A combined experimental and theoretical investigation on the role of halide ligands on the catecholase-like activity of mononuclear nickel(II) complexes with a phenol-based tridentate ligand

Article abstract of DOI:10.1021/ic401819t

Three new mononuclear nickel(II) complexes, namely, [NiL¹(H ₂O)₃]I₂·H₂O (1), [NiL ¹(H₂O)₃]Br₂·H₂O (2), and [NiL¹(H₂O)₃

Full text of DOI:10.1021/ic401819t

Article
pubs.acs.org/IC
A Combined Experimental and Theoretical Investigation on the Role
of Halide Ligands on the Catecholase-like Activity of Mononuclear
Nickel(II) Complexes with a Phenol-Based Tridentate Ligand
†
†
†
‡
§
Jaydeep Adhikary, Prateeti Chakraborty, Sudhanshu Das, Tanmay Chattopadhyay, Antonio Bauza,
́
⊥
⊥
,∥
,§
Shyamal Kumar Chattopadhyay, Bipinbihari Ghosh, Franz A. Mautner,* Antonio Frontera,*
,
†
and Debasis Das*
†
Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India
Department of Chemistry, Panchakot Mahavidyalaya, Sarbari, Purulia, Pin 723121, India
‡
§
Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma (Baleares), Spain
⊥
Department of Chemistry, Bengal Engineering and Science University, Howrah 711 103, India
∥
Institut fuer Physikalische und Theoretische Chemie, Technische Universitaet Graz, 8010 Graz, Austria
*
S Supporting Information
ABSTRACT: Three new mononuclear nickel(II) complexes,
1
1
namely, [NiL (H O) ]I ·H O (1), [NiL (H O) ]Br ·H O (2),
2
3
2
2
2
3
2
2
1
1
and [NiL (H O) ]Cl ·2H O (3) [HL = 2-[(2-piperazin-1-
2
3
2
2
ylethylimino)methyl]phenol], have been synthesized and
structurally characterized. Structural characterization reveals
1
2+
that they possess similar structure: [NiL (H O) ] complex
2
3
cations, two halide counteranions, and lattice water molecules.
One of the nitrogen atoms of the piperazine moiety is
protonated to provide electrical neutrality to the system, a
consequence observed in earlier studies (Inorg. Chem. 2010,
4
9, 3121; Polyhedron 2013, 52, 669). Catecholase-like activity
has been investigated in methanol by a UV−vis spectrophoto-
metric study using 3,5-di-tert-butylcatechol (3,5-DTBC) as the model substrate. Complexes 1 and 2 are highly active, but
surprisingly 3 is totally inactive. The coordination chemistries of 1 and 2 remain unchanged in solution, whereas 3 behaves as a
1:1 electrolyte, as is evident from the conductivity study. Because of coordination of the chloride ligand to the metal in solution,
it is proposed that 3,5-DTBC is not able to effectively approach an electrically neutral metal, and consequently complex 3 in
solution does not show catecholase-like activity. Density functional theory (DFT) calculations corroborate well with the
experimental observations and thus, in turn, support the proposed hypothesis of inactivity of 3. The cyclic voltametric study as
well as DFT calculations suggests the possibility of a ligand-centered reduction at −1.1 V vs Ag/AgCl electrode. An electron
paramagnetic resonance (EPR) experiment unambiguously hints at the generation of a radical from EPR-inactive 1 and 2 in the
presence of 3,5-DTBC. Generation of H O during catalysis has also been confirmed. DFT calculations support the ligand-
2
2
centered radical generation, and thus a radical mechanism has been proposed for the catecholase-like activity exhibited by 1 and
. Upon heating, 2 and 3 lose water molecules in two steps (first lattice waters, followed by coordinating water molecules),
2
whereas 3 loses four water molecules in a single step, as revealed from thermogravimetric analysis. The totally dehydrated species
are red, in all cases having square-planar geometry, and have amorphous nature, as is evident from a variable-temperature powder
X-ray diffraction study.
1
. INTRODUCTION
ions, mainly as small synthetic analogues of metallobiosites like
catechol oxidase, phosphatase, and cyt P-450 and as possible
Nickel(II) has very rich coordination chemistry owing to its
inherent ability to adopt various geometries that are often
interconvertible, and it is very well documented that such
configurational or conformational changes are generally
7
−15
mimics of nucleases.
During our study to develop synthetic
analogues of catechol oxidase, we synthesized a small dinuclear
nickel(II) complex of 2,6-bis(N-ethylpiperazinyliminomethyl)-
1
−6
4
-methylphenolate, and to our surprise, we observed an
associated with color changes.
Interestingly, how a change
extraordinary catalytic activity of the complex to oxidize not
in the coordination chemistry of nickel(II) may influence their
catalytic property has not yet been properly addressed in the
literature. Our group is continuously engaged in studying
Schiff-base complexes of transition- and post-transition-metal
Received: July 15, 2013
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
only 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlor-
ocatechol, a substrate that is very difficult to oxidize. Critical
°C using a Perkin-Elmer RXI FT-IR spectrophotometer with KBr
pellets. Electronic spectra (1400−200 nm) were obtained at 27 °C
using a Shimadzu UV-3101PC spectrometer with methanol as the
solvent and reference. Thermal analyses (TG−DTA) were carried out
on a Mettler Toledo (TGA/SDTA851) thermal analyzer in flowing
dinitrogen (flow rate: 30 cm min ). Ambient-temperature magnetic
susceptibility measurements were performed with Magway MSB Mk1
magnetic susceptibility balance. Conductance of the methanolic
solution of complexes was measured using a SYSTRONICS 306
conductivity meter. EPR experiments were performed at liquid-
nitrogen temperature (77 K) in methanol, using a JEOL JES-FA200
spectrometer at X band (9.13 GHz). Cyclic and differential pulse
voltammograms were recorded in MeCN solutions containing 0.1 M
TEAP as the supporting electrolyte, using a CH1120A potentiostat
with glassy carbon as the working electrode, platinum wire as the
counter electrode, and Ag/AgCl/saturated KCl as the reference
electrode. The ferrocene/ferrocenium couple was observed at E°
8
analysis revealed that the extra positive charge on the ligand
backbone might be instrumental for such extraordinary activity.
That unprecedented finding inspired us to study the catalytic
property of analogous mononuclear nickel(II) complexes. In
that study again we noticed that complexes with ligand having
extra positive charge (e.g., 2-[(2-piperazin-1-ylethylimino)-
3
1
1
methyl]phenol; HL ) after complexation exhibited catalytic
15
activity in oxidizing 3,5-DTBC. Incidentally, in all those cases,
the counteranion was nitrate. So, we were not in a position to
explore the role of the counteranion in manipulating the
coordination chemistry of nickel(II) species and, consequently,
their catalytic efficiency, particularly to catalyze the aerobic
oxidation of 3,5-DTBC. In order to get faithful answers to all of
those queries, as our first attempt, we synthesized (Scheme 1)
(
ΔE ) = 0.48 V (100 mV) under our experimental conditions. Powder
p
X-ray diffraction (PXRD) was performed on a XPERT-PRO
Scheme 1. Synthetic Route of Complexes 1−3
diffractometer with monochromated Cu Kα radiation (40.0 kV and
3
0.0 mA) at room temperature. Nickel was estimated gravimetrically
with dimethylglyoxime. All chemicals were obtained from commercial
sources and used as received. Solvents were dried according to
standard procedure and distilled prior to use. Salicylaldehyde, N-(2-
aminoethyl)piperazine, nickel(II) chloride hexahydrate, nickel(II)
bromide hydrate, nickel(II) iodide, and 3,5-di-tert-butylcatechol (3,5-
DTBC) were purchased from Aldrich.
