Characterization and dioxygen reactivity of a new series of coordinatively unsaturated thiolate-ligated manganese(II) complexes

Article abstract of DOI:10.1021/ic300192q

The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese(II) complexes ([Mn^II(S^Me2N₄(6-Me-DPEN))](BF₄) (1), [Mn^II(S^Me2N₄(6-Me-DPPN))](BPh ₄)·MeCN (3), [Mn^II(S^Me2N ₄(2-QuinoPN))](PF₆)·MeCN·Et₂O (4), and [Mn^II(S^Me2N₄(6-H-DPEN)(MeOH)](BPh ₄) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn^II(S^Me2N₄(2-QuinoEN))](PF ₆) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc.2011, 133, 12470). Dioxygen addition to complexes 1-5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: [Mn^III(S^Me2N₄(6-Me-DPEN)] ₂-(μ-O)(BF₄)₂·2MeOH (6), [Mn ^III(S^Me2N₄(QuinoEN)]₂-(μ-O) (PF₆)₂·Et₂O (7), [Mn ^III(S^Me2N₄(6-Me-DPPN)]₂-(μ-O) (BPh₄)₂ (8), [Mn^III(S^Me2N ₄(QuinoPN)]₂-(μ-O)(BPh₄)₂ (9), and [Mn^III(S^Me2N₄(6-H-DPEN)] ₂-(μ-O)(PF₆)₂·2MeCN (10). Labeling studies show that the oxo atom is derived from ¹⁸O₂. Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3-5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.

Full text of DOI:10.1021/ic300192q

Article
pubs.acs.org/IC
Characterization and Dioxygen Reactivity of a New Series of
Coordinatively Unsaturated Thiolate-Ligated Manganese(II)
Complexes
Michael K. Coggins, Santiago Toledo, Erika Shaffer, Werner Kaminsky,† Jason Shearer,‡
and Julie A. Kovacs*
The Department of Chemistry, University of Washington, Campus Box 351700, Seattle, Washington 98195-1700, United States
S
* Supporting Information
ABSTRACT: The synthesis, structural, and spectroscopic
characterization of four new coordinatively unsaturated
mononuclear thiolate-ligated manganese(II) complexes
([MnII(SMe2N4(6-Me-DPEN))](BF4) (1), [MnII(SMe2N4(6-
Me-DPPN))](BPh4)·MeCN (3), [MnII(SMe2N4(2-
QuinoPN))](PF6)·MeCN·Et2O (4), and [MnII(SMe2N4(6-H-
DPEN)(MeOH)](BPh4) (5)) is described, along with their
magnetic, redox, and reactivity properties. These complexes
are structurally related to recently reported [MnII(SMe2N4(2-
QuinoEN))](PF6) (2) (Coggins, M. K.; Kovacs, J. A. J. Am.
Chem. Soc. 2011, 133, 12470). Dioxygen addition to complexes
1−5 is shown to result in the formation of five new rare
examples of Mn(III) dimers containing a single, unsupported oxo bridge: [MnIII(SMe2N4(6-Me-DPEN)]2-(μ-O)(BF4)2·2MeOH
(6), [MnIII(SMe2N4(QuinoEN)]2-(μ-O)(PF6)2·Et2O (7), [MnIII(SMe2N4(6-Me-DPPN)]2-(μ-O)(BPh4)2 (8),
[MnIII(SMe2N4(QuinoPN)]2-(μ-O)(BPh4)2 (9), and [MnIII(SMe2N4(6-H-DPEN)]2-(μ-O)(PF6)2·2MeCN (10). Labeling studies
show that the oxo atom is derived from 18O2. Ligand modifications, involving either the insertion of a methylene into the
backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic
and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6−10 are
moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable
intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2
and dioxygen. Complexes 3−5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively
minor ligand modifications have upon the stability of metastable dioxygen-derived species.
loenzymes, including the heme-containing cytochrome
P45027,28 and nonheme superoxide reductase.29,30 Reactivity
studies involving small molecule thiolate-ligated transition-
metal complexes and dioxygen, or dioxygen-derived oxidants
have increased our understanding of the structural, electronic,
and reactivity properties of these metastable enzyme inter-
mediates. It has also been demonstrated that thiolate ligands
help to promote the activation of strong N−O and RCN
bonds,31−33 as well as facilitate the reduction of dioxygen and
superoxide.34−37 Thiolate ligands have been shown to stabilize
coordinatively unsaturated M(II) complexes (M = Mn, Fe, Co,
Ni, Cu, Zn),38,39 impart rich spectroscopic properties,30
significantly lower redox potentials,30,34 lower the activation
barrier to dioxygen binding,40 and help create potent high-
valent metal-oxos capable of activating C−H bonds.28,41
INTRODUCTION
■
Nature utilizes manganese ions to promote a wide variety of
oxidative transformations. The active sites of these metal-
loenzymes, examples of which include manganese lipoxygenase
(MnLO),1−4 ribonucleotide reductases,5 manganese superoxide
dismutases (MnSOD),6−9 catalases,10 and the oxygen evolving
complex of photosystem II,11,12 vary in terms of Mn ion
nuclearity and oxidation state. Despite these structural and
electronic differences, a unifying characteristic of this broad
class of metalloenzymes is that reactive Mn-superoxo, -peroxo,
or -oxo intermediates are proposed to form upon reaction with
−
dioxygen, or reduced derivatives (O2 , H2O2, H2O) thereof.
There are, however, very few well-characterized biological or
synthetic examples of such species.13−26 By exploring the
dioxygen chemistry of Mn2+ we should be able to improve our
understanding of the characteristic properties of Mn-dioxygen
intermediates and their thermodynamically favored products, as
well as the mechanisms by which they form.
To further our understanding of the influence that thiolate
ligands have upon biologically relevant transition metals, we
Metastable O2-derived intermediates have also been
established to form in a variety of cysteinate-ligated metal-
Received: January 25, 2012
Published: May 29, 2012
© 2012 American Chemical Society
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Article
g, 244.1 mmol) was slowly added to the solution over a period of 1 h.
The resulting mixture was allowed to slowly warm to room
temperature overnight, followed by evaporation of all volatiles to
result in a pink solid. The pink solid was redissolved in warm EtOH
(30 mL) and slowly layered with cool Et2O (80 mL) to cause the rapid
precipitation of the title compound as a white solid. The product was
isolated via filtration and dried under vacuum overnight to result in
have begun to explore the structural and reactivity properties of
thiolate-ligated Mn(II) complexes. Described herein are the
syntheses and X-ray crystal structures of a new series of
coordinatively unsaturated mononuclear alkyl thiolate-ligated
Mn(II) complexes constructed from related N-heterocyclic
containing ligand frameworks. Magnetic, electronic, and redox
properties are also described, as well as the properties of their
corresponding oxo derivatives.
1
99% yield (7.13 g, 40.2 mmol). H NMR (300 MHz, CDCl3): δ 8.27
(t, 1H), 7.89 (d, 1H), 7.60 (d, 1H), 3.50 (s, 2H), 3.05 (s, 3H).
Synthesis of N-(tert-Butyloxycarbonyl)-N′,N′-[bis(6-methyl-
2-pyridilmethyl)ethane-1,2-diamine] (6-Me-DPENBoc). NNBoc
(1.20 g, 7.5 mmol) was dissolved in 5 M NaOH (15 mL) and added to
a stirring solution of 2-(chloromethyl)-6-methylpyridine hydrochloride
(2.27 g, 12.7 mmol) that was also dissolved in 5 M NaOH (15 mL).
The solution was allowed to stir at room temperature for 4 days. Water
(25 mL) was then added to the reaction mixture, followed by the
extraction of crude organics with CH2Cl2 (3 × 50 mL). The combined
organics were washed with brine (3 × 100 mL), dried over Na2SO4,
and dried under vacuum to afford an orange residue. The residue was
chromatographed on silica gel using a 92:8 acetone:MeOH eluent
mixture. Concentration of all fractions containing the desired product
resulted in isolation of the title compound as an orange oil in 63%
yield (1.75 g, 4.7 mmol). 1H NMR (300 MHz, CDCl3): δ 7.51 (t, 2H),
7.21 (d, 2H), 7.00 (d, 2H), 3.81 (s, 4H), 3.20 (m, 2H), 2.68 (t, 2H),
2.55 (s, 6H), 1.44 (s, 9H). ESI-MS: Expected m/z for C21H30N4O2 =
370.5, found m/z = 371.5.
