A heptadentate ligand possessing two phenol groups: Its diverse coordination chemistry and the catalytic behaviors of its transition complexes towards benzene oxidation

Article abstract of DOI:10.1016/j.poly.2014.07.038

Reacting the heptadentate ligand 2,2′-(((pyridine-2,6- diylbis(methylene))bis((pyridin-2-ylmethyl)-azanediyl))bis(methylene))diphenol (H₂L) with one equivalent of CuCl₂·6H₂O, NiCl₂·6H₂O and FeCl₃

Full text of DOI:10.1016/j.poly.2014.07.038

Polyhedron 81 (2014) 743–748
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A heptadentate ligand possessing two phenol groups: Its diverse
coordination chemistry and the catalytic behaviors of its transition
complexes towards benzene oxidation
a
a,b,
Xiuli You a,c, Zhenghong Wei a, , Beibei Xu , Xiaoming Liu
⇑
⇑
a Department of Chemistry, Nanchang University, Nanchang 330031, China
b College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
c Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China
a r t i c l e i n f o
a b s t r a c t
Reacting the heptadentate ligand 2,20-(((pyridine-2,6-diylbis(methylene))bis((pyridin-2-ylmethyl)-aza-
nediyl))bis(methylene))diphenol (H2L) with one equivalent of CuCl2ꢀ6H2O, NiCl2ꢀ6H2O and FeCl3ꢀ6H2O,
followed by anion exchange with PF6ꢁ, BPh4ꢁ and ClOꢁ4 formed three mononuclear complexes
[Cu(H2L)](Cl)(PF6) (1), [Ni(HL)](BPh4) (2) and [Fe(L)](ClO4) (3), respectively. Their structures in the solid
state were determined by X-ray crystallography. The three metal centers in complexes 1–3 adopt
distorted tetragonal pyramidal (Cu2+), octahedral (Ni2+) and pentagonal bipyramidal (Fe3+) geometries,
respectively. In addition, the electrochemistry and their catalytic activity in the oxidation reaction of
benzene to phenol by H2O2 were investigated.
Article history:
Received 4 June 2014
Accepted 14 July 2014
Available online 1 August 2014
Keywords:
Transition metal complex
Multidentate ligand
Phenolate derivatives
Electrochemistry
Ó 2014 Elsevier Ltd. All rights reserved.
Benzene hydroxylation
1. Introduction
enzyme [23]. Thus, exploring the functional features of a complex
focuses largely on ligand design and synthesis, as well as the selec-
In recent decades, multidentate ligands and their transition
metal complexes have been one of the intensely investigated areas
owning to their diverse potential applications in many aspects,
such as in catalysis [1,2], solar cells [3,4], molecular devices
[5–7], mimics of metalloenzymes [8–10], and so on [11,12]. The
properties or functionalities of the obtained complexes depend
very much on the coordinated geometries and donor atoms, as well
as the metal centres. In catalytic reactions catalyzed by transition
metal complexes, an unsaturated coordinating site or an easy
leaving group on the metal centre is essential [13–21].
In nature, the required structural features are achieved via the
tertiary structure imposed by protein folding. Through such a
structural design, natural catalytic systems can make some reac-
tions proceed under mild conditions, otherwise harsh conditions
are necessary. For example, methane mono-oxygenases (MMO)
which are categorized into two types, soluble MMO (sMMO) and
particulate MMO (pMMO) can selectively oxidize the very stable
C–H bond in methane to methanol at ambient temperature and
pressure [22]. The former is an iron enzyme and the latter a copper
tion of suitable metals. In this sense, the chemistry of multidentate
organic ligands and their transition metals have always been the
focus for manipulation.