1
Synthesis of Complex [NiL (H O) ]I ·H O (1). A methanolic
2
3
2
2
solution (5 mL) of N-(2-aminoethyl)pipeazine (0.258 g, 2 mmol) was
added dropwise to a hot methanolic solution (10 mL) of
salicylaldehyde (0.244g, 2 mmol), and the resulting solution was
refluxed for 0.5 h. Then, a methanolic solution (5 mL) of NiI (0.625g,
2
2
mmol) was added, and the resulting solution was stirred for 3−4 h.
The green solution was filtered and kept in a CaCl desiccator in the
2
dark, and after a few days, crystals of complex 1, suitable for X-ray data
analysis, were obtained. Yield: 73%. Anal. Calcd for C H N O I Ni:
13
27
3
5 2
C, 25.27; H, 4.4; N, 6.8; Ni, 9.46. Found: C, 25.21; H, 4.49; N, 6.72;
Ni, 9.42. UV−vis−near-IR (methanol, nm): λ = 381, 629, 753, 924.
max
1
Synthesis of Complex [NiL (H O) ]Br ·H O (2). Complex 2 has
2
3
2
2
been prepared following the same procedure as that for 1, where
NiBr ·xH O (0.437g, 2 mmol) was used in place of NiI . The resulting
2
2
2
1
green solution was filtered and kept in a CaCl
2
desiccator. A few days
mononuclear nickel halide complexes, namely, [NiL (H O) ]I ·
2
3
2
later green single crystals were obtained that were suitable for X-ray
data collection. Yield: 70%. Anal. Calcd for C H N O Br Ni: C, 29.8;
1
1
H O (1), [NiL (H O) ]Br ·H O (2), and [NiL (H O) ]Cl ·
H O (3). All three complexes have been structurally
2
2
3
2
2
2
3
2
13
27
3
5
2
2
2
H, 5.19; N, 8.02; Ni, 11.16. Found: C, 29.72; H, 5.24; N, 7.98; Ni,
characterized. Comprehensive investigations of their solid-
state thermal properties, coordination behavior in solution, and
catalytic activity to oxidize 3,5-DTBC have been performed to
find out whether counteranions also have some role in
influencing the catalytic activity of the complex. In spite of
the very similar structural features of all three complexes in the
solid state, complex 3 shows amazingly different coordination
behavior in solution. Surprisingly, complex 3 shows inactivity in
catalyzing aerobic oxidation of 3,5-DTBC, whereas the other
two exhibit excellent catecholase-like activity. A conductance
study followed by detailed density functional theory (DFT)
calculations has been carried out to rationalize the unprece-
dented observation. Cyclic voltammetric (CV), differential
pulse voltammetric (DPV), and electron paramagnetic
resonance (EPR) studies have been performed to find out
the origin of the catecholase-like activity exhibited by 1 and 2,
and DFT calculations further have been done to propose the
possible mechanistic pathway involved in that activity.
1
1.08. UV−vis−near-IR (methanol, nm): λ = 381, 627, 751, 924.
max
1
Synthesis of Complex [NiL (H O) ]Cl ·2H O (3). Complex 3
2
3
2
2
was prepared by adopting the same procedure as that for 1 using
NiCl ·6H O (0.475g, 2 mmol) in place of NiI . X-ray-suitable green
crystals of complex 3 were obtained from the resulting solution after 1
2
2
2
week. Yield: 73%. Anal. Calcd for C H N O Cl Ni: C, 34.47; H,
13
29
3
6
2
6
.45; N, 9.28, Ni, 12.91. Found: C, 34.41; H, 6.53; N, 9.22; Ni, 12.83.
UV−vis−near-IR (methanol, nm): λ = 379, 646, 761, 971.
max
Detection of Hydrogen Peroxide in the Catalytic Reactions.
The formation of H O during the catalytic reaction was detected by
following the development of the characteristic band for I
spectrophotometrically (λ
upon reaction with I .
presence of different catalysts were carried out as in the kinetic
experiments ([complex] = 2.5 × 10 M; [3,5-DTBC] = 50 × 10
M). After 1 h of reaction, an equal volume of water was added, and the
quinone formed was extracted three times with dichloromethane. The
aqueous layer was acidified with H SO to pH 2 to stop further
2
2
−
3
= 353 nm; ε = 26000 M− cm ),
The oxidation reactions of 3,5-DTBC in the
1
−1
max
−
16,17
−5
−5
2
4
oxidation, and 1 mL of a 10% solution of KI and 3 drops of a 3%
solution of ammonium molybdate were added. In the presence of
hydrogen peroxide occurs the reaction H O + 2I + 2H → 2H O +
−
+
2
2
2
2
. EXPERIMENTAL SECTION
I , and with an excess of iodide ions, the triiodide ion is formed
2
−
−
Physical Methods and Materials. Elemental analyses (carbon,
according to the reaction I (aq) + I → I . The reaction rate is slow
2 3
hydrogen, and nitrogen) were performed using a Perkin-Elmer 240C
elemental analyzer. IR spectra (4000−500 cm ) were recorded at 27
but increases with increasing concentrations of acid, and the addition
of an ammonium molybdate solution renders the reaction almost
−1
B
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Inorganic Chemistry
Article
Table 1. Crystallographic Data and Processing Parameters of Complexes 1−3
1
2
3
empirical formula
fw
C H I N NiO
C H Br N NiO
C H Cl N NiO
6
1
3
27 2
3
5
13 27
2
3
5
13 29
2
3
617.87
523.87
452.98
system
monoclinic
monoclinic
monoclinic
space group
a (Å)
P2 /n
P2 /c
P2 /c
1
1
1
9.5132(9)
12.5144(13)
17.8254(18)
90
9.4906(9)
9.5733(9)
b (Å)
15.7080(14)
13.7970(13)
90
15.4155(14)
13.9557(12)
90
c (Å)
α (deg)
β (deg)
101.734(16)
90
109.536(14)
90
109.816(14)
90
γ (deg)
V (Å3)
2077.8(4)
4
1938.4(4)
4
1937.6(3)
4
Z
T (K)
100(2)
100(2)
100(2)
μ (mm−1)
Dcalc (Mg m−3)
data collected
unique reflns/Rint
param/restraints
GOF on F2
R1/wR2 (all data)
residual extrema (e Å−3)
3.931
5.150
1.310
1.975
1.795
1.553
16193
14784
15136
4236/0.0313
247/10
3889/0.0275
247/10
1.034
3940/0.0253
262/12
1.079
1.186
0.0362/0.0781
1.10/−0.63
0.0239/0.0605
0.63/−0.42
0.0261/0.0679
0.37/−0.35
Figure 1. UV−vis spectra of (a) a 10− M solution and (b) a 10−3 M solution of complexes 1−3 in a methanol medium.