EXPERIMENTAL SECTION
■
General Methods. All manipulations were performed using
Schlenk line techniques or under an N2 atmosphere in a glovebox.
Reagents and solvents were purchased from commercial vendors, were
of the highest available purity, and were used without further
purification unless otherwise noted. MeOH (Na), MeCN (CaH2),
and CH2Cl2 (CaH2) were dried and distilled prior to use. Et2O was
rigorously degassed and purified using solvent purification columns
housed in a custom stainless steel cabinet and dispensed by a stainless
steel Schlenk-line (GlassContour). 1H NMR spectra were recorded on
either a Bruker AV 301 or Bruker AV 300 FT NMR spectrometer at
ambient temperature and were referenced to residual deuterated
solvent. Chemical shifts are listed in parts per million (ppm). UV/vis
spectra were recorded on a Varian Cary 50 spectrophotometer
equipped with a fiber optic cable connected to a “dip” ATR probe (C-
technologies). A custom-built two-neck solution sample holder
equipped with a threaded glass connector was sized specifically to fit
the “dip” probe. Electrospray-ionization mass spectra were obtained on
a Bruker Esquire Liquid Chromatograph-Ion Trap mass spectrometer.
Electron paramagnetic resonance (EPR) spectra were recorded on a
Bruker E580 CW-EPR spectrometer operating at X-band frequency
between 4 and 7 K with an Oxford helium cryostat. EPR spectra were
collected with the following spectrometer parameters: frequency =
9.396 GHz, power = 2.008 mW, attenuation = 20 dB, sweep width =
6,000 G, gain = 1 × 103, conversion time = 5.4 ms, time constant = 5.4
ms. Magnetic moments (solid state) were obtained with polycrystal-
line samples in gel-caps from 5 to 300 K by zero-field cooling
experiments using a Quantum Design MPMS S5 SQUID magneto-
meter. Pascal’s constants were used to correct for diamagnetic
contributions to the experimental magnetic moment. Solution
magnetic moments were calculated by Evans method, with temper-
ature correction made in the manner described by Van Geet.42,43
Synthesis of N,N-Bis(6-methyl-2-pyridilmethyl)ethane-1,2-
diamine (6-Me-DPEN, L1). 6-MeDPENBoc (5.50 g, 14.8 mmol)
was dissolved in CH2Cl2 (10 mL) at room temperature and added to a
25 mL pear-shaped flask charged with a stirbar. Slow addition of
trifluoroacetic acid (17.00 g, 149.1 mmol) to the stirring solution
resulted in a brown mixture that was allowed to continue stirring
overnight. Evaporation of all volatiles resulted in a viscous brown oil
that was mixed with 5 M NaOH (20 mL) and extracted with CH2Cl2
(3 × 20 mL). The combined organics were washed with brine (3 × 50
mL) and dried over Na2SO4. Removal of all volatiles in vacuo afforded
the title compound as a pale orange oil in 89% yield (3.57 g, 13.2
1
mmol). H NMR (300 MHz, CDCl3): δ 7.54 (t, 2H), 7.33 (d, 2H),
7.00 (d, 2H), 3.81 (s, 4H), 2.77 (t, 2H), 2.64 (t, 2H), 2.53 (s, 6H).
ESI-MS: Expected m/z for C16H22N4 = 270.2, found m/z = 271.3.
Synthesis of N-[tert-Butyloxycarbonyl]-N′-N′-[bis(6-methyl-
2-pyridilmethyl)]propane-1,3-diamine] (6-Me-DPPNBoc). Fol-
lowing a similar procedure described for the preparation of 6-
MeDPEN(Boc), NPNBoc (1.30 g, 7.5 mmol) was reacted with 2-
(chloromethyl)-6-methylpyridine hydrochloride (2.40 g, 13.5 mmol)
to afford a crude product that was chromatographed on silica gel using
a 94:6 acetone/MeOH eluent mixture. The title compound was
Cyclic voltammograms were recorded in MeCN (100 mM Bun N-
4
(PF6) supporting electrolyte) on a PAR 263A potentiostat utilizing a
glassy carbon working electrode, platinum auxiliary electrode, and a
Ag+/AgNO3 reference electrode. X-ray crystallography data was
recorded on either a Bruker APEX II single crystal X-ray diffractometer
with Mo-radiation or a Bruker SMART Apex CCD diffractometer with
Mo Kα radiation. Elemental analyses were performed by Galbraith
Atlantic Microlabs, Norcross, GA. 3-Methyl-3-mercapto-2-butanone,
1-(tert-butyloxycarbonyl)ethyldiamine (NNBoc), N,N-[bis(2-
quinolinemethyl)]ethane-1,3-diamine (2-QuinoEN, L2), and
[MnII(SMe2N4(2-QuinoEN))](PF6)·Et2O (2) were prepared as
previously described.13
Synthesis of 1-(tert-Butyloxycarbonyl)propyldiamine
(NPNBoc). To a stirred solution of 1,3-diaminopropane (3.10 g,
41.9 mmol) in CH2Cl2 (25 mL), a 100 mL solution of di-tert-butyl
dicarbonate (1.52 g, 6.9 mmol) was added via an addition funnel at
room temperature over 3 h. The resulting solution was allowed to
continue to stir for a total of 24 h. White insolubles were filtered, and
the solution was washed with saturated Na2CO3 (2 × 100 mL), brine
(2 × 100 mL), and finally dried over Na2SO4. The solvent was then
removed under reduced pressure, and the title compound was afforded
as a viscous clear oil in 63% (0.76 g, 4.3 mmol). 1H NMR (300 MHz,
CDCl3): δ 3.21 (q, 2H), 2.75 (t, 2H), 1.60 (t, 2H), 1.43 (s, 9H). ESI-
MS: Expected m/z for C8H18N2O2 = 174.241, found m/z = 175.3.
Synthesis of 2-(Chloromethyl)-6-methylpyridine Hydro-
chloride. A solution of 6-methyl-2-(hydroxymethyl)pyridine hydro-
chloride (5.00 g, 40.6 mmol) in CH2Cl2 (10 mL) was cooled in an ice
water bath to 0 °C under an inert atmosphere. Thionyl chloride (24.15
1
obtained as an orange oil in 66% yield (1.89 g, 4.9 mmol). H NMR
(300 MHz, CDCl3): δ 7.52 (t, 2H), 7.30 (d, 2H), 7.00 (d, 2H), 3.74
(s, 4H), 3.13 (m, 2H), 2.58 (t, 2H), 2.53 (s, 6H), 1.67 (q, 2H), 1.41 (s,
9H). ESI-MS: Expected m/z for C22H32N4O2 = 384.5, found m/z =
385.6.
Synthesis of N,N-Bis(6-methyl-2-pyridilmethyl)propane-1,3-
diamine (6-Me-DPPN, L3). Following a similar procedure used for
the preparation of 6-MeDPEN, 6-MeDPPNBoc (5.20 g, 13.5 mmol)
was reacted with trifluoroacetic acid (17.00 g, 149.1 mmol) to afford
the title compound as a pale orange oil in 88% yield (3.38 g, 11.9
1
mmol). H NMR (300 MHz, CDCl3): δ 7.54 (t, 2H), 7.35 (d, 2H),
7.00 (d, 2H), 3.77 (s, 4H), 2.71 (t, 2H), 2.59 (t, 2H), 2.52 (s, 6H),
1.69 (q, 2H). ESI-MS: Expected m/z for C17H24N4 = 284.2, found m/z
= 285.3.
Synthesis of N-(tert-Butyloxycarbonyl)-N′-N′-[bis(2-
quinolinemethyl)]propane-1,3-diamine (2-QuinoPNBoc).
Using the procedure outlined in the preparation of 6-Me-DPENBoc,
NPNBoc (1.20 g, 6.9 mmol) was reacted with 2-(chloromethyl)-
quinoline hydrochloride (2.70 g, 12.7 mmol) resulting in a crude oil
that was chromatographed on silica gel using a 92:8 acetone/MeOH
eluent mixture. The title compound was obtained as a dark orange
solid in 64% yield (2.19 g, 4.8 mmol). 1H NMR (300 MHz, CDCl3): δ
8.09 (d, 4H), 7.76 (d, 2H), 7.66 (m, 4H), 7.48 (t, 2H), 5.79 (bs, 1H),
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Article
3.98 (s, 4H), 3.18 (m, 2H), 2.70 (t, 2H), 1.74 (m, 2H), 1.37 (s, 9H).
