SPIN-CROSSOVER IN NOVEL DIHYDROBIS(1-PYRAZOLY)BORATE [H2B(PZ)(2)]-CONTAINING IRON(II) COMPLEXES - SYNTHESIS, X-RAY STRUCTURE, AND MAGNETIC-PROPERTIES OF [FEL(H2B(PZ)(2))(2)] (L=1,10-PHENANTHROLINE AND 2,2'-BIPYRIDINE)

The first example of spin crossover iron(II) complexes based on dihydr obis(1-pyrazolyl)borate are presented here. The complexes {Fe[H2B(Pz)( 2)](2)(phen)} (phen = 1,10-phenanthroline), 1, and (Fe[H2B(Pz)(2)](2)( bipy)) (bipy = 2,2'-bipyridine), 2, have been synthesized and their st ructures determined by X-ray diffraction methods. Crystals 1 and 2 are monoclinic, space group C2/c, Z = 4 with a 17.448(4) Angstrom, b = 16 .101(4) Angstrom, c = 10.611(2) Angstrom, and beta = 112.47(2)degrees for 1 and a = 16.307(2) Angstrom, b = 15.075(4) Angstrom, c = 11.024(4 ) Angstrom, and beta = 114.95(5)degrees for 2 at 293 K. The crystal st ructure of 2 was also determined at 139 K in order to detect the struc tural changes associated with the S = 0 <-> S = 2 spin conversion. 2 r etains the same space group upon spin conversion with a 16.086-(6) Ang strom, b = 14.855(6) Angstrom, c = 10.812(2) Angstrom, and beta = 114. 18(3)degrees. The structures of 1 and 2 are made up of mononuclear neu tral species where the positive charge of iron(II) is neutralized thro ugh the coordination of two chelate bidentate dihydrobis (pyrazolyl)b orate anions, and phen or bipy neutral ligands are used to fill the ir on(II) coordination sphere. The molecular structures for both compound s are very similar, with Fe-N bond lengths in the 2.212-2.158 Angstrom range for the high-spin phase. The structural modifications associate d with the spin change in 2 mainly consist of a large reorganization o f the metal environment: the Fe-N decreases by 0.15 Angstrom (mean val ue) when the temperature is lowered from 290 to 139 K and a more regul ar shape of the [FeN6] octahedron is achieved through a slight modific ation of the trigonal deformation angle from 5.3 degrees to 3.2 degree s along with remarkable variations of the N-Fe-N angles. The thermodyn amic model of Slichter and Drickamer was applied to account for the ma gnetic data. The intermolecular interaction parameter and the enthapy and entropy changes associated with the spin transition were estimated as Gamma = 3.3 kT mol(-1), Delta H = 13.4 kJ mol(-1), and Delta S = 8 1.9 J mol(-1) K-1 for 1 and Gamma = 1.7 kT mol(-1), Delta H = 13.4 kJ mol(-1), and Delta S = 83.9 J mol(-1) K-1 for 2, respectively.