Divalent First-row Transition Metal Ions Research Articles

Research on three complex compounds of divalent first-row transition metal ions having the Schiff base derived from sulfanilamide and N-acetylisatin as bidentate ligand

The present paper aims to describe the research performed on three complex compounds formed by divalent first-row transition metal ions – namely M = Co(II), Ni(II) and Zn(II) (representing the complex kernels) with the Schiff base derived from the condensation of sulfanilamide with N-acetylisatin (which proved itself to be able to act as a bidentate ligand in all these cases). This Schiff base is N-(1-acetyl-2-oxoindolin-3-ylidene)-4-aminobenzenesulfonamide, which appears twice in each complex, but is not the only ligand in the structure, as two water molecules are also part of the coordination sphere; there also is an ionization sphere, represented by two chloride ions whose role is to equilibrate the electrical charge of the complex kernel (all the ligands being charge-free) – the general molecular formula of the complexes being thus: [ML2(H2O)2]Cl2. From the electrical point of view, we dealt with three complex compounds that all showed themselves to be electrolytes. As far as magnetic properties are concerned, two of them proved themselves to be paramagnetic, whereas the last one was found to be diamagnetic. After adding different spectral data, corroborating them with the prior information and with the elemental analysis – we have drawn the conclusion that the complexes are six-coordinated, so they all will tend to adopt an octahedral geometry.

The effect of pH and transition metal ions on cysteine-assisted gold aggregation for a distinct colorimetric response.

Colorimetric detection is a promising sensing strategy that is applicable to qualitative and quantitative determination of an analyte by monitoring visually detectable color changes with the naked eye. This study explored the cysteine (Cys)-induced aggregation of gold nanoparticles (AuNPs) in order to develop a sensitive colorimetric detection method for Cys. For this purpose, we systematically investigated the colorimetric response of AuNPs to Cys with varying particle sizes and concentrations. The AuNPs with various diameters ranging from 26.5 nm to 58.2 nm were synthesized by the citrate reduction method. When dispersed in water to have the same surface area per unit volume, the smaller AuNPs (26.5 nm) exhibited a more sensitive response to Cys compared to a larger counterpart (46.3 nm). We also examined the effect of divalent first-row transition metal ions (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+) on the Cys-induced aggregation of AuNPs. Among the tested metal ions, the addition of Cu2+ provided the highest enhancement in sensitivity to Cys regardless of pH between 3.5 and 7. The significant increase in the sensitivity caused by Cu2+ could be attributed to the capability of Cu2+ to form a highly stable chelate complex with surface-immobilized Cys, facilitating the aggregation of AuNPs. For the AuNPs–Cu2+ system at pH 7, the detection limit for Cys was determined to be 5 nM using UV-vis spectroscopy. The reported strategy showed the potential to be used for a rapid and sensitive detection of Cys and also metal ions that can facilitate Cys-mediated aggregation of AuNPs.

Transition Metal Sequestration by the Host-Defense Protein Calprotectin.

In response to microbial infection, the human host deploys metal-sequestering host-defense proteins, which reduce nutrient availability and thereby inhibit microbial growth and virulence. Calprotectin (CP) is an abundant antimicrobial protein released from neutrophils and epithelial cells at sites of infection. CP sequesters divalent first-row transition metal ions to limit the availability of essential metal nutrients in the extracellular space. While functional and clinical studies of CP have been pursued for decades, advances in our understanding of its biological coordination chemistry, which is central to its role in the host-microbe interaction, have been made in more recent years. In this review, we focus on the coordination chemistry of CP and highlight studies of its metal-binding properties and contributions to the metal-withholding innate immune response. Taken together, these recent studies inform our current model of how CP participates in metal homeostasis and immunity, and they provide a foundation for further investigations of a remarkable metal-chelating protein at the host-microbe interface and beyond.

The Hexahistidine Motif of Host-Defense Protein Human Calprotectin Contributes to Zinc Withholding and Its Functional Versatility.

