Spectroscopy and Photochemistry of Metal-Oxo Complexes

Abstract

Electronic spectroscopic, photophysical, and photochemical investigations of two types of metal-oxo complexes are described. The electronic absorption spectra of various molybdenyl ions (MoO3+ were measured in crystals and solutions. These spectra clearly support 2B1(x2-y2) ← 2B2(xy) and 2E(xz, yz) ← 2B2(xy) assignments for the two weak bands near 25,000 and 14,000 cm-1, respectively, in these molecules. A Franck-Condon analysis of the 2B1 ← 2B2 band in the 5 K single crystal absorption spectrum of (Ph4As)MoOCl4 indicated a 0.07(1) A elongation of the metal-chloride bond in the excited state. Crystalline (Ph4As)MoOCl4 luminesces from the lower energy component of a split 2E(xz, yz) state with a lifetime of 160 ns at 300 K, increasing to 1.4 µs at 5 K. No luminescence was detected from any molybdenyl ion in solution. The electronic absorption and emission spectra of trans-ReO2L4z (L = CN-, z = 3-; L = 1/2(ethylenediamine), z = +; L = pyridine, z = +; L = 4-picoline, z = +; L = 4-t-butylpyridine, z = +) were also measured in crystals and solutions. The lowest energy absorption bands are assigned to components of a split 3Eg[(eg)1(b2g)1] state (ground state 1Ag1g[(b2g)2]). Emission from at least two of these components was identified in the cyanide and pyridine complexes and only the ethylenediamine complex failed to luminesce. Franck-Condon analyses of the emission spectra indicated ca. 0.1 A elongations of each Re-O bond in the 3Eg excited states. The lifetime of the 3Eg state varies from 10 to 300 µs in crystals and is ca. 10 µs for trans-ReO2L4z species in aprotic solvents. Protons efficiently quench the luminescence of these molecules. Experiments directed toward photoinduced oxygen atom transfer chemistry are also described.