Intrinsic Coordination Properties of Iron: Gas-Phase Ligation of Ground-State Fe+ with Alkanes, Alkenes, and Alkynes and Intramolecular Interligand Interactions Mediated by Fe+

The kinetics and mode of coordination of the electronic ground state Fe + ( 6 D) have been investigated in the gas phase with the organic molecules methane, ethane, propane, butane, ethylene, allene, propene, 1,3-butadiene, isobutene, acetylene, propyne, and diacetylene. Reaction rate coefficients and product distributions for sequential ligation were measured with the selected-ion flow tube (SIFT) technique operating at 294 ± 3 K and a helium buffer-gas pressure of 0.35 ± 0.01 Torr. Also, bond connectivities in the ligated species were probed with multicollision-induced dissociation (CID) experiments. Rates of ligation with a single ligand were found to increase with an increasing number of degrees of freedom, or size, of the ligand and to follow the reactivity order alkynes > alkenes > alkanes. Ligation with at least two, at least three, and at least five molecules was observed with alkanes, alkenes and alkynes, respectively. Possible modes of bonding in the multiply-ligated Fe + cations are briefly described. The CID results provide evidence for the occurrence of intramolecular interactions between ligands mediated by Fe +, resulting in C−C bond formation in the ligated ions Fe(1,3-C 4 H 6 ) 4 +, Fe(C 2 H 2 ) 3 + and Fe(C 2 H 2 ) 5 +, Fe(CH 3 C 2 H) 2 + and Fe(CH 3 C 2 H) 4 +, and Fe(C 4 H 2 ) 2 + and Fe(C 4 H 2 ) 4 + . The postulated interligand interactions are attributed to cyclization or oligomerization reactions leading to the formation of benzene, dimethylcyclobutadiene, and diethynylcyclobutadiene in Fe(C 2 H 2 ) 3 +, Fe(CH 3 C 2 H) 2 +, and Fe(C 4 H 2 ) 2 +, respectively, and the formation of a dimer of 1,3-butadiene in Fe(1,3-C 4 H 6 ) 4 + .