Ligands For Transition Metals Research Articles

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Our Inaugural Early Career Advisory Board

Isatin Derived Mixed Ligand Metal Complexes as Antimicrobial, Antioxidant Agents and Catalyst for Oxidation of Styrene

A novel series of mixed ligand transition metal complexes of Cu(II), Zn(II), Ni(II) and Co(II) have been synthesized from the Schiff base ligand, 3-(naphthalen-1-ylimino)indolin-2-one (achieved by the condensation of isatin and naphthylamine) and 1, 10-phenonthroline. The structural features were corroborated by means of elemental analyses, molar conductance, IR, UV-Vis, 1H NMR and mass spectral studies. The data displays that these complexes have the composition of [MLX] type where X = 1, 10-phenonthroline. The UV-Vis. and magnetic susceptibility data of the complexes suggest a square-planar geometry around the central metal ion. The IR spectral data of the ligand exhibits the presence of iminic group and its complexes manifest that the imine group of the Schiff base coordinate with the metal ion and decreased value of carbonyl group indicates which coordinate to the metal. The proton NMR spectrum of the ligand shows the presence of imine group. The mass spectral data also confirms the metal complexes as [MLX] type. Molar conductance values of the complexes have been found in the range of 5–14 ohm−1cm2mol−1, suggesting the non-electrolytic nature of the complexes. The antimicrobial study reveals that copper and cobalt complexes are better antimicrobial agents than the standard drug streptomycin and Nystatin. The copper and cobalt complexes exhibit remarkable antioxidant potential than the other complexes. The catalytic oxidation of styrene by H2O2 was carried out in acetonitrile in the presence of mixed ligand metal complex as catalyst. The result of the catalytic study reveals that the zinc complex effectively catalyses the oxidation of styrene.

Synthesis of Arylstannanes via Palladium‐Catalyzed Decarbonylative Coupling of Aroyl Fluorides

AbstractAryl stannanes are valuable precursors in organic transformations, but their synthetic methods are limited. Here we present a Pd‐catalyzed decarbonylative stannylation of acid fluorides in the absence of exogenous base. Various aryl stannanes were efficiently prepared from bench‐stable transition metal catalyst and ligand with broad functional group compatibility and substrate scope including natural products and pharmaceuticals. This protocol was also successfully used to a late‐stage diversification of an existing uricosuric drug probenecid.magnified image

Base Metal Catalyzed Isocyanide Insertions

AbstractIsocyanides are diverse C1 building blocks considering their potential to react with nucleophiles, electrophiles, and radicals. Therefore, perhaps not surprisingly, isocyanides are highly valuable as inputs for multicomponent reactions (MCRs) and other one‐pot cascade processes. In the field of organometallic chemistry, isocyanides typically serve as ligands for transition metals. The coordination of isocyanides to metal centers alters the electronic distribution of the isocyano moiety, and reaction pathways can therefore be accessed that are not possible in the absence of the metal. The tunable reactivity of the isocyanide functional group by transition metals has evolved into numerous useful applications. Especially palladium‐catalyzed isocyanide insertion processes have emerged as powerful reactions in the past decade. However, reports on the use of earth‐abundant and cheap base metals in these types of transformations are scarce and have received far less attention. In this Minireview, we focus on these emerging base metal catalyzed reactions and highlight their potential in synthetic organic chemistry. Although mechanistic studies are still scarce, we discuss distinct proposed catalytic cycles and categorize the literature according to 1) the (hetero)atom bound to and 2) the type of bonding with the transition metal in which the (formal) insertion occurs.

Base Metal Catalyzed Isocyanide Insertions.

