Ligand-Induced Tuning of the Oxidase Activity of μ-Hydroxidodimanganese(III) Complexes Using 3,5-Di-tert-butylcatechol as the Substrate: Isolation and Characterization of Products Involving an Oxidized Dioxolene Moiety

Article abstract of DOI:10.1021/acs.inorgchem.7b00147

Oxidase activities of a μ-hydroxidodimanganese(III) system involving a series of tetradentate capping ligands H₂LR₁,R₂ with a pair of phenolate arms have been investigated in the presence of 3,5-di-tert-butylcatechol (H

Full text of DOI:10.1021/acs.inorgchem.7b00147

Article
pubs.acs.org/IC
Ligand-Induced Tuning of the Oxidase Activity of
μ‑Hydroxidodimanganese(III) Complexes Using 3,5-Di-tert-
butylcatechol as the Substrate: Isolation and Characterization of
Products Involving an Oxidized Dioxolene Moiety
Dhrubajyoti Mondal,† Sanchita Kundu,† Mithun Chandra Majee,† Atanu Rana,† Akira Endo,‡
and Muktimoy Chaudhury*,†
†Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
‡Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo
102-8554, Japan
S
* Supporting Information
ABSTRACT: Oxidase activities of a μ-hydroxidodimanganese(III)
system involving a series of tetradentate capping ligands H2LR ,R
1
2
with a pair of phenolate arms have been investigated in the presence
of 3,5-di-tert-butylcatechol (H2DBC) as a coligand cum-reductant.
The reaction follows two distinctly different paths, decided by the
substituent combinations (R1 and R2) present in the capping ligand.
With the ligands H2Lt‑Bu,t‑Bu and H2Lt‑Bu,OMe, the products obtained
are semiquinonato compounds [MnIII(Lt‑Bu,t‑Bu)(DBSQ)]·2CH3OH
(1) and [MnIII(Lt‑Bu,OMe)(DBSQ)]·CH3OH (2), respectively. In the
process, molecular oxygen is reduced by two electrons to generate
H2O2 in the solution, as confirmed by iodometric detection. With
the rest of the ligands, viz., H2LMe,Me, H2Lt‑Bu,Me, H2LMe,t‑Bu, and
+
H2LCl,Cl, the products initially obtained are believed to be highly reactive quinonato compounds [MnIII(LR ,R )(DBQ)] , which
1
2
undergo a domino reaction with the solvent methanol to generate products of composition [MnIII(LR ,R )(BMOD)] (3−6)
1
2
involving a nonplanar dioxolene moiety, viz., 3,5-di-tert-butyl-3-methoxy-6-oxocyclohexa-1,4-dienolate (BMOD−). This novel
dioxolene derivative is formed by a Michael-type nucleophilic 1,4-addition reaction of the methoxy group to the coordinated
+
quinone in [MnIII(LR ,R )(DBQ)] . During this reaction, molecular oxygen is reduced by four electrons to generate water. The
products have been characterized by single-crystal X-ray diffraction analysis as well as by spectroscopic methods and magnetic
measurements. Density functional theory calculations have been made to address the observed influence of the secondary
coordination sphere in tuning the two-electron versus four-electron reduction of dioxygen. The semiquinone form of the
dioxolene moiety is stabilized in compounds 1 and 2 because of extended electron delocalization via participation of the
appropriate metal orbital(s).
1
2
literature that control the two-electron versus four-electron
reduction of O2 catalyzed by metal-ion activators. These
include their optimized coordination geometry, steric factors,
and donor-atom combination and the net charge provided by
the primary coordination sphere that surrounds the metal
center.7
Nature uses various transition-metal ions for the activation of
O2, which is otherwise kinetically inert.8 Both iron and copper
have shown their preference for catalyzing those oxidation
reactions.9,10 Manganese also exhibits rich O2 chemistry in both
biological and synthetic systems. Its useful redox properties
have been utilized by nature in accumulating manganese in a
host of redox-active metalloenzymes.11 A number of key
INTRODUCTION
■
Activation of dioxygen (O2) by transition-metal ions plays a
pivotal role in many biological and industrial oxidation
processes.1 Oxidases are a group of ubiquitous metalloenzymes
that use such activated O2 as an electron acceptor to carry out
the proton-assisted selective oxidation of organic substrates.2 In
the process, oxygen is often reduced by four electrons to
generate water or by two electrons to generate hydrogen
peroxide (H2O2) depending upon the types of the metal-ion
activators involved.3 The former type of oxidation plays an
essential role in the sustenance of life processes (aerobic
respiration).4 Industrially, also oxidation of this type is
harnessed quite efficiently in the development of new fuel
cell technology.5 A two-electron O2 reduction, on the other
hand, generates H2O2, a versatile oxidizing agent ideal for doing
green chemistry.6 Various factors have been reported in the
Received: January 17, 2017
© XXXX American Chemical Society
A
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Article
reactions involving O2 and its reduced forms (water, peroxide,
and superoxide) are promoted by many nanganese-dependent
enzymes that include lipoxygenase,12 ribonucleotide reduc-
tase,13 superoxide dismutase,14 catalases,15 and the oxygen-
evolving complex of photosystem II.16 A large number of
bioinspired model manganese complexes are known to mimic
the structure and/or reactivity of the aforementioned
enzymes.17,18
between two and four electrons during the course of these
oxidase actions.
EXPERIMENTAL SECTION
■
Materials. All reactions were carried out in an aerobic environment
unless stated otherwise, with chemicals available from commercial
sources and used as received. The solvents were of reagent grade and
were dried by a standard procedure20 and distilled under nitrogen
prior to their use. The tetradentate bisphenol N2O2 ligands H2LR ,R
1
2
Herein we report the oxidase activity of a new manganese-
(III) system involving a family of facially coordinating N2O2
(summarized in Scheme 1) were prepared following a modified
ligands with a pair of appended phenolate arms (H2LR ,R
,
procedure of a reported method.21
1
2
Synthesis of Ligands. A modified procedure for the synthesis of
N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-dimethylethylene-
1,2-diamine (H2Lt‑Bu,t‑Bu) is described here as a prototype. To a
solution of 2,4-di-tert-butylphenol (4.12 g, 20 mmol) in methanol (30
mL) were added N,N-dimethylethylenediamine (0.88 g, 10 mmol) and
paraformaldehyde (60 mg, 20 mmol). The resulting solution was
refluxed for 15 h. It was then cooled to room temperature and rotary-
evaporated to ca. 10 mL volume. The resulting white precipitate was
filtered off, washed with 20 mL of cold methanol, and recrystallized
from a dichloromethane/methanol (1:1, v/v) mixture. Yield: 3.7 g
(70%). Mp: 131 °C. Anal. Calcd for C34H56N2O2: C, 77.75; H, 10.94;
shown in Scheme 1) that combine with [Mn(ClO4)2]·6H2O in
Scheme 1. Synthesis Protocol for Complexes 1−6
1
N, 5.31. Found: C, 77.84; H, 10.80; N, 5.30. H NMR (400 MHz,
CDCl3, δ/ppm): 1.27 (s, 18H, CH3 of t-Bu), 1.39 (s, 18H, CH3 of t-
Bu), 2.32 (s, 6H, CH3 of N(CH3)2), 2.58 (m, J = 7.0 Hz, 4H,
(Me)2NCH2CH2−), 3.61 (s, 4H, ArCH2), 6.88 (d, 4JHH = 2.0 Hz, 2H,
4
aryl), 7.20 (d, JHH = 2.0 Hz, 2H, aryl), 9.81 (s, 2H, phenolic OH).
UV−vis [CH3CN; λmax, nm (ε, L mol−1 cm−1)]: 280 (5600), 228
(13700). ESI-MS (positive) in CH3CN: m/z 525.50 (100%, M + H+).
