Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic and Spectroscopic Features and Applications for Catalysis of the Henry Reaction

Szilvia Bunda, N ó ra V. May,* D ó ra Bonczidai-Kelemen, Antal Udvardy, H. Y. Vincent Ching, Kevin Nys,

Article

Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic

and Spectroscopic Features and Applications for Catalysis of the

Henry Reaction

Mohammad Samanipour, Sabine Van Doorslaer, Ferenc Jo ó ,* and Norbert Lihi*

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sı Supporting Information

ABSTRACT: Copper(II) complexes formed with sulfonated salan

ligands (HSS) have been synthesized, and their coordination

chemistry has been characterized using pH-potentiometry and

spectroscopic methods [UV−vis, electron paramagnetic resonance

(

EPR), and electron−electron double resonance (ELDOR)-

detected NMR (EDNMR)] in aqueous solution. Several bridging

moieties between the two salicylamine functions were introduced,

e.g., ethyl (HSS), propyl (PrHSS), butyl (BuHSS), cyclohexyl (cis-

CyHSS, trans-CyHSS), and diphenyl (dPhHSS). All of the

investigated ligands feature excellent copper(II) binding ability via

the formation of a (O ,N,N,O ) chelate system. The results

indicated that the cyclohexyl moiety signiﬁcantly enhances the

stability of the copper(II) complexes. EPR studies revealed that the

−

arrangement of the coordinated donor atoms is more symmetrical around the copper(II) center and similar for HSS, BuHSS,

CyHSS, and dPhHSS, respectively, and a higher rhombicity of the g tensor was detected for PrHSS. The copper(II) complexes of

the sulfosalan ligands were isolated in solid form also and showed moderate catalytic activity in the Henry (nitroaldol) reaction of

aldehydes and nitromethane. The best yield for nitroaldol production was obtained for copper(II) complexes of PrHSS and BuHSS,

although their metal binding ability is moderate compared to that of the cyclohexyl counterparts. However, these complexes possess

larger spin density on the nitrogen nuclei than that for the other cases, which alters their catalytic activity.

INTRODUCTION

amines, which are stable to hydrolysis but still remain good

complexing agents. These ligands, called salans (Scheme 1),

have already served as catalysts for homogeneous trans-

formations, and their use in aqueous reaction systems can be

facilitated by appending to them ionic or polar substituents. An

example is the use of hydrogenated sulfonated salens (i.e.,

sulfosalans or HSS-type ligands), which are highly soluble in

water while showing pronounced stability against hydrolytic

decomposition.

■

,2-Bis(salicylaldiminato)ethane (salen) is probably one of the

most widely studied ligands in coordination chemistry. Because

it contains a potential O,N,N,O donor atom set for

tetradentate coordination, this ligand has been used mainly

for the complexation of hard metal ions such as nickel(II),

cobalt(II/III), and iron(II/III), although complexes with many

other metal ions are also known. Metal-salen complexes also

play a signiﬁcant role in homogeneous catalysis. With the

recent upsurge of aqueous organometallic catalysis, there is a

growing need to apply metal-salen complexes in aqueous

systems; however, this is often hindered by the insolubility of

such complexes in water. A possible solution to this problem is

the attachment of ionic or highly polar substituents (such as

sulfonate, carboxylate, ammonium, phosphonium, or oli-

Several studies have been published in the literature on the

complex formation and biological and catalytic activities of

certain salan complexes with metal ions such as titanium,

vanadium, chromium, and iron, and the state-of-the-art was

expertly reviewed in 2019. Variation of the linker group

goether groups) to the salen ligands.

Even then, the

Received: April 26, 2021

Published: July 12, 2021

hydrolytic instability of the imine-type ligands still remains a

major obstacle in their use in aqueous media. It has been

unequivocally demonstrated that extended use in aqueous

reaction mixtures leads to the hydrolytic decomposition of

metal-salen complexes. Elimination of the CN moiety in

salen derivatives by catalytic hydrogenation yields secondary

2021 American Chemical Society

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Article

Scheme 1. General Formulas of salen and salan-Type Ligands and the Structure of the Sulfonated salans Used in This Study

(

H L)

between the secondary nitrogen atoms in the salan molecules

may give the possibility of ﬁne-tuning the electronic properties

of the complexed metal ion and also the steric characteristics of

the metal complex with obvious consequences on the physical

however, most of our investigations were directed toward the

catalytic properties of Pd -salan complexes in C−C cross-

10,12,13

coupling reactions in aqueous reaction mixtures.

It was

assumed that knowledge of the distribution of various complex

species as a function of the pH could help in ﬁnding the

optimal reaction conditions and might be instructive for the

use of similar complexes of earth-abundant (instead of

precious) metals in homogeneous catalysis. Indeed, we

succeeded in developing the ﬁrst process of hydrogenation

and redox isomerization of allylic alcohols catalyzed by a

(

e.g., spectroscopic) and chemical (e.g., catalytic) behavior of

the molecule. With that aim in mind, a few studies of salan

−11

complexes included also variation of the bridging unit.

Among the metal ions not previously reviewed, copper(II) is

particularly useful for investigation of the formation and

electronic and steric properties of its salan complexes because,

in addition to the usual spectroscopic methods [UV−vis,

NMR, mass spectrometry (MS)], such complexes can also be

probed by electron paramagnetic resonance (EPR) and its

related hyperﬁne techniques, a sensitive tool to study

paramagnetic systems.

nickel(II) complex with an HSS-type ligand. In another study,

palladium(II) complexes formed with an extensive series of

sulfosalan ligands with various bridging units were applied for

the Suzuki−Miyaura cross-coupling reaction of aryl halides

10,12

with arylboronic acid derivatives.

In our laboratories, we have prepared a series of sulfosalans

Encouraged by the mentioned results of parallel coordina-

tion chemistry studies and catalytic investigations, we decided

on a closer investigation of the eﬀect of the various linkers in

the water-soluble sulfosalans on complex formation. A detailed

with various bridging units between the secondary nitrogen

,10,12,13

atoms.

Complex formation equilibria of some of these

,14

ligands were studied with palladium(II)

and nickel(II);

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EPR study has already been reported for chiral Cu -salen

Article

using the same concentration range as that in the pH-potentiometric

titrations. The CD spectra were registered with a Jasco J-810

spectropolarimeter using 1 mm and/or 1 cm cells in the 250−800 nm

wavelength range. Individual samples were prepared for the UV−vis

and CD experiments. IR spectra were recorded on a PerkinElmer

Spectrum Two FTIR spectrometer in attenuated-total-reﬂectance

mode.

Continuous-Wave (CW)-EPR Measurements and Simulation

of the Spectra. All CW-EPR spectra were recorded with a Bruker

ElexSys E500 spectrometer (microwave frequency 9.45 GHz,

microwave power 13 mW, modulation amplitude 5 G, and

modulation frequency 100 kHz). Anisotropic EPR spectra were

recorded for aqueous solutions containing 5 mM copper(II) chloride

and 5 mM sulfosalan ligands. A NaOH solution was added to the

stock solution to adjust the pH of the samples. An 0.1 mL aliquot of

the sample solution was introduced into a quartz EPR tube, and then

complexes; however, Cu -sulfosalan complexes have not

been investigated before using simultaneous equilibrium and

EPR techniques. Below we report the equilibrium and

structural study of copper(II) complexes with several HSS-

type ligands, as shown in Scheme 1. Furthermore, the catalytic

activity of these complexes was also studied in aqueous media

in the Henry or nitroaldol reaction of aldehydes and

nitroalkanes, a synthetically important C−C bond-forming

6−20

process.

