Naphthalene complexes: V. Arene exchange reactions in naphthalenechromium complexes

Di-η⁶-naphthalenechromium(0) (1) reacts at 150°C with benzene to yield (η⁶-naphthalene)(η⁶-benzene)chromium(0) (3) in 76% yield. In the presence of THF, 1 undergoes Lewis base catalyzed arene exchange at 80°C. Reactions of 1 with substituted arenes yield the mixed sandwich complexes 4 and 6–10 (arene = 1,4-C₆H₄Me₂, 1,3,5-C₆H₃Me₃, C₆Me₆, 1,4-C₆H₄(OMe)₂, 1,4-C₆H₄F₂ and 1,4-C₁₀H₆Me₂). In all but one case (with 1,4-dimethylnaphthalene) exchange of a single naphthalene ligand is observed. In marked contrast to the lability of 1, dimesitylenechromium(0) (5) is inert to arene displacement in benzene up to 240°C. The molecular structure of 3 has been determined by X-ray crystallography. The crystal data are as follows: a 7.784(1), b 13.411(2), c 22.772(5) Å, Z = 8, space group Pbca. The structure was refined to a R_w value of 0.043. The naphthalene ligand in 3 is nearly planar and parallel to the approximately eclipsed benzene ring. Metal atom-ring distances are 1.631(9) and 1.611(4) Å for naphthalene and benzene, respectively. Catalyzed and uncatalyzed naphthalene exchanges in the sandwich complex are compared to the analogous reactions with the Cr(CO)₃ complex 2. Naphthalene exchange in 2 in benzene is 10³ to 10⁴ times faster than arene exchange in other arenetricarbonylchromium compounds. The mild conditions for Lewis base catalyzed naphthalene exchange make 2 a good precursor of other arenetricarbonylchromium compounds. Examples include the Cr(CO)₃ complexes of styrene, benzocyclobutene, 1-ethoxybenzocyclobutene, 1,8-dimethoxy-9,10-dihydroanthracene and 1,4-dimethylnaphthalene.