Electrochemical synthesis and structural characterization of Co(II), Ni(II) and Cu(II) complexes of N,N-bis(4,5-dimethyl-2-hydroxybenzyl)-N-(2- pyridylmethyl)amine

Article abstract of DOI:10.1039/b907539a

The electrochemical oxidation of anodic metal (cobalt, nickel or copper) in a cell containing an acetonitrile solution of the ligand N,N-bis(4,5-dimethyl- 2-hydroxybenzyl)-N-(2-pyridylmethyl)amine (H₂L) affords complexes [Co₂L₂]·H₂O (1), [Ni₃L ₃] (2) and [Cu₂L₂] 3H₂O (4). On using nickel as the anode and the addition to the solution electrolytic phase of the amount of water necessary to saturate the solution, the electrolytic process gave rise to the new compound [Ni₂L₂(H ₂O)_1.5]·CH₃CN (3). Compounds 1 and 4 are dimeric and the metal atoms are pentacoordinated. Compound 3 also consists of dimeric neutral molecules with the nickel atoms in both penta- and hexacoordinated environments. The crystal structure of 2 shows the presence of a trimeric compound in which the nickel atoms are hexacoordinated. Electronic, IR and FAB spectra of the complexes are discussed and related to the structural information. The magnetic behavior of 1-4 denotes the occurrence of intramolecular antiferromagnetic interactions. The values obtained for the coupling constant J are -4.2 cm^-1, -5.3 cm^-1, -30 cm ^-1 and -4.7 cm^-1 for 1, 2, 3 and 4, respectively. These values are in full agreement with the structural characteristics of the compounds. The catalytic activity of the complexes towards the decomposition of hydrogen peroxide (catalase activity) was also studied.

Full text of DOI:10.1039/b907539a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Electrochemical synthesis and structural characterization of Co(II), Ni(II)
and Cu(II) complexes of N,N -bis(4,5-dimethyl-2-hydroxybenzyl)-
N-(2-pyridylmethyl)amine†
Elena Labisbal,a Laura Rodr´ıguez,a Oscar Souto,a Antonio Sousa-Pedrares,a Jose´ Arturo Garc´ıa-Va´zquez,*a
Jaime Romero,a Antonio Sousa,a Matilde Ya´n˜ez,b Francisco Orallob and Jose´ A. Realc
Received 16th April 2009, Accepted 4th August 2009
First published as an Advance Article on the web 24th August 2009
DOI: 10.1039/b907539a
The electrochemical oxidation of anodic metal (cobalt, nickel or copper) in a cell containing an
acetonitrile solution of the ligand N,N -bis(4,5-dimethyl-2-hydroxybenzyl)-N-(2-pyridylmethyl)amine
(H2L) affords complexes [Co2L2]·H2O (1), [Ni3L3] (2) and [Cu2L2] 3H2O (4). On using nickel as the
anode and the addition to the solution electrolytic phase of the amount of water necessary to saturate
the solution, the electrolytic process gave rise to the new compound [Ni2L2(H2O)1.5]·CH3CN (3).
Compounds 1 and 4 are dimeric and the metal atoms are pentacoordinated. Compound 3 also consists
of dimeric neutral molecules with the nickel atoms in both penta- and hexacoordinated environments.
The crystal structure of 2 shows the presence of a trimeric compound in which the nickel atoms are
hexacoordinated. Electronic, IR and FAB spectra of the complexes are discussed and related to the
structural information. The magnetic behavior of 1–4 denotes the occurrence of intramolecular
antiferromagnetic interactions. The values obtained for the coupling constant J are -4.2 cm-1,
-5.3 cm-1, -30 cm-1 and -4.7 cm-1 for 1, 2, 3 and 4, respectively. These values are in full agreement with
the structural characteristics of the compounds. The catalytic activity of the complexes towards the
decomposition of hydrogen peroxide (catalase activity) was also studied.
contain a potentially bridging phenoxo oxygen and nitrogen
donor sets have been widely used in the synthesis of dinuclear
Introduction
complexes of copper,17 manganese,18 cobalt,19 iron20 and zinc.19
As in hydroxo- and alkoxo-bridged metal complexes, the nature
of the magnetic interactions in phenoxo-bridged metal systems
is primarily determined by the M–O–M angle and the M ◊ ◊ ◊ M
separation.21,22
As part of our research in this area, we report here the
preparation, structure determination, magnetic properties and
“catalase-like” activities of cobalt, nickel and copper complexes
of the sterically hindered tripodal ligand N,N -bis(4,5-dimethyl-2-
hydroxybenzyl)-N-(2-pyridylmethyl)amine [H2L]) (Scheme 1).
Much of the interest in transition metal complexes of chelating
alkoxide and aryl-oxide ligands results from their potential
relevance as catalytic systems for the polymerization of alpha-
olefins.1 A substantial amount of work has been carried out
with monodentate phenolate2,3 and chelating phenolate4–6 ligands,
mainly on group IV and V metals. Dianionic amine bis(phenolate)
ligands were recently used as an approach to increase the hy-
drophobic nature of the coordinating ligands.7 However, the use of
chelating phenolates in other transition-metal groups is still rare.8,9
In addition, interest in the structure and reactivity of transition
metal complexes with this type of ligand is related, in part, to the
fact that they can be used as mimetic small molecular models for
the active sites of several redox and hydrolytic enzymes.10–14
The ability of aryl-oxide functionalized ligands to facilitate
efficiently the magnetic coupling between two paramagnetic
metal ions represents another source of interest in the realm
of molecular magnetism.15,16 Indeed, dinucleating ligands that
aDepartamento de Qu´ımica Inorga´nica, Universidad de Santiago de Com-
postela, 15782, Santiago de Compostela, Spain. E-mail: josearturo.garcia@
usc.es; Fax: +34-(9)81547102; Tel: +34-(9)81 563100, ext. 14950
bDepartamento de Farmacolog´ıa, Universidad de Santiago de Compostela,
15782, Santiago de Compostela, Spain
cInstitut de Ciencia Molecular/Departamento de Qu´ımica Inorga´nica,
Universitat de Valencia, 46071, Valencia, Spain
† Electronic supplementary information (ESI) available: The crystal pa-
rameters, experimental details for data collection and bond distances and
angles for 1–4 compounds. CCDC reference numbers 710317–710320. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b907539a
Scheme 1
8644 | Dalton Trans., 2009, 8644–8656
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reacts (with a stoichometry of 1:1) with H2O2 in the presence
of HRP to produce the highly fluorescent oxidation product,
resorufin.
Experimental
General considerations
R
To measure H2O2, equal volumes of sample and Amplexꢀ
All solvents, 3,4-dimethylphenol, 2-(2-aminomethyl)pyridine and
37% aq. formaldehyde were commercial products (Aldrich) and
were used as supplied. Cobalt, nickel and copper (Ega Chemie)
were used as plates (ca. 2 ¥ 2 cm). The chemicals used in the
catalase-like properties experiments were the tested compounds,
catalase from bovine liver and dimethylsulfoxide (purchased from
Sigma-Aldrich), resorufin sodium salt, H2O2, sodium phosphate
Red-HRP working solution were mixed. The final reaction
R
mixture contained 50 mM Amplexꢀ Red reagent and 0.1 units
(U)/mL HRP. After 10 min, the amount of resorufin generated
was quantified in a fluorescent plate reader (FLx800, Bio-Tek
Instruments, Inc., Winooski, Vermont, USA), using excitation at
545 nm with emission detection at 590 nm. Fluorescence was
converted to H2O2 concentration by using a curve generated
from standard samples containing known amounts of H2O2. The
assay was linear in the range from 0 to 5 mM H2O2. Control
experiments were carried out simultaneously by replacing the
tested compounds with appropriate dilutions of the vehicles. In
addition, the possible ability of the tested complexes to modify
the fluorescence generated in the reaction mixture due to non-
R
and HRP (supplied in the Amplexꢀ Red MAO assay kit from
Molecular Probes, Inc., Eugene, Oregon, USA). Appropriate dilu-
tions of the above substances were prepared every day immediately
before use in deionized water from the following concentrated
stock solutions kept at -20 ◦C: test compounds (0.1 M) in
DMSO; resorufin, H2O2 and HRP (0.1 M) in deionized water.
