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 Mossbauer 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 Fe-57 Mossbauer 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 approximate to 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. Delta H and Delta S 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 Delta H = 3.0 +/- 0.1 kJ mol(-1) and Delta 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 Angstrom 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.