Metal complexes of a novel heterocyclic benzimidazole ligand formed by rearrangement-cyclization of the corresponding Schiff base. Electrosynthesis, structural characterization and antimicrobial activity

Article abstract of DOI:10.1039/c8dt00532j

The electrochemical oxidation of anodic metals (M = cobalt, nickel, copper, zinc and cadmium) in a solution of the ligand 1H-anthra[1,2-d]imidazol-6,11-dione-2-[2-hydroxyphenyl] [H₂L] afforded homoleptic [ML] compounds. The addition to the elec

Full text of DOI:10.1039/c8dt00532j

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Metal complexes of a novel heterocyclic
benzimidazole ligand formed by rearrangement-
cyclization of the corresponding Schiff base.
Electrosynthesis, structural characterization and
antimicrobial activity†
Cite this: DOI: 10.1039/c8dt00532j
I. Casanova,a M. L. Durán, *a J. Viqueira,a A. Sousa-Pedrares, a F. Zani,b
a
J. A. Realc and J. A. García-Vázquez
*
The electrochemical oxidation of anodic metals (M = cobalt, nickel, copper, zinc and cadmium) in a solu-
tion of the ligand 1H-anthra[1,2-d]imidazol-6,11-dione-2-[2-hydroxyphenyl] [H2L] afforded homoleptic
[ML] compounds. The addition to the electrochemical cell of coligands (L’) such as 2,2’-bipyridine (bpy) or
1,10-phenanthroline (phen) allowed the synthesis, in one step, of heteroleptic [MLL’] compounds. The
crystal structures of H2L (1), [CoL(MeOH)]2 (2), [CoL(phen)]2 (3), [NiL(bpy)]2 (4), [CuL(bpy)] (5), [CuL(phen)]
(6) and [CdL(bpy)]2 (7) have been determined by X-ray diffraction techniques. The crystal structures of 2,
3, 4 and 7 consist of dimeric species in which both metallic atoms are connected through two phenolate
bridges in a penta-coordinated (2) or hexa-coordinated (3, 4 and 7) environment. Copper compounds 5
and 6 are monomeric species with the metal in a pentacoordinated [N4O] environment. In all the com-
pounds, the main interactions responsible for the crystal packing are classic (N–H⋯O, O–H⋯N and
O–H⋯O) and non-classic (C–H⋯O and C–H⋯N) hydrogen bond interactions, and π interactions
(π–π-stacking and C–H⋯π). All compounds were also characterized by microanalysis, IR spectroscopy,
1
FAB mass spectrometry and H NMR spectroscopy. Magnetic susceptibility data were measured for 2–4
over the temperature range 2–300 K, and their analysis has revealed the occurrence of intramolecular
antiferromagnetic coupling for 2 (J = −2 cm−1) and ferromagnetic coupling for 3 (J = 7.8 cm−1) and 4
(J = 2.8 cm−1) [J being the isotropic magnetic coupling parameter]. The nature of the magnetic coupling
in 2–4 is correlated with the magnitude of the M–Ophenolate–M angle between the phenolate bridge and
the metallic centers [M(II) = Co, Ni]. The in vitro antimicrobial properties of the novel ligand and its metal
complexes were detected against Gram positive and Gram negative bacteria and fungi. [NiL(bpy)]2 and all
tested Cd(II) complexes were the most active compounds, showing the highest inhibitory effect against
bacilli (MIC 1.5–3 µg mL−1) and Sarcina, Streptococci and Haemophilus influenzae bacterial strains (MIC
12–50 µg mL−1), while almost no antifungal properties were observed.
Received 8th February 2018,
Accepted 13th February 2018
DOI: 10.1039/c8dt00532j
rsc.li/dalton
their ability to form dinuclear complexes which are of great
importance not only in the development of new catalysts,1 but
Introduction
The coordination chemistry of dinucleating ligands has been also for their use for modelling the chemical environment in
widely investigated. The interest in these ligands arises from many metalloenzymes and metalloproteins that contain two
metal ions in their active site.2–4 Dinuclear metalloproteins are
very common in nature and despite their variety of functions,
most of them share important structural characteristics.
Regardless of the nature of the metal atoms, in the majority of
dinuclear metalloenzymes and metalloproteins the metals are
bridged by at least one oxo species (aqua, hydroxido, oxido
ligands), derived from exogenous water.5 Thus, for example,
the di-iron enzyme methane monooxygenase, which selectively
oxidises methane to methanol using dioxygen,6 and manga-
nese catalase, a dinuclear manganese containing enzyme
aDepartamento de Química Inorgánica, Universidad de Santiago de Compostela,
15782 Santiago de Compostela, Spain. E-mail: josearturo.garcia@usc.es;
Fax: +34-(9)81547102
bDepartamento di Farmacia, Parco Area delle Scienze, 27/A, 43124 Parma, Italy
cInstitut de Ciencia Molecular Departament de Química Inorgánica, Universitat de
Valencia, 46071 Valencia, Spain
†Electronic supplementary information (ESI) available. CCDC 1584238–1584244.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt00532j
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which decomposes hydrogen peroxide,7 present an oxo-bridge
and a carboxylate bridge between the metal atoms, which is
also very common in dinuclear metalloproteins. Examples of
di-copper enzymes are tyrosinase,8 which uses dioxygen in the
hydroxylation of monophenols to o-diphenols and for the two-
electron oxidation of o-diphenols to o-quinones, and cathechol
oxidase, which catalyzes the oxidation of catechols to quinones
without acting on tyrosine.9 Both of them present a dinuclear
active site with three hystidine ligands per copper atom and an
additional µ-hydroxo bridge between the metal ions.10 Urease,
a dinuclear nickel enzyme which catalyzes the hydrolysis of
urea to produce ammonia and carbamate,11,12 presents a water
bridge and a carboxylate bridge between the two nickel atoms.
Similarly, the active site of metalloprotease methionine amino-
peptidase contains two cobalt ions bridged by two carboxylate
groups from the protein and by a hydroxyl group (or a water
molecule).13 The same type of water and carboxylate bridges
between metal ions can be found in various hydrolytic zinc-
enzymes that mediate the cleavage of amides, phosphates,
antibiotics (β-lactams), and other biologically important
substrates.12,14
Scheme 1
presence in the electrolytic cell of the ligand H2L and
additional coligands, such as 2,2′-bipyridine or 1,10-phenan-
throline, allows the synthesis of heteroleptic compounds in
one step. This method has been successfully used for the syn-
thesis of metal compounds with other ligands having weak
acid groups, such as thiol,26,27 NH28 or hydroxyl groups.18,29
The new ligand and its corresponding metal complexes were
investigated for their antimicrobial properties against some
types of Gram positive and Gram negative bacteria, yeasts and
mould.
The presence of oxygen-donor bridges between the metal
atoms in most natural di-metallic proteins has inspired the
production of small-molecule biomimetic dinuclear complexes
with bridging oxygen groups, mainly carboxylate15 and phe-
noxo15c,d,16 bridges, although different bridging N-donors have
also been explored.15b,17 The phenolate bridge, in particular,
with a strong σ- and π-donating oxygen atom, is an excellent
bridging group that gives rise to dinuclear complexes with
Experimental
General
short metal–metal distances, which is important to mimic the Cobalt, nickel, copper, zinc and cadmium (Aldrich Chemie)
behavior of some metalloenzymes, like di-copper catechol were used as plates (ca. 2 × 2 cm). All other reagents, including
oxidase.8
solvents, 2,2′-bipyridine, 1,10-phenanthroline, 1,2-diamino-
Besides their biological interest, the dinuclear complexes anthraquinone and salicylaldehyde, were commercial products
with phenolate bridging ligands between paramagnetic metal and were used as supplied.
ions often show interesting magnetic properties. In particular,
dinucleating ligands that contain potentially bridging microanalyzer. IR spectra were recorded as KBr mulls on a
phenoxo oxygen and nitrogen donor atom sets have been Bruker IFS-66 V spectrophotometer. The 1H spectra of the
widely used in the synthesis of dinuclear complexes of ligand and complexes were recorded on Bruker WM
Microanalyses were performed using a PerkinElmer 240B
a
a
copper,18,19 manganese,20 cobalt,21 iron22 and zinc.21 As in 350 MHz spectrometer using the DMSO-d6 solvent. The chemi-
other hydroxo- and alkoxo-bridged metal complexes, the cal shifts were recorded against TMS as the internal standard.
nature of the magnetic interactions in phenoxo-bridged metal FAB mass spectra were recorded on a Micromass Autospec
systems is primarily determined by the M–O–M angle and the instrument, using 3-nitrobenzyl alcohol (3-NBA) as the matrix
M⋯M separation.23 The understanding of the magnetic pro- material. Variable temperature magnetic susceptibility
perties of such dinuclear species is important in order to measurements were carried out using microcrystalline
explain, for example, the role of these metal ions in living samples (20–60 mg) of compounds 2–4, using a Quantum
systems.4,24
Design MPMS2 SQUID susceptometer equipped with a 5.5 T
For these reasons, and as a result of our continuing interest magnet, operating at 0.1–0.5 T and at temperatures from
in the chemistry of dinuclear metal complexes, we now report 300–1.8 K. Experimental susceptibilities were corrected for
the synthesis of metal(II) complexes with the 1H-anthra[1,2-d] diamagnetism of the constituent atoms by the use of Pascal’s
imidazol-6,11-dione-2-[2-hydroxyphenyl] ligand [H2L], which constants.
