Cyclobutadiene complexes properties

Bunz et al. explored the possibility of doping PPE chains covalently with small amounts of fluorescence-quenching cyclobutadiene complexes, in order to endow their optical properties to the base polymer, PPE [80]. Due to their extensive experience of cyclobutadiene complexes in polymer synthesis [81], the authors prepared several polymers PAE-CoCpl-5 (Table 4) containing different contents of CoCp complexes. The quantum yields were determined by simple comparison of the intensities of the emitted light to that of a standard... 

The double-addition products of type XVI are surprisingly stable. Complex 36 i, e.g., can be crystallized from CH2Cl2/MeOH in the presence of hydrochloric acid without substantial decomposition. The most striking property of 36i is the conversion into the cyclobutadiene complex 33 (R = Ph), which is more conveniently prepared from 28a and PhC2Ph at 150 °C. As already mentioned, the reaction proceeds by a stepwise mechanism through complexes XIV—XVI. This reaction offers a new, facile preparation of 33. The cyclo-dimerisation of alkynes other than diphenylacetylene could not been substantiated, however. 

Diphenylacetylene with Mo(CO)a (298) in a sealed tube at 160°-170°C produces, in addition to two cyclobutadiene complexes, a yellow compound with the empirical formula [C9(CgH5)gO]Mo(CO)2, the infrared spectrum and chemical properties of which suit the tetraphenyl-cyclopentadienone complex of structure (11). On the other hand, the interaction of 3-hexyne with (CH3CN)gMo(CO)3 yields only the alkyne complex (570). 

Schimanke, H., and Gleiter, R. (1998). Synthesis and Electrochemical Properties of Butadiyne-Bridged Cyclopentadienylcobalt-Cyclobutadiene Complexes. Organometallics, 17,113-111. 

On treatment with aqueous silver nitrate it forms a stable crystalline complex (78), from which 75 can be recovered almost quantitatively by addition of ammonia ". Anhydrous nickel(ii) bromide converts cyclooctyne into the trimer (79) with a quantitative yield . However, when the reaction was carried out in the presence of a trace of water, the dimeric cyclobutadiene-nickel bromide complex (80) was obtained in 9-4% yield, together with 79 (85%) . The spectroscopic properties of 80 showed close similarity with those of the tetramethylcyclobutadiene-nickel chloride complex . 

Syntheses and properties of cyclobutadiene-transition metal complexes have been discussed in detail by Maitlis 167). Brown 168), and others 169) have reviewed the metal-ligand bond in terms of the MO approximation. The main bonding in these complexes is due to an overlap of the two degenerate nonbonding cyclobutadiene orbitals with spd hybrid metal atomic orbitals. 

One of the most interesting properties of the complex concerns its reactions with electrophilic reagents. It is found that these reactions lead to substituted cyclobutadiene-iron tricarbonyl complexes and, in this sense, the complex is classified as aromatic just as ferrocene may be so classified. The substitution reactions which have been performed so far are summarized below. 

Little meaningful physical data on these complexes has appeared yet. The study of their reactivity is complicated by the effects of the substituents on the cyclobutadiene ring and the other ligands present. Until more information is available on the properties of the recently prepared unsubstituted cyclobutadieneiron tricarbonyl (XVIII) 38) and similar molecules, it is hard to be certain which properties are due to the presence of a cyclobutadiene group. Thus while cyclobutadieneiron tricarbonyl (XVIII) is easily oxidized by ferric chloride in ethanol (as are other diene-iron tri-carbonyl complexes, albeit to different types of product), tetraphenyl-cyclobutadieneiron tricarbonyl (XIII) is very resistant to this reagent, and indeed to most others, presumably mainly due to the steric hindrance of the phenyls. 

Cyclobutadiene is expected to have strong donor characteristics, and the cyclobutadiene generated by the photolysis of (8) does seem to be complexed with the phthallic anhydride which is also formed. The 1240 cm" band, in this case, has a shoulder at 1238 cm" and the 570 cm" band is broadened, possibly because the four out-of-plane C—H vibrations are no longer degenerate in the complexed species. Complex formation is also reflected in changes in the properties of the phthallic anhydride, as the anhydride vibrations are distinctly different from those of free phthallic anhydride observed under the same conditions. The photolysis product also shows a u.v. absorption consistent with complex formation. 

Carbonyl complexes exhibit the presence of v(CO) bands in the IR spectrum. Their position is dependent, among other factors, on the donor properties of ligands. For [Fe(C4H4 Me )(CO)3] complexes, the v(CO) frequency becomes lower when X increases because the donor properties of cyclobutadiene are also increased. The spectra... 

Co, Rh and Ir - The synthesis, properties and reactions of (T]5-bicyclo[3.2.01hcpta-l,3-dienyl)(il -COD)cobalt(I) have been reported, including the X-ray crystal structure of the tetraphenylcyclobutadiene complex, (17), the product of its reaction with diphenylethyne. The reaction of [(Ti5-C5H4R)Co( n -COD)] with 1,6-cyclodecadiyne afforded the (q. C5H4R)cobalt-capped cyclobutadiene superphane. The yield of the reaction was reported to vary with the substituent R. 

Full details of the isolation of 1,4-adducts from the products of thermal reactions between perfluorobut-2-yne and benzene, toluene, and o-, m-, and p-xylenes have now been published. Photochemical reaction of the but-2-yne with benzene in the vapour phase yields a complex mixture, the major components of which are l,2-bis(trifluoromethyl)cyclo-octatetraene and compound (59), whereas the main product under liquid-phase conditions is a 2 1 adduct with spectroscopic properties consistent with structure (60). Vapour-phase photolysis of tricarbonylcyclobutadieneiron in the presence of a high concentration of perfluorobut-2-yne gives a good yield of 1,2-bis-(trifluoromethyl)benzene, a result compatible with efficient scavenging of cyclobutadiene produced initially ... 

Properties. In the crystal structures of the complexes 3.8 and 3.10, the cyclobutadiene ring is square-planar and the methyl or phenyl substituents are displaced out of the plane of the ring, away from the metal atom. 

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