Molecular magnets based on chain polymer complexes of copper(II) bis(hexafluoroacetylacetonate) with isoxazolyl-substituted nitronyl nitroxides

N-Alkylimidazol-5-yl-substituted Nitronyl Nitroxides and Their Mononuclear Cu(II) Complexes: Synthesis, Structure and Magnetic Properties

A novel synthetic approach has been employed to synthesize a series of new nitronyl nitroxides: 2-(1-propyl-1H-imidazol-5-yl)- (Ln-Pr ), 2-(1-isopropyl-1H-imidazol-5-yl)- (Li-Pr ) and 2-(1-butyl-1H-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (Ln-Bu ). The reaction of Cu(hfac)2 with LR in a 1 : 2 ratio yields mononuclear heterospin complexes [Cu(hfac)2 (LR )2 ] (LR =Ln-Pr , Li-Pr , Ln-Bu ), which have a similar crystal structure to the "jumping" crystals [Cu(hfac)2 (LMe )2 ] that exhibit chemomechanical activity. It was shown that an increase in the alkyl substituent R leads to changes in the crystal packing of the molecules and the absence of chemomechanical activity. Furthermore, it was found that two polymorph modifications of the heterospin complex [Cu(hfac)2 (Ln-Pr )2 ] can be obtained, and magnetic properties of [Cu(hfac)2 (Ln-Pr )2 ] strongly depend on the angle between the planes of the paramagnetic fragment O•-N-C=N→O and the imidazole ring in Ln-Pr .

Catalytic System for Cross-Coupling of Heteroaryl Iodides with a Nitronyl Nitroxide Gold Derivative at Room Temperature

A simple and highly effective methodology for the cross-coupling of heteroaryl iodides with NN-AuPPh3 at room temperature is reported. The protocol is based on a novel catalytic system consisting of Pd2(dba)3·CHCl3 and the phosphine ligand MeCgPPh having an adamantane-like framework. The present protocol was found to be well compatible with various heteroaryl iodides, thus opening new horizons in directed synthesis of functionalized nitronyl nitroxides and high-spin molecules.

Cu(hfac)2 Complexes with Nitronyl Ketones Structurally Mimicking Nitronyl Nitroxides in Breathing Crystals

Breathing crystals based on polymer-chain complexes of Cu(hfac)2 (hfac = hexafluoroacetylacetonate) with nitronyl nitroxides represent a new type of molecular magnetic sensors, exhibiting thermally and light-induced structural rearrangements in the spin clusters Cu ← (O∸N<)n accompanied by magnetic anomalies. To shed light on the driving forces of the rearrangements a method for the synthesis of sterically hindered 4-oxo-3,4-dihydro-2H-pyrrole-N-oxides (nitronyl ketones) structurally mimicking nitronyl nitroxides in breathing crystals has been determined. This method employs palladium-catalyzed cross-coupling of 3,3,4-trimethyl-4-nitropentanoic acid chloride with 4-stannylpyrazoles, leading to the formation of nitroketones followed by soft reduction into hydroxylamines, that then undergo self-initiated cyclization into the corresponding nitrones. The radical oxidation of the latter finally yields the target nitronyl ketones. An X-ray diffraction analysis of the nitronyl ketones confirms that their structural characteristics are close to those of the corresponding nitronyl nitroxides. Moreover, upon reaction with Cu(hfac)2, diamagnetic nitronyl ketones form polymer-chain complexes with a ‘head-to-tail’ motif and a structure similar to their analogues containing paramagnetic nitronyl nitroxides. Finally, it has been found that one of the complexes with nitronyl ketones does manifest a purely structural phase transition similar to its copper-nitroxide analogue, but at the same time it does not manifest any signs of a magneto-structural transition characteristic for breathing crystals where a Jahn–Teller axis in the coordination units CuNO4(O∸N<) is flipped. This observation strongly confirms the crucial role of the exchange interactions between spins of copper and nitroxide for the origin of magneto-structural anomalies in breathing crystals.