Activation of C-H, N-H, and O-H Bonds via Proton-Coupled Electron Transfer to a Mn(III) Complex of Redox-Noninnocent Octaazacyclotetradecadiene, a Catenated-Nitrogen Macrocyclic Ligand

Article abstract of DOI:10.1021/jacs.8b10250

Reaction of 1,3-diazidopropane with an electron-rich Mn(II) precursor results in oxidation of the metal center to a Mn complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-1,4,8,11-tetraido (OIM) ligand. Although describable as a Werner Mn(V) complex, analysis by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, and density functional theory calculations suggest an electronic structure consisting of a Mn(III) metal center with a noninnocent OIM diradical ligand. The resulting complex, (OIM)Mn(NH^tBu), reacts via proton-coupled electron transfer (PCET) with phenols to form phenoxyl radicals, with dihydroanthracene to form anthracene, and with (2,4-ditert-butyltetrazolium-5-yl)amide to extrude a tetrazyl radical. PCET from the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-ditert-butyltetrazolium-5-yl)amido ligand. Electron paramagnetic resonance and density functional theoretical analyses suggest an electronic structure wherein the manganese atom remains Mn(III) and the OIM ligand has been reduced by one electron to a monoradical noninnocent ligand. The result indicates PCET processes whereby the proton is transferred to the axial ligand to extrude ^tBuNH₂, the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.

Full text of DOI:10.1021/jacs.8b10250

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Article
Activation of C-H, N-H, and O-H bonds via proton-coupled
electron transfer to a Mn(III) complex of redox-noninnocent
octaazacyclotetradecadiene, a catenated-nitrogen macrocyclic ligand.
Shivaiah Vaddypally, Warren Tomlinson, Owen T. O'Sullivan, Ran Ding, Megan
Van Vliet, Bradford B. Wayland, Joseph P. Hooper, and Michael J. Zdilla
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10250 • Publication Date (Web): 11 Mar 2019
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Journal of the American Chemical Society
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Activation of C-H, N-H, and O-H bonds via proton-coupled
electron transfer to a Mn(III) complex of redox-noninnocent
octaazacyclotetradecadiene, a catenated-nitrogen
macrocyclic ligand.
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b
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Shivaiah Vaddypally, Warren Tomlinson, Owen T. O’Sullivan, Ran Ding, Megan M. Van Vliet,
a
b
a
Bradford B. Wayland, Joseph P. Hooper,* Michael J. Zdilla*
a
th
Department of Chemistry, Temple University, 1901 N. 13 St., Philadelphia, PA 19122
bDepartment of Physics, Naval Postgraduate School, 833 Dyer Rd., Monterey, CA 93943
KEYWORDS Energetic materials · non-innocent ligands · redox active ligands · valence tautomerism
ABSTRACT: Reaction of 1,3-diazidopropane with an electron-rich Mn(II) precursor results in oxidation of the metal center to a Mn
complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-1,4,8,11-
tetraido (OIM) ligand. Although describable as a Werner Mn(V) complex, analysis by XRD, magnetic measurements, XPS, CV, and
DFT calculations suggest an electronic structure consisting of a Mn(III) metal center with a non-innocent OIM diradical ligand. The
t
resulting complex, (OIM)Mn(NH Bu) reacts via proton-coupled electron transfer with phenols to form phenoxyl radicals, with
dihydroanthracene to form anthracene, and with (2,4-di-tert-butyltetrazolium-5-yl)amide to extrude a tetrazyl radical. PCET from
the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-di-tert-
butyltetrazolium-5-yl)amido ligand. EPR and density functional theoretical analysis suggests an electronic structure wherein the
manganese atom remains Mn(III) and the OIM ligand has been reduced by one electron to a mono-radical non-innocent ligand. The
t
result indicates proton-coupled electron transfer processes whereby the proton is transferred to the axial ligand to extrude BuNH
the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.
2
,
Introduction.
Tetrazene complexes at metal centers are well established.24-
3
7
and formation most commonly occurs by the assembly of
pairs of organic azides about the metal center with extrusion of
Novel ligand platforms to mediate difficult redox reactions
are of interest for synthetic homogeneous catalytic systems as
well as biomimetic studies. Due to its prevalence in biological
2
3, 31, 34, 35
N
2
.
In addition to our own previous report on
non-innocent behavior in iron tetrazene
2
3
1
-3
manganese,
redox metalloenzymes, the reactivity of Mn in various ligand
3
6-38
platforms is a frequent subject of study.4-13 In these systems,
complexes from several other groups are worthy of note.
proton-coupled electron transfer (PCET)—the coordinated (but
not necessarily simultaneous) movement of electrons and
protons between reactants and products—is of central interest
in a number of industrially and biologically relevant reactions
The vast majority of tetrazene complexes exhibit only a
single tetrazene ligand, with only a few exceptions to our
knowledge where complexes of two tetrazene ligands have been
23, 38
reported.
In this report, we describe a related assembly of a
14-18
mediated by transition metals.
In addition to the study of
pair of tetrazene units embedded in a macrocyclic ligand around
manganese, using as a synthon the disubstituted 1,3-
biomimetic reaction chemistry at Mn, our group has a particular
interest in nitrogen-rich analogues of classical ligands with the
goal of generating novel, metal-containing energetic
diazidopropane.
The
result
is
a
novel
octaazacyclotetradecadiene macrocycle (OIM) ligand, a partly
unsaturated octanitrogen topological analogue of the previously
reported 2,3,9,10-tetraazacyclotetradeca-1,3-diene macrocycle
1
9-22
materials.
the first manganese complexes of tetrazene, the N
the classic redox-non-innocent α-diimine ligand (Figure 1).
