New nickel complexes with an S4 coordination sphere; Synthesis, characterization and reactivity towards nickel and iron compounds

Article abstract of DOI:10.1016/j.ica.2004.02.003

Three new nickel complexes have been synthesized with the ligands Hbss (4-mercapto-2-thia-1-butylbenzene) and Hbsms (2-(benzylsulfanyl)-2-methyl-1- propanethiol). [Ni(bss)₂] is a mononuclear complex with an S ₄ coordination environme

Full text of DOI:10.1016/j.ica.2004.02.003

Inorganica Chimica Acta 357 (2004) 2687–2693
www.elsevier.com/locate/ica
New nickel complexes with an S4 coordination sphere;
synthesis, characterization and reactivity towards nickel and
iron compounds
a
Johanna A.W. Verhagen , Michela Beretta , Anthony L. Spek , Elisabeth Bouwman
a
b
a,
*
a
Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received 23 January 2004; accepted 7 February 2004
Available online 27 February 2004
Abstract
Three new nickel complexes have been synthesized with the ligands Hbss (4-mercapto-2-thia-1-butylbenzene) and Hbsms (2-
(benzylsulfanyl)-2-methyl-1-propanethiol). [Ni(bss)2] is
a mononuclear complex with an S4 coordination environment.
[Ni3(bss)4](BF4)2 and [Ni3(bsms)4](BF4)2 are linear trinuclear complexes that can be synthesized either directly from the ligands Hbss
and Hbsms in a reaction with Ni(BF4)2, or via the mononuclear complexes [Ni(bss)2] and [Ni(bsms)2] in a reaction with Ni(BF4)2. These
reactions have been monitored with ligand field spectroscopy. Crystals suitable for X-ray diffraction were obtained for
[Ni3(bss)4](BF4)2. The complex crystallizes in the space group P21=c. The nickel centers are in a square-planar environment; two
peripheral nickel centers with an S2S02 (S ¼ thiolato; S0 ¼ thioether) coordination environment and the central nickel ion with an S4
coordination environment.
The mononuclear nickel complexes [Ni(bss)2] and [Ni(bsms)2] were reacted with FeCl2, resulting in the hetero-tetranuclear nickel–
iron complexes [Ni(bss)2FeCl2]2 and [Ni(bsms)2FeCl2]2. All complexes were characterized by analytical and spectroscopic methods.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Nickel; Iron; S ligands; Electronic structure; Ligand field spectroscopy
1. Introduction
[Fe]-hydrogenases [2]. Extensive studies and character-
izations, including single-crystal X-ray diffraction, have
revealed the structure of several [NiFe]-hydrogenases
[3,4]. The X-ray studies have shown that the active sites
of the [NiFe] hydrogenases isolated from Desulfovibrio
gigas [3] and D. vulgaris [4] contain the hetero-dinuclear
site [(Cys-S)2NiII(l-S-Cys)2FeII(CN)2(CO)] in which the
nickel center has an S4 coordination sphere. Two cy-
steines are bridging the nickel center to the iron center.
Furthermore, the iron center is coordinated by three
diatomic molecules.
In recent years considerable interest has been shown in
hydrogenases, because of the fact that possible future
energy problems may be solved using a hydrogen econ-
omy [5]. This recent interest in hydrogenases is also shown
in the variety of structural models published [2,6–12].
However, most of the structural models contain a
mixed donor environment around nickel. A variety of
Nickel plays an important role in hydrogenases,
zureases, carbon-monoxide-dehydrogenase, acetyl-CoA-
synthase, superoxide dismutase and methyl-S-coen-
zyme-M methylreductase. Hydrogenases are widespread
among several classes of anaerobic and, occasionally,
even aerobic bacteria [1]. These enzymes catalyze the
reversible oxidation of dihydrogen, which allows the cell
to generate or dispose of reducing equivalents during
respiration. In vitro, hydrogenases also catalyze the Hþ/
D2 exchange [2].
