The crystal and molecular structure of 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetato(2−)cobalt(III) nitrate monohydrate—a complex exhibiting large 13C NMR steric shifts

Abstract

Past studies have shown major 13C NMR chemical shift differences between methylene carbon atoms proximal (In a γ position) and distal to a ligand methyl substituent in [Co(mddda)] + (mddda = 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetate(2−), NH 2CH 2CH 2N(CH 2COO −)CH 2CH 2CH(CH 3)N(CH 2COO −)CH 2CH 2NH 2). Since the chemical shift differences have been attributed to γ steric shifts associated with the methyl group on the hexadentate ligand, X-ray crystal structure determination has been carried out on [Co(mddda)]NO 3·H 2O to examine the steric features. The compound crystallizes in the space group P2 1/c with a = 9.756(2) Å, b = 14.912(4) Å, c = 11.783(2)°, Å, β = 95.70(2)°, and Z = 4. Intensities were collected on an automated diffractometer and the structure was refined to a conventional R factor of 0.058. The crystal contains a racemic mixture of ΔΛΔ(R) and ΛΔΛ(S) enantiometric cationic complexes, where the R and S labels designate the absolute configurations of the chiral methyl-substituted carbon atom of the ligands. These diastereomers are those required to maintain an equatorial methyl group on the central six-membered chelate ring, whose conformations are λ skew-boat for the ΔΛΔ isomer and δ skew-boat for the ΛΔΛ isomer. The respective conformations of the two skewed five-membered chelate rings are δδ and λλ. Though most corresponding bond lengths and bond angles in the two portions of the molecule are nearly the same, those differences which are observed indicate nonbonded interactions between the methyl substituent and adjacent methylene groups. Consideration of these interactions and geometrical variations indicates that bond angle distortion may play an important role in the γ shifts observed.