A representative 1H NMR spectrum of the ligand H2Lt‑Bu,OMe is
displayed in Figure S1 (in the Supporting Information). Detailed
characterization data of the remaining ligands are also summarized in
the Supporting Information.
Preparation of Complexes. Safety Note! Perchlorate salts of metal
complexes are potentially explosive and should be handled only in small
quantities with sufficient care.22
[MnIII(Lt‑Bu,t‑Bu)(DBSQ)]·2CH3OH (1).19 To a stirred solution of
H2Lt‑Bu,t‑Bu (131 mg, 0.25 mmol) in methanol (25 mL) was added
Et3N (0.5 mmol), followed by [Mn(ClO4)2·6H2O] (90 mg, 0.25
mmol), and the solution was refluxed for 1 h, during which a dark-
brown solution was obtained. It was cooled to room temperature, and
3,5-di-tert-butylcatechol (55 mg, 0.25 mmol) was added in small
portions over a period of 1 h with constant stirring. The precipitated
dark-brown microcrystalline solid was isolated by filtration, dried in air,
and finally recrystallized from a dichloromethane/methanol (1:1, v/v)
mixture. Yield: 104 mg (50%). Some of these crystals were of
diffraction quality and were used directly for X-ray crystal structure
analysis. Anal. Calcd for C50H82N2O6Mn: C, 69.66; H, 9.59; N, 3.25.
Found: C, 69.61; H, 9.39; N, 3.32. FT-IR bands (KBr pellet, cm−1):
2956 vs, 2904 s, 2856 m, 1480 s, 1465 vs, 1442 vs, 1409 m, 1359 m,
1259 s, 1244 s, 1203 m, 835 m, 557 m. UV−vis [CH2Cl2; λmax, nm (ε,
L mol−1 cm−1)]: 292 (20000), 370 (7900), 488 (4200), 700 (1350).
μeff (solid, 25 °C): 3.94 μB
[MnIII(Lt‑Bu,OMe)(DBSQ)]·CH3OH (2). This compound was prepared
following essentially the same procedure as that described for
compound 1 using the ligand H2Lt‑Bu,OMe. The X-ray-quality crystals
of this compound were grown from a methanol solution at 4 °C. Yield:
45%. Anal. Calcd for C43H66N2O7Mn: C, 66.39; H, 8.55; N, 3.60.
Found: C, 66.52; H, 8.49; N, 3.67. FT-IR bands (KBr pellet, cm−1):
2954 s, 2869 s, 1490 s, 1461 vs, 1427 s, 1357 m, 1242 s, 1205 s, 1062
m, 821 m, 811 m. UV−vis [CH2Cl2; λmax, nm (ε, L mol−1 cm−1)]: 303
(24200), 377 (7400), 495 (4000), 724 (1550). μeff (solid, 25 °C): 3.89
μB
the mandatory presence of 3,5-di-tert-butylcatechol (H2DBC)19
as a reductant to generate either a semiquinonato or a
quinonato product (1−6) due to a two- or four-electron
reduction of O2, respectively. The products have been
characterized by X-ray diffraction analysis as well as
spectroscopic methods. Density functional theory (DFT)
calculations have been made to identify the causative factor(s)
that tune the changeover of the stoichiometry of O2 reduction
[MnIII(LMe,Me)(BMOD)]·2CH3OH (3).19 Method A. This compound
was prepared following essentially the same procedure as that
described for compound 1 using the ligand H2LMe,Me. X-ray-quality
crystals were grown from a methanol solution at 4 °C. Yield: 47%.
Anal. Calcd for C39H61N2O7Mn: C, 64.62; H, 8.48; N, 3.86. Found: C,
64.38; H, 8.61; N, 3.90. FT-IR bands (KBr pellet, cm−1): 2956 m,
B
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Article
2908 m, 1606 vs, 1473 vs, 1427 s, 1361 m, 1247 s, 1161 m, 1070 m,
956 m, 823 m, 609 m, 557 m. UV−vis [CH2Cl2; λmax, nm (ε, L mol−1
cm−1)]: 281 (18500), 373 (6650), 505 (2900), 661 (660). μeff (solid,
25 °C): 4.54 μB
of these crystals were of diffraction-quality and were used directly for
X-ray crystal structure analysis. Anal. Calcd for C39H61N2O7Mn: C,
64.62; H, 8.48; N, 3.86. Found: C, 64.58; H, 8.19; N, 3.80. This
compound shows an overlapping IR spectrum with compound 3,
prepared by method A.
[MnIII(LMe,t‑Bu)(BMOD)]·CH3OH (4). This compound was prepared
following essentially the same procedure as that described for
compound 1 using the ligand H2LMe,t‑Bu instead of H2Lt‑Bu,t‑Bu. The
X-ray-quality crystals were grown from a methanol solution at 4 °C.
Yield: 50%. Anal. Calcd for C44H69N2O6Mn: C, 68.02; H, 8.95; N,
3.61. Found: C, 67.87; H, 9.07; N, 3.66. FT-IR bands (KBr pellet,
cm−1): 2958 s, 1606 vs, 1475 vs, 1361 m, 1267 m, 1218 m, 1072 m,
950 m, 835 m, 605 m, 549 m. UV−vis [CH2Cl2; λmax, nm (ε, L mol−1
cm−1)]: 277 (17400), 374 (5700), 509 (2300). μeff (solid, 25 °C): 4.88
μB
Physical Measurements. IR spectroscopic measurements were
made on samples pressed into KBr pellets using a Shimadzu 8400S
Fourier transform infrared (FT-IR) spectrometer, while for UV−vis
spectral measurements, a PerkinElmer Lambda 950 UV−vis−near-IR
spectrophotometer was employed. Elemental analyses (for C, H, and
N) were performed at IACS on a PerkinElmer model 2400 series II
CHNS analyzer. The electrospray ionization mass spectrometry (ESI-
MS) spectra in positive-ion mode were measured on a Micromass
QTOF model YA 263 mass spectrometer. The 1H NMR spectra were
recorded on a Bruker model Avance DPX-400 spectrometer using
tetramethylsilane as the internal reference. Magnetic moments at room
temperature were measured on a Gouy balance (Sherwood Scientific,
Cambridge, U.K.). Diamagnetic contributions were estimated using
Pascal’s constants.
Cyclic voltammetry in dichloroethane (DCE) was recorded on a
BAS model 100 B/W electrochemical workstation using a glassy
carbon working electrode and a platinum wire counter electrode.
Silver/silver chloride (Ag/AgCl) was used for the reference electrode
and ferrocene/ferrocenium as the internal standard. Solutions were
∼1.0 mM in samples and contained 0.1 M tetrabutylammonium
perchlorate (TBAP) as the supporting electrolyte.
[MnIII(Lt‑Bu,Me)(BMOD)] (5). This compound was prepared following
essentially the same procedure as that described for compound 1 using
the ligand H2Lt‑Bu,Me instead of H2Lt‑Bu,t‑Bu. The X-ray-quality crystals
were grown from a methanol solution at 4 °C. Yield: 43%. Anal. Calcd
for C43H65N2O5Mn: C, 69.33; H, 8.79; N, 3.76. Found: C, 69.03; H,
8.94; N, 3.77. FT-IR bands (KBr pellet, cm−1): 2954 vs, 2908 s, 1604
vs, 1463 vs, 1442 s, 1251 m, 1157 m, 1078 m, 825 m, 609 m, 555 m.
