Nickel(II), Copper(II) and Palladium(II) Complexes with Bis-Semicarbazide Hexaazamacrocycles: Redox-Noninnocent Behavior and Catalytic Activity in Oxidation and C-C Coupling Reactions

and Catalytic Activity in Oxidation and C −C Coupling Reactions

Article

Nickel(II), Copper(II) and Palladium(II) Complexes with Bis-

Semicarbazide Hexaazamacrocycles: Redox-Noninnocent Behavior

Anatolie Dobrov, Anastasia Fesenko, Alexander Yankov, Iryna Stepanenko, Denisa Darvasiova,

Martin Breza, Peter Rapta,* Lu í sa M. D. R. S. Martins,* Armando J. L. Pombeiro, Anatoly Shutalev,*

and Vladimir B. Arion*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01119

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sı Supporting Information

ABSTRACT: Nickel(II), copper(II), and palladium(II) com-

plexes ML , where M = Ni (1), Cu (2), Pd (3), and ML^O^M^e,

where M = Ni (4), Cu (5), Pd (6), have been prepared by

reactions of NiCl ·6H O, Cu(OAc) ·H O, and PdCl (MeCN)

with 14-membered bis-semicarbazide hexaazamacrocycles H L

OMe

and H₂L

in dimethylformamide (DMF). The compounds were

characterized by elemental analysis, ESI mass spectrometry, IR,

UV−vis, and 1D ( H, C) and 2D ( H- H COSY, H- H TOCSY,

H- H NOESY, H- C HSQC, H- C HMBC) NMR spectra (1,

, 4, and 6), and X-ray diﬀraction (2, 4−6). The complexes with

M N coordination environment have S = 0, 1/2, 0 ground states

for Ni, Cu, and Pd, respectively. The electrochemical behavior of 1−6 was investigated in detail. The electronic structures of 1e-

oxidized species were studied by EPR, UV−vis−NIR spectroelectrochemistry, and DFT calculations, indicating the redox-

noninnocent behavior of the ligands. Compounds 1−6 were tested in the oxidation of styrene and C−C coupling (Henry and

Knoevenagel condensations). Compounds 2 and 5 selectively catalyze the microwave-assisted oxidation of neat styrene to

benzaldehyde (up to 88% yield), whereas the 1 and 4 catalytic systems aﬀorded up to 99% β-nitroethanol yield with an appreciable

diastereoselectivity toward the formation of the anti isomer.

INTRODUCTION

the Lewis acidity of the metal, and by assisting the formation/

breaking of substrate covalent bond electron transfer events.

Cobalt complexes with macrocyclic ligands, namely porphyrins

■

The ligand-centered reactivity of transition-metal complexes

with redox-noninnocent ligands continues to be one of the

most exciting topics of coordination chemistry.

−9

and TAMLH (tetraamido macrocyclic ligand) (Chart 1), were

Two IC

reported to assist in the formation of catalytically competent

Fischer-type nitrenes, key intermediates in amination and

aziridination of organic substrates, via electron transfers from

both the metal and the ligand or via two ligand-to-substrate

Forums appeared in the past decade: i.e., on redox-active

ligands and on applications of metal complexes with ligand-

centered radicals. Base-metal complexes with ligand-centered

radicals became of particular interest in multielectron

25,26

2−14

single-electron-transfer events.

catalysis

inspired by metalloenzyme systems such as, for

The rational design of eﬃcient catalysts for important

chemical reactions requires an understanding of their

structures and spectroscopic properties, as well as the search

for new systems supported by redox-active ligands. S-

substituted isothiosemicarbazides (H NNC(SR)NH ) and

example, galactose oxidase, which requires close cooperation

between redox-active copper(I/II) with a redox-active

phenoxyl/phenolate ligand to catalyze the two-electron

.15

oxidation of alcohols to aldehydes,

attesting to the quick

extension of this area of research. These studies attracted

considerable attention about 30 years ago and were aimed at

an understanding of the electronic structure and bonding of

metal complexes with redox-noninnocent ligands and their use

isothiosemicarbazones (R R CNNC(SR)NH ) are known

Received: April 16, 2020

16−20

in stoichiometric reactions.

In recent years their use in

catalytic reactions has rapidly increased; catalytic functionaliza-

1−24

tion of C−H bonds are well-documented reactions.

Redox-active ligands can participate in the catalytic trans-

formations by storing and providing electrons, by modifying

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Chart 1. Redox-Noninnocent Macrocyclic Ligands:

Attempts to obtain metal-free macrocycles by demetalation

failed, precluding the investigation of the coordination

chemistry of other transition metals with these types of

Tetraphenylporphyrin (Left) and TAMLH (Right)

ligands.

Quite recently, some of us developed a stereoselective

synthesis of previously unknown 14-membered cyclic bis-

semicarbazones IX that can be considered as novel potentially

redox noninnocent ligands. A key step in the synthesis involved

an acid-catalyzed cyclization of the hydrazones of 4-(3-

32−34

oxobutyl)semicarbazides VII (Scheme 1).

The semi-

carbazides VII were prepared via a four-step strategy involving

amidoalkylation of the sodium enolate of pentane-2,4-dione

with N-(tosylmethyl)carbamates IVa to give the products of

the tosyl group substitution, compounds Va, followed by a

base-promoted retro-Claisen reaction and treatment of the

obtained β-carbamato ketones VIa with reﬂuxing hydrazine.

The main disadvantage of this approach was the low isolated

yields (29−42%) for substitution of the ethoxy group in VIa by

the hydrazine fragment due to the harsh reaction conditions.

Subsequently, the synthesis of hydrazones VII was signiﬁcantly

improved by using novel semicarbazone-based amidoalkylation

reagents IVb prepared by the three-component condensation

for their redox-noninnocent properties in metal complexes.

number of nickel(II) complexes with 14-membered macro-

cycles with two or only one isothiosemicarbazide moiety were

prepared via the condensation reaction of pentane-2,4-dione

and S-alkylisothiosemicarbazidium iodide by using nickel(II)

as a template. The use of isothiosemicarbazides ([H NNHC-

SR)NH ]I) as building blocks in template condensation

(

reactions with β-diketones allowed for the stepwise building of

8,29

macrocycles with a cis arrangement of −SR groups (e.g. I),

of semicarbazones with aldehydes and p-toluenesulﬁnic acid.

as well as macrocycles with a trans arrangement of −SR groups

These reagents were converted in two steps via Vb into VIb,

which reacted with excess hydrazine in reﬂuxing EtOH to give

VII. All of the above reactions proceeded in excellent yields.

During this study, it was found that hydrazones of β-(4-

semicarbazono)ketones VIII, readily formed by the treatment

of VIb with hydrazine under mild conditions, can be also

transformed into macrocycles IX by using catalytic amounts of

a strong acid. This cyclization is characterized by very high

trans selectivity (trans:cis ≥ 97:3). The developed approach

allows for a relatively rapid synthesis of macrocycles IX on a

multigram scale. In contrast to reported examples of 14-

30,31

(

e.g., II) assembled in a single reaction step (Chart 2).

Chart 2. Line Drawing of the 14-Membered

Hexaazamacrocyclic Nickel(II) Complexes with cis and

trans Arrangements of −SMe Groups

membered macrocycles based on isothiosemicarbazides,

isothiocarbazides

36,37

IX are conformationally more ﬂexible,

providing a rather dynamic binding cavity. However, their

application as ligands in coordination chemistry is hampered

by the extremely low solubility of these macrocycles in

common solvents, including DMSO, DMF, water, etc. The low

solubility can be explained by a high stability of the crystal

structure due to strong H-bonding between H−N−CO

Scheme 1. Two Approaches to 14-Membered Hexaazamacrocycles IX Starting from Ethyl Carbamate IIIa or Aldehyde

Semicarbazone IIIb

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

fragments of diﬀerent molecules, providing the formation of

zigzag polymer-like chains. Clearly, selective alkylation at the

hydrazine nitrogen atom of these fragments could result in

macrocycles which are expected to be soluble in organic

solvents and retain the ability for complex formation.

resulted colorless mixture was stirred for 3 h. During the ﬁrst 1 h the

gas evolution faded, the initial precipitate dissolved, and needlelike

crystals precipitated. After completion of the reaction, to the obtained

suspension were added H O (15 mL) and petroleum ether (10 mL),

and the resulting mixture was cooled to 0 °C. The oily precipitate was

ﬁltered, and washed with ice-cold H O (6 × 3 mL) and petroleum

Herein we describe the synthesis of the two semicarbazide-

ether to result in a ﬁne powder, which was washed with cold diethyl

based racemic 14-membered hexaazamacrocycles H L and

ether (2 × 3 mL) and dried to give H L (0.447 g, 88%) as a white

OMe

^H2^L

(Chart 3), which are soluble in organic solvents, by

solid. The analytically pure sample was obtained after crystallization

from boiling toluene. Mp: 178−179 °C (toluene). Anal. Calcd for

C H N O : C, 69.47; H, 8.16; N, 16.20. Found: C, 69.55; H, 8.25;

Chart 3. Proligands and Complexes Reported in This

Work

30 42

^N^,¹⁶^.¹⁸^.^I^R⁽^K^B^r⁾^:^νmax 3414 br s (ν(NH)), 3086 w, 3061 w, 3032 w

(

ν(CH_a_r_o_m)), 1661 vs (amide-I), 1636 m (ν(CN)), 1606 w, 1584 w

ν(CC_a_r_o_m)), 1510 vs (amide-II), 1059m (ν(N−N)), 744 s, 698 s

−

δ(CH_a_r_o_m)) cm . The recorded IR spectrum is shown in Figure S1 .

