Iron(II) complexes of dimethyltriazacyclophane

Abstract

Treatment of the ortho-triazacyclophane 1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-triene [(C₆H₅)₃(NH)(NCH₃)₂, L1] with Fe[N(SiMe₃)₂]₂ yields the dimeric iron(II) complex bis(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido)bis[(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido)iron(II)], [Fe(C₂₀H₁₈N₃)₄] or Fe₂(L1)₄ (9). Dissolution of 9 in tetrahydrofuran (THF) results in solvation by two THF ligands and the formation of a simpler monoiron complex, namely bis(μ-1,4-dimethyltribenzo[b,e,h][1,4,7]triazacyclonona-2,5,8-trien-7-ido-κN⁷)bis(tetrahydrofuran-κO)iron(II), [Fe(C₂₀H₁₈N₃)₂(C₄H₈O)₂] or (L1)₂Fe(THF)₂ (10). The reaction is reversible and 10 reverts in vacuo to diiron complex 9. In the structures of both 9 and 10, the monoanionic triazacyclophane ligand L1^- is observed in only the less-symmetric saddle conformation. No bowl-shaped crown conformers are observed in the solid state, thus preventing chelating κ³-coordination to the metal as had been proposed earlier based on density functional theory (DFT) calculations. Instead, the L1^- ligands are bound in either a η²-chelating fashion through the amide and one amine donor (for one of the four ligands of 9), or solely through their amide N atoms in an even simpler monodentate η¹-coordination mode. Density functional calculations on dimer 9 revealed nearly full cationic charges on each Fe atom and no bonding interaction between the two metal centers, consistent with the relatively long Fe...Fe distance of 2.912 (1) Å observed in the solid state.

Keywords: DFT calculations; azacyclophane; benzo-fused TACN; computational chemistry; crystal structure; diiron complex; ipso interaction.