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Cyclic M2(RL)2 Coordination Complexes of 5-(3-[N-tert-Butyl-N-aminoxyl]phenyl)pyrimidine with Paramagnetic Transition Metal Dications

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posted on 19.09.2005, 00:00 by Martha Baskett, Paul M. Lahti, Armando Paduan-Filho, Nei F. Oliveira
5-(3-(N-tert-Butyl-N-aminoxyl)phenyl)pyrimidine (RL = 3NITPhPyrim) forms isostructural cyclic M2(RL)2 cyclic dimers with M(hfac)2 (M = Mn, Co, Cu; hfac = hexafluoroacetylacetonate). Mn2(hfac)4(RL)2 exhibits strong antiferromagnetic Mn−RL exchange, with weak ferromagnetic exchange (0.7 cm-1) between Mn−RL units that is consistent with a spin polarization exchange mechanism. The magnetic moment of Co2(hfac)4(RL)2 at higher temperatures is consistent with strongly antiferromagnetic exchange within the Co−NIT units and tends toward zero below 50 K at lower magnetic fields. Cu2(hfac)4(RL)2 shows more complex behavior, with no high-temperature plateau in χT(T) up to 300 K but a monotonic decrease down to about 100 K. The Cu(II)−nitroxide bonds decrease by 0.2−0.3 Å over the same temperature range, corresponding to a change of nitroxide coordination from axial to equatorial. This thermally reversible Jahn−Teller distortion leads to a thermally induced spin state conversion from a high-spin, paramagnetic state at higher temperature to a low-spin state at lower temperature. This spin state conversion is accompanied by a reversible solid-state thermochromic change between dull yellow-brown at room temperature and green at 77 K.