Synthesis and ligand non-innocence of thiolate-ligated (N4S) iron(II) and nickel(II) bis(imino)pyridine complexes

Article abstract of DOI:10.1021/ic4013558

The known iron(II) complex [Fe^II(LN₃S)(OTf)] (1) was used as starting material to prepare the new biomimetic (N ₄S(thiolate)) iron(II) complexes [Fe^II(LN ₃S)(py)](OTf) (2) and [Fe^II(LN₃S)(DMAP)](OTf) (3), where LN₃S is a tetradentate bis(imino)pyridine (BIP) derivative with a covalently tethered phenylthiolate donor. These complexes were characterized by X-ray crystallography, ultraviolet-visible (UV-vis) spectroscopic analysis, ¹H nuclear magnetic resonance (NMR), and Moessbauer spectroscopy, as well as electrochemistry. A nickel(II) analogue, [Ni^II(LN₃S)](BF₄) (5), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed 1-3 and 5 undergo a single reduction process with E_1/2 between -0.9 V to -1.2 V versus Fc⁺/Fc. Treatment of 3 with 0.5% Na/Hg amalgam gave the monoreduced complex [Fe(LN₃S)(DMAP)] ⁰ (4), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron paramagnetic resonance (EPR) spectroscopy (g = [2.155, 2.057, 2.038]), and Moessbauer (δ = 0.33 mm s^-1; ΔE_Q = 2.04 mm s^-1) spectroscopy. Computational methods (DFT) were employed to model complexes 3-5. The combined experimental and computational studies show that 1-3 are 5-coordinate, high-spin (S = 2) Fe^II complexes, whereas 4 is best described as a 5-coordinate, intermediate-spin (S = 1) Fe^II complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (S_total = 1/2) ground state. Complexes 2 and 3 are shown to react with O₂ to give S-oxygenated products, as previously reported for 1. In contrast, the monoreduced 4 appears to react with O ₂ to give a mixture of sulfur oxygenates and iron oxygenates. The nickel(II) complex 5 does not react with O₂, and even when the monoreduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O₂.

Full text of DOI:10.1021/ic4013558

Article
pubs.acs.org/IC
Synthesis and Ligand Non-Innocence of Thiolate-Ligated (N4S)
Iron(II) and Nickel(II) Bis(imino)pyridine Complexes
Leland R. Widger,† Yunbo Jiang,† Maxime A. Siegler,† Devesh Kumar,⊥ Reza Latifi,§ Sam P. de Visser,*,§
Guy N. L. Jameson,*,‡ and David P. Goldberg*,†
†Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
‡Department of Chemistry, and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, PO Box 56,
Dunedin 9054, New Zealand
§Manchester Institute of Biotechnology and School of Chemical Engineering and Analytical Science, The University of Manchester,
131 Princess Street, Manchester M1 7DN, United Kingdom
⊥Department of Applied Physics, School for Physical Sciences, Babasaheb Bhimrao Ambedkar University, Vidya Vihar, Rae Bareilly
Road, Lucknow (U. P.) 226 025, India
S
* Supporting Information
ABSTRACT: The known iron(II) complex [FeII(LN3S)-
(OTf)] (1) was used as starting material to prepare the new
biomimetic (N4S(thiolate)) iron(II) complexes [FeII(LN3S)-
(py)](OTf) (2) and [FeII(LN3S)(DMAP)](OTf) (3), where
LN3S is a tetradentate bis(imino)pyridine (BIP) derivative with
a covalently tethered phenylthiolate donor. These complexes
were characterized by X-ray crystallography, ultraviolet−visible
1
(UV-vis) spectroscopic analysis, H nuclear magnetic reso-
nance (NMR), and Mossbauer spectroscopy, as well as
̈
electrochemistry. A nickel(II) analogue, [NiII(LN3S)](BF4)
(5), was also synthesized and characterized by structural and
spectroscopic methods. Cyclic voltammetric studies showed 1−3 and 5 undergo a single reduction process with E1/2 between
−0.9 V to −1.2 V versus Fc+/Fc. Treatment of 3 with 0.5% Na/Hg amalgam gave the monoreduced complex
[Fe(LN3S)(DMAP)]0 (4), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron
paramagnetic resonance (EPR) spectroscopy (g = [2.155, 2.057, 2.038]), and Mossbauer (δ = 0.33 mm s−1; ΔE = 2.04 mm
̈
Q
s−1) spectroscopy. Computational methods (DFT) were employed to model complexes 3−5. The combined experimental and
computational studies show that 1−3 are 5-coordinate, high-spin (S = 2) FeII complexes, whereas 4 is best described as a 5-
coordinate, intermediate-spin (S = 1) FeII complex antiferromagnetically coupled to a ligand radical. This unique electronic
configuration leads to an overall doublet spin (Stotal = 1/2) ground state. Complexes 2 and 3 are shown to react with O2 to give S-
oxygenated products, as previously reported for 1. In contrast, the monoreduced 4 appears to react with O2 to give a mixture of
sulfur oxygenates and iron oxygenates. The nickel(II) complex 5 does not react with O2, and even when the monoreduced nickel
complex is produced, it appears to undergo only outer-sphere oxidation with O2.
exogenous phenylthiolate (PhS−) ligand to the iron(II) center,
INTRODUCTION
■
yielding FeII(N3S) complexes in both cases. These complexes
were among the first mononuclear thiolato-iron(II) complexes
to selectively react with O2 to give S-oxygenated products,
reproducing the gross reactivity of the enzymatic system.
However, the Cys-bound iron(II) form of CDO contains an
iron center held in an (N4S) donor set, comprised of three His
ligands and one Cys substrate chelated through both the amino
and side-chain S groups. Thus, we are currently interested in
the design and synthesis of FeII(N4S) complexes in which a
fourth neutral N donor is included in the first coordination
Thiolate-ligated non-heme iron sites play key roles in biology,
exhibiting diverse structural features as well as contributing to a
wide range of reactivity.1−7 The mononuclear, thiolate-ligated
non-heme iron enzymes superoxide reductase,8−12 nitrile
hydratase,13−15 and cysteine dioxygenase (CDO)16−23 utilize
single iron sites coordinated in mixed N/S ligand environ-
ments. Work by some of us has focused on the preparation of
synthetic analogues of these sites, including some of the first
structural and functional models of CDO.1,24−33 A bis(imino)-
pyridine (BIP) ligand scaffold was employed, leading to the
synthesis of FeII(LN3S) complexes that mimicked certain
structural and functional features of CDO.24,25 The key thiolate
donor was incorporated into these complexes by either covalent
attachment to the BIP backbone, or by addition of an
Received: May 29, 2013
© XXXX American Chemical Society
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sphere to more closely match the active site of CDO prior to S-
oxygenation.
Herein, we describe the synthesis, spectroscopic character-
ization and O2 reactivity of a new series of FeII(N4S) complexes
(Figure 1) that contain a covalently linked, BIP-derived ligand
metal can be employed together for the activation of O2, and
how this combination will affect subsequent sulfur- or metal-
based oxygenation steps.
The new complexes [FeII(LN3S)(py)]OTf (2), [FeII(LN3S)-
(DMAP)]OTf (3) and [NiII(LN3S)]BF4 (5) were shown to
accept a single electron to give the respective monoreduced
products. In one case, successful crystallization of the
monoreduced species was achieved, and the structure of
[Fe(LN3S)(DMAP)]0 (4) was obtained by X-ray crystallo-
graphy. A detailed experimental and computational (DFT)
investigation into the electronic and structural properties of 4,
as well as the other monoreduced analogues, was conducted,
revealing several insights regarding the interplay between the
structures and the electronic configurations of these complexes.
The influence of the strong field thiolate donor, the extent of
non-innocence in the BIP ligand, and the oxidation and spin
state of the metal center are addressed. A major finding from
these studies is that complex 4, based on experimental and
computational results, is a monoreduced bis(imino)pyridine
complex with an intermediate-spin iron(II) center coupled with
a stable ligand-based radical.
The O2 reactivity of these complexes was also examined, and
for the starting iron(II) complexes 2 and 3, selective S-
−
oxygenation to give sulfonato (RSO3 ) products was observed,
as seen previously for [FeII(LN3S)(OTf)] (1).24 In contrast,
the monoreduced complex 4 reacts with O2, leading to a
mixture of products that suggest both iron oxygenation and
sulfur oxygenation readily occur for this monoreduced species.