UV−vis [CH2Cl2; λmax, nm (ε, L mol−1 cm−1)]: 289 (14900), 375
(5650), 511 (1550). μeff (solid, 25 °C): 4.55 μB
[MnIII(LCl,Cl)(BMOD)]·H2O (6). This complex was also prepared in a
manner similar to that for 1 using the ligand H2LCl,Cl. The X-ray-
quality crystals were grown from a methanol solution at 4 °C. Yield:
40%. Anal. Calcd for C33H43N2O6Cl4Mn: C, 52.12; H, 5.70; N, 3.68.
Found: C, 52.52; H, 5.64; N, 3.72. FT-IR bands (KBr pellet, cm−1):
2960 vs, 1606 s, 1458 vs, 1303 s, 1245 m, 1070 m, 865 m, 767 m, 757
m, 597m. UV−vis [CH2Cl2; λmax, nm (ε, L mol−1 cm−1)]: 287
(13000), 376 (4150), 523 (1080). μeff (solid, 25 °C): 4.79 μB
[(LMe,MeMnIII)2(μ-OMe)]BPh4 (7). To a stirred solution of H2LMe,Me
(131 mg, 0.25 mmol) in methanol (25 mL) was added Et3N (0.5
mmol), followed by [Mn(ClO4)2·6H2O] (90 mg, 0.25 mmol), and the
solution was refluxed for 1 h, during which a dark-brown solution was
obtained and then cooled to room temperature. To this was added a
solution of sodium tetraphenylborate (86 mg, 0.25 mmol) in methanol
(5 mL) under constant stirring. The resulting solution was stirred for
another 15 min and then filtered. The filtrate was kept in a refrigerator
at 4 °C. A very thin needle-shaped microcrystalline compound was
grown from this solution after ca. 2 days. A few of these crystals were
barely of diffraction-quality and were used directly for X-ray data
collection. Yield: 240 mg (72%). Anal. Calcd for C69H83N4O5BMn2: C,
70.89; H, 7.16; N, 4.79. Found: C, 70.68; H, 6.99; N, 4.75. FT-IR
bands (KBr pellet, cm−1): 2920 m, 2869 s, 1471 vs, 1249 s, 1161 m,
832m, 821 m, 734m, 705 m, 611m. ESI-MS (positive) in CH3CN:
835.62 [(LMe,MeMnIII)2(μ-OH)]+, 849.63 [(LMe,MeMnIII)2(μ-OMe)]+,
409.32 [LMe,MeMnIII]+.
Detection of Hydrogen Peroxide (H2O2). Accumulation of
H2O2 in the reaction mixture during the synthesis of complexes 1−6 in
an aerobic environment was monitored spectrophotometrically by
−
following the development of the characteristic band at 353 nm for I3
(ε = 26000 L mol−1 cm−1), generated because of oxidation of I− by
H2O2.23,24 In a typical experiment, the ligand H2Lt‑Bu,t‑Bu (32 mg, 0.06
mmol) was added to a solution of Mn(ClO4)2·6H2O (45 mg, 0.06
mmol) in methanol (15 mL), followed by triethylamine (NEt3; 12 mg,
0.12 mmol). The solution was refluxed for 1 h and then cooled to
room temperature. To this cold solution was added H2DBC19 (55 mg,
0.25 mmol), and the resulting solution was stirred for ca. 15 min to
generate the semiquinonato compound 1. The solution was rotary-
evaporated to ca. 5 mL volume. Dichloromethane (20 mL) and an
equal volume of water were added subsequently in order to extract
compound 1 into the dichloromethane layer, leaving H2O2 in water.
The process was repeated three times. The aqueous layer was acidified
with H2SO4 (ca. 10−2 M) to pH 2 to stop further oxidation, and 1 mL
of a 10% aqueous solution of KI and 3 drops of a 3% solution of
ammonium molybdate were added. In the presence of H2O2, iodide is
quantitatively oxidized to liberate iodine, which is stabilized as a I3− ion
in the solution. The rate of this oxidation is usually quite slow but
becomes almost instantaneous after the addition of a catalytic amount
of molybdate solution.25 The growth of a band at 353 nm due to the
[(LMe,t‑BuMnIII)2(μ-OMe)]BPh4 (8). This compound was prepared
following essentially the same procedure as that described for
compound 7 using the ligand H2LMe,t‑Bu. Yield: 68%. Anal. Calcd for
C81H107N4O5BMn2: C, 72.74; H, 8.06; N, 4.19. Found: C, 72.61; H,
7.87; N, 4.23. FT-IR bands (KBr pellet, cm−1): 2960 vs, 2923 s, 1479
vs, 1263 m, 1217 s, 837 m, 734 m, 705 m, 613 m. ESI-MS (positive) in
CH3 CN: 1003.24 [(LM e , t ‑ B u MnI I I )2 (μ-OH)]+ , 511.08
[LMe,t‑BuMnIII(OH)] + H+, 493.02 [LMe,t‑BuMnIII]+.
Compounds 3−6 have been also synthesized following an
alternative procedure (method B). Details of this procedure have
been described below for compound 3 as a representative example.
Compound 3. Method B. To a stirred solution of H2LMe,Me (89 mg,
0.25 mmol) in methanol (25 mL) was added Et3N (0.5 mmol),
followed by [Mn(ClO4)2·6H2O] (90 mg, 0.25 mmol), and the
solution was refluxed for 1 h in an anaerobic environment, during
which a dark-brown solution was obtained and then cooled to room
temperature. 3,5-Di-tert-butyl-o-benzoquinone (55 mg, 0.25 mmol)
was then added in small portions over a period of 1 h with constant
stirring. The resulting solution was filtered over a Celite bed; the
filtrate volume was reduced to about 15 mL by rotary evaporation and
then kept in a refrigerator at 4 °C. The product was obtained as a
brown crystalline solid within 2−3 days. Yield: 100 mg (56%). Some
−
formation of I3 was monitored. Because iodide ion in an acidic
solution is oxidized by aerial O2, blank experiments were also done to
check the experimental data.
DFT Calculations. All calculations were performed in the
Inorganic Chemistry HPC cluster at IACS. The geometries were
optimized using the 6-31g* basis set on Mn, C, N, O, and H atoms
using Gaussian 03, version C03, using the B3LYP functional and spin-
unrestricted formalism.26−29 The fully optimized structures were
confirmed by doing the frequency calculations to show that no
imaginary modes were present for all of these compounds. The final
energy calculations were performed using the 6-311+g* basis set on all
atoms.
X-ray Crystallography. Suitable crystals of 1 (dark-brown block,
0.23 × 0.21 × 0.15 mm3), 2 (dark-brown block, 0.15 × 0.15 × 0.12
mm3), 3 (brown block, 0.24 × 0.20 × 0.15 mm3), 4 (brown block, 0.25
× 0.18 × 0.15 mm3), 5 (brown block, 0.15 × 0.20 × 0.14 mm3), 6
(brown block, 0.18 × 0.15 × 0.12 mm3), 7 (brown needle, 0.15 × 0.06
× 0.05 mm3), and 8 (brown needle, 0.16 × 0.08 × 0.06 mm3) were
mounted on glass fibers coated with perfluoropolyether oil before
mounting. Intensity data for the aligned crystals were measured by
employing a Bruker SMART APEX II CCD diffractometer equipped
C
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Article
Figure 1. Molecular structure and atom-labeling scheme for complex 7 with thermal ellipsoids drawn at the 30% probability level. H atoms have
been omitted for clarity.
with a monochromatized Mo Kα radiation (λ = 0.71073 Å) source at
293(2) K except for compounds 1 and 4, which were measured at
150(2) K. No crystal decay was observed during data collection. In all
cases, absorption corrections based on multiscans using the SADABS
software30 were applied. The structures were solved by direct
methods31 and refined on F2 by a full-matrix least-squares procedure
In order to have a better understanding of these complicated
redox reactions, we decided to reevaluate the whole synthetic
procedure by carrying out the reactions in two consecutive
steps. In the initial step, the capping ligand H2LR ,R in refluxing
methanol was allowed to react with Mn(ClO4)2·6H2O in an
aerobic environment in the presence of Et3N. The products in
all cases, unfortunately, were intractable solids. ESI-MS spectra
of these solids indicate that these are all cationic complexes
1
2
2
based on all data minimizing wR = [∑[w(Fo22 − Fc2)2]/∑(Fo )2]1/2, R
= ∑||Fo| − |Fc||/∑|Fo|, and S = [∑[w(Fo − Fc2)2]/(n − p)]1/2
.
