Supported and Non-Supported Ruthenium(II)/Phosphine/[3-(2-Aminoethyl)aminopropyl]trimethoxysilane Complexes and Their Activities in the Chemoselective Hydrogenation of trans-4-Phenyl-3-butene-2-al

Syntheses of four new ruthenium(II) complexes of the [RuCl2(P)2(N)2] type using 2-(diphenylphosphino)ethyl methyl ether (P~O) as ether-phosphine and triphenylphosphine (PPh3) as monodentate phosphine ligands in the presence of [3-(2-aminoethyl)aminopropyl]trimethoxysilane as diamine co-ligand are presented for the first time. The reactions were conducted at room temperature and under an inert atmosphere. Due to the presence of the trimethoxysilane group in the backbone of complexes 1 and 2 they were subjected to an immobilization process using the sol-gel technique in the presence of tetraethoxysilane as cross-linker. The structural behavior of the phosphine ligands in the desired complexes during synthesis were monitored by 31P{1H}-NMR. Desired complexes were deduced from elemental analyses, Infrared, FAB-MS and 1H-, 13C- and 31P-NMR spectroscopy, xerogels X1 and X2 were subjected to solid state, 13C-, 29Si- and 31P-NMR spectroscopy, Infrared and EXAF. These complexes served as hydrogenation catalysts in homogenous and heterogeneous phases, and chemoselective hydrogenation of the carbonyl function group in trans-4-phenyl-3-butene-2-al was successfully carried out under mild basic conditions.


Introduction
Reduction of aldehydes and ketones to the corresponding alcohols is a core technology in fine chemicals synthesis, particularly for pharmaceuticals, agrochemicals, flavors and fragrances, which requires a high degree of stereochemical precision [1][2][3]. Asymmetric hydrogenations of C=C, C=O, and C=N functionalities have found important applications in organic synthesis and in the fine chemical business [3][4][5][6][7][8]. A high turnover frequency (TOF) can be obtained by designing suitable molecular catalysts and reaction conditions. Preferential reduction of a C=O function over a coexisting C=C linkage is an important and difficult task. Although there are many examples of highly efficient catalysts for olefin and ketone reduction, imine hydrogenation is still a challenge in terms of both the turnover frequency and the lifespan of the active catalyst [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. One of the best transition-metal complexes for ketone hydrogenation that has been discovered is the chiral Ru(II)-diphosphine/1,2diamine complex, which was developed by Noyori [3]. This system was found to be active in the chemoselective hydrogenation of carbonyl functional groups in the presence of olefins and in the reduction of imines [5][6][7][8]. The immobilization of metal complexes enables the longterm use of expensive or toxic catalysts and provides a clean and straightforward separation of the product [26][27][28][29][30][31][32][33].
It is interesting to investigate the chemical properties and structures of new ruthenium(II) complexes containing C=C functional groups in the backbone of the phosphine ligand and indirect to the phosphorus atom [1,1-bis(diphenylphosphinomethyl)ethane, dppme] ligands to see how these properties are related to the chemical behavior of complexes by monitoring any changes by 31 P{ 1 H}-NMR spectroscopy [21].
Recently we have synthesized a number of ruthenium(II) complexes of the [RuCl 2 (P) 2 (N) 2 ] type using both mondentate or bidentate phodsphine and amine ligands, and these complexes were tested as hydrogenation catalysts for functionalized carbonyl compounds. Our ongoing research interest is in synthesizing supported and non-supported phosphine/diamine Ru(II) complexes, then examining their activity for catalytic hydrogenation in both homogenous and heterogeneous phase [9,10,17].
In this work a set of ruthenium(II)/phosphine/diamine complexes were made available by using monodentate triphenylphosphine and monodetate/bidentate ether-phosphine ligands in the presence of the [3-(2-aminoethyl)aminopropyl]trimethoxysilane as diamine co-ligand. The presence of Si(OEt) 3 anchoring groups in the backbone of the these complexes enabled the immobilization process through a simple sol-gel reaction using Si(OEt) 4 as cross-linker. The desired complexes served as catalysts for selectivity hydrogenation of trans-4-phenyl-3-butene-2-al in both homogenous and heterogeneous phases under mild conditions.

