Oxidative C-C bond cleavage of α-keto acids by cobalt(II) complexes of nitrogen donor ligands

Article abstract of DOI:10.1002/ejic.201200663

Four cobalt(II) complexes, [(6Me₃TPA)Co^II(BF)] (BPh₄) (1), [(TPA)Co^II(BF)](BPh₄) (2), [{(6Me₃TPA)Co^II}₂(PP)](BPh₄) ₂ (3), and [(TPA)Co^II(PPH)](BPh₄) (4) [where 6Me₃TPA = tris(6-methyl-2-pyridylmethyl)amine, TPA = tris(2-pyridylmethyl)amine, BF = monoanionic benzoylformate, PP = dianionic phenylpyruvate, and PPH = monoanionic phenylpyruvate], of α-keto acid derivatives have been isolated to show their versatile reactivity with dioxygen. The X-ray crystal structure of 2 suggests a five-coordinate cobalt(II) center coordinated by a monodentate benzoylformate and a tetradentate nitrogen-donor supporting ligand. Conversely, complex 3 is a dinuclear cobalt complex where two cobalt(II) centers are bridged by PP. While complex 1 is unreactive towards dioxygen, 2 reacts slowly with oxygen to exhibit quantitative decarboxylation of coordinated benzoylformate to benzoate. An active cobalt-oxygen intermediate, intercepted by external substrates, is proposed to initiate the decarboxylation reaction. Complex 3 also reacts with dioxygen but to cleave the C2-C3 bond of PP with concomitant formation of benzaldehyde and an oxalate-bridged dicobalt(II) complex [{(6Me₃TPA)Co^II}₂(oxalate)](BPh ₄)₂ (5). The mononuclear PPH-cobalt(II) complex (4), unlike 2 and 3, does not undergo oxidative decarboxylation or C-C bond cleavage of PPH. In the reaction with dioxygen, 4 is oxidized to a PP-cobalt(III) complex, [(TPA)Co^III(PP)](BPh₄) (6), as established from the X-ray single-crystal structure. Cobalt(II) complexes of N₄ donor ligands oxidatively cleave the C-C bond of α-keto acids in the presence of dioxygen. Copyright

Full text of DOI:10.1002/ejic.201200663

FULL PAPER
DOI: 10.1002/ejic.201200663
Oxidative C–C Bond Cleavage of α-Keto Acids by Cobalt(II) Complexes of
Nitrogen Donor Ligands
Biswarup Chakraborty,[a] Partha Halder,[a] Priya Ranjan Banerjee,[a] and
Tapan Kanti Paine*[a]
Keywords: Cobalt / Nitrogen ligands / C–C bond cleavage / O–O activation / Decarboxylation
Four cobalt(II) complexes, [(6Me3TPA)CoII(BF)](BPh4) (1),
[(TPA)CoII(BF)](BPh4) (2), [{(6Me3TPA)CoII}2(PP)](BPh4)2 (3),
and [(TPA)CoII(PPH)](BPh4) (4) [where 6Me3TPA = tris(6-
dioxygen, 2 reacts slowly with oxygen to exhibit quantitative
decarboxylation of coordinated benzoylformate to benzoate.
An active cobalt-oxygen intermediate, intercepted by exter-
nal substrates, is proposed to initiate the decarboxylation re-
action. Complex 3 also reacts with dioxygen but to cleave the
C2–C3 bond of PP with concomitant formation of benzalde-
hyde and an oxalate-bridged dicobalt(II) complex
[{(6Me3TPA)CoII}2(oxalate)](BPh4)2 (5). The mononuclear
PPH-cobalt(II) complex (4), unlike 2 and 3, does not undergo
oxidative decarboxylation or C–C bond cleavage of PPH. In
the reaction with dioxygen, 4 is oxidized to a PP-cobalt(III)
complex, [(TPA)CoIII(PP)](BPh4) (6), as established from the
X-ray single-crystal structure.
methyl-2-pyridylmethyl)amine,
TPA
=
tris(2-pyridyl-
methyl)amine, BF = monoanionic benzoylformate, PP = dian-
ionic phenylpyruvate, and PPH = monoanionic phenylpyr-
uvate], of α-keto acid derivatives have been isolated to show
their versatile reactivity with dioxygen. The X-ray crystal
structure of 2 suggests a five-coordinate cobalt(II) center co-
ordinated by a monodentate benzoylformate and a tetraden-
tate nitrogen-donor supporting ligand. Conversely, complex
3 is a dinuclear cobalt complex where two cobalt(II) centers
are bridged by PP. While complex 1 is unreactive towards
dioxygen and mimic the α-ketoglutarate-dependent en-
zymes. Additionally, α-keto acid substrate phenylpyruvic
acid (PPH2) has relevance to a non-heme iron enzyme 4-
hydroxyphenylpyruvate dioxygenase (HPPD). HPPD cata-
lyzes the conversion of 4-hydroxyphenylpyruvate to 2,5-di-
hydroxyphenyl acetate. Phenylpyruvate (PPH), unlike ben-
zoylformate, tends to enolize depending upon the nature of
the metal ion and on the supporting ligand. The enolization
of PPH has been documented in iron(II) complexes of tetra-
dentate nitrogen donor ligands. The iron(II)–phenylpyr-
uvate complexes have been shown to undergo C–C bond
cleavage of phenylpyruvate in the reaction with di-
oxygen.[24,25] Aliphatic C–C bond cleavage of substrates
with enolate groups has been observed in β-diketone dioxy-
genase (Fe),[26–28] quercetin-2,3-dioxygenase (Cu)[29], and
acireductone dioxygenase (Ni).[30,31]
In some α-ketoglutarate-dependent enzymes, it has been
shown that the replacement of iron(II) by other divalent
metal ions like CoII, NiII, CuII, and ZnII leads to partial or
complete loss of activity.[32] Recently, copper(I)–benzoyl-
formate complexes have been shown to react with dioxygen
to carry out oxidative decarboxylation of benzoylformate at
low temperature in the presence of external substrates.[33]
Compared to a large number of model α-keto acid–iron(II)
complexes presented in the literature, α-keto acid complexes
of other metal ions are rare. Studies on other metal com-
plexes may provide additional information to understand
the mechanism of dioxygen activation by model α-keto acid
Introduction
α-Ketoglutarate-dependent dioxygenases constitute
a
large family of non-heme iron enzymes that catalyze the
two-electron oxidation of organic substrates with concomi-
tant decarboxylation of the α-keto acid in the presence of
dioxygen.[1–3] This class of enzymes is essential in many im-
portant biological functions such as DNA and RNA repair,
biosynthesis of antibiotics, herbicide degradation, and oxy-
gen sensing etc.[4–7] Despite their differences in biological
function, this family of enzymes shares a common 2-His-1-
carboxylate facial triad motif at the active site.[8,9] It has
now been established that an iron(II) center activates di-
oxygen to form an iron(IV)-oxo intermediate as the active
oxidant to affect the substrate oxidation.[1,10–15] In biomi-
metic chemistry, several α-keto acid–iron(II) complexes of
tetradentate ligands have been synthesized and charac-
terized.[16–24] In most of the cases, benzoylformic acid or
(phenyl)pyruvic acid have been used as model substrates
that bind to the metal center with different binding motifs.
