Tungsten hydride complex as a template in organic–inorganic hybrid materials

Solid State Sciences 5 (2003) 519–523 www.elsevier.com/locate/ssscie

Tungsten hydride complex as a template in organic–inorganic hybrid materials Isilda Montinho, Victor Boev, António M. Fonseca, Carlos J.R. Silva, Isabel C. Neves ∗ IBQF/Departamento de Química, Universidade do Minho, 4710-057 Braga, Portugal Received 14 October 2002; accepted 30 October 2002

Abstract A tungsten hydride complex, [WH2 (η2 -OOCCH3 )(Ph2 PCH2 CH2 PPh2 )2 ][BPh4 ], was dispersed in a hybrid matrix synthesized by a sol– gel process. The host matrix of the so-called ureasil is a network of silica to which oligopolyoxyethylene chains [POE, (OCH2 CH2 )n ] are grafted by means of urea cross-links. The free complex and sol–gel materials were characterized by thermal analysis (DSC) and spectroscopic methods (FT-IR and UV/Vis). The data gathered indicate that the tungsten(IV) complex is immobilized in the host matrix, and it exhibits structural properties different from those of the free form. These differences could arise either from distortions caused by steric effects imposed by the structure of hybrid matrix or by interactions with the matrix.  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Tungsten(IV) complex; Organic–inorganic materials; DSC; FTIR

1. Introduction One of the current interesting areas of catalysis research are the immobilization of metal complexes onto solid supports such as zeolites and other porous-structured materials like hybrid organic–inorganic materials produced by the sol– gel process [1]. This heterogenization often combines the homogeneous catalytic properties of metal complexes with some characteristics of the heterogeneous catalysts, such as shape selectivity and/or separation from reaction media. The host matrix, designated as U(600), is an organic– inorganic network material, classed as a ureasil that combines a reticulated siliceous backbone linked by short polyether-based segments. Urea bridges form the link between these two components, and the polymerisation of silicate substituted terminal groups generates the inorganic network. Metal complexes have been used as templates for the structure of host inorganic–organic matrix, inducing the interaction with macromolecular host structure and contributing to increase the cohesion and order of the resulting material [2].

* Corresponding author: Departamento de Química, Escola de Ciências, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal. E-mail address: [email protected] (I.C. Neves).

The sol–gel method is an attractive route to obtain new materials with high purity and homogeneity because it is performed at lower processing temperature. The materials offer substantial advantages over other techniques, in particular: (i) high surface area and (ii) low densities, together with a wide thermal and chemical stability [3]. A new class of hybrid organic–inorganic materials called di-ureasil (ureasilicates) were used in this work [4]. The organic component comprises poly(oxyethylene) [POE], chains covalently bonded via urea linkages to a siliceous backbone. In the reaction the organic-modified alkoxide (ICPTES) and the amine-substituted oligopolyoxyethylene (Jeffamine, ED-600®), are made to react in a mole proportion of 2 to 1.

Fig. 1. Molecular structure of tungsten hydride complex.

1293-2558/03/$ – see front matter  2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(03)00018-9

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Fig. 2. Structure of the ureasil precursor.

In this present paper, we describe the preparation of new materials obtained by dispersing a tungsten hydride complex, [WH2 (η2 -OOCCH3 )(Ph2PCH2 CH2 PPh2 )2 ][BPh4 ], (Fig. 1) into a high molecular weight di-ureasil structure, U(600), which contains about 8.5 oxyethylene repeating units (Fig. 2). The macromolecular structure is composed by oligopolyoxyethylene chains grafted onto a siliceous network through urea bridges [–NHC(=O)NH–] [5]. The material obtained was an amorphous, transparent monolith and was characterized by thermal analysis (DSC), infrared (DRIFT) and UV-visible spectroscopy. Our data indicate that the tungsten hydride complex is immobilized inside the matrix, and the guest exhibits structural properties different from those of the free complex. These differences could arise from inter-molecular interactions between the metal complex and some groups/atoms of the hybrid host matrix.

UPTES solution. The immobilization of the metal complex is achieved simultaneously by the hydrolysis and condensation reactions. The addition of a small amount of water (0.5 ml) to the obtained solution was done with the objective to start the hydrolysis and condensation reactions [7].

2. Experimental

2.2.2. Step 2. Immobilization of tungsten hydride complex in the hybrid matrix An appropriate mass of metal complex (Table 1) was dissolved in THF (5.0 ml) by using an ultrasound bath for 10 min. Different volumes of metal complex solution (0.0021 M) were added to the solution prepared in step 1. The mixture was stirred for 30 min and when this time had elapsed 0.5 ml of water were added. The mixture was stirred for 30 min, and then poured in a Teflon® mould, covered with Parafilm® and left in an atmosphere of ammonia for 24 h. Once the gelification had begun, the mould was kept in a fume cupboard, for a period of 96 h. The obtained material was a flexible, non-rigid and brittle homogeneous transparent film with pink coloration. The ureasil samples, which correspond to the pure matrix, are denominated as U(600). Using the same nomenclature the prepared material containing the immobilized metal complexes is denominated as U(600)n [WH2 (η2 -OOCCH3 ) (dppe)2][BPh4 ] where n represents the molecular ratio be-

