Three novel Bi(iii) complexes with in situ generated anilate ligands: Unusual oxidation of cyclohexanedione to dihydroxy benzoquinone

Article abstract of DOI:10.1039/c1dt11258a

Three novel bismuth(iii) complexes with in situ synthesized dihydroxybenzoquinone (anilic acid) ligands, namely [Bi(H₃dhbqdc) (H₂dhbqdc)]·2.5dmf (1), [Bi₂(mdhbqdc)(ox) ₂(dmf)₄] (2) and [Bi₂(mdhbqdc) ₂(ox)(dmf)₄] (3) (H₄dhbqdc = 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid; H₂mdhbqdc = dimethyl 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylate, ox = oxalate, dmf = dimethyl formamide) were prepared under ambient conditions. 1 features 3-D diamond-like structure with 2-D channels filled by dmf molecules. 2 and 3 are geometrically different but topologically identical. 2 shows a brick-wall layer in which ratio of mdhbqdc to ox is 1:2 while 3 has a herringbone layer in which the ratio of mdhbqdc to ox is 2:1. Bismuth-assisted aerobic in situ oxidation of dimethyl 1,4-cyclohexanedione-2,5-dicarboxylate (H₂dmchddc) gave H₂mdhbqdc, which underwent either ester hydrolysis to give H ₄dhbqdc or carbon-carbon cleavage to form oxalate. Comparative experiments revealed that both Bi(iii) and nitrate are important in the unusual oxidation. Possible reaction pathways for the formation of the dihydroxy benzoquinone (dhbq) ligands from their cyclohexanedione precursor were proposed.

Full text of DOI:10.1039/c1dt11258a

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PAPER
Three novel Bi(III) complexes with in situ generated anilate ligands: unusual
oxidation of cyclohexanedione to dihydroxy benzoquinone†
Hui Gao and Xian-Ming Zhang*
Received 1st July 2011, Accepted 21st October 2011
DOI: 10.1039/c1dt11258a
Three novel bismuth(III) complexes with in situ synthesized dihydroxybenzoquinone (anilic acid)
ligands, namely [Bi(H3dhbqdc)(H2dhbqdc)]·2.5dmf (1), [Bi2(mdhbqdc)(ox)2(dmf)4] (2) and
[Bi2(mdhbqdc)2(ox)(dmf)4] (3) (H4dhbqdc = 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid;
H2mdhbqdc = dimethyl 3,6-dihydroxy-2,5-benzoquinone-1,4-dicarboxylate, ox = oxalate, dmf =
dimethyl formamide) were prepared under ambient conditions. 1 features 3-D diamond-like structure
with 2-D channels filled by dmf molecules. 2 and 3 are geometrically different but topologically
identical. 2 shows a brick-wall layer in which ratio of mdhbqdc to ox is 1 : 2 while 3 has a herringbone
layer in which the ratio of mdhbqdc to ox is 2 : 1. Bismuth-assisted aerobic in situ oxidation of dimethyl
1,4-cyclohexanedione-2,5-dicarboxylate (H2dmchddc) gave H2mdhbqdc, which underwent either ester
hydrolysis to give H4dhbqdc or carbon–carbon cleavage to form oxalate. Comparative experiments
revealed that both Bi(III) and nitrate are important in the unusual oxidation. Possible reaction pathways
for the formation of the dihydroxy benzoquinone (dhbq) ligands from their cyclohexanedione precursor
were proposed.
On the other hand, the quest for efficient catalysts for the
selective insertion of oxygen atoms remains a difficult challenge
Introduction
in organic chemistry.14 Bismuth, the 83rd element, is the heaviest
stable element on the periodic table and generally considered to
be non-toxic and noncarcinogenic.15 Bismuth and its compounds
have been used in the pharmaceutical field for over four hundred
years. With increasing environmental concerns and the need for
‘green reagents’, the weakly acidic Bi(III) has been widely used in
organic syntheses such as oxidation, reduction and carbon–carbon
bond forming reactions.16 We became interested in exploring
the role of Bi(III) reagents such as BiNO3 in the aerobic oxida-
tion of simple organic substrates under solvothermal conditions
to generate novel ligands. Dimethyl 1,4-cyclohexanedione-2,5-
dicarboxylate (dmchddc) was chosen for this purpose based the
consideration that it possesses an array of enolizable C–H bonds
for oxidative functionalization.
