**3. Synthetic procedures of (olefin)Pt(PPh3)2 complex**

The literature has described a number of synthetic methods [11, 17] that led to the production of complexes of the type (olefin)Pt(PPh3)2. There are few basic procedures for the synthesis of these organometals. The usual starting substance is either chloride of bis(triphenylphosphine)platinum(II), ethylene‐*bis*(triphenylphosphine)platinum(0), tris(triphenylphosphine) platinum, or tetrakis(triphenylphosphine) platinum. In particular, the following reactions are involved: the reduction reaction of Pt(II)‐complexes (e.g., chloride of bis(triphenylphos‐ phine)platinum) and the exchange of alkenic ligands in Pt(0)‐coordination compounds, such as [Pt(PPh3)2(C2H4)]. The majority of such synthesized complexes, however, exhibited a very low stability (**Figure 4**).


**Figure 2.** Orbital description of the bonds in the olefin-metal models: (1) Dewar-Chatt-Duncanson model derived for

The literature has described a number of synthetic methods [11, 17] that led to the production of complexes of the type (olefin)Pt(PPh3)2. There are few basic procedures for the synthesis of these organometals. The usual starting substance is either chloride of bis(triphenylphos-

organometallic compounds; (2) Newns-Anderson model derived for surface complex substrate-catalyst.

252 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 3.** General order of coordinated olefins stability.

**3. Synthetic procedures of (olefin)Pt(PPh3)2 complex**


**Figure 4.** A scheme of preparation methods of (olefin)Pt(PPh3)2 complexes.

A series of model complexes of (olefin)Pt(PPh3)2 type were prepared. The preparation proce‐ dure consisted in reductive reactions of Pt (II)‐complexes (*cis*‐dichloro‐bis(triphenylphos‐ phane) platinum(II)), alternatively in the exchange of olefinic ligands in Pt‐(0)‐coordination compounds η‐ethylene‐*bis*(triphenylphosphane) platinum(0)). The syntheses were carried out in the absence of atmospheric oxygen and moisture, using the so‐called Schlenk techniques (**Figure 5**). In **Figure 6**, the ligands of the synthetically prepared complexes of the type (olefin)Pt(PPh3) are depicted, and clearly imply that the prepared complexes could be charac‐ terized as substituted ethylene with electron‐accepting substituents, which tend to stabilize the produced complex by shifting the energy levels of HOMO/LUMO in olefin toward the levels of Pt(PPh3)2. By contrast, the groups increasing the electron density in this place destabilize the complex to such a degree that its preparation is not possible. The example could be complexes (alkenic alcohol)Pt(PPh3) that have never been spectroscopically proven in the reaction mixtures. The reason could be sought in a low stability of these complexes probably caused by a hydroxyl group present in molecules of an alkenic ligand, which by increasing the electron density on C=C bond destabilizes the potentially produced complex. **Figure 6** captured the theoretically calculated stability order of the group of ethylene derivatives bearing typical smaller substituents, showing a good correlation with the stability of the prepared complexes, which were experimentally verified by NMR "*in situ*" competitive experiments.

**Figure 5.** The apparatus used for the synthesis of (olefin)Pt‐(0)‐(PPh3)2 complexes.

NMR spectroscopy is one of the most important techniques for the study of substances in a solution. It may therefore be somewhat surprising to combine NMR in the liquid phase with heterogeneous catalysis, but this connection is possible and the results obtained by NMR liquid phase can be used for the interpretation of phenomena related to heterogeneous catalysis. However, it is necessary to find a suitable system by which the interaction of active site‐reaction center can be approximated, and which also satisfies the condition of NMR experiments in liquid phase. If this condition is met, it is possible to monitor what is happening at the atomic level, thereby obtaining valuable information for heterogeneous catalysis.

