Synthesis Of Ionic P.N-hybrid Ligands And The Correspondin Pd/Rh/Au Transition Metal Complexes

P,N-hybrid ligands are known as one type of hemilabile ligands in which the soft P-donor firmly bind to the transition metal center for protection and stabilization while the hard N-donor can reversibly bind to the transition metal center. The release of N donor from the metal center can liberate a vacant site for the substrate insertion. The presence of the hemilabile ligand in a transition metal complex will significantly influence the corresponding catalytic performance. Hence, P,N-hybrid ligands and their applications in coordination chemistry have drawn much attention of the chemists in homogeneous catalysis.In this thesis, a serials of ionic P,N hybrid imidazolium-based ligands in type-Ⅰ hace been synthesized for the first time (1-6, Scheme1), in which each phosphorus atom in1-6is neighbored to the positive charged imidazolium ring. All these ionic P,N-hybrid ligands were fully characterized by1H/31P NMR, FT-IR and single crystal X-ray diffraction. As for1-6, due to intensive electro-withdrawing effect from the positive-charged imidazolium ring, the resultant ionic phosphines are featured with both a-donoting ability and π-accepting ability. In addition, the introduction of a different substituent at3-N position of imidazolium can synergetically influence the configuration and the catalytic performance of the corresponding transition metal complexes. It is found that the introduction of the substituent like pyridyl with the electron-withdrawing effect in3-N position of imidazolium can increase the bond length between2-C of imidazolium and phosphorus, giving rise to the possible formation of the N-heterocyclic carbene (NHC) ligand, and the conjugative effect between phosphorus and the positive-charged imidazolium can partially transfer the positive charges to phosphorus atom, resulting in the electrostatic interaction between N-donor and P-donor.Based on the availability of the prepared ionic P,N-hybrid ligands, the corresponding Pd/Rh/Au complexes were synthesized (Scheme2), which were fully characterized by1H/31P NMR, FT-IR, TG/DTQ CHN-elemental and single crystal X-ray diffraction. It is found that the π-backing bonding interactions from the metal to the phosphorus are universally present in the corresponding Pd/Rh/Au complexes, due to the π-accepting ability of the involved phosphine vicinal to the positive-charged imidazolium. On the other hand, the steric hindrance effect and electronic effect of N-donor in3-N position of imidazolium ring can significantly influence the C-P bonding interaction and consequently change the geometry, stability and catalytic performance of the corresponding transition metal complex. As a result, a novel NHC-ligated Pd-complex of12could be obtained under the controlled reaction conditions.The catalytic performance of the developed Pd-complexes was investigated in carbonylative Heck reactions of iodobenzene with styrene. The results show that, the NHC-ligated Pd-complex of12exhibits more efficient activity than the other Pd-complexes including traditional trans-PdCl2(PPh3)2. However,12is not stable enough and susceptibly undergoes deactivation in the reaction process, due to the dehydration of hydroxyl of PPhaOH.The catalytic performance of the developed ionic Rh-complexes was investigated in hydroformylation of1-octene. The relationship between catalytic performance and the structure of catalyst was carefully investigated. The results show that the shorter Rh-P bond length in the Ru-complex can suppress the replacement of the phosphine ligand by CO, which favors the selective formation of nonanals in hydro formylation of1-octene. Hence, the ionic Rh-complexes of21and23containing the shortest Rh-P bonds lengths correspond to the best yields of nonanals.