has the ability to form dinuclear metal species through oxygen
Synthesis
of
1H-anthra[1,2-d]imidazole-6,11-dione-2-[2-
atoms as bridges between two metal atoms. Due to the pres- hydroxyphenyl] (H2L). The ligand (H2L) was synthesized by
ence in the potential dinucleating ligand of weakly acidic N–H reacting 1,2-diaminoanthraquinone (1.0 g, 2.93 mmol) in
and O–H groups (Scheme 1), the homoleptic complexes were absolute ethanol (175 mL) with salicylaldehyde (0.44 mL,
prepared using an electrochemical procedure where the metal 4.10 mmol) using p-toluenesulfonic acid (30 mg) as the catalyst
is the anode of a cell containing the ligand in solution.25 The (Scheme 2). The reaction mixture was heated to reflux with
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0.49 mol F−1. At the end of the experiment, the brown solid
obtained was isolated, washed with ether and dried. Yield:
0.0518 g (0.120 mmol, 65%). Crystals of [CoL(MeOH)]2 (2) suit-
able for X-ray studies were obtained by crystallization of the
initial product from MeOH : CH2Cl2 (1 : 1). Anal. found: C,
61.25; H, 2.93; N 6.76%. Calc. for C22H14CoN2O4 (429.02): C,
61.53; H, 3.29; N, 6.53. IR (KBr, cm−1): 1663(s), 1594(s),
1301(s), 717(s). MS(FAB), m/z; 662 [Co2L2 − N2CHPhO], 646
[Co2L2 − N2CHPhO2], 398 [CoL]. UV-visible (cm−1): 7280,
10 870, 14 900, 17 550.
Synthesis of [CoL(bpy)]. A similar experiment to those
described above (8 V, 10 mA, 1 h) with H2L (0.0634 g,
0.186 mmol) and 2,2′-bipyridine (0.030 g, 0.186 mmol) in
acetone (50 cm3) led to the dissolution of 11.4 mg of metal,
Ef = 0.52 mol F−1. The crystals deposited in the cell were iso-
lated, washed with acetonitrile and ether and dried under
vacuum. Yield: 0.0981 g (0.161 mmol, 87%). The brown solid
was characterised as [CoL(bpy)]·3H2O. Anal. found: C, 60.77; H,
3.83; N, 8.98%. Calc. for C31H24CoN4O6 (607.49): C, 61.29; H,
3.98; N, 9.22. IR (KBr, cm−1): 1651(m), 1324(s), 1292(s), 763(s),
722(w). MS(FAB), m/z 1107 [Co2L2(bpy)2], 950 [Co2L2(bpy)], 554
[CoL(bpy)], 398 [CoL]. UV-visible (cm−1): 7930, 15 720, 17 420.
Synthesis of [CoL(phen)]. A solution of the ligand (0.0634 g,
0.186 mmol) and 1,10-phenanthroline (0.033 g, 0.186 mmol)
in acetone (50 cm3) was electrolyzed at 10 V and 10 mA for 1 h
and 10.6 mg of metal was dissolved from the anode, Ef =
0.48 mol F−1. At the end of the reaction the purpura solid
obtained was filtered, washed with acetone and dried under
vacuum. Yield: 0.0782 g (0.126 mmol, 73%). Crystals of
[CoL(phen)] (3) suitable for X-ray studies were obtained from
the mother liquor. Anal. found: C, 68.13; H 3.07; N, 9.30%.
Calc. for C33H18CoN4O3 (577.07): C, 68.62; H, 3.14; N, 9.71%. IR
(KBr, cm−1): 1668(s), 1518(s), 1329(s), 1292(s), 841(w), 717 (m).
Scheme 2
constant stirring, changing color from yellow to brown. The
water produced during the synthesis process was periodically
purged from the Dean–Stark trap. The reaction was followed
by thin layer chromatography, using a mixture of hexane/ethyl
acetate (5 : 1) as the eluent, until the complete disappearance
of the starting products. Thus, after 12 hours the heating was
stopped and the solution was cooled to room temperature. The
greenish precipitate formed was filtered, washed with absolute
ethanol and diethyl ether and dried under vacuum. Yield:
1.20 g (3.53 mmol, 84%). Suitable crystals for X-ray diffraction
were obtained by crystallization of the compound from a
1 : 1 mixture of MeOH and CH2Cl2. Anal. found: C, 73.72; H,
3.59; N, 8.26. Calc. for C21H12N2O3 (340.3): C, 74.11, H 3.55, N
8.23. IR (KBr, cm−1): 3354(s); 1666 (s), 1629 (w), 1589(s),
1523(s), 1478(s), 1327(m), 1295(s). 1H NMR (DMSO-d6, ppm):
12.8 (s, 1H), 8.4 (s, 1H), 8.2 (s, 2H), 8.1 (s, 2 H), 7.8 (s, 2H), 7.6
(t, 1H), 6.9 (t, 2H), 4.1 (s, 1H). MS (FAB), m/z: 340 [H2L].
Electrochemical synthesis
The complexes were obtained following an electrochemical MS(FAB), m/z 1155, [Co2L2(phen)2], 975 [Co2L2(phen)], 578
procedure.25 The cell consisted of a 100 mL tall-form beaker [CoL(phen)], 398 [CoL]. UV-visible (cm−1): 7450, 15 650, 17 513.
fitted with a rubber bung through which the electrochemical
leads entered. A solution of either the ligand or the ligand/coli- (0.0634 g, 0.186 mmol) in acetone (40 mL) at 9.2 V and 10 mA
gand (1,10-phenanthroline or 2,2′-bipyridine) mixture contain- for 1 h dissolved 10.2 mg of nickel from the anode, Ef
Synthesis of [NiL]. Electrolysis of a solution of the ligand
=
ing about 10 mg of tetraethylammonium perchlorate as a 0.47 mol F−1. At the end of the experiment, the blue solution
current carrier was electrolyzed using a platinum wire as the obtained was concentrated under vacuum and the blue solid
cathode and a metal plate suspended from another platinum obtained was isolated, washed with acetonitrile and ether and
wire as the sacrificial anode (Caution: perchlorate compounds dried under vacuum. Yield: 0.0706 g (0.163 mmol, 88%). The
are potentially explosive and should be handled in small quan- red solid was characterised as [NiL]·2H2O. Anal. found: C,
tities and with great care). Direct current was supplied by 58.48; H, 3.16; N, 6.07. Calc. for C21H14NiN2O5 (432.02): C,
using a purpose-built d.c. power supply. Applied voltages of 58.33; H, 3.27; N, 6.48; IR (KBr, cm−1): 1665(s), 1558(s),
5–15 volts allowed sufficient current flow for smooth dis- 1294(s), 714(s). MS(FAB), m/z; 660[Ni2L2 − N2CHPhO], 644
solution of the metal. In all cases, hydrogen was evolved at the [Ni2L2 − N2CHPhO2], 398 [NiL]. UV-visible (cm−1): 9025, 15 530.
cathode. The cell can be summarized as Pt(−)/solvent + H2L +
Synthesis of [NiL(bpy)]. A solution of the ligand (0.0634 g,
L′/M(+), where H2L represents the quinone ligand, L′ is the 0.186 mmol) and 2,2′-bipyridine (0.030 g, 0.186 mmol) in
additional ligand and M is either Co, Ni, Cu Zn or Cd. After acetone (50 cm3) was electrolyzed at 11 V and 10 mA for 1 h;
electrolysis, the crystalline solids obtained were filtered, 10.3 mg of nickel metal was dissolved from the anode, Ef =
washed with acetonitrile and diethyl ether and dried under 0.47 mol F−1. At the end of the experiment, the compound
vacuum.
obtained as a solid at the bottom of the cell was isolated,
Synthesis of [CoL]. Electrolysis of a solution of the ligand washed with acetone and ether and dried under vacuum.
