Coordination chemistry and catalytic applications of Pd(II)–, and Ni(II)–sulfosalan complexes in aqueous media

Article abstract of DOI:10.1016/j.jinorgbio.2019.110945

With the aim of identifying new types of water-soluble catalyst precursors for modification of biological membranes by homogeneous hydrogenation in aqueous solution and under mild conditions, we have performed detailed equilibrium and spectroscopic charac

Full text of DOI:10.1016/j.jinorgbio.2019.110945

Journal Pre-proof
Coordination chemistry and catalytic applications of Pd(II)–, and
Ni(II)–sulfosalan complexes in aqueous media
Norbert Lihi, Szilvia Bunda, Antal Udvardy, Ferenc Joó
PII:
S0162-0134(19)30640-3
DOI:
https://doi.org/10.1016/j.jinorgbio.2019.110945
JIB 110945
Reference:
To appear in:
Journal of Inorganic Biochemistry
Received date:
Revised date:
Accepted date:
24 September 2019
18 November 2019
20 November 2019
Please cite this article as: N. Lihi, S. Bunda, A. Udvardy, et al., Coordination chemistry
and catalytic applications of Pd(II)–, and Ni(II)–sulfosalan complexes in aqueous media,
Journal
of
Inorganic
Biochemistry
(2019),
https://doi.org/10.1016/
j.jinorgbio.2019.110945
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Journal Pre-proof
Coordination Chemistry and Catalytic Applications of Pd(II)–,
and Ni(II)–Sulfosalan Complexes in Aqueous Media
1
2,3
2
1,2
Norbert Lihi, * Szilvia Bunda, * Antal Udvardy, and Ferenc Joó
Affiliations
1
MTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group,
Debrecen, Hungary
2
University of Debrecen, Department of Physical Chemistry, Debrecen, Hungary
3
University of Debrecen, Doctoral School of Chemistry
Mail
4
002 Debrecen P.O.Box 400, Hungary
E-mail
lihi.norbert@science.unideb.hu
bunda.szilvia@science.unideb.hu

Journal P2 re-proof
ABSTRACT
With the aim of identifying new types of water-soluble catalyst precursors for modification of
biological membranes by homogeneous hydrogenation in aqueous solution and under mild
conditions, we have performed detailed equilibrium and spectroscopic characterization of
complex formation between nickel(II) or palladium(II) and salan-type ligands sulfonated in
their aromatic rings (N,N’-bis(2-hydroxy-5-sulfonatobenzyl)-1,4-diaminoethane (HSS), N,N’-
bis(2-hydroxy-5-sulfonatobenzyl)-1,4-diaminopropane (PrHSS) and N,N’-bis(2-hydroxy-5-
sulfonatobenzyl)-1,4-diaminobutane (BuHSS)) in the slightly acidic – alkaline pH range. The
stability constants of the metal complexes were determined using pH-potentiometry. The
catalytic activities of the [Ni(HSS)] and [Pd(HSS)] complexes in hydrogenation and redox
isomerization of oct-1-en-3-ol were also studied. The results indicate, that all of the
investigated ligands exhibit excellent nickel(II) and palladium(II) binding ability via the
–
–
formation of (O ,N,N,O ) linked chelate system. Both [Ni(HSS)] and [Pd(HSS)] catalyze the
hydrogenation and redox isomerization of oct-1-en-3-ol. [Pd(HSS)] shows excellent activity
and the reaction was highly selective to the formation of octan-3-ol. [Ni(HSS)] is also an
active and selective catalyst for this hydrogenation reaction and to the best of our knowledge,
[
Ni(HSS)] is the first nickel(II)-based, hydrolytically stable, water-soluble catalyst bearing
sulfonated salan moiety.
KEYWORDS
sulfonated salan, nickel(II), palladium(II), stability, hydrogenation, allylic alcohols

Journal P3 re-proof
INTRODUCTION
Organometallic catalysis in aqueous media [1-5] have been widely used for studies on
biological processes.[6] Such studies include modification of biological samples, both model
systems (such as liposomes), isolated cell organelles, and live cells in order to gain insight on
the role of biomembranes in various physiological processes.[6-8] Hyperpolarization of
suitable biomolecules such as pyruvate via hydrogenation with para-H is used for in vivo
2
imaging. [9] In addition to other strict requirements (no toxicity!), the catalyst should be able
to operate in aqueous environment, and solubility in water is often achieved by attaching polar
or ionic substituents to the ligands of the catalytically active metal complex.
In addition to the studies on biochemistry or cell physiology, aqueous organometallic
catalysis has also contributed very importantly to development of green chemical processes.
Such processes facilitate reactions of environmentally important substrates (hydrogenation of
carbon dioxide and various constituents of biomass) and also enable aqueous-organic liquid
biphasic technologies. The latter are characterized by the use of a minimum amount of (or no)
volatile organic solvents and by simple separation of products and catalysts, leading to highly
efficient catalyst recycling and low metal contamination of the products. [2-4,8,10]
Salen (N,N’-ethylenebis(salicylimine)) is one of the most widely used ligands in
coordination chemistry and its transition metal complexes show useful catalytic properties in
non-aqueous media in reactions such as oxidation, hydrogenation, various C-C couplings,
etc.). In order to capitalize on the versatile catalytic properties of salen complexes also in
water, this ligand and its derivatives were made water-soluble by appending on it ionic or
polar substituents such as, for example, sulfonate [11,12], carboxylate [13,14], ammonium
[
15], and phosphonium [16] groups. With respect to aqueous applications, however, a major
drawback is in the imine character of the ligand, which often results in the hydrolysis and
deterioration of the complexes in water especially under the sometimes harsh conditions of

