Cyclobutadiene complexes spectra

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). 

The C NMR spectrum of cyclobutadiene-iron tricarbonyl consists of a singlet at 209.0 ppm for the carbonyl carbons and a doublet centered at 61.0 ppm, Jc-H = 191 Hz (193). The large C-H coupling constant suggests hybridization at carbon between sp and sp which is consistent with the strained cyclobutadiene ring. In a plot of C versus chemical shifts the cyclobutadiene resonances fall on the same line as resonances for aromatic molecules, their ions and their metal n complexes. 

Following the suggestion made in 1956 that cyclobutadiene should form stable complexes with transition metals [7], a variety of such complexes has been prepared the first to be described (in 1959) was a tetramethylcyclobutadiene nickel complex C4Me4NiCl2 [8], This compound formed reddish-violet crystals soluble in water and in chloroform and its n.m.r. spectrum showed that all its twelve hydrogen atoms were equivalent. This complex decomposed when heated under reduced pressure to form a dimer of tetramethylcyclobutadiene. 

The proton NMR spectra of some of these complexes have been determined. The spectrum of cyclobutadieneiron tricarbonyl (XVIII) shows a singlet at 6.09r and that of benzocyclobutadieneiron tricarbonyl (XIX) a singlet at 5.98r due to the cyclobutadiene protons, as well as a multiplet due to the aromatic protons at 3.05r (3S). The NMR spectra of monosubstituted cyclobutadieneiron tricarbonyls (see Appendix) show the equivalence of the two cyclobutadiene ring protons adjacent to the substituent. This implies that the four-membered ring must be square (39a). Tetramethylcyclo-butadienenickel chloride in water shows only a single resonance due to the 12 equivalent methyl protons (32). The spectra of the tetraphenylcyclo-butadiene-metal complexes are those due to phenyl protons and are usually complex. In the (cyclopentadienyl)(tetraphenylcyclobutadiene)nickel and -palladium bromides (XLIV), however, sharp single phenyl proton resonances are obtained at 2.39r (65). The reason for the apparent equivalence of all the phenyl protons in (XLIV) is not clear. 

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... 

Excess iron nonacarbonylFe2(CO)9 was stirred with 3-chloro-(2-chloromethyl) propene in ether at room temperature for 12 hr. The pale yellow complex, mp 28-29°, was isolated from the reaction mixture. Propose a plausible structure from the following data Analysis C, 43.29 and H, 2.90 mass spectrum m/e 194, 166, 138, and 110 IR 1998 and 2064 cm NMR 8.00t(s) 7r-cyclobutadiene iron tricarbonyl 6.09t 7r-allyl-7r-cyclopentadienyl iron monocarbonyl 7.33t (syn), and 9.32t (anti). 

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