FeIII bipyrrolidine phenoxide complexes and their oxidized analogues

Article abstract of DOI:10.1021/ic500663x

Fe^III complexes of the symmetric (2S,2S)-[N,N-bis(1-(2-hydroxy- 3,5-di-tert-butylphenylmethyl))]-2,2-bipyrrolidine (H₂L¹) and dissymmetric (2S,2S)-[N,N-(1-(2-hydroxy-3,5-di-tert-butylphenylmethyl))-2- (pyridylmethyl)]-2,2-bipyrrolidine (HL²) ligands incorporating the bipyrrolidine backbone were prepared, and the electronic structure of the neutral and one-electron oxidized species was investigated. Cyclic voltammograms (CV) of FeL¹Cl and FeL²Cl₂ showed expected redox waves corresponding to the oxidation of phenoxide moieties to phenoxyl radicals, which was achieved by treating the complexes with 1 equiv of a suitable chemical oxidant. The clean conversion of the neutral complexes to their oxidized forms was monitored by UV-vis-NIR spectroscopy, where an intense φ-φ* transition characteristic of a phenoxyl radical emerged ([FeL¹Cl]^+?: 25 500 cm^-1 (9000 M ^-1 cm^-1); [FeL²Cl₂] ^+?: 24 100 cm^-1 (8300 M^-1 cm ^-1). The resonance Raman (rR) spectra of [FeL¹Cl] ^+? and [FeL²Cl₂]^+? displayed the characteristic phenoxyl radical _7a band at 1501 and 1504 cm^-1, respectively, confirming ligand-based oxidation. Electron paramagnetic resonance (EPR) spectroscopy exhibited a typical high spin Fe ^III (S = 5/2) signal for the neutral complexes in perpendicular mode. Upon oxidation, a signal at g ≈ 9 was observed in parallel mode, suggesting the formation of a spin integer system arising from magnetic interactions between the high spin Fe^III center and the phenoxyl radical. Density functional theory (DFT) calculations further supports this formulation, where weak antiferromagnetic coupling was predicted for both [FeL¹Cl] ^+? and [FeL²Cl₂]^+?.

Full text of DOI:10.1021/ic500663x

Article
pubs.acs.org/IC
FeIII Bipyrrolidine Phenoxide Complexes and Their Oxidized
Analogues
Linus Chiang,† Didier Savard,† Yuichi Shimazaki,‡ Fabrice Thomas,§ and Tim Storr*,†
†Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
‡College of Science, Ibaraki University, Bunkyo, Mito, 310-8512, Japan
§Dep
́ ́ ́
artement de Chimie Moleculaire, Chimie Inorganique Redox (CIRE), UMR-5250, Universite Grenoble Alpes, BP 53, 38041
Grenoble Cedex 9, France
S
* Supporting Information
ABSTRACT: FeIII complexes of the symmetric (2S,2′S)-
[N,N′-bis(1-(2-hydroxy-3,5-di-tert-butylphenylmethyl))]-2,2′-
bipyrrolidine (H2L1) and dissymmetric (2S,2′S)-[N,N′-(1-(2-
hydroxy-3,5-di-tert-butylphenylmethyl))-2-(pyridylmethyl)]-
2,2′-bipyrrolidine (HL2) ligands incorporating the bipyrroli-
dine backbone were prepared, and the electronic structure of
the neutral and one-electron oxidized species was investigated.
Cyclic voltammograms (CV) of FeL1Cl and FeL2Cl2 showed
expected redox waves corresponding to the oxidation of
phenoxide moieties to phenoxyl radicals, which was achieved
by treating the complexes with 1 equiv of a suitable chemical oxidant. The clean conversion of the neutral complexes to their
oxidized forms was monitored by UV−vis-NIR spectroscopy, where an intense π−π* transition characteristic of a phenoxyl
radical emerged ([FeL1Cl]+•: 25 500 cm−1 (9000 M−1 cm−1); [FeL2Cl2]+•: 24 100 cm−1 (8300 M−1 cm−1). The resonance
Raman (rR) spectra of [FeL1Cl]+• and [FeL2Cl2]+• displayed the characteristic phenoxyl radical ν7a band at 1501 and 1504 cm−1,
respectively, confirming ligand-based oxidation. Electron paramagnetic resonance (EPR) spectroscopy exhibited a typical high
spin FeIII (S = 5/2) signal for the neutral complexes in perpendicular mode. Upon oxidation, a signal at g ≈ 9 was observed in
parallel mode, suggesting the formation of a spin integer system arising from magnetic interactions between the high spin FeIII
center and the phenoxyl radical. Density functional theory (DFT) calculations further supports this formulation, where weak
antiferromagnetic coupling was predicted for both [FeL1Cl]+• and [FeL2Cl2]+•.
cooperative capabilities in synthetic models are salen ligands
(salen is a common abbreviation for N2O2 bis(Schiff base)-
bis(phenolate) ligands).6 These tetradentate ligands are
attractive targets due to their relative ease of synthesis and
their ability to form stable complexes with many metals in a
variety of oxidation states, several of which are robust catalysts
for organic transformations.7 Importantly, many metallosalen
complexes have been shown to exist in the [M(Salen)•]+
electronic state upon oxidation, demonstrating the utility of
salens as redox active ligands.4a,6a,b,f,i,k,m,n,r,8 Specifically, Fe
salen complexes have received significant attention as synthetic
models of heme systems.8,9 In certain cases, the ligand radical
formed was found to be delocalized over the π system of the
aromatic ring, generating a phenoxyl radical species. Numerous
FeIII phenoxide complexes have also been investigated and are
capable of undergoing successive oxidations to afford multiple
ligand radical species.10
1. INTRODUCTION
Correlating the electronic structure and reactivity of inorganic
entities is of considerable interest, both in nature1 and in
synthetic model systems.2 In certain metalloenzyme systems,
the transition metal functions in a cooperative manner with a
redox active ligand to promote multielectron reactions. One
such enzyme is galactose oxidase,1,3 which catalyzes the two-
electron oxidation of primary alcohols to aldehydes. To drive
this chemistry, galactose oxidase utilizes two one-electron redox
cofactors: a Cu center and a post-translationally modified
tyrosine residue. This modification, in the form of a Tyr-Cys
cross-link, is a key determinant in lowering the Tyr•/Tyr redox
potential, leading to stabilization of the radical resting state of
the enzyme. Numerous structural and functional synthetic
models of galactose oxidase have been studied, owing to the
simplicity of the active site.4 Other notable examples include
heme proteins, such as cytochrome P450, catalase, and
peroxidase, which form a FeIV-oxo heme radical cation (also
known as Compound I) to promote oxidation reactions.5
Depending on the energies of the redox-relevant orbitals,
complexes with pro-radical ligands ([Mn+L]) can undergo
ligand-based ([Mn+L•]+) or metal based ([M(n+1)L]+) oxidation.
