Spin Crossover in the 2,2‘-Bipyrimidine- (bpym-) Bridged Iron(II) Complexes [Fe(L)(NCX)2]2(bpym) (L = 2, 2‘-Bithiazoline (bt) and bpym; X = S, Se). X-ray Absorption Spectroscopy, Magnetic Susceptibility, Calorimetric, and Mössbauer Spectroscopy Studies

This contribution is concerned with the synthesis and characterization of two new iron(II) spin-crossover dinuclear compounds of the formula [Fe(bpym)(NCSe) 2 ] 2 (bpym) ( 2 ) and [Fe(bt)(NCSe) 2 ] 2 (bpym) ( 4 ), where bpym = 2,2‘-bipyrimidine and bt = 2,2‘-bithiazoline. Evidence for a S = 2 (high spin, HS) ↔ S = 0 (low spin, LS) spin-crossover behavior is provided by variable-temperature magnetic susceptibility measurements and 57 Fe Mössbauer spectrometric data. Compound 2 shows only one step which involves 50% of the iron(II) ions. The transition is very abrupt and takes place at T c ≈ 120 K when cooling and shows a hysteresis loop of 2.5 K. A two-step spin transition takes place for 4 . The first one occurs at 223 K and the second one at 265 K. Differential scanning calorimetry exhibits two peaks in the case of 4, a sharp one at 225 K and a broad one with a maximum at 262 K. Δ H and ΔS variations, per mol of dinuclear units, have been evaluated accurately for the first peak only, 5.7 ± 0.5 kJ mol - 1 and 25.5 ± 1 J mol - 1 K - 1, respectively. A rough estimate of the global variations gave 16 kJ mol - 1 and 65 J mol - 1 K - 1, respectively. Thermodynamic parameters, per mol of dinuclear units, for 2 are Δ H = 3.0 ± 0.1 kJ mol - 1 and Δ S = 25 ± 1 J mol - 1 K - 1 . X-ray absorption spectroscopy has been carried out for 2 and 4 as well as for the parent compounds [Fe(bpym)(NCS) 2 ] 2 (bpym) ( 1 ) and [Fe(bt)(NCS) 2 ] 2 (bpym) ( 3 ). The X-ray absorption near edge structures spectra of the two spin isomers are interpreted and the extended X-ray absorption fine structures spectra are analyzed quantitatively at the first coordination shell. The average Fe−N bond distance is close to 0.2 Å larger for the HS state than that of the LS state. The spin transitions in these dimeric compounds are interpreted in terms of LS−LS ↔ LS−HS and LS−HS ↔ HS−HS intramolecular processes.