Polypyrazolates of the heavier group 13 and 14 elements: A review

Heavy group 13 and 14 analogs of the poly(pyrazolyl)borates are the subject of this review. Within these, the most extensive research has been performed on the study of polypyrazolylgallates Me₂Ga(R₂pz)₂]⁻ (R = H or Me) and [MeGa(pz)₃]⁻, and their complexes with transition metals. In this review, the common features shared with the boron analogs are presented and contrasted. Other recently reported series of group 13 analogs are the alkali polypyrazolylaluminates Na[(R₂pz)₃] with R = Me, ^tBu; R′ = M. The complexes with one or two pyrazolyl ligands on aluminum display anagostic Al-CH₃?Na interactions, these interactions are persistent even if complexes were obtained from solutions with THF that normally coordinate alkali ions. The polypyrazolylsilane ligands Me₂Si(R₂pz)₂ and MeSi(R₂pz)₃ with R = H, Me are remarkably easy to obtain and isolate, in contrast is the fact that the carbon analogs are much harder to obtain and isolate in reasonable yields. Therefore it is surprising that the chemistry of the former ligands is not as developed as could be anticipated. Nevertheless there are examples of the use of these silanes as ligands with the following transition metals: Cr, Mn, Cu, Zn, Sc and Zr. The heavier group 14 analogs with Ge and Sn display coordination patterns with alkali and alkaline ions that resemble those observed with the borates and aluminates. The formation of cationic bimetallic cages of the type [E₂(R₂pz)₃]⁺ and neutral complexes [E₂(R₂pz)₄] has also been observed that can be consider formal isomers of the alkenes. The use of these compounds as ligands has been recently reported.     

New tripodal hydroxypyridinone based chelating agents for Fe(III), Al(III) and Ga(III): Synthesis, physico-chemical properties and bioevaluation

Two new tris-hydroxypyridinone based compounds (KEMPPr(3,4-HP)₃ and KEMPBu(3,4-HP)₃) have been developed and studied as strong sequestering agents for iron and the group III of metal ions, aimed as potential pharmacological applications on metal-chelation therapy. Their structure is based on the KEMP acid scaffold to which three 3-hydroxy-4-pyridinone chelating moieties are attached via two different size spacers. After the preparation and characterization of the compounds their physico-chemical properties were studied, in relation with their metal binding affinity and lipophilicity. The KEMPPr(3,4-HP)₃ ligand was also bioassayed to evaluate its in vivo metal sequestering capacity from most organs using an animal model overload with ⁶⁷Ga. These studies showed that, for both in solution and in vivo conditions, the compounds have higher metal chelating efficacy than Deferriprone, the commercially available iron chelator in medical application, thus some perspectives are envisaged as potential pharmaceutical drug candidates for chelating therapy.     

Transformation of (per)chlorate into chloride by a newly isolated bacterium: reduction and dismutation

 Bacterial strain GR-1 was isolated from activated sludge for its ability to oxidize acetate with perchlorate as electron acceptor. Sequencing of 16S rDNA revealed the isolate to belong to the β subgroup of Proteobacteria. When strain GR-1 was grown on acetate and perchlorate, the release of chloride was proportional to the disappearance of perchlorate, showing that this compound was completely reduced. In addition to perchlorate, strain GR-1 used chlorate, oxygen, nitrate and Mn(IV) as electron acceptor. The oxidation of acetate is coupled to the reduction of perchlorate and chlorate, whereas chlorite reduction is not affected by the addition of acetate. Strain GR-1 disproportionates chlorite into molecular oxygen and chloride. As a consequence, the strain oxidizes acetate by simultaneously reducing perchlorate to chlorite and molecular oxygen to water. Comparison of growth yields with oxygen, chlorate and perchlorate and calculated ΔG ^0′ values confirms this finding. Received: 26 June 1995/Received revision: 11 October 1995/Accepted: 16 October 1995     

Anion effects on construction of Zn compounds with a chelating ligand bis(2-pyridylmethyl)amine and their catalytic activities

