Ruthenium(III) Based diimine Complexes; Synthesis, Characterization, PXRD study and Catalytic Hydrogenation of Cyclohexene

This study deals with preparation and characterization of a group of Ru III -chelates contains tetradentate diimine ligands. These quadridentate ligands are derived from 2-OH-1-naphthaldehyde and a number of aliphatic diamines where the number of methylene groups between the two azomethine nitrogen donors varied from two to six are the components of quadridentate ligands. The pure isolated compounds were subjected to several physicochemical investigations to assign their structures. Spectral and magnetic measurements suggested a distorted octahedral arrangement of the six coordinate diimine ruthenium(III) complexes. The structural optimization for one of the current Ru III complexes was determined based on the processing of powder X-ray diffraction (PXRD) data by the computer program Expo 2014 PXRD. As well DFT calculations were applied to optimize the geometry in the case of complexes 1 . The newly synthesized ruthenium(III) diimines were tested as catalysts for hydrogenation of cyclohexene. The effect of the catalyst structure and the type of catalysis as well as the nature and amount of the solvent used on the catalytic performance of the current catalysts were studied. Catalytic experiments reported that the ongoing ruthenium(III) complexes are promising precatalysts that have successfully catalyzed hydrogenation of cyclohexene by hydrogen gas under moderate process conditions. The results obtained allowed to establish a mechanism for the studied catalytic hydrogenation reactions.


Introduction
Catalytic hydrogenation of olefins by molecular hydrogen plays a central role in many industries, such as pharmaceuticals, petrochemical, food, specialty chemicals, commodity chemicals, and agrochemicals [1a-e]. The literature includes numerous studies related to the catalytic hydrogenation of simple olefins by means of transition metal complexes [2,4]. The metal complexes showed a distinction in catalytic performance over the metal itself in the hydrogenation processes in terms of higher activity and work under milder conditions [2]. In this respect, a wide range of metal complexes containing d 8 -metals are widely used to catalyze the hydrogenation of the unsaturated hydrocarbons by H2 [3]. In the same context, 1961 saw the first use of ruthenium complexes in homogeneous catalysis processes to activate H2 [3f].
Catalytic processes in homogeneous systems are simpler from the chemical and kinetic point of view than the heterogeneous catalytic systems. Ruthenium complex, [RuCl2(PPh3)3], is most commonly used for H2 activation processes by the heterolytic splitting mechanism. The heterolytic splitting of molecular hydrogen to produce a metal monohydride and proton is well known for many catalytic hydrogenation processes of unsaturated hydrocarbons, especially in polar solvents [5]. Presently, many ruthenium complexes with distinct catalytic activity are present in hydrogenation processes for unsaturated bonds of olefins [6]. In particular, monohydride ruthenium-complexes were reported as catalysts with high activity and selectivity towards terminal alkenes [7].
The metal complexes of Schiff's bases play major roles in the progression of metal complexes chemistry. Among these metal complexes worth mentioning in this regard and which has been used as a catalyst in hydrogenation of simple alkene by H2 is the palladium(II) complex containing N2O2 donor sites of the tetradentate Schiff base ligand (salen) [8]. Within our knowledge the use of ruthenium(III) Schiff base complexes as catalysts for hydrogenation of simple alkenes by H2 is not yet known. Only one investigation could be found reporting the reductive carbonylation of nitrobenzene to phenyl urethane catalyzed by ruthenium(III) Schiff bases of the general formula [Ru III LCl] or [Ru III LCl2], where L is Schiff base with N2O2, N4 and NS donor groups [9].
Among the complexes tested (Ru-SolphCl2) showed the highest catalytic activity but kinetic studies and mechanism elucidations were not reported.
The aim of the current study is to synthesis and characterize a new series of ruthenium(III) complexes with tetradentate Schiff bases containing N2O2 donors for catalytic hydrogenation of the simple olefin, cyclohexene, by H2.

Chemicals and Materials:
All the materials used in the present study are of a high degree of purity as they were purchased from reliable sources. The current diimine ligands were prepared according to the method reported elsewhere [10].

