Indenyl effect

In organometallic chemistry, the indenyl effect refers to the enhanced rates of substitution displayed by η⁵-indenyl complexes vs the related η⁵-cyclopentadienyl complexes.

Mechanism

Associative substitution occurs by the addition of a ligand to a metal complex followed by dissociation of an original ligand. Associative pathways are not typically seen in 18-electron complexes due to requisite intermediates having more than 18 electrons. 18 electron indenyl complexes; however, have been shown to undergo substitution via associative pathways quite readily. This is attributed to the relative ease η⁵ to η³ rearrangement due to stabilization by the arene. This stabilization is responsible for substitution rate enhancements of about 10⁸ for the substitution of indenyl complexes compared to the corresponding cyclopentadienyl complex.

Kinetic data supports two proposed mechanisms for associative ligand substitution. The first mechanism, proposed by Hart-Davis and Mawby, is a concerted attack by the nucleophile and η⁵ to η³ transition followed by loss of a ligand and a η⁵ to η³ transition.

In a second mechanism proposed by Basolo, η⁵ to η³ transitions are in rapid equilibrium. The rate-limiting step occurs with the attack of the nucleophile on a η³ intermediate.

η⁵ to η³ Rearrangement in Other Ligands

Indenyl like effects are also observed in a number of non indenyl substituted metal complexes. In fluorenyl complexes, associative substitution is enhanced even further than indenyl compounds. The substitution rate of Mn(η⁵-C₁₃H₉)(CO)₃ is about 60 times faster than that of Mn(η⁵-C₉H₇)(CO)₃

Veiros conducted a study comparing the rate of substitution on [(η⁵-X)Mn(CO)₃] where X is cyclopentadienyl, indenyl, fluorenyl, cyclohexadienyl, and 1-hydronaphthalene. Unsurprisingly, it was found that the ease of η⁵ to η³ haptotropic shift correlated to the strength of the Mn-X bond.

History

The indenyl effect is a term given by Fred Basolo to a phenomenon first reported by Adam J. Hart-Davis and Roger J. Mawby in 1969. Hart-Davis and Mawby found that the rate of conversion of (η⁵-C₉H₇)Mo(CO)₃CH₃ to the phosphine-substituted acetyl complex followed bimolecular kinetics. This rate law was attributed to the ability of the indenyl ligand to undergo a η⁵ to η³ haptotropic rearrangement, which abets associative attack on the metal. The corresponding reaction of tributylphosphine with (η⁵-C₅H₅)Mo(CO)₃CH₃ was 10 times slower. [A.J. Hart-Davis, R.J. Mawby, "Reactions of -indenyl complexes of transition metals. Part I. Kinetics and mechanisms of reactions of tricarbonyl--indenylmethylmolybdenum with phosphorus(III) ligands" "J. Chem. Society A" 1969, 2403-6 DOI|10.1039/J19690002398]

Subsequent work by Hart-Davis, Mawby, and White compared CO substitution by phosphines in Mo(η⁵-C₉H₇)(CO)₃X and Mo(η⁵-C₅H₅)(CO)₃X (X = Cl, Br, I) and found the cyclopentadienyl compounds to substitute by an S_N1 pathway and the indenyl compounds to substitute by both S_N1 and S_N2 pathways. Mawby and Jones later studied the rate of CO substitution with P(OEt)₃ with Fe(η⁵-C₉H₇)(CO)₂I and Fe(η⁵-C₅H₅)(CO)₂I and found that both occur by an S_N1 pathway with the indenyl substitution occurring about 575 times faster. Hydrogenation of the arene ring in the indenyl ligand resulted in CO substitution at about half the rate of the cyclopentadienyl compound.

Work in the early 1980s by Fred Basolo found the S_N2 replacement of CO in Rh(η⁵-C₉H₇)(CO)₂ to be 10⁸ times faster than in Rh(η⁵-C₅H₅)(CO)₂. Shortly afterwards, Basolo tested the effect of the indene ligand on Mn(η⁵-C₉H₇)(CO)₃, the cyclopetadienyl analogue of which having been shown to be inert to CO substitution. Mn(η⁵-C₉H₇)(CO)₃ did undergo CO loss and was found to substitute via an S_N2 mechanism.

References

*C. White, R.J. Mawby, A.J. Hart-Davis, "Inorg. Chim. Acta", 1970, 4, 261.
*Fred Basolo, "Coord. Chem. Rev.," 1982, 43, 7.
*Mark E. Rerek, Liang-Nian Ji, and Fred Basolo." J. Chem. Soc. Chem. Commun.", 1983.
*Mark E. Rerek, Fred Basolo, "J. Am. Chem. Soc.," 1984, 106, 5908.
*Luis F. Veiros, "Organometallics," 2000, 19, 3127.
*M.J. Calhorda, Luis F. Veiros, "Chem. Eur. J.," 2002, 8, 4.