New Isolable Dialkylsilylene and Its Isolable Dimer That Equilibrate in Solution

The new isolable dialkylsilylene <b>3</b> bearing a bidentate alkyl substituent was synthesized. Recrystallization of silylene <b>3</b> gave yellow crystals of <b>3</b> and orange-red crystals of tetraalkyldisilene <b>4</b>, a dimer of <b>3</b>. In the solid state, <b>3</b> exists as a monomer with a closest distance of 6.745 Å between dicoordinate silicon atoms, while disilene <b>4</b> has a remarkably long SiSi double bond distance of 2.252 Å. An equilibrium between <b>3</b> and <b>4</b> in solution was observed by NMR and UV–vis spectroscopies, and the thermodynamic parameters of the equilibrium were estimated to be Δ<i>H</i> = −36 ± 3 kJ mol<sup>–1</sup> and Δ<i>S</i> = −170 ± 15 J mol<sup>–1</sup> K<sup>–1</sup>. Analysis of the percent buried volume, a measure of the steric demand around the divalent silicon, showed that the flexible steric bulkiness of the alkyl substituent of <b>3</b> and <b>4</b> allows the reversible dimerization of silylene <b>3</b> to disilene <b>4</b> and the isolation of both species.