Hexanuclear [Ni6L12] metallacrown framework consisting of NiS4 square-planar and NiS5 square-pyramidal building blocks

Article abstract of DOI:10.1039/b713668b

The hexanuclear [Ni₆L₁₂] (2) wheel-type cluster adopts an unusual structural motif whereby four NiS₄ square-planar and two NiS₅ square-pyramidal units are conjoined by edge sh? the NiS₅ units resemble the Ni centre of the inactive state in the [NiFe] hydrogenase. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b713668b

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Hexanuclear [Ni6L12] metallacrown framework consisting of NiS4
square-planar and NiS5 square-pyramidal building blocks†‡
Raja Angamuthu,a Huub Kooijman,b Martin Lutz,b Anthony L. Spekb and Elisabeth Bouwman*a
Received 5th September 2007, Accepted 12th September 2007
First published as an Advance Article on the web 19th September 2007
DOI: 10.1039/b713668b
The hexanuclear [Ni6L12] (2) wheel-type cluster adopts an
unusual structural motif whereby four NiS4 square-planar
and two NiS5 square-pyramidal units are conjoined by edge
sharing; the NiS5 units resemble the Ni centre of the inactive
state in the [NiFe] hydrogenase.
of the first example of a hexanuclear [Ni6L12] cluster having square-
planar as well as square-pyramidal coordinated nickel ions in the
same molecule.
The reaction of Ni(acac)2 with two equivalents of the thiouro-
nium salt 1 (Scheme 1) in the presence of two equivalents of tetram-
ethylammonium hydroxide led to an immediate colour change to
deep brown and the thiolate-bridged hexanuclear nickel complex
2 was isolated.§ The X-ray crystal structure determination of
reddish–brown rectangular-shaped crystals revealed a hexanuclear
metallacrown [Ni6L12] framework (Fig. 1) containing four square-
planar NiS4 and two square-pyramidal NiS5 units joined by
edge sharing. The whole molecule resides on a crystallographic
inversion centre. Although the ligand has been synthesized to act
as a bidentate chelate, it is coordinating only via the thiolate sulfur
in 10 out of the 12 ligands that are present in 2. Each nickel ion is
surrounded by four thiolate sulfurs of the l-SCH2CH2SC6H4Cl
Metal thiolates, including nickel thiolates, are of interest in the
context of their rich redox chemistry1,2 and structural diversity
in supramolecular architectures;3 moreover, as synthetic models
for environmentally and industrially significant enzymes like
hydrogenases.4–6 Despite the fact that there are plenty of exam-
ples for the ubiquitous O- and/or N-bridged metallacrowns,7–15
few literature reports are available concerning S-bridged
supramolecules.16–19 Cyclic structures [{Ni(l-SR)2}n] (n = 4–11)
with a range of monodentate thiolates and bidentate (NS,17,18
S219) thiolates are known since the first report16 of a hexanuclear
wheel. Noticeably all these reported cyclic structures are composed
of square-planar NiS4 units as building blocks. Interestingly, an
example of a decanuclear cluster crystallized with an encapsulated
benzene molecule as a guest has recently been reported.3 We herein
report the synthesis (Scheme 1), reactivity and structural features
˚
ligands with Ni–S distances of 2.1895(17)–2.2161(16) A in a
distorted square-planar fashion. However, one of the four ligands
coordinated to Ni(3) acts as a chelating bidentate ligand; its
thioether sulfur is coordinated at the apical position making
the coordination geometry of Ni(3) square-pyramidal with a s
value20 of 0.15. The two NiS5 units resemble the Ni centre of
the oxidised inactive state in the [NiFe] hydrogenase.21 The nickel
ions approximately form a hexagon, with Ni · · · Ni separations
˚
in the range of 2.8220(11)–3.1022(12) A. Two l-S bridges from
two ligands connect adjacent nickel ions. The twelve l-S atoms
form double crowns, one above and the other below the Ni6 ring.
