Gas-phase metal ion ligation: Collision-induced dissociation of Fe(H₂O)_x⁺ and Fe(CH₄)_x⁺ (x = 1-4)

Guided-ion beam mass spectrometry is used to study the collision-induced dissociation (CID) of Fe(H₂O)_x⁺ and Fe(CH₄)_x⁺ ions, x = 1-4. Ions are produced in a flow tube source to ensure thermalization. By measuring the CID thresholds, we obtain the following 0 K bond dissociation energies (BDEs) for Fe(H₂O)_x⁺ clusters: D°₀[Fe⁺-(H₂O)] = 30.6 ± 1.2 kcal/mol, D°₀[(H₂O)Fe⁺-(H₂O)] = 39.3 ± 1.0 kcal/mol, D°₀[(H₂O)₂Fe⁺-(H₂O)] = 18.2 ± 0.9 kcal/mol, and D°₀[(H₂O)₃Fe⁺-(H₂O)] = 19.6 ± 1.2 kcal/mol. Our values for x = 1 and 2 are compared to and contrasted with prior literature measurements and calculations. We also report the first measurements of the BDEs for Fe(CH₄)_x⁺ clusters, x = 1-4, obtaining the following 0 K values: D°₀[Fe⁺-(CH₄)] = 13.7 ± 0.8 kcal/mol, D°₀[(CH₄)Fe⁺-(CH₄)] = 23.3 ± 1.0 kcal/mol, D°₀[(CH₄)₂Fe⁺-(CH₄)] = 23.6 ± 1.4 kcal/mol, and D°₀[(CH₄)₂Fe⁺-(CH₄)] = 17.6 ± 1.4 kcal/mol. The most surprising observation here is that D°₀[(CH₄)₂Fe⁺-(CH₄)] exceeds D°₀[(H₂O)₂Fe⁺-(H₂O)]. This result and other trends in these BDEs are discussed by examining FeL_x⁺ bonding in terms of the nature of the ligand and the electronic structure of the ion.