New gold(I) and silver(I) complexes of sulfamethoxazole: Synthesis, X-ray structural characterization and microbiological activities of triphenylphosphine(sulfamethoxazolato-N2)gold(I) and (sulfamethoxazolato)silver(I)

Article abstract of DOI:10.1016/j.inoche.2007.06.005

Sulfamethoxazole (SMTZ) reacts with Ph₃PAuCl and AgCl in methanol/triethylamine to give [Ph₃PAu(SMTZ_-1H+)] (SMTZ_-1H+ = sulfamethoxazolato anion) (1) and [Ag(SMTZ_-1H+)] (2). While the lattice of 1 contains single molecules with linear N-Au-P bonds, compound 2 comprises a two-dimensional polymeric assembly of the deprotonated SMTZ ligand and silver ions, which are coordinated by one oxygen and three nitrogens in a distorted tetrahedral array. The microbiologic activities (Mueller-Hinton broth dilution tests) of 1 and 2 were determined in relation to free sulfamethoxazole.

Full text of DOI:10.1016/j.inoche.2007.06.005

Inorganic Chemistry Communications 10 (2007) 1083–1087
www.elsevier.com/locate/inoche
New gold(I) and silver(I) complexes of sulfamethoxazole: Synthesis,
X-ray structural characterization and microbiological activities of
triphenylphosphine(sulfamethoxazolato-N2)gold(I) and
(sulfamethoxazolato)silver(I)
a
a,
a
Lenice L. Marques , Gelson Manzoni de Oliveira *, Ernesto Schulz Lang ,
b
a
Marli Matiko Anraku de Campos , Lara Regina Soccol Gris
a
ˆ
Departamento de Quı´mica, Laborato´rio de Materiais Inorganicos, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
b
Departamento de Ana´lises Clı´nicas e Toxicolo´gicas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
Received 14 May 2007; accepted 11 June 2007
Available online 15 June 2007
Abstract
Sulfamethoxazole (SMTZ) reacts with Ph3PAuCl and AgCl in methanol/triethylamine to give [Ph3PAu(SMTZ-1H+)] (SMTZ-1H+
=
sulfamethoxazolato anion) (1) and [Ag(SMTZ-1H+)] (2). While the lattice of 1 contains single molecules with linear N–Au–P bonds, com-
pound 2 comprises a two-dimensional polymeric assembly of the deprotonated SMTZ ligand and silver ions, which are coordinated by
one oxygen and three nitrogens in a distorted tetrahedral array. The microbiologic activities (Mueller–Hinton broth dilution tests) of 1
and 2 were determined in relation to free sulfamethoxazole.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Sulfamethoxazole; Au and Ag complexes; Antimicrobial activity
It is well known that sulfonamide derivatives, through
exchanges of different functional groups without modifica-
tion of the structural S(O)2N(H) feature, can exhibit a wide
variety of pharmacological activities, such as antidiabetic,
antibacterial and antitumor [1–4]. In addition, some metal
complexes of these ligands have been found to promote
rapid healing of burns: the Ag(I)-sulphadiazine complex
is used for human burn treatment [5,6] and the Zn(II) com-
plex, for the prevention of bacterial infection in burned ani-
mals [7,8]. The effectiveness of these compounds does not
depend solely on the slow release of Ag(I) or Zn(II), but
rather depends strongly on the nature of the material to
which the metal ion is bound [7].
arthritis [9,10]. The explosive growth of gold chemistry
related to the antiarthritic activity of its compounds, in
the last decade, has also shown that some gold drugs seem
to be effective in the treatment of diseases such as tumors,
psoriasis and AIDS [11,12]. Of particularly great chemo-
therapeutical potential in cancer treatment are gold(I)
complexes with bidentate phosphanes [13,14] such as
[Au(dppe)2]Cl (dppe = Ph2PC2H4PPh2, 1,2-diphenylphos-
phinethane). Complementing the large spectrum of medic-
inally active gold compounds, recently we have described
[15] the synthesis and the characterization of [{(SDA-
Z)Au}2(l-dppe)] (SDAZ = sulphadiazinide anion), further
also the new sulfathiazole complexes of Au(I) and Ag(I)
[(sulfathiazolato)AuPPh3] and [Ag(sulfathiazolato)]2 (sul-
fathiazole = N1-2-thiazolyl-sulfanilamide) [16]. Given the
ability of sulfonamide derivatives to coordinate to metal
atoms in different manners, considerable interest in the syn-
thesis and structural aspects of new complexes has arisen
Gold(I) complexes containing sulphur ligands have been
extensively used in the medical treatment of rheumatoid
*
Corresponding author. Tel.: +55 55 3220 8757; fax: +55 55 3220 8031.
