Heterocyclic donor influences on the binding and activation of CO, NO, andO-2 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-pyridyl methyl, L-5MePy; 6-phenyl-2-pyridylmethyl, L-6PhPy; 2-quinolylmethyl, L-Qui n) were prepared and characterized by CHN analysis, NMR and FTIR spectrosco py, cyclic voltammetry, and mass spectrometry. An X-ray crystal structure o f [(LCu)-Cu-6PhPy]SbF6 was determined and compared to that previously repor ted for [(LCu)-Cu-Py]O3SCF3; similar distorted trigonal bipyramidal geometr ies are adopted with the ligands coordinated in eta(4) fashion. The complex es [LCu](+) (L = L-Py or L-Quin) form adducts with CO(g) in which the heter ocyclic appendage is displaced. With NO(g), [(LCu)-Cu-Py]O3SCF3 reacts in a disproportionation process to yield N2O and [(LCu)-Cu-Py(ONO)]O3SCF3, whic h was structurally defined by X-ray crystallography. Upon reaction with O-2 (g) at - 75 degrees 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 Rama n spectroscopy. In contrast, spectroscopy indicates that low temperature ox ygenation of [(LCu)-Cu-6PhPy]SbF6 yields a bis(mu-oxo)dicopper core, postul ated to be capped by eta(3)-L-6PhPy (pyridyl appendage not coordinated). De composition of the trans-1,2-peroxo compounds results in hydroxylation of t he ligand at the benzylic position of the heterocyclic appendage, but the b is(mu-oxo) complexes decay to give products resulting from N-dealkylation o f the heterocycle arm. The different fates of the Cu(I) complexes of L-Py a nd L-5MePy versus that of L-6PhPy upon oxygenation may be traced to the coo rdination of the heterocycle; in the former cases, the pyridyl unit remains coordinated, favoring trans-1,2-peroxo generation, whereas pyridyl dissoci ation facilitated by the sterically bulky 6-phenyl group on L-6PhPy yields a [eta(3)-(LCu)-Cu-6PhPy](+) fragment amenable to bis(mu-oxo) core formatio n. The steric properties of the heterocyclic components of the ligands used in this study thus are important determinants of the reactivity of their C u(I) complexes with small molecules. (C) 2000 Elsevier Science S.A. All rig hts reserved.