Revealing the Structure of Transition Metal Complexes of Formaldoxime

Article abstract of DOI:10.1021/acs.inorgchem.0c03362

Aerobic reactions of iron(III), nickel(II), and manganese(II) chlorides with formaldoxime cyclotrimer (tfoH3) and 1,4,7-triazacyclononane (tacn) produce indefinitely stable complexes of general formula [M(tacn)(tfo)]Cl. Although the formation of formaldoxime complexes has been known since the end of 19th century and applied in spectrophotometric determination of d-metals (formaldoxime method), the structure of these coordination compounds remained elusive until now. According to the X-ray analysis, [M(tacn)(tfo)]+ cation has a distorted adamantane-like structure with the metal ion being coordinated by three oxygen atoms of deprotonated tfoH3 ligand. The metal has a formal +4 oxidation state, which is atypical for organic complexes of iron and nickel. Electronic structure of [M(tacn)(tfo)]+ cations was studied by XPS, NMR, cyclic (CV) and differential pulse (DPV) voltammetries, M?ssbauer spectroscopy, and DFT calculations. Unusual stabilization of high-valent metal ion by tfo3- ligand was explained by the donation of electron density from the nitrogen atom to the antibonding orbital of the metal-oxygen bond via hyperconjugation as confirmed by the NBO analysis. All complexes [M(tacn)(tfo)]Cl exhibited high catalytic activity in the aerobic dehydrogenative dimerization of p-thiocresol under ambient conditions.

Full text of DOI:10.1021/acs.inorgchem.0c03362

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Article
Revealing the Structure of Transition Metal Complexes of
Formaldoxime
Ivan S. Golovanov, Roman S. Malykhin, Vladislav K. Lesnikov, Yulia V. Nelyubina, Valentin V. Novikov,
Kirill V. Frolov, Andrey I. Stadnichenko, Evgeny V. Tretyakov, Sema L. Ioffe,
and Alexey Yu. Sukhorukov*
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ABSTRACT: Aerobic reactions of iron(III), nickel(II), and manganese(II) chlorides with formaldoxime cyclotrimer (tfoH3) and
1,4,7-triazacyclononane (tacn) produce indefinitely stable complexes of general formula [M(tacn)(tfo)]Cl. Although the formation
of formaldoxime complexes has been known since the end of 19th century and applied in spectrophotometric determination of D-
metals (formaldoxime method), the structure of these coordination compounds remained elusive until now. According to the X-ray
analysis, [M(tacn)(tfo)]+ cation has a distorted adamantane-like structure with the metal ion being coordinated by three oxygen
atoms of deprotonated tfoH3 ligand. The metal has a formal +4 oxidation state, which is atypical for organic complexes of iron and
nickel. Electronic structure of [M(tacn)(tfo)]+ cations was studied by XPS, NMR, cyclic (CV) and differential pulse (DPV)
3−
̈
voltammetries, Mossbauer spectroscopy, and DFT calculations. Unusual stabilization of high-valent metal ion by tfo ligand was
explained by the donation of electron density from the nitrogen atom to the antibonding orbital of the metal−oxygen bond via
hyperconjugation as confirmed by the NBO analysis. All complexes [M(tacn)(tfo)]Cl exhibited high catalytic activity in the aerobic
dehydrogenative dimerization of p-thiocresol under ambient conditions.
Given the unusual composition of metal-formaldoxime
complexes and their use in analytical chemistry and
extraction, numerous studies were performed to establish
their structure. In the 1960s, the composition of the metal-
formaldoxime anion was revised to [M(ONCH2)6]2−,13−15
which was shown to be general at least for M = Mn, Fe, Ni,
V, and Ce (Scheme 1a).16 Several metal-formaldoxime
complexes were isolated and characterized by elemental
analysis, UV−vis, IR spectra, and electrochemical meth-
ods.13−15 However, the structure of the [M(ONCH2)6]2−
dianion and related metal-formaldoxime species was a matter
of debate (Scheme 1b), and surprisingly, it was not
determined yet.15,17−19
INTRODUCTION
■
In 1898, Dunstan and Bossi1 observed that formaldehyde
oxime (H2CNOH) forms intensively dark-colored species
upon reaction with transition metal salts (Mn, Fe, Ni, Cu) in
alkaline solutions. In 1913, Hofmann and Ehrhardt2
succeeded in isolating some of these species, in particular
manganese, iron, and nickel complexes, for which chemical
formulas Mn(ONCH2)3, Na2Fe(ONCH2)5, and Na3Ni-
(ONCH2)6 were proposed based on elemental analysis.
These authors disclosed that oxygen was required to form
these complexes in order to oxidize the metal ion. In the
reaction with formaldoxime or its cyclotrimer tfoH3,
manganese gave the most intensive coloring, which allowed
the analytical determination of this metal at very low
concentrations.3−5 Since then, the formaldoxime method
has become one of the most reliable techniques for
colorimetric or spectrophotometric determination of man-
ganese in the presence of other metals.6−11 Moreover, the use
of formaldehyde oxime as an effective extracting agent of D-
metals from several types of ores, such as pyrolusite,
malachite, heterogenite, and deep-sea manganese nodules,
was suggested.12
In this paper, we succeeded in the preparation and full
structural characterization (X-ray, HRMS, NMR, IR, Raman,
Received: November 12, 2020
Published: April 7, 2021
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tfoH3 (taken in the form of stable hydrochloride salt tfoH3·
HCl), the same metal complexes are formed as with
formaldoxime.13 On the basis of these experimental facts,
we concluded that structure A is unlikely, and metal-
formaldoxime complexes might possess a hexahydrotriazinane
ring as the ligand. This was confirmed by DFT calculations at
the DFT-D3 BP86/jorge-atzp level of theory, which predicted
structure C to be more stable than A and B by 58.2 and 79.6
kcal/mol, respectively (for M = Fe, see Supporting
Information).
Scheme 1. Metal-Formaldoxime Complexes: Background
Synthesis of Formaldoxime Complexes. To verify this
hypothesis, we generated several metal-formaldoxime com-
plexes Na2[M(ONCH2)6] (M = Mn, Fe, Ni) following the
13
́
̌
̌
procedures of Okac and Bartusek and Andersen and
Jensen17 from tfoH3·HCl or formaldoxime. However, these
compounds proved to be amorphous solids, which underwent
decomposition to form insoluble material upon attempts to
prepare suitable crystals for X-ray analysis. Our attempts to
get crystalline complexes Cat2[M(ONCH2)6] containing
organic cations derived from amine bases (Et3N, py) were
also not successful. Moreover, in the reaction of FeCl3 with
tfoH3·HCl and pyridine (py), a blue complex Fe(ON
CHNO)2py2 (1) was isolated in a small yield (Scheme 2).
Scheme 2. Formation of Iron(II) Nitrosolate Complex 1
̈
XPS, and Mossbauer spectroscopy) of closely related
complexes [M(ONCH2)3L]+Hal− (L, neutral polycyclic
triamine) derived from manganese, iron, and nickel. These
experimental studies together with high-level DFT calcu-
lations provide insights into the structure of metal-formal-
doxime complexes, which have been known for more than
120 years. Our results also provide background for the design
of structurally well-defined high-valent D-metal complexes for
applications in analytical chemistry and aerobic oxidation
catalysis.
RESULTS AND DISCUSSION
■
Three possible constitutional isomers shown in Scheme 1b
have been proposed for the dianion [M(ONCH2)6]2−.17−19
In structure A, the metal ion is coordinated by six
formaldoxime anions. In the other two structures B and C,
the deprotonated cyclic trimer of formaldoxime serves as a
ligand through coordination with nitrogen or oxygen atoms.
Moreover, isomeric structures with mixed coordination
modes of the ligand cannot be ruled out. In most of the
literature, structure A is postulated, yet some evidence in
support of structures B18 and C19 was obtained from IR
spectra.
According to the X-ray analysis of complex 1, iron(II) was
coordinated by two deprotonated nitrosoformaldoximes
(nitrosolate anions21) and two pyridine molecules. The
formation of nitrosolate complexes in reactions of formal-
doxime or tfoH3 with transition metals was never observed
previously. Interestingly, tfoH3 or formaldoxime are not
oxidized to nitrosoformaldoxime with O2, suggesting that an
iron-promoted process may take place.
Since the lability of [M(ONCH2)6]2− complexes could
be attributed to the formation of oligomeric and polymeric
species, we reasoned that substitution of three formaldoxime
anions for a tripodal capping ligand could result in
stabilization of the complex. To test this idea, 1,4,7-
triazacyclononane (tacn) was used as a capping ligand. We
were happy to find that the reaction of Na2[Fe(ONCH2)6]
with tacn·3HCl in methanol afforded an isolable deep violet
crystalline complex [Fe(tacn)(ONCH2)3]Cl (2, Scheme 3).
A remarkable experimental observation is that [M(ON
CH2)6]2− complexes are formed only with formaldoxime,
while substituted oximes (even acetaldehyde oxime) are inert.
Recently, when studying the complexation of oximes with
boronic acids,20 we have shown that substituted oximes do
not undergo reversible cyclotrimerization as readily as
formaldoxime does.20 Furthermore, with formaldoxime trimer
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In a similar fashion, an intensively wine-colored manganese
complex [Mn(tacn)(tfo)]Cl (4) was prepared from Mn-
(OAc)3 or MnCl2 under air. The composition of 4 was
established on the basis of HRMS and elemental analysis.
Although the crystals were of poor quality, the X-ray analysis
showed that the manganese ion is coordinated by three
oxygen atoms of formaldoxime trimer as in the corresponding
iron and nickel complexes 2 and 3. Attempts to get better
crystals by the metathesis of the anion were not successful.
We, therefore, tried to use other tripodal N3-ligands (1,3,5-
trimethyl-1,3,5-triazacyclohexane, 1,4,7-triazacyclononane,
1,5,9-triazacyclododecane, trispyrazolylborate).23 Finally, we
were able to get a good quality X-ray structure by using the
N,N′,N″-trimethyl-1,4,7-triazacyclononane (Me3tacn) as a
capping ligand (complex 5, Scheme 5).
Scheme 3. Synthesis of Iron(IV)-tfo Complex
[Fe(tacn)(tfo)]Cl (2)
Scheme 5. Synthesis of Manganese(IV)-tfo Complexes
[Mn(tacn)(tfo)]Cl (4) and [Mn(Me3tacn)(tfo)]Cl·MeOH
(5)
The same complex was prepared in 42% yield directly from
FeCl3 by the reaction with hydrochlorides of tacn and tfoH3
in the presence of a proton sponge (1,8-bis(dimethylamino)-
naphthalene) under air. The use of formaldoxime (generated
in situ from formaldehyde and hydroxylamine) instead of its
trimer tfoH3 in this reaction led to the same result. The
composition of complex 2 was proved by HRMS (electro-
spray ionization) and elemental analysis. UV−vis and FT-IR
spectra of 2 were similar to those reported previously for
Cs2[Fe(ONCH2)6] (see the Supporting Information),
indicating the presence of the same structural motif in
these two complexes. For crystals of complex 2, the X-ray
analysis could be performed, which revealed that the iron
atom is coordinated not by formaldoxime (as postulated
previously) but by its trimer (tfo). The N−OH groups of the
ligand are fully deprotonated, and oxy groups are bonded to
iron forming an adamantane-like structure [Fe(tacn)(tfo)]+.
