Metal(II) complexes with monodentate S and N ligands as structural models for zinc-sulfur DNA-binding proteins

Article abstract of DOI:10.1016/S0020-1693(98)00351-X

The reaction of several metal salts with various amounts of the monodentate ligands N-(2-thiophenyl)-2,5-dimethylpyrrole (H-tpdp) and N-methylimidazole in the presence of a base provided a series of novel complexes. They are of the general formulas [M^II(tpdp)₄]^2-, [M^II(tpdp)₂(N-MeIm)₂] (M = Zn, Co, Cd) and [M^II(tpdp)₃(N-MeIm)]^- (M = Zn, Co). The products were characterized by X-ray structure analyses as well as by elemental analyses and infrared spectroscopy, and for the cobalt complexes by additional UV-Vis spectroscopy. Crystallographic parameters: [Et₃NH]₂[Zn(tpdp)₄]·MeCN (1), [Et₃NH]₂[Co(tpdp)₄]·MeCN (2), [Zn(tpdp)₂(N-MeIm)₂] (6) and [Co(tpdp)₂(N-MeIm)₂] (7) are monoclinic, space group P2₁/c, with Z = 4; [Cd(tpdp)₂(N-MeIm)₂] (8) is monoclinic, space group P2₁/n, with Z = 4; [Ph₄P]₂[Cd(tpdp)₄] (3), [Me₄N][Zn(tpdp)₃(N-MeIm)] (4), and [Me₄N][Co(tpdp)₃(N-MeIm)] (5) are triclinic, space group P1, with Z = 2. All compounds have tetragonally compressed metal cores. The complexes are significant as models for many zinc-containing DNA-binding proteins.

Full text of DOI:10.1016/S0020-1693(98)00351-X

Inorganica Chimica Acta 285 (1999) 262±268
Metal(II) complexes with monodentate S and N ligands as
structural models for zinc±sulfur DNA-binding proteins
*
È
Jens Otto, Ingo Jolk, Torres Viland, Ralf Wonnemann, Bernt Krebs
Anorganisch-Chemisches Institut der Universitat Munster, Wilhelm-Klemm-Straûe 8, D-48149 Munster, Germany
È
È
È
Received 21 May 1998; accepted 24 July 1998
Abstract
The reaction of several metal salts with various amounts of the monodentate ligands N-(2-thiophenyl)-2,5-dimethylpyrrole (H-tpdp) and
N-methylimidazole in the presence of a base provided a series of novel complexes. They are of the general formulas [MII(tpdp)4]2
,
[MII(tpdp)2(N-MeIm)2] (M ˆ Zn, Co, Cd) and [MII(tpdp)3(N-MeIm)] (M ˆ Zn, Co). The products were characterized by X-ray structure
analyses as well as by elemental analyses and infrared spectroscopy, and for the cobalt complexes by additional UV±Vis spectroscopy.
Crystallographic parameters: [Et3NH]2[Zn(tpdp)4]ÁMeCN (1), [Et3NH]2[Co(tpdp)4]ÁMeCN (2), [Zn(tpdp)2(N-MeIm)2] (6) and
[Co(tpdp)2(N-MeIm)2] (7) are monoclinic, space group P21/c, with Z ˆ 4; [Cd(tpdp)2(N-MeIm)2] (8) is monoclinic, space group P21/n,
ꢀ
with Z ˆ 4; [Ph4P]2[Cd(tpdp)4] (3), [Me4N][Zn(tpdp)3(N-MeIm)] (4), and [Me4N][Co(tpdp)3(N-MeIm)] (5) are triclinic, space group P1,
with Z ˆ 2. All compounds have tetragonally compressed metal cores. The complexes are signi®cant as models for many zinc-containing
DNA-binding proteins. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Zinc complexes; Cobalt complexes; Cadmium complexes; Sulfur and nitrogen ligand complexes
1. Introduction
new compounds with the ligands N-(2-thiophenyl)-2,5-
dimethylpyrrole (H-tpdp) and N-methylimidazole. In
order to get further information about the environment of
the metal core in zinc-containing proteins, zinc is often
replaced by cobalt or cadmium [21], so that other spectro-
scopic examinations like UV±Vis and 113Cd NMR spectro-
scopy are possible. Accordingly, we synthesized not
only zinc, but also cobalt and cadmium complexes enabling
us to investigate differences and similarities in the direct
metal environment. Herein we report the syntheses and
crystal structures of the eight new complex compounds
[MII- (tpdp)4]2 , [MII(tpdp)2(N-MeIm)2] with M ˆ Zn,
Co, Cd, and [MII(tpdp)3(N-MeIm)] with M ˆ Zn, Co.
