Thermal properties of metal-metal bonded Pd ( I ) complexes supported onto porous Vycor glass

Thermal behavior of the complexes Pd2(dppm)2Cl2, Pd2(dppm)2(SnCl3)Cl and Pd2(dppm)2(SnCl3)2 (dppm = bis[diphenylphosphino(methane)], ((C6H5)2PCH2P(C6H5)2) in the solid state and immobilized onto porous Vycor glass was studied. Similar decomposition mechanisms were observed for the solid and immobilized complexes, with a small thermal stabilization upon immobilization. The decomposition products were characterized by X-ray diffractometry, Raman and diffuse reflectance infrared spectroscopy, which indicated the presence of a mixture of metallic palladium and oxidized species such as PdO, condensed phosphates, SnO2 and SnP2O7. According toX-ray diffractometry, the decomposition products of the immobilized complexes presented higher amounts of PdO than the solid-state residues, probably as an effect of interactions with silanol groups present in the glass surface.


INTRODUCTION
The study of supported organometallics and transition metal complexes is motivated mainly by applications as catalysts (Ando et al. 2004, Kureshi et al. 2004) and molecular precursors to advanced materials such as gas sensors (Alves et al. 2005, Silva et al. 2006), semiconductors, and metallic particles (Sunil et al. 1993).In particular, heterogeneous catalysts containing Pd or Pt have been studied owing to their activity in oxidation of CO, hydrogenation, (Evrard et al. 2004) and electrochemical oxidations (Yang and McElwee-White 2004).A previous paper by Richmond and co-workers reported the preparation of dinuclear phosphine-bridged palladium(I) species and their silica-bound analogues as catalysts for the cyclization of aminoalkynes, showing that in some cases the catalytic activity is improved by the higher thermal stability of the supported catalyst (Richmond et al. 2002).
The family of complexes of interest here includes the complex containing metal-metal bonded Pt(I) atoms Pt 2 (dppm) 2 Cl 2 (Glocking and Pollock 1972) and its palladium analogue, which can be used as homogeneous catalysts in several reactions (Kirss and Eisenberg 1989).The derivatives of SnCl 2 insertion into Pd-Cl bonds of Pd 2 (dppm) 2 Cl 2 also catalyze the alkoxycarbonylation of terminal alkenes (Nguyen et al. 2005).In this context, the thermal behavior of free and supported complexes is important both to evaluate the thermal stability and to identify decomposition products.However, for the complexes of interest here this aspect was still unexplored.
The interest in the reactivity of single bonded Pd 2 moieties for sensor phases is a growing area and previous attempts to immobilize similar com-pounds onto polystyrene for application in separation of gases were unsuccessful due to steric demands (Lee et al. 1986).Recently we described the immobilization of Pd 2 (dppm) 2 Cl 2 , Pd 2 (dppm) 2 (SnCl 3 )Cl, and Pd 2 (dppm) 2 (SnCl 3 ) 2 onto porous Vycor glass (Gimenez andAlves 2002, 2004), which proved to be suitable precursors for gas sensing systems (Alves et al. 2005, Silva et al. 2006).We were further interested in studying the thermal stability of these immobilized species, since the gas insertions into the metal-metal bond can be reverted both by heating and inert-gas flow.In this paper we report the thermogravimetric study of the complexes Pd 2 (dppm) 2 Cl 2 , Pd 2 (dppm) 2 (SnCl 3 )Cl, and Pd 2 (dppm) 2 (SnCl 3 ) 2 , both in the solid state and immobilized onto porous Vycor glass, as well as the characterization of the decomposition products by XRD, DRIFT, and Raman spectroscopy.

THERMOGRAVIMETRIC ANALYSIS
Thermogravimetric analyses were carried out in a TA Instruments 2960, using platinum sample holders, under the dynamic flow of dry synthetic air (20 mL min -1 ), at a heating rate of 10 K min -1 .

PYROLYSIS EXPERIMENTS
All pyrolysis experiments of solid and immobilized complexes were carried out in platinum crucibles under air for 24 h at specified temperatures.

RAMAN SPECTROSCOPY
Raman spectra were recorded on a Renishaw Raman Imaging Microscope System 3000, coupled to an optical microscope with resolution 1.5µm and a He-Ne (λ = 632.8nm) laser source.Sampling was accomplished by scanning different surface regions of the sample placed onto glass sheets.

FOURIER TRANSFORMED DIFFUSE REFLECTANCE INFRARED ANALYSIS (DRIFT)
Diffuse reflectance infrared spectra were obtained with a Nicolet 520 spectrophotometer in the spectral range 4000-400 cm -1 using KBr as reference.

