Synthesis of a tetrazine-based catecholamide derivative and its evaluation as a chelating agent for removal of Cd(II), Co(II), and Cu(II)

Article abstract of DOI:10.1080/00958972.2017.1340644

The synthesis and structural characterization of a tetrazine-based catecholamide (CAM) ligand, N,N′-bis(N″-(aminoethyl)-2,3-bis(hydroxy)benzamide)-1,2,4,5-tetrazine-3,6-diamine (5), were investigated. All compounds were characterized by ¹H NMR spectroscopy, ¹³C NMR spectroscopy, and FTIR spectroscopy. The protonation equilibria of 5 and complexation capacities (log β_pqr) of Cd²⁺, Co^{2 +}, and Cu²⁺ complexes of 5 were evaluated through potentiometric titration and spectrophotometric titration, respectively. Species independent pM value (=?log [M]_free) was used to compare metal affinities with the final sequence Cu²⁺?>?Cd²⁺?>?Co²⁺. Results show that 5 has potential for heavy metal removal.

Full text of DOI:10.1080/00958972.2017.1340644

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis of a tetrazine-based catecholamide
derivative and its evaluation as a chelating agent
for removal of Cd(II), Co(II), and Cu(II)
Shan Lei, Bo Jin, Rufang Peng, Qingchun Zhang & Shijin Chu
To cite this article: Shan Lei, Bo Jin, Rufang Peng, Qingchun Zhang & Shijin Chu (2017):
Synthesis of a tetrazine-based catecholamide derivative and its evaluation as a chelating
agent for removal of Cd(II), Co(II), and Cu(II), Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1340644
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1340644
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Date: 17 June 2017, At: 00:29

Journal of Coordination Chemistry, 2017
ꢁꢁpꢂ://ꢃꢄꢅ.ꢄꢆg/10.1080/00958972.2017.1340644
ꢀ
Synthesis of a tetrazine-based catecholamide derivative and
its evaluation as a chelating agent for removal of Cd(II), Co(II),
and Cu(II)
a
a,b
a,b
a
a
Shan Lei , Bo Jin , Rufang Peng , Qingchun Zhang and Shijin Chu
ꢇ
sꢁꢇꢁꢈ Kꢈꢉ lꢇbꢄꢆꢇꢁꢄꢆꢉ Cꢊꢋꢁꢅvꢇꢁꢅꢄꢌ Bꢇꢂꢈ ꢍꢄꢆ nꢄꢌꢎꢈꢁꢇꢋ Cꢄꢎpꢄꢂꢅꢁꢈꢂ ꢇꢌꢃ fꢊꢌcꢁꢅꢄꢌꢇꢋ mꢇꢁꢈꢆꢅꢇꢋꢂ, sꢄꢊꢁꢀwꢈꢂꢁ
b
uꢌꢅvꢈꢆꢂꢅꢁꢉ ꢄꢍ scꢅꢈꢌcꢈ ꢇꢌꢃ tꢈcꢀꢌꢄꢋꢄgꢉ, mꢅꢇꢌꢉꢇꢌg, Cꢀꢅꢌꢇ; dꢈpꢇꢆꢁꢎꢈꢌꢁ ꢄꢍ Cꢀꢈꢎꢅꢂꢁꢆꢉ, scꢀꢄꢄꢋ ꢄꢍ mꢇꢁꢈꢆꢅꢇꢋꢂ
scꢅꢈꢌcꢈ ꢇꢌꢃ eꢌgꢅꢌꢈꢈꢆꢅꢌg, sꢄꢊꢁꢀwꢈꢂꢁ uꢌꢅvꢈꢆꢂꢅꢁꢉ ꢄꢍ scꢅꢈꢌcꢈ ꢇꢌꢃ tꢈcꢀꢌꢄꢋꢄgꢉ, mꢅꢇꢌꢉꢇꢌg, Cꢀꢅꢌꢇ
ABSTRACT
ARTICLE HISTORY
The synthesis and structural characterization of a tetrazine-based
catecholamide (CAM) ligand, N,N′-bis(N″-(aminoethyl)-2,3-bis(hydroxy)
benzamide)-1,2,4,5-tetrazine-3,6-diamine (5), were investigated. All
rꢈcꢈꢅvꢈꢃ 12 nꢄvꢈꢎbꢈꢆ 2016
accꢈpꢁꢈꢃ 25 mꢇꢉ 2017
KEYWORDS
Cam ꢋꢅgꢇꢌꢃ;
ꢎꢈꢁꢇꢋ cꢄꢎpꢋꢈxꢈꢂ;