EXPERIMENTAL SECTION

Materials and Methods. The CuCl stock solution was prepared

■

from the highest available grade, and its concentration was checked

gravimetrically and by the inductively coupled plasma method. The

concentration of the sulfosalan ligands were checked by pH

potentiometry. For solution equilibrium and spectroscopic studies,

doubly deionized and ultraﬁltered (ELGA Purelab Classic system)

water was used.

.025 mL of methanol (MeOH) was added to avoid water

crystallization upon freezing. Frozen-solution EPR spectra were

measured in a Dewar container ﬁlled with liquid nitrogen at 77 K.

The measured spectra were corrected by the baseline spectrum

measured in the same way and simulated using a designated EPR

Synthesis of HSS-Type Ligands. The synthesis of the ligands

was performed according to the methods established in the literature.

HSS and BuHSS were prepared as described in ref 14, and cis-CyHSS,

program. To describe the isotropic spectra, the parameters g and

63,65

A₀of copper hyperﬁne ( Cu, I = / ) couplings were ﬁtted and

two equivalent a

nitrogen ( N, I = 1) superhyperﬁne couplings

trans-CyHSS, and dPhHSS were obtained as described earlier. The

synthesis of PrHSS was carried out analogously to the preparation of

have been taken into account, with the parameters determined by

electron−electron double resonance (ELDOR)-detected NMR

(EDNMR) measurements. The relaxation parameters α, β, and γ

HSS; the details were reported in our previous paper. All ligands

were identiﬁed on the basis of their H and C NMR spectra, and

deﬁned the line widths through the equation σMI = α + βM

+ γM

they were in excellent agreement with those reported earlier. The

purity of the ligands was also checked by pH-potentiometric titrations.

where M denotes the magnetic quantum number of the paramagnetic

metal ions. For the trans-CyHSS and dPhHSS spectra, the four lines

were ﬁtted with separate line-width values (ω −ω ).

The anisotropic EPR spectra, recorded at 77 K, were also analyzed

with the EPR program. A rhombic g tensor (g , g , g ) and a rhombic

^c^o^p^p^e^r^h^y^p^e^r^ﬁⁿ^e^A^t^eⁿ^s^o^r⁽^Ax ^,^Ay ^,^Az ) were ﬁtted to simulate

the spectra. A rhombic nitrogen hyperﬁne tensor (a , a , a )

obtained by EDNMR was used in the simulation without ﬁtting these

parameters. For a description of the line width, the orientation-

dependent α, β, and γ parameters were used to set up each

component spectrum. In order to check the correctness of the

evaluation, the solution spectra have been described also by using the

anisotropic EPR values and ﬁtting the rotational correlation times.

Because natural copper(II) chloride was used for the measurements,

both the isotropic and anisotropic spectra were calculated as the sum

of the spectra of Cu and Cu weighted by their natural abundances.

The hyperﬁne and superhyperﬁne coupling constants and the

relaxation parameters were obtained in ﬁeld units (Gauss = 10 T).

Pulsed-EPR Measurements. ESE-detected EPR and ELDOR

Synthesis and Isolation of the Cu -sulfosalan Complexes.

General Procedure. A total of 43.6 mg (0.24 mmol) of copper(II)

acetate [Cu(OAc) ] and 0.24 mmol of the appropriate sulfosalan

ligand (L) were dissolved in water (4 mL). The pH was adjusted to

.0−8.5 with concentrated sodium hydroxide (NaOH), and the

reaction mixture was stirred at 60 °C for 14 h. Then the solution was

cooled to room temperature in ice water, and the Na [Cu(L)]

product was precipitated by the addition of 30 mL of ice-cold ethanol.

The solid was ﬁltered, washed with absolute ethanol, and dried under

vacuum. The products were characterized by UV−vis and Fourier

transform infrared (FTIR) spectroscopies and by the high-resolution

(

HR) electrospray ionization (ESI)-MS method. For details of the

synthesis of Na [Cu(HSS)] (1), Na [Cu(PrHSS)] (2), Na [Cu-

(

BuHSS)] (3), Na [Cu(cis-CyHSS)] (4), Na [Cu(trans-CyHSS)]

(5), and Na [Cu(dPhHSS )] (6 ) and for the respective spectroscopic

data, see the Supporting Information.

−

pH Potentiometry. The protonation constants (pK ) of the

sulfosalan ligands and the overall stability constants (log β_p_q_r) of the

copper(II) complexes were determined by the pH-potentiometric

titration method using a carbonate free KOH solution (ca. 0.5 M).

The carbonate contamination was determined using the appropriate

(electron electron double resonance)-detected NMR (EDNMR)

27,28

measurements

were carried out with a W-band (95 GHz) Bruker

ELEXSYS E680 spectrometer in conjunction with a split-coil Oxford

T superconducting magnet equipped with an Oxford ﬂow cryostat

Gran functions. Aliquots (10 mL) of the ligands (ca. 4 mM) were

titrated, the ionic strength of the samples was adjusted to 0.2 M KCl,

and the equilibrium measurements were carried out at 298 K. The

samples were stirred using a magnetic stirrer. During the titrations, the

headspace and sample were purged with argon in order to ensure the

absence of oxygen and carbon dioxide. The pH measurements were

made using a computer-controlled Metrohm 785 DMP Titrino

automatic titrator, and the instrument was equipped with a Metrohm

and a Bruker cylindrical cavity at 6 K. For the EDNMR

^m^e^a^s^u^r^e^m^eⁿ^t^s^,^t^h^e^p^u^l^s^e^s^e^q^u^eⁿ^c^e^w^a^s^H^T^Amw2−T−π/

²mw1−τ−π −τ−echo, with the length of the high-turning angle

mw1

(

HTA) pulse 20 μs, t = 200 ns, and t = 400 ns. Delay times of T =

.4 μs and τ = 848 ns were applied. Spectra were recorded at 5−6

π/2 π

diﬀerent observer positions, with 20−45 scans for each spectrum

depending on the echo intensity. The samples were prepared in water

using a 0.83 mM complex concentration, and glycerol was used to

avoid water crystallization. NaOH was added to the samples by

adjusting the pH (around pH 8.0).

.0262.100 combination glass electrode. The pH reading was

converted to hydrogen-ion concentration, as described by Irving et

al. The protonation constants (pK ) of the ligands and the overall

stability constants of the metal complexes, β

= [Cu L H ]/

The EDNMR and ESE-detected EPR spectra were simulated using

the EasySpin open-source MATLAB toolbox (version 5.2.28).

