Generation and Reactivity of a One-Electron-Oxidized Manganese(V) Imido Complex with a Tetraamido Macrocyclic Ligand

Article abstract of DOI:10.1002/chem.201902405

The synthesis and X-ray structure of a new manganese(V) mesitylimido complex with a tetraamido macrocyclic ligand (TAML), [Mn^V(TAML)(N-Mes)]^? (1), are reported. Compound 1 is oxidized by [(p-BrC₆H₄)₃N]^+.[SbCl₆]^? and the resulting Mn^VI species readily undergoes H-atom transfer and nitrene transfer reactions.

Full text of DOI:10.1002/chem.201902405

A Journal of
Accepted Article
Title: Generation and Reactivity of a One-electron Oxidized
Manganese(V) Imido Complex bearing a Macrocyclic Tetraamido
Ligand
Authors: Tai-Chu Lau, Huatian Shi, Jianhui Xie, William W. Y. Lam,
Wai-Lun Man, Chi-Keung Mak, Shek-Man Yiu, and Hung-Kay
Lee
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To be cited as: Chem. Eur. J. 10.1002/chem.201902405
Link to VoR: http://dx.doi.org/10.1002/chem.201902405
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Generation and Reactivity of a One-electron Oxidized
Manganese(V) Imido Complex bearing a Macrocyclic Tetraamido
Ligand
Huatian Shi,[a] Jianhui Xie,[b] William W. Y. Lam,[c] Wai-Lun Man,[d] Chi-Keung Mak,[a] Shek-Man Yiu,[a]
Hung Kay Lee,[e] and Tai-Chi Lau*[a]
Abstract: We report the synthesis and X-ray structure of a new
manganese(V) mesitylimido complex bearing macrocyclic
tetraamido ligand (TAML), [MnV(TAML)(N-Mes)] (1). 1 is readily
oxidized by [(p-BrC6H4)3N][SbCl6]and the resulting species readily
undergoes H-atom transfer and nitrene transfer reactions.
We report herein the synthesis and characterization of a new
manganese(V) imido complex bearing the macrocyclic tetraamido
a
ligand TAML, [MnV(TAML)(N-Mes)] (Mes
= mesityl). The
tetraanionic TAML ligand is known to stabilize metal complexes
in high oxidation states.9 The oxo analog of this complex,
[MnV(TAML)(O)], has been reported by Fukuzumi and Nam.9e
[MnV(TAML)(N-Mes)] is highly stable and unreactive towards
common organic substrates. However, it readily undergoes one-
electron oxidation and the resulting species is highly reactive
towards nitrene transfer and HAT reactions.
Manganese(V) imido complexes have been postulated as
active intermediates in many useful organic transformation
reactions.1-6 For instance, manganese(V) nitrido complexes
bearing porphyrin or salen ligands react with trifluoroacetic
anhydride (TFAA) to generate manganese(V) acylimido species,
which can transfer the CF3CON group to alkenes.1-3 p-
Toluenesulfonic anhydride is also used to convert chiral
managnese(V) salen nitrido complexes to imido species, which
can perform asymmetric aziridination of alkenes.4 Trifluoroacetic
acid and BF3 have also been employed to convert manganese(V)
salen nitrido complexes to imido species which can react with
alkenes to give the parent aziridines.5 Manganese
phthalocyanines catalyze intra- and intermolecular C(sp3)-H
amination reactions and manganese imido/nitrene species are
proposed to be active intermediates.6 In order to directly study
the reactivity of these active species, there is a continuing interest
in isolating manganese (V) imido complexes. Surprisingly, so far
the only isolated and well-characterized manganese(V) imido
species are those with corrole and corrolazine ligands. These
include [Mn(tpfc)(NAr)] (tpfc = tris(pentafluorophenyl)corrole, Ar =
2,4,6-(CH3)3C6H2 (Mes), 2,4,6-Cl3C6H2 or p-toluenesulfonyl),
[Mn(Br8tpfc)(NAr) and [Mn(TBP8Cz)(NMes)] (TBP8Cz = octakis(p-
tert-butylphenyl)corrolazinato).7,8 Nitrene transfer as well as
hydrogen atom transfer (HAT) reactions of these imido complexes
have been demonstrated.7,8
The manganese(V) mesitylimido complex, [MnV(TAML)(N-
Mes)] (1) is prepared according to Scheme 1.
