31P and 1H NMR characterization of stereoisomeric macrocyclic nickel(II) complexes with a tetraphosphane ligand

Abstract

Several studies on metal complexes formed by macrocyclic ligands have shown that these compounds provide stimulating examples for studying the conformational properties of the molecules. Recent synthetic developments in this laboratory have provided some ideal ligands for investigating the configurational properties of polyphosphane macrocyclic complexes. As an example, the results of 31P and 1H NMR investigations on some stereoisomeric nickel(II) derivatives of the 4,7,13,16-tetraphenyl-1,10-dioxa-4,7,13,16-tetraphoscyclooctadecane {∣18∣aneP 4O 2} are here reported. The ∣18∣aneP 4O 2 ligand can exist in five basic structural forms (Fig. 1), as determined by the distribution of phosphorus atom configurations with respect to the aliphatic chains spanning the phosphorus atoms themselves. Three of these forms [(a), (b), and (c)] are nonenantiomeric, whereas (d) and (e) forms exist together their respective enantiomers. The five forms are interconvertible diastereoisomers; however, because of their high temperature of ▪ Fig. 1. The five possible diastereoisomers of ∣18∣aneP 4O 2 ligand. racemization, they can, in principle, be isolated as free ligands at room temperature. Two isomeric forms I and II of the ∣18∣aneP 4O 2 ligand have been isolated and their nickel(II) tetrafluoroborate derivatives have been synthesized and characterized. Conductometric, magnetic and spectroscopic measurements indicate that the latter compounds contain square planar {Ni(∣18∣aneP 4O 2)} 2+ cations, the nickel(II) ion being coordinated to the four phosphorus atoms. The 31P NMR spectrum of isomer I is reported in Fig. 2 and shows a pattern of ▪ Fig. 2. 31P NMR spectrum at 30 °C of d 6-DMSO solution of {Ni(∣18∣aneP 4O 2I)} 2+ resonances which indicates the presence of four nonequivalent phosphorus atoms. The (d) configuration, therefore, is proposed for this stereoisomer. Both 1H and 31P NMR spectra indicate a symmetric configuration for isomer II. Since form 1H NMR trace it appears that the ethylenic chains spanning the phosphorus atoms adopt the gauche conformation, only configurations (b) and (e) can be proposed for this stereoisomer. Preliminary X-ray diffractometric data indicate that the configuration (b) is to be assigned to the isomer II. Attempts to isolate and characterize all the possible isomers are actually in progress.