Electron-Poor Rhenium Allenylidenes and Their Reactivity toward

Article pubs.acs.org/Organometallics

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

Dipartimento di Scienze del Farmaco, Università degli Studi G. d’Annunzio, Via Dei Vestini, 31, I-66100 Chieti, Italy Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy § Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy ‡

S Supporting Information *

ABSTRACT: The reaction of 1-(phenyl)-1-(p-nitrophenyl)-2-propyn-1-ol with the Re(I) precursor [(triphos)(CO)2Re(OTf)] in dichloromethane at 0 °C afforded the cationic allenylidene complex [(triphos)(CO)2Re{CCC(C6H5)(p-C6H4NO2)}]+ (3) as a dark burgundy red triflate salt after solvent evaporation. The reaction of 3 with 1.2 equiv of the phosphine PMePh2 at −40 °C led first to the γ-phosphonioalkynyl complex [(triphos)(CO)2Re{CCCPh(p-C6H4NO2)(PMePh2)}]+ (5) (observed as a pair of distinct rotamers, 5a,b) and then, on slow increase of the temperature to 0 °C, to the α-phosphonioallenyl complex [(triphos)(CO)2Re{C(PMePh2)CCPh(p-C6H4NO2)}]+ (6). On the other hand, the reaction of 3 with the more nucleophilic PMe3 at −60 °C led to its complete transformation into a compound, suggested to be the α-phosphonioallenyl derivative [(triphos)(CO)2Re{C(PMe3)CC(C6H5)(p-C6H4NO2)}]+ (7). To study the effect due to the strongly electron withdrawing p-nitrophenyl substituent on the allenylidene geometry, electronic structure, and reactivity with phosphines, we performed theoretical calculations on 3 and other hypothetical p-nitro-substituted allenylidenes as well as on the products and plausible intermediates of its reaction with PMe3 and PMePh2. Finally, theoretical methods were applied to shed light on the nature of the two rotamers observed for the γ-phosphonioalkynyl complex 5.

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INTRODUCTION The reaction of propargylic alcohols, HCCCRR′(OH), with unsaturated transition-metal fragments, first reported by Selegue,1 opened the way to the easy synthesis of a great variety of metal allenylidenes, which form following intramolecular water elimination from intermediate γ-hydroxyvinylidene species (Scheme 1). The mechanism of the reaction has been completely clarified, and in a few cases, both the π-alkyne and the hydroxyvinylidene intermediates preceding the formation of the allenylidene moiety have been intercepted and isolated in the solid state.2 Metal allenylidenes are a robust platform to carry out several regiochemical transformations which rise from the unequal distribution of the electron density along the MCCC assembly. Thus, while nucleophiles add selectively to Cγ or Cα, electrophiles are generally inclined to attack the β-position of the polycarbon chain.3 This peculiar and easily controlled reactivity has been largely used to promote both catalytic reactions4 and selective stoichiometric structural elaborations of the allenylidene moiety,5 which have found application in several areas of synthetic organic chemistry and materials science. © 2011 American Chemical Society

Scheme 1

When a nucleophile attacks the allenylidene ligand, it is not always possible to anticipate whether the reaction takes place with α or γ regioselectivity, and examples of both attacks have Received: January 29, 2011 Published: December 7, 2011 57

dx.doi.org/10.1021/om200083h | Organometallics 2012, 31, 57−69

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been experimentally documented. In the case of the rhenium(I) allenylidene [(triphos)(CO)2Re(CCCPh2)]+ (triphos = MeC(CH2PPh2)3), a careful experimental study has shown that the attack of phosphines PMe3−xPhx (x = 1, 2) occurs at the γ-carbon under complete kinetic control. Once formed at lower temperature, the γ-phosphonioalkynyl cations [(triphos)(CO)2Re{CCCPh2(PMe3−xPhx)}]+ thermally tautomerize to afford the thermodynamically more stable α-phosphonioallenyl species [(triphos)(CO) 2 Re{C(PMe 3−x Ph x )C CPh2}]+.6 The mechanism of this intriguing phosphine migration from Cγ to Cα has been recently investigated by theoretical methods.7 Within the study, in the case of PMe3 addition, we found that the lowest energy path corresponds to an incomplete detachment of the phosphine moiety that moves stepwise from Cγ to Cα carbons while remaining weakly bound to the allenylidene chain. However, on the potential energy surface for this tautomerization process, we identified also a high-energy minimum corresponding to the zwitterionic β-phosphoniovinylidene structure that easily relaxes to a more stable β-phosphoniocyclopropenyl species which was proposed as a possible key intermediate. The Cβ-bound product of PMe3 addition exists as a minimum, probably due to the stabilization of the zwitterionic vinylidene resonance forms reported in Chart 1a, a picture confirmed by the optimized structure and

Theoretical calculations were performed on the synthesized allenylidene and extended to the whole series of the p-nitrosubstituted allenylidenes [(triphos)(CO) 2 Re(CC CRR′)]+ (R = Ph, R′ = p-C6H4NO2; R = H, R′ = pC6H4NO2; R = R′ = p-C6H4NO2) as well as on the products and plausible intermediates of their reaction with both PMe3 and PMePh2.

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RESULTS AND DISCUSSION

Synthesis and Characterization of [(triphos)(CO)2Re{CCC(C6H5)(p-C6H4NO2)}]OTf (3). The propargyl alcohol 1 was prepared in quantitative yields by reaction of (p-nitrophenyl)(phenyl)methanone with a known excess of a commercially available solution of ethynylmagnesium bromide in tetrahydrofuran at room temperature following a general and well-established method for the preparation of such alkynols (Scheme 2).9 Compound 1 is a new propargylic alcohol that Scheme 2

Chart 1. (a) Main Zwitterionic Resonance Form of the β-Phosphoniovinylidene Species and (b) the Resonance Form Responsible for Its Stabilization via Charge Delocalization onto a Nitrophenyl Substituent on Cγ

has been characterized by elemental analysis and conventional spectroscopic methods. The relevant chemicophysical data for 1 and for the rhenium complexes reported in this article are given in the Experimental Section. The reaction of 1 with the Re(I) precursor [(triphos)(CO)2Re(OTf)] (2)10 in dichloromethane at 0 °C affords a purple solution, from which the cationic allenylidene 3 is obtained as a dark burgundy red triflate salt after solvent evaporation (Scheme 3). The formation of 3 from the rhenium Scheme 3

the charge analysis. It is, however, a high-energy metastable species and collapses to the more stable β-phosphoniocyclopropenyl intermediate through bond formation between the Cα and Cγ atoms. Delocalization of the negative charge of Cγ on its substituents, which can be easily reached, for instance, by the use of p-nitrophenyl groups (see Chart 1b) could provide further stabilization of the zwitterionic species, thus favoring the formation of the β-phosphoniocyclopropenyl intermediate. Prompted by these observations, we decided to synthesize and characterize a novel rhenium(I) allenylidene complex bearing a p-nitrophenyl group and to investigate its reactivity toward tertiary phosphines. Our intent was to investigate the widely unexplored chemistry of d6 allenylidenes with strong electron-attracting substituents on the terminal carbon atom and, possibly, to intercept the still unknown exotic β-phosphoniocyclopropenyl species, for which preliminary theoretical calculations indicated a thermodynamic stability close to that of the γ-phosphonioalkynyl product. We could synthesize only the rhenium allenylidene with one p-nitrophenyl group, since any attempt to prepare the bis-pnitrophenyl complex failed.8

synthon 2 and a propargylic alcohol is straightforward and follows the general protocol which has been used to prepare other mononuclear Re(I) allenylidenes of formula [(triphos)(CO)2Re(CCCRR′)]Y (R = R′ = Ph; R = H, R′ = Ph).11,12 The reaction goes to completion by the Selegue scheme, where π coordination of the alkyne is followed by 1,2-tautomerization to the γ-hydroxyvinylidene species.1 From this latter derivative the allenylidene 3 is obtained after intramolecular elimination of water (Scheme 1). The formation of the allenylidene unsaturated carbene ligand in 3 was unequivocally inferred from the analysis of NMR (31P{1H}, 13C{1H}, 1H) and IR spectra, which parallel those reported for [(triphos)(CO)2Re(CCCPh2)]+ (4) and the other known rhenium(I) allenylidene complexes (see the Experimental Section).11 The presence of the strong electron-withdrawing nitro group on one of the phenyl substituents at the allenylidene Cγ is likely 58


