Preparation of pyranylidene complexes of ruthenium †

The reaction of the chloro-complex RuCl(η-C5H5)(PPh3)[P(OMe)3] with alkylpropiolates HCuCCOOR1 in alcohol R2OH affords pyranylidene derivatives [Ru(η-C5H5){vC(COOR1)C(H)C(H)C(OR1)O}(PPh3){P(OMe)3}]BPh4 (1, 3) and alkoxycarbene complexes [Ru(η-C5H5){vC(OR2)(CH2COOR1)}(PPh3){P(OMe)3}]BPh4 (2, 4). A reaction path for the formation of compounds 1–4, involving reactions on a vinylidene intermediate complex, is also discussed. The complexes were characterized spectroscopically (IR and H, C, P NMR) and by X-ray crystal structure determination of [Ru(η-C5H5){vC(COOMe)C(H)C(H)C(OMe)O}(PPh3){P(OMe)3}]BPh4 (1).


Introduction
3][4][5] This fact is somewhat surprising, given the increasing interest in their use in ring-opening reactions, 6 Diels-Alder reactions 4b,7 and 1,6 addition.4a Several methods for the synthesis of pyranylidene have been developed, mainly including the reaction of preformed carbene complexes with pyridinium ylides, 2b,c enol ethers 8 and 1,3-diketones, 3 or multicomponent species 9 such as alkynyl esters and N-methylmorpholine-N-oxide.Alternatively, pyranylidene complexes can be prepared from the reaction of M(CO) 5 L (M = Cr, Mo, W; L = THF, NEt 3 ) with β-alkynyl α,β-unsaturated carbonyl compounds 4 and from the reaction of M(CO) 6 with 1-lithio-1,3-dienes. 5 However, in only one case the simplest method for preparing a pyranylidene complex, involving dimerization of alkylpropiolate HCuCCOOR on a metal fragment, has been reported. 9e were interested in the reactivity of half-sandwich complexes containing phosphite ligands 10 and, in the course of our studies, found that the reaction of the complex RuCl(η 5 -C 5 H 5 )(PPh 3 )[P(OMe) 3 ] with alkylpropiolate leads to the first pyranylidene complexes of ruthenium.Our results, including the synthesis and characterization of pyranylidene and alkoxycarbene complexes of Ru(II), are reported here.
Crucial for successful syntheses was the use of alcohol containing the salt NaBPh 4 as a solvent, which probably favors substitution of the chloride ligand in the starting complex, yielding the two types of complexes 1, 3 and 2, 4.
The formation of both pyranylidene and alkoxycarbene derivatives in the reaction of RuCl(η 5 -C 5 H 5 )(PPh 3 )[P(OMe) 3 ] with alkylpropiolate is somewhat surprising, but may be explained according to the reaction path shown in Scheme 2, which involves the initial formation of a vinylidene intermediate [A].Reaction of this intermediate with alkylpropiolate gives rise to the dimerization of HCuCCOOR1, affording pyranylidene derivatives 1, 3. Parallel nucleophilic attack by the oxygen atom of R2OH on the Cα of the vinylidene, followed by proton-transfer, yields the final alkoxycarbene derivatives 2, 4.
In order to verify this reaction path, we attempted to isolate the vinylidene intermediate [A] or, at least, to identify it in the reaction mixture.At first, we treated the compound RuCl(η 5 -C 5 H 5 )(PPh 3 )[P(OMe) 3 ] with HCuCCOOR1, in the presence of NaBPh 4 , in a solvent other than alcohol such as dichloromethane, but no reaction was observed, probably owing to the insolubility of NaBPh 4 in this solvent.We therefore used a different strategy, involving a reaction of the starting chlorocomplex first with silver triflate, to form the triflate intermediate Ru(κ 1 -OTf )(η 5 -C 5 H 5 )(PPh 3 )[P(OMe) 3 ], and then with an excess of methylpropiolate in dichloromethane as a solvent (Scheme 3).
