Tetracarboxylate ligands as new chelates supporting copper(II) paddlewheel-like structures

Article abstract of DOI:10.1021/ic403077q

Two new ligands N,N,N′,N′-tetrakis(2-methylbenzoic acid)-1,4-diaminomethylbenzene, 5H₄, and N,N,N′,N′- tetrakis(2-methylbenzoic acid)-4,4′-diaminomethyldiphenyl, 6H₄, carrying four carboxylate groups suitable for bridging dinuclear centers have been prepared and their paddlewheel complexes with copper(II) prepared. The phenyl-bridged ligand 5H₄ gives a cyclic octanuclear species [(Cu₂)₄(5)₄], while the diphenyl-bridged ligand 6H₄ gives a lantern-like tetranuclear species [(Cu₂) ₂(6)₂]; both were characterized by X-ray crystallography. If the amine functions of 5 are protonated, intramolecular hydrogen bonds position the four carboxylates in such a way as to allow formation of the unusual compound [Cu₄(5H₂)₂Cl]³⁺ in which a Cu₄ square centered by a chloro ligand is sandwiched between two (5H₂)^2- ligands. The magnetic properties of this compound have been studied and show antiferromagnetic coupling between adjacent coppers (J = -33.7 cm^-1).

Full text of DOI:10.1021/ic403077q

Article
pubs.acs.org/IC
Tetracarboxylate Ligands as New Chelates Supporting Copper(II)
Paddlewheel-Like Structures
Antoine Gomila,†,‡ Sylvain Duval,†,§,‡ Celine Besnard,∥ Karl W. Kramer,⊥ Shi-Xia Liu,⊥ Silvio Decurtins,⊥
́
̈
and Alan F. Williams*,†
†Department of Inorganic and Analytical Chemistry, University of Geneva, 30 quai Ernest Ansermet, CH 1211 Geneva 4, Switzerland
∥Laboratory for X-ray Crystallography, University of Geneva, 24 quai Ernest Ansermet, CH 1211 Geneva 4, Switzerland
⊥Departement fur Chemie und Biochemie, Universitat Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Two new ligands N,N,N′,N′-tetrakis(2-methylbenzoic acid)-1,4-diaminome-
thylbenzene, 5H4, and N,N,N′,N′-tetrakis(2-methylbenzoic acid)-4,4′-diaminomethyldiphenyl,
6H4, carrying four carboxylate groups suitable for bridging dinuclear centers have been
prepared and their paddlewheel complexes with copper(II) prepared. The phenyl-bridged
ligand 5H4 gives a cyclic octanuclear species [(Cu2)4(5)4], while the diphenyl-bridged ligand
6H4 gives a lantern-like tetranuclear species [(Cu2)2(6)2]; both were characterized by X-ray
crystallography. If the amine functions of 5 are protonated, intramolecular hydrogen bonds
position the four carboxylates in such a way as to allow formation of the unusual compound
[Cu4(5H2)2Cl]3+ in which a Cu4 square centered by a chloro ligand is sandwiched between
two (5H2)2− ligands. The magnetic properties of this compound have been studied and show
antiferromagnetic coupling between adjacent coppers (J = −33.7 cm−1).
ethylenediaminetetraacetate (EDTA) complexes. Our approach
INTRODUCTION
■
is therefore to use aromatic acids and a partly aromatic linker to
give rigidity, with methylene−amino−methylene linkers to
allow some flexibility. This led us to N,N,N′,N′-tetrakis(2-
methylbenzoic acid)-1,4-diaminomethylbenzene (5) as a
possible ligand. To study the effect of changing the spacer we
also prepared N,N,N′,N′-tetrakis(2-methylbenzoic acid)-4,4′-
diaminomethyldiphenyl (6).
Both ligands were obtained in three steps following the same
route (Scheme 1). The first step involved bromination of
methyl-2-methylbenzoic acid 1 to form 2.32 Compound 2
undergoes an N-alkylation reaction with 1,4-diaminomethyl-
benzene or 4,4′-diaminomethyl-biphenyl33 to form, respec-
tively, the tetraesters 3 or 4. The yield of the second step is only
around 40% due to an intramolecular annelation reaction
occurring in parallel with the N-alkylation, decreasing
significantly the yield (see Scheme S1, Supporting Informa-
tion).34 Hydrolysis of these esters affords the desired
tetracarboxylate ligands 5 and 6 in a hexaprotonated form.35
Ligands 3 and 5 (as a hydrochloride salt) were characterized by
X-ray crystallography (see below).
The ability of copper(II) to form paddlewheel-like structures of
general formula Cu2(O2CR)4 with carboxylate ligands has been
used in formation of a wide range of molecules such as discrete
assemblies,1−3 polymeric chains,4−9 and MOFs (metal organic
frameworks).7,10−19 These materials can show useful properties
in domains such as catalysis,20 gas storage,21−24 and magnet-
ism.4,25−27 Polycarboxylate ligands such as 1,4-benzene
dicarboxylic acid or 1,3,5-benzenetricarboxylic acid28 are
frequently used, and the design of linkers between the
carboxylic acids has been discussed.29 In general, the four
carboxylate ligands of a paddlewheel complex belong to
different ligands, but one could imagine joining two or more
carboxylates to give discrete M2(dicarboxylate)2 or
M2(tetracarboxylate) species. Dicarboxylate ligands occupying
two carboxylate sites of a paddlewheel have indeed been
reported previously.9 A suitably designed tetracarboxylate
ligand could, in principle, strongly stabilize the paddlewheel
structure, acting as a tetrabridging ligand and allowing synthesis
of paddlewheel complexes with metals other than the
traditional ions (Mo(II), Rh(II), and Cu(II)).30,31 This article
reports the synthesis of two new tetracarboxylate ligands
conceived with this in view and reports their chemistry with
copper(II).
Copper Complexation Studies. Complexation of the
ligands with copper(II) in solution was followed by observing
the d−d transition of the d9 copper(II) ion. Since it is necessary
to deprotonate the carboxylate functions to allow complexation,
the reaction was followed by titrating a 2:1 copper/ligand
mixture with triethylamine as a base.
