Neutral, cationic, and anionic low-spin iron(III) complexes stabilized by amidophenolate and iminobenzosemiquinonate radical in N, N, O ligands

Article abstract of DOI:10.1021/ic401985d

A brownish-black complex [Fe^III(L)₂] (1) (S = 0), supported by two tridentate redox-active azo-appended o-amidophenolates [H ₂L = 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol], has been synthesized and structurally characterized. In CH₂Cl₂ 1 displays two oxidative and two reductive 1e^- redox processes at E_1/2 values of 0.48 and 1.06 V and -0.42 and -1.48 V vs SCE, respectively. The one-electron oxidized form [1]⁺ isolated as a green solid [Fe^III(L)₂][BF₄] (2) (S = 1/2) has been structurally characterized. Isolation of a dark ink-blue one-electron reduced form [1]^- has also been achieved [Co^III(η⁵- C₁₀H₁₅)₂][Fe^III(L)₂] (3) (S = 1/2). Moessbauer spectral parameters unequivocally establish that 1 is a low-spin (LS) Fe^III complex. Careful analysis of Moessbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major LS Fe^III and a minor LS Fe^II component (redox isomers): [Fe^III{(L^ISQ)^-?}₂]⁺ and [Fe^II{(L^IBQ)⁰}{(L^ISQ) ^-?}]⁺ (2) and [Fe^III{(L^AP) ^2-}₂]^- and [Fe^II{(L ^ISQ)^-?}{(L^AP)^2-}]^- (3). Notably, for both at 8 K mainly the major component exists. Broken-Symmetry (BS) Density Functional Theory (DFT) calculations at the B3LYP level reveals that in 1 the unpaired electron of LS Fe^III is strongly antiferromagnetically coupled with a π-radical of o- iminobenzosemiquinonate(1-) (L^ISQ)^-? form of the ligand, delocalized over two ligands providing 3- charge (X-ray structure). DFT calculations reveal that the unpaired electron in 2 is due to (L ^ISQ)^-? [LS Fe^III (S_Fe = 1/2) is strongly antiferromagnetically coupled to one of the (L^ISQ) ^-? radicals (S_rad = 1/2)] and 3 is primarily a LS Fe^III complex, supported by two o-amidophenolate(2-) ligands. Time-Dependent-DFT calculations shed light on the origin of UV-vis-NIR spectral absorptions for 1-3. The collective consideration of Moessbauer, variable-temperature (77-298 K) electron paramagnetic resonance (EPR), and absorption spectral behavior at 298 K, and DFT results reveals that in 2 and 3 the valence-tautomerism is operative in the temperature range 80-300 K.

Full text of DOI:10.1021/ic401985d

Article
pubs.acs.org/IC
Neutral, Cationic, and Anionic Low-Spin Iron(III) Complexes
Stabilized by Amidophenolate and Iminobenzosemiquinonate
Radical in N,N,O Ligands
Amit Rajput,† Anuj K. Sharma,† Suman K. Barman,† Debasis Koley,§ Markus Steinert,‡
and Rabindranath Mukherjee*,†,§
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
‡Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstrasse 4, D-37077 Gottingen, Germany
̈
̈
̈
̈
§Department of Chemistry, Indian Institute of Science Education and Research Kolkata, Mohanpur Campus, Mohanpur 741 252,
India
S
* Supporting Information
ABSTRACT: A brownish-black complex [FeIII(L)2] (1) (S =
0), supported by two tridentate redox-active azo-appended o-
amidophenolates [H2L = 2-(2-phenylazo)-anilino-4,6-di-tert-
butylphenol], has been synthesized and structurally charac-
terized. In CH2Cl2 1 displays two oxidative and two reductive
1e− redox processes at E1/2 values of 0.48 and 1.06 V and
−0.42 and −1.48 V vs SCE, respectively. The one-electron
oxidized form [1]+ isolated as a green solid [FeIII(L)2][BF4]
(2) (S = 1/2) has been structurally characterized. Isolation of a dark ink-blue one-electron reduced form [1]− has also been
achieved [CoIII(η5-C H ) ][FeIII(L) ] (3) (S = 1/2). Mossbauer spectral parameters unequivocally establish that 1 is a low-spin
̈
10 15
2
2
(LS) FeIII complex. Careful analysis of Mossbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major
̈
LS FeIII and a minor LS FeII component (redox isomers): [FeIII{(LISQ)−•}2]+ and [FeII{(LIBQ)0}{(LISQ)−•}]+ (2) and
[FeIII{(LAP)2−}2]− and [FeII{(LISQ)−•}{(LAP)2−}]− (3). Notably, for both at 8 K mainly the major component exists. Broken-
Symmetry (BS) Density Functional Theory (DFT) calculations at the B3LYP level reveals that in 1 the unpaired electron of LS
FeIII is strongly antiferromagnetically coupled with a π-radical of o-iminobenzosemiquinonate(1−) (LISQ −•
)
form of the ligand,
delocalized over two ligands providing 3− charge (X-ray structure). DFT calculations reveal that the unpaired electron in 2 is due
to (LISQ −• [LS FeIII (SFe = 1/2) is strongly antiferromagnetically coupled to one of the (LISQ −• radicals (Srad = 1/2)] and 3 is
)
)
primarily a LS FeIII complex, supported by two o-amidophenolate(2−) ligands. Time-Dependent-DFT calculations shed light on
the origin of UV−vis−NIR spectral absorptions for 1−3. The collective consideration of Mossbauer, variable-temperature (77−
̈
298 K) electron paramagnetic resonance (EPR), and absorption spectral behavior at 298 K, and DFT results reveals that in 2 and
3 the valence-tautomerism is operative in the temperature range 80−300 K.
This ligand is unique in the sense that electron-rich o-
amidophenolate(2−) unit will donate electron-density to the
metal-ion but a π-accepting azo group is expected to withdraw
electron-density to impart highly delocalized metal−ligand
interaction. It is a generally accepted notion that the electron
delocalization always reduces the molecular total energy and
hence stabilizes a molecule.16 In a molecule, a set of
overlapping delocalized electronic states across the entire
molecule is necessary for electronic conduction and hence such
systems have the potential to function as molecular electronic
devices.17,18 Moreover, recent works on donor−acceptor
properties19 utilizing metal complexes with both kinds of
ligands in a given complexthe o-amidophenolates14 on one
hand to act as donors and 2-(phenylazo)pyridines13 on the
other hand to act as acceptors, due to its azo functionhave
INTRODUCTION
■
Transition-metal complexes of redox-active ligands have long
been recognized in coordination chemistry.1 Because of redox-
active character these ligands profoundly influence the
electronic structural properties of the resulting complexes.2
Notably, synergistic metal- and organic radical-based redox
processes participate in enzymatic multielectron redox
reactions.3,4 Transition-metal complexes with redox-active
ligands have generated much interest in recent years owing
to an increased curiosity about the prospective role of various
ligand-based oxidation levels and in catalysis.4,5 Among a
variety of redox-active ligand platforms6−14 o-amidophenola-
tes1c,14 occupy a unique place via their observed coordination
chemistry and spectroelectrochemical properties.
As a part of our interest in metal-coordinated radicals,15 we
have synthesized a new potentially tridentate o-aminophenol
ligand which carries an additional redox-active azo group, 2-(2-
phenylazo)-anilino-4,6-di-tert-butylphenol H2(LAP) (Figure 1).
Received: April 1, 2013
Published: December 10, 2013
© 2013 American Chemical Society
36
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2H; Ph-H2), 1.46 (s, 9H; tert-butyl), 1.28 (s, 9H; tert-butyl). Positive
ESI-MS 402.25 (100% peak M+ + 1 ion peak).
Synthesis of Complexes. [FeIII(L)2] (1). To a suspension of H2L
(0.3 g, 0.748 mmol) in MeOH (20 mL) was added Et3N (0.151 g,
1.496 mmol), and stirred for 5 min. Solid [FeIII(acac)3] (0.132 g, 0.374
mmol) was then added, and the reaction mixture was refluxed for 8 h.
After cooling the dark precipitate that formed was filtered, washed with
MeOH, and dried in vacuum. Yield: 0.298 g, ∼47%. The crude solid
was redissolved in CH2Cl2-MeOH (15 mL; 4:1 v/v) and left for slow
evaporation, affording needles suitable for X-ray diffraction studies.