However, both the NiII complex 5, and its monoreduced
analog, do not exhibit any sulfur-based or metal-based
oxygenation in the presence of O2, even after prolonged
reaction times. This lack of reactivity is in contrast to the iron
analogues, as well as some nickel-thiolate complexes that do
react with O2 to give sulfur-oxygenated sulfinato and sulfonato
products, although these reactions can be sluggish.46−54
Figure 1. Complexes described in the present study.
providing an N3S(thiolate) donor set (LN3S−), and a fourth N
donor derived from exogenous pyridine derivatives. The
general synthetic strategy was to incorporate a fourth N
donor without the need for covalent modification of the BIP
backbone. This strategy proved productive, and the new
iron(II) complexes [FeII(LN3S)(py)](OTf) (2) and
[FeII(LN3S)(DMAP)](OTf) (3) were synthesized and charac-
terized by X-ray crystallography, NMR, electrochemistry,
EXPERIMENTAL SECTION
■
Mossbauer spectroscopy and density functional theory (DFT)
̈
calculations. A nickel(II) analogue, [NiII(LN3S)](BF4) (5), was
also prepared for comparative spectroscopic and reactivity
studies.
General Procedures. All reagents were purchased from
commercial vendors and used without further purification, unless
noted otherwise. All reactions were carried out under an atmosphere
of N2 inside a glovebox or under an argon atmosphere by standard
Schlenk and vacuum line techniques. Ultraviolet−visible (UV-vis)
spectra were recorded on an Agilent 8453 photodiode array
spectrophotometer. Electron paramagnetic resonance (EPR) spectra
were obtained on a Bruker EMX EPR spectrometer that was
controlled using a Bruker ER 041 X G microwave bridge at 15 K.
The EPR spectrometer was equipped with a continuous-flow liquid
helium cryostat and an ITC503 temperature controller made by
Oxford Instruments, Inc. Nuclear magnetic resonance (NMR) was
performed on a Bruker Avance 400 MHz FT-NMR spectrometer at 25
°C. Elemental analysis was performed by Atlantic Microlab, Inc.
(Norcross, GA). LDI-ToF mass spectra were obtained using a Bruker
Autoflex III Maldi ToF/ToF instrument (Billerica, MA). Samples were
dissolved in CH2Cl2 and deposited on the target plate in the absence
of any added matrix. Samples were irradiated with a 355-nm UV laser
and mass-analyzed by ToF mass spectrometry in reflectron mode.
FeII(LN3S)(OTf) (1) was prepared according to a literature
procedure.24 57Fe Mossbauer spectra were recorded on a Mossbauer
The BIP platform is also well-known to behave as a non-
innocent ligand, allowing for the storage of multiple reducing
equivalents on the ligand backbone. The BIP framework has
been shown to accept up to three electrons, which are stabilized
by delocalization over the conjugated ligand scaffold.34−45
However, the reduced analogues of these BIP complexes are
generally restricted to derivatives in which only minor changes
to the aryl substitution patterns on the imino positions have
been made, with no major modification of the chelating abilities
of the BIP framework. There is significant interest in the
development of non-innocent, polydentate ligands for their use
as redox-active partners in combination with metal ions to assist
with various chemical transformations such as the reduction of
small-molecules (e.g., CO2, N2) and other redox reactions.
With the thiolate-modified LN3S system in hand, we thus
wanted to determine if it could also function as a non-innocent
ligand, perhaps stabilizing one or more reducing equivalents
across the ligand backbone. A question to be addressed
concerns the influence of the additional, covalently linked
phenylthiolate arm on the stability, spin delocalization, and
spectroscopic and redox properties of possible (multi)reduced
ligand−metal complexes. It is also of interest to determine if
multiple reducing equivalents stored on both the ligand and the
̈
̈
spectrometer from SEE Co. (Science Engineering & Education Co.,
Edina, MN) equipped with a closed-cycle refrigerator system from
Janis Research Co. and Sumitomo Heavy Industries, Ltd. (SHI)).
Approximately 10 mg of sample was placed in a custom sample holder
made from Teflon and specifically designed for solid samples. Data
were collected in constant acceleration mode in transmission geometry
with an applied field of 47 mT parallel to the γ-rays. The zero velocity
of the Mossbauer spectra refers to the centroid of the room
̈
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temperature spectrum of a 25 μm metallic iron foil. Analysis of the
spectra was conducted using the WMOSS program (SEE Co., formerly
WEB Research Co., Edina, MN).
(17 μL, 0.155 mmol) and triethylamine (22 μL, 0.155 mmol) in EtOH
(1 mL) was added to the reaction mixture. The reaction was allowed
to stir at 80 °C for 24 h before being cooled to room temperature and
concentrated. The resulting solid residue was brought into a glovebox
where it was redissolved in a minimum amount of CH2Cl2 and filtered
through Celite. Slow vapor diffusion of diethyl ether into the filtrate
yielded 96 mg (94% yield) of 5 as dark red crystals suitable for X-ray
analysis, together with colorless crystals of Et3NH·BF4. Samples for
elemental analysis were prepared by manual separation of crystals of 5
from the colorless Et3NH·BF4 crystals. 1H NMR (CD2Cl2): δ 8.30 (br,
1H), 7.89 (br, 2H), 7.35 (t, 1H), 7.22 (d, 2H), 7.09 (d, 1H), 6.92 (t,
1H), 6.82−6.72 (m, 2H), 3.18 (br), 2.69 (s, 3H), 2.14 (s, 3H), 1.50 (d,
6H), 1.33 (br), (1.17 (d, 6H). LDI-MS (+): m/z 488.3 [5-BF4]+. Anal.
Calc. for [NiII(LN3S)](BF4)·CH2Cl2 (C28H32BCl2F4N3NiS): Pre-
dicted: C, 51.03; H, 4.89; N, 6.38. Found: C, 51.28; H 4.76; N, 6.66.
O2 Reactivity. Reactions of 2, 3, and 5 with excess O2 were
conducted as previously reported for 1 in CH2Cl2.24 The one-electron-
reduced complexes were reacted with O2 by freshly preparing solutions
of 4 or reduced 5 via Na/Hg amalgam in Et2O, followed by filtration
through Celite, as described for the synthesis of 4. Reaction mixtures
following O2 addition were then analyzed directly by LDI-MS.
Computational Methods. Density functional theory calculations
were performed on complexes 3−5, using methods and procedures
that we have tested and calibrated previously.55,56 The complexes were
calculated in several possible spin states. The spin state ordering of
transition-metal complexes can be dependent on the density functional
theory (DFT) functional chosen, and thus we tested several different
methods for our calculations. In an initial set of calculations, the
unrestricted hybrid density functional method B3LYP was used in
combination with an LACVP basis set on the metal and 6-31G on the
rest of the atoms (referenced as basis set B1) for geometry
optimizations and frequencies.57−59 Optimized structures were
employed as the starting point for single-point calculations using (a)
the LACV3P+ basis set on the metal and 6-311+G* on the rest of the
atoms; (b) the M06/B1 method;60 and (c) B3LYP/B1 with the
polarized continuum model with a dielectric constant of ε = 5.7. In a
second set of calculations, we used the unrestricted BP86 density
functional method61,62 in combination with an LACV3P+ basis set on
the metal and 6-311+G* on the rest of the atoms (referenced as basis
set B2). Full geometry optimizations were run with the Jaguar 7.6
program, followed by a frequency calculation using these methods.63
Single-point calculations with dispersion-corrected DFT were
performed as proposed by Schwabe and Grimme;64 however, this
had a negligible effect on the spin state ordering and relative energies.
Synthesis of [FeII(LN3S)(py)]OTf (2). Crystalline [FeII(LN3S)-
(OTf)] (1) (100 mg, 0.16 mmol) was dissolved in CH2Cl2 and
pyridine (15 μL, 0.19 mmol) was added. After stirring for 2 h, the
solution was filtered through Celite. Slow vapor diffusion of diethyl
ether or layering with pentane gave the desired product as dark brown
plates suitable for X-ray diffraction (XRD) analysis in 97% yield (110
mg). 1H NMR (CD2Cl2): δ 107.0, 95.4, 91.6, 45.2, 13.1, 11.6, 3.1, 1.3,
−4.1, −10.3, −16.2, −30.4. LDI-MS (+): m/z 484.2 ([2−py−OTf]+).
