Synthesis and Crystal Structure of <em>cis</em>-[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

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](ClO4)(2) with 1,2-diaminoethane (en) gives the folded cis-complex [NiL(en)](ClO4)(2). en .H2O 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&lt;-&gt;[NiL(OH2)(2)](2+) is represented by K-1 (25 degreesC) = 0.167, DeltaH degrees= - 55 +/- 3 kJ mol(-1) and DeltaS degrees = - 198 +/- 30 J K-1 mol(-1). The planar-cis-folded equilibrium: [NiL](2+) + en&lt;-&gt;cis-[NiL(en)](2) + is governed by K-2 (25 degreesC) = 417 dm(3) mol(-1), DeltaH degrees = -38.5 +/- 2 kJ mol(-1) and DeltaS degrees = -79 +/- 5 J K-1 mol(-1). The mechanism of the planar-folded octahedral equilibrium is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.