Evaluation of the Thermodynamic Properties of H_2 Binding in Solid State Dihydrogen Complexes [M(η^2‑H_2)(CO)dppe_2][BArF^(24)] (M = Mn, Tc, Re): An Experimental and First Principles Study

The solid state complex [Mn(CO)dppe_2][BArF^(24)] was synthesized, and the thermodynamic behavior and properties of the hydrogen absorption reaction to form the dihydrogen complex [Mn(η^2-H_2)dppe_2][BArF^(24)] were measured over the temperature range 313–373 K and pressure range 0–600 Torr using the Sieverts method. The absorption behavior was accurately described by Langmuir isotherms, and enthalpy and entropy values of ΔH° = −52.2 kJ/mol and ΔS° = −99.6 J/(mol K) for the absorption reaction were obtained from the Langmuir equilibrium constant. The observed binding strength was similar to metal hydrides and other organometallic complexes, despite rapid kinetics suggesting a site-binding mechanism similar to physisorption materials. Electronic structure calculations using the LANL2DZ-ECP basis set were performed for hydrogen absorption over the organometallic fragments [M(CO)dppe_2]^+ (M = Mn, Tc, Re). Langmuir isotherms derived from calculation for absorption onto the manganese fragment successfully simulated both the pressure–composition behavior and thermodynamic properties obtained from experiment. Results from calculations for the substitution of the metal center reproduced qualitative binding strength trends of 5d > 3d > 4d previously reported for the group 6 metals.