Direct Observation of Activated Hydrogen Binding to a SupportedOrganometallic Compound at Room Temperature

Current interest in the use of hydrogen as a transportation fuel has driven extensive research into novel gas storage materials.[1] Although physisorption materials can possess technologically viable storage capacities, their isosteric heats are generally below 10 kJmol1, limiting these materials to cryogenic temperatures. Chemisorbers, such as metal hydrides or complex hydrides, can store large amounts of hydrogen but require elevated temperatures to release the gas; isosteric heats for hydride systems are typically larger than 40 kJmol1. The optimum conditions for viable room-temperaturehydrogen storage require materials that possess isosteric heats of adsorption in between that of standard physisorbers and chemisorbers, typically in the 20-30 kJmol1 regime.[2] Theoretical work[3, 4] has shown that the incorporation of transition-metal atoms onto a porous support can provide such binding energies with multiple hydrogenmolecules adsorbed. However, despite the very large number of theoretical papers, there is no direct experimental proof of these predictions yet. An early experimental exampleis the gas-phase hydrogen reaction with Ti-ethylene complexes,[5] where the gravimetrically measured hydrogen uptake agrees well with theoretical predictions but details of structure, dynamics, and the local chemistry are absent. Herein, we present direct experimental evidence for dihydrogen- Ti binding on a silica-supported TiIII organometallic complex (hereafter referred to as Ti-HMS) using detailed sorption and inelastic neutron scattering (INS) measurements. Our experimental findings are further supported by extensive first-principles DFT and reaction path calculations. We show that the TiIII ion is essential for the formation of a dihydrogen complex, and its presence is confirmed by EPR spectroscopy (see Figure S1 in the Supporting Information).Surprisingly, we discover that the H2-Ti binding is a thermally activated process; exposing the supported organometallic to hydrogen below 150 K results in only physisorption, while near room temperature it forms H2-Ti moieties that are stable for extended periods of time. Such an activation barrier was missed in earlier DFT calculations, which predicted only the formation of dihydrogen complexes. Though this particular sample does not represent a viable storage material due to its modest uptake and nonoptimized support, it does offer a useful benchmark for understandingthe underlying hydrogen coordination chemistries.