7; Au, 16.9.
[Au(2L)(2L•)]‚CH2Cl2 (2). To a degassed solution of 1 (35 mg;
0.03 mmol) in CH2Cl2 (5 mL) was added ferrocenium hexafluo-
rophosphate, [Fc]PF6 (11 mg; 0.03 mmol), under an Ar blanketing
atmosphere. The mixture was stirred at 20 °C for 2 h, after which
time a green solution was obtained. Green microcrystals precipitated
upon layering of the solution with dry methanol. Yield: 21 mg.
Anal. Calcd for C44H52S4Au: C, 58.32; H, 5.78; S, 14.15; Au, 21.74.
Found: C, 58.1; H, 5.7; S, 14.0; Au, 21.6.
[Au(3L•)2](BF4) (3). The synthesis of [N(n-Bu)4][AuIII(3L)2] has
been executed in a fashion analogous to that described above for 1
by using 4,4′-diphenylbenzoin as starting material. The compound
has not been characterized in detail but was used for the following
oxidation reaction. To a degassed, dry solution of [N(n-Bu)4][AuIII-
(3L)2] (30 mg; 0.024 mmol) in CH2Cl2 (4 mL) was added [NO]-
BF4 (8 mg; 0.07 mmol) under an Ar blanketing atmosphere. The
mixture was stirred at 20 °C for 1.5 h, yielding a dark green solution
from which upon layering with n-hexane a green precipitate was
obtained. Yield: 0.01 g (40%). ESI-mass spectrum: pos. ion mode
m/z ) 985 {Au(3L)2}+; negative ion mode m/z ) 87 [BF4]-. Anal.
Calcd for C52H36S4AuBF4: C, 58.21; H, 3.38; S, 11.96. Found: C,
58.4; H, 3.3; S, 11.8.
4‚0.5Toluene
1‚2.25CH2Cl2
2‚CH2Cl2
4‚0.5toluene
chem formula
C62.25H92.5AuCl4.5NS4 C45H54AuCl2S4 C47.5H56PdS4
fw
1339.60
990.99
861.56
space group
a, Å
b, Å
c, Å
P-1, No. 2
17.7270(4)
17.8283(4)
24.6899(6)
69.394(5)
89.774(5)
65.875(5)
6575.9(3)
4
P41212, No. 92 Pna21, No. 33
11.705(1)
11.705(1)
64.778(5)
90
11.9728(5)
35.8262(14)
11.1766(4)
90
R, deg
â, deg
90
90
γ, deg
90
90
V, Å
Z
8875.0(13)
8
4794.1(3)
4
T, K
100(2)
1.353
181 310/70.0
100(2)
1.483
17 723/45.0
4515/2630
254/7
14.83/0.71073 5.90/0.71073
0.0746/1.029
0.0971
100(2)
1.194
25 566/52.0
8749/7571
503/1
F calcd, g cm-3
refln collected/2θmax
unique refln/I > 2σ(I) 57 536/50 115
no. of params/restr.
1405/77
µ, cm-1/λ, Å
25.84/0.71073
0.0338/1.033
0.0784
R1a/GOFb
0.0395/1.041
0.0804
+0.59/-0.64
wR2c (I > 2σ(I))
residual density, e Å-3 +3.18/-2.69
+0.86/-1.04
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b GOF )
2
2
2
2
2
[∑[w(Fo - Fc )2]/(n - p)]1/2
.
c wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
,
2
2
2
where w ) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc )/3.
contains three crystallographically independent complex anions of
which two reside on a center of inversion, two NBu4 cations and
4.5 dichloromethane solvent molecules. One of the NBu4 cations
in 1 was found to be disordered and was split on two positions in
a ratio of about 83:17. Two tertiary butyl groups attached to C(42)
and C(66) were also split in ratios of 57:43 and 59:41, respectively.
The dichloromethane molecule containing C(900) was given an
occupation factor of 0.5 because it is found to be disordered next
to a crystallographic inversion center. Geometries of the split models
were restrained using EADP, SAME, and SADI instructions of
ShelXL9711 (77 restraints). The dichloromethane molecule in 2 is
disordered and was split in a ratio of 59:41. Displacement
parameters and C-Cl and Cl-Cl distances were restrained to be
equal. The toluene solvent molecule in crystals of 4 was not fully
occupied, and an occupation factor of 0.5 was used.
Calculations. All calculations in this work were performed with
the electronic structure program ORCA.12 The geometry optimiza-
tions were carried out at the BP86 level of DFT.13 Because this
work is concerned with heavy transition metal complexes of Au
and Pd, scalar relativistic corrections were included using the
second-order Douglas-Kroll-Hess (DKH2)14 and zeroth-order
regular approximation (ZORA)15a method.
[Pd(2L•)2] (4). 4,4′-tert-Butylbenzoin (1.37 g; 4 mmol) was
refluxed for 3 h with 1.25 g (2.82 mmol) of P4S10 in 8 mL of xylene.
PdCl2 (0.176 g; 1 mmol) suspended in 20 mL of a methanol/water
mixture (10:1) was added to the cooled and filtered solution. After
8 h of stirring at room temperature, the precipitated brown powder
was filtered off and washed with methanol. Yield: 0.38 g (47%).
Anal. Calcd for C44H52S4Pd: C, 64.80; H, 6.43. Found: C, 64.85;
H, 6.58. ESI mass spectrum (CH2Cl2 solution): m/z ) 814.4 [Pd-
(L)2]+. Single crystals of [Pd(L)2]‚0.5toluene suitable for X-ray
crystallography were grown from a toluene solution by slow
evaporation of the solvent.
X-ray Crystallographic Data Collection and Refinement of
the Structures. A yellow single crystal of 1‚2.25CH2Cl2, brown
crystals of 2‚CH2Cl2, and 4‚0.5toluene were coated with perfluo-
ropolyether and picked up with a glass fiber. The crystals were
immediately mounted in the nitrogen cold stream (100 K) of a
Nonius Kappa-CCD diffractometer equipped with a Mo-target
rotating-anode X-ray source and a graphite monochromator (Mo
KR, λ ) 0.71073 Å). Final cell constants were obtained from least-
squares fits of all measured reflections. Crystallographic data of
the compounds are listed in Table 1. The Siemens ShelXTL10
software package was used for solution and artwork of the structure,
and ShelXL9711 was used for the refinement. The structures were
readily solved by direct and Patterson methods and subsequent
difference Fourier techniques. All non-hydrogen atoms in 1 and 4
were refined anisotropically. The crystals of 2 were extremely small,
and, due to a poor data-parameter ratio, only gold, sulfur, and
chlorine atoms were anisotropically refined. Hydrogen atoms were
placed at calculated positions and refined as riding atoms with
isotropic displacement parameters. The asymmetric unit in 1
In the geometry optimizations, the one-center approximation was
used, which eliminates DKH2 contributions to the analytic gradients.