Hydrogenases can be divided into two main classes,
based on their metal content: [NiFe]-hydrogenases and
* Corresponding author. Tel.: +31-71-527-4550; fax: +31-71-527-
4451.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.02.003

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J.A.W. Verhagen et al. / Inorganica Chimica Acta 357 (2004) 2687–2693
square-planar nickel complexes have been synthesized
with a NiS4 chromophore [13], and several nickel–iron
complexes have been synthesized relevant for hydroge-
nase modeling [2,11,14–18]. So far only a few hetero-
dimetallic nickel iron complexes have been reported
containing an S4 coordination environment around nickel
[16,19]. Recently, we have reported two new nickel com-
plexes with a NiS4 chromophore, [Ni(xbsms)] and
[Ni(bsms)2] [20]. The reactivity of the former nickel
complex towards several iron complexes has been inves-
tigated, which led to new interesting nickel–iron com-
plexes [19]. In this paper we describe the reactivity
towards iron compounds of two nickel complexes with
didentate ligands containing one thioether and one
thiolate donor, namely [Ni(bsms)2] and [Ni(bss)2] [20],
(Hbsms ¼ 2-(benzylsulfanyl)-2-methyl-1-propanethiol,
Hbss ¼ 4-mercapto-2-thia-1-butylbenzene). The thiolate
donors in these complexes constitute a possible site for
further reactivity.
2.3. 2-(benzylthio)ethanol (1)
To a solution of benzylmercaptan (10.0 ml, 10.58 g,
0.085 mol) in 50 ml ethanol, was added subsequently, 2-
chloroethanol (6.85 g, 0.085 mol) and a solution of
NaOH (3.41 g, 0.085 mol) in 25 ml water. After stirring
for 1 h and adding an additional amount of water, the
product was extracted with 3 ꢁ 25 ml chloroform. The
combined fractions were washed with 2 ꢁ 50 ml water to
remove the ethanol. After drying with MgSO4 and fil-
tration, the chloroform was evaporated in vacuo to
obtain a yellow oil in a yield of 11.46 g (80%). dH [199.56
MHz, CDCl3, 298 K] 6.69 (m, 5H, phenyl ring), 3.12 (s,
2H, Ph–CH2–S–), 3.06 (t, 3J ¼ 6:15 Hz, 2H, S–CH2–
CH2–OH), 2.33 (s, 1H, –OH), 2.01 (t, 3J ¼ 6:15 Hz, 2H,
–CH2–CH2–OH). dC [50.18 MHz, CDCl3, 298 K] 137.80
(Ph–C1), 128.55 (Ph–C2, Ph–C6), 128.26 (Ph–C3, Ph–
C5), 126.83 (Ph–C4), 60.13 (–CH2– CH2–OH), 35.52
(Ph–CH2–S–), 33.80 (–S–CH2–CH2–).
2.4. {[(2-chloroethyl)thio]methyl}benzene (2)
2. Experimental section
To a solution of 1 (11.46 g, 0.068 mol) in 20 ml chlo-
roform was slowly added a solution of excess thionyl
chloride (10 g, 0.085 mol) in 30 ml chloroform. After
stirring the reaction mixture for 1 h the chloroform and
the excess of thionyl chloride were evaporated in vacuo, to
yield 12.76 g of a yellow oil (100%). dH [199.56 MHz,
CDCl3, 298 K] 7.24 (m, 5H, phenyl ring), 3.69 (s, 2H, Ph–
CH2–S–), 3.46 (t, 3J ¼ 7:80 Hz, 2H, –S–CH2–CH2–), 2.70
2.1. Chemicals
All preparations were carried out in reagent grade
solvents. All chemicals used in the syntheses were ob-
tained from Acros or Aldrich and were used without
further purification. The complexes were synthesized in
an argon atmosphere using standard Schlenk tech-
niques. Solvents were deoxygenated by bubbling
through a stream of argon or by the freeze–pump–thaw
method and were dried over molecular sieves. The
complex [Ni(bsms)2] [20] was synthesized according to
published methods.