One class of ligand that has been studied extensively for their
The Fe complexes outlined above utilize a series of different
amine backbones as starting materials, from macrocyclic10a−c,11
to linear amines.9d,10d,11,12 Of particular interest are chiral
Received: March 21, 2014
Published: May 9, 2014
© 2014 American Chemical Society
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diamines, highlighted by Jacobsen’s catalyst, a MnIII salen that
employs trans-1,2-cyclohexanediamine as the backbone. It was
found that the chirality of the backbone can induce stereo-
selectivity during catalysis.7a,d However, it has also been shown
that reduced bis-alkoxy ligands employing the cyclohexanedi-
amine backbone bind metal ions as a mixture of diaster-
eomers,13 likely due to ligand flexibility. To this end, the use of
a more constrained backbone, such as 2,2′-bipyrrolidine, has
been shown to form metal complexes diastereospecifically.14 As
such, there have been an increasing number of catalysts
reported that employ this backbone.15 In particular, the recent
use of Fe complexes incorporating the bipyrrolidine backbone
for selective C−H oxidation and asymmetric epoxidation
chemistry is impressive.15b−e However, reports of ligands with
this backbone have been limited to symmetric structures, where
the pendant arms are either both phenolates or both pyridines.
Therefore, we aimed to synthesize the first example of
dissymmetric FeIII complexes using the 2,2′-bipyrrolidine
scaffold with one redox active phenoxide and one pyridine
donor appended onto the chiral backbone (HL2) and compare
this to the symmetric FeIII complex where both pendant arms
are redox active phenoxides (H2L1, Chart 1). We aimed to
performed by Mr. Farzad Haftbaradaran and Mr. Paul Mulyk at Simon
Fraser University on a Carlo Erba EA1110 CHN elemental analyzer.
All electron paramagnetic resonance (EPR) spectra were collected
using a Bruker EMXplus spectrometer operating with a premiumX X-
band microwave bridge and a dual mode resonant cavity (∼9.39 GHz
in parallel mode, 9.63 GHz in perpendicular mode). Low temperature
measurements of frozen solutions used an Oxford Instruments helium
temperature-control system and continuous flow cryostats. Samples for
X-band measurements were placed in 4 mm outer-diameter sample
tubes with sample volumes of approximately 300 μL. Resonance
Raman spectra were obtained on a MC-100DC spectrometer (Ritsu
Oyo Kogaku) with a Beamlok Kr-ion laser (Spectra-Physics) for 406.7
nm excitation, a holographic supernoch filter (Kaiser Optical Systems),
and Symphony CCD detector (HORIBA Jobin Yvon) cooled with
liquid N2. Spectra of solvated samples were collected in spinning cells
(0.5 cm diameter, 330 rpm) at −50 °C at an excitation wavelength νex
= 406.7 nm (20 mW), 135° back scattering geometry, and 5 min data
accumulation time. Peak frequencies were calibrated relative to indene
standards (accurate to 1 cm−1).
2.2. X-ray Structure Determination. Single crystal X-ray
crystallographic analysis of FeL1Cl and FeL2Cl2 was performed on a
Bruker X8 APEX II diffractometer with graphite monochromated
Mo−Kα radiation. A dark purple block (FeL1Cl·MeCN) or dark blue
prism (FeL2Cl2·CH2Cl2) crystal was mounted on a glass fiber. The
data were collected at 293 0.1 K to a maximum 2θ value of 55.0°.
Data were collected in a series of ϕ and ω in 0.50° widths with 10.0 s
exposures. The crystal-to-detector distance was 50 mm. The structure
was solved by direct methods (SIR92)19 and refined by least-squares
procedures using CRYSTALS (v14.40b)20 or ShelXle.21 The materials
crystallize with one molecule of solvent in the asymmetric unit. All
non-hydrogen atoms were refined anisotropically. All C−H hydrogen
atoms were placed in calculated positions but were not refined. All
crystal structure plots were produced using ORTEP-3 and rendered
with POV-Ray (v.3.6.2).22 A summary of the crystal data and
experimental parameters for structure determinations is given in Table
2. The structure of (R,R)-FeL2Cl2 was solved in a unit cell of the space
group P212121, which is a part of a larger unit cell of the same space
group.
Chart 1. Symmetric (H2L1) and Dissymmetric (HL2)
Ligands
2.3. Oxidation Protocol. Under an inert atmosphere at 198 K,
400 μL of a CH2Cl2 solution of the metal complex (4.6 mM) was
diluted into 3.0 mL of CH2Cl2. Monitored by UV−vis-NIR, a
saturated solution of [NC6H3Br2)3]+• [SbF6]− in CH2Cl2 was added in
20 μL additions resulting in clean conversion to the respective one-
electron oxidized species.
2.4. Calculations. Geometry optimizations were performed using
the Gaussian 09 program (Revision D.01),23 the B3LYP functional,24
and the 6-31G(d) basis set on all atoms. Frequency calculations at the
same level of theory confirmed that the optimized structures were
located at a minimum on the potential energy surface. Single point
calculations for energetic analysis were performed with the B3LYP
functional and the TZVP basis set of Ahlrichs on all atoms.25 Broken-
symmetry (BS) density functional theory (DFT) calculations were
performed with the same functional and basis set.26 The
corresponding orbital transformation (COT) was used to determine
the magnetic orbitals for the oxidized systems.27 AOMix was used for
determining atomic orbital compositions employing Mulliken
population analysis.28
characterize the oxidized forms of these Fe complexes and in
particular the generation and stability of ligand radical species.