H NMR (600.13 MHz, DMSO-d ): δ 7.26−7.31 (2 × 4H, m, CH in

two Ar), 7.16−7.19 (2 × 1H, m, CH in two Ar), 7.08 (2 × 1H, d, J =

.5 Hz, two NH), 5.05 (2 × 1H, ddd, J = 8.5, J = 6.4, J = 4.6 Hz,

two CH-N), 3.40−3.46 (2 × 1H, m, H in two NCH H ), 2.95−3.01

(

2 × 1H, m, H in two NCH H ), 2.96 (2 × 1H, dd, J = 16.3, J =

B A B

2 3

.4 Hz, H in two CH H ), 2.91 (2 × 1H, dd, J = 16.3, J = 4.6 Hz,

A A B

H in two CH H ), 1.87 (2 × 3H, s, CH in two CH CN), 0.88−

.14 (2 × 4H, m, CH in two CH C H ), 0.71 (2 × 3H, t, J = 7.3 Hz,

2 2 2

CH in two CH CH ) (see also Figure S2). C NMR (150.90 MHz,

DMSO-d ): δ 171.80 (Cq, CN), 157.76 (Cq, CO), 143.13 (Cq

in Ar), 127.84 (CH in Ar), 126.13 (CH in Ar), 125.91 (CH in Ar),

Underlined compound numbers indicate compounds studied by X-

0.51 (CH−N), 47.23 (NCH H ), 43.08 (CH H ), 27.86 (CH in

ray crystallography.

CH CH ), 19.54 (CH in CH CH ), 18.88 (CH CN), 13.56 (CH

in CH CH ) (see also Figure S3 ). The solubility of the compound in

DMSO is about 10 mg/mL.

selective alkylation of the corresponding unsubstituted

trans-2,9-Dibutyl-7,14-dimethyl-5,12-bis(4-methoxyphen-

precursors IX in Scheme 1 (Ar = Ph, 4-MeOC H ) and their

yl)-1,2,4,8,9,11-hexaazacyclotetradeca-7,14-diene-3,10-dione

OMe

nickel(II), copper(II), and palladium(II) complexes 1−6

(H L ). Method A. The reaction of macrocycle trans-IX (Ar = 4-

(

Chart 3). In addition, the structure and spectroscopic

MeOC

.96 mmol and butyl iodide (1.029 g, 5.59 mmol) in DMF (4.6 mL)

rt, 3 h) and subsequent workup were performed as described for

H in Scheme 1) (0.430 g, 0.92 mmol) with NaH (0.047 g,

6 4

properties of their 1e-oxidized species are reported. The ability

of these complexes to catalyze the microwave-assisted

oxidation of neat styrene to benzaldehyde as well as the

Henry C−C coupling of benzaldehyde with nitroethane and

the ultrasound-assisted Knoevenagel condensation of benzal-

dehyde and malononitrile was tested, and the results are

discussed.

(

H L . Yield: 0.468 g, 88% of white solid. Mp: 203.5−204.5 °C

(toluene). Anal. Calcd for C H N O : C, 66.41; H, 8.01; N, 14.52.

32 46 6 4

Found: C, 66.45; H, 8.15; N, 14.62. IR (KBr): ν_m_a_x3418 br s, 3370 br

sh (ν(NH)), 3068 w, 3059 w, 3037 w (ν(CH_a_r_o_m), 1666 vs (amide-I),

615 m (ν(CN)), 1584 m (ν(CC_a_r_o_m)), 1510 vs (amide-II), 1244 s

(

ν(C−O)), 1061 m (ν(N−N)), 1032 s (ν(C−O)), 836 s, 809 s

δ(CH )) cm . The full IR spectrum is shown in Figure S4 . H

−

arom

EXPERIMENTAL SECTION

NMR (600.13 MHz, DMSO-d ): δ 7.18−7.21 (2 × 2H, m, CH in two

■

Ar), 7.03 (2 × 1H, d, J = 8.6 Hz, two NH), 6.82−6.86 (2 × 2H, m,

Materials and Methods. All starting materials were purchased

from Sigma-Aldrich, Acros, and TCI and used as received unless

speciﬁed otherwise. The racemic macrocycles trans-IX (Ar = Ph, 4-

MeOC H ) were prepared by following the literature proce-

CH in two Ar), 5.00 (2 × 1H, ddd, J = 8.6, J = 6.4, J = 4.4 Hz, two

CH-N), 3.70 (2 × 3H, s, CH in two OCH ), 3.41−3.47 (2 × 1H, m,

H in two NCH H ), 2.95−3.01 (2 × 1H, m, H in two NCH H ),

2−35

.92 (2 × 1H, dd, J = 16.3, J = 6.4 Hz, H in two CH H ), 2.86 (2

A A B

2 3

dures.

1D ( H, C) and 2D ( H− H COSY, H− H TOCSY,

× 1H, dd, J = 16.3, J = 4.4 Hz, H in two CH H ), 1.86 (2 × 3H, s,

H− H NOESY, H− C HSQC, H− C HMBC) NMR spectra of

the proligands and their nickel(II) and palladium(II) complexes were

B A B

^C^H3 in two CH CN), 0.88−1.15 (2 × 4H, m, CH in two

CH C H ), 0.71 (2 × 3H, t, J = 7.3 Hz, CH in two CH CH ) (see

measured on Bruker spectrometers (500 and 600 MHz) in DMSO-d₆

also Figure S5 ). C NMR (150.90 MHz, DMSO-d ): δ 171.67 (Cq,

or CDCl at 25 °C. Chemical shifts are quoted relative to the solvent

CN), 157.75 (Cq, CO), 157.68 (Cq in Ar), 135.00 (Cq in Ar),

signals. Atom labeling used in the NMR resonance assignment is

shown in Scheme S1 . UV−vis absorption spectra were recorded on a

PerkinElmer Lambda 650 spectrophotometer. ESI mass spectra were

measured on a Bruker Esquire 3000 mass spectrometer by using

MeCN−MeOH as the solvent. Infrared spectra were obtained on a

Bruker Vertex 70 FT-IR spectrometer by means of the attenuated

total reﬂection technique or on a Bruker Alpha-T spectrometer on

KBr pellets. All elemental analyses were performed at the Micro-

analytical Laboratory of the University of Vienna with a PerkinElmer

26.97 (CH in Ar), 113.24 (CH in Ar), 54.91 (OCH ), 49.92 (CH−

N), 47.26 (NCH H ), 43.12 (CH H ), 27.89 (CH in CH CH ),

⁹^.⁵⁸⁽^C^H2 in CH CH ), 18.97 (CH CN), 13.53 (CH in

CH CH ) (see also Figure S6 ). H, C-HSQC, H, C-HMBC and

2 3

H, H-NOESY spectra are shown in Figures S7 −S11, respectively.

The solubility of the compound in DMSO is about 10 mg/mL.

OMe

Method B. H

NaH (0.070 g, 2.91 mmol), macrocycle trans-IX (Ar = 4-MeOC

Scheme 1) (0.626 g, 1.34 mmol) and butyl bromide (1.107 g, 8.08

(0.668 g, 86%, white solid) was prepared from

400 CHN Elemental Analyzer.

trans-2,9-Dibutyl-7,14-dimethyl-5,12-diphenyl-1,2,4,8,9,11-

mmol) in DMF (5 mL) (rt, 3 h) as described for H L .

hexaazacyclotetradeca-7,14-diene-3,10-dione (H L ). To a

Synthesis of Complexes. NiL (1). NiCl ·6H O (48 mg, 0.2

mixture of NaH (0.051 g, 2.13 mmol) and macrocycle trans-IX (Ar

mmol) and H L (104 mg, 0.2 mmol) in DMF (1.5 mL) were stirred

Ph in Scheme 1) (0.397 g, 0.98 mmol) was added anhydrous DMF

at 95 °C under argon for 3 h. The mixture was cooled to room

temperature, and the red crystals that formed were ﬁltered oﬀ, washed

with water, and dried in vacuo. Yield: 96 mg, 83%. Anal. Calcd for

C H NiN O (M = 575.37): C, 62.62; H, 7.01; N, 14.61. Found: C,

(

4.2 mL), and the obtained suspension was stirred at room

temperature for 5 min. Slow gas evolution was observed, and then

butyl iodide (1.097 g, 5.96 mmol) was added in one portion. The

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Crystal Data and Details of Data Collection for CuL ·2EtOH (2·2EtOH), H L^O^M^e, NiL^O^M^e(4), CuL^O^M^e(5), and

OMe

PdL

(6)

H₂L^O^M^e

NiL^O^M^e

CuL^O^M^e

PdL^O^M^e

CuL ·2EtOH

empirical formula

C H CuN O

C H N O

C H NiN O

C H CuN O

C H N O Pd

32 46

32 44

672.36

578.75

Pbca

12.46906(5)

20.80694(13)

635.44

Pbca

12.4254(9)

12.0463(8)

