Structural, spectroscopic, and electrochemical properties of tri- and tetradentate N3 and N3S copper complexes with mixed benzimidazole/thioether donors

Article abstract of DOI:10.1039/c2dt30756a

Cupric and cuprous complexes of bis(2-methylbenzimidazolyl)(2- methylthiophene)amine (L¹), bis(2-methylbenzimidazolyl)benzylamine (L²), bis(2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (L³), bis(1-methyl-2-methylbenzimidazolyl)benzylamine (Me ₂L²), and bis(1-methyl-2-methylbenzimidazolyl)(2,4- dimethylphenylthioethyl)amine (Me₂L³) have been spectroscopically, structurally, and electrochemically characterised. The thioether-containing ligands L³ and Me₂L³ give rise to complexes with Cu-S bonds in solution and in the solid state, as evidenced by UV-vis spectroscopy and X-ray crystallography. The Cu²⁺ complexes [L¹CuCl₂] (1), [L²CuCl₂] (2) and [Me₂L³CuCl]ClO₄ (3^Me,ClO4) are monomeric in solution according to ESI mass spectrometry data, as well as in the solid state. Their Cu⁺ analogues [L¹Cu]ClO ₄, [L²Cu]ClO₄, [L³Cu]ClO₄ (4-6), [BOC₂L¹Cu(NCCH₃)]ClO₄ (4^BOC), [Me₂L²Cu(NCCH₃) ₂]PF₆ (5^Me) and [Me₂L ³Cu]₂(ClO₄)₂ (6^Me) are also monomeric in acetonitrile solution, as confirmed crystallographically for 4^BOC and 5^Me. In contrast, 6^Me is dimeric in the solid state, with the thioether group of one of the ligands bound to a symmetry-related Cu⁺ ion. Cyclic voltammetry studies revealed that the bis(2-methylbenzimidazolyl)amine-Cu²⁺/Cu⁺ systems possess half-wave potentials in the range -0.16 to -0.08 V (referenced to the ferrocenium-ferrocene couple); these values are nearly 0.23 V less negative than those reported for related bis(picolyl)amine-derived ligands. Based on these observations, the N₃ or N₃S donor set of the benzimidazole-derived ligands is analogous to previously reported chelating systems, but the electronic environment they provide is unique, and may have relevance to histidine and methionine-containing metalloenzymes. This is also reflected in the reactivity of [Me₂L²Cu(NCCH ₃)₂]⁺ (5^Me) and [Me ₂L³Cu]⁺ (6^Me) towards dioxygen, which results in the production of the superoxide anion in both cases. The thioether-bound Cu⁺ centre in 6^Me appears to be more selective in the generation of O₂^- than 5^Me, lending evidence to the hypothesis of the modulating properties of thioether ligands in Cu-O₂ reactions.

Full text of DOI:10.1039/c2dt30756a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 9394
www.rsc.org/dalton
PAPER
Structural, spectroscopic, and electrochemical properties of tri- and
tetradentate N3 and N3S copper complexes with mixed
benzimidazole/thioether donors†
Ivan Castillo,*a Víctor M. Ugalde-Saldívar,b Laura A. Rodríguez Solano,a Brenda N. Sánchez Eguía,a
Erica Zeglioc and Ebbe Nordlanderc
Received 7th April 2012, Accepted 16th May 2012
DOI: 10.1039/c2dt30756a
Cupric and cuprous complexes of bis(2-methylbenzimidazolyl)(2-methylthiophene)amine (L1),
bis(2-methylbenzimidazolyl)benzylamine (L2), bis(2-methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)-
amine (L3), bis(1-methyl-2-methylbenzimidazolyl)benzylamine (Me2L2), and bis(1-methyl-2-
methylbenzimidazolyl)(2,4-dimethylphenylthioethyl)amine (Me2L3) have been spectroscopically,
structurally, and electrochemically characterised. The thioether-containing ligands L3 and Me2L3 give rise
to complexes with Cu–S bonds in solution and in the solid state, as evidenced by UV-vis spectroscopy
and X-ray crystallography. The Cu2+ complexes [L1CuCl2] (1), [L2CuCl2] (2) and [Me2L3CuCl]ClO4
(3Me,ClO4) are monomeric in solution according to ESI mass spectrometry data, as well as in the solid
state. Their Cu+ analogues [L1Cu]ClO4, [L2Cu]ClO4, [L3Cu]ClO4 (4–6), [BOC2L1Cu(NCCH3)]ClO4
(4BOC), [Me2L2Cu(NCCH3)2]PF6 (5Me) and [Me2L3Cu]2(ClO4)2 (6Me) are also monomeric in acetonitrile
solution, as confirmed crystallographically for 4BOC and 5Me. In contrast, 6Me is dimeric in the solid state,
with the thioether group of one of the ligands bound to a symmetry-related Cu+ ion. Cyclic voltammetry
studies revealed that the bis(2-methylbenzimidazolyl)amine-Cu2+/Cu+ systems possess half-wave
potentials in the range −0.16 to −0.08 V (referenced to the ferrocenium–ferrocene couple); these values
are nearly 0.23 V less negative than those reported for related bis(picolyl)amine-derived ligands. Based
on these observations, the N3 or N3S donor set of the benzimidazole-derived ligands is analogous to
previously reported chelating systems, but the electronic environment they provide is unique, and may
have relevance to histidine and methionine-containing metalloenzymes. This is also reflected in the
reactivity of [Me2L2Cu(NCCH3)2]+ (5Me) and [Me2L3Cu]+ (6Me) towards dioxygen, which results in
the production of the superoxide anion in both cases. The thioether-bound Cu+ centre in 6Me appears
to be more selective in the generation of O2˙− than 5Me, lending evidence to the hypothesis of the
modulating properties of thioether ligands in Cu–O2 reactions.
organic substrates.1 In the case of copper-containing metallo-
enzymes, the study of their remarkable selectivity has been aided
Introduction
Chemical and biochemical oxidations promoted by the Cu2+/+
redox couple are the subject of intense research due to their
involvement in the selective functionalisation of a wide range of
by the development of synthetic models, which facilitate an
understanding of the factors that govern the intimate details of
dioxygen activation and subsequent substrate oxidation. Most
model complexes incorporate pyrazolyl-,2 pyridyl-,1c,3 and
amine-based ligands;1,4 recent examples feature the biologically
relevant imidazolyl groups,5 which are closer structural and elec-
tronic analogues to the histidine donors in proteins. An initial
assessment of the donor capability of nitrogen ligands can be
established based on the pKa values of the conjugate acids.
Among heterocyclic species, the corresponding values for pyra-
zole,6 pyridine,7 imidazole,8 and histidine9 vary considerably,
with values of 2.48, 5.25, 6.95 and 6.17, respectively. These
differences are also reflected in the association constants with
Cu2+, which follow the same general trend of log K: 2.34, 2.60
and 3.76 for pyrazole,6 pyridine,10 and imidazole;11 chelate
formation in the case of free histidine precludes a direct
aInstituto de Química, Universidad Nacional Autónoma de México,
Circuito Exterior, CU, México, D. F. 04510, México.
E-mail: joseivan@unam.mx; Fax: +5255 5616 2217;
Tel: +5255 5622 4449
bFacultad de Química, División de Estudios de Posgrado, Universidad
Nacional Autónoma de México, Circuito Exterior, CU, México,
D. F. 04510, México. E-mail: vmus@unam.mx
cInorganic Chemistry Research Group, Chemical Physics, Center for
Chemistry and Chemical Engineering, Lund University, Box 124,
SE-221 00 Lund, Sweden. E-mail: Ebbe.Nordlander@chemphys.lu.se
†Electronic supplementary information (ESI) available: Spectroscopic,
mass spectrometry, and voltammetry data. CCDC 873058–873063. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30756a
9394 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012

View Article Online
comparison. Benzimidazoles have seldom been employed as
donors in chelating ligands for copper,12 with the notable excep-
tion of the binucleating scaffolds developed by Casella, Costas
and co-workers.13 For polydentate ligand assembly, substitution
at the 2-benzimidazole position results in a pKa of 6.10 for
2-methylbenzimidazole,14 which is closer to that of histidine
than imidazole. Moreover, the 2-methyl substituent does not
have a detrimental steric effect on Cu2+ binding, in contrast to
the effect on pyridine-based systems (log K for benzimidazole/
2-methylbenzimidazole is 3.34/4.43,15 while for pyridine/
2-methylpyridine it is 2.60/1.69).10
systems within the context of copper monooxygenases, we
herein report the synthesis as well as spectroscopic, redox, and
structural characterisation of Cu2+ and Cu+ complexes with bis
(2-methylbenzimidazolyl)amine ligands. For the purposes of
comparison, the ligands employed incorporate a thiophene sulfur
atom with poor donor properties, a non-coordinating but
sterically comparable benzyl group, and a thioether sulfur atom
with a coordination potential towards copper ions (Chart 1).
