Nickel(II) tri-tert-butoxysilanethiolates with N-heterocyclic bases as additional ligands: Synthesis, molecular structure and spectral studies

Article abstract of DOI:10.1016/j.poly.2011.06.028

Three heteroleptic, neutral nickel(II) tri-tert-butoxysilanethiolates with monodentate heterocyclic bases (pyridine, 2-methylpyridine and 3,5-dimethylpyridine) serving as additional ligands have been prepared following the same synthetic procedure. The complexes were characterized by single crystal X-ray structure determination and elemental analysis. For complexes 1 and 2, FT-IR and UV-Vis spectroscopy have been additionally recorded. Three different coordination motifs have been observed in these complexes. Molecules building tetragonal crystals of [Ni{SSi(O^tBu)₃} ₂(C₅H₅N)] (1) feature Ni(II) coordinated by two S,O-chelating tri-tert-butoxysilanethiolato residues and one N atom of pyridine in a strongly distorted trigonal bipyramidal environment. The complex [Ni{SSi(O^tBu)₃}₂(C₆H ₇N)₂] (2) forms triclinic crystals and its core atoms adopt a planar geometry with Ni(II) in the middle of the N₂S ₂ plane. Molecules of complex [Ni{SSi(O^tBu) ₃}₂(C₇H₉N)₂(H ₂O)] (3) form orthorhombic crystals with penta-coordinated Ni(II) in a distorted tetragonal pyramidal NiN₂OS₂ environment. Complex 2 roughly mimics one of the two metal centers in the active site of the ACS/CODH enzyme.

Full text of DOI:10.1016/j.poly.2011.06.028

Polyhedron 30 (2011) 2400–2405
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nickel(II) tri-tert-butoxysilanethiolates with N-heterocyclic bases as additional
ligands: Synthesis, molecular structure and spectral studies
⇑
Łukasz Ponikiewski, Agnieszka Pladzyk , Wiesław Wojnowski, Barbara Becker
Department of Inorganic Chemistry, Chemical Faculty, Gdan´sk University of Technology, Narutowicza Str. 11/12, 80-233 Gdan´sk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Three heteroleptic, neutral nickel(II) tri-tert-butoxysilanethiolates with monodentate heterocyclic bases
(pyridine, 2-methylpyridine and 3,5-dimethylpyridine) serving as additional ligands have been prepared
following the same synthetic procedure. The complexes were characterized by single crystal X-ray struc-
ture determination and elemental analysis. For complexes 1 and 2, FT-IR and UV–Vis spectroscopy have
been additionally recorded.
Three different coordination motifs have been observed in these complexes. Molecules building tetrag-
onal crystals of [Ni{SSi(OtBu)3}2(C5H5N)] (1) feature Ni(II) coordinated by two S,O-chelating tri-tert-but-
oxysilanethiolato residues and one N atom of pyridine in a strongly distorted trigonal bipyramidal
environment. The complex [Ni{SSi(OtBu)3}2(C6H7N)2] (2) forms triclinic crystals and its core atoms adopt
Received 19 May 2011
Accepted 15 June 2011
Available online 7 July 2011
Keywords:
Nickel(II)
Tri-tert-butoxysilanethiolate
Crystal structure
N ligand
a
planar geometry with Ni(II) in the middle of the N2S2 plane. Molecules of complex [Ni{SSi
Spectroscopic study
(OtBu)3}2(C7H9N)2(H2O)] (3) form orthorhombic crystals with penta-coordinated Ni(II) in a distorted
tetragonal pyramidal NiN2OS2 environment. Complex 2 roughly mimics one of the two metal centers in
the active site of the ACS/CODH enzyme.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
around the metal ion to square pyramidal when the enzyme is in
a resting state [15]. The transcription factor NikR, which plays a role
Nickel complexes with a thiolate or sulfido coordination sphere
have been intensively studied for their applications. Monolayered
nickel alkanethiolates, for instance, show antiferromagnetic cou-
pling [1,2], while planar tetracoordinated low-spin nickel(II) thiol-
ates oxidize with conversion to S-bonded sulfinato complexes by
the incorporation of dioxygen [3–7].