E-mail address: manzoni@quimica.ufsm.br (G. Manzoni de Oliveira).
1387-7003/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.06.005

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L.L. Marques et al. / Inorganic Chemistry Communications 10 (2007) 1083–1087
[17–20]. The chemical significance of metal complexes of
the derivative sulfamethoxazole [21] (5-methyl-3-isoxazolyl
sulfanilamide) (A) has been early recognized and many
examples have been reported [22–25].
O
CH3
N
O
S
H2N
NH
O
A
Fig. 1. Molecular structure of [Ph3PAu(SMTZ-1H+)] (1) (hydrogen atoms
˚
omitted). Selected bond lengths (A) and bond angles (deg): Au–
This ligand has shown a tendency to coordinate to the
metal ion through the isoxazolic nitrogen atom, even in
cases of deprotonation of the sulfonamidic nitrogen atom.
Some new results [26–28] corroborate the fact that the abil-
ity of the sulfonamides to act as ligands is based upon the
acidity of the S(O)2N–H function, allied with the presence
of vicinal nitrogen or oxygen atoms of the substituents as
potential coordination sites. Thus, the deprotonation of
the NH group yields an anionic donor ligand, and, in case
of sulfamethoxazole, the isoxazole ring affords the stereo-
chemical requisites for the achievement of complexes with
a monodentate, chelating or bridging ligand [29,30].
N(2) = 2.057(3), Au–P = 2.225(1), S–O(2) = 1.432(3), S–O(3) = 1.437(4),
S–N(2) = 1.615(4), S–C(11) = 1.753(5), P–C(51) = 1.802(4), P–C(31) =
1.802(4), P–C(41) = 1.815(4), N(2)–C(21) = 1.368(6), N(3)–C(14) =
1.401(7), N(2)–Au–P = 176.96(1), O(2)–S–O(3) = 117.4(2), O(2)–S–
N(2) = 104.80(2), O(3)–S–N(2) = 111.7(2), N(2)–S–C(11) = 107.3(2),
C(21)–N(2)–S = 119.3(3), C(51)–P–C(31) = 106.3(2), C(51)–P–C(41) =
105.3(2), C(31)–P–C(41) = 106.5(2), C(51)–P–Au = 111.34(2), C(31)–P–
Au = 111.57(1), C(41)–P–Au = 115.23(1), C(21)–N(2)–Au = 122.2(3),
S–N(2)–Au = 118.5(2).
With the aim to provide information about the chemis-
try of gold(I) and silver(I) complexes of sulfamethoxazole,
and to enhance the knowledges on the antibacterial activity
of transition metal complexes of sulfonamides [25,31,32],
we report now on the preparation [33], X-ray structural
features [34–36] and pharmacological activities (chemical
toxicity against some microorganisms) [37,38] of
[Ph3PAu(SMTZ-1H+)] (1) and [Ag(SMTZ-1H+)]n (2).
Fig. 2. Asymmetric unit of [Ag(SMTZ-1H+)] (2).
