Four pulse electron spin echo modulation studies of Cu(II) complexes in MCM-41 silica tube materials

The coordination geometry of Cu(II) complexes with water and ammonia has been studied by four pulse electron spin echo modulation spectroscopy in siliceous (L)Cu-MCM-41 and in aluminum-containing (L)Cu-AlMCM-41 where (L) denotes Cu(II) incorporation by liquid phase ionexchange. An analysis of the proton sum peaks in the echo modulation pattern of the water and ammonia ligands reveals significant differences in the Cu(II) coordination between MCM-41 and AlMCM-41. In the aluminum-containing material (L)Cu-AlMCM-41, Cu(II) coordinates to two molecules of water or ammonia and three framework oxygens in a square-based pyramidal coordination geometry. The base of the pyramid is formed by two adsorbate molecules together with two framework oxygens. A third framework oxygen is located at the apex of the pyramid. The cupric ion site is slightly shifted from the plane of the pyramid base towards the apex resulting in an off-plane position. In the siliceous material (L)Cu-MCM-41, [Cu(H₂O)₆]²⁺ and [Cu(NH₃)₄]²⁺ complexes are observed. The results of four pulse electron spin echo modulation experiments support a distorted octahedral coordination geometry for the [Cu(H₂O)₆]²⁺ complex in (L)Cu-MCM-41.