Synthesis of Stable Potassium Phosphinophosphides and Reaction with Organosilyl Halides and Chlorophosphanes.

The synthesis of sterically demanding 2,6-bis(2,4,6-trimethylphenyl)phenyl (Ter)-stabilized and H-substituted diphosphanes TerHP-PR2 (4a-4c) via conversion of the phosphide TerPHK (2) with secondary chlorophosphanes ClPR2 (3a-3c, where R = iPr, Ph, and tBu, respectively) is described. The diphosphanes 4a-4c were deprotonated using KH in tetrahydrofuran, selectively yielding the potassium phosphinophosphides K[TerP-PR2] (5a-5c). These phosphinophosphides are stable in solution as well as in the solid state and can be further functionalized via salt-metathesis reactions. Reaction with organosilyl halides selectively yields the silylated diphosphanes Ter(SiR12R2)P-P(iPr)2 (6a and 6b, where R1 = R2 = CH3 and R1 = CH3, R2 = Ph, respectively), whereas conversion with chlorophosphanes selectively yields the triphosphanes R12P-P(Ter)-P(iPr)2 (7a and 7b, where R = iPr and Ph, respectively).

N-Heterocyclic Carbenes Carrying Weakly Coordinating Anions.

In this Concept article we provide a brief overview of the design and preparation of N-heterocyclic carbenes carrying weakly coordinating anions (WCA-NHCs). The anionic charge in these ligand systems is located on an exocyclic group, for example, B(C6 F5 )3 , tethered to the backbone of the imidazole ring, thus resembling a weakly coordinating moiety. With the general guiding principle behind the application of WCA-NHCs being the conversion of otherwise cationic NHC complexes into their overall neutral congeners, numerous transition metal as well as main group element complexes were isolated during the last decade, which are summarized herein.

Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N-Heterocyclic Carbenes.

The lithium salts of anionic N-heterocyclic thiones and selones [{(WCA-IDipp)E}Li(toluene)] (1: E=S; 2: E=Se; WCA=B(C6 F5 )3 , IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene), which contain a weakly coordinating anionic (WCA) borate moiety in the imidazole backbone were reacted with Me3 SiCl, to furnish the silylated adducts (WCA-IDipp)ESiMe3 (3: E=S; 4: E=Se). The reaction of the latter with [(η5 -C5 Me5 )MCl2 ]2 (M=Rh, Ir) afforded the rhodium(III) and iridium(III) half-sandwich complexes [{(WCA-IDipp)E}MCl(η5 -C5 Me5 )] (5-8). The direct reaction of the lithium salts 1 and 2 with a half or a full equivalent of [M(COD)Cl]2 (M=Rh, Ir) afforded the monometallic complexes [{(WCA-IDipp)E}M(COD)] (9-12) or the bimetallic complexes [µ2 -{(WCA-IDipp)E}M2 (COD)2 (µ2 -Cl)] (13-16), respectively. The bonding situation in these complexes has been investigated by means of density functional theory (DFT) calculations, revealing thiolate or selenolate ligand character with negligible metal-chalcogen π-interaction.

Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species.

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1-dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch-Buttenberg-Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4-tetramorpholino-1-buten-3-yne (4), which was used for the synthesis of four-membered cyclic bent allene (CBA) transition-metal complexes of the type [(CBA)MLn ] (5-7; MLn =AuCl, RhCl(COD), RhCl(CO)2 ; CBA=1,3,4,4-tetramorpholino-1,2-cyclobutadiene; COD=1,5-cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4-tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4-tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C4 N4 core as shown by X-ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts.

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe3 and (NHC)PH with the dinuclear ruthenium and osmium complexes [(η6-p-cymene)MCl2]2 (M = Ru, Os) afforded the half-sandwich complexes [(η6-p-cymene){(NHC)P}MCl] and [(η6-p-cymene){(NHC)PH}MCl2] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(η6-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 °C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(η6-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

Isoselective Polymerization of rac-Lactide by Aluminum Complexes of N-Heterocyclic Carbene-Phosphinidene Adducts.