ꢂpꢈcꢁꢆꢄpꢀꢄꢁꢄꢎꢈꢁꢆꢅc ꢁꢅꢁꢆꢇꢁꢅꢄꢌ;
cꢄꢎpꢋꢈxꢇꢁꢅꢄꢌ cꢇpꢇcꢅꢁꢅꢈꢂ
1
13
compounds were characterized by H NMR spectroscopy, C NMR
spectroscopy, and FTIR spectroscopy. The protonation equilibria
2
+
2+
2+
of 5 and complexation capacities (log β ) of Cd , Co , and Cu
pqr
complexes of 5 were evaluated through potentiometric titration and
spectrophotometric titration, respectively. Species independent pM
value (=−log [M] ) was used to compare metal affinities with the
free
2
+
2+
2+
final sequence Cu > Cd > Co . Results show that 5 has potential
for heavy metal removal.
CONTACT Bꢄ Jꢅꢌ
jꢅꢌbꢄ@ꢂwꢊꢂꢁ.ꢈꢃꢊ.cꢌ; rꢊꢍꢇꢌg Pꢈꢌg
pꢈꢌgꢆꢊꢍꢇꢌg@ꢂwꢊꢂꢁ.ꢈꢃꢊ.cꢌ
sꢊppꢋꢈꢎꢈꢌꢁꢇꢋ ꢃꢇꢁꢇ ꢍꢄꢆ ꢁꢀꢅꢂ ꢇꢆꢁꢅcꢋꢈ cꢇꢌ bꢈ ꢇccꢈꢂꢂꢈꢃ ꢇꢁ ꢀꢁꢁpꢂ://ꢃꢄꢅ.ꢄꢆg/10.1080/00958972.2017.1340644.
©
2017 iꢌꢍꢄꢆꢎꢇ uK lꢅꢎꢅꢁꢈꢃ, ꢁꢆꢇꢃꢅꢌg ꢇꢂ tꢇꢉꢋꢄꢆ & fꢆꢇꢌcꢅꢂ Gꢆꢄꢊp

2
S. LEI ET AL.
1
. Introduction
Heavy metal ions in living systems have received attention because of their potential con-
tamination and diverse uses in many fields, such as cigarettes [1], seafood [2], and cosmetics
[
3]. Once ingested into the human body, some heavy metals disrupt physiological systems
by binding to protein groups even at low concentrations [4]. For example, cadmium can
cause cell apoptosis, kidney damage, and cancer [5], and cobalt may be carcinogenic to
humans [6]. Copper is an essential trace element in physiological processes; however, exces-
sive copper causes deposition in the brain, jaundice, and liver enlargement [7]. These metals
threaten human health.
The use of chelating agents is the most effective heavy metal removal method, forming
complexes with these metals. Numerous chelating agents, including catecholamides [8],
hydroxylpyridones [9], and azamacrocyclic derivatives [10], have been synthesized and inves-
tigated for their complexing capacities with metal ions. Catecholamides are effective che-
lators because the amide carbonyl at the ortho position relative to one of the chelating
oxygens can improve the affinity with metal ions [11].
Chemical characteristics of azacyclic metal complexes of cadmium [12], cobalt [13] and
copper [14] have been studied extensively. Azacyclic compounds are an essential part of
coordination chemistry, and formation constants of metal complexes are usually greater
than ordinary ligands due to the amine scaffold [15]. Tetrazine is one of the azacyclic struc-
tures which is used to complex metal ions [16]. Thus, a new tetradentate catecholaminde
ligand linked to tetrazine was investigated for its potential as a chelator for heavy metal ions.
