A copper-cyclen coordination complex associated with a polyoxometalate anion: Synthesis, crystal structure and electrochemistry of [Cu(cyclen)(MeCN)] [W6O19]

Article abstract of DOI:10.1016/j.inoche.2010.06.013

The reaction between Cu(NO₃)₂?3H₂O and a tetra-aza macrocycle, more specifically, cyclen in 1:1 MeCN-MeOH solvent mixture forms a Cu²⁺-cyclen coordination complex in situ, that has been reacted with an isopolyanion [W₆O₁₉]^2- in a slow diffusion technique, resulting in the isolation of an ion-pair solid [Cu(cyclen)(MeCN)][W₆O₁₉] (1). Single crystal structural investigation on 1 shows a square pyramidal geometry around the metal centre (copper ion) with an axially bound MeCN solvent molecule. The title compound 1 is the first crystallographically characterized ion-pair compound, in which a transition metal coordination complex of a tetra-aza-crown ether (cyclen) has been associated with a polyoxometalate cluster anion. This communication deals with synthesis, spectroscopic, structural and electrochemical analyses of compound 1.

Full text of DOI:10.1016/j.inoche.2010.06.013

Inorganic Chemistry Communications 13 (2010) 1114–1117
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Inorganic Chemistry Communications
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A copper–cyclen coordination complex associated with a polyoxometalate anion:
Synthesis, crystal structure and electrochemistry of [Cu(cyclen)(MeCN)][W6O19]
⁎
Monima Sarma, Tanmay Chatterjee, Samar K. Das
School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad 500046, Andhra Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction between Cu(NO3)2·3H2O and a tetra-aza macrocycle, more specifically, cyclen in 1:1 MeCN–
MeOH solvent mixture forms a Cu2+–cyclen coordination complex in situ, that has been reacted with an
Received 29 April 2010
Accepted 2 June 2010
Available online 9 June 2010
2−
isopolyanion [W6O19
]
in a slow diffusion technique, resulting in the isolation of an ion-pair solid [Cu
(cyclen)(MeCN)][W6O19] (1). Single crystal structural investigation on 1 shows a square pyramidal geometry
around the metal centre (copper ion) with an axially bound MeCN solvent molecule. The title compound 1 is
the first crystallographically characterized ion-pair compound, in which a transition metal coordination
complex of a tetra-aza-crown ether (cyclen) has been associated with a polyoxometalate cluster anion. This
communication deals with synthesis, spectroscopic, structural and electrochemical analyses of compound 1.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Polyoxometalate
Cyclen
Crystallography
Spectroscopy
Electrochemistry
1,4,7,10-tetraazacyclododecane (commonly known as cyclen) is a
well known aza-crown ether in the area of coordination chemistry
owing to its affinity to form coordination complexes with transition
metals [1]. Coordination complexes of cyclen have versatile applica-
tions, for example, in the areas of medical science [2–4], biology [5–7],
photochemistry and sensor materials [8–11] etc. Till date, several
transition metal complexes of simple or functionalized cyclen have been
reported [1–11]. Interestingly, aza-crown macrocycles have the poten-
tial to reduce the toxicity of toxic metal environment/surrounding
through their strong coordination with concerned metal ions. However,
such supramolecular complex species are generally soluble in common
laboratory solvents, as a result of which, separation of the concerned
metal ions from solution becomes difficult. In this context, polyoxome-
talate (POM) cluster anions can play an important role in the sense that,
they can easily be associated with such soluble supramolecular complex
cations (ion-pair association/adduct formation) which are generally
insoluble in common solvents. In other words, combination of the aza-
crown-ethers with POM anions can be used to separate specific metal
ions (depending on cavity size of the aza-crown ethers) from an
aqueous solution by precipitation method. Usage of the POM cluster
anions in combination with the crown ethers has been described in the
separation of radioactive wastes [12–14]. Structural, topological and
electronic diversities are the key aspects of the chemistry associated
with POM anions [15–18]. Inorganic solids, based on POM counter
anions and supramolecular and/or coordination complexes of crown
ethers [19–28] or porphyrins [29] as cations, have drawn considerable
interest in contemporary research as new functional materials, in
addition to the separation aspect. However, no structurally character-
ized coordination complexes of the aza-crown ethers associated with a
POM counter anion have been demonstrated yet in literature [30]. The
major difficulty in isolating such crystalline materials lies in the fact that,
very often the physical mixture of the components ends up with an
immediate formation of precipitate that inhibits complete character-
ization of such synthesized materials. Our preliminary investigation on
coordination complexes of tetra-aza-crown ethers associated with POM
anions has prompted us to synthesize an ion-pair compound [Cu
(cyclen)(CH3CN)][W6O19] (1) through a slow diffusion technique.