The N-heterocyclic carbene-phosphinidene adducts (NHC)PH were reacted with AlMe3 in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe3 ] and [{(IMes)PH}AlMe3 ] (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). In contrast, the dialuminum complex [{(Me IMes)PH}(AlMe3 )2 ] was obtained for Me IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethylimidazolin-2-ylidene. These complexes served as initiators for the efficient ring-opening polymerization of rac-lactide in toluene at 60 °C. High degrees of isoselectivity were found for the poly(rac-lactide) obtained in the presence of the monoaluminum complexes (Pm up to 0.92, Tm up to 191 °C), whereas almost atactic polymers were produced by the dialuminum complex. Detailed mechanistic studies reveal that the polymerization proceeds via a coordination-insertion mechanism with the carbene-phosphinidene ligands acting as stereodirecting groups.

Halogen Complexes of Anionic N-Heterocyclic Carbenes.

The lithium complexes [(WCA-NHC)Li(toluene)] of anionic N-heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA-NHC) reacted with iodine, bromine, or CCl4 to afford the zwitterionic 2-halogenoimidazolium borates (WCA-NHC)X (X=I, Br, Cl; WCA=B(C6 F5 )3 , B{3,5-C6 H3 (CF3 )2 }3 ; NHC=IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, or NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The iodine derivative (WCA-IDipp)I (WCA=B(C6 F5 )3 ) formed several complexes of the type (WCA-IDipp)I⋅L (L=C6 H5 Cl, C6 H5 Me, CH3 CN, THF, ONMe3 ), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA-IDipp)I(NHC)] and [PPh4 ][(WCA-IDipp)I(WCA-NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh4 ][(WCA-IDipp)2 Br] was isolated as a rare example of a hypervalent (10-Br-2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

Heterobimetallic Coinage Metal-Ruthenium Complexes Supported by Anionic N-Heterocyclic Carbenes.

The lithium complexes [(WCA-NHC)Li(toluene)] of anionic N-heterocyclic carbenes with a weakly coordinating borate moiety (WCA-NHC, WCA=B(C6 F5 )3 , NHC=IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were used for the preparation of silver(I) or copper(I) WCA-NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono- and dicarbene complexes with solvated lithium counterions [Li(THF)n ]+ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA-NHC)M(PPh3 )] and [(WCA-NHC)M(η2 -toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido- and iodido-bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA-NHC)M(µ-X)2 Ru(PPh3 )(η6 -p-cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA-NHC transmetalation.

Determination of the π-Accepting Properties of Borate-, Aluminate-, and Gallate-Functionalized N-Heterocyclic Carbenes by 77Se NMR Spectroscopy.

Anionic N-heterocyclic carbenes with weakly coordinating borate, aluminate, and gallate moieties of the type [(F5C6)3E-NHC]- (E = B, Al, Ga) were isolated as lithium salts by the lithiation of 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp) followed by the addition of E(C6F5)3 (E = B, Al, Ga). Treatment with elemental selenium afforded the lithium salts of the corresponding anionic selenourea derivatives [{(F5C6)3E-NHC}Se]- (NHC = IMes, E = B; NHC = IDipp, E = B, Al, Ga), which were examined, among other things, by means of 77Se{1H} NMR spectroscopy to assess the π-accepting properties of the WCA-NHC ligands in comparison with their neutral NHC congeners.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry.

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

N-Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes.

