Oxidative C-N bond cleavage of (2-pyridylmethyl)amine-based tetradentate supporting ligands in ternary cobalt(ii)-carboxylate complexes

Article abstract of DOI:10.1039/c9dt04438h

Three mononuclear cobalt(ii)-carboxylate complexes, [(TPA)Co^II(benzilate)]⁺ (1), [(TPA)Co^II(benzoate)]⁺ (2) and [(iso-BPMEN)Co^II(benzoate)]⁺ (3), of N4 ligands (TPA = tris(2-pyridylmethyl)amine and iso-BPMEN = N¹,N¹-dimethyl-N²,N²-bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate (η¹) binding of the carboxylates to the metal centre favours the five-coordinate cobalt(ii) complexes (1-3) for dioxygen activation. Complex 1 slowly reacts with dioxygen to enable the oxidative decarboxylation of the coordinated α-hydroxy acid (benzilate). Prolonged exposure of the reaction solution of 2 to dioxygen results in the formation of [(DPA)Co^III(picolinate)(benzoate)]⁺ (4) and [Co^III(BPCA)₂]⁺ (5) (DPA = di(2-picolyl)amine and HBPCA = bis(2-pyridylcarbonyl)amide), whereas only [(DPEA)Co^III(picolinate)(benzoate)]⁺ (6) (DPEA = N¹,N¹-dimethyl-N²-(pyridine-2-ylmethyl)-ethane-1,2-diamine) is isolated from the final oxidised solution of 3. The modified ligand DPA (or DPEA) is formed via the oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the -CH₂- moiety to -C(O)- takes place in the transformation of DPA to HBPCA on the cobalt(ii) centre. Labelling experiments with ¹⁸O₂ confirm the incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with ¹⁶O₂ and H₂O¹⁸ strongly support the involvement of water in the C-N bond cleavage pathway. A comparison of the dioxygen reactivity of the cobalt complexes (1-3) with those of several other five-coordinate mononuclear complexes [(TPA)Co^II(X)]^+/2+ (X = Cl, CH₃CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of the carboxylate co-ligands in the activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.

Full text of DOI:10.1039/c9dt04438h

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DOI: 10.1039/C9DT04438H
Table of Contents Entry
Cobalt(II)-carboxylate complexes of N4 ligands containing (2-pyridylmethyl)amine
backbone activate dioxygen to carry out the oxidative C-N bond cleavage of the ligand.
The coordination of carboxylate co-ligand plays important role to carry out the ligand
oxidation.

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DOI: 10.1039/C9DT04438H
ARTICLE
Oxidative C-N Bond Cleavage of (2-Pyridylmethyl)amine-Based
Tetradentate Supporting Ligands in Ternary Cobalt(II)-Carboxylate
Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Biswarup Chakraborty, Ivy Ghosh, Rahul Dev Jana, and Tapan Kanti Paine*
DOI: 10.1039/x0xx00000x
Three mononuclear cobalt(II)-carboxylate complexes, [(TPA)CoII(benzilate)]+ (1), [(TPA)CoII(benzoate)]+ (2) and [(iso-
BPMEN)CoII(benzoate)]+ (3) of N4 ligands (TPA
= tris(2-pyridylmethyl)amine, iso-BPMEN =
N1,N1-dimethyl-N2,N2-
bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate
(η1) binding of the carboxylates to the metal centre favours the five-coordinate cobalt(II) complexes (1-3) for dioxygen
activation. Complex 1 slowly reacts with dioxygen to exhibit oxidative decarboxylation of the coordinated -hydroxy acid
(benzilate).
A
prolonged exposure of the reaction solution of
2
to dioxygen results in the formation of
[(DPA)CoIII(picolinate)(benzoate)]+ (4) and [CoIII(BPCA)2]+ (5) (DPA
=
di(2-picolyl)amine and HBPCA bis(2-
=
pyridylcarbonyl)amide), whereas only [(DPEA)CoIII(picolinate)(benzoate)]+ (6) (DPEA = N1,N1-dimethyl-N2-(pyridine-2-
ylmethyl)-ethane-1,2-diamine) is isolated from the final oxidised solution of 3. The modified ligand DPA (or DPEA) is
formed via oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the –CH2- moiety to –C(=O)- takes
place in the transformation the DPA to HBPCA on cobalt(II) centre. Labelling experiments with 18O2 confirm the
incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with 16O2 and
H2O18 strongly support the involvement of water in the C-N bond cleavage pathway. A comparison of the dioxygen
reactivity of the cobalt complexes (1-3) with several other five-coordinate mononuclear complexes [(TPA)CoII(X)]+, (X = Cl,
CH3CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of carboxylate co-ligands in the
activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.
methylene groups.19-21 Unusual reactivity of the -CH2- groups
with metal complexes of tripodal (2-pyridylmethyl)amine-type
ligands has been documented.10, 17, 22-25
The stability of cobalt-oxygen species makes cobalt complexes
as ideal candidates to get information about O2-derived
intermediates in enzymatic reactions.26 Cobalt (II) complexes
Introduction
Tripodal tetradentate nitrogen ligands1-3 have received
considerable attention in studies related to dioxygen
activation at metal centre and to stabilise metal ions in
unusual oxidation states.4-11 Transition metal complexes of
nitrogen rich (2-pyridylmethyl)amine-based ligands are used
extensively as catalysts for a variety of oxidation reactions.12-16
In the catalytic pathway, metal based oxidants have been
proposed to affect the oxidation reactions. These ligands
contain reactive C-H bonds and are unstable under oxidising
conditions. As a result, many of these ligands suffer oxidative
degradation in catalytic reactions.17,
hydroxylation by C-H activation has been reported with metal
complexes of nitrogen donor ligands containing active
of tripodal nitrogen donor ligands containing reactive
27-29
methylene groups
have been reported to activate
dioxygen to form different cobalt-oxygen adducts.30-38 We
have investigated the dioxygen reactivity of cobalt(II)
complexes of (2-pyridylmethyl)amine-derived ligands with an
objective to gain insights into the nature of intermediate
generated in the reductive activation of dioxygen by analogous
iron(II)-α-hydroxy acid complexes.39 During the course of this
investigation, a new mode of reactivity of the cobalt(II)
complexes was observed. The cobalt(II)-α-hydroxy acid
18
Oxidative ligand
complex, [(TPA)CoII(benzilate)]+ (1) (TPA
=
tris(2-
School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A &
2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India, Fax: +91-33-2473-
2805; Tel: +91-33-2473-4971. Email: ictkp@iacs.res.in
pyridylmethyl)amine) exhibits oxidative decarboxylation of
benzilate in the reaction with dioxygen similar to that
observed with the corresponding iron(II)-benzilate complex.
