Indenyl effect
Encyclopedia
In organometallic chemistry
, the indenyl effect refers to the enhanced rates
of substitution
displayed by η5-indenyl
complexes vs the related η5-cyclopentadienyl
complexes.
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 chemical equilibrium
. The rate-limiting step occurs with the attack of the nucleophile
on a η3 intermediate.
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.
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.
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 SN1 pathway and the indenyl compounds to substitute by both SN1 and SN2 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 SN1 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 SN2 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 SN2 mechanism.
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
, the indenyl effect refers to the enhanced rates
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
of substitution
Substitution reaction
In a substitution reaction, a functional group in a particular chemical compound is replaced by another group. In organic chemistry, the electrophilic and nucleophilic substitution reactions are of prime importance...
displayed by η5-indenyl
Indene
Indene is a flammable polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring. This aromatic liquid is colorless although samples often are pale yellow...
complexes vs the related η5-cyclopentadienyl
Cyclopentadienyl
In organic chemistry, cyclopentadienyl is a cyclic group of atoms with the formula C5H5. Cyclopentadienyl are closely related to cyclopentadiene. Cyclopentadienyl have five carbon atoms bonded together in a pentagonal planar ring, all five of which are bonded to individual hydrogen atoms...
complexes.
Mechanism
Associative substitutionNucleophilic substitution
In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...
occurs by the addition of a ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
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
Indene
Indene is a flammable polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring. This aromatic liquid is colorless although samples often are pale yellow...
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
Arene
Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...
. This stabilization is responsible for substitution rate enhancements of about 108 for the substitution of indenyl complexes compared to the corresponding cyclopentadienyl
Cyclopentadienyl complex
A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
complex.
Kinetic data supports two proposed mechanisms for associative ligand substitution. The first mechanism, proposed by Hart-Davis and Mawby, is a concerted
Concerted reaction
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup...
attack by the nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
and η5 to η3 transition followed by loss of a ligand and a η5 to η3 transition.
Fred Basolo
Fred Basolo was an American inorganic chemist. He received his Ph.D. at the University of Illinois at Urbana-Champaign in 1943 with John C. Bailar. Basolo spent his entire professional career at Northwestern University...
, η5 to η3 transitions are in rapid chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
. The rate-limiting step occurs with the attack of the nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
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)3Cyclopentadienyl complex
A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
, 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 BasoloFred Basolo
Fred Basolo was an American inorganic chemist. He received his Ph.D. at the University of Illinois at Urbana-Champaign in 1943 with John C. Bailar. Basolo spent his entire professional career at Northwestern University...
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.
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 SN1 pathway and the indenyl compounds to substitute by both SN1 and SN2 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 SN1 pathway with the indenyl substitution occurring about 575 times faster. Hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of the arene
Arene
Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...
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 SN2 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 SN2 mechanism.