Nitronyl nitroxides are functional building blocks in cutting-edge research fields, such as the design of molecular magnets, the development of redox and photoswitchable molecular systems and the creation of redox-active components for organic and hybrid batteries. The key importance of the nitronyl nitroxide function is to translate molecular-leveloptimized structures into nano-scale devices and new technologies. In spite of great importance, efficient and versatile synthetic approaches to these compounds still represent a challenge. Particularly, methods for the direct introduction of a nitronyl nitroxide moiety into aromatic systems possess many limitations. Here, we report gold derivatives of nitronyl nitroxide that can enter Pd(0)-catalysed cross-coupling reactions with various aryl bromides, affording the corresponding functionalized nitronyl nitroxides. Based on the high thermal stability and enhanced reactivity in catalytic transformation, a new reagent is suggested for the synthesis of radical systems via a universal cross-coupling approach.
Gold(I) derivatives of nitronyl nitroxide (NN) stabilized by complexation with phosphine ligands were designed to promote dimerization of the paramagnets in the solid state via Au–Au bonding. The complexes were successfully synthesized by the reaction of the corresponding phosphine [n-Bu3P or (4-FC6H4)3P], AuCl(THT), and NN–H under basic conditions in high yields. The gold(I) complexes were characterized by cyclic voltammetry, electron spin resonance, infrared, and UV/vis spectroscopy supported by quantum chemical calculations. Crystallographic analyses of the complexes showed that they possess Au–Au bonds (2.930 and 3.095 Å). The Au–Au bonding was confirmed by density functional theory calculations and quantum theory of “atoms in molecules” and natural bond orbital analyses, which made it possible to determine bond critical points (3, −1) and Au–Au bond orders (0.36 and 0.23) as well as to estimate the orbital contribution to the energy of Au–Au bonds (12.0 and 8.6 kcal/mol).
The heterospin solid phases of the chain polymer [Cu(hfac)2LEt]∞ and bicyclic molecule [Cu(hfac)2LEt]2-I (LR = pyrazolyl-substituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu(hfac)2LEt]2-II. The single-crystal to single-crystal (SC–SC) transformation of [Cu(hfac)2LEt]2-I into [Cu(hfac)2LEt]2-II was recorded by X-ray diffraction analysis of the crystal as a function of time. At 255–277 K, the [Cu(hfac)2LEt]2-I → [Cu(hfac)2LEt]2-II SC–SC transformation proceeded for 12–18 h. The [Cu(hfac)2LEt]∞ → [Cu(hfac)2LEt]2-II SC–SC phase transformation was accompanied by a change in the crystal shape, spontaneous mechanical displacements of crystals, and a change in color from orange to dark green. This process started, to a certain extent, already in the crystals lying under the layer of the mother solution. After the crystals were separated from the solution, the SC–SC transformation [Cu(hfac)2LEt]∞ → [Cu(hfac)2LEt]2-II occurred completely within 4 h at room temperature. Under normal conditions, [Cu(hfac)2LPr]2-I also undergoes transformation into [Cu(hfac)2LPr]2-II. At the macro level, the transformation [Cu(hfac)2LPr]2-I → [Cu(hfac)2LPr]2-II is accompanied by spontaneous fragmentation of crystals, visualized as a scatter of small particles of the formed phase in different directions. The reverse transformation [Cu(hfac)2LPr]2-II → [Cu(hfac)2LPr]2-I occurs when [Cu(hfac)2LPr]2-II is cooled below 225 K. When [Cu(hfac)2LPr]2-II was heated above 300 K, the irreversible SC–SC phase transformation [Cu(hfac)2LPr]2-II → [Cu(hfac)2LPr]∞ was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu(hfac)2LPr]∞ single crystal at 303 K on a diffractometer for 1 day or more caused partial melting of the starting crystal, disappearance of X-ray diffraction reflections from the sample under study, and appearance of reflections corresponding to the formation of the new polymer complex [Cu(hfac)2L*Pr]∞, where L*Pr is the product of transformation of the radical including the oxidation of LPr and migration of the nitroxide O atom to the heterocycle, leading to the formation of 5-(tert-butylimino)-1-propyl-1,5-dihydro-4H-pyrazol-4-one (L*Pr). The results of the X-ray diffraction study of the phase transformations completely agreed with the data of magnetochemical measurements for the complexes. Having replaced the acyclic nitroxides LEt and LPr by their diamagnetic structural analogues LPEt (2,2-dimethyl-1-(1-ethyl-1H-pyrazol-5-yl)propan-1-one) and LPPr (2,2-dimethyl-1-(1-propyl-1H-pyrazol-5-yl)propan-1-one), we obtained the complexes [Cu(hfac)2LPEt]∞, [Cu(hfac)2(LPPr)2], and [(Cu(hfac)2)3(LPPr)2], for which the transformations are absolutely not characteristic. It was also found that polymorphic transformations are also uncharacteristic of complexes of other metals with the acyclic nitroxides under study ([Zn(hfac)2LEt]2, [Zn(hfac)2LPr]...
New antioxidants are commonly evaluated via two main approaches, i.e., the ability to donate an electron and the ability to intercept free radicals. We compared these approaches by evaluating the properties of 11 compounds containing both antioxidant moieties (mono- and polyphenols) and auxiliary pharmacophores (pyrrolidone and caprolactam). Several common antioxidants, such as butylated hydroxytoluene (BHT), 2,3,5-trimethylphenol (TMP), quercetin, and dihydroquercetin, were added for comparison. The antioxidant properties of these compounds were determined by their rates of reaction with 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and their oxidation potentials from cyclic voltammetry. Although these methods test different chemical properties, their results correlate reasonably well. However, several exceptions exist where the two methods give opposite predictions! One of them is the different behavior of mono- and polyphenols: polyphenols can react with DPPH more than an order of magnitude faster than monophenols of a similar oxidation potential. The second exception stems from the size of a “bystander” lactam ring at the benzylic position. Although the phenols with a seven-membered lactam ring are harder to oxidize, the sterically nonhindered compounds react with DPPH about 2× faster than the analogous five-membered lactams. The limitations of computational methods, especially those based on a single parameter, are also evaluated and discussed.
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