An improved understanding of the electronic signature of the
ligand radical forms will inform future studies in oxidative
catalysis.
2. EXPERIMENTAL SECTION
2.1. Materials and Methods. All chemicals used were of the
highest grade available and were further purified whenever necessary.16
The aminium radical chemical oxidant [N(C6H3Br2)3]+•[SbF6]− was
synthesized according to published protocols.17 3,5-Di-tert-butyl-2-
hydroxybenzyl bromide (3) was prepared from commercially available
2,4-di-tert-butylphenol in two steps by reported procedures.18
Electronic spectra were obtained on a Cary 5000 spectrophotometer
with a custom-designed immersion fiber-optic probe with variable
path-length (1 and 10 mm; Hellma, Inc.). Constant temperatures were
maintained by a dry ice/acetone bath. Solvent contraction was
accounted for in all variable-temperature studies. Cyclic voltammetry
(CV) was performed on a PAR-263A potentiometer, equipped with an
Ag wire reference electrode, a platinum disk working electrode, and a
2.5. Synthesis. 2.5.1. (2S,2′S)-[N-(2-(Pyridylmethyl)]-2,2′-bipyr-
rolidine (2). To a solution of (2S,2′S)-2,2′-bipyrrolidine (1) (500 mg,
3.6 mmol) in 1,2-dichloroethane (10 mL) was added K2CO3 (493 mg,
14.4 mmol), followed by 2-pyridinecarboxaldehyde (385 mg, 3.6
mmol). The reaction mixture was stirred at room temperature for 2 h
and then filtered and concentrated in vacuo. The product was dissolved
in anhydrous EtOH (30 mL), followed by the addition of 10% Pd/C
(150 mg). The reaction mixture was stirred under a H2 atmosphere for
3 days. This mixture was filtered over Celite, and the filtrate was
concentrated in vacuo to afford a yellow oil. The crude product was
subject to flash column chromatography using neutral alumina as the
stationary phase (eluent: 20:1 CH2Cl2/MeOH) to afford a pale yellow
n
Pt counter electrode with Bu4NClO4 (0.1 M) solutions in CH2Cl2.
Decamethylferrocene was used as an internal standard. Electrolysis was
performed by using a PAR-273A potentiostat at −40 °C using a large
Pt grid as the working electrode. Rotating disk electrode (RDE)
voltammetry was performed to monitor electrochemical oxidation
using a radiometer TCV101 speed control unit and a EDI 101
electrode. 1H NMR spectra were recorded on a Bruker AV-500
instrument. Mass spectra (positive ion) were obtained on an Agilent
6210 TOP ESI-MS instrument. Elemental analyses (C, H, N) were
1
oil that crystallizes over time as the amine 2. Yield: 645 mg, 81%. H
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a
Scheme 1. Synthetic Scheme for Dissymmetric Ligand HL2
NMR (CDCl3, 500 MHz): δ = 8.46−8.48 (ddd, 1H, Ar−H, J = 6.2,
2.0, 1.0), 7.65−7.69 (dt, 1H, Ar−H, J = 9.6, 2.2), 7.19−7.22 (m, 1H,
Ar−H), 7.18 (d, 1H, Ar−H, J = 9.8), 4.11 (d, 1H, Ar−CH, J = 19.7),
3.85 (d, 1H, Ar−CH, J = 19.7), 3.49−3.55 (m, 1H, N−CH), 3.40−
3.46 (m, 1H, N−CH), 3.11−3.21 (m, 2H, N−CH), 2.93−2.98 (m,
1H, N−CH), 2.63−2.69 (m, 1H, CH), 1.98−2.13 (m, 3H, CH),
1.84−1.94 (m, 3H, CH), 1.53−1.64 (m, 2H, CH); MS (ESI): m/z
(%): 232.1798 (100) [2 + H]+.
2.5.2. (2S,2′S)-[N,N′-(1-(2-Hydroxy-3,5-di-tert-butylphenylme-
thyl))-2-(pyridylmethyl)]-2,2′-bipyrrolidine (HL2). To a solution of
amine 2 (661 mg, 2.9 mmol) in MeCN (15 mL) was added K2CO3
(395 mg, 3.1 mmol), followed by a solution of benzyl bromide 3 (855
mg, 2.9 mmol) in MeCN (20 mL) dropwise at 0 °C. The resultant
mixture was warmed to room temperature, stirred for 16 h, and then
filtered, and the filtrate was concentrated in vacuo. The crude product
was subjected to flash column chromatography using silica gel as the
stationary phase (eluent: 20:1 CH2Cl2/MeOH) to afford a pale yellow
solid of ligand HL2. Yield: 855 mg, 67%. 1H NMR (CDCl3, 500
MHz): δ = 8.52−8.53 (d, 1H, Ar−H, J = 4.3), 7.61−7.64 (dt, 1H, Ar−
H, J = 7.6, 0.9), 7.38 (d, 1H, Ar−H, J = 7.7), 7.19 (d, 1H, Ar−H, J =
1.8), 7.13−7.15 (dd, 1H, Ar−H, J = 6.5, 5.5), 6.83 (d, 1H, Ar−H, J =
1.5), 4.22 (d, 1H, Ar−CH, J = 13.4), 4.03 (d, 1H, Ar−CH, J = 14.1),
3.50 (d, 1H, Ar−CH, J = 14.1), 3.39 (d, 1H, Ar−CH, J = 13.5), 3.03−
3.07 (m, 1H, N−CH), 2.97−3.00 (m, 1H, N−CH), 2.88−2.91 (m,
1H, N−CH), 2.74−2.78 (m, 1H, N−CH), 2.20−2.29 (m, 2H, N−
CH), 1.68−1.98 (m, 8H, CH), 1.40 (s, 9H, t-Bu), 1.27 (s, 9H, t-Bu);
MS (ESI): m/z (%): 450.3579 (100) [HL2 + H]+; elemental analysis:
calculated (found) for C29H43N3O·0.1 CH2Cl2: C 76.29, H 9.50, N
9.17; Found: C 76.57, H 9.51, N 9.25.
a
(i) 2-Pyridinecarboxaldehyde, 1,2-dichloroethane; (ii) H2, 10% Pd/C,
EtOH; (iii) K2CO3, MeCN.
