MnIV-Oxo complex of a bis(benzimidazolyl)-containing N5 ligand reveals different reactivity trends for MnIV-oxo than FeIV-oxo species

Article abstract of DOI:10.1039/c9dt00308h

Using the pentadentate ligand (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, 2pyN2B), presenting two pyridyl and two (N-methyl)benzimidazolyl donor moieties in addition to a central tertiary amine, new Mn^II and Mn^IV-oxo complexes were generated and characterized. The [Mn^IV(O)(2pyN2B)]²⁺ complex showed spectroscopic signatures (i.e., electronic absorption band maxima and intensities, EPR signals, and Mn K-edge X-ray absorption edge and near-edge data) similar to those observed for other Mn^IV-oxo complexes with neutral, pentadentate N₅ supporting ligands. The near-IR electronic absorption band maximum of [Mn^IV(O)(2pyN2B)]²⁺, as well as DFT-computed metric parameters, are consistent with the equatorial (N-methyl)benzimidazolyl ligands being stronger donors to the Mn^IV center than the pyridyl and quinolinyl ligands found in analogous Mn^IV-oxo complexes. The hydrogen- and oxygen-atom transfer reactivities of [Mn^IV(O)(2pyN2B)]²⁺ were assessed through reactions with hydrocarbons and thioanisole, respectively. When compared with related Mn^IV-oxo adducts, [Mn^IV(O)(2pyN2B)]²⁺ showed muted reactivity in hydrogen-atom transfer reactions with hydrocarbons. This result stands in contrast to observations for the analogous Fe^IV-oxo complexes, where [Fe^IV(O)(2pyN2B)]²⁺ was found to be one of the more reactive members of its class.

Full text of DOI:10.1039/c9dt00308h

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MnIV-Oxo complex of a bis(benzimidazolyl)-
containing N5 ligand reveals different reactivity
Cite this: DOI: 10.1039/c9dt00308h
trends for MnIV-oxo than FeIV-oxo species†
Melissa C. Denler,‡a Allyssa A. Massie,‡a Reena Singh,b Eleanor Stewart-Jones,a
a
Arup Sinha,b Victor W. Day,a Ebbe Nordlander b and Timothy A. Jackson
*
Using the pentadentate ligand (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine,
2pyN2B), presenting two pyridyl and two (N-methyl)benzimidazolyl donor moieties in addition to a
central tertiary amine, new MnII and MnIV-oxo complexes were generated and characterized. The
[MnIV(O)(2pyN2B)]2+ complex showed spectroscopic signatures (i.e., electronic absorption band maxima
and intensities, EPR signals, and Mn K-edge X-ray absorption edge and near-edge data) similar to those
observed for other MnIV-oxo complexes with neutral, pentadentate N5 supporting ligands. The near-IR
electronic absorption band maximum of [MnIV(O)(2pyN2B)]2+, as well as DFT-computed metric para-
meters, are consistent with the equatorial (N-methyl)benzimidazolyl ligands being stronger donors to the
MnIV center than the pyridyl and quinolinyl ligands found in analogous MnIV-oxo complexes. The hydro-
gen- and oxygen-atom transfer reactivities of [MnIV(O)(2pyN2B)]2+ were assessed through reactions with
hydrocarbons and thioanisole, respectively. When compared with related MnIV-oxo adducts, [MnIV(O)
(2pyN2B)]2+ showed muted reactivity in hydrogen-atom transfer reactions with hydrocarbons. This result
stands in contrast to observations for the analogous FeIV-oxo complexes, where [FeIV(O)(2pyN2B)]2+ was
found to be one of the more reactive members of its class.
Received 22nd January 2019,
Accepted 15th March 2019
DOI: 10.1039/c9dt00308h
rsc.li/dalton
field strength (Fig. 1).6 The N4py derivatives previously considered
were DMMN4py and 2pyN2Q (Fig. 1), which respectively feature
Introduction
The use of earth-abundant metal catalysts to effect hydro- electron-rich di-3,5-methyl-4-methoxy-pyridyl groups and bulky
carbon oxidation has grown significantly in recent years. These quinolinyl groups that serve to increase (for DMMN4py) and
efforts have led to new non-porphyrinoid Fe- and Mn-based decrease (for 2pyN2Q) the equatorial ligand field of the
catalysts for C–H bond functionalization.1–5 Catalytic mecha- Mn center relative to the parent N4py ligand. A DFT structure of
nisms for these systems typically feature substrate oxidation by [MnIV(O)(2pyN2Q)]2+ showed that the steric bulk of the quinolinyl
hydrogen- or oxygen-atom transfer steps (HAT and OAT, groups also crowds the oxo coordination position, causing a
respectively) initiated by high-valent metal-oxo species. Our
Naxial–MnvO angle of 170°, with the oxo ligand tilted away from
understanding of these catalysts is aided by structure–reactivity the quinolinyl moieties (Fig. 1). DFT structures for the corres-
relationships obtained through investigations of stoichio- ponding [MnIV(O)(N4py)]2+ and [MnIV(O)(DMMN4py)]2+ complexes
metric reactions of isolable metal-oxo species. For example, showed Naxial–MnvO angles of almost exactly 180°. The [MnIV(O)
we recently reported that MnIV-oxo adducts supported by (2pyN2Q)]2+ complex showed reaction rates with a variety of hydro-
derivatives of the pentadentate N4py ligand display HAT and carbons that were ca. 100-fold faster than those of [MnIV(O)
OAT reaction rates that are dependent on the equatorial ligand
(
DMMN4py)]2+ (Fig. 1). A larger, 4000-fold rate enhancement was
observed for thioanisole oxidation by [MnIV(O)(2pyN2Q)]2+ com-
pared to [MnIV(O)(DMMN4py)]2+.
More recently, Que and co-workers reported similar rate
enhancements for the analogous FeIV-oxo series,7 where
[FeIV(O)(2pyN2Q)]2+ reacts with hydrocarbons ca. 20–50-fold
faster than [FeIV(O)(DMMN4py)]2+.8 An X-ray crystal structure of
[FeIV(O)(2pyN2Q)]2+ showed a Naxial–FevO angle near 170°,
consistent with our DFT predictions from the [MnIV(O)
(2pyN2Q)]2+ complex. This angle contrasts with the analogous
aThe University of Kansas, Department of Chemistry and Center for Environmentally
Beneficial Catalysis, 1567 Irving Hill Road, Lawrence, KS 66045, USA.
E-mail: taj@ku.edu; Tel: +1 (785) 864-3968
bLund University, Chemical Physics, Department of Chemistry, Box 124,
SE-221 00 Lund, Sweden
†Electronic supplementary information (ESI) available. CCDC 1874429. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt00308h
N
axial–FevO angle found in the X-ray structure of [FeIV(O)
‡These authors made contributions of equal merit.
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Fig. 1 MnIV-Oxo complexes and ligands based on the N4py framework. Ligand abbreviations are as follows: N4py is N,N-bis(2-pyridylmethyl)-N-bis
(2-pyridyl)methylamine; DMMN4py is N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2Q is N,N-bis(2-
pyridyl)-N,N-bis(2-quinolylmethyl)methanamine; and 2pyN2B is N,N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)-amine.