Results and discussion
In addition to the well-known nitrogen-based ligands present
in the aforementioned systems, interest in thioether donors in the
context of biomimetic systems has emerged in recent years due
to the involvement of such ligands (methionine) in copper-
containing metalloenzymes.16 Among these, dioxygen activation
at the monocopper CuM site of peptidylglycine α-hydroxylating
monooxygenase (PHM) and dopamine β-monooxygenase
Ligand synthesis
Preparation of the bis(benzimidazolyl)amine ligands L1–L3 has
been reported by our research group,20 starting from the corre-
sponding benzimidazole-N-BOC protected analogues.21 Due to
the limited solubility of the Cu+ complexes in several solvent
systems tested for crystallisation, we resorted to the protected
ligand BOC2L1, as well as the benzimidazole-N-methyl
analogues Me2L2 and Me2L3, which are characterised by
improved solubility. The latter ligands were prepared in an
analogous fashion to the BOC-protected precursors, using
1-methyl-2-(chloromethyl)benzimidazole in the reactions with
benzylamine and 2-(2,4-dimethylphenylthio)ethylamine, respect-
ively (Scheme 1). All ligands feature benzimidazole moieties
with 2-substitution, thus possessing comparable basicity to that
of the imidazole group of histidine.10
(DβM) enzymes, with
a common (histidine)2(methionine)
coordination environment, represents a unique case due to the
presence of the methionine sulfur atom that could be potentially
oxidised.16d,e Although the presence of sulfur donors around
copper centres is known to modulate the redox potential of the
Cu2+–Cu+ couple, resulting in high redox potentials for sulfur-
rich coordination environments, the role of thioether donors in
biological and chemical oxidation systems has not been firmly
established.17
Inorganic model systems aimed at understanding dioxygen
activation at monocopper sites include a vast array of tripodal
ligands with nitrogen-based donor sets. Related ligands that
provide N3S and N2S coordination environments have been
developed, most of which contain pyridine and/or aliphatic
amines,17,18 with the exception of the bis(imidazole)thioether
ligands developed by the group of Nicholas,5 and an early
example of a bis(benzimidazole)thioether featuring a central
sulfur atom connecting the nitrogen heterocycles.19 In order to
investigate the properties of benzimidazole-derived copper
Synthesis and spectroscopic characterisation of the complexes
Cupric complexes were obtained by refluxing solutions of the
appropriate ligand with an equimolar amount of CuCl2·2H2O in
methanol, followed by trituration with diethyl ether. The blue–
green solids [LnCuCl2] (n = 1–3; complexes 1–3, cf. Chart 1)
and [Me2L3CuCl2] (3Me) were initially characterised by IR and
UV-vis spectroscopy. IR spectra present ligand-derived C–H
stretching bands between 3150 and 2770 cm−1, as well as aro-
matic C–C and C–N stretches around 1450–1275 cm−1. In the
latter spectral region the bands at 1450 and 1380 cm−1 shift to
higher frequencies relative to the free ligands (at ca. 1433 and
1343 cm−1). UV-vis spectroscopy revealed the presence of a
broad absorption band centred around 700 nm assigned to d–d
transitions, consistent with a square pyramidal coordination geo-
metry.22 Complex 3 has an additional band at 451 nm, while 3Me
exhibits a shoulder at 411 nm, which due to their intensities (ε =
316 and ∼408 M−1, Fig. S1 and S2 in the ESI†) were assigned
to the S → Cu charge transfer.18 The absence of a comparable
LMCT absorption for 1, which has a spectrum that is virtually
identical to that of 2, indicates that there is no Cu–S(thiophene)
Chart 1 Ligands employed in this work.
Scheme 1
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9394–9404 | 9395

View Article Online
interaction in solution. Electrospray ionisation mass spectrometry
(ESI-MS, Fig. S3–S6, ESI†) indicates that all the complexes are
monomeric in solution, with peaks at m/z consistent with
[LnCuCl]+ species in an acetonitrile–methanol solution in all
cases.
The corresponding cuprous complexes [LnCu(ClO4)]
(n = 1–3; complexes 4–6) were prepared by the addition of one
molar equivalent of [Cu(CH3CN)4]ClO4 to solutions of the
ligands in 1 : 1 acetonitrile–THF mixtures. The IR spectra
present ligand-based absorption bands similar to those observed
for their Cu2+ counterparts. Likewise, ESI-MS data are consist-
ent with monomeric [LnCu]+ complexes in acetonitrile–
methanol solutions (Fig. S5–S8, ESI†). The related complexes
[R2LnCu(ClO4)] (n = 1, R = BOC; n = 2, 3, R = Me; complexes
4BOC, 5Me, and 6Me, see Chart 1) were also prepared in order to
facilitate their characterisation in solution due to their increased
solubility. All the cuprous complexes were satisfactorily identified
Fig. 1 Mercury diagram at the 50% probability level of complex 1,
color code: gray, carbon; blue, nitrogen; turquoise, copper; green, chlor-
ine; yellow, sulfur. All hydrogens and minor-occupancy atoms were
removed for clarity.
1
by H and 13C NMR spectroscopy, accounting for the ligand-
derived signals in deuterated acetonitrile. The only significant
1
features of the H NMR spectra involve complexes 6 and 6Me
,
for which a fluxional process may be responsible for the broad-
ening of the benzimidazolyl–CH2–N(amine) and thioether
S–CH2CH2–N(amine) resonances around δ 4.0 and 3.0 ppm in
the former. The effect is more pronounced in the latter complex,
resulting in even broader resonances for all the above-mentioned
signals that give rise to a single resonance for both types of
protons in the S–CH2CH2–N(amine) groups around δ 3.06 ppm,
and a broad singlet for the benzimidazolyl–CH2–N(amine)
protons centred around δ 4.17 ppm (see Fig. S12, ESI†). Since
the mass spectrometry measurements are consistent with mono-
meric species in acetonitrile solution, the line broadening in the
1H NMR spectra may be the consequence of a concentration-
dependent dimerisation exchange process, which due to its
higher solubility results in the more drastic changes observed in
the spectra of 6Me at the high concentrations required (ca.
15 mM) for NMR spectroscopic measurements. Alternatively,
paramagnetic line broadening may occur due to the presence of a
small amount of Cu2+, generated by adventitious oxidation of
the complexes.
Fig. 2 Mercury diagram at the 50% probability level of complex 2.
Hydrogen atoms were removed for clarity.
In the latter orientation, the Cu⋯S distance is longer than the
sum of the van der Waals radii [3.47(1) vs. 3.20 Å].
Despite the poor donor properties of the aromatic sulfur atom
in 1, the corresponding thioether sulfur in L3 can act as a ligand,
as we recently confirmed in the structure of a complex closely
related to 3, which features one chloride and one perchlorate
anion (3ClO4).20 The analogous complex with methyl substitu-
ents at the benzimidazole N-atoms ([Me2L3CuCl]ClO4, complex
3Me,ClO4) was prepared from the cuprous complex [Me2L3Cu]-
ClO4 (6Me) by oxidation with chloroform, thus allowing a com-
parison with 3ClO4 in terms of the benzimidazole NH vs. NMe
structural effects (Fig. 3). Although the bond angles are all very
similar around the square pyramidal Cu2+ centres, the Cu–S and
Cu–Cl bond lengths of 2.727(1) and 2.247(1) Å in 3Me,ClO4 are
shorter than those in 3ClO4 [2.749(1) and 2.277(1) Å, respect-
ively]. Conversely, the distances to the benzimidazole nitrogen
donors are longer for the methylated complex: Cu–N1 and
Cu–N21, 1.954(2) and 1.961(2) Å for 3Me,ClO4, compared to
1.948(4) and 1.950(4) Å for 3ClO4. Nonetheless, the net differ-
ences in the structural parameters between the NH and NMe
complexes can be considered small, as additionally corroborated
by the similar degree of deviation from the square pyramidal
geometry (τ = 0.17 for 3Me,ClO4, 0.18 for 3ClO4).
Solid-state structures
The coordination environment around the copper centres in the
Cu2+ complexes 1 and 2 was established by single-crystal X-ray
diffraction; crystals were obtained from concentrated methanolic
solutions by slow evaporation. Both complexes are characterised
by a square pyramidal coordination geometry (τ = 0.05 for 1,
0.06 for 2, with τ = 0 for an idealized square pyramid23), with
the three nitrogen donors from the ligands coordinated in basal
positions (Fig. 1 and 2). The chlorides coordinate in the remain-
ing basal, and the apical position, the former with considerably
shorter Cu–Cl bond distances [2.211(2) vs. 2.680(1) Å in 1, and
2.213(1) vs. 2.658(1) Å in 2, Table 1]. The location of the sulfur
atom in the thiophene moiety of 1 is disordered over two posi-
tions, with occupancy factors of approximately 80% and 20%
each. The Cu⋯S distance in the major occupancy orientation (cf.
Fig. 1) of L1 is 4.794(3) Å, while the one with minor occupancy
is oriented towards the vacant coordination site of the Cu2+ ion.
