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

Abstract

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.

1. 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/1,2-diamine 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 ³¹P{¹H}- NMR spectroscopy [21].

Phosphorus–oxygen hemilabile ligands like 2-(diphenylphosphino)ethyl methyl ether ( P~O), reacts with various metals of catalytic relevance due to their ability to act as both a chelate ligand, stabilizing the metal complex, and a monodentate ligand providing a free coordination site for an incoming substrate (through the labilization of the weakly bonded oxygen atom) [12,13,14,15,16,17,18,19].

By the introduction of T-functionalized into the diamine ligands coordinated complexes, these complexes can be easily immobilized as atypical interphase to a polysiloxane matrix by sol-gel process [9,10,17,26,27,28,29,30,31,32,33]. In such interphases the stationary phase (comprising active centers, polymer and spacer) and a mobile component (gas, liquid or dissolved reactants) penetrate each other on a molecular scale without forming a homogeneous phase [26,27,28,29,30,31,32,33]. When such interphases are provided with a swellable polymer, they may imitate homogeneous conditions as the active centers become highly mobile, simulating the properties of a solution [9,10,17,26,27,28,29,30,31,32,33].

Recently we have synthesized a number of ruthenium(II) complexes of the [RuCl₂(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.

2. Results and Discussion

2.1. Ruthenium(II) complexes 1 and 2 synthetic investigation and structural behavior

Treating each of Cl₂Ru(P^⏜O)₂ and Cl₂Ru(PPh₃)₃ individually with an equivalent amount of [3-(2-aminoethyl)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.

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, ¹H-, ¹³C{¹H}− and ³¹P{¹H}−NMR spectroscopy data.

The stepwise formation of complex 1 is monitored by ³¹P{¹H}-NMR spectroscopy, in an NMR tube experiment, where addition of [3-(2-aminoethyl)aminopropyl]trimethoxysilane to a CDCl₃ solution containing Cl₂Ru(P^⏜O)₂ complex as starting material leads to the disappearance of the red color of the Cl₂Ru(P^⏜O)₂ complex and the singlet of this complex at δ_p = 64.40 ppm and the appearance of a new AB ³¹P{¹H}-NMR pattern at δ_p = 38.87, 35.64 ppm due to the formation of complex 1 with a trans-Cl₂Ru(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.

Figure 1. Time-dependent ³¹P{¹H}-NMR spectroscopic of Cl₂Ru(P^⏜O)₂ at δ_p = 64.4 ppm mixed with equivalent of diamine co-ligand in CDCl₃ in the NMR tube to produce complex 1 at δ_p = 38.87, 35.64 ppm a) before co-ligand addition, b) the first shot ~ 0.5 min. and c) the second shot ~1 min. after the co-ligand addition.

Figure 1. Time-dependent ³¹P{¹H}-NMR spectroscopic of Cl₂Ru(P^⏜O)₂ at δ_p = 64.4 ppm mixed with equivalent of diamine co-ligand in CDCl₃ in the NMR tube to produce complex 1 at δ_p = 38.87, 35.64 ppm a) before co-ligand addition, b) the first shot ~ 0.5 min. and c) the second shot ~1 min. after the co-ligand addition.

The weak ruthenium-oxygen bonds in bis(chelate)ruthenium(II) complexes of the Cl₂Ru(P^⏜O)₂ type are easily cleaved by an incoming molecule such as the [3-(2-aminoethyl)aminopropyl]-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.

2.2. Oxidative decomposition of complexes 1 and 2 by oxygen or H₂O₂

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 ³¹P{¹H}-NMR. The spectra showed that complex 1 (indicated by peaks at δ_p = 38.9, 35.6 ppm) was decomposed to form phosphine oxide [Ph₂P(=O)CH₂CH₂OCH₃] 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 ³¹P{¹H}-NMR.

Figure 2. Time-dependent ³¹P{¹H} NMR spectroscopic of complex 1 at δ_p = 38.9, 35.6 ppm in an open atmosphere a) fresh synthesis b) after 25 min. c) after 60 min.

