A direct hydrogen-1 and phosphorus-31 nuclear magnetic resonance cation solvation study of Al(ClO4)3, Ga(ClO4)3, In(ClO4)3, UO2(ClO4)2, and UO2(NO3)2 in water-acetone-dimethylsulfoxide and water-acetone-hexamethylphosphoramide mixtures

A competitive solvation study of Al(ClO₄)₃, Ga(ClO₄)₃, In(ClO₄)₃, UO₂(ClO₄)₂, and UO₂(NO₃)₂ in water-acetone-dimethylsulfoxide (DMSO) and water-acetone-hexamethylphosphoramide (HMPT) mixtures has been carried out by direct H¹ and P³¹ nuclear magnetic resonance (NMR) techniques. At low temperature, proton and ligand exchange are slow enough in these systems to permit the observation of signals for bulk and coordinated molecules of water and the organic bases (DMSO and HMPT). Both DMSO and HMPT compete effectively with water for coordination sites in the Al³⁺, Ga³⁺, and In³⁺ systems, with steric effects dominating the HMPT results. Both Al³⁺ and In³⁺ are able to bind a maximum of two to three HMPT molecules, for example. In contrast, UO²⁺ is solvated selectively by the organic molecules to the allowed maximum of 4 molecules per cation. H¹ and P³¹ NMR spectral results support the formation of only the mono-, tri-, and tetra-HMPT solvation complexes.     

Synthesis,crystal structure,electrochemical and magnetic properties of dinuclear complexes with strong electron-drawing groups in the diphenoxo-tetraaza macrocyclic ligand

Three symmetrical macrocyclic dinuclear complexes [M₂L(H₂O) _n ](ClO₄)₂ (M²⁺ = Cu²⁺, Ni²⁺, Mn²⁺ and n = 0, 2) have been synthesized by cyclocondensation between 2,6-diformyl-4-fluorophenol and 1,4-diaminobutane in the presence of M²⁺ cations. The crystal structure of [Cu₂L](ClO₄)₂ was determined by X-ray diffraction techniques. The electronic and magnetic properties of the complexes were studied by cyclic voltammetry and magnetic susceptibility. The results confirm that the complexes obtain electrons easily and there are very strong antiferromagnetic couplings between two copper(II) ions in [Cu₂L](ClO₄)₂. The strong electron-drawing groups of fluorine attached to the phenyl ring of a macrocyclic complex enhances the antiferromagnetic exchange of the complex and makes it more easily reduced than its analogs.     

Dimethyl sulfoxide as an O-donor ligand in tetravalent actinide complexes

Four new perchlorate complexes of tetravalent actinides with dimethyl sulfoxide (DMSO) molecules (An⁴⁺ = Th, U, Np, Pu) are synthesized and studied. According to the X-ray diffraction data, compounds [Th(DMSO)₉](ClO₄)₄ · 2CH₃CN (I), [U(DMSO)₈](ClO₄)₄ · CH₃CN (II), [Np(DMSO)₈](ClO₄)₄ · CH₃CN (III), and [Pu(DMSO)₈](ClO₄)₄ · CH₃CN (IV) crystallize in the triclinic crystal system (space group P1). The crystals of compounds II–IV are isostructural. The absorption spectra of the complexes in the IR and visible regions are measured. All compounds exhibit a decrease in the frequencies of stretching vibrations ν(SO) over the spectrum of free DMSO, indicating the formation of the O-bonded complexes of An⁴⁺. The optical spectra of the crystalline compounds exhibit shifts of the bands of electronic f-f transitions of the An⁴⁺ ions relative to the hydrated ions: the bathochromic shifts for the U and Np complexes and the hypsochromic shift for the Pu complex. The first coordination sphere of the actinide atoms in the studied complexes is highly stable.     