Journal P4 re-proof
catalysis (high temperature, presence of metal ions, bases or acids). [17] The problem of
catalyst decomposition can be eliminated by hydrogenation of the C=N bonds leading to
hydrolytically more stable complexes of N,N’-di(2-hydroxybenzyl)ethylenediamine (salan), or
those of HSS = hydrogenated, sulfonated salen (Figure 1). [17,18] An excellent recent review
discusses the synthesis and properties of salan complexes (both water-soluble and water-
insoluble) as well as their biomedical and catalytic applications hitherto investigated. [19]
Hydrogenation of salen-type ligands (e.g. those with various linker groups between the two
salicylidene moieties) is – in principle – generally suitable also for the synthesis of
hydrolytically stable sulfonated salan-type ligands (sulfosalans), however up till now this was
only scarcely attempted. [10,19-21]
The use of water as solvent allows to carry out catalytic reactions at various pH which can be
adjusted and controlled by addition of simple acids and bases (pH-static conditions are also
used [22]). Variations in the pH of a reaction mixture may strongly influence the formation of
various metal complex species (both their compositions and concentrations) and this may be
reflected in the results of a catalytic reaction in terms of product(s) yield(s) and selectivities.
n+
[
22] It is therefore important to learn the coordination chemistry of a given M /L system
even if the catalyst (or its precursor) is applied as a well-defined, isolated metal complex. For
speciation studies in aqueous solutions, one of the most reliable and widely applicable
methods is pH-potentiometry which may give information on the concentration distribution
and the stoichiometry of the various metal complex species as the function of pH. On the
basis of such speciations, it may be possible to identify the most abundant metal complex
species under the conditions of catalysis (although this may not necessarily be identical to the
real catalytic species).
(
(Please insert Figure 1 here))

Journal P5 re-proof
In this paper, we report a detailed pH-potentiometric study of complex equilibria established
2
+
2+
in aqueous solutions of Pd and Ni complexes of three sulfosalan ligands (N,N’-bis(2-
hydroxy-5-sulfonatobenzyl)-1,4-diaminoethane (HSS), N,N’-bis(2-hydroxy-5-
sulfonatobenzyl)-1,4-diaminopropane (PrHSS) and N,N’-bis(2-hydroxy-5-sulfonatobenzyl)-
,4-diaminobutane (BuHSS), see Figure 1). Catalytic activities of the [Ni(HSS)] and
Pd(HSS)] complexes in hydrogenation and redox isomerization of oct-1-en-3-ol (as a
1
[
representative allylic alcohol) are also reported.
EXPERIMENTAL
Synthesis and characterization of the sulfosalan ligands
1
13
HSS and BuHSS were prepared as described in [21] and were identified by their H and C
NMR spectra, respectively. Synthesis of PrHSS was carried out analogously to the
1
13
preparation of HSS and BuHSS; details of the synthesis and characterization ( H, C NMR
and ESI-TOF MS data) can be found in the Supplementary. The precursor of PrHSS was
prepared following the procedure described elsewhere. [23]
pH-potentiometry
2
–
[
PdCl
water using 2 equivalents of HCl to avoid hydrolytic processes. The NiCl
prepared from the highest available grade and its concentration was checked gravimetrically.
4
]
stock solutions were prepared by dissolving Na
2
[PdCl
4
] (Pressure Chemicals) in
2
stock solution was
2
–
The purity and the concentration of the sulfosalan ligands and the concentration of [PdCl ]
4
stock solutions were checked by pH-potentiometry. For solution equilibrium studies, triple
deionized and ultrafiltered (Milipore Q system) water was used.

Journal P6 re-proof
The protonation constants of the HSS ligands (pK
a
) and the overall stability constants of the
metal complexes (logβpqrs) were determined by pH-potentiometric method. 15 mL aliquots of
the ligands (ca. 2.1 mM concentration) were titrated with KOH solution of known
concentration (ca. 0.16 M). The carbonate contamination (less than 0.1 %) and the
concentration of the KOH solution were determined using the appropriate Gran functions.
[
24] During the titrations, the sample was purged with argon in order to ensure the absence of
oxygen and carbon dioxide. The ionic strength of the samples was adjusted to 0.2 M KCl and
the measurements were carried out at 298 K. The samples were stirred using a magnetic
stirrer. The pH-potentiometric titrations were made using a computer-controlled Mettler
Toledo T50 automatic titrator and the instrument was equipped with a DG 114-SC combined
glass electrode (Mettler Toledo). The pH readings were converted to equilibrium hydrogen
ion concentration as described by Irving et al. [25] The protonation constants and the overall
p
q
r
s
stability constants of the metal complexes, βpqrs = [M
p
L
q
H
r
Cl ]/[M] [L] [H] [Cl] (where “M”
s
stands for nickel(II) or palladium(II)) were calculated using the general computational
programs (SUPERQUAD [26] and PSEQUAD [27]).
Spectroscopic measurements
1
13
H and C NMR spectra were recorded in D
2
O solutions on a Bruker Avance 360
spectrometer. When required, the pH of the samples was adjusted with the use of NaOD and
1
13
DCl. H and C NMR chemical shifts are reported relative to DSS. ESI-TOF-MS
(
Electrospray Ionization Time-of-Flight Mass Spectrometry) measurements were carried out
on a Bruker maXis II MicroTOF-Q instrument (Bruker Daltonik, Bremen, Germany) in
negative ion mode. Details of the measurements can be found in the Supplementary.

