Synthesis, crystal structure and magnetic properties of a new cyanide-bridged iron(III)–nickel(II) ferromagnetic chain

The reaction of the iron(III) unit fac-[Fe{HB(pz)3}(CN)3]− [ HB ( pz ) 3 - = hydrotris ( 1 - pyrazolyl ) borate ] as the lithium salt (1) with the nickel(II) complex mer-[Ni(dpt)(H2O)3](ClO4)2 [dpt = dipropylenetriamine] in water affords the heterometallic compound of formula {[FeIII{HB(pz)3}(CN)3]2[NiII(dpt)]}n · 3nH2O (2). The structure of 2 has been determined by X-ray diffraction on single crystals and their magnetic properties have been investigated in the temperature range 1.9–300 K. Compound 2 is a zigzag chain compound with regular alternating bis-monodentate [Fe(1){HB(pz)3}(CN)3]− units and [Ni(dpt)]2+ cations, the six coordination around the nickel atom being achieved by the coordination of another [Fe(2){HB(pz)3}(CN)3]− group acting as a monodentate end-cap ligand. Magnetic data of 2 show the occurrence of intrachain ferromagnetic coupling (J = +11.4 cm−1 across bridging cyanide) and weak interchain antiferromagnetic interactions (J′ ca. −0.07 cm−1). These last ones are responsible for the observed metamagnetic behaviour of 2, the value of the critical field being Hc = 700 G. The incipient frequency-dependence of the out-of-phase signal of 2 under applied dc field H > Hc that is observed at T < 4.5 K supports the occurrence a slow relaxation of the magnetization, a feature which is reminiscent of the behaviour of the single chain magnets (SCMs). The use of the heteroleptic species [Fe{HB(pz)3}(CN)3]− as a ligand toward fully solvated metal ions and coordinatively unsaturated preformed complexes to prepare heterometallic complexes with new spin topologies is analyzed and discussed in the light of the available magneto-structural data.