Synthesis and crystal structure of cis-[NiL(en)](ClO4)2·en·H2O (L=3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane) and solution studies of the planar-cis-folding equilibrium

Abstract The reaction of formaldehyde and ethanolamine with the nickel(II) complex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) in the presence of triethylamine gives the planar nickel(II) complex of the penta-azamacrocycle-3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane (L) which can be readily isolated as the perchlorate salt. The reaction of [NiL](ClO 4 ) 2 with 1,2-diaminoethane (en) gives the folded cis -complex [NiL(en)](ClO 4 ) 2 ·en·H 2 O containing a water and a 1,2-diaminoethane of crystallisation. The nickel(II) centre in the crystal structure is distorted octahedral with Ni–N bond distances to the N(1), N(2), N(3) and N(4) atoms of the macrocycle in the range 2.099(7)–2.129(7) A which are typical of octahedral nickel(II) complexes with other such azamacrocycles ligands. The cis orientated Ni–N bond distances to the two N atoms of the 1,2-diaminoethane ligand are however somewhat longer at 2.168(7) and 2.161(7) A. The hydroxyethyl group is axial with the ligand having the trans V stereochemistry of the chiral sec -NH centres. The six-membered chelate rings have a chair conformation and the five-membered chelate rings are gauche . The planar- trans -octahedral diaqua equilibrium; [NiL] 2+ +2H 2 O↔[NiL(OH 2 ) 2 ] 2+ is represented by K 1 (25°C)=0.167, Δ H °=−55±3 kJ mol −1 and Δ S °=−198±30 J K −1 mol −1 . The planar- cis -folded equilibrium; [NiL] 2+ +en↔ cis -[NiL(en)] 2+ is governed by K 2 (25°C)=417 dm 3 mol −1 , Δ H °=−38.5±2 kJ mol −1 and Δ S °=−79±5 J K −l mol −l . The mechanism of the planar-folded octahedral equilibrium is discussed.