Anal. Calc. for FeII(LN3S)(py)(OTf)·CH2Cl2 (C34H37Cl2-
F3FeN4O3S2): C, 51.20; H, 4.68; N, 7.02. Found: C, 51.21; H, 4.88;
N, 7.67.
RESULTS AND DISCUSSION
■
Synthesis of Iron(II) and Nickel(II) Complexes. The
synthesis of the unsymmetrical, thiolate-ligated complex
[FeII(LN3S)(OTf)] (1) was accomplished by reacting the
imino-ketone precursor65 2-(OCMe)-6-(2,6-(iPr2−
C6H3NCMe)-C5H3N with the commercially available 2-
aminothiophenol and Fe(OTf)2 in a metal-assisted template
reaction as previously described.24 We sought to determine if
this complex could be used as a precursor to generate FeII
complexes with biologically relevant [N4S] donor sets via
substitution at the OTf− position. Displacement of the OTf−
ligand in 1 was an attractive route when compared to the more
challenging strategy involving multistep organic synthesis to
produce covalently linked N4S-donating ligands.26,66−68 The
addition of 1 equiv of either pyridine or N,N′-dimethylamino-
pyridine (DMAP) to dark red-purple 1 in CH2Cl2 at room
temperature does not result in any color change. However,
vapor diffusion of diethyl ether into the reaction mixture after
stirring for 2 h afforded single crystals as dark brown plates of
the pyridine-ligated [FeII(LN3S)(py)](OTf) (2), and black
plates in the case of the DMAP-ligated [FeII(LN3S)(DMAP)]-
(OTf) (3). These crystals yielded the X-ray structures of 2 and
3 (Figures 2 and 3, vide infra), confirming that the OTf− anion
was displaced by the pyridine derivatives. For large-scale
preparations of 2 and 3, layering of pentane was used in place
of vapor diffusion of Et2O to give both 2 and 3 in high yield.
Synthesis of [FeII(LN3S)(DMAP)]OTf (3). Crystalline
[FeII(LN3S)(OTf)] (1) (110 mg, 0.17 mmol) was dissolved in
CH2Cl2 and DMAP (25 mg, 0.21 mmol) was added. After stirring for
2 h, the solution was filtered through Celite. Slow vapor diffusion of
diethyl ether or layering with pentane gave 3 as black plates suitable
for XRD analysis in 95% yield (125 mg). 1H NMR (CD2Cl2): δ 128.0,
102.4, 92.8, 37.6, 19.4, 10.4, 8.4, 3.1, 1.4, −4.9, −7.7, −10.4, −13.7,
−22.8. LDI-MS (+): m/z 484.2 ([3−DMAP−OTf]). Anal. Calc. for
FeII(LN3S)(DMAP)(OTf) (C35H40F3FeN5O3S2): C, 55.63; H, 5.34;
N, 9.27. Found: C, 55.37; H, 5.29; N, 9.07.
Synthesis of [FeII(LN3S)(DMAP)]·2Et2O (4·2Et2O). Crystalline
[FeII(LN3S)(DMAP)](OTf) (3) (125 mg, 0.17 mmol) was combined
with 1.5 equiv of 0.5% Na/Hg amalgam (6.0 mg Na0, 1.15 g Hg) and
suspended in 5 mL of Et2O. Upon stirring, the insoluble starting
material (3) slowly dissolves into solution as it reacts, giving a dark
green/brown solution. After stirring for 2 h, the reaction mixture was
filtered through Celite, layered with pentane, and placed in the freezer
for several days, giving 4·2Et2O as dark brown crystals suitable for
XRD in 37% yield (38 mg). UV-vis (THF) (nm): 566, 625, 785 (ε =
890 M−1 cm−1). 1H NMR (THF-d8): δ 95.7, 33.8, 24.7, 5.8. EPR: g =
2.155, 2.057, 2.038. Mossbauer: Γ = 0.8 mm s−1, δ = 0.33 mm s−1,
̈
L=R
ΔEQ = 2.04 mm s−1. Anal. Calc. for [4 + Et2O + 0.25 NaOTf]
(C38.25H50F0.75FeN5Na0.25O1.75S1.25): C, 63.48; H, 6.96; N, 9.68.
Found: C, 62.85; H, 6.74; N, 9.73. Crystalline samples of 4·2Et2O
coprecipitate with a small amount of white solid impurity that could
not be separated from 4·2Et2O. The impurity was identified as NaOTf
by 19F NMR (see Figure S2). Inclusion of 0.25 equiv of NaOTf was
necessary to obtain a good fit for elemental analysis.
Synthesis of [NiII(LN3S)]BF4 (5). An amount of 2-(OCMe)-6-
(2,6-(iPr2−C6H3NCMe)-C5H3N (50 mg, 0.155 mmol) and
NiII(BF4)2·6H2O (55 mg, 0.163 mmol) was suspended in EtOH (5
mL) and heated at 80 °C for 30 min. The solids slowly dissolved to
give a deep green solution, and then a solution of 2-aminothiophenol
Figure 2. Displacement ellipsoid plot for the cation of complex 2 (50%
probablility level). The H atoms are removed for clarity.
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shorter than the imine N−Fe distances in both 2 and 3,
whereas the Fe−S bonds are nearly identical. There is minimal
effect of substitution of the para position of the pyridine
derivatives on the bond lengths in 2 and 3. An analysis of the
bond angles for 2 and 3 and their τ values (τ = 0.15 for 2, and τ
= 0.18 for 3; where τ = 0 for square pyramidal and τ = 1 for
trigonal bipyramidal)69 show that, in both cases, the FeII ions
are held in a distorted square pyramidal geometry, with the
pyridine derivatives occupying the axial position. The molecular
structure of 1 has been reported,24 and selected bond distances
are reproduced in Table 1. The Fe−N distances are very similar
to those found in 2 and 3, whereas the Fe−S distance appears
slightly shorter. Complex 1 exhibits a more idealized square
pyramidal geometry than 2 or 3, with τ = 0.12, and a
comparison of the τ values for 1−3 reveals that the geometry
becomes more distorted from square pyramidal with increasing
axial ligand donor strength (τ(OTf) = 0.12 < τ(py) = 0.15 <
τ(DMAP) = 0.18). In all three complexes 1−3, the pyridine
diimine backbone remains planar, while the ferrous ion sits
0.276 Å above the N3 plane for 1, but less out of plane for 2
(0.116 Å) and 3 (0.200 Å). In addition, the thiolate donor
never sits in the ideal equatorial position, but rather lies well
below the N3 plane. For 1 and 2, the Nplane−S distances are
0.824 and 1.18 Å, respectively, while for 3, it is significantly
more distorted with d(Nplane−S) = 1.8 Å. The Cimine−N and
Cimine−Cipso bond distances for the ligand are similar for
complexes 1−3 (Table 1), and are consistent with a neutral-
ligand bis(imino)pyridine backbone (LN3S−).34,43
Figure 3. Displacement ellipsoid plot for the cation of complex 3 (50%
probablility level). The H atoms are removed for clarity.
The synthesis of the NiII analogue of 1 was also targeted for
comparison of spectroscopic features and its reactivity toward
O2. The use of NiII(BF4)2 in place of FeII(OTf)2 in Scheme 1
results in a dark green solution. Vapor diffusion of Et2O
afforded dark red crystals, together with clusters of large
colorless needles. The red crystals were separated by hand and
analyzed by X-ray crystallography, revealing the NiII complex
[NiII(LN3S)](BF4) (5). The colorless crystals were presumed
to be Et3NH+BF4 , the expected byproduct of the template
−
reaction. Peaks corresponding to Et3NH+BF4 were confirmed
−
in the 1H NMR of 5. The coprecipitation of Et3NH+BF4− could
not be avoided even upon successive recrystallizations;
therefore, it was necessary to isolate complex 5 by manual
separation of crystalline material for elemental analysis and
further reactivity studies.