2.1. Reagents and solvents Jeffamine ED-600 (Fluka), a polyethylene oligopolymer with substituted amine terminal groups and a silicon alkoxide with a substituted isocyanate group, the 3-isocyanatepropyltriethoxysilane (ICPTES, Aldrich) were used in the preparation of the matrix. Both of the reagents were dried under dynamic vacuum during 30 min immediately before used. Ethanol (CH3 CH2 OH, Merck) and tetrahydrofuran (Merck) were stored over molecular sieves. The tungsten hydride complex, [WH2 (η2 -OOCCH3 )(Ph2 PCH2 CH2 PPh2 )2 ][BPh4 ] was prepared according to the published procedures [6]. High purity water with an electric resistivity of around 18 M S−1 cm−1 was used in all experiments. 2.2. Preparation of hybrid matrix containing the complex In the first stage of the synthesis of sol–gel material, a covalent bond between the alkoxysilane precursor (ICPTES) and the oligopolyoxyethylene was formed by the reaction of the isocyanate group (from ICPTES) with the terminal functional amines (Jeffamine) in tetrahydrofuran, THF. A urea cross-linked organic-inorganic hybrid precursor socalled ureapropyltriethoxysilane, UPTES was thus obtained (Fig. 2). In the second stage of synthesis, a controlled volume of a metal complex solution in THF was incorporated in the

2.2.1. Step 1. Synthesis of the ureasil precursor, UPTES The synthesis was performed in a fume cupboard. A stoichiometric proportion of 1 mol of O,O  - bis(2-aminopropyl)-polyethylene glycol-600, (Jeffamine ED-600®, 0.5 ml, 0.875 mmol) to 2 mol of 3-isocyanatepropyltriethoxysilane (ICPTES, 0.435 ml, 1.76 mmol) was dissolved in tetrahydrofuran (THF, 2.0 ml). The solution was stirred in a sealed glass flask for 30 min. The grafting process was monitored by infrared spectroscopy by using the extinction of the strong band at 2277 cm−1 , assigned to the stretching vibration of the R3 –Si–(CH2 )3 –NCO group [5].

Table 1 Metal complex concentration in the prepared sol–gel materials Samples U(600)95 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] U(600)237 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] U(600)474 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] U(600)709 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] U(600)945 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ]

Mole of complex V (µl) (10−6 mol) 9.23 3.69 1.85 1.24 0.92

4395 1759 880 588 440

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tween the Jeffamine (the limiting reagent used in the ureasil synthesis) and the metal complex (Table 1).

3. Results and discussion 3.1. Thermal analysis Complementary studies using differential scanning calorimetry (DSC) technique contributed to the better understanding of the effect of tungsten complex concentration on the materials final properties. Samples obtained from ureasil films were sealed within a 0.2 ml aluminum pan inside a preparative glovebox under argon atmosphere. Thermal analyses were carried out with a Mettler TC11 controller and a DSC20 Mettler oven equipped with a cooling accessory under high purity argon supplied at a constant 50 ml min−1 flow rate. All samples were subjected to a 10 ◦ C min−1 heating rate and were characterized between 25 and 350 ◦ C. The DSC thermograms of U(600)n [WH2 (η2 -OOCCH3 ) (dppe)2][BPh4 ] materials and pure hybrid matrix U(600) (n = ∞) are presented in Fig. 3. The DSC scan obtained for the hybrid matrix U(600), (n = ∞) shows a broad endothermic peak around 60 ◦ C due to water evaporation and a smooth line up to 350 ◦ C, is observed with increasing temperature. The presence of one exothermic peak around 180 ◦ C may be assigned to the reorganization of partially fused oxyethylene chains.

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This result indicates that the materials are thermally stable throughout the temperature range studied. The DSC curve obtained on a second scan, traced immediately after the sample cooled down to 25 ◦ C, does not show the broad endothermic peak, assigned to evaporation, neither an exothermic peak observed at 180 ◦ C. On the same thermogram is observed a small intensity endothermic peak at 30 ◦ C and signs of oxidative decomposition of the material at temperatures higher then 300 ◦ C. In all DSC traces of the samples, the presence of a sharp peak recorded at a temperature around 30 ◦ C may be associated with the fusion of macromolecules containing (CH2 CH2 O) segments of the polymer [5]. Samples with higher tungsten complex concentrations (n = 95 and n = 237) also show an endothermic peak in the temperature range between 80 and 120 ◦ C. The peak position shifts with complex concentration. Others authors [7] had confirmed that similar pattern was observed on identical materials assigning this behavior to strong interactions between the polymer and inorganic compounds. Future studies are necessary to be done to confirm if similar interactions are also responsible for the nature of the observed features. DSC scans of samples with lower tungsten complex concentration (n = 947 and n = 709) show an exothermic peak at 180 ◦ C, identical to that observed on the first DSC scan of the hybrid matrix U(600) (n = ∞). This behavior similarity may be explained by the fact that the tungsten complex is so low that material thermal properties are dominated by matrix morphology. At temperatures above 300 ◦ C DSC scans indicate that several thermal processes take place, with higher intensities that those observed from the pure matrix, suggesting that the tungsten complex contributes to matrix destabilization. From the obtained results we observed that at low tungsten complex concentration the extension of thermaloxidative (exothermic) processes are more intense than recorded for the pure matrix, but at higher concentrations thermal processes are less intense and predominately endothermic. This behavior suggest that the presence of tungsten complex contribute to matrix stabilization and this probably due to the increase of localized chemical interactions between atoms from the tungsten complex and those from the matrix structure. 3.2. Infra-red spectroscopy