Isocyanides are diverse C1 building blocks considering their potential to react with nucleophiles, electrophiles, and radicals. Therefore, perhaps not surprisingly, isocyanides are highly valuable as inputs for multicomponent reactions (MCRs) and other one-pot cascade processes. In the field of organometallic chemistry, isocyanides typically serve as ligands for transition metals. The coordination of isocyanides to metal centers alters the electronic distribution of the isocyano moiety, and reaction pathways can therefore be accessed that are not possible in the absence of the metal. The tunable reactivity of the isocyanide functional group by transition metals has evolved into numerous useful applications. Especially palladium-catalyzed isocyanide insertion processes have emerged as powerful reactions in the past decade. However, reports on the use of earth-abundant and cheap base metals in these types of transformations are scarce and have received far less attention. In this Minireview, we focus on these emerging base metal catalyzed reactions and highlight their potential in synthetic organic chemistry. Although mechanistic studies are still scarce, we discuss distinct proposed catalytic cycles and categorize the literature according to 1) the (hetero)atom bound to and 2) the type of bonding with the transition metal in which the (formal) insertion occurs.

Intramolecular Hydrogen Bonding Facilitates Electrocatalytic Reduction of Nitrite in Aqueous Solutions.

This work reports a combined experimental and computational mechanistic investigation into the electrocatalytic reduction of nitrite to ammonia by a cobalt macrocycle in an aqueous solution. In the presence of a nitrite substrate, the Co(III) precatalyst, [Co(DIM)(NO2)2]+ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene), is formed in situ. Cyclic voltammetry and density functional theory (DFT) calculations show that this complex is reduced by two electrons, the first of which is coupled with nitrite ligand loss, to provide the active catalyst. Experimental observations suggest that the key N-O bond cleavage step is facilitated by intramolecular proton transfer from an amine group of the macrocycle to a nitro ligand, as supported by modeling several potential reaction pathways with DFT. These results provide insights into how the combination of a redox active ligand and first-row transition metal can facilitate the multiproton/electron process of nitrite reduction.

Concise Pyrimido[4,5‐d]pyrimidine Synthesis via Direct Annulation of N‐Uracil Amidines with Benzaldehydes Under Transition Metal Free Conditions

AbstractA novel and concise route towards the synthesis of pyrimido[4,5‐d]pyrimidines via a direct annulation reaction of N‐uracil amidines with benzaldehydes under transition metal free conditions has been described. The requisite precursors N‐uracil amidines are obtained via SNAE reaction on 6‐chlorouracil with amidines. A series of pyrimido[4,5‐d]pyrimidines has been synthesized in good to excellent yields by the direct annulation of N‐uracil amidines with benzaldehydes using Cs2CO3 as base and molecular oxygen as oxidant in tBuOH at 100 0C. This synthetic protocol has a number of advantages such as‐ operational simplicity, use of green oxidant, avoidance of transition metal catalysts or ligands, and easy accessibility of the starting materials, making it a highly practical approach to access various pyrimido[4,5‐d]pyrimidines of biological interest.

Half- and mixed-sandwich metallacarboranes for potential applications in medicine

Abstract Today, medicinal chemistry is still clearly dominated by organic chemistry, and commercially available boron-based drugs are rare. In contrast to hydrocarbons, boranes prefer the formation of polyhedral clusters via delocalized 3c2e bonds, such as polyhedral dicarba-closo-dodecaborane(12) (closo-C2B10H12). These clusters have remarkable biological stability, and the three isomers, 1,2- (ortho), 1,7- (meta), and 1,12-dicarba-closo-dodecaborane(12) (para), have attracted much interest due to their unique structural features. Furthermore, anionic nido clusters ([7,8-C2B9H11]2−), derived from the neutral icosahedral closo cluster 1,2-dicarba-closo-dodecaborane(12) by deboronation followed by deprotonation are suitable ligands for transition metals and offer the possibility to form metallacarboranes, for example via coordination through the upper pentagonal face of the cluster. The isolobal analogy between the cyclopentadienyl(–1) ligand (Cp−) and [C2B9H11]2− clusters (dicarbollide anion, Cb2−) is the motivation in using Cb2− as ligand for coordination to a metal center to design compounds for various applications. This review focuses on potential applications of half- and mixed-sandwich-type transition metal complexes in medicine.

