CO2 fixation by dicopper(ii) complexes in hypodentate framework of N8O2

Article abstract of DOI:10.1039/c3dt53497a

A new ligand with N₈O₂ donors containing three potential metal-binding sites (H₂L) and its tricopper(ii) complex 1 are synthesized. The tricopper species is found to be formed from a hypodentate dicopper(ii) complex 2 in basic solutions. Complex 2 may be isolated from the reaction of H₂L with a copper source under acidic conditions. Complex 2 can undergo CO₂-abstraction to yield an octacopper(ii) complex 3. The single crystal structures of complexes 2 and 3 are characterized by X-ray crystallography. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53497a

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CO2 fixation by dicopper(II) complexes in
hypodentate framework of N8O2†
Cite this: Dalton Trans., 2014, 43,
6287
Yi-Hsueh Ho, Mu-Chieh Chang, Kuo-Hsuan Yu, Yi-Hung Liu, Yu Wang,
Yuan-Chung Cheng* and Jwu-Ting Chen*
Received 13th December 2013,
Accepted 28th January 2014
DOI: 10.1039/c3dt53497a
www.rsc.org/dalton
A new ligand with N8O2 donors containing three potential metal-
binding sites (H2L) and its tricopper(II) complex 1 are synthesized.
The tricopper species is found to be formed from a hypodentate
dicopper(II) complex 2 in basic solutions. Complex 2 may be iso-
lated from the reaction of H2L with a copper source under acidic
conditions. Complex 2 can undergo CO2-abstraction to yield an
octacopper(II) complex 3. The single crystal structures of com-
plexes 2 and 3 are characterized by X-ray crystallography.
Scheme 1 Synthesis of the N8O2 polydentate ligand.
Polydentate ligands containing multi-metal coordination sites
are interesting research targets either in metalloproteins or for
designing novel structure–reactivity relationships. Investi-
gation into p-MMO metalloenzymes has disclosed a tricopper(II)
complex with a rarely known triangular subunit in a cyclic
skeleton of (CuO)3.1 The constitution of the ligand in this tri-
copper(II) complex is composed of three tetradentate com-
ponents of N2O2 donors with the assistance of two alkoxides
and one oxide ion. All three copper centers are in a square
planar configuration, although two kinds of coordination sites
are involved.1
Although trinuclear copper(II) complexes with multidentate
ligands in other configurations have been previously reported,
the detailed coordination chemistry of such triangular tri-
copper(II) species remain elusive.2,3 In order to understand the
fundamental properties of coordination in such compounds,
we introduced bipyridinyl amine which provides a tridentate
coordinating mode and is supposed to offer better coordinat-
ing power into the ligand.4
component, for such a purpose has been reported.6 Herein, we
report a new polydentate ligand with N8O2 donors that exhibits
a hypodentate dinuclear complex without a hydroxide-bridging
moiety, and can show reactivity towards CO2 fixation.
The synthesis of [(2-Py2CH2)2NCH2CH(OH)CH2]2(c-C4H8N2)
(H2L) was succeeded first by the reaction of piperazine with
epichlorohydrin in double molar amounts to form the alcohol
derivative, and was then followed by the reactions with bis-
((pyridine-2-yl)methyl)amine to yield the desired product in
33% yields (Scheme 1).2
The reaction of H2L and three molar amounts of hexaaquo-
copper(II) perchlorate in acetonitrile with the addition of four
molar amounts of triethylamine readily results in the for-
mation of a deep green complex 1. The electronic spectra show
λmax appearing at 697 nm (ε, 196 M−1 cm−1) and a shoulder
at ca. 860 nm (ε, ∼180 M−1 cm−1). Such spectral data imply
that complex 1 may consist of copper(II) centers in both
square planar and five-coordinate trigonal bipyramidal
configurations.7
In addition, CO2 fixation has recently been becoming a
more important topic in coordination chemistry.5 Using
The measurement of the molar conductivity for 1 in aceto-
nitrile (5 × 10−4 M) at 25 °C was 353 Ω−1 cm2 mol−1, support-
ing that the complex salt is a trivalent perchlorate.8 A peak at
998.0325 m/z found by HR-ESI-MS matches with a tricopper
cation in the form of [LCu3(OH)(ClO4)2]+, in which a hydroxide
presumably bridges two copper ions. In the infrared spectra,
the uncoordinated perchlorate anions were evidenced at 1088
copper complexes, particularly with
a Cu(II)–O(H)–Cu(II)
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
E-mail: jtchen@ntu.edu.tw; Tel: +886 233661659
†Electronic supplementary information (ESI) available: The characterization and
and 624 cm−1
.
synthetic procedure of the new compounds; UV, CV and EPR spectral data; crys-
tallographic data of 2 and 3 in CIF format; magnetic properties of 1 and 3; ESI
titration spectrum of the ligand mixing with copper. CCDC 976265–976266. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3dt53497a
The half-wave potential (E1/2) of complex 1 was measured by
cyclic voltammetry in deoxygenated acetonitrile at 25 °C. Two
cathodic waves at −0.58 V and −0.80 V correspond to the
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Fig. 1 Optimized DFT structures for complex 1 (R,S-, left; R,R-, right).
Scheme 2 Synthesis of the N8O2 hypodentate dicopper(II) complex and
tricopper(II) complex.
reduction of three Cu(II) atoms.9 The smaller reversible signal
at −0.80 V may be assigned to the central component with
N2O2 coordination and the larger irreversible peak at −0.58 V