The X-ray structure for the NiII complex [NiII(LN3S)](BF4)
(5) is shown in Figure 4. Selected bond distances and angles for
5 are given in Table 3. The molecular structure of 5 shows a
square planar geometry for the NiII ion. The observed Ni−N
distances are 1.8151(14)−1.9159(15) Å, and are somewhat
shorter (∼0.1−0.4 Å) than other BIP-derived nickel(II)
complexes.45,70−73 The Ni−S distance of 2.1386(5) Å is
significantly shorter than the FeII−S distances seen for 1−3, as
are the Ni−N distances. The bond angles about the NiII center
deviate slightly from the idealized square-planar geometry
(82.44°−102.37°), likely due to the constraints imposed by the
three 5-membered chelate rings. The NiII ion and S atom sit
directly in the plane of the bis(imino)pyridine ligand as seen by
Ni−N3(plane) and S−N3(plane) distances of 0.030 Å and
0.082 Å, respectively. This structure contrasts those found for
1−3, where both the metal ion and sulfur donor lie significantly
X-ray Structures of the Iron(II) and Nickel(II) Com-
plexes. The X-ray structures for the iron(II) complexes
[FeII(LN3S)(py)](OTf) (2) and [FeII(LN3S)(DMAP)](OTf)
(3) are shown in Figures 2 and 3. Selected bond distances and
angles are given in Tables 1 and 2. The structures reveal that
the triflate ligand has been displaced in each case by the
heterocyclic N donor to afford the 5-coordinate FeII(N4S-
(thiolate)) products. There is a clear preference for forming the
5-coordinate complexes over the 6-coordinate species, in which
the OTf− would be retained in the first coordination sphere.
The Fe−N distances of 2.073(5)−2.196(5) Å for 2 and
2.0667(14)−2.2179(15) Å for 3 are indicative of high-spin (S =
2) FeII complexes. The pyridine-derived Fe−N bonds are
Scheme 1. Synthesis of Iron(II) Complexes (1−3), and Nickel(II) Complex (5)
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a
Table 1. Selected Bond Distances for [Fe(LN3S)]n+ Complexes
Bond Distances (Å)
a
[FeII(LN3S)(OTf)]
[FeII(LN3S)(py)]OTf
[FeII(LN3S)(DMAP)]OTf
[FeII(LN3S)(DMAP)]0
Fe1−N1
Fe1−N2
Fe1−N3
Fe1−N4
Fe1−S1
Fe−O1
N1−C7 (imine)
C7(imine)− C9(ipso)
C9(ipso)−N2(pyr)
N3−C14(imine)
C14(imine)−C13(ipso)
C13(ipso)−N2(pyr)
2.188(2)
2.072(2)
2.184(2)
N/A
2.173(5)
2.090(5)
2.196(5)
2.073(5)
2.3246(18)
N/A
2.1859(15)
2.0960(14)
2.2179(15)
2.0667(14)
2.3295(5)
N/A
1.9255(15)
1.8278(15)
1.9179(16)
2.0508(16)
2.2179(5)
N/A
2.2942(8)
2.0870(18)
1.293(3)
1.480(4)
1.343(3)
1.284(3)
1.483(4)
1.334(3)
1.298(8)
1.485(9)
1.342(8)
1.275(7)
1.494(8)
1.334(8)
1.292(2)
1.486(2)
1.341(2)
1.290(2)
1.487(3)
1.343(2)
1.347(2)
1.432(3)
1.376(2)
1.341(2)
1.426(3)
1.381(2)
a
Data taken from ref 24.
Table 2. Selected Bond Angles for [Fe(LN3S)]n+ Complexes
Table 3. Selected Bond Distances and Angles for Complex 5
Bond Angles (deg)
Bond Distances (Å)
[FeII(LN3S)
(py)]OTf
[FeII(LN3S)
[FeII(LN3S)
(DMAP)]0
Ni1−N1
Ni1−N2
Ni1−N3
Ni1−S1
N1−C7 (imine)
C7(imine)−C9(ipso)
C9(ipso)−N2(pyr)
N3−C14(imine)
C14(imine)−C13(ipso)
C13(ipso)−N2(pyr)
1.8981(15)
1.8151(14)
1.9159(15)
2.1386(5)
1.312(2)
1.480(2)
1.341(2)
1.299(2)
1.480(2)
1.338(2)
(DMAP)]OTf
N4−Fe1−N2
N4−Fe1−N1
N2−Fe1−N1
N4−Fe1−N3
N2−Fe1−N3
N1−Fe1−N3
N4−Fe1−S1
N2−Fe1−S1
N1−Fe1−S1
N3−Fe1−S1
109.79(19)
95.3(2)
112.91(6)
99.69(6)
73.08(5)
99.82(6)
73.94(6)
146.17(5)
107.20(4)
135.18(4)
81.40(4)
118.28(4)
99.92(6)
95.84(6)
81.26(7)
99.88(6)
80.65(7)
157.78(7)
105.25(5)
153.58(5)
88.27(5)
102.49(5)
74.0(2)
99.29(19)
73.4(2)
147.24(19)
106.05(15)
138.08(14)
81.79(15)
121.28(13)
Bond Angles (deg)
N2−Ni1−N1
N2−Ni1−N3
N1−Ni1−N3
N2−Ni1−S1
N1−Ni1−S1
N3−Ni1−S1
83.56(6)
82.44(6)
165.91(6)
175.17(5)
91.64(5)
102.37(5)
para pyridyl backbone protons, with the most intense
resonance (82.8, 95.4, and 92.8 ppm for 1−3, respectively)
likely arising from the meta protons that are apparently not split
in the local symmetry. We tentatively assign the third resonance
to a proton on the phenylthiolate ring. No peaks appear in the
spectrum for 1 between 80 ppm and 20 ppm, whereas complex
2 shows a broad peak at 45.2 ppm and complex 3 exhibits a
sharp peak at 37.6 ppm and a broad feature at 43.5 ppm. The
latter peaks for 2 and 3 can be assigned to the axial pyridine
ligands, which are absent in 1. This assignment was confirmed
by the generation in situ of 2 with the addition of 1 equiv of
pyridine-d5 to 1. As seen in Figure 5, the spectrum for
deuterated 2-d5 is a good match for 2, except for the
disappearance of the resonance at 45.2 ppm. The broadness
of the py peak and the absence of distinct signals for the ortho,
meta, and para protons suggest that the py ligand in 2 is labile
on the NMR time scale. In contrast, two peaks are observed for
3 in the region of the axial DMAP donor, and one is relatively
sharp, suggesting that DMAP is significantly less labile than py;
this observation is consistent with it being a more strongly
donating py derivative. Further upfield, the diisopropyl methyl
groups can easily be assigned to the intense singlets between
1.20 ppm and 1.36 ppm for 1−3, and the peaks found from
−14 ppm to −16 ppm are assigned to the para C−H on the
Figure 4. Displacement ellipsoid plot for the cation of complex 5 (50%
probablility level). The H atoms are removed for clarity.
out of the bis(imino)pyridine plane for these 5-coordinate
complexes.
1
NMR Spectroscopy. The paramagnetic H NMR spectra
for complexes 1−3 in CD2Cl2 are shown in Figure 5. Each
complex exhibits sharp, paramagnetically shifted peaks over a
broad range (from 130 ppm to −30 ppm), indicative of high-
1
spin (S = 2) FeII complexes. Detailed H NMR assignments
have been made for bis(imino)pyridine iron(II) complexes that
carry the same aryl imino substituents.34,65,74,75 These data can
be used as a guide for interpreting the spectra for 1−3, although
1
the unsymmetrical structures of 1−3 make their H NMR
patterns difficult to definitively assign. For all three complexes,
the downfield region (130−80 ppm) contains three sharp
peaks, with the exception of 3, in which the most downfield
resonance at 130 ppm is significantly broadened. In comparison
to symmetrical (BIP)FeX2 complexes, two of the three
resonances in this region can be assigned to the meta and
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1
Figure 5. H NMR spectra of complexes 1−3 in CD2Cl2 at 25 °C.
iPr2Ph substituent based on earlier work.34,65,74,75 The upfield
resonances at −21.9, −30.3, and −22.8 ppm for 1, 2, and 3,
respectively, likely arise from the isopropyl methine protons.
The 1H NMR spectrum of the diamagnetic Ni(II) complex 5
is shown in Figure 6. The peaks for the pyridyl protons are seen
Electrochemistry. Cyclic voltammetry of the iron(II) and
nickel(II) complexes was measured to gain insights regarding
the influence of the exogenous py derivatives and covalently
tethered phenylthiolato donor on the redox properties of these
complexes. The cyclic voltammograms for compounds 1, 2, 3,
and 5 are shown in Figure 7. The triflate complex 1 and the
Figure 7. Cyclic voltammograms of compounds 1−3 and 5 in MeCN.