Herein, we report a heptadentate ligand, 2,20-(((pyridine-2,6-
diylbis(methylene))bis((pyridin-2-ylmethyl)azanediyl))bis(methy-
lene))diphenol (H2L), and its transition metal complexes. The
ligand possesses two phenol and three pyridinyl groups. The coor-
dinating capability of the phenolic oxygen atoms can be tuned by
their deprotonation [24–30]. Reaction of the ligand with Cu2+
,
Ni2+ and Fe3+ salts produced three mononuclear complexes,
[Cu(H2L)](Cl)(PF6) (1), [Ni(HL)](BPh4) (2) and [Fe(L)](ClO4) (3),
respectively, in which the two phenol groups vary from fully coor-
dinated to fully uncoordinated. The electrochemistry and catalytic
efficiency on the direct oxidation of benzene to phenol by H2O2
were also investigated.
2. Experimental
2.1. Instrument and materials
⇑
Corresponding authors. Address: Department of chemistry, Nanchang
The reactions were performed under an argon atmosphere
using standard Schlenk techniques when necessary. Solvents were
freshly distilled by using appropriate drying agents prior to use.
University, Nanchang 330031, China. Tel./fax: +86 (0)573 83643937.
E-mail addresses: weizh@ncu.edu.cn (Z. Wei), xiaoming.liu@mail.zjxu.edu.cn
(X. Liu).
http://dx.doi.org/10.1016/j.poly.2014.07.038
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

744
X. You et al. / Polyhedron 81 (2014) 743–748
1H and 13C NMR spectra were recorded on an AVANCE DRX 400
(Bruker) spectrometer in CDCl3. UV–vis spectra were collected on
a Shimadzu UV-2550 spectrophotometer. Infrared spectroscopic
data were recorded on an FT-IR spectrometer (Scimitar 2000,
Varian). Micro-analysis was performed on a Heraeus CHNꢁOꢁRapid
instrument. The catalytic reactions were analyzed by GC (gas chro-
matography) using an Agilent instruments model 6890N network
GC system equipped with a DBꢁWAX capillary column (column
length: 30 m; internal diameter: 0.32 mm). Electrochemical
measurements were performed on an Echo Chemie Autolab
PGESTAT 30 in dry acetonitrile using [NBu4][BF4] as a supporting
electrolyte; platinum and Ag/AgCl were used as working and
reference electrodes [31], respectively. The compounds 2,6-bis
(bromomethyl)pyridine [32], and 4-methyl-2-(((pyridin-2-ylmethyl)
amino)methyl)phenol [33] were synthesized using literature
procedures.
1270 (s), 732 (s), 704 (s), 610 (m). UV–Vis (DMF, kmax/nm
/mol Lꢁ1 cmꢁ1)): 227 (27300), 267 (22640), 318 (8600).
(e
2.2.4. Complex 3
To a solution of the ligand H2L (559 mg, 1 mmol) in methanol
(10 mL) was added Et3N (0.5 mL). After being stirred for five min-
utes, FeCl3ꢀ6H2O (270 mg, 1 mmol) was added into the solution to
give a deep purple solution. The reaction was further stirred for
five minutes and LiClO4 (106 mg, 1 mmol) was added to accrue a
purple precipitate, which was collected via filtration, washed with
methanol and dried in vacuum. Recrystallisation of the product in
dichloromethane gave purple crystals of 3 after one week (413 mg,
58%). Anal. Calc. for C35H35N5O6ClFe (Fw = 713.0): C, 58.91; H, 4.91;
N, 9.82. Found: C, 58.58; H, 4.93; N, 9.64%. IR (cmꢁ1): 3403 (s),
3011 (w), 2921 (2), 2861 (w), 1605 (s), 1486 (s), 1442 (m), 1270
(s), 1154 (s), 1083 (s), 1015 (s), 795 (m), 639 (m), 545 (m).
UV–vis (DMF, kmax/nm
(e
/mol Lꢁ1 cmꢁ1)): 220 (24360), 260
2.2. Synthesis
(21460), 356 (1495), 585 (890).
2.2.1. The ligand H2L
2.3. X-ray crystallography [34]
2,6-Bis(bromomethyl)pyridine (265 mg, 1.0 mmol), 4-methyl-2-
(((pyridin-2-ylmethyl)amino)-methyl)phenol (684 mg, 3.0 mmol)
and K2CO3 (690 mg, 5.0 mmol) were mixed in acetonitrile (80 mL)
in a flask (250 mL). After being refluxed for 36 h, the reaction was fil-
tered, and the filtrate was concentrated using a rotary evaporation to
give the crude product. Further purification was performed by flash
chromatography on a silica gel column with ethyl acetate/petro-
leum ether (10:1) as the eluent to give a white solid (340 mg, 61%).
