synthesis, structure, and catalytic

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Duczmal, W. (1996). Competitive dehydrogenative silylation and hydrogenative dimerization of vinyltriethoxysilane catal- ysed by the (Ni(acac)2+PPh3) system, ...
Chemical Papers 62 (3) 268–274 (2008) DOI: 10.2478/s11696-008-0022-2

ORIGINAL PAPER

Complexes of transition metals bonded to silica via β-diketonate groups – synthesis, structure, and catalytic activity a

a Faculty b Faculty

Iwona Rykowska*,

a,b

Wlodzimierz Urbaniak

of Chemistry, Adam Mickiewicz University, ul. Grunwaldzka 6, 60 780 Pozna´ n, Poland

of Technology and Engineering, University of Technology and Life Sciences, ul. Seminaryjna 3, 85 326 Bydgoszcz, Poland Received 15 April 2007; Revised 10 November 2007; Accepted 13 November 2007

Transition metal complexes bonded to silica via silanes with β-diketonate groups can be used as packings for complexation gas chromatography or as immobilized homogenous metal complex catalysts. On basis of elemental analysis and the determination of surface area, possible structures of the complexes formed on the silica surface have been proposed. The possibility of using the immobilized complexes as catalysts has been indicated. Especially nickel complexes were taken into consideration. These immobilized complexes were used previously as packings for complexation gas chromatography. c 2008 Institute of Chemistry, Slovak Academy of Sciences  Keywords: supported metal complexes, β-diketonate complexes, hydrosilylation

Introduction Complexes of the transition metals bonded to the silica surface via silanes and involving hydrocarbon chains which terminate in a functional group capable of coordination with metals are widely used in catalysis as so-called immobilized homogeneous transition metal complex catalysts (Allum et al., 1976; Hartley, 1985; Panster & Wieland, 1996), as well as in the complexation gas chromatography (Cagniant, 1992; Pool & Pool, 1992; Wasiak & Wawrzyniak, 2005). As for the latter case, the mentioned complexes are used as packing components showing a particular ability to create π-type interactions with adsorbate molecules characterized by electron-donor-acceptor properties. As concluded from our previous research, some chromatographic packings of this kind can be applied directly as immobilized homogenous catalysts. Reactivity of organic compounds in catalytic processes as well as in chromatographic separation of organic compounds is typically characterized by the determination of the composition and structure of metal

complexes bonded to the silica surface. Supported transition metal complexes have been carefully characterized using different usually spectroscopic methods. In particular, some interesting results have been obtained for rhodium complexes immobilized on silica carriers via phosphine ligands (Gao & Angelici, 1999; Lindener et al., 2000). However, supported transition metal complexes bonded to silica via diketone groups, which show perfect chelating properties and little sensitivity to oxygen, have not been widely investigated yet. Such complexes have been successfully applied as catalysts for hydrosililation, hydrogenation, the synthesis of trisubstituted olefins (Čapka et al., 1992; Werner & M¨ohring, 1994), and as selective stationary phases for complexation gas chromatography (Wasiak et al., 1992; Wasiak & Rykowska, 1996, 1998, 1999). This paper reports on the synthesis and structural investigation of immobilized β-diketonate Ni, Co, Pd, and Cu complexes. Similar research regarding the usage of Ag(I) and Ni(II) bis-β-diketonates as sorbents for sulfur-containing compounds was reported in Wen-

*Corresponding author, e-mail: [email protected]

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I. Rykowska & W. Urbaniak/Chemical Papers 62 (3) 268–274 (2008)

zel et al. (1989). Additionally, research on the selectivity improvement analyzing weakly bonded complexes (e.g. with alkanes, alkenes, aldehydes, ketones, etc.), lanthanide metal chelates were proposed (Picker & Stevens, 1981). Several successful applications of the immobilized complexes under study used as packings for gas chromatography were previously reported (Wasiak et al., 1992; Wasiak & Rykowska, 1996, 1998, 1999). In this paper, the possibility of using these complexes as immobilized homogenous transition-metal complex catalysts is shown, with special attention paid to nickel complexes. Complexes of the other above-mentioned metals are used as references to provide a more complete picture of the whole group of immobilized complexes under study.

