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-[Os^IIILCl₂]⁺, which could be oxidized to trans-[Os^VIL(O)₂]²⁺ by H₂O₂. The trans-[Os^VIL(O)₂]²⁺ complexes show one intense ν_asym(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 (d_xy)²→(d_xy)¹(d_π*)¹transitions (d_π*=d_xz′d_yz). Room-temperature photoluminescence with emission maxima in the region 620–710 nm from trans-[Os^VIL(O)₂]²⁺ has been observed. In acetonitrile, trans-[Os^VIL(O)₂]²⁺ displays reversible Os^VI–Os^V and Os ^V–Os^IV couples with E^⊖ ranging from –0.67 to –0.73 and –1.48 to –1.74 V vs. ferrocenium–ferrocene, respectively. The complexes trans-[ Os^VL(O)₂]⁺ have been characterized by u.v.–visible spectroscopy and magnetic susceptibility measurements. In aqueous solutions, pH-dependent and reversible Os^VI–Os^III couples have been observed. Thermally, trans-[Os^VIL(O)₂]²⁺ complexes are poor oxidants for alkene oxidation.