R
Due to the photosensitivity of some chemicals (e.g., Amplexꢀ
R
enzymatic inhibition (e.g., by direct reaction with Amplexꢀ Red
Red reagent), all experiments in which these compounds were
used were performed in the dark. In all assays, neither deionized
reagent) was determined by adding these complexes to solutions
R
containing only the Amplexꢀ Red reagent in a sodium phosphate
R
water (Milli-Qꢀ, Millipore Ibe´rica S.A., Madrid, Spain) nor
buffer.
appropriate dilutions of the vehicle used (DMSO) had significant
pharmacological effects.
For details of data presentation and statistical analysis see the
ESI.†
Physical measurements
Ligand synthesis
Elemental analyses were performed using Carlo-Erba EA 1112
and Carlo-Erba EA 1108 microanalysers. IR spectra were recorded
on KBr discs using a Bruker IFS 66V spectrophotometer and an
The ligand H2L was prepared by the one-pot Man-
nich condensation25,26 between 3,4-dimethylphenol (6.11 g,
50 mmol), formaldehyde 37% (51.4 mL, 50 mmol) and 2-
(aminomethyl)pyridine (22.6 mL, 25 mmol) in ethanol as solvent.
The reaction mixture was refluxed during 6 h and then cooled.
The resulting white solid was filtered off, washed with water
1
ABB Bomen 102 spectrophotometer. H and 13C NMR spectra
were recorded on Bruker AMX350 MHz and Bruker AC-300
instruments using CDCl3 as solvent and chemical shifts were
determined against TMS. Electrospray mass spectral data were
obtained with 5 ¥ 10-4 M solutions (MeOH or CMe2Cl2) by
flow injection into a Hewlett-Packard 1100 series MSD. The
carrier solvent was 49% MeOH/49% H2O/2% formic acid or
98% CH2Cl2/2% formic acid. Electrospray ionization conditions
were as follows: nitrogen drying gas flow 10.0 L min-1; nebulizer
pressure, 40 psig; drying gas temperature 350 ◦C; capillary voltage,
4 kV. The capillary exit voltage was varied from 0 to 150 V. Variable
temperature magnetic susceptibility measurements were carried
out using microcrystalline samples (20–60 mg) of compounds
1–4, using a Quantum Design MPMS2 SQUID susceptometer
equipped with a 5.5 T magnet, operating at 0.1–0.5 T and at
temperatures from 300–1.8 K. The susceptometer was calibrated
with (NH4)2Mn(SO4)2·12H2O. Experimental susceptibilities were
corrected for diamagnetism of the constituent atoms by the use of
Pascal’s constants.
1
and crystallized from dichloromethane (8.75 g, 93%). H NMR
(350 Mz, CDCl3, 25 ◦C, TMS, ppm): d = 8.64, 7.70 and 7.14
(1H, pyridine ring); 6.79 and 6.69 (s, 2H, phenyl); 3.86 (s, 2H,
-CH2-py); 3.74 (s, 4H, -CH2-); 2.17 and 2.74 (s, 6H, -CH3). 13C
NMR (300 Mz, 25 ◦C, TMS, CDCl3, ppm) d 152 (C–OH); 137.8–
118.4 (C-aromatic); 56.1 (N–CH2), 55.7 (N–CH2, py) 19.8–18.9
(-CH3); IR (KBr) n = 1002(vs), 1241(s), 1294(s), 1504(vs),
1597(vs), 1630(s), 3100–3350(vbr) cm-1; EI MS m/z 377.4 (M+):
elemental analysis calcd (%) for C24H28N2O2: C 76.5, H, 7.4, N,
7.5; found: C 76.6, H 7.5, N 7.4.
Electrochemical synthesis of the metal complexes
The complexes were obtained using an electrochemical procedure
(Scheme 2).27 The cell consisted of a tall-form beaker (100 mL)
fitted with a rubber bung through which the electrochemical
leads entered. An acetonitrile solution of the ligand, containing a
few mg of tetramethylammonium perchlorate as a current carrier,
was electrolyzed using a platinum wire as the cathode and a
metal plate as the sacrificial anode (Caution: Although problems
were not encountered in this work, all perchlorate compounds are
potentially explosive, and should be handled in small quantities and
with great care!). The applied voltages (10–20 V) allowed sufficient
current flow for smooth dissolution of the metal. The current was
maintained at 5 mA for 1 h. In all cases, during the electrolysis
hydrogen was evolved at the cathode. Under these conditions the
cell can be summarized as M(+)/H2L + CH3CN/Pt(–).
Potential catalase-like properties of the synthesized compounds
were evaluated under physiological conditions, i.e., in aqueous
solution at pH = 7.2–7.4 and at micromolar concentrations of the
catalyst and hydrogen peroxide (H2O2). The assay procedure was
performed at 37 ◦C in a total volume of 1 mL containing 50 mM
sodium phosphate buffer, 100 mM H2O2 and the appropriate
amount of the tested compounds. At different time intervals,
aliquots of the reaction mixture were suitably diluted (1/100) in
sodium phosphate buffer and the remaining H2O2 present in these
R
diluted aliquots was quantified by the Amplexꢀ Red–Hydrogen
Peroxide method.23,24 This is a fluorescence assay in which
R
the Amplexꢀ Red reagent (10-acetyl-3,7-dihydroxyphenoxazine)
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Scheme 2 Schematic representation of the electrochemical synthesis of complexes.
Synthesis of [Co2L2]·H2O(1). Electrochemical oxidation of
a cobalt anode in a solution of H2L (0.035 g, 0.093 mmol)
in acetonitrile (80 cm3) at 10 V and 5 mA for 1 h caused
5.5 mg of cobalt to be dissolved, Ef = 0.50 mol F-1. During the
electrolysis process, hydrogen was evolved at the cathode. The
resulting solid was filtered off, washed with acetonitrile and ether
and characterized as [Co2L2] H2O (1) (0.032 g, 79%). Elemental
analysis: calcd (%) for C48H54Co2N4O5:C, 65.2; H, 6.2, N, 6.3;
found: C 65.1, H 6.2, N 6.3. IR (KBr, n, cm-1) 1610(s), 1553(m),
1494(s), 1454(m), 1309(vs), 1210(s), 1102(vs), 1021(m), 962(m),
874(s), 769(m); EI MS m/z; 867 [Co2L2]+, 434 [CoL]+, 377 [H2L]+.
Air concentration of the resulting solution yielded a dark violet
crystalline solid suitable for X-ray studies.
Synthesis of [Ni2L2(H2O)1.5]·CH3CN (3). A solution of the lig-
and (0.035 g, 0.093 mmol) and tetramethylammonium perchlorate
(ca. 10 mg) in acetonitrile containing 10% in volume of water
(80 cm3) was electrolyzed at 8 V and 5 mA during 1 h; 5.3 mg of
nickel metal was dissolved from the anode, Ef = 0.48 mol F-1. After
the electrolysis the clear solution was filtered to remove any solid
impurities and crystals of [Ni2L2(H2O)1.5]·CH3CN (3) suitable for
X-ray studies were obtained by air concentration (0.036 g, 84%).
Elemental analysis: calcd (%) for C50H58Ni2N5O5.5: C, 64.3, H, 6.3,
N, 7.5; found: C 64.2, H 6.2, N 7.5. IR (KBr, n, cm-1) 1609(s),
1556(m), 1489(vs), 1455(m), 1410(m), 1323(s), 1107(vs), 1025(m),
971(m). EI MS m/z; 867 [Ni2L2]+; 377 [H2L]+.
