Improvement of the activity, stability, and chemoselectivity of alkyne-metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBu ) ] (1, Mes=2,4,6-trimethylphenyl, OtBu =hexafluoro-tert-butoxy) onto partially dehydroxylated silica gave a well-defined silica-supported active alkyne-metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBu ) ] (1/SiO ). Both 1 and 1/SiO showed very high activity, selectivity, and stability in the self-metathesis of a variety of carefully purified alkynes, even at parts-per-million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.

The reaction of dipiperidinoacetylene (pipC Cpip, pip = NC 5 H 10 , 1a) with [Cp 2 Ti(η 2 -btmsa)] (2) or with [Cp 2 Zr(η 2 -btmsa)(py)] (4) (btmsa = bis(trimethylsilyl)acetylene, py = pyridine) afforded the metallacyclopentadienes [Cp 2 M(C 4 pip 4 )] (3, M = Ti; 5, M = Zr), which in the solid state exhibit twisted five-membered metallacycles with an unusual half-chair conformation. In contrast, the sterically more demanding decamethyltitanocene (Cp* 2 Ti) and -zirconocene (Cp* 2 Zr) complex fragments can only accommodate one alkyne ligand. Thus, the titanacyclopropene [Cp* 2 Ti(C 2 pip 2 )] ( 7) was isolated from the reaction of 1a with [Cp* 2 Ti(η 2 -btmsa)] ( 6) or with [Cp* 2 TiCl] in the presence of magnesium, whereas the zirconacyclopropenes [Cp* 2 Zr(C 2 X 2 )] (8a, X = pip; 8b, X = NC 5 H 9 -4-Me; 8c, X = NEt 2 ) were prepared by the reduction of [Cp* 2 ZrCl 2 ] with magnesium in the presence of 1a, bis(4-methylpiperidino)acetylene (1b), and bis(diethylamino)acetylene (1c), respectively. NMR studies showed that complexes 8 are in equilibrium with their tucked-in tetramethylpentafulvene−diaminovinyl isomers [Cp*(η 6 -C 5 Me 4 CH 2 )Zr(CX=CHX)] (9) in solution, which are formed by intramolecular C−H-bond activation and hydrogen transfer from one Cp* methyl group to the alkyne ligand. Thermodynamic and kinetic parameters were derived by variabletemperature NMR spectroscopy and DFT experiments. The molecular structures of 3, 5, 7, 8a, [8a•MgCl 2 ] 2 , 8b, and 8c were established by X-ray diffraction analyses.

The indenyl effect has been introduced to pentadienyl ("open cyclopentadienyl") chemistry by preparation of the phenylmethallyl ("open indenyl") ligand oInd(Me). The reaction of its potassium salt K(oInd(Me)) with [(η(5)-C(5)Me(5))RuCl](4) afforded the sandwich complex [(η(5)-C(5)Me(5))Ru(η(5)-oInd(Me))] (1), which, upon treatment with PMe(3), CO, and 2,6-dimethylphenyl isocyanide (CN-o-Xy), easily underwent η(5)-η(3) hapticity interconversion and formed the complexes [(η(5)-C(5)Me(5))Ru(η(3)-oInd(Me))(L)] (2, L = PMe(3); 3, L = CO; 4, L = CN-o-Xy). In these complexes, the η(3)-bound phenylmethallyl ligand adopts an anti-conformation with regard to the relative positions of the phenyl and methyl substituents. For the PMe(3) complex anti-2, slow conversion to the syn-isomer was observed, and this equilibrium reaction was monitored by NMR spectroscopy at 50 °C to determine a first order rate constant of k(323 K) = 6.57 × 10(-6) (± 0.02 × 10(-6)) s(-1) and an activation barrier of ΔG° = 26.8 kcal mol(-1). DFT calculations afforded a stabilization of syn-2 and syn-3 by ΔG(298) = -1.54 and -1.74 kcal mol(-1) over the respective anti-isomer.

The reactions of the molybdenum alkylidyne complex [MesC[triple bond, length as m-dash]Mo{OCMe(CF)}] (1) with the diaminoacetylenes RNC[triple bond, length as m-dash]CNR (2, NR = 4-methylpiperidinyl; 3, NR = NEt; Mes = 2,4,6-trimethylphenyl) afforded the metallacyclobutadiene (MCBD) complexes 4 and 5. In contrast to all other MCBD complexes, 4 and 5 are paramagnetic and best described as Mo(iv) species containing an anionic diaminodicarbene of the type [(RN)CC(Mes)C(NR)].

Improvement of the activity, stability, and chemoselectivity of alkyne‐metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBuF6)3] (1, Mes=2,4,6‐trimethylphenyl, OtBuF6=hexafluoro‐tert‐butoxy) onto partially dehydroxylated silica gave a well‐defined silica‐supported active alkyne‐metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBuF6)2] (1/SiO2‐700). Both 1 and 1/SiO2‐700 showed very high activity, selectivity, and stability in the self‐metathesis of a variety of carefully purified alkynes, even at parts‐per‐million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1/SiO2‐700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1/SiO2‐700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.