(2pyN2B)]2+, for which the value is 177° (2pyN2B is an N4py 9,10-dihydroanthracene (DHA) and xanthene were recrystal-
derivative with two pyridyl and two N-(methyl)benzimidazolyl lized from ethanol.10 All other chemicals were used as
functions (Fig. 1)).7 The FeIV-oxo rate enhancements were also received, with the exception of MeCN and diethylether, which
correlated with the FeIV ligand-field strength, as a linear relation- were sparged with argon and dried as described previously.11
ship was observed between the HAT energy barrier (from log(k2)) MnII(OTf)2·2CH3CN, used for preparation of metal complexes,
and the average Fe–N distance from the X-ray crystal structures of was synthesized according to a previously reported pro-
the FeIV-oxo complexes. The [FeIV(O)(2pyN2B)]2+ complex showed cedure.12 For O2-free reactions, solvents were dried following
HAT and OAT reaction rates approximately 1–2 orders of published procedures and degased by freeze–pump–thaw
magnitude faster than the parent [FeIV(O)(N4py)]2+ but 2–10 methods.10 Iodosobenzene (PhIO) was prepared from iodoso-
times slower than that of [FeIV(O)(2pyN2Q)]2+.7,9
benzene diacetate following a published procedure.13
To determine the influence of N-(methyl)benzimidazolyl
Electronic absorption spectra for formation and kinetic
ligation on a MnIV-oxo unit, we describe here the MnIV-oxo reactions were obtained on a Varian Cary 50 Bio or an Agilent
adduct supported by the 2pyN2B ligand, ([MnIV(O)(2pyN2B)]2+; 8453 spectrophotometer. Both spectrophotometers were inter-
see Fig. 1). To assess the influence of the 2pyN2B ligand on faced with Unisoku cryostats (USP-203-A), capable of maintain-
MnIV-oxo electronic structure and reactivity, we performed ing temperatures of 25 °C for all reactions. Electrospray-ioniza-
thorough spectroscopic investigations of [MnIV(O)(2pyN2B)]2+, tion mass spectrometry experiments were performed using an
including characterization by electronic absorption, EPR, and LCT Primers Micromass electrospray time-of-flight instrument.
Mn K-edge X-ray absorption near-edge spectroscopies. The NMR data were collected on a Bruker AVIIIHD 400 MHz NMR
reactivity of [MnIV(O)(2pyN2B)]2+ was explored using thio- instrument. EPR data were collected between 5 and 10 K on a
anisole and 9,10-dihydroanthracene (DHA) as model sub- 9 GHz Bruker EMXPlus spectrometer with an Oxford ESR900
strates for OAT and HAT reactions, respectively. A comparison continuous-flow liquid helium cryostat controlled by an
of the rates of [MnIV(O)(2pyN2B)]2+ with those of related MnIV- Oxford ITC503 temperature system. Perpendicular-mode data
oxo and FeIV-oxo complexes reveals a distinctly different were collected in a dual-mode Bruker ER4116DM cavity.
relationship between ligand structure and reactivity for the Recording conditions were 9.637 GHz microwave frequency,
MnIV-oxo adducts as compared to their FeIV-oxo analogues.
2.0 mW microwave power, 6 G modulation amplitude, 100 kHz
modulation frequency, and 141 ms time constant.
Experimental methods
Synthesis and characterization
Materials and instrumentation
2pyN2B was synthesized by a previously published method.9
All chemicals and solvents were purchased from commercial The [MnII(OH2)(2pyN2B)](OTf)2 complex (C31H29F6MnN7O7S2)
vendors and were ACS reagent-grade quality or better. was prepared by reacting equimolar amounts of the 2pyN2B
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free ligand and MnII(OTf)2·2CH3CN in CH3CN under an argon methyl groups in the ether molecule were placed with ideal-
atmosphere and stirring the resultant solution for 30 minutes ized “staggered” orientations and the rigid rotor methyl group
to an hour. Solid [MnII(OH2)(2pyN2B)](OTf)2 was obtained by in the metal complex utilized C–H bond lengths of 0.97 Å.
slow diffusion of diethyl ether into the [MnII(OH2)(2pyN2B)] Crystal data and structure refinement details are given in
(OTf)2 acetonitrile solution under argon. Repeated recrystalli- Table S1.†
zations yielded the [MnII(OH2)(2pyN2B)](OTf)2 salt in 52%
Preparation of XAS sample
yield. ESI-MS data for [MnII(OH2)(2pyN2B)](OTf)2 dissolved in
MeCN shows a single peak at 563.13 corresponding to A 15 mM sample of [MnII(OH2)(2pyN2B)](OTf)2 was prepared
[MnII(2pyN2B)(Cl)]+, in which the triflate ion was replaced by a by dissolving 9.3 mg (0.012 mmol) [MnII(OH2)(2pyN2B)](OTf)2
chloride in the mass spectrometer (Fig. S2†). The effective in 0.8 mL of ethanol, transferring the solution to an XAS
magnetic moment (µeff) of [MnII(OH2)(2pyN2B)](OTf)2 was sample holder, and flash freezing the solution in liquid nitro-
found to be 5.91µB, which is in good agreement with the gen. A 10 mM XAS sample of [MnIV(O)(2pyN2B)]2+ was pre-
expected µeff of 5.92µB for high-spin MnII centers. The perpen- pared by dissolving 13.3 mg (0.016 mmol) [MnII(OH2)
dicular-mode EPR spectrum of a 1 mM solution of [MnII(OH2) (2pyN2B)](OTf)2 in 1.6 mL of a TFE solution containing
(2pyN2B)](OTf)2 was collected at 10 K (Fig. S2†). The inter- 35.2 mg PhIO (0.16 mmol). The reaction mixture was placed
mediate [MnIV(O)(2pyN2B)]2+ (C29H27MnN7O) was prepared into a 0.2 cm cuvette, where the formation of [MnIV(O)
in situ by reacting 1.7 mg (0.002 mmol) [MnII(OH2)(2pyN2B)] (2pyN2B)]2+ was monitored by electronic absorption spec-
(OTf)2 with 4.4 mg (0.02 mmol) PhIO in 2.0 mL 2,2,2-trifluoro- troscopy. At maximum formation, two XAS samples were pre-
ethanol (TFE). An ESI-MS sample of [MnIV(O)(2pyN2B)]2+ was pared and flash frozen in liquid nitrogen.
prepared by diluting 20 µL of the solution in pre-cooled MeCN
XAS data collection
(−40 °C). ESI-MS data for isotopically-labeled [MnIV(O)
(2pyN2B)]2+ was obtained by incubating [MnIV(O)(2pyN2B)]2+ Mn K-edge XAS data for [MnIV(O)(2pyN2B)]2+ were collected at
with 15 µL H218O for 5 minutes to allow for water exchange. beamline 9-3 at the Stanford Synchrotron Radiation Light
Because the stability of [MnIV(O)(2pyN2B)]2+ is greatly reduced Source (SSRL). XAS spectra for [MnIV(O)(2pyN2B)]2+ were
in MeCN, the dominant peaks in the mass spectra come from obtained as fluorescence excitation spectra using a Canberra
MnII decay products (Fig. S3†).
100-element Ge array detector. Data for [MnII(OH2)(2pyN2B)]
(OTf)2 was collected at beamline 2-2 at SSRL using a Si(111)
monochromator and a 13-element Ge array fluorescence detec-
X-ray crystallography of [MnII(OH2)(2pyN2B)](OTf)2
Crystals suitable for characterization by X-ray diffraction were tor. Mn K-edge X-ray absorption spectra were collected over an
obtained by vapor diffusion of diethyl ether into an aceto- energy range of 6.3 to 7.4 keV (Si(220) monochromator). The
nitrile solution of [MnII(OH2)(2pyN2B)](OTf)2. A full hemi- frozen samples were maintained at 7 K during data collection
sphere of diffracted intensities (740 15-second frames with a by an Oxford liquid He cryostat. A reference spectrum of a
scan width of 1.00°) was measured for a single-domain speci- manganese foil was collected for each scan and an internal
men using graphite-monochromated Mo Kα radiation (λ = calibration was performed by setting the zero crossing of the
0.71073 Å) on a Bruker SMART APEX CCD Single Crystal second derivative of the K-edge energy of the reference spectra
Diffraction System.14 The integrated data15 were corrected to 6539.0 keV. The high flux at beamline 9-3 lead to photo-
empirically for variable absorption effects using equivalent reduction for [MnIV(O)(2pyN2B)]2+. When multiple scans were
reflections. The Bruker software package SHELXTL was used to collected on a single spot of the frozen sample for [MnIV(O)
solve the structure using “direct methods” techniques. All (2pyN2B)]2+, there was a noticeable shift of the edge to a lower
stages of the weighted full-matrix least-squares refinement energy. To reduce the effects of photoreduction, the beam was
2
were conducted using Fo data with the SHELXTL XL v2014 moved to different spots on the sample and only one scan was
software package.16 The final structural model incorporated collected per spot. Three scans were collected for [MnIV(O)
anisotropic thermal parameters for all nonhydrogen atoms (2pyN2B)]2+. Compared to conditions at beamline 9-3, beam-
and isotropic thermal parameters for all hydrogen atoms. All line 2-2 has reduced X-ray flux. These conditions required the
hydrogen atoms for the metal complex were located with a use of ethanol as the solvent rather than TFE, as the latter
difference Fourier and initially included in the structural solvent leads to diminished X-ray fluorescence. The relatively
model as independent isotropic atoms whose parameters were low flux and smaller number of detectors at this beamline
allowed to vary in least-squares refinement cycles. The methyl required the signal averaging of eighteen scans for [MnII(OH2)
group was eventually included as an idealized sp3-hydridized (2pyN2B)](OTf)2.
