Mariya V. Edeleva

pH-Sensitive C―ON Bond Homolysis of Alkoxyamines of Imidazoline Series with Multiple Ionizable Groups As an Approach for Control of Nitroxide Mediated Polymerization

Recently, a new concept of pH-switchable agents for reversible addition-fragmentation chain transfer (RAFT) polymerization has been introduced by Benaglia et al. (J. Am. Chem. Soc.2009, 131, 6914-6915). In this paper we extended the concept of pH-switchable mediators to nitroxide mediated polymerization (NMP) by employing nitroxides with basic or acidic groups as controlling agents. Four alkoxyamines, the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, have been prepared. The influence of pH on alkoxyamine homolysis rate constants (k(d)) and on the nitroxide-alkyl radical recombination rate constants (k(c)) was studied. All alkoxyamines under study as well as the parent nitroxides have several basic groups, which under pH variation can undergo consecutive protonation. It was shown that the k(d) value under basic conditions are significantly (up to 15-fold) higher than in acidic solution at the same temperature, whereas the k(c) value in basic solutions decrease by a factor of 2 only. The efficiency of NMP is known to be dependent on k(d) and k(c), both constants being dependent on the monomer structure; therefore the performance of NMP of different monomers in the controlled mode requires different conditions. It is shown that the pH value crucially affects the polymerization regime, changing it from the controlled to the uncontrolled mode. The controlled regime of NMP of different hydrophilic monomers (sodium 4-styrenesulphonate and acrylamide) in aqueous solution under mild conditions (90 °C) can be achieved using the same alkoxyamine by the variation of the pH value. The chain length of polymers depends on pH value during the polymerization.

Chemically Triggered C–ON Bond Homolysis in Alkoxyamines. Part 2: DFT Investigation and Application of the pH Effect on NMP

In recent work, a 15-fold increase in the C-ON bond homolysis rate constant kd of 4-pyridylethyl-SG1-based alkoxyamine was observed upon protonation of the pyridyl moiety in organic solvent. In this report, the pH dependence of kd (pKa = 4.7) is investigated in D2O/CD3OD (v/v 1:1). A 64-fold increase in kd is observed at acidic pH. Calculations show that the increase in kd upon protonation is due to both an increase in the stabilization of the protonated 4-pyridylethyl radical and an increase of the destabilization of the starting materials through an increase in the polarity of the alkyl fragment. This new alkoxyamine is applied to NMP of styrene and sodium styrene sulfonate.

Octahedral molybdenum cluster complexes with aromatic sulfonate ligands

This article describes the synthesis, structures and systematic study of the spectroscopic and redox properties of a series of octahedral molybdenum metal cluster complexes with aromatic sulfonate ligands (nBu4N)2[{Mo6X8}(OTs)6] and (nBu4N)2[{Mo6X8}(PhSO3)6] (where X- is Cl-, Br- or I-; OTs- is p-toluenesulfonate and PhSO3- is benzenesulfonate). All the complexes demonstrated photoluminescence in the red region and an ability to generate singlet oxygen. Notably, the highest quantum yields (>0.6) and narrowest emission bands were found for complexes with a {Mo6I8}4+ cluster core. Moreover, cyclic voltammetric studies revealed that (nBu4N)2[{Mo6X8}(OTs)6] and (nBu4N)2[{Mo6X8}(PhSO3)6] confer enhanced stability towards electrochemical oxidation relative to corresponding starting complexes (nBu4N)2[{Mo6X8}X6].

On the synthesis and characterisation of luminescent hybrid particles: Mo6 metal cluster complex/SiO2

Photoluminescent silica-based materials are used in applications in photonics, sensing, and biological and medical sciences. Specifically, hybrid particles based on silica doped by photoluminescent octahedral molybdenum metal cluster complexes are inexpensive and readily available via the Stober process and thus are promising materials for diverse applications. We evaluated design of photoluminescent materials based on silica and {Mo6X8}4+ clusters (where X = Cl, Br, I), including how synthesis conditions (chemical composition of metal cluster precursors (Bu4N)2[{Mo6X8}(NO3)6], loading of the precursor and presence of a surfactant) influence key parameters of the final materials, such as phase composition, size and morphology of the particles and photophysical characteristics. Our study revealed that hydrolysis of the molybdenum cluster precursors during the Stober process strongly affects both morphology and photophysical parameters of the materials, especially at high loadings. At relatively low loadings of the precursors, materials doped by {Mo6I8}4+ clusters demonstrated the most promising set of properties—the highest photoluminescence quantum yields and efficient singlet oxygen generation—while particle size and morphology remained the same as for undoped SiO2 materials.