3
(t, J ¼ 7:80 Hz, 2H, –CH2–CH2–Cl). dC [50.18 MHz,
CDCl3, 298 K] 137.31 (Ph–C1), 128.32 (Ph–C2, Ph–C6),
128.17 (Ph–C3, Ph–C5), 126.80 (Ph–C4), 42.44 (Ph–CH2–
S–), 35.99 (–S–CH2–CH2–), 32.84 (–CH2–CH2–Cl).
2.5. 4-thiouroneum-1-phenyl-2-thiabutane hydrochloride
(3)
2.2. Physical measurements
IR spectra were recorded on a Perkin–Elmer FT-IR
Paragon 1000 spectrophotometer equipped with a
golden gate ATR device, using the reflectance technique
(4000–300 cmꢀ1, resolution 4 cmꢀ1). Elemental analyses
were carried out on a Perkin–Elmer series II CHNS/O
analyzer 2400. Ligand field spectra were obtained on a
Perkin–Elmer Lambda 900 spectrophotometer. The
diffuse reflectance technique, with MgO as a reference
was used for the solid compounds. Ligand field spectra
of the solutions were obtained with the used solvent in
the reference beam. NMR spectra were taken on a
Bruker WM 300 MHz spectrometer and on a Jeol FX-
200 Teqmac. Proton and carbon chemical shifts are in-
dicated in ppm relative to tetramethylsilane (TMS).
Room temperature magnetic susceptibility measure-
ments were performed on a Johnson Matthey magnetic
susceptibility balance type MK1 or with the NMR
method [21].
To a solution of 2 (12.76 g, 0.068 mol) in 30 ml eth-
anol was added a solution of thiourea (5.16 g, 0.068
mol) in 50 ml ethanol. After 5 h of reflux the solvent was
evaporated under reduced pressure. Addition of chlo-
roform to the resulting oil induced precipitation of the
product as a white solid in a yield of 16.80 g (94%). dH
[199.56 MHz, DMSO-d6, 298 K] 9.34 (s, 4H, –S–
C(NH2)2 þClꢀ), 7.31 (m, 5H, phenyl ring), 3.83 (s, 2H,
Ph–CH2–S–), 3.46 (t, 3J ¼ 7:18 Hz, 2H, –CH2–CH2–S–),
2.64 (t, 3J ¼ 7:18 Hz, 2H, –S–CH2–CH2–). dC [50.18
MHz, CDCl3, 298 K] 174.32 (–S–C(NH2)2 þClꢀ),
142.85 (Ph–C1), 133.57 (Ph–C2, Ph–C6), 133.04 (Ph–
C3, Ph–C5), 131.56 (Ph–C4), 39.29 (–S–CH2–CH2–),
34.97 (–CH2–CH2–S–), 34.47 (Ph–CH2–S–). IR: mmax
(cmꢀ1) ¼ 3037s, 2729m, 1644vs, 1544m, 1495w, 1453m,
1406s, 1301w, 1276w, 1246w, 1117w, 1086w, 931w,
856vw, 815w, 764w, 725m, 704s, 607w, 637vw, 563w,
468m, 390vw.

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2.6. 4-Mercapto-2-thia-1-butylbenzene (Hbss) (4)
Crystals suitable for X-ray diffraction were obtained
from acetone/hexane.
To a solution of 3 (6.0 g, 0.023 mol) in 150 ml water
was added a solution of NaOH (1.5 g, 0.038 g) in 10
ml water. The reaction mixture was refluxed for 30 min
after which it was cooled to room temperature. Con-
centrated hydrochloric acid was added until the mix-
ture was neutral. The mixture was extracted with
3 ꢁ 25 ml portions of dichloromethane. The combined
fractions were dried over MgSO4. After filtration, the
filtrate was evaporated in vacuo to obtain the product
as an oil in a yield of 2.65 g (63%). dH [199.56 MHz,
CDCl3, 298 K] 7.30 (m, 5H, phenyl ring), 3.73 (s, 2H,
Ph–CH2–S–), 2.63 (m, 4H, –S–CH2–CH2–SH), 1.66 (t,
1H, –SH). dC [50.18 MHz, CDCl3, 298 K] 128.84 (Ph–
C2, Ph–C6), 128.72 (Ph–C1) 128.63 (Ph–C3, Ph–C5),
127.20 (Ph–C4), 36.17 (Ph–CH2–S–), 35.33 (–S–CH2–
CH2–), 24.43 (–CH2–CH2–SH). IR: mmax (cmꢀ1) ¼
3060s, 3026s, 2919s, 2845s, 2551m, 1950m, 1882m,
1807m, 1759w, 1711vw, 1601s, 1583m, 1493s, 1452s,
1428s, 1273s, 1240s, 1210s, 1136m, 1070s, 1028s, 966m,
947w, 917m, 876w, 844w, 804w, 768s, 700vs, 620w,
564s, 472s, 409vw, 380w.
