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, (mu-3:eta-2:eta-2:eta-2-C6H6), that accurately models the coordination geometry of benzene chemisorbed nondissociatively on the surface of a close-packed metal lattice. [Os3(CO)9(mu-3:eta-2:eta-2:eta-2-C6H6)] (1), the parent complex, is elaborated through stepwise "dehydrogenation" of the triply-bridging cyclohexadienyl compound [(mu-H)Os3(CO)9(mu-3:eta-2:eta-1:eta-2-C6H7)], and an X-ray diffraction analysis reveals intriguing multiple bond fixation within the coordinated arene: Im, a = 8.412 (i) angstrom, b = 35.449 (4) angstrom, c = 8.877 (1) angstrom, beta = 92.44 (1)-degrees, V = 2644.7 angstrom3, Z = 6, R = 3.2%, R(w) = 3.4% for 2465 reflections with F(o) > 4-sigma(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) angstrom). The preparation of an analogous toluene complex, [Os3(CO)9(mu-3:eta-2:eta-2:eta-2-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)(mu-3:eta-2:eta-2:eta-2-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)(mu-3:eta-2:eta-2:eta-2-C6H6)] that retain the face-capping benzene moiety. The molecular structures of [OS3(CO)8(PPh3)(mu-3:eta-2:eta-2:eta-2-C6H6)] and [Os3-(CO)9(eta-2-CH2CH2)(mu-3:eta-2:eta-2:eta-2-C6H6)] 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 pi-bound ethylene ligand, respectively. The Cluster [Os3(CO)8(PPh3)(mu-3:eta-2:eta-2:eta-2-C6H6] crystallizes in space group P2(1)/c with a = 13.381 (5) angstrom, b = 14.894 (5), c = 16.896 (7) angstrom, beta = 113.33 (3)-degrees, V = 3092.0 angstrom3, Z = 4, R = 3.9%, R(w) = 4. 1 % for 4747 reflections with F(o) > 4-sigma(F(o)). The cluster [Os3(CO)8(eta-2-CH2CH2)(mu-3:eta-2:eta-2:eta-2:-C6H6)] crystallizes in space group P2(1)/n with a = 8.876 (1) angstrom, b = 14.665 (2) angstrom, c = 14.178 (2) angstrom, beta = 92.56 (2)-degrees, V = 1843.7 angstrom3, Z = 4, R = 4.8%, R(w) = 4.8% for 1890 reflections with F(o) > 4-sigma(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 pi-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 pi-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.