The strange case of metal coordination in disubstituted-biquinolines: competition between ligands and formation of a hybrid organic/inorganic Cu(II)/Cu(I) ternary cocrystal

The synthesis of a new Cu(II) derivative, [(biq-OH)CuCl2], (4,4-bis(hydroxymethyl)- 2,2-biquinoline (biq-OH)), and its reaction with potassium tropolonate (KTrop) is herein reported and the surprising results are discussed. Due to the steric requirements of the biq moiety, the tetrahedral geometry adopted by [(biq-OH)CuCl2] derivative makes it an unstable species, causing an overall rearrangement after reaction with KTrop, with competition between ligands and reduction of the copper ion. Indeed, a heteroleptic Cu(II) complex, [(biq- OH)Cu(Trop)Cl], and a ternary cocrystal, f[Cu(Trop)2][Cu(biq- OH)2]Clg(biq-OH), in which homoleptic Cu(II) and Cu(I) species coexist, could be isolated from the reaction mixture and characterized. Single crystal X-ray analysis of both the biq-OH ligand and the ternary cocrystal f[Cu(Trop)2][Cu(biq-OH)2]Clg(biq-OH) are reported and a deep analysis of the intermolecular interactions of their supramolecular assemblies in the solid crystalline state, conducted by Hirshfeld surface and contact enrichment tools, are thus reported.