Introducing a new azoaromatic pincer ligand. Isolation and characterization of redox events in its ferrous complexes

Article abstract of DOI:10.1021/ic500355f

The isolation and complete characterization of a new bis-azoaromatic ligand, 2,6-bis(phenylazo)pyridine (L), are described, and its coordination to iron(II) is reported. A pseudo-trigonal-bipyramidal mixed-ligand complex of iron(II), FeLCl₂ (1), and a homoleptic octahedral iron complex, mer-[Fe(L)₂]ClO₄ [2]ClO₄, have been synthesized from L and FeCl₂ or hydrated Fe(ClO₄)₂, respectively, in boiling methanol. Determination of the X-ray crystallographic structure together with magnetic data (≈ 5.06 μ_B) and Moessbauer analysis of 1 established a high-spin Fe(II) complex ligated by one neutral 2,6-bis(phenylazo)pyridine ligand. The X-ray crystallographic structure (showing d_N-N > 1.30 A), Moessbauer data, and magnetic susceptibility measurements (≈ 1.65 μ_B) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a low-spin Fe(II) complex with a one-electron-reduced radical ligand coordination in [2]ClO₄. The ligand and the metal complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox events. The uncoordinated ligand, L, displays a reversible one-electron wave at -1.07 V and a quasi-reversible wave at -1.39 V. The partially reduced ligand L^?- shows a single-line EPR spectrum at g = 2.001, signifying that L^?- is a free radical. While complex 1 shows a reversible reduction at -0.08 V and an irreversible cathodic response at -0.98 V, the bis-chelate [2]ClO₄ undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at -0.18, -0.88, and -1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent monocationic complex [2]⁺ and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis(phenylazo)pyridine ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L]⁰), monoanionic monoradical ([L^?] ^-), and dianionic diradical ([L^??]^2-) forms.

Full text of DOI:10.1021/ic500355f

Article
pubs.acs.org/IC
Introducing a New Azoaromatic Pincer Ligand. Isolation and
Characterization of Redox Events in Its Ferrous Complexes
Pradip Ghosh,† Subhas Samanta,†,‡ Suman K. Roy,† Serhiy Demeshko,‡ Franc Meyer,‡
and Sreebrata Goswami†,*
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
‡Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The isolation and complete characterization of
a new bis-azoaromatic ligand, 2,6-bis(phenylazo)pyridine (L),
are described, and its coordination to iron(II) is reported. A
pseudo-trigonal-bipyramidal mixed-ligand complex of iron(II),
FeLCl2 (1), and a homoleptic octahedral iron complex, mer-
[Fe(L)2]ClO4 [2]ClO4, have been synthesized from L and
FeCl2 or hydrated Fe(ClO4)2, respectively, in boiling
methanol. Determination of the X-ray crystallographic
structure together with magnetic data (≈ 5.06 μB) and
Mossbauer analysis of 1 established a high-spin Fe(II) complex
̈
ligated by one neutral 2,6-bis(phenylazo)pyridine ligand. The
X-ray crystallographic structure (showing dN−N > 1.30 Å),
Mossbauer data, and magnetic susceptibility measurements (≈
̈
1.65 μB) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a
low-spin Fe(II) complex with a one-electron-reduced radical ligand coordination in [2]ClO4. The ligand and the metal
complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox
events. The uncoordinated ligand, L, displays a reversible one-electron wave at −1.07 V and a quasi-reversible wave at −1.39 V.
The partially reduced ligand L•− shows a single-line EPR spectrum at g = 2.001, signifying that L•− is a free radical. While
complex 1 shows a reversible reduction at −0.08 V and an irreversible cathodic response at −0.98 V, the bis-chelate [2]ClO4
undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at −0.18, −0.88,
and −1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent
monocationic complex [2]+ and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by
using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis(phenylazo)pyridine
ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L]0), monoanionic
monoradical ([L•]−), and dianionic diradical ([L••]2−) forms.
design of suitable ligands containing multiple azo functions is a
natural choice for exploration. However, to date, the chemistry
of azoaromatics has been dominated by ligands containing a
single azo function. As far as we are aware, there is only one
example of a bis-azo ligand, viz. 1,3-bis(phenylazo)benzene,
whose coordination has been studied for quite some time.4
However, since it coordinates as an anionic NCN donor and
expectedly reductions of its complexes occur at high cathodic
potentials, it seems that these are less promising for sequential
redox-induced events. Consequently, we have been in search of
a neutral ligand that contains multiple reducible azo functions
to serve our purpose of multiple redox processes in its
complexes. We are further encouraged by a recent report5 on
the superior redox properties of fomazanates over β-
diketiminates.
INTRODUCTION
■
The favorable properties of azoaromatics as redox-active ligands
have been quite well substantiated in recent times. This
primarily is due to the low-lying π* orbitals of the coordinated
azo function, resulting in multiple electron transfer as a
dominant electron sink in their metal complexes.1,2 For
example, six successive redox processes within an accessible
range of potentials were noted recently in Rh/Ir complexes2a,b
of the 2-(arylazo)pyridine ligand. Similar multielectron redox
phenomena have been noted in several other complexes of the
above and related3 ligands. In fact, in contemporary research,
the studies of redox properties, the isolation of metal complexes
in various redox states, and the discovery of different levels of
degeneracy2a,b in redox series have opened up new possibilities
in terms of chemical catalysis and electronic application
feasibilities.
Since the azo function in azoaromatics is primarily
responsible for the rich redox events in the complexes, the
Received: February 13, 2014
Published: April 17, 2014
© 2014 American Chemical Society
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1
In this report we wish to disclose the successful isolation of a
neutral bis-azo ligand, viz. 2,6-bis(phenylazo)pyridine (L), and
its coordination to Fe(II). There is virtually no literature on this
ligand except for a German patent6 describing the use of its 3d
ion metal complexes in the rubber-making industry. Fur-
thermore, this ligand has a close resemblance to another
tridentate ligand, 2,6-bis(arylimino)pyridine (L′) (Chart 1),
distribution pattern. Its H NMR spectrum is shown in Figure
1.
