Synthesis, crystal structure and catalytic activity in reductive amination of dichlorido(η6-p-cymene)(2′-dicyclohexylphosphanyl-2,6-dimethoxybiphenyl-κP)ruthenium(II)

The synthesis, crystal structure and catalytic activity in reductive amination reactions of a new ruthenium complex are described.


Chemical context
The design of new organometallic complexes is important for the development of new catalytic processes as well as for understanding those already known. Recently, a new methodology for reductive amination in the presence of carbon monoxide as the reducing agent, catalysed by rhodium (Chusov & List, 2014;Afanasyev et al., 2016;Yagafarov et al., 2015), iridium Molotkov et al., 2017) and ruthenium (Kolesnikov et al., 2015;Afanasyev et al., 2017) has been described. This protocol is based on the deoxygenation potential of CO and does not require an external hydrogen source. This methodology is therefore potentially more selective for those substrates bearing groups that are sensitive to hydrogenation. As a result of the high cost of rhodium and iridium, the development of new catalytic systems based on more abundant metals is important. It has previously been shown that addition of phosphines to ruthenium systems, which were supposed to stabilize catalytic species, dramatically decreases the activity of the catalytic system. To further understand this process and the role of phosphines, the title complex, I, was synthesized and its crystal structure and catalytic properties are reported herein. ISSN 2056-9890 Such 6 -arene Ru II complexes with piano-stool coordination are known to be active catalysts in different processes (Therrien, 2009), including hydrogenation (Moldes et al., 1998), hydroboration (Kaithal et al., 2016), transfer hydrogenation (Aznar et al., 2013;Ceró n-Camacho et al., 2006;Clavero et al., 2016) and isomerization of allylic alcohols (Díaz-Á lvarez et al., 2006;Baraut et al., 2015). Moreover, such complexes have shown promising medicinal properties (Nazarov et al., 2014), including anticancer activity (Chuklin et al., 2017).

Structural commentary
The title compound, I, crystallizes in the monoclinic space group P2 1 /c with two crystallographically independent molecules (A and B, comprising Ru1 and Ru2, respectively) in the asymmetric unit (Fig. 1). The geometries of both molecules are very similar, as illustrated in Fig. 2, showing the molecular overlap of the inverted molecule B on molecule A [r.m.s. deviation of 0.227 Å ; Mercury (Macrae et al., 2008)]. They are distinguished only by the twist angles of the two benzene rings in the phosphine substituents [89.54 (14) for A and 78.36 (14) for B].

Figure 1
A view of the molecular structure of compound I, with atom labelling. Displacement ellipsoids are shown at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Supramolecular features
In the crystal of I, molecules are linked by a C-HÁ Á ÁCl hydrogen bond and C-HÁ Á Á interactions forming -A-B-A-B-chains propagating along [100]; details are shown in Fig. 3 and Table 1. The overall packing in the crystal structure of I is illustrated in Fig. 4. There are no other significant intermolecular interactions present in the crystal structure.

Catalytic activity
The catalytic activity was investigated in a model reductive amination reaction between p-tolualdehyde and p-anisidine in conditions similar to those reported previously for ruthenium systems (Fig. 5). We were delighted to find out that complex I was active and furnished the desired amine in 61% yield. The catalytic activity of this complex can be explained by the lability of the p-cymene ligand, which can be replaced by twoelectron ligands such as CO or amine. The role of the phos-phine ligand is in the stabilization of catalytically active species [RuCl 2 SPhosL x ]. Interestingly, the dimeric precursor of I -[Ru(p-cymene)Cl] 2 Cl 2 -was two times less active (the amine yield is 34%), which can be explained by dissociation of the p-cymene ligands followed by aggregation of non-stabilized RhCl species. In summary, complex I is an active catalyst for reductive amination, and further tuning of phosphine ligands may result in even more active complexes.

Procedure for reductive amination
A glass vial in a 10 ml stainless steel autoclave was charged with 0.5 mol% of the catalyst, CH 3 CN, 1.2 equiv. of the p-anisidine and 1 equiv. of the p-tolualdehyde (the use of a glass vial is crucial: interaction of the catalyst with the metal surface inside the autoclave can lead to decreased catalytic activity). The autoclave was sealed, flushed three times with 5 bar of carbon monoxide (CO), and then charged with 50 bar of CO. The reactor was placed in an oil bath preheated to 413 K. After the indicated time, the reactor was cooled to room temperature and depressurized. The residue was purified by flash chromatography on silica gel using dichloromethane as eluent. A view of the C-HÁ Á ÁCl hydrogen bonds (dashed lines) and the C-HÁ Á Á interactions (blue arrows) leading to the formation of chains along [100]; see Table 1 for details. Only the H atoms involved in these interactions are shown, and the centroid in molecule A is red, while the centroid in molecule B is blue.

Figure 4
A view along the b axis of the crystal packing of compound I. The intermolecular interactions are shown as dashed lines (see Table 1), and only those H atoms involved in these interactions have been included.

Synthesis and crystallization
To a dichloromethane (7 ml) solution of [(p-cymene)RuCl 2 ] 2 (0.050 g, 0.082 mmol) was added SPhos (69 mg, 0.168 mmol). The dark-orange solution was stirred at room temperature for 24 h. The mixture was partially evaporated under reduced pressure, and the complex precipitated with diethyl ether (10 ml) to give a dark-orange solid (37 mg, 66%). Darkorange prismatic crystals were obtained by slow diffusion of pentane into the dichloromethane solution of complex I.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were placed in calculated positions and refined using a riding model: C-H = 0.95-1.00 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.
The X-ray diffraction study was carried out on the 'Belok' beamline of the National Research Center Kurchatov Institute (Moscow, Russian Federation) using a Rayonix SX165 CCD detector.
A rather large number of reflections (ca 100) were omitted in the final cycles of refinement for the following reasons: (1) In order to achieve better I/ statistics for high-angle reflections we selected exposure times to allow a small fraction of intensity overloads in the low-angle part of the detector. These low-angle reflections with imprecisely measured intensities were excluded from the final cycles of refinement.
2) In the present setup of the synchrotron diffractometer, the low-temperature device eclipses a small region of the image-plate detector near the high-angle limit. This small shadowed region was not masked during integration of the diffraction frames, which erroneously resulted in zero intensity of some reflections.
3) The quality of the single crystal chosen for the diffraction experiment was not perfect. Some systematic intensity distortions may be due to extinction and defects present in the crystal.

Funding information
This work was supported in part by the RUDN University Program (No. 5-100). Synchrotron radiation-based singlecrystal X-ray diffraction measurements were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007). A model reductive amination reaction between p-tolualdehyde and panisidine catalyzed by complex I.  SHELXL2014 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL (Sheldrick, 2015b), PLATON (Spek, 2009) and publCIF (Westrip, 2010).