Nickel complexes with N2O donor ligands: Syntheses, structures, catalysis and magnetic studies

Article abstract of DOI:10.1002/ejic.200700500

Two new terephthalato-bridged tetranuclear polymeric Ni^II complexes, namely [Ni₄L₄¹(μ-tp- κ₄-O)(H₂O)₂(μ-tp-κ₂- O)]· 2C₂H₅OH·CH₃OH·3H ₂O (1) and [Ni₄L₄²(μ-tp- κ₄-O)(H₂O)₂(μ-tp-κ₂- O)]·3H₂O (2) [L¹ = N-(3-aminopropyl)-5- bromosalicylaldimine and L² = N-(3-aminopropyl)salicylaldimine], are reported along with the syntheses and structures of the dicyanoargentate-bridged polymeric complexes [Ni(L¹)(H₂O)-{Ag(CN) ₂}]α (3) and [Ni(L³)(MeOH){Ag(CN) ₂}]α (4) [L³ = N-(3-amino-2,2-dimethylpropyl)-5- bromosalicylaldimine]. All four complexes are found to be effective heterogeneous catalysts for the epoxidation of alkenes such as styrene, α-methylstyrene and cyclohexene in the presence of tert-butyl hydroperoxide. The variable-temperature magnetic susceptibility measurements (300-2 K) of complex 1 show a fair degree of antiferromagnetic coupling between the Ni^II centers. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700500

FULL PAPER
DOI: 10.1002/ejic.200700500
Nickel Complexes with N2O Donor Ligands: Syntheses, Structures, Catalysis
and Magnetic Studies
Jishnunil Chakraborty,[a] Mahasweta Nandi,[b] Heike Mayer-Figge,[c] William S. Sheldrick,[c]
Lorenzo Sorace,[d] Asim Bhaumik,[b] and Pradyot Banerjee*[a]
Keywords: Nickel / N,O ligands / Epoxidation / Heterogeneous catalysis / Magnetic properties
Two new terephthalato-bridged tetranuclear polymeric NiII
complexes, namely [Ni4L41(µ-tp-κ4-O)(H2O)2(µ-tp-κ2-O)]·
2C2H5OH·CH3OH·3H2O (1) and [Ni4L42(µ-tp-κ4-O)(H2O)2(µ-
complexes are found to be effective heterogeneous catalysts
for the epoxidation of alkenes such as styrene, α-methylsty-
rene and cyclohexene in the presence of tert-butyl hydroper-
tp-κ2-O)]·3H2O (2) [L1 = N-(3-aminopropyl)-5-bromosalicyl- oxide. The variable-temperature magnetic susceptibility
aldimine and L2 = N-(3-aminopropyl)salicylaldimine], are re- measurements (300–2 K) of complex 1 show a fair degree of
ported along with the syntheses and structures of the dicya- antiferromagnetic coupling between the NiII centers.
noargentate-bridged polymeric complexes [Ni(L1)(H2O)-
{Ag(CN)2}]α (3) and [Ni(L3)(MeOH){Ag(CN)2}]α (4) [L3 = N-(3-
amino-2,2-dimethylpropyl)-5-bromosalicylaldimine]. All four
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
to a variety of connection modes with transition metal cen-
ters and provide abundant structural motifs. Furthermore,
it can act not only as a hydrogen-bond donor but also as
an acceptor due to the existence of protonated and/or de-
protonated carboxylate groups. The terephthalate ligand is
therefore regarded as an excellent candidate for the con-
struction of multidimensional coordination frameworks.
The introduction of another kind of both N- and O-do-
nor chelating ligand, such as a tridentate Schiff base, in the
{M/tp} (M = transition metal) system may induce new
structural evolution as the introduction of N-donor chelat-
ing ligands into the metal sites may inhibit the expansion
of the polymeric frameworks and lead to the desired low-
dimensional coordination polymers. However, the synthesis
of complexes containing mixed ligands is expected to be
more difficult to control than that of complexes containing
one ligand for several reasons, such as the different solubil-
ity of the organic ligands and competition between these
ligands to coordinate the transition metal. In order to gain
insight into the parameters that control these variables we
have synthesized two new terephthalate-bridged tetranu-
Introduction
Low-dimensional coordination polymers, including one-
dimensional (1D) chain-like and two-dimensional (2D)
layer-like structures, have received much attention in recent
years owing to their intriguing structural features and
unique electroconductive, nonlinear optical, and magnetic
properties, which differ from those of three-dimensional
(3D) coordination polymers.[1] A great variety of low-di-
mensional coordination polymers based on d-block transi-
tion metal ions and organic ligands have been prepared to
date, many of which have potential applications in the field
of functional materials.[2–6] The organic components con-
taining N- or O- donors in the framework offer great poten-
tial for chemical and structural diversity.[7]
Organic aromatic polycarboxylates are some of the most
widely employed multidentate O-donor ligands for the
preparation of such coordination polymers with multidi-
mensional networks. In this respect, terephthalate (tp) has
been studied extensively as a rigid and versatile bridging
ligand[8] as it contains multiple bridging moieties that lead
1
clear polymeric NiII complexes, namely [Ni4L4 (µ-tp-κ4-O)-
(H2O)2(µ-tp-κ2-O)]·2C2H5OH·CH3OH·3H2O
(1)
and
[a] Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
2
[Ni4L4 (µ-tp-κ4-O)(H2O)2(µ-tp-κ2-O)]·3H2O [2; L1 = N-(3-
aminopropyl)-5-bromosalicylaldimine and L2 = N-(3-ami-
nopropyl)salicylaldimine]. NiII forms a phenolate-bridged
binuclear unit in these complexes and two such units are
connected by a terephthalate moiety to form a tetranuclear
nickel(II) complex. The crystal structure of complex 1
shows that the tetranuclear units are connected to each
other by the terephthalate moiety in a bis(monodentate)
mode to form a one-dimensional network. The variable-
Jadavpur, Kolkata-700 032, India
Fax: +91-33-2473-2805
E-mail: icpb@mahendra.iacs.res.in
[b] Department of Materials Science and Center for Advanced Ma-
terials, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata-700 032, India
[c] Lehrstuhl für Analytische Chemie, Ruhr-Universität Bochum,
Universitätsstrasse 150, 44780 Bochum, Germany
[d] Laboratory for Molecular Magnetism (LAMM), Dipartimento
di Chimica, Università di Firenze,
Firenze, Italy
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P. Banerjee et al.
