Structural, spectral, and electrochemical properties of nickel(II), copper(II), and zinc(II) complexes containing 12-membered pyridine- and pyridol-based tetra-aza macrocycles

Article abstract of DOI:10.1021/ic402119s

The structural, electronic, and electrochemical properties of a series of novel 12-membered pyridine- and pyridol-based tetra-aza transition-metal (Ni, Cu, Zn) complexes {[M^II(L1)Cl](ClO₄), [M ^II(L2)Cl](ClO₄), a

Full text of DOI:10.1021/ic402119s

Article
pubs.acs.org/IC
Structural, Spectral, and Electrochemical Properties of Nickel(II),
Copper(II), and Zinc(II) Complexes Containing 12-Membered
Pyridine- and Pyridol-Based Tetra-aza Macrocycles
Kimberly M. Lincoln, Michael E. Offutt, Travis D. Hayden, Ryker E. Saunders, and Kayla N. Green*
Department of Chemistry, Texas Christian University, Fort Worth, Texas 76109, United States
S
* Supporting Information
ABSTRACT: The structural, electronic, and electrochemical
properties of a series of novel 12-membered pyridine- and pyridol-
based tetra-aza transition-metal (Ni, Cu, Zn) complexes {[MII(L1)-
Cl](ClO4), [MII(L2)Cl](ClO4), and [MII(L3)Cl](ClO4)} are
described (L1 (Pyclen) = 1,4,7,10-tetra-aza-2,6-pyridinophane; L2
= 3,6,9,15-tetraazabicyclo[9.3.1]penta-deca-1(15),11,13-trien-13-ol;
L3 = 3,6,9,15-tetra-azabicyclo[9.3.1]penta-deca- 1(15),11,13-trien-
12-ol). The subtle variations in the chemical properties of these
complexes were investigated using X-ray crystallography, UV−vis
and NMR spectroscopy, and cyclic voltammetry. In the solid-state,
the Ni(II) complexes adopt a unique bimetallic and cis-octahedral
(μ-Cl)2 coordination sphere, and the electronic studies provide
further evidence for the existence of a six-coordinate Ni(II) species
in solution. The pyridol-based Cu(II) and Zn(II) complexes contain
five-coordinate (N4Cl) geometries in the solid-state, in which the four N-donor atoms are not coplanar. Hydroxylation of the
pyridine ring was found to increase the amount of π electronic charge density residing throughout the aromatic system of the
ligand backbone, increase the strength of the M−Cl and M−N (pyridine) basal x- and y-plane interactions, and decrease the axial
M−N bonding interaction. The electrochemical studies demonstrate that (i) the Lewis-acidity of the metal center systematically
decreases across the series {[CuII(L3)Cl](ClO4) > [CuII(L1)Cl](ClO4) > [CuII(L2)Cl](ClO4)}, and (ii) the aromatic
backbones allow access to both Cu(I) and Cu(III) species in solution. Overall, the experimental findings are consistent with the
idea that p-hydroxylation enhances the Lewis-basicity of pyridine-based macrocycle and decreases the Lewis-acidity of the metal-
ion, while m-hydroxylation decreases the electron-donating ability of the backbone and increases the metal-ion Lewis-acidity.
coordination of the first-row divalent transition metal-ions
because the cavity size is spacious enough to allow coplanar
(N4) coordination between the ligand and metal-ion.15,20
Accordingly, ligands derived from smaller 12- and 13-
membered cavities restrict the coplanar coordination of the
metal-ion, forcing the ligand backbone to adopt a cis-folded
geometric arrangement where the metal-ion lies outside of the
N4 plane.11,18,19,22,24 The folding of the ligand backbone
induces ring strain throughout the structural framework of the
macrocycle, consequentially promoting distorted square-
pyramidal, trigonal-bipyramidal, or pseudo-octahedral geo-
metric arrangements about the metal center.3,4,7,15,18,19,23,24
The deviation of the macrocyclic ligand from planarity
significantly affects the electrochemical properties of the
complexes and leads to the stabilization of rather uncommon
metal oxidation states, and this behavior is well-docu-
mented.2,4,7,18,19,21,23,28 Transition metal complexes derived
from tetra-aza ligands, for example, tend to display rather
unusual redox behavior. Oxidation states such as Ni(I)/Ni(III)
INTRODUCTION
■
Tetra-aza macrocycles represent a unique family of ligands that
are utilized across a wide variety of fields, stemming from the
ability to form stable complexes with a wide range of both
transition and lanthanide metal cations.1−10 The versatile
reactivity displayed by these molecules is fundamentally
attributed to the promiscuous metal-binding nature of the
cyclic tetra-aza backbone.15 The specific function for which a
particular complex may play a role is largely dependent upon
the species of metal bound and whether or not the metal center
is redox active.2−4,7−18 Factors such as the cavity size,2,15,19−24
denticity,3−6,8−10 flexibility,2,18,21,22,25,26 and the presence of
substituents2,10,13,14,21,27−29 are all important structural ele-
ments which deeply influence the geometry of the metal
coordination sphere and corresponding redox activity of the
metal center. Tetra-aza macrocycles have a strong preference to
coordinate 3d transition metal-ions in a coplanar manner,
where the strength of the in-plane metal−nitrogen interactions
is modulated according the size of the ligand cav-
ity.2,15,19,20,23,28,29
Bush and Fabbrizzi have each established that size of the fully
saturated 14-membered cyclam cavity is an ideal host for
Received: September 5, 2013
© XXXX American Chemical Society
A
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and Cu(I)/Cu(III) are often produced easily within the tetra-
aza core, and the stability of these valence states can be tuned
according to the structure of the ligand back-
bone.2,13,14,16−18,20−23,25,26,28−30 Electrochemical studies con-
ducted on pyridine-based tetra-aza complexes containing Fe
and Cu are noteworthy and have been recognized in the fields
of biomimetic and organometallic catalysis, respec-
tively.13,14,17,18 Lastly, investigations concerning the redox-
structure relationship between the macrocyclic ligand and
metal-ion have established that the incorporation of unsatura-
tion into the structure of the tetra-aza backbone is necessary for
the stabilization of low-valence transition metal species in
solution.20,21,26,28 For instance, complexes derived from Schiff-
base and pyridine-based backbones permit the electrochemical
formation of stable Ni(I) and Cu(I) species in solu-
tion.16,25,26,28
antioxidant nature of the pyridol-based ligand, we were curious
to know whether the antioxidant activity of the pyridol system
would be reduced by a change in the position of the hydroxyl
group from p- to m-substituted. We hypothesized that the m-
hydroxyl ligand would display reduced antioxidant activity,
versus the p-hydroxyl derivative, arising from the fact that the
hydroxyl substituent in L3 lies in closer proximity to the N-
pyridine atom. The m-hydroxyl ligand (L3) was produced, and
the radical reducing capability was measured. Surprisingly, we
found that the radical-scavenging capability of the m-hydroxyl
substituted ligand was equivalent to the p-hydroxyl derivative
(Figure S1, Supporting Information). We then set out to
explore the chemical characteristics of a series of transition
metal complexes derived from the N-heterocyclic ligands, in
order to determine if the electronic nature of L2 and L3 could
be differentiated from one another.