Intriguingly, the formal oxidation state of iron is +4 in this
complex, assuming that tfo ligand is a trianion (vide infra).
Inspired by this result, we then tried to prepare mixed tfo-
tacn complexes with other D-metals. However, attempts to
isolate tfo-tacn complexes with cobalt, copper and vanadium
failed.22 Surprisingly, the reaction of NiCl2 with tacn and
tfoH3 hydrochlorides in the presence of a proton sponge in
methanol under air afforded a stable brown-colored nickel
complex [Ni(tacn)(tfo)]Cl·MeOH (3, Scheme 4). According
to the X-ray analysis, this complex had the same metal-
loadamantane motif (nickel coordinated by three N-oxy
groups of tfo3−) as the parent iron complex 2.
It is noteworthy that metal(IV)-tfo complexes were not
formed under the argon atmosphere, indicating that oxygen is
required for the oxidation (vide infra).
The obtained metal-tfo complexes 2−5 are well-soluble in
methanol, water, and DMF and poorly soluble in Et2O, THF,
and CH2Cl2.
X-ray Structure of Metal-tfo Complexes, FT-IR and
NIR Spectroscopy. The geometry and electronic structure
of complexes 2−5 are unprecedented and require special
discussion. It is surprising that at least three D-metals with
different electronic configurations tend to form stable
complex cations of the same composition [M(tacn)(tfo)]+
with a formal +4 oxidation state of the metal. Stable
nonbiomimetic high-valent iron(IV)24−31 and nickel(IV)32−38
complexes with organic ligands are scarcely known in the
literature. Moreover, the formation of high-valent nickel and
iron species by oxidation with molecular oxygen is extremely
rare.24,31 To our knowledge, compounds 2 and 3 are the first
structurally characterized organic nitroxy complexes of
iron(IV) and nickel(IV), which are formed under ambient
conditions by a spontaneous oxidation with air. Somewhat
related iron(IV) complexes with macrocyclic amides such as
TAMLs28−30 and hexahydrazide cages24 have been reported
in the literature. However, these complexes are stabilized by
donor effects of deprotonated amides as well as by a shielding
effect of clathrochelate,24 which is not the case in tfo
complexes.
Scheme 4. Synthesis of Nickel(IV)-tfo Complex
[Ni(tacn)(tfo)]Cl·MeOH (3)
The geometrical parameters of [M(tacn)(tfo)]+ cations are
very similar for manganese, iron, and nickel (Figure 1). All
three structures consist of a distorted metalloadamantane
framework formed by coordination of the metal ion with
three axial (with respect to triazinane ring) oxy groups of the
deprotonated tfoH3 ligand. The other three coordination
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Figure 1. Principal structural motifs and details of geometries around a metal center in complexes 2, 3, and 5. Hydrogens at carbon atoms are
omitted for clarity.
vacancies on the metal are occupied by the triazacyclononane
ligand (tacn or Me3tacn). The coordination geometry around
the metal ion is close to an ideal octahedron, as follows from
the angles of distortion toward a trigonal prism in the range
49.41(4)−51.54(10) (φ = 0° for an ideal trigonal prism, φ =
60° for an ideal octahedron). To better quantify this
distortion, continuous symmetry measures were used39 that
measure how close the shape of the coordination polyhedron
is to a reference shape, such as an ideal octahedron (OC)
and an ideal trigonal prism (ITP). For all the complexes
synthesized, the symmetry measures S(OC) and S(TPR)
evaluated from the X-ray diffraction data confirm a very small
deviation of the shape of the coordination polyhedron from
an ideal octahedron hinted by the above distortion angles
(Table 1).
Table 1. Main Geometric Parameters and Continuous
Symmetry Measures As Obtained from X-ray Diffraction
at 120 K for 2, 3, and 5
a
2
3
5
ϕ (deg)
h (Å)
S(OC)
S(TPR)
50.91(17)
2.3353(3)
0.172
51.54(10)
2.3435(12)
0.100
49.41(4)
2.4339(10)
0.295
16.185
15.784
14.847
a
φ is the distortion angle of the OC-TPR polyhedron, h is the height
of this polyhedron; S(OC) and S(TPR) are octahedral and trigonal
prismatic measures, respectively.
Å), and N−O bonds are considerably shorter (1.345−1.352
Å) compared to manganese and iron complexes. Axial N−O
bonds in the ligand are longer (1.455 Å) compared to metal-
tfo complexes as shown by X-ray analysis of tfoH3·HCl.
Remarkable features of all three structures are reduced
pyramidality of nitrogen atoms (deviation of nitrogen from
The M−O and N−O bond distances in iron and
manganese complexes are close and lie in the range of
1.86−1.89 Å and 1.39−1.41 Å, respectively. In [Ni(tacn)-
(tfo)]+, the Ni−O bonds are somewhat longer (1.909−1.917
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Figure 2. Curve fitted Mn 2p, Mn 3s, Ni 2p, and Fe 2p photoelectron lines for metal-tfo complexes 2, 3, and 5 (colored peaks show
deconvolution of the experimental spectra on individual components).
the mean plane formed by substituents is ca. 0.4 Å) and
shortening of N−O bonds compared to noncoordinated
derivatives of tfoH3 (1.42−1.47 Å40−42). These features are
usually an indication of hyperconjugation interactions
involving a lone electron pair (n → σ* donation, vide
infra).43 Reduced pyramidality of nitrogen atoms in these
structures can also be explained by the distortion of bond
angles C−N−O attributed to the presence of a large D-metal
in the adamantane cage.
indicative of the spin density being mostly localized on the
metal ion, thus ruling out the existence of the nitroxide
radical moiety in the molecule.
The magnetic susceptibilities χT of the iron and
manganese complexes (1.2 and 1.9 cm3·mol·K−1 respectively)
estimated by the Evans method48 (see Table S3 in
Supporting Information), often used for this purpose, are
close to spin-only values for S = 1 and S = 3/2 (1.00 and
1.87, respectively). This corresponds to two unpaired
electrons for a d4 configuration of the iron(IV) ion and to
three unpaired electrons for a d3 configuration of the
manganese(IV) ion, thus confirming the oxidation state of
+4 for the metal ions for both paramagnetic complexes.
FT-IR spectra of [M(tacn)(tfo)]+ complexes display similar
features. Characteristic bands attributed to CH2 stretching,
CH2 bending, CH2 twisting, C−N−C stretching, N−C−N
bending, and M−O stretching were observed. In all spectra,
two bands appearing at ca. 1010−920 cm−1 are the most
intensive. On the basis of the isotope frequency shifts in
complexes having 15N-labeled and deuterated tfo ligands,
DFT calculations, and previous IR studies on [M(ON
CH2)6]2− complexes,19 the higher-frequency band was
attributed to an almost pure N−O stretching, while the
lower one corresponds to CH2 rocking (see Tables S4−S6 in
the Supporting Information). The position of ν(N−O)
stretching frequency is characteristic for a single nitrogen−
oxygen bond observed in hydroxylamanines44 and metal
hydroxamates45−47 (1100−900 cm−1). In the NIR region
(1100−2000 nm), no charge transfer bands were observed.
NMR Spectroscopy and Evans Method. The low-spin
nature of the nickel(IV) complex with a d6 electronic
configuration was confirmed by its diamagnetic NMR spectra.
1H and 13C NMR spectra show clear signals of coordinated
tfo and tacn having diastereotopic CH2−protons. In contrast,
the NMR spectra of iron and manganese complexes were
dominated by the paramagnetic effects (see section 7 in
Supporting Information), i.e., the contact shifts that depend
on the spin density distribution in a molecule and the
pseudocontact shifts that arise from dipole−dipole electron−
nucleus interactions. While a full assignment of these signals
is outside the scope of this manuscript, the signals being
relatively narrow even for the manganese complex are
̈
XPS, Mossbauer Spectroscopy, and DFT Studies of
Metal-tfo Complexes. Additionally, the electronic structure
of [M(tacn)(tfo)]Cl complexes was studied by X-ray
̈
photoelectron spectroscopy (XPS), Mossbauer spectroscopy,
and DFT calculations. XPS data confirmed the composition
of complexes 2, 3, and 5 and the presence of key structural
motifs (see Supporting Information). XP spectra with respect
to the metal 2p and 3s regions are shown in Figure 2. In all
complexes, the metal atom is present in a single oxidation
state; i.e., the samples were redox pure. Although there is not
much XPS data for high-valent metal complexes with organic
ligands,49 some evidence on the metal oxidation state can still
be obtained. For complex 5, the binding energy at maximum
peak height Mn 2p3/2, doublet splitting (2p3/2−2p1/2 = 11.6
eV), and the position and intensity (27% of Mp2p main line)
of satellite peaks are consistent with known high-valent
manganese complexes, in particular, Mn(IV) complexes with
amino alcohol ligands,50 Ni[Mn(CN)6],51 and K3[Mn-
(CN)6)],51 as well as with some Mn(III) complexes such
as β-diketonates.52 The position of Mn 3s peaks is
intermediate between Mn(IV) and Mn(III) complexes with
N,O ligands.49,53 For nickel complex 3, the positions of main
and satellite peaks in the Ni 2p XPS spectrum are very
similar to KNiIO6 (855.4, 861.1, 873.0, 880.2 eV54), in which
Ni(IV) has an octahedral environment and the Ni−O bond
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length (1.88 Å) is close to [Ni(tacn)(tfo)]Cl. The observed
2p3/2 binding energy values are somewhat smaller than those
reported for other high-valent inorganic compounds of nickel,
such as K2NiF6 and Ni2O3.49 The Fe 2p XPS spectrum of
[Fe(tacn)(tfo)]Cl is more consistent with inorganic Fe(III)
compounds.49,55 However, the electronic effect of the organic
ligand is known to influence the binding energy state of the
electron significantly by changing the effective charge on the
metal atom.56 Thus, a direct comparison of XPS data for iron
and nickel tfo complexes with high-valent inorganic
compounds of these metals may not be reliable for
interpreting the oxidation state.
and 20.7 kcal/mol, respectively. The optimized geometry of
the triplet cation [Fe(tacn)(tfo)]+ is in excellent agreement
with the experimental X-ray data, while the high-spin
structure shows significant deviations (see Supporting
̈
Information). Calculated Mossbauer isomer shifts and
quadrupole splitting ΔEQ calculated both for DFT-optimized
(S = 1) and X-ray structures are in good agreement with
experimental data (Table 2). Calculated δ and ΔEQ values for
̈
Table 2. Experimental and Calculated (DFT) Mossbauer
Parameters of [Fe(tacn)(tfo)]Cl (2)
approach
S
T, K
δ (mm/s) ΔEQ (mm/s) Γ (mm/s)
̈
For iron complexes, Mossbauer spectra are more
experiment
experiment
295
90
0.155(1)
0.218(1)
0.19
0.20
0.16
|3.007(1)|
|3.007(1)|
−2.00
−2.21
−1.56
0.228(2)
0.256(1)
informative and reliable in interpreting the metal oxidation
57
̈
state. The Fe Mossbauer spectrum of [Fe(tacn)(tfo)]Cl (2)
a b
,
calculation
calculation
calculation
calculation
1
1
0
2
shows a quadrupole doublet with an isomer shift δ =
0.155(1) mm/s and a quadrupole splitting |ΔEQ| = 3.007(1)
mm/s at room temperature (Figure 3, Table 1). Given a
a c
,
a c
,
a c
,
0.39
−1.51
a
RHO was calculated with the B3LYP DFT functional using the
CP(PPP) basis set on Fe and the def2-TZVP basis set on other atoms.