Zinc cysteine coordination is a feature in many important
DNA-binding proteins. The number of cysteine residues
coordinated to zinc in these proteins varies from 2 to 4.
Zinc ®nger proteins contain a Zn(S-cys)2(N-his)2 centre
which plays a structural role in creating DNA-binding
®ngers [1±10] enabling the transcription of DNA. Other
zinc-containing transcription factors are steroid receptors
which activate the ®xation of a steroid hormone to a special
gene. These steroid receptors contain two separated Zn(S-
cys)4 centres [6,11,12]. The Ada protein of Escherichia coli
also employs a Zn(S-cys)4 site to repair deoxyribonucleic
acid alkyl phosphotriester lesions [13±15]. In the last ®ve
years there has been an increasing interest in the investiga-
tion of retroviral-type zinc ®nger proteins with a Zn(S-
cys)3(N-his) centre [16,17]. A better understanding of these
nucleocapsid proteins may open the path to the development
of a new anti-HIV strategy [18±20].
2. Experimental
All procedures were carried out under an argon or nitro-
gen atmosphere using standard Schlenk and glovebox tech-
niques. Solvents were dried and degassed. Elemental
analyses were carried out at the Institute of Organic Chem-
istry, University of MuÈnster. 1H NMR spectra were recorded
on a Bruker WH 300 spectrometer. Infrared spectra were
measured on a Bruker FTIR IFS 48 instrument. KBr pellets
were used. The UV±Vis spectra were recorded on a Hewlett
Packard HP-8453 instrument in DMSO.
As part of a study to provide structural and spectroscopic
models for zinc±cysteine centres in DNA-binding proteins,
we report the syntheses and crystal structures of a series of
*Corresponding author. Tel.: +49-251-833-3131; fax: +49-251-833-
8366; e-mail: krebs@uni-muenster.de
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00351-X

J. Otto et al. / Inorganica Chimica Acta 285 (1999) 262±268
263
2.1. Synthesis of the ligand N-(2-thiophenyl)-2,5-
dimethylpyrrole (H-tpdp)
ZnCl2 (3 mmol). The resulting white precipitate was dis-
solved through further addition of 25 ml MeOH and 1 h of
stirring. Next, 0.246 g N-methylimidazole (3 mmol) were
added. The solution was stirred again for 1 h and Me4NCl
(0.329 g, 3 mmol) was added. The white precipitate was
collected, washed with MeOH and dried in vacuo (79%
yield, 1.98 g). Recrystallization with acetone yielded col-
ourless crystals after one day. Decomposition occurs at
2108C. Anal. Calc. for C44H54N6S3Zn: C, 63.8; H, 6.6;
N, 10.1. Found: C, 63.8; H, 6.6; N, 10.2%. IR (cm 1):
3043w, 3017w, 2969m, 2917s, 1580vs, 1472vs, 764m,
663vs, 478vs.
È
According to Buu-Hoõ et al. [22], 50 g of freshly distilled
2-aminothiophenol (0.4 mol) and 60 g of acetonylacetone
(0.52 mol) were dissolved in 250 ml benzene and treated
with 1.5 ml acetic acid. The mixture was re¯uxed for 10 h.