Synthesis of [Ni3L3] (2). An experiment similar to that de-
scribed above was performed, with nickel as the anode and a
solution of H2L (0.035 g, 0.093 mmol) and tetramethylammonium
perchlorate (ca. 10 mg) in acetonitrile (80 cm3), at 5 mA and
6 V for 1 h, dissolved 5.4 mg of nickel (Ef = 0.49). A green
solid characterized as [Ni3L3] (2) was obtained (0.033 g, 82%).
Elemental analysis: calcd (%) for C72H78Ni3N6O6: C 66.6, H 6.1,
N 6.3; found C 66.6, H 6.1, N 6.4. IR (KBr, n, cm-1): = 1609(s),
1497(vs), 1457(m), 1409(m), 1322(s), 1103(vs), 1053(m), 1021(m),
952(m), 872(m), 761(m); EI MS m/z; 1300 [Ni3L3]+; 867 [Ni2L2]+;
433 [NiL]+; 377 [H2L]+;. Crystals suitable for X-ray studies were
obtained by crystallization from acetonitrile/dichloromethane.
Synthesis of [Cu2L2]·3H2O (4). Electrolysis of a solution of
the ligand (0.035 g, 0.093 mmol) and tetramethylammonium
perchlorate (ca. 10 mg) in acetonitrile (80 cm3) at 8 V and 5 mA
for 1 h dissolved 11.4 mg of copper from the anode, Ef = 0.96 mol
F-1 was performed. The solid obtained in the cell was collected by
filtration washed with acetonitrile and ether and characterized as
[Cu2L2]·3H2O (4) (0.033 g, 76%). Elemental analysis calcd. (%) for
C48H58Cu2N4O7: C, 62.0., H, 6.3, N, 6.0; found: C 62.6, H 6.1, N
6.2. IR (KBr, n, cm-1) 1612(s), 1487(s), 1456(s), 1410(m), 1317(s),
1100(vs), 1029(m), 791(m); EI MS m/z: 876 [Cu2L2]+; 437 [CuL]+;
377 [H2L]+. Slow air evaporation of the solvent from mother liquor
provided brown crystals suitable for X-ray studies.
8646 | Dalton Trans., 2009, 8644–8656
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X-ray crystallographic studies
molecule of acetonitrile (22 electrons) on a general position in the
space group P-1. Likewise, in the crystal structure of compound 4
three molecules of water, hydrogen-bonded to the copper complex,
could be refined. The remaining solvent molecules are located in
voids of the lattice and were removed using the Squeeze program.
The electron count for the voids (125 electrons) agrees with the
presence of one molecule of water (10 electrons) and one molecule
of acetonitrile (22 electrons) on a general position in the space
group P2(1)/c. Finally, the crystal structure of compound 2 also
presents highly disordered solvent molecules located in the voids
of the crystal lattice. The disorder for these molecules was so severe
that they were removed using Squeeze. The total electron count
for the voids in the cell (795 electrons) refer to one molecule of
acetonitrile (22 electrons) and two molecules of dichloromethane
(42 electrons) on a general position in the space group C2/c.
Pertinent details of the data collections and structure refine-
ments are summarized in Table 1. Important geometrical data
for all compounds are listed in Tables 1–5. Further details
regarding the data collections, structure solutions and refinements
are included in the ESI.† Ortep332 drawings with the numbering
schemes used are shown in Figs. 2–5.
Intensity data for compounds 1–4 were collected using a
Smart-CCD-1000 Bruker diffractometer (Mo-Ka radiation, l =
˚
0.71073 A) equipped with a graphite monochromator. Compound
1 was measured at 100 K and data for compounds 2–4 were
collected at 120 K. The w scan technique was employed to measure
intensities in all crystals. Decomposition of the crystals did not
occur during data collection. The intensities of all data sets were
corrected for Lorentz and polarization effects. Absorption effects
in all compounds were corrected using the program SADABS.28
The crystal structures of all compounds were solved by direct
methods. Crystallographic programs used for structure solution
and refinement were those of SHELX97.29 Scattering factors
were those provided with the SHELX program system. Missing
atoms were located in the difference Fourier map and included
in subsequent refinement cycles. The structures were refined
by full-matrix least-squares refinement on F2, using anisotropic
displacement parameters for all non-hydrogen atoms. Hydrogen
atoms not involved in hydrogen bonding were placed geometrically
and refined using a riding model with Uiso constrained at 1.2 (for
non-methyl groups) and 1.5 (for methyl groups) times Ueq of the
carrier C atom. In the last cycles of refinement of all structures
a weighting scheme was used, where weights were calculated
Results and discussion
using the following formula w = 1/[s (Fo2) + (aP)2 + bP], where
2
Synthesis of the complexes
2
2
P = (Fo + 2Fc )/3.
In compound 3, the refinement of oxygen O(6) as one full
oxygen atom gives a Uiso value that is too large in comparison
to the rest of the atoms in the complex. The large Uiso suggests
that this position is only partially occupied by an oxygen atom.
This oxygen atom O(6) is part of a water molecule coordinated
to the nickel atom Ni(2). The Ni–O bond length for this water,
The new metal complexes were obtained by electrochemical
oxidation of the appropriate cobalt, nickel or copper metal in
a cell containing an acetonitrile solution of the ligand. Elemental
analysis shows that metal ions react with the ligands in a 1:1
molar ratio to afford complexes of the bis(deprotonated) ligand
(L2-). The use of nickel as the anode in a cell containing a solution
of the ligand in acetonitrile containing 10% in volume of water
allowed the synthesis of [Ni2L2(H2O)1.5]·CH3CN (3).
In the synthesis of nickel and cobalt complexes the electrochem-
ical efficiency values, Ef, were close 0.5 mol F-1 (see experimental
part) and these are consistent with the following reaction scheme:
Cathode: [H2L] + 2e- → H2(g) + [L2-]
˚
2.324(8) A, indicates that this water is only weakly bound to the
nickel atom and therefore subject to higher vibration. In order
to obtain the value for the occupancy parameter for this oxygen,
Uiso was fixed at a value of 0.045, which is comparable to the
value of the terminal methyl groups. The occupancy parameter
obtained was 0.4687. This value was then rounded to 0.5 and used
in subsequent refinement cycles, giving a Uiso of 0.06368. This
compound 3 has disordered solvents in the void of the crystal
lattice. The contents of this void are half acetonitrile molecule and
two half independent water molecules. The solvent molecules inthe
void have been refined isotropically. Hydrogen atoms for the two
lattice water molecules were not modelled. The half acetonitrile
molecule is very close to the weakly bound water, O(6), and the
chemical meaning is that the weakly bound water is only present
in half the complexes, so half of the unit cells have a dimer with
two six-coordinated nickel atoms and two water molecules in a
void, while the other half of the unit cells have a dimer with one
six-coordinated nickel atom and one five-coordinated nickel atom
along with an acetonitrile molecule in a void.
The crystal structure of compound 1 presents one molecule
of water hydrogen-bonded to the cobalt complex. This molecule
could be located and refined. The structure of compound 1 also
presents one molecule of acetonitrile per cell in the voids of the
crystal lattice which does not interact with the complex and is
highly disordered. This molecule was removed using the Squeeze
program30 implemented in Platon.31 The electron count for the
voids in the cell (40 electrons) agrees with the presence of a
Anode: M → M2+ + 2 e-
Overall: [H2L] + M → [ML] + H2
M = Co and Ni.
