Metalloradical complexes of manganese and chromium featuring an oxidatively rearranged ligand

Article abstract of DOI:10.1021/ic801219u

Redox events involving both metal and ligand sites are receiving increased attention since a number of biological processes direct redox equivalents toward functional residues. Metalloradical synthetic analogues remain scarce and require better definition of their mode of formation and subsequent operation. The trisamido-amine ligand [(RNC₆H₄)₃N] ^3-, where R is the electron-rich 4-t-BuPh, is employed in this study to generate redox active residues in manganese and chromium complexes. Solutions of [(L¹)Mn(II)-THF]^- in THF are oxidized by dioxygen to afford [(L¹_re-1)Mn(III)-(O)₂-Mn(III)(L ¹_re-1)]^2- as the major product. The rare dinuclear manganese (III,III) core is stabilized by a rearranged ligand that has undergone an one-electron oxidative transformation, followed by retention of the oxidation equivalent as a π radical in an o-diiminobenzosemiquinonate moiety. Magnetic studies indicate that the ligand-centered radical is stabilized by means of extended antiferromagnetic coupling between the S = 1/2 radical and the adjacent S = 2 Mn(III) site, as well as between the two Mn(III) centers via the dioxo bridge. Electrochemical and EPR data suggest that this system can store higher levels of oxidation potency. Entry to the corresponding Cr(III) chemistry is achieved by employing CrCl₃ to access both [(L ¹)Cr(III)-THF] and [(L¹_re-1)Cr(III)-THF(CI)], featuring the intact and the oxidatively rearranged ligands, respectively. The latter is generated by ligand-centered oxidation of the former compound. The rearranged ligand is perceived to be the product of an one-electron oxidation of the intact ligand to afford a metal-bound aminyl radical that subsequently mediates a radical 1,4-(N-to-N) aryl migration.

Full text of DOI:10.1021/ic801219u

Inorg. Chem. 2008, 47, 10998-11009
Metalloradical Complexes of Manganese and Chromium Featuring an
Oxidatively Rearranged Ligand
Remle C¸ elenligil-C¸ etin,‡ Patrina Paraskevopoulou,† Nikolia Lalioti,¶ Yiannis Sanakis,⊥
Richard J. Staples,§ Nigam P. Rath,# and Pericles Stavropoulos*,†
Department of Chemistry, Missouri UniVersity of Science and Technology, Rolla, Missouri 65409,
Department of Chemistry, Boston UniVersity, Boston, Massachusetts 02215, Department of
Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138,
Department of Chemistry, UniVersity of Patras, Patras 26500, Greece, Department of Chemistry
and Biochemistry, UniVersity of Missouri-St. Louis, St. Louis, Missouri 63121, and the Institute of
Materials Science, NCSR “Demokritos”, Ag. ParaskeVi 15310, Greece
Received July 2, 2008
Redox events involving both metal and ligand sites are receiving increased attention since a number of biological processes
direct redox equivalents toward functional residues. Metalloradical synthetic analogues remain scarce and require better
definition of their mode of formation and subsequent operation. The trisamido-amine ligand [(RNC6H4)3N]3-, where R is
the electron-rich 4-t-BuPh, is employed in this study to generate redox active residues in manganese and chromium
complexes. Solutions of [(L1)Mn(II)-THF]- in THF are oxidized by dioxygen to afford [(L1re-1)Mn(III)-(O)2-Mn(III)(L1re-1)]2-
as the major product. The rare dinuclear manganese (III,III) core is stabilized by a rearranged ligand that has undergone
an one-electron oxidative transformation, followed by retention of the oxidation equivalent as a π radical in an
o-diiminobenzosemiquinonate moiety. Magnetic studies indicate that the ligand-centered radical is stabilized by means of
extended antiferromagnetic coupling between the S ) 1/2 radical and the adjacent S ) 2 Mn(III) site, as well as between
the two Mn(III) centers via the dioxo bridge. Electrochemical and EPR data suggest that this system can store higher
levels of oxidation potency. Entry to the corresponding Cr(III) chemistry is achieved by employing CrCl3 to access both
[(L1)Cr(III)-THF] and [(L1re-1)Cr(III)-THF(Cl)], featuring the intact and the oxidatively rearranged ligands, respectively. The
latter is generated by ligand-centered oxidation of the former compound. The rearranged ligand is perceived to be the
product of an one-electron oxidation of the intact ligand to afford a metal-bound aminyl radical that subsequently mediates
a radical 1,4-(N-to-N) aryl migration.
Introduction
most recently, in ribonucleotide reductase (class IV RNR),5
has been the center of significant attention. In this context,
the Mn(µ-O)2Mn core is a well-recognized feature of
manganese chemistry, encompassing the more rare Mn(III)(µ-
O)2Mn(III) oxidation level and the more common, mixed-
valent Mn(III)(µ-O)2Mn(IV) and high-valent Mn(IV)(µ-
O)2Mn(IV) counterparts.3b,6 While most of the discussion
is currently centered on metal-residing oxidation equivalents,
the involvement of tyrosyl radicals in the oxygen-evolving
PSII center,7 and the recent discovery of a cysteinyl radical
associated with a dioxygen-generated, Mn(IV)-Fe(III)-
Many manganese1 and chromium2 reagents are well
established as oxidizing entities, owing to the oxophilicity
of these metals and their rich redox capabilities. These
attributes also contribute, either beneficially or detrimentally,
to multiple engagements of both metals in critical biological
functions.3,4 Much more is known about manganese,3 whose
involvement, among others, in catalases, superoxide dismu-
tases, the oxygen-evolving complex of photosystem II, and
* To whom correspondence should be addressed.
‡
Boston University.
Missouri University of Science and Technology.
University of Patras.
NCSR “Demokritos”.
Harvard University.
University of Missouri-St. Louis.
(1) (a) Larrow, J. F.; Jacobsen, E. N. Topics Organomet. Chem. 2004, 6,
123–152. (b) Lee, T. V. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Eds.; Pergamon: Oxford, 1990; Vol. 7, pp 291-303.
(2) (a) Luzzio, F. A. Org. React. 1998, 53, 1–221. (b) Ley, S. V.; Maddin,
A. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon, Oxford, 1990; Vol 7, pp 251-289.
†
¶
⊥
§
#
10998 Inorganic Chemistry, Vol. 47, No. 23, 2008
10.1021/ic801219u CCC: $40.75  2008 American Chemical Society
Published on Web 10/21/2008

