Evaluation of the Thermodynamic Properties of H₂ Binding in Solid State Dihydrogen Complexes [M(η²-H₂)(CO)dppe₂][BArF²⁴] (M = Mn, Tc, Re): An Experimental and First Principles Study

Abstract

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(η2-H2)dppe2][BArF24] 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)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.

Spin-State Effects on the Thermal Dihydrogen Release from Solid-State [MH(η2-H2)dppe2]+ (M = Fe, Ru, Os) Organometallic Complexes for Hydrogen Storage Applications

“…The absorption enthalpy for the singlet iron fragment [FeHdppe 2 ] + is lower than that of the manganese 19 fragment [Mn(CO)dppe 2 ] + , which significantly underestimates the binding strength in the iron complex relative to our experimental observations and reported behavior in the solid state, suggesting a strong effect on the binding enthalpy due to spin effects despite previous reports of the nearly isoenergetic nature of the singlet and triplet states for the iron complex. 27 To investigate this effect further, MP2 calculations were performed on [FeHdppe 2 ] + in both the singlet and triplet states using the LANL2DZ-ECP basis set using the ROHF and UHF shell approximations.…”

“…19 However, an additional complication to the absorption behavior arises in complexes containing a hydride trans to the dihydrogen binding site. Dissociation of the bound dihydrogen molecule and recombination in the resulting trihydride [MH 3 dppe 2 ] + can lead to inversion of the binding site within the molecule.…”

“…To address the entropy arising from the inversion potential of the molecules, an adjustment to the formulation of the partition function used in the statistical Langmuir isotherm 19 can be performed. Because the unbound states are unable to alter their configuration, this inversion only affects the entropy of the bound states.…”

High‐Pressure Raman and Calorimetry Studies of Vanadium(III) Alkyl Hydrides for Kubas‐Type Hydrogen Storage

“…Assuming Henry's adsorption as T increases andt he TDS term begins to dominate relative to DH,s aturation is reacheda t higher and higher pressures. Thus, with enthalpies from 30 to 50 kJ mol À1 and an expected DS of 100 JK À1 [22] and T = 298 K, saturation would typically be reached at pressures below 40 bar,a so bserved previously for H 2 adsorption isothermso f Kubas complexes,s uch as [Mn(h 2 -H 2 )(CO)dppe 2 ][BArF 24 ]. [22] The fact that saturation is not reachedu ntil 120 bar is consistent with am echanism in which ac oncurrent endothermic process, possibly related to structurald eformation (as shown by computational methods to be necessary to achieve optimal Kubas binding in related transition metal hydrazides), [23] buffers the 30-50 kJ mol À1 expected forK ubas binding to give as cenario in which saturation cannot be reached until higher pressures are achieved.…”

“…Thus, with enthalpies from 30 to 50 kJ mol À1 and an expected DS of 100 JK À1 [22] and T = 298 K, saturation would typically be reached at pressures below 40 bar,a so bserved previously for H 2 adsorption isothermso f Kubas complexes,s uch as [Mn(h 2 -H 2 )(CO)dppe 2 ][BArF 24 ]. [22] The fact that saturation is not reachedu ntil 120 bar is consistent with am echanism in which ac oncurrent endothermic process, possibly related to structurald eformation (as shown by computational methods to be necessary to achieve optimal Kubas binding in related transition metal hydrazides), [23] buffers the 30-50 kJ mol À1 expected forK ubas binding to give as cenario in which saturation cannot be reached until higher pressures are achieved. Moreover, aM OF with flexible linkers has previously been shown to distort under pressure to open up porosity for the uptake of H 2 , [24] and perhaps as imilarf lexing of the amorphouss tructure is also important in opening up diffusion pathways through the 3m 2 g À1 nanostructure of V(III)-150H 2.…”