In the context of the ZORA approximation, it was shown that the
one-center approximation only introduces minor errors into a final
geometries.15b Large uncontracted Gaussian basis sets were used
at the metal center, which were derived from the well-tempered
basis sets of Huzinaga.16 For the remaining atoms, the all-electron
polarized triple-ê (TZVP)17 Gaussian basis sets of the Ahlrichs
(12) Neese, F. ORCA an Ab Initio, Density Functional and Semiempirical
Electronic Structure Program Package, Version 2.4, revision 36; Max-
Planck-Institut fu¨r Bioanorganische Chemie: Mu¨lheim/Ruhr, Germany,
May 2005.
(13) (a) Becke, A. D. J. Chem. Phys. 1988, 84, 4524. (b) Perdew, J. P.
Phys. ReV. B 1986, 33, 8522.
(14) Hess, B. A.; Marian, C. M. In Computational Molecular Spectroscopy;
Jensen, P., Bunker, P. R., Eds.; John Wiley & Sons: New York, 2000;
p 169 ff.
(15) (a) van Lenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys.
1996, 105, 6505. (b) van Lenthe, E.; Faas, S.; Snijders, J. G. Chem.
Phys. Lett. 2000, 328, 107.
(10) ShelXTL, V.5; Siemens Analytical X-Ray Instruments, Inc., 1994.
(11) Sheldrick, G. M. ShelXL97; Universita¨t Go¨ttingen, 1997.
1102 Inorganic Chemistry, Vol. 46, No. 4, 2007

Molecular Structure of Square-Planar Gold Complexes
group were used in an uncontracted form to allow for a distortion
of the inner shell orbitals in the presence of the relativistic potential.
All calculated structures were verified to represent local minima
by calculation of harmonic frequencies. No negative frequencies
were observed. The s2 expectation value for [Au(1L)(1L•)]0 is
0.75118, and for [Au(1L)2]2- it is 0.75152.
The property calculations at the optimized geometries were done
with the B3LYP functional.18 In this case, the same basis sets were
used, but the scalar-relativistic ZORA method was used because
in this formalism magnetic properties are more readily formulated.19a
For the ZORA calculations, the method of van Wu¨llen19b is
implemented in ORCA. Care was taken to ensure accurate
numerical integration in the presence of steep basis functions.
Details of the methods used to calculate the g-tensor and metal
hyperfine couplings can be found in ref 20a-c. Because the recently
implemented SOMF (spin orbit mean-field) orperator20c is presently
only available in the Breit-Pauli version, which is known to be
unsatisfactory for heavy elements, we have resorted to the effective
charge SOC (spin orbit coupling) Hamiltonian of Koseki et al.20d,e
However, because we have performed all-electron calculations using
scalar relativistic corrections, the effective nuclear charges for Pd
and Au needed to be adjusted. From comparison between calculated
SOC matrix elements with effective one-electron SOC constants,
the values Zeff(Pd) ) 14.3 and Zeff(Au) ) 27.5 were determined,
which lead to ênd values of ∼1300 cm-1 (Pd) and ∼4500 cm-1
(Au), respectively. All other effective nuclear charges had their
default values. TD-DFT calculations were carried out according to
ref 20f. Natural population analysis (NPA)21a-c was done through
an interface of ORCA to the Gennbo program version 5.0.21d
Isocontour plots were done with the ChemBats3D program.22
Figure 1. Infrared spectra of solid 1 (in black), 2 (in red), and 3 (in blue)
in the range 500-1800 cm-1 (KBr disks).
Results and Discussion
Syntheses and Crystal Structures. Upon heating a 1,4-
dioxane solution of 4,4′-di-tert-butylbenzoin or 4,4′-diphen-
ylbenzoin and P4S10 for 3 h to reflux, the ligand 1,2-di(4-
tert-butylphenyl)ethylene-1,2-dithiolene, 2LH2, or 3LH2 was
generated in situ.8,9 The addition of Na[AuCl4]‚2H2O and
[N(n-Bu)4]Br and heating to reflux initiated the precipitation
of yellow, diamagnetic crystals of [N(n-Bu)4][Au(2L)2] (1).
If PdCl2 was used as a starting material, the blue neutral
complex [Pd(2L•)2] (4) was obtained in crystalline form.
Complex 4 was also diamagnetic as was judged from its 1H
NMR spectrum.
The reaction of 1 with 1 equiv of ferrocenium hexafluo-
rophosphate in CH2Cl2 yielded green crystals of [Au(2L)-
(2L•)]‚CH2Cl2 (2). This complex is paramagnetic. Temperature-
dependent (4-300 K) magnetic susceptibility measurements
in an 1.0 T magnetic field on a solid sample of 2 revealed
a temperature-independent (50-300 K) magnetic moment
Physical Measurements. Electronic spectra of complexes in
CH2Cl2 solution were recorded at Mu¨lheim on a scanning double
beam UV-vis-NIR spectrophotometer (Perkin-Elmer, Lambda 19)
in the range 200-2000 nm, and at Vermont on an Olis Cary-14
spectrometer. Cyclic voltammograms and coulometric measure-
ments were performed by using an EG&G potentiostat/galvanostat
(model 273A). X-band EPR spectra at Mu¨lheim were recorded on
a Bruker ELEXSYS E500 spectrometer equipped with a He flow
cryostat (Oxford Instruments ESR 910) and Hewlett-Packard
frequency counter HP 5253B. Those recorded at Vermont were
obtained with a Bruker 300E instrument. The spectra were simulated
on the basis of a spin-Hamiltonian description of the electronic
ground state with S )1/2:
1
of 1.72 µB, indicating an S ) /2 ground state for 2.
Figure 1 displays the infrared spectra of 1, 2, and 3 (KBr
disks) in the range 500-1800 cm-1. The spectra of 1 and 2
exhibit many features in common. Notable differences
include very strong bands at 1375, 1149, and 857 cm-1 in
the spectrum of 2. The bands at 1149 and 1143 cm-1 are
unique in the spectra of 2 and 3, respectively; it is assigned
to a predominantly ν(CdS•) stretching mode.1,5 This mode
is indicative of the presence of a coordinated π radical
monoanion (L•)1-; it has also been identified for mono-
anionic, S,S-coordinated 3,5-di-tert-butyl-benzene-1,2-di-
thiolate(1-) π radicals.5,24,25 It has previously been observed
for [AuIII(LBu)(LBu•)] T [AuIII(LBu•)(LBu)] at 1085 cm-1. The
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
Hˆ ) µBBB‚g‚BS - gNµNBB‚BI + BS‚A‚BI + BI‚P‚BI
by using the simulation package XSOPHE.23
(16) (a) Huzinaga, S.; Miguel, B. Chem. Phys. Lett. 1990, 175, 289. (b)
Huzinaga, S.; Klobukowski, M. Chem. Phys. Lett. 1993, 212, 260.
(17) (a) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.
(b) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,
5289. (c) Ahlrichs, R.; May, K. Phys. Chem. Chem. Phys. 2000, 2,
943-945.
(18) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(19) (a) van Lenthe, E.; van der Avoird, A.; Wormer, P. E. S. J. Chem.