Metalloradical Complexes of Mn and Cr
Chart 1
containing cofactor in the RNR of the bacterium Chlamydia
trachomatis, is expected to fuel interest in the generation
and function of redox-active residues in tandem with Mn
centers.5
We have previously shown8 that compound [(L1)Fe(II)-
THF]-, where L1 is the trisamido-amine ligand [(4-t-Bu-
C6H4)NC6H4]3N3- (Chart 1), can be oxidized by dioxygen
in THF to afford [(L1re-1)Fe-O-Fe(L1re-1)] as a minor product.
The ligand in this diferric µ-oxo species has undergone one-
electron oxidative rearrangement, while the oxidation equiva-
lent is retained as a π radical. In the present study we
demonstrate that the corresponding Mn(II) complex can
generate a rare Mn(III)(µ-O)2Mn(III) core-containing ana-
logue, as the major product of its reaction with dioxygen.
Moreover, the [(L1re-1)Mn(III)] oxidation level can be
replicated in a mononuclear [(L1re-1)Cr(III)]-containing
compound, using CrCl3 as oxidant.
using Schlenk techniques on an inert gas/vacuum manifold or in a
drybox (O2, H2O < 1 ppm). Anhydrous diethyl ether, methylene
chloride, acetonitrile, tetrahydrofuran, hexane, pentane, and toluene
were purchased from Sigma-Aldrich. Acetone was distilled over
drierite. Solvents were degassed by three freeze-pump-thaw
cycles. Unless otherwise noted, all other reagents were purchased
at the highest purity available. Potassium hydride was provided as
dispersion in mineral oil and was thoroughly washed prior to use
with copious amounts of tetrahydrofuran followed by hexane. The
synthesis of the ligand L1H3 has been previously described.9 1H
NMR and 13C NMR spectra were recorded on Varian XL-400,
Varian INOVA/UNITY 400 MHz, Varian 300 Unity Plus, and
Varian 600 MHz FT-HR-NMR spectrometers. FT-IR spectra were
obtained on Nicolet Nexus 470, 670, and Magna 750 FT-IR ESP
spectrometers and on a Bruker EQUINOX 55 spectrometer with a
single reflection accessory (DuraSamplIR II by SensIR Technolo-
gies) equipped with a diamond element. UV-vis spectra were
obtained on a Hewlett-Packard 8452A diode array spectrometer
and a Varian Cary 50 spectrophotometer. EI and CI mass spectra
were obtained on a Finnigan MAT-90 mass spectrometer. HRMS
(ESI) data were collected on an Applied Biosystems QStar Pulsar
instrument with a microspray ion source, located at the University
of Missouri-Columbia. Microanalyses were done by Quantitative
Technologies Inc., Whitehouse, NJ, Galbraith Laboratories, Inc.,
Knoxville, TN, and on an in-house Perkin-Elmer 2400 CHN
analyzer.
Dual mode X-band EPR spectra were recorded at the Institute
of Materials Science, NCSR “Demokritos”, on an extensively
upgraded former Bruker ER-200D spectrometer interfaced with a
personal computer running appropriate software in the LabView
programming environment. The spectrometer was equipped with
an Oxford ESR 900 cryostat, an Anritsu MF76A frequency counter,
and a Bruker 035 M NMR gaussmeter. The perpendicular mode
spectra were obtained with the 4102ST cavity, whereas the parallel
mode spectra were obtained with the dual mode cavity, 4116 DM.
Simulations of the spectra were performed with the software
SpinCount kindly provided to us by Prof. M. P. Hendrich,
Department of Chemistry, Carnegie Mellon University, Pittsburgh,
PA, USA. X-band EPR spectra were recorded on a Bruker
Instruments Model ESP 300 with an EMX upgrade, supported by
an APD Cryogenics Co. Cryostat HELI-TRAN model LTR-3,
located at the Chemistry Department of Dartmouth College, NH.
[(L1)Mn(II)-THF][K(THF)x] (1a) and [(L1)Mn(II)-THF]-
(Ph4P)·3THF (1b). Ligand L1H3 (0.343 g, 0.5 mmol) was dissolved
in degassed THF (15.0 mL), to which KH (0.060 g, 1.5 mmol)
was added. The mixture was stirred overnight until all KH was
dissolved. MnCl2 (0.0625 g, 0.5 mmol) was then added as solid to
the resulting yellow THF solution. This mixture was stirred for
another 10 h to give a pale yellow solution. This solution was
Experimental Section
General Considerations. All operations were performed under
anaerobic conditions under a pure dinitrogen or argon atmosphere
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Inorganic Chemistry, Vol. 47, No. 23, 2008 10999

Çelenligil-Çetin et al.
refrigerated overnight to allow settling of KCl, and was carefully
filtered. The filtrate was reduced to 2.0 mL and pentane was allowed
to slowly diffuse into this solution at -20 °C to give microcrys-
talline 1a at an estimated yield of 60%. The solid was extracted
with THF (3.0 mL), and excess (Ph4P)Cl was added as a solid to
this solution until some precipitate was formed. This was filtered
and the filtrate was mixed with just enough hexane to initiate
precipitation. The precipitate was filtered and the filtrate was
allowed to stand at room temperature for a few days to afford X-ray
quality orange crystals of 1b (0.150 g, 25%). UV-vis (THF): λmax
(ε (M-1 cm-1)) 470 (1450), 520 (820). Elemental Analysis: Calcd.
for C76H79MnN4OP (1-3THF): C, 79.35; H, 6.92; N, 4.87. Found:
C, 79.34; H, 6.34; N, 4.84.
(M-1 cm-1)) 450 (5380), 500 (4080), 630 (3025). Elemental
Analysis: Calcd. for C52H59ClCrN4O (4-0.75C6H14): C, 74.04; H,
7.05; N, 6.64. Found: C, 74.57; H, 7.01; N, 6.69.
[(L1)Cr(III)-THF] (5). Ligand L1H3 (0.343 g, 0.5 mmol) was
dissolved in degassed THF (15.0 mL), to which KH (0.060 g, 1.5
mmol) was added. The mixture was stirred overnight until all KH
was dissolved. CrCl3 (0.079 g, 0.5 mmol) was added to this yellow
solution as solid, and the solution was allowed to stir overnight to
give a dark brown solution. This solution was evaporated to dryness
under vacuum, and the residue was extracted with hexane several
times and filtered to remove as much of 4 as possible. Once the
extracts started to become pale and almost transparent, the remaining
residue was extracted with diethyl ether to give a dark green
solution. This solution was allowed to stand at room temperature
to give dark-green, diamond-shaped crystals of 5 (0.120 g, 30%),
which are also air sensitive. UV-vis (THF): λmax (ε (M-1 cm-1))
430 (5760), 660 (3700), 800 (sh). Elemental Analysis: Calcd. for
C52H59CrN4O (5): C, 77.29; H, 7.36; N, 6.93. Found: C, 76.89; H,
7.12; N, 6.72.
Other Physical Measurements. Cyclic voltammetry was carried
out with a Bipotentiostat AFCBP1 from Pine Instrument Company
fitted in a Dry Box and controlled with the PineChem 2.7.9
software. Cyclic voltammetry was performed using a gold disk
working electrode (1.6 mm diameter) and a Ag/Ag+ (0.01 M
AgNO3 and 0.5 M [(n-Bu)4N]PF6 in acetonitrile) nonaqueous
reference electrode (Bioanalytical Systems, Inc.) with a prolonged
bridge (0.5 M [(n-Bu)4N]PF6 in acetonitrile). A thin Pt foil or gauge
(8 cm2, Sigma-Aldrich) was employed as counter electrode. The
working electrode was polished using successively 6, 3, 1 µm
diamond paste on a DP-Nap polishing cloth (Struers, Westlake,
OH), washed with water, acetone and air-dried. The Pt foil and
[(L1re-1)Mn(III)-(O)2-Mn(III)(L1re-1)][K(OEt2)]2 ·C6H14
(2).
Ligand L1H3 (0.343 g, 0.5 mmol) was dissolved in degassed THF
(15.0 mL), to which KH (0.060 g, 1.5 mmol) was added. The
mixture was stirred overnight until all KH was dissolved. MnCl2
(0.0625 g, 0.5 mmol) was added to this yellow solution as solid
and the mixture was allowed to stir for another 10 h to give a pale
yellow solution. Dioxygen (“extra dry”, 99.999%) was then bubbled
through this solution for 2.0 min to generate a dark green-brown
solution. This solution was evaporated to dryness under vacuum,
extracted with hexane, and filtered. The extract was evaporated to
dryness and redissolved in a minimum amount of diethyl ether.
The resulting solution was layered with hexane and allowed to stand
at room temperature to afford dark green crystals of 2 (0.200 g,
24%). IR (KBr, cm-1): 1591, 1570, 1549, 1527, 1513, 1499, 1481,
1466, 1448, 1409, 1391, 1361, 1315, 1267, 1191, 1158, 1106, 1043,
1017, 924, 866, 830, 739, 707, 675, 636, 612, 589, 583, 555, 493,
479, 467, 450, 435, 412, 404. UV-vis (THF): λmax (ε (M-1 cm-1))
276 (4550), 390 (sh), 496 (1810), 720 (sh). Elemental Analysis:
Calcd. for C110H136K2Mn2N8O4: C, 72.50; H, 7.52; N, 6.15. Found:
C, 71.78; H, 7.83; N, 6.27.
gauge electrodes were cleaned in a H2O2/H2SO4(conc) solution (1:4
v:v) and oven-dried. The concentration of the samples was 3 mM
and that of [(n-Bu)4N]PF6 (supporting electrolyte) was 0.5 M. The
potential sweep rate varied between 10-1000 mV/s. Unless stated
otherwise, all potentials are reported versus the ferrocenium/
ferrocene (Fc+/Fc) couple.
Magnetic susceptibility data were obtained at the Materials
Science Department of MIT on a Quantum Design AC MRMS-5S
SQUID magnetosusceptometer equipped with a 5.5 T magnet. Data
were collected in the temperature range 2-300 K at a field of 1.0
T within the linear response of the magnetic moment vs field
relationship. Compound 2 (0.0152 g) was charged in a capsule,
which was suspended in a straw. Corrections were made by
recording magnetic susceptibility vs temperature data of a blank
sample (capsule/straw) and for the diamagnetic contributions with
the assistance of Pascal tables.
Crystallographic data were collected at the Department of
Chemistry and Chemical Biology at Harvard University by using
a Bruker SMART CCD (charge coupled device) based diffracto-
meter equipped with an Oxford Cryostream low-temperature
apparatus operating at variable low temperatures. A suitable crystal
was chosen and mounted on a glass fiber using grease. Data were
measured using omega scans of 0.3° per frame for 30 s, such that
a hemisphere was collected. A total of 1271 frames were collected
with a maximum resolution of 0.75 Å. The first 50 frames were
recollected at the end of data collection to monitor for decay. Cell
parameters were retrieved using SMART software and refined using
SAINT on all observed reflections. Data reduction was performed
using the SAINT software, which corrects for Lp and decay. The
structures are solved by the direct method using the SHELXS-97
program and refined by least-squares method on F2, SHELXL-97,
incorporated in SHELXTL-PC V 5.10. All non-hydrogen atoms
1,3,5′-Tris(4-tert-butylphenyl)-1,3-dihydro-5′H-spiro[ben-
zo[d]imidazole-2,1′-phenazine] ·Et2O (3). Ligand L1H3 (0.343 g,
0.5 mmol) was dissolved in degassed THF (15.0 mL), to which
KH (0.060 g, 1.5 mmol) was added. The mixture was stirred
overnight until all KH was dissolved. MnCl2 (0.0625 g, 0.5 mmol)
was added to this yellow solution as solid and the mixture was
stirred for another 10 h to give a pale yellow solution. Dioxygen
was bubbled through the solution for approximately 15 min until a
purple color solution was generated. This solution was evaporated
to dryness under vacuum, extracted with diethyl ether and filtered.
The filtrate was reduced to a minimum amount and allowed to stand
at room temperature to give dark purple crystals of 3 (0.035 g,
10%). 1H NMR(THF-d8, 1.73 ppm): δ 7.604 (d, 2H, J ) 8.4 Hz),
7.237 (dd, 10H, J ) 40.8 Hz, J′ ) 8.4 Hz), 6.928 (d, 3H, J ) 8.4
Hz), 6.779 (t, 1H, J ) 7.2 Hz), 6.526 (d, 4H, J ) 3.0 Hz), 6.074
(d, 1H, J ) 8.4 Hz), 5.978 (dd, 1H, J ) 9.6 Hz, J′ ) 6.6 Hz),
5.667 (d, 1H, J ) 9.6 Hz), 1.367 (s, 27H, tert-butyl). UV-vis
(THF): λmax (ε (M-1 cm-1)) 296 (3830), 336 (2800), 522 (600).
HRMS (ESI): Calcd. for C48H51N4 ([M + H]+): 683.4108. Found:
683.4241.
[(L1re-1)(Cl)Cr(III)-THF] ·0.75C6H14 (4). Ligand L1H3 (0.343
g, 0.5 mmol) was dissolved in degassed THF (15.0 mL), to which
KH (0.060 g, 1.5 mmol) was added. The mixture was stirred
overnight until all KH was dissolved. Pink-colored CrCl3 (0.079
g, 0.5 mmol) was added to this yellow solution as solid and the
mixture was allowed to stir overnight to afford a dark brown
solution. This solution was evaporated to dryness under vacuum,
and the residue was extracted with hexane and filtered. The extract
was allowed to stand at room temperature to give dark-brown,
needle-like crystals of 4 (0.120 g, 28%). UV-vis (THF): λmax (ε
11000 Inorganic Chemistry, Vol. 47, No. 23, 2008

