Copper(II) complexes of pyridyl-appended diazacycloalkanes: Synthesis, characterization, and application to catalytic olefin aziridination

摘要

As part of an ongoing effort to rationally design new copper catalysts for olefin aziridination, a family of copper(II) complexes derived from new tetradentate macrocyclic Ligands are synthesized, characterized both in the solid state and in solution, and screened for catalytic nitrene transfer reactivity with a representative set of olefins. The pyridylmethyl-appended diazacycloalkane ligands L-6(py)(2), L-7(py)(2), and L-8(py)(2) are prepared by alkylation of the appropriate diazacycloalkane (piperazine, homopiperazine, or diazacyclooctane) with picolyl chloride in he presence of triethylamine. The ligands are metalated with Cu(ClO4)(2). 6H(2)O to provide the complexes [(L-6(py)(2))Cu(OClO3)]ClO4 (1), [(L-7(py)(2))Cu(OClO3)]ClO4 (2), and [(L-8(py)(2))Cu](ClO4)(2) (3), which, after metathesis with NH4PF6 in CH3CN; afford [(L-6(py)(2))Cu(CH3CN)](PF6)(2) (4), [(L-7(py)(2))Cu(CH3CN](PF6)(2) (5) and [(L-8(py)(2))Cu](PF6)(2) (6). All six complexes are characterized by X-ray crystallography, which reveals that complexes supported by L-6(py)(2) and L-7(py)(2) (1,2,4, 5) adopt square-pyramidal geometries, while complexes 3 and 6, ligated by L-8(py)(2), feature tetracoordinate, distorted-square-planar copper ions. Tetragonal! geometries in solution and d(x(2) - y(2)) ground states are confirmed for the complexes by a combination of UV-visible and EPR spectroscopies. The divergent flexibility of the three supporting ligands influences the Cu(II)/Cu(I) redox potentials within the family, such that the complexes supported by the larger ligands L-7(py)(2) and L-8(py)(2) (5 and 8) exhibit quasi-reversible electron transfer processes (E-1/2 similar to -0.2 V vs Ag/AgCl), while the complex supported by L-6(py)(2) (4), which imposes a rigid tetragonal geometry upon the central copper(II) ion, is irreversibly reduced in CH3CN solution. Complexes 4-6 are efficient catalysts tin 5 mol % amounts) for the aziridination of styrene with the iodinane PhINTs (in 80-90% yields vs PhINTs),while only 4 exhibits significant catalytic nitrene;transfer reactivity with l-hexene and cyclooctene. [References: 39]