ESI-MS: Expected m/z for C28H32N4O2 = 456.6, found m/z = 457.6.
Synthesis of N,N-[Bis(2-quinolinemethyl)]propane-1,3-dia-
mine (2-QuinoPN, L4). Following the procedure outlined in the
preparation of 6-MeDPEN, 2-QuinoPNBoc (2.10 g, 4.6 mmol) was
reacted with trifluoroacetic acid (17.00 g, 149.1 mmol) to afford the
title compound as a pale orange solid in 91% yield (1.63 g, 4.6 mmol).
1H NMR (300 MHz, CDCl3): δ 8.14 (d, 2H), 8.06 (d, 2H), 7.78 (d,
(2 mL) and added to the solution, which was then permitted to
continue stirring at room temperature for 2 days. At the end of this
period, all volatiles were removed in vacuo to afford a dark yellow
solid. The solid was triturated with Et2O (20 mL) via stirring at room
temperature for 30 min, followed by recovery of the solid material by
filtration. The solid material was then redissolved in MeCN (10 mL)
and stirred at room temperature for 1 h. The solution was filtered, and
the resulting MeCN solution was concentrated to a minimal volume
(3−5 mL). Et2O (7−8 mL) was carefully layered on top of the MeCN
solution, and the two layers were allowed to diffuse together overnight
at −30 °C. Filtration of the mixture resulted in 3 as a yellow crystalline
solid in 64% yield (0.99 g, 1.3 mmol). Electronic absorption spectrum:
λmax (nm) (ε (M−1 cm−1)): (MeCN): 282(1655); (MeOH):
279(770). IR(Nujol): νCN 1651 cm−1. EPR spectrum (MeOH/
EtOH glass): g = 1.99. Redox potential (MeCN, 298 K): E1/2 = 520
mV; Epa = 580 mV, Epc = 460 mV vs SCE. Magnetic moment (solid
state, 5−300 K): 5.57 μB; (solution, MeOH, 298 K): 5.78 μB. ESI-MS:
Expected m/z for [C22H31N4SMn]+ = 438.5, found m/z = 438.2.
Elemental Analysis for C48H54BN5SMn ; Calculated: C, 72.17; H, 6.81;
N, 8.77. Found: C, 72.25; H, 6.41; N, 8.67.
2H), 7.71 (d, 2H), 7.68 (m, 2H), 4.01 (s, 4H), 2.70 (m, 4H), 1.72 (q,
2H). ESI-MS: Expected m/z for C23H24N4 = 356.5, found m/z =
357.5.
Synthesis of N-(tert-Butyloxycarbonyl)-N′,N′-[bis(2-
pyridilmethyl)ethane-1,2-diamine] (6-H-DPENBoc). Using the
procedure outlined in the preparation of 6-Me-DPENBoc, NNBoc
(1.20 g, 7.5 mmol) was reacted with 2-picolyl chloride hydrochloride
(2.70 g, 12.7 mmol) resulting in a crude oil that was chromatographed
on silica gel using a 95:5 acetone/MeOH eluent mixture. The title
compound was obtained as a dark orange solid in 65% yield (1.67 g,
1
4.9 mmol). H NMR (300 MHz, CDCl3): δ 8.54 (d, 2H), 7.63 (t,
2H), 7.41 (d, 2H), 7.14 (t, 2H), 5.79 (bs, 1H), 3.86 (s, 4H), 3.21 (m,
2H), 2.70 (t, 2H), 1.44 (s, 9H). ESI-MS: Expected m/z for
C19H26N4O2 = 342.4, found m/z = 343.1.
Synthesis of [MnII(SMe2N4(2-QuinoPN))](PF6)·MeCN·Et2O (4).
A procedure similar to that employed in the preparation of 3 was
conducted with the following reagent amounts; sodium methoxide
(0.11 g, 2.0 mmol), 3-mercapto-3-methyl-2-butanone (0.24 g, 2.0
mmol), L4 (0.73 g, 2.0 mmol), manganese sulfate monohydrate (0.34
g, 2.0 mmol), and sodium hexafluorophosphate (0.34 g, 2.0 mmol).
Complex 4 was isolated as a dark orange crystalline solid in 71% yield
(0.95 g, 1.5 mmol). Electronic absorption spectrum: λmax (nm) (ε
(M−1 cm−1)) (MeCN): 320(2570); (MeOH): 321(1070). IR (Nujol):
νCN 1603 cm−1. EPR spectrum (MeOH,EtOH glass): g = 1.98.
Redox potential (MeCN vs SCE): Epa= 525 mV. Magnetic moment
(solid state, 5−300 K): 5.91 μB; (solution, MeOH, 298 K): 5.98 μB.
ESI-MS: Expected m/z for [C28H31N4SMn]+ = 510.6, found m/z =
510.3. Elemental Analysis for C28H31N4F6PSMn; Calculated: C, 51.30;
H, 4.77; N, 8.55. Found: C, 51.46; H, 5.00; N, 8.64.
Synthesis of N,N-Bis(2-pyridilmethyl)ethane-1,2-diamine (6-
H-DPEN, L5). Following the procedure outlined in the preparation of
6-MeDPEN, 6-H-DPENBoc (5.06 g, 14.8 mmol) was reacted with
trifluoroacetic acid (16.80 g, 149 mmol) to afford the title compound
as a pale orange solid in 85% yield (3.04 g, 12.6 mmol). 1H NMR (300
MHz, CDCl3): δ 8.45 (d, 2H), 7.56 (t, 2H), 7.40 (d, 2H), 7.06 (t,
2H), 2.71 (t, 2H), 2.58 (t, 2H). ESI-MS: Expected m/z for C14H18N4
= 242.3, found m/z = 242.2.
Synthesis of [MnII(SMe2N4(6-Me-DPEN))](BF4) (1). Sodium
methoxide (0.210 g, 3.9 mmol), 3-methyl-3-mercapto-2-butanone
(0.460 g, 3.9 mmol), and L1 (1.02 g, 3.8 mmol) were individually
dissolved in MeOH (2 mL) and cooled for 30 min at −30 °C in a
drybox. The sodium methoxide and 3-mercapto-3-methyl-2-butanone
solutions were then combined, and the resulting mixture was allowed
to stir at room temperature for 30 min. This solution was then slowly
added to a Schlenk flask charged with manganese sulfate monohydrate
(0.630 g, 3.7 mmol) and a stirbar. The methanolic solution of L1 was
then slowly added, producing a yellow mixture that was allowed to stir
at room temperature overnight. Sodium tetrafluoroborate (0.41 g, 3.7
mmol) was dissolved in MeOH (2 mL) and added to the solution,
which was then permitted to continue stirring at room temperature for
2 days. Removal of all volatiles in vacuo afforded a yellow solid that
was redissolved in MeCN (10 mL), filtered through a fine frit, and
concentrated to minimal volume (2−3 mL). The resulting solution
was carefully layered with Et2O (5 mL), and the mixture was allowed
to cool at −30 °C overnight. Filtration of the mixture resulted in 1 as a
pale yellow crystalline solid in 53% yield (1.02 g, 2.0 mmol). Electronic
absorption spectrum: λmax (nm) (ε (M−1 cm−1)) (MeCN): 317(350);
(EtCN): 317(305); (MeOH): 297(477); (CH2Cl2): 327(350). IR
(Nujol): νCN 1605 cm−1. EPR spectrum (9:1 MeOH/EtOH glass): g
= 2.00. Redox potential (MeCN vs SCE, 298 K): E1/2(MnIII/II) = 410
mV. Magnetic moment (solid state, 5−300 K): 5.83 μB; (solution,
MeOH, 301 K): 5.89 μB. ESI-MS: Expected m/z for [C21H29N4SMn]+
= 424.4, found m/z = 424.3. Elemental Analysis for C21H29BN4F4SMn;
Calculated: C, 49.33; H, 5.72; N, 10.96. Found: C, 48.36; H, 5.58; N,
10.72.