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Scheme 4

approaching the rotational barrier for their interconversion (Figure 1). A deeper NMR investigation of this dynamic process was hampered, however, as at −20 °C the irreversible isomerization of 5a,b into a single isomer of the αphosphonioallenyl complex [(triphos)(CO)2Re{C(PMePh2) CCPh(p-C6H4NO2)}]+ (6) took place. A mixture of 5a,b and 6 was observed between −10 and 0 °C, at which point the irreversible isomerization of the two γ-rotamers into 6 was complete. Scheme 4 shows the reaction transforming 3 into 6 via the intermediate formation of 5a,b rotamers. Due to the presence of four different substituents on the allene moiety, the final α-phosphonioallenyl complex 6 is chiral and was obtained as a racemic mixture. Complex 6 is stable in solution and does not decompose after several days. The 31P{1H} NMR signals due to 6 (CD2Cl2, 20 °C) consist of a AMM′X spin system: δX 26.35 (dt, JXM = JXM′ = 8.9 Hz, JXA = 7.2 Hz); signals for δM/M′ −16.10 (br m), δA −18.22 (ddd, JAM/M′ = 22.7 Hz, JAM′/M = 17.8 Hz), δM′/M −19.06 (br). No signal ascribable to the desired β-phosphonium cyclopropenyl intermediate was observed, likely because either it is high in energy (more than 2−3 kcal mol−1 above the γ-alkynylphosphonio species) or the barrier leading to the most stable α-phosphonioallenyl product is very low. When allenylidene 3 was reacted with 1.2 equiv of the less sterically demanding phosphine PMe3 in an NMR tube (CD2Cl2; 0.6 mL) at −70 °C, an immediate change of the color from purple to dark red was observed. 31P{1H} NMR monitoring of the reaction (−60 °C) showed the complete consumption of the starting material and its transformation into a single compound, which we propose to be the α-phosphonioallenyl derivative 7 (Scheme 5 and Figure 2). This is characterized by 31P{1H} NMR signals (CD2Cl2, −60 °C) consisting of a AMM′X spin system: δX 33.4 (dt); δA −15.7 (dd); δM/δM′ −15.8 to −16.5 (m); JAX = 1.9 Hz, JMX = JM′X = 4.5 Hz, JAM/M′ = 25.0 Hz, JAM′/M = 18.1 Hz. These values are in agreement with the pattern observed for 6. As 7 was rather unstable in solution, no attempt was made to isolate it. For the same reason, acquisition of 13C{1H} NMR spectra was not possible, as the product decomposed to undefined products during the overnight experiment (long acquisition time required). The immediate formation of 7 from the low-temperature reaction of 3 with PMe3 can be related to the presence of the nitro group, which affects the general reactivity of the ReC CC moiety. Noticeably, reaction of the γ,γ′-diphenylsubstituted allenylidene 4 with PMe3 also produced the α-phosphonioallenyl species, but the slow formation of the thermodynamic isomer was preceded by the immediate appearance of the γ-phosphonioalkynyl species at room temperature. The transformation was complete only after prolonged reflux in DCM.6 The different kinetic behavior of 3 and 4 with respect to the nucleophilic addition of PMe3 is remarkable and indicates that the presence of an electronwithdrawing substituent makes the allenylidene much more

responsible for the pronounced instability of 3 in solution. Thus, at variance with [(triphos)(CO)2Re(CCCPh2)]+, complex 3 rapidly decomposes on standing in DCM at room temperature within few minutes, giving a complicated mixture of rhenium-containing species whose 31P{1H} NMR spectra deny any simple interpretation. Due to the instability of 3 in solution a reliable 13C{1H} NMR spectrum was obtained only by running the experiment for a short time and at low temperature. In spite of this, we were able to isolate 3 in the solid state by addition of cold n-pentane to a freshly prepared DCM solution of 3, from which deep purple microcrystals of the allenylidene species could be collected in good yield. However, in all the subsequent reactions, compound 3 has been used immediately after being generated in situ from the reaction of 1 with 2. Reaction of 3 with Tertiary Phosphines. In order to verify whether the presence of the electron-withdrawing p-nitro group on the phenyl substituent at the Cγ carbon of 3 has some effect on the reactivity of the allenylidene moiety toward nucleophiles, we decided to investigate the reactivity of 3 with a pair of tertiary phosphines differing in either the cone angle or the electronic properties (PMePh2 and PMe3). Although different nucleophiles have been studied as reagents toward rhenium allenylidene species,5b,12 the reaction with tertiary phosphines was preferred, as the mechanism of the process was previously elucidated by in situ NMR studies6 and theoretically modeled by DFT calculations.7 In the first experiment, 3 was reacted with PMePh2, in order to verify whether it could be possible to detect the kinetic product and eventually the formation of the hypothesized β-phosphonium cyclopropenyl intermediate traversing the final transformation to the α-phosphonioallenyl derivative.7 The allenylidene 3 was mixed with 1.2 equiv of PMePh2 in CD2Cl2 (1.0 mL) at −85 °C in a 5 mm NMR tube. The 31 1 P{ H} NMR spectrum, recorded at −40 °C, showed that the reaction was already initiated, yielding signals ascribable to the γ-phosphonioalkynyl complex [(triphos)(CO)2Re{CCCPh(p-C6H4NO2)(PMePh2)}]+ (5; Scheme 4). Remarkably, the 31P{1H} NMR spectrum clearly showed that complex 5 exists as a pair of distinct rotamers, 5a,b (ca. 1:1.3 ratio), probably due to the existence of a high rotational barrier around either the C−C(PMePh2)RR′ or the CC(RR′)− PMePh2 bond. The signals were observed as AMM′X spin systems: δX 24.44 (s, 5b), 17.88 (s, 5a); signals for δA −9.20 (t, JAM = JAM′ = 18.9 Hz, 5b); −10.09 (t, JAM = JAM′ = 19.2 Hz, 5a); signals for δM+M′ −16.2 to −17.7 (m, 5a,b). After 15 min of standing at this temperature only the two γ rotamers were present in solution in the same ratio as above. A 13C{1H} NMR spectrum recorded overnight at −40 °C confirmed the formation of the pair of γ-alkynylphosphonio rotamers (see the Experimental Section). When the temperature was slowly increased to −20 °C, the signals of 5a,b broadened, 59


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Figure 1. 31P{1H} NMR variable-temperature (VT) monitoring of the reaction of 3 with PMePh2.

