Nickel(II) Complexes with Redox Noninnocent Octaazamacrocycles as Catalysts in Oxidation Reactions

Article abstract of DOI:10.1021/acs.inorgchem.9b01700

Nickel(II) complexes with 15-membered (1-5) and 14-membered (6) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide were prepared by template synthesis. The compounds were characterized by elemental analysis, electrospray ionization mass spectrometry, IR, UV-vis, ¹H NMR spectroscopies, and X-ray diffraction. The complexes contain a low-spin nickel(II) ion in a square-planar coordination environment. The electrochemical behavior of 1-6 was investigated in detail, and the electronic structure of 1e-oxidized and 1e-reduced species was studied by electron paramagnetic resonance, UV-vis-near-IR spectroelectrochemistry, and density functional theory calculations indicating redox noninnocent behavior of the ligands. Compounds 1-6 were tested in the microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce the industrially significant mixture of cyclohexanol and cyclohexanone (i.e., A/K oil). The results showed that the catalytic activity was affected by several factors, namely, reaction time and temperature or amount and type of catalyst. The best values for A/K oil yield (23%, turnover number of 1.1 × 10²) were obtained with compound 6 after 2 h of microwave irradiation at 100 °C.

Full text of DOI:10.1021/acs.inorgchem.9b01700

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nickel(II) Complexes with Redox Noninnocent Octaazamacrocycles
as Catalysts in Oxidation Reactions
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‡
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Anatolie Dobrov, Denisa Darvasiova, Michal Zalibera, Lukas Bucinsky, Ingrid Puskarova,
Peter Rapta,‡ Sergiu Shova,§ Dan Dumitrescu,∥ Luísa M. D. R. S. Martins,*,⊥ Armando J. L. Pombeiro,⊥
and Vladimir B. Arion*,†
†
̈
Institute of Inorganic Chemistry, University of Vienna, Wahringer Strasse 42, A-1090 Vienna, Austria
‡Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology
́
in Bratislava, Radlinskeho 9, SK-81237 Bratislava, Slovak Republic
§Inorganic Polymers Department, “Petru Poni” Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A, 700487 Iasi,
Romania
∥Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14-km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy
⊥
́
Centro de Química Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
S
* Supporting Information
ABSTRACT: Nickel(II) complexes with 15-membered (1−
5) and 14-membered (6) octaazamacrocyclic ligands derived
from 1,2- and 1,3-diketones and S-methylisothiocarbohydra-
zide were prepared by template synthesis. The compounds
were characterized by elemental analysis, electrospray
ionization mass spectrometry, IR, UV−vis, 1H NMR spectros-
copies, and X-ray diffraction. The complexes contain a low-
spin nickel(II) ion in a square-planar coordination environ-
ment. The electrochemical behavior of 1−6 was investigated
in detail, and the electronic structure of 1e-oxidized and 1e-
reduced species was studied by electron paramagnetic resonance, UV−vis−near-IR spectroelectrochemistry, and density
functional theory calculations indicating redox noninnocent behavior of the ligands. Compounds 1−6 were tested in the
microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce the industrially significant
mixture of cyclohexanol and cyclohexanone (i.e., A/K oil). The results showed that the catalytic activity was affected by several
factors, namely, reaction time and temperature or amount and type of catalyst. The best values for A/K oil yield (23%, turnover
number of 1.1 × 102) were obtained with compound 6 after 2 h of microwave irradiation at 100 °C.
Chart 1. Line Drawing of the Complex Anions I and II in
(PPN)2[Ni(cyclo-Gly-β-Ala)2-4H+] and (PPN)2[Ni(cyclo-α-
OH-Gly-β-Ala)2-4H+], Respectively
INTRODUCTION
■
Investigation of transition-metal complexes with redox non-
innocent ligands has been an active area of research over the
last three decades.1−9 The main focus was initially on a
detailed understanding of electronic structures of these
compounds and their use in stoichiometric reactivity
studies.10−13 In particular, a cyclic tetrapeptide or macrolactam
prepared by template reaction of nickel(II) salt with 2 mol
equiv of dipeptide ester in methanol in the presence of
NaOMe as base followed by addition of [PPN]Cl as
c o u n t e r c a t i o n s u p p l i e r ( P P N
= t r i p h e n y l -
(triphenylphosphoranylidene)amino)phosphonium), and iso-
lated as (PPN)2[Ni(cyclo-Gly-β-Ala)2-4H+] (I in Chart 1),14
was found to react in acetonitrile (ACN) in air with formation
of deep red solution. α-Hydroxylation of the glycine segment
occurred to give (PPN)2[Ni(cyclo-α-OH-Gly-β-Ala)2-4H+] (II
in Chart 1) as confirmed by X-ray crystallography.15 The
formation of nickel(III)-oxyl species as intermediates in
stoichiometric transformations was suggested via generation
of nickel(III) and a glycine radical in the first steps of the
reaction.
Nowadays, the use of first-row transition-metal complexes
with noninnocent ligands in catalytic reactions is rising.
Catalytic functionalization of C−H bonds and activation of
Received: June 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01700
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Inorganic Chemistry
Article
small molecules are well-documented in the literature.16−19
Redox-active ligands can participate in the catalytic cycle by
accepting or donating electrons and modifying the Lewis
acidity of the metal and in the formation/breaking of substrate
covalent bond electron transfer events upon bond activation
serving as an electron reservoir.20
crocyclic ligands 1−6 (Chart 4), their structure, and
spectroscopic properties of 1e-oxidized and 1e-reduced species.
a
Chart 4. Complexes Reported in This Work
The rational design of efficient oxidation catalysts requires
an understanding of the structure and spectroscopic properties,
as well as the reactivity of the key reactive species. Nickel(II)
complexes have attracted the interest of researchers in selective
hydroxylation of saturated hydrocarbons. In particular,
NiII(TPA) (TPA = tris(2-pyridylmethyl)amine, Chart 2) was
Chart 2. Line Drawings of TPA and TMG3tren
a
Underlined numbers indicate complexes studied by X-ray crystallog-
found effective for cyclohexane hydroxylation with m-
chloroperbenzoic acid (m-CPBA). A possible contribution of
the intermediate NiIII-oxido species was suggested.21,22 The
formation of NiIII−oxygen intermediates upon the reaction of
raphy.
The ability of these complexes to catalyze the microwave-
assisted solvent-free oxidation of cyclohexane by tert-butyl
hydroperoxide (TBHP) to the industrially significant mixture
of cyclohexanol and cyclohexanone (i.e., A/K oil) was tested,
and the results are discussed.