We have previously reported on the assembly of
4
analogue of
2
3
39
(TIM), a redox non-innocent, unsaturated variant of the
e-
e-
extensively reported tetraazacyclotetradecane (Cyclam)
4
0-45
ligand.
This ligand also represents a more saturated
RN
RN
NR
NR
RN
RN
NR
NR
RN
RN
NR
NR
analogue of the tetraazaannulene ligand and derivatives
e-
e-
46-48
N
N
N
N
N
N
thereof, which has been a focus of our work
and that of
49-53
others, including examples of manganese chemistry.
We
report the synthesis of manganese complexes of OIM, their
electronic structures, and PCET reactions via activation of C-H,
N-H, and O-H bonds.
Figure 1. Analogy between redox non-innocent -diimine (top)
and tetraazadiene (bottom).
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Page 2 of 12
) (2), and (OIM)Mn (NH(CN
4 2
tBu ) (3)a
V
t
V
IV
Table 1. Selected bond lengths (Å) from (OIM)Mn (NH Bu) (1), (OIM)Mn (N(SiMe
from single-crystal XRD and DFT calculations.
3
)
2
1
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1
1
1
1
1
1
1
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3
3
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1
1 (DFT)b
2
3
3 (DFT)b
Mn(1)-N(1)
Mn(1)-N(2/5)
Mn(1)-N(6/9)
N(3/7)-N(4/8)
N(2/4)-N(3/5)
N(6/8)-N(7/9)
a
1.786(4)
1.873
1.859(2)
1.9567(19)
2.198
1.851(3)/1.850(4)
1.853(4)/1.848(3)
1.281(6)/1.290(6)
1.327(5)/1.333(6)
1.328(6)/1.336(5)
1.869/1.877
1.875/1.881
1.299/1.303
1.310/1.306
1.304/1.305
1.874(2)/1.861(2)
1.867(2)/1.864(2)
1.300(3)/1.301(3)
1.338(3)/1.336(3)
1.340(3)/1.337(3)
1.844(2)/1.860(2)
1.848(2)/1.8598(19)
1.302(3)/1.299(3)
1.349(3)/1.338(3)
1.353(3)/1.349(3)
1.877/1.877
1.892/1.892
1.271/1.291
1.339/1.339
1.319/1.319
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Divided entries refer to separate, related atoms and their associated metrics in the order given, e.g., N(3/7)-N(4/8) denotes two bonds:
N(3)-N(4) and N(7)-N(8). Structures optimized using the MN15 functional and aug-cc-pVTZ basis set.
b
state59 and more thorough electronic structure investigation is
warranted.
Scheme 1
Figure 2. Thermal ellipsoid plots of (OIM)Mn(L) complexes.
The apical ligand is assigned as a protonated amide ligand
Ellipsoids set at 50% probability level. C-H hydrogen atoms
t
-
-
based upon the apical bond length between Mn and the tert-
butylamide nitrogen of 1.788 Å, signifying a monoanionic
apical ligand with partial double-bond character. Further, the
3 2
omitted for clarity. 1: L = [NH Bu] , 2: L = [N(SiMe ) ] , 3: L =
t
NH(CN
4 2
Bu ).
Results and Discussion
-1
presence of an N-H stretch at 3200 cm in the infrared spectrum
Synthesis and Characterization of 1 and 2. We have
previously shown that coordination of Mn(II) precursors in situ
by the strong σ-donor tert-butyl amide destabilizes the low
signifies the presence of an amide proton, as does the successful
crystallographic refinement of a single proton on the apical
amido nitrogen atom. While charge balance suggests a formal
oxidation state of Mn(V), this is a relatively uncommon
oxidation state. Even more uncommon is an isolable Mn(V)
complex without a terminal multiply bonded imide, oxide, or
2
3, 54, 55
oxidation state, favoring metal oxidation.
Reaction of 1,3-
mixture of
5
6
diazidopropane
with
a
5
7
bis[bis(trimethylsilyl)amidomanganese(II) and excess tert-
butyl amine results in assembly of a formally manganese(V)
Werner complex of the OIM ligand with an apical tert-
5
, 60-64
nitride ligand.
1 does not have a discernible NMR
spectrum, suggesting the resonances are relaxed by a
paramagnetic metal center. Evans method magnetic
measurements gives a μeff of 2.37 Bohr magneton) most
consistent with a high-spin S = 1 system (spin only value of μeff
= 2.83 Bohr magneton). In acetonitrile-d3 solution, interaction
of the acetonitrile residual protiosolvent with the open axial
coordination site in a 20 mM solution of 1 relaxes the
protiosolvent pentet to a singlet, suggesting a fast-relaxing, and
rapidly fluctuating magnetic S = 1 system. Mn(V) complexes in
analogous planar macrocyclic templates are most frequently
low-spin due to high-lying dxz and dyz orbitals in the presence of
strongly π-donating axial ligands. In this system, such a triplet
V
t
butylamido ligand, (OIM)Mn (NH Bu) (1, Scheme 1, Figure 2)
in 50% yield. The OIM ligand presumably forms from the well-
23, 31, 34, 35
established
condensation of two azides into tetrazene
(Scheme 1). X-ray
with concomitant extrusion of N
crystallographic analysis (Table 1) shows that the central N-N
bond on each side of the macrocycle is slightly shorter (1.28-
.29 Å) than the outer N-N bonds (1.32-1.34 Å). Such a “long-
short-long” pattern is often associated with the fully reduced
2
1
5
8
oxidation state of this potentially redox-non-innocent ligand,
though these particular bonds are all still in an intermediate
range between the expected lengths of N-N single (~1.47 Å) and
double (~1.24 Å) bonds, suggesting the possibility of redox-
non-innocent radical character on the ligands. Further, N-N
bond lengths are not a conclusive measure of ligand redox
65
electronic state is less common. An EPR signal in parallel
mode was not seen, which may be due to large zero-field
splitting. However, the existence and magnitude of 3s splitting
in the Mn XPS provides spectroscopic support for the spin state
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which arises from proposed redox non-innocence in the ligands
assignment (vide infra). Compound 1 has a low reduction
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potential of -718 mV vs. ferrocene as evidenced by a
quasireversible signal in the cyclic voltamagram (see
supporting information). For a metal-based reduction of a
Mn(V) ion, a higher reduction potential would be expected. For
instance, even with high-oxidation-state-stabilizing corrolazine
ligand and triply-bonded axial oxo ligands, the Mn(V)=O
of 1. This is in contrast to Mn-O calibrants, for which electronic
spin is relegated entirely to the metal ion, and the spin state is S
= 2. 3s splitting is the result of two possible ionization products
in paramagnetic manganese ions resulting from the ionization
of either the α- or the β-3s electron, which gives a product with
the remaining 3s electron either paired with the 3d electrons
(Hund-type system) or against (non-Hund). Thus, the value of
3s splitting is largest for traditional S=5/2 weak-field Mn(II)
6
6
system of Goldberg had a reduction potential ~320 mV higher
than 1. The low reduction potential of 1 thus suggests the
reduction event may occur at a Mn(V) ion, and may not even be
metal-based. The preponderance of unusual features for a
Mn(V) mononuclear system suggests the likelihood that the
complex is better described by an alternative valence tautomer,
and that the redox non-innocent ligand may hold one or more
redox equivalents.
