Time-Resolved Infrared Spectroscopy Studies of Olefin Binding in Photogenerated CpRu(CO)X (X = Cl, I) Transients

Abstract

The mechanism and energetics of ligand (L) substitution (L = THF, cyclopentene, cyclohexene, cyclooctene) from photogenerated CpRu-(CO)(X)L (X = Cl, I) complexes were studied using time-resolved infrared spectroscopy. The reactions proceed through a dissociative mechanism, and the Ru−L binding enthalpies were estimated. The trend in the bond enthalpies, Ru− (η 2 -cyclooctene) ≈ Ru−(η 2 -cyclopentene) > Ru−(η 2 -cyclohexene), is correlated with the strain energy of the cycloalkene ring. For all ligands investigated, CpRu(CO)(Cl)−L binding enthalpies were lower than those for the analogous CpMn(CO) 2 −L and BzCr(CO) 2 −L complexes. DFT calculations indicate that the lower binding enthalpy for the Ru−L complexes is due to a greater reorganizational energy for the CpRu(CO)Cl fragment as it adopts a configuration suitable for interaction with the ligand. ■ INTRODUCTIONTransition-metal-containing organometallic compounds have been shown to catalyze a variety of organic transformations. 1 Complexes containing the ruthenium metal center are particularly active catalysts in a variety of reactions. For example, many key reactions such as alkene isomerization, hydroformylation, C−H activation of arenes, CO 2 fixation, and others have been successfully catalyzed by derivatives of the Ru 3 (CO) 12 cluster. 2 Other Ru carbonyls such as Ru-(CO) 3 (PR 3 ) 2 have also been used as catalysts for the hydrodesulfurization of benzothiophenes. 3 The hydroformylation of 1-octene by CpRu(CO) 2 Cl has also been reported, and despite its high catalytic activity, there are no mechanistic studies that have been undertaken to identify and study the chemistry of the relevant intermediates. 4 Photoinduced time-resolved infrared spectroscopy is a powerful tool for identifying and studying the reactivity of intermediates that may be formed during a catalytic cycle. 5 A vacant coordination site on a metal complex, necessary for catalytic activity, can be generated by photolytic loss of a ligand. Upon binding of the substrate, the reactivity of the resulting intermediate can be probed with infrared light over a range of time scales. Metal carbonyls tend to be the best candidates for such studies, since photolytic loss of CO often proceeds with high quantum efficiency and the remaining CO's attached to the metal center are ideal infrared tags for probing the temporal profile of the resulting intermediates. This technique is therefore well suited to study the reactions of Ru carbonyl intermediates, given their importance in promoting several catalytic processes.As mentioned above, the reaction of CpRu(CO) 2 Cl with 1-octene in the presence of H 2 /CO leads to hydroformylation of the alkene with high selectivity and yield. 4 The mechanism of the reaction was not determined, although it was found that a key species was most likely the CpRu(CO) 2 H complex, presumably formed upon reaction of the parent compound with H 2 . Since this 18-electron coordinatively saturated hydride complex is unlikely to be the active species, it is reasonable to suggest ...

A unified set of experimental organometallic data used to evaluate modern theoretical methods

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Estimating the strength of the M–H–B interaction: a kinetic approach

“…During the past several years, we have utilized a kinetic method for estimating the bond dissociation enthalpies (BDE's) of several weakly coordinated M-L complexes. [30][31][32][33] These complexes are generated by photolyzing a metal precursor in the presence of L followed by ligand displacement by an incoming substrate. The reactions can be followed from the μs to hours timescale using time resolved IR spectroscopy.…”

Transition Metal-Olefin Bonding Interactions: A Density Functional Theory Study [M(CO) x (Æ? 2 - C 2 H 3 - C 6 H 4 - Y)]

“…Time-resolved infrared spectroscopy is a powerful tool for probing the reactivity of intermediates which may be formed during metal-olefin catalysis. 25 Metal complexes containing carbonyl ligands, in particular, have been shown to serve as appropriate models for investigating metal-olefin coordination as the photolytic loss of CO often occurs selectively in solution and with high quantum efficiency. 26 The production of metal-olefin complexation following photodissociation can then be monitored using FTIR spectroscopy since remaining CO ligands attached to the metal center are ideal infrared tags for probing the transient profile of the resulting intermediates.…”

“…26 The production of metal-olefin complexation following photodissociation can then be monitored using FTIR spectroscopy since remaining CO ligands attached to the metal center are ideal infrared tags for probing the transient profile of the resulting intermediates. [25][26][27] One method for experimentally measuring metal-olefin bond dissociation enthalpy consists of tracking the rate of chemical decomposition using FTIR spectroscopy over a range of time scales. Typically, a competitive reaction scheme will be set up between the olefin and a ligand that is expected to coordinate more strongly.…”