Rhenium Allenylidenes and Their Reactivity toward Phosphines: A

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Organometallics 2010, 29, 5982–5993 DOI: 10.1021/om100759c

Rhenium Allenylidenes and Their Reactivity toward Phosphines: A Theoretical Study Cecilia Coletti,† Luca Gonsalvi,‡ Antonella Guerriero,‡ Lorenza Marvelli,§ Maurizio Peruzzini,*,‡ Gianna Reginato,‡ and Nazzareno Re*,† †

Dipartimento di Scienze del Farmaco, Universit a degli Studi G. d’Annunzio, Via Dei Vestini, 31, I-66100 Chieti, Italy, ‡Istituto di Chimica dei Composti Organometallici, CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy, and §Dipartimento di Chimica, Universit a di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy Received August 3, 2010

Density functional and local MP2 calculations have been performed to study the electronic structure of the rhenium(I) allenylidene [(triphos)(CO)2Re(dCdCdCRR0 )]þ species [triphos = MeC(CH2PPh2)3; R, R0 = aryl group] and its reactivity toward tertiary phosphines. The calculated electronic structure shows a relatively electron-rich nature of the [(triphos)(CO)2Re]þ synthon in agreement with the experimental behavior of the corresponding allenylidene complex [(triphos)(CO)2Re(dCdCdCPh2)]þ (R = R0 = Ph). Both the kinetics and the thermodynamics of the nucleophilic addition of tertiary phosphines PMe3-xPhx (x = 0, 1, 2, and 3) have been considered. The results indicate lower activation energies for the phosphine attack to Cγ, which leads, however, to products higher in energy than those of the attack to CR. The computed behavior agrees with the experimental evidence showing that the products of the attack to Cγ are kinetically favored, while the products of the attack to CR are thermodynamically favored. Finally, we addressed the mechanism of phosphine migration from Cγ to CR, finding a low-energy path corresponding to an incomplete detachment of the phosphine moiety that then shifts from the Cγ to the CR atoms while remaining weakly bound to the allenylidene unit.

1. Introduction Transition metal allenylidenes, [LnMdCdCdCRR0 ]nþ,1 have received much attention in recent years for their applications to organic synthesis,2 homogeneous catalysis,3 and the *Corresponding authors. E-mail: [email protected] (N.R.); mperuzzini@ iccom.cnr.it (M.P.). (1) (a) Werner, H. Chem. Commun. 1997, 903. (b) Bruce, M. I. Chem. Rev. 1998, 98, 2797–2858. (c) Cadierno, V.; Gamasa, M. P.; Gimeno, J. Eur. J. Inorg. Chem. 2001, 571–591. (d) Bruce, M. I. Coord. Chem. Rev. 2004, 248, 1603–1625. (e) Rigaut, S.; Touchard, D.; Dixneuf, P. H. Coord. Chem. Rev. 2004, 248, 1585–1601. (f) Winter, R. F.; Zalis, S. Coord. Chem. Rev. 2004, 248, 1565–1583. (g) Cadierno, V.; Gimeno, J. Chem. Rev. 2009, 109, 3512–3560. (2) (a) Touchard, D.; Dixneuf, P. H. Coord. Chem. Rev. 1998, 178-180, 409–429. (b) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311–323. (c) Baya, M.; Buil, M. L.; Esteruelas, M. A.; Lopez, A. M.; O~ nate, E.; Rodriguez, J. R. Organometallics 2002, 21, 1841–1848. (d) Conejero, S.; Díez, J.; Gamasa, M. P.; Gimeno, J.; García-Granda, S. Angew. Chem., Int. Ed. 2002, 41, 3439–3442. (e) Werner, H.; Wiedemann, R.; Laubender, M.; Windm€uller, B.; Wolf, J. Chem.;Eur. J. 2001, 7, 1959– 1967. (f) Ilg, K.; Werner, H. Organometallics 2001, 20, 3782–3794. (g) Esteruelas, M. A. ; L opez, A. M. In Recent Advances in Hydride Chemistry; Peruzzini, M. Poli, R., Eds.; Elsevier: SA, Amsterdam, NL, 2001; Ch. 7, p 189, and references therein. (h) Buil, M. I.; Esteruelas, M. A.; Lopez, A. M.; O~ nate, E. Organometallics 2003, 22, 162–171. (i) Metal Vinylidenes and Allenylidenes in Catalysis: From Reactivity to Applications in Synthesis; Bruneau, C., Dixneuf, P. H., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (3) (a) Nishibayashi, Y.; Yoshikawa, M.; Inada, Y.; Hidai, M.; Uemura, S. J. Am. Chem. Soc. 2002, 124, 11846–11847. (b) Saoud, M.; Ma~ nas Carpio, S.; Romerosa, A.; Gonsalvi, L.; Peruzzini, M. Eur. J. Inorg. Chem. 2003, 1614–1619. (c) Abdallaoui, I. A.; Semeril, D.; Dixneuf, P. H. J. Mol. Catal. A: Chem. 2002, 182/183, 577–583. (d) Akiyama, R.; Kobayashi, S. Angew. Chem., Int. Ed. 2002, 41, 2602–2604. (e) Bruneau, C.; Dixneuf, P. H. Angew. Chem., Int. Ed. 2006, 45, 2176–2203. pubs.acs.org/Organometallics

Published on Web 10/06/2010

design of new materials.4 Indeed, the MdCdCdC moiety, with its unsaturated carbon chain and its alternating array of electrophilic/nucleophilic carbon sites, makes allenylidene complexes unique organometallic reagents for use in both fundamental and applied chemistry, especially in processes whereby the formation of a C-C or C-heteroatom bond is sought.2,3 The large majority of known allenylidene complexes contain d6 transition metal ions, particularly ruthenium(II) and, to a lesser extent, iron(II) and osmium(II).1 Experimental evidence1 and theoretical calculations5 have shown that for these complexes the σ component of the metal-allenylidene bond is stronger than the π one and that the R- and γ-carbon atoms of the allenylidene chain are electrophilic centers, while the β-carbon atom is nucleophilic. Moreover, the reactivity of the MC3 moiety may be further modulated by additional tunable factors, including (4) (a) Dembiski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J. Am. Chem. Soc. 2000, 122, 810–822. (b) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178/180, 431–509. (c) Bunz, U. H. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 969–971. (d) Roth, G.; Fischer, H.; Meyer-Friedrichsen, T.; Heck, J.; Houbrechts, S.; Persoons, A. Organometallics 1998, 17, 1511–1516. (5) (a) Re, N.; Sgamellotti, A.; Floriani, C. Organometallics 2000, 19, 1115–1122. (b) Marrone, A.; Re, N. Organometallics 2002, 21, 3562–3571. (c) Marrone, A.; Coletti, C.; Re, N. Organometallics 2004, 23, 4952–4963. (d) Kostic, N. M.; Fenske, R. F. Organometallics 1982, 1, 974–982. (e) Esteruelas, M. A.; Gomes, A. V.; Lopez, A. M.; Modrego, J.; O~nate, E. Organometallics 1997, 16, 5826–5835. (f) Cadierno, V.; Gamasa, M. P.; Gimeno, J.; Gonzales-Cueva, M.; Lastra, E.; Borge, J.; Garca-Granda, S.; Perez-Carre~no, E. Organometallics 1996, 15, 2137–2147. (g) Auger, N.; Touchard, D.; Rigaut, S.; Halet, J.-F.; Saillard, J.-Y. Organometallics 2003, 22, 1638–1644. (h) Grime, R. W.; Helliwell, M.; Hussain, Z. I.; Lancashire, H. N.; Mason, C. R.; McDouall, J. J. W.; Mydlowski, C. M.; Whiteley, M. W. Organometallics 2008, 27, 857–871. r 2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 22, 2010