Nickel plays various roles in biological systems. At least eight
nickel-dependent enzymes that represent a wide range of interest-
ing chemistry have been identified and characterized. Amidst them
is carbon monoxide dehydrogenase (CODH) [8], which intercon-
verts CO and CO2, and acetyl-coenzyme A synthase/carbon mono-
oxide dehydrogenase (ACS/CODH) [9–12], which converts CO gas
into a carbon source. NiFe hydrogenase catalyzes the reversible
oxidation of hydrogen gas [13].
Nickel proteins present diverse types of coordination modes on
the metal ion, however general insight into four nickel-binding sites
that have been so far identified in enzymatic structures shows the
tendency to adopt a square-planar geometry on the metal ion
[14]. For instance, nickel superoxide dismutase (NiSOD) in the fully
reduced form contains Ni(II) with a square planar geometry. The
oxidation state changes to Ni(III) and the spatial arrangement
of a cytoplasmic nickel sensor in Escherichia coli and Helicobacter
pylori, is built up of four units and each of them contains Ni(II) in
a square-planar coordination environment consisting of two histi-
dines and one cysteine from one NikR subunit and another histidine
from the adjacent subunit. The structure of the whole molecule of
this protein in the apo form suggests that the role of nickel is crucial
for the proper conformation and activity of the protein [16,17].
A large number of studies devoted to nickel proteins are dedi-
cated to the analysis of the structure of the A-cluster active site of
ACS/CODH enzyme. This particular protein attracts attention be-
cause of its significant role in the global carbon cycle. The active site
of the A-cluster of ACS/CODH consists of a Fe4S4 cubane bridged by a
cysteine thiolate to a dinuclear M(l
-S)M0 cluster where M is nickel
and M0 may be Ni(II), Zn(II) or Cu(I). The coordination of the nickel
center is completed by two backbone carboxamido nitrogen atoms,
resulting in a NiN2S2 square-planar arrangement [9–12].
In recent years, our work has been targeted towards biomimetic
studies and the prevalent part of these was devoted to zinc thiol-
ates [18]. Now we have turned our attention to nickel complexes
with S-thiolate and N-donor ligands that may serve as biomimetic
models for the active sites of nickel-containing proteins.
The synthesis of transition metal complexes with thiolate li-
gands presents some difficulties because of the strong bridging
tendency of thiolate sulfur, leading to the formation of oligo- and
polymeric species [19]. One of the strategies which help to avoid
⇑
Corresponding author. Tel.: +48 583472329; fax: +48 583472694.
E-mail address: agnieszka.pladzyk@pg.gda.pl (A. Pladzyk).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.06.028

Ł. Ponikiewski et al. / Polyhedron 30 (2011) 2400–2405
2401
acceptor. Water insoluble products were collected and recrystal-
lized from organic solvents. Complexes 1 and 2 are stable and
can be stored under atmospheric conditions for months, in contrast
to the unstable complex 3. The bases used for the syntheses were
chosen because of their different basicities (pKa: pyridine 5.20,
2-methylpyridine 5.90, 3,5-dimethylpyridine 6.15) and spatial
requirements.
The reaction with pyridine leads to compound 1. Its composi-
tion was initially established with the use of elemental analysis,
which pointed to a 1:2 molar ratio of nitrogen and sulfur, and thus
two silanethiolate residues and one pyridine molecule coordinated
to the metal ion. Also the FT-IR solid state spectrum of 1 showed
the presence of frequencies typical for a heterocyclic base and
Si–O–C bonding from the silanethiolate residue. The composition
and structure of the complex was finally established with the use
of X-ray analysis, showing the metal ion was coordinated by one
molecule of pyridine and two O, S-chelating silanethiolato residues
in a NiNO2S2 type of arrangement. Such a type of complex was syn-
thesized previously for Co(II) and Zn(II) [20,21,23,24].
Scheme 1. Obtained complexes 1–3.
such undesirable side processes is the use of sterically hindered
thiolate ligands.
The exchange of pyridine for more basic and more sterically
hindered 2-methylpyridine gave another product with the formula
[Ni{SSi(OtBu)3}2(C6H7N)2] (2). Again, its composition and structure
was established by elemental analysis, FT-IR spectroscopy and sin-
gle crystal X-ray crystallography. Surprisingly, unlike 1, we found
in 2 two N-donor ligands within the Ni(II) coordination sphere.