The solvent was removed in vacuo. Fractional distillation of
the remaining oil yielded 68.8 g (85%) of H-tpdp
1
[b.p. ˆ 748C (0.1 mm)] as a colourless liquid. NMR: H,
ꢀ ˆ 7.19±7.39 (m, Ph), 5.95 (s, C=CH), 2.92 (s, SH), 1.95 (s,
CH3). IR (cm 1): 3102m, 3063m, 2977s, 2918vs, 2885s,
2856m, 2550s, 1586s, 1522s, 1481vs, 1440vs, 1398vs,
1321s, 1261m, 1219m, 1091m, 1036m, 766vs, 738vs,
2.2.5. Synthesis of [Me4N][Co(tpdp)3(N-MeIm)] (5)
The synthesis was conducted analogously to 4. After
recrystallization from acetonitrile green prisms of 5 were
obtained at 158C. 82% yield. Decomposition occurs at
1418C. Anal. Calc. for C44H54N6S3Co: C, 64.3; H, 6.6; N,
10.2. Found: C, 64.0; H, 6.6; N, 10.4%. IR (cm 1): 3050w,
2974m, 2917m, 1579m, 1472vs, 737vs, 662m, 481m. UV±
Vis (DMSO), ꢁmax in nm (" in cm 1 M 1): 716 (3834), 698
(981), 681 (1040), 606 (421).
‡
473m. MS (m/e (%)): 203 (66) M .
2.2. Syntheses of the Zn(II), Co(II) and Cd(II) complexes
2.2.1. Synthesis of [Et3NH]2[Zn(tpdp)4]ÁMeCN (1)
0.068 g (0.5 mmol) of dry ZnCl2 dissolved in 5 ml MeOH
was added dropwise to a solution of Et3N (0.4 g, 4 mmol)
and H-tpdp (0.61 g, 3 mmol) in 15 ml MeOH. After 30 min
of stirring, the white precipitate was collected, washed with
MeOH and dried in vacuo (88% yield, 492 mg). 1 was
recrystallized from hot acetonitrile to yield colourless
prisms. M.p. 1838C. Anal. Calc. for C62H83N7S4Zn: C,
66.5; H, 7.5; N, 8.8. Found: C, 66.4; H, 7.5; N, 8.8%. IR
(cm 1): 3441m, 3096w, 3048w, 2975m, 2919w, 2669w,
1582s, 1474vs, 759m, 662vs, 487vs, 477vs.
2.2.6. Synthesis of [Zn(tpdp)2(N-MeIm)2] (6)
0.2 g Et3N (2 mmol), 0.406 g H-tpdp (2 mmol) and
0.164 g N-methylimidazole (2 mmol) were dissolved in
35 ml MeOH. Dry ZnCl2 (0.136 g, 1 mmol) in 10 ml MeOH
was added and big colourless crystals were obtained. The
crystals were ®ltered off, washed with MeOH and dried in
vacuo (82% yield, 0.52 g). 6 was recrystallized from hot
toluene to give colourless needles. M.p. 2158C. Anal. Calc.
for C32H36N6S2Zn: C, 60.6; H, 5.7; N, 13.2. Found: C, 60.5;
H, 5.7; N, 13.3%. IR (cm 1): 3131w, 3057w, 2967w, 2928w,
1582s, 1472vs, 763vs, 655vs, 479vs.
2.2.2. Synthesis of [Et3NH]2[Co(tpdp)4]ÁMeCN (2)
The synthesis was conducted analogously to 1. Green
prisms were obtained in 85% yield. Decomposition at 988C.
Anal. Calc. for C62H83N7S4Co: C, 66.9; H, 7.5; N, 8.8.
Found: C, 66.5; H, 7.7; N, 8.6%. IR (cm 1): 3096w, 3046m,
2975m, 2919m, 1579m, 1474vs, 760s, 664m, 481m. UV±
Vis (DMSO), ꢁmax in nm (" in cm 1 M 1): 716 (3875), 701
(862), 681 (930), 613 (421).
2.2.7. Syntheses of [Co(tpdp)2(N-MeIm)2] (7) and of
[Cd(tpdp)2(N-MeIm)2] (8)
The syntheses of 7 and 8 were conducted analogously to 6.
7 was recrystallized from benzene to give dark green
crystals at 48C within 5 days. 33% yield. M.p. 1508C. Anal.
Calc. for C32H36N6S2Co: C, 61.2; H, 5.8; N, 13.4. Found: C,
61.1; H, 5.6; N, 13.7%. IR (cm 1): 3129m, 3048m, 2967m,
2915w, 1581m, 1471vs, 755vs, 654vs, 484vs. UV±Vis
(DMSO), ꢁmax in nm (" in cm 1 M 1): 733 (4520), 658
(5360), 565 (5480), 502 (5560).
