STEREOCHEMICAL STUDIES OF COBALT(III) COMPLEXES OF SEXIDENTATE AMINOPOLYACIDS USING CARBON-13 CHEMICAL SHIFTS

Abstract

Abstract Detailed assignments of the 13C NMR spectra of two series of cobalt(III) complexes have been made with the aid of methyl substitution at the terminal carbon of the diamine backbone ring and other standard techniques. One series of complexes includes ligands ethylenediaminetetraacetate ion (edta), trimethylenediaminetetraacetate ion (1,3-pdta), and 1,3-butanediaminetetraacetate ion (1,3-mpdta). The other series of complexes includes the ligands tri-ethylenetetraaminediacetate ion (ttda), 1,9-diamino-3,7-diazanonane-3,7-diacetate ion (ddda), and 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetate ion (mddda). Spectral similarities reveal that these two types of complexes have similar bonding characteristics. Methyl substitution on the 6-membered backbone ring affects the overall complex structural parameters only slightly but the effects for a complex with a 5-membered backbone ring are obvious. The induced chemical shift changes for inductive (through-bond) and steric (through-space) perturbation of carbons caused by methyl substitution are consistent with those obtained for simple organic molecules. Comparison of carbon chemical shifts of complexes of 5-membered and 6-membered backbone ring reveals the strain effects and the results are explained partially in relation to those obtained by other spectroscopic methods. The strain effects are considered in reference to the chemical shift ranges of similar carbons in related complexes. A more strained environment makes the out-of-plane ring methylene carbon resonance more similar to that of the in-plane ring. The chemical shifts, in this case, seem to correlate with the C[sbnd]N[sbnd]C bond angles made between the methylene carbons of the side ring and of the backbone ring at the coordinated nitrogen.