FULL PAPER
temperature magnetic behavior of complex 1 is also investi-
gated.
Results and Discussion
Like terephthalate, cyanometalate anions have been ex-
tensively used as building blocks in supramolecular coordi-
nation polymers.[9,10] Compared to the higher coordination
Synthesis
The monocondensed ligands L1–L3 were prepared as
counterparts of various transition metals, there has been their nickel complexes Ni(L1)2, Ni(L2)2, and Ni(L3)2 by tre-
considerably less investigation of two-coordinate linear cya- ating 5-bromosalicylaldehyde or salicylaldehyde with
nometalate building blocks for the construction of coordi- Ni(OAC)2·4H2O and the corresponding amine (see Experi-
nation polymers. We have chosen to examine the ability of mental Section). Complex 1 was prepared by the reaction
the linear dicyanoargentate ion [Ag(CN)2]– to form supra- of Ni(L1)2 with equimolar amounts of Ni(ClO4)2·6H2O and
molecular frameworks. The syntheses and structures of two the disodium salt of terephthalic acid in EtOH/MeOH/H2O
dicyanoargentate-bridged polymeric networks, namely to produce the µ2-phenolate and terephthalate-bridged tet-
1
[Ni(L1)(H2O){Ag(CN)2}]α (3) and [Ni(L3)(MeOH){Ag- ranuclear complex [Ni4L4 (µ-tp-κ4-O)(H2O)2(µ-tp-κ2-O)]·
(CN)2}]α [4; L3 = N-(3-amino-2,2-dimethylpropyl)-5-bromo- 2C2H5OH·CH3OH·3H2O (Scheme 1). Complex 2 was pre-
salicylaldimine], are therefore also included in this paper.
pared in the same way as described for complex 1 but with
Intrigued by a recent study concerning the use of (salen)- salicylaldehyde for the Schiff base condensation instead of
NiII complexes as homogeneous/heterogeneous epoxidation 5-bromosalicylaldehyde. Complex 3 was prepared by the re-
catalysts,[11] we extended our study to the epoxidation of action of Ni(L1)2 with equimolar amounts of Ni(ClO4)2·
alkenes such as styrene, α-methylstyrene and cyclohexene 6H2O and [NaAg(CN)2] in MeOH/H2O to produce the ar-
in the presence of tert-butyl hydroperoxide heterogeneously gentocyanide-bridged heterobinuclear polymeric complex
catalyzed by complexes 1–4.
[Ni(L1)(H2O){Ag(CN)2}]α (Scheme 2). Complex 4 was syn-
Scheme 1. Synthetic routes to complexes 1 and 2.
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Nickel Complexes with N2O Donor Ligands
Scheme 2. Synthetic routes to complexes 3 and 4.
thesized in dmf/MeOH by treating Ni(L3)2 with equimolar firming the presence of the coordinated CH3OH molecule.
amounts of Ni(ClO4)2·6H2O and [NaAg(CN)2] to yield the The strong band in the range 1585–1602 cm–1 observed for
argentocyanide-bridged polymeric species [Ni(L3)(MeOH)- all complexes is due to the ν(CO) absorption of the phenol-
{Ag(CN)2}]α.
ate-containing ligands.
IR Spectra
Structure of Complex 1
The IR spectra of 1 and 2 show a broad absorption band
A view of 1 is shown in Figure 1 and key bond lengths
in the range 3321–3504 cm–1 which can be assigned to the and angles are listed in Table 1. The crystal structure of this
O–H stretching vibrations of the coordinated and noncoor- complex shows that the asymmetric unit contains a tetranu-
1
dinated water molecules and noncoordinated MeOH and clear molecule of the type [Ni4L4 (µ-O2CC6H4CO2)(H2O)2-
EtOH molecules in the case of complex 1. The strong band (O2CC6H4CO2)] in addition to two ethanol, one methanol,
in the range 1637–1645 cm–1 can be assigned to ν(CNiminic), and three water molecules. The tetranuclear unit is formed
which suggests coordination of the ligands to the metal cen- by two NiII atoms (Ni1 and Ni2) bridged by two µ2-phe-
ters through the imine nitrogen atoms. The tp unit coordi- nolato oxygen atoms (O1 and O2) of the Schiff-base ligands
nates the metal ions in both complexes in both its bis(mono- and a terephthalato ligand in its bis(bidentate) mode that
dentate) and bis(bidentate) coordination modes. The val- connects the four NiII centers. The tridentate ligand L1 pro-
ues of ∆ν, indicating the difference between the asymmetric vides an N2O2 donor set around each metal atom. The
stretching νa(COO–) and the symmetric stretching basal NiN2O2 plane for the Ni1 center is formed by the two
νs(COO–), for 1 are 148 and 29 cm–1, whereas those for nitrogen atoms (N1 and N2) and the two oxygen atoms (O1
complex 2 are 142 and 27 cm–1.
and O2) of the bridging phenolato ligands while the basal
The strong band at 2162 cm–1 in the IR spectrum of com- plane of the Ni2 center is formed by two different nitrogen
plex 3 is due to the ν(CN) absorption of the N-bonded ar- atoms (N3 and N4) of another L1 ligand and the same phe-
gentocyanide unit and the band at 3346 cm–1 is due to the nolate oxygens (O1 and O2). The O3 atom of the terminal
water molecule coordinated to the NiII center. Complex 4 water molecule and the O5 atom of the bridging bis(bident-
also exhibits a strong band at 2164 cm–1 due to the N- ate) terephthalato ligand occupy the axial positions of the
bonded argentocyanide unit. The broad band at 3437 cm–1 Ni1 center. The coordination environment of the Ni2 center
is assigned to an O–H stretching vibration, thereby con- is somewhat different to that of the Ni1 center. Thus, for
Figure 1. A view of the molecule of complex 1.
Eur. J. Inorg. Chem. 2007, 5033–5044
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P. Banerjee et al.
