Influence of the first and second coordination spheres on the diverse phenoxazinone synthase activity of cobalt complexes derived from a tetradentate Schiff base ligand

Article abstract of DOI:10.1039/c7nj02015e

This paper describes syntheses and structural characterization of four new cobalt(iii) compounds (1-4) derived from a N₃O donor Schiff base ligand, a condensation product of N,N-dimethyldipropylenetriamine and o-vanillin, and their catalytic activity relating to the function of phenoxazinone synthase. X-ray crystallography reveals that the Schiff base can coordinate the metal centre either in a tetradentate fashion through the monoanionic deprotonated form using all four donor sites (in 1 and 2) or in a tridentate fashion using the zwitterionic form of the Schiff base ligand, leaving the quaternary amine nitrogen free from coordination (in 3). The monoanionic deprotonated ligand can also bind the metal centre in a tridentate fashion where the pendent tertiary amine nitrogen is engaged in intramolecular hydrogen bonding in 4. Moreover, the triamine part of the Schiff base ligand can bind a metal centre both facially and meridionally. Therefore, all these versatilities associated with this triamine make it appealing for the development of coordination chemistry with diverse structures. All complexes are active functional models for phenoxazinone synthase, and as expected the availability of labile sites at the first coordination sphere for the substrate, o-aminophenol, binding is responsible for higher catalytic activity in 1 and 2. The importance of a proton abstraction site at the second coordination sphere behind the facile oxidation of the substrate is also explored (reactivity of 3vs.4). The remarkable finding from the mass spectral study discloses several important intermediates, and thereby provides significant information relating to the mechanistic pathway of the functioning phenoxazinone synthase activity of the synthetic models.

Full text of DOI:10.1039/c7nj02015e

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Influence of the first and second coordination
spheres on the diverse phenoxazinone synthase
activity of cobalt complexes derived from a
tetradentate Schiff base ligand†
Cite this:DOI: 10.1039/c7nj02015e
a
a
b
Anangamohan Panja, * Narayan Ch. Jana and Paula Brandao
˜
This paper describes syntheses and structural characterization of four new cobalt(III) compounds (1–4)
derived from a N O donor Schiff base ligand, a condensation product of N,N-dimethyldipropylenetriamine
3
and o-vanillin, and their catalytic activity relating to the function of phenoxazinone synthase. X-ray crystallo-
graphy reveals that the Schiff base can coordinate the metal centre either in a tetradentate fashion through
the monoanionic deprotonated form using all four donor sites (in 1 and 2) or in a tridentate fashion using
the zwitterionic form of the Schiff base ligand, leaving the quaternary amine nitrogen free from coordination
(in 3). The monoanionic deprotonated ligand can also bind the metal centre in a tridentate fashion where
the pendent tertiary amine nitrogen is engaged in intramolecular hydrogen bonding in 4. Moreover, the
triamine part of the Schiff base ligand can bind a metal centre both facially and meridionally. Therefore,
all these versatilities associated with this triamine make it appealing for the development of coordination
chemistry with diverse structures. All complexes are active functional models for phenoxazinone
synthase, and as expected the availability of labile sites at the first coordination sphere for the substrate,
o-aminophenol, binding is responsible for higher catalytic activity in 1 and 2. The importance of a proton
abstraction site at the second coordination sphere behind the facile oxidation of the substrate is also
explored (reactivity of 3 vs. 4). The remarkable finding from the mass spectral study discloses several
important intermediates, and thereby provides significant information relating to the mechanistic pathway
of the functioning phenoxazinone synthase activity of the synthetic models.
Received 6th June 2017,
Accepted 31st July 2017
DOI: 10.1039/c7nj02015e
rsc.li/njc
triplet ground state of O ; this high-energy barrier is nature’s way
2
Introduction
of protecting organic compounds (typically in the singlet ground
3
–6
Oxidation reactions play an important role in the synthesis state) from destructive oxidation. Nature has also developed
of valuable organic compounds, namely pharmaceuticals, agro- elegant mechanisms to make controlled aerobic oxidations
1
,2
chemicals, and other fine chemicals. Continuous pressure under ambient conditions in which metalloenzymes activate
from society demands the adaptation of sustainable and envir- molecular dioxygen and thereby facilitate the spin forbidden
onmentally benign processes in industrial synthesis by the interaction between dioxygen and organic matter for biochemical
7
–9
replacement of traditional oxidants which are toxic and causing oxidation processes. Bioinorganic chemists have made much
environmental pollution. Today, there is an increasing demand effort to investigate enzyme active-site chemistry by the use of a
1
0–12
for the selective oxidation of organic compounds utilizing synthetic analogue approach.
These enzymatic models can
dioxygen as a primary oxidant for industrial application because furnish reference compounds to get insight into the structures of
of its economic and environmental benefits. However, the direct active sites and reactive intermediates and the mechanistic
oxidation of organic substrates by molecular oxygen is rare details of dioxygen activation and oxidation reactions occurring
13–19
because of its kinetic inertness, which is associated with the at the active sites.
This type of information and insight is
often not available from direct enzymatic studies. Moreover,
highly efficient biomimetic models may provide new reagents
or efficient catalysts for the alternative of traditional toxic
inorganic (mostly heavy metals) catalysts for industrial processes.
