Building complexity in O2-binding copper complexes. site-selective metalation and intermolecular O2-binding at dicopper and heterometallic complexes derived from an unsymmetric ligand

Article abstract of DOI:10.1021/ic501951f

A novel unsymmetric dinucleating ligand (L^N3N4) combining a tridentate and a tetradentate binding sites linked through a m-xylyl spacer was synthesized as ligand scaffold for preparing homo- and dimetallic complexes, where the two metal ions are bound in two different coordination environments. Site-selective binding of different metal ions is demonstrated. L^N3N4 is able to discriminate between Cu^I and a complementary metal (M′ = Cu^I, Zn^II, Fe^II, Cu^II, or Ga^III) so that pure heterodimetallic complexes with a general formula [Cu^IM′(L^N3N4)]ⁿ⁺ are synthesized. Reaction of the dicopper(I) complex [Cu^I₂(L^N3N4)]²⁺ with O₂ leads to the formation of two different copper-dioxygen (Cu₂O₂) intermolecular species (O and ^TP) between two copper atoms located in the same site from different complex molecules. Taking advantage of this feature, reaction of the heterodimetallic complexes [CuM′(L^N3N4)]ⁿ⁺ with O₂ at low temperature is used as a tool to determine the final position of the Cu^I center in the system because only one of the two Cu₂O₂ species is formed.

Full text of DOI:10.1021/ic501951f

Article
pubs.acs.org/IC
Building Complexity in O2‑Binding Copper Complexes. Site-Selective
Metalation and Intermolecular O2‑Binding at Dicopper and
Heterometallic Complexes Derived from an Unsymmetric Ligand
Joan Serrano-Plana, Miquel Costas,* and Anna Company*
̀ ̀ ̀
Grup de Química Bioinorganica, Supramolecular i Catalisi (QBIS-CAT), Institut de Química Computacional i Catalisi (IQCC),
Departament de Química, Universitat de Girona, Campus Montilivi, E17071 Girona, Catalonia, Spain
S
* Supporting Information
ABSTRACT: A novel unsymmetric dinucleating ligand (LN3N4) combining a tridentate
and a tetradentate binding sites linked through a m-xylyl spacer was synthesized as ligand
scaffold for preparing homo- and dimetallic complexes, where the two metal ions are
bound in two different coordination environments. Site-selective binding of different metal
ions is demonstrated. LN3N4 is able to discriminate between CuI and a complementary
metal (M′ = CuI, ZnII, FeII, CuII, or GaIII) so that pure heterodimetallic complexes with a
general formula [CuIM′(LN3N4)]n+ are synthesized. Reaction of the dicopper(I) complex
[CuI2(LN3N4)]2+ with O2 leads to the formation of two different copper-dioxygen (Cu2O2)
T
intermolecular species (O and P) between two copper atoms located in the same site
from different complex molecules. Taking advantage of this feature, reaction of the
heterodimetallic complexes [CuM′(LN3N4)]n+ with O2 at low temperature is used as a tool
to determine the final position of the CuI center in the system because only one of the two
Cu2O2 species is formed.
ever, the number of model systems featuring unsymmetric
dinuclear sites is rather limited, most likely due to the difficulty
in establishing control over the distribution of the two types of
metals in the system.1,13,25,26 Apart from the inherent difficulty
in the challenging synthesis of unsymmetric dinucleating
ligands, the preparation of heterometallic complexes might be
hampered by the formation of homometallic complexes during
the synthesis. One efficient strategy to overcome this problem
is the design of dinucleating ligands with two differentiated
binding sites that exhibit markedly distinctive affinities for the
two metals involved.26,27 Although the synthesis of these
ligands is not straightforward, unsymmetric dicopper sys-
tems28−30 and synthetic models of cytochrome c oxidase have
been successfully prepared following this methodology.31
In this line, we recently described the preparation of a
dicopper(I) complex bearing a dinucleating ligand that confers
different coordination environments to the two metal centers.32
Reaction of this dicopper complex toward O2 afforded an
unsymmetric trans-1,2-peroxodicopper(II) core, which exhibits
reactivity patterns distinct from symmetric analogues.33
Following these studies, in the present work we explore the
coordination chemistry of a novel polyamine ligand scaffold
designed with the aim to provide two distinct binding sites. The
ligand was devised suitable for preparing well-defined
heterometallic complexes combining copper(I) and another
metal.
INTRODUCTION
■
Unsymmetric dinuclear active centers are commonly found in
metalloproteins.1 They can be either dimetallic centers where
two metal ions of different nature act together to afford activity
(e.g., FeZn purple acid phosphatases2) or homometallic sites in
which the asymmetry originates from a markedly different
coordination environment around each of the metal sites
(hemerythrin3). Especially remarkable is the structural diversity
found in O2-activating enzymes relying on copper.4,5 Some
copper-containing dinuclear proteins have a heterometallic
active site, so that a Cu center shares the active site with a
second metal such as iron (cytochrome c oxidase)6 or zinc
(superoxide dismutase).7 In addition, enzymes such as
peptidylglicine-α-hydroxylating monooxygenase (PHM),8 dop-
amine-β-hydroxylase (DβH),8,9 and tyrosinase10 bear two
copper atoms in distinct coordination environments. While
for PHM and DβH the residues coordinated to each of the
copper ions are different, in the case of tyrosinase, the
differences of the second coordination sphere render the two
metal ions inequivalent. The lack of equivalence enables the
two metals to perform different essential tasks in the enzymatic
reaction.8,9,11,12 The development of well-defined synthetic
models with unsymmetrical bimetallic centers is highly
desirable to understand these enzymatic processes, and it may
be relevant for the development of catalytic systems.13−18
Modeling copper−dioxygen chemistry occurring in enzymes
has attracted the attention of many research groups over the
last three decades, and it is well-stablished that the ligand
architecture surrounding the copper center determines the
structure of the resulting Cu−dioxygen species.5,19−24 How-
Received: August 14, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthetic Route toward the Preparation of LN3N4 and Schematic Representation of LN4N4
The designed ligand (LN3N4) contains two distinct binding
sites (N3 and N4) connected through a meta-xylyl moiety
(Scheme 1). Both donor sets were used independently to
support CuI/O2 chemistry. The tridentate binding site (N3)
corresponds to a triazacyclononane ring (tacn), which forms
highly stable complexes with a number of metals, which are
chemically robust and resistant to oxidative degradation.34 In
this coordination environment, copper(I) is facially bound
through the three aliphatic amines, and free coordination sites
are available for interaction with other molecules. Tolman et al.
