Hydride Alkenylcarbyne Osmium Complexes versus Cyclopentadienyl

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Hydride Alkenylcarbyne Osmium Complexes versus Cyclopentadienyl Type Half-Sandwich Ruthenium Derivatives Alba Collado, Miguel A. Esteruelas,* and Enrique O~nate* Departamento de Química Inorganica, Instituto de Ciencia de Materiales de Aragon, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain

bS Supporting Information ABSTRACT: The dihydride complex OsH2Cl2(PiPr3)2 (1) reacts with 2-methyl-1-hexen-3-yne and 2,4-dimethyl-1,3-pentadiene to give the hydride alkenylcarbyne derivatives OsHCl2{tCC(Me)dCHR}(PiPr3)2 (R = nPr (2), iPr (3)), which have been characterized by X-ray diffraction analysis. DFT calculations (B3PW91) suggest that the enyne is initially hydrogenated to afford a conjugated diene. The latter evolves into the hydride alkenylcarbyne derivative by means of two hydrogen migrations. The first migration is a 1,4-hydrogen shift within the diene (from the terminal CH2 group to the internal double bond) which takes place through the metal center. The second migration is a 1,2hydrogen shift from the terminal CH2 group to the osmium atom. In contrast to the case for 1, the ruthenium counterpart RuH2Cl2(PiPr3)2 (16) reacts with 2-methyl-1-hexen-3-yne to give a complex mixture of compounds, from which the derivatives Ru(η5-C5HR1R2R3R4)Cl(PiPr3) (17; R1 = C(CH3)dCH2, R2 = Et, R3 = nPr, R4 = Me) and RuCl2{dC(Et)CHdCMe2}(PiPr3)2 (18, traces) are isolated. Both 17 and 18 have been also characterized by X-ray diffraction analysis. DFT calculations (B3PW91) on the formation of 17 suggest that in the ruthenium case the hydrogenation of the enyne leads to an alkenylcarbene intermediate, which reacts with a second enyne molecule to afford the tetrasubstituted cyclopentadienyl group.

’ INTRODUCTION Osmium is more reducing than ruthenium and prefers coordination saturation and redox isomers with more metal carbon bonds.1 As a consequence of this, there are marked differences in chemistry between both metals, including some catalytic reactions. For instance, the half-sandwich osmium complex [Os(η5-C5H5)(py)3]PF6 efficiently catalyzes the 7-endo-heterocyclization of aromatic alkynols into benzoxepines, whereas the ruthenium counterpart and tungsten and rhodium precursors give no or low conversions.2 The differences in chemistry between osmium and ruthenium are particularly evident in the behavior of the MH2Cl2(PiPr3)2 (M = Os, Ru) complexes toward terminal alkynes and disubstituted allenes. The osmium complex OsH2Cl2(PiPr3)2 reacts with terminal alkynes to give the saturated hydride carbyne derivatives OsHCl2(tCCH2R)(PiPr3)2.3 Under the same conditions, the ruthenium counterpart affords mixtures of the unsaturated carbenes RuCl2(dCHCH2R)(PiPr3)2 and vinylidenes RuCl2(dCdCHR)(PiPr3)2.4 The saturated carbynes OsHCl2(tCCH2R)(PiPr3)2 are the oxidized osmium counterparts of the unsaturated carbene ruthenium derivatives RuCl2(dCHCH2R)(PiPr3)2. The reactions of OsH2Cl2(PiPr3)2 with 1-methyl-1-(trimethylsilyl)allene and 1,1dimethylallene yield initially the π-allene intermediates OsCl2(η2CH2dCdCRMe)(PiPr3)2, which evolve into the hydride alkenylcarbyne derivatives OsHCl(tCCHdCRMe)(PiPr3)2 (R = Me, Me3Si). The ruthenium compound, as does the osmium complex, reacts with these allenes to afford π-allene analogues, RuCl2(η2CH2dCdCRMe)(PiPr3)2. However, in this case, they evolve to r 2011 American Chemical Society

the unsaturated disubstituted vinylidenes RuCl2(dCdCRMe)(PiPr3)2 via the alkenylcarbene intermediates RuCl2(dCHCHd CRMe)(PiPr3)2 (the reduced ruthenium counterparts of OsHCl(tCCHdCRMe)(PiPr3)2), which undergo a metathesis reaction with a second molecule of allene.5 Osmium hydride alkenylcarbyne complexes have been shown to be versatile intermediates in organometallic synthesis. On deprotonation of the alkenyl substituent, they afford osmacyclopentapyrroles by assembly of the resulting allenylidene with a terminal alkyne and an acetonitrile molecule.6 Furthermore, they can be transformed into dienylcarbene species, which evolve to five-membered cyclic alkenylcarbene species by means of a Nazarov-type cyclization via η1-cyclopentadienyl intermediates.7 Due to the importance of the construction of five-membered rings in the synthesis of natural products,8 we are interested in the development of new types of osmium hydride alkenylcarbyne derivatives. The osmium hydride alkenylcarbyne complexes previously reported have been prepared by starting from allenes5,9 and by reaction of alkynols HCtCC(OH)R2 with dihydrogen and dihydride compounds, which afford dihydrogen species by coordination of electron-poor Lewis bases.3,10 In the search for new substrates for the preparation of hydride alkenylcarbyne complexes, we have now explored the reactions of the dihydrides Received: December 23, 2010 Published: March 18, 2011 1930

dx.doi.org/10.1021/om1011962 | Organometallics 2011, 30, 1930–1941

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MH2Cl2(PiPr3)2 (M = Os, Ru) with 2-methyl-1-hexen-3-yne and 2,4-dimethyl-1,3-pentadiene. This paper reports (i) the preparation of osmium hydride alkenylcarbyne complexes starting from enynic and diolefinic substrates, (ii) DFT calculations on the formation pathway of the new osmium compounds, (iii) the preparation of a 16-valenceelectron tetrasubstituted cyclopentadienyl ruthenium derivative starting from an enynic substrate, and (iv) DFT calculations on the formation pathway of the new ruthenium compound.

