Selective Nickel-Catalyzed Hydrodefluorination of Amides Using Sodium Borohydride.

Hydrodefluorination selective to the ortho position to amides is accomplished under mild conditions using sodium borohydride and a nickel catalyst. The facile formation of a nickelacycle intermediate with a specific geometry ensures selectivity without the need for electronic directing groups, and fluorine atoms in other positions remain intact. This method avoids the use of stoichiometric silanes which are typical for most other defluorination reactions, resulting in virtually no organic waste byproducts.

Reactivity of terminal imido complexes of group 4-6 metals: stoichiometric and catalytic reactions involving cycloaddition with unsaturated organic molecules.

Imido complexes of early transition metals are key intermediates in the synthesis of many nitrogen-containing organic compounds. The metal-nitrogen double bond of the imido moiety undergoes [2+2] cycloaddition reactions with various unsaturated organic molecules to form new nitrogen-carbon and nitrogen-heteroatom bonds. This review article focuses on reactivity of the terminal imido complexes of Group 4-6 metals, summarizing their stoichiometric reactions and catalytic applications for a variety of reactions including alkyne hydroamination, alkyne carboamination, pyrrole formation, imine metathesis, and condensation reactions of carbonyl compounds with isocyanates.

Coordination of 2,2'-(Trifluoroazanediyl)bis(N,N'-dimethylacetamide) with U(VI), Nd(III), and Np(V): A Thermodynamic and Structural Study.

Thermodynamic properties of the complexation of 2,2'-(trifluoroazanediyl)bis(N,N'-dimethylacetamide) (CF3ABDMA) with U(VI), Nd(III), and Np(V) have been studied in 1.0 M NaNO3 at 25 °C. Equilibrium constants of the complexation were determined by potentiometry and spectrophotometry. In comparison with a series of structurally related amine-bridged diacetamide ligands, including 2,2'-(benzylazanediyl)bis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA), CF3ABDMA forms weaker complexes with U(VI), Nd(III), and Np(V) due to the lower basicity of the center N atom in CF3ABDMA resulting from the attachment of the strong electron-withdrawing CF3- moiety. The complexation strength of CF3ABDMA with the three metal ions follows the order: UO22+ > Nd3+ > NpO2+, consistent with the order of the "effective" charges of the metal ions. Structural information on the U(VI)/CF3ABDMA complexes in solution and in solid was obtained by theoretical computation, single crystal X-ray diffractometry, 19F NMR, and electrospray ionization mass spectrometry. The structural data indicate that, similar to the three previously studied amine-bridged diacetamide ligands (BnABDMA, ABDMA, and MABDMA), the CF3ABDMA ligand coordinates to UO22+ in a tridentate mode, through the center nitrogen and the two amide oxygen atoms.

α-Diimine-Niobium Complex-Catalyzed Deoxychlorination of Benzyl Ethers with Silicon Tetrachloride.

α-Diimine niobium complexes serve as catalysts for deoxygenation of benzyl ethers by silicon tetrachloride (SiCl4) to cleanly give two equivalents of the corresponding benzyl chlorides, where SiCl4 has the dual function of oxygen scavenger and chloride source with the formation of a silyl ether or silica as the only byproduct. The reaction mechanism has two successive trans-etherification steps that are mediated by the niobium catalyst, first forming one equivalent of benzyl chloride along with the corresponding silyl ether intermediate that undergoes the same reaction pathway to give the second equivalent of benzyl chloride and silyl ether.

Hydrodehalogenation of alkyl halides catalyzed by a trichloroniobium complex with a redox active α-diimine ligand.

A high-valent d0 niobium(v) complex, (α-diimine)NbCl3 (1), bearing a dianionic redox-active α-diimine ligand served as a catalyst for a hydrodehalogenation reaction of alkyl halides in the presence of PhSiH3. During the catalytic reaction, the redox-active α-diimine ligand allowed the complex to reversibly release and accept one-electron through switching its coordination mode between a dianionic folded form and a monoanionic planar one.

Siderophore-inspired chelator hijacks uranium from aqueous medium.