Water-soluble hybrid materials based on {Mo6X8}4+ (X = Cl, Br, I) cluster complexes and sodium polystyrene sulfonate

The development of water-soluble forms of octahedral molybdenum clusters {Mo6X8}4+ (X = Cl, Br, I) is strongly motivated by the tremendous potential that these complexes have for biological applications, namely as agents for bioimaging and photodynamic therapy. In this work, we report the first water-soluble hybrid materials, which represent sodium polystyrene sulfonate doped by molybdenum clusters, and the evaluation of their photophysical and biological properties (dark and photoinduced cytotoxicity and cellular uptake) with the use of cervical cancer (HeLa) and human epidermoid larynx carcinoma (Hep-2) cell-lines as models.

pH-sensitive C-ON bond homolysis of alkoxyamines of imidazoline series: a theoretical study.

The pH-dependent kinetics of C-ON bond homolysis reactions of the four alkoxyamines (viz., the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl) in water solution have been scrutinized using DFT calculations (M06-2X/6-311++G(2df,p) level of theory with the PCM model). On the basis of computations, the experimental results obtained before (J. Org. Chem. 2011, 76, 5558) have been rationalized. The concentration dependence of all possible protonated forms of the four alkoxyamines was obtained from pH measurements. The contributions of particular protonated forms into the overall rate constants of thermolysis were estimated using the DFT calculated Gibbs free energies Δ(r)⁰G of C-ON bond homolysis reactions. The computations revealed that the observed rate constants of thermolysis of the two species at several pH values are dominated by decomposition reactions of two or even three protonated forms. The observed trends in reactivity of the alkoxyamines were mainly attributed to destabilization of the radical products of C-ON bond scission reactions. A linear correlation between the sum of radical stabilization energies (RSEs) of products of thermolysis and the calculated Gibbs free energies of reactions was found for various protonated forms of the species studied. Apart from this, the linear correlation exists between the relative RSE and nitrogen hyperfine constants aN of various protonated forms of the nitroxide radical products.

Trityl-based alkoxyamines as NMP controllers and spin-labels

Recently, new applications of trityl-nitroxide biradicals were proposed. In the present study, attachment of a trityl radical to alkoxyamines was performed for the first time. The rate constants kd of C-ON bond homolysis in these alkoxyamines were measured and found to be equal to those for alkoxyamines without trityl. The electron paramagnetic resonance (EPR) spectra of the products of alkoxyamine homolysis (trityl-TEMPO and trityl-SG1 biradicals) were recorded, and the corresponding exchange interactions were estimated. The decomposition of trityl-alkoxyamine showed more than an 80% yield of biradicals, meaning that the C-ON bond homolysis is the main reaction. The suitability of these labelled initiators/controllers for polymerisation was exemplified by means of successful nitroxide-mediated polymerisation (NMP) of styrene. Thus, this is the first report of a spin-labelled alkoxyamine suitable for NMP.

C–ON bond homolysis of alkoxyamines triggered by paramagnetic copper(II) salts

The metal complexation reactions of bis(hexafluoroacetylacetonato)copper(II) (Cu(hfac)2) with alkoxyamines (diethyl(2,2-dimethyl-1-(tert-butyl-(1-(pyridine-4-yl)ethoxy)amino)propyl)phosphonate and diethyl (2,2-dimethyl-1-(tert-butyl-(1-(pyridine-2-yl)ethoxy)amino)propyl)phosphonate) were studied. According to X-ray analysis, the molecular and crystal structures of 1:1 complexes depend on the configuration of the free alkoxyamines, that is dimeric (RSSR) and chain-polymeric (RR/SS) structures for para-pyridyl-substituted alkoxyamines, and cyclic unimeric (RS/SR) structure for ortho-pyridyl derivative. The complex (2:1 ratio Cu(hfac)2/alkoxyamine) for ortho-pyridyl-substituted alkoxyamine is not resolved. Upon warming, ortho complexes decomposed into free alkoxyamines and only a weak activation was observed. Upon warming, para complexes decomposed into their corresponding unimers, and then, a 21-fold increase in the rate constant of the C–ON bond homolysis was observed compared to the corresponding free alkoxyamines. Tuning of the homolysis rate constant of the C–ON bond via addition of pyridine is also reported.

Dual-initiator alkoxyamines with an N-tert -butyl- N -(1-diethylphosphono-2,2-dimethylpropyl) nitroxide moiety for preparation of block co-polymers

Correction for ‘Dual-initiator alkoxyamines with an N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide moiety for preparation of block co-polymers’ by Gerard Audran et al., RSC Adv., 2017, 7, 4993–5001.

Coordination-Initiated Nitroxide-Mediated Polymerization (CI-NMP)

Preparation of materials by nitroxide-mediated polymerization (NMP) is well known nowadays. To increase the possible usefulness of NMP for the production of hybrid materials or polymer-decorated complexes, coordination-initiated NMP (CI-NMP) was developed and investigated here. CI-NMP was exemplified using the instantaneous and spontaneous reaction of alkoxyamines carrying a pyridyl moiety on the alkyl group and the Zn(hfac)2 (hfac: hexafluoroacetylacetonate) complex as a metal centre. NMP of styrene and n-butyl acrylate was carried out with either previously or in situ-prepared complexes. Both approaches afforded NMP of the same quality. The positive influence of metal centre coordination is highlighted by efficient NMP at 90°C.