The reactions of L with two ferrous salts, viz. FeCl2 and
hydrated Fe(ClO4)2, at different metal to ligand ratios (Scheme
1) have been studied, and pure material could be isolated in
Scheme 1
Chart 1
two cases. The reaction between equimolar quantities of FeCl2
and L resulted in a brown crystalline product of composition
FeLCl2 (1). The reaction between hydrated Fe(ClO4)2 and 2
equiv of L resulted in a crystalline green product of
composition [Fe(L)2]ClO4 ([2]ClO4). The ESI MS of the
complex [2]ClO4 in acetonitrile showed an intense peak at m/z
630 amu assigned to [2]+; its simulated isotopic distribution
pattern matched well with the experimental pattern (see the
Supporting Information, Figure S2). Elemental analyses
(Experimental Section) of the ligand and both complexes
corroborate the formulations given above.
whose coordination chemistry, in general,7,8 and its redox
chemistry, in particular, received a tremendous boost in recent
years due to extensive catalytic applications9,10 of base metal−
L′ (Fe and Co in particular) complex catalysts in several useful
organic transformations. This indeed is another driving force
for taking up the exploration of the coordination chemistry of
the reference bis(phenylazo)pyridine ligand with a primary aim
of developing, in the long term, potent catalysts9,10 with
synergistic participation of metal and ligand redox activities.
The synthesis and complete characterization of the ligand L
and its two iron complexes are the primary concerns of this
report. To this end, X-ray crystallography, electrochemistry, and
various spectroscopic methods have been applied. Density
functional theory (DFT) has been used to address the
ambiguity in the oxidation state formalism.
Characterization: X-ray Crystallography, Magnetic
̈
Susceptibility, and Mossbauer Spectroscopy. The free
ligand and its two iron complexes have been characterized using
single-crystal X-ray diffraction. Crystallographic details are
collected in Table 1, and key bond lengths are compiled in
Table 2.
Single crystals suitable for X-ray crystallography of the ligand
L were grown by slow evaporation of a dichloromethane
solution of the compound. Its ORTEP and atom-numbering
scheme are shown in Figure 2. The X-ray crystallographic
analysis reveals a planar structure of the free ligand L, with two
RESULTS AND DISCUSSION
d
N−N values of 1.225(8) and 1.241(8) Å, respectively; these are
■
Synthesis. The neutral tridentate ligand L was prepared
using a procedure modified from that described for closely
related compounds with a single azo group.11 The synthesis
involves condensation of 2,6-diaminopyridine with nitro-
sobenzene in a 1:2 ratio under highly alkaline conditions. An
electrospray ionization (ESI) mass spectrum (MS) of the ligand
corroborates its formulation: the ESI MS of L in acetonitrile
(see the Supporting Information, Figure S1) displayed a peak at
m/z 288 amu for [L + H]+, with the expected isotopic
indicative12 of double-bond character of the azo functions.
Single crystals of complex 1 suitable for X-ray diffraction
analysis were obtained by slow evaporation of its solution in a
dichloromethane−hexane solvent mixture. The ORTEP and
atom-numbering scheme are shown in Figure 3a. The
coordination geometry of complex 1 is best described as a
distorted trigonal bipyramid with two N(azo) atoms (N1 and
N5) occupying the apical positions with the angle N1−Fe−N5
= 143.53(11)°. The extended azoaromatic ligand and the
1
Figure 1. H NMR spectrum of L in d6-DMSO (*1, H2O; *2, solvent). Inset: aromatic proton resonances.
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half of the molecule identical with the other half. In this
structure the most significant observation is that the N−N
bond lengths are elongated considerably (>1.30 Å), which is
taken as an indication that the ligand(s) is (are) reduced2d in
[2]+. Moreover, the average Fe−N(azo) bond lengths in [2]+
(average 1.975(3) Å) are significantly shorter than those in 1
(average 2.222(3) Å).
Table 1. Crystallographic Data for L, 1, and [2]PF6
L
1
[2]PF6
empirical formula
C17H13N5
287.32
C17H13Cl2FeN5
414.07
C34H26FeN10F6P
775.47
molecular mass
(amu)
temp (K)
cryst syst
space group
a (Å)
293
293
293
monoclinic
P21/c
orthorhombic
Pbca
orthorhombic
Fddd
The iron complexes are paramagnetic, and variable-temper-
ature magnetic susceptibility measurements were performed on
polycrystalline samples of 1 and [2]ClO4 in the temperature
range 295−2 K to confirm the spin states and to assess the
magnetic properties of the present systems. The temperature
dependence of the magnetic behavior as χMT versus T, χM
being the molar magnetic susceptibility, is shown in Figure 4.
At 295 K, the value of the product χMT for 1 is 3.23 cm3 mol−1
K, which is close to the spin-only value for high-spin Fe(II)
with S = 2 (3.00 cm3 mol−1 K). The χMT value of [2]ClO4 at
295 K is 0.34 cm3 mol−1 K, indicating the presence of one
unpaired electron (S = 1/2). Data analysis using a fitting
procedure to the appropriate spin Hamiltonian for zero-field
splitting and Zeeman interaction and including a term for
temperature-independent paramagnetism (TIP) provided the
values g = 2.07, |D| = 7.1 cm−1, and TIP = 12.8 × 10−4 cm3
mol−1 for 1. The data for [2]ClO4 were well simulated with g =
2.0 (fixed based on EPR results; see below), TIP = 5.3 × 10−4
cm3 mol−1, and some diamagnetic impurity (DI) of 7.4% (DI
had to be included since the g value was fixed at 2.0). The
decrease of χMT at temperatures below 10 K could be simulated
assuming the Weiss temperature Θ = −3.7 K, which reflects
weak intermolecular interactions between radical ligands.
Notably, weak intermolecular π−π interactions can be
identified in the solid-state structure of [2]ClO4.