The molecular structure of [Ph3PAu(SMTZ-1H+)] is rep-
resented in Fig. 1. Fig. 2 displays the asymmetric unit of
[Ag(SMTZ-1H+)]. Fig. 3 shows the supramolecular, bidi-
mensional assembling of 2 in the ab-plane, Fig. 4 exhibits
the zigzag configuration of the lattice of 2 in the bc-plane,
along the a-axis. Complex 1 crystallizes as single molecules,
with gold(I) bonded to the sulfonamidic nitrogen atom of
the sulfamethoxazolato anion and to the phosphorus atom
of the triphenylphosphine group. The bond distances are
formed in the crystal structure (Fig. 3) and the chelated sil-
ver is bound to N20 of the preceding SMTZ ligand. The
final donor atom is provided by N3000 from a STMZ ligand
in a parallel chain. The silver-N3 linkages between chains
form a zigzag chain approximately perpendicular to N2-
linked chains (Fig. 4). The nearly tetrahedral bonds on
the silver ions present variable distances: 2.183(3) {Ag–
N(2)}, 2.241(4) {Ag–N(1)0}, 2.571(4) {Ag–O(2)0} and
˚
2.057(3) {Au–N(2)} and 2.2251(11) A {Au–P}. The N(2)–
000
0
˚
Au–P bonds are nearly linear, with an angle of
176.96(10)ꢁ. The coordination number (2) of the Au(I)
ion is not increased through direct interactions with other
SMTZ ligands, as in the case of the silver complex 2. The
silver complex involves a complex two-dimensional poly-
meric assembly of the deprotonated SMTZ ligand and sil-
ver ions (Figs. 2–4). Each silver ion is coordinated by one
oxygen and three nitrogens in a distorted tetrahedral array.
Each ligand is coordinated to three different silver ions,
using four donor atoms, N1, O2, N2 and N3. Each silver
ion is chelated by N1 and O2 of a single SMTZ ligand to
form a six-membered ring. The N2 of this ligand is bound
to an adjacent silver ion in such a way that linear chains are
2.473(3) A {Ag–N(3) }. Although the Ag–O(2) and Ag–
N(3)000 bond distances are somewhat longer, they represent
also typical primary bonds, since the sums of the Ag/O and
˚
Ag/N van der Waals radii [39] are quite over 3 A. The lat-
tice of [Ag(SMTZ-1H+)]n holds bidimensional sheets (par-
tially shown in Fig. 3) along the c-axis.
The data regarding the minimal inhibitory concentration
(MIC) of the compounds SMTZ, [Ph3PAu(SMTZ-1H+)]
and [Ag(SMTZ-1H+)] against Escherichia coli, Staphylococ-
cus aureus and Pseudomonas aeruginosa are shown in Table
1. Table 2 presents the minimal inhibitory concentration of
the metal compounds [AuCl(PPh3)] and AgCl versus the
same strains. The Au(I) complex was greatly active versus

L.L. Marques et al. / Inorganic Chemistry Communications 10 (2007) 1083–1087
1085
˚
Fig. 3. Supramolecular, bidimensional assembling of [Ag(SMTZ-1H+)]n in the ab-plane, view along the c-axis. Selected bond lengths (A) and bond angles
(deg): Ag–N(2) = 2.183(3), Ag–N(1)0 = 2.241(4), Ag–O(2)0 = 2.571(4), Ag–N(3)000 = 2.473(3), N(2)–S = 1.594(3), N(2)–C(21) = 1.376(4), S–
O(2) = 1.459(3), S–O(3) = 1.443(3), O(3)–S–O(2) = 115.0(1), C(21)–N(2)–S = 120.9(2), S–O(2)–Ag00 = 116.8(1), C(21)–N(2)–Ag = 121.0(2), S–N(2)–
Ag = 117.9(1),
N(2)–Ag–N(1)0 = 138.6(1),
N(2)–Ag–N(3)000 = 105.5(1),
N(2)–Ag–O(2) = 138.92(9),
O(2)–Ag–N(3)000 = 80.83(8),
O(2)–Ag–
N(1)0 = 77.10(8), N(1)0–Ag–N(3)000 = 99.0(1). Symmetry transformations used to generate equivalent atoms: (0) 1 + x, y, z; (00) ꢀ1 + x, y, z; (000) 2 ꢀ x,
0.5 + y, 0.5 ꢀ z.
Table 2
Minimal inhibitory concentrations (MIC, in lg mLꢀ1) of the metal
compounds of Au(I) and Ag(I) against Gram-negative and Gram-positive
bacteria
E. coli (ATCC S. aureus (ATCC P. aeruginosa
25922)
25923)
(ATCC 27853)
AgCl
[AuCl(PPh3)] 512
256
>512
4
256
256
E. coli (8) and S. aureus (2 lg mLꢀ1) – last related to a mix-
ture 5:1 of SMTZ/trimetoprim, since the MIC value of the
Au(I) complex against this strain, related to free SMTZ and
even to [AuCl(PPh3)], had been early quantified as the best.
