The Synthesis of Iridabenzenes by the Coupling of Iridacyclopentadienes and Olefins

Abstract

The iridacyclopentadiene [(TpMe2)Ir{C1(R)=C(R)C(R)=C4(R)}(H2O)(Ir–C4)] (1; R = CO2Me) reacts with propene at 60 °C to give the iridabenzene [(TpMe2)Ir{C1(Et)C(R)C(R)C(R)C5(R)}(H)(Ir–C5)] (6), which also contains a hydride ligand, as the main product, accompanied by complexes 7 and 8 derived from the insertion of propene into the Ir–C bonds of 1. In contrast, the closely related metallacycle [(TpMe2)Ir{C1(R)=C(H)C(R)=C4(R)}(H2O)(Ir–C4)] (1‐H) gives the iridabenzene [(TpMe2)Ir{C1(Me)C(R)C(H)C(R)C5(R)}(Me)(Ir–C5)] (13), which contains an iridium‐bonded methyl group, upon treatment with propene at 20 °C. The proposed mechanism for the formation of metallabenzenes 6 and 13 postulates Ir–propene → Ir=CHEt or Ir–propene → Ir=CMe2 isomerisations, followed by insertion of the alkylidene ligand into an Ir–C(R) bond (regioselective for 1‐H) and α‐H or α‐Me elimination from the resulting alkyliridium moiety. At 25 °C, neat acetonitrile induces stereoselective hydride migration onto both the Ir[pb2]C bonds of 6 to give a kinetic mixture (6:1 ratio) of the isomeric adducts [(TpMe2)Ir{C1H(Et)C(R)=C(R)C(R)=C5(R)}(MeCN)(Ir–C5)] (9) and [(TpMe2)Ir{C1H(R)C(R)=C(R)C(R)=C5(Et)}(MeCN)(Ir–C5)] (10), with the latter being the thermodynamically preferred product. In contrast, complex 13 is stable in acetonitrile up to 100 °C, where it mainly experiences ring fragmentation to give the iridacyclopentadiene 1‐H as its MeCN adduct. All new compounds have been fully characterised by microanalysis and spectroscopy. Additionally, the solid‐state structures of 5, 6 and 13 have been determined by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Activation of Small Molecules by the Metal–Amido Bond of Rhodium(III) and Iridium(III) (η5-C5Me5)M-Aminopyridinate Complexes

“…Figure shows the solid-state structure of the molecules of this complex, corroborating the formulation anticipated in Scheme on the basis of spectroscopic data. The Ir1–C20 bond to the carbene carbon atom has a length of 1.911(12) Å, very similar to the distances found for other cationic and neutral carbene complexes of iridium­(III) reported by our group. − The Ir1–N1 bond to the pyridine ring has a distance of 2.113(8) Å, which is also comparable to the Ir–N pyridine distances found in related complexes described in this paper or previously. , …”

“…By analogy with previously reported studies, the α-H elimination that leads to complex 6-Ir + is expected to be reversible. − Indeed, observation of the hydride resonance of 6-Ir + as a broad singlet (vide supra) may be suggestive of fast equilibration of this complex with undetectable concentrations of the cyclic alkylpyridine solvent species 6-Ir·CD 2 Cl 2 + (Scheme ), resulting from fast migratory insertion of the carbene ligand of 6-Ir + into the Ir–H bond, promoted by coordination of CD 2 Cl 2 . Reaction of 6-Ir + with an excess of PMe 3 triggered the anticipated 1,2-H shift from iridium to the carbene carbon atom, although subsequent chemical changes must be invoked to account for the chemical constitution of the resulting product 7-Ir + .…”

Metallaaromatic Chemistry: History and Development

“…Furthermore, because in 98 and 99 the CH moiety of the iridacycle is at the para-position, the alkenyl carbon insertion must be highly regioselective, involving the only insertion into the IrC adjacent to the CH moiety (Scheme 35). 170,171 However, it is still not clear why, when 96 was used, C(CH 3 )R 1 appears to be favored over CHCH 2 R 1 , while, in the case of 92, the situation is contrary. Interestingly, when heated at 100 °C in CH 3 CN, complex 98 was converted to 97, demonstrating that the process is reversible.…”

Recent Advances in Metallaaromatic Chemistry

“…Since these early reports the study of metallabenzenes has flourished . The third row transition metals are well‐represented with examples incorporating osmium, iridium, platinum and, most recently, rhenium . Metallabenzenes of one second row transition metal, ruthenium, are also known .…”