This was accompanied by a change of bonding mode of both tri-
tert-butoxysilanethiolato ligands from chelating in 1 to strictly
monodentate in 2, and thus the formation of a NiN2S2 central core.
Syntheses performed previously for Co(II) and Zn(II) tri-tert-but-
oxysilanethiolates with the same pyridine derivative resulted,
amongst others, in the penta-coordinated [M{SSi(OtBu)3}2(C6H7N)]
[20,32].
Finally, we used 3,5-dimethylpyridine and following the same
synthetic procedure once more obtained another type of product
– deep violet crystals of the complex [Ni{SSi(OtBu)3}2(C7H9N)2
(H2O)(C6H6)] (3). Its structure was solved by X-ray crystallography,
revealing Ni(II) coordinated by two monodentate silanethiolato
residues, two N-donor bases and one water molecule, with the cen-
tral core adopting a NiN2OS2 form. We have noticed that at room
temperature after a short period of time the crystals become opa-
que, which may be caused by evaporation of the solvent from this
sample or it may suggest degradation of the complex. Heteroleptic
tri-tert-butoxysilanethiolates of Co(II) and Zn(II) with 3,5-dimeth-
ylpyridine as an additional ligand have been prepared by us previ-
ously [20,27–29], but in both cases they had the formula
[M{SSi(OtBu)3}2(C7H9N)] and chelating tri-tert-butoxysilanethiola-
to ligands, thus they resembled 1 much more then 3.
For many years we have been studying the chemistry of silan-
ethiols as a special type of thiols. Our research group showed that
tri-tert-butoxysilanethiol, when used as a source of a very bulky
thiolate ligand, clearly reduces the tendency of sulfur promoted
polymerization and allowed us to obtain numerous mono- and
binuclear heteroleptic silanethiolate complexes [20–33]. More-
over, unlike many others, this particular silanethiol is resistant to-
wards hydrolysis, and so the syntheses may be carried out under
atmospheric conditions [34,35]. We have obtained and structurally
characterized not only stable tri-tert-butoxysilanethiolates of diva-
lent cobalt, zinc and cadmium, but also those of manganese(II), and
recently iron(II) [26,33]. In some complexes, the immediate envi-
ronment on the metal center follows that found in the active site
of alcohol dehydrogenase (ADH) [32,33a], and one complex,
[Zn{SSi(OtBu)3}2(C7H9NO)], was tested for its catalytic properties
[36]. We have also affirmed that some heteroleptic cobalt(II) tri-
tert-butoxysilanethiolates are isomorphous to the related zinc(II)
complexes and can be considered as spectral models of ADH pro-
tein in solution [31].
So far the majority of nickel(II) thiolates serving as structural
mimics of the active sites of NiSOD, NikR and ACS/CODH enzymes
employ N, S ligands of the chelating type [37–40]. Only a few
examples of Ni(II) complexes with monodentate N- and S-ligands
are known [41–45]. For these reasons we directed our investiga-
tions towards heteroleptic nickel(II) thiolates with the use of
monodentate N-donor ligands. Our previous results prompted us
to attempt the synthesis of complexes with a NiN2S2 core, related
to the coordination mode present in ACS/CODH protein. In this pa-
per we report the synthesis, structural and spectroscopic charac-
terization of three heteroleptic tri-tert-butoxysilanethiolate
complexes of Ni(II) bearing additional pyridine related ligands of
different steric hindrance and different basicity. The synthetic
strategy presented here has led to complexes with three different
coordination geometries: trigonal bipyramidal in 1, square-planar
in 2 and tetragonal pyramidal in 3 (Scheme 1).
2.2. Crystal and molecular structure determination
The molecular structures of complexes 1–3 with the atom label-
ing schemes and their crystal packing are illustrated in Figs. 1–4
and 1S–3S (Supplementary data). Selected bond lengths and angles
are listed in Table 2. In order to estimate the geometric shape of the
obtained complexes, the Addison angular structural parameter (
has been calculated [54,55]. For four and five-coordinate com-
plexes the parameters s4 and 5, respectively are defined as
5 = ( and b are
s)
2. Results and discussion
s
s
4 = [360° ꢁ (
a
ꢁ b)]/141° and
s
a
ꢁ b)/60°, where
a
2.1. Synthesis
the largest bond angles in the complex. For complexes with coor-
dination number CN = 5, s5 becomes unity for a perfect trigonal
All
three
complexes,
[Ni{SSi(OtBu)3}2(C5H5N)]
(1),
bipyramidal geometry and
For four coordinated complexes a perfect square planar geometry
leads to 4 = 0 and a tetrahedral geometry to 4 = 1 [55].
s5 = 0 for a tetragonal pyramid [54].