Anal. Calcd for C52H58N6O2Fe (1): C, 73.06; H, 6.84; N, 9.83. Found:
1
C, 72.96; H, 6.74; N, 9.77. H NMR (CDCl3, 500 MHz): δ (ppm)
8.14 (d, J = 7.90 Hz, 4H; Ph-H7), 7.97 (s, 2H; Ph-H1), 7.08 (s, 2H;
Ph-H2), 6.86 (t, J = 0.35 Hz, 8H; Ph-H4 and Ph-H5), 6.63 (t, J = 0.25
Hz, 6H; Ph-H8 and Ph-H9), 1.56 (s, 9H; tert-butyl), 1.33 (s, 9H; tert-
butyl).
[FeIII(L)2][BF4] (2). To a solution of [FeIII(L)2] (0.1 g, 0.117 mmol)
in CH2Cl2 (10 mL), solid AgBF4 (0.023 g, 0.117 mmol) was added.
The mixture was stirred at 298 K for 10 min and filtered (removal of
metallic Ag). Solvent was then removed under reduced pressure, and
the black solid thus formed was recrystallized from CH2Cl2-Et2O (1:3,
v/v). Black crystals that obtained were found to be suitable for X-ray
structural study. Yield: 0.07 g, ∼64%. Anal. Calcd for
C52H58N6O2FeBF4 (2): C 66.32, H 6.21, N 8.92. Found C 66.21, H
Figure 1. Structure of H2L.
given an additional impetus to design our new ligand system
and to explore the electronic structural properties of the
resulting complexes. The works of Goswami13c and Verani17b
deserve special mention with regard to the viability of molecular
electronics, based on radical-containing coordination com-
plexes. The net metal−ligand interaction is expected to be
interesting and hence worthy of investigation. From this
background we report here on the synthesis, structural
characterization, and investigation of electronic structural
properties, through a series of spectroscopic techniques, of an
interesting new complex [FeIII(L)2] (1), with two ligands
providing 3− charge. As the ligand is expected to be redox-
active we investigated the redox properties of 1. We also
describe the synthesis and characterization, including electronic
structural aspects, of the monocation [1]+ and the monoanion
[1]− forms, in the successful synthesis of [FeIII(L)2][BF4] (2)
and [CoIII(η5-C10H15)2][FeIII(L)2] (3), respectively. We report
here also the structural properties of 2. Results of Density
Functional Theory (DFT) calculations at the B3LYP level of
theory have been utilized to assign the correct oxidation level of
the metal ion and also the coordinated ligands. Such results are
presented here, along with the results of Time-Dependent
(TD)-DFT calculations to throw light on the origin of UV−
vis−NIR spectral absorptions for all the three complexes.
−
6.19, N 8.82. IR (KBr, cm−1, selected peak): 1052 ν(BF4 ).
[CoIII(η5-C10H15)2]FeIII(L)2] (3). To a solution of [FeIII(L)2] (0.1 g,
0.117 mmol) in CH2Cl2 (10 mL), bis(pentamethylcyclopentadienyl)-
cobalt(II) (0.039 g, 0.117 mmol) was added, under anaerobic
conditions (Mbraun glovebox). The color of the solution immediately
changed from black to ink-blue. After stirring for 4 h in the glovebox,
the volume of the solution was reduced to ∼2 mL, and dry degassed
Et2O (15 mL) was added with vigorous stirring. This resulted in the
separation of a bluish-black solid, which was collected by filtration,
washed with Et2O, and dried in vacuum. Yield: 0.076 g, ∼65%. Anal.
Calcd for C62H68N6O2FeCo: C, 71.33; H, 6.56; N, 8.05. Found: C,
71.16; H, 6.49; N, 7.98.
Physical Measurements. Elemental analyses were obtained using
Thermo Quest EA1110 CHNS-O, Italy. IR spectra (KBr, 4000−600
cm−1) were recorded on a Bruker Vector 22 spectrophotometer. UV−
vis spectra were recorded using an Agilent 8453 diode-array
spectrophotometer. NIR absorption spectra were recorded using a
1
JACSO V-670 (Japan) spectrophotometer. H NMR spectra (CDCl3)
were obtained on a JEOL JNM LA 500 (500 MHz) spectrometer.
Chemical shifts are reported in ppm referenced to TMS. ESI-MS
spectra were recorded on a Waters-HAB213 spectrometer. X-band
electron paramagnetic resonance (EPR) spectra were recorded by
using either a Bruker EMX 1444 EPR spectrometer (fitted with a
quartz Dewar for measurement at 77 K) operating at 9.455 GHz or a
Magnettech GmbH MiniScope MS400 spectrometer (equipped with a
temperature controller TC H03), where the microwave frequency was
measured with a frequency counter FC400. The EPR spectra were
calibrated with diphenylpicrylhydrazyl, DPPH (g = 2.0037). Spectra
were treated by using the Bruker WinEPR software and simulated
using the Bruker SIMFONIA software.
Cyclic voltammetric (CV) experiments were performed at 298 K by
using CH instruments, Electrochemical Analyzer/Workstation model
600B series. The cell contained a Beckman M-39273 platinum-inlay
working electrode, a Pt wire auxiliary electrode, and a saturated
calomel electrode (SCE), as reference electrode. Details of the cell
configuration are as described before.23,24 For coulometry, a platinum
wire-gauze was used as the working electrode. The solutions were ∼1.0
mM in complex and 0.1 M in supporting electrolyte, TBAP.
Spectroelectrochemical measurements were performed using a
custom-made cell (Model EF-1350) from Bioanalytical Systems Inc.,
U.S.A.
EXPERIMENTAL SECTION
■
General Considerations. All reagents were obtained from
commercial sources and used as received. Solvents were dried/purified
as reported previously.20 2-(Phenylazo)aniline21 and [FeIII(acac)3]22
(acac− = 2,4-pentanedione anion) were synthesized following reported
procedures. Tetra-n-butylammonium perchlorate (TBAP) was pre-
pared and purified as before.23
Synthesis of H2L. To a stirring solution of 3,5-di-tert-butylcatechol
(0.2 g, 0.9 mmol) in n-heptane (1 mL) was added Et3N (0.12 mL) and
a solution of 2-(phenylazo)aniline (0.177 g, 0.9 mmol) in n-heptane (1
mL), under a dinitrogen atmosphere. The reaction mixture was heated
to reflux for 6 h, followed by stirring for 12 h. Solvent was then
removed under reduced pressure, and the crude solid thus obtained
was purified by silica-gel column chromatography. The dark red band
that eluted first [ethylacetate: n-hexane, 5:95 (v/v)] was collected.
Solvent removal afforded pure ligand as a dark red solid. Yield: 0.155 g,
43%. 1H NMR (CDCl3, 500 MHz): δ (ppm) 7.93 (d, J = 6.40 Hz, 2H;
Ph-H6), 7.88 (d, J = 7.35 Hz, 4H; Ph-H7), 7.50 (t, J = 0.60 Hz, 4H; Ph-
H8), 7.44 (t, J = 0.25 Hz, 2H; Ph-H9), 7.09 (s, 2H; Ph-H1), 6.92 (t, J =
0.50 Hz, Ph-H4 and Ph-H5), 6.65 (d, J = 8.25 Hz, 2H; Ph-H3), 6.15 (s,
Under our experimental conditions, the E1/2 and peak-to-peak
separation (ΔEp) values in CH2Cl2 for [FeIII(η5-C5H5)2]+/[FeII(η5-
C5H5)2] (Fc+/Fc) couple were 0.49 V vs SCE and 120 mV,
respectively.24
37
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Room-temperature magnetic susceptibility measurements were
made on polycrystalline samples (powder form) of [FeIII(L)2][BF4]
2 and [CoIII(η5-C10H15)2][FeIII(L)2] (3) by the Faraday method using
a locally built magnetometer.25 Effective magnetic moment was
calculated from μeff = 2.828[χMT]1/2, where χM is the corrected molar
susceptibility. The diamagnetic corrections were applied to the
susceptibility data.26 Solution-state magnetic susceptibilities were
obtained by the NMR technique of Evans27 in CH2Cl2 with a JEOL
JNM LA 500 (500 MHz) spectrometer and made use of the
paramagnetic shift of the methylene protons of CH2Cl2 as the
measured NMR parameter.
lations employed the B3LYP functional31 using all-electron Gaussian
basis sets. The carbon and hydrogen were treated with split-valence
plus polarization (SVP) basis sets of 6-31G* quality32 and triple-ζ
quality basis sets (TZVP) were used for iron, nitrogen, and oxygen.33
The symmetry-broken,34 singlet-diradical wave function of 1 was
optimized in Gaussian 09.30 Initially, the stability analysis of the
restricted DFT wave function was performed using the “stable=opt”
keyword. If any symmetry-broken DFT solution exists then the
program automatically finds the lowest energy wave function. This is
further verified by taking the restricted DFT wave function and
performing optimization with “guess=mix” keyword. The symmetry-
broken solution is measured from the expectation value of the spin
operator (⟨S2⟩), which is different from zero. For the monocation [1]+
the optimization was carried out starting from the X-ray structural
coordinates of 1 with S = 1/2 and a positive charge. Then the stability
analysis of the wave function leads to the broken-symmetry solution.
