Organometallics 1994, 13, 2010-2023
2010
Mechanism of Acetylene Cyclotrimerization Catalyzed by the fac-IrP3+Fragment: Relationship between Fluxionality and Catalysis Claudio Bianchini,**tKenneth 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 a t 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 = 16.471(6) A, b = 17.126(6) A, c = 12.030(4) A, 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), 2, which can be converted back to 1with C2H2 in the presence of the chloride scavenger T1PF6. Ethyne will displace C6H6from 1 a t 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 31Psite 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 m e r 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 metallacyclopentadiene, 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 (20electron species) but passes through unsaturated 73- and g2-benzenecoordination 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 configurations. It is undoubtedly the case that no single t Istitutoper lo Studiodella Stereochimicaed Energeticadei Composti di Coordinazione. t Indiana University. 8 Universite de Paris-Sud. Abstract published in Aduance A C S Abstracts, April 15, 1994.
mechanism governs cyclotrimerization at 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 precursors containing the (triphos)Ir+substructure (triphos = MeC(CH2PPh2)s). At the same time, we have reported on ~~
(1) Smith, D.P.; Strickler, J. R.; Gray, S. D.; Bruck, M. A.; Holmes, R. S.; Wigley, D. E. Organometallics 1992,11, 1275. (2) Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F. Organometallics 1991,10, 645.
0276-7333/94/2313-2010~04.50/0 0 1994 American Chemical Society
Acetylene Cyclotrimerization Catalyzed by fac-IrP3+
the structure, bonding, and reactivity of the Ir(PMeZPh)3+ moiety bound to alkynes, which shows no acetylene cyclotrimerization activity under the same condition^.^ We report here a study of the structures and fluxionality of species derived from (triphos)Mn+ with HC=CH, together with a molecular orbital study of the likely individual steps in a cyclotrimerization mechanism? This work shows the close relationship between those intramolecular migrations which lead to fluxionality and those which accomplish product (arene) release and return to the catalytic cycle by acetylene binding.
Experimental Section General Information. All reactions and manipulations were routinely performed under nitrogen, unless otherwise stated, by using Schlenk techniques. Tetrahydrofuran (THF) was purified by distillation from LiAlH4 under nitrogen. All other solvents and reagents were reagent grade and were used as received. Bis(tripheny1phosphine)iminiumchloride (PPNC1) was purchased from Aldrich. Literature methods were used for the preparation of [(triphos)Ir(CzH4)z]BPh45 and [(triphos)IrCl(C2H4)].s The solid complexeswere collected on sintered-glass frits and washed with appropriate solvents before being dried under a stream of nitrogen. Infrared spectra were recorded on a Perkin-Elmer 1600 Series FTIR spectrophotometer using samples mulled in Nujol between KBr plates. lH and 3lP(lH) NMR were recorded on either a Varian VXR 300 (299.94- and 121.42-MHz) or a Bruker ACP 200 (200.13-and 81.01-MHz)spectrometer. lH NMR shifts were measured relative to the residual lH NMR resonance in the deuterated solvent (CD2C12, 6 5.32; CDCl3, 6 7.13). slP NMR shifts are relative to external 85 % Hap04 with downfield values reported as positive. 13C(lH)NMR spectra were recorded on the Bruker instrument operating a t 50.32 MHz. Chemical shifts are relative to the deuterated solvent resonance (CDzC12, 6 53.1; CDC13,6 77.7). Broad band and selective 3lP(lH) NMR experiments were carried out on the Bruker ACP 200 instrument equipped with a 5-mm inverse probe and a BFX-5 amplifier. 