Substituent effects of fused Hammick silylenes via density functional theory survey

We compared and contrasted the substituent effects on Delta Epsilon(s-t) (singlet-triplet energy difference), band gap (Delta Epsilon(HOMO-LUMO)), aromaticity, atomic polar tensor (APT) charge distribution and reactivity of 54 singlet and triplet fused Hammick silylenes derived from silabenzopyridine-4-ylidene (1s-27s, and 1t-27t), and the synthesized dialkylsilylene by Kira, respectively. In this investigation we found the following: (1) The most and the least thermodynamic stability compared to Kira's silylene is demonstrated by fusion of two thiophene rings in para-position and two phosphole rings in ortho-position, respectively (Delta Epsilon(s-t) = 331.0 and 115.1 vs. 138.6 kJ/mol, respectively). (2) Although fusion of benzene, pyrrole, furan, thiophene, and phosphole ring kinetically stabilizes the corresponding silylenes, fusion of pyridine and phosphinine ring destabilizes their silylenes versus the isolated silylene's Kira (with the range of Delta EHOMO-LUMO from 312.8 to 410.2 vs. 351.7 kJ/mol). (3) The trend of substituent effects per number and size of rings is two five-membered rings > one five-membered rings > one six-membered rings > two six-membered rings. (4) Almost all cases, singlet Hammick silylenes reveal greater nucleophilicity, lesser electrophilicity, lower chemical potential, and greater global hardness than the scrutinized silylene by Kira. (5) We are waiting for experimental testing and verifications that place the heteroatom in the silapyridine's plane in a favorable spatial position to act not only as pi electron donor but also acceptor of sigma electron density.