Figure 2. Time-dependent ³¹P{¹H} NMR spectroscopic of complex 1 at δ_p = 38.9, 35.6 ppm in an open atmosphere a) fresh synthesis b) after 25 min. c) after 60 min.

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 H₂O₂ as another oxidizing agent. By adding very small testing drop of H₂O₂ 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 [H₂O₂:complex] were mixed.

2.3. Ruthenium(II) complexes1 and2 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 xerogelsX1 and X2 they were subjected to solid state measurements like NMR, IR and EXAF.

2.4. ³¹P-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]trimethoxy-silane caused the loss of the C2 axis, resulting in a splitting of the ³¹P{¹H}-NMR resonances of 1, 2, X1 and X2 into AB patterns [14,27]. The phosphorous chemical shifts and the ³¹P - ³¹P 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.

Figure 3. ³¹P{¹H}-NMR spectroscopic data of complex 2 at δ_p = 39.9, 44.1 ppm and their coupling constant J_pp = 31.6 Hz.

Figure 3. ³¹P{¹H}-NMR spectroscopic data of complex 2 at δ_p = 39.9, 44.1 ppm and their coupling constant J_pp = 31.6 Hz.

2.5. ¹H and ¹³C-NMR investigations

In the ¹H-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 ¹H-NMR study. The integration of the ¹H resonances confirms that the phosphine to diamine ratios are in agreement with the compositions of the desired complexes. A comparison the ¹H-NMR spectra of the free diamine co-ligand [3-(2-aminoethyl)amino-propyl]trimethoxysilane, individual starting material Cl₂Ru(P^⏜O)₂ complex and complex 1 (after mixing Cl₂Ru(P^⏜O)₂ with [3-(2-aminoethyl)aminopropyl]trimethoxysilane) is shown in Figure 4.

Figure 4. ¹H-NMR of: a) free ligand [3-(2-aminoethyl)aminopropyl]trimethoxysilane; b) complex Cl₂Ru(P^⏜O)₂ starting material and c) complex 1 in CDCl₃ at room temperature.

Figure 4. ¹H-NMR of: a) free ligand [3-(2-aminoethyl)aminopropyl]trimethoxysilane; b) complex Cl₂Ru(P^⏜O)₂ starting material and c) complex 1 in CDCl₃ at room temperature.

In the ¹³C-NMR spectra of complexes 1 and 2 and xerogels X1 and X2 characteristic sets of signals were observed, which are attributed to the PPh₃ and P~O phosphine ligands as well as the [3-(2-amino-ethyl)aminopropyl]trimethoxysilane diamine co-ligand. Their assignment was supported by free ligand ¹³C-NMR studies. Several sets of aliphatic and aromatic carbons related to the phosphine and diamine were assigned with the help of 135 DEPT ¹³C-NMR to differentiate between the odd and even C-types, CH, CH₃ up axis singlet, CH₂ down axis singlet, and C no singlet. As a typical example, the 135 DEPT ¹³C-NMR spectra of the free [3-(2-aminoethyl)-aminopropyl]trimethoxysilane, complex 1, complex 2, and solid state ¹³C-CP-MAS-NMR of xerogels X2 are shown in Figure 5.

Figure 5. Dept 135 ¹³C-NMR of: a) free [3-(2-aminoethyl)aminopropyl]trimethoxysilane; b) complex 1 in CDCl₃; c) complex 2 in CDCl₃; compared by solid state ¹³C-CP-MAS-NMR; d)X2 xerogel.

Figure 5. Dept 135 ¹³C-NMR of: a) free [3-(2-aminoethyl)aminopropyl]trimethoxysilane; b) complex 1 in CDCl₃; c) complex 2 in CDCl₃; compared by solid state ¹³C-CP-MAS-NMR; d)X2 xerogel.

Examination of the ¹³C-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 CH₃O peak at δ_C = 49.92 belonging to (CH₃O)₃Si 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 ²⁹Si-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 T² at δ_Si = -56.8 ppm and T³ at δ_Si = - 69.1 ppm as expected, Q silicon sites due to Si(OEt)₄ condensation agent were also recorded to Q³ at δ_Si = -101.2 and Q⁴ at δ_Si = -109.5 ppm silicon sites of the silica framework, as seen in Figure 6.

