No abstract

A new non-planar tridentate ligand 2-[3-(2Ј-pyridyl)pyrazol-1-ylmethyl]pyridine (L 6 ) and its mononuclear bis-ligand complexes [M(L 6 ) 2 ][ClO 4 ] 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-N pyrazole bond length is appreciably shorter than the M-N pyridine bond lengths. Due to the weak field nature of L 6 (absorption spectral analyses) towards Fe(), Co() and Ni() the complexes are uniformly high-spin at room-temperature. Temperaturedependent magnetic susceptibility measurements reveal that while Co() and Ni() complexes remain high-spin over the entire range investigated (6-300 K for 2 and 63-300 K for 3), the Fe() 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 K eq [ 1 A 1 (ls) ↔ 5 T 2 (hs)] vs. 1/T relationship (190-250 K) the derived thermodyanamic parameters are: ∆H = 9.7 ± 0.04 kJ mol Ϫ1 and ∆S = 56 ± 0.18 J K Ϫ1 mol Ϫ1 . While complex 1 displays in MeCN an irreversible M III /M II redox process [anodic peak potential, E pa = 1.06 V vs. SCE], for complex 2 such a redox process is reversible [E 1/2 = 0.74 V vs. SCE and ∆E p = 80 mV].D a l t o n T r a n s . ,

The title compound [Fe (II)(L) 2][ClO 4] 2.C 7H 8 (L = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [Fe (II)(L) 2][ClO 4] 2, published earlier ( Dalton Trans. 2003, 3392-3397). Magnetic susceptibility measurements, as well as Mossbauer and calorimetric investigations on polycrystalline samples of [Fe(L) 2][ClO 4] 2.C 7H 8 revealed the occurrence of an abrupt HS ( (5) T 2) <--> LS ( (1) A 1) transition with steep and narrow (2 K) hysteresis at approximately 232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale.