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Abstract

Nickel(II) ion reacts with 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) (H₄L¹) at room temperature with formation of cis-[Ni(L¹)]^2- complex. The complex was crystallised in several forms in dependence on conditions of isolation (temperature, pH, solvent mixture). Crystal structures of cis-O,O-[Ni(H₂L¹)] (1), cis-O,O-[Ni(H₂L¹)]·2H₂O (2), cis-O,O-[Ni(H₃L¹)]Cl·H₂O (3) and cis-O,O-[Ni(H₄L¹)]Cl₂·2H₂O (4) were determined by X-ray diffraction. In all the compounds, Ni²⁺ is in octahedral cis-O1,O2-trans-N1,N8-cis-O1,N4-[Ni(L¹)]^2- (hereafter abbreviated as cis-O,O-[Ni(L¹)]^2-) arrangement with V conformation of the cyclam ring. Complexes 3 and 4 exhibit very unusual coordination through the phosphoryl group of double-protonated phosphonate moiety. Tetraethylester of H₄L¹ having only poorly coordinating phosphoryl oxygen atoms also forms nickel(II) complex 5 with cis arrangement. Corresponding trans-O,O-[Ni(L¹)]^2- complex is formed only in strongly acidic medium through conversion of the cis complex; it was characterised by X-ray diffraction as dihydrochloride dihydrate, trans-O,O-[Ni(H₄L¹)]Cl₂·2H₂O (6). The analogous compound (NH₄)₂(trans-O,O-[Ni(L²)])·H₂O (7) is obtained quantitatively by reaction of Ni²⁺ with 4,11-dibenzyl-1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) (H₄L²). Formation of this complex 7 in the III cyclam ring conformation is preferred to the cis complex due to steric hindrance of bulky benzyl groups in cis-[Ni(L²)]^2- complex with the V cyclam ring conformation.

Keywords: Azacrown compounds; Cyclam; Phosphonates; Nickel(II) complexes; cis/trans Isomerism; Conformation analysis; Crystal structure determination; X-Ray diffraction; Macrocyclic ligands.

References: 34 live references.