Anilicacids (or 2,5-dihydroxy-p-benzoquinones) are strong
dibasic acids that have been used as ligands to coordinate various
transition metals, and as counteranions in conducting or paramag-
netic charge-transfer complexes, since they can undergo reversible
redox processes.17,18 The recent discovery of rare organic ferroelec-
tricity in hydrogen-bonding systems of organic dibasic bases and
anilic acids further highlighted the interest in these compounds.19
However, all of these investigations are focused on anilic acid
itself and a few of its derivatives such as halo-, cyano- and alkyl
anilic acids. However, it should be noted that dianion of 3,6-
dihydroxy-2,5-benzoquinone-1,4-dicarboxylic acid (H4dhbqdc),
a derivative of cyclohexanehexaone 1,4-dimethide, has never
been synthesized to date. A mesomeric form of H4dhbqdc,
Solvothermal in situ ligand synthesis has rapidly developed in
the crystal engineering of coordination complexes and in organic
synthesis over the past ten years due to its effectiveness, simplicity
and environmentally friendliness.1,2 In the crystal engineering
of coordination complexes, in situ ligand synthesis is suitable
to prepare novel coordination complexes from metal ions and
organic precursors and most of these complexes are not accessible
from a direct reaction of metal ions and ligands. In organic
synthesis, solvothermal in situ ligand syntheses have become very
useful pathways to organic ligands that are difficult to obtain by
routine synthetic methods. It has been previously reported that
in situ rearrangement,3 hydrolysis of carboxylate esters, organic
nitriles, and aldehydes,2 cleavage of carbon–carbon, carbon–
nitrogen, carbon–oxygen and carbon–sulphur bonds,4 cleavage
and formation of disulfide bonds,5 substitution of aromatic
groups,6 decarboxylation,7 carbon–carbon coupling,8 hydroxyla-
tion of aromatic rings,9 both C–C coupling and hydroxylation,10
cycloaddition of organic nitriles with azide to form tetrazoles,11
[2 + 2 + 1] addition of organic nitriles and ammonia to form
triazoles,12 and transformation of inorganic and organic sulphur,13
can occur under solvothermal conditions.
School of Chemistry & Material Science, Shanxi Normal University, Linfen,
041004, P. R. China. E-mail: zhangxm@dns.sxnu.edu.cn
† Electronic supplementary information (ESI) available. CCDC reference
numbers 805102–805104. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11258a
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3,6-bis(dihydroxymethylene)cyclohexane-1,2,4,5-tetraone,
only been alluded to in a theoretical study of the diradical
dianion.20
In the article, we present our observation of in situ for-
mation of H4dhbqdc and its ester dimethyl 3,6-dihydroxy-2,5-
benzoquinone-1,4-dicarboxylate (H2mdhbqdc) via oxidization of
dmchddc, and the isolation of three novel Bi(III) complexes,
namely [Bi(H2.5dhbqdc)2]·2.5dmf (1), [Bi2(mdhbqdc)(ox)2(dmf)4]
(2) and [Bi2(mdhbqdc)2(ox)(dmf)4] (3) (ox = oxalate).
has
cooling to room temperature, dark red block single crystals were
obtained in the yield of 18% (based on dmchhddc). Anal. Calcd
for C34H38Bi2N4O24: C, 31.30; H, 2.94; N, 4.29. Found: C, 31.01;
H, 3.12; N, 4.44. IR (KBr, cm-1): 3434 s, 2926 w, 2373 w, 1629 s,
1505 s, 1375 s, 1201 m, 1040 m, 977 w, 778 w, 549 w.
Crystal structure determination
Data collections were performed using a Bruker-AXS SMART
CCD area detector diffractometer with Mo-Ka radiation with a
˚
w-scan mode (l = 0.71073 A). Structure solutions and refinements
Experimental
were performed with the SHELXL-97 package (G. M. Sheldrick,
SHELXS-97 and SHELXL-97, Go¨ttingen University, Go¨ttingen,
Germany, 1997). The structures were solved by direct methods and
refined by full-matrix least squares refinements based on F2. Multi-
scan corrections were applied using SADABS. All non-hydrogen
atoms were anisotropically refined. In 1, the hydrogen atoms of
carboxylic groups are not included in the refinement. The dmf
molecules filled in channels can not be located in Fourier map
and thus are not included in the refinement. The contribution
of the solvent to the diffraction pattern was subtracted from
the observed data by the SQUEEZE method implemented in
PLATON. In 2 and 3, the hydrogen atoms of organic ligands
are geometrically placed and refined isotropically. Crystal data
and structure refinement parameters are shown in Table 1, and
selective bond lengths are shown in Table 2.
