Unusual cis/trans isomerism in octahedral nickel(II) complexes with 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) as a ligand

Nickel(II) ion reacts with 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methyl phosphonic acid) (H4L1) at room temperature with formation of cis-[Ni(L-1)] (2-) complex. The complex was crystallised in several forms in dependence o n conditions of isolation (temperature, pH, solvent mixture). Crystal struc tures of cis-O,O-[Ni(H2L1)] (1), cis-O,O-[Ni(H2L1)]. 2H(2)O (2), cis-O,O-[N i(H3L1)]Cl .H2O (3) and cis-O,O-[Ni(H4L1)]Cl-2. 2H(2)O (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(L-1)](2-) thereafter abbreviated as cis-O,O-[Ni( L-1)](2-) arrangement with V conformation of the cyclam ring. Complexes 3 a nd 4 exhibit very unusual coordination through the phosphoryl group of doub le-protonated phosphonate moiety. Tetraethylester of H4L1 having only poorl y coordinating phosphoryl oxygen atoms also forms nickel(II) complex 5 with cis arrangement. Corresponding trans-O,O-[Ni(L-1)](2-) complex is formed o nly 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)]Cl-2. 2H(2)O (6). The analogous compound (NH4)(2)(trans-O,O-[Ni( L-2)]).H2O (7) is obtained quantitatively by reaction of Ni2+ with 4,11-dib enzyl-1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) (H4L 2). Formation of this complex 7 in the III cyclam ring conformation is pref erred to the cis complex due to steric hindrance of bulky benzyl groups in cis-[Ni(L-2)](2-) complex with the V cyclam ring conformation.