Herein the syntheses, structural, electronic, and electro-
chemical properties of L1−L3 and the corresponding metal
complexes [NiII(L1)(Cl)]2(ClO4)2 (1), [NiII(L2)-
(Cl)]2(ClO4)2 (2), [NiII(L3)(Cl)]2(ClO4)2 (3), [CuIIL1Cl]-
(ClO4) (4), [CuIIL2Cl](ClO4) (5), [CuIIL3Cl](ClO4) (6),
[ZnIIL1Cl](ClO4) (7), [ZnIIL2Cl](ClO4) (8), and [ZnIIL3Cl]-
(ClO4) (9) are described. This work is significant because it
represents a first report on the chemical properties of a series of
transition-metal complexes derived from a pyridol-based (12-
membered) macrocyclic skeleton. The aim of this work is to
understand how the electronic nature of the metal center is
altered by a subtle change in the structure of the pyridine ring
and in particular to determine if the hydroxyl group acts as an
electron-donating or electron-withdrawing substituent, with
respect to the metal center. This type of information is useful
because it provides fundamental insight into how m- and p-
hydroxylation of the pyridine ring modifies the electronic
nature of the π-system and why this subtle change affects the
electronic behavior of the metal center.
Chemical interest in our research group is largely centered
on the 12-membered pyridine- and pyridol-based tetra-aza
macrocycles L1−L3 (Figure 1) (L1 (Pyclen) = 1,4,7,10-tetra-
Figure 1. The 12-membered pyridine- and pyridol-containing tetra-aza
(N4) macrocycles investigated in this study.
aza-2,6-pyridinophane; L2 = 3,6,9,15-tetraazabicyclo[9.3.1]-
penta-deca-1(15),11,13-trien-13-ol; L3 = 3,6,9,15-tetra-
azabicyclo[9.3.1]penta-deca-1(15),11,13-trien-12-ol). Initial
studies were focused on the use of the free macrocycles as
therapeutic compounds aimed specifically toward neuro-
degenerative disorders characterized by Cu-ion misregulation,
in conjunction with oxidative stress. Previously, we reported
that the macrocycles L1 and L2 are capable of both disrupting
and preventing the Cu(II)-induced formation of beta-amyloid
plaques in vitro.31,32 In addition, the N-heterocyclic backbones
possess antioxidant and radical scavenging properties and
demonstrate the ability to protect cells (Friedreich’s ataxia
fibroblasts) against death induced by reactive oxygen species.
Notably, the p-hydroxyl substituted ligand (L2) displays a
superior antioxidant and radical reducing capability compared
to the pyridine-based macrocycle (L1).32 Given the enhanced
EXPERIMENTAL SECTION
■
Caution! Perchlorate salts are explosive and should be handled in small
quantities. In particular, such compounds should never be heated as
solids. All chemical reagents were purchased from either Sigma Aldrich
or Alfa Aesar and used without further purification.
Chemical Preparation of the 12-Membered Pyridine- and
Pyridol-Based Tetra-aza Macrocycles L1−L3. The ligands
investigated in this study were prepared according to established
procedures, which involves the (1 + 1) condensation reaction between
the nosylated (Ns) amine segment and the respective benzyl
dichloride.32 Scheme 1 shows the general reaction conditions used
for the macrocyclization step and subsequent amine deprotection. The
synthetic details regarding L1 and L2 have been reported elsewhere,
and the full preparative details for the novel ligand m-hydroxyl ligand
Scheme 1. General Reactions Conditions Employed for the Marcocyclization Step and Amine Deprotection
B
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Table 1. Crystal Data, Intensity Collections and Structure Refinement Parameters for [NiII(L1)(Cl)]2(ClO4)2 (1),
[NiII(L2)(Cl)]2(ClO4)2 (2), [CuII(L2)(Cl)](ClO4)2 (5), [CuII(L3)(Cl)](ClO4) (6), [ZnII(L2)(Cl)](ClO4) (8), and
[ZnII(L3)(Cl)](ClO4) (9)
complex
formula
1
2
5
6
8
9
Ni2C22H36Cl4N8O8
Ni2C22H36Cl4N8O10
CuC11H18Cl2N4O5
CuC11H20Cl2N4O6
ZnC11H18Cl2N4O5 Zn2C22H36Cl4N8O10·
H2O
M.W.
799.77
monoclinic
P121/c1
7.1569(4)
18.703(10)
11.304(6)
90
831.77
420.87
monoclinic
P121/c1
11.454(5)
8.985(4)
18.166(6)
90
438.76
422.57
monoclinic
P121/n1
11.509(5)
9.0075(4)
15.471(7)
90
863.14
monoclinic
P21/c
unit cell
triclinic
triclinic
space group
P1
̅
P1
̅
a (Å)
7.065(4)
10.65(7)
11.79(7)
72.958(16)
80.072(16)
78.071(17)
825.2(9)
1
8.0118(10)
9.0865(11)
13.227(17)
109.532(3)
93.412(4)
107.352(3)
852.55(18)
2
7.783(2)
26.23(8)
17.07(5)
90
b (Å)
c (Å)
α
β
97.055(2)
90
122.113(19)
90
97.410(2)
90
104.446(13)
90
γ
volume (Å3)
1501.24(14)
4
1583.5(11)
4
1590.48(12)
4
3376.0(17)
4
Z
D
calc (g/cm3)
1.783
1.674
1.777
1.701
1.777
1.698
reflections collected
37488
64453
5146
13671
91677
7744
independent
reflections
3797
3671
3545
3617
8871
5483
Rint
0.0603
99.5
0.0142
95.9
0.0255
98.5
0.0186
95.7
0.0392
99.3
0.0458
100
completeness to θ
(%)
GOF
1.090
1.001
1.024
1.038
1.019
1.054
R1, wR2 [I > 2σ(I)]
R1, wR2
0.0363, 0.0889
0.0426, 0.0918
0.0401, 0.1121
0.0413, 0.1136
0.0245, 0.0638
0.0277, 0.0660
0.0407, 0.1179
0.0453, 0.1221
0.0453, 0.1106
0.0684, 0.1251
0.0551, 0.1646
0.0822, 0.1863
(L3) can be found in the Supporting Information (Schemes S1−S4).