General
All of the starting materials were purchased commercially at
reagent grade and used without further purification. Elemental
analyses were performed on a vario MACRO cube elemental
analyzer. The Fourier transform infrared spectroscopy (FT-IR)
spectra were recorded from KBr pellets in the range 400–4000 cm-1
on a Nicolet 5DX spectrometer. XRPD data were recorded in a
Bruker D8 ADVANCE diffractometer. The phase purity of 1–3
is confirmed by its powder X-ray diffraction patterns, which are
in good agreement with the XRD patterns simulated from the
single crystal structures (Fig. S1–3†). Thermogravimetric analyses
(TGA) were carried out in an air stream using SETARAM
LABSYS equipment with a heating rate of 10 ◦C min-1.
Synthesis of [Bi(H3dhbqdc)(H2dhbqdc)]·2.5dmf (1)
Results and discussion
A mixture of Bi(NO3)3·5H2O (195 mg, 0.402 mmol), dmchddc
(69 mg, 0.302 mmol) and HNO3 (600 mg, 1mol L-1) in a mole
ratio of 4 : 3 : 4.2 were dissolved in a dmf (3 mL)/CH3CN (3 mL)
mixture solution and sealed in a 12 mL Teflon-lined stainless
container, which was heated to 80 ◦C for 72 h. After cooling
to room temperature, deep red elongated octahedron-like single
crystals were obtained in the yield of 81% (based on dmchhddc).
Anal. Calcd for C23.5H22.5N2.5BiO18.5: C, 33.41; H, 2.68; N, 4.14.
Found: C, 34.51; H, 2.86; N 3.53. Combined EA and TGA (Fig.
S4†) indicate that there are about 2.5 free dmf molecules per
formula of 1. IR (KBr, cm-1): 3426 m, 3128 s, 1705 s, 1487 s,
1372 s, 1203 s, 1303 m, 973 m, 884 w, 829 w, 768 m, 534 m.
Structures
Compound 1 crystallizes in tetragonal space group I41/a, and the
asymmetric unit consists of one Bi(III) and half dhbqdc ligand
(Fig. 1a). The Bi atom lies on a fourfold inversion axis with
site occupancy of 0.25, which shows coordination symmetry of
triangular dodecahedron, chelated by eight hydroxyl oxygen atoms
from four dhbqdc groups. Charge balance requires that there is
one singly deprotonated H3dhbqdc and one doubly deprotonated
H2dhbqdc per formula. Only one crystallographically independent
dhbqdc indicates that one proton must be disordered over two
ligand molecules. The Bi(1)–O(1) and Bi(1)–O(2) bond lengths are
Synthesis of [Bi2(mdhbqdc)(ox)2(dmf)4] (2)
˚
2.457(6) and 2.463◦(7) A, and O–Bi(1)–O angles are in the range
˚
of 65.0(2) 148.2(2) . The C(1)–C(2) distance of 1.538(12) A in 1
A mixture of Bi(NO3)3·5H2O (195 mg, 0.402 mmol), dmchddc
(69 mg, 0.302 mmol) in a mole ratio of 4 : 3 were dissolved in dmf
(3 mL)/CH3CN (3 mL) mixture solutions and was sealed in a
is comparable with C–C lengths, largely indicating single bond
behaviour.21 The C(1)–C(3d) and C(2)–C(3) distances are close
˚
to 1.40 A and are comparable with C–C bond lengths of benzene,
◦
12 mL Teflon-lined stainless container, which is heated to 80 C
pointing to approximate 1.5 bond order. The C(1)–O(1) and C(2)–
O(2) distances (1.23–1.25 A) lie between single and double C–O
for 72 h. After cooling to room temperature, orange block single
crystals were obtained in the yield of 83% (based on dmchhddc).
Anal. Calcd for C26H34Bi2N4O20: C, 27.38; H, 3.00; N, 4.91. Found:
C, 27.09; H, 2.85; N, 4.75. IR (KBr, cm-1): 3429 s, 2355 w, 1635 s,
1362 m, 1095 s, 983 m, 866 w, 779 w, 618 w, 543 m.
