Synthetic, electrochemical and kinetic studies of phosphinites and their rhodium(I) complexes

Abstract

English: Organophosphorus ligands of the type, C6H5XPPh2, where X = O, S and NH, meta- and para-Ph2POC6H4OPPh2, were synthesised by the reaction between chlorophenylphosphine and the desired phenyl substituent in the presence of a base. The rhodium(I) complexess, [Rh(acac)CO(C6H4XPPh2)] where X = O, S and NH, as well as [(Rh(acac)CO(Ph2POC6H4-m-OPPh2)] and [(Rh(acac)CO(Ph2POC6H4-p-OPPh2)], were obtained by treating [Rh(acac)(CO)2] with the appropriate organophosphorus ligands. Kinetic results for the oxidative addition of methyl iodide to the rhodium(I) complexes of the type, [Rh(acac)CO(C6H4XPPh2)] where X = O, S and NH, revealed that the reaction exhibited only the oxidative addition. The reaction was found to be first order dependent on the concentration of the methyl iodide. The large negative activation entropy vales that were obtained, suggested an associative mechanism and no observable solvent pathway was found. It was found that as the Pauling electronegativity of O, S and N, increased, the first order rate constant of the oxidative addition decreased. The cyclic voltammetry of the organophosphorus ligands, C6H5XPPh2 where X = S and NH, meta- and para-Ph2POC6H4OPPh2, was found to be chemically and electrochemically irreversible. Only one oxidation and one reduction peak was observed and the oxidation peak was assigned to the one-electron oxidation of the free electron-pair on the phosphorous moiety, while the reduction peak was assigned to the reduction of the radical cation back to the neutral form. The rhodium complexes to which the phosphorus ligand was bound, showed two oxidation peaks of which the first one, O1, was assigned to the rhodium metal center and the second, O2, to the phosphorus moiety. Similar to the ligands system, the rhodium complex system is both chemically and electrochemically irreversible. The reduction peaks (R1) and (R2) assigned reduction of the Rh(III) back to Rh(I) in a two electron process and the second reduction peak (R2), which is the reduction of the radical cation of the organophosphorus ligand back to the neutral form (with the free electron-pair on the phosphorous moiety), respectively.

Afrikaans: Organofosfaat ligande van die vorm, C6H5XPPh2, waar X = O, S en NH, meta- en para-Ph2POC6H4OPPh2 is gesintetiseer deur die reaksie tussen chlorofenielfosfien en die gewenste feniel substituent in die teenwoordigheid van 'n basis. Die rodium (I) komplekse, [Rh(acac)CO(C6H4XPPh2)] waar X = O, S en NH, sowel as [(Rh(acac)CO(Ph2POC6H4-m-OPPh2)] en [(Rh(acac)CO(Ph2POC6H4-p-OPPh2)], is verkry deur die behandeling van [Rh(acac)(CO)2] met die toepaslike organofosfaat ligande. Kinetika resultate vir die oksidatiewe addisie van metiel jodied aan die rodium (I) komplekse van die formaat, [Rh(acac)CO(C6H4XPPh2)] waar X = O, S en NH, het aan die lig gebring dat die reaksie slegs oksidatiewe addisie ondergaan het. Daar is gevind dat die reaksie eerste orde afhanklikheid toon van die konsentrasie van die metiel jodied. Die groot negatiewe aktiverings entropie wat verkry is, stel 'n assosiatiewe meganisme voor en geen waarneembare oplosmiddel pad is gevind nie. Daar is gevind dat soos die Pauling elektronegatiwiteit van O, S en N, toegeneem het, die eerste orde tempo konstante van die oksidatiewe addisie afneem. Die sikliese voltammetrie van die organofosfaat ligande, C6H5XPPh2 waar X = S en NH, meta- en para-Ph2POC6H4OPPh2, toon aan dat hulle chemiese en elektrochemiese onomkeerbare redolsprosesse ondergaan. Slegs een oksidasie en een reduksie piek is waargeneem. Die oksidasie piek is aan die een-elektron oksidasie van die vrye elektron-paar op die fosfor groep toegeken, terwyl die reduksie piek aan die reduksie van die gesolveerde ontbinde radikaalkatioon toegeken kan word. In die geval van die rodium komplekse is twee oksidasie pieke waargeneem waarvan die eerste een, O1, toegeskryf word aan die rodium metaal sentrum en die tweede, O2, aan die fosfor eenheid. Soortgelyk aan die ligande, was redolsprosesse van die rodium komplekse beide chemies en elektrochemies onomkeerbaar. Die reduksie piek R1 is toegeken aan die reduksie van „n Rh(III) spesie terug na Rh (I) in 'n twee-elektron oordrag proses en die tweede reduksie piek (R2) is die reduksie van die metal-gestabiliseerde radikaalkatioon van die organofosfaat ligand terug na die neutrale vorm (met die vrye elektron - paar op die fosfor eenheid), onderskeidelik.