9396 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 1 Selected distances (Å) and angles (°) for the complexes
Distance (Å)
Angle (°)
Distance (Å)
Angle (°)
1
2
Cu1–Cl1
Cu1–Cl2
Cu1–N1
Cu1–N11
Cu1–N21
2.680(1)
2.211(1)
1.985(2)
1.971(2)
2.149(2)
Cl1–Cu1–Cl2
Cl1–Cu1–N1
Cl1–Cu1–N11
Cl1–Cu1–N21
Cl2–Cu1–N1
Cl2–Cu1–N11
Cl2–Cu1–N21
N1–Cu1–N11
N1–Cu1–N21
N11–Cu1–N21
109.61(3)
89.44(6)
87.84(6)
91.44(6)
99.02(7)
98.90(7)
158.94(6)
161.75(9)
80.23(8)
81.80(8)
Cu1–Cl1
Cu1–Cl2
Cu1–N1
Cu1–N11
Cu1–N21
2.658(1)
2.213(1)
1.974(2)
1.987(2)
2.141(2)
Cl1–Cu1–Cl2
Cl1–Cu1–N1
Cl1–Cu1–N11
Cl1–Cu1–N21
Cl2–Cu1–N1
Cl2–Cu1–N11
Cl2–Cu1–N21
N1–Cu1–N11
N1–Cu1–N21
N11–Cu1–N21
109.16(3)
87.88(6)
90.32(6)
92.91(6)
98.74(7)
98.94(7)
157.93(6)
161.82(9)
81.55(8)
80.47(8)
3Me,ClO4
Cu1–S1
Cu1–Cl1
Cu1–N1
Cu1–N11
Cu1–N21
4BOC
2.727(1)
2.247(1)
1.954(2)
1.961(2)
2.140(2)
S1–Cu1–Cl1
S1–Cu1–N1
105.61(3)
92.19(7)
93.30(7)
83.20(7)
98.50(8)
97.53(7)
171.14(7)
161.0(1)
81.8(1)
Cu1–N2
Cu1–N3
Cu1–N4
Cu1–N13
2.4764(2)
2.036(2)
1.917 (2)
2.009(2)
N2–Cu1–N3
N2–Cu1–N4
N2–Cu1–N13
N3–Cu1–N4
N3–Cu1–N13
N4–Cu1–N13
73.57(6)
140.49(7)
75.00(6)
113.62(7)
121.54(7)
122.02(7)
S1–Cu1–N11
S1–Cu1–N21
Cl1–Cu1–N1
Cl1–Cu1–N11
Cl1–Cu1–N21
N1–Cu1–N11
N1–Cu1–N21
N11–Cu1–N21
80.76(9)
5Me
6Me
Cu1–N1
Cu1–N11
Cu1–N31
Cu1–N34
1.974(4)
2.012(4)
2.135(7)
1.974(5)
N1–Cu1–N11
N1–Cu1–N31
N1–Cu1–N34
N11–Cu1–N31
N11–Cu1–N34
N31–Cu1–N34
114.5(2)
105.2(2)
121.2(2)
106.5(2)
110.3(2)
96.2(3)
Cu1–S1
Cu1–N1
Cu1–N11
Cu1–N21
2.254(1)
1.977(2)
2.029(2)
2.657(3)
S1–Cu1–N1
125.42(7)
108.66(7)
139.80(6)
125.5(1)
71.56(9)
71.29(9)
S1–Cu1–N11
S1–Cu1–N21
N1–Cu1–N11
N1–Cu1–N21
N11–Cu1–N21
Fig. 3 Mercury diagram at the 50% probability level of complex
3Me,ClO4; hydrogen atoms and a perchlorate anion were removed for
clarity.
Related cupric analogues with axial Cu–S(thioether) distances
vary from 2.528(1) to 2.655(1) Å.18 The shorter distances
observed in those complexes are likely attributable to the weaker
donor properties of the pyridyl groups of the ligands, relative to
the benzimidazolyl-derived ligands of 3Me,ClO4 and 3ClO4. The
cationic moiety [Me2L3CuCl]+ in 3Me,ClO4 is chiral due to the
stereogenic sulfur atom, but the complex crystallises as a race-
mate with the other enantiomer present in the structure being
generated by a crystallographic inversion centre.
Fig. 4 Mercury diagram at the 50% probability level of complex
4BOC; hydrogen atoms, the BOC groups (except for the N-bound
C atoms), perchlorate anion, and solvent molecules have been removed
for clarity.
benzimidazole-N-methylated analogues 5Me and 6Me from aceto-
nitrile and THF, respectively.
The corresponding Cu+ complexes were also structurally
characterized, albeit using modified ligands: BOC2L1
was employed for the preparation of 4BOC, which has improved
solubility over that of 4, and allowed recrystallisation from
acetonitrile. In the case of 5 and 6, their low solubility in aceto-
nitrile, acetone and THF demanded recrystallisation of the
4BOC has a highly distorted tetrahedral geometry defined by
the three ligand-derived nitrogen donors, and a molecule of
acetonitrile, with bond angles around the cuprous ion ranging
from 73.58(7) to 140.49(8)° (Fig. 4). Alternatively, the coordi-
nation geometry can be considered trigonal if the rather long
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9394–9404 | 9397

View Article Online
bond to the central amine is disregarded (Cu1–N2 2.476(2) Å,
nearly 0.50 Å longer than the rest of the Cu–N interactions). The
bond angles to the benzimidazole and acetonitrile N atoms of
N3–Cu1–N4 113.63(8), N3–Cu1–N13 121.55(7) and N4–Cu1–
N13 122.01(8)° are consistent with this geometry (see Table 1).
Although the thiophene sulfur atom is directed towards the Cu+
centre, the distance is too long to contemplate a Cu–S interaction
[Cu1⋯S1 3.398(1) Å].
Preparation of the methylated complex 5Me allowed the deter-
mination of its solid-state structure. The copper–nitrogen dis-
tance to the central amine N atom in 5Me [Cu1⋯N23 3.941(4)
Å] is much longer than that in 4BOC, such that the amine does
not coordinate to the Cu+ centre. As a result, the tetrahedral
coordination environment around the cuprous ion is defined by
the two benzimidazole groups and two molecules of acetonitrile,
with angles varying from 96.2(3)° between the two acetonitrile
ligands, to 121.2(2)° between one acetonitrile and one benzimi-
dazole N-donor (Fig. 5).
The Cu–N bond lengths are comparable to those observed for
4BOC, with an average of 1.974(5) Å to the benzimidazole
groups, and 2.074(6) Å to the acetonitrile molecules; complex
5Me features a non-interacting hexafluorophosphate counter-
anion. The methylated version of the thioether ligand also
allowed the structural characterisation of its cuprous complex
6Me, which is a centrosymmetric dimer in the solid state.
The thioether ligation to the Cu+ ion of the adjacent cationic
moiety of 6Me is reminiscent of the structure of the bis(2-picolyl)-
ethylthioethylamine–Cu+ complex reported by Karlin and co-
workers,18b although the geometry around the copper ion is
described as pyramidal in the latter complex, while in the case of
6Me it can be best viewed as trigonal considering the very long
Cu1–N21 distance to the central amine of 2.657(3) Å, which is
even longer than that for 4BOC (Fig. 6). In contrast, the corres-
ponding Cu–N distances in bis(2-picolyl)amine-derived ligands
range from 2.18 to 2.26 Å; coordination of the aliphatic amine to
the Cu+ centres does not appear to be necessary in the bis(benz-
imidazolyl)amine systems, likely due to the better donor proper-
ties of the benzimidazole moieties towards copper.18b,c,24 This
Fig. 6 Mercury diagram at the 50% probability level of complex 6Me
hydrogen atoms and perchlorate anions were removed for clarity.
;
Table 2 Redox data of copper complexes
Redox potentials (V vs. Fc+–Fc)
Complex
Eap
Ecp
E1/2
4
5
−0.01
0.04
−0.06
−0.02
−0.06
−0.16
−0.19
−0.17
−0.20
−0.26
−0.09
−0.08
−0.12
−0.11
−0.16
6
5Me
6Me
does not affect Cu1–S1 bonding, which at a distance of 2.254(1)
Å is comparable to that reported for related systems.18b,c,24
Electrochemical studies
Characterisation of the ligands by cyclic voltammetry at a con-
centration of 1 mM in acetonitrile solution and a scan rate
of 0.200 V s−1 revealed several oxidation processes for all of
L1–L3, Me2L2 and Me2L3 but the peaks are clearly distinct
from those centred at copper.20 The redox potentials for the
Cu2+–Cu+ couples were determined under anaerobic conditions
starting from the Cu+ complexes, see Table 2. The observed
redox waves behave quasi-reversibly, allowing the determination
of the anodic peak, cathodic peak, and half-wave potentials
(E1/2 relative to the ferrocenium–ferrocene [Fc+–Fc] couple, see
Fig. 7 and S13†).
Fig. 5 Mercury diagram at the 50% probability level of complex 5Me
hydrogen atoms and a hexafluorophosphate anion were removed for
;
clarity.
9398 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012

View Article Online
spectrometry. The presence of an oxidation product of Me2L3
during the oxidation of 6Me was also confirmed by 1H NMR
spectroscopy and FAB-MS (ESI Fig. S14 and S15†). The latter
technique allowed the detection of a species at m/z = 486, corre-
sponding to a mass of [Me2L3 + O]. ESI-MS analysis of the
oxidation products of 5Me by exposure of acetonitrile solutions
of the complex to O2 gave rise to a prominent peak at m/z =
493.2 (Fig. S16, ESI†), consistent with the aquo/hydroxo Cu2+
complex [Me2L2CuOH(H2O)]+, as well as unreacted 5Me. The
initial behaviour of 6Me under the same conditions was analo-
gous to that of 5Me, resulting in the formation of the aquo/
hydroxo complex detected at m/z = 567.2 [Me2L3CuOH-
(H2O)]+. Longer reaction times resulted in a significant amount
(ca. 20%) of a monocopper species at m/z = 723.3, tentatively
assigned to a complex derived from the association of 6Me
with the oxidative ligand decomposition fragment C10H13N3O
(2-methylbenzimidazolyl-aminomethanol
Fig. S17†).
+
[Me2L3Cu]+,
Fig. 7 Representative cyclic voltammograms of 1 mM solutions of
complex 5Me obtained with a glassy carbon electrode (ϕ = 7.1 mm2) in
0.1 M Bu4NPF6 in acetonitrile, at different inversion anodic potential
values (E+λ).
Further O2-reactivity studies were carried out with complexes
5Me and 6Me due to their improved solubility in acetonitrile and
THF relative to the non-methylated analogues. Voltammetry
measurements on 1 mM acetonitrile solutions of 5Me and 6Me in
the presence of O2 (Fig. S18–S20†) allowed us to evaluate the
Cu–dioxygen interaction in solution. For 6Me, the E1/2 value of
−0.16 V indicated that below this potential the presence of O2
must result in a Cu2+-superoxide species, which is characterised
by a second signal at Ecp = −1.64 V. Thus, while formal 1e−
reduction of O2 to the coordinated superoxide anion in [Cu–O2]+
occurred at a small negative potential, this species is stable
towards further reduction to coordinated peroxide relative to the
1e− reduction of free O2 to superoxide at Ecp = −1.45 V
(Fig. S19†). Moreover, 1e− reduction of the initially formed
Cu2+–O2˙− could result in an unstable Cu+–O2˙− complex,
releasing a free superoxide anion (vide supra) and regenerating
The half-wave potentials of complexes 4–6 had been reported
by our group from the complexes prepared in situ; the new data
obtained for the methylated analogues 5Me and 6Me at E1/2
=
−0.11 and −0.16 V indicate that the N-methyl substituents on
the benzimidazole donors result in a 30–40 mV shift to a nega-
tive potential, relative to 5 and 6. This implies that the methy-
lated complexes are more easily oxidised to the corresponding
cupric species under the working conditions. While the shift in
the half-wave potentials is small enough to consider the redox
properties of both types of complexes as nearly identical, it
nonetheless indicates that the N-methylation of the ligands
results in more electron-rich donors that facilitate oxidation at
copper.
