Catalytic performance and mechanism of Cu(II)-hydrazone complexes as models of galactose oxidase

Article abstract of DOI:10.1016/j.ica.2014.06.031

As simulants of galactose oxidase (GO), three mononuclear Cu(II) complexes with 1-((3-t-Bu,5-R1)-salicylidenehydrazono),2-((3-t-Bu,5-R2)- salicylidenehydrazono)-1,2-diphenylethane (H₂L^Bu,Bu: R1 = R2 = t-butyl; H₂L^MeO,

Full text of DOI:10.1016/j.ica.2014.06.031

Inorganica Chimica Acta 421 (2014) 446–450
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Catalytic performance and mechanism of Cu(II)-hydrazone complexes
as models of galactose oxidase
⇑
Huatian Shi, Yegao Yin
Department of Chemistry, Shantou University, 243 University Road, Shantou 515063, China
a r t i c l e i n f o
a b s t r a c t
Article history:
As simulants of galactose oxidase (GO), three mononuclear Cu(II) complexes with 1-((3-t-Bu,5-R1)-sali-
cylidenehydrazono),2-((3-t-Bu,5-R2)-salicylidenehydrazono)-1,2-diphenylethane (H2LBu,Bu: R1 = R2 = t-
butyl; H2LMeO,MeO: R1 = R2 = methoxyl; H2LBu,MeO: R1 = t-butyl, R2 = methoxyl) as ligands were synthe-
sized. X-ray diffraction defined their structures, like GO, all having a distorted square N2O2-coordinated
Cu(II) center, and catalytic experiments confirmed their abilities to enable the aerobic oxidation of benzyl
alcohol to benzaldehyde under room temperature with turnover numbers up to 823–1036. Voltammetric
measurements indicated that the cupric phenolates are electroactive, in the range of 0.3–0.9 V (vs EFc+/Fc),
all giving two anodic peaks symbolizing the formation of +1 and +2 charged complex radicals. The pres-
ence of radicals was proven by the thianthrene perchlorate titration UV–Vis spectra of 1–3 with showing
two new absorptions typical for phenoxy radicals and by the electronic spin resonance spectra with
revealing an antiferromagnetic coupling of phenoxy radicals with Cu(II) (s = 1/2).
Received 7 March 2014
Received in revised form 12 June 2014
Accepted 20 June 2014
Available online 9 July 2014
Keywords:
Galactose oxidase
Copper(II)
Benzyl aldehyde
Catalysis mechanism
Model
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
[Cu(II)(LMeO,MeO)] (2) and [Cu(II)(LBu,MeO)] (3), of the ligands dis-
criminated by 5,50-substitution of salicyaldehyde, and investigated
Molecularly modeling the metalloproteins that catalyze certain
biochemical processes is one of the major interests of modern
coordination chemistry [1–7]. Of aim to rationally design the
model systems, a hard effort has focused on understanding the cat-
alytic mechanism of metalloproteins, the correlation between the
structures and properties of mimics and their operation mecha-
nism [8–13]. Such being the case of GO, after an intensive investi-
gation of their structure, spectroscopy and catalytic process [14–
19], their phenoxyl radical-involving mechanism has been well
understood [20], which has led to the emergence of a large sum
of phenolate-coordinated Cu(II) complexes to stimulate the active
site and function of GO [21–28]. As highlight of this topical
research, Pierre, Stack and Halcrow have tactfully used salicylalde-
hyde Schiff bases to mimic the proteinic environment of Cu(II) in
GO, thereby revealing that the mimics operation through providing
the phenoxyl radicals of virtue intermediating the hydrogen
abstraction [24–37] and that the performance of model complexes
is subject to the phenol substitution [31–34].
their catalytic performance and their responses to oxidative condi-
tions. The results show that the complexes catalyze the aerobic
oxidation of benzyl alcohol under solventless and room tempera-
ture conditions through a nucleophilic hydrogen abstraction.
2. Experiment
2.1. Materials and methods
2.1.1. General information
The chemicals, including 3,5-di-tert-butylsalicylaldehyde, and
solvents used were available commercially. Oxidant, thianthrene
perchlorate (ThClO4) was prepared by a literature method [38].