On the contrary, the cobalt(II)-carboxylate complexes
[(TPA)CoII(benzoate)]+ (2) and [(iso-BPMEN)CoII(benzoate)]+ (3)
 Contributed equally.
Electronic Supplementary Information (ESI) available: [Spectral and
Crystallographic data]. See DOI: 10.1039/x0xx00000x
(iso-BPMEN
=
N1,N1-dimethyl-N2,N2-bis((pyridin-2-
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yl)methyl)ethane-1,2-diamine) undergo oxidative C-N bond symmetry.44, 45 The composition of the cobalt(IIV)-iecwaArrbtioclexyOlnalitnee
cleavage/oxygenation of the supporting N4 ligands. The role of complex 2 in solution was further verifiDeOdI:b1y0.1o0p3t9i/cCa9lDtTit0r4a4t3io8Hn
46
oxygen and water during the oxidative reactions is established of [(TPA)CoII(CH3CN)](ClO4)2 with benzoate in acetonitrile at
by labelling experiments. The influence of a carboxylate co- 25C. With incremental addition of benzoate, three new bands
ligand on the reactivity of cobalt(II) complexes is discussed in at 461 nm, 607 nm and 624 nm appear whereas the band at
this work and compared with those of a series of previously 552 nm slowly disappears (Fig. 1). No spectral change is
reported cobalt(II) complexes of the same supporting ligand.
observed after addition of 0.9 equiv. of benzoate. The spectral
changes clearly suggest the coordination of carboxylate to the
metal-ligand complex with the formation of about 90%
complex in solution. The final spectrum matches perfectly with
that of an acetonitrile solution of 2.
Scheme 1 Five-coordinate cobalt(II)-carboxylate complexes of N4 ligands.
Results and discussion
Synthesis and characterization
The cobalt(II) complexes were isolated from the reactions of
equimolar amounts of the tetradentate ligand, cobalt(II)
perchlorate hydrate, carboxylic acid (benzilic acid or benzoic
acid) and triethylamine (Experimental Section). The complexes
were isolated as either perchlorate or tetraphenylborate salts.
Fig. 1 Optical spectral changes upon addition of a mixture of benzoic acid
The FT-IR spectra of the complexes show sharp bands in the and triethylamine into a solution of [(TPA)CoII(CH3CN)]2+ in acetonitrile (1
regions 1627-1604 cm-1 and 1367-1339 cm-1 attributable to
mM) at 298 K.
νa(COO) and νs(COO), respectively. The energy difference
Structure of Co(II)-carboxylate complexes
Δν(COO) of >200 cm-1 is consistent with the monodentate
binding mode of the carboxylate groups.40 The ESI-mass
spectra of the complexes show molecular ion peaks at m/z
575.38, 470.10, and 450.25 with the isotope distribution
X-ray structures of 1 and 2 (Table S1) reveal that in each of the
monomeric cobalt complexes, the metal centre is ligated by
four nitrogen donors from the neutral tetradentate ligand and
one carboxylate oxygen (O1) of the monoanionic carboxylate
(benzilate for 1 and benzoate for 2) resulting in distorted
trigonal bipyramidal (TBP) coordination geometry (Fig. 2). The
axial positions of the distorted TBP geometry are occupied by
the amine nitrogen N3 of TPA and the carboxylate oxygen (O1)
with the N3–Co1–O1 angles of 167.40(11)° and 170.10(7)° for
1 and 2, respectively (Table 1). The equatorial positions are
occupied by three pyridine nitrogens (N1, N2 and N4) of the
supporting ligand. The Co-N bond distances are observed in
the range of 2.0605(19)–2.2109(18) Å similar to that observed
in reported five-coordinate high-spin cobalt(II) complexes.41-43
Several attempts to isolate single crystals of complex 3 failed.
However, DFT optimisation of 3 indicates a five-coordinate
geometry with monodentate carboxylate being energetically
more favourable than the six-coordinate complex (Fig. S3 and
Table S2). Furthermore, the bond parameters (Table S2) of the
five-coordinate geometry of 3 match well with those of the
structurally characterised complexes 1 and 2.
patterns
calculated
for
and
[(TPA)Co(benzilate)]+,
[(iso-BPMEN)Co(benzoate)]+,
[(TPA)Co(benzoate)]+
respectively. All the cobalt(II) complexes exhibit broad and
1
paramagnetically shifted H NMR spectra in solution typical of
high-spin cobalt(II) complexes (Figure S1, supporting
Information, SI). The room temperature magnetic moment
values of the complexes are found in the range of 4.06-4.32 µB.
A large orbital contribution of the metal ion results in higher
magnetic moments than calculated for the spin-only value of
1
high-spin d7 system. The magnetic moment values and the H
NMR spectra unambiguously support the high-spin nature of
the mononuclear cobalt(II) complexes.