8 was recrystallized from hot acetonitrile to yield colour-
less needles. Anal. Calc. for C32H36N6S2Cd: C, 55.5; H, 5.5;
N, 12.4. Found: C, 55.9; H, 5.3; N, 12.3%. IR (cm 1):
3120m, 3040w, 2950w, 2920m, 2870w, 1575m, 1420m,
1400s, 1310m, 1280m, 1230s, 1035m, 990s, 820s, 735s,
650s, 470m.
2.2.3. Synthesis of [Ph4P]2[Cd(tpdp)4] (3)
0.95 g (4.7 mmol) H-tpdp was dissolved in 5 ml DMF and
i
45 ml PrOH. Et3N (1 ml), Ph4PCl (0.74 g, 1.2 mmol) and
0.18 g CdCl2 (1 mmol) were added to the solution. After
stirring overnight, the white precipitate was collected and
dried in vacuo (86% yield, 1.37 g). 3 was recrystallized from
a DMF/iPrOH solution (1/10 vol./vol.) to yield pale yellow
crystals after cooling the solution to 08C for 24 h. M.p.
2078C. Anal. Calc. for C96H88N4P2S4Cd: C, 72.1; H, 5.5; N,
3.5. Found: C, 72.5; H, 5.6; N, 3.6%. IR (cm 1): 3040m,
2960m, 2900m, 2880m, 1665m, 1575s, 1510m, 1460s,
1425s, 1390s, 1310m, 1245w, 1220w, 1100s, 1070s,
1030m, 990s, 740s, 660w, 470m.
2.3. Crystal structure determination
2.2.4. Synthesis of [Me4N][Zn(tpdp)3(N-MeIm)] (4)
0.207 g Sodium (9 mmol) was dissolved in 25 ml MeOH
and treated with 1.827 g H-tpdp (9 mmol) and 0.409 g dry
Intensity data were collected on a STOE imaging-
plate diffraction system (3,6,7,8) (Mo Ka radiation,

264
J. Otto et al. / Inorganica Chimica Acta 285 (1999) 262±268
Table 1
Crystallographic data for 1, 2, 4, 5
1
2
4
5
Formula
C
62H83N7S4Zn
C62H83N7S4Co
1113.52
170(2)
C44H54N6S3Zn
828.48
C44H54N6S3Co
822.04
Formula weight
Temperature (K)
Diffractometer
Crystal size (mm)
Crystal system
Space group
1119.96
170(2)
150(2)
170(2)
Siemens P3
0.35 Â 0.41 Â 0.32
monoclinic
P21/c
Siemens P3
0.40 Â 0.25 Â 0.18
monoclinic
P21/c
Syntex P21
0.28 Â 0.22 Â 0.35
triclinic
Siemens P3
0.25 Â 0.28 Â 0.20
triclinic
ꢀ
P1
ꢀ
P1
Ê
a (A)
Ê
b (A)
Ê
c (A)
22.302(5)
13.631(3)
20.326(4)
90
22.306(4)
13.646(3)
20.311(4)
90
11.850(2)
13.236(3)
16.102(3)
89.26(3)
71.19(3)
64.81(3)
2140(1)
2
11.973(2)
13.283(3)
16.171(3)
89.35(3)
71.07(3)
64.68(3)
2175(1)
2
ꢂ (8)
ꢃ (8)
ꢄ (8)
91.87(2)
90
91.91(2)
90
Ê 3
V (A )
6176(2)
4
6197(2)
4
Z
3
)
Dc (g cm
F(000)
1.205
2392
1.197
2380
1.285
876
1.256
870
1
)
ꢅ (cm
2ꢆ Range (8)
hkl Range
5.76
4.56
7.58
5.76
4.00 ꢀ 2ꢆ ꢀ 54.00
28 ꢀ h ꢀ 18
1 ꢀ k ꢀ 17
25 ꢀ l ꢀ 25
13685
4.00 ꢀ 2ꢆ ꢀ 50.10
26 ꢀ h ꢀ 1
16 ꢀ k ꢀ 16
24 ꢀ l ꢀ 24
11179
4.00 ꢀ 2ꢆ ꢀ 54.04
10 ꢀ h ꢀ 15
15 ꢀ k ꢀ 16
19 ꢀ l ꢀ 20
9735
4.02 ꢀ 2ꢆ ꢀ 50.00
0 ꢀ h ꢀ 14
14 ꢀ k ꢀ 15
18 ꢀ l ꢀ 19
7917
No. data collected
No. unique data
Rint
13344
10880
9278
7519
0.0481
0.0381
0.0201
0.0260
No. unique data with I ꢁ 2ꢇ(I)
R1 (I > 2ꢇ(I))
wR2 (all data)
GOF
8773
6774
6880
4710
0.0354
0.0815
0.0391
0.0887
0.0319
0.0827
0.0609
0.1812
0.849
0.835
0.939
0.964
3
Ê
Residual density (e A
)
0.680
0.538
0.488
1.680
Ê
ꢁ ˆ 0.71073 A, graphite monochromator) with a sample-to-
plate distance of 60 mm (3,6,8) and 70 mm (7), respectively.