MS and NMR Spectroscopy. Time-of-ﬂight (TOF)-ESI-MS

measurements were made with a Bruker maXis II MicroTOF-Q-type

Qq-TOF-MS instrument (Bruker Daltonik, Bremen, Germany) both

in negative and positive modes. The instrument was equipped with an

electrospray ion source where the spray voltage was 4 kV. N was

utilized as a drying gas, and the drying temperature was 200 °C. The

spectra were accumulated and recorded using a digitalizer at a

pqr

[

Cu] [L] [H] , were calculated by using the dedicated computational

programs SUPERQUAD and PSEQUAD. The distribution curves

of the complexes formed between copper(II) and the ligands were

calculated by using the computational program MEDUSA.

UV−Vis, Circular Dichroism (CD), and FTIR Spectroscopic

Methods. UV−vis spectra of the copper(II) complexes were

recorded with an Agilent Technologies Cary 60 UV−vis xenon

pulse lamp spectrophotometer in the 250−800 nm wavelength range

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σ standard deviations are in parentheses.

Article

Table 1. Stepwise Deprotonation Constants (pK ) of the Ligands (I = 0.2 M KCl; T = 298 K)

HSS

PrHSS

BuHSS

cis-CyHSS

trans-CyHSS

dPhHSS

H₄L

H₃L

H₂L

6.00

7.44

8.75

10.64

32.83

6.93

7.82

9.57

11.28

35.60

6.94

7.81

10.08

11.47

36.30

4.32(5)

7.71(4)

9.10(4)

11.42(2)

32.55

4.45(4)

7.65(4)

9.13(4)

11.37(2)

32.60

6.13(6)

6.68(7)

9.33(6)

9.86(6)

32.00

∑

H_iL

σ standard deviations are in parentheses. Data are taken from ref 9.

Table 2. Stability Constants of the Complexes Formed between Copper(II) and the sulfosalan Ligands (I = 0.2 M KCl; T =

298 K)

HSS

PrHSS

BuHSS

cis-CyHSS

trans-CyHSS

dPhHSS

[

CuLH]⁻

24.25(1)

20.34(1)

8.43

24.82(1)

18.98(1)

5.74

22.04(1)

15.70(1)

3.67

26.25(1)

21.96(5)

8.76

26.70(1)

22.65(2)

9.51

CuL]²

−

17.75(6)

6.42

pCu

CuLH

CuL

3.91

5.84

6.34

4.29

4.05

Figure 1. Distribution of the complexes formed in the 1:1 Cu /HSS (left) and Cu /trans-CyHSS (right) systems and the λ values at the d−d

max

band (■) obtained by UV−vis spectroscopy as a function of the pH (I = 0.2 M KCl; T = 298 K). c = 2 mM.

sampling rate of 2 GHz. The MS spectra were calibrated externally

using the exact masses of sodium formate clusters. The spectra were

evaluated using DataAnalysis 4.4 software from Bruker.

7.0−8.5 with 1 M NaOH, and the solution was stirred at 60 °C for 2

h. Aliquots of such stock solutions were used to catalyze the Henry

coupling reaction. The ESI-MS spectra of these stock solutions were

identical with those prepared by the dissolution of isolated complexes.

The 360 MHz H NMR spectra were recorded on a Bruker Avance

DRX spectrometer at 298 K. The chemical shifts were referenced to

sodium (trimethylsilyl)propanesulfonate dissolved in the sample, and

RESULTS AND DISCUSSION

D O was used as a solvent.

■

Single-Crystal X-ray Diﬀraction Measurements. Diﬀraction

measurements of K [Cu(PrHSS)]·3H O were made on a Bruker D8

Equilibrium and Structural Studies in Solution. Acid−

Base Equilibria. The deprotonation constants of HSS, PrHSS,

and BuHSS were already published earlier, and the diﬀerent

VENTURE dual-source X-ray diﬀractometer with Mo Kα (λ =

.71073 Å) radiation. The structure was solved and reﬁned with the

2,9

sets of data show excellent agreement. Moreover, a thorough

30,31

use of SHELX program sets

analyzed by PLATON, and graphics were prepared with the

Mercury and OLEX2 software.

managed by OLEX2. Data were

analysis of the deprotonation microprocesses was also

published for these ligands. The acid dissociation constants

of the ligands were determined by ﬁtting the pH-potentio-

metric data and are collected in Table 1.

Catalytic Henry Reactions: Typical Procedure. In a Schlenk

tube, 0.025 mmol of Cu -sulfosalan catalyst was dissolved in 2 mL of

In accordance with the structures of the ligands, the sulfonic

acid groups, the two phenolic −OH groups, and the secondary

ammonium functions are involved in acid−base processes.

From these groups, deprotonation of the sulfonic acid groups

occurs below pH < 1.5; thus, it falls out from the

potentiometrically measurable pH range. Consequently, H₄L

denotes the ligands where the two phenolic −OH groups and

the secondary ammonium functions are protonated.

It was found that the acidity of the secondary ammonium

group is signiﬁcantly enhanced for the cyclohexyl-bridged

ligands (cis-CyHSS and trans-CyHSS). This is most probably

due to the electron-donating eﬀect of the cyclohexyl moiety.

The formation of an intramolecular hydrogen bond between

solvent [water or a 1:1 (v/v) water/MeOH mixture], followed by

addition of 135 μL (2.5 mmol) of nitromethane and 51 μL (0.5

mmol) of benzaldehyde. The reaction mixtures were magnetically

stirred at 75 °C in air. After the desired reaction time, the mixture was

cooled in ice water and extracted with 2 mL of dichloromethane. The

phases were separated, and the organic phase was dried by passing

through a short Na SO plug and evaporated to dryness on a rotary

evaporator. The residue was taken up in CDCl and subjected to H

NMR measurement. Conversions were determined by integration of

the formyl proton [H(CO)] and alcoholic OH (C−OH) proton

signals in benzaldehyde and in the nitroaldol product, respectively.

For catalysis of the Henry reaction with in situ prepared complexes,

.24 mmol of the appropriate sulfosalan and 43.60 mg (0.24 mmol) of

Cu(OAc) were dissolved in water (4 mL). The pH was adjusted to

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Article

the secondary amine group and protonated phenolic hydroxyl

group is more pronounced in these systems than for HSS,

due to the size of the linked chelate system [(6,5,6) for HSS vs

(6,7,6) for BuHSS]. Although dPhHSS forms a (6,5,6) linked

chelate system with copper(II), its stability is signiﬁcantly

lower than that of HSS. This is most probably due to the

presence of two aromatic moieties, which results in repulsive

forces between the neighboring aromatic systems.

PrHSS, BuHSS, and dPhHSS, respectively (Table 1, H L).

Consequently, the ﬁrst deprotonation step is separated from

the subsequent deprotonation processes. Such behavior has

already been published for sulfosalan ligands containing a

tertiary amine moiety. Upon an increase in the pH, the

further deprotonation steps signiﬁcantly overlap. On the basis

of earlier literature data, it is reasonable to assume that the

subsequent process is mainly associated with deprotonation of

The overall basicity of the investigated ligands diﬀers (Table

1); therefore, a direct comparison of the stability constants

does not provide essential information on the metal-binding

ability of the sulfosalan ligands. In order to obtain reliable

information on this feature, conditional stability constants and

theoretical distribution curves were calculated. On the basis of

the second hydroxyl group, while the highest pK value is

assigned to deprotonation of the secondary ammonium group.