Scheme 1. Reactivity of the [Cp(CO)(PPr 3)Ru(CdCdCPh2)]BF4 Allenylidene with PMe(3-x)Phx Phosphines i

Scheme 2. Reactivity of [(Ind)Ru(L)2{CdCdC(Ph)R}]PF6 Allenylidenes with PMe(3-x)Phx Phosphines

the electronic nature of the metallic center and the ancillary ligands, the overall charge of the compound, and, not last for importance, the steric and electronic properties of the Cγ substituents.1 The general behavior of these ruthenium cationic allenylidene complexes with neutral nucleophiles can be illustrated by their reactivity with phosphines.1,6,7 Complete regioselective attack at CR has been reported by Esteruelas and co-workers for the reactions of the cyclopentadienyl derivatives [CpRu(CO)(PPri3)(CdCdCPh2)]BF4 with PHPh2, PMePh2, and PPh3, yielding R-phosphonioallenyl complexes [CpRu(CO)(PPri3){η1-C-C(PRPh2)dCdCCPh2}]BF4 (R = H, Me, Ph) (Scheme 1).6 These results led to the conclusion that the addition of bulky phosphines at the CR atom of these complexes is kinetically and thermodynamically favored over the alternative pathway involving the direct attack by the nucleophile at the γ-carbon atom. Regioselectivity has also been observed by Gimeno and co-workers for the nucleophilic addition of PMe3-xPhx phosphines (x = 0-3) to the Cγ carbon atom of the indenyl complexes [(Ind)Ru(LL0 ){CdCdC(Ph)R}]PF6 (Ind=C9H7-; L, L0 =PPh3, CO or LL0 =dppm, dppe; R=H, Ph), yielding cationic γ-phosphonioalkynyl derivatives [(Ind)Ru(LL0 ){Ct CC(Ph)R(PMe3-xPhx)}]PF6 (Scheme 2).7 For the more π-accepting, i.e., L, L0 =CO/PR3, or less sterically demanding, i.e., LL0 = dppm, combinations of ancillary ligands, the γ-phosphonioalkynyl derivatives of PMe3 rearrange to the R-phosphonioallenyl isomers. Noticeably, this latter isomer is regioselectively obtained from the attack of PMe2Ph to the corresponding allenylidenes.7 These results suggest that the addition of phosphines to the Cγ atom is kinetically and thermodynamically favored for these indenyl complexes, but (6) Esteruelas, M. A.; G omez, A. V.; L opez, A. M.; Modriego, J.; O~ nate, E. Organometallics 1998, 17, 5434–5436. (7) Cadierno, V.; Gamasa, M. P.; Gimeno, J.; L opez-Gonzalez, M. C.; Borge, J.; Garcia-Granda, S. Organometallics 1997, 16, 4453– 4463.

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Chart 1. Allenylidene Complex of the [(triphos)(CO)2Re]þ Synthon

the addition to the CR atom may be thermodynamically, and also kinetically, favored when the metal center is less sterically congested or more electrophilic. Steric congestion at Cγ, due to the presence of two bulky phenyl substituents, has been considered responsible for the lack of reactivity of the ruthenium diphenylallenylidene derivative with either PPh3 or PMePh2. This behavior has allowed the classification of the cationic allenylidene complexes of the iron triad into two groups,8 depending on the regioselectivity of the addition of nucleophiles to the C3 chain of cationic allenylidenes and on the electronic nature of the metallic fragment: R-electrophile complexes that are based on an electron-poor metal fragment and undergo nucleophilic addition at the CR atom, such as [CpRu(CO)2(Cd CdCPh2)]BF4 or [CpRu(CO)(PR3)(CdCdCPh2)]BF4,6 and γ-electrophile complexes that are based on an electron-rich metal fragment and undergo nucleophilic addition on the Cγ atom, exemplified by [(Ind)Ru(PR3)2(CdCdCPh2)]PF6 or [Cp*Ru(PR3)2(CdCdCPh2)]BF4.7-9 In recent studies we have shown that the π-donor rhenium(I) synthon [Re(triphos)(CO)2]þ [triphos = MeC(CH2PPh2)3] is able to stabilize a variety of allenylidene complexes of the formula [(triphos)(CO)2Re(CdCdCRR0 )]þ (see Chart 1), whose diverse chemistry rivals that of the analogous and isoelectronic d6-ruthenium counterparts.10,11 In spite of the presence of two π-acceptor carbonyl ligands, this rhenium(I) allenylidene behaves as relatively electron rich, as indicated by its lack of reactivity toward weak nucleophiles such as water or alcohols, and can be considered in between the two R- and γ-electrophile classes.10 In an attempt to rationalize the regioselectivity of the addition of tertiary phosphines to metal allenylidenes, we investigated in detail the reaction of [(triphos)(CO)2Re(Cd CdCPh2)]OTf (1) with tertiary phosphines PMe3-xPhx (x = 0, PMe3; x=1, PMe2Ph; x=2, PMePh2; x=3, PPh3), differing from each other in either the nucleophilicity or the steric hindrance.11e It has been observed that, while the bulkier PPh3 does not react, the other phosphines primarily attack (8) Baya, M.; Crochet, P.; Esteruelas, M. A.; Gutierrez-Puebla, E.; L opez, A. M.; Modrego, J.; O~ nate, E.; Vela, N. Organometallics 2000, 19, 2585–2596. (9) Bustelo, E.; Jimenez-Tenorio, M.; Mereiter, K.; Puerta, M. C.; Valerga, P. Organometallics 2002, 21, 1903–1911. (10) Mantovani, N.; Marvelli, L.; Rossi, R.; Bianchini, C.; de los Rios, I.; Romerosa, A.; Peruzzini, M. J. Chem. Soc., Dalton Trans. 2001, 2353– 2361. (11) (a) Bianchini, C.; Mantovani, N.; Marchi, A.; Marvelli, L.; Masi, D.; Peruzzini, M.; Rossi, A.; Romerosa, A. Organometallics 1999, 18, 4501–4508. (b) Bianchini, C.; Mantovani, N.; Marvelli, L.; Peruzzini, M.; Rossi, A.; Romerosa J. Organomet. Chem. 2001, 617/618, 233–241. (c) Mantovani, N.; Marvelli, L.; Rossi, R.; Bertolasi, V.; Bianchini, C.; de los Ríos, I.; Peruzzini, M. Organometallics 2002, 21, 2382–2394. (d) Bertolasi, V.; Mantovani, N; Marvelli, L.; Rossi, R.; Akbayeva, D. N.; Bianchini, C.; de los Rios, I.; Peruzzini, M. Inorg. Chim. Acta 2003, 344, 207–213. (e) Peruzzini, M.; Barbaro, P.; Bertolasi, V.; Bianchini, C.; de los Rios, I.; Mantovani, N.; Marvelli, L.; Rossi, R. J. Chem. Soc., Dalton Trans. 2003, 4121–4131. (f) Mantovani, N.; Brugnati, M.; Gonsalvi, L.; Grigiotti, E.; Laschi, F.; Marvelli, M.; Peruzzini, M.; Reginato, G.; Rossi, R.; Zanello, P. Organometallics 2005, 24, 405–418. (g) Mantovani, N.; Bergamini, P.; Marchi, A.; Marvelli, L.; Rossi, R.; Bertolasi, V.; Ferretti, V.; de los Rios, I.; Peruzzini, M. Organometallics 2006, 25, 416–426.

