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 reaction 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 both steps. Recrystallization of the salts from nitromethane yielded a mixture from which bright yellow crystals were separated for structure determination. In addition, from the recrystallized selenocyanate, deep red-brown crystals 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 identified as high spin with average Fe-N distances of 2.16 and 2.17, respectively. In the unit cell of [Fe(bpp)(2)][NCSe](2).H2O .0.25 CH3NO2, there are 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 between pyrazole >NH groups, anions and solvate molecules is observed. The structure of the uncoordinated ligand was also determined, the molecule being found 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 bridges in the chelate rings. [Fe(bpp)(2)][NCS](2). 2H(2)O: triclinic, space group P (1) over bar, a 8.302(6), b 8.446(6), c 21.531(13), alpha 78.78(5), beta 82.80(5), gamma 89.85(4)degrees, Z 2. [Fe(bpp)(2)][NCSe](2): triclinic, space 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 16.933(14), beta 90.91(4)degrees, Z 8. Uncoordinated bpp: orthorhombic, space group Pna2(1), a 8.075(3), b 22.479(9), c 5.525(1), Z 4.