Various synthetic routes/strategies have been developed, so far, to
obtain cyclen in small to bulk quantities [31–34]. We have taken up
one of them. First, diethanolamine and diethylenetriamine have
undergone base mediated tosylation reaction to yield the tosylated
derivatives 2a and 2b respectively.
The next step involves the base mediated cyclization between –OTs
and –NHTs groups of the tri-tosylated diethanolamine (2a) and tri-
tosylated diethylenetriamine (2b) under “high dilution condition” to
obtain the cyclic product cyclen tetratosylate (2c). It has been then
protonated with conc. H2SO4 (see Scheme 1). Direct mixing of a
solution (MeCN, MeOH etc.) containing a Cu2+ salt and cyclen (i.e. Cu–
cyclen coordination complex) with [Bu4N]2[W6O19] results in the
immediate formation of a blue precipitate, which cannot be charac-
terized unambiguously. Thus, single crystals of the compound [Cu
(cyclen)(CH3CN)][W6O19] (1), suitable for X-ray crystallography, have
been grown by a slow diffusion technique [35], more specifically, in a
U-tube set-up as shown in Fig. 1. In this technique, two different
solutions containing the reacting substances are allowed to diffuse
⁎
Corresponding author. Tel.: +91 40 2301 1007; fax: +91 40 2301 2460.
E-mail address: skdsc@uohyd.ernet.in (S.K. Das).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.06.013

M. Sarma et al. / Inorganic Chemistry Communications 13 (2010) 1114–1117
1115
Characteristic IR absorptions of the hexatungstate POM anion appear as
strong bands centred at 983 cm−1 and 808 cm−1 that correspond to the
W=O stretching and W–Ob–W bending motions of the cluster anion
respectively. A broad band in the visible region, centred at 610 nm in the
electronic absorption spectrum of 1 (DMSO, ca. 10−4 M), is assigned to
the d–d transition in a Cu2+ (d9) system (see Supplementary material).
The four-component hyperfine ESR spectrum of 1 (frozen DMSO,
−140 °C) with g|| =2.28 (A|| =179–198G) and g⊥=2.07 suggests the
tetragonally elongated environment around Cu2+ (d9) ion with g|| Ng⊥
[37–39] (see Fig. 2). Furthermore, the diffuse reflectance spectrum of
compound 1 shows a very weak absorption at 1458 nm that might be
assigned to the characteristic peak for a square pyramidal Cu2+ complex
[40]. Based on the above spectroscopic observations, it can be inferred
that the inorganic-solid 1 comprises of a square pyramidal CuII–cyclen as
the cation which might be axially coordinated to an acetonitrile molecule
2−
and the cation is associated with a POM anion [W6O19
]
.
Single crystal X-ray structure determination on a dark blue crystal
of compound [Cu(cyclen)(CH3CN)][W6O19] (1), indeed, illustrates
good convergence with the spectroscopic structure prediction. The
crystal structure of compound 1 is elucidated by the assembly of the
molecules obeying C2/c space symmetry with half of the [Cu(cyclen)
2−
(MeCN)]2+ cation and half of the [W6O19
]
isopolyanion in its
asymmetric unit (Z′=½). The crystal structure of the compound 1 has
been shown in Fig. 3. As shown in Fig. 3, the coordination geometry
around copper is square pyramidal in which the four nitrogen atoms
of cyclen provide the basal coordination and a solvent acetonitrile
molecule defines the axial coordination of the square pyramid. The
POM anion is positioned at the centre of the crystallographic [100]
axis. O(10) i.e. the central oxygen atom of the hexatungstate anion is
located at the crystallographic inversion centre (0.5, 0.0, 0.0) with half
occupancy in the asymmetric unit. Cu(1)–N(3)–C(5)–C(6) axis of the
square pyramid projects parallel to the two-fold rotational axis of a
monoclinic crystal system, as a result of which, these atoms are found
to have half occupancies in the asymmetric unit of the concerned
crystal structure. The methyl protons of the MeCN molecule are
symmetrically disordered over two positions with respect to the two-
fold rotational symmetry passing through the Cu–MeCN bond. Thus,
these atoms have been refined after fixing their occupancies to 0.5
(AFIX 133). The cavity size of the tetra-aza-crown ether cyclen is not
large enough for complete encapsulation of Cu2+ due to the larger size
of the cation. Thus, in the crystal structure of compound 1, the Cu2+
ion is located 0.564(8)Å away from the basal {N4} plane of the
macrocycle (Fig. 3) and the macrocycle is found to be bent outwards
from Cu2+ ion for facile Cu–N coordination. In the relevant crystal
Scheme 1. Synthetic route used to obtain the macrocycle, cyclen.