rigid rotor that was allowed to rotate freely about its C–C bond
in refinement cycles. Hydrogen atoms for the ether molecule
XAS data analysis
of crystallization were placed at idealized riding model posi- XAS data analysis was performed using the Demeter software
tions with C–H bond lengths of 0.97–0.98 Å and isotropic package.17 Individual scans of raw data were analyzed and
thermal parameters that were 1.2 (nonmethyl) or 1.5 (methyl) combined using Athena. Fits of the pre-edge area was per-
times the equivalent isotropic thermal parameter of the formed using the FityK software.18 Pre-edge intensity was nor-
carbon atom to which they were covalently bonded. The malized relative to the tail of the fluorescence signal.
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EPR sample preparation
Cyclic voltammetry experiments
A 1 mM sample of [MnII(OH2)(2pyN2B)](OTf)2 was prepared A glassy carbon working electrode, a platinum auxiliary elec-
and flash frozen using liquid nitrogen in a 4 mm quartz EPR trode, and a Ag/AgCl quasi-reference electrode were used with
tube. A 10 mM sample of [MnIV(O)(2pyN2B)]2+ was prepared by an external ferrocene/ferrocenium potential as an external
thawing the XAS sample at −20 °C and then immediately reference. Cyclic voltammograms were recorded on an Epsilon
transferring 200 μL of this solution into a pre-cooled EPR tube potentiostat. These experiments used 16.5 mg (0.02 mmol)
and flash freezing the tube in liquid nitrogen.
[MnII(OH2)(2pyN2B)](OTf)2, with 22.0 mg PhIO (0.1 mmol) in
TFE with 0.1 M Bu4NPF6 electrolyte at 298 K. Fewer equivalents
of PhIO were used in electrochemical experiments due to
limited solubility of PhIO in TFE.
Electronic structure computations
All computations were performed using the ORCA 3.0.3 soft-
ware package.19 DFT geometry optimizations of [MnIV(O)
(2pyN2B)]2+ and [MnIV(O)(N4py)]2+ employed the TPSS func-
tional,20 TZVP (Mn, O, and N) and SVP (C, F, and H) basis
sets,21–23 and the COSMO solvation model (for TFE).24 Tight
optimization and SCF criteria were invoked using the TightOpt
and TightSCF keywords. The RI approximation, with TZVP/J
and SVP/J auxiliary basis sets, was used for geometry optimi-
zations. This level of theory replicates that employed for pre-
viously reported geometries of [MnIV(O)(2pyN2Q)]2+ and
[MnIV(O)(DMMN4py)]2+,6 allowing for a balanced structural
comparison of these complexes. Structures were converged to
the S = 3/2 spin states. Cartesian coordinates for [MnIV(O)
(2pyN2B)]2+ and [MnIV(O)(N4py)]2+ are included in Tables S4
and S5.†
Results and discussion
X-ray diffraction structure of [MnII(OH2)(2pyN2B)](OTf)2
The crystal structure of [MnII(OH2)(2pyN2B)](OTf)2 shows a
mononuclear MnII center in a distorted octahedral environ-
ment with an axial water ligand and the 2pyN2B ligand bound
in its expected pentadentate mode (Fig. 2). The axial Mn–OH2
distance of 2.087(3) Å is comparable to that observed for
[MnII(OH2)(2pyN2Q)](OTf)2 (2.091(2) Å).6 However, the water
ligand in [MnII(OH2)(2pyN2B)](OTf)2 is not as distorted from
the idealized octahedral position as in [MnII(OH2)(2pyN2Q)]
(OTf)2 (Naxial–Mn–OH2 angles of 177.9° and 163.8°, respect-
ively). This difference in bond angles reflects the greater steric
influence of the quinolinyl moieties in 2pyN2Q on the axial
coordination position. In support, the closest contacts for the
benzimidazolyl carbon atoms and the oxygen atom of the
water ligand of [MnII(OH2)(2pyN2B)]2+ are 3.807 Å. These sep-
arations lead to corresponding CH⋯OH2 interaction distances
of 3.093 Å. In contrast, the previously reported XRD structure
of [MnII(OH2)(2pyN2Q)](OTf)2 shows Cquinolinyl⋯OH2 separ-
ations of 3.453 and 3.515 Å, which leads to much shorter
CH⋯OH2 interaction distances of 2.529 and 2.616 Å. The
greater steric influence of the 2pyN2Q ligand as compared to
2pyN2B was also observed in the recently reported XRD struc-
tures for the FeIV-oxo complexes (vide supra).7 As discussed by
Que and coworkers, the steric influence exerted by the quinoli-
nyl substituents leads to long metal–Nquinoline distances and,
consequently, weak quinolinyl donation to the metal.2,7
HAT and OAT reactivity studies
In a typical reaction, when 1.0 mM of [MnIV(O)(2pyN2B)]2+ was
fully formed, an aliquot of a solution containing substrate
(thioanisole in TFE and DHA or xanthene in CH2Cl2) was
added to the cuvette for a final solution volume of 2.1 mL. The
decay of the 940 nm feature of [MnIV(O)(2pyN2B)]2+ was moni-
tored, and the resulting time trace was fit to a pseudo-first
order model. The observed rate constants were plotted against
the substrate concentration to obtain a second-order rate con-
stant. For DHA, 10–40 equivalents of substrate were added to
determine the second-order rate constant for HAT reactivity.
The reaction of [MnIV(O)(2pyN2B)]2+ with deuterated [D4]-DHA
was used to determine a primary kinetic isotope effect. For
xanthene, 20–50 equivalents of substrate were added to deter-
mine the second-order rate constant for HAT reactivity. 10 to
60 equivalents of thioanisole were used to determine the
second-order rate constant for the OAT reaction.
The two Mn–Nbenzimidazolyl distances in [MnII(OH2)
(2pyN2B)](OTf)2 are 2.193(2) Å, which are ca. 0.06–0.08 Å
shorter than the corresponding Mn–Npyridyl distances
of
Mn–N3,5-dimethyl-4-methoxypyridyl bond lengths of [MnII(OTf)
DMMN4py)]+ (Table 1).6,26 These short Mn–Nbenzimidazolyl dis-
[MnII(OTf)(N4py)]+,
but
similar
to
the
Product analysis experiments
Analysis of the product of the reaction between [MnIV(O)
(
(2pyN2B)]2+ and DHA was performed by removing the solvent tances are accordant with the greater basicity of benzimidazole
under vacuum and then re-dissolving the isolated residue in than pyridine.27 The Mn–Npyridyl distances of [MnII(OH2)
cyclohexane. The product anthracene was observed and quan- (2pyN2B)](OTf)2 (2.298(2) Å) are comparable to those observed
tified using the published extinction coefficient for anthracene for the other MnII complexes (Table 1). Overall, the average
in cyclohexane.25 Analysis of the product of the reaction of Mn–Nequatorial distance for [MnII(OH2)(2pyN2B)](OTf)2 is
[MnIV(O)(2pyN2B)]2+ and thioanisole was performed by slightly less than that of [MnII(OTf)(DMMN4py)]2+ (2.246 and
1H-NMR experiments following previously described 2.251 Å, respectively), making the 2pyN2B ligand the strongest
methods.6 The only modification was the use of toluene as equatorial donor of this series. The largest Mn–ligand bond
internal standard, which was added directly to the 1H-NMR deviation for [MnII(OH2)(2pyN2B)](OTf)2 is in the Mn–Namine
sample before analysis.
bond distance (2.375(3) Å), which is nearly 0.09 Å longer than
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Fig. 2 Molecular structures of [MnII(OH2)(2pyN2B)]2+ and [MnII(OH2)(2pyN2Q)]2+ cations. Thermal ellipsoids are shown at 50% probability and most
hydrogen atoms and triflate counter anions have been removed for clarity.