corresponds to the two terminal coordinations with N3O2. One
quasi-reversible anodic wave at +1.42 V, that may be assigned
to the oxidation of two Cu(II) atoms in an N3O2 environment,
was also observed.
The X-band EPR spectrum for 1 as a powdered sample
shows a ground state of S = 1/2 at either 298 or 77 K, although
without clear hyperfine splitting.2 An axial pattern with g||
=
2.218 > 2.1 > g⊥ = 2.04 > 2.00 may be explained by a tricopper(II)
species with the spin localized on the dx2−y2 orbital of an
unpaired copper(II) ion in square planar or square pyramidal
geometry.10,11 The strong antiferromagnetic coupling in 1 is
confirmed by SQUID.
Fig. 2 ORTEP plot and selected atom-labeling scheme of the cationic
part of 2 at 150 K, shown with 50% probability displacement ellipsoids.
H-atoms and counteranions have been omitted for clarity.
DFT calculations for the structure of 1, performed with the
Gaussian 09 package using B3LYP and the 6-31G* basis set,12
provide the expected structure, in which three Cu(II) ions in a
triangular arrangement are bridged by three oxygen atoms to
constitute a (CuO)3 ring subunit which is surrounded by L as a
decadentate. There is one copper center in a distorted square
planar configuration with N2O2 coordination, and the other
two copper(II) centers, which hold N3O2 coordination and
share a hydroxide bridge, are in trigonal bipyramidal/distorted
trigonal bipyramidal configurations with τ = 0.94 and 0.57,
respectively.13 Two possible diastereomers are considered
(Fig. 1). The R,S-form is only 0.6 kcal per mole more stable
than the R,R-form.14 The TD-DFT calculated electronic spec-
trum of 1 with the optimized structure shows similar UV peaks
to the experimental data.
same reaction gives a peak at m/z 1021 amu, corresponding to
a hypodentate dinuclear cation that may fit the formula [(H2L)-
Cu2(ClO4)3]+. The formation of 1 was not observed. For syn-
thetic purposes, indeed, in a reaction of H2L and Cu(ClO4)2 in
acidic conditions, a dinuclear complex in the form of [H4LCu2]-
(ClO4)6 (2) has been successfully isolated. Treating 2 with
sufficient NaOH or triethylamine can result in the formation
of 1 (Scheme 2).
Single crystals suitable for X-ray crystallographic analysis
were obtained from cosolvents of dichloromethane–aceto-
nitrile. The structure of 2 in C2/c symmetry has an inversion
center (Fig. 2). The distance between the copper ions is
extended to 12.9 Å.11 The Addison–Reedijk parameter, τ, is
0.08, which agrees with the UV-Vis data.15 The Cu–N bonds
(2.0–2.1 Å) are of normal lengths.16 However, the long Cu–O
bonds (2.4 Å) are likely from alcohol coordination.16 The para-
An N8 ligand which comprises the combination of pipera-
zine and bisimidazolyl amine was reported to also show
coordination with di- and tri-copper.4 On comparing this tri-
copper complex with 1, the latter may form a distorted hexago-
nal (CuO)3 ring skeleton with two alkoxides in L and one
bridging hydroxide, as shown in Fig. 1.
magnetic complex
2 has six perchlorate counteranions,
suggesting that the two nitrogen atoms of piperazine should
be protonated.
The hypodentate dinuclear complex 2 is expected to be
active due to its available coordination capacity. It is found
that exposing 1 to air under neutral or acidic conditions
results in the formation of a new species. In the reaction of 2
and Cu(ClO4)2 either under atmospheric conditions or in
the presence of NaHCO3, a product labeled as 3 is identified
Spectrophotometric titrations for the reaction of H2L and
[Cu(H2O)6](ClO4)2 in the ratio of 1 : 3 in acetonitrile without
the addition of Et3N show that the solution color becomes
dark blue at pH 5.30. A maximum absorption grows at 623 nm
(ε, 152 M−1 cm−1), which is consistent with a square planar or
square pyramidal configuration.7 The ESI-MS spectrum for the
6288 | Dalton Trans., 2014, 43, 6287–6290
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Conclusions
In conclusion, a new polydentate ligand with N8O2 donors has
been successfully synthesized from the condensation of two
bispyridyl amines and a piperazine bisalcohol derivative. Such
a ligand can bind Cu(II) ions first through the bispyridyl moi-
eties to yield a new hypodentate dinuclear complex in the form
of [H4LCu2](ClO4)6. With the assistance of hydroxide,
[H4LCu2]6+ can react with another Cu(II) ion to generate a tri-
copper complex in which the metal and oxygen atoms form a
(MO)3 ring skeleton, which is interesting to metalloenzymes.
Exposing [H4LCu2](ClO4)6 to CO2 results in a new complex in
the form of [L2Cu8(CO3)2(OH)2]6+, which is interesting for CO2
capture and storage.
Acknowledgements
This work was supported by the National Science Council,
Taiwan, ROC. We thank Y.-C. Su for the SQUID measurement,
S.-Y. Sun and P.-Y. Lin for the high resolution ESI-MS spectra,
and Y.-C. Wu for the EPR spectra.
Scheme 3 The formation pathway for 3.
by HR-ESI-MS with a peak at m/z 714.3063, which may
be
assigned
as
a
complex
with
the
formula
[(C34H42N8O2)2Cu8(OH)2(CO3)2(ClO4)4]3+. In an alternative feas-
ible process, H2L and Cu(ClO4)2 are mixed in acetonitrile in
the presence of dry ice. After 10 minutes, the resulting solution
may be introduced into H2O to form the desired light blue
product (Scheme 3).
Notes and references
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6290 | Dalton Trans., 2014, 43, 6287–6290
This journal is © The Royal Society of Chemistry 2014