Synthesis and structure of gold(III) complexes of asymmetric dithiolene ligands

Article abstract of DOI:10.1016/j.poly.2005.09.002

Four new [Au(dithiolene)₂]^- complexes, each involving two asymmetrically substituted dithiolene ligands (SC(H)C(R)S; R = phenyl, pyridin-2-yl, pyridin-3-yl, or pyridin-4-yl), have been synthesized by reacting K[AuCl₄] with the corresponding dithiolene, generated by base hydrolysis of the thione-protected 1,3-dithiolate. Each complex has been isolated as its [PPh₄]⁺ salt and the structures of these [PPh₄][Au(dithiolene)₂] compounds have been determined by X-ray crystallography; each anion possesses a square planar {AuS₄} core with a cis-arrangement of the R groups for R = phenyl and pyridin-2-yl complexes, but a trans-arrangement for R = pyridin-3-yl and pyridin-4-yl. In each compound, the lengths of the C-C (1.34 ± 0.01 A?) and C-S (1.75 0.03 A?) bonds of the metallocycle are consistent with the ene-1,2-dithiolate form of the dithiolene, i.e., these are [Au(III)bis(ene-1,2- dithiolate)]^- complexes. Each compound is diamagnetic and ¹H NMR studies indicate that, in solution in d⁶-acetone, both the cis and trans isomers of the [Au(SC(H)C(R)S)₂]^- anion are present and do not interconvert at room temperature. Each [Au(dithiolene)₂]^- complex undergoes an oxidation (80a reduction (-1880+/[Fc]); the particular potentials observed correlate with the electron withdrawing ability of the dithiolene substituent (R), as expressed by the Hammett σ_p-parameter. Comparison of the information obtained in these studies with that determined for related systems leads to the conclusion that the oxidation is a ligand-based process and that reduction involves addition of an electron to an orbital that possesses both metal and ligand character.

Full text of DOI:10.1016/j.poly.2005.09.002

Polyhedron 25 (2006) 591–598
www.elsevier.com/locate/poly
Synthesis and structure of gold(III) complexes of asymmetric
dithiolene ligands
Josephine M. Tunney, Alexander J. Blake, E. Stephen Davies, Jonathan McMaster,
*
Claire Wilson, C. David Garner
School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
Received 4 July 2005; accepted 1 September 2005
Available online 19 October 2005
We are pleased to mark Professor Malcolm ChisholmÕs many significant contributions to Inorganic Chemistry, using the occasion of his 60th birthday to
congratulate him on his achievements, wishing him health, happiness, and continued scientific success.
Abstract
Four new [Au(dithiolene)2]ꢀ complexes, each involving two asymmetrically substituted dithiolene ligands (SC(H)C(R)S; R = phe-
nyl, pyridin-2-yl, pyridin-3-yl, or pyridin-4-yl), have been synthesized by reacting K[AuCl4] with the corresponding dithiolene, gen-
erated by base hydrolysis of the thione-protected 1,3-dithiolate. Each complex has been isolated as its [PPh4]+ salt and the structures
of these [PPh4][Au(dithiolene)2] compounds have been determined by X-ray crystallography; each anion possesses a square planar
{AuS4} core with a cis-arrangement of the R groups for R = phenyl and pyridin-2-yl complexes, but a trans-arrangement for
˚
˚
R = pyridin-3-yl and pyridin-4-yl. In each compound, the lengths of the C–C (1.34 0.01 A) and C–S (1.75 0.03 A) bonds of the
metallocycle are consistent with the ene-1,2-dithiolate form of the dithiolene, i.e., these are [Au(III)bis(ene-1,2-dithiolate)]ꢀ com-
plexes. Each compound is diamagnetic and H NMR studies indicate that, in solution in d6-acetone, both the cis and trans isomers
1
of the [Au(SC(H)C(R)S)2]ꢀ anion are present and do not interconvert at room temperature. Each [Au(dithiolene)2]ꢀ complex under-
goes an oxidation ð80 < Epa < 230 mVÞ and a reduction (ꢀ1880 < Epc ðredÞ < ꢀ2020 mV; versus [Fc]+/[Fc]); the particular potentials
observed correlate with the electron withdrawing ability of the dithiolene substituent (R), as expressed by the Hammett rp-param-
eter. Comparison of the information obtained in these studies with that determined for related systems leads to the conclusion that
the oxidation is a ligand-based process and that reduction involves addition of an electron to an orbital that possesses both metal
and ligand character.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Gold; Dithiolene; Structure; Redox properties
1. Introduction
related [Au(dithiolene)2]z (z = 0 or 2ꢀ) complexes identi-
fied [2–4,7,8,12,15–18,23,25,26,30,31]; the electronic struc-
ture of these systems, notably the role of the dithiolene
ligands in the redox processes, has been probed by ab initio
and density functional calculations [21,32].
There are a range of strategies available to synthesize
dithiolene complexes [33]. We have adapted the procedure
developed by Davies et al. [34,35] to synthesize four new
[Au(dithiolene)2]ꢀ complexes, each of which involves the
metal coordinated by two asymmetrically substituted
dithiolenes.
An extensive range of bis-dithiolene complexes of Au
and their properties have been reported [1–31]; the majority
are [Au(dithiolene)2]ꢀ complexes with a square planar
{AuS4} core, diamagnetic, and generally considered to in-
volve a Au(III) (d8) centre. The redox properties of
[Au(dithiolene)2]ꢀ complexes have been investigated and
*
Corresponding author. Tel.: +44 115 951 4188; fax: +44 115 951 3563.
E-mail address: Dave.Garner@nottingham.ac.uk (C.D. Garner).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.09.002

592
J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
2. Experimental
potassium O-isopropyl xanthate (5.57 g, 32.0 mmol) and
H2SO4 (10 cm3, 98%), and obtained as a cream colored so-
lid (1.14 g, 22%). Elemental Anal. Calc. for C8H5NOS2: C,
49.27; H, 2.58; N, 7.18. Found: C, 48.88; H, 2.52; N, 6.97%.
Anhydrous N,N-dimethylformamide (DMF), over
molecular sieves, was purchased from Fluka and used as
received. Potassium O-isopropyl xanthate [36], 4-(phenyl)-
1,3-dithiol-2-one [37], 2-bromoacetylpyridine hydrobro-
mide, 3-bromoacetylpyridine hydrobromide and 4-bromo-
acetylpyridine hydrobromide [38,39] were prepared by the
literature methods or modifications thereof.