Table 1 Manganese-ligand bond distances (Å) for MnII complexes supported by the 2pyN2B, N4py, DMMN4py, and 2pyN2Q ligands
[MnII(OH2)(2pyN2B)]2+
[MnII(OTf)(N4py)]+ a
[MnII(OTf)(DMMN4py)]+ b
[MnII(OH2)(2pyN2Q)]2+ b
Mn–O
2.087(3)
2.193(2)
2.193(2)
2.298(2)
2.298(2)
2.375(3)
2.246
2.125(3)
2.255(4)
2.271(4)
2.271(4)
2.278(4)
2.289(4)
2.269
2.114(3)
2.189(4)
2.225(3)
2.296(4)
2.293(3)
2.278(4)
2.251
2.091(2)
2.250(2)
2.265(2)
2.297(2)
2.303(2)
2.279(2)
2.279
c
Mn–NX c
Mn–NX
Mn–Npyridyl
Mn–Npyridyl
Mn–Namine
Mn–Nequatorial
d
e
Mn–Ntotal
2.271
2.273
2.256
2.279
a From ref. 26. b From ref. 6. c NX is Nbenzimidazolyl for [MnII(OH2)(2pyN2B)]2+, Npyridyl for [MnII(OTf)(N4py)]+, N3,5-dimethyl-4-methoxypyridyl for
[MnII(OTf)(DMMN4py)]+, and Nquinolinyl for [MnII(OH2)(2pyN2Q)]2+ d Average of the Mn–N bond distances in the equatorial positions. e Average of
.
all Mn–N bond distances.
the corresponding distances for the other MnII structures tallography experiments, and it is therefore expected that the
(Table 1). The FeII–Namine distance in [FeII(2pyN2B)(NCMe)] Fe–Nequatorial distances should be less sensitive to perturbations
(ClO4)2 showed a similar elongation of 0.067 Å relative to in the equatorial ligand field than the high-spin (S = 5/2) MnII
[FeII(N4py)(NCMe)](ClO4)2.9 This variation in bond length was complexes, in which the metal 3dx2−y2 MO, which is
attributed to different angular requirements of the five-mem- metal–Nequatorial σ-antibonding, will be populated.8,9,28
bered benzimidazolyl and six-membered pyridyl rings.9 The
Formation and characterization of [MnIV(O)(2pyN2B)]2+
same rationale can be applied to account for the elongated
Mn–Namine bond of [MnII(OH2)(2pyN2B)](OTf)2. When this The addition of PhIO to a TFE solution of [MnII(OH2)(2pyN2B)]
elongated Mn–Namine distance is considered, the average (OTf)2 at 25 °C led to the formation of new electronic absorp-
Mn–N distances of [MnII(OH2)(2pyN2B)](OTf)2 (Mn–Ntotal in tion bands at 940, 460, and 410 nm (Fig. 3). These bands
Table 1) are just slightly less than those of [MnII(OTf)(N4py)]+ reached maximum intensity when 10 equivalents of PhIO were
and [MnII(OH2)(2pyN2Q)]2+, and longer than that of [MnII(OTf) used. The energy and intensity of the near-IR absorption band
(
DMMN4py)]+ (Table 1). Collectively, these metric data show at 940 nm (ε = 250 M−1 cm−1, assuming 100% formation) is
that 2pyN2B provides an equatorial ligand field comparable in quite similar to that observed for MnIV-oxo complexes of the
strength to that of DMMN4py and stronger than that of N4py N4py, DMMN4py, and 2pyN2Q ligands (λmax = 920–1180 nm; ε =
and 2pyN2Q but with markedly attenuated axial donation.
179–290 M−1 cm−1; Table 2).6,26 On the basis of this similarity,
The trend in Mn–Nequatorial bond length observed for the and strongly supported by additional data described below, we
manganese(II) complexes with the 2pyN2B, N4py, and assign the chromophore formed by the reaction of [MnII(OH2)
DMMN4py ligands is more dramatic than that observed for the (2pyN2B)](OTf)2 with PhIO to [MnIV(O)(2pyN2B)]2+. In previous
corresponding iron(II) complexes. Specifically, [FeII(NCMe) investigations of [MnIV(O)(N4py)]2+ and its derivatives, the
(2pyN2B)](ClO4)2, [FeII(NCMe)(N4py)](ClO4)2, and [FeII(NCMe) near-IR absorption bands of these complexes were attributed
(
DMMN4py)](OTf)2 showed average Fe–Nequatorial distances within to the 4B1 → 4E transition, which corresponds to a one-electron
the narrow range of 1.971 Å to 1.978 Å. This 0.007 Å length span excitation from the quasi-degenerate MnIV 3dxz and 3dyz MOs
is much smaller than the 0.038 Å variation seen for the corres- to the unoccupied 3dx2−y2 MO.6,29 These MOs are MnvO
ponding MnII complexes (Table 1). The FeII complexes are all π-antibonding and Mn–Nequatorial σ-antibonding, respectively.
low spin (S = 0) at the temperatures employed for the X-ray crys- Previous DFT computations for [MnIV(O)(N4py)]2+ and its
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Fig. 3 Top: Formation of [MnIV(O)(2pyN2B)]2+ from PhIO oxidation of [MnII(OH2)(2pyN2B)]2+. Bottom right: Electronic absorption spectra showing
the final spectra of [MnIV(O)(2pyN2B)]2+ following the addition of 1 to 10 equivalents PhIO to [MnII(OH2)(2pyN2B)](OTf)2 (1.0 mM in TFE at 25 °C).
The inset shows the intensity of the near-IR absorption band as a function of equivalents PhIO. Bottom left: Perpendicular-mode X-band EPR
spectra of [MnIV(O)(2pyN2B)]2+ at 10 K.
Table 2 Electronic absorption near-IR band maxima (nm), extinction coefficients (M−1 cm−1), t1/2 values in TFE at 25 °C (hours), EPR g-values,
hyperfine couplings (mT), XAS edge and pre-edge energies (eV), XAS pre-edge areas, and CV peak potentials (V) for MnIV-oxo complexes
ab
Edge
Pre-edge
Area
Ep,c
a
c
Complex
λmax
ε
t1/2
geff
[MnIV(O)(DMMN4py)]2+
[MnIV(O)(2pyN2B)]2+
[MnIV(O)(N4py)]2+
920
940
950
1180
290
250
230
179
6
4.38
5.70
4.35
4.30
7.7
7.0
7.6
7.2
6550.3
6549.6
6550.8
6549.2
6541.3
6541.4
6541.6
6541.2
19.6
14.6
18.9
17.0
0.61
0.47
0.80
1.01
2.7
0.5
0.2
[MnIV(O)(2pyN2Q)]2+
a The geff value of the most intense, positive signal at low field (see Fig. S6). b Hyperfine splitting of six-line signals observed near g = 5.7–5.8 (see
Fig. S6). c Cathodic peak potential versus SCE. Data for [MnIV(O)(N4py)]2+ comes from ref. 26 and data for [MnIV(O)(DMMN4py)]2+ and [MnIV(O)
(2pyN2Q)]2+ comes from ref. 6.
derivatives showed that variations in the energy of this elec- (2pyN2B)]2+ show an ion peak at m/z of 272.08 (Fig. S3†),
tronic transition arise predominantly from perturbations in which agrees with that predicted for the MnIV-oxo adduct (m/z
Mn–Nequatorial bonding.6,29 As shown in Table 2, the near-IR of 272.08). When [MnIV(O)(2pyN2B)]2+ is incubated with
absorption maxima of [MnIV(O)(2pyN2B)]2+ (940 nm; H218O, the ESI-MS shows a new peak at 273.08, which indi-
10 640 cm−1) lies between those of [MnIV(O)(DMMN4py)]2+ cates water exchange to give the MnIV–18O adduct (Fig. S3†). At
(920 nm; 10 870 cm−1
)
and [MnIV(O)(N4py)]2+ (950 nm; 25 °C in TFE, a 1.0 mM solution of [MnIV(O)(2pyN2B)]2+
10 530 cm−1). This position is consistent with the benzimida- showed a half-life of ∼2.7 hours (Fig. S4†), which falls between
zolyl moieties being better σ-donors than an unmodified pyri- those of [MnIV(O)(2pyN2Q)]2+ and [MnIV(O)(DMMN4py)]2+
dine, as also manifested in the MnII–NX distances of the (half-lives of 0.2 and 6 hours, respectively). The thermal decay
corresponding MnII complexes (Table 1).
product of [MnIV(O)(2pyN2B)]2+ is EPR silent and shows
Even when using 10 equivalents PhIO, the rate of formation prominent ESI-MS peaks at 662.15 and 726.18, which corres-
of [MnIV(O)(2pyN2B)]2+ was relatively slow, taking about pond to [MnIII(2pyN2B)(OCH2CF3)(Cl)]+ and [MnIII(2pyN2B)
20 minutes to reach completion. ESI-MS data for [MnIV(O) (OCH2CF3)2]+, respectively (Fig. S5†).