˚
bond. These bond parameters are close to those in other reported
anilate complexes. The coordination polymer formed in 1 features
a Bi(III) center that coordinates with a total of eight oxygen
atoms from the four anilate ligands, each of which connects with
two Bi(III) atoms in bis-bidentate mode, generating diamond-like
network (Fig. 1b). To our knowledge, only one example of anilate
complex with diamond network is known to date.18 There are 2-D
channels along the a- and b-axis directions, which are occupied
Synthesis of [Bi2(mdhbqdc)2(ox)(dmf)4] (3)
A mixture of Bi(NO3)3·5H2O (195 mg, 0.402 mmol), dmchddc
(69 mg, 0.302 mmol) in a mole ratio of 4 : 3 were dissolved in
dmf (3 mL)/CH3CN (3 mL) mixture solution and was sealed in
a 12 mL Teflon-lined stainless container under the high-purity
nitrogen atmosphere, which is heated to 80 ◦C for 72 h. After
by dmf molecules. A calculation using the PLATON reveals that
the total potential solvent area volume is 834.8 A per unit cell, or
30.5% of the total volume.22
3
˚
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Table 1 Crystal data and structure refinement parameters for 1–3
Compound
1
2
3
Formula
Fw
Crystal system
C16H9BiO16
666.14
Tetragonal
C26H34Bi2N4O20
1142.53
C34H38Bi2N4O24
1304.64
Monoclinic
Triclinic
¯
Space group
I41/a
P1
P21/n
˚
a/A
9.8323(3)
9.8323(3)
28.2700(13)
90
9.6935(6)
10.0302(6)
11.5844(7)
65.9780(10)
66.7680(10)
63.3430(10)
887.93(9)
1
10.1663(5)
20.7198(10)
11.3749(5)
90
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
90
90
112.5330(10)
90
2213.13(18)
2
1.958
8.031
3
˚
V/A
2732.98(17)
4
1.597
6.514
1228
Z
r
calc,(g cm-3)
2.133
9.984
544
m, (mm-1)
F(000)
1256
Size/mm
Reflections
0.22 ¥ 0.10 ¥ 0.06
6783/1488
0.9980/0.7283
1488/24/94
1.186
0.0535/0.0610
0.1628/0.1702
2.407/-0.915
0.20 ¥ 0.12 ¥ 0.08
4771/3748
0.9680/0.6980
3748/0/235
1.083
0.0288/0.0332
0.0645/0.0668
1.228/-0.646
0.28 ¥ 0.28 ¥ 0.08
8865/4661
0.5659/0.2120
4661/6/289
1.021
0.0329, 0.0718
0.0484, 0.0796
1.938/-0.659
T
max/Tmin
Data/restraints/parameters
S
R1
a
b
wR2
Largest Peak & hole (e A-3)
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ꢀFo| - |Fcꢀ/ |Fo|, b wR2 = [ w(Fo - Fc2)2/ w(Fo2)2]1/2
.
Table 2 Selective bond lengths for 1–3
Compound 1
Bi(1)–O(1)
Bi(1)–O(1a)
Bi(1)–O(1b)
Bi(1)–O(1c)
2.457(6)
2.457(6)
2.457(6)
2.457(6)
Bi(1)–O(2b)
Bi(1)–O(2)
Bi(1)–O(2c)
Bi(1)–O(2a)
2.463(7)
2.463(7)
2.463(7)
2.463(7)
Symmetry codes : a) y + 1/4, -x + 3/4, -z - 1/4; b) -y + 3/4,x - 1/4, -z
-1/4; c)-x + 1, -y + 1/2, z; d) -x + 1, - y + 1, -z.
Compound 2
Bi(1)–O(8a)
Bi(1)–O(6)
Bi(1)–O(1)
Bi(1)–O(7)
2.361(4)
2.371(3)
2.378(4)
2.449(4)
Bi(1)–O(9)
Bi(1)–O(5b)
Bi(1)–O(2c)
Bi(1)–O(10)
2.493(4)
2.494(4)
2.494(4)
2.558(4)
Symmetry codes: a) -x + 1, -y + 1, -z; b) -x, -y + 1, -z + 1; c)-x, -y + 1, -z.