EI-MS
m/z:
195 ðC8H5NOS2þÞ,
167 ðC7H5NS2þÞ,
122 ðC6NSH4þÞ, 78 (C5H4N+). 1H NMR (300 MHz,
CDCl3, 298 K): dH 8.67 (dd, 2H, H2, H6, JH–H 4.5,
1.7 Hz), 7.30 (dd, 2H, H3, H5, JH–H 4.4, 1.6 Hz), 7.15 (s,
1H, Hdithio).
2.1. Synthesis of 4-(pyridin-2-yl)-1,3-dithiol-2-one
2.4. Synthesis of [PPh4][Au(sdt)2] (1)
2–Bromoacetylpyridine hydrobromide (8.41 g, 29.9 mmol)
was suspended in CH2Cl2 (60 cm3) and triethylamine
(3.64 g, 36.0 mmol) added to give a pink solution. Potassium
O-isopropyl xanthate (6.26 g, 36.0 mmol) was added and the
orange suspension stirred for 2 h. The reaction was
quenched by the addition of water (100 cm3) and the CH2Cl2
layer separated. The aqueous phase was washed with
CH2Cl2 (2 · 10 cm3), the CH2Cl2 phases were combined,
dried (MgSO4), filtered, and the filtrate evaporated in vacuo
to give a brown solid. The solid was treated with H2SO4
(10 cm3, 98%) resulting in an exothermic reaction. After
5 min, ice was added to the reaction until no further heat
was generated. The solution was diluted with water and then
neutralised with NaHCO3. The pale precipitate was ex-
tracted into CH2Cl2, the organic phase dried (MgSO4),
and the filtrate evaporated to dryness to yield a pale brown
solid. Recrystallization from hot EtOH afforded a cream
colored solid (0.80 g, 14%). Elemental Anal. Calc. for
C8H5NOS2: C, 49.27; H, 2.58; N, 7.18. Found: C,
49.18; H, 2.57; N, 6.98%. EI-MS m/z: 195 ðC8H5NOS2þÞ,
167 ðC7H5NS2þÞ, 123 ðC6NSH5þÞ, 78 (C5H4N+). 1H NMR
(300 MHz, CDCl3, 298 K): dH 8.60–8.58 (m, 1H, H6),
7.79–7.73 (m, 1H, H3), 7.53–7.50 (m, 1H, H4), 7.36 (s, 1H,
Hdithio), 7.28–7.24 (m, 1H, H5).
4-Phenyl-1,3-dithiol-2-one (0.10 g, 0.5 mmol) and
NaOH (0.10 g, 2.5 mmol) were suspended in EtOH
(10 cm3). A solution of K[AuCl4] (0.10 g, 0.3 mmol) in
water (10 cm3) was added and a dark yellow colored solu-
tion was obtained. This solution was heated at reflux for
3 h and then allowed to cool to room temperature. A solu-
tion of [PPh4]Br (0.20 g, 0.5 mmol) in water (10 cm3) was
then added and a pale brown solid precipitated. The vol-
ume of the solvent was reduced in vacuo and the precipitate
collected by filtration. This solid was recrystallized from a
CH2Cl2 solution layered with n-hexane; the product was
isolated by decantation of the mother liquor and then dried
in vacuo to yield a golden crystalline solid (0.11 g, 51%).
Elemental Anal. Calc. for AuS4PC40H32: C, 55.29; H,
3.71. Found: C, 54.35; H, 3.69%. FAB-MS m/z: 530
1
[Au(sdt)2]ꢀ. H NMR (300 MHz, (CD3)2CO, 298 K): dH
8.03–7.94 (m, 4H, PPh4), 7.88–7.82 (m, 16H, PPh4), 7.57
(m, 4H, H2, H6), 7.19 (m, 4H, H3, H5), 7.08 (m, 2H, H4),
6.63, 6.62 (2s, 2H, Hdithio).
2.5. Synthesis of [PPh4][Au(2-pedt)2] (2)
This compound was prepared as described for 1 from 4-
(pyridin-2-yl)-1,3-dithiol-2-one (0.10 g, 0.5 mmol), NaOH
(0.10 g, 2.5 mmol) and K[AuCl4] (0.10 g, 0.3 mmol). Addi-
tion of a solution of [PPh4]Br (0.20 g, 0.5 mmol) in water
(10 cm3) to the dark yellow solution resulted in the precip-
itation of a pale brown solid. The volume of the solvent
was reduced in vacuo, the precipitate was collected by fil-
tration and then recrystallized from a solution in CH2Cl2
layered with n-hexane. The product was isolated by decan-
tation of the mother liquor and then dried in vacuo to yield
a golden crystalline solid (0.04 g, 18%). Elemental Anal.
Calc. for AuS4N2PC38H30: C, 52.41; H, 3.47; N, 3.22.
Found: C, 52.21; H, 3.34; N, 3.04%. FAB-MS m/z: 532
2.2. Synthesis of 4-(pyridin-3-yl)-1,3-dithiol-2-one
This compound was prepared as for 4-(pyridin-2-yl)-1,3-
dithiol-2-one from 3-bromoacetylpyridine hydrobromide
(8.11 g, 28.9 mmol), triethylamine (3.51 g, 34.8 mmol),
potassium O-isopropyl xanthate (6.06 g, 34.8 mmol) and
H2SO4 (10 cm3, 98%), and obtained as a cream colored so-
lid 0.90 g (16%). Elemental Anal. Calc. for C8H5NOS2: C,
49.27; H, 2.58; N, 7.18. Found: C, 49.26; H, 2.52; N,
7.18%. EI-MS m/z: 195 ðC8H5NOS2þÞ, 167 ðC7H5NS2þÞ,
122 ðC6NSH þÞ, 78 (C5H4N+). 1H NMR (300 MHz,
[Au(2-pedt)2]ꢀ. H NMR (300 MHz, (CD3)2CO, 298 K):
1
4
CDCl3, 298 K): dH 8.73–8.72 (m, 1H, H2), 8.63–8.60 (m,
1H, H6), 7.73–7.69 (m, 1H, H4), 7.38–7.33 (m, 1H, H5),
6.94 (s, 1H, Hdithio).
dH 8.32 (m, 2H, H6), 8.03–7.96 (m, 4H, PPh4), 7.89–7.81
(m, 16H, PPh4), 7.62, 7.61 (2s, 2H, Hdithio), 7.60–7.58 (m,
4H, H3, H4), 6.99 (m, 2H, H5).
