High activity of an indium alkoxide complex toward ring opening polymerization of cyclic esters

Abstract

Abstract. An indium complex supported by a ferrocene-derived Schiff base ligand has an unprecedented high activity toward ε-caprolactone, δ-valerolactone, and β-butyrolactone. L-lactide, D,L-lactide, and trimethylene carbonate polymerizations also showed moderate to high activity.Over the past two decades, the ring-opening polymerization of cyclic esters has been increasingly studied because of the promise to produce biodegradable polymers from biomass. In our recent studies, a cerium complex supported by a Schiff base ligand with a ferrocene backbone polymerized L-lactide in a controlled fashion. 24 Phosphinimine analogues with yttrium and indium also demonstrated good activity in lactide and trimethylene carbonate polymerizations. 22 Therefore, we decided to combine indium's activity with the Schiff base, ferrocene-derived ligand in order to study the activity of the resulting complexes toward a broad range of cyclic ester polymerizations. Additional motivation was found from recent studies that point to the biocompatibility, robustness, and high activity of indium catalysts in the ring opening polymerization of cyclic esters. 10,25 Herein, we report a ferrocene-derived Schiff base indium complex that possesses a remarkable range of activity toward cyclic esters and exceptionally fast polymerization rates with lactones. Compound (salfen)In(O t Bu) (salfen = 1,1'-di(2,4-di-tert-butyl-6-iminephenoxy)ferrocene) represents the first indium precatalyst capable of δ-valerolactone polymerization and the most active indium catalyst to date in the polymerization of β-butyrolactone and ε-caprolactone. The polymerization rates of ε-caprolactone are even competitive with those of current industrial catalysts.Compound (salfen)In(O t Bu) was synthesized from the reaction of (salfen)InCl and freshly sublimed KO t Bu. In turn, (salfen)InCl was synthesized by combining InCl3 with K2(salfen), which was generated from H2(salfen) and two equivalents of KH (Eq 1). Needle crystals of (salfen)InCl were isolated from a diethyl ether solution. The solid state molecular structure (Figure 1) shows a distorted octahedral indium center and a THF molecule coordinated trans to the chloride ligand. Similarly to the phosphinimine indium chloride analogue previously reported by us, 22 a long In-Fe distance (3.98 Å) and a staggered configuration of the Cp rings were apparent. Elongation of the In-heteroatom distances compared to other compounds, such as (salen)InCl (salen = N,N ' -bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (rac-or (R,R)-H2(ONNO))) 9 and (phosfen)In(OPh) (phosfen = 1,1'-di(2-tert-butyl-6-diphenylphosphiniminophenoxy)ferrocene), 22 by about 0.25 Å possibly compensates for the distortion caused by the staggered Cp rings.

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…Group 4 metal complexes supported by salfan, thiolfan, and thiolfan* ligands were prepared in a similar way using Zr(OtBu) 4 or Ti(OiPr) 4 as metal precursors. 43 In addition, the group 13 metal complex (salfen)In(OtBu) was prepared through salt metathesis between InCl 3 and K 2 (salfen) followed by treating (salfen)InCl with KOtBu, 38 while late transition metal complexes such as M(salfen) (M = Co, Zn) were also prepared by salt metathesis from the corresponding metal chlorides. 87 C. C-H Activation, Dearomatization, and Ring-Opening Reactions of aromatic N-heterocycles Scheme 3: Reactivity of rare-earth metal alkyl complexes supported by ferrocene diamide ligands toward aromatic N-heterocycles: (a) C-H activation of pyridine followed by C-C coupling and formation of bipyridine radical anion; 63 (b) dearomatization of isoquinoline; 65 (c) C-H activation of 2-phenylpyridine followed by hydrogen migration; 54 (d) ring-opening of 1-methylimidazole; 82 (e) reactivity of pincer-type pyridine diamide complexes.…”

Synthesis, physical and antimicrobial studies of ferrocenyl-N-(pyridinylmethylene)anilines and ferrocenyl-N-(pyridinylmethyl)anilines

“…37 In addition, liquid crystalline ferrocenyl Schiff bases also known as ferrocenomesogens, show interesting magnetic traits such as paramagnetism and control of the molecular orientation in magnetic fields. 38,39 Moreover, there is high interest in transition metal complexes with electronically communicating metal centres spanned by conjugated systems that make ferrocenyl metalloligands appealing.…”

Switchable Ring‐Opening Polymerization by a Ferrocene Supported Aluminum Complex

“…The solid state molecular structure shows a pentacoordinate aluminum center with an eclipsed Cp configuration. The metal‐Fe distance was measured as 3.87 Å, which is shorter than in (thiolfan*)Al(O t Bu) at 4.61 Å, (salfen)In(O t Bu) at 3.98 Å, (salfen)Zr(O i Pr) 2 at 4.05 Å, but comparable to that of (salfen)Co at 3.72 Å . The Al‐heteroatom distances are similar to those in other tetradentate ONNO‐type salen (salen= N,N′ ‐bis(salicylidene)ethylenediamine) supported aluminum complexes…”

“…Yield: 0.312 g, 53 %. 1 H NMR (500 MHz, 25°C, C 6 D 6 ) δ (ppm): 7.85 (s, 2H, N=CH), 7.65 (s, 2H, ArH), 6.78 (s,2H,ArH),5.42 (s,2H,5.32 (m,1H,CH(CH 3 ) 2 ), 3.77-3.96 (d and s, 6H, fc-H), 1.73 (d,6H,CH(CH 3 ) 2 ), 1.54 (s, 18H, C(CH 3 )), 1.28 (s, 18H, C (CH 3 )). 13 Calcd: C,70.48;H,7.84;N,3.82.…”

“…Related supported ligands could also be used with zirconium as redox switchable catalysts: (salfan)Zr(O t Bu) 2 (salfan=1,1′‐di(2‐ tert ‐butyl‐6‐ N ‐methylmethylenephenoxy)ferrocene, Figure b) had the capability to synthesize an ABC type triblock of PBL‐PCHO‐PLA‐O t Bu, while (salfen)Zr(O i Pr) 2 (salfen=1,1′‐di(2,4‐bis‐ tert ‐butyl‐salicylimino)ferrocene, Figure c) could synthesize an ABA type PLA‐PCHO‐PLA triblock via two redox switches. When the indium analogue, (salfen)In(O t Bu) (Figure d), was examined, a high ring‐opening polymerization activity toward several lactones was observed with the reduced compound, but the catalyst was not redox switchable.…”