(0.0634 g, 0.186 mmol) in acetone (50 cm3) at 12 V and 10 mA Yield: 0.0954 g (0.167 mmol, 90%). The red solid was charac-
for 1 h dissolved 10.7 mg of cobalt from the anode, Ef
=
terised as [NiL(bpy)]·H2O. Anal. found: C, 64.82; H, 3.43; N,
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9.57%. Calc. for C31H20NiN4O4 (571.21): C, 65.17; H, 3.53; N,
Synthesis of [ZnL]. A solution of the ligand (0.1268 g,
9.81%. IR (KBr, cm−1): 1665(s), 1326(s), 1294(s), 754(s), 714(s). 0.373 mmol) in acetonitrile was electrolysed at 10 mA for two
MS(FAB), m/z 1107[Ni2L2(bpy)2], 951[Ni2L2(bpy)], 553 hours, dissolving 23.6 mg of the zinc from the anode, Ef =
[NiL(bpy)], 397 [NiL]. UV-visible (cm−1): 8860, 15 800, 20 400. 0.48 mol F−1. At the end of the electrolysis, the solid obtained
Crystallization of the compound from methanol produced was recovered, washed with diethyl ether and dried under
crystals of the methanol solvate suitable for X-ray studies, vacuum. Yield: 0.113 g (0.28 mmol, 75%). The brown solid was
[NiL(bpy)]2·2.25(MeOH)·0.5(H2O) (4).
characterised as [ZnL]. Anal. found: C, 63.02; H, 2.93; N,
Synthesis of [NiL(phen)]. A solution of the ligand (0.0634 g, 7.06%. Calc. for C21H10ZnN2O3 (403.71): C, 62.48; H, 2.50; N,
0.186 mmol) and 1,10-phenanthroline (0.033 g, 0.186 mmol) 6.94%. IR (KBr, cm−1): 1667(s), 1590(m), 1327(s), 717(s).
in a 1 : 1 mixture of methanol and benzene (40 mL) was elec-
Synthesis of [ZnL(bpy)]. A solution of the ligand (0.1268 g,
trolysed at 10 mA for 1 h., dissolving 10.7 mg of the metal 0.373 mmol) and 2,2′-bipyridine (0.0582 g, 0.373 mmol) in
from the anode, Ef = 0.48 mol F−1. At the end of the reaction methanol (40 mL) was electrolysed for 2 hours at 10 mA, dis-
the violet solid obtained was filtered, washed with acetone and solving 25.3 mg of metal, Ef = 0.51 mol F−1. The insoluble
dried under vacuum. Yield: 0.0903 g (0.143 mmol, 77%). The solid obtained was filtered, washed with diethyl ether and
solid was characterised as [NiL(phen)]·3H2O. Anal. found: C, dried under vacuum. Yield: 0.173 g (0.31 mmol, 80%). The
62.38; H, 3.68; N, 8.76%. Calc. for C33H24NiN4O6 (631.27): C brown solid was characterised as [ZnL(bpy)]. Anal. found: C,
62.79; H, 3.83; N, 8.88%. IR (KBr, cm−1): 1647(m), 1515(m), 66.34; H, 3.28; N, 9.87%. Calc. for C31H18ZnN4O3 (559.89): C,
1324(m), 841(m), 717(s). MS(FAB), m/z 1155 [Ni2L2(phen)2], 66.50; H, 3.24; N, 10.01% IR (KBr, cm−1): 1660(s), 1326(m),
975 [Ni2L2(phen)], 577[NiL(phen)], 397 [NiL]. UV-visible 1293(s), 755(m), 723(s). MS(FAB), m/z 1119 [Zn2L2(bpy)2], 965
(cm−1): 7660, 9025, 15 620, 17 820.
[Zn2L2(bpy)], 809 [Zn2L2], 403 [ZnL].
Synthesis of [CuL]. Electrolysis of a solution of the ligand
Synthesis of [ZnL(phen)]. A solution of the ligand (0.1268 g,
(0.1268 g, 0.373 mmol) in methanol (40 mL) for 1 h at 10 mA 0.373 mmol) and 1,10-phenanthroline (0.0671 g, 0.373 mmol)
dissolved 23.5 mg of copper, Ef = 0.99 mol F−1. The brown in methanol (40 mL) was electrolysed for 2 hours at 10 mA, dis-
solid formed was filtered, washed with methanol and diethyl solving 24.9 mg of zinc, Ef = 0.51 mol F−1. The insoluble solid
ether and dried under vacuum. Yield: 0.126 g (0.30 mmol, obtained was filtered, washed with diethyl ether and dried
80%). The solid was characterised as [CuL]·H2O. Anal. found: under vacuum. Yield: 0.176 g (0.28 mmol, 76%). The reddish
C, 60.31; H, 3.00; N, 6.94%. Calc. for C21H12CuN2O4 (419.88): solid was characterised as [ZnL(phen)]·2H2O. Anal. found: C,
C, 60.07; H, 2.88; N, 6.67%. IR (KBr, cm−1): 1648(s), 1601(m), 63.97; H, 3.58; N, 8.94%. Calc. for C33H22ZnN4O5 (619.94): C,
1314(s), 718(s). MS(FAB), m/z 402 [CuL]. UV-visible (cm−1): 63.93; H, 3.58; N, 9.04% IR (KBr, cm−1): 1663(m), 1517(m),
14 900, 16 300.
1320(m), 1293(s), 844(m), 722(m). 1H NMR (DMSO-d6, ppm):
Synthesis of [CuL(bpy)]. An acetone solution of the ligand 8.3–6.50 (m, 10H), 8.75 (d, 2H), 8.73 (d, 2H), 8.54 (d, 2H), 8.58
(0.1268 g, 0.373 mmol) and 2,2′-bipyridine, (0.0582 g, (s, 2H).
0.373 mmol) was electrolysed at 10 mA for 1 hour, dissolving
Synthesis of [CdL]. A solution of the ligand (0.1268 g,
24.2 mg of metal from the anode, Ef = 1.02 mol F−1. The brown 0.373 mmol) in acetonitrile (40 mL) was electrolysed at 10 mA
solid obtained at the end of the reaction was filtered, washed for 2 h, dissolving 40.7 mg of cadmium from the anode, Ef =
with ether and dried under vacuum. Yield: 0.173 g (0.31 mmol, 0.48 mol F−1. At the end of the experiment, the violet solid de-
83%). The solid was characterised as [CuL(bpy)]. Anal. found: posited at the bottom of the cell was filtered, washed with
C, 66.40; H, 3.54; N, 9.58%. Calc. for C31H18CuN4O3 (558.06): diethyl ether and dried under vacuum. Yield: 0.146
g
C, 66.72; H, 3.25; N, 10.04%. IR (KBr, cm−1): 1666(s), 1327(m), (0.32 mmol, 87%). The solid was characterised as [CdL]. Anal.
1295(s), 770(w), 715(s). MS(FAB), m/z 402 [CuL], 341 [L]. UV- found: C, 55.58; H, 2.50; N, 6.34%. Calc. for C21H10CdN2O3
visible (cm−1): 15 720, 22 300. Crystals of [CuL(bpy)]·MeOH (5) (450.73): C, 55.96; H, 2.24; N, 6.22%. IR (KBr, cm−1): 1667(s),
suitable for X-ray studies were obtained by crystallization from 1590(s), 1327(m), 714(s). 1H NMR (DMSO-d6, ppm): 8.3–6.7 (m,
methanol.
10H).
Synthesis of [CdL(bpy)]. A solution of the ligand (0.1268 g,
Synthesis of [CuL(phen)]. A solution of the ligand (0.1268 g,
0.373 mmol) and phenanthroline (0.0671 g, 0.373 mmol) was 0.373 mmol) and 2,2′-bipyridine (0.0582 g, 0.373 mmol) in
electrolysed at 10 mA for 1 hour, dissolving 23.3 mg of metal methanol (40 mL) was electrolysed at 10 mA for 2 h, dissolving
from the anode, Ef = 0.98 mol F−1. At the end of the reaction 41.1 mg of metal dissolved, Ef = 0.49 mol F−1. At the end of the
the solid obtained was filtered, washed with diethyl ether and experiment, the solid obtained was filtered, washed with
dried under vacuum. Yield: 0.184 g (0.30 mmol, 80%). The red diethyl ether and dried under vacuum. Yield: 0.204
g
solid was characterised as [CuL(phen)]·2H2O. Anal. found: C, (0.34 mmol, 90%). The brown solid was characterised as [CdL
64.01; H, 3.48; N, 8.96%. Calc. for C33H22CuN4O5 (618.10): C, (bpy)]. Anal. found: C, 61.62; H, 3.05; N, 8.95%. Calc. for
64.12; H, 3.59; N, 9.06%. IR (KBr, cm−1): 1648(m), 1516(m), C31H18CdN4O3 (606.91): C, 61.35; H, 2.99; N, 9.23% IR (KBr,
1330(s), 842(s), 724(s). MS(FAB), m/z 582 [CuL(phen)], 242 cm−1): 1665(m), 1324(m), 750(m), 715(s). MS(FAB), m/z 607
[Cu(phen)]. UV-visible (cm−1): 15 600, 21 000. Red crystals of [CdL(bpy)], 451 [CdL]. Crystals of [CdL(bpy)]2·3(MeOH) (7) suit-
[CuL(phen)]·(C3H6O) (6) suitable for X-ray studies were able for X-ray studies were obtained by crystallization from
obtained by crystallisation of the product from acetone.
methanol.