It is to be noted that the extraordinary catalytic activity of
metalloenzymes is mainly associated with the crucial role of
protein chains that help in substrate recognition and stabilisation
a
b
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS,
WB 721152, India. E-mail: ampanja@yahoo.co.in
Department of Chemistry, CICECO-Aveiro Institute of Materials,
University of Aveiro, 3810-193 Aveiro, Portugal
†
Electronic supplementary information (ESI) available: Fig. S1–S6. CCDC
543047–1543050 for 1–4. For ESI and crystallographic data in CIF or other
1
electronic format see DOI: 10.1039/c7nj02015e
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20–22
of the intermediate through various noncovalent forces.
amine (both secondary and tertiary), imine, pyridine (from
Therefore, development of synthetic analogues beyond the first pyridine-2-aldehyde part) or phenol (from salicylaldehyde part),
coordination sphere could be the right way to develop efficient are able to provide different donor sites allowing fine tuning of
biomimetic catalysts.
the ligand field around the metal centre, and thereby may lead
We have been developing biomimetic catalysts for the oxida- to significant improvement in the catalytic activity. Surprisingly,
tion of 3,5-di-tert-butylcatechol and o-aminophenol by molecular only single report on Cu(II) complexes with a Schiff base derived
2
3–25
dioxygen, mimicking the function of catechol oxidase.
phenoxazinone synthase,
and from N,N-dimethyldipropylenetriamine is available in the
2
6,27
40
respectively. The latter, a multi- literature. Therefore, we are interested in the systematic devel-
copper metalloenzyme, catalyses the oxidative coupling of a opment of coordination chemistry of the transition metals
wide variety of substituted o-aminophenols to phenoxazinone derived from this triamine, with the goal of their potential
4
1
chromophores through catalytic activation of dioxygen in the applications in diverse fields, such as magnetic materials
2
8
final step for the biosynthesis of actinomycin D, which is used and biorelevant catalysis. Accordingly, in this present endeavour,
clinically for the treatment of many tumours, including Wilm’s Schiff base HL (Scheme 1) derived from N,N-dimethyldipropylene-
tumour, where the phenoxazinone chromophore intercalates triamine and o-vanillin was used for the synthesis of cobalt
with DNA base-pairs, thereby strongly inhibiting DNA dependent complexes in the presence of pseudohalides (azide, thiocyanate
2
9,30
RNA synthesis.
From the active site structures of several and cyanate). In continuation of our long-standing interest in the
23–27
oxidase enzymes and from the reports in the literature on field of bioinspired catalysis,
various model complexes with transition metals,
herein, we report the synthesis
it is and structures of four new cobalt complexes, [Co(L)(N
]Á
clear that the active site of the functional models must have 0.5CH CN (1), [Co(L)(NCS) ]Á0.5H O (2), [Co(HL) ][Co(NCS) ]ÁNCS
2
3–28,31–39
3 2
)
3
2
2
2
4
labile or free coordination sites available for substrate binding. (3) and [Co(L) ] [Co(NCO) ] (4), derived from Schiff base ligand
2
2
4
Therefore, the ligands having a lower number of donor atoms HL and pseudohalides, and their biomimetic catalytic activity
are more promising. Coordination compounds derived from related to the function of phenoxazinone synthase. Interestingly,
the Schiff base ligands have been extensively studied as the influence of solvents and pseudohalides on the diverse
enzymatic models mainly because of their synthetic simplicity coordination chemistry of cobalt was observed. Moreover, the
and tremendous structural diversities, and thereby fine tuning different donor properties of the Schiff base associated with
of the catalytic activity can be carried out by modification of the N,N-dimethyldipropylenetriamine part were also explored.
coordination environment at the metal centre by judicious Furthermore, emphasis was given to gain insight into the
choice of amine and carbonyl parts of the Schiff bases.
A triamine protected at one end, namely N,N-dimethyl- tigations of the synthesized complexes.
dipropylenetriamine, may produce N or N O donor Schiff base
structure–reactivity correlation through detailed kinetic inves-
4
3
ligands by reacting with pyridine-2-aldehyde or salicylaldehyde,
respectively, for the development of coordination compounds
having either vacant or labile position(s) available for substrate
binding. Moreover, the presence of bis(propylenic) linkers in
Experimental section
Materials and physical measurements
this amine may increase the flexibility of the ligands so that Cobalt(II) nitrate hexahydrate, o-aminophenol (OAPH), o-vanillin,
substrate binding to the metal centre becomes possible. and N,N-dimethyldipropylenetriamine were commercially avail-
Furthermore, such ligands including several functions, namely able reagents and were used as received. Other chemicals and
Scheme 1 Schiff base HL.
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2 11 4 5
solvents were purchased from commercial sources and used 358 mg (72%). Anal. calcd for C37H54Co N O S : C 44.66%,
without further purification.
H 5.47%, N 15.48%. Found: C 44.51%, H 5.48%, N 14.39%. FTIR
À1
Caution! Azide salts of metal complexes especially with (cm , KBr): n(NCS) 2067 s, n(CQN) 1615 s.
organic ligands are potentially explosive. Only a small amount
of the material should be prepared and it should be handled synthesized from an acetonitrile/water (20 : 1) mixture adopting
with great care. an identical procedure as described for 1, except that NaNCO
Elemental analyses (carbon, hydrogen and nitrogen) were was used instead of NaN . Colour: dark brown, yield: 295 mg
performed using a Perkin-Elmer 240C elemental analyser. IR (78%). Anal. calcd for C68
Synthesis of complex [Co(L) ] [Co(NCO) ] (4). Complex 4 was
2 2 4
3
H
104Co
3
N
16
O
12: C 53.93%, H 6.92%,
À1
spectra were recorded as KBr pellets on a PerkinElmer Spectrum N 14.80%. Found: C 54.06%, H 6.95%, N 14.69%. FTIR (cm
,
À1
Two FTIR spectrophotometer in the range of 400 to 4000 cm
.
KBr): n(NCO) 2217 vs, n(CQN) 1626 s.
