Energetics of the Reactions of (η⁶-C₆H₆)Cr(CO)₃ with n-Heptane, N₂, and H₂ Studied by High-Pressure Photoacoustic Calorimetry

Abstract

The enthalpy changes associated with the photosubstitution of (η6-C6H6)Cr(CO)3 by n-heptane, H2, and N2 have been measured by high-pressure photoacoustic calorimetry (PAC). The kinetics of the reaction of (η6-C6H6)Cr(CO)3 to form (η6-C6H6)Cr(CO)2L (L = H2 and N2) under high pressures of H2 and N2 have been monitored by time-resolved infrared spectroscopy (TRIR), showing that the increased concentration of ligand at high pressure greatly accelerates the reaction, making it possible to use the PAC technique to study these reactions. PAC experiments in n-heptane show the enthalpy change for the reaction of (η6-C6H6)Cr(CO)3 + H2 → (η6-C6H6)Cr(CO)2(η2-H2) + CO to be +91 ± 4 kJ mol-1. The PAC data indicate that the strength of the Cr−L bond in (η6-C6H6)Cr(CO)2L (L = H2 and N2) is similar to that in the corresponding Cr(CO)5L complexes. We estimate the Cr−H2 bond dissociation enthalpy (BDE) in (η6-C6H6)Cr(CO)2(η2-H2) to be 60 ± 4 kJ mol-1 and the Cr−N2 BDE to be 6 kJ mol-1 stronger (66 ± 4 kJ mol-1). The thermal equilibrium between (η6-C6H6)Cr(CO)2(η2-H2) and (η6-C6H6)Cr(CO)2(N2) was studied by FTIR in a polyethylene matrix pressurized with a mixture of H2 and N2 (130 bar). Conversion of (η6-C6H6)Cr(CO)2(N2) to (η6-C6H6)Cr(CO)2(η2-H2) was achieved by using a high pressure of H2 at 22 °C.

Chemistry of Reactive Organometallic Compounds at Low Temperatures and High Pressures: Reactions of M(CO)6(M = Cr, Mo, W), (η6-C6H3Me3)M(CO)3(M = Cr and Mo), and W(CO)5CS with H2and N2in Polyethylene Matrices

“…Such experiments were possible because the enormous difference in vapor pressure of H 2 and Xe makes it relatively easy to vent H 2 from lXe solution. Recent high-pressure photoacoustic measurements of Cr−N 2 and Cr−H 2 bond dissociation energies suggest that the reverse exchange should be observable in the presence of a great excess of H 2 . Such experiments are not usually possible in lXe because, once N 2 has been added to lXe solution, it is extremely difficult to remove again.…”

Hydrogen Storage in Microporous Metal–Organic Frameworks with Exposed Metal Sites

“…Unfortunately, the low efficiency of the photolysis in solid‐state samples precluded further decarbonylation of these products, such that other, more efficient means to remove carbonyl ligands need to be developed to take advantage of all three metal binding sites on the half‐sandwich units. Nevertheless, given that the H 2 binding energy for the hydrogenated species is expected to be in the vicinity of 60–70 kJ mol −1 , as measured for [(C 6 H 6 )Cr(CO) 2 (H 2 )]26 and [(C 6 H 5 Me)Cr(CO) 2 (H 2 )],27 these results suggest that an approach involving the functionalization of the organic bridging ligands could produce materials with very high H 2 affinity.…”

New Directions in Inorganic and Metal-Organic Coordination Chemistry in Supercritical Fluids

“…This stability was not entirely unexpected because the (C n H n )M(CO) 2 moiety was already known to form extremely stable dinitrogen complexes . Attempts have been made to quantify the strength of the M−H 2 bond dissociation energies in such compounds. , The first approach involved an Arrhenius analysis of the rate of reaction of (toluene)Cr(CO) 2 (H 2 ) with CO in scXe, exploiting the miscibility of H 2 , CO, and scXe to keep the relative concentrations of gases constant as the temperature was varied. , The activation energy 73 ± 2 kJ mol -1 was close to the M−H 2 bond dissociation energies, later measured for Cr(CO) 5 (H 2 ) 439 and (C 6 H 6 )Cr(CO) 2 (H 2 ) 437 using high-pressure photoacoustic calorimetry (PAC) in n- heptane solution. PAC is an appealingly simple technique in which the energy released in a reaction is measured acoustically. , However, it is not easy to apply to high-pressure systems 107,437,439,442 and, so far at least, has proved extremely difficult to use on supercritical solutions…”