The electric field gradient was calculated using the TPSS DFT
functional with the DKH-def2-QZVPP basis set on Fe and the def2-
TZVP basis set on other atoms was used. Relativistic effects were
taken into account by requesting a Douglas−Kroll−Hess second
b
order scalar relativistic calculation. Calculations were performed for
c
the X-ray structure. Calculations were performed for the DFT-
optimized structure (DFT-D3 level of theory with BP86 functional
was used for geometry optimization and calculations of thermochem-
istry).
low and high-spin structures substantially differ from the
experiment (Table 2). According to a Mulliken population
analysis of the triplet state structure, the spin density is
almost fully localized on the iron atom (1.79 of 2 electrons,
Figure 4a). These computational data provide strong
evidence for the correct assignment of the ground state
electron configuration close to d4 for the iron atom.
̈
Figure 3. Mossbauer spectra of [Fe(tacn)(tfo)]Cl (2).
distorted octahedral coordination geometry of [Fe(tacn)-
Figure 4. Calculated spin density in iron(IV)-tfo and manganese-
(IV)-tfo complexes 2 and 5.
̈
(tfo)]Cl, the observed hyperfine parameters of Mossbauer
spectra are consistent with the values expected for the triplet
Fe(IV) complexes.24,26−28,57,58 A large |ΔEQ| indicates a
relatively enhanced value of the electric field gradient
presumably arising from deformation (e.g., stretching along
the axis of the gradient) of the coordination octahedron
around the Fe(IV) atom. No signs of decomposition were
Similar DFT calculations of manganese complex [Mn-
(Me3tacn)(tfo)]+ revealed that the high-spin complex (S = 3/
2) is more stable than the low-spin structure (S = 1/2) by
18.4 kcal/mol. The optimized geometrical parameters of the
high-spin complex are very close to those derived from the X-
ray analysis. The molecular orbital picture of this complex
̈
observed in the Mossbauer spectra of [Fe(tacn)(tfo)]Cl after
40 days at room temperature.
2
2
2
According to DFT calculations performed at the DFT-D3
level of theory with the BP86 functional, the triplet state was
predicted to be most stable for complex [Fe(tacn)(tfo)]+.
Hypothetical singlet and pentet states were less stable by 9.0
shows that three SOMOs are almost pure dz , dx −y , and dxy
Mn-derived orbitals (Figure 5). All three unpaired electrons
are localized on a manganese atom according to Mulliken
population analysis supporting the d3 electronic configuration
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Figure 5. Calculated SOMO orbitals of [Mn(Me3tacn)(tfo)]+.
Figure 6. Electrochemical data for a 1 mM dimethylformamide solution of the iron complex [Fe(tacn)(tfo)]Cl (2) with 0.1 M (n-
(C4H9)4N)PF6 as a supporting electrolyte: (a) cyclic voltammogram for the first reduction process, (b) cyclic voltammogram for the first
oxidation process.
(Figure 4b). Interestingly, DFT calculations revealed that the
mide/TBAP solution (Figure 6 and Figures S6−S8 in the
Supporting Information). For the iron and nickel complexes
2 and 3, one irreversible oxidation wave (with E0 equal to
0.23 and 0.21 V referenced to the Fc/Fc+ couple) and one
irreversible reduction wave (at E0 = −1.0 and −1.2 V) were
detected by CV voltammetry; the manganese complex 4
features several irreversible reductions (Figure S8 in the
Supporting Information). All the above waves show an almost
irreversible behavior at a scan rate of 0.1 V s−1; the increase
in the scan rate leads to a quasi-reversible behavior in the
case of the iron- and nickel-centered processes. The linear
dependence of the peak current on the square root of the
scan rate (Figures S6d, S7d in Supporting Information)
suggests a diffusion-limited process. Note that the observed
redox potentials are very close to those previously reported
for a clathrochelate iron(IV) complex with a similar trigonal
prismatic geometry.24 Thus, ligand tfo3− provides very
efficient stabilization of the highly oxidized metal center.
The Nature of Stabilization of High-Valent Metals in
Complexes with tfo Ligand. Geometrically, [M(tacn)-
(tfo)]+ cations are somewhat similar to complexes of
deprotonated all-cis-cyclohexanetriol and related ligands (in
particular, 1,3,5-triamino-1,3,5-trideoxy-inositol taci).62,63
However, these ligands do not form complexes with late D-
metals in high oxidation states. Due to the presence of M−
O−N motifs, [M(tacn)(tfo)]+ complexes can be considered
as analogs of metal hydroxamates, in particular hydroxamate
2
2
2
order of d orbitals (dz the lowest, then dx −y and dxy) is
different from typical octahedral splitting; the same splitting
caused by the trigonal symmetry is characteristic of
complexes with an encapsulated metal ion (clathrochelates59),
such as iron(IV),24 and often results in unusual magnetic
properties, such as giant magnetic anisotropy60 and single
molecule magnet behavior.61
For nickel complex [Ni(tacn)(tfo)]+, we were able to
localize only one structure with d6 configuration. This
structure has the singlet (S = 0) ground state and is in
good agreement with the X-ray data for complex 3.
Interestingly, two complexes having d7 and d8 configurations
with large spin densities on N−O groups were also identified
as local minima (see Supporting Information). The optimized
geometry of both high-spin complexes showed significant
distortion of the metalloadamantane unit, in which one M−O
distance was elongated (2.37 Å for d7, 3.50 Å for d8)
compared to the other two bonds (1.91 Å for d7, 1.98 Å for
d8). According to Mulliken population analysis, the longer
M−O distances are formed with nitroxide radical groups.
However, these high-spin complexes were separated from the
parent diamagnetic d6 complex by 22.6 and 41.3 kcal/mol,
respectively.
Electrochemical Properties. Redox properties for the
complexes obtained were accessed by cyclic (CV) and
differential pulse (DPV) voltammetries in a dimethylforma-
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sideophores.64 Although anionic hydroxamate ligands can
form M(IV) complexes, the latter are derived from early
transition metals.65−69 Therefore, tfo3− provides some
additional electronic stabilization for the high-valent metal
center.70 One could assume that tfo3− is a noninnocent
ligand, which is oxidized by the metal. Indeed, hydroxyl-
amines are known to be redox-active, forming corresponding
nitroxide radicals or oxoammonium cations upon oxidation.
Characteristic examples are nitroxide species TEMPO,
AZADO, PROXYL, PINO, etc.71 From this viewpoint,
three canonical structures A1, B1, and C1 could be proposed
to describe the electronic structure of [M(tacn)(tfo)]+
complexes (Figure 7). These structures differ in the formal
to the structure of [M(tacn)(tfo)]+ complexes. This is also
confirmed by DFT calculations, which show very low spin
density on the nitrogen and oxygen atoms (see Figure 4 and
Supporting Information).
As for the canonic structure C1, the formation of a chelate
would lead to a severe distortion of the sp2 geometry of the
nitrogen atom. For this reason, we believe that the
contribution of the C1 form in the structure of [M(tacn)-
(tfo)]+ is also small. On the other hand, canonical structure
C1 clearly demonstrates the mechanism of additional
stabilization of the D-metal ion by donation of the electron
density from the n-orbital of nitrogen to metal d orbitals. The
experimentally observed flattening of nitrogen atoms can be a
consequence of these anomeric interactions. Hyperconjuga-
tion effects are known to be strong in organic adamantane
derivatives due to their conformationally rigid structure and
optimal geometry with staggered bonds.43,91,92 Indeed, NBO
analysis of [Ni(tacn)(tfo)]+ revealed the presence of strong
nN → σNi−O* and interactions nN → σC−N* with energies of
4.8 and 6.6 kcal/mol.
Thus, the stabilization of high-valent metals in [M(tacn)-
(tfo)]+ complexes can be explained by a strong σ-donor
capacity of three anionic N-oxy groups of tfo3− and additional
donation of the electron density on the metal center from
nitrogen atoms via hyperconjugation. These electronic effects
are multiplied due to a tridentate nature of the tfo3− ligand
and the formation of a stable conformationally rigid
adamantane structure.
Oxidation Reactivity and Catalytic Activity of Metal-
tfo Complexes. Given the high oxidation state of the metal
center, [M(tacn)(tfo)]+ complexes are expected to exhibit
oxidative properties. Indeed, iron(IV), manganese(IV), and
nickel(IV) complexes are known to be reactive intermediates
in biological and artificial catalytic oxidation processes.26,27
However, [M(tacn)(tfo)]+Cl− complexes were found to be
indefinitely stable in the solid state and in solution (methanol
and water). The UV−vis spectra monitoring showed no signs
of decomposition of complexes 2, 3, and 5 in methanol and
water at least within a week (c = 1 mM). More surprisingly,
they were relatively stable in the acidic medium even in the
presence of 165 equiv of trifluoroacetic acid. Complexes 2, 3,
and 5 were resistant toward the action of oxidizable organic
substrates such as hydroquinone, triphenylphoshine, and
naphthalene (2 equiv of reducing agent, UV−vis control).
However, with sodium ascorbate, dithionite, and sulfide,
substantial changes in UV−vis spectra were observed (see
Supporting Information). For example, the addition of 2
equiv of Na2S to the nickel complex 3 resulted in an
immediate disappearance of brown color. Surprisingly, after a
few minutes under air, the initial brown color was restored.
The recovery of the complex was confirmed by UV−vis
monitoring shown in Figure 8. Similar behavior was observed
for iron and manganese complexes 2 and 5.
Figure 7. Canonical structures of metal(IV)-tfo complexes.
oxidation state of metal (from +2 to +4) and the oxidation
level of the nitroxyl unit. Indeed, complexes of D-metals with
hydroxylamines, nitroxide radicals, and oxoammonium cations
having M−O−N, M−(*O−N), and M−(ON+) motifs,
respectively, have been described in the literature.72−79 In
structures B1 and C1, the tfo ligand exhibits a mixed-valence
state,80,81 which may be apparent in the N−O bond
stretching frequency in the IR spectra. The N−O bond
stretching frequency strongly depends on the oxidation level
of the nitroxide group (1000−900 cm−1 for hydroxylamines44
and metal hydroxamates,45,46 ca. 1350 cm−1 for nitroxide
radicals82 and ca. 1600 cm−1 for oxoammonium cations83,84).
The position of ν(N−O) at 1015−963 cm−1 in [M(tacn)-
(tfo)]+ complexes is indicative of a single nitrogen−oxygen
bond, while the mixed valency of the ligand would result in a
much higher frequency (intermediate between hydroxylamine
and nitroxide radical or oxoammonium cation).