Similarly, for the monoanion [1]− the optimization was carried out
starting from the X-ray coordinates of 1 with S = 1/2 and a negative
charge. Stability analysis revealed that the wave function was already
stable. Corresponding orbitals and spin-density plots were made using
VMD35a and Chemcraft35b Visualization programs. The wave
functions of optimized geometries (at B3LYP functional) for all the
complexes were used in Time-Dependent (TD)-DFT calculations
employing the BP86 functional and the polarizable continuum model,
CPCM (CH2Cl2 as solvent).36 TD-DFT-derived electronic spectra
were plotted using GaussSum.37
57Fe Mossbauer spectra at 298 K were recorded by using a Wissel
̈
1200 spectrometer and a proportional counter. 57Co(Rh) in a constant
acceleration mode was used as the radioactive source. Temperature-
dependent Mossbauer spectra were recorded with a conventional
̈
spectrometer with alternating constant acceleration of the source,
either from Wissel or with a home-built setup (Georg-August-
Universitat Gottingen).28
̈
̈
Crystal Structure Determination. Single-crystals of suitable
dimension were used for data collection. Diffraction intensities were
collected on a Bruker SMART APEX CCD diffractometer, with
graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation at 100(2)
K. The data were corrected for absorption. For 1, out of 28 207
reflections measured, 10 647 were unique (Rint = 0.0648), and 6013
were used (I > 2σ(I)) for structure solution. The corresponding values
for 2 are 52 888, 8148 (0.0813), and 6264. The structures were solved
by SIR-97, expanded by Fourier-difference syntheses, and refined with
the SHELXL-97 package incorporated in WinGX 1.64 crystallographic
package.29 The position of the hydrogen atoms were calculated by
assuming ideal geometries, but not refined. All non-hydrogen atoms
were refined with anisotropic thermal parameters by full-matrix least-
squares procedures on F2. Pertinent crystallographic parameters are
summarized in Table 1.
RESULTS AND DISCUSSION
■
Synthesis of a New Ligand H2L and [FeIII(L)2] 1. The
ligand H2L was synthesized by stoichiometric reaction between
3,5-di-tert-butylcatechol and 2-(phenylazo)aniline in n-heptane
in the presence of Et3N, under a dinitrogen atmosphere.
Purification was achieved by silica-gel column chromatography,
Computational Details. All calculations were performed using the
Gaussian 09 program.30 Density Functional Theory (DFT) calcu-
1
affording a dark red solid. Purity was checked by its H NMR
(Figure S1, Supporting Information) and ESI-MS spectra.
Wieghardt and co-workers9,38 unequivocally established that o-
amidophenolates are redox-active O,N-coordinated ligands
which can exist in different protonation and oxidation levels
in coordination compounds. Other groups have provided
examples to strengthen this trend.39−41 The expected redox
behavior of (LAP)2− from both o-amidophenolate and azo
functionality is shown in Scheme 1.
Table 1. Data Collection and Structure Refinement
Parameters for [FeIII(L)2] 1 and [FeIII(L)2][BF4] 2
1
2
empirical formula
formula weight
crystal color, habit
temperature (K)
wavelength (Å)
crystal system
space group
crystal size (mm3)
a (Å)
C52H58FeN6O2
854.89
C52H58FeN6O2BF4
1005.99
black, prism
100(2)
black, block
100(2)
Aerobic reaction of [FeIII(acac)3] with H2L and Et3N in
MeOH under refluxing conditions afforded, after work up and
recrystallization from CH2Cl2-MeOH, X-ray quality black
crystals of [FeIII(L)2] 1. Complex 1 exhibits in CDCl3 a clean
1H NMR spectrum in the δ 0−10 ppm range, attesting to its
diamagnetic character (Figure S2, Supporting Information).
Positive ESI-MS of 1 in CH2Cl2 showed a peak at m/z = 855.4
corresponding to the species {[FeIII(L)2] + [H]+}, based on the
simulated mass and isotopic distribution pattern (Figure S3,
Supporting Information).
Redox Properties of 1. To investigate the possibility of
identifying metal-centered redox and/or accessibility of various
ligand redox levels, cyclic voltammetry (CV) experiments on 1
were carried out. The CV of 1 in CH2Cl2 solution displays
(Figure 2) four redox processes: two reductions at E1/2 values
of −0.42 V (peak-to-peak separation, ΔEp = 180 mV) and
−1.48 V (ΔEp = 240 mV) and two oxidations at E1/2 values of
0.48 V (ΔEp = 160 mV) and 1.06 V (ΔEp = 300 mV) vs SCE.
Both the first oxidation and the first reduction processes are
chemically reversible (ratio of cathodic and anodic peak
current, ipc/ipa ≈ 1)24 and electrochemically quasireversible
electron-transfer reactions. The most cathodic and most anodic
redox processes were not investigated any further.
0.71073
0.71073
orthorhombic
Pca21 (no. 29)
0.10 × 0.08 × 0.06
22.475(3)
12.0625(16)
16.508(2)
90.0
trigonal
R3c (no. 161)
0.12 × 0.10 × 0.08
30.9863(9)
30.9863(9)
28.9032(16)
90.0
b (Å)
c (Å)
α (deg)
β (deg)
90.0
90.0
γ (deg)
90.0
120.0
volume (Å3)
4475.4(10)
4
24033.4(17)
18
Z
densitycalc (g cm−3
μ (mm−1
)
1.220
1.251
)
0.385
0.348
no. reflcns collcd
no. unique reflcns
28207
52888
10647 (Rint = 0.0648) 8148 (Rint = 0.0813)
no. reflcns used [I > 2σ(I)] 6013
6264
goodness-of-fit on F2
0.99
1.048
ab
,
final R indices [I > 2σ(I)]
0.0682 (0.1303)
0.1601 (0.2124)
0.0857 (0.2255)
0.1135 (0.2573)
ab
,
R indices (all data)
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(|Fo|2 − |Fc|2)2]/∑[w(|
Fo|2)2]}1/2
.
38
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Inorganic Chemistry
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Scheme 1
Figure 2. Cyclic voltammogram (100 mV/s) of a 1.0 mM solution of [FeIII(L)2] 1 in CH2Cl2 (0.1 M in TBAP) at a platinum working electrode.
Indicated peak potentials are in V vs SCE.
Synthesis and Characterization of [FeIII(L)2][BF4] 2 and
[CoIII((η5-C10H15)2][FeIII(L)2] 3. Coulometric oxidation of 1 in
CH2Cl2 (0.1 M TBAP; 298 K) at 0.7 V vs SCE established that
the wave at E1/2 = 0.48 V corresponds to a one-electron
process. The oxidized form [1]+ was found to be stable on the
time scale of CV (Figure S4, Supporting Information). In fact,
this behavior finally led to designed chemical synthesis of 2.
Controlled-potential coulometry of 1 in CH2Cl2 (0.1 M TBAP;
298 K) at −0.7 V vs SCE established that the wave at −0.42 V
corresponds to a one-electron reduction process, yielding [1]−.
The reduced form of 1 was found to be stable under anaerobic
conditions, which finally led to designed chemical synthesis of
3. Electrochemical reoxidation of such solutions (Figure S5,
Supporting Information) produced quantitatively the starting
complex 1.
C10H15)2],43,44 under anaerobic conditions. Usual workup of
this solution produced ink-blue air-sensitive microcrystals of
[CoIII(η5-C10H15)2][FeIII(L)2] (3).