(S,S)-FeL1Cl and (S,S)-FeL2Cl2 were prepared by treating an
acetonitrile solution of the corresponding ligand with FeCl2 in
air, and the metal complexes were isolated in bulk following the
removal of solvents in vacuo. Recrystallization of (S,S)-FeL1Cl
(slow evaporation of acetonitrile) and (S,S)-FeL2Cl2 (slow
diffusion of diethyl ether into dichloromethane) afforded X-ray
suitable crystals in moderate yield. Solution magnetic
susceptibility measurements (1H Evan’s method) revealed
that (S,S)-FeL1Cl (μeff = 5.98) and (S,S)-FeL2Cl2 (μeff
=
5.89) both contain a high spin, S = 5/2 FeIII center. The (R,R)
analogues of these complexes were prepared in a similar
manner, starting with (2R,2′R)-2,2′-bipyrrolidine (Figures S1
and S2, Supporting Information). For the remainder of the
manuscript, FeL1Cl and FeL2Cl2 refers to the (S,S) complexes.
3.2. X-ray Analysis of FeL1Cl and FeL2Cl2. The molecular
structures of FeL1Cl and FeL2Cl2 are presented in Figure 1 and
2.5.3. Synthesis of FeL1Cl. To a solution of ligand H2L1 (340 mg,
0.6 mmol) in MeCN (15 mL) was added FeCl2 (77 mg, 0.6 mmol)
under aerobic conditions. The solution immediately turned purple
upon addition and was stirred at room temperature for 2 h, after which
it was concentrated in vacuo. The purple powder was recrystallized
under an inert atmosphere by slow evaporation of a concentrated
solution of FeL1Cl in MeCN to afford dark purple crystals. Yield: 143
mg, 36%. MS (HRMS): m/z: 666.3599; elemental analysis: calculated
(%) for C38H58N2O2ClFe·MeCN: C 67.88, H 8.69, N 6.10; found: C
67.82, H 8.83, N 6.24. Solution magnetic moment (1H Evan’s
Method): μeff = 5.98.
2.5.4. Synthesis of FeL2Cl2. To a solution of ligand HL2 (110 mg,
0.2 mmol) in MeCN (4 mL) was added FeCl2 (31 mg, 0.2 mmol)
under aerobic conditions. The solution immediately turned dark blue
upon addition and was stirred at room temperature for 2 h, after which
it was concentrated in vacuo. The blue powder was recrystallized under
an inert atmosphere by the diffusion of diethyl ether into a
concentrated solution of FeL2Cl2 in CH2Cl2 to afford dark blue
crystals. Yield: 89 mg, 63%. MS (HRMS): m/z: 574.2041; elemental
analysis: calculated (%) for C29H42N3OCl2Fe·1.2 CH2Cl2/C 53.55, H
6.61, N 6.20; found: C 53.33, H 6.64, N 6.30. Solution magnetic
moment (1H Evan’s Method): μeff = 5.89.
Figure 1. ORTEP plot of FeL1Cl (50% probability) using POV-Ray,
excluding hydrogen atoms. Selected interatomic distances [Å] and
angles [°]: Fe(1)−O(1): 1.865, Fe(1)−O(2): 1.866, Fe(1)−N(1):
2.246, Fe(1)−N(2): 2.132, Fe(1)−Cl(1): 2.217, C(1)−O(1): 1.333,
C(2)−O(2): 1.317; angles: N(1)−Fe(1)−O(1): 88.6, N(1)−Fe(1)−
N(2): 78.0, N(2)−Fe(1)−O(2): 86.2, O(1)−Fe(1)−O(2): 92.9,
N(1)−Fe(1)−O(2): 159.1, N(2)−Fe(1)−O(1): 134.8, N(1)−
Fe(1)−Cl(1): 96.1, N(2)−Fe(1)−Cl(1): 109.0, O(1)−Fe(1)−Cl(1):
115.4, O(2)−Fe(1)−Cl(1): 102.1.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Solid State Characterization of
Ligands and Complexes. (S,S)-H2L1 was prepared by
reported methods.14b,c The synthetic scheme of the dissym-
metric ligand (S,S)-HL2 is presented in Scheme 1. (S,S)-HL2
was prepared starting with the condensation of commercially
available chiral (2S,2′S)-2,2′-bipyrrolidine (1) with 2-pyridine-
carboxaldehyde, where the formation of the aminal inter-
Figure 2 respectively, and select crystallographic data are shown
in Table 1. The solid state structure for FeL1Cl exhibits a
slightly distorted square pyramidal geometry with the expected
N2O2 coordination sphere from the ligand and an apical
chloride ligand, affording an overall neutral complex. The Fe-
amine (Fe1−N1:2.245 Å, Fe1−N2:2.131 Å) bond distances are
in line with other structurally similar 5-coordinate diamine-
bisphenoxide FeIII complexes.9d,11,12 Interestingly, the slight
difference in bond distances between the metal ion and the two
bipyrrolidine amine nitrogens have also been observed for other
square pyramidal bipyrrolidine complexes.14c,29 This is possibly
a result of the sterically demanding ortho tert-butyl moieties,
which distorts the ligand donors from an ideal square pyramidal
1
mediate was confirmed by H NMR, but was not isolated due
to its limited stability. This product was immediately subjected
to hydrogenation conditions to form amine 2, which was then
reacted with 1 equiv of benzyl bromide 3 to afford
dissymmetric ligand (2S,2′S)-[N,N′-(1-(2-hydroxy-3,5-di-tert-
butylphenylmethyl))-2-(pyridylmeth-yl)]-2,2′-bipyrrolidine
((S,S)-HL2).