Please note that each of the ligands utilized were isolated as the amine
HCl salts, prior to complexation with the metal-ions.
crystalline form suitable for X-ray analysis but failed. ESI-MS (m/z):
found: 279.1142 [NiII(L3)-H]+ (100%), theoretical: 279.0834
[NiII(L2)-H]+. UV−vis λmax/nm (ε/M−1·cm−1): 220 (448), 285
(271), 552 (6), 788 (sh), and 947 (12).
Preparation of the [(NiII(N4)(Cl)]2(ClO4)2, [CuII(N4)(Cl)](ClO4),
and [ZnII(N4)(Cl)](ClO4) Complexes (1−9) Derived from the
Tetra-aza (N4) Macrocyclic Ligands of L1−L3. The ligand−metal
complexes were all prepared in a similar manner. A 50 mg sample of
L1, L2, or L3 (HCl salt) was dissolved in 10 mL of deionized water,
and the pH of the ligand in solution was adjusted to 10.0 using
concentrated potassium hydroxide. Next, a one molar equivalent of the
respective [MII](ClO4)2·6H2O solid salt was added to a stirring
solution containing the ligand. The pH of the complex in solution was
adjusted to 5.0 with potassium hydroxide, and the complex mixture
was stirred overnight at 50 °C. The next day, the solvent was removed
under reduced pressure, yielding the crude inorganic complexes as
powdery solids. The crude Cu(II) and Zn(II) complexes (4−9) were
taken up in methanol and filtered through a 45 μm syringe to remove
the potassium chloride byproduct. The solvent was removed under
reduced pressure which yielded the blue Cu(II) and off-white Zn(II)
complexes as powdery solids. The crude Ni(II) complexes (1−3) were
first washed with methanol and then filtered through a 45 μm syringe
(the observed purple solid is insoluble in methanol). Next, the filter
containing the purple precipitate was rinsed with 5 mL of methanol,
followed by 10 mL of deionized water in order to elute the desired
complex from the filter. The solvent was removed under reduced
pressure, which gave complexes 1−3 as purple powdery solids.
[NiII(L1)(Cl)]2(ClO4)2 (1). Yield = 58% (33.8 mg, 0.042 mmol). ESI-
MS (m/z): found: 263.0017 [NiII(L1)-H]+ (100%), theoretical:
263.0885 [NiII(L1)-H]+. UV−vis λmax/nm (ε/M−1·cm−1): 264
(137), 271 (sh), 563 (14), 779 (sh), and 955 (41). Purple crystals
suitable for X-ray structural analysis were obtained by the slow
evaporation of water.
[CuII(L1)(Cl)](ClO4) (4). Yield = 91% (75.4 mg, 0.186 mmol). The
crystal structure for this complex has been reported previously by
Busch et al.33 ESI-MS (m/z): found: 304.0550 [CuII(L1)(Cl)]+
(100%), theoretical: 304.0516 [CuII(L1)(Cl)]+. UV−vis λmax/nm (ε/
M−1·cm−1): 261 (4353), 275−330 (sh), and 713 (109).
[CuII(L2)(Cl)](ClO4) (5). Yield 97% (55.5 mg, 0.131 mmol). Aqua
blue crystals suitable for X-ray structural analysis were obtained from
the slow evaporation of methanol. ESI-MS (m/z): found: 320.0950
[CuII(L2)(Cl)]+ (100%), theoretical: 320.0465 [CuII(L2)(Cl)]+. UV−
vis λmax/nm (ε/M−1·cm−1): 249 (8779), 284−330 (sh), and 699
(107).
[CuII(L3)(Cl)](ClO4) (6). Yield 95% (47.6 mg, 0.113 mmol). Teal blue
crystals suitable for X-ray analysis were obtained in a manner similar to
5. ESI-MS (m/z): found: 320.0618 [CuII(L3)(Cl)]+ (100%),
theoretical: 320.0465 [CuII(L3)(Cl)]+. UV−vis λmax/nm (ε/M−1·
cm−1): 208 (12454), 246 (7841), 268−350 (sh), and 696 (121).
[ZnII(L1)(Cl)](ClO4) (7). Yield 91% (85.3 mg, 0.223 mmol). The
crystal structure for this complex has been reported previously by
Busch et al.33 ESI-MS (m/z): found: 305.0231 [ZnII(L1)(Cl)]+
(100%), theoretical: 305.0511 [ZnII(L1)(Cl)]+. 13C NMR (D2O) δ:
154.3, 141.2, 121.3, 51.9, 51.8, 50.0, 49.9, and 45.3.
[ZnII(L2)(Cl)](ClO4) (8). Yield 93% (45.2 mg, 0.106 mmol). Yellow
crystals suitable for X-ray analysis were obtained from the slow
evaporation of methanol. ESI-MS (m/z): found: 321.0919 [ZnII(L2)-
(Cl)]+ (100%), theoretical: 321.0461 [ZnII(L2)(Cl)]+. 13C NMR
(D2O) δ: 168.6, 156.0, 109.2, 51.6, 48.9, 49.0, 45.5, and 45.4.
[ZnII(L3)(Cl)](ClO4) (9). Yield 91% (50.1 mg, 0.118 mmol). Yellow
crystals suitable for X-ray analysis were obtained in a similar manner as
8. ESI-MS (m/z): found: 321.0942 [ZnII(L3)(Cl)]+ (100%),
theoretical: 321.0461 [ZnII(L3)(Cl)]+. 13C NMR (D2O) δ: 149.8,
145.0, 141.8, 126.8, 121.8, 51.3, 50.5, 49.8, 48.9, 48.2, 45.3, and 45.1.
X-ray Crystallography. Crystal diffraction data were collected at
100 K on a Bruker SMART X2S diffractometer, equipped with
monochromatic Mo Kα radiation (λ = 0.71073 Å). Data collection,
frame integration, data reduction (multiscan), and structure
[NiII(L2)(Cl)]2(ClO4)2 (2). Yield = 95% (42.9 mg, 0.051 mmol). ESI-
MS (m/z): found: 279.1183 [NiII(L2)-H]+ (100%), theoretical:
279.0834 [NiII(L2)-H]+. UV−vis λmax/nm (ε/M−1·cm−1): 246
(200), 556 (16), 784 (sh), and 951 (41). Purple crystals suitable for
X-ray structural analysis were obtained in a similar manner as 1.
[NiII(L3)(Cl)]2(ClO4)2 (3). Yield = 88% (56.0 mg, 0.067 mmol).
