Synthesis, structure, and electrochemistry of pyridinecarboxamide cobalt(III) complexes; the effect of bridge substituents on the redox properties

Article abstract of DOI:10.1016/j.poly.2008.05.031

Two series of complexes of the types trans-[Co^III(Mebpb)(amine)₂]ClO₄ {Mebpb^2- = N,N-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a), morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[Co^III(cbpb)(amine)₂]X {cbpb^2- = N,N-bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine = pyrrolidine (prldn), X = PF₆ (1b), piperidine (pprdn), X = PF₆ (2b), morpholine (mrpln), X = ClO₄ (3b), benzylamine (bzlan), X = PF₆ (4b)} have been synthesized and characterized by elemental analyses, IR, UV-Vis, and ¹H NMR spectroscopy. The crystal structure of 1a has been determined by X-ray diffraction. The electrochemical behavior of these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on the redox properties, is also reported. The reduction potential of Co^III, ranging from -0.53 V for (1a) to -0.31 V for (3a) and from -0.48 V for (1b) to -0.22 V for (3b) show a relatively good correlation with the σ-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox processes.

Full text of DOI:10.1016/j.poly.2008.05.031

Polyhedron 27 (2008) 2771–2778
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure, and electrochemistry of pyridinecarboxamide cobalt(III)
complexes; the effect of bridge substituents on the redox properties
b
a
a
Soraia Meghdadi a, Mehdi Amirnasr a, , Mohammad H. Habibi , Ahmad Amiri , Vahid Ghodsi ,
*
Azadeh Rohani a, Ross W. Harrington c, William Clegg c
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
c School of Chemistry, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Two series of complexes of the types trans-[CoIII(Mebpb)(amine)2]ClO4 {Mebpb2ꢀ = N,N-bis(pyridine-2-
carboxamido)-4-methylbenzene dianion, and amine = pyrrolidine (prldn) (1a), piperidine (pprdn) (2a),
morpholine (mrpln) (3a), benzylamine (bzlan) (4a)}, and trans-[CoIII(cbpb)(amine)2]X {cbpb2ꢀ = N,N-
bis(pyridine-2-carboxamido)-4-chlorobenzene dianion, and amine = pyrrolidine (prldn), X = PF6 (1b),
piperidine (pprdn), X = PF6 (2b), morpholine (mrpln), X = ClO4 (3b), benzylamine (bzlan), X = PF6 (4b)}
have been synthesized and characterized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy.
The crystal structure of 1a has been determined by X-ray diffraction. The electrochemical behavior of
these complexes, with the goal of evaluating the effect of axial ligation and equatorial substitution on
the redox properties, is also reported. The reduction potential of CoIII, ranging from ꢀ0.53 V for (1a) to
ꢀ0.31 V for (3a) and from ꢀ0.48 V for (1b) to ꢀ0.22 V for (3b) show a relatively good correlation with
Article history:
Received 27 March 2008
Accepted 26 May 2008
Available online 18 July 2008
Keywords:
Bispyridylamide complexes of Co(III)
Substituted N4-amido ligand
Crystal structure
Cyclic voltammetry
the
r-donor ability of the axial ligands. The methyl and chloro substituents of the equatorial ligand have
a considerable effect on the redox potentials of the central cobalt ion and the ligand-centered redox
processes.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Complexes of somewhat similar ligands containing two pyri-
dine-2-carboxamide groups have been extensively studied in re-
The binding of metal ions by proteins and peptides is of funda-
mental interest due to the importance of metal ions in biological
systems [1]. The carboxamide [–C(O)NH–] group, ubiquitous
throughout nature in the primary structure of proteins, is an
important ligand construction unit for coordination chemists. Pyri-
dinecarboxamides, a burgeoning class of multidentate ligands con-
taining this linkage, are available from condensation reactions
between pyridyl-bearing amine or carboxylic acid precursors, pro-
moted by coupling agents such as 1,10-carbonyldiimadazole,
diphenylphosphoryl azide or triphenylphosphite [2–6]. These li-
gands have found use in asymmetric catalysis [3,7], molecular
receptors and dendrimer synthesis [8–10], and platinum(II) com-
plexes with antitumor properties [5]. Transition metal complexes
of dianionic bis(amide) ligands, derived from picolinic acid and
2,6-pyridinedicarboxylic acid, are subject to electrochemical oxi-
dation at both metal and ligand sites to form metal- and ligand-
centered oxidation products, respectively [6,11–16].
cent years [17–31]. The deprotonated amide is a strong r-donor
capable of stabilizing early as well as late metal ions in high oxida-
tion states [21–23], making high-valent metal complexes of the li-
gands suitable as Lewis acid catalysts. Although a large number of
metal complexes have been characterized and their electrochemi-
cal, spectroscopic and magnetic properties as well as their ligand
exchange ability studied, it is only recently that they have found
more extensive applications in catalysis [27,28]. One factor which
makes the ligands attractive for catalytic applications is the sim-
plicity whereby the structure of the ligands can be modified by a
modular approach. The electronic and steric properties can conve-
niently be modified by altering the diamine backbone and by the
introduction of suitable substituents in the pyridine nuclei [12].
Since the pyridine nitrogen atom resembles imidazole nitrogen,
metal complexes of bispyridylamides have also been employed
as models for metalloenzymes [27].
As part of our continued interest in the transition-metal chem-
istry of dianionic bispyridylamide ligands [25,30,31], herein we de-
scribe the synthesis and spectroscopic characterization of a group
of diaxially ligated octahedral complexes of CoIII with Mebpb2ꢀ
and cbpb2ꢀ as equatorial ligands (Scheme 1). The crystal structure
* Corresponding author. Tel.: +98 311 391 2351; fax: +98 311 391 2350.
E-mail address: amirnasr@cc.iut.ac.ir (M. Amirnasr).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.05.031

2772
S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
was filtered off, washed with ethanol (20 mL), and recrystallized
from a mixture of methanol and water (2:1 v/v). Yield (70%).
m.p. 168–169 °C. FT-IR (KBr, cmꢀ1
max: 3302 (s, N–H), 1689,
1659 (s, C@O), 1589 (m, C@C), 1520 (m, C–N). UV–Vis: kmax (nm)
, L Mꢀ1 cmꢀ1) (CH3CN): 300 (9427), 267 (15662), 219 (26563).