Three Zn^II complexes containing bispicam ligands (bispicam = bis(2-pyridylmethyl)amine), [Zn(bispicam)₂](NO₃)₂·2CH₃OH 4A, [Zn(bispicam)(NO₃)₂] 4B, and [Zn(bispicam)₂](OTf)₂6, were obtained, and their structures were determined by X-ray crystallography. Complexes of the general formulation [Zn(bispicam)₂]X₂ (X = Cl⁻ (1), Br⁻ (2), I⁻ (3), NO₃⁻ (4A), ClO₄⁻ (5), and OTf⁻ (6)) show fac geometric isomers (a) or enantiomers (c) and (d) according to anions. Moreover, complexes 4-6 could carry out the catalytic transesterification of a range of esters with methanol under the mild conditions. Importantly, the catalyst 4B with an unsaturated structure has shown better efficiency than the catalysts, 4A, 5, and 6, having saturated structures. To explain this reactivity difference, two different reaction mechanisms have been proposed (metal-based vs. amide N-H-based).     

Reaction of bis(bis(trimethylsilyl)amido)mercury(II) with 3,3′-disubstituted binaphthols: Cyclization via an intramolecular electrophilic aromatic substitution reaction

Reaction of the mercury(II) amide Hg[N(SiMe₃)₃]₂ with 3,3′-disubstituted binaphthols (HO)₂C₂₀H₁₀(R)₂-3,3′ (R = SiMe₃, SiMe₂Ph, SiMePh₂, SiPh₃) in a 2:1 stoichiometric ratio furnishes four hexacyclic 1,7-disilylsubstituted derivatives of peri-xanthenoxanthene (PXX). Reaction of these two reagents in a 1:1 ratio results in a mixture of the hexacyclic products as well as the related pentacyclic species which contain one hydroxyl group and only one C-O-C ring fusion. The structures of three of the hexacyclic products (R = SiMe₃, SiMe₂Ph, SiMePh₂) and one of the pentacyclic products (R = SiMe₃) have been obtained. The reaction of Hg[N(SiMe₃)₃]₂ with the 3,3′-disubstituted binaphthols proceeds via an intramolecular electrophilic aromatic substitution reaction and several intermediates in this process have been detected using NMR (¹H and ¹⁹⁹Hg) spectroscopy.     

[Benzilato(2−)-O,O]bis[1,3-diphenylpropane-1,3-dionato(1−)-O,O]silicon(IV): a neutral heteroleptic hexacoordinate silicon(IV) complex with an SiO6 skeleton

Reaction of tetramethoxysilane with 1,3-diphenylpropane-1,3-dione and benzilic acid (molar ratio 1:2:1) in tetrahydrofuran/n-pentane yielded the neutral heteroleptic hexacoordinate silicon(IV) complex [benzilato(2−)-O¹,O²]bis[1,3-diphenylpropane-1,3-dionato(1−)-O,O]silicon(IV) (5). Compound 5 was structurally characterized by single-crystal X-ray diffraction, solid-state VACP/MAS NMR spectroscopy (²⁹Si), and solution NMR spectroscopy (¹H, ¹³C, ²⁹Si). The chiral silicon(IV) complex, with its octahedral SiO₆ coordination polyhedron, is configurationally stable in solution on the NMR time scale (solvent: CDCl₃; maximum temperature studied: 58 °C).     

A family of titanium (IV) alkoxo complexes with N,O and O,O chelating ligands. Crystal structure of [Ti(O-i-Pr)2{2-(−)-menthoxo-pyridine}2]