have shown that the alkyl substituents in the nitrogen atoms of
the triazacyclonane ring determine the outcome of the reaction
of copper(I) with O2. While iPr-substituted tacn (iPr3tacn)
leads to a mixture of bis(μ-oxo)dicopper(III) (O) and μ−η2:η2-
peroxodicopper(II) (sP) species,35 the less bulky benzyl or
methyl substituents (Bn3tacn and Me3tacn) afford exclusively
bis(μ-oxo)dicopper(III) (O) intermediates.36−38 The second
site (N4) offers a tetracoordinating environment composed of
two aliphatic nitrogen atoms and two pyridines (uns-penp). In
contrast to tacn, uns-penp supports the formation of trans-1,2-
peroxodicopper(II) (TP) species upon reaction with O2,39,40 as
occurs with other N-based tetradentate ligands.41−44 The
markedly different denticity of the two binding sites in LN3N4 is
envisioned to translate into different binding constants that in
turn should the preparation of heterobimetallic complexes
combining copper(I) and another metal without contamination
by the homometallic analogues.
copper centers. The nature of the Cu2O2 unit can be
interrogated by its distinctive spectroscopic properties, and
this in turn can be used to identify which is the binding site (N3
or N4) of CuI and M′ in the heterometallic complex. This
analysis demonstrates that metal binding in LN3N4 is site-
selective.
RESULTS AND DISCUSSION
■
Synthesis of LN3N4 and LN4N4. LN3N4 was obtained after
four reaction steps with moderate yields starting from Me2tacn·
3HBr and 3-(bromomethyl)benzaldehyde (Scheme 1, see
Experimental Section for details). The meta-xylyl linker was
chosen because it has been successfully applied in the past for
the preparation of dicopper complexes that intramolecularly
bind O2 and that can be seen as models of O2-activating
dicopper proteins.5 The use of the unsymmetric platform 3-
(bromomethyl)benzaldehyde, prepared by reductive hydrolysis
of the commercially available 3-(bromomethyl)benzonitrile,45
enables the connection of two differentiated donor sets in the
same molecule. The linkage of the N3 set (tacn) occurs by
reaction of 3-(bromomethyl)benzaldehyde with equimolar
amounts of Me2tacn·3HBr through a nucleophilic substitution
reaction to give compound a. The N4 coordination site is
introduced by a condensation reaction between the aldehyde
functionality and the primary amine of uns-penp to give an
imine (b). Its hydrogenation and posterior methylation affords
the final ligand LN3N4, which is purified by column
chromatography.
Thus, in this work, we report the synthesis of the
dinucleating ligand LN3N4 and we explore its ability to
coordinate to copper(I) together with a second metal ion to
give the corresponding heterobimetallic complexes
[CuIM′(LN3N4)]n+ (M′ = CuI, ZnII, FeII, CuII, or GaIIII).
Heterometallic complexes [CuIM′(LN3N4)]n+ exhibit fast
reactivity with O2 to form tetrametallic [(LN3N4)M′Cu(O2)-
CuM′(LN3N4)]2n+ species where the O2 molecule binds two
For the sake of comparison, we also synthesized the
symmetric ligand LN4N4 (Scheme 1 and Supporting Informa-
tion, Scheme S1) bearing two identical tetracoordinating
binding sites connected through a meta-xylyl moiety. LN4N4
was prepared through a similar synthetic route as the one
followed to obtain LN3N4 (see Supporting Information for
further details).
B
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
1
Figure 1. H NMR monitoring of the titration of LN3N4 with Zn(CF3SO3)2 in acetone-d6 at 240 K. For clarity, only the aromatic region of the H
NMR spectrum is shown. The aliphatic region of the spectra is shown in Supporting Information, Figure S1. The tridentate and tetradentate sites are
represented by a yellow triangle and a purple square, respectively.
Figure 2. (a) UV−vis absorption spectra for the reaction of [CuI2(LN3N4)]2+ (3) (0.30 mM) and O2 at −90 °C in acetonitrile/acetone 1:19. (inset)
Time traces at 405 and 530 nm (3O/TP). (b) CSI-MS spectra at −90 °C for the reaction of 3 with O2 in acetonitrile:acetone 1:19 leading to the
formation of a dimeric species (3O/TP).
Homodimetallic Complexes. Titration of LN3N4 with
Zn(CF3SO3)2. To explore the ability of LN3N4 to selectively bind
a metal ion in one of its two binding sites, we performed a H
ion is occurring between N3 and N4 sites. When more than 1
equiv of ZnII is added the signals corresponding to Hα, Hβ, and
Hγ of the free ligand (LN3N4) completely disappear, and new
well-defined signals clearly arise (8.85, 8.74, and 8.18 ppm,
Figure 1). Similar information can be extracted by analyzing the
aliphatic region of the NMR spectrum (Supporting Informa-
tion, Figure S1). Addition of a total of 2 equiv of ZnII affords
1
NMR titration of the ligand in deuterated acetone (acetone-d6)
by adding small amounts of a metal salt (Figure 1). In
particular, we chose Zn(CF3SO3)2 because of its diamagnetic
character and its stability under air enables its manipulation in
the open atmosphere. The 1H NMR spectrum of the free ligand
(LN3N4) in acetone-d6 shows readily identifiable signals in the
aromatic region corresponding to the two equivalent pyridine
units in the tetradentate N4 site (α, β, and γ protons) along
with the less well-defined signals of the meta-xylyl moiety.
Addition of 0.5 equiv of Zn(CF3SO3)2 causes a broadening of
these signals, but no apparent shift is observed, which suggests
that ZnII preferentially binds to the N3 site and not to N4.
Addition of 0.5 equiv more of ZnII causes severe broadening of
the NMR lines, so that most likely some exchange of this metal
the homodimetallic complex [ZnII (LN3N4)]4+ (1), which was
2
further characterized by high-resolution mass spectrometry
(HR-MS) (Figure 1 and Supporting Information, Figure S2).
Remarkably, the aromatic region of the 1H NMR spectrum of 1
resembles that of the complex with the symmetric ligand
[ZnII (LN4N4)](CF3SO3)4 (2) (Supporting Information, Figure
2
S4). Thus, it seems that the first equivalent of zinc preferentially
binds to the tacn ring (N3) of LN3N4 and that the second
equivalent binds to the N4 binding site to form 1. This
observation suggests that LN3N4 can be potentially used to hold
C
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
heterobimetallic complexes generated by sequential addition of
two different metals.
Scheme 2. Representation of the Possible Structures of
a
3O/TP
Synthesis of [CuI2(LN3N4)]2+ (3) and Its Reactivity with O2.
The dicopper(I) complex [CuI2(LN3N4)]2+ (3) was prepared in
the glovebox by reaction of LN3N4 with 2 equiv of
[CuI(CH3CN)4](CF3SO3) in acetonitrile affording a bright
yellow solution. All attempts to isolate this compound by
precipitation or crystallization using diethyl ether were
unsuccessful due to the tendency of the resulting complex to
disproportionate, as evidenced by a color change to deep green
(CuII) and the formation of copper mirror (Cu0) upon addition
of diethyl ether into solutions of 3. Synthesis of 3 in any solvent
different from acetonitrile such as CH2Cl2, tetrahydrofuran
(THF), or acetone also led to its immediate disproportionation.