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Table 2. Selected Crystallographic Data for FeL1Cl and
FeL2Cl2
FeL1Cl
FeL2Cl2·CH2Cl2
formula
C38H58ClN2O2Fe
666.16
H3
C30H44Cl4N3OFe
660.33
P212121
9.0064(9)
14.6395(15)
24.409(2)
90
formula weight
space group
a (Å)
27.55(2)
27.55(2)
12.754(11)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
90
γ (deg)
120
90
V [Å3]
8382(16)
9
3218.3(6)
4
Z, Dcalc [g/cm3]
T (K)
296
296
ρcalcd (g cm−3
λ (Å)
)
1.188
1.363
Figure 2. ORTEP plot of FeL2Cl2 (50% probability) using POV-Ray,
excluding hydrogen atoms and solvent. Selected interatomic distances
[Å] and angles [°]: Fe(1)−N(1): 2.243, Fe(1)−N(2): 2.253, Fe(1)−
N(3): 2.289, Fe(1)−O(1): 1.876, Fe(1)−Cl(1): 2.321, Fe(1)−Cl(2):
2.314, C(1)−O(1): 1.338; angles: N(1)−Fe(1)−Cl(2): 169.4, N(2)−
Fe(1)−Cl(1): 158.2, N(3)−Fe(1)−O(1): 170.6, N(1)−Fe(1)−N(2):
77.2, N(2)−Fe(1)−Cl(2): 92.3, Cl(2)−Fe(1)−Cl(1): 99.5, Cl(1)−
Fe(1)−N(1): 90.9, N(3)−Fe(1)−N(2): 72.7, N(2)−Fe(1)−O(1):
97.9, O(1)−Fe(1)−Cl(1): 99.6, Cl(1)−Fe(1)−N(3): 89.7, N(3)−
Fe(1)−Cl(2): 86.6, Cl(2)−Fe(1)−O(5): 93.5, O(1)−Fe(1)−N(1):
86.5, N(1)−Fe(1)−N(3): 91.7.
0.710 73
0.510
0.710 73
0.828
μ (cm−1
)
a
R indices with I > 2σ(I) (data)
0.0424
0.1096
0.0488
1.015
0.0304
0.0767
0.0361
1.055
wR2
R1
goodness-of-fit on F2
a
Goodness-of-fit on F.
bipyrrolidine backbone, coordinates to the metal in a Λ helical
manner, as previously observed for other metal complexes
employing this enantiomer as the backbone.14b Conversely, the
metal center is coordinated in a Δ helical manner when the
enantiomer of this ligand was prepared using (R,R)-
bipyrrolidine as the backbone (Figure S2, Supporting
Information), demonstrating the induction of chirality from
ligand to metal depending on backbone stereochemistry.14c
The Fe−Namine bond distances are similar to those found for
FeL1Cl and are significantly longer than reported Fe−Nimine
bond distances for Fe(Salen)Cl.30a The Fe1−O1 and C1−O1
bond distances are similar to those in FeL1Cl, supporting the
FeIII-phenoxide formulation. Interestingly, the two Cl ligands
occupy a cis-α conformation; it has been shown that labile
ligands in this orientation are critical to catalytic activity and
stereoselectivity.15b,c
geometry. As expected, these bond distances are longer than
the Fe−Nimine bond distance for Fe(Salen)Cl (2.102, 2.087
Å).30 The Fe1−O1 and Fe1−O2 bond distances (1.864, 1.866
Å) are similar to that of Fe−Ophenoxide in Fe(Salen)Cl (1.882
Å),30 while the C1−O1 bond distance is also typical of a
phenoxide moiety. The donor atoms in FeL1Cl are forced from
the mean equatorial plane by 0.21−0.22 Å, and the metal center
is positioned 0.54 Å above this plane. These values are greater
than those observed in a similar diamine-bisphenoxide
complex,9d,12 suggesting a stronger distortion at the metal
center. This was reflected in the calculated trigonality index31
τ
of 0.41, which was defined by the equation τ = (β − α)/60,
where β is the N1−Fe1−O2 angle and α is the N2−Fe1−O1
angle. Perfectly trigonal bipyramidal and square pyramidal
complexes have τ values of 1 and 0, respectively. The larger τ
value observed for FeL1Cl, in comparison to other 5-coordinate
diamine-bisphenoxide FeIII complexes with backbones such as
ethylenediamine (τ = 0.31)9d,12 or homopiperazine (τ = 0.20)11
is likely a result of the more rigid bipyrrolidine, leading to a
more distorted metal center.
3.3. Theoretical Analysis of FeL1Cl and FeL2Cl2. DFT
calculations were carried out on neutral FeL1Cl and FeL2Cl2.
In both cases, a high spin S = 5/2 FeIII center was predicted to
be the lowest in energy, in line with solution state magnetic
data. The S = 5/2 high spin state was favored over the S = 3/2
intermediate spin state (FeL1Cl: +15 kcal/mol; FeL2Cl2: +18
kcal/mol) and the S = 1/2 low spin state (FeL1Cl: +27 kcal/
mol; FeL2Cl2: +22 kcal/mol). With the exception of the Fe−
Namine bonds, which can be overestimated by DFT
calculations,32 optimized structures of FeL1Cl and FeL2Cl2
The solid state structure of FeL2Cl2 exhibits a slightly
distorted octahedral geometry with the expected N3O
coordination sphere of the ligand and two chloride ligands to
afford an overall neutral complex. The ligand, featuring a (S,S)-
Table 1. Experimental and Calculated (in Parentheses) Coordination Sphere Metrical Parameters for the Fe Complexes (in Å)
amines
complex
FeL1Cl
Fe1−O1
Fe1−O2
Fe1−N1
Fe1−N2
Fe1−N3
Fe1−Cl1
Fe1−Cl2
C1−O1
C2−O2
1.865
(1.857)
(1.824)
1.876
1.866
2.246
(2.334)
(2.219)
2.243
2.132
(2.225)
(2.227)
2.253
2.217
(2.274)
(2.236)
2.321
1.333
(1.340)
(1.347)
1.338
1.317
(1.865)
(2.010)
(1.335)
(1.278)
[FeL1Cl]+
FeL2Cl2
2.289
2.314
(1.859)
(2.014)
(2.340)
(2.337)
(2.341)
(2.289)
(2.335)
(2.202)
(2.320)
(2.268)
(2.314)
(2.259)
(1.332)
(1.276)
[FeL2Cl2]+
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using the B3LYP functional and 6-31G(d) basis set are in good
agreement with the solid state data, predicting coordination
bond lengths to within 0.06 Å (Table 1). In addition, the
slight asymmetry in the coordination sphere for FeL1Cl
observed in the X-ray structure was also well predicted,
where the bulky ortho tert-butyl groups forces the two
phenoxide moieties out of the plane from one another. The
trigonality index of this optimized structure is 0.23, further
supporting a strongly distorted square pyramidal geometry
around the metal center. Of note, a hydrogen bonding
interaction was also predicted between the Cl atom and a
benzylic proton (Cl···H: 2.73 Å), which may further contribute
to the asymmetric coordination sphere.
resonance Raman studies (vide infra). A second irreversible
oxidative process for FeL2Cl2 was observed at ca. 1.4 V at the
limit of the solvent window (Figure S4, Supporting
Information).
