Coordination induced 2-(2′-hydroxyphenyl) imidazolidine ring hydrolysis of dinucleating amine–imine–phenol ligands: X-ray structures of hardness-matched mononuclear cobalt(III) complexes as end products having isomeric N4O2 coordination spheres

Two dinucleating ligands H3LIx (subscript I for imidazolidine; x = 1, 2) having pendant imidazolidine rings have been reacted with cobalt salts in aqueous methanol at room temperature to obtain the mononuclear complexes [CoIIIL1]Cl · 2H2O (1 · 2H2O) and [CoIIIL2]ClO4 · H2O (2 · H2O) of hydrolyzed ligands H2Lx. Expected dicobalt complexes were not obtained due to the coordination induced hydrolytic cleavage of the central substituted imidazolidine rings. The complexes 1 · 2H2O and 2 · H2O were structurally characterized by X-ray crystallography. Both mer- and fac-ON2 binding modes of the two halves of the hexadentate ligands have been identified in these complexes. The mononuclear complexes have CoN4O2 coordination spheres with a N4O2 donor atom set of the two hexadentate ligands, forming isomeric distorted octahedral coordination geometries around the metal atoms. As a result, the imine nitrogen pairs coordinated trans to each other in [CoL1]+, whereas the phenolic oxygens are trans in [CoL2]+. In complex 1 · 2H2O the average Co–N(imine), Co–N(amine), and Co–O(phenolate) distances are 1.910, 1.959, 1.894 Å, respectively, and in 2 · H2O the corresponding average distances are 1.944, 1.991 and1.887 Å, respectively. Both the cobalt(III) complexes are diamagnetic. The UV–Vis spectra of the end products, after the imidazolidine ring hydrolysis, is different from the spectrum of the initial reaction mixture containing the μ-bis(tetradentate) ligands, H3LIx(x=1,2) and metal salts.

Graphical abstractTwo dinucleating ligands having pendant imidazolidine rings have been reacted with cobalt salts in aqueous methanol at room temperature to obtain the mononuclear complexes [CoIIIL1]Cl · 2H2O (1 · 2H2O) and [CoIIIL2]ClO4 · H2O (2 · H2O) of hydrolyzed ligands. Anticipated dicobalt(II/II or III/III) complexes were not obtained due to a coordination driven hydrolytic cleavage of the substituted central imidazolidine rings of the ligands. Structurally characterized hardnessmatched complexes 1 · 2H2O and 2 · H2O, show mer- and fac-ON2 binding modes of the two tridentate halves of the parent hexadentate ligands.Figure optionsDownload full-size imageDownload as PowerPoint slide