6.1995(13)
19.910(4)
24.478(5)
90
13.334(2)
14.982(3)
17.158(3)
90
14.521(5)
27.079(5)
32.807(5)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
96.343(6)
90
90
90
90
90
3002.9(11)
8
3427.7(10)
8
12900(5)
16
Z
Dcalcd (g/cm3)
1.271
1.605
1.597
cryst dimens (mm) 0.12 × 0.15 × 0.11 × 0.13 ×
0.10 × 0.12 × 0.13
0.17
0.15
θ range for data
collecn (deg)
1.3−17.2
2.4−20.8
1.7−26.5
GOF
1.00
1.05
0.78
no. of rflns collected 12061
19566
1772
40804
3340
no. of unique rflns
1830
final R indices (I >
2σ(I))
R1
0.0363
0.0875
0.0327
0.0851
0.0362
0.1339
wR2
Table 2. Selected Bond Distances (Å) of L, 1, and [2]PF6
N(1)−
N(4)−
Fe(1)−
Fe(1)−
Fe(1)−
complex
N(2)
N(5)
N(1)
N(3)
N(5)
L
1.225(8)
1.283(5)
1.302(3)
1.241(8)
1.278(4)
1.311(3)
The electronic structures of 1 and [2]ClO4 were further
1
2.198(3)
1.971(2)
1.995(3)
1.848(2)
2.246(3)
1.980(4)
authenticated by zero-field 57Fe Mossbauer spectroscopy
̈
[2]PF6
(Figure 5). The spectrum of 1 at 80 K shows a doublet with
isomer shift δ = 0.67 mm/s and quadrupole splitting ΔEQ
=
1.85 mm/s. These values are consistent with the presence of
high-spin Fe(II) in a five-coordinate covalent ligand environ-
ment.8i The isomer shift of 1 is somewhat lower than δ ≈ 0.9
mm/s for the related complexes FeL′Cl2 (L′ = 2,6-bis-
(arylimino)pyridine),14 which is in accordance with L being a
better π acceptor than L′. For [2]ClO4 the Mossbauer
̈
parameters are δ = 0.14 mm/s and ΔEQ = 1.49 mm/s,
suggesting that the iron(II) ion is in its low-spin d6
configuration.8a Consequently, the spin carrier in the case of
[2]ClO4 should be the radical ligand, with [FeII(L)(L•−)]+
being a reasonable description of the electronic structure. One
should note that a much larger quadrupole splitting ΔEQ would
be expected for low-spin iron(III),8a and also the N−N bond
lengths in [2]+ (dN−N = 1.302(3)/ 1.311(3) Å) are most
compatible with an average (caused by crystallographic
symmetry) of one neutral L ligand (dN−N ≈ 1.28 Å) and one
singly reduced L•− ligand (dN−N ≈ 1.35 Å); see Electronic
Structure Analysis for further discussion.
Cyclic Voltammetry, EPR, and Spectroelectrochemis-
try. The redox behavior of the free ligand and its complexes
was studied by cyclic voltammetry and related electrochemical
techniques; the data are collected in Table 3. The cyclic
voltammogram of the ligand L shows two successive one-
electron reductions, [L]0 → [L•]− → [L••]2− at −1.07 and
−1.39 V (vs Ag/AgCl), reflecting the sequential filling of the
lowest π* orbitals (Figure 6). Further two-step reduction-
Figure 2. ORTEP of L with ellipsoids at the 30% probability level.
Hydrogen atoms are omitted for clarity.
bound metal ion are roughly located in a plane, with no atom
deviating by more than 0.04 Å from the mean plane. The dN−N
values for the complex are 1.278(4) and 1.283(5) Å, clearly
elongated in comparison to those of the free ligand; this may be
attributed to the effect of dπ(Fe) → π*(L) back-bonding
interactions.1a,13
The perchlorate salt of the cationic complex [2]+ is
microcrystalline; however, its hexafluorophosphate salt [2]PF6
formed crystals suitable for a single-crystal X-ray structure
determination. The geometry of the cation [2]+ is a distorted
octahedron comprising two meridionally coordinating ligands
that orient the two N(py) atoms in mutually trans positions. Its
ORTEP and atom-numbering scheme are shown in Figure 3b.
Overall the molecule [2]+ has crystallographic C2 symmetry
(noncrystallographic D2d symmetry with an S4 axis) that makes
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Figure 3. ORTEP of (a) 1 and (b) [2]+ with ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity.
Figure 4. χMT vs T plots of 1 (circles, top) and [2]ClO4 (triangles,
bottom). The solid lines represent the best simulations.
s2a−d,15 to [L•]3− and [L]4−, though likely possible, have not
been observed under the experimental conditions.
The ligand L can be reduced on the bulk scale by exhaustive
electrolysis: the one-electron-reduced product [L•]−, generated
by partial electrolysis, is green and showed an isotropic EPR
signal (g = 2.001) signifying the formation of an anion radical
(Figure 7).
This creates a mixed-valent situation within the organic
framework with reduction of only one of two azo functions. A
low-energy transition was observable at 680 nm. TD-DFT
calculations (see below) indicated the two low-energy
Figure 5. Zero-field 57Fe Mossbauer spectra of (a) 1 and (b) [2]ClO
at 80 K. The solid lines represent simulations with Lorentzian
doublets.
transitions SOMO(α) → LUMO(α) and SOMO(α) →
LUMO+1(α) & LUMO+3(α), in which the SOMO(α) is
localized on half of the ligand and the LUMO is localized on
the other half. Higher energy orbitals, viz. LUMO+1(α) and
LUMO+3(α), have mainly π* character of the phenyl groups
(Supporting Information, Figures S3a and S3b). The calculated
lowest energy transition is broad and appeared in the low-
energy IR region (outside of our experimental range).
However, the absorption at 680 nm observed in our experiment
is in the range of the calculated energy (610 nm) for the above
transition SOMO(α) → LUMO+1(α) & LUMO+3(α). Visible
range spectroelectrochemistry of L showed a gradual generation
of [L•]−, also evidenced by color changes (Figure 8). The two-
̈
4
electron-reduced product [L••]2− was hypersensitive, and no
meaningful characterization was possible.