Organometallics: Exploration Tool for Surface Phenomena in Heterogeneous Catalysis http://dx.doi.org/10.5772/65337 255

(**Figure 5**). In **Figure 6**, the ligands of the synthetically prepared complexes of the type (olefin)Pt(PPh3) are depicted, and clearly imply that the prepared complexes could be charac‐ terized as substituted ethylene with electron‐accepting substituents, which tend to stabilize the produced complex by shifting the energy levels of HOMO/LUMO in olefin toward the levels of Pt(PPh3)2. By contrast, the groups increasing the electron density in this place destabilize the complex to such a degree that its preparation is not possible. The example could be complexes (alkenic alcohol)Pt(PPh3) that have never been spectroscopically proven in the reaction mixtures. The reason could be sought in a low stability of these complexes probably caused by a hydroxyl group present in molecules of an alkenic ligand, which by increasing the electron density on C=C bond destabilizes the potentially produced complex. **Figure 6** captured the theoretically calculated stability order of the group of ethylene derivatives bearing typical smaller substituents, showing a good correlation with the stability of the prepared complexes, which were experimentally verified by NMR "*in situ*" competitive

254 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 5.** The apparatus used for the synthesis of (olefin)Pt‐(0)‐(PPh3)2 complexes.

level, thereby obtaining valuable information for heterogeneous catalysis.

NMR spectroscopy is one of the most important techniques for the study of substances in a solution. It may therefore be somewhat surprising to combine NMR in the liquid phase with heterogeneous catalysis, but this connection is possible and the results obtained by NMR liquid phase can be used for the interpretation of phenomena related to heterogeneous catalysis. However, it is necessary to find a suitable system by which the interaction of active site‐reaction center can be approximated, and which also satisfies the condition of NMR experiments in liquid phase. If this condition is met, it is possible to monitor what is happening at the atomic

experiments.

**Figure 6.** Synthetically prepared complexes of (olefin)Pt(PPh3) type and their relative stability determined by NMR "*in situ*" experiments.

The developed NMR *"in situ"* method of competitive production of model (olefin)Pt(PPh3) complexes is a significant progress toward assessing the stability of adsorbed complexes, particularly allowing to express the relative stability of a complex to be produced. The homogeneous complex of ethylene *bis*(triphenylphosphine)platinum(0) was used as the suitable model for studying the interaction of active site‐reaction center, which was capable of forming coordination compounds with ligands of the CC double bond. As mentioned above, ethylene coordinated to platinum could be exchanged for other unsaturated substrates. To test this concept, experiments with esters of fumaric acid and maleic were primarily conducted, that is, with dimethyl fumarate and diethyl maleate. Using these substrates, the influence of *cis*/*trans* isomerism on the CC multiple bond was studied. Both substrates were able to displace ethylene from the complex and form a stable complex with platinum, which was manifested by a splitting of the previous singlet signal of olefinic (**Figure 7**).

**Figure 7.** Fragment of 1 H NMR spectra of diethyl maleate bis(triphenylphosphine)platinum(0) complex.

Wherever the equimolar mixture of both ligands was added to the solution of ethylene *bis*(triphenylphosphine)platinum(0), the formation of the equimolar quantities of both com‐ plexes of esters with platinum was prevented, but a complex with dimethyl fumarate was preferentially produced. It could be assumed that the resulting ratio reflected the different adsorption properties of both ligands. In order to verify that it was the case of a behavior resulting from an inter‐displacement of ligands, an experiment was conducted, in which first the complex with diethyl maleate was prepared and subsequently dimethyl fumarate added. Even under these circumstances, the ligand (diethyl maleate) displacement took place as well as the stabilizing of a ratio corresponding to the situation of a simultaneous addition of both ligands (**Figure 8**). This method has a certain limit related to a small dy‐ namic range of NMR measurements, and thus it is necessary to compare substrates as de‐ scribed above with near adsorptivities. On the other hand it implies that even minor changes in the absorptivity of substrate molecules can be registered using this procedure. In a series of synthesized complexes of (olefin)Pt(PPh3), the complexes with a near stability have always been selected, and NMR "*in situ"* competitive measurements performed. Based on them, a relative stability of the prepared complexes was determined. As shown below, this sequence very well correlated with calculated bonding energies of the listed complexes.

Organometallics: Exploration Tool for Surface Phenomena in Heterogeneous Catalysis http://dx.doi.org/10.5772/65337 257

**Figure 8.** Selected 1 H NMR spectra in the course of competitive displacement of ligands (diethyl maleate and dimethyl fumarate).
