Evaluation of the Thermodynamic Properties of H(2) Binding in Solid State Dihydrogen Complexes...

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 313K-373K 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.