Effects of substituents and counterions on the structures of silenolates: a computational investigation

The structures and charge distributions of substituted silenolates [H2SiC(O)X]− (X=H, SiH3, Me, t-Bu, OMe, NMe2; group A), [Y2SiC(O)H]− (Y=H, F, Me, Ph, SiH3, SiMe3; group B), and [Y2SiC(O)X]− (Y=Me, X=t-Bu, and Y=SiMe3; X=t-Bu, OMe, NMe2; group C) were examined through density functional theory calculations. The effects of the solvated counterion (K+, Li+, or MgCl+) and coordination site (O or Si) on the properties of group C silenolates were also studied. The variation in the degree of π-conjugative reverse SiC bond polarization, ΣΦRP(π), calculated by natural resonance theory, was determined. The ΣΦRP(π) correlated with r(SiC) for both group A and B silenolates, and the correlation between ΣΦRP(π) and the sum of valence angles at Si, Σα(Si), was good for group A but poor for group B due to strong influence of the inductive effect. The SiC charge difference correlated well with ΣΦRP(π) for group A, but not for group B, again an effect of inductive substituent effects. The group C silenolates were coordinated to Li(THF)3+, MgCl(THF)4+, and K(THF)5+ either via the O or Si atom. The coordination energies show that coordination to the hard O is preferred for Li+ and MgCl+, but the K+ ion coordinated simultaneously to Si and O. Coordination of the solvated metal ion to O resulted in shorter SiC bond length, an increased Σα(Si) value, and lower Δq(SiC) when compared to the naked silenolate. Choice of counterion and substituent provides a means to extensively vary the properties of silenolates such as their reactivity.