In the case of FeL1Cl, two quasi-reversible one-electron
oxidation waves were observed, with the first wave assigned to a
ligand-centered oxidation similarly to FeL2Cl2. The nature of
the second oxidative wave is currently under investigation. A
structurally analogous bisphenoxide FeIII salen complex
reported by Fujii et al. exhibited similar electrochemical
behavior, where two one-electron ligand-based oxidative
waves were observed at 0.85 and 0.96 V vs Fc+/Fc.8 FeL1Cl
displays considerably lower oxidation potentials in comparison
to Fe(Salen)Cl, likely due to the increased electron-donating
nature of the amine moieties in comparison to imines.
The electrochemical data for Fe(Salen)Cl,8 and several
reports of FeIII/FeIV couples at >0.9 V vs Fc+/Fc,33 suggest that
the second oxidative wave of FeL1Cl is likely a ligand-based
oxidation. No further oxidation waves were observed when
scanning toward more oxidizing potentials (Figure S5,
Supporting Information). As such, the difference between the
first and second redox potentials (ΔEox) can provide insight
regarding the amount of electronic coupling between the two
redox-active phenoxide moieties, as well as the degree of
electronic delocalization upon oxidation. Theoretically, in the
absence of electronic coupling, a symmetric complex with two
redox active moieties should exhibit two redox processes
separated by ΔEox = [RT/F] ln 4),34 which is often observed as
a single, two-electron redox couple due to limited resolution.
This was not observed experimentally (Figure 3). The relative
stability toward comproportionation of the monooxidized
species (Kc) can be described by eqs 1−3 using ΔEox.
3.4. Electrochemistry. Redox processes for FeL1Cl and
FeL2Cl2 were probed by cyclic voltammetry (CV) in CH2Cl2
by using tetra-n-butyl-ammonium perchlorate (nBu4NClO4) as
the supporting electrolyte (Figure 3). The redox potentials
••
•
[ML] + [ML ] ⇌ 2[ML]
(1)
(2)
(3)
Figure 3. Cyclic voltammogram of FeL1Cl (black) and FeL2Cl2
n
• 2
(blue). Conditions: 2.5 mM complex, 0.1 M Bu4NClO4, scan rate
[ML]
100 mV s−1, CH2Cl2, 233 K.
Kc =
••
[ML][ML ]
versus ferrocenium/ferrocene (Fc+/Fc) are reported in Table 3.
An irreversible reduction wave was observed for both FeL1Cl
and FeL2Cl2, corresponding to a metal-based reduction from
FeIII to FeII in line with previous reports.10b,e We examined
whether loss of a chloride ligand upon reduction leads to the
irreversible redox process; however, examining the reduction
using a large excess of chloride (Et4NCl as the supporting
electrolyte) did not improve the reversibility of this redox wave.
This result is in contrast to reported 5- or 6-coordinate FeIII
amine-phenolate complexes which display reversible FeIII/FeII
redox waves and is likely due to structural reorganization upon
reduction in this study.10b,e A quasi-reversible one-electron
redox process was observed for FeL2Cl2 at 0.59 V vs Fc+/Fc,
which was assigned to ligand-centered oxidation from
phenoxide to the corresponding phenoxyl radical species
based on comparison to the CV of the ligand HL2 (Figure
S3, Supporting Information) and is further confirmed by
⎛
⎜
⎝
⎞
⎟
⎠
ΔEoxF
RT
Kc = exp
The values of Kc and ΔEox for FeL1Cl, reported in Table 3,
suggest moderate coupling between the two redox-active
phenoxides. This pattern has also been observed for the Cu
complex of H2L1, which exhibits a ΔEox of 0.14 V and a Kc of
1.1 × 103.35 In addition, Fujii et al. have hypothesized that the
increased charge of the metal center (MIII vs MII) leads to a
contraction of the metal d orbital manifold, which in turn limits
delocalization of the ligand radical due insufficient orbital
overlap.6k This is also in good agreement with our experimental
results.
3.5. Electronic Spectroscopy. The electronic absorption
spectra of FeL1Cl and FeL2Cl2 are typical of high spin FeIII
phenoxide complexes (Figure 4).11,12 A characteristic phen-
oxide to FeIII charge transfer band was observed in the visible
a
Table 3. Redox Potentials of Fe Complexes versus Fc+/Fc Peak to Peak Separation in Parentheses
1
2
2
compound
Ered (V)
E1/2 (V)
E1/2 (V)
ΔEox (E1/2 − E1/21) (V)
Kc (233 K)
FeL1Cl
−1.16 (irr.)
0.59 (0.10)
0.85
0.86 (0.13)
0.96
0.27
0.11
6.9 × 105
2.4 × 102
b
Fe(Salen)Cl
FeL2Cl2
−1.01 (irr.)
0.59 (0.10)
a
b
Peak to peak difference for Fc+/Fc couple at 233 K is 0.07 V. Ref 8.
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Table 4. Spectroscopic Properties of the Fe Complexes in
a
CH2Cl2 Solution
complex
FeL1Cl
λmax, cm−1 (ε × 103, M−1 cm−1
29300 br (4.4), 17600 (3.5)
)
[FeL1Cl]+•
FeL2Cl2
25500 (9.0), 18700 sh (2.9), 16700 (2.8), 11100 w (0.52)
25800 br (3.7), 15500 br (3.8)
[FeL2Cl2]+• 24100 (8.3), 21500 (3.6), 19200 (3.0), 17500 (3.1),
14400 (2.1), 12600 sh (1.2), 6900 (1.0)
a
Conditions: 0.6 mM for FeL1Cl and FeL2Cl2, 0.4 mM for [FeL1Cl]+•
and [FeL2Cl2]+•, CH2Cl2, 198 K.