In view of the wide applicability and versatility of titanium based Lewis acids in selective organic synthesis including asymmetric synthesis, we have synthesized a family of mono and polyatomic titanium derivatives. The polymetallic complexes prepared are bridged by pyridimine, quinone and triazine based ligands. The synthesis of [{Ti(O-i-Pr)₃(Oddbf)}₂] (1), [Ti(O-i-Pr)₂(Oddbf)₂] (2), [{Ti(O-i-Pr)₂(Oddbf)(OMent)}₂] (3) (ddbfO = 2,3-dihydro-2,2-dimethyl-benzofuranoxo; MentO = (1R,2S,5R)-(−)-menthoxo), [{Ti(O-i-Pr)₃(OMenpy)}₂] (4), [Ti(O-i-Pr)₂(OMenpy)₂] (5) (MenpyO = (1S,2S,5R)-(−)-menthoxo-pyridine); [{(Ti(OR)₃)₂L}_n] (RO = isopropoxo, (1R,2S,5R)-(−)-menthoxo) (6-11) and [{(Ti(O-i-Pr)₃)₃L}_n] (12) was accomplished from a Lewis acid such as Ti(O-i-Pr)₄, [{Ti(O-i-Pr)₃(OMent)}₂] or [Ti(OMent)₄] and chelating ligands (ddbfOH = 2,3-dihydro-2,2-dimethyl-benzofuranol; MenpyOH = (1R,2S,5R)-(−)-5-methyl-2-isopropyl-1-(2′-pyridinyl)cyclohexan-1-ol; LH₂ = 4,6-dihydroxy-2,5-diphenyl-pyrimidine, 2,4-dihydroxy-5,6-dimethyl-pyrimidine, 5,8-dihydroxy-1,4-napthoquinone, 2,5-dihydroxy-1,4-benzoquinone and LH₃ = cyanuric acid) that provide a rigid framework for the metal centre. The molecular structure of 5 has been determined by single crystal X-ray diffraction studies.     

Synthesis and theoretical analysis of the structures and bonding in five new neutral square planar bis[2,4-di(aryl)-1,3,5-triazapentadienato]nickel(II) complexes

Solvothermal reactions in methanol of nickel acetate tetrahydrate, Ni(OAc)₂ · 4H₂O, with benzonitrile derivatives NC(C₆H₄)X, where X is one of the electron withdrawing substituents -CN, -NO₂, or -CF₃, located at the m- or p-positions relative to -CN, yield complexes of the general formula Ni{HNC(R)-NC(R)-NH}₂. More specifically, 3-nitrobenzonitrile, 4-nitrobenzonitrile, 1,3-dicyanobenzene, 1,4-dicyanobenzene, and ααα-trifluoro-p-toluonitrile are found to react with Ni(OAc)₂ · 4H₂O to yield Ni{HNC(R)-NC(R)-NH}₂, where R = 3-(NO₂)C₆H₄, 4-(NO₂)C₆H₄, 3-(CN)C₆H₄, 4-(CN)C₆H₄, or 4-(CF₃)C₆H₄, respectively. Analogous reactions of nitriles lacking electron withdrawing groups do not occur under similar conditions. Solid-state structures have been determined for the complexes with p-NO₂, p-CN, and p-CF₃ substituents on the phenyl rings. In addition, we describe density functional theory (DFT) and natural bonding orbital theory (NBO) studies on a simplified analog of these compounds, aimed at understanding their molecular bonding. It is shown that the new compounds for which solid-state structures have been determined are model examples of coordination compounds containing robust ω-bonds.     

Diazo complexes of osmium: preparation of binuclear derivatives with bis(aryldiazene) and bis(aryldiazenido) bridging ligands

Depending on experimental conditions and the nature of the phosphite, the reaction of OsH₂P₄ [P=P(OEt)₃ and PPh(OEt)₂] with bis(aryldiazonium) salts [N₂Ar-ArN₂](BF₄)₂ [Ar-Ar=4,4^′-C₆H₄-C₆H₄, 4,4^′-(2-CH₃)C₆H₃-C₆H₃(2-CH₃), 4,4^′-C₆H₄-CH₂-C₆H₄ and 1,5-C₁₀H₆] afford the cis and the trans binuclear [{OsHP₄}₂(μ-HNNAr-ArNNH)](BPh₄)₂1, 2 aryldiazene derivatives. These complexes 1, 2 further react with the mono(diazonium) (4-CH₃C₆H₄N₂)BF₄ salt to give the bis(aryldiazene) [{Os(4-CH₃C₆H₄NNH)P₄}₂(μ-HNNAr-ArNNH)](BPh₄)₄3, 4 derivatives. Binuclear bis(aryldiazenido) [{OsP₄}₂(μ-N₂Ar-ArN₂)](BPh₄)₂ (6) [P=P(OEt)₃; Ar-Ar=4,4^′-C₆H₄-C₆H₄, 4,4^′-C₆H₄-CH₂-C₆H₄] complexes were prepared by deprotonating with NEt₃ the nitrile-diazene [{Os(4-CH₃C₆H₄CN)P₄}₂(μ-HNNAr-ArNNH)](BPh₄)₄ (5) derivatives. The aryldiazenido compounds 6 react with HCl to give the new aryldiazene [{OsClP₄}₂(μ-HNNAr-ArNNH)](BPh₄)₂ (7) derivatives. The characterisation of the complexes by IR and ¹H, ³¹P, ¹⁵N NMR data is also discussed. The reaction of the hydride OsH₂(PPh₂OEt)₄ with mono(diazonium) salts was also studied and led exclusively to the mono(aryldiazene) [OsH(ArN NH)(PPh₂OEt)₄]BPh₄ (8) (Ar=C₆H₅, 4-CH₃C₆H₄) derivatives. Spectroscopic data (¹H, ³¹P, ¹⁵N NMR) on ¹⁵N-labelled derivatives suggest the presence of two isomers with the N-bonded and the π-bonded ArNNH ligand, respectively.     