Experimental Materials and reagents Silica (Porasil C, 80–100 mesh) was obtained from Waters Associates (Milford, MA, USA) and 3-(3triethoxysilylpropyl)-2,4-petanedione (TESPA) was prepared as described in literature (Urbaniak & Schubert, 1991). Nickel complexes Ni(propA) and Ni(TESPA)2 used as homogenous catalysts were prepared according to procedure given for Ni(acac)2 (Charles & Pawlikowski, 1958), using 3-propyl-2,4pentanodione (propA) and 3-(3-triethoxysilylpropyl)2,4-pentanodione (TESPA) as ligands, respectively. Other transition metal salts and complexes were obtained from Aldrich (Sigma Aldrich Ltd.) and used without further purification. Triethoxysilane and vinyltriethoxysilane were distilled prior to use. Xylene, hexane, and tetrahydrofuran (THF) were distilled over metallic sodium prior to use. These chemicals were also obtained from Aldrich (Sigma Aldrich Ltd.). Elemental analysis of modified silica was performed on a Perkin-Elmer 2400 CHN Elemental Analyser. Specific surface areas of the modified supports under study were measured on a Micrometritics ASAP 2010 sorptometer. The measurements were carried out via the BET method using nitrogen as the adsorbate. Preparation of immobilized transition-metal complexes Approximately 50 g of dry silica (Porasil C) was immersed in a mixture consisting of 100 ml of anhydrous xylene and 5 ml of 3-(3-triethoxysilylpropyl)2,4-pentanodione (TESPA). The mixture was boiled under continuous stirring for 12 hours in a vessel with a reflux condenser and was protected from moisture. Unreacted silane was extracted with xylene and hexane in a Soxhlet apparatus. After this operation, silica was subjected to drying at 120 ◦C under vacuum and reacted with hexamethyldisilazane to deac-

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tivate free silanol groups remaining on the silica surface (so-called “end-capping”). Accurate trimethylsilylation of silanol groups is a prerequisite for obtaining stable supported complexes, since otherwise would the metal acetylacetonates be readily hydrolyzed by acidic silanol groups (Čapka et al., 1992). In the next step, silica, altered as described above, was modified with chlorides, acetylacetonates, and hexafluoroacetylacetonates of Ni (II), Cu (II), Co (II), and Pd (II). To do so, dry modified silica was immersed in a solution of anydrous tetrahydrofuran and one of the aforementioned metal salts or complexes. The solution was then allowed to stand at room temperature for 7 days, protected from moisture. Silica was then filtered off, excess of the precursor salts and complexes was extracted with tetrahydrofuran in a Soxhlet apparatus, and silica was again subjected to drying. Scheme of the reactions taking place during the synthesis is presented in Fig. 1, where MX2 stands for CuCl2 , Cu(acac)2 , Cu(hfac)2 , CoCl2 , Co(acac)2 , Co(hfac)2 , NiCl2 , Ni(acac)2 , Ni(hfac)2 , and PdCl2 , respectively, “acac” stands for acetylacetone and “hfac” stands for hexafluoroacetylacetone. Immobilized NiCl2 and Ni(acac)2 , denoted as SiO2 -A-NiCl2 and SiO2 -A-Ni(acac)2 , respectively, were chosen as catalysts for the investigation. Please note that the bonding between metal and silica-bonded diketone groups was confirmed in our previous research using the UV-VIS (reflection technique) and diferential scanning calorimetry (DSC) analysis (Wasiak & Rykowska, 1998). Hydrosilylation reaction (general procedure) Nickel catalyst was placed in a glass ampoule that was filled with a mixture of vinyltriethoxysilane and triethoxysilane. All manipulations were carried out using standard Schlenk and high vacuum-line techniques. Sealed glass ampoules were heated at given temperature (120 ◦C). Products were identified by the gas chromatography (GC) – mass spectrometry (MS) analysis (Varian 3300 gas chromatograph equipped with a DB-1/30 m capillary column and a Finnigan Mat 800 ion trap detector), comparing the spectra and the retention times of peaks with those of previously described standards (Marciniec & Maciejewski, 1993; Maciejewski et al., 2000). Distribution of substrates and products, conversion of substrates, and yield of the reaction products were determined by the GC analysis. Valuable information on modified silica supports is usually obtained on basis of elemental analysis. In particular, the density of surface coverage may be calculated from the total carbon content. To characterize modified silica, elemental analysis was performed and surface area was measured using the BET method. On