R
)
m
(e
O
O
N
H
N
H
2.2.2. H2cbpb
H2cbpb was synthesized by a procedure similar to that for
H2Mebpb, but 4-chloro-1,2-phenylenediamine was used instead
of 4-methyl-1,2-phenylenediamine. The product was recrystallized
from ethanol. Yield (70%). m.p. 196–198 °C. Anal. Calc. for
N
R
N
Ligand
C
18H13N4O2Cl: C, 61.28; H, 3.71; N, 15.88. Found: C, 61.28; H,
3.61; N, 15.81%. FT-IR (KBr, cmꢀ1
max: 3297 (s, N–H), 1697,
1661 (s, C@O), 1588 (m, C@C), 1506 (s, C–N). UV–Vis: kmax (nm)
L Mꢀ1 cmꢀ1
(CH3CN): 300 (10735), 267 (18627), 211
(35942). 1H NMR (CDCl3, 300 MHz): d = 10.32, 10.19 (2H, NHamidic
CH3
Cl
H2Mebpb
H2cbpb
)
m
(e
,
)
Scheme 1.
,
s), 8.56 (2Ha, dd, J = 9.8, 4.4), 8.31 (2Hd, d, J = 7.8), 8.03 (1Hg, d,
J = 7.8), 7.92 (2Hc, dd, J = 7.7, 7.7), 7.81 (1He, d, J = 8.6), 7.48 (2Hb,
m), 7.26 (1Hf, d, J = 8.0).
of a representative complex, trans-[Co(Mebpb)(prldn)2]ClO4, has
been determined. The IR, UV–Vis, and 1H NMR spectra of the com-
plexes are also reported and discussed. The electrochemical behav-
ior of the complexes has also been investigated with the goal of
evaluating the effects of axial ligation and equatorial substitution
on their redox properties.
2.2.3. Synthesis of trans-[Co(Mebpb)(prldn)2] ClO4 (1a)
To a solution of Co(CH3COO)2 ꢁ 4H2O (62.3 mg, 0.25 mmol) in
methanol (20 mL) was added a boiling solution of H2Mebpb
(83.1 mg, 0.25 mmol) in methanol (30 mL). To this solution was
added dropwise 9.07 mmol (0.75 mL) of pyrrolidine and air was
bubbled through the reaction mixture for 3 h. The final reaction
mixture was filtered, a solution of 0.25 mmol (30.6 mg) of Na-
ClO4 ꢁ H2O in methanol was slowly added to the filtrate, and the
resulting solution was stirred for 5 min. Dark blue crystals were
obtained by slow evaporation of methanol. The crystals were iso-
lated by filtration, washed with methanol, and dried in vacuum.
Yield: 75%. Anal. Calc. for C27H32N6O6ClCo: C, 51.4; H, 5.11; N,
2. Experimental
2.1. Materials and general methods
All solvents and chemicals were of commercial reagent grade
and used as received from Aldrich and Merck. Elemental analyses
were performed by using a Perkin–Elmer 2400II CHNS-O elemental
analyzer. UV–Vis spectra were recorded on a Jasco V-570 spectro-
photometer. Infrared spectra (KBr pellets) were obtained on a Jasco
680-plus FT-IR spectrophotometer. The 1H NMR spectra of the
13.32. Found: C, 50.73; H, 5.29; N, 13.11%. FT-IR (KBr, cmꢀ1
3168 (s, N–H), 1626 (s, C@O), 1599 (s, C@C), 1571 (s, C–N), 1109,
) mmax:
1087 (s, Cl–O). UV–Vis: kmax (nm) (e
, L Mꢀ1 cmꢀ1) (CH3CN): 625
complexes were obtained on
a
Bruker AVANCE DR X500
(180), 418 (6686). 1H NMR (CD3CN, 500 MHz): d = 1.29 (8Hi, m),
1.55, 2.12 (8Hh, m), 2.39 (3H, CH3, s), 3.30 (2H, amine NH, br s),
6.94 (1Hf, d, J = 8.2), 8.00–8.39 (6H b,c,d, m), 8.71 (1Hg, s), 8.75
(1He, d, J = 8.2), 9.46 (2Ha, dd, 5.2, 5.2).
(500 MHz) spectrometer. Proton chemical shifts are reported in
parts per million (ppm) relative to an internal standard of Me4Si.
Cyclic voltammograms were recorded by using a Sama Research
Analyzer M-500. Three electrodes were utilized in this system, a
glassy carbon working electrode, a platinum disk auxiliary elec-
trode, and Ag wire as reference electrode. The glassy carbon work-
ing electrode (Metrohm 6.1204.110) with 2.0 0.1 mm diameter
2.2.4. Synthesis of trans-[Co(cbpb)(prldn)2] PF6 (1b)
To a solution of Co(CH3COO)2 ꢁ 4H2O (62.3 mg, 0.25 mmol) in
methanol–ethanol (2:1 v/v) (20 mL) was added a boiling solution
of H2cbpb (88.2 mg, 0.25 mmol) in methanol–ethanol (2:1 v/v)
(30 mL). To this solution was added dropwise 9.07 mmol
(0.75 mL) of pyrrolidine and air was bubbled through the reaction
mixture for 3 h. The volume of the reaction mixture was increased
to 75 mL by adding methanol and the final solution was filtered. A
solution of 0.25 mmol (40.6 mg) of ammonium hexafluorophos-
phate in methanol–ethanol (2:1v/v) (25 mL) was slowly added to
the filtrate and the resulting solution stirred for 5 min. Green-blue
crystals were obtained by slow evaporation of solvent. The crystals
were isolated by filtration, washed with cold methanol, and dried
in vacuum. Yield: 75%. Anal. Calc. for C29H29N6O2ClPF6Co: C,
44.81; H, 4.19; N, 12.05. Found: C, 44.54; H, 4.09; N, 11.91%. FT-
was manually cleaned with 1 lm alumina polish prior to each scan.