These model complexes exhibit versatile reactivity towards
[a] Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032,
India
Fax: +91-33-2473-2805
E-mail: ictkp@iacs.res.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200663
Eur. J. Inorg. Chem. 2012, 5843–5853
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B. Chakraborty, P. Halder, P. R. Banerjee, T. K. Paine
FULL PAPER
complexes. With this objective we have investigated the re-
ESI-MS of mononuclear complexes 1, 2, and 4 (Figures
activity of cobalt(II) complexes of α-keto acid derivatives S2 and S3) display molecular ion peaks at m/z = 540.36,
supported by tetradentate nitrogen donor ligands. 498.05, and 511.98 with isotopic distribution patterns calcu-
As a result of our investigation, we report herein the syn- lated for [(6Me3TPA)Co(BF)]+, [(TPA)Co(BF)]+, and
thesis and characterization of four cobalt(II) complexes [(TPA)Co(PPH)]+, respectively. Dinuclear complex (3) exhi-
[(6Me3TPA)CoII(BF)](BPh4) (1), [(TPA)CoII(BF)](BPh4) bits an ion peak at m/z = 471.95 with an expected isotope
(2), [{(6Me3TPA)CoII}2(PP)](BPh4)2 (3), and [(TPA)CoII- distribution pattern for the complex cation [{(6Me3TPA)-
(PPH)](BPh4) (4), where BF = benzoylformate anion, PP = Co}2(PP)]2+ (Figure S3). Ion peaks other than molecular
dianionic phenylpyruvate enolate, and PPH = monoanionic ion peaks are observed in the mass spectra (Figures S2 and
phenylpyruvate (Scheme 1). The diverse reactivity of model S3). Room-temperature magnetic moment values for the
complexes with dioxygen that lead to the oxidative C–C mononuclear complexes (1, 2, and 4) fall in the range 4.17–
bond cleavage of α-keto acids is discussed.
4.94 μB, indicative of the high-spin nature of the complexes.
The magnetic moments are higher than those calculated for
a spin-only high-spin d7 ion, indicating a large orbital con-
tribution to the magnetic moment. A lower value (6.35 μB)
of magnetic moment than expected for two uncoupled co-
balt(II) centers in 3 indicates that the cobalt(II) centers are
exchange coupled. The complexes display d-d transitions in
the visible region associated with the high-spin cobalt(II)
complex (Figure S4). While the reported α-keto acid–
iron(II) complexes of N4 donor ligands exhibit iron(II)-to-
keto charge-transfer bands,[35] the cobalt(II) complexes (1,
2, and 4) do not display such charge-transfer bands. The
optical spectroscopic data are consistent with the noncoor-
dination of carbonyl oxygen from α-keto acid in 1, 2, and
4. The optical spectra of cobalt(II)–benzoylformate com-
plexes (1 and 2) differ in the position of d-d bands implying
different ligand field strengths engendered by the support-
ing ligands. A higher intensity of the absorption bands in 2
is attributed to geometry less than octahedral symmetry
(five-coordinate in 2 vs. six-coordinate in 1). An intense
charge-transfer transition at around 330 nm is observed in
the optical spectrum of 3 which may be assigned to co-
balt(II)–PP interaction.
Scheme 1. Syntheses of cobalt(II) complexes.
Results and Discussion
To investigate the compositions of cobalt(II)–benzoyl-
The cobalt(II) complexes were prepared by mixing the formate complex 2 in solution, UV/Vis titration of [(TPA)-
ligands with cobalt(II) salt and sodium salt of α-keto acids. CoII(CH3CN)]2+[36] with sodium benzoylformate was car-
The complexes were isolated as tetraphenylborate or per- ried out in acetonitrile at room temperature (Figure 1). It is
chlorate salts (Scheme 1). All the complexes were charac- very clear from the optical spectral changes during the ti-
terized by several spectroscopic and analytical techniques tration that one equivalent of sodium benzoylformate is
including elemental analysis, FTIR, ESI-MS, and room- needed for maximum formation of 2. No spectral change
temperature magnetic measurements. The IR spectrum of 1 is observed upon further addition of benzoylformate. The
exhibits νasym(COO) and νsym(COO) stretching frequencies
at 1632 and 1450 cm–1, respectively, whereas those of 2 ap-
pear at 1634 and 1391 cm–1. Additionally, 1 and 2 show a
carbonyl stretching (of the α-keto group) frequency at 1672
and 1688 cm–1, respectively. The spectroscopic data (see
Supporting Information, Figure S1) suggest that the oxygen
atom from the α-keto group of benzoylformate remains
noncoordinated in both the complexes. It appears from the
carboxylate stretching frequencies that benzoylformate
binds in monodentate fashion through one of the carboxyl-
ate oxygen atoms in 2 and in bidentate fashion via both the
carboxylate oxygen atoms in 1.[34] IR spectrum of complex
4 exhibits carbonyl stretching frequency of phenylpyruvate
at 1680 cm–1. The absence of this characteristic stretching
vibration in complex 3 suggests a possible enolization of
Figure 1. Optical spectral changes during the titration of [(TPA)-
CoII(CH3CN)]2+ in acetonitrile (c = 1 mm) with sodium benzoyl-
PPH (Scheme 1).
formate.
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Oxidative C–C Bond Cleavage by Co Complexes
absence of a metal-to-keto charge-transfer transition in the cobalt(II) center. The bond parameters in 2 show resem-
optical spectrum together with the analytical data support blance to those reported in a five-coordinate cobalt(II) sali-
the binding of benzoylformate to the cobalt(II) center via a cylate complex of the same ligand.[37]
carboxylate oxygen atom in 2.
Table 1. Selected bond lengths [Å] and bond angles [°] for 2.
To confirm the structures of mononuclear α-keto acid–
cobalt(II) complexes in solid state, efforts were made to Co(1)–O(1)
1.977(2)
2.057(2)
2.070(2)
1.255(3)
1.530(3)
Co(1)–N(1)
Co(1)–N(3)
C(19)–O(2)
C(20)–O(3)
2.058(2)
2.193(2)
1.222(3)
1.207(3)
Co(1)–N(2)
grow the single crystals of 1, 2, and 4. Unfortunately, single
Co(1)–N(4)
crystals of 1 and 4 suitable for X-ray diffraction could not
C(19)–O(1)
be isolated even after several attempts. However, from the
C(19)–C(20)
analytical and spectroscopic data discussed above, and by
analogy with a related cobalt(II)–salicylate complex[37] of
the tetradentate 6Me3TPA ligand, a six-coordinate co-
balt(II) complex with a bidentate binding motif of benzo-
ylformate through both the carboxylate oxygen atoms may
be proposed for 1. Conversely, single crystals of 2 were
grown by vapor diffusion of diethyl ether into a solution of
the complex in acetonitrile and tetrahydrofuran. The single
crystal structure of 2 reveals a five-coordinate mononuclear
complex cation where the metal ion is coordinated by four
nitrogen donors from the ligand and one carboxylate oxy-
gen (O1) of monoanionic benzoylformate (Figure 2).