RESULTS
■
Ligand Design and Synthesis. Ligand design requires a
certain degree of flexibility to allow the ligand to wrap around
the M2 unit, but it must be sufficiently rigid to prevent
formation of simple mononuclear species such as the classic
Received: December 16, 2013
Published: February 20, 2014
© 2014 American Chemical Society
2683
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
a
Scheme 1. Synthetic Pathway for Ligands 5 and 6
a
(i) NBS (1 equiv), AiBN (0.03 equiv), CCl4, reflux; (ii) 1,4-diaminomethylbenzene (0.2 equiv), K2CO3 (3 equiv), CH3CN, reflux; (iii) 4,4’-
diaminomethyldiphenyl (0.2 equiv), K2CO3 (3 equiv), CH3CN, reflux; (iv) KOH (20 equiv), H2O, MeOH, reflux, then HCl 6 M.
Figure 1. Changes in the visible spectrum upon addition of base (triethylamine) to a mixture of 1 equiv of the salt 6H6Cl2 with 2 equiv of copper(II)
perchlorate in DMF. (Left) Change in spectrum; (right) change in extinction at 700 nm.
Figure 2. Changes in the visible spectrum upon addition of base (triethylamine) to a mixture of 1 equiv of the salt 5H6Cl2 with 2 equiv of copper(II)
perchlorate in DMF. (Left) Change in spectrum; (right) change in extinction at 700 nm.
Ligand 6 showed fairly simple behavior: no change in
spectrum was observed for the first equivalent of base, but after
2 equiv per ligand, a strong band (ε = 200 L mol−1 cm−1 per
copper), typical of the dicopper−tetracarboxylate chromo-
phore,36 grew at 700 nm, reaching a plateau after 4 equiv
corresponding to deprotonation of the four carboxylate groups
(Figure 1). From the DMF solution containing an excess of
base, blue crystals of [Cu4(6)2(DMF)4], 9, could be isolated.
The infrared spectrum of compound 9 shows several νco
vibration bands going from 1587 to 1664 cm−1, establishing
the coordination of the carboxylate functions to the copper
atoms. The visible spectrum of the solid showed a maximum at
710 nm in agreement with the solution studies and formation
of a paddlewheel.
The behavior with ligand 5 was different and showed clear
evidence of an intermediate with a maximum around 790 nm,
giving a green color to the solution. The dicopper−
tetracarboxylate paddlewheel band at 700 nm grows in only
after 4 equiv of base per ligand have been added, when
deprotonation of the amine functions begins. A maximum is
attained when the amines are fully deprotonated after adding 6
equiv of base (Figure 2).
From solutions containing 3 and 10 equiv of base we were
able to isolate green crystals of 7, [Cu4(μ4-Cl)(5H2)2(DMF)4]-
2684
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
(ClO4)3·DMF, and blue crystals of 8, [Cu8(5)4(DMF)8]·
(Et2O)4, respectively. Use of the stoichiometric quantity of base
(4 equiv) to prepare 7 did not give a crystalline product. The
infrared spectra of these compounds show several carbonyl
stretching bands in the ranges from 1573 to 1669 cm−1 and
1603 to 1651 cm−1, respectively, confirming the coordination of
the carboxylate functions to the copper atoms.37 Compound 7
shows an absorption maximum at 763 nm in the solid state and
at 790 nm in solution, while 8 shows an absorption maximum
at 710 nm in the solid state and at 715 nm in solution. We
conclude that the isolated species are essentially the same as
those observed in solution. In high-resolution ESI mass
spectrometry compound 7 shows the presence of several
polynuclear species with up to three copper atoms, suggesting
partial fragmenting of the complex characterized by X-ray
crystallography (see below).
Crystallographic Studies. The crystal structure of 3
(Figure S1, Supporting Information) shows no particular
features of interest. No hydrogen-bonding interactions are
seen, but there are stacking interactions between benzoates of
different molecules. There appears to be no favored orientation
of the ester units. The crystal structure of the tetraacid 5 was
obtained in the form of the hydrochloride salt [5H6]-
(Me2NH2)Cl3·2DMF and shows significant differences from
that of 3. All carboxylate and amine functions are protonated,
and there is an intramolecular bifurcated hydrogen bond
between the N−H+ group and the carbonyl oxygens of the four
acids. This hydrogen bond holds the two toluic acid functions
bound to each amine roughly parallel (Figure 3). There are
stacking interactions between toluic acid functions of
neighboring molecules. The three chloride ions are hydrogen
bonded to protons of the carboxylic acid functions, the fourth
carboxylic acid being hydrogen bonded to a disordered DMF
molecule. Two of the chlorides are further hydrogen bonded to
+
the same Me2NH2 cation (Figure S2, Supporting Informa-
tion), which we presume to have been generated by
decomposition of DMF during crystallization. Details of
hydrogen bonding are given in Table S1, Supporting
Information.
The structure of [Cu4(μ4-Cl)(5H2)2(DMF)4](ClO4)3·DMF,
7, was rather unexpected (Figure 4). It may be regarded as a
sandwich containing a Cu4 square with a chloride ion in the
center as a filling. This essentially planar unit lies in between
two 5H22− units where the four deprotonated carboxylate units
each bridge one side of the Cu4 square. The four copper sites
are virtually identical. The copper is bound to four oxygen
atoms, each belonging to a different carboxylate group with a
mean Cu−O distance of 1.957 Å. The four oxygens are
essentially planar (maximum deviation 0.02 Å), but the copper
is displaced by an average distance of 0.27 Å from the O4 plane
toward the central chloride ion, with a mean Cu−Cl distance of
2.57 Å. Each pair of copper ions is bridged by two carboxylates.
Four DMF molecules are weakly coordinated by oxygen (Cu−
O > 2.5 Å). The fifth DMF and the three perchlorate anions lie
around the complex. The DMF molecules are disordered.
Copper−copper distances lie between 3.596 and 3.652 Å.
The amine functions are still protonated, and the bifurcated
N−H···OC hydrogen bonds seen in [5H6](Me2NH2)-
Cl3.2DMF are still present, although the conformation of the
ligand has changed to allow metal binding (Figure 5). The
complex as a whole has approximate D2d symmetry. Details of
hydrogen bonds and selected bond distances and angles are
given in the Supporting Information.
Square Cu4O systems have been reported in the literature on
a number of occasions,38−43 but the closest to the Cu4Cl system
reported here is the [Cu4(μ4-OH)(CO3)8]9− ion reported by
Abrahams et al.44 in which the four copper ions are bridged by
eight carbonate ions compared with eight carboxylates as seen
here and the μ4-bridge occupies an axial position in a square
pyramidal (SP) geometry with neighboring Cu planes roughly
Figure 3. Structure of the ligand 5H62+. Bifurcated hydrogen bonds
between the protonated amine function and the carbonyl oxygens are
shown as red dashed lines.