Tetrabutylammonium hexafluorophosphate (TBAH) was used as
supporting electrolyte. Acetonitrile was dried over CaH2. The solu-
tions were deoxygenated by purging with Ar for 5 min. All electro-
chemical potentials were calibrated versus internal Fc+/0
(E0 = 0.40 V versus SCE) couple under the same conditions [32].
Caution: Perchlorate salts with organic compounds are poten-
tially explosive and should be handled with care.
2.2. Synthesis
2.2.1. H2Mebpb
4-Methyl-1,2-phenylenediamine was recrystallized from hot
ligroin before use. The ligand was prepared according to the meth-
od described in the literature with some modification [18]. To a
solution of 2-pyridinecarboxylic acid (7.38 g, 60 mmol) in pyridine
IR (KBr, cmꢀ1
1563 (s, C–N), 842 (s, PF6). UV–Vis: kmax (nm) (
)
m
max: 3171 (s, N–H), 1629 (s, C@O), 1600 (s, C@C),
e
, L Mꢀ1 cmꢀ1
)
(CH3CN): 611 (188), 410 (7354). 1H NMR (CD3CN, 500 MHz):
d = 1.30 (8Hi, m), 1.52, 2.12 (8Hh, m), 3.28 (2H, NHamine, br s),
7.11 (1Hf, bd, J = 8.6), 7.99–8.41 (6Hb,c,d, m), 8.82–8.83 (2H, He,
Hg), 9.42 (2Ha, bd, J = 4.8).
(25 mL) in
a two-necked round-bottom flask equipped with
mechanical stirrer was added a solution of 4-methyl-1,2-phenyl-
enediamine (3.66 g, 30 mmol) in 5 mL pyridine, and the mixture
stirred at 40 °C for 40 min. Triphenylphosphite (15.6 mL, 60 mmol)
was then added dropwise and the reaction mixture was stirred at
90–100 °C for 4 h. Upon cooling to room temperature the product
precipitated as a microcrystalline solid. The resulting precipitate
2.2.5. Synthesis of trans-[Co(Mebpb)(pprdn)2]ClO4 (2a)
To a solution of Co(CH3COO)2 ꢁ 4H2O (62.3 mg, 0.25 mmol) in
20 mL ethanol–methanol (1:2 v/v) was added a boiling solution
of H2Mebpb (83.1 mg, 0.25 mmol) 30 mL ethanol–methanol (1:2

S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
2773
v/v). To this solution was added dropwise 7.58 mmol (0.75 mL) of
piperidine and air was bubbled through the reaction mixture for
3 h. The final reaction mixture was filtered and a solution of
0.25 mmol (30.6 mg) of NaClO4 ꢁ H2O in methanol was added
slowly to the filtrate. Dark green-blue crystals were obtained by
slow evaporation of solvent. The crystals were isolated by filtra-
tion, washed with a mixture of ethanol and methanol (1:2 v/v),
and dried in vacuum. Yield: 80%. Anal. Calc. for C29H36N6O6ClCo:
C, 52.85; H, 5.51; N, 12.75. Found: C, 52.32; H, 6.1; N, 12.2%. FT-
kmax (nm) (
NMR (CD3CN, 500 MHz): d = 1.61, 2.98 (8Hh, m), 3.55 (2H, NHamine
br s), 3.21 (8Hi, m), 7.09 (1Hf, m), 7.81–8.48 (6Hb,c,d, m), 8.82–8.91
e
, L Mꢀ1 cmꢀ1) (CH3CN): 663 (232), 405 (7061). 1H
,
(2He,g, m), 9.39 (2Ha, br d, J = 4.8).
2.2.9. Synthesis of trans-[Co(Mebpb)(bzlan)2] ClO4 (4a)
To a solution of Co(CH3COO)2 ꢁ 4H2O (24.9 mg, 0.1 mmol) in
methanol (20 mL) was added a boiling solution of H2 Mebpb
(33.2 mg, 0.1 mmol) in methanol (30 mL). To this solution was
added dropwise 6.86 mmol (0.75 mL) benzylamine and air was
bubbled through the reaction mixture for 3 h. The final reaction
mixture was filtered and a solution of 0.1 mmol (12.2 mg) of Na-
ClO4 ꢁ H2O in methanol was added slowly to the filtrate. Dark
green-blue crystals were isolated by filtration, washed with a mix-
ture of ethanol and diethyl ether (9:1 v/v), and dried in vacuum.
Yield: 65%. Anal. Calc. for C33H32N6O6ClCo: C, 56.38; H, 4.59; N,
IR (KBr, cmꢀ1
) mmax: 3162 (s, N–H), 1626 (s, C@O), 1599 (s, C@C),
1565 (s, C–N), 1111, 1087 (s, Cl–O). UV–Vis: kmax (nm) (e,
L Mꢀ1 cmꢀ1) (CH3CN): 682 (193), 417 (6532). 1H NMR (CD3CN,
500 MHz): d = 0.91–1.28 (12Hi,j, m), 1.40–2.22 (8Hh, m), 2.39 (3H,
CH3, s), 2.45 (2H, NHamine, br s), 6.93 (1Hf, d, J = 7.9), 8.00–8.41
(6Hb,c,d, m), 8.71 (Hg, s), 8.75 (He, d, 8.2) 9.39 (2Ha, br d, J = 5.0).