O(1)–Co(1)–N(1) 109.97(7)
O(1)–Co(1)–N(3) 170.95(7)
N(1)–Co(1)–N(2) 114.62(7)
N(1)–Co(1)–N(4) 113.43(7)
N(2)–Co(1)–N(4) 119.69(7)
O(1)–Co(1)–N(2) 93.75(7)
O(1)–Co(1)–N(4) 102.02(7)
N(1)–Co(1)–N(3) 78.08(7)
N(2)–Co(1)–N(3) 78.80(7)
N(3)–Co(1)–N(4) 77.68(7)
Conversely, the solid-state structure of 3 exhibits a dico-
balt complex cation and tetraphenylborate counteranions.
The dianionic phenylpyruvate (PP) forms a bridge between
the two cobalt centers supported by the tetradentate ligand
(Figure 3). In the asymmetric dimer, Co1 displays a six-co-
ordinate distorted octahedral geometry with four nitrogen
donors from the ligand and two oxygen donors (O1 and
O2) from the dianionic phenylpyruvate. The other cobalt
center Co2 has a five-coordinate distorted square-pyrami-
dal (τ = 0.47) coordination geometry ligated by four nitro-
gen atoms of the ligand and the carboxylate oxygen O3.
The complex cation is found to be isostructural with an
iron(II) phenylpyruvate complex of the same ligand and all
the bond lengths are close to the reported iron analogue.[24]
The enolization of PPH upon coordination at the iron cen-
ter is evident from the C44–O2 and C44–C45 distances of
1.318(3) and 1.360(3) Å, respectively (Table 2).
Figure 2. ORTEP plot of the complex cation of 2 with 40% thermal
ellipsoid parameters. The counterion and all H atoms are omitted
for clarity.
The carbonyl oxygen O3 of benzoylformate is not coordi-
nated to the metal center and supports the spectroscopic
features in solution. Such a binding motif of benzoylform-
ate has been observed in [(TPA)FeII(BF)(MeOH)]+[16] and
in [(TptBu,iPr)FeII(BF)] [TptBu,iPr = hydrotris(3-tert-butyl-5-
isopropyl-1-pyrazolyl)borate][18] complexes. In 2, the amine
nitrogen N3 of the ligand and the carboxylate oxygen O1
of benzoylformate occupy the axial positions with an N3–
Co1–O1 angle of 170.95°(7), whereas three pyridine nitro-
gen atoms (N1, N2, and N4) of the ligand occupy the equa-
torial plane with NPy–Co–NPy angles ranging from
113.43°(7)–119.69°(7) (Table 1). A longer Co1···O2 distance
Figure 3. ORTEP plot of the cationic part of 3 with 40% ellipsoid
probability. The tetraphenylborate counterion and all H atoms are
omitted for clarity.
The reported cobalt(II) complexes of TPA, [(TPA)-
of 3.263 Å clearly indicates the noncoordination of the CoII(CH3CN)](ClO4)2,[39]
[(TPA)CoII(Cl)](ClO4),
and
carboxylate oxygen O2 with the metal center. This particu- [Co2(TPA)2(μ-tp)](ClO4)2 (tp = dianionic terephthalate)[40]
lar disposition of donor atoms constitutes a distorted trigo- all exhibit five-coordinate distorted trigonal bipyramidal
nal bipyramidal coordination geometry (τ = 0.85)[38] at the coordination geometry at the metal center. By analogy with
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Table 2. Selected bond lengths [Å] and bond angles [°] for gen atoms of the monoanionic phenylpyruvate. The C20–
3·CH2Cl2.
C21 distance of 1.524 Å and C20–O3 distance of 1.229 Å
support the coordination of phenylpyruvate in the keto
form. The bond parameters (Table 3) obtained from the op-
timized geometry are close to those of 2 and other reported
five-coordinate cobalt(II) complexes.[37,39,40]
Co(1)–O(1)
Co(1)–N(1)
Co(1)–N(3)
Co(2)–O(3)
Co(2)–N(6)
Co(2)–N(8)
C(44)–C(45)
C(43)–O(1)
O(2)–Co(1)–N(2) 168.61(7)
N(2)–Co(1)–O(1) 88.66(6)
N(2)–Co(1)–N(4) 81.75(7)
O(2)–Co(1)–N(3) 102.80(7)
O(1)–Co(1)–N(3) 91.17(6)
O(2)–Co(1)–N(1) 102.64(7)
O(1)–Co(1)–N(1) 90.41(7)
N(3)–Co(1)–N(1) 154.40(7)
O(3)–Co(2)–N(6) 165.29(7)
O(3)–Co(2)–N(7) 92.78(7)
N(6)–Co(2)–N(7) 75.37(8)
N(8)–Co(2)–N(5) 109.82(8)
N(7)–Co(2)–N(5) 136.82(8)
2.162(2)
2.305(2)
2.200(2)
1.978(2)
2.135(2)
2.076(2)
1.360(3)
1.258(3)
Co(1)–O(2)
Co(1)–N(2)
Co(1)–N(4)
Co(2)–N(5)
Co(2)–N(7)
C(44)–O(2)
C(43)–O(3)
C(43)–C(44)
O(2)–Co(1)–O(1)
O(2)–Co(1)–N(4) 109.41(6)
O(1)–Co(1)–N(4) 167.17(7)
N(2)–Co(1)–N(3) 77.75(7)
N(4)–Co(1)–N(3) 95.08(7)
N(2)–Co(1)–N(1) 76.74(7)
N(4)–Co(1)–N(1) 79.14(7)
O(3)–Co(2)–N(8) 111.12(7)
N(8)–Co(2)–N(6) 80.01(7)
N(8)–Co(2)–N(7) 99.98(8)
O(3)–Co(2)–N(5) 104.24(7)
N(6)–Co(2)–N(5) 79.77(7)
1.974(2)
2.130(2)
2.192 (2)
2.161(2)
2.161(2)
1.318(3)
1.269(3)
1.511(3)
79.96(6)
Reaction of Cobalt(II) Complexes with Oxygen
Unlike the iron(II)–benzoylformate complex of the tripo-
dal 6Me3TPA ligand, the corresponding cobalt(II) complex
1 is unreactive towards dioxygen. Conversely, the five-coor-
dinate cobalt(II)–benzoylformate complex 2 reacts with
oxygen in acetonitrile slowly over a period of five days dur-
ing which time the green solution turns orange. The final
reaction solution exhibits an absorbance at 460 nm suggest-
ing the formation of a low-spin cobalt(III) complex (Figure
S5). Unfortunately, the final reaction solution exhibits a
complicated 1H NMR spectrum implying a mixture of spe-
cies. However, ESI-MS of the reaction solution of 2 after
two days displays an ion peak at m/z = 470.01 with the
isotopic distribution pattern calculated for [(TPA)Co-
(benzoate)]+ which signify the oxidative decarboxylation of
benzoylformate to benzoate during the reaction (Figure 5).
the structure of 2 and of other cobalt(II) complexes of TPA,
a five-coordinate PPH-cobalt(II) complex is expected for 4.