2
instantaneous. The formation of I3− could be monitored spectrophoto-
was prepared and a 0.1 M KOH solution was used as the titrant. The
ionic strength was maintained with a 0.1 M KCl solution. The
temperature was measured before and after each experiment and
remained constant at a value of 298 ± 0.5K. Absorbances at different
pH values were recorded using a Shimadzu UV-3101PC spectropho-
−
metrically because of the development of the characteristic I band
3
−1
−1 18
(
λmax = 353 nm; ε = 26000 M cm ).
X-ray Crystal Structure Analysis. The X-ray single-crystal data of
complexes 1−3 were collected on a Bruker-AXS SMART CCD
diffractometer. The crystallographic data, conditions retained for the
intensity data collection, and some features of the structure
refinements are listed in Table 1. The intensities were collected with
Mo Kα radiation (λ = 0.71073 Å). Data processing, Lorentz
polarization, and absorption corrections were performed using the
tometer. pK values were calculated following the literature
a
2
3
procedure.
Theoretical Methods. The geometries and energies of all
complexes included in this study were computed at the BP86-D3/
def2-TZVPD level of theory within the program TURBOMOLE,
19
2
4
SAINT, SMART, and SADABS computer programs. The structures
version 6.4. The solvent effects were taken into account by using the
25
were solved by direct methods and refined by full-matrix least-squares
COSMO (Conductor like Screening Model) solvation model. For
the calculations, we used the BP86 functional with the latest available
correction for dispersion (D3). The orbital and spin density plots were
performed at the B3LYP/6-31+G* level of theory by means of the
2
20
methods on F , using the SHELXTL program package. All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms
were located from difference Fourier maps, assigned with isotropic
displacement factors, and included in the final refinement cycles by use
of either geometrical constraints (HFIX for hydrogen atoms with
parent carbon atoms) or restraints (DFIX for hydrogen atoms with
parent nitrogen or oxygen atoms). Molecular plots were performed
26
Gaussian 09 package.
3
. RESULTS AND DISCUSSION
21
22
with the Mercury program, and the PLATON program package was
used for hydrogen-bonding analysis.
Syntheses and FT-IR and UV−Vis Spectra of the
1
Complexes. The Schiff-base ligand (L ) is prepared through
the classical method where salicylaldehyde and N-(2-
aminoethyl)piperazine is refluxed in a methanol medium for
0.5 h. This ligand is further treated with nickel(II) chloride/
bromide/iodide in situ separately to prepare complexes 1−3,
Potentiometric Titration. Potentiometric studies were carried out
with a SYSTRONICS μ pH SYSTEM 361 pH meter fitted with blue-
+
glass and Ag/AgCl reference electrodes, calibrated to read −log [H ]
directly, designated as the pH in methanol/water (90:10, v/v). In a
−4
typical titration experiment, 10 mL of a 1 × 10 M complex solution
C
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Inorganic Chemistry
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 1−3
Complex 1
Ni1−N1
Ni1−O1
Ni1−O3
N1−C7
1.994(4)
2.030(3)
2.076(3)
1.273(6)
Ni1−N2
Ni1−O2
Ni1−O4
O1−C1
2.249(4)
2.099(3)
2.078(3)
1.333(5)
N1−Ni1−O1
N1−Ni1−O3
O1−Ni1−O3
N1−Ni1−O4
O1−Ni1−O4
O3−Ni1−O4
N1−Ni1−O2
O1−Ni1−O2
90.43(14)
90.03(14)
89.82(13)
176.08(14)
90.08(12)
86.08(12)
99.05(14)
85.56(12)
O3−Ni1−O2
O4−Ni1−O2
N1−Ni1−N2
O1−Ni1−N2
O3−Ni1−N2
O4−Ni1−N2
O2−Ni1−N2
169.83(12)
84.87(12)
82.30(14)
171.26(13)
95.02(14)
97.50(13)
90.77(13)
Figure 2. Perspective view (50% probability ellipsoids) of complex 1
with the atom numbering scheme.
Complex 2
Ni1−N1
Ni1−O1
Ni1−O3
N1−C7
2.0072(19)
2.0307(16)
2.0700(16)
1.276(3)
Ni1−N2
Ni1−O2
Ni1−O4
O1−C1
2.1998(19)
2.0621(16)
2.0723(16)
1.320(3)
N1−Ni1−O1
N1−Ni1−O2
O1−Ni1−O2
N1−Ni1−O3
O1−Ni1−O3
O2−Ni1−O3
N1−Ni1−O4
O1−Ni1−O4
90.04(7)
174.87(7)
94.64(6)
98.05(7)
88.10(6)
84.23(6)
91.49(7)
90.24(7)
O2−Ni1−O4
O3−Ni1−O4
N1−Ni1−N2
O1−Ni1−N2
O2−Ni1−N2
O3−Ni1−N2
O4−Ni1−N2
86.40(7)
170.31(6)
82.94(7)
172.98(7)
92.38(7)
92.94(7)
89.87(7)
Figure 3. Perspective view (50% probability ellipsoids) of complex 2
with the atom numbering scheme.
Complex 3
Ni1−N1
Ni1−O1
Ni1−O3
N1−C7
1.9988(14)
2.0491(11)
2.0837(12)
1.278(2)
Ni1−N2
Ni1−O2
Ni1−O4
O1−C1
2.2186(13)
2.0635(12)
2.0993(12)
1.3322(19)
N1−Ni1−O1
N1−Ni1−O2
O1−Ni1−O2
N1−Ni1−O3
O1−Ni1−O3
O2−Ni1−O3
N1−Ni1−O4
O1−Ni1−O4
89.12(5)
175.04(5)
95.81(5)
89.96(5)
88.71(5)
89.54(5)
96.64(5)
88.13(5)
O2−Ni1−O4
O3−Ni1−O4
N1−Ni1−N2
O1−Ni1−N2
O2−Ni1−N2
O3−Ni1−N2
O4−Ni1−N2
84.16(5)
172.64(5)
82.45(5)
171.57(5)
92.62(5)
91.21(5)
92.89(5)
Figure 4. Perspective view (50% probability ellipsoids) of complex 3
with the atom numbering scheme.
8
for octahedral d ions. In the case of complex 3, all three bands
are red-shifted with slightly higher ε values. Complexes 1−3
also exhibit two highly intense bands at the regions ∼310 nm
respectively. All three complexes are characterized by routine
physicochemical techniques as well as by X-ray single-crystal
structure analyses. All of the complexes show IR bands due to
(ε/dm3 mol
−1
cm ; 1417, 1842, and 2589 for 1−3,
−1
−1
3
−1
−1
CN stretching in the range 1640−1650 cm and skeleton
respectively) and ∼380 nm (ε/dm mol cm = 1384,
1762, and 2372 for 1−3, respectively) most likely due to ligand-
to-metal charge transfer. Solid-state electronic spectra of
complexes 1−3 are very similar compared to their respective
electronic spectrum in methanol, suggesting retention of the
octahedral geometry in the solid state (Supporting Information,
Figure S4).
−1
vibrations in the range 1590−1600 cm . A broad band in the
−1
range 3300−3500 cm is due to a hydrogen-bonded O−H
group of a coordinated water molecule (Supporting Informa-
tion, Figures S1−S3). Magnetic susceptibility measurements
(
μ
= 3.2 μ at 298 K) suggest that nickel(II) possesses
eff B
octahedral configuration in all three complexes. Electronic
spectra (Figure 1) recorded in methanol also reveal that the
coordination environment around nickel(II) in each case is
Description of the Crystal Structures of 1−3.
