Organometallic chemistry in supercritical fluids: The generation and detection of dinitrogen and nonclassical dihydrogen complexes of group 6, 7, and 8 transition metals at room temperature

Article abstract of DOI:10.1021/ja00168a027

The complete miscibility of N₂ and H₂ with supercritical fluids is used to stabilize unstable, and previously unknown, dinitrogen and dihydrogen complexes which would normally decay rapidly at ambient temperatures. UV photolysis of a series of (C_nR_n)M(CO)₃ compounds [M = Fe, Mn, Re or Cr; R = H for n = 4; R = H or Me for n = 5 or 6] dissolved in supercritical Xenon (scXe) under a high pressure of N₂ or H₂ leads to the formation of (C_nR_n(CO)₂(N₂) and (C_nR₂)M(CO)2(H2) complexes at room temperature. Re and Fe undergo facile substitution of more than one CO group by N₂, while Cr and Mn do not. For all elements except Re, the spectroscopic evidence suggests that the (C_nR_n)M(CO)₂(H₂) complexes contain the nonclassical η² H₂ ligand. No IR bands directly associated with η²-H₂ in these compounds have been observed, but the experiments were sufficiently sensitive to detect the ν(H-H) band of W(CO)₅(H₂) under similar conditions. Most of these dihydrogen complexes decompose by reaction with CO to regenerate the (C_nR_n)M(CO)₃ starting material, but significant amounts of Fe(CO)₅ are formed in the decay of (C₄H₄)Fe(CO)₂(H₂). Kinetic measurements between 11 and 80 °C show that (C₆H₅Me)Cr(CO)₂(H₂) reacts with CO with an activation energy of 70 ±5 kj mol^-1, which may correspond to the Cr-(η²-H₂) bond dissociation energy.