Synthesis of [MnII(SMe2N4(6-H-DPEN)(MeOH)](BPh4) (5). A
procedure similar to that employed in the preparation of 1 was
conducted with the following reagent amounts: sodium methoxide
(0.10 g, 1.9 mmol), 3-mercapto-3-methyl-2-butanone (0.22 g, 1.9
mmol), L5 (0.43 g, 1.8 mmol), manganese sulfate monohydrate (0.30
g, 1.8 mmol), and sodium tetraphenylborate (0.61 g, 1.8 mmol). 5 was
isolated as a white solid in 21% yield (0.28 g, 0.37 mmol). Electronic
absorption spectrum (MeCN): λmax (nm) (ε (M−1 cm−1)):
283(1690). IR (Nujol): νCN 1600 cm−1. EPR spectrum (MeOH,E-
tOH glass): g = 1.99. Redox potential (MeCN vs SCE, 298 K): Epa
=
413 mV, Epc = 0 mV. Magnetic moment (solid state, 5−300 K): 5.88
μB; (solution, MeOH, 298 K): 5.79 μB. ESI-MS: Expected m/z for
[C19H25N4SMn]+ = 395.9, found m/z = 396.5.
Synthesis of [MnIII(SMe2N4(6-Me-DPEN)]2-(μ-O)(BF4)2·2MeOH
(6). 1 (0.500 g, 0.48 mmol) was dissolved in MeCN (5 mL) and
allowed to stir in air for 30 min, during which time the solution turned
from yellow to dark purple. The purple solution was then added
dropwise into pentane (10 mL), causing the immediate precipitation
of a purple solid. The solid material was isolated via filtration,
redissolved in MeCN (2 mL), and carefully layered with Et2O (10
mL). The mixture was allowed to diffuse together at room temperature
overnight, resulting in dark purple crystals suitable for X-ray diffraction
studies in 97% yield (0.49 g, 0.47 mmol). Electronic absorption
spectrum (MeCN): λmax (nm) (ε (M−1 cm−1)): 325(3690), 557(520).
IR (Nujol): νCN 1602 cm−1. Redox potential (MeCN vs SCE, 298
K): E1/2= +465 mV. ESI-MS: Expected m/z for [C42H58N8OS2Mn2]2+
Synthesis of [MnII(SMe2N4(6-Me-DPPN))](BPh4)·MeCN (3).
Sodium methoxide (0.11 g, 2.0 mmol), 3-mercapto-3-methyl-2-
butanone (0.24 g, 2.0 mmol), and L3 (0.58 g, 2.0 mmol) were
individually dissolved in MeOH (4 mL) and cooled for 30 min at −30
°C in a drybox. The sodium methoxide and 3-mercapto-3-methyl-2-
butanone solutions were then combined, and the resulting mixture was
allowed to stir at room temperature for another 30 min. This solution
was then added dropwise to a Schlenk flask charged with manganese
sulfate monohydrate (0.35 g, 2.0 mmol) and a stirbar. The methanolic
solution of L3 was then added dropwise, producing a dark yellow
mixture that was allowed to stir at room temperature overnight.
Sodium tetraphenylborate (0.70 g, 2.0 mmol) was dissolved in MeOH
=
432.5, found m/z = 432.6. Elemental Analysis for
C44H66B2N8O3F8S2Mn2; Calculated: C, 47.93; H, 6.03; N, 10.16.
Found: C, 46.18; H, 5.62; N, 10.34.
Synthesis of [MnIII(SMe2N4(2-QuinoEN)]2-(μ-O)(PF6)2·Et2O (7).
A procedure similar to that employed in the preparation of 7 was
conducted with the following reagent amounts; 2 (0.50 g, 0.78 mmol).
7 was obtained as a dark purple solid in 98% yield (0.50 g, 0.38 mmol).
Electronic absorption spectrum: λmax (nm) (ε (M−1 cm−1)) (MeCN):
345(3450), 562(550); (MeOH): 332(2000), 606(240). IR (Nujol):
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νCN 1603 cm−1. Redox potential (MeCN vs SCE, 298 K): Epa= +780
mV (irrev). ESI-MS: Expected m/z for [C54H58N8OS2Mn2]2+ = 504.5,
found m/z = 504.4. Elemental Analysis for C54H58N8OF12P2S2Mn2
Calculated: C, 49.93; H, 4.50; N, 8.63. Found: C, 50.93; H, 4.63; N,
8.77.
The crystal-to-detector distance was set to 40 mm, and the exposure
time was 20 s per degree for all sets of exposure. The scan width was
0.5°. Data collection was 99.7% complete to 25.0° in ϑ. A total of
61,040 merged reflections were collected covering the indices h = −18
to 17, k = −23 to 20, l = −21 to 22. A total of 8,766 reflections were
symmetry independent, and the Rint = 0.0432 indicated that the data
was good (0.07). Indexing and unit cell refinement indicated a
primitive monoclinic lattice with the space group P21/n (No. 14). The
complex and counterion are accompanied by a 2:1 mixture of MeCN
and Et2O that occupy the same space. The associated disorder
required a few stabilizing restraints to ensure convergence of the
thermal parameters for the three solvents molecules.
A yellow crystal of 5 was cut down to dimensions 0.6 × 0.6 × 0.6
mm3 and mounted on a glass capillary with oil. Data was collected at
−143 °C. The crystal-to-detector distance was set to 30 mm, and the
exposure time was 15 s per degree for all sets of exposure. The scan
width was 2.0°. Data collection was 98.5% complete to 25.0° in ϑ. A
total of 48881 merged reflections were collected covering the indices h
= −13 to 13, k = −19 to 19, l = −19 to 19. A total of 7339 reflections
were symmetry independent, and the Rint = 0.039 indicated that the
data was better than average quality (0.07). Indexing and unit cell
Synthesis of [MnIII(SMe2N4(6-Me-DPPN))]2(μ-O)(BPh4)2 (8). An
8 mL EtCN solution of 3 (0.200 g, 0.26 mmol) was prepared under an
inert atmosphere in a drybox. The resulting yellow solution was
removed from the drybox, cooled to −80 °C, and exposed to air
overnight. During this period the solution was found to turn from
yellow to dark brown. Cold Et2O (−80 °C, 10 mL) was carefully
layered on top of the EtCN solution, and the resulting mixture was
maintained at −80 °C for a few weeks to permit the formation of small
crystals suitable for X-ray diffraction studies. Electronic absorption
spectrum (MeCN): λmax (nm) (ε (M−1 cm−1)): 374 (750), 594 (190).
ESI-MS: Expected m/z for [C44H62N8OS2Mn2]2+ = 446.5, found m/z
= 446.3.
Synthesis of [MnIII(SMe2N4(2-QuinoPN))]2(μ-O)(BPh4)2 (9). A
procedure similar to that described for the synthesis of 8 was
employed with a 4 mL EtCN solution of the BPh4− salt of 4 (0.400 g,
0.482 mmol). Small gray crystals of 9 were obtained in extremely low
yields from crystallization attempts. Electronic absorption spectrum
(MeCN): λmax (nm): 388, 573 (br). ESI-MS: Expected m/z for
[C56H62N8OS2Mn2]2+ = 518.6, found m/z = 518.3.
refinement indicated a triclinic P-lattice with the space group P1 (No.
̅
2).
A black block of 6 with dimensions 0.30 × 0.20 × 0.15 mm3 was
mounted on a glass capillary with oil. Data was collected at −173 °C.
The crystal-to-detector distance was set to 40 mm, and the exposure
time was 10 s per degree for all sets of exposure. The scan width was
0.5°. Data collection was 98.7% complete to 25.0° in ϑ. A total of
21,006 merged reflections were collected covering the indices h = −16
to 13, k = −23 to 24, l = −9 to 14. A total of 5,905 reflections were
symmetry independent, and the Rint = 0.041 indicated that the data
was excellent (0.07). Indexing and unit cell refinement indicated a
monoclinic P lattice with the space group P21/c (No. 14). One
methanol molecule was found with half-site occupancy in the
structure.