Human calprotectin (CP, S100A8/S100A9 oligomer, MRP-8/MRP-14 oligomer) is an abundant host-defense protein that is involved in the metal-withholding innate immune response. CP coordinates a variety of divalent first-row transition metal ions, which is implicated in its antimicrobial function, and its ability to sequester nutrient Zn(II) ions from microbial pathogens has been recognized for over two decades. CP has two distinct transition-metal-binding sites formed at the S100A8/S100A9 dimer interface, including a histidine-rich site composed of S100A8 residues His17 and His27 and S100A9 residues His91 and His95. In this study, we report that CP binds Zn(II) at this site using a hexahistidine motif, completed by His103 and His105 of the S100A9 C-terminal tail and previously identified as the high-affinity Mn(II) and Fe(II) coordination site. Zn(II) binding at this unique site shields the S100A9 C-terminal tail from proteolytic degradation by proteinase K. X-ray absorption spectroscopy and Zn(II) competition titrations support the formation of a Zn(II)-His6 motif. Microbial growth studies indicate that the hexahistidine motif is important for preventing microbial Zn(II) acquisition from CP by the probiotic Lactobacillus plantarum and the opportunistic human pathogen Candida albicans. The Zn(II)-His6 site of CP expands the known biological coordination chemistry of Zn(II) and provides new insight into how the human innate immune system starves microbes of essential metal nutrients.

Role of the Metal in the Bonding and Properties of Bimetallic Complexes Involving Manganese, Iron, and Cobalt

A multidentate ligand platform is introduced that enables the isolation of both homo- and heterobimetallic complexes of divalent first-row transition metal ions such as Mn(II), Fe(II), and Co(II). By means of a two-step metalation strategy, five bimetallic coordination complexes were synthesized with the general formula M1M2Cl(py3tren), where py3tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. The metal-metal pairings include dicobalt (1), cobalt-iron (2), cobalt-manganese (3), diiron (4), and iron-manganese (5). The bimetallic complexes have been investigated by X-ray diffraction and X-ray anomalous scattering studies, cyclic voltammetry, magnetometry, Mössbauer spectroscopy, UV-vis-NIR spectroscopy, NMR spectroscopy, combustion analyses, inductively coupled plasma optical emission spectrometry, and ab initio quantum chemical methods. Only the diiron chloride complex in this series contains a metal-metal single bond (2.29 Å). The others show weak metal-metal interactions (2.49 to 2.53 Å). The diiron complex is also distinct with a septet ground state, while the other bimetallic species have much lower spin states from S = 0 to S = 1. We propose that the diiron system has delocalized metal-metal bonding electrons, which seems to correlate with a short metal-metal bond and a higher spin state. Multiconfigurational wave function calculations revealed that, indeed, the metal-metal bonding orbitals in the diiron complex are much more delocalized than those of the dicobalt analogue.

Recognition Preference of Rhodamine-Thiospirolactams for Mercury(II) in Aqueous Solution

This work presents the design, syntheses, photophysical properties and Hg(2+)-binding of the red-emitting rhodamine derivatives RS1, RS2, and RS3 with different coordination ability and different spatial effects that derived from rhodamine thiohydrazone chromophores and respective carboxaldehydes (benzaldehyde, pyridine-2-carboxaldehyde, ferrocenecarboxaldehyde). Chemosensors RS2 and RS3 afford turn-on fluorescence enhancement and display high brightness in water with the EC(50) for Hg(2+) of 0.5 ppb. The fluorescence intensities are nearly proportional to the amount of Hg(2+) at ppb level, when employing 100 nM probes in water. The fluorescence responses of these two chemosensors are Hg(II) specific, and the probes are selective for Hg(II) over alkali, alkaline earth metals, divalent first-row transition metal ions, and Group 12 congeners Zn(II) and Cd(II), as well as heavy metals Pb(II) and Ag(I). X-ray crystal structure analyses exhibit the thioether derivative of the spirolactone in these compounds. Hg(II)-specific binding in water would make the opening of the spirolactam ring and consequently causes the appearance of strong absorption at visible range, and the obvious and characteristic color change from colorless to pink. Compared to the thioamides, the improved selectivity for Hg(2+) is attributed to the poorer coordination affinity of the thioether over other interference metal ions.