slowly into each other through a Grade-4 silica crucible in a U- or H-
shaped shell. The slow mixing of the reacting components results in
lower molecular concentration of the resulting product (which is less
soluble) that helped us to isolate good crystalline materials of 1,
suitable for facile crystallographic characterization. The title com-
pound 1 has been isolated as a deep blue crystalline solid and its
structure has been determined through spectroscopic analyses and
unambiguously by single crystal X-ray structure determination [36].
The infrared spectrum of compound 1 (KBr disc) shows the
characteristic features for the tetra-aza-crown ether and the POM
anion. Strong IR band at 3250 cm−1 is assigned to the asymmetric and
symmetric N–H stretching vibrations of cyclen whereas, the band for –C
(sp3)–H stretching at 2949 cm−1 is weakly intense. The medium
intensity and sharp IR band at 2310 cm−1 might be attributed to Cu
coordinated MeCN molecule (see crystallographic discussions, vide infra).
Fig. 1. Slow diffusion between two components in a U-shaped cell for the growth of
good quality crystals suitable for single crystal XRD analysis.
Fig. 2. ESR spectrum of 1 recorded at −140 °C (frozen DMSO).

1116
M. Sarma et al. / Inorganic Chemistry Communications 13 (2010) 1114–1117
structure, the bond lengths of cyclen are observed as an average C–C
1.512(20) Å and average C–N 1.431–1.507(19) Å. The average Cu–N
bond length for the [Cu(cyclen)(MeCN)]2+ complex cation is found to
be 2.019(12) Å for cyclen coordination whereas, the apical MeCN
molecule is located at a distance of 2.071(14)Å from Cu(1) (see
caption of Fig. 3 for the relevant bond distances). Extensive C–H⋯O
and N–H⋯O supramolecular hydrogen bonding interactions between
[Cu(cyclen)(MeCN)]2+ counter cation and the POM anion have been
observed in the crystal lattice of compound 1 (see Fig. 3). Along with
the coulombic interactions between the cationic and anionic counter
parts of compound 1, the supramolecular C–H⋯O and N–H⋯O
hydrogen bonding interactions between them assemble six POM
cluster anions around the cationic complex (see Fig. 3 and caption of
Fig. 3 for the geometrical parameters for the relevant supramolecular
interactions). The Cu–cyclen complexes pack in such a way that a
channel-like arrangement is formed in the crystal lattice of 1 (Fig. 3,
bottom) in which the POM cluster anions occupy the voids through
2−
coulombic and H-bonding interactions. The isopolyanion [W6O19
]
(also known as Lindqvist type isopolyanion) comprises of six edge
shared {WO6} octahedra. All the W atoms in the isopolyanion reside in
a distorted octahedral geometry bonded to one terminal (Ot), four
bridging (Ob) and one central (Oc) oxygen atoms. Thus, the W–O
bonds in the isopolyanion can be grouped into three categories and
these bond lengths are observed as W–Ot 1.686(9)–1.700(9) Å, W–Ob
1.916(7)–1.948(9) Å and W–Oc 2.317(1)–2.339(4) Å in the crystal
structure of compound 1.