Scheme 1. Synthesis of 1.
+
1 is isolated as the PPh4 salt. The complex is diamagnetic,
as evidenced by the sharp resonances in the normal region in the
1H NMR spectrum, consistent with its formulation as a d2 Mn(V)
complex (Figure S1). The electrospray ionization mass spectrum
(ESI/MS) of 1 in CH3CN exhibits a predominant parent peak (M-)
at m/z = 558.3 (Figure S2). The cyclic voltammetry (CV) of 1 in
+/0
CH3
,
ΔE
(Figure S3).
[a]
[b]
[c]
H. Shi, C. K. Mak, S. M. Yiu, T. C. Lau*
Department of Chemistry, City University of Hong Kong
Tat Chee Avenue, Kowloon Tong, Hong Kong SAR, China.
E-mail: bhtclau@cityu.edu.hk
The molecular structure of 1 has been determined by X-ray
crystallography (Figure 1). The complex has a distorted square
pyramidal geometry, with the TAML ligand in the equatorial plane
and a mesitylimido ligand in the axial position. The Mn center is
0.56 Å out of the plane defined by four N atoms of the TAML ligand.
The Mn-Nimido distance is 1.610 (2) Å, which is similar to those of
[(tpfc)Mn(NMes)] (1.613 Å)7a and [(TBP8Cz)Mn(NMes)] (1.595 Å
and 1.611 Å),8 consistent with a M≡NMes triple bond. In
accordance with this triple bond character, the Mn1-N5-C20 angle
is close to linear (175.6o).
J. Xie
School of Chemistry and Chemical Engineering
Hefei University of Technology
Hefei 230009, P. R. China.
W. W. Y. Lam
Department of Food and Health Sciences
Technological and Higher Education Institute of Hong Kong
Tsing Yi Road, Hong Kong SAR, China.
W. L. Man
Department of Chemistry, Hong Kong Baptist University
Kowloon Tong, Hong Kong SAR, China.
H. K. Lee
[d]
[e]
Department of Chemistry, The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong SAR, China.
Supporting information for this article is given via a link at the end of
the document.
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Experimental
Simulated
2800
3000
3200
3400
3600
3800
Field (Gauss)
Figure 1. ORTEP plot (30% probability level) of [PPh4]1. The PPh4 counter ion
and hydrogens have been omitted for clarity.
Figure 2. Experimental and two-component simulated EPR spectra of 2.
Component 1: Mn(VI) with gxx = gyy = 2.001, gzz = 1.993; Axx = Ayy = 134.9 MHz,
Azz = 386.7 MHz; Component 2 (ligand based radical: g = 1.991). The ratio
between Mn(VI) species [MnVI(TAML)(N-Mes)] and ligand based radical
[MnV(TAML+)(NMes)] is 40 : 1.
1 is stable in the solid state and in CH3CN or CH2Cl2 solution
for days at room temperature. A solution of 1 in CH3CN or CH2Cl2
is also unreactive towards various organic substrates, such as
PPh3 or phenol for >24 h at 25 ºC, as monitored by UV/Vis
spectrophotometry. Evidently, the presence of the tetraanionic
TAML ligand stabilizes the Mn(V) imido complex. Since the CV of
1 shows a reversible oxidation wave at +0.49 V vs. Fc+/0, this
suggests that it is possible to oxidize 1 using a suitable oxidant,
which should make the resulting imido species more reactive.