7
1
complexes due to the increased exchange energy for the spin-
aligned Hund-type product, and the increased electron-electron
repulsion for the non-Hund product. In Mn-O calibrants, this
splitting decreases as the Mn atom is oxidized to higher
oxidation states due to the corresponding decrease in spin
quantum number and resultant decreased exchange energy for
the Hund-like configuration, and decreased electron-electron
repulsion for the non-Hund configuration. However, since
compound 1 is not a traditional S = 2 Werner-type Mn(III)
complex with closed-shell ligands, but rather a paramagnetic
complex with an open-shell redox non-innocent ligand, the
normal expectations for the 3s splitting will not necessarily hold
if the ligand based SOMO orbitals have significant metal
character. The measured spin state of S = 1 based on solution
magnetic susceptibility suggests the ligand radicals are
antiferromagnetically coupled to the metal center. This
decreased overall spin is expected to decrease the 3s splitting
due to lower exchange energy, which explains the low
measured splitting of 4.6 eV. Thus, the 3s and 2p regions of the
Mn XPS are consistent with a Mn(III) ion antiferromagnetically
coupled to ligand-based radicals. Further, these observations
provide a precautionary tale on the use of 3s splittings to assign
metal oxidation states in complexes with redox non-innocent
ligands: 3s splitting in non-innocent complexes cannot be
directly compared to Mn-O calibrants due to differences in
quantum spin.
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An analogous synthesis performed with
a smaller,
stoichiometric amount of tert-butylamine resulted in the
isolation of the analogous compound with an apical
trimethylsilylamido ligand in place of tert-butyl amide,
V
(
OIM)Mn (N(SiMe
3
)
2
) (2, Scheme 1, Fig. 2) albeit in only
trace yields. Bond metrics in the macrocyclic 2 for the inner and
external N-N bonds (1.29-1.30 and 1.33-1.35 Å respectively)
are similar to those in 1 (Table 1) again suggesting possible
intermediate N-N bond order, though the apical Mn-N bonds
are slightly longer, at 1.84-1.86 Å, but still indicative of a
monoanionic axial nitrogen.
Given the unusual spin state and structural features in these
compounds, we examined the electronic structure of the
manganese atom in more detail using X-ray photoelectron
spectroscopy (XPS) to measure the binding energies of the core
2p, and the 3s electronic splitting, which are often invoked as a
means to identify manganese oxidation state. In this analysis,
higher oxidation states correspond to larger 2p electron binding
energies and smaller 3s electronic splitting, for instance, in
6
7
manganese oxides. The complexes presented here are not
oxides however, and the covalency of the metal ligand bond
does slightly affect the position of the 2p signals, with increased
68
3/2
M-L covalency tending to decrease binding energy. The 2p
XPS peak of 1 occurs at 641 eV, (Figure 3) and its low energy
and shape are inconsistent with Mn(IV) or higher oxidation
6
7
states. While this value of binding energy is also slightly low
for Mn(III) oxides, the absence of the satellite shakeup feature
6
7
at 647 eV suggests it is not assignable as Mn(II). The signal is
thus most consistent with a Mn(III) ion whose donor ligands are
stronger than oxide; similar ionization energies have been seen
for Mn(III)-acetonylacetate-type complexes with electron-rich
6
8
(
tert-butyl) donating groups on the ligand periphery, and for
6
9
surface Mn(III)-nitride species. The spectrum is thus an
excellent match to the expected position for nitrogen-ligated
Mn(III) in 1. Correspondingly, fitting of the 2p spectra to the
3/2
expected multiplet patterns in Mn according to the method of
7
0
Nesbitt gave a superior fit for Mn(III) (see SI). The
assignment of a Mn(III) metal center therefore suggests a
partially oxidized dianionic diradical OIM ligand.