Perspective views together with the atom numbering schemes
for complexes 1−3 are presented in Figure 2−4, respectively,
and selected bond parameters are given in Table 2. The
2
7,28
roughly octahedral.
Complexes 1 and 2 display three weak
3
−1
absorption bands at ∼625, ∼750, and ∼925 nm (ε/dm mol
−1
3
3
3
2+
cm , 3−10) assigned to spin-allowed A → T (P), A →
structures consists of mononuclear [Ni(C H N O)(H O) ]
2
g
1g
2g
13 19
3
2
3
3
3
3
T (F), and A → T (F) transitions, respectively, expected
complex cations, two halide counteranions, and lattice water
1
g
2g
2g
D
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Inorganic Chemistry
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Table 3. Hydrogen-Bonding System for Compounds 1−3
D−H···A
a
D−H···A
symmetry of A
D···A (Å)
(deg)
Complex 1
N3−H90···O5 [−x + 1, y, z]
2.720(5)
143(4)
1
1
1
N3−H91···I1
O2−H92···I1
[−x + / , y + / , z − / ] 3.557(4)
163(4)
163(4)
174(5)
166(4)
2
2
2
1
3
1
[x − / , −y + / , z − / ] 3.521(3)
2
2
2
O2−H93···O1 [−x, −y + 2, −z]
2.685(4)
1
1
O3−H94···I2
[−x + / , y + / , −z +
2 2
1
3.443(3)
/ ]
2
O3−H95···I1
O4−H96···I2
O4−H97···O1 [−x, −y + 2, −z]
O5−H98···I1
3.438(3)
169(3)
168(3)
156(4)
147(4)
175(4)
1
1
1
[−x+ / , y + / , −z + / ] 3.542(3)
Figure 6. UV−vis spectra of (i) complex 2 and (ii) 3,5-DTBC and (iii)
changes in the UV−vis spectra of complex 2 upon the addition of 3,5-
DTBC observed after each 5 min interval.
2
2
2
2.696(4)
3.509(4)
3.524(4)
3
1
O5−H99···I2
[−x + / , y + / , −z +
2 2
1
/
2
]
Complex 2
1
1
N3−H90···Br1 [−x, y − / , −z + / ]
3.175(2)
167(2)
166(2)
2
2
1
1
N3−H91···Br2 [−x + 1, y − / , −z + / ] 3.312(2)
2
2
O2−H92···O1 [−x + 1, −y, −z + 1]
2.754(2)
151.6(19)
O2−H93···Br1 [−x, −y, −z + 1]
O3−H94···O1 [−x + 1, −y, −z + 1]
O3−H95···O5
O4−H96···Br1
O4−H97···Br2 [x − 1, y, z]
O5−H98···Br2
3.4685(18)
2.751(2)
156(2)
168(3)
173(2)
176(2)
172(2)
160(3)
161(2)
2.722(3)
3.2871(18)
3.2639(18)
3.2812(19)
3.3511(19)
O5−H99···Br1 [−x + 1, −y, −z + 1]
Figure 7. UV−vis spectra of (i) complex 3 and (ii) 3,5-DTBC and (iii)
changes in the UV−vis spectra of complex 3 upon the addition of 3,5-
DTBC observed after each 5 min interval.
Complex 3
1
1
N3−H90···Cl2 [−x, y − / , −z + / ]
3.2551(16)
2.709(2)
155.9(17)
2
2
N3−H91···O6 [−x, −y, −z]
149.9(16)
161.5(18)
168.6(16)
O2−H92···Cl1 [−x + 1, −y, −z + 1]
O2−H93···O1 [−x + 1, −y, −z + 1]
O3−H94···Cl1 [x − 1, y, z]
O3−H95···Cl2
3.1576(14)
2.7069(17)
3.1260(14)
3.0918(13)
3.1823(14)
2.7016(16)
3.2295(19)
3.1961(18)
3.2166(15)
Table 4. Kinetic Parameters for Complexes 1 and 2
catalyst
Vmax (M s−1)
KM (M)
kcat (h−1
)
167(2)
2.5747 × 10−
2.3562 × 10−
6
1.1053 × 10−4
0.88159 × 10
9.2689 × 10
8.4825 × 10
1
170.8(18)
164.0(18)
164.7(19)
1
2
6
−4
1
O4−H96···Cl1
O4−H97···O1 [−x + 1, −y, −z + 1]
1
1
O5−H98···Cl2 [x + 1, −y + / , z + / ]
1592)
2
2
mer conformation. The Ni−N1, Ni−N2, Ni−O1, and Ni−O
1
1
aq
O5−H99···Cl1 [x, −y + / , z + / ]
168(3)
157(2)
2
2
bond distances vary in the ranges 1.994(4)−2.0072(19),
.1998(19)−2.249(4), 2.030(3)−2.0491(11), and
O6−H100···
2
Cl2
1
1
2.0621(16)−2.0993(12) Å, respectively. The mean N1−Ni−
O1, N1−Ni−N2, and O1−Ni−N2 bond angles are 89.86,
82.56, and 171.93°, respectively. The piperazine moieties adopt
chair conformation. The N3 atoms are not ligated to the
nickel(II) center but form hydrogen bonds to one halide anion
and one water molecule in the case of complexes 1 and 3 and to
two bromide anions in the case of complex 2 (Table 2). In
addition, the three aqua ligands and the lattice water molecules
form hydrogen bonds of the types O−H···X and O−H···O to
generate supramolecular network structures (Table 3 and
Supporting Information, Figures S5−S7).
O6−H101···
[−x + 1, y + / , −z + / ] 3.2145(15)
170(3)
2
2
Cl1
a
D = donor; A = acceptor.
Catechol Oxidase Activity. In order to confirm the ability
−4
of the nickel(II) complexes to oxidize 3,5-DTBC, 1 × 10 mol
−3
−2
dm methanolic solutions of 1−3 were treated with 1 × 10
−3
mol dm (100 equiv) of 3,5-DTBC under aerobic conditions.
The course of the reaction was followed by UV−vis
spectroscopy, and the time-dependent spectral scans of the
three complexes are depicted in Figures 5−7. From the figures,
it is evident that a band ∼390 nm is observed to increase with
time after the addition of 3,5-DTBC because of the gradual
increment of the concentration of 3,5-DTBQ (3,5-DTBQ
exhibits λmax ∼ 400 nm in methanol) in the cases of 1- and 2-
catalyzed reactions, whereas nearly no change is noticed in the
spectral pattern with complex 3. These data unambiguously
Figure 5. UV−vis spectra of (i) complex 1 and (ii) 3,5-DTBC and (iii)
changes in the UV−vis spectra of complex 1 upon the addition of 3,5-
DTBC observed after each 5 min interval.
molecules (one in the case of complexes 1 and 2 and two water
molecules in the case of 3). The nickel(II) centers are ligated
by N1, N2, and O1 donor atoms of the C H N O ligand and
1
3
19
3
three aqua ligands in a distorted octahedral environment with
E
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Table 5. Kinetic Data for Catecholase-like Activity of
Different Mono- and Dinuclear Copper and Nickel
Complexes
complexes have very similar structural features in the solid state.
