Generation, Characterization, and Reactivity of the Transition Metal−o-Benzyne Analog of Pyrazine (Fe⁺−2,3-Didehydropyrazine) in the Gas Phase:  An Experimental and Theoretical Study

Abstract

Fe+−2,3-didehydropyrazine (2) has been generated and its reactivity with simple olefins and alkynes studied by using Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. 2, which is prepared by dehalogenation of chloropyrazine by Fe+, undergoes a simple adduct formation (no ligand coupling) with ethene and ethene-d 4. Ethyne also yields adduct formation; however, ligand coupling has clearly occurred in this reaction. Interestingly, reactions with propene and isobutene yield the same products with similar product distributions. Again, ligand coupling is involved and a metal-centered mechanism featuring activation of the allylic carbon−hydrogen bonds is proposed. Propyne, allene, 1-butene, and cis-2-butene yield a variety of products with 2. However, 2 yields exclusive dehydrogenation with 1,3-butadiene to generate FeC8H6N2 +. CID results suggest that this FeC8H6N2 + ion consists of quinoxaline bound to Fe+. Ligand displacement reactions yield a bond dissociation energy of 47 ± 5 kcal/mol for D°(Fe+−quinoxaline). FeC4H2N2 + ion (2) has also been investigated theoretically. Density functional calculations predicted that the ground state of 2,3-didehydropyrazine is the triplet state, with the singlet state being 9.9 kcal/mol higher than the triplet state. 2 has C 2v symmetry with the metal center coplanar with the 2,3-didehydropyrazine ring. 2 has a sextet ground state with doublet and quartet states 4.9 and 6.8 kcal/mol higher, respectively. The bond dissociation energy D°(Fe+−C4H2N2) for the sextet ground state is computed to be 87 ± 10 kcal/mol.

A Multireference Ab Initio Study of the Diradical Isomers of Pyrazine

“…At this level of theory the ortho (2,3) isomer was reported not to exist. A DFT-based study by Chen et al using the B3LYP functional and the LANL2DZ basis set showed that the ortho (2,3) isomer exists as an intermediate and predicts the ground state to be the triplet state, in disagreement with our findings . For the para (2,5) isomer, the Δ E (ST) splitting computed with QCISD is 8.7 kcal/mol.…”

“…In the 2,6-pyridine singlet isomer, Debbert et al 49 found that the singlet state is destabilized because the antibonding orbital associated with the interaction of the nitrogen lone pair with the dehydrocarbons is comparable in energy to the σ* orbital. A similar antibonding character is found in the σ orbital for meta (2,6) and may be the cause of the destabilization of the singlet state.…”

“…5 Additionally, heteroarynes containing nitrogen are often intermediates in synthetic organic and inorganic chemistry. For instance, it has been shown experimentally that 2,3-didehydropyrazine is an intermediate in the elimination of hydrochloric acid from chloropyrazine by iron 6 and is thought to be an intermediate in the pyrolysis of pyrazine-2,3-dicarboxylic acid anhydride. 7 This study will focus on the characterization of 2,3-didehydropyrazine (ortho), 2,5- didehydropyrazine (para), and 2,6-didehydropyrazine (meta) diradical isomers (Figure 1).…”

Simplified Treatment of Electronic Structures of the Lowest Singlet and Triplet States of Didehydropyrazines

“…4 Moreover, a spin contamination due to the admixture of multiple spin states is often involved in conventional SR-based calculations on open-shell diradical systems. Density functional calculations with B3LYP functional and the LANL2DZ basis by Chen et al 63 predicted that the ground state of 2,3-didehydropyrazine is the triplet state rather than singlet, with the singlet state being 9.9 kcal/mol higher than the triplet state. Therefore, observations made by Chen et al are not in accordance with the findings of the present work and of Scott et al As have already been discussed, different levels of theories have predicted the ground states of the three isomers of pyrazine diradical to be having either singlet or triplet nature.…”

Reactions of Metal Ions and Their Clusters in the GaN Phase Using Laser Ionization-Fourier Transform Mass Spectrometry

“…Chen, H. Chen, J. M. Delgado, R. Li, H. Liu, V. K. Nanayakkara, G. E. Patterson Ph. D. Theses (1998 and1999) Quan Chen5/98 ''Applications of Fourier Transforn~Ion Cyclotron Resonance (lWICR) Mass Spectrometry and Density Functional Theory to the Studies of Gas-Phase Metal Ion-Ligand Structures and Reactivities" Ruomei Li 8/99" Studies of Organometallic Ions in the Gas-Phase by using Fourier Transform Ion CycIotron Resonance Mass Spec@ometry" Huiping Chen 8/99" A Gas-Phase Study of the Structure, Reactivity and Thermochemistry of Transition Metal-Ligand Systems by Fourier Transform Ion CycIotron Resonance Mass Spectrometry and Density Functional Theory" Summa rv of Research Accomplished (a). Overview…”