6Me
.
With respect to related tripodal ligands, the benzimidazole-
containing chelates described herein afford copper complexes
with less negative half-wave potentials than their pyridine-based
counterparts. For example, the copper complex of the N3S-type
1-[bis(2-picolyl)amino]-2-(methylthio)ethane (BPMMEA)25 is
characterised by E1/2 = −0.35 V (vs. Fc+–Fc), thus making it
more easily oxidisable from Cu+ to Cu2+ than the closely related
complexes 6 and 6Me. This observation can be rationalised based
on the lower-lying LUMO orbitals of benzimidazoles compared
to pyridines, which may act as π-acceptors and stabilise the
reduced Cu+ ion.26
Analysis of the reactions of 3 mM acetonitrile (−40 °C) or
THF (−78 °C) solutions of 5Me and 6Me with O2 by ESR
revealed signals consistent with axial Cu2+ centres, g⊥ = 2.060,
g|| = 2.251, and A|| = 162 Gauss for 5Me (Fig. S21†), and A|| =
168 G for 6Me at −170 °C. The solutions were kept at a low
temperature and measured again at 0.5 h intervals. In the case of
6Me, the ESR spectrum obtained in THF displayed an additional
sharp signal after 1 h of O2 exposure with g = 2.004, which is
consistent with the presence of the superoxide anion
(Fig. S22†).27 This was confirmed by employing the radical trap
5,5-dimethyl-1-pyrroline-N-oxide (DMPO),27,28 giving rise to a
characteristic pattern centered at g = 2.006 (J1 = 13.4, J2 = 10.3,
J3 = 0.9 G, Fig. S23†). O2˙− was also detected indirectly during
the oxygenation of 5Me upon addition of DMPO. It has been pro-
posed that the initial step in the reactions of Cu+ with O2 is the
formation of superoxide complexes, which have been described
as ESR silent.18b,29 In our system, the putative superoxide
[Me2LnCu–O2]+ evolved to the ESR active [Me2LnCuOH
(H2O)]+ detected by ESI-MS, and free superoxide; thioether
donating methionine ligands in the active sites of PHM and
DβM have been proposed to favour the formation of the 1e−-
reduced superoxide over the 2e−-reduced peroxide by stabilising
the corresponding Cu2+-superoxides,30 accounting for the prefer-
ential formation of O2˙− by 6Me. The free superoxide anion
Reactivity studies
Cu+ complexes 4–6, 5Me and 6Me (80–100 mg) were oxidised in
air as acetonitrile, THF or acetone solutions prepared in an inert
atmosphere glovebox, and demetalated by treatment with con-
centrated NH4OH. Examination of the recovered ligands L1–L3,
Me2L2 and Me2L3 by thin layer chromatographic (TLC) com-
parison with original samples of the ligands revealed that the
compounds remained mostly unchanged, as also confirmed by
recovery of the solids, which were quantified by weight in
85–92% yields. Confirmation of their identity was made by
1H NMR spectroscopy and electron-ionisation (EI) mass
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9394–9404 | 9399

View Article Online
230 mV compared to the related pyridyl-containing complexes
represent a significant change in the electronic properties.
The reaction of the cuprous complexes [Me2L2Cu]+ (5Me) and
[Me2L3Cu]+ (6Me) with dioxygen at low temperatures leads to
the formation of the superoxide anion, and thus presumably to
unstable Cu2+–O2˙− intermediates, as indicated by trapping of
the superoxide ion using the spin trap DMPO. The final
oxidation products detected are Cu2+(OH)(H2O) complexes in
both cases; mass spectrometry indicates additional oxidative
ligand degradation in the case of 6Me. Nonetheless, this latter
complex appears to catalyse the formation of superoxide both in
acetonitrile and THF solutions, as evidenced by ESR and cyclic
voltammetry. In contrast, 5Me generates superoxide only at
the initial stages of the reaction with dioxygen, since the
DMPO–O2˙− adduct disappears within 0.5 h of O2 exposure.
These observations support the notion that thioether ligands
direct dioxygen activation by Cu+ centres to selectively generate
the superoxide anion without further reduction, thus resulting
in the C–H activation observed in PHM and DβM.30 Further
analysis of the Cu–O2 species formed during the course of the
reaction between the thioether-containing complex 6Me and
related derivatives and dioxygen, as well as reactivity studies
with exogenous substrates are currently under way.
Scheme 2 Reactivity of Cu+ complexes with O2.
detected by ESR may be released from the Cu2+-superoxo
complex in a net electron transfer (ET) process. Alternatively,
the copper-superoxide complexes can abstract hydrogen atoms
from solvent molecules to afford hydroperoxo complexes Cu2+–
OOH, as shown in Scheme 2. The mechanism for further
H-abstraction required for the formation of the final identified
Cu2+ products [Me2LnCuOH(H2O)]+ remains to be elucidated.
Conclusions
We present a systematic study of the spectroscopic, structural,
and electrochemical properties of a series of bis(2-methylbenzi-
midazolyl)amine-derived ligands and their copper complexes.
Both benzidmidazole-NH and NMe ligands support Cu2+ and
Cu+ complexes of the type [LnCuCl2] and [LnCu(NCCH3)x]
ClO4 (x = 0–2), with the N-methylated ligands being more
amenable for structural characterisation, particularly when the
cuprous analogues are concerned. The spectroscopic properties
for both types of complexes do not vary significantly, and
neither do the geometric parameters as determined for the
thioether-containing methylated complex [Me2L3CuCl]ClO4
(3Me,ClO4) with respect to the previously reported cupric
complex with benzimidazolic NH groups [L3CuCl]ClO4.20
The Cu2+-thioether complexes are characterised by a distinctive
S → Cu charge transfer band around 400–450 nm, which indi-
cates that this interaction is present in solution as well as in the
solid state.
Experimental
Materials and methods
Air-sensitive compounds were manipulated under an atmosphere
of dinitrogen in a glovebox or by vacuum-line and Schlenk tech-
niques. Solvents were dried by standard methods and distilled
under N2 prior to use. The starting material 1-methyl-benzi-
midazole-2-carbaldehyde was purchased from MolPort.
CuCl2·4H2O, [Cu(CH3CN)4]PF6, and Bu4NPF6 were used as
31
received from Aldrich Chemical Co. [Cu(CH3CN)4]ClO4 and
the ligands BOC2L1, and L1–L3 were prepared according to the
literature procedures (Warning! Metal perchlorates are poten-
tially explosive and should be handled with extreme care).
Melting points were determined on an Electrothermal Mel-Temp
apparatus and are uncorrected. Infrared spectra were recorded
Regarding the solid-state structures, the coordination environ-
ment of the Cu+ centres in all crystallographically characterised
complexes [R2LnCu(NCCH3)x]ClO4 (R = BOC, n = 1, complex
4BOC; R = Me, n = 2, 3, complexes 5Me and 6Me) is defined by
the two benzimidazole donors, without the participation of the
aliphatic amine. This observation appears to indicate that the
benzimidazolyl groups are better donors towards Cu+ than the
nitrogen-based pyridyl groups of related complexes, affording
either tetrahedral or trigonal coordination geometries in com-
plexes 4BOC, 5Me and 6Me. The thioether group of 6Me coordi-
nates to an adjacent Cu+ centre, generating a dimeric structure in
the solid state.
The differences in donating properties between the benzimida-
zole-NH and NMe ligands are relatively small, and give rise to
only marginal changes in the spectroscopic and structural proper-
ties of their copper complexes, while the small difference in the
redox properties accounts for the more negative half-wave poten-
tials of 5Me and 6Me by 30–40 mV relative to 5 and 6. The less
negative E1/2 values of the Cu2+–Cu+ redox couple by about
with
a
Bruker Tensor 27 spectrophotometer in the
4000–400 cm−1 region as KBr disks. NMR spectra were
recorded on a JEOL Eclipse 300 spectrometer in CDCl3 with
tetramethylsilane as an internal standard, or in CD3CN referenced
relative to the residual solvent protons at 300 (1H) or 75 MHz
(13C). Mass spectra were obtained on a JEOL JMS-SX-102A
mass spectrometer at an accelerating voltage of 10 kV with a
nitrobenzyl alcohol matrix and Xenon atoms at 6 keV (FAB+), a
JEOL JMS-AX505HA spectrometer (Electron Ionization), or
a
Bruker Daltonics Esquire 6000 spectrometer with ion
trap (Electrospray). Elemental analyses were performed at the
microanalytical facility of the Instituto de Química, or USAI –
Facultad de Química.