Figure 4. Structure of the [Cu4(μ4-Cl)(5H2)2(DMF)4]3+ cation in 7. Hydrogens and the DMF molecules weakly bound on the Cl−Cu axis have
been omitted. (Left) View perpendicular to the S4 axis; (right) view along the S4 axis.
2685
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
2−
Figure 5. Binding mode of the 5H2 ligand in 7 showing the
Figure 7. Structural representation of compound [Cu4(6)2(DMF)4],
9. Only one of the orientations of the disordered external DMF
molecules is shown, and hydrogens are omitted.
persistence of the bifurcated hydrogen bonds in the complex.
perpendicular. The copper−copper distances in the carbonate
anion are slightly shorter (3.155 Å) than those observed here,
but this is probably an effect due to the larger central chloride
ion in our compound.
Full deprotonation of the ligand leads to formation of 8,
[Cu8(5)4(DMF)8].(Et2O)4, a tetrameric structure with four
Cu2 units bridged by four ligands 5 (Figure 6). The amine(bis-
collinear. Selected bond distances and angles are given in the
Supporting Information.
Magnetic Properties of the Sandwich Complex
[Cu4(μ4-Cl)(5H2)2(DMF)4](ClO4)3·DMF, 7. There have been
some magnetic studies on the Cu4O systems mentioned
above39−41 which concluded that antiferromagnetic coupling
was present. We felt it would be of interest to study the Cu4Cl
system prepared here. The χmT(T) plot (Figure 8) of a
Figure 8. Thermal variation of χmT for 7 (solid line is a fit according to
eq 2).
polycrystalline sample of 7 exhibits decreasing χmT values on
cooling, starting from 1.37 cm3 K mol−1 at 300 K and
essentially leveling off at a low value of 0.01 cm3 K mol−1 at 1.9
K; the still slightly positive values at low temperatures are due
to a paramagnetic single ion contribution. The high-temper-
ature χmT value is lower than expected for four noninteracting
Cu(II) ions (∼1.5 cm3 K mol−1 with g = 2), which is due to
antiferromagnetic exchange interactions; the latter is indicated
also by the decreasing χmT values with lowering temperature.
The low-temperature χmT values correspond to a diamagnetic
ground state (S = 0).
The inverse magnetic susceptibility curve, given in Figure 9,
shows a linear behavior in the temperature range 200−300 K,
which results in a Weiss constant Θ = −51(2) K, in good
agreement with the negative slope of the χmT vs T curve.
For the tetranuclear entity we can use the following isotropic
Hamiltonian to describe the intracluster magnetic exchange
interactions (eq 1)
Figure 6. Complex [Cu8(5)4(DMF)8] in 8 looking along the S4 axis.
Hydrogens and disordered ether molecules in the channels are not
shown.
toluic acid) units act as chelates, and the Cu2(O2CR)4 unit is
thus built up from two coppers and two ligands. The
paddlewheel unit shows typical geometry (Cu−Cu distance
2.6346(7) Å, mean Cu−O 1.966(10) Å), and two DMF
molecules are bound in the axial positions. The tetramer has
crystallographic S4 symmetry. Disordered diethyl ether
molecules occupy the channels in the center of the tetramer.
Selected bond distances and angles are given in the Supporting
Information.
The diphenyl-bridged ligand 6 forms a centrosymmetric
double paddlewheel complex [Cu4(6)2(DMF)4], 9 (Figure 7).
The two ligands act as bridges between the Cu2 units, each
ligand providing two carboxylates to each dinuclear unit. The
paddlewheel unit shows typical geometry (Cu−Cu distance
2.6152(4) Å, mean Cu−O 1.970(7) Å). The axial sites are
occupied by DMF molecules; those on the outside of the cage
are disordered. The two Cu2 units are aligned in the same
direction as a result of the center of inversion but are not
H = −2J(S1·S2 + S1·S4 + S2·S3 + S3·S4)
(1)
This model does not consider “diagonal” coupling constants,
which avoids an overparameterization; the quality of the fitting
does not depend on it, so that J can be obtained with
2686
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
consequently no overlap between the magnetic orbitals and the
chloro bridge, and we therefore assumed this interaction to be
negligible. This is supported by the fact that the data is well
fitted by the model neglecting diagonal coupling as mentioned
above. Coupling through the carboxylate bridges is well known,
most famously in the parent compound dicopper tetraacetate
dihydrate where the J value is −286 cm−1.45 An example of a
dicarboxylate bridged system, tricopper hexa(2,4,6-triisopropyl-
benzoate), has been reported by Cler
́
ac, Cotton et al.46 In this
triangular system a J value of −127 cm−1 was reported. This
substantial decrease compared to the tetracarboxylate bridge
may be attributed to (1) the presence of only two carboxylate
bridges, (2) the longer Cu−Cu distances (3.131 Å compared
with 2.64 Å), and (3) the fact that the magnetic orbitals are no
longer parallel but inclined at 60°. If we move from this system
to the one studied here, the number of carboxylate bridges is
identical but the Cu−Cu distance increases to around 3.6 Å and
the magnetic orbitals are now mutually perpendicular. The still
further reduced J value of −33.7 cm−1 is thus physically
reasonable. It is close to the values obtained by Alzuet et al.47 of
−22.5 and −29 cm−1 for a system with four coppers bridged by
either two sulfathiazolato or two hydroxo bridges.
Figure 9. Inverse magnetic susceptibility vs temperature for 7 (solid
line represents a Curie−Weiss fit in the temperature range 200−300
K).
reasonable accuracy from the susceptibility data. Consequently,
the Hamiltonian gives rise to six spin states comprising the total
spin values (ST) of 2, 1, 0 with the corresponding energy levels
in terms of the magnetic coupling constant J as given below
E1(ST = 2) = −2J
E2(ST = 1) = + 2J
E3(ST = 1) = 0
E4(ST = 1) = 0
E5(ST = 0) = +4J
E6(ST = 0) = 0
Magnetic studies have been made on two Cu4−μ4-hydroxo
complexes.42,43 Both show overall antiferromagnetism, but the
two systems are significantly different from each other in their
χT against T curves. They are not directly comparable to our
2
2
complex since the magnetic orbitals (dx −y ) lie in the Cu4(OH)
plane and overlap at the OH bridge.
