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

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-de 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 degrees(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 degrees(Fe+-C4H2N2) for the sextet ground state is computed to be 87 +/- 10 kcal/mol.