Crystal Structures and Magnetic Properties of MCl42− (M = Mn(II) and Co(II)) Salts of m- and p-N-Methylpyridinium Nitronyl Nitroxides

Organic radical cations, 4,4,5,5-tetramethyl-2-(1-methyl-3 or 4-pyridinio)-3-oxide-4,5-dihydro-1H-1-imidazolyloxyl (or, m- and p-N-methylpyridinium nitronyl nitroxides, abbreviated as m- and p-MPYNN+, respectively), were found to crystallize with magnetic counter anions, MCl42− (M = Mn2+ (S = 5/2) and Co2+ (S = 3/2)), although the MnCl42− salts were rather air-sensitive. X-Ray full-crystal analyses were carried out on the stable CoCl42− salts. The structure of (m-MPYNN+)2CoCl42− crystallizes in the monoclinicP21/c space group, while that of (p-MPYNN+)2CoCl42− belongs to the triclinic space group. Although (m-MPYNN+)2MnCl42− and (p-MPYNN+)2MnCl42− were not stable enough for an X-ray full data collection, they were indicated to be isostructural to the corresponding CoCl42− salts, respectively, by their diffractions of 20 > θ > 25°. Variable-temperature magnetic susceptibility measurements reveal a clear contrast between the (m-MPYNN+)2MCl42− and (p-MPYNN+)2MCl42− salts. The two (m-MPYNN+)2MCl42− salts exhibit a ferromagnetic behavior independently of MCl42−, which is attributable to a m-MPYNN+ dimer. However, the magnetic properties of two (p-MPYNN+)2MCl42− salts are strongly dependent on the MCl42− anion: (p-MPYNN+)2MnCl42− exhibits an antiferromagnetic interaction in opposition to a ferromagnetic one in (p-MPYNN+)CoCl42−. The magnetic difference between them can be qualitatively understood in terms of a charge-transfer interaction between MCl42− and p-MPYNN+, in which the difference in the electronic structure between the Mn2+ and Co2+ ions is reflected.