Synthesis and crystal structure of aqua(2.2.2-cryptand)(perchlorato-O)lead(II) diaqua(2.2.2-cryptand)lead(II) tris(perchlorate)

A mixed complex aqua(2.2.2-cryptand)(perchlorato-O)lead(II) diaqua(2.2.2-cryptand)lead(II) tris(perchlorate), [Pb(2.2.2-Crypt)(CIO₄)(H₂O)]⁺ [Pb(2.2.2-Crypt)(H₂O)₂]²⁺ (ClO ₄ ^? )₃, is synthesized and studied by X-ray diffraction analysis. The crystals are orthorhombic: space group Pbca, a = 19.118 Å, b = 15.360 Å, c = 39.020 Å, Z = 8. The structure is solved by a direct method and refined by the full-matrix least-squares method in the anisotropic approximation to R = 0.089 for 7712 reflections (CAD-4 automated diffractometer, λMoK _α radiation). In each of the two complex cations of the host-guest type in the structure, the Pb²⁺ cation is coordinated by all the eight heteroatoms (6O + 2N) of the cryptand ligand and by two O atoms of the water molecule and ClO ₄ ^? anion or by two O atoms of two water molecules. In the crystal, alternating complex cations and ClO ₄ ^? anions are linked into infinite chains (along the z axis) through interionic hydrogen bonds O-H···O-Cl.     

Protonation of the acyclic tetraaza metallocomplex of gold(III) [Au(C9H19N4)]2+ in aqueous solution. Synthesis and crystal and molecular structure of [Au(C9H20N4)](H5O2)(ClO4)4

Protonation equilibrium has been studied for the acyclic gold(III) tetraaza metallocomplex [AuB]²⁺ [B = N, N′-bis(2-aminoethyl)-2,4-pentanediiminato(1−)] in aqueous solution. The synthetic procedure is described. The crystal and molecular structure of the protonated form of the [AuHB](H₅O₂)(ClO₄)₄ complex has been determined. Monoclinic crystals with unit cell dimensions a = 11.964(2) Å, b = 13.789(3) Å, c = 15.496(3) Å, β = 109.00(3)°, V = 2417.1(8) ų, Z = 4, ρcalc = 2.243 g/cm³, space group P2₁/n. The structure is built of nearly planar [Au(C₉H₂₀N₄)]³⁺ complex cations, (H₅O₂)⁺ cations, and [ClO₄]⁻ anions. The gold atom coordinates four nitrogen atoms of the ligand, forming a square plane. The six-membered chelate ring of the ligand is protonated at the central β-carbon atom and contains imine C=N bonds. The oxygen atoms of the perchlorate ions are hydrogen bonded to the (H₅O₂)⁺ dihydroxonium ion and to the nitrogen atoms of the NH₂ groups of the [AuHB]³⁺ cation. Original Russian Text Copyright ? 2005 by V. A. Afanasieva, L. A. Glinskaya, R. F. Klevtsova, and I. V. Mironov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 46, No. 5, pp. 909–915, September–October, 2005.     

Tuning the Electronic Properties in Ruthenium–Quinone Complexes through Metal Coordination and Substitution at the Bridge

A rare example of a mononuclear complex [(bpy)₂Ru(L¹_?H)](ClO₄), 1 (ClO₄) and dinuclear complexes [(bpy)₂Ru(μ‐L¹_?2H)Ru(bpy)₂](ClO₄)₂, 2 (ClO₄)₂, [(bpy)₂Ru(μ‐L²_?2H)Ru(bpy)₂](ClO₄)₂, 3 (ClO₄)₂, and [(bpy)₂Ru(μ‐L³_?2H)Ru(bpy)₂](ClO₄)₂, 4 (ClO₄)₂ (bpy=2,2′‐bipyridine, L¹=2,5‐di‐(isopropyl‐amino)‐1,4‐benzoquinone, L²=2,5‐di‐(benzyl‐amino)‐1,4‐benzoquinone, and L³=2,5‐di‐[2,4,6‐(trimethyl)‐anilino]‐1,4‐benzoquinone) with the symmetrically substituted p‐quinone ligands, L, are reported. Bond‐length analysis within the potentially bridging ligands in both the mono‐ and dinuclear complexes shows a localization of bonds, and binding to the metal centers through a phenolate‐type “O^?” and an immine/imminium‐type neutral “N” donor. For the mononuclear complex 1 (ClO₄), this facilitates strong intermolecular hydrogen bonding and leads to the imminium‐type character of the noncoordinated nitrogen atom. The dinuclear complexes display two oxidation and several reduction steps in acetonitrile solutions. In contrast, the mononuclear complex 1 ⁺ exhibits just one oxidation and several reduction steps. The redox processes of 1 ¹⁺ are strongly dependent on the solvent. The one‐electron oxidized forms 2 ³⁺, 3 ³⁺, and 4 ³⁺ of the dinuclear complexes exhibit strong absorptions in the NIR region. Weak NIR absorption bands are observed for the one‐electron reduced forms of all complexes. A combination of structural data, electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structures of the complexes. Our DFT results indicate that the electronic natures of the various redox states of the complexes in vacuum differ greatly from those in a solvent continuum. We show here the tuning possibilities that arise upon substituting [O] for the isoelectronic [NR] groups in such quinone ligands.     