Moreover, the nature of the d-d band assigned for cobalt(II)
and the stretching vibrations of the keto and carboxylate
groups in the FTIR spectrum are comparable for both 2
and 4. Furthermore, the cationic part of complex 4 (Fig-
ure 4) was optimized by DFT calculation which confirmed
the five-coordinate geometry at the metal center. As in com-
plex 2, the metal ion is coordinated by four nitrogen donors
of the tetradentate ligand and one of the carboxylate oxy-
Figure 5. ESI-MS (positive ion mode in acetonitrile) of the solution
after reaction of 2 with dioxygen. Inset: experimental and simulated
isotope distribution patterns of the peak at m/z = 470.01.
To identify the organic products formed during the reac-
1
tion with oxygen, acidic workup and subsequent H NMR
analysis of the oxygenated solution was carried out using
the perchlorate salt of 2 (Figure 6). A quantitative conver-
sion of benzoylformate to benzoate was observed after 5 d.
Unfortunately, all attempts to isolate the oxidized complex
from the reaction solution failed. It is important to mention
here that no decarboxylation of benzoylformate was ob-
served when a control experiment was carried out with a
1:1 mixture of cobalt(II) perchlorate and sodium benzo-
ylformate under O2 in the absence of ligand. The benzoic
acid formed in the reaction of 2 with O2 was esterified with
Figure 4. DFT-optimized geometry of the cationic part of complex
4.
Table 3. Selected bond lengths [Å] and bond angles [°] of the DFT
optimized geometry for complex 4.
Co–N(1)
Co–N(3)
Co–O(1)
C(20)–C(21)
N(1)–Co–N(3)
N(2)–Co–N(4)
O(1)–Co–N(1)
O(1)–Co–N(3)
2.0595
2.2622
2.0286
1.5236
76.786
126.436
91.989
168.706
Co–N(2)
Co–N(4)
C(20)–O(3)
N(1)–Co–N(2)
N(1)–Co–N(4)
N(3)–Co–N(4)
O(1)–Co–N(2)
O(1)–Co–N(4)
2.0488
2.0789
1.2290
109.466
108.783
77.759
107.082
107.569
α-bromoacetophenone according to
a reported pro-
cedure.[41] GC–MS of the ester derivative further confirms
the formation of benzoic acid in the reaction of 2 with oxy-
gen (Figure S6).
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Oxidative C–C Bond Cleavage by Co Complexes
served percentage of 18O incorporation into oxalate may be
attributed to the exchange of 18O with residual water in the
reaction medium.
Figure 6. Formation of benzoic acid with time, monitored by 1H
NMR spectroscopy (500 MHz in CDCl3 at 298 K), during the reac-
tion of [(TPA)CoII(BF)](ClO4) (2·ClO4) with dioxygen. The peak
marked as * represents the resonance due to the ortho protons of
the phenyl ring of benzoic acid and the # marked peak represents
the residual solvent peak.
Figure 8. ESI-MS (positive ion model in acetonitrile) of the solu-
tion after reaction of 3 with (A) 16O2, (B) 18O2.
To confirm the nature of the oxidized metal complex, X-
ray-quality single crystals of 5 were grown from a solvent
mixture of dichloromethane and methanol. The solid-state
structure of the dicationic complex (Figure 9) reveals a sym-
metrical oxalate-bridged dinuclear cobalt complex. The
asymmetric unit contains only half of the overall dicationic
unit. Two cobalt centers are symmetry related with an inver-
sion point at the middle point of the oxalate C–C bond.
Each of the cobalt is hexacoordinated by four nitrogen do-
nors (N1–N4) of the ligand and two carboxylate oxygen
atoms (O1 and O2) of the bridging oxalate ligand. The Co–
N distances (Table 4) lie in the range 2.146(2)–2.225(2) Å
whereas the Co1–O1 and Co1–O2 distances are 2.036(2)
and 2.209(1) Å, respectively. The structure of the oxalate-
bridged dinuclear species bears resemblance to the reported
iron(II) oxalate complex.[24]
Conversely, the green solution of 3 in acetonitrile
changes slowly under an oxygen atmosphere over a period
of 7 h during which time the charge-transfer bands decay
slowly (Figure 7). The decay of the charge-transfer band in-
dicates the oxidative transformation of PP. This observation
is in line with the PP-iron(II) complex of the same ligand.
To understand the nature of PP cleavage products, the reac-
tion solution after removal of metal ion was analyzed by
1
1H NMR spectroscopy. H NMR spectroscopic data con-
firms the formation of about 40% benzaldehyde.
Figure 7. Optical spectral changes during the reaction of 3 (0.1 mm)
1
with dioxygen at room temperature. Inset: H NMR spectrum of
the reaction solution after removal of the metal ion.
ESI-MS of the final reaction solution exhibits a predomi-
nant peak at m/z = 435.15 with an isotope distribution
pattern calculated for [{(6Me3TPA)CoII}2(oxalate)]2+ (5)
(Figure S7). ESI-MS and optical spectroscopic data suggest
C–C bond cleavage of PP in the reaction with oxygen. The
ESI-MS spectrum of the oxidized solution after reaction of
3 with 18O2 shows a predominant peak at m/z = 436.15
Figure 9. ORTEP plot of the complex cation of 5 with 40% thermal
ellipsoids. Hydrogen atoms and counterions have been removed for
clarity.
The green solution of complex 4 in acetonitrile showing
along with a peak at 435.15 (Figure 8), indicating that one absorption bands at 304 nm and 322 nm slowly changes to
of the oxygen atoms of 18O2 is incorporated into oxalate. an orange solution during the reaction with O2 for 3 h. ESI-
About 60% incorporation of 18O into oxalate is calculated MS (Figure 10, inset) of the orange solution displays a pre-
from the isotope distribution patterns of the peaks. The ob- dominant molecular ion peak at m/z = 511.12 with an iso-
Eur. J. Inorg. Chem. 2012, 5843–5853
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5847

B. Chakraborty, P. Halder, P. R. Banerjee, T. K. Paine
FULL PAPER
Table 4. Selected bond lengths [Å] and bond angles [°] for complex gen donor O2 from phenylpyruvate occupy the axial posi-
5.
tions with an N2–Co1–O2 angle of 179.10(7)°.
