Synthesis and EPR characterisation of triphenylmethane derivatives carrying N-tert-butyl nitroxide radical moieties: use of the diradical as a ligand for a complex with MnII(hfac)2

Abstract

Oxidation of the tris(hydroxylamine) 3 with various oxidants has been monitored by EPR spectroscopy and found to show stepwise formation of the corresponding mononitroxide radical 4, dinitroxide radical 5 and trinitroxide radical 6, which was finally converted into the quinonoid diradical 8. The robust triplet nature of diradical 8 was corroborated by its effective magnetic moment of 2.7µ_B at 300 K. When mixed with bis(hexafluoroacetylacetonato)manganese(II), Mn(hfac)₂, diradical 8 gave black–violet solids 9 of formula [Mn(hfac)₂]₃·8₂. The temperature dependence of the reciprocal of the molar susceptibility of the complex indicated a ferromagnetic interaction with a Curie constant C= 7.86 emu K mol^–1 and a Weiss temperature θ= 87.5 K. At 5 K, hysteresis with a coercive force of 17 Oe and a remaining magnetisation of 910 emu G mol^–1 was observed, confirming the phase transition to a magnet.