It is now quite urgent to know whether the observed solid-state
structures remain intact in solution. In order to get an idea
regarding the solution structure of the complexes, we first of all
performed a conductivity study of them in methanol (vide
infra). The kinetics of the 3,5-DTBC oxidation was determined
by monitoring the increase of the concentration of the product
k
cat
(h
a
−1
catalyst
solvent
Mononuclear Ni(II) Compounds
1) [NiL (H O) ](NO ) methanol
)
ref (year)
2
(
(
(
52.60 15
2
3
3 2
3
,5- DTBQ. The kinetics for the oxidation of the substrate 3,5-
(
2012)
2012)
2011)
DTBC was determined by the initial rate method at 25 °C. The
concentration of the substrate 3,5-DTBC was always kept 10
times higher than that of the complex, and the increase of the
respective quinone concentration was determined at 390 nm
wavelength for each complex. Solutions of substrates of
Dinuclear Ni(II) Compounds
acetonitrile
1
2) [Ni (L ) (NCS) ]
64.1
29
2
2
2
(
Mononuclear Cu(II) Compounds
methanol
7
3) [Cu(L )Cl]Cl
11.16 30
(
−3
concentration ranging from 0.001 to 0.05 mol dm were
Dinuclear Cu(II) Compounds
prepared from a concentrated stock solution in methanol. A
total of 2 mL of the substrate solution was poured into a 1 cm
spectrophotometer quartz cell thermostatted at 25 °C. Then
0.04 mL of a 0.005 mol dm− complex solution was quickly
added to it so that the ultimate concentration of the complex
(
(
(
(
4) [Cu (H L)(μ-OH)](ClO )
4 2
methanol
methanol
methanol
28.74 31
2
2
(
2008)
5) [Cu L (ClO ) ]
93.6
55
32
2
2
4 2
3
(
2012)
H,H
6) [Cu (L −O)(OH)(MeCN)2]
33
(2012)
2
[
ClO4]2
−4
−3
became 1 × 10 mol dm . The dependence of the initial rate
on the concentration of the substrate was spectrophotometri-
cally monitored at the respective wavelength. Moreover, the
initial rate method showed a first-order dependence on the
complex concentration and exhibited saturation kinetics at
higher substrate concentrations. Because both complexes 1 and
7) Cu diep
water−
63
34
2
methanol
(2012)
1
a
2
HL (1) = 2-[(2-piperazin-1-yl-ethylimino)methyl]phenol; HL (2) =
-[1-(3-methylaminopropylamino)ethyl]phenol; HL7 (3) = 6-(bis-
2
pyrazol-1-ylmethylamino)hexan-1-ol; H L (4) = 2,6-bis[{{(2-
3
hydroxybenzyl)(N′,N′-(dimethylamino)ethyl)}amino}methyl]-4-
2
showed saturation kinetics, a treatment based on the
methylphenol; HL (5) = 2-[[2-(diethylamino)ethylamino]methyl]-
H,H
phenol; L
(6) = 1,3-bis[(N,N-dimethylaminoethyl)aminomethyl]-
Michaelis−Menten model seemed to be appropriate. The
benzene; diep (7) = 2,8-dimethyl-5,11-bis(pyridin-2-ethyl)-
,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-
binding constant (K ), maximum velocity (V ), and rate
M
max
1
constant for dissociation of the substrates (i.e., turnover
diazecine.
number, k ) were calculated for the complexes using the
cat
Lineweaver−Burk graph of 1/V versus 1/[S] (Supporting
demonstrate that 1 and 2 are active catalysts for the aerial
oxidation of 3,5-DTBC to 3,5-DTBQ, whereas 3 turns out to
be inactive. The inactivity of complex 3 is apparently quite
surprising because structure analyses reveal that all three
Information, Figures S8−S10), with the equation 1/V = (K /
M
V
)(1/[S]) + 1/V , and the kinetic parameters are
max
max
presented in Table 4. Table 5 represents the kcat values of
some mono- and dinuclear complexes of copper(II) and
Figure 8. EPR spectra of (a) complex 1, (b) a mixture of complex 1 and 3,5-DTBC, (c) complex 2, and (d) a mixture of complex 2 and 3,5-DTBC.
F
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Table 6. E° Values in CV and DPV for Complexes 1−3
complex
E°/Epeak values from CV
E° values from DPV
a
a
a
b
b
3
2
0.97, 1.25, 1.6, −1.10 (220), −1.53 (120)
0.89, 1.16, 1.3, 1.5, −1.14, −1.51, −2.0
0.8, 1.13, 1.38, −1.14, −1.56, −2.0
0.4, 0.76, 1.1, −1.23, −2.0
b
b
b
0.95, 1.19 (90), 1.45, −1.15 (160), −1.44 (250)
b
a
a
b
1
0.5, 0.75 (200), 1.05, −1.17 (200), −1.51 (200)
a
b
Irreversible. E°/V (ΔE /mV), where ΔE = E − Epc
p
p
pa
Figure 9. Bottom: DFT model and X-ray structure of compound 1. Top: optimized structures of [NiLX(OH ) ] complexes (X = Cl, Br, and I; left,
2
2
middle, and right, respectively).
Scheme 2. Reaction Used To Evaluate the Substitution
Ability of the Halides
Scheme 3. Proposed Catalytic Cycle of the Oxidation of 3,5-
+
Dimethylcatechol by the [NiL(OH ) ] Complex
2
3
nickel(II) reported recently. Upon comparison of Tables 4 and
5
, it may be stated that complexes 1 and 2 belong to the highly
efficient catalyst group, where the order of their activity is 1 > 2.
Here it is noted that the catalytic reaction performed under
an inert atmosphere does not show the occurrence of 3,5-
DTBQ. However, the formation of 3,5-DTBQ was immediately
detected upon exposure of the reaction mixture to a dioxygen
atmosphere. It is now essential to know whether dioxygen
reduces to water or H O during the oxidation process. The
Conductivity Study. To rationalize the composition of the
complexes in solution, a molar conductivity study of the 10−
3
M
methanolic solution of complexes 1−3 has been performed.
The conductance values at 298 K for complexes 1−3 are 175,
2
2
−
−
−1
2
−1
oxidation of I to I followed by the generation of I , as is
165, and 110 Ω cm M , respectively. Critical analysis of the
values suggests that complex 3 behaves as a 1:1 electrolyte,
whereas complexes 1 and 2 behave as a 2:1 electrolyte, as
expected from their solid-state structure (vide supra). The 1:1
electrolytic behavior of complex 3 may be rationalized by
2
3
evident from the UV−vis spectral study of the solution
(
Supporting Information, Figure S11) obtained after proper
workup of the mixture of catechol, complex, and KI (see the
Experimental Section), clearly hints that dioxygen is reduced to
2
9,35
−
H O , as reported by other investigators also.
considering that in solution one Cl ion remains in the outer
2
2
G
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Figure 10. HOMO and LUMO of A and spin densities in B and C computed at the UB3LYP/6-31+G* level of theory.
sphere whereas the other one coordinates with the nickel(II)
center. It is now reasonable to consider that the affinity to make
the metal−substrate complex in the case of complex 3 is less
irreversibly for the other two complexes. The oxidative waves at
0.4 V for the iodo complex, 1.4 V for the bromo complex, and
1.3−1.5 V for the chloro complex probably correspond to
−
−
1
because one Cl is in the coordination sphere, which leads to
halide oxidations (X to / X ).
