Structure and dynamics of iron pentacarbonyl

The dynamics of CO ligand scrambling in Fe(CO)5 has been investigated by linear infrared spectroscopy in supercritical xenon solution. The activation barrier for the Berry pseudorotation in Fe(CO)5 was determined experimentally to be Ea = 2.5 ± 0.4 kcal mol–1 by quantitative analysis of the temperature-dependent spectral line shape. This compares well with the range of Ea/(kcal mol–1) = 2.0 to 2.3 calculated by various DFT methods and the value of 1.6 ± 0.3 previously obtained from 2D IR measurements by Harris et al. ( Science 2008, 319, 1820). The involvement of Fe(CO)5···Xe interactions in the ligand scrambling process was tested computationally at the BP86-D3/AE2 level and found to be negligible

Density functional theoretical studies of the Re-Xe bonds in Re(Cp)(CO) (PF3)Xe and Re(Cp)(CO)(2)Xe

Density functional calculations have been used to probe the electronic structures of Re(Cp)(CO)(2)Xe and Re(Cp)(CO)(PF3)Xe. The calculated CO stretching frequencies compare favorably with those determined experimentally. Our calculations of delta(Xe) and (3)J(Xe-F) for Re(Cp)(CO)(PF3)Xe represent the first for a well-characterized transition metal-noble gas compound and demonstrate that DFT using the BP86 and SAOP functionals reproduces these parameters to within 1% and 8% of their experimentally determined values. The calculated Re-Xe bond dissociation energies for Re(Cp)(CO)(2)Xe (12.3 kcal mol(-1)) and Re(Cp)(CO)(PF3)Xe (11.9 kcal mol(-1)) are also in excellent agreement with the lower limits for these energies estimated from the activation parameters for the reaction of the complexes with CO in supercritical Xe. A topological analysis of the electron density in Re( Cp)( CO) 2Xe and Re(Cp)(CO)(PF3) Xe reveals positive del(2)rho(r) at the critical points (del rho(r(c)) = 0.1310 and 0.1396 e angstrom(5) for Re(Cp)(CO)(2)Xe and Re(Cp)(CO)(PF3)Xe, respectively, indicating that the Re-Xe interaction is essentially closed-shell in both complexes. Fragment and overlap density of states analyses show that the orbital interactions in these compounds is dominated by overlap involving the Xe p orbitals and the orbitals of the Cp, CO, and/or PF3 ligands; the Re d orbitals appear to contribute little to the orbital interactions between the Re(Cp)(CO)(2) and Re(Cp)(CO)(PF3), and Xe fragments

Simulating the Pyrolysis of Polyazides: a Mechanistic Case Study of the [P(N₃)₆]⁻ Anion

Pyrolysis of the homoleptic azido complex [P­(N₃)₆]⁻ was simulated using density functional theory based molecular dynamics and analyzed further using electronic-structure calculations in atom-centered basis sets to calculate the geometries and electronic structures. Simulations at 600 and 1200 K predict a thermally induced and, on the simulation time scale, irreversible dissociation of an azido anion. The ligand loss is accompanied by a barrierless (free-energy) transition of the geometry of the complex coordination sphere from octahedral to trigonal bipyramidal. [P­(N₃)₅] is fluxional and engages in pseudorotation via a Berry mechanism

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)(CH(4)) and M(η(5) [Image: see text] C(5)R(5))(CO)(2)(L) [where M = Mn or Re, R = H or CH(3) (Re only); and L = CH(4) or C(2)H(6)]. 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)(CH(4)) and Re(Cp*)(CO)(2)(L) [Cp* = η(5) [Image: see text] C(5)(CH(3))(5) and L = CH(4), C(2)H(6)] 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 scCH(4) and demonstrated that the initially generated Re(Cp*)(CO)(2)(CH(4)) forms an equilibrium mixture of Re(Cp*)(CO)(2)(CH(4))/Re(Cp*)(CO)(2)(CH(3))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)(C(2)H(6)):Re(Cp)(CO)(2)(C(2)H(5))H > Re(Cp*)(CO)(2)(C(2)H(6)):Re(Cp*)(CO)(2)(C(2)H(5))H ≈ Re(Cp)(CO)(2)(CH(4)):Re(Cp)(CO)(2)(CH(3))H > Re(Cp*)(CO)(2)(CH(4)):Re(Cp*)(CO)(2)(CH(3))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 CH(4) binding enthalpies to the metal center of W [Image: see text] CH(4) (30 kJ·mol(−1)), Mn [Image: see text] CH(4) (39 kJ·mol(−1)), and Re [Image: see text] CH(4) (51 kJ·mol(−1)) bonds in W(CO)(5)(CH(4)), Mn(Cp)(CO)(2)(CH(4)), and Re(Cp)(CO)(2)(CH(4)), 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 delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy

The organometallic alkane complexes Re(Cp)(CO)(PF3)(alkane) and Re(Cp)(CO)(2)(alkane) have been detected after the photolysis of Re(Cp)(CO)(2)(PF3) in alkane solvent. NMR and time-resolved IR experiments reveal that the species produced by the interaction of n-pentane with [Re(Cp)(CO)(PF3)] are an equilibrium mixture of Re(Cp)(CO)(PF3)(pentane) and Re(Cp)(CO)(PF3)(pentyl)H. The interaction of cyclopentane with [Re(Cp)(CO)(PF3)] most likely results in a similar equilibrium between cyclopentyl hydride and cyclopentane complexes. An increasing proportion of alkane complex is observed on going from n-pentane to cyclopentane to cyclohexane, where only a small amount, if any, of the cyclohexyl hydride form is present. In general, when [Re(Cp)(CO)(PF3)] reacts with alkanes, the products display a higher degree of oxidative cleavage in comparison with [Re(CP)(CO)(2)], which favors alkane complexation without activation. Species with the formula Re(Cp)(CO)(PF3)(alkane) have higher thermal stability and lower reactivity toward CO than the analogous Re(Cp)(CO)(2)(alkane) complexes

Characterization of an organometallic xenon complex using NMR and IR spectroscopy

Photolysis of Re((i)PrCp)(CO)(2)(PF(3)) in liquid or supercritical Xe yields two new compounds [Re((i)PrCp)(CO)(2)Xe and Re((i)PrCp)(CO)(PF(3))Xe]. Re((i)PrCp)(CO)(PF(3))Xe has been characterized by NMR and IR spectroscopies. The compound is an organometallic Xe complex that has been characterized by using NMR spectroscopy and is shown to be longer-lived than other organometallic Xe complexes by IR spectroscopy. (19)F, (31)P, and (129)Xe chemical shifts have been determined. The (129)Xe chemical shift of Re((i)PrCp)(CO)(PF(3))Xe, δ –6,179, is a Xe shift that is significantly shielded, on the order of 1,000 ppm, with respect to free Xe. The coupling constants between coordinated (129)Xe and both the (19)F and (31)P nuclei present have been extracted, confirming the identity of the compound. Observed line widths give a lower limit to the lifetime of the coordinated Xe of 27 ms at 163 K

Probing the mechanism of carbon-hydrogen bond activation by photochemically generated hydridotris(pyrazolyl)borato carbonyl rhodium complexes: New experimental and theoretical investigations

Fast time-resolved infrared (TRIR) experiments and density functional (DFT) calculations have been used to elucidate the complete reaction mechanism between alkanes and photolytically activated hydridotris(pyrazoly-1-yl)boratodicarbonylrhodium. TRIR spectra were obtained after photolysis of Rh(TP4-tBu-3,5-Me)(CO)(2) in n-heptane, n-decane, and cyclohexane and of Rh(Tp(3,5-Me))(CO)(2) in n-heptane and cyclohexane. Initial photolysis produces a coordinatively unsaturated, 16-electron monocarbonyl species that vibrationally relaxes to an intermediate with v(CO) of 1971 cm(-1) in n-heptane solution (species A). DFT calculations on Rh(Tp(3,5-Me))(CO)-RH (RH = C2H6, C6H12) suggest that A is the triplet state of a five-coordinate, square-pyramidal Rh(k(3) -Tp(3,5-Me))(CO)-RH, in which the alkane is weakly bound. Within the first 2 ns, a new transient grew in at 1993 cm(-1) (species B). The calculations show that the observed species B is the singlet state of a four-coordinate Rh(K-2-Tp(3,5-Me))(CO)(RH), in which the alkane is strongly bound and one pyrazolyl ring is rotated, decoordinating one N. The transient due to B grew at the same rate as A partially decayed. However, A did not decay completely, but persisted in equilibrium with B throughout the time up to 2500 ps. The v(CO) bands due to A and B decayed at the same rate as a band at 2026 cm(-1) grew in (tau ca. 29 ns, n-heptane). The latter band can be readily assigned to the final alkyl hydride products, Rh(K-3-Tp(4-tBu-3,5-Me))(CO)R(H) and Rh(K-3-Tp(3,5-Me))(CO)R(H) (species D). The experimental data do not allow the elucidation of which of the two alkane complexes, A or B, is C-H activating, or whether both of the complexes react to form the final product. The calculations suggest that a third intermediate (species Q is the C-H activating species, that is, the final product D is formed from C and not directly from either A or B. Species C is nominally a five-coordinate, square-pyramidal Rh(K-21/2-T-p3,T-5-Me)(CO)(RH) complex with a strongly bound alkane and one pyrazolyl partially decoordinated, but occupying the apical position of the square pyramid. Intermediate C is unobserved, as the calculations predict it possesses the same CO stretching frequency as the parent dicarbonyl. The unobserved species is predicted to lie on the reaction path between A and B and to be in rapid equilibrium with the four-coordinate species B