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 Seff = 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 Δr/kB = 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.