Can self-assembly of copper(ii) picolinamide building blocks be controlled?

Abstract

Four copper(II) derivatives with picolinamide (piaH), [Cu(N₃)₂(piaH)₂] (1), [Cu(NCO)₂(piaH)₂] (2), [Cu(pia)₂]·2H₂O (3) and [Cu(pia)(piaH)(ClO₄)]₂ (4) are reported. 1 and 2 were isolated as powders shortly after precipitation, becoming crystalline (3) after prolonged standing in mother liquor. The single crystal X-ray analysis revealed the deprotonation of the amide nitrogen (pia) in both 3 (mononuclear square planar [Cu(pia-N,N)₂] building blocks) and 4 (pia acting as a bridge between copper(II) ions within the dimer). MAGSUS χ_MT versus T curves suggest only weak magnetic interactions between the copper(II) species in 3 and 4, though 4 is dinuclear. The magnetic coupling was reproduced by DFT calculations on the experimentally determined structures. The mechanism of formation of compounds 3 and 4 was analyzed by DFT calculations. Models of 1 and 2 underwent transfer of two hydrogen atoms from the piaH ligands to the two axial ligands N₃⁻ or NCO⁻ that were released yielding 3. In the reaction leading to 4, the perchlorate is a very weak base that cannot deprotonate the piaH ligand and after the first hydrogen transfer to acetate the dimer 4 is formed. This reactivity, favoring alternative pathways, may switch off an expected self-assembly process.