Figure 4. Electronic spectra of (a) FeL1Cl (black), [FeL1Cl]+• (red),
and (b) FeL2Cl2 (black) and [FeL2Cl2]+• (red) in CH2Cl2 at 198 K.
Intermediate gray lines measured during the oxidation titration with
[N(C6H3Br2)3]+•[SbF6]−.
region (FeL1Cl: 17 600 cm−1 (3500 M−1 cm−1)); FeL2Cl2: 15
500 cm−1 (3800 M−1 cm−1)).10b The broad feature at higher
energies is tentatively assigned to an amine to FeIII charge
transfer band.10a,b
Chemical oxidation using aliquots of the aminium radical
oxidant [N(C6H3Br2)3]+•[SbF6]− (Eox = 1.1 V vs Fc+/Fc in
CH2Cl2) under an inert atmosphere at 198 K resulted in new
spectra with isosbestic points (FeL1Cl: 20 600 and 15 300
cm−1; FeL2Cl2: 12 400 cm−1), suggesting direct conversion of
the neutral to the oxidized species. Substantial changes were
observed upon oxidation, most notably the appearance of a new
and intense band in the visible region ([FeL1Cl]+•: 25 500
cm−1 (9000 M−1 cm−1); [FeL2Cl2]+•: 24 100 cm−1 (8300 M−1
cm−1)), which was assigned to a π−π* transition of a phenoxyl
radical species as previously observed for other phenoxyl
radical-containing compounds.10b In addition, a bathochromic
shift and decrease in the intensity of the phenoxide to FeIII CT
band was also observed (FeL1Cl: 17 600 cm−1 (3500 M−1
cm−1); [FeL1Cl]+•: 16 700 cm−1 (2800 M−1 cm−1); (FeL2Cl2:
15500 cm−1 (3800 M−1 cm−1); [FeL2Cl2]+•: 14 400 cm−1
(2100 M−1 cm−1)). A similar pattern of these bands has been
observed for an analogous FeIII-phenoxyl radical complex.10b
Half-lives at room temperature ([FeL1Cl]+•: t1/2 = 6 h;
[FeL2Cl2]+•: t1/2 = 12 min) precluded isolation of the oxidized
species in the solid state for X-ray analysis.
Figure 5. Resonance Raman (rR) spectra of (a) FeL1Cl (black),
[FeL1Cl]+• (red); (b) FeL2Cl2 (black), [FeL2Cl2]+• (red). The rR
spectrum for the neutral complexes were collected in CH2Cl2, and
their one-electron oxidized forms in 10:1 CH2Cl2/MeCN mixture. λex
= 406.7 nm, T = 223 K.
This observation suggests a ligand-based oxidation, further
confirming the formation of FeIII-phenoxyl radical species for
both complexes. A similar series of bands has been observed in
the rR spectrum of a related FeIII-phenoxyl radical com-
plex.10b,36 It should be noted that the phenoxyl radical ν8a band,
which is expected to be found at ca. 1600 cm−1, could not be
detected due to significant fluorescence of the sample (beyond
1600 cm−1).
3.7. Continuous Wave X-band Electron Paramagnetic
Resonance. The perpendicular mode X-band EPR spectra of
FeL1Cl and FeL2Cl2, and their one-electron oxidized forms
[FeL1Cl]+• and [FeL2Cl2]+•, were collected at 10 K in CH2Cl2
with 0.1 M nBu4NClO4 (Figures S6a and S7, Supporting
Information). The EPR spectrum of FeL1Cl display resonances
3.6. Resonance Raman Spectroscopy. Resonance
Raman spectroscopy (rR) of complexes FeL1Cl and FeL2Cl2,
and their one-electron oxidized forms [FeL1Cl]+• and
[FeL2Cl2]+•, were carried out at the excitation wavelength of
406.7 nm (Figure 5). The rR spectra exhibit a new peak upon
oxidation: ([FeL1Cl]+•: 1504 cm−1; [FeL2Cl2]+•: 1501 cm−1),
which we tentatively assign as the phenoxyl radical ν7a band.
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at g effective (geff) values of 2, 3.6, 4.3, 4.9, and 9.4 (Figure 6a).
These features are consistent with a high-spin (S = 5/2) FeIII
[FeL2Cl2]+• are tentatively assigned as integer spin systems
by EPR that arise from antiferromagnetic interactions between
the phenoxyl radical (S = 1/2) and the high spin FeIII (S = 5/2)
center.
3.8. Theoretical Analysis of [FeL1Cl]+• and [FeL2Cl2]+•.
As their short half-lives precluded isolation of the one-electron
oxidized species [FeL1Cl]+• and [FeL2Cl2]+•, DFT calculations
were completed to provide valuable insight into their electronic
structure. High spin (S = 3, ligand radical ferromagnetically
coupled to the high spin FeIII center) and broken symmetry (S
= 2, ligand radical antiferromagnetically coupled to the high
spin FeIII center) solutions were considered. Using the
B3LYP24/ TZVP25 level of theory, the two spin states were
found to be essentially isoenergetic for both complexes, with
the broken symmetry solutions being slightly favored in both
cases (0.03 and 0.7 kcal/mol for [FeL1Cl]+• and [FeL2Cl2]+•,
respectively). In addition, the exchange coupling J was
calculated using the Yamaguchi formula (eq 4), which is
applicable for systems from the strong to weak exchange
limit.37
Figure 6. Parallel mode X-Band EPR spectrum of (a) 0.5 mM solution
of electrochemically generated [FeL1Cl]+• in CH2Cl2 with 0.1 M
nBu4NClO4. Conditions: Frequency, 9.385 GHz; power: 20.8 mW and
(b) 0.5 mM solution of electrochemically generated [FeL2Cl2]+• at 0.5
mM in CH2Cl2 (containing 0.1 M nBu4NClO4). Conditions:
Frequency, 9.390 GHz; power, 8.4 mW; modulation frequency, 100
kHz; amplitude, 1 mT; T = 10 K.