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The X-band, perpendicular-mode EPR spectrum for
[MnIV(O)(2pyN2B)]2+ is characterized by a broad, positive
feature centered at geff = 5.7, a weak derivative signal near geff
=
2.9, and a multiline signal around geff = 2 (Fig. 3, bottom left).
The signal at geff = 5.7 shows a six-line hyperfine splitting with
a = 7.0 mT. The multiline signal at geff = 2 likely derives from a
multi-nuclear Mn species, which we assign as a MnIIIMnIV
dimer (ESI†).30 Although the geff-values for [MnIV(O)
(2pyN2B)]2+ are in the range expected for mononuclear, S = 3/2
MnIV species,31 the spectrum is perturbed compared to those
observed for the other N4py-based oxomanganese(IV)
complexes (Fig. S6† and Table 2).6,26 The down-field shift of
the geff = 5.7 signal for [MnIV(O)(2pyN2B)]2+, and the appear-
ance of the weak signal at geff = 2.9, suggests that this oxo-
manganese(IV) complex has a larger rhombic component to its
zero-field splitting tensor (i.e., larger E/D parameter, where D
and E are the axial and rhombic zero-field splitting para-
meters, respectively). The EPR spectrum of [MnIV(O)
(2pyN2B)]2+ is reminiscent of that observed for [MnIV(O)(OH)
(Me2EBC)]+ (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo
[6.6.2]hexadecane), which had two overlapping six-line hyper-
fine features at geff = 5.95 and 4.99, and a derivative signal at
geff = 2.62.31
Fig. 4 Experimental XANES data for frozen samples of [MnII(OH2)
(2pyN2B)](OTf)2 in ethanol (dashed traces) and [MnIV(O)(2pyN2B)]2+ in
TFE (solid traces).
The XAS spectrum of [MnIV(O)(2pyN2B)]2+ shows a promi-
nent pre-edge feature centered at ca. 6541 eV (Fig. 4) with an
area of 14.6 units (Fig. S8†). This feature is readily attributed
to a MnIV 1s-to-3d transition, which gains intensity through
MnIV 3d–4p mixing induced by the short MnvO distance. The
previously described [MnIV(O)(N4py)]2+, [MnIV(O)(DMMN4py)]2+,
and [MnIV(O)(2pyN2Q)]2+ complexes also showed pre-edge fea-
tures near 6541 eV (Fig. S9†),6,26 with areas ranging from
17.0–19.6 units.
Cyclic voltammetry (CV) data for [MnIV(O)(2pyN2B)]2+ show
an irreversible one-electron reduction at −0.07 V vs. Cp2Fe/
Cp2Fe+ (+0.47 V vs. SCE; see Fig. S7†), which is attributed to
the MnIII/IV couple. This process is irreversible, even at scan
rates up to 1000 mV s−1. The observed peak potential is
slightly less than that of [MnIV(O)(DMMN4py)]2+ (0.06 V vs.
Electronic structure computations for [MnIV(O)(2pyN2B)]2+
Cp2Fe/Cp2Fe+ or +0.61 V vs. SCE), and over 0.5 V less than DFT calculations were carried out to further understand the
[MnIV(O)(2pyN2Q)]2+, which has the highest potential for this geometric and electronic structure of [MnIV(O)(2pyN2B)]2+. The
series (Table 2).6 The low peak potential for [MnIV(O) DFT-optimized structure of [MnIV(O)(2pyN2B)]2+ has a MnvO
(2pyN2B)]2+ is consistent with the short Mn–Nbenzimidazolyl dis- bond length of 1.671 Å, which is very similar to those predicted
tances observed in the X-ray structure of [MnII(OH2)(2pyN2B)] for the MnIV-oxo complexes of the DMMN4py, N4py, and
(OTf)2 (Fig. 2), which suggest that the benzimidazolyl ligands 2pyN2Q ligands (1.673–1.678 Å; see Fig. 5 and Table S3†).
are stronger donors than the pyridyl or quinolinyl ligands. While the Mn–Nequatorial distances of [MnIV(O)(2pyN2B)]2+ are
Although peak potentials have not been reported for the analo- the shortest of the series (2.011 Å; see Fig. 5 and Table S3†),
gous [FeIV(O)(2pyN2B)]2+ complex, we can compare the shift they are only slightly shorter than the Mn–Nequatorial distances
observed between [MnIV(O)(2pyN2B)]2+ and [MnIV(O)(N4py)]2+ of [MnIV(O)(DMMN4py)]2+ and [MnIV(O)(N4py)]2+ (2.015 and
with that reported for their FeII analogues. In that case, the 2.018 Å, respectively). These predictions are consistent with
[FeII(NCMe)(2pyN2B)]2+ complex had a potential 0.45 V less than the similar near-IR λmax values of these complexes (Table 2),
that of [FeII(NCMe)(N4py)]2+ (0.398 and 0.85 V versus Cp2Fe/ which we had previously shown reflect the equatorial ligand-
Cp2Fe+, respectively).9 The 0.33 V lower potential of [MnIV(O) field strength.6,29 Further, the Mn–Nequatorial distances of
(2pyN2B)]2+ than [MnIV(O)(N4py)]2+ is comparable (Table 2).
[MnIV(O)(2pyNQ)]2+ are significantly elongated at 2.054 Å, con-
Mn K-edge X-ray absorption data for [MnII(OH2)(2pyN2B)]+ sistent with the larger perturbation in the near-IR λmax of this
and [MnIV(O)(2pyN2B)]2+ show a 2.5 eV increase in edge energy complex (Table 2). These variations in metal–nitrogen equator-
for the latter complex (Fig. 4), which is consistent with the ial bond lengths are qualitatively similar to that observed in
higher Mn oxidation state. The edge energy of [MnIV(O) the crystal structures of the corresponding FeIV-oxo com-
(2pyN2B)]2+ (6549.6 eV) is also very similar to those observed plexes.7 In that case, the average FeIV–Nequatorial bond length of
for other MnIV-oxo complexes with neutral pentadentate [FeIV(O)(2pyN2Q)]2+ was significantly elongated (2.046 Å), while
ligands (6549.2–6550.8 eV).6,26 The observation that the edge those of [FeIV(O)(2pyN2B)]2+ and [FeIV(O)(N4py)]2+ were quite
energy of [MnIV(O)(2pyN2B)]2+ is on the lower end of those similar (1.971 and 1.956 Å, respectively). Nonetheless, for the
observed for related MnIV-oxo adducts is congruent with the FeIV-oxo complexes, N4py provides a stronger equatorial ligand
EPR data for this complex, which shows a weak multiline field than 2pyN2B, whereas DFT computations for the corres-
signal (Fig. 3) due to the presence of some multinuclear ponding MnIV-oxo complexes predicts 2pyN2B to be a stronger
species (potentially a MnIIIMnIV dimer).
equatorial donor (Fig. 5 and Table S3†).
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Fig. 5 Selected bond lengths (Å), bond angles (°) from DFT computations. Nequatorial is the average of the four equatorial Mn–N bond lengths.