Cyclic voltammetric experiment on compound 1 has been
performed in DMSO (c.a. 10−3 M) at 298 K. The relevant experiment
includes three electrode assembly consisting of Pt as the working
electrode, Ag/AgCl as the reference electrode, a Pt gauze as the
counter electrode and tetrabutylammonium perchlorate (TBAP) as
the reference electrolyte. The relevant cyclic voltammogram has been
shown in Fig. 4. In order to comprehend any variation in red-ox
properties of the metal macrocyclic complex in presence of a POM
anion in solution, we have also performed cyclic voltammetry on the
starting precursors, Cu(cyclen)(NO3)2 and [Bu4N]2[W6O19] (see
Supplementary material for the relevant voltammograms). The Cu
(cyclen)(NO3)2 complex shows an almost irreversible reductive
response at −0.75 V vs Ag/AgCl, which has been assigned to the
red-ox couple [Cu(cyclen)]2+/[Cu(cyclen)]+. But, in the cyclic
voltammogram of compound 1 (Fig. 4) two important phenomena
have been observed: (i) the almost reversible nature of the [Cu
Fig. 3. Top: thermal ellipsoidal plot of compound 1 in 20% probability level. Only the
asymmetric unit has been labeled. Only some of hydrogen atoms have been shown for
clarity. Middle: hydrogen bonding environment around the Cu–cyclen coordination
complex. Bottom: perspective view of the crystal packing viewed down the
crystallographic c-axis. Geometrical parameters for hydrogen bonding interactions: C
(1)–H(1A)⋯O(5) 0.97, 2.68 and 3.601(15) Å, 159.4° (−x+0.5, y+0.5); C(2)–H(2A)⋯O
(3) 0.97, 2.50 and 3.315 (14) Å, 141.4° (−x+1, y, −z+0.5); N(1)–H(1 N)⋯O(7) 0.88
(11), 2.44(11) and 3.301(13) Å, 169(10)° (x, −y, z+0.5); N(2)–H(2 N)⋯O(4) 0.79(11),
2.46(12) and 3.175(14) Å, 151(11)°. Selected bond lengths: C(1) N(1) 1.432(17), C
(2) N(1) 1.503(16), C(2) C(3) 1.511(17), C(3) N(2) 1.432(18), C(4) N(2) 1.508(18), C
(5) N(3) 1.11(2), C(5) C(6) 1.44(2), Cu(1) N(2) 2.004(12), Cu(1) N(1) 2.035(11), Cu
(1) N(3) 2.070(14).
Fig. 4. Cyclic voltammogram of 1 in DMSO at 25°( 2) with a scan rate of 0.1 V s−1
.

M. Sarma et al. / Inorganic Chemistry Communications 13 (2010) 1114–1117
1117
(cyclen)]2+/[Cu(cyclen)]+ couple and (ii) considerable shift of the
red-ox potential of the [Cu(cyclen)]2+/[Cu(cyclen)]+ couple (from
[5] S. Aoki, E. Kimura, Chem. Rev. 104 (2004) 769 and the references therein.
[6] K.L. Haas, K.J. Franz, Chem. Rev. 109 (2009) 4921 and the references therein.
[7] S. Mukhopadhyay, S.K. Mandal, S. Bhaduri, W.H. Armstrong, Chem. Rev. 104
(2004) 3981 and the references therein.
[8] J. Svoboda, B. König, Chem. Rev. 106 (2006) 5413 and the references therein.
[9] K. Binnemans, Chem. Rev. 109 (2009) 4283 and the references therein.
[10] R. Martínez-Máñez, F. Sancenón, Chem. Rev. 103 (2003) 4419 and the references
therein.
−0.75 V to −0.45 V vs Ag/AgCl), while the red-ox response
2−
corresponding to the POM cluster anion (W6O ) remains almost
19
constant. Previously reported electrochemical studies on Cu–cyclen
complexes have shown that voltammograms for the [Cu(cyclen)]2+
/
[Cu(cyclen)]+ couple were mostly irreversible [41–43]. Because of
some conformational change/change of geometry around copper ion
in the Cu–cyclen complex (e.g., from square planar to tetrahedral), the
cyclen cavity cannot bind and/or stabilize the Cu1+ oxidation state
[44–46]. It has been observed in some cases that solvent effects can
overcome this problem of irreversibility by its coordination to Cu1+
centre when the chemical processes associated with these structural
changes are fast enough on the electrochemical scale thereby showing
reversibility [46]. But in the case of compound 1, probable reason for
the reversible nature of the concerned couple in presence of a POM
anion in solution is not so clear to us.