Scheme 5

electron-withdrawing p-nitro group on the terminal phenyl substituents on the geometry, the electronic structure, and the reactivity with phosphines of the [(triphos)(CO)2Re(C CCPh2)]+ complex and to shed light on the nature of the two distinct rotamers observed for the product of the nucleophilic attack of PMePh2 at the Cγ allenylidene atoms: i.e., the γ-phosphonioalkynyl complex [(triphos)(CO)2Re{C CCPh(p-C6H4NO2)(PMePh2)}]+ species (5). For the sake of completeness we did not limit our attention to the experimentally investigated [(triphos)(CO)2Re(CC CRR′)]+ allenylidene with R = Ph, R′ = p-C6H4NO2 but considered also the hypotetical complexes with R = H, R′ = p-C6H4NO2 and with R = R′= p-C6H4NO2, which could not be experimentally prepared.8

reactive. In this view, it is likely that the initial γ-trimethylphosphonioalkynyl isomer of 7 cannot be intercepted, even when the reaction is carried out at very low temperature, due to fast isomerization. DFT Calculations. Theoretical calculations have been performed to study the effects of the presence of a strong 60


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Figure 2. 31P {1H} NMR (CD2Cl2, −60 °C) signals of 7: (left) PX signals (PMe3); (right) triphos signals (PA, PM).

contributes to the γ,γ′-diphenyl-substituted allenylidene 4 (see Chart 2) and is enhanced by replacing a phenyl substituent with a hydrogen atom.

In a recently performed theoretical study on the parent rhenium(I) diphenyl allenylidene 4,7 we obtained geometries in good agreement with X-ray data and showed a relatively electron rich nature of the [(triphos)(CO)2Re]+ synthon, consistent with the reactivity pattern of the corresponding allenylidene. Geometry, Bonding, and Electronic Structure. Following the same approach, we first performed geometry optimizations of the [(triphos)(CO)2Re(CCCRR′)]+ allenylidenes (R = Ph, R′ = p-C6H4NO2; R = H, R′ = p-C6H4NO2; R = R′ = p-C6H4NO2). The main geometrical parameters are compared with those of the parent diphenyl allenylidene in Table 1 and

Chart 2. Cumulene and Alkynyl Resonance Forms of the γ,γ′-Diphenyl-Substituted Allenylidene Complex 4

We have then calculated the bond dissociation energy between the [(triphos)(CO)2Re]+ fragment and the CC CRR′ ligand 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. The results are presented in Table 2 and show that the presence of a p-nitro group on one

Table 1. Main Geometrical Parameters, Calculated at the PBE/BS1 Level of Theory, for the [(triphos)(CO)2Re(C CCRR′)]+ Allenylidenes (R = R′ = Ph; R = Ph, R′ = p-C6H4NO2; R = R′ = p-C6H4NO2; R = H, R′ = p-C6H4NO2) bond distance (Å) R, R′ substituents on Cγ Ph, Ph Ph, p-C6H4NO2 p-C6H4NO2, p-C6H4NO2 H, p-C6H4NO2

Cα−Cβ

Cβ−Cγ

Cγ−C(R)

Cγ−C(R′)

2.000 1.999 1.981

1.268 1.270 1.272

1.374 1.372 1.369

1.471 1.471 1.475

1.477 1.481 1.480

1.973

1.275

1.355

-

1.451

Re−Cα

Table 2. Bond Dissociation Energies (kcal mol−1), Calculated at the PBE/BS4//PBE/BS3 Level of Theory, for the [(triphos)(CO)2Re(CCCRR′)]+ Allenylidenes (R = R′ = Ph; R = Ph, R′ = p-C6H4NO2; R = R′ = p-C6H4NO2; R = H, R′ = p-C6H4NO2)

indicate that the introduction of one or two p-nitro groups on the terminal phenyl substituents progressively shifts the ReCCC unit toward a more uniform cumulene structure, shortening the Re−Cα and Cβ−Cγ bonds and lengthening the Cα−Cβ bonds. This effect is due to the destabilization of the alkynyl resonance form that significantly 61

R, R′

E*

ERRe

ERC3RR′

De

Ph, Ph Ph, p-C6H4NO2, p-C6H4NO2, p-C6H4NO2, H, p-C6H4NO2

92.3 89.8 88.0 89.8

3.7 4.1 4.2 4.3

0.2 0.2 0.4 0.2

88.0 85.5 83.4 85.3


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or two of the terminal phenyl substituents causes only a small decrease of the bond dissociation energy, by 2−5 kcal mol−1. A more detailed insight into the nature of the Re− allenylidene bond, based on a bond analysis in Cs symmetry employing the energy decomposition scheme developed by Ziegler and Rauk,14 allowed us to separate the σ and π contributions to the bond energy, see Computational Details and the Supporting Information. This analysis shows that the increase of the orbital interaction energy observed upon the inclusion of a p-nitro group is almost completely due to the contribution from π back-donation, that from σ donation being essentially constant. This is consistent with our previous calculations on a series of metallacumulenes LmM(C)nR2 with different substituents, showing that the π back-donation contribution increases for π-accepting substituents due to the energy lowering of the empty acceptor allenylidene π* MOs, which approach in energy the occupied metal donor dπ orbitals.13b Finally, to provide a rationale for the effects of the p-nitro groups on the reactivity of the [(triphos)(CO)2Re(CC CPh2RR′)]+ allenylidene, we have calculated 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 for the three nitrosubstituted complexes with those for [(triphos)(CO)2Re (C CCPh2)]+. This analysis gives essentially the same results obtained in previous theoretical investigations on allenylidene and cumulenylidene d6 metal complexes of groups VI−VIII, indicating that the reactivity of these complexes toward both electrophilic and nucleophilic attack is mostly determined by frontier orbital factors, in particular by the high-lying HOMO and a low-lying LUMO, which determine the regioselectivity of, respectively, the electrophilic and nucleophilic attack.13 For all the considered allenylidenes, irrespective of the terminal substituents, the HOMO has contributions mainly from the metal and the carbon atom in an even position along the chain (Cβ), while the contributions to the LUMO come mainly from the carbon atoms in odd positions (Cα and Cγ), determining respectively their electrophilic or nucleophilic character (see Table S2 in the Supporting Information). However, the energy of the LUMOs of the p-nitro derivatives is significantly lower, by 0.4−0.7 eV, thus indicating an increased reactivity toward nucleophilic attack. This picture is perfectly consistent with the experimentally observed reactivity pattern of both the diphenyland phenyl−p-nitrophenyl-substituted compounds, the latter being much more reactive. Reactivity toward Phosphines. Geometry optimizations were carried out on the products of the addition of PMe3 to the Cα, Cβ, and Cγ atoms of the three considered p-nitrosubstituted allenylidenes and on the corresponding βphosphoniovinylidene and β-phosphoniocyclopropenyl intermediates. Several conformations can be envisaged for each of these products (see Stereoisomers and Rotamers for the Phosphine Adducts): the most plausible ones were optimized, and only the results for the global minima have been considered and reported below. The geometries obtained for the Cα and Cγ adducts, reported in Figure 3 for the {CC C(Ph)(p-C6H4NO2)} derivative, show the expected αphosphonioallenyl and γ-phosphonioalkynyl structures. On the other hand, the optimization of the β-phosphoniovinylidene collapsed to the β-phosphoniocyclopropenyl intermediate (see Figure 3), indicating that the p-nitro-substituted β-phosphoniovinylidene structures are not minima on the PES, at variance

Figure 3. Calculated geometries of the (a) α-phosphonioallenyl and (b) γ-phosphonioalkynyl products of PMe3 addition to 3 and of (c) the β-phosphoniocyclopropenyl intermediate. The rhenium allenylidene complex has been modeled including only two out of the six phenyl groups of the triphos ligand.

with what is observed for the diphenyl parent, for which, however, they correspond to high-energy metastable minima. The calculated reaction energies, enthalpies, and free energies for the phosphine addition to the Cα and Cγ atoms and for the formation of the corresponding β-phosphoniocyclopropenyl intermediate are reported in Table 3 and compared with the corresponding values for the addition to the parent diphenyl allenylidene 4. The results are reported both in the gas phase and in dichloromethane solution, and a first analysis 62