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Synthesis of [CdL(phen)]. A solution of the ligand (0.1268 g, molecule of methanol per asymmetric unit that was removed
0.373 mmol) and 1,10-phenanthroline (0.0671 g, 0.373 mmol) using the Squeeze program. The crystal structure of compound
in methanol (40 mL) was electrolysed at 10 mA for 2 h, dissol- 5 contains a disordered molecule of methanol that was removed
ving 41.3 mg of cadmium, Ef = 0.49 mol F−1. The solid using the Squeeze program. The asymmetric unit of the crystal
obtained was filtered, washed with diethyl ether and dried structure of 6 contains two copper complexes and two acetone
under vacuum. Yield: 0.188 g (0.29 mmol, 80%). The brown molecules. Compound 7 did not give detectable diffraction
solid was characterised as [CdL(phen)]. Anal. found: C, 62.52; above θ = 25.35°, which is responsible for the low bond pre-
H, 2.75; N, 8.46%. Calc. for C33H18CdN4O3 (630.93): C, 62.82; cision. The crystal structure of 7 contains four methanol mole-
H, 2.88; N, 8.88% IR (KBr, cm−1): 1665(m), 1515(m), 1326(m), cules per asymmetric unit, two of which are disordered over two
1290(m), 850(m), 722(m). 1H NMR (DMSO-d6, ppm): 8.2–6.8 positions with 55 : 45 and 65 : 35 occupancies.
(m, 14H), 9.0 (d, 2H), 8.8 (d, 2H). MS(FAB), m/z 1262
[Cd2L2(phen)2], 1082 [Cd2L2(phen)], 630 [CdL(phen)].
Table S1† summarizes pertinent details of the data collec-
tion and the structure refinement of the crystal structures of
compounds 1–7. Selected bond lengths and angles are listed
in Tables S2–S6 of the ESI.† The program ORTEP334 was used
X-Ray crystallography
Intensity data sets for all compounds except 4 were collected at to generate all the pictures. CCDC reference numbers:
293 K with the use of a Smart-CCD-1000 Bruker diffractometer 1584238–1584244.†
(MoKα radiation, λ = 0.71073 Å) equipped with a graphite
Antimicrobial activity
monochromator. The omega scans technique was employed to
measure the intensities. The intensity data set for 4 was col- The in vitro antimicrobial activity assessment of the new com-
lected at 120 K with the use of a FR591-Kappa-CCD-2000 pounds was carried out determining their minimum inhibi-
Bruker-Nonius diffractometer (CuKα radiation, λ = 1.54180 Å) tory concentrations (MICs) against a range of bacteria and
equipped with a graphite monochromator. The omega and phi fungi by means of the broth two-fold dilution procedure.35 For
scans technique was employed to measure the intensities. No antibacterial tests, Gram positive Bacillus megaterium ATCC
decomposition of the crystals occurred during data collection. 19213, Bacillus subtilis ATCC 6633, Sarcina lutea ATCC 9341,
The intensities of all data sets were corrected for Lorentz and Staphylococcus aureus ATCC 25923, clinical isolates of
polarization effects. Absorption effects in all compounds were Staphylococcus haemolyticus, Streptococcus agalactiae and
corrected using the program SADABS.30 The crystal structures Streptococcus faecalis as well as Gram negative Escherichia coli
of all compounds were solved by direct methods. ATCC 8739, Haemophilus influenzae ATCC 19418 and
Crystallographic programs used for structure solution and Salmonella typhimurium ATCC 14028 bacteria were selected.
refinement were those of SHELXL-201431 installed on a PC Among fungi, Candida tropicalis ATCC 1369 and Saccharomyces
clone. Scattering factors were those provided with the SHELX cerevisiae ATCC 9763 yeasts and Aspergillus niger ATCC
program system. Missing atoms were located in the difference 6275 mould were used as test microorganisms.
Fourier map and included in the subsequent refinement
The compounds were dissolved in dimethyl sulfoxide. The
cycles. The structures were refined by full-matrix least-squares solutions were diluted in the media (Haemophilus test
refinement on F2, using anisotropic displacement parameters medium for Haemophilus influenzae, Mueller Hinton broth for
for all non-hydrogen atoms. Hydrogen atoms not involved in the other bacteria, and Sabouraud liquid medium for fungi)
classical hydrogen bonding were placed geometrically and (Oxoid, Basingstoke, UK) in the final concentration range from
refined using a riding model, including free rotation about 200 µg mL−1 to 0.003 µg mL−1. In all cases, DMSO never
C–C bonds for methyl groups. For all compounds, hydrogen exceeded the amount of 1% v/v and blank tubes containing
atoms were refined with Uiso constrained at 1.2 (for non- DMSO were also prepared for each microbial species and used
methyl groups) and 1.5 (for methyl groups) times Ueq of the as the control for solvents in the experiments. Aliquots of the
carrier C atom. Compound 3 presents two severely disordered bacterial and fungal suspensions were mixed with the chemi-
molecules of acetone in the voids of the crystal lattice. This cals to obtain an inoculum size of 5 × 105 colony forming units
solvent was removed using the Squeeze program,32 per mL and 5 × 103 cells per mL, respectively. Bacteria were
implemented in Platon.33 Compound 4 was a very weak diffrac- then incubated at 37 °C for 24 h and fungi at 30 °C for 48 h.
tor. It was measured with a rotating-anode source diffracto- Ampicillin and miconazole were tested under identical con-
meter (CuKα radiation) to theta(max) = 65.14° with 93% com- ditions to those of antibacterial and antifungal positive refer-
pleteness (data 98% complete to 58.93°), as it did not give ences, respectively. In order to determine the minimum inhibi-
detectable diffraction above that angle. This is reflected in the tory concentration values (MIC, µg mL−1), the assessment of
low precision of the structural parameters. The asymmetric unit the lowest concentration of the compound that inhibits visible
of the crystal structure of 4 contains two nickel dimers, three growth of the tested microorganisms was performed.
and a half molecules of methanol and a molecule of water. One
The minimum bactericidal concentrations (MBCs) and the
of the methanol molecules is disordered over two positions. minimum fungicidal concentrations (MFCs) were determined
The disorder was handled by introducing split positions for the by subculturing on fresh sterile medium 100 μL of culture
disordered molecule in the refinement with the respective occu- until each sample remained clear. MBC and MFC values rep-
pancies (70 : 30). This structure also contains a very disordered resented the lowest concentration of the compound (µg mL−1
)
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showing 99% inhibition after the incubation period of bacteria Anode:
at 37 °C for 24 hours and of fungi at 30 °C for 48 hours.
M ! M2þ þ 2 eꢀ
Each experiment was performed under sterile conditions
and repeated at least three times.
Overall:
or
H2L þ M ! ½MLꢁ þ H2ðgÞ
Results and discussion
H2L þ M þ L′ ! ½MLL′ꢁ þ H2ðgÞ
Synthesis and spectral properties
M = Co, Ni, Zn or Cd and L′ = bpy or phen.
The final product of the condensation of 1,2-diaminoanthra-
quinone and salicylaldehyde in the presence of p-toluenesulfo-
nic acid as the catalyst is not the expected Schiff base (rep-
resented as HL1 in Scheme 2), but the ligand 1H-anthra[1,2-d]
imidazol-6,11-dione-2-[2-hydroxyphenyl] (H2L). The isolation
of the ligand H2L indicates that the reaction involves the for-
mation of the Schiff base (HL1) in a first step, followed by an
intramolecular nucleophilic attack on the iminic carbon atom
by the nitrogen atom of the amino group (Scheme 2). The
green solid obtained was characterized by elemental analysis,
In the synthesis of the copper complexes, the values
obtained for the electrochemical efficiency were always close to
1.0 mol F−1. This fact together with the evolution of hydrogen
gas from the cathode shows that I oxidation state species are
the initial products of the electrochemical process, in agree-
ment with the following reaction scheme:
Cathode:
H2L þ 2eꢀ ! H2ðgÞ þ L2ꢀ
1
Anode:
IR and H NMR spectroscopy and mass spectrometry. Crystals
2Cu ! 2Cuþ þ 2eꢀ
of H2L(1) suitable for X-ray studies were obtained (Fig. 1) by
crystallization of the solid from a mixture of dichloromethane
and methanol.
However, the analytical data show that the final products
are compounds in which the copper atom is in II oxidation
state. This suggests a subsequent oxidation in solution from
Cu(I) to Cu(II) as soon as it is formed. In these cases, the
overall reaction can be represented by:
The new metal complexes were obtained by electrochemical
oxidation of the appropriate metal (cobalt, nickel, copper, zinc
or cadmium) in an electrolytic cell containing a solution of the
ligand. This method represents a simple alternative to other
standard chemical procedures.25 Heteroleptic complexes with
2,2′-bipyridine or 1,10-phenanthroline as co-ligands were also
obtained by addition of the corresponding coligand into the
cell. In all cases, the elemental analysis shows that the metal
ions react with the ligand at a 1 : 1 molar ratio to afford com-
plexes of the bi-deprotonated ligand (L2−).
