Electronic, Magnetic, and Redox Properties and O2 Reactivity of Iron(II) and Nickel(II) o-Semiquinonate Complexes of a Tris(thioether) Ligand: Uncovering the Intradiol Cleaving Reactivity of an Iron(II) o-Semiquinonate Complex

Article abstract of DOI:10.1021/acs.inorgchem.7b01491

The iron(II) semiquinonate character within the iron(III) catecholate species has been proposed by numerous studies to account for the O₂ reactivity of intradiol catechol dioxygenases, but a well-characterized iron(II) semiquinonate species that exhibits intradiol cleaving reactivity has not yet been reported. In this study, a detailed electronic structure description of the first iron(II) o-semiquinonate complex, [PhTt^tBu]Fe(phenSQ) [PhTt^tBu = phenyltris(tert-butylthiomethyl)borate; phenSQ = 9,10-phenanthrenesemiquinonate; Wang et al. Chem. Commun. 2014, 50, 5871-5873], was generated through a combination of electronic and M?ssbauer spectroscopies, SQUID magnetometry, and density functional theory (DFT) calculations. [PhTt^tBu]Fe(phenSQ) reacts with O₂ to generate an intradiol cleavage product, diphenic anhydride, in 16% yield. To assess the dependence of the intradiol reactivity on the identity of the metal ion, the nickel analogue, [PhTt^tBu]Ni(phenSQ), and its derivative, [PhTt^tBu]Ni(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate), were prepared and characterized by X-ray crystallography, mass spectrometry, ¹H NMR and electronic spectroscopies, and SQUID magnetometry. DFT calculations, evaluated on the basis of the experimental data, support the electronic structure descriptions of [PhTt^tBu]Ni(phenSQ) and [PhTt^tBu]Ni(3,5-DBSQ) as high-spin nickel(II) complexes with antiferromagnetically coupled semiquinonate ligands. Unlike its iron counterpart, [PhTt^tBu]Ni(phenSQ) decomposes slowly in an O₂ atmosphere to generate 14% phenanthrenequinone with a negligible amount of diphenic anhydride. [PhTt^tBu]Ni(3,5-DBSQ) does not react with O₂. This dramatic effect of the metal-ion identity supports the hypothesis that a metal(III) alkylperoxo species serves as an intermediate in the intradiol cleaving reactions. The redox properties of all three complexes were probed using cyclic voltammetry and differential pulse voltammetry, which indicate an inner-sphere electron-transfer mechanism for the formation of phenanthrenequinone. The lack of O₂ reactivity of [PhTt^tBu]Ni(3,5-DBSQ) can be rationalized by the high redox potential of the metal-ligated 3,5-DBSQ/3,5-DBQ couple.

Full text of DOI:10.1021/acs.inorgchem.7b01491

Article
pubs.acs.org/IC
Electronic, Magnetic, and Redox Properties and O Reactivity of
2
Iron(II) and Nickel(II) o‑Semiquinonate Complexes of a Tris(thioether)
Ligand: Uncovering the Intradiol Cleaving Reactivity of an Iron(II)
o‑Semiquinonate Complex
†
‡
§
†
†
Peng Wang, Michelle M. Killian, Mohamed R. Saber, Tian Qiu, Glenn P. A. Yap,
∥
†
§
‡
Codrina V. Popescu, Joel Rosenthal, Kim R. Dunbar, Thomas C. Brunold,
,†
and Charles G. Riordan*
†
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States
Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
‡
§
∥
*
S Supporting Information
ABSTRACT: The iron(II) semiquinonate character within
the iron(III) catecholate species has been proposed by
numerous studies to account for the O reactivity of intradiol
2
catechol dioxygenases, but a well-characterized iron(II)
semiquinonate species that exhibits intradiol cleaving reactivity
has not yet been reported. In this study, a detailed electronic
structure description of the first iron(II) o-semiquinonate
tBu
tBu
complex, [PhTt ]Fe(phenSQ) [PhTt
= phenyltris(tert-
butylthiomethyl)borate; phenSQ = 9,10-phenanthrenesemi-
quinonate; Wang et al. Chem. Commun. 2014, 50, 5871−5873],
was generated through a combination of electronic and
̈
Mossbauer spectroscopies, SQUID magnetometry, and density
tBu
functional theory (DFT) calculations. [PhTt ]Fe(phenSQ)
reacts with O to generate an intradiol cleavage product, diphenic anhydride, in 16% yield. To assess the dependence of the
2
tBu
intradiol reactivity on the identity of the metal ion, the nickel analogue, [PhTt ]Ni(phenSQ), and its derivative,
PhTt ]Ni(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate), were prepared and characterized by X-ray
crystallography, mass spectrometry, H NMR and electronic spectroscopies, and SQUID magnetometry. DFT calculations,
evaluated on the basis of the experimental data, support the electronic structure descriptions of [PhTt ]Ni(phenSQ) and
PhTt ]Ni(3,5-DBSQ) as high-spin nickel(II) complexes with antiferromagnetically coupled semiquinonate ligands. Unlike its
iron counterpart, [PhTt ]Ni(phenSQ) decomposes slowly in an O atmosphere to generate 14% phenanthrenequinone with a
negligible amount of diphenic anhydride. [PhTt ]Ni(3,5-DBSQ) does not react with O . This dramatic effect of the metal-ion
tBu
[
1
tBu
tBu
[
tBu
2
tBu
2
identity supports the hypothesis that a metal(III) alkylperoxo species serves as an intermediate in the intradiol cleaving reactions.
The redox properties of all three complexes were probed using cyclic voltammetry and differential pulse voltammetry, which
indicate an inner-sphere electron-transfer mechanism for the formation of phenanthrenequinone. The lack of O reactivity of
2
tBu
[
PhTt ]Ni(3,5-DBSQ) can be rationalized by the high redox potential of the metal-ligated 3,5-DBSQ/3,5-DBQ couple.
1
. INTRODUCTION
Our interest in transition-metal dioxolene complexes stems
from their biological significance. An iron(III) catecholate
species was identified in the active sites of intradiol catechol
dioxygenases, enzymes that catalyze the oxidative cleavage of
Transition-metal dioxolene complexes continue to attract much
interest, in part because of their applications as redox-active
ligands and their relevance to catechol dioxygenase enzymes.
As a well-established redox-active ligand, dioxolene can exist in
three different oxidation states: neutral (quinone), mono-
anionic (semiquinonate), and dianionic (catecholate) (Scheme
1
−3
16,25
the C1−C2 bond of catechols.
The form of the enzyme
that react with O contains a five-coordinate iron(III) site. One
2
of the proposed mechanisms for how the iron(III) catecholate
species activates O2 invokes the mixing of an iron(II)
1). All three redox forms have been explored in transition-metal
dioxolene chemistry, which led to various intriguing observa-
4
−7
8−14
tions such as valence tautomerization,
spin-crossover,
Received: June 12, 2017
15−18
19−24
biomimetic properties,
and catalytic activities.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01491
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Redox Series of Quinone (Q)−Semiquinonate
(phenSQ). In addition, studies of the O2 reactivity of
[PhTt ]Fe(phenSQ) led to the discovery of the first synthetic
tBu
(
SQ)−Catecholate (Cat)
iron(II) o-semiquinonate species exhibiting intradiol cleaving
tBu
reactivity. For comparison to [PhTt ]Fe(phenSQ), two nickel
tBu -
o-semiquinonate analogues were prepared, namely, [PhTt ]
tBu
Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ). The solid-state
structures, electronic and magnetic properties, and redox
behaviors of these new nickel complexes are presented and
discussed. Finally, comparative O reactivity studies of the
semiquinonate state into the iron(III) catecholate ground state.
According to this mechanism (Scheme 2), iron(II) o-semi-
2
iron(II) and nickel(II) o-semiquinonate complexes highlight
the fundamental effect of the metal ion on the intradiol cleaving
reactivity.
quinonate is the “active form” that reacts with O to generate
2
26
an iron(III) alkylperoxo intermediate with a monodentate
dioxolene moiety. The iron(III) alkylperoxo intermediate then
undergoes a Criegee rearrangement, followed by ring opening
to subsequently yield the intradiol cleavage product. This
2. EXPERIMENTAL AND COMPUTATIONAL METHODS
2
.1. General Information. All air- and moisture-sensitive
27−30
proposal has been supported by theoretical calculations
reactions were performed under N using standard Schlenk techniques
2
and the observed correlations between the spectroscopic
features and intradiol reactivity of iron(III) catecholate model
or carried out under an Ar or N2 atmosphere in a Vacuum
Atmospheres glovebox equipped with a gas purification system. Unless
otherwise noted, all reagents were purchased from commercial sources
3
1−35
compounds.
Nevertheless, to the best of our knowledge,
direct observation of the intradiol cleaving reactivity of a well-
defined iron(II) semiquinonate species had not previously been
reported.
and used without further purification. Anhydrous NiI was purchased
2
from Strem Chemicals. Solvents were of reagent-grade or better and
were dried by passage through activated alumina and then stored over
4
Å molecular sieves prior to use. Deuterated solvents were purchased
In a recent communication, some of us reported the
preparation and characterization of the first mononuclear
from Cambridge Isotope Laboratories and stored over 4 Å molecular
tBu
37
38
38
sieves. [PhTt ]Tl, Ti(phenSQ), and Tl(3,5-DBSQ) were
tBu
iron(II) o-semiquinonate complex, namely, [PhTt ]Fe-
prepared following published procedures.