In conclusion, we have shown, for the first time, a crystallographic
evidence for the co-existence of a transition metal aza-crown ether
complex with a polyoxoanion in compound [Cu(cyclen)(CH3CN)]
[W6O19] (1). We have demonstrated that the red-ox potential of the
couple [Cu(cyclen)]2+/[Cu(cyclen)]+ has been altered (as compared
to that of same couple in bare Cu–cyclen complex) in presence of an
isopolyanion. This study clearly suggests that the title complex is a
better oxidizing agent when it is supramolecularly associated with a
POM anion. Rigorous investigations on electrochemical and catalytic
behaviours of transition metal aza-crown ether complexes containing
POMs as the counteranions are in progress in our laboratory.
[11] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515 and the references therein.
[12] P. Rajec, V. Švec, V. Mikulaj, R. Hanzel, J. Radioanal. Nucl. Chem. 229 (1998) 9.
[13] S.–R. Sheen, J.–S. Shih, Analyst 117 (1992) 1691.
[14] L.A. Fernando, M.L. Miles, L.H. Bowen, Anal. Chem. 52 (1980) 1115.
[15] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, 1983.
[16] M.T. Pope, A. Müller, Polyoxometalate Chemistry, Kluwer, Dordrecht, 2001.
[17] D. Hagrman, P.J. Hagrman, J. Zubieta, Angew. Chem., Int. Ed. 38 (1999) 2638.
[18] Topical issue on polyoxometalates: C.L. Hill, Guest Ed. Chem. Rev. 98 (1998)
1–389 and the references therein.
[19] Y. Li, N. Hao, E. Wang, M. Yuan, C. Hu, N. Hu, H. Jia, Inorg. Chem. 42 (2003) 2729.
[20] W. You, E. Wang, Y. Xu, Y. Li, L. Xu, C. Hu, Inorg. Chem. 40 (2001) 5468.
[21] W.S. You, E. Wang, H. Zhang, L. Xu, Y.B. Wang, J. Mol. Struct. 554 (2000) 141.
[22] W.S. You, E. Wang, L. Xu, Z.M. Zhu, Y.P. Gu, J. Mol. Struct. 605 (2002) 41.
[23] W.S. You, E. Wang, Q.L. He, L. Xu, Y. Xing, H.Q. Jia, J. Mol. Struct. 524 (2000) 133.
[24] Y.G. Li, E. Wang, S.T. Wang, Y. Lu, C.W. Hu, N.H. Hu, H.Q. Jia, J. Mol. Struct. 607
(2001) 133.
[25] P.C.R. Soares-Santos, L.C. Silva, F.L. Sousa, H.I.S. Nogueira, J. Mol. Struct. 888
(2008) 99.
[26] C. Streb, T. McGlone, O. Brücher, D.-L. Long, L. Cronin, Chem. Eur. J. 14 (2008) 8861.
[27] Y. Zhao, Z. Shi, S. Ding, N. Bai, W. Liu, Y. Zou, G. Zhu, P. Zhang, Z. Mai, W. Pang,
Chem. Mater. 12 (2000) 2550.
[28] T. Akutagawa, D. Endo, F. Kudo, S.-I. Noro, S. Takeda, L. Cronin, T. Nakamura, Cryst.
Growth Des. 8 (2008) 812.
[29] A. Falber, B.P. Burton-Pye, I. Radivojevic, L. Todaro, R. Saleh, L.C. Francesconi, C.M.
Drain, Eur. J. Inorg. Chem. (2009) 2459 and the references therein.
[30] CSD version 5.30, November 2008 at University of Hyderabad.
[31] J.M. Wilson, F. Giordani, L.J. Farrugia, M.P. Barrett, D.J. Robins, A. Sutherland, Org.
Bimol. Chem. 5 (2007) 3651.
[32] G. Hervé, H. Bernard, N. Le Bris, J.-J. Yaouanc, H. Handel, L. Toupet, Tet. Lett. 39
(1998) 6861.
[33] D.P. Reed, G.R. Weisman, Org. Synth. 10 (2004) 667.
Acknowledgements
[34] T.J. Atkins, J.E. Richman, W.F. Oettle, Org. Synth. 6 (1998) 652.
[35] To a 10 ml (1:1 mixture of CH3CN and CH3OH) solution of cyclen (51.6 mg,
0.3 mmol) was added Cu(NO3)2 (69.8 mg, 0.3 mmol) and the reaction mixture
The authors thank the Department of Science and Technology
(DST) (Project No. SR/S1/IC-23/2007), India and Center For Nano-
technology (CFN) in the University of Hyderabad for financial support.