Cu þ H2L ! ½CuLꢁ þ H2ðgÞ
or
Cu þ H2L þ L′ ! ½CuLL′ꢁ þ H2ðgÞ
where L′ is bpy or phen.
A similar mechanism involving the electrochemical oxi-
dation of Cu to Cu(I) followed by a further oxidation of Cu(I) to
Cu(II) was also postulated for other Cu(II) complexes obtained
via electrochemical oxidation of copper anodes.18,29
The electrochemical efficiency Ef, defined as the amount of
metal dissolved per Faraday of charge, was calculated for all
the electrochemical processes. In the case of the Co, Ni, Zn or
Cd complexes the values of Ef were close to 0.5 mol F−1 (see
the Experimental part). These data and the evolution of hydro-
gen gas from the cathode are consistent with the following
reaction scheme:
The synthesis of [ML] complexes can be readily accom-
plished in good yields, by the simple one-step electrochemical
procedure described above. The IR spectra of these complexes
do not show the bands attributed to ν(O–H) and ν(N–H), which
appear at 3354 cm−1 in the IR spectra of the free ligand. In
addition, the band attributed to ν(C–O), which appears at
1295 cm−1 in the IR spectra of the free ligand, is shifted to
higher wavenumbers (1300–1330 cm−1) in the complexes.
These facts show that the amide and phenol protons are lost
during the electrochemical synthesis and that the ligand is in
the dianionic form in the complexes. The room temperature
1H NMR spectrum of the cadmium complex only exhibits mul-
tiplet resonances in the aromatic region, δ 8.3–6.6 ppm, as the
signals attributed to the NH and OH hydrogens, which appear
at δ 4.10 and 12.80 ppm, respectively, in the NMR spectrum of
the free ligand, are now absent.
Cathode:
H2L þ 2eꢀ ! H2ðgÞ þ L2ꢀ
The FAB mass spectra of the cobalt, nickel, and copper
[ML] compounds show peaks due to the [ML] molecular ions,
with the appropriate isotope distribution. In addition, peaks
Fig. 1 Ortep diagram of 1.
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+
associated with fragments of the dinuclear species [ML]2 are transitions in a distorted octahedral geometry for the metal. In
also observed in each case (see the Experimental section), both cases, the ν1 band is split probably because of the distor-
which can be taken as an indication of the dimeric structure tion from the regular symmetry.36 This geometry was con-
of these compounds.
firmed in the case of the complex [CoL(phen)] by X-ray diffrac-
The solid state electronic spectrum of the complex [CoL tion. The electronic spectra in the solid state of the heterolep-
(MeOH)], obtained by crystallization of [CoL] from CH2Cl2/ tic compounds of Ni(II) show bands in the regions 8860–7700,
CH3OH, is consistent with a pentacoordinated environment 15 800–15 600, and 17 820–20 400 cm−1. These bands are
around the cobalt(II).36 The bands at 7280, 10 870, 14 900 and within the expected range for octahedral Ni2+ complexes and
3
3
17 550 cm−1 are assigned to 4A2(F) → 4B2; 4A2(F) → 4E(F), can be assigned to the transitions: A2g
→ →
3T2g(ν1), A2g
4A2(F) → 4E(P) and 4A2(F) → 4A2(P) transitions in a square 3T1g(F)(ν2) and A2g → T1g(P)(ν3), respectively, assuming a dis-
pyramidal geometry for the metal.36 However, the spectrum torted octahedral geometry for the complex. The ν1 band of
may be interpreted as a trigonal bipyramidal geometry case the phenanthroline derivative (7660, 9025 cm−1) is split prob-
given that the compound [CoL(MeOH)] has an intermediate ably as a consequence of its distorted octahedral symmetry.36
structure between both symmetries (vide infra). The diffuse Diffuse reflectance spectra of the heteroleptic complexes of
reflectance spectra of [NiL] and [CuL] show two bands at copper(II) show two broad bands at 15 700 and 22 300 cm−1 for
9025 cm−1 (ν2) and 15 530 cm−1 (ν3) for the nickel compound (5) and at 15 600 and 21 000 cm−1 for (6), which suggests a
and at 14 900 and 16 300 cm−1 for the copper complex, which five-coordinate geometry of these complexes.36
3
3
3
3
3
3
can be attributed to the T1(F) → A2 (ν2) and T1(F) → T1(P)
(ν3), transitions, respectively, for tetrahedral nickel(II)
X-ray crystal structure study
a
1
1
1
1
complex and to the A1g(F) → B1g and A1g → Eg, transitions
for a square planar copper(II) complex.36
The molecular structure of the ligand 1 and the metal complexes
2–6 was determined by single crystal X-ray diffraction analysis.
The incorporation of additional bidentate ligands into the
cell allows the synthesis of heteroleptic complexes of general
formula [ML(bpy)] and [ML(phen)], M = Co, Ni, Cu, Zn, and
Cd. These complexes show IR bands characteristic of the co-
ordinated L2− ligand and also bands at around 770 and
715 cm−1 or at around 1515, 850 and 710 cm−1 typical of co-
ordinated 2,2′-bipyridine and 1,10-phenanthroline, respect-
ively. The 1H NMR spectra of the heteroleptic complexes of
zinc and cadmium show, besides the signals attributed to the
quinone ligand, the signals due to coordinated 2,2′-bypiridine
or 1,10-phenanthroline. In the case of the heteroleptic com-
plexes of cobalt, nickel, zinc and cadmium, the FAB spectra
show the peaks corresponding to the dinuclear species
[ML(bpy)]2 and [ML(phen)]2 , which are the structures that
have been confirmed by X-ray diffraction in many instances
(vide supra). In most cases the FAB spectra also show peaks
corresponding to the monomeric species and other signals
due to fragments formed by the loss of anthraquinone, bipyri-
dine or phenanthroline moieties from the dinuclear species.
In the case of the copper heteroleptic compounds, only in the
spectrum of the phenanthroline derivative 5 the peak corres-
ponding to the monomeric species at m/z 582 appears, which
is the structure that has been confirmed for 5 by X-ray diffrac-
tion (vide infra). The mass spectrum of 6 shows the peak
corresponding to the ion resulting from the loss of the anthra-
quinone ligand from the initial complex (m/z = 402) and the
peak due to the free ligand, although the peak corresponding
to the molecular ion is not observed.
Description of the crystal structures of 2, 3, 4, 5, 6, and 7
The asymmetric units of the crystal structures of 4 and 6 have
two independent molecules. In these cases, only one of the
complex molecules will be used for the structural discussion,
as they are both very similar.
The cobalt(II) compounds 2 and 3, the nickel(II) compound
4 and the cadmium(II) compound 7 are dimeric molecules
with the metal atoms linked by phenoxo bridges (Fig. 2, 3, 4
and 5, respectively). In all dimers, the bridge is slightly asym-
metric and the distance between the two metal atoms is high
enough to suggest the absence of a binding interaction
between them. In the case of the structures that involve tran-
sition metals, 2, 3 and 4, the four-membered ring formed by
the two metal atoms and the two bridging oxygen atoms is
flat, although in the case of the cadmium(II) complex 7 this
ring is bent [dihedral angle: 25.52(12)°].
+
+
In all dimers, each dianionic terdentate ligand (L)2− che-
lates one of the metal atoms through the imidazoline nitrogen
atom and the keto and phenolate oxygen atoms, bridging the
Diffuse reflectance spectra of the heteroleptic complexes of
cobalt(II), [CoL(bpy)] and [CoL(phen)], are typical of a d7
system in a high-spin octahedral environment. Thus, the com-
plexes present in the UV-visible region three spin-allowed tran-
sitions in the ranges 7930–7450, 15 720–15 650 and
17 520–17 420 cm−1 which are assigned respectively to the
4
4
4
4
4
4T1g(F) → T2g(ν1), T1g(F) → A2g(ν2) and T1g(F) → Tlg(P)(ν3) Fig. 2 ORTEP diagram of the molecular structure of 2.
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other metal center through the phenolate oxygen atom. In
compounds 2, 4 and 7, each one of the two (L)2− ligands che-
lates a different metal, giving rise to similar coordination
environments for both metal centers. In the case of the cobalt
dimer 2, each cobalt atom completes its coordination sphere
coordinating a methanol (solvent) molecule, forming five-coor-
dinate coordination entities, [CoO4N], with an intermediate
geometry between a trigonal bipyramid and a square pyramid
(trigonality index τ = 0.49).37 In the case of the nickel dimer 4
and of the cadmium dimer 7, each metal atom achieves six-
coordination, [MO3N3], by binding an (N, N) chelating 2,2′-
bipyridine coligand, although the final geometries are very
different. In compound 4 (Fig. 4), both nickel atoms show octa-
hedral geometry, which is distorted due to the low value of the
N–Ni–N bite angle forced by the chelating 2,2′-bipyridine coli-
gand and the formation of the phenoxo bridge. In compound
7 (Fig. 5), the coordination geometry is different for both
cadmium atoms. The values of the twist angles suggest dis-
torted triangular prism geometry for Cd1 and a highly dis-
torted octahedral geometry for Cd2 (see figures and data in the
ESI, Fig. S1†).
