Copper(II) Complexes of Pyridyl-Appended Diazacycloalkanes: Synthesis, Characterization, and Application to Catalytic Olefin Aziridination

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 L6(py)2, L7(py)2, and L8(py)2 are prepared by alkylation of the
appropriate diazacycloalkane (piperazine, homopiperazine, or diazacyclooctane) with picolyl chloride in the presence
of triethylamine. The ligands are metalated with Cu(ClO4)2·6H2O to provide the complexes [(L6(py)2)Cu(OClO3)]ClO4 (1), [(L7(py)2)Cu(OClO3)]ClO4 (2), and [(L8(py)2)Cu](ClO4)2 (3), which, after metathesis with NH4PF6 in
CH3CN, afford [(L6(py)2)Cu(CH3CN)](PF6)2 (4), [(L7(py)2)Cu(CH3CN)](PF6)2 (5), and [(L8(py)2)Cu](PF6)2 (6).
All six complexes are characterized by X-ray crystallography, which reveals that complexes supported by L6(py)2 and L7(py)2 (1, 2, 4, 5) adopt square-pyramidal geometries, while complexes 3 and 6, ligated by L8(py)2,
feature tetracoordinate, distorted-square-planar copper ions. Tetragonal geometries in solution and d(x2 − y2)
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 L7(py)2 and L8(py)2 (5 and 6) exhibit quasi-reversible
electron transfer processes (E1/2 ∼ −0.2 V vs Ag/AgCl), while the complex supported by L6(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 (in 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
1-hexene and cyclooctene.