Structural Adaptation of Ni₄O₄ Units To Form Cubane, Open Dicubane, Dimeric Cubane, and One-Dimensional Polymeric Cubanes: Magnetostructural Correlation of Ni₄ Clusters

The complexation reactions of a tripodal chelating ligand, [3,5-bis­(2-amino-ethyl)-[1,3,5]­triazinan-1-yl]-methanol (<b>L</b>), which is produced by the <i>in situ</i> transformation of 1,3,6,8-tetraazatricyclo­[4.4.1.1^3,8]­dodecane (<b>L</b>^<b>1</b>) with Ni­(NO₃)₂·6H₂O has been explored in the presence of ammonium salts of inorganic and organic anions. These reactions resulted in four crystalline complexes [Ni₄(<b>L</b>)₂­(μ₃-OH)₂­(NCS)₄]·4H₂O (<b>1</b>), [Ni₄(<b>L</b>)₂­(μ₂-N₃)₄­(N₃)₂]·2H₂O (<b>2</b>), [Ni₈(<b>L</b>)₄­(μ₃-OH)₄­(BDC)₃­(H₂O)₄]·BDC·28­(H₂O) (<b>3</b>, BDC = 1,4-benzene dicarboxylate) and {[Ni₄(<b>L</b>)₂­(μ₃-OH)₂­(NDS)₂­(H₂O)₂]·NDS·11­(H₂O)}_<i>n</i> (<b>4</b>, NDS = naphthalene-1,5-disulfonate). The crystal structure analyses of <b>1</b>–<b>4</b> reveal that all contain Ni­(II) clusters, which act as secondary building units to generate higher order aggregates. The complexes <b>1</b>, <b>3</b>, and <b>4</b> contain exclusively Ni₄ cubane units: a discrete cubane in <b>1</b>, a dimer of cubanes linked by BDC in <b>3</b>, and cubanes linked in one dimension by NDS to form a 1D-coordination polymer in <b>4</b>. Interestingly, complex <b>2</b> exhibits an open dicubane with two missing vertices. Although a plethora of water molecules had been included in their crystal lattices, the crystals were found to be stable even at room temperature. The water molecules govern the overall crystal packing by the formation of strong hydrogen bonds and clusters. Large clusters of water such as (H₂O)₂₈ and (H₂O)₁₆ were observed in <b>3</b> and <b>4</b>, respectively, while dimers of water were observed in <b>1</b> and <b>2</b>. Magnetic susceptibility (χ_M) measurements in the temperature range of 2–300 K on <b>1</b>–<b>3</b> reveal that the metal centers are ferromagnetically coupled in all three depending on their respective exchange pathways. Interestingly, the room temperature (300 K) χ_M<i>T</i> values increase as the molecular aggregation increases from discrete cubane (5.5 cm³ K mol^–1) to face sharing open dicubane (6.21 cm³ K mol^–1) to connected dicubane (11.36 cm³ K mol^–1). The modes of bridging by ^–OH, N₃^–, and BDC and their bond angles with paramagnetic Ni­(II) centers clearly explained the overall ferromagnetism operating in the spin clusters