Isostructural Potassium and Thallium Salts of Sterically Crowded Triazenes: A Structural and Computational Study

Because of their similar cationic radii, potassium and thallium(I) compounds are usually regarded as closely related. Homologous molecular species containing either K+ or Tl+ are very rare, however. We have synthesized potassium and thallium salts MN3RR′ derived from the biphenyl- or terphenyl-substituted triazenes Tph2N3H (1a), Dmp(Mph)N3H (1b), Dmp(Tph)N3H (1c), and (Me4Ter)2N3H (1d) (Dmp = 2,6-Mes2C6H3 with Mes = 2,4,6-Me3C6H2; Me4Ter = 2,6-(3,5-Me2C6H3)2C6H3; Mph = 2-MesC6H4; Tph = 2-TripC6H4 with Trip = 2,4,6-iPr3C6H2). The potassium complexes 2a−d were obtained in almost quantitative yield from the reaction of 1a−d with potassium metal in n-heptane. Metalation of 1a−d with TlOEt afforded the thallium triazenides 3a−d in high yields. All new compounds have been characterized by 1H and 13C NMR spectroscopy, elemental analysis, and X-ray crystallography and for selected species by melting point (not 3b), IR spectroscopy (2a, 2d, 3a, 3c, 3d), and mass spectrometry (2a, 3c). In the solid-state structures of monomeric 2a and 3a, quasi-monomeric 2b, 3b, 2c, and 3c, and dimeric 2d and 3d additional metal-ηn-π-arene-interactions to the flanking arms of the biphenyl- and terphenyl groups in the triazenide ligands of decreasing hapticity n are observed. Remarkably, all homologous potassium and thallium complexes crystallize in isomorphous cells. For 2a and 3a, the nature of the M−N and M···C(arene) bonding was studied by density functional theory calculations.