Synthesis and properties of [MII(L^6)2][ClO4]2(M = Fe, Co and Ni): structures of Co and Ni complexes and spin-state transition by Fe complex (L^6= 2-[3-(2-pyridyl)pyrazol-1-ylmethyl]pyridine)

A new non-planar tridentate ligand 2-[3-(2ʹ-pyridyl)pyrazol-1-ylmethyl]pyridine (L^6) and its mononuclear bis-ligand complexes [M(L6)2] [ClO4]2[M = Fe (1), Co (2) and Ni (3)] have been synthesized. In the dications of six-co-ordinate complexes of 2 and 3, each L^6 is arranged in a meridional mode. Structural analyses reveal that the M-Npyrazole bond length is appreciably shorter than the M-Npyridine bond lengths. Due to the weak field nature of L^6(absorption spectral analyses) towards Fe(II), Co(II) and Ni(II) the complexes are uniformly high-spin at room-temperature. Temperature-dependent magnetic susceptibility measurements reveal that while Co(II) and Ni (II) complexes remain high-spin over the entire range investigated (6-300 K for 2 and 63-300 K for 3), the Fe(II) complex exhibits a temperature-induced (5.2-300 K) spin transition in going from the S= 2 state at 300 K to predominantly an S= 0 state below 80 K. From the linear ln Keq[1A1(ls)(left right arrow)5T2(hs)]vs. 1/T relationship (190-250 K) the derived thermodyanamic parameters are: (delta)H= 9.7 (plus-minus) 0.04 kJ mol^-1 and (delta)S= 56 (plus-minus) 0.18 J K^-1 mol^-1. While complex 1 displays in MeCN an irreversible MIII/MII redox process [anodic peak potential, Epa= 1.06 V vs. SCE], for complex 2 such a redox process is reversible [E1/2= 0.74 V vs. SCE and (delta)Ep= 80 mV].