The minimum bactericidal concentrations (MBC) of the
complexes of Au(I) and Ag(I) against E. coli and P. aerugin-
osa show values with two dilution steps under the MIC, i.e.,
twofold more concentrated than the MIC values. Against S.
aureus no bactericidal activity was observed for the test
compounds. The differentiated behavior of the Gram-nega-
tive and Gram-positive strains in relation to the test drugs
should be attributed, at first, to the different morphology
of these bacteria classes. The more complex cellular wall
of the Gram-negative strains should impart different perme-
ability toward the test compounds, since their solubility is
not identical.
Fig. 4. Zigzag configuration of the lattice of [Ag(SMTZꢀ1H)]n in the bc-
plane, viewed along the a-axis.
Table 1
Minimal inhibitory concentration (MIC, in lg mLꢀ1) of the sulfameth-
oxazole (SMTZ) compounds
E. coli
(ATCC
25922)
S. aureus
(ATCC
25923)
P. aeruginosa
(ATCC 27853)
SMTZ
[Ag(SMTZ-1H+)]n
[Ph3PAu(SMTZ-1H+)]
512
64
8
>512
64
512
16
256
2a
a
Related to SMTZ/trimetoprim (5:1). All the others are related to
SMTZ only.

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L.L. Marques et al. / Inorganic Chemistry Communications 10 (2007) 1083–1087
[11] F. Bachechi, A. Burini, R. Galassi, B.R. Pietroni, M. Severini, J.
To understand, to a certain extent, the described micro-
Organomet. Chem. 575 (1999) 269.
biological behavior of the studied metal complexes, it
should be proper to remember that it has been suggested
that the truly active antibacterial species is the ionic form
of sulfonamides, therefore, this form must penetrate cells
[40]. However, the small lipid solubility of ionic sulfon-
amide is supposed to inhibit efficient penetration across
the lipoidal bacterial membrane. Alternatively, the molecu-
lar form presents higher lipid solubility, but it would not be
active unless it ionizes to some degree inside the cell. Thus,
sulfonamides would penetrate bacterial cells in the union-
ized form, but once they are inside a cell, their antibacterial
action would be due to the ionized form [41]. Further
experiments are in course to determine the solubility and
ionization patterns of the complexes in the blood plasma.
Finally, it is to date well known that overall, different
complexes of gold(I) have been studied as drug agents in
a variety of areas. Auranofin is used as an anti-inflamma-
tory agent for rheumatoid arthritis, and is now also being
studied as an antitumor agent [42]. Triphenylphosphine-
gold(I) complexes have also been tested as cancer-fighting
agents, along with bis(dppe)gold(I) complexes [43], as we
have already pointed out in the introduction of this work.
Nevertheless, we think that basic biological research on
new sulfamethoxazole complexes of Au and Ag, such as
the experiments described in this work, can also be signifi-
cant because it contribute to the biological knowledges
about the potential uses of this kind of compounds.
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CCDC 634754 and 634753 contain the supplementary
crystallographic data for 1 and 2. The data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.
ac.uk.
[33] [Ph3PAu(SMTZ-1H+)] (1) Sulfamethoxazole (0.126 g, 0.5 mmol) was
dissolved in methanol under moderate heating, and triethylamine
(1 mL) was added to the fully transparent solution. At a pH close to
11.5, and after the temperature stabilization at 65 ꢁC, solid Ph3PAuCl
(0.247 g, 0.5 mmol) was added. The gray suspension was refluxed
overnight, turning the color pink. After 23 h the reaction system was
switched off and the hot mixture was filtered. The precipitate was
washed with several portions of methanol and stored in a desiccator.
After drying, the solid was redissolved in CH2Cl2 and an upper layer
of petroleum ether was slowly added. Colorless crystals were formed
after two days. Melting point 218 – 220 ꢁC. (The preparation was
alternatively carried out starting from 0.3 mmol of [AuCl(PPh3)]
(0.148 g), silver acetate (0.050 g) and sulfamethoxazole (0.076 g) in the
moderate absence of light achieved by covering the reaction vessels
with aluminum foil. [AuCl(PPh3)] and silver acetate were dissolved
in ꢁ5 mL of benzene and stirred at 75 ꢁC. After 1 h the reaction
was interrupted and AgCl was separated by filtration. The temper-
ature and stirring were restored and sulfamethoxazole was added.
