LOS JUEGOS Y LA RESOLUCIÓN DE PROBLEMAS

The binding strength of the H2 molecule in dihydrogen complexes is typically evaluated from the stretching of the H-H bond when bound to the metal center. This property is a measure of the electron density removed from the bonding σ orbital of the hydrogen molecule and donated to the anti-bonding σ∗ orbital by the metal center, representing the degree of bond breaking within the H2 molecule and the strength of the electronic interaction. H-H distances less than ∼1.0 ˚A, representing the “true” dihydrogen complexes, are considered weakly bound and easily removed from the dihydrogen complex. Bond distances of greater than∼1.0 ˚A, representing complexes with

1_{Parts of this section have been reproduced with permission fromEvaluation of the Thermodynamic Properties}

of H2 Binding in Solid State Dihydrogen Complexes [M(η2-H2)(CO)dppe2][BArF24] (M = Mn, Tc, Re): An Ex- perimental and First Principles Study by David G. Abrecht and Brent Fultz. Copyright 2012 American Chemical Society.

more dihydride character, are considered more tightly bound and less likely to reversibly absorb the dihydrogen complex.112, 113

X-ray diffraction and neutron diffraction provide the only truly definitive method for determin- ing H-H bond distance. However, x-ray scattering from the low mass hydrogen atom relative to large metal atoms is often weak and indeterminate, and neutron diffraction experiments are ex- pensive and difficult to perform. Estimates of the H-H bond separation are available from NMR spectroscopy measurements, and are the most commonly reported method of approximating the binding strength. Maltby121 _{and Luther}122 _{independently developed relationships correlating the} H-H bonding distance from x-ray and neutron diffraction measurements to the solid-state NMR coupling constant,JHD, of isotopomeric HD complexes. The relationship given by Maltby is:

dHH= 1.42−0.0167JHD

where dHH is the H-H bond distance, in angstroms, and JHD is given in hertz. The equation given by Luther is the same as the equation above, with values within experimental error of those found by Maltby. These relationships have also been shown to hold approximately in solution NMR studies, despite the potential for interference from solvent molecules and increased molecular motion. Because of the ease and clarity of solution state NMR spectra relative to solid-state NMR spectra or organometallic crystallographic methods, this method has come to dominate the literature for estimates of binding strength.

While the interatomic distances can provide relative measures of the dihydrogen binding strength in similar families of complexes, these values only provide approximations to the binding strength and are not universally applicable as measures of thermodynamic properties. The dihydrogen molecule often exists in equilibrium with its corresponding dihydride, with a low energy barrier separating the two states. Removal of dihydrogen may be possible through the transient formation of a weak dihydrogen complex, as seen in W(CO)3(P(i_{-Pr)3)2(H2), despite the complete dissociation of the} H2 molecule to form the dihydride.120 _{The existence of additional hydrides ligated to the metal} surface can also add entropic stabilization to the complex through intramolecular site exchange,

stabilizing weaker complexes.96 _{Varying ratios ofσ and π donation ability created by the metal-} ligand pairs lead to poor correlation between the observed lability of the dihydrogen ligand and the H- H distance for molecules with widely disparate chemistries. Additionally, the bond distance provides no measure of the stability of the dehydrogenated complex; rearrangement of the molecular geometry after dehydrogenation can strongly affect the thermodynamic properties of hydrogen absorption and release, and consequently the equilibrium pressure and temperature.88

Thermodynamic information on dihydrogen complexes has largely been focused on the heterolytic cleavage of the dihydrogen molecule and the protonation ability of the complexes,96 _{in order to ob-} tain greater information on the activity of hydrogenases.88 _{These interactions are unfortunately} detrimental to direct hydrogenation hydrogen storage applications, as they represent a potentially irreversible side reaction to form stable hydride complexes, and thermodynamic values presented in these studies do not typically include the unsaturated complex for assistance in evaluating direct hydrogenations. The available thermodynamic data on direct hydrogenations to form dihydrogen complexes remains scarce. Calorimetric studies have reported solution-state binding enthalpies of ∆H◦_{= -41.8 and -46.9 kJ/mol for the complexes W(CO)}

3(PCy3)2and W(CO)3(P-i-Pr3)2in toluene, respectively.123, 124 _{A similar study in THF, attempting to develop relationships between the group} 6 metals in the complexes M(CO)(PCy3)2(M=Cr, Mo, W), found ∆H◦ between -27.2 kJ/mol and -41.8 kJ/mol, and entropy values ∆S◦ between -100 and -110 J/mol-K.95_{Solid-state measurements} are limited and show deviations from solution behavior, with one study giving ∆H◦= -13.22 kJ/mol and ∆S◦ = -9.62 J/mol-K for hydrogen absorption over the complex125[Ir(cod)(PPh3)2]SbF6, rep- resenting a significant reduction in binding strength and an unexpected increase in the entropy of the bound state. While most of the data are consistent with values obtained from hydrogen storage materials, the current measurements represent a wide range of metals and ligands providing different chemical properties that, apart from the systematic study performed by Gonzalez, et al. for the group 6 complexes, do not provide chemical trends to aid in the design of new materials.

Difficulties in obtaining systematic thermodynamic data for dihydrogen complexes arise from their inherent stability, with the equilibrium pressures of most known materials falling outside of

acceptable ranges for traditional chemistry techniques. New techniques for measuring the thermody- namic properties of dihydrogen complexes, especially at elevated pressures, are necessary to evaluate their potential as hydrogen storage materials. In the next chapters, the mechanism of pressurized hydrogenation to form dihydrogen complexes in the solid state is examined, and techniques for establishing the thermodynamic properties of these materials are established.

Chapter 3

Studies of

[M(η

2

−

H

₂

)(CO)dppe

₂

]

+

(M = Mn, Tc, Re) Complexes

1

As discussed in the previous chapter, technical difficulties have largely prevented direct quantification of thermodynamic properties for dihydrogen binding in organometallic complexes. To help resolve these difficulties and enable the generation of systematic thermodynamic data relevant to the hy- drogen storage community, we have used the Sieverts method to obtain experimental isotherm mea- surements on the interaction of hydrogen gas with the dihydrogen complex [Mn(CO)dppe2][BArF24_] (dppe = 1,2-bis(diphenylphosphino)ethane, BArF24 _{= tetrakis-(3,5-trifluoromethyl)phenylborate).} In addition, we report electronic structure calculations on the hydrogenation of the fragments [M(CO)dppe2]+ (M = Mn, Tc, Re) to examine trends in the binding energy within the group 7 metals. We interpret our results in terms of the solid-state binding mechanism for hydrogen in these materials, and show a relatively rapid and facile means to quantitatively evaluate thermodynamic properties and establish chemical trends that are useful for materials design.