Structural Diversity in a New Series of Halogenated Quinolyl Salicylaldimides-Based Fe^III Complexes Showing Solid-State Halogen-Bonding/Halogen···Halogen Interactions

A new series of tridentate <i>N</i>-8-quinolyl-salicylaldimine ligands, Hqsal-5,7-X₂ [X = Cl­(<b>1</b>), Br­(<b>2</b>), I­(<b>3</b>)], halo-substituted at the 5,7 position of the aminoquinoline moiety and their corresponding complexes with Fe^III were synthesized and formulated as [Fe­(qsal-5,7-X₂)­(NCS)­(MeO)]₂·solv. (X = Cl (<b>1a</b>), Br (<b>2a</b>), I (<b>3a</b>, solv = ¹/₂MeOH), [Fe₄(qsal-5,7-X₂)₄­(NCS)₂­(MeO)₂]­·solv. (X = Br (<b>2b</b>), I (<b>3b</b>; solv = 4CH₂Cl₂)) by single-crystal X-ray diffraction analysis. <b>1a</b> and <b>2a</b> are isostructural dimers where each Fe^III metal ion, showing a distorted octahedral environment, is bound by a N,N,O tridentate (qsal-5,7-X₂)⁻ (X = Cl and Br) ligand, a N-coordinated SCN^– anion, and a bridging methanolate anion. <b>2b</b> and <b>3b</b> are centrosymmetric tetramers where each Fe^III is bound by three nitrogen atoms and three oxygen atoms derived from a tridentate (qsal-5,7-X₂)⁻ (X = Br and I), a SCN^–, a bridging methanolate anion, and a bridging μ²-oxy moiety. In <b>3b</b>, the iodine atoms dominate the packing interactions through the establishment of a halogen-bonding network. The magnetic behavior of <b>1a</b>–<b>3a</b> dimers and <b>2b</b>–<b>3b</b> tetramers indicate the presence of strong antiferromagnetic interactions between Fe^III centers (<i>S</i> = 5/2), mediated by the alkoxy bridges. Experimental data can be modeled with an isotropic Hamiltonian, <i>H</i> = −2<i>J</i>(<i>S</i>₁ · <i>S</i>₂) for dimers (<i>J</i> = −15 cm^–1) and <i>H</i> = −2<i>J</i>(<i>S</i>₂ · <i>S</i>₃) – 2<i>J</i>′(<i>S</i>₁ · <i>S</i>₂ + <i>S</i>₃ · <i>S</i>₄), for tetramers (<i>J</i> = −24 cm^–1, <i>J</i>′ = −11 cm^–1). The magnetic behavior of <b>1a</b>–<b>3a</b> dimers indicates the presence of strong antiferromagnetic interactions between Fe^III centers (<i>S</i> = 5/2), mediated by the alkoxy bridges. <b>2b</b> and <b>3b</b> show the same magnetic behavior since they contains analogous bridges between paramagnetic centers, but for a linear tetramer. Density functional theory (DFT) calculations, based on hybrid functional mPW1PW paralleled by the Def2SVP all-electron split-valence basis sets, support the experimental results, showing that monomers could possibly show a spin crossover (SCO) behavior, even though the formation of complexes with an even number of metal ions results in the strong antiferromagnetic interactions. Accordingly, broken symmetry DFT calculations carried out on <b>1a</b> clearly show that the antiferromagnetic coupling of the two HS Fe^III centers results in the lowest energy electron configuration of the complex