Tantalum isocyanide complexes: TaI(CNDipp)6 (Dipp is 2,6-diisopropylphenyl) and ionic [Ta(CNDipp)7][Ta(CNDipp)6], a formal disproportionation product of the 17-electron Ta0 metalloradical Ta(CNDipp)6.

Treatment of tetraethylammonium hexacarbonyltantalate, [Et4N][Ta(CO)6], with 1.1 equivalents of molecular iodine (I2) in tetrahydrofuran (THF) at 200 K, followed by the addition of 6.0 equivalents of 2,6-diisopropylphenyl isocyanide (CNDipp) and slow warming to 293 K over a 24 h period gave the tantalum(I) iodide derivative hexakis(2,6-diisopropylphenyl isocyanide-κC)iodidotantalum(I), [TaI(C13H17N)6] or TaI(CNDipp)6, 1. Recrystallization of this substance from pentane provided deep-red nearly black parallelepipeds of the product, which was characterized by single-crystal X-ray diffraction. Addition of 1 in THF at 200 K to a suspension of an excess (5.8 equivalents) of caesium graphite (CsC8), followed by warming, filtration, and solvent removal, afforded a dark-green oily solid of unknown composition, from which several red-brown rhombohedral plates of the ditantalum salt heptakis(2,6-diisopropylphenyl isocyanide-κC)tantalum hexakis(2,6-diisopropylphenyl isocyanide-κC)tantalate, [Ta(C13H17N)7][Ta(C13H17N)6] or [Ta(CNDipp)7][Ta(CNDipp)6], 2, were harvested. Salt 2 is a unique substance, as it is the only known example of a salt containing a homoleptic cation, [MLx]+, and a homoleptic anion, [MLy]-, with the same transition metal and π-acceptor ligand L. In solution, 2 undergoes full comproportionation to afford the recently reported 17-electron paramagnetic zerovalent tantalum complex Ta(CNDipp)6, the only known isolable TaL6 complex of Ta0.

Cu(i), Ag(i), Ni(ii), Cr(iii) and Ir(i) complexes with tritopic NimineCNHCNamine pincer ligands and catalytic ethylene oligomerization.