Journal P7 re-proof
UV-visible spectra of the nickel(II) complexes were recorded with an Agilent Technologies
Cary 8454 UV-VIS diode array spectrophotometer in the 200 – 800 nm wavelength range
using the same concentration range as in the pH-potentiometric titrations.
Typical procedure of hydrogenation and redox isomerization of oct-1-en-3-ol:
−7
−7
A solution of [Pd(HSS)] (1.25×10 to 2.5×10 mol) in water (100 − 200 μL), oct-1-en-3-ol
−
4
(
2.5×10 mol), and 3 mL of 0.2 M acetate buffer of appropriate pH (I=0.2 M KCl) were
placed into a high-pressure tube. The tube was repeatedly evacuated and filled with H and
finally pressurized with H to reach 1 − 5 bar total pressure. The reaction vessel was
2
2
immersed into a thermostated bath (25 – 80 °C), and the mixture was stirred for the desired
reaction time. At room temperature the products were extracted with 2 mL of toluene, dried
over MgSO
4
, and subjected to gas chromatography.
In case of [Ni(HSS)] catalyst, the same procedure was used, however, the reaction mixture
−
5
contained an aqueous solution of [Ni(HSS)] (1.07×10 mol in 1 mL of water), oct-1-en-3-ol
−
4
(
2.5×10 mol), and further 2 mL of water and the reactions were run at 80 °C.
The reaction mixtures were analyzed by gas chromatography using a HP5890 Series II
equipment (Supelcowax 10 - Fused Silica 30 m×0.32 mm×0.25 µm; FID; carrier gas: argon).
Composition of the product mixtures are expressed as area %. The peaks in the
chromatograms were identified by their order and standard samples and the composition of
the product mixture was calculated from the peak areas.
RESULTS AND DISCUSSION
Acid-base properties of the ligands
The protonation constants (pK
a
) of the ligands were determined by pH-potentiometric
titrations and the data are collected in Table 1. The stepwise protonation constants of HSS

Journal P8 re-proof
have already been published [17] and the two sets of data show excellent agreement. It
follows from the structure of the investigated ligands, that in principle the sulfonic acid
groups, the two phenolic-OH groups and the secondary amine groups may take part in
deprotonation processes. However, the pH-potentiometric data clearly show, that the
deprotonation of sulfonic acid groups takes place below pH < 1.5 which value falls out the
potentiometrically measurable pH range. Therefore, four deprotonation constants have been
1
13
determined by pH-potentiometry while the pH-dependent H and C NMR experiments
offered a possibility to assign the pK
a
values of the individual groups.
a
Table 1. Stepwise Deprotonation Constants (pK
I = 0.2 M KCl, T = 298 K)
HSS
a
) of the Ligands
(
b,c
PrHSS
6.93(8)
7.82(8)
9.57(7)
11.28(4)
35.60
BuHSS
6.94(4)
7.81(4)
10.08(3)
11.47(2)
36.30
BHBDPA
5.90
H
H
H
4
3
2
L
L
L
6.00(8)
7.44(7)
8.75(6)
10.64(4)
32.83
8.47
10.16
11.13
35.66
HL
ΣH
i
L
a
3
σ standard deviations are in parentheses.
b
c
BHBDPA: (N,N’-bis(2-hydroxybenzyl)-2,3-diamino-propionic acid
Data are taken from Ref. [28]
1
3
Selected part of the proton decoupled C NMR spectra of HSS as a function of pH are shown
in Figure 2 and the chemical shifts of the carbon atoms as a function of pH are illustrated in
1
3
1
the Supplementary, Figure S7. The C{ H} NMR spectra of the examined systems are
available in the Supplementary (Figure S8, S9).
(
(Please insert Figure 2 here))
Based on the thorough analysis of NMR data it can be concluded, that the lowest, measurable
pK value belongs to the deprotonation of one of secondary amino groups. In slightly acidic
a

