Wheel-Shaped Icosanuclear Homo- and Heterometallic Complexes of Ni ^II, Co ^II, and Cu ^II ions supported by unsymmetrical aminoalcohol ligands

Reactions of M(OAc) ₂·4H ₂O (M = Ni, Co) with 3-[benzyl(2-hydroxyethyl)amino]-1-propanol (H ₂L) in the presence of pyridine or triethylamine afforded novel homometallic icosanuclear wheel-shaped complexes [M ₂₀L ₄(HL) ₄(OAc) ₂₈] (M = Ni (1), Co (2)), which consist of a central M ^II ₁₂ single-stranded, nearly planar loop with four peripheral [M ₂(HL)(OAc) ₂] fragments attached in an S ₄ symmetrical fashion. The complexes can alternatively be recognized as saddle-shaped wheel structures, in which four tetranuclear units of [M ₄L(HL)(OAc) ₇] ^2- are connected by four M ²⁺ ions (M5). The tetranuclear unit itself can be derived from an ideal C ₂ symmetrical [M ₄(HL) ₂(μ-η ²-OAc) ₄(μ- η ¹,η ¹-OAc) ₂(η ¹, η ¹-OAc)] ^- structure through deprotonation of the HL ^- ligand, and is composed of two plane-shared M ₃O ₄ incomplete cubanes in which the M2 and M3 atoms are involved in the central fused plane and the M1 and M4 atoms are disposed at the apex sites. Mixed-metal icosanuclear complexes [Ni _xM _20-xL ₄(HL) ₄(OAc) ₂₈] (3, M = Co, x = 9.5) and [Ni ₁₂M ₈L ₄(HL) ₄(OAc) ₂₈] (4, M = Cu) were also synthesized by using equimolar amounts of Ni ^II and M ^II ions, and were shown to have similar structures to 1 and 2. X-ray crystallographic and fluorescent analyses revealed that complex 3 contains nonstoichiometric amounts of Ni ²⁺ and Co ²⁺ ions in the ratio of 9.5:10.5 and that these are disordered at every metal site. In striking contrast, complex 4 has a stoichiometric formula of Ni ₁₂Cu ₈, which was confirmed by the Jahn-Teller elongation of Cu ²⁺ ions, and consequently, the M2 and M5 positions are occupied exclusively by the Cu ²⁺ ions. The temperature-dependent direct current (dc) magnetic susceptibility data showed the presence of ferromagnetic exchange interactions in the Ni homometallic (1) and NiCu bimetallic (4) complexes, while the Co homometallic (2) and NiCo bimetallic (3) complexes exhibited antiferromagnetic interactions due to spin-orbit coupling effects of the octahedral Co ^II ions. The present results demonstrate that the unsymmetrical aminoalcohol ligand H ₂L is quite effective in organizing the homo- and heterometallic icosanuclear wheel-shaped metal arrangements.