Modifizierung von Schichtsilicaten mit sterisch anspruchsvollen Metallkomplexen. 3. Synthese und Intercalation der planar-quadratischen Komplexe [Cu(bppep)(H2O)](ClO4)2 und [Ni(bppep)(Cl)Cl] (bppep = 2,6-Bis[1-phenyl-1-(pyridin-2-yl)ethyl]pyridin) in Hectorit

Modification of Layer Silicates by Sterically Demanding Metal Complexes: Synthesis and Intercalation of the Square Planar Complexes [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl (bppep = 2,6-Bis[1-phenyl-1-(pyridine-2-yl)ethyl]pyridine) in Hectorite Sodium-aqua hectorite reacts with [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl with exchange of the sodium-aqua cations against the complex cations [Cu(bppep)(H₂O)]²⁺ and [Ni(bppep)(Cl)]⁺, respectively. In addition, cation-anion pairs of [Cu(bppep)(H₂O)](ClO₄)₂ and [Ni(bppep)(Cl)]Cl are also intercalated between the hectorite layers (intersalation). On the other hand, it is possible to synthesize [Cu(bppep)(H₂O)]²⁺ or [Ni(bppep)(H₂O)]²⁺ modified hectorites without additional ion-pair intercalation (intersalation) by reaction of nickel- and copper-hectorites with the bppep ligand.     

Anion‐Dependence of Ytterbium Complexes and Their NIR Luminescence

The reaction of 2‐aldehyde‐8‐hydroxyquinoline, histamine, and YbX₃ · 6H₂O (X = NO₃^–, ClO₄^–) affords two ytterbium complexes [Yb(nma)₂] · ClO₄ · 2CH₂Cl₂ ( 1 ) and [Yb(nma)(NO₃)₂(DMSO)] · CH₃OH ( 2 ) (Hnma = N‐(2‐(8‐hydroxylquinolinyl)methane(2‐(4‐imidazolyl)ethanamine))). The crystal structures were determined by X‐ray diffraction and it has been revealed that the anions have played important role in the assembly. In the case of 1 , the Yb³⁺ ions are completely encapsulated by two nma ligands with uncoordinated perchlorate anion balancing the positive charge. In the case of 2 , the Yb³⁺ ions are ligated by the ligand, oxygen atoms of the nitrate ion, and DMSO. Both complexes exhibit essential NIR luminescence of Yb³⁺ ions.     

Low‐dimensional compounds containing cyano groups. XII.)copper(II) perchlorate

The title compound, [Cu(C₂N₃)(C₃H₁₀N₂)₂]ClO₄, is made up of [Cu(tn)₂{N(CN)₂}]⁺ complex cations (tn is 1,3‐diamino­propane) and ClO₄⁻ anions. The Cu^II atom is coordinated by four N atoms of two equatorial tn ligands, with an average distance of 2.041 (7) Å, and one nitrile N atom of the dicyanamide anion in an axial position, at a distance of 2.236 (3) Å, in a manner approaching square‐planar coordination geometry. The complex has C_s symmetry, with the mirror plane lying through the central C atoms of both tn ligands and the dca ligand. The ClO₄⁻ anion might be considered as very weakly coordinated in the opposite axial position [Cu—O = 2.705 (3) Å], thus completing the Cu^II coordination to asymmetric elongated octa­hedral (4+1+1*). The Cu atom and the perchlorate anion both lie on mirror planes.     