20.6886(13)

640.27

683.13

Pbca

11.1119(3)

12.5051(3)

21.8330(6)

space group

a, Å

b, Å

P2 /c

8.2547(6)

9.8118(5)

12.0730(7)

90.282(3)

105.555(3)

113.128(3)

859.57(9)

7.8910(3)

12.4896(4)

16.1132(5)

c, Å

α, deg

β, deg

γ, deg

V, Å³

102.2179(15)

3235.01(3)

3096.7(4)

1552.07(9)

3033.81(14)

λ, Å

0.71073

1.299

0.48 × 0.40 × 0.20

100(2)

0.681

0.0417

0.1208

1.060

1.54184

1.188

0.59 × 0.33 × 0.26

296(2)

0.638

0.0777

0.2513

1.072

0.71073

1.363

0.08 × 0.06 × 0.01

100(2)

0.673

0.0330

0.71073

1.370

0.15 × 0.07 × 0.03

100(2)

0.750

0.0509

0.71073

1.496

0.10 × 0.08 × 0.07

100(2)

0.660

0.0311

ρ_c_a_l_c_d, g cm⁻

cryst size, mm³

T, K

−1

μ, mm

wR2

GOF

0.0795

1.028

0.1078

1.132

0.0708

1.010

R1 = ∑||F | − |F ||/∑|F |. wR2 = {∑[w(F − F ) ]/∑[w(F ) ]} . GOF = {∑[w(F_o− F ) ]/(n − p)}¹^/², where n is the number of

1/2

2 2

reﬂections and p is the total number of parameters reﬁned.

−1

2.65, 62.57; H, 7.09, 7.12; N, 14.53, 14.54. ESI MS (positive ion

H] , 645 [M + Na] . IR (most characteristic bands, cm ): 2958 m,

mode for Ni isotope): m/z 575 [M + H] , 597 [M + Na] . IR (most

characteristic bands, cm ): 3407 br, 2956 m, 2929 m, 2871 m, 1639

2928 m, 2868 m, 1644 s, 1606 vs, 1494 m, 1381 vs, 1362 vs, 1333 vs,

1067 vs, 751 s, 699 vs, 511 s. H NMR (500 MHz, CDCl ): δ 7.43

−

vs, 1604 vs, 1580 s, 1493 s, 1446 s, 1390 vs, 1360 vs, 1075 s, 766 s,

(2H, d, J = 7.3 Hz, CH in Ar), 7.28 (2H, t, J = 7.0 Hz, CH in Ar), 7.20

52 s, 699 vs. H NMR (600 MHz, CDCl ): δ 7.46 (2H, d, J = 6.7

(1H, t, J = 7.3 Hz, CH in Ar), 5.00 (1H, s, CH−N), 3.93 (1H, brs, H

Hz, H + H , Ar), 7.31 (2H, t, J = 6.5 Hz, H + H , Ar), 7.20 (1H, t, J

in NCH H ), 3.30 (1H, d, J = 18.3 Hz, H in CH H ), 3.21 (1H, d, J

6.5 Hz, H , Ar), 4.75 (1H, s, CH-N), 3.75 (1H, brs, H_Ain

= 17.6 Hz, H in CH H ), 3.14−3.02 (1H, m, H in NCH H ), 2.14

NCH H ), 3.18 (1H, d, J = 18.0 Hz, H in CH H ), 3.11 (1H, d, J =

(3H, s, CH in CH CN), 1.18 (2H, brs, CH in CH CH ) 1.06

8.7 Hz, H in CH H ), 2.77 (1H, brs, H in NCH H ), 2.15 (s, 3H,

(1H, brs, CH in CH CH ), 0.89 (1H, brs, CH in CH CH ), 0.74 (3H,

2 2 2 2

CH in CH CN), 1.21 (1H, brs, H in NCH H CH H ), 1.11

t, J = 7.1 Hz, CH₃in CH CH ). The complex is soluble in

(

1H, brs, CH in CH CH ), 0.99 (1H, brs, CH in CH CH ), 0.78

dichloromethane and acetonitrile.

OMe

1H, brs, H in NCH H CH H ), 0.74 (3H, t, J = 6.7 Hz, CH in

NiL

(4). NiCl ·6H O (48 mg, 0.2 mmol) and H L

(120 mg,

CH CH ). C NMR (151 MHz, CDCl ): δ 169.64 (Cq, CN),

0.21 mmol) in DMF (2 mL) were stirred at 105 °C for 22 h. The

mixture was cooled to room temperature, and the red crystals that

formed were ﬁltered oﬀ, washed with water, and dried in air. Yield:

112 mg, 88%. Anal. Calcd for C H NiN O (M = 635.42): C,

65.90 (Cq, CO), 142.14 (C in Ar), 128.24 (CH, C +C , Ar),

26.61 (CH, C , Ar), 126.25 (CH, C + C , Ar), 51.21 (NCH H ),

4 2 6 A B

6.80 (CH, CH−N), 44.72 (CH H ), 26.09 (CH in

32 44

NCH H CH H ), 24.27 (CH CN), 20.02 (CH in CH CH ),

60.49; H, 6.98; N, 13.93. Found: C, 60.68, 60.80; H, 7.10, 7.31; N,

3.63 (CH in CH CH ). The complex is soluble in chloroform,

13.27, 13.40. ESI MS (positive ion mode for Ni isotope): m/z 635

−1

dichloromethane, acetonitrile, and methanol.

[M + H] , 657 [M + Na] . IR (most characteristic bands, cm ):

CuL ·2H O (2·2H O). Copper(II) acetate monohydrate (42 mg,

3054 w, 2955 m, 2933 m, 2883 w, 1648 vs, 1616 vs, 1509 s, 1442 m,

.21 mmol) and H₂L (110 mg, 0.21 mmol) in DMF (2 mL) were

1357 s, 1234 vs, 1166 s, 1070 s, 1019 s, 848 vs, 607 s. H NMR (500

stirred at 80 °C under argon for 2.5 h. The mixture was cooled to

room temperature and placed in the refrigerator overnight. The

crystals that formed were ﬁltered oﬀ, washed with water, and dried in

air. Yield: 115 mg, 94% of brownish red crystals. Anal. Calcd for

C H CuN O ·2H O (M = 616.25): C, 58.47; H, 7.20; N, 13.64.

MHz, CDCl ): δ 7.35 (2H, d, J = 7.5 Hz, CH in Ar), 6.85 (2H, d, J =

7.7 Hz, CH in Ar), 4.71 (1H, s, CH−N), 3.79 (1H, brs, H in

NCH H ), 3.77 (3H, s, CH in OCH ), 3.16 (1H, d, J = 19.1 Hz, H

in CH H ), 3.10 (1H, d, J = 20.5 Hz, H in CH H ), 2.79 (1H, t, J =

12.6 Hz, H in NCH H ), 2.17 (3H, s, CH in CH CN), 1.24 (1H,

Found: C, 58.11, 58.34; H, 7.09, 7.21; N, 13.51, 13.57. ESI MS

brs, CH in CH CH ), 1.14 (1H, brs, CH in CH CH ), 1.01 (1H, brs,

2 2 2 2

(

positive ion mode for Cu isotope): m/z 580 [M + H] , 602 [M +

CH in CH CH ), 0.82 (1H, brs, CH in CH CH ), 0.75 (3H, t, J = 7.0

2 2 2 2

−1

Na] . IR (most characteristic bands, cm ): 3466 br, 3397 br, 2954 m,

Hz, CH₃in CH CH ). The complex is soluble in chloroform,

2 3

928 m, 2870 m, 1633 s, 1590 vs, 1579 s, 1448 s, 1385 vs, 1335 vs,

069 s, 765 s, 700 vs, 595 vs, 520 vs. The complex is soluble in

OMe

methanol, ethanol, chloroform, dichloromethane, and acetonitrile.

PdL ·3H O (3·3H O). Bis(acetonitrile)palladium(II) chloride (52

mg, 0.2 mmol), H₂L (104 mg, 0.2 mmol), and sodium acetate

trihydrate (26 mg, 0.19 mmol) in DMF (1.5 mL) were stirred at 90

°C for 3 h. The mixture was cooled to room temperature and placed

in the refrigerator overnight. The crystals that formed were ﬁltered oﬀ,

washed with acetonitrile−water 1:1, and dried in air. Yield: 70 mg,

6% of yellowish crystals. Anal. Calcd for C H N O Pd·3H O (M =

77.14): C, 53.21; H, 6.85; N, 12.41. Found: C, 53.42; H, 6.37; N,

2.33%. ESI MS (positive ion mode for Pd isotope): m/z 623 [M +

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

861 w, 1648 vs, 1624 vs, 1579 s, 1509 vs, 1359 s, 1323 vs, 1235 vs,

020 s, 846 vs, 526 vs. The product is very soluble in chloroform

oxidizing agent purchased from Alfa Aesar was used as received. An

Avantes Model AvaSpec-2048x14-USB2 spectrometer served for

recording UV−vis−NIR spectra in both spectroscopic and

spectroelectrochemical studies. Halogen and deuterium lamps were

used as light sources (Avantes, Model AvaLight-DH-S-BAL). The cell

was positioned in a CUV-UV Cuvette Holder (Ocean Optics)

connected to the diode-array UV−vis−NIR spectrometer by optical

ﬁbers. UV−vis−NIR spectra were processed using the AvaSoft 7.7

software package. EPR spectra were recorded with an EMXplus X-

band EPR spectrometer (Bruker, Germany).