[NiI I (TMG3 tren)]2 + (TMG3 tren
= tris[2-(N-
tetramethylguanidyl)ethyl]amine, Chart 2) with m-CPBA at
low temperature was evidenced by electron paramagnetic
resonance (EPR) and UV−vis spectroscopic measurements.23
Tetraazamacrocyclic nickel(II) complexes with highly basic
ligand structures containing carboxamidate groups (e.g., III in
Chart 3) have been discovered to oxidize the cyclohexane to
cyclohexanone and cyclohexanol via high-valent NiIII−oxyl
radical intermediate.24,25
EXPERIMENTAL SECTION
■
New, more convenient synthetic protocols providing higher yields are
described below for previously communicated compounds 2, 3, and
6.29,30 Diphenylglyoxal-bis(S-methylisothiocarbohydrazonato)nickel-
(II) was synthesized by following a published protocol.30 1-(4-
Pyridinyl)butane-1,3-dione was obtained as described in the
literature.31
Chart 3. Nickel(II) Bis(amidate) Complex III
Synthesis of Complexes. Complex 1. To a hot solution of
NiCl2·6H2O (3.60 g, 15.1 mmol), S-methylisothiocarbohydrazidium
iodide (7.5 g, 30.2 mmol) and pyridine (5.0 g, 63.0 mmol) in water
(200 mL) was added dropwise over 30 min a solution of freshly
distilled diacetyl (1.3 g, 15.1 mmol) in water (30 mL). The mixture
was heated at 100 °C for 10 min, and after it cooled, the precipitate
formed was filtered off, washed thoroughly with water, and dried in
vacuo. The solution of this raw product in dimethylformamide (DMF;
80 mL) was added dropwise over 80 min to the boiling pentan-2,4-
dione (50 mL). The mixture was refluxed for 1 h, and after it cooled,
the precipitate formed was filtered off, washed with DMF, until the
washing solution became almost colorless, and then with methanol,
and dried in vacuo. Yield: 2.84 g (46%) of dark red crystals. Anal.
Calcd for C13H20NiN8S2 (Mr = 411.18): C, 37.97; H, 4.90; N, 27.25;
S, 15.60%. Found, %: C, 37.80; H, 4.72; N, 26.88; S, 15.21. IR (most
characteristic bands, cm−1): 1623m, 1512s, 1458s, 1414s, 1295s,
Nickel(II) complexes with S-substituted isothiosemicarba-
zides (H2NNC(SR)NH2) and isothiosemicarbazones
(R1R2CNNC(SR)NH2) are known for their redox non-
innocent properties.26 The isothiocarbohydrazides (NH2NC-
(SR)NHNH2) and isothiocarbohydrazones (R1R2CNNC-
(SR)NHNH2) are closely related derivatives and might also
exhibit redox noninnocent behavior.27 Moreover, vanadium-
(V) complexes with Schiff bases of carbohydrazone
(H2NNHCONHNH2) were found to be efficient in
peroxidative oxidation of cyclohexane in cyclohexanol and
cyclohexanone under solvent-free low-power microwave
(MW) irradiation. The key role of the carbohydrazone ligand,
which might undergo reduction to an active form of the
catalyst, was suggested from density functional theory (DFT)
calculations.28 Herewith we report on the template synthesis of
nickel(II) complexes with 15- and 14-membered octaazama-
1
1265s, 1159s, 1008s. H NMR CDCl3, δ, ppm: 2.10 (s, 3H), 2.30 (s,
3H), 2.44 (s, 6H), 2.51 (s, 3H), 2.60 (s, 3H), 3.60 (s, 2H). ESI MS
(positive ion mode for 58Ni): m/z 411 [M + H]+. UV−vis (CH2Cl2),
λmax, nm (ε, M−1 cm−1): 502 (5250), 463 (5073), 390 (3177),
323(11 730).
Complex 2. Diphenylglyoxal-bis(S-methylisothio-
carbohydrazonato)nickel(II) (1.0 g, 2.1 mmol) and pentan-2,4-
dione (4 mL, 39 mmol) were stirred at 130 °C under argon for 8 h.
The volatile components of the reaction mixture were removed under
reduced pressure. The residue was washed with ethanol until the
washing solution become almost colorless and purified by column
B
DOI: 10.1021/acs.inorgchem.9b01700
Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data and Details of Data Collection for 3−5 and 6·2DMF
compound
3
4
5
6·2DMF
empirical formula
fw
C28H26N8NiS2
597.40
C33H28N8NiS2
659.46
C27H25N9NiS2
598.39
C38H40N10NiO2S2
791.64
space group
a [Å]
b [Å]
P21/n
Pbcn
P21/c
P21/c
14.5257(14)
9.1384(5)
21.4350(18)
16.396(3)
22.138(4)
16.751(3)
15.2262(6)
11.3590(5)
16.4526(8)
11.0723(2)
9.3423(2)
18.1275(4)
c [Å]
α [deg]
β [deg]
γ [deg]
V [Å3]
Z
107.792(3)
112.421(2)
92.317(2)
2709.2(4)
4
6080(2)
8
2630.4(2)
4
1873.59(7)
2
λ [Å]
ρcalcd [g cm−3
0.710 73
1.465
0.700 00
1.441
0.710 73
1.511
1.541 86
1.403
]
crystal size [mm]
T [K]
0.12 × 0.09 × 0.03
100(2)
0.905
0.0600
0.1106
0.03 × 0.005 × 0.005
100(2)
0.777
0.0553
0.1597
0.13 × 0.06 × 0.03
100(2)
0.933
0.0450
0.0934
0.12 × 0.08 × 0.03
100(2)
2.186
0.0329
0.0804
μ [mm−1
]
a
R1
wR2
b
c
GOF
0.980
1.019
0.996
0.934
a
b
c
2
2
2
R1 = ∑||F0| − |Fc||/∑|F0|. wR2 = {∑[w(F0 − Fc2)2]/∑[w(F0 )2]}1/2. GOF = {∑[w(F0 − Fc2)2]/(n − p)}1/2, where n is the number of
reflections, and p is the total number of parameters refined.
chromatography on silica using chloroform as eluent. The second red
fraction collected was evaporated; the product was washed with
ethanol and dried in vacuo. Yield: 0.85 g (75%) of dark red crystals.
Anal. Calcd for C23H24NiN8S2 (Mr = 536.32): C, 51.60; H, 4.52; N,
20.93; S, 11.98%. Found: C, 51.74; H, 4.40; N, 20.63; S, 11.48%. IR
(most characteristic bands, cm−1): 1538m, 1505s, 1493s, 1441s,
659 [M + H]+. UV−vis (CH2Cl2), λmax, nm (ε, M−1 cm−1): 535
(16 734), 473sh, 410sh, 365(42 594), 335 (43 436).
Complex 5. A mixture of diphenylglyoxal-bis(S-methylisothio-
carbohydrazonato)nickel(II) (0.2 g, 0.42 mmol) and 1-(4-pyridinyl)-
butane-1,3-dione (0.8 g, 4.9 mmol) was heated at 130 °C under argon
for 8 h. After it cooled, the separated precipitate was filtered off and
washed with diethyl ether. The raw product was purified on silica by
using chloroform as eluent. The third red fraction was collected, and
the solvent was removed under reduced pressure. The product was
washed with ethanol (3 mL) and dried in vacuo. Yield: 0.11 g (44%)
of red crystals. Anal. Calcd for C27H25NiN9S2 (Mr = 598.37): C,
54.20; H, 4.21; N, 21.07; S, 10.72%. Found, %: C, 53.81; H, 4.13; N,
20.90; S, 10.41. IR (most characteristic bands, cm−1): 1589m, 1534s,
1487s, 1407vs, 1324vs, 1251s, 1208vs, 948vs, 701vs. 1H NMR CDCl3,
δ: 2.01 (s, 3H), 2.13 (s, 3H), 2.19 (s, 3H), 4.06 (s, 2H), 7.08, 7.18
(10H), 7.53 (s, 2H), 8.78 (s, 2H). ESI MS (positive ion mode for
58Ni): m/z 620 [M + Na]+, (negative ion mode): m/z 596 [M−H]−.