The splitting of the Mn 3s XPS signal is related to the number
of unpaired electrons, and is frequently cited as another measure
of manganese oxidation state. Despite poor signal to noise ratio
in the 3s region of the spectrum, a splitting of approximately 4.6
eV is measurable (Figure 3), and somewhat lower than the
expected splitting (5.5 eV) in Mn(III) oxide calibrants, and
Figure 3. XPS spectra showing ionization potentials of core
manganese electrons. Top: 2p spectral region, showing diagnostic
3
/2
2p signature at 641 eV. Bottom: 3s spectrum (black) with two-
7
1
Lorentzian fit (red) showing an approximate 3s electronic splitting
of 4.6 eV.
closer to the value of Mn(IV) calibrants. This discrepancy is
explainable by virtue of the experimental S = 1 spin state of 1,
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Density Functional Theoretical Calculations on 1. While 1
Page 4 of 12
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might be formally described as a Mn(V) complex with a
dianionic tetrazadiene ligand, several unusual experimental
features discussed above suggest this description may be
electronically invalid. These observations include the absence
of a strongly π -donating axial ligand, an unusual S = 1
electronic spin state, OIM N-N bond lengths indicative of
intermediate bond order, and XPS spectra all suggestive of the
possibility of ligand redox non-innocent behavior. For further
support we examined the electronic structure using density
functional theoretical methods. The structure of 1 was
calculated in the gas phase with the unrestricted MN15
functional and a triple-zeta aug-cc-pVTZ basis set. A number
of physical parameters, notably the spin density localized on the
Mn and the Mn-apical N bond length, are sensitive to the
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2, 73
functional and basis set. The Supporting Information
provides full details about the downselection of a computational
method; the Minnesota family of functionals with a large
percentage of exact exchange and a large triple-zeta basis set
were found to be the most accurate for these systems.
Accordingly, the MN15/aug-cc-pVTZ combination is used for
all calculations presented here. Details are also given in the SI
on the examination of alternative spin states, which are closely
energetically spaced. Calculated bond lengths for 1 are
generally consistent with X-ray data (Table 1) with the
exception of the Mn(1)-N(1) apical bond. All DFT methods
examined overpredicted its length, and thus as a check,
electronic structure calculations were also performed for all
systems fixed at the experimental X-ray geometry. Calculated
N-N bond lengths in OIM otherwise agree reasonably well with
experiment and support an intermediate state between N-N
single and double bonds.
Figure 5. Four singly occupied d-character orbitals in 1.
The electron localization function (ELF)74 is a useful analysis for
the visualization of bonding and the assignment of oxidation state.
The ELF takes advantage of the Pauli exclusion principle to assign
basins with high probability of paired electrons, which correspond
to the locations of bonds, non-bonding “lone pairs”, or core
electrons, offering an analogy to Lewis theory. Further, the electron
count within these basins can serve as a computationally rigorous
analog of traditional electron counting approaches in inorganic
chemistry used for oxidation state assignment. The ELF for 1
(Figure 6) also supports a Mn(III) oxidation state. In complex 1
there are five disynaptic valence basins between Mn and the ligand
nitrogen atoms (Figure 6, green), which may be considered as
bonds, along with a single monosynaptic valence basin on the
apical N representing a nitrogen lone pair (Figure 6, blue).
However, there is significant overlap between the mono- and
disynaptic basins, and all are located between the Mn and the five
nearest nitrogens; this suggests these are all participating in the
Mulliken spin density and topological analysis suggests that
1
is best described as an Mn(III) center with an overall S = 1
spin state and non-innocent features on the OIM ligand. The
calculated spin density for the Mn atom in the optimized
-
4
III
complex is +3.88 e consistent with a d Mn ion, and
corresponding spin density plots are shown in Figure 4. A small
-
amount of spin (+0.03 e ) has delocalized from the Mn onto the
-
apical N. Close to 2 e of unpaired spin-down is localized on the
four nitrogens of the OIM ligand. When fixed in the
experimental geometry, the spin density decreases slightly to
-
+
3.64 e . Biorthogonalized orbital analysis was conducted and
bonding
process.
indicates the presence of four predominately d-character α -
4
SOMO orbitals, consistent with a d Mn(III) ion. Figure 5
shows the energies of these four orbitals relative to the HOMO
for the complex.
Figure 6. ELF for 1 (left) and 3 (right). The f = 0.88 isosurface is
shown. Core electron basins are shown in red, protonated valence
basins in cyan, monosynaptic basins (lone pairs) in blue and
disynaptic valence basins (bonds) in green.
Figure 4. Spin density plots of 1 (left) and 3 (right).
All six basins mentioned above appear to contribute to N-Mn
bonding and are significantly shifted away from the central
metal towards the nitrogen atoms. This, combined with the lone
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bond lengths for the interior and exterior N-N bonds of the
pair character of the monosynaptic basin, suggests that the bulk
1
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of the electron density within the basins is associated with the
nitrogen atoms. Thus, to a first approximation, all electrons in
these basins may be considered as either lone pairs or as ligand
donor pairs coordinating to the metal ion. The total population
macrocyclic OIM ligand are similar (1.3 and 1.34-1.35Å
respectively) to those of 1 and 2, though longer on average,
signifying possible reduction of the OIM ligand. The
intermediate values of the N-N bond lengths again suggest the
possibility of an intermediate redox state of the ligands, though
bond lengths alone are a poor indicator of exact ligand redox
state. With the neutral zwitterionic axial ligand, the reaction
stoichiometry suggests a one-electron reduction has occurred
concomitant with the protolytic removal of tert-butyl amide
(Scheme 1).
-
within these six basins in 1 is 17.72 e . Considering that each of
the five nitrogen atoms provides three valence electrons to be
paired in covalent bonds (15 total), then these population counts
5
9
-
indicate that the Mn has transferred 17.72 – 15 = 2.72 e to the
five ligand nitrogens in 1, and thus has a formal charge of +
2
.72, consistent with a Mn(III) metal center.