Thus, characterization of 3 was carried out in solution in
1
acetonitrile-containing mixtures. The H NMR spectrum of 3
exhibits broad signals even at 240 K (Supporting Information,
Figure S5),46 while HR-MS shows the expected peaks for this
dicopper(I) complex (Supporting Information, Figure S6).
As 3 could not be isolated, studies of its reaction with O2
were carried out starting from freshly prepared solutions by
mixing LN3N4 with 2 equiv of the copper(I) source. A
concentrated stock solution of 3 in acetonitrile (18 mM) was
prepared, and after a 60-fold dilution with dry acetone, the
reaction with O2 was monitored by UV−vis spectroscopy at
−90 °C. Upon exposure to O2, two intense absorption bands
concurrently developed within 1 min (Figure 2a): the
absorption at λmax 530 nm (ε = 2000 M−1 cm−1) reached its
maximum absorbance after 45 s of reaction, and then it
decayed, while another band at λmax 405 nm (ε = 15 000 M−1
cm−1) was formed approximately over the same period of time,
but it remained stable for at least 15 min at −90 °C.
Remarkably, the latter quickly decomposed upon warming the
solution, and none of the bands were detected when the
reaction of 3 with O2 was performed at room temperature.
Finally, it is worth highlighting that the oxygenation of 3 was
irreversible, as recovery of the starting dicopper(I) complex was
not achieved when vacuum was applied after reaction with O2.
The fact that these two bands follow completely different
decomposition time courses indicates that they arise from two
independent chromophores. The reported O species obtained
upon reaction of two mononuclear [CuI(Me3tacn)]+ units with
O2 exhibits very characteristic absorption bands at 307 nm (ε =
16 000 M−1 cm−1) and 412 nm (ε = 18 000 M−1 cm−1),37 while
TP species originating from the reaction of [CuI(uns-penp)]+
with O2 presents a UV−vis absorption at 535 nm (ε = 7000
M−1 cm−1).40 Comparison of these reported data with the
observed reactivity of 3 with O2 suggests that the unstable
intermediate with λmax = 530 nm may correspond to a TP
species formed by interaction of two copper centers
coordinated to the N4 sites of two different molecules.
Similarly, the intense band observed at 405 nm suggests the
formation of an O species by interaction of copper centers
located in the N3 sites of two different molecules, giving rise to
a complex structure containing two different Cu2O2 cores
(3O/TP). Because of the intermolecular character of O2 binding
in 3, both a dimeric species or an oligomeric/polymeric
structure are plausible structures for 3O/TP (Scheme 2a,b,
respectively). Cryospray ionization mass spectrometry (CSI-
MS) turned out to provide conclusive information to
distinguish between the two possibilities. Analysis of the initial
stages of the reaction of 3 with O2 by CSI-MS at 183 K (Figure
2b) clearly showed a major peak at m/z 1795.3 with a mass
a
(a) Dimeric species. (b) Oligomeric/polymeric species.
value and an isotopic pattern corresponding to {[(LN3N4)-
CuIICuIII(μ-O)2(μ-1,2-O2)CuIICuIII(LN3N4)](CF3SO3)3}+,
which is consistent with the formation of a dimeric species in
3O/TP (Scheme 2a). No peak that could be attributed to an
oligomeric/polymeric species was detected.
T
To gain more evidence about the formation of a putative P
species by interaction of two copper(I) centers in two different
N4 sites, we synthesized the symmetric dicopper(I) complex
using LN4N4 [CuI2(LN4N4)]2+ (4) (Scheme 1 and Supporting
Information, Figure S7). Indeed, upon reaction of 4 with O2 at
−90 °C in acetonitrile/acetone 1:19 the exclusive formation of
the band at λmax = 530 nm (ε = 4400 M−1 cm−1) was observed
(Supporting Information, Figure S8).
Heterobimetallic Complexes. Despite the fact that both
binding sites in LN3N4 are nitrogen-based, the coordination
number, donor set, and binding constants were envisioned to
be different enough to enable the preparation of hetero-
bimetallic complexes in which each of the metal ions is
selectively coordinated to one of the binding sites, that is, N3 or
N4. In this line and as shown above, titration of LN3N4 with ZnII
indicates that this ligand is potentially a good candidate to hold
heterobimetallic complexes.
Synthesis of [MIICuI(LN3N4)]3+ (M = Zn (5), Cu (6), Fe (7))
and Their Reactivity with O2. To test the ability of LN3N4 to
give well-defined heterobimetallic complexes, initial experi-
ments were carried out by combining copper(I) and zinc(II).
The use of these diamagnetic metal ions enables the proper
characterization of the resulting complexes by 1H NMR
spectroscopy.
[ZnIICuI(LN3N4)]3+ (5) was prepared in the glovebox by
sequential addition of 1 equiv of [CuI(CH3CN)4](CF3SO3)
and 1 equiv of Zn(CF3SO3)2 in an acetonitrile solution of
LN3N4. No matter the order of addition of the two metals, in any
1
case the H NMR spectrum of the product was the same. The
aromatic region of the 1H NMR spectrum of 5 presents a great
similarity with that of 1 and 2 (Figure 3 and Supporting
Information, Figure S4), meaning that zinc coordinates to the
N4 site. If, instead, CuI were located in the pyridine site,
1
broader NMR signals, like those observed in the H NMR
spectrum of 3 or 4, would be observed (Figure 3 and
Supporting Information, Figure S5). Thus, according to H
NMR, in compound 5 zinc binds to the N4 site, while copper is
1
bound to the N3 unit. It is worth highlighting that the addition
D
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
Figure 3. H NMR spectrum of 1, 3, and 5 in CD3CN/acetone-d6 1:5 at 240 K. Only the aromatic region is shown for clarity.
Figure 4. HR-MS spectrum of 5. The peak at 878.2141 corresponds to the mononuclear {[Zn(LN3N4)](CF3SO3)2H}+ complex (LN3N4 = C31H45N7).
of 1 equiv of CuI after the addition of 1 equiv of ZnII displaces
the latter from the N3 site to the N4 site (as ascertained in the
titration experiment, the first equivalent of zinc coordinates to
N3 site).
M−1 cm−1) corresponding to the intermolecular O species
[(LN3N4)ZnIICuIII(μ-O)2CuIIIZnII(LN3N4)]6+ (5O). No growth
of any band at 530 nm characteristic of the P species was
observed, meaning that all the copper is located in the tacn
tridentate binding site (Figure 5a). Remarkably, formation of
5O was observed independently of the order of addition of the
two metallic salts to LN3N4 during the complex synthesis, which
agrees with the formation of the same species as ascertained by
NMR (see above).
