Iron(II) complexes of 2,6-di(1-alkylpyrazol-3-yl)pyridine derivatives - The influence of distal substituents on the spin state of the iron centre

2,6-Di(1-methyl-pyrazol-3-yl)pyridine (L), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (L), 2,6-di(1-benzyl-pyrazol- 3-yl)pyridine (L) and di(1-isopropyl-pyrazol-3-yl)pyridine (L ) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(L)] complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(L)]X (X = BF , ClO and PF ), [Fe(L)][BF ], [Fe(L)][BF] and [Fe(L)][PF ] have been determined at 150 K. All of these contain high-spin iron centres except [Fe(L)][BF ]·xHO, which is predominantly low-spin at that temperature. All the complexes are high-spin between 5 and 300 K as solvent-free bulk powders, and are also high-spin in (CD) CO solution between 193 and 293 K. This was unexpected, since the parent complex [Fe(3-bpp)] undergoes spin-crossover in the same solvent with T = 247 K [40]. The high-spin nature of the [Fe(L )] complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand 'R' substituents.