(

≥50 mg/mL), has good solubility in methanol (≥13 mg/mL), is

moderately soluble in acetonitrile (≥1 mg/mL), and is sparingly

soluble in dimethylformamide (≤0.5 mg/mL).

OMe

PdL ·0.5H O (6·0.5H O). A mixture of bis(acetonitrile)-

OMe

palladium(II) chloride (66 mg, 0.25 mmol) and H₂L

(134 mg,

.23 mmol) in DMF (1.5 mL) was stirred at 65 °C for 21 h. The

volatile components of the reaction mixture were removed under

reduced pressure. The residue was washed on a glass microﬁber ﬁlter

with acetonitrile (20 mL) and then with dichloromethane (20 mL).

The acetonitrile solution was passed through a silica column by using

acetonitrile as eluent. The fraction containing the palladium complex

was evaporated, combined with dichloromethane solution, and

recrystallized. Yield: 30 mg, 19% of yellow crystals. Anal. Calcd for

C H N O Pd·0.5H O (M = 692.16): C, 55.53; H, 6.55; N, 12.14.

42−47

Computational Details. Standard B3LYP/6-311+G*

geom-

H 0

etry optimization of neutral [NiL ] in the singlet spin state and

OMe q

[ML ] , M = Ni, Cu, in various charge q and spin states (with spin

multiplicity m) was performed using the Gaussian16 program

package. The unrestricted DFT formalism has been used for all

complexes. Solvent eﬀects in dichloromethane were approximated by

Found: C, 55.57, 55.73; H, 6.53, 6.45; N, 11.99, 11.99. ESI MS

the solvation model based on density (SMD) modiﬁcation of the

integral equation formalism polarizable continuum model as

implemented in Gaussian16. The stability of the optimized structures

was conﬁrmed by vibrational analysis (no imaginary vibrations).

Excited-state energies with corresponding electron transitions were

(

positive ion mode for Pd isotope): m/z 683 [M + H] , 705 [M +

−1

Na] . IR (most characteristic bands, cm ): 3055 w, 2954 m, 2932 m,

879 w, 1651 vs, 1621 vs, 1509 s, 1369 s, 1235 vs, 1055 s, 1020 vs,

46 vs, 603 s. H NMR (500 MHz, CDCl ) δ 7.34 (2H, d, J = 8.5 Hz,

0−52

CH in Ar), 6.81 (2H, d, J = 8.6 Hz, CH in Ar), 4.94 (1H, s, CH−N),

evaluated using the time-dependent DFT method

for 70 states.

.95 (1H, m, H in NCH H ), 3.76 (3H, s, CH in OCH ), 3.24 (1H,

The MOLEKEL package has been used for the visualization of

molecular orbitals and spin densities. The atomic charges and d

electron populations on the central metal atom have been obtained

dd, J = 17.4, 2.5 Hz, H in CH H ), 3.16 (1H, dd, J = 17.7, 4.2 Hz,

H in CH H ), 3.09 (1H, m, H in NCH H ), 2.14 (3H, s, CH in

54−56

CH CN), 1.23 (1H, brs, CH in CH CH ), 1.16 (1H, brs, CH in

via the natural population analysis (NPA)

Gaussian16.

as implemented in

CH CH ), 1.08 (1H, brs, CH in CH CH ), 0.95 (1H, brs, CH in

CH CH ), 0.76 (3H, t, J = 7.2 Hz, CH in CH CH ). UV−vis

Catalytic Studies. The microwave-assisted peroxidative oxidation

of neat styrene was performed in a focused Anton Paar Monowave

300 reactor, using a 10 mL capacity Pyrex tube with a 13 mm internal

diameter and equipped with a rotational system and an IR

temperature detector. Styrene (1.0 mmol), complex 2 or 5 (5

mmol), 30 wt % aqueous H O (4 mmol), and chlorobenzene as an

CH Cl ), λ , nm (ε, M⁻cm ): 502 (5250), 463 (5073), 390

3177), 323(11730). The complex is soluble in dichloromethane and

−1

(

2 2 max

acetonitrile.

Crystallographic Structure Determination. X-ray diﬀraction

quality single crystals of 2·2EtOH were grown from ethanol, 4 and 5

were obtained by slow evaporation of chloroform−ethanolic

solutions, and 6 was obtained from dichloromethane. The measure-

ments were performed on Gemini (H₂L ), Bruker D8 Venture (2·

diﬀractometers. Single crystals were positioned at 55, 30, 27, 30,

and 24 mm from the detector, and 4222, 1122, 641, 1122, and 444

frames were measured, each for 3/12, 3, 24, 30, and 10 s over 1, 1, 0.5,

internal standard (100 μL) were placed in the tube, and the tube was

sealed. The reaction mixture was MW-irradiated with stirring at 80 °C

for 15−60 min. To follow the reaction evolution, 100 μL aliquots

were withdrawn at the desired reaction times and, after they were

cooled, the samples were centrifuged (to remove the catalyst, which is

insoluble in the reaction media at room temperature) and analyzed by

gas chromatography (GC).

GC measurements were carried out using a Perkin−Elmer Clarus

600 C with an FID detector and a capillary column (DB-WAX,

column length 30 m, internal diameter 0.32 mm) and the Jasco-

Borwin v.1.50 software. The temperature of the injector was 200 °C.

The initial temperature was maintained at 90 °C for 2 min, then

raised up to 150 °C (10 °C/min), and then directly up to 250 °C (50

°C/min) and ﬁnally kept at this temperature for 8 min. Helium was

used as the carrier gas. The products were identiﬁed by comparison of

their retention times with known reference compounds, and the

styrene conversion values (result of two concordant assays) were

determined by the internal standard method.

OMe

EtOH and 6), and Bruker X8 APEXII CCD (4 and 5)

OMe

.5 and 0.5° scan width for H₂L , 2·2EtOH, and 4−6, respectively.

The data were processed using SAINT and STOE X-RED

38,39

software.

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; structure reﬁnement,

SHELXL-2014; molecular diagrams, ORTEP; computer, Intel

CoreDuo. CCDC ﬁ le numbers 1996061 (H ₂ L ), 1996062 (5),

OMe

996063 (4), 2005715 (2·EtOH), and 1996065 (6).

The catalyst reusability in consecutive runs was tested by separating

the used catalyst from the reaction mixture by centrifugation followed

by ﬁltration of the supernatant solution, washing with acetonitrile/

water, and drying in air. The new run was initiated by addition of the

fresh reagents (see above) in addition to the used catalyst. After

completion of each run, the products were analyzed by GC as

described above.

For the nitroaldol C−C couplings an OS1025 STEM Omni

(Electrothermal) reactor was used. In a Teﬂon-capped 10 × 25 mL

Pyrex tube were placed benzaldehyde (1.0 mmol), nitroethane (4.0

mmol), 1, 3, 4 or 6 (5.0 μmol, 0.5 mol % vs. benzaldehyde), and the

chosen solvent (2.0 mL). The reaction mixture was stirred (600 rpm)

under ambient conditions up to 48 h. At the desired time, the reaction

was stopped and the reaction mixture centrifuged to remove the

solids. Subsequent evaporation of the solvent yielded the crude

product. When water was used as the solvent, the mixture was dried

Electrochemistry and Spectroelectrochemistry. Cyclic vol-

tammograms of 0.5 mM DCM and ACN solutions of 1−6 containing

.1 M nBu NPF supporting electrolyte were measured in a

homemade miniature electrochemical cell using a platinum-disk or

glassy-carbon-disk working electrode (from Ionode, Australia), a

platinum wire as the counter electrode, and silver wire as a pseudo

reference electrode. Ferrocene served as the internal potential

standard, and the potentials were determined vs the 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 under an

argon atmosphere in a spectroelectrochemical cell kit (AKSTCKIT3)

with a Pt microstructured honeycomb working electrode, purchased

from Pine Research Instrumentation. UV−vis−NIR measurements of

chemically oxidized compounds were performed in an oxygen-free

glovebox using quartz cells of 1.0 cm path length from Agilent

under vacuum. A sample was dissolved in DMSO-d and analyzed at

room temperature by H (300 MHz) NMR on a Bruker Avance II+

Technologies. Nitrosonium tetraﬂuoroborate (NOBF , 98%) as an

300 (UltraShieldMagnet) spectrometer using tetramethylsilane (Si-

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

−3 , respectively, with selected bond distances and bond

Article

(

CH ) ) as an internal reference. The yield of the β-nitroethanol

X-ray Crystallography. The results of X-ray diﬀraction

OMe

products (relative to benzaldehyde) was established using 1,2-

dimethoxyethane as an internal standard. The values of the vicinal

coupling constants between the α-N−C−H and α-O−C−H protons

studies of 2·EtOH, H L , and 4−6 are shown in Figures

(

J = 7−9 and 3.2−4 Hz, respectively) helped in the identiﬁcation of

the syn or anti isomer.

For the catalyst recycling experiments, the used catalyst was

separated, washed with several portions of water, and dried in an oven

at 60 °C. Each cycle was initiated after the preceding one upon

addition of new typical portions of all other reagents (see above).