Phys. 1998, 108, 4783. (b) van Wu¨llen, C. J. Chem. Phys. 1998, 109,
392.
(20) (a) Neese, F. J. Chem. Phys. 2001, 115, 11080. (b) Neese, F. J. Chem.
Phys. 2003, 118, 3939. (c) Neese, F. J. Chem. Phys. 2005, 122, 034107.
(d) Koseki, S.; Gordon, M. S.; Schmidt, M. W.; Matsunaga, N. J.
Phys. Chem. 1995, 99, 12764. (e) Koseki, S.; Schmidt, M. W.; Gordon,
M. S. J. Phys. Chem. A 1998, 102, 10430. (f) Neese, F.; Olbrich, G.
Chem. Phys. Lett. 2002, 362, 170.
(21) (a) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066. (b)
Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735. (c) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV.
1988, 88, 899. (d) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.;
Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold F. NBO
5.0; Theoretical Chemistry Institute, University of Wisconsin: Madi-
son, WI, 2001; http://www.chem.wisc.edu/∼nbo5.
(22) Chem.Bats3d, version 6.0; CambridgeSoft.com, 2000.
(23) Hanson, G. R.; Gates, K. E.; Noble, C. J.; Griffin, M.; Mitchell, A.;
Benson, S. J. Inorg. Biochem. 2004, 98, 903.
(24) Ray, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. Inorg. Chem.
2005, 44, 5345.
(25) Petrenko, T.; Ray, K.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc.
2006, 128, 4422.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1103

Kokatam et al.
Figure 3. Structure of the neutral [PdII(2L•)2] molecule in crystals of 4.
Figure 2. Structure of the anion [AuIII(2L)2]- in crystals of 1. The structure
of the neutral species [AuIII(2L)(2L•)] T [AuIII(2L•)(2L)] in crystals of 2 is
similar and not shown.
Scheme 2
IR spectrum of 4 displays this ν(CdS•) band also at 1149
cm-1; for [Pd(S2C2Ph2)2], it has been observed at 1136
cm-1.9c
When the above oxidation reaction was carried out on
[N(n-Bu)4][AuIII(3L)2] in CH2Cl2 solution with 2-3 equiv
of [NO]BF4, a dark green solution was obtained from which
upon layering with n-hexane a dark green precipitate of
[AuIII(3L•)2](BF4) (3) was obtained in 40% yield. Complex
3 is diamagnetic and displays an intense ν(CdS•) stretching
mode at 1149 cm-1 (Figure 1), which is absent in the IR
spectrum of [N(n-Bu)4][AuIII(3L)2]. The electronic spectrum
of 3 recorded in CH2Cl2 displays the same set of absorption
maxima as that of electrochemically generated [AuIII(2L•)2]+
(see below and Table 3).
Table 2. Selected Average Bond Distances and Standard Deviations in
1, 2, and 4 in angstroms
complex
1
2
4
M-S
C-S
2.3148(5)
1.769(2)
1.355(3)
2.291(5)
1.749(20)
1.36(2)
2.251(1)
1.708(4)
1.399(5)
C
1-C2
tions discussed below show that one-electron oxidation of 1
to 2 does not involve the central AuIII ion (no AuIV in 2).
On the other hand, the av C-S bond distance in 1 at 1.769
( 0.006 Å is long and indicates a closed-shell dianion (2L)2-,
whereas in 4 it is short at 1.708 ( 0.01 Å. A straightforward
explanation of this trend is as follows: in 1 there are an
AuIII center and two closed-shell dithiolato(2-) ligands
present.24 The observed long C-S distance is in excellent
agreement with a number of structures containing bis-
(dithiolato)gold(III) monoanions reported recently by Garner
et al.2 In 4, there are a PdII central ion and two monoanionic
π radical ligands present with a short av C-S bond at 1.71
( 0.01 Å. Scheme 2 summarizes the important C-S and
olefinic C-C bond distances in the closed-shell dianionic
form (1L)2- and its π radical monoanion (1L•)1- as was
determined by X-ray crystallography.26 In excellent agree-
ment with the above model is the behavior found for the
olefinic C-C bond distances in 1 and 4. In 1, this bond
length is at an av of 1.35 ( 0.01 Å; it has considerable double
bond character. In contrast, this bond in 4 is found at an av
1.40 ( 0.01 Å, which is significantly longer than in 1. The
C-C bonds in 2 cannot be discussed due to the low X-ray
Figure 2 shows the structure of the monoanion [AuIII(2L)2]-
in crystals of 1; that of the neutral molecule [AuIII(2L•)(2L)]
T [AuIII(2L)(2L•)]- in crystals of 2 is similar and not shown.
We note that the quality of the crystal structure determination
of neutral 2 is below standards. The estimated standard
deviations (esd) are too large to allow a discussion of C-C
and C-S bond lengths, but the atom connectivity and square-
planar coordination geometry is unequivocally established.
Table 2 summarizes important bond lengths and angles in 1
and 2. The structure of the neutral molecule [PdII(2L•)2] in
crystals of 4 is shown in Figure 3; selected bond lengths are
again listed in Table 2.
(26) [Pt(1L•)2]: Dessy, G.; Fares, V.; Bellito, C.; Flamini. Cryst. Struct.
Commun. 1982, 11, 1743. [Ni(1L•)2]: Kuramoto, N.; Asao, K. Dyes
Pigm. 1990, 12, 65. [Ni(1L•)2]: Megnamisi-Belombe, M.; Nuber, B.
Bull. Chem. Soc. Jpn. 1989, 62, 4092.
It is noteworthy that the average Au-S bond lengths in 1
at 2.315 ( 0.002 Å and in 2 at 2.291 ( 0.015 Å are very
similar indeed. This observation together with the calcula-
1104 Inorganic Chemistry, Vol. 46, No. 4, 2007

Molecular Structure of Square-Planar Gold Complexes
Figure 4. Cyclic voltammogram of 1 in CH2Cl2 solution (0.10 M [N(n-
Bu)4]PF6) at 25 °C at a scan rate of 200 mV s-1 (conditions: glassy carbon
working electrode; ferrocene (Fc) internal standard).
Figure 6. Electronic spectra of 1 (red), and 2 (blue dashed line), and the
electrochemically generated monocation [AuIII(2L•)2]+ (black dotted line)
in CH2Cl2 solution (0.2 M [N(n-Bu)4]PF6) at -25 °C.
2
E1/2 ) -1.07 V as indicated in eq 2. Both correspond to
successive one-electron reductions generating a mono- and
then a dianion, eq 2. A third quasi-reversible (oxidation)
wave is observed at E1/20 ) +0.60 V, which may correspond
to the [Pd(L)2]1+/[Pd(L)2]0 couple. This process has not been
investigated further.