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the Cγ-allenylidene carbon of 1 to give γ-phosphonioalkynyl species [(triphos)(CO)2Re{CtCCPh2(PMe3-xPhx)}]OTf, which thermally and irreversibly isomerize to the thermodynamically stable R-phosphonioallenyl species [(triphos)(CO)2Re{C(PMe3-xPhx)dCdCPh2}]OTf. Therefore, for these rhenium(I) allenylidenes it is experimentally verified that the phosphine nucleophiles first attack the allenylidene-Cγ carbon atom to give kinetic γ-phosphonioalkynyl products, which thermally transform into thermodynamic R-phosphonioallenyl derivatives. The kinetics of the γ-phosphonioalkynyl f Rphosphonioallenyl isomerization is determined by both the steric hindrance and the nucleophilicity of the attacking PMe3-xPhx phosphine: for the bulkier and less nucleophilic PMePh2 the isomerization is fast even at -20 °C, while for the less bulky and more nucleophilic PMe3 the isomerization is very slow at room temperature and completes only upon prolonged reflux in CH2Cl2. Although several calculations have been performed on specific ruthenium and osmium allenylidene complexes,5 to the best of our knowledge no theoretical study has been carried out on the analogous rhenium complexes, nor has the intriguing mechanism of the isomerization transforming the γ-phosphonioalkynyl species into the R-phosphonioallenyl derivative been theoretically analyzed in spite of its general relevance for the chemistry of metal allenylidenes. In this work, we have carried out density functional calculations aimed at studying the electronic structure of the rhenium allenylidene complexes [(triphos)(CO)2Re(CdCdCRR0 )]þ and understanding their reactivity toward nucleophiles, notably tertiary phosphines PMe3-xPhx (x = 0-3). In keeping with the experimental results, we found that, irrespective of the tertiary phosphine investigated, the products of the phosphine attack to CR are thermodynamically favored over those of the attack to Cγ, while the latter attack always shows lower activation energies, in agreement with the experimental evidence. Moreover, we theoretically investigated in detail the possible tautomerization mechanisms to account for phosphine migration from Cγ to CR, showing that the lowest energy path corresponds to an incomplete detachment of the phosphine moiety that then shifts from the Cγ to the CR atoms while remaining weakly bound to the allenylidene unit.

2. Computational and Methodological Details All calculations were carried out using density functional theory12 and LMP2 as implemented in the Jaguar 7.5 suite13 of ab initio quantum chemistry programs and the Amsterdam Density Functional 2007.03 package (ADF).14 Geometries were optimized with Jaguar, using different exchange correlation functionals, i.e., BP86,15 BLYP,16 PBE,17 and B3LYP.18 The 6-31G** basis set was employed for the main group atoms,19 (12) (a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (b) Ziegler, T. Chem. Rev. 1991, 91, 651–667. (13) Jaguar 7.5; Schr€odinger, L.L.C.: New York, 2007. (14) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931–967. (15) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (16) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (17) Perdew, J. P.; Burke, K.; Enzerhof, M. Phys. Rev. Lett. 1996, 67, 3865–3868. (18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (19) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2256–2261.

Coletti et al. and the Los Alamos LACVP** basis,20 including relativistic effective core potentials, for rhenium (BS1). The energies were reevaluated by additional single-point calculations at each optimized geometry using the triple-zeta basis set 6-311þþG**,21 while rhenium was described by a modified version of LACVP**, designated as LACV3Pþþ**, where the exponents were decontracted and a diffuse function added (BS2). Since recent studies have shown the low accuracy of DFT methods for highly conjugated π systems and, in particular, their failure in predicting the energy difference of cumulene and polyyne isomers, which are involved in the considered systems,22 we decided to employ also a more reliable wave function method such as MP2. The LMP223 rather than the canonical MP2 calculations were actually chosen because of the lower computational cost and reduced BSSE since previous calculations showed that LMP2 with triple-zeta plus polarization basis sets accurately reproduces the MP2 results for several kinds of systems.24 Even using the computationally cheaper LMP2 approach, the size of the allenylidene complex and attacking phosphine molecules and the large basis sets required to achieve a sufficient accuracy (with up to 104 atoms and 1520 basis functions) prevented a full systematic optimization of all considered minima and transitions states, so that we performed single-point LMP2/BS2 calculations on the PBE/BS1 geometries. Test calculations on the smallest model (see below) have shown that a further optimization at the LMP2/BS1 level does not significantly affect either the geometries of the allenylidene complex 1 and the products of the attack of the PMe3 nucleophile to CR and Cγ atoms or the corresponding reaction energies (at the LMP2/BS2 level). Vibrational frequency calculations based on analytical second derivatives at the PBE/BS1 level of theory were performed to confirm proper convergence to local minima and maxima for equilibrium and transition-state geometries, respectively, and to derive the zero-point-energy (ZPE) and vibrational entropy corrections at room temperature, allowing to calculate reaction and activation enthalpies and free energies. Solvation energies were evaluated by a self-consistent reaction field (SCRF) approach,25 based on accurate numerical solutions of the Poisson-Boltzmann equation,26 using the basis BS2 and employing a dielectric constant (ε) of 8.93 for CH2Cl2. We make use of the restricted spin formalism throughout the whole study, since all molecules and fragments discussed in this work are closed-shell species. Bond dissociation energies between the nucleophiles and the allenylidene complexes have been calculated as energy difference between the energies of the addition products and those of the optimized reagents. The ADF program was employed to analyze the Re(I)allenylidene bond dissociation energies and separate the contributions from σ donation and π back-donation, according to the extended transition state (ETS) decomposition scheme derived and implemented by Ziegler and Rauk.27 As to the (20) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (21) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650–654. (22) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2006, 110, 10478– 10486. (23) Saebo, S.; Tong, W.; Pulay, P. J. Chem. Phys. 1993, 98, 2170– 2175. (24) Kaminski, G. A.; Maple, J. R.; Murphy, R. B.; Braden, D. A.; Friesner, R. A. J. Chem. Theory Comput. 2005, 1, 248–254. (25) (a) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027–2094. (b) Cramer, C. J.; Truhlar, D. G. Chem. Rev. 1999, 99, 2161–2200. (26) (a) Tannor, D. J.; Marten, B.; Murphy, R. B.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Ringnalda, M. N.; Goddard, W. A., III.; Honig, B. J. Am. Chem. Soc. 1994, 116, 11875–11882. (b) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. (27) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1–10. (b) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755–1759. (c) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558–1565.

