Ethenedione

Ethenedione (ethylenedione, caihon monoxide dimer, C2O2), 1, is an interesting molecule for at least three reasons (1) it is a small, simple-looking molecule with no immediately obvious reason why it should not be able to exist, yet it has defied decades of attempts to prepare it [1, 2, 3, 4], (2) it is a dimer of the well-known carbon monoxide, and unknown dimers, oligomers and polymers of well-known stable molecules have been of interest for some time [5, 6, 7, 8, 9, 10, 11, 12, 13, 14,15], (3) theory indicates that the ground state shotrld be a triplet [1,2,16,17,18, 19], a characteristic which would put it in the small class of simple molectrles like O2 which are groimd state triplets despite the fact that a straightforward closed-shell Kekule structure (with all electrons represented as bonds or as lone pairs) can be written for them. 

There are surprisingly many papers which, directly or somewhat adventitiously, address the ethenedione problem, and only those which seem to bear directly on our discussion will be given here these should provide entry to the relatively few others. 

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Fig. 9.1 The (valence) molecular orbitals of ethenedione. For the correlations with the MOs of carbon monoxide only one set of CO orbitals is shown thus for example MO 1 actually results from [0(2s)+C(2s)] + [0(2s)+C(2s)], and M02 results from [0(2s)+C(2s)] - [0(2s)+C(2s)]. These AMI MO energies are very approximate. For the construction of such MO diagrams see [55] Unlike the case of 2 CO, bringing two CH2 groups together does not lead to a triplet because CHj lacks degenerate orbitals and thus so does ethene [55]...

After all these negative findings one might be forgiven being heartened by the synthesis of the oxime of ethenedione, 8, which Maier and coworkers were able to prepare by matrix isolation photolysis at 10Kof9,10, and 11 [35],... 

In conventional, wet, chemistry, an oxime is merely a masked ketone, and said ketone can usually be hberated by hydrolysis of the oxime. However, such an approach to ethenedione appears to be quixotic under conventional solution conditions not only would the (probable) ultrashort lifetime of the ketone be an impediment to its spectroscopic observation, but even the generation of this molecule seems unlikely here, because of the sensitivity of the required intermediates along the Itydrolysis pathway, e.g. 0=C=C(OH)NHOH. The synthesis of ethenedione oxime, while clearly interesting, probably ties off the path to the elusive diketone. 

To help put ethenedione in perspective, a quick look is warranted at the state of our knowledge of these related species isomers of ethenedione, the cation and anion of neutral ethenedione, the sulfur analogues OCCS and SCCS, and the imines OCCNH and HNCCNH. 

Fig. 9.2 Results of MP2/6-31G calculations on some C2O2 molecules. The energies do not include the ZPEs (whieh are all 33 5 kJ mol ). Singlet ethenedione and D2h (planar) C2O2 had an imaginary frequeney and are thus (at least at this calculalional level) transition states rather than relative minima. Author s calculations...

The cation and anion of neutral ethenedione are well-known. The radical cation, OCCO, was prepared by the reaction of CO with CO, isolated in a neon matrix and studied by ESR supplemented by ab initio calculations [38], and it has also been generated by mass spectrometry [39, 40]. The dication is known too [41, 42]. The radical anion was been reported and subjected to extensive theoretical analysis... 

Before considering possible approaches to ethenedione, let us consider the bicyclic alkene 12 (above) and the tetrahedral ether 18 (Fig. 2). They are dioxa derivatives of, respectively, bicyclo[1.1.0]but-l(3)-ene (19) and tetrahedrane (20). 

The carbon monoxide dimer 0=C=C=0, ethenedione, appears to dififer fundamentally from dimers etc. (oligomers and polymers) based on nitrogen, on carbon... 

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