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PCET Reactions. Reactivity tests demonstrate 1 is capable of
The generation of this protonated, one-electron reduced
t
formally abstracting C-H, N-H, and O-H hydrogen atoms from
substrates with relatively weak X-H bonds, resulting in
reduction of the metal complex by a single electron and
concomitant protonation and labilization of the axial tert-butyl-
amide ligand as an amine. Motivated by our work in novel
energetic materials, we sought to generate a metal coordination
complex with greater nitrogen content by replacement of the
axial tert-butylamido ligand of 1 with a nitrogen-rich tetrazolate
ligand. The neutral, zwitterionic(1,3-di-tert-butyl-1H-tetrazol-
complex from NH(CN
4
Bu
2
) suggests the involvement of two
equivalents of the tetrazole reagent with different roles (Scheme
1): Half of the reagent performs a PCET reaction whereby the
tetrazolylamido proton protolyzes tert-butyl amide, releasing it
as tert-butylamine and extrudes a tetrazolyl radical, providing a
single electron, reducing the complex. The organic byproduct,
tetrazolyl radical, is analogous to common aromatic radicals
7
7-81
formed from phenols or aryl amines,
and is detectable by
EPR (Figure S4). To our knowledge, this is the first example of
a metal-mediated PCET reaction at a tetrazole, though reports
7
5
3
-ium-5-yl)amide reagent was intended to protolyze the axial
9
0-92
tert-butylamido ligand, releasing it as tert-butylamine, and
resulting in simple ligand exchange to give the isovalent
manganese complex with axially ligated (1,3-di-tert-butyl-1H-
tetrazol-3-ium-5-yl)imide. Instead, a PCET reaction ensued, as
evidenced by the appearance of detectable 1,3-di-tert-butyl-1H-
tetrazol-3-ium-5-yl radical by EPR spectroscopy (Figure S4),
the expected product of hydrogen atom abstraction (Scheme 1).
Density functional theoretical calculations on this di-tert-
butyltetrazolamide substrate (MN15/aug-cc-pVTZ) indicate a
bond dissociation free energy of 419 kJ/mol at 298 K, higher
of stable of tetrazolyl radicals exist.
the tetrazole-based reagent, being
The remaining half of
stronger-donating
a
zwitterionic amide ligand, simply replaces the now labile tert-
butyl amine ligand, and coordinates the Mn ato, giving
compound 3 in 42% yield according to equation 1:
V
t
tBu
(
OIM)Mn (NH Bu) + 2 NH(CN
4
2
) 
2
)N· + BuNH (1)
IV
tBu
tBu
t
(
OIM)Mn (NH(CN
4
2
) + (CN
4
2
3
Compound 3 is a high-spin S = / system based upon its EPR
2
7
6
than that of 2,6-di-tert-butylphenol (399 kJ/mol) as expected
due it’s being a stronger N-H bond, and electron-enriched due
to its monoanionic protonation state.
spectrum, which shows a rhombically distorted axial signature
(Figure 7). This spin state is possibly consistent with a Mn(IV)
3
d complex, however our past characterization of a related
2
3
manganese tetrazene complex also exhibited a S = 3/2
signature, but was better described as a Mn(III) complex with a
delocalized ligand radical. Though this previously reported
EPR signature exhibited a very different shape than that in
Figure 7, this is to be expected due to a different
pseudotetrahedral coordination geometry than that of square
planar 3. Thus, as in the case of 1, the electronic structure
warrants careful investigation.
As compound 1 was able to activate the N-H bond of the
aminotetrazole substrate, further substrate tests were performed
to explore what other bonds types could be activated by PCET.
As expected, tri-tert-buyl phenol, a common arene-based PCET
77-81
substrate
was converted to the phenoxyl radical by reaction
with 1 as detected by EPR spectroscopy (Figure S5). Finally,
the possibility of C-H activation by compound 1 was explored
by reaction with excess dihydroanthracene. Gas
chromatographic analysis of the reaction product mixture
shows the formation of anthracene, in 40% yield based on 1,
confirming the ability of 1 to activate the C-H bond of
dihydroanthracene (Figure S6). Other C-H bond activation at
8
2-86
87-89
high-valent manganese-oxygen
and manganese-imido
systems are worthy of note. Our system is unusual in that it
represents an example of a hydrogen atom transfer to an amido
(
not imido) ligand to form an amine-based product, tert-butyl
amine.
Characterization of product 3. Instead of the intended ligand-
exchanged product, the formally manganese(IV) complex, with
a protonated, neutral (zwitterionic) tetrazolamido ligand,
t
(OIM)Mn[NH(CN
4
Bu
2
)] (3, Figure 2) was isolated from the
di-tert-butyltetrazoliumamide. The
Figure 7. Experimental and simulated EPR spectra of 3. gx = 1.90,
gy = 2.01, gz = 4.43. MW Freq. 9.633014 GHz, MW power 20.12
mW, Mod. Freq. 100 kHz, Mod. Amplitude, 6.00 G.
reaction of 1 with
protonated state of the tetrazole ligand is evidenced by the
identification and successful refinement of a proton on the
apical amide nitrogen, and the relatively long contact with the
metal atom at 1.96 Å in comparison with the shorter apical
bonds in compounds 1 and 2 (1.79 and 1.86 respectively). The
Density Functional Theoretical Calculations on 3. While the
ground spin state of 3 is consistent with a simple Mn(IV)
tetrazene complex with a dianionic tetrazene ligand (Figure 1-
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left), it is worth noting that, like in the case of 1, the N-N bond
Page 6 of 12
Figure 8. Four singly occupied d-character orbitals in 3.
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lengths in the OIM ligand, though they exhibit the expected
long-short-long (1.34-1.30-1.35 Å) pattern of the fully reduced
ligand, possess values in an intermediate range between the
expected lengths of N-N single (~1.47 Å) and double (~1.24 Å)
bonds, suggesting the possibility of redox-non-innocent radical
character on the ligands. This would be consistent with our
Quantum Theory of Atoms in Molecules (QTAIM) analysis
is another useful tool for examining bonding environments in
donor-acceptor complexes from quantum calculations.
