Structurally diverse copper(II) complexes of polyaza ligands containing 1,2,3-triazoles: Site selectivity and magnetic properties

Article abstract of DOI:10.1021/ic2021319

Copper(II) acetate mediated coupling reactions between 2,6-bis(azidomethyl) pyridine or 2-picolylazide and two terminal alkynes afford 1,2,3-triazolyl- containing ligands L¹-L⁶. These ligands contain various nitrogen-based Lewis basi

Full text of DOI:10.1021/ic2021319

Article
pubs.acs.org/IC
Structurally Diverse Copper(II) Complexes of Polyaza Ligands
Containing 1,2,3-Triazoles: Site Selectivity and Magnetic Properties
Pampa M. Guha, Hoa Phan, Jared S. Kinyon, Wendy S. Brotherton, Kesavapillai Sreenath,
J. Tyler Simmons, Zhenxing Wang, Ronald J. Clark, Naresh S. Dalal,* Michael Shatruk,* and Lei Zhu*
Department of Chemistry and Biochemistry, Florida State University (FSU), Tallahassee, Florida 32306-4390, United States
S
* Supporting Information
ABSTRACT: Copper(II) acetate mediated coupling reactions
between 2,6-bis(azidomethyl)pyridine or 2-picolylazide and
two terminal alkynes afford 1,2,3-triazolyl-containing ligands
L1−L6. These ligands contain various nitrogen-based Lewis
basic sites including two different pyridyls, two nitrogen atoms
on a 1,2,3-triazolyl ring, and the azido group. A rich structural
diversity, which includes mononuclear and dinuclear complexes
as well as one-dimensional polymers, was observed in the
copper(II) complexes of L1−L6. The preference of copper(II)
to two common bidentate 1,2,3-triazolyl-containing coordination sites was investigated using isothermal titration calorimetry and,
using zinc(II) as a surrogate, in 1H NMR titration experiments. The magnetic interactions between the copper(II) centers in three
dinuclear complexes were analyzed via temperature-dependent magnetic susceptibility measurements and high-frequency electron
paramagnetic resonance spectroscopy. The observed magnetic superexchange is strongly dependent on the orientation of magnetic
orbitals of the copper(II) ions and can be completely turned off if these orbitals are arranged orthogonal to each other. This work
demonstrates the versatility of 1,2,3-triazolyl-containing polyaza ligands in forming metal coordination complexes of a rich
structural diversity and interesting magnetic properties.
Scheme 1. AAC To Afford a 1,2,3-Triazole under
Acceleration of Cu(OAc)2
INTRODUCTION
■
a
,
15
The copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC)
reaction provides an important method in molecular conjugation
chemistry.1−4 The heterocyclic 1,2,3-triazolyl ring is known for
its large dipole moment (∼5 D) and the ability to accept
hydrogen bonds, which are beneficial in applications such as drug
development5,6 and the production of peptide mimetics.7,8 In the
past few years, 1,2,3-triazole has also been recognized as a potent
hydrogen-bond donor9 and a metal coordination ligand.10,11
These discoveries cemented the status of 1,4-disubstituted 1,2,3-
triazole as a functionally versatile molecule. In conjunction with
the synthetic prowess of the CuAAC reaction, one can anticipate
the rapid adoption of 1,2,3-triazolyl as an important functional
entity, in addition to a molecular linker, that provides capabilities
of hydrogen bonding as both a donor and an acceptor, metal
coordination, and π−π interactions in various supramolecular
constructs.
Our group is interested in incorporating the 1,2,3-triazolyl
group in polydentate ligands for developing fluorescent
indicators for metal ions, in particular zinc(II).12,13 1,2,3-Triazole
has been commonly observed to bind a metal ion via the N3
(Nγ in this paper, Scheme 1) atom.10 Recently, we14−16 and
others17−23 have demonstrated the coordination of metal ions by
a 1,2,3-triazolyl group via the N2 (Nβ in this paper) atom in
single-crystal structures. In most reported 1,2,3-triazolyl-contain-
ing ligands, a single multidentate binding pocket containing the
triazolyl moiety can be readily identified. In the current work, we
are curious how 1,2,3-triazolyl participates in metal coordination
a
This procedure works particularly well when R2 is a copper(II)
binding ligand, e.g., 2-picolyl group. The numbering of 1,2,3-triazole is
shown. We opt to use α, β, and γ, instead of 1, 2, and 3, to number the
nitrogen atoms to avoid the confusion in describing the X-ray crystal
structures involving 1,2,3-triazoles.
in a more challenging structural context, in which other ligands
compete in binding. Herein, we report our initial findings on the
coordination chemistry of 1,2,3-triazolyl-containing ligands that
include differently positioned pyridyl and azido functionalities.
The paramagnetic copper(II) is the metal ion of choice in this
study because of our inherent interest in the magnetic properties
of polynuclear copper(II) complexes.24−28 Structural elucidation
of a series of mononuclear, binuclear, and one-dimensional chain
structures reveals fascinating effects of the substituents at the
1 and 4 positions of the 1,2,3-triazolyl ring and the counteranions
on the coordination mode of 1,2,3-triazole molecules.
Received: October 4, 2011
Published: March 7, 2012
© 2012 American Chemical Society
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Figure 1. Structures of L1−L6. “1” and “2” indicate two binding pockets: a nonplanar pocket involving Nβ and a planar one involving Nγ, respectively.
a
Scheme 2. Reaction Leading to Monotriazolyl (L3 and L4) and Ditriazolyl (L5 and L6) Ligands
a
The stoichiometric ratio is 1:1, as shown in blue. Reaction yields are based on the conversion of diazide.
the monotriazole.31 Similarly, in our case, after formation of the
RESULTS AND DISCUSSION
■
monotriazole L3 in the reaction involving phenylacetylene, L3
Synthesis. Ligands L1−L6 (Figure 1) could be prepared via
may hold onto the copper(I) ion in pocket 1 (see Figure 2A) to
the typical CuAAC protocols developed by the groups of Fokin/
Sharpless and Meldal.1,2 Our group found that organic azides with
a pendant metal coordination ligand (i.e., chelating azides)
undergo facile 1,2,3-triazole formation accelerated by Cu(OAc)2
without the addition of a reducing agent (Scheme 1).29 Inci-
dentally, utilization of chelating azides in the synthesis affords
triazolyl-containing multidentate ligands that utilize Nβ (pockets 1
in Figure 1), rather than the more Lewis basic Nγ,30 in 1,2,3-
triazolyl in metal chelation.14,15 This procedure [Cu(OAc)2-AAC;
the typical copper(I)-catalyzed reaction is abbreviated as Cu(I)-
AAC in this paper] not only increases the rate of the reaction to
afford ligands L1−L6 but simplifies the purification processes.
Ligands L1, L2, L5, and L6 were prepared using the Cu(OAc)2-
AAC approach, as reported in our previous work.14,16 These four
ligands were also reported by Crowley et al. using a one-pot
Cu(I)-AAC reaction.18,19 Interesting observations were made
during the syntheses of monotriazolyl ligands L3 and L4
(Scheme 2). Phenylacetylene or 2-ethynylpyridine and 2,6-
bis(azidomethyl)pyridine in a 1:1 ratio underwent Cu(OAc)2-
AAC reactions upon the slow addition of the alkyne to the
diazide. The reaction, judging by consumption of the alkyne
component, to produce L3 was shorter (1 h) than that of L4
(3 h). However, L4 was obtained in higher abundance than L3
(44% vs 17%) when both reactions were performed under the
same conditions.
