Azido-mediated intermolecular interactions of transition metal complexes

The coordinated azido ligand has a variety of ways to establish intermolecular contacts whose nature is computationally analysed in this work on dimers of the [N3-Hg(CF3)] complex with different interactions involving only N⋯N contacts, or with an additional Hg⋯N contact. The applied tools include the molecular electrostatic map of the monomer, an energy decomposition analysis (EDA), a topological AIM analysis of the electron density and the study of NCI (non-covalent interactions) isosurfaces. The interactions between two azido ligands are found to be weakly stabilizing (by 0.2 to 2.7 kcal mol-1), topology-dependent and require dispersion forces to complement orbital and electrostatic stabilization. Those interactions are supplemented by the formation of simultaneous Hg⋯N secondary interactions by about -1 kcal mol-1, and by the ability of the monomer to simultaneously interact with several neighbours in the crystal structure.

The First Alkaline-Earth Azidoaurate(III), Ba[Au(N3 )4 ]2 ⋅ 4 H2 O

Transparent, dark orange Ba[Au(N₃ )₄ ]₂  ⋅ 4 H₂ O was synthesized by reaction of Ba(N₃ )₂ and AuCl₃ or HAuCl₄ in aqueous solution. The novel barium tetraazidoaurate(III) tetrahydrate crystallizes in the monoclinic space group Cc (no. 9) with a=1813.68(17) pm, b=1737.95(11) pm, c=682.04(8) pm and β=108.849(4)°. The predominant structural features of Ba[Au(N₃ )₄ ]₂  ⋅ 4 H₂ O are two crystallographically independent discrete anions [Au(N₃ )₄ ]^- with gold in a tetragonal planar coordination by nitrogen. Vibrational spectra show good agreement with those of other azidoaurates(III). Upon drying, this salt was shown to be a highly explosive material.

Tale of Three Molecular Nitrides: Mononuclear Vanadium (V) and (IV) Nitrides As Well As a Mixed-Valence Trivanadium Nitride Having a V3N4 Double-Diamond Core

Transmetallation of [VCl₃(THF)₃] and [TlTp^tBu,Me] afforded [(Tp^tBu,Me)VCl₂] (1, Tp^tBu,Me = hydro-tris(3-tert-butyl-5-methylpyrazol-1-yl)borate), which was reduced with KC₈ to form a C_3v symmetric V^II complex, [(Tp^tBu,Me)VCl] (2). Complex 1 has a high-spin (S = 1) ground state and displays rhombic high-frequency and -field electron paramagnetic resonance (HFEPR) spectra, while complex 2 has an S = 3/2 ⁴A₂ ground state observable by conventional EPR spectroscopy. Complex 1 reacts with NaN₃ to form the V^V nitride-azide complex [(Tp^tBu,Me)V≡N(N₃)] (3). A likely V^III azide intermediate en route to 3, [(Tp^tBu,Me)VCl(N₃)] (4), was isolated by reacting 1 with N₃SiMe₃. Complex 4 is thermally stable but reacts with NaN₃ to form 3, implying a bis-azide intermediate, [(Tp^tBu,Me)V(N₃)₂] (A), leading to 3. Reduction of 3 with KC₈ furnishes a trinuclear and mixed-valent nitride, [{(Tp^tBu,Me)V}₂(μ₄-VN₄)] (5), conforming to a Robin-Day class I description. Complex 5 features a central vanadium ion supported only by bridging nitride ligands. Contrary to 1, complex 2 reacts with NaN₃ to produce an azide-bridged dimer, [{(Tp^tBu,Me)V}₂(1,3-μ₂-N₃)₂] (6), with two antiferromagnetically coupled high-spin V^II ions. Complex 5 could be independently produced along with [(κ₂-Tp^tBu,Me)₂V] upon photolysis of 6 in arene solvents. The putative {V^IV≡N} intermediate, [(Tp^tBu,Me)V≡N] (B), was intercepted by photolyzing 6 in a coordinating solvent, such as tetrahydrofuran (THF), yielding [(Tp^tBu,Me)V≡N(THF)] (B-THF). In arene solvents, B-THF expels THF to afford 5 and [(κ₂-Tp^tBu,Me)₂V]. A more stable adduct (B-OPPh₃) was prepared by reacting B-THF with OPPh₃. These adducts of B are the first neutral and mononuclear V^IV nitride complexes to be isolated.

