EQUILIBRIUM AND STRUCTURAL STUDIES OF NICKEL(II) COMPLEXES OF 1,4,8,11-TETRAAZACYCLOTETRADECANE AND 1,4,8,11-TETRAMETHYL-1,4,8,11-TETRA-AZACYCLOTETRADECANE IN COORDINATING SOLVENTS AND NITROMETHANE SOLUTION - ISOLATION OF SOME BIS-(SOLVENTO)-ADDUCTS OF 1,4,8,11-TETRA-AZACYCLOTETRADECANICKEL(II) PERCHLORATE

Two isomers (trans-I and trans-III) of the diamagnetic, square-planar complexes [Ni(Lb)](ClO4)2 (Lb = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane), and a single isomer (trans-III) of [Ni(La)](ClO4)2 (La = 1,4,8,11-tetra-azacyclotetradecane) have been investigated in co-ordinating solvents (H2O, DMF, DMSO, MeCN) and in non-coordinating nitromethane. In aqueous solution, equilibrium constants (K) and related thermodynamic parameters (ΔH°, ΔS°) associated with diamagnetic ⇄ paramagnetic solvation equilibria ([Ni(L)]2+ + nH2O ⇄ [Ni(L)(OH2)n]2+ have been determined from visible spectra and magnetic susceptibility measurements; at 298 K, K = 0.25, 1.05 and 1.14, -ΔH°/kJ mol-1 = 24 ± 2,11 ± 2 and 44 ± 2, ΔS°/JK-1 mol-1 = -94 ± 10, -39 ± 10 and -143 ± 10 for complexes of trans-III-La (n = 2), trans-I-Lb (n =1) and trans-III-Lb respectively. The structurally related trans-III-[Ni(L)]2+ ions (L = La and Lb) form octahedral bis-solvento-adducts, whereas the trans-I-[Ni-(Lb)]2+ ion is sterically constrained such that it forms a five co-ordinate paramagnetic mono-(solvento) adduct only. Spectrophotometric pH-titration of the aqua-complexes was undertaken, and their pKa′s determined. Unstable solid bis-(solvento)-adducts of [Ni(La)](ClO4)2 with DMF, DMSO and MeCN have been isolated and characterized by a diffuse reflectance spectroscopy. The solid complexes with DMSO and MeCN were also examined by differential scanning calorimetry which confirms that MeCN is co-ordinated more strongly than CMSO, in agreement with the visible spectra of the solids. The rod-like MeCN molecule is also found to be a better ligand for trans-I-[Ni(Lb)]2+ ion, solution visible and 1H n.m.r. spectra showing complete formation of the mono- (acetonitrile) adduct, but only partial formation of mono-(solvento)-adducts in DMSO, DMF and H2O. Low temperature 1H n.m.r. studies of the paramagnetic, five-co-ordinate trans-I-[Ni(Lb)Cl]+ ion indicates that it is trigonal bipyramidal(tbp) in nitromethane solution, but a distinction between square-pyramidal and tbp geometries is not possible from 1H n.m.r. studies of the corresponding [Ni(Lb)(OH2)]2+ ion in aqueous solution. 13C n.m.r. spectra of the three square planar diamagnetic [Ni(L)](ClO4)2 complexes (L = La or Lb) were also obtained in nitromethane solution. © 1979.