In this study, N,N’-bis(N’’-(aminoethyl)-2,3-bis(hydroxy)benzamide)-1,2,4,5-tetrazine-3,6-
diamine was synthesized and investigated as a chelating agent for Cd(II), Co(II), and Cu(II).
2
. Experimental
2
.1. Materials and instrumentation
The organic reagents were commercially available from Aladdin and used without purifica-
tion. Anhydrous solvents were purchased from Chengdu Kelong Chemical Reagents Co.;
CH Cl was distilled before use. Flash chromatography was carried out on 300–400 mesh
2
2
1
silica gel from Qingdao Hailang. NMR spectra were recorded with Bruker AVANCE 600 ( H:
00 MHz, C: 150 MHz) and Bruker AVANCE 300 ( H: 300 MHz, C: 75 MHz) spectrometers
1
3
1
13
6
in CDCl or CD OD. FTIR spectra were recorded on a Nicolet 380 FTIR spectrophotometer
3
3
−1
−1
(
Thermo Fisher Nicolet, USA) in KBr with a resolution of 4 cm from 400 to 4000 cm . A
UV–Vis spectrophotometer (Thermo Scientific Evolution 201, USA) with a double beam light
source from 190 to 1100 nm was used.
2
.2. Potentiometric and spectrophotometric titrations
All titrations were carried out on a ZDJ-4B automatic potentiometric titrator (Shanghai INESA
Scientific Instrument Co., Ltd.) at 25 ± 0.1 °C and I = 0.1 M (KCl) under N , and spectropho-
2
tometric titrations were simultaneously performed with a Thermo Scientific Evolution 201
UV–Vis spectrophotometer. Solid reagents were weighed on a Sartorius BT25S analytical
balance. Ultrapure water was distilled using a ULUP-IV ultrapure water system (Sichuan
ULUPURE Technology Co., Ltd.) and degassed by ultrasonic device. The 0.1 M nitric acid

JOURNAL OF COORDINATION CHEMISTRY
3
standard solution and 0.1 M KOH standard solution were commercially available from
Aladdin. Stock solutions of 0.01 M metal ions were made from Cd(NO ) ·4H O, CoCl ·6H O,
3
2
2
2
2
and Cu(NO ) ·3H O. The metal ion stock solutions were obtained using certain amounts of
3
2
2
respective metal salts which were directly diluted with 0.1 M KCl aqueous solution to 10 mL.
The ligand stock solution was 2 × 10− M. KCl aqueous solution (0.1 M) was prepared to 1 L
in volumetric flask. A certain amount of the chelating agent was weighed accurately with a
starting ethanol volume of 1%, dissolved, and diluted with 0.1 M KCl aqueous solution to
4
250 mL in volumetric flask.
The protonation constants of 5 were investigated with 25 mL ligand stock solution, and
.01 M diluted KOH standard solution was continuously dropped into the titration vessel.
0
2
+
2+
2+
To study the formation constants of Cd , Co , and Cu complexes, a mixture of 0.5 mL
metal ions stock solutions, 1.5 mL nitrate standard solution (reducing pH value of the titration
solution), and 25 mL ligand stock solution were added to the titration vessel; the titrations
were measured with 0.01 M KOH solution gradually adding into the titration vessel from low
to high pH. All titrations were repeated thrice.