To a mixed liquid of HClO4 (0.8 ml) and acetic anhydride (50 ml)
was added
a CCl4 solution (100 ml) of thianthrene (0.5 g,
0.046 mol). The solution was stood overnight and then gave the
crystals of ThClO4. The product was collected by filtration, followed
by washing with CCl4 and drying in vacuum, in yield ca 90% (UV–
As another probe to the effect of varied phenol substitution and
Vis absorption in CH2Cl2: kmax = 550 nm,
e
= 8500 Mꢀ1 cmꢀ1).
extended p-conjugation of ligands on the function of such mimics,
we prepared three complexes, namely [Cu(II)(LBu,Bu)] (1),
2.1.2. Single crystal determination
The single crystals of 1–3 for structural analyses were obtained
from evaporating the reaction solutions of ligands and CuCl2ꢁ4H2O
in ethanol. Single-crystal X-ray diffraction data for 1–3 were
⇑
Corresponding author.
E-mail address: ygyin@stu.edu.cn (Y. Yin).
http://dx.doi.org/10.1016/j.ica.2014.06.031
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

H. Shi, Y. Yin / Inorganica Chimica Acta 421 (2014) 446–450
447
collected on an Agilent Technologies Gemini A System (Mo K
a
,
recrystallizing the precipitates in ethanol, white needle crystals
as dihydrazone were collected in yield of 60% (8 g).
k = 0.710 73 Å) at 298 K. The data were processed using CrysAlis-
Pro.1 and corrected and scaled using the SCALE3 ABSPACK scaling
algorithm. The structures of 1–3 were solved by direct method [39]
and refined by a least-square fitting method using Olex 2 Program
[40].
2.2.3. Synthesis of H2LBu,Bu and H2LMeO,MeO
The symmetric ligands were prepared in parallel with refluxing
the mixture of dihydrazone (1.19 g, 0.005 mol) and 2 eqs of corre-
sponding substituted salicylaldehydes in 100 ml alcohol for 6 h.
For H2LBu,Bu, the dosage of aldehyde is 2.34 g (0.01 mol) and the
yield is 80% (3 g). 1HNMR (400 MHz, CDCl3): d 11.24 (s, 2H, OH),
8.73 (s, 2H, CH = N), 7.90 (m, 4H, ArH), 7.43 (m, 6H, ArH), 7.33 (d,
J = 2.4 Hz, 2H, ArH), 7.07 (d, J = 2.4 Hz, 2H, ArH), 1.32 (s, 18H, 3-
C(CH3)3), 1.25 (s, 18H, 5-C(CH3)3), and, for H2LMeO,MeO, the amount
of aldehyde is 2.08 g (0.01 mol) and the yield is 88% (3 g). 1H NMR
(400 MHz, CDCl3) d 11.04 (s, 2H, –OH), 8.68 (s, 2H, N@CH), 7.96–
7.81 (m, 4H, ArH), 7.52–7.34 (m, 6H, ArH), 6.92 (d, J = 3.0 Hz,
2H,ArH), 6.57 (d, J = 3.0 Hz, 2H, ArH), 3.72 (s, 6H, –OCH3), 1.27 (s,
18H, C(CH3)3).
2.1.3. Cyclic voltammetry
The electrochemical measurements were performed on a Zah-
ner Im6ex system with a standard three-electrode cell, consisting
of a platinum disk as working electrode, a platinum wire as auxil-
iary and a Ag/AgCl as reference. The samples were made of
1.0 ꢂ 10ꢀ3 M CH2Cl2 solutions with adding 0.1 M (n-Bu)4NPF6 as
supporting electrolyte. The potentials were recorded in reference
to E1/2(ferrocenium/ferrocene) (shortened for EFc+/Fc) in a scan rate
100 mV/s.
