SiCl 3+ and SiCl + affinities for pyridines determined by using the kinetic method with multiple stage mass spectrometry: Agostic effects in the gas phase

Abstract

Cluster ions, Py1SiCl 3 (+) Py2 and Py1SiCl(+)Py2, where Py1 and Py2 represent substituted pyridines, formed upon reactive collisions of mass-selected SiCl 3 (+) or SiCl(+) cations with a mixture of pyridines, are shown to have loosely bound structures by multiple stage mass spectrometry experiments in a pentaquadrupole mass spectrometer. The fragment ion abundance ratio, ln([Py1SiCl n (+) ]/[Py2SiCl n (+) ]) (n=1 or 3) is used to estimate the relative SiCl 3 (+) or SiCl(+) affinities of the constituent pyridines by the kinetic method. In the case of clusters comprised of meta- and/or para-substituted pyridines (unhindered pyridines), the SiCl 3 (+) and SiCl(+) affinities are shown to display excellent linear correlations with the proton affinities (PAs). On the assumption that the effective temperatures of the SiCl 3 (+) - and SiCl(+)-bound dimers are 555 K (i. e., the same as those of the corresponding Cl(+)-bound dimers), SiCl 3 (+) and SiCl(+) affinities of the substituted pyridines, relative to pyridine, are estimated to be 3-MePy (2.1 kcal/mol), 4-MePy (3.2 kcal/mol), 3-EtPy (3.7 kcal/mol), 4-EtPy (4.2 kcal/mol), 3,5-diMePy (4.8 kcal/mol), and 3,4-diMePy (5.4 kcal/mol). The SiCl 3 (+) and SiCl(+) cation affinities are related to the proton affinities by the expressions: relative (SiCl 3 (+) ) affinity = 0.95 ΔPA and relative (SiCl(+)) affinity = 0.60 ΔPA. The smaller constant in the relationship between the relative SiCl affinity and the relative proton affinity is the result of weaker bonding.Steric effects between the ortho-substituted alkyl group and the central SiCl 3 (+) cation reduce the SiCl 3 (+) affinities of dimers that contain ortho-substituted pyridines. The magnitude of the steric acceleration of fragmentation is used to measure a set of gas-phase steric parameters (S (k)). The steric effects in the SiCl 3 (+) dimers are similar in magnitude to those in the corresponding Cl(+)-bound dimers but weaker than those produced by the bulky [OCNCO](+) group. An inverted steric effect is observed in those SiCl(+)-bound dimers that incorporate ortho-substituted pyridines and is ascribed to auxiliary Si-H-C bonding, which stabilizes the ortho-substituted pyridine-SiCl(+) bond. This auxiliary bonding appears to correspond to agostic bonding, which is well characterized in solution and occurs in competition with steric effects that weaken the pyridine-SiCl(+) interaction.Ion-molecule reactions of pyridines with halosilicon radical cations SiCl 2 (+) and SiCl 4 (+) as well as alkylated halosilicon cations Si(CH3)2Cl(+) and Si(CH3)Cl 2 (+) also are investigated. In these cases, charge exchange and associated reactions are the main reaction channels, and clustering is not observed.