Nam and coworkers recently reported that one-electron oxidation
of [MnV(TAML)(O)]10 and FeV(TAML)(NTs)]9g and the oxidized
species become much more reactive.
The structure of [MnVI(TAML)(N-Mes)] was investigated by DFT
calculation at BP86/def2-TZVP level of theory. The result shows
that the structure with doublet ground state is more stable (Table
S1). The Mn-Nimido bond length and Mn-Nimido-C angle of the
equilibrium structure are 1.611 Å and 176.42 °, respectively
(Figure S6). The Mulliken spin population analysis shows that 0.9
spin density (α – β) is located on the Mn center. On the other
hand, only 0.057 spin density is on the N atom of imido group
(Figure S7), indicating that there is negligible nitrene radical
character of the imido group. The calculation results together with
the EPR data indicate that the oxidation of 1 is predominantly
metal-based and hence 2 may be formulated as a manganese(VI)
imido complex, [MnVI(TAML)(N-Mes)], mixed with a small portion
of Mn(V) TAML based radical complex, [Mn(V)(TAML+)(N-Mes)].
These two species are probably in equilibrium as a result of
valence tautomerization.15
Addition of 1 equiv. of the oxidant [(p-BrC6H4)3N]+[SbCl6]- (E0
= 0.67 V vs. Fc+/0 11
in CH3CN to 1 at 40 °C to 25 ºC produced
)
the one electron oxidized species 2, with immediate discharge of
the blue color of the radical cation (λmax at 700 nm) (Figure S4).
The X-band electron paramagnetic resonance (EPR) spectrum of
2 (CH3CN, 4K, Figure 2) shows a S = 1/2 signal with an axial
splitting pattern that is typical of a manganese(VI) species and is
similar to the EPR of the manganese(VI) salen nitrido complex
[Mn(SaltBu)N]+ reported by Storr13a as well as two manganese(VI)
nitrido complexes reported by Wieghardt.14 In contrast, one-
electron oxidation of the oxo analogue of 1, [MnV(TAML)(O)]
occurs at the TAML ligand to generate [MnV(TAML+)(O)]; in this
case its EPR spectrum shows a typical isotropic ligand radical
signal centered at g = 2.002.10 The EPR spectrum of 2 was
simulated using a two-component model (Mn(VI) and ligand-
based radical, Figure S5) with the EasySpin program developed
by Stoll.16 The simulation results show that 2 is predominantly the
manganese(VI) species [MnVI(TAML)(N-Mes)] with around 2.5%
of the ligand-based radical [MnV(TAML+)(NMes)].
The oxidized species 2 decays slowly at room temperature,
as monitored by UV/Vis spectrophotometry (Figure S8). The
initial rate of the decomposition is second-order with respect to [2]
(Figure 3), indicating that it is a bimolecular process.
Figure 3. Plot of initial rate vs [2]2 [slope = (1.27 ± 0.09) × 104; r2 = 0.978].
A neutral dark brown complex 3 could be separated from the
decay mixture of 2 by silica column chromatography, with 10%
yield based on Mn. The molecular structure of 3 has been
determined by X-ray crystallography (Figure 4). It shows the
addition of two NMes nitrene groups to the o-phenylene ring of
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the TAML ligand in 2, resulting in its conversion into an o-
benzoquinonediimine moiety. The two imine C-N distances are
different; C5-N5 = 1.315 Å, C6-N6 = 1.300 Å. The Mn-Nimido
distance is 1.609 (2) Å, which is the same as that of 1. 3 is
diamagnetic as it shows sharp resonances in the normal range in
the 1H NMR spectrum (Figure S9), consistent with a Mn(V)
complex. As the complex is neutral, it requires the presence of an
H+ ion to balance the charge, which is tentatively assigned to N5
since it has a longer C-N bond and a difference peak was
successfully assigned to and refined as a hydrogen atom near to
N5 in crystal structure refinement. In the proton NMR, the
resonance at 9.90 ppm is assigned to the N-H proton. The
presence of N-H is also evidenced by a stretch in the IR at 3221
cm-1. The ESI/MS of 3 shows a predominant peak at m/z = -822.9
(Figure S10), which is assigned to [3-H]-. MS/MS of the peak
shows a fragment ion at m/z -689.5, which is attributed to the loss
of NMes bound to the Mn enter (Figure S11).