After completion of each run, the products were analyzed by NMR as

described above.

The ambient ultrasound (US) assisted Knoevenagel reactions were

carried out in an Elma Transsonic 600/H ultrasonic bath. In a capped

round-bottom Pyrex ﬂask were placed benzaldehyde (1.0 mmol),

malononitrile (2.0 mmol), 1, 3, 4 or 6 (10.0 μmol, 1 mol % vs.

benzaldehyde), and acetonitrile (2 mL) under ambient conditions.

The ﬂask was immersed in the ultrasonic bath in an open atmosphere

for up to 3 h. Then, the reaction mixture was centrifuged and the

solids were removed by ﬁltration. The ﬁltrate was evaporated, and the

residue was dissolved in CDCl₃and analyzed by H NMR (see

above).

Blank experiments, in the absence of 1−6, were performed for all

(

oxidation, and Henry and Knoevenagel C−C couplings) reaction

types under the optimized conditions, and no signiﬁcant reagent

conversion was detected (e.g., 2% for the β-nitroethanol products).

Moreover, experiments with the respective precursor salts (NiCl₂·

H O, Cu(OAc) ·H O, and [PdCl (NCMe) ]), also run using the

2 2 2 2 2

optimized conditions, led to yields much lower than those found for

−6.

Figure 1. ORTEP plot of CuL ·2EtOH (2·2EtOH) with the atom-

labeling scheme and thermal ellipsoids drawn at the 50% probability

level. The cocrystallized solvent is not shown. Selected bond distances

(Å) and bond angles and dihedral angles (deg): Cu−N1 =

RESULTS AND DISCUSSION

Synthesis of Hexaaza Macrocyclic Proligands. The

macrocyclic compounds trans-IX (Scheme 1) were prepared in

several steps as reported previously.

■

.9671(17), Cu−N3 = 1.8699(17) Å; N1−Cu−N3 = 82.69(7),

N1 −Cu−N3 = 97.31(7), θ

= −55.4(2), θ

N3−C1−C2−C3

C1−C2−C3−N1

1.9(3) (i denotes an equivalent atom generated by the symmetry

transformation −x + 1, −y + 1, −z + 1).

2−35

In contrast to

thiosemicarbazide, which exists in solution in the two

tautomeric forms thione and thiol, the one−ol tautomerism

is not typical for semicarbazide and macrocycles IX (Ar = Ph,

angles quoted in the legends. All compounds crystallized as

racemates: the copper(II) complex 2·EtOH in the triclinic

-MeOC H ) in Scheme 1. The alkylation of the latter does

6 4

centrosymmetric space group P1, 5 in the monoclinic space

not lead to alkylation of the chalcogen in the semicarbazide (or

semicarbazone) fragment as occurs in the case of thio(seleno)-

semicarbazides or thio(seleno)semicarbazones. Instead, a

OMe

group P2 /c, and H L , 4, and 6 in the orthorhombic space

group Pbca. The nickel(II), copper(II), and palladium(II) in 2

and 4−6 adopt a square-planar coordination geometry. The

selective butylation of hydrazinic nitrogen atoms occurred in

OMe

structures of H L , 2·EtOH, and 4−6 are centrosymmetric.

the presence of NaH in DMF with the generation of H L and

OMe

^Iⁿ^H2^L

the inversion center is in the middle of the

H₂L

(see Chart 3). The formation of these products was

macrocycle, while in 2·EtOH and 4−6 the central metal ions

lie on the inversion center. The conformation adopted by the

does not change markedly upon complex

formation, as shown for example for NiL

The Ni−N1 bond distance is signiﬁcantly (by ca. 0.025 Å)

longer than Ni−N3 in nickel(II) complex 4. The diﬀerence

between these two bonds in the nickel(II) complex with IX

conﬁrmed by 1D ( H, C) and 2D ( H- H COSY, H- H

TOCSY, H- H NOESY, H- C HSQC, H- C HMBC)

OMe

free ligand H₂L

NMR spectra, ESI mass spectrometry, and single-crystal X-ray

OMe

in Figure 2.

diﬀraction of H₂L

(vide infra).

Synthesis of Complexes. Complexes 1 and 4 were

OMe

prepared by reactions of NiCl ·6H O with H L and H L

in a 1:1 molar ratio upon heating in DMF in 88 and 83%

where Ar = Ph (Scheme 1) with Ni−N

= 1.900(11) Å

hydrazine

yields, respectively. Compounds 2·2H O and 5 were

and Ni−N

= 1.860(11) Å is not signiﬁcant. In the case of

synthesized similarly starting from Cu(OAc) ·H O in 94 and

amide

copper(II) complexes 2·EtOH and 5 the bond length

diﬀerence is quite signiﬁcant and amounts to ca. 0.1 Å, while

0% yields, respectively. The reaction of PdCl (CH CN) with

2 3 2

H L and NaOAc as a base in a 1:1:1 molar ratio in DMF

the diﬀerence in Pd−N

and Pd−N

interatomic

aﬀorded 3·3H O in 56% yield, while that of PdCl (CH CN)

hydrazine

amide

OMe

distances has vanished in 6 (statitically equal within 3σ). This

is not the case for palladium complexes with tridentate

semicarbazonates, i.e. 2.014(3) and 1.966(3) Å in the

dipalladium complex with the trianion of 2-hydroxy-3-

methoxybenzaldehyde semicarbazone Pd

2.033(3) and 1.976(3) Å in Pd(HL)PPh

hydroxyacetophenone semicarbazone. The trend M−N1 >

M−N3 is typical for ﬁrst-row transition-metal complexes with

isothiosemicarbazides, where the electronically distinct termi-

with H₂L

in a 1:1 molar ratio in DMF led to crude 6, which

was puriﬁed by column chromatography on silica to give pure

·0.5H O in 19% yield. All six complexes in ESI mass spectra

measured in positive ion mode gave strong peaks attributed to

[

M + H] and [M + Na] ions (see Figures S12 −S14 ). H

(L)Cl(PPh or

, where H L = 2-

)

NMR spectra were in accordance with C molecular symmetry

for 1, 3, 4, and 6. The formation of nickel(II), copper(II), and

OMe

palladium(II) complexes with H L and H L

was further

conﬁrmed by single-crystal X-ray diﬀraction studies.

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 2. (a) ORTEP plot of H₂L^O^M^ewith the atom-labeling scheme

and thermal ellipsoids drawn at the 20% probability level. Selected

bond distances (Å) and selected torsion angles (deg): N1−N2 =

Figure 3. (a) ORTEP plot of CuL^O^M^e(5) with the atom-labeling

scheme and thermal ellipsoids drawn at the 50% probability level.

Selected bond distances (Å) and bond angles and torsion angles

(deg): Cu−N1 = 1.9623(18), Cu−N3 = 1.8666(17); N1−Cu−N3 =

97.17(7) (i denotes an equivalent atom generated by the symmetry

.424(3), N2−C4 = 1.400(3), C4−N3 = 1.337(3), C4−O1 =

.218(3), N3−C1 = 1.448(3), C1−C2 = 1.534(4), C2−C3 =

.508(4) Å; θ

= −59.3(3), θ

= 21.0(3) (i

N3−C1−C2−C3

C1−C2−C3−N1

denotes an equivalent atom generated by the symmetry trans-

transformation −x, −y + 1, −z), N1−Cu−N3 = 82.83(7),

OMe

formation −x + 1, −y + 1, −z + 1). (b) ORTEP plot of NiL

with the atom-labeling scheme and thermal ellipsoids drawn at the

0% probability level. Selected bond distances (Å) and bond angles

and torsion anlgles (deg): Ni−N1 = 1.8570(18), Ni−N3 =

.8329(16); N1−Ni−N3 = 96.52(7) (i denotes an equivalent atom

generated by the symmetry transformation −x, −y, −z + 1), N1−Ni−

(4)

= −59.4(2), θ

= 26.4(3). (b) ORTEP plot

N3−C1−C2−C3

C1−C2−C3−N1

OMe

of PdL

(6) with the atom-labeling scheme and thermal ellipsoids

drawn at the 50% probability level. Selected bond distances (Å) and

bond angles and torsion angles (deg): Pd−N1 = 1.9624(15), Pd−N3

= 1.9545(15); N3 −Pd−N1 = 98.65(6) (i denotes an equivalent atom

generated by the symmetry transformation −x, −y, −z + 1), N1−Pd−

N3 = 83.48(7), θ

= −58.9(2), θ

= 27.9(3).

N3 = 81.35(6), θ

= −64.6(2), θ

= 42.5(2).

N3−C1−C2−C3

C1−C2−C3−N1

N3 −C1−C2−C3

C1−C2−C3−N1

nal amine groups produce a somewhat diﬀerent anisotropic

ligand ﬁeld.

to study the spectroelectrochemical properties of 1−6 and

identify, if possible, the site of electron transfer by performing,

in addition, DFT calculations.

Upon coordination of the macrocyclic ligands to metals two

almost ﬂat ﬁve-membered chelate rings and two six-membered

rings with a puckered or envelope conformation are formed.