Element Anal. Calc. for C35H37N5O2 (FW = 559.7): C, 75.11; H, 6.66;
N, 12.51. Found: C, 75.58; H, 6.93; N, 12.64%. 1H NMR (d, ppm,
400 M, CDCl3): 10.71 (s, 2H, OH), 8.57 (d, J = 4.8 Hz, 2H, Py), 7.62
(m, 2H, Py), 7.56 (m, 1H, Py), 7.32 (d, 2H, Py), 7.26 (m, 2H, Py), 7.16
(m, 2H, Py), 6.98 (d, J = 8.1 Hz, 2H, Ph), 6.86 (s, 2H, Ph), 6.79 (d,
J = 8.1 Hz, 2H, Ph), 3.86 (s, 8H, CH2), 3.75 (s, 4H, CH2), 2.23 (s, 6H,
CH3). 13C NMR: 158.2, 157.8, 155.1, 148.9, 137.4, 136.7, 130.6,
129.4, 127.9, 123.2, 122.4, 122.2, 121.7, 116.2, 59.2, 59.0, 57.1,
The procedures for the data collection of X-ray single crystal dif-
fraction data were the same as described in our previous work [35].
All non-hydrogen atoms were modeled anisotropically. Except for
the hydroxyl hydrogen atoms, which were added from a Fourier
map, all other hydrogen atoms were placed in idealized positions
and treated as riding atoms. Details of the data collection and
refinement are given in Table 1.
2.4. Catalytic oxidation of benzene with H2O2 in acetonitrile
A typical procedure for the catalytic oxidation of benzene has
been reported in previous work [21]. To a solution of the catalyst
(20 lmol), benzene (10 mmol) and CH3CN (3 mL) in a reaction
flask (25 mL), an aqueous solution of H2O2 (2 mL, 30%) was added.
The reaction mixture was heated to 80 °C and stirred vigorously for
3 h. After being cooled to room temperature, the reaction mixture
was filtered, and the filtrate formed into two layers. The sample
taken from the upper organic layer was analyzed by gas
chromatography.
20.4. UV–Vis (DMF, kmax/nm (e
/mol Lꢁ1 cmꢁ1)): 219 (24360), 267
(28690).
2.2.2. Complex 1
To a solution of the ligand H2L (559 mg, 1 mmol) in methanol
(10 mL) was added CuCl2ꢀ6H2O (242 mg, 1 mmol) to give a blue
solution. After being stirred for five minutes, NH4PF6 (163 mg,
1 mmol) was added to produce a blue precipitate, which was col-
lected via filtration, washed with methanol and dried in vacuum.
Recrystallisation of the solid in a MeCN/MeOH/Et2O system gave
blue crystals of 1 after two weeks (522 mg, 65%). Anal. Calc. for
3. Results and discussion
3.1. Synthesis and characterization
C
35H37N5O2PF6ClCu (Mw = 803.6): C, 52.26; H, 4.60; N, 8.71.
The reaction of the ligand with Cu2+, Ni2+ and Fe3+ is summa-
rized in Scheme 1. In our work, we found its reaction with three
equivalents of Cu2+ in the presence of excess Et3N afforded a hexa-
nuclear cluster, [Cu6L2(OH)4](ClO4)4, in which the six Cu2+ centres
were bridged by the oxygen atoms of the phenolate groups of the
ligand and four OHꢁ groups. However, when no base was added,
the reaction of the ligand H2L with either one or two equivalents
of CuCl2 produced only a mononuclear complex, [Cu(H2L)](Cl)(PF6)
(1), in which the two phenol groups were pendant, without being
deprotonated. Under similar conditions, only oily products, which
were unsuitable for further characterization, were obtained when
both Ni2+ and Fe3+ were employed. When a base such as Et3N
was used, the reaction of H2L with either one or two equivalents
of Ni2+ and Fe3+ gave rise to two mononuclear complexes,
[Ni(HL)](BPh4) (2) and [Fe(L)](ClO4) (3), respectively. In complex
2, only one phenol group was deprotonated and subsequently
coordinated to Ni2+, whereas in complex 3, both phenol groups
were completely deprotonated and coordinated to the Fe3+ ion to
form a seven-coordinated complex.
