Coordinative flexibility of 1,2-bis[1,4,7-triazacyclonon-1-yl]propan-2-ol in mononuclear and binuclear Ni(ii) complexes

Abstract

Reaction of 1,2-bis[1,4,7-triazacyclonon-1-yl]propan-2-ol hexabromide (T₂PrOH.6HBr) with Ni(ClO₄)₂·6H₂O and adjustment of the pH to 7 resulted in the crystallization of pink and blue products from the one reaction mixture. The analytical data and X-ray structure determinations establish compositions corresponding to [Ni(T₂PrOH)]Br(ClO₄)·H₂O (1, pink crystals) and [Ni₂(T₂PrO)(OH₂)₃Br]Br(ClO₄)·2H₂O (2, blue crystals). A repeat synthesis of the latter yielded the diperchlorate monohydrate [Ni₂(T₂PrO)(OH₂)₃Br](ClO₄)₂·H₂O (3). In the mononuclear complex (1), the 2-propanol group connecting the two 1,4,7-trizacyclononane (tacn) rings is protonated, the six nitrogen donors from the T₂PrOH ligand coordinating to a single Ni(II) centre in a distorted octahedral geometry. In the binuclear complexes (2) and (3), three coordination sites on each distorted octahedral Ni(II) centre are occupied fac by three nitrogen donors from the one tacn ring, the two metal centres being linked by an endogenous alkoxo bridge. A notable common feature of the two identical cations of (2) and (3) is that for one Ni(II) centre the remaining two sites are occupied by two water ligands, while in the other a bromo ligand replaces one ligated water. Similar binuclear systems have been recently defined [Zn₂(T₂PrO)X(H₂O)₂](ClO₄)₂ (X = Cl, Br), two complexes that exhibit coordination asymmetry with one pseudo-octahedral and one pseudo-square pyramidal Zn(II) centre. The weak antiferromagnetic coupling in 2 and 3 is discussed and compared to di-phenoxo-bridged Ni(II) examples.