E1/2: 1 = −1.1 V, 2 = −1.1 V, 3 = −1.2 V, and 5 = −0.9 V; ΔEp: 1 =
120 mV, 2 = 130 mV, 3 = 130 mV, and 5 = 66 mV. Working electrode,
glassy carbon; counter electrode, platinum; and reference electrode,
Ag/Ag+. Electrolyte: LiClO4, 100 mM. Scan rate = 100 mV/s.
Figure 6. 1H NMR spectrum of 5 in CD2Cl2 at 25 °C. The
−
Et3NH+BF4 impurity signal is denoted by an asterisk (*).
pyridine complex 2 exhibit quasireversible waves at E1/2 = −1.1
V, while the DMAP complex 3 reveals a more negative E1/2
value (−1.2 V). No other obvious redox processes for these
complexes are present within the solvent window for CH3CN.
The fact that the E1/2 values for 1 and 2 are the same suggests
that the axial pyridine donor in 2 does not exhibit any influence
on the redox potential, but is also consistent with the pyridine
ligand dissociating to some extent in solution, as indicated by
the NMR data. Substitution of py for DMAP does have a clear
impact, shifting the redox potential by ∼100 mV more negative
in 3, compared to 2. This result is consistent with DMAP being
a more strongly donating ligand than unsubstituted pyridine.
at δ 8.30 and 7.88, and the diisopropylphenyl peaks are
assigned to δ 7.35 (t, 1H) and 7.22 (d, 2H). Other peaks in the
aromatic region include δ 7.09 (d, 1H), 6.92 (t, 1H), and
overlapping peaks at 6.79 (d, 1H) and 6.75 (t, 1H), which can
be attributed to the phenylthiolate ring. The peaks found at
−
3.18 and 1.33 ppm come from a small amount of Et3NH+BF4
impurity (vide supra), and the peak at 3.18 ppm likely masks
the isopropyl methine C−H peaks for 5. The ketimine CH3
resonances are located at δ 2.70 (s, 3H) and 2.14 (s, 3H).
There are two distinct isopropyl CH3 peaks found at δ 1.50 (d,
6H) and 1.17 (d, 6H), indicating restricted rotation of the
diisopropylphenyl substituent about the Nimino−Cipso bond.
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Scheme 2. Reduction of Thiolate-Ligated MII Bis(imino)pyridine Complexes
The nickel(II) complex is the easiest to reduce in the series,
with E1/2 = −0.9 V.
Non-innocent BIP ligands have been shown previously to
undergo up to three sequential reductions in which the
electrons are delocalized over the BIP backbone.34−45 The
negative E1/2 values for 1−3 are consistent with ligand-based
reduction, and covalent attachment of the phenylthiolate group
exerts a strong influence on the redox properties, in comparison
to other BIP complexes.25 The non-thiolate-ligated BIP
complexes typically show two reversible reduction waves,
whereas 1−3 exhibit only one reduction process, suggesting
that the next reduction for 1−3, if it exists, lies outside the
solvent window. Similarly, the nickel(II) analogue 5 exhibits
only one redox process at −0.9 V, which is within the range of
Figure 8. UV-vis spectral change for the conversion of 3 (brown trace)
to 4 (green trace) with 0.5% Na/Hg amalgam in tetrahydrofuran
(THF).
ligand-based reductions for the iron complexes, but is more
positive by ≥200 mV. These data show that the BIP-derived
ligand of the NiII complex 5 is significantly easier to reduce than
1−3.
and will decompose upon prolonged standing, even at −35 °C
in a drybox freezer, as seen by spontaneous conversion of dark
green solutions of 4 to a brown precipitate. Crystals for XRD
can be grown from layering of pentane into Et2O at −35 °C,
but the crystals must be obtained within 48 h or decomposition
occurs and no crystalline product will form. Crystalline 4 is also
difficult to isolate outside of the mother liquor without further
decomposition. Attempts to scale up the reaction failed to give
crystalline product: instead, it led to more-rapid decomposition.
Many reduced Fe(BIP) complexes have been reported, most
involving doubly reduced species relative to the MII complexes
analogous to 1−3. However, a few examples of monoreduced
and triply reduced Fe(BIP) complexes have been de-
scribed.37−39 The monoreduced Fe(BIP) complexes were
prepared by either reduction of iron(II) starting materials
(e.g., [(iPrBIP)FeIICl2] + e− → [(iPrBIP)FeCl]0), or oxidation of
doubly reduced (formally Fe0) Fe(BIP) complexes (e.g.,
[(iPrBIP)Fe(N2)]0 → [(iPrBIP)Fe(OEt2)]+ + e−) and, in both
cases, yield 4-coordinate iron complexes.37−39 The 5-coordinate
complexes [(iPrBIP)Fe(Br)(THF)] and [(iPrBIP)Fe(CO)2]+,
were prepared via one-electron oxidation of their respective
doubly reduced precursors, while [(EtBIP)Fe(Cl)(Et2O)] was
prepared by reduction of the starting material.37,40,78 However,
the doubly reduced species generally are more easily obtained
than the monoreduced analogs, and careful stoichiometric
control must be used to synthesize monoreduced complexes. In
contrast, the putative doubly reduced analog of 3 is not
accessible as seen by electrochemistry, and thus reduction of 3
leads to the selective isolation of a rare 5-coordinate,
monoreduced complex.
One-Electron Reduction of the Iron(II) and Nickel(II)
Complexes. A summary of the possible redox transformations
for the phenylthiolate-appended [M(LN3S)(L)]+ complexes
are shown in Scheme 2. The CV data for 1−3 and 5 each show
well-separated, quasi-reversible reduction waves, indicating that
one-electron-reduced complexes should be synthetically acces-
sible. The lack of a second reduction process suggests that the
one-electron-reduced complexes can be isolated without the
complication of forming two-electron-reduced species, in
contrast to other BIP complexes.39 Several methods have
been employed to reduce Mn+(BIP) complexes, including
treatment with aluminum/lithium alkyl reagents, NaBEt3H,
KC8, and Na/Hg amalgam.35,70,76,77 The latter method was a
convenient choice for the controlled reduction of the thiolate-
ligated complexes described here.
Reduction of the DMAP complex 3 was accomplished by
stirring a suspension of 3 in Et2O over a slight excess of 0.5%
Na/Hg amalgam, affording the one-electron-reduced [Fe-
(LN3S)(DMAP)]0 (4), as shown in Scheme 3. The progress
Scheme 3. Synthesis of the Monoreduced Complex
[Fe(LN3S)(DMAP)]0 (4)
Attempted reductions of the iron(II) complexes 1 and 2
suspended in Et2O led to deep green solutions similar to those
seen for 4, but the products were highly soluble, even in
mixtures with pentane, and crystals could not be isolated. The
nickel(II) complex 5, which is insoluble in Et2O, reacts similarly
with Na/Hg amalgam to give a homogeneous bright green
solution which presumably contains the monoreduced nickel
analogue. This product was also highly soluble and could not be
of the reaction is easily followed by the colorless ethereal
suspension of 3 converting to a deep green, homogeneous
solution. The UV-vis spectrum following the Na/Hg reduction
is shown in Figure 8, where the solvent is THF instead of Et2O
to maintain the solubility of 3. Distinct features appear at 566,
625, and 785 nm that are indicative of 4. It should be noted that
solutions of 4 in either THF or Et2O are extremely air-sensitive,
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crystallized, but provides a useful comparison to iron-
containing 4; therefore, it was prepared in situ from 5 and
characterized by EPR spectroscopy (vide infra).
X-ray Structure of Monoreduced [Fe(LN3S)(DMAP)]0
(4). The crystal structure of 4 (Figure 9) shows an overall
Figure 10. Comparison of selected ligand bond lengths for 3 (left) and
4 (right).
3 to 4. A careful comparison of the bond distances in 4 to those
reported in the literature suggests that the ligand in 4 is similar
to a two-electron reduced BIP ligand. However, 4 would need
to be formulated as an FeIII complex with a doubly reduced
(LN3S)3− ligand, which is unlikely, given that iron(III)
complexes are often not stable in the presence of monoanionic
thiolate donors. Further evidence from Mossbauer spectroso-
̈
copy and DFT calculations point to 4 as being an iron(II)
complex with a one-electron-reduced ligand (vide infra). The
phenylthiolate-appended LN3S ligand appears capable of
behaving as a non-innocent ligand by accommodating a
reducing equivalent delocalized over the BIP backbone.
Figure 9. Displacement ellipsoid plot for 4 (50% probablility level).