Extensive attempts were carried out to obtain this complex in a
C
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determination were carried out using APEX2 software. Structural
refinements were performed with XShell (v 6.3.1), by the full-matrix
least-squares method. All non-hydrogen atoms were refined using
anisotropic thermal parameters, while the hydrogen atoms were
treated as mixed. The ORTEP molecular plots (50%) were produced
using Olex 2. The crystallographic details are combined into Table 1.
Mass Spectrometry. ESI mass spectral analysis were obtained on
an Agilent 1200 series 6224 TOF LC/MS spectrometer at 175 V (+
ESI scan) in aqueous solution.
m-substituted ligand displays a greater amount of charge
transfer character versus either L1 or L2 and indicates that the
electronic nature of the aromatic system of L3 has an increased
level of perturbation. This behavior is attributed to the
asymmetric nature of the m-hydroxyl substituted backbone.
The 13C NMR spectra for L1−L3 are represented in Figure
3a. The most significant feature to be noted is that the signals
pertaining to L1 (124.1 ppm to 149.9 ppm) and L3 (125.3
ppm to 151.5 ppm) lie within close proximity to each other,
while those of L2 (111.5 ppm to 166.2 ppm) show a greater
separation. This feature suggests that the hydroxyl substituent
exerts an electron-donating (resonance) effect that is depend-
ent upon the substitution pattern, with respect to the N-
pyridine atom. To understand the chemical nature behind these
shifts, the resonance structures which represent the delocaliza-
tion of an oxygen atom lone pair into the aromatic system of L2
and L3 must be considered (Figure 3b). For the p-substituted
ligand (blue), a lone pair on the oxygen atom can be
delocalized onto C(2), C(4), and the pyridine nitrogen atom,
but for the m-substituted ligand (red) the electronic charge
density can only delocalize onto atoms C(1), C(3), and C(5).
This difference is significant because the lone pair never resides
directly on the nitrogen atom in L3, and as a result the negative
charge is stabilized by a lesser extent, when compared to L2. A
correspondent upfield shift is observed for L2 at 111.5 ppm,
arising from the delocalization of the lone pair onto C(2) and
C(4). Concomitantly, in the spectrum of L2, there is a
considerable downfield shift at 166.2 ppm assigned to C(1) and
C(5) as the lone pair cannot delocalize onto these atoms.
Structural Properties of the Six-Coordinate Com-
plexes [NiII(L1)(Cl)]2(ClO4)2 (1) and [NiII(L2)(Cl)]2(ClO4)2
(2). Solid-state structures containing the first-row 3d transition
metals Mn(III), Fe(III), Cu(II), and Zn(II) coordinated to L1
have been reported previously.33 The structure for the Ni(II)
complex is yet to be reported; therefore, crystals suitable for X-
ray diffraction analysis were obtained for comparison to the
pyridol-based systems. Repeated attempts to isolate complex 3
as a crystalline solid suitable for X-ray analysis were
unsuccessful and further complicated by the fact that 1−3 are
strictly water-soluble. This anomaly is potentially a result of the
chiral nature of the metal complex formed, where the solid-state
structure of 3 would lack a mirror plane and an inversion
center. The molecular structures of the Ni(II) complexes 1 and
2 are depicted in Figure 4, with the selected bond lengths and
angles listed in Table 2. The structures of complexes 1 and 2
are observed to be dimeric species in the solid-state. The Ni(II)
ion is ligated by the N atoms N(1), N(2), N(3), and N(4) of
the ligand backbones and bridged by two Cl atoms [Cl(1) and
Cl(1′)], giving rise to a symmetrically identical complex. The
macrocyclic ligands L1 and L2 present cis-folded (N4)
configurations, giving precedence to the idea that the 12-
membered cavity is not spacious enough to allow the coplanar
coordination of the Ni(II) ion by the ligand.18,23,29,34 All of the
Ni−N bond lengths for the pyridine-based (1) complex
(2.133−1.997 Å) are shorter than those for the corresponding
pyridol-based (2) complex (2.146−2.013 Å). This contraction
indicates that the bonding interaction between the Ni(II) and
nitrogen atoms weakens slightly. Likewise, the distances
between the neighboring Ni(II) and Cl atoms are increased
for complex 2 and is consistent with a decrease in Lewis-acidity
of the metal center as result of p-hydroxyl functionalization.
Electronic Properties of the Six-Coordinate Com-
plexes [NiII(L1)(Cl)]2(ClO4)2 (1), [NiII(L2)(Cl)]2(ClO4)2 (2),
UV−vis Spectrophotometry. The electronic absorption spectra
were obtained between 200 and 1100 nm on an 8453 UV−vis
spectrophotometer (Agilent) using a 3 mL quartz cuvette with a 1 cm
path length. The extinction coefficients of each complex were
calculated according to the Beer−Lambert law.
NMR Spectroscopy. 1H and 13C NMR spectra were carried out in
deuterated solvents at 25 °C. The spectra of the ligands (L1−L3) and
the respective Zn(II) complexes (7−9) were measured in D2O. All
spectra were obtained on a Bruker AvanceIII (400 MHz) High
Performance Digital NMR spectrometer.
Cyclic Voltammetry. All experiments were performed using a Basi
C3 cell stand under a blanket of N2 associated with a conventional
three-electrode electrochemical cell. The system was composed of a
carbon working electrode, an Ag/AgNO3 reference electrode, and a Pt
counter electrode. All experiments reported were performed at a
potential sweep rate of 100 mV·s−1. For electrochemical analysis, 10.0
mg of each complex was dissolved in a 10 mL solution of anhydrous
dimethylformamide (DMF) containing tetrabutylammonium tetra-
flouroborate (10.0 mg) as the supporting electrolyte.
RESULTS AND DISCUSSION
■
Electronic Properties of the 12-Membered Pyridine-
and Pyridol-Based Tetra-aza Macrocyclic Ligands. The
electronic properties of L1−L3 were characterized using UV−
vis and NMR spectroscopy in aqueous solution. As shown in
Figure 2, there is a bathochromic shift in the absorption
Figure 2. Electronic absorption spectra of the free macrocycles
measured in water, at pH 6.0 (25 °C). Extinction coefficients [λmax/nm
(ε/M−1·cm−1)] for L1: 260 (2549), 266 (2203); L2: 257 (7125); L3:
220 (7233), 248 (sh), and 284 (4168).