Synthesis of [MnIII(SMe2N4(6-H-DPEN)]2(μ-O)(PF6)2·(MeCN)2
(10). A procedure similar to that employed in the preparation of 6
was conducted with [MnII(SMe2N4(6-H-DPEN)(MeOH)](PF6) (0.20
g, 0.35 mmol). 10 was obtained as a light purple solid in 95% yield
(0.19 g, 0.17 mmol). Electronic absorption spectrum: λmax (nm) (ε
(M−1 cm−1)) (MeCN): 327 (4463), 332 (4470), 533 (286). IR
(Nujol): νCN 1600 cm−1. Magnetic moment (solid state, 300 K):
4.89 μB/Mn; (solution, MeOH, 298 K): 4.01 μB/Mn. Redox potential
(MeCN vs SCE, 298 K): E1/2= +295 mV. ESI-MS: Expected m/z for
[C38H50N8OS2Mn2]2+ = 404.3, found m/z = 404.3. Elemental Analysis
for C38H50N8OF12P2S2Mn2 Calculated: C, 41.54; H, 4.59; N, 10.20.
Found: C, 41.31; H, 4.68; N, 10.36.
Alternative Synthetic Route to 6, 7, and 10 Using
Iodosylbenzene. Addition of 1.1 equiv of solid iodosylbenzene to
a saturated MeCN solution (2−4 mL typical volume) of either 1, 2, or
5 under an inert atmosphere resulted in the immediate formation of 6,
7, or 10, respectively. The remaining solids were removed by filtration,
and the resulting MeCN solution was added dropwise into pentane
(10 mL) to precipitate the desired product. Isolation of the
precipitated solid material via filtration resulted in yields very close
to those using O2.
X-ray Crystallographic Structure Determination. A red prism
of 1 with dimensions 0.48 × 0.29 × 0.24 mm3 was mounted on a glass
capillary with oil. Data was collected at −143 °C. The crystal-to-
detector distance was set to 40 mm, and the exposure time was 60 s
per degree for three total sets of exposure. The scan width was set to
1.0°. Data collection was 90.7% complete to 29.53° in ϑ and 99.6%
complete to 25°. A total of 32,825 partial and complete reflections
were collected covering the indices h = −13 to 13, k = −19 to 19, l =
−22 to 22. 5,980 reflections were symmetry independent and the Rint
= 0.038 indicated that the data was above average quality (0.07).
Indexing and unit cell refinement indicated an orthorhombic lattice
with the space group P212121 (No. 19).
A crystal of 7 with dimensions 0.30 × 0.07 × 0.05 mm3 was
mounted on a glass capillary with oil. Data was collected at −173 °C.
The crystal-to-detector distance was set to 49.4 mm, and the exposure
time was 20 s per degree for all sets of exposure. Data collection was
99.0% complete to 27.50° in ϑ. A total of 29,448 reflections were
collected covering the indices h = −26 to 26, k = −29 to 28, l = −17 to
16. A total of 6,872 reflections were symmetry independent with Rint
=
0.0553. Indexing and unit cell refinement indicated a monoclinic lattice
with the space group C2/c.
A colorless plate of 8 with dimensions 0.30 × 0.20 × 0.15 mm3 was
mounted on a glass capillary with oil. Data was collected at −173 °C.
The crystal-to-detector distance was set to 40 mm, and the exposure
time was 10 s per degree for all sets of exposure. The scan width was
0.5°. Data collection was 99.7% complete to 28.36° in ϑ. A total of
123,964 merged reflections were collected covering the indices h =
−38 to 38, k = −15 to 15, l = −33 to 34. A total of 10,165 reflections
were symmetry independent, and the Rint = 0.0732 indicated that the
data was acceptable (average quality 0.07). Indexing and unit cell
refinement indicated a monoclinic C lattice with the space group C2/c
(No. 15).
A crystal of 9 cut to dimensions 0.10 × 0.10 × 0.05 mm3 was
mounted on a glass capillary with oil. Data was collected at −173 °C.
The crystal-to-detector distance was set to 40 mm, and the exposure
time was 60 s per degree for all sets of exposure. The scan width was
1.0°. Data collection was 49.5% complete to 20° in ϑ. A total of 69,414
reflections were collected covering the indices h = −25 to 25, k = −49
to 48, l = −14 to 14. A total of 9,478 reflections were symmetry
independent with Rint = 0.6592. Indexing and unit cell refinement
indicated a monoclinic P lattice with the space group P21/c (No.14).
A plate of 10 was cut to dimensions 0.59 × 0.40 × 0.40 mm3 was
mounted on a glass capillary with oil. Data was collected at −173 °C.
The crystal-to-detector distance was set to 30 mm, and the exposure
time was 45 s per degree for all sets of exposure. The scan width was
1.0°. Data collection was 99.3% complete to 25° in ϑ. A total of 78,553
reflections were collected covering the indices h = −21 to 21, k = −26
A pale-yellow prism of 3 with dimensions 0.20 × 0.17 × 0.10 mm3
was mounted on a glass capillary with oil. Data was collected at −163
°C. The crystal-to-detector distance was set to 40 mm, and the
exposure time was 20 s per degree for all sets of exposure. The scan
width was 0.5°. Data collection was 98.7% complete to 25.0° in ϑ. A
total of 114,668 merged reflections were collected covering the indices
h = −17 to 17, k = −20 to 20, l = −22 to 22. A total of 16,232
reflections were symmetry independent, and the Rint = 0.0343
indicated that the data was good (0.07). Indexing and unit cell
refinement indicated a triclinic P lattice with the space group P1 (No.
̅
2).
A brown prism of 4 with dimensions 0.18 × 0.15 × 0.10 mm3 was
mounted on a glass capillary with oil. Data was collected at −163 °C.
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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for [MnII(SMe2N4(6-Me-DPEN)](PF6) (1), [MnII(SMe2N4(2-
QuinoEN))](PF6) (2), [MnII(SMe2N4(6-Me-DPPN)](BPh4) (3), [MnII(SMe2N4(2-QuinoPN))](PF6) (4), [MnII(SMe2N4(6-H-
DPEN)(MeOH)](BPh4)·MeOH (5), [MnIII(SMe2N4(6-Me-DPEN)]2(μ-O)(BF4)2 (6), [MnIII(SMe2N4(2-QuinoEN)]2(μ-O)(PF6)2
(7), [MnIII(SMe2N4(6-Me-DPPN))]2(μ-O)(BPh4)2 (8), and [MnIII(SMe2N4(6-H-DPEN)]2(μ-O)(PF6)2·2MeCN (10)
1
2
3
4
5
6
7
8
10
Mn−S(1)
Mn−N(1)
Mn−N(2)
Mn−N(3)
2.3710(6)
2.186(2)
2.297(2)
2.222(2)
2.239(2)
N/A
2.3835(9)
2.170(2)
2.274(2)
2.225(3)
2.200(3)
N/A
2.3742(3)
2.1909(10)
2.2476(10)
2.1830(10)
2.2787(10)
N/A
2.3626 (5)
2.2106 (15)
2.2424 (16)
2.1886 (16)
2.2689 (14)
N/A
2.4597(6)
2.2215(16)
2.3595(16)
2.2790(18)
2.2463(17)
2.2767(7)
1.999(3)
2.151(2)
2.581(2)
2.501(2)
1.7602(4)
1.7602(4)
3.520
2.292(1)
2.010(3)
2.130(3)
2.543(3)
2.370(3)
1.7599(6)
1.7599 (6)
3.512
2.2547(16)
2.0660(16)
2.2008(16)
2.3524(16)
2.796
2.2968(15)
2.011(4)
2.190(4)
2.243(5)
2.223(5)
1.771(4)
1.750(4)
3.515
Mn−N(4)
a
Mn(1)−O(1)
Mn(1′)−O(1)
Mn(1)···Mn(1′)
S(1)−Mn−N(1)
S(1)−Mn−N(2)
S(1)−Mn−N(3)
S(1)−Mn−N(4)
N(1)−Mn−N(3)
N(1)−Mn−N(4)
N(3)−Mn−N(4)
S(1)−Mn−O(1)
N(1)−Mn−O(1)
Mn−O(1)−Mn
τ-value
2.183(1)
1.7941(6)
1.7941(6)
3.433
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
81.15(5)
156.98(5)
117.36(5)
117.38(5)
121.37(7)
120.58(7)
100.00(7)
N/A
82.20(7)
156.10(7)
106.98(7)
123.09(7)
118.53(9)
119.04(9)
104.16(9)
N/A
82.03(3)
154.06(3)
127.47(3)
93.97(3)
114.15(4)
140.58(4)
99.40(4)
N/A
82.66(4)
155.06(4)
124.91(5)
100.31(4)
104.28(6)
152.29(6)
96.55(5)
N/A
78.77(4)
155.39(4)
108.79(5)
108.23(4)
84.76(6)
104.19(6)
142.93(6)
82.08(8)
164.17(7)
106.19(6)
106.84(6)
86.85(6)
92.68(13)
146.57(6)
95.37(2)
177.21(9)
180.0
82.60 (8)
163.90(9)
101.40(8)
109.23(8)
94.92(11)
84.75(11)
149.03(10)
99.20(3)
177.02(12)
172.3(2)
N/A
82.43(5)
169.44(5)
117.64(5)
N/A
83.4(1)
164.2(1)
101.2(1)
106.6(1)
86.81(6)
84.4(1)
86.81(6)
N/A
N/A
151.7(2)
98.9(1)
a
97.35(4)
89.99(4)
172.35(6)
146.15(11)
N/A
a
N/A
N/A
N/A
N/A
171.07(6)
176.5(2)
173.0(2)
N/A
N/A
N/A
N/A
N/A
N/A
0.47
0.59
0.55
0.22
0.05
N/A
a
In this case, O(1) = MeOH oxygen.