SHELXL-2013 was used for both structure solutions and refine-
ments.32 A summary of the relevant crystallographic data and the final
refinement details is given in Table S1.
III
+
involving the [(LR ,R Mn )2(μ-OH)] molecular ion. However,
using a bulky anion, viz., tetraphenylborate, in the reaction
mixture, we became successful to isolate the OMe-bridged
binuclear MnIII complexes 7 and 8 as crystalline solids when
H2LMe,Me and H2LMe,t‑Bu, respectively, were used as the capping
ligands. Interestingly, for an acetonitrile solution of compound
7, the mass spectral data in the positive-ion mode display the
presence of both μ-OH (m/z 835.62) and μ-OMe (m/z
849.63) binuclear MnIII species, with the former being a
product of hydrolysis in solution (Figure S2). For compound 8,
however, the OMe-bridged form in an acetonitrile solution
exists exclusively as a hydroxido-bridged product,
[(LMe,t‑BuMnIII)2(μ-OH)]BPh4, as revealed from its ESI-MS
(positive) data involving a molecular-ion peak at m/z 1003.24
([M+]; Figure S3). In the case of ligand H2Lt‑Bu,t‑Bu, the product
obtained is a powdery solid that also conforms as a μ-hydroxido
compound, as revealed by ESI-MS (Figure S4). Isolation of the
solid product remained elusive with the remaining ligands.
Nevertheless, their solutions in methanol all correspond to μ-
hydroxido products, as observed for compound 8 (Figure S3).
In the second step, the OMe-bridged (7 and 8) or
hydroxydo-bridged (with ligand H2Lt‑Bu,t‑Bu) precursor com-
pounds were allowed to react at room temperature in methanol
under constant stirring with 2 mol equiv of H2DBC.35 For the
OMe-bridged precursors, the addition of Et3N (1 equiv) is
essential at this stage for completion of this reaction. The
presence of aerial O2 is also mandatory in this step. We believe
this extra amount of base is used up to hydrolyze the
1
2
All non-H atoms were refined anisotropically. The H atoms were
calculated and isotropically fixed in the final refinement [d(C−H) =
0.95 Å, with the isotropic thermal parameter of Uiso(H) = 1.2Uiso(C)].
The SMART and SAINT software packages33 were used for data
collection and reduction, respectively. Crystallographic diagrams were
drawn using the Diamond software package.34 The CCDC reference
numbers for the crystal structures of complexes 1−7 are 1548156−
1548162.
RESULTS AND DISCUSSION
■
Synthesis. A new family of facially coordinating tetradentate
diphenol ligands (H2LR ,R , shown in Scheme 1) have been
synthesized. These ligands in an aerobic environment react with
Mn(ClO4)2·6H2O in methanol in the presence of an added
base (NEt3) and H2DBC as the coligand. The isolated products
1−6 are of two different types, each involving an oxidized form
of the dioxolene coligand, and appear to be controlled by the
substituent combinations (R1 and R2) present in the capping
1
2
ligand (H2LR ,R ) framework. Thus, in compounds 1 and 2, in
which the capping ligands have the substitution combinations
of t-Bu,t-Bu (in 1) and t-Bu,OMe (in 2), the added coligand is
oxidized to the 3,5-di-tert-butylsemiquinonato anion (DBSQ−)
and remains coordinated to the MnIII center. In the rest of the
products (3−6), oxidation of the coligand proceeds further,
leading to the generation of a new monoanionic ligand, viz.,
3,5-di-tert-butyl-3-methoxy-6-oxocyclohexa-1,4-dienolate
(BMOD−) in which the aromaticity of the precursor DBC2−
coligand is lost. A methoxy group from the solvent methanol is
accommodated in the oxidized dioxolene ring, forming a σ-
bond with the lone sp3-hybridized C center in the ring (Scheme
1).
1
2
III
+
[(LR ,R Mn )2(μ-OMe)] species to the corresponding hydrox-
1
2
ido species [(LR ,R Mn )2(μ-OH)] , which is the active
intermediate in this case. With the remaining ligands, the
precursor μ-hydroxido compounds generated in situ in
III
+
1
2
D
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methanol were allowed to react with H2DBC exactly under
similar conditions, as mentioned above except that the addition
of base was not required at all in the latter cases. The products,
including their yields obtained in all cases, were grossly the
same when the reactions were carried out in a single step, as
mentioned in the Experimental Section.
It may be mentioned here that the isolation of complexes 1−
6 is only possible when methanol is used as a solvent, as
reported in their synthesis protocol. The compounds are stable
at room temperature and have reasonably good solubility in
dichloromethane.
The IR spectra of the complexes show all of the characteristic
bands of the coordinated ligands. In Figure S5 are shown the IR
spectra of the two representative compounds 1 and 3 in the
1750−1100 cm−1 region. Two strong and sharp bands
appearing at 1606 and 1473 cm−1 in the spectrum of compound
3 correspond to ν(CO) and ν(O−CH3) stretching modes,
respectively, due to the incorporation of a carbonyl group and a
methoxy group in the BMOD− ring of the oxidized dioxolene
moiety. The spectra of 1 and 2 include strong bands at 1480
and 1490 cm−1, respectively, which can be assigned to a C−O
stretching vibration of the attached semiquinonato ligand.36
Description of the Crystal Structures. Attempts have
been made to determine the crystal and molecular structures of
the precursor methoxy-bridged compounds 7 and 8. A
perspective view of the molecular structure of 7 is displayed
in Figure 1. Unfortunately, because of poor crystal quality, the
data collected with several alternative crystals of compound 8
are found to be not enough to generate a molecular structure of
publishable quality. However, an ORTEP view of a poorly
refined structure is displayed in Figure S6 for the purpose of its
authentication. Complex 7 crystallizes in triclinic space group
Figure 2. Molecular structure and atom-labeling scheme for complex 1
with thermal ellipsoids drawn at the 30% probability level. Inset:
Observed bond lengths in the coordinated dioxolene moiety in 1.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 1
and 2
1
2
Bond Lengths (Å)
1.986(5)
Mn−O1
Mn−O2
Mn−O3
Mn−O4
Mn−N1
Mn−N2
C1−O1
C6−O2
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C1
2.042(2)
2.108(2)
1.893(2)
1.869(2)
2.112(2)
2.304(3)
1.304(4)
1.271(3)
1.404(4)
1.362(5)
1.414(5)
1.364(4)
1.437(4)
1.447(4)
P1 with two molecular mass units accommodated per unit cell
̅
2.191(7)
(Table S1) and relevant metrical parameters, as summarized in
Table S2. The asymmetric unit consists of one
[(LMe,MeMnIII)2(μ-OMe)]+ complex cation and one tetraphe-
nylborate anion. The MnIII centers in this cation are
surrounded by N2O4 donor sets, providing distorted
octahedral geometry for the metal centers. These include
N2O2 coordination sites, all coming from the capping ligand
[LMe,Me]2−, together with a pair of O donor atoms coming from
a bridging phenolate and a bridging methoxy anion,
respectively. The basal plane around Mn1 is made up of N1,
O1, O3, and O4 donor atoms (N3, O2, O3, and O5 for Mn2),
and the apical sites are occupied by N2 and O2 atoms (N4 and
O1 for Mn2). The two Mn centers are connected by a bridging
methoxo (O3) and two bridging phenoxo (O1 and O2). O
atoms The Mn1···Mn2 separation is 2.9393(12) Å. The
methoxo O3 atom is almost equidistant [1.951(3) and
1.958(3) Å] from the individual Mn centers, which are high-
spin d4 species showing Jahn−Teller axial elongations along the
N2−Mn1−O2 and N4−Mn2−O1 axes, as revealed from the
metrical data summarized in Table S2.