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Table 1. Characterization of chromatographic packings with diketonate groups and their metal complexes bonded to the silica surface

Composition, wi /% No.

SBET

Surface concentration/

Number of

(µmol m−2 )

diketonate

n(silane)/

groups

n(metal)

Compound C

H

Cl

Metal

m2 g−1

Silane

Metal

per nm2

1

TESPA

3.03

0.47





75

4.20



2.52



2 3 4

NiCl2 Ni(acac)2 Ni(hfac)2

1.69 2.46 1.84

0.40 0.47 0.33

– – –

0.69 0.11 0.17

97 74 71

1.81 2.13 1.66

1.21 0.25 0.42

1.09 1.28 1.00

1.50 8.52 3.95

5 6 7

CuCl2 Cu(acac)2 Cu(hfac)2

2.84 2.89 2.84

0.78 0.44 0.42

0.27(0.93*) – –

0.49 0.10 0.21

82 71 102

3.60 2.61 1.78

0.94 0.22 0.32

2.17 1.57 1.07

3.83(0.99**) 7.14 5.56

8 9 10

CoCl2 Co(acac)2 Co(hfac)2

1.06 2.08 2.26

0.38 0.44 0.43

– – –

0.71 0.10 0.15

94 71 79

1.17 1.88 1.83

1.28 0.24 0.32

0.70 1.13 1.10

0.92 7.83 5.72

11

PdCl2

2.94

0.75

0.25(0.87*)

0.75

81

3.78

0.87

2.28

4.43(1.00**)

*Surface concentration in µmol m−2 . ** Chlorine to metal mole ratio.

CH3 Si

OH

+ Si

(EtO)3Si

OH

OH

CH3 Si

xylene

(CH2)3C

OEt

O

Si

O

O

CH3 OH

Si

Si

O

Si

O

OEt

O

Si

(CH2)3C

(CH2)3CH

O CH3

O

CH3

CH3

MX2 THF

CH3 Si

OEt

O

Si

O

CH3 O

Si (CH2)3C

MX

Si

O

Si

O

OEt Si

O

CH3 O

(CH2)3C O

CH3

EtO

O

CH3

O

Si

O

Si

Si

M

C(CH2)3 O CH3

Fig. 1. Preparation of complexes immobilized on silica via β-diketonate groups.

basis of the obtained results, the surface concentration of bonded silane was determined from the formula proposed by Berendsen et al. (1980) and developed by Buszewski et al. (1998). The surface concentration of bonded siloxane molecules (denoted by α/(µmol m−2 )) µwas calculated from the carbon content according to the following equation α=

6

%C × 10 (100 n 12 − %C M )SBET

cents, n number of carbon atoms in the molecule of bonded silane, M molecular mass of siloxane, and SBET specific surface area (m2 g−1 ).