Steric and electronic effects on Si-Ge bond lengths: An experimental and theoretical structural study of Me2Ge(SiCl3)2 and Me3GeSiCl3

The molecular structures of dimethylbis(trichlorosilyl)germane [Me₂Ge(SiCl₃)₂] and trimethyl(trichlorosilyl)germane (Me₃GeSiCl₃) have been determined in the gas phase. Me₃GeSiCl₃ was found ab initio to possess C_3v symmetry, with a low-lying torsional motion of the SiCl₃ group relative to the GeMe₃ group. The gas electron diffraction data were modelled with C₃ symmetry, although little deviation from C_3v symmetry was observed. Me₂Ge(SiCl₃)₂ (C_2v symmetry) was also found to have a very low-lying vibrational frequency relating to the rotation of the SiCl₃ groups. This led to the gas electron diffraction data being modelled in C₂ symmetry, with the observed combined deviation of the SiCl₃ groups being 14° from the eclipsed structure calculated ab initio. The Ge-C bond lengths were unaffected by the addition of an extra SiCl₃ group to the central germanium atom, but the Ge-Si bond lengths were observed to increase by over 1 pm.     

Photoluminescence and electrogenerated chemiluminescence of a bis(bipyridyl)ruthenium(II)-porphyrin complex

The photoluminescence (PL) and electrogenerated chemiluminescence (ECL) of [H₂(MPy3,4DMPP)Ru(bpy)₂Cl](PF₆), where H₂MPy3,4DMPP = meso-tris-3,4-dimethoxyphenyl-mono-(4-pyridyl)porphyrin and bpy = 2,2′-bipyridine, are reported in acetonitrile. The compound has a complex absorbance spectrum with bands characteristic of both the porphyrin and ruthenium moieties. PL emission maxim are observed at 655 nm when excited at the maximum absorption intensity corresponding to the porphyrin Soret π → π^∗ band, and around 600 nm when excited at wavelengths corresponding to Ru(dπ)-bpy (π^∗) MLCT transition. The photoluminescence efficiency (?_em) of the 655 nm emission is 0.039 and that of the free porphyrin is 0.69 compared to at 0.042.[H₂(MPy3,4DMPP)Ru(bpy)₂Cl](PF₆) displays complex electrochemical behavior, with one electrochemically reversible Ru^II-Ru^III oxidation and two quasi-reversible waves at more cathodic potentials corresponding to the porphyrin moiety. Oxidative ECL was generated using the coreactant tri-n-propylamine (TPrA). ECL efficiencies (?_ecl) were 0.14 for [H₂(MPy3,4DMPP)Ru(bpy)₂Cl]⁺ and 0.099 for H₂MPy3,4DMPP using as the standard (?_ecl = 1). ECL intensity was linear with respect to concentration from 1 to 0.001 μM.The ECL intensity peaks at potentials corresponding to oxidation both the ruthenium and porphyrin moieties as well as TPrA, indicating that multiple pathways for formation of the excited state are possible. However, an ECL spectrum shows a band similar in energy and shape to that of the Soret emission (655 nm for the PL and 656 nm for the ECL, respectively), indicating the same excited state is formed in each experiment.     

