Ultralarge 3d/4f Coordination Wheels: From Carboxylate/Amino Alcohol-Supported {Fe₄Ln₂} to {Fe₁₈Ln₆} Rings

Abstract

A family of wheel-shaped charge-neutral heterometallic {Fe^III₄Ln^III₂}- and {Fe^III₁₈M^III₆}-type coordination clusters demonstrates the intricate interplay of solvent effects and structure-directing roles of semiflexible bridging ligands. The {Fe₄Ln₂}-type compounds [Fe₄Ln₂(O₂CCMe₃)₆(N₃)₄(Htea)₄]·2(EtOH), Ln = Dy (1a), Er (1b), Ho (1c); [Fe₄Tb₂(O₂CCMe₃)₆(N₃)₄(Htea)₄] (1d); [Fe₄Ln₂(O₂CCMe₃)₆(N₃)₄(Htea)₄]·2(CH₂Cl₂), Ln = Dy (2a), Er (2b); [Fe₄Ln₂(O₂CCMe₃)₄(N₃)₆(Htea)₄]·2(EtOH)·2(CH₂Cl₂), Ln = Dy (3a), Er (3b) and the {Fe₁₈M₆}-type compounds [Fe₁₈M₆(O₂CCHMe₂)₁₂(Htea)₁₈(tea)₆(N₃)₆]·n(solvent), M = Dy (4, 4a), Gd (5), Tb (6), Ho (7), Sm (8), Eu (9), and Y (10) form in ca. 20-40% yields in direct reaction of trinuclear Fe^III pivalate or isobutyrate clusters, lanthanide/yttrium nitrates, and bridging triethanolamine (H₃tea) and azide ligands in different solvents: EtOH for the smaller {Fe₄Ln₂} wheels and MeOH/MeCN or MeOH/EtOH for the larger {Fe₁₈M₆} wheels. Single-crystal X-ray diffraction analyses revealed that 1-3 consist of planar centrosymmetric hexanuclear clusters built from Fe^III and Ln^III ions linked by an array of bridging carboxylate, azide, and aminopolyalcoholato-based ligands into a cyclic structure with a cavity, and with distinct sets of crystal solvents (2 EtOH per formula unit in 1a-c, 2 CH₂Cl₂ in 2, and 2 EtOH and 2 CH₂Cl₂ in 3). In 4-10, the largest 3d/4f wheels currently known, nearly linear Fe₃ fragments are joined via mononuclear Ln/Y units by a set of isobutyrates and amino alcohol ligands into virtually planar rings. The magnetic properties of 1-10 reveal slow magnetization relaxation for {Fe₄Tb₂} (1d) and slow relaxation for {Fe₄Ho₂} (1c), {Fe₁₈Dy₆} (4), and {Fe₁₈Tb₆} (6).