DISCUSSION
■
In the case of antiferromagnetic coupling (J = negative), the
singlet E5 is the ground state. Applying these energy values to
the van Vleck equation gives the following analytical expression
(eq 2)
While the two ligands prepared here do form paddlewheel
complexes, they do not act as tetradentate ligands to one
dinuclear unit as was hoped. If we examine the structures of the
two paddlewheel complexes 8 and 9, the two
Cu2(dicarboxylate)2 units are virtually identical (Figure 10),
χm = [2Nβ2g2/kT·(A/B)](1 − ρ) + (Nβ2g2/kT)·ρ
(2)
A = 5exp(−E1/kT) + exp(−E2/kT) + exp(−E3/kT)
+ exp(−E4/kT)
and
B = 5exp(−E1/kT) + 3exp(−E2/kT) + 3exp(−E3/kT)
+ 3exp(−E4/kT) + exp(−E5/kT) + exp(−E6/kT)
where ρ is the portion of paramagnetic impurity that follows
the Curie law. In a straightforward manner it can be deduced
that the experimental magnetic susceptibility data can be well
represented in the whole temperature range with the
parameters g = 2.10, J = −33.7 cm−1, and ρ = 0.01 (see Figure
8). The value of the coupling constant J corresponds to a
moderate antiferromagnetic intracluster coupling. There is no
indication for magnetic intercluster interactions. In summary,
the isotropic HDVV model is a reliable basis for describing this
tetranuclear Cu(II) cluster and provides a good overall picture
of the spin states.
Figure 10. syn-Cu2(dicarboxylate)2 unit in complex 9.
the only difference being that one dicarboxylate unit in 8 is
rotated about a C2 axis perpendicular to the dicopper axis to
give an anti conformation, while in 9 both units are syn.
This suggests that the aminebis(toluic acid) units is a good
choice for the dicarboxylate unit but that the linker between the
two units needs to be improved. The four single bonds between
the aryl bridge and the arylcarboxylates offer considerable
conformational flexibility as shown by the structures presented
here. Rather than a monoligand−dimetal unit, we observe a bis-
ligand bis-dimetal unit in 9 and a tetrakis-ligand tetrakis-dimetal
compound in 8. Clearly, since the ligand cannot wrap around
the dimetal unit, formation of bis or tetrakis compounds is
preferred. The bis−bis compound 9 is a simple solution, but
examination of Figure 7 shows no obvious reason why ligand 5
We now turn to the magnetostructural discussion. The four
2
2
magnetic orbitals of the Cu(II) ions are the dx −y orbitals
directed roughly along the Cu−O bonds and are perpendicular
to the Cu−Cl bond. Magnetic exchange could be mediated
either by the bridging chloride ion or by the carboxylate
2
2
bridges. The Cu−Cl bonds are quite long, and the dx −y orbitals
have δ symmetry with respect to these bonds. There is
2687
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
Table 1. Crystallographic Details
ligand 3
C44H44N2O8
ligand 5
complex 7
complex 8
complex 9
formula
fw
C48H60Cl3N5O10
973.36
C95H103Cl4Cu4N9O33
2294.82
C203.4H231.4Cu8N18.2O45.4
C116H128Cu4N12O24
2328.46
728.81
4165.79
190
temp./K
cryst syst
space group
a/Å
220
192
180
180
triclinic
monoclinic
P21
triclinic
tetragonal
monoclinic
P21/n
P-1
P-1
I4
̅
11.6522(5)
12.1177(7)
13.7807(8)
104.260(5)
94.848(4)
91.927(4)
1876.11(18)
2
10.9478(3)
15.3695(3)
15.1896(4)
90
14.2102(6)
17.3347(9)
21.6372(9)
95.097(4)
102.273(4)
106.039(4)
4943.1(4)
2
31.4137(6)
31.4137(6)
10.8904(3)
90.00
12.86902(13)
18.90939(14)
23.3517(2)
90.00
b/Å
c/Å
α/deg
β/deg
94.351(2)
90
90.00
97.2196(8)
90.00
γ/deg
vol./Å3
90.00
2548.47(11)
2
10746.9(4)
2
5637.47(9)
2
Z
ρcalcd (mg/mm3)
m/mm−1
F(000)
1.290
1.268
1.542
1.287
1.372
0.721
2.117
2.712
1.473
1.482
772.0
1028.0
2368.0
4349.0
2432.0
2Θ range for data
6.648−147.184°
5.836−146.634°
5.372−133.194°
5.62−148.1°
6.04−146.66°
collection
no. of reflns collected
11 918
10 029
30 386
11 334
20 713
no. of independent reflns 7299 [Rint = 0.0262]
7204 [Rint = 0.0383]
7204/141/601
1.055
30 386 [twinned]
30 386/400/1486
1.037
7956 [Rint = 0.0270]
7956/150/699
1.040
10 996 [Rint = 0.0213]
10 996/34/759
1.031
data/restraints/params
7299/350/630
1.038
goodness-of-fit on F2
final R indexes [I ≥ 2σI] R1 = 0.0524, wR2 =
R1 = 0.0512, wR2 =
0.1434
R1 = 0.0981, wR2 =
0.2609
R1 = 0.0433, wR2 = 0.1139 R1 = 0.0383, wR2 =
0.1321
0.0989
final R indexes [all data] R1 = 0.0782, wR2 =
R1 = 0.0553, wR2 =
0.1486
R1 = 0.1505, wR2 =
0.3101
R1 = 0.0483, wR2 = 0.1199 R1 = 0.0486, wR2 =
0.1522
0.1074
largest diff. peak/hole/e
0.28/−0.22
0.54/−0.24
1.29/−0.96
0.49/−0.27
0.88/−0.29
Å−3
2[Cu4(μ ‐Cl)(5H2)2]3+ ↔ [{Cu2(5)}4] + 8H+ + 2Cl−
should not form a similar structure unless it is to avoid
repulsion between DMF molecules coordinated axially to the
dicopper. This repulsion between axial ligands may explain why
5 forms a tetrameric structure in which the dicopper axes are
perpendicular to the main axis of the ligands.
4
(3)
CONCLUSIONS
■
Both ligands presented here act as dibridging but not as
tetrabridging. With ligand 6 a lantern-type complex48 is found
as might be expected. The behavior of ligand 5 is less
predictable. When 5 is fully deprotonated, a cyclic tetramer is
formed rather than a lantern-type tetranuclear species, possibly
to allow the coordination of the axial DMF ligands. The
partially deprotonated ligand is surprising, since the intra-
molecular hydrogen bonding holds the ligand in a con-
formation suitable to template a Cu4(μ4-Cl) species, with a
structure quite different from previously reported Cu4(μ4-OH)
complexes. The magnetic properties of the compound are
significantly different from the previously reported compounds,
but this is explicable on careful examination of the structure.