Journal P9 re-proof
pH range, a drastic downfield shift was observed in the aliphatic region of the NMR spectra
confirming the aforementioned deprotonation process. This effect is well demonstrated in
Figure S7, where the chemical shift of C8 carbon atom – assigned as one of the aliphatic
carbon atoms – decreases by increasing the pH which corroborates that the deprotonation
process occurs on the secondary amino group. The aromatic carbon atoms remain almost
intact in this pH range. This, relatively low pK value for a secondary amine group, suggests
a
the formation of an intramolecular hydrogen bond between the deprotonated amine and a
protonated phenolic hydroxyl group.[28] A similar observation has already been reported in
the case of (N,N’-bis(2-hydroxybenzyl)-2,3-diamino-propionic acid ligand (BHBDPA). [28]
Upon increasing the pH, further deprotonation processes significantly overlap, however,
1
3
based on the downfield shift in the aromatic part of the C NMR spectra, these processes can
be assigned to the deprotonation of phenolic hydroxyl groups, which can be easily followed
on the chemical shift of C1 atom (Figure S7). In strongly alkaline solutions, the highest pK
a
value belongs to the deprotonation of the other amine group resulting in further downfield
1
3
shift in the aliphatic part of the C NMR spectrum. The micro- and macroprocesses of HSS
deprotonation are summarized in Scheme S1 (see Supplementary).
The results of pH-potentiometric measurements have also demonstrated, that the increasing
size of the bridging unit between the two salicylamine moieties in PrHSS and BuHSS
significantly enhances the basicity of the phenolic hydroxyl and the secondary amine groups
resulting in higher basicity compared to that of HSS. This effect is also revealed by NMR
spectroscopy where the downfield shift in the aliphatic region occurs in more alkaline pH for
PrHSS and BuHSS than that obtained for HSS.
Pd(II) and Ni(II) complexes of the sulfosalan ligands

Journal 1P0 re-proof
The equilibrium studies on the Pd(II)/Ni(II) – sulfosalan ligand systems were performed using
pH-potentiometric titrations and the stability constants of the corresponding complexes are
collected in Table 2. It is important to note, that chloride ion was used as a competitor during
2
–
–
the titrations of Pd(II)/sulfosalan systems because the formation of [PdCl
4
] and [PdCl ]
3
species shifts the Pd(II)/sulfosalan complex formation into the measurable pH range. In the
absence of chloride ion, the complexation occurs already under very acidic conditions
hindering the determination of stability constants. [29]
a
Table 2. Stability Constants (logβpqrs) of the Complexes Formed between Palladium(II) or
b
Nickel(II) and the Ligands (I = 0.2 M KCl, T = 298 K)
HSS
37.31(5)
31.2(2)
18.7(1)
13.00(4)
6.11
PrHSS
36.98(6)
BuHSS
37.3(5)
[
[
[
[
PdHLCl]
PdL]
29.5(2)
19.07(2)
12.15(2)
7.48
31.7(5)
15.98(5)
7.89(2)
5.60
NiLH]
NiL]
ꢁ
ꢂꢄꢃꢅꢆ
ꢀꢁ
ꢀ
ꢂꢃ
ꢇꢈꢄꢃ
5.70
6.92
8.09
ꢇꢈꢃ
a
Chloro complexes of palladium(II) were taken into account in the calculations of stability
+
–
constants. These are as follows: [PdCl] (logβ = 5.08); [PdCl
2
] (logβ = 8.88); [PdCl
] (logβ = 12.18). Data are taken from Ref. [30]
σ standard deviations are in parentheses.
3
] (logβ
2
–
=
11.30); [PdCl
4
b
3
As a representative example, the molar distribution of the complex species in the Pd(II):HSS
:1 system are shown in Figure 3 while the distributions of the Pd(II):PrHSS 1:1 and the
1
Pd(II):BuHSS 1:1 systems are reported in the Supplementary (Figure S10 and S11). In
general, the complex formation processes with palladium(II) were quite fast; the titrations
were performed in equilibrium controlled mode and the waiting time between two titration
points fell into the range of 90 – 360 s.
For palladium(II), the complex formation in all cases starts with the ternary PdHLCl complex
in slightly acidic pH range. In these complexes, the coordination sphere of Pd(II) is
–
accommodated by the (O ,N,N) donor set and the square-planar environment is completed by

Journal 1P1 re-proof
the binding of the chloride ion. Although the amino group is the most basic site in the ligands,
–
–
the (O ,N,O ) coordination mode is unlikely. This can be explained by considering the fact,
that the donor atoms in the ligands are in chelate position. Therefore, the coordination of the
–
–
individual donors occurs in a cooperative manner. If the coordination mode is (O ,N,O ), a
macrochelate complex forms in the PdHLCl species. With respect to thermodynamics, the
formation of the coupled chelate system is more favourable than the formation of
macrochelate species.
(
(Please insert Figure 3 here))
Upon increasing the pH, a further deprotonation process results in the formation of PdL
complexes. In these complexes, the chloride ion is replaced by the phenolate O donor to form
coupled chelate systems with ring sizes of (6,5,6), (6,6,6,), and (6,7,6) in case of HSS, PrHSS,
and BuHSS, respectively. Stability constants of the three PdL complexes are approximately
the same, although Pd(II) prefers the (6,5,6) membered chelate systems. [31] This effect
concerning the stabilities can be easily explained by considering the fact, that PrHSS and
BuHSS have higher basicity than that of HSS resulting in increased stabilities. To compare
the palladium(II) binding abilities of the sulfosalan ligands, theoretical distribution curves
were calculated (Figure 4).
(
(Please insert Figure 4 here))
The calculated concentration distribution curves in
a
system containing
Pd(II):HSS:PrHSS:BuHSS in 1:1:1:1 concentration ratio clearly demonstrate that HSS
behaves as a more effective Pd(II) chelator than PrHSS or BuHSS. Under the conditions of
Figure 4, HSS dominantly coordinates Pd(II) in the entire pH-range which is due to the
formation of (6,5,6) membered chelate systems. Significant competition between HSS and