Osmotic coefficients of aqueous rare-earth perchlorates and nitrates

Osmotic coefficients of aqueous solutions of La(ClO₄)₃, Pr(ClO₄)₃, Nd(ClO₄)₃, Sm(ClO₄)₃, Gd(ClO₄)₃, Dy(ClO₄)₃, Ho(ClO₄)₃, Er(ClO₄)₃, Tm(ClO₄)₃, Yb(ClO₄)₃, Lu(ClO₄)₃, Nd(NO₃)₃, Sm(NO₃)₃, and Gd(NO₃)₃ in the molality range from 0.1 mol kg^?1 nearly to saturation at 298.15 K have been determined by the isopiestic method. The results obtained agree very well with those of Rard et al. published recently. A distinct two-series effect has been observed for the constant-molality values of rare-earth perchlorates. The results are discussed in terms of hydration of the cations and ionic association.     

Synthesis and characterization of Co(II), Ni(II), Zn(II) and Cu(II) complexes with a new tetraazamacrocyclic Schiff base ligand containing a piperazine moiety: X-ray crystal structure determination of the Co(II) complex

Two macrocyclic Schiff base ligands, L¹ [1+1] and L² [2+2], have been obtained in a one-pot cyclocondensation of 1,4-bis(2-formylphenyl)piperazine and 1,3-diaminopropane. Unfortunately, because of the low solubility of both ligands, their separation was unsuccessful. In the direct reaction of these mixed ligands (L¹ and L²) and the appropriate metal ions only [CoL¹(NO₃)]ClO₄, [NiL¹](ClO₄)₂, [CuL¹](ClO₄)₂ and [ZnL¹(NO₃)]ClO₄ complexes have been isolated. All the complexes were characterized by elemental analyses, IR, FAB-MS, conductivity measurements and in the case of the [ZnL¹(NO₃)]ClO₄ complex with NMR spectroscopy.     

Dinuclear nickel complexes of phenolbased dinucleating macrocycles: Synthesis,structure, and properties

Abstract

Dinuclear Ni₂(II,II) complexes with the formula [Ni₂(R^m,n)](ClO₄)₂ ((m,n)= (2,2) (1), (2,3) (2), (2,4) (3)) have been obtained where (R^m,n)^2- denotes the macrocycles containing two 2,6-bis(iminomethyl)-4-methylphenolate entities combined through two lateral chains, -(CH₂)_m- and -(CH₂)_n-, at the imino nitrogens. [Ni₂(R^2,2)](ClO₄)₂ (1) crystallizes in the triclinic crystal system, space group P 1, with Z=1, a=8.396(2) Å, b= 10.021(2) Å, c=8.104(2) Å, α=109.56(2)°, β=99.40(2)°, γ=79.89(2)°, V=628.5(3) ų and Z=1. The refinement converges with R=0.0384 and R_w=0.0415 for 2075 reflections with | Fo | > 3[sgrave](| Fo |). In the centrosymmetric [Ni₂(R^2,2)]²⁺, a pair of Ni(II) ions are bridged by two phenolic oxygens with the Ni···Ni separation of 2.801(1) Å. Each Ni assumes a planar configuration with Ni-O bond distances of 1.842(3) and 1.838(3) Å and Ni-N bond distances of 1.814(3) and 1.823(3) Å. In the solid state, 1 is diamagnetic (S₁=S₂=0) whereas [Ni₂(R^2,3)]-(ClO₄)₂ (2) and [Ni₂(R^2,4)](ClO₄)₂ (3) are of a mixed-spin (S₁=0, S₂=1). In DMSO and pyridine all the complexes assume high-spin (S₁=S₂=1). The Ni₂(II,II) complexes are electrochemically reduced in DMSO or pyridine to Ni₂(I,II) and Ni₂(I,I) complexes. The conproportionation constants of the Ni₂(I,II) complexes are determined to be 3.4X10⁴-1.2X10⁵ in DMSO and 1.6X10³-2.6X10⁵ in pyridine. The Ni₂(I,II) and Ni₂(I,I) complexes of 1–3 have been prepared by electrolysis in DMSO. The mixed-valent complexes of 1 and 2 are characterized by an intervalence (IV) transition band at 790 and ～ 700 nm, respectively, and belong to Class II using the classification of Robin and Day. The Ni₂(I,II) complex of 3 shows no IT band (Class I). The Ni₂(I,II) complexes of 1-3 show well-resolved ESR spectra due to the spin-coupled S_T = 1/2 ground-state. The Ni₂(I,I) complexes of 1-3 are all ESR- innocent probably due to the strong antiferromagnetic interaction.     

