Heterocyclic donor influences on the binding and activation of CO, NO, and O₂ by copper complexes of hybrid triazacyclononane-pyridyl ligands

Copper(I) complexes of 1,4-diisopropyl-7-R"-1,4,7-triazacyclononane (R"=2-pyridylmethyl, L^Py; 6-methyl-2-pyridylmethyl, L^6MePy; 5-methyl-2-pyridylmethyl, L^5MePy; 6-phenyl-2-pyridylmethyl, L^6PhPy; 2-quinolylmethyl, L^Quin were prepared and characterized by CHN analysis, NMR and FTIR spectroscopy, cyclic voltammetry, and mass spectrometry. An X-ray crystal structure of [L^6PhPyCu]SbF⁶ was determined and compared to that previously reported for [L^PyCu]O₃SCF₃; similar distorted trigonal bipyramidal geometries are adopted with the ligands coordinated in η⁴ fashion. The complexes [LCu]⁺ (L = L(Py) or L(Quin)) form adducts with CO(g) in which the heterocyclic appendage is displaced. With NO(g), [L(Py)Cu]O₃SCF₃ reacts in a disproportionation process to yield N₂O and [L(Py)Cu(ONO)]O₃SCF₃, which was structurally defined by X-ray crystallography. Upon reaction with O₂(g) at-75°C the Cu (I) complexes of L^Py and L^5MePy yield trans-1,2-peroxo species [LCuOOCuL]²⁺ as determined by UV-Vis and resonance Raman spectroscopy. In contrast, spectroscopy indicates that low temperature oxygenation of [L(6PhPy)Cu]SbF₆ yields a bis(μ-oxo)dicopper core, postulated to be capped by η³-L^6PhPy (pyridyl appendage not coordinated). Decomposition of the trans-1,2-peroxo compounds results in hydroxylation of the ligand at the benzylic position of the heterocyclic appendage, but the bis(μ-oxo) complexes decay to give products resulting from N-dealkylation of the heterocycle arm. The different fates of the Cu(I) complexes of L(Py) and L(5MePy) versus that of L(6PhPy) upon oxygenation may be traced to the coordination of the heterocycle; in the former cases, the pyridyl unit remains coordinated, favoring trans-1,2-peroxo generation, whereas pyridyl dissociation facilitated (6PhPy) by the sterically bulky 6-phenyl group on L yields a [η³-L(6PhPy)Cu]⁺ fragment amenable to bis(μ-oxo) core formation. The steric properties of the heterocyclic components of the ligands used in this study thus are important determinants of the reactivity of their Cu(I) complexes with small molecules.