In our view, A1, B1, and C1 can be better viewed as
resonance forms with the real structure of [M(tacn)(tfo)]+
being likely a superposition of these canonical structures (this
treatment makes the formal oxidation state of the metal
meaningless). The relative contribution of these forms is
different and may depend on the nature of the metal.
High-resolution mass spectra of the dark-red solution
obtained by the reduction of [Fe(tacn)(tfo)]Cl (2) with
sodium ascorbate contained a single cation [Fe(tacn)(tfo)-
H]+, which corresponds to a protonated iron(III) complex.
The same ion was observed in the reaction of FeCl3 with
The geometrical parameters of octahedral Mn, Ni, and Fe
complexes with nitroxide radicals substantially differ from
those observed in [M(tacn)(tfo)]+ complexes: M−O bond
distances are much longer (>1.9 Å), and N−O bonds are
considerably shorter (1.2−1.3 Å).75,77,85,86 By these structural
metrics as well as ν(N−O) frequencies, [M(tacn)(tfo)]+
cations are closer to complexes with reduced TEMPO,73,76
deprotonated hydroxylamines,87 hydroxamic acids,88 and N-
oxides of tertial amines.89,90 Thus, the nitroxide radical
structure B1 does not seem to make a significant contribution
tacn and tfoH3 under an inert atmosphere (along with
57
iron(IV) complex [Fe(tacn)(tfo)]+). The Fe Mossbauer
̈
spectra of both samples obtained by evaporation of these
solutions contained a quadrupole doublet with an isomer shift
of ca. δ 0.32 mm/s and a quadrupole splitting |ΔEQ| of ca.
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disulfide. Moreover, in the presence of N-tert-butylmethani-
mine N-oxide, spin-trapping of thiyl radical was not observed
by ESR. These experiments suggest that the oxidation of the
thiol group proceeds via an ionic mechanism rather than a
radical pathway.93
Catalytic aerobic oxidation of thiols received some
attention in recent years as it proves an efficient and waste-
less method for the production of disulfides compared to
conventional oxidizing reagents.93−97 Although numerous
aerobic methods have been developed based on noble and
non-noble metal catalysts, they usually require high catalyst
loadings,93,97 elevated temperatures,94−96 additives,94 or
photochemical activation.97 To the best of our knowledge,
iron complex [Fe(tacn)(tfo)]Cl (2) exhibits much higher
activity in the oxidation of p-thiocresol compared to
previously reported catalysts. We believe this can be a good
starting point to develop a new generation of non-noble
metal catalysts for aerobic oxidation of thiols and related
sulfur compounds.
CONCLUSIONS
Figure 8. UV−vis monitoring of the reaction between complex 3
and Na2S under an air atmosphere.
■
In conclusion, the structure of transition-metal-formaldoxime
complexes (iron, nickel, and manganese) was revealed for the
first time. According to the X-ray analysis, deprotonated
formaldoxime cyclotrimer (tfo3−) binds to the metal via three
oxygen atoms forming an adamantane-like cage structure.
The other three coordination vacancies are occupied by a
macrocyclic triamine (tacn), which was used as a supporting
ligand. The metal in [M(tacn)(tfo)]Cl has a formal +4
oxidation state, which is atypical for late transition metal
complexes with organic ligands. The assignment of the
ground state electronic configuration was confirmed by XPS,
0.9 mm/s at room temperature. Compared to [Fe(tacn)(tfo)]
Cl (2), the signal has a more positive isomer shift (Δδ = 0.17
mm/s) indicating a metal-centered reduction resulting in the
formation of the high-spin (S = 5/2) iron complex. These
̈
Mossbauer parameters are in reasonable agreement with
calculated values for electroneutral iron(III) complex [Fe-
(tacn)(tfo)] with S = 5/2. Upon exposure to air, this doublet
disappears, and the formation of the [Fe(tacn)(tfo)]Cl
complex is observed. However, our attempts to obtain single
crystals for this complex failed. Further research will be
performed to isolate and characterize metal(III)-tfo com-
plexes.
̈
Mossbauer spectroscopy, NMR (Evans method), FT-IR
spectroscopy, and DFT calculations. Interestingly, these
indefinitely stable high-valent metal complexes are formed
from the corresponding Fe(III), Ni(II), and Mn(II) salts by
oxidation with air.
The reversible character of reduction suggests that
[M(tacn)(tfo)]+ complexes may be catalytically active in
the aerobic oxidation of sulfur compounds. Indeed, all three
complexes (2, 3, and 5) exhibited catalytic activity in the
aerobic oxidation of p-thiocresol to the corresponding
disulfide with air under ambient conditions in methanol
(Table 3). Iron complex 2 showed the highest catalytic
activity with TON = 2000 and TOF = 83. With nickel
complex 3, 1 equiv of triethylamine was needed to achieve
full conversion of thiol. The addition of TEMPO to the iron-
catalyzed reaction did not result in a drop of the yield of
We believe that well-defined high-valent metal-formal-
doxime complexes such as [M(tacn)(tfo)]Cl prepared in
this work may find interesting applications in catalysis
(aerobic oxidation reactions) and analytical chemistry
(spectrophotometric determination of transition metals). As
an illustration of this, we have shown that [M(tacn)(tfo)]Cl
complexes exhibit high catalytic activity in the aerobic
dehydrogenative dimerization of p-thiocresol under ambient
conditions.
EXPERIMENTAL SECTION
■
Table 3. Aerobic Oxidation of p-Thiocresol Catalyzed by
Metal(IV)-tfo Complexes
General Methods and Instrumentation. All reactions were
carried out in oven-dried (150 °C) glassware. CH2Cl2, CHCl3, and
Et3N were distilled over CaH2; THF was distilled over LiAlH4.
Na2[Fe(ONCH2)6] was prepared according to a previously
described protocol.17 1,4,7-Triazacyclononanetrihydrochloride (tacn
•3HCl), N,N′,N″-trimethyl-1,4,7-triazacyclononane (Me3tacn), 1,8-
bis(dimethylamino)naphthalene (proton sponge), p-thiocresol, naph-
thalene, Ph3P, hydroquinone, sodium ascorbate, and all inorganic
reagents were commercial grade and used as received. NMR spectra
were recorded at room temperature with residual solvent peaks as
internal standards. Multiplicities are indicated by s (singlet), d
(doublet), m (multiplet), and br (broad). Melting points were
determined on a Kofler heating stage and were not corrected.
HRMS experiments were performed on a mass-spectrometer with
electrospray ionization and a time-of-flight (TOF) detector. Peaks in
FT-IR spectra data are reported in cm−1 with the following relative
isolated yield of
complex
mol % additive conversion
disulfide
[Fe(tacn)(tfo)]Cl (2) 0.1%
>99%
>99%
80%
78%
[Ni(tacn)(tfo)]Cl
0.5%
0.5%
Et3N
Et3N
(3)
[Mn(Me3tacn)(tfo)]
Cl (5)
>99%
81%
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intensities: s (strong), m (medium), w (weak), br (broad), and sh
(shoulder). UV−vis spectra were recorded with the use of a Jasco V-
770 spectrophotometer or SF2000 spectrophotometer for the
solutions of the investigated compounds. Raman spectra were
measured on Princeton Instruments SpectraPro with a CCD
detector at laser wavelength λ = 671.7 nm, spot size ca. 2 μm,
intervals 200−900 cm−1 and 800−1400 cm−1, with 6 min of
exposition on each interval. GC-MS was performed on a Chromatec
5000 with an Agilent DB-1MS column 122−0132.
(CH2Cl2, c = 2.3 × 10−3 M) peaks λ nm: 600 (max. extinction
coefficient εmax = 0.8 × 103). ESI-HRMS m/z [M + H]+ calcd for
[C12H13FeN6O4]+: 361.0342. Found: 361.0339.
[Fe(tacn)(tfo)]Cl (2). To a mixture of tfoH3·HCl (34 mg, 0.2
mmol), tacn·3HCl (48 mg, 0.20 mmol), anhydrous FeCl3 (32 mg,
0.20 mmol), and proton sponge (428 mg, 2.0 mmol) was added 5
mL of methanol. The reaction mixture was stirred for 1 h at room
temperature under air and then kept for an additional 24 h with a
closed cap. The precipitate was centrifuged off, and a clear solution
containing complex 2 was concentrated in a vacuum. The residue
was dried at 0.1 Torr for 30 min, and a mixture of Et2O (10 mL)
and CH2Cl2 (30 mL) was added to the crude product. The mixture
was left to stay in a refrigerator at 4 °C for 4 days. Then precipitate
was separated from the mother liquor and dried at 0.1 Torr for 30
min. The residue was placed in a centrifuge cup and centrifuged
with CH2Cl2 (3 × 10 mL) and THF (2 × 10 mL). The residual
solid was dissolved in 3 mL of methanol (dark maroon solution)
and centrifuged. Clear solution was separated from a small amount
of undissolved material and concentrated in a vacuum. This
operation was repeated twice. Solid material was dried in a vacuum
at 0.1 Torr to give 31 mg (yield 42%) of 2 as a black solid. Mp.
above 280 °C. FT-IR (KBr): 3449 (br), 3240 (s), 3053 (s), 2978
(s), 2924 (m), 2883 (s), 2828 (w), 1631 (br), 1485 (w), 1450 (s),
1419 (m), 1360 (m), 1329 (w), 1261 (m), 1229 (w), 1167 (m),
1101 (s), 1042 (s), 972 (s), 929 (s), 874 (m), 830 (m), 779 (s),
746 (s), 632 (s), 604 (m), 530 (s), 460 (s), 431 (m). UV−vis
spectrum (MeOH, c = 6.4 × 10−6 M), λ nm: 273 (max. extinction
Detailed information on the X-ray diffraction experiments,
paramagnetic NMR, electrochemical measurements, XPS, 57Fe
̈
Mossbauer absorption spectra, and DFT calculations is provided
in the Supporting Information.
Synthesis of tfoH3·HCl. Method 1. To a stirred formaldehyde
solution (37 wt % in H2O, 4.4 mL, 59 mmol) at 0 °C was added a
hydroxylamine hydrochloride (4.0 g, 57 mmol). The mixture was
stirred for 5 min at 0 °C, warmed up to room temperature, and
stirred for an additional 2 h. The resulting solution was concentrated
in a vacuum (80 Torr, 40 °C). Then, 5 mL of ethanol was added to
the residue, and the mixture was left to stay in a refrigerator at 4 °C
for 24 h. Then, another 10 mL of ethanol was added, and solid
material was filtered off and was dried in a vacuum (0.2 Torr, 25
°C) to give 2.0 g (yield 61%) of tfoH3·HCl as a white solid.