IR: mmax (cmꢀ1) ¼ 2998w, 1602w, 1496s, 1456s, 1413s,
1285m, 1242m, 1168m, 1051vs, 1038vs, 1014vs, 994vs,
935s, 880w, 857s, 841m, 766s, 700vs, 678m, 620w, 565m,
520s, 478s, 404w, 377w, 353w, 312w. Elemental analysis
Anal. Calc. for C36H44B2F8Ni3S8 (1083.3): C, 39.93; H,
4.10; S, 23.68. Found: C, 39.92; H, 4.11; S, 23.16%.
2.9. Ni3(bsms)4](BF4)2 (7)
The product can be synthesized in the same two
methods as for [Ni3(bss)4](BF4)2. In method 1 the brown
product was obtained in a yield of 87%. In method 2 the
brown product was obtained in a yield of 82%. dH
3
[300.13 MHz, acetone-d6, 323 K] 7.75 (d, J ¼ 7:32 Hz,
8H, Ph–C2, Ph–C6), 7.51 (m, 12H, Ph–C3, Ph–C4, Ph–
2
C5), 4.32 (d, J ¼ 12:72 Hz, 4H, Ph–CHH–S–), 4.23 (d,
2
2J ¼ 12:72 Hz, 4H, Ph–CHH–S–), 2.92 (d, J ¼ 13:14
Hz, 4H, –C(CH3) 2–CHH–S), 2.65 (d,2J ¼ 13:14 Hz,
4H, –C(CH3)2–CHH–S), 1.69 (s, 12H, –C(CH3)(CH3)–),
1.59 (s, 12H, –C(CH3) (CH3)–). IR: mmax
(cmꢀ1) ¼ 2967w, 1603w, 1496s, 1466m, 1455s, 1420w,
1395w, 1378s, 1368m, 1272w, 1228w, 1199w, 1142m,
1045vs, 1033vs, 929w, 882w, 856w, 815w, 770s, 745s,
721s, 700vs, 620w, 594w, 546w, 519s, 488m, 458s, 409w,
2.7. [Ni(bss)2] (5)
To a solution of Hbss (0.63 g, 3.4 mmol) in 30 ml
toluene was added [Ni(acac)2] (0.43 g, 1.7 mmol). After
2 h stirring, the solvent was partly evaporated in vacuo.
Upon addition of diethyl ether a brown precipitate was
formed, which was collected by filtration in a yield of
0.59 g (81%). dH [300.13 MHz, acetone-d6, 298 K] 7.40
(m, 5H, phenyl ring), 4.08 (s, 2H, Ph–CH2–S–), 2.78 (m,
4H, –S–CH2–CH2–S). IR: mmax (cmꢀ1) ¼ 3027w, 2921m,
1601m, 1518w, 1493s, 1452s, 1411s, 1242m, 1194s,
1112s, 1070m, 1026m, 1002w, 941m, 916w, 866w, 838w,
744m, 697vs, 666m, 619m, 564s, 462s, 402s, 352w, 327m.
Elemental analysis Anal. Calc. for C18H22NiS4 (425.3):
C, 50.83; H, 5.21; S, 30.15. Found: C, 49.97; H, 5.35; S,
29.60%.
340w.
Elemental
analysis
Anal.
Calc.
for
C44H60B2F8Ni3S8 (1193.5): C, 44.22; H, 5.06; S, 21.46.
Found: C, 43.42; H, 5.01; S, 21.13%.