2.3. Synthesis of 4-(pyridin-4-yl)-1,3-dithiol-2-one
2.6. Synthesis of [PPh4][Au(3-pedt)2] (3)
This compound was prepared as for 4-(pyridin-2-yl)-1,3-
dithiol-2-one from 4-bromoacetylpyridine hydrobromide
(7.63 g, 27.2 mmol), triethylamine (3.30 g, 32.7 mmol),
This compound was prepared as described for 1 from
4-(pyridin-3-yl)-1,3-dithiol-2-one (0.10 g, 0.5 mmol), NaOH
(0.10 g, 2.5 mmol) and K[AuCl4] (0.10 g, 0.3 mmol).

J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
593
Addition of a solution of [PPh4]Br (0.20 g, 0.5 mmol) in
3. Physical characterization
water (10 cm3) to the dark yellow solution resulted in the
precipitation of a pale brown solid. The volume of the
solvent was reduced in vacuo, the precipitate was collected
by filtration and then recrystallized from a solution in
CH2Cl2 layered with n-hexane. The product was isolated
by decantation of the mother liquor and then dried in vacuo
to yield a golden crystalline solid (0.08 g, 37%). Elemental
Anal. Calc. for AuS4N2PC38H30: C, 52.41; H, 3.47; N,
3.22. Found: C, 52.26; H, 3.49; N, 3.10%. FAB-MS m/z:
532 [Au(3-pedt)2]ꢀ. 1H NMR (300 MHz, (CD3)2CO,
298 K): dH 8.80 (d, 2H, H2, JH–H 2.1 Hz), 8.30 (dd, 2H,
H6, JH–H 4.9, 1.6 Hz), 8.03–7.97 (m, 4H, PPh4), 7.89–7.83
(m, 18H, PPh4, H4), 7.19 (m, 2H, H5) 6.79, 6.78 (2s, 2H,
Hdithio).
Elemental analyses of the compounds isolated in these
studies were accomplished by the Microanalytical Lab-
oratory of the School of Chemistry, the University of
Nottingham, and by the Elemental Analysis Service at
London Metropolitan University. FAB and EI mass spec-
tra were recorded on a Fisons VG Trio 200 or a Fisons VG
1
Autospec spectrometer. H NMR spectra were recorded
on a Bruker DPX300 NMR spectrometer and UV/Vis
spectra were recorded on a Perkin–Elmer Lambda 16
spectrophotometer.
The cyclic voltammogram of each compound (1 mM) in
DMF, containing [NnBu4][BF4] (0.2 M) as the background
electrolyte, was recorded at room temperature using a
glassy carbon working electrode, a Pt wire secondary
electrode, and a saturated calomel reference electrode. To
avoid overlapping redox couples, the [(Me5C5)2Fe]+/
[(Me5C5)2Fe] ([Fc*]+/[Fc*]) couple was used as the internal
reference and the potentials of the redox processes observed
were referenced to that of the [Fc]þ+/[Fc] couple by an
independent calibration ðDE1=2; ½Fcꢁ =½Fcꢁ versus ½Fcꢂ2ꢁþ=
½Fcꢂ2ꢁ; ¼ 0:475 VÞ. Measurements were performed using an
Autolab PGSTAT20 potentiostat and compensation for
internal resistance was not applied.
Golden, single crystals of 1, 2, 3 and 4 were grown by dis-
solution of the corresponding compound in CH2Cl2 fol-
lowed by the slow diffusion of n-hexane into the solution.
Details of the unit cell dimensions, data collection, and
refinement are given in Table 1. Data for the structures of
1, 2 and 3 were collected on a Bruker SMART 1000 CCD
area detector diffractometer, whilst data for 4 were collected
on a Bruker SMART APEX detector diffractometer. Both
systems were equipped with an Oxford Cryosystems open
flow cryostat. Structures 1 and 2 were solved by direct
methods and structures 3 and 4 by Patterson heavy-atom
2.7. Synthesis of [PPh4][Au(4-pedt)2] (4)
This compound was prepared as described for 1 from
4-(pyridin-4-yl)-1,3-dithiol-2-one
(0.10 g,
0.5 mmol),
NaOH (0.10 g, 2.5 mmol) and K[AuCl4] (0.10 g,
0.3 mmol). Addition of a solution of [PPh4]Br (0.20 g,
0.5 mmol) to the dark yellow solution resulted in the pre-
cipitation of a pale brown solid. The volume of the sol-
vent was reduced in vacuo, the solid was separated by
filtration and then recrystallized from a solution in
CH2Cl2 layered with n-hexane. The product was isolated
by decantation of the mother liquor and then dried in
vacuo to yield a golden crystalline solid (0.04 g, 18%).
Elemental Anal. Calc. for AuS4N2PC38H30: C, 52.41; H,
3.47; N, 3.22. Found: C, 51.99; H, 3.48; N, 3.15%.
FAB-MS m/z: 532 [Au(4-pedt)2]ꢀ. 1H NMR (300 MHz,
(CD3)2CO, 298 K): dH 8.36 (dd, 4H, H2, H6, JH–H 4.7,
1.6 Hz), 8.03–7.94 (m, 4H, PPh4), 7.89–7.83 (m, 16H,
PPh4), 7.49 (dd, 4H, H3, H5, JH–H 4.9, 1.6 Hz), 7.15,
7.14 (2s, 2H, Hdithio).
