Novel silicon super bases at DFT level of theory: effects of fused benzene rings on the basicity of 2,4,6-cycloheptatrienesilylene

Abstract

Super bases are extremely important compounds with high proton affinities (PAs) and many applications in organic, inorganic, polymer, and photochemistry. Here, we have compared and contrasted the basicity of 2,4,6-cycloheptatrienesilylene (1), with its benzo-substituted derivatives including: 4,5-benzo-2,4,6-cycloheptatrienesilylene (2), 3,4-benzo-2,4,6-cycloheptatrienesilylene (3), 2,3-benzo-2,4,6-cycloheptatrienesilylene (4), dibenzo[a,c]-2,4,6-cycloheptatrienesilylene (5), dibenzo[a,d]-2,4,6-cycloheptatrienesilylene (6), dibenzo[a,e]-2,4,6-cycloheptatrienesilylene (7), dibenzo[a,b]-2,4,6-cycloheptatrienesilylene (8), and tribenzo[a,c,e]-2,4,6-cycloheptatrienesilylene (9), at B3LYP/6-311++G** level. All scrutinized silylenes (1–9) and their corresponding protonated forms (1H–9H) appear as minima on their energy surfaces. The conductor-like polarizable continuum model is applied to predict the pka values for nucleophilic silylenes (1–9) in dimethyl sulfoxide, using thermodynamic cycles of Gibbs free energies. In most cases, the scrutinized 1–9 show relatively high basicity, which qualify them for being categorized as super bases or proton sponges. The overall trend of basicity (7 > 6 > 3 > 1 > 2 > 4 > 5 > 8 > 9) appears consistent with both proton affinity in solution phase (PA2) and nucleophilicity (N). Among our scrutinized silylenes, 7 shows the highest basicity, Mulliken electronegativity ( $${\mathcal{X}}$$ ), pka, N, PA, the lowest singlet–triplet energy gap (ΔEs–t), absolute chemical hardness (ηabs), band gap (ΔEH–L), divalent angle (C–Si–C, $${\hat{\text{A}}}$$ ), and Si–C bond length ( $${\dot{\text{A}}}$$ ). The least basic silylene turns out to be 9, which is the most non-planar structure. It shows the lowest dipole moment (D), nucleus independent chemical shift value (NICS (1)), N, PA, the widest dihedral angle (C–Si–C–C, $${\hat{\text{D}}}$$ ), and the highest ΔEH–L. Our investigation introduces novel silylenic super bases with possible applications in organic chemistry. Following up on our previous report (Kassaee et al. [41]), we investigate the basicity of novel one-, two-, three-, and four-cyclic conjugated silylenes (1–9). Most of these silylenes turn out as a super base for showing high pKa and proton affinity.