The deviations of atom C1 from the mean plane through M,

Electrochemistry and Spectroelectrochemistry. The

redox properties of 1−6 were investigated by cyclic

voltammetry in dichloromethane (DCM) or in acetonitrile

(ACN) containing tetrabutylammonium hexaﬂuorophosphate

as the supporting electrolyte. One oxidation wave was observed

for all investigated complexes 1−6 in DCM in the potential

window available, as shown in cyclic voltammograms in Figure

4. Nickel(II) complexes exhibit both electrochemical and

N3 (N3 ), C2, C3 and N1 (N1) are summarized in Table S1

in the Supporting Information.

A comparison of torsion angles quoted in the legends to

OMe

Figures 2 and 3 for H₂L , NiL , CuL , and PdL

shows that insertion of palladium(II) into the macrocycle

causes its largest conformational change. This was expected,

since it is known that 4d metals have ionic radii about 0.1−0.2

−

chemical reversibility upon anodic oxidation at 100 mV s

with the half-wave potentials E₁_/₂= +0.60 and +0.57 V vs Fc /

Fc for 1 and 4, respectively. The redox waves for copper(II)

complexes 2 and 5 are quasi-reversible and with the lowest

Å greater than those of 3d metals.

Another feature of note is the lack of proton donors in the

molecules of the metal complexes, which makes intermolecular

interactions via hydrogen bond formation impossible. There

are only van der Waals interactions between the molecules of 2

and 4−6. In contrast, the nickel(II) complex of trans-IX (Ar =

Ph) is involved in strong intermolecular interactions of lactam

groups via centrosymmetric N−H···O hydrogen bonds, as

redox potentials of the oxidative response with E = +0.45 and

+0.42 V vs Fc /Fc, respectively. In addition, a small new

counterpeak is observed upon a backward scan, indicating the

lower stability of the oxidized state for copper(II) complexes in

comparison to the nickel(II) analogues. Note that complexes

4−6 with R = OMe show slightly lower redox potentials of the

aforementioned oxidation processes. In contrast to nickel(II)

and copper(II) complexes, the palladium(II) complexes 3 and

6 reveal a fully irreversible oxidation peak with much higher

redox potentials of the oxidative response at +0.83 and +0.82 V

shown in Figure S15. As mentioned previously, this might be

a reason for the low solubility of the nickel(II) complexes in

common organic solvents and water.

As 3d metal complexes with isothiosemicarbazide- and

(

iso)thiocarbazide-based ligands (open chain and macrocyclic)

vs Fc /Fc, respectively. Similar redox behavior was observed in

61,27

revealed noninnocent behavior,

it was of particular interest

acetonitrile solutions at a GC working electrode with E₁_/₂=

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 4. Cyclic voltammograms of 1−6 in the region of the ﬁrst

anodic peak (1, dark cyan trace; 2, red trace; 3, dark yellow trace; 4,

blue trace; 5, black trace; 6, magenta trace), all in 0.1 M nBu NPF /

−

DCM at a Pt working electrode (scan rate 100 mV s ).

0.57 V for 1, E₁_/₂= +0.56 V for 4, E = +0.38 V for 2, E =

pa pa

0.36 V for 5, E = +0.81 V for 3, and E = +0.79 V for 6 (all

vs Fc /Fc).

Upon in situ anodic oxidation of 1 in a spectroelectrochem-

ical cell with a Pt microstructured honeycomb working

electrode, a nearly reversible redox couple in the corresponding

cyclic voltammogram at 10 mV s⁻in nBu NPF /DCM was

observed (see inset in Figure 5a) with a simultaneous

evolution of new absorption bands with maxima at 324, 427,

Figure 5. In situ spectroelectrochemistry of 1 in 0.1 M nBu NPF /

−1

DCM at 10 mV s . (a) UV−vis−NIR spectra detected simulta-

neously upon the in situ oxidation in the region of the ﬁrst anodic

voltammetric peak (blue lines). Inset: the corresponding cyclic

voltammogram. (b) 3D view of the potential dependence of UV−vis−

NIR spectra measured during a cyclic voltammetric scan. Inset: EPR

spectrum registered in the region of the ﬁrst anodic wave at 298 K.

The color-highlighted potential regions in the voltammogram shown

in (a) correspond to the color-highlighted optical spectra depicted in

24, and 874 nm (Figure 5a). At the same time, an isotropic X-

band EPR spectrum was observed with a high g value of 2.1004

and the line width ΔH = 28.5 G at room temperature,

indicating a strong noninnocent behavior of the ligand (see

inset in Figure 5b). The same UV−vis−NIR and EPR

spectroelectrochemical responses were found for the nickel(II)

analogue 4 (Figure S16 ), conﬁrming the very small inﬂuence of

(b).

the OCH group of the ligand on the redox behavior of the

investigated complexes.

intensities of the simulated EPR spectra, which ﬁt the

experimental EPR spectrum at 100 K well (see dark cyan

trace in Figure 6a), determined by double integration, indicate

that the ratio of the Ni(III) species to the Ni(II)−ligand

radical species is 0.3:1 at 100 K.

To investigate the possibility of tautomerism between

nickel(III)−ligand and nickel(II)−ligand radical states for

our oxidized 1, we performed an ex situ EPR spectroelec-

trochemical experiment, in which 1 was electrolyzed in 0.1 M

nBu NPF /DCM at the ﬁrst anodic peak. After bulk

As expected, less reversible or irreversible spectroelectro-

chemical response was observed for copper(II) and palladium-

(II) complexes, as illustrated in Figure S17. Upon oxidation of

electrolysis an EPR tube with the sample under argon was

immediately immersed into liquid nitrogen, and EPR spectra

were recorded. Figure 6a shows the EPR spectrum of the one-

electron-oxidized 1 at 100 K (black trace).

Two anisotropic EPR signals with diﬀerent g values are

clearly seen, in contrast to the single isotropic EPR signal (S =

−

5 at a 5 mV s scan rate the voltammogram was nearly

irreversible (Figure S17a ) and a new absorption band at 370

nm arose (Figure S17c ). This band only partially disappeared

after a backward scan, conﬁrming irreversible changes upon

oxidation. In the case of 2 the band appeared at 358 nm (not

shown). Even more irreversible behavior was found for

palladium(II) complexes 3 and 6, as seen in Figure S17b,d .

Chemical oxidation of 0.5 mM solutions of complexes 4 and 5

in DCM with a stoichiometric amount of NOBF (E° = 1.00

/2) measured at 298 K. In the latter case the g value of 2.1004

is between those for nickel(III)−ligand and nickel(II)−ligand

radical species, indicating the formation of a ligand radical with

a marked delocalization of the unpaired spin onto orbitals of

the nickel ion. Simulation of this EPR signal (see blue trace in

Figure 6a) yielded g = 2.215(0), 2.079(8), 2.055(3). An

additional EPR signal emerged at low temperatures (see red

trace in Figure 6a), whose intensity increased by a decrease in

the temperature, gave g = 2.269(8), 2.246(7), 2.020(5), which

can be attributed to Ni(III) species. The g matrix deduced for

1/2

V vs Fc /Fc) cleanly aﬀorded one-electron-oxidized products

with characteristic UV−vis−NIR spectra (Figure S18).

In the cathodic part the reduction processes are character-

istic only for copper(II) complexes 2 and 5. The best

electrochemical reversibility was observed when a GC working

electrode was used (Figure S19). A reversible voltammetric

the Ni(III) species is characteristic of a low-spin 3d electronic

conﬁguration with a (d ,d ,d ) d ground state. The

response with E₁_/₂= −1.63 V vs Fc /Fc was registered for both

xy yz xz

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 6. (a) X-band EPR spectrum of oxidized 1 (black trace)

simulated as described in the text (the same response was observed

for 4, not shown). Experimental EPR parameters: X-band, frequency

.448 GHz, center ﬁeld 3130 G, sweep width 1000 G, modulation

amplitude 2 G, modulation frequency 100 kHz, time constant 10.24

Figure 7. In situ spectroelectrochemistry of 5 in 0.1 M nBu NPF /

4 6

ms, receiver gain 1 × 10 , temperature 100 K. (b) Spin density

−

ACN at 10 mV s . (a) 3D view of the potential dependence of UV−

vis−NIR spectra measured upon a cyclic voltammetric scan. Inset: the

corresponding cyclic voltammogram. (b) UV−vis−NIR spectra

detected simultaneously upon the in situ reduction in the region of

the ﬁrst cathodic voltammetric peak (red lines). Inset: EPR spectra

registered in the region of the ﬁrst cathodic wave at 298 K.

distribution calculated for the paramagnetic monocharged species

[

4] .

complexes. Upon in situ cathodic reduction of 5 (Figure 7) the

reversible redox couple in the corresponding cyclic voltammo-

−1

gram at 10 mV s in 0.1 M nBu NPF /ACN (inset in Figure

a) was accompanied by simultaneous evolution of a broad

absorption band with a maximum at 420 nm (Figure 7b). A

strong decrease in the initial copper(II) (d , S = 1/2) EPR

signal (Figure S20) was observed in the analogous

spectroelectrochemical experiment directly in the EPR cavity

using a large platinum working electrode (see inset in Figure

spin state, and the same holds for [CuL^O^M^e]⁺. In Figure 8a,

experimental electronic absorption spectra for the nickel(II)

complex 4 (black trace in Figure 8a) and those for [4] (red

trace), generated electrochemically, are compared with

theoretical TD-DFT electronic transitions calculated for 4 in

b), indicating the reduction of Cu(II) with the formation of a

a singlet state (blue columns in Figure 8a) and [4] in a

diamagnetic d EPR-silent Cu(I) species. Additionally, upon a

voltammetric reverse scan, reoxidation and a nearly full

recovery of the initial optical bands were observed, attesting

to the chemical reversibility of the cathodic reduction (Figure

doublet spin state (pink columns). Theoretical calculations

using CH Cl solvent eﬀects within the solvation model based

on density (SMD) predict very well the experimental

transitions found for 4 as well as the existence of three low-

a). As expected, a very similar spectroelectrochemical

energy transitions observed for [4] . For 4, the ﬁrst transition

response was also found for 2 (Figure S21).