2.1.4. EPR spectroscopy
Electron paramagnetic resonance (EPR) spectra were deter-
2.2.4. Synthesis of ligand H2LBu,MeO
mined on a Bruker EMX spectrometer at 293 K using 1 ꢂ 10ꢀ3
M
The asymmetric ligand was prepared by first dropwise adding
the ethanol solution (100 ml) of 3,5-di-tert-butyl-2-hydroxyl-
phenzyl aldehyde (1.17 g, 0.005 mol) into a solution (20 ml) of
enzyl dihydrazone (1.19 g, 0.005 mol) at ambient temperature
under stirring within 2 hs, and then a solution (10 ml) of 3-tert-
butyl-5-methoxylsalicylaldehyde (1.04 g, 0.005 mol). After that,
the solution was refluxed for another 2 h and then cooled down
to room temperature. Evaporation of the solution gave the yellow
solids of H2LBu,MeO in yield of 77%. 1H NMR (400 MHz, CDCl3) d
11.23 (s, 1H, –OH), 11.05 (s, 1H, –OH), 8.73 (s, 1H, CH@N), 8.68
(s, 1H, CH@N), 7.93–7.87 (m, 4H, ArH), 7.49–7.37 (m, 6H, ArH),
7.33 (d, J = 2.4 Hz, 1H, ArH), 7.07 (d, J = 2.4 Hz, 1H, ArH), 6.92 (d,
J = 3.0 Hz, 1H,ArH), 6.57 (d, J = 3.0 Hz, 1H, ArH), 3.72 (s, 3H, –
OCH3), 1.32 (s, 9H, C(CH3)3), 1.30 (s, 9H, C(CH3)3), 1.25 (s, 9H,
C(CH3)3).
CH2Cl2 solutions of samples. The frequency of microwave was kept
at 9.810511 GHz, the illuminated energy at 20 mW and the scan
range as 1000 G – 5000 G. Spin quantitation was carried out by
double integration of derivative EPR spectra and normalized to
the intensity before the addition of 1 eq ThClO4.
2.1.5. UV–Vis spectroscopy
The titration UV–Vis spectra of 1–3 were recorded on a Perkin
Elmer Lambda 950 UV–Vis Spectrometer by each time adding
30 ll of saturated ThClO4 solution until twice the equivalent of
complexes.
2.1.6. Catalytic performance
The catalytic performance experiments were performed in the
Schlenk-flask under pure O2 sphere. NaOH (0.2 g) was dissolved
into Benzyl alcohol (20 ml) before the addition of complex
(2 mg). The blank experiment was assembled the same above
except without adding the complex. The mixture resulted with
O2 bubbling and stirring in 20 h was analysed by a Shimadzu
GC–MS spectrometer and the production of benzyl aldehyde was
determined on a Anglient 1100 series HPLC equipment using the
detecting wavelength at 248 nm, with mobile phases CH3CN:H2-
O = 45:55. The results are the average of three runs.
2.2.5. Syntheses ef model compounds 1–3
These complexes were synthesized similarly as follows: To an
alcoholic solution (25 ml) of ligand (0.1 mmol) and NaOH
(0.2 mmol) was added solution (5 ml) of CuCl2ꢁ2H2O (0.02 g,
0.2 mmol). The mixed solution was heated to boil and then cooled
down to room temperature. The solution was evaporated in room
for about 2 days and then gave the black block crystals of complex.
1 (ESI-MS(M+1): 732. Anal. Calc. C, 72.131; N, 7.650; H, 7.104.
Found: C, 71. 93; N, 7.65; H, 7.42%) in yield of 95% (0.07 g) based
on amount of ligand. 2 (ESI-MS(M+1): 680. Anal. Calc. C, 67.059;
N, 8.235; H, 5.882. Found: C, 66.87; N, 8.19; H, 6.176%) in yield
of 88% (0.06 g) and 3 (ESI-MS(M+1): 706. Anal. Calc. C, 69.688; N,
7.932; H, 6.516. Found: C, 69.72; N, 8.16; H, 6.579%) in yield of
89% (0.063 g).