The cobalt(II)-carboxylate complexes display three ligand field
transitions in the visible region (460-650 nm) with low molar
extinction coefficients (Fig. S2), typical of high-spin cobalt(II)
complexes.41-43 Moreover, the energies of these bands are very
similar to those observed in five-coordinate high-spin cobalt(II)
complexes Co(Et4-dien)X2 (X = Br, NCS, N3) in solution. The
bands originate from the spin-allowed transitions from the
microstates of two 4F and 4P ground state terms of Co(II) in C3v
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT04438H
Fig. 2 ORTEP plots of the complex cations of (a) 1 and (b) 2 with 30% thermal
ellipsoid parameters. The counter ions have been omitted for clarity.
Fig. 3 Five-coordinate TBP geometry at the cobalt centre of cobalt(II)-
complexes where the central atom is situated below the equatorial plane
(yellow triangle formed by N1, N2 and N4 atoms, whereas the red arrow
indicates the distance of the Co(II) atom from the plane).
Table 1. Selected bond lengths (Å) and angles (°) of 1 and 2.
Bond distances/angles
Complex 1
Complex 2
Co(1)–O(1)
1.985(3)
1.9680(15)
2.0809(19)
2.0648(18)
2.2109(18)
2.0605(19)
93.23(7)
Co(1)–N(1)
2.089(3)
Co(1)–N(2)
2.088(3)
Co(1)–N(3)
2.195(3)
Co(1)–N(4)
2.087(3)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(2)
O(1)–Co(1)–N(3)
O(1)–Co(1)–N(4)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(4)
N(2)–Co(1)–N(3)
N(2)–Co(1)–N(4)
N(3)–Co(1)–N(4)
92.18(11)
103.56(12)
167.40(11)
112.90(12)
114.43(12)
76.44(12)
113.45(12)
76.92(12)
117.05(12)
77.18(12)
105.76(7)
170.10(7)
108.40(7)
113.26(7)
76.96(7)
115.36(7)
77.45(7)
Fig. 4 Optical spectral change of 1 (1mM in acetonitrile) during the reaction
with dioxygen at 298 K.
117.34(7)
77.53(7)
Reaction of the Co(II) complexes with dioxygen
Between the two N4 ligands, the iso-BPMEN ligand with one
flexible arm (-CH2-CH2-NMe2) allows more distortion at the
cobalt centre (τ = 0.64) in 3 compared to 1 and 2 (τ values
around 0.88). Of note, almost ideal TBP geometries (τ; 0.96-
The cobalt(II) complexes are stable in the solid state and in
solution under inert atmosphere but are air-sensitive in
solution. Complex 1 in acetonitrile reacts with oxygen over a
time period of one week during which the d-d bands slowly
disappear with concomitant formation of a broad band with
0.99) are observed for [(TPA)CoIICl]+ and [(TPA)CoIICH3CN]2+
46
complexes.43,
However, cobalt(II) centre in the five-
1
absorption maxima at 470 nm (Fig. 4). The H NMR signals in
coordinate complexes (1-3) and in the previously reported
complexes
([(TPA)CoIICl]+, [(TPA)CoIICH3CN]2+,
[(TPA)CoII(salicylate)]+, [(TPA)CoII(benzoylformate)]+, and [(iso-
BPMEN)CoII(salicylate)]+) is situated 0.39-0.47 Å below the
equatorial plane formed by three nitrogen donors (N1-N2-N4)
of the ligands (Fig. 3).
the downfield region vanishes with time with the appearance
of three new signals at 7.8 ppm, 7.6 ppm and 7.5 ppm. The 1H
NMR spectrum of the oxidised solution suggest the formation
of a low-spin cobalt(III) species (Figs. 5 and S5). The three
signals in diamagnetic region are attributed to the aromatic
protons of non-coordinated benzophenone. Other peaks in the
aromatic region originate from the protons of pyridine rings of
the TPA ligand. The ESI-mass spectrum of the oxidised solution
of 1 shows an ion peak at m/z 349.22 calculated for [(TPA)Co]+
(Fig. S4). Therefore, oxidative transformation of the metal-
bound benzilate to benzophenone takes place in the reaction
with dioxygen. A control experiment with an equimolar
mixture of cobalt(II) perchlorate hexahydrate, benzilic acid and
Et3N in acetonitrile does not yield any decarboxylated product.
Thus the oxidative decarboxylation of benzilate by the cobalt
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complex is similar to that reported for iron(II)-benzilate ppm and 12.5 ppm typical of a low-spin cobalt(III) complex
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complexes of tridentate and tetradentate nitrogen ligands.39, (Fig. 7). The resonance of -NHamine proton of DPA is shifted
DOI: 10.1039/C9DT04438H
47-49
down-field to 12.2 ppm and appears as a broad singlet. A
sharp singlet for one of the protons of picolinate unit is
observed at 9.8 ppm. The α-protons of the pyridine rings of
DPA appear as a doublet at 8.4 ppm. Other aromatic protons
are observed between 7 ppm and 8 ppm. Two multiplet signals
of the aliphatic -CH2 protons of DPA appear at around 5 ppm.
1
Fig. 5 H NMR spectral (500 MHz, CD3CN, 298 K) changes with time ((a) 0 h
(b) 3 d (c) 1 week) during the reaction of 1 with dioxygen. The peaks marked
as # originate from residual CH3CN and H2O in NMR solvent, and traces of
CH3OH added to improve the solubility of the complex.
Complex 2 in acetonitrile also reacts with dioxygen at 298 K.
The d-d bands disappear over a period of 3 weeks with the
formation of a broad band showing absorption at 470 nm (Fig.
S6a). In the ESI-mass spectrum of the oxidised solution of 2,
ion peaks at m/z 256.01, 379.05 and 511.02 with the isotope
distribution patterns calculated for [(DPA-H2)Co]+, [(DPA-
H)Co(picolinate)]+ and [(BPCA)2Co]+, (DPA = di(2-picolyl)amine
and HBPCA = bis(2-pyridylcarbonyl)amide) respectively, are
detected (Fig. 6). The results suggest the formation of at least
two cobalt(III) complexes of DPA and BPCA ligands after the
oxidation of 2 (Scheme 2).
