The relationships of catalytic activity of metal Schiff base catalysts and the Hammett constants of their anion-cationic substituents on ligand

Article abstract of DOI:10.1002/poc.3451

For quaternary ammonium, pyridinium, or imidazolium anion-cationic substituents, a linear relationship can be established between the reactivity of metal Schiff base catalysts and the Hammett constants of their anion-cationic substituents with multi-atomic anions on ligand based on the theoretical and experimental works.

Full text of DOI:10.1002/poc.3451

Research article
Received: 12 October 2014,
Revised: 14 March 2015,
Accepted: 24 March 2015,
Published online in Wiley Online Library: 6 May 2015
(wileyonlinelibrary.com) DOI: 10.1002/poc.3451
The relationships of catalytic activity of metal
Schiff base catalysts and the Hammett
constants of their anion–cationic
substituents on ligand
Hang Chena, Lu Jiaa, Jia Yaoa, Jinghui Hub, Kexian Chena,b,
Zhirong Chena and Haoran Lia,b*
For quaternary ammonium, pyridinium, or imidazolium anion–cationic substituents, a linear relationship can be established
between the reactivity of metal Schiff base catalysts and the Hammett constants of their anion–cationic substituents with
multi-atomic anions on ligand based on the theoretical and experimental works. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: DFT calculation; electronic effect; Hammett constant; ionic-pair substituent; metal Schiff base
catalytic reactivity qualitatively.[14,17,29,30] The performances of
Cu Salen catalysts with ionic-pair substituents on the oxidation
INTRODUCTION
The transition metal complexes are extensively investigated be-
cause of their importance in catalytic oxidation.[1–5] Various con-
ventional organic substituents on the ligand of metal complexes
result in different catalytic effects.[3,6,7] Unfortunately, these
homogeneous catalysts have an intrinsic disadvantage, for
example, unrecoverablility, and were not easily to be overcome
by the conventional organic substituents.[8] As one of the
alternative approaches for tackling these problems, ionic-pair
substituents had been employed as the promising groups to
modify the metal complexes.[9,10] Guillemine et al.[11]and Yao
et al.[12]reported separately that ionic-pair-substituted ruthenium
carbene can be reused by 10 times in ring-closing metathesis.
Furthermore, the catalytic activities could be conveniently tuned
by combining different cations and anions.[13–17] Such as, the
ionic-pair-substituted Salen–Mn catalysts were found to be more
effective than the unsubstituted one in the enantioselective ep-
oxidation of styrene.[18,19] Grela et al. prepared Hoveyda–Grubbs
catalysts with pyridinium or quaternary ammonium ionic-pair
substituents and used in olefin metathesis. The results showed
that they were highly active catalysts in ring closing metathe-
sis.[20,21] When pyridinium or imidazolium-substituted acetyl
acetone metal compounds were used in the C–H activation
reactions, our group found that the catalysts with ionic-pair
substituents have higher activity than the corresponding
unsubstituted counterpart.[13–15]
of cyclohexene showed that the more nucleophilic the anion of
the ionic-pair substituent on the ligand is, the higher the
catalytic reactivity would be.[14] More recently, a pyridinium
substituted metal Schiff base catalyst Co-[Salen-Py][PF6]2 showed
more excellent catalytic performance in the oxidation of 4-methyl
guaiacol to vanillin compared with other unsubstituted catalysts.
It was believed that the pyridinium substituents could decrease
the electron density of the metal center, thereby enhancing the
catalytic activity of the Schiff base catalyst.[17] As far as we know,
there are few works about the structure and reactivity of catalyst
with ionic-pair substituents quantitatively.[8,28] By investigating
the structure–reactivity relationships of acetyl acetone–Fe cata-
lysts with ionic-pair substituents in C–H activation reaction, our
group found that the introduction of ionic-pair substituents alters
the spin density carried by Fe/O atoms (SDFe/SDO), the charge
carried by O atom (QO), and the isotropic Fermi contact couplings
of O atom (IFCCO) in the Fe = O part, thereby influencing the reac-
tivity of the catalyst.[8] The introduction of the ionic-pair substitu-
ents that increases the SDO, QO, and IFCCO or decreases the SDFe
can make the catalyst more powerful. Although all these can be
understood in terms of the electronic effect of ionic-pair substit-
uents, it is still not easy to predict the influence of the ionic-pair
substituents on the catalytic reactivity quantitatively.