1.882(5)
1.914(5)
2.086(6)
2.335(7)
1.304(11)
1.326(10)
1.428(11)
1.361(13)
1.441(12)
1.397(11)
1.413(13)
1.455(13)
Bond Angles (deg)
178.1(3)
O3−Mn1−O4
O2−Mn1−N2
O1−Mn1−N1
173.55(9)
177.46(9)
173.93(9)
167.5(2)
171.3(3)
representative example for these two compounds. The
asymmetric unit consists of a neutral mononuclear entity
[Mn(Lt‑Bu,t‑Bu)(DBSQ)]19 and two methanol molecules as
solvents of crystallization. The MnIII center in this complex
has a distorted octahedral geometry. The basal positions are
taken up by O3, N1, and O4 donor atoms, all coming from the
N2O2 ligand, together with the O1 atom from the dioxolene
moiety, while the apical positions are taken up by the donor
atom O2, the remaining O donor atom from the dioxolene and
N2 from the tetradentate ligand. The trans angles N2−Mn−O2
167.5(2)° [177.46(9)° for 2], N1−Mn−O1 171.3(3)°
[173.93(9)°], and O3−Mn−O4 178.1(3)° [173.55(9)°] are
The molecular structures and atom-labeling schemes for the
semiquinonato complexes 1 and 2 are shown in Figures 2 and
S7, respectively. Their metrical data provide confirmatory
evidence in support of the semiquinonato status of the
dioxolene moieties in these compounds. Identical atom-labeling
schemes have been adopted for both structures for an easy
comparison of their relevant metrical parameters (Table 1).
The crystal structure of complex 1, which crystallizes in
monoclinic space group P2 (P1 for 2) with two molecular
̅
1
mass units accommodated per unit cell, is described here as a
E
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Table 2. Selected Bond Distances (Å) and Angles (deg) for 3−6
3
4
5
6
Bond Lengths (Å)
1.9531(18)
2.2102(19)
1.8977(19)
1.8671(19)
2.113(2)
Mn−O1
Mn−O2
Mn−O3
Mn−O4
Mn−N1
Mn−N2
C1−O1
C6−O2
C1−C2
C2−C3
C3−C4
C4−C5
C5−C6
C6−C1
1.964(3)
2.254(3)
1.875(2)
1.861(2)
2.109(3)
2.310(3)
1.315(4)
1.226(4)
1.339(5)
1.485(6)
1.517(5)
1.315(5)
1.473(5)
1.496(5)
1.941(5)
2.211(5)
1.905(4)
1.888(4)
2.098(5)
2.309(6)
1.315(8)
1.239(8)
1.352(10)
1.476(12)
1.486(13)
1.359(11)
1.500(10)
1.432(10)
1.921(4)
2.223(4)
1.882(4)
1.863(4)
2.091(5)
2.291(5)
1.323(6)
1.242(7)
1.317(8)
1.482(9)
1.465(10)
1.346(9)
1.476(8)
1.475(8)
2.308(2)
1.331(3)
1.244(3)
1.350(4)
1.494(4)
1.497(4)
1.337(4)
1.474(4)
1.476(4)
Bond Angles (deg)
175.53(8)
174.93(8)
176.56(9)
110.5(3)
O3−Mn1−O4
O2−Mn1−N2
O1−Mn1−N1
O5−C3−C2
O5−C3−C4
O5−C3−C29
C2−C3−C29
C4−C3−C29
C2−C3−C4
C1−C2−C3
C1−C6−C5
C2−C1−C6
C4−C5−C6
C5−C4−C3
176.94(12)
175.16(11)
174.44(11)
111.0(4)
108.8(4)
105.4(4)
110.8(4)
110.4(4)
110.4(4)
125.4(4)
119.4(4)
119.0(4)
117.1(4)
128.2(4)
174.7(2)
167.74(19)
171.1(2)
111.7(8)
107.4(8)
100.0(6)
113.6(9)
110.6(8)
112.7(7)
124.6(8)
120.2(6)
119.2(6)
118.7(7)
124.3(8)
174.06(11)
177.11(9)
174.03(10)
111.2(3)
108.8(3)
103.8(3)
110.0(3)
109.9(4)
112.8(3)
123.5(4)
120.6(3)
119.6(3)
115.7(3)
127.2(3)
107.9(2)
104.5(2)
110.9(3)
110.3(3)
112.4(3)
123.4(3)
120.5(2)
119.4(3)
116.7(3)
126.8(3)
close to linearity, as expected for an octahedral geometry. The
average Mn−O and Mn−N distances in the basal plane are well
within the expected range.37 The apical Mn−O2 2.191(7) Å
[2.108(2) Å] and Mn−N2 2.335(7) Å [2.304(3) Å] distances,
on the other hand, are significantly elongated because of high-
spin MnIII being a Jahn−Teller ion.37
schemes have also been adopted here for all of these structures
in order to have a straightforward comparison of their relevant
metrical data. Again, for the sake of brevity, a generic
description of these structures will be provided considering
compound 3 as a representative example. A perspective view of
the molecular structure of 3 is depicted in Figure 3, and those
For a coordinated dioxolene moiety, the average C−O
distance often provides decisive clues as to the oxidation state
of this noninnocent ligand. The observed C−O distances are
usually short (ca. 1.28 Å)38,39 when the ligand is in the quinone
or semiquinone state compared to a fully reduced catecholate
ligand (ca. 1.34 Å).40 The average C−O distance observed here
is 1.315 Å (1.287 Å). Considering that the crystal structure of 1
has been determined at 150 K41 and that of 2 at a much higher
temperature of 293 K, the C−O distance in 1 is somewhat in
the upper range (due to charge transfer from a near phenolate
as discussed in the mechanistic part; see later). Apart from this,
the observation of a two short−four long intraring C−C bond
pattern (quinoid distortion) provides confirmatory evidence
(inset, Figure 2) in support of the semiquinone status for the
dixolene moiety39a in compounds 1 and 2.
Compounds 3−6 have a different set of structures, as
established from their X-ray crystallographic analysis. These are
all isostructural molecules. While 3 crystallizes in the triclinic
space group P1 with two molecular mass units accommodated
̅
per cell, compounds 4−6, on the other hand, have the
monoclinic space group P21/c in all cases with four molecular
mass units in their respective unit cell. Their relevant metrical
parameters are summarized in Table 2. Identical atom-labeling
Figure 3. Partially labeled POV-Ray (in ball-and-stick form) diagram
showing the atom-labeling scheme in complex 3. H atoms have been
omitted for clarity.