Table 1
Unit cell dimensions, data collected and refined for the crystal structures of [PPh4][Au(sdt)2] (1), [PPh4][Au(2-pedt)2] (2), [PPh4][Au(3-pedt)2] (3), and
[PPh4][Au(4-pedt)2] (4)
Compound
1
2
3
4
Empirical formula
M
C
868.83
40H32 Au P S4
C38H30 Au N2 P S4
870.82
C38H30 Au N2 P S4
870.82
C38H30 Au N2 P S4
870.82
Crystal system
Space group
a (A)
monoclinic
P21/n
14.7438(11)
13.7638(10)
17.0507(13)
90
92.199(2)
90
3457.6(4)
4
monoclinic
P21/n
14.3534(8)
13.8681(7)
17.0646(9)
90
91.873(1)
90
3395.0(5)
4
monoclinic
I2/a
18.187(4)
17.332(4)
21.346(5)
90
90.960(4)
90
6728(3)
8
monoclinic
I2/a
18.169(2)
17.452(2)
21.532(2)
90
94.417(2)
90
6807(2)
8
˚
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
l (mmꢀ1
)
4.572
4.658
4.701
4.646
Reflections collected
Independent reflections
Rint
30951
8584
0.032
19494
8226
0.038
20743
8115
0.062
19270
7918
0.033
Final R1 [I > 2r(I)], wR2
0.0264, 0.0571
0.0344, 0.0803
0.0540, 0.135
0.0473, 0.115

594
J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
methods, using SHELXS-97 [40]. In each case, a full-matrix
least-squares refinement was carried out using SHELXL-97
[41]. All hydrogen atoms were placed in geometrically
calculated positions and thereafter refined using a riding
model with Uiso(H) = 1.2Ueq(C), unless otherwise stated.
All non-hydrogen atoms were refined with anisotropic
atomic displacement parameters (adps), except partially
occupied atoms of 2 and 4. Compound 2 had a disorder
in the pyridine ring containing the atoms C13–C18. The
initial refined values of the isotropic adp (Uiso) suggested
a misassignment of the carbon and nitrogen atoms C18
and N2. However, further investigation suggested an elec-
tron density intermediate between C and N at both sites.
Therefore, the ring was modeled with two coincident
half-occupied sites, with attendant half-occupied hydrogen
atom positions corresponding to those of the C atoms.
Compound 4 had a disorder in the anion containing
Au2. All atoms of this anion, except the gold and sulfur,
were positioned in two sites with occupancies of 0.70 and
0.30. Geometric restraints were applied to the distances
in all the partial occupancy disorder models, such that
the bond lengths were restrained to be the same in each
component. Hydrogen atoms were geometrically placed.
lene = sdt or 2-pedt) anion, each of which has a cis-
configuration (Fig. 1(a) and (b)). The asymmetric unit of
compounds 3 and 4 each comprises a [PPh4]+ cation and
two independent half [Au(dithiolene)2]ꢀ (dithiolene = 3-
pedt or 4-pedt) anions, the Au atoms of which are situated
on an inversion center, i.e., each anion has a trans-configu-
ration (Fig. 1(c) and (d)). The dimensions of each
[Au(dithiolene)2]ꢀ complex are presented in Table 2. In
each anion, the AuS4 center is essentially square planar:
the length of the four Au-S bonds is essentially the same
˚
(2.30 0.02 A); the S–Au–S angles are all close to 90ꢁ;
and the AuS4 array is precisely planar in compounds 3
4. Results and discussion
4.1. Syntheses
Each of the [Au(SC(H)C(R)S)2]ꢀ (R = phenyl, pyridin-
2-yl, pyridin-3-yl, or pyridin-4-yl: dithiolene abbreviations =
sdt, 2-pedt, 3-pedt, and 4-pedt, respectively) complexes have
been prepared (Scheme 1) by reacting K[AuCl4] with the
appropriate dithiolene, generated by base hydrolysis of the
corresponding thione-protected 1,3-dithiolate. Each [Au-
(SC(H)C(R)S)2]ꢀ complex has been isolated as its [PPh4]+
salt (compounds 1, 2, 3, and 4, respectively) in a reasonable
(18–51%) yield.
4.2. Structural information
The asymmetric unit of compounds 1 and 2 each com-
prises a [PPh4]+ cation and a [Au(dithiolene)2]ꢀ (dithio-
H
R
H
R
R
H
S
S
S
S
S
S
(1) NaOH
[PPh4]
Au
O
(2) K[AuCl4]
(3) [PPh4]Br
2
2
3
6
N
6
3
4
2
N
6
R =
5
N
6
3
4
5
5
4
5
Scheme 1. Route used for the synthesis of [PPh4][Au(dithiolene)2]
(dithiolene = SC(H)C(R)S, R = phenyl (sdt), pyridin-2-yl (2-pedt), pyri-
din-3-yl (3-pedt) and pyridin-4-yl (4-pedt)) compounds. The numbering
scheme for the various ligand substituents has been used in the assignment
of the 1H NMR spectra of these compounds.
Fig. 1. Structure of: (a) [Au(sdt)2]ꢀ; (b) [Au(2-pedt)2]ꢀ; (c) [Au(3-pedt)2]ꢀ;
and (d) [Au(4-pedt)2]ꢀ in the corresponding [PPh4][Au(dithiolene)2]
compound (1, 2, 3 and 4, respectively).