Heterotetranuclear Oxalato-Bridged ReIV3MII (M = Mn, Fe, Co, Ni, Cu) Complexes: A New Example of a Single-Molecule Magnet (M = Ni)

The use of the mononuclear species (NBu(4))(2)[Re(IV)Cl(4)(ox)] (NBu(4)(+) = tetra-n-butylammonium cation; ox = oxalate dianion) as a ligand toward fully solvated divalent first-row transition-metal ions affords the tetranuclear complexes (NBu(4))(4)[{Re(IV)Cl(4)(mu-ox)}(3)M(II)] with M = Mn (1), Fe (2), Co (3), Ni (4), and Cu (5). Their structure is made up of discrete [{ReCl(4)(mu-ox)}(3)M](4-) anions and bulky NBu(4)(+) cations. The complexes 2-5 crystallize in the triclinic system with space group P1; 2 and 5 as well as 3 and 4 are isostructural. The Re and M atoms exhibit somewhat distorted ReCl(4)O(2) and MO(6) octahedral surroundings, with the oxalate groups adopting the bis-bidentate bridging mode. Magnetic susceptibility measurements on polycrystalline samples of 1-5 in the temperature range 1.9-300 K show the occurrence of intramolecular antiferromagnetic [J = -1.30 cm(-1) (1)] and ferromagnetic couplings [J = +1.62 (2), +3.0 (3), +16.3 (4), and +4.64 cm(-1) (5)], with the Hamiltonian being defined as H = -J[S(M)(S(Re1) + S(Re2) + S(Re3))]. Compound 4 is the first example of an oxalato-bridged heterometallic species that behaves as a single-molecule magnet with a ground-state spin S = (11)/(2) and D = -0.8(1) cm(-1), as shown by the study of its static and dynamic magnetic properties and a high-frequency electron paramagnetic resonance study on polycrystalline samples together with detailed micro-SQUID measurements on single crystals.

Structural modification of rhodamine-based sensors toward highly selective mercury detection in mixed organic/aqueous media

In virtue of the sulfurphilic nature of Hg(2+), three new sensors RN1, RN2 and RST1 that combine a thiophene group and one or two rhodamine choromophores, or a thiospirolactam rhodamine chromophore, were designed and prepared for the selective detection of Hg(2+) in aqueous media, respectively. These sensors all displayed good brightness and fluorescence enhancement following Hg(2+) coordination with limits of detection for Hg(2+) at the ppb level. Thus, they have the potential for distinguishing between safe and toxic levels of inorganic mercury in drinking water. RN1 exhibited chromogenic and fluorogenic selectivity over alkali, alkaline earth metals, divalent first-row transition metal ions as well as heavy metals, but the presence of Cu(2+) had a small but significant influence on the absorption detection of Hg(2+). Compared to RN1, the introduction of sufficient sulfur atoms could increase the binding capability of RST1 towards Hg(2+) relative to the sensor RN1, but decrease its Hg specific ability. The existence of some heavy and transition metal ions, such as Pb(2+), Ag(+), Cu(2+) enhance the silent absorption spectra of RST1. Spectral evidence and X-ray structural investigations of the mercury complex revealed a possible 1:2 complexation behaviour between the Hg(2+) ion and the sensor RN1 or RST1. Sensor RN2 which contains two rhodamine carboxhydrazone arms exhibited better selectivity, compared to those of RN1 and RST1. The addition of Cu(2+) only caused a small interference for the absorption detection of Hg(2+) under the same conditions, demonstrating the efficiency of the robust bis-chelating mode with regard to the selectivity for Hg(2+).

Turn-on and ratiometric mercury sensing in water with a red-emitting probe.