2,2′‐Bipyridinium bis(perchlorate)

The title compound, [H₂bipy](ClO₄)₂ or C₁₀H₁₀N₂²⁺·2ClO₄^?, was obtained at the interface between an organic (2,2′‐bi­pyridine in methanol) and an aqueous phase (perchloric acid in water). The compound crystallizes in space group P and comprises discrete diprotonated trans‐bipyridinium cations, [H₂bipy]²⁺, and ClO₄^? anions. The cations and anions are connected through N—H?O and C—H?O hydrogen bonds [distances N?O 2.817 (4) and 2.852 (4) Å, and C?O 3.225 (6)–3.412 (5)Å]. The C—C bond distance between the two rings is 1.452 (5) Å. The bipyridinium cation has a trans conformation and the N—C—C—N torsion angle is 152.0 (3)°.     

Synthesis, crystal structure, hydrogen bonding and spectroscopy of transition-metal complexes with the ligand (N,N′-bis(2-pyridylmethyl)-1,3-propanediamine)

Six mononuclear complexes are reported with the tetradentate ligand N,N′-bis(2-pyridylmethyl)-1,3-propanediamine, (abbreviated as pypn) i.e. [Cu(pypn)(ClO₄)₂](H₂O)_1/2 (1), [Fe(pypn)Cl₂](NO₃) (2), [Zn(pypn)Cl](ClO₄) (3), [Co(pypn)(NCS)₂](ClO₄) (4), [Co(pypn)(N₃)₂](ClO₄) (5), [Zn(pypn)(NCS)₂] (6). The synthesis and X-ray crystal structures of all six compounds and their spectroscopic properties are presented.The geometry of the Cu²⁺, Co³⁺, Zn²⁺, Fe³⁺ ions is essentially octahedrally based, with the mm conformation (for Cu) and m_sf conformations for the other 3 metal ions; in compound 3 the geometry around the Zn²⁺ is distorted trigonal bipyramidal. The stabilisation of the crystal lattices is maintained by interesting, relative strong hydrogen bonds.     

Crystal structure of NaBa2[Co(Edta)]2(ClO4)3 · 9H2O

Crystals of Ba Na ethylenediaminetetraacetatocobaltate(III) perchlorate, NaBa₂[Co(Edta)]₂(ClO₄)₃ · 9H₂O, contain two crystallographically nonequivalent Ba²⁺ cations and two complex cations [Co(Edta)]^? (Edta^4? is ethylenediaminetetraacetic acid anion), the latter anions showing opposite chirality. The nearest surrounding of the Ba²⁺ ions involves three water molecules (including two bridging water molecules), six O atoms of four complexes [Co(Edta)]^? and the perchlorate O atom. Tetrameric fragments Ba₂(H₂O)₄[Co(Edta)]₂(ClO₄)₂ are united through the Ba-O bonds into layers with the Ba atoms in the middle of the layers and the perchlorate ions and complex anions at the periphery; in the latter anions, noncoordinated O atom of one of the R-metallocycles is directed outside. The Na atom of the Na(H₂O)(ClO₄) group located between the layers is bonded to these O atoms of the neighboring layers.     

Oxidative DNA cleavage by Cu(II) complexes of 1,10-phenanthroline-5,6-dione

A dimeric dichloro-bridged copper(II) complex [Cu₂(pdon)₂Cl₄] · 2DMF (1) and two mononuclear copper(II) complexes [Cu(pdon)(DMSO)Cl₂] · DMSO · H₂O (2) and [Cu(pdon)₃] · (ClO₄)₂ · 2.25CH₃CN · 6H₂O (3) (pdon = 1,10-phenanthroline-5,6-dione) have been synthesized and characterized. Variable-temperature magnetic susceptibility studies indicate the existence of weak anti-ferromagnetic coupling in the binuclear complex. The interaction of these complexes with CT-DNA (calf thymus DNA) has been studied using absorption and emission spectral methods. The apparent binding constants (K _app) for 1, 2 and 3 are 5.20 × 10⁵, 2.68 × 10⁵ and 7.05 × 10⁵ M^?1, respectively, showing moderate intercalative binding modes. All of these complexes cleave plasmid DNA to nicked DNA in a sequential manner as the concentration or reaction time is increased. The cleavage mechanism between the complex and plasmid DNA is likely to involve singlet oxygen ¹O₂ and ?OH as reactive oxygen species.     