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calculations with ADF, we carried out geometry optimizations with the PBE functional and using a polarized double-zeta STO basis set on main group atoms and a triple-zeta for rhenium and ruthenium (BS3). Energies were then evaluated on the optimized geometries using the same functional and a polarized triple-zeta STO basis set on all atoms (BS4). Relativistic effects on Re are included using the scalar “zeroth-order regular approximation” (ZORA)28 as implemented in ADF. According to the Ziegler and Rauk decomposition scheme, the bond dissociation energy has been decomposed into a number of contributions:

ΔEðRe- allenylideneÞ ¼ - ½ΔEstr þ ΔE elst þ ΔE Pauli þ ΔE orb The first term, ΔEstr, is the strain energy necessary to convert the fragments from their equilibrium geometries to the conformation they assume in the optimized structure of the overall complex and corresponds to the sum of the fragment strain energies, Estr[(triphos)(CO)2Re] þ Estr[CdCdCR2]. ΔEelst represents the electrostatic interaction of the nuclear charges and the unmodified electronic charge density of one fragment with those of the other fragment, while EPauli is the four-electron destabilizing interaction between occupied orbitals (Pauli repulsion), and together they correspond to the steric repulsion between the two fragments. ΔEorb, known as the orbital interaction term, represents the attractive orbital interaction that gives rise to the energy lowering upon coordination. This term may be broken up into contributions from the orbital interactions within the various irreducible representations Γ of the overall symmetry group of the system, according to the decomposition scheme proposed by Ziegler.27b This decomposition scheme is particularly useful in the considered complexes with Cs pseudosymmetry, as it allows one to separate the energy contributions corresponding to σ donation (EA0 ) and to π back-donation (EA0 0 ). Indeed, the ligand-to-metal σ donation takes place into the A0 representation, while the metal-toligand π back-donation takes place into the A00 representation. To perform such an analysis, we first reoptimized all complexes in Cs symmetry: the main geometrical parameters and bond dissociation energies show small deviations from the corresponding values obtained without any symmetry constraint. Models Employed. Due to the large size of the Re(I) allenylidene complex 1, comprising eight phenyl groups, six from the ancillary tridentate tripodal ligand and two sitting on the Cγ carbon of the allenylidene moiety, and in an attempt to reduce the large computational load, we tested three different models of increasing size, which progressively approach the real complex cation: a model where all the eight phenyl groups were replaced by hydrogen atoms (model 1), a model explicitly including only the two phenyl groups on the terminal carbon atom of the allenylidene chain (model 2), and finally a model where we included four phenyl rings, namely, the two on the allenylidene unit and the two on the triphos ligand pointing toward the allenylidene moiety and thus responsible for possible steric clashes with the substituents on the phosphine nucleophiles attacking the allenylidene unit (model 3), respectively (a), (b), and (c) in Figure 1. No attempt was made to include the large tetraphenylborate counteranion as (i) the X-ray data show that it is placed far from the allenylidene chain interacting only (28) (a) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597–4610. (b) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281–293.

Figure 1. Optimized geometries, at the PBE/BS1 level, of the three considered models for the rhenium(I) [(triphos)(CO)2Re(CdCdCPh2)]þ allenylidene complex cation including (a) none, (b) two, and (c) four out of the eight phenyl groups (see text). weakly with the terminal phenyl groups and (ii) the reactivity properties of these rhenium allenylidene complexes are not affected by the nature of the counteranions.11c The comparison of the geometrical parameters calculated with the X-ray data shows a good agreement with the experimental data only if the largest model (model 3) is employed, with little differences among the considered BP86, BLYP, PBE, and B3LYP exchange-correlation functionals; see Table S1. The three models were also benchmarked for the nucleophilic attack of trimethylphosphine, PMe3, to 1, performing geometry optimizations with all considered functionals on both possible products, i.e., the R-phosphonioallenyl and the γ-phosphonioalkynyl complexes, and calculating the reaction energies

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Coletti et al. Table 1. Calculated Bond Dissociation Energies (kcal mol-1) for the Allenylidene Complexes 1, [Cp(CO)2Ru(CdCdCPh2)]þ, [Cp*(PMe3)2Ru(CdCdCPh2)]þ, and [Cp(PMe3)(CO)Ru(CdCdCPh2)]þ N þa

[(triphos)(CO)2Re(CdCdCPh2)] [(triphos)(CO)2Re(CdCdCPh2)]þb [(triphos)(CO)2Re(CdCdCPh2)]þc [Cp(CO)2Ru(CdCdCPh2)]þc [Cp(PMe3)(CO)Ru(CdCdCPh2)]þc [Cp*(PMe3)2Ru(CdCdCPh2)]þc

E*

ERMLn

ERC3Ph2

De

90.8 92.3 92.5 105.1 101.5 94.5

1.0 3.7 4.7 3.0 0.1 9.8

0.2 0.2 0.7 4.4 3.5 4.5

89.6 88.0 87.6 97.7 98.0 80.6

a At the LMP2/BS2//PBE/BS1 level. b At the PBE/BS2//PBE/BS1 level. c At the PBE/BS4//PBE/BS3 level, with ADF.

Figure 2. Comparison of the HOMO energy of the [(triphos)(CO)2Re]þ synthon with those of [Cp*(PMe3)2Ru]þ, [Cp(PMe3)(CO)Ru]þ, and [Cp(CO)2Ru]þ fragments. for the phosphine addition to the CR and Cγ atoms. In view of the drawbacks of DFT methods with conjugated systems,22,29,30 we calculated the reaction energies for the addition of a phosphine nucleophile to the CR and Cγ atoms of 1 also with a more reliable correlated wave function approach, performing LMP2/ BS2 single-point calculations on the PBE/BS1 geometries. On the basis of the above benchmarking (see Supporting Information for details), we concluded that only LMP2 calculations used in combination with the largest model, 3, give a reasonable agreement with the available experimental data for the addition of the considered nucleophiles to 1, and the LMP2/ BS2//PBE/BS1 level of theory will be therefore employed in all following calculations to evaluate the electronic energies, unless differently stated.

3. Results and Discussion 3.1. Electronic Structure of Rhenium(I) Allenylidenes. In an attempt to characterize the electronic nature of the rhenium(I) [(triphos)(CO)2Re]þ fragment and compare it with that of the most common metal fragments involved in allenylidene chemistry, we have calculated the energies and the nature of its frontier orbitals along with those of [Cp*(PMe3)2Ru]þ, [Cp(PMe3)(CO)Ru]þ, and [Cp(CO)2Ru]þ, which are quite common moieties in the chemistry of cationic Ru(II) allenylidenes and representative of typical electron-rich (the former) and electron-poor (the latter two) metal synthons.1,5-8 The four fragments share the same frontier orbital pattern, with a LUMO of dz2 character involved in the σ bonding to the allenylidene moiety and a HOMO of dxz character involved in the π backdonation to allenylidene1,5 and whose energy determines the extent of their electron richness. The axes are oriented so that the xz plane bisects the OC-Re-CO angle with the allenylidene unit pointing along the z axis. Figure 2 shows that the HOMO orbital of the [(triphos)(CO)2Re]þ fragment is in between that of [Cp*(PMe3)2Ru]þ (at high energy) and those of [(Cp(PMe3)(CO)Ru]þ and [Cp(CO)2Ru]þ (both at low energy), suggesting that, in spite of the presence of two π-acceptor carbonyl ligands, this rhenium(I) synthon is relatively electron rich, thus forming an allenylidene complex with intermediate character between Esteruelas’ R- and γ-electrophile classes8 and relatively stable (29) Quintal, M. M.; Karton, A.; Iron, M. A.; Daniel Boese, A.; Martin, J. M. L. J. Phys. Chem. A 2006, 110, 709–716. (30) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101– 194118.