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the Ni2 center the axial positions are occupied by the O4 terephthalate moiety in its bis(monodentate) mode to form
atom of the bridging terephthalato ligand and the O6 atom an infinite one-dimensional network where symmetry-re-
of another bridging terephthalato ligand in its bis(mono- lated O6 terephthalato-oxygen atoms bind two Ni2 centers
dentate) coordination mode. Each nickel atom has a dis- in a zig-zag structure along the a-axis The simultaneous
torted octahedral environment. The phenolate-bridged presence of two terephthalate ligands with different bridg-
nickel atoms are separated by 3.076 Å with two Ni1–O1– ing modes gives rise to an unprecedented coordination
Ni2 and Ni1–O2–Ni2 bridging angles of 97.4(4)° and chain. It is interesting to note that the adjacent layers are
96.5(3)°, respectively; two Ni2N4O2 moieties are separated not parallel when viewed along the c-axis but are inclined
from each other by 10.996 Å. The basal bond lengths to each other with an angle of 39.17° (Figure 3). The dihe-
around Ni1 are within the range 2.020(8)–2.089(10) Å while dral angle formed between the anion and the equatorial
those around Ni2 are in the range 2.044(8)–2.092(10) Å. plane is 62.14°, and the carboxylate group O6/C61/O7 devi-
The apical Ni1–O3, Ni1–O5, Ni2–O4, and Ni2–O6 bond ates from coplanarity with the phenyl ring (11.65°). This
lengths are 2.143(6), 2.077(8), 2.091(6), and 2.133(8) Å, deviation is uniform throughout the chain. The Ni···Ni sep-
respectively. The O3–Ni1–O5 bond angle is 176.8(3)° while aration within a linear chain is 11.296 Å. This distance is
that for O4–Ni2–O6 is 177.8(3)°. The Ni–Otp bond lengths
are similar to those found in other terephthalate-bridged
nickel complexes.[12,13]
The packing diagram of complex 1 (Figure 2) shows that
the tetranuclear NiII units are joined to each other by the
Table 1. Selected bond lengths [Å] and angles [°] in complex 1.
Bond lengths [Å]
Ni1–O1
Ni1–O2
Ni1–O3
Ni1–O5
Ni1–N1
Ni1–N2
2.020(8)
2.079(8)
2.143(6)
2.077(8)
2.030(11)
2.089(10)
Ni2–O1
Ni2–O2
Ni2–O4
Ni2–O6
Ni2–N3
Ni2–N4
2.075(8)
2.044(8)
2.091(6)
2.133(8)
2.047(10)
2.092(10)
Bond angles [°]
O1–Ni1–O2
O1–Ni1–O3
O1–Ni1–O5
O1–Ni1–N1
O1–Ni1–N2
O2–Ni1–O3
O2–Ni1–O5
O2–Ni1–N1
O2–Ni1–N2
O3–Ni1–O5
O3–Ni1–N1
O3–Ni1–N2
O5–Ni1–N1
O5–Ni1–N2
N1–Ni1–N2
Ni1–O1–Ni2
79.7(3)
88.0(3)
94.5(3)
89.0(4)
174.6(4)
93.9(3)
88.7(3)
168.6(4)
94.9(4)
176.8(3)
86.9(4)
92.8(3)
91.0(4)
85.0(4)
96.5(4)
97.4(4)
O1–Ni2–O2
O1–Ni2–O4
O1–Ni2–O6
O1–Ni2–N3
O1–Ni2–N4
O2–Ni2–O4
O2–Ni2–O6
O2–Ni2–N3
O2–Ni2–N4
O4–Ni2–O6
O4–Ni2–N3
O4–Ni2–N4
O6–Ni2–N3
O6–Ni2–N4
N3–Ni2–N4
Ni1–O2–Ni2
79.2(3)
86.8(3)
95.4(3)
165.5(4)
97.0(3)
93.0(3)
86.9(3)
87.6(4)
176.3(3)
177.8(3)
87.7(4)
86.3(4)
90.1(3)
93.9(3)
96.0(4)
96.5(3)
Figure 3. A view of the packing fragment showing the crossing of
the 1D networks in 1. The polyhedra represent the Ni2N4O6 core.
Figure 2. Packing diagram of complex 1. The polyhedra represent the Ni2N4O6 core.
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Nickel Complexes with N2O Donor Ligands
shorter than in other terephthalate-bridged nickel com- The central nickel atom lies slightly above the equatorial
plexes such as [Ni(pyrazole)4(tp)]n [11.446(2) Å],[14] plane (0.048 Å).
[Ni(C12H30N6O2)(C8H4O4)]n·4nH2O [11.514(1) Å],[12] where
Table 2. Selected bond lengths [Å] and angles [°] in complex 3.
n
is ϱ, and [(µ-terephthalato){Ni(dca)(dpt)(H2O)}2]
[11.379(10) Å].[15] The one-dimensional chains are con-
nected together by intermolecular hydrogen-bonding inter-
actions between the oxygen atom of uncoordinated meth-
anol and the coordinated amine of the Schiff base
[N2···O300 3.33(3) Å]. In addition, one lattice ethanol
molecule takes part in hydrogen bonding with another co-
ordinated amine group of the Schiff base [N4···O200
3.12(3) Å]. There is also an intramolecular hydrogen bond
connecting an amine group with a bis(monodentate)
terephthalato oxygen atom [N4···O7 2.889(14) Å]. The two
C–O bond lengths of the carboxylate group are practically
identical even though one is coordinated to the Ni2 center
[1.253(16) Å] and the other is not [1.253(19) Å]. In general,
the C–O bond lengths of coordinated carboxylate groups
fall in the range 1.266–1.279 Å and are longer than the free
carbonyl C–O bonds (1.226–1.255 Å) when terephthalate
coordinates a metal ion in a monodentate fashion.[12]
Bond lengths [Å]
Ni1–O1
Ni1–O2
Ni1–N1
Ni1–N2
Ni1–N3
2.015(4)
N3–C8
Ag1–C8
Ag1–C9
N4–C9
1.142(9)
2.064(7)
2.065(7)
1.139(9)
2.152(4)
2.045(4)
2.076(5)
2.115(5)
Bond angles [°]
O1–Ni1–O2
O1–Ni1–N2
O2–Ni1–N1
O2–Ni1–N3
N1–Ni1–N3
C8–Ag1–C9
87.66(16)
175.28(17)
175.72(19)
84.29(17)
92.05(19)
176.4(3)
O1–Ni1–N1
O1–Ni1–N3
O2–Ni1–N2
N1–Ni1–N2
N2–Ni1–N3
89.90(18)
86.61(19)
87.91(17)
94.42(19)
91.4(2)
Hydrogen bonds [Å, °]
D–H–A
O2–H2···O1
D···A
H···A
1.78(5)
ЄD–H···A
176(7)
2.608(6)
The crystal packing of complex 3 (Figure 5) shows that
the [Ag(CN)2]– units display an almost linear coordination
[C8–Ag1–C9 176.4(3)°] with Ag1–C8 and Ag1–C9 bond
lengths of 2.064(7) and 2.065(7) Å, respectively, and form
endless one-dimensional chains involving the [NiL1(H2O)]+
moiety. These chains lie parallel to each other with diagonal
distances a and b of 8.12 and 5.95 Å, respectively, and an
Ag···Ag distance of about 10.6 Å. The minimum Ag···Ag
distance found between two successive layers is 4.514 Å,
which is greater the van der Waals diameter of silver
(3.44 Å)[16] and hence no direct Ag···Ag interactions are
present. In addition to the argentocyanide bridging, there
Structure of Complex 3
The coordination environment of the NiII ion in complex
3 is shown in Figure 4 along with the atom-numbering
scheme. Key bond lengths and angles are listed in Table 2.