Preparation of diimine ruthenium(III) complexes 1 -4
An ethanol solution (50 mL) containing 0.2 mol of ligand diamine was heated for 30 minutes and followed by adding an equivalent amount of ruthenium salt (RuCl3 3H2O) dissolved in 50 ml of absolute ethanol. After that, the precipitate formed was filtered, washed with alcohol, ether, and then placed in a desiccator over P2O5 for a week. In most cases addition of ether was desirable to complete precipitation of the metal chelate.
Significant purity of metal chelates was achieved through further washing with the Soxhlet process with ethanol as the solvent. The composition of the pure isolated ruthenium(III) complexes was primarily established by elemental analysis which is recorded in Table 1.

2.3.
Physical measurements, the catalytic hydrogenation process that includes the hydrogenation apparatus, experiments, the analytical process and computational details are given in the Supplementary Information S1.

Characterization of Ru III -based diimines
The present study aims to prepare and characterize a group of Ru IIIbased complexes containing diimine ligands derived from 2-OH-1-naphthaldehyde and a number of aliphatic diamines with carbon chain in which the number of carbon atoms ranges from two to six. The reaction of the hydrated ruthenium trichloride salt (RuCl3 3H2O) in ethanolic solution with these Schiff bases gave a family of ruthenium (III) chelates which was subjected to several physicochemical investigations to assign their structures. In this respect, the results of elemental analysis in Table 1 demonstrated that the molar ratio of the newly prepared ruthenium(III) diimine complexes is 1:1. The molar conductance measurements in DMF at room temperature as shown in Table 1 indicate their nonelectrolytic behavior [12]. The full structural characterization of these ruthenium(III) Schiff base complexes was completed via comprehensive spectroscopic studies. The analytical data and the incoming spectral results demonstrate that the present Schiff bases behave as quadridentate dibasic ligands providing the chromophore N2O2 to coordinate with ruthenium(III) ion. Determination of the thermal stability of metal complexes is also an important factor, particularly when these metal complexes participate in catalytic applications under variable thermal conditions. Therefore, thermal analysis (TGA and DTA) was performed for the ruthenium(III) complexes under study. All thermal measurements were performed in an inert atmosphere of N2 at a temperature range of 50 to 1000 °C that begins with room temperature and ends when the weight loss constant has been reached as shown in the supplementary materials S2-S5. The corresponding thermal data such as the mass loss, temperature ranges and assigning the chemical compositions of both the thermally lost portion and the residual, as well as differential thermal features during the successive pyrolysis stages are given in Table 2.
As is evident from the thermograms curves of the existing ruthenium ( The estimated metal content values from the remaining metal oxide correspond to those estimated by the analytical methods, confirming the validity of the proposed molecular formulae for these ruthenium(III) diimine complexes.
Differential thermal analysis (DTA) curves show that each stage of weight loss is accompanied by an exothermic peak at a definite temperature value that given in Table 2.
In this respect, except in the case of complex 4, no endothermic peaks were observed, indicating that the current ruthenium(III) complexes are not subject to melting or suffer from any changes in the network prior to decomposition. This finding is in an agreement with the fact that the existing ruthenium(III) chelates decompose without melting.