Caution! The product starts to decompose above 50 °C in a vacuum
1
(0.1 Torr). H NMR spectrum of tfoH3·HCl is in agreement with
the literature data.98 1H NMR (300 MHz, HSQC, D2O, δ, ppm):
4.53 (s, 6 H, 3 CH2). 13C NMR (75 MHz, HSQC, D2O, δ, ppm):
74.3 (CH2). ESI-HRMS m/z [M-Cl]+ calcd for [C3H10N3O3]+:
136.0717. Found: 136.0718.
coefficient εmax = 5.4 × 104), 345 (εmax = 2.8 × 104), 492 (εmax
=
1.9 × 104), 578 (εmax = 7.7 × 103). Raman spectrum: 217 (intensity:
340, Fe−O),99 289 (intens.: 161), 362 (intens.: 190), 387 (intens.:
202), 429 (intens.: 209), 462 (intens.: 120), 500 (intens.: 508,
vsFe−O and Fe−N),99 524 (intens.: 792), 538 (intens.: 358), 632
(intens.: 227), 748 (intens.: 105), 976 (intens.: 148), 1250 (intens.:
2354, twisting CH2 from tfo and tacn),99 1357 (intens.: 115), 1424
(intens.: 5818, scissoring CH2 from tfo and tacn).99−101 ESI-HRMS
m/z [M-Cl]+ calcd for [C9H21FeN6O3]+: 317.1019. Found:
317.1021. Anal. Calcd for C9H21ClFeN6O3·0.5CH3OH: C, 30.95;
H, 6.29; N, 22.80. Found: C, 31.54; H, 5.95; N, 22.82. For synthesis
and characterization of isotope-labeled complexes 2, see Supporting
Information.
Method 2 (Used to Prepare 15N-Labeled and Deuterated tfoH3·
HCl). To a stirred hydroxylamine hydrochloride solution (100 mg,
1.44 mmol) in H2O (0.1 mL) was added a paraformaldehyde (43
mg, 1.44 mmol). The mixture was refluxed in a closed vial for 15
min until the formation of clear solution. The resulting solution was
concentrated in a vacuum (80 Torr, 40 °C). Then, ethanol (150
uL) was added to the residue, and the mixture was left to stay in a
refrigerator at 4 °C for 24 h. Then precipitate was separated from
mother liquor, washed with ethanol (3 × 150 μL), and dried in a
vacuum (0.2 Torr, 25 °C) to give 34 mg (yield 41%) of tfoH3·HCl
as a white solid. Mp. 108−112 °C. Caution! The product starts to
1
decompose above 50 °C in a vacuum (0.1 Torr). H NMR spectrum
[Ni(tacn)(tfo)]Cl·MeOH (3). To a mixture of tfoH3·HCl (34 mg,
0.20 mmol), tacn·3HCl (48 mg, 0.20 mmol), anhydrous NiCl2 (26
mg, 0.20 mmol), and proton sponge (428 mg, 2.0 mmol) was added
5 mL of methanol. The reaction mixture was stirred for 1 h at room
temperature under air and then for an additional 24 h with a closed
cap. The precipitate was centrifuged off, and clear solution was
concentrated in a vacuum. The residue was dried at 0.1 Torr for 30
min, and a mixture of Et2O (10 mL) and CH2Cl2 (30 mL) was
added to the crude product. The mixture was left to stay in a
refrigerator at 4 °C for 2 days. Then, the precipitate was separated
from the mother liquor (off-white solid was not collected) and dried
in a vacuum at 0.1 Torr for 30 min. The residue was placed in a
centrifuge cup and centrifuged with CH2Cl2 (5 × 8 mL) and THF
(2 × 10 mL). Solid residue was dissolved in 3 mL of methanol
(dark maroon solution) and concentrated in a vacuum. The
resulting crude material was dissolved in 1 mL of methanol; clear
solution was separated from a small amount of off-white solid and
evaporated again. This operation was repeated with 0.4 mL of
methanol and then with 0.2 mL of methanol (until clear solution is
formed upon dissolution in methanol). The residue was dried in a
vacuum at 0.1 Torr to give 22 mg (yield 28%) of 3 as a black solid.
of tfoH3·HCl is in agreement with the literature data.98 1H NMR
(300 MHz, D2O, δ, ppm): 4.61 (s, 6 H, 3 CH2). FT-IR (KBr):
3242 (s,br), 3034 (s,sh), 2835 (s,br), 2622 (s), 2523 (m), 2419 (w),
1541 (s), 1405 (s,br), 1348 (m), 1307 (w), 1253 (w), 1203 (s),
1178 (s), 1131 (s), 1039 (s), 973 (s), 947 (s), 889 (s), 796 (s), 709
(m), 622 (br), 557 (s), 503 (m), 431 (s). Anal. Calcd for
C3H10ClN3O3: C, 21.00; H, 5.87; N, 24.49. Found: C, 21.16; H,
5.78; N, 23.86. Single crystals of tfoH3·HCl, suitable for X-ray
diffraction, were obtained by recrystallization from hot ethanolic
solution. Using 15N-hydroxylamine hydrochloride or D2-paraformal-
dehyde, 15N-labeled and deuterated samples of tfoH3·HCl were
prepared.
Fe(ONCHNO)2py2 (1). To a solution of anhydrous FeCl3 (150
mg, 0.93 mmol) in methanol (5 mL) were consequently added
tfoH3·HCl (159 mg, 0.93 mmol) and pyridine (0.63 mL, 7.8 mmol).
The purple-brown solution was stirred at room temperature
overnight under air. The resulting dark-olive solution was
concentrated under reduced pressure. The residue was dried in a
vacuum to remove the unreacted pyridine and then dissolved in
water (20 mL). The aqueous layer was washed with a mixture of
EtOAc-Et2O until the organic layer became colorless. Then, CHCl3
(20 mL) was added to the aqueous layer and the mixture was stirred
overnight with access to air. The resulting deep-blue chloroform
layer was separated and concentrated in a vacuum to give 7 mg of
dark blue crystals of complex 1 (yield: 2%). Mp. above 280 °C. FT-
IR (KBr): 3480 (br), 3074 (w), 2922 (w), 2851 (w), 1605 (m),
1487 (w), 1449 (s), 1407 (s), 1385 (s), 1373 (s), 1357 (s), 1238
(m), 1219 (m), 1096 (m), 1066 (m), 1030 (s), 876 (w), 830 (m),
799 (w), 763 (m), 695 (m), 643 (w), 606 (w). UV−vis spectrum:
1
Mp. above 280 °C. H NMR (300 MHz, HSQC, COSY, D2O, δ,
ppm, J/Hz): 2.73 (d, J = 11.6 Hz, 3 H, CH2), 3.04 (m, 6 H, CH2−
CH2), 3.30 (m, 6 H, CH2−CH2), 3.38 (br. s, CH3OH), 3.97 (br, 3
H, NH), 5.13 (d, J = 11.6 Hz, 3 H, CH2). 13C NMR (75 MHz,
HSQC, D2O, δ, ppm): 45.9 (CH2-CH2), 86.0 (CH2). FT-IR (KBr):
3434 (br), 3255 (s, br), 3195 (s, br), 2944 (m), 2893 (br), 1631
(m, br), 1454 (m), 1428 (m), 1366 (m), 1327 (m), 1278 (w), 1220
(m), 1103 (s), 1049 (s), 1015 (s), 955 (m), 925 (s), 874 (m), 831
(m), 762 (m), 625 (s), 592 (m), 539 (w), 508 (m), 445 (s), 419
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(w). UV−vis spectrum (MeOH, c = 8.5 × 10−6 M), λ nm: 211
(max. extinction coefficient εmax = 2.5 × 104), 256 (εmax = 1.7 ×
104), 303 (εmax = 4.8 × 104), 367 (εmax = 2.8 × 104). ESI-HRMS
m/z [M-Cl]+ calcd for [C9H21N6NiO3]+: 319.1023. Found:
319.1016. Anal. Calcd for C9H21ClNiN6O3·CH3OH: C, 31.00; H,
6.50; N, 21.69. Found: C, 30.74; H, 6.13; N, 21.29. For synthesis
and characterization of isotope-labeled complexes 3 see Supporting
Information.
[Mn(tacn)(tfo)]Cl (4). To a mixture of tfoH3·HCl (64 mg, 0.38
mmol), tacn·3HCl (90 mg, 0.38 mmol) and Mn(OAc)3·2H2O (102
mg, 0.38 mmol) was added 3 mL of methanol. After 15 min of
stirring, Et3N (0.53 mL, 3.8 mmol) was added. The reaction mixture
was stirred for 1 h at room temperature under air and then for an
additional 24 h with a closed cap. The resulting mixture was
concentrated at 80 Torr. The residue was triturated with a CHCl3-
Et2O mixture (5 mL: 5 mL). The solid material was separated from
the mother liquor and dried in a vacuum for 8 h at 100 °C (off-
white solid was not collected). The resulting crude material was
triturated with 5 mL of CHCl3 and dried at 0.1 Torr to give 47 mg
(yield 35%) of 4 as a black solid. Mp. above 250 °C. ESI-HRMS m/
z [M-Cl]+ calcd for [C9H21MnN6O3]+: 316.1050. Found: 316.1047.
Anal. Calcd for C9H21ClMnN6O3: C, 30.74; H, 6.02; N, 23.90.
Found: C, 30.74; H, 6.07; N, 23.89.
784 (s), 738 (s), 661 (w), 619 (s), 591 (m), 509 (s), 451 (s). UV−
vis spectrum (MeOH, c = 5.8 × 10−6 M), λ nm: 211 (max.
extinction coefficient εmax = 2.7 × 104), 252 (εmax = 3.7 × 104), 297
(εmax = 3.6 × 104), 451 (εmax = 2.6 × 104), 542 (εmax = 6.8 × 103).
ESI-HRMS m/z [M-Cl]+ calcd for [C12H27MnN6O3]+: 358.1520.
Found: 358.1525. Anal. calcd for C12H27ClMnN6O3·2CH3OH·H2O:
C, 35.34; H, 7.84; N, 17.66. Found: C, 34.89; H, 7.47; N, 17.42.
Aerobic Oxidation of p-Thiocresol Catalyzed by Fe[(tacn)-
(tfo)]Cl. A solution of p-thiocresol (25 mg, 0.2 mmol, 1000 equiv)
in methanol (250 μL) was added to a 1 mM solution of
Fe[(tacn)(tfo)]+Cl− in methanol (200 μL, 0.0002 mmol, 1 equiv).
The mixture was stirred in a closed vessel equipped with a magnetic
stirrer and an air-filled balloon for 24 h. The resulting solution was
concentrated under reduced pressure. Hexane (2 mL) and water (1
mL) were added to the residue. The organic phase was collected,
and the aqueous layer was washed with hexane (2 × 2 mL). The
combined organic phase was dried over anhydrous sodium sulfate,
concentrated under reduced pressure, and dried to a constant weight
to give 19.7 mg (80%) of p-tolyl disulfide as a white solid. Mp. 46
°C (lit.102 43−46 °C). GC-MS: r.t. 10.1 min; m/z = 246 ([M]+*).
1H NMR spectrum of p-tolyl disulfide is in agreement with the
literature data.102 1H NMR (300 MHz, CDCl3, δ, ppm): 2.32 (s, 6
H), 7.09 (m, 4 H), 7.37 (m, 4 H).
Safety Note! Formaldehyde is highly toxic upon inhalation/ingestion
and corrosive upon skin and eye contact. All operations must be
conducted in a fume hood with suitable protection and precautions
taken.