In the synthesis of the copper complex, an Ef value close to
1.0 mol F-1 is indicative that the anionic oxidation leads initially
to the Cu(I) metal. However, the analytical data show that the
final compound is [CuL]. This suggests a subsequent oxidation in
solution from Cu(I) to Cu(II) as soon as it is formed, according to
the following reaction scheme:
1
Cathode: [H2L] + e- → H2(g) + [HL-]
2
Anode: Cu → Cu++ e-
1
2
Solution: [HL-] + Cu+ → [CuL] + H2(g)
This behavior has been observed previously in the synthesis of
other Cu(II) complexes with related ligands using an electrochem-
ical procedure.33
The neutral complexes are sparingly soluble in common organic
solvents but they are soluble in polar coordinating solvents such
as DMSO and DMF. The compounds appear to be stable in
the solid state and in solution. The structural studies (see below)
show the presence of the dimeric species [Co2L2]·H2O (1) and
[Cu2L2]·3H2O (4). In the case of nickel, the compound is the
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Table 1 Summary of crystallographic data and structure refinement
Compound
1
2
3
4
Empirical formula
Formula weight
Crystal size, mm
Temperature, K
Wavelength
Crystal system
Space group
C48H54Co2N4O5
884.81
0.50 ¥ 0.41 ¥ 0.23
100(2)
0.71073
Triclinic
P-1
C72H78N6Ni3O6
1299.53
0.38 ¥ 0.31 ¥ 0.30
120(2)
0.71073
Monoclinic
C 2/c
C49H58.50N4.5Ni2O6.5
931.92
0.17 ¥ 0.17 ¥ 0.09
120(2) K
0.71073
Monoclinic
P 2(1)/n
C48H58Cu2N4O7
930.06
0.21 ¥ 0.17 ¥ 0.09
120(2)
0.71073
Monoclinic
P 2(1)/c
Unit cell dimens.
˚
a, A
10.886(4)
11.807(4)
20.893(7)
85.537(6)
75.688(5)
63.367(5)
2324.3(14)
2
51.673(9)
13.205(2)
21.820(4)
90
101.747(3)
90
14577(4)
8
0.818
14.083(6)
17.120(7)
19.394(8)
90.
104.000(7)
90
4537(3)
4
0.885
38942
10.694(2)
22.640(4)
20.273(4)
90
95.394(3)
90
4886.9(16)
4
0.921
˚
b, A
˚
c, A
a/◦
b/◦
g /◦
3
˚
Volume, A
Z
m, mm-1
0.761
29164
No. reflections collectec.
No. of independent reflections
Data/restraints/parameters
Goodness-of-fit
62460
77473
10551 [R(int) = 0.0296]
10551/1/546
1.066
14836 [R(int) = 0.0418]
14836/0/797
1.093
9334 [R(int) = 0.1172]
9334/4/582
0.932
10130 [R(int) = 0.1015]
10130/6/576
1.078
Final R indices [I > 2s(I)]
R1a = 0.0318
R1 = 0.0346
R1 = 0.0597
R1 = 0.0502
wR2b = 0.0799
wR2 = 0.0875
wR2 = 0.1266
wR2 = 0.1187
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = [|Fo| - |Fc|/ |Fo]; b wR2 = [| (Fo - Fc2)/ (Fo2)]1/2
.
2
trimeric species [Ni3L3] (2), although when the synthesis was
carried out in acetonitrile/water solution the species obtained was
the dimer [Ni2L2(H2O)1.5]·CH3CN (3).
Description of structures. Compounds 1–4 were studied by
X-ray diffraction.†
and two phenoxo oxygen atoms of a bis(deprotonated) Mannich
base ligand and also by another bridging phenoxo oxygen atom
from another similar fragment.
Analysis of the shape-determining bond angles, using the
geometrical parameter t [t = (b - a)/60]34 gives values of 0.65
and 0.71 for the two cobalt atoms and suggests that they are in
an environment close to a [CoN2O3] trigonal bipyramid (t = 1)
with the two phenoxo oxygen atoms and the pyridine nitrogen
atom of a tetradentate ligand in the equatorial plane and the other
nitrogen atom of the same ligand and the bridging oxygen atom
from the other ligand in the apical positions. The two coordination
polyhedra are joined through the edge that contains the two
bridging oxygen atoms.
Structure of [Co2L2]·H2O (1). The molecular structure of 1 is
shown in Fig. 1 together with the atom labeling scheme adopted. A
selection of bond distances and angles, with the estimated standard
deviations, is given in Table 2.
The four-membered ring formed by the two cobalt atoms and
the two oxygen atoms, i.e. Co(1)–O(1)–O(3)–Co(2), is nearly
planar (r.m.s. deviation of 0.0882). However, the bridge is
slightly asymmetric, with two shorter bond distances [Co(1)–O(1)
˚
1.9744(12) and Co(2)–O(3) 1.9965(12) A] and two longer distances
˚
[Co(1)–O(3) 2.0820(12) and Co(2)–O(1) 2.0427(12) A]. These
bond distances are slightly longer than those corresponding to the
metal and oxygen phenoxo terminal atoms, Co(1)–O(2) 1.9341(12)
˚
and Co(2)–O(4) 1.9222(12) A, respectively. However, all bond
distances are in the range of average values for Co–O(phenoxo)
bonds observed in other pentacoordinated Co(II) complexes.35–38
˚
The two cobalt(II) centers are separated by 3.1192(8) A, a value
that is similar to those found in other phenoxo-bridged dinuclear
cobalt compounds.35,36
The two Co–N(amine) distances are similar, Co(1)–N(1)
˚
˚
2.1982(14) A and Co(2)–N(3) 2.1699(14) A, and are close to the
Fig. 1 ORTEP diagram of the molecular structure of 1 with 50% thermal
ellipsoid probability.
same bond distances in other pentacoordinated cobalt complexes,
i.e. in the range 2.092(4)–2.197(5) A.
bonds, 2.0865(14) and 2.0710(15) A, are slightly shorter but are
35–38,39
˚
The Co–N(pyridine)
˚
also in the normal range observed in pentacoordinated cobalt(II)
The molecular structure of 1 consists of a dinuclear species
with the cobalt atom pentacoordinated by two nitrogen atoms
40
˚
complexes of pyridine derivatives [2.068–2.174 A].
8648 | Dalton Trans., 2009, 8644–8656
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˚
Table 2 Selected bond lengths (A) and angles (deg) for [Co2L2]·H2O (1)
Co(1)–O(2)
Co(1)–O(1)
Co(1)–O(3)
Co(1)–N(2)
Co(1)–N(1)
Co(1)–Co(2)
1.9341(12)
1.9744(12)
2.0820(12)
2.0865(14)
2.1982(14)
3.1192(8)
Co(2)–O(4)
Co(2)–O(3)
Co(2)–O(1)
Co(2)–N(4)
Co(2)–N(3)
1.9222(12)
1.9965(12)
2.0427(12)
2.0710(15)
2.1699(14)
O(2)–Co(1)–O(1)
O(2)–Co(1)–O(3)
O(1)–Co(1)–O(3)
O(2)–Co(1)–N(2)
O(1)–Co(1)–N(2)
O(3)–Co(1)–N(2)
O(2)–Co(1)–N(1)
O(1)–Co(1)–N(1)
O(3)–Co(1)–N(1)
N(2)–Co(1)–N(1)
Co(1)–O(1)–Co(2)
117.83(5)
103.79(5)
78.10(5)
115.89(5)
125.17(5)
99.28(5)
91.02(5)
89.75(5)
164.01(5)
79.14(5)
101.86(5)
O(4)–Co(2)–O(3)
O(4)–Co(2)–O(1)
O(3)–Co(2)–O(1)
O(4)–Co(2)–N(4)
O(3)–Co(2)–N(4)
O(1)–Co(2)–N(4)
O(4)–Co(2)–N(3)
O(3)–Co(2)–N(3)
O(1)–Co(2)–N(3)
N(4)–Co(2)–N(3)
Co(1)–O(3)–Co(2)
116.10(5)
101.70(5)
78.54(4)
119.18(6)
124.21(5)
96.99(6)
91.02(5)
92.78(5)
166.80(5)
79.57(6)
99.75(5)
The bond lengths and angles for the ligand are as expected,
with values close to those found in other complexes with related
ligands. In all cases, the C–O bond lengths have intermediate values
O(3) acts as terminal and a third oxygen atom O(4) as triple oxo-
bridge. Finally, the third ligand is bonded to the metals through
both amine N(5) and pyridine N(6) nitrogen atoms, a phenoxo
oxygen atom O(5) acts as a triple bridge between the three metal
atoms and, finally, the other oxygen atom O(6) bridges two metal
atoms.