Metalloradical Complexes of Mn and Cr
Table 1. Summary of Crystallographic Data for Compounds 1-5
1b
2
3
4
5
formula
Mr
crystal system
space group
a (Å)
C88H103MnN4O4P
1366.65
monoclinic
P2(1)/n
10.7305(12)
24.248(3)
29.792(3)
90
98.768(2)
90
7661.0(15)
4
C
110H136K2Mn2N8O4
C
104H120N8O2
C
56.50H69.50ClCrN4O
C52H59CrN4O
808.03
1822.35
triclinic
1514.08
monoclinic
C2/c
39.025(8)
12.187(2)
22.221(4)
90.00
122.573(4)
90.00
8906(3)
4
908.11
triclinic
triclinic
j
j
j
P1
P1
P1
12.6764(17)
13.4406(18)
16.108(2)
79.940(3)
67.018(3)
83.947(3)
2485.7(6)
1
10.4052(6)
16.2284(10)
16.2419(10)
81.0380(10)
83.8510(10)
75.6920(10)
2618.3(3)
2
11.691(2)
13.995(3)
15.370(3)
62.987(3)
85.304(4)
84.835(4)
2228.9(7)
2
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dcalcd (g cm-3
T (K)
)
1.185
293(2)
1.217
99(2)
1.129
193(2)
1.152
293(2)
1.204
213(2)
λ (Å)
0.71073
0.246
0.1113
0.71073
0.393
0.0778
0.71073
0.067
0.1683
0.2538
0.71073
0.310
0.1417
0.71073
0.298
0.1942
µ (mm-1
)
a
R1 (4σ data)
b
wR2 (4σ data)
0.1849
0.1441
0.1823
0.1569
a R1 ) Σ|Fo| - |Fc||Fo|. b wR2 ) [Σw(Fo - Fc )2/Σw(Fo )2]1/2
.
2
2
2
Scheme 1
were refined anisotropically. Hydrogens were calculated by geo-
metrical methods and refined as a riding model. The crystals used
for diffraction studies showed no decomposition during data
collection. All drawings are done at 50% ellipsoids unless otherwise
stated. Pertinent crystallographic data are collected in Table 1.
ometry. The closest analogues are the corresponding iron(II)
compounds [(L1)Fe(II)-THF][K(THF)3]8 and [(L3)Fe(II)-
THF][K(THF)2],10 and the Mn(II) compounds [(L3)Mn(II)-
THF][K(THF)2] and [(L5)Mn(II)-THF][K(THF)4],10 where
L3 and L5 are the 3,5-(CF3)2 and 3,5-Cl2 substituted ligands,
respectively, in lieu of the 4-t-Bu substitution in L1.
Results and Discussion
Ligand Synthesis. Scheme 1 summarizes the key steps
leading to the synthesis of the tripodal trisamido amine ligand
L1H3. The synthetic protocol relies on the preparation of
2,2′,2′′-triaminotriphenylamine,9 which is further function-
alized by virtue of Pd-catalyzed Hartwig-Buchwald arylation
of amines. A family of ligands has thus been generated,
which can provide a tetradentate C3-symmetric environment
for metallation purposes. Details on the synthesis and
characterization of this ligand family have already been
published.9 Here we concentrate on aspects of oxidative
reactivity involving Mn(II) and Cr(III) precursor species
featuring the prototypical L1H3 ligand, in which both metal
and ligand are engaged in redox transformations.
Synthesis of Mn(II) Precursor Complexes. The reaction
of anhydrous MnCl2 (beads, 99.99%) with 3 equiv of K3L1,
prepared in situ by deprotonation of the ligand L1H3 with
KH in THF, leads to generation of yellow-orange solutions
from which crystals of [(L1)Mn(II)-THF][K(THF)x] (1a) are
isolated either from concentrated THF solutions or upon
layering pentane over THF solutions of 1a. The compound
is exceedingly air-sensitive (see below) and also loses solvent
easily, thus hampering efforts to further define its stoichi-
Figure 1. Solid-state structure of [(L1)Mn(II)-THF](Ph4P)·3THF (1b)
showing 50% probability ellipsoids and the atom labeling scheme (the
countercation (Ph4P)+ and the solvated THF molecules have been omitted
for clarity). Selected interatomic distances (Å) and angles (deg): Mn(1)-N(3)
2.084(3), Mn(1)-N(4) 2.088(3), Mn(1)-N(2) 2.113(3), Mn(1)-O(1)
2.228(3),Mn(1)-N(1)2.316(3),N(3)-Mn(1)-N(4)114.92(12),N(3)-Mn(1)-
N(2) 115.19(11), N(4)-Mn(1)-N(2) 114.68(12), N(3)-Mn(1)-O(1)
109.57(11), N(4)-Mn(1)-O(1) 99.73(11), N(2)-Mn(1)-O(1) 100.23(11),
N(3)-Mn(1)-N(1)76.91(10),N(4)-Mn(1)-N(1)77.03(11),N(2)-Mn(1)-N(1)
76.39(10).
Inorganic Chemistry, Vol. 47, No. 23, 2008 11001

