Gas-phase reactions of cobalt(III)β-ketoenolates with Brønsted acid reagents

Abstract

The gas-phase reactions of a variety of 3-substituted pentane-2,4-dionate cobalt(III) complexes of formula [Co{MeC(O)CR¹C(O)Me}₂{MeC(O)CR²C(O)Me}](R¹= R²= H a; R¹= H, R²= Cl b, Br c, NO₂d, SCN e, Me f or Et g; R²= H, R¹= Cl h, Br i, NO₂j, SCN k, Me l or Et m) with various Brønsted acid systems, e.g.[CH₅]⁺, [t-C₄H₉]⁺, [H₃]⁺, [D₃]⁺, [NH₄]⁺ and [ND₄]⁺, were studied by chemical ionization (CIMS) and ion-trap mass spectrometry (ITMS). In all cases the following main reactions were observed: (i) ion-to-molecule charge transfer with formation of the corresponding molecular ions, [Co{MeC(O)CR¹C(O)Me}₂{MeC(O)CR²C(O)Me}]˙⁺; (ii) unprecedented [H₂]⁺(or [D₂]⁺) transfer from the gaseous Brønsted acids to the neutral cobalt complexes with the formation of the ions [M+ 2H]⁺ or [M+ 2D]⁺, respectively, which give rise by fragmentation to the ions [MeC(O)CRC(O)Me + 2H]⁺ or [MeC(O)CRC(O)Me + 2D]⁺, which in many cases, are the most abundant species; (iii) H⁺(or D⁺) transfer from the Brønsted acids to the neutral cobalt complexes with the formation of the ions [M+ H]⁺ or [M+ D]⁺, respectively. The ITMS study of the gas-phase reaction between the ion [Co(acac)₂]⁺(acac = pentane-2,4-dionate) and complexes a–g revealed two processes, the ion-to-molecule charge transfer which results in the formation of the radical cations [Co(acac)₂{MeC(O)CRC(O)Me}]˙⁺, and a largely predominant process which leads to two bimetallic ions, [Co₂(acac)₄]⁺ and [Co₂(acac)₃]⁺, which, according to kinetic studies, arise from two parallel reactions. The results are discussed in terms of analogies and differences between the gas- and the solution-phase reactions of tris(β-ketoenolato)cobalt(III) complexes with Brønsted acids. The preparation of the complexes e, g and k–m, not previously reported, is described; furthermore, f has been obtained in higher yields through an alternative one-step synthesis. All new compounds have been characterized by elemental analysis, mass and ¹H NMR spectrometry.