From Mononuclear Compounds to [2 × 2] Metallogrids: Ferromagnetically Coupled Systems Built by Nickel(II) and 3,6-Bis(2′-pyridyl)pyridazine (dppn)

Mono-, di-, tri-, and tetranuclear compounds of nickel­(II) of formula [Ni­(dppn)₃]­(NCS)₂·0.5dppn (<b>1</b>), [{Ni­(dppn)­(NCS)}₂(μ-dppn)­(μ-NCS)]­NCS (<b>2</b>), [Ni₃(dppn)₂(N₃)₂(μ-dppn)₂(μ-N₃)₂]­(ClO₄)₂·CH₃CH₂OH·2H₂O (<b>3</b>), and [Ni₄(μ-dppn)₄(μ-N₃)₄]­Cl₄·5H₂O (<b>4</b>) [dppn = 3,6-bis­(2′-pyridyl)­pyridazine] have been prepared and their structures determined by single crystal X-ray diffraction. Compound <b>1</b> is made up of mononuclear tris-chelated [Ni­(dppn)₃]²⁺ units, thiocyanate counterions, and dppn molecules of crystallization. <b>2</b> contains cationic dinuclear units, the six coordination around each metal center being achieved with a pyridazine-dppn and one end-on thiocyanate nitrogen as bridges, a didentate dppn ligand and a terminally bound <i>N</i>-thiocyanate. The electroneutrality in <b>2</b> is reached by free thiocyanato anions. <b>3</b> contains bent trinuclear cationic units where all nickel­(II) ions exhibit a NiN₆ chromophore with two bridging bis-didentate dppn and two end-on azide groups connecting the metal centers. Didentate dppn and monodentate azide ligands complete the six coordination at the peripheral nickel­(II) ions. The charge in <b>3</b> is counterbalanced by perchlorate anions and ethanol and water molecules of crystallization are also present. <b>4</b> exhibits a [2 × 2] grid-type of nickel­(II) ions where each pair of metal atoms are connected across a pyridazine-dppn fragment and one end-on azido group. Free chloride anions neutralize the positive charge of the grid and water molecules of crystallization are also present. Cryomagnetic susceptibility measurements on crushed crystals of <b>1</b>–<b>4</b>, in the temperature range 1.9–300 K, reveal the occurrence of a Curie law for <b>1</b> and overall ferromagnetic behavior for <b>2</b>–<b>4</b> [<i>J</i> = +4.52(2) (<b>2</b>), +1.32(1) (<b>3</b>), and +10.50(2) and +9.10(2) cm^–1 (<b>4</b>)]. The dominant ferromagnetic contribution across the single end-on thiocyanato (<b>2</b>) or azido bridges (<b>3</b> and <b>4</b>) versus the antiferromagnetic one through the pyridazine (<b>2</b>–<b>4</b>) is at the origin of this behavior. The values of the intramolecular ferromagnetic interactions in <b>3</b> and <b>4</b> were substantiated by DFT-type calculations. No ac signals were observed for <b>2</b>–<b>4</b> either in the lack of or under nonzero applied dc fields.