Self-assembly of a [Ni8] carbonate cube incorporating four μ4-carbonato linkers through fixation of atmospheric CO 2 by ligated [Ni2] complexes

Article abstract of DOI:10.1039/c3dt52999a

The communication reports the synthesis, characterization, and magnetic behavior of a novel μ₄-carbonato supported and imidazole capped ligated nickel cage [Ni₈(μ-H₂bpmp)₄(μ ₄-CO₃)₄

Full text of DOI:10.1039/c3dt52999a

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Self-assembly of a [Ni8] carbonate cube
incorporating four µ4-carbonato linkers through
fixation of atmospheric CO2 by ligated [Ni2]
complexes†
Cite this: Dalton Trans., 2014, 43,
1970
Received 24th October 2013,
Accepted 18th November 2013
Aloke Kumar Ghosh,a Moumita Pait,a Michael Shatruk,b Valerio Bertolasic and
DOI: 10.1039/c3dt52999a
www.rsc.org/dalton
Debashis Ray*a
The communication reports the synthesis, characterization, and
magnetic behavior of a novel µ4-carbonato supported and imid-
azole capped ligated nickel cage [Ni8(µ-H2bpmp)4(µ4-CO3)4(ImH)8]-
(NO3)4·2H2O (1) through self-assembly of ligand bound ferro-
magnetic Ni2 building blocks. Structural analysis indicates newer
geometrical features for the coordination cage formation and
dominant interdimer antiferromagnetic coupling resulting in a dia-
magnetic ground state.
Chart 1
The self-assembly of dinuclear transition-metal complexes to
high nuclearity supramolecular aggregates through control of
reaction conditions has become an exciting area of research in
synthetic coordination chemistry because of the observation
that several of these compounds show single-molecule magnet
(SMM) behavior.1–11 Over the past two decades, the field has
been enriched by the synthesis and characterization of a large
number of polymetallic entities of first-row transition metals,
lanthanides, and actinides having diverse and exciting
structures.12 Several aesthetically pleasing architectures (e.g.,
squares,13 rectangles,1 and pentagons14) have been assembled
in recent years. These are discrete polynuclear coordination
compounds assembled from mono-, bi- or trinuclear building
blocks. In several known phenoxido bound dinuclear nickel(II)
complexes the NiII ions are coupled ferromagnetically to yield
S = 2 states.15–17 These ferromagnetically coupled dinuclear
entities can be assembled into larger magnetically coupled
systems by means of bridging ligands capable of mediating
magnetic exchange interaction. In particular, such an endeavor
may be accomplished during fixation of atmospheric CO2 by a
mildly basic aqueous methanolic reaction mixture of Ni(II)
ions and co-ligands, due to the in situ generation of carbonato
linkers. The geometry of the ensuing multinuclear edifices is
dictated by the nature of the co-ligands used in the reaction.
A
multidentate ligand 2,6-bis-[(3-hydroxy-propylimino)-
methyl]-4-methyl-phenol (H3bpmp,18 Chart 1) has been suc-
cessfully utilized in the isolation of many d-block transition-
metal clusters.19
We have recently isolated mono- and trinuclear Zn com-
plexes of H3bpmp and its close congener for varying aggregat-
ing performances.18 To proceed with paramagnetic metal ions,
we have chosen a mixed-ligand approach via the reactions of
H3bpmp with a variety of co-ligands, in this case with ImH
and in situ generated CO32−. The reaction of Ni(NO3)2·6H2O,
H3bpmp, imidazole, and NEt3 in a methanolic solution led to
the isolation of a green block like solid after slow evaporation
after 9 days. This resulted in a novel octanuclear nickel cluster
of the general formula [Ni8(µ-H2bpmp)4(µ4-CO3)4(ImH)8]-
(NO3)4·2H2O (1) in 74% yield (eqn (1)) [see the ESI† for full
experimental details].
aDepartment of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India.