Found: C, 52.58; H, 4.33; N, 8.64%. IR (cmꢁ1): 3400 (s), 3101 (w),
2919 (w), 2865 (w), 1608 (s), 1489 (s), 1445 (m), 1265 (s), 1132
(s), 1108 (s), 1087 (s), 843 (s), 758 (m), 667 (m), 551 (m). UV–Vis
(DMF, kmax/nm (e
/mol Lꢁ1 cmꢁ1)): 223 (21360), 261 (20690), 289
(7900), 323 (4356), 650 (434).
2.2.3. Complex 2
To a solution of the ligand H2L (559 mg, 1 mmol) in methanol
(10 mL) was added Et3N (0.2 mL). After being stirred for five min-
utes, NiCl2ꢀ6H2O (474 mg, 2 mmol) was added to give a green solu-
tion. The reaction was further stirred for five minutes and Na[BPh4]
(342 mg, 1 mmol) was added to produce a light-green precipitate
which was collected via filtration, washed with methanol and dried
in vacuum. Recrystallisation of the product in DMF/Et2O gave light-
green crystals of 2 after four days (402 mg, 43%). Anal. Calc. for
C
59H56N5O2BNi (Mw = 936.6): C, 75.59; H, 5.98; N, 7.47. Found:
C, 75.18; H, 5.53; N, 7.64%. IR (cmꢁ1): 3405 (s), 3051 (m), 2919
(w), 2865 (w), 1607 (s), 1578 (s), 1489 (s), 1449 (s), 1422 (m),

X. You et al. / Polyhedron 81 (2014) 743–748
745
Table 1
Crystallographic data for complexes 1–3.
Complex
1
2
3
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
35H37N5O2PF6ClCu
C59H56N5O2BNi
936.59
monoclinic
P21/c
12.413(3)
25.374(7)
17.030(3)
90
C35H35N5O6ClFe
712.98
monoclinic
P21/n
11.0642(10)
20.4959(19)
15.8545(15)
90
803.67
monoclinic
P21/n
9.2432(7)
24.3108(18)
16.1907(12)
90
b (Å)
c (Å)
a
(°)
b (°)
103.382(1)
90
3539.4(5)
4
1.508
1652
117.602(13)
90
4753.4(19)
4
1.309
1976
108.277(1)
90
3414.0(5)
4
1.387
1484
c
(°)
V (Å3)
Z
Dcalc (g cmꢁ3
F(000)
)
l
(Mo K
a
) (mmꢁ1
)
0.810
0.459
0.572
Total number of reflections
33068
44095
31854
hmax
28.35
28.28
28.26
Number of unique reflections (Rint
Number of observed reflections
Number of variables
R1, wR2 (obsd data)
R1, wR2 (all data)
)
8801 (0.041)
5416
485
0.0509, 0.1268
0.1003, 0.1522
1.026
0.521, ꢁ0.329
11679 (0.156)
5578
615
0.0700, 0.1329
0.1621, 0.1686
0.951
1.077, ꢁ0.531
8407 (0.024)
6324
435
0.0396, 0.1083
0.0588, 0.1228
1.026
0.388, ꢁ0.309
GOF, S
Maximum/minimum peak (e Åꢁ3
)
Scheme 1. Synthetic routes to the mononuclear complexes 1–3.
The absorption spectra of free ligand H2L and complexes 1–3 in
DMF are shown in Fig. 1. For the ligand H2L, the strong absorption
3.2. Structural analysis of complexes 1–3
peaks at 219 and 267 nm are assigned to
p–
p⁄ transitions of the
The cationic structures of complexes 1–3 are shown in Fig. 2.
