Are there analogues of the indenyl effect in larger ring systems: a DFT study of hydride attack on [Mn(CO)3(naphthalene)]+ and [Cr(CO)3(benzotropylium)

Abstract

Two pairs of complexes, [Mn(CO)3(benzene)]+/[Mn(CO)3(naphthalene)]+ and [Cr(CO)3(tropylium)]+/[Cr(CO)3(benzotropylium)]+, have been used as a platform to establish the extent to which the well-known ‘indenyl effect’ translates into other bicyclic ligand systems. Density functional theory (DFT) suggests that the ‘naphthalene effect’ is minimal, the pathway for hydride reduction of [Mn(CO)3(naphthalene)]+ resembling closely that for the benzene analogue. In the benzotropylium system, in contrast, stabilisation of an η5 coordination mode through aromatisation of the six-membered ring plays a similar role to stabilisation of η3 in the indenyl effect. The greater influence of aromatisation in the five- and seven-membered ring systems stems from the presence of formal charge on the ligands in these cases: localisation of this charge on a subset of the available carbon atoms enhances the electrostatic component of the metal-ligand bond. This is particularly dramatic in the benzotropylium case, where the η7–η5 slippage corresponds to a formal 2-electron reduction of the ligand from [C7H7]+ to [C7H7]−.