2.10. [Ni(bss)2FeCl2]2 (8)
To a solution of [Ni(bss)2] (0.080 g, 0.19 mmol) in 30
ml acetonitrile was added solid FeCl2 ꢂ 4H2O (0.037 g,
0.19 mmol). After 1 h stirring, the acetonitrile was partly
evaporated in vacuo until a precipitate was formed. This
precipitate was collected by filtration to obtain a brown
product in a yield of 0.025 g (24%). IR: mmax
(cmꢀ1) ¼ 2980w, 2920w, 1496m, 1455s, 1428m, 1410m,
1277w, 1240m, 1204w, 1156w, 1072m, 1028w, 918m,
840m, 826m, 768s, 700vs, 674s, 628m, 565m, 475s,
352m, 306vs. Elemental analysis Anal. Calc.: for
C18H22Cl2FeNiS4 (552.05): C, 39.16; H, 4.02; S, 23.23.
Found: C, 38.74; H, 3.90; S, 22.23%.
2.8. [Ni3(bss)4](BF4)2 (6)
Method 1: To a solution of Hbss (1.05 g, 5.7 mmol) in
10 ml ethanol was added a solution of [Ni(H2O)6](BF4)2
(1.46 g, 4.3 mmol) in 10 ml ethanol. Immediately the
solution turned dark brown and a precipitate was
formed. After 30 min stirring the formed brown pre-
cipitate was collected by filtration and washed with
ethanol. Yield: 1.20 g (78%).
Method 2: To a solution of [Ni(bss)2] (0.17 g, 0.4
mmol) in 30 ml acetone was added a solution of
[Ni(H2O)6](BF4)2 (0.070 g, 0.2 mmol) in 15 ml ethanol.
After 2 h of stirring, the acetone was evaporated in
vacuo to obtain a brown precipitate, which was col-
lected by filtration in a yield of 0.17 g (80%).
2.11. [Ni(bsms)2FeCl2]2 (9)
To a solution of [Ni(bsms)2] (0.100 g, 0.21 mmol) in
40 ml acetonitrile was added FeCl2 ꢂ 4H2O (0.041 g, 0.25
mmol). After 1 h of stirring, the unreacted starting
products were removed by filtration. The acetonitrile of
the filtrate was partly evaporated in vacuo. After adding
diethyl ether, the formed precipitate was collected by
filtration to obtain a brown product in a yield of 0.045 g
(27%). IR: mmax (cmꢀ1) ¼ 2957w, 2922w, 1495m, 1455s,
1418m, 1366m, 1267m, 1245m, 1196m, 1141m, 1082m,

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Table 1
Crystal and structure refinement data for the trinuclear nickel complex
[Ni3(bss)4](BF4)2 (6)
complexes can be synthesized: the mononuclear complex
[Ni(bss)2] (5) and the trinuclear complex [Ni3(bss)4]
(BF4)2 (6). Both the mononuclear and the trinuclear
complex are air stable in the solid state. Similarly,
[Ni3(bsms)4](BF4)2 (7) has been synthesized.
Two new nickel–iron complexes were synthesized: by
a reaction of [Ni(bss)2] or [Ni(bsms)2] with FeCl2, the
complexes [Ni(bss)2FeCl2]2 (8) and [Ni(bsms)2FeCl2]2
(9) are formed. Both complexes are moderately air sta-
ble and only slightly soluble in polar solvents.
Empirical formula
Molecular weight
Crystal dimensions (mm)
Crystal system
C36H44B2F8Ni3S8
1082.96
0.03 ꢁ 0.20 ꢁ 0.20
monoclinic
P21=c
19.2282(17)
11.6797(8)
10.0485(17)
103.609(7)
2193.3(4)
2
Space group
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
b (°)
V (A )
3
ꢀ
3.2. Structures of the nickel complexes
Z
Dcalc: (mg mꢀ3
)
1.6398(3)
1.72
3738
Absorption coefficient l (mmꢀ1
Reflections measured
Reflections collected
)
A projection of the structure of the cationic complex
[Ni3(bss)4]2þ is shown in Fig. 1. Crystal data are given in
Table 1 and selected bond distances and angles are given
in Table 2. [Ni3(bss)4](BF4)2 crystallizes in the space
group P21=c with two molecules in the unit cell. The
structure is trinuclear with the three nickel centers on a
row. The central nickel ion, Ni(2) is located on an in-
version center and has an S4 coordination environment
consisting of four thiolate sulfurs. The peripheral nickel
ions, Ni(1) and Ni(1a) are coordinated by two bss li-
gands in a cis orientation. The overall shape of the tri-
nuclear cation can be described as a zig-zag chain
consisting of three square planes. The angle formed
between the planes, Ni(1)S2S02–Ni(2)S4 is 68.24(19)°.