The triflate complex quickly reacted with methylpropiolate by changing color in the solution, from which we were not able to isolate a solid, but only an oily product.Its 13 C NMR spectrum showed a doublet of doublets at 199.71 ppm ( J CP = 15.9,J CP = 11.6 Hz), which might be attributed to the Cα carbon atom of a vinylidene species [A].However, this value is at lower frequency than those observed for the Cα of several known vinylidene derivatives. 11As suggested by a reviewer, this resonance may be attributed to the Cα of the vinyl species [Ru(η 5 -C 5 H 5 ){C(OTf )vC(H)COOMe}(PPh 3 )[P(OMe) 3 ] [C], formed by nucleophilic attack of the triflate ion on the carbene carbon atom of the vCvC(H)COOMe ligand.In an HMBC experiment, the 13 C signal at 199.7 ppm is correlated with the multiplet at 5.47 ppm of the 1 H spectrum, attributable to the vC(H)COOMe of the vinyl ligand, fitting the proposed formulation for [C].The triflate ligand is labile in the complex Ru(κ 1 -OTf )(η 5 -C 5 H 5 )(PPh 3 )[P(OMe) 3 ] and can be substituted by alkyne, which then tautomerizes 11 on the metal center to give vinylidene intermediate [A].Reaction with the triflate ion OTf − can give the vinyl intermediate [C], which may be in equilibrium with [A] (Scheme 3).DFT calculations on model systems, where P-donor ligands are replaced by PH 3 and PF 3 , support this hypothesis, the ΔH difference between [Ru(η 5 -C 5 H 5 ){C(OTf )vC(H)COOMe}(PH 3 )(PF 3 )] and [Ru(η 5 -C 5 H 5 ){vCvC(H)COOMe}(PH 3 )(PF 3 )]OTf being only about 1.2 kcal mol −1 in favor of the latter.The fact that vinylidene [A] and/or vinyl [C] complexes are really intermediates of the reaction path proposed in Scheme 2 was confirmed by treatment with ethanol, which gave ethoxycarbene 2b as the final product, the addition of methanol affording methoxycarbene 2a.Instead, addition of alkylpropiolate to vinylidene intermediate [A] only yielded traces of the pyranylidene complex, and it was only with the addition of alcohol to the solution of [A] that the reaction started, affording a mixture of pyranylidene 1 and alkoxycarbene 2 complexes.However, the greater amount of carbene 2 with respect to pyranylidene (4 : 1 ratio) suggests a faster reaction of vinylidene [A] with alcohol than that with propiolate.
The need for alcohol as a solvent is explained by the fact that the formation of pyranylidene from the reaction of vinylidene [A] with alkylpropiolate must involve a hydrogen shift, which may be a proton transfer strongly favored by protic solvents such as alcohols (Scheme 4).
In fact, plausible mechanisms for the formation of the pyranylidene probably involve cyclization reactions between an alkylpropiolate molecule and coordinated vinylidene or η 2alkyne, affording intermediates [B1] ( path 1) or [B2] ( path 2), respectively.Subsequent hydrogen transfers in these intermediates gave the final pyranylidene complexes 1, 3. DFT calculations on a simplified system, in which [Ru] = [Ru(η 5 -C 5 H 5 ) (PH 3 )(PF 3 )] + and R1 = Me (see ESI †), indicated the formation of intermediate [B2], which is about 20.7 kcal mol −1 more stable than [B1].The final formation of the pyranylidene derivative is strongly favored, as its Gibbs energy is lower than that of [B2] by about 53.8 kcal mol −1 .DFT calculations ruled out intramolecular pathways for the hydrogen shift from [B2] to the final product, the estimated energy barrier being unreasonably high.On the other hand, the computed proton affinity of [B2], 232 kcal mol −1 , enforces the hypothesis that the hydrogen atom could migrate by acid-base equilibria with the solvent. 12ath 2 requires preliminary tautomerization of vinylidene to η 2 -alkyne, but the energy difference between these species is quite low, about 1.5 kcal mol −1 in favor of the vinylidene complex.Equilibrium mixtures containing η 1 -vinylidene and η 2 -alkyne complexes have previously been reported. 13However, the greater stability of vinylidene may partially explain the competitive reaction with alcohols and the formation of the corresponding carbenes as prevailing species.
The reaction path involving [B2] was more deeply investigated from a computational point of view.It should be noted that, for the previously reported multicomponent syntheses of Cr(0) and W(0) pyranylidene derivatives 9 from the reaction of carbene complexes with alkylpropiolate in the presence of NMO (N-methylmorpholine-N-oxide), the proposed mechanism involved the formation of a propadienylidene intermediate of the type [M]vCvCvC(O − ) (OR1), the reaction of which with alkylpropiolate yielded the final pyranylidene complexes.In our case, transformation of vinylidene intermediate [A] in a propadienylidene species, like that proposed for Cr(0) and W(0), is improbable owing to the absence of a base.
Nucleophilic attack of alcohol on the Cα carbon of ruthenium vinylidene complexes affording alkoxycarbene derivatives has previously been reported with unsubstituted [Ru]v CvCH 2 compounds. 14The reaction of our mixed-ligand halfsandwich derivatives with alkylpropiolate highlights a new example of the formation of alkoxycarbene species.