A scan range from 0 to 1808 with an exposure time of 2 min
per 28 increment was used. Intensity data for compounds 1,
2, 4 and 5 were collected on four-circle diffractometers
(same radiation, etc.) using the w-scan technique. Further
details on the data collections are summarized in Tables 1
and 2. The structures were solved using the Patterson
method (program XS) [23]. A series of full-matrix least-
squares re®nement cycles on F2 (program SHELXL 93) [24]
followed by Fourier syntheses gave all the remaining atoms.
The hydrogen atoms were included at ®xed positions with a
common isotropic temperature factor, except for the four
triethylammonium protons of structures 1 and 2. All non-
hydrogen atoms of the complexes 1±8 were re®ned aniso-
tropically.
metal complexes with monodentate ligands modelling the
environment of zinc in N,S-coordinated active centres of
enzymes. The reaction of several metal salts with various
amounts of the ligands H-tpdp and N-MeIm in the presence
of a base provided the series of complexes, [MII(tpdp)4]2
,
[MII(tpdp)2(N-MeIm)2] (M ˆ Zn, Co, Cd) and [MII(tpdp)3-
(N-MeIm)] (M ˆ Zn, Co). All these compounds were
characterized by X-ray structure analyses, elemental ana-
lyses and IR absorption spectroscopy. In addition the Co
compounds were characterized by UV±Vis spectroscopy.
The attempt to synthesize the missing [Cd(tpdp)3(N-
MeIm)] complex was not successful. Here we obtained
the dinuclear complex [Cd2(tpdp)6]2 , which will be pub-
lished later.
3.1. Crystal structures of [Et3NH]2[Zn(tpdp)4]ÁMeCN (1),
[Et3NH]2[Co(tpdp)4]ÁMeCN (2) and
[Ph4P]2[Cd(tpdp)4] (3)
3. Results and discussion
A projection of the [Zn(tpdp)4]2 anion is depicted in
Fig. 1 as an example for this series. Relevant bond length
and bond angle information for the three complexes is given
in Table 3. In all of these complexes the metal ion is in a
distorted tetrahedral environment. Compounds 1 and 2 are
isomorphous and isostructural. The average M±S bond
The ligand H-tpdp is similar to the often used ligand
thiophenol [25,26]. The bulky substituent 2,5-dimethylpyr-
role in ortho-position should prevent the formation of higher
condensate aggregates as known from pure zinc thiophe-
nolates [27±29]. The main goal was to produce tetrahedral

J. Otto et al. / Inorganica Chimica Acta 285 (1999) 262±268
265
Table 2
Crytallographic data for 3, 6, 7, 8
3
6
7
8
Formula
C96H88N4P2S4Cd
1600.28
C32H36N6S2Zn
634.16
C32H36N6S2Co
627.72
C32H36N6S2Cd
681.19
Formula weight
Temperature (K)
Diffractometer
Crystal size (mm)
Crystal system
Space group
293(2)
293(2)
293(2)
293(2)
STOE IPDS
0.31 Â 0.20 Â 0.25
triclinic
STOE IPDS
0.27 Â 0.30 Â 0.17
monoclinic
P21/c
STOE IPDS
0.38 Â 0.31 Â 0.23
monoclinic
P21/c
STOE IPDS
0.21 Â 0.25 Â 0.19
monoclinic
P21/n
ꢀ
P1
Ê
