Carbonyl-phosphine hetero-ligated half-metallocene iron(II) catalysts for living radical polymerization: concomitant activity and stability

Two neutral ligands, carbonyl (CO) and phosphine, were cooperatively incorporated into half-metallocene iron(II) complexes (CpFeBr(CO)(PR 3 ); Cp=C 5 H 5 ; PR 3 =PPh 3 , P(OPh) 3 , PMePh 2 , PMe 2 Ph, P( n -Bu) 3 ) for more active and versatile systems in transition metal-catalyzed living radical polymerization. For methyl methacrylate (MMA) with a bromide initiator [Me 2 C(CO 2 Me)CH 2 C(Me)-(CO 2 Me)Br; Me=CH 3 ] [H-(MMA) 2 -Br], these hetero-ligated catalysts are superior, in terms of catalytic activity and molecular weight control, to similar homo-ligated half-metallocenes carrying two identical ligands such as CpFeBr(CO) 2 and CpFeBr(PR 3 ) 2 . Among the CpFeBr(CO)(PR 3 ) complexes examined, CpFeBr(CO)(PMePh 2 ) showed the highest activity and the best controllability (>90% conversion within 24 h; M w / M n =1.29), and the ‘living’ character of the polymerizations therewith was proved by sequential monomer addition experiments. In spite of the high activity, the Fe(II) complex is stable and robust enough to be handled under air, rendering it suitable for practical use. The concomitant high activity and high stability were attributed to the in situ generation of a real active catalyst with a 16-electron configuration by the irreversible release of the CO group from CpFeBr(CO)(PR 3 ) on the activation of a terminal C–Br bond, as confirmed by the Fourier transform infrared monitoring of model reactions with the initiator as a dormant-end model compound. Two neutral ligands, carbonyl and phosphine, were cooperatively incorporated into half-metallocene iron(II) complexes [CpFeBr(CO)(PR 3 )] for more active and versatile systems in transition metal-catalyzed living radical polymerization. Among the CpFeBr(CO)(PR 3 ) complexes examined, CpFeBr(CO)(PMePh 2 ) showed the highest activity and the best controllability, and the ‘living’ character of the polymerizations therewith was proved by sequential monomer-addition experiments. In spite of the high activity, the Fe(II) complex is stable and robust enough to be handled under air, rendering it suitable for practical use.