Journal 1P2 re-proof
BuHSS occurs only in strongly alkaline pH-range, while only negligible complexation of
Pd(II) by PrHSS is seen in the entire pH range; both effects are explained by the differences
between the basicities of the studied sulfosalan ligands.
In general, palladium(II) forms more stable complexes than nickel(II) which is clearly shown
by the stability constants in Table 2. For the nickel(II) – sulfosalan systems, the equilibrium
features are the same than those of Pd(II) – sulfosalans, however, the stabilities and the
amounts of the corresponding complexes are different. In all cases, the complex formation
processes start with formation of an [NiHL] complex which, however, is only a minor species
and its formation significantly overlaps with the formation of [NiL]. The distribution of the
complexes formed in the Ni(II):HSS 1:1 system as a function of pH is shown in Figure 5. The
–
–
coordination of an (O ,N,N,O ) donor set results in the formation of square-planar complexes
which is confirmed by UV-Vis spectroscopy. The coordination of nickel(II) to the ligands is
accompanied by significant changes in the absorbance at 495 nm, which d-d band is
–
–
characteristic for square-planar nickel(II) complexes. [32] The amount of the (O ,N,N,O )
coordinated species reaches a maximum around at pH 8 confirming the equilibrium model
obtained by pH-potentiometry in the Ni(II):HSS 1:1 system (Figure 6). The same tendencies
were observed at 366 nm where the nickel(II)-phenolate MLCT band is characteristic. This
–
corroborates that the NiHL complex features a (N,N,O ) donor set which is completed by the
binding of the phenolate group. In strongly alkaline solutions, there is no indication for the
formation of mixed hydroxido species and the titration curves do not show any extra base
consumption processes, thus these ligands are excellent chelators for nickel(II) and able to
hinder the hydrolysis of the metal ion.
(
(
(Please insert Figure 5 here))
(Please insert Figure 6 here))

Journal 1P3 re-proof
Comparison of the stability constants of NiL – sulfosalan complexes with HSS, PrHSS and
BuHSS ligands, confirms that the (6,5,6)-membered chelate system is more favourable for
nickel(II) than the (6,6,6)- or (6,7,6)-membered chelates; this results in high nickel binding
ability (see Table 2; the theoretical species distributions are available in the Supplementary,
Figure S12).
Catalytic applications of Pd(II)- and Ni(II)-sulfosalan complexes in hydrogenation/redox
isomerization of allylic alcohols
Catalytic redox isomerization of allylic alcohols is a valuable, 100 % atom-economic
synthetic process and various metal complexes are used as catalysts in this reaction. However,
in most cases, such catalysts contain tertiary phosphine or N-heterocyclic carbene ligands [33-
3
9], furthermore, application of Ni(II)-based catalysts for transposition of allylic alcohols is
rare [40]. Pd(II)-complexes of HSS have already been applied successfully as catalysts in
aqueous-phase hydrogenation and redox isomerization of allylic alcohols [20], as well as in
Sonogashira [21] and Suzuki–Miyaura [10] C-C cross-coupling reactions. Isolated [Pd(HSS)]
proved to be an active and selective catalyst for hydrogenation of ,-unsaturated aldehydes
(
such as cinnamaldehyde). [41] More importantly for biological applications, [Pd(HSS)]
could be effectively used for hydrogenation of phospholipids in aqueous dispersions
liposomes) and also for the hydrogenation of unsaturated fatty acyl residues in the bacterial
(
cell membranes of Pseudomonas putida F1. [41] No such reactions were performed with
Ni(II)-sulfosalan complexes. Furthermore, no attempt has been made to pinpoint the actual
catalytic species in such reactions with the use of methods of coordination chemistry for
identifying the possible species present in solution of the transition metal – sulfosalan
(
pre)catalysts. In aqueous organometallic catalysis, pH-potentiometry was conceived as a
straightforward method to determine the various metal-sulfosalan species as a function of the

Journal 1P4 re-proof
pH. Indeed, as described above, stability constants and molar distributions were determined
for Pd(II)- and Ni(II)-complexes with HSS, PrHSS and BuHSS ligands.
The catalytic activity of [Ni(HSS)] was assessed in the reaction of oct-1-en-3-ol under H . In
2
principle, the reaction can yield the hydrogenated product, octan-3-ol, and the product of
redox isomerization octan-3-one (Scheme 1). These reactions were run at pH=7.04, where the
[
Ni(HSS)] species is far dominant and no free Ni(II) is present (Figure 5) – the pH of the
reaction mixture did not change during the process. Under the conditions of Figure 7, the
reaction proceeded smoothly and was highly selective for the formation of the saturated
alcohol (C=C hydrogenation). It is remarkable that at a [substrate]/[catalyst] = 23.4 ratio,
conversion of oct-1-ene-3-ol to octan-3-ol reached 80% in 3 h, which corresponds to a
–
1
catalyst turnover frequency TOF = 6.2 h
(TOF = mol reacted substrate × (mol
–
1
catalyst × time) ). In comparison, under similar, albeit not identical conditions, the use of
–
1
[
Pd(HSS)] led to a TOF = 650 h . [20]
(Please insert Scheme 1 here))
(Please insert Figure 7 here))
(
(
The hydrogenation/redox isomerization of oct-1-en-3-ol catalyzed by [Ni(HSS)] was strongly
influenced by the hydrogen pressure. As it is shown in Figure 8, there was only a 5 %
conversion at P(H
2
) = 1 bar, which steadily increased with the pressure to reach 86 % at
P(H ) = 5 bar.
2
(
(Please insert Figure 8 here))
These observations demonstrate, that [Ni(HSS)] is an active and selective catalyst for the
hydrogenation of the C=C double bond in allylic alcohols with no significant selectivity
towards formation of saturated ketones. According to the species distribution curve (Figure

