Photochemistry of [M(η5-C5H5)(CO)3Et](M = Mo or W): a mechanistic study using time-resolved infrared spectroscopy and matrix isolation

Abstract

Matrix isolation at 13 K and time-resolved infrared (TRIR) spectroscopy at room temperature have been used to study the mechanism of the photochemical conversion of [M(η5-C5H5)(CO)3Et] into [M(η5-C5H5)(CO)3H](M = Mo or W). Ultraviolet photolysis of [M(η5-C5H5)(CO)3Et] isolated in methane matrices produces two distinct dicarbonyl species, [M(η5-C5H5)(CO)2Et ⋯ CH4]1 and [M(η5-C5H5)(CO)2(CH2CH2-µ-H)]2, together with cis-[(M)η5-C5H5)(CO)2(C2H4)H]3(for M = W only), and eventually trans-[M(η5-C5H5)(CO)2(C2H4)H]4. In the matrix 1 and 2 can be interconverted using selective photolysis with visible wavelengths. These two species have also been identified in room-temperature heptane solution, using TRIR spectroscopy. For the tungsten system TRIR has also confirmed the existence of a fast equilibrium between 2 and 3. An activation energy measured for the decay of 2 suggests that the rate-determining step in this decay is the isomerization of 3 to 4. Although 3 was not observed in either matrix or in TRIR experiments with the molybdenum system, the existence of a further equilibrium between [Mo(η5-C5H5)(CO)2(CH2CH2-µ-H)] and trans-[Mo(η5-C5H5)(CO)2(C2H4)H] has been observed. These results are of relevance to current studies on ‘agostic’ hydrogen interactions.