MECHANISM OF ACETYLENE CYCLOTRIMERIZATION CATALYZED BY THE FAC-IRP3+ FRAGMENT - RELATIONSHIP BETWEEN FLUXIONALITY AND CATALYSIS

Reaction of [(triphos)Ir(C2H4)2](BPh4) with C2H2 at 25-degrees-C gives [(triphos)Ir(eta4-C6H6)](BPh4), 1, which was shown to have this Ir/benzene connectivity by single crystal X-ray diffraction. Crystal data (-155-degrees-C): a = 16.471(6) angstrom, b = 17.126(6) angstrom, c = 12.030(4) angstrom, alpha = 101.22(2)-degrees, beta = 93.61(2)-degrees, and gamma = 75.46(1)-degrees with Z = 2 in space group Pi. This species reacts with C2H2 in the presence of Cl- to give (triphos)IrCl(eta2-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-degrees-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 DELTAH = 10.7(3) kcal/mol and DELTAS = -9.5(6) kcal-1 mol-1. The benzene ring H-1 NMR spectra are also temperature-dependent, and the fluxionality can be accounted for by the same activation parameters appropriate to P-31 site exchange; the same physical mechanism thus accomplishes both site exchanges. The structural study indicates that eta4-C6H6, which is nonplanar, is a stronger pi-acceptor than is butadiene itself. A multistep mechanism has been studied with extended Huckel 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 pi 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 eta4-benzene complex, can be achieved in a concerted manner and leads directly to the product. The C-C bond lengths within the eta4-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 eta4-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 eta6-coordination (20-electron species) but passes through unsaturated eta3- and eta2-benzene coordination modes. These unsaturated species (notably the eta2 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 eta4-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 pi 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.