Owing to the geometrical restrictions introduced by the bridging
Scheme 1 Synthesis of ligand, complexes 2 and 3 from thiouronium salt
1, and the chemical oxidation of 2 by iodine.
aLeiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University,
P.O. Box 9502, 2300 RA Leiden, The Netherlands. E-mail: bouwman@
chem.leidenuniv.nl; Fax: +31-71-527-4451; Tel: +31-71-527-4550
bBijvoet Center for Biomolecular Research, Crystal and Structural Chem-
istry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
† CCDC reference numbers 655322 and 655323. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b713668b
‡ Electronic supplementary information (ESI) available: Experimental
details on the synthesis of 1, crystallographic determinations and stacking
plots. See DOI: 10.1039/b713668b
Fig. 1 Perspective view of 2. Ni, blue; S, red; Cl, green; C, grey.
Hydrogens are omitted for clarity. Symmetry operation a: −x, 1 − y,
˚
−z. Selected bond lengths (A): Ni–S, 2.1895(17)–2.2161(16); Ni · · · Ni,
2.8220(11)–3.1022(12).
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sulfur atoms, the NiS4 units are not strictly planar; the nickel ions
in dichloromethane, which resulted in a colour change of the
dichloromethane solution from dark brown to deep greenish
brown.¶ Filtration and slow evaporation of the solvent in air
yielded dark brown hexagonal plates of the complex [Ni2L2I2]
3 suitable for X-ray diffraction. The ability of the ligand to
coordinate via the thioether sulfur is evidenced by the formation
of 3 and the X-ray crystal structure determination revealed
a binuclear structure with crystallographic twofold symmetry
(Fig. 3), in which two ligands bridge the two Ni ions through the
thiolate sulfurs, and the thioether sulfurs and iodide ions occupy
the terminal positions of the nickel ions. Thus, two NiS3I square-
planar units are conjoined at an edge, and the Ni2S2 rhombus is
folded with a dihedral angle of 66.28(2)◦ with an intramolecular
˚
Ni(1), Ni(2) and Ni(3) are 0.121, 0.026 and 0.119 A above their
corresponding S4 planes, respectively. The S–Ni–S cis bond angles
range from 81.88(6) to 98.31(6)◦. The ellipsoidal Ni6 ring with Ni–
Ni–Ni vertex angles of 118.84(3)–129.39(4)◦, has a distance range
˚
between 5.4081(12) and 6.3316(13) A for opposite nickel ions.
˚
The Ni(2) · · · Ni(2) distance is noticeably shorter (5.4081(12) A)
˚
in comparison with the Ni(1) · · · Ni(1) distance (6.3316(13) A).
The ligands that are coordinated to Ni(1) are involved in p–p
stacking to a neighbouring hexanuclear molecule with a stacking
˚
distance of 3.854(4) A. On the other hand, the chelating ligand
coordinated to the Ni(3) ion shows a comparatively weaker
˚
stacking (3.902(4) A) while the non-chelating ligand coordinated
˚
to the same nickel ion is involved in a comparatively stronger
Ni · · · Ni separation of 2.8602(4) A. The Ni–S distances are quite
˚
˚
stacking interaction (3.771(4) A). These asymmetric p–p stacking
normal in 3, the Ni–Sthioether distance (2.1940(7) A) is slightly longer
than the Ni–Sthiolate distances (2.1658(7) and 2.1902(7) A). The
crystal packing of 3 along the c axis shows a one-dimensional chain
formed by the intermolecular interaction between the chlorides
˚
interactions may contribute to the ellipsoidal nature of the Ni6
hexagon and the orientation of the ligands as present in the solid
state structure (Fig. S1–S3).
To investigate whether the solid-state structure of the [Ni6L12]
complex is retained in solution, the 1H NMR spectra of the
complex has been recorded in CD2Cl2 solution (Fig. 2). Despite the
fact that the ligands are quite different in the context of structural
orientations as revealed by the crystal structure, the NMR spectra
of this Ni6 complex demonstrates a remarkably simple pattern that
suggests that there are only two types of ligands present in equal
and the electron-deficient centroids (C–Cl · · · Cg(p-ring)) at a
22,23
1H and 13C NMR spectra of
˚
distance of 3.696 A (Fig. S4).
3 in CD2Cl2 reveal more or less the same chemical shift values as
interpreted for the chelating ligand part of the NMR of 2.