3
6
(
phenSQ) (Figure 1). On the basis of the average C−O
tBu
2
.2. Synthesis. 2.2.1. [PhTt ]NiI. Anhydrous NiI (1.250 g, 4.0
2
distance of the dioxolene ligand, 1.285(2) Å, the redox state of
the dioxolene ligand was assigned as a semiquinonate.
Furthermore, the existence of a five-coordinate high-spin
iron(II) ion was inferred on the basis of the ligand-field (LF)
mmol) was ground into a fine powder and suspended in 100 mL of
tBu
tetrahydrofuran (THF). After stirring for 6 h, [PhTt ]Tl (1.204 g, 2.0
mmol) was added in small portions. After stirring for 16 h, the reaction
mixture was filtered through Celite, removing TlI, and the solvent was
removed in vacuo. The residue was extracted with pentane (2 × 40
mL), and the resulting red solution was filtered through Celite.
−1
−1
transition at 935 nm (539 M cm ; Figure 1). Given the
scarcity of mononuclear iron(II) o-semiquinonate species and
Removal of the solvent under reduced pressure afforded a brown solid
their proposed involvement in the mechanism of O activation
2
1
(
919 mg, 69%). H NMR (C D ): δ 15.2 (27H, br, C(CH ) S), 11.1
6
6
3 3
by intradiol catechol dioxygenases and their model compounds,
(
2H, s, m-(C H )B), 8.5 (2H, s, o-(C H )B), 8.3 (1H, s, p-(C H )B).
6
5
6
5
6
5
it is highly desirable to gain insight into the electronic structure
−1
−1
UV−vis [toluene; λmax, nm (ε, M cm )]: 311 (1518), 375 (sh), 413
(sh), 452 (6036), 567 (972), 841 (427), 903 (432). Anal. Calcd for
C H BINiS : C, 43.26; H, 6.57. Found: C, 43.50; H, 6.80.
tBu
of [PhTt ]Fe(phenSQ) and to examine its reactivity with O .
2
Herein, we report spectroscopic and magnetic studies in
conjunction with density functional theory (DFT) calculations
21
38
3
tBu
2.2.2. [PhTt ]Ni(phenSQ). A suspension of Tl(phenSQ) (83 mg,
tBu
to further interrogate the electronic structure of [PhTt ]Fe-
0.20 mmol) in 30 mL of THF was added dropwise over 20−30 min to
Scheme 2. Proposed Mechanism for Intradiol catechol Dioxygenases Highlighting Iron(II) o-Semiquinonate as the Active Form
Responsible for O Activation
2
B
DOI: 10.1021/acs.inorgchem.7b01491
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. X-ray structure of [PhTttBu]Fe(phenSQ) (left) and electronic absorption spectrum measured in THF (right).
a stirring solution of [PhTttBu]NiI (117 mg, 0.20 mmol) in 10 mL of
THF. A yellow precipitate gradually formed as the color of the
solution turned from orange-red to purple. The mixture was stirred for
sample warmed). The solution was concentrated to 2−3 mL in vacuo,
and 0.5 M HCl (2 mL, 0.5 mmol) was added to dissociate the metal
from the organic products. After the mixture was stirred for 5 min, 15
6
h and then filtered through a pad of Celite. THF was removed in
mL of H O was added, and the sample was extracted with ethyl acetate
2
vacuo, and the residue was washed with pentane (2 × 4 mL) and then
extracted with pentane/diethyl ether (1:1, v/v). Filtration of the
extract through a Celite pad, followed by solvent removal in vacuo,
yielded a dark-purple powder (106 mg, 80%). Slow evaporation of a
concentrated pentane/diethyl ether (2:1, v/v) solution of the product
(3 × 20 mL). The organic extracts were combined and dried over
anhydrous Na SO . After removal of ethyl acetate in vacuo, the
2
4
1
products were obtained and analyzed by H NMR spectroscopy
(CDCl ) and GC.
3
tBu
2.5. Examining the Reaction of [PhTt ]Ni(PhenSQ) with O
2
1
1
yielded crystals suitable for X-ray diffraction (XRD) analysis. H NMR
by Electronic Absorption Spectroscopy, H NMR Spectrosco-
tBu
py, and GC. [PhTt ]Ni(PhenSQ) (2.5 mg, 0.0038 mmol) was
(
λ
C D ): δ 52.6 (br), 9.4 (br, C(CH ) S), −4.1 (br). UV−vis [toluene;
max
6
6
3 3
−1
−1
dissolved in 100 mL of toluene in an Ar-filled glovebox, and 3 mL of
the solution was transferred to a cryostat UV−vis cuvette charged with
a magnetic stir bar. The cuvette was sealed with a rubber septum and
, nm (ε, M cm )]: 316 (9131), 398 (12877), 536 (11375), 586
(
10011), 846 (710), 966 (sh). LIFDI-MS. Calcd for C H BNiO S
35 46 2 3
+
[
(M) , 100%]: m/z 663.2113. Found: m/z 663.2093. μeff (C D ):
6 6
removed from the glovebox. With slow stirring, 1 atm of dry O was
2
.18(3) μ . Anal. Calcd for C H BNiO S : C, 63.27; H, 6.98. Found:
2
B
35 46
2 3
bubbled gently into the solution through a needle for 5 min at room
temperature. Electronic absorption spectra were collected every 15
min until no further spectral changes were observed (∼27 h).
For the H NMR spectroscopy and GC measurements, [PhTttBu]-
Ni(phenSQ) (30 mg, 0.045 mmol) was dissolved in 10 mL of toluene
in a 100 mL Schlenk flask charged with a magnetic stir bar. The
C, 63.31; H, 7.18.
.2.3. [PhTttBu]Ni(3,5-DBSQ). A solution of Tl(3,5-DBSQ) (178 mg,
.42 mmol) in 50 mL of THF was added dropwise over 30 min to a
2
0
1
tBu
stirring solution of [PhTt ]NiI (233 mg, 0.40 mmol) in 20 mL of
diethyl ether. A yellow precipitate gradually formed as the color of the
solution turned from orange-red to red-purple. The mixture was
stirred for 6 h and then filtered through a pad of Celite. The solvent
was removed in vacuo, and the residue was extracted with pentane.
Filtration of the extract through Celite followed by solvent removal in
vacuo yielded a dark-brown powder (230 mg, 85%). Slow evaporation
of a concentrated pentane solution of the product yielded crystals
solution was removed from the glovebox and 1 atm of dry O was
2
bubbled into the solution through a needle for 5 min. Then, a balloon
filled with dry O was connected to the flask through a needle and a
2
syringe. The solution was stirred for 36 h and subsequently
concentrated to 2−3 mL in vacuo, and then 0.5 M HCl (2 mL, 0.5
mmol) was added. After the mixture was stirred for 5 min, 15 mL of
1
suitable for XRD analysis. H NMR (C D ): δ 10.3 (sh), 9.2 (br,
6
6
−
1
−1
H O was added and the sample was extracted with ethyl acetate (3 ×
C(CH ) S), −7.1 (br). UV−vis [toluene; λmax, nm (ε, M cm )]:
00 (7328), 366 (sh), 522 (5811), 566 (sh), 842 (823), 966 (sh).
2
3
3
2
0 mL). The organic extracts were combined and dried over
3
+
anhydrous Na SO . After removal of ethyl acetate in vacuo, the
products were obtained and analyzed by H NMR spectroscopy
LIFDI-MS. Calcd for C H BNiO S [(M) , 100%]: m/z 675.3052.
2
4
35
58
2 3
1
Found: m/z 675.3093. μeff (C D ): 2.25 μ .
6
6
B
2
.3. Examining the Reaction of [PhTttBu]Fe(PhenSQ) with O2
3
(
CDCl ) and GC.
tBu
by Liquid-Injection Field Desorption Ionization Mass Spec-
2.6. Examining the Reaction of [PhTt ]Ni(3,5-DBSQ) with
trometry (LIFDI-MS). In an Ar-filled glovebox, [PhTttBu]Fe-
tBu
O
by Electronic Absorption Spectroscopy. [PhTt ]Ni(3,5-
2
(
PhenSQ) (3.0 mg, 0.0045 mmol) was dissolved in 10 mL of toluene,
DBSQ) (10.5 mg, 0.0155 mmol) was dissolved in 100 mL of toluene
in an Ar-filled glovebox. A 3 mL aliquot of the solution was transferred
to a cryostat UV−vis cuvette charged with a magnetic stir bar. The
cuvette was sealed with a rubber septum and removed from the
and 4 mL of the solution was transferred to a 20 mL scintillation vial
charged with a magnetic stir bar. The vial was then sealed with a
rubber septum and removed from the glovebox. The solution was
degassed by two freeze−pump−thaw cycles, and the vial was then
immersed in a dry ice/acetone bath (−78 °C). With stirring, the
solution was exposed to 1 atm of dry O at −78 °C. After 5 min, the
solution was warmed to room temperature and the LIFDI-MS
spectrum was taken.
glovebox. With gentle stirring, 1 atm of dry O was bubbled gently into
2
the solution through a needle for 5 min at room temperature.