9
3
QTAIM uses basins of electron density to assign bond critical
points (BCPs) that reveal the locations and properties of bonds
from electron density, the Laplacian of electron density, and
energy density at these BCPs, and is particularly useful as a
measure of bonding analogy between structurally similar
complexes. QTAIM analysis reveals the presence of BCPs
between the manganese atom and the five closest nitrogen
atoms for both 1 and 3. For 1, density metrics at the BCPs
former report of
a four-coordinate neutral manganese
bistetrazene complex which was described as a Mn(III) ion with
2
3
delocalized radical character on the ligands.
Like 1, compound 3 was optimized with the unrestricted
MN15 functional and the triple-zeta aug-cc-pVTZ basis set.
Mulliken spin density and molecular orbital analysis indicate
that the manganese atom in 3 (like in 1) is most consistent with
a Mn(III) oxidation state with a delocalized ligand radical.
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indicate a weak, shared metal-ligand interaction with a negative
energy density 퐻푏 and a positive electron density Laplacian ∇2
휌푏. The four Mn-N bonds in 3 connecting the metal to the OIM
ligand are topologically almost identical to the five Mn-N bonds
in 1, demonstrating the electronic analogy between the two
complexes, despite the differing formal oxidation state. The
-
Calculated spin density on the Mn atom is +4.40 e , comparable
-
to the value of +3.88 e for 1, but slightly higher due to overall
4
reduction of the molecule, but sttll consistent with a d Mn(III)
ion. Spin density plots are shown in Figure 4. A small amount
of spin (+0.02 e for 3) has delocalized from the Mn onto the
apical N. As expected, the delocalization is smaller in 3 than in
BCP connecting the metal to the apical nitrogen in the weaker
t
NHCN
4
( Bu)
2
ligand, however, is different as expected. The
2
values for 휌푏 and ∇ 휌푏 are reduced by a factor of two and 퐻푏 is
reduced by nearly a factor of four. While this bond is chemically
similar to the other bonds, it is significantly weaker. Table 2
shows average metrics for the five Mn-N BCPs in 1 and the four
Mn-N OIM BCPs in 3 as well as the metrics for the Mn-apical
N BCP.
1
due to the increased bond length (2.20 vs. 1.87 Å). Roughly 1
-
e of spin-down density is present on the four nitrogens adjacent
to the Mn on the OIM ligand. Calculations fixed in the
experimental geometry show a spin density on the Mn atom of
+
4.37, very close to the value calculated at the optimized
geometry.
Table 2. QTAIM metrics for Mn-N bonds in 1 and 3.
As in the case with 1, biorthogonalized orbital analysis was
conducted and indicates the presence of four predominately d-
character singly-occupied orbitals for both 1 and 3, consistent
with a description of a Mn(III) center for both. The HOMO in
both cases has predominately Mn dz2 character. Thus, besides a
reordering of the deeper-lying d-orbitals, the electronic
structure at Mn remains surprisingly unperturbed following
one-electron reduction. Figure 8 plots the energies of these four
orbitals relative to the HOMO. Figure S10 compares the
biorthogonalized orbitals of 1 and 3 side-by-side.
∇ퟐ흆풃
BCPs
흆풃
푯풃
Mn-N (1)
Mn-N (3)
Mn-N1 (3)
0.1085
0.1151
0.0566
0.3888
0.3929
0.2180
-0.0353
-0.0399
-0.0101
All values given in atomic units.
The ELF for 3 also supports a Mn(III) oxidation state. In 3,
there are four disynaptic valence basins (bonds) and three
monosynaptic basins (lone pairs) between the metal and
adjacent nitrogen atoms, and like in 1, there is significant
overlap between the mono- and disynaptic basins, and all are
located between the Mn and the four nearest nitrogens; all are
participating in the bonding process. Figure 6 displays the ELF
for both complexes. As in 1, all basins contribute to N-Mn
bonding and are significantly shifted away from the central
metal towards the nitrogen atoms, and the bulk of the electron
density within the basins is associated with the nitrogen atoms.
The total population within seven basins in 3 is 18.47e
comparable to the total of 17.72e in 1). Subtraction of this
value from the theoretical 15 valence electrons available among
5 donor nitrogens gives a difference of 3.47e ,
corresponding to a formal charge of +3.47 on nitrogen, also
consistent with an Mn(III) metal center.
-
-
(
-
the
Between the comparable biorthogonalized orbitals, metal
spin densities, bond metrics, ELF, and QTAIM analyses, the
results are suggestive of a remarkably similar electronic
structure at the Mn atom between complexes 1 and 3, despite 3
being formally reduced by a full electron. Indeed, the spin- and
charge-density changes upon reduction of 1 to 3 by proton
coupled electron transfer suggest that the reduction is centered
on the redox non-innocent OIM ligand, not the metal center.
Thus, the operative PCET process involves a mechanism where
the proton is transferred to the axial tert-butyl amide ligand, and
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commercial sources (Aldrich, Strem). Anhydrous solvents such
the electron is transferred to the equatorial OIM ligand, with
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very little alteration of the electronic structure of the metal atom
itself, as evidenced by spin-density, QTAIM, and ELF analysis.