The absence of ν(O−H) and ν(N−H) in the IR spectra of
1−3 confirmed that the ligand is coordinated in these
complexes in the doubly deprotonated form. Elemental
analyses and IR data are in agreement with the above
formulations. Effective magnetic moment (μeff) values of solid
samples of 2 and 3 measured at 298 K were determined to be
1.89 μB and 1.91 μB, respectively. Solution-state values of
coulometrically generated one-electron oxidized [1]+ and one-
electron reduced [1]− forms were calculated to be 1.91 μB and
1.84 μB, respectively. Thus [1]+/2 and [1]−/3 are paramagnetic
with respect to one unpaired electron. The stability of 2 and 3
in CH2Cl2 solutions was investigated by their clean ESI-MS
pattern. Positive and negative ESI-MS of 2 (Figure S6,
Supporting Information) displayed peaks at m/z = 854.40
and 87, respectively. Based on the simulated mass and isotopic
distribution pattern these peaks correspond to the species
[FeIII(L)2]+ and [BF4]−, respectively. Positive and negative ESI-
MS of 3 (Figure S7, Supporting Information) showed peaks at
Clean one-electron chemical oxidation of 1 in CH2Cl2 at 298
42
K by 1 equiv of AgBF4 was achieved. Solvent removal
afforded a solid, which on usual workup led to the isolation of a
greenish-black solid [FeIII(L)2][BF4] 2. Chemical reduction of
1 could be achieved in CH2Cl2 at 298 K by 1 equiv of
bis(pentamethylcyclopentadienide)cobalt(II) [CoII(η5-
39
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Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) in [FeIII(L)2] 1 and [FeIII(L)2][BF4] 2
1
2
1
2
Fe−N1
Fe−N3
Fe−O1
1.896(4)
1.927(4)
1.957(3)
1.315(6)
1.426(7)
1.397(7)
1.404(7)
1.401(8)
1.387(7)
1.440(7)
1.368(6)
1.402(6)
1.433(7)
1.422(7)
1.366(8)
1.377(9)
1.384(8)
1.394(7)
1.378(7)
1.280(6)
1.454(7)
1.379(8)
1.372(9)
1.372(10)
1.376(10)
1.397(8)
1.390(8)
Fe−N1
Fe−N3
Fe−O1
1.889(6)
C33−C38
C33−C34
C34−C35
C35−C36
C36−C37
C37−C38
N5−C33
N5−N6
N6−C32
C27−C32
C27−C28
C28−C29
C29−C30
C30−C31
C31−C32
1.436(7)
1.419(8)
1.353(8)
1.387(9)
1.376(8)
1.418(7)
1.379(7)
1.283(6)
1.451(7)
1.378(8)
1.408(9)
1.363(11)
1.375(11)
1.394(9)
1.382(8)
C33−C38
C33−C34
C34−C35
C35−C36
C36−C37
C37−C38
N5−C33
N5−N6
N6−C32
C27−C32
C27−C28
C28−C29
C29−C30
C30−C31
C31−C32
1.396(10)
1.424(10)
1.378(12)
1.388(11)
1.374(10)
1.409(10)
1.374(10)
1.267(8)
1.923(6)
1.955(5)
O1−C26
C13−C26
C13−C14
C14−C15
C15−C20
C20−C21
C21−C26
N1−C12
N1−C13
C7−C12
C7−C8
O1−C26
C13−C26
C13−C14
C14−C15
C15−C20
C20−C21
C21−C26
N1−C12
N1−C13
C7−C12
C7−C8
1.278(9)
1.436(10)
1.414(10)
1.360(11)
1.412(10)
1.368(11)
1.453(10)
1.381(11)
1.366(9)
1.449(9)
1.388(10)
1.353(12)
1.409(14)
1.381(14)
1.366(12)
1.384(11)
1.405(11)
1.421(10)
1.397(13)
1.376(13)
1.368(11)
1.406(11)
1.395(10)
1.271(8)
C8−C9
C8−C9
C9−C10
C10−C11
C11−C12
N2−C7
N2−N3
N3−C6
C6−C1
C1−C2
C2−C3
C3−C4
C9−C10
C10−C11
C11−C12
N2−C7
N2−N3
N3−C6
C6−C1
C1−C2
C2−C3
C3−C4
Fe−N1−C13
Fe−N1−C12
C12−N1−C13
N1−Fe−N3
N1−Fe−N4
N1−Fe−N6
N3−Fe−N6
N3−Fe−N4
N4−Fe−N6
N2 -N3Fe
N5 -N6Fe
Fe−N4−C38
Fe−N4−C39
C38−N4−C39
N1−Fe−O1
N1−Fe−O2
N3−Fe−O1
N3−Fe−O2
N4−Fe−O1
N4−Fe−O2
N6−Fe−O1
N6−Fe−O2
O1−Fe−O2
109.8(3)
Fe−N1−C13
Fe−N1−C12
C12−N1−C13
N1−Fe−N3
N1−Fe−N4
N1−Fe−N6
N3−Fe−N6
N3−Fe−N4
N4−Fe−N6
N2 −N3-Fe
N5 −N6-Fe
Fe−N4−C38
Fe−N4−C39
C38−N4−C39
N1−Fe−O1
N1−Fe−O2
N3−Fe−O1
N3−Fe−O2
N4−Fe−O1
N4−Fe−O2
N6−Fe−O1
N6−Fe−O2
O1−Fe−O2
110.3(4)
125.7(5)
124.0(6)
89.9(2)
170.5(2)
96.5(3)
92.4(2)
96.1(3)
90.6(2)
129.6(5)
128.6(5)
123.8(5)
110.9(4)
125.2(6)
82.9(2)
89.1(2)
171.9(2)
92.8(2)
90.6(2)
83.3(2)
91.9(2)
172.3(2)
83.58(19)
125.8(3)
124.4(3)
90.73(19)
169.33(17)
96.21(18)
92.96(18)
96.59(18)
91.18(18)
128.7(4)
1.465(9)
1.377(11)
1.370(12)
1.385(14)
1.429(15)
1.386(12)
1.379(11)
C4−C5
C5−C6
C4−C5
C5−C6
128.6(3)
125.3(3)
Fe−N4
Fe−N6
Fe−O2
1.886(4)
1.928(4)
1.955(3)
1.307(6)
1.430(7)
1.404(7)
1.383(7)
1.419(9)
1.381(8)
1.439(7)
1.364(6)
1.379(6)
Fe−N4
Fe−N6
Fe−O2
1.880(6)
1.927(6)
1.929(5)
1.302(9)
1.441(10)
1.394(10)
1.377(10)
1.447(10)
1.338(10)
1.428(10)
1.368(9)
1.389(9)
109.9(3)
124.7(4)
83.87(16)
88.30(16)
173.17(17)
90.72(17)
88.22(16)
83.86(16)
91.78(17)
174.13(17)
84.96(15)
O2−C52
C52−C39
C39−C40
C40−C41
C41−C46
C46−C47
C47−C52
N4−C39
N4−C38
O2−C52
C52−C39
C39−C40
C40−C41
C41−C46
C46−C47
C47−C52
N4−C39
N4−C38
m/z = 329.17 and 854.40, respectively. Simulated mass and
isotopic distribution pattern of these peaks correspond to the
species [CoIII(η5-C10H15)2]+ and [FeIII(L)2]−, respectively.
Complexes 1 and 2 have been structurally characterized (see
Crystal Structure section). In spite of our sincere efforts so far
we have not succeeded in growing single-crystals of 3 suitable
for crystallographic studies.
of both diamagnetic dianionic o-amidophenolate (LAP)2− and
−•
paramagnetic (LISQ
)
coordinated to FeIII ion.
o-Amidophenolates(2−) are redox-active and can exist in
three redox levels, and Scheme 1 shows the geometric features
observed. Wieghardt and others1c,9,14,38−42 firmly established
that it is possible to unambiguously discern these forms in a
complex by high-quality X-ray crystallography, if the estimated
experimental error of the C−C, C−O, and C−N bond lengths
is not larger than 0.015 Å (3σ). For (LAP)2− the C−O and C−
N bonds are ∼1.35 Å and ∼1.37 Å, respectively. For (LISQ)−•,
where the six-membered ring is not equivalent (quinoid-type
distortion) comprising a short, a long, and another short C−C
bond followed by three long ones and, in addition, both the C−
O and C−N distances are significantly shorter (∼1.30 and
Description of the Structures. To confirm the structure
of the complexes and to extract information on the oxidation
level of both iron and coordinated ligands single-crystal X-ray
structure determination of the parent complex [FeIII(L)2] 1 and
its one-electron oxidized counterpart [FeIII(L)2][BF4] 2 were
carried out. Selected bond length and bond angles are listed in
Table 2.