a (A)
Ê
b (A)
Ê
c (A)
13.087(1)
13.475(1)
26.821(3)
95.92(1)
93.14(1)
116.13(1)
4197(1)
2
15.030(3)
15.818(3)
14.210(3)
90
15.186(3)
15.925(3)
13.947(3)
90
7.510(1)
15.090(1)
28.488(3)
90
ꢂ (8)
ꢃ (8)
ꢄ (8)
90.73(3)
90
90.88(3)
90
95.68(1)
90
Ê 3
V (A )
3378(1)
4
3373(1)
4
3213(1)
4
Z
3
)
Dc (g cm
F(000)
1.266
1668
1.247
1328
1.236
1316
1.408
1400
1
)
ꢅ (cm
2ꢆ Range (8)
hkl Range
4.46
8.80
6.62
8.41
10.20 ꢀ 2ꢆ ꢀ 56.10
17 ꢀ h ꢀ 17
17 ꢀ k ꢀ 17
35 ꢀ l ꢀ 35
33928
10.22 ꢀ 2ꢆ ꢀ 56.04
19 ꢀ h ꢀ 18
20 ꢀ k ꢀ 20
18 ꢀ l ꢀ 18
27873
9.14 ꢀ 2ꢆ ꢀ 51.88
18 ꢀ h ꢀ 18
19 ꢀ k ꢀ 19
17 ꢀ l ꢀ 17
22486
10.42 ꢀ 2ꢆ ꢀ 56.10
9 ꢀ h ꢀ 9
19 ꢀ k ꢀ 18
37 ꢀ l ꢀ 37
22 934
No. data collected
No. unique data
Rint
18179
7682
6373
7598
0.0537
0.0671
0.0794
0.0905
No. unique data with I > 2ꢇ(I)
R1 (I > 2ꢇ(I))
wR2 (all data)
GOF
12917
5025
4878
5496
0.0513
0.1348
0.0509
0.1337
0.0438
0.1159
0.0515
0.1528
1.113
1.110
1.071
1.054
3
Ê
Residual density (e A
)
0.606
0.345
0.238
0.907
Table 3
Ê
Selected interatomic distances (A) and angles (8) for the series
[MII(tpdp)4]2
M ˆ Zn (1)
M ˆ Co (2)
M ˆ Cd (3)
M±S(1)
M±S(2)
M±S(3)
M±S(4)
2.328(1)
2.332(1)
2.338(1)
2.350(1)
2.296(1)
2.304(1)
2.297(1)
2.318(1)
2.536(1)
2.537(1)
2.551(2)
2.523(1)
S(1)±M±S(2)
S(1)±M±S(3)
S(1)±M±S(4)
S(2)±M±S(3)
S(2)±M±S(4)
S(3)±M±S(4)
115.13(2)
115.93(3)
97.40(2)
94.66(2)
119.38(2)
115.82(2)
114.38(3)
116.65(3)
96.68(3)
94.19(3)
118.83(3)
117.70(3)
119.50(4)
92.32(4)
124.40(4)
124.79(4)
87.61(3)
111.36(4)
interactions between two sulfur atoms of the ligand and the
proton of one triethylammonium as shown in Fig. 2. The
HÁ Á ÁS distances are 2.44 (HÁ Á ÁS(2)), 2.61 (HÁ Á ÁS(1)), 2.89
Fig. 1. Molecular anion of complex 1 with vibrational ellipsoids at the
50% probability level.
Ê
(HÁ Á ÁS(4)) and 3.34 A (HÁ Á ÁS(3)) for 1 and 2.41 (HÁ Á ÁS(2)),
Ê
lengths are 2.337(1) A for 1, 2.304(1) A for 2 and
Ê
Ê
2.65 (HÁ Á ÁS(1)), 2.83 (HÁ Á ÁS(4)) and 3.32 A (HÁ Á ÁS(3)) for
Ê
2.537(1) A for 3. They show no signi®cant differences to
2. In the literature N±HÁ Á ÁS hydrogen bonds are listed for
Ê
the distances in other known metal±sulfur complexes
[30,31], but the bond angles deviate signi®cantly from
the ideal Td symmetry. The angles vary from 97.40(2) to
119.38(2)8 for 1, from 94.19(3) to 118.83(3)8 for 2 and from
87.61(3) to 124.79(4)8 for 3. In 1 and 2 there are strong
HÁ Á ÁS distances as being signi®cantly shorter than 3 A
[32,33]. Thus, for the complexes 1 and 2, three of these
interactions can be described as hydrogen bonds. In both
cases the sulfur atoms S(1) and S(4) form a tricentric
hydrogen bond with the proton. These strong interactions

266
J. Otto et al. / Inorganica Chimica Acta 285 (1999) 262±268
Fig. 2. Orientation of the cations in 1 towards the anion (metal donor set)
Ê
with HÁ Á ÁS distances (A).