2.2.6. Synthesis of trans-[Co(cbpb)(pprdn)2]PF6 (2b)
11.95. Found: C, 56.10; H, 4.52; N, 11.84%. FT-IR (KBr, cmꢀ1
3251, 3214, 3135 (m, m, s, N–H), 1628 (s, C@O), 1601 (s, C@C),
1566 (s, C–N), 1094, 1081 (s, Cl–O). UV–Vis: kmax (nm) (
L Mꢀ1 cmꢀ1) (CH3CN): 610 (235), 414 (7013). 1H NMR (CD3CN,
500 MHz): d = 2.42 (3H, CH3, s), 2.74 (4Hh, m), 2.91 (4H, NH2amine
) mmax:
This complex was prepared by the same method as for 1b ex-
cept that piperidine was used instead of pyrrolidine. Dark green-
blue crystals were obtained after 5 days; they were filtered off,
washed with a small amount of cold methanol, and dried in vac-
uum. Yield: 65%. Anal. Calc. for C28H33N6O2ClPF6Co: C, 46.39; H,
4.58; N, 11.59. Found: C, 45.52; H, 4.52; N, 11.25%. FT-IR (KBr,
e
,
,
br s), 6.74 (4Hi, bd), 7.09–7.15 (6Hj,k, m), 6.96(1Hf,d, J = 8.2),
7.94–8.35 (6Hb,c,d, m), 8.75 (1Hg, s), 8.78 (1He, d, J = 8.2), 9.41
(2Ha, dd, J = 4.6, 4.6, 4.6).
cmꢀ1
C–N), 841 (s, PF6). UV–Vis: kmax (nm) (
) mmax: 3173 (s, N–H), 1627 (s, C@O), 1600 (s, C@C), 1562 (s,
e
, L Mꢀ1 cmꢀ1) (CH3CN):
656 (205), 409 (7434). 1H NMR (CD3CN, 500 MHz): d = 0.9–1.29
(12Hi,j, m), 1.35–2.22 (8Hh, m), 2.47 (2H, NHamine, br s), 7.12 (1Hf,
d, J = 8.6), 7.95–8.45 (6Hb,c,d, m), 8.83–8.87 (2H, He, Hg), 9.36
(2Ha, br d, J = 4.8).
2.2.10. Synthesis of trans-[Co(cbpb)(bzlan)2] PF6 (4b)
This complex was prepared by the same method as for 1b ex-
cept that benzylamine was used instead of pyrrolidine. Dark
green-blue crystals were obtained after one week; they were fil-
tered off, washed with a mixture of ethanol and diethyl ether
(9:1 v/v), and dried in vacuum. Yield: 65%. Anal. Calc. for
2.2.7. Synthesis of trans-[Co(Mebpb)(mrpln)2] ClO4 (3a)
To a solution of Co(CH3COO)2 ꢁ 4H2O (62.3 mg, 0.25 mmol) in
methanol (5 mL) was added a solution of H2Mebpb (83.1 mg,
0.25 mmol) in acetone (50 mL). To this solution was added drop-
wise morpholine (8.61 mmol, 0.75 mL) and air was bubbled
through the reaction mixture for 3 h. The final reaction mixture
was filtered and a solution of 0.25 mmol (30.6 mg) of NaClO4 ꢁ H2O
in acetone was added slowly to the filtrate. Dark green-blue crys-
tals were isolated by filtration, washed with a mixture of acetone
and methanol (1:9 v/v), and dried in vacuum. Yield: 70%. Anal. Calc.
for C27H32N6O8ClCo: C, 48.92; H, 4.86; N, 12.68. Found: C, 48.01; H,
C
32H29N6O2ClPF6Co: C, 49.98; H, 3.80; N, 10.93. Found: C, 49.11;
H, 3.80; N, 10.66%. FT-IR (KBr, cmꢀ1
max: 3213, 3133 (m, N–H),
1629 (s, C@O), 1601 (s, C@C), 1562 (s, C–N), 843 (s, PF6). UV–Vis:
kmax (nm) (
, L Mꢀ1 cmꢀ1) (CH3CN): 607 (224), 408 (7210). 1H
)
m
e
NMR (CD3CN, 500 MHz): d = 2.74 (4Hh, m), 3.05 (4H, NH2amine, br
s), 6.75 (4Hi, bd, 7.6), 7.05–7.16 (7Hj,k,f, m), 7.96–8.33 (6Hb,c,d, m),
8.73–8.76 (2He,g, m), 9.41 (2Ha, br d).
2.3. X-ray crystallography
5.18; N, 12.64%. FT-IR (KBr, cmꢀ1
C@O), 1599 (s, C@C), 1566 (m, C–N), 1111, 1091 (s, Cl–O). UV–
)
mmax: 3141 (m, N–H), 1626 (s,
Dark blue crystals of 1a suitable for X-ray crystallography were
obtained from methanol. Data were collected at 150 K, on a Nonius
KappaCCD diffractometer (Mo K
Vis: kmax (nm) (
e
, L Mꢀ1 cmꢀ1) (CH3CN): 689 (198), 412 (8134).
a
radiation, k = 0.71069 Å) using
1H NMR (CD3CN, 500 MHz): d = 1.65–1.95 (8Hh, overlaps with sol-
vent signals), 2.39 (3H, CH3, s), 2.97 (2H, NHamine, br s), 3.20 (8Hi,
m), 6.95 (1Hf, d, J = 7.9), 8.02–8.44 (6Hb,c,d, m), 8.73 (1Hg, s), 8.75
(1He, d, J = 8.2), 9.42 (2Ha, br d, J = 4.9).