-e
-e
[PdII(2L•)2]0
[PdII(2L•)(2L)]1-
[PdII(2L) ]2- (2)
\x
2
+e
w
w
+e
1
2
E1/2
E1/2
The oxidation of [Au(mnt)2]- was investigated in CH2Cl2,
using either 0.1 M [NBu4][PF6] or [NBu4][B(C6F5)4] as the
supporting electrolyte with identical results. A diffusion-
controlled Nernstian-shaped one-electron oxidation was
observed (E1/2 ) 0.91 V), the electron-transfer stoichiometry
being confirmed by bulk coulometry. As the coulometric
oxidation proceeded, the solution turned a dark gray, which
is assumed to be the color of neutral [Au(mnt)(mnt•)].
Solutions taken at this point gave optical and EPR spectra
consistent with this premise (vide infra). It was noticed that
a dark gray precipitate formed after completion of the
electrolysis and that this was accelerated at lower tempera-
tures. However, our attempts at isolation of this very fine
precipitate resulted only in its spontaneous reduction back
to the Au(III) monoanion, [Au(mnt)2]-. It has been noted
previously that neutral [M(mnt•)2] complexes (M ) Ni group)
generally display poor solubilities, because of the likely
formation of weakly coordinated oligomers.27 Slow formation
of oligomers would also account for the poor solubility of
the corresponding Au complex.
Figure 5. Cyclic voltammogram of 4 in CH2Cl2 solution (0.10 M [N(n-
Bu)4]PF6) at 25 °C at a scan rate of 400 mV s-1 (conditions: glassy carbon
working electrode; ferrocene internal standard).
quality of the crystals and, consequently, the large error of
(0.1 Å. In Garner’s complexes in ref 2, the av C-C distance
is reported at 1.34 ( 0.01 Å corresponding to an [AuIII(L)2]-
electronic structure with two dithiolato(2-) ligands.
Electrochemistry. Cyclic voltammograms (cv) of 1 and
4 have been recorded in CH2Cl2 solutions containing 0.10
M [(n-Bu)4]PF6 supporting electrolyte at 20 °C at a glassy
carbon working electrode. All potentials are referenced versus
the ferrocenium/ferrocene (Fc+/Fc) couple. The cv of 1 is
shown in Figure 4, and that of 2 is identical to that of 1.
Three fully reversible one-electron-transfer waves have
1
2
been observed at E1/2 ) +0.45 V, E1/2 ) -0.11 V, and
3
E1/2 ) -2.09 V in the cv of 1 and 2. Controlled potential
coulometric measurements established that monoanionic 1
undergoes two successive one-electron oxidations and a one-
electron reduction at a very negative potential, eq 1.
Optical Spectroscopy. Electronic spectra of 1, 2, and the
electrochemically generated monocation [AuIII(2L•)2]+ have
been recorded at -25 °C in CH2Cl2 solutions. The CH2Cl2
solution spectrum of 3 is very similar to that of the
aforementioned cation. The spectrum is shown in Figure 6
and summarized in Table 3. The spectrum of 1 displays two
weak d-d transitions in the visible; no charge transfer (CT)
bands are observed >600 nm in the near-infrared (NIR).
-e
-e
-e
[AuIII(2L•)2]+
[AuIII(2L•)(2L)]0
[AuIII(2L) ]-
\x w\
2
+e +e
w
w
x
+e
1
2
3
E1/2
E1/2
E1/2
[AuII(2L)2]2- (1)
1
The cv of 3 displays the same three redox waves at E1/2
)
+0.51 V, E1/22 ) -0.07 V, and E1/23 ) -2.03 V. The cv of
4 is displayed in Figure 5. Interestingly, only two reversible
electron-transfer waves are observed at E1/21 ) -0.41 V and
(27) Geiger, W. E.; Barrie`re, F.; LeSuer, R. J.; Trupia, S. Inorg. Chem.
2001, 40, 2472.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1105

Kokatam et al.
Table 3. Electronic Spectra of Complexes in CH2Cl2 Solution
complex
λ
max, nm (104 ꢀ, M-1 cm-1
)
ref
[AuIII(2L)2]-
696(0.04), 441(0.4), 331sh
this work
this work
this work
this work
this work
this work
this work
5
[AuIII(2L•)(2L)]0
[AuIII(2L•)2]+
[AuIII(3L•)2]+
[PdII(2L•)2]0 a
[PdII(2L•)(2L)]- a
[PdII(2L)2]2- a
[AuIII(LBu)2]- b
1495(2.12), 1184(0.2), 426(0.84)
1066(4.25), 640(0.6), 383sh
1142(4.2), 687(0.69), 407(3.5)
915(3.7), 270(6.0)
1250(1.7), 296(5.5), 265(5.2)
303(4.3), 245(5.3)
618(0.008), 415(0.02)
[AuIII(LBu•)(1LBu)]0 b 1452(2.7), 1023(0.2), 400(0.6)
5
24
[Pd(LBu•)2]0 b
850(5.0), 620(0.8), 379(1.2)
1140(2.5), 894(0.5), 408(0.1), 340(7.7) 24
[Pd(LBu•)(LBu)]1- b
[Pd(LBu)2]2- b
392(0.6), 350(0.9)
490(0.3), 1110(∼1.0)
24
this work
[Au(mnt•)(mnt)]0
a At -25 °C. b (LBu
)
and (LBu•
)
are derivatives of 3,5-di-tert-
2-
1-
butylbenzene-1,2-dithiol, H2(LBu).
Figure 8. X-band EPR spectra of (a) [Pd(2L•)(2L)]-, (b) [AuIII(2L•)(2L)]0,
and (c) [AuIII(mnt)(mnt•)]0 in frozen CH2Cl2 (at 30 K (a), 90 K (b), and
120 K (c)). Experimental conditions: mw-frequency, (a) 9.43219, (b)
9.42972, (c) 9.46025 GHz; mw-power, (a) 100, (b) 4, (c) 2 µW; modulation
amplitude, (a) 0.7, (b) 0.2, (c) 0.5 mT. The solid lines represent spin-
Hamiltonian simulations using parameters given in Table 4. The line in the
spectrum of (a) is the hyperfine split contribution from 105Pd (I ) 5/2; 22.2%
abundance); electric quadrupole interactions were neglected for 105Pd.
intensity of the LLCT maximum of [M(L•)2]n complexes is
at higher energy and twice as intense as the IVCT band for
Figure 7. Electronic spectra of neutral 4 (black), the monoanion
[PdII(2L•)(2L)]- (red), and the dianion [PdII(2L)2]2- (blue), of which the latter
two were electrochemically generated in CH2Cl2 solution (0.10 M [N(n-
Bu)4]PF6).
the corresponding one-electron reduced form [M(L•)(L)]n-1
.
As reported above for [AuIII(2L)2]-, the corresponding
complex [Pd(2L)2]2- does not exhibit any absorption maxima
>400 nm.
Similar spectra have been reported for other diamagnetic,
square-planar AuIII complexes.
From the above analysis of the electronic spectra, it
appears that the monocationic species [AuIII(2L•)2]+ (and
[AuIII(3L•)2]+) is isostructural and isoelectronic with the
neutral complex [PdII(2L•)2]. Similarly, neutral [AuIII(2L•)-
(2L)] and monoanionic [PdII(2L•)(2L)]1- possess the same
electronic structure as do [AuIII(2L)2]- and [PdII(2L)2]2-.
