Omar S. Jina

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W(CO)5(CH4) and M(η5C5R5)(CO)2(L) [where M = Mn or Re, R = H or CH3 (Re only); and L = CH4 or C2H6]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous n-heptane complexes. Re(Cp)(CO)2(CH4) and Re(Cp*)(CO)2(L) [Cp* = η5C5(CH3)5 and L = CH4, C2H6] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re(Cp*)(CO)3 in scCH4 and demonstrated that the initially generated Re(Cp*)(CO)2(CH4) forms an equilibrium mixture of Re(Cp*)(CO)2(CH4)/Re(Cp*)(CO)2(CH3)H within the first few nanoseconds (τ = 2 ns). The ratio of alkane to alkyl hydride complexes varies in the order Re(Cp)(CO)2(C2H6):Re(Cp)(CO)2(C2H5)H > Re(Cp*)(CO)2(C2H6):Re(Cp*)(CO)2(C2H5)H ≈ Re(Cp)(CO)2(CH4):Re(Cp)(CO)2(CH3)H > Re(Cp*)(CO)2(CH4):Re(Cp*)(CO)2(CH3)H. Activation parameters for the reactions of the organometallic methane and ethane complexes with CO have been measured, and the ΔH‡ values represent lower limits for the CH4 binding enthalpies to the metal center of WCH4 (30 kJ·mol−1), MnCH4 (39 kJ·mol−1), and ReCH4 (51 kJ·mol−1) bonds in W(CO)5(CH4), Mn(Cp)(CO)2(CH4), and Re(Cp)(CO)2(CH4), respectively.

Understanding the factors affecting the activation of alkane by Cp′Rh(CO)2 (Cp′ = Cp or Cp*)

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rates of activation of alkanes by Cp′Rh(CO) (Cp′ = η5-C5H5 or η5-C5Me5). We have monitored the kinetics of C─H activation in solution at room temperature and determined how the change in rate of oxidative cleavage varies from methane to decane. The lifetime of CpRh(CO)(alkane) shows a nearly linear behavior with respect to the length of the alkane chain, whereas the related Cp*Rh(CO)(alkane) has clear oscillatory behavior upon changing the alkane. Coupled cluster and density functional theory calculations on these complexes, transition states, and intermediates provide the insight into the mechanism and barriers in order to develop a kinetic simulation of the experimental results. The observed behavior is a subtle interplay between the rates of activation and migration. Unexpectedly, the calculations predict that the most rapid process in these Cp′Rh(CO)(alkane) systems is the 1,3-migration along the alkane chain. The linear behavior in the observed lifetime of CpRh(CO)(alkane) results from a mechanism in which the next most rapid process is the activation of primary C─H bonds (─CH3 groups), while the third key step in this system is 1,2-migration with a slightly slower rate. The oscillatory behavior in the lifetime of Cp*Rh(CO)(alkane) with respect to the alkane’s chain length follows from subtle interplay between more rapid migrations and less rapid primary C─H activation, with respect to CpRh(CO)(alkane), especially when the CH3 group is near a gauche turn. This interplay results in the activation being controlled by the percentage of alkane conformers.

A systematic approach to the generation of long-lived metal alkane complexes: combined IR and NMR study of (Tp)Re(CO)2(cyclopentane)

Short wavelength photolysis of (Tp)Re(CO)(3) (Tp = tris(pyrazol-1-yl)borate) at low-temperature in cyclopentane yielded (Tp)Re(CO)(2)(cyclopentane), an alkane complex with three nitrogen ligands that was characterised by NMR spectroscopy.

Mechanistic study of the photofragmentation of the clusters [Os3(CO)10(diene)] (diene=cis-1,3-butadiene, 1,3-cyclohexadiene): direct observation of the open-triangle primary photoproduct with nanosecond time-resolved infrared and UV–Vis spectroscopy

Abstract The photochemistry of the clusters [Os 3 (CO) 10 (diene)] (diene= cis -1,3-butadiene, 1,3-cyclohexadiene) was studied in detail. Upon near-UV irradiation photofragmentation occurs to give mononuclear [Os(CO) 3 (diene)]. The second fragmentation product [Os 2 (CO) 7 ] was trapped by reaction with CO, or with alkenes at low temperatures, to produce the dinuclear complexes [Os 2 (CO) 7 (L) 2 ] (L=CO, ethene, 1-octene). The fragmentation was studied further with nanosecond time-resolved UV–Vis and IR spectroscopy and was found to proceed via the initial formation of a triosmium photoproduct with a lifetime of about 100 ns in hexane. From the spectroscopic data we infer that this intermediate has a coordinatively unsaturated open-triangle structure with two bridging carbonyl groups, one bridging the split (CO) 4 OsOs(CO) 2 (diene) bond.

Do early and late transition metal noble gas complexes react by different mechanisms? A room temperature time-resolved infrared study of (η5-C5R5)Rh(CO)2(R = H or Me) in supercritical noble gas solution at room temperature

Using fast time-resolved infrared spectroscopy, the late transition metal noble gas complexes CpRh(CO)(L) and Cp*Rh(CO)(L) (Cp = η5-C5H5; Cp* = η5-C5Me5; L = Xe and Kr) have been characterized at room temperature in supercritical noble gas solution. The krypton complexes are more reactive than the corresponding xenon compounds. There is a significant difference in reactivity with CO between CpRh(CO)(L) and Cp*Rh(CO)(L) with CpRh(CO)(Xe) (kCO = 6.7 × 105 dm3 mol−1 s−1) being ca. 20 times less reactive than Cp*Rh(CO)(Xe) (kCO = 1.4 × 107 dm3 mol−1 s−1). Similarly CpRh(CO)(Kr) (kCO = 1.3 × 108 dm3 mol−1 s−1) is also less reactive than Cp*Rh(CO)(Kr) (kCO = 4.3 × 108 dm3 mol−1 s−1). This should be contrasted to the early transition metal noble gas complexes, where for a given metal–noble gas complex, CpM(CO)2(L) and Cp*M(CO)2(L) (M = Mn or Re) have similar reactivity towards CO. The activation parameters for the reaction of CpRh(CO)(L) and Cp*Rh(CO)(L) with CO have been determined above room temperature, and suggest that the reaction proceeds by an associative mechanism.

Two photochemical pathways in competition: matrix isolation, time-resolved and NMR studies of cis-[Ru(PMe3)4(H)2]

cis-[Ru(PMe3)4(H)2] (1) reacts by two distinct photochemical pathways resulting in the formation of [Ru(PMe3)4] and [Ru(PMe3)3(H)2]; derivatives of these intermediates are generated in the presence of CO and Ph2SiH2.