The ability of these ligands to act as doubly bridging species is
currently under investigation in our laboratory.
Formation of the sandwich compound 7 was unexpected, but
the spectroscopic titrations clearly show that the compound is
formed in solution and that 7 is not just an artifact of
crystallization. We presume that the partial deprotonation of
the ligand gives fully deprotonated carboxylates and protonated
amine functions allowing formation of the bifurcated N−H···
(OC)2 hydrogen bond. A search of the Cambridge Structural
Database (version February 2013) showed that this motif is
quite common, 41 examples with H···O distances between 1.9
and 2.2 Å. The effect of this hydrogen bond is to pull together
two carboxylate groups. With a suitable conformational change,
this can bring all four carboxylate groups of ligand 5 to one side
of the phenyl spacer with the four O−O vectors of the
carboxylates forming a square. It is thus ideally arranged to bind
four copper atoms. This is not a possibility for ligand 6 since
the diphenyl spacer results in the two pairs of carboxylates
being too far apart to form a square.
The competition between metal and proton for a metal
binding site is a trivial example of pH-modifying complexation,
but in the present case, the protonation/deprotonation
equilibrium serves to modify the conformation of the ligand,
which binds in both cases using four carboxylates. We
confirmed that the changes are reversible. Adding 4 equiv of
base (NEt3) to a solution of 7 gives the spectrum of the tetra-
paddlewheel complex 8. Similarly, 8 equiv of paratoluene−
sulfonic acid added to a solution of 8 give the spectrum of 7:
EXPERIMENTAL SECTION
General. All chemicals were obtained from Aldrich or Strem and
used without further purification. Silica gel (70−230 mesh) for flash-
column chromatography was purchased from Aldrich.
■
1H and 13C NMR spectra were recorded on a Bruker Advance 400
spectrometer (400 MHz) at room temperature. Chemical shifts are
given with respect to tetramethylsilane. High-resolution mass spectra
were obtained on a QSTAR XL (AB/MSD Sciex) instrument on an
ESI positive mode by the Mass Spectrometry Laboratory, University of
Geneva. Microanalyses were performed using a Varian MICRO Cube
instrument at the Microchemical Laboratory of the University of
Geneva. IR spectra were recorded on a Perkin-Elmer Spectrum-One
spectrometer equipped with a Specac Golden Gate ATR instrument.
2688
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
UV−vis spectra were recorded on a Perkin-Elmer Lambda 900
spectrometer with an integrating sphere attachment for solid samples.
Vis−NIR spectrophotometric titrations were recorded in solution with
a Perkin-Lambda 5 spectrophotometer interfaced to a PC and using a
probe of 1.0 cm path length. Automated titrations were carried out at
25 °C using a Metrohm buret with a 5 mL syringe and performed by
addition of 0.1 and then 0.2 mL aliquots of triethylamine solution
(13.0 or 26.0 mM in DMF, respectively, for ligand 5 or 6) to a mixture
of ligands 5H6Cl2 or 6H6Cl2 (1.2 × 10−5 or 2.4 × 10−5 mol,
respectively) and Cu(ClO4)2·6H2O (2.6 × 10−5 or 5.2 × 10−5 mol,
respectively, for ligand 5 or 6). X-ray crystallography: Intensity
measurements were made using an Agilent Supernova diffractometer
equipped with a CCD bidimensional detector using monochromatic
Cu Kα radiation (λ = 1.54184 Å). Crystal data and structure
refinement details are given in Table 1.
for 1 h, and the solid was filtered and dried under vacuum for one
night at 50 °C. Yield: 0.51 g (92%). This product (10 mg) was
dissolved in 2 mL of DMF, and diethyl ether was allowed to diffuse
into the solution to give white crystals suitable for X-ray
1
crystallography. H NMR (400 MHz/DMSO-d6, ppm): 7.76 (d, 4H,
Har), 7.58 (d, 4H, Har), 7.54 (t, 4H, Har), 7.38 (t, 4H, Har), 7.29 (s, 4H,
Har), 4.39 (s, 8H, −CH2−), 4.29 (sh, 4H, −CH2−). 13C NMR (100
MHz/DMSO-d6, ppm): 168.86, 132.77, 131.06, 130.40, 130.04,
129.53, 58.52, 57.76. IR (solid, ν, cm−1): 3371 w br, 1683 s, 1600
w, 1578 w, 1495 w, 1454 m, 1268 s, 1084 m, 1020 w, 912 w, 843 w,
743 s, 699 m, 651 m, 514 m. MS (ESI-POS-HR): m/z [C40H36O8N2 +
+
H+] = 673.2545 (calculated for C40H37O8N2 673.2544). Anal. Calcd
for [C40H38O8N2]Cl2·9H2O found (calcd): C, 53.37 (52.92); N, 2.96
(3.09); H, 5.64 (6.22). The water content found from elemental
1
analysis is consistent with a broad water peak in the H NMR (3.43
Warning: Organic salts of perchlorate ion are potentially explosive and
ppm) and a band in the infrared spectrum.
care should be taken.
4,4′-diaminomethyldiphenyl, C14H16N2.33 4,4′-Dibromo-1,1′-bi-
phenyl (1 g, 2.94 mmol) was dissolved in a THF/EtOH/H2O
(16:12:4 32 mL) mixture. NaN3 (10.4 g, 160 mmol) in 140 mL of
water was added to the solution. The mixture was heated for 1 h. After
cooling to room temperature, PPh3 (1.54 g, 5.88 mmol) was slowly
added. The solution was refluxed 30 min and cooled to room
temperature, and then concentrated HCl was added until pH = 1, and
the solution was heated a further 2 h. Ph3PO was eliminated by
filtration, and the mixture was washed with CHCl3. The aqueous phase
was basified with NaOH 5 M and extracted with CHCl3. The organic
phase was dried over anhydrous Na2SO4 and evaporated to afford a
Magnetic susceptibility measurements were made on a Quantum
Design MPMS SQUID-XL magnetometer under an applied magnetic
field of 2000 Oe between 300 and 1.9 K. The sample was prepared in a
gelatin capsule. Diamagnetic corrections were made for the sample
using the approximation −0.45 × molecular weight (1997 g/mol) ×
10−6 cm3 mol−1, and the sample holder was corrected for by measuring
directly the susceptibility of the empty capsule.
Syntheses. The synthetic procedure for the ligands was shown in
Scheme 1.