Fig. 3 ORTEP diagram of the molecular structure of 3.
In the case of the cobalt dimer 3, both (L)2− ligands chelate
the same metal atom and bridge the other one, which is also
bonded to the two chelating phenanthroline coligands (Fig. 3).
Thus, the coordination environments for the cobalt atoms are
different, [CoO4N2] and [CoN4O2], with distorted octahedral
geometries. The degree of distortion is higher for the one
bonded to the two phenanthroline coligands, as the main
source of distortion is their small chelation angle, 77.03(9) and
77.14(10)°. These values are similar to those found in other
six-coordinated cobalt(II) complexes with chelating phenanthro-
line ligands.38 The complexes 5 and 6 consist of monomer units
in which the copper atom is pentacoordinated to a (O, N, O)
terdentate chelating dianionic ligand (L)2− and to a (N, N) chelat-
ing neutral coligand; 2,2′-bipyridine for 5 and 1,10-phenanthro-
line for 6 (Fig. 6). In both cases, the geometry is much closer to a
square pyramid than to a trigonal bipyramid, as suggested by the
geometric parameter τ (0.08 for 5 and 0.06 for 6),37 with one of
the nitrogen atoms of the coligand in the apical position.
Fig. 4 ORTEP diagrams of the molecular structures of 4.
The M–O bond distances involving the phenolic oxygen
atoms are always shorter than the ones involving the ketonic
group (Tables S3–S6†), showing a stronger interaction, as
Fig. 6 ORTEP diagram of the molecular structure of 5 (left) and 6
Fig. 5 ORTEP diagrams of the molecular structures of 7.
(right).
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expected. Thus, for example, the Co–O(phenoxo) distances in ligands of the pentacoordinated cobalt dimer 2. In the hexa-
compound 2, 2.000(2) Å and 2.0850(19) Å, are within the range coordinated dimers 3, 4 and 7 the distortion is higher:
of 1.886–2.168 Å found in other pentacoordinated cobalt(II) 11.48(9)° and 29.72(7)° for the asymmetric cobalt dimer 3;
complexes.18,39 These Co–O(phenoxo) distances are slightly 10.12(9)° and 21.29(8)° (unit 1) and 14.62(9) and 23.57(9)°
longer in the octahedral compound 3, 2.0511(19)–2.0990(18) Å, (unit 2) for the nickel dimer 4; and 16.55(19)° and 25.44(16)°
as found for other six-coordinated cobalt(II) complexes with for the cadmium dimer 7. In the case of the monomer copper
bridging phenoxy groups [2.0323–2.112 Å].40 The weaker Co–O compounds 5 and 6, the four-fused-ring moiety and the
(ketonic) interactions of both complexes are reflected in longer phenol moiety remain almost coplanar, like in the free ligand
distances, 2.1723(19)–2.1817(19) Å, which are normal com- [6.30(23)° for 5 and 3.21(20)° (unit 1) and 10.80(84)° (unit 2)
pared to similar ligands reported in the literature.41 In the for 6].
case of compounds 4 and 7, the values of M–O(ketonic) and
In all cases, the C–O distances of the ketone groups of the
M–O(phenolic) distances, M = Ni, Cd, can also be considered quinone moiety [range 1.216(7)–1.257(3) Å] have similar values
normal, as they are similar to those found in other six-co- to those found in the free ligand [average value 1.227(3) Å],
ordinated complexes of nickel(II)42 and cadmium(II).43 The although the bond distance involving the coordinated oxygen
mononuclear copper complexes follow the same trend, with atom is always slightly longer. This fact suggests that the
Cu–O(phenolate) distances typical of pentacoordinate copper(II) ligand is in its completely oxidized form, as other metal com-
complexes,44 and longer Cu–O(ketone) bond distances. plexes with the ligand in its semiquinone or catecholate forms
However, the Cu–O distances involving ketonic oxygen atoms show C–O distances in the ranges 1.27–1.30 Å and 1.34–1.36 Å,
[2.0622(19) Å in 5 and 2.101(2) Å in 6] are longer than usual, respectively. Thus, the electrochemical process has only
1.908–1.969 Å.45
involved the deprotonation of the ligand.
The Co–N(imidazole) bond distances of compounds 2 and
A description of the crystal packing of compounds 1–7 is
3 are very similar [range 1.982(2)–2.010(2) Å] but shorter than given in the ESI (Fig. S2–S19 and Tables S7–S9†).
those found in related cobalt(II) complexes [2.128–2.163 Å].46
Comparison of the crystal structures
This is also observed for the M–N distances of the mono-
nuclear copper complexes 5 and 6, with shorter Cu–N(imid- The dianionic ligand (L)2− is a (ONO) tridentate chelating
azole) distances than the normal range found in other penta- ligand. The coordination of (L)2− to a M(II) metal ion is enough
coordinated copper(II) complexes, 1.953–1.978 Å.47 In the case to balance its charge but does not produce a coordinatively
of the six-coordinated dimeric compounds 4 and 7, these M–N saturated complex. The dimerization through the formation of
distances are similar to those found in similar Ni(II)48 or a μ-phenoxo bridge increases the coordination number to four.
Cd(II)49 complexes.
However, the geometry imposed by the planar (ONO) ligand
In compound 2, the value of the bond distance that involves (L)2− implies that the final geometry of the metal in the dimer
the coordinated methanol molecule, Co–O(4): 1.985(2) Å, is will be square planar. Incidentally, this is the proposed geome-
shorter than the range found for other pentacoordinated try for the dimeric complex [CuL], as suggested by the diffuse
cobalt(II) compounds containing methanol as
[2.056–2.261 Å],50 suggesting the presence of a stronger bond ions that do not give stable square planar geometry (Co2+, Ni2+,
compared to the literature compounds.
Cd2+) the μ-phenoxo dimer has to be stabilized by coordination
a ligand reflectance spectrum (vide supra). In the case of other M(II)
The M–N distances involving the bipyridine (4, M = Ni; 7, of other ligands. Coordination of monodentate ligands, like
M = Cd) or phenantroline (3, M = Co) coligands of the metal methanol solvent molecules, produces five-coordinate metal
dimers are all normal and similar to those found in related centers, as observed for the Co2+ ions in the crystal structure of
six-coordinated Co(II),38 Ni(II),51 and Cd(II)52 complexes. In the 2. The use of bidentate (N, N) chelating coligands, such as bpy
case of the mononuclear square pyramidal copper complexes 5 and phen, can complete the six-coordination for the metal
(bpy) and 6 (phen), the Cu–N bond length involving the nitro- atom. The analysis of the molecular structures of 4 and 7
gen atom on the basal plane [2.024(2) Å for 5 and 2.019(3) Å (vide supra) shows that the chelating ligand (L)2− is capable of
for 6] is shorter than the one involving the nitrogen atom on stabilizing different geometries around the six-coordinated
the apical position [2.256(2) Å for 5 and 2.271(3) Å for 6]. This metal atom.
behavior has been found in other pentacoordinated copper
complexes with 2,2-bipyridine53,54 or 1,10-phenanthroline54,55 gand also produces six-coordinate (octahedral) geometry for
coligands. both cobalt(II) atoms. However, the final structure is un-
In the case of compound 3, the use of a phenantroline coli-
Regarding the conformation of the coordinated dianionic expected as it shows asymmetrical environments for the cobalt
ligand (L)2−, the formation of the phenoxo bridges in the atoms, [N4O2] and [O4N2]. This is the result of the coordination
metal dimers produces a twist of the planar four-fused-ring of both dianionic (L)2− ligands to one cobalt atom, forming a
moiety with respect to the planar phenol moiety, which is [CoII(L)2]2− entity, and the coordination of the two phenantro-
more pronounced with increasing coordination number line coligands to the other cobalt atom, forming
a
around the metal center. Thus, although these two moieties [CoII(phen)2]2+ entity. An analysis of the CCDC database
are almost coplanar in the free ligand [dihedral angle: (CCDC Version 5.38, November 2016) reveals the rarity of this
4.33(9)°], they twist to 13.55(6)° in both symmetry equivalent structure 3. The search shows that there are only 6 structurally
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characterized Co(II) dimers with two terdentate ligands with a
lateral phenoxo group that bridges the cobalt atoms.56,57 In all
cases, the structures are symmetrical, with a monodentate
ligand completing five coordination56 as in complex 2 or with
two monodentate ligands per cobalt atom, completing six
coordination.57 In these latter cases described in the literature,
the final structures are not as those found in complexes 4 or 7,
as the [Co2L2] system forms a plane with the monodentate
ligands in apical positions. In any case, it is important to
stress the symmetrical nature of the related literature com-
plexes. The only related asymmetrical cobalt(II) dimers
included in the CCDC database are two examples of complexes
of the type [Co(ter-L)2][CoCl2], where (ter-L) is a monoanionic
terdentate ligand, in which the two terdentate ligands coordi-
nate an octahedral cobalt atom, as in 3, and the phenoxo
groups bridge a [CoCl2] unit, with a tetrahedral cobalt
atom.58,59 However, in these cases the terdentate ligands are
monoanionic and not planar, favoring facial coordination.