Selected bond lengths and angles are tabulated in Table 2. In com-
plex 1, the cation [Cu(H2L)]2+ is jointly balanced by both Clꢁ and
PFꢁ6 anions. As shown in Fig. 2 (top), in the cation of complex 1,
the Cu(II) atom adopts a distorted tetragonal pyramidal geometry,
coordinated by three pyridinyl N atoms and two tertiary amine N
atoms of the H2L ligand, with the N1-N4 atoms forming the basal
plane and N5 occupying the axial position, leaving the two phenol
groups pendant. The retained H atoms in the hydroxyl group and
aromatic rings or n–p⁄ transitions. For complexes 1–3, in addition
to the ligand-based absorption bands at about 220 and 260 nm,
metal-based absorption bands are also observed. Compared with
complex 1, whose spectral profile in the short wavelength region
shows no significant difference with that of the free ligand, the
intensity of the absorption bands of complexes 2 and 3 in ultra-vio-
let region was weakened. This suggests that the coordination of the
phenolate O atom to the metal ions alters greatly the frontier
orbitals of the two complexes. The new absorption bands, with
shoulder peaks at 323 nm for complex 1 (Cu2+), 308 nm for com-
plex 2 (Ni2+) and 356 nm for complex 3 (Fe3+), are possibly
assigned to the ligand to metal charge transfer (LMCT) absorption
[24]. In the long wave length range, the weak absorptions between
510 and 800 nm for complex 1 (Cu2+), 390 and 500 nm for complex
2 (Ni2+) complex, and 420 and 800 nm for complex 3 (Fe3+) are
assigned to d–d transitions, Fig. 1 (inset).
the Clꢁ anions form a dimer, [Cu(H2L)(
l-Cl)]2, through four inter-
molecular hydrogen bonds, O–Hꢀ ꢀ ꢀCl, Fig. S1. This may explain
why not all the Clꢁ anions are replaced by PFꢁ6 anions in the anions
metathesis. Complex 2 is composed of a [Ni(HL)]+ cation and a
[BPh4]ꢁ anion. As shown in Fig. 2 (middle), in the cation of complex
2, the Ni2+ atom takes
a distorted octahedral geometry,
coordinated by three pyridinyl N atoms, two tertiary amine N
atoms and one O atom from one phenolate group, in which the

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X. You et al. / Polyhedron 81 (2014) 743–748
Table 2
0
Selected bond lengths (ÅA) and angles (°) in complexes 1–3.
Complex 1 (Cu)
Complex 2 (Ni)
Complex 3 (Fe)
Cu–N(1) = 2.001(3)
Cu–N(3) = 1.925(2)
Cu–N(5) = 2.339(3)
Cu–N(2) = 2.065(2)
Cu–N(4) = 2.127(3)
N(2)–Cu(1)–
Ni–N(1) = 2.115(3)
Ni–N(3) = 2.036(3)
Ni–N(5) = 2.105(3)
Ni–N(2) = 2.351(3)
Ni–N(4) = 2.145(3)
Ni–O(1) = 2.066(2)
Fe–N(1) = 2.2822(17)
Fe–N(3) = 2.2529(16)
Fe–N(5) = 2.3351(18)
Fe–N(2) = 2.3048(16)
Fe–N(4) = 2.2841(16)
Fe–O(1) = 1.9025(13)
N(4) = 161.93(10)
N(3)–Cu(1)–
N(4)–Ni(1)–
Fe–O(2) = 1.9153(13)
N(2) = 82.34(10)
N(1)–Cu(1)–
N(2) = 150.60(11)
N(3)–Ni(1)–
N(1)–Fe(1)–
N(4) = 112.49(11)
N(1)–Cu(1)–
O(2) = 166.46(10)
N(3)–Ni(1)–
N(2) = 70.93(6)
N(3)–Fe(1)–
N(2) = 83.02(10)
N(3)–Cu(1)–
N(4) = 76.94(11)
O(2)–Ni(1)–
N(4) = 70.58(6)
N(1)–Fe(1)–
N(4) = 80.41(11)
N(3)–Cu(1)–
N(2) = 119.71(9)
N(3)–Ni(1)–
N(5) = 78.83(6)
N(3)–Fe(1)–
N(5) = 103.73(11)
N(3)–Cu(1)–
N(1) = 160.77(11)
N(1)–Cu(1)–
N(2) = 73.68(10)
O(2)–Ni(1)–
N(4) = 89.69(10)
N(5)–Ni(1)–
N(2) = 71.14(6)
N(4)–Fe(1)–
N(5) = 69.82(6)
O(1)–Fe(1)–
Fig. 1. UV–Vis absorptions of the ligand H2L and complexes 1–3 (1.0 ꢂ 10ꢁ4
mol Lꢁ1 in DMF).