Over millennia, nature has evolved an ability to selectively recognize and sequester specific metal ions by employing a wide variety of supramolecular chelators. Iron-specific molecular carriers-siderophores-are noteworthy for their structural elegance, while exhibiting some of the strongest and most selective binding towards a specific metal ion. Development of simple uranyl (UO22+) recognition motifs possessing siderophore-like selectivity, however, presents a challenge. Herein we report a comprehensive theoretical, crystallographic and spectroscopic studies on the UO22+ binding with a non-toxic siderophore-inspired chelator, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2BHT). The optimal pKa values and structural preorganization endow H2BHT with one of the highest uranyl binding affinity and selectivity among molecular chelators. The results of small-molecule standards are validated by a proof-of-principle development of the H2BHT-functionalized polymeric adsorbent material that affords high uranium uptake capacity even in the presence of competing vanadium (V) ions in aqueous medium.

Solution Thermodynamics and Kinetics of Metal Complexation with a Hydroxypyridinone Chelator Designed for Thorium-227 Targeted Alpha Therapy.

The solution chemistry of a chelator developed for 227Th targeted alpha therapy was probed. The compound of interest is an octadentate ligand comprising four N-methyl-3-hydroxy-pyridine-2-one metal-binding units, two tertiary amine groups, and one carboxylate arm appended for bioconjugation. The seven p Ka values of the ligand and the stability constants of complexes formed with Th(IV), Hf(IV), Zr(IV), Gd(III), Eu(III), Al(III), and Fe(III) were determined. The ligand exhibits extreme thermodynamic selectivity toward tetravalent metal ions with a ca. 20 orders of magnitude difference between the formation constant of the Th(IV) species formed at physiological pH, namely [ThL]-, and that of its Eu(III) analogue. Likewise, log ß110 values of 41.7 ± 0.3 and 26.9 ± 0.3 (T = 25 °C) were measured for [ThL]- and [FeIIIL]2-, respectively, highlighting the high affinity and selectivity of the ligand for Th ions over potentially competing endogenous metals. Single crystal X-ray analysis of the Fe(III) complex revealed a dinuclear 2:2 metal:chelator complex crystallizing in the space group P1̅. The formation of this dimeric species is likely favored by several intramolecular hydrogen bonds and the protonation state of the chelator in acidic media. LIII edge EXAFS data on the Th(IV) complexes of both the ligand and a monoclonal antibody conjugate revealed the expected mononuclear 1:1 metal:chelator coordination environment. This was also confirmed by high resolution mass spectrometry. Finally, kinetic experiments demonstrated that labeling the bioconjugated ligand with Th(IV) could be achieved and completed after 1 h at room temperature, reinforcing the high suitability of this chelator for 227Th targeted alpha therapy.

Complexation-assisted reduction: complexes of glutaroimide-dioxime with tetravalent actinides (Np(iv) and Th(iv)).

Glutaroimide-dioxime forms strong complexes with tetravalent Th(iv) and Np(iv) in aqueous solution. In conjunction with literature data on the complexation of glutaroimide-dioxime with other metal ions, it was found that the complexes become weaker as the effective charge density on the metal ions decreases: V5+ > Th4+ ≈ Fe3+ > UO22+ > Eu3+/Nd3+ > Cu2+ > Pb2+ > NpO2+ > Ca2+/Mg2+. In the glutaroimide-dioxime complexes with Th(iv) and Np(iv), deprotonation of the imide group and relocation of the two hydrogen atoms from oxygen to nitrogen of the oxime groups result in a large conjugated system (-O-N-C-N-C-N-O-) that coordinates strongly to the metal center in a tridentate mode via the central imide nitrogen atom and the two oxime oxygen atoms. Because the stability of glutaroimide-dioxime complexes with Np(iv) is much higher than those with Np(v), the redox potential of the Np(v)/Np(iv) couple is expected to be shifted significantly. As a result, crystals of glutaroimide-dioxime complexes with Np(iv) were readily obtained from initial solutions containing Np(v). A mechanism of complexation-assisted reduction integrating the thermodynamic and structural data from this work is discussed.

Complexation of NpO2+ with Amine-Functionalized Diacetamide Ligands in Aqueous Solution: Thermodynamic, Structural, and Computational Studies.