[Ni{SSi(OtBu)3}2(C6H7N)2] (2) and [Ni{SSi(OtBu)3}2(C7H9N)2(H2O)]ꢀ
(C6H6) (3), were obtained following the same synthetic procedure.
To a solution of NiCl2 in water, the respective heterocyclic base was
added followed by addition of silanethiol. The reagents were taken
in a 1:4:2 molar ratio, with the base serving as a ligand and HCl
s
s
Crystals of complex 1 consist of eight molecules in the unit cell
(Figs. 1 and 1S). The central Ni(II) atom is penta-coordinated by
two S atoms and two O atoms, derived from two O, S-chelating

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Ł. Ponikiewski et al. / Polyhedron 30 (2011) 2400–2405
Fig. 1. Molecular structure of [Ni{SSi(OtBu)3}2(C5H5N)] 1 with the atom labeling
t
scheme (hydrogen atoms of BuO groups are omitted, thermal ellipsoids are drawn
at 30% probability).
Fig. 3. Molecular structure of [Ni{SSi(OtBu)3}2(C7H9N)2(H2O)]ꢀC6H6 3 with the atom
labeling scheme (hydrogen atoms of tBuO groups are omitted, thermal ellipsoids are
drawn at 30% probability).
Fig. 4. A comparison of the geometries of the cores of complexes 1 and 3 (thermal
ellipsoids are drawn at 30% probability).
NiN2OS2 kernel. The Addison parameter s5 presents a value of
0.18. Hence, the geometry is a definitely closer to a square pyramid
than to a trigonal bipyramid, but it is far from a perfect one. The
base of the pyramid is formed by two S atoms, an O atom from
water and a N atom from one of the 3,5-dimethylpyridines, while
the second N-base molecule forms the peak of the pyramid
(Fig. 4). The nickel atom deviates by 0.301 Å from the basal plane.
The bond lengths found in complex 3 are similar to those found in
complex 1 and other penta-coordinated nickel thiolate complexes
[46–53]. The coordinated molecule of water is engaged in the for-
mation of two intramolecular hydrogen bonds, O–Hꢀ ꢀ ꢀO, to oxygen
atoms from adjacent tert-butoxy groups (Table 3). These should
help to keep the water in place and to provide some structural
stabilization, although the effect does not seem to be a strong
one, as evidenced by the lability of the compound.
Fig. 2. Molecular structure of [Ni{SSi(OtBu)3}2(C6H7N)2] 2 with the atom labeling
scheme (tBu groups and hydrogen atoms of the aromatic ring are omitted for clarity,
thermal ellipsoids are drawn at 30% probability).
silanethiolate residues, and one N atom of pyridine. The parameter
s5 calculated for complex 1 has a value of 0.55 and thus its geom-
etry is unequivocal, being about halfway between a square pyra-
mid and a trigonal bipyramid, albeit slightly closer to the latter,
with the oxygen atoms occupying apical positions. The Ni1–O3
and Ni1–O6 distances (2.239(9) and 2.269(9) Å, respectively)
clearly exceed the sum of covalent radii for Ni and O, which is
equal to 1.90 Å, but are nonetheless shorter than those found in
similar transition metal silanethiolates where the (tBuO)3SiS resi-
due behaves as a bidentate ligand [20,24,27–30].
Complex 2 crystallizes with one molecule in the unit cell (Figs. 2
and 2S). Although the geometry of its NiN2S2 core may be
2.3. Spectral studies
approximated as square-planar (s4 = 0) with the S1–Ni1–S2 and
The UV–Vis spectra were measured for 1 and 2 in n-hexane
N1–Ni1–N2 angles being the ideal value of 180°, the two N–Ni–S
angles differ by ca. 10°. The Ni–S bond distance of 2.253(2) Å in 2
is slightly longer than those found in square-planar NiN2S2
complexes with scorpionate-type N2S2 ligands [9–12].