E-mail: dray@chem.iitkgp.ernet.in; Fax: (+91) 3222-82252; Tel: (+91) 3222-283324
bDepartment of Chemistry & Biochemistry, Florida State University, Tallahassee,
FL 32306, USA
4H3bpmp þ 8NiðNO3Þ2 ꢀ 6H2O þ 8ImH þ 4CO2 þ 4NEt3
! ½Ni8ðμ-H2bpmpÞ4ðμ4-CO3Þ4ðImHÞ8ꢁðNO3Þ4 ꢀ 2H2O
cDipartimento di Chimica e Centro di Strutturistica Diffrattometica, Università di
þ 4ðHNEt3ÞðNO3Þ þ 8HNO3 þ 42H2O…
ð1Þ
Ferrara, via Borsari 46, 44100 Ferrara, Italy
†Electronic supplementary information (ESI) available: Detail synthesis pro-
2−
Direct addition of CO3 as potassium carbonate can also
provide 1 in an equivalent yield reported above. Complex 1
(Fig. 1, below) forms prismatic green crystals and crystallizes
cedure, characterization data, and crystallographic data in CIF. CCDC 869968.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52999a
1970 | Dalton Trans., 2014, 43, 1970–1973
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binding to four Ni2 fragments present, and it appears to be
directing the positions of these fragments. The eight Ni(II)
ions are thus positioned in vertices of a distorted cube com-
pressed along the 4-fold axis.
Recently we have shown the formation of a Co5 cluster
which had been seen to encapsulate bridging CO32− ligands in
the absence of any carbonate addition.6–8,22 Similar to this
2−
earlier report, in the present case the CO3 ligand is most
probably derived from the fixation of atmospheric CO2, which
results in the fascinating Ni8 coordination cage. The CO32− ion
(vis-à-vis NO3−) was identified via charge balance consider-
ation, solution molar conductivity determination23 and cau-
tious consideration of the X-ray diffraction data (ESI†). The
FTIR spectrum of 1 displays strong absorptions at 1548 and
1331 cm−1 due to ν
(CO ) and νsym(CO2) vibrational modes
¯
2
¯
asym
of the carbonate ligand, respectively.24 The transformation of
2−
atmospheric CO2 to CO3 is similar to the function of carbo-
nic anhydrase, forming HCO3 in animal tissues at high CO2
−
concentration. Four such carbonato ligands provide appropri-
ate binding geometry along the four faces of the cube to
support the formation of the cluster cube.
Fig. 1 (above) ORTEP20 view of the asymmetric unit and (below) ball
and stick presentation of the octanuclear unit [Ni8(µ-H2bpmp)4(µ4-
CO3)4(ImH)8](NO3)4·2H2O in 1. Color code: Ni, light and dark green; N,
blue; O, red; C, black.
Around the four faces of the carbonate-stabilized metallic
core lie organic blocking groups made up of singly deproto-
nated H3bpmp and imidazole ligands. The former displays
bridging [3 + 3] mode of bonding, while the latter functions as
a terminal ligand. The crystallographically unique atoms Ni1
and Ni2 are six-coordinate with distorted octahedral geome-
tries, with a cis-N2O4 coordination environment (ESI, Fig. S3
and S4†). The remaining six Ni atoms of the cube display the
same symmetry related geometry. The average Ni–O and Ni–N
bond lengths are 2.070 Å and 2.060 Å, respectively. The Ni–OH
bonds from the alcohol ends are longer at 2.080 (av) Å. The
corresponding lengths for the carbonato ligand range from
2.030 to 2.101 Å. The bond angles are within the normal range
for the octahedral coordination environment. Within the Ni
cube, the distance between adjacent metal centers along one
edge of the cube, within the dimer, is 3.012 Å. The average
2−
Fig. 2 (left) Side view of coordination environments around Ni atoms in
1 and (right) observed ligand bonding modes.
distance along the other edges, across the CO3 bridge, is
5.322 Å.
2−
While the CO3 bridging modes present in this complex
in the tetragonal space group P421c, with the asymmetric unit
(Fig. 1, below) containing the Ni2 repeating unit as well as a
nitrate counter anion and solvent H2O molecule. The structure
displays disorder on the propanol chain C13–C14–C15–O3
which was refined over two sites (Fig. 1, below).