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the α-phosphonioallenyl derivative is the thermodynamically most stable product. More interestingly, the presence of p-nitro groups stabilizes the β-phosphoniocyclopropenyl intermediates, which, for the bis-p-nitrophenyl allenylidene, is only 4−5 kcal mol−1 less stable than the Cγ product. A different situation has been found for the {CCCH(p-C6H4NO2)}complex, for which a significant stabilization of both the Cα and Cγ phosphine adducts has been observed, ca. 3−8 kcal mol−1, while the phosphoniocyclopropenyl intermediate is strongly destabilized and is 17−18 kcal mol−1 above the Cγ adduct, both effects being probably due to the release of the steric strain upon the replacement of a phenyl group with a hydrogen atom. We then estimated the energy barriers for the nucleophilic addition of PMe3 to the Cα and Cγ atoms of the three considered p-nitro-substituted allenylidenes. Preliminary energy scan calculations for the attack at either Cα or Cγ atoms of the mono- or di-p-nitro-substituted species, using as reaction coordinate the C−P distance, indicate an initial decrease of the energy upon the phosphine approach, suggesting the formation of stable adducts between the phosphine moiety and the p-nitro-substituted diphenylallenylidenes, as already observed for the unsubstituted diphenyl species7 (see Figure S1 in the Supporting Information). Geometry optimization calculations lead to stable minima much deeper for the attack at the Cγ (5−10 kcal mol−1) than at the Cα atom (less than 2 kcal mol−1). The formation energies, enthalpies, and free energies of the noncovalent adducts for the attack on the Cγ atom are reported in the Supporting Information (Table S3). We could also identify the transition states for the attack at the Cα and the Cγ atoms. A different situation has been observed for the {C CCH(p-C6H4NO2)} complex, for which the energy scan shows a monotonous decrease for the phosphine attack at both the Cα and Cγ atoms, without any minimum or maximum, indicating that this attack is practically barrierless. The activation energies, enthalpies, and free energies calculated for the attack of PMe3 are reported in Table 4 and compared with the corresponding values for the diphenylallenylidene. For the attack at the Cγ atom, Table 4 also reports (in parentheses) the activation energies, enthalpies, and free energies relative to the noncovalent allenylidene−phosphine adducts rather than to the allenylidenes and phosphine infinitely apart. The results indicate activation free energies (enthalpies) for the mono- and

Table 3. Reaction Energies, Enthalpies, and Free Energies (at the LMP2/BS2//PBE/BS1 Level; kcal mol−1) for the Addition of PMe3, to the Cα and Cγ Atoms of the Considered [(triphos)(CO)2Re(CCCRR′)]+ Allenylidenes (R = R′ = Ph; R = Ph, R′ = p-C6H4NO2; R = R′ = p-C6H4NO2; R = H, R′ = p-C6H4NO2) and for the Formation of the β-Phosphoniocyclopropenyl Intermediate adduct

ΔEgas

ΔEsol

R = R′ = Ph α-phosphonioallenyl −34.2 −35.2 γ-phosphonioalkynyl −33.3 −32.1 β-phosphoniocyclopropenyl −21.8 −21.6 R = Ph, R′ = p-C6H4NO2 α-phosphonioallenyl −36.0 −36.2 γ-phosphonioalkynyl −29.8 −29.5 β-phosphoniocyclopropenyl −21.4 −22.5 R = R′ = p-C6H4NO2 α-phosphonioallenyl −36.2 −37.6 γ-phosphonioalkynyl −31.1 −29.4 β-phosphoniocyclopropenyl −23.8 −25.5 R = H, R′ = p-C6H4NO2 α-phosphonioallenyl −41.7 −41.2 γ-phosphonioalkynyl −36.4 −35.8 β-phosphoniocyclopropenyl −18.0 −18.5

ΔHsol

ΔGsol

−33.5 −31.0 −20.3

−19.0 −14.6 −4.7

−33.7 −28.0 −20.4

−20.0 −14.0 −7.0

−35.7 −27.6 −23.6

−22.0 −14.6 −9.8

−38.5 −33.7 −16.1

−26.4 −22.4 −3.7

shows (i) a small effect due to solvation, (ii) a small increase due to the ZPE and thermal contributions, and (iii) quite large and unfavorable entropy contributions, by 13−17 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 phosphine addition. With regard to the effects of the p-nitro substituents on the terminal phenyl groups, Table 3 shows small effects on the stability of Cα and Cγ phosphine adducts, within 3 kcal mol−1. In particular, the presence of the p-nitro groups leads to a small stabilization of the Cα adduct by 1−3 kcal mol−1, as expected on the basis of the increased electrophilic character of the allenylidene caused by the presence of such an electronattracting group, while the stability of the Cγ derivative remains substantially unchanged. As for the unsubstituted complex 4, the Cα adduct is always more stable than the Cγ adduct by 4− 8 kcal mol−1 in both enthalpy and free energy, indicating that

Table 4. Activation Energies, Enthalpies, and Free Energies (at the LMP2/BS2//PBE/BS1 Level; kcal mol−1) for the Addition of PMe3 to the Cα and Cγ Atoms of the Considered [(triphos)(CO)2Re(CCCRR′)]+ Allenylidenes (R = R′ = Ph; R = Ph, R′ = p-C6H4NO2; R = R′ = p-C6H4NO2; R = H, R′ = p-C6H4NO2)a adduct

ΔE⧧gas

α-phosphonioallenyl γ-phosphonioalkynyl

+6.2 −3.4 (+2.2)


+5.2 −3.7 (+3.5)


+3.2 −4.4 (+5.4)


barrierless barrierless

ΔE⧧sol R = R′ = Ph +8.4 −0.8 (+2.6) R = Ph, R′ = p-C6H4NO2 +6.8 −2.1 (+2.9) R = R′ = p-C6H4NO2 +5.8 −3.0 (+3.2) R = H, R′ = p-C6H4NO2 barrierless barrierless

ΔH⧧sol

ΔG⧧sol

+8.0 −0.4 (+3.2)

+21.2 +12.8

+7.1 −0.9 (+1.8)

+20.0 +11.3

+6.1 −3.7 (+0.2)

+19.1 +11.0



a

The values in parentheses for the attack at the Cγ atom are relative to the noncovalent allenylidene−phosphine adducts rather than to the allenylidene and phosphine infinitely apart. 63


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We also estimated the energy barriers for the attack of PMePh2 at both the Cα and Cγ atoms in 3, using the same approach employed for PMe3, and the calculated activation energies, enthalpies, and free energies are reported in Table 6 and compared with those for the attack of PMe3. The results show that the activation free energies calculated for the attack of PMePh2 are significantly higher than those calculated for PMe3, 25.9 vs. 20.0 kcal mol−1 for the attack at Cα and 21.1 vs. 11.3 kcal mol−1 for the attack at Cγ. It is worth noting that the activation free energy for the attack at Cα is not only much higher than those for the attack at Cγ, 25.9 vs. 21.1 kcal mol−1, but has an absolute value high enough to prevent the direct formation of the thermodynamically most stable α-phosphonioallenyl product. Therefore, the combined results of the calculated reaction and activation energies, enthalpies, and free energies for the nucleophilic attack of PMe3 and PMePh2 at the considered allenylidenes show that, although the attack at Cγ is always kinetically favored while that at Cα leads to the thermodynamically most stable product, only for the most sterically demanding PMePh2 phosphine is the height of the barrier for the attack to Cα sufficiently high to allow the initial formation of the kinetic γ-phosphonioalkynyl adductat least at low temperaturesand its subsequent isomerization to the thermodynamic α-phosphonioallenyl product. These results are consistent with the experimental studies showing that, while the nucleophilic attack of PMe3 at 3 quantitatively leads to the formation of the α-phosphonioallenyl product even at −70 °C, the attack of PMePh2 at −40 °C leads to the formation of the γ-phosphonioalkynyl adduct 5, which only above −20 °C irreversibly isomerizes to the α-phosphonioallenyl product 6. Stereoisomers and Rotamers for the Phosphine Adducts. The presence of large phenyl groups on the allenylidene Cγ atom and on the triphos ligand may lead to large steric hindrance for the rotation around single bonds in the final products of the addition of the tertiary phosphines and, therefore, to significant barriers for the interconversion among their low-energy conformations. Moreover, the addition of sterically demanding phenylphosphines (passing from PMe3 to PMePh2) and the use of asymmetric Ph and p-PhNO2 substituents on the allenylidenes could bring on an interesting stereochemistry featuring the possibility of distinct rotamers and stereoisomers. Rotation is in principle possible around Re−Cα and Cα−P bonds in the α-phosphonioallenyl and around the Re−Cα Cβ−Cγ pseudoaxis and the Cγ−P bond in the γ-phosphonioalkynyl product. The rotation around these bonds is strongly influenced by the asymmetric nature of the rhenium coordination sphere around the unsaturated C3 ligand, characterized by one sterically crowded region with two phenyl groups protruding from the triphos ligand and one sterically empty region with two carbonyl groups. We will discuss here only the possible conformers of the γ-phosphonioalkynyl species which have been observed as a pair of distinct rotamers, 5a,b, while the conformations for the α-phosphonioallenyl species are reported in the Supporting Information. In the γ-phosphonioalkynyl species, the rotation around the Re−CαCβ−Cγ pseudoaxis leads to three possible relative orientations of the phosphonio and the two phenyl groups on Cγ with (i) the sterically most demanding phosphonio group pointing between the triphos phenyl groups (and the two phenyl groups toward the carbonyl ligands), (ii) the

