Evaluation of the Thermodynamic Properties of H2 Binding in Solid State Dihydrogen Complexes [M(η~2-H2)(CO)dppe2][BArF~(24)] (M = Mn, Tc, Re): An Experimental and First Principles Study

摘要

The solid state complex [Mn(CO')dppe2]-[BArF24] was synthesized, and the thermodynamic behavior and properties of the hydrogen absorption reaction to form the dihydrogen complex [Mn(η~2-H2)dppe2][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)dppe2]~+ (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.