Fig.
7
1H NMR (300 MHz, CD3CN, 298K) spectrum of
[(DPA)Co(picolinate)(benzoate)](ClO4) (4).
The X-ray crystal structure of 4 (Table S3) shows a monomeric
six-coordinate cobalt centre ligated by three nitrogen donors
of di-2-picolylamine (DPA), one carboxylate oxygen (O1) of
benzoate and, one carboxylate oxygen (O3) and the pyridine
nitrogen (N4) of picolinate (Fig. 8). The axial positions of the
distorted octahedron are occupied by the carboxylate oxygens
(O1 and O3) with the O1-Co1-O3 angle of 173.90(12)° whereas
the equatorial plane is composed of the nitrogen donors N1,
N2, N3 and N4. The metal-ligand bond distances (Table S4) are
found in the range between 1.904(3) Å and 1.939(4) Å, typical
of low-spin cobalt(III) complexes. An intramolecular hydrogen
bonding interaction is observed between the amine nitrogen
(N2) of DPA and the carboxylate oxygen atom O2 of benzoate
with the N2···O2 distance of 2.736 Å.
Fig. 6 ESI-mass spectrum (positive ion mode in acetonitrile) of the oxidised
solution of 2.
A
red crystalline solid of one of the complex
[(DPA)Co(picolinate)(benzoate)](ClO4) (4) was isolated from
the oxidised solution of 2 after three weeks. The 1H NMR
spectrum of the crystalline solid exhibits signals between 4
Fig. 8 ORTEP plot of the cationic part of 4 with 30% ellipsoid probability.
Counter anion, solvent molecules and hydrogen atoms except H2A are
4 | J. Name., 2012, 00, 1-3
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omitted for clarity. Red dotted line represents the intramolecular hydrogen
bonding interaction.
Effect of carboxylate co-ligand
View Article Online
DOI: 10.1039/C9DT04438H
Although all the three cobalt(II) complexes have five-
coordinate geometry, their reaction towards dioxygen is
controlled by the axial ligand present at the fifth coordination
site. Complex 1 undergoes oxidative decarboxylation of the
axial co-ligand benzilate to benzophenone, whereas the
reactions of complexes 2 and 3 (with benzoate as co-ligand)
with dioxygen lead to the oxidative C-N cleavage of the ligand
backbone. In the ligand oxidation pathway, one of the
pyridylmethyl arms of the TPA/iso-BPMEN is oxidatively
cleaved to form the modified ligands DPA, DPEA, BPCA and
picolinate (Scheme 2). In addition, the dioxygen reactivity of
Crystalline
solid
of
another
cobalt(III)
complex,
[(BPCA)2Co](BPh4) (5) was isolated from the oxidised solution
of 2 in the presence of sodium tetraphenylborate. The 1H NMR
spectrum of 5 displays resonance signals between 6.5 ppm and
8.5 ppm (Fig. S7). Two doublet signals and two triplet signals
are observed above 7.5 ppm attributable to the aromatic
protons of the pyridine rings of BPCA ligand. The integration
patterns of the peaks indicate the presence of a four-fold
symmetry in the cationic part of the complex in solution. X-ray
crystal structure of the cationic complex displays
a
two
other
five-coordinate
cobalt(II)
complexes
mononuclear six-coordinate cobalt centre coordinated by two
monoanionic BPCA (Fig. S8). The cationic part of 5 is iso-
structural with the reported complex [CoIII(BPCA)2](ClO4) with
comparable bond parameters (Table S5).50
The cobalt(II)-benzoate complex (3) of the iso-BPMEN ligand
upon prolonged exposure to dioxygen in acetonitrile results in
a broad band at 475 nm (Fig. S6b). The ESI-mass spectrum of
the oxidised solution exhibits ion peaks at m/z 359.09 and
481.13 along with the isotope distribution patterns
[(TPA)CoII(Cl)](ClO4) and [(TPA)CoII(CH3CN)](ClO4)2 that lack any
carboxylate donor was also investigated. We reported two
biomimetic
cobalt(II)-α-keto
acid
complexes
[(TPA)CoII(BF)](BPh4) and [(TPA)CoII(PPH)](BPh4) (BF
=
benzoylformate an PPH = phenylpyruvate). The reaction of
[(TPA)CoII(PPH)](BPh4)
[(TPA)CoIII(PP)](BPh4),
with
dioxygen
affords
whereas [(TPA)CoII(benzoate)]+ is
formed upon oxidative decarboxylation of [(TPA)CoII(BF)](BPh4)
over the course of 5 days.41 A prolonged exposure of
attributable
[(DPEA)Co(picolinate)(benzoate)]+ (where DPEA
dimethyl-N2-((pyridin-2-yl)methyl)ethane-1,2-diamine)
to
[(DPEA-H)Co(picolinate)]+
and
N1,N1-
(Fig.
[(TPA)CoII(BF)]+ to dioxygen also forms
[(DPA)CoIII(picolinate)(benzoate)]+ and [CoIII(BPCA)2]+ (Fig. S10).
The cobalt(II)-acetate complex [(TPA)CoII(acetate)](BPh4)51
a mixture of
=
S9). In the 1H NMR spectrum, the proton resonances are
observed in the region between 2.5 ppm and 12.5 ppm
suggesting the diamagnetic nature of the cobalt species. The
1H NMR (Fig. 9) and ESI-mass spectra of the red solid isolated
from the oxidised solution of 3 confirms the formation of a
displays
similar
reactivity
as
observed
with
[(TPA)CoII(benzoate)](BPh4) (Fig. S11). On the other hand, the
cobalt(II)-chloro complex [(TPA)CoII(Cl)](ClO4)2 is stable in air
and does not react with dioxygen but the cobalt(II)-acetonitrile
complex [(TPA)CoII(CH3CN)](ClO4)2 forms a dimeric cobalt(III)-
hydroxo complex [(TPA)CoIII(OH)](ClO4)2}2 (7).
cobalt(III)
complex
with
the
molecular
formula
[(DPEA)CoIII(picolinate)(benzoate)](BPh4) (6) as the only
species. Herein, picolinate is exclusively formed upon oxidative
C-N bond cleavage of one of the pyridylmethyl arm of the iso-
BPMEN ligand. Thus the C-N bond of the aliphatic arm (-CH2-
CH2-NMe2) is less reactive than that of the pyridylmethyl arm (-
CH2-Py).