1
amounts in solution. The simple pattern of the H NMR and
the axial coordination present in the crystal structure led us to
postulate that the solution structure of the Ni6 cluster is as given
in the inset of Fig. 2. All of the six axial positions in the outer
side of the Ni6 ring are occupied by the coordination of thioether
sulfurs of the ligands alternately directed from above and below
the Ni6 ring. The two equally distributed sets of two doublets (7.31
and 7.24 ppm) and one singlet (7.12 ppm) in the aromatic region
1
of the H NMR spectra belong to the chelating ligands and the
monodentate ligands, respectively.
Fig. 3 Perspective view of 3. Ni, blue; S, red; Cl, green; I, violet; C, grey.
Hydrogens are omitted for clarity. Symmetry operation a: 1 − x, y, 0.5 − z.
˚
Selected bond lengths (A): Ni(1)–S(1), 2.1658(7); Ni(1a)–S(1), 2.1902(7);
Ni(1)–S(2), 2.1940(7); Ni(1) · · · Ni()1a, 2.8602(4).
The electrochemistry of 2 in a dichloromethane solution shows
only an irreversible oxidation process, observed at 0.77 V vs.
Ag/AgCl (Fig. 4). The chemical oxidation of 2 has been achieved
by adding three equivalents of iodine to a dichloromethane
solution of 2 in an electrochemical cell. The CV of the unstable
oxidised species has been recorded immediately. The quasi-
reversible reduction of the NiIII species is observed at 0.60 V with
reoxidation at 0.66 V. Within a few minutes a white sediment is
deposited at the bottom of the cell, which by NMR proved to be
the disulfide of the ligand. The complex 3 shows a quasi-reversible
oxidation at 0.740 V, of which the reduction occurs at 0.46 V.
In conclusion, the hexanuclear nickel(II)-thiolato cluster
[Ni6L12] 2 has been synthesized, with NiS4 square-planar and
NiS5 square-pyramidal units in the same molecule. The NiS5 units
resemble the Ni centre of the inactive state in the [NiFe] hydro-
genase.21 The chemical oxidation of the [Ni6L12] complex with
iodine yields the dinuclear complex 3 via an unstable intermediate
as observed in the electrochemistry. Reactions of 2 and 3 with a
series of iron complexes are under consideration to make models
for [NiFe] hydrogenase.
Fig. 2 1H NMR of 2 in CD2Cl2 solution and the schematic diagram of
the solution structure of 2 (inset). R indicates the 4-chlorophenyl groups of
the ligand; *, signals from monodentate ligands; ᭺, signals from chelating
ligands; +, signal of CD2Cl2.
In order to shed light on the solution structure and to make
use of the reactive axial sites of the nickel ions of complex
2, oxidation with three equivalents of iodine was performed
4642 | Dalton Trans., 2007, 4641–4643
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¶ Preparation of 3. To a solution of 2 (0.28 g, 0.1 mmol) in 30 ml of
dichloromethane was added a solution of three equivalents of iodine
(0.076 g, 0.3 mmol) in 30 ml of dichloromethane and the reaction mixture
was stirred for one hour at room temperature. After filtration of the white
precipitate and evaporation of the solvent in open air for two days dark
brown block-shaped crystals suitable for X-ray diffraction were isolated in
83% yield corresponding to 3. 1H NMR dH [300.13 MHz, CD2Cl2, 298 K]
7.30 (dd, 8H, phenyl ring), 3.19 (t, 4H, Ph-S-CH2-CH2-S-), 2.83 (t, 4H,
Ph-S-CH2-CH2-S-). 13C NMR dC [75.47 MHz, CD2Cl2, 298 K] 131.26 (Ph-
C2), 129.32 (Ph-C3), 37.23 (Ph-S-CH2-CH2-S-), 32.03 (Ph-S-CH2-CH2-
S-). Elemental analysis (%), calculated for C16H16Cl2I2Ni2S4 (778.65): C
24.68, H 2.07, S 16.47, found C 24.53, H 1.98, S 16.29. Crystallographic
data for 3: C16H16Cl2I2Ni2S4, Fw = 778.65, yellow plate, 0.36 × 0.21 ×
0.03 mm3, monoclinic, C2/c (no. 15), a = 16.4582(5), b = 15.0185(6),
◦
3
˚
˚
c = 9.8037(2) A, b = 110.134(2) , V = 2275.16(13) A , Z = 4, Dx
=
2.273 g cm−3, l = 4.97 mm−1. 20 584 Reflections were measured up to a
resolution of (sin h/k)max = 0.65 A−1. An absorption correction based on
˚
multiple measured reflections was applied (0.33–0.0.86 correction range).