Figure 2. Preferred coordination mode of copper(I/II) to ligands L3
and L4.
rapidly turn over the second azido group that is in the proximity.
Therefore, monotriazole L3 is only a minor component (17%)
in the product mixture compared to the ditriazole molecule L5.
When 2-ethynylpyridine is used, however, formation of the
monotriazole compound L4 presents a second, thermodynami-
cally more favorable binding pocket for copper (as supported by
the X-ray crystal structure of complex 4 in Figure 10). The
translocation of the catalytic copper from pocket 1 to pocket 2
(Figure 2B) in L4 eliminates the coordination of the copper
center to the second azido group, thus inhibiting formation of
the ditriazole L6. Consequently, a relatively large abundance
of L4 (44%) was observed in the product mixture. The loss
of azido−copper interaction because of the translocation of
copper in L4 also accounts for the longer reaction involving
2-ethynylpridine.
It was reported that when certain diazides take part in Cu(I)-
AAC reactions, the ditriazole formation is favored over that of
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Complex [Cu2(L1)2(OAc)4]·4CH3CN (1) was prepared by
the reaction of Cu(OAc)2·H2O and the monotriazole ligand L1
in CH3CN. The reaction was first attempted at room
temperature, from which only Cu(OAc)2·H2O crystallized out
after prolonged stirring. Upon heating of the solution up to
45 °C for 4 h, its color changed from blue to dark green, indicating
the occurrence of the reaction. Slow evaporation of the CH3CN
solution afforded deep-green single crystals of complex 1. The
ditriazole ligand L5 did not react with Cu(OAc)2 after prolonged
stirring in refluxing CH3CN. The steric effect due to substitu-
tions at both the 2 and 6 positions of pyridine may have
hampered crystallization of the complex. We tried to synthesize
the copper(II) complexes with L3 using perchlorate and chloride
as counteranions but failed to obtain single crystals suitable for
X-ray diffraction after several trials. The similar reaction with L4
afforded a few single crystals of complexes [Cu(L4)2(ClO4)2] (4)
and [Cu(L4)Cl2]n (5), but most of the product was of an oily
form. Other copper(II) complexes, [Cu(L2)2(ClO4)2] (2),
[Cu2(L2)2(CH3CN)2][Cu2(L2)2(CH3CN)4](ClO4)8·2H2O (3),
[Cu(L5)(NO3)2] (6), [Cu(L6)2(H2O)2](ClO4)2 (7), and
[Cu2(L6)(OAc)4(H2O)]·10H2O (8), were obtained with high
yields and purity from the reactions of respective ligands and
copper(II) salts in suitable solvents. All of the complexes were
characterized by X-ray crystallography (Table 1), elemental
analysis, and UV/vis spectroscopy.
[Cu2(L2)2(CH3CN)2][Cu2(L2)2(CH3CN)4](ClO4)8·2H3O (3).
By reducing the ligand-to-metal ratio from 2:1 to 1:1, copper(II)
was found in both pockets in the dinuclear complex 3. The
asymmetric unit shows two discrete dinuclear copper(II)
complexes (3A and 3B). In each structure, the two ligands
are trans to each other. The central 1,2,3-triazolyl groups bridge
two copper(II) ions, which are bound in the five-membered ring
pocket of one ligand and the six-membered ring pocket of the
other. Complexes 3A and 3B differ in the coordination sphere
of copper(II). In 3A, the two copper(II) centers are square-
pyramidal, where the axial position of each copper(II) is
occupied by a CH3CN molecule (Figure 5). The copper(II)
centers in 3B are octahedral with two axial CH3CN molecules
on each metal ion (Figure S1 in the Supporting Information).
There is a slight difference in the Cu−Cu distance in the two
dinuclear units [4.079(1) Å in 3A and 4.053(2) Å in 3B]. In
both 3A and 3B, the six-membered, nonplanar chelate ring
adopts a boat conformation, typical of what has been observed
in other complexes containing pocket 1.14,15,18,21
The structure of dinuclear copper(II) complex 3 is reminiscent
of a dinuclear silver(I) complex of L2 reported by Crowley and
Bandeen.19 In the silver(I) complex, two tetrahedral silver(I)
centers take up all eight Lewis basic nitrogen atoms in two L2
ligands in the same manner as that observed in complex 3. The
triazolyl moiety bridges the two silver(I) centers, which are
separated by 4.98 Å. This value is larger than the Cu−Cu
distance in 3 (4.05−4.08 Å) because of the larger ionic radius of
silver(I). Bridging 1,2,3-triazolyl groups have also been observed
in copper(I) complexes where Cu−Cu distances are 3.61817 and
3.329 Å,22 respectively. These distances are shorter than the
Cu−Cu distance in 3, which can be attributed to the stronger
repulsive electrostatic interactions between two copper(II) ions
in 3 than that in the dinuclear copper(I) complexes.
[Cu2(L1)2(OAc)4]·4CH3CN (1). Ligand L1 has only one
bidentate pocket 1 (Figure 1). It has been reported that L1 forms
mononuclear complexes with Cu(ClO4)214 and CuCl2.18,21 The
axial positions of an octahedrally coordinated copper(II) are
occupied by an oxygen atom of perchlorate and a nitrogen atom
of CH3CN in complex [Cu(L1)(CH3CN)(ClO4)](ClO4)14. The
equatorial positions are taken by pyridine nitrogen and Nβ of the
triazolyl in a bidentate chelation of two L1 ligands. Structurally
similar complexes of L1 with other metal ions have also appeared
in the literature in the past year.18,21,32−34
Coordination Chemistry of L2 in Solution. The
coordination chemistry of L2 in CH3CN was investigated
Ligand L1 upon reacting with Cu(OAc)2·H2O displaces the
water molecules at axial positions of the dinuclear paddlewheel
[Cu2(OAc)4(H2O)2] and coordinates as a monodentate ligand
via pyridyl nitrogen N1 (Figure 3). The triazolyl moiety is not
coordinated, which suggests the coordination preference of
copper(II) to the pyridyl over the triazolyl. The asymmetric
unit of complex 1 consists of an isolated dinuclear complex
[Cu2(L1)2(OAc)4] with the center of symmetry in the midpoint
between the two copper(II) centers. The Cu−Cu distance
[2.6218(3) Å] and axial Cu−N distance [2.207(1) Å] are
typical for axially substituted [Cu2(μ-carboxylate)4] com-
plexes.35−39 This structure bears resemblance to the complex
1
using H NMR. The zinc(II) ion was chosen as a diamagnetic
1
analogue of copper(II) in the H NMR titration experiment.
The peaks in spectrum 1 (Figure 6) were assigned based on the
two-dimensional COSY experiment (Figure S2 in the
Supporting Information). The color coding scheme is shown
in the ChemDraw structure of L2 in Figure 6, where the
protons in red and blue are associated with the coordination
pockets 1 and 2, respectively.