RESULTS AND DISCUSION
Figure 1 shows the TG curves for the solid samples, while their corresponding data are displayed in Table I.The decomposition of 1 starts at 471 K with an abrupt mass loss up to 837 K corresponding to a 62% loss (calculated 62%), attributed to elimination of one P(Ph) 2 CH 2 Cl 2 active radical and one dppm molecule.Formation of similar radical species was reported by Zayed and co-workers in the thermal decomposition of mononuclear Pt(dppm)Cl 2 complexes in the range 393-773 K (Zayed et al. 1999).In the present case a further decomposition step can be observed above 837 K, with a mass loss of 13% attributed to the elimination of residual phosphine species and reduction of PdO eventually formed on the surface.The formation of metallic Pd residues or Pd/PdO mixtures rely on the pyrolysis conditions as well as on the presence of groups in the molecular structure able to generate reducing conditions during decomposition.
An Acad Bras Cienc (2008) 80 (2) In the TG curve of compound 2, the mass gain starting from 474 K may be attributed to oxygen uptake.The mass increase at relatively low temperatures by oxygen absorption is commonly observed for Pd and Pt complexes such as [Pd(PPh 3 ) 4 ] and [Pt(PPh 3 ) 4 ], rendering species such as CO 2− 3 and Ph 3 PO (phosphinoxide) (Barbieri et al. 1995).As verified in Figure 1, the mass gain is followed by a gradual mass loss from 546 K to 968 K.No clear plateau can be defined but the curve derivative shows a broad feature from 546 to 822 K corresponding to a 49% loss.Probably the decomposition involves elimination of P(Ph) 2 CH 2 Cl 2 plus one entire dppm (expected loss: 52%).
For compound 3 the initial 2% loss due to elimination of water suggests the presence of SnCl 2 (hygroscopic) as an impurity.When this compound is put into CH 2 Cl 2 solution the following chemical equilibrium takes place (Olmstead et al. 1979): A large SnCl 2 excess is necessary in the synthesis leading to the presence of SnCl 2 as a product impurity.Exhaustive purification attempts by recrystallization will form compound 2 and, in fact these limitations precluded the crystal structure determination for this compound, since its earliest report (Olmstead et al. 1979).According to TG curve, compound 3 degrades with continuous mass loss from 413 K up to 916 K.In the range 586-683K the mass loss observed (24%) is coherent with loss of a P(Ph) 2 CH 2 Cl 2 radical plus a Cl 2 molecule (calculated 24%).The decomposition of the residue is gradative and incomplete with a final mass percent of 48%.The decomposition products were characterized by XRD, DRIFT and Raman.According to X-ray diffraction, Figure 2, the main decomposition product of solid complex 1 is metallic palladium (2θ = 40.1 • , 46.6 • and 68.1 • (JCPDS)).For the complexes 2 and 3, residues contain SnO 2 (2θ = 26.5 • , 34 DRIFT and micro-Raman spectroscopies suggested the presence of additional phases, mostly in the case of 1.For 1, the Raman spectrum (Fig. 3) shows the typical features of PdO (Chan and Bell 1984), probably not detected by X-ray diffraction due to its concentration on the surface.For compounds 2 and 3, SnO 2 bands near 630 cm -1 dominate the Raman spectra (Fig. 3).For all samples, spectra of some surface regions show a pair of broad bands at 1330 cm -1 and 1580 cm -1 indicating amorphous carbon residues (Macedo et al. 2008).DRIFT spectrum of 1 (not shown) showed a broad band around 1000 cm -1 suggesting the presence of condensed phosphates.For 2 and 3, SnP 2 O 7 already detected by XRD was confirmed due to the bands at 1160 cm -1 and 1026 cm -1 , assigned to ν sym (P-O) and ν asym (P-O) from terminal PO 2− 3 groups, respectively, and at 748 cm -1 assigned to ν sym (P-O) from P-O-P bridges (Hubin andTarte 1967, Guan et al. 2005).Also a strong band at 1280 cm -1 can be assigned to intra-chain PO 2 moieties from condensed phosphates.
Thermal behavior of immobilized complexes was also studied.Figure 4 shows the TG curves of the PVGsupported complexes and for all samples the initial mass loss from 298 K to 423 K is due to elimination of adsorbed water from the glass surface.Up to 25% of PVG "dry mass" can be composed by adsorbed water as the pore surface is composed mainly by the very reactive silanol (Si-OH) groups (Hood andNordberg 1938, 1942).At higher temperatures all mass losses are attributed to decomposition of the complexes and from these curves as well as from the mass loss observed for solid samples, it is possible to estimate the amount of adsorbed complexes.The related data are displayed in Table II.The thermal decomposition of PVG/1 starts at 449 K with loss of 1%, followed by 1%.The initial decomposition temperature of the supported complex is slightly higher than in the solid state, indicating a stabi- lization of the supported complex.The thermal decomposition of PVG/2 starts at 510 K, with a deflection at 563 K and a gradual mass loss up to 919 K. Finally, for PVG/3 mass losses at 488 K, 573 K and 740 K can be observed, slightly higher than in the solid state.As glass signals dominated the DRIFT and Raman spectra of PVG samples, only XRD was able to detect residues of the complexes.Interestingly in this case PdO peaks are observed in addition to metallic Pd for all samples, suggesting that the close contact of the complexes with silanol groups facilitates the formation of PdO, in contrast to the behavior of the solid complexes.For PVG/2 and PVG/3 the peaks of metallic palladium are weak in comparison to SnO 2 and SnP 2 O 7 peaks.
From the results of thermal analysis and characterization of the decomposition products, we conclude that the thermal stability of the complexes is improved by immobilization onto porous Vycor glass and the composition of the residues presents some differences.The decomposition mechanism is complex and involves concomitant processes such as elimination of P(Ph) 2 CH 2 Cl 2 radicals and Cl 2 molecules.The residues, both in the solid state and embedded in the glass, consist of a mixture of compounds such as oxides, phosphates and metallic palladium.