Dinitrogen addition to c - C 5 H 5 Fe + , C 6 H 6 Fe + and FeO + in the gas phase

Results of an experimental study using the selection-ion flow tube technique are reported for reactions of bare Fe + and iron containing FeX ÷ cations (X = C6H 6 (or B), c-C5n 5 (or Cp), O, (Cp)2, B 2) with dinitrogen at 294 _+ 3 K and at a helium buffer-gas pressure of 0.35 _+ 0.01 Torr. Fe z, B2Fe + and Cp2Fe + do not react with dinitrogen. A very slow sequential addition of two N z molecules was observed with FeO +. CpFe + and BFe + reacted without the subsequent addition of a second N 2 molecule. These results provide insight into the bonding of N 2 as a ligand with Fe as the coordination centre in the gas-phase, and into intrinsic kinetic aspects of dinitrogen addition. Iron is the only transition metal common to all biological systems of dinitrogen fixation [1,2]. Recent experimental and theoretical studies [3-11] of transition metal ca t ion-dini t rogen and other small l igand complexes in the gas-phase have found that the binding energies of these complexes are much larger than theoretical values based on electrostatic bonding alone. It has been shown that charge transf e r / cha rge -quadrupo le interaction and covalent bonding are also important [4-7]. We present here the first experimental evidence for the coordination of unsaturated FeX + cations, in particular cC s H s F e + (CpFe+) , CaH6Fe + (BFe +) and FeO ÷ with dinitrogen as a ligand to iron in the gas-phase at 294 ± 3 K. Also, kinetic measurements are reported here which provide the first insight into the intrinsic eff iciency of dinitrogen addition reactions. The results reported here (see Table 1) were obtained using a selected-ion flow tube (SIFT) which has been described previously [12,13]. Al l measurements were performed at 294 _ 3 K and at a helium buffer-gas pressure of 0.35 _ 0.01 Torr. The reactant cations Fe z, CpFe ÷ and CpzFe ÷ were produced in a low pressure ionization source by 2 0 7 0 eV electron-impact ionization of Cp2Fe vapour. In the experiments with BFe + and BEFe + cations were produced from Fe z all the way upstream in the flow tube by adding benzene (1.1 × 1017 molecule s -1 ) through the aspirator gas inlet together with the helium buffer gas. FeO + was derived from the reaction Fe z + N20 --~ FeO + + N 2 in a mixture of Fe(CO) 5 and N20 in a high-pressure ionization source. Al l the regent cations were allowed to thermalize upstream of the flow tube by = 4 × 10 4 collisions with He atoms before adding N 2. The adduct complexes BFe ÷ and CpFe ÷, which have a pyramidal structure [14,15], were observed to react with dinitrogen and formed adducts without the subsequent addition of a second molecule of N 2. Rate coefficients are summarized in Table 1 and a 0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00547-2 340 V.L Baranov et al. / Chemical Physics Letters 239 (1995) 339-343