̌
of L1 with Rh(OAc)2 recently reported by Kosmrlj et al.,33
where L1 binds to the dinuclear rhodium(II) paddlewheel at the
axial positions.
[Cu(L2)2(ClO4)2] (2). Ligand L2 has two binding sites that
can form either five- or six-membered rings upon coordination
to copper(II), involving the Nγ or Nβ atom of the 1,2,3-triazolyl
(Figure 1), respectively. Complex 2 was obtained from the
reaction of Cu(ClO4)2·6H2O and L2 in 2:1 (ligand-to-metal)
stoichiometry. It is a mononuclear complex where copper(II) is
in an octahedral environment (Figure 4). Two bidentate L2
ligands occupy the basal plane where Cu−N2 (Nγ) is slightly
shorter than Cu−N1 (Npy). The axial positions are occupied
by perchlorate oxygen atoms with Cu−O1 at 2.419(1) Å. The
two L2 ligands are in a trans configuration. Copper(II) shows a
preference to the planar, five-membered chelation pocket 2
over the nonplanar, six-membered pocket 1.
Figure 3. ORTEP view of complex 1. CH3CN solvent molecules are
omitted for clarity. Thermal ellipsoids are at 50% probability. Selected
distances [Å]: N1−Cu1 2.207(1), Cu1−Cu2 2.6218(3).
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Figure 4. ORTEP view of complex 2. Thermal ellipsoids are at 50% probability. Selected distances [Å]: N1−Cu1 2.038(1), N2−Cu1 2.027(1),
O1−Cu1 2.419(1).
(ligand-to-metal) complexes with copper(II).14,18,21 In this work,
ligand L7 also predictably forms a 2:1 (ligand-to-metal) complex
with Cu(ClO4)2 (Figure 8). The planar bidentate sites of two L7
ligands constitute the square plane of a copper(II)-centered
octahedron. The two perchlorate ions are loosely situated at the
axial positions. The observed bidentate binding mode of L7 is
similar to the reported L7 complexes with ruthenium(II),40
rhenium(I),41 and silver(I).18
The thermodynamics of L1 and L7 binding to Cu(ClO4)2 was
studied using isothermal titration calorimetry (ITC), which
lately has been applied in small-molecule/metal-coordination
systems.13,42−46 Exothermic binding was observed as
Cu(ClO4)2 was titrated into the CH3CN solution of ligand
L1 or L7 (Figure 9). In each case, a major transition occurs at
Figure 5. ORTEP view of 3A. Thermal ellipsoids are at 50%
probability. Selected distances [Å]: N1−Cu1 2.004(4), N2−Cu1
0.5 mol equiv of the titrant, indicating the formation of a
1.997(4), N3−Cu2 2.037(4), N4−Cu2 2.003(4), N5−Cu1 2.146(5),
2:1 (ligand-to-metal) complex, which is consistent with the
Cu1−Cu2 4.079(1).
reported solid-state structures of such complexes.14,18 There
are minor transitions in the case of ligand L7 that precede the
major change at 0.5 mol equiv (Figure 9A), which suggests
Upon the initial addition of Zn(ClO4)2, the hydrogen atoms
from the 2-picolyl component (red in Figure 6), which is
the formation of a 3:1 (ligand-to-metal) complex. This transition
included in pocket 1, retain their splitting patterns while the
was not analyzed quantitatively in this report because of its small
pyridyl hydrogen atoms in pocket 2 (blue) show significant
magnitude. Because of the presence of the preceding minor
shifting and broadening (spectrum 2). As the zinc(II) addition
thermal transition, the thermodynamic parameters of Cu(ClO4)2/
continues, the broadened signals reemerge in spectrum 3, where
L7 complex formation need to be considered as “estimated”.
the peaks that undergo the most changes are the protons of
Consequently, the level of accuracy for the set of data pertaining
pocket 2 (e.g., HF and HI). HE, which is the methylene hydrogen in
to L7 is lower than that pertaining to ligand L1.
pocket 1, shows no change at this point nor do HB and HD.
The one-site binding model provided in the software by
Further addition of zinc(II) leads to a downfield shift of HE and the
separation of HB and HD, indicating the occupation of pocket 1 by
zinc(II). All other peaks undergo slight downfield shifts, as
anticipated in a metal coordination process. The two-step
MicroCal was used to fit the titration traces to obtain the
association constant per binding site (K) and enthalpy (ΔH°),
from which the association entropy (TΔS°) was calculated.47 In
the fitting process, the copper(II) ion is considered as a “receptor”
sequential binding is the most obvious when the shift of HA
capable of binding two ligands at identical sites in a 2:1 (ligand-to-
(green triangles in Figure 6) is followed. The solution coordination
receptor) molar ratio. The thermodynamic parameters of the
association of copper(II) to a single ligand site are reported
behavior of L2 mirrors the observations in the solid state where the
preferred coordination site for copper(II) and zinc(II) is pocket 2.
Pocket 1 can be filled at high metal-ion concentrations.
(Figure 9). The affinity (K) of the five-membered ring planar
pocket 2 (ligand L7) is ∼100-fold higher than that of the six-
membered ring nonplanar pocket 1 (ligand L1), which is
Investigators of 1,2,3-triazole coordination chemistry have
invoked the higher Lewis basicity of Nγ than that of Nβ to
explain the observed favorability of Nγ in metal coordination.23,30
We examined the thermodynamic origin of the formation of
copper(II) complexes at pockets 2 and 1 separately using
isomeric ligands L1 and L7 (Figure 7). Ligand L1 forms 2:1
1
consistent with the observations in the solid state and in the H
NMR titration experiment (Figure 6).
The separation of the enthalpic and entropic contributions to
the affinity sheds more light on the binding process. The forma-
tion enthalpy of the L7/copper(II) complex is 5.6 kcal·mol−1
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Figure 6. 1H NMR spectra (500 MHz, CD3CN) of compound L2 in the presence of increasing concentrations of Zn(ClO4)2. Spectra 1−8: [Zn2+]/
[L2] = 0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.90, and 1.06. Green triangles represent the evolution of HA.
patternsNP1/Nβ,14 NP1/NZ,14 and NP2/Nγ (this work,
Figure 8)have been verified in the copper(II) complexes
of the respective bidentate ligands. Ligand L4 upon reacting
with Cu(ClO4)2 affords mononuclear complex 4 (Figure S3
in the Supporting Information) where copper(II) locates in
Figure 7. Structures of isomeric ligands L1 and L7 used in ITC studies.
pocket 2 of L4 in a five-membered ring chelate similar to that
of complex [Cu(L7)2(ClO4)2] (Figure 8). The unconjugated
pyridyl and azido groups are left unbound. Once again, the
five-membered chelation pocket 2 shows a higher affinity to
copper(II) than the six-membered chelation pocket 1.
[Cu(L4)Cl2]η (5). Same as in complex 4, ligand L4 upon
reacting with CuCl2·2H2O shows a consistent coordination
preference to pocket 2 to result in complex 5 (Figure 10).