Cobalt(II) "Scorpionate" complexes as electronic ground state models for cobalt-substituted zinc enzymes: Structure investigation by magnetic circular dichroism

Zinc centers in pseudo-tetrahedral geometry are widely found in biology, often with three histidine ligands from protein. The trispyrazolylborate "scorpionate" ligand is used as a model for this tris(histidine) motif, and spectroscopically active Co^II is often used as a substitute for spectroscopically silent Zn^II. In this work, four pseudo-tetrahedral scorpionate complexes with the formula (Tp^t-Bu,Tn)CoL, where Tp^t-Bu,Tn = hydrotris(3-tert-butyl, 5-2'-thienyl-pyrazol-1-yl)borate anion and L = Cl^-, N₃^-, NCO^-, or NCS^-, were studied using variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The major goal was to determine the axial and rhombic zero field splitting (ZFS) parameters (D and E, respectively) of these S = 3/2 systems and compare these ZFS parameters to those determined previously by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on the same (L = Cl^- and NCS^-) or closely related complexes. Additionally, HFEPR studies were undertaken here on the complexes with L = N₃^-, NCO^-. Crystal structures for these two complexes are also first reported here. The values of D determined by VTVH MCD were + 12.8 and + 3.6 cm^-1 for the L = Cl^- and NCS^- complexes, respectively. These values are in close agreement with those for the same complexes as previously determined by HFEPR. The values of D determined by VTVH MCD were + 3.0 and + 6.6 cm^-1 for the L = N₃^- and NCO^- complexes, respectively. These values were not as close to those determined by HFEPR in the present study, which are 4.2 cm^-1 ≤ |D| ≤ 5.6 cm^-1 in Tp^t-Bu,TnCoN₃, and 8.3 cm^-1 ≤ |D| ≤ 11.0 cm^-1 in Tp^t-Bu,TnCoNCO. The bands in MCD spectra of these complexes were assigned in C_3v symmetry and a complete ligand-field analysis of the MCD data was made using the Angular Overlap Model (AOM), which is compared to previous results.

The Uranium(VI) Oxoazides [UO2 (N3 )2 ⋅CH3 CN], [(bipy)2 (UO2 )2 (N3 )4 ], [(bipy)UO2 (N3 )3 ]- , [UO2 (N3 )4 ]2- , and [(UO2 )2 (N3 )8 ]4

The reaction between [UO₂ F₂ ] and an excess of Me₃ SiN₃ in acetonitrile solution results in fluoride-azide exchange and the uranium(VI) dioxodiazide adduct [UO₂ (N₃ )₂ ⋅CH₃ CN] was isolated in quantitative yield. The subsequent reaction of [UO₂ (N₃ )₂ ⋅CH₃ CN] with 2,2'-bipyridine (bipy) resulted in the formation of the azido-bridged binuclear complex [(bipy)₂ (UO₂ )₂ (N₃ )₄ ]. The triazido anion [(bipy)UO₂ (N₃ )₃ ]^- was obtained by the reaction of [UO₂ (N₃ )₂ ⋅CH₃ CN] with stoichiometric amounts of bipy and the ionic azide [PPh₄ ][N₃ ]. The reaction of [UO₂ (N₃ )₂ ] with two equivalents of the [PPh₄ ][N₃ ] resulted in the formation of the mononuclear tetraazido anion [UO₂ (N₃ )₄ ]^2- as well as the azido-bridged binuclear anion [(UO₂ )₂ (N₃ )₈ ]^4- . The novel uranium oxoazides were characterized by their vibrational spectra and in the case of [(bipy)₂ (UO₂ )₂ (N₃ )₄ ]⋅CH₃ CN, [PPh₄ ][(bipy)UO₂ (N₃ )₃ ], [PPh₄ ]₂ [UO₂ (N₃ )₄ ], [PPh₄ ]₂ [UO₂ (N₃ )₄ ]⋅2CH₃ CN, and [PPh₄ ]₄ [(UO₂ )₂ (N₃ )₈ ]⋅4CH₃ CN by their X-ray crystal structures.