’ RESULTS AND DISCUSSION Reactions of OsH2Cl2(PiPr3)2 with 2-Methyl-1-hexen-3yne and 2,4-Dimethyl-1,3-pentadiene: Formation of Hydride Alkenylcarbyne Complexes. Treatment of a toluene

solution of OsH2Cl2(PiPr3)2 (1) with 2.2 equiv of 2-methyl-1hexen-3-yne for 24 h at 353 K leads to the hydride alkenylcarbyne derivative OsHCl2{tCC(Me)dCHnPr}(PiPr3)2 (2), which is isolated as a brown solid in 55% yield, according to eq 1.

Figure 1 gives a view of the geometry of 2. The coordination around the osmium atom can be rationalized as a distorted octahedron with the phosphorus atoms of the phosphine ligands occupying trans positions (P(1)OsP(2) = 167.55(10)°). The perpendicular plane is formed by the cis-disposed chloride ligands (Cl(1)OsCl(2) = 87.13(10)°), the hydride disposed trans to Cl(1) (H(01)OsCl(1) = 172(3)°), and the carbyne group disposed trans to Cl(2) (C(1)OsCl(2) = 173.9(4)°). The most conspicuous feature of the structure is the very short OsC(1) bond length of 1.728(11) Å, which is fully consistent with an OsC(1) triple-bond formulation.11 The alkenylcarbyne proposal is supported by the bond lengths and angles within the η1-carbon donor ligand: e.g., C(1) and C(2) are separated by 1.434(15) Å and C(2) and C(4) by 1.359(17) Å, whereas the angles around C(2) and C(4) are in the range 115127°. The 1H, 13C{1H}, and 31P{1H} NMR spectra of 2 in benzened6 at room temperature are consistent with the structure shown in Figure 1. In agreement with the presence of a hydride ligand in the complex, the 1H NMR spectrum in the high-field region contains at 6.57 ppm a triplet with a HP coupling constant of 16.6 Hz. In the low-field region the most noticeable signal is another triplet at 6.70 ppm, which shows a HH coupling constant of 7.3 Hz, corresponding to C(4)H. In the 13C{1H} spectrum the OsCR resonance appears at 257.5 ppm, also as a triplet but with a CP coupling constant of 11 Hz. The C(sp2) resonances due to C(2) and C(4) are observed at 146.2 and 146.6 ppm, respectively, as singlets. As expected for equivalent phosphines, the 31P{1H} NMR spectrum shows a singlet at 18.9 ppm. At first glance, the formation of 2 could imply the reduction of the CC triple bond of the enyne and the subsequent activation of both terminal C(sp2) bonds of the conjugated diene. One of the hydrogen atoms should migrate to the CHEt carbon atom of the diolefin (C(5) in Figure 1), while the other atom should go to the metal center. In order to confirm this, we carried out the reaction of 1 with 2,4-dimethyl-1,3-pentadiene.

Figure 1. Molecular diagram of 2. Selected bond lengths (Å) and angles (deg): OsC(1) = 1.728(10), OsH(01) = 1.48(3), OsP(1) = 2.410(3), OsP(2) = 2.410(3), OsCl(1) = 2.523(3), OsCl(2) = 2.480(3), C(1)C(2) = 1.434(15); P(1)OsP(2) = 167.55(10), Cl(1)OsCl(2) = 87.13(10), H(01)OsCl(1) = 172(3), C(1)OsCl(2) = 173.9(4).

Treatment of a toluene solution of 1 with 2.2 equiv of the diolefin for 72 h at 353 K, as expected, gives rise to the hydrogenation of 1 equiv of the diene to afford 2,4-dimethyl-2pentene and the formation of the hydride alkenylcarbyne derivative OsHCl2{tCC(Me)dCHiPr}(PiPr3)2 (3), which is isolated as a brown solid in 75% yield, according to eq 2. This reaction is consistent with previous results of Caulton and co-workers, who have observed that 1 reacts with styrene and propylene to give equimolecular amounts of OsHCl2{tCCH2R}(PiPr3)2 (R = Ph, Me) and the hydrogenated olefin.12

Complex 3 has been also characterized by X-ray diffraction analysis. Figure 2 gives a view of the geometry of this compound. The coordination around the metal center is like that of 2: i.e., a distorted octahedron with trans phosphines (P(1)OsP(2) = 167.34(3)Å) and cis chlorides (Cl(1)OsCl(2) = 87.87(2)°). The OsC(1) bond length of 1.734(3) Å agrees well with that of 2 and those reported for other osmium carbyne derivatives.13 The bond lengths and angles within the η1-carbon donor ligand are also consistent with the alkenylcarbyne formulation. C(1) and C(2) are separated by 1.443(4) Å and C(2) and C(4) by 1.332(5) Å, whereas the angles around C(2) and C(4) are in the range 115127°. The 1H, 13C{1H}, and 31P{1H} NMR spectra of 3 in benzened6 at room temperature are consistent with the structure shown in Figure 2 and agree well with those of 2. In the 1H NMR spectrum, the hydride resonance appears at 6.61 ppm as a triplet with a HP coupling constant of 16.5 Hz, whereas the C(4)H signal is observed at 6.53 ppm as a doublet with a HH coupling constant of 10.1 Hz. In the 13C{1H} NMR spectrum, the OsCR resonance appears at 257.6 ppm as a triplet with a CP coupling constant of 11 Hz, whereas the C(sp2) resonances due to C(2) and C(4) are observed at 143.5 and 153.1, respectively, 1931

dx.doi.org/10.1021/om1011962 |Organometallics 2011, 30, 1930–1941

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Figure 2. Molecular diagram of 3. Selected bond lengths (Å) and angles (deg): OsC(1) = 1.734(3), OsP(2) = 2.4150(7), OsP(1) = 2.4166(7), OsCl(2) = 2.4886(7), OsCl(1) = 2.5083(7), C(1)C(2) = 1.443(4), C(2)C(4) = 1.359(17); P(1)OsP(2) = 167.34(3), OsC(1)C(2) = 178.8(3),. 31