Table 2. Bond Dissociation Energy Analysis for the Allenylidene Complexes 1, [Cp(CO)2Ru(CdCdCPh2)]þ, and [Cp*(PMe3)2Ru(CdCdCPh2)]þa allenylidene [(triphos)(CO)2Re(CdCdCPh2)] [Cp(CO)2Ru(CdCdCPh2)]þ [Cp*(PMe3)2Ru(CdCdCPh2)]þ

E* þ

Ester

Eorb

EA0

EA0 0

-88.3 20.8 -109.1 -79.5 -29.6 -103.7 6.0 -109.7 -84.8 -24.9 -91.9 35.4 -127.3 -84.5 -42.8

a

E* is the snapping energy, and Ester is the sum of the electrostatic interaction (ΔEelst) and Pauli repulsion (ΔEPauli); see text. EA0 and EA0 0 are energy contributions corresponding to σ donation and π backdonation, respectively.

toward nucleophilic attack, in agreement with the experimental findings. We have then calculated the bond dissociation energy between the [(triphos)(CO)2Re]þ fragment and the CdCdCPh2 ligand in 1 according to the fragment-oriented approach of the ADF program, in which we first evaluate the “snapping energy” between the two fragments, E*(Re-C3Ph2), and then the relaxation energies gained by the fragments when they are allowed to relax from the geometry they assume in the complex to their equilibrium geometry. For the purpose of consistency with the following bond analysis (vide infra), the calculations have been performed with the ADF program. However, the results are almost identical with those calculated with the Jaguar package employing the same exchange-correlation potential (PBE) and the larger basis set and very close to those calculated at the LMP2 level of theory; see Table 1. For more detailed insight into the nature of the Reallenylidene bond, we also performed a bond analysis employing the energy decomposition scheme developed by Ziegler and Rauk in Cs symmetry,27 which allows separating the orbital interaction contributions to the bond energy within the irreducible representations A0 and A00 , corresponding to σ and π interactions; see Computational Details. The results are presented in Tables 1 and 2 together with the analogous bond dissociation energies and bonding contributions calculated at the same level of theory for the [Cp*(PMe3)2Ru(CdCdCPh2)]þ, [Cp(PMe3)(CO)Ru(CdCdCPh2)]þ, and [Cp(CO)2Ru(CdCdCPh2)]þ cationic Ru(II) allenylidenes as representative of typical Esteruelas’ R- and γ-electrophile classes, respectively, to allow a more thorough comparison between [(triphos)(CO)2Re(CdCdCPh2)]þ and these well-characterized fragments. The bond dissociation energy of the allenylidene unit in 1 is 87.6 kcal mol-1 (88.0 kcal mol-1 at the PBE/BS2//PBE/BS1 level and 89.6 kcal mol-1 at the LMP2/BS2//PBE/BS1 level with Jaguar), a value in between those calculated for [Cp(CO)2Ru(CdCd CPh2)]þ, 97.7 kcal mol-1, [Cp(PMe3)(CO)Ru(CdCdCPh2)]þ, 98.0 kcal mol-1, and [Cp*(PMe3)2Ru(CdCdCPh2)]þ, 80.6 kcal

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Table 3. Mulliken Charges and Breakdown of the Contribution from Rhenium and Allenylidene Carbon Atoms to the HOMO and LUMO of 1 in Cs Symmetry complex [(triphos)(CO)Re- Mulliken charges (CdCdCPh2)]þ ε orbital -7.49 HOMO -6.40 LUMO

M

CR

0.35 -0.39 33.3 8.4

8.0 21.3

Cβ

Cγ

0.08 -0.17 21.7 26.8

mol-1, showing that the considered rhenium allenylidene has a similar thermodynamical stability to that of the most common cationic Ru(II) allenylidenes. Small strain energies are observed in all cases, showing that the bond dissociation energies are close to the snapping energies. The bond analysis indicates small steric and preparation energies so that the bonding dissociation energy is mainly due to the orbital interaction. The analysis of the contribution from the A0 and A00 irreducible representations to the orbital interaction energy shows that the energy contribution from σ donation is more important than that from π back-donation, respectively 79.5 vs 29.6 kcal mol-1. Of particular interest is the analysis of the [Cp(CO)2Ru(CdCdCPh2)]þ and [Cp*(PMe3)2Ru(CdCd CPh2)]þ allenylidenes, which show similar contributions from σ donation, respectively 84.8 and 84.5 kcal mol-1, while the corresponding contributions from π back-donation, 24.8 and 42.8 kcal mol-1, are much higher for the latter electron-rich fragment than for the former electron-poor synthon. This is consistent with our previous calculations on a series of metallacumulenes LmM(dC)nR2 with metal fragments LmM of different electron richness, showing that the π back-donation contribution increases with the electron richness of the metal fragment due to the energy increase of the metal donor dπ orbital, which approaches in energy the accepting allenylidene π* MO.5c The comparison of the results of the bonding analysis for 1 with that for these two reference Ru(II) allenylidenes indicates that 1 has a similar or slightly lower contribution from σ donation and an intermediate contribution from π back-donation, thus confirming our findings from the analysis of the frontier orbital energies of the corresponding metal fragments (vide supra) showing that 1 has an intermediate behavior between Esteruelas’ R- and γ-electrophile classes. Finally, to provide a rationale of the reactivity of the [(triphos)(CO)2Re(CdCdCPh2)]þ allenylidene cation and in particular to better explain the regioselectivity of the addition of nucleophiles,10,11 we have collected in Table 3 the Mulliken gross atomic charges on the metal and the carbon atoms of the allenylidene unit and the breakdowns of the contribution of the same atoms to the HOMO and LUMO. This analysis confirms essentially the results obtained in previous theoretical investigations on allenylidene and cumulenylidene d6 metal complexes of groups VI-VIII, mainly Ru(II), including [Cp*(PMe3)2Ru(CdCd CPh2)]þ and [Cp(PMe3)(CO)Ru(CdCdCPh2)]þ,5 thus stressing once more the electronic similarities existing between ruthenium(II) and rhenium(I) cumulenylidenes. In particular, the results of a Mulliken population analysis show no significant charge differences among the three carbon atoms of the allenylidene chain, indicating that charge distribution is not important in determining the regioselectivity of either electrophilic or nucleophilic attack. This is consistent with the molecular orbital energies (see the orbital interaction diagram in Figure 3 showing relatively high-lying HOMOs and low-lying LUMOs) and suggesting that the reactivity of these complexes toward both electrophilic and nucleophilic attack is mostly determined by frontier orbital factors. Moreover, since 1 has a

Figure 3. Orbital interaction diagram for the [(triphos)(CO)2Re]þ and [CdCdCPh2] fragments in 1.