Electronic absorption spectra were measured using an Agilent
Carry-60 diode array UV-vis spectrophotometer at room tempera- X-ray crystallography
ture. Electrospray ionization mass spectrometry (ESI-MS positive)
Single crystal X-ray diffraction data for 1–4 were collected using a
Bruker Kappa Apex-II CCD diffractometer, equipped with mono-
chromated Mo-Ka radiation (l = 0.71073 Å). Data were collected
in a wide range of j and o angles to increase the number of
redundant reflections. The images obtained during the data
collection for all complexes were processed with the software
SAINT+, and the absorption effects were corrected by using the
was performed using a Micromass Q-tof-Micro Quadruple mass
spectrophotometer. Cyclic voltammetric experiments were carried
out at room temperature in methanol using tetrabutylammonium
perchlorate as a supporting electrolyte on a CH Instrument
electrochemical workstation model CHI630E with a three-
electrode assembly comprising a platinum working electrode,
a platinum wire auxiliary electrode and a Ag/AgCl reference
electrode. The powder X-ray diffraction (PXRD) data were
collected using a Bruker D8 Advance X-ray diffractometer using
Cu-Ka radiation (l = 1.5418 Å) operating at 40 kV and 40 mA.
Synthesis of the Schiff-base ligand (HL). A tetradentate
ligand (HL) was synthesized via the Schiff base condensation
method. Briefly, 1.0 mmol of N,N-dimethyldipropylenetriamine
42
multi-scan method implemented in SADABS. The structures
were solved using direct methods and refined by successive full-
2
43
matrix least-squares cycles on F using SHELXL-v.2013. All the
non-hydrogen atoms were successfully refined using anisotropic
displacement parameters. Hydrogen atoms attached to carbon
atoms were placed at geometrically idealized positions, and
included in subsequent refinement cycles in riding-motion
approximation with isotropic thermal displacement parameters
fixed at 1.2 or 1.5 times that of the respective parent atoms. All
other hydrogen atoms attached to the oxygen and nitrogen
atoms were found in the Fourier difference maps, their distances
and thermal displacement parameters were fixed in some cases
and were allowed to refine with a riding model. Additional
information related to crystallographic data collection and struc-
ture refinement details are summarized in Table 1.
(159 mg) and 1.0 mmol of o-vanillin (152 mg) were combined in
20 ml of methanol (or acetonitrile). The resulting mixture was
allowed to reflux for ca. 1 h, and then cooled. The in situ
prepared Schiff base ligand was directly used for the synthesis
of the metal complexes adopting a general procedure as follows.
Synthesis of [Co(L)(N
.0 mmol) and Schiff base ligand HL (1.0 mmol) were combined in
0 ml acetonitrile, and to the mixture 2 ml of an aqueous solution of
3
)
2
]Á0.5CH
3
CN (1). Co(NO
3
)
2
Á6H
2
O (291 mg,
1
4
sodium azide (130 mg, 2.0 mmol) was added with stirring. The
resulting mixture was heated to reflux for about 30 min during
which time the colour of the solution changed to dark brown. The
reaction mixture was then filtered and kept at ambient temperature
for slow evaporation. Analytically pure dark-brown crystals suitable
for X-ray diffraction were separated out from the solution within
a week, which were collected by filtration and washed with
methanol/ether and air dried. Yield: 392 mg (86%). Anal. calcd
for C H Co N O : C 44.79%, H 6.08%, N 29.19%. Found:
Catalytic oxidation of o-aminophenol
Catalytic oxidation of o-aminophenol (OAPH) was carried out by the
À2
À5
reaction of 1.0 Â 10 M of OAPH with 2 Â 10 M of the complexes
in air-saturated methanol at room temperature. The reaction was
followed spectrophotometrically by monitoring the progressive
increase in the absorbance as a function of time at 434 nm
À1
À1 44
(e = 9095 M
cm ), which is characteristic of 2-amino-
3
4
55
2 19 4
phenoxazin-3-one in methanol. The catalytic efficiency of the com-
plexes was examined by measuring time course spectral profiles
using variable concentrations of the substrate with a fixed concentra-
À1
C 44.64%, H 6.05%, N 29.06%. FTIR (KBr, cm ): n(N ) 2033 vs;
3
n(CQN) 1619 s.
Synthesis of complex [Co(L)(NCS)
was synthesized from an acetonitrile/water (20 : 1) mixture
2
]Á0.5H
2
O (2). Complex 2
À5
tion (2 Â 10 M) of the complexes under pseudo-first order condi-
tions. All the kinetic measurements were carried out for the period of
following a very similar procedure as described for 1, except
10 min and the initial rate of the reaction was determined by linear
that NH
4
SCN was used instead of NaN
3
. Colour: dark brown,
regression from the slope of the absorbance versus time plot.
yield: 395 mg (83%). Anal. calcd for C36H54Co N O S : C 45.37%,
2 10 5 4
H 5.71%, N 14.70%. Found: C 45.46%, H 5.65%, N 14.59%.
À1
FTIR (cm , KBr): n(NCS) 2108 s, n(CQN) 1623 s.