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as singlets. A singlet at 18.8 ppm in the P{ H} NMR spectrum is also characteristic of this compound. Theoretical Calculations on the Formation Mechanism of the Hydride Alkenylcarbyne Complexes. In an effort to gain insight into the mechanistic details of the formation of 2 and 3, we have carried out DFT calculations (B3PW91) on the hydrogenation of the enyne to the diene and the double-hydrogen migration in the latter, using PMe3 and 2-methyl-1-penten-3-yne as models of PiPr3 and enyne. The changes in free energy (ΔG) and electronic energy (ΔE) have been computed at 298.15 K and P = 1 atm. Figure 3 shows the energy profile for the hydrogenation step. Chart 1 collects the optimized structures and selected structural parameters. The coordination of the CC triple bond of the enyne to the model starting complex OsH2Cl2(PMe3)2 (1t) affords the OsH2(π-alkyne) (dH1H2 = 1.435 Å) species 4t with the H1H2 vector and the coordinated C1C2 bond parallel and perpendicular, respectively, to the POsP direction. The migration of H1 from the metal center to the C1 carbon atom of the carboncarbon triple bond leads to the intermediate 5t, which is 17.0 kcal mol1 more stable than 4t. This species can be described as a hydride osmacyclopropene derivative. The OsC1, OsC2, and C1C2 bond lengths of 2.142, 1.915, and 1.430 Å, respectively, compare well with those found by X-ray diffraction analysis for the reported osmacyclopropene complexes.14 The hydrogen migration takes place via the transition state TS1, which lies 10.0 kcal mol1 above 4t. Intermediate 5t evolves into the pentadienyl species 6t, as a consequence of the slippage of the metal from C1C2 to C2C3C4. The OsC2, OsC3, OsC4, C2C3, and C3C4 distances of 2.116, 2.221, 2.196, 1.424, and 1.422 Å, respectively, support the η3 coordination of the organic fragment to the metal center.15 Intermediate 6t is 0.5 kcal mol1 less stable than 5t. The transition state connecting both intermediates, TS2, lies 15.7 kcal mol1 above 4t and can be described as an unsaturated η1-pentadienyl species coordinated through the internal carbon atom C2 of the dienyl unit (dOsC2 = 2.033 Å). The migration of the hydride ligand H2 from the metal center to C2 leads to 7t, containing a pentadiene group coordinated by the terminal C3C4 double bond and the generated

Figure 3. Relative energies (ΔG (ΔE) in kcal mol1) for the transformation of 4t into 8t. Legend: red H, H1; blue H, H2; [Os], OsCl2(PMe3)2.

C(sp2)H bond (C2H2). This intermediate is 1.5 kcal mol1 less stable than 6t. The transition state TS3 connecting 6t and 7t lies 1.6 kcal mol1 above 6t and results from the approach of the hydride ligand to C2. In 7t the coordinated C3C4 double bond is disposed almost parallel to the POsP direction, whereas C2H2 is perpendicular. The release of the latter from the coordination sphere of the metal allows the C3C4 double bond to turn toward the plane perpendicular to the POsP direction. As a result of this, the η2-pentadiene complex 8ta is formed through the transition state TS4, which lies 5.8 kcal mol1 above 7t. The hydrogenation product 8ta is 22.3 kcal mol1 more stable than 4t but 13.6 kcal mol1 less stable than the final hydride alkenylcarbyne compound (vide infra). The rotation of the C1C2 double bond of 8ta around of the 3 C C2 single bond affords 8tb, which is 1.1 kcal mol1 less stable than 8ta. Then, the migrations of the hydrogen atoms of the coordinated terminal CH2 group of the diene occur. The first of them is a 1,4-hydrogen shift (from C4 to C1) which takes place via the metal center (C4fOsfC1), whereas the second one is a 1,2-hydrogen shift from the coordinated terminal carbon atom to the metal (from C4 to Os). Figure 4 shows the energy profile for both migrations. Chart 2 collects the optimized structures of the intermediates and transition states, as well as selected structural parameters. The 1,4-hydrogen shift is initiated by the migration of H3 from the coordinated terminal carbon atom of the diene to the metal. The CH bond activation affords the unsaturated hydride η1-1,3pentadienyl intermediate 9t. It is 9.1 kcal mol1 more stable than 4t and 12.1 kcal mol1 less stable than 8tb. The migration takes place via TS5, which lies 8.8 kcal mol1 above 4t. This transition state results from the approach of H3 to the metal center. At the same time, C3 moves away from the osmium atom. The electronic saturation of the metal center of 9t is achieved by coordination of the C2H2 bond of the pentadienyl ligand. The entry of this bond in the coordination sphere of the metal produces a strong destabilization of the system. The resulting intermediate 10t is 11.2 kcal mol1 less stable than 4t and 20.3 kcal mol1 less stable than 9t. The destabilization appears to be a consequence of the necessary distortion in the coordination polyhedron to make space for the incoming bond. Thus in the transition state TS6, 1932