Figure 6. ²⁹Si CP/MAS NMR spectrum of X2, prepared by condensate of complex 2 with 10 equivalent of Si(OEt)₄ cross-linkers.

Figure 6. ²⁹Si CP/MAS NMR spectrum of X2, prepared by condensate of complex 2 with 10 equivalent of Si(OEt)₄ cross-linkers.

2.6. FAB mass spectra of the complex 1 and 2

Complexes 1 and 2 were subjected to FAB-MS, which showed molecular ion peaks M⁺ [Cl₂Ru(PP)NN]⁺ at m/z 972.2 and 882.1 respectively, which revealed their exact calculated mass. For comparison the FAB-MS spectrum of complex 1 is presented in Figure 7.

The first three fragments are the most important and were assigned as M⁺ = [Cl₂Ru(P~O)2NN]⁺ at m/z 882.1, the second molecular ion was formed by the loss of HCl to give a fragment ion peak at m/z 847.2 belonging to M⁺-HCl =[RuCl₂(P~O)2NN-HCl]⁺, the third fragment at m/z 662.0, which was the most stable one, was formed by loss of diamine ligand from the structure of complex 1 to give the starting material M⁺ -NN = [Cl₂Ru(P^⏜O)₂]⁺, as seen in Figure 7.

Figure 7. FAB-Mass spectrum of complex 1.

Figure 7. FAB-Mass spectrum of complex 1.

2.7. 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.

Figure 8. Infra-red spectrum cm^-1 per well-known function group of complex 1.

Figure 8. Infra-red spectrum cm^-1 per well-known function group of complex 1.

2.8. 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 k³ 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.

Figure 9. Experimental (solid line) and calculated (dotted line) EXAFS functions (a) and their Fourier transforms (b) for xerogel X2 measured at Ru K-edge.

Figure 9. Experimental (solid line) and calculated (dotted line) EXAFS functions (a) and their Fourier transforms (b) for xerogel X2 measured at Ru K-edge.

Table 1. EXAFS determined structural parameters of xerogel X2.

A-Bsa	Nb	rc [Å]	σd [Å]	ΔE0e [eV]	k-range [Å-1]	Fit-Index
Ru – N	2	2.17 ± 0.02	0.050 ± 0.005	22.26		22.06
Ru – P	2	2.27 ± 0.02	0.067 ± 0.007		3.0–18.0
Ru – Cl	2	2.42 ± 0.03	0.054 ± 0.006

^a absorber (A) – backscatterers (Bs), ^bcoordination number N, ^cinteratomic distance r, ^dDebye-Waller factor σ with its calculated deviation and ^e shift of the threshold energy ΔE_0.

Table 1. EXAFS determined structural parameters of xerogel X2.

A-Bsa	Nb	rc [Å]	σd [Å]	ΔE0e [eV]	k-range [Å-1]	Fit-Index
Ru – N	2	2.17 ± 0.02	0.050 ± 0.005	22.26		22.06
Ru – P	2	2.27 ± 0.02	0.067 ± 0.007		3.0–18.0
Ru – Cl	2	2.42 ± 0.03	0.054 ± 0.006

^a absorber (A) – backscatterers (Bs), ^bcoordination number N, ^cinteratomic distance r, ^dDebye-Waller factor σ with its calculated deviation and ^e shift of the threshold energy ΔE_0.

2.9. 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. Unwanted and hence of minor interest both the hydrogenation of the C=C double bond, leading to the saturated aldehyde B and the full hydrogenation of C=O and C=C bonds resulting the formation of saturated alcohol C. The hydrogenation reactions using complexes 1 and 2, and xerogel X1 and X2 as catalysts were carried out under identical conditions: 35 °C with a molar substrate:catalyst (TON, S/C) ratio of 1,000:1, under 2 bar of hydrogen pressure, in 50 mL of 2-propanol [Ru: Co-catalysts (KOH, tBuOK and K₂CO₃): trans-4-phenyl-3-butene-2-al] [1:10:1,000], the results are listed in 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.