After 3 h a discolored precipitate is removed by filtration and
recrystallized from CH2Cl2.): C28H25AuN3O3PS (711.51). C,H,N,S-
Analysis, Found: C, 47.01; H, 3.68; N, 5.88; S, 4.98. Calc.: C, 47.26;
H, 3.54; N, 5.91; S, 4.51%. IR (KBr): 1122.5 [s (strong), ms (SO2)],
1324.6 [s, mas (SO2)], 3365.7 [m (medium), ms (NH2)], 3473.6 [m, mas
(NH2)]. [Ag(SMTZ-1H+)] (2): Sulfamethoxazole (0.126 g, 0.5 mmol)
was dissolved in methanol and the temperature was kept constant at
50 ꢁC. After the addition of triethylamine (1 mL) the solution
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L.L. Marques et al. / Inorganic Chemistry Communications 10 (2007) 1083–1087
1087
remained fully transparent with the pH in the range of 10–11. The
reaction vessel was wrapped in aluminum foil, then AgCl (0.0716 g,
0.5 mmol) was added. The mixture was refluxed for 4 h and the gray
precipitate was filtered and dried (the mother solution was stored).
The solid was dissolved in a 25% aqueous solution of ammonia and
recrystallized, yielding colorless crystals suitable for X-ray analysis.
(From the mother solution a slight amount of a residue, probably
triethylammonium chloride, was obtained by solvent evaporation.)
Melting point 277–281 ꢁC. C10H10AgN3O3S (360.14). C,H,N,S-Anal-
ysis, Found: C, 32.78; H, 3.40; N, 11.15; S, 8.58. Calc.: C, 33.26; H,
3.07; N, 11.64; S, 8.88%. IR (KBr): 1120.2 [s, ms (SO2)], 1322.5 [s,
mas(SO2)], 3323.2 [m, ms (NH2)], 3388.2 [m, mas (NH2)].
[37] Antimicrobial activity: Broth microdilution tests were used to
determine the minimal concentration of the antimicrobial agent
required to inhibit the microorganisms (MIC). We have employed
the methods prescribed in the M7-A6 document for aerobical
bacteria from the National Committee for Clinical Laboratory
Standards (NCCLS). The microbiologic activity of the sulfameth-
oxazole complexes of Au(I) and Ag(I) and their salts was measured
in relation to free sulfamethoxazole. The standard strains used were
the Gram-positive Staphylococcus aureus ATCC 25923 and the
Gram-negatives Escherichia coli ATCC 25922 and Pseudomonas
aeruginosa ATCC 27853. Preparation of the standard solutions of the
testing compounds: all the standard solutions for the metal com-
plexes and related compounds were prepared in a 1:1 mixture of
dimethyl sulfoxide (DMSO)/methanol, in a concentration of 10 mg/
mL. DMSO and methanol are useful solvents to determine the
biologic activity of relative insoluble substances, like some antibi-
otics. Also the mixture DMSO/H2O is widely used. Dilution series: a
sterile 96-well microtitration plate was prepared containing 100 lL
of the Mueller–Hinton broth per well. 100 lL of a 2048 lg mLꢀ1
drug-containing Mueller–Hinton broth was added in the first well. A
twofold dilution series was prepared from this first well. The final
drug concentrations in the wells were respectively 512, 256, 128, 64,
32, 16, 8, 4, 2, 1and 0.5 lg mLꢀ1. The plate was incubated at 37 ꢁC
for 18–24 h. All tests and inoculations were performed in triplicate.
MIC and MBC determination: after the incubation times and
addition of the indicator to the wells, the lowest concentration of
drug inhibiting microbial growth was selected as the minimal
inhibitory concentration (MIC). The minimum bactericidal concen-
tration (MBC) was determined by transferring 100 lL from the
lowest growth well for subculture in agar Mueller–Hinton plates
which were incubated aerobically at 37 ꢁC for 18–24 h. The MBC
was read as the lowest concentration of drug which reduces the
count of viable cells by 99.9% from the initial inoculum.