Infra red spectra
To illustrate the bonding pattern of the present Schiff bases with the ruthenium (III) ion, the infrared spectra were measured as KBr disks of both the Schiff base bonds and their corresponding metal chelates. Based on the data obtained from the charts in S6-S13 the spectral assignments of the diimines along with the ruthenium (III) chelate were recorded in Table 3. Observed band shifts can be correlated to changes in the ligand system by coordination and thus give information about the bonding and arrangement in the metal complexes. The spectral features shown in S6-S13 and the frequency data in Table 3 demonstrate that the coordination pattern of Schiff bases with ruthenium(III) center is fairly identical for all metal chelates. The spectra of the complexes reveal the disappearance of the characteristic OH-bands of the ligands due to the destruction of the intermolecular hydrogen bond as a result of the coordination of phenolic oxygen to ruthenium(III) ion [13].
The evanescence of the distinctive peaks to OH group from the spectra of the ruthenium(III) chelates is consistent with those observed for the palladium(II) and nickel(II) complexes with the same current diimine ligands [2f, 4]. The remarkable shift for the υ(C-O) band at 1415 -1450 cm -1 to higher wavenumbers at 1450 -1500 cm -1 in the spectra of the complexes indicates the bonding of the phenolic oxygen to ruthenium(III) center [14]. In the same regard, the characteristic band of the Schiff base linkage is shifted to higher wavenumbers (1620 -1680 cm -1 ) as shown in table 3 due to bonding of the imine nitrogen to Ru III -center [14]. The bonding of ruthenium(III) ion to the current ligands through the nitrogen and oxygen is supported by the emergence of new peaks at 490-570 and 380-450 cm -1 attributable to the υ(M-N) and υ (M-O) respectively [14].
The results of elemental and thermogravimetric analysis showed, that the present Schiff base ligands behave as a dibasic acid which is then bound with ruthenium(III) ion as dianionic tetradentate ligands [13,15]. Since ruthenium(III) ion attains the coordination number six, the remaining coordination sites would be occupied by water molecule and the chloride ion. The broad band appearing at 3400 -3450 cm -1 range (Table 3) is an indication of the existence of H2O in the metal chelate molecule. The present TG measurements indicated that the present ruthenium(III) complexes have constant weight till 150 °C at which they begin to decompose. This fact excluded the surface nature of the water content and supports that the water molecule is coordinated to ruthenium(III) ion.
This fact rule out the superficial nature of the water content and supports the coordination bonding of H2O to the ruthenium (III) ion. Further emphasis on the coordination bonding of the water content of the current ruthenium(III) complexes comes from bending vibration patterns observed at 900 -930, 790 -800 and 660 -675 cm -1 which are characteristic of wagging, twisting and rocking absorptions [15]. Participation of chloro ligand in the coordination chromophore around ruthenium(III) ion is inferred from the new band appeared in the spectra of the metal complexes at wavenumber range of 300 -380 cm -1 assignable to υ(M-Cl) [14].
The composition of the current ruthenium(III) imine complexes appears to depend mainly on the number of carbon atoms of the alkyl bridge between the two azomethine groups of the diimine molecule. For L 1 its ruthenium(III) complexes is a monomeric in nature as evidenced from its magnetic moment value because the resulting chelate ring is five membered as illustrated in Scheme I. Concerning ruthenium(III) complexes of L 2 , L 3 and L 4 that contain more than three methylene groups in the bridge between two Schiff base linkages a dimeric or polymeric structure is formed (scheme I) [13]. This is to be expected because the resulting chelating ring will be somewhat unstable because it is larger than six membered and therefore the N2O2 donors cannot be given by a single diimine molecule.
Based on analytical data, measurements of both TG and molar conductance in addition to spectral investigations, the existing ruthenium(III) chelates can be formulated as shown in For n = 4, 5 and 6 the resulting ruthenium(III) complexes 2, 3 and 4 are dimeric or polymeric Scheme I: Structures of ruthenium(III)-based diimine complexes Table 3: FTIR spectra (cm -1 ) of diimines and Ru III -based chelates 1-4  Table 1; **the characteristic stretching of the coordinated water υ(H2O)