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Chart 1

electron hydride alkenylcarbyne 15t, by means of the 1,2-hydrogen shift of H4 from C4 to the metal center, in agreement with the osmium preference for coordination saturation and the redox isomers with more metalcarbon bonds.1 The hydride carbyne product is 6.3 kcal mol1 more stable than its carbene isomer and 35.9 kcal mol1 more stable than the starting η2-diene complex 4t. The transition state TS11 connecting 14t and 15t lies 23.1 kcal mol1 above 14t but 6.5 kcal mol1 below 4t. It results from the approach of H4 to the osmium atom. As a consequence of the approach process, the osmium carbene unit

connecting 9t and 10t, the ClOsCl and ClOsH3 angles of 80.2 and 55.7°, respectively, result from strong reductions of the respective angles of 9t (129.3 and 74.2°, respectively). The slippage of the osmium atom of 10t toward the C1C2 double bond of the organic fragment affords 11t, via the transition state TS7. Intermediate 11t is 3.7 kcal mol1 less stable than 4t but 7.5 kcal mol1 more stable than 10t. The subsequent insertion of the coordinated C1C2 double bond of 11t into the OsH3 bond gives 12t, where the saturated character of the metal center is maintained by means of the agostic coordination of the formed H3C1 bond. Intermediate 12t is 6.2 kcal mol1 more stable than 11t. The insertion takes place via the transition state TS8, which lies 4.3 kcal mol1 above 4t. The dissociation of the agostic H3C1 bond from the osmium atom leads to the osmacyclobutene species 13t, through the transition state TS9. Intermediate 13t, which is 5.6 kcal mol1 more stable than 12t, evolves into the carbene derivative 14t, via the transition state TS10. Complex 14t is 8.4 kcal mol1 more stable than the η2-diene species 8tb and 29.6 kcal mol1 more stable than 4t. The 16-valence-electron alkenylcarbene intermediate 14t isomerizes into the 18-valence-

forms an OsdC(R) H system. The OsC4, OsH4, and H4C4 distances of 1.808, 2.063, and 1.198 Å, respectively, as well as the C4OsH4 angle of 35.3° support this distorted η2-coordination mode,5,10b which has been observed in alkylidene complexes of electron-deficient transition metals.16 The activation barrier for the second hydrogen migration, the carbene to hydride carbyne transformation, is significantly lower than that for the C4 f C1 hydrogen migration (14.7 versus 36.3 kcal mol1). This suggests that the latter is the rate-determining 1933


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Figure 4. Relative energies (ΔG, (ΔE) in kcal mol1) for the transformation of 8tb into 15t. Legend: red H, H1; blue H, H2; purple H, H3; green H, H4, [Os], OsCl2(PMe3)2.

step for the diene to hydride alkenylcarbyne transformation promoted by the osmium fragment OsCl2(PiPr3)2. However, starting from 4t, the isomerization of 5t into 6t is the highest barrier. Thus, the hydrogenation of the CC triple bond of the enyne is the rate-determining step for the overall enyne to hydride alkenylcarbyne transformation. These high activation barriers are consistent with an experimental reaction temperature higher than room temperature. Reaction of RuH2Cl2(PiPr3)2 with 2-Methyl-1-hexen-3-yne. Because ruthenium is more oxidizing than osmium, the ruthenium(IV) counterpart RuH2Cl2(PiPr3)2 (16) is significantly less stable than the osmium(IV) dihydride complex 1. As a consequence of this, the reactions of the ruthenium(IV) dihydride that need high activation energy vie with decomposition processes of the starting compound. Thus, treatment of toluene solutions of 16 with 2-methyl-1-hexen-3-yne leads to complex mixtures of compounds containing the half-sandwich derivative Ru(η5-C5HR1R2R3R4)Cl(PiPr3) (17; R1 = C(CH3)d CH2, R2 = Et, R3 = nPr, R4 = Me). Its amount depends upon the experimental conditions, increasing with a decrease of the reaction temperature and the ruthenium/enyne molar ratio used. The best result was obtained when the reaction was performed for 15 min at room temperature in the presence of 6.0 equiv of organic substrate. From the resulting mixture, the derivatives 17 and RuCl2{dC(Et)CHdCMe2}(PiPr3)2 (18) were isolated as a blue solid in 60% yield and as a few blue crystals, respectively (eq 3).

Complex 17 was characterized by elemental analysis, 1H, C{1H}, and 31P{1H} NMR spectroscopy, and an X-ray crystallographic study. Figure 5 gives a view of the molecule. The

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structure proves the formation of the tetrasubstituted cyclopentadienyl ligand, which is a result of the assembly of two enyne molecules and a hydride ligand. The geometry around the ruthenium atom is the typical two-legged piano stool for fivecoordinated half-sandwich complexes,17 with a P(1)RuCl(1) angle of 92.79(2)°. The plane defined by the atoms P(1), Ru, and Cl(1) is almost perpendicular to the five-membered ring. The angle between the centroid of the ring, the metal, and the centroid of the P(1)RuCl(1) unit is 177.7°. The phosphine ligand is disposed transoid to the carbon atoms C(6) and C(9) with P(1)RuC(6) and P(1)RuC(9) angles of 154.10(7) and 157.80(7)°, respectively. In agreement with this, the 13C{1H} NMR spectrum in benzene-d6 at room temperature shows at 90.6 and 89.7 ppm doublets with CP coupling constants of 8 and 6 Hz, corresponding to these atoms, whereas the resonances due to C(2) (P(1)RuC(2) = 116.13(7)), C(13) (P(1)Ru C(13) = 119.42(7)°), and C(1) (P(1)RuC(1) = 101.59(7)°) appear at 70.1, 68.0, and 63.3 ppm as singlets. This indicates that the disposition of the phosphine ligand with regard to the carbon atoms of the C5 ring in the solid state is maintained in benzene solution at room temperature: i.e., the C5 ring does not rotate around the centroidmetal axis. A singlet at 55.4 ppm in the 31 1 P{ H} NMR spectrum is also characteristic of 17. The carbene complex 18 was characterized by X-ray diffraction analysis. Figure 6 gives a view of the molecule. The geometry around the metal center can be rationalized as a square pyramid with the alkylidene in the apex and trans phosphines (P(1) RuP(2) = 157.27(3)°) and trans chlorides (Cl(1)RuCl(2) = 167.06(3)°). The four atoms P(1), P(2), Cl(1), and Cl(2) forming the base are approximately in one plane, whereas the ruthenium is located 0.37 Å above this plane toward the apical position. The most conspicuous features of the structure are first the RuC(1) bond length of 1.865(4) Å, which is consistent with a RuC(1) double-bond formulation,18 and second the RuC(1)C(2), RuC(1)C(4), and C(2)C(1)C(4) angles of 122.8(3), 120.6(3), and 116.5(3)°, respectively, which clearly indicate sp2 hybridization for C(1). The bond lengths and 1934