X-ray crystal structure of 7 (Table S3) reveals a hydroxo-
bridged dimeric cobalt(II) complex, where the two metal ions
are separated by 2.9591(15) Å (Fig. 10). Each of the metal ion
in 7 is in distorted octahedral coordination geometry with four
nitrogen donors from ligand and two oxygen atoms of the
bridging hydroxo ligands. The axial positions are occupied by a
hydroxo oxygen (O1′) and a pyridine nitrogen (N3) with the
O1′-Co1-N3 angle of 177.75(18)°. The Co-N distances in the
range 1.915(5)–1.932(4) Å are typical of cobalt(III) complexes
(Table S6).
Fig.
9
1H NMR spectrum (500 MHz, CD3CN, 298 K) of
[(DPEA)CoIII(picolinate)(benzoate)](BPh4) (6).
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complexes.50 Formation of BPCA from TPA via the dissociation
of one of the 2-pyridylmethyl arm and the oxidation at the
Fig. 10 ORTEP plot of 7 with 40% ellipsoid probability. Counter anions and
hydrogen atoms are omitted for clarity. Symmetry code (′): ½-x, ½-y, z.
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DOI: 10.1039/C9DT04438H
methyl carbons possibly by atmospheric oxygen on a rhenium
From the results described above, it is clear that the oxidation
of the methylene groups of TPA (and of iso-BPMEN) is
associated with the dioxygen activation at the metal centre
and can only be observed when one carboxylate co-ligand is
coordinated. The products, formed upon ligand cleavage,
remain coordinated to the metal centre in the oxidised
complexes 4 and 5. The ESI-mass spectrum of the oxidised
solution of 2 after the reaction with dioxygen in the presence
of excess benzilic acid (20 equiv) and triethylamine shows ion
peak at m/z 349.09 for [(TPA)Co]+ (Fig. S12). Thus, the
oxidative cleavage of a ligand arm can be avoided in the
presence of excess amount of substrate such as benzilate,
which does not form carboxylate ligand upon oxidation.
Fig. 7 ESI-mass spectra of the oxidised solution after reaction of 2 with 16O2
(a and b) and with 18O2 (c and d).
complex has been documented.52
[Co(II)(mebpena)]+
(mebpena
A
cobalt(II) complex
N-methyl-N,N’-bis(2-
=
pyridylmethyl)ethylene diamine-N’-acetate) of a monoanionic
N4O ligand bearing a pendant carboxymethyl group has been
reported to react with O2 to oxidatively cleave one
pyridylmethyl arm of the ligand.35 35 In the latter, a cobalt(III)-
superoxide species has been proposed to initiate the C-N bond
cleavage reaction via abstraction of an H atom from a
methylene carbon atom of the ligand. Reactivity studies on a
series of cobalt(II) complexes stressed the importance of a
coordinatively unsaturated complex for oxidative ligand
Scheme 2 Reactions of the five-coordinate cobalt(II) complexes of TPA and
iso-BPMEN with dioxygen.
35
dissociation by cobalt.35
The observed reactivity of the
complexes reported here is attributed to the presence of
vacant coordination site for reaction with O2. For the cobalt(II)-
benzoate complexes, oxygen activation may take place at the
metal(II) centre to affect ligand oxidation via H-atom
abstraction pathway by a cobalt(III)-superoxide.53-56 The role of
carboxylate co-ligand in the oxidative cleavage of the
supporting ligand indicates a dimeric pathway in which the
carboxylate may bridge between the two cobalt centre.
Involvement of cobalt-oxo type active oxidant, generated via
peroxo-bridged dicobalt (III) species, in the oxidative
degradation of the ligand is more likely. The incorporation of
oxygen atom from labeled water indicates the involvement of
cobalt-oxo species in the oxygenation of ligand backbone.
However, further investigation is necessary to unravel the
exact nature of the active oxidant and the mechanism of the C-
N bond cleavage pathway.
Mechanistic studies
To establish the role of oxygen and water in the
decarboxylation reaction of benzilate on 1 and C-N bond
cleavage reaction on 2, reactions were performed with 18O2 in
the presence or absence of H218O. The result of labelling
experiment with 1 and 18O2 confirms no incorporation of
labeled oxygen into benzophenone. However, the ESI-mass
spectrum of the oxidised solution of 2 after the reaction with
18O2 shows ion peaks at m/z 381.05 and 383.05, supporting the
incorporation of two 18O into [(DPA-H)Co(picolinate)]+ (Figs. 7a
and 7c). In addition, the presence of ion peaks at m/z 513.05,
515.02, 517.02 and 519.02 support the incorporation of up to
four 18O atom into [Co(BPCA)2]+ (Fig. 7c, Scheme 2).
Furthermore, a mixed labelling experiment with 2 and 16O2 in
the presence of H2O18 demonstrates the incorporation of 18O
from H2O18 into [(DPA-H)Co(picolinate)]+ and [Co(BPCA)2]+ (Fig.
S13) highlighting the involvement of water in the C-N bond
cleavage pathway.
