A face-capping bonding mode for benzene in triosmium carbonyl cluster complexes

Article abstract of DOI:10.1021/ja00033a026

This paper describes the synthesis of trinuclear metal complexes containing benzene in a new face-capping bonding mode, (μ₃:η²:η²:η²-C ₆H₆), that accurately models the coordination geometry of benzene chemisorbed nondissociatively on the surface of a close-packed metal lattice. [Os₃(CO)₉(μ₃:η²:η ²:η²-C₆H₆)] (1), the parent complex, is elaborated through stepwise dehydrogenation of the triply-bridging cyclohexadienyl compound [μ-H)O₃s(CO)₉(μC₆H₃η ²:η¹:η²-C₆H₇)], and an X-ray diffraction analysis reveals intriguing multiple bond fixation within the coordinated arene: Im, a = 8.412 (1) A?, b = 35.449 (4) A?, c = 8.877 (1) A?, β= 92.44 (1)°, V = 2644.7 A?³, Z = 6, R = 3.2%, R_w = 3.4% for 2465 reflections with F_o 4σ(F_o). The ring symmetrically caps a trimetal face and shows Kekule-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) A?). The preparation of an analogous toluene complex, [Os₃(CO)₉(μ:η²:η ²η²)] 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)(μ₃η ²: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(NCMe)(μ:η²:η ²:η²-C₆H₆)] that retain the face-capping benzene moiety. The molecular structures of [Os₃(CO)₈(PPh₃)(μ₃η ²:η²:β²-C₆H₆)] and [Os₃(CO)₈(η²-CH₂CH ₂)(μ₃:η²:η²:η ²-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:η ²:η²:η²-C₆H₆)] crystallizes in space group P2_1/c with a = 13.381 (5) A?, b = 14.894(5),c= 16.896 (7) A?, β= 113.33(3)°, V= 3092.0 A?³, Z = 4, R = 3.9%, R_w = 4.1% for 4747 reflections with F_o >4σ(F_o). The cluster [Os₃(CO)₈-(η²-CH₂CH ₂(μ₃:η²:η²:η ² crystallizes in space group P2_1/nwith a = 8.876 (1) A?, b = 14.665 (2) A?, c = 14.178 (2) A?, β= 92.56 (2)°, V= 1843.7 A?³, 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 x-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.