Several compounds have been reported to have a similar
structure [10,25–30].
3027
Reflections observed (I > 2rðIÞ) 1212
Ra
wR2
0.0893
b
0.192
0.975
259
Sc
Number of refined parameters
P
P
a R ¼ ðjFoj ꢀ jFcjÞ= jFoj:
P
2
2 ꢀ1
.
b w ¼ ½ ðFoÞ þ ð0:0594 ꢁ PÞ ꢃ
P
2
2
2
1=2
c S ¼ ½ wðjFoj ꢀ jFcj Þ =ðNo ꢀ NvÞꢃ
:
1070m, 1028w, 1003w, 953w, 919w, 870w, 812w, 781s,
700vs, 620w, 586w, 548w, 481s, 468m, 408w, 383w,
340m, 310s. Elemental analysis Anal. Calc. for
C22H30Cl2FeNiS4 (607.2): C, 43.45; H, 4.97; S, 21.09.
Found: C, 42.88; H, 4.99; S, 20.40%.
Because of the inversion center Ni(2) has a perfect
square-planar surrounding. The peripheral nickel cen-
ters have only a small tetrahedral distortion with an
interplanar angle of 5.9(2)° between the planes S(6)–
Ni(1)–S(9) and S(26)–Ni(1)–S(29). The thiolates are
slightly pushed to each other resulting in an angle of
only 81.25(17)° for S(6)–Ni(1)–S(26), which is probably
due to steric hindrance of the cis oriented phenyl groups.
The Ni(1)–S–Ni(2) angles of 79.45(15)° and 79.53(17)°
together with the Ni–S distances result in a Ni(1)–Ni(2)
2.12. Crystal structure determination of [Ni3(bss)4]
(BF4)2
X-ray data were collected for a red–brown crystal on
an Enraf Nonius TurboCAD4 diffractometer on rotat-
ing anode (Mo Ka radiation, graphite monochromator,
ꢀ
k ¼ 0:71073 A, T ¼ 150 K). Reflection profiles were
highly structured and only a limited data set up to 23° in
could be collected. The structure was solved with Patt-
erson techniques using the program DIRDIF96 [22] and
refined on F 2 using SHELXL96 [23]. Hydrogen atoms
were introduced at calculated positions and refined
riding on their carrier atoms. Geometrical calculation,
including the ORTEP illustration and structure valida-
tion were done with PLATON [24]. In Table 1 relevant
numerical crystal and refinement data are collected.
ꢀ
distance of 2.784(2) A. In NMR spectroscopy only
3. Results and discussion
3.1. Synthesis
The new ligand Hbss was synthesized in an overall
yield of 48%. The synthesis of this ligand is similar to the
synthesis of Hbsms as described earlier [20]. Starting
from this didentate ligand Hbss, two different nickel
Fig. 1. Displacement ellipsoid plot of [Ni3(bss)4]2þ (6), drawn at the
50% probability level. Hydrogen atoms are omitted for clarity.