bis-p-nitro-substituted allenylidenes lower than those for the diphenylallenylidene: 19−20 vs 21.2 kcal mol−1 (6−7 vs 8 kcal mol−1) for the attack at Cα and ca. 11 vs 12.8 kcal mol−1 (0−2 vs 3.2 kcal mol−1) for the attack at Cγ. The lower activation free energies are consistent with the electronic structure analysis above, showing lower LUMO energies for the p-nitrosubstituted allenylidenes and suggesting a higher reactivity, and match with the experimentally observed reactivity of the phenyl(p-nitrophenyl)allenylidene 3, much higher than that of the diphenyl-substituted allenylidene 4. It is worth noting that, although in the p-nitro-substituted allenylidenes the activation free energies for the attack at Cγ are lower than those for the attack at Cα, as observed for the diphenylallenylidene, the absolute values of the activation free energy for the attack at Cα in the nitro-substituted derivatives is low enough to allow the direct formation of the thermodynamically most stable α-phosphonioallenyl product. This result supports the conclusion that product 7, directly obtained from the reaction of 3 with PMe3 even at −70 °C, is indeed the α-phosphonioallenyl derivative. For the experimentally investigated {CCC(Ph)(pC6H4NO2)} allenylidene 3, we addressed the effect of the use of the more sterically demanding PMePh2 phosphine on the relative stabilities of the α-phosphonioallenyl, γ-phosphonioalkynyl, and β-phosphoniocyclopropenyl species and on the energy barriers for the nucleophilic attack at the Cα and Cγ atoms. Geometry optimizations were performed on the most plausible conformers of the α-phosphonioallenyl and γphosphonioalkynyl products as well as on the β-phosphoniocyclopropenyl intermediate and the results for the global minima allowed to calculate their formation energies, enthalpies, and free energies, which are reported in Table 5 Table 5. Reaction Energies, Enthalpies, and Free Energies (at the LMP2/BS2//PBE/BS1 Level, kcal mol−1) for the Addition of PMe3 and PMePh2 to the Cα and Cγ Atoms of the [(triphos)(CO)2Re(CCCRR′)]+ Allenylidene (R = Ph, R′ = p-C6H4NO2) and for the Formation of the Corresponding β-Phosphoniocyclopropenyl Intermediate adduct α-phosphonioallenyl γ-phosphonioalkynyl β-phosphoniocyclopropenyl α-phosphonioallenyl γ-phosphonioalkynyl β-phosphoniocyclopropenyl

ΔEgas PMe3 −36.0 −29.8 −21.4 PMePh2 −35.6 −23.4 −12.1

ΔEsol

ΔHsol

ΔGsol

−36.2 −29.5 −22.5

−31.7 −28.0 −20.4

−20.0 −14.0 −7.0

−30.7 −17.4 −7.4

−28.2 −15.8 −5.4

−13.9 −1.4 +10.0

and compared with those for the attack of PMe3. Table 5 shows a fairly large decrease of the stabilities of these species on increasing the phosphine cone angle from PMe3 to PMePh2, by 4−15 kcal mol−1 in enthalpy and 6−17 kcal mol−1 in free energy, which is more evident for the γ-phosphonioalkynyl and β-phosphoniocyclopropenyl species. The Cα adduct is still more stable than the Cγ adduct, and this difference augments on increasing the phosphine cone angle: from 3.7 kcal mol−1 in enthalpy and 6.0 kcal mol−1 in free energy for PMe3 to 12.4 kcal mol−1 in enthalpy and 12.5 kcal mol−1 in free energy for PMePh2. 64


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Table 6. Activation Energies, Enthalpies, and Free Energies (at the LMP2/BS2//PBE/BS1 Level; kcal mol−1) for the Addition of PMe3 and PMePh2 to the Cα and Cγ Atoms of the [(triphos)(CO)2Re(CCCRR′)]+ Allenylidene (R = Ph, R′ = p-C6H4NO2)a adduct

ΔE⧧gas


+5.2 −3.7 (+3.5)


+7.7 +0.4

ΔE⧧sol PMe3 +6.8 −2.1 (+2.9) PMePh2 +12.6 +5.7

ΔH⧧sol

ΔG⧧sol

+7.1 −0.9 (+1.8)

+20.0 +11.3

+12.8 +6.2

+25.9 +21.1

a

The values in parentheses for the attack at the Cγ atom are relative to the noncovalent allenylidene−phosphine adducts rather than to the allenylidene and phosphine infinitely apart.

phosphonio group pointing between the carbonyl ligands (and the phenyl groups toward the triphos phenyl groups), or (iii) the phosphonio group lying between one CO and one triphos phenyl group (and the phenyl groups toward one CO and one triphos phenyl groups) (F, G, or H, respectively, in Chart 3).

Chart 4. Possible Orientations of the Phosphonio Group with Respect to the Alkynyl Unit through Rotation around the Cγ−P Bond in the γ-Phosphonioalkynyl Producta

Chart 3. Possible Orientations of the Phosphonioalkynyl Ligand with Respect to the [(triphos)(CO)2Re]+ Metal Fragment through Rotation around the Re−CαCβ−Cγ Pseudoaxis in the γ-Phosphonioalkynyl Product

The orientation around the Cγ−P bond is expected to be invariably staggered with only one possible conformation for the products of the addition of the symmetric PMe3 or PPh3 phosphines and three distinct conformations for the products of the addition of the asymmetric PMe2Ph or PMePh2 phosphines (I−K and L−N, respectively, in parts a and b of Chart 4). These forecasts were checked against a detailed analysis of all the minima obtained from geometry optimizations of the most plausible orientations of the rotatable bonds of the phosphine addition (see Reactivity toward Phosphines). Moreover, in order to determine the existence of different rotamers and estimate the energy barriers involved in their interconversion, we performed an energy scan around each of the corresponding rotatable bonds, and the results are reported in Figures 4 and 5 and discussed in the following. Indeed, for atropoisomerism to be observed, the involved rotamers have to be separated by sufficiently high barriers for both the clockwise and anticlockwise rotation of the corresponding dihedral angle. All the scans were performed using the experimental {CC C(Ph)(p-C 6 H 4 NO 2 )} species with the bulkier PMePh 2 phosphine, the only one experimentally showing the presence of different rotamers at low temperature. The scans were carried out at the PBE/BS1 level of theory, and whenever necessary, the energies corresponding to minima and transition states were also re-evaluated at the LMP2/BS2 level of theory. The analysis of the geometry optimizations performed for the most plausible orientations of both rotatable bonds for the addition products to Cγ of the experimentally investigated PMe3 and PMePh2 phosphines indicates that the most stable

a

Only the orientations for the product of PMe2Ph and PMePh2 addition to the Cγ atom of [(triphos)(CO)2Re(CCCPh2)]+ (a and b, respectively) and of PMePh2 to [(triphos)(CO)2Re{CC C(Ph)(p-C6H4NO2)}]+ (c) are shown.