Electronic absorption spectra were collected every 15 min over a 12 h
period. No spectral changes were observed.
2
2.7. Physical Methods. NMR spectra were recorded on a Bruker
AVIII 400 spectrometer. Chemical shifts (δ) were referenced to
residual protons in the deuterated solvents. Electronic absorption
spectra were recorded on a Varian Cary 50 UV−vis spectrophotometer
using screw-top quartz cuvettes with a 1 cm path length. Solution-state
2
.4. Examining the Reaction of [PhTttBu]Fe(PhenSQ) with O2
1
by H NMR Spectroscopy and Gas Chromatography (GC). In an
Ar-filled glovebox, [PhTt ]Fe(PhenSQ) (30 mg, 0.045 mmol) was
tBu
dissolved in 10 mL of toluene in a 100 mL Schlenk flask charged with
a magnetic stir bar. The solution was removed from the glovebox and
degassed by two freeze−pump−thaw cycles. The flask was then
3
9−41
magnetic moments were determined using the Evans method.
LIFDI-MS
4
2,43
was performed on a Waters GCT Premier mass
tBu
immersed in a dry ice/acetone bath (−78 °C), and 1 atm of dry O
spectrometer. The static magnetic properties of [PhTt ]Fe-
2
tBu
tBu
was added through a needle. The solution was stirred at −78 °C for 30
(phenSQ), [PhTt ]Ni(phenSQ), and [PhTt ]Ni(3,5-DBSQ) were
measured on samples of ground crystals using a Quantum Design
MPMS-XL SQUID magnetometer operating over the temperature
min, then warmed to room temperature, and stirred for an additional
3
0 min (the pressure of O was released via an outlet needle as the
2
C
DOI: 10.1021/acs.inorgchem.7b01491
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
range 1.8−400 K at a 1000 Oe direct-current field. The data were
modeled as a Gaussian band with a full width at half-maximum of 2500
−
1
corrected for diamagnetic contributions using Pascal constants.
cm .
Low-field (0.04 T), variable-temperature (5−200 K) Mo
̈
ssbauer
2.8.1. [PhTttBu]Fe(phenSQ). Starting coordinates for the initial
tBu
spectra were recorded on a closed-cycle refrigerator spectrometer,
model CCR4K, equipped with a 0.04 T permanent magnet,
maintaining temperatures between 5 and 300 K. The samples
consisted of solid powders (or crystalline material) suspended in
Nujol, placed in Delrin 1.00 mL cups, and frozen in liquid nitrogen.
The isomer shifts are quoted at 5 K with respect to the iron metal
models of [PhTt ]Fe(phenSQ) were taken from X-ray crystallo-
graphic data. Because the magnetic susceptibility [μeff = 4.65(2) μB]
and electronic absorption data indicate a high-spin metal center, the
3
spin state of this complex was set to S = / (high-spin iron(II)
2
−
coupled antiferromagnetically to an SQ ligand radical, models
Fe(phenSQ)1 [B3LYP] and Fe(phenSQ)2 [BP86]). Because model
spectra recorded at 298 K. Mo
software WMOSS (Thomas Kent, See Co., Edina, MN).
̈
ssbauer spectra were analyzed using the
Fe(phenSQ)1 converged to an unreasonable electronic structure, a
3
third S = /
model was generated using the optimized geometry and
2
Electrochemical experiments were performed using a CHI-620D
molecular orbital (MO) descriptions from Fe(phenSQ)2 as the
starting point for a geometry optimization with the B3LYP functional
potentiostat/galvanostat under an N atmosphere. Cyclic voltammetry
2
(
CV) and differential pulse voltammetry (DPV) were performed using
̈
[model Fe(phenSQ)3]. Mossbauer parameters were derived for
a standard three-electrode configuration. The working electrode was a
polished glassy carbon electrode (GCE; 3.0 mm diameter, CH
Instruments), the auxiliary electrode was a platinum wire, and a Ag -
coated silver wire was used as a pseudoreference electrode.
Fe(phenSQ)3 using the same functionals and basis sets as described
above. The DFT calibration for the isomer shift (mm/s) relative to α-
iron at 295 K (eq 1) was performed using experimental data for a test
+
5
4
set of molecules, as was previously described.
Decamethylferrocene (Fc*, 1 mM) was used as the internal standard,
+
δ = α(ρ − C) + β = −0.298(ρ − 11580) + 1.118
and all potentials were referenced to Fc /Fc through the relationship
o
o
(
1)
+
+
44
Fc /Fc = 427 mV + Fc* /Fc*.
The computed charge density at the iron nucleus, ρ(0), for
Fe(phenSQ)3 from this calculation was 11581.0971, which yields δ
0.791 mm/s using eq 1. The same DFT calculation yielded a
Single-crystal XRD data were obtained by mounting crystals using
viscous oil onto plastic mesh and cooling them to the data collection
temperature. Data were collected on a Bruker-AXS APEX II CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
=
quadrupole splitting of ΔE = 2.486 mm/s.
Q
tBu
2
.8.2. [PhTt ]Ni(phenSQ). The starting coordinates for the initial
model of [PhTt ]Ni(phenSQ) were taken from X-ray crystallo-
graphic data. Because the magnetic susceptibility measurement [μ
0
0
.71073 Å). Unit cell parameters were obtained from 36 data frames,
.3° ω, from three different sections of the Ewald sphere. The data sets
tBu
=
eff
were treated with absorption corrections based on redundant
multiscan data. The systematic absences and unit cell parameters
1
2
.18(3) μ ] indicates an S = / system, the spin state of the complex
B
2
1
was modeled as S = / (high-spin nickel(II) coupled antiferromagneti-
cally to an SQ ligand radical or low-spin nickel(II) coordinated by
SQ , models Ni(phenSQ)1 [B3LYP] and Ni(phenSQ)2 [BP86]).
tBu
2
were consistent with Cc and C2/c for [PhTt ]Ni(3,5-DBSQ) and,
−
tBu
tBu -
uniquely, with Pbca for [PhTt ]NiI and with P2 /n for [PhTt
]
−
1
Ni(PhenSQ). The solution in the centrosymmetric space group
option, C2/c, for [PhTttBu]Ni(3,5-DBSQ) yielded chemically reason-
able and computationally stable results of refinement. The structures
were solved using direct methods and refined with full-matrix least-
Because model Ni(phenSQ)1 converged to an unreasonable electronic
1
structure, a third S = / model was generated using the optimized
2
geometry and MO descriptions from Ni(phenSQ)2 as the starting
point for a geometry optimization with the B3LYP functional [model
Ni(phenSQ)3].
2
tBu
squares procedures on F . The iodine atom in [PhTt ]NiI was found
to be disordered in two positions with a refined site occupancy ratio of
.8.3. [PhTttBu]Ni(3,5-DBSQ). The starting coordinates for models of
2
7
5:25. Slight disorder was observed in a tert-butyl group in
tBu
[
PhTt ]Ni(3,5-DBSQ) were taken from X-ray crystallographic data.
tBu
[
PhTt ]Ni(3,5-DBSQ) but could not be modeled satisfactorily.
1
The spin state of this complex was set to S = / (high-spin nickel(II)
2
The disordered groups were refined with three-dimensional U rigid-
−
ij
coupled antiferromagnetically to an SQ ligand radical or low-spin
group restraints. All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were treated as idealized
contributions. Atomic scattering factors are contained in the SHELXL
−
nickel(II) coordinated by SQ , models Ni(3,5-DBSQ)1 [B3LYP] and
Ni(3,5-DBSQ)2 [BP86]). Because model Ni(3,5-DBSQ)1 converged
1
to an unreasonable electronic structure, a third S = / model was
4
5
2
2
013−2014 program libraries. The CIFs have been deposited with
the Cambridge Crystallographic Database as CCDC 1541366−
541368.
.8. Computational Methods. Computational models were
generated using the optimized geometry and MO descriptions from
Ni(3,5-DBSQ)2 as the starting point for a geometry optimization with
the B3LYP functional [model Ni(3,5-DBSQ)3].
1
2
tBu
II
generated for each [PhTt ]M SQ complex and analyzed against
the available experimental data. Unless otherwise stated, the initial
atomic coordinates for each [PhTt ]M SQ model were imported
from crystallographic data. DFT geometry optimizations and single-
point calculations were performed with the ORCA, version 2.9.0 or
3
. RESULTS AND DISCUSSION
tBu
II
tBu -
3.1. Electronic and Magnetic Properties of [PhTt ]
Fe(phenSQ). To further interrogate the electronic structure of
tBu
[
̈
PhTt ]Fe(phenSQ), solid-state Mossbauer spectra were
46
2
.9.1, software package developed by Dr. Frank Neese. Both high-
collected in an applied field of 0.04 T at a range of
temperatures. The zero-field spectrum recorded at 100 K is
displayed in Figure 2. The spectra at 6−100 K exhibit a single
sharp quadrupole doublet, with an isomer shift (δ) of 0.92 mm/
II
spin and low-spin M models of each complex with a semiquinone
radical ligand were considered, and the spin multiplicity of the
complex was specified, rather than metal and ligand oxidation states.
