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 = L^HAmMe, X = ClO₄; L = L^MeAmH, X = PF₆) and [L^PivCu(O₃-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 [L^MeAmR'Cu(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).