Experimental
Aerial oxidation of the -CH2- group of N-(2-picolyl)picolinamide
on cobalt(III) and iron(III) centre has been reported to
Methods and equipment
generate [MIII(BPCA)2](ClO4) (where
M = Co and Fe)
6 | J. Name., 2012, 00, 1-3
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All reagents and solvents were purchased from commercial (4H, benzilate-m-H), 7.4 (2H, benzilate-p-H) and -1.2 (3H,
View Article Online
DOI: 10.1039/C9DT04438H
sources and were used without further purification, unless PyHγ).
otherwise noted. Solvents were purified and dried prior to use. [(TPA)CoII(benzoate)](BPh4) (2). Single crystals were obtained
Acetonitrile was dried over calcium hydride. Triethylamine was by diffusion of diethyl ether into a THF-CH3OH (1:1) solution of
dried over sodium prior to use. Preparation and handling of the complex. Yield: 0.55 g (70%). Elemental analysis calcd (%)
air-sensitive materials were carried out under an inert for C49H43BCoN4O2 (789.63 g/mol): C, 74.53; H, 5.49; N, 7.10.
atmosphere by using standard Schlenk techniques or in a glove Found: C, 74.25; H, 5.41; N, 6.98. IR (KBr, cm-1): 3053(m), 1610
box. Although no problem was encountered during the (s), 1570(m), 1481(m), 1437(m), 1366(s), 1267(w), 1026(w),
synthesis of these complexes from Co(ClO4)2, perchlorate salts 766(m), 733(s), 708(s). ESI-MS (in positive ion mode, CH3CN):
are potentially explosive and should be handled with care!57 m/z 470.10 (100%, [(TPA)Co(benzoate)]+). UV-vis in CH3CN
The ligands12 and the five-coordinate cobalt(II) complexes (λmax): 461 nm (ε = 170 M-1cm-1), 607 nm (ε = 115 M-1cm-1) and
[(TPA)CoII(Cl)](ClO4),43
[(TPA)CoII(OAc)](BPh4)51
[(TPA)CoII(CH3CN)](ClO4)46
and 624 nm (ε = 110 M-1cm-1). Magnetic moment µeff (298 K): 4.32
1
[(TPA)CoII(benzoylformate)](BPh4)41 µB. H NMR in CD3CN (500 MHz, 298 K); δ (ppm) 143.5 (3H,
were synthesized according to the procedure reported in PyHα), 96.8 (6H, CH2Py), 49.8 (3H, PyHβ), 45.1 (3H, PyHβ′), 21.6
literature. (2H, benzoate-o-H), 14.1 (2H, benzoate-m-H), 10.3 (1H,
Fourier transform infrared spectroscopy on KBr pellets was benzoate-p-H), 7.3 (8H, BPh4-Ar-o-H), 6.9 (8H, BPh4-Ar-m-H),
performed on a Shimadzu FT-IR 8400S instrument. Elemental and 6.7 (4H, BPh4-Ar-p-H) and -0.2 (3H, PyHγ).
analyses were performed on a Perkin Elmer 2400 series II CHN [(iso-BPMEN)CoII(benzoate)](BPh4) (3). Green solid. Yield: 0.51
analyzer. Electron-spray ionization mass spectra were g (67%). Elemental analysis calcd (%) for C47H47BCoN4O2
1
recorded with a Waters QTOF Micro YA263. H NMR spectra (769.65 g/mol): C, 73.35; H, 6.16; N, 7.28. Found: C, 72.79; H,
were measured at room temperature on Bruker DPX-300 or 6.12; N, 7.23. IR (KBr, cm-1): 3053(m), 2984(m), 2924(m),
500 MHz spectrometer. Solution electronic spectra were 1607(s), 1564(m), 1481(m), 1441(m), 1377(s), 1362(s),
measured on an Agilent 8453 diode array spectrophotometer. 1026(m), 843(m), 758(s), 735(s), 708(s). ESI-MS (in positive ion
Room temperature magnetic data were collected on a Gouy mode,
CH3CN):
m/z
450.25
(100%,
[(iso-
balance (Sherwood Scientific, Cambridge, UK). Diamagnetic BPMEN)Co(benzoate)]+). UV-vis in CH3CN (λmax): 468 nm (ε =
contributions were estimated for each compound by using 160 M-1 cm-1), 602 nm (ε = 120 M-1 cm-1). Magnetic moment
1
Pascal’s constants. GC-MS measurements were carried out µeff (298 K): 4.06 µB. H NMR in CD3CN (500 MHz, 298 K); δ
with a Perkin Elmer Clarus 600 using Elite 5 MS (30 m x 0.25 (ppm) 153.8 (2H, PyHα), 126.9 (4H, CH2Py), 113.9 (3H, N-CH3),
mm x 0.25 μm) column with a maximum temperature of 105.0 (3H, N-CH3), 71.1 (2H, CH2), 59.6 (2H, CH2), 52.4 (2H,
300°C. Labelling experiments were carried out with 18O2 gas PyHβ), 47.0 (2H, PyHβ′), 24.5 (2H, benzoate-o-H), 15.0 (2H,
(99 atom %) or H2O18 (98 atom %) purchased from Icon benzoate-m-H), 11.8 (1H, benzoate-p-H), 7.3 (8H, BPh4-Ar-o-H),
Services Inc., USA.
7.0 (8H, BPh4-Ar-m-H) and 6.8 (4H, BPh4-Ar-p-H).
Syntheses of cobalt(II) complexes. A solution of cobalt(II)
Isolation of cobalt(III) complexes. Pure oxygen gas was
bubbled through an acetonitrile solution of cobalt (II) complex
(1-3,
[(TPA)CoII(OAc)](BPh4) and [(TPA)CoII(benzoylformate)](BPh4))
for 5 min and was kept for stirring at room temperature. The
resulting green solution slowly turned orange-red over a
period of 3-5 d except for [(TPA)CoII(Cl)](ClO4). The oxidised
solution was allowed to stand for few days until crystalline
compound separated out from the solution.
perchlorate hexahydrate (0.37 g, 1 mmol) in 5 mL methanol
was added to a solution of ligand (1 mmol) in 5 mL methanol.