Results and discussion
Syntheses and general characterization
Synthesis of complex [Co(HL) ][Co(NCS) ]ÁNCS (3). Complex
2
4
3
was synthesized from a methanol/water (20 : 1) mixture follow-
ing the same methodology applied for the synthesis of 1, but The condensation reaction of N,N-dimethyldipropylenetriamine
NH SCN was used in place of NaN . Colour: dark brown, yield: with o-vanillin in a 1 : 1 molar ratio in methanol resulted
4
3
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Table 1 Crystal data and structure refinement parameters of complexes 1 to 4
1
2
3
4
Empirical formula
Formula weight (g mol
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
C
34
H
55Co
2
N
19
O
4
C
18
H
26.5CoN
5
O
2.25
S
2
C
37
H
54Co
2
N
11
O
4
S
5
C
68 3 16 12
H104Co N O
À1
)
911.83
298(2)
Monoclinic
C2/c
25.636(3)
10.3538(4)
19.712(2)
90
126.446(16)
90
471.99
150(2)
Triclinic
P1%
7.9958(4)
10.1437(5)
13.4435(7)
85.118(2)
83.482(2)
89.535(2)
1079.38(9)
2
995.07
150(2)
Monoclinic
P2 /c
18.977(3)
11.8646(17)
21.938(3)
90
108.078(4)
90
1514.46
298(2)
Tetragonal
P4 /n
17.0179(4)
17.0179(4)
12.9851(4)
90
1
2
90
90
g (1)
Volume (Å )
3
4208.8(7)
4
1.439
4695.6(11)
4
1.408
3760.60(17)
2
1.337
Z
D
À3
calc (mg m
)
1.452
1.013
À1
m (mm
)
0.850
0.978
0.722
F(000)
1912
1.98–28.29
8285
4591(0.0250)
3874
493
2.56–29.20
37 750
5801(0.0270)
5130
2076
2.12–26.51
121 708
9653(0.0837)
6908
1602
y range (1)
Reflections collected
Independent reflections (Rint
1.69–25.40
55 064
3462(0.0832)
2261
)
Observed reflections [I 4 2s(I)]
Restraints/parameters
Goodness-of-fit on F
0/272
1.039
0/269
1.043
0/555
1.108
0/231
1.049
2
Final R indices [I 4 2s(I)]
R
wR
1
= 0.0407
= 0.0972
= 0.0507
= 0.1030
0.437/À0.350
R
wR
1
= 0.0253
= 0.0646
= 0.0315
= 0.0677
0.402/À0.335
R
wR
R
wR
1
= 0.0549
= 0.1409
= 0.0862
= 0.1767
1.143/À1.063
R
wR
R
1
wR
1
= 0.0380
= 0.0832
= 0.0738
= 0.0931
0.170/À0.256
2
2
2
2
R indices (all data)
R
1
R
1
1
wR
2
wR
2
2
2
À3
Largest diff. peak/hole (e Å
)
dark-orange Schiff base HL (Scheme 1). Treatment of this in situ
generated Schiff base with cobalt(II) nitrate hexahydrate in the bands in the range of 1615–1626 cm , which are characteristic
The IR spectra of all complexes show strong and sharp
À1
À
À
À
presence of pseudohalides (N3 for 1, NCS for 2 and 3, and NCO
for 4) in a 1 : 1 : 2 molar ratio afforded [Co(L)(N
]Á0.5CH
1), [Co(L)(NCS) O (2), [Co(HL) ][Co(NCS)
Co(L) [Co(NCO)
stretching vibrations of the azomethine bond of the Schiff base.
3
)
2
3
CN In the spectrum of complex 1, a strong absorption band is
]ÁNCS (3) and observed at 2033 cm , which is a characteristic stretching
À1
(
[
2
]Á0.5H
2
2
4
4
5
2
]
2
4
] (4), in high yield. Although the stoichio- band of an azide ion. Complexes 2 and 3 show two sharp
À1
metry of the reactants is found to be insensitive to the synthesis absorption bands for thiocyanate ions, at 2108 and 2067 cm
of these complexes, the distinguishing role of the solvents respectively.
,
4
1,46
The relatively lower stretching vibration of the
is observed for the synthesis of thiocyanate complexes. For thiocyanate ion in 3 is associated with the greater p acceptor
instance, when the reaction is carried out in an acetonitrile/ ability of the thiocyanate from a Co(II) ion compared to its p
À
water mixture in the presence of NCS , the only isolable acceptor ability from a Co(III) centre in 2. A similar characteristic
À1 45
product is 2, but when the same reaction was performed in a stretching band for a cyanate ion in 4 is observed at 2217 cm
methanol/water mixture, we isolated only compound 3 in high
yield. Isolation of both 2 and 3 from the same reactants
.
Structural descriptions
suggests that both of them are present in equilibrium Crystal structures of complexes 1–4 were determined by single-
in solution, where the relative population in a particular crystal X-ray diffraction; thermal ellipsoid plots of the struc-
solvent system governs the identity of the species that tures of 1 and 2 with selected atom numbering schemes are
crystallizes from a given solution. To check whether they are depicted in Fig. 1, while Fig. 2 and 3 represent the molecular
interconvertible, we re-dissolved 2 and
3 in methanol structures of 3 and 4, respectively. Important bond distances
and acetonitrile solvents, respectively, but upon standing at for the metal coordination sphere for all structures are given in
ambient temperature for a long period of time, no spectral Table 2. Neutral complexes 1 and 2 crystallize on general
chance is noticed (Fig. S1, ESI†). These results indicate that positions – 1 incorporates a disordered acetonitrile molecule
both of them are thermodynamically stable products and with half occupancy on a 2-fold axis, while 2 crystallizes with
formation of them is controlled by the solvents used for their a water molecule of half occupancy on a general position.
synthesis. However, in all the complexes, the metal centre The structures of both 1 and 2 are eventually similar in that
bonded to the Schiff base ligand is oxidized to Co(III) by the Co(III) centre is hexacoordinated with distorted octahedral
aerial oxygen. All these complexes are air-stable and moisture geometry as reflected from the significant deviation of the
insensitive, and are soluble in common organic solvents like cisoid (83.83(9)–94.46(5)1) and transoid (173.65(4)–176.62(5)1)
methanol, acetonitrile and DMF. The purity of the bulk angles from the ideal values. The deprotonated Schiff base
sample was verified by elemental analysis as well as by PXRD ligand (L) is bonded to the metal centre with phenolate-O and
measurements (Fig. S2, ESI†).
three nitrogen atoms (one each of imine, secondary amine and
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Fig. 1 Crystal structures of 1 and 2 showing selected atom numbering schemes. Thermal ellipsoids are drawn at 30% probability.
Table 2 Selected bond distances (Å) for complexes 1–4
Bond
1
2
3
4
Co(III)–N(imine)
1.9302(18) 1.9298(11) 1.943(3) 1.9336(18)
.941(3)
Co(III)–N(secondary amine) 2.0115(18) 1.9816(11) 2.044(3) 2.0117(19)
.043(3)
2.1038(11) —
1.8862(14) 1.8608(9) 1.886(2) 1.8884(15)
.896(2)
1.940(19) 1.9187(11) —
.9508(18) 1.9235(11)
1
2
Co(III)–N(tertiary amine)
Co(III)–O(phenolate)
2.112(2)
—
1
Co(III)–N(pseudohalide)
Co(II)–N(pseudohalide)
—
1
—
—
1.946(4) 1.948(3)
1
1
1
.947(4)
.962(4)
.968(4)
Fig. 2 Crystal structure of one of the crystallographically independent
complex cations of 3 with the selected atom labelling scheme. Ellipsoids
are drawn at 30% probability.
