Tuning Magnetic Relaxation in a Tb-Nitronyl Nitroxide Complex by Using Cocrystalline Paramagnetic Complex

New 2p–4f and 2p–3d–4f compounds [Tb­(hfac)3(NIT-PhNO2)2]·0.5C7H16 (1) and [Ln­(hfac)3(NIT-PhNO2)2]2[Cu­(hfac)2(NIT-PhNO2)2] (LnIII = Gd 2, Tb 3; hfac = hexafluoroacetylacetonate; NIT-PhNO2 = 2-(p-nitrophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been obtained. Complex 1 consists of mononuclear trispin [Tb­(hfac)3(NIT-PhNO2)2] units in which two radical ligands are ligated to the Tb­(III) ion as monodentate ligands through the NO groups, while complexes 2 and 3 contain two kinds of trispin moieties, namely, [Ln­(hfac)3(NIT-PhNO2)2] and [Cu­(hfac)2(NIT-PhNO2)2]. In the [Cu­(hfac)2(NIT-PhNO2)2] moiety, the radicals are bonded to the copper­(II) ion in the axial positions via the nitroxides. For three compounds, 1D supramolecular chains are formed via the π–π stacking interactions involving the radical ligands. Magnetic investigations show that both Tb complexes exhibit slow relaxation of magnetization at low temperature; strikingly, complex 3 displays a higher energy barrier than that of 1. It represents the first example to use the paramagnetic complex to tune magnetic relaxation of 4f-based compounds.