The synthesis, photophysical properties, and Hg(II) binding of a red-emitting sensor for mercuric ion are presented. 2-[11-[(2-[[Bis-(2-ethylsulfanylethyl)amino]methyl]phenylamino)methyl]-3-hydroxy-10-oxo-10H-benzo[c]xanthen-7-yl]benzoic acid (MS5) is based on the seminaphthofluorescein chromophore and employs a thioether-rich metal-binding unit. This sensor affords both turn-on and single-excitation dual-emission ratiometric Hg(II) detection in aqueous solution. The fluorescence response of MS5 is Hg(II)-specific, and the probe is selective for Hg(II) over alkali and alkaline earth metals, most divalent first-row transition metal ions, and the Group 12 congeners Zn(II) and Cd(II). MS5 binds Hg(II) reversibly and can be recycled. The EC50 for 1 microM MS5 is 910 nM, and a lower detection limit of 50 nM is obtained when employing 500 nM probe. X-ray crystallographic studies using a salicylaldehyde-based model of MS5 are also presented. 2-[(2-[[Bis-(2-ethylsulfanylethyl)amine]methyl]phenylamine)methyl]phenol coordinates Hg(II) with two thioether sulfur atoms, two amino nitrogen atoms, and a phenol oxygen atom arranged in a distorted trigonal bipyramidal geometry. Studies of natural water samples spiked with mercuric salts indicate that MS5 can rapidly detect Hg(II) in such complex solutions and demonstrate its potential utility in the field.

Properties of a new 4-imidazolyl derivative of a 14-membered tetraazamacrocyclic chelating agent

A new bis-N,N'-(5-methylimidazol-4-ylmethyl) derivative of a 14-membered tetraazamacrocycle, L1, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with divalent first-row transition metal ions and Fe3+ were determined at 298.2 K in aqueous 0.10 mol dm(-3) KNO3. Compound L1 exhibits high overall basicity, which is mainly conferred by the imidazolyl groups. The complexes of the divalent first row-transition metal ions of L1 follow the Irving-Williams order of stability with the maximum for Cu2+ as expected, but a steep fall of constants is verified for the Mn2+, Fe2+ and Co2+, in one side, and for the Zn2+ complexes, in the other side. Additionally, L1 shows a large affinity for Fe3+, and the relative stability constants for its Cd2+ and Pb2+ complexes indicate that L1 may be useful for the complexometric determination of these two toxic metal ions in solutions containing both metal ions. These studies together with NMR, UV-vis and EPR spectroscopic data indicated the presence of mononuclear complexes, which adopt distorted pyramidal or octahedral geometries depending on the metal centre. The X-ray crystal structure of [Cu(HL1)](PF6)2(NO3).H2O showed that the coordination sphere of the copper centre can be described as a distorted square pyramid with the basal plane defined by three nitrogen donors of the macrocycle backbone and one nitrogen atom from one imidazolyl pendant arm. The apical position is occupied by the nitrogen atom of the macrocycle trans to the pyridine ring. To achieve this coordination environment, the macrocycle is folded along the axis defined by the two N atoms contiguous to the pyridine ring. The free methylimidazolyl arm points away from the metal centre leading to an intramolecular Cu[dot dot dot]N distance of 5.155(1) A.

Synthesis, crystal structure and magnetic properties of two-dimensional malonato-bridged cobalt(ii) and nickel(ii) compounds

Two isostructural malonato-bridged complexes of formula {[M(H2O)2][M(mal)2(H2O)2]}n [M = Co(II) (1), Ni(II) (2); H2mal = malonic acid] have been synthesised and characterized by X-ray diffraction. Their structure consists of corrugated layers of trans-diaquabismalonatemetalate(II) and trans-diaquametal(II) units bridged by carboxylate–malonate groups in the anti–syn conformation. Two crystallographycally independent metal atoms occur in 1 and 2. The malonate anion acts simultaneously as a bidentate and bis-monodentate ligand. Variable-temperature (1.9–295 K) magnetic susceptibility measurements indicate the occurrence of weak antiferro- (1) and ferromagnetic (2) interactions between the cobalt(II) (1) and nickel(II) ions (2) through the anti–syn caboxylate–malonate bridge. A brief discussion on the structural diversity and crystal engineering possibilities of the malonate complexes with divalent first-row transition metal ions other than copper(II) is carried out.