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.

Table 2. Hydrogenation of trans-4-phenyl-3-butene-2-al by Ru(II) complexes.

Run	Catalyst	Co-catalyst	Conversion (%)a	Selectivity (%) a	TOF b
1	1	tBuOK	>99 c	>99 A	1,160
2	2	tBuOK	>99 c	>99 A	1,050
3	1	KOH	>99 c	>99 A	1,210
4	2	KOH	>99 c	>99 A	1,070
5	1 or 2	K2CO3	0 d	-	-
6	X1	tBuOK	95 d	93 A	80
7	X2	tBuOK	90 d	90 A	75
8	X1	KOH	94 d	92 A	78
9	X2	KOH	92 d	91 A	76

^a Yield and selectivity were determined by GC. ^b Turnover frequency: mole of product per mole of catalyst per hour, h^-1. ^c The reaction was conducted to the hydrogenation for one hour. ^d The reaction was conducted to the hydrogenation for 12 hours

Table 2. Hydrogenation of trans-4-phenyl-3-butene-2-al by Ru(II) complexes.

Run	Catalyst	Co-catalyst	Conversion (%)a	Selectivity (%) a	TOF b
1	1	tBuOK	>99 c	>99 A	1,160
2	2	tBuOK	>99 c	>99 A	1,050
3	1	KOH	>99 c	>99 A	1,210
4	2	KOH	>99 c	>99 A	1,070
5	1 or 2	K2CO3	0 d	-	-
6	X1	tBuOK	95 d	93 A	80
7	X2	tBuOK	90 d	90 A	75
8	X1	KOH	94 d	92 A	78
9	X2	KOH	92 d	91 A	76

^a Yield and selectivity were determined by GC. ^b Turnover frequency: mole of product per mole of catalyst per hour, h^-1. ^c The reaction was conducted to the hydrogenation for one hour. ^d The reaction was conducted to the hydrogenation for 12 hours

These catalysts were only effective in the presence of excess hydrogen in 2-propanol and a strong basic co-catalyst like KOH andtBuOK, since when weakly basic K₂CO₃ 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.

Figure 10. Hydrogenation reaction of trans-4-phenyl-3-butene-2-al using complex 1 and xerogel X1 catalysts under the above mild conditions.

Figure 10. Hydrogenation reaction of trans-4-phenyl-3-butene-2-al using complex 1 and xerogel X1 catalysts under the above mild conditions.