Evaluation of a SiPM array coupled to a Gd3Al2Ga3O12:Ce (GAGG:Ce) discrete scintillator

PurposeIn this study, we present the results of the evaluation of the SensL ArraySL-4 photo-detector, coupled to a 6 × 6 GAGG:Ce scintillator array, with 2 × 2 × 5 mm³ crystal size elements for possible applications in medical imaging detectors with focus in PET applications. Experimental evaluation was carried out with ²²Na and ¹³⁷Cs radioactive sources and the parameters studied were energy resolution and peak to valley ratio.MethodsArraySL-4 is a commercially available, 4 × 4 array detector covering an active area of 13.4 mm². The GAGG:Ce scintillator array used in this study has 0.1 mm thickness BaSO₄ reflector material between the crystal elements. A symmetric resistive voltage division matrix was applied, which reduces the 16 outputs of the array to 4 position signals. A Field Programmable Gate Array was used for triggering and digital processing of the signal pulses acquired using free running Analog to Digital Converters.ResultsRaw images and horizontal profiles of the 6 × 6 GAGG:Ce scintillator array produced under 511 keV and 662 keV excitation are illustrated. Moreover, the energy spectra obtained with ²²Na and ¹³⁷Cs radioactive sources from a single 2 × 2 × 5 mm³ GAGG:Ce scintillator are shown. The peak to valley ratio and the mean energy resolution values are reported.ConclusionsThe acquired raw image of the GAGG:Ce crystal array under 511 keV excitation shows a clear visualization of all discrete scintillator elements with a mean peak to valley ratio equal to 40. The mean energy resolution was measured equal to 10.5% and 9% respectively under 511 keV and 662 keV irradiation.     

Oxidation of L-serine and L-threonine by bis(hydrogen periodato)argentate(III) complex anion: a mechanistic study

Oxidation of l-serine and l-threonine by a silver(III) complex anion, [Ag(HIO(6))(2)](5-), has been studied in aqueous alkaline medium. The oxidation products of the amino acids have been identified as ammonia, glyoxylic acid and aldehyde (formaldehyde for serine and acetaldehyde for threonine). Kinetics of the oxidation reactions has been followed by the conventional spectrophotometry in the temperature range of 20.0-35.0 degrees C and the reactions display an overall second-order behavior: first-order with respect to both Ag(III) and the amino acids. Analysis of influences of [OH(-)] and [periodate] on the second-order rate constants k' reveals an empirical rate expression: k(')=(k(a)+k(b)[OH(-)])K(1)/([H(2)IO(6)(3-)](e)+K(1)), where [H(2)IO(6)(3-)](e) is equilibrium concentration of periodate, and where k(a)=6.1+/-0.5M(-1)s(-1), k(b)=264+/-6M(-2)s(-1), and K(1)=(6.5+/-1.3)x10(-4)M for serine and k(a)=12.6+/-1.7M(-1)s(-1), k(b)=(5.5+/-0.2)x10(2)M(-2)s(-1), and K(1)=(6.2+/-1.5)x10(-4)M for threonine at 25.0 degrees C and ionic strength of 0.30M. Activation parameters associated with k(a) and k(b) have also been derived. A reaction mechanism is proposed to involve two pre-equilibria, leading to formation of an Ag(III)-periodato-amino acid ternary complex. The ternary complex undergoes a two-electron transfer from the coordinated amino acid to the metal center via two parallel pathways: one pathway is spontaneous and the other is assisted by a hydroxide ion. Potential applications of the Ag(III) complex as a reagent for modifications of peptides and proteins are implicated.     

Synthesis and potent ionophoric activity of a squaramide-linked bis(choloyl) conjugate

A squaramide-linked bis(choloyl) conjugate was synthesized and fully characterized on the basis of NMR (¹H and ¹³C) and ESI MS (LR and HR) data. Fluorescence and chloride ion selective electrode assays indicate that this compound exhibits potent ionophoric activity across egg-yolk l-α-phosphatidylcholine-based liposomal membranes, presumably via an anion-modulating anion–cation co-transport/symport process. A Hill analysis reveals that three molecules of this compound are assembled into the transport-active species.     