Journal 1P5 re-proof
5
), at the pH = 7.04 of aqueous solutions of the [Ni(HSS)] catalyst, the dominant species is
2
–
–
[
NiL] albeit minor amounts of [NiHL] are also present. It should be considered, though,
that the species distribution was calculated for T = 25.0 °C and I = 0.2 M (KCl) and it should
not be taken directly relevant for the reaction conditions. Clearly, further detailed studies are
needed to close the gap between the conditions of equilibrium and kinetic studies.
Nevertheless, [Ni(HSS)] is reported here as the first nickel(II)-based, hydrolytically stable,
water-soluble catalyst containing a sulfonated salan ligand, which – due to its significant
catalytic activity in hydrogenation of allylic alcohols – deserves further scrutiny.
In aqueous solutions, [Pd(HSS)] catalyzed the hydrogenation /redox isomerization of oct-1-
-1
en-3-ol with excellent activity (TOF up to 1894 h at 40 °C), and the reaction was highly
selective to the formation of octan-3-ol (maximum yield 85.6 %). It is important to note, that
in separate experiments under the conditions of Figure 9, there was no hydrogenation of
octan-3-one to octan-3-ol, meaning that the reaction involves hydrogenation of the C=C bond
in oct-1-en-3-ol and the redox isomerization followed by C=O hydrogenation route can be
excluded.
The reaction was run at several pH, and the results are shown in Figure 9. It was found, that
raising the pH from 3.6 to 4.6 resulted in a drop of the conversion of oct-1-en-3-ol from
3
6.5 % to 11.0 %, however, further increase of the pH led to a steep increase of the
conversion reaching 94.7 % at pH 8.2. The molar distribution of the complexes formed in the
Pd(II): HSS 1:1 system as a function of pH is shown in Figure 3. It can be seen that at
2
–
2
5.0 °C, the dominant species both at pH 3.6 and 4.6 is [PdHLCl] which is gradually
2
-
replaced by [PdL] upon increasing the pH up to 8.2. This leads to the conclusion, that in this
2
–
system the catalytically most active species is [PdL] , and that the hydrogenation of oct-1-en-
2
–
3
-ol at pH 4.2 and below may be due to [PdL] which is present in about 10 % at pH 4.2,
and/or to the ternary chlorido complexes which can be found in small but not negligible

Journal 1P6 re-proof
concentrations at pH 3.6. It should be emphasized here, too, that the conditions of the
equilibrium (speciation) studies and those of the catalytic measurements were not exactly the
same, although in these investigations 0.2 M KCl was applied in all measurements and the
difference of the temperatures used for equilibrium and catalytic experiments was only 15 °C.
CONCLUSIONS
In summary, with the use of pH potentiometry, we have determined the protonation constants
of three sulfonated salan ligands, HSS, PrHSS, and BuHSS, as well as the equilibrium
2
+
2+
constants for the stepwise formation of their complexes with Pd and Ni . In a parallel study
we determined the catalytic activity of [Ni(HSS)] and [Pd(HSS)] complexes in the
hydrogenation and redox isomerization of oct-1-en-3-ol which revealed significant activity
2
–
also in the case of the non-precious metal complex [NiL] . The catalytic activity of
Pd(HSS)] was also determined as a function of the pH. Comparison of the pH-dependence of
the catalytic activity to the species distribution determined by pH-potentiometry led to the
[
2
-
conclusion that [PdL] possesses the highest catalytic activity in this reaction.
Although this approach may help in identifying actual catalytic species in homogeneous
catalysis in aqueous solutions, nevertheless, the results should be evaluated with due care,
especially when there is a considerable gap between the experimental conditions of speciation
and catalytic studies.
ABBREVIATIONS
HSS
N,N’-bis(2-hydroxy-5-sulfonatobenzyl)-1,4-diaminoethane
N,N’-bis(2-hydroxy-5-sulfonatobenzyl)-1,4-diaminopropane
N,N’-bis(2-hydroxy-5-sulfonatobenzyl)-1,4-diaminobutane
PrHSS
BuHSS