With dianionic terdentate bridging phenoxo ligands, this type
of {CoII[(ter-L)]2}2− unit has only been observed in cobalt(II)
trimers of general formula CoII{CoII[(ter-L)]2}, in which the
central cobalt atom tetrahedrally coordinates the four bridging
oxygen atoms, giving rise to symmetrical trimers.59,60 However,
in these trimers the ligands are not planar and also favor facial
coordination.
It is interesting to note that the literature does not present
examples of cobalt(II) μ-phenoxo dimers with bidentate coli-
gands, so no cobalt analogs of the nickel dimer 4 or the
cadmium dimer 7 are known. In any case, the symmetrical
dimer was the expected structure for 3.
In the case of the Cu(II) complexes 5 (bpy) and 6 (phen), the
coordination of the dianionic ligand (L)2− and of a (N, N)
coligand gives rise to stable five-coordinate mononuclear com-
plexes, so no dimerization is observed.
Magnetic properties
To assess the nature and extent of magnetic exchange between
the two cobalt(II) ions in 2 and 3 and between the two nickel(II)
ions in 4, variable-temperature (2–300 K) magnetic suscepti-
bility measurements were carried out on powder samples.
Fig. 7(a–c) display the χMT product vs. T for 2, 3 and 4 (where
χM is the magnetic susceptibility per mol of the compound
and T is the temperature). The values of χMT at 300 K are 4.35,
4.65 and 2.35 cm3 K mol−1 for 2, 3 and 4, respectively. These
values are consistent with the expected ones for two uncoupled
cobalt(II) or nickel(II) with some orbital contribution. Upon
Fig. 7 Thermal dependence of the product χMT for compounds 2 (a), 3
(b) and 4 (c). The circles represent experimental data and the solid line
the fit of the data.
cooling, the χMT value of 2 is almost constant in the temperature maximum value of 6.89 cm3 K mol−1 at 10 K. The decrease of the
interval 300–150 K, and then decreases more markedly at temp- χMT product below this temperature can be attributed to the
eratures below 100 K reaching a value of ca. 1.0 cm3 K mol−1 at occurrence of zero-field splitting (ZFS) and/or intermolecular
2 K. This behavior indicates the occurrence of weak antiferro- antiferromagnetic exchange interactions. Similarly to 3, the
magnetic coupling between the two cobalt(II) ions in 2. However, χMT value for 4 continuously increases to reach a value of
no maximum was observed for the χM vs. T plot most probably 3.44 cm3 K mol−1 at 2 K also indicating the presence of intra-
due to very weak magnetic coupling and/or the presence of some molecular ferromagnetic coupling.
paramagnetic impurities. In contrast to 2, the χMT values of 3
High-spin cobalt(II) in regular octahedral surroundings has
gradually increase with decreasing T indicating the occurrence of a T1g ground state. Because of the presence of first-order spin
ferromagnetic exchange interaction. The χMT vs. T plot displays a orbit coupling in strict or nearly pure octahedral geometry the
4
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magnetic properties of complexes with T ground terms are [Co2L]·1CH3CN (H2L = N,N-bis(3,4-dimethyl-2-hydroxybenzyl)-
often found to show considerable temperature dependence, N′,N′-dimethylethylenediamine) (J = −4.2 cm−1), which shows
and interpreting them is difficult.2,61 However, when the lower- a Co–Ophenolate–Co angle slightly larger (102.04°) than that for
ing of the symmetry is strong enough to remove the orbital 2.18 In contrast, the ferromagnetic coupling (J = 7.8 cm−1
)
4
degeneracy of the T term only second-order spin orbit coup- observed for complex 3 can be associated with the smaller
ling is operative and the magnetic properties can be straight- Co–Ophenolate–Co angle of 97.05°. These data are consistent
forwardly interpreted. This is clearly the situation for 2, which with the critical angle predicted for switching the sign of the
is a centro-symmetric species made up of two five-coordinated coupling constant J in phenoxide-bridged compounds which is
cobalt(II) ions defining a geometry half-way between a square around 96–98°.65,70,71 Given that the average Ni–O–Ni angle
planar pyramid and a trigonal bipyramid. Although the is 97.1° for 4, the small positive J value obtained for the
cobalt(II) ions are hexacoordinated in 3, the octahedra are dis- dinuclear nickel(II) derivative is explained in the same terms as
torted enough to eliminate the effects of first-order spin orbit for 3.
coupling. Thus, the magnetic coupling of 2 and 3 has been sat-
isfactorily analyzed for dinuclear species with S1 = S2 = 3/2
Antimicrobial activity
from the isotropic Heisenberg–Dirac–van Vleck (HDVV) spin The in vitro antimicrobial properties of the novel ligand and its
exchange Hamiltonian H = −JS1S2 + Σi giβHSi (i = 1,2) where metal complexes were studied targeting the determination of
J is the intradimer exchange interaction and Si is the quantum their minimum inhibitory concentration (MIC) against several
spin operator.62,63 Similarly, the experimental data of 4 were bacterial and fungal strains. In order to compare the effects,
fitted to the χMT expression derived from the HDVV both 2,2′-bipyridine and 1,10-phenanthroline coligands were
Hamiltonian for S1 = S2 = 1.
also included in the evaluation. The results are summarized in
A least-squares fit of the data leads to J = −2 cm−1, g = 2.17 Table 1.
and ρ = 0.08 (ρ accounts for monomeric paramagnetic impuri-
The H2L ligand was completely devoid of antibacterial
ties)62 for 2 and J = 7.8 cm−1 and g = 2.19 for 3. Although this activity up to the concentration of 200 µg mL−1. On the other
simple approach is perfectly correct for 3 in the interval hand, a good inhibition was exhibited by the 1,10-phenanthroline
300–12 K, it does not allow simulating the data below 12 K, coligand towards both Gram positive and Gram negative tested
which essentially reflect the ZFS of the dinuclear states. The bacteria (MIC 12–50 µg mL−1) and, only in higher concentrations,
best fit between experimental and calculated data was by the other coligand 2,2′-bipyridine against Gram negative
obtained for J = 2.8 cm−1 and g = 2.14 for 4. The solid lines in Escherichia coli and Heamophilus influenzae (MIC 100 µg mL−1
Fig. 7(a–c) correspond to the calculated curves using the and Gram positive Bacillus subtilis (MIC 200 µg mL−1).
)
obtained parameters and show an excellent agreement
between the experimental and theoretical χMT data.
Among the metal complexes, all tested Cd(II) compounds
and [NiL(bpy)]2·2H2O showed strong antibacterial activity
One important difficulty in making general statements towards Gram positive Bacillus subtilis, with MIC values
about trends in the magnitude of the coupling constant is the ranging from 1.5 to 3 µg mL−1, and good effectiveness for the
small number of structurally and magnetically characterized Gram negative Heamophilus influenzae strain that was inhibited
μ-phenoxo-bridged
dinuclear
cobalt(II)
complexes.64 at concentrations of 12–25 µg mL−1. The same microorgan-
Furthermore, the combination of the phenoxo bridge with isms, as well as Staphylococcus aureus, were sensitive also to
other additional bridges (usually acetato) in the same mole- the Cu(II) complexes [CuL]·H2O and [CuL(phen)]·2H2O at
cular unit makes this analysis more difficult.65,66 In this 50–100 µg mL−1 and to the Co(II) derivative [CoL(phen)]2 at
respect, as far as we know, the number of μ-phenoxo-dibridged 200 µg mL−1. Identical results were detected for [CoL(MeOH)]2
cobalt(II) complexes so far reported is even smaller.67–69 The against both S. aureus and H. influenzae. The Gram negative
coupling constant in these compounds is commonly rational- Escherichia coli strain was resistant to all the tested complexes
ized in terms of the Co–Ophenoxo–Co angles and Co⋯Co intra- up to the concentration of 200 µg mL−1
.
dimer distance.65 Additional geometrical factors considering
With regard to the results of antifungal activity, the new
the fold angle along the O–O axis, the solid angle at the compounds had no significant effect on the growth of yeasts
oxygen bridge, which affect the s/p character of the hybrid and mould under investigation. Only [NiL(bpy)]2·2H2O and all
orbitals on the bridging oxygen atom, and electronic effects of Cd(II) complexes showed
a
moderate inhibition of
the substituents on the phenol ring have been taken into Saccharomyces cerevisiae at 100–200 µg mL−1, although both
account.67 The weak antiferromagnetic J value, −2 cm−1, found parent coligands were found to be active against all tested
for
[Co2L2Cl2(CH3OH)2] (HL = 3-[(furan-2-ylmethylimino)methyl]-
2-hydroxy-5-methylbenzaldehyde) (J
−4.22 cm−1 68
2
is close to that observed for the complexes fungi at 12–100 µg mL−1
.
