Butadiene-Iron carbonyl complexes

Diaza-1,3-butadiene (a-Diimine) Ligands Their Coordination Modes and the Reactivity of Their Metal Complexes, 21, 152 Diene-Iron Carbonyl Complexes, 1, 1... 

In the presence of iron carbonyl complexes 372), the dimerization of the butadiene can be substantially suppressed ... 

Even though the X-ray data suggest the presence of two a bonds and one 7T bond between metal and olefin as was postulated for the butadiene-iron carbonyls, the UV spectra of the triphenyltropone complexes closely resemble that of the free olefin (80) suggesting that the bonding of the complex may indeed be intermediate between that of structure (96) and simple rr bonding to two olefinic double bonds (80). This concept is more fully discussed in the section on cobalt (Section VII, A). 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

An enhancement of the selectivity in the dicarboxylation of butadiene has been claimed by Tkatchenko and coworkers [83]. When an ether such as tetrahydrofurane or dimethoxyethane is used as solvent and catalytic amounts of an iron carbonyl complex such as Cp2Fe2(C0)4 are added, the electrocarboxylation of butadiene is achieved with an overall current yield of 76 % and a selectivity of the cis- and trans 3-hexenedioic acid up to 75 %. Possibly, this interesting combination of electrochemistry and homogeneous catalysis will enable further enhancements in the C-C linkage of dienes and carbon dioxide. 

Structural characterization of (butadiene)tricarbonyliron complexes has been carried out by microwave spectro-scopy/" " solution calorimetry/" pulsed-electron high pressure mass spectrometry/ infrared spectro-scopy/ and vibrational overtone spectroscopy/ Theoretical investigations using DFT have also been reported/ " Extended Hiickel calculations have been employed to study structure and stereodynamic relationships in a variety of ( 7" -diene)iron carbonyl complexes/ ... 

The olefin-iron carbonyl complexes were first introduced by two entirely different synthetic methods. In 1930 Reihlen and co-workers (I) obtained butadiene-iron tricarbonyl by a reaction of butadiene with iron penta-carbonyl and in 1953 Reppe and Vetter (2) reported organoiron compounds, since shown to be diene-iron carbonyl complexes, following reaction of acetylene with iron carbonyls. 

Acyclic Series. The first complex in the acyclic series was prepared from butadiene by the thermal method. Heating isoprene and pentacarbonyliron at high temperature, however, is inefficient due to competitive Diels-Alder dimerization. Despite the formation of some bis(diene) iron carbonyl complexes on prolonged irradiation, the photochemical method is superior in this case. Complexation of acyclic dienes by Fe(CO)3 is limited to those that can adopt a cisoid conformation, with the syn substitution pattern normally preferred. 2,4-Hexadienolc acid, for example, ean be conveniently complexed by a photolytic procedure. Trialkylsilyl-substituted dienes have also been complexed. ... 

Many diene iron tricarbonyl complexes are known.f Butadiene iron tricarbonyl was first prepared in 1930 by Reihlen and co-workers [60], by treatment of iron pentacarbonyl with butadiene under pressure. It is a typical diene iron carbonyl complex being a yellow-brown oil which distils slowly at 60° in high vacuum. It is soluble in the common organic solvents and reacts with chlorinated hydrocarbons. The pure oil and its solutions are oxidized in air within hours, forming iron oxides. The diene system resists hydrogenation and does not undergo the Diels-Alder reaction [5]. 

Reactions of acyclic derivatives with carbon electrophiles have also been examined.33,34 An illustrative reaction involving methylation of the unsubstituted complex [MnCr 4-butadiene)(CO)3], (19), is shown in Scheme 16. Again, the reaction is presumed to occur via a methylmanganese species (20) and after methyl migration the unsaturated metal center is stabilized by formation of a Mn—H—C bridge (isomers 21a and 21b). Deprotonation of equilibrating (21a and 21b) yields the [Mn(l-methylbutadiene)(CO>3]-complex (22), which has exclusively trans stereochemistry.34 This sequence represents alkylation of the terminal carbon of butadiene and complements the iron carbonyl chemistry, where terminal acylation has been achieved as described above. Unpublished results indicate that a second methylation of (22) occurs... 