With their N-amine and N'-imine substituents, the imidazolium chlorides [(ImH){C(Me)[double bond, length as m-dash]NDipp}(C2NMe2)]Cl (1) and [(ImH){C(Me)[double bond, length as m-dash]NDipp}(C3NMe2)]Cl (2) are suitable precursors to pincer-type tritopic NimineCNHCNamine ligands for various transition metals and allow an investigation of the influence of (i) the functionalities associated with the NHC donor and (ii) the length of the spacer connecting the amine group to the heterocycle. The mononuclear Cu(i) complexes [CuCl{Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (3) and [CuCl{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (4) were prepared by the reaction of 1 and 2 with mesitylcopper(i), respectively. The reactions of 1 and 2 with Ag2O, in the presence of molecular sieves, afforded the Ag(i) complexes [AgCl{Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (5) and [AgCl{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (6), respectively. A dinuclear Cu(i) complex containing a ladder-type skeleton formed by copper and iodine atoms, [Cu4(μ2-I)2(μ3-I)2{μ-κ2-CNHC,Namine(Im){C(Me)[double bond, length as m-dash]NDipp}(C3NMe2)}2] (7), was obtained by deprotonation of 2 using sodium trimethylsilylamide/CuI. A transmetallation reaction from 5 to [NiBr2(dme)] afforded an unexpected mixed-metal tetranuclear, centrosymmetric complex [NiCl{Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}{μ2-κ3-Nimine,CNHC,NamineIm[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)}AgBr2]2 (8), which contains two mononuclear Ni(ii) units, one with a κ1CNHC donor and the other with a κ2-Nimine,CNHC chelating and μ2-CNHC,Namine bridging ligand. These two monocationic complexes are connected by the dianonic, disilver unit [Ag2Br4]2-. Furthermore, the mononuclear tritopic Ni(ii) pincer complexes [Ni(NCMe){Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ3Nimine,CNHC,Namine}][BF4]2 (9) and [Ni(NCMe){Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ3Nimine,CNHC,Namine}][BF4]2 (10) were obtained when [Ni(NCMe)6](BF4)2 was used as a precursor in transmetallation reaction with 9 and 10, respectively. Transmetallation from the silver complexes 5 and 6 was also applied to the synthesis of chromium complexes and whether [CrCl2(THF)2] or [CrCl3(THF)3] was used as a precursor, the same chromium(iii) complexes, mer-[CrCl3{Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ3Nimine,CNHC,Namine}] (11) and mer-[CrCl3{Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ3Nimine,CNHC,Namine}] (12), respectively, were isolated. Moreover, silver transmetalation reactions from 5 and 6 were successfully applied to the Ir(i) precursor [Ir(μ-Cl)(cod)]2 (cod = 1,5-cycloocadiene) and the Ir(i) complexes [IrCl(cod){Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ1CNHC}] (13) and [IrCl(cod) {Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ1CNHC}] (14), respectively, containing a monodentate NHC ligand were isolated as yellow crystals in good yields. The cationic, Nimine,CNHC-chelated Ir(i) complexes [Ir(cod){Im[C(Me)[double bond, length as m-dash]NDipp](C2NMe2)-κ2Nimine,CNHC}]BF4 (15) and [Ir(cod){Im[C(Me)[double bond, length as m-dash]NDipp](C3NMe2)-κ2Nimine,CNHC}]BF4 (16) were obtained by halide abstraction from 13 and 14, respectively. The following 10 complexes 3, 6, 7, 8·2Et2O, 9-11, 12·2C7H8, 14 and 16·THF were characterized by X-ray diffraction analysis. In catalytic ethylene oligomerization, the Ni complex 10 showed a productivity of 14 600 gC2H4 per (gNi h), giving 65% butenes and 30% hexenes. With the Cr complex 12, the productivity was up to 9200 gC2H4 per (gCr h) and a very high selectivity for α-olefins was observed, from C4 to C8.

Two-dimensional π-conjugated metal-organic nanosheets as single-atom catalysts for the hydrogen evolution reaction.

The design and synthesis of electrocatalysts with controllable active sites are highly desirable for producing hydrogen. Inspired by the experimental realization of two-dimensional metal-organic framework (2D-MOF) nanosheets, a computational screening was performed to find out the optimal single-atom catalysts for the hydrogen evolution reaction (HER) from 36 2D-MOFs composed of transition metal and benzene-like molecular ligands. Through first-principles calculations, six configurations were identified as potential catalysts for the HER with performance comparable to those of Pt-based catalysts due to the near-zero Gibbs free energy and favourable activation barriers originating from their appropriate d-orbital bonding mechanism with atomic hydrogen.

Two d10 Metal–Organic Frameworks as Low-Temperature Luminescent Molecular Thermometers

The assemblies of a p-conjugated organic luminophore ligand (p-tr2Ph) and d10 transition metal cations produce two metal–organic frameworks (MOFs), [Zn(μ4-p-tr2Ph)(μ2-NO3)]·NO3 (1), and {[Cd2(μ4-1,4-BDC)2(μ4-p-tr2Ph)(μ2-DMF)(H2O)2]}·2H2O (2) (p-tr2Ph = 1,4-phenylene-4,4′-bis(1,2,4-triazole), DMF = N,N′-dimethylformamide, 1,4-H2BDC = 1,4-dicarboxybenzene). Compound 1 is a new three-dimensional (3D) cationic framework with left-handed helical channels constructed from a right-handed helical chain [Zn(μ2-tr)2(μ2-NO3)]n. Compound 2 possesses a 3D 3-fold interpenetrated neutral framework with one-dimensional distorted hexagonal channels in each MIL-88 type single-net. The compounds 1 and 2 both exhibit blue fluorescence. Notably, the intensity-based luminescent thermometer measurements reveal that compound 2 is extremely more linear with temperature, has finer reversibility within three cycles, and a wider temperature range than those of compound 1, indicating that compound 2 is a more desirable low-temperatur...