2622 Reflections were unique (Rint = 0.0311), of which 2179 were observed
[I > 2r(I)]. 118 Parameters were refined with no restraints. R1/wR2
[I > 2r(I)]: 0.0203/0.0387. R1/wR2 [all refl.]: 0.0319/0.0413. S = 1.056.
Fig. 4 Cyclic voltammograms of 1 mM solutions of 2 (−), 2 + 3I2 (---)
−3
and 3 ( · · · ) in CH2Cl2 containing 0.1 M (NBu4)PF6. Scan rate 200 mV s−1
.
˚
Residual electron density between −0.42 and 0.37 e A
.
Pt disc working, Pt wire counter electrodes used with a Ag/AgCl reference
electrode.
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Notes and references
§ Preparation of 2. To a suspension of [Ni(acac)2] (0.51 g, 2 mmol) in 30 ml
of toluene was added two equivalents of 1 (1.13 g, 4 mmol). After the
addition of two equivalents of NMe4OH (0.72 g, 4 mmol), the solution
was refluxed for 2 h. The resulting brown solution was filtered and the
solvent was evaporated under reduced pressure to give a dark brown oil.
Ethanol (5 ml) was added to the dark brown oil yielding 0.63 g of a reddish–
brown solid (yield 72%). Rectangular shaped tiny reddish–brown crystals
suitable for X-ray diffraction were isolated from a solution of methanol
and acetone (1 : 1) after standing for one day at room temperature. 1H
NMR dH [300.13 MHz, CD2Cl2, 298 K]) 7.31 (d, 2H, phenyl ring, chelate),
7.24 (d, 2H, phenyl ring, chelate), 7.12 (s, 4H, phenyl ring, monodentate),
3.18 (t, 2H, Ph-S-CH2-CH2-S-, chelate), 2.87 (t, 2H, Ph-S-CH2-CH2-S-,
monodentate), 2.32 (t, 2H, Ph-S-CH2-CH2-S-, chelate), 2.01 (t, 2H,
Ph-S-CH2-CH2-S-, monodentate). 13C NMR dC [75.47 MHz, CD2Cl2,
298 K] 131.32 (Ph-C2, chelate), 130.64 (Ph-C2, monodentate), 129.58
(Ph-C3, chelate), 129.42 (Ph-C3, monodentate), 37.42 (Ph-S-CH2-CH2-S-,
chelate), 34.59 (Ph-S-CH2-CH2-S-, monodentate), 32.25 (Ph-S-CH2-CH2-
S-, chelate), 26.52 (Ph-S-CH2-CH2-S-, monodentate). Elemental analysis
(%), calculated for C96H96Cl12Ni6S24 (2796.95): C 41.23, H 3.46, S
27.51, found C 40.97, H 3.53, S 27.24. Crystallographic data for 2:
C96H96Cl12Ni6S24, Fw = 2796.95, brown needle, 0.28 × 0.04 × 0.02 mm3,
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¯
˚
triclinic, P1 (no. 2), a = 11.8667(12), b = 13.9186(12), c = 18.843(2) A,
◦
3
˚
a = 75.996(11), b = 76.080(11), c = 74.991(13) , V = 2863.7(5) A ,
Z = 1, Dx = 1.622 g cm−3, l = 1.73 mm−1. 55 395 Reflections were
measured up to a resolution of (sin h/k)max = 0.60 A−1. An absorption
˚
correction based on multiple measured reflections was applied (0.64–
0.0.97 correction range). 10 355 Reflections were unique (Rint = 0.1398),
of which 6121 were observed [I > 2r(I)]. 622 Parameters were refined
with no restraints. R1/wR2 [I > 2r(I)]: 0.0596/0.0863. R1/wR2 [all refl.]:
22 T. J. Mooibroek, S. J. Teat, C. Massera, P. Gamez and J. Reedijk, Cryst.
Growth Des., 2006, 6, 1569.
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0.1311/0.1048. S = 1.049. Residual electron density between −0.47 and
−3
˚
0.54 e A
.
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The Royal Society of Chemistry 2007
Dalton Trans., 2007, 4641–4643 | 4643
©