The coordination environment for each nickel center is best
described by a distorted octahedral geometry. Two nitrogen
atoms, a phenolate oxygen atom of the tridentate Schiff-
base ligand L1, and an oxygen atom of the coordinated
water molecule, with Ni1–N1, Ni1–N2, Ni1–O1, and Ni1–
O2 distances of 2.045(4), 2.076(5), 2.015(4), and 2.152(4) Å,
respectively, define the equatorial plane around the nickel
atom. The axial sites are occupied by two nitrogen atoms
of the bridging argentocyanide moiety with Ni1–N3 and
Ni1–N4 bond lengths of 2.115(5) and 2.101 Å, respectively.
Figure 4. A view of the molecule of complex 3.
Figure 5. Packing diagram of complex 3.
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P. Banerjee et al.
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are intermolecular hydrogen-bonding interactions between nolato oxygen atoms (O1) in one layer and the methanol
the phenolate oxygen of the L1 moiety in one layer and the oxygen atom (O2) in the adjacent layer (O1···O2 2.586 Å)
coordinated water oxygen of the next layer which form to form a 2D supramolecular network (Figure 8). It is note-
stable eight-membered rings [O2···O1 2.608(6) Å; ЄO2– worthy that this network adopts a wavelike layer (Figure 9)
H20···O1 176(7)°] in an extended two-dimensional ladder- due to greater deviation of the bond angles [ЄNi1–N3–C8
like network (Figure 6).
166.4(2)° and ЄNi1_a–N4–C9 166.6(2)°] from planarity in
the bridging argentocyanide moieties along the b-axis. The
same bond angles in complex 3 are close to planarity, which
means that the formation of a wavelike network can be
ruled out and a chainlike network is obtained instead.
Figure 7. A view of the molecule of complex 4.
Table 3. Selected bond lengths [Å] and angles [°] in complex 4.
Bond lengths [Å]
Ni1–O1
Ni1–O2
Ni1–N1
Ni1–N2
Ni1–N3
2.0316(18)
2.1135(19)
2.045(2)
2.088(3)
2.084(2)
N3–C8
Ag1–C8
Ag1–C9
N4–C9
1.147(4)
2.050(3)
2.061(3)
1.142(4)
Figure 6. Packing fragment of complex 3 showing the ladder-like
network.
Bond angles [°]
O1–Ni1–O2
O1–Ni1–N2
O2–Ni1–N1
O2–Ni1–N3
N1–Ni1–N3
C8–Ag1–C9
95.50(9)
178.07(8)
177.40(9)
90.66(8)
90.61(9)
175.27(13)
O1–Ni1–N1
O1–Ni1–N3
O2–Ni1–N2
N1–Ni1–N2
N2–Ni1–N3
86.77(9)
89.69(9)
85.91(9)
91.86(9)
88.98(9)
Structure of Complex 4
A perspective view of 4 together with the atom-labeling
scheme is shown in Figure 7. Each nickel site is coordinated
by two nitrogen atoms, a phenoxo oxygen atom of the tri-
dentate L2 ligand, an oxygen atom of the methanol mole-
cule, and two bridging nitrogen atoms of the [Ag(CN)2]–
moiety. The average Ni–N bond length is 2.069 Å and the
Ni–Ophenolato and Ni–Omethanol bond lengths are 2.0316(18)
and 2.1135(19) Å, respectively. The N(O)–Ni–O(N) bond
angles range from 86.77(9)° to 178.07(8)° (see Table 3),
which means that the nickel centers display a distorted octa-
hedral coordination geometry. The argentocyanide ion acts
as a bridging ligand that connects the [NiL3(MeOH)]+ moi-
eties to form endless one-dimensional chains. The Ni···Ni
separation within a chain is 10.414 Å, whereas the Ni···Ni
separation between two successive chains is 5.052 Å. This
distance is uniform throughout two successive layers. More-
over, adjacent chains are connected together through non-
covalent interactions between the two coordinated phe- Figure 8. Packing diagram of complex 4.
5038 Eur. J. Inorg. Chem. 2007, 5033–5044
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Nickel Complexes with N2O Donor Ligands
Table 4. Epoxidation of styrene[a] catalyzed by complexes 1–4.
Entry Catalyst Conversion [%][b] Epoxide selectivity TON[c]
1
2
3
4
1
2
3
4
39
56
41
66
81
82
91
83
121
36
127
205
[a] Solvent: CH3CN; temperature: 333 K; complex 2 was used as a
solid catalyst whereas the other catalysts were impregnated on a
2D hexagonal mesoporous silica. [b] Reaction time: 24 h; conver-
sion and TON were measured by GC using n-decane as internal
standard. [c] TON: mol of substrate converted per mole of NiII
present in catalyst used for the reaction.