Results and discussion Characteristics of modified silica and immobilized metal complexes

(1)

where %C denotes the carbon contribution in per-

Silica suitable for immobilization of metal complexes, in turn used as heterogenized catalysts, should

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have an appropriate texture, sufficient surface area, and pore volume, reasonable particle size, and good mechanical properties. Beside these properties, it should be resistant to solvents, high temperatures, etc. (Allum et al., 1976; Hartley, 1985; Panster & Wieland, 1996; Cagniant, 1992; Pool & Pool, 1992). The silica surface should be mesoporous or macroporous (with the optimum mean pore diameter equal to 10–30 nm), ideally free of micropores smaller than 2 nm, as they are not accessible for bulky metal complexes and their presence slows the mass transfer down (Novak et al., 1990; Buszewski et al., 1998). Moreover, pores at the silica surface should be of identical or similar shape. These requirements are met by a silica gel used as a support in column packings for chromatographic systems (Buszewski et al., 1998). Our previous research has shown that such characteristics are demonstrated by Porasil C, characterized by the mean pore radius of approximately 15 nm, and homogenous porosity (Rykowska & Wasiak, 2003). The surface coverage with silane and metal complexes was calculated, and the results obtained are shown in Table 1. These results were used to determine and furthermore to describe the structures of complexes formed on the surface. The total number of coordinating groups bonded to the silica surface is limited by the number of available silane groups. In dried silica gels, the concentration of OH groups is assumed to be about 4.6 groups per 1 nm2 (8 µmol per 1 m2 ) (Hartley, 1985; Cagniant, 1992; Pool & Pool, 1992; Novak et al., 1990). However, for steric reasons, only a part of these groups may be used in the reaction with a silyl group. A detailed study has shown that silanes with three reactive groups on the silicon atom, e.g. with three alkoxy groups, react with the surface with two groups at the most, and the average number of groups involved varies from 1.5 to 2 (Vansant et al., 1995). Therefore in practice, when a monolayer is formed, the maximum concentration of coordinating groups on the silica surface is equal to 1/2 groups per 1 nm2 of the surface, which corresponds to 1.7/3.3 µmol of bonded silane per 1 m2 (Vansant et al., 1995; Buszewski et al., 1998). In the modified silica gels under study, the number of silica-bonded coordinating groups was in the typical range. This fact leads to the conclusion that silane bonded to the silica surface forms a monolayer. After the introduction of metal, the concentration of coordinating groups decreased slightly. This influence was, however, much greater than expected. A possible explanation is that after the silane to silica bonding, unreacted silane excess was not fully washed out or the bond was weak and, later on, silane was separated from the surface. A factor undoubtedly favoring this process is the presence of metal salts and side products formed upon the metal complexation. In case of the chloride salts, hydrochloric acid can be a side prod-

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uct. It breaks the siloxane bond, which is clearly visible with NiCl2 and CoCl2 . When the leaving agent was acetylacetone or its fluorine derivative, the concentration decrease of surface coordinating groups was smaller. Comparison of the amount of bonded silane with that of deposited metal showed that the number of coordinating groups was in two- or fourfold excess compared to the amount demanded stoichiometrically to form the complex, i.e. two diketone groups for a single metal atom. Direct verification of one or two coordinating groups being involved in the bonding remains a problem of further research. Considering the average number of coordinating groups per 1 nm2 and the length of carbon skeleton between the support and the metal (the sum of atomic diameters being greater than 1 nm), it can be expected that two complex groups coordinate the metal. However, a careful analysis of the results given in Table 1 suggests that the dominant species involved only one group, and that the surface complex had the structure of type I as shown in Fig. 1. For further applications, this phenomenon is beneficial due to the incomplete coordinating saturation allowing a stronger interaction with other organic compounds (Cagniant, 1992; Pool & Pool, 1992; Wasiak & Rykowska, 1996). A proof of the formation of these kinds of structures can be the comparison of the amount of bonded metal and metal-bound chlorine after the bonding reaction. In case of CuCl2 and PdCl2 , the molar ratio metal to chlorine was very near to 1:1, only one chlorine atom was replaced by a diketone group, while the other chlorine atom remained unused. For CoCl2 and NiCl2 , the amount of chlorine was not determined. However, as follows from the mole ratio of bonded silane to metal, only one coordinating group can bond the latter one. It could be expected that a similar case occurs when a metal complex with acetylacetone (M(acac)2 ) or hexafluoroacetylacetone (M(hfac)2 ) is bonded to the silica surface. Although the number of free ligands (bonded to silica) per metal atom is much higher for these complexes compared to chlorine salts immobilization, the exchange of ligands is much less effective since the amount of bonded metal is threeto fivefold smaller than for chlorides. This means that ligands bonded to silica exchange chlorine groups bonded to metal (in chlorine salts) much more easily than acetylacetone ligands in (M(acac)2 ) or hexafluoroacetylacetone ligands in (M(hfac)2 ). It has been reported that the exchange of the first diketone group in metal complexes is always faster and easier than that of the second group (Siedle, 1987; Mehrotra et al., 1978). Therefore, and also due to the steric effect, the exchange of more than one group (i.e. any group containing a metal ion in the compound bonded to the carrier) and the same metal ion is less probable.