Cyclic voltammetry measurements were made under N2 in
anhydrous acetonitrile with a potentiostat–galvanostat EG&G
PAR model 263A with a glassy carbon working electrode and a
platinum wire auxiliary electrode. Potentials were recorded
versus
a pseudo-reference electrode of AgCl(s)-Ag(wire)
immersed in a 0.1 M (C4H9)4NCl acetonitrile solution. All
9400 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012

View Article Online
Synthesis of (1-Me-2-CH2C7H5N2)2(CH2C6H5)N (Me2L2). To
an acetonitrile solution (15 mL) of 1-methyl-2-(chloromethyl)-
benzimidazole (500 mg, 2.70 mmol) was added benzylamine
(170 μL, 1.38 mmol), K2CO3 (1.12 g, 8.10 mmol), and a cataly-
tic amount of NaI (ca. 10 mg). After 12 h of stirring the benzi-
midazole was consumed, and the reaction mixture was filtered
through celite, washed with 2 portions of 10 mL of dichloro-
methane, and slow evaporation of the volatiles afforded colour-
less crystals of Me2L2. Yield: 443 mg (83%), mp 185–187 °C.
IR (KBr): 3063, 3032, 2935, 2877, 2814, 1615, 1510, 1474,
1440, 1401, 1331, 1288, 1228, 1151, 1105, 994, 969, 909, 855,
voltammograms were started from the current null potential
(Ei=0) and were scanned in both directions, positive and negative,
and obtained at a scan rate of 0.200 V s−1. In agreement with
IUPAC convention, the voltammogram of the ferrocenium–ferro-
cene (Fc+–Fc) system was obtained to establish the values of the
half wave potentials (E1/2) from the expression E1/2 = (Eap
+
Ecp)/2. To obtain the normalised current for each complex, the
measured current was divided by the exact molar concentration
of the electroactive species.
1
767, 744, 699, 656, 574, 537, 485, 444, 405 cm−1. H NMR
X-Ray crystallography
(CDCl3, 300 MHz, 20 °C): δ 7.65 (m, 2 H, Bzim), 7.17 (m, 11
H, Bzim, Ph), 3.89 (s, 4 H, NCH2Bzim), 3.65 (s, 2 H, NCH2Ph),
3.28 (s, 6 H, NCH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz,
20 °C): δ 151.32 (Bzim), 142.27 (Bzim), 137.90 (Bzim), 136.08
(Bzim), 129.80 (Bzim), 128.42 (Bzim), 127.65 (Bzim), 122.75
(Ph), 122.09 (Ph), 119.72 (Ph), 109.15 (Ph), 59.21 (NCH2Ph),
50.71 (NCH2Bzim), 29.57 (NCH3) ppm. Anal. Calcd for
C25H25N5: C, 75.92; H, 6.37; N, 17.71. Found: C, 76.29; H,
6.24; N, 17.37.
Single crystals were mounted on a Bruker SMART diffracto-
meter equipped with an Apex CCD area detector. Frames were
collected by omega scans, and integrated with the Bruker
SAINT software package using the appropriate unit cell. The
structures were solved using the SHELXS-97 program, and
refined by full-matrix least-squares on F2 with SHELXL-97.
Weighted R-factors, Rw, and all goodness-of-fit indicators, S,
were based on F2. The observed criterion of F2 > 2σF2 was used
only for calculating the R-factors. All non-hydrogen atoms were
refined with anisotropic thermal parameters in the final cycles of
refinement. Hydrogen atoms were placed in idealised positions,
Synthesis of (1-Me-2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N
(Me2L3). To an acetonitrile solution (40 mL) of 1-methyl-2-
(chloromethyl)benzimidazole (535 mg, 2.96 mmol) was added
2-(2,4-dimethylphenylthio)ethylamine (388 mg, 1.48 mmol),
with isotropic thermal parameters assigned to the values Uiso
=
1.2 times the thermal parameters of the parent non-hydrogen
atom. Selected crystallographic data are presented in Table 3.
Table 3 Crystallographic data
1
2
3Me,ClO4
4BOC
5Me
6Me
Formula
Molecular weight 507.91
Crystal system
Space group
C21H19Cl2CuN5S C23H21Cl2CuN5 C28H31Cl2CuN5O4S C37H38ClCuN8O8S C29H31CuF6N7P C56H62Cl2Cu2N10O8S2
501.89
668.08
Triclinic
859.85
Triclinic
686.12
Orthorhombic
Pbca
1265.26
Triclinic
Orthorhombic
Pbca
Orthorhombic
Pbca
ˉ
P1
ˉ
P1
ˉ
P1
Wavelength (Å) 0.71073
0.71073
Blue
298(2)
0.71073
Green
298(2)
0.71073
Colorless
173(2)
0.71073
Colorless
298(2)
0.71073
Colorless
123(2)
Crystal colour
T (K)
Blue
298(2)
Crystal size
(mm)
a (Å)
0.10 × 0.29 × 0.43 0.09 × 0.29 ×
0.30
0.06 × 0.10 × 0.31
0.20 × 0.10 × 0.08
0.19 × 0.24 × 0.29 0.06 × 0.12 × 0.17
18.3323(16)
9.2808(8)
25.476(2)
90
18.583(2)
9.5012(11)
25.292(3)
90
11.3107(15)
11.6824(16)
13.0796(17)
66.608(2)
70.800(2)
71.327(2)
1461.7(3)
−13 ≤ h ≤ 13
−14 ≤ k ≤ 14
−15 ≤ l ≤ 15
1.518
12.9077(15)
13.4737(15)
14.0582(16)
73.803(2)
66.247(2)
66.342(2)
2029.6(4)
−15 ≤ h ≤ 15
−16 ≤ k ≤ 16
−16 ≤ l ≤ 16
1.407
17.106(3)
14.758(2)
25.087(4)
90
11.355(2)
11.7958(16)
12.583(2)
67.661(2)
63.781(2)
87.360(2)
1383.3(3)
−13 ≤ h ≤ 13
−14 ≤ k ≤ 14
−15 ≤ l ≤ 15
1.519
b (Å)
c (Å)
α (o)
β (o)
90
90
90
90
γ (o)
90
90
V (Å3)
hkl Ranges
4334.4(7)
−22 ≤ h ≤ 22
−11 ≤ k ≤ 11
−30 ≤ l ≤ 30
1.557
4465.6(9)
−22 ≤ h ≤ 22
−11 ≤ k ≤ 11
−30 ≤ l ≤ 30
1.493
6333.3(18)
−20 ≤ h ≤ 20
−17 ≤ k ≤ 17
−30 ≤ l ≤ 30
1.439
ρcalc (g cm−3
)
Z
8
2072
1.369
8
2056
1.238
2
690
1.045
2
896
0.715
8
2816
0.807
1
656
1.006
F(000)
μ (mm−1
)
θ range (°)
Absorption corr. Empirical
Tmax, Tmin
Refinement
1.95 to 25.33
1.95 to 25.37
Empirical
0.8961, 0.7098
1.95 to 25.37
Empirical
0.9349, 0.7498
1.67 to 25.37
Empirical
0.8740, 0.7278
Full-matrix least-
squares on F2
7443
2.00 to 25.38
Empirical
0.8628, 0.6651
1.89 to 25.40
Empirical
0.9424, 0.8008
0.8772, 0.6076
Full-matrix least-
squares on F2
3956
Full-matrix least- Full-matrix least-
Full-matrix least- Full-matrix least-
squares on F2
4086
squares on F2
5363
squares on F2
5815
squares on F2
5061
Reflections
Data/restr./param. 3956/172/323
Goodness-of-fit 1.049
4086/1/286
1.051
5363/81/402
1.018
7443/130/551
1.045
5815/0/401
1.012
5061/0/365
1.028
R
Rw
0.0372
0.0883
0.0371
0.0910
0.590,−0.290
0.0372
0.0931
0.434,−0.255
0.0368
0.0972
0.523,−0.265
0.0625
0.1596
0.542,−0.329
0.0410
0.0897
0.477,−0.264
Peak, hole (e Å3) 0.493, −0.298
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9394–9404 | 9401

View Article Online
K2CO3 (409 mg, 29.6 mmol), and a catalytic amount of NaI (ca.
40 mg). After refluxing for 12 h the benzimidazole was con-
sumed, and the reaction mixture was filtered through celite,
washed with 2 portions of 10 mL of dichloromethane, and slow
evaporation of the volatiles afforded colourless crystals of
Me2L3. Yield: 374 mg (54%), mp 146–149 °C. IR (KBr): 3057,
3030, 2973, 2921, 2811, 1512, 1478, 1433, 1402, 1360, 1331,
1283, 1237, 1172, 1128, 1098, 1058, 1033, 989, 951, 882, 863,
(KBr): 3379, 3243, 3068, 3032, 2907, 2856, 2769, 2629, 2483,
1617, 1592, 1535, 1451, 1381, 1320, 1276, 1235, 1118, 1047,
996, 954, 865, 745, 700, 626, 496, 426 cm−1. Mass spectrum
ES (acetonitrile–methanol): m/z = 466 [L2CuCl]+ (100%). UV-
vis (methanol): λmax 716 nm (145 M−1). Anal. Calcd for
C23H23Cl2CuN5O: C, 53.13; H, 4.46; N, 13.47. Found: C,
52.99; H, 4.03; N, 13.53.
[{(2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N}CuCl2] (3). L3
(50 mg, 0.12 mmol), CuCl2·2H2O (20 mg, 0.12 mmol), green
solid. Yield: 50 mg (67%), mp 160–164 °C. IR (KBr): 3403,
3102, 3037, 2971, 2913, 2765, 2639, 1622, 1597, 1544, 1475,
1450, 1384, 1324, 1276, 1224, 1090, 1048, 1000, 921, 810,
748, 550, 494, 436 cm−1. Mass spectrum ES (acetonitrile–
methanol): m/z = 540 [L3CuCl]+ (100%). UV-vis (methanol):
λmax 451 (316 M−1), 694 nm (89 M−1). Anal. Calcd for
C26H31Cl2CuN5O2S: C, 51.02; H, 5.10; N, 11.44. Found: C,
51.48; H, 5.32; N, 10.77.
808, 790, 762, 741, 653, 569, 539, 485, 442, 401 cm−1 1H
.