Figure 9. A view of the packing fragment of 4 showing the 2D
wave-like network.
Table 5. Epoxidation of α-methylstyrene[a] catalyzed by complexes
1–4.
Entry Catalyst Conversion [%][b] Epoxide selectivity TON[c]
Heterogeneous Catalysis
1
2
3
4
1
2
3
4
56
76
69
61
86
93
78
89
153
43
189
167
The use of nickel complexes with cyclam and salen li-
gands as active catalysts in the epoxidation of alkenes[17–22]
has been reported in the literature. Some examples include
the use of (salen)Ni-type complexes in faujasites for the oxi-
dation of phenol with H2O2,[23] the epoxidation of cyclohex-
[a] Solvent: CH3CN; temperature: 333 K; complex 2 was used as a
solid catalyst whereas the other catalysts were impregnated on a 2D
hexagonal mesoporous silica. [b] Reaction time: 24 h; conversion
ene and 1-hexene by NaOCl,[24,25] as well as the use of [Ni- and TON were measured by GC using n-decane as internal stan-
dard. [c] TON: mol of substrate converted per mole of NiII present
(salen)] immobilized on several solid supports as a catalyst
in catalyst used for the reaction.
for hydrogenation reactions.[26] More recently, Ferreira et
al.[11] have carried out an extensive study of (salen)NiII
Table 6. Epoxidation of cyclohexene[a] catalyzed by complexes 1–4.
complexes as homogeneous catalysts and, after immobiliza-
Entry Catalyst Conversion [%][b] Epoxide selectivity TON[c]
tion in zeolites (X and Y; following the “ship-in-a-bottle”
procedure), as heterogeneous catalysts in the epoxidation of
trans-β-methylstyrene by NaOCl (considered to be environ-
mentally more friendly).
The heterogeneous oxidation reactions were carried out
using 1.0 g of substrate and 0.200 g of loaded catalyst
(0.050 g of solid complex in the case of complex 2) in 10 mL
of CH3CN whilst stirring in a two-necked, round-bottomed
flask fitted with a water condenser in an oil bath at 333 K.
tert-Butyl hydroperoxide (equimolar with respect to the
substrate) was added immediately before the start of the
reaction. The products were collected from the reaction
mixture after different time intervals and analyzed by gas
1
2
3
4
5
6
1
2
3
4
4
4
82
89
81
89
87
33
82
82
87
84
79
29
323
72
319
350
[a] Solvent: CH3CN; temperature: 333 K; complex 2 was used as a
solid catalyst whereas the other catalysts were impregnated on a
2D hexagonal mesoporous silica. [b] Reaction time: 24 h; conver-
sion and TON were measured by GC using n-decane as internal
standard. [c] TON: mol of substrate converted per mole of NiII
present in catalyst used for the reaction.
chromatography. They were identified by comparison with hexene shows the highest conversion and TON; the epoxide
known standards. selectivities for styrene and α-methylstyrene are also very
Tables 4, 5, and 6 show the results obtained with for the good.
various Ni complexes immobilized on silica. Acetonitrile
Microporous and mesoporous titanium silicates have
was used as the solvent in all cases. The major products been widely employed in the epoxidation of aromatic and
of the reactions are epoxides and the selectivities for these alicyclic unsaturated compounds[27] in the presence of dilute
compounds are very high. The very high TONs obtained aqueous H2O2 as oxidant. The intermediate titanium hy-
for the oxidation of styrene, α-methylstyrene, and cyclohex- droperoxo species formed play a crucial role in catalyzing
ene suggest a very high catalytic efficiency for the nickel the epoxidation reaction. The water molecules present in 1–
hydroperoxo species that could form in the presence of tert- 3 and the methanol moiety in 4 could help the Ni complex
butyl hydroperoxide. Small amounts of the corresponding to form such Ni hydroperoxide species in presence of H2O2,
diols were also obtained for all substrates.
Complex 2 is insoluble in all common solvents therefore in the catalysis of these epoxidation reactions (Scheme 3).
instead of immobilizing it on mesoporous silica this pure One control experiment was carried out for cyclohexene
similar to the titanium silicate surface, and this could help
complex was used as a solid catalyst. Our experimental re- over immobilized catalyst 4 to check the efficiency of subse-
sults show that the TONs obtained with the immobilized quent catalytic cycles and whether Ni leaches from these
catalysts are better than the pure nickel complex 2 for all catalysts (Table 6, entry 4). After the initial experiment the
the olefin substrates and that among these substrates cyclo- catalyst was washed thoroughly with acetonitrile and then
Eur. J. Inorg. Chem. 2007, 5033–5044
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5039

P. Banerjee et al.
FULL PAPER
epoxidation of styrene catalyzed by the four NiII complexes.
The conversions follow a sigmoid curve and reach maxi-
mum conversion after 24 h. It is apparent from these experi-
mental observations that immobilized Ni complex 4 is most
Scheme 3. Proposed mechanism for the partial oxidation of styrene,
α-methylstyrene, and cyclohexene by tert-butyl hydroperoxide with
complexes 1–4 as catalyst.
Figure 11. Bar diagram showing the catalytic activity of complexes
1–4 with α-methylstyrene (CH3CN, 333 K).
treated with 0.1  HCl solution in EtOH for 8 h at 343 K
and finally dried at 373 K for 2 h. It was then re-used as
catalyst for a second experiment (Table 6, entry 5). The
catalytic activity was found to remain practically the same,
with only a marginal difference in the epoxide selectivity.
This suggests that there is almost no Ni leaching into the
liquid phase from these immobilized catalysts. A blank re-
action of the oxidation of cyclohexene in the absence of any
solid catalyst under otherwise identical reaction conditions
was also carried out (Table 6, entry 6). The results show
very poor conversion and epoxide selectivity in this case,
thereby confirming the catalytic role of the Ni complex and
the immobilized catalysts in these epoxidation reactions.
Figures 10, 11, and 12 show bar diagrams for the epoxid-
ation of the alkenes over different immobilized catalysts at
333 K. It is clear from these figures that all the NiII com-
plexes act as efficient epoxidation catalysts in the presence
of tert-butyl hydroperoxide as oxidant. Figure 13 shows the
alkene conversion as a function of reaction time for the
Figure 12. Bar diagram showing the catalytic activity of complexes
1–4 with cyclohexene (CH3CN, 333 K).