high-lying HOMO and a low-lying LUMO quite isolated in energy from the other frontier MOs, these two orbitals play the main role in controlling the regioselectivity of, respectively, the electrophilic and nucleophilic attack. The breakdowns of the contribution from the metal and the allenylidene carbon atoms to the HOMO and LUMO (see Table 3) show that the HOMO has contributions mainly from the metal and the carbon atom in even position along the chain (Cβ), while the contributions to the LUMO come mainly from the carbon atoms in odd positions (CR and Cγ), determining, respectively, their electrophilic or nucleophilic character. This picture perfectly agrees with the experimentally observed reactivity of 1. The comparison with the analogous Mulliken population analysis and breakdown of the HOMO and LUMO contributions from the metal and the allenylidene carbon atoms for the two reference Ru(II) allenylidenes shows essentially the same qualitative results, in agreement with our previous theoretical studies,5a-c where we found that the localization of HOMO and LUMO on the even/odd-numbered carbons of allenylidene and cumulenylidene complexes is essentially unaffected by variations in the electron richness; that is, no changes on the regioselectivity are foreseen, although the energy of the LUMOs of the complexes rises with the electron richness, thus reducing the reactivity toward nucleophilic attack. Our calculations also show that the LUMO of 1 is close in energy to that of the electron-rich complexes, suggesting for this species a reduced reactivity toward nucleophilic attack in agreement with the experimentally observed reactivity trends.10,11 In conclusion, the positive combination of the ancillary ligand donor set provided by the two non-sterically demanding π-acceptor terminal CO groups and the sterically demanding σ-donor facial tripodal triphosphine together with the relatively high energy of the rhenium(I) orbitals apparently leads to an optimization of both electronic and

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Coletti et al. Table 4. Bonding Energies, Enthalpies, and Free Energies of the PMe3, PMe2Ph, PMePh2, and PPh3 Nucleophiles to the Cr and Cγ Atoms of 1 Calculated with Model 3 at the LMP2/BS2//PBE/ BS1 Level of Theory (kcal mol-1) adduct

ΔEgas

ΔEsol

ΔHsol

ΔGsol

-35.2 -32.1 þ25.5 -21.6

-33.5 -31.0 þ25.5 -20.3

-19.0 -14.6 þ42.5 -4.7

-28.2 -23.3

-28.4 -23.6

-12.1 -8.0

-26.6 -18.0

-26.7 -16.7

-9.8 -0.5

-20.8 -11.2

-21.0 -11.4

-2.8 þ6.7

PMe3 R-phosphonioallenyl γ-phosphonioalkynyl β-phosphoniovinylidene phosphoniocyclopropenyl

-34.2 -33.3 þ27.7 -21.8 PMe2Ph

R-phosphonioallenyl γ-phosphonioalkynyl

-30.6 -28.0 PMePh2


-31.4 -23.8 PPh3


-27.8 -19.0

Thermodynamics. The calculated reaction energies, enthalpies, and free energies for the phosphine addition to the CR and Cγ atoms of the allenylidenes moieties,

½ðtriphosÞðCOÞ2 ReðCdCdCPh2 Þþ þ PMe3 - x Phx f ½ðtriphosÞðCOÞ2 RefCðPMe3 - x Phx ÞdCdCPh2 gþ ½ðtriphosÞðCOÞ2 ReðCdCdCPh2 Þþ þ PMe3 - x Phx f ½ðtriphosÞðCOÞ2 RefCtC;CPh2 ðPMe3 - x Phx Þgþ

Figure 4. Optimized geometries, at the PBE/BS1 level, of (a) the R-phosphonioallenyl and (b) the γ-phosphonioalkynyl products, for the four-phenyl model (model 3) and PMe3.

steric effects, allowing for the stabilization of the cationic rhenium(I) allenylidene without depressing the C3 ligandcentered reactivity, which can be considered in between the two R- and γ-electrophile classes. 3.2. Reactivity of Rhenium(I) Allenylidenes toward Phosphines. Geometry. Geometry optimizations were carried out, at the PBE/BS1 level and employing the four-phenyl model 3, on the products of the addition of the PMe3, PMe2Ph, PMePh2, and PPh3 nucleophiles to the CR and Cγ atoms of 1. Stable adducts could be obtained for all considered phosphines, and the corresponding geometries, reported in Figure 4 for PMe3, show the expected R-phosphonioallenyl and γ-phosphonioalkynyl structures. Small differences are observed in the bonding parameters of the ReC3P core on increasing the phosphine cone angle from PMe3 to PPh3, while significant variations of the dihedral angles can be discerned. The main geometrical parameters are in line with the known X-ray data available for these kinds of metal complexes.

evaluated both in gas phase and in dichloromethane solution are reported in Table 4 and are equivalent to the corresponding CR-P and Cγ-P bond dissociation energies, enthalpies, and free energies changed in sign. These values show a small but significant decrease due to the solvent effects, an effect growing with the size of the phosphine (from 1 to 7 kcal mol-1); the ZPE and thermal contributions are almost negligible, while the entropy contributions are quite large and unfavorable (by 1317 kcal mol-1) due to the loss of one translational degree of freedom and the overall weakening of the bonds within the metal-allenylidene core upon phosphines addition. Table 4 shows a steady decrease on increasing the phosphine cone angle from PMe3 to PPh3, with a positive value of the free energy for the addition of the bulkier PPh3, which is therefore predicted to be unfavorable, in keeping with the experimental finding.11e More importantly, the results in Table 4 show that the CR adduct is always more stable than the Cγ adduct, by 3 to 10 kcal mol-1 in enthalpy or 4 to 10 kcal mol-1 in free energy, thus indicating that the phosphine attack to CR leads to the thermodynamically most stable product, i.e., the R-phosphonioallenyl derivative. This energy difference, which is slightly enlarged by solvation and entropy contributions, increases with the size of the phosphine, consistently with the experimental evidence. Kinetics. To estimate the energy barriers for the nucleophilic addition of the considered phosphines to the CR and Cγ atoms of 1, we first performed a series of energy scan calculations for the attack of each of the PMe3, PMe2Ph, and PMePh2 phosphines to either CR and Cγ atoms, using as reaction coordinate the C-P

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Figure 5. Energy scan for the attack of PMe3 to the Cγ atom of 1, using as reaction coordinate the P-Cγ distance.

distance. The energy of the allenylidene-phosphine adduct has been calculated for several C-P distances, decreasing from 5.0 A˚, a distance corresponding to a negligible interaction between the phosphine and the allenylidene complex, to 1.8 A˚, an average of the C-P bond length estimated for the final products of the phosphine attack, whereby optimizing all the remaining coordinates. The results indicate an initial decrease of the energy upon the phosphine approach, followed by a slight increase up to a maximum around a C-P value of 2.2-2.4 A˚ and then a sudden decrease to the well of the C-P bond. The initial energy decrease is particularly tangible for the attack to the Cγ atom and is lowered by the increase of the steric crowding of the phosphine group, as exemplified by the energy scan for the attack of PMe3 to the Cγ atom of 1, reported in Figure 5, which shows (i) a wide minimum at a C-P value around 3.5 A˚ with an energy of ca. 6 kcal mol-1 and (ii) a low-energy maximum at a C-P value of 2.8 A˚ with an energy of ca. 3 kcal mol-1 below the energy of the allenylidene and the phosphine reagents infinitely apart. On the other hand, barely distinguishable minima have been calculated for the attack to the CR atom of 1, with energies only 1 kcal mol-1 below the allenylidene and phosphine infinitely apart. These energy scans indicate the formation of stable adducts between the phosphine moiety and 1, ascribable to an electrostatic interaction between the negatively polarized phosphorus atom and the electrophilic Cγ center. Such a behavior has been confirmed by geometry optimization calculations, which, as expected since the C-P distance is a good reaction coordinate, lead to stable minima with geometries and energies very close to those obtained by the energy scan. The formation energies, enthalpies, and free energies of these noncovalent allenylidenephosphine adducts are reported in Table 5, which shows deeper minima on increasing the number of phenyl substituents on the phosphine moiety (due to π-π interactions between the phosphine phenyl rings and those on the terminal allenylidene Cγ atom), although this trend is reduced by solvation and entropic contributions. For the same reason, the maximum in the energy scan curve;in principle an upper limit;can be considered a good approximation of the transition state for the phosphine attack, and, indeed,

