An in solution adsorption characterization technique based on the response to an external magnetic field of porous paramagnetic materials: application on supramolecular metal–adenine frameworks containing heterometallic heptameric clusters

Abstract

Herein we explore the opportunities arising from combining magnetic properties and porosity in metal–organic materials. In this sense, we have prepared an adenine based homometallic wheel-shaped heptameric [Cu₇(μ-adeninato)₆(μ₃-OH)₆(μ-OH₂)₄]²⁺ entity containing two metal coordination environments: CuO₆ at the core of the wheel with an unusually modest Jahn–Teller distortion and six peripheral CuN₂O₄ with a more pronounced elongation. The difference in the coordination environments of this compound facilitates the selective replacement of the central metal position by other metal centers (Zn^II, Ni^II, Co^II and Cr^III) and boosts the magnetic properties of the homometallic heptameric entity. The nature of the central metal modulates the complex net of ferro- and antiferromagnetic superexchange pathways within the heptameric entity to tune the total spin (S_T = 3 (Cu₆Zn), 5/2 (Cu₆Cu), 2 (Cu₆Ni), 3/2 (Cu₆Co), and 9/2 (Cu₆Cr)). No evidence of single-molecule magnet behavior has been observed at 2 K, but at room temperature, where these compounds are still in the paramagnetic regime, the attraction force exerted by an external magnetic field (H) on particles immersed in a liquid is enough to keep them attached to an electromagnet pole. The 4S(S + 1) value of the central metal follows a linear dependence with respect to the 1/[H·∇(H)] value at which the particles are detached from the pole of the electromagnet. There is also a linear dependence of the H·∇(H) term with respect to the adsorbate mass incorporated inside the pores of the paramagnetic adsorbent which has allowed performing straightforward sorption selectivity experiments on Cu₆Cu directly in solution, which are based on a property of the adsorbent and not as usually based on an indirect assessment of the adsorbate remaining in solution.