The three nickel atoms are in a distorted octahedral [N2O4]
environment and, on the whole, the structure can be considered
as consisting of a central Ni(3) octahedron sharing two contigu-
ous triangular faces, with O(2)O(4)O(5) belonging to the Ni(1)
octahedron and O(4)O(5)O(6) to the Ni(2) octahedron. Thus, the
Ni(1), Ni(2) and Ni(3) octahedra share a common edge, O(4)–O(5)
(Fig. 3).
41
=
between a typical C–O single bond and a C O double bond.
This compound crystallizes with one non-coordinated water
molecule. This molecule is hydrogen bonded to the two phenoxo
oxygen atoms not involved in the Co2O2 bridge, O(2) and O(4).
Structure of [Ni3L3] (2). The molecular structure of 2 is shown
in Fig. 2 along with the atom labeling scheme adopted. A selection
of bond lengths and angles with the estimated standard deviations
are given in Table 3.
Fig. 2 ORTEP diagram of the molecular structure of 2 with 50% thermal
Fig. 3 A partial view of the coordination sphere of the metals in 2.
ellipsoid probability.
The structure consists of trimeric species in which three nickel
atoms are coordinated to three bis(deprotonated) Mannich bases
with four phenoxo oxygen atoms bridging the nickel atoms.
The ligands are all tetradentate but show different coordination
behavior. A first ligand acts as an N(1)N(2) chelate, an oxygen
atom O(1) acts as terminal and O(2) acts as a bridge between two
nickel atoms. A second ligand also acts as an N(3)N(4) chelate, the
The bond distances between the nickel atoms and the phenoxo
˚
oxygens are in the range 2.0053(14)–2.2386(14) A and these are
significantly different to one another, with the longest distances
corresponding to the oxygen atoms O(5) and O(4), which establish
triple bridges between the metal centers. The shortest distances are
very similar to those observed in other dinuclear and trinuclear
Ni(II) derivatives containing oxygen bridging atoms.42–46
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˚
Table 3 Selected bond distances (A) and angles (deg) for [Ni3L3] (2)
Ni(1)–O(2)
Ni(1)–O(1)
Ni(1)–O(4)
Ni(1)–N(2)
Ni(1)–N(1)
Ni(2)–O(4)
Ni(3)–O(6)
Ni(3)–N(6)
Ni(3)–N(5)
Ni(1)–Ni(3)
2.0103(14)
2.0313(14)
2.1801(13)
2.0598(17)
2.1245(16)
2.0734(13)
2.0053(14)
2.0555(17)
2.0612(17)
2.9047(5)
Ni(1)–O(5)
Ni(2)–O(3)
Ni(2)–O(6)
Ni(2)–N(4)
Ni(2)–N(3)
Ni(2)–O(5)
Ni(3)–O(2)
Ni(3)–O(5)
Ni(3)–O(4)
Ni(2)–Ni(3)
2.1956(14)
2.0195(14)
2.0452(14)
2.0480(17)
2.0676(17)
2.2386(14)
2.0241(14)
2.0582(13)
2.1939(14)
2.8195(5)
O(2)–Ni(1)–O(1)
O(2)–Ni(1)–N(2)
O(1)–Ni(1)–N(2)
O(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
N(2)–Ni(1)–N(1)
O(2)–Ni(1)–O(4)
O(1)–Ni(1)–O(4)
N(2)–Ni(1)–O(4)
N(1)–Ni(1)–O(4)
O(1)–Ni(1)–O(5)
N(2)–Ni(1)–O(5)
N(1)–Ni(1)–O(5)
O(4)–Ni(1)–O(5)
O(2)–Ni(1)–O(5)
O(6)–Ni(3)–O(2)
O(2)–Ni(3)–N(6)
O(2)–Ni(3)–O(5)
O(6)–Ni(3)–N(5)
N(6)–Ni(3)–N(5)
O(6)–Ni(3)–O(4)
N(6)–Ni(3)–O(4)
N(5)–Ni(3)–O(4)
Ni(2)–O(4)–Ni(1)
Ni(1)–O(2)–Ni(3)
Ni(3)–O(5)–Ni(1)
Ni(3)–O(5)–Ni(2)
163.18(6)
112.05(6)
84.71(6)
91.74(6)
89.58(6)
80.75(6)
79.38(5)
99.71(5)
99.93(6)
170.71(6)
83.27(5)
161.70(6)
112.86(6)
68.70(5)
80.77(5)
158.75(6)
100.23(6)
83.89(5)
94.22(6)
84.11(7)
80.00(5)
113.70(6)
160.86(6)
105.41(5)
92.10(6)
86.07(5)
81.90(5)
O(3)–Ni(2)–O(6)
O(3)–Ni(2)–N(4)
O(6)–Ni(2)–N(4)
O(3)–Ni(2)–N(3)
O(6)–Ni(2)–N(3)
N(4)–Ni(2)–N(3)
O(5)–Ni(2)–O(4)
O(3)–Ni(2)–O(4)
O(6)–Ni(2)–O(4)
O(4)–Ni(2)–N(4)
O(4)–Ni(2)–N(3)
O(3)–Ni(2)–O(5)
O(6)–Ni(2)–O(5)
N(4)–Ni(2)–O(5)
N(3)–Ni(2)–O(5)
O(6)–Ni(3)–N(6)
O(6)–Ni(3)–O(5)
N(6)–Ni(3)–O(5)
O(2)–Ni(3)–N(5)
O(5)–Ni(3)–N(5)
O(2)–Ni(3)–O(4)
O(5)–Ni(3)–O(4)
Ni(1)–O(5)–Ni(2)
Ni(1)–O(4)–Ni(3)
Ni(2)–O(4)–Ni(3)
Ni(2)–O(6)–Ni(3)
172.50(6)
87.92(6)
93.79(6)
90.54(6)
96.90(6)
83.97(7)
69.74(5)
96.64(6)
82.03(5)
174.71(6)
93.26(5)
89.25(5)
83.36(5)
113.15(6)
162.85(6)
88.34(6)
89.15(5)
174.25(6)
105.90(6)
90.91(6)
78.76(5)
70.91(5)
99.50(5)
83.22(5)
82.66(5)
88.22(5)
The Ni–N(amine) bond distances, although slightly differ-
from a tetradentate bi-deprotoned Mannich base ligand. The
coordination is completed by an oxygen atom from another similar
fragment, which acts as a bridge between metal atoms. In the
environment of Ni(1) there is an oxygen atom from a water
˚
ent to one another [Ni(1)–N(1) 2.1245(16) A, Ni(2)–N(3)
˚
˚
2.0676(17) A and Ni(3)–N(5) 2.0612(17) A], are analogous to
those found in hexacoordinated trinuclear Ni(II) complexes,46–49
e.g. in [Ni3L’2(OAc)2(NCS)2] (L’ = 2-butyliminomethyl-4-methyl-
6{[methyl-(2-pyridin-2-ethyl)amino]methyl}phenol)50 or in the
dimeric species 3 described in this work. The Ni–N(pyridine) bond
˚
molecule that, with a bond distance of 2.096(3) A, establishes
a normal covalent bond with the metal atom. Within the network
half of the cells contain a second water molecule, the oxygen atom
of which is coordinated to the Ni(2) atom with a bond distance
˚
distances, 2.0480(17)–2.0598(17) A, are the same as those found
˚
in other hexacoordinated Ni(II) complexes with ligands derived
from pyridine.51–54
In the dianionic Mannich ligand the bond distances and angles
are as expected and are similar to the distances and angles found
in other complexes that contain this ligand.
of 2.328(7) A. This indicates a weaker interaction in this case. As
such, it can be considered that Ni(1) is always hexacoordinated
while Ni(2) is hexacoordinated in only half of the cases and
pentacoordinated in the other half.