X-Band EPR Spectroscopy. The cw-X-band EPR spectra
of the neutral gold complexes [Au(2L•)(2L)] (2) and [Au-
(mnt)(mnt•)] in frozen CH2Cl2 solution (0.10 M [N(n-Bu)4]-
PF6) display both anisotropic signals around g ≈ 2 that are
typical of an S ) 1/2 system with rhombic g factors (Figure
8, panels b and c, Table 4).
In contrast, the spectrum of 2 displays a very intense
absorption maximum in the NIR at 1495 nm (ꢀ ) 2.1 × 104
M-1 cm-1) and a weaker transition at 1184 nm (ꢀ ) 0.2 ×
104 M-1 cm-1). These bands have been previously assigned
to an intervalence transition of type [AuIII(L•)(L)] T [AuIII-
(L)(L•)], which corresponds to a spin allowed transition 1b1u
f 2b2g for [AuIII(L•)(L)] (in D2h symmetry). For the
corresponding benzene-1,2-dithiolato complex, this IVCT
band has been observed at 1452 nm (ꢀ ) 2.7 × 104 M-1
cm-1).5 The optical spectrum of [Au(mnt)(mnt•)] has an
intense near-IR absorption at 1110 nm (ꢀ ≈ 104 M-1 cm-1).
It is now quite remarkable that the spectra of the
monocations [AuIII(2L•)2]+ and [AuIII(3L•)2]+ display an
intense ligand-to-ligand CT in the NIR at both 1066 nm (ꢀ
) 4.2 × 104 M-1 cm-1) and 1142 nm (4.2 × 104 M-1 cm-1),
respectively. This is the first time that this transition has been
observed for a gold complex. These LLCT absorption
maxima have been reported for neutral bis(benzene-1,2-
dithiolate)metal(II) complexes (M ) Ni, Pd, Pt).24 The
spectrum of 4 in Figure 7 displays this maximum at 915 nm
(ꢀ ) 3.7 × 104 M-1 cm-1), whereas its reduced form
[PdII(2L•)(2L)]- exhibits this band at 1250 nm (ꢀ ) 1.7 ×
104 M-1 cm-1). It has been noted previously24 that the
The spectra show remarkable hyperfine splitting from the
3
197Au nucleus (I ) /2, 100% abundance), which deviates
from the more normal appearance of the most observed
multiplets in having unusual spacing and intensity distribu-
tion of the hyperfine lines. Similar anomalies have been
observed previously for [AuII(dithio-carbamate)2]28,29 and the
[Au(mnt)2]2- dianion.6,30,32 The unusual features are caused
by a sizable electric field gradient (efg) at the 197Au nucleus
(28) Va¨ngard, T.; Akerstro¨m, S. Nature 1959, 184, 183.
(29) Willigen, H. V.; Rens, J. G. M. Chem. Phys. Lett. 1968, 2, 283.
(30) Rens, J. G. M.; Boers, E. Mol. Phys. 1970, 19, 745.
1106 Inorganic Chemistry, Vol. 46, No. 4, 2007

Molecular Structure of Square-Planar Gold Complexes
Table 4. EPR Parameters of [Pd(2L)(2L•)]-, [Au(2L)(2L•)] (2), and
Table 5. Selected Optimized Structural Parameters of Gold Complexes
(Experimental Data in Parentheses)
[Au(mnt)(mnt•)]
tensor
[Pd(2L)(2L•)]- c [Au(2L)(2L•)] (2) [Au(mnt)(mnt•)]
G
gx
gy
gz
1.965(1.960)
2.046(2.041)
2.066(2.051)
1.944
2.030
2.065
1.928
2.039
2.075
A × 104, cm-1 Ax -6.0(5.0)
-6.8
-8.1
complex
Au-S, Å
C-S, Å
C
1-C2, Å
1.362
Ay -6.0(6.6)
-6.7
-6.3
-6.6
-7.0
-8.1
-7.7
Az -7.6(9.2)
[Au(1L)2]2-
2.410
(2.416)a
2.362
1.780
(1.73)
1.788
(1.770)
1.756
a
A0 -6.5
(1.37)
1.359
(1.355)
1.383
(1.36)
1.408
P × 104, cm-1 Px
n.d.
n.d.
n.d.
-150
-150
[Au(1L)2]1-
[Au(1L)(1L•)]0
[Au(1L•)2]1+
Py
Pz
+50
+100
90
+50
+100
120
(2.315)b
2.360
T, Kb
30
(2.291)c
2.357
(1.749)
1.736
a Isotropic value derived from the anisotropic tensor. b Temperature.
c Values in brackets refer to [Pd(1L)(1L•)]- from: Bowmaker, G. A.; Boyd,
P. D. W.; Campbell, G. K. Inorg. Chem. 1983, 22, 1208.
a From ref 32 for [AuII(mnt)2]2-
.
b From this work for [AuIII(2L)2]-.
c From this work for [Au(L)(L•)] (2).
a hypothetical Au(II) 5d9 configuration,32 one can infer that
the magnetic orbitals are having less than 10% gold 5d-
character31 and the complexes are best described as Au(III)
compounds with a ligand radical.
and the corresponding strong quadrupole hyperfine interac-
tion with the principal axes pointing in a different direction
from those of the g matrix or of the magnetic hyperfine
tensor. The misorientation of quantization axes induces
mixing of hyperfine levels, line shifts, and “forbidden”
transitions in the EPR spectra. In the analysis of our spectra
of the neutral [Au(L)(L•)] (2) and [Au(mnt)(mnt•)] com-
plexes, we followed the previous work and simulated the
X-band powder spectra by using mixed magnetic and electric
hyperfine interactions.
Excellent fits were obtained with the g-, A-, and P-tensor
components summarized in Table 4 and shown in Figure 8.
Minor details, like small relative rotations of the different
tensors principal axes system, could not be resolved, but these
are difficult to interpret in quantitative chemical terms
anyhow. The signs of the A-values also could not be
determined from experiment, but were taken from the DFT
calculations (see below). However, a comparison of the
anisotropic A values of [Au(mnt)(mnt•)] with the result of a
measurement performed in liquid solution at 160 K reveals
that the three tensor components must have the same sign.
This is inferred from the close match of the isotropic value
calculated from the anisotropic values given in Table 4 and
the value measured at 160 K, |A0|exp ) 7.8 × 10-4 cm-1.