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Chart 2

angles within the alkylidene ligand are consistent with the alkenylcarbene proposal;19 e.g., C(1) and C(4) are separated by 1.484(6) Å and C(4) and C(5) by 1.339(6) Å and the angles around C(4) and C(5) are in the range 110131°. The presence of 18 in the reaction mixture is revealed by a triplet with a CP coupling constant of 7.5 Hz at 276.7 ppm in the 13C{1H} NMR spectrum of the crude reaction mixture. Theoretical Calculations on the Formation Mechanism of the Half-Sandwich Complex. The reaction of 16 with 2-methyl1-hexen-3-yne to give 17 has been also studied by DFT calculations

(B3PW91) using PMe3 and 2-methyl-1-penten-3-yne as models of PiPr3 and enyne. As in the osmium case, the process has been divided into two parts: the hydrogenation of the triple bond and the coupling of the resulting organic fragment with a second enyne molecule. Figure 7 shows the energy profile for the hydrogenation. Chart 3 collects the optimized structures and selected structural parameters of the involved intermediates and transition states. The coordination of the CC triple bond of the enyne to the model starting complex RuH2Cl2(PMe3)2 (16t) affords the 1935


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Figure 5. Molecular diagram of 17. Selected bond lengths (Å) and angles (deg): RuCl(1) = 2.3571(7), RuP(1) = 2.3697(7), RuC(1) = 2.158(3), RuC(2) = 2.137(2), RuC(6) = 2.154(2), RuC(9) = 2.167(3), RuC(13) = 2.135(3), C(1)C(2) = 1.436(4), C(2)C(6) = 1.460(4), C(6)C(9) = 1.422(4), C(9)C(13) = 1.444(4), C(13) C(1) = 1.426(4), C(3)C(5) = 1.340(4); P(1)RuCl(1) = 92.79(2).

Figure 6. Molecular diagram of 18. Selected bond lengths (Å) and angles (deg): RuCl(1) = 2.4017(10), RuCl(2) = 2.3863(10), RuP(1) = 2.4103(10), RuP(2) = 2.4372(11), RuC(1) = 1.865(4), C(1) C(4) = 1.484(6), C(4)C(5) = 1.339(6); P(1)RuP(2) = 157.27(3), Cl(1)RuCl(2) = 167.06(3), RuC(1)C(2) = 122.8(3), Ru C(1)C(4) = 120.6(3), C(2)C(1)C(4) = 116.5(3).

dihydrogen (dH1H2 = 0.891 Å) π-alkyne species 19t with the coordinated H1H2 and C1C2 bonds parallel and perpendicular, respectively, to the POsP direction. Ruthenium is a poorer π-back-bonder than osmium because the osmium valence orbitals have better overlap with the ligand orbitals.20 As a consequence of this, the H1H2 and C1C2 bond lengths in 19t are respectively about 0.50 and 0.02 Å shorter than in the osmium counterpart 4t. The migration of H1 from the metal center to the C1 carbon atom of the carboncarbon triple bond leads to the intermediate 20t, which is 9.5 kcal mol1 more stable than 19t. As for its osmium counterpart 5t, intermediate 20t can be described as a metallacyclopropene derivative. The hydrogen migration takes place via the transition state TS12, which lies 9.0 kcal mol1 above 19t. Intermediate 20t undergoes reductive elimination by migration of H2 to C1 with a very low activation barrier. The transition state for the migration, TS13, is only 0.6

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Figure 7. Relative energies (ΔG (ΔE) in kcal mol 1) for the transformation of 19t into 21t. Legend: red H, H1, blue H, H2; [Ru], RuCl2(PMe3)2.

kcal mol1 above 20t. The resulting alkenylcarbene species 21t is 30.4 kcal mol1 more stable than 19t. The migration of H2 from the osmium atom of 5t to C1 is also possible. However, the resulting alkenylcarbene, the osmium counterpart to 21t, is 14.9 kcal mol1 less stable than the η2-diene hydrogenation product 8ta and 28.5 kcal mol1 less stable than the hydride carbyne 15t. The experimental alkenylcarbene 18, related to 21t, should be the result of the migration of the hydride ligands of the starting complex to the C2 and C4 carbon atoms of the model enyne. The coordination of a second molecule of enyne to the ruthenium atom of the alkenylcarbene hydrogenation product should promote the release of one of the phosphine ligands. This substitution process is typical for the catalytic olefin metathesis reactions, including those analyzed by DFT calculations.21 Figure 8 gives the energy profile for the formation of the halfsandwich product 17t, starting from the resulting alkenylcarbene π-enyne intermediate 22t. Chart 4 collects the optimized structures and selected structural parameters of the involved intermediates and transition states. The migration of the alkenylcarbene ligand of 22t from the ruthenium atom to the C6 carbon atom of the coordinated C5C6 triple bond of the enyne gives rise to the alkenyldienylcarbene intermediate 23t via the transition state TS14. Intermediate 23t, which contains the generated C2C6 double bond of the dienyl substituent coordinated to the ruthenium atom, is 15.7 kcal mol1 more stable than 22t. The transition state TS14 lies 15.1 kcal mol1 above the latter. Complex 23t undergoes an electronic distribution within the dienyl-carbene moiety to afford the isomer 24t, where the resulting organic fragment is coordinated to the ruthenium atom through a RuC5 single bond (dRuC5 = 1.947 Å) and η3-allyl via C2C3C4. The allyl moiety coordinates in an asymmetrical fashion, with the separation between C2 and the metal (2.260 Å) being longer than those between the metal and C3 (2.185 Å) and C4 (2.182 Å). The carboncarbon distances within the allyl unit, 1.442 Å for C2C3 and 1.425 Å for C3C4, agree well with those found by X-ray diffraction analysis for reported allyl complexes.15,22 The isomerization, which has an activation barrier of 8.8 kcal mol1 (TS15), is an exothermic process by 15.7 kcal mol1. The 1936


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Chart 3

Figure 8. Relative energies (ΔG, (ΔE) in kcal mol1) for the transformation of 21t into 17t. Legend: red H, H1; blue H, H2; purple H, H3; green H, H4; [Ru], RuCl(PMe3).