Fig. 3. DSC traces of sol–gel materials with tungsten hydride complex.

The IR spectra of free complex and sol–gel materials were obtained from powered samples without dilution at room temperature, using a Bomem MB104 spectrometer in the 4000–400 cm−1 ranges. The samples were recorded using a reflectance (DRIFT) cell with a maximum resolution of 4 cm−1 by averaging 20 scans. The reflectance spectra were calculated as the ratio (R) of sample reflectance to that

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Fig. 4. Infrared spectra of (a) U(600), (b) U(600)709 [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] and (c) [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ].

of background and expressed as f (R) = (1 − R)2 /2R, the Kubelka–Munk function. The infra-red spectra of hybrid matrix U(600), U(600)709[WH2 (η2 -OOCCH3 )(dppe)2][BPh4 ] and free complex in the spectral region 2500 to 600 cm−1 are shown in Fig. 4. The spectrum of free complex is dominated by the bands characteristic of diphenylphosphine ligands (1480, 1435, 735 and 695 cm−1 ) and the BPh4 anion (1100, 744 and 720 cm−1 ) [8]. Other important bands are observed at: 1580 and 1412 cm−1 , which are assigned to carboxylate groups and the broad bands at 1955–1815 cm−1 are attributed to hydride groups. The spectra of the sol–gel materials are dominated by bands due to the hybrid matrix U(600): the strongest band at 1160 cm−1 is attributed to N–CO–N stretching and bands corresponding to the vibration modes associated with the matrix are observed in the spectral region between 1050– 800 cm−1 . Others bands have been described elsewhere [9– 11]. The material containing the immobilized complex did not show any change in the bands assigned to matrix. This result indicates that the metal complex has been immobilized in the hybrid host and primarily appears to have kept its integrity. The IR bands of sol–gel materials with the complex occur at frequencies shifted around 10–15 cm−1 from those of the free complex and the spectrums show the absence of characteristic groups, like: the BPh4 anion and C–H deformation from diphenylphosphine. We observed the same shift in the spectra for different concentrations of the metal complex. These variations in band frequency can also be attributed to (i) the distortion of the metal complex and (ii) the interactions between the ether oxygen groups of the

polyethylene chain segments of the hybrid matrix with the complex ligands. 3.3. UV/vis spectroscopy Reflectance UV/vis spectra of free complex and sol–gel materials were obtained from solid sample films and were recorded on a Shimadzu UV/2501PC spectrophotometer at room temperature in the range 800–200 nm using a matrix as a reference. The UV/vis spectroscopy technique can provide information on the immobilized metal complexes and on the ureasil structure. Electronic data for the free complex and the sol–gel materials are summarized in Table 2. The electronic spectrum of tungsten complex exhibits one intense, high-energy band at λmax = 320 nm (14100 dm3 mol−1 cm−1 ) and a less intense broad band at λmax = 500 nm (5100 mol dm3 mol−1 cm−1 ). The band of higher energy was assigned to π –π ∗ ligand transitions and the broad band in visible region at to attributed to metal-ligand charge transfer transitions, dπ (WII ) → π ∗ (LD) [LD = diphenylphosphine ligand]. Room temperature electronic spectra of hybrid matrix U(600) are very similar to those observed in sol–gel materials with similar matrices [12]. Table 2 Electronic spectral data for the samples Samples

λmax (nm)

[WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] U(600) 250a U(600)n [WH2 (η2 -OOCCH3 )(dppe)2 ][BPh4 ] 250a

300a

a π –π ∗ electronic transitions and

320a 300a

500b

320a

b d –π ∗ electronic transitions. π

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When the spectra of sol–gel materials with tungsten hydride complex is compared with those of the free complex, the high-energy band is observed and the band of lower energy is not observed probably due to the low concentration of the metal complex in the matrix.

4. Conclusion By combining thermal analysis and spectroscopic data it was possible to conclude that when the tungsten hydride complex was used a template it allowed us to obtain a new material by sol–gel technique. The films were found to show good mechanical proprieties and were highly amorphous transparent monoliths. Their excellent optical transparency was maintained over a large range of tungsten complex concentration. Furthermore, the analysis data suggest that the structure exhibits a framework topology similar to that of the ureasil structure. However, there is some evidence for host–guest interaction between the tungsten complex and the matrix. References [1] G.-J. Kim, J.-H. Shin, Tetrahedron Lett. 40 (1999) 6827;

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