2
.3. Synthesis of N-(aminoethyl)-2,3-bis(hydroxy)benzamide (4)
The mixture of N-(aminoethyl)-2,3-bis(benzyloxy)benzamido-trifluoroacetate 2 (2 g,
.08 mmol) and 20 mL 1.25 M NaOH aqueous solution was stirred for 20 min. After extraction
by CH Cl (20 mL × 3), the organic layer was dried over anhydrous Na SO , and then the
4
2
2
2
4
filtrate was evaporated to dryness to obtain N-(aminoethyl)-2,3-bis(benzyloxy)benzamide
as clear oil (1.34 g, 87%). A mixture of 3 (1.34 g, 3.55 mmol) and 10% Pd/C (200 mg) was
3
−1
dissolved in 50 mL ethanol and stirred under H (130 mL min ) atmosphere for 4 h. The
2
resulting mixture was filtered, and the filtrate was concentrated in vacuo to obtain
1
N-(aminoethyl)-2,3-bis(hydroxy)benzamide 4 as yellow solid (0.69 g, 99%). H NMR (600 MHz,
CD OD, 298.0 K) δ (ppm): 7.27 (dd, J 7.8 and 1.8 Hz, 1H, Ar-H), 6.83 (dd, J 7.8 and 1.8 Hz, 1H,
3
1
3
Ar-H), 6.50 (t, J 7.8 Hz, 1H, Ar-H), 3.56 (t, J 6 Hz, 2H, CH ), 2.97 (t, J 6 Hz, 2H, CH ); C NMR
2
2
(
150 MHz, CD OD, 298.0 K) δ (ppm): 172.48 (C=O), 154.90 (Ar-C), 149.14 (Ar-C), 119.81 (Ar-
3
−1
CH), 117.43 (Ar-CH), 117.25 (Ar-CH), 116.44 (Ar-C), 41.76 (CH ), 40.51 (CH ); FTIR (KBr) v (cm ):
2
2
3
432 (N–H and O–H), 2974, 2926 (C-H from -CH ), 1623 (C=O), 1552 (C–N–H), 1445 (benzene
2
ring), 1049 (C–N), 746.
2
1
.4. Synthesis of N,N′-bis(N″-(aminoethyl)-2,3-bis(hydroxy)benzamide)-
,2,4,5tetrazine-3,6-diamines (5)
A mixture of 3,6-dichlorotetrazine 1 (0.08 g, 0.53 mmol), N-(aminoethyl)-2,3-bis(hydroxy)
benzamide 4 (0.21 g, 1.06 mmol), and 2,4,6-trimethylpyridine (0.13 g, 1.06 mmol) was dis-
solved in 30 mL distilled CH Cl and stirred at room temperature for 2 h. The solution was
2
2
concentrated in vacuo, and the residue was chromatographed on a silica gel column with
a mixture of ethanol and CH Cl as the mobile phase to derive 5 as orange solid (0.25 g, 80%).
2
2
1
R = 0.5. H NMR (300 MHz, CD OD, 298.0 K) δ (ppm): 7.78 (s, H, CO-NH), 7.46 (s, H, CO-NH),
f
3
7
.14 (dd, J 8.1 and 1.5 Hz, 1H, Ar-H), 6.90 (dd, J 7.8 and 1.5 Hz, 1H, Ar-H), 6.69 (t, J 7.8 Hz, 1H,
1
3
Ar-H), 3.74 (m, 2H, CH ), 3.68 (m, 2H, CH ); C NMR (75 MHz, CD OD, 298.0 K) δ (ppm): 171.82
2
2
3
(
1
C=O), 163.10 (C–N), 160.67 (C–N), 150.05 (Ar-C), 146.99 (Ar-C), 119.53 (Ar-CH), 119.40 (Ar-CH),
−1
18.42 (Ar-CH), 116.24 (Ar-C), 41.72 (CH ), 38.98 (CH ); FTIR (KBr) v (cm ): 3432 (N–H and
2 2

4
S. LEI ET AL.
Scheme 1. sꢉꢌꢁꢀꢈꢁꢅc ꢆꢄꢊꢁꢈ ꢁꢄ 3,6-ꢃꢅcꢀꢋꢄꢆꢄꢁꢈꢁꢆꢇzꢅꢌꢈ 1.
Scheme 2. sꢉꢌꢁꢀꢈꢁꢅc ꢆꢄꢊꢁꢈ ꢁꢄ 5.
O–H), 2926 (C–H from –CH ), 1638 (C=O and C=N), 1457 (benzene ring), 1384, 1330 (C–N
2
from tetrazine ring), 794, 741.
3
. Results and discussion
.1. Synthesis
,6-Dichlorotetrazine 1 was synthesized according to the reported procedure [17] shown in
3
3
1
3
Scheme 1, and the C NMR spectrum of 1 matched with that reported previously.