C4C5 coupling in 24t leads to the cyclobutadiene derivative 25t with one of the C4H hydrogen atoms (H3 in Chart 4) involved in an agostic interaction with the metal center (dRuH3 = 1.872 Å).

The C4C5 bond formation takes place through the transition state TS16, with a low energy barrier of 4.1 kcal mol1. Intermediate 25t is 27.2 kcal mol1 more stable than 24t. The 1937


Organometallics

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Chart 4

rupture of the C4H3 bond involved in the agnostic interaction affords the ruthenium(IV) cyclopentadienyl intermediate 26t. The reaction is barrierless and proceeds through the transition state TS17. Complex 26t is 5.6 kcal mol1 more stable than 25t. Finally, the reductive elimination of H3Cl1 from 26t gives 17t. The process occurs in two steps. The chloride ligand Cl1 migrates from the ruthenium atom to the H3 hydride to afford 27t, which dissociates H3Cl1. The first step is at least thermoneutral (þ0.9 kcal mol1). It takes place via the transition state TS18, which lies 6.7 kcal mol1 above 26t and can be described as an Ru(η2HCl) species with a H3Cl1 separation of 1.515 Å and RuH3 and RuCl1 distances of 1.752 and 2.869 Å, respectively. The dissociation of H3Cl1 is an exothermic process by 5.3 kcal mol1. The reaction product 17t is 68.6 kcal mol1 more stable than 22t.

’ CONCLUDING REMARKS This study has revealed that hydride alkenylcarbyne osmium complexes can be prepared by reaction of the starting compound

OsH2Cl2(PiPr3)2 with enynes containing an internal CC triple bond and a terminal CC double bond. Initially the CC triple bond adds both hydride ligands of the starting compound to afford a conjugated diene. Subsequently, the resulting metal fragment OsCl2(PiPr3)2 promotes a 1,4-hydrogen shift through the metal center, from the terminal CH2 group to the internal CC double bond of the diene. The migration generates a five-coordinate alkenylcarbene intermediate. As expected for the osmium preference for coordination saturation and the redox isomers with more metalcarbon bonds, the latter isomerizes into the 18valence-electron hydride alkenylcarbyne product. In accordance with the initial hydrogenation of the CC triple bond, conjugated dienes with a terminal CH2 group are also useful organic fragments to prepare hydride alkenylcarbyne osmium complexes. The ruthenium complex RuH2Cl2(PiPr3)2, in contrast to its osmium counterpart, reacts with the enyne 2-methyl-1-hexen-3yne to give a five-coordinate half-sandwich derivative with a tetrasubstituted cyclopentadienyl ligand, as the main product of the reaction. In this case, the hydrogenation of the enyne leads to 1938


Organometallics an alkenylcarbene intermediate. The addition of the alkenylcarbene ligand to a second enyne molecule generates the tetrasubstituted cyclopentadienyl group. In conclusion, this paper expands the range of useful organic substrates for the synthesis of hydride alkenylcarbyne osmium complexes starting from OsH2Cl2(PiPr3)2 and shows elegant and new evidence of the marked differences in reactivity between the latter and its ruthenium counterpart.