Solvation Structures and Solvent Exchange Reactions of Metal Ions in Various Coordinating Solvents

Abstract The solvation structures and the reaction mechanisms of the solvent exchange reactions of metal ions are essential aspects of the interactions of the metal ions with the solvent molecules and information about them is necessary to understand the reactivities of the metal ions in solution. The solvation structures in various coordinating solvents were discussed on the basis of the structure parameters determined by the extended X-ray absorption fine structure technique, referring to the ionic size, ionic charge, and electronic configuration of the metal ions and the bulkiness and electron donating ability of the solvent molecules. It was shown that the deficient bulkiness of 1,1,3,3-tetramethylurea leads to a unique variation in the solvation number for a series of 3d-block metal ions and that the strong electron donating ability of propylamine reduces the solvation number. The solvent exchange mechanisms of various metal ions were discussed on the basis of the activation parameters determined by the nuclear magnetic resonance technique, and the effects of the ionic charge, ionic size, and electronic configuration of the metal ions were regularized. The kinetic results for the solvent exchange of the Ni2+ and Mn2+ ions in a series of nitriles revealed the effect of the solvent bulkiness on the variation in the mechanism. The kinetic chelate effect was pointed out on the basis of the activation parameters for the solvent exchange of the divalent first-row transition metal ions in bidentate solvents such as ethylenediamine and trimethylenediamine.

Fluorescence sensing of ionic analytes in water: from transition metal ions to vitamin B13.

The fluorescence chemosensor ATMCA has been realised by appending an anthrylmethyl group to an amino nitrogen of TMCA (2,4,6-triamino-1,3,5-trimethoxycyclohexane), a tripodal ligand selective for divalent first-row transition metal ions in water. The ATMCA ligand can act as a versatile sensor for ZnII and CuII ions. Its sensing ability can be switched by simply tuning the operating conditions. At pH 5, ATMCA detects copper(II) ions in aqueous solutions by the complexation-induced quenching of the anthracene emission. Metal ion concentrations < 1 microM can be readily detected and very little interference is exerted by other metal ions. At pH 7, ATMCA signals the presence of ZnII ions at concentrations < 1 microM by a complexation-induced enhancement of the fluorescence. Again the sensor is selective for ZnII over several divalent metal ions, with the exception of CuII, CoII and HgII. Most interestingly, the [ZnII(atmca)]2+ complex can act as a fluorescence sensor for specific organic species, notably selected dicarboxylic acids and nucleotides, by the formation of ternary ligand/zinc/substrate complexes. The oxalate anion is detected in concentrations <0.1 mM; however, no effects on the system's fluorescence is observed in the presence of monocarboxylic acids and long-chain dicarboxylic acids. Among the nucleotides, those containing an imide or amide function are readily detected and an unprecedented high sensitivity for guanine derivatives allows the determination of this nucleotide for 0.05-0.5 mM solutions. Moreover, [ZnII(atmca)]2+ is a very effective and selective sensor in the case of vitamin B13 (orotic acid) in sub-micromolar concentrations. The operative features of the systems investigated are also clearly suitable for intracellular analyses. The factors at the source of organic substrate recognition, here briefly discussed, are of paramount importance for further developments in the applicability of these sensing systems.

Tetracyanomanganate(II) and its salts of divalent first-row transition metal ions.

The first known paramagnetic, tetrahedral cyanide complex, [Mn(II)(CN)(4)](2)(-), is formed by the photoinduced decomposition of [Mn(IV)(CN)(6)](2)(-) in nonaqueous solutions or by thermal decomposition in the solid state. In acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (lambda(max) = 25 700 cm(-1), epsilon = 3700 cm(-1) M(-1)) causes the homolytic cleavage of cyanide radicals and reduction of Mn(IV). Free cyanide in dichloromethane leads to the isolation of polycyanide oligomers such as [C(12)N(12)](2)(-) and [C(4)N(4)](-), which was crystallographically characterized as the PPN(+) salt C(40)H(30)N(5)P(2): monoclinic space group = I2/a, a = 18.6314(2) A, b = 9.1926(1) A, c = 20.8006(1), beta =106.176(2) degrees, Z = 4]. In the solid state Mn(IV)-CN bond homolysis is thermally activated above 122 degrees C, according to differential scanning calorimetry measurements, leading to the reductive elimination of cyanogen. The [Mn(II)(CN)(4)](2-) ion has a dynamic solution behavior, as evidenced by its concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and lability of [Mn(II)(CN)(4)](2-) in solution, its reaction with divalent first-row transition metal cations leads to the formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the metal ions and multiple structural and cyano-linkage isomers. alpha-Mn(II)[Mn(II)(CN)(4)] has an interpenetrating sphalerite- or diamond-like network structure with a unit cell parameter of a = 6.123 A (P43m space group) while a beta-phase of this material has a noninterpenetrating disordered lattice containing tetrahedral [Mn(II)(CN)(4)](2-). Linkage isomerization or cyanide abstraction during formation results in alpha-Mn(II)[Co(II)(CN)(4)] and Mn(II)[Ni(II)(CN)(4)] lattice compounds, both containing square planar tetracyanometalate centers. alpha-Mn(II)[Co(II)(CN)(4)] is irreversibly transformed to its beta-phase in the solid state by heating to 135 degrees C, which causes a geometric isomerization of [Co(II)(CN)(4)](2)(-) from square planar (nu(CN) = 2114 cm(-1), S = (1)/(2)) to tetrahedral (nu(CN) = 2158 cm(-1), S = (3)/(2)) as evidenced by infrared and magnetic susceptibility measurements. Mn(II)[Ni(II)(CN)(4)] is the only phase formed with Ni(II) due to the high thermodynamic stability of square planar [Ni(II)(CN)(4)](2)(-).

