The mechanism of reversible-deactivation radical polymerization (RDRP) mediated by chloro-substituted amine−bis(phenolate)iron complexes was studied via a combination of Mossbauer, EPR, NMR, and online Vis/NIR spectroscopy. It was found that styrene polymerization operates via an atom-transfer radical polymerization (ATRP) mechanism when alkyl halide is present, whereas an interplay between ATRP and organometallic-mediated radical polymerization (OMRP) equilibria occurs during MMA polymerization. The preparation of the amine−bis-(phenolate)iron(II) complex, [O 2 NN′]Fe II , allowed polymerizations to be performed under an OMRP-only regime free of alkyl halide that yields dispersities as low as 1.29 for MMA polymerization. ■ INTRODUCTIONReversible-deactivation radical polymerization (RDRP) enables the synthesis of polymeric materials with precisely tailored topologies, architectures, chain lengths, functionalities, and narrow molar-mass distributions. 1−3 Fe-based systems are particularly attractive for use in RDRP due to the low toxicity and broad availability of Fe. 4,5 Fe catalysts operate in two RDRP techniques, i.e., atom-transfer radical polymerization (ATRP) and organometallic-mediated radical polymerization (OMRP). 6−11 Both methods are based on a metal-mediated activation−deactivation equilibrium of propagating radicals involving the Fe catalyst in different oxidation states.Fe-mediated ATRP has been reported using iron(II) and iron(III) halides with amines, imines, and phosphines as ligands for in situ catalyst formation. 6,11,12 ATRP mediated by iron halides has also been performed without the addition of external ligands but by using coordinating solvents or by adding ammonium salts for the purpose of charge separation. 13−18 Femediated OMRP has been studied far less frequently, with acetylacetonate-, porphyrin-, and Schiff base-ligated Fe complexes offering reasonable levels of control. 19,20 Even in apparently well-defined Fe-mediated RDRP systems, interesting mechanistic features have been observed under closer experimental and computational scrutiny. For instance, an interplay between ATRP and catalytic chain transfer (CCT) was reported for a series of α-diimine iron complexes, R1,R2 [NN]FeCl 2 . 21−27 In this case, the nature of the ligand was found to determine the dominant polymerization mechanism, with electron-donating groups favoring ATRP and yielding halogen-terminated polymers with well-controlled molar masses. Conversely, ligands with electron-withdrawing groups were found to favor CCT, with the instability of the organometallic species resulting in β-hydrogen elimination to yield low molar mass, olefin-terminated polymers.Amine−bis(phenolate)iron complexes, [O 2 NN′]FeCl, facilitate efficient RDRP of substituted styrenes and methyl methacrylate (MMA) with dispersities as low as 1.07. 28,29 The kinetic insight obtained from these reactions suggests a dual-mechanistic control, involving both ATRP and OMRP, which is in agreement with computational studies. 30 However, the expected Fe species...

A new library of silver(I)-N-heterocyclic carbene complexes prepared from the natural products caffeine, theophylline and theobromine is reported. The complexes have been fully characterised using a combination of NMR spectroscopy, mass spectrometry, elemental analysis and X-ray diffraction analysis. Furthermore, the hydrophobicity of the complexes has been measured. The silver(I)-N-heterocyclic carbenes have been evaluated for their antiproliferative properties against a range of cancer cell lines of different histological types, and compared to cisplatin. The data shows different profiles of response when compared to cisplatin in the same panel of cells, indicating a different mechanism of action. Furthermore, it appears that the steric effect of the ligand and the hydrophobicity of the complex both play a role in the chemosensitivity of these compounds, with greater steric bulk and greater hydrophilicity delivering higher cytotoxicity.

An electrochemical approach for the preparation of copper(I) N-heterocyclic carbene complexes has been developed to include a diverse range of ligand precursors. Importantly, the method is effective for a ligand precursor that contains several acidic protons and for which traditional methods of carbene formation are not suitable.

The preparation of a series of imidazolium salts bearing N-allyl substituents, and a range of substituents on the second nitrogen atom with varying electronic and steric properties, is reported.

A library of pyridyl- and picolyl-substituted imidazolium salts have been synthesized and coordinated to copper, via transmetalation from silver(I)–N-heterocyclic carbenes (NHCs), to prepare several copper(I)– and copper(II)–NHC complexes. The copper(I)–NHCs are complexes of the type Cu(NHC)Br, with the solid-state structures revealing a variety of coordination environments around the copper centers. The stability of the copper(II) complexes is particularly unusual, given the absence of a “hard” anionic tethering group appended to the ligands. The stability has been attributed to the pyridyl substituent, with the complexes being extremely stable, while those with an appended anionic group tend to be more sensitive to air/moisture. The ligands and complexes have been examined in an Ullmann-type etherification reaction and exhibit improved activity in comparison to copper in the absence of a ligand or the common Cu(I)–NHC complexes Cu(IMes)Cl and [Cu(IMes)2]PF6, indicating stabilization of higher oxidation state species by the ligands during the catalytic cycle.