Complex 3 crystallizes with eight molecules in the unit cell
(Figs. 3 and 3S). The nickel coordination sphere contains five inde-
pendent ligands – two silanethiolate residues, two molecules of
3,5-dimethylpyridine and one molecule of water, which form a
solution and Table 4 lists the electronic absorption data. The li-
gands show the expected the
p ?
p⁄ transitions of aromatic rings
and n ? p⁄ transitions of the pyridines and silanethiolate residues
in the range 210–380 nm [20,23,27–31,36]. Nickel thiolates exhibit
ligand-to-metal charge transfer within the range 250–350 nm, as-
signed as S ? Ni LMCT [56,57].
The absorption spectra of 1 and 2 in the UV region exhibit intense
bands at 212 nm which can be attributed to the
r
(Si–S) ? r⁄(Si–S)

Ł. Ponikiewski et al. / Polyhedron 30 (2011) 2400–2405
2403
Table 1
Table 2
Crystal data and structure refinement for 1–3 complexes.
Selected bond lengths (Å) and angles (°) for complexes 1–3.
1
2
3
1
2
3
Empirical
formula
Formula
weight
T (K)
k (Å)
Crystal
system
Space group
C
29H59NiNO6S2Si2
C36H68NiN2O6S2Si2 C44H80NiN2O7S2Si2
Bond lengths
Ni1–N1
Ni1–N2
Ni1–S1
Ni1–S2
Ni1–O3
Ni1–O6
Ni1–O7
S1–Si1
2.025(2)
1.907(3)
2.253(2)
2.048(5)
2.069(5)
2.385(4)
2.420(4)
696.78
803.93
928.11
2.302(8)
2.311(8)
2.239(9)
2.269(9)
120(2)
0.71073
tetragonal
120(2)
150(2)
0.71073
triclinic
0.71073
orthorhombic
2.095(4)
2.068(2)
2.050(2)
1.659(4)
1.631(4)
1.632(4)
1.647(4)
1.657(4)
1.629(4)
ꢀ
I4
Pbca
2.075(2)
2.071(2)
1.622(2)
1.633(2)
1.665(2)
1.635(2)
1.623(2)
1.662(2)
2.097(5)
P1
S2–Si2
Unit cell dimensions
Si1–O1
Si1–O2
Si1–O3
Si2–O4
Si2–O5
Si2–O6
1.630(3)
1.627(3)
1.638(3)
a (Å)
b (Å)
c (Å)
29.900(4)
9.1388(6)
9.4625(6)
14.6865(11)
77.324(6)
75.767(6)
62.410(7)
1082.78(13)
1
16.1204(4)
20.5424(4)
31.5906(11)
90
90
90
29.900(4)
8.8450(18)
90
90
90
7908(2)
8
1.171
0.692
3008
a
(°)
b (°)
c
(°)
V (Å3)
10461.3(5)
8
Bond angles
Z
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–S(1)
N(1)–Ni(1)–S(2)
N(2)–Ni(1)–S(1)
N(2)–Ni(1)–S(2)
S(1)–Ni(1)–S(2)
Si(1)–S(1)–Ni(1)
Si(2)–S(2)–Ni(1)
S(1)–Ni(1)–O(3)
S(1)–Ni(1)–O(6)
N(1)–Ni(1)–O(3)
N(2)–Ni(1)–O(6)
O(3)–Ni(1)–O(6)
O(7)–Ni(1)–N(1)
O(7)–Ni(1)–N(2)
O(7)–Ni(1)–S(1)
O(7)–Ni(1)–S(2)
180.00(10)i
85.45(10)
94.55(10)i
94.55(10)i
85.45(10)i
180.00(18)i
111.71(5)
96.78(19)
106.83(13)
93.91(13)
84.28(13)
90.58(13)
159.07(6)
117.74(8)
04.83(7)
qcalc (g/cm3)
1.233
1.179
110.61(8)
108.53(8)
l
(mmꢁ1
)
0.641
0.541
F(0 0 0)
Crystal size
(mm)
h Range (°)
Index ranges
434
4016
0.21 ꢂ 0.18 ꢂ 0.14 0.45 ꢂ 0.07 ꢂ 0.04 0.20 ꢂ 0.15 ꢂ 0.09
140.74(3)
84.25(4)
84.21(3)
78.30(5)
97.69(5)
94.41(8)
91.54(8)
173.71(7)
2.0395–32.4692
ꢁ30 6 h 6 38
ꢁ38 6 k 6 37
ꢁ11 6 h 6 11
31 523
2.4400–32.3144
ꢁ7 6 h 6 10
ꢁ10 6 k 6 11
ꢁ17 6 h 6 17
6368
2.0550–32.5425
ꢁ12 6 h 6 21
ꢁ27 6 k 6 27
ꢁ41 6 h 6 41
79 318
Reflections
collected
Independent
reflections
Rint
Completeness 26.99° (99.8%)
to h
Refinement
method
8572
3670
12 617
92.83(17)
170.37(18)
92.19(12)
89.55(12)
0.0350
0.0460
0.1385
25.00° (96.4%)
28.00° (99.9%)
Full-matrix least-squares on F2
i
Symmetry codes: 2 ꢁ x, 1 ꢁ y, 1 ꢁ z.