2.2. Synthesis of ligands and complexes
2.2.1. Synthesis of 3-tert-butyl-5-methoxylsalicylaldehyde
The intermediate was prepared by a known method [41]. A
mixture of 3-tert-butyl-4-hydroxyanisole (7.35 g, 40 mmol) and
urotropine (11.25 g, 80 mmol) in glacial acetic acid (40 ml) was
heated at 110 °C for 2 h. After disappearance of 3-tert-butyl-4-
hydroxyanisole, a H2SO4 solution (33%, 40 ml) was added to it at
75 °C. The resulting mixture was heated again at 110 °C for another
3 h and then was extracted with 100 ml of diethyl ether. The
extract was washed with two portions of 100 ml water, then two
portions of 100 ml saturated Na2CO3 solution and finally 100 ml
saturated NaCl solution. The organic layer was dried over MgSO4,
and then the solvent was removed by evaporation under a reduced
pressure. The residue dissolved in CH2Cl2 (10 ml) was passed
through a silica column using CH2Cl2 as eluent to give the aldehyde
as yellow oil in yield of about 60% (5 g).
3. Results and discussion
3.1. Structures and catalytic performance of 1–3
X-ray diffraction defined the structures of 1–3 as the mononu-
clear complexes crystallizing in a monoclinic P21/c space group
(Table S1). Fig. 1 specifies their assembly, indicating that the mim-
ics, like GO [20], all have a N2O2-coordinated Cu(II) center, and,
owing to the repulsion between the phenyls of enzyl, their coordi-
nation centers are obviously distorted from a planar square. Of
note is that, despite the alteration of R1 and R2, the metal-involv-
ing bond facts of 1–3 are not drastically different (Table S2), in a
way, rationalizing the ascription of the function diversity of 1–3
to the effect of substitution. Moreover, it is noticed that the obtuse
N–Cu–O angles of 1–3, as those of salen-type mimics [28], are
2.2.2. Synthesis of benzyl dihydrazone
The intermediate was harvested as white precipitates from
refluxing the ethanolic solution (100 ml) of enzyl (10.5 g,
0.05 mol) and excessive hydrazine (20 ml, 80%) for 6 h. With

448
H. Shi, Y. Yin / Inorganica Chimica Acta 421 (2014) 446–450
Fig. 1. The ORTEP drawings of 1–3 (from left to right) with hydrogen atoms omitted for clarity(Carbon: blue; Oxygen: red; Nitrogen: purple; Copper: brown). (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
larger than 150°, and so we expect the Cu(II) sites to be open,
allowing the attack of alcohol substrate.
1.43 ꢂ 105, for [2⁄]+ is 3.95 ꢂ 104). Logically, the easier formation
and less stability of [2⁄]+ are stemmed in the effect of more elec-
tron-donating of MeO because it enriches the electron on pheno-
On this consideration, we evaluated the catalytic activities of
1–3 by determining the yields of benzyl aldehyde from bubbling
benzyl alcohol containing 0.2 g NaOH with air in the presence of
2 mg complexes for 20 h. Chromatographic analysis verified that
the catalyzed reactions are high yielding with turnover numbers
lates. This is agreed by the CV curve of 3 with giving an Ep1a
,
relating to oxidation of MeO substituted phenolate, close to that
of 2, and a E2pa, relating to oxidation of butylated phenolate, close
to that of 1 [33,43]. This pattern also comes to a conclusion that
these oxidations are ligand-base [33].
as 823, 1036 and 979 (turnover frequency(TOF): 0.0114 sꢀ1
,
0.0144 sꢀ1, 0.0136 sꢀ1), average data of three independent catalytic
runs, and there is no any benzoic acid found in the blank and
catalytic experiment, which means that no further oxidation of
benzyl aldehyde happens in the system. This result comparable
with the reported [28], and so promises the application of 1–3 as
catalysts for aerobic oxidation of benzyl alcohol in solventless
and room temperature conditions.