Çelenligil-Çetin et al.
Scheme 2
Addition of excess (Ph4P)Cl to yellow-orange solutions
one-electron oxidation, followed by migration of a phenylene
ring (bridging between atoms N1 and N4) from atom N1 to
atom N3 (Scheme 2). Furthermore, the oxidizing equivalent
that initiates this radical 1,4-(N-to-N) phenyl migration12 is
retained as a radical site on a newly formed o-diiminosemi-
benzoquinonate (o-disq-) moiety.13
of 1a in THF affords bright orange solutions from which
crystals of [(L1)Mn(II)-THF](Ph4P) · 3THF (1b) can be
isolated in good yields upon layering with hexane.
The structure of 1b reveals a distorted trigonal bipyramidal
geometry around the Mn(II) site, featuring two similar and
a slightly longer Mn(II)-Namido bonds with an average value
of 2.095 (3) Å, whereas the Mn(II)-Namine bond is signifi-
cantly longer at 2.316(3) Å. The distortion is also exemplified
by the position of the Mn(II) atom at a distance of 0.479 Å
from the plane defined by the three amido nitrogen atoms
and toward the coordinated THF molecule, generating an
average Namine-Mn-Namido angle of 76.78° consistent with
the geometric requirements of five-member rings. All metri-
cal parameters associated with the bond distances noted
above are longer by comparison to those found in the
analogous Fe(II) complex,8 reflecting the larger ionic radius
of Mn(II).
The dual mode X-band EPR spectrum of 1a in frozen THF
(5 mM) (Figure S1 in the Supporting Information and Figure
5 below) consists of signals in a wide field region in the
perpendicular mode (g ) 14.7, 5.10, 3.21, 2.06) and a
characteristic signal in parallel mode. Both are very similar
to those previously observed for mononuclear Mn(II) com-
plexes,11 albeit slightly shifted. The axial component of the
zero-field splitting parameter |D| (on the order of 0.1 cm-1
for 1a) and the rhombicity (E/D ∼ 0.15-0.25) are also
similar to reported values.
Oxidation of Mn(II) Precursor with Dioxygen. Yellow-
orange solutions of [(L1)Mn(II)-THF][K(THF)x] in THF are
exceedingly sensitive to dry dioxygen, turning green-brown
instantaneously upon quick exposure (1-3 min). Exhaustive
extraction with hexane and crystallization from Et2O/hexane
provides dark green-brown crystals of [(L1re-1)Mn(III)-(O)2-
Mn(III)(L1re-1)][K(OEt2)]2 · C6H14 (2). Following extraction
with hexane, a small amount of a remaining brown com-
pound can be obtained from diethyl ether extracts. Despite
numerous attempts, the brown species has not provided
suitable crystalline samples and remains as yet uncharacter-
ized. However, glimpses to its possible nature are discussed
in subsequent sections.
The ligand rearrangement compromises the integrity of
the C3-symmetric ligand and alleviates the steric hindrance
imposed by the three 4-t-butyl-phenyl arms. As a conse-
quence, the structure of compound 2 is dimeric (Figure 2),
featuring a Mn(µ-O)2Mn core at the rare Mn(III) oxidation
level, and a radical-containing ligand (S ) 1/2) per Mn. The
coordination of the asymmetric core of the centrosymmetric
structure of 2 is best viewed in Figure 3. Each Mn(III) site
possesses a distorted trigonal bipyramidal coordination
geometry, defined with respect to Mn(1) by the equatorial
atoms O(1A), N(2) and N(4) and the axial atoms N(1) and
O(1) (N(1)-Mn(1)-O(1) ) 164.38(7)°). The position as-
signment of the symmetry related O and O(1A) atoms
reverses for Mn(1A). The purely amine-type atoms N(3) and
N(3A) lie outside the coordination sphere of Mn(1) and
Mn(1A), respectively (Mn(1)-N(3) ) 3.417 Å). The elonga-
tion of the Mn(1)-N(4) bond (2.1270(18) Å) associated with
this genuine Namido atom is most likely due to the Jahn-Teller
effect, which is typical of Mn(III), d4 sites, and its axis tends
to lie perpendicular to the planar Mn2O2 core. The semi-
imino nature of atoms N(1) and N(2) associated with the
o-disq- moiety is supported by the average C-N bond
distance (1.375(3) Å) that lies between a single and a double
bond, the pattern of four long and two short C-C bonds, as
well as the longer than expected Mn-N bond distances for
genuineNamido atoms(Mn(1)-N(1))2.0311(19),Mn(1)-N(2)
) 2.0114(18) Å).
Metrical parameters for the Mn(III)(O)2Mn(III) core of all
isolable compounds known to date are collected in Table
2.14-20 Compound 2 possesses the longest Mn-Mn distance
and one of the longest Mn-O-Mn angles, possibly reflecting
(12) Studer, A.; Bossart, M. Tetrahedron 2001, 57, 9649–9667.
(13) (a) Herebian, D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc. 2003,
125, 10997–11005. (b) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe,
E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123,
2213–2223.
(14) Gultneh, Y.; Yisgedu, T. B.; Tesema, Y. T.; Butcher, R. J. Inorg.
Chem. 2003, 42, 1857–1867.
(15) Mikata, Y.; So, H.; Yamashita, A.; Kawamura, A.; Mikuriya, M.;
Fukui, K.; Ichimura, A.; Yano, S. Dalton Trans. 2007, 3330–3334.
(16) Goodson, P. A.; Oki, A. R.; Glerup, J.; Hodgson, D. J. J. Am. Chem.
Soc. 1990, 112, 6248–6254.
(17) Goodson, P. A.; Hodgson, D. J. Inorg. Chem. 1989, 28, 3606–3608.
(18) Kitajima, N.; Singh, U. P.; Amagai, H.; Osawa, M.; Moro-oka, Y.
J. Am. Chem. Soc. 1991, 113, 7757–7758.
Compound 2 features a ligand that has been rearranged
with respect to the intact C3-symmetric ligand of 1. As
indicated below, the ligand reorganization is triggered by a
(10) Pinnapareddy, D.; Paraskevopoulou, P.; Stavropoulos. P., manuscript
in preparation.
(11) Pierce, B. S.; Elgren, T. E.; Hendrich, M. P. J. Am. Chem. Soc. 2003,
125, 8748–8759.
11002 Inorganic Chemistry, Vol. 47, No. 23, 2008