Ruthenium(II) complexes 1 and 2 synthetic investigation and structural behavior
Treating each of Cl 2 Ru(P ∩ O) 2 and Cl 2 Ru(PPh 3 ) 3 individually with an equivalent amount of [3-(2aminoethyl)aminopropyl]trimethoxysilane in dichloromethane resulted in the formation of complexes 1 and 2, respectively, as shown in Scheme 1.

Scheme 1.
The synthetic route to prepare complexes 1-2 and xerogels X1 and X2. High melting yellow powders were obtained in very good yields. These complexes are soluble in chlorinated solvents such as chloroform or dichloromethane and insoluble in polar or non-polar solvents like water, methanol, diethyl ether and n-hexane. The structures of the desired complexes have been deduced from elemental analysis, infrared spectroscopy, FAB-mass spectrometry, 1 H-, 13 C{ 1 H}− and 31 P{ 1 H}−NMR spectroscopy data.
The stepwise formation of complex 1 is monitored by 31 P{ 1 H}-NMR spectroscopy, in an NMR tube experiment, where addition of [3-(2-aminoethyl)aminopropyl]trimethoxysilane to a CDCl 3 solution containing Cl 2 Ru(P ∩ O) 2 complex as starting material leads to the disappearance of the red color of the Cl 2 Ru(P ∩ O) 2 complex and the singlet of this complex at δ p = 64.40 ppm and the appearance of a new AB 31 P{ 1 H}-NMR pattern at δ p = 38.87, 35.64 ppm due to the formation of complex 1 with a trans-Cl 2 Ru(dppme)NN formula, together with the appearance of the yellow color of the latter, confirming the hemilabile cleavage of 2Ru-O to form the 2Ru-N complex 1 in a very short time and without side products, as seen in Figure 1. trimethoxysilane co-ligand. Due to the hemilabile character the oxygen donor in the ether-phosphine ligand is regarded as an intramolecular solvent impeding decomposition of the complex by protection of vacant coordination sites, which accelerates and stabilizes the synthesis of complex 1 without any side products.

Oxidative decomposition of complexes 1 and 2 by oxygen or H 2 O 2
The desired complexes showed some sensitivity toward oxygen in solution, and the colour changed from yellow to green if oxygen allowed to enter the reaction, or if the reactions were carried out in an open atmosphere. To examine the stability of complex 1 toward oxygen, 0.10 g was dissolved in 20 mL of dichloromethane in an open atmosphere, and several samples were taken over time and subjected to 31 P{ 1 H}-NMR. The spectra showed that complex 1 (indicated by peaks at δ p = 38.9, 35.6 ppm) was decomposed to form phosphine oxide [Ph 2 P(=O)CH 2 CH 2 OCH 3 ] with δ p = 30.8 ppm and other green oily ruthenium complexes in around one hour, as shown in Figure 2. These complexes are mostly free of phosphine or paramagnetic ruthenium(III) species because nothing except the phosphine oxide were detected by 31 P{ 1 H}-NMR. Under identical conditions complex 2 displayed more stability toward oxygen compared with complex 1, 2 hours are required to decompose the same quantity of complex 2 completely, which confirmed that phosphine complexes were more stable than the ether-phophine ones. The oxidatative decomposition processes of complexes 1 and 2 were accelerated by addition of H 2 O 2 as another oxidizing agent. By adding very small testing drop of H 2 O 2 to solution of the same quantity of complexes 1 or 2 the colors changed to green immediately. In general, only seconds were required to ensure the complete decomposition of the more stable phosphine complex 2 when 1:1 (mole:mole) of [H 2 O 2 :complex] were mixed.

Ruthenium(II) complexes 1 and 2 and synthetic investigation of xerogels X1 and X2
Complexes 1 and 2 are very important in interphase chemistry. They can be converted as primary complexes to prepare stationary phases via the sol gel technique in order to support complexes that transform the system from homogenous to heterogeneous phase or interphase catalysts [33][34][35]. Complexes 1 and 2 were subjected to a typical sol-gel polymerization process at room temperature in the presence of 10 equivalents of Si(OEt) 4 as cross-linker using methanol/THF/water to prepare polysiloxane xerogels X1 and X2, as shown in Scheme 1. Due to the poor solubility of the xerogels X1 and X2 they were subjected to solid state measurements like NMR, IR and EXAF.