Electronic absorption spectra
In the absence of suitable crystals for the structural analysis of metal complexes, spectroscopic techniques and magnetic investigations are an alternative to determine the geometry of metal complex. Accordingly, the UV-Vis spectra of the current ruthenium(III) chelates were recorded as KBr discs. The spectra recorded for all complexes are approximately the same, indicating that their stereochemistry is identical and presented in S14 while the relevant energy values are explained and listed in Table 4. Table 4 displays that the strong bands with high frequency values are assignable to charge transfer of the type π → t2g (π*) transition where π is the HOMO orbital of the donor atoms and π* is the LUMO orbital, namely the incomplete orbital of the metal ion, t2g [16].
In the absence of any crystal field, metal complexes with d 5configuration have 6 S ground state term symbol. For the hexa-coordinated d 5 -metal complexes the ground state becomes 6 A1g and 2 T2g in both the weak and strong ligand field respectively.
Ruthenium(III) ion belongs to d 5 system where 2 T2g is the ground state for the hexacoordinated d 5 -metal complexes in the low-spin state of the octahedral symmetry. The corresponding electronic configuration is t 5 2g and the first excited doublet levels in the power-up arrangement are 2 A2g and 2 T2g respectively [17]. In the d 5 system and especially for ruthenium(III) ion in an octahedral geometry the likely spin-allowed ligand field transitions are: 2 T2g → 4 T1g, 2 T2g → 4 T2g and 2 T2g → 2 A2g, corresponding to υ1, υ2 and υ3, respectively [28]. The spectra of the current ruthenium(III) chelates (S14) exhibit three bands related to the expected d-d spin allowed transitions of the six coordinate d 5 -system in an octahedral stereochemistry of low spin ruthenium(III) complexes [17]. These bands appear in the low frequency zones at wavenumber values of 13333 -13513, 14492 -14705 and 18691 -19047 cm -1 which are assigned to υ1, υ2 and υ3, respectively [18]. An analysis of these spectral data allows us to compute the ligand field stabilization energy 10Dq, interelectronic Racah repulsion B and Nephelauxetic ratio β. 10Dq was determined from the energy difference between the ground state 2 T2g and the excited state 2 A2g based on the relation: 2 T2g → 2 A2g = 10Dq -3F2 -20F4, with F2 = 10F4 = 1000 cm -1 [18]. In the same regard the parameters B and C were computed from the relations [19]: The data in Table 4 show that the values of B (144.87 -160.37 cm -1 ) are less than the corresponding free ion Bo value (630 cm -1 ). The overlap of the orbits of both the ligand and the metal and the penetration of the free lone pairs of ligand to the d orbits of the metal ion leads to blocking or weakening the effect of the positive charge on the metal core, which is reflected in the observed decrease in the value of the electron repulsion [17]. This is reflected in the observed decrease in the value of electron repulsion which in turn causes the electron cloud to expand in d orbitals and consequently the interelectronic repulsion decreases. It is understood that as the valence state of a metal ion increases, its volume decreases and the value of Bo increases. However, this marked decrease in B compared to Bo indicates the predominance of covalent bonding in the ruthenium(III) complex molecule and thus leads to an increase in values of the ligand field stabilization energy (10Dq). This increase in the values of 10Dq is generally related to a significant electron delocalization [20].  Magnetic susceptibility measurements at 22 ºC of the current ruthenium(III) diimines (Table 5) demonstrate the low spin state of the d 5 ruthenium(III) ion. The data in Table 5 indicate that the μeff-values of Ru III ion are related to t 5 2g configuration in the octahedron stereochemistry. For complex 1, its μeff-value is greater than 1.73 BM suggesting appreciable spin -orbit coupling arising from incomplete quenching of the orbital contribution to the magnetic moment [21]. On the other hand, for complexes 2, 3 and 4 the values of μeff are lower than the spin only value. This may be an indication of spinspin interactions between the low spin neighboring Ru III centers in the dimeric or polymeric structures. As well this observed decrease in the magnetic moments could also arise from an extensive electron delocalization or lowering in the symmetry of ligand fields [22]. In the same regard, for metal complexes of 4d and 5d metals their room temperature magnetic moments are often found below the spin -only values and this behavior could be ascribed to high spin -orbit coupling constants. In this respect, the paramagnetism that can be expected from the unpaired electrons alone is reduced because the spin orbit coupling aligns the vectors L and S in opposite directions [23].
However, the magnetic moment values (Table 5) and the electronic absorption spectral properties of the current ruthenium(III) Schiff bases complexes are characteristic of the octahedral structure and are comparable with other ruthenium(III) complexes [24].