Figure 4. Energy scan for the rotation around the Re−CαCβ−Cγ axis for the γ-phosphonioalkynyl product of the addition of PMePh2 to the asymmetric {CCC(Ph)(p-C6H4NO2)} allenylidene. Angles are given in deg and energies in kcal mol−1. 65


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(ca. 2 kcal mol−1). The energy barriers separating these three rotamers are significantly larger than those corresponding to the rotation around the Re−CαCβ−Cγ axis (Figure 4), amounting to ca. 18−19 kcal mol−1. These results are thus fully consistent with the experimentally observed existence of two atropisomers with similar population (1:1.3) that are stable up to −20 °C and give us the confidence to identify them with rotamers M′ and N′. Remarkably, for the experimentally studied α-phosphonioallenyl and γ-phosphonioalkynyl species derived from the asymmetric {CCC(Ph)(p-C6H4NO2)} allenylidene 3, the stereochemistry is further complicated following the possibility of optical activity. Indeed, due to the presence of four different substituents on the Cα and on the Cγ terminal carbon atoms of the α-phosphonioallenyl or of four different substituents on the tetrahedral Cγ atom of the γ-phosphonioalkynyl, these species are chiral and may exist as two enantiomers. However, due to lack of any asymmetric promoter along the synthetic procedures used, these enantiomers cannot be resolved.

■

Figure 5. Energy scan for the rotation around the Cγ−P bond for the γ-phosphonioalkynyl product of the addition of PMePh2 to the asymmetric {CCC(Ph)(p-C6H4NO2)} allenylidene. Angles are given in deg and energies in kcal mol−1.

CONCLUSIONS In this study we synthesized and characterized a novel rhenium(I) allenylidene complex bearing an electron-withdrawing p-nitrophenyl group and studied its reactivity toward tertiary phosphines. Our intent was to investigate the widely unexplored chemistry of d6 allenylidenes with strong electronattracting substituents on the terminal carbon atom and, possibly, to intercept the so far elusive β-phosphoniocyclopropenyl species, whose existence is suggested by a recent theoretical study. The cationic allenylidene [(triphos)(CO)2Re{CCC(C6H5)(p-C6H4NO2)}]+ (3) was prepared from the reaction of 1-phenyl-1-(p-nitrophenyl)-2-propyn-1-ol (1) with the Re(I) precursor [(triphos)(CO)2Re(OTf)] (2) in dichloromethane at 0 °C and obtained as a dark burgundy red triflate salt after solvent evaporation. In order to study the effect of the electron-withdrawing p-nitro substituent on the Cγ carbon of 3 on the reactivity of the allenylidene moiety toward nucleophiles, we investigated the reaction of 3 with both PMePh2 and PMe3, differing in either the cone angle or the electronic properties. The reaction of 3 with the sterically demanding phosphine PMePh2 at −40 °C led first to the γ-phosphonioalkynyl complex [(triphos)(CO) 2 Re{CCCPh(p-C 6 H 4 NO 2 )(PMePh 2 )}] + (5)observed as a pair of distinct rotamersand then, on increasing slowly the temperature to about 0 °C, to the α-phosphonioallenyl complex [(triphos)(CO)2Re{C(PMePh2)CCPh(pC6H4NO2)}]+ (6). On the other hand, reaction with PMe3 at −60 °C led to its complete consumption and transformation into the α-phosphonioallenyl derivative [(triphos)(CO)2Re{C(PMe3)CC(C6H5)(p-C6H4NO2)}]+ (7). To study how a strong electron-withdrawing nitro group on the allenylidene substituents affects the geometry, the electronic structure, and the reactivity with phosphines of the [(triphos)(CO)2Re( CCCPh2)]+ complex as well as to shed light on the nature of the two rotamers observed for the γ-phosphonioalkynyl complex 5, we also performed theoretical calculations on 3 and other hypothetical p-nitro-substituted allenylidenes and modeled the products and plausible intermediates of their reaction with PMe3 and PMePh2. On the whole, both experimental and theoretical studies indicate that the presence of an electron-withdrawing p-nitro substituent on the Cγ carbon of the allenylidene moiety causes

conformations assume only orientation G (see Chart 3), with the phosphonio group pointing between the CO ligands and the two phenyl groups on Cγ stabilized by the π−π stacking with the two triphos phenyls, while orientations F and H are higher in energy or do not even correspond to equilibrium structures. Instead, similar energies have been calculated for the three possible staggered orientations, L−N, around the Cγ−P bond in the products of the addition of PMePh2, reported in Chart 4b, the higher steric repulsion expected when a phenyl group on P is directed between the two phenyl groups on Cγ (as in M and N) being partially compensated by energetically more favorable π stacking. In agreement with the analysis of the geometry optimizations, the energy scan for the rotation around the Re−Cα Cβ−Cγ axis performed for the experimental {CCC(Ph)(p-C 6H 4NO 2)} species shows a global minimum with G orientation, two minima with H, and one with F orientation, all within 3 kcal mol−1 from G. These minima are separated by barriers of, at most, 9 kcal mol−1 (see Figure 4), so that the atropisomerism experimentally observed for the product of the nucleophilic attack of PMePh2 at the Cγ allenylidene atom of 3 is not expected to be due to hampered rotation around this axis. The rotation around the Cγ−P bond for the γ-phosphonioalkynyl product of the addition of PMePh2 to the asymmetric allenylidene 3, which has been experimentally thoroughly studied, was investigated through an energy scan of the corresponding dihedral angle (see Figure 5), which allowed us to perform a detailed conformational analysis calculating the energies of the three possible atropisomers arising from the restricted rotation (i.e. L′−N′ in Chart 4c) and the barrier for their interconversion. Because the energy barriers in this case are significantly larger (see the following), all the energies along the scan have also been re-evaluated at the LMP2/BS2 level of theory. Our calculations indicate that rotamers M′ and N′, with the Me group on P synclinal to the alkynyl group on Cγ, are essentially isoenergetic (differing by approximately 0.2 kcal mol−1), while the antiperiplanar rotamer L′ is higher in energy 66


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an increase of the reactivity toward nucleophiles, attributed both to a higher thermodynamic stability of the α-phosphonioallenyl and γ-phosphonioalkynyl derivatives and to lower activation energies for the nucleophilic attack at the Cα and Cγ carbon atoms. The cone angle and the basicity of the attacking phosphine also have important effects on the outcome of the nucleophilic attack: while for smaller and more basic nucleophiles such as PMe3 the barriers for the attack at the Cα and Cγ carbon atoms are both quite low and only the formation of the thermodynamic α-phosphonioallenyl product is observed, even at temperatures as low as −60 °C, for less basic and sterically more demanding phosphines, such as PMePh2, the barrier for the attack at Cα is high enough to allow the isolation, at least below −20 °C, of the metastable γ-phosphonioalkynyl derivative. Our calculations also allowed us to attribute the atropoisomerism observed for the γphosphonioalkynyl derivative to a hampered rotation around the Cγ−P bond and to identify the two corresponding rotamers. Finally, the putative β-phosphonium cyclopropenyl intermediate could not be intercepted in either case, mainly because it is still high in energy (7 and 11.5 kcal mol−1 above the γ-alkynylphosphonio derivatives, respectively, for PMe3 and PMePh2) and probably also because of the low barrier leading to the thermodynamic α-phosphonioallenyl product.