a
to 26, l = −15 to 19. A total of 910 reflections were symmetry
independent with Rint = 0.114. Indexing and unit cell refinement
indicated a orthorhombic P lattice with the space group P21212
(No.18).
Scheme 1
A red plate of 11 cut to dimensions 0.50 × 0.30 × 0.10 mm3 was
mounted on a glass capillary with oil. Data was collected at −143 °C.
The crystal-to-detector distance was set to 30 mm, and the exposure
time was 20 s per degree for all sets of exposure. The scan width was
2.0°. Data collection was 98.0% complete to 25° in ϑ. A total of 68,741
reflections were collected covering the indices h = −21 to 21, k = −28
to 27, l = −19 to 18. A total of 10,722 reflections were symmetry
independent with Rint = 0.1380. Indexing and unit cell refinement
indicated a monoclinic C lattice with the space group Cc (No.9).
The data for 1, 5, 10, and 11 were integrated and scaled using hkl-
SCALEPACK. Data for 3, 4, 6, 7, 8, and 9 were integrated and scaled
using SAINT, SADABS within the APEX2 software package by Bruker,
and solutions were made by direct methods (SHELXS, SIR97) to
produce complete heavy atom phasing models. Structures were
completed by difference Fourier synthesis with SHELXL97 or
SHELXTL 6.10 (7). All non-hydrogen atoms were refined anistropi-
cally by full-matrix least-squares methods, while all hydrogen atoms
were placed using a riding model. Crystal data for 1 and 3−10 are
summarized in Table 1 below and metrical parameters for 1−8 and 10
are provided in Table 2. A complete list of the crystal data and metrical
parameters for 11 are provided in the Supporting Information.
RESULTS AND DISCUSSION
■
Syntheses and Structural Characterization of Com-
plexes 1−5. Ligand scaffolds containing various combinations
of N-heterocyclic moieties (pyridine, 6-methyl-pyridine, quino-
line) and alkyl spacers (ethyl, propyl) used in this study were
constructed via the synthetic route shown in Scheme 1.
Maintaining a single primary amine in L1−L5 was a key
prerequisite to forming the desired thiolate-containing ligands,
which are constructed via Mn(II)-templated Schiff-base
condensations between the primary amine and 3-mercapto-3-
methyl-2-butanone. The alkyl thiolate ligated complexes
[MnII(SMe2N4(6-Me-DPEN)](PF6) (1), [MnII(SMe2N4(2-
a
Reaction conditions: (i) 6.0 equiv of SOCl2, CH2Cl2, inert
atmosphere (N2), 0 °C → ambient temperature, 16 h. (ii) 0.17
equiv of Boc2O, CH2Cl2, inert atmosphere (N2), ambient temperature,
24 h; (iii) 1.7−1.9 equiv of 2-(chloromethyl)-6-methylpyridine
hydrochloride (L1 and L3), 2-(chloromethyl)quinoline hydrochloride
(L2 and L4), or 2-(chloromethyl)-pyridine hydrochloride (L5), 5 M
NaOH (aq), room temperature, 3−5 days; (iv) 10−32 equiv of
trifluoroacetic acid, CH2Cl2, 12 h.
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QuinoEN))](PF6) (2), [MnII(SMe2N4(6-Me-DPPN)]-
(BPh4)·MeCN (3), [MnII(SMe2N4(2-QuinoPN))]-
(PF6)·MeCN·Et2O (4), and [MnII(SMe2N4(6-H-DPEN)-
(MeOH)](BPh4)·MeOH (5) were all prepared in a similar
fashion by combining equimolar amounts of MnSO4·H2O,
ligands L1−L5 (respectively), NaOMe, 3-methyl-3-mercapto-2-
butanone, and either NaBF4, NaPF6, or NaBPh4 in MeOH,
under an inert atmosphere. This general method is analogous
to that employed previously by our group for the synthesis of a
variety of divalent thiolate-ligated transition metal complexes
built upon the tris(2-aminoethyl)amine (tren) ligand scaf-
fold.38,39 Selected metrical parameters for complexes 1−5 are
assembled in Table 2. The X-ray structure of 2 was reported
elsewhere.13
As shown in the ORTEP diagrams of Figure 1, monocationic
complexes 1, 3, and 4 are five-coordinate complexes despite
Figure 2. Chemdraw diagrams of [MnII(SMe2N4(6-Me-DPEN))]+ (1),
[MnII(SMe2N4(2-QuinoEN))]+ (2), [MnII(SMe2N4(6-Me-DPPN))]+
(3), [MnII(SMe2N4(2-QuinoPN))]+ (4), and [MnII(SMe2N4(6-H-
DPEN)(MeOH)]+ (5).
Substitution of the pyridine 6-position has been found in a
number of cases to have dramatic effects upon the properties
and reactivity of various small molecule compounds containing
first-row transition metals.45−47 Solvent coordination to 5 is
presumably more facile due to the absence of these bulkier
substituents. The coordination geometries of complexes 1−5
are found to vary from distorted trigonal bipyramidal to square
pyramidal (τ(1) = 0.59, τ(2) = 0.55, τ(3) = 0.22, τ(4) = 0.05,
and τ(5) = 0.47),48 indicating that modification of the alkyl
spacer length from an ethylene (1, 2, and 5) to propylene (3
and 4) unit (Figure 2) has a considerable impact upon the
coordination geometry. This effect has previously been
observed when comparing the structures of five-coordinate
bis-thiolate-ligated [FeI I I (S2 M e 2 N3 (Et,Pr))]+ and
[FeIII(S2Me2N3(Pr,Pr))]+, although the addition of an extra
methylene unit in the latter complex was found to promote a
more trigonal bipyramidal geometry.49
Figure 1. ORTEP diagrams of [MnII(SMe2N4(6-Me-DPEN))]+ (1),
[MnII(SMe2N4(6-Me-DPPN))]+ (3), [MnII(SMe2N4(2-QuinoPN))]+
(4), and [MnII(SMe2N4(6-H-DPEN)(MeOH)]+ (5) with counterions,
solvents of crystallization, and hydrogen atoms omitted.
The Mn−S(1) bond distances in complexes 1−4 range from
2.3710(6) Å to 2.3835(9) Å (Table 2), and are very similar to
the Mn−SR bonds found in aromatic thiolate-ligated
[MnII (SPh)10]50 and [MnII{HB-(3,5-Me2pz)3}(SC6F5)]51
4
being synthesized and crystallized from coordinating solvents
(MeOH or MeCN). These observations are consistent with the
X-ray crystal structure of previously reported five-coordinate
2.13 The primary coordination spheres of each complex contain
a thiolate sulfur (S(1)), imine nitrogen (N(1)), tertiary amine
(N(2)), and two N-heterocyclic moieties (N(3) and N(4)).
Complex 5, on the other hand, was found to contain a
methanol ligand bound in an available coordination site trans to
the imine nitrogen (Figure 2). The presence of a single
counterion per Mn complex in the X-ray crystal structure of 5
supports the assignment of this exogenous ligand as methanol
and not methoxide.