The H atoms are removed for the sake of clarity.
EPR Spectroscopy. Further information regarding the
electronic structure of the monoreduced 4 was obtained by
EPR spectroscopy. Crystalline 4 was dissolved in Et2O and EPR
data were recorded at 15 K. A nearly axial EPR spectrum was
observed, and a good simulation was obtained for an S = 1/2
system with g values of 2.155, 2.057, and 2.038 (Figure 11).
neutral iron complex with no additional counterions. Therefore,
charge balance indicates that this complex is the one-electron-
reduced analogue of the cation of 3. The DMAP is retained in
the axial position and, together with the LN3S ligand, gives a 5-
coordinate Fe center. Selected bond distances and angles are
given in Tables 1 and 2. The Fe and S atoms in 4 have moved
toward the equatorial plane of the bis(imino)pyridine backbone
(Fe−N3(plane) = 0.254 Å and S−N3(plane) = 0.411 Å),
compared to 3. This flattening of the geometry around the Fe
center is characterized by a change in τ value from τ = 0.18 for
3 to τ = 0.08 for 4. There is a significant shortening of the Fe−
N and Fe−S bonds for LN3S, with the Fe−S bond decreasing
by 0.1 Å and the Fe−N bonds decreasing by 0.3 Å. These
changes are opposite to what would be expected for a metal-
centered reduction to give a formal high-spin iron(I) complex,
and they are more consistent with ligand-based reduction.
It has been shown that the C−C and C−N bond distances in
BIP complexes are indicative of ligand reduction, and an
analysis of ligand backbone distances is essential when assigning
the locus of reduction in these complexes.37−43 To gain insight
into the electronic structure of 4, it is useful to compare
metrical parameters with other structurally characterized 5-
coordinate monoreduced complexes reported in the literature.
The complexes [Fe(iPrBIP)(Br)(THF)]78 and [Fe(EtBIP)-
Figure 11. X-band EPR spectrum of 4 (2.4 mM) in Et2O at 16 K
(black line). Inset: full-range experimental spectrum. Parameters:
frequency, 9.479 GHz; microwave power, 2 mW; modulation
amplitude, 10 G; modulation frequency, 100 kHz. Parameters for
simulation (red line): S = 1/2; g = [2.155, 2.057, 2.038]; Lorentzian
linewidths, W = 33, 32, and 30 G.
(Cl)]40 exhibit similar Cimine−Nimine, Cimine−Cipso, and Cipso
−
Cpyridyl bond distances in the BIP backbone that are consistent
with one-electron ligand reduction. In contrast, [Fe(iPrBIP)-
(CO)2](BArF ),37 another monoreduced complex, has bond
4
distances closer to a non-reduced BIP ligand and has been
assigned as an iron(I) complex. The critical metrical parameters
for 4 are highlighted in the structural fragments shown in
Figure 10, in which 4 can be compared with its non-reduced
precursor 3. The C−Nimine and C−Npyridyl bonds are elongated,
while the Cimine−Cipso bonds are shortened in 4, compared to 3.
These changes in the ligand backbone clearly point to the
complex undergoing ligand-based reduction upon conversion of
The S = 1/2 ground state could arise from a low-spin (ls)
iron(I) center resulting from metal-based reduction, a ls-
iron(II) (S = 0) center with a ligand-based radical, or an
intermediate-spin (is) iron(II) (S = 1) center antiferromagneti-
cally coupled to a ligand-based radical. An additional possibility
is a high-spin (hs) iron(II) (S = 2) antiferromagnetically
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coupled to a ligand-based radical, but this configuration would
give rise to a total spin ground state of S = 3/2, which should
exhibit g values spread over a much wider range than that
observed in Figure 11.38 To make a definitive assignment, it is
useful to draw comparisons with other monoreduced BIP-Fe
complexes. For the monoreduced complex [(iPrBIP)Fe(CO)2]-
(BArF ), a rhombic EPR spectrum with g = 2.111, 2.043, 1.994
4
is observed and attributed to a low-spin FeI configuration with
an S = 1/2 ground state.37 In contrast, the monoreduced
[(iPrBIP)Fe(OEt2)](BArF ) exhibits an EPR spectrum assigned
4
to an S = 3/2 species derived from a high-spin FeII ion
antiferromagnetically coupled to the BIP radical anion.38 The
spectrum in Figure 11 does not match the spectra for either of
the former complexes. These results suggest that the LN3S
complex 4, which contains a covalently tethered phenylthiolate
group, exhibits a new electronic configuration, consisting of
either ls-FeII and a ligand-based radical or an is-FeII center
antiferromagnetically coupled to a ligand-based radical, not
previously observed in other monoreduced complexes.
The in situ reduction of the nickel complex 5 helps clarify the
EPR properties and electronic ground states of the LN3S
complexes described in this work. The monoreduced analogue
of 5, [Ni(LN3S)]0, was generated in situ and its EPR spectrum
is shown in Figure 12. A sharp singlet at g = 2.00 (Δpp = 20 G)
Figure 13. Mossbauer spectra at 5.3 K in a weak applied field (47 mT)
̈
of 3 (starting material) and 4 (monoreduced). Both spectra consist of
broad quadrupole doublets (ΓL=R = 0.8 mm s−1). Complex 3 has
parameters that can be assigned to a high-spin iron(II) species: (δ =
0.87 mm s−1; ΔEQ = 2.70 mm s−1), while 4 is consistent with an
intermediate-spin iron(II) species coupled to a S = 1/2 radical: (δ =
0.33 mm s−1; ΔEQ = 2.04 mm s−1).
samples for 3 and 4 were mixtures of both crystalline and
amorphous solids. The spectrum of 3 can be fitted to a
quadrupole doublet with parameters that are highly distinc-
tive79 for a high-spin iron(II) ion (δ = 0.87 mm s−1; ΔEQ = 2.70
mm s−1). However, the isomer shift is on the low end of the
expected range, and this lowering is likely caused by both
covalency of the thiolate iron bond and the five-coodinate
nature of the complex. We previously reported an analogous
iPrBIP complex, [FeII(iPrBIP)(SPh)Cl], which contains an
exogenous thiolate donor in place of the tethered PhS−
group in 1−3 and Cl− as the fifth ligand, and its Mossbauer
̈
spectrum was collected and is displayed in Figure S7 in the
Supporting Information.25 Complex 3 and [FeII(iPrBIP)(SPh)-
Cl] exhibit similar isomer shifts and quadrupole splittings,
suggesting that substitution of the neutral DMAP ligand with
Figure 12. X-band EPR spectrum of 5 (3.0 mM) + Na/Hg (0.5%)
amalgam in THF at 16 K. Parameters: frequency, 9.479 GHz;
microwave power, 2 mW; modulation amplitude, 10 G; modulation
frequency, 100 kHz.
the anionic Cl− ligand has little effect on the Mossbauer spectra
̈
is observed and can be assigned to a purely ligand-based
radical.70 Such a spectrum would be expected for 4 if this
complex contained an ls-FeII ion and a single, unpaired e−
delocalized over the LN3S ligand. Taken together, these data
suggest that 4 may be best described as an intermediate-spin
iron(II) ion antiferromagnetically coupled with a ligand radical,
giving rise to the axial EPR spectrum in Figure 11.
and, hence, electronic structure. These complexes can also be
compared favorably with data for the thiolate-ligated non-heme
iron enzymes CDO20 and SOR.81
We next examined the one-electron-reduced complex 4 by
Mossbauer spectroscopy. Analysis of crystalline 4 affords a
̈
Mossbauer spectrum distinct from 3, with a significant decrease
̈
in both isomer shift and quadrupole splitting (δ = 0.33 mm s−1;
ΔEQ = 2.04 mm s−1). Comparison with the monoreduced and
doubly reduced complexes in Table 5 shows that 4 exhibits
̈
Mossbauer Spectroscopy. Mossbauer spectra of micro-
crystalline samples of complexes 3 and 4 with natural
̈
abundance 57Fe were collected at 5.3 K and with a weak
Mossbauer parameters with values between those of mono-
̈
applied field (47 mT) and are given in Figure 13. Mossbauer
reduced [(BIP)Fe(CO)2]+, which contains two strong-field CO
donors and an ls-FeI center, and monoreduced [(BIP)Fe-
(OEt2)]+, which has a single, weak-field Et2O donor and an hs-
FeII ion. Surprisingly, the isomer shift and quadrupole splitting
values for 4 are, in fact, closest to those seen for the doubly
̈
parameters for these complexes, together with complexes from
the literature, are given in Tables 4 and 5. Both 3 and 4
produced spectra that consisted of broad quadrupole doublets.