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Table 2. Conversion of vinyltriethoxysilane and product distribution of its reactions with triethoxysilane catalyzed by homogenous and immobilized nickel complexesa III

IV

V

VI

Catalysts

Vinyltriethoxysilane conversion/%

Y b /%

S c /%

Y b /%

S c /%

Y b /%

S c /%

Y b /%

S c /%

Ni(acac)2 Ni(propA)2 Ni(TESPA)2 SiO2 -A · Ni(acac)2 SiO2 -A · NiCl2

100 100 98 21 18

7 68 59 12 11

7 68 59 57 65

28 15 16 4 3

28 15 16 20 17

30 2 6 3 2

30 2 6 14 11

35 15 17 2 2

35 15 18 9 11

a) Glass ampoules, 2 h, 120 ◦C; n(CH2 — —CHSi–): n(HSi–): n(cat.) = 1 : 1 : 1 × 10−3 ; b) yield; c) selectivity.

Investigation of catalytic activity Considering the acetylacetone complexes studied, nickel complexes have been proved to exhibit particularly interesting catalytic properties, especially in hydrosilylation reactions (Marciniec, 2005). Previous

(EtO)3SiH + CH2 = CHSi(OEt)3

study by Marciniec et al. (1994, 1996) and Maciejewski et al. (2000) on the reaction of vinyltriethoxysilane with triethoxysilane catalysed by Ni(acac)2 reported the formation of a mixture of few products according to scheme

(EtO)3SiCH = CHSi(OEt)3 + CH3CH2Si(OEt)3 III

(2)

(EtO)3SiCH2CH2CH2CH2Si(OEt)3 (EtO)3SiCH(CH3)CH2CH2Si(OEt)3 IV

(3)

(EtO)3SiCH2CH2Si(OEt)3 V

(5)

(EtO)2SiH2 + (EtO)4Si VIa VIb

Bis(triethoxysilyl)ethene (III ), two isomers of bis (triethoxysilyl)butane (IV ), and bis(triethoxysilyl) ethane (V ) are obtained as a result of the reactions Eqs. (2)–(4), respectively. Moreover, disproportionation of hydrosilane Eq. (5) occurred. The main pro-

(EtO)3SiH + (EtO)3SiCH = CHSi(OEt)3

(4)

cesses are accompanied by dehydrogenative silylation Eq. (6) and hydrosilylation Eq. (7) of the unsaturated product obtained by the reaction Eq. (2), according to the equations

(EtO)3SiCH = C[Si(OEt)3]2

(6)

(EtO)3SiCH2CH[Si(OEt)3]2

(7)