Figure 13. Conversion vs. time plot for the epoxidation of styrene
with tert-butyl hydroperoxide catalyzed by complexes 1–4 (CH3CN,
333 K).
Figure 10. Bar diagram showing the catalytic activity of complexes
1–4 with styrene (CH3CN, 333 K).
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Nickel Complexes with N2O Donor Ligands
reactive for the epoxidation of all three substrates with high
TONs (205, 167, and 350 for styrene, α-methylstyrene, and
cyclohexene, respectively).
Magnetic Behavior of 1
The temperature dependencies of χMT and χM for com-
plex 1 are plotted in Figure 14. The value of χMT observed
at room temperature (4.54 cm3 mol–1 K at 300 K) is typical
for four uncoupled NiII ions with g Ͼ 2.00, as expected. On
lowering the temperature the χMT product decreases
smoothly, reaching 0.05 cm3 mol–1 K at 2 K. This behavior
Figure 15. M vs. H curve for 1 measured at 2.7 (squares) and 5.1 K
clearly points to the existence of moderate antiferromag- (triangles).
netic interactions between the NiII centers, which result in
a diamagnetic ground state. This is confirmed by the shape
The obtained value of JЈ appears to be relatively large
of the χM vs. T curve, which passes through a maximum at
and higher than would be expected on the basis of literature
data.[29] This might, in principle, be due to the neglecting
of the single-ion zero-field-splitting terms that affect the
shape of the calculated temperature dependence of the
susceptibility curve in the same way as an intermolecular
coupling.[30] We therefore fitted the data by including these
terms using the Ginsberg formula for dinuclear NiII centers
with interdimer interactions.[30,31] The best fit parameters
obtained with this approach were J = 14.5Ϯ0.2 cm–1, in
perfect agreement with the simple isotropic approach, 2zJЈ
= 3.4Ϯ0.6 cm–1, Nα = (4.5Ϯ2)ϫ10–4 emumol–1, g =
2.17Ϯ0.02, and D = 1–8 cm–1, with the latter parameter
having only a weak effect on the calculated curve. We can
conclude on the basis of the two fits that the Ni–Ni interac-
tion in each dinuclear moiety is close to 14.5 cm–1 and that
a non-negligible interaction is transmitted by the tereph-
thalato ligand, in contrast to what is normally assumed.[32]
The value of the Ni–Ni coupling constant in the dinuclear
moiety is somewhat larger than would be expected on the
basis of simple magnetostructural correlations derived for
centrosymmetric bis(phenolate)-bridged dinuclear nickel
complexes derived by Thompson’s group[33] [Equation (2)].
around 22 K.
Figure 14. Temperature-dependence of χMT (left scale, full squares)
and χM (right scale, full triangles) for 1 and best fit curves obtained
with the parameters reported in the text. The theoretical curves
calculated with the two different models cannot be discerned.
An initial quantitative analysis of the results obtained
was performed using a simple isotropic Hamiltonian that
considers all the possible exchange interactions in the tetra-
mer [Equation (1)].
J = +7.27θ – 704.06
(2)
where θ is the bridging Ni–O–Ni angle. Using the average
value of the bridging angle (96.94°) for θ would result in a
predicted value of J = 0.8 cm–1 for 1, far from what is ob-
served experimentally. We attribute this discrepancy to the
effect of the carboxylato ligand, which bridges the two
nickel ions in a syn-syn fashion and provides an additional
pathway for the antiferromagnetic coupling between the
two centers.[34] Finally, we note that the importance of the
interdimer exchange coupling provided by the terephthalato
ligand lies in the large π-connectivity it may provide, thus
providing a relatively efficient pathway whilst keeping the
interacting metal centers far apart.
ˆ
H = J(S1S2 + S3S4) + JЈ(S1S3 + S1S4 + S2S3 + S2S4) + gβSH (1)
where J is the exchange coupling within each dinuclear moi-
ety, in other words the exchange interactions transmitted by
the two phenolato bridges and the carboxylato one, and JЈ
describes the coupling through the terephthalato bridging
ligand. The fit was implemented using the CLUMAG pack-
age,[28] including a temperature-independent paramagne-
tism term (Nα = (8Ϯ2)ϫ10–4 emumol–1), which yields the
best fit parameters J = 14.5Ϯ0.6 cm–1, JЈ = 3.8Ϯ1.2 cm–1,
and g = 2.15Ϯ0.02. This set of parameters results in a dia-
magnetic ground state, as expected, with the first triplet ly-
ing 14.5 cm–1 higher in energy. This is in fair agreement
with the M vs. H curve (Figure 15), which shows the begin-
ning of a singlet–triplet field induced transition at the high-
est measured field.
Conclusions
We have prepared four polymeric nickel(II)/Schiff-base
complexes with terephthalate and argentocyanide as bridg-
Eur. J. Inorg. Chem. 2007, 5033–5044
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5041

P. Banerjee et al.
FULL PAPER
stirring for 30 min. After 2 h constant stirring, tetraethylammo-
nium hydroxide was added dropwise until the color of the solution
turned white. The pH value of the final mixture was about 11.0.
The resultant mixture was aged overnight whilst stirring at room
temperature and then transferred into a polypropylene bottle and
heated at 80 °C for 72 h without stirring. The solid produced was
recovered by filtration, washed several times with water, and dried
at room temperature. The resulting powder was calcined at 703 K
to remove the organic surfactants. The molar ratio of the various
constituents was TEOS/CTAB/SDS/H2O = 50:25:5:4500. For load-
ing moieties. Complex 1 is tetranuclear and its X-ray crystal
structure shows that each tetranuclear unit contains both
phenolate and terephthalate bridges. These tetranuclear
units are connected together by the terephthalate units in a
bis(monodentate) coordination mode to form an infinite
1D network. Variable-temperature magnetic measurements
show that this complex exhibits moderate antiferromagnetic
coupling. Complex 2 has the same structure as complex 1,
although we were unable to obtain crystals suitable for X-
ray diffraction. Both complexes 3 and 4 form 2D polymeric ing of the NiII complexes, 1.0 g of the calcined solid was suspended
in 50 mL of dmf containing 0.2 g of complex. The mixture was
stirred for 4 h at room temperature, then filtered off and washed
several times with dmf and dried in vacuo at ambient temperature.
