Alkynic crystal structure

For the alkyne ligand two bonding modes are possible for dimetallic complexes, MAS and MA6, which are known as perpendicular and parallel acetylene complexes, respectively. The bonding mode of MAS is the most common (Table 16). These two modes have been studied in detail by Hoffmann and coworkers . The perpendicular MAS alkyne is seen as a 4-electron donor of 2 Ti-electrons to each metal, whilst the parallel alkyne ligand donates 2 electrons via a bonds. At the time of publication of References 78 and 79 (1982) only a few dimetallic alkyne crystal structures had been determined. However, Hoffmann... 

Both these hydrides insert alkenes and alkynes the crystal structure of [Rh(NH3)5(C2H5)]2+Br2 shows the ethyl group has a trans-influence comparable to that of the hydride [93]. 

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

Reactions of the Ir1 complexes (289) and (290) with H2 and subsequent reaction of the dihydride products with dimethyl acetylenedicarboxylate, Me02CC CC02Me (dma), give the Ir11 complexes (291), (292), (293), and (294), as shown in Reaction Scheme 26.482 The X-ray crystal-structure determination of (294) confirms the double insertion of the alkyne into each of the Ir—H bonds of (293). 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

The crystal structure of this compound shows a slightly elongated C=C bond of the alkyne and the usual deviation from linearity at the two carbon atoms of the triple bond.18,19... 

Figure 8 The preparations and crystal structures of copper(I) alkene and alkyne complexes... 

To our knowledge there is no experimental evidence for n bonding between a BeR2 unit and a simple alkene or alkyne ligand. However a n interaction has been found in the X-ray crystal structure of the dimeric dipropynylberyllium trimethylamine adduct (48). The unit cell of this molecule contains two independent centrosymmetric dimers in which the alkynyl groups exhibit different types of interaction with the beryllium atoms, one of them forming an effectively electron-precise dimer unit by n interaction as portrayed in XX. 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

X-Ray Crystal Structure Determinations of Molybdenum(II) and Tungsten(II) Halocarbonyl Alkyne Complexes Reported since 1987... 

The crystal structures of in excess of 70 hexacarbonylalkyne-dicobalt complexes have been determined by X-ray diffraction. In all cases the basic structural features are the same and can be illustrated by the structure of [Co -FjCQCFjXCO), shown in Fig. 1,Z7 The molecule has pseudo C2v symmetry in which the two cobalt atoms and the two acetylenic carbon atoms form a quasi-tetrahedral core. The carbon-carbon bond of the alkyne is in a perpendicular orientation relative to the cobalt-cobalt bond, as opposed to the parallel orientation which is also observed in dinuclear complexes with bridging alkyne ligands. The coordination around the cobalt atoms is distorted octahedral and the two tricarbonylcobalt moieties are eclipsed. 

Connelly and Orpen have used the complexes [Co2( -RC2R)(/a-dppm)2(CO)2] (R = Me, Ph, or C02Me) to test the molecular orbital description of [Co2(jU-alkyne)(CO)6] outlined in Section II,B.63a The X-ray crystal structures of [Co2( U.-MeC2Me)( U,-dppm)2(CO)2] (see Fig. 10) and [Co2(jU-PhC2Ph)(jU,-dppm)2(CO)2] show the alkyne to be orthogonal to the cobalt-cobalt bond. Oxidation affords the monocations [Co2(jU-RC2R) (/a-dppm)2(CO)2]+, and the crystal structure of [Co2(,u-MeC2Me)... 

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