Low-Lying Excited States and Primary Photoproducts of [Os_3(CO)_(10)-(s-cis-L)] (L=Cyclohexa-l,3-diene,Buta-l,3-diene)] Clusters Studied by Picosecond Time-Resolved UV/Vis and IR Spectroscopy and by Density Functional Theory

摘要

Combined picosecond transient absorption and time-resolved infrared studies were performed,aimed at characterising low-lying excited states of the cluster [Os_3(CO)_(10)(s-cis-L)] (L=cyclohexa-l,3-diene,1) and monitoring the formation of its photoproducts.Theoretical (DFT and TD-DFT) calculations on the closely related cluster with L=buta-l,3-diene (2') have revealed that the low-lying electronic transitions of these [Os_3(CO)_(10)(s-cis-1,3-diene)] clusters have a predominant sigma(core)pi(CO) character.From the lowest sigmapi excited state,cluster 1 undergoes fast Os-Os(l,3-diene) bond cleavage (tau=3.3ps) resulting in the formation of a coordinatively unsatu-rated primary photoproduct (la) with a single CO bridge.A new insight into the structure of the transient has been obtained by DFT calculations.The cleaved Os-Os(l,3-diene) bond is bridged by the donor 1,3-diene ligand,compensating for the electron deficiency at the neighbouring Os centre.Because of the unequal distribution of the electron density in transient la,a second CO bridge is formed in 20 ps in the photoproduct [Os_3(CO)_8(mu-CO)_2-(cyclohexa-l,3-diene)] (1b).The latter compound,absorbing strongly around 630 nm,mainly regenerates the parent cluster with a lifetime of about 100ns in hexane.Its structure,as suggested by the DFT calculations,again contains the 1,3-diene ligand coordinated in a bridging fashion.Photoproduct 1b can therefore be assigned as a high-energy coordination isomer of the parent cluster with all Os-Os bonds bridged.