maxima following the series L2 < L1 < L3. This observation
agrees with density functional theory (DFT) theoretical
calculations (Figure S2, Supporting Information) that show
the difference in energy between the HOMO−LUMO
molecular orbitals of the ligands follows a similar trend: L3
(ΔE = 249 kJ/mol) > L2 (ΔE = 421 kJ/mol) ≥ L1 (ΔE = 491
kJ/mol). Hydroxylation of the pyridine ring gives rise to
significantly more intense π → π* charge transfer bands,
suggesting that the π-bonding interactions are stronger in
pyridol systems of L2 [257 nm (7125)] and L3 [20 nm (7233)
and 284 nm (4168)], versus the pyridine system [(L1) 260 nm
(2549) and 266 nm (2203)]. Additionally, the spectrum of the
D
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Figure 3. (a) Overlay of the 13C NMR spectra of L1−L3, measured in D2O solution, (pH = 6.0). L1: δ = 149.9, 140.3, 124.1, 49.1, 43.5, 43.0; L2: δ
= 166.2, 151.5, 111.5, 49.0, 43.7, 43.1; L3: δ = 151.5, 139.7, 138.2, 125.9, 125.3, 48.6, 45.6, 44.4, 43.4, 43.1, 42.5. (b) Shown are the structures of the
intermediate compounds which contribute to the resonance hybrid of L2 (red) and L3 (blue), resulting from the delocalization of an oxygen atom
lone pair into the aromatic system.
and [NiII(L3)(Cl)]2(ClO4)2 (3). The absorption spectra of the
violet Ni(II) complexes were obtained in aqueous solution, and
are shown in Figure 5. No color changed was observed when
the complexes were dissolved in water. A blue-shift in the
absorbance maxima is observed across the series from 3 [285
nm (271)], to 1 [264 nm (137)], to 2 [246 nm (200)], which
is consistent with the spectrum obtained for L1−L3. Again, the
pyridol-based complexes (2 and 3) have larger extinction
coefficients (π → π*) than the pyridine-based complex (1),
suggesting the π-bonding interactions are strengthened in
response to hydroxylation. Additionally, complex 3 contains an
intense high energy band at 220 nm (448) also noted to be
unique in the spectra of L3 (Figure 5). Furthermore, two lower
energy transitions ranging from 450 to 1100 nm are observed in
the spectra of the Ni(II) derivatives. The pyridine-based (1)
[563 (14) and 955 (41)] and p-hydroxyl (2) [556 (16) and 951
(41)] complexes have larger extinction coefficients than the m-
hydroxyl derivative (3) [552 (6) and 947 (12)].
It should be noted, however, that Ni(II) complexes derived
from tetra-aza ligands containing a ring size greater than 12-
membered exist in solution as an equilibrium mixture of six-
coordinate (blue), and four-coordinate (yellow) spe-
cies.19−21,23,29,35−40 The electronic spectra of the four-
coordinate Ni(II) species are characterized by the presence of
absorption bands that are confined between 300 to 600 nm and
normally contain λmax values not to exceed 500
nm.20,21,27,29,30,37−41 The complexes described here lack the
presence of absorption bands in the range between 320 to 490
nm and are inconsistent with the electronic spectra defined by a
four-coordinate Ni(II) species. Furthermore, Busch and co-
workers have established that most six-coordinate Ni(II) tetra-
aza complexes are paramagnetic, green or violet, with
characteristically low extinction coefficients.35,36 These attrib-
utes are consistent with the complex systems reported here, and
to date not a single report exists that provides evidence for the
existence of a four-coordinate Ni(II) complex derived from a
12-memebred tetra-aza ligand.29 It is currently postulated that
the six-coordinate Ni(II) species that were isolated in the solid-
state exist in solution as well. However, it is plausible that an
exchange reaction between the chlorine ligand and solvent
molecule could occur, resulting from the dissociation of the
dimeric Ni(II) species in solution.
Electrochemical Properties of the Six-Coordinate
Complexes [NiII(L1)(Cl)]2(ClO4)2 (1), [NiII(L2)(Cl)]2(ClO4)2
(2), and [NiII(L3)(Cl)]2(ClO4)2 (3). Electrochemical experi-
ments carried out on complexes 1−3 in DMF and aqueous
solvents yielded ill-defined and irreproducible cyclic voltam-
metry (CVs) results, respectively. The former result is likely
due to poor solubility of the Ni(II) complexes in DMF.
Fabbrizzi et al. reported similar inconsistent electrochemical
behavior for an octahedral, 12-membered dioxo tetra-aza
macro-cyclic Ni(II) complex.2 Ghachtonli and co-workers also
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pyridol-based complexes 5, 6, 8, and 9 are five-coordinate and
deviate between square-pyramidal and trigonal-bipyramidal
geometries.33 Calculation of the angular geometric parameter
(τ) was utilized in order to quantitate the percentage by which
the [CuII(N4)Cl]+ and [ZnII(N4)Cl]+ complexes are distorted
away from square-pyramidal.43−45 The τ-value was calculated
according to the formula defined by Addison [τ = (β − α)/60],
where α° = N(4)−M−Cl(1) and β° = N(1)−M−N(3) are the
angles that define the basal plane.45 Please note that for a
perfect square-pyramidal species τ = 0.00, and for a perfect
trigonal-bipyramidal species, τ = 1.00. The molecular structures
of the pyridol-based Cu(II) and Zn(II) complexes are depicted
in Figure 6, and the selected metric parameters of 4−9 are
listed in Table 3. The basal plane is defined by the position of
the donor atoms N(1), N(3), N(4), Cl(1), with N(2) as the
axial donor.
As shown in Figure 6, the pyridol-derived complexes each
present cis-folded geometric arrangements where the metal-ion
lies outside the N4 plane.46,47 The bond lengths for complexes
4−9 follow the expected periodic trends, increasing from
Cu(II) to Zn(II), with an exception occurring for the axial Zn−
N(2) bond lengths (Table 3). The axial Cu−N(2) and Zn−
N(2) bond lengths increase by 0.132 Å in 5, 0.096 Å in 6, 0.039
Å in 8, and 0.029 Å in 9 upon hydroxylation of the pyridine
ring. This result indicates that the axial M−N(2) bonding
interaction is destabilized in response to hydroxylation; this
effect is more pronounced for the complexes derived from L2.
Another significant trend to be noted is that the basal M−Cl(1)
and M−N(4) bond distances are shorter for the pyridol-derived
complex systems. This contraction indicates that the π-bonding
interactions are stronger for the pyridol-based complexes (5, 6,
8, and 9) versus the pyridine-based complexes (4 and 7). The
Cu−Cl(1) bond lengths decrease, significantly, by 0.140 Å in 5,
and 0.101 Å in 6, while the Zn−Cl(1) bond lengths decrease
only slightly.