Each bridging phenoxo oxygen atom is more or less symmet-
rically bound, with Ni–O bond distances in the range between
˚
2.080(3) [Ni(1)–O(4)] and 2.034(3) A [Ni(2)–O(4)]. These distances
Structure of [Ni2L2(H2O)1.5]·CH3CN (3). In compound 3 the
water molecule O(5) located above Ni(1) is a whole and normal
water molecule. However, within the network only half of the cells
contain Ni(2) weakly coordinated by a second water molecule O(6)
and, as such, this can be considered as half a water molecule. The
molecular structure of the complex that contains the two water
molecules is shown in Fig. 4 along with the atom labeling scheme
adopted. A selection of bond lengths and angles with the estimated
standard deviations are given in Table 4.
are fairly similar to those found between the nickel and the
terminal phenoxo oxygen atoms [Ni(1)–O(1) = 2.017(3), Ni(2)–
˚
O(3) = 1.968(3) A]. Neither of these distances falls outside the
range of average Ni–O(phenoxo) distances observed in other hex-
42–46
˚
acoordinated nickel(II) complexes [1.971(2)–2.102(7) A].
nickel atoms are separated by 3.2076(13) A, a distance that is close
to those found in other similar dinuclear nickel compounds.47–49
The
˚
˚
The Ni(1)–O(5) bond distance [2.096(3) A] is as expected and
is similar to those described above. However, the Ni(2)–O(6)
The structure consists of binuclear species in which each metal
atom is coordinated to two nitrogen and two oxygen atoms
˚
bond distance [2.328(7) A] is considerably longer and this is
8650 | Dalton Trans., 2009, 8644–8656
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˚
Table 4 Selected bond lengths (A) and angles (deg) for [Ni2L2(H2O)1.5]·CH3CN (3)
Ni(1)–O(1)
Ni(1)–O(4)
Ni(1)–N(2)
Ni(1)–N(1)
Ni(2)–O(2)
Ni(2)–N(3)
Ni(1)–Ni(2)
2.017(3)
2.080(3)
2.096(4)
2.126(4)
2.035(3)
2.126(4)
3.2076(13)
Ni(1)–O(2)
Ni(1)–O(5)
Ni(2)–O(3)
Ni(2)–O(4)
Ni(2)–N(4)
Ni(2)–O(6)
2.060(3)
2.096(3)
1.968(3)
2.034(3)
2.039(4)
2.328(7)
O(1)–Ni(1)–O(2)
O(2)–Ni(1)–O(4)
O(2)–Ni(1)–O(5)
O(1)–Ni(1)–N(2)
O(4)–Ni(1)–N(2)
O(1)–Ni(1)–N(1)
O(4)–Ni(1)–N(1)
N(2)–Ni(1)–N(1)
O(3)–Ni(2)–O(2)
O(3)–Ni(2)–N(4)
O(2)–Ni(2)–N(4)
O(4)–Ni(2)–N(3)
N(4)–Ni(2)–N(3)
O(4)–Ni(2)–O(6)
N(4)–Ni(2)–O(6)
Ni(1)–O(2)–Ni(2)
172.72(11)
76.32(11)
87.37(12)
91.47(12)
100.46(13)
93.10(12)
168.41(12)
80.61(14)
94.15(12)
107.83(14)
103.71(13)
93.90(12)
81.31(14)
90.9(2)
O(1)–Ni(1)–O(4)
O(1)–Ni(1)–O(5)
O(4)–Ni(5)–O(5)
O(2)–Ni(1)–N(2)
O(5)–Ni(1)–N(2)
O(2)–Ni(1)–N(1)
O(5)–Ni(1)–N(1)
O(3)–Ni(2)–O(4)
O(4)–Ni(2)–O(2)
O(4)–Ni(2)–N(4)
O(3)–Ni(2)–N(3)
O(2)–Ni(2)–N(3)
O(3)–Ni(2)–O(6)
O(2)–Ni(2)–O(6)
N(3)–Ni(2)–O(6)
Ni(1)–O(4)–Ni(2)
98.40(11)
87.32(12)
86.75(12)
94.42(12)
172.79(14)
92.10(12)
92.36(13)
159.65(13)
77.91(11)
92.32(13)
91.97(13)
170.45(12)
69.3(2)
82.5(2)
92.9(2)
102.46(12)
173.5(2)
103.09(13)
Fig. 4 ORTEP diagram of the molecular structure of 3 with 50% thermal ellipsoid probability.
indicative of a weak interaction between the two atoms. Finally, the
Ni–N(amine) bond distances and Ni–N(pyridine) bond distances
are normal and similar to those found in the trimer complex
described above. As such these data do not warrant any further
comment.
The compound contains an acetonitrile molecule of crystal-
lization that does not interact with the cobalt complex in any
significant manner.
pyridine nitrogen and two oxygen atoms of a bis(deprotonated)
ligand. The coordination is completed by a bridging oxygen atom
from another similar fragment. The Cu2O2 bridge is essentially
planar (r.m.s. 0.0843). The bridge is clearly asymmetric, with
two longer bond distances [Cu(1)–O(2) 2.149(2), Cu(2)–O(4)
˚
2.144(2) A] and two shorter distances [Cu(1)–O(4) 1.930(2),
˚
Cu(2)–O(2) 1.950(2) A]. The two metal centers are separated by
˚
3.1342 A.
The values of 0.30 for Cu(1) and 0.25 for Cu(2) found for
the trigonality index t suggest that the metal atoms are in an
environment close to square pyramidal,34 with bridging oxygen
atoms occupying apical positions and the oxygen atoms not
involved in the bridge in a syn disposition with respect to the
Cu2O2 core. Both square pyramids share the common basal edge
defined by the bridging O2 and O4 and their bases are inverted.
Crystal Structure of [Cu2L2]·3H2O (4). A view of compound
4 is shown in Fig. 5 together with the atomic labeling scheme
used. A selection of bond distances and angles (with the estimated
standard deviations) are given in Table 5.
The structure of the complex consists of [Cu2L2] dimers in which
each metal atom is pentacoordinated by an amine nitrogen, a
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˚
Table 5 Selected bond distances (A) and angles (deg) for [Cu2L2].3H2O (4)
Cu(1)–O(4)
Cu(1)–O(1)
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–O(2)
1.930(2)
1.939(2)
2.009(3)
2.027(3)
2.149(2)
Cu(2)–O(2)
Cu(2)–O(3)
Cu(2)–N(4)
Cu(2)–N(3)
Cu(2)–O(4)
1.950(2)
1.958(2)
2.015(3)
2.046(3)
2.144(2)
O(4)–Cu(1)–O(1)
O(4)–Cu(1)–N(2)
O(1)–Cu(1)–N(2)
O(4)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(4)–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
N(2)–Cu(1)–O(2)
N(1)–Cu(1)–O(2)
Cu(2)–O(2)–Cu(1)
96.61(10)
89.69(11)
150.66(11)
168.66(10)
94.39(10)
82.00(11)
79.29(9)
104.11(9)
105.21(10)
95.40(10)
99.66(9)
O(2)–Cu(2)–O(3)
O(2)–Cu(2)–N(4)
O(3)–Cu(2)–N(4)
O(2)–Cu(2)–N(3)
O(3)–Cu(2)–N(3)
N(4)–Cu(2)–N(3)
O(2)–Cu(2)–O(4)
O(3)–Cu(2)–O(4)
N(4)–Cu(2)–O(4)
N(3)–Cu(2)–O(4)
Cu(1)–O(4)–Cu(2)
98.04(10)
91.30(11)
151.38(10)
166.43(10)
93.65(11)
81.56(12)
79.00(9)
103.00(9)
105.33(10)
91.72(10)
100.48(9)
1309–1323 cm-1 attributable to n(CO) (at 1294 cm-1 in the free
ligand), indicates that phenolic hydrogen atoms are lost during the
electrochemical synthesis and that the bis(deprotonated) ligand
is coordinated to the metal ions through both phenoxo oxygen
atoms.