T
Compound 5 was also characterized by HR-MS (Figure 4).
Two main peaks were observed at m/z = 828.1501 and
942.1346 with mass values and isotopic patterns fully consistent
with the pure heterodimetallic species {[CuIZnII(LN3N4)]-
(CF3SO3)(Cl)}+ and {[CuIZnII(LN3N4)](CF3SO3)2}+. Remark-
ably, no peaks of the possible homodimetallic dizinc(II) or
dicopper(I) complexes 1 and 3, respectively, were detected.
To obtain more experimental evidence in favor of the
heterobimetallic complex 5, we studied its reactivity with O2 at
cryogenic temperatures. UV−vis monitoring of the reaction of
5 with O2 at −90 °C in acetonitrile/acetone 1:19 showed the
formation within 20 min of a new band at 405 nm (ε = 11 900
The formation of 5O was further confirmed by CSI-MS at
183 K, which afforded a spectrum with peaks at m/z 2065.2049,
1951.2226, and 1839.2400 whose mass value and distribution
pattern were consistent with {[(LN3N4)ZnIICuIII(μ-
O)2CuIIIZnII(LN3N4)(CF3SO3)5}+, {[(LN3N4)ZnIICuIII(μ-
O)2CuIIIZnII(LN3N4)](CF3SO3)4Cl}+, and {[(LN3N4)-
ZnIICuIII(μ-O)2CuIIIZnII(LN3N4)](CF3SO3)3Cl2}+, respectively
E
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. (a) UV−vis absorption spectra for the reaction of 5 (0.3 mM) with O2 in acetonitrile/acetone 1:19 at −90 °C. (inset) Time trace at 405
nm. (b) CSI-MS spectra at −90 °C corresponding to the reaction of 5 with O2 in acetonitrile/acetone 1:19 to give 5O (LN3N4 = C31H45N7).
(Figure 5b). Remarkably all the identified peaks in the ESI-MS
spectrum corresponded to pure heterobimetallic species
containing both copper and zinc, which further renders LN3N4
as a privileged platform to selectively hold two different metals.
Since both ZnII and CuI have a d10 electronic configuration,
none of them has stabilization of the crystal field according to
the crystal field theory. Thus, it is remarkable that CuI is located
T
in only one site, avoiding mixtures of O and P species upon
reaction with O2.
Preparation of [CuIICuI(LN3N4)]3+ (6) and [FeIICuI-
(LN3N4)]3+ (7) was also achieved through the sequential
addition of 1 equiv of [CuI(CH3CN)4](CF3SO3) and then 1
equiv of CuII(CF3SO3)2 or [FeII(CF3SO3)2(CH3CN)2] to
L
N3N4 in acetonitrile. These compounds could be characterized
by HR-MS (Supporting Information, Figures S9 and S10).
Attempts to characterize 6 and 7 by means of 1H NMR proved
unsuccessful, as only broad, not informative bands were
observed in the spectra. UV−vis monitoring of the reaction
of 6 and 7 with O2 at −90 °C in acetonitrile/acetone 1:19 at
−90 °C indicated the exclusive formation in both cases of an O
species, 6O (λmax = 405 nm, ε = 12 000 M−1 cm−1), and 7O
(λmax = 405 nm, ε = 12 300 M−1 cm−1), most likely arising from
the intermolecular interaction between two copper(I) centers
coordinated to N3 sites of two different molecules (Figure 6
and Supporting Information, Figure S11). Interestingly, no
heterometallic intramolecular O2 activation occurred in
[FeIICuI(LN3N4)]3+ (7), where the redox active iron(II) center
could potentially be involved in such activation processes.47
Instead, formation of the intermolecular O species seems to be
the thermodynamic driving force that dictates the fate of the O2
activation. Overall, divalent metals such as ZnII, FeII or CuII
coordinate to the N4 site in LN3N4 leaving the N3 site available
for copper(I), which gives intermolecular O species upon
reaction with O2.
Comparison of the formation and decay rates of the four
bis(μ-oxo) species described in this manuscript (3O/TP, 5O, 6O,
and 7O) might give insight into the influence of the second
metal in the formation of this Cu2O2 species. Interestingly,
compounds 5, 6, and 7, which contain CuI in the trivalent site
(N3) together with a divalent spectator metal such as ZnII, CuII,
and FeII in the tetradentate site (N4), behave in an analogous
fashion, and full formation of the corresponding bis(μ-oxo)
species (5O, 6O, and 7O) is achieved after 10 min at −75 °C. In
Figure 6. UV−vis absorption spectra for the reaction of
[CuICuII(LN3N4)]3+ (6) (0.15 mM) and O2 at −90 °C in
acetonitrile/acetone 1:19. (inset) Time trace at 405 nm (6O).
sharp contrast, formation of 3O/TP is much faster, and this
species is fully developed only 1 min after starting the reaction
of 3 with O2 (Supporting Information, Figure S12a). On the
other hand, the decay rates of 5O, 6O, and 7O are small (only
15% decomposition after 2 h at −60 °C) compared to the
bis(μ-oxo) unit in 3O/TP, which decomposes in only 20 min at
−60 °C (Supporting Information, Figure S12b). The
acceleration in the decay rate of 3O/TP might be related to
the strain exerted by a putative interaction between the CuII
centers resulting from the decomposition of the thermally
unstable trans-peroxide unit. Such acceleration in the formation
and decay rates of Cu2O2 species has been previously
documented in Cu2O2 species derived from strained dinucleat-
ing ligands.48 ESI-MS analyses of the final species derived from
the thermal decomposition of 3O/TP, 5O, 6O, and 7O showed the
exclusive formation of metal-hydroxide species coordinated to
the intact LN3N4 ligand. No ligand oxidation was detected in any
case.
Synthesis of [GaIIICuI(LN3N4)]4+ (8) and Its Reactivity with
O2. The use of GaIII completely changes the position of the
copper center within ligand LN3N4. [GaIIICuI(LN3N4)]4+ (8) was
prepared analogously to the previously described heterometallic
complexes but using GaCl3 as the complementary metal source
F
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. HR-MS spectrum of 8. Peaks at 806.1911 and 818.0461 correspond to mononuclear GaIII species (LN3N4 = C31H45N7).
to copper(I). In the HR-MS spectrum of 8 peaks at m/z
754.1316 and 868.1114 corresponding to the heterodimetallic
complex {[CuIGaIII(LN3N4)](Cl)3}+ and {[CuIGaIII(LN3N4)]-
(Cl)2(CF3SO3)}+ were observed (Figure 7).
