Gas phase agostic bonding in pyridine SiF n+ (n = 1, 3) cluster ions investigated by the kinetic method

Abstract

Loosely bonded cluster ions, Py 1SiF 3 +Py 2 and Py 1SiF +Py 2, where Py 1 and Py 2 represent substituted pyridines, are formed by ion/molecule reactions between mass-selected SiF 3 + or SiF + and a mixture of pyridines. The clusters are shown to have loosely bound symmetric structures by MS 3 experiments and ab initio calculations. The SiF 3 +/pyridine dimer is shown to have a trigonal bipyrimidal structure. Relative SiF 3 + and SiF + affinities of the constituent pyridines are measured by the kinetic method, and excellent linear correlations with the proton affinity of meta- and para-substituted pyridines are observed. Gas-phase stereoelectronic parameters ( S k ) for SiF 3 + and SiF + are also experimentally measured and show that the binding of the ortho-substituted pyridines is governed by two opposing effects, steric hindrance and agostic bonding. Agostic bonding of the form C–H --- Si +, is evident in the SiF + system, just as it is in the corresponding SiCl +/pyridine dimers. On the other hand, steric hindrance plays a key role in weakening the strength of the interaction of the central SiF 3 + ion and the ortho-substituted pyridines compared with that in SiF +-bound cluster ions. The relatively larger Lewis acidity of fluorinated siliconium ions compared with the corresponding chlorinated species shortens the Si–N bond and makes overall steric effects larger in the SiF n + (n = 1, 3) systems than in the SiCl n + (n = 1, 3) systems. The potential application of the kinetic method in recognizing agostic bonding in transition metal systems in the gas phase is also demonstrated in this study.