Ligand L4 and one of the chlorides act as bridging ligands to
provide a six-coordinate environment for the copper(II) ion
and to afford a one-dimensional coordination polymer that
propagates along the crystallographic c axis (Figure 11). The
axial positions of the copper(II) center are occupied by a
bridging chloride [Cu−Cl = 2.946(2) Å] and an azido group
[Cu−N = 2.568(3) Å], while the equatorial plane is formed by
more favorable than that of the L1/copper(II) complex. This
observation is consistent with the argument that Nγ involved in
pocket 2 in L7 is more Lewis basic than Nβ in pocket 1 in L1.
However, the favorable enthalpy of the L7/copper(II) complex
formation is diminished by ∼50% because of a more severe entropic
penalty than the process involving L1. The opposite contributions
of enthalpy and entropy to the overall affinity can be considered as a
form of “enthalpy−entropy compensation”, as is often observed in
small-molecule host−guest association processes.48
[Cu(L4)2(ClO4)2] (4). Ligand L4 contains five different
nitrogen atoms capable of coordination: two different pyridyl
nitrogen atoms (NP1 in pocket 1 and NP2 in pocket 2), Nβ and
Nγ of the 1,2,3-triazolyl, and the alkylated nitrogen atom of
the azido group (NZ). All three possible bidentate binding
Figure 8. ORTEP view of complex [Cu(L7)2(ClO4)2]. Thermal ellipsoids are at 50% probability. Selected distances [Å]: Cu1−N1 (Npy) 2.035(2),
Cu1−N2 (Nγ) 2.011(2), Cu1−O1 2.429(2).
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Figure 9. ITC data: (A) a solution of Cu(ClO4)2 (2.0 mM) is titrated into a solution of ligand L7 (0.4 mM) in CH3CN at 298 K. The one-site model
was used to fit the data. n (molar ratio) = 0.42 0.04, K = (1.00 0.14) × 107 M−1 (per binding site), ΔH° = −(22.0 0.1) kcal·mol−1, and
TΔS° = −12.5 kcal·mol−1; (B) a solution of Cu(ClO4)2 (2.0 mM) is titrated into a solution of ligand L1 (0.3 mM) in CH3CN at 298 K. The one-site
model was used to fit the data. n (molar ratio) = 0.48 0.03, K = (1.01 0.06) × 105 M−1 (per site), ΔH° = −(16.4 0.7) kcal·mol−1, and TΔS° =
−9.5 kcal·mol−1.
Figure 10. ORTEP view of complex 5. Thermal ellipsoids are at 50% probability. Selected distances [Å]: Cu1−N1 (Npy) 2.057(3), Cu1−N2 (Nγ)
2.022(3), Cu1−Cl1 2.247(1), Cu1−Cl2 2.271(1).
mononuclear compounds where copper(II) takes the only
coordination pocket 1 (Figure 1).16 A 2:1 (ligand-to-metal)
complex containing an octahedral copper(II) forms in the case
of a perchlorate salt, whereas CuCl2 leads to a 1:1 complex
containing a trigonal-bipyramidal copper(II). Cu(NO3)2·H2O
also forms a 1:1 complex with L5 but with a different copper(II)
Figure 11. Perspective view of the one-dimensional polymeric chain
structure of complex 5. Cu−Cu distance in one dinuclear core:
geometry. The asymmetric unit consists of a copper(II) center
3.841(1) Å.
that is octahedrally coordinated by a pyridyl nitrogen atom
(N1), two triazolyl Nβ atoms (N2 and N3), and two nitrate
anions (Figure S4 in the Supporting Information). One nitrate
anion is bidentate and coordinates copper(II) through atoms
O1 and O2, whereas the other nitrate is monodentate.
pocket 2, a bridging chloride, and a terminal chloride. The
Cu−Cu distance in each dinuclear copper(II) unit is 3.841(1) Å.
[Cu(L5)(NO3)2] (6). Our group previously reported that the
tridentate L5 upon reacting with Cu(ClO4)2 and CuCl2 gave
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[Cu(L6)2(H2O)2](ClO4)2 (7). Ligand L6 contains one
tridentate nonplanar coordination pocket 1 and two bidentate
planar pockets 2. Similar to complexes 2, 4, and 5, the five-
membered chelate pocket 2 of L6 is preferred by the copper(II)
ion. Cu(ClO4)2·6H2O upon reacting with L6 affords a
mononuclear complex 7 with a ligand-to-metal ratio at 2:1
(Figure S5 in the Supporting Information). Copper(II)
occupies one of the two pockets 2 in L6, leaving the other
two binding sites vacant. Two water molecules coordinate at
the axial positions of the copper(II) ion. The noncoordinating
component of the ligand curves over the apical water ligand
where a hydrogen bond between the water ligand and the free
conjugated pyridyl is observed [d(O1···N3) = 2.797(3) Å].
[Cu2(L6)(OAc)4·H2O]·10H2O (8). Ligand L6 upon reacting
with Cu(OAc)2·H2O affords a completely different complex (8;
Figure 12). The asymmetric unit includes two copper(II) ions
Magnetic Properties of Dinuclear Complexes 1, 3, 5, and
8. The magnetic properties of dinuclear copper(II) complexes 1,
3, 5, and 8 were studied by means of SQUID magnetometry on
polycrystalline samples and electron paramagnetic resonance
(EPR) spectroscopy on pellet samples (to avoid partial alignment
in the high magnetic field). For complex 1, the χT value at 320 K
is 0.44 emu·K·mol−1, which is smaller than the spin-only value of
1
0.75 emu·K·mol−1 expected for two noninteracting S = /2
copper(II) ions. Upon lowering the temperature, χT decreases
rapidly and nearly linearly to a value only slightly above 0 and
remains essentially constant below 70 K (Figure 13A). The
observed behavior indicates a strong antiferromagnetic super-
exchange between the copper(II) ions, which is in agreement with
the well-established behavior of paddlewheel-type dicopper(II)
tetraacetate complexes.50−52 The superexchange can be described
by the Heisenberg−Dirac−van Vleck Hamiltonian
̂
̂
2
̂
H = −2JS ·S
(1)
1
where J is the magnetic exchange coupling constant and S1 and S2
are the interacting spin states. This leads to the Bleaney−Bowers
equation,53 which describes the temperature dependence of χT:
2
2
2Ng μ
1
B
χT =
+ (χT)
imp
k
3 + exp(−2J/kT)
(2)
where the second term was introduced to account for the presence
of a small amount of mononuclear copper(II) paramagnetic
impurity and to achieve a satisfactory fit at low temperatures. The
best fit to the experimental data was obtained with an isotropic g
factor of 2.05, the magnetic exchange constant J = −161 cm−1, and
0.9 mol % paramagnetic impurity (R2 = 0.9973).