Dioxatriazamacrocycle-N,N′,N″-triacetic Acids: Synthesis, Protonation Constants, and Metal-Complex Studies. Crystal Structure of Hydrogen [1,4-Dioxa-7,10,13-triazacyclopentadecane-7,10,13-triacetato(4)-κN7,κN11,κN13,κO7]copper(1 -) Hydrate (2 : 1) ([Cu(HL1)]⋅0.5 H2O)

Two new dioxatriazacyclopentadecanetriacetic acids were synthesized, i.e. 1,4-dioxa-7,10,13-triazacyclopentadecane-7,10,13-triacetic acid (1; H3L1) and 1,4-dioxa-7,10,14-triazacyclohexadecane-7,10,14-triacetic acid (2; H3L2). The protonation constants of these compounds and the stability constants of complexes of both ligands with the alkaline-earth metal ions, Mn2+ to Zn2+, Cd2+, and Pb2+ were determined by potentiometric methods at 25° in 0.10M tetramethylammonium nitrate solution. Both ligands exhibit two high-value protonation constants and two low-value ones. Only mononuclear complexes were found for both ligands with the alkaline earth metal ions, and their stability constants are surprisingly low, suggesting the involvement of only two N-atoms of the macrocycles and two carboxylate groups in the coordination to these metal ions (or a very weak interaction with all the carboxylates). Mono- and dinuclear species were found in solution for most of the divalent first-row transition-metal ions, Cd2+, and Pb2+. Ligand 1 (H3L1) formed mononuclear complexes that were thermodynamically more stable, while 2 (H3L2) stabilized the dinuclear species better due to the larger cavity size of the macrocycle. Electronic and EPR-spectroscopic studies in solution revealed that the Co2+, Ni2+, and Cu2+ complexes are six-coordinate, and that the three N-atoms of the macrocycles are involved in the coordination. EPR Spectra of the copper(II) dimer of 2 show resonances corresponding to the ΔMs=1 and ΔMs=2 transitions. The structure of [Cu(HL1)]⋅0.5 H2O, obtained from Cu2+ and 1, was determined by single-crystal X-ray diffraction. The complex adopts a distorted compressed trigonal-bipyramidal geometry, with the macrocycle in a folded conformation. The basal plane is formed by two N- and one O-atoms of the macrocycle backbone, and the apical positions are occupied by the other N-atom of the ring and one of the O-atoms of one carboxylate group. Electronic and EPR-spectroscopic studies show that the same complex exists in solution in a six-coordination symmetry with tetragonal elongation.

