19497-23-5Relevant articles and documents
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
Buckner, Steven W.,Gord, James R.,Freiser Ben S.
, p. 6606 - 6612 (2007/10/02)
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 ΔHf(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.
Carbon-Hydrogen Bond Dissociation Energies in Alkylbenzenes. Proton Affinities of the Radicals and the Absolute Proton Affinity Scale
Meot-Ner (Mautner), Michael
, p. 5 - 10 (2007/10/02)
Rate constants (k) were measured for proton-transfer reactions from alkylbenzene ions RH+ to a series of reference bases B, i.e., RH+ + B -> BH+ + R*.For exothermic reactions (ΔH -1.For example, the reaction C6H5CH3+ + B -> BH+ + C6H5CH2* is fast (reaction efficiency = k/kcol >/= 0.5) when B = MeO-t-Bu or stronger bases, but k/kcol is significantly smaller when B is n-Pr2O or weaker bases.From the falloff curve of reaction efficiency vs.PA(B), we find PA(n-Pr2O) = PA(C6H5CH2*) + 0.8 kcal mol-1 = 200.0 kcal mol-1.Since PA(C6H5CH2*) is obtained from known thermochemical data, this relation defines the absolute PA of n-Pr2O.Through a ladder of known PA, we then obtain PA(i-C4H8) = 186.8 kcal mol-1; we also obtain the absolute PAs of other oxygen bases.Falloff curves of reaction efficiencies of 3-FC6H4CH3+, C6H5C2H5+, C6H5-n-C3H7+, and C6H5-i-C3H7+ with these reference bases give then the following PAs of R* and R-H bond dissociation energies (Do) (all in kcal mol-1) as R*, PA(R*), Do(R-H): 3-FC6H4CH2*, 197.2, 89.4; , 197.9, 86.2; , 199.1, 86.1; , 199.6, 86.1.In similar manner, rate constants for H+ transfer from C6H5NH2+ to reference pyridines and amines yield PA(C6H5NH*) = 221.5 and Do(C6H5NH-H) = 85.1 kcal mol-1 (1 kcal mol-1 = 4.18 kJ mol-1).
