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 Cu^II, Ni^II, 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.