Parameters
S (Goodness-
of-fit)
388
1.16
233
529
1.119
1.101
Table 3
R1 [I > 2
r
(I)]
R1 = 0.0389
wR2 = 0.0946
R1 = 0.0482
wR2 = 0.0979
differences in
peak and hole
R1 = 0.0542
R1 = 0.0864
Hydrogen bond lengths (Å) and angles (°) for 3.
wR2 = 0.1377
R1 = 0.0812
wR2 = 0.2100
R1 = 0.1678
R indices (all
data)
Largest
D–Hꢀ ꢀ ꢀA
O7–H 7Dꢀ ꢀ ꢀO5
O7–H 7Fꢀ ꢀ ꢀO1
D–H
Hꢀ ꢀ ꢀA
2.03(2)
1.91(3)
Dꢀ ꢀ ꢀA
2.859(5)
2.743(6)
\D–Hꢀ ꢀ ꢀA
168(6)
164(7)
wR2 = 0.1662
0.694 and ꢁ0.944
wR2 = 0.2533
0.625 and ꢁ0.898
0.84(2)
0.85(2)
(e Åꢁ3
)
1.430 and
ꢁ0.970
Table 4
Summary of electronic spectra of complexes 1 and 2.
UV–Vis: kmax (nm) (e )
, Mꢁ1 cmꢁ1
transitions from the tri-tert-butoxysilanethiolato residue [58]. Rely-
ing on our results concerning the spectral analysis of cobalt(II) silan-
ethiolates [28–31], we suppose that the band at 252 nm present in
the spectra of both complexes may correspond to N-ligand ? Ni CT
transitions. The spectrum of 1 contains additionally two less intense
bands at 336 and 380 nm, which can be ascribed as S ? Ni charge
transfer transitions of the silanethiolato residue. Surprisingly, there
are no such two bands present in the spectrum of 2, which may sug-
gests that complex 2 is not sufficiently stable in diluted solutions
1
2
CT transitions
212 (7126)
252 (5870)
336 (1272)
380 (1768)
212 (7126)
252 (5870)
d ? d transitions
484 (154)
548 (140)
668 (89)
504 (52)
576 (55)
716sh (75)
and undergoes dissociation. We recently observed such
a
tendency to dissociate for a solution of the cobalt(II) complex
[Co{SSi(OtBu)3}2(C6H7N)2] [28–30a].
Complex 1 shows absorptions at 484 and 548 nm and a broad
band at 668 nm with a shoulder at 716 nm. It may suggest that
in the solution complex 1 changes its geometry from strongly dis-
torted trigonal bipyramidal to square-pyramidal. Neutral nickel
complexes with a square-planar geometry exhibit relatively weak
features in the region (ca. 500–650 nm) expected for ligand field
transitions of planar Ni(II) chromophores and are essentially trans-
parent at lower energies. Accordingly, complex 2 shows only two
absorbance bands at 508 and 576 nm.
Nickel(II) complexes are numerous and many of their spectral
studies have been presented in the literature. The most common
are complexes with a trigonal bipyramidal geometry, mainly be-
cause of the frequent use of tripod-like ligands. The spectra of
these complexes in the visible region are very characteristic, with
two bands, often asymmetric with weak absorptions. Square pyra-
midal Ni(II) complexes are less common and their spectra in the
visible region exhibit a broad band which contains at least three
electronic transitions [57].