As follows from the obtained best-fit parameters, the S = 0
ground state appears at 322 cm−1 (−2J) below the S = 1 excited
state. High-frequency EPR (HFEPR) measurements were used
to obtain the magnetic parameters of the excited state, i.e., the g
values and zero-field-splitting parameters D and E.53 The EPR
spectrum of 1 recorded at 275 GHz and 300 K reveals a pattern
characteristic of a zero-field split S = 1 spin state with a uniaxial
g tensor (Figure 14, top). The spectrum was simulated with gx =
gy = 2.0665(5), gz = 2.3650(5), D = 0.345 cm−1, and E = 0. The
axial symmetry of the g tensor is consistent with the nearly
tetragonal local symmetry of the copper(II) ions in the crystal
structure of 1 (Figure 3). The zero-field splitting D can be
expressed as the sum of the exchange and dipolar components
(Dex and Ddip):
Figure 12. ORTEP view of complex 8. The 10 noncoordinating water
molecules are omitted for clarity. The thermal ellipsoids are at 50%
probability. Selected distances [Å]: Cu1−O1 1.941(2), Cu1−O2
1.939(2), Cu1−O3 2.316(2), Cu1−N1 2.062(2), Cu1−N2 1.993(2),
Cu2−O3 2.643(2), Cu2−O4 1.970(2), Cu2−O5 1.950(2), Cu2−O6
2.334(2), Cu2−N3 1.985(2), Cu2−N4 2.024(2), Cu1−Cu2 4.701(1).
D = D + D
in two pockets 2 of the same ligand, leaving pocket 1 unbound.
The two copper(II) centers are not equivalent. Cu1 is in a
square-pyramidal environment where Nγ of triazolyl (N2), N1
of pyridyl, and O1 and O2 from two monodentate acetates
constitute the square plane. O3 from the bridging acetate takes
the apical position. On the other side, Cu2 is in an octahedron
whose square plane consists of the same set of donor atoms in
the other pocket 2 [Nγ of triazole (N3) and N4 of pyridyl], O5
of a monodentate acetate, and O4 from the bridging acetate.
O6 of a water molecule and O3 of the bridging acetate occupy
the axial positions. It should be noted that the Cu2−O3
distance [2.643(2) Å] is relatively long, which may or may not
qualify as a coordinative bond.49 The four acetate anions are
not coordinatively equivalent. Three of them are monodentate
anions, and the last one acts as a monodentate ligand toward
Cu1 and a bidentate ligand toward Cu2, thus bridging the two
copper(II) centers through O3 (Figure 12).
ex
dip
(3)
⎡
⎤
1
8
1
4
2
2
D
=
J
(g − 2) − (g − 2)
z x
⎢
⎣
⎥
⎦
ex
(4)
2
2
2
−(2g + g )β
z
⊥
D
=
dip
3
(5)
2r
Inserting the experimentally determined J and g values in
eqs 3−5 yields the calculated D value of 0.40 cm−1 (Dex
=
0.58 cm−1; Ddip = −0.18 cm−1), which is in agreement with the
measured value of 0.345 cm−1.
The magnetic behavior of 3 is similar to that of 1, although
the decrease in χT with the temperature is slower (Figure 13B),
indicating a weaker antiferromagnetic superexchange via the
1,2,3-triazolyl bridges in 3 relative to that via the carboxylate
bridge in 1. Indeed, fitting the experimental data to eq 2
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Figure 13. Temperature dependences of χT for complexes 1 (A), 3 (B), and 5 (C). The red solid lines represent the best fit to the experimental data
(circles) using the Bleaney−Bowers equation (2). See the text for the best-fit parameters.
Note that the same J value was used for both dimers present in
the crystal structure of 3, taking into account the similarity of
the Cu−Nbridge bond lengths, Cu−Nbridge−Nbridge bond angles,
and Cu−Cu distances in these dimers.
The EPR spectrum of 3 does not exhibit a zero-field-splitting
pattern (Figure 14, middle) in contrast to 1. The spectrum was
simulated with a rhombic g tensor, resulting in gx = 2.0457(5),
gy = 2.0770(5), and gz = 2.2502(5). The rhombic symmetry of
the g tensor is consistent with the lower symmetry of the local
coordination environment of the copper(II) ions in the crystal
structure of 3.
The χT value of the chloride-bridged copper(II) dimeric unit
of coordination polymer 5 is 0.86 emu·K·mol−1 at 300 K, which is
higher than the spin-only value of 0.75 emu·K·mol−1 expected for
two noninteracting copper(II) ions. The deviation is explained by
a slight orbital contribution to the total magnetic moment. The
χT value remains almost constant down to 45 K and decreases
rapidly below this temperature, indicating a weak antiferromag-
netic superexchange mediated by the bridging chloride ions
(Figure 13C). Fitting the experimental data to the Bleaney−
Bowers equation (2) resulted in g = 2.16 and J = −1 cm−1. These
values are in line with those of reported bis(μ-chloro)dicopper(II)
complexes.54−56
Complex 8 exhibits an essentially constant χT value in the
Figure 14. Experimental (EXPT, cornflower) EPR spectra of 1, 3, and
2−300 K temperature range (Figure 15A), indicating negligible
8 recorded at 275 GHz and 300 K and their theoretical simulations
magnetic coupling between the copper(II) ions in the dimer.
The average value of χT is 0.98 emu·K·mol−1, higher than
the spin-only value of 0.75 emu·K·mol−1 expected for
(SIM, garnet).
resulted in the best-fit parameters of g = 2.09, J = −39 cm−1,
and 1.5% mononuclear copper(II) impurity (R2 = 0.9938).
1
two noninteracting S = /2 copper(II) ions. The deviation is
Figure 15. Temperature dependence of χT (A) and field dependence of magnetization at 1.8 K (B) for complex 8. The solid line represents the best
fit to the Brillouin function for two noninteracting copper(II) ions with gav = 2.24.
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explained by a slight orbital contribution to the total magnetic
moment. The field-dependent magnetization curve measured at
1.8 K is best fit by the Brillouin function for two noninteracting
copper(II) ions with gav = 2.24 (Figure 15B). A satisfactory
simulation of the experimental EPR spectrum (Figure 14,
bottom) was obtained with a uniaxial g tensor: gx = gy =
2.0605(5), and gz = 2.2865(5).
orbitals to the superexchange pathway leads to negligible
magnetic coupling in 8. The reported copper(II) coordination
chemistry with ligands L1−L6 may extend to other transition-
metal ions with similar coordination geometrical preference, as
demonstrated in a H NMR experiment with zinc(II). The
investigations along these lines are ongoing in our laboratories
and will be reported in due course.
1
The much weaker magnetic exchange between the copper(II)
ions in the monoacetate-bridged dinuclear complex 8, as
compared to the strong coupling observed in tetraacetate-
bridged complex 1, can be attributed to the change in the orbital
superexchange pathway. In both complexes, the coordination
environment of the copper(II) ion exhibits various extents of
axial elongation due to the Jahn−Teller distortion, which leaves
EXPERIMENTAL SECTION
■
Materials and General Methods. Warning! Low-molecular-weight
organic azides and copper(II) perchlorate used in this study are potentially
explosive. Appropriate protective measures should always be taken when
handling these compounds. Reagents and solvents were purchased from
various commercial sources and used without further purification
unless otherwise stated. Analytical thin-layer chromatography (TLC)
was performed using precoated TLC plates with silica gel 60 F254
(EMD). Flash-column chromatography was performed using 40−63 μm
2
2
the dx −y orbital directed along the shorter equatorial bonds to
act as the magnetic orbital. In complex 1, this orbital participates
in the direct σ−σ overlaps with acetate ligands involved in the
superexchange pathway (Figure 3), thus resulting in the strong
magnetic coupling between the copper(II) ions. In contrast, in
the structure of 8 (Figure 12), the bridging acetate has a strong
1
(230−400 mesh ASTM) silica gel (EMD). H NMR spectra were
recorded at 300 or 400 MHz, and 13C NMR spectra were collected at
100 or 125 MHz (on a 400 or 500 MHz spectrometer). All chemical
shifts were reported in δ units relative to tetramethylsilane. CDCl3 was
treated with alumina gel prior to use. Mass spectrometry (MS) spectra
(ESI) were obtained on a JEOL AccuTOF spectrometer at the Mass
Spectrometry Laboratory at FSU. IR spectra were recorded on a
PerkinElmer Spectrum 100 series FT-IR spectrometer, equipped with
a Universal ATR sampling accessory. The absorption spectra were
recorded on a Varian Cary 100 Bio UV/vis spectrophotometer.