Related examples of hydrogen atom transfer reactions that do
not involve redox changes at the metal include recent work on
as acetonitrile, pentane were purified using an Innovative
Technology, Inc. Pure Solv. system. Tetrahydrofuran and
TM
hexamethyldisiloxane
benzophenone ketyl under a nitrogen atmosphere. BuNH
were
distilled
from
sodium
is
1,3-
t
2
9
4
57
Ru(V) porhyrinoids from the Goldberg group wherein a
hydrogen atom is transferred to the porphyrin ligand without a
change in oxidation number. Additionally, work in this group
involved the transfer of hydrogen atoms to a valence tautomeric
Mn(IV)-corrolazine radical cation where the electron reduces
the corrolazine ligand, and the axial oxo ligand becomes
protonated without a formal change in manganese oxidation
3 2
purchased from Aldrich and MnN(Si(Me )) ,
5
6
t
75
Diazidopropane, di- butylaminotetrazole were synthesized
according to literature protocol. Anhydrous solvents were used
1
throughout all experiments. H-NMR spectra were recorded on
a Bruker Avance 400 MHz spectrometer. FT-IR spectra were
-1
recorded in the range of 400-4000 cm on a Nicolet iS5 FT-IR
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with a iD5 ATR accessory. EPR spectra were recorded on a
Bruker EMX X-band spectrometer equipped with an Oxford
Cryosystems low temperature cryostat. Spectra were recorded
at 5 K. UV-visible spectra were recorded on a Shimadzu UV-
9
5
state. A related example from Abu-Omar exhibited two PCET
events where only one of the two electrons reduces the metal
center but both protons protonate the axial oxo ligand.96
1
800 UV spectrophotometer in the range of 200-900 nm. High
resolution mass spectrometry was performed on an Agilent
520 Accurate Mass Measurement Q-ToF mass spectrometer.
Conclusion
We report here the synthesis of an unsaturated octaaza
topological analogue of the TIM, Cyclam, and tetraazaannulene
6
Single crystal and powder X-ray diffraction were performed on
a Bruker KAPPA APEX II DUO diffractometer. Single crystals
were mounted on a MiTeGen loop using N-Apezion and
powder XRD, samples were caked onto the end of a glass fiber
using N-Apezion. Crystal data was collected and processed
ligands:
1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-
1,4,8,11-tetraido macrocycle (OIM). The ligand features two
redox non-innocent tetrazene-based bidentate ligand motifs
within a 14-member macrocycle. The formation of a pseudo
square pyramidal motif is observed, with a formal oxidation
state of Mn(V), but which is better described as a Mn(III)
complex with a partially oxidized OIM ligand diradical. PCET
to this complex may be initiated by reaction with various weak
O-H, N-H, or C-H donors. In the case of reaction with 2,4-di-
tert-butyltetrazolium-5-yl)amide, isolated is a formal Mn(IV)
complex that forms by PCET. Electronic structure analysis
indicates that this complex is also best described as a Mn(III)
complex, and that the reduction has occurred on the OIM
ligand, with the proton being transferred to the axial amide
ligand.
97
using the Bruker Suite and solved and refined using the
9
8
99
SHELXTL suite with Olex2 as a GUI. CHN lemental
analysis was carried out at Rochester University under the
CENTC instrumental facility. X-ray Photoelectron
Spectroscopy (XPS) was performed using a Thermo Fisher K-
Alpha+ with an Al-Kα monochromatic X-ray source. Data
collected for the Mn 2p3/2 and O 1s spectral regions were peak
fitted using Casa XPS software. To fit the Mn 2p3/2 region the
7
0
procedure used by Nesbitt was employed that takes into
account the multiplet structure resulting from the presence of
unpaired valence electrons in the 3d orbitals of manganese in
the birnessite sample. This fitting procedure relies on the
It is curious to note that in combination with our previous
2
3
report, there now exist a total of five separate manganese
tetrazene complexes over a range of three different oxidation
state (formally between III and V), yet electronic structure
analysis suggests that all complexes thus far reported are best
described as Mn(III) complexes with varying ligand oxidation
state. Future efforts will attempt to further elucidate the
mechanism of PCET, including whether electron transfer
precedes, coincides with, or follows proton transfer in this
PCET with divergent proton and electron movement.
theoretical calculations by Gupta of the expected XPS spectra
100, 101
from the three Mn(IV), Mn(III), and Mn(II) ions.
The
prior work showed that the calculated XPS spectra for the free
ions (each containing 5 multiplet peaks) allowed the accurate
fitting of experimental XPS Mn 2p3/2.. Peaks with a 50:50
Gaussian:Lorentzian contribution were used in the procedure
and the full width half maximum for each peak was between
0
.79 and 1.57 eV. Using these parameters the spectral data was
fitted by varying the relative contribution of individual set of
multiplet peaks.
Experimental Section
Precautions
Synthesis
t
The low-carbon azide and tetrazene molecules are explosive
materials. They should be stored cold in solutions of
concentration of no more than 0.5 M, or isolated only in few-
milligram quantities. Shock, spark, heat, or in the case of 3 the
exotherm from exposure to air can result in explosion.
Weighing should be performed with a plastic spatula, with the
experimenter electrically grounded by a wire attached to the
wrist in contact with skin. Proper precautions should be taken,
including use of standard laboratory PPE plus explosion proof
mask, shield, Kevlar gloves, use of plastic spatulae, and
electrical grounding while handling these materials. All
syntheses were performed in microscale for maximum safety.
Synthesis of [Mn(OIM)(HN Bu)] (1)
3 2
MnN(Si(Me )) (0.0375g, 0.100mmol) was dissolved in 5mL
of pentane and followed by addition of 1,3-diazidopropane (0.1
g, 0.8mmol) resulting in a light-yellow solution. 1mL of tert-
butylamine was added to reaction mixture dropwise which
turned to dark brown solution. This reaction mixture is stirred
further 2.5 h and filtered through a 0.45 PTFE syringe filter.
