User:Circador/Cyclobutadieneiron tricarbonyl

Lead
Cyclobutadieneiron tricarbonyl is an organoiron compound with the formula Fe(C4H4)(CO)3. The molecule is a low melting point, yellow solid that is soluble in organic solvents, and sublimes at high vacuum. Initially postulated by Longuet-Higgins and Orgel in 1956, it is notable as the first isolable example of an unsubstituted cyclobutadiene species. Through studies of structure and reactivity, it has helped uncover fundamental information about the ground state of cyclobutadiene, contribute to the theory of metalloaromaticity, and enable new reactions in organic chemistry as a precursor for cyclobutadiene, which is an elusive species in the free state.

Via cyclobutene derivatives
It was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and diiron nonacarbonyl:


 * C4H4Cl2 + 2 Fe2(CO)9 →  (C4H4)Fe(CO)3 + 2 Fe(CO)5 + 5 CO + FeCl2

Pettit, Amiet, and Reeves further generalized this synthesis by using the sodium salt of iron carbonyl anion Na2Fe(CO)4 to make it, which could be extended to other metals and substituted dihalocyclobutenes.

C4H4Cl2 + NaFe(CO)4 →  (C4H4)Fe(CO)3 + CO + 2NaCl In 1967, it was prepared by reaction of iron pentacarbonyl with photo-α-pyrone via brief irradiation by Rosenblum and Gatsonis.

Via alkynes and alkadiynes
Although the tetraphenyl-substituted cyclobutadieneiron tricarbonyl was prepared in 1959 by Hubel, in 1973 Buhler was able to cyclodimerize acetylene directly at high temperature and pressure of 110 °C and 9,000 atm in order to make cyclobutadieneiron tricarbonyl among other byproducts. Fe(CO)5 + 2HCCH → (C4H4)Fe(CO)3 + 2 CO

Substituted cyclobutadieneiron iron tricarbonyl complexes can also be prepared via reacting alkadiynes with iron carbonyls, such as in 1972 by Eavenson.

Via π-ligand transfer reactions
Tetramethyl and tetraphenyl-substituted cyclobutadiene iron complexes have also been synthesized in low to moderate yields by transferring the substituted cyclobutadiene ligand from Ni, Pd, or Pt cyclobutadiene complexes under reflux.

Structure and Bonding
Cyclobutadieneiron tricarbonyl is an example of metalloaromaticity, in which the Fe 3dxz and 3dyz orbitals interact with the π-system of cyclobutadiene to form a 4-membered aromatic ring in which electrons are delocalized outside the plane. This may be understood by considering the 2 d electrons of the metal as strongly interacting with the cyclobutadiene Pi system, such that there are 4n+2 electrons total. This aromaticity and equivalency of the C-C bonds has been characterized by NMR spectroscopy, x-ray crystallography, gas-phase electron diffraction, in which the C-C bonds are equivalent with an extremely low or nonexistent barrier to rotation. The C-C distances are 1.426 Å. Additionally, the cyclobutadiene ligand may be understood as a sigma donor, interacting with the Metal dz2, s, and pz orbitals, and delta-accepting ligand, allowing for backbonding interactions from the dx2-y2 orbital. In the local symmetry of the CBDFe, three interactions are qualitatively depicted in figure 3.

Meanwhile, the bottom half of the compound features a tripodal carbonyl, making it an example of a piano stool complex. Here, the CO ligands are best classified as sigma donating and pi-acceptors, more strongly interacting with the dz2, dx2-y2 and dxy orbitals. Compared to traditional CO stretching frequencies, IR spectroscopy reveals two signals. In the local C3v symmetry of Fe(CO)3, these interactions are depicted.

Although formally the compound has a Cs symmetry, by examining the local symmetry of the ligands on the top and the bottom half of the molecule as C4v and C3v, ligand field theory can help understand the bonding as a perturbation on a C4H4Fe fragment by carbonyl ligands, indicating the unique nature of the CBDFe fragment. This can be especially contrasted with CpFe, in which Cp- is aromatic independent of its role as a metal ligand, in which both C-C bond distances are shorter and C-M bond distances are shorter in CpFe. Using aromaticity markers, researchers have even found that isolobal fragments to Fe(CO)3. CH+ and Be

Reactivity
Reactions not involving cyclobutadiene

Ligand substitution with dimethyl fumarate, dimethyl maleate, n-carboethoxyazepine, NO

dimerization,

Reactions that change cyclobutadiene

can change to eta 3, react with CF2=CF2

can react with ring expansion

reactions that only affect cyclobutadiene

can add COCH3, CHO, Ch2Cl, CH2NMe2, HgCl, D exchange, SO3H, COCCl3, COSMe for monosubstituted, which can then continue to transformed

di and polysubstituted, oxymercuration Cyclobutadieneiron tricarbonyl displays aromaticity as evidenced by some of its reactions, which can be classified as electrophilic aromatic substitution:


 * CyclobutadieneirontricarbonylReactions.png

It undergoes Friedel-Crafts acylation with acetyl chloride and aluminium chloride to give the acyl derivative 2, with formaldehyde and hydrochloric acid to the chloromethyl derivative 3, in a Vilsmeier-Haack reaction with N-methylformanilide and phosphorus oxychloride to the formyl 4, and in a Mannich reaction to amine derivative 5.

The reaction mechanism is identical to that of EAS:


 * Cyclobutadieneirontricarbonylreactionmechanism.png

Reactions that release cyclobutadiene
Oxidative decomplexation of cyclobutadiene is achieved by treating the tricarbonyl complex with ceric ammonium nitrate. The released cyclobutadiene is trapped with a quinone, which functions as a dienophile.

can be trapped with various dienophiles

Uses and Applications
Solid-supported cyclobutadieneiron tricarbonyl complexes have been developed and evaluated for use in rapid and parallel synthesis of rigid rings

Also led to a new synthesis of cubane,

Metal-stabilized carbonium ions

[2+2+1] intramolecular Pauson-Khand Type cycloaddition

Cycloadducts

Oligomeric compounds

radial cyclopentadienyl metals