Although 2 undergoes intermolecular nitrene transfer
reactions with itself, it also reacts readily with various organic
substrates if they are present in excess. For instance, it reacts
rapidly with 2,4-di-tert-butylphenol (20-fold excess) to give the
phenol dimer and 2,4,6-trimethylaniline in 25% and 56% yield,
respectively, as analyzed by GC-FID and GC-MS (Scheme
3a). The observation of the dimer product indicates that the
imido species abstracts a H atom from O-H to generate a
phenoxy radical which then rapidly dimerizes.17 H-atom
abstraction by the imido results in the formation of the free
aniline.
Scheme 3. HAT and nitrene transfer reaction of 2 with various organic
substrates.
2 also reacts readily with 9,10-dihydroanthracene (DHA);
monitoring of the reaction by UV/Vis spectrophotometry
reviewed the appearance of characteristic peaks of anthracene
(Figure S14). Anthracene (22%) and 2,4,6-trimethylaniline
(38%) were also detected and quantified by GC/MS and
GC/FID (Scheme 3b). The rate of the reaction is first-order in
both [2] and [DHA], and the second-order rate constant k2 =
4.79  10-2 M-1 s-1 at 25.0 °C was obtained. When d4-DHA was
used as substrate, k2D = 1.69  10-2 M-1 s-1, this gives a kinetic
isotope effect (KIE) of 2.8, which suggests that the rate-limiting
step involves CH bond cleavage, consistent with a HAT
mechanism (Figure 5).
Figure 4. ORTEP plot (30% probability level) of 3. Hydrogens have been
omitted for clarity.
Our results indicate that the decay of 2 occurs initially via
intermolecular nitrene transfer. In addition to [3-H]-, ESI/MS of
the decay mixture of 2 shows the presence of at least two Mn-
containing ions at m/z = -689.6 and -425.4, which are assigned
to complex 4 and [Mn(TAML)]-, respectively (Scheme 2,
Figure S12). In complex 4, the two NMes groups are proposed
to be both attached to the aromatic ring, since MS/MS does not
show the presence of fragment ions arising from the loss of
NMes (Figure S13). Attempts to isolate PPh4[4] from the
mixture were unsuccessful.
DHA
0.004
DHA-d4
0.003
0.002
0.001
0.000
0.00
0.02
0.04
0.06
0.08
[DHA] / M
Figure 5. Plots of kobs vs [DHA] (closed circle) and [DHA-d4] (open circle).
DHA: slope = (4.79 ± 0.08) × 10-2, y-intercept = -(0.01 ± 4.20) ×10-5, r2
Scheme 2. Decay products of 2.
=
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0.999. d4-DHA: slope = (1.69 ± 0.07) ×10-2, y-intercept = (1.22 ± 0.26) ×10-
4, r2 = 0.994].
Conclusions
The kinetics for the reaction of 2 with DHA were also
carried out as a function of temperature; from the Erying plot of
ln(k2/T) versus 1/T, ΔH‡ and ΔS‡, are found to be (14.7 ± 0.5)
kcal mol-1 and – (15 ± 1) cal mol-1 K-1, respectively (Figure
S15). The negative ΔS‡ value is consistent with the ordered
transition state of a HAT reaction.
The reaction of 2 with various other hydrocarbons with
weak C-H bonds were also investigated (Figure S16-S19), and
the rate constants were compiled in Table S2. Log k2’ (k2’ = k2
divided by the number of active H) shows a linear correlation
with the C-H bond dissociation energy of the substrates, which
supports a HAT mechanism for the reaction of 2 with the
organic substrates (Figure 6).