Cyclic voltammetry measurements of complexes 1 and [2]+
indicated that, upon coordination of the ligand to Fe(II) in the
complexes, the reduction potentials of the azo chromophore
underwent considerable anodic shifts. For example, complex 1
shows one reversible reductive response at −0.08 V and an
irreversible response at −0.98 V (Figure 6). Exhaustive
electrolysis at −0.3 V confirms a one-electron reduction: 1 →
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a
d
Table 3. Cyclic Voltammetric and UV−Vis Spectral Data of L, 1, and [2]ClO4
b
compound
cyclic voltammetry E1/2
,
V (ΔEp, mV)
UV−vis λ (ε, 10−4 M−1 cm−1
)
c
L
−1.07 (150), −1.39 (200)
440 (0.18), 320 (3.2)
c
e
e
1
−0.08 (100), −0.98
890 (0.1), 540, 410, 335 (1.1)
e
[2]ClO4
0.54 (80), −0.18 (80), −0.88 (110), −1.2 (120)
Conditions: acetonitrile solution for L and 1 (supporting electrolyte [Et4N]ClO4), dichloromethane solution for complex [2]+ (supporting
630 (0.39), 415, 340 (3.5)
a
b
electrolyte [Bu4N]ClO4), working electrode platinum, reference electrode Ag/AgCl. E1/2 = 0.5(Epa + Epc), where Epa and Epc are anodic and
c
d
cathodic peak potentials, respectively, ΔEp = Epa − Epc,. Scan rate: 50 mV s−1. Quasi-revesible or irreversible. Wavelengths in nm and molar
e
extinction coefficients in M−1 cm−1. In acetonitrile solvent for L and 1 and dichloromethane solvent for [2]ClO4. Shoulder.
Figure 6. Segmented cyclic voltammograms of L (red), 1 (black), and
[2]ClO4 (green). Conditions: potentials vs Ag/AgCl; L and 1
Figure 8. Electronic spectra of gradual generation of the one-electron-
measured in CH3CN/[Et4N]ClO4 solution; [2]+ measured in
reduced ligand [L•]− in acetonitrile.
CH2Cl2/[Bu4N]ClO4 solution.
Figure 7. X-band EPR spectrum of [L•]− in frozen CH3CN solution at
Figure 9. X-band EPR spectrum of [2]+ in frozen CH2Cl2 solution at
120 K. Conditions: microwave frequency 9.136 GHz; power 0.998
120 K. Conditions: microwave frequency 9.124 GHz; power 0.998
mW; modulation amplitude 0.1 mT.
mW; modulation amplitude 0.1 mT.
1−. The solution of 1− is green and showed a broad transition
near 660 nm (Supporting Information, Figure S4). The EPR
spectrum of coulometrically generated 1− shows a slightly axial
(almost isotropic; see Supporting Information, Figure S5)
signal at g = 1.998/1.965, confirming the formation of a radical
upon reduction. The anion radical complexes of transition
metals are known to often have a slight axial character and g
values slightly deviating from 2.0023 due to metal−ligand
interactions.16 [2]+ displayed a nearly isotropic EPR signal with
a small metal contribution16 at g = 1.968 (Figure 9).
Accordingly, it is reasonable that unpaired spin resides primarily
on the ligands. Unfortunately, however, no 14N hyperfine
coupling is observed in the EPR spectrum, either at 120 K or at
room temperature. The cationic complex [2]+ thus belongs to a
rare class of stable radical complexes. Normally the ligand
radical spin in such complexes is quenched via coupling with
other unpaired spin(s) within the system.
The cationic complex, [2]+ shows one reversible one-
electron oxidation at 0.54 V and three successive reversible one-
electron reductions at −0.18, −0.88, and −1.2 V, respectively
(Figure 6). The nature of the redox processes in [2]+ was
studied by EPR spectroscopy and density functional theory
(DFT) calculations (see below). Coulometrically generated
oxidized and reduced compounds [2]2+ (electrolysis at 0.8 V)
and [2]0 (electrolysis at −0.4 V) are EPR silent. The oxidized
compound [2]2+ was further studied by NMR and Mossbauer
̈
1
spectroscopy. It is diamagnetic and displays a well-resolved H
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NMR spectrum; the low number of resonances reflects the high
symmetry (D2d) in solution (see the Supporting Information,
are in slightly better agreement with experimental values than
parameters calculated for FeIII(L•−)Cl2. The spin density plot of
1 is shown in Figure 11, and the calculated parameters are
collected in Table S2 (Supporting Information). This
description is also consistent with its physicochemical behavior
(vide supra).
Figure S6). Its Mossbauer parameters (Supporting Information,
̈
Figure S7) are δ = 0.15 mm/s and ΔEQ = 1.41 mm/s: i.e., very
similar to those of parent [2]+. This suggests that the iron(II)
ion remains in its low-spin d6 configuration. Thus, the results
together clearly indicate that the iron center is unaffected and
oxidation occurs at the radical ligand center. Since both the
oxidized and reduced forms are moderately stable, we have
attempted to study their electronic spectral properties, which
are depicted in Figure S8 (Supporting Information). Upon
oxidation the charge transfer transition at 630 nm of [2]+
disappeared, while on reduction the band was shifted to the red
(20 nm) with augmentation of intensity (Supporting
Information, Figure S8). This is an indication8a that reduction
of the second ligand has occurred in [2]0, which should thus be
described as [FeII(L•−)2]0. The corresponding bis-ligated
iron(II) complex8a with 2,6-bis(arylimino)pyridine is diamag-
netic and showed a reversible oxidation (assigned to a metal-
based Fe(II)/Fe(III) couple) and two reversible one- reductive
waves (assigned to ligand-based redox processes) at relatively
high potential in comparison to that of complex [2]+.
Figure 11. Spin density plot of 1 derived from DFT calculations.
Experimental magnetic data for [2]+ are consistent with
either of the following two formulations: (i) low-spin FeIII (SFe
= 1/2) with two ligand radicals (SL = +1) and one ligand radical
antiferromagnetically coupled to a single metal center spin or
(ii) low-spin FeII (SFe = 0) and one unpaired electron (SL =
+1/2) either localized on a single ligand or delocalized over
both. DFT calculations for [2]+ favor a low-spin iron(II)
description (Figure 12) for the complex: one unpaired spin is
Electronic Structure Analysis. In order to have a closer
look into the electronic structure of the ligand and complexes,
we have performed a series of calculations using DFT at the
B3LYP level. The observation that the free ligand L may be
reduced by at least two electrons leading to [L•]− and [L••]2−
with doublet and triplet ground states, respectively, is also
evident from the DFT calculations. The calculated geometries
and metrical parameters of the ligand in its three oxidation
states are collected in Table S1 (Supporting Information). The
one-electron-reduced ligand [L•]− has a doublet ground state
(Figure 10), and the net spin density is delocalized primarily
Figure 12. Spin density plot of [2]+ derived from DFT calculations.
