Cyano-bridged pentanuclear FeIII3MII2 (M = Ni, Co, Fe) clusters:: Synthesis, structures, and magnetic properties

The reactions of [ M-II( Tpm(Me))( H2O)(3)](2+) ( M) Ni, Co, Fe; Tpm(Me)) tris( 3,5-dimethyl-1-pyrazoyl) methane) with [ Bu4N]-[( Tp) Fe-III( CN)(3)] ( Bu4N+) tetrabutylammonium cation; Tp = tris( pyrazolyl) hydroborate) in MeCN - Et2O afford three pentanuclear cyano-bridged clusters, [( Tp)(3)( Tpm(Me))(2)(Fe3M2II)-M-III( CN)(9)] ClO4 center dot 15H(2)O ( M) Ni, 1; M) Co, 2) and [( Tp)(3)- TpmMe)(2)(Fe3Fe2II)-Fe-III( CN) 9] BF4 center dot 15H(2)O ( 3). Single-crystal X-ray analyses reveal that they show the same trigonal bipyramidal structure featuring a D-3h-symmetry core, in which two opposing Tpm(Me)-ligated MII ions situated in the two apical positions are linked through cyanide bridges to an equatorial triangle of three Tp-ligated FeIII ( S = 1/2) centers. Magnetic studies for complex 1 show ferromagnetic coupling giving an S = 7/2 ground state and an appreciable magnetic anisotropy with a negative D-7/2 value equal to -0.79 cm(-1). Complex 2 shows zero-field splitting parameters deducted from the magnetization data with D = -1.33 cm-1 and g = 2.81. Antiferromagnetic interaction was observed in complex 3.