The enolization of coordinated PPH, upon oxidation of
4 with oxygen, is evident from the bond parameters. A
shorter Co1–O2 bond with a length of 1.866(2) Å along
with the double bond character of C20–C21 [length
1.344(3) Å] (Table 5) confirm the enolate character of the
phenylpyruvate moiety coordinated to the cobalt center.
The metal–nitrogen distances are in good agreement with
low-spin cobalt(III) complexes.[37]
Co(1)–O(1)
Co(1)–N(1)
Co(1)–N(3)
O(1)–Co(1)–N(2) 172.99(8)
N(2)–Co(1)–N(1) 78.75(9)
N(2)–Co(1)–N(4) 75.91(8)
2.036(2)
2.175(2)
2.225(2)
Co(1)–O(2)
Co(1)–N(2)
Co(1)–N(4)
O(1)–Co(1)–N(1) 104.79(8)
O(1)–Co(1)–N(4) 97.44(7)
N(1)–Co(1)–N(4) 98.96(7)
N(2)–Co(1)–O(2) 98.39(8)
N(4)–Co(1)–O(2) 81.47(7)
N(2)–Co(1)–N(3) 79.08(8)
N(4)–Co(1)–N(3) 145.33(9)
2.209(2)
2.146(2)
2.203(2)
O(1)–Co(1)–O(2)
78.18(6)
N(1)–Co(1)–O(2) 176.88(8)
O(1)–Co(1)–N(3) 106.01(8)
N(1)–Co(1)–N(3) 99.28(7)
O(2)–Co(1)–N(3) 78.85(7)
Table 5. Selected bond lengths [Å] and bond angles [°] for complex
6·2CH3CN.
topic distribution pattern calculated for [(TPA)Co(PP)]+
(Figure S7). The orange solution exhibits two strong ab-
sorptions at 322 nm and 335 nm (Figure 10).
Co(1)–O(1)
Co(1)–N(1)
Co(1)–N(3)
O(2)–C(20)
1.880(2)
1.921(2)
1.917(2)
1.340(2)
1.312(3)
87.46(6)
90.46(7)
176.69(7)
95.09(7)
170.27(7)
179.10(7)
85.15(7)
85.15(7)
Co(1)–O(2)
Co(1)–N(2)
Co(1)–N(4)
C(20)–C(21)
1.866(2)
1.944(2)
1.921(2)
1.344(3)
1.223(3)
94.60(7)
93.98(7)
92.39(7)
89.21(7)
87.70(7)
91.68(7)
86.89(7)
C(19)–O(1)
C(19)–O(3)
O(2)–Co(1)–O(1)
O(1)–Co(1)–N(3)
O(1)–Co(1)–N(4)
O(2)–Co(1)–N(1)
N(3)–Co(1)–N(1)
O(2)–Co(1)–N(2)
N(3)–Co(1)–N(2)
N(1)–Co(1)–N(2)
O(2)–Co(1)–N(3)
O(2)–Co(1)–N(4)
N(3)–Co(1)–N(4)
O(1)–Co(1)–N(1)
N(4)–Co(1)–N(1)
O(1)–Co(1)–N(2)
N(4)–Co(1)–N(2)
The two cobalt(II)–phenylpyruvate complexes react with
dioxygen but afford different compounds (Scheme 2). While
3 undergoes oxidative C–C bond cleavage of PP, 4 is oxid-
ized to form a PP-cobalt(III) complex upon loss of a Cβ-H
proton of PPH.
Figure 10. Optical spectral changes of 4 (concentration: 0.5 mm) in
the presence of molecular oxygen in acetonitrile at room tempera-
ture. Inset: ESI-MS of the oxidized solution of 4.
To investigate the mechanism of the reaction of co-
balt(II) complexes with dioxygen, reactions were performed
at low temperature. No intermediate species was observed
in the optical spectrum at low temperature. The reaction of
2 with dioxygen was monitored by X-band EPR at 77 K
using a superoxide trap DMPO.[42] Unfortunately, no EPR
signal of the DMPO-superoxide spin adduct was observed.
Therefore, interception studies were carried out with 2,4-di-
tert-butylphenol and 2,4,6-tri-tert-butylphenol to intercept
oxygen-derived intermediate species involved in the reaction
pathway (Scheme 3). The formation of cobalt(III) superox-
ide species in the reaction of cobalt(II) complexes with di-
oxygen is well established.[43] It is also known that the for-
mation of biphenol from phenol takes place via H atom
abstraction by a metal-superoxide intermediate followed by
C–C coupling.[16,23,44–46] In the reaction of 2 with dioxygen,
an almost stoichiometric amount of 4,4Ј,6,6Ј-tetra-tert-but-
ylbiphenol is formed as observed in the 1H NMR spectrum
of the organic product derived from 2,4-di-tert-butylphenol
(see Exp. Section). An active oxygen species initiates the
reaction via hydrogen atom abstraction to form an interme-
diate phenoxyl radical, which dimerizes immediately to
form biphenol. Control experiments suggest that both the
cobalt(II) complex and dioxygen are needed for the forma-
tion of biphenol from phenol.
Single crystals of the oxidized complex 6 were grown by
vapor diffusion of diethyl ether into the reaction solution.
The solid-state structure of the cationic complex (Figure 11)
shows a mononuclear hexa-coordinate distorted octahedral
cobalt complex ligated by four nitrogen donors (N1–N4) of
the ligand and two oxygen atoms (O1 and O2) of the dian-
ionic phenylpyruvate (PP). The amine nitrogen N2 and oxy-
The formation of 2,4,6-tri-tert-butylphenoxyl radical in a
small amount as observed by X-band EPR spectroscopy
during the reaction of 2 with 2,4,6-tri-tert-butylphenol and
Figure 11. ORTEP plot of the cationic part of complex 6 with 40%
thermal ellipsoids. Hydrogen atoms and counterions have been re-
moved for clarity.
5848
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 5843–5853

Oxidative C–C Bond Cleavage by Co Complexes
Scheme 2. Reactivity of cobalt(II) phenylpyruvate complexes.
unit in 4 stabilizes the keto form of PPH. The C–C bond
cleavage of PP in complex 3, upon reaction with dioxygen,
proceeds via a dioxetane-type intermediate as proposed ear-
lier for PP-iron(II) complexes of the 6Me3TPA ligand.[24]
The reaction of complex 4 with dioxygen results in a co-
balt(III) complex that spontaneously deprotonates the co-
ordinated PPH to stabilize a PP-cobalt(III) complex, 6. The
reaction of 2 and 3 with dioxygen is slower with respect to
the corresponding iron analogues, however, the oxidative
decarboxylation of benzoylformate is not inhibited by the
TPA-cobalt(II) complex.