Figure 10. Spin density plot of the one-electron-reduced ligand [L•]−
derived from DFT calculations.
delocalized17,18 over the two ligands with only a small part
(∼7%) residing on metal orbitals, which is in line with the
observed slight anisotropy in the EPR spectrum of [2]+. It
should be noted, however, that DFT generally has a tendency
to overemphasize electron delocalization, and hence a more
localized mixed-valent [FeII(L)(L•−)]+ description would be
appropriate. The one-electron-reduced compound [2]0 was
optimized in three states: namely singlet closed shell, singlet
broken symmetry (BS), and triplet open shell. Among the
different optimized structures, the triplet (S = 1) open shell
calculated geometry with parallel spin densities (ρL = +1.8) on
the two ligands is energetically the lowest (Supporting
Information, Table S3) and favors a triplet ground state
description. Its spin density plot is depicted in Figure S9
(Supporting Information), and the optimized parameters
(Table S3) along with Cartesian coordinates are included in
the Supporting Information. The difference in calculated
over the pyridyl ring and two azo chromophores. As
anticipated, N−N bond lengths of the azo groups are elongated
in the reduced ligand (calculated dN−N = 1.292 Å in [L•]−
versus 1.259 Å in L; see the Supporting Information, Table S1).
DFT calculations for the iron complexes considering multiple
metal/ligand oxidation or spin state combinations were
performed to discount their alternative electronic structures.
The observed magnetic moment of the complex 1 is, in
principle, consistent with two formulations, including (i) high-
spin-FeII (SFe = 2) with a neutral ligand (SL = 0) and (ii) high-
spin FeIII (SFe = 5/2) with one unpaired spin on the ligand (SL =
1/2) coupled antiferromagnetically, giving rise to the exper-
imentally observed S = 2 ground state description. The latter
description is energetically higher than the former by 2.7 kcal/
mol. Furthermore, metrical parameters calculated for FeII(L)Cl2
4683
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Article
Synthesis. Preparation of L. The ligand, L was synthesized11 and
purified by following a procedure slightly modified from that for 2-
(phenylazo)pyridine, with 2,6-diaminopyridine was used in place of 2-
aminopyridine in a 1:2 molar proportion. Yield: 30%. IR (KBr, cm−1):
1571 (ν(CN) + ν(CC)), 1448 (ν(NN)). Anal. Calcd for
C17H13N5: C, 71.06; H 4.56; N 24.37. Found: C, 70.94; H, 4.80; N,
23.98. ESI-MS: m/z 288 [M + H]+. 1H NMR (d6-DMSO, 400 MHz):
8.31 (t, 1H, J = 7.6 Hz), 8.03−8.00 (m, 4H), 7.92 (d, 2H, J = 7.6 Hz),
7.67 (t, 6H, J1 = 3.6 Hz, J2 = 2.8 Hz) ppm.
energies between the last two is small (∼0.7 kcal/mol). Hence,
a triplet description [Fe(L•−)2] appears reasonable and is in
accordance with the orthogonal arrangement of the two ligands.
Oxidation of [2]+ is a ligand-based redox process, and the
dicationic complex [2]2+ is best described as a low-spin
1
[FeII(L)2]2+ (S = 0) complex, which is consistent with the H
NMR and Mossbauer spectral data (vide supra).
̈
Preparation of FeLCl2 (1). A mixture of 126 mg (1 mmol) of FeCl2
and 287 mg (1 mmol) of L in 50 mL of methanol was refluxed for 4 h.
The resulting brown solution was evaporated to dryness. The crude
residue was subsequently extracted with dichloromethane and
recrystallized from dichloromethane−hexane solution. Yield: 91%
(375 mg). IR (KBr, cm−1): 1580 (ν(CN) + ν(CC)), 1430
(ν(NN)). Anal. Calcd for C17H13Cl2FeN5: C, 49.31; H, 3.16; N,
16.91. Found: C, 49.74; H, 3.20; N, 17.12.
Preparation of Fe(L)2ClO4 ([2]ClO4). A mixture of 180 mg (0.5
mmol) of Fe(ClO4)2·6H2O and 287 mg (1 mmol) of L in 50 mL of
methanol was refluxed for 3 h. The resulting green solution was
evaporated to dryness. The crude residue was subsequently extracted
with dichloromethane and recrystallized from dichloromethane−
hexane solution. Yield: 78% (285 mg). IR (KBr, cm−1): 1580
(ν(CN) + ν(CC)), 1350 (ν(NN)). Anal. Calcd for
C34H26FeN10ClO4: C, 55.95; H, 3.59; N, 19.19; Found: C, 55.53; H,
3.59; N, 19.13. ESI-MS: m/z 630 [2]+. The corresponding
hexafluorophosphate salt, [2]PF6, was obtained in >90% yield by the
addition of a saturated aqueous solution of [NH4]PF6 to a solution of
[2]ClO4 in methanol.
CONCLUSIONS
■
In summary, we have introduced the new pincer-type ligand L
and the first two metal complexes of that ligand. A major
highlight lies in the highly redox noninnocent character of the
ligand, which is superior to those of bis(imino)pyridine and
related pincer ligands, as anticipated.7 In particular, L has two
easily reducible azo functions that enable the ligand to serve as
a multielectron redox reservoir. Thorough crystallographic,
spectroscopic, and DFT analysis of two FeII complexes allowed
us to elucidate the coordination mode of L and the electronic
structure of the complexes in various overall oxidation states.
The new ligand seems far more trivial and readily applicable in
comparison to many common systems, and its complexes are
expected to have great potential in the growing areas of
chemical as well as electronic applications of molecular systems
with multiple accessible redox levels. Our efforts in these areas
are ongoing and will be reported in due course.