(
in 1) and thiocyanate (in 2) ions. Again, in both cases, three
nitrogen atoms from the triamine part of the Schiff base occupy
one meridional position, while two nitrogen atoms of pseudo-
halides along with the phenolate-O atom of the Schiff base
occupy other meridional position. The Co–N distances of the
pseudohalides and the Co–O distances of the phenolate group
of the tetradentate L ligand in 1 and 2 vary in the range of
1.8608(9)–1.8862(16) and 1.9187(11)–1.952(2) Å, respectively,
which are comparable to the bond distances observed in
4
7–49
analogues low-spin octahedral Co(III) complexes.
In both
complexes, the Co–N bond distances reveal that secondary
amine bonds are somewhat longer than the imine moieties in
both complexes (see Table 2), which is consistent with the state
of hybridization of respective nitrogen atoms. But the signifi-
cantly longer bond distance of the tertiary amine over the
secondary amine groups found in these structures probably
arises from the increased steric congestion around the metal
centre due to the presence of two methyl substitutions at the
tertiary nitrogen, thereby leading to the weaker binding ability
4
1
of the tertiary amine in these complexes. Furthermore, the
N–N distances of azide ions [1.181(3)–1.192(3) Å] between the
central nitrogen and the nitrogen bonded to the metal centre
are somewhat longer than the terminal N–N bonds [1.142(3)–
Fig. 3 Crystal structure of the complex cation of 4 with the selected atom
numbering scheme. Ellipsoids are drawn at 30% probability.
tertiary amine nitrogen atoms) and the two remaining positions 1.144(3) Å], which is typical for coordination through the
4
7,49
are occupied by two nitrogen atoms from two terminal azide anionic terminal of an azide ion.
Again, as found in similar
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108.44(8)–111.55(17)1 for 3 and 4, respectively, are in agreement
with the tetrahedral geometry. In the complex cations, the
geometry of a cobalt(III) centre is found to be pseudo-
octahedral and it is coordinated by two Schiff base ligands,
each of which occupies a facial position and binds the metal
centre through the phenolate-O, imine-N and secondary amine-N.
The Co–N distances of the imine and secondary amine and the
Co–O distances of the phenolate group of the Schiff base ligand
in 3 and 4 vary in the range of 1.9338(18)–1.943(3), 2.0116(19)–
2.044(3) and 1.886(2)–1.896(2) Å, respectively, which are com-
parable to bond distances observed in 1 and 2 and also in
analogues low-spin octahedral Co(III) complexes with similar
4
7–49
coordination environments.
As observed in 1 and 2, the
Fig. 4 A part of the packing showing hydrogen bonding and C–HÁ Á Áp
interactions in 2.
imine bonds are somewhat shorter than the secondary amine
nitrogen in 3 and 4, which is the consequence of their respec-
tive state of hybridization. The solid state structures of 3 and 4
complexes, the thiocyanate is coordinated through the nitrogen are stabilized by noncovalent interactions like hydrogen bond-
4
1,46
end in 2.
The crystal packing of 1 is stabilized by hydrogen ing and C–HÁ Á Áp interactions (Table S2, ESI†). The most
bonding interactions (Table S1, ESI†) involving the secondary striking difference in the noncovalent interactions in these
amine and a solvent acetonitrile molecule as shown in Fig. S3 compounds mainly arises from the hydrogen bonding inter-
(ESI†). The stability in the solid state is further reinforced by actions. Whereas the tertiary amine nitrogen in the Schiff base
C–HÁ Á Áp interactions between the adjacent molecules (Table S2, ligand is not protonated in 4, thereby it is involved in intra-
ESI†). The solid state structure of 2 is stabilized by multiple molecular hydrogen bonding with the secondary amine from
hydrogen bonding interactions (Table S1, ESI†) in which a the same ligand (Fig. S4, ESI†). The tertiary amine nitrogen in 3
crystallized water molecule simultaneously interacts with the is on the other hand protonated, which brings different hydro-
sulphur atom of a thiocyanate ion and the methoxy oxygen gen bonding patterns in the system (Table S1, ESI†). Careful
atom from two different complex molecules (Fig. 4). Another inspection of the solid state packing of 3 shows that the
thiocyanate ion is also connected with a secondary amine group protonated tertiary amine nitrogen is engaged in strong hydro-
from a neighbouring molecule through hydrogen bonding. gen bonding with the nitrogen atom of a thiocyanate counter
C–HÁ Á Áp interaction (Table S2, ESI†) involving two adjacent ion that is simultaneously involved in hydrogen bonding with
complexes further stabilizes the overall supramolecular struc- the protonated tertiary amine of the adjacent complex cation
ture of 2 in the ab plane.
Unlike 1 and 2, both 3 and 4 are ionic compounds. Complex along the a axis (Table S1, ESI†). This supramolecular chain
crystallizes in the monoclinic P2 /c space group such that the structure is further supported by the noncovalent interactions
though a sulphur atom, leading to a 1D chain propagating
3
1
2
À
asymmetric unit includes two crystallographically independent offered by Co(NCS)4 which is involved in N–HÁ Á ÁS hydrogen
3
+
complex cations of composition [Co(HL)2]
on inversion bonding and N–HÁ Á Áp interactions with two different complex
2À
À
centres and two different anions Co(NCS)
general positions. Complex 4 on the other hand crystallizes in involving NCO ions in Co(NCO)
4
and NCS on cations as displayed in Fig. S5 (ESI†). The C–HÁ Á Áp interaction
À
2À
4
and the azomethine C–H
the tetragonal space group P4
2
/n where the complex cation, of the complex cation further contribute to the solid state
+
[
Co(L)
Co(NCO)
and 4 are apparently similar, they are significantly different binding ability of the present Schiff base ligand in these
from each other from the charge neutralization point of view. systems. It can either coordinate the metal centre in a tetra-
2
] , sits on an inversion centre, while the complex anion packing in 4 (Fig. S4, ESI†).