UV−vis (CH2Cl2), λmax, nm (ε, M−1 cm−1): 542 (11 042), 467sh,
405sh, 365 (26 792).
Complex 6. A mixture of diphenylglyoxal-bis(S-methylisothio-
carbohydrazonato)nickel(II) (1.0 g, 2.1 mmol) and benzil (2.0 g, 9.5
mmol) was heated at 140 °C under argon for 6 h. The residue was
washed thoroughly with acetone, DMF, and acetone, until almost
colorless washing solution, and dried in vacuo. Yield: 0.95 g (69%) of
dark green powder. Anal. Calcd for C32H26NiN8S2 (Mr = 645.43): C,
59.55; H, 4.06; N, 17.36; S, 9.94%. Found, %: C, 59.52; H, 4.04; N,
17.23; S, 10.25. IR (most characteristic bands, cm−1): 1486s, 1383s,
1302s, 1250s, 1208vs, 1093s, 949vs, 709vs, 691vs. 1H NMR CDCl3, δ,
ppm: 2,16 (s, 3H), 7.00−7.20 (20H). ESI MS (positive ion mode for
58Ni): m/z 645 [M + H]+. UV−vis (CH2Cl2), λmax, nm (ε, M−1
cm−1): 608 (8402), 562 (5882), 523sh, 448 (10 372), 429 (10 299),
400 (12 954), 377sh, 344 (18 238).
1
1398s, 1239vs, 1244s, 1247s, 1205vs, 944vs, 914s, 694vs. H NMR
CDCl3, δ, ppm: 2.13 (s, 3H), 2.16 (s, 3H), 2.45 (s, 3H), 2.50 (s, 3H),
3.72 (s, 2H), 7.03−7.12 (m, 6H), 7.14−7.19 (m, 4H). ESI MS
(positive ion mode for 58Ni): m/z 535 [M + H]+. UV−vis (CH2Cl2),
λmax, nm (ε, M−1 cm−1): 519 (11 200), 468sh, 402 (8207), 340
(19 615).
Complex 3. Diphenylglyoxal-bis(S-methylisothio-
carbohydrazonato)nickel(II) (1.0 g, 2.1 mmol) and excess of
benzoylacetone (1.7 g, 10.4 mmol) were heated at 140 °C under
argon for 6 h. Then the unreacted benzoylacetone was removed under
reduced pressure. The residue was purified on silica by using
chloroform as eluent. The main red fraction collected was evaporated,
and the product was washed with ethanol and dried in vacuo. Yield:
0.92 g (66%) of red crystals. Anal. Calcd for C28H26NiN8S2 (Mr =
597.39): C, 56.30; H, 4.39; N, 18.76; S, 10.74%. Found, %: C, 55.93;
H, 4.33; N, 18.32; S, 10.44. IR (most characteristic bands, cm−1):
1504s, 1458s, 1440s, 1399s, 1324s, 1292vs, 1247s, 1208vs, 949s, 926s,
1
699vs. H NMR CDCl3, δ, ppm: 1.94 (s, 3H), 2.14 (s, 3H), 2.15 (s,
3H), 4.05 (s, 2H), 6.97−7.57 (m, 15). ESI MS (positive ion mode for
58Ni): m/z 597 [M + H]+. UV−vis (CH2Cl2), λmax, nm (ε, M−1
cm−1): 534 (10 949), 469sh, 409 (12 623), 362 (25 017).
Complex 4. A mixture of diphenylglyoxal-bis(S-methylisothio-
carbohydrazonato)nickel(II) (1.0 g, 2.1 mmol) and dibenzoyl-
methane (1.5 g, 6.7 mmol) was heated at 130 °C under argon for
6 h. After it cooled, the residue was stirred in hot EtOH (10 mL) for
20 min, filtered off, washed with ethanol and dried, then dissolved in
CHCl3, and purified by column chromatography (SiO2, eluent
CHCl3). The second red fraction was collected, and the volatiles
were evaporated; the product was washed with ethanol and dried in
vacuo. Yield: 0.92 g (66%) of red crystals. Anal. Calcd for
C33H28NiN8S2 (Mr = 659.45): C, 60.10; H, 4.28; N, 16.99; S,
9.73%. Found, %: C, 59.85; H, 4.22; N, 16.88; S, 9.81. IR (most
characteristic bands, cm−1): 1494s, 1461s, 1441s, 1396s, 1295s,
Crystallographic Structure Determination. X-ray diffraction
quality single crystals of 3−5 were grown by slow evaporation of
chloroform−ethanolic solutions. Crystals of 6·2DMF were obtained
by recrystallization of 6 from DMF. The measurements were
performed on a Bruker X8 APEXII CCD (3 and 5) and STOE (6·
2DMF) diffractometers. Single crystals were positioned at 35, 35, and
40 mm from the detector, and 512, 1645, and 7610 frames were
measured, each for 20, 40, and 2/12 s over 0.5, 0.5, and 0.25° scan
width for 3, 5, and 6·2DMF, respectively. The data were processed
using SAINT and STOE X-RED software.32 Compound 4 was
1
1209vs, 934s, 691vs. H NMR CDCl3, δ, ppm: 2.19 (s, 3H), 2.21 (s,
3H), 4.55 (s, 2H), 7.07(s, 6H), 7.11−7.23(m, 6H), 7.44−7.49(m,
6H), 7.56−7.59(m, 2H). ESI MS (positive ion mode for 58Ni): m/z
C
DOI: 10.1021/acs.inorgchem.9b01700
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measured at the XRD2 structural biology beamline, Elettra
synchrotron. The beamline is equipped with an Arinax MD2S
diffractometer and a Pilatus 6 M area detector, and monochromated
X-rays are provided by a super conducting wiggler followed by a dual
crystal monochromator. Frame integration was performed using the
XDS package.33 Up to 12 crystals of 4 were tested. The sample
generally showed poor crystallinity, and, while most data sets could be
processed to reasonable Rint values, the structure could not be refined
to an R factor below 10%. Twin analysis was inconsistent, and it was
concluded that the measured crystals were aggregates. At this point a
small crystal of 5 × 5 × 30 μm was selected. The crystal was weakly
diffracting, and the resolution of the collected X-ray data has been
estimated to be 1.1 Å. Nevertheless the structure could be solved, and
electron density maps allowed for the determination of the atomic
connectivity. Crystal data, data collection parameters, and structure
refinement details are given in Table 1. The structures were solved by
direct methods and refined by full-matrix least-squares techniques.
