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.