Conclusions
We have synthesized and characterized four cobalt(II)
complexes using two different α-keto acids. All the co-
balt(II) complexes, except 1, react with molecular oxygen to
exhibit a versatile reactivity pattern. The cobalt(II) benzo-
ylformate complex 2 reacts with dioxygen to carry out oxi-
dative decarboxylation of coordinated benzoylformate to
benzoate. The dicobalt(II) complex 3 undergoes C–C bond
cleavage of PP in the reaction with dioxygen to afford benz-
aldehyde and an oxalate-bridged dicobalt(II) complex 5.
One of the oxygen atoms from molecular oxygen is incorpo-
rated into oxalate and the other oxygen possibly into benz-
aldehyde. The mononuclear PPH-cobalt(II) complex 4, in
the reaction with dioxygen, forms a PP-cobalt(III) complex.
Interception studies with external reagents suggest the in-
volvement of an active cobalt-oxygen species in the decar-
boxylation of 2. The effect of supporting ligands together
with the role of the metal ion in directing the reactivity of
model cobalt(II) complexes have been demonstrated. The
reactions described in this work have relevance in many C–
C bond-cleaving dioxygenases.
Scheme 3. Reaction of 2 with dioxygen in the presence of phenolic
substrates.
subsequent decay of the radical species to 2,6-di-tert-butyl-
1,4-benzoquinone further supports the involvement of an
active cobalt-oxygen species (Figure S8).[47,48] Unlike in the
case of 2, no evidence for the formation of 4,4Ј,6,6Ј-tetra-
tert-butylbiphenol or 2,6-di-tert-butyl-1,4-benzoquinone
from the corresponding phenol was obtained in the reaction
of other cobalt(II) complexes with dioxygen. Substrates like
thioanisole, triphenylphosphane, and dimethyl sulfoxide
have no effect on the oxidative decarboxylation of benzo-
ylformate in 2, C–C bond cleavage of PP in 3, and β-hydro-
gen elimination from PPH in 4. Moreover, no oxo atom
transfer occurs to these substrates with all the cobalt(II)
complexes.
The bidentate binding of benzoylformate results in the
formation of a coordinatively saturated cobalt(II) benzo-
ylformate complex 1 which hampers the coordination of di-
oxygen and makes it unreactive. Conversely, in complex 2
oxygen can be activated at the vacant site to form a cobalt-
oxygen intermediate to initiate the oxidative de-
carboxylation of coordinated benzoylformate. The
(6Me3TPA)CoII unit in 3 facilitates the enolization of PPH
Experimental Section
Materials and Physical Measurements: Commercial-grade chemi-
cals were used for synthetic purposes and solvents were distilled
and dried before use. Although no problem was encountered during
the synthesis of the complexes, perchlorate salts are potentially ex-
to coordinate as a dianionic ligand, whereas the (TPA)CoII plosive and should be handled with care![49] Air-sensitive complexes
Eur. J. Inorg. Chem. 2012, 5843–5853
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5849

B. Chakraborty, P. Halder, P. R. Banerjee, T. K. Paine
FULL PAPER
were prepared and stored in an inert atmosphere glove box. Li-
gands were synthesized according to the protocol reported in the
literature.[50] Complex [(TPA)CoII(CH3CN)](ClO4)2 was synthe-
sized following a reported procedure.[36]
optimized by spin unrestricted formalism using the BP86 func-
tional.[54,55] Optimization was performed with all the coordinates
using the 6-31G* basis set for all the atoms except hydrogen, for
which the 6–21G* basis set was used.[56]
Fourier transform infrared spectroscopy on KBr pellets was per-
formed with a Shimadzu FT-IR 8400S instrument. Elemental
analyses were performed with a Perkin–Elmer 2400 series II CHN
analyzer. Electro-spray ionization mass spectra were recorded with
a Waters QTOF Micro YA263. 1H NMR spectra were measured at
room temperature with a Bruker DPX-500 spectrometer. Solution
electronic spectra were measured with an Agilent 8453 diode array
spectrophotometer. Room temperature magnetic data were col-
lected on a Gouy balance (Sherwood Scientific, Cambridge, UK).
Diamagnetic contributions were estimated for each compound by
using Pascal’s constants. X-band EPR measurements were per-
formed with a JEOL JES-FA 200 instrument.
General Procedure for the Syntheses of BF-Cobalt(II) Complexes:
Cobalt(II) perchlorate hexahydrate (0.37 g, 1 mmol) was added to
a solution of the ligand (1 mmol) in methanol (5 mL). To the re-
sulting solution sodium benzoylformate (0.17 g, 1 mmol) in meth-
anol (2 mL) was added. The solution was then stirred at room tem-
perature for 5 h. A solid compound was isolated from the reaction
solution upon addition of 1 mmol sodium tetraphenylborate. The
compound was then filtered, washed with methanol, and dried.
[(6Me3TPA)CoII(BF)](BPh4) (1): Yellow solid; yield 0.55 g (63%).
1·H2O: C53H51BCoN4O4 (877.74 g/mol): calcd. C 72.52, H 5.86, N
6.38; found C 72.61, H 5.68, N 6.22. IR (KBr): ν = 3053, 3036,
˜
2926 (m), 1672 (s), 1632 (s), 1607 (m), 1580 (m), 1450 (s), 1234
(m), 787 (m), 737 (s), 706 (s) cm–1. ESI-MS (in positive ion mode,
CH3CN): m/z (%) = 333.36 (30) [6Me3TPA + H]+, 540.36 (70)
[(6Me3TPA)Co(BF)]+, 582.45 (100) [(6Me3TPA)Co(BF)(CH3-
CN)]+. UV/Vis in CH3CN: λmax (ε, m–1 cm–1) 390 (sh), 452 (sh), 568
(34) nm. Magnetic moment μeff (298 K): 4.94 μB.
X-ray Crystallographic Data Collection and Refinement of the Struc-
tures: Diffraction data for 2, 3·CH2Cl2, 5, and 6·2CH3CN were
collected with a Bruker Smart APEX II (Mo-Kα radiation, λ =
0.71073 Å). Details of the data collection and structure refinements
are provided in Table 6. Cell refinement, indexing and scaling of
the data set were carried out with the APEX2 v2.1-0 software.[51]
The structures were solved by direct methods and subsequent Fou-
rier analyses and refined by the full-matrix least-squares method
based on F2 with all observed reflections.[52] In all the complexes
the hydrogen atoms were fixed.
[(TPA)CoII(BF)](BPh4) (2): Green solid. Single crystals suitable for
X-ray diffraction were obtained by vapor diffusion of diethyl ether
into a tetrahydrofuran-acetonitrile solution of the complex; yield
0.65 g (79%). C50H43BCoN4O3 (817.62 g/mol): calcd. C 73.45, H
5.30, N 6.85; found C 73.32, H 5.33, N 6.57. IR (KBr): ν = 3051
˜
CCDC-865288 (for 2), -865289 (for 6·2CH3CN), -865290 (for 5),
and -865291 (for 3·CH2Cl2) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(m), 2986 (m), 1687 (m), 1633 (s), 1609 (s), 1576 (m), 1481 (s), 1431
(s), 1391 (s), 1230 (s), 766 (m), 733 (s), 706 (S) cm–1. ESI-MS (in
positive ion mode, CH3CN): m/z (%) = 498.05 (100) [(TPA)-
Co(BF)]+). UV/Vis in CH3CN: λmax (ε, m–1 cm–1) 470 (220), 605
(160), 625 (sh) nm. Magnetic moment μeff (298 K): 4.51 μB.