Non-H atoms were refined with anisotropic displacement parameters.
H atoms were inserted in calculated positions and refined with a
riding model. The following computer programs and hardware were
used: structure solution, SHELXS-2014 and refinement, SHELXL-
2014;34 molecular diagrams, ORTEP;35 computer, Intel CoreDuo.
See Supporting Information for more information about CCDC
1921435 (3), 1921438 (4), 1921436 (5), and 1921437 (6·2DMF).
Electrochemistry and Spectroelectrochemistry. The cyclic
voltammetric studies were performed in a homemade miniature
electrochemical cell using platinum-disk working electrode (from
Ionode, Australia), a platinum wire as the counter electrode, and silver
wire as pseudoreference electrode. Ferrocene served as the internal
potential standard, and the potentials were determined versus
ferricenium/ferrocene couple. A Heka PG310USB (Lambrecht,
Germany) potentiostat with a PotMaster 2.73 software package was
used in cyclic voltammetric and spectroelectrochemical studies. In situ
spectroelectrochemical measurements were performed on a spec-
trometer Avantes, model AvaSpec-2048 × 14-USB2, under an argon
atmosphere in a spectroelectrochemical cell kit (AKSTCKIT3) with
the Pt-microstructured honeycomb working electrode, purchased
from Pine Research Instrumentation. Halogen and deuterium lamps
were used as light sources (Avantes, model AvaLight-DH-S-BAL).
The cell was positioned in the CUV-UV Cuvette Holder (Ocean
Optics) connected to the diode-array UV−vis−NIR (NIR = near-
infrared) spectrometer by optical fibers. UV−vis−NIR spectra were
processed using the AvaSoft 7.7 software package. EPR spectra were
recorded with the EMXplus X-band EPR spectrometer (Bruker,
Germany).
1−20 μmol of 1−6, 2.50 mmol of cyclohexane, and 5.00 mmol of 70%
aqueous (aq) tert-butyl hydroperoxide (TBHP) were added. The tube
was placed in the MW reactor, and the system was stirred (800 rpm)
and irradiated (10 W) for 0.5−3 h at 60 to 100 °C. After the reaction,
the mixture was cooled to room temperature, and a sample was
analyzed by gas chromatography (GC). For the MW assays in the
presence of a radical trap, NHPh2 in a stoichiometric amount relative
to TBHP was further added to the reaction mixture. For the reactions
performed under conventional heating, the reagents (as above) were
introduced into a round-bottomed flask (25 mL) adapted to a reflux
condenser, immersed in an oil bath (60−100 °C).
A FISONS Instruments GC 8000 gas chromatograph equipped
with a DB-624 (J&W) capillary column (with flame ionization
detector (FID)) and Jasco-Borwin v.1.50 software were used for the
GC measurements. Helium was the carrier gas. The temperature of
the injection was 240 °C. The initial temperature (120 °C) of the
column was maintained for 1 min, then raised 10 °C/min to 200 °C,
and held at this temperature for 1 min. The attribution of the GC
peaks was accomplished by comparison with the chromatograms of
pure cyclohexane, cyclohexanol, and cyclohexanone. Quantification by
the internal standard method was performed on the basis of
calibration curves constructed with known concentrations of samples
of cyclohexane, cyclohexanol, cyclohexanone, and nitromethane.
Subsequently, following a method developed by Shul’pin,53−59 an
excess of triphenylphosphine was added to the sample, and the
resulting mixture was analyzed again to estimate the amount of the
cyclohexyl hydroperoxide primary product formed. Gas chromatog-
raphy−mass spectrometry (GC-MS) analyses were performed using a
PerkinElmer Clarus 600 C instrument (using helium as the carrier
gas). The ionization voltage was 70 eV. GC experiments were
conducted in the temperature-programming mode, using an SGE
BPX5 column (30 m × 0.25 mm × 0.25 μm). The products were
identified by comparison of their retention times with pure reference
compounds, as well as of their mass spectra to fragmentation patterns
obtained from the National Institute of Standards and Technology
(NIST) spectral library of the software of the mass spectrometer.
Blank experiments, in the absence of 1−6, were performed under
the optimized reaction conditions, and no cyclohexane conversion
was detected. Experiments with NiCl2·6H2O were run using the
optimized reaction conditions found for 1−6. The obtained A/K oil
yield under such conditions was 0.8%.
RESULTS AND DISCUSSION
Synthesis of Complexes. Complexes 1−6 were prepared
in two steps as shown in Scheme 1. First, the reaction of
■
Computational Details. Standard B3LYP/6-311G*36−42 geom-
etry optimization in various spin states was performed using the
Gaussian0943 program package. For open-shell systems, the
unrestricted DFT formalism was used; that is, B3LYP implies
UB3LYP. Solvent effects in dichloromethane, acetonitrile, and
dimethylformamide were approximated by the integral equation
formalism polarizable continuum model (IEFPCM)44,45 as imple-
mented in Gaussian09. The stability of the optimized structures was
confirmed by vibrational analysis (no imaginary vibrations). The g
tensors46,47 were evaluated using the ORCA 3.0.3 software package.48
Calculation in ORCA accounted for the COSMO49 solvent model of
dichloromethane using BLYP and B3LYP functionals. Hyperfine
interaction constants were approximated by the Fermi contact
interaction as given in the Gaussian09 output files. The IQmol
package50 was used for the visualization of the molecular orbitals and
spin densities using the Gaussian09 fchk. The atomic d-populations
on the central nickel atom and the atomic orbital populations of
localized orbitals were obtained via the Mulliken population analysis
(MPA) as implemented in Gaussian09 and ORCA packages. Atomic
charges and bond characteristics were obtained from the Quantum
Theory of Atoms in Molecules (QTAIM)51 using the AIMAll
package.52
Scheme 1. Synthesis of 1−6
diacetyl and/or diphenylglyoxal with 2 mol equiv of S-
methylisothiocarbohydrazidium iodide in water or ethanol in
the presence of pyridine or ammonia as a base afforded the
nickel(II) complexes with open-chain tetradentate ligands
(species A in Scheme 1).
Then the reactions of the latter with glyoxal, acetylacetone
(Hacac), benzoylacetone (Hbzac), dibenzoylmethane, 1-(4-
pyridinyl)-butane-1,3-dione, and diphenylglyoxal afforded the
products 1−6, respectively, in different yields (11 to 69%).