J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
595
Table 2
Dimensions (bond length/A; interbond angle/ꢁ) of the [Au(dithiolene)2]
anions of [PPh4][Au(sdt)2] (1), [PPh4][Au(2-pedt)2] (2), [PPh4][Au(3-pedt)2]
(3) and [PPh4][Au(4-pedt)2] (4)
the present cases – the dithiolene ring is not fused to an
ꢀ
˚
aromatic ring. For sp2 hybridized carbon atoms, typical
˚
˚
bond lengths are: C–C, 1.43–1.48 A; C@C, 1.30–1.36 A;
˚
˚
C–S, 1.71–1.75 A; and C@S 1.67–1.68 A [43]. Thus, the
length of both the C–C (1.34 0.01 A) and the C–S
(1.75 0.03 A) bonds of the ligands in each of the com-
Structure
1
2
3
4
˚
Au1–S1
Au1–S2
Au(A)–S3
Au(A)–S4
S1–C1
2.3126(9)
2.3141(9)
2.3199(9)
2.3149(9)
1.737(4)
1.777(3)
1.744(3)
1.770(3)
1.336(5)
1.337(4)
2.3124(12)
2.3173(12)
2.3083(12)
2.3116(12)
1.732(4)
1.768(5)
1.732(5)
1.769(4)
1.335(6)
2.297(2)
2.298(2)
2.301(2)
2.299(2)
1.732(9)
1.770(8)
1.730(9)
1.759(9)
1.349(10)
1.337(12)
2.308(2)
2.3054(18)
2.311(2)
2.2817(19)
1.741(8)
1.777(8)
1.766(8)
1.749(8)
1.335(10)
1.343(16)
˚
pounds 1–4 are consistent with the ene-1,2-dithiolate form
of the dithiolene. Therefore, these [Au(dithiolene)2]ꢀ an-
ions can, to a first approximation, be formulated as Au(III)
bis(ene-1,2-dithiolate) complexes, as proposed for [Au(1,2-
C6H4S2)2]ꢀ by Ray et al. on the basis of the C–S bond
S2–C2
S3–C11
S4–C12
C1–C2
˚
lengths of this complex (average 1.762 A) [31]. Consistent
with this proposal, the electronic spectrum of compound
C11–C12
1.343(7)
1
contains two weak absorptions at ca. 720 nm
S1–Au1–S2
S3–Au(A)–S4
Au1–S1–C1
Au1–S2–C2
Au(A)–S3–C11
Au(A)–S4–C12
S1–C1–C2
S2–C2-C1
S3–C11–C12
S4–C12–C11
89.66(3)
89.95(3)
89.80(4)
89.90(4)
89.95(8)
89.62(8)
101.7(3)
102.6(3)
101.4(3)
102.8(3)
125.4(6)
120.3(7)
125.7(7)
120.4(7)
89.97(7)
90.00(8)
101.6(3)
102.1(2)
98.4(5)
105.2(5)
125.1(6)
121.3(6)
129.1(10)
116.2(10)
(e ꢃ 100 Mꢀ1 cmꢀ1) and ca. 465 nm (e ꢃ 260 Mꢀ1 cmꢀ1),
similar to those of [Au(1,2-C6H4S2)2]ꢀ and related
[Au(dithiolene)2]ꢀ complexes [21,31].
101.15(12)
102.32(11)
100.82(12)
101.82(10)
126.2(3)
120.3(3)
125.6(3)
121.3(2)
101.25(17)
101.59(15)
101.39(15)
101.71(17)
125.4(4)
121.7(4)
125.2(4)
121.3(4)
1
4.3. H NMR spectroscopy
The dithiolene proton of these [Au(SC(H)C(R)S)2]ꢀ
complexes provides a valuable probe of the nature of these
1
complexes in solution. In the H NMR spectrum of each
The labelling of the atoms is shown in Figure 1 and Au(A) corresponds to
Au(1) for 1 and 2 (the asymmetric unit of which involve one [Au(dithio-
lene)2]ꢀ anion) and Au(2) for 3 and 4 (the asymmetric unit of which
involve two independent half [M(dithiolene)2]ꢀ anions).
[PPh4][Au(dithiolene)2] (dithiolene = sdt, 2-pedt, 3-pedt,
or 4-pedt) compound in (CD3)2CO at 298 K the dithiolene
(Hdithio) resonances were observed as two singlets, consis-
tent with the presence of both cis and trans isomers of
the square-planar anion in solution. The resonances ob-
served and the ratio of the integrals were: [PPh4][Au(sdt)2],
6.63 and 6.62 ppm, ca. 1:1; [PPh4][Au(2-pedt)2], 7.62 and
7.61 ppm, ca. 1:1; [PPh4][Au(3-pedt)2], 6.79 and 6.78 ppm,
ca. 9:1; and [PPh4][Au(4-pedt)2], 7.16 and 7.15, ca. 1:1.
These Hdithio resonances vary with the nature of the dithio-
lene as 2-pedt > 4-pedt > 3-pedt > sdt, as observed for the
[MO(dithiolene)2]2ꢀ (M = Mo or W) complexes of these li-
gands. This order is consistent with nature of the ligands, in
respect of: (i) inductive effects and (ii) the relative ability of
the aromatic substituent to stabilize a resonance form in
which a positive charge is localized at the C atom carrying
the dithiolene proton [34,35].
and 4 and very close to planar in compounds 1 and 2 (the
maximum deviation of an atom from the AuS4 plane is
˚
˚
0.044 A in 1 and 0.043 A in 2). Also, each {AuSCCS} met-
allodithiolene ring is essentially planar with the maximum
˚
displacement from the mean plane being 60.05 A. These
structural details are in good agreement with those
reported for other [Au(dithiolene)2]ꢀ anions [6,9,13,
˚
16,19,20,22,29–31], as are the C–S (1.75 0.03 A) and
dithiolene C–C (1.34 0.01 A) bond lengths (provided
˚
that the C–C bond is not part of an aromatic ring).
The four crystal structures described herein are the first
such examples of gold bound to two asymmetrically substi-
tuted dithiolenes; also, [Au(sdt)2]ꢀ and [Au(2-pedt)2]ꢀ rep-
resent the first cis-isomers of a bis-dithiolene complex to be
structurally characterized. The essential equivalence of the
four M–S and four C–S bonds of each [Au(dithiolene)2]ꢀ
(dithiolene = sdt, 2-pedt, 3-pedt, or 4-pedt) complex indi-
cates that the asymmetric nature of the dithiolene does
not significantly affect the nature of the metal–sulfur
interactions. However, the asymmetric nature of the
dithiolene ligands is reflected in the S–C–C interbond
angles; the S–C(H)–C(R) angles (125.1(6)–126.2(3)ꢁ) are sig-
nificantly larger than the S–C(R)–C(H) angles (116.2(10)–
121.7(4)ꢁ).
The resolution of two environments for the dithiolene
1
protons of compounds 1–4 contrasts with the H NMR
spectra observed for [Ni(sdt)2], in CD2Cl2; at >198 K a sin-
gle dithiolene resonance is manifest but, at 198 K two res-
onances, of relative intensity ca. 1:1 and separated by
9.8 Hz are resolved. This latter information is consistent
with a rapid interconversion of the cis and trans isomers
of [Ni(sdt)2] at >198 K and has been attributed to a facile
rotation of the ligand(s) around the metal [44]. Noting
1
these results, the H NMR spectrum of [PPh4][Au(sdt)2]
in (CD3)2SO was recorded at ca. 298, 320, 330, 340, 355
and 365 K. At each temperature, the dithiolene proton res-
onances appeared as two singlets with a relative intensity of
ca. 1:1. The splitting between these two singlets decreased
slightly as the temperature was increased, from 3.28 Hz
at 298 K to 2.96 Hz at 365 K, suggesting that coalescence
may occur at >365 K. However, it is clear that the
An important aspect to the nature of a dithiolene com-
plex is the ‘‘non-innocence’’ of this ligand, the two extreme
forms being the ene-1,2-dithiolate (ꢀS–C@C–Sꢀ) and the
dithioketone (S@C–C@S) [42]. Thus, the lengths of the
C–C and C–S bonds can be used to comment on the nature
of a dithiolene ligand in a complex, especially when – as in

596
J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
activation energy for the interconversion of the cis and
trans isomers of [Au(sdt)2]ꢀ is significantly greater than
that for the corresponding process of [Ni(sdt)2] [44]; this
could be due to stronger metal–ligand bonds for Au (a
5d-metal) than Ni (a 3d-metal).