The filtrate was recrystallized by vapor diffusion at room
temperature
in
a
double-vial
apparatus
with
hexamethyldisiloxane as precipitant. Dark brown crystals are
obtained after 5 days. The crystalline material was isolated by
decanting the mother liquor and washing with pentane. Yield:
General Methods
3
0.016g (0.050 mmol), Yield: 50% based on MnN(Si(Me ))2.
-1
FT-IR: [cm ] 3204 (N-H stretch) 2943, 2844, (C-H stretch);
421, 1394, 1336 (C-H bend) 1259, 1200 (M=NR stretch). Unit
Cell (XRD): Orthorhombic P. a = 7.5991(15) Å b = 9.5866(19)
All operations were performed under a rigorous dry,
anaerobic atmosphere of nitrogen gas using standard Glove Box
and Schlenk line techniques. All reagents were purchased from
1
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Page 8 of 12
3
Å c = 20.193(4) Å, V = 1471.1(5) Å . CHN analysis Calcd for
: C, 37.153%; H, 6.859%; N, 38.994%. Found: C,
36.976%, H, 6.512%; N, 38.815%.
Synthesis of [Mn(OIM)(N(SiMe
UV-Visible, EPR, HRMS, and FTIR spectra, gas chromatograms,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
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4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
C
10
H
22MnN
cyclic voltamagrams, XPS fitting, details of density functional
calculations, and crystallographic information files; also available
from CCDC under deposition numbers 1830593-1830595. This
material is available free of charge via the Internet at
http://pubs.acs.org.
9
3 2
) ] (2)
The synthesis of compound 2 is performed exactly as
compound 1 except that only 100µL of of tertiarybutylamine
was added. A solid, insoluble product was obtained in reaction
mixture after one day. The mother liquor was decanted and the
residue discarded. The mother liquor was transferred to a
double aparatus with hexamethyldisiloxane as precipitant. A
trace yield of light orange crystals is obtained in 2 days. Unit
Cell (XRD): Triclinic P. a = 8.3841(11) Å b = 15.402(2) Å c =
AUTHOR INFORMATION
Corresponding Author
*
Michael J. Zdilla – mzdilla@temple.du
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Author Contributions
The manuscript was written through contributions of all authors.
1
7.081(2) Å, α = 73.318(3)°, β = 81.288(3)°, γ = 89.959(3)°, V
3
=
2086.4(5) Å .
Funding Sources
Synthesis of [Mn(OIM)(C
9
H
19
N
5
)] (3)
Support of this work by the Office of Naval Research under award
number N00014-16-1-2055 and by the National Science
Foundation under award 1362016 is gratefully acknowledged.
DFT calculations were supported by the Office of Naval Research
Multi-University Research Initiative Grant No.
Compound 1 (0.010g, 0.031mmol) was dissolved in 3mL of
tetrahydrofuran followed by the addition of (0.017g,
.086mmol) of di-tert-butyltetrazole which gave a brown
solution. This reaction mixture is stirred for 24-48 hours and
filtered using a 0.45 PTFE filter and kept in vial containing
0
N0001417WX00357. The CENTC elemental analysis center at
Rochester University is supported by NSF under award number
0
pentane as diffusing solvent at low temperature of -35 C.
0
650456. NMR instrumentation at Temple University is
Brown needles are obtained after two days, and isolated by
decanting the mother liquor and washing with pentane. Weight:
0.006g (0.013 mmol), Yield: 40% based on compound 1. CHN
combustion analysis indicates low C and N, signifying
incomplete combustion. The identity was confirmed by high-
resolution mass spectroscopy and the purity of the crystalline
material confirmed using PXRD (See Supporting Information).
ESI-MS. Theory: 448.22057, Found: 448.2217. CHN analysis
supported by a CURE grant from the Pennsylvania Department of
Health. The XPS measurements carried out at the University of
Delaware surface analysis facility were supported by the NSF
under grant 1428149 and by the NIH NIGMS COBRE program
under grant P30-GM110758.
ACKNOWLEDGMENT
We thank Anastasia Gallo for assistance with Mass Spectroscopic
Measurements. We also thank Ian G. McKendry for helpful
discussions about XPS interpretations.
9
Calcd for C10H22MnN : C, 40.176%; H, 6.968%; N, 40.605%.
Found: C, 36.141%, H, 6.545%; N, 38.564%.
t
Reaction of [Mn(OIM)(HN Bu)] (1)
tert-butylphenol:
with 2,4,6-tri-
ABBREVIATIONS
0
.003g of compound 1 dissolved in 1 mL toluene followed
by addition of 0.012g of solid 2,4,6-tri-tert-butylphenol. After
hours, the reaction was analyzed by EPR spectroscopy for
phenoxyl radical.
PCET, proton-coupled electron transfer; OIM, 1;2;3;4;8;9;10;11-
octaazacyclotetradeca-2;9-diene-1;4;8;11-tetraido
TIM, 2,3,9,10-tetraazacyclotetradeca-1,3-diene
macrocycle;
macrocycle;
2
Cyclam, tetraazacyclotetradecane; DFT, density functional theory;
NMR, nuclear magnetic resonance; EPR, electron paramagnetic
resonance; XPS, X-ray photoelectron spectroscopy; QTAIM,
quantum theory of atoms in molecules; BCP, bond critical point;
HOMO, highest-occupied molecular orbital; SOMO, singly-
occupied molecular orbital; ELF, electron localization function;
ATR, attenuated total reflectance;
t
Reaction of [Mn(OIM)(HN Bu)] (1)
dihydroanthracene:
with
9,10-
0
.004g of compound 1 was dissolved in 2mL of acetonitrile
and followed by the addition of 0.001g of 9,10-
dihydroanthracene, stirred for 24 hours in inert atmosphere. The
crude reaction mixture was spiked with a known quantity of
naphthalene (typically 1.0-2.0 mg weighed on an analytical
balance) and this solution was analyzed by GC analysis.
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