The similarity of the EPR parameters of the neutral gold
complexes [Au(2L)(2L•)] (2) and [Au(mnt)(mnt•)] indicates
only minor differences in the basis electronic structure of
the two compounds. A first interpretation of the ground state
can be derived from the low value of the magnetic hyperfine
coupling constants. Because the isotropic part A0 is less than
10% of that theoretically expected for the Fermi contact for
The frozen solution EPR spectrum of the monoanion
[Pd(2L)(2L•)]- also shows a rhombic derivative line pattern
with g values close to g ) 2. Weak shoulders at the low-
field lines indicate the presence of hyperfine splitting from
105Pd (I ) 5/2, 22.2% abundance). The spectrum was
reasonably well simulated with the g- and A-values given in
Table 4. The low anisotropy of both tensors as well as the
low isotropic part of the magnetic hyperfine coupling again
indicates predominant ligand radical character of the mag-
netic orbital. (Here, we refrained from simulating quadrupole
effects because of the low resolution of the complex
hyperfine pattern.)
Calculations. (a) Structure Optimizations. Density
functional theoretical (DFT) calculations have been carried
out by employing BP86 and B3LYP functionals for the
dianion [Au(1L)2]2-, the monoanion [Au(1L)2]1-, the neutral
complex [Au(1L)(1L•)]0, and the monocation [Au(1L•)2]1+,
where (1L)2- represents a truncated version of (2L)2-, the
1,2-diphenyl-ethylene-1,2-dithiolate ligand, and (1L•)1- is the
truncated π radical anion. Table 5 summarizes important
bond distances of all of these species. The agreement of the
experimental and calculated data for the monoanion and the
neutral complex is reasonably good. The parameters of the
ligands, in particular, are quite accurately predicted with a
typical error in bond length not exceeding 0.02 Å. The
overestimation of the Au-S bond distances varying 0.05-
0.06 Å is typical of DFT functionals and has been observed
in a variety of systems previously.24,34,35
(31) A0 only gives the s-orbital contribution and is not directly informative
of the d-orbital makeup. The theoretical A0 value for 6s gold is 1026
× 10-4 cm-1 (p 536 of Weil’s 1994 book, EPR), so our measured
value just tells us that we have less than 1% Au 6s. Alternatively, the
contribution of the metal orbital can be estimated from the anisotropy
of the A-tensor by using the uniaxial constant, B0, which parameterizes
the spin-dipole contribution for d-orbitals. Its theoretically value is
18.8 × 10-4 cm-1 for d9 gold (Weil, J. A.; Bolton, J. R.; Wertz, J. E.
Electron Paramagnetic Resonance; John Wiley & Sons, Inc.: New
York, 1994; p 536). If we are to use the anistotropic A values from
Table 4, we obtain B ) 1.3 × 10-4 cm-1 and % Au (d) ) 7%.
(32) Kirmse, R.; Kampf, M.; Olk, R.-M.; Hildebrand, M.; Krautscheid, H.
Z. Anorg. Allg. Chem. 2004, 630, 1433.
Interestingly, the optimized structures of the monoanion,
the neutral complex, and the monocation feature similar
Au-S bond distances at an av 2.36 Å. In contrast, the same
bond in the corresponding dianion is longer at 2.41 Å. This
is in excellent agreement with experiment. The Au-S bond
length in [N(n-Bu)4]2[Au(mnt)2] is reported at 2.416 Å.32 It
(34) (a) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem.
2002, 41, 4179. (b) Lauterbach, C.; Fabian, J. Eur. J. Inorg. Chem.
1999, 1995.
(35) Herebian, D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2003,
125, 10997.
(33) Schiødt, N. C.; Sommer-Larsen, P.; Bjørnholm, T.; Folmer-Nielsen,
M.; Larsen, J.; Bechgaard, K. Inorg. Chem. 1995, 34, 3688.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1107

Kokatam et al.
5d manifold lies very deep in energy due to the high effective
nuclear charge of gold. Thus, the metal d and the ligand p
orbitals are well separated from each other and the super-
exchange interaction observed in the corresponding
[M(LBu)(LBu•)]1- (M ) Ni, Pd, Pt) complexes24 is reduced
to a minimum. The singly occupied MO (SOMO) in [AuIII-
(1L)(1L•)]0 is, therefore, predominantly ligand-based with only
5% of Au 5dxz character. In contrast, the four doubly
2
2
2
occupied orbitals 1a′ (dx -y ), 2a′ (dyz), 1a′′ (dxz), and 3a′ (dz )
are of mostly metal d origin, and, hence, the oxidation state
of the gold ion is best represented as Au(III) (d8).
The LUMO of the neutral complex is the antibonding
combination of the metal dxy and the ligand orbitals. Because
of the ligand geometry, the overlap between these two
orbitals is favorable, providing an efficient pathway for
ligand-to-metal σ donation.
Upon one-electron reduction of [AuIII(1L)(1L•)]0, the ad-
ditional electron enters the 3a′′ orbital, which becomes the
HOMO of the monoanion [Au(1L)2]1-. Because the electron
enters an orbital that is almost Au-S nonbonding, the Au-S
bond length does not change upon going from the neutral to
the monoanionic species. The electronic structure of the
monoanion is, therefore, adequately described as a Au(III)
ion attached to two closed-shell (1L)2- dianions.24,33
Further reduction of the mono- to the corresponding
dianion [Au(1L)2]2- results in the single occupancy of the
4a′′ (dxy) orbital. This orbital is strongly Au-S antibonding,
and, consequently, the bond length increases on going from
the mono- to the dianion. The ground state of the [Au(1L)2]2-
complex is calculated to be 2A′′ with the following electron
configuration: (1a′)2(1a′′)2(2a′)2(3a′)2(4a′)2(2a′′)2(5a′)2(3a′′)2-
(4a′′)1.
Figure 9. MO-scheme for the neutral molecule [AuIII(1L)(1L•)]0 T [AuIII-
(1L•)(1L)]0 as obtained from a ZORA B3LYP DFT calculation. Point group
Cs is considered for this complex; in parentheses, the corresponding orbitals
are labeled in D2h symmetry.
is also noteworthy that the olefinic C-C bond distance
increases, whereas the average C-S distance decreases on
going from the monoanionic to the neutral complex and to
the cation [Au(1L•)2]1+. However, these metrical parameters
of the ligands do not change on going from the mono- to
the dianion. These findings are in excellent agreement with
the notion that the one-electron-transfer steps in the series
[Au(L)2]1-/0/1+ are ligand-based processes, whereas the
reduction of the monoanion to the dianion is metal-based.
The calculated average C-S bond length in the neutral
species at 1.756 Å corresponds to the arithmetric mean of
the C-S bond distance at 1.788 Å in the monoanion and
1.736 Å for the monocation. Thus, the neutral complex may
be described as delocalized ligand mixed valent [AuIII(1L)-
(1L•)] T [AuIII(1L•)(1L)] (class III), whereas the monocation
contains two π radical monoanions and a trivalent Au
center: [AuIII(1L•)2]+.
(b) Bonding Scheme and Ground-State Properties. For
the MO description of the complexes within the Cs point
group, we chose the coordinate system shown in Figure 9,
which also exhibits a qualitative bonding scheme obtained
from the spin unrestricted B3LYP DFT calculation on the
neutral [AuIII(1L)(1L•)] complex. The compositions of the
important MOs are given in Table S1 of the Supporting
Information.
