Enolate chelating N-heterocyclic carbene complexes of Fe(II): Synthesis, structure and their catalytic activity for ring-opening polymerization of ɛ-caprolactone

Abstract

respectively. Attempt to synthesize mono-ligand Fe(II) bromide by the 1:1 molar ratio of NaL to FeBr 2 is unsuccessful, and the same complexes of 3 and 4 were obtained. Both 3 and 4 have been depicted by elemental analysis and X-ray structure determination. Preliminary studies show that both 3 and 4 can be used as single-component catalyst for the ring-opening polymerization of ε-caprolactone, and the catalytic activity of 3 is higher than that of 4. iron complex, functionalized N-heterocyclic carbene, ε-caprolactone, ring-opening polymerization Over the past decade, the application of N-heterocyclic carbenes (NHCs), as alternatives to traditional phosphines, in ligand designs and modifications has played a crucial role for developing late transition metal-based catalysts [1,2] . Up to date, the most of current attention has been focused on ruthenium, rhodium, palladium, and nickel-based systems. However, the chemistry of NHC systems of iron is considerably less developed. To the best of our acknowledge, only several kinds of ironbased NHC complexes have been reported, including the mononuclear Fe(II) halides of Fe(NHC) 2 X 2 [3] , the pianostool Fe(II) complexes of CpFe(CO) 2 (NHC)X [4] , the pincer Fe(II)/Fe(III) complexes of Fe[2,6-bis(NHC)-pyridine] X n (n = 2 or 3) [5,6] , and binuclear Fe(II) complexes of Fe 2 (μ-S(CH 2 ) 3 S)(CO) 6-x (NHC) x (x = 1, 2) [7] . Among these complexes, only the complex of Fe(NHC) 2 X 2 has been found to show good catalytic activity for the atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate [3] . Considering the currently increasing attention on iron-based systems [8,9] , further study on the synthesis of iron-based complexes with NHC ligand and their reactivity in homogeneous catalysis is valuable.Very recently, we have designed a novel kind of phenolate chelating NHC ligands [10] and synthesized the first anionic functionalized NHC complex of Fe(II) [11] . It is also found that this kind of iron complex could be used as single-component initiator for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) [11] . However, the complex of this type is extremely sensitive to air and moisture, resulting in difficulty to detailed reactivity investigation. Frequently, stability is one of the significant points for the purpose of constructing a new kind of metal-based catalyst. This prompted us to modify the reported phenolate chelating NHC ligand to an enolate form (Scheme 1). Recently, enolate chelating

N‐Heterocyclic Carbene Ligands and Iron: An Effective Association for Catalysis

“…Due to intramolecular transesterification reactions, the control of the chain length and the polydispersity was low during the polymerization. New Fe‐NHC complexes chelated by an enolate moiety ( C7 and C8 , Figure 4) gave similar results in ε‐caprolactone polymerization 19…”

Iron Catalysis in Organic Synthesis

“…841 One of the complexes provided the polyester with a number-average molecular weight of 4.0 Â 10 4 in 100% yield after 9 h reaction time in toluene at 80°C. The molecular weight distribution of 3.1 was rather high indicating that inter-or intramolecular transesterification reactions may occur.…”

Markovnikov-Selective, Activator-Free Iron-Catalyzed Vinylarene Hydroboration

“…31 These were deprotonated and reacted with FeBr2 to give the five iron(II) complexes 1a-c and 2a, b, respectively (Scheme 2). 32,33 It is worth noting at this point, that even with ligand synthesis, these iron catalysts were prepared in 2steps and at considerably less cost than even commercially available Wilkinson's catalyst. 34 Single-crystal X-ray analysis revealed that these bis-ligated complexes all adopted a similar distorted tetrahedral structure featuring an anchoring iron-carbene bond and the potentially activating group in the Fe-O motif (see ESI for details).…”

“…31 These were deprotonated and reacted with FeBr 2 to give the five iron(II) complexes 1a−c and 2a,b, respectively (Scheme 2). 32,33 It is worth noting at this point, that even with ligand synthesis, these iron catalysts were prepared in 2-steps and at considerably less cost than even commercially available Wilkinson's catalyst. 34 Initial investigations into alkene hydroboration found success using catecholborane (HBcat).…”