The benzylidyne complex [PhC≡W{OSi(OtBu) } ] (1) catalyzed the cross-metathesis between 1,4-bis(trimethylsilyl)-1,3-butadiyne (2) and symmetrical 1,3-diynes (3) efficiently, which gave access to TMS-capped 1,3-diynes RC≡C-C≡CSiMe (4). Diyne cross-metathesis (DYCM) studies with C-labeled diyne PhC≡ C- C≡CPh (3*) revealed that this reaction proceeds through reversible carbon-carbon triple-bond cleavage and formation according to the conventional mechanism of alkyne metathesis. The reaction between 1 and 3* afforded the 3-phenylpropynylidyne complex PhC≡ C- C≡W{OSi(OtBu) } ] (5*), indicating that alkynylalkylidyne complexes are likely to act as catalytically active species. Attempts to isolate 5* from mixtures of 1 and 3* afforded crystals of the ditungsten 2-butyne-1,4-diylidyne complex [(tBuO) SiO} W≡ C- C≡ C- C≡W{OSi(OtBu) } ] (6*), which was additionally characterized by X-ray diffraction analysis. Depolymerization-macrocyclization of a carbazole-butadiyne polymer, obtained from 3,6-diethynyl-9-dodecylcarbazole (7) under copper-catalyzed Hay coupling conditions, was also efficiently catalyzed by 1 and afforded a mixture of mono-, diyne- and triyne-containing tetrameric macrocycles, revealing that diyne disproportionation into monoynes and triynes occurs as a slow side reaction that interferes with a high diyne metathesis selectivity. Potential catalytic pathways were studied by means of quantum-chemical calculations, and kinetic studies were performed to substantiate an α,α-mechanism for the catalytic diyne metathesis reaction, which involves intermediate alkynylalkylidyne and α,α'-dialkynylmetallacyclobutadiene intermediates.

Several rare-earth-metal complexes involving both imidazolin-2-iminato and dicarbollide ligands were prepared by the reaction of imidazolin-2-iminato rare-earth-metal dichlorides [(ImArN)MCl2(THF)3] with 1 equiv of Na2[C2B9H11] in THF to afford the complexes [(ImArN)M(η5-C2B9H11)(THF)2] (2a, M = Sc; 2b, M = Y; 2c, M = Lu) in high yields. Treatment of [(ImArN)M(CH2SiMe3)2(THF) n ] with 1 equiv of the zwitterionic [nido-(Me2NHCH2CH2)C2B9H11] in THF afforded the donor-functionalized dicarbollide complexes [(ImArN)M{σ:η5-(Me2NCH2CH2)C2B9H10}(THF)] (4a, M = Sc; 4b, M = Y; 4c, M = Lu) in good yields. In a similar manner, complexes [(ImArN)Y{σ:σ:η 5 -(Me2NCH2)2C2B9H9}(THF)] (5) and [(ImArN)Y{σ:σ:η 5 -(Me2NCH2CH2)(MeOCH2CH2)C2B9H9}] (6) with two tethered donor moieties were synthesized. All complexes were characterized by various spectroscopic techniques and elemental analyses. Their structures were further confirmed by single-crystal X-ray analyses, revealing that the metal−nitrogen bonds in these complexes are either the shortest or among the shortest ever reported for scandium−, yttrium−, or lutetium−nitrogen systems.

Reaction of anhydrous rare earth metal halides MCl(3) with 2 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im(Dipp)NH) and 2 equiv of trimethylsilylmethyl lithium (Me(3)SiCH(2)Li) in THF furnished the complexes [(Im(Dipp)N)(2)MCl(THF)(n)] (M = Sc, Y, Lu). The molecular structures of all three compounds were established by single-crystal X-ray diffraction analyses. The coordination spheres around the pentacoordinate metal atoms are best described as trigonal bipyramids. Reaction of YbI(2) with 2 equiv of LiCH(2)SiMe(3) and 2 equiv of the imino ligand Im(Dipp)NH in tetrahydrofuran did not result in a divalent complex, but instead the Yb(III) complex [(Im(Dipp)N)(2)YbI(THF)(2)] was obtained and structurally characterized. Treatment of [(Im(Dipp)N)(2)MCl(THF)(n)] with 1 equiv of LiCH(2)SiMe(3) resulted in the formation of [(Im(Dipp)N)(2)M(CH(2)SiMe(3))(THF)(n)]. The coordination arrangement of these compounds in the solid state at the metal atoms is similar to that found for the starting materials, although the introduction of the neosilyl ligand induces a significantly greater distortion from the ideal trigonal-bipyramidal geometry. [(Im(Dipp)N)(2)Y(CH(2)SiMe(3))(THF)(2)] was used as precatalyst in the intramolecular hydroamination/cyclization reaction of various terminal aminoalkenes and of one aminoalkyne. The complex showed high catalytic activity and selectivity. A comparison with the previously reported dialkyl yttrium complex [(Im(Dipp)N)Y(CH(2)SiMe(3))(2)(THF)(3)] showed no clear tendency in terms of activity.

Invited for the cover of this issue is the group of Matthias Tamm at the Technische Universität Braunschweig. The image depicts an ambitious karateka opting to break the carbon–carbon triple bond rather than the single bond. Read the full text of the article at https://doi.org/10.1002/chem.201801651.

Benzylidyne tungsten systems bearing a combination of alkoxide and amide ligands were readily obtained by partial alcoholysis of amido‐supported tungsten complexes. Benzylidyne tris(dimethylamido)tungsten was treated with fluorinated alcohols Me2(CF3)COH, Me(CF3)2COH, and (CF3)3COH, and also with silanols (tBuO)3SiOH, and Ph3SiOH, all of which resulted in complexes of the type [PhC≡W(NHMe2)(NMe2)(OR)2]. Full displacement of the amido ligands was also achieved in [PhC≡W(NHMe2){OC(CF3)2Me}{OSi(O‐tBu)3}2] and [PhC≡W(NHMe2)(OSiPh3)3]. In addition, reaction of the three fluorinated alcohols with hexakis(dimethylamido)ditungsten yielded isomeric mixtures of bimetallic complexes [W2(NMe2)4(OR)2], which bear two electron‐donating ligands and one electron‐withdrawing ligand per tungsten atom. All amido‐substituted compounds are active in the self‐metathesis of 5‐benzyloxy‐2‐pentyne, although [W2(OR)2(NMe2)4] complexes require longer initiation times depending on the degree of fluorination of the tert‐butoxide ligand.