CONCLUSIONS
■
In this work we have developed a novel dinuclear unsymmetric
ligand scaffold (LN3N4) that contains two highly differentiated
coordination environments: a tridentate site (N3) composed of
a tacn ring and a tetradentate environment (N4) offered by two
aliphatic amines and two pyridines. It is shown that LN3N4 is
potentially a good candidate for the synthesis of hetero-
dimetallic complexes (Scheme 3). Reaction of the homodinu-
clear copper(I) complex (3) with O2 at low temperature affords
Interestingly, UV−vis monitoring of the reaction of this
complex with O2 at low temperature only exhibited the
T
formation of the characteristic bands of P species (8TP) with
an absorption band centered at 530 nm (Figure 8). No trace of
T
a mixture of O and P species arising from the intermolecular
interaction between two copper centers in the same binding
environment but placed in two different complex molecules
(Table 1). Taking advantage of the ligand asymmetry, a set of
heterodimetallic complexes containing CuI and a divalent or
trivalent complementary metal has been prepared as
ascertained by 1H NMR and HR-MS analyses. Since CuI
forms kinetically labile complexes, its binding site in the
heterometallic complexes is finally dictated by the donor set
preferences of the complementary metal. Divalent metals such
as ZnII, CuII, or FeII bind preferentially at the N4 site and force
the coordination of CuI to the N3 site. Instead, a trivalent metal
such as GaIII preferentially bind to the tacn site, thus
positioning CuI in the N4 site. Since the nature of the O2-
bound species at copper sites depends in their ligand donor
set,5,19,20 it follows that reaction between the heterodimetallic
complexes (5−8) and O2 afforded the exclusive formation of
Figure 8. UV−vis absorption spectra for the reaction of
[GaIIICuI(LN3N4)]4+ (8, 0.79 mM) with O2 in acetonitrile/acetone
1:19 at −90 °C. (inset) Time trace at 530 nm (8TP).
T
either O or P, depending on the donor set of the CuI site
the band at 405 nm characteristic of the O compound was
detected. Such result indicates that the trivalent metal is tightly
bound to the tacn site blocking the access of copper(I) in the
starting material, which is displaced to the N4 site. Remarkably,
the use of redox active trivalent metals such as FeIII caused the
immediate oxidation of the copper(I) site to copper(II), thus
preventing further studies of O2 activation.
(Scheme 3, Table 1). When CuI is bound at the tridentate site
T
O species form upon reaction with O2, while formation of P
species occurs upon reaction of O2 with complexes where CuI is
bound at the N4 site. Indeed, the exclusive formation of O or
TP when 5−8 react with O2 could be regarded as evidence for
site-selective binding to LN3N4. Furthermore, the position of the
G
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 3. Representation of the Complexes and the O2-Bound Species Generated upon Reaction with O2 Described in This
Work
was achieved with a cryostat from Unisoku Scientific Instruments,
Japan
Table 1. Summary of the Spectroscopic Features of the O
and TP Species Formed upon Reaction of [MII/IIICuI(L)]3+/4+
with O2 at −90°C in Acetonitrile/Acetone 1:19
Materials and Synthesis. Only the synthesis of the unsymmetric
ligand LN3N4 is described below. A detailed synthesis procedure for the
symmetric ligand LN4N4 can be found in the Supporting Information.
Reagents and solvents used were of commercially available reagent
quality unless otherwise stated. Solvents were purchased from
Scharlab, Acros, or Sigma-Aldrich and were used without further
purification.
UV−vis features
λmax, nm
Cu2O2
species
a
ligand compound N4 site N3 site
(ε, M−1 cm‑1)
LN3N4
3
CuI
CuI
405 (15 000),
O + TP
530 (2000)
405 (11 900)
405 (12 000)
405 (12 300)
5
6
7
8
4
ZnII
CuII
FeII
CuI
CuI
CuI
CuI
CuI
O
Synthesis of LN3N4. LN3N4 was obtained as a yellow oil after a total
of four reaction steps with moderate yields starting from 1,4-dimethyl-
1,4,7-triazacyclononane trihydrobromide salt (Me2tacn·3HBr), (2-
aminoethyl)bis(2-pyridylmethyl)amine (uns-penp), and 3-
(bromomethyl)benzaldehyde. Me2tacn·3HBr was synthesized after
three reaction steps starting from 1,4,7-tritosyl-1,4,7-triazacyclono-
nane. 3-(bromomethyl)benzaldehyde and uns-penp were synthesized
from commercially available reagents after one and two steps,
respectively, following previously described procedures.49,50 See
Supporting Information for the correspondence of 1H NMR and
13C NMR signal assignments.
O
O
GaIII 530 (4050)
CuI
530 (4400), 600 (sh)
TP
TP
LN4N4
a
ε values were calculated taking into account the expected maximum
concentration of Cu2O2 unit.
copper(I) center within the ligand framework can be tuned by
the complementary metal.
Synthesis of a. Me2tacn·3HBr (1.26 g, 6.3 mmol) and 3-
(bromomethyl)benzaldehyde (2.53 g, 6.3 mmol) were mixed in a
two-necked flask in anhydrous CH3CN (40 mL), leading to a yellow
mixture. Na2CO3 (4.69 g, 44 mmol) and tetrabutylammonium
bromide (TBABr, 0.05 g, 0.16 mmol) were then added directly as
solids. Nitrogen atmosphere was established, and the mixture was
refluxed for 20 h. Then it was cooled to room temperature, and a
yellow solution with a suspended white solid was obtained. The
solution was filtered to remove unreacted Na2CO3, and the filter cake
was washed with CH2Cl2. The solvent from the combined filtrates was
removed under reduced pressure, and an orange oil was obtained.
NaOH (1 M, 15 mL) was added, and the product was extracted with
CH2Cl2 (3 × 10 mL). The organic layers were combined and dried
over MgSO4, and the solvent was removed under reduced pressure. n-
Pentane (50 mL) was added, and the mixture was stirred overnight.
After filtration the solvent from the filtrates was removed under
reduced pressure, and 1.13 g (4.1 mmol, 65%) of a were obtained. 1H
NMR (CDCl3, 400 MHz, 298 K) δ, ppm: 10.02 (s, 1H, Hj), 7.86 (s,
1H, Hi), 7.76 (d, J = 7.6 Hz, 1H, Hh), 7.66 (d, J = 7.6 Hz,1H, Hf), 7.46
(t, J = 7.6 Hz, 1H, Hg), 3.74 (s, 2H, He), 2.80 (s, 4H, Hb), 2.77−2.75
(m, 4H, Hd), 2.68−2.65 (m, 4H, Hc), 2.35 (s, 6H, Ha). 13C NMR
(CDCl3, 100 MHz, 298 K) δ, ppm: 192.55 (Cl), 141.55 (Ch), 136.43
(Cf), 135.26 (Ck), 130.14 (Cg), 128.88 (Cj), 128.45 (Ci), 62.82 (Ce),
57.11, 56.92 (Cd,Cc), 56.02 (Ca), 46.66 (Cb). ESI-MS (m/z): 276.2
[M + H]+
Thus, ligand LN3N4 has been proven to be a good platform for
the synthesis of heterodimetallic complexes. The reactivity of
some of the O and TP species described in this work is
currently being investigated in our lab. Moreover, synthetic
efforts are devoted to the preparation of new asymmetric ligand
scaffolds that can force the intramolecular O2 activation
performed by two different metals akin to the reactivity
exhibited by some enzymes.