Table 5. Energies, Enthalpies, and Free Energies for the Formation of the Adducts of PMe3, PMe2Ph, PMePh2, and PPh3 to the Cγ Atom of 1, Calculated at the LMP2/BS2//PBE/BS1 Level of Theory (kcal mol-1) nucleophile

ΔEgas

ΔEsol

ΔHsol

ΔGsol

PMe3 PMe2Ph PMePh2 PPh3

-5.6 -6.7 -7.2 -9.6

-3.4 -3.0 -2.9 -3.9

-3.6 -3.2 -3.0 -3.6

11.5 10.6 13.2 14.7

Table 6. Activation Energies, Enthalpies, and Free Energies for the Attack of PMe3, PMe2Ph, PMePh2, and PPh3 to the Cr and Cγ Atoms of 1 Calculated at the LMP2/BS2//PBE/BS1 Level of Theory (kcal mol-1)a attack site

ΔE‡gas

ΔE‡sol

ΔH‡sol

ΔG‡sol

þ8.0 þ3.2

þ21.2 þ12.8

þ9.3 þ4.4

þ20.4 þ13.8

þ9.9 þ4.8

þ25.8 þ19.6

þ11.8 þ6.9

þ29.4 þ24.0

PMe3 CR Cγ

þ6.2 -3.4(2.2)

CR Cγ

þ3.4 -2.1(3.8)

þ8.4 0.8(þ2.6) PMe2Ph þ7.8 þ2.0(þ4.3) PMePh2

CR Cγ

þ4.7 -1.2(þ4.4)

þ10.0 þ3.8(þ5.3) PPh3

CR Cγ

þ4.0 -1.6(11.2)

þ11.1 þ5.8(þ9.7)

a The values in parentheses refer to the phosphine adducts to the Cγ atom of 1; see the text.

transition-state optimizations starting from these maximum geometries easily led to transition states with nearby structures and energies. The activation energies, enthalpies, and free energies, with respect to the allenylidenes and phosphine infinitely apart, so calculated for the attack of PMe3, PMe2Ph, PMePh2, and PPh3 to both the CR and Cγ atoms are resumed in Table 6, which also reports (in parentheses) the activation energies,

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Scheme 3. Possible Mechanisms of Phosphine Migration from the Cγ to the Cr Atom: (a) Direct 1,3 Migration from Cγ to Cr, and (b) Stepwise 1,2 Migration from Cγ to Cβ and Then from Cβ to Cr

enthalpies, and free energies relative to the noncovalent adducts. The results for PMe3, PMe2Ph, and PMePh2 indicate relatively low activation free energies, 12-19 kcal mol-1, for the attack to Cγ and significantly higher values, 20-25 kcal mol-1, for the attack to CR. The low values calculated for the attack to Cγ are consistent with the experimental evidence showing that the nucleophilic addition to this atom occurs easily even below room temperature.10,11 The significantly higher activation free energies for the attack to CR, by ca. 5-12 kcal mol-1, are also in agreement with the observed reactivity pattern of these phosphines, with 1 leading to a complete regioselectivity with initial formation of the Cγ adduct at low temperature. Higher values are calculated for the attack of the sterically most hindering phosphine, PPh3, 30 and 24 kcal mol-1 for the attack to the CR and Cγ atoms, respectively, in keeping with the experimentally observed unreactivity of this phosphine. The combined results of the calculation of the reaction energies and barriers for the nucleophilic attack of PMe3, PMe2Ph, and PMePh2 clearly show that the attack to Cγ is kinetically favored, while that to CR leads to the thermodynamically most stable product, in perfect agreement with the experimental evidence.11e 3.3. Mechanism of the Cγ to Cr Phosphine Migration: A Rationale for the Phosphonioalkynyl to Phosphonioallenyl Tautomerization. We further took into account the possible mechanisms of phosphine migration from the Cγ to the CR atom of the initially formed γ-phosphonioalkynyl species, leading to the final R-phosphonioallenyl. Two mechanisms may be envisaged to account for this tautomerization reaction: (a) the direct 1,3 phosphine migration from Cγ to CR, and (b) the stepwise double 1,2 phosphine migration from Cγ to Cβ and then from Cβ to CR; see Scheme 3. Due to the high computational load required, calculations have been performed only for the migration of the simplest phosphine, PMe3, assuming that the migration of the aryl-substituted phosphines PMe(2-x)Ph(1þx) (x = 0, 1) follows the same mechanism, differing only in the height of the tautomerization barriers. To evaluate the energy barriers for the direct 1,3 migration, path A, we first performed a bidimensional energy scan calculation for the shift of the PMe3 moiety from the Cγ to the CR atom, using the Cγ-P and CR-P distances as reaction coordinates. The energy of the allenylidene-phosphine adduct has been calculated for a grid of Cγ-P and CR-P distances in the range from 1.5 to 4.0 A˚, allowing the exploration of the PES in a

region containing the minima corresponding to the formation of both the Cγ-P and CR-P bonds and the noncovalent adducts of the phosphine around the Cγ and CR atoms and extending up to distances where the interaction between 1 and the phosphine is negligible, whereby optimizing all the remaining coordinates. Since the computational cost of such calculations is very expensive, the bidimensional scan was carried out on the smaller model 2 (including only two phenyl groups on the terminal carbon atom of the allenylidene chain). The results obtained within this model are intended to be qualitative, but can be readily extended to the larger model 3 case to give an estimate of the feasibility of the considered mechanism (see below). The PES section on this grid is reported in Figure 6 and indicates a possible low-energy path for the phosphine migration, consisting of (i) the partial phosphine detachment from the initial γ-phosphonioalkynyl species (Cγ-P = 1.9 A˚) to give the noncovalent PMe3 adduct at Cγ (Cγ-P = 3.2 A˚) at nearly constant large CR-P distances around 4.0 A˚, (ii) the approach of PMe3 from Cγ to CR along a shallow channel leading from the noncovalent PMe3 adduct at Cγ (Cγ-P=3.2 A˚, CR-P=4.0 A˚) to that at CR (Cγ-P= 4.0 A˚, CR-P=3.1 A˚) with an increase of the Cγ-P and a decrease of the CR-P distance, and (iii) the overreach of a maximum/barrier close to the transition state for the direct attack of PMe3 to the CR atom. This path is explicitly indicated by a dark line in Figure 6a, and its energy profile reported in Figure 6b indicates that steps (i) and (ii) proceed at energies below that of 1 and PMe3 infinitely apart and that the maximum has about the same energy as that found for the direct attack of PMe3 to the CR atom. Our calculations thus indicate that the lowest energy path for the direct 1,3 phosphine migration initially consists of a partial detachment of the phosphine moiety that then shifts from the Cγ to the CR atoms while remaining still weakly bound to the allenylidene unit. These results can be extended to the four-phenyl model 3, allowing establishing as an upper limit for the Cγ to CR isomerization activation enthalpy (free energy) the enthalpy (free energy) difference between the transition state for the attack to the CR atom and the enthalpy (free energy) of the phosphine bound to the Cγ species, which amounts to 39.0 (35.8) kcal mol-1. When considering path B, we first checked if the species with the phosphine bound to Cβ is a real intermediate or a transition state. A preliminary DFT optimization indicates a high-energy metastable species, a minimum on the potential energy surface but thermodynamically unstable with respect to dissociation, 27.7 kcal mol-1 higher than the separate