■

C{ 1 H} NMR (acetone-d 6, 20 °C): δ 154.0 (C ipso C6 H 4), 148.2 (Cipso NO2), 145.7 (Cipso C6H5), 129.3 (m-C6H5), 128.7 (o-C6H4), 128.0 (p-C6H5), 126.8 (o-C6H5), 124.2 (m-C6H4), 86.8 (CH), 77.6 (CCH), 74.0 (Cq). IR (KBr, cm−1): ν(CH) 3255, ν(CC) 2109. Anal. Found (Mw = 253.26): C, 71.22; H, 4.26; N, 5.51. Calcd for C15H11NO3: C, 71.14; H, 4.38; N, 5.53. Synthesis of [(triphos)(CO)2Re{CCC(C6H5)(p-C6H4NO2)}]OTf (3). A 5 mm screw-cap NMR tube was charged with 30 mg (0.118 mmol) of 1, 120 mg (0.119 mmol) of [(triphos)Re(CO)2(OTf)], and 0.8 mL of CD2Cl2 cooled at 0 °C. The purple solution was allowed to react for 6 h at 0 °C. 31P{1H} NMR monitoring of the reaction showed the quantitative formation of allenylidene 3. Addition of cold n-pentane (5 mL) and workup gave deep purple microcrystals of the allenylidene species in about 85% isolated yield. 1H NMR (CD2Cl2, −60 °C): δ 8.2−6.9 (m, 39 H aromatic), 2.81−2.75 (m, 4H, CH2,eq(triphos)), 2.50 (br s, 2H, CH2,ax(triphos)), 1.63 (br s, 3H, CH3(triphos)). 13C{1H} NMR (CD2Cl2, −60 °C): δ 296.6 (dt, JCPA = 35.7 Hz, JCPM = 11.7 Hz, Re CCC); 222.4 (d, JCPA = 17.8 Hz, ReCCC), 191.2 (m, 13

EXPERIMENTAL SECTION

General Information. The synthesis and the manipulation of the air-sensitive reaction mixtures were performed using vacuum-line and Schlenk techniques. Preparation, purification, and manipulation of the rhenium complexes were carried out under an atmosphere of dry nitrogen using distilled and deoxygenated solvents. Tetrahydrofuran (THF) was dried and degassed by passing it through a double column under a stream of nitrogen, with the MBraun SPS solvent purification system. Dichloromethane was purified by distillation onto P2O5. The rhenium precursor [(triphos)(CO)2Re(OTf)] (2) was prepared as reported in the literature.10 Unless otherwise stated, all the other reagents and chemicals were used as received by commercial suppliers. Deuterated acetone and dichloromethane for NMR measurements (Aldrich) were dried over molecular sieves (4 Å). 1H, 13 C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker Avance DRX-300 spectrometer (operating at 300.13, 75.48, and 121.50 MHz, respectively) and a Bruker DRX-400 instrument (operating at 400.13, 100.61, and 161.97 MHz, respectively). All chemical shifts (δ) are reported in ppm relative to TMS (1H, 13C) and were calibrated against the deuterated solvent multiplet (13C) or the residual protiated solvent (1H). 31P{1H} NMR chemical shifts were measured relative to external 85% H3PO4 with downfield values taken as positive. IR spectra (KBr pellets) were recorded on a PerkinElmer Spectrum BX Series FT-IR spectrometer. Mass spectra were obtained at a 70 eV ionization potential and are reported in the form m/z (intensity relative to base 100). Elemental analyses (C, H, N) were performed using a Carlo Erba Model 1106 elemental analyzer by the Microanalytical Service of The University of Florence. Synthesis of 1-(4-Nitrophenyl)-1-phenylprop-2-yn-1-ol (1). A solution of 1-(4-nitrophenyl)-1-phenylmethanone (200 mg, 0.9 mmol) in dry THF (4.5 mL) was cooled to 0 °C and treated with a solution of ethynylmagnesium bromide (0.5 M in THF; 3.5 mL, 1.8 mmol). The solution was warmed to room temperature and stirred for 2 h before being quenched with an aqueous HCl solution (0.1 M, 5 mL) and extracted with ethyl acetate (3 × 8 mL). The organic phase was collected, dried over anhydrous Na2SO4, and evaporated to dryness to afford 208 mg of 1-(4-nitrophenyl)-1-phenylprop-2-yn-1-ol (93% yield) as a pale yellow solid. MS: m/z 252 (12) [M + H]+; 53 (100). 1H NMR (acetone-d6, 20 °C): δ 8.22 (d, JHH = 9.1 Hz, 2H, Ar), 7.91 (d, JHH = 9.1 Hz, 2H, Ar), 7.67 (d, JHH = 8.2 Hz, 2H, Ar), 7.39−7.26 (m, 3H, Ar), 6.16 (br s, 1H, OH), 3.53 (s, 1H, CH).

CO), 156.5 (s, ReCCC), 39.4 (s, CH3Ctriphos), 39.0 (br s, CH3C(triphos)), 33.0−32.1 (m, CH2(triphos)). 31P{1H} NMR (CD2Cl2, −60 °C): AM2 spin system, δA −20.09 (d, JAM = 24.4 Hz); δM −18.54 (t). IR (KBr, cm−1): ν(CO) 2000, 1941; ν(CC C) 1916; ν(NO) 1342. Anal. Found (Mw = 1251.22): C, 56.82; H, 4.01; N, 1.26. Calcd for C59H48NF3O7P3ReS: C, 56.64; H, 3.87; N, 1.12. In Situ NMR Reaction of 3 with PMePh2. A solution of the allenylidene 3 (99 mg, 0.079 mmol) in CD2Cl2 (0.8 mL) was prepared in a 5 mm screw-cap NMR tube and cooled to −85 °C. Neat PMePh2 (18 μL, 0.095 mmol) was added through the screw cap via microsyringe, which causes an immediate color change from purple to red. The tube was then inserted into the NMR probe precooled to −80 °C, and the progress of the reaction was monitored by 31P{1H} NMR spectroscopy. The first spectrum run at −40 °C showed that the reaction between 3 and the phosphine had already started, yielding a mixture of the two γ-phosphonioalkynyl rotamers (5a,b). On standing at this temperature for additional 15 min, no other NMR resonances, apart from those of 5a,b, were observed in the spectrum. When the

temperature was increased to −20 °C, new resonances ascribable to the α-phosphonioallenyl isomer [(triphos)(CO)2Re{(C(PMePh2) CC(C6H5)(p-C6H4NO2)}]OTf (6) appeared in the spectrum and the mixture of 5a,b and 6 was observed in different ratios up to −10 °C. At 0 °C the isomerization of 5a,b into 6 was complete and only the NMR signals of the α-allenyl isomer were observed. Addition of cold n-pentane (5 mL) gave 6 as a dark red powder, which was collected by filtration and washed with cold n-pentane; yield 86%. 67


