Synthesis, stereochemistry, and spin transition of heterometal dinuclear Fe(II)-Cr(III) complex bridged by 3,5-bis(pyridin-2-yl)pyrazolate

We have synthesized and characterized a new heterometal dinuclear complex, [(nta)Cr-III(mu -bpypz)Fe-II(picen)]BF4 (1), where nta = nitritotriacetate; bpypz(-) =3,5-bis(pyridin-2-yl)-pyrazolate; and picen = N,N'-bis-(2-pyridy lmethyl)ethylenediamine. The X-ray structure of the Fe(II)-Cr(III)(.)H(2)O( .)3dmf demonstrated that this has a dinuclear structure with a highly disto rted octahedron around a high-spin Fe(II) and intramolecular bifurcated thr ee-centered hydrogen bonds between the carboxylate oxygens of the ma and an amine proton of the picen with an abnormal conformation. A magnetic suscep tibility measurement and/or the Mossbauer spectra of 1 substantiated a grad ual spin-transition from 130 K to 300 K in a heating process centered aroun d 250 K with a small hysteresis dip in the Fe(H) complex. The spin-transiti on temperature of 1 is lower than that of the mononuclear complex, [Fe-II(p icen)(Hbpypz)](BF4)(2)(2), resulting from the weaker ligand field of 1 with a distorted octahedron than that of 2.