DFT Calculations: All calculations were performed with the
Gaussian 03 program package.[53] The geometry of complex 4 was
[(TPA)CoII(BF)](ClO4) (2·ClO4): To a solution of cobalt(II) per-
chlorate hexahydrate (0.37 g, 1 mmol) in acetonitrile (5 mL) was
Table 6. Crystallographic data for 2, 3·CH2Cl2, 5, and 6·2CH3CN.
2
3·CH2Cl2
5
6·2CH3CN
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C50H43BCoN4O3
817.62
C100H96B2Cl2Co2N8O3
1668.23
C92H88B2Co2N8O4
1509.18
monoclinic
P2(1)/c
9.698(3)
17.644(5)
24.316(6)
90.00
112.469(9)
90.00
3844.9(19)
2
C55H50BCoN6O3
912.75
monoclinic
P2(1)/c
11.4009(8)
9.0805(7)
41.472(3)
90.00
103.840(2)
90.00
4168.8(5)
4
1.303
298(2)
0.460
1708
triclinic
monoclinic
P2(1)/c
16.3195(10)
11.6310(7)
26.1992(13)
90.00
116.087(3)
90.00
4466.3(4)
4
1.357
150(2)
0.438
1912
¯
P1
12.1803(7)
16.2584(10)
21.9791(14)
89.732(2)
78.343(2)
88.092(2)
4260.4(4)
2
1.300
120(2)
0.509
1748
α [°]
β [°]
γ [°]
V [Å3]
Z
ρcalcd. [Mg/m3]
T [K]
1.304
150(2)
0.491
1584
μMo-Kα [mm–1]
F(000)
θ range [°]
Reflections collected
Reflections unique
R(int)
1.01–26.01
40580
8151
0.0487
6228
4.20–29.86
68801
23574
0.0509
16066
1060
0.984
1.47–21.47
26168
4371
0.0489
3509
490
1.663
1.39–27.92
48856
10511
0.0396
8209
582
0.967
Data [I Ͼ 2σ(I)]
Parameters refined
532
Goodness-of-fit on F2 0.919
R1 [I Ͼ 2σ(I)]
wR2
0.0403
0.1183
–0.404, 0.281
0.0522
0.1436
–1.488, 1.151
0.0343
0.0498
–0.218, 0.191
0.0472
0.1237
–0.857, 0.473
Residuals [eÅ–3]
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Oxidative C–C Bond Cleavage by Co Complexes
added a solution (5 mL) of the ligand (0.29 g, 1 mmol) in acetoni-
trile. To the resulting solution solid sodium benzoylformate (0.17 g,
1 mmol) was added and the resulting mixture was stirred for 5 h.
The solution was concentrated to 1 mL and diethyl ether (10 mL)
was added. The mixture was then stirred for 1 h to precipitate a
green solid which was isolated by filtration, washed 2–3 times with
diethyl ether, and dried; yield 0.45 g (76%). C26H23ClCoN4O7
(597.87 g/mol): calcd. C 52.23, H 3.88, N 9.37; found C 52.54, H
[(TPA)CoIII(PP)](BPh4) (6): Complex 4 (0.021 g, 0.025 mmol) was
dissolved in acetonitrile (5 mL) and dry oxygen was bubbled
through the solution for 2 min. The resulting solution was stirred
under oxygen for 3 to 4 h during which time the green solution
slowly changed to brown. The brown solution was concentrated to
obtain a brown solid. Single crystals were isolated upon recrystalli-
zation of the solid from a solvent mixture of acetonitrile and meth-
anol; yield 0.017 g (82%). C51H44BCoN4O3 (830.66 g/mol): calcd.
3.79, N 9.57. IR (KBr): ν = 3433 (m), 2922–2853 (w), 1665 (m), C 73.74, H 5.34, N 6.74; found C 73.57, H 5.67, N 6.89. IR (KBr):
˜
1609 (s), 1483 (m), 1441 (m), 1230 (s), 1144–1088 (vs), 771 (m), 631 ν = 3443 (m), 3051 (s), 2924 (m), 2853 (m), 1643 (s), 1609 (s), 1481
˜
(s) cm–1.
(m), 1446 (m), 1375 (m), 1285 (m), 773 (m), 735 (s), 706 (s) cm–1.
ESI-MS (in positive ion mode, CH3CN): m/z (%) = 349.37 (70)
[(TPA)Co]+, 511.12 (100) [(TPA)Co(PP)]+. UV/Vis in CH3CN: λmax
(ε, m–1 cm–1) 322 (22000 m–1 cm–1), 335 (19000 m–1 cm–1) nm.
[{(6Me3TPA)CoII}2(PP)](BPh4)2 (3): To a solution of the ligand
(0.33 g, 1 mmol) in methanol (5 mL), cobalt (II) chloride hexahy-
drate (0.24 g, 1 mmol) in methanol (2 mL) was added. To the re-
sulting solution was added solid sodium phenylpyruvate (NaPPH)
(0.18 g, 1 mmol) and the solution was stirred for 5 h. Treatment of
the solution with NaBPh4 (0.34 g, 1 mmol) resulted in the isolation
of a light green solid. Single crystals suitable for X-ray diffraction
were grown from a solvent mixture of dichloromethane and meth-
anol; yield 0.76 g (95%). 3·CH2Cl2: C100H96B2Cl2Co2N8O6
(1668.23 g/mol): calcd. C 71.99, H 5.80, N 6.72; found C 71.82, H
Reactivity with Dioxygen: Pure oxygen was bubbled through a solu-
tion of the cobalt(II) benzoylformate complex (0.02 mmol) in dry
acetonitrile (10 mL) for 2 min. The solution was continuously
stirred under oxygen for 7 d. The orange solution was dried to re-
move the solvent and the residue was treated with 2-m HCl solu-
tion. The organic products were then extracted with diethyl ether,
and dried with anhydrous Na2SO4. After removing the solvent the
solid mass was analyzed by 1H NMR spectroscopy in CDCl3. A
control experiment with cobalt(II) perchlorate and sodium benzo-
ylformate in acetonitrile solution under oxygen was performed to
elucidate the role of the ligand. No appreciable decomposition of
benzoylformate was observed after 7 d.