Metalloradical Complexes of Mn and Cr
coordination of the Namine atom as a result of steric hindrance.
It is worthwhile noting that each potassium ion makes close
contact to one of the oxo bridges as shown in Scheme 2,
and is further supported by a diethyl ether molecule and
several electron-rich aromatic carbons and nitrogen atoms
that are within van der Waals distance.21
The stretching vibrations of the Mn-O bonds associated
with the Mn(O)2Mn core are obscured by a broad ligand-
derived absorption band, but a difference spectrum with
respect to the ligand as well as the analogous [(L1re-1)Fe(III)-
O-Fe(III)(L1re-1)] complex,8 reveals that a residual band at
730 cm-1 can be tentatively assigned to the stretching of
the Mn(O)2Mn core (Figure S2 in the Supporting Informa-
tion). This is at slightly higher wavenumber than the reported
range (670-700 cm-1) for similar compounds containing the
Mn(O)2Mn core.
A similar diferric µ-oxo compound ([(L1re-1)Fe(III)-O-
Fe(III)(L1re-1)]) carrying the same oxidatively rearranged
ligand has been previously isolated.8 The iron congener is
distinguished with respect to 2 by virtue of a single oxo
bridge, a close Fe-Namine contact (2.458(4) Å) and a strong
Fe-Namido bond (1.960(5) Å). These features stabilize the
iron compound with respect to further oxidation by dioxygen,
whereas compound 2 evolves further upon prolonged expo-
sure to dioxygen (see below).
Figure 2. Solid-state structure of [(L1re-1)Mn(III)-(O)2-Mn(III)-
(L1re-1)][K(OEt2)]2 ·C6H14 (2) showing 50% probability ellipsoids and the
atom labeling scheme. Selected interatomic distances (Å) and angles (deg)
are Mn(1)-O(1) 1.8328(15), Mn(1)-O(1A) 1.8592(15), Mn(1)-N(2)
2.0114(18),Mn(1)-N(1)2.0311(19),Mn(1)-N(4)2.1270(18),O(1)-Mn(1)-O(1A)
84.30(7),K(1)-O(1)2.6932(16),K(1)-O(3S)2.748(2),Mn(1)-O(1)-Mn(1A)
95.70(7), O(1)-Mn(1)-N(2) 95.63(7), O(1A)-Mn(1)-N(2) 142.82(7),
O(1)-Mn(1)-N(1)164.38(7),O(1A)-Mn(1)-N(1)91.41(7),N(2)-Mn(1)-N(1)
78.77(7), O(1)-Mn(1)-N(4) 97.98(7), O(1A)-Mn(1)-N(4) 117.00(7),
N(2)-Mn(1)-N(4)99.89(7),N(1)-Mn(1)-N(4)97.35(7),O(1)-Mn(1)-Mn(1A)
42.52(5), O(1A)-Mn(1)-Mn(1A) 41.78(5), N(2)-Mn(1)-Mn(1A) 127.36(5),
N(1)-Mn(1)-Mn(1A) 131.42(6), N(4)-Mn(1)-Mn(1A) 113.66(5).
Magnetics. Magnetic susceptibility data for compound 2
were collected in the temperature range 5-300 K at a field
strength of 1.0 T, and were corrected for the magnetization
of the sample holder. The measured molar magnetic sus-
ceptibility for 2 as a function of temperature is shown in
Figure 4. The product ꢁMT, where ꢁM is the molar suscep-
tibility, decreases with decreasing temperature. The spin
Hamiltonian and the expression used to fit the susceptibility
data in the temperature range 10-300 K are given in eqs 1
and 2, where S1 ) S4 ) 1/2 (ligand radical) and S2 ) S3 )
2 (Mn(III)) and F is the paramagnetic impurity of a Mn(III)
ion. The expression (eq 2) used to fit the magnetic suscep-
tibility data incorporates the contribution of the molar
susceptibility of compound 2 as determined from the
Hamiltonian (first term of eq 2) plus the contribution of a
fraction F of paramagnetic impurities due to a Mn(III) ion
according to the Curie law (second term of eq 2).22 The best
fit is shown in the same figure as a solid line and corresponds
to g ) 2.0 (fixed), JR-Mn > -600 cm-1 (undefined) for the
coupling between each Mn(III) and ligand-centered radical,
and JMn-Mn ) -120(3) cm-1 for the coupling between the
two Mn(III) via the oxo bridges and F ) 3.3%. Although an
S ) 0 ground-state is expected, the nonzero susceptibility at
low temperatures is due to a paramagnetic impurity of a
manganese ion. The more drastic decrease below 8 K is due
to zero-field effects of this impurity. It was not possible to
Figure 3. Detail of the solid-state structure of 2 showing the Mn(O)2Mn
core and the first coordination sphere. Bond distances (Å) related to the
o-disq- moiety are as follows: N(1)-C(1) 1.362(3), N(2)-C(2) 1.352(3),
C(1)-C(2) 1.441(3), C(2)-C(3) 1.425(3), C(3)-C(4) 1.370(3), C(4)-C(5)
1.412(4), C(5)-C(6) 1.374(3), C(6)-C(1) 1.419(3).
the steric encumbrance around each Mn(III) site. Otherwise,
the Mn-O bond distances of 2 are within the range expected
for Mn(III) and, on average, longer than those known for
the much more common mixed-valent Mn(III)(O)2Mn(IV)
and Mn(IV)(O)2Mn(IV) cores.6 Presumambly, the Mn(III)-
(O)2Mn(III) core is stabilized in 2 due to the weaker ligand
field imposed by the semiimino N atoms and the non-
(21) (a) Huffman, J. C.; Green, M. A.; Kaiser, S. L.; Caulton, K. G. J. Am.
Chem. Soc. 1985, 107, 5111–5115. (b) Bouwkamp, M. W.; Lobkovsky,
E.; Chirik, P. J. Inorg. Chem. 2006, 45, 2–4.
(22) (a) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993.
(b) Andersen, N. H.; Døssing, A.; Mølgaard, A. Inorg. Chem. 2003,
42, 6050–6055.
(19) Glerup, J.; Goodson, P. A.; Hazell, A.; Hazell, R.; Hodgson, D. J.;
McKenzie, C. J.; Michelsen, K.; Rychlewska, U.; Toftlund, H. Inorg.
Chem. 1994, 33, 4105–4111.
(20) Hitomi, Y.; o, A.; Matsui, H.; Ito, T.; Tanaka, T.; Ogo, S.; Funabiki,
T. Inorg. Chem. 2005, 44, 3473–3478.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11003