The synthetic route of N,N′-bis(N″-(aminoethyl)-2,3-bis(hydroxy)benzamide)1,2,4,5-
tetrazine-3,6-diamine 5 is presented in Scheme 2. N-(aminoethyl)-2,3bis(benzyloxy)
benzamido-trifluoroacetate 2 was prepared according to our previous work [18]. After
N-(aminoethyl)-2,3-bis(benzyloxy)benzamido-trifluoroacetate 2 stirring in 10 wt. % NaOH
aqueous solution, 3 was obtained in good yield (87%). The protecting groups were removed
−
1
by 130 mL·min H and Pd/C in ethanol at room temperature to produce 4 in a 99% yield.
2
The deprotecting compound 4 was treated with 3,6-dichlorotetrazine 1 in the presence of
2,4,6-trimethylpyridine to obtain the desired tetradentate ligand 5 in a 80% yield.
3
.2. Ligand protonation
The protonation constants of 5 were investigated by potentiometric titration [19] beyond
the 5.9−11.0 pH range. The fitting analysis by the Hyperquad 2013 program [20] presented
the global protonation constants (log β) which allowed calculation of the stepwise

JOURNAL OF COORDINATION CHEMISTRY
5
h ꢇ
Table 1. sꢁꢈpwꢅꢂꢈ pꢆꢄꢁꢄꢌꢇꢁꢅꢄꢌ cꢄꢌꢂꢁꢇꢌꢁꢂ (ꢋꢄg K ) ꢄꢍ 5, ꢃꢄpꢇꢎꢅꢌꢈ ꢇꢌꢃ dhi ꢍꢄꢆ cꢄꢎpꢇꢆꢅꢂꢄꢌ.
ꢅ
log K H
log K2H
log K3H
log K4H
Ligand
1
5
10.50(2)
13.11
13.09
9.89(4)
9.59
9.54
7.94(2)
–
–
7.06(3)
–
–
b
ꢃꢄpꢇꢎꢅꢌꢈ
b
dhi
ꢇ
h
ꢅ
I = 0.1 m KCꢋ, 25.0 ± 0.1ꢏC. K = [h l] / ([h l] [h]).
ꢅ
ꢅ-1
b
rꢈꢍ. [19b].
Figure 1. mꢄꢋꢈcꢊꢋꢇꢆ ꢂꢁꢆꢊcꢁꢊꢆꢈꢂ ꢄꢍ ꢃꢄpꢇꢎꢅꢌꢈ ꢇꢌꢃ dhi.
H
i
protonation constants (log K , i = 1, 2, 3, 4) presented inTable 1. Ligand 5 has four dissociable
protons because of the two dihydroxybenzene cores of catechol, and the first two protona-
tion constants are 10.50 and 9.89, respectively. The gap of 0.61 logarithm units for the first
two protonation constants can be explained by the electrostatic repulsion between positive
H
i
charges in the protonation species [21]. The average log K values of the first two protonation
constants (10.50 and 9.89) and the last two protonation constants (7.94 and 7.06) are 10.20
and 7.50, respectively, and lower than the values of 4-(2-aminoethyl)benzene-1,2-diol (dopa
-
mine, Figure 1) and 5,6-dihydroxyindole (DHI, Figure 1) [15(b)], indicating that 5 has more
acidic property as a result of the electron-withdrawing effect of the amide carbonyl groups
[
11]. For this reason, the four protonation constants of 5 can be calculated only by potenti-
ometric titration without the use of spectrophotometric titration.
The concentration distribution curve was obtained with the HySS program [22]. The first
two protonation constants correspond to the meta hydroxyl groups (high pH) and the last
two protonation constants correspond to the ortho hydroxyl groups (low pH). At pH < 5, 5
displays the fully protonated form H L, where L is ligand. As pH increased, the ligand loses
4
−
2−
3−
4−
its protons from hydroxyl oxygens to form H L , H L , HL , and L species, respectively.
Among these species, H L is the main form at neutral pH as shown in Figure 3(A). The pro-
3
2
−
3
posed forms of 5 are shown in Figure 2, in agreement to previous research [23].