F
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of 4−6 are shown in Figures S8−S10, respectively. The Mn
center is also six-coordinated here using a doubly deprotonated
facially coordinating N2O2 ligand, providing N1, N2, O3, and
O4 donor atoms and a mononegative dioxolene derivative
(BMOD−)19 with O1 and O2 donors to complete the
octahedral geometry. The Mn−O [1.861(2)−1.964(3) Å]
and Mn−N [2.091(5)−2.113(2) Å] distances in the basal plane
in these compounds are all in the expected range.37 The
dioxolene O2 atom occupies the Jahn−Teller axis, together
with the imino N2 atom of the tetradentate ligand, to form a
trans O2−Mn−N2 angle of 175.16(11)°. The elongation along
this axis is clearly reflected in the bond lengths Mn−O2
[2.254(3)−2.210(2) Å] and Mn−N2 [2.310(3)−2.291(5) Å],
compared to the relevant bond distances in the equatorial plane
(Table 2).
breaks the aromaticity of this transformed dioxolene ring, which
now includes a C3 center that makes the angles C2−C3−C4
110.4(4)°, C2−C3−O5 111.0(4)°, and C4−C3−C29
110.4(4)°, all deviating substantially from 120°, the expected
angle at a C sp2 center (Figure 4). The remaining angle at C3,
O5−C3−C29, is 105.4(4)°, which is also close to an ideal
tetrahedral bond angle of 109.5°. The data thus provide a clear
indication that the hybridization status of the C3 atom in the
ring has been changed to sp3 from its initial sp2 state in
complexes 3−6 during the course of this oxidation, followed by
a methanolysis reaction (see later).
Magnetic Susceptibility Data. The magnetic moment
data obtained at room temperature for complexes 1−6 are put
together in the Experimental Section. The semiquinonato
compounds 1 and 2 display subnormal magnetic moments of
3.94 and 3.89 μB (per Mn), respectively, at 298 K. The
observed decrease in the effective magnetic moments for these
manganese(III) complexes below their S = 2 spin-only value of
4.86 μB is attributable to the strong antiferromagnetic
interactions of the single unpaired electron of the coordinated
semiquinone (DBSQ) moiety with the four manganese(III)
Perhaps the most striking feature observed in the structures
of compounds 3−6 is the unprecedented type of dioxolene
moiety observed in these molecules. As displayed in Figure 4,
3
unpaired electrons, producing an S = /2 ground state with an
expected spin-only moment of 3.92 μB.42 Susceptibility data for
the remaining compounds [MnIII(LR ,R )(BMOD)] (3−6)
containing a quinone derivative BMOD− as the coligand, vary
in the range 4.54−4.88 μB. The results are well in conformity
with molecules behaving as a simple paramagnet with an S = 2
ground state.
1
2
Electronic Spectra. The electronic spectra of complexes
1−6 have closely similar features (Figure 6), each displaying a
number of moderate-to-high-intensity bands in a dichloro-
methane solution, as summarized in Table 3. Spectra with
similar features involving multiple charge-transfer bands are not
uncommon for metal semiquinonato compounds.36 Complexes
1 and 2 show a band of moderate intensity with ε = 1350 L
mol−1 cm−1 (1550 L mol−1 cm−1 for 2) in the visible region at
700 nm (724 nm) with a vibronic progression that is diagnostic
of a coordinated semiquinone ligand in these molecules.43 The
remaining strong band in the visible region at 488 nm (495
nm) with ε = 4200 L mol−1 cm−1 (4000 L mol−1 cm−1)
possibly arises from the PhO− → MnIII ligand-to-metal charge-
trasnfer transition arising from a pπ orbital on the phenolate O
atom to the half-filled MnIII dπ* orbitals.44 This band in
complexes 3−6 is slightly red-shifted and appears in the form of
a shoulder. All other bands appearing in the UV region in these
complexes are due to ligand internal transitions.
Figure 4. Bond lengths in the dearomatized dioxolene ring and bond
angles at the sp3-hybridized C3 center in complex 3.
the intraring C−C distances and angles in the dioxolene ring
changed drastically in these molecules during the course of
their aerobic oxidation, followed by methanolysis. The C2−C3
1.485(6) Å, C3−C4 1.517(5) Å, C5−C6 1.473(5) Å, and C6−
C1 1.496(5) Å bond lengths in 3 are almost of single-bond
character, while those between C1−C2 1.339(5) Å and C4−C5
1.315(5) Å are close to double-bond character. Also, the C6−
O2 distance 1.226(4) Å is more like a double bond, providing a
planar quinoid structure that includes half of this ring (plane a
in Figure 5) involving the C2, C1, C6, C5, and C4 atoms. The
remaining half containing C2, C3, and C4 centers lie in a
different plane (plane b in Figure 5), and together they provide
a “half-chair”-like conformation for this ring, with planes a and
b making a dihedral angle of 5.61°. The observed nonplanarity
Figure 5. Dihedral angle between planes a and b in complex 3 and the half-chair conformation of the transformed dioxolene ring.
G
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Figure 6. UV−vis spectra of the manganese(III) complexes (1− 6) in
a dichloromethane solution (concn ∼ 10−4 M).
Figure 7. Cyclic voltammogram of compound 1 in DCE (0.1 M
TBAP) at a scan rate of 100 mV s−1 using a glassy carbon working
electrode (potentials vs Ag/AgCl). Inset: Cyclic voltammogram of the
corresponding free ligand H2Lt‑Bu,t‑Bu in DCE.
Table 3. Electronic Spectra of Complexes 1−6
compound
λ, nm (ε, L mol−1 cm−1
)
[Mn(Lt‑Bu,t‑Bu)(DBSQ)]·
2CH3OH (1)
292 (20000), 370 (7900), 488 (4200), 700
(1350)
[Mn(Lt‑Bu,OMe)(DBSQ)]·
CH3OH (2)
303 (24200), 377 (7400), 495 (4000), 724
(1550)
126 mV) [0.44 V (184 mV) for 2], followed by an irreversible
oxidation (process II) at Epa = 1.12 V (0.75 V). The data, as
expected, clearly indicate that the oxidation processes are much
easier with compound 2 than with compound 1. Taking a cue
from the electrochemical data of the free ligands, one can
conclude that process I in this case is most likely to originate
from a ligand-based oxidation involving one of the phenolate
[Mn(LMe,Me)(BMOD)]·
2CH3OH (3)
281 (18500), 373 (6650), 505 (2900), 661
(660)
[Mn(LMe,t‑Bu)(BMOD]·CH3OH 277 (17400), 374 (5700), 509 (2300), 660
(4)
(550)
[Mn(Lt‑Bu,Me)(BMOD)] (5)
289 (14900), 375 (5650), 511 (1550)
[Mn(LCl,Cl)(BMOD)]·H2O (6) 287 (13000), 376 (4150), 523 (1100)
moieties of the capping ligand [Lt‑Bu,t‑Bu 2− (or [Lt‑Bu,OMe]2−).
]
The absence of the cathodic response in process II confirms the
onset of a decomposition process after the initial oxidation. In
the cathodic region, the cyclic voltammograms of 1 and 2
involve a couple of irreversible reductions. In order to avoid
speculation, we have decided not to make any further
comments about these electrochemical processes.
Electrochemical Properties. Because the substituent
combinations R1 and R2 in the capping ligand H2LR ,R play
the domineering role in deciding the course of these redox
reactions, we thought it would be prudent to study the
electrochemical behaviors of complexes 1−6, just to explore
how these substituents in the capping ligand influence their
overall redox performances. Cyclic voltammograms of the
complexes as well as their free associated capping ligands have
been recorded at a glassy carbon working electrode under an
atmosphere of purified dinitrogen in a DCE solution (0.1 M
TBAP) at 25 °C in the potential range of −1.0 to +1.8 V versus
Ag/AgCl reference electrode. The voltammetric features of the
free ligands H2Lt‑Bu,t‑Bu and H2Lt‑Bu,OMe are quite similar, with
both undergoing a single oxidation process in the anodic
potential range and with the degree of reversibility being
compromised upon going from H2Lt‑Bu,OMe to H2Lt‑Bu,t‑Bu. We
believe that the quasi-reversible voltammogram observed at E1/2
= +0.72 V (ΔE = 143 mV) with the ligand H2Lt‑Bu,t‑Bu [Figure 7
(inset)] is due to phenolate → phenoxyl oxidation. The
corresponding oxidation for H2Lt‑Bu,OMe is grossly reversible,
appearing at a less positive potential of 0.45 V (ΔE = 80.2 mV).
