The electronic structure of trimethylenemethane was discussed in 1948. It is a neutral four-carbon molecule containing four pi molecular orbitals. When trapped in a solid matrix at about 90 K, the six hydrogen atoms of the molecule are equivalent. Thus, it can be described either as zwitterion, or as the simplest conjugated hydrocarbon that cannot be given a Kekulé structure. It can be described as the superposition of three states: It has a tripletground state, and is therefore a diradical in the stricter sense of the term. Calculations predict a planar molecule with three-fold rotational symmetry, with approximate bond lengths 1.40 Å and 1.08 Å. The H–C–H angle in each methylene is about 121 degrees. Of the three singlet excited states, the first one, 1, is a closed shell diradical with flat geometry and fully degenerate threefold symmetry. The second one, 1, is an open-shell radical with a D3h-symmetric equilibrium between three equal geometries; each has a longer C–C bond and two shorter ones, and is flat and bilaterally symmetric except that the longer methylene is twisted 79 degrees out of the plane. The third singlet state, 2/', is also a D3h-symmetric equilibrium of three geometries; each is planar with one shorter C–C bond and two longer ones. The next higher energy states are degenerate triplets, 1 and 2, with one excited electron; and a quintet state, , with the p orbitals occupied by single electrons and D3h symmetry.
Preparation
Trimethylenemethane was first obtained from photolysis of the diazo compound 4-methylene-Δ1-pyrazoline with expulsion of nitrogen, in a frozen dilute glassy solution at −196 C. It was also obtained from photolysis of 3-methylenecyclobutanone, both in cold solution and in the form of a single crystal, with expulsion of carbon monoxide. In both cases, trimethylenemethane was detected by electron spin resonance spectroscopy. Trimethylenemethane has been obtained also by treating potassium with 2-iodomethyl-3-iodopropene|isobutylene diiodide 2C= in the gas phase. However the product quickly dimerizes to yield 1,4-dimethylenecyclohexane, and also 2-methylpropene by abstracting two hydrogen atoms from other molecules.
Organometallic chemistry
A number of organometallic complexes have been prepared, starting with Fe3, which was obtained by the ring-opening of methylenecyclopropane with diiron nonacarbonyl. The same complex was prepared by the salt metathesis reaction of disodium tetracarbonylferrate with 1,1-bisethylene. Related reactions give M4. The reaction leading to Mo4 also gives Mo3 containing a dimerized TMM ligand. TMM complexes have been examine for their potential in organic synthesis, specifically in the trimethylenemethane cycloaddition reaction with only modest success. One example is a palladium-catalyzed cycloaddition of trimethylenemethane.