Chloride abstraction from the complexes [(η6 -p-cymene){(IDipp)P}MCl] (2 a, M=Ru; 2 b, M=Os) and [(η5 -C5 Me5 ){(IDipp)P}IrCl] (3 b, IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF ) in the presence of trimethylphosphine (PMe3 ), 1,3,4,5-tetramethylimidazolin-2-ylidene (Me IMe) or carbon monoxide (CO) afforded the complexes [(η6 -p-cymene){(IDipp)P}M(PMe3 )]BArF ] (4 a, M=Ru; 4 b, M=Os), [(η6 -p-cymene){(IDipp)P}Os(Me IMe)]BArF ] (5) and [(η5 -C5 Me5 ){(IDipp)P}IrL][BArF ] (6, L=PMe3 ; 7, L=Me IMe; 8, L=CO). These cationic N-heterocyclic carbene-phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low-field 31 P NMR resonances and short metal-phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal-phosphorus bond can be described in terms of an interaction between a triplet [(IDipp)P]+ cation and a triplet metal complex fragment ligand with highly covalent σ- and π-contributions. Crystals of the C-H activated complex 9 were isolated from solutions containing the PMe3 complex, and its formation can be rationalized by PMe3 dissociation and formation of a putative 16-electron intermediate [(η5 -C5 Me5 )Ir{P(IDipp)}I][BArF ], which undergoes C-H activation at one of the Dipp isopropyl groups and addition along the iridium-phosphorus bond to afford an unusual η3 -benzyl coordination mode.

Well-Defined Alkyne Metathesis Catalysts: Developments and Recent Applications.

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 and thoroughly studied alkyne metathesis catalysts is reviewed, which includes novel well-defined, in situ formed and heterogeneous systems. Various alkyne metathesis methodologies, including alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), cyclooligomerization, acyclic 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) and diyne cross-metathesis (DYCM).

Isolation of Carbene-Stabilized Arsenic Monophosphide [AsP] and its Radical Cation [AsP]+. and Dication [AsP]2.

Arsenic monophosphide (AsP) species supported by two different N-heterocyclic carbenes were prepared by reaction of (IDipp)PSiMe3 (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with (IMes)AsCl3 (2) (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) to afford the dichloride [(IMes)As(Cl)P(IDipp)]Cl (3), which upon reduction with KC8 furnished heteroleptic [(IMes)AsP(IDipp)] (4). The corresponding mono- and dications [(IMes)AsP(IDipp)][PF6 ], [5]PF6 , and [(IMes)AsP(IDipp)][GaCl4 ]2, [6][GaCl4 ]2 , respectively, were prepared by one-electron oxidation of 4 with ferrocenium hexafluorophosphate, [Fc]PF6, or by chloride abstraction from 3 with two equivalents of GaCl3 , respectively. Compounds 4-6 represent rare examples of heterodiatiomic interpnictogen compounds, and X-ray crystal structure determinations together with density functional theory (DFT) calculations reveal a consecutive shortening of the As-P bond lengths and increasing bond order, in agreement with the presence of an arsenic-phosphorus single bond in 4 and a double bond in 62+ . The EPR signal of the cationic radical [5]+. indicates a symmetric spin distribution on the AsP moiety through strong hyperfine coupling with the 75 As and 31 P nuclei.

Dimerisation of Dipiperidinoacetylene: Convenient Access to Tetraamino-1,3-Cyclobutadiene and Tetraamino-1,2-Cyclobutadiene Metal Complexes.

The reaction of 1,2-dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2 ⋅dioxane afforded the 1,2,3,4-tetrapiperidino-1,3-cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2 , which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4-tetrapiperidino-3-buten-1-yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2 ⋅dioxane furnished the 1,3,4,4-tetrapiperidino-1,2-cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition-metal complexes containing this novel four-membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2 ]2 , and [(Me3 N)W(CO)5 ] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2 ] (6), and [(CBA)W(CO)5 ] (7). The molecular structures of all compounds 2-7 were determined by X-ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a.

Directed Iridium-Catalyzed Hydrogen Isotope Exchange Reactions of Phenylacetic Acid Esters and Amides.