Metal atoms and the immediately ligated oxygen and sulfur atoms
4
7
s and a quadrupole splitting (ΔE ) of 2.29 mm/s, which are
were described with Ahlrich’s polarized triple-ζ-valence basis set,
Q
while the remaining atoms were modeled with the polarized split-
consistent with high-spin iron(II). Temperatures up to 200 K
48
valence basis set. For each spin state, computations were carried out
using the spin-unrestricted formalism and either Becke’s three-
parameter hybrid functional for exchange along with the Lee−
did not significantly affect the quadrupole splitting. A five-
coordinate high-spin ferrous complex with redox-innocent
tBu
ligands, {[PhTt ]FeCl} , which was previously characterized
4
9,50
2
Yang−Parr functional for correlation (B3LYP)
or Becke’s
55
in our laboratories, exhibited Mo
̈
ssbauer parameters of δ =
= 3.45 mm/s, similar to those of
PhTt ]Fe(phenSQ). Recently, Fiedler, Popescu, and co-
workers have explored the oxidation of iron(II) catecolate
complexes with Mossbauer spectroscopy, showing that metal-
based oxidation to iron(III) catecholate leads to a significantly
functional for exchange along with Perdew’s functional for correlation
51−53
0.96 mm/s and ΔE
Q
(
BP86).
Tight self-consistent-field convergence criteria were
tBu
[
specified with an integration grid of 302 Lebedev points. For further
analyses of the computed electronic structures, single-point DFT and
time-dependent DFT (TD-DFT) calculations were performed on the
optimized geometries. The TD-DFT results were used to simulate
absorption spectra, whereby each of the 40 computed transitions was
̈
56
̈
lower isomer shift (0.5 mm/s). Thus, the Mossbauer data of
D
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Inorganic Chemistry
Article
7,58
5
paramagnetic resonance (EPR) and DFT data.
The
ferromagnetic coupling observed in the iron(II) p-semi-
−1
quinonate complexes (DFT-computed J values of ∼65 cm ,
H = −2JS ·S ) was attributed to the orthogonal orientation of
1
2
the magnetic orbitals.
Below 50 K, the χT values dropped sharply, which may be
attributed to zero-field splitting (ZFS) and/or intermolecular
interactions. The field-dependent magnetization data collected
at 1.8 K (Figure S7) slowly increased at lower fields, which is
indicative of intermolecular antiferromagnetic interactions. At
fields above 5 T, magnetization approached saturation at 2.1 μB,
well below the expected value, which is also indicative of ZFS
and/or intermolecular interactions. The susceptibility data were
ssbauer spectrum of [PhTttBu]Fe(phenSQ) at
̈
2
2
2
Figure 2. Zero-field Mo
00 K. The solid red line is a least-squares fit obtained with the
parameters shown.
fitted to the Hamiltonian H = −2J(S ·S ) + DS + E(S − S )
1
2
z
x
y
1
59
+
gβB·S using PHI software. The χT values in the high-
temperature regime (300−50 K) were used to calculate a
−1
coupling constant of −127 cm with g = 2.22. Fits of the low-
temperature range were used to estimate the ZFS parameters of
the iron(II) ion (D and E) and intermolecular interactions (zJ).
tBu
[
PhTt ]Fe(phenSQ) strongly support its description as a
high-spin iron(II) semiquinonate species.
To study the magnetic interaction between the high-spin
iron(II) and the o-semiquinonate ligand, the temperature-
−1
−1
Best fits were obtained for D = +14.5 cm , E = 0.001 cm ,
−1
and zJ = −0.23 cm . No satisfactory fits were obtained without
tBu
dependent magnetic susceptibility of [PhTt ]Fe(phenSQ) in
including intermolecular interactions. The field-dependent
magnetization curves at different temperatures (1.8−4.5 K)
are nonsuperimposable, indicating the presence of ZFS (Figure
S8).
the solid state was probed by SQUID magnetometry. As shown
3
−1
in Figure 3, the χT value of 2.58 cm K mol at 300 K (μeff
=
DFT calculations were performed using the broken-
symmetry approach to provide a detailed description of the
tBu
electronic structure of [PhTt ]Fe(phenSQ). Starting from the
tBu
molecular structure of [PhTt ]Fe(phenSQ) determined by
3
XRD, geometry optimizations for the S = / spin state were
2
performed using both the BP86 and B3LYP functionals. The
metric parameters and electronic structure descriptions
obtained from the experimental data and DFT calculations
are provided in Table 1. While model Fe(phenSQ)1 fails to
Table 1. Selected Bond Distances (Å) for
tBu
[
PhTt ]Fe(phenSQ) Obtained by XRD and DFT, along
with Electronic Structure Descriptions Derived from
a
Experimental Data and DFT Calculations
Fe(phenSQ)1
Fe(phenSQ) Fe(phenSQ)
2 (BP86, 3 (B3LYP,
(
B3LYP, S =
3
3
3
bond distance
XRD
/
2
)
S = /
2
)
2
S = / )
Figure 3. Temperature dependence of χT (blue circles) for
tBu
[
PhTt ]Fe(phenSQ). The susceptibility data were fitted (black and
Fe1−O1
Fe1−O2
O1−C22
O2−C35
C35−C22
Fe1−S2(ax)
Fe1−S1(eq)
Fe1−S3(eq)
2.064(2)
2.015(2)
1.285(2)
1.285(2)
1.435(3)
2.388(1)
2.456(1)
2.506(1)
1.932
1.926
1.311
1.311
1.412
2.544
2.387
2.372
2.062
2.150
−1
green lines) as described in the text, yielding J = −127 cm (g = 2.22),
D = +14.5 cm , E = 0.001 cm , and zJ = −0.23 cm .
2.033
1.302
1.304
1.444
2.307
2.474
2.509
2.029
1.279
1.292
1.446
2.439
2.509
2.591
−1
−1
−1
4
.54 μ ) comports with the solution magnetic moment at room
B
temperature [μ = 4.65(2) μ ], confirming the same spin state
eff
B
in the solid state and in solution. As the temperature was
lowered from 300 to 50 K, the χT value slowly decreased to
3
−1
electronic
structures
high-spin FeII intermediate-
with AF-
coupled SQ
high-spin FeII high-spin FeII
with AF-
coupled SQ
2
.19 cm K mol , which is slightly higher than the spin-only
II
spin Fe with
F-coupled SQ
with AF-
coupled SQ
3
−1
3
value of 1.88 cm K mol for an S = / , g = 2.0 system but
2
much lower than what is expected for the uncoupled (i.e., S = 2
a
AF = antioferromagnetic. F = ferromagnetic.
1
3
−1
tBu
and / ) system of 3.38 cm K mol . Thus, [PhTt ]Fe-
(
2
3
phenSQ) adopts an S = /2 ground state derived from
antiferromagnetic coupling between the high-spin iron(II) (S =
1
2
) and the o-semiquinonate radical (S = / ). The increase in
predict the correct spin state of iron, the geometries and
ground-state electronic structures of models Fe(phenSQ)2 and
Fe(phenSQ)3 are consistent with the experimental data.
However, because the TD-DFT-calculated electronic absorp-
tion spectrum for Fe(phenSQ)2 agrees poorly with the
experimental data (Figure S11), only the results obtained for
model Fe(phenSQ)3 will be discussed further.
2
χT as the temperature was raised from 50 to 300 K is likely due
to the thermal population of the S = /2 excited state.
Interestingly, Fiedler’s mononuclear five-coordinate iron(II) p-
semiquinonate complexes exhibit ferromagnetic coupling
between the high-spin iron(II) center and the p-semiquinonate
radical to yield an S = / ground state, as indicated by electron
5
5
2
E
DOI: 10.1021/acs.inorgchem.7b01491
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Inorganic Chemistry
The DFT-computed Mo
Article
1
−1
̈
ssbauer parameters for Fe-
the broad band at 935 nm (539 M− cm ) in the experimental
(
phenSQ)3, δ = 0.79 mm/s and ΔE = 2.49 mm/s, are in
spectrum.
Q
good agreement with the experimental isomer shift δ = 0.92
The good agreement between the geometric structures and
tBu
mm/s and quadrupole splitting ΔE = 2.29 mm/s (Table 2).
spectroscopic data obtained experimentally for [PhTt ]Fe-
Q
−1
Likewise, the calculated coupling constant, J = −79.7 cm ,
agrees reasonably well with the experimental value, J = −127
(phenSQ) and predicted computationally for model Fe-
(phenSQ)3 warrants a closer inspection of the calculated
electronic structure. The qualitative MO diagram for model
Fe(phenSQ)3 derived from the DFT results (Figure 5) reveals
that the iron(II) ion is high-spin (S = 2), with one electron pair
occupying the Fe d -based MO and the four unpaired spin-up
electrons occupying the remaining Fe d-based MOs. The spin-
up electron in the Fe d -based MO is coupled to the spin-
−
1
cm (Table 2).