[FeIII(L)2] 1. A perspective view of the metal coordination
environment in 1 is shown in Figure 3. Given the neutral nature
of 1, both the tridentate ligands can exist either as a one-
∼1.35 Å, respectively) than those in (LAP)2−. The trend
−•
observed for (LISQ
)
is further amplified upon oxidation by
one more electron to the (LIBQ)0 level, with C−O and C−N
bond distances of ∼1.24 and ∼1.30 Å, respectively.
electron paramagnetic o-iminobenzosemiquinonate(1−) π
−•
radical (LISQ
)
coordinated to FeII ion or as a combination
40
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Inorganic Chemistry
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Figure 3. Perspective view of [FeIII(L)2] 1.
Figure 4. Perspective view of [FeIII(L)2][BF4] 2.
The sensitive C−O, C−N, and intraring bond distances
associated with the coordinated ligands in 1 (Table 2) reveal
the following important features. (i) The C−O and C−N bond
distances exhibit a rather complex relationship where the C−O
1.366(9) Å, respectively. For the other ligand, the correspond-
ing distances are 1.302(9) Å and 1.368(9) Å, respectively. As
that observed for 1, the C−O and C−N bond distances do not
fall under the set benchmark: (i) the C−O distance of one
distances [1.307(6) and 1.315(6) Å] are closer to the localized
ligand is shorter than localized (LISQ −•
)
and for the other it is
−• distance. (ii) The C−N distance
of both the ligands corresponds to either the localized (LAP)2−
−•
(LISQ
)
distance, whereas the C−N distance of one ligand
closer to the localized (LISQ
or the localized (LISQ −•
form. Clearly, the behavior can be
)
1.364(6) Å is in between the distances for localized (LAP)2− and
(LISQ −• forms and for the other ligand the distance 1.402(6) is
)
)
larger than even the localized (LAP)2− form. Given the set
benchmark by Wieghardt and others,1c,9,14,38−42 the observed
metric parameters and the accuracy level of observed bond
lengths (3σ value: 0.021 for C−C, 0.018 for both C−O
and C−N), it is not possible to unambiguously assign the
oxidation level of the coordinated ligands in 1. The observed
parameters, however, suggest that two ligands provide 3−
charge and a radical is delocalized over two ligands.41a
Admittedly, it is not exact, but it is a reasonable approximation
that one ligand is in the (LAP)2− form and the other in the o-
attributed to the effect of delocalization of metal and ligand.41a
The accuracy level of bond length determination (3σ values:
0.030 Å for C−C and 0.027 Å for both C−O and C−N
bonds) and hence the observed metric parameters (Table 2) do
not allow us to unambiguously assign the oxidation level of the
coordinated ligands in 2. However, the composition and
observed metric parameters of this monocation suggest that
two ligands provide together 2− charge, and each is present in
−•
the o-iminobenzosemiquinonate(1−) π radical (LISQ
)
form
(Scheme 1). Understandably, it is a reasonable approximation.
The observed Fe−O and Fe−N bond distances are Fe−O1
1.955(5) and 1.929(5) Å and Fe−N1 1.889(6) and 1.880(6) Å.
The Fe−N(azo) distances are Fe−N3 1.923(4) and Fe−N6
1.927(6) Å. The N−N bond distances are N2−N3 1.271(8)
and N5−N6 1.267(8) Å, attesting that as in 1 not significant
resonance delocalization is present in the azo part of the
present ligand.
iminobenzosemiquinonate(1−) π radical (LISQ −• form. The Fe
)
atom must then be ascribed a +3 oxidation state. The observed
Fe−O [Fe−O1 1.957(3) and 1.955(3) Å] and Fe−N [Fe−N1
1.896(4) and 1.886(4) Å] bond distances support this view
with iron in the low-spin (LS) state (see Mossbauer section).
̈
The Fe−N(azo) distances are the longest [Fe−N3 1.927(4)
and Fe−N6 1.928(4) Å], revealing that azo group has not
pulled much electron density, meaning thereby not significant
resonance delocalization in the azo part of the present ligand
(N−N bond distances N2−N3 1.280(6) and N5−N6 1.283(6)
Å). The N−N distance is an excellent indicator of the charge
on an azo function. From ∼1.24 Å for free ligands the
coordination by back-donating metals may shift this value to
about 1.25−1.30 Å. Authentic one-electron reduced (i.e., anion
radical) ligands have N−N distance of ∼1.35 Å.41b,45,46 It is
worth mentioning here that in high-spin tris-complex of
Mossbauer Spectra. The zero-field 57Fe Mossbauer
̈
̈
spectra of [FeIII(L)2] 1 were recorded at 298 and 200 K
(Figure S8, Supporting Information), and 80 K displaying
(Figure S9, Supporting Information) uniformly a symmetric
quadrupole doublet. The parameters (Table 3) are in complete
accord with the d5 LS FeIII.47 To elucidate the electronic
structure of its one-electron oxidized [1]+ and one-electron
reduced [1]− forms we have recorded temperature-dependent
Mossbauer spectra of 2 and 3, respectively (Table 3).
̈
bidentate 2-anilino-4,6-di-tert-butylphenolate in which the
The spectrum of 2 at 80 K could be satisfactorily fitted by
two subspectra (Figure 5): an intense symmetric quadrupole
doublet and a minor symmetric doublet. The relative intensity
of the minor doublet increases at 200 K (Figure 5). At 8 K the
minor doublet completely disappears (Figure S9, Supporting
Information). The minor doublet could be due to either a LS
FeII or a high-spin (HS) FeIII (spin-state transition) species.
−•
ligands are in the (LISQ
)
oxidation level, the distances are
Fe−O 2.015(3), 2.018(3), and 2.009(3) Å and Fe−N 2.057(4),
2.149(4), and 2.091(4) Å.38b
[FeIII(L)2][BF4] 2. A perspective view of the metal
coordination environment in 2 is shown in Figure 4. The C−
O and C−N bond distances for one ligand are 1.278(9) and
41
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a
,
Table 3. Zero-Field Mossbauer Parameters of [FeIII(L)2] 1, [FeIII(L)2][BF4] 2, and [CoIII(η5-(C10H15)2][FeIII(L)2] 3
b
̈
LS FeIII
A(1)
LS FeII
A(2)
complex
T, K
δ, mms−1
ΔEQ, mms−1
δ, mms−1
ΔEQ, mms−1
relative intensity A(1):A(2), %
1
1
1
2
2
2
3
3
3
298
200
80
0.04
0.09
0.11
0.13
0.14
0.16
0.07
0.13
0.14
0.14
2.13
1.94
1.97
1.87
1.85
1.90
1.99
1.99
2.03
2.03
100
100
100
200
80
0.47
0.73
0.73
1.06
72:28
93:7
100
8
200
80
0.56
0.61
1.13
1.06
80:20
86:14
100
8
0.63
1.05
96:4
a
b
Isomer shift vs α-Fe at 20 °C. Quadrupole splitting.
s−1 (43% relative intensity), and a minor symmetric doublet at
δ = 0.75 mm s−1 and ΔEQ = 1.13 mm s−1 (8% relative
intensity). Given the data at hand (see EPR section) we believe
that two subspectra (Figure 5) represent the right fit for 2.
The spectrum of 3 recorded at 80 K also point toward the
presence of two species. The data could be satisfactorily fitted
(Figure 6) by two subspectra: an intense symmetric quadrupole
doublet and a minor symmetric doublet. The relative intensity
of the minor doublet increases at 200 K (Figure 6), as that
observed for 2. Here also at 8 K the minor doublet completely
Figure 5. Zero-field 57Fe Mossbauer spectra (solid) of [FeIII(L) ]-
̈
2
[BF4] 2 (a) at 200 K (fitting of two subspectra) and (b) 80 K (fitting
of two subspectra).
Given the metal−ligand bonding parameters of LS FeIII in 2
obtained at 100 K (Table 2), the spin-transition to HS FeIII at
80 K is not feasible. On a similar ground and also given the
spectral parameters we rule out the possibility of HS FeII. The
LS FeII is definitely a possibility.48,49 At 8 K, the spectrum of 2
is a pure LS FeIII complex (Figure S9, Supporting Information).