The niobium oxoazides [NbO(N3)3], [NbO(N3)3·2CH3CN], [(bipy)NbO(N3)3], Cs2[NbO(N3)5] and [PPh4]2[NbO(N3)5]

Niobium oxotriazide, [NbO(N3)3], was prepared in a fluoride-azide exchange reaction between [NbOF3] and an excess of Me3SiN3 in SO2 solution. In acetonitrile solution, the fluoride-azide exchange resulted in the isolation of the adduct [NbO(N3)3·2CH3CN]. The subsequent reaction of [NbO(N3)3] with 2,2'-bipyridine (bipy) resulted in the isolation of the bipyridine adduct [(bipy)NbO(N3)3]. The pentaazido anion [NbO(N3)5](2-) was obtained by the reaction of [NbO(N3)3] with two equivalents of ionic azide. The novel niobium oxoazides were fully characterized by their vibrational spectra, impact, friction and thermal sensitivity data and, in the case of [(bipy)NbO(N3)3], Cs2[NbO(N3)5], and [PPh4]2[NbO2(N3)5] by their X-ray crystal structures.

Taming Tin(IV) Polyazides

The first charge-neutral Lewis base adducts of tin(IV) tetraazide, [Sn(N3)4(bpy)], [Sn(N3)4(phen)] and [Sn(N3)4(py)2], and the salt bis{bis(triphenylphosphine)iminium} hexa(azido)stannate [(PPN)2Sn(N3)6] (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline; py = pyridine; PPN = N(PPh3)2) have been prepared using covalent or ionic azide-transfer reagents and ligand-exchange reactions. The azides were isolated on the 0.3 to 1 g scale and characterized by IR and NMR spectroscopies, microanalytical and thermal methods and their molecular structures determined by single-crystal XRD. All complexes have a distorted octahedral Sn[N]6 coordination geometry and possess greater thermal stability than their Si and Ge homologues. The nitrogen content of the adducts of up to 44% exceed any Sn(IV) compound known hitherto.

The First Molybdenum(VI) and Tungsten(VI) Oxoazides MO2(N3)2, MO2(N3)2⋅2 CH3CN, (bipy)MO2(N3)2, and [MO2(N3)4](2-) (M=Mo, W)

Molybdenum(VI) and tungsten(VI) dioxodiazide, MO2(N3)2 (M=Mo, W), were prepared through fluoride-azide exchange reactions between MO2F2 and Me3SiN3 in SO2 solution. In acetonitrile solution, the fluoride-azide exchange resulted in the isolation of the adducts MO2(N3)2⋅2 CH3CN. The subsequent reaction of MO2(N3)2 with 2,2'-bipyridine (bipy) gave the bipyridine adducts (bipy)MO2(N3)2. The hydrolysis of (bipy)MoO2(N3)2 resulted in the formation and isolation of [(bipy)MoO2N3]2O. The tetraazido anions [MO2(N3)4](2-) were obtained by the reaction of MO2(N3)2 with two equivalents of ionic azide. Most molybdenum(VI) and tungsten(VI) dioxoazides were fully characterized by their vibrational spectra, impact, friction, and thermal sensitivity data and, in the case of (bipy)MoO2(N3)2, (bipy)WO2(N3)2, [PPh4]2[MoO2(N3)4], [PPh4]2[WO2(N3)4], and [(bipy)MoO2N3]2O by their X-ray crystal structures.

The Vanadium(V) Oxoazides [VO(N3)3], [(bipy)VO(N3)3], and [VO(N3)5](2-)

Vanadium(V) oxoazide [VO(N3)3] was prepared through a fluoride-azide exchange reaction between [VOF3] and Me3SiN3 in acetonitrile solution. When the highly impact- and friction-sensitive compound [VO(N3)3] was reacted with 2,2′-bipyridine, the adduct [(bipy)VO(N3)3] was isolated. The reaction of [VO(N3)3] with [PPh4]N3 resulted in the formation and isolation of the salt [PPh4]2[VO(N3)5]. The adduct [(bipy)VO(N3)3] and the salt [PPh4]23[VO(N3)5] were characterized by vibrational spectroscopy and single-crystal X-ray structure determination, making these compounds the first structurally characterized vanadium(V) azides.