The loading (immobilization) of the Ni complexes on the mesopo-
rous host was confirmed by UV/Vis solid state spectroscopy.
networks utilizing the bridging argentocyanide units and
strong noncovalent interactions.
Alkene epoxidation studies have been performed with all
the complexes as active heterogeneous catalysts. Styrene, α-
methylstyrene, and cyclohexene are all efficiently converted
into their corresponding epoxides. The experimental results
show that immobilized Ni complex 4 is the most active cat-
alyst for the epoxidation of all three substrates (TON values
of 205, 167, and 350 for styrene, α-methylstyrene, and cyclo-
hexene, respectively).
Bis(3-aminopropyl-5-bromosalicylideneiminato)nickel(II) [Ni(L1)2]:
This complex was prepared following the method of Elder.[36] Thus,
a solution of 1,3-diaminopropane (8.88 g, 120 mmol) in ethanol
was heated at 60 °C and a solution of 5-bromosalicylaldehyde
(4.02 g, 20 mmol) in ethanol added dropwise with stirring for
30 min. After the addition was complete, solid Ni(CH3COO)2·
4H2O (2.49 g, 10 mmol) was added at once. The solid dissolved
rapidly to give a dark solution, which was refluxed for 20 min at
60 °C. The green precipitate that formed was filtered off, washed
with a small amount of ethanol and diethyl ether, and dried in air.
Experimental Section
Starting Materials: Salicylaldehyde, 5-bromosalicylaldehyde, 1,3-di-
aminopropane, 1,3-diamino-2,2-dimethylpropane, terephthalic
acid, and NiII salts were purchased form commercial sources and
used as received. Sodium terephthalate was readily prepared by the
reaction of terephthalic acid with a stoichiometric amount of
NaOH in water. All other chemicals (sodium hydroxide, AgNO3,
and NaCN) were of reagent grade. Ethanol, methanol, and dmf
were of reagent grade and used without further purification. Dou-
bly distilled water was used throughout. The acetonitrile used in the
epoxidation reaction was of HPLC grade. tert-Butyl hydroperoxide
purchased form Aldrich was used as oxidant. The substrates sty-
rene, α-methylstyrene, and cyclohexene were also obtained from
Aldrich.
Bis(3-aminopropylsalicylideneiminato)nickel(II) [Ni(L2)2]: This com-
plex was also prepared by the method described by Elder.[36]
Bis(3-aminopropyl-5-bromo-2,2-dimethylsalicylideneiminato)nickel-
(II) [Ni(L3)2]: This complex was prepared in a similar manner to
Ni(L1)2. Thus, a solution of 1,3-diamino-2,2-dimethylpropane
(6.13 g, 60 mmol) in ethanol was heated at 60 °C and a solution
of 5-bromosalicylaldehyde (2.01 g, 10 mmol) in ethanol was added
dropwise with stirring for 30 min. The deep yellow precipitate ob-
tained dissolved after the addition of solid Ni(CH3COO)2·4H2O
(1.25 g, 5 mmol) at once. The solid dissolved rapidly to give a dark
brown solution, which was refluxed for 30 min at 60 °C. The green
precipitate obtained was filtered off, washed with a small amount
of ethanol and diethyl ether, and dried in air.
Physical Measurements: Microanalyses (CHN) were performed
with a Perkin–Elmer 240C elemental analyzer. FT-IR spectra were
obtained on a Nicolet, MAGNA-IR 750 spectrometer with samples
prepared as KBr pellets. A Perkin–Elmer 2380 AAS was used for
wet chemical analysis. GC analysis was carried out with an Agilent
Technologies 6890N network GC system equipped with a fused
silica capillary column (30 mϫ0.32 mm) and a FID detector. The
column temperature was increased from 80 °C (4 min) to 220 °C at
a rate of 5 °Cmin–1. The injection temperature was 200 °C. The
solid-state electronic spectra were measured with a Shimadzu UV
2401PC spectrophotometer using a BaSO4 pellet as background
standard. Magnetic measurements were performed with a Cryo-
genic SQUID S600 magnetometer operating between 2 and 300 K
with applied magnetic fields of up to 65 kOe. Raw data were cor-
rected for diamagnetic contribution of the sample holder and the
intrinsic diamagnetism of the sample estimated by Pascal’s con-
stants.
[Ni4L14(µ-tp-κ4-O)(H2O)2(µ-tp-κ2-O)]·2C2H5OH·CH3OH·3H2O
(1): A solution of [Ni(L1)2] (0.057 g, 0.1 mmol) in 10 mL of hot
ethanol was added to a methanolic solution (5 mL) of Ni-
(ClO4)2·6H2O (0.036 g, 0.1 mmol) with constant stirring. The color
of the solution turned deep green. An aqueous solution of Na2tp
(0.021 g, 0.1 mmol) was then added to this mixture. The resulting
solution was stirred for 30 min, then filtered. Slow evaporation at
room temperature yielded green crystals of 1 suitable for X-ray
analysis after 4 d. Yield: 0.040 g (45%). C61H82Br4N8Ni4O20
(1800.8): calcd. C 40.65, H 4.55, N 6.22; found C 39.40, H 4.70, N
6.40.
[Ni4L2 (µ-tp-κ4-O)(H2O)2(µ-tp-κ2-O)]·3H2O (2):
A solution of
4
[Ni(L2)2] (0.083 g, 0.2 mmol) in 10 mL of hot ethanol was added
to a methanolic solution (5 mL) of Ni(ClO4)2·6H2O (0.073 g,
0.2 mmol) with constant stirring. The color of the solution turned
General Procedure for the Preparation of the Immobilized Catalysts: deep green. An aqueous solution of Na2tp (0.042 g, 0.2 mmol) was
For the synthesis of the mesoporous silica,[35] the cationic surfac-
tant cetyl trimethylammonium bromide (2.96 g) was mixed with an
aqueous solution containing sodium dodecyl sulfate (0.47 g) and
vigorously stirred at room temperature for 30 min. Gel formation
occurred immediately leading to a viscous suspension. Tetraethyl
orthosilicate (3.5 g) was added to this suspension with continuous
then added to this mixture. The resulting solution was stirred for
30 min, then filtered. This solution yielded dark green crystals of 2
on standing at room temperature for 3 d. Unfortunately, it was not
possible to grow single crystals of X-ray diffraction quality. Yield:
0.085 g (63%). C56H70N8Ni4O17 (1360.8): calcd. C 49.38, H 5.14,
N 8.23; found C 49.29, H 5.06, N 8.17.