N(5) = 93.74(11)
N(1) = 164.37(11)
O(2) = 174.98(6)
two tertiary amine N atoms and two terminal pyridinyl N atoms
are trans to each other, and the coordinated phenolate O atom is
trans to the middle pyridinyl N atom. The other phenol group is left
intact. The H atom on the non-deprotonated hydroxyl group, O1–
H1, interacts with O2 to form an intramolecule hydrogen bond,
O1–H1ꢀ ꢀ ꢀO2. Complex 3 is seven-coordinated with a distorted pen-
tagonal bipyramidal geometry, in which all the seven ligating
atoms are involved in coordination to the metal centre, Fig. 2 (bot-
tom). In the crystal structure, the five N atoms approximately com-
pose the pentagonal plane and the two phenolate O atoms occupy
the two apical positions, with the \O1–Fe1–O2 angle deviating
slightly from 180°, by about 5°, Fig. S2.
As shown in Table 2, in complexes 1 and 2, the M–N(amine)
bond lengths are longer than those of M–N(py) due to strong back
donating
p bonding between M and the pyridinyl moiety. While in
complex 3, the Fe–N(amine) bond lengths are comparable to those
of Fe–N(py), suggesting that back donating bonding in the
Fe–N(py) bonds is weak. This is attributed to that the Fe3+ (d5)
ion has a much lower electron density compared to that of the
Cu2+ (d9) and Ni2+ (d8) ions. As shown in Table 2, the M–N bond
lengths in complex 3 are generally longer than those in the other
two complexes. This may not only be ascribed to the weak donat-
ing discussed above, but also to the crowded coordination around
the Fe3+ center.
3.3. Electrochemical investigation
The cyclic voltammograms for the ligand H2L and complexes 1–
3 were collected in MeCN solution. The free ligand H2L shows one
oxidation peak at 1.0 V, as displayed in Fig. S3. Complex 1 exhibits
two reduction processes, a reversible one at 0.015 V and an irre-
versible one at ꢁ0.82 V. The first reduction is assigned to the
Cu2+/Cu+ couple and the second to the process Cu+/Cu0. The assign-
ment of the latter process is supported by the observation of a
sharp stripping peak at ꢁ0.23 V during reverse scanning [36],
Fig. 3 (top). The peak potential separation of the first process is
0.097 V, which is supportive of the one-electron assignment. The
nearly one unit ratio of the cathodic to anodic peak currents (ipc
/
ipa), 0.66, suggests the quasi-reversibility of the first redox process.
Complex 2 exhibits two irreversible electrochemical responses, as
shown in Fig. 3 (middle). The reduction process at the negative
position ꢁ1.5 V is undoubtedly assigned to Ni2+/Ni+, but the
Fig. 2. The cationic structures of complexes 1 (top), 2 (middle) and 3 (bottom) with
ellipsoids at the 30% probability level.

X. You et al. / Polyhedron 81 (2014) 743–748
747
Table 3
a
Catalytic oxidation of benzene to phenol by H2O2
.