’ EXPERIMENTAL SECTION All reactions were carried out with rigorous exclusion of air using Schlenk-tube or glovebox techniques. Solvents were dried by standard procedures and distilled under argon prior to use. The starting materials OsH2Cl2(PiPr3)2 (1)23 and RuH2Cl2(PiPr3)2 (16)4a were prepared by the published methods. All reagents were obtained from commercial sources. 1H, 31P{1H}, and 13C{1H} NMR spectra were recorded on a Varian Gemini 2000, a Bruker Avance 300 MHz, or a Bruker Avance 400 MHz instrument. Chemical shifts (expressed in parts per million) are referenced to residual solvent peaks (1H, 13C{1H}) or external H3PO4 (31P{1H}). Coupling constants, J and N, are given in hertz. Spectral assignments were achieved by 1H1H COSY, 13C APT, 1 H13C HSQC, and 1H13C HMBC experiments. Attenuated total reflection infrared spectra (ATR-IR) of solid samples were run on a Perkin-Elmer Spectrum 100 FT-IR spectrometer. C and H analyses were carried out in a Perkin-Elmer 2400 CHNS/O analyzer. Highresolution electrospray mass spectra were acquired using a MicroTOFQ hybrid quadrupole time-of-flight spectrometer (Bruker Daltonics, Bremen, Germany). Preparation of OsHCl2{tCC(Me)dCHnPr}(PiPr3)2 (2). A brown suspension of 1 (200 mg, 0.343 mmol) in 7 mL of toluene was treated with 2-methyl-1-hexen-3-yne (94 μL, 0.754 mmol). The mixture was heated at 353 K. After 24 h the solution was filtered through Celite and the solvent was removed in vacuo. The addition of n-pentane to the residue led to a brown solid, which was washed with n-pentane at 243 K and dried in vacuo. Yield: 128 mg (55%). The 31P{1H} NMR spectrum of the reaction mixture shows a conversion of 80%. The low yield is due to the high solubility of this complex in n-pentane. Anal. Calcd for C25H54OsCl2P2: C, 44.23; H, 8.02. Found: C, 44.59; H, 7.71. MS: m/z 643 [M Cl]þ. IR (cm1): ν(OsH) 2145 (m), ν(CdC) 1590. 1H NMR (300 MHz, C6D6, 293 K): δ 6.7 (t, JHH = 7.3, 1H, CH), 2.67 (m, 6H, PCH), 1.61 (s, 3H, tCCCH3), 1.47 (dvt, N = 12.0, JHH = 6.0, 18H, PCHCH3), 1.37 (dvt, N = 12.0, JHH = 6.0, 18H, PCHCH3), 1.35 (dCCH2 overlapped with the CH3 of PiPr3 assigned indirectly by HSQC), 1.03 (m, 2H, CH2CH3), 0.65 (t, JHH = 7.2, 3H, CH2CH3), 6.57 (t, JPH = 16.6, 1H, OsH). 31P{1H} NMR (121.4 MHz, C6D6, 243 K): δ 18.9 (s). 13C{1H}-APT NMR plus HMBC and HSQC (75.4 MHz, C6D6, 293 K): δ 257.5 (t, JCP = 11, tC), 146.6 (s, CH), 146.2 (s, tCC), 31.5 (s, dCCH2), 26.7 (vt, N = 26, PCH), 20.7 (s, dCCH2CH2), 20.1 and 19.9 (both s, PCHCH3), 13.7 (s, CH2CH3), 10.0 (s, tCCCH3). Preparation of OsHCl2{tCC(Me)dCHiPr}(PiPr3)2 (3). A brown suspension of 1 (300 mg, 0.514 mmol) in 7 mL of toluene was treated with 2,4-dimethyl-1,3-pentadiene (150 μL, 1.131 mmol). The mixture was heated at 353 K for 72 h. Then, the solution was filtered through Celite and the solvent was removed in vacuo. The addition of npentane to the residue led to a brown solid, which was washed with npentane at 243 K and dried in vacuo. A GC-MS analysis of an aliquot of the crude reaction mixture showed the presence of 2,4-dimethyl-2pentene. Yield: 260 mg (75%). Anal. Calcd for C25H54OsCl2P2: C, 44.23; H, 8.02. Found: C, 44.28; H, 8.20. MS: m/z 643 [M Cl]þ. IR (cm1): ν(OsH) 2159(m), ν(CdC) 1595. 1H NMR (300 MHz, C6D6, 293 K): δ 6.53 (d, JHH = 10.1, 1H, dCH), 2.65 (m, 6H, PCH), 2.03 (m, 1H, dCCH), 1.52 (s, 3H, tCCCH3), 1.45 (dvt, N = 12.0,

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JHH = 6.0, 18H, PCHCH3), 1.35 (dvt, N = 12.0, JHH = 6.0, 18H, PCHCH3), 0.60 (d, JHH = 6.6, 6H, (CH3)2), 6.61 (t, JPH = 16.5, 1H, OsH). 31P{1H} NMR (121.4 MHz, C6D6, 243 K): δ 18.8 (s). 13 C{1H}-APT NMR plus HMBC and HSQC (75.4 MHz, C6D6, 293 K): δ 257.6 (t, JCP = 11, tC), 153.1 (s, dCHCH), 143.5 (s, tCC), 28.6 (s, dCHCH), 26.7 (vt, N = 26, PCH), 20.1 and 19.9 (both s, PCHCH3), 19.7 (s, (CH3)2), 9.7 (s, tCCCH3).

Reaction of RuH2Cl2(PiPr3)2 with 2-Methyl-1-hexen-3-yne: Preparation of Ru(η5-C5HR1R2R3R4)Cl(PiPr3) (17; R1 = C(CH3)dCH2, R2 = Et, R3 = nPr, R4 = Me). Complex 2 reacted with

2-methyl-1-hexen-3-yne to afford a mixture of complexes, from which complex 17 was isolated as the main product and the carbene derivative RuCl2{dC(Et)CHdCMe2}(PiPr3)2 was also obtained as blue crystals. The cyclopentadienyl derivative was prepared as follows: an orange suspension of 2 (175 mg, 0.354 mmol) in 7 mL of toluene was treated with 2-methyl-1-hexen-3-yne (265 μL, 2.123 mmol). The mixture was stirred for 15 min at room temperature. After this time, the solvent was removed under vacuum. The residue was treated with n-pentane, filtered, and vacuum-dried to give a blue solid. RuCl[(1-{C(CH3)d CH2}-2-(CH2CH3)-3-(CH2CH2CH3)-4-(CH3))cyclopentadienyl]PiPr3 was obtained as the main product in the reaction mixture Yield: 101 mg (59%). Anal. Calcd for C23H42RuClP: C, 56.76; H, 8.71. Found: C, 56.39; H, 8.42. 1H NMR (300 MHz, C6D6, 293 K): δ 5.39 and 5.20 (both s, 2H, dCH2), 3.80 (s, 1H, η5-C5H), 2.78 (m, 2H, 2-(CH2CH3)), 2.65 (m, 2H, 3-(CH2CH2)), 2.10 (m, 3H, PCH), 2.04 (s, 3H, 1-(C(CH3))), 1.45 (m, 2H, 3-(CH2)), 1.20 (s, 3H, 4-(CH3)), 1.09 (dd, JHP = 14.1, JHH = 7.7, 9H, PCHCH3), 1.07 (dd, JHP = 14.5, JHH = 7.7, 9H, PCHCH3), 1.05 (2-(CH2CH3)), overlapped with the CH3 of PiPr3 assigned indirectly by HSQC and HMBC), 0.88 (t, JHH = 7.0, 3-(CH2CH2CH3)). 31P{1H} NMR (121.4 MHz, C6D6, 243 K): δ 55.4 (s). 13C{1H}-APT NMR plus HMBC and HSQC (75.4 MHz, C6D6, 293 K): δ 142.5 (s, C3d), 115.5 (s, dC5H2), 90.6 (d, JCP = 8, C6), 89.7 (d, JCP = 6, C9) 70.1 (s, C2), 68.0 (s, C13), 63.3 (s, C1H), 28.1 (s, C10H2), 25.3 (d, JCP = 17, PCH), 24.5 (s,-C4H3), 23.6 (s, C11H2), 20.6 and 20.3 (both s, PCHCH3), 19.1 (s, C7H2), 15.0 (s, C8H3), 14.8 (s, C12H3), 13.7 (s, C14H3).