Table 1
Crystal data and structure refinement for (1a)
ꢀ
Chemical formula
[C27H32CoN6O2]+ClO4
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
631.0
triclinic
2.2.8. Synthesis of trans-[Co(cbpb)(mrpln)2] ClO4 (3b)
ꢀ
P1
To a solution of Co(CH3COO)2 ꢁ 4H2O (24.9 mg, 0.1 mmol) in
methanol–ethanol (2:1 v/v) (20 mL) was added a solution of
H2cbpb (35.3 mg, 0.1 mmol) in methanol–ethanol (2:1 v/v)
(30 mL). To this solution was added dropwise morpholine
(8.61 mmol, 0.75 mL) and air was bubbled through the reaction
mixture for 3 h. The volume of the reaction mixture was increased
to 75 mL by adding methanol and the final solution was filtered. A
solution of 0.1 mmol (12.2 mg) of NaClO4 ꢁ H2O in methanol–etha-
nol (2:1 v/v) (25 mL) was slowly added to the filtrate and the
resulting solution stirred for 5 min. Green-blue crystals were ob-
tained by slow evaporation of solvent. The crystals were isolated
by filtration, washed with mixture of diethyl ether and methanol
(1:9 v/v) and dried in vacuum. Yield: 65%. Anal. Calc. for
9.382(5)
11.788(5)
13.361(5)
84.155(5)
81.886(5)
82.339(5)
1444.6(11)
2, 1.451
a
(°)
b (°)
c
(°)
V (Å3)
Z, DCalc (g cmꢀ3
)
Crystal size (mm)
0.40 ꢂ 0.40 ꢂ 0.35
l
(mmꢀ1
)
0.738
Reflections collected
Independent reflections (Rint
Transmission factors
Refined parameters
29046
)
5671 (0.0293)
0.757–0.782
381
1.108
0.0680
0.2070
1.89 and ꢀ1.22
Goodness-of-fit on F2
R (F2 > 2
r
)
C
26H29N6O8Cl2Co: C, 45.69; H, 4.28; N, 12.29. Found: C, 45.35; H,
4.29; N, 12.00%. FT-IR (KBr, cmꢀ1
max: 3169 (m, N–H), 1626 (s,
C@O), 1600 (s, C@C), 1562 (s, C–N), 1119, 1089 (s, ClO4). UV–Vis:
Rw (F2, all data)
Maximum and minimum electron density (e Åꢀ3
)
m
)

2774
S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
Table 2
the COLLECT [33] program. Cell refinement and data reduction were
performed with EVALCCD [34]. Crystal data, together with other rele-
vant information on structure determination, are listed in Table 1.
An empirical absorption correction, based on repeated and equiva-
lent reflections, was carried out using SADABS [35]. The structure was
solved by SIR97 [36] and refined, against F2, by SHELXTL [37]. Aniso-
tropic displacement parameters were assigned to all the non-
hydrogen atoms. Hydrogen atoms were placed in idealized posi-
tions and allowed to ride on their respective parent atoms. The ma-
jor residual electron density features are close to the anion, which
is probably subject to unresolved disorder.
Selected bond lengths (Å) and angles (°) for (1a)
Bond lengths
Co–N1
Co–N2
Co–N3
Co–N4
1.997(3)
1.892(3)
1.892(3)
1.994(3)
1.992(3)
2.012(3)
Co–N5
Co–N6
Bond angles
N1–Co–N2
N1–Co–N3
N1–Co–N4
N1–Co–N5
N1–Co–N6
N2–Co–N3
N2–Co–N4
N2–Co–N5
N2–Co–N6
N3–Co–N4
N3–Co–N5
N3–Co–N6
N4–Co–N5
N4–Co–N6
N5–Co–N6
82.88(13)
166.85(14)
110.63(13)
90.16(13)
89.36(13)
84.10(14)
166.28(13)
92.86(13)
88.43(13)
82.46(14)
88.69(13)
92.08(13)
89.59(13)
89.32(13)
178.57(13)
3. Results and discussion
3.1. Synthesis
All complexes were prepared by the following general method.
To a solution of Co(CH3COO)2 ꢁ 4H2O in an appropriate solvent was
added an equimolar amount of ligand in an appropriate solvent. A
vigorous stream of air was passed through this solution, to which
was gradually added excess amine. The air oxidation was contin-
ued for a period of 3 h, during which the color changed from
red-brown to green-blue. Dark green-blue crystals of these com-
plexes were obtained in good yield (65–80%).
6
1.892(3)–2.012(3) Å, and are indicative of a low-spin (t2g
)
config-
uration of the central CoIII ion [22].
3.2. Description of structure of trans-[Co(Mebpb)(prldn)2]ClO4 (1a)
Of the four Co–N bonds in the equatorial plane, the Co–Namide
bond distances [Co–N2 = 1.892(3) Å and Co–N3 = 1.892(3) Å],
which are comparable to the Co–Namide values [1.885(2) Å and
1.891(1) Å] in trans-[CoIII(bpb)(prldn)2]NCS, are indicative of
strong bonding between the deprotonated amide nitrogens and
the cobalt center. This is in agreement with the fact that the depro-
To the best our knowledge, this is only the second reported
structure of a [CoIII(bpb)(amine)2]+ complex containing neutral N-
donor axial ligands. Fig. 1 shows the molecular structure of 1a with
the atomic numbering scheme. The structure exhibits a distorted
octahedral environment around cobalt. Selected bond angles and
distances are listed in Table 2. As expected, the ligand (Mebpb2ꢀ
)
tonated amide nitrogen is a very strong
r-donor. The deprotonated
in 1a binds to the trivalent metal ion (CoIII) via two pyridine N
atoms and two amido N atoms in the equatorial plane of the octa-
hedron. There are two pyrrolidine molecules attached to the metal
to fill its axial sites. All Co–N bond distances are within the range
mode of coordination of the amide nitrogen atoms is also evident
from the absence of the amide proton signals in the 1H NMR spec-
trum of complex 1a (vide infra). The bond distances to the pyridine
nitrogen atoms [Co–N1 = 1.997(3) Å and Co–N4 = 1.994(3) Å] are
longer than those to N2 and N3, which is in accord with the lower
donor ability of the pyridine nitrogen atoms. The N–Co–N bond an-
gles in the equatorial plane consist of one which is larger than 90°
[N1–Co–N4 = 110.63(13)°] and three smaller bite angles [N1–Co–
N2 = 82.88(13)°,
N2–Co–N3 = 84.10(14)°,
and
N3–Co–
N4 = 82.46(14)°]. This indicates that there might be some strain
in the coordination due to the rigid geometry of the ligand. The
dihedral angle between the two pyridyl rings of the ligand is
15.0°. The dihedral angles between the central benzene plane
and the two pyridyl rings are 10.7° and 5.5°. This observation can
be attributed to the close intramolecular contact that occurs be-
tween the two H atoms in the 6-positions of the pyridyl rings;
the H1ꢁ ꢁ ꢁH18 distance of 2.05 Å is significantly less than the sum
of the van der Waals radii (2.40 Å). The effect would be expected
to be more pronounced if the ligand were perfectly planar [38].
