THE ARENE-EXCHANGE REACTION IN NAPHTHALENE-CR(CO)3 AND PYRENE-CR(CO)3

The arene-exchange reaction has been studied for a number of labile (arene)Cr(CO)3 substrates in hydrocarbon solvents. A two-term rate law was established of the form -d[S]/dt = k(A)[S] + k(B)[S][arene'], where S is the (arene)Cr(CO)3 substrate and toluene was used as the external, arene, ligand. The lability of S follows the ordering (2,6-dimethylpyridine)- > (pyrene)- > (naphthalene)- > (2,5-dimethylthiophene)Cr(CO)3. Activation parameters for (naphthalene)Cr(CO)3 were DELTA-H double-ended dagger A = 28 (1) kcal mol-1, DELTA-S double-ended dagger A = -14 (2) cal K-1 mol-1, DELTA-H double-ended dagger B = 26 (1) kcal mol-1, and DELTA-S double-ended dagger B = -23 (3) cal K-1 mol-1. For (pyrene)Cr(CO)3 they were determined to be DELTA-H double-ended dagger A = 25 (1) kcal mol-1, DELTA-S double-ended dagger A = -19 (3) cal K-1 mol-1, DELTA-H double-ended dagger B = 24 (1)kcal mol-1, and DELTA-S double-ended dagger B = -22 (2) cal K-1 mol-1. The substitutional affinity of (pyrene)Cr(CO)3 for arene' was found to be C6H5CF3 < C6H5Me < m-C6H4Me2 almost-equal-to C6H5Bu(t) < C6M6 < 1,3,5-cycloheptatriene. It is suggested that the arene'-independent path in the above rate law proceeds via an (eta-y-arene)Cr(CO)3 intermediate and the other path through an (eta-x-arene)Cr(CO)3(eta-6-x-arene') intermediate with y < x. Molecular orbital calculations at the extended Huckel level were used to construct full potential energy surfaces for ring slippage in benzene, naphthalene, and (pyrene)MnCp. Selected regions were refined for the Cr(CO)3 derivatives. An eta-6 --> eta-2 path was found to be most favourable for the naphthalene system, whereas an eta-6 --> eta-1 path required the least energy for pyrene. In (benzene)Cr(CO)3 both paths were found to be comparable in energy. The calculations correctly predict arene lability to be in the order benzene << naphthalene < pyrene. No evidence for a discrete eta-4 intermediate was found for the arene'-independent route.