Syntheses, spectroscopy, and electrochemistry of high-valent osmium(V) and -(VI) oxo complexes of macrocyclic tertiary amine ligands

Abstract

Osmium has been inserted into macrocyclic tertiary amine ligands [L = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, 1,4,8,12-tetramethyl-1,4,8,12-tetra-azacyclopentadecane, 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane, and meso-2,3,7,11,12-pentamethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17), 13,15-triene] to give trans-[OsIIILCl2]+, which could be oxidized to trans-[OsVIL(O)2]2+ by H2O2. The trans-[OsVIL(O)2]2+ complexes show one intense νasym(OsO2) stretch at 870-880 cm-1. Their u.v.-visible spectra display two vibronic structured electronic absorption bands centred at ca. 310 and 350 nm, assignable to the spin-allowed and spin-forbidden (dxy)2 → (dxy)1(dπ*)1 transitions (dπ* = dxz′dyz). Room-temperature photoluminescence with emission maxima in the region 620-710 nm from trans-[OsVIL(O) 2]2+ has been observed. In acetonitrile, trans-[OsVIL(O)2]2+ displays reversible OsVI-OsV and Os V-OsIV couples with E⊖ ranging from -0.67 to -0.73 and -1.48 to -1.74 V vs. ferrocenium-ferrocene, respectively. The complexes trans-[ OsVL(O)2]+ have been characterized by u.v.-visible spectroscopy and magnetic susceptibility measurements. In aqueous solutions, pH-dependent and reversible OsVI-OsIII couples have been observed. Thermally, trans-[OsVIL(O)2]2+ complexes are poor oxidants for alkene oxidation.