This is in contrast to the suggested 2A2g (in D2h symmetry)
ground state for [Au(mnt)2]2-.6 Our result supports van Rens
assignment of a b1g (D2h symmetry) orbital symmetry of the
1
ground state of [Au(mnt)2]2- (S ) /2).7a
It is now interesting that the presently calculated spin
population at the gold ion in [Au(1L)2]2- is significantly
larger (∼25%) than has been observed experimentally (10%)
from a single-crystal EPR study on [N(n-Bu)4]2[Au(mnt)2].7a,b
The electronic structure of the [Au(mnt)2]2- dianion has
previously been described in terms of a central Au(II) (d9)
attached to two closed-shell ligand dianions.32 Based on our
present calculations, it appears to be more realistic to describe
the electronic structure of the dianion [Au(1L)2]2- as
intermediate AuII/AuIII with a metal-ligand delocalized
SOMO (25% Au 5d, 75% 3p orbital of four S atoms). We
note that previous EHT-MO calculations have also predicted
a 20% Au character for this SOMO.7b
The electronic structure of the neutral, square-planar
complexes [NiII(L•X,X)2]0, where (L•X,X 1- represents a semi-
)
quinonate(1-), (L•O,O)1-, an analoguous π radical anion
1-
(L•N,N
)
derived from o-phenylenediamide, or the corre-
The ground-state electronic configuration of [AuIII(1L)-
(1L•)]0 is as follows (in Cs symmetry): (1a′)2(1a′′)2(2a′)2(3a′)2-
(4a′)2(2a′′)2(5a′)2(3a′′)1(4a′′)0.
Previously, a very similar bonding scheme24 has been
calculated for [Au(LBu)(LBu•)]0 (in D2h symmetry).The Au
sponding benzene-1,2-dithiolate(1-) π radical (L•S,S)1-, has
recently been shown to be best described as singlet diradicals
containing a diamagnetic Ni(II) ion (d8) and two ligand π
radicals, which couple antiferromagnetically yielding the
observed St ) 0 ground state.34 It was shown that a single
1108 Inorganic Chemistry, Vol. 46, No. 4, 2007

Molecular Structure of Square-Planar Gold Complexes
Figure 10. Qualitative MO scheme for the monocation [AuIII(1L•)2]+ as derived from a broken symmetry ZORA B3LYP DFT calculation. The doubly
occupied MO’s are canonical orbitals, and the singly occupied MO’s result from a corresponding orbital transformation.37 The coordinate system is the same
as in Figure 9.
determinant, closed-shell approximation of the DFT method
is not an appropriate starting point for a quantitative
description of these complexes.35 Therefore, the broken
symmetry formalism as developed by Noodleman36 was
applied (here for the monocation [Au(1L•)2]+) to determine
the diradical index.24,35 The broken symmetry B3LYP wave
Table 6. Comparison of the Charge and Spin Populations at the Gold
Ion As Calculated from a Natural Population Analysis of the
One-Electron Density of the Ground State Obtained from Scalar
Relativistic ZORA-B3LYP DFT Calculations
complex
electron-5d
electrons-6s
spin-5d
[Au(1L)2]2-
[Au(1L)2]1-
[Au(1L)(1L•)]0
[Au(1L•)2]1+
9.34
8.91
8.89
8.84
0.62
0.58
0.60
0.64
0.25
0.00
0.05
0.00
functions of the neutral complexes show an increasing
1-
diradical character from (L•S,S)1-, to (L•N,N
)
to (L•O,O)1-.
At these levels of theory, the diradical character of [NiII-
(L•S,S)2] is practically zero,34a and it does not break symmetry
spontaneously. A large antiferromagnetic coupling between
the two unpaired electrons of the two radical ligands is
considered to be responsible for the negligible diradical
character. In subsequent papers,24,35 it was shown that the
superexchange interaction through the central metal ion
contributes strongly to the antiferromagnetism. The extent
of the superexchange interaction depends on the effective
nuclear charge of the central metal ion in the square-planar
[M(L•S,S)2] complexes. Because of the small effective nuclear
charge of the Ni(II) ion, the Ni d-orbitals are placed very
close to the sulfur orbitals, resulting in considerable mixing
of metal and ligand orbitals, and, hence, significant super-
exchange interaction leading to a very small diradical
character. Therefore, for [Ni(L•S,S)2], no broken symmetry
solution has been found.24,38
On the other hand, the Au(III) ion possesses a much larger
effective nuclear charge and the Au d-orbitals are situated
deep in energy and are well-separated from the ligand
orbitals. The antiferro-magnetic coupling between the ligand
radicals mediated by the superexchange interaction through
the central Au(III) ion is expected to be small. Accordingly,
a broken symmetry solution has been found for the mono-
cation [AuIII(1L•)2]1+. A qualitative bonding scheme derived
from such a B3LYP broken symmetry DFT calculation is
shown in Figure 10 for this species. There are six doubly
occupied canonical MOs, four of which are of predominantly
Au character and are situated very low in energy (Au(III),
d8). The remaining two orbitals are at higher energy and
ligand-based. Analysis of the corresponding orbitals37 yields
a spin-coupled (magnetic) pair formed between two ligand-
based orbitals. The overlap between these two orbitals is 0.35.
A diradical character of 88% is calculated by using proce-
dures described in ref 33 with a singlet-triplet (-2JGS) gap
of 1024 cm-1. The electronic structure of [Au(III)(1L•)2]1+
can therefore be described in terms of a central, diamagnetic
Au(III) ion (d8) attached to two antiferromagnetically coupled
ligand π radicals.
Interestingly, similar calculations performed on the iso-
electronic complex [PdII(1L•)2] did not produce a broken
symmetry solution. Because of the lower effective nuclear
charge of Pd(II) as compared to Au(III), the Pd orbitals are
situated closer in energy to the ligand-based orbitals. A Pd
d-orbital contribution of ∼20% has been calculated for the
SOMO (HOMO).
Table 6 gives the results of a natural population analysis
of the B3LYP densities for the gold complexes. Interestingly,
the d population remains constant in the series [Au(1L)2]1+/0/1-
but changes significantly upon reduction to the dianion. This
is again consistent with ligand-based electron-transfer pro-
cesses in the above series of complexes and a significant
metal contribution to the SOMO in the dianion.
(36) (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (b) Noodleman,
L.; Davidson, E. R. Chem. Phys. 1986, 109, 131.
(37) Neese, F. J. Phys. Chem. Solids 2004, 65, 781.
(38) Lim, B. S.; Fomitchev, D. V.; Holm, R. H. Inorg. Chem. 2001, 40,
4257.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1109

Kokatam et al.
Table 7. Calculated (Experimental Data in Parentheses) EPR
Parameters As Obtained from the Scalar Relativistic ZORA-B3LYP
DFT Calculations
This is assigned to a 2a′ (1au in D2h) f 3a′′ (2b2g in D2h)
transition, which is experimentally observed at 7800 cm-1
(oscillator strength 0.025).