The broad spectra may be attributed to the fact that the
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Table 4. 57Fe Mossbauer Parameters for Selected High-Spin Iron(II) Complexes
̈
δ (mm s−1
)
ΔEQ (mm s−1
)
ground state, S
source
[FeII(LN3S)(DMAP)](OTf) (3)
[(iPrBIP)FeCl2]
0.87
0.89
0.92
0.78
0.90
2.70
2.40
2.40
2.62
1.12
2
2
2
2
2
this work
ref 39
[(iPrBIP)FeCl2]
this work
this work
ref 80
[(iPrBIP)Fe(SPh)Cl]
[(MesBIP)FeCl2]
Table 5. 57Fe Mossbauer Parameters for Monoreduced and Doubly Reduced BIP-Iron Complexes
̈
δ (mm s−1
)
ΔEQ (mm s−1
)
Stotal
SFe
Sligand
source
Monoreduced Complexes
[FeII(LN3S)(DMAP)] (4)
0.33
0.17
1.13
0.77
2.04
1/2
1/2
3/2
3/2
1
1/2
2
1/2
0
this work
ref 37
[(iPrBIP)Fe(CO)2](BArF )
0.62
4
[(iPrBIP)Fe(OEt2)](BArF )
2.51
1/2
1/2
ref 38
4
[(iPrBIP)FeCl]
0.73
2
ref 39
Doubly Reduced Complexes
[(iPrBIP)Fe(DMAP)]
0.31
0.30
0.03
1.94
2.04
1.17
0
0
1
1
1
0
1
1
1
ref 39
ref 82
a
[(iPrBAP)Fe(DMAP)]
[(iPrBIP)Fe(CO)2]
refs 39, 82
a
BAP = bis(aldimino)pyridine.
Figure 14. Optimized UB3LYP/B1 geometries of 5[FeII(LN3S)(DMAP)]+ and 1[NiII(LN3S)]+, and their reduced complexes (bond lengths given in
angstroms). Also given are group spin densities.
5
reduced complexes, which have been assigned as intermediate-
distance of 2.407 Å for 3 in Figure 14 is slightly elongated,
compared to the X-ray structure (Fe−S = 2.3246(18) Å). Note
that when geometry optimizations for 3 were run with a pure
density functional such as BP86 instead of B3LYP, a different
spin state ordering was found, in which a degenerate singlet/
triplet ground state was obtained and the quintet spin state was
calculated to be much higher in energy (see the Supporting
Information). Consequently, pure density functional methods
such as BP86 give results that are inconsistent with the
experimental findings, and therefore we did not proceed further
with these types of methods.
spin (is) iron(II) species with diradical ligands. The Mossbauer
̈
data for 4 thus point to an is-FeII center as the best assignment
for this complex, and this conclusion is in good agreement with
both the structural and EPR data. The strong-field thiolate
ligand in 4, coupled with the relatively weak-field DMAP donor,
gives rise to this unique electronic configuration.
Computational Studies. The iron(II) and nickel(II)
complexes, and their monoreduced analogues, were studied
by DFT calculations to gain further insight into their structures
and electronic configurations. The optimized geometries of
[(LN3S)FeII(DMAP)]+ (3) and [NiII(LN3S)]+ (5) were
calculated with the B3LYP functional on all low-lying spin
states and are given in Figure 14. A quintet spin ground state (S
= 2, hs-FeII) was found for 3. At ΔE+ZPE level of theory, the
quintet state (53) for the iron(II) complex is more stable than
either the triplet state (S = 1, is-FeII, (33)) or the singlet state
(13), by 13.8 and 17.0 kcal mol−1, respectively. Inclusion of
dispersion corrections further increases the energy gaps to 21.6
and 32.7 kcal mol−1, respectively. These results indicate that the
singlet and triplet spin states should be inaccessible at room
temperature, and match nicely with the experimental
For the nickel(II) complex 5, a closed-shell singlet spin state
(S = 0) was found to be the ground state. We also calculated
the triplet and quintet spin states for 5, but we found these
1
states to be higher in energy than 5, by 4.2 and 33.3 kcal
mol−1, respectively. No changes in spin state ordering were
found for this complex when alternative density functional
methods were employed. The optimized geometry for the
ground state singlet is approximately square planar, consistent
1
with the X-ray structure. The Ni−N distances for 5 are in
excellent agreement with those derived from the crystal
structure (Table 3), although the Ni−S distance is slightly
elongated (2.201 Å via the DFT method, 2.1386(5) Å
according to the experimental data), similar to the trend seen
1
observations (structural parameters, H NMR) that show 3
has an hs-FeII ground state. The Fe−N bond lengths in the
5
5
DFT-optimized structure of 3 are within the range of 2.092−
for the Fe−S distance in 3 versus the X-ray structure. The
2.279 Å, and they are in good agreement with the X-ray
DFT calculations at the B3LYP level of theory often slightly
structure (Fe−N for 3: 2.073(5)−2.196(5)). The Fe−S
overestimate metal−sulfur bond lengths.83 The other main
J
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Inorganic Chemistry
Article
5
2
Figure 15. Orbital diagrams of [FeII(LN3S)(DMAP)]+ (left) and [FeII(LN3S)(DMAP)]0 (right).
structural featuresand, importantly, the spin ground states
are well-reproduced by the calculations.
4 (Table 1), but the Fe−S distance is somewhat longer,
following the trend seen for 3 and 5. In addition the Fe−N
distance for the axial DMAP ligand is also slightly elongated
(DFT, 2.170 Å; expt, 2.0508(16) Å).
The monoreduced complex 4 was also studied at the B3LYP
level of theory, taking into consideration the possible doublet
(S = 1/2) and quartet (S = 3/2) spin states for this complex.
Calculations were initiated by using the coordinates from the X-
ray structure of 4 as a starting point, and the results led to an
overall doublet spin ground state (S = 1/2) for the
monoreduced complex. The doublet spin state is found to be
stabilized by 6.3 kcal mol−1 over the quartet spin state, and
solvent corrections raise this energy gap to 6.5 kcal mol−1;
however, dispersion corrections reduce it to 0.6 kcal mol−1. The
computationally derived S = 1/2 ground state thus matches that
derived from the EPR spectrum observed in Figure 9 for
complex 4. The bond distances for the doublet spin ground
state of 4 are shown in Figure 14, and both the Fe−N and Fe−
S distances are significantly shortened compared to the starting
complex 3. The Fe−N bond lengths for the in-plane LN3S
ligand are in good agreement with the structural parameters for
The molecular orbital diagram for the doublet spin ground
state of 4 is given in Figure 15, together with the MO diagram
for the quintet spin ground state of starting complex 3. For
complex 4, the computational results are consistent with an is-
FeII (SFe = 1) center antiferromagnetically coupled with a
ligand-based radical to give an overall doublet spin (Stotal = 1/2)
ground state. As seen in Figure 15, two α spin electrons reside
in metal-based orbitals, while a predominantly ligand-based
orbital (π*L) contains one β spin electron. The two unpaired
electrons on the metal are reflected by a calculated spin density
of ρFe = 1.96, whereas the unpaired electron on the ligand leads
to ρLN3S = −1.00 (Figure 14). In contrast, the MO diagram for
3 reveals four half-filled (α-spin) metal-based orbitals resulting
in ρFe = 3.74, as expected for a hs-FeII center. The calculations
show that one-electron reduction of 3 occurs by the filling of a
K
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Inorganic Chemistry
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low-lying virtual ligand (π*L) orbital in preference to filling of
an empty metal-based orbital, confirming the non-innocent
nature of the LN3S ligand. The spin state change that occurs at
the Fe center upon reduction of 3 (hs-FeII) to 4 (is-FeII)
appears to be a consequence of the destabilization of the σ*xy
orbital. This destabilization is likely brought on by an increase
in ligand field strength for the one-electron-reduced LN3S
ligand and a consequent shortening of the Fe−N and Fe−S
bonds. Destabilization of σ*xy leads to depopulation of this
orbital and occupation of the lower-lying π*yz orbital, resulting
in an overall is-FeII center. Finally, the ligand-based β electron
in 4 is mainly located on the bis(imino)pyridine backbone,
which leads to the significant bond length perturbations shown
in Figure 16, where the backbones for 53 and 24 are compared.
doubly oxygenated sulfinato-iron(II) complex.26 We thus
sought to test the new N4S(thiolate) iron(II) complexes 2
and 3 to determine their reactivity toward O2.