̈
Table 2. Experimental and DFT-Calculated Mossbauer
Parameters and Coupling Constant, J, Derived from Model
yz
Fe(phenSQ)3
xz
δ (mm/s)
ΔEQ (mm/s)
J (cm−1)
down electron in the SQ π*-based MO with a modest spatial
overlap of S = 0.28, giving rise to the antiferromagnetic
coupling between the high-spin iron(II) and the SQ radical
observed by SQUID magnetometry.
experimental
calculated
0.92
0.79
2.29
2.49
−127
−79.7
3.2. O Reactivity of [PhTttBu]Fe(phenSQ). Numerous
The TD-DFT-calculated electronic absorption spectrum for
model Fe(phenSQ)3 is in good agreement with the
experimental spectrum (Figure 4). The transitions associated
2
iron(III) catecholate complexes modeling the reactivity of
intradiol catechol dioxygenases have been reported over the last
15−17,60−68
3
0 years.
While the intradiol cleaving reactivity of the
enzymes and the model complexes was often attributed to the
iron(II) semiquinonate character within the iron(III) catecho-
late species, we are unaware of any well-characterized iron(II)
semiquinonate species exhibiting intradiol cleaving reactivity.
Thus, our well-characterized iron(II) o-semiquinonate complex,
tBu
[
PhTt ]Fe(phenSQ), afforded a unique opportunity to test
the “iron(II) semiquinonate” mechanistic hypothesis by
reacting it with O2.
tBu -
Dry O was introduced to a toluene solution of [PhTt ]
2
Fe(phenSQ) at −78 °C, and the reaction mixture was allowed
to warm to ambient temperature. The mass spectrum of the
products (Figure 6a) was compared to the calculated mass
spectra of phenanthrenequinone (Figure 6b) and diphenic
anhydride (Figure 6c). The results obtained are consistent with
the formation of diphenic anhydride, the intradiol cleavage
product, as well as phenanthrenequinone. To further character-
ize the organic products, the reaction was performed on a
preparative scale. The organic products were dissociated from
the iron via the addition of 0.5 M HCl followed by extraction
into ethyl acetate. Signals of diphenic anhydride were
1
identifiable in the H NMR spectrum of the products, although
they partially overlapped with the signals of other products
(
Figure 6d). The yields of diphenic anhydride (16%) and
Figure 4. Comparison of the experimental electronic absorption
spectrum of [PhTt ]Fe(phenSQ) and the TD-DFT-calculated
spectrum for model Fe(phenSQ)3. EDDMs for the relevant
transitions are shown on top of the computed spectrum.
phenanthrenequinone (4%) were determined by GC (Figure
S22). The reaction of [PhTt ]Fe(phenSQ) with O is
depicted in Scheme 3. Notably, the formation of diphenic
anhydride through oxygenation of [PhTt ]Fe(phenSQ)
tBu
tBu
2
tBu
represents the first example of intradiol cleaving reactivity of
a synthetic iron(II) semiquinonate complex. Furthermore, it
with the major features in the computed spectrum were
assigned on the basis of electron density difference maps
validates our previous assertion that the observed intermedi-
tBu
ate(s) of the reaction of [PhTt ]Fe(phenSQ) with O at low
2
36
(
EDDMs; Figures 4 and S16). The transitions responsible for
temperature is relevant to the intradiol reactivity. The reason
for the low yield of the intradiol cleavage product is not clear.
We propose that boronic esters derived from the phenSQ
ligand may be generated because a similar product was
−1
−1
the two intense features at 384 nm (16736 M cm ) and 412
nm (14784 M− cm ) have primarily phenSQ-to-Fe charge-
1
−1
II
II
transfer character and both phenSQ-to-Fe charge-transfer and
tBu
phenSQ π → π* character, respectively. The transitions
identified in the oxygenation reaction of [PhTt ]Co(3,5-
−
1
36
associated with the weaker features at 571 nm (3575 M
DBSQ). Indeed, in addition to diphenic anhydride and
−1
−1
−1
cm ) and 592 nm (3693 M cm ) have large contributions
from Fe -to-phenSQ and [PhTt ]-to-phenSQ charge-transfer
excitations. These features match with the experimental
phenanthrenequinone, signals consistent with 2-
phenylphenanthro[9,10-d][1,3,2]dioxaborole (m/z 296.10)
and 2-phenoxyphenanthro[9,10-d][1,3,2]dioxaborole (m/z
312.12) were observed in the LIFDI-MS spectrum collected
II
tBu
−1
−1
II
transition at 600 nm (868 M cm ). Finally, the Fe d /
xz
tBu
2
2
d to d
(
transition gives rise to the weak feature at 874 nm
4 M cm ) in the calculated spectrum, which corresponds to
on the reaction mixture of [PhTt ]Fe(phenSQ) and O2
yz
x −y
−
1
−1
without treatment by acid (Figure S24).
F
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Inorganic Chemistry
Article
Figure 5. Qualitative MO diagram derived from a DFT calculation for model Fe(phenSQ)3.
Figure 6. Analysis of the organic products of the reaction of [PhTttBu]Fe(phenSQ) with O . (a) LIFDI-MS spectrum of the organic products. (b)
2
1
Calculated mass spectrum of phenanthrenequinone. (c) Calculated mass spectrum of diphenic anhydride. (d) H NMR spectra of the organic
products (top) and authentic diphenic anhydride (bottom).
.3. Synthesis and X-ray Structures of [PhTttBu]Ni-
findings of the intradiol cleaving reactivity of [PhTt ]Fe-
tBu
3
tBu
(
phenSQ) and [PhTt ]Ni(3,5-DBSQ). Inspired by the
(phenSQ), as well as the previously reported intradiol reactivity
G
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Inorganic Chemistry
Article
Scheme 3. Reaction of [PhTttBu]Fe(phenSQ) with O2
Table 3. Selected Bond Distances (Å) for
tBu
tBu
[PhTt ]Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ)
Obtained by XRD and DFT, along with Electronic Structure
Descriptions Derived from Experimental Data and DFT
a
Calculations
Ni(phenSQ) Ni(phenSQ) Ni(phenSQ)
1
(B3LYP,
2 (BP86,
3 (B3LYP,
1
1
1
bond distance
XRD
S = / )
S = / )
S = / )
2
2
2
tBu
36
of [PhTt ]Co(3,5-DBSQ), we sought to assess the role of
Ni−O1
Ni−O2
2.015(2)
1.986(2)
1.280(3)
1.285(3)
1.433(3)
2.303(1)
2.381(1)
2.358(1)
2.017
1.994
1.286
1.288
1.440
2.482
2.398
2.357
2.043
2.052
the metal ion in directing O reactivity. To this end, we
2
2.033
1.293
1.293
1.452
2.296
2.384
2.379
2.050
1.282
1.283
1.447
2.373
2.441
2.438
prepared, characterized, and explored the O reactivity of two
2
O1−C22
O2−C35
C35−C22
Ni1−S2(ax)
Ni1−S1(eq)
Ni1−S3(eq)
tBu
nickel(II) semiquinonate complexes, [PhTt ]Ni(phenSQ)
tBu
and [PhTt ]Ni(3,5-DBSQ).
tBu
The strategy employed for the preparation of [PhTt ]Ni-
tBu
(
phenSQ) and [PhTt ]Ni(3,5-DBSQ) was similar to that
tBu
used for the preparation of [PhTt ]Fe(phenSQ). Metathesis
tBu
tBu
of [PhTt ]NiI with Tl(phenSQ) yielded [PhTt ]Ni-
II
II
II
II
electronic
structures
high-spin Ni low-spin Ni
high-spin Ni high-spin Ni
tBu
(
phenSQ) in 80% yield. Metathesis of [PhTt ]NiI with
with AF-
coupled
SQ
with SQ
with AF-
coupled
SQ
with AF-
coupled
SQ
tBu
Tl(3,5-DBSQ) yielded [PhTt ]Ni(3,5-DBSQ) in 85% yield.
Both complexes are five-coordinate, as revealed by their X-ray
crystal structures, with the phenSQ and 3,5-DBSQ ligands
Ni(3,5-
Ni(3,5-
Ni(3,5-
DBSQ)1
DBSQ)2
DBSQ)3
binding to the nickel center in a bidentate fashion (Figure 7).
(B3LYP,
(BP86,
(B3LYP,
1
1
1
tBu
bond distance
XRD
S = / )
S = / )
S = / )
2
2
2
[
=
PhTt ]Ni(phenSQ) adopts a square-pyramidal geometry (τ
5
tBu
0.00), while the geometry of [PhTt ]Ni(3,5-DBSQ) is
Ni1−O1
Ni1−O2
O1−C22
O2−C27
C22−C27
Ni1−S2(ax)
Ni1−S1(eq)
Ni1−S3(eq)
2.026(1)
1.971(1)
1.282(2)
1.287(2)
1.458(2)
2.301(1)
2.382(1)
2.393(1)
2.007
2.064
2.084
6
9
1.986
1.293
1.290
1.464
2.506
2.396
2.362
1.986
1.299
1.302
1.472
2.293
2.372
2.410
2.017
1.285
1.287
1.472
2.381
2.435
2.455
distorted square-pyramidal (τ5 = 0.34). Selected bond
distances are listed in Table 3. The average C−O distances
for [PhTt ]Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ) are
tBu
tBu
1
.283(3) and 1.285(2) Å, respectively, characteristic of
semqiuinonate ligands (1.27−1.31 Å). The average Ni−O
tBu
tBu
distances for [PhTt ]Ni(phenSQ) and [PhTt ]Ni(3,5-
DBSQ) are 2.001(2) and 1.999(1) Å, respectively. These
distances are nearly identical with the average Ni−O distance
II
II
II
II
electronic
structures
high-spin Ni low-spin Ni high-spin Ni high-spin Ni
with AF-
coupled SQ
with SQ
with AF-
coupled SQ
with AF-
coupled SQ
[
2.002(1) Å] of a trigonal-bipyramidal high-spin nickel(II)
70
semiquinonate complex reported by Shultz and co-workers,
a
AF = antiferromagnetic. F = ferromagnetic.
tBu
tBu
indicating that both [PhTt ]Ni(phenSQ) and [PhTt ]Ni-
3,5-DBSQ) are high-spin (S = 1) nickel(II) complexes.
