Trends in the unimolecular reactions of triaryl pnictogen radical cations Ar3E center dot+ (E = N, P, As, Sb, Bi): Reductive elimination versus E-C bond cleavage

The unimolecular reactions of the molecular ions M.+ of the triphenyl pnict ogens (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) spectromet ry to select ions of low internal excess energy. Molecular ions of tripheny lamine 1 both of high and low internal energy prefer fragmentation by losse s of one and two H atoms, which proceeds by a well-known reaction between t wo phenyl ligands resulting in fragment ions of an azafluorene structure. I n contrast, the molecular ions of the heavier triphenyl pnictogens 3, 4, an d 5 dissociate at high and low internal energies almost completely by gener ating 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 in ternal 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 r adicals, which is discussed in the literature, is definitely excluded. Dire ct 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 sp ectra of the molecular ions, giving positive evidence for the reductive eli mination process. Formation of B.+ at low and high excess energies with exc lusion 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 t he benzene rings. The fast isomerization of the heavier triaryl pnictogen r adical 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.+, gener ated by loss of two aryl radicals, is distinctly below the AE of ions A(+), generated by loss of only one aryl radical.