Trends in the reactions of gaseous phenylpnictogen radical cations C6H5EH2˙+ (E = N, P, As)

Dalton Transactions Pub Date: 2008-05-30 DOI: 10.1039/B801970C

Abstract

In context of an analysis of the effect of the central atom E of gaseous radical cations of phenyl pnictogens C6H5EH2, E = N (1), P (2), and As (3), the mass spectrometric reactions of phenyl phosphane 2 have been re-investigated by D-labeling and by using methods of tandem mass spectrometry. The 70 eV mass spectrum of 2 shows the base peak for ion [M-2H]˙+ and significant peaks for ions [M-H]+, [M-(2C,3H)]+, [M-PH]˙+, and [M-(C,P,2H)]+. Metastable 2˙+ fragments exclusively by loss of H2, and the investigation of deuterated 2-d2 shows that excessive H/D migrations occur before fragmentation. Other significant fragment ions in the mass spectrum of 2 arise by losses of C2H2,P, or HCP from the ion [M-H]+. This mass spectrometric behavior puts the radical cation 2˙+ in between the fragmentation reactions of aniline radical cation 1˙+ (loss of H and subsequent losses of C2H2, or HCN) and phenyl arsane radical cation 3˙+ (elimination of H2 and loss of As from ion [M-H]+). The fragmentation mechanisms of the radical cations 1˙+–3˙+ and of related ions were analyzed by calculations of the enthalpy of relevant species at the stationary points of the minimum enthalpy reaction pathways using the DFT hybrid functionals UBHLYP/6–311+G(2d,p)//UBHLYP/6–311+G(d). The results show that, in contrast to ionized aniline 1˙+, the reactions of the derivatives 2˙+ and 3˙+ of the heavier main group elements P and As are characterized by an easy elimination of H2via a reductive elimination of group C6H5-E (E = P, As) and by a special stability of bicyclic isomers of 2˙+ and 3˙+. Thus, while 1˙+ rearranges by ring expansion and formation an 7-aza-tropylium cation by loss of H·, the increased stability of bicyclic intermediates in the rearrangement of 2˙+ and in particular of 3˙+ results in separate rearrangement pathways. The origin of these effects is the more extended and diffuse nature of the 3p and 4p AO of P and As.

Graphical abstract: Trends in the reactions of gaseous phenyl pnictogen radical cations C6H5EH2˙+ (E = N, P, As)
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