2
À
4
resides on a 4-fold axis. Although the structures of
As a whole, the structural analyses reveal the versatile
3
2À
À
Crystallization of both anions Co(NCS)4 and NCS on general dentate fashion through the monoanionic deprotonated form
positions requires the protonation of the tertiary amine-N of using all four donor sites (in 1 and 2) or in a tridentate fashion
the Schiff base ligands for balancing the charge in 3, while it is using the zwitterionic form of the Schiff base ligand leaving the
not protonated in complex 4. Although in both cases, the quaternary amine nitrogen free from coordination (in 3). More-
potentially tetradentate Schiff base ligand binds the metal over, the monoanionic deprotonated ligand can also binds the
centre in a tridentate fashion, the zwitterionic form of the metal centre in a tridentate fashion where the pendent tertiary
Schiff base is coordinated to the metal centre in 3, while it is amine nitrogen is involved in intramolecular hydrogen bond-
monoanionic in 4 but bonded with the metal centre using three ing in 4. Furthermore, in our recent report with a Ni(II) system,
donor sites leaving the tertiary amine group free from coordi- it was observed that the triamine part of the Schiff base ligand
2
À
2À
41
nation. The geometry of both Co(NCS)
4
and Co(NCO)
4
is binds the metal centre facially, while it is meridionally
tetrahedral in which the pseudohalides are coordinated to coordinated with the metal centre in the present systems
5
0–52
cobalt(II) ions through the nitrogen atoms.
The N–Co–N (1 and 2). Therefore, all these above versatilities associated
bond angles varying in the range of 104.10(17)–115.12(17) and with this triamine suggest that the ligands derived from this
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triamine could be useful in developing coordination chemistry
with diverse structures.
Electrochemical studies
The redox potential of the active site of a metalloenzyme plays
an important role in the catalytic oxidation of biochemical
compounds in natural systems. Therefore, an electrochemical
study especially under identical conditions to the catalytic
study of the synthetic analogues could provide useful informa-
tion about the catalytic ability of these compounds. In this
context, the electrochemical behaviours of all complexes have
been examined in methanol in the presence of 0.1 M tetra-
butylammonium perchlorate as a supporting electrolyte at
ambient temperature. Representative cyclic voltammograms
of 2 and 3 are depicted in Fig. 5 and 6, respectively. Scanning
in the cathodic direction, an irreversible reduction response
was found at À0.77 and À0.71 V for 1 and 2, respectively, which
is a consequence of the reduction of Co(III) to Co(II) at the
electrode surface. This irreversible electrode process is due to a
significant modification of the reduced species at the electrode
surface in terms of loss of azide or thiocyanate ions from the
metal centres. Unlike 1 and 2, the cyclic voltammograms of
Fig. 6 Cyclic voltammograms of 3 in methanol using a platinum working
electrode in the presence of tetrabutylammonium perchlorate as a sup-
À1
porting electrolyte at room temperature with a scan rate of 100 mV s
.
used substrate o-aminophenol (OAPH) UV-vis spectrophotome-
3
6
trically in dioxygen-saturated methanol at room temperature.
At first, it is important to confirm the ability of the synthesized
À5
3
3
and 4 consist of an irreversible oxidative response (0.99 V for
and 1.01 V for 4) in addition to the reductive response
complexes to oxidize OAPH. Accordingly, 2.0 Â 10 M methanolic
solutions of these complexes were combined with a 0.01 M
solution of OAPH under aerobic conditions. The progress of the
reaction was monitored by UV-vis spectrophotometry, and time-
dependent spectral profiles, scanning with 5 min time intervals,
after the addition of OAPH are presented in Fig. 7–9 for 1, 2 and 4
respectively, and in Fig. S6 (ESI†) for 3. The spectral scans reveal
the cumulative growth of the peak intensity at ca. 434 nm,
characteristic of the phenoxazinone chromophore, suggesting
the catalytic oxidation of OAPH to 2-aminophenoxazin-3-one under
aerobic conditions. These spectral behaviours unambiguously
(
À0.92 V for 3 and À0.94 V for 4). The reduction process is
responsible for the reduction of Co(III) to Co(II) of the complex
cations at the electrode surface, whereas the oxidative response
is due to the oxidation of Co(II) to Co(III) of the complex anions
2
À
of general formula [CoX4]
(X = SCN for 3 and NCO for 4).
Electrochemical irreversibility again suggests the instability of
the oxidized species at the electrode surface.
Phenoxazinone synthase mimicking activity
Phenoxazinone synthase like activity of the complexes was demonstrate that all complexes are active catalysts for the aerial
examined by monitoring the catalytic oxidation of commonly
Fig. 7 Time dependent UV-vis spectral changes for the oxidation of
À2
À5
Fig. 5 Cyclic voltammograms of 2 in methanol using a platinum working
o-aminophenol (1.0 Â 10 M) catalysed by 1 (2 Â 10 M) in dioxygen-
saturated methanol. The spectra were recorded at 5 min intervals under
aerobic conditions at room temperature.
electrode in the presence of tetrabutylammonium perchlorate as a sup-
À1
porting electrolyte at room temperature with a scan rate of 100 mV s
.
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Fig. 10 Initial rate versus substrate concentration plot for the oxidation
of OAPH in dioxygen-saturated methanol catalysed by 1–4 at room
temperature. Symbols and solid lines represent the experimental and
simulated profiles, respectively.
Fig. 8 Time dependent spectral scan showing growth of 2-
À2
aminophenoxazin-3-one at 434 nm upon addition of 1.0 Â 10 M OAPH
À5
to a solution containing 2 (2 Â 10 M) in dioxygen-saturated methanol.
