A Face-Capping Bonding Mode for Benzene in Triosmium Carbonyl Cluster Complexes

This paper describes the synthesis of trinuclear metal complexes containing benzene in a new face-capping bonding mode, (μ₃:η²:η²:η²-C₆H₆), that accurately models the coordination geometry of benzene chemisorbed nondissociatively on the surface of a close-packed metal lattice. [Os₃(CO)₉(μ₃:η²:η²:η²-C₆H₆)] (1), the parent complex, is elaborated through stepwise “dehydrogenation” of the triply-bridging cyclohexadienyl compound [(μ-H)Os₃(CO)₉(μ₃:η²:η²:η²-C₆H₆)], and an X-ray diffraction analysis reveals intriguing multiple bond fixation within the coordinated arene: Im, a = 8.412 (1) Å, b = 35.449 (4) Å, c = 8.877 (1) Å, β = 92.44 (1)°, V = 2644.7 ų, Z = 6, R = 3.2%, R_w = 3.4% for 2465 reflections with F_o> 4σ(Fo). The ring symmetrically caps a trimetal face and shows Kekulé-type distortion toward the hypothetical cyclohexa-1,3,5-triene (on average, C-C bond lengths alternate between 1.41 (3) and 1.51 (4) Å). The preparation of an analogous toluene complex, [Os₃(CO)₉(μ₃:η²:η²:η²-C₆H₅Me)] is also reported. Oxidative decarbonylation of 1 by trimethylamine N-oxide in the presence of MeCN affords the lightly-ligated complex [Os₃(CO)₈(NCMe)(μ₃:η²:η²:η²-C₆H₆)] (9). Two-electron donor ligands (e.g., CO, PR₃, C₅H₅N, olefins) readily displace the labile nitrile ligand from 9, giving derivatives [Os₃(CO)₈(L)(μ₃:η²:η²:η²-C₆H₆)] that retain the face-capping benzene moiety. The molecular structures of [Os₃(CO)₈(PPh₃)(μ₃:η²:η²:η²-C₆H₆)] and [Os₃(CO)₈(η²-CH₂-CH₂)(μ₃:η²:η²:η²-C₆H₆)] have been determined by X-ray crystallography and are derived from that of complex 1, with an equatorial carbonyl ligand being replaced by triphenylphosphine and a π-bound ethylene ligand, respectively. The cluster [Os₃(CO)₈(PPh₃)μ₃:η²:η²:η²-C₆H₆)] crystallizes in space group P2₁/c with a = 13.381 (5) Å, b = 14.894 (5), c = 16.896 (7) Å, β = 113.33 (3)°, V = 3092.0 Å3, Z = 4, R = 3.9%, R_w = 4.1% for 4747 reflections with F_o > 4σ(F_o). The cluster [Os₃(CO)₈(PPh₃)(μ₃:η²:η²:η²-C₆H₆)] crystallizes in space group P2₁/n with a = 8.876 (1) Å, b = 14.665 (2) Å, c = 14.178 (2).β = 92.56 (2)°, V= 1843.7 Å3, Z = 4, R = 4.8%, R_w = 4.8% for 1890 reflections with F_o > 4σ(F_o). Semiempirical molecular orbital calculations provide useful insights into the structure and bonding of complex 1. Primary contributions to metal-arene bonding arise from overlap of the benzene HOMO with the LUMO of the cluster fragment and through π-back donation to the benzene LUMO from a high-lying cluster-based molecular orbital. The trigonal ring distortion may be traced to an internal mixing of the benzene π system that leads to increased overlap in the C-C bonds eclipsing the metal atoms, at the expense of the alternate noneclipsing bonds which consequently become elongated. Possible relationships between these compounds and aromatic adsorbate complexes of transition metal surfaces are also explored.