Silacyclopropenylidenes matrix isolation

S-37 (see above) it is also possible to prepare and to matrix-isolate the silicon species 124, 125, and 126, which again exist in a photoequilibrium. Our first entry to 1-silacyclopropenylidene (124) was the pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane (123).71,72 Even though the structure of educt molecule 123 suggests formation of ethynylsilylene (125), the isolated product was 124. Obviously 125 had already thermally isomerized to the most stable isomer 124 before the products were condensed at 10 K. 

A particle whose connectivities are indicative of structure 104 has earlier been detected by neutralization-reionization mass spectrometry134. Furthermore, 1-silacyclopropenylidene (104) was recently identified by microwave spectroscopy135. 1-Silacyclopropenylidene (104) (kmax = 286 nm136 a delocalized three-center jr-bond orbital128) generated from 102 and matrix-isolated is transformed into ethynylsilylene... 

The observed product distributions reflect the influence of both factors. In the parent compound 102 and in the silyl substituted 104, the initially formed open-chain silylenes 103 and 146 are relatively sensitive and the migration tendency of the group R is quite large, and therefore the silacyclopropenylidenes 104 and 145 are found. In contrast, the methyl and the ethynyl compound 138 and 150 are stable enough to pass the pyrolysis tube and both can be matrix-isolated upon pyrolysis of the appropriate precursors 137 and 149. Due to the small migration tendencies of the methyl and the ethynyl group, the rearrangement product 135 is not found at all and 151 is detected only in relatively minor amounts. 

Among the cyclic silylenes 4-7, SiC2 (4) [2] and 1-silacyclopropenylidene (5) [3] have already been investigated by matrix spectroscopy. This series is now completed by silylene 6, the formal hydrogenation product of 5. We also report on the isolation of 7, which can be derived from 5 by formal substitution of one CH unit by a nitrogen atom. 

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