Compound 3
Bi(1)–O(11a)
Bi(1)–O(3b)
Bi(1)–O(4b)
Bi(1)–O(12)
2.341(4)
2.367(4)
2.379(4)
2.419(4)
Bi(1)–O(2)
Bi(1)–O(1)
Bi(1)–O(9)
Bi(1)–O(10)
2.475(4)
2.507(4)
2.556(5)
2.576(4)
Symmetry codes: a) -x + 1, -y + 2, -z; b) x - 1/2, -y + 3/2, z - 1/2.
¯
Complex 2 crystallizes in triclinic space group P1, and the
asymmetric unit consists of one Bi(III), half mdhbqdc, two
half oxalates and two dmf molecules. The Bi(III) center adopts
triangular dodecahedral geometry, coordinated by eight oxygen
atoms from one mdhbqdc, two oxalates and two dmf molecules.
Fig. 1 View of (a) the coordination environments of Bi(III) and (b) 3-D
structure along a-axis direction in 1. For clarity, carboxylate groups of
dhbqdc are omitted in (b).
˚
The Bi–O bond lengths are in the range of 2.361(4)–2.558(4) A,
in which ratio of mdhbqdc to ox is 1 : 2 (Fig. 2). Adjacent layers
are packed in an overlapped mode with repeat sequence of AA
fashion into 3-D array. Different from 1, Bi center in 2 is also
coordinated by two dmf molecules as well as an oxalate, and the
and O–Bi(1)–O angles are in the range of 65.77(12)–145.95(13)◦.
Each Bi(III) in 2 is connected to three neighbours via bis-bidentate
ox and mdhbqdc to give 2-D brick-wall layer with (6,3) topology
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neighbours via bis-chelate ox and mdhbqdc to give 2-D (6,3)-
topological herring-bone layer (Fig. 3).23 Different from 2, the
molar ratio of mdhbqdc to ox in 3 is 2 : 1 rather than 1 : 2.
Fig. 3 View of the coordination environments of Bi(III) ions and 2-D
herring-bone layer in 3. For clarity, dmf molecules and methyl carboxylate
groups of mdhbqdc are omitted.
Fig. 2 View of the coordination environments of Bi(III) ions and 2-D
brick-wall layer in 2. For clarity, dmf molecules and methyl carboxylate
groups of mdhbqdc are omitted.
Honeycomb, brick-wall and herringbone represent three 2-D
(6,3) nets, which are geometrically different but topologically
identical.24 For anilate complexes, Cambridge Structural Database
study shows that only honeycomb net has been reported. Besides,
it should be noted that geometrically different (6,3) nets of brick-
wall and herringbone in 2 and 3 are also reflected by molar ratio
of mdhbqdc to ox.
latter is apparently formed from oxidative cleavage of part ester
anilate ligand. It should be noted that two methyl ester groups in
the anilate ligand in 2 remains intact, whereas these groups in 1 are
hydrolyzed into the free carboxylic acids in the presence of nitric
acid.
Compound 3 crystallizes in monoclinic space group P21/n,
and the asymmetric unit consists of one Bi(III), one mdhbqdc,
half oxalate and two dmf molecules. The Bi(III) in 3 also adopts
triangular dodecahedral geometry but is coordinated by eight
oxygen atoms from two mdhbqdc, one oxalate and two dmf
molecules. The Bi–O bond lengths are in the range of 2.341(4)–
Syntheses, thermal analyses and possible mechanism of
hydroxylation
Synthetic conditions for 1–3 are shown in Scheme 1. Complex 1
was obtained by ambient solvothermal treatment of dmchddc with
Bi(NO3)3 in the presence of HNO3 in the mixed dmf/acetonitrile
solution at 80 ◦C for 3 days. A similar reaction in the absence
˚
2.576(4) A, and O–Bi–O angles are in the range of 64.76(12)–
154.23(13)◦. Similar to 2, oxidative cleavage of the part ester anilate
ligands resulted in oxalate. Each Bi(III) in 3 is connected to three
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the enolizable C–H bonds of 1 to give teraol 1¢, which undergoes
acid-catalyzed dehydration to form anilate H2mdhbqdc (Scheme
2). Hydrolysis of the methyl esters in the presence of external
HNO3 gives anilic acid H4dhbqdc. It should be noted that the
hydrolysis of the ester groups may occur at any stage during
the reaction. Further investigation revealed that the reaction is
specific to Bi(NO3)3 as attempts to reproduce the same in situ
reactions with BiCl3 in place of Bi(NO3)3 were fruitless. Formation
of Bi(III) complex 3, albeit in low yield, under anaerobic conditions
suggests that the starting Bi(III) reagent alone is strong enough
an oxidant to fulfil the oxidation sequence and a Bi(IV) species
is not necessarily involved. As mentioned by Hanna,25 the real
oxidant possibly concerns Bi(V) or Bi(II) rather than Bi(IV) or
Bi(I)). The formation of the oxalate ligand can be rationalized
by the further in situ oxidation of H2mdhbqdc to give a diol
intermediate and its subsequent oxidative cleavage mediated by
Bi(III).