Synthesis of [Mn(tacn)(tfo)]Cl (4) from MnCl2. To a mixture
of tfoH3·HCl (34 mg, 0.20 mmol), anhydrous MnCl2 (25 mg, 0.20
mmol), tacn·3HCl (48 mg, 0.20 mmol), and proton sponge (428
mg, 2.0 mmol) was added 5 mL of methanol. The reaction mixture
was stirred for 1 h at room temperature under air and then for an
additional 48 h with a closed cap. The precipitate was centrifuged
off, and the solution was concentrated in a vacuum. The residue was
dried at 0.1 Torr for 1 h, and a mixture of Et2O (20 mL) and
CH2Cl2 (30 mL) was added to the crude product. The mixture was
kept at 4 °C for 3 days. Then, the precipitate was separated from
the mother liquor and dried at 0.1 Torr for 30 min (off-white solid
was not collected). The resulting brown solid was placed in a
centrifuge cup and centrifuged with CH2Cl2 (3 × 10 mL). The
residual solid was dissolved in 3 mL of methanol (dark maroon
solution) and centrifuged. Clear solution was separated from a small
amount of undissolved material and concentrated in a vacuum. To
the residual solid was added 1 mL of methanol; the clear solution
was separated from insoluble solid and concentrated in a vacuum.
This operation was repeated with 0.4 mL of methanol (until clear
solution is formed upon dissolution in methanol). The residue was
dried at 0.1 Torr to give 20 mg (yield 26%) of complex 4 as a black
solid. This procedure was used to prepare isotope-labeled complexes
4 (for details, see Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03362.
■
sı
Detailed experimental procedures, crystallographic data
and characterization data for all compounds, copies of
̈
XPS, Mossbauer, FT-IR, UV−vis, NMR, Raman and
mass spectra, details of DFT calculations, and electro-
chemical studies (PDF)
Animated N−O stretching frequency of compound 2
(AVI)
Animated N−O stretching frequency of compound 3
(AVI)
Animated N−O stretching frequency of compound 4
(AVI)
[Mn(Me3tacn)(tfo)]Cl·MeOH (5). To a mixture of tfoH3·HCl
(34 mg, 0.20 mmol), anhydrous MnCl2 (25 mg, 0.20 mmol), and
proton sponge (300 mg, 1.40 mmol) were added 5 mL of methanol
and Me3tacn (0.39 mL, 0.20 mmol). The reaction mixture was
stirred for 1 h at room temperature under air and then for an
additional 24 h with a closed cap. The precipitate was centrifuged
off, and the solution was concentrated in a vacuum. The residue was
dried at 0.1 Torr for 1 h, and a mixture of Et2O (20 mL) and
CH2Cl2 (30 mL) was added to the crude product. The mixture was
left to stay in a refrigerator at 4 °C for 4 days. Then, precipitate was
separated from the mother liquor and dried at 0.1 Torr for 30 min
(off-white solid was not collected). The resulting brown solid was
placed in a centrifuge cup and centrifuged with a CH2Cl2-Et2O
mixture (5 mL: 5 mL) and with a CH2Cl2-Et2O mixture (4.5 mL:
2.5 mL). To the residual solid was added 3 mL of methanol; the
clear solution was separated from insoluble crystals and concentrated
in a vacuum. The last operation was repeated with 1 mL of
methanol and then with 0.4 mL of methanol (until clear solution is
formed upon dissolution in methanol). The residue was dried at 0.1
Torr to give 14 mg (yield 15%) of 5 as a black solid. Mp. above 280
°C. FT-IR (KBr): 3382 (br), 2994 (m), 2934 (m), 2849 (m), 2624
(w;br), 1697 (w), 1658 (w) 1599 (w), 1501 (m), 1462 (s), 1415
(m), 1382 (m), 1345 (w), 1290 (s), 1252 (w), 1202 (w), 1160 (s),
1121 (w), 1058 (s), 1003 (s), 970 (s), 931 (s), 901 (m), 868 (w),
Accession Codes
CCDC 2035754−2035757 and 2061527 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Alexey Yu. Sukhorukov − N. D. Zelinsky Institute of
Organic Chemistry, Russian Academy of Sciences, Moscow,
Russia 119991; Plekhanov Russian University of Economics,
Moscow, Russia 117997; orcid.org/0000-0003-4413-
9453; Email: sukhorukov@ioc.ac.ru
Authors
Ivan S. Golovanov − N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow, Russia
119991; orcid.org/0000-0001-7219-2409
5533
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Article
(5) Goto, K.; Komatsu, T.; Furukawa, T. Rapid colorimetric
determination of manganese in waters containing iron:
modification of the formaldoxime method. Anal. Chim. Acta 1962,
27, 331.
(6) Youngvises, N.; Suwannasaroj, K.; Jakmunee, J.; AlSuhaimi, A.
Multi-reverse flow injection analysis integrated with multi-optical
sensor for simultaneous determination of Mn(II), Fe(II), Cu(II)
and Fe(III) in natural waters. Talanta 2017, 166, 369.
(7) Grebel, J. E.; Charbonnet, J. A.; Sedlak, D. L. Oxidation of
organic contaminants by manganese oxide geomedia for passive
urban stormwater treatment systems. Water Res. 2016, 88, 481.
(8) Kaewwonglom, N.; Jakmunee, J. Sequential injection system
with multi-parameter analysis capability for water quality measure-
ment. Talanta 2015, 144, 755.
Roman S. Malykhin − N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow, Russia
119991
Vladislav K. Lesnikov − N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow, Russia
119991
Yulia V. Nelyubina − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow, Russia 119991
Valentin V. Novikov − A. N. Nesmeyanov Institute of
Organoelement Compounds, Russian Academy of Sciences,
Moscow, Russia 119991; orcid.org/0000-0002-0225-
0594
A
Kirill V. Frolov − Shubnikov Institute of Crystallography of
FSRC “Crystallography and Photonics,” Russian Academy of
Sciences, Moscow, Russia 119991
(9) Zhu, M.; Ginder-Vogel, M.; Parikh, S. J.; Feng, X.-H.; Sparks,
D. L. Cation Effects on the Layer Structure of Biogenic Mn-Oxides.
Environ. Sci. Technol. 2010, 44, 4465.
Andrey I. Stadnichenko − Boreskov Institute of Catalysis,
Siberian Branch of Russian Academy of Sciences,
Novosibirsk, Russia 630090
Evgeny V. Tretyakov − N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow, Russia
119991; orcid.org/0000-0003-1540-7033
Sema L. Ioffe − N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow, Russia
119991
(10) Koeber, K.; Kottelwesch, H.; Schneider, D. In Mn Manganese;
Demmer, H., Kottelwesch, H., Schleitzer-Rust, E., Eds.; Springer-
Verlag: Berlin, 1987; p 232.
(11) Henriksen, A. An automatic, modified formaldoxime method
for determining low concentrations of manganese in water
containing iron. Analyst 1966, 91, 647.
(12) Derie, R. Formaldehyde oxime leaching of metals from deep-
sea manganese nodules and other ores. Nature 1986, 324, 660.
̈
̌
̌
(13) Bartusek, M.; Okác, A. Uber die Komplexsalze des
Formaldoxims II. Isolierung von Nickel-, Mangan-und Kobaltkom-
plexen. Collect. Czech. Chem. Commun. 1961, 26, 883.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03362
̈
̌
̌
(14) Bartusek, M.; Okác, A. Uber die Komplexsalze des
Formaldoxims. Collect. Czech. Chem. Commun. 1961, 26, 52.
̈
̌
̌
(15) Bartusek, M.; Okác, A. Uber komplexsalze des formaldoxims
III. Komplexverbindungen des nickels, mangans und kobalts. Collect.
Czech. Chem. Commun. 1961, 26, 2174.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
̌
(16) Becka, J.; Jokl, J. Papierelektrophorese von Komplexverbin-
dungen des Formaldoxims mit Mangan, Nickel, Eisen und Vanadin.
Collect. Czech. Chem. Commun. 1971, 36, 2467.
Notes
(17) Andersen, F. A.; Jensen, K. A. Infrared spectra of transition
metal coordination compounds with the formaldoximate ion. J. Mol.
Struct. 1980, 60, 165.
(18) Andersen, F. A.; Jensen, K. A. Infrared spectroscopic study of
some transition metal compounds with coordinated formaldoximate
ion. J. Mol. Struct. 1982, 79, 357.
(19) Andersen, F. A.; Jensen, K. A. The configuration of transition
metal coordination compounds containing the anion of hexahydro-
1,3,5-triazine-1,3,5-triol. J. Mol. Struct. 1986, 141, 441.
(20) Golovanov, I. S.; Sukhorukov, A. Y.; Nelyubina, Y. V.;
Khomutova, Y. A.; Ioffe, S. L.; Tartakovsky, V. A. Synthesis of
B,O,N-Doped Adamantanes and Diamantanes by Condensation of
Oximes with Boronic Acids. J. Org. Chem. 2015, 80, 6728.
(21) Gouzerh, P.; Jeannin, Y.; Rocchiccioli-Deltcheff, C.; Valentini,
F. Synthesis, properties and structure of tris(ethylnitrosolato)
complexes of manganese, iron, and cobalt. J. Coord. Chem. 1979,
9, 221.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The reported study was funded by the Russian Foundation
for Basic Research (project # 20-33-70188). X-ray diffraction
data were collected using the equipment of the Center for
Molecular Composition Studies of INEOS RAS with financial
support from the Ministry of Science and Higher Education
of the Russian Federation. Mossbauer and Raman spectros-
copy studies were supported by the Russian Ministry of
Science and Higher Education within the State assignment of
the FSRC “Crystallography and Photonics” of RAS. The
authors acknowledge Dr. Ivan A. Troyan (Shubnikov Institute
of Crystallography of FSRC “Crystallography and Photonics”
of RAS) for the Raman spectroscopy measurements and
Vladislav M. Korshunov (P. N. Lebedev Physical Institute of
RAS) for the UV−vis spectroscopy measurements.
(22) With cobalt, cation [Co(tacn)(tfo)]+ was observed in ESI-
HRMS; however, attempts to crystallize this complex failed.
(23) With trispyrazolylborate, HRMS revealed the formation of the
[Mn(trispyrazolylborate)2]+ complex.
(24) Tomyn, S.; Shylin, S. I.; Bykov, D.; Ksenofontov, V.;
Gumienna-Kontecka, E.; Bon, V.; Fritsky, I. O. Indefinitely stable
iron(IV) cage complexes formed in water by air oxidation. Nat.
Commun. 2017, 8, 14099.
(25) Malischewski, M.; Adelhardt, M.; Sutter, J.; Meyer, K.;
Seppelt, K. Isolation and structural and electronic characterization of
salts of the decamethylferrocene dication. Science 2016, 353, 678.
(26) McDonald, A. R.; Que, L. High-valent nonheme iron-oxo
complexes: Synthesis, structure, and spectroscopy. Coord. Chem. Rev.
2013, 257, 414.
REFERENCES
■
(1) Dunstan, W. R.; Bossi, A. L. XXXV.The preparation and
properties of formaldoxime. J. Chem. Soc., Trans. 1898, 73, 353.
(2) Hofmann, K. A.; Ehrhardt, U. Innerkomplexe Metallsalze der
̈
Oxalsaure-Derivate und des Triform-oxims. Ber. Dtsch. Chem. Ges.
1913, 46, 1457.
(3) Marczenko, Z. Dosage spectrophotometrique de differents
metaux (Mn, Ce, V, Ni, Fe) au moyen de la formaldoxime. Anal.