2 2
neutralization of the effective positive charge on nickel(II) to a
greater extent compared to complexes 1 and 2, where the
counteranions are in the outer sphere in solution also. The
direct effect is the ineffectiveness of 3 in catalyzing the
oxidation of 3,5-DTBC. Now it is essential to know the most
probable reason behind the activity of the complexes to catalyze
the oxidation of 3,5-DTBC, i.e., whether metal-centered redox
participation or a radical pathway is responsible for that activity,
and for that purpose, we have performed EPR and CV studies.
EPR Study. The EPR study was performed at 77 K
temperature under a 9.13 GHz magnetic field immediately after
On the negative side of the reference electrode, two quasi-
reversible reductions are observed at −1.1 to −1.2 V and at
−1.4 to −1.5 V. The first reduction process is undoubtedly
ligand-centered, which is conclusively supported by the DFT
calculations (vide infra). The second reduction is also probably
a ligand-based process, although nickel(II)/nickel(I) reduction
cannot be ruled out at this potential. At more negative potential
at −2.0 V, another reductive peak with a large current height is
observed in DPV, which is again assigned to ligand-centered
reduction. The CV and DPV data are summarized in Table 6,
and figures are given in the Supporting Information (Figures
S12−S14).
DFT Study. As aforementioned, three new complexes have
been synthesized and X-ray characterized (see Scheme 1).
Moreover, it has been demonstrated that complex 1 shows very
high catecholase activity and complex 2 is moderately active. In
sharp contrast, complex 3 is totally inactive in catalyzing the
aerobic oxidation of DTBC. X-ray single-crystal structure
determination reveals that all complexes are cationic nickel(II)
complexes with two halides as counteranions.
−3
mixing 3,5-DTBC with a 10 M methanolic solution of
complexes 1 and 2 in an inert atmosphere. The EPR study
reveals that the complexes are EPR-inactive, whereas a sharp
signal at g ∼ 2 (Figure 8) is generated when the spectra are
taken after the addition of 3,5-DTBC to the bromide and
iodide complexes, suggesting the formation of a free
2
9,36,37
radical.
However, under the experimental conditions,
the free ligand and 3,5-DTBC mixture or nickel salt and 3,5-
DTBC mixture are EPR-silent. This implies that oxidation of
3
,5-DTBC is occurring via a radical pathway only when nickel
Interestingly, conductance measurement of the three
complexes in methanol suggests that complexes 1 and 2
behave as a 2:1 electrolyte, as expected accordingly to the solid-
state structure. However, the conductance measurement for
complex 3 agrees with the formation of a 1:1 electrolyte,
complexes are used as catalysts. The generation of a ligand-
centered radical has further been established by the DFT study.
CV Study. All complexes show ligand-centered oxidation at
0
.70−1.0 V, with the oxidations being irreversible for the chloro
−
and bromo complexes and quasi-reversible for the iodo
complex. At around 1.1−1.2 V, the oxidative response is
probably due to nickel(II) to nickel(III) oxidation, which shows
quasi-reversible behavior for the bromo complex but behaves
suggesting that in methanol solution one Cl ion is coordinated
with the nickel metal center (vide supra). Therefore, because of
the presence of chloride coordinated to the metal in solution,
DTBC is not able to approach the metal and consequently the
H
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size of the ligand, as represented in Figure 9 and Scheme 2. In
the DFT model, we have used a dimethylamino group instead
of the protonated piperazine residue (LH; see Scheme 2). It
can be observed that the optimized structure is very similar to
the solid-state geometry. The computed equatorial metal−
ligand distances agree well with the experimental ones. The
Ni−OH distances are slightly larger in the theoretical model
2
than in the X-ray structure. We have also optimized the
complexes by replacing one water molecule by a chloride,
bromide, or iodide anion, and the geometries are shown in the
top part of Figure 9. We have replaced the equatorial water
molecule by the halide because this is the substitution that
yields the most stable complex. These complexes have been
used to evaluate the energetic cost of replacing one water
molecule by the halide. It should be mentioned that the
calculation in the gas phase gives very favorable energies for this
substitution because of the strong electrostatic attraction
between the positive metal center and the corresponding
counterion. However, when solvent effects are taken into
account, the results are very different. In particular, we have
used three explicit solvent molecules in conjunction with a
continuum model using COSMO mimicking methanol as the
solvent. The results are shown in Scheme 2, and it can be
observed that only the chloride anion is able to substitute the
equatorial water molecule of the complex, while the other two
halides are not able to do this substitution. This likely explains
the formation of a 1:1 electrolyte in a methanol solution of this
−
complex, where one Cl ion is coordinated to the nickel metal
center, preventing the DTBC approach to the metal.
Theoretical Study of the Catecholase Mechanism of
the Nickel Complex. It is well established that catechol may
bind with metal in different modes of coordination where
monodentate and chelated bidentate modes (considering a
single metal center) are more common. Here, the important
issue is which mode of catechol binding is happening in our
case. The pK values of the substrate and those of the
a
complexes may be useful to address that issue. The pK values
a
3
4,35,38
of 3,5-DTBC are reported as 10.4 and 14.7.
Complexes 1
and 2 exhibit pK values of 5.9 and 5.93 for first M−OH (1)
a
2
and 8.67 and 8.73 for second M−OH (2), respectively
2
(
Supporting Information, Figure S15). The above data clearly
demonstrate that at the experimental conditions deprotonation
of a second water molecule of 1 and 2 is not feasible. So, a
monodentate mode of catechol coordination with the nickel(II)
center is more probable. The proposed mechanism for the
catalytic cycle for the oxidation of 3,5-dimethylcatechol (as a
+
model of 3,5-DTBC by the [NiL(OH ) ] complex) is shown
2
3
in Scheme 3. We have computed all of the intermediates
involved in the cycle, and we have focused our attention on the
key intermediates A−C, analyzing the frontier orbitals in A and
spin densities in B and C in order to further confirm the
mechanistic proposal. In the optimized complexes, an ancillary
interaction between the nickel metal center and oxygen atom of
the phenolic −OH group in A and the oxygen atom of the
carbonyl group in B is observed and has been represented as a
dashed line. The electron transfers are highlighted in red and
the proton transfers in blue.
Figure 11. Variable-temperature PXRD pattern of complexes (a) 1,
(
b) 2, and (c) 3.
In Figure 10, we show the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
of compound A, taking into account that there are two
chloride complex does not show catecholase activity. We have
performed a DFT study in order to verify this hypothesis. We
have used the BP86-D3 functional and def2-TZVPD basis set
for the study (see the Theoretical Methods section for details).