The electrospray mass spectra of the compounds show the
peaks associated with dimer [M2L2]+ (m/z 867, 867 and 876,
respectively, for 1, 3 and 4 or the trimer [Ni3L3]+ (m/z 1306), with
the appropriate isotope distribution. The spectra also show peaks
associated with the loss of one ML unit from the initial species
(m/z 434, 870 and 437, respectively, for 1, 2 and 4 and the loss
of a second NiL unit in the case of the trimeric nickel complex
(m/z 433). In all cases the peak due to the free ligand at m/z 377
is also observed.
Magnetic properties. The magnetic behavior of 1, expressed in
the form of cMT and cM versus T, where cM is the molar magnetic
susceptibility and T the temperature, is depicted in Fig. 6. cMT
at 300 K is equal to 4.68 cm3 K mol-1 (cM = 1.58 ¥ 10-2 cm3 K
mol-1) and this value is consistent with two Co(II) ions in the high-
spin state (S = 3/2, g = 2.23). cMT decreases as T decreases, first
smoothly down to 75 K and then more rapidly to reach a value of
Fig. 5 ORTEP diagram of the molecular structure of 4 with 50% thermal
ellipsoid probability.
The bond lengths between the copper and oxygen atoms,
although different as a consequence of the asymmetry of the
bridge, can be considered as normal and are similar to those
found in other pentacoodinated copper(II) complexes containing
a bridging phenoxo oxygen.55 The Cu–N(amine), 2.027(3) and
˚
˚
2.046(3) A, and Cu–N(pyridine), 2.009(3) and 2.015(3) A, bond
lengths, respectively, are similar and are within the normal range
observed in other pentacoordinated copper(II) complexes of
amine55 and/or pyridine33 ligands.
This compound crystallizes with three non-coordinated water
molecules. One of these molecules, O(1 s), is hydrogen-bonded to
the two phenoxo oxygen atoms not involved in the Cu2O2 bridge,
O(1) and O(3). A second molecule, O(2 s) is hydrogen-bonded to
one of the phenoxo oxygen atoms not involved in the Cu2O2 bridge,
O(3). Finally, the third molecule, O(3 s) is hydrogen-bonded to the
water molecule O(2 s).
IR spectroscopy and mass spectrometry. In the IR spectra
of complexes, the n(O–H) (at 3350–3100 cm-1) and d(OH) (at
1241 cm-1) vibrations corresponding to the free ligand are not
observed. This fact, along with the presence of a band at
Fig. 6 Magnetic behavior of 1.
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0.25 cm3 K mol-1 at 2 K. cM increases continuously from 1.58 ¥ 10-2
at 300 K to reach a maximum, 0.182 cm3 mol-1, at ca. 10 K. Below
this temperature cM decreases rapidly. This behavior, reminiscent
of an intramolecular antiferromagnetic interaction between two
Co(II) ions, has been analyzed using an expression of cM based
on the isotropic Hamiltonian H = -JSASB with SA = SB = 3/2.56
The best fit for calculated and experimental data (solid line in
Fig. 6) was achieved for J = -4.2 cm-1, g = 2.26 with R = 8 ¥
10-4. R corresponds to the agreement factor, which is defined as
Ri[(cM)iexptl - [(cM)icalc]2/[(cM)iexptl]2).
The cMT vs. T plot for 2 is shown in Fig. 7. At 297 K, the cMT
value of 3.39 cm3 K mol-1 is consistent for three Ni(II) ions with
S = 1 (g = 2.13), in agreement with the structural data, which
confirm the presence of [NiII]3 trinuclear species. cMT decreases
smoothly down to 50 K, then decreases more rapidly to reach a
value of 0.86 cm3 K mol-1 at 2 K, denoting the occurrence of an
antiferromagnetic interaction.
of the magnetic susceptibility to improve the quality of the fit,
usually, it is associated to the intermolecular interactions but here
implicitly reflects the occurrence of zero-field splitting of the Ni(II)
S = 1 ground state.
The cMT vs. T plot for 3 is shown in Fig. 8. cMT is equal to
2.05 cm3 K mol-1 at 297 K, a value close to that expected for the
contribution of two S = 1 spin centers without orbital contribution
(g = 2). On cooling, cMT decreases continuously almost to zero at
2 K while cM increases from 6.9 ¥ 10-3 cm3 mol-1 at 297 K to reach
a maximum value of 2.13 ¥ 10-2 cm3 mol-1 at 39 K. Below 39 K,
cM decreases again to reach a minimum at ca. 9 K, from which a
new increase is observed.
Fig. 8 Magnetic behavior of 3.
The behavior described above is consistent with the occurrence
of antiferromagnetic coupling between the two S = 1 centers and
the existence of a small paramagnetic impurity. In this case, the
occurrence of zero-field splitting in the S = 1 local spin state was
explicitly taken into account as reported by Ginsberg et al.57 The
best fit for calculated and experimental data was achieved for J =
-30 cm-1, D = 4.7 cm-1 (the zero-field splitting parameter), g =
2.14, r (paramagnetic impurities) = 0.05 and R = 2 ¥ 10-3. The
calculated values are represented as a solid line in Fig. 8.
Fig. 7 Magnetic behavior of 2.
This magnetic behavior was analyzed using a spin Hamiltonian
H = -J(SA1SB + SA2SB) - J’(SA1
SA2) characterized by coupling
constants J and J’, which correspond to two identical pathways
Ni(A)–Ni(B) and Ni(A)–Ni(A, respectively (see Scheme 3).56
The cMT vs. T plot for 4 is shown in Fig. 9. At 300 K, cMT
has a value close to 0.83 cm3 K mol-1, which is in the range
expected for two Cu(II) ions (g = 2.10). In the temperature
Scheme 3 View of the Ni3 core of compound 2 indicating the magnetic
coupling pathway.
The best fit for calculated and experimental cMT data was
achieved for J = -5.3 cm-1, J’ =–2.7 cm-1, g = 2.16, q = -0.05 cm-1
and R = 3 ¥ 10-4. The parameter q was introduced in the expression
Fig. 9 Magnetic behavior of 4.
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range 300–50 K, cMT remains almost constant while below 50 K
it decreases rapidly down to 0.06 cm3 K mol-1, suggesting the
occurrence of very weak antiferromagnetic interactions. These
is determined by the dihedral angle defined by the intersection
of two connected Ni(II) octahedral basal planes (around 140◦).
Departure of this angle from 180◦ reduces drastically the overlap
between the orbitals containing the unpaired electrons. This
structural parameter could account for the magnitude and even
the sign of the coupling constant J involving the Ni(1)–Ni(3) and
Ni(2)–Ni(3) pairs. A more definitive correlation between J and the
structure of 2 would be expected, however, the available number
of magneto-structurally characterized phenoxo-bridged trinuclear
Ni(II) complexes is still small.
magnetic data were analyzed using the isotropic Heisenberg
1
2
interaction with SA = SB
=
described for 1. The resulting cM
versus T expression, the so-called Bleaney–Bowers expression, was
implemented to take into account the paramagnetic impurities
r. The parameters determined by least-squares fit minimizing J,
g, and r are -4.7 cm-1, g = 2.12, r = 0.08 and R = 8 ¥ 10-4.