J.A.W. Verhagen et al. / Inorganica Chimica Acta 357 (2004) 2687–2693
2691
Table 2
Selected bond distances (A) and angles (°) in [Ni3(bss)4] (BF4)2 (6)
ꢀ
Ni(1)–S(6)
Ni(1)–S(9)
2.162(5)
2.196(2)
2.164(5)
Ni(2)–S(6)
Ni(2)–S(26)
Ni(1)ꢂ ꢂ ꢂ Ni(2)
2.194(4)
2.188(5)
2.784(2)
Ni(1)–S(26)
Ni(1)–S(29)
2.188(5)
S(6)–Ni(1)–S(9)
S(6)–Ni(1)–S(26)
S(6)–Ni(1)–S(29)
S(9)–Ni(1)–S(26)
S(9)–Ni(1)–S(29)
S(26)–Ni(1)–S(29)
92.21(18)
81.25(17)
170.90(18)
173.43(19)
94.75(17)
91.71(17)
S(6)–Ni(2)–S(26)
S(6)–Ni(2)–S(26a)
80.0(16)
100.0(16)
Ni(1)–S(6)–Ni(2)
Ni(1)–S(26)–Ni(2)
79.45(15)
79.53(17)
a ¼ symmetry position ꢀx; ꢀy; ꢀz.
broad signals are obtained, indicating paramagnetism,
which is probably due to fluxionality of the nickel cen-
ters between square planar and tetrahedral.
Ni(2) center. The several isosbestic points show that no
intermediate species is observed and the formation of
the trinuclear complex is instantaneous. The parent
mononuclear nickel complexes can be formed again in a
reaction of the trinuclear complexes with 1,10-phenan-
throline, resulting in half an equivalent of side product
[Ni(phen)3](BF4)2.
The reactivity of the mononuclear complexes to form
the linear trinuclear complexes comprises a rearrange-
ment of the ligands from trans to cis. Thus, despite the
trans orientation of the didentate ligands in the mono-
nuclear nickel complexes these may still be useful in the
pursuit for hetero-dinuclear nickel–iron complexes.
The structure of [Ni3(bsms)4](BF4)2 (7), containing
three low-spin Ni(II) centers is expected to be similar to
the structure of [Ni3(bss)4](BF4)2, described above. In
NMR spectroscopy a clear spectrum is obtained only at
323 K. 1H NMR shows separate resonances for the
protons within a CH2 group and for the methyl groups
on the same carbon atom, confirming a rigid structure of
the ligands. Only the phenyl groups have no hindrance
in rotating, as only three signals are observed for each
phenyl group.
The complex [Ni(bss)2] (5), containing a low-spin
Ni(II) center, is expected to be similar to the structure of
[Ni(bsms)2] with both ligands in trans positions [20].
3.4. Structures of the nickel–iron complexes
The structures of [Ni(bss)2FeCl2]2 (8) and [Ni(bsms)2-
FeCl2]2 (9) are expected to be similar to [Ni(xbsms)-
FeCl2]2 based on elemental analysis and IR [19]. Room
temperature magnetic susceptibility experiments show
that the iron centers are high spin, with a leff of 4.20 per
iron center for 8 and a leff of 4.35 per iron for 9. These
values are lower than that reported for [Ni(xbsms)-
FeCl2]2 (leff ¼ 5:35) [19].
3.3. Reactivity of the mononuclear complexes
The reactions of [Ni(bss)2] and [Ni(bsms)2] with
Ni(BF4)2 to form the trinuclear complexes can be fol-
lowed in ligand field spectroscopy. In Fig. 2 the titration
curve of the reaction of [Ni(bss)2] with Ni(BF4)2 is
shown. The titration curve for [Ni(bsms)2] is very simi-
lar. As can be observed in Fig. 2, the maximum change
is reached at 0.5 equivalents of Ni(BF4)2, confirming the
symmetrical structure of two Ni(1) centers and one
3.5. UV–Visible–NIR spectroscopy of the complexes
The square-planar surrounding of the nickel centers
in all of the complexes is reflected in their ligand field
spectra. The data are presented in Table 3 for the nickel
complexes and in Table 4 for the nickel–iron complexes.
All complexes are brown solids and yield pale brown
solutions in acetonitrile, acetone or chloroform. For
[Ni(bss)2] all absorption bands are similar to the ab-
sorption bands of the comparable complex [Ni(bsms)2]
[20].
