Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH)(CF)}] (MoF, x = 0; MoF, x = 1; MoF, x = 2; MoF, x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF < MoF < MoF before sharply decreasing for MoF, with a similar effect for the supported systems (MoF ≈ MoF < MoF < MoF). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π*(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF, prior to a sharp decrease in reactivity for MoF due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their C NMR chemical shift tensors.

The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesCM{OC(CF 3 ) n Me 3−n } 3 ] (M = Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M = W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared by the reaction of the tribromides [MesCMBr 3 (dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC(CF 3 ) n Me 3−n . The molecular structures of all complexes were established by X-ray diffraction analysis. The catalytic activity of the resulting alkylidyne complexes in the homometathesis and ring-closing alkyne metathesis of internal and terminal alkynes was studied, revealing a strong dependency on the fluorine content of the alkoxide ligand. The different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne. Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C 3 Et 3 )Mo{OC(CF 3 ) 3 } 3 ], which was characterized spectroscopically.

Although alkyne metathesis has been known for 50 years, rapid progress in this field has mostly occurred during the last two decades. In this article, the development of several highly efficient andt horoughly studied alkyne metathesis catalysts is reviewed, which includes novel welldefined, in situ formed and heterogeneous systems. Various alkyne metathesis methodologies, including alkyne crossmetathesis (ACM), ring-closing alkyne metathesis (RCAM), cy-clooligomerization,a cyclic diyne metathesis polymerization (ADIMET), and ring-opening alkyne metathesis polymerization (ROAMP), are presented, and their application in natural product synthesis, materials science as well as supramolecular and polymer chemistry is discussed. Recent progress in the metathesis of diynes is also summarized, which gave rise to new methods such as ring-closing diyne metathesis (RCDM)a nd diyne cross-metathesis(DYCM). Scheme1.Mechanism of alkyne metathesis.Scheme2.Synthesis of Schrock's tungsten alkylidyne complex 3.[a] H. Ehrhorn, Prof. Dr.M.T amm Scheme9.Synthesis of molybdenumb enzylidynecomplexes 22 with triphenylsilanolate ligands ;phen = 1,10-phenanthroline.Scheme10. Synthesis of trisilanolate tungsten and molybdenum benzylidyne complexes 25.

The molybdenum and tungsten 2,4,6-trimethylbenzylidynes [MesCM{OC(CF 3 ) 2 Ph} 3 ] (12a, M = Mo; 12b, M = W) were prepared and structurally characterized as related complexes to already known [MesC M{OC(CF 3 ) 2 Me} 3 ] (MoF6, M = Mo; WF6, M = W). While treatment of 12a with 3-hexyne yielded the propylidyne complex [EtCMo{OC(CF 3 ) 2 Ph} 3 ] (13), the tungsten congener 12b formed isolable metallacyclobutadiene (MCBD) species 14−16 by reaction with 3-hexyne, 1-phenyl-1-propyne, and 2,4-hexadiyne, which can be correlated with the higher electrophilicity of the tungsten complex. Furthermore, the labile MCBD [(C 3 Et 3 )Mo{OC(CF 3 ) 2 Me} 3 ] ( 17) was isolated at low temperature from the reaction of the highly active MoF6 catalyst with 3-hexyne and could be characterized by X-ray diffraction analysis. At room temperature, the same reaction afforded [EtCMo{OC(CF 3 ) 2 Me} 3 ] ( 18), and the equilibrium reaction with 3-hexyne to form 17 was additionally studied by variable temperature NMR spectroscopy, which allowed determining ΔH°and ΔS°for the formation of MCBD 17. The experimental thermodynamic data were used to set the benchmark for DFT calculations. Moreover, the deprotiometallacyclobutadiene complex (DPMCBD) [{C 3 (Mes)(Ph)}Mo-{OC(CF 3 ) 3 } 2 ] (19), prepared from [MesCMo{OC(CF 3 ) 3 } 3 ] (MoF9) and phenylacetylene, was isolated and structurally characterized as a decomposition product of terminal alkyne metathesis and employed in the polymerization of phenylacetylenes.

The molybdenum and tungsten complexes M2(OR)6 (Mo2F6, M = Mo, R = C(CF3)2Me; W2F3, M = W, R = OC(CF3)Me2) were synthesized as bimetallic congeners of the highly active alkyne metathesis catalysts [MesC≡M{OC(CF3)nMe3− n}] (MoF6, M = Mo, n = 2; WF3, M = W, n = 1; Mes = 2,4,6-trimethylphenyl). The corresponding benzylidyne complex [PhC≡W{OC(CF3)Me2}] (W Ph F3) was prepared by cleaving the W≡W bond in W2F3 with 1-phenyl-1-propyne. The catalytic alkyne metathesis activity of these metal complexes was determined in the self-metathesis, ring-closing alkyne metathesis and cross-metathesis of internal and terminal alkynes, revealing an almost equally high metathesis activity for the bimetallic tungsten complex W2F3 and the alkylidyne complex W Ph F3. In contrast, Mo2F6 displayed no significant activity in alkyne metathesis.