Mechanism of acetylene cyclotrimerization catalyzed by the fac-IrP3+ fragment: Relationship between fluxionality and catalysis

Article abstract of DOI:10.1021/om00017a067

Reaction of [(triphos)Ir(C₂H₄)₂](BPh₄) with C₂H₂ at 25°C gives [(triphos)Ir(η⁴-C₆H₆)](BPh₄), 1, which was shown to have this Ir/benzene connectivity by single crystal X-ray diffraction. Crystal data (-155°C): a = 16.471(6) A?, b = 17.126(6) A?, c = 12.030(4) A?, α = 101.22(2)°, β = 93.61(2)°, and γ = 75.46(1)° with Z = 2 in space group P1. This species reacts with C₂H₂ in the presence of Cl^- to give (triphos)IrCl(η²-C₄H₄), 2, which can be converted back to 1 with C₂H₂ in the presence of the chloride scavenger TlPF₆. Ethyne will displace C₆H₆ from 1 at 60°C in THF, thus completing a catalytic cyclotrimerization of C₂H₂ to benzene. While the phosphorus nuclei in 1 form an AM₂ spin system, these undergo site exchange with activation parameters ΔH^? = 10.7(3) kcal/mol and ΔS^? = -9.5(6) kcal^-1 mol^-1. The benzene ring ¹H NMR spectra are also temperature-dependent, and the fluxionality can be accounted for by the same activation parameters appropriate to ³¹P site exchange; the same physical mechanism thus accomplishes both site exchanges. The structural study indicates that η⁴-C₆H₆, which is nonplanar, is a stronger π-acceptor than is butadiene itself. A multistep mechanism has been studied with extended Hu?ckel calculations. It is shown that the C-C bond formation between the first two alkynes to give the unsaturated metallacyclopentadiene is permitted when the three spectator ligands are in a fac geometry but is forbidden when they are in a mer geometry, which explains the puzzling difference of reactivity between monodentate triphosphine and tripodal complexes. It is shown that this unsaturated metallacycle is highly reactive toward an incoming ligand since it is not strongly stabilized by conjugation within the π system. This explains why it can be isolated by trapping with a Lewis base. The addition of the third alkyne to the metallacyclopentadiene, leading to the η⁴-benzene complex, can be achieved in a concerted manner and leads directly to the product. The C-C bond lengths within the η⁴-benzene are shown to be due to the presence of a potent metal donor and to the nonplanarity of the benzene ring. The fluxionality of the η⁴-benzene, which makes all carbons of the ring and the three phosphine ligands equivalent on the NMR time scale, is suggested to be due to an easy displacement/ rotation of the IrP₃⁺ fragment around the ring. This displacement avoids η⁶-coordination (20-electron species) but passes through unsaturated η³- and η²-benzene coordination modes. These unsaturated species (notably the η² one) have the proper low-lying LUMO to coordinate an additional alkyne. This leads back to the monoalkyne complex and benzene production. Fluxionality and reactivity of the η⁴-benzene ring are therefore interrelated. The efficiency of the catalysis is suggested to be due to the fact that all intermediates are reactive 16-electron species stabilized by additional donation from the conjugated π system of the organic ligand. The presence of an enforced fac arrangement of the three spectator ligands avoids the thermodynamic trap of the trigonal bipyramidal bis(alkyne) complex.