The spectra were recorded at 5 min intervals under aerobic conditions at
room temperature.
concentration plots show rate saturation kinetics as depicted
in Fig. 10. These observations unambiguously conclude that
an intermediate complex–substrate adduct is formed in a pre-
equilibrium stage followed by the irreversible redox transforma-
tion of the intermediate in the rate determining step of the
catalytic cycle. The Michaelis–Menten model as well as its linear
double reciprocal Lineweaver–Burk plot can be applied to analyse
the values of the parameters Vmax, K , and Kcat. The observed and
M
simulated nonlinear initial rate versus substrate concentration plot
and the Lineweaver–Burk plot for all complexes are shown in
Fig. 10 and Fig. S7 (ESI†), respectively. Analysis of the experimental
À2
data yielded Michaelis binding constant (K
M
À2
) values of 1.35 Â 10
,
À2
À3
1
.24 Â 10 , 8.87 Â 10 and 1.02 Â 10 M for 1–4, respectively,
À7 À7 À8
and Vmax values of 3.00 Â 10 , 2.70 Â 10 , 4.65 Â 10 and
À7
À1
1
.45 Â 10 M s for 1–4, respectively. The turnover number
(k
) value is obtained by dividing V
by the concentration
max
cat
À5
of the complex used (2.0 Â 10 M), and is calculated to be
À1
5
4.0, 48.6, 26.1 and 8.37 h for 1–4, respectively.
Fig. 9 The time resolved spectral profile showing oxidation of o-aminophenol
À2
À5
(
1.0 Â 10 M) catalysed by 4 (2 Â 10 M) in dioxygen-saturated methanol. The Plausible mechanism and comparative catalytic activity
spectra were recorded at 5 min intervals under aerobic conditions at room
temperature.
From the detailed kinetic analysis, it is clear that complexes
and 2 are efficient catalysts for the aerobic oxidation of
o-aminophenol, while compounds 3 and 4 are relatively less
1
oxidation of OAPH to the corresponding phenoxazinone chromo- reactive. These pseudohalide bound complexes favour the sub-
phore. Spectral profiles further suggest that although complexes 1, strate binding by the loss of two coordinated pseudohalides,
2
and 4 are efficient catalysts for OAPH oxidation, the reactivity of which is the main reason behind the excellent reactivity of
complex 3 is significantly low. Detailed reaction kinetics was both 1 and 2. Slightly greater reactivity of the azide complex
further performed to understand the extent of the catalytic effi- compared to the thiocyanate analogue is probably due to the
À5
ciency. For this purpose, a 2.0 Â 10 M solution of the complexes better lability of azide over a thiocyanate ion. The mass spectral
was treated with at least 10-fold excess of the substrate to follow study could provide valuable information from which the most
the pseudo-first-order conditions, and the time scan at the max- possible mechanism of the catalytic cycle can be framed.
imum band (434 nm) of 2-aminophenoxazin-3-one was carried out Therefore, the electrospray ionization mass spectrum (ESI-MS
for a period of 10 min, and the initial rate was determined by positive) of a representative compound 2 in the presence of
linear regression from the slope of the absorbance versus time plot, 50 equivalents of o-aminophenol in acetonitrile was recorded
considering the extinction coefficient of 2-aminophenoxazin-3-one after 15 min of mixing, and the spectrum is depicted in Fig. 11.
À1
À1
44
as 9095 M cm in methanol. The initial rate versus substrate Interestingly, the mass spectrum is clean enough as it only
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Fig. 11 Electrospray ionization mass spectrum (ESI-MS positive) of a 1 : 50 mixture of complex 2 and o-aminophenol in acetonitrile recorded after
5 min of mixing.
1
comprises a small numbers of peaks and all of them are well available from most of the preceding mass spectral studies
assignable. Remarkably, the base peak at m/z = 459.20 is a 1 : 1 of the reported synthetic models of phenoxazinone synthase,
5
3
complex–substrate aggregate of a monocationic species of and in some cases it is wrongly assigned. Therefore, the
III
+
composition [Co (L)(OAP)] (calculated m/z = 459.18), estab- present finding with proper assignment could provide signifi-
lishing the formation of a stable complex–substrate adduct. cant information in the mechanistic study of phenoxazinone
The relative population of the original complex cation generated synthase.
III
+
from the loss of a coordinated thiocyanate ion, [Co (L)(NCS)]
Now at this stage, both the mass spectral study and rate
(calculated m/z = 409.11; found at m/z = 409.23), is about only 1% saturation kinetics unambiguously conclude that the catalytic
compared to the most abundant peak of the complex–substrate cycle for the aerobic oxidation of o-aminophenol catalysed by
aggregate, and thereby establishes the ease of formation of the 1 and 2 starts with the formation of a complex–substrate aggre-
complex–substrate aggregate, exploiting the stronger chelating gate by the replacement of the coordinated pseudohalides by
ability of an o-aminophenolate ion. Two other cobalt containing o-aminophenol. This complex–substrate intermediate at the
species are located at m/z = 351.29 and 414.28 and these peaks rate determining step generates an OAP radical through inner
II
+
II
+
are assigned to [Co (L)] and [NaCo (L–H)] , respectively. sphere electron transfer from o-amionophenolate to the metal
Furthermore, a lithium adduct of ligand HL is observed at centre. The OAP radical thereafter may generate o-benzoquinone
m/z = 300.14. The peaks at m/z = 173.12 and 200.14 are assignable monoamine (BQMI) in many ways including the disproportiona-
to the sodium adduct of the substrate along with solvent tion of the OAP radical itself. This BQMI is then reacted with
+
acetonitrile, [Na(OAPH)(MeCN)] , and an aggregate of the proto- another OAPH to generate a reactive intermedia I-B as suggested
nated OAPH and five water molecules, respectively. The peak at by the mass spectral study, which thereafter undergoes
m/z = 248.18 is related to a peak of the molecular ion of the two electron oxidation with molecular oxygen probably by the
product 2-aminophenoxazin-3-one combined with two water assistance of the complexes to produce another reactive inter-
molecules. The second highest abundant peak at m/z = 216.15 mediate II-B, again supported by the mass spectral study.