The low lying ji -orbitals of conjugated dienes explain the stability of many diene metal carbonyl complexes e.g. butadiene iron tricarbonyl 382>. Photochemical preparation of the latter 286> is superior to the thermal procedure 272>. Mercury can be used as a sensitizer in the photoreaction of metal carbonyls with dienes I09.i7o,i76,24i)... 

The 18-electron rule is often of value in predicting likely structures for their complexes. 1,3-Butadiene, for example forms the iron carbonyls (C Hj)Fe(CO) and (QHs)Fe(CO)j. In the former the butadiene is linked to iron through only one of the double bonds, whereas in the latter it is ri bonded. 

The largest group of diene complexes which have been investigated are iron carbonyl compounds. When 1,3-butadiene reacts with Fe lCO), the initial product is a labile f/ -complex, which readily loses carbon monoxide to yield (f/ -C4Hg)Fe(CO)3. Butadiene(tricarbonyl)iron is a yellow, essentially air stable complex, m.p. 19 "C. It was first prepared by Reihlen in 1930 by heating FeiCO), with butadiene in a tube under pressure, but this was an isolated discovery which did not arouse much interest at the time. On account of the ease of preparation and handling of these diene complexes, however, an enormous amount of work has been done in this area since the late 1950s. Only modest precautions are required to protect solutions from oxidation and some operations can be carried out even in air. 

Butadiene-iron tricarbonyl reacts with various esters of phenylphos-phinous acid to give displacement of one carbonyl group and formation of complexes of the type C4Hj-Fe(CO)2 P (OR)2 (87). 

Carboxylic acids, a-bromination of 55, 31 CARBOXYLIC ACID CHLORIDES, ketones from, 55, 122 CARBYLAMINE REACTION, 55, 96 Ceric ammonium nitrate [Ammonium hexa mtrocerate(IV)[, 55, 43 Chlorine, 55, 33, 35, 63 CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Cinnamomtnle, a-phenyl- [2-Propeneni-tnle 2,3-diphenyl-], 55, 92 Copper(l) iodide, 55, 105, 123, 124 Copper thiophenoxide [Benzenethiol, copper(I) salt], 55, 123 CYCLIZATION, free radical, 55, 57 CYCLOBUTADIENE, 55, 43 Cyclobutadieneiron tricarbonyl [Iron, tn-carbonyl(r)4-l,3-cyclo-butadiene)-], 55,43... 

There is a notable tendency to form oligomers when acetylenic substances interact with compounds of metals, and this tendency is also shown by butadiene 117) (see Section IV, B,d). This is particularly so with the carbonyls of iron and cobalt, and the oligomerization reactions are favored with nickel 121) and with palladium compounds 113, 122, 123). This phenomenon may be related to the hydropolymerization of acetylenes on metal surfaces, and it may be that such polymerization processes would be better described in terms of ir-complexes. 

The diene ligands transform to the cisoid form because of the thermodynamic stability of the complex. Because the bond distances of Cj—and C2—Cy in the butadiene ligand are 1.45 and 1.46 A, respectively, the double bond character between C2—C3 and the bond order alternation is still insignificant [43]. This may be due to the strong K-acidity of three carbonyl ligands which reduces n-back donation from iron to the butadiene ligand. (Cyclohexadiene)tricarbonyliron complexes are also noteworthy. Fe(l,3-cyclo-hexadiene)(CO)3 was prepared by the reaction of 1,3-cyclohexadiene with Fe(CO)5 [44] (eq (13)) and more stable Fe(l,4-cyclohexadiene)(CO)3 was also prepared [45]. 

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