High and Low Spin Mononuclear and Dinuclear Iron(II) Complexes of 4-Amino and 4-Pyrrolyl-3,5-di(2-pyridyl)-4H-1,2,4-triazoles

The first dinuclear iron(II) complexes of any 4-substituted 3,5-di(2-pyridyl)-4H-1,2,4-triazole ligands, [Fe-2(II)(adPt)(2)(H2O)(1.5)(CH3CN)(2.5)](BF4)(4) and [Fe-2(II)(pldpt)(2)(H2O)(2)(CH3CN)(2)](BF4)(4), are presented [where adpt is 4-amino-3,5-di(2-pyridyl)4H-1,2,4-triazole and pldpt is 4-pyrrolyl-3,5-di(2-pyridyl)-4H-1,2,4-triazole]. Both dinuclear complexes feature doubly triazole bridged iron(II) centers that are found to be [high spin-high spin] at all temperatures, 4-300 K, and to exhibit weak antiferromagnetic coupling. In the analogous monometallic complexes, [Fe-II(Rdpt)(2)(X)(2)](n+), the spin state of the iron(II) center was controlled by appropriate selection of the axial ligands X. Specifically, both of the chloride complexes, [Fe-II(adpt)(2)(Cl)(2)]. 2MeOH and [Fe-II(pldpt)(2)(Cl)(2)center dot 2MeOH center dot H2O, were found to be high spin whereas the pyridine adduct [Fe-II(adpt)(2)(py)(2)](BF4)(2) was low spin. Attempts to prepare [Fe-II(pldpt)(2)(py)(2)](BF4)(2) and the dinuclear analogues [Fe-2(II)(RdPt)(2)(py)(4)](BF4)(4) failed, illustrating the significant challenges faced in attempts to develop control over the nature of the product obtained from reactions of iron(II) and these bis-bidentate ligands.