Çelenligil-Çetin et al.
Table 2. Physicochemical Parameters of the Mn(III)-(O)2-Mn(III) Corea
E(Mn2III,III/Mn2III,IV),
IV,IV
Mn· · ·Mn Mn-O(1) Mn-O(2) Mn-O-Mn O(1)-Mn-O(2)
J
E(Mn2III,IV/Mn2
)
compound
(Å)
(Å)
(Å)
(deg)
(deg)
(cm-1
)
(V vs Ag/AgCl)
ref
16
[L2MnO]2(ClO4)2 ·H2O
2.676(3)
1.814(8)
1.830(9)
1.818(9)
1.853(9)
1.863(8)
1.841(6)
94.5(4)
92.1(4)
93.9(3)
86.3(4)
87.1(4)
86.1(3)
-86.4 ( 0.2 0.568, 1.337
0.678, 1.446
[L3MnO]2(NO3)2 ·6H2O
[L3MnO]2(ClO4)2
2.674(4)
2.686(1)
15
15
15
15
[L4MnO]2(ClO4)3
0.090, 0.903
no electrochemistry
in the region of -0.5 -
(+1.8) V
[L5MnO]2(ClO4)2 ·7H2O
1.855(4)
1.851(4)
93.3(2)
85.7(2)
87.0(2)
1.839(4)
1.806(5)
1.808(6)
1.846(4)
1.824(4)
1.813(6)
1.787(6)
1.851(4)
93.9(2)
96.5(3)
97.0(3)
93.8(2)
[L6MnO]2
2.696(2)
2.699(2)
2.81 µB/mol
-101(1)
17
[L7MnO]2(ClO4)2 ·5H2O
86.2(2)
18
13
13
[L8MnO]2(ClO4)2
2.7042(5) 1.8264(12) 1.8285(12) 95.44(6)
84.56(6)
84.33(12)
84.20(12)
86.15(8)
86.34(12)
85.96(12)
89.1(1)
0.87, 1.70
0.78, 1.58
[L9MnO]2(ClO4)2 ·2.2MeCN 2.7067(11) 1.822(3)
1.830(2)
1.829(3)
95.67(12)
95.80(12)
2.7119(12) 1.826(3)
2.6649(5) 1.8238(19) 1.8251(19) 93.83(9)
2.6675(9) 1.824(2)
1.830(2)
2.656(2)
[L10MnO]2(ClO4)2 ·2DMF
[L11MnO]2(ClO4)2 ·H2O
-97.5
-82.9
0.79, 1.61
0.80, 1.64
14
14
1.821(2)
1.828(2)
1.861(3)
93.77(11)
93.92(11)
90.9(1)
[L12MnO]2(BPh4)2
[(L1re-1)MnO]2[K(OEt2)]2 ·
C6H14 (2)
1.866(3)
19
this work
2.7373(7) 1.8328(15) 1.8592(15) 95.70(7)
84.30(7)
-120
see text
a L2: N,N,′-bis((6-methylpyrid-2-yl)methyl)ethane-1,2-diamine; L3: N,N,′-bis((6-methylpyrid-2-yl)methyl)-N-2-pyridylmethylamine; L4: N,N,′-bis((4-
methylpyrid-2-yl)methyl)ethane-1,2-diamine; L5: N,N,′-bis(2-methylpyrazyl)ethane-1,2-diamine; L6: hydrotris(3,5-diisopropyl-1-pyrazolyl)borate; L7: N,N,′-
bis(2-pyridylmethyl)-N,N,′-dimethyl-1,2-ethanediamine; L8: tris(6-methyl-2-pyridylmethyl)diamine; L9: 2-(2-pyridyl)ethylbis(6-methyl-2-pyridylmethyl)amine;
L10: N,N,′-bis(2-quinolylmethyl)-N,N,′-dimethyl-1,2-ethanediamine; L11: N,N,′-bis(2-quinolylmethyl)-N,N,′-diethyl-1,2-ethanediamine; L12: tris(pyrid-2-ylmethyl)-
amine.
JMn-Mn for 2 is somewhat larger than that reported for other
[Mn(III)(O)2Mn(III)]-containing compounds,14-20 but the
available data remain limited for any meaningful magneto-
structural correlations23 (Table 2). The related [Mn2(5-
MeOsaltmen)2(DCNNQI)2] compound24 (5-MeOsaltmen2-
) N,N′-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylide-
neiminate); DCNNQI ) N,N-dicyano-1,4-naphthoquinone-
diiminate radical), which features two phenolate bridges,
exhibits strong antiferromagnetic coupling between the
Mn(III) ion and the ligand-residing radical, but weak
ferromagnetic Mn(III)/Mn(III) coupling via the phenolate
bridges.
Other Species in the Reaction of Mn(II) and Dio-
xygen. The progression of the reaction of THF solutions of
1a (5 mM) with dry dioxygen was monitored by X-band
EPR spectroscopy. At certain time intervals batches of
Figure 4. Temperature dependence of the susceptibility data of compound
samples (∼200 µL) were removed, transferred to EPR tubes
2 in the form of ꢁMT vs T (solid stars) along with the fitting results (solid
and plunged into liquid nitrogen. Interaction with O2 for
approximately 0.5 min leads to the disappearance of the
Mn(II) signal noted above. At 4.2 K, the loss of the feature
at g ∼ 5.1 from the Mn(II) monomer signal is accompanied
by the appearance of a new signal of different shape
consisting of a relatively broad absorption with an apparent
peak at roughly the same g-value (Figure 5, left). Its
temperature dependence suggests that it arises from a ground
spin state. This feature is consistent with an S ) 3/2 system
with a zero-field splitting parameter |D| . hν(∼0.3 cm-1 at
X-band) and rhombic distortion (E/D ∼ 0.2). Such an S )
3/2 derived signal has been observed in mononuclear Mn(IV)
line) according to eqs 1 and 2. See text for details.
simulate this parameter from the above fitting procedure since
the above model will be overparametrized.
Hexc ) -2JR-Mn(S1 · S2 + S3 · S4) - 2JMn-Mn(S2 · S3) +
4
Σ µBB · gi · Si (1)
i)1
Ng2µ2BS(S + 1)
ꢁ ) (1 - F)ꢁexc + F
(2)
3κBT
The magnetic susceptibility data of [(L1re-1)Fe(III)-O-
Fe(III)(L1re-1)] show a similar trend to that observed with 2,
featuring a strong antiferromagnetic coupling between the
metal and the ligand-based radical (JR-Fe > -500 cm-1
(undefined)) and a more modest antiferromagnetic coupling
between the two metals (JFe-Fe ) -123 cm-1). The value of
(23) Law, N. A.; Kampf, J. W.; Pecoraro, V. L. Inorg. Chim. Acta 2000,
297, 252–264.
(24) Kachi-Terajima, C.; Miyasaka, H.; Sugiura, K.-I.; Cle´rac, R.; Nojiri,
H. Inorg. Chem. 2006, 45, 4381–4390.
11004 Inorganic Chemistry, Vol. 47, No. 23, 2008

Metalloradical Complexes of Mn and Cr
Figure 5. Perpendicular mode X-band EPR spectra of a 5 mM solution of [(L1)Mn(II)-THF][K(THF)x] (1a) in THF after exposure to dioxygen for 0, 0.5,
3.0, and 20 min. EPR parameters: (Left) T ) 4.2 K, microwave power 2.2 mW, modulation amplitude 25 Gpp, microwave frequency 9.42 GHz; (Right) T
) 4.2 K, microwave power 0.15 mW, modulation amplitude 5 Gpp, microwave frequency 9.42 GHz.
complexes,25 but may also arise from strong antiferromag-
netic coupling between a Mn(III) ion (S ) 2) and an S )
1/2 radical-bearing moiety, or between a Mn(II) ion (S )
5/2) and two S ) 1/2 radical residues.26 Electrochemical data
(see below) favor the Mn(III)-L• scenario.
spectrum with an S ) 1/2 system interacting with two 55Mn(I
) 5/2) nuclei indicate anisotropic hyperfine tensors for both
55Mn sites with apparent isotropic values of A1,iso ∼ 296 MHz
and A2,iso ∼ 171 MHz. If we assume a Mn(III)-Mn(IV)
dimer, A1 is consistent with the Mn(III) site and A2 with the
Mn(IV) site, although these values are relatively small by
comparison with known examples of mono-oxo or dioxo
bridged Mn(III)-Mn(IV) complexes.29 A possible explana-
tion for these reduced 55Mn hyperfine values is that ap-
preciable spin density delocalizes toward the ligand due to
covalency.29b It is also conceivable that the signal may be
better accommodated by a [Mn(III)-Mn(III)(L•)] species, a
precursor of the EPR-silent compound 2.
As the time of interaction of 1a with O2 increases to 3.0
min, the S ) 1/2 multiline signal noted above decreases to
ca. 35% of that observed at 0.5 min and at longer times is
completely lost. However, a new multiline signal with very
similar properties is evident at 3.0 min, which also decays
slowly over time (at 20 min only 35% of the intensity
observed in the 3.0 min spectrum is observed). This new
multiline signal exhibits slow relaxation and comparable
width (970 G) with respect to the previous S ) 1/2 multiline
signal, but is more complicated exhibiting more than 16 lines.
If we assume an S ) 1/2 system interacting with two 55Mn(I
) 5/2) nuclei as before, the smaller width suggests smaller
values for the hyperfine tensors. This signal is associated
with the second minor species isolated from ethereal extracts
as noted above, and is again likely to contain a [Mn(III)-
Mn(III)(L•)] core.
A second feature that grows in as the Mn(II) signal
disappears is a multiline signal at g ) 2.0 (Figure 5, right).
The general appearance of the signal is that of a pseudo 16-
line spectrum,27 which is strongly reminiscent of the signals
observed from Mn(III)-Mn(IV) dimers. The temperature
dependence indicates that the spin state responsible for this
signal is an S ) 1/2 ground state, well isolated from other
spin manifolds of the coupled system, implying strong
exchange coupling. That the ground-state is well-isolated
from other spin states (as the result of strong exchange
coupling) is further supported by the slow relaxation proper-
ties of the signal at 4.2 K as suggested by its facile saturation.
Such a property is generally observed in mono- or dioxo
bridged Mn(III)-Mn(IV) dimers as opposed to relevant
Mn(II)-Mn(III) dimers.28 However, the width of the ob-
served signal (∼1000 G) is narrower than that reported for
mono-oxo or dioxo-bridged Mn(III)-Mn(IV) compounds
(∼1190 and 1250 G, respectively).29 Simulations of the
(25) (a) Kessissoglou, D. P.; Li, X.; Butler, W. M.; Pecoraro, V. Inorg.
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Finally, the decrease of the aforementioned signals is
accompanied by the emergence of a relatively narrow (∆Hpp
) 12 G) free-radical signal centered at g ) 2.0048 (Figure
S3 in the Supporting Information, after 20 min interaction
with O2 at room temperature).
Exposing bulk solutions of 1a (or 2) in THF to O2 for
15-20 min results in precipitation of MnO2 and generation
Inorganic Chemistry, Vol. 47, No. 23, 2008 11005

Çelenligil-Çetin et al.
Figure 7. Cyclic voltammogram of [(L1)Mn(II)-THF][K(THF)x] (1a) in
THF/[(n-Bu)4N]PF6 (0.5 M) with a Au disk electrode (1.6 mm in diameter);
scan rate 0.1 V/s. Arrow indicates direction of scan.
Figure 8. Cyclic voltammogram of [(L1re-1)Mn(III)-(O)2-Mn(III)-
(L1re-1)][K(OEt2)]2 (2) in THF/[(n-Bu)4N]PF6 (0.5 M) with a Au disk
electrode (1.6 mm in diameter); scan rate 0.1 V/s. Arrow indicates direction
of scan.
Figure 6. (Top) Solid-state structure of [L1re-4] (3) showing 50% probability
ellipsoids and the atom labeling scheme. (Bottom) Selected interatomic
distances (Å).
waves at 0.415 and 0.628 V and a cathodic wave at -0.294
V (vs the Fc+/Fc couple). Similar behavior has been
observed7 with the electrochemical oxidation of tetrahydro-
furan solutions of [(L1)Fe(II)-THF]-, albeit at more acces-
sible potentials, and assigned to independent one-electron
Fe(II)/Fe(III) (quasi-reversible wave) and two-electron (L1)-
Fe(II) f (L1re-1)-Fe(III) (first anodic wave) transformations.
It is thus likely that the 0.415 V anodic wave is due to (L1)-
Mn(II) f (L1re-1)-Mn(III) and that the S ) 3/2 EPR signal
which is consistent with a formal Mn(IV) mononuclear site
observed by EPR in the early stages of the reaction of Mn(II)
with dioxygen may be equivalent to the electrochemically
accessible (L1re-1)-Mn(III) species. Furthermore, the presence
of both Mn(III) and (L1re-1)-Mn(III) containing sites may
be responsible for providing the initial [Mn(III)Mn(III)(L•)]
compound as indicated by EPR spectroscopy.
of a pink solution, from which the diamagnetic, metal-free
species [L1re-4] (3) is isolated as purple crystals from diethyl
ether. Its spiro structure (Figure 6) indicates further oxidative
rearrangement of the ligand framework (see below for
details) although its stoichiometry is the same as that of the
deprotonated ligands [L1]3- or [L1
]
re-1
2- minus one hydrogen
atom. It can be perceived as the product of a formal four-
electron oxidation of [L1]3- (or three-electron oxidation
versus [L1re-1]2-) according to eq 3. The compound is unstable
in solution and slowly gives rise to unidentified S ) 1/2
radical species as noted by EPR spectroscopy.
[L1]3- f [Lr1e-4] (3) + 4e- + H+
(3)
Electrochemistry. The cyclic voltammogram of 1a in
THF (Figure 7) shows one wave at an E1/2 value of -0.228
V (∆E ) 111 mV, ip,a/ip,c ) 1.12), most likely due to the
Mn(II)/Mn(III) couple, followed by two-electron anodic
The oxidation of 2 was examined by cyclic voltammetry
in THF (Figure 8) and MeCN. In both cases the main feature
of the voltammograms are two anodic/cathodic waves at E1/2
values of -0.402 (∆E ) 63 mV) and -0.191 (∆E ) 102
11006 Inorganic Chemistry, Vol. 47, No. 23, 2008

Metalloradical Complexes of Mn and Cr
Scheme 3
Scheme 4
sites is similar to that encountered with Mn(III) complexes,
the deprotonated ligand K3L1 was reacted with anhydrous
CrCl3 in THF to afford dark brown-green solutions. Two
air-sensitive products can be isolated from this reaction.
Brown hexane extracts afford needle-like crystals of
[(L1re-1)Cr(III)-(Cl)(THF)]·0.75C6H14 (4), whereas the resi-
due can be crystallized from diethyl ether to provide dark
green, diamond-shaped crystals of [(L1)Cr(III)-(THF)] (5).
Hence, both the intact and the oxidatively rearranged ligand
are featured in 5 and 4, respectively. The quantitative
generation of 4 from 5 in THF according to eq 4 provides
an explanation for the formation of both species in the
mV) V. It is more likely that those represent ligand-centered
rather than metal-centered events, especially since the E1/2
values (vs Ag/AgCl)30 are low with respect to values reported
for Mn(III)Mn(III) f Mn(III)Mn(IV) and Mn(III)Mn(IV)
f Mn(IV)Mn(IV) couples (Table 2). In addition, an ir-
reversible wave at Ep,a ) 0.079 V and two cathodic waves
at -0.018 and -0.547 V are noted (Figure 8, marked with
asterisks), which are all dominant features in the cyclic
voltammogram of the metal-free species 3 (Figure S4 in the
Supporting Information). The intensity of the features
associated with 3 increases upon exposure of 2 to dioxygen
at the expense of the aforementioned waves at -0.402 and
-0.191 V.
reaction of L1 K and CrCl3.
3
[(L1)Cr(III) - (THF)] (5) + CrCl3 f [(L1re-1)Cr(III) -
(Cl)(THF)] (4) + CrCl2 (4)
Thus CrCl3 acts both as a ligand metallation source and
an oxidizing agent. The first oxidizing equivalent above the
Cr(III) level is therefore directed toward the ligand, in sharp
contrast to the well-established silyl-derivatized, TREN-
containing compound [Cr(III)(N3N)] which is reportedly
oxidized by CuCl2 to [Cr(IV)(N3N)Cl] (Scheme 4)31 as well
as by other oxidants such as AgX and halogens.32 The
Synthesis of Cr(III) Complexes. In order to establish
whether the behavior of ligand L1H3 with respect to Cr(III)
Figure 10. Solid-state structure of [(L1)Cr(III)-THF] (5) showing 50%
probability ellipsoids and the atom labeling scheme. Selected interatomic
distances (Å) and angles (deg): Cr(1)-N(1) 2.094(3), Cr(1)-N(2) 1.969(3),
Cr(1)-N(3) 1.943(3), Cr(1)-N(4) 1.965(3), N(1)-C(13) 1.474(5), N(1)-C(1)
1.479(4), N(1)-C(7) 1.486(4), N(2)-C(12) 1.373(5), N(2)-C(19) 1.427(5),
N(3)-C(6)1.398(4),N(3)-C(29)1.439(5),N(4)-C(18)1.397(4),N(4)-C(39)
1.423(5), N(3)-Cr(1)-N(4) 124.20(13), N(3)-Cr(1)-N(2) 125.96(14),
N(4)-Cr(1)-N(2)104.34(14),N(3)-Cr(1)-O(1)94.95(12),N(4)-Cr(1)-O(1)
102.30(12), N(2)-Cr(1)-O(1) 96.58(12), N(3)-Cr(1)-N(1) 82.14(12),
N(4)-Cr(1)-N(1) 82.78(13), N(2)-Cr(1)-N(1) 81.90(12).
Figure 9. Solid-state structure of [(L1re-1)Cr(III)-(Cl)(THF)]·2C6H14 (4)
showing 50% probability ellipsoids and the atom labeling scheme. Selected
interatomic distances (Å) and angles (deg): Cr(1)-N(1) 1.932(4), Cr(1)-N(2)
1.959(4), Cr(1)-N(4) 2.019(4), Cr(1)-O(1) 2.102(3), Cr(1)-N(3) 2.216(3),
Cr(1)-Cl(1) 2.3452(13), N(1)-Cr(1)-N(2) 81.19(16), N(1)-Cr(1)-N(4)
94.76(15), N(2)-Cr(1)-N(4) 94.14(15), N(1)-Cr(1)-N(3) 80.63(14),
N(2)-Cr(1)-N(3)160.57(15),N(3)-Cr(1)-N(4)80.62(13),N(1)-Cr(1)-Cl(1)
88.70(11), N(2)-Cr(1)-Cl(1) 92.97(11), N(3)-Cr(1)-Cl(1) 93.41(9),
N(4)-Cr(1)-Cl(1) 172.50(12), N(1)-Cr(1)-O(1) 175.31(14).
Inorganic Chemistry, Vol. 47, No. 23, 2008 11007