Only small differences in absorption maxima between
a solution in acetone and a solution in acetonitrile are
observed, which excludes solvent coordination. Due to
d–d transitions three absorption bands are found, be-
tween 13,000 and 26,000 cmꢀ1. Compared to the
mononuclear complexes, for the trinuclear complexes an
extra absorption band is observed around 24,000 cmꢀ1
Fig. 2. Titration curve of [Ni(bss)2] with [Ni(BF4)2] yielding [Ni3(bss)4]
(BF4)2 in acetone.

2692
J.A.W. Verhagen et al. / Inorganica Chimica Acta 357 (2004) 2687–2693
Table 3
Electronic absorption maxima for the Ni(II) complexes
m/cmꢀ1 (e/molꢀ1 l cmꢀ1
)
Solid statea
Acetone
Acetonitrile
Chloroform
[Ni(bss)2]
17,200
22,700
27,200
32,900
38,000
14,900 (99)
21,600 (sh)
14,700 (62)
22,100 (sh)
28,200 (5159)
31,400 (8799)
28,300 (8087)
31,400 (11,500)
[Ni3(bss)4](BF4)2
13,300
19,000
23,400
30,200
36,700
13,200 (217)
19,800 (2172)
25,600 (9037)
13,100 (287)
19,800 (2163)
25,600 (10,306)
36,100 (84,817)
[Ni3(bsms)4](BF4)2
a Diffuse reflectance.
13,800
19,300
22,900
29,800
36,700
13,400 (224)
20,200 (3904)
23,200 (9059)
20,200 (1879)
25,600 (7943)
32,400 (28,683)
36,700 (29,240)
Table 4
Electronic absorption maxima for the Ni(II)–Fe(II) complexes
m/cmꢀ1 (e /molꢀ1 l cmꢀ1
)
Solid statea
Acetonitrile
Chloroform
[Ni(bss)2FeCl2]2(8)
16,200
19,300
24,500
30,200
36,700
20,000 (sh)
26,500 (sh)
33,500 (sh)
18,200 (1191)
24,200 (2861)
30,800 (11,400)
[Ni(bsms)2FeCl2]2(9)
13,600 (116)
20,000 (sh)
13,800 (sh)
19,300 (1287)
19,500
21,000
23,900
24,200 (sh)
27,500 (sh)
30,600 (sh)
37,300 (17,199)
27,900 (sh)
33,400 (13,685)
38,000 (13,660)
35,500
a Diffuse reflectance.
arising from the extra nickel center. This absorption
band is at high energy due to the four electron-donating
thiolate sulfurs. The absorption maxima of the d–d
transitions of Ni(1) and Ni(1a) have shifted to lower
energy, around 13,500 and 20,000 cmꢀ1, due to the
bridging and therefore less electron-rich thiolate sulfurs.
The ligand field spectra of the [NiL2FeCl2]2 com-
plexes 8 and 9 are similar to the ligand field spectrum of
[Ni(xbsms)FeCl2]2, suggesting a similar hetero-tetranu-
clear structure [19]. The absorption maxima resulting
from ligand to metal charge transfer are different for a
solution in chloroform as compared with a solution in
acetonitrile, indicating possible coordination of MeCN.
nickel complexes [Ni3(bss)4](BF4)2 and [Ni3(bsms)4]
(BF4)2. The reactivity of the complexes is shown in the
trans to cis rearrangement of the ligands. Furthermore,
two new nickel iron complexes have been synthesized,
starting from both mononuclear complexes in a reaction
with FeCl2. These nickel–iron complexes show similar
behavior compared to the nickel–iron complexes with
the ligand xbsms [19].
The synthesized compounds have been made in a
search for new structural models of [NiFe]-hydrogenases.
In a subsequent study, the complexes will be screened for
hydrogenase-like activity by means of reaction of the
[NiFe]-complexes with dihydrogen or protons.
4. Conclusions and outlook
5. Supporting information
Three new nickel complexes have been synthesized,
the mononuclear complex [Ni(bss)2], and the trinuclear
The crystallographic data have been deposited with
the Cambridge Crystallographic Data Base, CCDC no

J.A.W. Verhagen et al. / Inorganica Chimica Acta 357 (2004) 2687–2693
2693
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