3. Experimental

3.1. General

All reactions were carried out in an inert atmosphere (argon) by using standard high vacuum and Schlenk-line techniques, unless otherwise noted. Prior to use CH₂Cl₂, n-hexane, and Et₂O were distilled from CaH₂, LiAlH₄, and from sodium/benzophenone, respectively, ether-phosphine ligand, Cl₂Ru(P^⏜O)₂ and Cl₂Ru(PPh₃)₂ were prepared according to literature methods [9,10,11,12,13]. [3-(2-aminoethyl)aminopropyl]trimethoxysilane and tetramethoxysilyl were purchased from Acros. Elemental analyses were carried out on an Elementar Vario EL analyzer. High-resolution liquid ¹H-, ¹³C{¹H}-, DEPT 135, and ³¹P{¹H}-NMR spectra were recorded on a Bruker DRX 250 spectrometer at 298 K. Frequencies are as follows: ¹H-NMR: 250.12 MHz, ¹³C{¹H}-NMR: 62.9 MHz, and ³¹P{¹H}-NMR 101.25 MHz. Chemical shifts in the ¹H- and ¹³C{¹H- NMR spectra were measured relative to partially deuterated solvent peaks which are reported relative to TMS. ³¹P-NMR chemical shifts were measured relative to 85% H₃PO₄. CP/MAS solid-state NMR spectra were recorded on Bruker DSX 200 (4.7 T) and Bruker ASX 300 (7.05 T) multinuclear spectrometers equipped with wide-bore magnets. Magic angel spinning was applied at 4 kHz (²⁹Si) and 10 kHz (¹³C, ³¹P) using (4 mm ZrO₂ rotors). Frequencies and standards: ³¹P, 81.961 MHz (4.7 T), 121.442 MHz (7.05 T) [85% H₃PO₄, NH₄H₂PO₄ (δ = 0.8) as second standard]; ¹³C, 50.228 MHz (4.7 T), 75.432 MHz (7.05 T) [TMS, carbonyl resonance of glycine (δ = 176.05) as second standard]; ²⁹Si, 39.73 MHz (4.7 T), 59.595 MHz (7.05 T, (Q8M8 as second standard). All samples were prepared with exclusion of molecular oxygen. . IR data were obtained on a Bruker IFS 48 FT-IR spectrometer. Mass spectra: EI-MS, Finnigan TSQ70 (200 °C) and FAB-MS, Finnigan 711A (8 kV), modified by AMD and reported as mass/charge (m/z). The analyses of the hydrogenation experiments were performed on a GC 6000 Vega Gas 2 (Carlo Erba Instrument) with a FID and capillary column PS 255 [10 m, carrier gas, He (40 kPa), integrator 3390 A (Hewlett Packard)]. The EXAFS measurements were performed at the ruthenium K–edge (22118 eV) at the beam line X1.1 of the Hamburger Synchrotronstrahlungslabor (HASYLAB) at DESY Hamburg, under ambient conditions, energy 4.5 GeV, and initial beam current 120 mA. For harmonic rejection, the second crystal of the Si(311) double crystal monochromator was tilted to 30%. Data were collected in transmission mode with the ion chambers flushed with argon. The energy was calibrated with a ruthenium metal foil of 20 μm thickness. The samples were prepared of a mixture of the samples and polyethylene.

3.2. General procedure for the preparation of the complex 1 and 2

Diamine (0.23 mmol, 5% excess) was dissolved in dichloromethane (5 mL), the solution was added dropwise to a stirred solution of Cl₂Ru(P^⏜O)₂ and Cl₂Ru(PPh₃)₃(0.22 mmol) in dichloromethane (10 mL) within 2 min. The mixture was stirred for ca. 2 h at room temperature while the color changed from brown to yellow, then the volume of the solution was concentrated to about 2 mL under reduced pressure. Addition of diethyl ether (40 mL) caused the precipitation of a solid which was filtered (P4), then dissolved again in dichloromethane (40 mL) and concentrated again under vacuum to a volume of 5 mL. Addition of n-hexane (80 mL) caused the precipitation of a solid which was filtered (P4), well washed with n-hexane each and dried under vacuum. Complex 1 and 2 were obtained in analytically pure form in very good yields. m.p. > 340 °C (dec.). Complex 1: ¹H-NMR (CDCl₃): δ (ppm) 0.11 (m, 2H, CH₂Si), 0.57 (m, 2H, SiCH₂CH₂), 1.45 (br, 2H, SiCH₂CH₂CH₂N), 2.02 (m, 2H, CH₂NCH₂CH₂N), 2.34 (m, 2H, CH₂NCH₂CH₂N), 2.44 (br, 4H, PCH₂), 2.88, 2.92 (2s, 6H, CH₃OCH₂), 3.08 (m, 4H, CH₂O), 3.48 (br, 9H, CH₃OSi), 3.55 (s, 3H, NH₂), 6.90–7.90 (m, 20H, C₆H₅); ³¹P{¹H}-NMR (CDCl₃): δ (ppm) 35.64, 38.87, dd, AB pattern with Jpp = 35.6 Hz, ¹³C{¹H}-NMR (CDCl₃): δ (ppm) 6.67 (s, C, CH₂Si), 21.11 (s, SiCH₂CH₂ ), 26.12, 27.03 (2m, 2C, PCH₂), 43.82 (s, C, HNCH₂CH₂NH₂), 48.43 (s, C, HNCH₂CH₂NH₂), 50.32 (s, 3C, SiOCH₃), 55.45 (s, C, SiCH₂CH₂CH₂NH), 57.93, 58.01 (2s, 2C, OCH₃), 69.31, 69.40 (2s, 2C, OCH₂), 127.20-134.0 (m, 24C, C₆H₅); FAB–MS; (m/z): 882.2 (M⁺); Yield 92% related to Ru(II); Anal. Calc. C, 51.70; H, 6.39; Cl, 8.03; N, 3.17; for C₃₈H₅₆Cl₂N₂O₅P₂RuSi: Found C, 51.55; H, 6.24; Cl, 8.23; N, 3.54 %. Complex 2: ¹H-NMR (CDCl₃): δ (ppm) 0.12 (m, 2H, CH₂Si), 0.82 (m, 2H, SiCH₂CH₂), 1.32 (br, 2H, SiCH₂CH₂CH₂N), 2.41 (m, 2H, CH₂NCH₂CH₂N), 2.81 (m, 2H, CH₂NCH₂CH₂N), 3.29 (s, 3H, NH₂), 3.46 (br, 9H, CH₃OSi), 6.90-7.60 (m, 30H, C₆H₅); ³¹P{¹H}-NMR (CDCl₃): δ (ppm) 39.9 , 44.1 dd, AB pattern with Jpp = 31.6 Hz, ¹³C{¹H}-NMR (CDCl₃): δ (ppm) 6.69 (s, C, CH₂Si), 21.82 (s, SiCH₂CH₂ ), 43.01 (s, C, HNCH₂CH₂NH₂), 49.17 (s, C, HNCH₂CH₂NH₂), 50.86 (s, 3C, SiOCH₃), 54.92 (s, C, SiCH₂CH₂CH₂NH), 127.20-135.60 (3m, 36C, C₆H₅); FAB–MS; (m/z): 918.1 (M⁺); Yield 83% related to Ru(II); Anal. Calc. C, 57.25; H, 7.01; Cl, 7.59; N, 3.08; for C₄₈H₅₈Cl₂N₂O₃P₂RuSi: Found C, 57.51; H, 5.70; Cl, 7.72; N, 3.05%.

