Electronic and structural properties of the spin crossover systems bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) thiocyanate and selenocyanate

Essentially high-spin [Fe(bpp)(2)][NCS](2). 2H(2)O and [Fe(bpp)(2)][NCSe](2 ) (bpp = 2,6-bis(pyrazol-3-yl)pyridine) were isolated from an aqueous react ion mixture. Both salts undergo an abrupt transition to low spin below room temperature, that for the thiocyanate occurring in two steps and the high- spin --> low-spin --> high-spin cycle being accompanied by hysteresis in bo th steps. Recrystallization of the salts from nitromethane yielded a mixtur e from which bright yellow crystals were separated for structure determinat ion. In addition, from the recrystallized selenocyanate, deep red-brown cry stals of composition [Fe(bpp)(2)][NCSe](2).H2O .0.25 CH3NO2 were obtained. Recrystallized [Fe(bpp)2][NCS](2). 2H(2)O and [Fe(bpp)(2)][NCSe](2) were id entified as high spin with average Fe-N distances of 2.16 and 2.17, respect ively. In the unit cell of [Fe(bpp)(2)][NCSe](2).H2O .0.25 CH3NO2, there ar e four independent iron atoms, three identified as low spin and the fourth as high spin. All salts crystallize in a layer-type array involving edge-to -face and face-to-face aryl-aryl-type interactions. Hydrogen bonding betwee n pyrazole >NH groups, anions and solvate molecules is observed. The struct ure of the uncoordinated ligand was also determined, the molecule being fou nd in a planar arrangement with the cis-cis configuration for the pyrazolyl groups relative to the central pyridyl and the >NH group being at the N 2 atom. Hydrogen bonding involving the >NH groups leads to stepped stacks of molecules. The principal difference in the geometry of coordinated and free bpp molecules is a contraction in the angles about the interannular bridge s in the chelate rings. [Fe(bpp)(2)][NCS](2). 2H(2)O: triclinic, space grou p P (1) over bar, a 8.302(6), b 8.446(6), c 21.531(13), alpha 78.78(5), bet a 82.80(5), gamma 89.85(4)degrees, Z 2. [Fe(bpp)(2)][NCSe](2): triclinic, s pace group P (1) over bar, a 8.354(4), b 8.409(4), c 19.918(9), alpha 87.02 ( 3), beta 83.15(3), gamma 88.86(3)degrees, Z 2. [Fe(bpp)(2)][NCSe](2).H2O .0.25 CH3NO2: monoclinic, space group Pn, a 16.425(12), b 20.774(9), c 16.9 33(14), beta 90.91(4)degrees, Z 8. Uncoordinated bpp: orthorhombic, space g roup Pna2(1), a 8.075(3), b 22.479(9), c 5.525(1), Z 4.