NMR (CDCl3, 300 MHz, 20 °C): δ 7.70 (m, 2 H, Bzim), 7.26
3
(m, 6 H, Bzim, Ph), 6.87 (d, J = 7.95 Hz, 1 H, Thioph), 6.82
(s, 1 H, Thioph), 6.63 (d, 3J = 7.95 Hz, 1 H, Thioph), 4.09
(s, 4 H, NCH2Bzim), 3.70 (s, 6 H, NCH3), 3.00 (s, 4 H,
NCH2CH2S), 2.16 (s, 6 H, ArCH3) ppm. 13C{1H} NMR
(CDCl3, 75 MHz, 20 °C): δ 150.87 (Bzim), 140.86 (Bzim),
137.85 (Bzim), 136.12 (Bzim), 135.76 (Bzim), 131.17 (Bzim),
130.99 (Bzim), 129.18 (Ar), 127.25 (Ar), 123.36 (Ar), 122.73
(Ar), 119.31 (Ar), 109.53(Ar), 54.16 (NCH2Bzim), 51.30
(NCH3), 31.22 (NCH2CH2S), 30.26 (NCH2CH2S), 20.97
(ArCH3), 20.39 (ArCH3) ppm. Anal. Calcd for C27H29N5S: C,
71.18; H, 6.42; N, 15.37. Found: C, 71.39; H, 6.45; N, 14.79.
Alternatively, 2-(2,4-dimethylphenylthio)ethylamine (388 mg,
1.48 mmol) was initially condensed with 1-methyl-benzimida-
zole-2-carbaldehyde (0.240 mg, 1.49 mmol) in a reductive
amination reaction: the two reactants were dissolved in dichloro-
methane (20 ml) and stirred at room temperature for 3 h, after
which sodium tris(acetoxy)borohydride (530 mg, 2.5 mmol) was
added to the reaction. The reaction mixture was stirred for an
additional two hours to generate a light yellow precipitate that
was filtered and dried in vacuo to give 2-((2,4-dimethylphenyl)-
thio)-N-((1-methyl-benzomidazol-2-yl)methyl)ethanamine.
Yield: 369 mg (76%), 1H NMR (CDCl3, 500 MHz, 20 °C):
δ 6.98–7.28 (m, 7 H, Bzim, Ph), 4.13 (s, 2 H, NCH2Bzim), 3.82
(s, 3 H, NCH3), 3.02 (t, 2 H, NCH2CH2S), 2.93 (t, 2 H,
NCH2CH2S), 2.34 (s, 3 H, ArCH3), 2.26 (s, 3 H, ArCH3). In
a subsequent condensation reaction, 2-((2,4-dimethylphenyl)-
thio)-N-((1-methyl-1H-benzomidazol-2-yl)methyl)ethanamine
(733 mg, 2.25 mmol), 1-methyl-2-(chloromethyl)benzimidazole
(407 mg, 2.25 mmol), K2CO3 (1.11 g, 8 mmol) and NaI (40 mg,
0.3 mmol) were dissolved in acetonitrile (30 mL) and the result-
ing solution was refluxed for 24 h. The solvent was removed
in vacuo and the residue was redissolved/triturated in chloroform
and filtered to obtain a clear solution. Removal of the solvent
under reduced pressure gave the product, Me2L3. Yield: 697 mg
(66%).
[{(1-Me-2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N}CuCl2]
(3Me). Me2L3 (255 mg, 0.54 mmol), CuCl2·2H2O (92 mg,
0.54 mmol), green solid. Yield: 209 mg (64%), mp 143–145 °C.
IR (KBr): 3151, 2922, 1737, 1613, 1500, 1483, 1455, 1375,
1325, 1293, 1238, 1053, 1009, 935, 811, 748, 571, 545,
435 cm−1. UV-vis (methanol): λmax 411 nm (408 M−1), 728 nm
(137 M−1). Anal. Calcd for C30H36Cl5CuN5OS: C, 47.69; H,
4.80; N, 9.27. Found: C, 47.46; H, 4.76; N, 8.85.
[{(1-Me-2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N}CuCl]-
(ClO4) (3Me,ClO4). Me2L3 (20 mg, 0.04 mmol) and [Cu-
(CH3CN)4]ClO4 (12 mg, 0.04 mmol) were dissolved in aceto-
nitrile (ca. 2 mL). To the colourless solution was added 1 mL of
chloroform; a gradual colour change to green was observed, and
after 48 h X-ray quality crystals formed. Yield: 24 mg (90%),
mp 273–276 °C. IR (KBr): 2978, 2945, 1594, 1504, 1483, 1452,
1384, 1321, 1292, 1249, 1158, 1085, 994, 960, 936, 916, 812,
750, 622, 541, 510, 472, 431 cm−1. UV-vis (methanol): λmax
661 nm (96 M−1). Anal. Calcd for C28H31Cl2CuN5O4S: C,
50.34; H, 4.68; N, 10.48. Found: C, 50.00; H, 4.53; N, 10.36.
Synthesis of Cu+ complexes. To solutions of L1–L3, BOC2L1,
Me2L2 or Me2L3 in THF was added an equimolar amount of the
appropriate Cu+ salt dissolved or suspended in THF–acetonitrile.
The mixtures were stirred for 4 h, and then volatiles were
removed under reduced pressure. The off-white solids were
washed with two portions of diethyl ether, and dried under
vacuum.
Synthesis of [{(2-CH2C7H5N2)2(2-CH2C4H3S)N}CuCl2] (1).
L1 (60 mg, 0.15 mmol), CuCl2·2H2O (26 mg, 0.15 mmol),
turquoise solid. Yield: 70 mg (94%), mp 214–216 °C. IR (KBr):
3429, 3078, 3028, 2976, 2901, 2852, 2769, 2628, 2482, 1615,
1534, 1467, 1450, 1373, 1318, 1273, 1112, 1046, 994, 953, 866,
826, 763, 705, 495, 427 cm−1. Mass spectrum ES (acetonitrile–
methanol): m/z = 471 [L1CuCl]+ (100%). UV-vis (methanol):
λmax 716 nm (134 M−1). Anal. Calcd for C21H19Cl2CuN5S: C,
49.69; H, 3.77; N, 13.96. Found: C, 49.84; H, 4.04; N, 13.87.
[{(2-CH2C7H5N2)2(2-CH2C4H3S)N}Cu]ClO4 (4). L1 (150 mg,
0.40 mmol), [Cu(CH3CN)4]ClO4 (130 mg, 0.40 mmol), colour-
less solid. Yield: 100 mg (45%), mp 226–232 °C (dec.). IR
(KBr): 3233, 3067, 2926, 2826, 1621, 1599, 1535, 1496, 1461,
1340, 1284, 1208, 1121, 1038, 965, 923, 851, 823, 746, 709,
622, 497, 434 cm−1 1H NMR (CD3CN, 300 MHz, 20 °C):
.
δ 10.97 (s, 2 H, NH), 7.78 (m, 2 H, Bzim), 7.50 (m, 2 H, Bzim),
7.28 (m, 5 H, Bzim, Thioph), 7.00 (s, 1 H, Thioph), 6.91 (s,
1 H, Thioph), 3.99 (s, 2 H, NCH2Thioph), 3.90 (s, 4 H,
NCH2Bzim) ppm. 13C{1H} NMR (CD3CN, 75 MHz, 20 °C):
δ 153.88 (Bzim), 142.28 (Bzim), 141.80 (Bzim), 134.56 (Bzim),
128.89 (Bzim), 128.15 (Bzim), 127.45 (Bzim), 125.04 (Thioph),
124.41 (Thioph), 119.00 (Thioph), 113.90 (Thioph), 54.82
Synthesis of [{(2-CH2C7H5N2)2(CH2C6H5)N}CuCl2] (2). L2
(200 mg, 0.55 mmol), CuCl2·2H2O (94 mg, 0.55 mmol), blue
microcrystals. Yield: 210 mg (75%), mp 244-247 °C (dec.). IR
9402 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012

View Article Online
(NCH2Thioph), 50.84 (NCH2Bzim) ppm. Anal. Calcd for
C21H21ClCuN5O5S: C, 45.49; H, 3.82; N, 12.63. Found: C,
45.96; H, 3.54; N, 12.46. Mass spectrum ES (acetonitrile–metha-
nol): m/z = 436 [L1Cu]+ (100%).
127.97 (Bzim), 127.75 (Bzim), 126.70 (Thioph), 126.06
(Thioph), 119.41 (Thioph), 116.48 (Thioph), 88.45 (C(CH3)3),
55.86 (NCH2Bzim), 55.76 (NCH2Thioph), 28.21 (C(CH3)3)
ppm. Anal. Calcd for C33H38ClCuN6O8S: C, 50.96; H, 4.92; N,
10.81. Found: C, 51.12; H, 4.87; N, 11.05.
[{(2-CH2C7H5N2)2(CH2C6H5)N}Cu]ClO4 (5). L2 (200 mg,
0.55 mmol), [Cu(CH3CN)4]ClO4 (180 mg, 0.55 mmol), colour-
less microcrystals. Yield: 160 mg (53%), mp 196–200 °C (dec.).