The presence of essentially equal amounts of the cis and
trans isomers of [Au(sdt)2]ꢀ, [Au(2-pedt)2]ꢀ, and [Au(4-
pedt)2]ꢀ in solution and the lack of any significant inter-
conversion of these isomers at room temperature suggests
that solid samples of their [PPh4]+ salts contain an essen-
tially equal amounts of the two isomers. However, from
the structural studies, it appears that one isomer of the an-
ion may preferentially crystallize and/or been present in the
crystal selected for X-ray analysis. In respect of [PPh4]-
[Au(3-pedt)2], we presume that the trans isomer identified
by X-ray crystallography predominates in the sample for
Fig. 2. Cyclic voltammogram of [PPh4][Au(sdt)2] (1) (1 mM), at room
temperature in DMF containing [NnBu4][BF4] (0.2 M), recorded using a
glassy carbon working electrode, a Pt wire secondary electrode, and a
saturated calomel reference electrode; potentials are vs. the [Fc]+/[Fc]
couple.
1
which the H NMR spectrum was recorded.
4.4. Redox behavior
The cyclic voltammogram of each [PPh4][Au(dithio-
lene)2] (dithiolene = sdt, 2-pedt, 3-pedt, or 4-pedt) com-
pound has been recorded in dmf solution containing
[NnBu4][BF4] (0.2 M) as the background electrolyte. Each
compound (Table 3) exhibits both an oxidation and a
reduction process.
an internal standard. Also, the oxidation was monitored
by UV/Vis spectroscopy and the profile seen to change
through a series of isosbestic points. However, application
of a potential sufficient to reverse the oxidation did not
reproduce the original UV/Vis spectrum (even at 243 K).
Therefore, the product of the oxidation of 1 is not stable
within the timescale of the bulk electrolysis experiment.
Also, oxidation of 1 with I2 did not produce a stable
product.
The oxidation of the compounds 1–4 occurs in the range
80 mV < EapðoxÞ < 230 mV (versus [Fc]+/[Fc]). The relative
values of these potentials (Table 3) correlates with the elec-
tron withdrawing nature of the substituent (R), as mea-
sured by the Hammett parameter, rp [45]; thus, 1
(R = Ph; rp = ꢀ0.01) is the easiest to oxidize ðEapðoxÞ ¼
80 mVÞ and 4 (R = pyridin-4-yl; rp = 0.44) is the most dif-
ficult to oxidize ðEapðoxÞ ¼ 230 mVÞ. For each compound
1–4, a variation in the scan rate (from 20 to 300 mV sꢀ1),
monitoring the wave profile, the separation of the peak
potentials (DE generally >70 mV), and the ratio of the
cathodic and anodic currents ðꢀIcp : Ipa ¼ 1:0Þ indicated
that the oxidation is not electrochemically reversible. The
most promising results were obtained for compound 1
(Fig. 2). At a scan rate >50 mV sꢀ1, DE is ca. 70 mV and
comparable with that of the [Fc*]+/[Fc*] couple used as
The reduction of the compounds 1–4 occurs in the range
ꢀ1880 mV < EcpðredÞ < ꢀ2020 mV (versus [Fc]+/[Fc]). As
for oxidation of these compounds, the relative values of
these reduction potentials (Table 3) correlate with the elec-
tron withdrawing nature of the dithiolene substituent (R),
as measured by the Hammett parameter, rp [45]; thus, 1
is the most difficult to reduce ðEcpðredÞ ¼ ꢀ2020 mVÞ and
4 is the easiest to reduce ðEcpðredÞ ¼ ꢀ1880 mVÞ. For each
compound the reduction wave is accompanied by a corre-
sponding oxidation wave upon reversal of the potential
(Fig. 2). The electrochemical characteristics of the process
were examined for a range of scan rates (20–300 mV sꢀ1
)
and, in each case, the reduction was not reversible. Also,
following the reduction of each compound, scans at a more
positive potential revealed other, relatively minor, oxida-
tion processes, suggesting that each reduced species is reac-
tive and/or unstable.
The cyclic voltammogram (CV) of 1 (Fig. 2) is
remarkably similar, in respect of both the overall profile
and the potentials of the redox processes, to the CV
of [Au(3,5-di-tert-butyl-1,2-benzenedithiolate)2]ꢀ ([Au(3,5-
(tBu)2bdt)2]ꢀ) [31]. This latter complex, in CH2Cl2 solution,
undergoes two, reversible, one-electron processes; an oxi-
dation at 74 mV and a reduction at ꢀ2280 mV (versus
[Fc]+/[Fc]). The correspondence in redox behavior and
the one-electron nature of both redox couples of [Au(3,5-
(tBu)2bdt)2]ꢀ implies that oxidation of 1 involves the
Table 3
Potentials (vs. [Fc]+/[Fc]) observed for the oxidation and reduction of
[PPh4][Au(sdt)2] (1), [PPh4][Au(2-pedt)2] (2), [PPh4][Au(3-pedt)2] (3), and
[PPh4][Au(4-pedt)2] (4) (1 mM) at room temperature in DMF containing
[NnBu4][BF4] (0.2 M)
Compound EapðoxÞ E1/2(ox) EcpðredÞ E1/2(red)
1
2
3
4
DE([Fc*]+/[Fc*])
80
120
180
230
50(70) ꢀ2020
ꢀ1970(90) 70
ꢀ1970
ꢀ1920(90) 80
ꢀ1880(90) 70
ꢀ1840(90) 70
ꢀ1920
200(70) ꢀ1880
The shape of the voltammogram observed for the oxidation of 2 and 3
does not allow an accurate determination of E1/2. For each compound,
other redox processes were observed outside the potential range 1000 to
ꢀ2100 mV.