(
tBu -
3
.4. Electronic and Magnetic Properties of [PhTt
]
The temperature-dependent magnetic properties of the
nickel(II) semiquinonate complexes were examined using
tBu
Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ). The signals in the
1
tBu
H NMR spectra of both nickel semiquinonate complexes are
SQUID magnetometry (Figure 9). For [PhTt ]Ni(3,5-
DBSQ), the χT value is 0.52 cm3 K mol at 300 K,
−1
broad and paramagnetically shifted, reflecting the paramagnetic
character of the complexes. The electronic absorption spectrum
corresponding to μeff = 2.03 μ , which is consistent with the
B
tBu
of [PhTt ]Ni(phenSQ) shows LF transitions at 846 nm (710
solution μeff of 2.25 μ at room temperature. This μ value is
B
eff
−
1
−1
tBu
M
cm ) and 966 nm (shoulder). Similarly, [PhTt ]Ni(3,5-
slightly higher than the spin-only value of 1.73 μ expected for
B
−1
−1
1
DBSQ) exhibits LF transitions at 842 nm (823 M cm ) and
an S = / ground state but much lower than that for the
2
1
9
66 nm (shoulder), as shown in Figure 8.
uncoupled (S = 1 and / ) system (μ = 3.32 μ ), suggesting
2
eff
B
II
The aforementioned LF transitions are present only when
strong antiferromagnetic coupling between the high-spin Ni (S
= 1) and 3,5-DBSQ radical (S = / ). The χT values are
II
1
the Ni ion is high-spin (S = 1). Indeed, LF transitions of
previously reported five-coordinate high-spin nickel(II) com-
plexes of tris(thioether) borate ligands are found at similar
wavelengths [866 nm (54 M cm ) for [PhTt ]Ni(NO )
and 845 nm (350 M cm ) for [PhTt ]Ni(O ) ].
2
effectively constant throughout the temperature range 2−300
K, which further supports the presence of very strong
−1
−1
tBu
71
antiferromagnetic coupling, leading to population of only the
3
−1
−1
Ad
72
1
S = / state. Above 300 K, χT gradually increased because of
2
2
Figure 7. Crystal structures of [PhTttBu]Ni(phenSQ) (left) and [PhTttBu]Ni(3,5-DBSQ) (right).
H
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Inorganic Chemistry
Article
even at low temperature, intermolecular interactions were not
considered in this fit.
Strong antiferromagnetic coupling was also observed for
tBu
[
[
PhTt ]Ni(phenSQ). The room temperature μeff for
tBu
PhTt ]Ni(phenSQ) in benzene-d is 2.18(3) μ , indicating
6
B
1
3
−1
an S = / ground state. The χT value is 0.505 cm K mol (μeff
2
=
2.0 μ ; g = 2.31) at 300 K. A slightly steeper increase in χT
B
was observed between 300 and 390 K. Additionally, below 15
tBu
K, the χT values for [PhTt ]Ni(phenSQ) dropped gradually
down to 0.31 cm3 K mol at 2 K, possibly signifying
−1
intermolecular interactions via π−π-stacking pathways in the
solid-state structure. The saturation behavior of the magnet-
1
ization data collected at 1.8 K further supports an S = /
2
coupled ground state (Figure S10). Fitting of the susceptibility
tBu
data revealed a smaller [relative to [PhTt ]Ni(3,5-DBSQ)]
Figure 8. Electronic absorption spectra of [PhTttBu]Ni(phenSQ) and
−1
exchange coupling constant of J = −320 cm and small
tBu
[
PhTt ]Ni(3,5-DBSQ) in toluene. The inset highlights the LF
−1
intermolecular dipolar coupling, zJ = −0.57 cm (g = 2.22,
transitions.
II
assuming J ≫ D). Notably, a five-coordinate Ni (3,5-DBSQ)
complex of a triazamacrocycle ligand reported by Dei and co-
73
workers exhibits strong antiferromagnetic coupling between
II
high-spin Ni and 3,5-DBSQ ligand with μ = 1.89 μ . The
eff
B
absolute value of the coupling constant was estimated to be
greater than 300 cm− because of the lack of excited-state
1
Cum,Me
population. Similarly, in Shultz’s Tp
Ni(nitronyl nitroxide
semiquinone), the calculated J
value is between −244 and
Ni‑SQ
−1
70
−
525 cm . To the best of our knowledge, our measurements
represent the first accurate determination of coupling constants
for five-coordinate high-spin nickel(II) complexes with
antiferromagnetically coupled semiquinonate ligands.
tBu
Following the approach employed for [PhTt ]Fe(phenSQ),
broken-symmetry DFT calculations were performed for
tBu
tBu
[
PhTt ]Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ) to obtain
detailed descriptions of the electronic structures and to
correlate the electronic structures with the magnetic properties.
Geometry optimizations based on the X-ray crystal structures
1
were performed for the S = / spin ground state using both the
2
BP86 and B3LYP functionals. When the B3LYP functional was
used, the calculated geometric and electronic structures of
tBu
tBu -
[
PhTt ]Ni(phenSQ) [model Ni(phenSQ)1] and [PhTt ]
Ni(3,5-DBSQ) [model Ni(3,5-DBSQ)1] did not agree with the
experimental data (Table 3). Although the models obtained
using the BP86 functional, Ni(phenSQ)2 and Ni(3,5-DBSQ)2,
accurately reproduce the experimental molecular structures and
are predicted to have the correct spin distribution, the TD-
DFT-calculated electronic absorption spectra are inconsistent
with the experimental data (Figures S12 and S13). However,
when models Ni(phenSQ)2 and Ni(3,5-DBSQ)2 were
subjected to further geometry optimizations using the B3LYP
functional, the resulting models, Ni(phenSQ)3 and Ni(3,5-
DBSQ)3, exhibited geometric and electronic structures and
afforded TD-DFT-calculated absorption spectra consistent with
the corresponding experimental data.
Figure 9. χT versus T plots (blue circles) for [PhTttBu]Ni(phenSQ)
tBu
(
top) and [PhTt ]Ni(3,5-DBSQ) (bottom). The susceptibility data
−1
were fitted (black line) as described in the text, yielding J = −320 cm
−1
tBu
The TD-DFT-calculated absorption spectrum for the
(g = 2.22) and zJ = −0.57 cm for [PhTt ]Ni(phenSQ) and J =
−1
tBu
Ni(phenSQ)3 model is in excellent agreement with the
−
382 cm (g = 2.24) for [PhTt ]Ni(3,5-DBSQ).
tBu
experimental spectrum of [PhTt ]Ni(phenSQ) (Figure 10).
Inspection of the EDDMs provided in Figures 10 and S17
3
−1
−1
the thermal population of the S = / excited state. The field-
reveals that the intense feature at 397 nm (15967 M cm ) in
the computed spectrum, which corresponds to the prominent
2
dependent magnetization data collected at 1.8 K show
saturation behavior at fields above 5 T, consistent with an S
feature at 398 nm (12877 M− cm ) in the experimental
spectrum, arises from a semiquinonate intraligand transition.
Toward lower energy, the transitions associated with the
1
−1
1
=
/2 coupled ground state (Figure S9). Fitting of the
2
susceptibility data to the Hamiltonian H = −2J(S ·S ) + DS
1
2
z
2
2
59
−1
−1
+
−
E(S − S ) + gβB·S using PHI software yielded a large J of
computationally predicted features at 547 (3497 M cm )
x
y
−1
−1
−1
II
382 cm (g = 2.24). Because the χT values remain constant
and 555 nm (4864 M cm ) have primarily Ni -to-phenSQ
I
DOI: 10.1021/acs.inorgchem.7b01491
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Inorganic Chemistry
Article
corresponds to the broad band at 846 nm (710 M− cm ) in
the experimental spectrum, arises from a nickel(II) LF
transition.
1
−1
The TD-DFT calculated absorption spectrum for model
tBu -
Ni(3,5-DBSQ)3 and the experimental spectrum of [PhTt ]
Ni(3,5-DBSQ) also agree quite well (Figure S14). Because
these spectra and the underlying transitions are similar to those
tBu
of [PhTt ]Ni(phenSQ) described above, they will not be
discussed further.