These deprotonation microprocesses are illustrated in Scheme

S1 .

the calculated pCu values (pCu = −log [Cu ]free of the

complexes at pH 7.4 using 10 μM copper(II) and ligand

concentrations), the investigated ligands exhibit a trend of

copper(II) ion aﬃnity, which is as follows: trans-CyHSS > cis-

CyHSS > HSS > dPhHSS > PrHSS > BuHSS.

In addition, the acid−base properties of the two cyclohexane

isomers are the same; therefore, the relative positions of the

amine groups do not have a signiﬁcant eﬀect on this feature.

Moreover, X-ray diﬀraction studies were reported on these

ligands. It was shown that the two cyclohexyl rings precisely

overlap and only the position of the rings is diﬀerent in the

The distribution curves of the multicomponent system

containing copper(II) and all of the investigated ligands in an

equimolar solution (Figure 2 ) unambiguously prove that the

solid phase.

Copper(II) Complexes. The complex formation processes

between copper(II) and the ligands were studied by the pH-

potentiometric method, and the stability constants of the

corresponding complexes are reported in Table 2. The

representative concentration distribution curves of the species

formed in the Cu /HSS and Cu /trans-CyHSS are shown in

Figure 1, while similar distributions for the systems examined

are reported in Figures S1 −S4 .

The equilibrium model used for the ﬁtting of the pH-

potentiometric data was conﬁrmed by pH-dependent UV−vis

titration. A comparison of the λ values of the d−d transition

max

(

vide infra) with the species distribution curves calculated on

the basis of the stability obtained by pH-potentiometric

titrations conﬁrms that the equilibrium model applied for

reﬁnement of the data set is correct. The equilibrium is

Figure 2. Theoretical distribution curves between copper(II) and all

of the investigated ligands at an equimolar solution as a function of

the pH. Copper(II) complexes of PrHSS and dPhHSS are not shown

2−

described by considering the formation of [CuL] and

−

monoprotonated [CuLH] species for all systems except that

of dPhHSS.

because of their negligible contribution to the metal binding. c_C_u= 2

In the slightly acidic pH range, the complex formation

reactions start with the [CuLH]⁻complexes. In these

complexes, the coordination sphere of copper(II) is accom-

mM.

most eﬀective copper chelators are the trans-CyHSS and cis-

CyHSS ligands. HSS and BuHSS also contribute to the metal

binding; however, this feature is negligible for PrHSS and

dPhHSS. It is an interesting issue why trans-CyHSS forms the

most stable complexes, and the copper(II) complexes of cis-

HSS possess lower stability. Most probably, the trans positions

of the amine groups are more favorable to accommodate the

metal ion than the cis positions, or the formation of

conformation isomers can also be envisioned. In order to

conﬁrm this presumption, further spectroscopic measurements

were carried out.

MS and Electronic Absorption Spectroscopy. First, the

MS spectra of the copper(II) complexes were registered at pH

∼ 8, where the [CuL] complexes are the unique species in all

systems examined in this work. In all cases, the corresponding

[CuL] complexes were observed, the MS spectra do not

exhibit any indication for the formation of dimeric or bis-ligand

species, and the major peaks are attributed to the mononuclear

copper complexes and ligands (Figures S5 −S10). The

observed and calculated isotope patterns were in good

agreement, conﬁrming the existence of the postulated species

(Table S1 ). Noticeably, the negative-ion mode used in these

−

modated by the (O ,N,N) donor set. The metal center

possesses a (6,5)-membered chelate system with HSS, cis-

CyHSS, trans-CyHSS, and dPhHSS, while (6,6)- and (6,7)-

membered chelate systems form with PrHSS and BuHSS,

respectively. It is noteworthy to mention that, although the

secondary amine groups are the most basic sites in the ligands,

−

the formation of (O ,N,O ) mode is unlikely. Such a

coordination mode hinders the formation of a linked chelate

system, which is not favorable with respect to the

thermodynamic point.

Upon an increase in the pH, the next process provides the

−

2−

formation of a (O ,N,N,O )-coordinated complex ([CuL] ).

The stepwise deprotonation constants for the formation of

2−

−

CuLH

[

CuL] (pK

in Table 2) are the highest for PrHSS and

CuL

2−

BuHSS. This feature can readily be explained by considering

the fact that the increasing size of the bridging unit between

the two salicylamine moieties in PrHSS and BuHSS shifts

deprotonation and coordination of the phenolic −OH group

into the less acidic pH range. In addition, the stability

constants of the corresponding copper(II) complexes exhibit

decreasing tendency by increasing the bridging unit, which is

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Inorganic Chemistry

the Cu /trans -CyHSS system are reported in Figure 3 .

Article

transition (around 600 nm, between 100 and 550 M⁻¹cm⁻¹)

and the copper(II) phenolate metal-to-ligand charge-transfer

experiments leads to the formation of Cu -sulfosalan

complexes. Such an outer-sphere-reduction mechanism has

already been observed for several copper(II) complexes in the

gas phase under MS conditions, which happens in the

electrospray source when a high electric ﬁeld is applied

band (around 390 nm, between 1000 and 2000 M⁻cm ).

The λ_m_a_xvalues with their corresponding molar absorptiv-

ities are reported in Table S2 , and the set of data are in good

agreement with those obtained for similar systems. It is clear

from Table S2 that the λmax values of the d−d transition are

within the range of noncentrosymmetric square-planar copper

−1

between the capillary and counter electrode.

Because the nonspherical symmetry and the Jahn−Teller

eﬀect of the d ground state of copper(II) can contribute to the

stereochemistry of copper(II) complexes, determination of the

geometry is not a trivial task. However, thorough analysis of

the electronic absorption spectra oﬀers the possibility of

studying the geometry of the complexes formed in an aqueous

−

complexes with a (O ,N,N,O ) donor set and are similar to

11,39

those reported for similar complexes.

Although the

−

copper(II) ion is accommodated by the (O ,N,N,O ) donors

in all cases, the λmax values as well as the molar absorptivity

values of the investigated complexes slightly diﬀer. This is due

to distortion of the geometry around the metal center and

symmetry considerations; if the distortion is small, the

absorption becomes sharper and more intense, while its

solution.

The electronic absorption spectra of the Cu -sulfosalan

ligands were registered as a function of the pH. As a

representative example, the pH-dependent UV−vis spectra of

maximum shifts to higher-energy values. The same situation

is expected for the copper(II) complex of trans-CyHSS; hence,

the distortion is small, resulting in a decrease in the λ_m_a_xvalue.

On the contrary, rhombic distortion excludes higher

absorption maxima for PrHSS.

It is important to note that the ligands (trans-CyHSS and

cis-CyHSS) and their copper(II) complexes contain stereo-

centers; thus, their copper(II) complexes may exhibit optical

activity. In an attempt, the CD spectra of these complexes were

registered. Unfortunately, no optical activity was observed by

using CD spectroscopy (data not shown), indicating that

complexation with copper(II) yielded racemic mixtures.

X-Band CW-EPR and W-Band Pulsed-EPR Studies.