For the first time, a catalytic protocol for a highly selective hydrogen isotope exchange (HIE) of phenylacetic acid esters and amides under very mild reaction conditions is reported. Using a homogeneous iridium catalyst supported by a bidentate phosphine-imidazolin-2-imine P,N ligand, the HIE reaction on a series of phenylacetic acid derivatives proceeds with high yields, high selectivity, and with deuterium incorporation up to 99 %. The method is fully adaptable to the specific requirements of tritium chemistry, and its effectiveness was demonstrated by direct tritium labeling of the fungicide benalaxyl and the drug camylofine. Further insights into the mechanism of the HIE reaction with catalyst 1 have been provided utilizing DFT calculations, NMR studies, and X-ray diffraction analysis.

(Benz)Imidazolin-2-iminato Aluminum, Zinc, and Magnesium Complexes and Their Applications in Ring Opening Polymerization of ε-Caprolactone.

Synthesis and characterization of a new family of aluminum, magnesium, and zinc complexes supported by (benz)imidazolin-2-iminato ligands are reported herein. Under different reaction conditions, (benz)imidazolin-2-imine ligands reacted with metal alkyl precursors to afford the corresponding complexes, displaying mononuclear, binuclear, and trinuclear structures. Reaction in 1:1 stoichiometry at room temperature afforded the simple Lewis acid-base adducts, which exhibited distorted tetrahedral geometries around the metal centers, whereas high temperature reactions gave rise to dimeric complexes, with each metal bridging two (benz)imidazolin-2-imine ligands to form a four-membered metallacycle ring at the center of the complexes. The catalytic ring opening polymerization (ROP) of ε-caprolactone was studied using these complexes as precatalysts in the presence of isopropanol, and very high activities were observed, affording high/medium molecular weight poly(ε-caprolactone) products with low polydispersity indices. Detailed kinetic studies revealed that the polymerizations proceeded in a living manner and molecular weights of the polymers could be precisely controlled.

Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties.

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(µ-I)]2 (1-M, M = Fe, Co; Cp' = η5-1,2,4-tri-tert-butylcyclopentadienyl) with [ImDippNLi]2 (ImDippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NImDipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe3)2}] (3-Co) and ImDippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by 57Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Formation of alkyne-bridged ferrocenophanes using ring-closing alkyne metathesis on 1,1'-diacetylenic ferrocenes.

Novel alkyne-bridged ferrocenophanes [fc{CO2(CH2) n C≡}2] (2a: n = 2; 2b: n = 3) were synthesized from the corresponding terminal diacetylenic ferrocenes [fc{CO2(CH2) n C≡CH}2] (1a: n = 2; 1b: n = 3) through ring-closing alkyne metathesis (RCAM) utilizing the highly effective molybdenum catalyst [MesC≡Mo{OC(CF3)2CH3}3] (MoF6; Mes = 2,4,6-trimethylphenyl). The metathesis reaction occurs in short time with high yields whilst giving full conversion of the terminal alkynes. Furthermore, the solvent-dependant reactivity of 2a towards Ag(SbF6) is investigated, leading to oxidation and formation of the ferrocenium hexafluoroantimonate 4 in dichloromethane, whereas the silver(I) coordination polymer 5 was isolated from THF solution.

Organoactinide-Catalyzed Monohydroboration of Carbodiimides.

The organoactinide-catalyzed monohydroboration of carbodiimides is reported herein. The catalytic reactions proceed under very mild conditions in a highly atom-efficient and highly selective fashion to afford the corresponding monohydroborated N-borylformamidine products in high yields. A plausible mechanism is proposed based on stoichiometric and kinetic studies.

Unraveling the Mechanism of 1,3-Diyne Cross-Metathesis Catalyzed by Silanolate-Supported Tungsten Alkylidyne Complexes.

The benzylidyne complex [PhC≡W{OSi(OtBu)3 }3 ] (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≡CSiMe3 (4). Diyne cross-metathesis (DYCM) studies with 13 C-labeled diyne PhC≡13 C-13 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≡13 C-13 C≡W{OSi(OtBu)3 }3 ] (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)3 SiO}3 W≡13 C-13 C≡13 C-13 C≡W{OSi(OtBu)3 }3 ] (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.