The formation of macrocyclic complexes 1−6 was confirmed
by the strong peaks in positive ion ESI mass spectra, which
Catalytic Studies. The catalytic tests for cyclohexane oxidation
were performed in a microwave Anton Paar Monowave 300 (25 W),
using a G10 MW borosilicate glass tube. To the G10 reaction tube,
D
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Figure 2. (a) ORTEP plot of 4 with atom-labeling scheme and
thermal ellipsoids drawn at 50% probability level. Selected bond
distances (Å), bond angles, and dihedral angles (deg): Ni−N2 =
1.851(7), Ni−N5 = 1.861(8), Ni−N10 = 1.882(7), and Ni−N13 =
1.798(8); N2−Ni−N5 = 83.7(4), N5−Ni−N10 = 102.0(4), N10−
Ni−N13 = 82.4(4), N2−Ni−N13 = 91.7(4), and N6−C7−C8−C9 =
−79.7(11). (b) ORTEP plot of 6 with atom-labeling scheme and
thermal ellipsoids drawn at 50% probability level. Selected bond
distances (Å) and bond angles (deg): Ni−N2 = 1.8183(15), Ni−N5
= 1.7928(15); N2−Ni−N5 = 84.82(7), N5i−Ni−N2 = 95.18(7). (i)
−x + 1, −y + 1, −z + 1.
Figure 1. (a) ORTEP plot of 3 with atom-labeling scheme and
thermal ellipsoids drawn at 50% probability level. Selected bond
distances (Å), bond angles, and dihedral angles (deg): Ni−N2 =
1.846(4), Ni−N5 = 1.872(4), Ni−N10 = 1.897(4), and Ni−N13 =
1.809(4); N2−Ni−N5 = 82.98(16), N5−Ni−N10 = 102.21(17),
N10−Ni−N13 = 82.62(16), N2−Ni−N13 = 92.18(17); N6−C7−
C8−C9 = 83.2(6); (b) ORTEP plot of 5 with atom-labeling scheme
and thermal ellipsoids drawn at 50% probability level. Selected bond
distances (Å), bond angles, and dihedral angles (deg): Ni−N2 =
1.8382(19), Ni−N5 = 1.861(2), Ni−N10 = 1.8812(19), and Ni−N13
= 1.806(2); N2−Ni−N5 = 83.83(8), N5−Ni−N10 = 102.24(8),
N10−Ni−N13 = 82.27(8), N2−Ni−N13 = 92.37(9); N6−C7−C8−
C9 = −81.3(3).
result in the formation of predominantly one geometric isomer
from the two possible (vide infra).
could be attributed to [M + H]+ or [M + Na]+ ions. 1H NMR
spectra were in accordance with the C1 molecular symmetry for
1−5. Notice that the spectrum of 6 is in agreement with the
C2h molecular symmetry suggested for this compound, at least
for some particular conformations. The thiomethyl group
protons were observed in the spectrum at 2.16 ppm.
The isolation of intermediates A was possible because of the
steric nonequivalence of end amine groups in S-methyliso-
thiocarbohydrazidium iodide60 (Figure S1). Independent of
the conformation adopted, the thiomethyl group creates steric
hindrance for one of the two end NH2 groups. Note that only
one geometric isomer from the two possibilities was isolated
for 3 and 5. The structures of the isomers shown below are
favored, since one of the SCH3 groups exerts steric hindrance
at the end noncoordinated NH2 group, whereas the second is
far away from the coordinated NH2 group and is not able to
affect the availability of this for further condensation reaction.
Distinct steric requirements for R3 and R4 in 1,3-diketone
X-ray Crystallography. The results of X-ray diffraction
studies of 3−5 and 6 are shown in Figures 1 and 2,
respectively, with selected bond distances and bond angles
quoted in the legends. All complexes studied, except 4,
crystallized in monoclinic space groups, 3 crystallized in P21/n,
and 5 and 6 crystallized in P21/c. Complex 4 crystallized in the
orthorhombic Pbcn space group. The crystals of complexes 3−
5 do not contain cocrystallized solvent, whereas 6 contains two
cocrystallized DMF molecules per molecule of 6. Complexes
3−5 contain 15-membered octaazaamacrocyclic ligands, while
6 has a 14-membered octaazamacrocyclic ligand. The 15-
membered ligands are not planar due to the presence of one
sp3-hybridized methylene group in the macrocycle, which
interrupts the conjugation, while the 14-membered macrocycle
in 6 is essentially flat. The nickel(II) in 3−5 and 6 adopts a
square-planar N4 coordination geometry due to a strong ligand
field of the tetradentate ligands. The bond lengths Ni−N2,
Ni−N5, Ni−N10, and Ni−N13 in 3 and 5 are very similar.
E
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Figure 3. Cyclic voltammograms for complexes (a) 1, (b) 2, (c) 4, and (d) 6 in DCM/nBu4NPF6 solutions (Pt-disc working electrode, scan rate
100 mV s−1).
Table 2. Electrochemical Data Versus Fc+/Fc for 1−6 in
arrangement of macrocyclic complexes and of cocrystallized
DMF molecules in the crystal of 6·2DMF.
a
DCM/nBu4NPF6 Solutions
Electrochemistry and Spectroelectrochemistry. Two
reversible oxidation waves and one or two reduction steps were
observed for all investigated NiIIL complexes 1−6 in the
potential window available, as shown in cyclic voltammograms
in Figure 3 for 1, 2, 4, and 6 in dichloromethane (DCM)
solutions. Very similar results were obtained in ACN solutions
of the complexes (not shown), although the solubility was
markedly lower. The UV−vis spectra of initial solutions of 1−6
are shown in Figure S6.
Electrochemical data for 1−6 derived from the cyclic
voltammetry in DCM/nBu4NPF6 solutions using Pt working
electrode and a scan rate of 100 mV s−1 are summarized in
Table 2.
In contrast to the cathodic reduction, the first anodic
oxidation step of 1−5 is much less dependent on the
macrocycle substitution (half-wave oxidation potential is in
the range from +0.53 to +0.69 V vs Fc+/Fc) suggesting a
partial contribution of the NiII ion to the redox behavior. Only
for 6, a higher oxidation potential at +0.79 V versus Fc+/Fc
was observed. The second oxidation peak is, for all complexes,
more dependent on the substitution pattern indicating a
ligand-centered second electron transfer. The oxidation of 1
shows electrochemically irreversible characteristics at 100 mV
s−1, but the cyclic voltammograms become more reversible at
red
ox1
ox2
complex
E1/2
E1/2
0.53
E1/2
0.82
1
2
3
4
5
6
−2.08
−1.90
−1.80
−1.62
−1.75
−1.38
−2.01
0.58
0.57
0.57
0.69
0.79
0.96
0.95
0.87
1.06
1.03
a
Electrochemical data in volts. Pt working electrode, scan rate 100
mV s−1.
Three kinds of chelate cycles, namely, five-, six-, and seven-
membered, are present in 3−5, and only two, five- and six-
membered, are in 6. A 10° increase of the bite angle is of note
when passing from five- to six- and to seven-membered chelate
rings in 3−5. The same incremental increase of the bite angle
is observed in 6 when going from five- to six-membered chelate
rings. A portion of the crystal structure of 3 showing the
formation of ribbons via weak intermolecular contacts C−H···
S of 3.74 Å is shown in Figure S2, while a fragment of one-
dimensional (1D) chain in the crystal structure of 4 is shown
in Figure S3. Figure S4 reveals the presence of dimers in the
crystal structure of 5 associated via weak intermolecular
contacts Ni···S of 3.42 Å, while Figure S5 shows a 1D
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Figure 5. In situ UV−vis−NIR and ex situ EPR spectroelectrochem-
istry of 2 in ACN/nBu4NPF6 under an argon atmosphere: (a) UV−
vis−NIR spectra measured in the region of the first anodic peak upon
forward scan with the corresponding cyclic voltammogram (see inset,
scan rate v = 10 mV s−1); (b) representative EPR spectrum (black
line) observed for [2]+ in frozen ACN solution at 110 K and its
simulation (red line, g1 = 2.169, g2 = 2.126, g3 = 1.986).