The orange solution was allowed to stir for a while and an
equimolar amount of carboxylic acid (benzilic acid or benzoic
acid) and Et3N (140 µL, 1 mmol) in 5 mL methanol was added
dropwise. The reaction mixture was further stirred for 4 h. The
perchlorate salts were isolated upon addition of diethyl ether
to a concentrated reaction solution. Addition of one equivalent
of NaBPh4 led to the isolation of tetraphenylborate salts of the
complexes.
[(TPA)CoII(Cl)](ClO4),
[(TPA)CoII(CH3CN)](ClO4),
[(DPA)CoIII(picolinate)(benzoate)](ClO4) (4). Pure oxygen gas
was bubbled through an acetonitrile solution of
[(TPA)CoII(benzoate)](ClO4) (57 mg, 0.10 mmol) for 5 min and
was kept for stirring at room temperature. The resulting green
solution slowly turned orange-red over a period of 3 d. Single
crystals suitable for X-ray diffraction were obtained from a
concentrated oxidised solution after 2 weeks. Yield: 19 mg
(32%). Elemental analysis calcd (%) for C25H22ClCoN4O8 (600.85
g/mol): C, 49.97; H, 3.69; N, 9.32. Found: C, 49.93; H, 3.73; N,
9.17. IR (KBr, cm-1): 3439(br), 3706(m), 2926(m), 1724(s),
1670(s), 1607(s), 1572(m), 1475(m), 1448(m), 1337(s),
1284(m), 1090(vs), 625(s). ESI-MS (in positive ion mode,
CH3CN): m/z 256.01 (100%, [(DPA-2H)Co]+), 379.05 (80%,
[(TPA)CoII(benzilate)](ClO4) (1). Green single crystals were
obtained by diffusion of diethyl ether into a THF-CH3OH (1:1)
solution of the complex. Yield: 0.34 g (51%). Elemental analysis
calcd (%) for C32H29ClCoN4O7 (675.98 g/mol): C, 56.86; H, 4.32;
N, 8.29. Found: C, 56.72; H, 4.35; N, 8.19. IR (KBr, cm-1):
3429(m), 2924(m), 1609(s), 1485(m), 1443(m), 1377(m),
1115(s), 1092(s), 762(m), 706(m), 623(m). ESI-MS (in positive
ion mode, CH3CN): m/z 575.38 (100%, [(TPA)Co(benzilate)]+).
UV-vis in CH3CN (λmax): 467 nm (ε = 150 M-1cm-1), 605 nm (ε =
110 M-1cm-1), 626 nm (ε = 90 M-1cm-1). Magnetic moment µeff
1
(298 K): 4.29 µB. H NMR in CD3CN (500 MHz, 298 K): δ (ppm)
[(DPA-H)Co(picolinate)]+),
501.00
(10%,
147.0 (3H, PyHα), 99.6 (6H, CH2Py), 52.3 (3H, PyHβ), 45.2 (3H,
PyHβ′), 15.6 (1H, benzilate-OH), 9.1 (4H, benzilate-o-H), 8.2
1
[(DPA)Co(picolinate)(benzoate)]+). H NMR (300 MHz, CD3CN,
298 K): δ (ppm) 12.17 (br, s, 1H), 9.73 (d, 1H, J = 4.8 Hz), 8.48
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
(m, 2H), 8.13 (d, 1H, J = 7.2 Hz), 7.98 (t, 2H, J = 7.5 Hz),7.81 (d, benzophenone (δ 7.81 ppm) with respect to the aromatic
View Article Online
2H, J = 7.5 Hz), 7.61 (t, 4H, J = 7.0 Hz), 7.45 (t, 1H, J = 7.2 Hz), protons of 2,4-di-tert-butylphenol at δDO6I:.1603.10p3p9m/C9iDnT0C4D43C8lH3.
1
7.33 (m, 4H), 5.06 (m, 2H), 4.87 (m, 2H).
Benzophenone: H NMR (500 MHz, CDCl3) δ 7.81 (d, 4H), 7.60
[(BPCA)2CoIII](BPh4) (5). Red crystals suitable for X-ray (t, 2H), 7.51 (t, 4H).
diffraction were isolated from a concentrated solution of the X-ray crystallographic data collection and refinement of the
oxidised species of 2 (78 mg, 0.10 mmol) after 3 weeks. Yield: structures. Crystallographic data for complexes 1, 2, 4·CH3CN,
16 mg (38%). Elemental analysis calcd (%) for C48H36BCoN6O4 5 and 7·2CH3CN are provided in Tables S1 and S3. Diffraction
(830.58 g/mol): C, 69.41; H, 4.37; N, 10.12. Found: C, 68.95; H, data were collected at 120 K for all the complexes on a Bruker
4.26; N, 10.11%. IR (KBr, cm-1): 3526(br), 3445(br), 3049(m), Smart APEX II (Mo-Kα radiation, λ
= 0.71073 Å). Cell
2928(m), 1720(s), 1651(m), 1606(m), 1337(s), 1296(m), refinement, indexing and scaling of the data set were carried
762(m), 736(m), 700(s). ESI-MS (in positive ion mode, CH3CN): out using the APEX2 v2.1-0 software.58 The structures were
1
m/z 511.02 (100%, [(BPCA)2Co]+). H NMR (300 MHz, CD3CN, solved by direct methods and subsequent Fourier analyses and
298 K): δ (ppm) 8.35 (d, 4H, J = 5.3 Hz), 8.21 (d, 4H, J = 7.3Hz), refined by the full-matrix least-squares method based on F2
8.09 (t, 4H, J = 7.7 Hz), 7.5 (t, 4H, J = 7.1 Hz), 7.3 (m, 8H), 7.0 (t, with all observed reflections.59 In all the complexes hydrogen
8H, J = 7.4 Hz), 6.8 (t, 4H, J = 7.2 Hz).