Chim. Acta 1964, 31, 224.
(4) Bradfield, E. G. An improved formaldoxime method for the
determination of manganese in plant material. Analyst 1957, 82, 254.
5534
https://doi.org/10.1021/acs.inorgchem.0c03362
Inorg. Chem. 2021, 60, 5523−5537

Inorganic Chemistry
pubs.acs.org/IC
Article
(27) Hohenberger, J.; Ray, K.; Meyer, K. The biology and
chemistry of high-valent iron-oxo and iron-nitrido complexes. Nat.
Commun. 2012, 3, 720.
(28) Chanda, A.; Shan, X.; Chakrabarti, M.; Ellis, W. C.; Popescu,
D. L.; Tiago de Oliveira, F.; Wang, D.; Que, L.; Collins, T. J.;
aqueous solution: Effects of pH and iron complexation. Geochim.
Cosmochim. Acta 2005, 69, 3237.
(46) Bhatt, K.; Agrawal, Y. K. Hydroxamic Acids and Their Metal
Complexes: Preparation, Properties, and Infrared Spectra. Synth.
React. Inorg. Met.-Org. Chem. 1972, 2, 175.
Mu
̈
nck, E.; Bominaar, E. L. (TAML)FeIVO Complex in Aqueous
(47) Yang, J.; Bremer, P. J.; Lamont, I. L.; McQuillan, A. J.
Infrared Spectroscopic Studies of Siderophore-Related Hydroxamic
Acid Ligands Adsorbed on Titanium Dioxide. Langmuir 2006, 22,
10109.
Solution: Synthesis and Spectroscopic and Computational Charac-
terization. Inorg. Chem. 2008, 47, 3669.
(29) Chanda, A.; Popescu, D.-L.; de Oliveira, F. T.; Bominaar, E.
L.; Ryabov, A. D.; Mu
̈
nck, E.; Collins, T. J. High-valent iron
(48) Evans, D. 400. The determination of the paramagnetic
susceptibility of substances in solution by nuclear magnetic
resonance. J. Chem. Soc. 1959, 2003.
(49) NIST X-ray Photoelectron Spectroscopy Database, NIST
Standard Reference Database Number 20, National Institute of
Standards and Technology: Gaithersburg, MD, 2000, 20899.
(50) Zhou, A.-J.; Liu, J.-L.; Herchel, R.; Leng, J.-D.; Tong, M.-L.
High-spin tetranuclear MnII2MnIV2 clusters with unique Mn(ii)-
Mn(iv) magnetic exchange: synthesis, structures and magnetism.
Dalton Trans. 2009, 3182.
(51) Buschmann, W. E.; Miller, J. S. Magnetic Ordering and Spin-
Glass Behavior in First-Row Transition Metal Hexacyanomanganate-
(IV) Prussian Blue Analogues. Inorg. Chem. 2000, 39, 2411.
(52) Buitendach, B. E.; Erasmus, E.; Niemantsverdriet, J. W.;
Swarts, J. C. Properties of Manganese(III) Ferrocenyl-β-Diketonato
Complexes Revealed by Charge Transfer and Multiplet Splitting in
the Mn 2p and Fe 2p X-Ray Photoelectron Envelopes. Molecules
2016, 21, 1427.
̈
complexes with tetraamido macrocyclic ligands: Structures, Moss-
bauer spectroscopy, and DFT calculations. J. Inorg. Biochem. 2006,
100, 606.
(30) Ghosh, A.; Tiago de Oliveira, F.; Yano, T.; Nishioka, T.;
̈
Beach, E. S.; Kinoshita, I.; Munck, E.; Ryabov, A. D.; Horwitz, C.
P.; Collins, T. J. Catalytically Active μ-Oxodiiron(IV) Oxidants from
Iron(III) and Dioxygen. J. Am. Chem. Soc. 2005, 127, 2505.
(31) Shylin, S. I.; Pavliuk, M. V.; D’Amario, L.; Mamedov, F.; Sá,
J.; Berggren, G.; Fritsky, I. O. Efficient visible light-driven water
oxidation catalysed by an iron(iv) clathrochelate complex. Chem.
Commun. 2019, 55, 3335.
(32) Meucci, E. A.; Ariafard, A.; Canty, A. J.; Kampf, J. W.;
Sanford, M. S. Aryl-Fluoride Bond-Forming Reductive Elimination
from Nickel(IV) Centers. J. Am. Chem. Soc. 2019, 141, 13261.
(33) Padamati, S. K.; Angelone, D.; Draksharapu, A.; Primi, G.;
Martin, D. J.; Tromp, M.; Swart, M.; Browne, W. R. Transient
Formation and Reactivity of a High-Valent Nickel(IV) Oxido
Complex. J. Am. Chem. Soc. 2017, 139, 8718.
(34) Martinez, G. E.; Ocampo, C.; Park, Y. J.; Fout, A. R.
Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via
Halogen and Halogen Surrogates. J. Am. Chem. Soc. 2016, 138,
4290.
(53) Fujiwara, M.; Matsushita, T.; Ikeda, S. Evaluation of Mn3s X-
ray photoelectron spectroscopy for characterization of manganese
complexes. J. Electron Spectrosc. Relat. Phenom. 1995, 74, 201.
(54) Mansour, A. N.; Melendres, C. A. Characterization of KNiIO6
by XPS. Surf. Sci. Spectra 1994, 3, 287.
́
(55) Fuentes, S.; Munoz, P.; Barraza, N.; Chávez-Angel, E.;
̃
Sotomayor Torres, C. M. Structural characterisation of slightly Fe-
doped SrTiO3 grown via a sol-gel hydrothermal synthesis. J. Sol-Gel
Sci. Technol. 2015, 75, 593.
(56) Bressan, M.; Furlani, C.; Polzonetti, G. XPS of coordination
compounds: additive ligand effect in some copper(I) and copper(II)
chelates with 1,2 phosphino (or phosphine/oxide)-sulfido ethane
ligands. Polyhedron 1983, 2, 523.
(35) Camasso, N. M.; Sanford, M. S. Design, synthesis, and
carbon-heteroatom coupling reactions of organometallic nickel(IV)
complexes. Science 2015, 347, 1218.
(36) Carnes, M.; Buccella, D.; Chen, J. Y.-C.; Ramirez, A. P.;
Turro, N. J.; Nuckolls, C.; Steigerwald, M. A Stable Tetraalkyl
Complex of Nickel(IV). Angew. Chem., Int. Ed. 2009, 48, 290.
(37) Singh, A. B. Spectroscopic and Electrochemical Studies of
Nickel(III) and Nickel(IV) Complexes derived From Oximato
Ligands. Synth. React. Inorg. Met.-Org. Chem. 1986, 16, 433.
(38) Baucom, E. I.; Drago, R. S. Nickel(II) and nickel(IV)
complexes of 2,6-diacetylpyridine dioxime. J. Am. Chem. Soc. 1971,
93, 6469.
(39) Alvarez, S. Relationships between Temperature, Magnetic
Moment, and Continuous Symmetry Measures in Spin Crossover
Complexes. J. Am. Chem. Soc. 2003, 125, 6795.
(40) Semakin, A. N.; Sukhorukov, A. Y.; Lesiv, A. V.; Ioffe, S. L.;
Lyssenko, K. A.; Nelyubina, Y. V.; Tartakovsky, V. A. Unusual
Intramolecular Cyclization of Tris(β-oximinoalkyl)amines. The First
Synthesis of 1,4,6,10-Tetraazaadamantanes. Org. Lett. 2009, 11,
4072.
(41) Semakin, A. N.; Nelyubina, Y. V.; Ioffe, S. L.; Sukhorukov, A.
Y. 2,4,9-Triazaadamantanes with “Clickable” Groups: Synthesis,
Structure and Applications as Tripodal Platforms. Eur. J. Org. Chem.
2020, 2020, 6723.
(42) Jerslev, B.; Brehm, L.; Gabrielsen, M. V. The Crystal
Structure and the Conformation of 1,3,5-Triacetoxyhexahydro-1,3,5-
triazine. Acta Chem. Scand. 1977, 31, 875.
(43) Semakin, A. N.; Sukhorukov, A. Y.; Nelyubina, Y. V.;
Khomutova, Y. A.; Ioffe, S. L.; Tartakovsky, V. A. Urotropine Isomer
(1,4,6,10-Tetraazaadamantane): Synthesis, Structure, and Chemistry.
J. Org. Chem. 2014, 79, 6079.
(44) Nightingale, R. E.; Wagner, E. L. The Vibrational Spectra and
Structure of Solid Hydroxylamine and Deutero-Hydroxylamine. J.
Chem. Phys. 1954, 22, 203.
̈
(57) Niarchos, D.; Petridis, D. High field Mossbauer effect studies
of some anionic iron (IV) dithiochelates. Chem. Phys. 1979, 41, 97.
(58) Sellmann, D.; Geck, M.; Knoch, F.; Ritter, G.; Dengler, J.
Transition-metal complexes with sulfur ligands. 57. Stabilization of
high-valent iron(IV) centers and vacant coordination sites by sulfur.
pi.-donation: syntheses, x-ray structures, and properties of [Fe-
(“S2“)2(PMe3)n] (n = 1, 2) and (NMe4)[Fe(“S2“)2(PMe3)-
2].CH3OH (“S2“2- = 1,2-benzenedithiolate(2-)). J. Am. Chem. Soc.
1991, 113, 3819.
(59) Voloshin, Y.; Belaya, I.; Krämer, R. Cage Metal Complexes:
Clathrochelates Revisited; Springer, 2017.
(60) Novikov, V. V.; Pavlov, A. A.; Belov, A. S.; Vologzhanina, A.
V.; Savitsky, A.; Voloshin, Y. Z. Transition Ion Strikes Back: Large
Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates. J.
Phys. Chem. Lett. 2014, 5, 3799.
(61) Novikov, V. V.; Pavlov, A. A.; Nelyubina, Y. V.; Boulon, M.-
E.; Varzatskii, O. A.; Voloshin, Y. Z.; Winpenny, R. E. P. A Trigonal
Prismatic Mononuclear Cobalt(II) Complex Showing Single-
Molecule Magnet Behavior. J. Am. Chem. Soc. 2015, 137, 9792.
(62) Hegetschweiler, K.; Kradolfer, T.; Gramlich, V.; Hancock, R.
D. The Design of Selective Chelating Agents: 1,3,5-Trideoxy-1,3,5-
tris-(dimethylamino)-cis-inositol, a Powerful Ligand for Hard and
Highly Charged Metal Ions. Chem. - Eur. J. 1995, 1, 74.
(63) Morgenstern, B.; Steinhauser, S.; Hegetschweiler, K.;
Garribba, E.; Micera, G.; Sanna, D.; Nagy, L. Complex Formation
of Vanadium(IV) with 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol and
Related Ligands. Inorg. Chem. 2004, 43, 3116.
(45) Edwards, D. C.; Nielsen, S. B.; Jarzeç ki, A. A.; Spiro, T. G.;
Myneni, S. C. B. Experimental and theoretical vibrational spectros-
copy studies of acetohydroxamic acid and desferrioxamine B in
(64) Al Shaer, D.; Al Musaimi, O.; de la Torre, B. G.; Albericio, F.