Theoretical Studies. To get a deeper insight into the

at 360 nm with a signiﬁcant oscillator strength corresponds

mainly to that from HOMO to LUMO+1. The shape of the

HOMO orbital indicates the strong noninnocent character of

the ligand and also the ligand-centered site for oxidation.

Indeed, the calculated spin density distribution for para-

H 0

electronic structures of 1−6, the geometries of [NiL ] and

OMe q

[

ML ] , q = 0, +1, −1, were optimized by starting from the

available X-ray crystallographic data. The structural parameters

of the DFT-optimized structures match reasonably well with

the experimental metrical parameters from single-crystal X-ray

magnetic monocharged [1] and [4] is similar to the HOMO

shape and is located on the central nickel atom and two

diﬀraction data, as shown in Table S1. Unlike centrosymmetric

neigboring nitrogens (see Figure 6b). In the case of [4] , the

lowest energy intense electron transitions at 878 and 698 nm

(see pink columns in Figure 8a) correlate very well with the

experimentally observed optical bands. These transitions are

attributed to the β-HOMO to β-LUMO transitions (β-

HOMO-6 (−0.2735 au) → β-LUMO (−0.1726 au) for 698

OMe q

[

NiL ] complexes, their oxidized copper analogues (q =

1) undergo a symmetry decrease due to a pseudo-Jahn−

Teller eﬀect. From the obtained energetics of the optimized

geometries (Table S2 ), DFT calculations indicate that a

neutral-spin singlet for [NiL^O^M^e]⁰is more stable than its triplet

Inorg. Chem. XXXX, XXX, XXX−XXX

2 and 5 as the only product (Figure 9 ).

Article

Scheme 2. Reactions Catalyzed by 1−6

Abbreviations: MW, microwave; US, ultrasound.

heated leads to increased reaction rates, and therefore, the

desired transformations occur in a shorter period of time.

Compounds 2 and 5 selectively catalyze the microwave-

assisted oxidation of neat styrene to benzaldehyde using

hydrogen peroxide (30% aqueous solution) as the oxidizing

agent, under low MW irradiation (25 W) and in the absence of

any added solvent (Scheme 2). The reaction conditions were

optimized relative to several reaction parameters: namely, MW

irradiation power, time, temperature, and catalyst and oxidant

amounts. Under the optimized conditions (40 min of

irradiation at 80 °C), the catalytic systems aﬀorded

benzaldehyde in up to 88 and 81% yields (TOF, turnover

Figure 8. (a) Comparison of the experimental absorption spectra for

neutral (black trace) and one-electron-oxidized (red trace) nickel(II)

complex 4 and calculated electronic transitions for [4] (blue

columns) and [4] (pink columns). Inset: B3LYP orbitals

contributing to the TD-B3LYP electron transition for [4] at 878

−1

nm. (b) B3LYP orbitals contributing to the lowest TD-B3LYP

frequency, up to 2.6 × 10 and 2.4 × 10 h ), respectively, for

electron transition for [4] .

nm and β-HOMO-2 (−0.2610 au) → β-LUMO (−0.1726 au)

for 878 nm, with dominating charge transfer from the ligand to

the central atom (see the inset in Figure 8a). The relatively

small changes in the charge, d electron population, and spin

density at the Ni and Cu central atoms (see Table S3) during

the redox processes are in full agreement with the noninnocent

character of the ligand as well. For [5] the spin density is

located on the copper and four neigboring nitrogen atoms (see

the inset in Figure S22a ). Although the calculated optical

transitions for [5 ] match the experimental UV−vis bands

very well (Figure S22 ), this is not the case for transitions

computed for the charged states (Figure S22).

The documented redox behavior of metal complexes

justiﬁed further investigation of 1−6 in catalytic oxidation

and C−C coupling reactions.

Catalytic Studies. In the present study, the oxidation of

styrene to benzaldehyde and C−C coupling reactions at

benzaldehyde were chosen as benchmark reactions, on account

Figure 9. Benzaldehyde conversion and selectivity obtained from the

MW-assisted oxidation of neat styrene, catalyzed by 2 or 5.

64−67

of their industrial importance.

Metal(II) complexes 1−6

The selectivity exhibited by catalysts 2 and 5 is remarkable.

were tested, according to their features, as catalysts for the

above reactions under environmentally benign operation

conditions: (i) microwave-assisted oxidation of neat styrene

to benzaldehyde, (ii) room-temperature Henry C−C coupling

of benzaldehyde with nitroethane, and (iii) ultrasound-assisted

Knoevenagel condensation of benzaldehyde and malononitrile

In fact, several homo- and heterogeneous Cu(II) catalytic

70−73

systems previously used

for the oxidation of styrene were

not able to aﬀord selective benzaldehyde formation. Among

them are copper(II) complexes of a Schiﬀ base derived from

salicylaldehyde and o-aminobenzyl alcohol, which, encapsu-

lated in zeolite-Y, catalyze the oxidation of styrene by H O

(

Scheme 2). When heating was required (oxidation of

and, under the optimized reaction conditions, gave ﬁve

reaction products (styrene oxide, benzaldehyde, 1-phenyl-

ethane-1,2-diol, benzoic acid, and phenylacetaldehyde). Other

examples are copper(II) complexes with salen ligands tethered

styrene), the alternative microwave (MW) energy source was

chosen, as it is known

68,69

that MW irradiation improves the

yield and selectivity of a process relative to those achieved

through conductive heating (where heat from an external

source ﬁrst passes through the walls of the vessel to reach the

reaction mixture). Moreover, this eﬃcient method of trans-

ferring energy (based on dipole rotation and/or ionic

conduction) from the microwaves to the substances being

onto amino-functionalized graphene oxide (which require

acetonitrile as solvent and the sacriﬁcial coreductant

isobutyraldehyde) or polymer-anchored copper(II) that led

to styrene epoxide, benzaldehyde, and styrene oxide (84%

maximum selectivity for benzaldehyde). Recently, types of

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Cu(II) complexes bearing tris(pyrazolyl)methane C-homo-

scorpionate ligands immobilized on sucrose derived hydro-

chars were reported to selectively catalyze the conversion of

styrene to benzaldehyde in a homogeneous medium, where the

complex immobilization resulted in a decrease in the

conversion and selectivity toward benzaldehyde, relative to

the values achieved in homogeneous assays.

Even though the metal-catalyzed oxidation of alkenes with

hydrogen peroxide has been intensily studied, there is currently

no ﬁrm opinion on the mechanisms of these reactions. We

propose that the MW-assisted oxidation of neat styrene

catalyzed by the copper(II) complex 2 or 5 proceeds through

a free radical mechanism, where the availability of reversibly

reducible copper(II) complexes with bis-semicarbazide hexaaza

macrocycles (see Figure 7) by the hydroperoxide is crucial for

Figure 10. Yield of β-nitroalkanol obtained from the Henry coupling

of benzaldehyde with nitroethane, catalyzed by 1 (dashed lines) or 4

(solid lines).

the oxidation to occur. It could be initiated by the Cu-

assisted formation of the oxygen-centered radicals hydroxyl

+0.57 V vs Fc /Fc for 1 and 4, respectively (in DCM; see

and hydroperoxyl, similar to the Fenton chemistry. The

sequential addition of these radicals to the styrene double bond

Figure 4) and E1/2 = +0.57 and +0.56 V vs Fc /Fc for 1 and 4,

respectively (in ACN; see Figure S23), are not signiﬁcant,

while their optical spectra are very similar (not shown). In

addition, the small diﬀerence in HOMO−LUMO energy

separation for the two complexes of 0.008 eV (Table S2)

(4.261 eV for the optimized geometry of the neutral singlet

compound 1 and 4.253 eV for complex 4 in DCM) does not

explain the markedly higher catalytic eﬃcacy of 4.

gives the 1,2-hydroxyhydroperoxide PhCH(OH)CH OOH as

a key intermediate. Further radical-induced fragmentation of

the latter leads to the formation of benzaldehyde. In addition,

complexes 2 and 5 are much more active than their precursor

salt, copper(II) acetate monohydrate, which led to a

benzaldehyde maximum yield of 23%. This suggests a favorable

role of the macrocyclic ligand in this catalytic oxidation

reaction. Moreover, after the removal of catalyst 2 or 5 from

the reaction mixture (see the Experimental Section), the

reaction did not proceed toward a higher yield of the product

of the oxidation of styrene in solution, in accord with the

absence of a catalytically active species.

The Henry reaction is a powerful tool in modern synthetic

6−78

chemistry.