31 P-NMR investigation of complexes 1 and 2 and Xerogels X1 and X2
The use of an asymmetric diamine co-ligand such as [3-(2-aminoethyl)aminopropyl]trimethoxysilane caused the loss of the C2 axis, resulting in a splitting of the 31 P{ 1 H}-NMR resonances of 1, 2, X1 and X2 into AB patterns [14,27]. The phosphorous chemical shifts and the 31 P -31 P coupling constants (Jpp = 30-36 Hz) of the desired complexes suggest that the phosphine ligand was positioned trans to the diamine, with trans dichloro atoms, to form the kinetically favored trans-Cl2Ru(II) isomer, as seen in Figure 3.

1 H and 13 C-NMR investigations
In the 1 H-NMR spectra of complexes 1 and 2 characteristic sets of signals were observed, which are attributable to the phosphine as well as [3-(2-aminoethyl)aminopropyl]trimethoxysilane ligands. Their assignment was supported by a free ligand 1 H-NMR study. The integration of the 1   In the 13 C-NMR spectra of complexes 1 and 2 and xerogels X1 and X2 characteristic sets of signals were observed, which are attributed to the PPh 3 and P~O phosphine ligands as well as the [ ethyl)aminopropyl]trimethoxysilane diamine co-ligand. Their assignment was supported by free ligand 13 C-NMR studies. Several sets of aliphatic and aromatic carbons related to the phosphine and diamine were assigned with the help of 135 DEPT 13 C-NMR to differentiate between the odd and even C-types, CH, CH 3 up axis singlet, CH 2 down axis singlet, and C no singlet. As a typical example, the 135 DEPT 13 C-NMR spectra of the free [3-(2-aminoethyl)-aminopropyl]trimethoxysilane, complex 1, complex 2, and solid state 13 C-CP-MAS-NMR of xerogels X2 are shown in Figure 5. Examination of the 13 C-CP-MAS-NMR spectrum of the modified solids along with the solution phase spectrum of the corresponding molecular precursor led to the conclusion that the organic fragments in complex 2 and xerogel X2 remained intact during the grafting and subsequent workup without measurable decomposition (Figure 5d). The absence of the CH 3 O peak at δ C = 49.92 belonging to (CH 3 O) 3 Si in the [3-(2-aminoethyl)aminopropyl]trimethoxysilane co-ligand after the sol-gel process of complex 2 to furnish xerogel X2, were the major differences noted between spectra, which supported the immobilization of the desired hybrid Ru(II) complexes. The total disappearance of groups in X2 (Figure 5b), compared by complex 2 (Figure 5c and 5d), provides good confirmation of a sol-gel process gone to full completion [9,10,17].
Solid-state 29 Si-NMR provided further information about the silicon environment and the degree of functionalization [9,10,17,[27][28][29][30][31][32][33]. In all cases, the organometallic/organic fragment of the precursor molecule was covalently grafted onto the solid, and the precursors were, in general, attached to the surface of the polysiloxane by multiple siloxane bridges. The presence of T m sites in case of xerogel X1 and X2 in the spectral region of T 2 at δ Si = -56.8 ppm and T 3 at δ Si = -69.1 ppm as expected, Q silicon sites due to Si(OEt) 4 condensation agent were also recorded to Q 3 at δ Si = -101.2 and Q 4 at δ Si = -109.5 ppm silicon sites of the silica framework, as seen in Figure 6.

IR investigations of ruthenium complexes 1 and 2
The IR spectra of the desired complexes in particular show several peaks which attributed to stretching vibrations of the main function group, in the ranges 3,490-3,300 cm -1 (v NH ), 3,280-3,010 cm -1 (v PhH ) and 3,090-2,740 cm -1 (v CH ). All other characteristic bands due to the other function groups are also present in the expected regions, as seen in Figure 8. The IR spectrum which contained the chemical shifts of the main fragments represented the well-known function groups of complex 1 as an example was illustrated in Figure 8.

