 Trimethylenemethane, average of three configurations. Formally, the radial bonds have valency 4/3. Each terminal carbon has 2/3 of an unfilled valence bond.
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| Names | |
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Preferred IUPAC name
2-Methylidenepropane-1,3-diyl | |
| Other names
Trimethylenemethane biradical; Trimethylenemethane diradical
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| Identifiers | |
3D model (
JSmol)
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PubChem
CID
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| Properties | |
| C4H6 | |
| Molar mass | 54.092 g·mol−1 |
Except where otherwise noted, data are given for materials in their
standard state (at 25 °C [77 °F], 100 kPa).
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Trimethylenemethane (often abbreviated TMM) is a
chemical compound with
formula C
4H
6. It is a
neutral free molecule with two unsatisfied
valence bonds, and is therefore a highly reactive
free radical. Formally, it can be viewed as an
isobutylene molecule C
4H
8 with two
hydrogen atoms removed from the terminal
methyl groups.
Structure
The electronic structure of trimethylenemethane was discussed in 1948. [1] [2] It is a neutral four-carbon molecule containing four pi molecular orbitals. When trapped in a solid matrix at about 90 K (−183 °C), 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:
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It has a triplet ground state (3A2′/3B2), and is therefore a diradical in the stricter sense of the term. [3] Calculations predict a planar molecule with three-fold rotational symmetry, with approximate bond lengths 1.40 Å (C–C) and 1.08 Å (C–H). The H–C–H angle in each methylene is about 121°. [1]
Of the three singlet excited states, the first one, 11A1 (1.17 eV above ground), is a closed shell diradical with flat geometry and fully degenerate threefold (D3h) symmetry. The second one, 11B2 (also at 1.17 eV), is an open-shell radical with a D3h-symmetric equilibrium between three equal geometries; each has a longer C–C bond (1.48 Å) and two shorter ones (1.38 Å), and is flat and bilaterally symmetric except that the longer methylene is twisted 79° out of the plane (C2 symmetry). The third singlet state, 21A1/1A1′ (3.88 eV), is also a D3h-symmetric equilibrium of three geometries; each is planar with one shorter C–C bond and two longer ones (C2ν symmetry). [1]
The next higher energy states are degenerate triplets, 13A1 and 23B2 (4.61 eV), with one excited electron; and a quintet state, 5B2 (7.17 eV), with the p orbitals occupied by single electrons and D3h symmetry. [1]
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 (77 K). [3]
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. [3]
Trimethylenemethane has been obtained also by treating
potassium with 2-iodomethyl-3-iodopropene and isobutylene diiodide (IH
2C)2C=CH
2 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 (hydrocarbon or
potassium hydride).
[4]
Organometallic chemistry
A number of
organometallic complexes have been prepared, starting with Fe(C
4H
6)(CO)3, which was obtained by the ring-opening of methylenecyclopropane with
diiron nonacarbonyl (Fe
2(CO)9).
[3] The same complex was prepared by the
salt metathesis reaction of
disodium tetracarbonylferrate (Na
2Fe(CO)4) with
1,1-bis(chloromethyl)ethylene (H2C=C(CH2Cl)2).
[5] Related reactions give M(TMM)(CO)4 (M = Cr, Mo). The reaction leading to (TMM)Mo(CO)4 also gives Mo(C
8H
12)(CO)3 containing a dimerized TMM ligand.
[5]
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 [3+2] cycloaddition of trimethylenemethane. [6]
-
![Structure of Ru(trimethylenemethane)(CO)3, viewed down C3 axis.[7]](https://upload.wikimedia.org/wikipedia/commons/thumb/8/82/PETCEWtopView.png/120px-PETCEWtopView.png)
Structure of Ru(trimethylenemethane)(CO)3, viewed down C3 axis. [7] -
Structure of Ru(trimethylenemethane)(CO)3, viewed orthogonal to C3 axis.
![Structure of Ru(trimethylenemethane)(CO)3, viewed down C3 axis.[7]](/info/en/?search=File:PETCEWtopView.png)
References
- ^ a b c d Slipchenko Lyudmila V., Krylov Anna I. (2003). "Electronic structure of the trimethylenemethane diradical in its ground and electronically excited states: Bonding, equilibrium geometries, and Vibrational frequencies". Journal of Chemical Physics. 118 (15): 6874–6883. Bibcode: 2003JChPh.118.6874S. doi: 10.1063/1.1561052. S2CID 4204676.
- ^ C. A. Coulson (1948), Journal de Chimie Physique et de Physico-Chimie Biologique, volume 45, page 243. Cited by Slipchenko and Krylov (2003)
- ^ a b c d Paul Dowd (1972). "Trimethylenemethane". Accounts of Chemical Research. 5 (7): 242–248doi=10.1021/ar50055a003. doi: 10.1021/ar50055a003.
- ^ Skell Philip S., Doerr Robert G. (1967). "Trimethylenemethane". Journal of the American Chemical Society. 89 (18): 4688–4692. doi: 10.1021/ja00994a020.
- ^ a b J. S. Ward & R. Pettit (1970). "Trimethylenemethane complexes of iron, molybdenum, and chromium". Journal of the Chemical Society D (21): 1419–1420. doi: 10.1039/C29700001419.
- ^ Trost Barry M (1979). "New conjunctive reagents. 2-Acetoxymethyl-3-allyltrimethylsilane for methylenecyclopentane annulations catalyzed by palladium(0)". Journal of the American Chemical Society. 101 (21): 6429–6432. doi: 10.1021/ja00515a046.
- ^ Herberich, G. E.; Spaniol, T. P. (1993). "Trimethylenemethane Complexes of Ruthenium, Osmium and Rhodium Via the Compound CH2=C(CH2SnMe3)2". Journal of the Chemical Society, Dalton Transactions (16): 2471–2476. doi: 10.1039/DT9930002471.