No abstract

Determination of the energy and other characteristics of the frontier orbitals in complex molecules is a fundamental problem in photochemistry [1], coordination chemistry, and metal-complex catalysis. Experimental study of the properties of organometallic compounds of early transition metals [2,3] and their shortlived intermediates (catalysis [4], redox processes [5], including free radical ones, etc.) is a difficult task. In this work, the reversible isomerization of bridged metallocene was investigated for the first time; we also proposed an approach to comparative estimation of the factors that cause breaking of the structural integrity of d 0 -metal ion complexes upon single-photon excitation such as charge transfer from ligand to metal (LMCT), electrochemical transfer of an electron ( e -) to the lowest unoccupied MO (LUMO) (reduction), and generation of a vacancy ( h + ) on the highest occupied MO (HOMO) (oxidation). Earlier, it was demonstrated that the same MOs are involved in both optical and redox electron transfer in these metallocenes [2,[5][6][7].In the photolysis of Group IVB metallocenes with visible light in the absence of active additives, various photochemical reactions such as dissociation, isomerization, etc. can occur [2,5,8,9]. In this study, we discovered reversible photoisomerization of the ansa -metallocene complex C 6 H 10 ( IndH 4 ) 2 ZrCl 2 (Ind is indenyl) in media with different solvation abilities, particularly, in cyclopentadiene (most likely, the photoisomerization follows the dissociative mechanism through a biradical excited state). The complex C 6 H 10 ( IndH 4 ) 2 ZrCl 2 exists as rac-and meso -isomers. Its dissolved forms were examined by optical spectroscopy: single crystals of the rac -isomer (electron transfer between the frontier orbitals; λ LMCT = 366 nm) and a racemic mixture of rac-(55%) and meso -isomers (45%). The structure of the rac -isomer was confirmed by X-ray diffraction analysis; the composition of the racemic mixture was determined from NMR data (our unpublished data).The absorption and luminescence spectra of the racisomer are slightly shifted toward shorter wavelengths ( ∆ E 0-0 = 150, ∆ E Ï ‡ı ≈ 500 cm -1 ) as compared to the spectra of the racemic mixture containing the meso -isomer (the rac -isomer mainly absorbs at 300 nm, while the meso -isomer absorbs at 360 nm). The radiation lifetimes of the rac -isomer and the racemic mixture are the same within the instrumental error (i.e., τ rac ≥ τ meso ). When exposed to UV light with λ ≈ 310 nm, the dissolved rac -isomer transforms to the racemic mixture (more intense absorption peak at 360 nm), while exposure of the racemic mixture to UV light with λ = 366 nm makes it pass into the rac -isomer (more intense absorption peak at 300 nm). Such transitions occur repeatedly; however, prolonged irradiation of the complex results in its decomposition.Our calculations performed with the Latimer diagram [10] for Cp 2 MCl 2 (where Cp is cyclopentadienyl and M = Ti, Zr, and Hf) showed that π -complexes of early transitio...