Good analytical data were obtained for both pyranylidene 1, 3 and alkoxycarbene 2, 4 derivatives, which were isolated as yellow-orange solids stable in air and in solution of polar organic solvents, where they behave as 1 : 1 electrolytes. 15nfrared and NMR data support the proposed formulations,  1.The overall geometry of the half-sandwich pianostool complex is octahedral, as demonstrated by near 90°v alues for angles P-Ru-P and P-Ru-CO, between 88.26(9) and 96.56(3)°.The larger angle is that of P-Ru-P.The η 5 -coordination mode of the Cp ligand shows Ru-C bond distances between 2.241(3) and 2.265(4) Å.The average value for Ru-C bond lengths [2.257(4) Å] is slightly longer than the usual ones for CpRu moieties, 16 and the Ru-Ct distance [1.9136(2) Å] is also longer than that found, for example, at 1.9056(5) Å in Ru(Cp)(GeH 3 )[P(OMe) 3 ](PPh 3 ). 17he Ru-P bond distances [2.3244(8) Å for Ru-PPh 3 and 2.2413(8) Å for Ru-P(OMe) 3 ] are similar to those previously found in other phosphite-phosphine Ru(II) compounds, e.g., 2.2945(16) and 2.1933 (19) Å in Ru(Cp)(GeH 3 )[P(OMe) 3 ](PPh 3 ) 17 and 2.3113(4) and 2.2239(5) Å in Ru(Cp)Cl[P(OMe) 3 ](PPh 3 ). 18horter Ru-P bonds correspond, as usual, to the phosphite ligand. 19he pyran-2-ylidene ligand is bonded to the ruthenium metal with a Ru-C distance of 2.018(3) Å.This is shorter than the usual range for Ru-C(sp 2 ) single bonds [2.03-2.11Å] 20,21 although slightly longer than that found for other Fischer-type carbene complexes. 22The pyran-2-ylidene ring is essentially planar (rms of 0.016 Å).Distances in the ring (see Table 1) indicate a formal (although short) single bond between C(1) and O(3), 1.392(4) Å, and between C(1) and C(2), 1.402(5) Å. Electronic delocalization takes place in the rest of the ring, including the O(3)-C(5) bond, with values between 1.330(4) and 1.384(7) Å. 9 The methoxycarbonyl substituent on the pyranylidene ligand is almost perpendicular [dihedral angle between planes of 86.9(2)°] to the plane of the rings, proving the single-bond character of the C(2)-C(6) bond, with a length of 1.498(6) Å.The IR spectra of pyranylidene complexes 1, 3 show a medium-intensity band at 1731-1733 cm −1 , attributed to the ν CO of the ester substituent COOR1.Besides signals of the ancillary ligands and BPh 4 anions, the 1 H NMR spectra show an AB quartet at 7.06 and 5.44 ppm for 1 and at 7.14 and 5.41 ppm for 3 of the H3 and H4 protons of pyranylidene and the signals of methyl or ethyl groups of the COOR1 and OR1 substituents at C2 and C5 atoms, respectively.The methyls of 1 appear as singlets at 3.89 and 3.48 ppm, and the ethyls of 3 as quartets at 4.42 and 4.17 and triplets at 1.02 and 1.26 ppm.However, strong support for the presence of the pyranylidene ligand comes from the 13 C NMR spectra, which, at −30 °C, show the characteristic carbene carbon resonance as a broad multiplet at 332 ppm.The spectra show the signals of the C2, C3, C4 and C5 atoms of the heterocycle ligand between 172 and 94 ppm, as well as those of the COOR1 and OR1 substituents, fitting the proposed formulation for pyranylidene complexes.
The NMR proton spectra of alkoxycarbene complexes [Ru-(η 5 -C 5 H 5 ){vC(OR2)(CH 2 COOR1)}(PPh 3 ){P(OMe) 3 }]BPh 4 (2, 4) show an AB quartet at 4.59-3.89ppm of the diastereotopic CH 2 protons of the CH 2 COOR1 group of carbene and the signals of the substituents R1 and R2, which appear as singlets for methyls and quartets and triplets for ethyls.However, diagnostic of the presence of alkoxycarbene vC(OR2)CH 2 COOR1 are the 13 C NMR spectra, which show a doublet of doublets at 297.67-297.21ppm, attributed to carbene carbon resonance.Singlets at 61.01 for 2a, 61.42 for 2 and 62.85 ppm for 4a also appear in the spectra which, in an HMQC experiment, were correlated with the AB quartets at 4.59-3.89ppm in the proton spectra and were attributed to the methylene carbon resonance of the CH 2 COOR1 group.The signals of the COOR1 and OR2 substituents were also observed, whereas the 31 P NMR spectra are AB quartets, matching the proposed formulation.