Solution X-band CW-EPR spectra recorded in equimolar metal

and ligand concentrations at room temperature have been

simulated by the designated EPR program. The best spectral

ﬁt is shown in Figure 4, and the isotropic EPR parameters are

collected in Table 3 (the line-width parameters are reported in

Table S3 ).

Figure 3. pH-dependent UV−vis spectra for the 1:1 Cu /trans-

CyHSS system as a function of the pH. c = 2 mM.

The spectra were ﬁtted with isotropic g and A parameters,

taking into account two equivalent nitrogen atoms (a ) with

The UV−vis spectra of the investigated systems feature two

signiﬁcant absorptions, which can be assigned as the d −d

parameters obtained by the EDNMR results (vide infra). In

general, the obtained spin-Hamiltonian parameters are in good

Figure 4. Experimental (black) and simulated (red) X-band CW-EPR spectra of the CuL complexes formed in the Cu -sulfosalan systems at 298 K

in solution for ligands (a) HSS, (b) PrHSS, (c) BuHSS, (d) dPhHSS, (e) trans-CyHSS, and (f) cis-CyHSS.

1264

Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

Cu [3,5-di-tert -butylsalan-(S ,S )-cyclohexane].

Article

Table 3. EPR Parameters Obtained for the Cu -sulfosalan Complexes in an Aqueous Solution

isotropic EPR parameters

anisotropic EPR parameters

isotropic EPR parameters

A₀/×10⁻cm⁻¹

g , g , g

A_x^C^u,A_y^C^u, A_z^C^u/×10⁻⁴cm⁻¹

g /A /×10 cm⁻¹

−4

g₀_,_c_a_l_c

complex

g₀

−

[

Cu(HSS)]²

2.109

2.114

2.110

2.107

2.089

2.087

2.041, 2.055, 2.240

2.044, 2.065, 2.246

2.040, 2.055, 2.240

2.035, 2.053, 2.237

2.037, 2.051, 2.233

2.038, 2.050, 2.230

2.038, 2.057, 2.235

2.057, 2.033, 2.237

2.037, 2.055, 2.239

35, 15, 186

25, 28, 176

36, 25, 185

31, 25, 185

30, 23, 186

36, 15, 187

36.4, 10.6, 189.3

26.4, 18.5, 189.9

28.3, 22.5, 184.9

36.1, 7.8, 184.9

122

128

121

120

121

118

121

2.112

2.118

2.108

2.107

2.106

2.110

2.109

2.110

2.111

Cu(PrHSS)]²

−

Cu(BuHSS)]²

−

Cu(cis-CyHSS)]²

−

Cu(trans-CyHSS)]

Cu(dPhHSS)]²

−

2.038, 2.056, 2.238

−1

The experimental errors were ±0.001 for g and ±1.0 cm for A . Recorded in an 80% H O/MeOH solvent mixture. The experimental errors

−

−1

−4

−1

were ±0.002 for g and g , ±0.001 for g , ±2 × 10 cm for A and A , and ±1 × 10 cm for A . Calculated by the equation g

= (g + g +

0,calc

x y

g )/3 on the basis of anisotropic values. Data are taken from ref 11. The structures of the copper(II) complexes are shown in Scheme S2. 7:

Cu [salan-(R,R)-cyclohexane]; 8: Cu [methoxyvanilin-(R,R)-cyclohexane]; 9: Cu [methoxyvanilin-(S,S)-diphenylethane-1,2-diamine]; 10:

Figure 5. Frozen-solution X-band CW-EPR spectra (black) of the CuL complexes formed in the Cu -sulfosalan systems at 77 K: (a) HSS; (b)

PrHSS; (c) BuHSS; (d) dPhHSS in 67% dimethyl sulfoxide/water; (e) trans-CyHSS in 80% water/MeOH; (f) cis-CyHSS in 80% water/MeOH.

Simulated EPR spectra (red) calculated from the spin-Hamiltonian parameters (reported in Table 3) obtained by the ﬁtting of the spectra.

agreement with those reported for similar copper(II)

rotational inhibition induced by the large substituents. This

was also conﬁrmed by ﬁtting the rotation correlation times on

the basis of anisotropic EPR parameters (Figure S11). The

5,41,42

complexes.

Moreover, the relationship between the g

tensors reported in Table 3 (g > g , g > 2.0) indicates the

presence of a d

ground state of copper in a square-planar or

rotation correlation time for Cu /dPhHSS is approximately 1

x −y

square-pyramidal coordination environment, where, most

likely, the axial position is accommodated by the solvent

molecule.

order of magnitude higher than those obtained for other

complexes examined. Noticeably, the rotation correlation time

for Cu /trans-CyHSS is higher than that of the cis counterpart.

The spectra and the simulated EPR parameters show a high

similarity for the copper(II) complexes of HSS and BuHSS;

however, the data for the copper(II) complex of PrHSS diﬀer

signiﬁcantly. The higher g and lower A values reﬂect a lower

ligand ﬁeld around the copper(II) center in the case of the

PrHSS complex compared to those formed with HSS or

BuHSS. These results are coherent with the λ_m_a_xvalues

obtained by UV−vis spectroscopy and conﬁrm distortion of

the geometry for the copper(II) complex of PrHSS.

On the basis of the similar size of the ligands, this is unlikely;

instead, the possibility arises that the line broadening can

originate from the coexistence of structural isomers for the

trans-CyHSS complexes in solution at room temperature. In

order to investigate the coordination geometry in more detail,

anisotropic EPR spectra have been measured in frozen solution

at pH = 7.0. The anisotropic spectra were simulated with

rhombic g-tensor and copper hyperﬁne A-tensor parameters

(Table 3). Because the perpendicular part of the EPR spectra

of the copper(II) complexes (3200−3350 G in Figure 5) does

not allow an accurate determination of the rhombic nitrogen

The line broadening detected in the solution spectra of the

copper(II) complex of dPhHSS is most probably due to the

1265

Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

in the g -tensor anisotropy between the two complexes.

Article

Table 4. Nitrogen Hyperﬁne and Quadrupole Values (in MHz) Used in the Simulation of the W-Band EDNMR Shown in

Figure 7

complex

¹⁴N

A_x

A_y

A_z

Q_x

Q_y

Q_z

A₀= ∑ A_i/3

i=1

−

[

Cu(HSS)]²

32.0

31.0

36.0

35.0

32.0

31.0

30.0

26.0

27.0

26.5

32.0

28.0

30.0

26.0

25.5

28.0

31.0

25.5

−1.20

−1.0

0.73

0.60

0.73

0.47

0.40

0.47

29.2

27.5

30.3

30.2

32.7

30.3

29.2

27.5

Cu(PrHSS)]²

−

−1.0

Cu(BuHSS)]²

−

−1.20

Cu(cis-CyHSS)]²

−

0.73

The experimental error was ±1 MHz for A , ±1.5 MHz for A , ±1 MHz for A , and ±0.3 MHz for Q. Euler angles for both A and Q: α = 45 ±

0°; β, γ = 0 ± 10°. Euler angles for both A and Q: α = −45 ± 10°; β, γ = 0 ± 10°. Euler angles for both A and Q: α = 50 ± 15°; β, γ = 0 ± 15°.

Euler angles for both A and Q : α = −50 ± 15 °; β , γ = 50 ± 15 °.

