English abstract
A diverse range of iron(III) Schiff-base compounds with en-based salicylaldimines, ketimines and pyridylhydrazones have been synthesised, isolated and characterised by a cocktail of physical techniques pertinent to the spin-crossover phenomenon. These compounds are [Fe(happh)2]X (X = BF4, BPh4, C104 or C1), [Fe{(5-0Me) salph}2]X (X = BPh4, C104 or C1), [Fe{(3-OMe)-salph}2]BPh4, [Fe(hapEen)2]C104, [Fe(hapMen)2]C104, [Fe{(3-R1,5-R3)-salEen)}2]C104 (R' = R2 =H; R' = R2 = 'Buz; R' = OM, R? = Br; R=R2 = C1; R'= Me, R2 = H; R' = H, R2 = Me; R'=H, R2 = NO2; R' = H, R2 = OMe) and (Fe{(3-R',5-R3)-salMen}2]CIO4 (R' = R2 = 'Buz; R' = H, R2 = Me). The chemical identities of these compounds have been ascertained by elemental analyses and infrared spectroscopy. The X-ray crystal structures of the six-coordinate iron(III) complex cations (Fe(happh)2]* and [Fe{(3-R1,5-R2)-salEen}2]* (R' = R2 = H; R' = R2 = 'Buz; R' = H, R2 = Me; and R' = H, R2 = OMe) (with C104 as the counter anion) show the coordination sphere at the metal centre to consist of two tridentate Schiff-base ligands oriented perpendicularly to each other, with each set of donor atoms arranged meridionally to give a cis,cis, trans-coordination mode. The A1g 272g spin transformations were probed with the aid of magnetic susceptibility measurements, single-crystal X-ray structure determinations, ESR spectroscopy and electronic spectroscopy. A complex interplay of factors was demonstrated to influence the spin states of the compounds; these included the nature, size and position of a given substituent group as well as the type of counter ion. In the solid state, (Fe{(3,5-'Buz) salEen}2]C104 and [Fe(happh)2]BPh4 are high spin between 300 and 5 K whereas [Fe{(3,5-C12)-salEen}2]C104 and [Fe{(3-Me)-salEen}2]C104 are low-spin under the same conditions. The compounds [Fe(happh)2]C104-MeOH, [Fe(happh)2]BF4, [Fe(hapEen)2]C104, [Fe{(5-OMe)-salEen}z]C104, [Fe{(5,3-Br,OMe)-salEen}2]CI04 and [Fe{(5-NO2)-salEen}2]C104 exhibit incomplete spin transitions: the hydrazones are fully HS at room temperature, but do not convert completely to the LS state at low temperature; on the other hand, the ketimines and salicylaldimines are fully low spin below 100 K, but fail to convert completely to the HS state up to 300 K. [Fe(salEen)2]C104 and [Fe{(5-Me)-salEen}2]CIOs undergo complete S = 5/2 ++ S= 1/2 transformations: in the former compound the spin transition occurs gradually over a
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very wide temperature range whereas in the latter the crossover takes place abruptly in two steps. All the iron(III) compounds convert to the low-spin state in methanol solution at liquid nitrogen temperature as revealed by ESR spectroscopy. With the exception of [Fe{(3,5-'Buz)-salEen}2]C104, all the iron(III) compounds show interconversion of the HS and LS states in methanol at room temperature. The spin states were readily distinguishable by the energies of the associated LMCT absorptions in the visible spectra.