[Ag(NH3)2]ClO4: Kristallstrukturen,Phasenumwandlung, Schwingungsspektren

[Ag(NH₃)₂]ClO₄: Crystal Structures, Phase Transition, and Vibrational Spectra [Ag(NH₃)₂](ClO₄) is obtained from a solution of AgClO₄ in conc. ammonia as colourless single crystals (orthorhombic, Pnmn, Z = 4, a = 795.2(1) pm, b = 617.7(1) pm, c = 1298.2(2) pm, R_all = 0.0494). The structure consists of linearly coordinated cations, [Ag(NH₃)₂]⁺, stacked in a staggered conformation and of tetrahedral (ClO₄)^‐ anions. A first order phase transition was observed between 210 and 200 K and the crystal structure of the low‐temperature modification (monoclinic, P2/m, Z = 4, a = 789.9(5) pm, b = 604.1(5) pm, c = 1290.4(5) pm, β = 97.436(5)°, at 170 K, R_all = 0.0636) has also been solved. Spectroscopic investigations (IR/Raman) have been carried out and the assignment of the spectra is discussed.     

Solubility of aluminum,gallium, and indium perchlorates in water

Solubility polytherms of the system M(ClO₄)₃-H₂O (M³⁺ = Al³⁺, Ga³⁺, In³⁺) were measured. Positions of singular points and vitrification ranges were determined. Values of eutectic concentrations were compared with earlier known data on the inversion concentrations measured by the viscometry method. Compositions of found crystalline hydrates were calculated from the solubility polytherms. The concept of the ability of the studied aqua cations to show properties of aqua acids, of the structure of solutions of posteutectic concentrations (structurally forced processes and microheterogeneity), of the electronic structure of M³⁺ cations, and of the localized hydrolysis phenomenon were used to understand the existence of the polyaqua compounds.     

Hydrogen-bonded networks based on manganese(II), nickel(II), copper(II) and zinc(II) complexes of N,N′-dimethylurea

A project related to the crystal engineering of hydrogen-bonded coordination complexes has been initiatied and some of our first results are presented here. The compounds [Mn(DMU)₆](ClO₄)₂ (1), [Ni(DMU)₆](ClO₄)₂ (2), [Cu(OClO₃)₂(DMU)₄] (3) and [Zn(DMU)₆](ClO₄)₂ (4) have all been prepared from the reaction of N,N-dimethylurea (DMU) and the appropriate hydrated metal perchlorate salt. Crystal structure determinations of the four compounds demonstrate the existence of [M(DMU)₆]²⁺ cations and ClO₄ ^– counterions in (1), (2) and (4), whereas in (3) monodentate coordination of the perchlorate groups leads to molecules. The [M(DMU)₆]²⁺ cations and ClO₄ ^– anions self-assemble to form a hydrogen-bonded one-dimensional (1D) architecture in (1) and different 2D hydrogen-bonded networks in (2) and (4). The hydrogen bonding functionalities on the molecules of (3) create a 2D structure. The complexes were also characterised by room-temperature effective magnetic moments and i.r. studies. The data are discussed in terms of the nature of bonding and the known structures.     

Dinuclear complex of gold(III) with 1,3-diaminopropane (HL), [Au<Subscript>2</Subscript>(HL)(L)<Subscript>2</Subscript>](ClO<Subscript>4</Subscript>)<Subscript>3</Subscript>(OH): Synthesis and crystal and molecular structures

The dinuclear gold(III) complex, [Au₂(HL)(L)₂](ClO₄)₃(OH) (I) (HL = 1,3-diaminopropane), with two amide bridges has been synthesized for the first time. According to the X-ray diffraction data, the crystal structure of complex I consists of the complex cations [Au₂(HL)(L)₂]⁴⁺ and anions ClO₄⁻ and OH⁻. The coordination sites AuN₄ are insignificantly distorted squares. In the four-membered ring Au₂N₂, the gold atoms are bound by the bridging nitrogen atoms of the deprotonated primary amine.