Elemental analysis data were collected at Atlantic Microlab, Inc., after
samples were vacuum-dried for an extended amount of time (up to 18 h).
The magnetic susceptibility measurements were carried out on poly-
crystalline samples using a superconducting quantum interference
device (SQUID) magnetometer (Quantum Design MPMS-XL). The
direct-current susceptibility was measured in an applied field of 0.1 T
in the 1.8−300 K temperature range. Field-dependent magnetization
was obtained at 1.8 K, with the magnetic field varied from 0 to 7 T.
The data were corrected for diamagnetic contributions using tabulated
constants.58 The EPR measurements were made at the National High
Magnetic Field Laboratory, using the HFEPR spectrometers and
2
2
σ−σ overlap only with the dx −y orbital of the Cu2 center
(Figure 16), while the bond to the Cu1 ion is formed through
2
2
2
Figure 16. Arrangement of the dx −y and dz orbitals at the copper(II)
centers in dinuclear complex 8. Only the unpaired spins on the
2
2
magnetic dx −y orbitals are indicated.
procedures described earlier.59,60 The syntheses of ligands L1 and
14
2
2
2
an overlap with the dz orbital, leaving the dx −y orbital of this
ion essentially noninteracting (of δ type) with respect to the
ligand’s orbitals that provide the superexchange pathway. This
explains the negligible magnetic coupling between the two
acetate-bridged copper(II) ions in complex 8. These orbital
considerations complement other reports on ferromagnetically
coupled dinuclear copper(II) complexes.49,57
16
L5 were reported previously.
Synthesis of L2. 2-(Azidomethyl)pyridine (0.136 g, 1.0 mmol) and
t
2-ethynylpyridine (0.120 mL, 1.1 mmol) were dissolved in BuOH
(2.5 mL). To that solution was added Cu(OAc)2·H2O (25 μL, 0.4 M
in water). After 10 min, the reaction mixture was filtered through a
short silica plug with CH2Cl2/CH3OH to afford the pure product as a
yellow solid in 98% yield. 1H NMR (300 MHz, CDCl3): δ 8.6 (dd, J =
4.8 and 15.0 Hz, 2H), 8.27 (s, 1H), 8.19 (d, J = 8.4 Hz, 1H), 7.79 (td,
J = 1.5 and 7.8 Hz, 1H), 7.70 (td, J = 1.5 and 7.5 Hz, 1H), 7.30−7.22
(m, 3H), 5.74 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 154.4, 150.3,
145.0, 149.5, 148.9, 137.5, 137.0, 123.6, 123.0, 122.9, 122.5, 120.4,
55.9. HRMS ([M + H]+): m/z 238.1093 (calcd), 238.1093 (found).
Syntheses of L3 and L5. 2,6-Bis(azidomethyl)pyridine (38 mg,
0.20 mmol) and phenylacetylene (22 μL, 0.20 mmol) were dissolved
in CH3OH (4.0 mL). An aqueous solution of Cu(OAc)2·H2O (25 μL,
0.4 M, 5 mol %) was added and stirred at room temperature for 1 h.
The reaction mixture was then diluted with ethyl acetate and passed
through a short plug of silica. Solvent was removed under reduced
pressure, and the residue was purified by silica chromatography using a
1:1 hexanes/ethyl acetate mixture. The monotriazole product L3 was
isolated as a viscous oil with 17% (10 mg) yield. 1H NMR (300 MHz,
CDCl3): δ 7.97 (s, 1H), 7.83 (d, J = 7.2 Hz, 2H), 7.72 (t, J = 7.8 Hz,
1H), 7.41 (t, J = 7.2 Hz, 2H), 7.28−7.34 (m, 2H), 7.20 (d, J = 7.8 Hz,
1H), 5.70 (s, 2H), 4.45 (s, 2H). 13C NMR (125 MHz, CDCl3): δ
156.3, 154.8, 148.5, 138.6, 130.7, 129.0, 128.4, 125.9, 121.9, 121.8,
120.5, 55.7, 55.4. HRMS-ESI+ ([M + H+]): m/z 292.1311 (calcd),
292.1312 (found). IR (neat)/cm−1: 2097 (azido). The ditriazole
product L5 was isolated as a white solid. Yield: 40% (31 mg). 1H NMR
(300 MHz, CDCl3): δ 7.87 (s, 2H), 7.80 (d, J = 7.2 Hz, 4H), 7.71 (t,
J = 7.8 Hz, 1H), 7.30−7.42 (m, 6H), 7.20 (d, J = 7.8 Hz, 2H), 5.69 (s, 4H).
SUMMARY
■
In coordination with 1,2,3-triazolyl-containing polyaza ligands
that include pyridyl and azido groups, copper(II) prefers the five-
membered, planar chelation pocket involving pyridyl nitrogen
and Nγ in 1,2,3-triazolyl over the six-membered, nonplanar
pocket involving an unconjugated pyridyl group and Nβ in 1,2,3-
triazolyl. ITC suggests that this binding preference is primarily
enthalpy-driven, which is offset by a noticeable entropic penalty.
Depending on the nature of the counterion, mononuclear,
dinuclear, and one-dimensional chain structures result. In
addition to enriching the versatile coordination chemistry of
1,4-disubstituted 1,2,3-triazole molecules, we are interested in the
magnetic properties of polynuclear copper(II) complexes of the
triazolyl-containing ligands. The magnetic properties of three
dinuclear complexes, 1, 3, and 8, were investigated and shown to
be strongly dependent on the arrangement of the magnetic
orbitals at the interacting copper(II) centers. While strong anti-
ferromagnetic superexchange between copper(II) ions is
observed for 1 and 3, orthogonality of one of the magnetic
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13C NMR (125 MHz, CDCl3): δ 154.8, 148.4, 138.9, 130.5, 129.0, 128.4,
125.8, 122.1, 120.4, 55.4. HRMS (ESI+, [M + H+]): m/z 394.1780 (calcd),
394.1781 (found).
and filtered through a piece of glass microfiber. Vapor diffusion of
diethyl ether into the blue CH3CN solution afforded blue X-ray dif-
fraction quality single crystals of 4. Anal. Calcd for C28H24Cl2CuN16O8
(4): C, 39.70; H, 2.86; N, 26.46. Found: C, 39.84; H, 2.75; N, 26.22.
