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

Abstract

Two neutral ligands, carbonyl (CO) and phosphine, were cooperatively incorporated into half-metallocene iron(II) complexes (CpFeBr(CO)(PR 3 ); Cp¼C 5 H 5 ; PR 3 ¼PPh 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; Me¼CH 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 (490% conversion within 24 h; M w /M n ¼1.29), 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 Keywords: carbonyl ligand; half-metallocene; iron catalyst; living radical polymerization; metal catalysis; methacrylate; phosphine ligand INTRODUCTION A key component of transition metal-catalyzed chemical reactions is obviously a metal complex catalyst, which determines and controls critical parameters including rate, efficiency, selectivity, versatility and so on, 1 and it is particularly true for metal-mediated living radical polymerization (Scheme 1), which we have been pursuing for over a decade (for recent reviews on transition metal catalyzed living radical polymerization, see Kamigaito et al., 2,3 Ouchi et al. 4 and Matyjaszewski and Xja 5 ). In general, a metal complex consists of a transition metal center and ligands, and the two components are connected through coordination and sometimes through metal-carbon bonds formed from a vacant d-orbital of the former and s-, p-or n-electrons of the latter. The ligands thereby affect the electronic as well as steric environments of complexes and, in turn, their catalytic performance.Living radical polymerizations are now powerful tools to synthesize controlled polymeric architectures, because, unlike the ionic counterparts, they are simple and easy to execute, robust and reproducible under varying conditions, and, above all, versatile and applicable to a wide range of monomers including functional derivatives that are often required in biochemistry, materials science and other disciplines

Dicarbonyl pentaphenylcyclopentadienyl iron complex for living radical polymerization: Smooth generation of real active catalysts collaborating with phosphine ligand

“…In the MA polymerization, the carbon–halogen bond in the dormant species is tighter, and the growing radical species is more reactive than with MMA, and hence both of activation and deactivation need to be further promoted to control the polymerization. As for the analog complexes [(Cp′)Fe(CO)(L phos )Br; Cp′ = Cp, Cp*], the Cp* derivatives give a little broader MWDs ( M w / M n ∼ 1.3–1.4) and limited conversion (<60%),24 while the Cp cannot control the polymerization at all 23…”

“…From these practical viewpoints, iron (Fe) should be one of most ideal central metals for the catalyst 4. Since the first discovery with FeCl 2 (PPh 3 ) 2 by our group,5 a variety of iron complexes have been developed for the LRP catalysts;6–24 however, there are few examples to both of catalytic activity for variety of monomers and practical utilities with convenience in handling.…”

“…Quite recently, we have found that carbonyl‐phosphine heteroligated cyclopentadienyl complexes [(Cp′)Fe(CO)(L phos )Br; Cp′ = η‐C 5 H 5 (Cp)23 or η‐C 5 (CH 3 ) 5 (Cp*):24 L phos = phosphine], prepared via the ligand exchange for (Cp′)Fe(CO) 2 Br in the presence of phosphine ligand on heating or under UV irradiation, are active to catalyze LRP (Scheme ). Although they are stable even under oxygen, once they encounter an initiator for the metal‐catalyzed LRP at the polymerization temperature (>60 °C), they turned into unsaturated active species [(Cp′)Fe(L phos )Br] via the carbonyl release to catalyze the polymerization.…”

Iron-based catalytic systems in atom-transfer controlled-radical-polymerization processes

“…The known catalytic systems based on half sand wich iron(II) derivatives containing two carbonyl groups and the halogen atom (CpFeBr(CO) 2 ) were modified via incorporation of electron donor phos phorus containing ligand PR 3 into the complex [68].…”

Synthesis, reactivity, and some photochemistry of ortho-N,N-dimethylaminomethyl substituted aryl and ferrocenyl pentamethylcyclopentadienyl dicarbonyl iron complexes

“…In this regard, special attention is given to well-defined complexes possessing a cyclopentadienyl mono-or dicarbonyl iron moiety. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Among such catalysts, the importance of the neutral complex CpFe(CO) 2 Me (1, Cp: cyclopentadienyl = 1 5 -C 5 H 5 ) [14][15][16][17][18][19][20] and the cationic complex [CpFe(CO) 2 …”