Search

Oxidation of 5,15-dithiaporphyrin with meta-chloroperbenzoic acid afforded the corresponding S,S-tetraoxide in good yield. The resultant 5,5,15,15-tetraoxo-5,15-dithiaporphyrin exhibited the highly electron-deficient nature as elucidated by the electrochemical analysis and theoretical calculations. Treatment of tetraoxodithiaporphyrin with zinc(II) acetate and nickel(II) acetate provided the corresponding metal complexes efficiently. Owing to its enhanced Lewis acidity of the metal center by the electron-deficient ligand, the nickel complex underwent facile axial ligation to form pentacoordinate and hexacoordinate high-spin (S=1) complexes in solution and solid, respectively. The binding constant of pyridine to the NiII center was significantly higher than those of conventional porphyrin NiII complexes. Temperature-dependent magnetic susceptibility measurements of the high-spin NiII complex revealed the presence of weak ferromagnetic interactions.

A bis(NiII -porphyrinyl)aminyl radical with meso-C6 F5 groups was prepared as a spin-delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both NiII centers became hexacoordinated by accepting two axial pyridines, which triggered a spin-state change of the NiII centers from diamagnetic (S=0) to paramagnetic (S=1). The resulting high-spin NiII centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S=5/2). Importantly, this coordination-induced spin-state switching can be conducted in a reversible manner, in that washing of the high-spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.

Fused benzene rings to antiaromatic compounds generally improve their stability but attenuate their antiaromaticity. The opposite case is now reported. NiII benzonorcorroles were synthesized and the effect of benzo-fusion on the antiaromaticity was elucidated. The benzo-fusion resulted in significant decrease of the HOMO-LUMO gaps and enhancement of the paratropic ring current effect. Furthermore, the introduction of the benzo groups induced singlet diradical character in the antiaromatic porphyrinoid.

The spin states of microcrystalline samples of [Fe(OMTPP)L2]+ (L = DMAP or Py) have been examined and interpreted using Mossbauer spectroscopy, SQUID magnetometry and X-ray crystallography. The Mossbauer spectra of [Fe(OMTPP)(DMAP)2]+ and [Fe(OMTPP)Py2]+ showed that both of these complexes maintain the low-spin (S = 1/2) state over the 77−300 K temperature range. The spin states of these complexes were further confirmed by SQUID magnetometry. Thus, the magnetic behavior of [Fe(OMTPP)Py2]+ is quite different from that of the structurally related species [Fe(OETPP)Py2]+. The latter complex exhibits a novel spin crossover between the S = 3/2 and S = 1/2 states as revealed by the spectroscopic and magnetic measurements. In order to understand the reasons for the absence of the spin crossover process in [Fe(OMTPP)Py2]+, we have compared the crystal and molecular structures of [Fe(OMTPP)Py2]+ with those of the recently reported species [Fe(OETPP)Py2]+. In the case of [Fe(OMTPP)Py2]+, the Fe−Naxial bond lengths hardly change with temperature and are 2.058(6) and 2.024(4) A at 298 and 80 K, respectively. These results are in sharp contrast to those of the spin crossover complex [Fe(OETPP)Py2]+, in which the Fe−Naxial bonds contract from 2.201(3) A at 298 K to 1.993(3) A at 80 K. We have ascribed the difference in magnetic behavior between [Fe(OMTPP)Py2]+ and [Fe(OETPP)Py2]+ to the difference in molecular packing; the former adopts a densely packed cubic crystal system while the latter shows a less condensed monoclinic system. A cavity calculation has further confirmed the above mentioned assumption. While the cavity sizes around the pyridine ligands in [Fe(OETPP)Py2]+ are 32.08 and 28.88 A3 at 298 K, that in [Fe(OMTPP)Py2]+ is only 19.81 A3. Furthermore, the cavities contract by 17.7% in [Fe(OETPP)Py2]+ when the temperature is lowered from 298 to 80 K whereas the contraction is only 5.3% in the case of [Fe(OMTPP)Py2]+. On the basis of these results, we have concluded that the loosely packed crystal system and the wide cavities around the axial ligands are the important requirements for the occurrence of the spin crossover process in the solid state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes. Strong axial ligands (L), such as imidazole and 4-dimethylaminopyridine, lead to the formation of complexes with a pure S=1/2 state, while weak ligands, such as THF, give complexes with a pure S=3/2 state. Intermediate strength ligands, such as pyridine and 4-cyanopyridine, give complexes that show a novel spin crossover between the S=1/2 and S=3/2 states.

‘C-Aza-2-deoxy-L-lyxonucleosides’ in which a sugar ring oxygen is replaced with a nitrogen atom are synthesized from 2-deoxy-3,5-O-(tetraisopropyldisiloxane-1,3-diyl)-D-erythro-pentofuranose via a sequential procedure of the addition of lithium salts of aromatic heterocycles, Swern oxidation and reductive aminocyclization. Their structures are determined mainly by X-ray crystallography and NMR measurements. Their bioassay is also described.