We have used a theoretical model in which we have reduced the
2
2
2
unpaired electrons in A mainly located in the d
and dz
x −y
orbitals of the Ni2+ metal center; the HOMO that is
represented corresponds to the highest-energy orbital with
I
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double occupancy. Interestingly, the HOMO is located basically
in the catechol ring and the LUMO in the CN bond and the
aromatic ring of the ligand (L). Therefore, the electron transfer
proposed in the mechanism is supported by the orbital
distribution in A, and likely this electron transfer is facilitated
by the metal center. As a consequence, the spin-density
distribution in B shows, on the one hand, the two unpaired
electrons on the nickel metal center with some spin density
delocalized into the atoms directly bonded to the nickel and, on
the other hand, one unpaired electron mainly localized in the
CN bond and three carbon atoms of the aromatic ring of the
ligand and the other electron delocalized in the catechol ring
and oxygen atoms. This distribution agrees well with the
HOMO−LUMO shape observed in A. Finally, in C, the
distribution of the electron density (apart from the two
electrons of nickel) is again located in the CN bond and
three aromatic carbon atoms. The CN bond elongates from
Variable-Temperature PXRD Study. In order to gain a
better understanding of the thermal reactions of 1−3, we
performed a variable-temperature PXRD study (Figure 11).
From the PXRD patterns, it is evident that in all cases the
crystalline nature of the species remains intact up to 100 °C
and after 150 °C all species become amorphous. The most
interesting feature is obtained from complex 2; the variable-
temperature PXRD patterns clearly show a phase transition
after 80 °C. At present, the reason of this phase transition is not
clear to us. Variable-temperature FT-IR and UV−vis−near-IR
spectral studies and detailed DSC analysis, which are underway
with similar other systems in our laboratory, are very much
essential for a better description of that phase transition.
However, from the variable-temperature PXRD patterns, it is
obvious that without the coordinated water molecules the
crystalline nature of any of the species cannot be retained.
4
. CONCLUSION
1.30 to 1.38 Å upon electron transfer, as expected taking into
consideration the antibonding nature of the LUMO.
The century-old Werner’s coordination theorem becomes the
guideline to establish the structure−function relationship in
small coordination complexes of the nickel(II) Schiff-base
system demonstrated in the present work. All three nickel(II)
halide complexes of a tridentate uninegative Schiff-base ligand
From the above discussion, it may be stated that the
electrochemical study hints that the first reduction is ligand-
centered, which is confirmed by DFT studies. The EPR study
clearly demonstrates the formation of a radical in the presence
of 3,5-DTBC. DFT calculations support the ligand-centered
radical generation. Thus, at this moment, the radical pathway is
supposed to be the more probable reason for the catecholase
activity of complexes 1 and 2.
Thermogravimetric Analysis. Thermal studies of all three
complexes show well-defined stepwise decomposition (Sup-
porting Information, Figures S16−S18). Complexes 2 and 3
have similar types of thermograms, whereas complex 1 shows a
slightly different type of decomposition. The loss of five water
molecules present in complex 3 takes place in two consecutive
steps upon heating. In first step, the weight loss is 12.2% (calcd
1
(L ) in the solid state have similar structures with two halide
ions in the outer coordination sphere. However, in solution
they exhibit different coordination chemistries. In the case of
the chloride complex 3, upon dissolution one of the chloride
ions enters into the inner coordination sphere, whereas in the
case of the other two complexes, the halide ions remain as
counteranions. These particular features have been authenti-
cated by classical experimental verification and, more
interestingly, by DFT calculations to provide a new dimension
to the present study. Because of a change in the status of the
chloride ions from counteranions to coordinating ligands,
effective charge neutralization of the central metal ion takes
place to such an extent that enzyme−substrate adduct
formation, the key step in any catalysis, is not favored and
consequently complex 3 in solution does not exhibit any
catalytic activity to catalyze the aerobic oxidation of 3,5-DTBC.
On the other hand, the other two complexes show excellent
catecholase-like activity, and a radical mechanism has been
proposed to be the possible mechanistic pathway behind that
oxidation on the basis of experimental observations and DFT
calculations.
1
7
1.9%), and in second step, the weight loss is 7.7% (calcd
.9%), suggesting the loss of three and two water molecules,
respectively, between 27 and 199 °C. Complex 2 shows weight
losses of 6.8% (calcd 6.87%) and 6.9% (calcd 6.87%),
corresponding to the elimination of two water molecules in
each step in the temperature range of 31−212 °C. The species
obtained after the first step of deaquation of 2 and 3 via a
temperature arrest technique does not show any visual color
change, whereas the species generated after the second step of
deaquation is red in color. The red species turns green
immediately upon exposure to an open atmosphere. The color
change is supposed to be due to an octahedral to square-planar
structural transformation. In the case of complex 1, single-step
decomposition takes place in the temperature range of 27−185
ASSOCIATED CONTENT
■
*
S
Supporting Information
X-ray crystallographic data in CIF format of complexes 1−3,
FT-IR spectra, electronic spectra in the solid state, packing
°C. Here the weight loss is 13.14% (calcd 11.66%), which is
responsible for the loss of four water molecules. In this case
also, the dehydrated species is red-colored. Fortunately, this red
species takes a few minutes to convert to the green species even
upon exposure to an open atmosphere. The red species is
diamagnetic, and solid-state electronic spectral analysis
diagrams, kinetic plots, electronic spectra of the formation of
−
I , CV and DPV diagrams, spectrophotometric titration plots,
3
and thermograms. This material is available free of charge via
the Internet at http://pubs.acs.org.
(
Supporting Information, Figure S19) clearly suggests a
AUTHOR INFORMATION
■
*
*
*
square-planar environment around nickel(II). However, all
three dehydrated species upon further heating generate NiO as
the thermally stable end product (for complex 1, expt wt loss =
Corresponding Authors
E-mail: dasdebasis2001@yahoo.com.
E-mail: toni.frontera@uib.es.
E-mail: mautner@tugraz.at.
8
8.42% at 594 °C and theor wt loss = 87.79%.; for complex 2,
expt wt loss = 80% at 595 °C and theor wt loss = 85.77%, and
for complex 3, expt wt loss = 83.7% at 585 °C and theor wt loss
Notes
=
83.55%).
The authors declare no competing financial interest.
J
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Inorganic Chemistry
Article
(
23) Osor
́
io, R. E. H. M. B.; Peralta, R. A.; Bortoluzzi, A. J.; de
ACKNOWLEDGMENTS
■
Almeida, V. R.; Szpoganicz, B.; Fischer, F. L.; Terenzi, H.; Mangrich,
A. S.; Mantovani, K. M.; Ferreira, D. E. C.; Rocha, W. R.; Haase, W.;
Tomkowicz, Z.; Anjos, A.; Neves, A. Inorg. Chem. 2012, 51, 1569−
1589.
This work was supported by the Council of Industrial and
Scientific Research, New Delhi [CSIR Project 01(2464)/11/
EMR-II dated 16-05-2011 to D.D.], the DGICYT of Spain
(
Projects CTQ2011-27512/BQU and CONSOLIDER IN-
GENIO 2010 CSD2010-00065, FEDER funds), and the
Direccio General de Recerca i Innovacio del Govern Balear
Project 23/2011, FEDER funds). F.A.M. thanks Dr. J.
(24) Ahlrichs, R.; Bar
Phys. Lett. 1989, 162, 165−169.
25) Klamt, A.; Schu rmann, G. J. Chem. Soc., Perkin Trans. 2 1993,
99−805.
26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
̈ ̈
, M.; Hacer, M.; Horn, H.; Komel, C. Chem.
(
7
(
̈ ̈
u
́
́
(
Baumgartner (TU-Graz) for assistance.
