Gas-Phase Chemistry of Transition Metal-Imido and -Nitrene Ion Complexes. Oxidative Addition of N-H Bonds in NH3 and Transfer of NH from a Metal Center to an Alkene

Article abstract of DOI:10.1021/ja00228a002

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, reats with much higher efficiency than does kinetically excited Cr⁺.FeO⁺ reacts with NH3 to generate FeNH⁺ with loss of H2O.Thermochemical studies of VNH⁺ and FeNH⁺ involving ion-molecule reactions indicate values of D⁰(V⁺-NH) = 101 +/- 7 kcal/mol and D⁰(Fe⁺-NH) = 54 +/- 14 kcal/mol, the latter value in accord with D⁰(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 FeO⁺.Proton-transfer experiments indicate PA(VN) = 220 +/- 4 kcal/mol from which ΔH_f(VN) = 111 +/- 9 kcal/mol and D⁰(V-N) = 125 +/- 9 kcal/mol are obtained.VNH⁺ is unreactive with ethene and benzene, but FeNH⁺ transfers NH to ethene and benzene through metathesis and homologation reactions.A cyclic metalloaminobutane intermediate is consistent with the products of the FeNH⁺/ethene reaction.