J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
597
[Au(dithiolene)2]/[Au(dithiolene)2]ꢀ couple and reduction
involves the [Au(dithiolene)2]ꢀ/[Au(dithiolene)2]2ꢀ couple.
The nature of the frontier orbitals of [Au(dithiolene)2]ꢀ
complexes has been investigated by performing ab initio
calculations for [Au(bdt)2]ꢀ [21] and the results of density
functional calculations for [Au(bdt)2]z (z = 0 or 1) and re-
lated complexes of Ni, Pd, Pt, and Cu [32]. Schiødt et al.
[21] showed that, as observed by Schultz et al. [12], the
two highest occupied molecular orbitals (HOMOs) of
[Au(bdt)2]ꢀ have p-character and comprise the antisym-
metric and symmetric combination of the ligand HOMOs
with 4 and 0% contribution from Au orbitals, respectively;
each of these orbitals is C–S antibonding and C–C bonding
in character. The LUMO of [Au(bdt)2]ꢀ has r-character, is
the antibonding combination of the metal dxy and ligand
1b1g orbitals with significant contributions from both the
metal (Au dxy ca. 48%) and the ligand orbitals [21]. Similar
results were obtained by density functional calculations
accomplished for [Au(bdt)2]z (z = 0 or 1) [32].
As described in 4.2, the length of the C–C and C–S
bonds of the dithiolene ligands of compounds 1–4 are con-
sistent with formulation as [Au(III)(ene-1,2-dithiolate)2]ꢀ,
as indicated by structural [31] and theoretical [32] studies
of [Au(bdt)2]ꢀ. One-electron oxidation of a [Au(dithio-
lene)2]ꢀ complex, electrochemically or chemically (e.g.,
I2), yields the corresponding, paramagnetic, [Au(dithio-
lene)2] complex, examples of which have been structurally
characterized [12,16,30]. Dautel et al. [30] and Ray et al.
[31] have compared the dimensions of a [Au(dithiolene)2]ꢀ
complex with those of its a [Au(dithiolene)2] counterpart
and shown that conversion of the former to the latter
leaves the Au–S bond length essentially unchanged, but
the C–S bond shortens and the C–C bond lengthens. These
structural changes are consistent with the loss of an elec-
tron from the HOMO of a [Au(dithiolene)2]ꢀ complex,
i.e., this ligand-based orbital1 is Au–S non-bonding, C–S
antibonding, and C–C bonding [21,32].
A
comparison of redox potentials for [Au(L)2]ꢀ
(L = sdt, 2-pedt, 3-pedt or 4-pedt) with those of their
[M(L)2]ꢀ (M = Ni, Pd or Pt) analogs [43] indicates that
oxidation is less sensitive to a change in the metal than that
of the corresponding reduction process. This is consistent
with an oxidation being predominantly ligand-based and
reduction involving an orbital with both ligand and metal
character (vide infra). The trends within this series of
[Au(L)2]ꢀ complexes indicate a tuning of the ligand energy
levels by the appended aromatic substituents.
5. Conclusions
Four new [Au(dithiolene)2]ꢀ complexes, each involving
two asymmetrically substituted dithiolene ligands (SC(H)-
C(R)S; R = phenyl, pyridin-2-yl, pyridin-3-yl, or pyridin-
4-yl) have been synthesised, isolated as the [PPh4]+ salt
and structurally characterized. The lengths of the C–C
and C–S bonds of the ligands in each [Au(dithiolene)2]ꢀ
complex is consistent with the dithiolene being present as
an ene-1,2-dithiolate and, hence these are [Au(III)bis(ene-
1,2-dithiolate)]ꢀ complexes. Each compound is diamag-
1
netic and H NMR studies indicate that, in solution in
d6-acetone, both the cis and trans isomers of the
[Au(SC(H)C(R)S)2]ꢀ anion are present and do not inter-
convert at room temperature. Each [Au(dithiolene)2]ꢀ
complex undergoes a ligand-based oxidation process and
a reduction that involves the addition of an electron to
an orbital that possesses considerable ligand character.
These [PPh4][Au(dithiolene)2] compounds are suitable dia-
magnetic hosts that facilitate EPR studies of corresponding
[M(dithiolene)2]ꢀ (M = Ni, Pd, or Pt) complexes [46].
Acknowledgements
We thank the EPSRC for the provision of a studentship
to J.M.T.
One-electron reduction of a [Au(dithiolene)2]ꢀ complex
yields the corresponding [Au(dithiolene)2]2ꢀ complex that
possesses one unpaired electron and EPR spectra of several
of these systems have been reported [2,7,8,25,26]. A single-
crystal EPR-ENDOR study of 13C enriched [nBu4-
N]2[Au(mnt)2] diluted in [nBu4N]2[Ni(mnt)2] found that
the majority (ca. 64%) of the unpaired spin density is lo-
cated on the S-atoms [25,26]. This information is in quali-
tative agreement with the significant involvement of ligand
orbitals in the LUMO of [Au(dithiolene)2]ꢀ complexes
[21,32].
Appendix A. Supplementary data
Crystallographic data for the structure of compounds
1–4 have been deposited with the Cambridge
Crystallographic Data Centre, CCDC Nos. 275968,
275969, 275970 and 275971, respectively. Copies of the
information may be obtained from The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Tel.: +44
1223 336408; fax: +44 1223 336033). Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.poly.2005.09.002.
1
References
Ray et al. [31,32] favor a description of the oxidized species as
[AuIII(L)(Lꢀ)] (L = dithiolene). This implies that oxidation of a [Au(dithio-
lene)2]ꢀ complex removes an electron from one ligand, whereas the
theoretical results [12,21,32] indicate that the electron is lost from an
orbital that involves an equal contribution from each ligand and the
dimensions observed for [Au(dithiolene)2] complexes provide no such
distinction between the ligands [12,16,30].
[1] A. Davison, N. Edelstein, R.H. Holm, A.H. Maki, Inorg. Chem. 2
(1963) 1227.
[2] J.H. Waters, H.B. Gray, J. Am. Chem. Soc. 87 (1965) 3534.
[3] R. Williams, E. Billig, J.H. Waters, H.B. Gray, J. Am. Chem. Soc. 88
(1966) 43.

