The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition metal complex [Fe(II)(L(•))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(•) coordinate equatorially while the oxygen containing the radical from L(•) coordinates axially forming a linear O(•)··Fe(II)··O(•) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.

The first Fe(III) qsal-X complex exhibiting abrupt complete spin crossover at 228 K with a hysteresis of 8 K, [Fe(qsal-I)2]OTf is reported. Structural studies of the MeOH solvate in the LS and HS state and at the spin transition are described.

The reaction of Fe(NCS)3 prepared in situ in MeOH with Hqsal-X (Hqsal-X = 5-X-N-quinolylsalicylaldimine) in CH2Cl2 yields the FeIII complexes, [Fe(qsal-X)2]NCS·solvent (X = F 1; X = Cl, 2, Br, 3 solvent = MeOH; X = I, solvent = 0.25CH2Cl2·0.5MeOH 4) in moderate to good yields. IR spectroscopy confirms that NCS− acts as a counteranion only and that the qsal-X ligand is bound to the FeIII centre. SQUID magnetometric studies reveal stepped hysteretic spin crossover in 1 and 2, which is abrupt in both steps in latter compound. Mössbauer spectroscopic studies of 1 and 2 support these conclusions. The bromo derivative, 3, undergoes half spin crossover up to 340 K while 4 is low spin at all temperatures measured. The spin transition temperature, T1/2 is found to increase on moving from F to Br. UV-Vis and NMR spectroscopic studies indicate that 1–4 have spin states intermediate between HS and LS in solution. Structural studies show that 1, 2 and 3 crystallize in triclinic P while 4 is in monoclinic P21/c. Crystallographic studies of 1 at 100, 200 and 270 K show that spin crossover proceeds from a [LS–LS] state through a [LS–HS] intermediate to a [HS–HS] state (LS = low spin, S = 1/2, HS = high spin, S = 5/2). Similar results are found for 3 although this time a [LS–IS] state exists at 123 K while a [LS–HS] state is found at 295 K (IS = intermediate spin state where partial spin crossover has occurred). Both 2 and 4 are found to have LS FeIII centres although the latter contains two crystallographically independent FeIII centres in the asymmetric unit. The crystal packing in 1–4 consists of extensive π–π interactions through the planar qsal-X ligands and CH∙∙∙X (X = O, halogen) and/or X∙∙∙π (X = halogen) interactions which result in pseudo 3D supramolecular networks. This results in high cooperativity in 1 and 2 and is probably responsible for the hysteretic stepped spin crossover in these compounds.

The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.