Syntheses and magnetic properties of a bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine and its metal complexes

Abstract

A novel bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine, NIT-2-TrzPm (NIT-2-TrzPm = (2-(2′-triazolopyrimidine)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and six new transition metal complexes of this ligand, namely [M(hfac)₂(NIT-2-TrzPm)]·CH₂Cl₂ (M = Mn (1_Mn) and Co (2_Co)), [M(hfac)₂]₂(NIT-2-TrzPm) (M = Mn (3_Mn) and Co (4_Co)), [Mn(NIT-2-TrzPm)₂(MeOH)₂](ClO₄)₂·MeOH (5_Mn), and [Co(NIT-2-TrzPm)₂(MeOH)₂]₂(ClO₄)₄·4MeOH (6_Co) were prepared and characterized structurally and magnetically. These complexes can be selectively synthesized by controlling the reaction ratio of M(hfac)₂·2H₂O to the radical ligand (for 1_Mn to 4_Co) or using metal perchlorates as the starting materials (for 5_Mn and 6_Co). Single crystal X-ray crystallographic analyses confirmed that 1_Mn and 2_Co are isostructural 3d–2p M^II–radical complexes, in which the NIT-2-TrzPm radical acts as a terminal bidentate ligand chelating to one 3d ion, while 3_Mn and 4_Co are isostructural 3d–2p–3d M^II–radical–M^II complexes with the NIT-2-TrzPm radical acting as a bridging ligand between two 3d ions. For complexes 5_Mn and 6_Co, two NIT-2-TrzPm ligands from the equatorial positions coordinate with the metal center to form the 2p–3d–2p structures with the axial positions occupied by two methanol molecules. Magnetic analysis on the Mn^II complexes revealed the existence of a strong antiferromagnetic interaction between the Mn^II and the NIT radical spin, while weak ferromagnetic coupling for Mn⋯Mn and Rad⋯Rad in the Mn–NIT–Mn and Rad–Mn–Rad spins was confirmed. Interestingly, although the NIT-bridged complexes 3_Mn and 4_Co possess significantly different magnetic anisotropy, field-induced slow magnetic relaxation can be observed in both complexes, which was assigned to the phonon bottleneck effect for 3_Mn and field-induced SMM behavior for 4_Co. To the best of our knowledge, 3_Mn is the first example of the NIT-bridged binuclear Mn^II complex undergoing slow magnetic relaxation.