Namine refers to the amine nitrogen trans to the oxo ligand. Ntotal refers to the average of all Mn–N bond distances. Data for [MnIV(O)(DMMN4py)]2+
[MnIV(O)(2pyN2Q)]2+ and [MnIV(O)(N4py)]2+ have been published previously.6,26
,
The Mn–Namine bond in the DFT-optimized structure of ity model, complexes with lower-energy near-IR absorption
4
[MnIV(O)(2pyN2B)]2+ is elongated compared to the other MnIV- bands (lower E excited-state energies) should have lower tran-
oxo complexes (Fig. 5 and Table S3†). This result is in agree- sition states for HAT and OAT reactions. Alternatively, the
ment with the MnII XRD structures, where the Mn–Namine MnIII/IV reduction potential represents a portion of the driving
bond of [MnII(OH2)(2pyN2B)](OTf)2 was also longer than that force for the HAT and OAT process, with more positive poten-
of the other MnII complexes. The DFT structure also demon- tials being associated with a larger driving force. The [MnIV(O)
strates that the 2pyN2B ligand provides significantly less steric (2pyN2B)]2+ complex provides an interesting test for these corre-
crowding of the oxo compared to the 2pyN2Q ligand. This is lations, as this complex shows the lowest MnIII/IV Ep,c values for
exemplified by the O–Mn–Namine angle of 177.48°, which is this series, but has a near-IR absorption maximum between
less distorted than in [MnIV(O)(2pyN2Q)]2+ (O–Mn–Namine those of [MnIV(O)(N4py)]2+ and [MnIV(O)(DMMN4py)]2+ (Table 2).
angle of 170.14°). For comparison, we note that [MnII(OH2)
Treatment of 1.0 mM [MnIV(O)(2pyN2B)]2+ in TFE with an
(2pyN2B)](OTf)2 also showed a less distorted O–Mn–Namine excess of 9,10-dihydroanthracene (DHA) at 25 °C led to the
angle of 177.9°, while the corresponding angle in [MnII(OH2) decay of the MnIV-oxo electronic absorption signals by pseudo-
(2pyN2Q)](OTf)2 is 163.8°.
first-order and isosbestic behavior (Fig. 6, top). The pseudo-
first order rate constants, obtained by fitting the dis-
appearance of the 940 nm band of [MnIV(O)(2pyN2B)]2+ over
Hydrocarbon and thioanisole oxidation by [MnIV(O)(2pyN2B)]2+
To further compare [MnIV(O)(2pyN2B)]2+ with other MnIV-oxo time (Fig. 6, top inset), showed a linear increase as a function
adducts, we investigated the reactivity of this complex with of increasing DHA concentration (Fig. 6, center). This analysis
hydrocarbons and thioanisole. These reactions are expected to yielded a second-order rate constant (k2) of 0.27(2) M−1 s−1. To
occur by different mechanisms (HAT and OAT, respectively), determine any solvent influence on this rate, we performed
allowing us to understand the influence of the N-(methyl)ben- analogous kinetic investigations for the reaction of DHA with
zimidazolyl functions in [MnIV(O)(2pyN2B)]2+ on two separate [MnIV(O)(2pyN2B)]2+ in 1 : 1 (vol : vol) TFE : CH2Cl2, which
reaction types. Our prior investigation of [MnIV(O)(N4py)]2+, yielded a smaller k2 of 0.16(1) M−1 s−1 (Fig. S10†). This slight
[MnIV(O)(DMMN4py)]2+, and [MnIV(O)(2pyN2Q)]2+ revealed cor- influence of solvent on the rate is similar to that observed for
relations between HAT and OAT reaction rates with both the [MnIV(O)N4py]2+ 26
and reinforces other reports of modest
,
near-IR absorption energy and the MnIII/IV reduction potentials enhancements for HAT reaction rates in TFE.34 The rate of
of these complexes.6 In each case, faster rates were associated DHA oxidation by [MnIV(O)(2pyN2B)]2+ in 1 : 1 TFE : CH2Cl2 is
with a more positive MnIII/IV reduction potential and a lower- two-fold slower than that observed for [MnIV(O)(DMMN4py)]2+
energy near-IR absorption band. The basis for correlations (k2 = 0.34(2) M−1 s−1 in 1 : 1 TFE : CH2Cl2), which had shown
with the near-IR absorption band are provided by DFT compu- the slowest rate of DHA oxidation for MnIV-oxo complexes of
tations, which predict that HAT and OAT reactions for N4py and its derivatives (Table 3).6,26 To further compare the
[MnIV(O)(N4py)]2+ and related MnIV-oxo complexes involve HAT reactivity of [MnIV(O)(2pyN2B)]2+ with that of [MnIV(O)
crossing from the 4B1 ground state to a 4E excited state (arising
(
DMMN4py)]2+, we determined the rate of reaction of [MnIV(O)
from a MnIV e(dxz, dyz) → b1(dx2−y2) one-electron excitation).32,33 (2pyN2B)]2+ with xanthene, which has a weaker C–H bond
Our MCD investigations of [MnIV(O)(N4py)]2+ assigned the than DHA (bond dissociation free energies of 73.3 kcal mol−1
near-IR absorption maxima of this complex as an excitation to vs. 76.0 kcal mol−1, respectively).35 The oxidation of xanthene
the E excited state.29 Thus, according to this two-state reactiv- by [MnIV(O)(2pyN2B)]2+ gave k2 = 0.39(2) M−1 s−1 in 1 : 1
4
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formed using deuterated [D4]-DHA. The decay of the near-IR
feature of [MnIV(O)(2pyN2B)]2+ in the presence of [D4]-DHA was
significantly slower than that observed for DHA (Fig. S12 and
S13†), and a comparison of k2 values yielded an H/D kinetic
isotope effect of 12.5. This value is similar to that observed for
[MnIV(O)(DMMN4py)]2+ and [MnIV(O)(N4py)]2+ (10.2 and 11.2,
respectively)6,26 and is suggestive of a HAT mechanism.
Oxidation of 1.0 equivalent of DHA by [MnIV(O)(2pyN2B)]2+
produced 0.4 equivalents anthracene per equivalent MnIV-oxo
complex, also similar to that reported for DHA oxidation by
[MnIV(O)(N4py)]2+ and [MnIV(O)(DMMN4py)]2+.6,26 Additionally,
ESI-MS of the solution following the reaction showed two pro-
minent peaks at 563.14 and 662.15, corresponding to
[MnII(2pyN2B)(Cl)]+
and
[MnIII(2pyN2B)(OCH2CF3)(Cl)]+
respectively (Fig. S14†). Thus, DHA oxidation by [MnIV(O)
(2pyN2B)]2+ gives a mixture of MnII and MnIII products.
We also probed the reactivity of [MnIV(O)(2pyN2B)]2+ with
thioanisole. Unlike the hydrocarbons considered previously,
thioanisole should react with this MnIV-oxo complex by an
OAT mechanism. The addition of 40 equivalents thioanisole to
a 1.0 mM solution of [MnIV(O)(2pyN2B)]2+ in TFE at 25 °C led
to a rapid disappearance of the electronic absorption bands at
405 and 940 nm by a first-order process for up to 3 half-lives
(Fig. S16†). This reaction was accompanied by a slight increase
in absorption intensity at 610 nm, with isosbestic points at
540 and 720 nm. We observed an increase in the decay rate of
[MnIV(O)(2pyN2B)]2+ as a function of thioanisole concen-
tration, giving a second-order rate constant (k2) of 0.028(3)
M−1 s−1 in TFE (Table 3 and Fig. 6, bottom). This rate is inter-
mediate between those of [MnIV(O)(DMMN4py)]2+ and [MnIV(O)
(N4py)]2+ (k2 of 0.0023(4) and 0.060(3) M−1 s−1, respectively),
and is 300-fold slower than that of [MnIV(O)(2pyN2Q)]2+ (k2 =
9.2(9) M−1 −1).6,26 1H NMR analysis performed after the reac-
s
tion of [MnIV(O)(2pyN2B)]2+ with 60 equivalents of thioanisole
showed methylphenylsulfoxide in 50% yield relative to the
starting MnII concentration.
