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 (LMe), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (LAll), 2,6-di(1-benzyl-pyrazol- 3-yl)pyridine (LBz) and di(1-isopropyl-pyrazol-3-yl)pyridine (L iPr) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(LR)2]2+ complexes (R = Me, All, Bz and iPr). Crystal structures of [Fe(LMe)2]X2 (X- = BF4-, ClO4- and PF6-), [Fe(LAll)2][BF 4]2, [Fe(LBz)2][BF4] 2 and [Fe(LiPr)2][PF 6]2 have been determined at 150 K. All of these contain high-spin iron centres except [Fe(LMe)2][BF 4]2·xH2O, 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 (CD3) 2CO solution between 193 and 293 K. This was unexpected, since the parent complex [Fe(3-bpp)2]2+ undergoes spin-crossover in the same solvent with T = 247 K [40]. The high-spin nature of the [Fe(L R)2]2+ complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand 'R' substituents.