Hexa-, hepta- and dodeca-nuclear nickel(II) complexes of three Schiff-base ligands derived from 1,4-diformyl-2,3-dihydroxybenzene

Abstract

Three new closely-related diimine ligands, H₄L¹1, H₄L²22 and H₄L³3, were prepared by the condensation of 1,4-diformyl-2,3-dihydroxybenzene (1) with two equivalents of 2-aminoethanol, 3-aminopropanol or 2-aminophenol, respectively. Three polynuclear complexes, [Ni^II₇(H₂L¹1)₄(OAc)₆(H₂O)₂]·5H₂O (2·5H₂O), [Ni^II₆(H₂L²22)₂(OAc)₆(μ-OMe)₂(H₂O)₂]·5H₂O (3·5H₂O) and [Ni^II₁₂(L³3)₆(MeOH)₇(H₂O)₃]·4H₂O (4·4H₂O), were prepared by reaction of the appropriate ligand with 1.50/1.75, 1.75/3.00 or 2.00 equivalents, respectively, of nickel(II) acetate tetrahydrate, in methanol. The crystal structures of these complexes are very different to one another, despite what initially appear to be relatively minor ligand modifications. While [Ni^II₇(H₂L¹1)₄(OAc)₆(MeOH)₂]·7MeOH·4H₂O possesses C₂ symmetry and has a 4:7 L:M ratio, [Ni^II₆(H₂L²22)₂(OAc)₆(μ-OMe)₂(MeOH)₂]·0.5MeOH·0.5H₂O·Et₂O and [Ni^II₁₂(L³3)₆(MeOH)_6.5(H₂O)_3.5]·solvent have no internal symmetry and have L:M ratios of 2:6 and 6:12, respectively. Two of the four ligands in [Ni^II₇(H₂L²22)₄(OAc)₆(MeOH)₂] are in similar planes, whilst the two remaining ligands are near perpendicular to these planes. The six ligands in [Ni^II₁₂(L³3)₆(MeOH)_6.5(H₂O)_3.5] adopt an offset grid-like arrangement with eight of the twelve metal ions at the intersections of the perpendicular, offset, ligands. In contrast, [Ni^II₆(H₂L²22)₂(OAc)₆(μ-OMe)₂(MeOH)₂] features an [Ni^II₄(OR)₄] cubane core with wing tips of the remaining two nickel(II) ions. All three complexes were subsequently prepared in stoichiometric syntheses, in yields of 50% (2·5H₂O), 48% (3·5H₂O) and 62% (4·4H₂O). The magnetic properties of 2·5H₂O, 3·5H₂O, 4·4H₂O are consistent with intra-complex antiferromagnetic coupling dominating overall, and therefore all three complexes display low spin ground states.