The C–O, Cpyridyne–N, and Ccarboxy–N bond distances agree well with
those reported for related complexes [16,20,39–41].
All cis Neq–Co–Nax bond angles are within 3° of the ideal right-
angle. The trans-axial N5–Co–N6 unit with a bond angle of
178.57(13)° is essentially linear. The distances between the cobalt
atom and the two axial nitrogen donor atoms differ only slightly
[Co–N5 = 1.992(3) Å and Co–N6 = 2.012(3) Å] and compare well
with the Co–N distances found in trans-[CoIII(bpb)(prldn)2]NCS
[25].
A further interesting feature of the structure of 1a is the inter-
molecular hydrogen bonding (Table 3). The structure consists of
chains of cations linked by rather strong N–Hꢁ ꢁ ꢁO@C hydrogen
bonds zig-zagging along the c axis (Fig. 2); the anions are not in-
volved in hydrogen bonding.
Fig. 1. The asymmetric unit of complex 1a with the atom labeling scheme.
Displacement ellipsoids are drawn at the 50% probability level; H atoms are omitted
for clarity.

S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
2775
Table 3
the ligands coordinate in their deprotonated form [17]. The bands
corresponding to the amine N–H stretching vibrations are shifted
to lower frequencies by about 98–265 cmꢀ1 relative to the uncoor-
dinated amines, which is in accord with expectation. Two sharp
C@O stretching vibration bands at 1689 and 1659 cmꢀ1 for
H2Mebpb and 1661 and 1697 cmꢀ1 for H2cbpb are shifted to lower
frequencies upon coordination of the deprotonated amide, which is
in agreement with the data reported for related complexes [31].
The C–N stretching vibration of medium intensity appearing at
1520 cmꢀ1 for H2Mebpb and at 1506 cmꢀ1 for H2cbpb undergoes
a significant shift to higher frequencies in the corresponding com-
plexes. This displacement is to be expected because of the reso-
nance enhancement in the deprotonated amide which in turn
leads to strengthening of the C–N bond [17]. The stretching vibra-
tions due to the perchlorate and hexafluorophosphate anions
appear in the 1080–1120 cmꢀ1 and 840–846 cmꢀ1 regions, respec-
tively [42].
Hydrogen bond lengths (Å) and angles (°) for (1a)
D–Hꢁ ꢁ ꢁA
N5–H5ꢁ ꢁ ꢁO2A
N6–H6ꢁ ꢁ ꢁO1B
D–H (Å)
Hꢁ ꢁ ꢁA (Å)
2.06(5)
2.07(5)
Dꢁ ꢁ ꢁA (Å)
2.847(4)
2.896(4)
D–Hꢁ ꢁ ꢁA (°)
159(4)
158(4)
0.83(5)
0.87(5)
Symmetry operations for equivalent atoms: (A) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z and (B) 1 ꢀ x,
1 ꢀ y, ꢀz.
3.3. Spectroscopic studies
The octahedral geometry of complex 1a is evident from the X-
ray structure analysis (Fig. 1). An octahedral trans structure for
other complexes (2a–4a, 1b–4b) can be inferred based on the sim-
ilarity of the spectroscopic data of these complexes with those of
complex 1a. The FT-IR data of the complexes are listed in Section
2. Meaningful information regarding the bonding sites of the ligand
molecules can be obtained by comparing the IR spectra of the co-
balt complexes with those of the uncomplexed ligands. In the IR
spectra of H2Mebpb and H2cbpb a band characteristic of the NH
group is observed in the 3295–3305 cmꢀ1 region. The disappear-
ance of this band in the IR spectra of the complexes confirms that
The UV–Vis data are presented in the Experimental section. The
electronic spectra of the trans-[CoIII(Mebpb)(amine)2]ClO4 com-
plexes (1a–4a) consist of ligand field transitions with low intensity
(180 <
e < 235) which, depending on the axial amine ligand field
strength, appear in the 610–689 nm region, and intense LMCT
Fig. 2. A view, along the b axis, of the packing of 1a. Hydrogen bonding is shown as dashed lines. Anions have been omitted, together with H atoms not involved in the
hydrogen bonding.
+
h
h
amine
i
R
N
H
Hf
N
Co
Hg
He
O
O
Hd
N
i
Co
H
N
N
Hc
Co
N
j
Ha
Hb
amine
i
h
H
Compound
R
Amine
N
Co
O
CH3
Cl
j
i
Pyrrolidine
Piperidine
1a 1b
2a 2b
h
k
Morpholine
Benzylamine
3a
3b
4a 4b
H
N
Co
H
Scheme 2.

2776
S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
transitions appearing in the 412–418 nm region. Similar transi-
tions have been observed in the electronic absorption spectra of
trans-[CoIII(cbpb)(amine)2]X complexes (1b–4b) with a blue shift
of about 3–25 nm in the position of all the absorption bands rela-
tive to those of trans-[CoIII(Mebpb)(amine)2]ClO4 complexes (d–d
transitions, 607–656 nm and CT transitions, 405–410 nm).
The 1H NMR spectroscopic measurements were performed on
samples in CD3CN and CDCl3 solutions. Scheme 2 displays the
labeling system. The assignments are based on comparisons with
corresponding rhodium(III), iridium(III), organocobalt(III) and
ruthenium(II) complexes and by integrated area ratios [17,18,43–
45].
The absence of the amido N–H resonance in the spectra of the
complexes indicates that the ligand is coordinated to cobalt in its
deprotonated form. The proton signals of the equatorial amide li-
gand in the complexes are generally shifted downfield relative to
the free ligand. As expected, the most significant downfield shifts
correspond to the protons ortho to the pyridyl nitrogen, Ha, and
ortho to the nitrogen substituent on the diamido benzene group,
He,g [44]. The broad doublet of doublet at ca. d = 9.39–9.46 is as-
signed to Ha.
Fig. 3. Cyclic voltammogram of H2Mebpb ligand in acetonitrile solution at 298 K.