Potential of Food Hydrolyzed Proteins and Peptides to Chelate Iron or Calcium and Enhance their Absorption.

Iron and calcium are two essential micronutrients that have strong effects on nutrition and human health because of their involvement in several biological and redox processes. Iron is responsible for electron and oxygen transport, cell respiration, and gene expression, whereas calcium is responsible for intracellular metabolism, muscle contraction, cardiac function, and cell proliferation. The bioavailability of these nutrients in the body is dependent on enhancers and inhibitors, some of which are found in consumed foods. Hydrolyzed proteins and peptides from food proteins can bind these essential minerals in the body and facilitate their absorption and bioavailability. The binding is also important because excess free iron will increase oxidative stress and the risks of developing chronic diseases. This paper provides an overview of the function of calcium and iron, and strategies to enhance their absorption with an emphasis on hydrolyzed proteins and peptides from foods. It also discusses the relationship between the structure of peptides and their potential to act as transition metal ligands.

Spin‐Crossover and Field‐Induced Single‐Molecule Magnet Behaviour in Metal(II)‐Dipyrazolylpyridine Complexes

The reaction between the 2,6‐di(pyrazol‐1‐yl)‐4‐(bromomethyl)pyridine (L) ligand and divalent transition metal salts led to the formation of three mononuclear complexes of formula [M(L)2](BF4)2·xMeNO2 [M = FeII (1·xMeNO2), x = 0 and 4, M = CoII, x = 2 (2·2MeNO2) and]. Static magnetic measurements highlighted thermal spin crossover at high temperature (320–340 K) for the two FeII solvatomorphs nevertheless both the solvation degree and crystal packing had drastic influence on the shape and width of the hysteresis loop. Thus the 1 displayed abrupt and straight (6 K) hysteresis loop while for 1·4MeNO2 the thermal spin transition could be described as a six‐step gradual transition with the widest magnetic bistability range (35 K) for this kind of complexes. Irradiation at 530 nm of 1·4MeNO2 provoked an efficient LIESST effect at low temperature (TLIESST = 55 K). Finally dynamic magnetic measurements of 2·2MeNO2 shown a field‐induced mononuclear Single‐Molecule Magnet behavior with Δ = 14.6(4) cm–1 and τ0 = 6.28(8) × 10–7 s.

Copper(II) complexes of bidentate mixed ligands as artificial nucleases: Synthesis, crystal structure, characterization and evaluation of biological properties

Mixed ligand transition metal complexes play a significant role in biological processes by interacting with small molecules and activating the enzymes for nuclease activity. Four mixed ligand copper(II) complexes [Cu(5-nsal)(bipy)(ClO4)] (1), [Cu(2-hnap)(bipy) (ClO4)] (2), [Cu(5-nsal)(biim)(ClO4)] (3) and [Cu(2-hnap)(biim)(ClO4)] (4) (bipy = 2,2′-bipyridyl, biim = 2,2′-bi-1H-imidazole, 5-nsal = 5-nitrosalicylaldehyde and 2-hnap = 2-hydroxynaphthaldehyde) were successfully synthesized and physico-chemically characterized. The X-ray quality single crystals of copper(II) complexes 1 and 2 showed that the copper atom was penta-coordinated with square-pyramidal geometry. The physicochemical characterizations of the copper(II) complexes also confirmed the formation of anticipated structures. The redox potentials of the complexes assessed by cyclic voltammetry showed a quasi-reversible redox behavior consistent with Cu(II)/Cu(I) reduction in the cathodic region. The results of ctDNA binding studies using absorption, fluorescence and circular dichroism spectra exposed that the synthesized complexes interact via intercalation mode. The molecular docking studies (in silico) showed that complexes interact via major grooves which supported the results obtained from in vitro DNA binding studies. The nuclease activity of the complexes showed a moderate cleave of the supercoiled plasmid pBR322 DNA in presence of reducing agent (MPA) via hydroxy radical formation. All the copper (II) complexes exhibited a significant antimicrobial activity against two gram-positive, two gram-negative bacterial stains and two fungal species.

