Gas-phase chemistry of transition metal-imido and -nitrene ion complexes. Oxidative addition of nitrogen-hydrogen bonds in ammonia and transfer of NH from a metal center to an alkene

Abstract

We report here on the gas-phase chemistry of a number of bare transition metal-nitrene and -imido ion complexes, MNH+. Group 3, 4, and 5 atomic metal ions react with NH3 at thermal energies to generate MNH+ via dehydrogenation. A reaction mechanism is proposed involving initial oxidative addition to an N-H bond, in analogy to mechanisms proposed for reactions of gaseous atomic metal ions with hydrocarbons. Cr+ reacts with NH3 via slow condensation to form CrNH3+, as do all group 6-11 atomic metal ions investigated. However, excited-state Cr+ reacts with NH3 via bond-insertion reactions to form CrNH2+ and CrNH+. An unidentified metastable electronic state of Cr+, produced by direct laser desorption of chromium foil, reacts with much higher efficiency than does kinetically excited Cr+. FeO+ reacts with NH3 to generate FeNH+ with loss of HjO. Thermochemical studies of VNH+ and FeNH+ involving ion-molecule reactions indicate values of D°(V+-NH) = 101 ± 7 kcal/mol and £)°(Fe+-NH) = 54 ± 14 kcal/mol, the latter value in accord with £>°(Fe+-NH) = 61 ± 5 kcal/mol obtained from photodissociation. The high bond strength for VNH+ indicates multiple bonding, analogous to that in the isoelectronic VO+, while the weaker bond strength for FeNH+ indicates a single bond, analogous to that in the isoelectronic

Iron-Mediated Amination of Hydrocarbons in the Gas Phase

“…0.4 for Reaction 2. The formation of FeNH via Reaction 2a is the predominating channel and has been reported previously [13], whereas the side Reaction 2b to yield the FeOH cation has not been mentioned before. The required FeO precursor is most conveniently generated in one of the two following reactions [34].…”

“…Indeed, the splitting of these states is narrow (within 0.2 eV), and the accuracy of the computational approach used does not permit a definite assignment of the ground state for the dication. Interestingly, the vertical ionization energies with respect to FeNH ( [13], who observed CÀN coupling products for both substrates. Encouraged by this important observation, we probed the versatility of FeNH as an imine-transferring agent in the gas phase by reacting FeNH with some selected inorganic and organic substrates.…”

“…In fact, Freiser and coworkers have demonstrated, in a pioneering study about transition-metal imines, that in the gas phase FeNH is able to transfer the imine unit to benzene and ethene [13]. Here, we report some spectroscopic properties of FeNH derived from a combination of massspectrometric experiments with high-level ab initio calculations, and examine the chemical behavior of FeNH towards a series of small organic and inorganic substrates experimentally.…”

“…The OÀO bond cleavage as well as NÀO coupling can be described in terms of a metathesis mechanism, followed by reductive elimination (Reaction 8a) or H-migration and NO loss (Reaction 8b). Although the neutral products cannot be detected directly, thermochemical data predict that the formation of nitrous acid in Reaction 8a is much more exergonic than that of NO OH, for example 13 ). However, a detailed theoretical analysis of a related system has demonstrated that the mechanistic course of metal-mediated O 2 activations can be rather complex [51].…”

“…In fact, the systems FeNH /H 2 and FeO /H 2 are quite similar with respect to the reaction efficiencies Helvetica Chimica Acta ± Vol. 81 (1998) 2359 13 ) Thermochemical data predict the reaction HNO 2 3 . This difference may be attributed to the energy demand for achieving a five-coordinated C-atom en route to the activation of CH 4 .…”