Naphthalene complexes: V. Arene exchange reactions in naphthalenechromium complexes

Abstract

Di-η 6-naphthalenechromium(0) ( 1) reacts at 150°C with benzene to yield (η 6-naphthalene)(η 6-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-C 6H 4Me 2, 1,3,5-C 6H 3Me 3, C 6Me 6, 1,4-C 6H 4(OMe) 2, 1,4-C 6H 4F 2 and 1,4-C 10H 6Me 2). 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 10 3 to 10 4 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.