A qualitative MO diagram for model Ni(phenSQ) derived
3
from our DFT results is shown in Figure 11. Consistent with
II
our experimental data, the Ni is high-spin, with two unpaired
2
2 2
spin-up electrons occupying the Ni(d )- and Ni(d - )-based
z
x y
2
z
MOs. The spin-up electron in the Ni(d )-based MO is coupled
to the spin-down electron in the phenSQ π*-based MO with
spatial overlap of S = 0.31, leading to the strong
antiferromagnetic coupling observed by SQUID magnetometry.
−1
The calculated coupling constant, J = −273 cm , is in good
−1
agreement with the SQUID-derived value, J = −320 cm .
As expected on the basis of the experimental and TD-DFT-
computed absorption spectra, the MO diagram for model
Ni(3,5-DBSQ)3 (Figure S15) closely resembles that obtained
Figure 10. Comparison of the experimental absorption spectrum of
tBu
for Ni(phenSQ)3 in Figure 11. Again, the spin-up electron in
[
PhTt ]Ni(phenSQ) and the TD-DFT calculated spectrum for
2
z
the Ni(d )-based MO is coupled to the spin-down electron in
model Ni(phenSQ)3. EDDMs for the relevant transitions are shown
on top of the computed spectrum.
the 3,5-DBSQ π*-based MO with spatial overlap of S = 0.31,
leading to strong antiferromagnetic coupling. The larger
coupling constant calculated for this complex, J = −282 versus
−273 cm− for [PhTt ]Ni(phenSQ), agrees with our SQUID
1
tBu
charge-transfer character. Lastly, the weak feature in the
−1
−1
tBu
computed spectrum at 719 nm (1533 M cm ), which
data, which show that [PhTt ]Ni(3,5-DBSQ) features
Figure 11. Qualitative MO diagram for model Ni(phenSQ)3 derived from the DFT results.
J
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
II
tBu
stronger antiferromagnetic coupling between the Ni ion and
the semiquinonate ligand. Similarly strong antiferromegnetic
coupling between high-spin nickel(II) and a ligand derived
radical, superoxide, has been proposed to best describe the
electronic structures of nickel−dioxygen adducts, including
[PhTt ]Ni(3,5-DBSQ) are summarized in Scheme 4. The
tBu
difference in the O reactivity between [PhTt ]Ni(phenSQ)
2
Scheme 4. O Reactivity of [PhTttBu]Ni(phenSQ) and
2
tBu
[PhTt ]Ni(3,5-DBSQ)
74,75
those prepared directly from nickel(I) and O .
2
tBu
tBu -
3
.5. O Reactivity of [PhTt ]Ni(phenSQ) and [PhTt ]
2
Ni(3,5-DBSQ). With a thorough understanding of the
tBu
tBu -
electronic structures of [PhTt ]Ni(phenSQ) and [PhTt ]
Ni(3,5-DBSQ) in hand, their O reactivity was investigated.
2
tBu
Stirring a toluene solution of [PhTt ]Ni(phenSQ) under O2
at room temperature resulted in the slow decomposition of the
complex, as indicated by the gradual loss of electronic
absorption bands at 536, 586, and 846 nm (Figure 12). To
tBu
and [PhTt ]Ni(3,5-DBSQ) implies that the dioxolene ligand
also plays an important role in affecting the O reactivity, which
2
was further interrogated through electrochemical studies (vide
infra).
3
.6. Redox Properties of [PhTttBu]Fe(phenSQ),
tBu
tBu
[
PhTt ]Ni(phenSQ), and [PhTt ]Ni(3,5-DBSQ). The
redox properties of [PhTt ]Fe(phenSQ), [PhTt ]Ni-
phenSQ), and [PhTt ]Ni(3,5-DBSQ) were probed using
tBu
tBu
tBu
(
CV and DPV. The cyclic voltammograms of the three
tBu
complexes are shown in Figure 13. The CV of [PhTt ]Fe-
Figure 12. Decomposition of [PhTttBu]Ni(phenSQ) in toluene under
an O2 atmosphere at room temperature monitored by electronic
absorption spectroscopy.
tBu
identify the organic products, the reaction of [PhTt ]Ni-
(
phenSQ) with O was conducted on a preparative scale and
2
monitored by electronic absorption spectroscopy. After
complete decomposition of [PhTt ]Ni(phenSQ), 0.5 M
tBu
HCl was added to liberate the ligand-derived products. The
1
H NMR spectrum of the organic products lacks resonances
due to diphenic anhydride. Indeed, GC indicated that
phenanthrenequinone was produced in 14% yield along with
a negligible amount of diphenic anhydride (ca. 0.5% yield;
Figure S23).
Figure 13. Cyclic voltammograms of 1.0 mM solutions of [PhTttBu]-
Fe(phenSQ), [PhTt ]Ni(phenSQ), and [PhTt ]Ni(3,5-DBSQ) in
tBu
tBu
tBu
The lack of intradiol cleaving reactivity of [PhTt ]Ni-
phenSQ) contrasts with the reactivity displayed by [PhTt ]
THF containing 0.1 M n-Bu NClO under an atmosphere of N (scan
tBu -
4
4
2
(
rate: 100 mV/s).
Fe(phenSQ), highlighting the crucial role of the metal ion in
effecting the intradiol cleavage. The general intradiol cleavage
mechanism invokes the formation of a metal(III) alkylperoxo
2
6,76,77
intermediate.
Because the redox potential for the
(phenSQ) shows irreversible oxidation and reduction events.
Interestingly, while the oxidation process is irreversible at scan
rates up to 1 V/s, detection of the anodic current associated
with the reduction couple is only observed at faster scan rates,
which suggests strongly that the one-electron reduction product
nickel(III)/nickel(II) couple is much higher than that for the
iron(III)/iron(II) couple, a nickel(III) alkylperoxo intermediate
would not form as readily as an iron(III) alkylperoxo
tBu
intermediate. Therefore, in the case of [PhTt ]Ni(phenSQ),
tBu
the rate of electron transfer from the phenSQ ligand to O2
outcompetes the rate of formation of the nickel(III)
alkylperoxo intermediate. As a result, phenanthrenequinone
was formed rather than diphenic anhydride.
of [PhTt ]Fe(phenSQ) is also relatively unstable (Figure
tBu
S25). By contrast, the cyclic voltammogram of [PhTt ]Ni-
(phenSQ) exhibits quasi-reversible oxidation and reversible
reduction processes, suggesting that the kinetic stability of the
tBu
tBu
In contrast to its phenSQ counterpart, [PhTt ]Ni(3,5-
oxidation and reduction products of [PhTt ]Ni(phenSQ) is
tBu
tBu
DBSQ) is inert toward O . No spectral change was observed
much greater than that of [PhTt ]Fe(phenSQ). [PhTt ]Ni-
(3,5-DBSQ) exhibits a reversible reduction and an irreversible
oxidation wave, indicating that on the time scale of the
voltammetry experiments the one-electron-oxidation product of
2
tBu
after stirring of a toluene solution of [PhTt ]Ni(3,5-DBSQ)
under an O atmosphere for 12 h (Figure S19). The results of
2
tBu
the O2 reactivity studies on [PhTt ]Ni(phenSQ) and
K
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 14. Differential pulse voltammograms of [PhTttBu]Fe(phenSQ), [PhTttBu]Ni(phenSQ), and [PhTttBu]Ni(3,5-DBSQ) in THF (1.0 mM)
containing 0.1 M n-Bu NClO . Left: oxidation side. Right: reduction side. Experimental parameters: 0.004 V potential increment, 0.05 V pulse
4
4
amplitude, 0.05 s pulse width, 0.0167 s sampling width, and 0.5 s pulse period.
tBu
[
PhTt ]Ni(3,5-DBSQ) is less stable than the one-electron-
On the reduction side of the differential pulse voltammo-
tBu
tBu
tBu
oxidation product of [PhTt ]Ni(phenSQ).
grams, [PhTt ]Fe(phenSQ) and [PhTt ]Ni(phenSQ) are
reduced at −0.92 and −1.00 V, respectively. These events are
assigned as the phenSQ/phenCat redox couple. The reduction
To garner information regarding the redox potentials at
thermodynamic equilibrium for the irreversible redox events,
differential pulse voltammograms were collected (Figure 14).
tBu
of [PhTt ]Ni(3,5-DBSQ) at −0.94 V is assigned to a 3,5-
tBu
tBu
Oxidation of [PhTt ]Fe(phenSQ) and [PhTt ]Ni(phenSQ)
DBSQ/3,5-DBCat redox couple. A comparison of the
tBu
tBu -
+
0
reduction potentials of [PhTt ]Ni(3,5-DBSQ) and [PhTt ]
Ni(phenSQ) reveals that it is easier to reduce 3,5-DBSQ to its
catecholate form, which explains the inability to isolate an
iron(II) semiquinonate complex with 3,5-DBSQ as the ligand.
was observed at −0.10 and 0.05 V (vs Fc /Fc ), respectively.
