Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives-The Influence of Ligand Geometry on Metal Ion Spin State

Seven [FeL2][BF4](2) complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-y1)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D-2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD3CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yppyridine, 2,6-cli(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-l-yl)pyrimidine derivatiVes often exhibit thermal spin-crossover behavior. Gas phase density functional, theory calculations confirm the high-spin form of [Fe(bpt)(2)](2+) and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fetpyrazolyl c-bonding interaction, which we attribute to a small narrowing of the &elate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence,the high-spin state of [Fe(bpt)(2)](2+) centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of 'the bpt derivatives was also synthesized.