To find out the cause of this tunability, most of the previous pa-
pers aimed at steric effect[22–26] and the interaction between the
ions and the solvents.[25–27] Besides, several reports focused on
the structural properties and reactivity of the catalysts with
ionic-pair substituents.[8,28] The most requisite features of these
metal complexes catalysts with ionic-pair substituents were that
the ionic-pair substituents should connect with the ligand by
conjugated double bond, consequently, the cation–anion inter-
action could easily deliver to the metal center.[14] Most studies
predicted the influence of the ionic-pair substituents on the
* Correspondence to: Haoran Li, Department of Chemistry, Zhejiang University,
Hangzhou 310027, China.
E-mail: lihr@zju.edu.cn
a H. Chen, L. Jia, J. Yao, K. Chen, Z. Chen, H. Li
Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University,
Hangzhou 310027, China
b J. Hu, K. Chen, H. Li
State Key Laboratory of Chemical Engineering, Department of Chemical and
Biological Engineering, Zhejiang University, Hangzhou 310027, China
J. Phys. Org. Chem. 2015, 28 570–574
Copyright © 2015 John Wiley & Sons, Ltd.

RELATIONSHIP BETWEEN ELECTRONIC EFFECT AND REACTIVITY
It is well known that the Hammett constant σ for a conven-
tional substituent group gives a measure of its electronic effect
on the reaction center.[31] Dissociation of substituted benzoic
acids is the experimental method to obtain Hammett constants
σ,[32–34] which was also chosen for the definition of σ,[35] primar-
ily from the pK values in water. Because of the difficulty of the
synthesis of some ionic substituted benzene, few Hammett con-
stants σ of ionic substituents, which are important to design and
improve the catalysts, can be found in literatures. Recently, we
established a convenient method to predict the Hammett
constants of ionic-pair substituents.[36] If a general relationship
between the catalytic activity of metal Schiff base catalysts and
the Hammett constants of their ionic-pair substituents on ligand
is built, it will be helpful and convenient to design the catalysts
and tune their reactivity. In this paper, we present our efforts
to explore the relationship between the parameters of the reac-
tivity, for example, reaction barrier, turnover frequency (TOF),
and the Hammett constants of its anion–cationic substituents
on ligand. By analyzing our previous works, a linear relation be-
tween the Hammett constants of its anion–cationic substituents
on ligand and the reaction barrier or TOF was observed. We also
report the metal Schiff base catalysts with imidazolium anion–
cationic substituents on the ligand Co-[Salen-Mim][X]2 (X = PFꢀ6 ,
ClOꢀ4 , BF4ꢀ, NO3ꢀ, and Brꢀ) in this paper. Then, the oxidation of
4-methyl guaiacol was carried out to investigate the electronic
effect of anion–cationic substituents on ligand on their catalytic
activities.
Co-[Salen-Mim][Br]2 (1 mmol) in ethanol (50 ml), and then the
mixture was stirred at room temperature for 8 h. The resulted
mixture was filtered, and the obtained solid was dried in vac-
uum to give Co-[Salen-Mim][X]2 (X = PFꢀ6 , ClO4ꢀ).
Co-[Salen-Mim][NO3]2: Yield: 91%. ICP-MS: Anal. Calcd. for
C20H28CoN8O8: Co, 10.39%. Found: Co, 10.36%. ESI-MS,
m/z = 505.15(M-NO3)+. FT-IR (KBr): 3436, 3142, 3093, 2959, 1716,
1582, 1472, 1428, 1387, 1285, 1204, 1098, 1066, 1017, 985, 936,
834, 765, 688, 627, 485 cmꢀ1
.
Co-[Salen-Mim][BF4]2: Yield: 93%. ICP-MS: Anal. Calcd. for
C20H28B2CoF8N6O2: Co, 9.55%. Found: Co, 9.47%. ESI-MS,
m/z = 530.17(M-BF4)+. FT-IR (KBr): 3439, 3141, 1606, 1386, 1283,
1201, 1097, 1026, 934, 829, 758, 683, 619, 486 cmꢀ1
.
Co-[Salen-Mim][ClO4]2: Yield: 89%. ICP-MS: Anal. Calcd. for
C20H28Cl2CoN6O10: Co, 9.18%. Found: Co, 9.22%. ESI-MS,
m/z = 542.13(M-ClO4)+. FT-IR (KBr): 3540, 3140, 3095, 2964,
1701, 1583, 1544, 1421, 1349, 1278, 1200, 1081, 933, 813, 754,
685 cmꢀ1
.
Co-[Salen-Mim][PF6]2: Yield: 96%. ICP-MS: Anal. Calcd. for
C20H28CoF12N6O2P2: Co, 8.04%. Found: Co, 8.07%. ESI-MS,
m/z = 588.12 (M-PF6)+. FT-IR (KBr): 3733, 3161, 1562, 1433, 1276,
1194, 1100, 1029, 935, 831, 750, 699 cmꢀ1
.
Density functional theory calculation
The density functional theory was applied at the B3LYP/6-
311++G* level. All calculations were performed with the
Gaussian03 programs package.[37] All the structures were opti-
mized without any restriction, and the final optimized structures
were therefore all at the lowest point when referred to energy.
To obtain a further understanding of electronic distribution,
natural bond orbital (NBO) analysis was also performed by using
the same basis set.
EXPERIMENTAL SECTION
Preparation and characterization of catalysts
The procedure to synthesize catalysts Co-[Salen-Mim][Br]2
(Scheme 1) and their characterization are listed in the SI.
The calculation method of Hammett constants of ionic
substituents
The AgNO3 or AgBF4 (2 mmol) was added to the Co-[Salen-
Mim][Br]2 (1 mmol) in ethanol (50 ml), and then the mixture
was stirred away from light at room temperature for 8 h. The
resulted mixture was filtered and the filtrate was evaporated
under reduced pressure at 60 °C, and the obtained solid was
dried in vacuum. Co-[Salen-Mim][X]2 (X = BFꢀ4 , NO3ꢀ) were pre-
pared. NH4PF6 or NaClO4 (2 mmol) was added to the
Based on the linear relationship between the para Hammett
constants σ and the natural population analysis (NPA) charge q
of the para carbon atom of the conventional monosubstituted
benzenes (σ = 3.762 + 18.467q, R2 = 0.94), the Hammett constants
of ionic-pair substituents were obtained by calculating the natu-
ral population analysis charge q of their para
carbon atom at the B3LYP/6-311++G* level
in the gas phase.[36]
Procedure for the oxidation of 4-methyl
guaiacol
In
a
typical reaction, 4-methyl guaiacol
(290 mmol), NaOH (957 mmol), ethylene glycol
(134.5 ml), water (25 ml) and Co-[Salen-Mim][X]2
(1.38 mmol) were added into a 250 ml four-
neck round bottom reactor, which was fitted
with an overhead stirrer and a reflux con-
denser. The reaction temperature was kept
constant in water bath. Oxygen was supplied
to the reactor from the top using a single
glass tube inlet. Timely samples were taken
and analyzed on a high-performance liquid
chromatography (HPLC) equipped with an
ultraviolet detector (λmax = 279 nm) on a 25 cm
RP-18 column.
Scheme 1. Synthesis of M-[Salen-Mim][X]2 (X = PFꢀ6 , ClO4ꢀ, BF4ꢀ, NO3ꢀ, and Brꢀ; M = Co2+, Cu2+
,
Ni2+, and Mn2+
)
J. Phys. Org. Chem. 2015, 28 570–574
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

H. CHEN ET AL.
RESULTS AND DISCUSSION
Correlation of Hammett constant with reactivity parameters
Because changing anion is easier than replacing cation from the
synthesis viewpoint,[38,39] the anion–cationic substituents have re-
ceived more attention in the reported works.[8,14,15,17,28,40,41]
Among them, quaternary ammonium, pyridinium, or imidazolium
were commonly used.[8] For example, the acetyl acetone–Fe cata-
lysts with anion-quaternary ammonium substituents were used
in oxidation of methane by our group.[8] A linear relationship was
found between the spin density carried by Fe/O atoms (SDFe/
SDO), or the charge carried by O atom (QO), or the isotropic Fermi
contact couplings of O atom (IFCCO) in the Fe=O part and the reac-
tivity of the analogous catalysts. Because the C–H activation pro-
cess undergoes electron transfer, SDFe/SDO, QO, and IFCCO are
terms of the electronic effect of ionic-pair substituents. We deduce
that there should also be a linear relationship between the
Hammett constants and the reactivity. The Hammett constants of
their anion–cationic substituents on ligand were calculated via
the method reported in our previous work,[36] and the reaction
barriers (RB) were used as the reactivity parameter. Totally 10
different anions (PFꢀ6 , AlCl4ꢀ, BFꢀ4 , AsFꢀ6 , SbFꢀ6 , AlF4ꢀ, CF3COꢀ2 ,
CF3SO3ꢀ, NO3ꢀ, and Clꢀ) were used in our previous work.[8] We
chose seven common anions (PFꢀ6 , AlClꢀ4 , BF4ꢀ, AlF4ꢀ, CF3COꢀ2 ,
CF3SOꢀ3 , and NOꢀ3 ) for correlation. The reasons are that the bond
between ionic-pair substituent ꢀNH2(CH3)X (X= SbF6, AsF6) and
matrix moiety was broken during the structure optimization.
