Synthesis and Characterization of a Family of Systematically Varied Tris(2-pyridyl)methoxymethane Ligands: Copper(I) and Copper(II) Complexes

An efficient modular protocol for synthesizing a series of facial-capping tris-pyridyl ligands, based on the tris(2-pyridyl)methoxymethane backbone, has been developed which allows for systematic variations of the steric
demands at the periphery of the ligand. The coordination chemistry of one such family of ligands that positions
0→3 methoxy groups at the periphery with Cu(I) and Cu(II) is presented. The ligands are tris(2-pyridyl)methoxymethane (L0), bis(2-pyridyl)(2-(6-methoxy)pyridyl)methoxymethane (L1), bis(2-(6-methoxy)pyridyl)(2-pyridyl)methoxymethane (L2), and tris(2-(6-methoxy)pyridyl)methoxymethane (L3). The ligand exchange behavior
and, to a lesser extent, the structures of the these complexes vary dramatically given the small perturbation of
introducing methoxy substituents. Two distinct coordination modes are observed for the Cu(I) complexes, both
in solution and the solid state. One is a pseudo-tetrahedral coordination comprised of the facial-capping, tris-pyridyl ligand and a monodentate ligand such as CH3CN, CO, or PPh3. The other structural type is also a pseudo-tetrahedral Cu(I) monomer formed by two tris-pyridyl ligands coordinated in a bidentate manner with preferable
binding by the nonmethoxy pyridyl subunits. With the exception of the most sterically hindered ligand, L3,
which only displays monoligation to Cu(I), all ligands form both types of Cu(I) complexes, and the formation is
controlled by stoichiometry. Both competitive ligand binding experiments and ligand substitution with CO(g)
show that the [(L0)2Cu]+ and [(L1)2Cu]+ complexes have nearly equivalent stability in aprotic solvent, and greater
stability than the [(L2)2Cu]+ complex due to inclusion of bulky methoxypyridines into the Cu(I) coordination
sphere. The Cu(II) complexes of the ligand series generate “bis-tris”, [(L0→3)2Cu]2+, complexes, with the Cu(II)
ligated in a tetragonally distorted octahedral coordination environment. The degree of bulk at the ligand periphery
dictates the Cu(II)-ligand bond lengths both in solution and the solid state. In these complexes, the bulky pyridyl
ring prefers to bind in the axial position. For the most sterically encumbered ligand, L3, the bisligated Cu(II)
complex is moisture sensitive, reacting to give a monoligated, tris-aqua species, [L3Cu(H2O)3]2+.