SiCl₃⁺ 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.

Energetics of hydride and electron pair attachment to EX30/+ (E=B, C, Al, Si and X=F, Cl, Br, I) and the study of bonding trends among EX30/+, EX32−/−, and EX3H−/0 by use of ELF and NBO analyses

“…All optimized geometries are in excellent agreement with available experimental [1,2,51] and previously calculated [5,38,49,[51][52][53][54][55] values. The calculated E-X bond length is systematically longer in EX 3 2À/À than in EX 3 0/+ , and is consistent with the greater E-X bond polarity that is anticipated for an anion in which the E atom is in a lower formal oxidation state.…”

“…The gas-phase reaction of SiF 3 + with NH 3 in a SIFT apparatus led to HF elimination instead of stable complex formation [48]. The Lewis acidity of SiCl 3 + was demonstrated by tandem and multiple-stage mass spectrometry experiments using different substituted pyridines as the ligands which showed that SiCl 3 + Á2(pyr 1 pyr 2 ) adducts formed [49]. In view of the foregoing discussion, there is a need for a Lewis acidity scale that is independent of the nature of the donor ligand and free of the BSSE error.…”

The kinetic method of making thermochemical determinations

“…45,46 The second point is emphasized by Drahos and Ve key,44 but also appears in the early experiments by McLuckey et al 45 Evidence for the third point has been discussed qualitatively in various recent experimental studies. 28,47 The elegant work of Drahos and Ve key44 enhances the understanding of e †ective temperature provided by Brauman and co-workers. 20 The latterÏs analysis indicates that Eqn (1) is most accurate when the cluster species is activated well above the average of the threshold energies for the two processes, that is, when the internal energy is high.…”

The Elusive Silyliumylidene [ClSi:]+ and Silathionium [ClSiS]+ Cations Stabilized by Bis(Iminophosphorane) Chelate Ligand

“…Very recently, Reid, Roesky, Stalke, and their respective co‐workers synthesized the cationic chlorogermyliumylidene10a and chlorostannyliumylidene10b complexes B through a Lewis base mediated autoionization of GeCl 2 and SnCl 2 in the presence of a neutral tridentate donor ligand (Scheme ). In 1996, Cooks and co‐workers reported a mass spectrometry study of bis(pyridine)‐supported [ClSi:] + 9e. However, an isolable complex that bears the chlorosilyliumylidene cation is still unknown.…”