598
J.M. Tunney et al. / Polyhedron 25 (2006) 591–598
[4] M.J. Baker-Hawkes, E. Billig, H.B. Gray, J. Am. Chem. Soc. 88
(1966) 4870.
[25] L. Ihlo, R. Bo¨ttcher, R.-M. Olk, R. Kirmse, Inorg. Chim. Acta 281
(1998) 160.
[5] A. Davison, D.V. Howe, E.T. Shawl, Inorg. Chem. 6 (1967) 458.
[6] J.H. Enemark, J.A. Ibers, Inorg. Chem. 7 (1968) 2636.
[7] R.L. Schlupp, A.H. Maki, Inorg. Chem. 13 (1974) 44.
[8] A. Domingos, R.T. Henriques, V. Gama, M. Almeida, A. Lopes, L.
Alcacer, Syn. Met. 27 (1988) B411.
[26] L. Ihlo, R. Stosser, W. Hofbauer, R. Bo¨ttcher, R. Kirmse, Z.
Naturforsch. 54 (1999) 597.
[27] M. Takahashi, N. Robertson, A. Kobayashi, H. Becker, R.H.
Friend, A.E. Underhill, J. Mater. Chem. 8 (1998) 319.
´
[28] E. Cerrada, J. Garrido, M. Laguna, N. Lardies, I. Romeo, Synth.
[9] M.A. Mazid, M.T. Razi, P.J. Sadler, Inorg. Chem. 20 (1981) 2872.
[10] P. Kuppusamy, N. Venkatalakshmi, P.T. Manoharan, J. Cryst.
Spectros. Res. 15 (1985) 629.
Met. 102 (1999) 1709.
[29] S. Lo Schiavo, F. Nicolo, R. Scopelliti, G. Tresoldi, P. Piraino,
`
Inorg. Chim. Acta 304 (2000) 108.
´
[11] J.C. Fitzmaurice, A.M.Z. Slawin, D.J. Williams, J.D. Woollins, A.J.
Lindsay, Polyhedron 9 (1990) 1561.
[30] O.J. Dautel, M. Fourmigue, E. Canadell, P. Auban-Senzier, Adv.
Funct. Mater. 12 (2002) 693.
´
[12] A.J. Schultz, H.H. Wang, L.C. Soderholm, T.L. Sifter, J.M. Williams,
K. Bechgaard, M.-H. Whangbo, Inorg. Chem. 26 (1987) 3757.
[13] T.W. Hijmans, W.P. Beyermann, Phys. Rev. Lett. 58 (1987)
2351.
[31] K. Ray, T. Weyhermuller, A. Goossens, M.W.J. Craje, K. Wieghardt,
¨
Inorg. Chem. 42 (2003) 4082.
[32] K. Ray, T. Weyermu¨ller, F. Neese, K. Wieghardt, Inorg Chem. 44
(2005) 5345.
[14] U. Geiser, A.J. Schultz, H.H. Wang, M.A. Beno, J.M. Williams,
Acta Crystallogr., Sect. C 44 (1988) 259.
[15] G. Matsubayashi, A. Yokozawa, J. Chem. Soc., Dalton Trans. (1990)
3535.
[33] T.B. Rauchfuss, Prog. Inorg. Chem. 52 (2003) 1.
[34] E.S. Davies, R.L. Beddoes, D. Collison, A. Dinsmore, A. Docrat,
J.A. Joule, C.R. Wilson, C.D. Garner, J. Chem. Soc., Dalton Trans.
(1997) 3985.
[16] G. Rindorf, N. Thorup, T. Bjørnholm, K. Bechgaard, Acta Crystal-
logr., Sect. C 46 (1990) 1437.
[17] S.P. Best, S.A. Ciniawsky, R.J.H. Clark, R.C.S. McQueen, J. Chem.
Soc., Dalton Trans. (1993) 2267.
[18] N.C. Schiødt, T. Bjørnholm, C.S. Jacobsen, K. Bechgaard, Synth.
Met. 56 (1993) 2164.
[35] E.S. Davies, G.M. Aston, R.L. Beddoes, D. Collison, A. Dinsmore,
A. Docrat, J.A. Joule, C.R. Wilson, C.D. Garner, J. Chem. Soc.,
Dalton Trans. (1998) 3647.
[36] J. Berger, I. Uldall, Acta Chem. Scan. 18 (1964) 1353.
[37] A.K. Bhattacharya, A.G. Hortmann, J. Org. Chem. 39 (1974) 95.
[38] K. Schank, Chem. Ber. 102 (1969) 383.
[19] M.C. Gimeno, P.G. Jones, A. Laguna, M. Laguna, R. Terroba,
Inorg. Chem. 33 (1994) 3932.
[39] W. Wunderlich, J. Prackt. Chem. 2 (1955) 302.
[40] G.M. Sheldrick, SHELXS-97, University of Go¨ttingen, Go¨ttingen,
Germany, 1997.
´
[20] R.M. Davila, R.J. Staples, J.P. Fackler Jr., Acta Crystallogr., Sect. C
50 (1994) 1898.
[41] G.M. Sheldrick, SHELXL-97, University of Go¨ttingen, Go¨ttingen,
Germany, 1997.
[21] N.C. Schiødt, P. Sommer-Larsen, T. Bjørnholm, M. Folmer Nielsen,
J. Larsen, K. Bechgaard, Inorg. Chem. 34 (1995) 3688.
[22] A. Charlton, A.E. Underhill, A. Kobayashi, H. Kobayashi, J. Chem.
Soc., Dalton Trans. (1995) 1285.
[23] N.C. Schiødt, T. Bjørnholm, K. Bechgaard, J.J. Neumeier, C.
Allgeier, C.S. Jacobsen, N. Thorup, Phys. Rev. B 53 (1996) 1773.
[24] M. Nakano, A. Kuroda, G. Matsubayashi, Inorg. Chim. Acta 254
(1997) 189.
[42] J.A. McCleverty, Prog. Inorg. Chem. 10 (1968) 49.
[43] F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpen,
R. Taylor, J. Chem. Soc., Perkin Trans. II (1987) S1.
[44] A. Sugimori, N. Tachiya, M. Kajitani, T. Akiyama, Organometallics
15 (1996) 5664.
[45] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 91 (1991) 165.
[46] J. Tunney, Ph.D. Thesis, The University of Nottingham, 2003.