Figure 4. ORTEP drawing of the six-coordinate Ni(II) complexes 1
(top) and 2 (bottom) showing the presence of a Ni(μ-Cl)2Ni bridge,
with the labeling scheme adopted. The two perchlorate counterions
have been omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for
the Six-Coordinate Complexes: [NiII(L1)(Cl)]2(ClO4)2 (1)
and [NiII(L2)(Cl)]2(ClO4)2 (2)
Crystal field theory predicts that the dz2 orbital is
significantly destabilized when the metal-ion is coordinated in
a trigonal-bipyramidal environment, arising from the fact that
the axial bonding interactions are stronger in the z-axis than in
the xy-plane. Conversely, a destabilized dx2−y2 orbital results
from the strengthened in-plane xy-bonding interactions in a
square-pyramidal environment. The degree by which the five-
coordinate complexes deviate away from square-pyramidal is
modulated by the strength of the in-plane M−N(4) and M−
Cl(1) bonds. As seen experimentally, complexes 5 (τ = 0.07), 6
(τ = 0.20), and 8 (τ = 0.16) are distorted closer toward square
pyramidal while 4 (τ = 0.63), 7 (τ = 0.42) and 9 (τ = 0.43)
deviate further away. This observation leads to the conclusion
that complexes that contain stronger basal M−N(4) and M−
C(1) bonds and weaker N−N(2) interactions present
distortions that lie closer toward square-pyramidal.28 For each
of these cases, the Cu(II) and Zn(II) complexes derived from
L2 are least distorted from square pyramidal and suggests that
p-hydroxylation imposes a stronger basal ligand-field about the
metal center compared to m-hydroxylation.
Lastly, the metric parameters related to the aromatic system
for the pyridol-based complexes reveal some noteworthy
points. The most striking feature to be noted is that the C−
O(1) bond lengths for the p-hydroxyl complexes (5 and 8) are
shorter by 0.101 Å and 0.014 Å, respectively, than those for the
complexes derived from the m-hydroxyl ligand (6 and 9). A
shorter C−O bond length is consistent with the idea that the
C−O bond of L2 has more partial double-bond character than
complex
Ni−N(1)
Ni−N(2)
Ni−N(3)
Ni−N(4)
Ni−Cl(1)
Ni−Cl(1′)
Ni(1)−Ni(1′)
Cl(1)−Cl(1′)
N(1)−Ni−N(3)
N(2)−Ni−Cl(1)
N(4)−Ni−Cl(1)
1
2
2.133(2)
2.077(2)
2.125(2)
1.997(1)
2.383(1)
2.462(7)
3.572
2.142(3)
2.087(2)
2.146(3)
2.013(2)
2.416(3)
2.482(14)
3.613
3.274
3.307
155.4(9)
176.0(6)
172.9(6)
155.0(9)
175.0(6)
174.0(6)
reported correspondent behavior for a six-coordinate Ni(II)
species derived from the cyclen backbone (12-membered).10
Conversely, Ni(II) complexes derived from larger 14-, 15-, and
16-membered tetra-aza (unsaturated) ligands are capable of
accommodating Ni(I) and Ni(III) valence states rather
efficiently, arising from the fact that the four-coordinate
complex species prevail in solution.16,20,21,23,29,30,42 Taken
together, these results suggests that the high coordination
number imposed by the 12-membered ring system prevents the
effective accommodation of Ni(I) and Ni(III) species in
solution.
Structural Properties of the Five-Coordinate [Cu(II)-
Cl](ClO4) and [Zn(II)Cl](ClO4) Complexes (4−9). In analogy
with the solid-state structures for complexes 4 and 7, the
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Figure 5. The electronic absorption spectra for [NiII(L1)(Cl)]2(ClO4)2 (1), [NiII(L2)(Cl)]2(ClO4)2 (2), and [NiII(L3)(Cl)]2(ClO4)2 (3) measured
in water, at pH 6.0 (25 °C). The inset shows the spectra in the range between 400 and 1000 nm. Extinction coefficients [λmax/nm (ε/M−1·cm−1)] for
(1): 264 (137), 271 (sh), 563 (14), 779 (sh), and 955 (41); (2): 246 (200), 556 (16), 784 (sh), and 951 (41); (3): 220 (448), 285 (268), 552 (6),
788 (sh), and 947 (12).
Figure 6. ORTEP representations of the five-coordinate pyridol-based Cu(II) and Zn(II) complexes, 5 (top left) exhibiting a 7%, 6 (top right) a
20% distortion, 8 (bottom left) a 16%, and 9 (bottom right) a 43% distortion from square-pyramidal, with labeling scheme adopted. The perchlorate
counterions and solvent molecules (H2O in 9) comprising the asymmetric units have been omitted for clarity.
that of L3 and indicates that the lone pair of the oxygen atom is
delocalized to a greater extent throughout the aromatic system
of L2. Correspondingly, the O(1)−H(1) bond distances are
shorter in the p-substituted complexes, compared to the m-
hydroxyl derivatives. These observations suggest that the
pyridol ring of complex 5 contains the greatest amount of
electronic charge density and is supported by the upfield shift
noted in the 13C NMR spectra of L2 (Figure 3a) and in the
corresponding Zn(II) complex spectra discussed in greater
detail below.
Electronic and Electrochemical Properties of the Five-
Coordinate [Cu(II)Cl](ClO4) and [Zn(II)Cl](ClO4) Com-
plexes (4−9). The electronic spectra of the Cu(II) complexes
were measured in water and are shown in Figure 7. Again, no
color change was observed when the complexes were dissolved
in aqueous solution. In the UV region of the spectra, complexes
4 [261 (4353)], 5 [249 (8779)], and 6 [208 (12454), 246
(7841)] contain broader and more intense π → π* transitions
compared to the spectra of L1−L3. This result signifies that the
π-bonding interactions become stronger in response to Cu(II)
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Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for the Five-Coordinate Complexes [CuII(L1)(Cl)](ClO4) (4),
[CuII(L2)(Cl)](ClO4) (5), [CuII(L3)(Cl)](ClO4) (6), [ZnII(L1)(Cl)](ClO4) (7), [ZnII(L2)(Cl)](ClO4) (8), and
a
[ZnII(L3)(Cl)](ClO4) (9)
complex
M−N(1)
M−N(2)
M−N(3)
M−N(4)
M−Cl(1)
C−O(1)
O(1)−H
N(1)−M−N(3)
N(4)−M−Cl(1)
4
5
6
7
8
9
2.052(4)
2.041(3)
2.045(4)
1.963(3)
2.365(1)
2.082(16)
2.173(19)
2.079(16)
1.939(16)
2.226(9)
1.340(4)
0.819(2)
158.5(6)
154.2(7)
2.060(2)
2.137(2)
2.063(2)
1.952(2)
2.264(7)
1.361(1)
0.920(1)
159.6(9)
147.6(6)
2.245(2)
2.048(1)
2.212(2)
2.055(2)
2.245(6)
2.207(17)
2.087(19)
2.209(19)
2.041(17)
2.214(6)
1.339(3)
0.840(2)
153.3(7)
143.5(5)
2.214(5)
2.060(5)
2.226(5)
2.059(4)
2.234(5)
1.353(3)
0.850(2)
145.5(15)
119.8(12)
153.3(1)
115.8(1)
145.1(6)
119.8(5)
τ-value
0.63
0.07
0.20
0.42
0.16
0.43
a
The structural parameters for 4 (CCDC # 639155) and 7 (CCDC # 628572) were obtained from the Cambridge Crystallographic Data Center.33
Figure 7. Electronic absorption spectra of [CuII(L1)(Cl)]+ (4), [CuII(L2)(Cl)]+ (5), and [CuII(L3)(Cl)]+ (6) measured in water, at pH 6.0 (25
°C). The inset shows the spectra in the range between 500 and 1000 nm. Extinction coefficients [λmax/nm (ε/M−1·cm−1)] for (4): 261 (4353), 275−
330 (sh), and 713 (109); (5): 249 (8779), 284−330 (sh), and 699 (107); (6): 208 (12454), 246 (7841), 268−350 (sh), and 696 (121).