Scheme 2 Proposed mechanism for the formation of H2mdhbqdc,
H4dhbqdc and oxalic acid.
Scheme 1 Schematic view of syntheses of bismuth complexes of in situ
generated derivatives of anilic acids H4dhbqdc and H2mdhbqdc under
aerobic (methods a and b) and anaerobic conditions (method c).
Conclusions
By using an environmentally friendly bismuth reagent and
under ambient conditions, in situ oxidation of dimethyl 1,4-
cyclohexanedione-2,5-dicarboxylate to give 3,6-dihydroxy-2,5-
benzoquinone-1,4-dicarboxylate or dimethyl 3,6-dihydroxy-2,5-
benzoquinone-1,4-dicarboxylate was first observed. Three novel
bismuth(III) complexes of anilate ligands featuring diamond,
brick-wall and herringbone nets have been synthesized and
characterized. Oxalates were also in situ generated in 2 and
3, which came from carbon–carbon cleavage of dimethyl 1,4-
cyclohexanedione-2,5-dicarboxylate. Geometrically different but
topologically identical 2 and 3, molar ratio of anilate to ox
is exactly reversed. Experimental results revealed that both
Bi(III) and nitrate are important in the unusual oxidation.
This may provide an efficient and environmentally friendly
route for the selective and controllable oxidization of ketone.
We are currently isolating 3,6-dihydroxy-2,5-benzoquinone-1,4-
dicarboxylate from its Bi(II) complexes, and preparation of
molecular magnets with this anilate ligand in radical form is under
way.
of HNO3 generated Bi(III) complex 2, whereas degassing of
the reaction vessel and refilling with nitrogen under otherwise
identical conditions resulted in the formation of the closely related
complex 3.
Thermal analysis for 1 in air atmosphere and under 1 atm.
◦
pressure at the heating rate of 10 C min-1 was performed on
a polycrystalline sample, which shows continuous weight loss of
71.6% (calc.72.4%) from 75 ◦C to 460 ◦C, corresponds to the
removal of free dmf molecules and anilate ligands. A total weight
loss of 58.8% (calc. 59.3%) is occurred to 2 from 165 ◦C to 540 ◦C,
in agreement with the removal of coordinated dmf molecules and
◦
anilate ligands. Compound 3 is stable up to 130 C, and weight
loss of 65.1% (calc. 65.3) from 130 ◦C to 480 ◦C is consistent with
the removal of coordinated dmf molecules and anilate ligands. The
higher thermal stability of 2 and 3 is due to the absence of free
dmf molecule in the structures. The final residues in all the three
compounds are poorly crystalline Bi2O3.
The in situ aerobic oxidation of dmchddc to form H4dhbqdc
accompanied by ester hydrolysis and H2mdhbqdc with concurrent
formation of oxalate by oxidative C–C bond cleavage in the work
is rather interesting. Although the use of bismuth reagents for
oxidation of organic compounds are well documented, mecha-
nistic interpretations for those oxidation processes rarely appear
in the literature.25 We propose that the formation of H2mdhbqdc
and H4dhbqdc proceeds via a stepwise global hydroxylation of
Acknowledgements
This work was financially supported by National Basic Research
Program of China (2012CB821701) and National Science Fund
for Distinguished Young Scholars (20925101). We thank professor
Q.-W. Yao for constructive discussion.
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14 E. I. Solomon, R. Sarangi, J. S. Woertink, A. J. Augustine, J. Yoon and
S. Ghosh, Acc. Chem. Res., 2007, 40, 581; R. Curci, L. D’Accolti and
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15 H. Suzuki, Organobismuth chemistry, Elsevier Science, Amsterdam;
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bismuth, 1st ed., Blackie Academic & Professional, London; New York,
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