Toward Dendritic Two-Dimensional Polycarbenes: Syntheses of ‘Starburst’-Type Nona- and Dodecadiazo Compounds and Magnetic Study of Their Photoproducts

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Abstract
‘Starburst’-type nona- and dodecadiazo compounds were designed and synthesized by using the cyclotrimerization of ethynyl ketones as a key step. Construction of the larger dodecadiazo compounds was facilitated by using a trimethylsilyl protecting group for an acetylenic moiety. The diazo compounds were photolyzed in MTHF solid solution at cryogenic temperatures and analyzed by means of Faraday magnetometry and ESR spectroscopy. From the field dependence of the magnetization, the nonacarbene, the photoproduct of nonadiazo compound, was evidenced to have a nonadecet (S = 9) ground state. This spin multiplicity is the highest value ever reported for purely organic materials. On the other hand, the spin multiplicity was low and antiferromagnetic interaction operated in the photoproduct of the dodecadiazo compound. This result was interpreted not in terms of the intermolecular antiferromagnetic interaction but of the intramolecular cross-linking between the carbene centers. In the ‘Starburst’-type polycarbenes with extended branching, the reactive carbene centers can become mutually too close to each other in space and so appear to recombine readily. This study showed one of the limitations of the extension of the carbene network by way of flexible ‘Starburst’-type structures.

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