IR (KBr): 3243, 3065, 2970, 2926, 2847, 1621, 1599, 1535,
1492, 1453, 1389, 1344, 1275, 1124, 1034, 968, 917, 848, 749,
701, 622, 494, 439 cm−1. 1H NMR (CD3CN, 300 MHz, 20 °C):
δ 10.85 (s, 2 H, NH), 7.89 (m, 2 H, Bzim), 7.61 (m, 2 H, Bzim),
[{(1-Me-2-CH2C7H5N2)2(CH2C6H5)N}Cu]PF6
(5Me). The
ligand Me2L2 (120 mg, 0.30 mmol) was dissolved in 10 mL of
a 1 : 1 mixture of anhydrous CH3CN–acetone in a Schlenk tube
under N2, and solid [Cu(CH3CN)4]PF6 (113 mg, 0.30 mmol)
was added. The initially colourless solution turned pale yellow
over the course of 12 h of stirring. This solution was evaporated
to dryness under reduced pressure, washed with 2 portions of
10 mL diethylether, and dried under vacuum to obtain 5Me as
colourless microcrystals. Yield: 143 mg (79%), mp 239–243 °C
(dec.). IR (KBr): 3062, 3031, 2953, 2852, 1680, 1616, 1490,
1455, 1358, 1331, 1296, 1273, 1242, 1213, 1159, 1106, 1075,
3
7.52 (d, J = 7.50 Hz, 2 H, Ph), 7.38 (m, 7 H, Bzim, Ph), 3.97
(s, 4 H, NCH2Bzim), 3.88 (s, 2 H, NCH2Ph) ppm. 13C{1H}
NMR (CD3CN, 75 MHz, 20 °C): δ 152.23 (Bzim), 140.58
(Bzim), 137.00 (Bzim), 132.79 (Bzim), 129.03 (Bzim), 128.10
(Bzim), 127.35 (Bzim), 123.40 (Ph), 122.62 (Ph), 117.68 (Ph),
111.63 (Ph), 58.88 (NCH2Ph), 49.33 (NCH2Bzim) ppm. Anal.
Calcd for C23H24ClCuN5O5.5: C, 49.55; H, 4.34; N, 12.56.
Found: C, 49.54; H, 4.54; N, 12.82. Mass spectrum ES (aceto-
nitrile–methanol): m/z = 430 [L2Cu]+ (100%).
1
1008, 969, 840, 745, 704, 621, 558, 482, 443 cm−1. H NMR
(CD3CN, 300 MHz, 20 °C): δ 7.84 (m, 2 H, Bzim), 7.52 (m,
4 H, Bzim), 7.33 (m, 7 H, Bzim, Ph), 4.01 (s, 4 H, NCH2Bzim),
3.89 (s, 2 H, NCH2Ph), 3.70 (s, 6 H, NCH3) ppm. 13C{1H}
NMR (CD3CN, 75 MHz, 20 °C): δ 153.97 (Bzim), 141.12
(Bzim), 138.22 (Bzim), 136.30 (Bzim), 130.69 (Bzim), 129.35
(Bzim), 128.87 (Bzim), 124.43 (Ph), 124.06 (Ph), 118.99 (Ph),
111.61 (Ph), 60.54 (NCH2Ph), 49.73 (NCH2Bzim), 31.03
(ArCH3) ppm. Anal. Calcd for C27H28CuF6N6P: C, 50.27; H,
4.38; N, 13.03. Found: C, 50.27; H, 4.30; N, 12.81.
[{(2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N}Cu]ClO4 (6). L3
(100 mg, 0.23 mmol), [Cu(CH3CN)4]ClO4 (80 mg, 0.23 mmol),
colourless solid. Yield: 50 mg (35%), mp 153–156 °C (dec.). IR
(KBr): 3261, 3061, 2964, 2922, 2863, 1621, 1601, 1535, 1451,
1384, 1318, 1275, 1226, 1101, 925, 851, 806, 745, 620, 546,
1
495, 434 cm−1. H NMR (CD3CN, 300 MHz, 20 °C): δ 10.93
(s, 2 H, NH), 7.80 (m, 2 H, Bzim), 7.50 (m, 2 H, Bzim), 7.26
[{(1-Me-2-CH2C7H5N2)2(2,4-Me2C6H3SCH2CH2)N}Cu]ClO4
(6Me). The ligand Me2L3 (112 mg, 0.24 mmol) was dissolved in
10 mL of anhydrous CH3CN in a Schlenk tube under N2, and
solid [Cu(CH3CN)4]ClO4 (79 mg, 0.24 mmol) was added. The
initially colourless solution turned pale yellow over the course of
3 h of stirring. This solution was evaporated to dryness under
reduced pressure, washed with two portions of 10 mL diethyl
ether, and dried under vacuum to obtain 6Me as colourless micro-
crystals. Yield: 133 mg (82%), mp 216–219 °C (dec.). A small
amount of the hexafluorophosphate complex was prepared exclu-
sively for combustion analysis employing the same methodo-
logy. IR (KBr): 3030, 2947, 2919, 2868, 1615, 1486, 1451,
1359, 1328, 1279, 1242, 1212, 1084, 1008, 975, 926, 873, 809,
3
(m, 4 H, Bzim), 7.00 (d, J = 7.20 Hz, 1 H, Ar), 6.91 (s, 1 H,
3
Ar), 6.59 (d, J = 7.20 Hz, 1 H, Ar) 3.98 (s, 4 H, NCH2Bzim),
2.95 (m, 4 H, NCH2CH2S), 2.10 (s, 6 H, ArCH3) ppm. 13C{1H}
NMR (CD3CN, 75 MHz, 20 °C): δ 154.48 (Bzim), 148.97 (Ar),
146.10 (Bzim), 138.10 (Bzim), 135.01 (Bzim), 132.42 (Bzim),
130.78 (Bzim), 128.39 (Bzim), 127.38 (Ar), 124.97 (Ar), 124.29
(Ar), 115.46 (Ar), 68.64 (NCH2Bzim), 55.36 (NCH2CH2S),
26.60 (NCH2CH2S), 21.23 (ArCH3), 20.82 (ArCH3) ppm. Anal.
Calcd for C26H31ClCuN5O6S: C, 48.75; H, 4.88; N, 10.93.
Found: C, 48.47; H, 5.07; N, 10.67. Mass spectrum ES (aceto-
nitrile–methanol): m/z = 504 [L3Cu]+ (100%).
t
[{(1-CO2 Bu-2-CH2C7H4N2)2(2-CH2C4H3S)N}Cu]ClO4 (4BOC).
1
749, 621, 546, 456, 430 cm−1. H NMR (CD3CN, 300 MHz,
The ligand BOC2L1 (300 mg, 0.52 mmol) was dissolved in
15 mL of anhydrous CH3CN in a Schlenk tube under N2, and
solid [Cu(CH3CN)4]ClO4 (170 mg, 0.52 mmol) was added. The
initially colourless solution turned pale yellow over the course of
12 h of stirring. This solution was evaporated to dryness under
reduced pressure, washed with 2 portions of 10 mL diethylether,
and dried under vacuum to obtain 1 as colourless microcrystals.
Yield: 280 mg (73%), mp >300 °C (dec. 192–195 °C). IR
(KBr): 3266, 2979, 2940, 2897, 1752, 1527, 1458, 1433, 1398,
1372, 1343, 1317, 1270, 1211, 1154, 1088, 988, 945, 879, 850,
20 °C): δ 7.88 (m, 2 H, Bzim), 7.55 (m, 2 H, Bzim), 7.36 (m,
3
4 H, Bzim), 7.13 (d, J = 7.80 Hz, 1 H, Thioph), 6.97 (s, 1 H,
Thioph), 6.68 (d, 3J = 7.80 Hz, 1 H, Thioph), 4.17 (s, 4 H,
NCH2Bzim), 3.72 (s, 6 H, NCH3), 3.06 (s, 4 H, NCH2CH2S),
2.30 (s, 3 H, ArCH3), 2.18 (s, 3 H, ArCH3) ppm. 13C{1H} NMR
(CD3CN, 75 MHz, 20 °C): δ 141.31 (Bzim), 139.51 (Bzim),
137.93 (Bzim), 132.16 (Bzim), 131.23 (Bzim), 130.98 (Bzim),
124.29 (Ar), 123.92 (Ar), 119.07 (Ar), 118.76 (Ar), 111.56 (Ar),
55.40 (NCH2CH2S), 52.08 (NCH2CH2S), 34.01 (NCH2Bzim),
20.91 (ArCH3), 20.57 (ArCH3) ppm. Anal. Calcd for
C28H31CuF6N5PS: C, 49.59; H, 4.61; N, 10.33. Found: C,
49.63; H, 4.58; N, 10.37.
756, 717, 621, 497, 473, 431 cm−1 1H NMR (CD3CN,
.
300 MHz, 20 °C): δ 7.99 (m, 2 H, Bzim), 7.80 (m, 2 H, Bzim),
3
7.43 (m, 4 H, Bzim), 7.34 (d, J = 5.10 Hz, 1 H, Thioph), 7.15
3
3
(d, J = 3.60 Hz, 1 H, Thioph), 6.96 (dd, J = 3.60, 5.10 Hz,
1 H, Thioph), 4.43 (s, 4 H, NCH2Bzim), 4.33 (s, 2 H,
NCH2Thioph), 1.05 (s, 18 H, tBu) ppm. 13C{1H} NMR
(CD3CN, 75 MHz, 20 °C): δ 156.92 (CvO), 149.11 (Bzim),
140.35 (Bzim), 138.87 (Bzim), 134.12 (Bzim), 130.33 (Bzim),
Ligand recovery after Cu+ complex oxidation
Solutions of the cuprous complexes 4–6, 5Me and 6Me (ca.
100 mg) in 10 mL of THF, acetone or a THF–acetone mixture
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9394–9404 | 9403

View Article Online
D. Cavagnino, G. Cerina, L. Santagostini and R. Ugo, Inorg. Chem.,
1996, 35, 7516; (c) I. Garcia-Bosch, A. Company, J. R. Frisch,
M. Torrent-Sucarrat, M. Cardellach, I. Gamba, M. Güell, L. Casella,
L. Que Jr., X. Ribas, J. M. Luis and M. Costas, Angew. Chem., Int. Ed.,
2010, 49, 2406.
prepared in the N2-filled glovebox were exposed to air for 24 h.
