Synthesis and copper coordination chemistry of hindered 1,4,7-triazacyclononane ligands with amide appendages

Copper complexes of the new ligands(1a) L-Piv and L-RAmR' that comprise 1,4-diisopropyl-1,4,7-triazacyclononanes linked to secondary and tertiary amide groups were prepared and characterized, with a particular view toward evaluating amide structural, spectroscopic, and potential hydrogen-bonding influences of relevance to ongoing copper-dioxygen reactivity studies. X-ray crystal structures of the Cu(I) complexes [LCu(CH3CN)]X (L = L-HAmMe, X = ClO4; L = L-Piv, X = CF3SO3) revealed typical 4-coordinate geometries with the amide dangling free, while those of the Cu(II) compounds [LCuCl]X (L = LHAmMe, X = ClO4; L = L-MeAmH, X = PF6) and [(LCu)-Cu-Piv(O-3-SCF3)]O3SCF3 showed 5-coordinate square pyramidal geometries with the amide coordinated to the metal via its carbonyl oxygen atom. Analysis of FTIR spectra of the aforementioned compounds and the carbon monoxide adducts [(LCu)-Cu-MeAmR'(CO)]SbF6 (R' = H or Me) allowed (i) identification of signatures of amide structural features, hydrogen bonding, and metal coordination and (ii) classification of the amide ligands as generally electron withdrawing relative to alkyl-substituted counterparts (e.g., 1,4,7-triisopropyl-1,4,7-triazacyclononane).