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Figure 6. (a) Bidimensional energy scan, with a grid of Cγ-P and CR-P distances in the range 1.5-4.0 A˚, showing the direct PMe3 migration from the Cγ to the CR atom. (b) Energy profile for the direct PMe3 migration from the Cγ to the CR atom. Chart 2. Resonance Structures of the β-Phosphoniovinylidene and β-Phosphoniocyclopropenyl Intermediates

allenylidene and phosphine molecules and 61 kcal mol-1 above the Cγ-bound isomer. The instability of this unusual Cβ-bound PMe3 adduct is in line with the difficulty to draw a stable resonance structure, the only possible one being the zwitterionic vinylidene resonance form depicted in Chart 2a. This picture is confirmed by the optimized structure of this species; see Figure 7a, showing Re-CR and CR-Cβ distances of 1.99 and 1.34 A˚, in the range of double rhenium-carbon and

carbon-carbon bonds, respectively, and a Cβ-Cγ distance of 1.51 A˚, typical of a single carbon-carbon bond, which are consistent with a β-phosphoniovinylidene structure. This high-energy structure corresponds to a shallow minimum, which comes out to be unstable toward small geometry perturbations. Indeed, a geometry optimization after small decreases of the CR-Cβ-Cγ angle leads to a new minimum corresponding to an intermediate featuring the phosphoniocyclopropenyl structure; see Chart 2b and Figure 7b. An energy of 21.8 kcal mol-1 below infinitely separated 1 and PMe3 has been calculated for this species, 59.5 kcal mol-1 lower than the β-phosphoniovinylidene intermediate. The formation of this stable species is easily understood if we consider that the allenylidene chain of the β-phosphoniovinylidene intermediate is bent by ca. 120° around Cβ (see Figure 7a), allowing an easy attack of the Cγ anionic center to the CR electrophilic species, with formation of a stable CR-Cγ bond. This is a facile process, as indicated by the energy of the transition state, calculated only 8 kcal mol-1 above the β-phosphoniovinylidene intermediate. These calculations thus indicate that the stepwise double 1,2 phosphine migration via Cβ involves the not yet described β-phosphoniocyclopropenyl rather than the β-phosphoniovinylidene structure as key intermediate. A joint theoreticalexperimental study aimed at possibly intercepting this putative

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Figure 8. Energy profile for the PMe3 stepwise migration from the Cγ to the CR atom via Cβ.

Figure 7. Optimized geometry of (a) the β-phosphoniovinylidene and (b) the phosphoniocyclopropenyl intermediates.

and intriguing intermediate is in progress and will be published in due time. We then calculated the energy barriers for the stepwise 1,2 phosphine migration mechanism, i.e., from Cγ to Cβ, leading to the β-phosphoniocyclopropenyl intermediate, and subsequently from Cβ to CR. A transition-state optimization along the Cγ to Cβ migration path led to a late transition state with a structure quite close to that of the β-phosphoniocyclopropenylidene intermediate and an enthalpy of 49.8 (a free energy of 45.3) kcal mol-1 above the initial γ-phosphonioalkynyl species. We also searched for a transition state along the second 1,2 phosphine shift from Cβ to CR, finding an early transition state with a structure close to that of the β-phosphoniopropenylidene intermediate 40.2 kcal mol-1 in enthalpy (36.8 in free energy) above it and 50.9 (46.7) kcal mol-1 above the initial γ-phosphonioalkynyl species. Figure 8 resumes the whole energy profile for the stepwise double 1,2 phosphine migration via Cβ. These results show that the highest enthalpy barrier along path B, consisting of two consecutive 1,2 shifts, from Cγ to Cβ and then from Cβ to CR, is of 50.9 kcal mol-1 (46.7 kcal mol-1 in free energy) and corresponds to the transition state for the Cγ to Cβ shift. This enthalpy barrier is significantly higher than the upper limit for the barrier of 39.0 (35.8 in free energy) kcal mol-1 estimated for the direct 1,3 phosphine migration and can be therefore ruled out. In summary, our calculations show that the phosphine migration from the Cγ to the CR atom follows a direct 1,3

phosphine migration with an upper limit to the activation enthalpy of 39.0 kcal mol-1 (free energy of 35.8 kcal mol-1). Furthermore, the formation of a stable adduct (Table 5) with the phosphine close to the allenylidene chain would lead to a large frequency factor for the isomerization reaction, and thus a larger rate constant than that expected from considerations based on only the activation energy. This activation enthalpy or free energy is consistent with the stability of the initially formed γ-phosphonioalkynyl species, whose complete conversion to the final R-phosphonioallenyl product requires, for the PMe3 case, heating in refluxing dichloromethane. The extension of this result to the attack of the phosphines PMe2Ph and PMePh2 gives an upper limit for the free energy of the corresponding isomerization processes of 28.4 and 26.3 kcal mol-1, respectively, This, together with the formation of more stable adducts when bulkier phosphines are involved, is in agreement with the experimental evidence of a much faster tautomerization occurring for the reaction with either PMePh2 or PMePh2.

4. Conclusions In this work we have carried out density functional and local MP2 calculations to study the electronic structure of the rhenium allenylidene complexes [(triphos)(CO)2Re(CdCdCRR0 )]þ and their reactivity toward PMe3-xPhx (x=0-3) tertiary phosphines. The calculated electronic structure indicates that the [(triphos)(CO)2Re]þ synthon, in spite of the presence of two carbonyl ligands, has a relatively electron-rich metal center, which is consistent with the experimental behavior of the corresponding allenylidene complexes, which are unreactive toward mild nucleophiles, like alcohols. We then considered both the kinetics and the thermodynamics of the nucleophilic addition of PMe3-xPhx. The results indicate lower activation energies for the phosphine attack to Cγ, while the products of the phosphine attack to CR are, however, lower in energy. The computed behavior agrees with the experimental evidence showing that the products of the attack to Cγ are kinetically favored while those of the attack to CR are thermodynamically favored.

Article

We finally considered the possible tautomerization mechanisms to account for phosphine migration from Cγ to CR and found that the lowest energy path corresponds to an incomplete detachment of the phosphine moiety that then shifts from the Cγ to the CR atoms while remaining weakly bound to the allenylidene unit. Our calculations also indicate the possible role played by a low-energy β-phosphoniocyclopropenyl structure as key intermediate. A joint theoreticalexperimental study aimed at possibly intercepting this putative and intriguing intermediate is in progress and will be published in due time.


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Acknowledgment. Italian Ministry for University and Research is acknowledged for financial support (contract 2006038520). Ente Cassa di Risparmio di Firenze (ECRF) is thanked through the project FIRENZE HYDROLAB for sponsoring a doctoral grant to A.G. Supporting Information Available: Details of the benchmarking of model molecules and exchange-correlation functionals (including Tables S1 and S2) and Cartesian coordinates for all the considered complexes and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

Rhenium Allenylidenes and Their Reactivity toward Phosphines: A

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