EXPERIMENTAL SECTION
Instrumentation. All the spectroscopic and chromatographic
analyses were carried out in Unitat d’Anal
■
́
̀
isi Quimica i Estructural
(UAQiE) or in the laboratories of the Bioinorganic and Supra-
molecular Chemistry Group (QBIS) at the University of Girona.
Elemental analyses were performed using a CHNS-O EA-1108
elemental analyzer from Fiscons. High-resolution mass spectra (HR-
MS) were recorded on a Bruker MicrOTOF-Q II instrument using
electrospray ionization (ESI) or cryospray ionization (CSI) sources at
̀
Serveis Tecnics of the University of Girona. Samples were introduced
into the mass spectrometer ion source by direct infusion using a
syringe pump and were externally calibrated using sodium formate. A
cryospray attachment was used for CSI-MS. Temperature of the
nebulizing and drying gases was set at −90 °C. The capillary voltage
was set at −4500 V, and the collision energy at 8−10 eV. The
instrument was operated in both positive and negative ion modes.
NMR experiments were performed on a Bruker Ultrashield Avance
III400 and Ultrashield DPX300 spectrometers. UV−vis spectroscopy
was performed with an Agilent 50 Scan (Varian) UV−vis
spectrophotometer with 1 cm quartz cells. Low-temperature control
Synthesis of b. A measured quantity of uns-penp (786 mg, 3.2
mmol) was dissolved in THF (5 mL) and cooled to 0 °C. A solution
of a (893 mg, 3.2 mmol) in THF (5 mL) was added dropwise. The
mixture was left to attain room temperature, and it was stirred for 15 h.
The solvent was removed under reduced pressure, and 1.95 g (3.88
mmol, >100%) of a pale yellow oil were obtained and used in the next
step without further purification. 1H NMR (CDCl3, 400 MHz, 298 K)
H
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
δ, ppm:8.52 (d, J = 4.8 Hz, 2H, Hq), 8.25 (s, 1H, Hj), 7.66 (s, 1H, Hf),
7.61−7.52 (m, 3H, Ho+Hn+Hg), 7.45 (d, J = 7.8 Hz, 1H, Hi), 7.37 (t, J
= 7.8 Hz, 1H, Hh), 7.14−7.11 (m, 2H, Hp), 3.93 (s, 4H, Hm), 3.83−
3.78 (m, 2H, Hk), 3.72 (s, 2H, He), 2.98−2.92 (m, 6H, Hl+Hb), 2.78
(br. s., 8H, Hd+Hc), 2.40 (s, 6H, Ha). 13C NMR (CDCl3, 100 MHz,
298 K) δ, ppm: 161.95 (Cp), 159.84 (Ci), 149.04 (Ct), 140.15 (Ch),
136.44 (Cq/Cr), 131.57 (Ck), 128.71, 128.45, 127.15 (Cj, Cg, Ci),
125.53 (Cf), 122.93 (Cq/Cr), 122.02 (Ci), 62.92 (Ce), 60.79 (Co),
59.49 (Cm), 56.42 (Cn), 55.77 (Cc+Cd), 54.89 (Cb), 45.83 (Ca). ESI-
MS (m/z): 500.3 [M + H]+, 276.2 [a+H]+, 243.1 [uns-penp+H]+.
Synthesis of c. Compound b (1.95 g, 3.9 mmol) was dissolved in
absolute ethanol (50 mL), and sodium borohydride (0.16 g, 4.2
mmol) was added as a solid in little portions. The reaction was stirred
for 12 h at room temperature, and then 10 mL of water was added to
remove the unreacted NaBH4. After removal of the solvent under
reduced pressure, CH2Cl2 (15 mL) and H2O (4 mL) were added. The
mixture was treated with CH2Cl2 (2 × 8 mL). The organic layers were
dried over MgSO4, and the solvent was removed under reduced
UV−vis Spectroscopy: Sample Preparation and Monitoring
of the Formed Species at Low Temperature. All the UV−vis
experiments were performed in acetonitrile/acetone 1:19 as the
solvent mixture. The final complex concentration ranged between 0.3
and 0.9 mM. For the preparation of a 0.9 mM sample, a UV−vis cell
was charged with 100 μL of the acetonitrile complex solution (∼18
mM) and 1.9 mL of dry acetone in the glovebox. The quartz cell was
capped with a septum, taken out of the box, and placed in a Unisoku
thermostated cell holder designed for low-temperature experiments at
183 K. After reaching thermal equilibrium, a UV−vis spectrum of the
starting complex was recorded. Dioxygen was injected into the cell
with a balloon and a needle through the septum causing immediate
reaction. For lower concentrations, the sample preparation was the
same as described before, but in this case, the appropriate volume of
the complex solution (∼18 mM) was taken, and CH3CN was added
until the total volume of the aliquot was 100 μL.
ASSOCIATED CONTENT
■
1
pressure to obtain 1.68 g (3.3 mmol, 86%) of a pale yellow oil. H
S
* Supporting Information
NMR (CDCl3, 400 MHz, 298 K) δ, ppm: 8.51 (d, 2H, J = 4.3 Hz, Hq),
7.64−7.60 (m, 2H, Ho), 7.45 (d, 2H, J = 7.7 Hz, Hn), 7.24−7.21 (m,
3H, Hf+Hi+Hh), 7.15−7.12 (m, 3H, Hg+Hp), 3.83 (s, 4H, Hm), 3.66
(s, 2H, Hj), 3.62 (s, 2H, He), 2.79 (s, 4H, Hb), 2.77−2.70 (m, 8H,
Hk+Hl+Hd), 2.65−2.63 (m, 4H, Hc), 2.34 (s, 6H, Ha). 13C NMR
(CDCl3, 100 MHz, 298 K) δ, ppm: 159.67 (Cp), 149.08 (Ct), 140.26
(Cf+Ch), 136.37 (Cr), 128.75, 128.12, 127.63 (Cg+Cj+Ck), 126.47
(Ci), 122.94 (Cq), 121.96 (Cs), 63.26 (Ce), 60.71 (Co), 57.07 (Cc),
56.81 (Cb), 55.90 (Cd), 54.101 (Cm), 53.80 (Cl), 46.68 (Cn). ESI-MS
(m/z): 502.4 [M + H]+, 251.6 [M+2H]2+
Detailed synthesis of LN4N4, NMR spectra of the different steps
in the synthetic routes of LN3N4 and LN4N4, aliphatic region of
the 1H NMR monitoring of the titration of LN3N4 with
Zn(CF3SO3)2, HR-MS spectra of 1−4, 6, and 7, UV−vis
monitoring of the reaction of 4 and 7 with O2, comparison of
the formation/decay rates of bis(μ-oxo) species (3O/TP, 5O, 6O,
7O). This material is available free of charge via the Internet at
http://pubs.acs.org.