X-ray Crystallography. Crystallographic data for compounds L, 1,
and [2]PF6 are collected in Table 1. Suitable X-ray-quality crystals of
these compounds were obtained by the slow evaporation of a
dichloromethane−hexane solution of the compound. All data were
collected on a Bruker SMART APEX-II diffractometer, equipped with
graphite-monochromated Mo Kα radiation (λ =0.71073 Å), and were
corrected for Lorentz polarization effects.
L: a total of 12061 reflections were collected, of which 1830 were
unique (Rint = 0.062), satisfying the I > 2σ(I) criterion, and were used
in subsequent analysis.
1: a total of 19566 reflections were collected, of which 1772 were
unique (Rint = 0.108), satisfying the I > 2σ(I) criterion, and were used
in subsequent analysis.
[2]PF6: a total of 40804 reflections were collected, of which 3340
were unique (Rint = 0.04).
EXPERIMENTAL SECTION
■
Materials and Measurements. All reagents and chemicals were
purchased from commercial sources and used without further
purifications. Tetrabutylammonium perchlorate was prepared and
recrystallized as reported earlier.19 Caution! Perchlorates have to be
handled with care and appropriate safety precautions! Syntheses of the
complexes were done in open atmosphere. UV−vis absorption spectra
were recorded on a Perkin-Elmer Lambda 950 UV−vis spectropho-
tometer and a J&M TIDAS instrument. Infrared spectra and NMR
spectra were obtained using a Perkin-Elmer 783 spectrophotometer
and a Bruker Avance 400/500 MHz spectrometer, respectively, and
SiMe4 was used as the internal standard. A Perkin-Elmer 240C
elemental analyzer was used to collect microanalytical data (C, H, N).
ESI mass spectra were recorded on a Micromass Q-TOF mass
spectrometer (Model No. YA263). Cyclic voltammetry potentials were
measured under a nitrogen atmosphere using a Ag/AgCl reference
electrode, with a Pt-disk working electrode and a Pt-wire auxiliary
electrode, in dichloromethane or acetonitrile containing 0.1 M
[Bu4N]ClO4 or 0.1 M [Et4N]ClO4 respectively. A Pt-gauge working
electrode was used for exhaustive electrolyses. The E1/2 value for the
ferrocenium−ferrocene couple under our experimental conditions was
The structures were solved by employing the SHELXS-97 program
package21 and were refined by full-matrix least squares based on F2
(SHELXL-97).22 All hydrogen atoms were added in calculated
positions.
Computational Details. All DFT calculations presented in this
paper were carried out using the Gaussian 09W program package.23
Full geometry optimizations were performed without symmetry
constraints. The vibrational frequency calculations were performed
to ensure that the optimized geometries represent the local minima
and that there are only positive eigenvalues. The hybrid B3LYP
exchange-correlation functional24 was used. The TZVP basis set25 of
triple-ζ quality with one set of polarization functions was used on Fe
and Cl atoms, and the 6-31G(d) basis set was used for the C, H, and N
atoms. The broken-symmetry approach26,27 was employed to establish
the singlet state S = 0 of the compound(s). The calculations of the
ground-state singlet states were performed using either spin-restricted
or spin-unrestricted approaches (in G09W, combined with
GUESS=MIX). Mulliken spin densities were used for analysis of the
spin populations on ligand and metal centers.28 Singlet excitation
energies based on the solvent-phase (CH3CN) optimized geometry of
the compound L were computed using the time-dependent density
functional theory (TDDFT) formalism29 in acetonitrile using the
conductor-like polarizable continuum model.30 GaussSum31 was used
to calculate the percentage contribution of ligand and metal to the
frontier orbital and the fractional contributions of various molecular
orbitals in the optical spectral transition.
0.40 V. Mossbauer spectra were recorded with a 57Co source in a Rh
̈
matrix using an alternating constant acceleration Wissel Mossbauer
̈
spectrometer operated in the transmission mode and equipped with a
Janis closed-cycle helium cryostat. Isomer shifts are given relative to
iron metal at ambient temperature. Simulation of the experimental
data was performed with the Mfit program (E. Bill, Max-Planck
Institute for Chemical Energy Conversion, Mulheim/Ruhr, Germany).
̈
Temperature-dependent magnetic susceptibility measurements of 1
and [2]ClO4 were carried out with a Quantum-Design MPMS-XL-5
SQUID magnetometer equipped with a 5 T magnet in the range from
295 to 2.0 K at a magnetic field of 0.5 T for 1 and 0.05 T for [2]ClO4.
The powdered sample was contained in a gel bucket and fixed in a
nonmagnetic sample holder. Each raw data file for the measured
magnetic moment was corrected for the diamagnetic contribution of
the sample holder and the gel bucket. The molar susceptibility data
were corrected for the diamagnetic contribution. Temperature-
independent paramagnetism (TIP) and a diamagnetic impurity (DI)
were included according to χcalcd = (1 − DI)χ + TIP. Simulation of the
experimental magnetic data was performed with the julX program.20
4684
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Inorganic Chemistry
Article
Ed. 2002, 41, 3873−3876. (d) Praneeth, V. K. K.; Ringenberg, M. R.;
Ward, T. R. Angew. Chem., Int. Ed. 2012, 51, 10228−10234. (e) Lu, C.
ASSOCIATED CONTENT
* Supporting Information
■
S
C.; Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem.
̈
Figures, tables, and CIF files giving ESI-MS spectra of L and
[2]+, orbitals involved in the electronic transition of [L•]−.
electronic spectra of 1 and 1−, EPR spectrum of the
Soc. 2008, 130, 3181−3197.
(4) Majumder, P.; Baksi, S.; Halder, S.; Tadesse, H.; Blake, A. J.;
Drew, M. G. B.; Bhattacharya, S. Dalton Trans. 2011, 40, 5423−5425
and referenecs therein.
electrogenerated reduced complex 1−, H NMR spectrum of
1
the electrogenerated oxidized compound [2]2+, Mossbauer
̈
(5) Chang, M.-C.; Dann, T.; Day, D. P.; Lutz, M.; Wildgoose, G. G.;
Otten, E. Angew. Chem., Int. Ed. 2014, 53, 4118−4122.