Close inspection of the metrical parameters (Table 2) for
methanol-bound 5 reveals a fairly dramatic elongation of the
Mn·····N(2) separation (2.3595(16) Å) to the point where it is
nearly 0.07 Å longer than the sum of the ionic radii for these
two atoms (2.29 Å).44 Complex 5 is, therefore, also best
described as a five-coordinate complex. This particular
compound is further distinguished in that the pyridine moieties
in this complex are unsubstituted and therefore impose less
steric congestion upon the open coordination site than do the
6-methylpyridine or quinoline moieties of 1−4 (Figure 2).
(Mn−SR = 2.38(1) Å (mean length) and 2.385(3) Å,
respectively), but shorter than those found in alkyl thiolate-
ligated [MnII(edt)2]2− and [MnII(SMe2N4(tren))]+ (Mn−SR =
2.432(7)Å and 2.412(3) Å, respectively).39,50 The π-accepting
N-heterocyclic moieties found in 1−4 should increase the metal
ion Lewis acidity relative to the structurally analogous aliphatic
primary amine-ligated [MnII(SMe2N4(tren))]+, which explains
the contraction of the Mn−S(1) bonds in 1−4 versus
[MnII(SMe2N4(tren))]+. Complex 5 has a longer Mn−S(1)
(2.4597(6) Å) bond relative to 1−4 due presumably to the
coordination of methanol (Table 2). The spin-states of
complexes 1−5 are all identical.
Magnetic, Spectroscopic, and Electrochemical Prop-
erties of Complexes 1−5. Fits to variable temperature solid-
state magnetic susceptibility data show that complexes 1−5
each contain high-spin Mn(II) ions, as illustrated for complex 1
in Figure 3a. Each complex exhibits Curie behavior from 5 to
300 K (Figures 3a, Supporting Information, Figures S-20, S-
25)13 with calculated effective magnetic moments between 5.57
μB and 5.91 μB, which is close to the theoretical spin-only
value (5.92 μB) for a S = 5/2 system. Solution magnetic
moments obtained using the Evans method in MeOH solution
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Figure 3. Inverse molar magnetic susceptibility (χM−1) vs temperature (T) plot (a) for [MnII(SMe2N4(6-MeDPEN))](BF4) (1) showing Curie
behavior consistent with an S = 5/2 ground spin-state. X-band EPR spectrum (b) of [MnII(SMe2N4(6-MeDPEN))](BF4) (1) in MeOH/EtOH (9:1)
glass at 7 K.
were very similar to the solid-state data (see Experimental
Section). As illustrated in Figure 3b for complex 1, low
temperature (4−7 K) X-band EPR spectra of 1−5 (Supporting
Information, Figures S-24, and S-29)13 display similar six-line
signals with g-values of approximately 2.00 and hyperfine
splitting constants ranging from 90 to 100 G, again consistent
with monomeric high-spin (S = 5/2) Mn(II) (IMn = 5/2). The
electronic absorption spectrum of each complex (Supporting
Information, Figures S-16, S-18, S-22, S-27) was found to be
featureless throughout the visible region in a wide range of
solvents (MeCN, EtCN, MeOH, CH2Cl2). This is expected
oxidation state (Supporting Information, Figures S-26 and S-
30). Given that some of these processes are irreversible, it is
difficult to draw any conclusions regarding the N-heterocyclic
ligand’s influence on redox potential. However, it does appear
that the trigonal bipyramidal complexes containing the ethyl
linker are easier to oxidize than the square pyramidal complexes
containing the propyl linker. No other redox processes, such as
a wave attributable to a Mn(IV/III) couple, were observed
within the MeCN solvent window. The relatively modest
potentials associated with the MnII/MnIII redox couple (460−
580 mV vs SCE) suggest that 1−5 could be oxidized by a
variety of biological oxidants, including O2.
́
given that the ligand field transitions are both Laporte and spin
Dioxygen Reactivity of Complexes 1−5. Complexes 1−
5 were found to be highly reactive with O2 in organonitrile
solvents (MeCN, EtCN), each to afford a rare example of an
unsupported oxo-bridged Mn(III) dimer (complexes 6−10).
Selected metrical parameters for these complexes,
[MnIII(SMe2N4(6-Me-DPEN)]2-(μ-O)(BF4)2·2MeOH (6),
[MnIII(SMe2N4(QuinoEN)]2-(μ-O)(PF6)2·Et2O (7),
[MnI I I (SM e 2N4(6-Me-DPPN)]2 -(μ-O)(BPh4)2 (8),
[MnIII(SMe2N4(QuinoPN)]2-(μ-O)(BPh4)2·MeCN (9), and
[MnIII(SMe2N4(6-H-DPEN)]2-(μ-O)(PF6)2·2MeCN (10), are
provided either in Table 2 or Supporting Information, Table S-
34. Dicationic μ-oxo bridged 6, 7 (Figure 5), and 10 (Figure 6)
were found to be extremely robust; each complex exhibits
indefinite stability in both the solid state and solution under
ambient conditions. Complexes 8 (Figure 7) and 9 (Supporting
Information, Figure S-38), on the other hand, were found to be
thermally unstable (τ1/2 for 8 and 9 is ∼15−20 min at −40 °C
in MeCN) and rapidly convert to intractable products upon
warming to ambient temperature. Because of this, the crystal
quality of complex 9 was poor resulting in a less-than desirable
resolution (R = 16%) X-ray structure, which does not lend itself
to a thorough consideration of metrical parameters. This
structure is, however, included in the following discussion
primarily to confirm that O2 addition to 4 results in the
formation of an unsupported oxo-bridged dimer.
forbidden for an S = 5/2 spin system. The absence of intense
S(π)→Mn(d) charge transfer bands in the visible region
implies that these transitions occur at higher energy (UV
region), in agreement with Solomon’s spectral analysis of four-
coordinate [MnII{HB(3,5-iPr2pz}3(SC6F5)].52 Ligand-centered
π→π* transitions involving the imine and N-heterocyclic
moieties likely contribute to the absorption features observed in
the near UV spectral region of each complex (see Experimental
Section).
The electrochemical properties of 1−5 were examined using
cyclic voltammetry in MeCN at ambient temperature.
Complexes 1 (Figure 4) and 3 (Supporting Information,
Figure S-21) display reversible and quasireversible Mn(III/II)
redox couples at E1/2 = 410 mV and 520 mV, respectively, while
complexes 2,13 4, and 5 are irreversibly oxidized to the +3
Only 0.25 equiv of O2 per equivalent of Mn(II) are required
to quantitatively convert 1, 2, and 5 to their corresponding μ-
oxo dimers 6, 7, and 10, at room temperature on a high-vacuum
line (with a calibrated known-volume bulb). Each dimer was
isolated in nearly quantitative yields (92−98%), suggesting that
metal-centered oxidation is the predominant O2 reaction
pathway, as opposed to sulfur oxidation, which frequently
occurs with thiolate-containing transition metal com-
Figure 4. Cyclic voltammogram of [MnII(SMe2N4(6-MeDPEN))]-
n
(BF4) (1) in MeCN vs SCE with 0.1 M Bu4 PF6 supporting electrolyte
and scan rate of 150 mV·s−1.
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Figure 5. ORTEP diagrams of {[MnIII(SMe2N4(6-Me-DPEN)]2-(μ-O)}2+ (6) and {[MnIII(SMe2N4(QuinoEN)]2-(μ-O)}2+ (7) with counterions,
solvents of crystallization, and hydrogen atoms omitted.
Figure 6. ORTEP diagram of {[MnIII(SMe2N4(6-H-DPEN)]2-(μ-O)}2+
(10) with counterions, solvents of crystallization, and hydrogen atoms
omitted.
Figure 7. ORTEP diagram of {[MnIII(SMe2N4(6-Me-DPPN)]2-(μ-
O)}2+ (10) with counterions, solvents of crystallization, and hydrogen
atoms omitted.
plexes.53−56 The instability of dimers 8 (Figure 7) and 9
(Supporting Information, Figure S-38) precluded a quantitative
determination of the amount of O2 necessary to form these
species. Iodosylbenzene (PhIO, 0.5 equiv) addition to 1, 2, and
5 also affords dimers 6, 7, and 10, respectively, in high yields at
ambient temperatures, suggesting that a high-valent Mn(IV)
O intermediate forms along the O2 reaction pathway.
