Metal Complexes of Severely Crowded Molecules. Synthesis, Structure, and Reactivities of the M(CO)₃ (M = Cr, Mo, and W) Complexes of Octamethylnaphthalene

The preparations of the metal tricarbonyl complexes (Cr, Mo, and W) of octamethylnaphthalene (OMN) are described. The structure of the chromium complex was determined by single-crystal X-ray crystallography [P1̄ space group, Z = 2, a = 9.326 (4) Å, b = 13.992 (3) Å, c = 7.919 (2) Å, α = 95.44 (2)°, β = 115.00 (3)°, γ = 76.40 (2)°]. The chromium tricarbonyl fragment is π bound to one the six-membered rings. The distortions of the metal-complexed OMN differ very little from the structure of the free ligand. The thermal and photochemical reactions of the Cr complex with phosphorus donor ligands differed. Photolysis of (OMN)Cr(CO)₃ in the presence of triphenylphosphine produces purple (OMN)Cr(CO)₂PPh₃, while heating (OMN)Cr(CO)₃ in neat trimethyl phosphite generates Cr(CO)₃[P(OCH₃)₃]₃. Maleic anhydride reacts with (OMN)Cr(CO)₃ to give two isomers resulting from a Diels-Alder addition to the free ring. The stereochemistry of one of the isomers was determined by X-ray crystallography [P2₁/c space group, Z = 4, a = 16.219 (6) Å b = 9.284 (3) Å c = 16.718 (6) Å β = 118.07 (3)°] and was found to result from attack of maleic anhydride on the side of the ring opposite the metal atom. The two isomers differ in the orientation (exo and endo) of the anhydride group with respect to the aromatic ring. A comparison of the equilibrium constants for the free ligand and the complex in the Diels-Alder reaction shows an enhancement of over 2 orders of magnitude for the metal complex. The difference is attributed to the increased strength of interaction of the metal with the planar ring of the cycloaddition product.