(6) Kerns, M. L.; Bowen, D. E., III; Rodewald, S. German Pat. Appl.
DE 10038214 A1, 2000.
(7) Zhu, D.; Budzelaar, P. H. M. Organometallics 2008, 27, 2699−
2705.
spectrum of the electrogenerated oxidized compound [2]2+,
electronic spectra of [2]+, [2]2+, and [2]0, spin density plots of
the one-electron-reduced compound [2]0 and the two-electron-
reduced ligand (L••)2−, frontier molecular orbitals of the
oxidized complex [2]2+, selected experimental and calculated
bond distances for L, [L•]− and [L••]2−, selected experimental
and calculated bond parameters for 1, [2]+, [2]2+, and [2]0,
Cartesian coordinates of the optimized structures of [2]+, [2]2+,
and [2]0, and crystallographic data for L, 1, and [2]PF6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(8) (a) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt,
̈
K. Inorg. Chem. 2000, 39, 2936−2947. (b) Budzelaar, P. H. M.; de
Bruin, B.; Gal, A. W.; Wieghardt, K.; van Lenthe, J. H. Inorg. Chem.
2001, 40, 4649−4655. (c) Stieber, S. C. E.; Milsmann, C.; Hoyt, J. M.;
Turner, Z. R.; Finkelstein, K. D.; Wieghardt, K.; DeBeer, S.; Chirik, P.
J. Inorg. Chem. 2012, 51, 3770−3785. (d) Bowman, A. C.; Milsmann,
C.; Bill, E.; Turner, Z. R.; Lobkovsky, E.; DeBeer, S.; Wieghardt, K.;
Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 17353−17369. (e) Bowman,
AUTHOR INFORMATION
Corresponding Author
*E-mail: icsg@iacs.res.in.
Notes
A. C.; Milsmann, C.; Bill, E.; Lobkovsky, E.; Weyhermuller, T.;
̈
■
Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2010, 49, 6110−6123.
(f) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.;
Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676−1684.
(g) Wile, B. M.; Trovitch, R. J.; Bart, S. C.; Tondreau, A. M.;
Lobkovsky, E.; Milsmann, C.; Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg.
Chem. 2009, 48, 4190−4200. (h) Bart, S. C.; Lobkovsky, E.; Bill, E.;
Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2007, 46, 7055−7063.
(i) Bart, S. C.; Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.;
Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128,
13901−13912. (j) Darmon, J. M.; Turner, Z. R.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2012, 31, 2275−2285.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support received from the Department of Science and
Technology (Project SR/S1/IC/0031/2010), New Delhi,
India, is acknowledged. S.G. thanks the DST for a J. C. Bose
fellowship. Crystallography was performed at the DST funded
National Single Crystal Diffractometer Facility at the Depart-
ment of Inorganic Chemistry, IACS. P.G. and S.K.R. thank the
Council of Scientific and Industrial Research for fellowship
support. S.S. thanks the Alexander von Humboldt foundation
for a postdoctoral fellowship.
(9) (a) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794−795.
(b) Tondreau, A. M.; Atienza, C. C. H.; Weller, K. J.; Nye, S. A.;
Lewis, K. M.; Delis, J. G. P.; Chirik, P. J. Science 2012, 335, 567−570.
(c) Hoyt, J. M.; Sylvester, K. T.; Semproni, S. P.; Chirik, P. J. J. Am.
Chem. Soc. 2013, 135, 4862−4877. (d) Russell, S. K.; Lobkovsky, E.;
Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858−8861. (d) Obligacion,
J. V.; Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 4133−
4136. (e) Obligacion, J. V.; Chirik, P. J. J. Am. Chem. Soc. 2013, 135,
19107−19110. (f) Obligacion, J. V.; Chirik, P. J. Org. Lett. 2013, 15,
2680−2683. (g) Hoyt, J. M.; Sylvester, K. T.; Semproni, S. P.; Chirik,
P. J. J. Am. Chem. Soc. 2013, 135, 4862−4877. (h) Yu, R. P.; Darmon,
J. M.; Hoyt, J. M.; Margulieux, G. W.; Turner, Z. R.; Chirik, P. J. ACS
Catal. 2012, 2, 1760−1764. (i) Monfette, S.; Turner, Z. R.; Semproni,
S. P.; Chirik, P. J. J. Am. Chem. Soc. 2012, 134, 4561−4564. (j) Russell,
S. K.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858−
8861. (k) Atienza, C. C. H.; Milsmann, C.; Lobkovsky, E.; Chirik, P. J.
Angew. Chem., Int. Ed. 2011, 50, 8143−8147. (l) Russell, S. K.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858−8861.
(m) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2005, 127, 9660−9661.
REFERENCES
■
(1) (a) Ghosh, B. K.; Chakravorty, A. Coord. Chem. Rev. 1989, 95,
239−294. (b) Kaim, W. Inorg. Chem. 2011, 50, 9752−9765. (c) Sarkar,
B.; Patra, S.; Fiedler, J.; Sunoj, R. B.; Janardana, D.; Mobin, S. M.;
Niemeyer, M.; Lahiri, G. K.; Kaim, W. Angew. Chem., Int. Ed. 2005, 44,
5655−5658. (d) Sarkar, B.; Patra, S.; Fiedler, J.; Sunoj, R. B.;
Janardanan, D.; Lahiri, G. K.; Kaim, W. J. Am. Chem. Soc. 2008, 130,
3532−3542. (e) Kaim, W. Coord. Chem. Rev. 2001, 219−221, 463−
488. (f) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem., Int. Ed. 1998, 37,
2403−2404.
(2) (a) Goswami, S.; Sengupta, D.; Paul, N. D.; Mondal, T. K.;
Goswami, S. Chem. Eur. J. 2014, DOI: 10.1002/chem.201304369.
(b) Paul, N. D.; Rana, U.; Goswami, S.; Mondal, T. K.; Goswami, S. J.
