Compartmental ligands. Part 2. Mononuclear transition-metal complexes of acyclic Schiff bases derived from 1,3,5-triketones

Abstract

A series of acyclic Schiff bases (1) derived from 1,3,5-triketones and α,ω-alkanediamines has been prepared, and their mononuclear complexes with CuII, NiII, UO2, and VO are described. Phenyl-substituted alicyclic bases are prepared by the reaction of 1-p-X-phenylhexane-1,3,5-trione (X = H, Br, Me, or OMe)(2 mol) and ethane-1,2-diamine (en)(1 mol) in ethanol; the condensation of 7,7-dimethyloctane-2,4,6-trione with en similarly yields 2,2,7,12,17,17-hexamethyl-8,11-diazaoctadeca-6,12-diene-3,5,14,16-tetraone (1 I). 6, 11-Dimethyl-7,10-diazahexadeca-5,11-diene-2,4,13,15-tetraone, (1 a), is prepared by acid hydrolysis of the macrocyclrc Schiff base 5,9,14,18-tetramethyl-1,4,10,13-tetra-azacyclo-octadeca-5,8,14,17-tetraene-7,16-dione (2a). Treatment of the alicyclic bases with metal acetates gives the corresponding mononuclear complexes. The ligands (1) are potentially tetra-anionic and have two available dissimilar compartments for complex formation, one comprising N2O2 and the other O2O2 donor sites. The co-ordination selectivity of (1) is established as a necessary prerequisite for the formation of pure heterobinuclear chelates of these ligands. The reaction of Cu[O2CMe]2 with (1a) gives three products: the two N2O2 and O2O2 positional isomers, (5a) and (6a)·H2O, and a homobinuclear complex. The mononuclear complexes of Ni, UO2, and VO show N2O2, O2O2, and O2O2 site occupancy respectively. The preference of Ni for the N2O2 site is reinforced in its reaction wth the phenyl-subsmuted bases, but here Cu shows a preference for O2O2 as do VO and UO2. The site selectivity of Ni is further exemplified in the reaction of (5a; M = Cu) with Ni[O2CMe]2 in which the Ni replaces the Cu in the ligand. The complexes have been characterised by chemical analysis and spectral measurements, and the potential role of magnetic studies in establishing the site occupancy has been investigated.