Organometallics

Article

5a,b (two rotamers: 1:3 ratio): 1H NMR (CD2Cl2, −40 °C) δ 8.4− 6.7 (m, 49H aromatic, 5a,b), 2.73 (d, JHPX = 15.0 Hz, 3H, P(CH3)Ph2, 5b), 2.70−2.30 (m, 6H, CH2(triphos), 5a,b), 2.51 (d, JHPX = 15.0, 3H, PCH3Ph2, 5a), 1.51 (br s, 3H, CH3(triphos), 5a), 1.46 (br s, 3H, CH3(triphos), 5b); 13C{1H} NMR (CD2Cl2, −40 °C) δ 199.1−198.2 (m, CO, 5a,b), 115.8 (m, Re−CC−C, 5a,b), 103.6 (m, Re−C C−C, 5b), 103.3 (m, Re−CC−C, 5a), 55.4 (d, JCPX = 44.9 Hz, Re− CC−C, 5a,b), 39.6 (m, CH3(triphos), 5a,b), 39.0 (m, CH3C(triphos), 5a,b), 34.0−32.1 (m, CH2(triphos), 5a,b), 11.3 (d, JCPX = 58.9 Hz, PCH3Ph2, 5a or 5b), 7.8 (d, JCPX = 47.4 Hz, PCH3Ph2, 5b or 5a); 31P{1H} NMR (CD2Cl2, −40 °C) AMM′X spin systems, δX 24.44 (s, 5b), 17.88 (s, 5a), δA −9.20 (t, JAM = JAM′ = 18.9 Hz, 5b), −10.09 (t, JAM = JAM′ = 19.2 Hz, 5a), δM+M′ −16.2 to −17.7 (m, 5a,b). 6: 1H NMR (CD2Cl2, 20 °C) δ 8.2−6.6 (m, 49H, aromatic), 2.89− 2.54 (m, 6H, CH2(triphos)), 2.35 (d, JHPX = 12.2 Hz, 3H, PCH3Ph2), 1.65 (s, 3H, CH3(triphos)); 13C{1H} NMR (CD2Cl2, 20 °C) δ 216.0 (dt, JCPX = 8.2 Hz, JCPM = JCPM′ = 3.8 Hz, Re−CCC), 198.9 (br t, JCPM = 49.8 Hz, JCPM′ = 41.7 Hz, CO), 100.7 (dd, JCPX = 25.2 Hz, JCPA = 2.5 Hz, Re−CCC), 72.8 (dt, JCPX = 26.5 Hz, JCPM = JCPM′ = 8.2 Hz, Re−CCC), 39.2 (q, JCP = 9.5 Hz, CH3(triphos)), 37.7 (q, JCP = 3.8 Hz, CH 3 C(triphos)), 36.8 (br t, J CPeq = 20.8 Hz, CH2Peq(triphos)), 32.5 (br d, JCPax = 23.3 Hz, CH2Pax(triphos)), 11.5 (d, JCPX = 58.6, PCH3Ph2); 31P{1H} NMR (CD2Cl2, 20 °C) AMM′X spin system, δX 26.35 (dt, JXM = JXM′ = 8.9 Hz, JXA = 7.2 Hz), δM/M′ −16.10 (br m), δA −18.22 (ddd, JAM/M′ = 22.7 Hz, JAM′/M = 17.8 Hz), δM′/M −19.06 (br); IR (KBr, cm−1) ν(CO) 1942, ν(CCC) 1877, ν(NO2) 1340. Anal. Found (Mw = 1451.44): C, 59,29; H, 4.34; N, 0.78. Calcd for C72H61NF3O7P4ReS: C, 59.58; H, 4.24; N, 0.97. In Situ NMR Reaction of 3 with PMe3. PMe3 (34 μL, 0.034 mmol) was syringed at −70 °C into a 5 mm screw-cap NMR tube containing a purple solution of the allenylidene 3 (35 mg, 0.028 mmol) in CD2Cl2 (0.6 mL) cooled to the same temperature. The solution immediately turned dark red, and the first 31P{1H} NMR recorded at −60 °C showed the complete disappearance of the allenylidene 3 and its transformation into the α-phosphonioallenyl derivative 7. No attempt was made to isolate 7 due to its fast decomposition. For this reason it was not possible to measure clear 13 C{1H} NMR spectra, as it required long acquisition times. 7: 31P{1H} NMR (CD2Cl2, −60 °C) AMM′X spin system, δX 33.4 (dt), δA −15.7 (dd), δM/δM′ −15.8 to −16.5 (m), JAX = 1.9 Hz, JMX = JM′X = 4.5 Hz, JAM/M′ = 25.0 Hz, JAM′/M = 18.1 Hz; IR (CD2Cl2, cm−1) ν(CO) + ν(CCC) 1940, 1878. Computational and Methodological Details. All calculations were carried out using density functional theory15 as implemented in the Jaguar 7.5 suite16 of ab initio quantum chemistry programs and the Amsterdam Density Functional 2007.03 package (ADF).17 Geometries were optimized by using Jaguar and employing the PBE18 exchange correlation functional with the 6-31G** basis set.19 Rhenium was represented by the Los Alamos LACVP** basis,20 which includes relativistic effective core potentials (BS1). The energies were reevaluated by additional LMP221 single-point calculations at each optimized geometry using the triple-ζ basis set 6-311++G**,22 while rhenium was described by a modified version of LACVP**, designated as LACV3P++**, where the exponents were decontracted and a diffuse function was added (BS2). In a recent paper we have shown that the PBE functional performs better than other functionals commonly employed in the calculations of organometallic compounds (BP86,23 BLYP,24 and B3LYP25) and that the use of a more reliable wave function method such as LMP2 is necessary to make up for the problems of the DFT methods for highly conjugated π systems and, in particular, their failure in predicting the energy difference of cumulenes and polyyne isomers, which are involved in the considered systems.26 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 us to calculate reaction and activation enthalpies and free energies. Solvation energies were evaluated by a self-consistent reaction field (SCRF) approach,27 based on accurate numerical

solutions of the Poisson−Boltzmann equation,28 using the basis BS2 and employing a dielectric constant 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 differences between the energies of the addition products and the energies of the optimized reactants using the ADF program, for purpose of consistency with the following bond analysis (vide infra). Indeed, ADF was also 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.14 As to the calculations performed with ADF, we carried out geometry optimizations with the PBE functional and a polarized double-ζ STO basis set for main-group atoms and triple-ζ for rhenium (BS3). Energies were then evaluated on the optimized geometries using the same functional and a polarized triple-ζ STO basis set on all atoms (BS4). Relativistic effects on Re are included using the scalar “zeroth-order regular approximation” (ZORA)29 as implemented in ADF. Although this approach and basis sets are not the same as those implemented in the Jaguar package, in a previous study on the diphenylallenylidene,7 we showed that the results with ADF at this level of theory are fully consistent with those calculated with Jaguar employing the same exchange-correlation potential at a similar PBE/ BS4//PBE/BS3 level and are very close to those calculated at the wave function LMP2/BS2//PBE/BS1 level. According to the Ziegler and Rauk decomposition scheme, the bond dissociation energy is decomposed into a number of contributions:

DE(Re−allenylidene) = − [ΔEstr + ΔEelst + ΔEPauli + ΔEorb] 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 fragments strain energies: Estr[(triphos)(CO)2Re] + Estr[CCCR2]. 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 ΔEPauli represents the four electron destabilizing interactions between occupied orbitals (Pauli repulsion) and together they correspond to the steric repulsion between the two fragments, ΔEster. ΔEorb, known as the orbital interaction term, represents the attracting orbital interactions which give 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.14b This decomposition scheme is particularly useful in the considered complexes of pseudo-C s symmetry, as it allows one to separate the energy contributions corresponding to σ donation (EA′) and to π back-donation (EA″). Indeed, the ligand to metal σ donation takes place into the A′ representation, while the metal to ligand π back-donation takes place into the A″ representation. To perform such an analysis, we first reoptimized all complexes in Cs symmetry: the main geometrical parameters and bond dissociation energies show only small deviations from the corresponding values obtained without any symmetry constraint.

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S Supporting Information * Text, tables, and figures giving additional experimental and theoretical details. This material is available free of charge via the Internet at http://pubs.acs.org.

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Organometallics

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (N.R.); [email protected] (M.P.).

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Electron-Poor Rhenium Allenylidenes and Their Reactivity toward

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