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Nickel Complexes with N2O Donor Ligands
Table 7. Crystal data and details for structure refinement for 1, 3, and 4.
1
3
4
Formula
C122H160Br8N16Ni8O40
3599.62
C12H14AgBrN4NiO2
492.76
C15H19AgBrN4NiO2
533.83
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
monoclinic
P21/n (no. 14)
12.073(2)
18.601(4)
16.998(3)
90/99.70(3)/90
3762.7(13)
1
monoclinic
P21/c (no. 14)
10.204(2)
21.706(4)
8.0562(16)
90/110.33(3)/90
1673.2(6)
4
triclinic
¯
P1 (no. 2)
8.2964(3)
10.4145(5)
11.3805(5)
96.918(4)/102.963(4)/96.348(4)
941.60(7)
2
1.883
α/β/γ [°]
V [Å3]
Z
Dcalcd [gcm–3]
1.553
3.180
1.956
4.691
µ [mm–1]
4.175
F(000)
Temperature [K]
λ [Å]
θmin–θmax [°]
Reflections collected
Independent reflections
Observed data [I Ͼ 2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
1764
294
0.71073 (Mo-Kα)
2.2–25.0
6266
6093
3140
6093/0/429
0.889
R1 = 0.0828
wR2 = 0.1480
R1 = 0.2370
wR2 = 0.1872
960
294
526
108
0.71073 (Mo-Kα)
2.1–30.0
5787
4857
2516
4857/2/196
0.922
R1 = 0.0566
wR2 = 0.1145
R1 = 0.1145
wR2 = 0.1324
0.71073 (Mo-Kα)
3.4–27.6
9565
4287
3373
4287/0/220
0.927
R1 = 0.0286
wR2 = 0.0635
R1 = 0.0392
wR2 = 0.0656
R indices (all data)
[Ni(L1)(H2O){Ag(CN)2}]α (3): A methanolic solution (5 mL) of
Ni(ClO4)2·6H2O (0.036 g, 0.1 mmol) was added to a methanolic 5214 reflections, respectively, and refined by least-square methods.
and 4 were determined from automatic centering of 14, 15, and
solution (10 mL) of [Ni(L1)2] (0.057 g, 0.1 mmol) and the resulting
green solution stirred for 10 min. An aqueous solution (5 mL) of
Na[Ag(CN)2] (0.018 g, 0.1 mmol) was then added to this mixture
and the solution stirred for 20 min. The solution was filtered to
6266 Reflections were collected for 1 in the range 2.2° Ͻ θ Ͻ 25.0°,
5787 reflections were collected in the range 2.1° Ͻ θ Ͻ 30.0° for 3,
and 7224 reflections were collected in the range 3.4° Ͻ θ Ͻ 27.6°
for 4. A total of 3140, 2516, and 3373 reflections were assumed as
remove any suspended material and kept at room temperature. observed [I Ͼ 2σ(I)] for complexes 1, 3, and 4 respectively. All
Slow evaporation of solvents yielded deep green crystals suitable calculations for data reduction, structure solution and refinement
for X-ray analysis after 7 d. Yield: 0.018 g (73%). were performed using standard procedures with SHELXS-97[37]
C12H14AgBrN4NiO2 (492.76): calcd. C 29.22, H 2.84, N 11.36;
found C 29.16, H 2.79, N 11.23.
and SHELXL-97.[38] All non-hydrogen atoms were refined aniso-
tropically by full-matrix least-squares techniques on F2. The H
atoms were constrained to idealized positions and refined iso-
tropically using a riding model.
[Ni(L3)(MeOH){Ag(CN)2}]α (4): A methanolic solution (5 mL) of
Ni(ClO4)2·6H2O (0.036 g, 0.1 mmol) was added to a methanolic
solution (10 mL) of [Ni(L3)2] (0.057 g, 0.1 mmol) and the resulting
green solution stirred for 10 min. A dmf solution (5 mL) of
Na[Ag(CN)2] (0.018 g, 0.1 mmol) was then added to this mixture
and the solution stirred for 20 min. It was then filtered and kept at
room temperature. Slow evaporation of the solvents yielded green
crystals suitable for X-ray analysis after 5 d. Yield: 0.021 g (40%).
C15H20AgBrN4NiO2 (534.83): calcd. C 33.66, H 3.74, N 10.47;
found C 33.59, H 3.70, N 10.41.
CCDC- 645841 (for 1), -645842 (for 3), and -645843 (for 4) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
M. N. gratefully acknowledges financial support from the Council
of Scientific and Industrial Research, New Delhi for a senior re-
search fellowship. AB wishes to thank the Department of Science
and Technology, Government of India, New Delhi for a Ramanna
Fellowship grant.
Crystal-Structure Determination: Single crystals of complex 1 were
grown from EtOH/MeOH/H2O and those of complexes 3 and 4
were grown from MeOH/H2O and dmf/MeOH medium, respec-
tively. The X-ray single-crystal data for complexes 1 and 3 were
collected with a Siemens P4 four-circle diffractometer whereas
those of complex 4 were collected on an Oxford Instruments Sap-
phire2-CCD diffractometer. Crystal size: 0.18ϫ0.21ϫ0.51,
0.30ϫ0.31ϫ0.52, and 0.08ϫ0.17ϫ0.22 mm3 for complexes 1, 3,
and 4 respectively. Unit cell dimensions and intensity data were
measured at 294 K for 1 and 3 and 108 K for 4. The crystallo-
graphic data, conditions used for the intensity data collection and
some features of the structure refinement are listed in Table 7.
Graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) was
used to collect the data sets. Accurate unit-cell parameters for 1, 3,
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