coordination.43 The absorbance maximum of 5 is blue-shifted
by 8 nm, versus the maxima for L2. The weak shoulder peak
that appears around 248 nm for the m-substituted ligand
becomes a well-defined electronic transition in complex 6,
indicating that metal complexation significantly perturbs the
energy of this transition. Furthermore, the extinction
coefficients pertaining to complexes 5 and 6 in the UV region
of the spectrum are much larger than in complex 4. This result
implies that there is a greater amount of π-density residing
throughout the pyridol systems versus the pyridine system, and
this effect is more pronounced for L2 than L3. In the visible
portion of the spectrum, the blue-colored Cu(II) complexes
each contain a single broad absorption band assigned as Cu-
based d−d electronic transitions. The absorption maxima for
the pyridol complexes blue-shift by 14 nm (5) and 17 nm (6),
with respect to the maximum for 4 (λmax = 713 nm). This
feature is consistent with the fact that 5 (τ = 0.07) and 6 (τ =
0.20) present similar geometries in the solid-state, compared to
4 (τ = 0.63), and suggests that the electronic behavior of the
Cu(II) ion is affected by the presence of the hydroxyl group.
These findings suggest that the lone pair from the oxygen atom
is delocalized throughout the aromatic system, and hydroxyl
functionalization of the pyridine ring effectively allows for an
extension of π-conjugation and is consistent with the
conclusions drawn above from the solid-state studies.
CV was used to the characterize the redox behavior of the
Cu(II) and Zn(II) complex systems, in order to determine if
the electronic nature of the metal center is affected by the
structural modifications made to L1. The electrochemical
activity of each complex was studied in both aqueous solution
and dimethylformamide (DMF). We found that the use of
DMF as a solvent gave rise to more reproducible CV scans,
versus the studies performed in aqueous solution (despite the
partial solubility of 5 and 6 in DMF). All of the complexes
reported here undergo quasi-reversible Cu(III)/Cu(II) and
Cu(II)/Cu(I) redox cycles, separated by a potential of ∼1.00 V,
when scanned in the cathodic direction, which are coupled to
ligand-based oxidations and reductions, respectively.16,21,28 The
more anodic charge transfer process is always coupled to a
ligand-based oxidation (Epa ≈ 800 mV) and is assigned as the
Cu(III)/Cu(II) redox cycle.2,16,21,22,25,28,48,49 Conversely, the
more cathodic charge transfer process is assigned as the
Cu(II)/Cu(I) redox cycle.16,21,26,28 The ligand-based redox
assignments were made based on the CVs of the Zn(II)
complexes (7−9). As expected, no Zn-based redox activity is
observed for complexes 7−9 in DMF solution; however, similar
ligand-based events noted for both the Cu(II) complexes are
present (shown in Figure S3, Supporting Information). Figure
8a,b shows the CVs obtained when complexes 4−6 were
scanned continuously between 1.10 V and −1.60 V starting
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Table 4. E1/2 Values (V) for 4−6 for the Cu(III)/Cu(II) and
a
Cu(II)/Cu(I) Redox Processes
complex
Cu(III)/Cu(II)
Cu(II)/Cu(I)
ligand [Epa (mV)]
4
5
6
−0.12
−0.083
−0.14
−0.96
−1.20
−1.05
761
735
790
a
The potential measurements were conducted using a glassy carbon
working electrode, a platinum auxiliary electrode, and Ag+/AgNO3
reference electrode, at a scan rate of 100 mV/s. The data reported here
has been normalized to the value of the half-wave potential for the
Fc(III)/Fc(II) redox couple.
occurs with the p-hydroxyl substituted ligand (Table 4 and
Figure 8c). A modification in the position or removal of the
hydroxyl substituent results in a more positive E1/2 value and is
a reflection of how the position of the hydroxyl moiety
influences the Lewis-acidity of the copper center. This result
implies that the Cu center of 5 (E1/2 = −0.083 V) is more
electron-rich than that of either 4 (E1/2 = −0.12 V) or 6 (E1/2
=
−0.14 V), suggesting that the electron-donating capacity of the
macrocycle increases in the order L2 > L1 ≥ L3. This trend
correlates with the blue-shift observed in the electronic spectra
of 4−6 and is postulated to be a consequence of the enhanced
π-bonding interactions of L2 between the aromatic system,
chlorine atom, and metal center. The stronger xy-plane
interactions displayed by 5 observed both in the solid-state
and in solution correlate with the strength of the Cu−N(4) and
Cu−(Cl) π-bonding interactions. The L2 derivative imposes a
stronger basal interaction with the Cu(II) center compared to
L1 and L3 and allows for an easier attainment of the Cu(III)
oxidation state.2,15,20,21,28,29,34
The Cu(II)/Cu(I) Redox Couple. As complex 4 is scanned
from the cathodic to the anodic direction the Cu(II)/Cu(I)
redox cycle becomes a more favorable process. As seen in
Figure 8b, the peak corresponding to the oxidation of Cu(I)
[Epa = −850 mV] is well-defined for the pyridine-based
complex 4 versus the pyridol-derived complexes, where the
peaks for complexes 5 and 6 are similar in shape and intensity
to those in Figure 8a. Moreover, consecutive cycles (two or
greater) gave rise to a significant reduction in the peak intensity
of 4 similar to the CV shown in Figure 8a. It should be noted
that the repeated cycles result in no loss of current intensity for
the remaining peaks. When the scan window of each complex is
restricted to a more cathodic potential (−0.40 V to −1.60 V)
the oxidation of Cu(I) becomes a more favorable process for 4
and 6, than for 5. This idea is evidenced by the significant
increase in the current intensity for 4 and 6 observed in Figure
8c, versus the current values in Figure 8a.