Journal 1P7 re-proof
DSS
Sodium trimethylsilylpropanesulfonate
Metal to Ligand Charge Transfer
Turnover Frequency
MLCT
TOF
ACKNOWLEDGEMENTS
The research was supported by the EU and co-financed by the European Regional
Development Fund (under the projects GINOP-2.3.2-15-2016-00008 and GINOP-2.3.3-15-
2
016-00004). N. L., A. U. and F. J. are grateful for the financial support of the Hungarian
National Research, Development and Innovation Office (NKFIH PD-128326 and FK-
28333). This work was also supported by ÚNKP-19-3 New National Excellence Program of
1
the Ministry for Innovation and Technology.
REFERENCES
[
[
1]
2]
F. Joó, Aqueous Organometallic Catalysis, Kluwer, Dordrecht, 2001.
B. Cornils, W. A. Herrmann (Eds.), Aqueous-Phase Organometallic Catalysis.
nd
Concepts and Applications, 2 edn., Wiley-VCH, Weinheim, 2004.
[
[
[
[
[
3]
4]
5]
6]
7]
D. J. Adams, P. J. Dyson, S. J. Tavener, Chemistry in Alternative Reaction Media,
Wiley, Chichester, 2004.
P. Dixneuf, V. Cadierno (Eds.), Metal-Catalyzed Reactions in Water, Wiley-VCH,
Weinheim, 2013.
L.-A. Schaper, S. J. Hock, W. A. Herrmann, F. E. Kühn, Angew. Chem. Int. Ed. 52
(
2013) 270–289; Angew. Chem. 125 (2013) 284–304.
I. Horváth, A. Glatz, V. Varvasovszki, Zs. Török, T. Páli, G. Balogh, E. Kovács, L.
Nádasdi, S. Benkő, F. Joó, L. Vígh, Proc. Natl. Acad. Sci. USA 95 (1998) 3513–3518.
P.J. Quinn, F. Joó, L. Vígh, Prog. Biophys. Mol. Biol. 53 (1989) 71–103.

Journal 1P8 re-proof
[
[
[
[
8]
9]
F. Joó, Acc. Chem. Res. 35 (2002) 738–745.
G. Papp, H. Horváth, F. Joó, ChemCatChem 11 (2019) 3000–3003.
10] Sz. Bunda, A. Udvardy, K. Voronova, F. Joó, J. Org. Chem. 83 (2018) 15486–15492.
11] Y.-S. Liu, N.-N. Gu, P. Liu, X.-W. Ma, Y. Liu, J.-W. Xie, B. Dai, Tetrahedron 71
(
2015) 7985–7989.
[
[
12] S. Noël, F. Perez, J. T. Pedersen, B. Alies, S. Ladeira, S. Sayen, E. Guillon, E. Gras,
C. Hureau, J. Inorg. Biochem. 117 (2012) 322–325.
13] J. C. Pessoa, S. Marcão, I. Correia, G. Gonçalves, Á. Dörnyei, T. Kiss, T. Jakusch, I.
Tomaz, M. M. C. A. Castro, C. F. G. C. Geraldes, F. Avecilla, Eur. J. Inorg. Chem.
(
2006) 3595–3606.
[
[
[
[
[
14] S. A. Samokhvalova, V. A. Ershov, D. A. Lukyanov, P. S. Vlasov, O. V. Levin, Russ.
J. Gen. Chem. 89 (2019) 852–855.
15] Y. A. Rulev, Z. Gugkaeva, V. I. Maleev, M. North, Y. N. Belokon, Beilstein J. Org.
Chem. 11 (2015) 1614–1623.
16] M. N. Dehkordi, A.-K. Bordbar, P. Lincoln, V. Mirkhani, Spectrochim. Acta A 90
(
2012) 50–54.
17] I. Correia, J. C. Pessoa, M. T. Duarte, M. F. M. da Piedade, T. Jakusch, T. Kiss, M. M.
C. A. Castro, C. F. G. C. Geraldes, F. Avecilla, Eur. J. Inorg. Chem. (2005) 732–744.
18] I. Correia, Á. Dörnyei, T. Jakusch, F. Avecilla, T. Kiss, J. C. Pessoa, Eur. J. Inorg.
Chem. (2006) 2819–2830.
[
[
19] J. C. Pessoa, I. Correia, Coord. Chem. Rev. 388 (2019) 227–247.
20] K. Voronova, M. Purgel, A. Udvardy, A. C. Bényei, Á. Kathó, F. Joó,
Organometallics 32 (2013) 4391–4401.
[
21] K. Voronova, L. Homolya, A. Udvardy, A. C. Bényei, F. Joó, ChemSusChem 7 (2014)
2
230–2239.

