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)CrIII(μ-bpypz)FeII(picen)]BF4 (1), where nta = nitritotriacetate; bpypz− =3,5-bis(pyridin-2-yl)-pyrazolate; and picen = N,N′-bis-(2-pyridylmethyl)ethylenediamine. The X-ray structure of the Fe(II)–Cr(III)·H2O·3dmf demonstrated that this has a dinuclear structure with a highly distorted octahedron around a high-spin Fe(II) and intramolecular bifurcated three-centered hydrogen bonds between the carboxylate oxygens of the nta and an amine proton of the picen with an abnormal conformation. A magnetic susceptibility measurement and/or the Mössbauer spectra of 1 substantiated a gradual spin-transition from 130 K to 300 K in a heating process centered around 250 K with a small hysteresis dip in the Fe(II) complex. The spin-transition temperature of 1 is lower than that of the mononuclear complex, [FeII(picen)(Hbpypz)](BF4)2(2), resulting from the weaker ligand field of 1 with a distorted octahedron than that of 2.