Evaluation of the thermodynamic properties of H ₂ binding in solid state dihydrogen complexes [M(ν 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(ν 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.