Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 3. Formation and Cleavage of C−C and C−O Bonds in the Reactions of the Complexes [Mo₂Cp₂(µ-COMe)(µ-COR)(µ-PCy₂)]BF₄(R = Me, Et)

Abstract

The reactions of the 30-electron complexes [Mo 2 Cp 2 (µ-COMe)(µ-COR)(µ-PCy 2 )]BF 4 (Cp ) η 5 -C 5 H 5 ; R ) Me, Et) with CO, CN t Bu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes, and [Mo 2 Cp 2 {µ-η 2 :η 2 -C(OMe)C(OMe)}(µ-PCy 2 )(µ-tedip)]BF 4 , respectively. These products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment. The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths (ca. 2.66 Å), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction of short intermetallic distances (2.75 Å). Demethylation of the dicarbonyl complexes leads to the new carboxycarbyne derivatives [Mo 2 Cp 2 {µ-C(CO 2 R)}(µ-PCy 2 )(CO) 2 ] (R ) Me, Et) as a result of the unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably formed first. This proposal is supported by DFT calculations, these predicting a moderate activation barrier (∆G q 298 ) 78 kJmol -1 ) for the spontaneous transformation (∆G 298 ) -51 kJ mol -1 ) of the ketenyl intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive toward the 16-electron fragment Ru(CO) 4 , to give the 46-electron cluster [Mo 2 RuCp 2 (µ-COMe) 2 (µ-PCy 2 )(CO) 4 ]BF 4 , which concentrates most of its electronic unsaturation at the dimolybdenum center (Mo-Mo ) 2.686(1) Å), in spite of the rearrangement of the carbyne ligands, both of them now bridging one of the Mo-Ru edges, according to an X-ray diffraction study.

C−C Bond Formation between the μ‐Alkylidyne Ligands in a Diruthenium Bis(μ‐alkylidyne) Complex; σ‐ and π‐Aromaticity of the Ru2C2 Core

“…Characteristic 13 C NMR resonances for the μ-alkylidyne carbon atoms were observed at δ 362.8 ppm for 2 a and at δ 363.5 and 369.7 ppm for the μ-CMe and μ-CEt ligands in 2 b, respectively; these are comparable to the resonances observed for 3 (δ 365.1). [18] These signals appeared to be considerably shielded in comparison to those of the Zn adduct 1 b (δ 383.9 and 388.4 ppm), [17] suggesting that the electron density at the Ru 2 C 2 core in 2 b increased with the liberation of the Lewis acidic ZnCl 2 moiety.…”

“…The increase in two electrons upon coordination with CO was offset by the transformation of the two μ-alkylidyne ligands into the μ-η 2 :η 2 -alkyne ligand. Coupling between two μ 3 -alkylidyne ligands in response to chemical reduction has been demonstrated for a tetrairon complex, [{(C 5 H 4 Me)Fe} 4 (μ 3 -CH) 2 (μ 3 -CO) 2 ] 2 + , by Okazaki et al [41] Ruiz and co-workers demonstrated that the bis(μ-alkylidyne) complexes of molybdenum, [(CpMo) 2 (μ-COMe) 2 (μ-PCy 2 )] + and [(CpMo) 2 (μ-CPh)(μ-COMe)(μ-PCy 2 )] + , underwent reversible carbyne-carbyne coupling via coordination and decoordination of CO. [12][13][14] Unlike the dimolybdenum complex, the CÀ C bond of the μ-η 2 :η 2 -alkyne ligand in 6 a was not broken upon irradiation, presumably because the μ-CO ligand was unreactive to photoirradiation.…”

“…1 H NMR spectra were referenced to tetramethylsilane as an internal standard. 13 C NMR spectra were referenced to the natural-abundance carbon signal of the solvent employed. IR spectra were recorded on a JASCO FT/IR-4200 spectrophotometer.…”

“…On the other hand, Ruiz and co-workers elucidated that bis (μ-alkoxycarbyne) complexes [Cp 2 Mo 2 (μ-COMe)(μ-COR)(μ-PCy 2 )] + (Cp=C 5 H 5 ; R=Me, Et; Cy = cyclohexyl), which contain an additional μ-PCy 2 ligand, undergo CÀ C bond formation between the μ-alkylidyne ligands, along with the incorporation of two 2electron donors. [12,13] The group also exhibited the μ-COR/μ-CPh (R=Me, H) coupling in [Cp 2 Mo 2 (μ-CPh)(μ-COR)(μ-PCy 2 )] + (R=Me, H). [14] Due to the μ-phosphanido ligand, the Mo 2 C 2 core adopted a puckered structure (dihedral angle,~130°).…”

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…27) that could be found, contain metalloid atoms rather than transition metals bonded to the carbene carbon (Scheme 27, 72) [124,125]. The only other relevant complexes, resembling the type of complex where the second transition metal is directly bonded to the heteroatom-substituted carbene carbon atom, are once again older examples of bridging carbyne complexes, albeit carbyne carbon atoms substituted by heteroatoms such as oxygen, nitrogen and sulphur [126][127][128]. …”