Oxo-bridged dimer formation is readily monitored by
electronic absorption spectroscopy, where, as shown in Figure
8 for thiolate-ligated 1, O2 addition results in the appearance of
a broad, low-intensity band in the visible region, and a color
change from pale yellow to a more intense purple. Electrospray
mass spectral data shows that the bridging oxo atom in
complexes 6, 7, and 10 is derived from 18O2 (Supporting
Information, Figures S-31−S-34). This, coupled with the fact
that 6−10 were also shown to form via PhIO addition to 1−5,
implies that the mechanism of μ-oxo dimer formation involves
O−O bond cleavage, via a metastable high-valent metal oxo
intermediate. Metastable intermediates are detected in the low
temperature (−40 °C) reaction between 1 and 2 and O2 by
electronic absorption spectroscopy and stopped-flow. The
properties of these intermediates and the kinetics of their
formation are currently being investigated, and will be discussed
in a future manuscript. In contrast to 1 and 2, intermediates are
not observed in the low temperature reaction between
complexes 3−5 and O2, even at temperatures as low as −100
°C.
Structural and Magnetic Properties of Oxo-Bridged 6,
7, 8, and 10. Although there are numerous examples of μ-
carboxylate-μ-oxo and bis-oxo Mn(III) dimers,59−67 there are
significantly fewer examples containing a single, unsupported
oxo bridge.57,58 The few reported examples display nearly linear
Mn−O−Mn bridging angles, Mn···Mn separations of 3.4−3.6
Å, and six-coordinate Mn ions.57,58 None of these examples
include thiolate ligands. Solid-state magnetic susceptibility (χM)
vs temperature data for 6, 7, and 10 were fit using julX70 to the
̂
̂
̂
exchange Hamiltonian H = −2J(S1·S2) (where S1 = S2 = 2 and g
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containing 1−3 to Mn(III)-containing 6−8 (Table 2). An
increase in coordination number could account for some of the
observed elongation, but typically this effect is not as
pronounced as the metal ion radius decrease caused by
oxidation. More likely an electronic stabilization gained upon
distortion (akin to the Jahn−Teller effect, which strictly only
applies to systems possessing Oh symmetry) provides a driving
force for the elongation of trans M····Npy,quin bonds. The Mn−
N(3) and Mn−N(4) distances for 6 and 7 are each significantly
longer than the sum of both the Shannon covalent and ionic
radii for a Mn(III)-N bond (2.105 Å),44 suggesting that these
two dimers are most accurately described as containing four-
coordinate Mn(III) ions. The metrical parameters of 6 and 7
are overall relatively similar to one another and suggest that
replacement of the 6-methylpyridine moieties in 6 with
quinoline moieties in 7 only introduces minor structural
perturbations. Sterics also appear to play a role in the trans
M····Npy,quin bond elongation, given that the less sterically
congested, unsubstituted (R= H) pyridine derivative 10 (Figure
6) does not have elongated Mn−Npy bonds (Table 2). This is
further supported by comparing the bond lengths of 10 with
those of [MnIII(SMe2N4(4-MeO-3,5-Me2-DPEN)]2-(μ-O)-
(PF6)2·2MeCN (11).69 Complex 11 (Figure 9) contains
Figure 8. Electronic absorption spectra of [MnII(SMe2N4(6-Me-
DPEN))](BF4) (1) (orange) and [MnIII(SMe2N4(6-Me-DPEN)]2-(μ-
O)(BF4)2·2MeOH (6) (blue) in MeCN at room temperature.
∼ 2) over the temperature range 5−300 K to afford J-values of
−125.6 cm−1 (6), −1.0 cm−1 (7), and −48.0 cm−1 (10), which
fall within the range seen with the few reported examples
unsupported μ-oxo bridged Mn(III) dimers.58,59 There does
not appear to be any obvious correlation between the
magnitude of this antiferromagnetic coupling constant and
any particular metrical parameter for the series of dimers
reported herein.
The Mn−S(1) bond lengths decrease by ∼0.1 Å (Table 2)
upon conversion of 1−5 to 6−10, consistent with an increase
in oxidation state from Mn(II) to Mn(III). The Mn(III)-SR
bond lengths of 6−10 are comparable to those of five-
coordinate Mn(III)-S(thiosalicylate) (2.2913(24) and
2.2752(25) Å),68 but are notably shorter than those of
dinuclear [Mn2(edt)4]2− (Mn(III)-RSb = 2.632(2), Mn(III)-
RSt= 2.32 Å; RSb = bridging, RSt = terminal).50 The oxo-
bridging ligand of complexes 6−10 is trans to an imine nitrogen
(Figures 5−7 and Supporting Information, Figure S-38), and
the Mn−O−Mn bond angles vary from 180.0° in 6 to
146.15(11)° in 8. The latter is, to our knowledge, the most
acute bridging angle for an unsupported μ-oxo Mn(III) dimer,
and is likely a consequence of steric congestion. Given that a
more acute bridging angle would preclude the formation of
strong, stabilizing π bonds between the oxo ligand and the Mn
ions, the highly bent Mn−O−Mn angle in 8 is likely
responsible for its instability. Complex 8 is also distinguished
from the other dimers in terms of the relative orientation of the
two halves of the dimer. In 6, 7, 9, and 10 each half is rotated
∼180° relative to one another, as shown by the relative
positioning of the sulfurs (Figures 5−6), and the dihedral angle
formed between the Mn(1)−S(1)−N(1)−N(2)−O(1) and
Mn(1′)−S(1′)−N(1′)−N(2′)−O(1) planes (0°, 16.0°, 18.5°,
and 7.1° for 6, 7, 9, and 10, respectively; Supporting
Information, Figures S-43−S-46). In complex 8, on the other
hand, the Mn(1)−S(1)−N(1)−N(2)−O(1) and Mn(1′)−
S(1′)−N(1′)−N(2′)−O(1) planes are nearly orthogonal 73.7°
(Figure 7).
Figure 9. ORTEP diagram of {[MnIII(SMe2N4(4-MeO-3,5-Me2-
DPEN)]2-(μ-O)}2+ (11) with counterions, solvents of crystallization,
and hydrogen atoms omitted. Selected distances (Å) and angles
(degrees): Mn−O(1)ave, 1.75(1); Mn−S(1)ave, 2.28(2); Mn−N(1)ave
,
2.01(4); Mn−N(2)ave, 2.18(2); Mn−N(3,4)ave, 2.23(2); Mn−O(1)−
Mn, 175.4(7).
substituents in the 3-, 4-, and 5-positions of the pyridine
rings, but not in the 6-position, and the Mn−N(3) (2.219 Å)
and Mn−N(4) (2.248 Å) bond distances are significantly
shorter than those in the more sterically encumbered 6 (Figure
5; R(6-position) = Me) or 7 (Figure 5; R(6-position) = fused
aryl), but comparable to those in less sterically encumbered 10
(R(6-position) = H). This demonstrates that the coordination
geometry and metrics are particularly sensitive to bulky
substituents in the 6-position of the pyridine ring. Noticeable
differences in redox potentials provide additional evidence to
support a higher coordination number in the less sterically
congested Mn(III) dimer 10 (R = H) in the solution state,
relative to the more bulky Mn(III) dimers 6 and 7. The redox
potential of 10 (+295 mV vs SCE) is significantly lower than all
Another structural feature that perhaps contributes to the
instability of dimeric 8 is the extrusion of one of the 6-
methylpyridine moieties, in each half of the dimer, to a distance
of 2.796 Å from the manganese center (Table 2, Figure 7). In
contrast to what one would expect, all of the other Mn−N
distances elongate (by nearly 0.5 Å) upon oxidation of Mn(II)-
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Summary and Conclusions. We have synthesized and
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Mn(II) complexes with N-heterocylic-ligand scaffolds, and
showed that each complex contains high-spin (S = 5/2) Mn(II)
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■
S
* Supporting Information
NMR and mass spec data for ligands L1−L5. Magnetic,
electrochemical, and electronic absorption data for complexes
1, 3−10. Crystallographic tables for 1, 3−10. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
(29) Katona, G.; Carpentier, P.; Niviere, V.; Amara, P.; Adam, V.;
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1427.
■
Corresponding Author
*Phone: (206)543-0713. Fax: (206)685-8665. E-mail: kovacs@
chem.washington.edu.
Notes
The authors declare no competing financial interest.
†UW Staff Crystallographers.
‡University of Nevada, Reno.
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ACKNOWLEDGMENTS
NIH funding (#RO1GM45881-19) is gratefully acknowledged.
■
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