From Mononuclear Compounds to [2 x 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 (1), [{Ni-(dppn)(NCS)}(2)(mu-dppn)(mu-NCS)]NCS (2), [Ni-3(dppn)(2)(N-3)(2)(mu-dppn)(2)(mu-N-3)(2)](ClO4)(2)center dot CH3CH2OH center dot 2H(2)O (3), and [Ni-4(mu-dppn)(4)(mu-N-3)(4)]Cl-4 center dot 5H(2)O (4) [dppn = 3,6-bis(2'-pyridyl)pyridazine] have been prepared and their structures determined by single crystal X-ray diffraction. Compound 1 is made up of mononuclear tris-chelated [Ni(dppn)(3)](2+) units, thiocyanate counterions, and dppn molecules of crystallization. 2 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 N-thiocyanate. The electroneutrality in 2 is reached by free thiocyanato anions. 3 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 3 is counterbalanced by perchlorate anions and ethanol and water molecules of crystallization are also present. 4 exhibits a [2 x 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 1-4, in the temperature range 1.9300 K, reveal the occurrence of a Curie law for 1 and overall ferromagnetic behavior for 2-4 [J = +4.52(2) (2), +1.32(1) (3), and +10.50(2) and +9.10(2) cm(-1) (4)]. The dominant ferromagnetic contribution across the single end-on thiocyanato (2) or azido bridges (3 and 4) versus the antiferromagnetic one through the pyridazine (2-4) is at the origin of this behavior. The values of the intramolecular ferromagnetic interactions in 3 and 4 were substantiated by DFT-type calculations. No ac signals were observed for 2-4 either in the lack of or under nonzero applied dc fields.