VIc

VId

Table 2 shows yield and selectivity of the three most important products, III–V, and the by-products VIIa–VIId formed by the reactions presented by Eqs. (5)–(7). These results indicate that selectivity towards the formation of dehydrogenated silylation product (III) under the conditions studied is much lower for Ni(acac)2 than for the other catalysts investigated. Such a high selectivity towards the product III, as that obtained by Ni(propA)2 , Ni(TESPA)2 , SiO2 -A · Ni(acac)2 , and SiO2 -A · NiCl2 , was previously obtained only by the Ni(0) complexes or binary catalytic systems of Ni(acac)2 with phosphine (Marciniec et al., 1996). Analogously, high selectivity obtained in the presence of simple catalytic systems indicate an appealing synthetic pathway for bis(silyl)ethenes pro-

duction, used as monomers or cross-linking agents in silicone rubber. Under the reaction conditions studied, the reduction of soluble nickel catalysts to nickel precipitate as a black solid was observed, as in the case of previously studied Ni(acac)2 . In the reaction studied, immobilized catalysts showed lower activity than soluble complexes, which can be explained by the support acting as a steric hindrance. This hindrance can significantly inhibit the formation of catalytically active intermediate compounds playing an important role in this reaction (Čapka et al., 1992; Michalska et al., 1994). Immobilization of the nickel complexes did, however, stabilize the system, enabling repetitive use of the catalysts. It should be noted, however, that af-

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I. Rykowska & W. Urbaniak/Chemical Papers 62 (3) 268–274 (2008)

ter three consecutive uses, activity of the immobilized nickel complexes decreased by about 30 % without a significant change in selectivity. No significant differences were observed in the activity, selectivity, and stability between NiCl2 and Ni(acac)2 anchored on silica via the β-diketonate groups. Similarly, only small differences were observed when the obtained immobilized nickel complexes were used as packings for complexation gas chromatography (Wasiak & Rykowska, 1996).

Conclusions In this study, a successful application of diketonatefunctionalized silanes in immobilization of transitionmetal complexes on a silica surface is demonstrated. Such complexes can be used as packings for the complexation gas chromatography or as immobilized homogeneous metal-complex catalysts. On basis of elemental analysis and surface area calculations, possible structures of the complexes formed on the silica surface were proposed. Proposed use of immobilized nickel complexes as catalysts was proven. The selectivity obtained in the reaction of triethoxysilane with vinyltriethoxysilane was similar to those obtained in the presence of the Ni(0) complexes and binary systems such as Ni(II) with phosphine. Although, the investigated immobilized catalysts exhibited lower yields, they are not decomposed to metallic nickel, as in the case of soluble complexes, thus offering the possibility of multiple use. Moreover, as demonstrated in our previous DSCanalysis (Wasiak & Rykowska, 1998), immobilization increased the thermal resistance of the complexes. The catalytic systems presented in this work allow the use of bis(triethoxysilyl) derivatives of ethane and butanes as monomers and cross-linking compounds in the production of silicone polymers and modification of organic polymers. References Allum, K. G., Hancock, R. D., Howell, I. V., Lester, T. E., McKenzie, S., Pitkethly, R. C., & Robinson, P. J. (1976). Supported transition metal complexes V.: Liquid phase catalytic hydrogenation of hexene-1, cyclohexane and isoprene under continuous flow conditions. Journal of Catalysis, 43, 331–338. DOI: 10.1016/0021-9517(76)90318-3. Berendsen, G. E., Pikaart, K. A., & de Galon, L. (1980). Preparation of various bonded phases for HPLC using monochlorosilanes. Journal of Liquid Chromatography, 3, 1437–1464. Buszewski, B., Jezierska, M., Welniak, M., & Berek, D. (1998). Survey and tretrends in the preparation of chemically bonded silica phases for liquid chromatographic analysis. Journal of High Resolution Chromatography, 21, 267–281. DOI: 10.1002/(SICI)1521-4168(19980501)21:53.0.CO;2-7. Cagniant, D. (1992). Complexation chromatography. New York: Marcel Dekker.

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