The ease of oxidation in the latter case is due to the strong
electron-donating influence of the OMe group in the para
position of the phenolate ring in the ligand. These two ligands
are known for their ability to form the semiquinonato
complexes 1 and 2. The cyclic voltammogram of complex 1
in the anodic potential range is displayed in Figure 7, which
involves two electrochemical processes (processes I and II).
Similar features are also observed with compound 2. Process I
is a quasi-reversible oxidation appearing at E1/2 = 0.53 V (ΔEp =
1
2
The electrochemical behaviors of complexes 3−6 are grossly
similar but differ widely from those of complexes 1 and 2. In
fact, the cyclic voltammograms of the free capping ligands
associated with these compounds show a lone irreversible
oxidation, with Epa lying in the range between 0.7 and 0.8 V.
The voltammogram of a representative ligand H2LMe,t‑Bu is
shown in the inset of Figure S11. The results suggest the
instability of the oxidized phenoxyl moiety of these ligands in
the voltammetric time scale and fail to reappear during the
reverse cathodic scan. The cyclic voltammogram of the
corresponding dioxolene compound 4 is also shown in Figure
S11 as a representative of the BMOD-type compounds. It
constitutes three irreversible oxidations in the observed
potential range. To avoid any speculative remarks, we have
decided to make no further comment about the origin of these
oxidation processes.
The results of the electrochemical study thus indicate that
the stability of the phenoxyl moiety of the oxidized capping
ligand in 1−6 is probably the key in deciding the course of
either of the two oxidation reactions. The ligands H2Lt‑Bu,t‑Bu
and H2Lt‑Bu,OMe, which can generate stable phenoxyl radicals at
least in the voltammetric time scale, are capable of producing
the semiquinato compounds 1 and 2. The rest of the ligands,
on the other hand, are incapable of generating the stable
phenoxyl radicals and guide the reaction to proceed to the
H
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generation of compounds 3−6, with BMOD as the oxidized
dioxolene moiety.
Ligand-Induced Tuning of the Oxidase Activity: Two-
Electron versus Four-Electron Reduction of Molecular
Oxygen. We are now quite certain that a binuclear μ-
acting as a four-electron oxidant that oxidizes the incoming
dioxolene moiety up to the quinone (DBQ)19 stage and itself is
reduced to water, as shown by eq 2. The coordinated o-quinone
in these compounds appears to be highly reactive46a and
initiates a domino reaction that involves an intermolecular
Michael-type 1,4-addition reaction46 with a methoxy group as a
nucleophile from the solvent methanol (eq 3), as outlined in
Scheme 2. The products obtained (3−6) are very interesting
III
hydroxidomanganese(III) cationic complex, [(LR ,R Mn )2(μ-
1
2
OH)]+, is initially formed as an intermediate when the ligand
H2LR ,R , together with H2DBC, is allowed to react with
Mn(ClO4)2·6H2O in methanol in the mandatory presence of a
base and molecular oxygen. This intermediate (its μ-OMe
derivative is isolable as a crystalline solid with the ligands
H2LMe,Me and H2LMe,t‑Bu using tetraphenylborate as the
counteranion) then participates in the oxidation of H2DBC in
the presence of molecular oxygen as an electron acceptor and is
reduced by two or four electrons (eqs 1 and 2) depending on
1
2
Scheme 2. Domino Reaction Involving a Michael-Type 1,4-
Addition of the CH3O− Group to the Orthoquinone Moiety
Generated during the Four-Electron Reduction of O2 by the
Catecholato Ligand That Led to the Formation of
a
Compounds 3−6
the substituent combination (R1 and R2) present in the
attached capping ligand [LR ,R ] . Thus, with the t-Bu,t-Bu and
t-Bu,OCH3 combinations, the incoming DBC2− ligand is
oxidized up to the semiquinone DBSQ− stage to generate
compounds 1 and 2, respectively, and in the process, molecular
oxygen is reduced to H2O2 (eq 1). The generation of H2O2
during this oxidation was monitored spectrophotometrically
following a reported iodometric detection procedure,23,24 as
outlined in the Experimental Section. This involves the
development of a characteristic UV band due to the πg →
2−
1
2
a
Participation of the MnIII ion in the reaction is not shown.
σu* transition45 for I3 (λmax = 353 nm; ε = 26 000 L mol−1
−
molecules, containing an unusual type of nonplanar oxidized
dioxolene moiety that forms part of the methyl ether derivative
BMOD−.
cm−1) when the generated H2O2 is allowed to react with I− at
pH = 2.0. Figure 8 depicts the time-interval spectra for the
[(LR1,R2MnIII)2(μ‐OH)]+ + 2[DBC]2− + O2
→ 2[(LR1,R2)MnIII(DBQ)]+ + 2O2−
(2)
[(LR1,R2)MnIII(DBQ)]+ + OCH3−
→ [(LR1,R2)MnIII(BMOD)]
(3)
In order to have further verification of our conjecture that the
formation of a quinoid-type molecule [LR ,R MnIII(DBQ)]+ is
1
2
the initial step of the domino reaction that leads to the final
III
products [LR ,R Mn (BMOD)] (3−6), we attempted the
syntheses of 3−6, following an alternative route (method B
in the Experimental Section) in which H2DBC was replaced by
DBQ19 as the precursor dioxolene moiety. The successful
isolation of 3−6 in moderate-to-high yields from a methanol
solution confirms the involvement of the quinoid compound
1
2
III
+
[LR ,R Mn (DBQ)] as the precursor to the final products 3−6.
1
2
We believe this two- versus four-electron reduction of O2 by
the μ-hydroxido intermediates [(LR ,R Mn )2(μ-OH)]
III
+
1
2
Figure 8. Time-interval spectra that show the indirect monitoring
(iodometric assay; details are in the text) of the gradual accumulation
of H2O2 in solution during the synthesis of complex 1. Inset:
Complete absence of H2O2 generation in solution when the capping
(Scheme 1) is fine-tuned by the secondary coordination sphere
around the MnIII centers, which differ among themselves by the
substituent combinations (R1 and R2) in the phenolate arms of
the capping ligands. It is well-known that both 2,4-di-tert-
butylphenol and 2-tert-butyl-4-methoxyphenol can efficiently
stabilize the phenoxyl radical because of their ability to accept
ligand is changed from H2Lt‑Bu,t‑Bu to H2LMe,t‑Bu or H2Lt‑Bu,Me
.
gradually generated I3− in solution over a period of ca. 15 min.
The data provide an indirect estimation of H2O2 accumulated
during the synthesis of compound 1 following eq 1.
the spin density from the associated ring.47 The presence of
these phenolate moieties in the attached [LR ,R
]
ligands in 1
2−
1
2
and 2 stabilizes the dioxolene moiety in the semiquinone form
(DBSQ) during their oxidation because of an extended charge
delocalization between the phenolate and semiquinone
moieties via participation of the appropriate metal orbital(s).
An equilibrium between the semiquinone/phenolate (form A)
and catecholate/phenoxyl (form B) tautomeric forms (Figure
9) provides extra stability to the semiquinoid structures in 1
and 2 and thus restricts the oxidase activity up to the two-
[(LR1,R2MnIII)2(OH)]+ + 2[DBC]2− + O2
R1,R2
2−
→ 2[(L )MnIII(DBSQ)] + O2
(1)
Interestingly, with the rest of these ligands under similar
experimental conditions, we failed to detect any H2O2 in the
reaction mixtures, as shown in Figure 8 (inset) for compounds
4 and 5. We believe molecular oxygen in the latter instant is
I
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Inorganic Chemistry
Article
corresponding bond distance (1.396 Å) in 3, providing a
clear indication that the phenolate ring in 1 is displaying a
partial quinoid distortion characteristic of a phenoxyl moiety.48
The shift of the odd phenolate electron to the semiquinone
ring is also reflected in elongation of the average C−O distance
(1.315 Å; Table 1) in the dioxolene moiety of compound 1.
