Pd(II) and Rh(I) chelate complexes of the bidentate phosphino–thiourea ligand PhNHC(S)NHCH2CH2PPh2: structural properties and activity in homogeneous and hybrid catalysis

The new bifunctional ligand PhNHC(S)NHCH2CH2PPh2 (Ptu), containing the thiourea and the phosphine functions, and its Pd(II) and Rh(I) complexes were prepared. Ptu behaves as a bidentate ligand forming seven-membered chelation rings in a boat-like conformation. In the case of palladium, this was ascertained by the X-ray determination of the structure of the complexes [Pd(Ptu)2]Cl2·6CHCl3 (1), [Pd(Ptu)2Cl]Cl·EtOH (2) and [Pd(Ptu)2][CoCl4].CHCl3·2EtOH (3). In complexes 1 and 3, the two ligands are related centrosymmetrically, whereas in 2 they are folded by the same side in an umbrella arrangement. The Rh(I) complexes [Rh(cod)(Ptu)]X (X=Cl or PF6; cod=1,5-ciclooctadiene) and [Rh(cod)(Ptu)]2[CoCl4] were characterized by 31P-NMR and FT-IR spectroscopy. The catalytic activity in homogeneous hydrogenation and hydroformylation reactions of the Pd(II) and Rh(I) complexes was dependent on the counter-anion, being very low in the case of Cl− and high in the case of non-coordinating anionic groups such as CoCl4− and PF6−. The related ligand (EtO)3Si(CH2)3NHC(S)NHCH2CH2PPh2 (SiPtu) was sol-gel processed giving hybrid inorganic–organic xerogels (XGPtu). Anchored Pd(II) and Rh(I) complexes were obtained by two procedures: by treating XGPtu with solution of precursor complexes or by sol-gel processing previously prepared SiPtu complexes. Palladium complexes of the thiourea ligand (EtO)3Si(CH2)3NHC(S)NHPh (Siphtu), with different S–Pd ratios, were also sol-gel processed. The rhodium containing xerogel was found to be an active catalyst for the hydroformylation of styrene, but metal leaching occurred, even if to a limited extent. In the case of Pd-containing materials, it was ascertained that the heterogeneous hydrogenating activity depends on the presence of colloidal metal particles, which form when the metal species are not adequately surrounded by the P,S- or S-ligands.