13-Membered macrocycles and their complexometric properties: study of 7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane

The protonation constants of L1[7,11-bis(carboxymethyl)-1,4-dioxa-7,11-diazacyclotridecane] and the stability constants of complexes formed by this ligand with the alkaline-earth metal ions, the divalent first-row transition metal ions, Cd 2+, and Pb 2+ were determined by potentiometric methods, at 25°C and ionic strength 0.10 M in tetramethylammonium nitrate. In general, the stability constant values for the complexes studied are lower than expected for a potential six-co-ordinate ligand with an N 2O 4 co-ordination sphere. The complexes of L1 with metal ions, forming mainly electrostatic interactions (the alkaline-earth metal ions, Mn 2+ and Pb 2+), have a sharp decrease of stability when compared with the corresponding complexes of 12-membered ligands, L3 and L4 [7,10-bis(carboxymethyl)-1,4-dioxa-7,10-diazacyclodecane and 4,10-bis(carboxymethyl)-1,7-dioxa-4, 10-diazacyclododecane, respectively]. However, the Cu 2+ and Ni 2+ complexes of L1 and L4 have about the same values of stability constants. Hence, the 13-membered ligand is more selective for the divalent first-row transition metal ions than the 12-membered ones are. To explain these results we suggest that probably all the donor atoms of the ligands ( L1, L3 and L4) are involved on the co-ordination to the metal ions when mainly electrostatic interactions take place (also to the Co 2+, as spectroscopic measurements have revealed) but the distances and principally the orientation of the lone-pairs of electrons of L1 are not in the better position for the co-ordination. Based on electronic and EPR spectroscopic measurements in solution and in magnetic moments of the complexes some explanations about the peculiarities of the Ni 2+ and Cu 2+ complexes of L1 and L4 were proposed. The Ni 2+ complexes of both ligands exhibit tetragonally distorted octahedral geometry, but this metal ion seems to fit better in the cage formed by L1 than that of L4, as indicated by the values of stability constants. Both ligands should adopt different arrangements around the metal ion, the distortion from the octahedron being more important for [Ni L4] reflecting the loss in terms of crystal field stabilisation energy. However, the Cu 2+ forms wit L1 a square pyramidal pentaco-ordinate complex while L4 prefers a distorted octahedral arrangement.

Design of selective macrocyclic ligands for the divalent first-row transition-metal ions †

The protonation constants of H2L1, 3,11-bis(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene and H3L2, 3,7,11-tris(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene, and stability constants of complexes formed by these macrocycles with Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Ga3+, Fe3+ and In3+ were determined at 25 °C and ionic strength 0.10 mol dm–3 in NMe4NO3. Both compounds are very selective for the divalent first-row transition-metal ions, exhibiting very high stability constants for Cu2+, fairly high values for Ni2+, but sharply decreasing ones for the remaining metal ions of this row. Their complexes with the alkaline-earth or larger metal ions, such as Pb2+, have low stability constants. The crystal structure of [CuL1]·4H2O was determined. The copper atom is encapsulated by the macrocycle in a distorted octahedral environment. The equatorial plane contains the four nitrogen atoms of the tetraaza ring and six-co-ordination is completed via two oxygen atoms of the appended carboxylate groups. The angles at the metal centre are close to the expected values of 90 and 180° for octahedral geometry. Molecular mechanics studies carried out for the cis and the trans octahedral [ML1] complexes were consistent with the structure found in the solid state. For a mean CuII–N distance of 2.01 A the experimentally observed trans isomer is 6.5 kcal mol–1 more stable than the cis one. On the other hand these calculations suggest that larger ions such as Pb2+, Ca2+ or Mn2+ can be accommodated by the macrocycle in a cis-octahedral environment. However, these ions allow co-ordination numbers higher than six and so other structures ought to be also considered. The low stability constants for metal complexes of Co2+ and Zn2+ indicate that these complexes do not have a trans-octahedral structure, while the molecular mechanics calculations reveal that the cis isomer is not the most stable form. Therefore, other structures with co-ordination numbers lower than six should be considered, implying that one or more donor atoms are not co-ordinated. Stability constants of metal complexes of (L2)3– and EPR studies suggest that not all the donor atoms in this macrocycle are co-ordinated when complexes are formed with first-row-transition divalent metal ions.