Figure 4. Cyclic voltammograms for 1 in the region of the first
oxidation peak (a) and going to the second oxidation wave (b) at
different scan rates in DCM/nBu4NPF6 (Pt-disc working electrode).
higher scan rates (1000 mV s−1), as shown in Figure 4,
indicating moderate stability of the generated cation 1+.
ACN was found to be the best solvent for spectroelec-
trochemistry of complexes 1−6 in the anodic part. Upon
anodic oxidation of 2 in ACN/nBu4NPF6, a reversible redox
couple was observed in the corresponding cyclic voltammo-
gram even at a low scan rate of 10 mV s−1. The oxidation of 2
was accompanied by a simultaneous evolution of new
absorption band at 640 nm via isosbestic points at 565 and
475 nm (Figure 5a).
Simultaneously, a single-line X-band EPR spectrum was
observed with giso value of 2.0995 and a line width ΔBpp = 4.3
mT at room temperature (Figure S6a). The shift of the giso
from the free electron value of 2.0023 indicates that the one-
electron oxidation of 2 generates a ligand-centered radical with
a pronounced delocalization of the unpaired spin onto the
nickel ion.61−64 A spectroelectrochemical experiment coupled
with X-band EPR measurement at 110 K allowed the EPR
characterization of electrochemically generated [2]+ in frozen
solution (Figure 5b). The rhombic EPR signal with a rather
large g-tensor anisotropy (g1 = 2.172, g2 = 2.125, and g3 =
1.979) suggests that, while [2]+ cannot be explicitly described
as a nickel(III) species, the contribution of the central nickel
ion is rather significant.64 Similar redox behavior and EPR
spectra were measured also for other 1e-oxidized nickel(II)
complexes as shown for 5 in Figure S7b.
In the cathodic part, the most negative half-wave reduction
potential (E1/2red) was observed for 1, featuring only methyl
substituents, at −2.08 V versus Fc+/Fc, while less negative
reduction potential at E1/2 = −1.75 V versus Fc+/Fc was
observed for 4 with four phenyl substituents instead of methyl
groups. Even less negative reduction potential was found for 5
at −1.62 V versus Fc+/Fc, where pyridyl was present instead of
one phenyl group. Generally, the reduction potential and
reversibility depend on the number of phenyl or methyl
substituents at the macrocycle indicating a ligand-based
reduction. Two nearly reversible cathodic waves were observed
only for 6 at half-wave potentials −1.38 and −2.01 V versus
Fc+/Fc. Fully reversible spectroelectrochemical response was
observed in the in situ UV−vis−NIR spectroelectrochemistry
of 6 (Figure 6), which contains the most extended π-system
within the macrocycle. New absorption bands at 490 and 910
nm appear in the region of the first cathodic peak via isosbestic
points and are typical for anion radicals with delocalized spin
within the π-conjugated system. A broad slightly anisotropic
intense EPR spectrum with g-value of giso = 2.0004 character-
istic for the ligand-based radical was observed confirming a
purely ligand-based reduction and delocalized spin within the
macrocycle.
G
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studied species (neutral, single oxidized/reduced, and/or
double reduced) and spin states are collected in Table 3.
The correlation trend for oxidation (Figures 7c,d) is less linear
than that for the reduction, with 5 (presence of pyridine
group) being the outlier, likely because the differences among
the complexes occur in a considerably smaller potential
window.
Following the B3LYP/6-311G* calculation we found the
1
singlet states to be preferred in the case of neutral [NiIIL]0
species and doublets in the case of [NiIIL·]− and [NiIIL·]+.
According to the B3LYP/6-311G* frontier orbitals and spin
densities, shown in Figure 8 for 4, the first oxidation and
reduction events are both mostly ligand-centered. However,
the oxidation shows a partial contribution of the central NiII
ion, which is reflected by the shapes and Ni atom populations
of B3LYP/6-311G* highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) of 4 (Figure 8a−d), as well as by the particular
spin densities of the corresponding radical cation and radical
anion (Figure 8e,f), albeit the B3LYP/6-311G* calculations
favor a ligand-centered redox process, unless the order of the
closely degenerate HOMO and HOMO−1 is not reversed due
to the choice of the functional used (vide infra).
2
2
Furthermore, the d-orbital Mulliken populations on Ni as
well as the QTAIM and/or Mulliken charges and spins of Ni
(see Table 4) of the neutral, oxidized, and reduced species
remain almost constant, being in line with the interpretation of
preferably ligand-centered redox processes. Nevertheless,
according to experimental findings, the oxidation process
(smaller effect of ligand substitution on the potential) as well
as 2[NiIIL·]+ itself (the EPR signal) are affected by the Ni ion.
HOMO−1 is close in eigenvalue with HOMO, and it has a dxy-
orbital population of 19%; hence, the choice of DFT functional
might affect the frontier orbitals order. For instance, the BLYP
functional HOMO has a 48% Ni character (not shown). In
addition, the B3LYP/6-311G* d-orbital populations shown in
Table 4 agree well with crystal field theory (CFT) ordering of
NiII d-orbitals of square-planar (D4h) coordination geometry;
that is, the CFT-ordered d-orbitals according to energy
correlate with the population on a given d orbital.
Figure 6. In situ UV−vis−NIR spectroelectrochemistry of 6 in DMF/
nBu4NPF6 in a thin layer cell: potential dependence of UV−vis−NIR
spectra with corresponding cyclic voltammogram (scan rate 20 mV
s−1) in (a) forward and (b) backward scans.
Following the B3LYP/6-311G* localized orbitals of all
species in Table 4, one finds eight d electrons on Ni; hence,
one can assign the formal oxidation state of nickel to 2+. An
additional discrepancy with CFT is the removed degeneracy of
dyz and dxz atomic orbitals (AOs) at the B3LYP/6-311G* level
of theory when considering the obtained populations. This is
likely related to the fact that the macrocyclic ligand is not of
square-planar (D4h) symmetry, and hence bonds along x and y
axes are different. This is further confirmed by the QTAIM
bond critical point (BCP) characteristics (see Table S2).