3
.3. Metal complexation
2+
2+
2+
Metal affinities of 5 were studied with Cd , Co , and Cu at a 1:1 ratio to form mononuclear
complexes. Spectrophotometric titrations [24] were carried out until there were no significant
changes on the UV–Vis curves. The raw data were analyzed by HypSpec 2014 program [25]
yielding cumulative stability constants (log βpqr) according to the equilibrium in solution,
and spectrophotometric curves are shown in Supporting Information. The concentration
distribution curves were determined with the HySS program, as shown in Figure 3.
2+
2+
2+
Figure 3(B–D) show that deprotonations of complexes of 5 with Cd , Co , and Cu occur
stepwise as pH increased. The five species, [MH L], [MH L], [MH L], [MHL], and [ML], are
4
3
2
formed in all three systems, but there are some differences between the amount of ligand

6
S. LEI ET AL.
Figure 2. Pꢆꢄpꢄꢂꢈꢃ ꢍꢄꢆꢎꢂ ꢄꢍ 5 ꢊꢌꢃꢈꢆ ꢇcꢅꢃꢅc ꢇꢌꢃ bꢇꢂꢅc cꢄꢌꢃꢅꢁꢅꢄꢌꢂ.
–
4
Figure 3. Cꢄꢌcꢈꢌꢁꢆꢇꢁꢅꢄꢌ ꢃꢅꢂꢁꢆꢅbꢊꢁꢅꢄꢌ cꢊꢆvꢈꢂ ꢄꢍ ꢍꢆꢈꢈ 5, C = 2.0 × 10 m (a) ꢇꢌꢃ ꢁꢀꢈ cꢄꢎpꢋꢈxꢈꢂ ꢍꢄꢆꢎꢈꢃ ꢅꢌ
l
2+
2+
2+
–4
ꢁ
ꢀꢈ ꢂꢉꢂꢁꢈꢎ 5-ꢎꢈꢁꢇꢋ ꢅꢄꢌꢂ ꢇꢁ ꢇ 1 : 1 ꢆꢇꢁꢅꢄ: Cꢃ (B), Cꢄ (C), ꢇꢌꢃ Cꢊ (d), C = C = 1.852 × 10 m. Cꢄꢌꢃꢅꢁꢅꢄꢌꢂ:
l m
T = 25.0 ± 0.1 ꢏC, I = 0.1 m KCꢋ.

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 4. mꢄꢋꢈcꢊꢋꢇꢆ ꢂꢁꢆꢊcꢁꢊꢆꢈꢂ ꢄꢍ 4-liCams, edta, ꢇꢌꢃ meCams.
Table 2. Cꢊꢎꢊꢋꢇꢁꢅvꢈ ꢂꢁꢇbꢅꢋꢅꢁꢉ cꢄꢌꢂꢁꢇꢌꢁꢂ (ꢋꢄg β ) ꢄꢍ cꢄꢎpꢋꢈxꢈꢂ ꢄꢍ 5 wꢅꢁꢀ Cꢃ2+, Cꢄ2+, ꢇꢌꢃ Cꢊ2+; pm vꢇꢋꢊꢈꢂ
pqꢆ
ꢍꢄꢆ 5 ꢇꢌꢃ ꢂꢄꢎꢈ ꢆꢈpꢄꢆꢁꢈꢃ ꢋꢅgꢇꢌꢃꢂ ꢇꢁ ph 7.4.ꢇ
log βpqr
Cd2+
Co2+
Cu2+
Compound
(p, q, r)
5
(1, 0, 1)
22.36(4)
29.39(3)
34.50(5)
38.43(4)
39.93(2)
17.10
–
18.68(2)
27.31(4)
34.49(3)
38.96(5)
40.20(2)
14.71
6.5
7.7
14.5
22.53(4)
29.09(5)
34.44(2)
38.43(3)
39.92(3)
17.17
13.6
16.9
16.9
(
(
(
(
1, 1, 1)
1, 2, 1)
1, 3, 1)
1, 4, 1)
pm
b
4-liCams
pm
pm
pm
b
meCams
–
–
edtac
ꢇ
−5
−6
I = 0.1 m KCꢋ, 25.0 ± 0.1ꢏC. pm = −ꢋꢄg [m]ꢍꢆꢈꢈ wꢅꢁꢀ C = 10 ꢇꢌꢃ C = 10 m. βpqꢆ = [m h l ] / ([m]p[h]q[l]ꢆ).
l
m
p q ꢆ
b
c
rꢈꢍ. [26ꢇ].
rꢈꢍ. [26b].