Figure 6. (a) Representation of the g values of the CuL complexes as a function of the λ values. (b) Comparison of the W-band ESE-detected

max

−

2−

EPR spectra recorded at 6 K for complexes [Cu(HSS)] and [Cu(PrHSS)] . The spectra are shown normalized to highlight the strong diﬀerence

Figure 7. Experimental (black) and simulated (red) W-band N EDNMR spectra recorded at 6 K for 80% water/glycerol solutions of the

−

2−

complexes (a) [Cu(HSS)] , (b) [Cu(PrHSS)] , (c) [Cu(BuHSS)] , and (d) [Cu(cis-CyHSS)] recorded at diﬀerent magnetic-ﬁeld settings.

Only the features of the high-frequency branch of the N nuclear transitions (M = − / manifold) are shown. A detailed explanation is given in

the main text. The simulations are done using the values reported in Table 4.

superhyperﬁne coupling values, these have been determined by

W-band EDNMR experiments and were kept constant during

the ﬁtting and simulation of the X-band EPR spectra (Table

for the copper(II) complexes of HSS (2.041 and 2.055) and

BuHSS (2.040 and 2.055). According to these values, it is

reasonable to assume that the arrangement of the coordinated

donor atoms is more symmetrical and similar in HSS and

BuHSS, and a higher rhombicity was detected for PrHSS.

The introduction of electron-donating substituents on the

sulfosalan backbone results in slight changes in the EPR

parameters. Somewhat lower g and g values can be due to the

). In a comparison of the obtained values, PrHSS also

−

exhibited a lower copper hyperﬁne A value (176.1 × 10

cm ). Higher values were found for HSS and BuHSS (186.2

−

−1

and 184.8 × 10 cm , respectively). Besides the lower A_z

value, the anisotropy of the g tensor is higher for the

copper(II) complex of PrHSS because g_xand g_ydiﬀer

considerably (2.044 and 2.065), while they are less anisotropic

eﬀect of electron donation in cis-CyHSS, trans-CyHSS, and

dPhHSS. According to the method established by Peisach−

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Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

Article

4,45

Blumberg,

the g /A ratio gives a rough measure for the

overlap between the N orbitals and the d

orbital of copper,

x −y

tetragonal distortion, although it must be noted that the g and

A_zvalues are also inﬂuenced by the coordination number, the

nature of the ligands, and the dielectric constant of the solvent

pointing to a less planar (CuONNO) segment. The diﬀerence

observed here between the two sets of copper complexes

suggests a higher tension induced by the bridging fragment in

the (6,5)-membered chelates versus the (6,6)- and (6,7)-

(

Table 3). All g /A values of the complexes under study fall in

z z

the range between 120 and 130, in agreement with the square-

planar or square-pyramidal (with axial coordination of the

solvent molecule) geometries of the copper(II) complexes. In a

membered chelates. The overall N spin density for the

studied complexes (and, hence, the N hyperﬁne values) is

only 65−80% of the earlier-reported spin density on the imine

comparison with the values reported for Cu -salen complexes

nitrogen atoms in related Cu -salen complexes. This can be

in noncoordinating solvents with and without the addition of

an amine ligand, the values indicate solvent ligation (vide

related to the diﬀerence between the imine and amine nitrogen

atoms (diﬀerence between the sp and sp hybridization of the

copper-ligating nitrogen) and, to a lesser extent, to the higher

planarity in the salen case. This is substantiated by the

infra). Figure 6a shows the plot of g as a function of λ_m_a_x,

which is correlated and shows a decrease in the ligand ﬁeld in

the order dPhHSS > cis-CyHSS > BuHSS ∼ HSS > PrHSS.

observation that the N spin density of the here-studied Cu -

salan complexes is also 60−70% of that reported for copper

complexes of phenolic oximes, of which some have slight

trans-CyHSS gave an outlier value; according to λ , it shows

max

the highest ligand ﬁeld, although the g value was found to be

6,47

similar to that of cis-CyHSS. Because λ_m_a_xwas measured at

tetrahedral distortion.

Similarly, the N isotropic hyper-

room temperature and g in a frozen solution, we believe that

ﬁne values of the pyrrole nitrogen atoms in planar copper

porphyrins and copper phthalocyanines are signiﬁcantly larger

than those of the Cu -salan complexes in this study.

the copper(II) complex of trans-CyHSS has structural isomers

appearing in solution. This would also explain the much wider

solution EPR spectrum of the trans form in comparison to the

cis isomer.

In order to compare the spin-Hamiltonian parameters of the

complexes in more detail, W-band ESE-detected EPR (Figures

These values are, however, close to those obtained for amino

nitrogen (−NH ) couplings of copper(II) dipeptides deter-

mined by EDNMR ( A = 32.0−32.3 MHz). Furthermore,

while the hyperﬁne values of the imine nitrogen atoms in Cu -

salen complexes are indistinguishable, the values of the two

ligating amine nitrogen atoms in the Cu -salan complexes are

b and S12 ) and EDNMR spectra have been recorded

(Figures 7 and S12 −S16 ). EDNMR is a pulsed-EPR technique,

based on the use of pulses with diﬀerent microwave

frequencies, that allows one to detect the nuclear transitions

and thus unravel the hyperﬁne and nuclear quadrupole

interactions of the magnetic nuclei interacting with the

slightly diﬀerent (Table 4), indicating small local out-of-plane

distortions.

The complex formation processes were examined at diﬀerent

pH values in a frozen solution in the case of HSS, PrHSS, and

BuHSS (Figures S17 −S19 ). Around pH ∼ 4, only a

unpaired electron. The positions of the peaks detected in

these spectra of the copper(II) complexes reﬂect the N and

copper hyperﬁne and nuclear quadrupole couplings. Experi-

ments were conducted at speciﬁc magnetic ﬁeld positions,

shown together with the high-ﬁeld ESE-detected EPR spectra

in Figure S12. Figure 7 shows the part of the EDNMR spectra

[Cu(aqua)] complex was found for both the HSS and

−

PrHSS ligands. The protonated [Cu(LH)] complex could be

detected for HSS, in addition to the [Cu(aqua)] complex at

pH = 4.50. At pH = 11.0, the same signal as that at pH = 7.0

was measured; thus, the formation of a mixed hydroxido

complex was not detected either under the conditions of EPR

spectroscopy or during the pH-potentiometric titrations. For

(

after baseline correction) reﬂecting the N nuclear transitions

within the M = − / manifold, together with the simulations

done with the parameters reported in Table 4. The hyperﬁne

Cu /PrHSS, the spectrum measured at pH = 5.39 can be ﬁtted

couplings are found to be strong; i.e., the signals are centered

well with consideration of two components in the simulation.