Methyl-2-bromomethylbenzoate,C8H9O2Br,32 2. Methyl-2-methyl-
benzoic acid, 1 (3.5 g, 23.3 mmol), NBS (4.2 g, 23.5 mmol), and AiBN
(0.1 g, 0.6 mmol) were heated together at reflux in 30 mL of CCl4 for
24 h under an N2 atmosphere. The mixture was cooled to 0 °C and
filtered. The organic solution was washed twice with 50 mL of Na2SO3
0.1 M, and the solvent was evaporated under reduced pressure. The
solid was taken up in 20 mL of diethyl ether, and the solution was
dried over Na2SO4 and filtered on 5 cm of silica gel. The resulting
solution is a mixture of the desired bromide compound, and the
starting material in an 80%/20% ratio (ca. from 1H NMR signal
intensity compared with the published spectrum32). The crude
product was used without purification for the second step.
1
white solid (m = 0.38 g, yield = 61%). H NMR (400 MHz/CDCl3,
ppm): 7.56 (d, 4H, Har), 7.38 (d, 4H, Har), 3.92 (s, 4H, −CH2−).
N,N,N′,N′-Tetrakis(methyl-2-methylbenzoate)-4,4′-diaminome-
thyldiphenyl, C50H48N2O8, 4. K2CO3 (1.72 g, 12.4 mmol) was added
to 4,4′-bis(aminomethyl)diphenyl (0.18 g, 0.83 mmol) under nitrogen
atmosphere in 30 mL of freshly distilled CH3CN. Crude methyl-2-
bromomethylbenzoate, 2 (1.14 g, 4.28 mmol) was dissolved in 60 mL
of distilled CH3CN and added to the first solution. The mixture was
refluxed for 2 days, and the solution was filtered while hot. The solvent
was evaporated under reduced pressure to give an orange-brown oil.
The oil was dissolved in 20 mL of CH2Cl2, washed twice with
Na2S2O3, and dried over Na2SO4. The solvent was evaporated, and the
mixture was purified by silica gel chromatography (AcOEt/hexane
1H NMR (400 MHz/CDCl3, ppm): 7.97 (d, 1H, Har), 7.49 (t, H,
Har), 7.47 (d, 1H, Har), 7.37 (t, 1H, Har), 4.97 (s, 2H, −CH2−), 3.97
(s, 3H, −CH3).
1
1:2) to afford the pure product. m = 0.28 g (yield = 42%). H NMR
(400 MHz/CDCl3, ppm): 7.86 (d, 4H, Har), 7.79 (d, 4H, Har), 7.47
(d, 4H, Har), 7.46 (t, 4H, Har), 7.39 (d, 4H, Har), 7.26 (t, 4H, Har),
4.00 (s, 8H, −CH2−), 3.85 (s, 12H, −CH3), 3.65 (s, 4H, −CH2−).
MSp (ESI-POS-HR): m/z [C50H48O8N2 + H+] = 805.3479 (calcd for
N,N,N′,N′-Tetrakis(methyl-2-methylbenzoate)-1,4-diaminome-
thylbenzene, C44H48N2O8, 3. K2CO3 (6 g, 43.5 mmol) was added to
1,4-bis(aminomethyl)benzene (0.373 g, 2.74 mmol) under nitrogen
atmosphere in 60 mL of freshly distilled CH3CN. The mixture is
refluxed for 30 min, and the crude methyl-2-bromomethylbenzoate, 2
(3.73 g, 14.0 mmol), was added in 30 mL of distilled CH3CN. The
solution was refluxed for 3−4 days, and the solution was filtered while
hot. Solvents were evaporated under reduced pressure to give an
orange-brown oil. The oil was dissolved in 20 mL of CH2Cl2, washed
twice with brine, and dried over Na2SO4. Solvent was evaporated, and
the mixture was purified on silica gel, first by hexane/AcOEt 2:1 and
then the second time by methanol gradient chromatography (CH2Cl2/
MeOH 1−5%) to afford the tetraester pure. Yield: 0.79 g (40%). This
product was dissolved in 5 mL of MeOH, and slow evaporation of this
solution yielded white crystals suitable for X-ray crystallography.
1H NMR (400 MHz/CDCl3, ppm): 7.88 (d, 4H, Har), 7.81 (d, 4H,
Har), 7.49 (t, 4H, Har), 7.30 (s, 4H, Har), 7.26 (t, 4H, Har), 4.02 (s, 8H,
−CH2−), 3.87 (s, 12H, −CH3), 3.61 (s, 4H, −CH2−). MS (ESI-POS-
HR): m/z [C44H44O8N2 + H+] = 729.3157 (calcd for C44H45O8N2+ at
729.3170). IR (solid, ν, cm−1): 2950 w br, 1715 s, 1601 w, 1576 w,
1484 w, 1433 m, 1364 w, 1255 s, 1191 w, 1129 m, 1074 s, 966 m, 799
w, 739 s, 672 w.
+
C50H49O8N2 at 805.3483).
N,N,N′,N′-Tetrakis(methyl-2-methylbenzoic acid)-4,4′-diamino-
methyldiphenyl Dihydrochloride C46H40N2O8, 6H6Cl2. The tetraester
4 (0.28 g, 0.35 mmol) was dissolved in 30 mL of MeOH, and 18 mL
of KOH 1 M was added to the solution. The mixture was refluxed for
24 h. The reaction was stopped, and the solvent was evaporated. The
solid was then dissolved in 20 mL of water, and the pH was adjusted to
2 using HCl 6M, resulting in precipitation of a white solid. The
solution was left for precipitation at 0 °C for 1 h, and the solid was
filtered and dried under vacuum for a night at 50 °C. Yield: 0.26 g
(90%). 13C NMR (100 MHz/DMSO-d6, ppm): 168.92, 134.61,
132.99, 132.04, 131.36, 130.81, 130.60, 130.24, 130.00, 126.64, 57.92,
56.55. IR (solid, ν, cm−1): 1695 m, 1610 w, 1586 w, 1452 w, 1398 w,
1264 m, 1084 w, 921 w, 812 w, 750 S, 727 w, 655 m, 515 m. 1H NMR
(400 MHz/DMSO-d6, ppm): 7.78 (d, 4H, Har), 7.68 (broad, 4H, Har),
7.57 (d, 4H, Har), 7.55 (t, 4H, Har), 7.43 (d, 4H, Har), 7.39 (t, 4H,
Har), 4.18 (very broad, 8H, −CH2−), 3.37 (very broad, 4H, −CH2−).