Synthesis of LN3N4. Compound c (1.68 g, 3.3 mmol) was dissolved
in formic acid 98% (13 mL); formaldehyde 37% (25 mL) was added,
and the mixture was refluxed for 24 h. The solvent was then removed
under reduced pressure, and NaOH 3 M (5 mL) was added to the
resultant yellow solid. The product was extracted with CH2Cl2 (3 × 50
mL). The organic phases were dried over MgSO4, and the solvent was
removed under reduced pressure. The resulting yellow oil was
extracted with hexane overnight. After filtration, the solvent from the
filtrates was removed under reduced pressure. The resulting yellow oil
was purified by column chromatography over silica using a mixture of
CH2Cl2/MeOH/NH3 80:20:4 as eluent. 770 mg (1.49 mmol, 44%) of
AUTHOR INFORMATION
■
Corresponding Authors
*Phone: +34 972 41 98 42. Fax: +34 972 41 81 50. E-mail:
miquel.costas@udg.edu. (M.C.)
*E-mail: anna.company@udg.edu. (A.C.)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
1
■
a yellow oil was obtained. H NMR (CDCl3, 400 MHz, 298 K) δ,
ppm: 8.52 (d, J = 4.6 Hz, 2H, Hq), 7.63 (td, J = 7.9 Hz, J′ = 1.8 Hz,
2H, Ho), 7.52 (d, J = 7.9 Hz, 2H, Hn), 7.23−7.20 (m, 3H, Hf+Hi+Hh),
7.15−7.11 (m, 3H, Hp+Hg), 3.84 (s, 4H, Hm), 3.61 (s, 2H, He), 3.44
(s, 2H, Hj), 2.79 (s, 4H, Hb), 2.75 (t, J = 6.4 Hz, 2H, Hl), 2.72−2.68
(m, 4H, Hd), 2.65−2.58 (m, 6H, Hc+Hk), 2.34 (s, 6H, Ha), 2.12 (s,
3H, Hr). 13C NMR (CDCl3, 100 MHz, 298 K) δ, ppm: 160.07 (Cp),
149.11 (Ct), 140.15, 138.98 (Cf,Ch), 136.46 (Cr), 129.90, 128.10,
127.87, 127.63 (Ci, Cj, Ck, Cg), 123.00 (Cq), 121.99 (Cs), 63.44 (Ce),
62.77 (Cl), 60.95 (Co), 57.11 (Cc), 56.83 (Cb), 56.02 (Cd), 55.58
(Cm), 52.36 (Cn), 46.62 (Ca), 42.65 (Cu). HR-MS (ESI time-of-flight)
m/z calc. for [M + H]+ 516.3809, found 516.3807.
Synthesis of Dimetallic Complexes. All the complexes
containing copper(I) used in this work were synthesized in acetonitrile
at room temperature under anaerobic conditions in a glovebox to
avoid the oxidation of the initial CuI centers. Because of the high
instability of these CuI centers, all the attempts to isolate the copper
complexes failed. Thus, every day freshly prepared solutions of the
complexes (∼18 mM) were used. NMR samples were prepared in an
analogous way directly in deuterated solvent in the glovebox. For the
preparation of heterodimetallic complexes, a general procedure was
followed: LN3N4 was dissolved inside the glovebox in acetonitrile, and 1
equiv of [Cu(CH3CN)4](CF3SO3) was added directly as a solid. After
10 min of stirring, 1 equiv of the desired metallic salt (Zn(CF3SO3)2,
Cu(CF3SO3)2, Fe(CF3SO3)2(CH3CN)2, or GaCl3) was directly added
as a solid, leading to an 18 mM solution of the complex with the
general formula of [MII/IIICuI(LN3N4)]3+/4+. From this solution, the
desired concentration for characterization or reactivity experiments
was obtained after dilutions with the appropriate amount of
acetonitrile or acetone.
Financial support for this work was provided by the European
Commission (FP7-PEOPLE-2011-CIG-303522 to A.C. and
ERC-2009-StG-239910 to M.C.), MINECO (CTQ2012-
37420-C02-01/BQU and CSD2010-00065 to M.C.) and
Generalitat de Catalunya (ICREA Academia Award to M.C.).
The Spanish Ministry of Science is acknowledged for a Ramon
y Cajal contract to A.C. We are thankul to Dr. X. Ribas for
́
financial support from INNPLANTA Project No. INP-2011-
0059-PCT-420000-ACT1. We also thank Dr. L. Go
́
mez
(Serveis Tecnics de Recerca, Universitat de Girona) for helpful
̀
advice in setting up the HR-MS experiments and for fruitful
discussions. We thank Catexel for a generous gift of 1,4,7-
tritosyl-1,4,7-triazacyclononane.
REFERENCES
■
(1) Belle, C.; Pierre, J.-L. Eur. J. Inorg. Chem. 2003, 4137−4146.
(2) Mitic,
́
N.; Smith, S. J.; Neves, A.; Guddat, L. W.; Gahan, L. R.;
Schenk, G. Chem. Rev. 2006, 106, 3338−3363.
(3) Stenkamp, R. E. Chem. Rev. 1994, 94, 715−726.
(4) Bento, I.; Carrondo, M. A.; Lindley, P. F. J. Biol. Inorg. Chem.
2006, 11, 539−547.
(5) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004,
104, 1013−1045.
(6) Kaila, V. R. I.; Verkhovsky, M. I.; Wikstrom, M. Chem. Rev. 2010,
̈
110, 7062−7081.
(7) Tainer, J. A.; Getzoff, E. D.; Richardson, J. S.; Richardson, D. C.
Nature 1983, 306, 284−287.
I
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(8) Chen, P.; Solomon, E. I. Proc. Nat. Acad. Sci. U.S.A. 2004, 101,
13105−13110.
(39) Weitzer, M.; Schatz, M.; Hampel, F.; Heinemann, F. W.;
Schindler, S. J. Chem. Soc., Dalton Trans. 2002, 686−694.
(40) Schatz, M.; Leibold, M.; Foxon, S. P.; Weitzer, M.; Heinemann,
F. W.; Hampel, F.; Walter, O.; Schindler, S. Dalton Trans. 2003,
1480−1487.
(9) Blackburn, N. J.; Pettingill, T. M.; Seagraves, K. S.; Shigeta, R. T.
J. Biol. Chem. 1990, 265, 15383−15386.
(10) Matoba, Y.; Kumagai, T.; Yamamoto, A.; Yoshitsu, H.;
Sugiyama, M. J. Biol. Chem. 2006, 281, 8981−8990.