might be one reason for the decrease of the corresponding
S±M±S angles.
To our knowledge the distortion of the tetrahedron in
complex 3 is the largest one observed for related compounds
with monodentate sulfur ligands. This rather large deviation
from the ideal Td symmetry cannot be explained with
packing effects caused by the bulky counterion Ph4P . In
similar complexes like [Ph4P]2[Cd(SPh)4] the tetrahedral
Fig. 3. Molecular anion of complex 4 with vibrational ellipsoids at the
50% probability level.
‡
angles vary only from 98.7(2) to 120.0(2)8 [26].
3.2. Crystal structures of [Me4N][Zn(tpdp)3(N-MeIm)] (4)
and [Me4N][Zn(tpdp)3(N-MeIm)] (5)
Coucouvanis et al. [30] published some structural prin-
ciples of [MII(SPh)4]2 complexes. Therein the phenyl ring
approaches coplanarity with its M±S±C plane; this orienta-
tion allows the overlap of the 3pp sulfur orbital with the
aromatic ring. The interaction of the coplanar MSPh groups
with the MS4 core results in the overall distortion of the MS4
tetrahedron in a systematic manner [26,30]. With respect to
this interaction there are only two possible different con-
formations for the M(SPh)4 unit, both of which have idea-
lized D2d symmetry: a S4 and a D2d isomer [34,35]. The S4
isomer is predicted to have a tetragonally compressed MS4
core while the D2d isomer is predicted to have a tetragonally
elongated MS4 core [26,30,34,35].
4 and 5 are isomorphous and isostructural. As an example
the thermal ellipsoid plot of complex 4 is depicted in Fig. 3.
Compounds 4 and 5 crystallize with two [MII(tpdp)3(N-
MeIm)] anions and two tetramethylammonium ions per
unit cell. The counterions are systematically disordered, but
will not be discussed in detail here. Selected interatomic
distances and angles of both complexes are given in Table 4.
The geometry of the metal coordination is approximately
Ê
tetrahedral. The average bond lengths are 2.328(1) A (4) and
Ê
Ê
2.295(2) A (5) for the M±S bonds and 2.082(2) A (4) and
Ê
2.065(4) A (5) for the M±N bonds. The angles show only
A series of [MII(S-2-Ph-C6H4)4]2 compounds, published
by Silver et al. [31], appear to follow the same rules in spite
of the fact that there were slight deviations (14±168) from
coplanarity. Although the H-tpdp ligand is quite similar to 2-
phenylbenzenethiole, complexes 1, 2 and 3 show even larger
deviations from coplanarity between the phenyl rings and
the M±S±C planes. The average torsion angles are 26.18 for
1, 24.88 for 2 and 20.58 for 3. The two S±M±S angles in 1
bisected by the pseudo S4 axis are 94.66(2) and 97.40(2)8
while the average of the remaining four angles is 116.578.