5.83, N 6.79. IR (KBr): ν = 3053 (s), 2984 (m), 2920 (m), 1607 (s),
˜
1576 (s), 1537 (s), 1468 (s), 1445 (s), 1421 (m), 1366 (m), 1124 (w),
787 (m), 735 (s), 706 (s) cm–1. ESI-MS (in positive ion mode,
CH3CN): m/z (%) = 333.06, (100) [6Me3TPA + H]+, 471.95 (40)
[{(6Me3TPA)Co}2(PP)]2+, 553.93 (50) [(6Me3TPA)Co(PP)]+). UV/
Vis in CH3CN: λmax (ε, m–1 cm–1) 330 (20100), 590 (128), 610
(127) nm. Magnetic moment μeff (298 K): 6.35 μB.
Ester Derivative of Benzoic Acid: Complex 2 (0.02 mmol) was dis-
solved in acetonitrile (10 mL) and pure oxygen was passed through
the solution for 2 min. Acidic workup of the oxidized solution re-
sulted in a white crude mass which was treated with α-bromoace-
tophenone (0.02 mmol) and potassium fluoride (0.04 mmol) in
DMF (2 mL). The compound was then isolated according to the
reported method and was analyzed by GC–MS.[41]
[(TPA)CoII(PPH)](BPh4) (4): To a solution of the ligand TPA
(0.29 g, 1 mmol) in methanol (5 mL) was added solid cobalt(II)
perchlorate hexahydrate (0.36 g, 1 mmol). The orange-red solution
was then treated with solid sodium phenylpyruvate (0.18 g,
1 mmol) and the resulting green solution was stirred for a further
5 h at room temperature. After treating the reaction solution with
NaBPh4 (1 mmol), a green solid was isolated. The solid was iso-
lated by filtration, washed with methanol, and dried; yield 0.65 g
(78%). C51H45BCoN4O3 (831.67 g/mol): calcd. C 73.65, H 5.45, N
Isolation of Benzaldehyde from the Reaction of 3 with O2: Cobalt(II)
phenylpyruvate complex 3 was dissolved in dry dichloromethane
(10 mL) and oxygen was bubbled through the solution for 2 min.
Stirring of the solution was continued for 6 h under oxygen. A
saturated solution of H2K2EDTA in water (10 mL) was added and
the resulting mixture was stirred overnight. The organic layer was
separated and dried with anhydrous Na2SO4. Solvent was removed
and the organic product was analyzed by 1H NMR spectroscopy
in CDCl3. Benzaldehyde was quantified with respect to the peak
assigned for the 6Me3TPA ligand.
6.74; found C 73.33, H 4.97, N 6.91. IR (KBr): ν = 3429 (m), 3053
˜
(s), 2926 (m), 2854 (m), 1679 (s), 1639 (m), 1608 (s), 1576 (m), 1481
(s), 1439 (m), 1367 (m), 1119, 1103 (m), 767 (m), 735 (s), 708
(s) cm–1. ESI-MS (in positive ion mode, CH3CN): m/z (%) = 349.01
(20) [(TPA)Co]+, 511.98 (100) [(TPA)Co(PPH)]+. UV/Vis in
CH3CN: λmax (ε, m–1 cm–1) 304 (7300), 322 (6600), 452 (475) and
603 (280) nm. Magnetic moment μeff (298 K): 4.17 μB.
Interception Studies: Cobalt(II) complexes (0.02 mmol) were dis-
solved in dry acetonitrile (10 mL) and 2,4-di-tert-butylphenol or
2,4,6-tri-tert-butylphenol (10 equiv.) was added. Oxygen was
bubbled through the solution and stirring was continued at room
temperature under oxygen for 5 d. The orange solution was dried
to remove the solvent and the residue was treated with 2 m HCl
solution. The organic products were then extracted with diethyl
ether, and dried with anhydrous Na2SO4. The solution was passed
through a silica column using diethyl ether as eluent. After remov-
ing the solvent the solid mass was analyzed by 1H NMR spec-
[{(6Me3TPA)CoII}2(oxalate)](BPh4)2 (5): Dry oxygen was bubbled
through an acetonitrile solution (10 mL) of complex 3 (0.041 g,
0.026 mmol) for 2 min and the solution was kept under an oxygen
atmosphere for 7 h. The solution was concentrated to yield a pink
solid. X-ray-quality single crystals were grown from a solvent mix-
ture of dichloromethane and methanol; yield 0.023 g (59%).
C92H88B2Co2N8O4 (1509.18): calcd. C 73.22, H 5.88, N 7.42; found
C 72.31, H 5.76, N 7.33. IR (KBr): ν = 3051 (m), 2926 (m), 1655
˜
(s), 1605 (s), 1575 (m), 1446 (m), 787 (m), 737 (s), 706 (s) cm–1. troscopy in CDCl3.
ESI-MS (in positive ion mode, CH3CN): m/z (%) = 435.15 (100)
1H NMR spectroscopic data of 4,4Ј,6,6Ј-tetra-tert-butyl-2,2Ј-bi-
phenol (500 MHz, 298 K, CDCl3): δ = 1.34 (s, 18 H), 1.47 (s, 18
H), 7.13 (d, J = 2.5 Hz, 2 H), 7.37 (d, J = 2.5 Hz, 2 H) ppm.
[{(6Me3TPA)Co}2(oxalate)]2+
,
480.15 (15) [{(6Me3TPA)Co-
(oxalate)} + H]+. UV/Vis in CH3CN: λmax (ε, m–1 cm–1) 495 (65),
520 (60), 555 (50) nm. Magnetic moment μeff (298 K): 6.46 μB.
1H NMR spectroscopic data of 2,6-di-tert-butyl-1,4-benzoquinone
(500 MHz, 298 K, CDCl3): δ = 1.27–1.28 (br., 18 H), 6.50 (s, 2
H) ppm. Control experiments with cobalt(II) perchlorate and 2,4-
di-tert-butylphenol or 2,4,6-tri-tert-butylphenol in acetonitrile solu-
Complex 5 was also prepared independently by reacting the ligand
(1 mmol) with cobalt(II) chloride (1 mmol), disodium oxalate
(0.5 mmol), and sodium tetraphenylborate in methanol.
Eur. J. Inorg. Chem. 2012, 5843–5853
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5851

B. Chakraborty, P. Halder, P. R. Banerjee, T. K. Paine
FULL PAPER
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4,4Ј,6,6Ј-tetra-tert-butyl-2,2Ј-biphenol or 2,6-di-tert-butyl-1,4-
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Supporting Information (see footnote on the first page of this arti-
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data for 2, 3·CH2Cl2, 5, and 6·2CH3CN in CIF file format.
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Acknowledgments
We gratefully acknowledge the Department of Science and Tech-
nology (DST), Government of India for financial support (project
number SR/S1/IC-51/2010). Single-crystal diffraction data for the
complexes were collected at the DST-funded National Single Crys-
tal Diffractometer Facility at the Department of Inorganic Chemis-
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and P. H. acknowledge the Council of Scientific and Industrial Re-
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Received: June 18, 2012
Published Online: October 8, 2012
Eur. J. Inorg. Chem. 2012, 5843–5853
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5853