The data for 2 at 80 K could be fitted by three subspectra as
well (Figure S10, Supporting Information): two symmetric
quadrupole doublets δ = 0.13 mm s−1, ΔEQ = 2.04 mm s−1
(49% relative intensity) and δ = 0.14 mm s−1, ΔEQ = 1.59 mm
Figure 6. Zero-field 57Fe Mossbauer spectrum (solid) of [CoIII(η5-
̈
C10H15)2][FeIII(L)2] 3 (a) at 200 K (fitting of two subspectra) and (b)
80 K (fitting of two-subspectra).
42
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Inorganic Chemistry
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disappears (Figure S9, Supporting Information). Given the
asymmetry of the spectrum at 8 K we fitted the data by two
subspectra: an intense symmetric quadrupole doublet (Figure
S11, Supporting Information) and a minor symmetric doublet
(Table 3; see EPR section). Clearly, the quality of the fitting
improves.
Temperature-dependent Mossbauer spectral behavior of 2
̈
and 3 could be explained if we invoke the occurrence of
thermally driven valence-tautomerism.2b,c,7a,10−12,42,50,51 In the
case of 2 we consider the equilibrium [FeIII{(LISQ)−•}2]+ (LS
FeIII) ⇌ [FeII{(LIBQ)o}{(LISQ)−•}]+ (LS FeII) in the solid state
−•
0
(see EPR section). In fact, the FeIII−(LISQ
)
⇌ FeII−(LIBQ
)
equilibrium is related by exchange of an electron between the
metal ion and the coordinated redox-active ligand, keeping the
total spin St = 1/2 invariant. We believe that this phenomenon
is operative in the 80−200 K range. The presence of strong
−•
FeIII−(LISQ
)
exchange interaction in 2 strengthens our
hypothesis and rules out the possibility of the existence of
another thermally driven valence-tautomer [FeIII{(LAP)2−}-
{(LIBQ)0}]+ (LS FeIII), and hence three-subspectra fitting is
discarded. For 3 we consider the equilibrium [FeIII{(LAP)2−}2]−
(LS FeIII) ⇌ [FeII{(LISQ)−•}{(LAP)2−}]− (LS FeII) in the solid
state (see EPR section). Recent theoretical studies have
concluded that owing to the energetic proximity of the metal
d-orbitals and the frontier ligand orbitals of redox-active
ligands, the transition-metal complexes of such ligands may
exhibit thermally induced reversible valence-tautomeric behav-
ior.51 The temperature-dependent Mossbauer spectral behavior
̈
of 2 and 3 is thus understandable.
Absorption Spectra. Measurements were recorded in
CH2Cl2 (Figure 7). For 3 the measurements were done strictly
under anaerobic conditions. Bluish-black solutions of 1 display
very intense (∼104 M−1 cm−1) absorptions at 420 nm (ε, 11
750 M−1 cm−1), 490 (13 000), 540 (12 970), 685 (17 800), and
1590 (13 650). Greenish-black solutions of 2 display (Figure 7)
intense (∼103 M−1 cm−1) absorptions at 435 (6500), 600
(5800), 860 sh (2700), 1010 sh (2600), 1130 (3000), 1580 sh
(600), 1890 (1100). Ink-blue solutions of 3 display (Figure 7)
very intense (∼103 M−1 cm−1) absorptions at 425 (6520), 570
(8900), 650 sh (8600), 755 sh (5480) 1060 (1910), 1590
(1600). The conversion of 1 to 3 is accompanied by the
appearance of a new peak at 1060 nm. Moreover, the
absorption band maxima in the 450−900 nm region are
shifted, and the intensities are lowered. In going from 1 to 2 a
new peak appears at 1890 nm, the absorption band maxima in
the said region are shifted, and the intensities are further
lowered. It is interesting to note that the absorption at ∼1590
nm remains invariant in going from 1 to [1]− or [1]+; however,
the intensity of the absorption monotonically decreases. This
observation does not corroborate well with the significant
change in the electronic structure of the ligand(s). In fact, it
correlates well with closely similar electronic structure. The
existence of valence-tautomeric equilibria2b,c,7a,10−12,42,50,51
[FeIII{(LISQ)−•}2]+ (LS FeIII) ⇌ [FeII{(LIBQ)o}{(LISQ)−•}]+
(LS FeII) and [FeIII{(LAP)2−}2]− (LS FeIII) ⇌ [FeII{(LISQ)−•}-
{(LAP)2−}]− (LS FeII) could explain the spectral behavior of 2
and 3, respectively. Given the delocalized nature of the
Figure 7. UV−vis−NIR spectra of (a) [FeIII(L)2] 1, (b) [FeIII(L)2]-
[BF4] 2, and (c) [CoIII(η5-C10H15)2][FeIII(L)2] 3 in CH2Cl2.
recorded in the 300−1000 nm region. Figure 8a shows the
changes in the absorption spectral feature during electro-
chemical oxidation of 1. Two clear isosbestic points are
observed. The resulting spectrum displays a distinct absorption
coordinated ligands (see X-ray structure and Mossbauer
̈
section) such a phenomenon is justifiable. The origin of
observed absorptions for 1, 2, and 3 is described later (see TD-
DFT section).
Spectroelectrochemistry. Experiments were done in
CH2Cl2 (0.1 M TBAP) at 298 K, and the spectra were
Figure 8. Changes in absorption spectral feature during spectroelec-
trochemical experiment: (a) coulometric oxidation at 0.7 V vs SCE
and (b) coulometric reduction at −0.7 V vs SCE of a 1 mM solution
(CH2Cl2 containing 0.1 M TBAP; 298 K) of [FeIII(L)2] 1.
43
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Inorganic Chemistry
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maximum at 600 nm (see Absorption Spectra section).
Electrochemical re-reduction of solutions of [1]+ produced
quantitatively the spectrum of 1. Figure 8b shows the
absorption spectral changes during electrochemical one-
electron reduction of 1. During reduction two clear isosbestic
points are observed. The resulting spectrum of [1]− exhibits an
absorption maximum at 570 nm and two shoulders at 650 and
755 nm (see Absorption Spectra section). Electrochemical
reoxidation of solutions of [1]+ produced quantitatively 1.
EPR Spectra. The diamagnetism of 1 indicates a strong
intramolecular antiferromagnetic coupling between the SFe = 1/
2 spin of the LS FeIII (see Mossbauer section) and the Srad = 1/
̈
2 spin distributed over two ligands (see X-ray and DFT
section). Thus 1 is EPR silent. Isolated 2 in CH2Cl2 at 77 K
exhibits an isotropic signal at g = 2.002 (Figure S12, Supporting
Information). However, CH2Cl2-toluene glass (77 K) of 2
(Figure S12, Supporting Information) displays a sharp signal,
with a slight anisotropy. From simulations of the latter spectra
the following g values are obtained: gx = gy = 2.003, gz = 1.98.
Clearly, the signal corresponds to a ligand-based radical. The
electronic structure of [1]+/2 could then be assigned as
[FeIII{(LISQ)−•}2]+ (S = 1/2 due to (LISQ)−•, considering
Figure 9. Temperature dependence of EPR spectra recorded for
[FeIII(L)2][BF4] 2 (a) solid and (b) CH2Cl2-toluene (1:1; v/v).
2.016, and 1.997 is due to [FeIII{(LAP)2−}2]− and the signal at g
= 2.064 as due to [FeII{(LISQ)−•}{(LAP)2−}]− (LS FeII). Solid of
3 behaves similarly. Temperature-dependent EPR spectral
behavior of 2 and 3 clearly point toward thermally driven
antiferromagnetic coupling between a LS FeIII center and a
−•
(LISQ
)
radical; see X-ray, Mossbauer, and DFT section).
valence-tautomerism2b,c,7a,10−12,42,50,51 (see Mossbauer and
̈
̈
Absorption Spectra sections), for 3 at 77 K [FeIII{(LAP)2−}2]−
Coulometrically generated (CH2Cl2, 0.1 M in TBAP) solutions
of [1]− at 77 K and also of 3 as CH2Cl2-toluene (1:1 v/v) glass
at 77 K (Figure S13, Supporting Information) display rhombic
EPR spectra, characterized by S = 1/2. From simulations the
following g values are obtained: gx = 2.123, gy = 2.007, gz =
1.987 (gav = 2.039). Notably, the gav value of 3 is considerably
lower than that of authentic six-coordinate LS FeIII complexes47
but is higher than that of [MII{(LISQ)−•}(bpy)][PF6] (bpy =
2,2′-bipyridine; M = Pd and Pt).38f The latter complexes exhibit
−•
-
is favored and at higher temperatures [FeII{(LISQ
) }
{(LAP)2−}]− is favored. We believe that intramolecular
electron-exchange occurring on the EPR time scale contribute
to the temperature dependence of the EPR spectra.