Tris(carboxymethyl)oxatriazamacrocycles and their metal complexes †

The protonation constants of H3L1 [4,8,12-tris(carboxymethyl)-1-oxa-4,8,12-triazacyclotetradecane] and of H3L2 [4,7,11-tris(carboxymethyl)-1-oxa-4,7,11-triazacyclotridecane] and the stability constants of the complexes formed by both with alkaline-earth metal ions, divalent first-row transition-metal ions, Cd2+, Pb2+ and Fe3+ were determined by potentiometric methods, at 25 °C and ionic strength 0.10 mol dm–3 in tetramethylammonium nitrate. The co-ordination properties of both ligands for alkaline-earth and divalent first-row transition-metal ions are quite different. The 13-membered macrocycle, (L2)3–, exhibits fairly high stability constants but unsatisfactory selectivity with the first-row transition-metal ions. The 14-membered ligand, (L1)3–, is very selective for the same series of metal ions, the difference in stability between its complexes of Cu2+ and Mn2+ being 10.96 log units and that of the complexes of Cu2+ and Zn2+ 7.29 log units, the constant of the complex of Mn2+ is sufficiently high for its possible quantitative determination by (L1)3–, at pH values higher than 7. However, the metal complexation behaviour of the (L1)3– is similar to that of the corresponding bis(carboxymethyl) derivative, therefore the difficult preparation of H3L1 does not compensate the benefits. Increase in size of the macrocycle cavity leads to a sharp decrease in stability of complexes of metal ions involved mainly in electrostatic interactions (the alkaline-earth metal ions, Mn2+ and Pb2+). Cobalt(II) complexes also undergo a significant decrease in stability with increase in cavity size and the constant for [ZnL1]– is much lower than that of [ZnL2]–. However, the complexes of Cu2+ and Ni2+ with both macrocycles have about the same values of the stability constants. To explain these results it is proposed that all the donor atoms of the ligands are involved in co-ordination to metal ions which form complexes mainly by electrostatic interactions, although the distances and principally the orientation of the lone pairs of electrons are gradually more disfavoured for co-ordination with increasing cavity size. Complexes of the first-row transition-metal ions undergo the same effects, each being able to choose the donor atoms from the ligands most appropriate for their strict co-ordination preferences. So Co2+, Ni2+ and Cu2+ adopt five- or six-co-ordination with these potentially seven-co-ordinate ligands, as shown by electronic and EPR spectroscopic measurements in solution and the magnetic moments of the complexes.

Theoretical study of the mono- and di-hydrated divalent ions of the first-row transition metals

Theoretical binding energies and MO distances have been obtained by ab initio quantum chemical calculations performed at the modified correlated pair functional (MCPF)level on the mono- and di-hydrated divalent first-row transition metal ions, M(H 2O) n 2+, n= 1,2,M = Ca, Sc, …, Zn. The results are discussed in terms of electronic factors and compared with the variation of the binding energies of the hexahydrated ions, M(H 2O) 6 2+, in order to evaluate the different effects of one or two ligands and of an octahedral ligand field. Also included is a discussion of correlation and relativistic effects, and a test of the internal water geometry, which has been carefully optimized. All dihydrated ions are linear, except Ca(H 2O) 2 2+, where the OCaO angle was found to be 130°. Strong correlations exist between binding energies and MO distances, showing the electrostatic origin of the bonding.

Ligand Interchange Controls Many Oxidations of Divalent First-Row Transition Metal Ions by Free Radicals

These studies suggest that the rate-determining step in the oxidation of a large variety of transition metal complexes by free radicals is the ligand exchange step. In other words it is proposed that free radicals behave in many reactions with transition metal complexes as incoming nucleophilic ligands. It seemed of interest to investigate this speculative statement in more detail. It was decided therefore to study the interaction of various Mn(II), Fe(II), and Co(II) complexes with a variety of free radicals. These ions were selected as it was shown that Mn(II) ligand exchange processes processed usually via the I[sub a] mechanism, whereas those of Fe(II) and Co(II) proceed usually via the I[sub d] mechanism. The selected reactions are such that they were all suggested to proceed via the inner-sphere mechanism as their rates of reaction are nearly independent of the nature of the free radical and can therefore be considered as typical complex formation reactions as shown earlier. The pressure dependence of the studied reactions enables a detailed comparison with the volumes of activation for solvent exchange and complex formation reactions of these divalent first-row transition elements. Futhermore, it allows the authors to comment on the general behavior of free radicalsmore » during complex formation reactions with different metal centers.« less