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
REFERENCES
■
(
1) Ferentinos, E.; Maganas, D.; Raptopoulou, C. P.; Terzis, A.;
Psycharis, V.; Robertson, N.; Kyritsis, P. Dalton Trans. 2011, 40, 169−
1
(
80.
2) Maganas, D.; Grigoropoulos, A.; Staniland, S. S.; Chatziefthimiou,
S. D.; Harrison, A.; Robertson, N.; Kyritsis, P.; Neese, F. Inorg. Chem.
010, 49, 5079−5093.
3) Li, J.; Song, H.; Cui, C.; Cheng, J.-P. Inorg. Chem. 2008, 47,
468−3470.
4) Rotthaus, O.; Thomas, F.; Jarjayes, O.; Saint-Aman, C. P. E.;
Pierre, J.-L. Eur. J. Inorg. Chem. 2006, 12, 6953−6962.
5) Mukhopadhyay, S.; Mandal, D.; Ghosh, D.; Goldberg, I.;
Chaudhury, M. Inorg. Chem. 2003, 42, 8439−8445.
6) Das, D.; Ray Chaudhuri, N.; Ghosh, A. Polyhedron 1996, 15,
919−3922.
7) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Bhattacharaya, S.;
Suresh, E.; Nethaji, M.; Zangrando, E.; Das, D. Inorg. Chem. 2008, 47,
083−7093.
8) Chattopadhyay, T.; Mukherjee, M.; Mondal, A.; Maiti, P.;
2
(
3
(
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
revision B.01; Gaussian, Inc.: Wallingford, CT, 2009.
(
(
27) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Science Publishers BV: Amsterdam, The Netherlands, 1984; p 553.
28) Buchanan, R. M.; Mashuta, M. S.; Oberhausen, K. J.;
Richardson, J. F. J. Am. Chem. Soc. 1989, 111, 4497−4498.
29) Biswas, A.; Das, L. K.; Drew, M. G. B.; Aromí, G.; Gamez, P.;
Ghosh, A. Inorg. Chem. 2012, 51, 7993−8001.
30) Marion, R.; Zaarour, M.; Qachachi, N. A.; Saleh, N. M.; Justaud,
F.; Floner, D.; Lavastre, O.; Geneste, F. J. Biol. Inorg. Chem. 2011, 105,
391−1397.
31) Banerjee, A.; Sarkar, S.; Chopra, D.; Colacio, E.; Rajak, K. K.
Inorg. Chem. 2008, 47, 4023−4031.
32) Biswas, A.; Das, L. K.; Drew, M. G. B.; Diaz, C.; Ghosh, A. Inorg.
Chem. 2012, 51, 10111−10121.
33) Mandal, S.; Mukherjee, J.; Lloret, F.; Mukherjee, R. N. Inorg.
Chem. 2012, 51, 13148−13161.
34) Mendoza-Quijano, M. R.; Ferrer-Sueta, G.; Flores-Alamo, M.;
Aliaga-Alcalde, N.; Gomez-Vidales, V.; Ugalde-Saldívara, V. M.;
Gasque, L. Dalton Trans. 2012, 41, 4985−4997.
35) Neves, A.; Rossi, L. M.; Bortoluzzi, A. J.; Szpoganicz, B.;
Wiezbicki, C.; Schwingel, E. Inorg. Chem. 2002, 41, 1788−1794.
36) Guha, A.; Chattopadhyay, T.; Paul, N. D.; Mukherjee, M.;
Goswami, S.; Mondal, T. K.; Zangrando, E.; Das, D. Inorg. Chem.
012, 51, 8750−8759.
37) Benisvy, L.; Wanke, R.; da Silva, M. F. C. G.; Pombeiro, A. J. L.
Eur. J. Inorg. Chem. 2011, 2791−2796.
38) Tyson, C. A.; Martell, A. E. J. Am. Chem. Soc. 1968, 90, 3379−
386.
(
3
(
(
(
7
(
(
Banerjee, A.; Banu, K. S.; Bhattacharya, S.; Roy, B.; Chattopadhyay, D.
J.; Mondal, T. K.; Nethaji, M.; Zangrando, E.; Das, D. Inorg. Chem.
1
(
2
(
010, 49, 3121−3129.
9) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Mukherjee, M.;
Bhattacharya, S.; Patra, G. K.; Zangrando, E.; Das, D. Dalton Trans.
009, 8755−8764.
10) Banu, K. S.; Chattopadhyay, T.; Banerjee, A.; Bhattacharya, S.;
Zangrando, E.; Das, D. J. Mol. Catal. A: Chem. 2009, 310, 34−41.
11) Guha, A.; Banu, K. S.; Banerjee, A.; Ghosh, T.; Bhattacharya, S.;
Zangrando, E.; Das, D. J. Mol. Catal. A: Chem. 2011, 338, 51−57.
12) Guha, A.; Banerjee, A.; Mondol, R.; Zangrando, E.; Das, D. J.
Coord. Chem. 2011, 22, 3872−3886.
13) Banerjee, A.; Guha, A.; Maiti, P.; Goswami, S.; Chattopadhyay,
(
2
(
(
(
́
(
́
(
(
(
T.; Mondal, T. K.; Bhattacharya, S.; Zangrando, E.; Das, D. Transition
Met. Chem. 2011, 36, 195−199.
14) Das, S.; Maiti, P.; Ghosh, T.; Zangrando, E.; Das, D. Inorg.
(
(
2
(
Chem. Commun. 2012, 15, 266−268.
15) Guha, A.; Banu, K. S.; Das, S.; Chattopadhyay, T.; Sanyal, R.;
Zangrando, E.; Das, D. Polyhedron 2013, 52, 669−678.
16) (a) Monzani, E.; Quinti, L.; Perotti, A.; Casella, L.; Gullotti, M.;
Randaccio, L.; Geremia, S.; Nardin, G.; Faleschini, P.; Tabbi, G. Inorg.
Chem. 1998, 37, 553−562. (b) Monzani, E.; Battaini, G.; Perotti, A.;
Casella, L.; Gullotti, M.; Santagostini, L.; Nardin, G.; Randaccio, L.;
Geremia, S.; Zanello, P.; Opromolla, G. Inorg. Chem. 1999, 38, 5359−
369.
17) Ackermann, J.; Meyer, F.; Kaifer, E.; Pritzkow, H. Chem.Eur. J.
002, 8, 247−258.
18) Monzani, E.; Quinti, L.; Perotti, A.; Casella, L.; Gullotti, M.;
(
(
3
(
5
(
2
(
Randaccio, L.; Geremia, S.; Nardin, G.; Faleschini, P.; Tabbi, G. Inorg.
Chem. 1998, 37, 553−562.
(
19) (a) SAINT, version 7.23; SMART; Bruker AXS Inc.: Madison,
WI, 2005 and 2006. (b) Sheldrick, G. M. SADABS, version 2;
University of Gottingen, Gottingen, Germany, 2001.
20) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
21) Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.;
̈
̈
(
(
Shields, G. P.; Taylor, R.; Towler, T.; van de Streek, J. J. Appl.
Crystallogr. 2006, 39, 453.
(
22) Spek, A. L. Acta Crystallogr., Sect. D 2009, 65, 148.
K
dx.doi.org/10.1021/ic401819t | Inorg. Chem. XXXX, XXX, XXX−XXX