EXAFS measurement of xerogel X2
The xerogel X2 was chosen as an example to determine the bond lengths between the metal center and the coordinating atoms of the ligand. The k 3 weighted EXAFS function of xerogel X2 can be described best by six different atom shells. The first intensive peak in the corresponding Fourier transform (Figure 9a) is mainly due to the nitrogen atoms. Chlorine and phosphorus atoms were found in the case of the most intense peak. For the most intense peak of the Fourier Transform, two equivalent phosphorus, two nitrogen atoms and two chlorine atoms with Ru-P, Ru-N and Ru-Cl bond distances of 2.27, 2.17 and 2.42 Å, respectively, were found ( Figure 9b and Table 1). These results reveal a good agreement between the experimental and the calculated functions.

Catalytic activity of complexes 1, 2, X1 and X2 in the hydrogenation of trans-4-phenyl-3-butene-2-al
To study the catalytic activity of the ruthenium(II) complexes, trans-4-phenyl-3-propene-2-al was selected, because three different rego-selective hydrogenation are expected (Scheme 2). The selective hydrogenation of the carbonyl group affords the corresponding unsaturated alcohol A.  Table 2.

Scheme 2. Different hydrogenation possibilities of trans-4-phenyl-3-butene-2-al:
Selective carbonyl function group hydrogenation to produce A, selective C=C function group hydrogenation to produce B, full hydrogenation path with no selectivity to produce C.  These catalysts were only effective in the presence of excess hydrogen in 2-propanol and a strong basic co-catalyst like KOH and tBuOK, since when weakly basic K 2 CO 3 was used as co-catalyst, no hydrogenation reaction was observed, even after longer reaction times. Complexes 1 and 2 are highly active under these mild conditions and gave rise to 99% conversion and selective hydrogenation of the C=O group in the presence of a C=C function. Complex 2 was slightly less active under identical conditions compared to complex 1, which can be attributed to the hemilability of the ether-phosphine ligand in the diamine/ruthenium(II) system.
The other ruthenium(II) precursors, xerogels X1 and X2, displayed high conversion ratios and selectivity (~ 90%) in the C=O selective hydrogenation of the trans-4-phenyl-3-butene-2-al using strong basic conditions. Expected constant decrease in the activity and the selectivity were observed by comparing the homogenous 1 and 2 with the heterogeneous X1 and X2 precursors under identical conditions. The hydrogenation reaction under the above conditions using complex 1 as catalyst was finished within one hour, as seen in Figure 10a, while xerogel X1 under the same condition takes ~ 12 hours to react to 95% conversion, as evident in Figure 10b. The GC-conversion of the hydrogenation process was plotted vs. reaction time in minutes as illustrated in Figure 10.

General procedure for sol-gel processing of xerogels X1 and X2
Complexes 1 and 2 (0.100 mmol) and Si(OEt) 4 (1 mmol,10 equivalents) in THF (5 mL) were mixed together. The sol-gel took place when a methanol/water mixture (2 mL, 1:1 v/v) was added to the solution. After 24 h stirring at room temperature, the precipitated gel was washed with toluene and diethyl ether (30 mL of each), and petroleum ether (20 mL). Finally the xerogel was ground and dried under vacuum for 24 h to afford after workup ~ 300 mg [yield ~45% based on Ru(II)] of a pale yellow powder were collected.

Conclusions
Four new diamine/phosphine/ruthenium(II) complexes were prepared. Complexes 1 and 2 were prepared by ligand exchange and hemilable cleavage methods, respectively. The presence of T-silyl functions on the diamine co-ligand backbone enabled the hybridization of these complexes in order to support them on a polysiloxane matrix through the sol-gel technique in order to produce xerogels X1 and X2. The structural behaviors of the phosphine ligands in the desired complexes during synthesis were monitored by 31 P{ 1 H}-NMR. The structure of complexes 1 and 2 described herein have been deduced from elemental analyses, infrared, FAB-MS and 1 H-, 13 C-, H, and 31 P-NMR spectroscopy data. The xerogel structuresof X1 and X2 were determined by solid state 13 C-, 29 Si-and 31 P-NMR spectroscopy, infrared spectroscopy and EXAFS. When these complexes were tested as catalysts for the hydrogenation of trans-4-phenyl-3-butene-2-al in both homogenous and heterogeneous phases, they showed a high degree of stability and activity as well as an excellent degree of carbonyl hydrogenation selectivity under mild conditions.