Catalysts
Yield (%)
TON
TOF (hꢁ1
)
cEp (V)
Complex 1
Complex 2
Complex 3
12.1
0
13.5
61
0
68
20
0
23
0.015
ꢁ1.50
0.037
a
ncatalyst = 20 lmol, nbenzene = 10 mmol, nH2O2 = 20 mmol, time = 3 h, T = 80 °C.
is tentatively assigned to a metal-based electron transfer. The
two peak currents are approximately of the same amplitude, which
implies that both redox processes involve the same number of
electrons. Complex 3 also shows two redox processes, with the first
reduction peak at 0.037 V and the second at ꢁ0.40 V, Fig. 3
(bottom). The first reduction is assigned to the Fe3+/Fe2+ couple
and the second irreversible redox process is probably due to the
reduction of the decomposed product from the first reduction.
3.4. Catalytic oxidation of benzene into phenol
It is well known that the direct homogeneous oxidative func-
tionalization of alkanes or benzene into alkyl alcohol and phenol
are very important reactions in organic chemistry [37]. The cata-
lytic performances of complexes 1–3 for the oxidation of benzene
into phenol by H2O2 were assessed and the results are shown in
Table 3. The reaction conditions used in the catalytic oxidation
were obtained in previous work [21]. Among the three complexes,
the Ni2+ complex shows no activity whatsoever whereas the other
two exhibit similar catalytic efficiency, but not as good as previ-
ously reported, namely [Cu(phen)Cl2] (phen = phenanthroline)
(yield 22%, TON = 242) [38] and [Fe(gma)(PBu3)] (gma = glyoxal-
bis(2-mercaptoanil)) (yield 20%, TON = 110) [21]. In the hydroxyl-
ation of benzene catalyzed by transition metal complexes, either
radical or high valent oxo-species or both are involved in the cata-
lytic process. In either pathway, it is believed that a direct interac-
tion between the oxidant and the metal centre is essential. The
coordination of the oxidant as a ligand to the metal centre ought
to be the correct first step. Such binding will depend highly on
the acidity of the metal centre. The more acidic or the more posi-
tive the reduction potential, the easier the interaction. Bearing this
argument in mind, we looked at the reduction potentials of the
three complexes, which indicate that the Ni2+ complex 2 is the
hardest to reduce and the other two have approximately the same
potentials. This may explain the complete inertness of complex 2
in the catalysis. Overall, the reduction potentials correlate well
with the activity of the complexes.
4. Conclusions
A heptadentate ligand bearing two phenol groups is reported.
Depending on whether the phenol groups bind to the metal centers
or not, the nature of the metal ions (Cu2+, Ni2+, Fe3+) and the
employment of a base, its reaction with the appropriate metal salts
gives rise to diverse coordination chemistry. Unlike Cu2+, both Ni2+
and Fe3+ react with the ligand to form mononuclear complexes (2
and 3) in the presence of Et3N. For Ni2+, only one phenol group is
coordinated, despite the usage of excessive base. The three
complexes 1, 2 and 3 show five- six- and seven-coordinated metal
centers, respectively. Electrochemically, both complexes 1 and 3
have similar reduction potentials, while complex 2 possesses the
most negative potential. The Ni2+ complex (2) is completely inert
in the catalysis. It seems that a higher ease of accepting electrons
is beneficial to the catalysis.
Fig. 3. Cyclic voltammograms of complex 1 (top), complex 2 (middle) and complex
3 (bottom) in 0.1 mol Lꢁ1 [NBu4]BF4/MeCN (scanning rate = 0.1 V sꢁ1; E/V vs. Ag/
AgCl).
assignment of the irreversible oxidation process at 0.95 V is some-
what difficult, based on the current electrochemical data. One may
argue that the oxidation process of the complex is ligand-based
since the oxidation process of ligand H2L appears nearby anyway
(1.04 V, Fig. S3). However, considering that the binding of the par-
tially deprotonated ligand HLꢁ to the Lewis acid Ni2+ would push
the potential of the ligand-based oxidation process positively,
means that the oxidation potential should be greater than 1.04 V
rather than 0.95 V. Therefore, the oxidation process of complex 2

748
X. You et al. / Polyhedron 81 (2014) 743–748
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The authors thank NSF of China (Grant No.: 20871064,
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