2
+
5
-Co complex and the other two complexes at different pH. At pH 3, [CoH L] exists at 90%
3
of the total metal concentration, while [CdH L] and [CuH L] exist at 80%. Moreover, [CoL] is
3
3
the main species at pH > 9, and [CdL] and [CuL] at pH > 7, indicating that 5 tends to complex
2
+
2+
Cd and Cu if the three metal ions co-exist under neutral condition.
Table 2 lists the cumulative stability constants of complexes of 5 with Cd , Co , and Cu
2+
2+
2+
and pM values for 5 and other reported ligands at pH 7.4. The greater pM value, the stronger
binding affinity is. Cumulative stability constants show that the deprotonated ligand is more
2
+
efficient for metal complexation. The pM values of 5 for Co (14.71) are slightly lower than
2
+
2+
2+
2+
that for Cd (17.10) and Cu (17.17) at pH 7.4, the values for Co and Cu are higher than
known ligands, such as 1,6-bis(2,3-dihydroxy-5-sulfobenzoyl)-1,6-diazahexa (4-LICAMS),
ethylenediaminetetraacetic acid (EDTA), and 1,3,5-tris(2,3-dihydroxy-5sulfobenzoyl)triami-
no-methyl-benzene (MECAMS) [26], whose structures are shown in Figure 4. There has been
2
+
no study performed for Cd complexes using the spectrophotometric titration method,
such that it is hard to compare the pCd value of 5 with other ligands. pCu and pCd values
at neutral pH are similar. This indicates that 5 can chelate cadmium effectively.

8
S. LEI ET AL.
Table 3. pm vꢇꢋꢊꢈꢂ ꢄꢍ cꢄꢎpꢋꢈxꢈꢂ ꢄꢍ 5 wꢅꢁꢀ Cꢃ2+, Cꢄ2+, ꢇꢌꢃ Cꢊ2+ ꢇꢁ ꢃꢅffꢈꢆꢈꢌꢁ ph.
a
2+
2+
2+
pM (M = Cd , Co , Cu )
Metal ion
pH 3.0
pH 7.4
pH 9.0
2+
2+
2+
Cꢃ
Cꢄ
Cꢊ
7.10
7.56
7.09
17.10
14.71
17.17
20.84
17.31
21.01
ꢇpm = −ꢋꢄg [m] wꢅꢁꢀ C = 10 ꢇꢌꢃ C = 10 m.
−5
−6
ꢍꢆꢈꢈ
l
m
The pM values are calculated at pH 7.4, but it is also valuable to study the values under
acidic and basic conditions in the treatment of environmental pollution. Therefore, pM values
2
+
2+
2+
of complexes of 5 with Cd , Co , and Cu at pH 3.0 and 9.0 are also investigated, as shown
in Table 3. The pCo value is generally higher than pCd and pCu at pH 3.0, suggesting that
some complexing atoms (N, O) are capable for metal complexation. At pH 7.4 and 9.0, the
2
+
2+
2+
pM values are similar with the sequence Cu > Cd > Co , indicating that the influential
moiety for complexation is phenolic hydroxyl.
4
. Conclusion
1
13
A new tetrazine-based catecholamide ligand 5 was prepared and characterized by H, C
NMR, and FTIR spectroscopy. Protonation constants were studied by potentiometric titration
showing more acidic property than bidentate dopamine and hexadentate MECAM.
Complexation capacities with cadmium(II), cobalt(II), and copper(II) were investigated by
spectrophotometric titration. The pM values of the complexes were calculated under acidic,
neutral, and basic conditions, and the values indicate that complexation capacities of 5 with
2
+
2+
2+
these metal ions are in the sequence Cu > Cd > Co at pH 7.4. Results show that 5 can
complex the three heavy metal ions effectively and may be used in metal removal.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the National Natural Science Foundation of China [grant number 21301142],
[
[
grant number 51572230]; the Southwest University of Science and Technology Researching Project
grant number 14tdfk05].
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