Heteroleptic Complexes of Ruthenium Nitrosyl with Pyridine and Bypiridine-Synthesis and Photoisomerization.

The reaction of [RuNO(Py)2Cl2OH] with bipyridine in water-ethanol media results in trans-(NO, OH)-[RuNO(Py)(Bpy)ClOH]+ with an acceptable yield (60-70%) as hexafluorophosphate salt. Further treatment of the hydroxy-complex with concentrated HF quantitatively leads to trans-(NO, F)-[RuNO(Py)(Bpy)ClF]+. Despite the chirality of both coordination spheres, the hexafluorophosphate salts crystallized as racemates. A NO-linkage isomerism study of the obtained complexes was performed at 80 K with different excitation wavelengths (405, 450, 488 nm). The most favorable wavelengths for the MS1 isomer (Ru-ON) formation were 405 and 450 nm, where the linkage isomer populations were 17% and 1% for [RuNO(Py)(Bpy)ClOH]PF6 and [RuNO(Py)(Bpy)ClF]PF6. The shift of the excitation wavelength to the green (488 nm) sharply decreased the MS1 population. The IR-spectral signatures of MS1 were registered. Reverse-transformation Ru-ON (MS1)-Ru-NO (GS) was investigated for [RuNO(Py)(Bpy)ClOH]PF6 using IR and DSC techniques that made it possible to determine the kinetic parameters (Ea and k0) and decay temperature.

Origin of Long-Range Hyperfine Couplings in the EPR Spectra of 2,2,5,5-Tetraethylpyrrolidine-1-oxyls.

Cyclic nitroxides with several bulky alkyl substituents adjacent to the nitroxide group are known to demonstrate a much higher stability to bioreduction than their tetramethyl analogues. Among these so-called "sterically shielded" nitroxides, the pyrrolidine derivatives are the most stable. The EPR spectra of some sterically shielded pyrrolidine-1-oxyls were reported to show one or two large additional doublet splittings with a hyperfine coupling (hfc) constant (ca. 0.2-0.4 mT). To determine the origin of these hfc, a series of 2-R-2,5,5-triethyl-3,4-bis(hydroxymethyl)-pyrrolidine-1-oxyls with methylene groups stereospecifically enriched with deuterium were prepared, and their CW EPR spectra were studied. In addition, these nitroxides were investigated using quantum chemical calculations on the UB3LYP/def2-TZVP level and NBO analysis. The apparent constants were assigned to hfc with γ-hydrogen in the side chain, with the contribution of the NBO orbital ßπ*(N-O) to the natural localized molecular orbital ßσ(C-H) playing the major role. This interaction is efficient if the ethyl substituent is in the pseudoaxial position of the ring and the CH2-CH3 bond is codirected with (parallel to) N-O. The apparent constant aH increases with the Boltzmann population of this conformation.