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Ł. Ponikiewski et al. / Polyhedron 30 (2011) 2400–2405
stirred for 15 min and then (tBuO)3SiSH was added (0.76 mL,
2.5 mmol). The reaction mixture was stirred for 2 h at room tem-
perature, during which time a green precipitate for 1 or deep-violet
for 2 and 3 was formed. The product was filtered off, dissolved in
20 ml of toluene (1 and 2) or benzene (3) and dried for 2 h with
anhydrous MgSO4. After filtration the solutions were concentrated
to ca. 5 mL and left for crystallization at 4 °C.
3.2.1. [Ni{SSi(OtBu)3}2(C5H5N)] 1
Violet crystals of
1 were obtained after 1 week. M.p.
118–120 °C; Anal. Calc. for C29H59NiNO6S2Si2: C, 49.94; H, 8.47;
N, 2.01; S, 9.19. Found: C, 50.01; H, 8.45; N, 2.00; S, 9.20%. IR
(KBr) m: 2974 (s), 2927 (s), 2900 (s, sh), 2867 (s), 1555 (w), 1540
(w), 1518 (vw), 1506 (vw), 1487 (vw), 1470 (w), 1447 (m), 1388
(m), 1364 (s), 1241 (m), 1201 (m, sh), 1184 (s), 1067 (vs), 1051
(vs, sh), 1025 (s) 984 (w, br), 816 (m), 803 (m), 791 (s, sh), 779
(m), 768 (s), 753 (s), 723 (m), 696 (m), 645 (w, sh), 635 (w), 548
(w), 530 (w), 499 (w), 470 (w, br), 420 (w).
3.2.2. [Ni{SSi(OtBu)3}2(C6H7N)2] 2
Violet crystals of 2 were isolated after a week. M.p. 151–152 °C;
Anal. Calc. for C36H68NiN2O6S2Si2: C, 53.81; H, 8.47; N, 3.48; S, 7.97.
Found: C, 53.71; H, 8.67; N, 3.46; S, 7.76%. IR (KBr) m: 2972 (vs),
2928 (s), 2902 (w, sh), 2870 (w), 1610 (w), 1571 (vw), 1490 (w),
1470 (w, sh), 1461 (w), 1387 (s), 1362 (s), 1301 (vw), 1238 (s),
1206 (s), 1188 (s), 1043 (vs), 1024 (vs), 1004 (vs), 947 (vw), 909
(vw), 845 (vw), 821 (m), 800 (m), 777 (w), 762 (m), 731 (w), 719
(w), 687 (m), 660 (w, sh), 643 (s), 580 (w), 544 (s), 511 (vw), 503
(vw), 493 (w), 477 (w), 460 (w), 451 (w), 422 (w).
Fig. 5. Solid state FT-IR spectra of complexes 1 and 2 in the range 1100–950 cmꢁ1
.
Vibrational spectra were recorded for complexes 1 and 2 in the
solid state in the range 4000–700 cmꢁ1. They are similar, neverthe-
less some differences are visible in the region 1100–950 cmꢁ1
(Fig. 5), with the frequencies red-shifted for complex 2. So far,
we have been unable to compare the currently presented spectra
with other nickel(II) silanethiolates, because such spectra have
not been published yet. Nevertheless, we have shown that all the
metal tri-tert-butoxysilanethiolates synthesized so far contain
superimposed bands in this range, assigned to Si–O–C bonding
from the silanethiolate residue. Furthermore some tri-tert-butoxy-
silanethiolate complexes of Mn(II) [26,33], Co(II) [20,28,29], Zn(II)
[21,23,24,31b] and Cd(II) [27,30b,c], where the oxygen atom from
the –Si(OtBu)3 group interacts with the metal center, exhibit an
3.2.3. [Ni{SSi(OtBu)3}2(C7H9N)2(H2O)]ꢀC6H6
3
After 1 month some dark-violet crystals of 3 were obtained.
M.p. 136–137 °C; Anal. Calc. for C44H80NiN2O7S2Si2: C, 56.94; H,
8.69; N, 3.02; S 6.91. Found: C, 53.62; H, 8.71; N, 3.29; S, 7.56%.
