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)]+
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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]−.
KeywordsIndenyl effect DFT Naphthalene effect Tropylium and hydride attack
The author is indebted to Professor J. E. McGrady and his group at theoretical chemistry laboratory, Oxford University, for their assistance throughout the work.
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