Ax,
MHz
Ay,
MHz
Az,
For the two [Au(1L)2]1-/2- species, the 3a′′ (2b2g in D2h)
orbital is doubly occupied, and, consequently, no transitions
are calculated in the NIR in agreement with experiment. Note
that the 2a′ (1au in D2h) f 4a′′ (1b1g in D2h) transition
expected for all three species, [Au(L)2]0/1-/2-, appears at high
energy and is not observed in the first 25 calculated states
(energy range 5000-25 000 cm-1) of these complexes.
As pointed out previously,35 the broken symmetry formal-
ism only crudely models the multireference character of the
diradical system [M(L•)2]n and is not an entirely satisfactory
substitute for a genuine multiconfigurational treatment.
Hence, no TD-DFT calculations were performed on the [Au-
(1L•)2]1+ complex.
complex
gx
gy
gz
MHz
[Au(1L)(1L•)]0
[Au(2L)(2L•)]a
2.028
2.023
1.962
-10
-15
-11
(2.065) (2.030) (1.944)
(-20) (-20) (-19)
[Au(mnt)(mnt•)]0 a (2.0758) (2.0381) (1.9279) (-16) (-91) (-24)
[Au(1L)2]2-
2.08
(2.02)
2.05
(2.01)
2.00
(1.98)
-203
-204
-193
[Au(mnt)2]2- b
(-123) (-121) (-119)
a This work. b Reference 7.
Table 8. Results of the ZORA-B3LYP TD-DFT Calculation on
[AuIII(1L)(1L•)] T [AuIII(1L•)(1L)]
energy, cm-1
oscillator strength
transition (Cs)
exp.
calc.
exp.
calc.
5a′ f 3a′′
2a′ f 3a′′
6666
7800
7020
7300
0.129
0.025
0.20
0.03
(c) Calculation of EPR Parameters. The calculated EPR
parameters (ZORA-B3LYP DFT level) are summarized in
Table 7. The calculated g-values and 197Au hyperfine splitting
parameters of [AuIII(1L)(1L•)] T [AuIII(1L•)(1L)] are in
reasonable agreement with the experimental ones for [AuIII-
(2L)(2L•)] T [AuIII(2L•)(2L)] (see above). The SOMO of this
species is predominantly ligand-based with only 5% of Au
dxz character. Consequently, together with the rather small
nuclear moment of the 197Au nucleus, the 197Au hyperfine
constants are small, both calculated and experimental. There
is also little angular momentum in the ground-state wave
function, and, thus, the calculated and observed g-shifts are
small and reflect the organic radical character of the ground
state.
Summary
The principal results of this investigation are summarized
as follows.
(1) For the first time, a four-membered, reversible electron-
transfer series [Au(L)2]z (z ) 1+, 0, 1-, and 2-) has been
characterized electrochemically and, in part, chemically.
Similarly, the three-membered, reversible series [Pd(L)2]z (z
) 0, 1-, 2-) has also been investigated. All members
possess a planar structure (Cs symmetry).
(2) By using DFT calculations at the B3LYP level, the
geometries of the above gold series [Au(1L)2]z with z ) 1+,
0, 1-, 2- and of [PdII(1L•)2]0 have been optimized and
compared to experimental X-ray data where possible. Agree-
ment is in general excellent. The observed trends in Au-S,
C-S, and C-C distances within the series are reliably
reproduced. Calculated EPR parameters for paramagnetic
[AuIII(1L•)(1L)]0 T [AuIII(1L)(1L•)] and [AuII(1L)2]2- are also
in excellent agreement with experiment. The electronic
spectrum of the above neutral species has also been
calculated, and, again, the data are in excellent agreement
with experiment.
3
On the other hand, large 197Au (I ) /2, 100% natural
abundance) hyperfine splittings are predicted by calculations
for the dianionic complex [AuII(1L)2]2-. The experimental
spectrum of [AuII(1L)2]2- has not been measured due to its
instability in solution. However, the EPR parameters for
[Au(mnt)2]2- 7b given in Table 7 agree reasonably well with
those calculated for [AuII(1L)2]2-.
(d) Excited-State Calculations. The electronic spectra of
the [M(LBu)2]0/1-/2- (M ) Ni, Pd, Pt) and of [Au(LBu)2]0/1-
complexes have been reported previously in detail.5,24
Assuming D2h symmetry, the 1b1u f 2b2g transition in the
[M(LBu)(LBu•)]z complexes is predominantly ligand-to-ligand
intervalence charge transfer (IVCT) in origin and is assigned
to the intense band in the NIR region. For the diradical
complexes [M(L•)2]z, the 1b1u f 2b2g transition is ligand-
to-ligand charge transfer (LLCT) in origin and also occurs
in the NIR region. As pointed out above, the intensity of
the LLCT band is approximately twice that of the IVCT
band.
(3) From these calculations, it is clearly established that
four doubly occupied, energetically low lying metal d-orbitals
exist, which allow the assignment of an Au(III) (d8) oxidation
state for the monocation, the neutral, and the monoanionic
gold species. Two molecular orbitals at significantly higher
energy can then be identified as being the electroactive
orbitals 5a′ (in Cs) or 1b1u (D2h symmetry) and 3a′′ (in Cs)
or 2b2g (in D2h), which are predominantly ligand-based. Thus,
independent of the overall charge z, these two orbitals have
0% and only 5% metal d-character, respectively. This is in
contrast to the corresponding series [Ni(S2C2Me2)2]0,1-,2-
,
The calculated spectra of complexes as obtained from
time-dependent DFT calculations are in reasonable agreement
with experiment as shown in Table 8. For the neutral [Au-
(1L)(1L•)]0 complex, the most intense transition in the NIR
region is an IVCT band and corresponds to the 5a′ (1b1u in
D2h) f 3a′′ (2b2g in D2h) transition. It is predicted at 7020
cm-1 and is observed at 6666 cm-1. The next transition is
calculated at 7300 cm-1 with a smaller oscillator strength.
where the 2b2g orbital has a much higher metal d-character
(∼20%).38 Similarly, the corresponding DFT calculations for
[Pd(1L•)2]0 show that the HOMO 3a′′ has ∼20% metal
character. It is thus clear that the redox steps for [Au(1L)2]z
(z ) 1+, 0, 1-) are nearly purely ligand-based. For the
monocation [AuIII(1L•)2]+, a diradical character of 88% has
been calculated from a broken symmetry DFT calculation
again in contrast to calculations on the corresponding neutral
1110 Inorganic Chemistry, Vol. 46, No. 4, 2007

Molecular Structure of Square-Planar Gold Complexes
complexes of Ni34,38 and Pd for which no broken symmetry
solutions were found.
Supporting Information Available: X-ray crystallographic files
for compounds 1, 2, and 4, and Table S1 containing the composition
of selected molecular orbitals of [Au(1L)2]z complexes. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support. S.K. and K.R. are grateful
for a Max-Planck stipend. The work at Vermont was funded
by the National Science Foundation, CHE-0092702.
IC061181U
Inorganic Chemistry, Vol. 46, No. 4, 2007 1111