Reactions of complexes 2 and 3 with excess O2 in CH2Cl2
were analyzed directly by LDI-MS. The mass spectral data
(Figures S3 and S4 in the Supporting Information) show
dominant ions that correspond to the triply oxygenated
sulfonato-iron(II) complex, [Fe(LN3SO3)]+ (m/z = 532).
This oxygenation pattern is the same as that seen for 1.24
Conversion of 1 to an N4S(thiolate) coordination environment
by addition of py or DMAP does not alter the O2 reactivity
such that sulfur oxygenation results in a doubly oxygenated
sulfinato product, as seen for our other N4S(thiolate) system.25
Some nickel(II) thiolate complexes can undergo sulfur
oxygenation, most likely through a mechanism involving direct
attack of O2 on the coordinated sulfur atom, while other NiII-
thiolates are inert toward O2, and orbital compositions have
been analyzed to explain these differences in O2 reactivity.46−54
We did not observe any reaction between complex 5 and excess
O2 in CH2Cl2, even after prolonged stirring for several days.
Solutions of the monoreduced nickel(II) complex [Ni(LN3S)]0
generated from Na/Hg amalgam in THF were next tested for
their reactivity toward O2. In this case, a rapid reaction with O2
was indeed observed by an immediate color change from bright
to dark green, but LDI-MS analysis of the reaction mixture
revealed only a peak (m/z = 488.3) for the starting material 5,
suggesting only outer-sphere oxidation had occurred. In
contrast, a related NiII(BIP) complex undergoes outer-sphere
reduction to give a one-electron-reduced product that then
reacts with O2 via an inner-sphere mechanism to afford ligand-
oxidized products.70
The monoreduced compound 4 also exhibits rapid reactivity
with O2. Exposure of solutions of 4 in Et2O to air, or bubbling
O2, leads to an immediate color change from dark green to
brown with concurrent formation of a brown precipitate.
Analysis by LDI-MS reveals a mixture that can be assigned to
iron oxygenates and sulfur oxygenates, with multiple major
peaks assigned to [SOFeIII−O−FeIIISO]2+ (m/z = 508.3),
[SO2FeIII−O−FeIIISO2]2+ (m/z = 524.4), and [SO3FeIII−O−
FeIII−SO3]2+ (m/z = 540.4). While complexes 1−3 exhibit S-
centered oxidation, the monoreduced complex 4 appears to
show significant oxidation at the Fe center. This represents a
significant change in the observed reactivity of these complexes,
as a result of the change in electronic structure.
Figure 16. Optimized UB3LYP/B1 geometries of 5[FeII(LN3S)-
(DMAP)]+ and 2[FeII(LN3S)(DMAP)]0, highlighting BIP ligand
backbone (bond lengths given in angstroms).
The trends in the perturbations provide a satisfying match to
what is seen experimentally in Figure 10 and help to further
validate the DFT results. The close agreement of the calculated
ligand backbone bond distances (Figure 16) with the X-ray
structure of 4 provides additional evidence for the assignment
of 4 as a one-electron reduced ligand radical coupled to an is-
FeII ion (ρFe = 1.96, ρL = −1.00; see Figure 14a).
1
The monoreduced product of 5 was also investigated by
DFT, giving the optimized geometry for the doublet spin
ground state of 2[NiII(LN3S)]0 shown in Figure 14. In this case,
there are only very minor perturbations on the metal−ligand
bond lengths, compared to the non-reduced starting material.
Analysis of the spin density shows that the extra electron clearly
lies on the LN3S ligand and not the closed-shell nickel center,
with an α electron occupying a π*L orbital (Figure S8 in the
Supporting Information). The DFT calculations for the
monoreduced nickel complex provide further confirmation
that LN3S functions easily as a non-innocent ligand, and the
ligand-based radical predicted by DFT is in full agreement with
the sharp organic radical seen in the EPR spectrum for
monoreduced 5.
CONCLUSIONS
■
The triflate-ligated ferrous complex [FeII(LN3S)(OTf)] (1)
serves as a useful precursor for the facile synthesis of the
biomimetic N4S(thiolate) complexes [FeII(LN3S)(DMAP)]-
OTf (2) and [FeII(LN3S)(DMAP)]OTf (3). Template
assembly with nickel(II) tetrafluoroborate leads to the NiII
analogue [NiII(LN3S)]BF4 (5). Structural and spectroscopic
methods show that the iron complexes are 5-coordinated, high-
spin (S = 2) iron(II) species, while the nickel complex is a 4-
coordinate square-planar nickel(II) species. Electrochemical
measurements revealed that these complexes uniformly exhibit
only one reversible reduction wave between −0.9 V and −1.2
V, vs Fc+/Fc, within the CH3CN solvent window. Bulk
chemical reduction of 3 by Na/Hg amalgam gives the extremely
air-sensitive, monoreduced complex [Fe(LN3S)(DMAP)]0 (4),
which was isolated and crystallographically characterized. No
evidence for a two-electron reduced product was observed,
which was consistent with the electrochemical analysis. This
O2 Reactivity. We previously found that 1 reacts with
24
excess O2 in CH2Cl2 to give an S-oxygenated sulfonate
complex, providing the first example of an FeII-thiolate complex
that reacts with O2 to give selective sulfur oxygenation. This
reaction mimicked some of the general features of the reaction
seen for CDO, but was lacking in the fact that the donor set at
the metal center was N3S(thiolate), not N4S(thiolate), and the
−
sulfur-oxygenated product was a triply oxygenated RSO3
complex, as opposed to the doubly oxygenated RSO2− product
generated by the enzymatic system. In a subsequent report, we
prepared an improved N4S(thiolate) iron(II) model complex
based on the tripodal N4Py ligand, [FeII(N3PyS)(solvent)]+,
and we found that it reacts with O2 to give a biomimetic,
L
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behavior contrasts other bis(imino)pyridine complexes, which
typically can house up to two or three reducing equivalents
beginning at the M2+(BIP0) redox level, and have been shown
to form two-electron reduced products from Na/Hg reduction.
It can be concluded that the anionic thiolate arm appended to
the new LN3S system likely causes multiple reductions to be
thermodynamically unfavorable. The X-ray structure of 4 shows
it is a 5-coordinated complex, with one DMAP ligand
remaining coordinated in the axial position following reduction.
The electronic structure of the monoreduced complex 4 was
firmly established through a combination of structural analysis,
calculated structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dpg@jhu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
EPR and Mossbauer spectroscopies as well as computational
̈
The National Institute of Health (NIH) is gratefully acknowl-
edged for financial support to D.P.G. (No. GM62309). The
National Service of Computational Chemistry Software
(NSCCS) is thanked for generous CPU time to S.P.d.V. D.K.
is a DST-Ramanujan Fellow (SR/S2/RJN-11/2008). G.N.L.J.
thanks the Marsden Fund and The International Mobility Fund
administered by the Royal Society of New Zealand.
(DFT) methods. The ground state of 4 is clearly a doublet state
that arises from an intermediate-spin (S = 1) iron(II) center
antiferromagnetically coupled to a ligand-based radical. The
spin state of the Fe center is reflected in the unusual Mossbauer
̈
parameters for this monoreduced BIP complex, with δ = 0.33
mm s−1 and ΔEQ = 2.04 mm s−1. These parameters differ
significantly from other monoreduced Fe(BIP) complexes that
exhibit either hs-FeII (S = 2) or ls-FeII (S = 0) configurations,
and they, in fact, are consistent with the doubly reduced
Fe(BIP) species, which have been previously assigned as is-FeII
complexes carrying diradical ligands. The computational studies
show that the doublet spin ground state is comprised of two α
electrons occupying metal-based orbitals of dz2 and dxz
parentage and one β electron residing on a ligand-based π*
orbital with most of the spin density localized on the
bis(imino)pyridine backbone and not on the phenylthiolate
arm. The DFT results are nicely validated by their agreement
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic details for [FeII(LN3S)(py)](OTf),
[FeII(LN3S)(DMAP)](OTf), [Fe(LN3S)(DMAP)]0, and
[NiII(LN3S)](BF4) in CIF format. H and 19F NMR spectra
1
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