DFT Calculations. Along with structural and spectroscopic
investigation, theoretical calculations were also performed to
understand the electronic structure of the neutral, mono-
cationic, and monoanionic form of the complexes, that is, to
make correct assignment of the redox level of the metal and the
coordinated ligands in [1], [1]+, and [1]−. DFT-calculated at
the B3LYP level (see Computational Details in the
Experimental Section) bond parameters, based on the
optimized structures of [1], [1]+, and [1]− (Tables S1−S3,
Supporting Information), match fairly well with the exper-
imental data (Table 2 and Table S4, Supporting Information).
DFT calculation for 1 leads to a broken-symmetry solution with
one unpaired electron on the metal center and another one
delocalized over two ligands.52 One unpaired electron on iron
implies that iron is present in the LS trivalent state. This is in
a S = 1/2 signal due to (LISQ −•
at giso = 2.002 for Pd and giso =
)
2.0 for Pt. The unpaired electron in 3 thus predominantly
resides in a metal d orbital (see DFT section). The electronic
structure of [1]−/3 could then be assigned as
[FeIII{(LAP)2−}2]−. Thus the first one-electron oxidation and
reduction of 1 are both ligand-centered processes, and the spin-
state of FeIII remains invariant.
To substantiate the phenomenon of valence-tautomerism
(see Mossbauer and Absorption Spectra section) temperature-
̈
dependent EPR measurements were done on 2 and 3 at 298,
200, 120, and 77 K. At 298 K the signal (g = 2.002) of 2 as a
solid is isotropic. As the temperature is lowered down to 120 K
a noticeable asymmetry develops, however, without any
measurable splitting (Figure 9). The spectra at 120 K (Figure
9) and 77 K (Figure S14, Supporting Information) are
identical. In CH2Cl2-toluene (1:1; v/v) 2 exhibits at 298 and
200 K an isotropic signal; however, at 120 K the signal is clearly
asymmetric (Figure 9). At 298 K complex 3 as polycrystalline
solid is characterized by the g values 2.120, 2.061, 2.055 br,
2.012, and 1.990. On lowering temperature down to 200 and
120 K the feature at g values of 2.061 and 2.055 is clearly
resolved, but the rest of the feature remains invariant (Figure
10a). At 77 K the spectrum is not well-resolved (Figure S14,
Supporting Information). In CH2Cl2-toluene (1:1; v/v) 3
exhibits at 298 K an isotropic signal at g = 2.048. At 200 and
120 K the signal is clearly anisotropic with g values of 2.126,
2.064, 2.016, and 1.997 (Figure 10b). Notably, for 3 as glass
(77 K) the signal at g = 2.064 is absent (Figure S13, Supporting
Information). We assign the rhombic signal of 3 as CH2Cl2-
toluene glass at 298, 200, and 120 K with g values of 2.126,
accordance with the Mossbauer results. Since 1 is neutral three
̈
negative charges are expected to be contributed by the two
ligands. This indicates that one ligand exists as dianionic form
and the other one exists as a o-iminobenzosemiquinonate (1−)
π radical. However, this unpaired electron (radical) is
delocalized over both the ligands. The Mulliken spin-density
plots and the spin-density values (Figure 11) revealed that the
unpaired electron on LS FeIII is antiferromagnetically coupled
with the unpaired electron delocalized over both the ligands.
The magnetic orbitals for 1 are displayed in Figure 12. The α-
HOMO orbital has substantial ligand character (∼86%) and the
β-HOMO orbital is predominantly metal-centered (∼73%).
Strong spatial overlap (S = 0.77) between these orbitals
accounts for its diamagnetism even at room temperature.
Analysis of the LUMO orbitals shows that the α-LUMO orbital
−•
has ∼95% ligand-character and is contributed by the (LISQ
)
part of the ligand. Thus upon first reduction the electron is
expected to be added to the o-iminobenzosemiquinonate(1−)
part of the ligand to form o-amidophenolate(2−). The β-
44
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Figure 10. Temperature dependence of EPR spectra recorded for [CoIII(η5-C10H15)2][FeIII(L)2] 3 (a) solid and (b) CH2Cl2-toluene (1:1; v/v).
LUMO orbital is primarily ligand-based with ∼32% metal-
character. Notably, α-LUMO+1 and α-LUMO+2 orbitals are
predominantly azo-based.
As revealed from the spin-density plot and spin-density
values (Figure 11), upon oxidation the spin-population on the
metal center in [1]+ remains unchanged. Thus oxidation takes
transfer (LLCT) and ligand-to-metal charge-transfer (LMCT)
transitions. The next absorptions calculated at lower wave-
lengths of 757 and 684 nm could be assigned as transitions
from donor amidophenolate(2−) to the acceptor azo unit,
along with minor metal-to-azo charge-transfer (MLCT)
component, as revealed from the analysis of the frontier
orbitals.
place at the ligand center, and both the ligands exist in the
−•
(LISQ
)
level. The unpaired electron on the metal center
Upon oxidation the absorption(s) in the near-IR region is/
are red-shifted and also the intensity decreased. TD-DFT
(Figure S15, Figure S17, and Table S6, Supporting
Information) results show that the near-IR spectral feature at
1636 nm corresponds to α-HOMO to α-LUMO transition with
lower oscillator strength as compared to 1 (f = 0.0269 in [1]+
vs 0.0786 in 1) than the lowest prominent excitation in 1. The
absorption at 1636 nm corresponds to LLCT and LMCT. The
α-HOMO corresponds to electron density from the ligands
scattered particularly on o-iminobenzosemiquinonate units
while the α-LUMO has both metal and ligand contribution.
Similar to what was observed for 1, in the case of [1]+ also
amidophenolate-to-azo charge-transfer (LLCT) and LMCT
both occurs at lower wavelength region, calculated at 1181,
1072, and 682 nm. The orbitals involved in such transitions are
β-HOMO and β-LUMO+2. Interestingly, we observe an
excitation of minor contribution (∼13%) at 682 nm from α-
HOMO-1 to α-LUMO+1 orbital that is metal to ligand type,
strengthening the existence of valence-tautomerism in [1]+ (cf.
couples antiferromagnetically with the unpaired electron on
one of the ligands leading to S = 1/2 state. The singly occupied
molecular orbital (SOMO) (Figure 12) has ∼99% ligand-
character which implies that the residual unpaired electron
resides on the other ligand center. Analysis of the β-LUMO
orbital reveals that it has ∼99% primarily o-iminobenzosemi-
quinonate-based ligand character. Thus, during the course of
1e− oxidation/reduction of [1]+ the (LISQ −• will be responsible
)
in the ligand redox level changes, while the metal center is not
expected to undergo any change in its oxidation level.
DFT calculations suggest that [1]− will have one unpaired
electron. From the spin-density plot and the spin-density values
(Figure 11) it is revealed that this electron resides on the metal
center. So iron is in LS +3 oxidation state. Since the complex
carries a negative charge and the metal ion is in the +3
oxidation state and there is no unpaired electron residing on
the ligands, thus both the ligands exist in the (LAP)2− redox
level. The SOMO (Figure 12) for [1]− has predominantly
metal-character (∼66%).
Mossbauer and EPR spectra).