Therefore, signiﬁcant eﬀorts have been

directed toward the development of better synthetic protocols

and the improvement of stereochemical outcomes. In recent

decades, the classical base-catalyzed procedure has been

complemented by metal-catalyzed processes which proceed

via various modes of reactant activation depending on the

metal, ligand(s), and additives. The metal cation is expected to

act as a Lewis acid center, activating nitroalkane (increasing its

acidity) and/or aldehyde (increasing its electrophilic charac-

ter). The ligand or additive, having a Lewis base character,

accepts a proton from nitroalkane to generate a nucleophilic

nitronate anion. Scheme 3 shows a plausible mechanism for

the Henry reaction of benzaldehyde with nitroethane catalyzed

by complex 1 or 4.

Although the selectivity toward benzaldehyde was main-

tained at least for three consecutive cycles of the recycling tests

(

see the Experimental Section), the styrene conversion

progressively decreased (46, 32, and 21%, respectively).

Therefore, an optimization of the catalytic system is still

required.

Compounds 1, 3, 4, and 6 were found to be catalysts for

both C−C cross-couplings (Scheme 2). The complexes were

tested for the Henry C−C coupling of benzaldehyde with

nitroethane, at room temperature, leading to a syn- and anti-β-

nitroethanol mixture. The reaction conditions were optimized

relative to several parameters (e.g., reaction time, reagent

ratios, type and amount of solvent, and amount of catalyst) in

order to promote the β-nitroethanol syn:anti diastereoselectiv-

ity. The optimized catalytic conditions were found to be

benzaldehyde (1 mmol), nitroethane (4 mmol), 1 or 4 (5

μmol, 0.5 mol % vs benzaldehyde), MeOH (2 mL), and

stirring (600 rpm) in air at room temperature for 48 h (Figure

Scheme 3. Proposed Mechanism for the Henry C−C

Coupling of Benzaldehyde with Nitroethane Catalyzed by

Complex 1 or 4

0).

Under the above conditions, the 1 and 4 catalytic systems

aﬀorded respectively up to 94 and 99% β-nitroethanol yields

based on benzaldehyde) with an appreciable diastereoselec-

(

tivity toward the formation of the anti isomer (anti:syn

selectivities 74:26 and 79:21 for 1 and 4, respectively), which

does not change along the reaction time. The palladium

compounds 3 and 6 led to similar yields, but the selectivity for

one of the isomers was very poor. In fact, anti:syn ratios of ca.

0:50 were observed.

The higher catalytic activity for the Ni(II) complex 4,

excellent even at room temperature, cannot be rationalized at

the moment. The diﬀerences in the electrochemical oxidation

potentials measured in diﬀerent solvents, i.e., E₁_/₂= +0.60 and

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

This mechanism involves the coordination of nitroethane to

the Ni(II) center to aﬀord the complex A with subsequent

proton abstraction from the activated nitroethane by one of the

two anionic semicarbazide fragments of the ligand. This

cooperative process gives the nucleophilic species B, which

reacts with benzaldehyde to yield C. The latter after the

intramolecular proton transfer converts into the ﬁnal complex

A series of six nickel(II), copper(II), and palladium(II)

OMe

complexes with hexaaza macrocycles H L and H L

were

synthesized and characterized by analytical and spectroscopic

techniques (IR, UV−vis, H and C NMR), ESI mass

spectrometry, single-crystal X-ray diﬀraction, and spectroelec-

trochemistry. EPR spectroelectrochemical experiments with

nickel(II) complexes 1 and 4 and DFT calculations indicate

the strong noninnocent character of the ligand. Copper(II)

complexes 2 and 5, which showed both oxidation and

reduction in the available window of potentials, proved to be

quite eﬀective catalysts in microwave-assisted neat styrene

oxidation in benzaldehyde. Nickel(II) and palladium(II)

complexes 1, 3, 4, and 6, which revealed only reversible or

quasi-reversible oxidation in the same tested potential range,

were found to catalyze the Henry coupling reaction of

benzaldehyde with nitroethane. Nickel(II) complexes 1 and

4 provided yields up to 99% and marked diastereoselectivity in

the formation of the anti isomer, in comparison to palladium-

(II) complexes, which aﬀorded anti:syn ratios of ca. 50:50. In

addition, the diamagnetic complexes 1, 3, 4, and 6 catalyzed

the Knoevenagel condensation reaction of benzaldehyde and

malononitrile to 2-benzylidenemalononitrile under sonochem-

ical conditions at room temperature with yields up to 56%.

However, superior yields were reported recently for other

homogeneous metal-based catalysts.

Recycling of 4 as a catalyst was tested (see the Experimental

Section), but the drastic decrease in product yield observed

after the second cycle hampered its further reuse.

The catalytic performance of 1 or 4 toward the Henry C−C

coupling of benzaldehyde with nitroethane is comparable to, or

79−81

superior to, those exhibited by other Ni(II) complexes

under various conditions such as under homogeneous,

solvent-free, or ionic liquid conditions. However, the

diastereoselectivity obtained in the case of 1 and 4 is diﬀerent

from that shown by recently prepared dinuclear and trinuclear

Ni(II) aroylhydrazone complexes that catalyze the Henry

coupling of benzaldehyde and nitroethane in methanol at room

temperature, with a maximum yield of 72% and a

diastereoselectivity toward the formation of the syn isomer of

4:36 syn:anti. Its improvement required solvent-free con-

ditions (89.2% yield; 72:28 syn:anti selectivity) and higher

temperatures (75 °C: 90.6% yield; 76:24 syn:anti selectivity).

The knowledge that ultrasound (US) can be used as an

activation method (by improving yields or selectivity and

reducing duration time) to perform chemical reactions led us

to test the catalytic activity of compounds 1, 3, 4, and 6 for the

Knoevenagel condensation of benzaldehyde and malononitrile

under sonochemical conditions. It was found that 1, 3, 4, and 6

are able to selectively catalyze, at room temperature, the above

US-assisted reaction to aﬀord 2-benzylidenemalononitrile

ASSOCIATED CONTENT

Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01119.

■

sı

Atom-labeling scheme used in the NMR resonance

assignment, IR, NMR, and ESI mass spectra, fragment of

the crystal structure of the Ni(II) complex with trans-IX

(

Scheme 2). The best reaction conditions, achieved by varying

(

Ar = Ph), in situ spectroelectrochemistry of 2 and 4−6,

reaction parameters such as time, type of solvent, and ratio of

reagents or catalyst amount, are benzaldehyde (1 mmol),

malononitrile (2 mmol), 10 μmol of 1, 3, 4, and 6, acetonitrile

absorption spectra of 1 and 5 and of NOBF oxidized

species, cyclic voltammograms of 1 and 4 and of 2 and 5

in the cathodic region, cyclic voltammograms of 1−6 in

the region of the ﬁrst anodic peak, EPR spectra of 2 and

(

2 mL) in air, room temperature and under US irradiation for

h. Under such conditions, similar yields (up to 56%) of the

, comparison of the experimental and calculated

C−C coupling product (selectivity >99%) were obtained for

the above compounds. US irradiation signiﬁcantly promoted

the reaction, allowing us to achieve the same yield in a

markedly shorter time (e.g. catalytic system 6 under 2 h of US

irradiation led to a 53% yield of 2-benzylidenemalononitrile,

whereas only 11% was reached under the same conditions for

the non-US-assisted reaction). However, since several

absorption spectra for neutral, one-electron-oxidized,

and one-electron-reduced complex 5, and details of the

conformations of six-membered rings in 2 and 4−6 and

of DFT calculations (PDF )

Accession Codes

CCDC 1996061 −1996063, 1996065 , and 2005715 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

83,84

homogeneous metal-based catalysts have been reported

to provide yields higher than those of the present work, further

studies were not performed.

CONCLUSIONS

■

223 336033.

A reliable multistep procedure for the synthesis of 14-

membered bis-semicarbazide macrocyclic compounds IX has

been developed previously by some of us. However, these

potential ligands for transition metals revealed poor solubility

in common organic solvents, mainly because of the presence of

strong hydrogen bonding involving lactam units in the crystal

structure. The low solubility limited their use in complex

formation reactions. Alkylation of IX did not result in

alkylation of semicarbazide moieties at the oxygen atom.

Instead, butylation at one of the hydrazinic NH groups took

place, which conferred solubility in common organic solvents

due to the lack of hydrogen-bonding interactions typical for IX.

AUTHOR INFORMATION

Corresponding Authors

■

Peter Rapta − Institute of Physical Chemistry and Chemical

Physics, Faculty of Chemical and Food Technology, Slovak

University of Technology in Bratislava, SK-81237 Bratislava,

Slovak Republic; Email: peter.rapta@stuba.sk

Luísa M. D. R. S. Martins − Centro de Quimica Estrutural,

Instituto Superior Tecnico, Universidade de Lisboa, 1049-001

Lisboa, Portugal ; Email: luisamargaridamartins@

tecnico.ulisboa.pt

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Anatoly Shutalev − N. D. Zelinsky Institute of Organic

Chemistry, Russian Academy of Sciences, 119991 Moscow,

Russian Federation; orcid.org/0000-0002-8038-8230;

Email: anatshu@gmail.com

Vladimir B. Arion − University of Vienna, Institute of Inorganic

Chemistry, A-1090 Vienna, Austria; orcid.org/0000-0002-

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