Crystal structure of azido-(η(5)-cyclo-penta-dien-yl)bis-(tri-phenyl-phosphane-κP)ruthenium(II) di-chloro-methane hemisolvate

The title solvated complex, [Ru(η(5)-C5H5)(N3){P(C6H5)3}2]·0.5CH2Cl2, displays a typical piano-stool geometry about the Ru(II) atom. The bond lengths and angles of the cyclo-penta-dienyl and phosphane ligands are very similar to that of the unsolvated complex [Taqui Khan et al. (1994 ▶). Acta Cryst. C50, 502-504]. The azide anion displays similar N-N distances of 1.173 (3) and 1.156 (3) Å and has an N-N-Ru angle of 119.20 (15)°, indicating a greater contribution of the canonical form Ru-N=N((+))=N((-)) for the bonding situation. An intra-molecular C-H⋯N hydrogen-bonding inter-action between one ortho H atom of a phosphane ligand and the N atom coordinating to the metal is observed. A similar inter-molecular inter-action is observed between a meta H atom of a phosphane ligand and the terminal azide N atom of a neighbouring complex. Finally, two C-H⋯N inter-actions exists between the H atoms of the di-chloro-methane solvent mol-ecule and the terminal N atom of two azide anions. The solvent mol-ecule is located about a twofold rotation axis and shows disorder of the Cl atoms with an occupancy ratio of 0.62 (3):0.38 (3).

Coordination adducts of niobium(V) and tantalum(V) azide M(N₃)₅ (M=Nb, Ta) with nitrogen donor ligands and their self-ionization

Several new donor-acceptor adducts of niobium and tantalum pentaazide with N-donor ligands have been prepared from the pentafluorides by fluoride-azide exchange with Me3SiN3 in the presence of the corresponding donor ligand. With 2,2'-bipyridine and 1,10-phenanthroline, the self-ionization products [MF4(2,2'-bipy)2](+)[M(N3)6](-), [M(N3)4(2,2'-bipy)2](+)[M(N3)6](-) and [M(N3)4(1,10-phen)2](+)[M(N3)6](-) were obtained. With the donor ligands 3,3'-bipyridine and 4,4'-bipyridine the neutral pentaazide adducts (M(N3)5)2⋅L (M=Nb, Ta; L=3,3'-bipy, 4,4'-bipy) were formed.

Probing platinum azido complexes by 14N and 15N NMR spectroscopy

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) azido complexes is extremely broad (W(1/2)≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic<py<NH(3)<Cl(-)<N(3)(-)<NO(2)(-)<SCN(-) (2-pic=2-methylpyridine). In addition, an axial Cl(-) appears to weaken an equatorial Pt(IV)-NH(3) bond to a greater extent than an axial OH(-) ligand.

Molecular structure of hydrazoic acid with hydrogen-bonded tetramers in nearly planar layers

Hydrazoic acid (HN(3))--potentially explosive, highly toxic, and very hygroscopic--is the simplest covalent azide and contains 97.7 wt % nitrogen. Although its molecular structure was established decades ago, its crystal structure has now been solved by X-ray diffraction for the first time. Molecules of HN(3) are connected to each other by hydrogen bonds in nearly planar layers parallel to (001) with stacking sequence A, B, ... The layer distance, at 2.950(1) Å, is shorter than that in 2H-graphite [3.355(2) Å]. The hydrogen bonds N-H···N are of great interest, since the azido group consists of three homonuclear atoms with identical electronegativity, but different formal charges. These hydrogen bonds are bifurcated into moderate ones with ≈2.0 Å and into weak ones with ≈2.6 Å. The moderate ones build up tetramers (HN(3))(4) in a nearly planar net of eight-membered rings. To the best of our knowledge, such a network of tetramers of a simple molecule is unique.

Recoupled-pair bonding and 4-electron 3-center bonding units

Consideration is given to recoupled-pair bonding and the origin of electronic hypervalence for formulations of the bonding for symmetric 4-electron 3-center ((4e,3c)) bonding units with one overlapping atomic orbital per atomic center. Molecular orbital and valence bond theory for symmetric (4e,3c) bonding units is redescribed and applied to aspects of the bonding for SF(6) and CLi(6). The results of minimal basis set calculations for CLi(6) provide support for a hypothesis that two Li-C-Li (3e,3c) bonding units rather than two (4e,3c) bonding units are preferred for this molecule. Brief comments are also made on the use of [Formula: see text] and [Formula: see text] as valence bond structures for the three electron bond.