̈
TD-DFT Results. Time-Dependent (TD)-DFT calculations
were done on 1, [1]+, and [1]− to get information about the
origin of various absorptions occurring in the 400−2000 nm
region and to get a trend within this series of complexes,
varying in the oxidation level of the coordinated ligands but the
oxidation state of the metal remaining invariant. The
calculations on 1 revealed (Figure S15, Figure S16, and Table
S5, Supporting Information) that the most allowed (f =
0.0786) lowest energy transition calculated at 1390 nm involves
excitation from (α/β)-HOMO to (α/β)-LUMO. Inspection of
frontier orbitals shows that this absorption in the near-IR
region corresponds to a combination of ligand-to-ligand charge-
For [1]− the moderately allowed (f = 0.0236) lowest energy
transition calculated by TD-DFT occurs at 1211 nm (Figure
S15, Figure S18, and Table S7, Supporting Information). This
excitation is amidophenolate-to-azo type. The next important
transition occurs at 978 nm comprising the β-HOMO and β-
LUMO+1 orbitals. Again similar excitations from both
amidophenolate-to-azo (LLCT) and amidophenolate-to-metal
(LMCT) were calculated. Additionally, metal-to-azo MLCT
transition is calculated at 704 and 624 nm. As that for [1]+, the
electronic transitions further substantiate the valence-tautomer-
ism, prevalent in monoanion [1]−. Given the fact that
absorption spectra of transition metal complexes is by far the
45
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Inorganic Chemistry
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Figure 12. Qualitative MO diagrams of the magnetic orbitals for (a)
[FeIII(L)2] 1, (b) [1]+, and (c) [1]−.
charges. Strong intramolecular antiferromagnetic coupling
between these two entities yields the observed diamagnetic
(S = 0) ground state for 1. The low-spin FeIII in 2 is stabilized
by two ligands each in the o-iminobenzosemiquinonate (1−) π
radical (LISQ −•
form, and it is strongly antiferromagnetically
)
coupled to one of the o-iminobenzosemiquinonato radicals (Srad
= 1/2), resulting in a doublet state due to other o-
iminobenzosemiquinonato radical (Srad = 1/2). The low-spin
FeIII in 3 is stabilized by two ligands each in the o-
amidophenolate(2−) (LAP)2− form. It is demonstrated that
the oxidation level of the coordinated ligands and LS state of
FeIII can be deduced by X-ray crystallographic metric
parameters of reasonable quality in conjunction with 1H
Figure 11. Spin-density plots of (a) [FeIII(L)2] 1 (on each ligand 0.39
and on FeIII −0.78), (b) [1]+ (on each ligand 0.89 and on FeIII −0.80),
and (c) [1]− (on each ligand 0.037 and on FeIII 0.93).
most difficult to calculate by DFT methods, we admit that the
remarks made in this section on valence-tautomerism are highly
speculative in nature.
NMR, mass, EPR, and Mossbauer, and UV−vis−NIR spectral
̈
SUMMARY AND CONCLUDING REMARKS
data, and cyclic voltammetric results. We have provided
evidence that valence-tautomerism is operative for 2 and 3.
Exploration of the generality and versatility of the
coordination behavior of this new tridentate ligand toward
other transition metal ions is underway.
■
The most salient features of the present study are summarized
here. A new complex [FeIII(L)2] 1 coordinated by two
tridentate redox-active azo-appended o-amidophenolate ligands
has been synthesized. The monocation [1]+ − [FeIII(L)2][BF4]
2 − has been isolated and structurally, magnetically, and
spectroscopically characterized. The monoanion [1]−
−
ASSOCIATED CONTENT
* Supporting Information
■
[CoIII(η5-C10H15)2][FeIII(L)2] 3 − has also been isolated and
magnetically and spectroscopically characterized. Structural
analysis of 1 and 2, room-temperature magnetic moment
values of 2 and 3, various spectroscopic data, and DFT
calculations provide the spin-state of all three complexes. It has
been conclusively deduced that the electronic structure of 1
must be described as a LS FeIII stabilized by a ligand π radical
delocalized over two ligands, providing together three negative
S
1H NMR spectra of H2L and [FeIII(L)2] 1 (Figures S1−S2);
positive ESI−MS spectrum of {[FeIII(L)2] + H+} (Figure S3);
CV scans of 1e− oxidized and 1e−reduced solutions of
[FeIII(L)2] 1 in CH2Cl2 (Figures S4−S5, respectively); positive
and negative ESI−MS spectra of [FeIII(L)2][BF4] 2 and
[CoIII(η5-C H ) ][FeIII(L) ] 3 (Figures S6−S7); Mossbauer
̈
̈
10 15
2
2
spectra (solid) of 1 at 298 and 200 K (Figures S8); Mossbauer
46
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Inorganic Chemistry
Article
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spectra (solid) of 1 at 80 K, 2 at 8 K, and 3 at 8 K (Figure S9);
Mossbauer spectrum (solid) of 2 at 80 K (fitted three
̈
subspectra) (Figure S10); Mossbauer spectrum (solid) of 3
̈
at 8 K (fitted two subspectra) (Figure S11); EPR spectra (77
K) of 2 in CH2Cl2 and in CH2Cl2-toluene glass (Figure S12);
EPR spectra (77 K) of coulometrically generated [1]− in
CH2Cl2 and 3 in CH2Cl2-toluene glass (Figure S13); EPR
spectra (77 K) of 2 and 3 as solid (Figure S14); TD-DFT-
calculated electronic spectra of 1, [1]+, and [1]− (Figure S15);
representative molecular orbitals involved in TD-DFT of 1,
[1]+, and [1]− (Figures S16−S18, respectively); DFT-
optimized Cartesian coordinates of 1, [1]+, and [1]− (Tables
S1−S3, respectively); X-ray structural and DFT-optimized
bond lengths of 1, [1]+, and [1]− (Tables S4); TD-DFT-
calculated electronic transitions of 1, [1]+, and [1]− (Tables
S5−S7, respectively). This material is available free of charge
via the Internet at http://pubs.acs.org. CCDC-927552 (1) and
950999 (2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: rnm@iitk.ac.in. Phone: +91-512-2597437. Fax: +91-
512-2597436.
■
(12) (a) Storr, T.; Wasinger, E. C.; Pratt, R. C.; Stack, T. D. P. Angew.
Chem., Int. Ed. 2007, 46, 5198−5201. (b) Storr, T.; Verma, P.; Pratt, R.
C.; Wasinger, E. C.; Shimazaki, Y.; Stack, T. D. P. J. Am. Chem. Soc.
2008, 130, 15448−15459. (c) Storr, T.; Verma, P.; Shimazaki, Y.;
Wasinger, E. C.; Stack, T. D. P. Chem.Eur. J. 2010, 16, 8980−8983.
(13) (a) Joy, S.; Pal, P.; Mondal, T. K.; Talapatra, G. B.; Goswami, S.
Chem.Eur. J. 2012, 18, 1761−1771. (b) Samanta, S.; Ghosh, P.;
Goswami, S. Dalton Trans. 2012, 41, 2213−2226. (c) Paul, N. D.;
Rana, U.; Goswami, S.; Mondal, T. K.; Goswami, S. J. Am. Chem. Soc.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the Department of Science &
Technology (DST), Government of India. R.M. sincerely
thanks DST for a J.C. Bose fellowship. A.R. gratefully
acknowledges the award of an SRF by UGC, and S.K.B.
acknowledges the award of an SRF by the Council of Scientific
& Industrial Research, Government of India. We are grateful to
(14) Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Coord.
Chem. Rev. 2009, 253, 291−324.
(15) (a) Mukherjee, A.; Lloret, F.; Mukherjee, R. Inorg. Chem. 2008,
47, 4471−4480. (b) Mukherjee, A.; Lloret, F.; Mukherjee, R. Eur. J.
Inorg. Chem. 2010, 1032−1042. (c) Mukherjee, A.; Mukherjee, R.
Indian J. Chem. 2011, 50A, 484−490 and references therein.
(16) Najafpour, J.; Foroutan-Nejad, C.; Shafiee, G. H.; Peykani, M. K.
Comput. Theor. Chem. 2011, 974, 86−91 and references therein.
(17) (a) Noro, S.-i.; Chang, H.-C.; Takenobu, T.; Murayama, Y.;
Kanbara, T.; Aoyama, T.; Sassa, T.; Wada, T.; Tanaka, D.; Kitagawa,
S.; Iwasa, Y.; Akutagawa, T.; Nakamura, T. J. Am. Chem. Soc. 2005,
127, 10012−10013. (b) Wickramasinghe, L. D.; Perera, M. M.; Li, L.;
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DOI: 10.1002/anie.201306765.
Dr. Serhiy Demeshko of Georg-August-Universitat Gottingen
̈
̈
for his help in temperature-dependent (200−8 K) Mossbauer
̈
spectral measurements. We sincerely thank Dr. Prasanta Ghosh
of R. K. Mission Residential College, Narendrapur, Kolkata, for
his help in temperature-dependent (298−120 K) EPR spectral
measurements in his laboratory. Comments of the reviewers
were very helpful at the revision stage.
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