Rh(I) complexes with N,N and N,P ligands immobilized on carbon surfaces – recyclable catalysts

Abstract

This thesis describes the study of the immobilization of Rh(I) complexes with N,N (bis(pyrazol-1-yl)methane (bpm), bis(N-methylimidazol-2-yl)methane (bim), and 4-(pyrazol-1-yl)methyl)-1-H-1,2,3-triazole (PyT)) and N,P (1-[2-(diphenylphosphino)ethyl]pyrazole (dmPyP) ligands onto carbon surfaces (glassy carbon electrode (GC), glassy carbon beads (GCB), carbon black (XC)) via robust carbon-carbon bonds. The immobilized Rh(I) complexes were employed as recoverable catalysts for hydromamination and hydroalkoxylation reactions. A series of aforementioned N,N ligands and one N,P ligand precursor were functionalized with aniline and aniline hydrochloride respectively. The treatment of the aniline-functionalized ligands and aniline hydrochloride-functionalized ligand precursor with HNO2 resulted in the transformation of the NH2 functionality into diazonium one. The electrochemical reduction of the diazonium functionality of the N,N ligands and N,P ligand precursor resulted in the surface attachment of these compounds onto glassy carbon electrode surfaces. The treatment of the surface-bound N,P ligand precursor with LiPPh2 resulted in the formation of the surface-bound N,P ligand (dmPyP). The complexation of [Rh(CO)2(μ-Cl)]2 with the immobilized N,N and N,P ligands led to the formation of immobilized Rh(I) complexes of these ligands. XPS confirmed the surface attachment of the N,N ligands and N,P ligand precursor as well as the success of the synthesis of the N,P ligand on the surface and complexation between immobilized ligands and the Rh precursor. The surface coverage of immobilized Rh(I) complexes was evaluated using CV and was found to be close to being a monolayer. The immobilized Rh(I) complexes were active as catalysts for the intramolecular hydroamination and dihydroalkoxylation reactions with extremely high TONs in comparison with their homogeneous counterparts. Studies of the recovered catalyst samples showed that Rh leached from the surface while the ligand remained intact. The aniline-functionalized N,N ligands were also attached onto glassy carbon electrodes without electrochemical assistance by dipping the electrode into a solution of aryl diazonium salt. Complexation of the ligands immobilized without electrochemical assistance and [Rh(CO)2(μ-Cl)]2 resulted in immobilized Rh(I) complexes with these ligands. XPS confirmed that the immobilized species had the same nature as those immobilized using electrochemical reduction and they were effective catalysts for dihydroalkoxylation. The surface coverage of these complexes was evaluated by means of CV and was found to be close to being a monolayer. The ligand bpm was also immobilized onto carbon powders (GCB and XC) without electrochemical assistance. The immobilized support-[Rh(bpm)(CO)2] complexes were obtained by the treatment of bpm-functionalized supports with [Rh(CO)2(μ-Cl)]2. XPS confirmed the ligand attachment onto the surface of carbon powders and complexation with Rh. TGA also confirmed the covalent immobilization of the ligand and was used to determine the surface coverage of ligand. The immobilized complex XC-[Rh(bpm)(CO)2] was active as a catalyst for the intramolecular hydroamination and dihydroalkoxylation of alkynes reactions. The recovered catalyst XC-[Rh(bpm)(CO)2] had significantly lower activity in the hydroamination and slightly decreased activity in the hydroalkoxylation reaction in comparison with the fresh catalyst. It was found that Rh leached from the surface of XC support during the catalytic reaction due to the cleavage of N-Rh bonds. The structure of a 13CO-labelled complex [Ir(PyP)(13CO)Cl] was examined using a series of solid-state NMR techniques. 1D 1H MAS, 13C and 31P CP MAS NMR spectra provided structural information similar to that obtained using NMR in solution. High resolution 2D solid-state correlation spectroscopy provided information about the dipolar coupling between protons and carbon/phosphorus. The use of the BABA pulse sequence to create and reconvert back double quantum coherences revealed the dipolar coupling contacts between immediately adjacent protons. The internuclear distance 13CO-31P was determined using REDOR. The combination of all of these techniques made it possible to obtain the additional information about the structure of [Ir(PyP(13CO)Cl] in solid state which was similar to that obtained using X-Ray crystallography.