(11) Klinman, J. P. Chem. Rev. 1996, 96, 2541−2561.
(12) Prigge, S. T.; Kolhekar, A. S.; Eipper, B. A.; Mains, R. E.; Amzel,
L. M. Science 1997, 278, 1300−1305.
(41) Comba, P.; Haaf, C.; Helmle, S.; Karlin, K. D.; Pandian, S.;
Waleska, A. Inorg. Chem. 2012, 51, 2841−2851.
(42) Borzel, H.; Comba, P.; Katsichtis, C.; Kiefer, W.; Lienke, A.;
̈
Nagel, V.; Pritzkow, H. Chem.Eur. J. 1999, 5, 1716−1721.
(43) Henson, M. J.; Vance, M. A.; Zhang, C. X.; Liang, H.-C.; Karlin,
K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 5186−5192.
(44) Lee, D.-H.; Wei, N.; Murthy, N. N.; Tyeklar, Z.; Karlin, K. D.;
Kaderli, S.; Jung, B.; Zuberbuehler, A. D. J. Am. Chem. Soc. 1995, 117,
12498−12513.
(13) Garcia-Bosch, I.; Ribas, X.; Costas, M. Eur. J. Inorg. Chem. 2012,
179−187.
(14) York, J. T.; Llobet, A.; Cramer, C. J.; Tolman, W. B. J. Am.
Chem. Soc. 2007, 129, 7990−7999.
(15) Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel, W.
W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10660−
10661.
(45) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. Tetrahedron 1997,
53, 6755−6790.
̀
(46) Costas, M.; Xifra, R.; Llobet, A.; Sola, M.; Robles, J.; Parella, T.;
(16) Fujita, K.; Schenker, R.; Gu, W.; Brunold, T. C.; Cramer, S. P.;
Riordan, C. G. Inorg. Chem. 2004, 43, 3324−3326.
(17) Yao, S.; Herwig, C.; Xiong, Y.; Company, A.; Bill, E.; Limberg,
C.; Driess, M. Angew. Chem., Int. Ed. 2010, 49, 7054−7058.
(18) Kundu, S.; Pfaff, F. F.; Miceli, E.; Zaharieva, I.; Herwig, C.; Yao,
S.; Farquhar, E. R.; Kuhlmann, U.; Bill, E.; Hildebrandt, P.; Dau, H.;
Driess, M.; Limberg, C.; Ray, K. Angew. Chem., Int. Ed. 2013, 52,
5622−5626.
Stoeckli-Evans, H.; Neuburger, M. Inorg. Chem. 2003, 42, 4456−4468.
(47) Kim, E.; Chufan, E. E.; Kamaraj, K.; Karlin, K. D. Chem. Rev.
́
2004, 104, 1077−1134.
(48) Karlin, K. D.; Lee, D.-H.; Kaderli, S.; Zuberbuhler, A. K. Chem.
̈
Commun. 1997, 475−476.
(49) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. Tetrahedron 1997,
53, 6755−6790.
(50) Schatz, M.; Leibold, M.; Foxon, S. P.; Weitzer, M.; Heinemann,
F. W.; Hampel, F.; Walter, O.; Schindler, S. Dalton. Trans. 2003,
1480−1487.
(19) Hatcher, L. Q.; Karlin, K. D. J. Biol. Inorg. Chem. 2004, 9, 669−
683.
(20) Hatcher, L. Q.; Karlin, K. D. In Advances in Inorganic Chemistry;
Eldik, R. v., Reedijk, J., Eds.; Academic Press: Waltham, MA, 2006;
Vol. Vol. 58, p 131−184.
(21) Dalle, K. E.; Gruene, T.; Dechert, S.; Demeshko, S.; Meyer, F. J.
Am. Chem. Soc. 2014, 136, 7428−7434.
(22) Mandal, S.; Mukherjee, J.; Lloret, F.; Mukherjee, R. Inorg. Chem.
2012, 51, 13148−13161.
(23) Park, G. Y.; Qayyum, M. F.; Woertink, J.; Hodgson, K. O.;
Hedman, B.; Narducci Sarjeant, A. A.; Solomon, E. I.; Karlin, K. D. J.
Am. Chem. Soc. 2012, 134, 8513−8524.
(24) Matsumoto, J.; Kajita, Y.; Masuda, H. Eur. J. Inorg. Chem. 2012,
2012, 4149−4158.
(25) Uyeda, C.; Peters, J. C. Chem. Sci. 2013, 4, 157−163.
(26) Halvagar, M. R.; Neisen, B.; Tolman, W. B. Inorg. Chem. 2013,
52, 793−799.
(27) Roth, A.; Spielberg, E. T.; Plass, W. Inorg. Chem. 2007, 46,
4362−4364.
(28) Tachi, Y.; Aita, K.; Teramae, S.; Tani, F.; Naruta, Y.; Fukuzumi,
S.; Itoh, S. Inorg. Chem. 2004, 43, 4558−4560.
(29) Tachi, Y.; Matsukawa, Y.; Teraoka, J.; Itoh, S. Chem. Lett. 2009,
38, 202−203.
(30) Murthy, N. N.; Mahroof-Tahir, M.; Karlin, K. D. Inorg. Chem.
2001, 40, 628−635.
(31) Kim, E.; Chufan
2004, 104, 1077−1134.
(32) Garcia-Bosch, I.; Company, A.; Frisch, J. R.; Torrent-Sucarrat,
́
, E. E.; Kamaraj, K.; Karlin, K. D. Chem. Rev.
M.; Cardellach, M.; Gamba, I.; Guell, M.; Casella, L.; Q, L., Jr.; Ribas,
̈
X.; Luis, J. M.; Costas, M. Angew. Chem., Int. Ed. 2010, 2406−2409.
(33) Kieber-Emmons, M. T.; Ginsbach, J. W.; Wick, P. K.; Lucas, H.
R.; Helton, M. E.; Lucchese, B.; Suzuki, M.; Zuberbuhler, A. D.; Karlin,
̈
K. D.; Solomon, E. I. Angew. Chem., Int. Ed. 2014, 53, 4935−4939.
(34) Wainwright, K. P. Coord. Chem. Rev. 1997, 166, 35−90.
(35) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young,
V. G.; Que, L.; Zuberbuhler, A. D.; Tolman, W. B. Science 1996, 271,
̈
1397−1400.
(36) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Cramer,
C. J.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1995, 117, 8865−
8866.
(37) Cole, A. P.; Mahadevan, V.; Mirica, L. M.; Ottenwaelder, X.;
Stack, T. D. P. Inorg. Chem. 2005, 44, 7345−7364.
(38) Mahadevan, V.; Hou, Z.; Cole, A. P.; Root, D. E.; Lal, T. K.;
Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 119, 11996−
11997.
J
dx.doi.org/10.1021/ic501951f | Inorg. Chem. XXXX, XXX, XXX−XXX