MS (ESI-POS-HR): m/z [C46H41O8N2 + H+] = 749.2870 (calcd for
+
C40H37O8N2 749.2857). Anal. Calcd for [C46H42O8N2]Cl2: found
N,N,N′,N′-Tetrakis(2-methylbenzoic acid)-1,4-diaminomethyl-
benzene Dihydrochloride C40H38N2O8Cl2, 5H6Cl2. The tetraester 3
(0.6 g, 0.82 mmol) was dissolved in 60 mL of MeOH, and 18 mL of
KOH 1 M was added to the solution. The mixture was refluxed for 24
h, after which the solvent was evaporated. The solid was then dissolved
in 20 mL of water, and the pH was adjusted to 2 with HCl 6 M,
resulting in precipitation of a white solid. The solution was left at 0 °C
(calcd): C, 69.17 (67.24); N, 3.27 (3.41); H, 5.41 (5.15).
[Cu4-μ4-Cl(5H2)2(DMF)4](ClO4)3·DMF·4H2O, 7. Suitable crystals for
X-ray diffraction were obtained by dissolving the acid 5H6Cl2 (10 mg,
0.015 mmol) and Cu(ClO4)2·6H2O (11 mg, 0.030 mmol) in 2 mL of
DMF. Triethylamine (6.2 μL, 0.045 mmol) was added to the solution
that turned from colorless to blue. Finally, diethyl ether was left to
2689
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
diffuse into the mixture to afford greenish-blue crystals (yield 5 mg,
yield = 32%). IR (solid, ν, cm−1): 1669 w, 1624 m, 1595 m, 1573 m,
1451 m, 1382 S, 1302 m, 1080 S, 867 m, 793 m, 762 S, 711 m, 672 m,
6 1 9 m , 4 9 3 m . A n a l . C a l c d f o r C u 4 C l -
(C40H34O8N2)2(H2O)4(ClO4)3DMF: found (calcd): C, 48.05
(48.04); N, 3.12 (3.38); H, 3.92 (4.00).
[Cu8(5)4(DMF)8]·Et2O, 8. Ligand 5H6Cl2 (10 mg, 0.015 mmol) and
Cu(ClO4)2·6H2O (11 mg, 0.030 mmol) were dissolved in 5 mL of
DMF with addition of NEt3 (21 μL, 0.15 mmol). Blue crystals suitable
for X-ray analysis appear after diffusing in diethyl ether (10.6 mg,
74%). IR (solid, ν, cm−1): 3400 br, 3043 w, 1610 m, 1590 s, 1561 s,
1451 m, 1370 s, 1289 w, 1151 m, 1089 m, 806 w, 749 s, 707 m, 670 m.
Anal. Calcd for [Cu8(C40H32O8N2)4(DMF)8]·12H2O: found (calcd):
C, 55.43 (55.47); N, 5.53 (5.63); H, 5.03 (5.26). The water content
found from elemental analysis is supported by the IR spectrum where
a large band due to water is observed and by the presence of channels
in the crystal structure.
[Cu2(6)(DMF)2]2, 9. The ligand 6H6Cl2 (15 mg, 0.021 mmol) and
Cu(ClO4)2·6H2O (15.4 mg, 0.042 mmol) were dissolved in 2 mL of
DMF with NEt3 (29 μL, 0.21 mmol). Slow diffusion of diethyl ether
into the solution affords blue crystals suitable for X-ray analysis. (14.5
mg, 52%). IR (solid, ν, cm−1): 2812 w br, 1668 m, 1613 s, 1494 w,
1453 w, 1412 s, 1384 w, 1255 w, 1155 w, 1093 m, 813 w, 796 w, 745
m, 672 m. Anal. Calcd for [Cu4(C46H40O8N2)2(DMF)4]·2DMF found
(calcd): C, 59.41 (60.32); N, 6.22 (6.39); H, 5.25 (5.61).
(6) Agterberg, F. P. W.; Provo
Oevering, H.; Buijs, W.; Lakin, M. T.; Spek, A. L.; Reedijk, J. Inorg.
Chem. 1997, 36, 4321−4328.
(7) Reger, D. L.; Debreczeni, A.; Reinecke, B.; Rassolov, V.; Smith,
M. D.; Semeniuc, R. F. Inorg. Chem. 2009, 48, 8911−8924.
(8) Das, A.; Todorov, I.; Dey, S. K.; Mitra, S. Inorg. Chim. Acta 2006,
359, 2041−2046.
(9) Dai, F.; He, H.; Gao, D.; Ye, F.; Qiu, X.; Sun, D. CrystEngComm
2009, 11, 2516−2522.
(10) Eubank, J. F.; Mouttaki, H.; Cairns, A. J.; Belmabkhout, Y.;
Wojtas, L.; Luebke, R.; Alkordi, M.; Eddaoudi, M. J. Am. Chem. Soc.
2011, 133, 14204−14207.
(11) Ohmura, T.; Usuki, A.; Fukumori, K.; Ohta, T.; Ito, M.;
Tatsumi, K. Inorg. Chem. 2006, 45, 7988−7990.
(12) Tsai, C.-C.; Luo, T.-T.; Yin, J.-F.; Lin, H.-C.; Lu, K.-L. Inorg.
Chem. 2009, 48, 8650−8652.
(13) Tonigold, M.; Volkmer, D. Inorg. Chim. Acta 2010, 363, 4220−
4229.
(14) Perman, J. A.; Cairns, A. J.; Wojtas, L.; Eddaoudi, M.;
Zaworotko, M. J. CrystEngComm 2011, 13, 3130−3133.
(15) Zhang, H.-X.; Yao, Q.-X.; Jin, X.-H.; Ju, Z.-F.; Zhang, J.
CrystEngComm 2009, 11, 1807−1810.
(16) Eddaoudi, M.; Kim, J.; Wachter, J. B.; Chae, H. K.; O’Keeffe, M.;
Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, 4368−4369.
(17) Eddaoudi, M.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem.
Soc. 2002, 124, 376−377.
(18) Pham, T.; Forrest, K. A.; Nugent, P.; Belmabkhout, Y.; Luebke,
R.; Eddaoudi, M.; Zaworotko, M. J.; Space, B. J. Phys. Chem. C 2013,
117, 9340−9354.
(19) Eubank, J. F.; Wojtas, L.; Hight, M. R.; Bousquet, T.; Kravtsov,
V. C.; Eddaoudi, M. J. Am. Chem. Soc. 2011, 133, 17532−17535.
(20) Hamza, F.; Kickelbick, G. Macromolecules 2009, 42, 7762−7771.