These components are the [Cu(PrHSS)] complex and

2−

around A/2 and split over twice the N Larmor frequency,

with the nuclear quadrupole coupling leading to further

broadening of the lines. In Figure 7, only the high-frequency

presumably an unidentiﬁed copper(II) complex possessing a

broad singlet spectrum. This is usually due to the line-

broadening eﬀect of close copper centers originating from

dimer complexes or precipitations. At pH = 7.0, the signal can

branch is shown. The corresponding N nuclear frequencies

of the M = / manifold are partially covered by the central

2−

Lorentzian hole of the EDNMR spectra as well as

be ﬁtted with one component ([Cu(PrHSS)] ), and the same

spectrum is preserved at pH = 11.46. In the case of BuHSS, the

63,65

contributions of the

Cu hyperﬁne lines (at observer

2−

positions corresponding to g_xand g ; see the exemplary

same [Cu(BuHSS)] spectra were recorded for all three

spectrum in Figure S15). The assignment of the lines in Figure

investigated pH values (7.0, 8.02, and 11.52).

to N could be done based on a number of spectra in which

Synthesis of Cu -sulfosalan Complexes in Solid Form

both branches were clearly resolved (see the example in Figure

S16 ). The higher g-tensor anisotropy detected in the X-band

CW-EPR spectra is reﬂected in the broader and asymmetric

high-ﬁeld peak of the W-band ESE-detected EPR spectrum of

and Their Catalytic Application in the Henry Reaction.

The copper(II) complexes of sulfonated salan ligands were

obtained in a straightforward method by heating equimolar

amounts of Cu(OAc) and the appropriate ligand (L) in a

2−

[

(

Cu(PrHSS)] corresponding to the g and g parameters

basic aqueous solution. Depending on the ligand, the pH was

adjusted to 7.0−8.5, at which, according to the equilibrium

measurements, all of the copper(II) ions were found in the

Figure 6b and Table S4 ).

From Table 4, a clear diﬀerence is observed between the

2−

hyperﬁne values of [Cu(HSS)] and [Cu(cis-HSS)] , on the

form of [Cu(L)] . Precipitation with cold ethanol and

one hand, and [Cu(PrHSS)]²and [Cu(BuHSS)] , on the

other hand. Moreover, the spin density on the nitrogen nuclei

is also increased in the latter case, as follows from the increase

−

2−

subsequent washings furnished the solid products as bluish-

green or blue solids in 59−82% yield. HR ESI-MS spectra were

in excellent agreement with the calculated masses of the

respective ions. In the FTIR spectra, all complexes exhibited

the characteristic strong absorptions of the −SO₃⁻group

in the N isotropic hyperﬁne contribution ( A ). A smaller

spin density on the nitrogen nuclei results from a smaller

1267

Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

around 1030 and 1180 cm , while the absorptions in the 1155

catalyst. The results are shown in Figure 8 , which also displays

Article

−

Table 5. Yield of the Nitroaldol Product in the Henry

Reaction of Benzaldehyde with Nitromethane with Various

Catalysts as a Function of Time

−

15 cm range can be likely assigned to the Ar−O vibrations.

UV−vis and EPR spectral features were discussed in the

previous sections of this manuscript.

yield (%) for select reaction times

Several attempts were made to obtain single crystals suitable

for X-ray diﬀraction measurements; however, these were

mostly unsuccessful. The notable exception was [Cu-

catalyst

no Cu(II)

Cu(OAc)

4 h

12 h

19 h

36 h

2−

(

PrHSS)] , single crystals of which were obtained from a

KOH solution as K [Cu(PrHSS )]·3H O. Details on this

[Cu(HSS)] (1)

structure are reported in the Supporting Information.

[Cu(PrHSS)] (2)

[Cu(BuHSS)] (3)

[Cu(dPhHSS)] (6)

Catalysis of the Henry Reaction by Cu -sulfosalan

Complexes in Aqueous Media. The reaction of nitroalkanes

with aldehydes or ketones is a synthetically useful reaction

resulting in β-nitro alcohols, which can yield important

compounds upon further transformations (e.g., nitroalkenes

Conditions: 0.5 mmol of benzaldehyde, 2.5 mmol of nitromethane, 5

mol % Cu(II) catalyst, 2 mL of water, 75 °C.

16−18,50,51

by dehydration or β-amino alcohols via reduction).

value around 8. According to the pH-potentiometric

investigations, above pH 8, all copper is in the form of

The reaction is catalyzed by various bases; however, transition-

metal catalysts are used increasingly, especially for the

enantioselective synthesis of β-nitro alcohols. Copper com-

plexes were found to be eﬀective catalysts in this

2−

[Cu(L)] with each studied sulfosalan ligand. Consequently,

we attribute the catalytic activities determined in the above

−

reactions exclusively to [Cu(L)]

complexes with no

9,52,53,20,54

reaction.

The Henry reaction of benzaldehyde

contribution from other possible forms of copper(II), which,

in principle, could be obtained by the dissociation of L. The

eﬀect of the pH on the catalytic activity was studied in the

reaction between benzaldehyde and nitromethane with 6 as the

with nitromethane (Scheme 2) is often studied to establish

55−57

the basic catalytic properties of new types of catalysts.

Scheme 2. Henry Reaction of Benzaldehyde and

Nitromethane

We have established that, in aqueous solution, the various

Cu -sulfosalan complexes eﬃciently catalyzed the reaction of

benzaldehyde and nitromethane under mild conditions and in

air. According to the H NMR spectra of the reaction mixtures,

the reactions yielded the corresponding nitroaldol product

exclusively, and only unreacted starting compounds could be

observed in addition (Figure S26). The originally blue

solutions of the Cu -sulfosalan complexes gradually turned

to yellow and later orange or light brown. However, the

reaction mixtures regained their blue color upon standing (in

air). No changes in the conversions were observed when the

reactions were run in an argon atmosphere. The conversions

were not aﬀected by the addition of trimethylamine, showing

that it did not act as either a base catalyst or a ligand for

copper(II). Table 5 shows the yield of the reaction with the

various catalysts as a function of time (see also Figure S27).

There was a slow reaction in the absence of a copper(II)

_F_i_g_u_r_e₈_._D_i_s_t_r_i_b_u_t_i_o_n_o_f_t_h_e_c_o_m_p_l_e_x_e_s_f_o_r_m_e_d_i_n_t_h_e₁_:₁_C_uII_/

dPhHSS systems and the λ_m_a_xvalues at the d−d band (■) obtained

by UV−vis spectroscopy as a function of the pH (c = 2 mM; T = 298

K). The open symbols (

□

and ○) represent the yield of the nitroaldol

product in the Henry reaction of benzaldehyde and nitromethane

after 19 and 36 h, respectively. Yields are expressed as mole fractions

of the initial benzaldehyde amount (conditions of the catalytic

reaction: 0.5 mmol of benzaldehyde, 2.5 mmol of nitromethane, 0.025

mmol of 6, 2 mL of water, 75 °C).

catalyst also, and Cu(OAc) itself also showed appreciable

catalytic activity. While the activity of 1 was equal to or only

slightly higher than that of Cu(OAc) , the catalysts 2, 3, and 6

the species distribution in the Cu /dPhHSS system as a

showed signiﬁcantly higher activity. The best result, 50% yield

function of the pH. As can be seen, by increasing the mole

−

corresponding to TON = 10 [TON = turnover number =

fraction of [Cu(dPhHSS)] (e.g., increasing the pH), the

yield of the nitroaldol product increases in parallel and levels

oﬀ at pH = 6 when all copper(II) ions are already bound to

dPhHSS. On the basis of this result, it may be concluded that

under neutral or basic conditions all catalysis can be attributed

−

(

moles of reacted benzaldehyde)(moles of catalyst) ], was

obtained with 6 in 36 h at 75 °C. This catalytic activity is close

to that (TON = 14) determined in the same reaction under

identical conditions for copper(II) tris(pyrazolylmethane)-

2−

sulfonate complexes but lags behind the activity (up to TON

7.3 in 48 h) of the copper(II) catalysts bearing a sulfonated

Schiﬀ base ligand, 2-(2-pyridylmethyleneamino)-

to [Cu(dPhHSS)] .