Am. Chem. Soc. 2012, 134, 6520−6523. (c) Ghosh, P.; Samanta, S.;
(10) (a) Ma, J.; Feng, C.; Wang, S.; Zhao, K.-Q.; Sun, W.-H.;
Redshaw, C.; Soland, G. A. Inorg. Chem. Front. 2014, 1, 14−34 and
references therein. (b) Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012,
2, 270−279. (c) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am.
Chem. Soc. 1998, 120, 4049−4050. (d) Small, B. L.; Brookhart, M. J.
Am. Chem. Soc. 1998, 120, 7143−7144. (e) Small, B. L.; Marcucci, A. J.
Organometallics 2001, 20, 5738−5744. (f) Small, B. L. Organometallics
2003, 22, 3178−3183.
Roy, S. K.; Joy, S.; Kramer, T.; McGrady, J. E.; Goswami, S. Inorg.
̈
Chem. 2013, 52, 14040−14049. (d) Samanta, S.; Ghosh, P.; Goswami,
S. Dalton Trans. 2012, 41, 2213−2226. (e) Joy, S.; Kramer, T.; Paul,
̈
N. D.; Banerjee, P.; McGrady, J. E.; Goswami, S. Inorg. Chem. 2011,
50, 9993−10004. (f) Paul, N.; Samanta, S.; Goswami, S. Inorg. Chem.
2010, 49, 2649−2655. (g) Sanyal, A.; Chatterjee, S.; Castineiras, A.;
̃
́ ̌
Sarkar, B.; Singh, P.; Fiedler, J.; Zalis, S.; Kaim, W.; Goswami, S. Inorg.
Chem. 2007, 46, 8584−8593. (h) Sanyal, A.; Banerjee, P.; Lee, G.-H.;
Peng, S.-M.; Hung, C.-H.; Goswami, S. Inorg. Chem. 2004, 43, 7456−
(11) (a) Campbell, N.; Henderson, A. W.; Taylor, D. J. Chem. Soc.
1953, 1281−1285. (b) Joy, S. Ph.D. Thesis, Jadavpur University,
Kolkata, India, 2012.
́ ̌
7462. (i) Samanta, S.; Singh, P.; Fiedler, J.; Zalis, S.; Kaim, W.;
Goswami, S. Inorg. Chem. 2008, 47, 1625−1633.
(12) Saha, A.; Das, C.; Goswami, S.; Peng, S.-M. Indian J. Chem.
(3) (a) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440−
1459. (b) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B.
Angew. Chem., Int. Ed. 2011, 50, 3356−3358. (c) Enright, D.;
Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew. Chem., Int.
2001, 40A, 198−201.
(13) Santra, B. K.; Thakur, G. A.; Ghosh, P.; Pramanik, A.; Lahiri, G.
K. Inorg. Chem. 1996, 35, 3050−3052. (b) Kar, S.; Pradhan, B.; Sinha,
4685
dx.doi.org/10.1021/ic500355f | Inorg. Chem. 2014, 53, 4678−4686

Inorganic Chemistry
Article
R. K.; Kundu, T.; Kodgire, P.; Rao, K. K.; Puranik, V. G.; Lahiri, G. K.
Dalton Trans. 2004, 1752−1760.
(14) Widger, L. R.; Jiang, Y.; Siegler, M. A.; Kumar, D.; Latifi, R.; de
Visser, S. P.; Jameson, G. N. L.; Goldberg, D. P. Inorg. Chem. 2013, 52,
10467−10480.
(15) Sinan, M.; Panda, M.; Banerjee, P.; Shinisha, C. B.; Sunoj, R. B.;
Goswami, S. Org. Lett. 2009, 11, 3218−3221.
(16) (a) Kaim, W. Coord. Chem. Rev. 1987, 76, 187−235. (b) Kaim,
W. Inorg. Chem. 1984, 23, 3365−3368.
(17) Lesh, F. D.; Lord, R. L.; Heeg, M. J.; Bernhard, H. S.; Verani, C.
N. Eur. J. Inorg. Chem. 2012, 463−466.
(18) DFT analysis indicates charge delocalization between two
coordinated ligands pointing to a formal charge of 0.5− for each
ligand. However, a localized description with two different oxidation
states, 1− and 0, is a more acceptable proposition, though such
oxidation states are impossible17 because the two resonating forms are
isoenergetic.
(19) Goswami, S.; Mukherjee, R. N.; Chakravorty, A. Inorg. Chem.
1983, 22, 2825−2832.
(20) Available through http://ewww/mpi-heim.mpg.de/bac/logins/
bill/julX_en.php.
(21) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467−473.
(22) Sheldrick, G. M. SHELXL 97, Program for the Refinement of
Crystal Structures; University of Gottingen, Gottingen, Germany, 1997.
̈
̈
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09, Revision
C.01; Gaussian, Inc., Wallingford, CT, 2010.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys.
1988, 37, 785−789. (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys.
1980, 58, 1200−1211. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski,
C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627.
(25) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,
̈
5829−5835.
(26) Ginsberg, A. P. J. Am. Chem. Soc. 1980, 102, 111−117.
(27) (a) Noodleman, L.; Case, D. A.; Aizman, A. J. Am. Chem. Soc.
1988, 110, 1001−1005. (b) Noodleman, L.; Davidson, E. R. Chem.
Phys. 1986, 109, 131−143. (c) Noodleman, L.; Norman, J. G., Jr.;
Osborne, J. H.; Aizman, C.; Case, D. A. J. Am. Chem. Soc. 1985, 107,
3418−3426. (d) Noodleman, L. J. Chem. Phys. 1981, 74, 5737−5743.
(28) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833−1840.
(29) (a) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,
454−464. (b) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem.
Phys. 1998, 109, 8218−8224. (c) Casida, M. E.; Jamoroski, C.; Casida,
K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439−4449.
(30) (a) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput.
Chem. 2003, 24, 669−681. (b) Cossi, M.; Barone, V. J. Chem. Phys.
2001, 115, 4708−4717. (c) Barone, V.; Cossi, M. J. Phys. Chem. A
1998, 102, 1995−2001. (d) O’Boyle, N. M.; Tenderholt, A. L.;
Langner, K. M. J. Comput. Chem. 2008, 29, 839−845.
(31) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P.
J. J. Chem. Phys. 1996, 105, 1052−1059.
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