λmax/nm (εmax/dm3·mol−1·cm−1, CH3CN): 698 (310).
Syntheses of L4 and L6. 2,6-Bis(azidomethyl)pyridine (38 mg,
0.20 mmol) and 2-ethynylpyridine (21 mg, 0.20 mmol) were dissolved
in CH3OH (4.0 mL). An aqueous solution of Cu(OAc)2·H2O (25 μL,
0.4 M, 5 mol %) was added and stirred at room temperature. After 3 h,
the reaction mixture was passed through a short plug of silica. Solvent
was removed under reduced pressure, and the residue was purified
by silica chromatography using CH3OH in CH2Cl2 (gradient 0−2%).
The monotriazole ligand L4 was isolated as a white solid in 44%
Synthesis of Complex [Cu(L4)Cl2]n (5). Method 1. Complex 5
was obtained by following a procedure similar to that of
complex 4. CuCl2·2H2O (0.17 g, 1.0 mmol) was used in place
of Cu(ClO4)2·6H2O. A few green single crystals suitable for
X-ray diffraction were obtained by vapor diffusion of diethyl
ether into a CH3CN solution of 5. The isolated yield was low.
Method 2. A CH3OH (1.0 mL) solution of CuCl2·2H2O (34 mg,
0.2 mmol) was added to a CH3OH solution (2.0 mL) of ligand L4
(58 mg, 0.2 mmol) and stirred for 20 min. The formed pale-green
precipitate was washed with CH3OH (3 × 3.0 mL) and then with
diethyl ether (5 × 2.0 mL). It was then dissolved in dimethyl sulfoxide
(DMSO; 1.0 mL) and diluted by adding CH3CN (5.0 mL). The
solution was filtered and kept for diethyl ether diffusion, which led to
the formation of microcrystals. The crystals were washed with diethyl
ether (5 × 2.0 mL) and dried. It was again dissolved in DMSO
(1.0 mL) and diluted by adding CH3OH (20 mL). The solution was
filtered and kept for diethyl ether diffusion for overnight. The obtained
crystals were verified via X-ray diffraction to be identical with those
obtained using method 1. The isolated yield was 44% (37 mg). Anal.
Calcd for C14H12Cl2CuN8 (5): C, 39.40; H, 2.83; N, 26.26. Found: C,
39.57; H, 2.81; N, 26.29. λmax/nm (εmax/dm3·mol−1·cm−1, CH3CN):
680 (308). IR (neat)/cm−1: 2085 (azido).
1
(26 mg). H NMR (300 MHz, CDCl3): δ 8.56 (d, J = 4.8 Hz, 1H),
8.26 (s, 1H), 8.17 (d, J = 7.8 Hz, 1H), 7.68−7.80 (m, 2H), 7.15−7.32
(m, 3H), 5.72 (s, 2H), 4.47 (s, 2H). 13C NMR (125 MHz, CDCl3): δ
156.4, 154.5, 150.3, 149.6, 149.0, 138.5, 137.1, 123.1, 122.8, 121.8,
121.7, 120.4, 55.8, 55.4. MS-ESI ([C14H12N8 + Na]+): m/z 315.14
(calcd), 315.10 (found). MS-ESI ([2L4 + Na]+): m/z 607.0 (calcd),
607.2 (found). IR (neat)/cm−1: 2084 (azido). The ditriazole product L6
was isolated as a white solid with 27% (22 mg) yield. 1H NMR (300 MHz,
CDCl3): δ 8.55 (d, J = 4.8 Hz, 2H), 8.27 (s, 2H), 8.17 (d, J = 7.8 Hz, 2H),
7.79 (t, J = 7.8 Hz, 2H), 7.68 (t, J = 7.8 Hz, 1H), 7.15−7.37 (m, 4H),
5.72 (s, 4H). 13C NMR (125 MHz, CDCl3): δ 154.8, 150.3, 149.6,
149.0, 138.9, 137.1, 123.1, 122.9, 122.1, 120.5, 55.6. HRMS-ESI+
([M + H+]): m/z 396.1685 (calcd), 396.1687 (found).
Synthesis of Complex [Cu2(L1)2(OAc)4](CH3CN)4 (1). A solution
of Cu(OAc)2·H2O (0.19 g, 1.0 mmol) in CH3CN (5.0 mL) was added to
a solution of ligand L1 (0.236 g, 1.0 mmol in CH3CN, 5.0 mL). The
resulting mixture was heated to 45 °C and stirred for 4 h. The color of
the solution becomes dark green. The solvent was subsequently removed
under reduced pressure. The resulting green solid was washed with
diethyl ether (3 × 20 mL) to afford the complex in powder form in 78%
yield (0.078 g). The product was dissolved in a minimal amount of
CH3CN and filtered through a piece of glass microfiber. Slow
evaporation of the CH3CN solution gave dark-green crystals that were
suitable for X-ray diffraction. Anal. Calcd for C36H36Cu2N8O8
([Cu2(L1)2(OAc)4]): C, 51.73; H, 4.34; N, 13.41. Found: C, 52.06; H,
4.37; N, 13.53. λmax/nm (εmax/dm3·mol−1·cm−1, CH3CN): 710 (250).
Synthesis of Complex [Cu(L2)2(ClO4)2] (2). Ligand L2 (50 mg,
0.21 mmol) was dissolved in CH3CN (∼1 mL). A solution of
Cu(ClO4)2·6H2O (0.038 g, 0.10 mmol) in CH3CN (∼1 mL) was added
dropwise to the ligand solution, and the mixture was stirred for several
minutes. The solvent was then removed under reduced pressure, and the
complex was rinsed with diethyl ether (3 × 10 mL). The complex was
dissolved in a minimal amount of 50:50 CH3CN/CH3OH, filtered
through a glass microfiber, and set up for vapor diffusion with diethyl
ether, which afforded blue X-ray diffraction quality single crystals of 2.
Anal. Calcd for C26H22Cl2CuN10O8 (2): C, 42.37; H, 3.01; N, 19.01.
Found: C, 42.59; H, 3.01; N, 19.08. λmax/nm (εmax/dm3·mol−1·cm−1,
CH3CN): 680 (310).
Synthesis of Complex [Cu(L5)(NO3)2] (6). Solutions of Cu-
(NO3)2·3H2O (0.19 g, 1.0 mmol in 5.0 mL of CH3OH) and ligand L5
(0.30 g, 1.0 mmol in 5.0 mL of CH3OH) were mixed and stirred. The
color of the solution turned to deep blue, and a deep-blue precipitate
separated out slowly upon keeping the mixture at room temperature for
overnight. The blue solid was then filtered and washed with diethyl
ether and redissolved in CH3OH. Vapor diffusion of diethyl ether into
the green solution resulted in the formation of deep-blue single crystals
of complex 6. Yield: 0.46 g, 80%. Anal. Calcd for C23H19CuN9O6 (6):
C, 47.55; H, 3.30; N, 21.70. Found: C, 47.81; H, 3.32; N, 21.61. λmax
/
nm (εmax/dm3·mol−1·cm−1): 670 (260).