13C-DEPT,2D-HETCOR,and 2D-COSY NMR experiments were run on the Bruker ACP 200 spectrometer. The computer simulation of NMR spectra was carried out with a locallydeveloped package containing the programs LAOCN36 and DAVINS? running on a Compaq Deskpro 386/25 personal computer. The initial choices of shifts and coupling constants were refined by iterative least-squares calculations using the experimental digitized spectrum. The line-shape analysis of the variable-temperature NMR spectra was accomplished by means of the DNMR3 program8 adapted for the Compaq computer. Errors in the calculated rate constants were estimated by varying the rate constant around the best-fit value, until an observable difference between simulated and experimental spectra, both displayed on the graphical terminal, could be detected. These errors proved to be 10% (only a t the coalescence point) or less. Conductivities were measured with an Orion Model 990101 conductance cell connected to a Model 101 conductivity meter. The conductivity data were obtained a t sample concentrations of ca. 103M in nitroethane solutions at room temperature. GC analyses were performed on a Shimadzu GC-8A gas chromato(3)(a) Marinelli, G.; Rachidi, I. E.-I.; Streib, W. E.; Eisenstein, 0.; Caulton, K. G. J. Am. Chem. SOC. 1989,111, 2346. (b) Marinelli, G.; Streib, W. E.; Huffman, J. C.; Caulton, K. G.; Gagn6, M. R.; Takata, J.; Dartiguenave, M.; Chardon, C.; Jackson, S. A.; Eisenstein, 0.Polyhedron 1990,9,1867. (4)Preliminary communication: Bianchini, C.; Caulton, K. G.; Chardon, C.; Eisenstein, 0.;Folting, K.; Johnson, T. J.; Meli, A.; Peruzzini, 1991,113, M.; Rauscher, D. J.; Streib, W. E.; Vizza, F. J.Am. Chem. SOC. 5127. (5)Barbaro, P. L.;Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Vizza, F. Organometallics 1991,IO, 2227. (6)Castellano, S.;Bothner-By, A. A. J. Chem. Phys. 1964,41,3863. (7)Stephenson, D.S.;Binsch, G. J.Magn. Reson. 1980,37,395. (8)Available from the Quantum Chemistry Program Exchange, Indiana University.
Organometallics, Vol. 13, No. 5, 1994 2011 graph fitted with a thermal conductivity detector and a 6-ft. 0.1 % SP-1000 80/100 Carbopack C l/s-in. stainless steel column (Supelco Inc.). Quantification was achieved with a Shimadzu C-R6A Chromatopac coupled with the chromatograph, operating with an automatic correct area normalization method. Synthesisof [(triphos)Ir(rl'-C~Hs)]BP4 (1). A. [(triphoe)Ir(C2H4)2]BPh, (0.41 g, 0.34 mmol) was dissolved in THF (30 mL) under ethyne (1atm) a t room temperature. After 1h, ethyne was replaced with nitrogen and the resulting orange solution was treated with ethanol (40 mL). On partial evaporation of the solvent, yellow crystals of 1 precipitated. They were filtered off and washed with n-pentane; yield 95%. Anal. Calcd for C71H@BIrP3: C, 70.23; H, 5.40; Ir, 15.83. Found: C, 70.11; H, 5.36; Ir, 15.71. AM = 48 cm2 i2-l mol-'. Spectral data have been reported.' B. An equimolar mixture of [(triphos)IrCl(C2H4)] (0.30 g, 0.34 mmol) and TlPFe (0.12 g, 0.34 mmol) in THF (30 mL) was stirred at room temperature under ethyne (1atm) for 2 h. After replacing ethyne with nitrogen, TlCl was removed by filtration. Addition of NaBPh, (0.17 g, 0.5 mmol) and n-heptane (60 mL) to the orange solution led to the precipitation of a yellow solid. Two recrystallizations from THFlethanolln-heptane gave pure samples of 1 in 65% yield. Reaction of [(triphos)Ir(q4-C6H6)]BPh,(1) with Ethyne. A THF (10 mL) solution of 1 (0.06 g, 0.05 mmol) was stirred under a 1-atm pressure of ethyne a t 60 "C for 24 h. At regular intervals (ca. every 30 min), the reaction mixture was sampled and monitored by GC analysis, showing formation of benzene (0.5 mol of benzene/mol of catalyst/h). slP(lH)NMR showed 1 to be the only phosphorus-containing compound present in solution during and at the end of the catalytic reaction. Compound 1 was quantitatively recovered a t the end of the reaction. When the reaction was carried out at room temperature, no benzene production was observed. Reaction of [ (triphos)Ir(q4-C6H6)]BPh4(1) with Ethyne in the Presence of PPNCl. A solution of 1 (0.30 g, 0.25 mmol) and PPNCl (0.14 g, 0.25 mmol) in THF (40 mL) was heated a t reflux temperature under ethyne (1atm). After 2 h, the resulting