Correlations for HAT and OAT reactions of MnIV-oxo species
Fig. 6 Top: Electronic absorption spectra showing the decay of 1.0 mM
[MnIV(O)(2pyN2B)]2+ (purple trace) upon addition of 40 equivalents DHA
in TFE at 25 °C. The inset shows the decay of the feature at 940 nm.
Center: Pseudo-first-order rate constants (kobs) vs. DHA concentration
The present data for HAT and OAT reactions of [MnIV(O)
(2pyN2B)]2+ allow us to revisit our prior correlations of these
reaction rates with the λmax of the near-IR absorbance feature
of the MnIV-oxo complexes and the MnIII/IV reduction poten-
tials.1,6 Fig. 7 shows the relationships between the barriers for
DHA oxidation by the MnIV-oxo complexes (represented as log
(k2)) vs. the MnIII/IV reduction potential and the near-IR tran-
sition energy. DHA oxidation by [MnIV(O)(2pyN2B)]2+ occurs at
a rate expected on the basis of the reduction potential of this
complex (Fig. 7, squares), thereby reinforcing this correlation.
In contrast, [MnIV(O)(2pyN2B)]2+ is an outlier for the previously
observed correlation between the near-IR transition energy
for MnIV-oxo species. Bottom: Pseudo-first-order rate constants (kobs
)
versus thioanisole concentration for oxomanganese(IV) species. For the
center and bottom plots, the lines represent best fits to the data used to
determine the second-order rate constants (k2). Data for [MnIV(O)
(
DMMN4py)]2+ [MnIV(O)(N4py)]2+ and [MnIV(O)(2pyN2Q)]2+ are from
, ,
ref. 6 and 26. As the rates of reaction for [MnIV(O)(2pyN2Q)]2+ are sub-
stantially faster than those of the other complexes, the data points are
off scale (see Fig. S16 and S19† for plots with an expanded scale).
TFE : CH2Cl2 (Fig. S11†). This rate is slightly slower than that and the rate of DHA oxidation via hydrogen atom transfer.
observed for [MnIV(O)(DMMN4py)]2+ in 1 : 1 TFE : CH2Cl2 (k2 of While the near-IR λmax of [MnIV(O)(2pyN2B)]2+ lies midway
0.62(6) M−1 s−1), showing that [MnIV(O)(2pyN2B)]2+ is the between those of [MnIV(O)(N4py)]2+ and [MnIV(O)(DMMN4py)]2+
slower oxidant for two hydrocarbons.
(Table 2 and Fig. 7), [MnIV(O)(2pyN2B)]2+ shows a rate of reac-
To probe the mechanism of hydrocarbon oxidation by tion with DHA that is slower than that of both these com-
[MnIV(O)(2pyN2B)]2+, analogous kinetic experiments were per- plexes. Thus, for HAT reactions of these MnIV-oxo complexes
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Table 3 Second-order rate constants (k2) for the reactions of several MnIV-oxo complexes with DHA and thioanisole
Complexa
k2(DHA)b
k2(thioanisole)b
Solvent
Temp. (°C)
Ref.
[MnIV(O)(2pyN2Q)]2+
[MnIV(O)(N4py)]2+
23(3)
9.2(9)
0.060(3)
0.0023(4)
0.028(3)
NRd
TFE
25
25
25
25
0
20
0
6
26
3.6(4)
0.34(6)
0.27(2)
0.12
TFE : CH3CN (19 : 1)
TFE : CH2Cl2 (DHA); TFE(thioanisole)
TFE
MeCN : H2O(9 : 1)
DMSO
[MnIV(O)(DMMN4py)]2+
[MnIV(O)(2pyN2B)]2+
[MnIV(O)(OH2)(BQCN)]2+
[MnIV(O)(H3buea)]−
[MnIV(O)(Bn-TPEN)]2+
6
c
36
37
38
0.026(2)
NRd
NRd
1.3
TFE : CH3CN (19 : 1)
a Ligand abbreviations as follows: BQCN is N,N′-dimethyl-N,N′-bis(8-quinolyl)cyclohexanediamine; H3buea is [tris[(N′-tert-butylureayl)-N-ethyl)]
amine; Bn-TPEN is N-benzyl-N,N′,N‘-tris(2-pyridylmethyl)-1,2-diaminoethane. b M−1 s−1. c This work. d Not reported.
Fig. 7 Experimental log(k2) values for DHA oxidation (HAT) by MnIV-oxo
Fig. 8 Experimental log(k2) values for thioanisole oxidation (OAT) by
complexes versus MnIII/IV reduction potentials (squares, black line,
bottom abscissa) and near-IR electronic absorption band maxima (dia-
monds, blue line, top abscissa). The fitted lines show the correlation
between these parameters.
MnIV-oxo complexes versus MnIII/IV reduction potentials (squares, black
line, bottom abscissa) and near-IR electronic absorption band maxima
(diamonds, blue line, top abscissa). The fitted lines show the correlation
between these parameters.
(
DMMN4py)]2+ and [MnIV(O)(N4py)]2+ (by 0.14 and 0.33 V,
with DHA, the MnIII/IV reduction potential is a better indicator
of reactivity than the 4E excited-state energy (taken as the near-
IR λmax value).
respectively), but reacts with thioanisole 10-fold faster than
[MnIV(O)(DMMN4py)]2+ and only 2-fold slower than [MnIV(O)
(N4py)]2+ (Table 2). The lack of a correlation between the thioa-
nisole oxidation rate and the MnIII/IV potentials of these MnIV-
oxo complexes is not unexpected, as thioanisole oxidation pre-
sumably occurs by an OAT mechanism, which is a two-electron
process.
A similar comparison of OAT reaction rates of MnIV-oxo
complexes vs. MnIII/IV reduction potential and near-IR λmax
values is presented in Fig. 8. For these OAT reactions, there is
a close correlation between the OAT reaction barrier and the
energy of the near-IR electronic absorption band (Fig. 8, dia-
monds). In particular, [MnIV(O)(2pyN2B)]2+ shows a near-IR
band quite similar in energy to that of [MnIV(O)(N4py)]2+,
which is in keeping with their similar rates of reaction with
Comparison of MnIV-oxo and FeIV-oxo reactivity at parity of
ligand coordination sphere
thioanisole (Fig. 8). This correlation is consistent with DFT The present kinetic investigations of [MnIV(O)(2pyN2B)]2+ also
predictions that the 4E excited state is involved in OAT reac- allow us to compare variations in HAT and OAT rates of these
tions of MnIV-oxo complexes.32 In contrast, [MnIV(O) MnIV-oxo adducts with those of FeIV-oxo complexes supported
(2pyN2B)]2+ has a rate substantially faster than expected on the by the same series of ligands. For the MnIV-oxo complexes,
basis of its reduction potential (Fig. 8, squares). Specifically, [MnIV(O)(2pyN2Q)]2+ displays the fastest rate of reaction with
[MnIV(O)(2pyN2B)]2+ has a lower potential than [MnIV(O) DHA, with [MnIV(O)(DMMN4py)]2+ and [MnIV(O)(2pyN2B)]2+
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OAT reactions clearly demonstrates that identical ligand per-
turbations tune the reactivity of FeIV-oxo and MnIV-oxo com-
plexes quite differently (Fig. 9).
The observation that the oxometal(IV) complexes with
2pyN2Q ligation provide the fastest rates could be related to
several unique features of these complexes. First, as shown in
the DFT structure of [MnIV(O)(2pyN2Q)]2+ and the X-ray crystal
structure of [FeIV(O)(2pyN2Q)]2+, the steric bulk of the quinoli-
nyl moieties in these complexes crowds the oxo binding site,
leading to a distortion in the Naxial–MvO bond angle from the
180° expected for an idealized six-coordinate complex.6,7 For
comparison, the [FeIV(O)(2pyN2B)]2+ and [MnIV(O)(2pyN2B)]2+
complexes show evidence of more modest steric interactions
between the N(methyl)benzimidazolyl groups and the oxo
ligand (Fig. 5). Respective DFT and X-ray crystallographic
results for these complexes show Naxial–MvO bond angles of
177°. The influence, if any, of this relatively small bond angle
distortion on the reactivity of these complexes is unclear at
present.