Temperature-independent paramagnetism (TIP) was considered
equal to 120 ¥ 10-6 cm3 mol-1.
Catalase-like activity. The catalytic decomposition of H2O2
was studied as a model reaction for the catalase. These studies
on disproportionation of the H2O2 into H2O and O2 by the
synthesized compounds were carried out under physiological
The antiferromagnetic behavior observed for 1, 3 and 4 is
consistent with their structural characteristics. These compounds
have quite similar structures, however, they display remarkably
different J values. It is well-known that direct comparison between
the magnetic coupling values of 1, 3 and 4 cannot be made because
Co(II), Ni(II) and Cu(II) ions differ in the number, n, of unpaired
electrons. Consequently, the expected J values can be related as
follows: J[Cu(II)] ª n2J[Ni(II) or Co(II)].56 Taking into account
that the experimental J value is -30 cm-1 for 3, the expected
values for 1 and 4 would be -13 and -120 cm-1, respectively, if
the molecular geometries of 1 and 4 were exactly the same as
for 3. The large differences between these values and those found
experimentally, -4.2 cm-1 (1) and -4.7 cm-1 (4), can be ascribed
to structural factors. The coordination geometry of Ni(II) in 3
clearly facilitates a larger overlap between the magnetic orbitals
through the bridges, which also define favorable angles Ni(1)–
O(2)–Ni(2) = 103.09(13)◦ and Ni(1)–O(4)–Ni(2) = 102.46(12)◦
for strong antiferromagnetic interaction. The geometry around
Co(II) in 1 is much more distorted and hence less appropriate for
magnetic exchange. In addition, the slightly smaller angles Co(1)–
O(1)–Co(2) = 101.86(5)◦ and Co(1)–O(3)–Co(2) = 99.75(5)◦ with
respect to 3, justify a smaller overlap of the magnetic orbitals at the
bridge. In the case of 4 the reason for the small experimental J value
R
conditions (see experimental part) by the Amplexꢀ Red-HRP
method.23,24
The synthesized compounds themselves were unable to directly
R
react with the Amplexꢀ Red reagent, which indicates that these
compounds do not interfere with the measurements. All of the
tested complexes 1–4 (1–25 mM) exhibited a significant catalase-
like activity in a concentration-dependent manner (Fig. 10).
Analysis of the data was carried out based on the Michaelis–
Menten model, which was originally developed for enzyme
kinetics. From the Lineweaver–Burk plots a number of parameters
were calculated and are listed in Table 5. Similar results have
been reported previously for a number of different inorganic
complexes.60–63
2
2
is essentially misorientation of the orbitals, dx , containing the
–y
unpaired electrons. Indeed, the coordination geometry of 4 favors
2
2
2
overlapping between the dx
and dz , and consistently with the
–y
2
structure it is expected for the dz orbital to bear very small spin
density.
Concerning the trinuclear complex 2, the antiferromagnetic
interaction observed between the triple-bridged peripheral and the
central Ni(II) ions [Ni(1)–Ni(3) and Ni(2)–Ni(3)] (J = -5.3 cm-1)
could be considered an unexpected result at first glance. The angles
Ni(1)–O–Ni(3) and Ni(2)–O–Ni(3), found in the interval 82–92◦
(Table 3), are in the range observed for related relevant examples in
which weak-medium ferromagnetic interaction occurs.58 However,
unlike 2, all the ferromagnetically coupled compounds referred
above are constituted of linear trinuclear Ni(II) species (except 58
d). This fact together with the strongly distorted Ni coordination
sites may be the cause of the sign of J found in 2. As far as
we know, only one phenoxo-bridged trinuclear Ni(II) complex
exhibiting antiferromagnetic interaction has been reported up
to now. In this linear trinuclear Ni(II) compound the larger
Ni–O–Ni angles, found in the 99.7–100.5◦ range, account for the
observed antiferromagnetic coupling between adjacent Ni(II) ions
(J ª -16 cm-1).59 These angles are close to those observed for the
doubly linked Ni(1) and Ni(2) ions in 2. The smaller magnitude of
the coupling constant (J’ = -2.7 cm-1) found in the latter could be
ascribed to the strongly bended Ni(1)O(4)O(5)Ni(2) moiety, which
Fig. 10 Effects of synthesized compounds (1–25 mM) on the degradation
of H2O2. Each bar represents the mean S.E.M. (indicated by vertical
bars) for five experiments. Experiments were carried out in a total volume
of 1 mL.
The Kcat values for all compounds were very similar and ranged
between 7.49 ¥ 10-3 2.67 ¥ 10-4 s-1 and 2.67 ¥ 10-2 0.89 ¥
10-3 s-1 (Table 6). In addition, the dimeric nickel(II) complex 3
was the most effective in degrading H2O2 since the corresponding
Kcat value was significantly higher than the corresponding Kcat
values for compounds 1, 2 and 4. Although these Kcat values
were relatively low, the corresponding catalytic efficiency (Kcat/Km)
values were higher, as shown in Table 6. All of these kinetics
correlated well with the Relative Catalase Activity (RCA), which
was calculated for comparative purposes as indicated in the
experimental part. The RCA values for all compounds were very
close and ranged between 19.15 0.86 mM and 27.70 1.49 mM
8654 | Dalton Trans., 2009, 8644–8656
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Table 6 Parameters of catalase-like activity for 1–4 compounds
[comp]
[H2O2]
V
Vmax
Kcat
K
cat/Km
RCA
Complex
(mM)
(mM)
(nmoles H2O2 degraded/min)
(nmoles H2O2 degraded/min)
102(s-1)
(M-1s-1)
(mM)
1
1
10
25
25
25
100
100
100
400
25
1.86 0.19
4.12 0.62
9.29 0.84
12.80 0.75
3.32 0.36
14.71 1.68
13.33 0.86
40.00 1.95
11.24 1.30
0.98 0.07
0.89 0.05
2.67 0.09*
0.75 0.03
169.46 11.76
227.40 6.34
129.95 4.52*
367.32 26.43
26.85 0.97
27.70 1.49
19.15 0.86*
24.42 1.27
2
3
4
1
10
25
25
25
100
100
100
400
25
1.86 0.15
5.11 0.48
8.82 0.62
13.22 0.98
5.24 0.44
1
10
25
25
25
100
100
100
400
25
2.32 0.06
5.21 0.21
12.54 0.64
28.08 1.03
4.35 0.05
1
10
25
25
25
100
100
100
400
25
0.46 0.12
6.50 0.74
9.27 1.23
10.61 0.98
7.96 0.87
Experiments were carried out in a total volume of 1 mL. Each value is the mean S.E.M. for five experiments. Level of statistical significance: *P < 0.05
with respect to the RCA, Kcat and Kcat/Km values of the compounds 1, 2 and 4, as determined by ANOVA/Dunnett’s.
(Table 6). In addition, the RCA value for compound 3 was slightly,
but significantly, lower (P < 0.05) than the corresponding RCA
values for compounds 1, 2 and 4, which indicates that compound
1999, 38, 428–447.
3 exhibits the highest catalytic activity.
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The electrochemical oxidation of a metal anode (cobalt, nickel or
copper) in a cell containing the potentially tetradentate Mannich
base ligand allowed us to synthesize neutral complexes with high
purities and good yields. The cobalt and copper complexes are
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with the dimeric nickel(II) the most efficient.
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This work was supported by the Ministerio de Ciencia y
Tecnologia (Spain) (grants CTQ2006-05298 and CTQ 2007-
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FISS PI061537) and by the Xunta de Galicia (Spain) (grants
PGIDIT07PXIB-203038PR and PGIDIT05BTF20302PR).
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