Çelenligil-Çetin et al.
Scheme 5
intricacies of assigning electronic structures to chromium
complexes featuring ligand-centered radicals are nicely
demonstrated in a recent publication by Wieghardt and
co-workers.33
defined by the three sp2 hybridized Namido atoms in the
direction of the coordinated THF. The average Cr-Namido
bond distance (1.957(3) Å) is similar to that encountered in
4, but significantly longer than the corresponding average
value of 1.877(3) Å reported for [Cr(III)(N3N)] due to the
reduced basicity of Ar2N- moiety. Conversely, the weakened
axial coordination of [Cr(III)(N3N)] features a longer
Cr-Namine bond distance (2.243(3) Å) than that observed in
5 (2.094(3) Å).
The structure of 4 (Figure 9) displays a distorted octahedral
geometry, in which the axis defined by O(1)-Cr(1)-N(1)
(175.31°) makes the closest approach to linearity. The ligand
is rearranged as in compound 2 and also retains the one-
electron oxidized o-disq- moiety between nitrogen atoms
N(1) and N(2), as indicated by the average C-N distance
of 1.361(6) Å and the pattern of four long (C(33)-C(34)
1.435(6), C(34)-C(35) 1.409(7), C(36)-C(37) 1.401(7),
C(33)-C(38) 1.409(6) Å) and two short (C(35)-C(36)
1.356(7), C(37)-C(38) 1.378(7) Å) C-C bonds. On the
other hand, the Cr(1)-N(1) and Cr(1)-N(2) bond distances
(1.932(4), 1.959(4) Å) associated with the semi-imino
residues are shorter than that observed for the genuine amido
moiety (Cr(1)-N(4) ) 2.019(4) Å). The shorter bonds can
be explained by taking into account the position of atoms
N(1) and N(2) trans to the weakly coordinating THF and
N(3)amine moieties, respectively, whereas the location of atom
N(4) is influenced by the strong trans effect of the coordi-
nated chloride.
Mechanistic Considerations. With reference to Scheme
5, the oxidative rearrangement of metallated ligand [L1]3-
to [L1
]
2- has been previously established to occur via an
re-1
initial one-electron transfer to an Namido residue to generate
an electrophilic aminyl radical. This N-centered radical then
undergoes a 1,4-(N-to-N) phenyl migration, while the initial
oxidation equivalent is retained in a newly formed disq- ring.
Downstream oxidative events, leading to the metal-free
[L1re-4], most likely follow a similar pattern, enabled by initial
oxidation of the genuine N(4)amido residue of 2, which is
electron-rich due to the weak coordination to Mn(III). The
ensuing five-member imidazolato ring is trapped in [L1re-4],
since radical coupling is apparently preferred over ring
opening. This process generates the central imino nitrogen
N(1) of [L1re-4], which then protects the adjacent carbon from
repetition of the same process by the newly formed aminyl
radical, generated by one-electron oxidation of the remaining
The structure of 5 (Figure 10) establishes that the ligand
remains intact and that the compound possesses a distorted
trigonal bipyramidal geometry with slight deviation from C3
symmetry. The chromium ion lies 0.27 Å above the plane
(32) Filippou, A. C.; Schneider, S.; Ziemer, B. Eur. J. Inorg. Chem. 2002,
2928–2935.
(30) Tsierkezos, N. G. J. Solution Chem. 2007, 36, 289–302.
(31) Schneider, S.; Filippou, A. C. Inorg. Chem. 2001, 40, 4674–4677.
(33) Lu, C. C.; DeBeer George, S.; Weyhermu¨ller, T.; Bill, E.; Bothe, E.;
Wieghardt, K. Angew. Chem., Int. Ed. 2008, 47, 6384–6387.
11008 Inorganic Chemistry, Vol. 47, No. 23, 2008

Metalloradical Complexes of Mn and Cr
amido residue. Instead, the aminyl radical adds to an ortho
position to form the six-member ring. The removal of the
bridgehead H atom is more contentious and is probably
driven thermodynamicaaly by dioxygen addition to the allylic
cyclohexenyl radical, followed by elimination of a hydro-
peroxyl radical. It is worthwhile noting that [L1re-4] possesses
structural elements of the celebrated Perkin’s mauveine-type
compounds.34
the case of manganese, one-electron oxidation above the
Cr(III) level is ligand-centered, causing the same ligand
rearrangement and retention of the oxidation equivalent as
a π radical.
(4) The oxidation of the ligand is perceived to occur via
an incipient metal-bound aminyl radical that is generated
upon a one-electron ligand oxidation. This electrophilic
radical is then engaged in rearrangement reaction, which
amounts to an internal 1,4(N-to-N) radical aryl migration.
The rearrangement gives rise to an o-diiminobenzosemi-
quinonate type moiety, which stabilizes the oxidation equiva-
lent as a π radical.
Conclusions
The following are the principal findings of the present
study:
Future studies will be directed toward elucidating the redox
events above the [[(L1re-1)-M(III)] oxidation level, as sug-
gested by current electrochemical results.
(1) Solutions of [(L1)Mn(II)-THF]- (1a) in THF can be
oxidized by dioxygen to afford the dioxo-bridged compound
[(L1re-1)Mn(III)-(O)2-Mn(III)(L1re-1)][K(OEt2)]2 (2). This
compound contains a ligand that has been oxidatively
rearranged, while retaining the oxidation equivalent as a π
radical in a newly formed o-disq- moiety. The reduced
basicity of the ligand and the steric encumbrance imposed
contribute toward stabilizing the rare Mn(III)(O)2Mn(III)
core.
Acknowledgment. We thank Drs. Vassilis Tangoulis,
Charles Barnes, Jeff R. Long, Louise Berben, Nicholas
Leventis, George D. Chryssikos, Spyros Koinis, and Thomas
Mavromoustakos for experimental assistance and useful
discussions. We gratefully acknowledge generous financial
support (to P.S.) by the NSF (CHE-0412959) and in part by
the NIH/NIEHS (5 P42 ES007381). We also acknowledge
NSF funding (CHE-0420497) for the purchase of the
diffractometer at the University of Missouri-St. Louis.
(2) Compound 2 exhibits antiferromagnetic coupling,
which is analyzed as being composed of a strong coupling
between the S ) 1/2 ligand-centered radical and the S ) 2
Mn(III) ion and a more modest coupling between the two
Mn(III) ions via the two oxo bridges. Compound 2 also
shows rich electrochemical behavior, indicating storage of
additional oxidation equivalents whose location is deemed
to be ligand centered.
Note Added after ASAP Publication. Due to a produc-
tion error Chart 1, Schemes 3-5, and Figure 6 have been
replaced in the version published ASAP October 21, 2008;
the corrected version published ASAP October 28, 2008.
(3) CrCl3 can provide the necessary oxidizing equivalent
to oxidize compound [(L1)Cr(III)-THF] (5), featuring the
intact C3-symmetric ligand, to the corresponding six-
coordinate species [(L1re-1)Cr(III)-(Cl)(THF)] (4). As with
Supporting Information Available: X-band EPR spectra of 1a
in THF (Figure S1), IR spectrum of 2 (Figure S2), X-band EPR
spectra of compounds 1a and 2 after exposure to dioxygen (Figure
S3), cyclic voltammogram of 3 (Figure S4), and X-ray crystal-
lographic data for compounds 1-5 in CIF format. The material is
available free of charge via the Internet at http://pubs.acs.org.
(34) (a) Seixas de Melo, J.; Takato, S.; Sousa, M.; Melo, M. J.; Parola,
A. J. Chem. Commun. 2007, 2624–2626. (b) Sousa, M. M.; Melo,
M. J.; Parola, A. J.; Morris, P. J. T.; Rzepa, H. S.; Seixas de Melo,
J. S. Chem. Eur. J. 2008, 14, 8507-8513.
IC801219U
Inorganic Chemistry, Vol. 47, No. 23, 2008 11009