atoms are fixed and all the non-hydrogen atoms are refined
[(DPEA)CoIII(picolinate)(benzoate)](BPh4) (6). After two weeks, anisotropically. One of the oxygen atoms (O8) of the
addition of diethyl ether to the oxidised solution of 3 (77 mg, perchlorate anion in 4·CH3CN was found disordered and fixed
0.10 mmol) results in the isolation of a red solid. Yield: 49 mg over three position. In complex 7·2CH3CN, hydrogen atom of
(62%). Elemental analysis calcd (%) for C47H46BCoN4O4 (800.64 the hydroxide coordinated to the cobalt(II) centre could not be
g/mol): C, 70.51; H, 5.79; N, 7.00. Found: C, 69.95; H, 5.66; N, located. CCDC 1964010, 1964012, 1964011, 1964013 and
7.11%. IR (KBr, cm-1): 3462(br), 3049(m), 2926(m), 1670(s), 1964014 contain the supplementary crystallographic data for
1603(s), 1475(m), 1387(s), 1348(m), 733(s), 708(s). ESI-MS (in 1, 2, 4, 5 and 7·2CH3CN, respectively. The data can be obtained
positive ion mode, CH3CN): m/z 236.01 (100%, [(DPEA- free of charge from the Cambridge Crystallographic Data
2H)Co]+), 359.09 (90%, [(DPEA-H)Co(picolinate)]+), 481.13 Centre via www.ccdc.cam.ac.uk/data_request/cif
1
(30%, [(DPEA)Co(picolinate)(benzoate)]+). H NMR (300 MHz, DFT Calculations. Density functional theory (DFT) calculations
CD3CN, 298 K): δ (ppm) 12.22 (br, s, 1H), 9.54 (d, 1H, J = 4.8 of the complexes were carried out using the Gaussian 09
Hz), 8.44 (t, 1H, J = 5.0 Hz), 8.28 (t, 1H, J = 7.2 Hz), 8.11 (d, 1H, J program package.60 The five-coordinate geometry of 1 and 2
= 5.0 Hz), 7.89 (d, 3H, J = 9.0 Hz), 7.51 (m, 3H, J = 7.0 Hz), 7.39 were optimised with the cartesian coordinates obtained from
(m, 2H, J = 7.2 Hz), 7.21 (m, 8H), 7.11 (t, 1H, J = 7.4 Hz), 6.94 (t, the crystal structures of the complexes whereas those of
8H, J = 7.5 Hz), 6.81 (t, 4H, J = 7.3 Hz), 4.88 (m, 2H), 4.48 (m, complex 3 were obtained from the previously reported
2H), 3.37 (m, 2H), 3.28 (m, 3H), 3.20 (m, 3H).
complex [iso-BPMEN)CoII(salicylate)]+.42 GaussView5 software
[{(TPA)CoIII(OH)}2](ClO4)4 (7). Dry oxygen gas was purged package was used for generating input file and for analysis of
through the solution [(TPA)CoII(CH3CN)](ClO4)2 (63 mg, 0.10 bond distances and angles around metal centre and different
mmol) in acetonitrile (10 mL) for 5 min and was kept under spin density plot. The local minima with positive eigenvalues
oxygen atmosphere. The pink solution slowly turned red over a were ensured for performing the vibrational frequency
week. Single crystals suitable for X-ray diffraction were calculations with the optimised geometries. The energies
isolated from the acetonitrile solution upon addition of 5 mL reported for the optimised geometries included zero-point and
methanol to the oxidised solution in diethyl ether. Yield: 47 mg free energy corrections.
(82%).
Elemental
analysis
calcd
(%)
for
C36H38Cl4Co2N8O18·CH3CN (1171.46 g/mol): C, 38.96; H, 3.53; N,
10.76. Found: C, 38.58; H, 3.60; N, 10.51. IR (KBr, cm-1):
3408(br), 3049(m), 2933(m), 1609(s), 1481(m), 1445(m),
1288(m), 1144, 1115, 1086(s), 775(m), 627(s).
Conclusions
Three five-coordinate cobalt(II) complexes of tripodal N4
ligands with benzilate or benzoate co-ligand were isolated and
characterised. The cobalt(II)-benzoate complexes activate
dioxygen to carry out oxidative ligand dissociation via C-N
bond cleavage of one of the pyridylmethyl arms. The cobalt(II)-
benzilate complex of the TPA ligand, which does not exhibit
ligand oxidation, undergoes decarboxylation of the
coordinated benzilate in the reaction with dioxygen. Labelling
experiments with 18O2 and H218O confirm the involvement of
dioxygen and water in ligand cleavage pathway. The results
described herein show the importance of carboxylate co-
ligands in the dioxygen activation at the coordinatively
unsaturated cobalt(II) centre and on the oxidative C-N bond
cleavage of the supporting ligand by a cobalt-oxygen oxidant.
This study reveals that cobalt-carboxylate complexes of (2-
pyridylmethyl)amine-based ligands are not stable under
Reaction of cobalt(II)-benzilate complex, 1 with oxygen and
analysis of organic products. Solid complex (0.10 mmol) was
dissolved in dry acetonitrile (15 mL) under nitrogen
atmosphere. Pure oxygen gas was bubbled through the
solution and was allowed to stir at room temperature for 4 h,
during which the initial light yellow solution slowly turned light
orange. The solution was concentrated and to the residue was
added 10 mL 2M HCl. The organic products were then
extracted with diethyl ether (3 × 20 mL), dried over sodium
sulphate and evaporated under reduced pressure. The
products were analysed and quantified by GC-MS and 1H NMR
spectroscopy with respect to internal standard. Quantification
of benzophenone (from benzilate complexes) was performed
by comparing the peak area of four aromatic ortho-protons of
8 | J. Name., 2012, 00, 1-3
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Further investigations to unravel the exact nature of the active
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Conflicts of interest
Authors declare no conflicts of interest.
Acknowledgements
TKP acknowledges the Council of Scientific and Industrial
Research (CSIR), India for the financial support (Project File No.
01(2776)/14/EMR-II). RDJ thanks CSIR for
fellowship.
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