The S±M±S angles of complexes 2 and 3 are in the same
range, as shown in Table 3. Accordingly, the D2d conforma-
tional isomer has been observed in all cases, the symmetry is
only approximate.1
slight deviations from ideal Td symmetry. The smallest
angles are the S(2)±MII±S(3) angles with 105.23(3)8 (4)
and 103.8 (1)8 (5) and the largest angles are the S(2)±MII±
N(4) angles with 114.41(5)8 (4) and 115.2 (1)8 (5), respec-
tively. Only very few model complexes for Zn(S-cys)3(N-
his) centre have been published so far. To our knowledge
there are only two other ZnS3N and CoS3N crystal struc-
tures each reported with monodentate ligands [36±39]. It is
Table 4
Ê
Selected interatomic distances (A) and angles (8) for the series
[Me4N][MII(tpdp)3(N-MeIm)]
M ˆ Zn (4)
M ˆ Co (5)
M±S(1)
M±S(2)
M±S(3)
M±N(4)
2.342(1)
2.327(1)
2.314(1)
2.082(2)
2.311(2)
2.294(1)
2.281(2)
2.065(4)
1A general remark (for which we are indebted to one of the referees)
concerning the possible causes for distortions of tetrahedral ML4
complexes such as M(SR)4 thiolates and especially of tetrahedral
M(S-Cys)4 sites in proteins should be added in this context. Tetrahedral
complexes are very easily distorted with little or no change of energy;
generally it is not necessary to find electronic explanations for these
distortions. Very often it is not adequate to make a big and incorrect point
of such electronic reasons in the interpretation of crystallographic results.
This is especially true for such, sometimes, inadequate discussions for
MS4 distortions in protein structures
S(1)±M±S(2)
S(1)±M±S(3)
S(1)±M±N(4)
S(2)±M±S(3)
S(2)±M±N(4)
S(3)±M±N(4)
108.25(3)
114.14(3)
106.46(5)
105.23(3)
114.41(5)
108.57(5)
107.8(1)
113.0(1)
106.9(1)
103.8(1)
115.2(1)
110.3(1)

J. Otto et al. / Inorganica Chimica Acta 285 (1999) 262±268
267
136.84(4)8 for 8. Consequently, in all these complexes the
tetrahedron is signi®cantly distorted. The causes of the
variation in bond lengths and angles of 6, 7 and 8 are the
different ion radii of the three metal ions. Several MS2N2
structures with substituted thiophenol and N-methylimida-
zole are known in recent literature [40±43].
4. Conclusion
The present study shows that the metal centres of the three
reported zinc model complexes are similar to the metal
centre of zinc-containing DNA-binding proteins [1±17].
Furthermore we report that all cobalt complexes described
herein are isomorphous and isostructural to the analogous
zinc complexes. The cadmium complexes show slight dif-
ferences in the crystal systems, but the direct metal envi-
ronments are quite similar to those in the zinc or cobalt
analogues. Thus, the present investigation clearly supports
results that there should be only minor changes in the direct
metal environment in proteins after replacing zinc by cobalt
or cadmium.
Fig. 4. Complex 6 with vibrational ellipsoids at the 30% probability level.
remarkable that only few model compounds for retroviral-
type zinc ®nger proteins are known although they are of
increasing interest due to their potential connection to a new
HIV-therapy [18±20].
3.3. Crystal structures of [Zn(tpdp)2(N-MeIm)2] (6),
[Co(tpdp)2(N-MeIm)2] (7) and [Cd(tpdp)2(N-MeIm)2]
(8)
5. Supplementary material
A thermal ellipsoid plot of complex 6 as an example for
this series of neutral complexes is given in Fig. 4. Again the
zinc (6) and cobalt (7) complexes are isomorphous and
isostructural. Relevant bond lengths and bond angles for the
three complexes are given in Table 5. With average M±S
Further details of the crystal structure investigation may
be obtained from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK, on
quoting the full journal citation.
Ê
bond lengths of 2.291(1) A (6), 2.275(1) A (7) and
Ê
Ê
Ê
2.467(1) A (8) and M±N bond lengths of 2.074(3) A (6),
Acknowledgements
Ê
Ê
2.014(2) A (7) and 2.286(4) A (8) the M-ligand bond
lengths are shorter than the ones in the structures described
before. This could be caused by the lower steric demand
of the two N-methylimidazole ligands. Concomitantly, the
N±M±N angles are the smallest, while the S±M±S angles are
the largest. The bond angles vary from 99.5(1) to 128.75(4)8
for 6, from 102.6(1) to 123.66(3)8 for 7 and from 93.3(2) to
È
We thank Mechtild Lage and Bettina Bremer for the
crystallographic data collections, and we thank Dr Fried-
helm Rogel for valuable discussions. This work was sup-
ported by the Fonds der Chemischen Industrie and the
Deutsche Forschungsgemeinschaft.
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Table 5
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