Structure solution by PXRD and DTF study
Due to the practical difficulty in obtaining a single crystal suitable for the present Ru III -based complexes, we could not perform a good structural analysis. However, the structural solution for metal complexes based on the processing of powder X-ray diffraction (PXRD) data by a structure solution computer program such as Expo 2014 is now common and accepted as an alternative of single crystal structural analysis technique [4]. In this context, the Rietveld Refinement technique was applied to achieve precise compatibility between the experimental results and computer processing of the X-ray pattern data of ruthenium(III) based complex 1 as shown in Figure 2; the corresponding PXRD pattern is shown in Figure 1.
The crystallographic data indicate triclinic system of the microcrystalline powder of complex 1 based on the space group value of P -1. The related the crystal lattice are 12.044, 9.946, 7.840Å for the dimensions a, b and c respectively ( Table 6). The corresponding angles, namely, α, β and γ have the values of 108.015°, 101.354° and 87.001° respectively. In the same regard, the cell volume (Å 3 ), volume per atom (Å 3 ) and the calculated density (g/cm 3 ) are 875.596, 14.123 and 1.976 respectively. Figure 3 shows the packing diagram of the unit cell incorporating the number of two molecules of the metallic complex.    Looking at the optimized octahedron geometry in the numbering adopted scheme  (Table 7). In the same context, the bonds distance between Ru(47) and the two axial sites O(49) and Cl (48)   The data in Table 7 indicate that the values of bond angles around the Ru(47) center almost of ≈ 90° characteristic to the hybridization of d 2 sp 3 of the octahedral stereochemistry. For the hexa coordinated metal complexes with the octahedral polyhedron, a question arises about the ideality of this geometry, is it an octahedron or a trigonal prism? Determination the geometrical index "τ6" gives answer for this question based on the relation τ6 = θ/60; where θ is the twist angle between the opposing trigonal faces in the octahedron. In this respect if τ6 equals one this means that the respective geometry is an ideal octahedron while for τ6 equals zero the corresponding structure is a perfect trigonal prism [25]. The calculated value of τ6 is 0.9 and approaches one which indicates that the geometry of the current ruthenium(III) complex is almost a perfect octahedron.   The DFT method comes to provide information about the geometry and hence these results were evaluated by comparison with their peers of experimental crystalline form.
The wB97XD/def2-SVPP model chemistry [26] was used for geometry optimizations in the gas phase as implemented in Gaussian 16 suite [27]. The optimized geometrical structures of the studied ruthenium complex, [RuL 1 Cl(H2O)] is reported in Figures 4 and   5. Interestingly, previous X-ray crystallography studies reported that similar Schiff base metal complexes possess trans and cis isomers, but the trans isomer is more common [28]. Accordingly, we calculated both isomers of [RuL 1 Cl(H2O)], and in agreement to the above mentioned references, it was found that the trans isomer is more stable than the cis one by 8 kcal/mol. In contrast, the both ruthenium(III) centers of the dimer prefer to adopt cis mode than the trans one by 6 kcal/mol. bond angles are within the ranges found for other related ruthenium(III) complexes whose exact structures have been determined by X-ray single crystal studies [27].
It should be noted here that the results obtained based on DFT calculations agree well with the results of X-ray powder diffraction data processing by Expo 2014, confirming the accuracy of the final structure of the ruthenium(III) complex under study.

Catalytic hydrogenation of cyclohexene
The To confirm the catalytic potential of the ruthenium(III) complexes under study, blank experiments were carried out in the absence of the catalyst. The results obtained showed that cyclohexane was not formed in these blank experiments, which confirms the catalytic potency of the ruthenium(III) complexes tested. The catalytic potential of studied ruthenium(III) complexes is evaluated using the following relationship: Yield percentage = [product / (reactants + product)] × 100 Influence of catalyst structure, type of catalysis, nature and quantity of co-solvents and solvents were taken into consideration during the evaluation of the catalytic activity.
Early studies reported that the catalytic hydrogenation of alkenes by molecular hydrogen in the presence of metal complexes is strongly influenced by the type and quantity of the solvent and the associated solvent [2e,f,g,h,4]. In this context, DMF proved to be the most widely used solvent in these catalytic reactions and was considered an influential factor in determining the course of many catalytic hydrogenation reactions. [29]. In light of this, DMF was used as a major component in the medium of catalytic reactions during this work. The data in S15 present the solubility degrees of H2 in the different types and quantities of solvents and co-solvents employed in the present study.
Cyclohexene is the best choice for measuring the catalytic potential of simple alkenes hydrogenation reactions. This is due to the fact that the product of the hydrogenation process is a single product that is not accompanied by by-products, and this will facilitate the study of the mechanism of this reaction. On the other hand, the use of the corresponding non-cyclic alkene (1-hexene) is not appropriate since the main product of the hydrogenation process is accompanied by the formation of many by-products. This is due to the transfer of the unsaturated bond through the carbon chain of this compound and the formation of a number of structural isomers, which therefore makes it difficult to accurately suggest the mechanism of this reaction.
The results in Table 9 indicate that the present ruthenium(III) diimine complexes catalyze the hydrogenation of cyclohexane by H2 with an efficiency ranging from 6 to 80% depending on the structure of catalyst and type solvent and co-solvents used. Among the complexes tested (  [4]. The current ruthenium(III) complexes are slightly soluble in ethanol, but completely soluble in DMF. In pure EtOH the activity of the complexes for the catalytic hydrogenation of cyclohexene is reduced ( [RuCl2P(Ph)3] [30].
With DMF excluded and under the prevailing practical conditions it is difficult to verify for a specific kind of catalysis. This is due to the always observed partial insolubility of the ruthenium complexes in the other solvents used in the study but the differences in catalytic activity are slight. However, the results in Table 9 demonstrate that the current ruthenium(III) based diimines could catalyze cyclohexene hydrogenation by both the homogeneous and heterogeneous catalysis. An overview of Table 9 shows that for a given ligand, both the type and quantity of the solvent control the catalytic hydrogenation of cyclohexane by H2.