3.3. General procedure for sol–gel processing of xerogels X1 and X2

Complexes1 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.

Xerogel X1: ³¹P-CP/MAS-NMR: δ = 35.64, 38.87, dd, AB pattern with J_pp = 35.6 Hz; ¹³C-CP/MAS NMR: δ (ppm) 5.44 (br, 1C, CH₂Si), 20.65 (m, 1C, CH₂CH₂Si), 27.25 (m, 2C, PCH₂), 43,87 (br, 1C, NH₂CH₂CH₂NH), 48.32 (br, 1C, NH₂CH₂), 55.72 (s, 1C, NHCH₂CH₂CH₂), 57.21 (br, 2C, OCH₃), 69.77 (br, 2C, OCH₂), 120.00-140.00 (m, 24C, C₆H₅); ²⁹Si CP/MAS NMR: δ = –67.2 ppm (T³), –57.4 ppm (T²), -101.8 ppm (Q³), -109.3 ppm (Q⁴).

Xerogel X2: ³¹P-CP/MAS-NMR: δ = 39.9 , 44.1 ppm. dd, J_pp =31.6 Hz; ¹³C-CP/MAS NMR: δ (ppm) 6.68 (br, 1C, CH₂Si), 22.12 (m, 1C, CH₂CH₂Si), 42.31 (s, 1C, NH₂CH₂CH₂NH), 48.44 (br, 1C, NH₂CH₂), 54.82 (s, 1C, NHCH₂CH₂CH₂), 120.00-140.00 (m, 36C, C₆H₅); ²⁹Si CP/MAS NMR: δ = –69.1 ppm (T³), –56.8 ppm (T²), -101.2 ppm (Q³), -109.5 ppm (Q⁴).

4. 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 ³¹P{¹H}-NMR. The structure of complexes 1 and 2 described herein have been deduced from elemental analyses, infrared, FAB-MS and ¹H-, ¹³C-, H, and ³¹P-NMR spectroscopy data. The xerogel structuresof X1 and X2 were determined by solid state ¹³C-, ²⁹Si- and ³¹P-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.