Control of the spin state of Fe(II) 2,6-di(pyrazol-1-yl)pyridine complexes by distal ligand substitution

The complexes [Fe(L1R)2]X2 (L1R = 2,6-bis-{3-iso-propylpyrazol-1-yl}pyridine[L1Pri], X−=BF4− or PF6−; L1R = 2,6-bis-{3-phenylpyrazol-1-yl}pyridine [L1Ph], X−=BF4−) are high-spin between 5 and 300 K in the solid, and between 180 and 300 K in acetone solution. In contrast, [Fe(L1Mes)2]X2 (L1Mes = 2,6-bis-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine, X−=BF4− or PF6−) are fully low-spin at 300 K in the solid and solution phases. This shows that the spin state of the Fe ion in these compounds depends on the inductive properties of the ligand 3,3″-substituents only, as long as those substituents are aromatic.