Thus, the overall structure of 1 at lower temperature (150 K) is
a combination of both forms A and B, with form B being the
major contributor. Also, the appearance of a distinct signal at
1631 cm−1 in the IR spectrum of compound 1 (Figure S5) due
to the ν(CC) stretching pattern provides an indication of the
presence of a phenoxyl moiety in this compound.49 Thus, of the
two tautomeric forms, the structure B probably contributes
more to the overall structures of compounds 1 and 2. For the
remaining compounds, O2 reduction is not arrested in the two-
electron stage because of the poor stability of the intermediate
semiquinonato compound. That probably explains why only a
few semiquinonato compounds of manganese(III) have been
reported thus far in the literature.43a,50 Dioxolene oxidation
during the formation of compounds 3−6 proceeds further up
to the quinonato stage with concomitant four-electron
reduction of O2.
Figure 9. Equilibrium between the tautomeric forms involving the
semiquinone/phenolate (form A) and catecholate/phenoxyl (form B)
combinations.
electron stage. A careful look at the crystal structure of the
phenolate moiety (Table S3) in compound 1 (Figure 10)
Krebs et al.18e recently reported a series of mononuclear
manganese(III) complexes using tetradentate tripodal ligands
with N4 and N3O donor combinations with closely related
structures. Some of these compounds satisfactorily model the
structure of a manganese-substituted form of an intradiol-
cleaving iron catechol dioxygenase. These complexes also
exhibit high catalytic activity for the oxidation of 3,5-di-tert-
butylcatechol to 3,5-di-tert-butylquinone, mimicking the role of
catechol oxidase model compounds showing saturation kinetics
at high substrate concentrations. Interestingly, the oxidized
dioxolene moiety did not bind the MnIII center in those
compounds, as observed with compounds 1−6.
Figure 10. Comparison between the relevant bond distances of the
phenolate moieties in complexes 1 and 3. Enlargement of the C7−C8
bond in complex 1 provides an indication of the partial phenoxyl
character of the concerned phenol ring.
DFT Calculations. Theoretical IR spectra for representative
types of these reported complexes have been constructed on
the basis of DFT calculations (Figure S12). Relevant vibrational
bands of these spectra have been used subsequently for
determined at lower temperature (150 K)41 indicates an
enlarged C8−C7 bond of 1.448 Å compared to the
Figure 11. Energy gaps between the phenolate-based HOMO and semiquinone-based LUMO in complexes 2 and 3.
J
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Inorganic Chemistry
Article
correlation with the corresponding experimental spectra. For
example, the C−O stretching vibration of the semiquinonato
moiety in compound 1 appears at 1497 cm−1 in its theoretical
spectrum (video S1). Unfortunately, in the experimental
spectrum, this region is quite congested because of the
appearance of three strong bands, including the one at 1480
cm−1, which shows a fair degree of concordance with the
theoretical C−O vibration. The DFT-optimized complex
[MnIII(LMe,Me)(DBQ)] interestingly exhibits an IR band at
1606 cm−1 due to the CO stretching vibration of the
associated quinone moiety (video S2), which is in complete
agreement with the experimental CO vibration of the
[MnIII(LMe,Me)(BMOD)] complex (3).
We also performed DFT calculations on complex 2 and the
optimized semiquinone structure of compound [MnIII(LMe,Me)-
(DBSQ)], only to see that the semiquinone ring in complex 2
is richer in electron density likely because of the shift of the
electron density from the associated phenolate ring. Accord-
ingly, the spin density on the phenolate ring is more for 2,
suggesting greater phenoxyl character in 2 relative to 3 (Figure
11). Further, it is found that the energy separation between the
phenolate-based highest occupied molecular orbital (HOMO)
and the semiquinone-based lowest unoccupied molecular
orbital (LUMO) is ∼6.5 kcal lower for 2 relative to 3. Thus,
the higher stability of compound 2 in its semiquinone form is
likely due to the extended delocalization of the electron density
over both phenolate and semiquinone rings. For complex 3,
however, the unstable semiquinonato intermediate undergoes
further oxidation, followed by a Michael-type addition reaction
to the final BMOD product.
1H NMR spectrum of the ligand H2Lt‑Bu,OMe (Figure S1),
ESI-MS spectra in the positive-ion mode of compounds
7 and 8 (Figures S2 and S3) and of the intermediate
[(Lt‑Bu,t‑BuMnIII)2(μ-OH)]+ (Figure S4), IR spectra of 1
and 3 (Figure S5), partially labeled perspective views of
the crystal structures of complexes 2, 4−6 and 8 (Figures
S6−S10), cyclic voltammograms of compound 4 and the
ligand H2LMe,t‑Bu (Figure S11), theoretical FT-IR spectra
of the DFT-optimized complexes 1 and [MnIII(LMe,Me)-
(DBQ)] (Figure S12), relevant crystallographic data
(Table S1), metrical parameters for compound 7 (Table
S2), selected bond distances of phenolate rings in 1−6
(Table S3), and details of the syntheses and character-
ization of the H2LR ,R ligands (PDF)
1
2
Video presentation of the C−O stretchings of the DFT-
optimized complex 1 (video S1) (AVI)
Video presentation of the C−O stretchings of the DFT-
optimized complex [MnIII(LMe,Me)(DBQ)] (video S2)
(AVI)
Accession Codes
CCDC 1548156−1548162 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: icmc@iacs.res.in.
■
ORCID
CONCLUDING REMARKS
■
Muktimoy Chaudhury: 0000-0002-6069-1072
Notes
The authors declare no competing financial interest.
In summary, we have reported here the oxidase activity of
binuclear μ-hydroxidodimanganese(III) cationic intermediates
III
+
[(LR ,R Mn )2(μ-OH)] , which, during their reaction with
H2DBC, exhibit either a two- or four-electron reduction of O2.
The course of these alternative reactions appears to be
controlled by the secondary coordination sphere surrounding
the MnIII centers in the μ-hydroxido intermediates (Scheme 1).
The presence of 2,4-di-tert-butylphenol and 2-tert-butyl-4-
1
2
ACKNOWLEDGMENTS
■
We thank Dr. Abhishek Dey for his help with DFT calculations.
We also thank the reviewers for providing many constructive
suggestions. This work was supported by the Council of
Scientific and Industrial Research (CSIR), New Delhi, India.
D.M. and M.C.M. also thank the CSIR for the award of
research fellowships. Single-crystal X-ray diffraction data were
recorded on an instrument supported by DST, New Delhi,
India, as a National Facility at IACS under the IRHPA program.
methoxyphenol moieties in the [H2LR ,R ] ligands in 1 and 2,
respectively, stabilizes the dioxolene moiety in the semiquinone
form47 during their oxidation because of an extended charge
delocalization between the phenolate and semiquinone
moieties (Figure 9). For the remaining compounds, O2
reduction is not arrested in the two-electron stage because of
the poor stability of the intermediate semiquinonato products.
The oxidation of dioxolene proceeds further up to the
quinonato stage with concomitant four-electron reduction of
O2. The reactive quinonato compounds then participate in a
domino reaction with the solvent methanol to provide
compounds 3−6 with an unusual dioxolene moiety (Scheme
2). The present work thus provides a unique example of the
influence of the secondary coordination sphere in controlling
the electron stoichiometry of the oxidase action.
1
2
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DOI: 10.1021/acs.inorgchem.7b00147
Inorg. Chem. XXXX, XXX, XXX−XXX