Scan rate: 100 mV sꢀ1, c = 2.8 ꢂ 10ꢀ3 M.
3.4.1. Electrochemistry of the ligands
Although the chemical shifts of Hf were found to be virtually
unaffected by the axial ligand, chemical shifts of He,g and Ha of
The cyclic voltammograms of the ligands and their complexes
were conducted at 25 °C under an argon atmosphere. It is evident
from the cyclic voltammogram of H2Mebpb, displayed in Fig. 3,
that the ligand is electroactive over a range from ꢀ0.5 to ꢀ2.2 V.
Two ligand-centered reductions are observed at ꢀ1.89 V and
ꢀ2.2 V (Fig. 3). These reductions are attributed to the pyridyl rings
and are shifted to more positive values in the corresponding com-
plexes (Table 4). The tendency of the ligand towards reduction is
apparently increased due to the transfer of electron density to
the metal center [46].
the cobalt complexes varied significantly. In general, d (He,g
)
changes in the same direction as d (Ha) but to a larger extent.
The changes in the chemical shift of Hc and Hd are in line with
the donor ability of the axial ligands and the UV–Vis spectral data.
The trend observed in
bzlan > prldn > pprdn > mrpln, conforms to the
d
(Hc, Hd) for complexes 1a–4a,
-donating
r
strength of the axial ligands, which in turn is evident from the elec-
trochemistry of these complexes.
The appearance of the signals due to the amine protons indi-
cates the coordination of two amine ligands in the axial positions.
Aside from the aromatic protons of benzylamine which appear at
about 6.74–7.12 ppm, other proton resonances of the axial amines
appear between about 0.91 and 3.55 ppm.
The cyclic voltammogram of H2cbpb shows features similar to
those observed for H2bpb, with an anodic shift in the peak posi-
tions resulting from the electron-withdrawing nature of the chloro
substitution in the H2cbpb ligand.
The 1H NMR spectra of trans-[CoIII(cbpb)(amine)2]+ complexes
(1b–4b) show features similar to those observed for trans-[CoIII-
(Mebpb)(amine)2]+ complexes. As expected, the protons on the
benzene ring exhibit slight downfield shifts in the peak positions
in the [CoIII(cbpb)(amine)2]+ complexes, demonstrating the elec-
tron-withdrawing effect of chloro substitution in the equatorial
cbpb2ꢀ ligand.
3.4.2. Electrochemistry of the complexes in the absence of amines
The reduction potentials for the cobalt complexes are summa-
rized in Table 4. The cyclic voltammogram of the representative
complex, trans-[CoIII(Mebpb)(pprdn)2]ClO4 (2a), is shown in Fig.
4a. An interesting observation in the cyclic voltammetric measure-
ments of these cobalt(III) complexes is the presence of a nearly
reversible oxidative response in the range of E1/2 = 0.98–1.01 V
(Fig. 4, Table 4) (the first oxidation process, which can be assigned
as mainly ligand-centered, occurs at virtually identical potentials).
The next reduction peak observed over the range of ꢀ0.31 to
ꢀ0.53 V in (1a–4a) complexes, and ꢀ0.22 to ꢀ0.49 V in (1b–4b)
complexes, is metal-centered. This is supported by the relative sen-
sitivity of CoIII/CoII reduction potentials to the nature of the axial
amine ligand. From the Epc1 values (Table 4), it is evident that
3.4. Electrochemistry
Cyclic voltammograms of the ligands and complexes in acetoni-
trile solutions with 0.1 M [N(n-Bu)4]PF6 as the supporting electro-
lyte were recorded at a glassy carbon working electrode. The
approximate concentrations of the compounds were 1 ꢂ 10ꢀ3
–
10ꢀ6 M.
Table 4
Redox potentials of (1a–4a) and (1b–4b) cobalt complexes in acetonitrilea
b
No.
Compound
E1/2
Epc1
Epa1
Epc2
Epa2
Epc3
Epa3
Epc4
Epa4
1a
2a
3a
4a
1b
2b
3b
4b
[Co(Mebpb)(Prldn)2]ClO4
[Co(Mebpb)(Pprdn)2]ClO4
[Co(Mebpb)(Mrpln)2]ClO4
[Co(Mebpb)(Bzlan)2]ClO4
[Co(cbpb)(Prldn)2]PF6
[Co(cbpb)(Pprdn)2]PF6
[Co(cbpb)(Mrpln)2]ClO4
[Co(cbpb)(Bzlan)2]PF6
1.01
1.03
1.04
0.98
1.16
1.16
1.19
1.12
ꢀ0.53
ꢀ0.42
ꢀ0.31
ꢀ.049
ꢀ0.48
ꢀ0.35
ꢀ0.22
ꢀ0.49
ꢀ0.14
ꢀ0.01
0.03
ꢀ0.05
ꢀ0.13
ꢀ0.06
0.33
ꢀ1.28
ꢀ1.24
ꢀ1.23
ꢀ1.26
ꢀ1.19
ꢀ1.17
ꢀ1.15
ꢀ1.13
ꢀ1.42
ꢀ1.39
ꢀ1.40
ꢀ1.38
ꢀ1.41
ꢀ1.38
ꢀ1.35
ꢀ1.43
ꢀ2.08
ꢀ2.06
ꢀ2.09
ꢀ2.05
ꢀ2.00
ꢀ2.00
ꢀ2.00
ꢀ2.01
ꢀ2.0
ꢀ1.99
ꢀ2.01
ꢀ1.97
ꢀ1.92
ꢀ1.92
ꢀ1.93
ꢀ1.92
ꢀ1.28
ꢀ1.23
ꢀ1.24
ꢀ1.25
ꢀ1.32
ꢀ1.32
ꢀ1.31
ꢀ1.32
ꢀ0.002
a
Potentials are vs. Fc+/0 in 0.1 M TBAH, T = 293 K. Scan rate 100 mV/s. Approximate concentrations: 1 ꢂ 10ꢀ3–10ꢀ6 M.
b
E1/2 = 0.5(Ec + Ea).