Besides, the interaction between the mono-atomic anion of ionic-
pair substituent in small size (such as Clꢀ or Brꢀ) and central metal
atom is direct, while the multi-atomic anion of ionic-pair substituent
in large size has an effect on center metal atom indirectly through
the direct interaction of ligand. The details of molecular structures
are shown in SI. Therefore, the multi-atomic anion of ionic-pair
substituent in large size is discussed in this paper.
Figure 1. The reaction barriera of the C-H activation process during
methane oxidation by the acetyl acetone–Fe catalysts with anion-quater-
nary ammonium substituentsb plotted against the Hammett constant σc.
a
b
The reaction barrier data was selected from reference. 8 The multi-
c
atomic anions of ionic-pair substituent in large size were chosen. The
Hammett constant were calculated according to reference. 36 RB,
reaction barriers
The RB of the C–H activation and C–O rebound processes were
correlated with the Hammett constants of their anion–cationic
substituents on ligand. (Fig. 1) A linear relation can be found be-
tween the Hammett constants and the reactivity. The correlation
coefficient R2 is 0.78.
RB ¼ ꢀ49:96σ þ 99:76 R2 ¼ 0:78
(1)
Figure 2. The reaction barriera of the C–H activation process during
methane oxidation by the metalloporphyrin catalysts with anion-quater-
nary ammonium substituentsb plotted against the Hammett constant σc.
Because all the ionic-pair substituents are electron-
withdrawing groups, the formula 1 suggests that the anion–
cationic substituents on ligand with larger Hammett constant
will make the catalyst more active. This result agrees with that
reported in the literature.[8] It suggested that less negative
charge carried by the O atom leads to higher reactivity.[8]
The methane C–H activation processes were also chosen to
test the reactivity of the metalloporphyrin catalysts with anion-
quaternary ammonium substituents in Hu’s report.[28] A linear
or almost linear relationship can also be found between the
Hammett constants and the reactivity (Fig. 2). The correlation co-
efficient R2 is 0.96.
a
b
The reaction barrier data was selected from reference. 28 The multi-
c
atomic anions of ionic-pair substituent in large size were chosen. The
Hammett constant were calculated according to reference. 36 RB, reac-
tion barriers
constants of its anion–cationic substituents on ligand (Fig. 3). A linear
relationship can be found between the Hammett constants and the
reactivity. The correlation coefficient R2 is 0.98.
TOF ¼ 153:41σ ꢀ 50:84 R2 ¼ 0:98
(3)
RB ¼ ꢀ81:05σ þ 133:98 R2 ¼ 0:96
(2)
The oxidation of 4-methyl guaiacol to test the metal Schiff
base catalysts with imidazolium anion–cationic substituents
The two metal Schiff base catalysts with ionic-pair substituents on
the aforementioned ligand were reported in theoretical works.[8,28]
The experimental data are also used to correlate the Hammett con-
stants with reactivity. The oxidation of cyclohexene by molecular ox-
ygen in the presence of Cu Salen catalysts with anion–pyridinium
substituents was studied by our group.[14] TOF was used as the reac-
tivity parameter. The TOF values were correlated with the Hammett
To better understand the relativity between the electronic effect
and reactivity, the metal Schiff base catalysts with imidazolium
anion–cationic substituents on ligand Co-[Salen-Mim][X]2
(X= PFꢀ6 , ClOꢀ4 , BF4ꢀ, NOꢀ3 , and Brꢀ) were synthesized to investigate
the relationship between the Hammett constants of their anion–
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Copyright © 2015 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2015, 28 570–574

RELATIONSHIP BETWEEN ELECTRONIC EFFECT AND REACTIVITY
Table 2. The Hammett constants of the Co-[Salen-Mim][X]2
Entry
Substituenta
TOF
Hammett constantb
1
2
3
4
5
ꢀ[Mim][Br]
22.3
22.7
23.1
26.0
26.9
0.25
0.14
0.36
0.37
0.39
ꢀ[Mim][NO3]
ꢀ[Mim][BF4]
ꢀ[Mim][ClO4]
ꢀ[Mim][PF6]
TOF, turnover frequency.
a[Mim]+: imidazolium [C4H6NN+].