In either case, the oxidation of Cu(I) is least favorable for
complex 5 and is independent of the scan window. This result
implies that the p-hydroxyl ligand is not well-suited to
accommodate the Cu(II)/Cu(I) redox cycle in the DMF
solution. Furthermore, the ligand-based oxidation that is
coupled to the Cu(II)/Cu(I) redox process is no longer
observed for complexes 4 and 6 but remains present in complex
5. This behavior suggests that when a more cathodic potential
is applied to the system, the oxidation of Cu(I) becomes a
more favorable or reversible process concomitant to the
absence of the ligand-based reduction. Lastly, the easiest
attainment of the Cu(I) oxidation state, which is defined by the
least negative half-wave potential, occurs with the pyridine-
based ligand (Table 4). The same trend noted for the Cu(III)/
Figure 8. Cyclic voltammograms obtained for the Cu(II) complexes
(4−6) in 10 mL of DMF containing tetrabutylammonium
tetrafluoroborate (10 mg) as the supporting electrolyte at a scan
rate of 100 mV/s. The first two figures show the CVs obtained for
each complex scanned from the (a) cathodic-to-anodic and (b)
anodic-to-cathodic direction (from 1.10 V to −1.60 V), and (c)
between 1.10 and 0.50 V, and 0.40 and 1.60 V. The potential values
presented here have been normalized to the half-wave potential of the
Fc(III)/Fc(II) redox couple (E1/2 = 0.00 V).
from the anodic and cathodic directions, respectively. The Cu-
based redox processes were also studied individually, as well,
and the resulting CVs are shown in Figure 8c. The E1/2 values
for each redox cycle were calculated according to the data
presented in Figure 8c and are listed in Table 4. These results
are discussed in greater detail below.
The Cu(III)/Cu(II) Redox Couple. An attempt to separate
the Cu(III)/Cu(II) redox processes from the more anodic
ligand-based oxidation resulted in the absence of a current
response. In each case examined when the window of the scan
was confined between 200 mV to −500 mV, the current of the
system remained essentially constant and the resulting CVs
lacked definition (data not shown). As shown in Figure 8c, the
Cu(III)/Cu(II) redox couple was only observed when a
potential greater than 800 mV was applied to each complex.
This result suggests that the ligands must first undergo
oxidation in order to accommodate the higher valent Cu(III)
species. The easiest attainment of the Cu(III) state, which is
defined by the more negative value of the half-wave potential,
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Figure 9. Overlay of the 13C NMR spectra of the Zn(II) complexes (7−9) measured in D2O solution (pH = 6.0). 7: δ = 154.3, 141.2, 121.3, 51.9,
51.8, 50.0, 49.9, 45.3; 8: δ = 168.6, 156.0, 109.2, 51.6, 49.0, 48.9, 45.5, 45.4; 9: δ = 149.8, 145.0, 141.8, 126.8, 121.8, 51.3, 50.5, 49.8, 48.2, 45.3, 45.1.
Cu(II) processes is observed for the Cu(II)/Cu(I) cycle. This
results of this study reveal that the Lewis-basicity of the 12-
membered pyridine- and pyridol-based tetra-aza macrocyclic
ligands increases across the series L2 > L1 > L3. The
asymmetric nature of the m-hydroxyl ligand masks any of the
electron-donating ability gained upon hydroxylation of the
pyridine ring. Future work will focus on isolation and
characterization of the Cu(III) species derived from 4−6, via
chemical oxidation and controlled potential electroly-
sis.2,28,49−51 Notably, the formation of intermediate Cu(III)
species is thought to be an essential component for the
assembly of C−C and C-heteroatom bonds for Cu/ligand-
catalyzed cross-coupling reactions.17,52 The ligand sets
presented herein display the ability to accommodate both
Cu(I) and Cu(III) oxidation states in solution and should be
considered toward catalysis of this type.
result implies that the Cu center of 5 (E1/2 = −1.20 V) is more
electron-rich than that of either 4 (E1/2 = −0.96 V) or 6 (E1/2
=
−1.05 V) and suggests that the electron-donating capacity of
the ligands increases across the series L2 > L1 > L3, which
correlates with the blue-shift observed in the electronic spectra
of 4−6.
The electronic properties of the Zn(II) complexes were
characterized via NMR spectroscopy in D2O solution, and the
resulting spectra are presented in Figure 9. The information
inferred from the spectra is consistent with the involvement of
the three aliphatic and single pyridine nitrogen atoms in a
nonplanar fashion, with respect to the metal cation. Notably,
the 13C NMR spectra of 7 and 8 show eight resonances, which
contain two additional peaks compared to the spectra for L1
and L2. The benzyl carbon atoms retain chemical equivalence,
but the four aliphatic carbon atoms composing the lower and
more flexible portion of the ligand lose chemical equivalence
upon coordination of Zn(II). The result is suggestive of a
nonplanar and less symmetric carbon framework in solution
versus the free macrocycles. These findings are consistent with
the results obtained in the solid-state studies, as the ligands
display a cis-folded conformation where the metal-ion lies
outside the N4 plane. In addition, the trends noted for the
resonance signals related to the aromatic carbon atoms of L1−
L3 are conserved in the 13C NMR spectra of complexes 7−9 as
well. As noted previously, the upfield shift in the resonance
signal observed for complex 8 (109.2 ppm) is supportive of the
conclusion that the p-pyridol system is more electron donating
that the m-substituted pyridol system and is reflected by the
electrochemical behavior of the Cu(II) complexes discussed
above.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data for 1, 2, 5, 6, 8, and 9 (CCDC #: 950044,
950046, 950045, 911993, 950047 and 947495, respectively) in
both CIF and table format. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kayla.green@tcu.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for generous financial support from the
TCU Andrews Institute of Mathematics & Science Education,
TCU Research and Creativity Grants (to K.G.), TCU SERC
(to M.E.O. and T.H.), and the Robert A. Welch Foundation (to
K.G., P-1760). We sincerely thank the group of Dr. Eric
Simanek for access to mass spectrometry and Dr. Tracy Hanna
for helpful conversations related to this project. In addition, we
are grateful to Zoltan Kovacs (UTSW) for the sole contribution
of L1 and Charles Campana for XRD advice.
CONCLUSION
■
Effect of p- versus m-Hydroxylation. The data presented
throughout this study support the idea that introduction of the
hydroxyl substituent leads to an increase in the amount of
electronic charge density throughout the pyridine ring. Clearly,
p-hydroxylation induces a larger electron-donating effect than
m-hydroxylation with respect to the metal center. Overall, the
J
dx.doi.org/10.1021/ic402119s | Inorg. Chem. XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ic402119s | Inorg. Chem. XXXX, XXX, XXX−XXX