Synthesis of Complex [Cu(L6)2(H2O)2](ClO4)2 (7). Solutions of
ligand L6 (0.39 g, 1.0 mmol, in 5.0 mL of CH3OH) and Cu(ClO4)2·
6H2O (0.37 g, 1.0 mmol, in 5.0 mL of CH3OH) were mixed and stirred
for 30 min. The color of the solution becomes deep blue. The solvent
was subsequently removed under reduced pressure. The resulting blue
solid was washed with diethyl ether (3 × 10 mL) to afford the complex
in powder form in 76% yield (0.827 g). The product was dissolved in a
minimal amount of CH3OH and filtered through a piece of glass
microfiber. Slow evaporation of a CH3OH solution afforded blue X-ray
diffraction quality single crystals of [Cu(L6)2(ClO4)2(H2O)2]. Anal.
Calcd for C42H38CuN18O10Cl2 ([Cu(L6)2(ClO4)2(H2O)2]): C, 46.31;
Synthesis of Complex [Cu2(L2)2(CH3CN)2](ClO4)4 (3A). The
ligand L2 (0.050 g, 0.21 mmol) was dissolved in CH3CN (∼1 mL).
A solution of Cu(ClO4)2·6H2O (0.077 g, 0.21 mmol) in CH3CN
(∼1 mL) was added dropwise to the ligand solution, and the mixture
was stirred for several minutes. The solvent was then removed, and the
complex was rinsed with diethyl ether (3 × 10 mL). The complex was
dissolved in a minimal amount of CH3CN, filtered through a glass
microfiber, and set up for vapor diffusion with diethyl ether. X-ray
diffraction analysis reveals two discrete dinuclear copper(II) units (3A
and 3B) in an asymmetric unit. The crystals were vacuum-dried before
elemental analysis, the result of which matches the composition of
structure 3A. Anal. Calcd for C30H28Cl4Cu2N12O16 (3A): C, 33.32; H,
H, 3.52; N, 23.14. Found: C, 46.43; H, 3.59; N, 23.14. λmax/nm (εmax
/
dm3·mol−1·cm−1, CH3CN): 690 (260).
Synthesis of Complex [Cu2(L6)(OAc)4(H2O)](H2O)10 (8). Com-
plex 8 was obtained by following a procedure similar to that of 7.
Cu(OAc)2·H2O (0.19 g, 1.0 mmol) was used in place of Cu(ClO4)2·
6H2O. Deep-blue single crystals suitable for X-ray diffraction were
obtained by slow evaporation of the CH3OH solution of 8. Yield:
0.72 g, 76%. Anal. Calcd for C29H51Cu2N9O19: C, 36.40; H, 5.37; N,
13.17. Found: C, 36.18; H, 5.03; N, 13.12. λmax (εmax/dm3·mol−1·cm−1,
CH3CN): 680 (285).
Synthesis of Complex [Cu(L7)2(ClO4)2]. Solutions of ligand L7
(0.24 g, 1.0 mmol in 5 mL of CH3CN) and Cu(ClO4)2·6H2O (0.37 g,
1.0 mmol in 5 mL of CH3CN) were mixed and stirred. The color of
the solution became deep blue. The solvent was subsequently removed
under reduced pressure. The resulting blue solid was washed with
diethyl ether (3 × 20 mL) to afford the complex in 79% yield (0.58 g).
The product was dissolved in a minimal amount of CH3CN and
filtered through a piece of glass microfiber. Vapor diffusion of CH2Cl2
into the blue CH3CN solution afforded blue X-ray diffraction quality
single crystals of [Cu(L7)2(ClO4)2]. Anal. Calcd for C28H24Cl2CuN8O8: C,
2.61; N, 15.54. Found: C, 33.43; H, 2.71; N, 15.27. λmax/nm (εmax
/
dm3·mol−1·cm−1,CH3CN): 675 (280).
Synthesis of Complex [Cu(L4)2(ClO4)2] (4). Solutions of ligand
L4 (0.29 g, 1.0 mmol, in 5.0 mL of CH3OH) and Cu(ClO4)2·6H2O
(0.37 g, 1.0 mmol, in 5 mL of CH3OH) were mixed and stirred. The
color of the solution became deep blue. The solvent was subsequently
removed under reduced pressure. The resulting blue solid was washed
with diethyl ether (3 × 20 mL) to afford the complex in 68% yield
(0.58 g). The product was dissolved in a minimal amount of CH3CN
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45.76; H, 3.29; N, 15.25. Found: C, 45.80; H, 3.22; N, 15.17. λmax/nm
(εmax/dm3·mol−1·cm−1, CH3CN): 650 (133).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dalal@chem.fsu.edu (N.S.D.), shatruk@chem.fsu.edu
(M.S.), lzhu@chem.fsu.edu (L.Z.).
Crystal Data Collection and Refinement. A suitable single
crystal was mounted on a goniometer head of a Bruker SMART APEX II
diffractometer using a nylon loop with a small amount of Paratone oil
(Hampton Research). The crystal was cooled to either −100 or −120 °C
in a cold stream of N2 gas. After a crystal that was indexed to give a
satisfactory unit cell was found, a full low-temperature data set was
recorded using a sample-to-detector distance of 6 cm. The number of
frames taken was typically 2400 using 0.3° ω scans with 20 s of frame
collection time. Data integration was performed using the program
SAINT, which is part of the Bruker suite of programs.61 Empirical
absorption correction was performed using SADABS.62 XPREP was used
to obtain an indication of the space group, and the structure was solved
by direct methods and refined by SHELXTL.63 All non-hydrogen atoms
were refined anisotropically, while hydrogen atoms were typically placed
in calculated positions and constrained to a riding model. In some
cases, the quality of the data was good enough to allow a direct hydrogen
assignment.
The nitrogen atoms of CH3CN in complex 1 have enlarged ellipsoids,
which could be modeled as disorder, but this has not been done. In
complex 3, O21 is a lone water molecule. However, none of the Q peaks
are appropriate to be assigned as the missing hydrogen atoms. The
structure of complex 4 is triclinic-solved as a racemic twin. The ellipsoids
of the perchlorate ions present show only a slight sign of disorder. The
extreme thinness of the plate of complex 5 is responsible for getting
incomplete data, but the data/parameter ratio is still above 10. In
complex 8, the hydrogen atoms of unbound water molecules were
refined with the O−H distances restrained using the DFIX command.
Because O27 and other water oxygen atoms appear to hydrogen bond to
four other oxygen atoms, disordered hydrogen atoms must be present.
Thus, at any given instant, the apparent 1.30 Å H−H distance would not
really be present. Refining half of the hydrogen atoms seems to push the
data too hard.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation (Grant CHE-0809201 to L.Z. and Grant CHE-
0911109 to M.S.). All HFEPR measurements were made at the
National High Magnetic Field Laboratory in Tallahassee, FL,
which is supported by NSF cooperative Grant DMR-0654118,
by the State of Florida, and by the DOE. We are grateful to
Dr. Hans van Tol for the help with the EPR measurements. The
authors also thank the Institute of Molecular Biophysics at FSU
for providing access to a VP-ITC microcalorimeter (Microcal).
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