We have reported a new manganese(V) imido complex
bearing the TAML ligand. One-electron oxidation of this
complex affords the first example of a manganese(VI) imido
species, which readily undergoes novel nitrene transfer
reaction with itself and with thioanisoles. It also abstracts H-
atom from phenol and hydrocarbons with weak C-H bonds.
Compounds of Mn(VI) are rare. Apart from Mn(VI) oxo species,
only a few Mn(VI) nitrido species are known.12-14 However, the
reactivity of these complexes towards various substrates has
not been investigated. Our work should be a significant
contribution to high-valent manganese imido chemistry.
Experimental Section
Detailed information on synthesis and general procedures can be found in
the Supporting Information. Crystallographic data of 1 and 3 are available
at the Cambridge Crystallographic Data Centre (CCDC 1868325 and
1868326).
1
BNAH
AcrH2
0
xanthene
-1
Acknowledgements
DHA
CHD
This research is supported by the Research Grants Council of
Hong Kong (CityU 11302917), the Hong Kong University Grants
Committee (AoE/P-03-08) and the National Natural Science
Foundation of China (21801058). We thank Dr. C. Y. Wong and
Dr. K. C. Lau for helpful discussions.
-2
68
70
72
74
76
78
BDE / kcal mol-1
Figure 6. Plot of log(k2’) vs BDEs for the reaction of 2 with various organic
substrates in CH3CN at 25.0 ºC. Slope = -0.3 ± 0.03. BNAH = 1-benzyl-1,4-
dihydronicotinamide. AcrH2 = 9,10-dihydro-10-methylacridine. CHD = 1,4-
cyclohexadiene.
Keywords: Manganese complex • Imido transfer • Hydrogen
atom transfer
2 also undergoes direct nitrene transfer to thioanisole to
produce the corresponding sulfilimine (Scheme 3c, Figure
S20). The rate of the reaction is first-order in [2] and [PhSMe],
the second order rate constant k2 = 1.87  101 M-1 s-1 at 25.0 °C
(Figure S21). The activation parameters ΔH‡ and ΔS‡ were
found to be (5.8 ± 0.1) kcal mol-1 and (33 ± 1) cal mol-1 M-1,
respectively (Figure S22). The large negative ΔS‡ value is
consistent with the ordered transition state of a direct nitrene
transfer reaction.
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Huatian Shi,[a] Jianhui Xie,[b] William W.
Y. Lam,[c] Wai-Lun Man,[d] Chi-Keung
Mak,[a] Shek-Man Yiu,[a] Hung Kay
Lee,[e] and Tai-Chi Lau*[a]
Page No. – Page No.
Generation and Reactivity of a One-
electron Oxidized Manganese(V)
Imido Complex bearing a Macrocyclic
Tetraamido Ligand
We report the synthesis and X-ray structure of a new Mn(V) mesitylimido(N-Mes)
bearing a tetraamido macrocyclic ligand (TAML), Mn(V)(TAML)(N-Mes) (1). Upon
one electron oxidation, a reactive species Mn(TAML)(N-Mes) (2) was generated,
which readily undergoes H-atom transfer and nitrene transfer reactions.
[a]
[b]
[c]
[d]
H. Shi, C. K. Mak, S. M. Yiu, T. C. Lau*
Department of Chemistry, City University of Hong Kong
Tat Chee Avenue, Kowloon Tong, Hong Kong SAR, China
E-mail: bhtclau@cityu.edu.hk
J. Xie
School of Chemistry and Chemical Engineering
Hefei University of Technology
Hefei 230009, P. R. China.
W. W. Y. Lam
Department of Food and Health Sciences
Technological and Higher Education Institute of Hong Kon
Tsing Yi Road, Hong Kong SAR, China.
W. L. Man
This article is protected by copyright. All rights reserved.
Department of Chemistry, Hong Kong Baptist University