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

Abstract

The structures and charge distributions of substituted silenolates [H 2SiC( O)X] − (X=H, SiH 3, Me, t-Bu, OMe, NMe 2; group A), [Y 2SiC( O)H] − (Y=H, F, Me, Ph, SiH 3, SiMe 3; group B), and [Y 2SiC( O)X] − (Y=Me, X= t-Bu, and Y=SiMe 3; X= t-Bu, OMe, NMe 2; 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, Σ Φ RP(π), calculated by natural resonance theory, was determined. The Σ Φ RP(π) correlated with r(SiC) for both group A and B silenolates, and the correlation between Σ Φ RP(π) and the sum of valence angles at Si, Σ α(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 Σ Φ RP(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 Σ α(Si) value, and lower Δ q(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.