bThe Hammett constant were calculated according to
reference.36
Figure 3. The turnover frequencya of the oxidation of cyclohexene by
the Cu Salen catalysts with anion–pyridinium substituentsb plotted
against the Hammett constant σc. a The turnover frequency was selected
from reference. 14 b The Hammett constant were calculated according to
reference. 36 c The multi-atomic anions of ionic-pair substituent in large
size were chosen. TOF, turnover frequency
cationic substituents on ligand and their catalytic activity. In this
section, the catalysts Co-[Salen-Mim][X]2 were evaluated in the ox-
idation of 4-methyl guaiacol to vanillin. According to our previous
report on the oxidation of 4-methyl guaiacol, Co metal and the sol-
vent (ethylene glycol/water (V/V) = 85:15) was the better choice for
this reaction.[17] As shown in Table 1, it was found that the reactiv-
ity of the catalyst Co-[Salen] without substituents was low (Entry 6),
in comparison to Co-[Salen-Mim][X]2. The introduction of ionic-pair
substituents could improve the reactivity. It was found that the
anions have a subtle effect on the catalytic activity (Entry 1–5).
The TOF value of Co-[Salen-Mim][PF6]2 (26.9) (Entry 5) is the largest
among the studied Co-[Salen-Mim][X]2 (Entry1–5). The catalytic
activities of the catalysts with different anions follow the order:
PFꢀ6 > ClO4ꢀ > BF4ꢀ > NOꢀ3 > Brꢀ. It is shown that the reactivity of
Co-[Salen-Mim][X]2 is increased with increasing of the Hammett
constant of its substituent. However, the mono-atomic anion Brꢀ
is an exception to this rule. The Hammett constants of their
anion–cationic substituents on ligand and the TOF values are col-
Figure 4. The turnover frequency of the oxidation of 4-methyl guaiacol
by the Co Salen catalysts with imidazolium anion–cationic substituentsa
a
plotted against the Hammett constant σb. The multi-atomic anions of
ionic-pair substituent in large size were chosen. b The Hammett constant
were calculated according to reference.36 TOF, turnover frequency
lected in Table 2. We chose the multi-atomic anions (PFꢀ6 , ClO4ꢀ,
BFꢀ4 , and NOꢀ3 ) for correlation. The correlation between TOF and
the Hammett constants in Fig. 4 suggests that there is a linear re-
lationship as follows:
Table 1. The results of the oxidation of 4-methyl guaiacol
with different catalystsa
TOF ¼ 13:39σ þ 20:67
R2 ¼ 0:69
(4)
Entry
Catalyst
Time (h) Conv. (%) TOF (hꢀ1 b
)
CONCLUDING REMARKS
1
2
3
4
5
6
Co-[Salen-Mim][Br]2
Co-[Salen-Mim][NO3]2
Co-[Salen-Mim][BF4]2
Co-[Salen-Mim][ClO4]2
Co-[Salen-Mim][PF6]2
Co-[Salen]
5
5
5
5
5
5
53
54
55
62
64
45
22.3
22.7
23.1
26.0
26.9
18.9
As far as we know, despite that they are important in designing
and improving the catalysts, few Hammett constants of ionic-
pair substituents were reported in literatures. In this paper, the
Hammett constants of their anion–cationic substituents on
ligand were calculated via the method reported in our previous
works,[36] then we explored the relationship between the param-
eters of the reactivity (such as reaction barrier or TOF) of metal
Schiff base catalysts and the Hammett constants of their
anion–cationic substituents on ligand. The anion–cationic sub-
stituents often contain quaternary ammonium, pyridinium, or
imidazolium cations.[8] Based on the analysis on the previous
theoretical and experimental works, it was found that the reac-
tivity was correlated with the Hammett constants of their
TOF, turnover frequency.
aReaction conditions: 4-methyl guaiacol (290 mmol), oxygen
flow rate (40 cm3/min), stirring rate rpm 1000, NaOH
(957 mmol), ethylene glycol (150 g), water 25 g,
temperature75 °C, and catalyst (1.38 mmol).
bTOF is calculated by expression of (moles of 4-methyl
guaiacol)/[(mole of catalyst)*time (h)].
J. Phys. Org. Chem. 2015, 28 570–574
Copyright © 2015 John Wiley & Sons, Ltd.
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H. CHEN ET AL.
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To better understand the relativity between the electronic effect
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anions and their reactivity. The quantitative electronic effect
and reactivity relationships proposed in the present study are
expected to be useful in designing a more powerful catalyst with
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Acknowledgements
This work was supported by the Program for Zhejiang Leading
Team of S&T Innovation (2011R50007), and the Fundamental
Research Funds of the Central Universities.
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J. Phys. Org. Chem. 2015, 28 570–574