These redox events are attributed to ligand-based phenSQ/
phenQ redox cycling, which occur at similar potentials for other
M (phenSQ) complexes.
phenSQ/phenQ couple for [PhTt ]Ni(phenSQ), compared
to that for [PhTt ]Fe(phenSQ), can be attributed to (a) a
greater LF stabilization energy (LFSE) for high-spin Ni d
versus high-spin Fe d and (b) a stronger antiferromagnetic
exchange in [PhTt ]Ni(phenSQ). A similar trend was
observed by Pierpont et al. in their studies of the [Tp
M(3,5-DBSQ) series (M = Zn , Cu , and Co ). In their case,
the zinc(II) analogue, which has no LFSE and no exchange
interaction between the diamagnetic Zn ion and the
semiquinonate radical, has an oxidation potential that is 0.15
and 0.17 V lower than those of the copper(II) and cobalt(II)
analogues, respectively. The oxidation of [PhTt ]Ni(3,5-
DBSQ) to [PhTt ]Ni(3,5-DBQ) occurs at 0.28 V, which is
.23 V higher than the potential at which [PhTt ]Ni-
phenSQ) is oxidized to [PhTt ]Ni(phenQ) . These data
II
78,79
The higher potential of the
tBu
tBu
4. CONCLUSIONS
II
8
The geometric and electronic structures of a previously
tBu
II
6
reported iron(II) o-semiquinonate complex, [PhTt ]Fe-
36
tBu
(
phenSQ), and two new nickel(II) o-semiquinonate com-
tBu
tBu
Cum,Me -
plexes, [PhTt ]Ni(phenSQ) and [PhTt ]Ni(3,5-DBSQ),
were elucidated by X-ray crystallography, optical and NMR
spectroscopies, SQUID magnetometry, and DFT calculations.
]
II
II
II 80
II
II
All three complexes contain high-spin M ions antiferromag-
netically coupled to the semiquinonate radicals, with [PhTt ]
tBu -
Ni(3,5-DBSQ) having the largest exchange interaction (J =
−
1
tBu
tBu
−382 cm ), followed by [PhTt ]Ni(phenSQ) (J = −320
−1 tBu −1
tBu
+
cm ) and then [PhTt ]Fe(phenSQ) (J = −127 cm ). The
trend of the magnitude of exchange interactions between the
M ions and the semiquinonate ligands is well reproduced by
tBu
0
(
II
tBu
+
the DFT calculations.
tBu
indicate that [PhTt ]Ni(3,5-DBSQ) is more resistant toward
oxidation of the semiquinonate ligand than [PhTt ]Ni-
phenSQ), which provides an explanation for why [PhTt ]
Ni(3,5-DBSQ) does not react with O , whereas [PhTt ]Ni-
The fact that [PhTttBu]Fe(phenSQ) exhibits intradiol
tBu
cleaving reactivity provides new evidence supporting the theory
that the iron(II) semiquinonate character within the iron(III)
tBu -
(
tBu
2
catecholate species could be responsible for the O reactivity of
2
(
phenSQ) does.
intradiol catechol dioxygenases. The O2 reactivity studies
further reveal the importance of the metal ion on the intradiol
The oxidation potentials of [PhTttBu]Fe(phenSQ) and
tBu
[
PhTt ]Ni(phenSQ) also provide clues as to whether the
II
cleaving reactivity because changing the metal ion from Fe to
oxidation product phenQ is generated through an outer-sphere
or inner-sphere electron-transfer mechanism. The redox
II
Ni abolishes the intradiol reactivity and makes the oxidation of
phenSQ to phenQ more pronounced. The metal-ion effect is
caused by the difference in the redox potentials of the
metal(III)/metal(II) couples and, likely, differences in the O2
binding affinity. When the metal(III)/metal(II) potential is
−
potential of O /O is around −1.0 V versus SCE (ca. − 1.5
2
2
+
0
81
V vs Fc /Fc ), which is much lower than the redox potential
of phenSQ/phenQ. Thus, direct electron transfer from phenSQ
tBu
II
to O is unlikely. Instead, phenQ may be generated through an
inner-sphere electron-transfer mechanism, which involves
binding of O to the metal, followed by electron transfer
from the phenSQ ligand to the M(O ) moiety. Under this
mechanism, the low O affinity of nickel(II) disfavors the O2
binding and, therefore, makes the formation of phenQ sluggish.
2
high, as in the case of [PhTt ]Ni(phenSQ), the Ni ion acts
as a mediator for the inner-sphere electron transfer from
2
phenSQ to O , generating phenQ. On the other hand, when
2
2
the midpoint potential of the metal(III)/metal(II) couple is
sufficiently low, the formation of the metal(III) alkylperoxo
intermediate outcompetes the inner-sphere electron transfer
2
L
DOI: 10.1021/acs.inorgchem.7b01491
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
and, thus, the reaction produces primarily the intradiol cleavage
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Valence Tautomerism in Co Complexes. Acc. Chem. Res. 2007, 40,
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product. Last, the lack of any O reactivity of [PhTt ]Ni(3,5-
2
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(
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DBSQ) indicates that the midpoint potential of the semi-
quinonate/quinone couple also affects the outcome of the O2
reactions. This assertion is further supported by the results of
the electrochemical studies, which show that the midpoint
potential for the 3,5-DBSQ/DBQ couple is higher than that of
the phenSQ/phenQ couple.
3
(
P. D. Synthesis, Characterization, and Photophysical Studies of an
Iron(III) Catecholate-Nitronylnitroxide Spin-Crossover Complex.
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ASSOCIATED CONTENT
■
̀
(9) Girerd, J. J.; Boillot, M. L.; Blain, G.; Riviere, E. An EPR
*
S
Supporting Information
investigation of the electronic structure of pseudo-octahedral and spin
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01491.
(10) Simaan, A. J.; Boillot, M. L.; Carrasco, R.; Cano, J.; Girerd, J. J.;
Mattioli, T. A.; Ensling, J.; Spiering, H.; Gutlich, P. Electronic,
̈
1H NMR and LIFDI-MS spectra, magnetization data,
calculated EDDMs, TD-DFT-calculated absorption
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iron system in the solid state: theoretical study of the electronic nature
of the doublet and sextet states. Chem. - Eur. J. 2005, 11, 1779−1793.
spectra for models Fe(phenSQ) , Ni(phenSQ) , and
2
2
Ni(3,5-DBSQ) , experimental TD-DFT-calculated ab-
(11) Floquet, S.; Simaan, A. J.; Rivier
̀ ́
e, E.; Nierlich, M.; Thuery, P.;
2
sorption spectra, MO diagram for model Ni(3,5-
Ensling, J.; Gutlich, P.; Girerd, J. J.; Boillot, M. L. Spin crossover of
̈
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3
tBu
structure, magnetic and Mossbauer investigations. Dalton Trans. 2005,
[
PhTt ]Fe(phenSQ) (PDF)
1
(
734−1742.
Accession Codes
12) Simaan, A. J.; Boillot, M.-L.; Rivier
̀
e, E.; Boussac, A.; Girerd, J.-J.
III
CCDC 1541366−1541368 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
A Two-Step Spin Crossover in [(TPA)Fe (cat)]BPh . Angew. Chem.,
4
Int. Ed. 2000, 39, 196−198.
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F.; Kru
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50−953.
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A.; Zimmer, M.; Gerhards, M.; van Wullen, C.; Kruger, H.-J.; Diller, R.
̈
ger, H. J. Temperature-induced spin-transition in a low-spin
9
(
̈
n,
̈
̈
AUTHOR INFORMATION
■
Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast
Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical
Complex. Chem. - Eur. J. 2017, 23, 2119−2132.
Corresponding Author
E-mail: riordan@udel.edu
*
(
15) Bugg, T. D. H. Dioxygenase enzymes: catalytic mechanisms and
chemical models. Tetrahedron 2003, 59, 7075−7101.
16) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Dioxygen
ORCID
Thomas C. Brunold: 0000-0001-6516-598X
Charles G. Riordan: 0000-0002-8572-4571
(
Activation at Mononuclear Nonheme Iron Active Sites: Enzymes,
Notes
Models, and Intermediates. Chem. Rev. 2004, 104, 939−986.
(
17) Palaniandavar, M.; Mayilmurugan, R. Mononuclear non-heme
The authors declare no competing financial interest.
iron(III) complexes as functional models for catechol dioxygenases. C.
R. Chim. 2007, 10, 366−379.
ACKNOWLEDGMENTS
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(18) Dey, S. K.; Mukherjee, A. Catechol oxidase and phenoxazinone
C.G.R. acknowledges financial support of the National Science
Foundation (NSF) through Award CHE-1112035. C.V.P.
acknowledges support from the NSF through Award CHE-
synthase: Biomimetic functional models and mechanistic studies.
Coord. Chem. Rev. 2016, 310, 80−115.
(19) Jastrzebski, R.; Weckhuysen, B. M.; Bruijnincx, P. C. Catalytic
oxidative cleavage of catechol by a non-heme iron(III) complex as a
green route to dimethyl adipate. Chem. Commun. 2013, 49, 6912−
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1445959. M.M.K. acknowledges financial support through an
NSF graduate fellowship. K.R.D. gratefully acknowledges the
Robert A. Welch Foundation (A-1449) for financial support.
(20) Chatterjee, S.; Sheet, D.; Paine, T. K. Catalytic and regiospecific
extradiol cleavage of catechol by a biomimetic iron complex. Chem.
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