Associating the electrochemical readings to the catalytic effi-
ciencies of 1–3, we come to an conclusion that the activities of
the model compounds are subject to the substitution of phenol,
or saying, an electron enrichment on phenol benefits the catalysis
of complexes [31]. The conclusion implicates that the hydrogen
abstraction of benzalcohol is actually a nucleophilic process. This
is supported by the fact that the tyrosyl radicals in photosystem
II and ribonucleotide reductase are hydrogen-bonded to acidic res-
idues [44,45]. Also, it is noticeable that the hydrazone adducts,
compared with those of the non-conjugating salen-type ligands,
are less oxidizable with the E1pa more positive [22,32]. To this
observation, a possible account is that extended conjugation of
3.2. Responses of 1–3 to oxidative conditions
It is well known that Cu(II)-phenolate systems are electroactive
[42]. To have an insight into the responses of 1–3 to aerobic condi-
tion (EO2/OH– = 0.61 V versus EFc+/Fc), we examined their redox
behavior in the range of ꢀ0.5–1.0 V, thereby obtaining the voltam-
mograms shown in Fig. 2, which revealed a variety of these model
compounds in oxidation propensity. It is easily seen that the plots
of 1 and 2 are similarly exhibiting two quasi-reversible redox pairs,
ligands disperses the electrons on phenol oxygen by p–p conjuga-
tion, which has been proved by the comparison of the oxidation
potentials of Schiff bases of saturated [22,32,33] and unsaturated
diamines [27,32]. In this sense, an extended conjugation of ligand
makes the formation of univalent radicals difficult, and hence low-
ers the activity of complexes.
which can be assigned to the processes [CuL] M [CuL⁄]+ M [CuL⁄⁄ 2+
]
[26,27,32]. However, a scrutiny reveals that the E1pa of 2 is less posi-
tive than that of 1, so meaning an easier formation of [2⁄]+ than
As evidence of the presence of phenoxy radicals under oxidative
condition, we examined the change of UV–Vis spectra of 1–3 with
titration by ThClO4 (E1/2 = 0.89 V versus EFc+/Fc) [46], an oxidant
that expectedly can bring the production of cationic radicals
(Scheme 1). Fig. 3 gives the spectra before addition (more details
are presented in Table S4), indicating that the complexes are akin
showing the bands arising from intraligand charge transfer (ILCT)
in the range of 250–300 nm, from ligand–metal change transfer
(LMCT) near 450 nm and from d–d transition at ca 670 nm [47].
Of the bands, LMCT is most diagnostic of the effect of substitution
since its energy is directly relating to the electronic affinity of
ligand [48]. This is proven true by the LMCT of 1 with highest
energy relating to weak electron-donation of t-Bu, of 2 with a low-
est energy to strong donation of MeO, and of 3 covering the peaks
of 1 and 2 to symbolize both the two transitions.
[1⁄]+, and the separation
D
Epa (E2pa – E1pa) of 2 is smaller (the
detailed information is presented in Table S3), indicating a less sta-
bility of [2⁄]+ (the disproportionation constant Kc for [1⁄]+ is
As represented by the spectra of 2 (Fig. 4) (those of 1 and 3 are
given in Fig. S1), with adding ThClO4 the LMCTs of 1–3 decrease
gradually and meanwhile the new bands attributable to the
absorptions of +1 charged radicals [49–54], are formed in the range
Fig. 2. The voltammograms of 1–3 with the peak potentials recorded in reference to
Scheme 1. The possible reactions of 1–3 (generally denoted as [CuL]) titrated by
EFc+/Fc
.
ThClO4.

H. Shi, Y. Yin / Inorganica Chimica Acta 421 (2014) 446–450
449
alyze the aerobic oxidation of benzyl alcohol to benzaldehyde
under solventless and ambient temperature conditions. Combined
electrochemical, spectroscopic and magnetochemical investiga-
tions proved their catalytic mechanism through providing mono-
phenoxy radicals as Lewis bases to intermediating hydrogen
abstraction.
Notes
The authors declare no competing financial interest.
Acknowledgements
This work is financially supported by the National Natural Sci-
ence Foundation (20971083) and the Natural Science Foundation
of Guangdong Province (9151503101000629).
Fig. 3. The UV–Vis absorption spectra of 1–3 in dichloromethane.
Appendix A. Supplementary material
CCDC 990098–990100 contains the supplementary crystallo-
graphic data for 1–3. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.06.031.
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