are quite common, the coordination driven supramolecular
assembly of cubic cluster 1 is rather exceptional. Intermole-
cular void spaces between the clusters are occupied by sol-
vation water molecules and nitrate ions (ESI, Fig. S5†). There
are intermolecular hydrogen bonds formed between the
cluster cube and the water molecules and counter ions via the
protonated alcohol terminals of the H2bpmp− ligands. In one
case the disordered arm is not coordinated to the NiII ion, but
is engaged in a hydrogen bonding interaction with one solvent
water molecule. Terminal imidazole groups were also engaged
in hydrogen bonding interactions with O atoms of the nitrate
groups (ESI, Fig. S6†). Selected bond lengths and angles are
given in Table S1 in the ESI.†
The complex consists of eight NiII ions, with the metallic
core displaying a cubic motif (Fig. S1†). It may best be
described as µ-H2bpmp− bound two NiII ions (Ni1 and Ni2)
occupying two corners of the cube and repeated via bridging
of carbonato groups in a cyclic manner (ESI, Fig. S2†).
Complex 1 is formed by four dinuclear [Ni2(H2bpmp)]3+ units
linked covalently by four μ4-carbonate ligands generated in situ
through fixation of atmospheric CO2 into the supramolecular
[Ni2(μ-CO3)]4 aggregate. It was found that at the four faces of
the cube four carbonato groups were present, which adopt a
µ4-η1:η1:η2-bonding mode (Fig. 2) ([4.211] in Harris notation21),
Magnetic properties were investigated on a polycrystalline
sample of 1. The temperature dependence of inverse magnetic
susceptibility (1/χ) is nearly linear in the 50–300 K range
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expected from the relatively low and competing values of the
magnetic exchange constants.
In summary, this work is one of the notable examples of
utilizing in situ generated carbonato linkers to form a discrete
cubic coordination cluster of fascinating magnetic behavior,
revealing the flexibility and utility of this linker for future syn-
thesis and magnetic material applications. A new octanuclear
cubic coordination cluster type has been prepared through the
assembly of H2bpmp− chelated Ni(II) ions and imidazole
capping ligands. The four faces of the cubic cluster contain
four µ4-carbonato ligands, which are derived from atmospheric
CO2 fixation. Magnetic studies indicate the presence of com-
peting magnetic interactions mediated by the carbonate
bridge. The antiferromagnetic exchange across the face diag-
onal of the cubic cluster dominates the magnetic behavior,
resulting in the spin-singlet ground state.
Fig. 3 Temperature dependence of χT for 1. The solid red curve rep-
resents the best simulation of the experimental data. Inset: temperature
dependence of inverse magnetic susceptibility of 1, with the Curie–
Weiss fit shown as a solid red line.
(Fig. 3, inset). Fitting this part of the data to the Curie–Weiss
law resulted in the Curie constant of 10.39(2) emu K mol−1 per
cubic cluster, i.e. ∼1.30(2) emu K mol−1 per Ni2+ ion. This
value is larger than the spin-only value of 1.00 emu K mol−1
expected for the S = 1 Ni2+ ion, indicating orbital contribution
to the total magnetic moment, as is well known for Ni(II) com-
plexes.25 The Weiss constant obtained from the linear fit is
negative, θ = –37.3(3) K, suggesting the presence of dominant
antiferromagnetic interactions between Ni2+ ions. Neverthe-
less, the magnetic exchange coupling in this cluster is far
more complicated. This conjecture can be made based on the
earlier detailed work by Fondo et al.15 on the magnetic pro-
perties of a tetranuclear carbonato-bridged Ni(II) complex with
bonding topology that essentially represents one face of the
Ni8 cubic cluster discussed herein. According to the report on
the tetranuclear complex and using the high symmetry of our
complex, we can identify three magnetic exchange pathways: a
ferromagnetic exchange within each Ni2 dimer (J1 > 0), a ferro-
magnetic exchange along the long cube edge (J2 > 0), and an
antiferromagnetic exchange along the face diagonal of the
cube (J3 < 0). The latter two interactions are mediated by the
Acknowledgements
AKG is thankful to the Council of Scientific and Industrial
Research, New Delhi, India for financial support. The authors
also thank DST, New Delhi, for providing the Single Crystal
X-ray Diffractometer facility in the Department of Chemistry,
IIT Kharagpur under its FIST program. VB acknowledges the
Italian Ministry of University and Scientific Research (MIUR,
Rome). MS is grateful to the National Science Foundation for
the partial support of this work (award CHE-0911109).
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