[38] NCCLS, Methods for Dilution Antimicrobial Susceptibility Tests for
Bacteria That Grow Aerobically: Approved Standard – Sixth Edition.
NCCLS document M7-A6 (ISBN 1-56238-486-4). NCCLS, 940 West
Valley Road, Suite 1400, Wayne, Pennsylvania 19087–1898 USA,
2003.
[34] Crystal structure analysis: the data were collected with a Bruker
APEX II CCD area-detector diffractometer and graphite-monochro-
matized Mo-Ka radiation. The structures were solved by direct
methods using SHELXS-97. Subsequent Fourier-difference map analy-
ses yielded the positions of the non-hydrogen atoms. Refinements
were carried out with the SHELXL-97 package. All refinements were
made by full-matrix least-squares on F2 with anisotropic displacement
parameters for all non-hydrogen atoms. Hydrogen atoms were
included in the refinement in calculated positions. The crystal data
and structure refinement for [Ph3PAu(SMTZ-1H+)] (1) and
[Ag(SMTZ-1H+)]n (2) are: Empirical formula: C28H25AuN3O3PS (1),
C10H10AgN3O3S (2); formula weight: 711.51 (1), 360.14 (2); crystal
system, space group: orthorhombic, Pna21 (1), monoclinic, P21/c (2);
˚
unit cell dimensions a, b, c (A): a = 19.0368(4) (1), 5.9653(4) (2);
b = 9.5820(2) (1), 15.6521(18) (2); c = 14.8461(3) (1), 13.1982(16) (2);
3
˚
a, b, c (ꢁ): a = b = c = 90 (1), b = 9.983(3) (2); V (A ): 2708.09(10) (1),
1213.7(2) (2); Z, Calc. density (g cmꢀ3): 4, 1.745 (1), 4, 1.971 (2);
crystal size (mm): 0.32 · 0.15 · 0.06 (1), 0.14 · 0.13 · 0.1 (2); h range
(ꢁ): 3.02–27.50 (1), 2.04–26.49 (2); index ranges: ꢀ24 6 h 6 24 (1),
ꢀ7 6 h 6 7 (2), ꢀ12 6 k 6 12 (1), ꢀ19 6 k 6 19 (2), ꢀ19 6 l 6 19 (1),
ꢀ16 6 l 6 16 (2); reflections collected: 29659 (1), 16152 (2): reflec-
tions unique: 6010 [Rint = 0.0495] (1), 2511 [Rint = 0.0275] (2);
completeness to theta max.: 99.8% (1), 99.6% (2); absorption
correction for 1 and 2: semi-empirical from equivalents; max. and
min. transmission: 1 and 0.234223 (1), 1 and 0.883415 (2); refinement
method: full-matrix least-squares on F2: data/restraints/parameters:
6010/1/335 (1), 2511/0/163 (2); goodness-of-fit on F2: 0.842 (1),
1.021(2); final R indices [I > 2r(I)]: R1 = 0.0248, wR2 = 0.0409 (1),
R1 = 0.0247, wR2 = 0.0792 (2); R indices (all data): R1 = 0.0438,
wR2 = 0.0429(1), R1 = 0.0407, wR2 = 0.1151 (2); largest diff. peak and
[39] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[40] A. Gringauz, Introduction to Medicinal Chemistry, Wiley-UCH,
New York, 1997, pp. 61–67.
[41] W.O. Foye, T.L. Lemke, D.A. Williams, Principles of Medicinal
Chemistry, fourth ed., Williams & Williams, 1995, pp. 709–713.
[42] E. Mizushima, K. Sato, T. Hayashi, M. Tanaka, Angew. Chem., Int.
Ed. 41 (2002) 4563.
hole (e Aꢀ3): 0.618 and ꢀ0.427 (1), 1.001 and ꢀ1.108 (2).
˚
[35] G.M. Sheldrick, SHELXS-97, Program for Crystal Structure Solution,
University of Go¨ttingen, Germany, 1997.
[36] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Go¨ttingen, Germany, 1997.
[43] C.F. Shaw III, A. Beery, G.C. Stocco, Inorg. Chim. Acta 123 (1986)
213.