Self-assembled monolayers of bis(salicylaldiminato)nickel(II) Schiff-base complexes: synthesis and structure

The synthesis of two derivatized Ni^II Schiff-base complexes is reported. Self-assembled monolayers (SAMs) have been obtained by reaction of coupling layers, functionalized glass substrates and the derivatized complex precursors. The self-assembled films have been characterized by contact angle measurements, X-ray photoelectron spectroscopy, and UV-Vis absorption spectroscopy. A structure of these SAMs is proposed on the basis of spectroscopic data and molecular metrical parameters.     

Synthesis and structural characterization of a pyridyl-substituted cyclodisilazane [(6-Me-Apy)2(μ-SiMe)2]; (6-Me-ApyH2=6-methyl-2-amino-pyridine)

Treatment of 6-Me-ApyH₂ (6-methyl-2-amino-pyridine) with one equivalent of nBuLi followed by addition of 0.5 equivalents of Cl₂SiMe₂ unexpectedly yields the cyclodisilazane [(6-Me-Apy)₂(μ-SiMe)₂]. [(6-Me-Apy)₂(μ-SiMe)₂] was characterised by ¹H, ¹³C and ²⁹Si NMR and IR spectroscopy, elemental analysis and X-ray crystallographic methods. The X-ray structure reveals a dinuclear compound having a four-membered Si₂N₂ core with coplanar aromatic rings.     

Spectroscopic properties of mono- and binuclear platinum(II) alkynyl complexes with phosphine ligands: A theoretical study

Based on their MP2 optimized structures in the ground states, we obtained solution absorption spectra for trans-[Pt^II(CCR)₂(PH₃)₂] (R = H (1) and Ph (2)) and trans-[Pt^II(CCH)₂(PH₂CH₂PH₂)]₂ (3) under the time-dependent density functional theory calculations. These absorptions agree with experimental observations. The unrestricted MP2 optimization performed for 3 in the lowest-energy triplet excited state shows that upon excitation the PtPt distance shortens about 0.347 Å with respect to the 3.188 Å one in the ground state. The UMP2 calculations estimated that its ³[σ^∗(d_z²)σ(p_z)] excited state produces the 531 nm emission, corresponding to the 580 nm one of trans-[Pt^II(CCPh)₂(PPh₂CH₂PPh₂)]₂ in the solid state at 298 K.     

Synthesis of a new tetradentate bis(imino-phosphine) ligand and its complexation with copper(I) ions

Schiff base condensation of m-phenylenediamine with two equivalents of o-(diphenylphophino)benzaldehyde products the potentially tetradentate molecule 1,3-(Ph₂P(o-C₆H₄)CHN)₂C₆H₄ (1) in high yield. The reaction of 1 and [Cu(NCMe)₄]BF₄ affords the dinuclear complex [(1,3-(Ph₂P(o-C₆H₄)CHN)₂C₆H₄)₂Cu₂](BF₄)₂ (2) through coordination of the imino-phosphine groups. The structure of 2 has been determined by an X-ray diffraction study.     

Germanium tetra(tertiary butoxide): Synthesis, structure and its utility as a precursor for the preparation of europium doped germanium oxide nanoparticles

Germanium tetra(tertiary butoxide), [Ge(O^tBu)₄], has been prepared by the reaction of GeCl₄ with KOBu^t in benzene. It is a monomeric crystalline solid having a distorted tetrahedral configuration, defined by the coordination of four OBu^t groups around germanium atom. The TG analysis showed that the compound is thermally stable and volatilizes at around 130 °C. Europium doped and un-doped germanium oxide nanoparticles were prepared based on the urea hydrolysis of Ge(O^tBu)₄/Eu(OOCCH₃)₃ in ethylene glycol medium at 150 °C followed by heating the resulting product at 750 °C. The nanoparticles were characterized by XRD, TEM and PL measurements. The europium doped nanoparticles, which were nearly monodispersed (∼30 nm), showed luminescence and the Eu³⁺ ions were occupying the surface of the GeO₂ nanoparticles.