Journal 1P9 re-proof
[
22] F. Joó, J. Kovács, A. C. Bényei, Á. Kathó, Angew. Chem. Int. Ed. 37 (1998) 969–970;
Angew. Chem. 110 (1998) 1024-1026.
[
[
[
[
[
23] H. Diehl, C. C. Hach, Inorg. Synth. 3 (1950) 196–201.
24] G. Gran, Analyst 77 (1952) 661–671.
25] H. M. Irving, M. G. Miles, L. D. Pettit, Anal. Chim. Acta 38 (1967) 475–488.
26] P. Gans, A. Sabatini, A. Vacca, J. Chem. Soc. Dalton (1985) 1195–1200.
27] L Zékány, I. Nagypál in: D. Leggett (Ed.), Computational Methods for the
Determination of Formation Constants, Plenum Press, New York, 1985, pp. 291–353.
28] A. Jancsó, Z. Paksi, S. Mikkola, A. Rockenbauer, T. Gajda, J. Inorg. Biochem. 99
[
[
(
2005) 1480–1489.
29] N. Lihi, D. Sanna, I. Bányai, K. Várnagy, I. Sóvagó, New J. Chem. 41 (2017) 1372–
379.
1
[
[
30] L. I. Elding, Inorg. Chim. Acta 7 (1973) 581–588.
31] L. Cattalini, A. Cassol, G. Marangoni, G. Rizzardi, E. Rotondo, Inorg. Chim. Acta, 3
(
1969) 681–684.
[
32] G. Sciortino, N. Lihi, T. Czine, J.-D. Maréchal, A. Lledós, E. Garribba, Int. J. Quant.
Chem. 118 (2018) e25655.
[
[
[
33] R. Uma, C. Crévisy, R. Grée, Chem. Rev. 103 (2003) 27–51.
34] V. Cadierno, P. Crochet, J. Gimeno, Synlett (2008) 1105–1124.
35] V. Cadierno, P. Crochet, J. Francos, S. E. García-Garrido, J. Gimeno, N. Nebra,
Green. Chem. 11 (2009) 1992–2000.
[
36] F. Scalambra, P. Lorenzo-Luis, I. de los Rios, A. Romerosa, Coord. Chem. Rev. 313
(
2019) 118–148.
[
[
37] P. Lorenzo-Luis, A. Romerosa, M. Serrano-Ruiz, ACS Catalysis 2 (2012) 1079–1086.
38] F. Scalambra, M. Serrano-Ruiz, A. Romerosa, Dalton Trans. 46 (2017) 5864–5871.

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[
[
[
39] J. Francos, S. E. García-Garrido, J. García-Álvarez, P. Crochet, J. Gimeno, V.
Cadierno, Inorg. Chim. Acta 455 (2017) 398–414.
40] H. Bricout, E. Monflier, J.-F. Carpentier, A. Mortreux, Eur. J. Inorg. Chem. (1998)
1
739–1744.
41] R. Gombos, B. Nagyházi, F. Joó, React. Kinet. Mech. Cat. 126 (2018) 439–451.

Journal 2P1 re-proof
Figure 1. Sulfonated tetrahydrosalen (sulfosalan) ligands used in this study.
1
3
Figure 2. Selected part of the C NMR spectra of HSS as a function of pH. cHSS = 10 mM.
Figure 3. The distribution of the complexes formed in the Pd(II):HSS 1:1 system as a
function of pH, c
Figure 4. Theoretical distribution curves calculated in the Pd(II):HSS:PrHSS:BuHSS 1:1:1:1
system at c = 2.0 mM.
L
= 2.0 mM.
L
Figure 5. Distribution of the complexes formed in the Ni(II):HSS 1:1 system and the
absorbance values obtained by UV-Vis spectroscopy at 366 nm as a function of pH. c
L
= 2.0
mM
Figure 6. Selected UV-Vis spectra in the Ni(II):HSS system at 1:1 metal to ligand ratio as a
function of pH. c = 2.0 mM
L
Figure 7. Time course of the hydrogenation and redox isomerization of oct-1-en-3-ol
catalyzed by [Ni(HSS)] (▲– oct-1-en-3-ol, ■ – octan-3-ol, ● – octan-3-one). Conditions:
-5
-4
n([Ni(HSS)]) = 1.07×10 mol, n(oct-1-en-3-ol) = 2.5×10 mol, 3mL water, pH = 7.04,
P(H ) = 5 bar, T = 80 °C.
2
Figure 8. The effect of hydrogen pressure on the hydrogenation and redox isomerization of
oct-1-en-3-ol catalyzed by [Ni(HSS)] (▲– oct-1-en-3-ol, ■ – octan-3-ol, ● – octan-3-one).
-5
-4
Conditions: n([Ni(HSS)]) = 1.07×10 mol, n(oct-1-en-3-ol) = 2.5×10 mol, 3 mL water, pH
=
7.04, t = 3 h, T = 80 °C.
Figure 9. Conversion of oct-1-en-3-ol (♦) and yields of octan-3-ol (■) and octan-3-one (●) in
hydrogenation and redox isomerization of oct-1-en-3-ol catalyzed by [Pd(HSS)] as a function
-7
-4
of pH. Conditions: n([Pd(HSS)]) = 2.5×10 mol, n(oct-1-en-3-ol) = 2.5×10 mol, 3 mL
acetate buffer, I = 0.2 M KCl, P(H ) = 1 bar, t = 30 min, T = 40 °C.
Scheme 1. Hydrogenation and redox isomerization of oct-1-en-3-ol catalyzed by [Ni(HSS)].
2

Journal 2P2 re-proof
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.

Journal 2P3 re-proof
Graphical abstract
Formation constants of nickel(II) and palladium(II) complexes of sulfonated salan ligands were
determined by pH-potentiometric titrations. In addition to hydrogenation of unsaturated lipids in
model and biomembranes, these water-soluble, selective catalysts proved outstandingly effective for
the hydrogenation and redox isomerization of oct-1-en-3-ol, too.

Journal 2P4 re-proof
HIGHLIGHTS



Organometallic catalysis in aqueous media to study biological processes
Sulfonated salan-type ligands are excellent water-soluble metal chelators
Homogeneous catalysis in aqueous media with Pd(II)- and Ni(II)-sulfosalan
complexes

High activity and selectivity in hydrogenation and redox isomerization

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5