The Cd complexes and [NiL(bpy)]2·2H2O, showing the
=
)
and highest antibacterial properties, were tested against a wide
[Co2L2]·H2O (H2L = N,N-bis(4,5-dimethyl-2-hydroxybenzyl)-N- spectrum of other bacteria including Gram positive, such as
(2-pyridylmethyl)amine) (J = −4.2 cm−1).19 The three com- Bacillus megaterium ATCC 19213, Sarcina lutea ATCC 9341,
pounds display a similar Co–Ophenolate–Co angle, 100.21°, 99.5° clinical isolates of Staphylococcus haemolyticus, Streptococcus
and 100.8°, respectively. A larger antiferromagnetic J value, agalactiae, Streptococcus faecalis and Gram negative, namely
−7.5 cm−1
,
was observed for the related complex Salmonella typhimurium ATCC 14028 (Table 2). Interestingly,
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Table 1 Antimicrobial activity, expressed as MIC (μg mL−1) and, in brackets, as MBC (μg mL−1) and MFC (μg mL−1
)
Bacteriaa
BS
Bacteriab
EC
Fungic
CT
Compound
SA
HI
SC
AN
H2L (1)
>200
>200
>200
25(>200)
200(>200)
>200
200(>200)
>200
>200
>200
50(200)
>200
50(100)
>200
>200
>200
>200
>200
>200
0.15(0.7)
—
>200
100(>200)
12(200)
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
6(12)
—
>200
>200
50(200)
>200
>200
50(200)
25(100)
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
—
2,2′-Bipyridine
1,10-Phenanthroline
[CoL(MeOH)]2 (2)
[CoL(bpy)]·3H2O
[CoL(phen)]2 (3)
[NiL]
[NiL(bpy)]2·2H2O (4)
[NiL(phen)]·3H2O
[CuL] H2O
[CuL(bpy)] (5)
[CuL(phen)]·2H2O (6)
[ZnL]
[ZnL(bpy)]
[ZnL(phen)]·2H2O
[CdL]
200(>200)
12(100)
>200
>200
200(>200)
>200
3(6)
>200
50(100)
>200
50(100)
>200
>200
>200
3(6)
1.5(6)
3(6)
100(>200)
12(200)
200(>200)
>200
200(>200)
>200
12(100)
>200
100(>200)
>200
25(200)
12(100)
>200
>200
>200
>200
>200
200(>200)
>200
>200
>200
>200
>200
>200
100(>200)
100(>200)
200(>200)
—
100(200)
200(>200)
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
—c
100>200
>200
>200
>200
25(100)
12(25)
12(50)
0.07(0.3)
—
[CdL(bpy)]2(7)
[CdL(phen)]
Ampicillin
0.03(0.3)
—
Miconazole
3(25)
12(50)
3(25)
a Gram positive bacteria: Bacillus subtilis ATCC 6633 (BS) and Staphylococcus aureus ATCC 25923 (SA); b Gram negative bacteria: Escherichia coli
ATCC 8739 (EC) and Haemophilus influenzae ATCC 19418 (HI). c Yeasts: Candida tropicalis ATCC 1369 (CT) and Saccharomyces cerevisiae ATCC
9763 (SC); mould: Aspergillus niger ATCC 6275 (AN).
Table 2 Spectrum of the inhibitory activity of the most active compounds, expressed as MIC (μg mL−1) and, in brackets, as MBC (μg mL−1) and
against several Gram positive and Gram negative bacteria
Bacteriaa
BM
Bacteriab
ST
Compound
SL
SH
SAG
SG
[NiL(bpy)]2·2H2O (4)
[CdL]
[CdL(bpy)]2 (7)
[CdL(phen)]
Ampicillin
3(6)
3(6)
1.5(3)
3(6)
0.07(0.15)
25(100)
25(50)
12(25)
50(100)
0.0015(0.007)
<200
12(25)
12(50)
12(25)
12(25)
25(>200)
25(100)
25(50)
50(100)
0.7(6)
>200
<200
>200
>200
1.5(3)
100(>200)
50(100)
>200
0.03(0.07)
0.03(0.07)
a Gram positive bacteria: Bacillus megaterium ATCC 19213 (BM), Sarcina lutea ATCC 9341 (SL), Staphylococcus haemolyticus clinical isolate (SH),
Streptococcus agalactiae clinical isolate (SAG), Streptococcus faecalis clinical isolate (SF). b Gram negative bacterium: Salmonella typhimurium ATCC
14028 (ST).
these complexes exhibited remarkable activity towards
Differential patterns of antimicrobial properties of the
B. megaterium at 1.5–3 µg mL−1 and towards Sarcina and both ligand and its metal complexes used in this work were
streptococci at 12–50 µg mL−1, whereas showed low growth observed. Specifically, the ligand and its nickel and zinc homo-
inhibition of S. haemolyticus and no effect on S. typhimurium.
leptic complexes were devoid of antimicrobial activity. In con-
To determine the antimicrobial activity pattern, minimum trast, the complexation of H2L with cadmium strongly
bactericidal and fungicidal concentrations were detected for improved the antimicrobial activity of the ligand against Gram
the active compounds. The MBC and MFC values reported in positive Bacillus subtilis and Gram negative Haemophilus influ-
Tables 1 and 2 were always higher than the corresponding enza and slightly affected the growth of the yeast
MICs, suggesting a bacteriostatic and fungistatic behaviour.
Saccharomyces cerevisiae. Along the same lines, a moderate
Moreover, all tested compounds were demonstrated to be increase in the inhibition of bacteria Staphylococcus aureus
lower antimicrobial agents, when compared to ampicillin and and Haemophilus influenzae was promoted by the complexation
miconazole, positive antibacterial and antifungal controls, of H2L with cobalt and copper metal ions and was also
respectively (Tables 1 and 2).
detected for [CuL]·H2O towards Bacillus subtilis.
Overall, the obtained results revealed that [CdL(bpy)]2 was
Collectively, 1,10-phenanthroline heteroleptic complexes
the best antimicrobial agent among the tested complexes and were shown to exhibit attenuation and less extensive anti-
that Gram positive bacilli were the most sensitive strains to microbial behavior, compared to the coligand alone, implying
these compounds.
that the metal(II)-bond decreases 1,10-phenanthroline anti-
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microbial properties. Similar observations were made in the magnetic coupling parameter (J) shows the occurrence of anti-
case of the cobalt, copper and zinc 2,2′-bipyridine heteroleptic ferromagnetic coupling (J = −2 cm−1) for 2, and ferromagnetic
complexes, whereas [NiL(bpy)]2·2H2O and [CdL(bpy)]2 exhibi- coupling for 3 (J = 7.8 cm−1) and 4 (J = 2.8 cm−1). These values
ted a remarkable increase of the antibacterial activity of the together with the M–Ophenoxo–M (M = NiII, CoII) angles found
parent coligand against Bacillus subtilis and Heamophilus for the three derivatives are perfectly consistent with magneto-
influenzae.
structural correlations recently reported.65,71 The in vitro anti-
Finally, no differential antimicrobial behavior was notice- microbial properties of the complexes were also studied.
able among homoleptic and heteroleptic complexes of each Biological studies show that the Cd complexes and
metal, suggesting that the introduction of the coligands into [NiL(bpy)]2·2H2O exhibit significantly higher antimicrobial
the homoleptic complexes had no great effect, even though selectivity against common Gram-positive bacteria. The anti-
coligands themselves were found to be antimicrobial agents. microbial behavior among homoleptic and heteroleptic com-
Exceptions were detected for [NiL(bpy)]2·2H2O that exhibited plexes of each metal is not very different, suggesting that the
an increased strong inhibition of B. subtilis and H. influenzae introduction of the coligands into the homoleptic complexes
when compared to [NiL], and for [CuL(bpy)] that, instead, was has no effect on the activity of the compounds.
completely devoid of the antibacterial activity shown by the
homoleptic complex, [CuL]·H2O.
Conflicts of interest
There are no conflicts to declare.
Conclusions
The one-pot-syntheses of new metal complexes (Co, Ni, Cu, Zn
and Cd) containing the novel heterocyclic benzimidazole
Acknowledgements
ligand
1H-anthra[1,2-d]imidazol-6,11-dione-2-[2-hydroxy-
This work was partially supported by the Spanish Ministerio
de Economía y Competitividad (MINECO) (Grant Unidad de
Excelencia María de Maeztu MDM-2015-0538).
phenyl] (H2L) were achieved by using an electrochemical pro-
cedure. The homoleptic complexes were obtained by electroly-
sis of solutions of the ligand H2L with an appropriate metal
anode. The addition of bidentate nitrogen donors such as bpy
or phen to the cell solutions containing the ligand H2L allows
the electrosyntheses of the corresponding heteroleptic com-
plexes in only one step with good overall yields. All these com-
plexes were fully characterized by a number of analytical tech-
niques and when possible, by X-ray single-crystal diffraction
analysis.
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