In the last decade a major theme of organometallic chemistry has been the design and development of alternative ligand systems capable of stabilizing monomeric metal complexes while provoking novel reactivity. Exploration of this field is driven by the potential use of these complexes in catalysis and organic synthesis. Examples of monoanionic chelating Ndonor ligands that have received much recent attention (Scheme 1) include the b-diketiminate (I) [1] and the amidinate (II) [2] ligand systems. Much less attention has been given to the closely related triazenides (III). [3, 4] This may be attributed to the lack of suitable ligands that are sterically crowded enough to prevent undesirable ligand redistribution reactions and allow better control of the electronic and steric properties at the metal center.Triazenides are weaker donors than the isoelectronic amidinates and the related b-diketiminates, and should induce greater electrophilicity at a bonded metal atom. [5] This is reflected by the results of an NBO (natural bond orbitals) analysis of the energy-minimized structures [6] of the model anions 1,3-diphenyl-1,3-diketiminate (I M ), 1,3-diphenyl-1,3-diazaallyl (II M ), and 1,3-diphenyltriazenide (III M ) (see Supporting Information), which shows an NPA (natural population analysis) charge for the chelating N atoms of À0.54, À0.60, and À0.38, respectively.We recently succeeded in the preparation of aryl-substituted, sterically crowded triazenes. Ligands of this type may be synthesized in excellent yields by the reaction of different substituted 2-lithiobiphenyls with the m-terphenyl azide 1, followed by hydrolysis (Scheme 2).In a first attempt to test their properties, we have used the obtained triazenes to stabilize pentafluorophenyl compounds of the heavier alkaline-earth metals calcium, strontium, and barium. The heteroleptic pentafluorophenyl triazenides are accessible in tetrahydrofuran as solvent by a convenient onepot transmetalation/deprotonation [7] reaction from the triazene 2 a (HN 3 ArAr'), bis(pentafluorophenyl)mercury, and the corresponding alkaline-earth metal (Scheme 3). After crystallization from n-heptane, either the THF-free compounds [M(C 6 F 5 )(N 3 ArAr')] (M = Sr (4), Ba (5)) or the solvate [Ca(C 6 F 5 )(N 3 ArAr')(thf)] (3) were isolated in good yields. It is remarkable that attempts to replace the pentafluorophenyl substituents by a second triazenide ligand have not been successful so far. Apparently, the steric bulk of the latter prevents further substitution or ligand redistribution and therefore formation of the homoleptic complexes. [8] Solutions of 4 or 5 in aromatic or aliphatic solvents show considerable thermal stability and can be stored at ambient

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-Mes 2C6H3 with Mes=2,4,6-Me3C6H2; Me4Ter=2,6-(3,5-Me2C6H3)2C6H3; Mph=2-MesC6H4; Tph=2-TripC6H4 with Trip=2,4,6-(i)Pr3C6H2). 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 (1)H and (13)C 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-eta (n)-pi-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.

The one-pot transmetalation/deprotonation reaction of the bulky triazene Dmp(Tph)N3H with bis(pentafluorophenyl)mercury and europium or ytterbium affords the structurally characterized unsolvated metal(II) pentafluorophenyl triazenides [Dmp(Tph)N3MC6F5] (M = Eu, Yb; Dmp = 2,6-Mes2C6H3 with Mes = 2,4,6-Me3C6H2; Tph = 2-TripC6H4 with Trip = 2,4,6-(i)Pr3C6H2) or, depending on the molar ratio, the solvated complex [Dmp(Tph)N3YbC6F5(THF)].

In the last decade a major theme of organometallic chemistry has been the design and development of alternative ligand systems capable of stabilizing monomeric metal complexes while provoking novel reactivity. Exploration of this field is driven by the potential use of these complexes in catalysis and organic synthesis. Examples of monoanionic chelating Ndonor ligands that have received much recent attention (Scheme 1) include the b-diketiminate (I) [1] and the amidinate (II) [2] ligand systems. Much less attention has been given to the closely related triazenides (III). [3, 4] This may be attributed to the lack of suitable ligands that are sterically crowded enough to prevent undesirable ligand redistribution reactions and allow better control of the electronic and steric properties at the metal center.Triazenides are weaker donors than the isoelectronic amidinates and the related b-diketiminates, and should induce greater electrophilicity at a bonded metal atom. [5] This is reflected by the results of an NBO (natural bond orbitals) analysis of the energy-minimized structures [6] of the model anions 1,3-diphenyl-1,3-diketiminate (I M ), 1,3-diphenyl-1,3-diazaallyl (II M ), and 1,3-diphenyltriazenide (III M ) (see Supporting Information), which shows an NPA (natural population analysis) charge for the chelating N atoms of À0.54, À0.60, and À0.38, respectively.We recently succeeded in the preparation of aryl-substituted, sterically crowded triazenes. Ligands of this type may be synthesized in excellent yields by the reaction of different substituted 2-lithiobiphenyls with the m-terphenyl azide 1, followed by hydrolysis (Scheme 2).In a first attempt to test their properties, we have used the obtained triazenes to stabilize pentafluorophenyl compounds of the heavier alkaline-earth metals calcium, strontium, and barium. The heteroleptic pentafluorophenyl triazenides are accessible in tetrahydrofuran as solvent by a convenient onepot transmetalation/deprotonation [7] reaction from the triazene 2 a (HN 3 ArAr'), bis(pentafluorophenyl)mercury, and the corresponding alkaline-earth metal (Scheme 3). After crystallization from n-heptane, either the THF-free compounds [M(C 6 F 5 )(N 3 ArAr')] (M = Sr (4), Ba (5)) or the solvate [Ca(C 6 F 5 )(N 3 ArAr')(thf)] (3) were isolated in good yields. It is remarkable that attempts to replace the pentafluorophenyl substituents by a second triazenide ligand have not been successful so far. Apparently, the steric bulk of the latter prevents further substitution or ligand redistribution and therefore formation of the homoleptic complexes. [8] Solutions of 4 or 5 in aromatic or aliphatic solvents show considerable thermal stability and can be stored at ambient