The differences result from the evaporation of benzene and are
comparable with those calculated without solvent (C38H74
NiN2O7S2Si2: C, 53.77; H, 8.73; N, 3.30; S 7.55%).
3.3. X-ray
Data were collected on a KUMA KM4 diffractometer with graph-
ite-monochromated Mo Ka radiation using a Sapphire-2 CCD
additional band at about 980 cmꢁ1
.
detector. The apparatus was equipped with an open flow thermo-
stat (Oxford Cryosystems) which enabled experiments to be
carried out at 120 K. The structures were solved by direct methods
and refined by full-matrix least-squares on F2 (all data) using the
SHELXL program package [59]. All non-hydrogen atoms were refined
anisotropically. H atoms bonded to C and O were included in calcu-
lated positions and refined as riding on their parent C atom with
C–H = 0.95 Å, Uiso(H) = 1.2 Ueq(C) for aromatic, C–H = 0.98 Å,
Uiso(H) = 1.5 Ueq(C) for methyl H atoms and O–H = 0.84 and
0.85 Å, Uiso(H) = 1.2 Ueq(O). Basic crystal data, descriptions of the
diffraction experiment and details of the structure refinement are
given in Table 1.
3. Experimental
3.1. General procedures
The elemental analyses (C, H, S and N contents) were performed
on an Elemental Analyser EA 1108 (Carlo Erba Instruments). Solu-
tion electronic spectra were recorded in the range 200–1000 nm on
a Unicam SP300 spectrometer. Quartz cuvettes and n-hexane as
solvent were used. The IR spectra were measured in the range
4000–700 cmꢁ1 with a Mattson Genesis II Gold spectrometer in
KBr pellets. (tBuO)3SiSH was prepared according to the procedure
described in [34].
4. Conclusion
3.2. Syntheses
In summary, we have prepared three novel Ni(II) tri-tert-but-
oxysilanethiolate complexes with N-heterocyclic bases as addi-
tional ligands. Complexes 1–3 were prepared using the same
procedure, which provides a facile means to obtain neutral com-
plexes with a different ligand composition around the metal ion
and different coordination geometries: tetragonal pyramidal 1,
square-planar 2 and trigonal bipyramidal 3. Complex 2, with a
NiN2S2 core, roughly mimics the nickel center in the ACS/CODH
All three complexes were obtained using the same synthesis
procedure. To a solution of NiCl2ꢀ6H2O (0.30 g, 1.25 mmol) in water
(10 mL), freshly distilled base was added dropwise (5 mmol) [pyr-
idine: 0.40 mL, 1; 2-methylpyridine: 0.49 mL, 2; 3,5-dimethylpyri-
dine: 0.57 mL, 3]. In each case the addition of base changed the
color of the mixture from green to blue. Next the solution was

Ł. Ponikiewski et al. / Polyhedron 30 (2011) 2400–2405
2405
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(2005) m2582.
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Chem. 35 (2010) 821.
active site. However its lability in solutions prompted us to put
more effort towards the stabilization of potential biomimetic com-
plexes. Our approach relies on synthesizing nickel silanethiolates
with other types of nitrogen ligands.
[29] A. Pladzyk, K. Baranowska, P. Hapter, Transition Met. Chem. 35 (2010) 373.
[30] (a) A. Pladzyk, K. Baranowska, Acta Crystallogr., Sect. E 62 (2006) m2602;
Appendix A. Supplementary data
´
(b) A. Dołe˛ga, K. Baranowska, J. Gajda, S. Kazmierski, M. Potrzebowski, Inorg.
Chim. Acta 360 (2007) 2973;
(c) A. Dołe˛ga, K. Baranowska, Z. Jarza˛bek, Acta Crystallogr., Sect. E 64 (2008)
m1515.
[31] (a) A. Dołe˛ga, A. Pladzyk, K. Baranowska, J. Jezierska, Inorg. Chim. Acta 362
(2009) 5085;
CCDC 813189, 813190 and 813191 contains the supplementary
crystallographic data for (1), (2) and (3). These 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. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.poly.2011.06.028.
(b) A. Dołe˛ga, A. Pladzyk, K. Baranowska, M. Wieczerzak, Inorg. Chem.
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