colorless solution was concentrated under reduced pressure to ca. 10mL and the precipitated PPNBPh, eliminated by filtration. The filtrate, treated with n-heptane (30 mL), led to the precipitation of sandy crystah of [(triphos)IrC1(+C4H4)] (2) in 80% yield. Anal. Calcd for C4sH43C11rP3:C, 59.76; H, 4.79; Ir, 21.25. Found: C, 59.74; H, 4.72; Ir, 21.09. Reaction of [(triphos)IrCl($-C4H,)] (2) with Ethyne in the Presence of T1PF6. A solid sample of 2 (0.30 g, 0.33 mmol) was added to a stirred THF (30-mL) solution of TlPFe (0.12 g, 0.34 mmol) under a steady stream of ethyne a t room temperature. Within a few minutes, the solid dissolved to give an orange solution. After 1 h, TlCl was removed by filtration and then NaBPh, (0.17 g, 0.50 mmol) in ethanol (30 mL) was added. On partial evaporation of the solvent under a stream of nitrogen, yellow crystals of 1 precipitated in 80% yield. Molecular Orbital Calculations. The extended Huckel calculations (EHT) were carried out by using the weighted Hij f o r m ~ l a . The ~ atomic parameters for Ir are taken from the literature.3b The geometries for the several model species were taken from the experimental data on related complexes: P31r(T4-C&)+, this work; PJr(CzR2)+, ref 3b, P&(C4R4)+, ref 10; P3Ir(C5H5),ref11(with the following changes: Ir-P = 2.3 A, C-C = 1.4 8, in the benzene complex, P-Ir-P = 90"). For the planar benzene complex, Ir-(C&Is plane) = 1.98,. The phosphines were modeled by PHs. X-ray Structure Determination of [(triphos)Ir(C6H6)]BPh&*THF. A crystal of suitable sizewas mounted in a nitrogen atmosphere glovebag using silicone grease and was transferred to a goniostat where it was cooled to -155 "C for characterization (9) Ammeter, J. H.; Btirgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. SOC. 1978,100,3086. (10)Thorn, D.L.;Hoffman, R. N o w . J. Chim. 1979,3,39. (11) Bleeke, J. R. Acc. Chem. Res. 1991,24,271.
2012 Organometallics, Vol. 13,No. 5, 1994 Table 1. Crystallographic Data for I( triphos)IrC~cHrlBPhd.l/~THF C71H65BIrP3e1/2THF 16.47 l(6) 17.126(6) 12.030(4) 101.22(2) 93.6 1 (2) 75.46( 1) 3221.68
Z
L
fw
1214.25
space group Pi T,OC -155 A, 8, 0.710 69 pcald, g cnr3 1.252 p(Mo Ka),cm-I 21.7 R 0.0845 R W 0.08 19
(Table 1)and data collection (6O < 29 < 45°).12A systematic search of a limited hemisphere of reciprocal space revealed no symmetry among the observed intensities. An initial choice of spacegroupPT was later proven correctby the fact that attempted refinement in space group P1 was unsatisfactory. Following complete intensity data collection and correctionfor absorption, data processing gave a residual of 0.047 for the averaging of 911 unique intensities which had been measured more than once. Four standards measured every 400 data showed no significant trends. The structure was solved using a combination of direct methods (MULTAN78)and Fourier techniques. The Ir and P positions were determined from an initial E map and the remaining non-hydrogen atoms were obtained from subsequent iterations of least-squares refinement and difference Fourier calculation. One group of residual peaks that formed a fivemembered ring was includedas five half-weightcarbonsto model a partial-occupancy THF solvent molecule. The remaining residual peaks were rejected as being chemically unreasonable. The possibility of additional severely disordered solvent cannot be ruled out. Hydrogens were included in fixed calculated positions for all except the solvent atoms. The largest peak in the final difference map was 4.9 e/A3 in a cage formed by Ir(l), P(2), P(15), P(28), and C(41)-C(44). A false peak this large is somewhat unusual; however, a second data set collected on a crystal of different shape from a different sample gave the same result. Results of the structuredeterminationare shown in Tables 2 and 3. Additional details are available as supplementary material.
Bianchini et al. spectrum consists of an A3 spin system (6 -22.75 at 40 "C), which transforms into an AM2 pattern in the slow exchange limit (