Ring-Walking in Catalyst-Transfer Polymerization.

Catalyst-transfer polymerization (CTP) has emerged as a useful method for synthesizing conjugated polymers with control over their length, sequence, and end-groups. However, the extent to which the polymerizations are living and chain-growth (or not) is highly catalyst and monomer dependent. Few studies have elucidated the impact of these identities on the stability and reactivity of the key intermediate, especially under polymerization-relevant conditions. We developed herein a simple experiment to identify catalyst stability and ring-walking ability using in situ-generated polymers. The combined results show that the ancillary ligand, metal, and polymer identity all play a crucial role. While each catalyst studied walks efficiently over large distances in poly(thiophene), the trends observed for poly(phenylene) highlight the differing roles of transition metal and ancillary ligand identities. The insights gained herein should be useful for extending CTP to other monomer and copolymer scaffolds.

A novel light-induced ATRP for the preparation of water dispersible fluorescent nanodiamonds and their biological imaging applications

Carbon-based fluorescent nanoparticles are considered to be new promising nontoxic fluorescence probes which suitable for various bioimaging and biomedical applications. However, the synthesis and production of such high-quality fluorescent carbon-based nanoparticles is still challenging, and its most application potential is unexplored. In this work, we introduce a facile light-induced surface-initiated atomic transfer radical polymerization (SI-ATRP) method to fabricate the nanodiamonds (NDs) based fluorescence probes in the absence of transition metal ligands. In this sample way, the Fluorescein was employed and played the role as the fluorescent signal and photocatalyst simultaneously for its photochemical property. Besides, the poor dispersibility of pristine NDs was also obviously improved by modifying with PEGMA, and the modified NDs (FNDs-polyPEGMA) display good dispersion and biocompatibility via characterization methods. A series of characterization techniques were also carried out to confirm the successful fabrication of fluorescent NDs after polymerization. Through our investigation, the as prepared FNDs-polyPEGMA nanoparticles with high dispersibility and stable green fluorescence could be the potentially used fluorescent probes in biomedical application.

Borylenes: An Emerging Class of Compounds.

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H2 , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.

Arylamines made easy

Organic chemists love their functional groups, and perhaps the most hotly pursued of these amours is the amine. Arylamines, for example, are often important components of pharmaceuticals or other biologically active molecules. But the reactions commonly used to make them require transition metal catalysts and ligands that can be time-consuming to screen and expensive to remove should they contaminate a final product. A new method for constructing both symmetrical and unsymmetrical diaryl-, arylalkyl-, and dialkylamines now gives chemists a general way to make these important molecular motifs without having to use transition metals and ligands. Umpolung—the fun-to-say chemical phenomenon in which a functional group’s polarity is reversed—underlies the new method. Chemists, led by László Kürti of Rice University, Daniel H. Ess of Brigham Young University, and Guigen Li of Nanjing University and Texas Tech University, have come up with a series of imines and an oxime—all derived from malonate—in which

Theoretical Assessment of Low-Valent Germanium Compounds as Transition Metal Ligands: Can They Be Better than Phosphines or NHCs?

We analyze the key transition metal ligand properties, σ-donor and π-acceptor ability, ligand-to-metal charge transfer, and steric parameters, using theoretical methods in order to examine 144 synthesized low-valent germanium compounds as potential ligands in homogeneous transition metal catalysis. We compare these features to those of currently widely used ligands (carbenes, phosphines). We find that several low-valent germanium compounds discovered lately, especially Ge(0) and double-donor-stabilized Ge(II) compounds, can easily outperform phosphine ligands in σ-donor strength or reach the strength of NHCs. We set up a databank in which one can find the most suitable germanium-based ligand to promote different homogeneous catalytic steps. We present the factors behind the favorable features of low-valent germanium compounds, which may help to design new low-valent germanium compounds with enhanced properties in the future.