A new synthesis of (8-quinolyl)-5-methoxysalicylaldimine (Hqsal-5-OMe) is reported and its crystal structure is presented. Two Fe(III) complexes, [Fe(qsal-5-OMe)(2)]Cl⋅solvent (solvent = 2 MeOH⋅0.5 H(2)O (1) and MeCN⋅H(2)O (2)) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal-5-OMe)(2)]Cl⋅2 MeOH⋅0.5 H(2)O (1) exhibits rare crystallographically independent high-spin and low-spin Fe(III) centres at 150 K, whereas [Fe(qsal-5-OMe)(2)]Cl⋅ MeCN⋅H(2)O (2) is low spin at 100 K. In both structures there are extensive π-π and C-H⋅⋅⋅π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped-spin crossover with T(1/2) = 245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half-spin crossover also with a hysteresis of 10 K. The two compounds are the first Fe(III) complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by (57) Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to Fe(II) at -0.18 V and irreversible oxidation of the coordinated qsal-5-OMe ligand at +1.10 V.

A series of FeIII complexes, [Fe(qsal‐5‐OMe)2]Y [Y = BF4 (1), PF6 (2), NCS (3), BPh4 (4)], have been prepared and structurally and magnetically characterized. The low‐temperature structures of 1 and 3 as solvent adducts were determined by X‐ray crystallography with LS FeIII centres found in both cases. π–π and C–H···π interactions between the cations lead to 2D sheets that are linked to one another through C–H···O and, in the case of 3, C–H···N/S interactions resulting in high cooperativity. Magnetic studies revealed abrupt and gradual spin crossover for 3 and 2, respectively. For 3, spin crossover with possible thermal hysteresis was observed although solvent loss (CH2Cl2) could occur. In contrast, 1 and 4 were found to be low and high spin only, respectively. The results of Mössbauer spectroscopic studies are consistent with the magnetic susceptibility data and indicate that there are two low‐spin FeIII centres in 2. Finally, electrochemical studies showed reversible reduction to FeII at –0.22 V, whereas reversible oxidation of the qsal‐5‐OMe ligand was found to occur between 1.04–1.06 V.

Reaction of a rigid tridentate ligand o-[(1H-imidazol-2-yl)methylideneamino]phenol (2-Himap) with Co(ClO) in the presence of NaN, or Co(NO) without a base yields [Co(2-Himap)] 1 and [Co(2-Himap)]NO·MeOH 2, respectively. Both complexes exhibit a mer-octahedral geometry with the cobalt centre being distorted along an octahedral-trigonal prismatic pathway. The packing in 1 and 2 is dominated by H-bonding forming 2D sheets and 1D chains, respectively. Detailed dc and ac magnetic studies indicate that 1 is a field-induced single-ion magnet (SIM) with D = 36.7 cm and E = 2.0 cm. Extensive ab initio calculations support these conclusions and suggest that relaxation of the magnetization occurs principally through direct quantum tunnelling in the ground state, with the Raman process dominant in an applied field. This contrasts with the recently reported series of mer-[Co(L)] (L = monoanionic NNO donor ligand; Inorg. Chem., 2017, 56, 6056-6066) complexes where D is negative, as these compounds have a more ambiguous geometry, and highlights the importance of supramolecular interactions in subtly altering the coordination sphere thereby impacting the magnetic behaviour.

A detailed investigation of the accuracy of different quantum mechanical methods for the study of iron(III) spin crossover complexes is presented. The energy spin state gap between the high and low spin states; ΔE(HS‐LS) of nine iron(III) quinolylsalicylaldiminate complexes were calculated with nine different DFT functionals, then compared. DFT functionals: B3LYP, B3LYP‐D3, B3LYP*, BH&HLYP, BP86, OLYP, OPBE, M06L, and TPSSh were tested with six basis sets: 3‐21G*, dgdzvp, 6‐31G**, cc‐pVDZ, Def2TZVP, and cc‐pVTZ. The cations from the X‐ray crystal structures of [Fe(qsal‐OMe)2]Cl·MeCN·H2O, [Fe(qsal‐OMe)2]Cl·2MeOH·0.5H2O, [Fe(qsal‐OMe)2]BF4·MeOH, [Fe(qsal‐OMe)2]NCS·CH2Cl2, [Fe(qsal‐F)2]NCS, [Fe(qsal‐Cl)2]NCS·MeOH, [Fe(qsal‐Br)2]NCS·MeOH, [Fe(qsal‐I)2]OTf·MeOH, and [Fe(qsal)2]NCS⋅CH2Cl2 were used as starting structures. The results show that B3LYP, B3LYP‐D3, OLYP, and OPBE with a 6‐31G**, Def2TZVP, and cc‐pVTZ basis set give reasonable results of ΔE(HS‐LS) compared with the experimental data. The enthalpy of [Fe(qsal‐I)2]+ calculated with an OLYP functional and cc‐pVTZ basis set (1.48 kcal/mol) most closely matches the experimental data (1.34 kcal/mol). B3LYP* yields an enthalpy of 5.92 kcal/mol suggesting it may be unsuitable for these Fe(III) complexes, mirroring recent results by Kepp (Inorg. Chem., 2016, 55, 2717–2727).

The new ligand Hqsal‐4‐OMe was synthesized, and four iron(III) complexes, [Fe(qsal‐4‐OMe)2]Y·CH2Cl2 [Hqsal‐4‐OMe = 4‐methoxy‐N‐(8‐quinolyl)salicylaldimine; Y = ClO4 (1), NO3 (2)] and [Fe(qsal‐4‐OMe)2]Y (Y = PF6 (3), OTf (4)] as well as the isomer [Fe(qsal‐5‐OMe)2]PF6·CH2Cl2 (5), were prepared. UV/Vis spectroscopic studies indicated that the complexes are high‐spin in solution and exhibit a ligand‐to‐metal charge‐transfer band around 400 nm. X‐ray crystallographic studies on [Fe(qsal‐4‐OMe)2]Y·CH2Cl2 (Y = ClO4, NO3) and [Fe(qsal‐4‐OMe)2]PF6 at 97 K revealed high‐spin FeIII centres with one of the qsal‐4‐OMe ligands significantly distorted from planarity. In contrast, in 5 the FeIII centre is low‐spin at low temperature, and the room‐temperature structure of 5 shows about 25 % spin crossover (SCO). The structure of [Fe(qsal‐4‐OMe)2]Y·sol shows weak π–π interactions between neighbouring cations, while 5 exhibits stronger π–π interactions that link it into a 1D chain. SQUID magnetometric studies revealed that 1–4 are essentially high‐spin, whereas 5 undergoes gradual and almost complete SCO up to 350 K. A combination of distortions to the qsal‐4‐OMe ligand and intermolecular C–H···O interactions involving both the coordinated phenolate oxygen atom and the OMe group seem to be responsible for the loss of SCO.