Syntheses, Structures, Magnetic Properties, and Density Functional Theory Magneto-Structural Correlations of Bis(μ-phenoxo) and Bis(μ-phenoxo)-μ-acetate/Bis(μ-phenoxo)-bis(μ-acetate) Dinuclear Fe^IIINi^II Compounds

The bis­(μ-phenoxo) Fe^IIINi^II compound [Fe^III(N₃)₂LNi^II(H₂O)­(CH₃CN)]­(ClO₄) (<b>1</b>) and the bis­(μ-phenoxo)-μ-acetate/bis­(μ-phenoxo)-bis­(μ-acetate) Fe^IIINi^II compound {[Fe^III(OAc)­LNi^II(H₂O)­(μ-OAc)]_0.6·[Fe^IIILNi^II(μ-OAc)₂]_0.4}­(ClO₄)·1.1H₂O (<b>2</b>) have been synthesized from the Robson type tetraiminodiphenol macrocyclic ligand H₂L, which is the [2 + 2] condensation product of 4-methyl-2,6-diformylphenol and 2,2′-dimethyl-1,3-diaminopropane. Single-crystal X-ray structures of both compounds have been determined. The cationic part of the dinuclear compound <b>2</b> is a cocrystal of the two species [Fe^III(OAc)­LNi^II(H₂O)­(μ-OAc)]⁺ (<b>2A</b>) and [Fe^IIILNi^II(μ-OAc)₂]⁺ (<b>2B</b>) with weights of 60% of the former and 40% of the latter. While <b>2A</b> is a triply bridged bis­(μ-phenoxo)-μ-acetate system, <b>2B</b> is a quadruply bridged bis­(μ-phenoxo)-bis­(μ-acetate) system. Variable-temperature (2–300 K) magnetic studies reveal antiferromagnetic interaction in <b>1</b> and ferromagnetic interaction in <b>2</b> with <i>J</i> values of −3.14 and 7.36 cm^–1, respectively (<i>H</i> = −2<i>JS</i>₁·<i>S</i>₂). Broken-symmetry density functional calculations of exchange interaction have been performed on complexes <b>1</b> and <b>2</b> and also on previously published related compounds, providing good numerical estimates of <i>J</i> values in comparison to experiments. The electronic origin of the difference in magnetic behavior of <b>1</b> and <b>2</b> has been well understood from MO analyses and computed overlap integrals of BS empty orbitals. The role of acetate and thus its complementarity/countercomplementarity effect on the magnetic properties of diphenoxo-bridged Fe^IIINi^II compounds have been determined on computing <i>J</i> values of model compounds by replacing bridging acetate and nonbridging acetate ligand(s) by water ligands in the model compounds derived from <b>2A</b>,<b>B</b>. The DFT calculations have also been extended to develop several magneto-structural correlations in these types of complexes, and the correlations focus on the role of Fe–O–Ni bridge angle, average Fe/Ni–O bridge distance, Fe–O–Ni–O dihedral angle, and out-of-plane shift of the phenoxo group