New Isolable Dialkylsilylene and Its Isolable Dimer That Equilibrate in Solution

The new isolable dialkylsilylene 3 bearing
a bidentate
alkyl substituent was synthesized. Recrystallization of silylene 3 gave yellow crystals of 3 and orange-red crystals
of tetraalkyldisilene 4, a dimer of 3. In
the solid state, 3 exists as a monomer with a closest
distance of 6.745 Å between dicoordinate silicon atoms, while
disilene 4 has a remarkably long SiSi double
bond distance of 2.252 Å. An equilibrium between 3 and 4 in solution was observed by NMR and UV–vis
spectroscopies, and the thermodynamic parameters of the equilibrium
were estimated to be ΔH = −36 ±
3 kJ mol–1 and ΔS = −170
± 15 J mol–1 K–1. 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 3 and 4 allows
the reversible dimerization of silylene 3 to disilene 4 and the isolation of both species.