Role of Alkylated 2,6‐bis(tetrazol‐5‐yl)pyridyl Ligands and Iron(II) Salts in Selecting Spin Crossover Complexes

The substituents in 2,6-bis(2-R-2H-tetrazol-5-yl)pyridyl neutral ligands R2btp (R = Me, tBu) revealed a prominent effect when reacting with iron(II) salts, in combination with the nature of the anion. According to metal:ligand molar ratio, reaction of R2btp with FeCl2 led to isolate [FeII(R2btp-k3N,N’,N”)2](FeIIICl4)2, R = Me (1a) and tBu (2a), where the octahedral iron(II) centres bring two tridentate ligands in mer coordination mode, but also [FeII(tBu2btp-k3N,N’,N”)(tBu2btp-kN)2(H2O-kO)](FeIIICl4)2 (2b), where one ligand is tridentate, while the other two coordinate the iron(II) through one tetrazolyl nitrogen atom, and the octahedral sphere is completed by one water molecule. In all cases, half of the iron ions are oxidised to iron(III) forming the paramagnetic tetrachloroferrato counterions. Reaction of tBu2btp with Fe(ClO4)2×6H2O led to the octahedral [FeII(tBu2btp-k3N,N’,N”)2](ClO4)2∙4DCM (4a∙4DCM), which shows solvent-dependent spin crossover behaviour: while 4a∙4DCM is blocked in the high spin state, its unsolvated form, 4a, undergoes spin transition to low spin in two subsequent steps at 206 K, with opening of a 23-K hysteresis (T1/2¯ = 194 K, T1/2­ = 217 K), and at 136 K (T1/2¯ = 135 K, T1/2­ = 137 K). The magnetic profile changes to an incomplete spin transition when the sample absorbs water molecules yielding 4a×1.5H2O.