along with a minor peak at m/z = 214.17 is the most remarkable Finally, 2-aminophenoxazin-3-one is produced from the oxida-
finding of this mass spectral study as these are related to the tive dehydrogenation of intermediate II-B by the complexes as a
molecular ion peaks of two important reactive intermediates catalyst as shown in Scheme 2.
generated during the course of oxidative coupling of o-aminophenol
The lower reactivity of compounds 3 and 4 compared to
as drawn in Scheme 2. This kind of information was not readily 1 and 2 is quite reasonable as the labile sites for substrate
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Scheme 2 Plausible mechanistic pathway for the aerobic oxidation of o-aminophenol catalysed by 1 and 2.
binding are not available for these systems, leading to catalytic prepared by adding a varying amount of a tetrabutylammonium
activity through the outer sphere electron transfer pathway. hydroxide base, and then these complex solutions were mixed
5
4
These types of complexes are often found to be inactive, but
in the present case, especially compound 4 is found to be
significantly catalytically active towards phenoxazinone synthase.
In our previous study, we observed that the positive charge at the
ligand backbone brings significantly enhanced catalytic activity in
which the positive charge of the pyridinium moiety at the ligand
backbone may attract the substrate OAPH towards the formation
of the complex–substrate adduct, leading to the enhanced
5
5
catalytic activity in that system. In the present case, the
pendent tertiary amine group is protonated in 3, while it is
not protonated in 4. As seen earlier, if simply the positive
charge at the second coordination sphere is the crucial factor,
then one can expect that compound 3 would show better catalytic
activity than complex 4. But in reality, we have observed that
compound 4 exhibits significantly higher activity than complex 3.
In order to explore how the protonation/deprotonation at the
second coordination sphere alter the catalytic activity in these
systems, a series of 2 Â 10 M solutions of 3 in methanol was oxidation of o-aminophenol (0.01 M) catalyzed by complex 3 (2 Â 10 M).
Fig. 12 Plot of initial rate vs. concentration of an added base for the aerobic
À5
À5
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Scheme 3 Stabilization of the intermediate through hydrogen bonding and facilitating oxidation of o-aminophenol by proton abstraction.
with 0.01 M substrate and the time course of the reaction is tridentate fashion (in 4) where the pendent tertiary amine
monitored UV-vis spectrophotometrically. As shown in Fig. 12, the nitrogen is utilized in the formation of intramolecular hydrogen
catalytic activity of the complex increases with an increase in the bonding. It is worth noting that, in our recent report with a Ni(II)
added base and finally the rate of the reaction of 3 becomes system, the triamine part of the Schiff base ligand binds the
comparable to the rate of 4. This observation establishes that the metal centre facially, while it is meridionally coordinated with
deprotonation of quaternary amine at the second coordination the metal centre in the present systems (1 and 2). Therefore,
sphere brings the higher activity in the system. This result all these versatilities associated with this triamine make it a
suggests that not the positive charge but the hydrogen bond promising synthon for the development of coordination chemistry
acceptor ability of the second coordination sphere is the foremost with diverse structures. All complexes exhibit strong phenoxazi-
criterion for the substrate recognition and stability of the inter- none synthase activity, and as expected the availability of
mediate for enhanced catalytic activity (see Scheme 3). Furthermore, binding sites at the first coordination sphere for the substrate,
tertiary amine nitrogen abstracts a proton from o-aminophenol and o-aminophenol, is responsible for significantly higher catalytic
facilitates the oxidation of OAPH, leading to the formation of the activity in 1 and 2. The presence of a hydrogen bond acceptor
OAP radical at the rate determining step. When the tertiary amine is site at the second coordination sphere in 4 not only stabilizes
protonated, such proton abstraction from the substrate is not the complex–substrate outer sphere association complex but
possible and thus oxidation of o-aminophenol is not favored. also abstracts a proton from o-aminophenol, thereby facilitat-
Therefore, the present study discloses that if the strong hydrogen ing oxidation of the substrate. From the ESI mass spectral data
bond acceptor center is present at the secondary coordination of 2 in the presence of 50 equivalents of o-aminophenol, several
sphere, it can not only recognize the substrate and stabilize the important intermediates have been assigned, which provides
complex substrate intermediate through hydrogen bonding, but significant information regarding the mechanistic pathway
also abstract a proton from the substrate and thereby facilitate of the functioning phenoxazinone synthase activity of the syn-
oxidation of a substrate like o-aminophenol. This result clearly thetic models. This result therefore directs the development of
justifies the fact that the incredible reactivity of metalloenzymes coordination compounds having both labile or vacant positions at
in biological systems is not only due to the presence of available the first coordination sphere for substrate binding and a hydro-
labile position(s) for substrate binding but also due to the crucial gen bond acceptor cum proton abstraction centre at the second
role of the protein chain that helps in substrate recognition and coordination sphere for better biomimetic catalysts that have a
stabilisation of the intermediate through various noncovalent forces. higher degree of structural resemblance with the metalloenzymes.
Conclusions
Acknowledgements
In summary, we have synthesized a new tetradentate N O donor
3
A. P. would like to thank the University Grant Commission,
ligand by the Schiff base condensation of N,N-dimethyldi-
New Delhi, for financial support. The authors thank Indian
propylenetriamine and o-vanillin. Using this Schiff base four
Institute of Chemical Biology, Kolkata for providing the Mass
new cobalt complexes have been prepared in the presence of
spectral facility.
pseudohalides in high yields, and the influence of the solvents
on their syntheses has been noticed. X-ray crystal structure
analyses reveal that the Schiff base can bind the metal centre
either in a tetradentate fashion through the monoanionic
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