Comparative photochemistry of some tricarbonyl(η4-polyene)iron complexes in frozen gas matrices at ca. 12 K

Abstract

Ultraviolet irradiation of [Fe(η⁴-polyene)(CO)₃] complexes [polyene = 1,3-butadiene (C₄H₆), cyclobutadiene (C₄H₄), 2,4-cyclopentadien-1-one (C₄H₄CO), cyclopentadiene (C₅H₆), 1,3-cyclohexadiene (C₆H₈), 1,3-cycloheptadiene (C₇H₁₀), 1,3-cyclo-octadiene (C₈H₁₂), or 1,3,5,7-cyclo-octatetraene (C₈H₈)] in frozen gas matrices at ca. 12 K results in two competing processes. One is partial dechelation of the polyene ligand to form [Fe(η²-polyene)(CO)₃] species and the other is dissociation of a CO ligand to produce co-ordinatively unsaturated [Fe(η⁴-polyene)(CO)₂] species. The photogenerated species were characterised by i.r. spectroscopy. Evidence is presented which indicates that [Fe(η²-polyene)(CO)₃] species are responsible for s-cis⇄s-trans, ‘tub’⇄‘chair’, and π⇄σ rearrangements in [Fe(η⁴-C₄H₆)(CO)₃], [Fe(η⁴-C₈H₈)(CO)₃], and [Fe(η⁴-C₄H₄)(CO)₃] respectively. The possibility of ring opening to form a metallocyclic intermediate is discussed in relation to the photoconversion of [Fe(η⁴-C₄H₄)(CO)₃] into [Fe(η⁴-C₄H₄CO)(CO)₃] in a carbon monoxide matrix. In a ¹³CO-doped methane matrix, facile ¹³CO/¹²CO exchange occurred for the Fe(CO)₃ fragment of [Fe(η⁴-C₄H₄CO)(CO)₃] but no exchange was found for the ketonic CO group. Ring-to-metal hydrogen migration forming [Fe(η⁵-C₅H₅)(CO)₂H] and [Fe(η⁵-C₇H₉)(CO)₂H] has been found to occur for [Fe(η⁴-C₅H₆)(CO)₃] and [Fe(η⁴-C₇H₁₀)(CO)₃] complexes via the co-ordinatively unsaturated fragments [Fe(η⁴-C₅H₆)(CO)₂] and [Fe(η⁴-C₇H₁₀)(CO)₂]. The intermediacy of η²-bound polyene species of the type [Fe(η²-polyene)(CO)₄] has been established in the photodissociation of polyene ligands and the formation of [Fe(CO)₅] in carbon monoxide matrices.