(21) Li, J.-R.; Yu, J.; Lu, W.; Sun, L.-B.; Sculley, J.; Balbuena, P. B.;
Zhou, H.-C. Nat. Commun. 2013, 4, 1538.
́
Kluit, H. A. J.; Driessen, W. L.;
ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme S1 and Figures S1 and S2; details of X-ray crystal
structures, hydrogen-bond data, and selected bond distances
and angles for compounds 7, 8, and 9. This material is available
free of charge via the Internet at http://pubs.acs.org. CCDC
973946−973950 contain the supplementary crystallographic
material for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(22) Jiang, H.-L.; Makal, T. A.; Zhou, H.-C. Coord. Chem. Rev. 2013,
257, 2232−2249.
(23) Liu, T.-F.; Chen, Y.-P.; Yakovenko, A. A.; Zhou, H.-C. J. Am.
Chem. Soc. 2012, 134, 17358−17361.
(24) Zhuang, W.; Yuan, D.; Liu, D.; Zhong, C.; Li, J.-R.; Zhou, H.-C.
Chem. Mater. 2012, 24, 18−25.
AUTHOR INFORMATION
■
Corresponding Author
(25) Mikuriya, M.; Azuma, H.; Nukada, R.; Handa, M. Chem. Lett.
1999, 28, 57−58.
*E-mail: Alan.Williams@unige.ch.
(26) Fontanet, M.; Popescu, A.-R.; Fontrodona, X.; Rodríguez, M.;
Present Address
§Unite
́
de Catalyze et Chimie du Solide (UCCS) UMR CNRS
8181, Universite Lille 1 Batiment C7, B.P. 90188, 59652
Romero, I.; Teixidor, F.; Vinas, C.; Aliaga-Alcalde, N.; Ruiz, E.
̃
Chem.Eur. J. 2011, 17, 13217−13229.
́
(27) Campbell, G. C.; Reibenspies, J. H.; Haw, J. F. Inorg. Chem.
1991, 30, 171−176.
Villeneuve d’Ascq cedex, France.
Author Contributions
(28) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.;
Williams, I. D. Science 1999, 283, 1148.
‡These authors contributed equally.
(29) Li, J.-R.; Yakovenko, A. A.; Lu, W.; Timmons, D. J.; Zhuang, W.;
Yuan, D.; Zhou, H.-C. J. Am. Chem. Soc. 2010, 132, 17599−17610.
(30) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal
Atoms; Oxford University Press: Oxford, 1993.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
́ ́
(31) Tong, L. H.; Clifford, S.; Gomila, A.; Duval, S.; Guenee, L.;
■
Williams, A. F. Chem. Commun. 2012, 9891−9893.
We thank the Swiss National Science Foundation for their
support of this work.
(32) Lee, M.; Kang, M.; Moon, B.; Oh, H. B. Analyst 2009, 134,
1706−1712.
(33) Bolognesi, M. L.; Bartolini, M.; Mancini, F.; Chiriano, G.;
Ceccarini, L.; Rosini, M.; Milelli, A.; Tumiatti, V.; Andrisano, V.;
Melchiorre, C. ChemMedChem 2010, 5, 1215−1220.
REFERENCES
■
(1) Prakash, M. J.; Zou, Y.; Hong, S.; Park, M.; Bui, M.-P. N.; Seong,
G. H.; Lah, M. S. Inorg. Chem. 2009, 48, 1281−1283.
(2) Li, J.-R.; Zhou, H.-C. Angew. Chem., Int. Ed. 2009, 48, 8465−
8468.
(34) Dolle, R. E.; MacLeod, C.; Martinez-Teipel, B.; Barker, W.;
Seida, P. R.; Herbertz, T. Angew. Chem., Int. Ed. 2005, 44, 5830−5833.
(35) Imbert, D.; Fatin-Rouge, N.; Bunzli, J.-C. G. Eur. J. Inorg. Chem.
̈
2003, 2003, 1332−1339.
(3) Li, J.-R.; Zhou, H.-C. Nat. Chem. 2010, 2, 893−898.
(4) Valentine, J. S.; Silverstein, A. J.; Soos, Z. G. J. Am. Chem. Soc.
1974, 96, 97−103.
(5) Bell, S. J.; Jennings, K. L.; Danielson, E. D.; Solomon, E. I.;
Musselman, R. L. J. Cryst. Growth 1995, 154, 108−112.
(36) Lewis, J.; Mabbs, F. J. Chem. Soc. 1965, 3894−3897.
(37) Mathey, Y.; Greig, D. R.; Shriver, D. F. Inorg. Chem. 1982, 21,
3409−3413.
(38) McKee, V.; Tandon, S. S. Chem. Commun. 1988, 385.
2690
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691

Inorganic Chemistry
Article
(39) Samuels, J. A.; Vaartstra, B. A.; Huffman, J. C.; Trojan, K. L.;
Hatfield, W. E.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 9623−
9624.
(40) Breeze, S. R.; Wang, S.; Greedan, J. E.; Raju, N. P. J. Chem. Soc.,
Dalton Trans. 1998, 2327−2333.
(41) Reim, J.; Werner, R.; Haase, W.; Krebs, B. Chem.Eur. J. 1998,
4, 289−298.
(42) Burkhardt, A.; Spielberg, E. T.; Simon, S.; Gorls, H.; Buchholz,
̈
A.; Plass, W. Chem.−Eur. J. 2009, 15, 1261−1271.
(43) Seppala, P.; Colacio, E.; Mota, A. J.; Sillanpaa, R. Inorg. Chem.
2013, 52, 11096.
̈
̈
̈
̈
(44) Abrahams, B. F.; Haywood, M. G.; Robson, R. Chem. Commun.
2004, 938−939.
(45) Figgis, B. N.; Martin, R. L. J. Chem. Soc. 1956, 3837−3846.
́
(46) Clerac, R.; Cotton, F. A.; Dunbar, K. R.; Hillard, E. A.;
Petrukhina, M. A.; Smucker, B. W. C. R. Acad. Sci., Ser. IIC: Chem.
2001, 4, 315.
(47) Alzuet, G.; Real, J. A.; Borras, J.; Santiago-Garcia, R.; Garcia-
Granda, S. Inorg. Chem. 2001, 40, 2420−2423.
(48) Li, J.-R.; Zhou, H.-C. Nat. Chem. 2010, 2, 893−898.
2691
dx.doi.org/10.1021/ic403077q | Inorg. Chem. 2014, 53, 2683−2691