Light-Induced Magnetostructural Anomalies in a Polymer Chain Complex of Cu(hfac)2 with tert-Butylpyrazolylnitroxides

Aromatic SNF-Approach to Fluorinated Phenyl tert-Butyl Nitroxides

The interaction of octafluorotoluene (1a), as well as pentafluorobenzonitrile (1b) with tert-butylamine, followed by the oxidation of thus formed tert-butylanilines (2a,b) with meta-chloroperoxybenzoic acid led to functionalized perfluorinated phenyl tert-butyl nitroxides [namely, 4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a) and 4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile (3b)] with nearly quantitative total yields. The molecular and crystal structures of nitroxide 3a were proved by single crystal X-ray diffraction analysis. The radical nature of both nitroxides was confirmed by ESR data. The interaction of Cu(hfac)₂ with the obtained nitroxides 3a,b gave corresponding trans-bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O′)bis{4-(N-tert-butyl(oxyl)amino)perfluoroarene-κO}copper (II) complexes ([Cu(hfac)₂(3a)₂] and [Cu(hfac)₂(3b)₂]). X-ray crystal structure analysis showed square bipyramid coordination of a centrally symmetric Cu polyhedron with the axial positions occupied by oxygen atoms of the nitroxide groups. Magnetic measurements revealed intramolecular ferromagnetic exchange interactions between unpaired electrons of Cu(II) ions and paramagnetic ligands, with exchange interaction parameters J_Cu–R reaching 53 cm⁻¹.

Light-Induced Spin Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)₂ L^R family (hfac=hexafluoroacetylacetonato, L^R =pyrazole-substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu ), mainly Cu(hfac)₂ L^Pr (1), Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₈ (2) and Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₀ (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These Cu^II -nitroxide based molecular magnets display thermally and optically induced switchable behavior and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperature-dependent exchange interaction within the spin triads formed by the nitroxide-copper(II)-nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light-induced spin switching for other members of this family. The 500 nm band of the low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitroxide and Cu bands.

Light-Induced Spin State Switching in Copper(II)-Nitroxide-Based Molecular Magnet at Room Temperature

Molecular magnets Cu(hfac)₂L^R exhibit an unusual type of photoinduced magnetostructural switching in exchange-coupled copper(II)-nitroxide clusters. Such photoswitching from strongly coupled to weakly coupled spin state (SS → WS) was recently found to be ultrafast, thus enhancing the interest in these systems and the scope of their potential applications. However, to date such SS → WS photoswitching was demonstrated only at cryogenic temperatures, being limited by the absence of suitable SS states and short relaxation times at T > 100 K. In this work we selected model compound Cu(hfac)₂L^iso-Pr residing in the mixed SS/WS state at room temperature and investigated it using femtosecond optical spectroscopy. Photoinduced spin dynamics was detected, and an ultrafast SS → WS photoswitching was for the first time demonstrated at room temperature, constituting an important milestone in the development of copper(II)-nitroxide molecular magnets for practical purposes.

Spin-state-correlated optical properties of copper(ii)–nitroxide based molecular magnets

Pronounced thermochromism of copper(ii)–nitroxide based molecular magnets is explained.

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.

Radicals in metal–organic frameworks

We discuss early advances in the incorporation of radicals into the ever evolving class of materials known as metal–organic frameworks.