Complex 2 was also applied as a catalyst for the Henry

reaction of nitromethane with other aldehydes (Table 6).

While 4-bromo, 4-ﬂuoro, and 2-ethoxy substituents changed

the reactivity of the respective benzaldehyde only slightly, the

benzenesulfonate. Aqueous solutions of the isolated Cu -

sulfosalan complexes used here for catalytic studies have a pH

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Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

Table 6. Henry Reaction of Nitromethane with Various

Benzaldehydes Catalyzed by 2 in Aqueous Media

> PrHSS. trans-CyHSS gave an outlier value and exhibits the

highest ligand ﬁeld.

The Cu -sulfosalan complexes were found to be catalytically

active in the Henry (nitroaldol) reaction between benzalde-

hyde and nitromethane. The water solubility of the copper(II)

complex catalysts allowed the use of water as the only solvent

in the reaction, in accordance with the pursuit of reducing

organic solvents’ use in chemical procedures.

aldehyde

yield (%)

aldehyde

yield (%)

benzaldehyde

2-ethoxybenzaldehyde

3-methylbenzaldehyde

-bromobenzaldehyde

-ﬂuorobenzaldehyde

Conditions: 0.5 mmol of benzaldehyde, 2.5 mmol of nitromethane, 5

mol % catalyst, 2 mL of water, 75 °C, 19 h.

Finally, our results unambiguously show that a multi-

disciplinary approach is necessary to design new homogeneous

catalysts. The combination of solution equilibrium and

sensitive spectroscopic techniques such as EPR provides

deeper insight into the thermodynamic and electronic features

of copper(II) complexes, which can inﬂuence the activity of

putative catalysts. In our case, the high thermodynamic

stability and symmetric square-planar geometry are not

favorable to catalyze eﬀectively the nitroaldol reaction between

aldehydes and nitromethane; however, moderate stability,

rhombic coordination geometry, and higher spin density on

the nitrogen nucleus may contribute to increasing their

eﬃciency. The Henry reaction includes the simultaneous

binding of the electrophile and nucleophile to the metal center.

For the most reactive transition state, the nucleophile should

be placed perpendicular to the ligand plane (axial positions),

while the electrophile should occupy one of the more Lewis

conversion of m-tolualdehyde was only one-ﬁfth of that of the

parent benzaldehyde.

The size of the nitroalkane chain also aﬀected the yield. In

the reaction of nitropropane and benzaldehyde, with the use of

mol % catalyst 2 at 75 °C in 19 h of reaction time, only 20%

nitroaldol yield (anti/syn molar ratio 44:56) was obtained

instead of 26% with nitromethane.

Henry reactions between benzaldehyde and nitromethane

were also performed with in situ prepared Cu -sulfosalan

catalysts. For this purpose, equimolar amounts of copper(II)

acetate and the appropriate sulfosalan ligand were heated in

water for 2 h, and aliquots of the resulting blue solutions were

used for catalysis. A comparison of the benzaldehyde

conversions obtained with the in situ prepared or isolated

complexes showed similar catalytic activities in the two cases

(

e.g., for 2 in 19 h: 26% isolated catalyst; 20% in situ prepared

catalyst).

In liquid biphasic catalytic systems, the addition of water-

acidic equatorial sites. For the sulfosalan complexes, the

donor groups are located in the equatorial positions; therefore,

those copper(II) complexes that exhibit higher rhombicity and

small local out-of-plane distortions [e.g., copper(II) complexes

PrHSS or BuHSS] may provide a better environment to

accommodate the reactants.

miscible organic cosolvents to the aqueous phase may be

necessary to facilitate mass transport between the organic

phase of a water-insoluble organic substrate and the catalyst-

containing aqueous phase. With the use of 1:1 (v/v) water/

MeOH mixtures as solvents, we observed no eﬀect on the

conversion of the benzaldehyde/nitromethane nitroaldol

reaction with catalysts 1 and 2; however, in the case of 3

and 6, the conversions fell signi ﬁ cantly (from 33 to 27% and

from 36 to 25%, respectively; Figure S28). In a related study

using copper(II) complexes of a sulfonated Schiﬀ base ligand

as catalysts, it was also found that Henry reactions proceeded

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01264.

■

Synthesis and isolation of the Cu -sulfosalan complexes,

scheme of the deprotonation microprocesses, distribu-

tion curves, MS spectra and measured and calculated m/

z values, table of absorption maxima and molar

absorption coeﬃcient of the copper(II) complexes,

experimental and calculated CW-EPR spectra and their

simulated parameters, W-band ESE-detected EPR

spectra, W-band EDNMR spectra, frozen-solution X-

band CW-EPR spectra, ORTEP diagram, crystal data,

and yields of nitroaldol production (PDF )

faster in water than in MeOH. Because the general

mechanism of the Henry reaction includes deprotonation/

protonation steps and it is assisted by strong hydrogen

bonding in aqueous solutions, the beneﬁcial eﬀect of water

on the reaction rate can be rationalized by the high polarity,

high proton solvation power of water, and propensity for

hydrogen-bond formation.

CONCLUSIONS

In summary, the solution equilibria and spectroscopic and

catalytic features of Cu -sulfosalan complexes were studied

Accession Codes

■

CCDC 2071125 contains 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 Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

with the use of pH potentiometry and several spectroscopic

methods. All of the investigated ligands form mononuclear

complexes with copper(II), and the metal ion is accommo-

−

dated by the (O ,N,N,O ) donor set. This coordination mode

is dominant in the entire pH range and hinders the formation

of mixed hydroxido species. The investigated ligands exhibit a

trend of copper(II) ion aﬃnity, which is as follows: trans-

CyHSS > cis-CyHSS > HSS > dPhHSS > PrHSS > BuHSS.

The copper(II) complexes possess a square-planar coordi-

nation environment; however, higher rhombicity was detected

AUTHOR INFORMATION

Corresponding Authors

■

Nóra V. May − Centre for Structural Science, Research Centre

for Natural Sciences, Budapest H-1519, Hungary;

Email: may.nora@ttk.mta.hu

for PrHSS than for the other species. On the basis of the g and

λ_m_a_xvalues of the complexes, a decrease of the ligand ﬁeld was

observed in the order dPhHSS > cis-CyHSS > BuHSS ∼ HSS

Ferenc Joó − Department of Physical Chemistry and MTA-

DE Redox and Homogeneous Reaction Mechanisms Research

Group, University of Debrecen, Debrecen H-4032, Hungary;

1269

Inorg. Chem. 2021, 60, 11259−11272

Inorganic Chemistry

orcid.org/0000-0001-9457-8038; Email: joo.ferenc@

Article

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