Conclusions
This paper reports the first pyran-2-ylidene carbene complexes of ruthenium prepared through dimerization of alkylpropiolate on a half-sandwich fragment.An alkoxycarbene derivative was also obtained.A reaction path for the formation of the pyranylidene and ethoxycarbene complexes, involving a vinylidene intermediate, is proposed.

Materials and physical measurements
All synthetic work was carried out under an appropriate atmosphere (Ar, N 2 ) by standard Schlenk techniques or in an inert atmosphere dry-box.All solvents were dried over appropriate drying agents, degased on a vacuum line, and distilled in vacuum-tight storage flasks.RuCl 3 •3H 2 O was a Pressure Chemical Co.(USA) product; other reagents were purchased from commercial sources at the highest available purity and were used as received.Infrared spectra were recorded on a Perkin-Elmer Spectrum-One FT-IR spectrophotometer.NMR spectra ( 1 H, 13 C, 31 P) were obtained on an AVANCE 300 Bruker spectrometer at temperatures between −90 and +25 °C, unless otherwise noted. 1 H and 13 C spectra are referred to internal tetramethylsilane. 31P{ 1 H} chemical shifts are reported with respect to 85% H 3 PO 4 , with downfield shifts considered positive.COSY, HMQC and HMBC NMR experiments were performed using standard programs.The iNMR software package 23 was used to treat NMR data.The conductivity of 10 −3 mol dm −3 solutions of the complexes in CH 3 NO 2 at 25 °C was measured on a radiometer CDM 83.Elemental analyses were performed in the Microanalytical Laboratory of the Dipartimento di Scienze del Farmaco, University of Padova (Italy).Melting points (m.p.) were determined in capillary on Büchi 535 apparatus.

Crystal structure determination
Crystallographic data for 1 were collected at room temperature using a Bruker Smart 6000 CCD detector and Cu-Kα radiation (λ = 1.54178Å) generated by an Incoatec microfocus source equipped with Incoatec Quazar MX optics.The software APEX2 25 was used for collecting frames of data, indexing reflections and the determination of lattice parameters, SAINT 25 was used for integration of the intensity of reflections, and SADABS 25 was used for scaling and empirical absorption correction.The crystallographic treatment was performed with the Oscail program. 26The structure was solved by Patterson methods and refined by full-matrix least-squares based on F 2 . 27Non-hydrogen atoms were refined with anisotropic displacement parameters.Hydrogen atoms were included in idealized positions and refined with isotropic displacement parameters.Figures were drawn with ORTEP-3 for Windows. 28etails of crystal data and structural refinement are given in Table 2. CCDC 1046060 contains the supplementary crystallographic data for this paper.

Computational details
The computational geometry optimization of the complexes was carried out without symmetry constraints, using the hyper-GGA M06 functional 29 in combination with a polarized triple-ζ quality basis set composed of the 6-311G(d,p) set on the light atoms and the ECP-based LANL2TZ(f ) set on the ruthenium centre. 30The "restricted" formalism was applied 31 and the CPCM implicit solvation model for ethanol was added to all the calculations. 32All the stationary points were characterized by IR simulation (harmonic approximation), from which thermodynamic data were computed, considering T = 298.15K. 31 The software used was Gaussian 09. 33Preliminary geometry optimizations and proton affinity calculations were performed in vacuo with the hybrid DFT functional EDF2 34 and the polarized double-ζ quality basis set LACVP** (LANL2DZ ECP on ruthenium). 35The software used for EDF2 calculations was Spartan 08. 36All the computational optimizations were performed on an Intel-based x86-64 workstation.
A coordinate-driving study based on the progressive elongation of the C(2)-C(3) bond of [B2] (see Scheme 4 for numbering) allowed us to find a possible transition-state geometry, which was then fully optimized and characterized by IR simulations (see the ESI † xyz file for details of the atomic coordinates).The activation Gibbs free energy (referred to [Ru]-η 2 -HCuCH + methylpropiolate) was 32.7 kcal mol −1 , as depicted in Scheme 5. Quite interestingly, the only imaginary frequency (i340 cm −1 ) of the transition state mainly concerns the formation of the C(2)-C(3) bond from [Ru]-η 2 -HCuCH and methylpropiolate.The carbon-oxygen bond of the six-membered heterocycle is instead not yet present in the transition state, the O(3)⋯C(1) distance being about 2.5 Å.This result suggests that the carbon-oxygen interaction occurs after the C(2)-C(3) bond formation, in a secondary step having low activation energy.This idea was supported by a further coordinate-driving simulation based on the progressive elongation of the O(3)-C(1) bond of [B2], which did not lead to any C-C bond break.