National single crystal XRD facility at University of Hyderabad by DST
is highly acknowledged. We thank Prof. S. Pal for permitting us and
Tulika Ghosh for helping us to record the cyclic voltammograms. MS
and TC thank UGC, New Delhi, India and CFN respectively for their
fellowships.
was stirred overnight at room temperature.
A solution of [Bu4N]2[W6O19]
(567 mg, 0.3 mmol) in CH3CN was prepared separately. These two solutions
were then charged onto two different arms of an U-tube setup (see Fig. 1) and
allowed to mix slowly through a G4 frit that joins two arms of the U-tube. Deep
blue needles of 1 were isolated after few days. IR (KBr, cm−1): 3250.35 (s, NH),
2949.43 (w, aliphatic CH), 2883.84 (w), 2310.93 (m, M–N≡C), 983.78 (s, W O),
808.25 (W–Ob–W). Anal. Calcd. For C10H23CuN5O19W6 (1683.97): C, 7.12; H, 1.37;
N, 4.16. Found: C, 7.21; H, 1.31; N, 4.32.
[36] Crystal data for 1: C10H23CuN5O19W6, M=1683.97, monoclinic, space group C2/c,
a=10.827(4)Å, b=17.180(6)Å, c=15.006(5)Å, β=94.008(6)°, U=2784.4(17)
Å3, Z=4, Dcalcd =4.017 gm cm−3 μ=25.517 mm−1
, , F(000)=2972, crystal
Appendix A. Supplementary material
size = 0.16 × 0.04 × 0.04 mm3, reflections collected/unique = 13206/2447
(Rint =0.0474), final R1/wR2=0.0372/0.0671, 195 parameters, highest diff.
Text and figures depicting spectroscopic studies (IR, UV–visible,
ESR) and elemental analysis, figures and tables related to the crystal
structure of 1. X-ray crystallographic data for 1 (CIF format) may be
obtained free of charge via www.ccdc.cam.ac.uk using the CCDC
depository number CCDC 771216. Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/j.
inoche.2010.06.013.
peak/hole=+1.222/–0.978 e.Å−3
.
[37] M.C. Styka, R.C. Smierciak, E.L. Blinn, R.E. Desimone, J.V. Passariello, Inorg. Chem.
17 (1978) 82.
[38] V. Shivaiah, S.K. Das, Inorg. Chem. 44 (2005) 8846.
[39] J.-P. Costes, F. Dahan, G. Novitchi, V. Arion, S. Shova, J. Lipkowski, Eur. J. Inorg.
Chem. (2004) 1530.
[40] B.J. Hathaway, in: G. Wilkinson, R.D. Gillard, J.A. McCleverthy (Eds.), Comprehen-
sive Coordination Chemistry, vol. 5, Pergamon Press, Oxford, England, 1987,
p. 533.
[41] K.S. Woodin, et al., Eur. J. Inorg. Chem. (2005) 4829.
[42] S.El Ghachtouli, C. Cadiou, I. Déchamps-Olivier, F. Chuburu, M. Aplincourt, V.
Turcry, M. Le Baccon, H. Handel, Eur. J. Inorg. Chem. (2005) 2658.
[43] S.-Y. Kim, I.-S. Jung, E. Lee, J. Kim, S. Sakamoto, K. Yamaguchi, K. Kim, Angew.
Chem. Int. Ed. 40 (2001) 2119.
[44] A. Bencini, L. Fabbrizzi, A. Poggi, Inorg. Chem. 20 (1981) 2544.
[45] A. Buttafava, L. Fabbrizzi, A. Perotti, A. Poggi, G. Poli, B. Seghi, Inorg. Chem. 25
(1986) 1456.
References
[1] L.F. Lindoy, The Chemistry of Macrocyclic Ligand Complexes, Cambridge
University Press, 1989.
[2] A.J.L. Villaraza, A. Bumb, M.W. Brechbiel, Chem. Rev. 110 (2010) 2921 and the
references therein.
[3] R.B. Lauffer, Chem. Rev. 87 (1987) 901 and the references therein.
[4] P. Caravan, J.J. Ellison, T.J. McMurry, R.B. Lauffer, Chem. Rev. 99 (1999) 2293 and
the references therein.
[46] G. Gasser, M.J. Belousoff, M.A. Bond, L. Spiccia, Inorg. Chem. 46 (2007) 3876.