EHS − EBS
J = −
2
2
̂
̂
⟨S ⟩HS − ⟨S ⟩BS
(4)
The computed exchange coupling were −2 cm−1 and −48
cm−1 for [FeL1Cl]+• and [FeL2Cl2]+•, respectively, correspond-
ing to weak antiferromagnetic (S = 2) exchange interaction for
these systems. The strength of coupling is reflected in the
amount of overlap between a metal-based SOMO and a ligand-
based SOMO, which were found to be 23% and 24% for
[FeL1Cl]+• (Figure 7) and [FeL2Cl2]+• (Figure 8), respec-
tively.28 For [FeL2Cl2]+•, the Fe1−O1 bond distance was
predicted to elongate in comparison to the neutral complex
(Table 1). This is indicative of the formation of a neutral
phenoxyl radical upon oxidation, which has decreased electron
donating ability relative to the anionic phenoxide moiety
center, where |D| ≫ hν, as commonly observed for such
complexes. This spectrum was analyzed using rhombograms,
which represents the geff of the three Kramers’ doublets | 1/2>,
| 3/2>, | 5/2> as a function of rhombicity E/D. The E/D
ratio of 0.20 for FeL1Cl signifies medium-to-strong rhombic
distortions and compares well with a structurally similar
complex.9d The EPR spectrum of FeL2Cl2 in CH2Cl2 is
markedly different in comparison to FeL1Cl (Figure S7,
Supporting Information). A dominant signal observed at g =
4.3 indicates larger rhombicity, which was reflected in the E/D
value of 0.33 and is expected for an octahedral high spin FeIII
complex. Electrochemical oxidation of FeL1Cl to [FeL1Cl]+•
induces quenching of the FeIII EPR signal (ca. 90%), likely
reflecting antiferromagnetic coupling between the radical and
FeIII center (Figure S6a, Supporting Information). In addition,
the appearance of a sharp new signal at g = 2.00 indicates a
minor uncoupled phenoxyl radical species (Figures S6a and
S6b, Supporting Information); theoretical calculations predict
elongation of the Fe−O bond upon oxidation to the ligand
radical (vide infra). Electrochemical oxidation of FeL2Cl2 to
[FeL2Cl2]+• also results in quenching of the FeIII EPR signal
(ca. 65%); however, no significant signal at g = 2.00 was
observed for this derivative (Figure S7, Supporting Informa-
tion).
Due to the presence of the prominent signal at g = 4.3 (likely
arising from a high spin FeIII contaminant) that superimposes
on the spectrum of [FeL1Cl]+•, X-band EPR analysis in parallel
mode was explored. Indeed, the signals at geff = 4.1 and 2.0 were
significantly attenuated, while new broad resonances were
observed at ca. 75 mT (g ≈ 9) and at 150−250 mT (Figure 6a).
The same behavior was observed for [FeL2Cl2]+•, which also
exhibited an intense resonance at 75 mT (Figure 6b). Such an
intense signal at 75 mT has been previously reported for an
octahedral FeIII-phenoxyl radical complex, where the signal was
attributed to the |ΔMs| = 2 transition of a (Stotal = 2) system
arising from antiferromagnetic interactions between the ligand
radical and FeIII center.10c Thus, both [FeL1Cl]+• and
Figure 7. Biorthogonalized Kohn−Sham molecular orbitals for the
broken symmetry solution of [FeL1Cl]+•. AOMix MO breakdown in
brackets; see Experimental Section for details.
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to the number of redox-active phenoxides in the complex. Both
complexes were shown to undergo clean one-electron oxidation
using a suitable chemical oxidant as measured by UV−vis-NIR
spectroscopy to afford [FeL1Cl]+• and [FeL2Cl2]+•. The locus
of oxidation was demonstrated to be ligand-based by resonance
Raman spectroscopy with the emergence of the ν7a phenoxyl
radical band. This formulation was well supported by EPR
spectroscopy and theoretical studies, which further revealed the
oxidized species contain a phenoxyl radical antiferromagneti-
cally coupled to the high spin FeIII center. Work involving the
replacement of the chloride ligands to promote catalytic
oxidation reactions is currently ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystal structures of (R,R)-FeL1Cl and (R,R)-FeL2Cl2,
and cif files of all structures, electrochemical data of H2L1 and
HL2, X-band EPR spectra of the neutral and one-electron
oxidized species, and further computational data. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tim_storr@sfu.ca.
■
Notes
Figure 8. Biorthogonalized Kohn−Sham molecular orbitals for the
broken symmetry solution of [FeL2Cl2]+•. AOMix MO breakdown in
brackets; see Experimental Section for details.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by a NSERC Discovery Grant (T.S.),
and a travel grant from the France-Canada Research Fund (T.S.
and F.T.). L.C. thanks NSERC for a Research Exchange Travel
Award. Compute Canada and Westgrid are acknowledged for
access to computational resources. Brian O. Patrick is thanked
for X-ray analysis of (R,R)-FeL2Cl2.
(FeL2Cl2: Fe−O 1.859 Å; [FeL2Cl2]+•: Fe−O 2.014 Å).
Additionally, the C−O bond distances of phenoxide/phenoxyl
moieties are often indicative of the oxidation state of the ligand
due to the formation of a quinoidal ring system upon oxidation.
In this case, a shortening of the C−O bond was predicted upon
oxidation (FeL2Cl2: 1.332 Å; [FeL2Cl2]+•: 1.276 Å), once again
suggesting the formation of a ligand radical species (Table 1).
A similar pattern was observed for [FeL1Cl]+•. Interestingly,
the electronic structure was predicted to contain a localized
phenoxyl radical coupled to the FeIII center and is highlighted
by the predicted changes in C1−O1 (FeL1Cl: 1.340 Å;
[FeL1Cl]+•: 1.347 Å) and C2−O2 bond distances (FeL1Cl:
1.335 Å; [FeL1Cl]+•: 1.278 Å). The shortening of the C2−O2
bond distance signifies phenoxyl radical formation on the ring
system containing C2−O2, while the C1−O1 ring remains
unchanged.
The formation of a localized phenoxyl radical in this
symmetric system concurs with the moderate coupling
observed by CV (Kc value), which has been used previously
to evaluate the degree of delocalization for oxidized bi-
sphenoxide complexes.6f,i,l This localized ligand radical
electronic structure matches the report by Fujii et al., in
which the increased metal charge (MnIII) leads to d-orbital
contraction, limiting delocalization of the ligand radical over the
salen ligand scaffold.6k
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