The solutions were diluted with ethyl acetate (10 mL), and
extracted with 10 mL of concentrated NH4OH solutions for
demetalation. The aqueous phase was further extracted with 2
portions of 10 mL ethyl acetate. The combined organic phases
were analysed by TLC by comparison with original samples of
L1–L3, Me2L2 and Me2L3, then dried with Na2SO4 and evapor-
ated to dryness to quantify the amount of ligand recovered by
weight (85–92%).
14 D. J. Rabiger and M. M. Joullié, J. Org. Chem., 1964, 29, 476.
15 B. Ülküseven, I. Kizilcikli, A. Tavman and B. Akkurt, Rev. Inorg. Chem.,
2001, 21, 369.
16 (a) L. A. Finney and T. V. O’Halloran, Science, 2003, 300, 931;
(b) E. I. Solomon, R. K. Szilagyi, S. D. George and L. Basumallick,
Chem. Rev., 2004, 104, 419; (c) D. B. Rorabacher, Chem. Rev., 2004,
104, 651; (d) S. T. Prigge, B. A. Eipper, R. E. Mains and L. M. Amzel,
Science, 2004, 304, 864; (e) J. P. Klinman, J. Biol. Chem., 2005, 281,
3013; (f) J. Shearer, P. Soh and S. Lentz, J. Inorg. Biochem., 2008, 102,
2103; (g) A. V. Davis and T. V. O’Halloran, Nat. Chem. Biol., 2008, 4,
148; (h) S. I. Chan and S. S.-F. Yu, Acc. Chem. Res., 2008, 41, 969.
17 (a) T. Osako, Y. Ueno, Y. Tachi and S. Itoh, Inorg. Chem., 2004, 43,
6516; (b) R. C. Pratt, L. M. Mirica and T. D. P. Stack, Inorg. Chem.,
2004, 43, 8030; (c) C. Belle, W. Rammal and J.-L. Pierre, J. Inorg.
Biochem., 2005, 99, 1929; (d) N. W. Aboelella, B. F. Gherman,
L. M. R. Hill, J. T. York, N. Holm, V. G. Young Jr., C. J. Cramer and
W. B. Tolman, J. Am. Chem. Soc., 2006, 128, 3445; (e) L. Q. Hatcher,
D.-H. Lee, M. A. Vance, A. E. Milligan, R. Sarangi, K. O. Hodgson,
B. Hedman, E. I. Solomon and K. D. Karlin, Inorg. Chem., 2006, 45,
10055; (f) G. Y. Park, Y. Lee, D.-H. Lee, J. S. Woertink, A. A. Narducci
Sarjeant, E. I. Solomon and K. D. Karlin, Chem. Commun., 2010, 46, 91.
18 (a) M. Kodera, T. Kita, I. Miura, N. Nakayama, T. Kawata, K. Kano and
S. Hirota, J. Am. Chem. Soc., 2001, 123, 7715; (b) D.-H. Lee,
L. Q. Hatcher, M. A. Vance, R. Sarangi, A. E. Milligan, A. A. Narducci
Sarjeant, C. D. Incarvito, A. L. Rheingold, K. O. Hodgson, B. Hedman,
E. I. Solomon and K. D. Karlin, Inorg. Chem., 2007, 46, 6056;
(c) Y. Lee, D.-H. Lee, G. Y. Park, H. R. Lucas, A. A. Narducci Sarjeant,
M. T. Kieber-Emmons, M. A. Vance, A. E. Milligan, E. I. Solomon and
K. D. Karlin, Inorg. Chem., 2010, 49, 8873.
19 (a) J. V. Dagdigian and C. A. Reed, Inorg. Chem., 1979, 18, 2623;
(b) J. V. Dagdigian, V. McKee and C. A. Reed, Inorg. Chem., 1982, 21,
1332; (c) R. P. Bonomo, E. Rizzarelli, M. Bressan and A. Morvillo,
Inorg. Chim. Acta, 1991, 186, 21.
20 L. A. Rodríguez Solano, I. Aguiñiga, M. López Ortiz, R. Tiburcio,
A. Luviano, I. Regla, E. Santiago-Osorio, V. M. Ugalde-Saldívar,
R. A. Toscano and I. Castillo, Eur. J. Inorg. Chem., 2011, 3454.
21 P. R. Martínez-Alanis, M. López Ortiz, I. Regla and I. Castillo, Synlett,
2010, 423.
22 R. Mukherjee, in Comprehensive Coordination Chemistry II, ed.
J. A. McCleverty and T. J. Meyer, Elsevier, Amsterdam, 2004, vol. 6,
pp. 747–910.
23 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C. Verschoor,
J. Chem. Soc., Dalton Trans., 1984, 1349.
Acknowledgements
The authors thank DGAPA-PAPIIT (Proyecto IN211509) and
the Swedish Research Council (VR) for financial support, Rubén
A. Toscano and Simón Hernández-Ortega for X-ray crystallogra-
phy, Virginia Gómez-Vidales for ESR, Carmen Márquez and
Luis Velasco for mass spectrometry, Rocío Patiño for IR spec-
troscopy, Eréndira García, Professor Juventino García, and
USAI-Facultad de Química, UNAM for combustion analysis.
Notes and references
1 (a) L. M. Mirica, X. Ottenwaelder and T. D. P. Stack, Chem. Rev., 2004,
104, 1013; (b) E. A. Lewis and W. B. Tolman, Chem. Rev., 2004, 104,
1047; (c) R. A. Himes and K. D. Karlin, Curr. Opin. Chem. Biol., 2009,
13, 119.
2 (a) C. Mealli, C. S. Arcus, J. L. Wilkinson, T. J. Marks and J. A. Ibers,
J. Am. Chem. Soc., 1976, 98, 711; (b) M. R. Malachowski, H. B. Huynh,
L. J. Tomlinson, R. S. Kelly and J. W. Furbee, Jr., J. Chem. Soc., Dalton
Trans., 1995, 31; (c) Z. Chen, N. Karasek, D. C. Craig and S. B. Colbran,
J. Chem. Soc., Dalton Trans., 2000, 3445.
3 (a) K. D. Karlin, S. Kaderli and A. D. Zuberbühler, Acc. Chem. Res.,
1997, 30, 139; (b) S. Itoh and S. Fukuzumi, Acc. Chem. Res., 2007, 40,
592.
4 (a) J. A. Halfern, S. Mahapatra, E. C. Wilkinson, S. Kaderli,
V. G. Young, Jr., L. Que, Jr., A. D. Zuberbühler and W. B. Tolman,
Science, 1996, 271, 1397; (b) S. Schindler, Eur. J. Inorg. Chem., 2000,
2311.
5 (a) L. Zhou, D. Powell and K. M. Nicholas, Inorg. Chem., 2007, 46,
2316; (b) L. Zhou, D. Powell and K. M. Nicholas, Inorg. Chem., 2007,
46, 7789; (c) L. Zhou and K. M. Nicholas, Inorg. Chem., 2008, 47, 4356.
6 T. R. Musgrave and E. R. Humburg, Jr., Adv. Heterocycl. Chem., 1987,
41, 250.
24 T. Ohta, T. Tachiyama, K. Yoshizawa, T. Yamabe, T. Uchida and
T. Kitagawa, Inorg. Chem., 2000, 39, 4358.
7 R. H. Linnell, J. Org. Chem., 1960, 25, 290.
25 E. A. Ambundo, M.-V. Deydier, A. J. Grall, N. Aguera-Vega,
L. T. Dressel, T. H. Cooper, M. J. Heeg, L. A. Ochrymowycz and
D. B. Rorabacher, Inorg. Chem., 1999, 38, 4233.
8 C. T. Bruice and G. L. Schmir, J. Am. Chem. Soc., 1958, 80, 148.
9 N. C. Li, B. E. Doody and J. M. White, J. Am. Chem. Soc., 1957, 79,
5859.
26 Handbook of Heterocyclic Chemistry, ed. A. R. Katritzky,
C. A. Ramsden, J. A. Joule and V. V. Zhdankin, Elsevier, Amsterdam,
3rd edn, 2010.
10 M. S. Sun and D. G. Brewer, Can. J. Chem., 1967, 45, 2729.
11 J. T. Edsall, G. Felsenfeld, D. S. Goodman and F. R. N. Gurd, J. Am.
Chem. Soc., 1954, 76, 3054.
27 J. R. Harbour and M. L. Hair, J. Phys. Chem., 1978, 82, 1397.
28 C. E. Diaz-Uribe, M. C. Daza, F. Martínez, E. A. Páez-Mozo,
C. L. B. Guedes and E. Di Mauro, J. Photochem. Photobiol., A, 2010,
215, 172.
29 T. Fujii, S. Yamaguchi, S. Hirota and H. Masuda, Dalton Trans., 2008,
164.
12 (a) L. K. Thompson, B. Ramaswamy and R. D. Dawe, Can. J. Chem.,
1978, 56, 1311; (b) A. W. Addison, H. M. J. Hendriks, J. Reedijk and
L. K. Thompson, Inorg. Chem., 1981, 20, 103; (c) R. Balamurugan and
M. Palaniandavar, Inorg. Chem., 2001, 40, 2246; (d) M. Vaidyanathan,
R. Balamurugan, U. Sivagnanam and M. Palaniandavar, J. Chem. Soc.,
Dalton Trans., 2001, 3498.
30 P. Chen and E. I. Solomon, J. Am. Chem. Soc., 2004, 126, 4991.
31 T. C. Deivaraj and J. J. Vittal, J. Chem. Soc., Dalton Trans., 2001, 322.
13 (a) L. Casella, M. Gullotti, R. Radelli and P. Di Gennaro, J. Chem. Soc.,
Chem. Commun., 1991, 1611; (b) L. Casella, E. Monzani, M. Gullotti,
9404 | Dalton Trans., 2012, 41, 9394–9404
This journal is © The Royal Society of Chemistry 2012