Search

Abstract: The coordination effects of the magnetic interaction between a paramagnetic transition metal or lanthanide ion and radical ligand(s) upon the spectroscopic (chiroptical and/or luminescence) properties will be described and discussed mainly for the heteroleptic complexes [M(L)(X)2] (M=Ni(II), Co(II), Cr(III), Ln(III); L=nitroxide radicals such as NIT2py(2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl) or IM2py (2-(2-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy), X=β-diketonates). For the Ni(II), Cr(III) or Co(II) complexes, we show how the orbital orthogonality or overlap between the dσ(eg) or dπ(t2g) of the metal ion and the SOMO (singly occupied antibonding π molecular orbital) of the nitroxides is relevant to the magnetic interactions associated with the spin-forbidden d–d transition intensity enhancement and/or the newly appeared d-SOMO metal-to-ligand charge transfer (MLCT) band. This is discussed in terms of the exchange mechanism or Valence Bond Configurational Interaction (VBCI) model. From the magnetic interactions for the Cr(III) with monodentate NITnpy and IMnpy (n =3 or 4) and their spectroscopic properties, the appearance of MLCT state is found to be a necessary and sufficient condition for the spin-forbidden transition intensity enhancement according to the spin-polarization mechanism. The lowest excited state magnetic interaction of the Cr(III) complexes was found to be larger than that of the ground state and discussed also on the basis of VBCI model. The magnetic circular dichroism (MCD) intensity enhancement of the intraligand singlet–triplet transitions in the ancillary acetylacetonate was examined in conjunction with the ligand-to-ligand charge transfer (LLCT) for the Ni(II) complexes. The intraligand transition of the nitroxide radicals was examined in connection with the luminescence behavior of the lanthanide(III) complexes. [Copyright &y& Elsevier]

Some lanthanide(III) complexes containing three types of a chelated n-methylpyridine-substituted imino nitroxide radical IM n Me2py( n =4,5,6) were synthesized with a small size of the multi-spin system. X-ray structural analysis of [Gd(hfac) 3 (IM6Me2py)], [Dy(hfac) 3 (IM4Me2py)] demonstrated that they are rigidly eight-coordinted with one IM n Me2py and three hfac ligands. Magnetic susceptibility measurements of the Gd(III) complexes indicate that the antiferromagnetic interaction between Gd(III) and imino nitroxide radical is different from the other lanthanide complexes containing nitronyl radicals. lanthanide imino nitroxide radical crystal structures magnetic interaction [ABSTRACT FROM AUTHOR]

Abstract: The magnetic interactions in a new series of isostructural imino nitroxide radical lanthanide(III) complexes, [Ln(hfac)3(IM2py)] (Ln=Gd–Yb: IM2py=2-(2′-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-1-oxy; hfac=1,1,1,5,5,5-hexafluoro-2,4-pentanedione), are examined by considering the intrinsic paramagnetic contribution of the Ln(III) ion from the corresponding [Ln(hfac)3(pybzim)] with a diamagnetic pybzim(2-(2-pyridyl)benzimidazole) ligand; the Ln(III)–IM2py interaction being antiferromagnetic for the 4f7 to 4f13 Ln(III) complexes and negligibly small for the other complexes. This series is the first example reverse to the previous cases for the series of Ln–Cu or Ln–aminoxyl(NIT) radical (4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-3-oxide-1-oxy) complexes, other than only a few examples of semiquinone Ln complexes. This reverse nature of the magnetic interaction, as compared with the NIT complexes, validates the empirical approach by O. Kahn et al. [Inorg. Chem. 38 (1999) 3692; J. Am. Chem. Soc. 122 (2000) 3413] in the spin-coupled systems for a series of Ln(III) complexes. [Copyright &y& Elsevier]

Abstract: Synthesis, characterization and magnetic properties of new lanthanide–radical complexes, [LnIII(hfac)3(IM2imH)] (Ln=Gd, Tb; IM2imH=2-(2-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy), are described. The molecular structure of the [Tb(hfac)3(IM2imH)] has been determined by the X-ray diffraction. The magnetic susceptibility data for [Gd(hfac)3(IM2imH)] show that the Gd–IM2imH magnetic interaction is antiferromagnetic with an exchange coupling constant J=−2.59 cm−1 in contrast to the ferromagnetic interaction in most of Gd(III) complexes containing paramagnetic center, which will be examined in connection with planarity of the IM2imH chelate. [Copyright &y& Elsevier]

Magnetic cimular dichroism and NIR luminescence of nitroxide radical complexes, [CrIII(β-diketonato)2(NIT2py or IM2py)]PF6, demonstrate that the energy gaps between the single (¹LD) and triplet (³LD) spin coupled levels in the lowest excited ²Eg,²T1g states are much larger than those in the ground state. This is the first observation of magnetic interactions in the excited states of radical complexes, which could be elucidated in terms of the exchange mechanism. [ABSTRACT FROM AUTHOR]

Abstract: A series of bis(4,4,4-trifluoro-1-R-1,3-butanedinato) Co(II) complexes with imino nitroxide radical(IM2py), [Co(Rtfc)2(IM2py)] (R=trifluoromethyl, phenyl, naphthyl, thienyl), was prepared and characterized by the X-ray analysis which demonstrated the stereospecific formation of the trans(CF3)-isomer with the trans disposition of the CF3 groups. The significant substituents effect on the UV–Vis spectra and magnetic properties were discussed in consideration of the inversion of the dπ(t2g) orbital levels leading to the orbital orthogonality or overlap with the SOMOπ∗ of IM2py in terms of the AOM parametrizations or spin-paring energy. [Copyright &y& Elsevier]