2010 Organometallics 1994, 13, 2010-2023 Mechanism of Acetylene Cyclotrimerization Catalyzed by the fac-IrP3+Fragment: Relationship between Fluxionality and Catalysis Claudio Bianchini,**t Kenneth G. Caulton,*?$ Catherine Chardon,§ Marie-Liesse Doublet,§ Odile Eisenstein,*J Sarah A. Jackson,$ Todd J. Johnson,$ Andrea Meli,t Maurizio Peruzzini,t William E. Streib,t Albert0 Vacca,? and Francesco Vizzat Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via J. Nardi 39, 50132 Firenze, Italy, and Laboratoire de Chimie Thborique, Bdtiment 490, Universitb de Paris-Sud, 91405 Orsay, France Received October 8, 1993" Reaction of [(triphos)Ir(C2H4)2] (BPh4) with C2H2 at 25 "C gives [(triphOS)Ir(?4-CsH6)] (BPhl), 1, which was shown to have this Ir/benzene connectivity by single crystal X-ray diffraction. Crystal data (-155 'C): a zyxwvutsr = 16.471(6) A, b = 17.126(6) A, c = 12.030(4) A, zyxw a = 101.22(2)', /3 = 93.61(2)', and y = 75.46(1)' with 2 = 2 in space group PI. This species reacts with C2H2 in the presence of C1- to give (triphos)IrC1(v2-C4H4), zyxwv 2, which can be converted back to 1 with C2H2 in the presence of the chloride scavenger T1PF6. Ethyne will displace C6H6from 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 AH* = 10.7(3) kcal/mol and AS* = -9.5(6) kcal-' mol-'. The benzene ring 'H 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 T4-C6H6, which is nonplanar, is a stronger .rr-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 P system. This explains why it can be isolated by trapping with a Lewis base. The addition of the third alkyne to the metallacy- clopentadiene, leading to the benzene complex, can be achieved in a concerted manner and leads directly to the product. The C-C bond lengths within the v4-benzeneare shown to be due to the presence of a potent metal donor and to the nonplanarity of the benzene ring. The fluxionality of the g4-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 $coordination (20- electron species) but passes through unsaturated 73- and g2-benzene coordination modes. These unsaturated species (notably the q2 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 q4-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 P 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(a1kyne) complex. Introduction The transition-metal catalyzed cyclotrimerization of alkynes is noteworthy as a process which forms three C-C bonds, yet it must stop there or run the risk of concurrently producing polyacetylene. It can be catalyzed by a wide variety of metals, oxidation states and d-electron con- figurations. It is undoubtedly the case that no single t Istitutoper lo Studiodella Stereochimicaed Energeticadei Composti zyxwvuts t Indiana University. 8 Universite de Paris-Sud. di Coordinazione. Abstract published in Aduance ACS Abstracts, April 15, 1994. 0276-7333/94/2313-2010~04.50/0 mechanism governs cyclotrimerizationat all of these varied catalytic centers. The evidence for various mechanisms has been thoroughly reviewed by Wigley,l with particular reference to low-valent early (electropositive) transition metal centers. We have reported2 a series of studies where acetylene cyclotrimerization is effected at 25 "C by catalyst precur- sors containing the (triphos)Ir+substructure (triphos = MeC(CH2PPh2)s). At the same time, we have reported on ~~ (1) Smith, zyxwvu D. P.; Strickler, J. R.; Gray, S. D.; Bruck, M. A.; Holmes, (2) Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F. R. S.; Wigley, D. E. Organometallics 1992, 11, 1275. Organometallics 1991, 10, 645. 0 1994 American Chemical Society