PUB - Publikationen an der Universität Bielefeld

The unimolecular reactions of the molecular ions M.+ of the triphenyl pnictogens (C6H5)(3)E (1, E = N; 2, E = P; 3, E = As; 4, E = Sb; 5, E = Bi) have been investigated using mass-analyzed ion kinetic energy (MIKE) spectrometry to select ions of low internal excess energy. Molecular ions of triphenylamine 1 both of high and low internal energy prefer fragmentation by losses of one and two H atoms, which proceeds by a well-known reaction between two phenyl ligands resulting in fragment ions of an azafluorene structure. In contrast, the molecular ions of the heavier triphenyl pnictogens 3, 4, and 5 dissociate at high and low internal energies almost completely by generating ions C6H5E.+ (B.+; E = As, Sb, Bi). Ionized triphenyl phosphane 2 is intermediate, showing loss of H as the main reaction at low internal energy. The ions B.+ (E = As, Sb, Bi) are generated from molecular ions of low internal energy in a one-step process without ions (C6H5)(2)E+ (A(+); E = As, Sb, Bi) being intermediates. The formation of B.+ by successive cleavages of the relatively weak E-C-aryl bonds and sequential losses of two phenyl radicals, which is discussed in the literature, is definitely excluded. Direct formation of B.+ from M.+ entails reductive elimination Of C6H5E.+ under ligand coupling to yield biphenyl as the neutral product. Biaryl ions have been detected earlier in 70 eV mass spectra of triaryl pnictogens, and in the case of tris(4-ethylphenyl)-arsane 6 and 4-methylphenylbisphenylarsane 7, the formation of biaryl-derived ions is the main reaction in the MIKE spectra of the molecular ions, giving positive evidence for the reductive elimination process. Formation of B.+ at low and high excess energies with exclusion of the formation of intermediate A(+) requires a fast and effortless rearrangement of the molecular ions into the structure of a complex which is well prepared for biaryl loss. It is assumed that this complex contains the biphenyl moiety and the fragment C6H5E rather loosely bound to one of the benzene rings. The fast isomerization of the heavier triaryl pnictogen radical cations explains also the puzzling phenomenon known for a long time that in the case of 3, 4, and 5 the appearance energy AE of ions B.+, generated by loss of two aryl radicals, is distinctly below the AE of ions A(+), generated by loss of only one aryl radical.