Palladium (II)–Salan Complexes as Catalysts for Suzuki–Miyaura C–C Cross-Coupling in Water and Air. Effect of the Various Bridging Units within the Diamine Moieties on the Catalytic Performance

Water-soluble salan ligands were synthesized by hydrogenation and subsequent sulfonation of salens (N,N’-bis(slicylidene)ethylenediamine and analogues) with various bridging units (linkers) connecting the nitrogen atoms. Pd (II) complexes were obtained in reactions of sulfosalans and [PdCl4]2−. Characterization of the ligands and complexes included extensive X-ray diffraction studies, too. The Pd (II) complexes proved highly active catalysts of the Suzuki–Miyaura reaction of aryl halides and arylboronic acid derivatives at 80 °C in water and air. A comparative study of the Pd (II)–sulfosalan catalysts showed that the catalytic activity largely increased with increasing linker length and with increasing steric congestion around the N donor atoms of the ligands; the highest specific activity was 40,000 (mol substrate) (mol catalyst × h)−1. The substrate scope was explored with the use of the two most active catalysts, containing 1,4-butylene and 1,2-diphenylethylene linkers, respectively.


Catalysis experiments and gas chromatographic analysis of the reaction mixtures
At the end of the reactions, the mixtures were allowed to cool to room temperature and then were extracted by chlorofom (2 mL). After separation of the phases (15-20 min) the organic phase was removed by a Pasteur pipette and filtered through a short MgSO4 plug.
Gas chromatographic determinations were carried out with the use of an Agilent Technologies 7890A type chromatograph, equipped with a flame ionization detector (FID) and an autosampler. HP-5 (30 m × 0.32 mm × 0.25 µm) and OPTIMA (30 m × 0.32 mm × 1.25 µm) capillary columns were used with the following temperature program: 130 °C for 5 min, ramp to 250 °C (60 °C/min), hold at this temperature for 6 min. All components were separated on the baseline. Samples of 1 μl were injected, and the detector was set to 300 °C. Gases, such as N2 (carrier) and H2 (for FID) were supplied by gas generators.
Products were identified by their retention times (based upon calibration with the corresponding standards). Product distribution was calculated from the integrated areas of the chromatographic peaks. Calibration of the detector sensitivity for all compounds was carried out in the full concentration range which allowed the measurements to be run without an internal standard. Conversions were calculated for the aryl-halide reactants.

Experimental details for molecular structure determinations of sulfonated salan ligands and Pd(II)-complexes by SC-XRD
Crystal data, and details of data collection and structure refinement are summarized in Table   S8. Crystals were mounted on MITEGEN loops, and diffraction intensity data were measured on a Bruker Venture D8 diffractometer (INCOATEC IµS 3.0 dual CuKα and MoKα sealed tube microsources, Photon II Charge-Integrating Pixel Array detector). The data sets were collected and integrated using the APEX3 software package [S1]. Multi-scan absorption corrections were performed using SADABS. The molecular structures were solved with the use of dual methods (SHELXT) [S2] and refined on F 2 using the SHELXL program [S3] incorporated into the Olex 2 Crystallographic Software & Services [S4] and WinGX suite [S5]. All non-hydrogen atoms were refined anisotropically. The crystal 1×2H2O was refined as a 2-component twin with a BASF factor of 0.589(5). C8 atom is disordered (59%, 41%) and the value of Z' is 0.5, only half of the formula unit is present in the asymmetric unit, with the other half consisting of symmetry equivalent atoms [Symmetry code: -x,1-y,-z]. In 3 and 4, Z'=0.5 and symmetry operators are -x,1-y,-z and 1-x,1-y,1-z, to complete the molecule.
The hydrogen atoms of the zwitterionic compounds were placed at idealized positions and refined using a riding model. The positions of hydrogen atoms of H2O were determined based on the electron density distribution. In compounds 2, 5b, 5cb, the protons of H2O molecules could not be located from the electron density map; here we present the best refinement result.
Crystals of 5ca were very small and low diffracting. 8 was also measured by Rigaku RAXIS-RAPID II diffractometer using a Mo-Kα source for crystals.
In 7 and 8, RIGU restraints were used, all crystal were twins, of poor quality, and all refinements were uncertain. Despite that, the connectivities in both Pd(II)-sulfosalan complexes were clearly defined; however the inorganic polymers were disordered and did not allow better refinement results.
Data were analyzed by using PLATON [S6] and figures and CIFs for the paper were prepared using the Mercury CSD 4.3.0 software [S7] and pubCIF [S8].
Data sets with structural factors were deposited in the Cambridge Crystallographic Data Centre (CCDC) numbers of 2020275-2020282 and 2020437.

Crystallographic characterization of Sulfonated salan ligands 1-5
Crystals of 1×2H2O were obtained from water and belong to the monoclinic P21/c space group.
Another difference in the structures of these solvomorphs is in that the aromatic rings are at 34.95° angle to each other (see the superposition of the two molecules in Figure S4). The powder diffraction pattern calculated from the cell parameters of the crystals obtained from water and the one measured experimentally on the powdery product yielded by the synthesis, are identical ( Figure S5), meaning that the microcrystalline product also contains two molecules of water for one HSS molecule.  (2) were also obtained from water at room temperature. The compound crystallizes in the orthorhombic Fdd2 space group, and, in addition to the zwitterionic ligand, the asymmetric unit contains five full water molecules and another one with half crystallographic occupancy ( Figure S6). Figure S6. Capped sticks representation of 2×5.5H2O, lattice water molecules are omitted for clarity (upper).ORTEP diagram of the asymmetric unit of 2×5.5 H2O showing the atom labelling scheme (lower).
This compound crystallized in the monoclinic C2/c space group, and the asymmetric unit contained the zwitterionic sulfosalan and six water molecules ( Figure S24). Crystals of 5ca synthesized with enantiomerically pure cis-1,2-diaminocyclohexane could also be obtained from water ( Figure S22). In this case, the crystals belong to the monoclinic P21/c space group and the asymmetric unit contained only two water molecules in addition to the zwitterionic sulfosalan ( Figure S25). Overlay of the structures of 5ca and 5cb ( Figure S26) shows, that the cyclohexyl rings precisely overlap and only the position of the aromatic rings are different. This may be due to the different number of water molecules in the asymmetric unit which allows different degree of rotation around the flexible bonds.   Table 7. (7)  Diffraction measurements were made on several crystals of both complexes at 150 K and at room temperature. Since the crystals were twinned and the polymer chains flexible, despite all our efforts, all R values were higher than 10 % together with wR2-s >25 %. Due to these errors, the bond lengths and angles determined for the complexes are not suitable for discussion.