The second influence of the quinolinyl functions is to
provide weaker metal–nitrogen bonds than the pyridyl and
N(methyl)benzimidazolyl donors in the other complexes. For
[MnIV(O)(2pyN2Q)]2+, this effect is apparent in both the extre-
mely low energy of the near-IR absorption band maxima
(reflecting the stabilization of MnIV 3dx2−y2 MO that is σ-anti-
bonding with respective to the equatorial ligands) and the
high MnIII/IV reduction potential (Table 2). For [FeIV(O)
(2pyN2Q)]2+, Fe–Nquinolinyl bond elongations relative to compar-
able Fe–N distances are readily apparent in the X-ray crystal
structure.7 As has been discussed above, this is suggested to
Fig. 9 Blue: Comparison of the rates of reaction of DHA with oxoman-
ganese(IV) species supported by N4py and its derivatives. Red:
Comparison of the rates of reaction of ethylbenzene (EtBn) with oxoiron
(IV) complexes supported by N4py and its derivatives. The sources of the
rate data are as follows: [MnIV(O)(DMMN4py)]2+ and [MnIV(O)(2pyN2Q)]2+
from ref. 6; [MnIV(O)(N4py)]2+ from ref. 26; [FeIV(O)(DMMN4py)]2+ from
ref. 8; [FeIV(O)(N4py)]2+ from ref. 39; [FeIV(O)(2pyN2B)]2+ from ref. 9,
[FeIV(O)(2pyN2Q)]2+ from ref. 7. For the oxoiron(IV) complexes, the reac-
tivity trend is slightly different with cumene (Fig. S19†). Errors for the
rates of the MnIV-oxo complexes are indicated in Table 3. Errors for the
FeIV-oxo rate constants have not all been reported.
showing roughly similar rates that are the slowest for the be a consequence of the steric interactions between the quino-
series (Fig. 9). These rates give an overall ligand ordering of linyl moieties and the FeIV(O) entity, leading to a weaker
2pyN2Q > N4py > DMMN4py ≈ 2pyN2B for HAT reactions of the effective field being exerted by these substituents. For the FeIV-
MnIV-oxo adducts. For the FeIV-oxo analogues, we can compare oxo series, the relationship between these bond elongations
ligand effects using reaction rates for ethylbenzene oxidation, and HAT and OAT reaction rates was displayed through a corre-
which show an ordering of 2pyN2Q > 2pyN2B > N4py ≈ lation between the average FeIV–N distances of these com-
DMMN4py (Fig. 9). In this case, [FeIV(O)(2pyN2Q)]2+ shows a plexes and the barriers (expressed as log(k2)) for cyclohexane
rate of reaction with ethylbenzene that is only 2-fold faster and thioanisole oxidation.7 A similar correlation, using experi-
than that of [FeIV(O)(2pyN2B)]2+.7 Additionally, [FeIV(O) mental Mn–N distances is not possible for the MnIV-oxo com-
(2pyN2B)]2+ reacts with ethylbenzene 12-fold faster than plexes, as experimental structural data are lacking for these
[FeIV(O)(N4py)]2+, with even more dramatic rate enhancements complexes. The only common set of experimental metric para-
observed for other substrates.9 The HAT reactivity of [FeIV(O) meters for the corresponding Mn systems comes from the
(
DMMN4py)]2+ and [FeIV(O)(N4py)]2+ are quite similar, X-ray crystal structures of the MnII complexes. Plots of the
with the latter complex showing a faster rate of reaction with average MnII–N distance from these structures vs. log(k2) for
ethylbenzene (Fig. 9) but
a
slower rate with cumene DHA and thioanisole oxidation show reasonable correlations,
albeit with a fair amount of scatter (Fig. 10, top-left panel). Far
(Fig. S20†).8,39
There is also data available to compare the OAT rates of less scatter, especially for the DHA reactions, is observed when
MnIV-oxo and FeIV-oxo complexes supported by the N4py, the average MnII–Nequatorial distance is plotted versus the
2pyN2B, and 2pyN2Q ligands. As was the case for the HAT log(k2) values (Fig. 10, top-right panel). Analogous correlations
reactions, both [MnIV(O)(2pyN2Q)]2+ and [FeIV(O)(2pyN2Q)]2+ developed using DFT-calculated MnIV–N distances show a fair
show greatly enhanced reactivity towards thioanisole degree of scatter (Fig. 10, bottom panels). It should be noted
(Fig. S20†). For the FeIV-oxo series, [FeIV(O)(2pyN2B)]2+ showed that the variation in average MnIV–N distances is quite small
the next highest reactivity with [FeIV(O)(N4py)]2+ having the (from 2.033 Å for [MnIV(O)(DMMN4py)]2+ to 2.065 Å for
slowest rate.9 In contrast, [MnIV(O)(2pyN2B)]2+ displayed a [MnIV(O)(2pyN2Q)]2+. For comparison, the average crystallo-
thioanisole oxidation rate slightly less than that of [MnIV(O) graphic Fe–N distances of [FeIV(O)(N4py)]2+ and [FeIV(O)
(N4py)]2+ (Table 3). Collectively, the rate data for both HAT and (2pyN2Q)]2+ vary by 0.08 Å (1.972 and 2.053 Å, respectively).
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Fig. 10 Comparison of the log(k2) for DHA (blue) and thioanisole (green) and the average Mn–N distances (left panels) and Mn–N equatorial dis-
tances (right panels) for manganese(II) species (top panels) and manganese(IV)-oxo species (bottom panels) supported by N4py and its derivatives.
Errors for the reaction rates can be found in Table 3.
Comba, Sastri, and co-workers have also observed
a
plexes considered in this present work. For example, Fig. 7
different pattern of reactivity for MnIV-oxo and FeIV-oxo com- shows a reasonable correlation between the HAT reaction
plexes at parity of ligand sphere.40,41 Their work employed pen- barrier and the MnIII/IV reduction potential, suggesting that
tadentate bispidine ligands (termed L1 and L2), whose this electronic parameter, and not some steric factor, can
different architectures place different amine functions trans to account for the reactivity ordering of these complexes.
the oxo binding site.41 Kinetic examination of thioanisole and Nonetheless, the basis for the disparate reactivity patterns of
2,4-di-tert-butylphenol oxidation by [MnIV(O)(L1)]2+ and these MnIV-oxo and FeIV-oxo species is unclear at present.
[MnIV(O)(L2)]2+ provided faster rates for the former complex. In Further experimental and computational work is needed to
contrast, the [FeIV(O)(L2)]2+ species is a more rapid oxidant fully explain these observations.
than [FeIV(O)(L1)]2+.40,42,43 DFT analysis of cyclohexane oxi-
dation by this set of metal-oxo species revealed that the FeIV-
oxo adducts react with this hydrocarbon through the so-called
σ-pathway (linear Fe–O–H angle in the transition state), while
Conclusions
the MnIV-oxo adducts utilize the π-pathway (bent Mn–O–H In this work, we have described new MnII and MnIV-oxo com-
angle in the transition state). It was further proposed that, plexes supported by the 2pyN2B ligand that features a mixture
because of the distinctly different transition state structures, of amine, pyridyl, and N(methyl)benzimidazolyl donors. The
electronic factors dominate in the FeIV-oxo species, whereas [MnIV(O)(2pyN2B)]2+ complex features a MnIV-oxo unit whose
steric factors are dominant for the MnIV-oxo species.41 While spectroscopic signatures resemble those of [MnIV(O)(N4py)]2+
intriguing, it is unclear whether this model can explain the and its analogues [MnIV(O)(DMMN4py)]2+ and [MnIV(O)
different reactivity patterns of the MnIV-oxo and FeIV-oxo com- (2pyN2Q)]2+. The near-IR electronic absorption band maxima
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of [MnIV(O)(2pyN2B)]2+, which corresponds to a MnIV e(dxz, dyz) Haukka at the University of Jyväskylä in Finland for structural
→ b1(dx2−y2) one-electron excitation, lies between those of data collection on CCDC entry 1891620.
[MnIV(O)(DMMN4py)]2+ and [MnIV(O)(N4py)]2+, illustrating the
higher donor strength of the N(methyl)benzimidazolyl func-
tions compared to unsubstituted pyridyl groups. With regard
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