S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
2777
Fig. 5. Cyclic voltammograms of [CoIII (Mebpb)(pprdn)2]ClO4, 2a, at different
piperidine concentrations ratios (0, black), (50, green), (100, blue) in
acetonitrile containing 0.1 M TBAH as supporting electrolyte at 298 K. Scan rate
100 mV sꢀ1, c = 1.5 ꢂ 10ꢀ3 M. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
Fig. 4. Cyclic voltammograms of [CoIII(Mebpb)(pprdn)2]ClO4, c = 1.8 ꢂ 10ꢀ3 M (a),
and [CoIII (cbpb)(bzlan)2]PF6, c = 1.2 ꢂ 10ꢀ3 M (b), in acetonitrile at 298 K. Scan rate,
100 mV sꢀ1
.
results have been reported by Che et al. [16,43] and Ray and Muk-
herjee [19].
stronger axial ligands shift the reduction potentials to more nega-
tive values. In general the CoIII/CoII reductions for all these com-
plexes are electrochemically irreversible in nature [19].
Accordingly, we assume that the lack of reversibility observed
(Fig. 4) is most likely due to the loss of axial ligands from the cobal-
t(II) complex, because the electron is added to the antibonding dz2
orbital [47].
3.4.3. Electrochemistry of the complexes in the presence of amines. It
is well established that in many trans-[CoIII(N2O2)(L)2]+ Schiff base
complexes, the first irreversible reduction process (CoIII/CoII) can
become quasi-reversible under controlled concentration condi-
tions [47,50]. In an attempt to examine the possibility of reversible
electrochemical behavior in analogous amido complexes, the elec-
trochemistry of [CoIII(Mebpb)(pprdn)2]ClO4 was carried out in the
presence of excess amine. In the absence of additional amine the
first reduction processes of (CoIII/CoII) is electrochemically irrevers-
MeCN
½CoIIIðMebpbÞðamineÞ2ꢃþ þ eꢀ ! ½CoIIðMebpbÞꢃ þ 2 amine
ð1Þ
ible (
wave of Co(II) is shifted towards more negative potentials (Fig. 5)
E = 107 mV), indicating the increased formation of six-coordi-
DE = 410 mV). Upon addition of excess amine, the oxidation
Owing to the very facile substitution of the axial ligands in the
reduced form of these cobalt(III) complexes, electrochemical
reversibility for the CoIII/CoII couple is not observed. The oxidation
of [CoII(Mebpb)(MeCN)] adduct occurs at much higher potentials.
The CoIII/CoII cathodic wave in all [CoIII (Mebpb)(amine)2]+ com-
plexes is followed by two overlapping redox waves, resulting from
the combination of the CoII/CoI couple [48] and the redox couple
corresponding to one of the pyridyl rings (Fig. 4a). In the [CoIII(cbp-
b)(amine)2]+ complexes, however, these couples are resolved (Fig.
4b). The final electrochemically reversible reduction couple ob-
served in the voltammograms is related to the second pyridyl ring,
that appears between E1/2 = ꢀ 2.01 and ꢀ2.05 V for [CoIII(Mebp-
b)(amine)2]+ and at E1/2 = ꢀ 1.96 V for [CoIII(cbpb)(amine)2]+ com-
plexes. Additional evidence produced in substantiation of this
fact are the results obtained from the cyclic voltammetric studies
of the Ni(Mebpb) complex, that shows only two reduction waves
in the range ꢀ1.57 to ꢀ2.04 V, attributed to the consecutive reduc-
tion of the two pyridyl rings [49]. Similar results have been re-
ported for Ni(bpb) and Ni(bpen) complexes by Fenton et al. [46].
(D
nated adducts; i.e. the electrode process is preceded by a reversible
chemical reaction
½CoIIðMebpbÞꢃ þ excess amine½CoIIIðMebpbÞðamineÞ2ꢃþ þ eꢀ
ð2Þ
The redox potential of the Co(II/I) couple is not significantly chan-
ged by the addition of excess amine and can be regarded as approx-
imately independent.
4. Conclusion
The combination of spectroscopic, crystallographic, and electro-
chemical measurements we have described has illustrated the
main features of the coordination and redox chemistry of the
trans-[CoIII(Mebpb)(amine)2]ClO4
and
trans-[CoIII(cbpb)(ami-
ne)2]PF6 complexes. The coordination geometry around the CoIII
center in these complexes is distorted octahedral. Electrochemical
studies reveal that the first reduction process CoIII/CoII is irrevers-
ible and strongly influenced by the nature of the axial amines.
When the axial ligands are changed from morpholine to pyrroli-
dine a change in the CoIII/CoII cathodic peak of 220 mV for Mebpb
complexes and 260 mV for cbpb complexes is observed. This pro-
cess becomes quasi-reversible upon the addition of excess amine.
The two redox waves, resulting from the combination of the CoII/
CoI couple and the redox couple corresponding to one of the pyri-
dyl rings, overlap in [Co(Mebpb)(amine)2]+, but are resolved in
[Co(cbpb)(amine)2]+ complexes. In general, the redox couples of
[Co(cbpb)(amine)2]+ complexes are shifted to more positive values
relative to those of the corresponding [Co(Mebpb)(amine)2]+ com-
The
r-donor strength of the axial ligands also has a significant
influence on the CoIII/CoII reduction process, and the tendency of
CoIII towards reduction increases as the axial ligand becomes a
weaker donor. These observations are compatible with the
H
NMR and UV–Vis spectroscopic data.
A similar behavior has been found for [CoIII(cbpb)(amine)2]+
complexes. Fig. 4b shows the cyclic voltammogram of [CoIII(cbpb)-
(bzlan)2]PF6 (4b) as an example. In general the redox couples of
[CoIII(cbpb)(amine)2]+ complexes are shifted to more positive val-
ues relative to those of the corresponding [CoIII(Mebpb)(amine)2]+
complexes, demonstrating the electron-withdrawing effect of
chloro substitution on the electron density of the system. Similar

2778
S. Meghdadi et al. / Polyhedron 27 (2008) 2771–2778
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