Calculated EPR B3LYP/6-311G* and BLYP/6-311G* g-
tensor parameters are shown in Table S3. The BLYP functional
was utilized because of a closer agreement with the anisotropy
of the oxidized species found experimentally. The reduced
2[4]− species has g-tensor components closer to a free electron
ge value. Still, the gxx component is below 2.000 what is in line
with a small contribution of the central ion. In the case of the
Note that the redox behavior in the cathodic part is much
more complex compared to the anodic oxidation, and this
aspect is a matter of more extended study that will be reported
separately. The results of these studies are in good agreement
with DFT calculations, which show that frontier molecular
orbitals of 4 are mainly ligand-centered. Both, 1e-reduction
and 1e-oxidation are expected to take place at the ligands
rather than at nickel(II) (vide infra).
Theoretical Studies. To get a deeper insight into the
electronic structures of 1−6, their geometries were optimized
using DFT (see Experimental Section). The structural
parameters of the DFT-optimized structures match reasonably
well with the experimental metrical parameters from single-
crystal X-ray diffraction data, as showcased for compounds 3−
6 in Table S1.
In addition, the correlation of calculated electronic structure
parameters relevant to the acceptance (εLUMO and electron
affinity) of an electron with the experimentally measured redox
behavior of 1−6 is quite good. One can see an almost perfect
linear dependence in the case of reduction (see Figure 7a,b);
the R factor is 0.9581 for εLUMO versus reduction potential
(Figure 7a). In the case of electron affinity versus reduction
potential, the R factor is 0.9819 (Figure 7b). The energies of all
2
oxidized species [2]+, the obtained electronic structure leads
to an underestimated g value compared to the experiment (see
Figure 5b), despite the BLYP functional choice, which leads to
a larger contribution of the Ni ion. To improve the agreement
2
with the larger anisotropy of [2]+ found experimentally, the
HOMO β [eigenvalue −0.2769; 30.5% Ni(d) character] and
H
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Figure 7. Correlation of theoretically determined parameters (a) εLUMO, (b) electron affinity, (c) εHOMO, and (d) ionization energy with the
red
ox1
experimentally determined reduction E1/2 and oxidation E1/2 potentials of 1−6.
a
Table 3. Total 6-311G* Energies of All Studied Species and Spin States
charge
MS
1
2
3
4
5
6
0
0
−1
−1
1
1
2
2
1
3
2
4
2
4
1
3
−3250.378 52
−3250.344 39
−3250.465 86
−3250.429 49
−3250.1861
−3250.154 32
−3249.947 67
−3249.9452
−3633.922 22
−3633.887 55
−3634.016 66
−3633.980 68
−3633.728 73
−3825.693 84
−3825.659 94
−3825.791 85
−3825.756 25
−3825.500 58
−4017.4710
−4017.4367
−4017.5709
−4017.534 99
−4017.2772
−4017.2447
−4017.0443
−4017.0462
−3841.733 06
−3841.674 92
−3841.842 21
−3841.806 52
−3841.536 85
−3978.156 14
−3978.113 09
−3978.273 07
−3978.2151
−3977.955 34
−3977.905 22
−3977.721 01
−3977.720 66
−3633.493 66
−3633.492 11
−3825.266 43
−3825.264 29
−3841.302 39
−3841.299 71
a
All values are in hartrees. Spin multiplicity is denoted MS
LUMO β [eigenvalue −0.2680; 4.8% Ni(d) character] were
rotated (BLYProt) to calculate the g-tensor values, which lead
to a further improvement in comparison to experiment.
Nevertheless, the agreement is still not perfect; that is, the two
largest g-components and the width of the spectrum are still
underestimated. The largest anisotropy is found after the
inclusion of solvent effects (COSMO − DCM) for the BLYProt
g-values. Similar results were also obtained for the 2[4]+
species, with the anisotropy and width of the spectrum being
larger compared to 2[2]+. The g-tensor is found ligand-
occupied molecular orbital (SOMO) is in agreement with
the experimental evidence contrary to [2]+ and [4]+. Even
though the macrocyclic ligands in 1−6 create a strong ligand
field favoring the formation of square-planar nickel(II)
complexes and making further axial coordination of coligands
unlikely, we observed dimeric associates in the crystal structure
of 5 hold together by weak Ni···S contacts of 3.42 Å.
Therefore, we decided to test the activity of 1−6 in catalytic
cyclohexane oxidation reactions.
2
2
Catalytic Studies. In the present study, the MW-induced
neat oxidation of cyclohexane to A/K oil (Scheme 2) was
chosen as a benchmark reaction. Nickel(II) complexes 1−6
were tested as catalysts for the above reaction, using aqueous
TBHP as oxidant.
2
centered in the case of [6]+. Hence, the HOMO−LUMO
rotation for 2[6]+ cannot be considered as appropriate.
2
Although the HOMO/LUMO orbitals of [6]+ are closely
degenerate, the dominant ligand character of the singly
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irradiation at 100 °C in the presence of 10 μmol of catalyst,
leading to a maximum A/K oil yield of 23% (Figure 9) with a
TON value of 1.1 × 102.
The ability of compounds 1−6 to catalyze the oxidation of
cyclohexane follows the trend found for their reduction
potentials (vide supra), with the total yields of cyclohexanol
and cyclohexanone, obtained under the optimized assayed
conditions, from 7 (for 1) to 23% (for 6) and TON from 37 to
113. The compound easiest to reduce, 6, exhibits the highest
catalytic activity. The catalytic activity decreases with the
decrease of reduction potential, with 1 exhibiting the most
negative reduction potential and lowest A/K oil yield obtained.
Thus, the significant effect of the substituents at the
macrocycle suggests the reduction potential as an important
factor for their catalytic behavior.
In fact, the cyclohexane oxidation mechanism appears to be
of radical nature. One evidence results from the performed
experiments to detect the formation of cyclohexyl hydro-
peroxide as a primary product, following a method proposed
by Shul’pin:53−59 the detected cyclohexanol amount signifi-
cantly increased after addition of triphenylphosphine to the
reaction mixture (due to the reduction of CyOOH by PPh3,
with the formation of phosphane oxide) and a decrease in the
amount of cyclohexanone. Moreover, the experiments
involving the addition of the oxygen radical trap (Ph2NH) to
the reaction mixture led to a significant yield drop of over 93%,
when compared to the reaction performed under the same
conditions in the absence of such a radical trap. Thus, the
MW-promoted oxidation of neat cyclohexane catalyzed by the
nickel(II) complexes 1−6 should proceed through a free-
radical mechanism, where the availability of reducible NiIIL
octaazamacrocyclic complexes (as proved by electrochemistry)
by the hydroperoxide would be crucial for the oxidation to
occur (see Scheme 3).28 Consequently, the Ni-catalyzed
formation of t-BuOO· and t-BuO· radicals is a key step for
the occurrence of the C−H abstraction from the cyclohexane,
leading to to the formation of cyclohexyl radical, the beginning
of the cyclohexane conversion, that proceeds by a radical path
to the desired products cyclohexanol and cyclohexanone. The
involvement of t-BuOOH in reduction-based chemistry was
evidenced by following the reaction of the latter with 6 by
UV−vis and EPR spectroscopies. A very small amount of
reduced 6 is clearly seen after addition of TBHP in Figure S8.
