**1. Introduction**

Salen (*N*,*N*'-*bis*(salicylaldiminato)-1,2-diaminoethane) and its derivatives, which can be easily obtained by condensation of salicylaldehyde and ethylendiamine or their various substituted analogues, have played prominent roles as ligands in coordination chemistry and catalysis throughout the years [1–5]. Salan (*N*,*N*'-*bis*(*o*-hydroxybenzyl)-1,2-diaminoethane) is the tetrahydro derivative of salen, usually obtained from the latter by reduction with NaBH4 [1,6–9]; however, direct synthesis via Mannich reaction is also known [10]. Salan has become a general name for analogous *N*,*N*'-*bis*(*o*-hydroxybenzyl)-α,ω-diaminoalkanes, too, which may have diverse linker groups between nitrogen atoms and/or variously substituted *o*-hydroxybenzyl moieties. As secondary amines, salans are much less vulnerable to hydrolysis than their diimine parent compounds, and for this reason, they are more suitable for applications in aqueous media [11,12]. Transition metal complexes of salans have earned important applications in catalysis of various reactions such as polymerization [13,14], sulfoxidation [15], oxygen transfer [9], fluorination and hydroxylation [16], to name a few. The promising biomedical and catalytic properties and applications of salan complexes have been reviewed recently [1].

Carbon–carbon cross-coupling reactions are of fundamental importance in organic synthesis as shown by the high number of publications (413 for the Suzuki–Miyaura reaction in 2019 (Scopus, Elsevier)) and can be conveniently practiced in fully organic media [17–19]. On the other hand, health and environmental safety requires the elimination of organic solvents from chemical processes as much as possible. A viable alternative to the use of organic solvents is the application of water as the reaction medium [20–22]. Organometallic catalysis in aqueous systems has great potential for green chemistry, and this approach has been extended to the field of C–C cross-couplings, too [23–29]. Not only the replacement of volatile and harmful organic solvents but also improved process characteristics (fire safety, catalyst recycling, etc.) and product quality are attractive features of aqueous procedures.

In homogeneously catalysed aqueous/organic biphasic reactions, such as the Pd-catalysed cross-coupling of aryl halides and arylboronic acids, the catalyst should be preferentially soluble in water. Hydrophilic palladacycles [30], complexes of tertiary phosphines [23,31,32], N-heterocyclic carbenes [33–35] and water-soluble complexes with salen ligands [2,36,37] have already been applied as catalysts in aqueous C–C cross-couplings. Alternatively, the reactants and the catalyst have to be incorporated into micelles formed by appropriate surfactants within the bulk aqueous phase [38–42]. Both methods allowed the design of outstandingly productive and robust catalytic procedures.

We have been interested in aqueous organometallic catalysis for several years [21] and employed as catalysts complexes of transition metals with water-soluble tertiary phosphine and/or N-heterocyclic carbene ligands. Recently, we launched a program to study in aqueous media the catalytic properties of sulfonated salan-based complexes in reactions such as hydrogenation of alkenes and ketones [43], redox isomerization of allylic alcohols [44,45] and carbon–carbon cross-coupling reactions [46]. In particular, some Pd (II)–salan complexes were found to be highly effective catalysts for the Sonogashira and the Suzuki–Miyaura cross-coupling reactions [46,47].

In contrast to what may be suggested by the simplified formulae in Scheme 1, the structure of even the simplest sulfosalan, HSS (1), deviates from planarity and the free rotation around the C–N bonds gives high flexibility to the ligands in coordination to a metal ion. This flexibility is largely influenced by the length of the bridging unit between the secondary amine nitrogens (e.g., C2 vs C4 alkyl chains). The structure, rigidity and steric requirements of the linker unit (e.g., ethyl, *cis*- or *trans*-1,2-cyclohexyl, 1,2-diphenylethyl linkers) similarly may have large effects on the coordination ability of the sulfosalan ligands, which may be manifested also in the catalytic properties of the resulting complexes. During our studies, we noted important differences in the catalytic activities of Pd (II)–sulfosalan complexes; therefore, we decided to perform a comparative study of a reasonably large series of such complexes. In this paper, we present the results of a comparative study of the catalytic performance of complexes **6**–**10** (Scheme 1) in Suzuki–Miyaura cross-coupling reactions. For the purpose of these studies, we synthesized the new ligands **4**, **5b** and **5c** and the new complexes Na2[Pd(PrHSS)] (**7**), Na2[Pd(dPhHSS)] (**9**), Na2[Pd(*trans*-CyHSS)] (**10b**) and Na2[Pd(*cis*-CyHSS)] (**10c**). To gain more insight into the structural features of the sulfosalan ligands and their Pd (II)–complexes, all sulfosalan ligands, **1**–**5,** as well as complexes **6** and **7** were studied in detail by single crystal X-ray diffraction (SC-XRD) (**1** and **3** by powder X-ray diffraction, too).

**Scheme 1.** Salan ligands (hydrogenated sulfonated salens, **1**–**5**) and their Pd (II) complexes (**6**–**10)** used in this study, together with the intermediates of their synthesis (salens **11**–**15** and hydrogenated salens **21**–**25**): ligands **1**–**5** were isolated as zwitterions, and complexes **6**–**10** were isolated as Na salts.

#### **2. Results and Discussion**

#### *2.1. Synthesis*

The new ligands, **4**, **5b** and **5c**, and the Pd (II) complexes **7**, **9**, **10b** and **10c**, were synthesized according to the procedure used by us earlier for the rest of the compounds, **1**–**3**, **6**, **8** and **10a** [44–47]. Briefly, the starting salens were obtained by condensation of salicylaldehyde and the appropriate diamine, and the latter were reduced to the hydrogenated salens with four equivalents of NaBH4 in methanol. The white hydrogenated salen products were sulfonated in an ice-cold 4:1 mixture of fuming sulfuric acid (20%) and concentrated (96%) sulfuric acid. Addition of the reaction mixtures to cold water and adjustment of the pH to 4 led to formation of white precipitates of the salan ligands (Figure 1).

**Figure 1.** Capped sticks representations of **1** × 2H2O. Symmetry code: (i) –x, 1–y, –z.

Na2[Pd(PrHSS)] (**7**), Na2[Pd(dPhHSS)] (**9**) and Na2[Pd(CyHSS)] (**10**) were prepared from equivalent amounts of the sulfosalan ligand and (NH4)2[PdCl4] in aqueous solutions adjusted to pH 7.5 with concentrated NaOH solution and kept at 60 ◦C for 10 h. The yellow complexes were precipitated from the cooled reaction mixtures with the addition of ice-cold ethanol.

All compounds showed the characteristic A1 sulfonate stretching frequency in the infrared spectrum within the 1029.0–1033.4 cm−<sup>1</sup> range and displayed the expected 1H and 13C-NMR signals, as well as the correct electrospray ionization (ESI) MS molecular ion peaks. Data are given in the Materials and Methods section, and the 1H and 13C{1H} NMR spectra are collected in the Supplementary Material.
