Order in disorder: solution and solid-state studies of [M III2M II5] wheels (MIII = Cr, Al; MII = Ni, Zn)

Abstract

A family of heterometallic Anderson-type ‘wheels’ of general formula [M^III₂M^II₅(hmp)₁₂](ClO₄)₄ (where M^III = Cr or Al and M^II = Ni or Zn giving [Cr₂Ni₅] (1), [Cr₂Zn₅] (2), [Al₂Ni₅] (3) and [Al₂Zn₅] (4); hmpH = 2-pyridinemethanol) have been synthesised solvothermally. The metallic skeleton common to all structures describes a centred hexagon with the M^III sites disordered around the outer wheel. The structural disorder has been characterised via single crystal X-ray crystallography, 1–3D ¹H and ¹³C solution-state NMR spectroscopy of the diamagnetic analogue (4), and solid-state ²⁷Al MAS NMR spectroscopy of compounds (3) and (4). Alongside ESI mass spectrometry, these techniques show that structure is retained in solution, and that the disorder is present in both the solution and solid-state. Solid-state dc susceptibility and magnetisation measurements on (2) and (3) reveal the Cr–Cr and Ni–Ni exchange interactions to be J_Cr–Cr = −1 cm⁻¹ and J_Ni–Ni,r = −5 cm⁻¹, J_Ni–Ni,c = 10 cm⁻¹. Fixing these values allows us to extract J_Cr–Ni,r = −1.2 cm⁻¹, J_Cr–Ni,c = 2.6 cm⁻¹ for (1), the exchange between adjacent Ni and Cr ions on the ring is antiferromagnetic and between Cr ions on the ring and the central Ni ion is ferromagnetic.