Suggested mechanism
Hydrogenation of cyclohexene by H2 could proceed via electrons flow from the HOMO of the unsaturated double bond of cyclohexene (πorbital) to the LUMO of the hydrogen molecule (σ*). Or the electrons flow can come from the HOMO of H2 (σ) to the LUMO for the unsaturated double bond of cyclohexene (π*orbital). For the first or second path to occur, there must be a complete overlap between the interacting orbits, and this will not be achieved on the ground. Accordingly, these symmetry limitations can be overcome with the use of an appropriate catalyst and this is the role of the present ruthenium (III) complexes. In this context, the catalysis of this symmetric-blocked reaction could be accomplished by activation of either H2 or cyclohexene as shown by pathways (a) and (b). To distinguish between these two possible pathways, the following practical procedure was implemented: In the absence of ruthenium(III) complex, the reaction vessel was saturated with hydrogen gas, but the reaction between hydrogen and cyclohexane did not occur. In the same regard, repeating the same experiment in the presence of the ruthenium(III) complexes in question and the absence of olefin resulted in a remarkable decrease in the hydrogen gas pressure by an amount proportional to the amount of the catalyst in the reaction vessel. In light of this practical procedure, we can assume that the first path (a) is the dominant pathway and, therefore, the catalytic course of this reaction can be visualized and discussed as follows.
Since the examined complexes [RuL n ClH2O]; n = 1-4, are hexa coordinate and thus have no free coordinated sites for binding the reactants to perform the catalytic process.
In this case, one of the axial good leaving ligands (Cl or H2O) must be dissociate to provide a vacant coordination site for binding either H2 or cyclohexene to initiate the hydrogenation process. Accordingly, [RuL n ClH2O] is a precatalyst and the actual active catalyst is generated in the reaction medium (in situ). According to the polarization theory, H2O is less polarized than chloro ligand, and therefore less bound to ruthenium(III) center. Thus the spark of this catalytic reaction is the substitution reaction between H2O and H2 to produce the intermediate [H2RuL n Cl] in a fast step. It is generally assumed that the insertion occurs by means of a synergistic reaction pathway, via a more or less polar circular transmission state involving fracture and formation of bonds simultaneously [32]. Electrophilic attack by a proton on the carbon atom bonded to the metal yields the saturated product and regenerates the catalyst in its free active form. The catalytic cycle proposed for the homogeneous hydrogenation of cyclohexene using the chosen complex, [RuL n ClH2O], may therefore be represented as shown in scheme II.

Conclusion
In this contribution, four diimine ligands were synthesized via Schiff condensation between 2-OH-1-naphthaldehyde and a number of aliphatic diamines with carbon chain ranging from two to six. The pure isolated ligands interacted with RuCl3 3H2O to afford a series of metal chelates. Several physicochemical and spectroscopic techniques were employed to characterize the structural formulae of the prepared compounds. Octahedral structure was assigned for the newly synthesized ruthenium(III) diimine based complexes.
Confirmation of the assigned structural formula of complex 1 was achieved by PXRD and DFT calculations. Catalytic hydrogenation of cyclohexene by H2 in presence of the Ru III -based complexes in question was studied. The catalytic investigations included effect of catalyst structure, type of catalysis and nature and amount of solvent and cosolvent on the yield of the catalytic hydrogenation processes. The results in Table 9 show that there is a relationship between both the type and quantity of the solvent used and the values of yield percentage of cyclohexane. Moreover, the results obtained allowed establishing that the catalytic hydrogenation reaction takes place through H2 activation pathway. As well the catalytic investigations of the current hydrogenation process allowed us to suggest a catalytic cycle.