New Type of Single Chain Magnet: Pseudo-One-Dimensional Chain of High-Spin Co(II) Exhibiting Ferromagnetic Intrachain Interactions

Two new six-coordinated high-spin Co­(II) complexes have been synthesized through the reactions of Co­(II) salts with dipyridylamine (dpamH) and 5-nitro-salicylaldehyde (5-NO₂-saloH) or 3-methoxy-salicylaldehyde (3-OCH₃-saloH) under argon atmosphere: [Co­(dpamH)₂(5-NO₂-salo)]­NO₃ (1) and [Co­(dpamH)₂(3-OCH₃-salo)]­NO₃·1.3 EtOH·0.4H₂O (2). According to the crystal packing of compound 1, two coordination cations are linked with two nitrate anions into a cyclic dimeric arrangement via N–H···O and C–H···O hydrogen bonds. In turn, these dimers are assembled into (100) layers through π–π stacking interactions between inversion-center related pyridine rings of the dpamH ligands. The crystal packing of compound 2 reveals a 1D assembly consisting solely from the coordination cations, which is formed by π–π stacking interactions between pyridine rings of one of the dpamH along the [010] and another 1D assembly of the coordination cations and nitrate anions through the N–H···O hydrogen-bonding interactions along the [001] direction. All complexes were magnetically characterized, and a new approximation method was used to fit the magnetic susceptibility data in the whole temperature range 2–300 K on the basis of an empirical expression which allows the treatment of each cobalt­(II) ion in axial symmetry as an effective spin S_eff = ¹/₂. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.5 K for compound 2. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ_r/k_B = 55(1) K for the [10–1488 Hz] frequency range. In order to reveal the importance of the crystal packing in the SCM behavior, a gentle heating process to 180 °C was carried out to remove the solvent molecules. The system, after heating, undergoes a major but not complete collapse of the network retaining to a small percentage its SCM character.