Hydroxamate siderophores: Natural occurrence, chemical synthesis,
5535
https://doi.org/10.1021/acs.inorgchem.0c03362
Inorg. Chem. 2021, 60, 5523−5537

Inorganic Chemistry
pubs.acs.org/IC
Article
iron binding affinity and use as Trojan horses against pathogens.
Eur. J. Med. Chem. 2020, 208, 112791.
(83) Niu, P.; Liu, X.; Shen, Z.; Li, M. Electrochemical Performance
of ABNO for Oxidation of Secondary Alcohols in Acetonitrile
Solution. Molecules 2019, 24, 100.
(65) Guérard, F.; Lee, Y.-S.; Tripier, R.; Szajek, L. P.; Deschamps,
J. R.; Brechbiel, M. W. Investigation of Zr(IV) and 89Zr(IV)
complexation with hydroxamates: progress towards designing a
better chelator than desferrioxamine B for immuno-PET imaging.
Chem. Commun. 2013, 49, 1002.
(66) Jones, K. E.; Batchler, K. L.; Zalouk, C.; Valentine, A. M.
Ti(IV) and the Siderophore Desferrioxamine B: A Tight Complex
Has Biological and Environmental Implications. Inorg. Chem. 2017,
56, 1264.
(67) Duckworth, O. W.; Bargar, J. R.; Sposito, G. Coupled
biogeochemical cycling of iron and manganese as mediated by
microbial siderophores. BioMetals 2009, 22, 605.
(68) Lee, H. B.; Bogart, J. A.; Carroll, P. J.; Schelter, E. J.
Structural and electrochemical characterization of a cerium(iv)
hydroxamate complex: implications for the beneficiation of light rare
earth ores. Chem. Commun. 2014, 50, 5361.
(84) Wang, X.; Ma, J.; Song, D.; Shen, Z.; Li, M.; Ma, C.
Characterization and Electrocatalytic Activity of Poly (4-thienylace-
tyl-oxy-2, 2, 6, 6-tetramethylpiperidin-1-yloxy) Prepared by Electro-
chemical Polymerization. ECS Electrochem. Lett. 2014, 3, H12.
(85) Caneschi, A.; Gatteschi, D.; Renard, J. P.; Rey, P.; Sessoli, R.
Magnetic coupling in zero- and one-dimensional magnetic systems
formed by nickel(II) and nitronyl nitroxides. Magnetic phase
transition of a ferrimagnetic chain. Inorg. Chem. 1989, 28, 2940.
(86) Zhang, C.-X.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Zhao, B.
Synthesis, crystal structure and magnetic properties of a nickel(II)
complex with pyridine-substituted nitronyl nitroxide radicals.
Transition Met. Chem. 2003, 28, 621.
(87) Samolova, E.; Premuzic, D.; Plociennik, S.; Hołynska, M.łg.
Bis(benzimidazole) as supramolecular building block in manganese-
(IV) chemistry. J. Mol. Struct. 2019, 1176, 366.
(88) Hou, Z.; Sunderland, C. J.; Nishio, T.; Raymond, K. N.
Preorganization of Ferric Alcaligin, Fe2L3. The First Structure of a
Ferric Dihydroxamate Siderophore. J. Am. Chem. Soc. 1996, 118,
5148.
(69) Buglyó, P.; Culeddu, N.; Kiss, T.; Micera, G.; Sanna, D.
Vanadium (IV) and vanadium (V) complexes of deferoxamine B in
aqueous solution. J. Inorg. Biochem. 1995, 60, 45.
(70) Anionic N-oxy-ligands such as deprotonated hydroxylamines,
hydroxamic acids, and oximes tend to stabilize high-valent metals, in
particular Ni(III), due to the strong s-donor capacity of these
ligands. As a consequence of this, nitroxyl ligands (such as TEMPO
and TEMPO-H) are known to promote various Cu- and Fe-
catalyzed oxidation reactions, e.g., oxidation of alcohols to ketones.
(71) Haugland, M. M.; Lovett, J. E.; Anderson, E. A. Advances in
the synthesis of nitroxide radicals for use in biomolecule spin
labelling. Chem. Soc. Rev. 2018, 47, 668.
(89) Nielsen, A.; Larsen, F. B.; Bond, A. D.; McKenzie, C. J.
Regiospecific Ligand Oxygenation in Iron Complexes of a
Carboxylate-Containing Ligand Mediated by a Proposed FeV-Oxo
Species. Angew. Chem., Int. Ed. 2006, 45, 1602.
(90) Wang, F.; Feng, L.; Dong, S.; Liu, X.; Feng, X. Chiral N,N’-
dioxide-iron(iii)-catalyzed asymmetric sulfoxidation with hydrogen
peroxide. Chem. Commun. 2020, 56, 3233.
(91) Wang, Y.; Wu, J. I. C.; Li, Q.; Schleyer, P. v. R. Why Are
Some (CH)4 X and (CH2)6 X Polyheteroadamantanes So Stable?
6
4
(72) Walroth, R. C.; Miles, K. C.; Lukens, J. T.; MacMillan, S. N.;
Stahl, S. S.; Lancaster, K. M. Electronic Structural Analysis of
Copper(II)-TEMPO/ABNO Complexes Provides Evidence for
Copper(I)-Oxoammonium Character. J. Am. Chem. Soc. 2017, 139,
13507.
(73) Kleinlein, C.; Bendelsmith, A. J.; Zheng, S.-L.; Betley, T. A.
C-H Activation from Iron(II)-Nitroxido Complexes. Angew. Chem.,
Int. Ed. 2017, 56, 12197.
(74) Nguyen, T.-A. D.; Wright, A. M.; Page, J. S.; Wu, G.; Hayton,
T. W. Oxidation of Alcohols and Activated Alkanes with Lewis Acid-
Activated TEMPO. Inorg. Chem. 2014, 53, 11377.
(75) Scepaniak, J. J.; Wright, A. M.; Lewis, R. A.; Wu, G.; Hayton,
T. W. Tuning the Reactivity of TEMPO by Coordination to a Lewis
Acid: Isolation and Reactivity of MCl3(η1-TEMPO) (M = Fe, Al). J.
Am. Chem. Soc. 2012, 134, 19350.
(76) Ahlers, C.; Dickman, M. H. Iron-Mediated Cleavage of
Coordinated 1,1,1,5,5,5-Hexafluoropentane-2,4-dione by the 2,2,6,6-
Tetramethylpiperidine-1-oxyl Nitroxyl Radical. Inorg. Chem. 1998,
37, 6337.
(77) Dickman, M. H.; Porter, L. C.; Doedens, R. J. Bis(nitroxyl)
adducts of bis(hexafluoroacetylacetonato)manganese(II). Prepara-
tion, structures, and magnetic properties. Inorg. Chem. 1986, 25,
2595.
(78) Dighe, S. U.; Chowdhury, D.; Batra, S. Iron Nitrate/TEMPO:
a Superior Homogeneous Catalyst for Oxidation of Primary
Alcohols to Nitriles in Air. Adv. Synth. Catal. 2014, 356, 3892.
(79) Iron, M. A.; Szpilman, A. M. Mechanism of the Copper/
TEMPO-Catalyzed Aerobic Oxidation of Alcohols. Chem. - Eur. J.
2017, 23, 1368.
Org. Lett. 2010, 12, 1320.
(92) Laube, T. First Crystal Structure Analysis of an Aliphatic
CarbocationStabilization of the 3,5,7-Trimethyl-1-adamantyl Cat-
ion by C-C Hyperconjugation. Angew. Chem., Int. Ed. Engl. 1986, 25,
349.
(93) Corma, A.; Ródenas, T.; Sabater, M. J. Aerobic oxidation of
thiols to disulfides by heterogeneous gold catalysts. Chem. Sci. 2012,
3, 398.
(94) Fernández-Salas, J. A.; Manzini, S.; Nolan, S. P. Efficient
ruthenium-catalysed S-S, S-Si and S-B bond forming reactions.
Chem. Commun. 2013, 49, 5829.
(95) Dhakshinamoorthy, A.; Primo, A.; Esteve-Adell, I.; Alvaro, M.;
Garcia, H. Boron Nitride Nanoplatelets as a Solid Radical Initiator
for the Aerobic Oxidation of Thiophenol to Diphenyldisulfide.
ChemCatChem 2015, 7, 776.
(96) Patra, A. K.; Dutta, A.; Pramanik, M.; Nandi, M.; Uyama, H.;
Bhaumik, A. Synthesis of Hierarchical Mesoporous Mn-MFI Zeolite
Nanoparticles: A Unique Architecture of Heterogeneous Catalyst for
the Aerobic Oxidation of Thiols to Disulfides. ChemCatChem 2014,
6, 220.
(97) Dethe, D. H.; Srivastava, A.; Dherange, B. D.; Kumar, B. V.
Unsymmetrical Disulfide Synthesis through Photoredox Catalysis.
Adv. Synth. Catal. 2018, 360, 3020.
(98) Jensen, K. A.; Holm, A. Investigations of formaldehyde oxime,
its polymers and coordination compounds. I. On the nature of the
so-called “triformoxime” and isolation of the authentic trimer, 1,3,5-
trihydroxyhexahydro-1,3,5-triazine. Mater.-Fys. Medd. - K. Dan.
Vidensk. Selsk. 1978, 40, 1.
(99) North, J. M.; Achey, R. M.; Dalal, N. S. Low-frequency
Raman modes of the single-molecule magnets Mn12-acetate and
Fe8Br8 and their analogs. Phys. Rev. B: Condens. Matter Mater. Phys.
2002, 66, 174437.
(80) Hankache, J.; Wenger, O. S. Organic Mixed Valence. Chem.
Rev. 2011, 111, 5138.
(81) Heckmann, A.; Lambert, C. Organic Mixed-Valence
Compounds: A Playground for Electrons and Holes. Angew.
Chem., Int. Ed. 2012, 51, 326.
(82) Rintoul, L.; Micallef, A. S.; Bottle, S. E. The vibrational group
frequency of the N-O stretching band of nitroxide stable free
radicals. Spectrochim. Acta, Part A 2008, 70, 713.
(100) Duraipandian, S.; Petersen, J. C.; Lassen, M. Authenticity
and Concentration Analysis of Extra Virgin Olive Oil Using
Spontaneous Raman Spectroscopy and Multivariate Data Analysis.
Appl. Sci. 2019, 9, 2433.
(101) DFT calculation of complex 2. See section 12.2 in the
Supporting Information for details.
5536
https://doi.org/10.1021/acs.inorgchem.0c03362
Inorg. Chem. 2021, 60, 5523−5537

Inorganic Chemistry
pubs.acs.org/IC
Article
(102) Iranpoor, N.; Firouzabadi, H.; Jamalian, A. Deoxygenation of
Sulfoxides and Reductive Coupling of Sulfonyl Chlorides, Sulfinates
and Thiosulfonates Using Silphos [PCl3-n(SiO2)n] as a Heteroge-
neous Phosphine Reagent. Synlett 2005, 2005, 1447.
5537
https://doi.org/10.1021/acs.inorgchem.0c03362
Inorg. Chem. 2021, 60, 5523−5537