Structural Analysis of Complexes 2, 3, 17, and 18. X-ray data were collected on a Bruker Smart APEX CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.710 73 Å). Data were collected over the complete sphere and were corrected for absorption by using a multiscan method applied with the SADABS program.23 The structures were solved by direct methods. Refinements of complexes were performed by full-matrix least squares on F2 with SHELXL97,24 including isotropic and subsequently anisotropic displacement parameters. The disordered groups observed in 2 (phosphines and carbyne ligands) were refined with two moieties, complementary occupancy factors (about 0.7(a):0.3(b)), and isotropic thermal parameters. Crystal data for 2: C25H54Cl2OsP2, Mw 677.72, irregular block, orange (0.10 0.08 0.02 mm), monoclinic, space group P21/c, a = 15.481(5) Å, b = 11.628(4) Å, c = 17.533(5) Å, β = 107.312(5)°, V = 3013.3(16) Å3, Z = 4, Dcalcd = 1.494 g cm3, F(000) = 1376, T = 100(2) K, μ = 4.526 mm1, 22 897 measured reflections (2θ = 358°, ω scans 0.3°), 5921 unique reflections (Rint = 0.1036), minimum/maximum transmission factors 0.613/0.915, final agreement factors R1 = 0.0680 (3761 observed reflections, I > 2σ(I)) and wR2 = 0.1397, 5921/29/246 1939


Organometallics data/restraints/parameters, GOF = 1.068, largest peak and hole 1.949 and 1.541 e/Å3. Crystal data for 3: C25H54Cl2OsP2, Mw 677.72, irregular block, red (0.10 0.08 0.08 mm), monoclinic, space group P21/c, a = 14.8549(8) Å, b = 11.9049(6) Å, c = 17.4551(9) Å, β = 101.6120(10)°, V = 3023.7(3) Å3, Z = 4, Dcalcd = 1.489 g cm3, F(000) = 1376, T = 100(2) K, μ = 4.511 mm1, 36 806 measured reflections (2θ = 358°, ω scans 0.3°), 7450 unique reflections (Rint = 0.0401), minimum/ maximum transmission factors 0.624/0.746, final agreement factors R1 = 0.0227 (6478 observed reflections, I > 2σ(I)) and wR2 = 0.0558, 7450/0/290 data/restraints/parameters, GOF = 1.012, largest peak and hole 0.973 and 0.579 e/Å3. Crystal data for 17: C23H42ClPRu, Mw 486.06, irregular block, blue (0.20 0.08 0.06 mm), monoclinic, space group P21/c, a = 20.9540(17) Å, b = 7.2718(6) Å, c = 16.6303(13) Å, β = 106.7800(10)°, V = 2426.1(3) Å3, Z = 4, Dcalcd = 1.331 g cm3, F(000) = 1024, T = 100(2) K, μ = 0.827 mm1, 28 679 measured reflections (2θ = 358°, ω scans 0.3°), 5925 unique reflections (Rint = 0.0541), minimum/ maximum transmission factors 0.730/0.862, final agreement factors R1 = 0.0350 (4836 observed reflections, I > 2σ(I)) and wR2 = 0.0824, 5925/0/245 data/restraints/parameters, GOF = 1.060, largest peak and hole 0.871 and 0.500 e/Å3. Crystal data for 18: C25H54Cl2P2Ru, Mw 588.59, irregular block, violet (0.10 0.06 0.02 mm), triclinic, space group P1, a = 8.4833(15) Å, b = 9.5113(17) Å, c = 19.066(4) Å, R = 82.446(3)°, β = 87.096(4)°, γ = 76.442(4)°, V = 1482.3(5) Å3, Z = 2, Dcalcd = 1.319 g cm3, F(000) = 624, T = 100(2) K, μ = 0.828 mm1, 13 845 measured reflections (2θ = 358°, ω scans 1.0°), 6958 unique reflections (Rint = 0.0510), minimum/maximum transmission factors 0.708/0.993, final agreement factors R1 = 0.0498 (5162 observed reflections, I > 2σ(I)) and wR2 = 0.1008, 6958/0/289 data/restraints/parameters, GOF = 0.995, largest peak and hole 0.692 and 0.551 e/Å3. Computational Details. The theoretical calculations were carried out on the model complexes by optimizing the structures at the b3pw91DFT levels with the Gaussian 03 program.25 The basis sets used were the LANL2DZ basis and pseudopotentials for Os and 6-31G(d,p) for the rest of the atoms. The transition states were found by carrying out potential energy surfaces of the processes following the reaction coordinates, optimizing the maxima, and confirming by frequency calculations (only one negative). The connections between the starting and final reactants were checked by slightly perturbing the TS geometry toward the minima geometries and reoptimizing. The numerical values shown in the figures correspond to calculated Gibbs energies (1 atm, 298.15 K). The relative energies (ΔG, ΔE) for 22t were calculated using the energies calculated for free phosphine and enyne ligands.

’ ASSOCIATED CONTENT Supporting Information. CIF files, figures, text, and tables giving computational details, orthogonal coordinates of theoretical structures, complete ref 26, details of the crystal structure determination, and crystal data for compounds 2, 3, 17, and 18. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the Spanish MICINN (Project numbers CTQ2008-00810 and Consolider Ingenio 2010 CSD2007-00006),

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the DGA (E35), and the European Social Fund is acknowledged. A.C. thanks the CSIC for her JAE grant.

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