EPR spin-trapping experiments with 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO) spin trap and TBHP in the presence of 6
revealed higher amount of reactive radicals than in the absence
of nickel(II) complex (Figure S9) attesting the catalytic activity
of 6 with involvement of radical-based reactions. Note that
similar behavior was reported previously for the systems with
H2O2.70,71
1
Figure 8. (a−d) Frontier orbitals of [4]0 (eigenvalues are shown in
parentheses; in hartrees), isosurface value is 0.06 e·bohr−3 and (e, f)
spin density of (e) [4]− and (f) [4]+, isosurface value is 0.002 e·
2
2
bohr−3.
Table 4. Mulliken d-Orbital Populations of Different
Species of 4 and MPA and QTAIM Charges and Spins of 4
in DCM
2
2
2
species
dx −y
dxy
dz
dyz
dxz
sσ
dtotal
1[4]0
2[4]−
2[4]+
1.05
1.04
1.08
1.66
1.67
1.62
1.85
1.84
1.86
1.86
1.87
1.87
1.93
1.93
1.93
0.57
0.61
8.36
8.35
8.36
0.53
MPA
QTAIM
charge
spin
0.00
charge
spin
1[4]0
2[4]−
2[4]+
1.02
0.98
1.05
0.97
0.95
0.99
0.00
0.01
−0.02
−0.01
−0.01
Scheme 2. Microwave-Assisted Neat Oxidation of
Cyclohexane to A/K Oil with tert-Butylhydroperoxide
Catalyzed by 1−6
As previously observed for other catalytic systems28,65−67 the
used low (25 W) power of MW irradiation provides a much
more efficient synthetic method than conventional (non-
pressurized refluxing) heating, allowing the fast achievement of
significantly higher yields (e.g., for compound 6, 23% A/K oil
yield by 2 h of MW irradiation in comparison with 6.4% using
an oil bath heating).
Our catalysts 1−6 are much more effective and selective
than the [Ni(tpa)] (tpa = tris(2-pyridylmethyl)amine))68 or
the [Ni(OAc)2(H2O)(LtBu)] (LtBu = N,N-bis(2-pyridylmeth-
yl)-N-{(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl}amine)69
complexes, which require the much stronger m-chloroperben-
zoic acid (mCPBA) as oxidant and lead to cyclohexanol,
The effect of several catalytic parameters, such as reaction
temperature and time, catalyst, and oxidant amount, on the
catalytic activity (in terms of turnover number (TON),
turnover frequency (TOF), and/or yield) was investigated.
The optimized reaction conditions were: 2 h of MW
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Figure 9. Total yield of cyclohexanol and cyclohexanone obtained by MW-induced neat oxidation of cyclohexane with THBP catalyzed by the
nickel(II) complexes 1−6 (10 μmol) at 100 °C.
unpaired spin into the orbitals of nickel ion. The EPR spectra
are neither typical nickel(III) nor organic radical, in character,
which indicates a system with a redox noninnocent ligand. The
found redox behavior was further confirmed by the shapes of
the HOMO and LUMO orbitals, visualization of spin densities,
and population analysis.
Scheme 3. Reaction Mechanism of Catalyzed Oxidation of
Cyclohexane in the Presence of Octaazamacrocyclic
Nickel(II) Complexes
Compounds 1−6 act as catalysts for the MW-induced and
solvent-free oxidation (with aq TBHP) of cyclohexane, via a
radical mechanism, affording selectively cyclohexanol and
cyclohexanone and in good yields (up to 23%), as a promising
alternative to the industrial aerobic process, which also
operates at higher temperature. The catalytic activity was
found to correlate well with the easy of reduction of the
nickel(II) complexes with noninnocent macrocyclic ligands,
with complex 6 as the most active in the series and easiest to
reduce.
cyclohexanone, and ε-caprolactone (from Baeyer−Villiger
reaction of cyclohexanone with mCPBA) mixtures. They are
also more efficient catalysts than the nickel(II) complexes of
N-heterocyclic carbene (NHC) ligands trans-[NiX2(NHC)2]
(X = Cl or I), which afford a maximum A/K-oil yield of 15%,
with a cyclohexane-to-TBHP ratio of 1:14 and 12 h reaction
time at 70 °C, thus in much more drastic conditions than those
used in the present study.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01700.
■
S
X-ray diffraction structure of S-methylisothiocarbohy-
drazidium iodide, details of crystal structures for 3−5
and 6·2DMF, EPR spectra of [2]+ and [5]+, details of
DFT calculations (PDF)
CONCLUSIONS
■
Stepwise template building of octaazamacrocyclic nickel(II)
complexes 1−6 was possible due to symmetry nonequivalence
of two end amine groups in S-methylisothiocarbohydrazide or
S-methylisothiocarbohydrazidium iodide. This symmetry non-
equivalence results from thiomethylation of thiol sulfur atom in
thiocarbohydrazide due to the formation of a double bond and
steric hindrance of one of the end NH2 groups by SCH3 group.
For the same reason, the template condensation reaction of
intermediate species A in Scheme 1 with benzoylacetone and
1-(4-pyridinyl)-butane-1,3-dione results mainly in the for-
mation of one geometric isomer, the structure of which was
confirmed by X-ray crystallography of 3 and 5.
Accession Codes
CCDC 1921435−1921438 contain the supplementary crys-
tallographic 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 Authors
*E-mail: luisamargaridamartins@tecnico.ulisboa.pt.
(L.M.D.R.S.M.)
*E-mail: vladimir.arion@univie.ac.at. (V.B.A.)
ORCID
Michal Zalibera: 0000-0002-6527-1982
■
For relatively stable radical cations the rhombic EPR signal
with a rather large g-tensor anisotropy, obtained at 110 K upon
one-electron oxidation, indicates the formation of a ligand-
centered radical with pronounced delocalization of the
K
DOI: 10.1021/acs.inorgchem.9b01700
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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Selective α-carbon hydroxylation of glycine in nickel(II) cyclo-
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́
̌
̌
́
Lukas Bucinsky: 0000-0002-0190-3231
Vladimir B. Arion: 0000-0002-1895-6460
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
́
The financial support of the Austrian Agency for International
Cooperation in Education and Research (OEAD) (Grant No.
SK01/2018) is acknowledged. This work was supported by the
Science and Technology Assistance Agency under Contract
Nos. APVV-15-0053, APVV-15-0079, and SK-AT-2017-0017,
by the Slovak Grant Agency VEGA under Contract Nos. 1/
0598/16, 1/0416/17, and 1/0466/18, by the Ministry of
Education, Science, Research and Sport of the Slovak Republic
by funding within the scheme “Excellent research teams”, and
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partially by the Fundaca̧ o para a Ciencia e a Tecnologia
̃
(FCT), Portugal, through the research project Nos. PTDC/
QUI-QUI/109846/2009, PTDC/QEQ-ERQ/1648/2014, and
UID/QUI/0100/2019. We thank the HPC centre at the
Slovak Univ. of Technology in Bratislava, which is a part of the
Slovak Infrastructure of High Performance Computing (SIVVP
Project, ITMS code 26230120002, funded by the European
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