Complexation of Np(V) with three structurally related amine-functionalized diacetamide ligands, including 2,2'-azanediylbis( N, N'-dimethylacetamide) (ABDMA), 2,2'-(methylazanediyl)bis( N, N'-dimethylacetamide) (MABDMA), and 2,2'-(benzylazanediyl)bis( N, N'-dimethylacetamide) (BnABDMA), in aqueous solutions was investigated. The stability constants of two successive complexes, namely, NpO2L+ and NpO2L2+, where L stands for the ligands, were determined by absorption spectrophotometry. The results suggest that the stability constants of corresponding Np(V) complexes follow the trend: MABDMA > ABDMA ≈ BnABDMA. The data are discussed in terms of the basicity of the ligands and compared with those for the complexation of Np(V) with an ether oxygen-linked diacetamide ligand. Extended X-ray absorption fine structure data indicate that, similar to the complexation with Nd3+ and UO22+, the ligands coordinate to NpO2+ in a tridentate mode through the amine nitrogen and two oxygen atoms of the amide groups. Computational results, in conjunction with spectrophotometric data, verify that the 1:2 complexes (NpO2(L)2+) in aqueous solutions are highly symmetric with Np at the inversion center, so that the f-f transition of Np(V) is forbidden and NpO2(L)2+ does not display significant absorption in the near-IR region.

Thermodynamic, Structural, and Computational Investigation on the Complexation between UO22+ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution.

The stability constants (log ß), enthalpies of complexation (ΔH), and entropies of complexation (ΔS) for the complexes of uranium(VI) with a series of amine-functionalized diaetamide ligands, 2,2'-benzylazanediylbis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-methylazanediylbis(N,N'-dimethylacetamide) (MABDMA), in aqueous solution were determined by potentiometry and calorimetry. Electronspray ionization mass spectrometry was used to verify the presence of uranium(VI) complexes in solution. The thermodynamic data indicate that the binding strengths of the three ligands with UO22+ follow the order BnABDMA < ABDMA < MABDMA, parallel to the order of the protonation constants as well as the order of the stability of the Nd3+ complexes, suggesting that the complexation of UO22+ with the ligands consist predominantly of electrostatic interactions. Denisty functional theory calculations were conducted to reveal the structures, electronic charge distribution, and energetics of the uranium(VI) complexes, providing insight into the thermodynamic trends of the complexation. Extended X-ray absorption fine structure spectroscopy was used to identify the structures of the uranium(VI) complexes in aqueous solution.

f-Block complexes of a m-terphenyl dithiocarboxylate ligand.

Straightforward syntheses are provided for the m-terphenyl dithiocarboxylic acid 2,6-(C6H4-4-tBu)2C6H3CS2H (TerphCS2H, 2) and its lithium and potassium salts, TerphCS2Li(Et2O)2 and TerphCS2K (1·Et2O and 4, respectively). These compounds can be isolated in good yields on multi-gram scales starting from Terph-I without isolating intermediates. Salt metathesis and protonolysis reactions provided access to the homoleptic actinide(iv) complexes (TerphCS2)4An (An = Th (5) and U (6)). Electrochemical and reactivity studies revealed that the dithiocarboxylate ligand is incompatible with U(iii). The homoleptic lanthanum(iii) complex (TerphCS2)3La and its η6-toluene adduct (7 and 7·tol, respectively) were also structurally characterized. Binding of toluene to 7 was shown to displace intramolecular La-Carene close contacts that are facilitated by a distortion from the usual geometry of bound dithiocarboxylate ligands.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater.

Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V5+ complex, exhibiting the highest stability constant value ever observed for the V5+ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.

New supporting ligands in actinide chemistry: tetramethyltetraazaannulene complexes with thorium and uranium.

We report the synthesis, characterization, and preliminary reactivity of new heteroleptic thorium and uranium complexes supported by the macrocyclic TMTAA ligand (TMTAA = Tetramethyl-tetra-aza-annulene). The dihalide complexes Th(TMTAA)Cl2(THF)2 (1), [UCl2(TMTAA)]2 (2) and U(TMTAA)I2 (3) are further functionalized to the Cp* derivatives ThCp*(TMTAA)Cl (4), UCp*(TMTAA)Cl (5) and UCp*(TMTAA)I (6) (Cp* = pentamethylcyclopentadienide). Compounds 4-6 are also obtained through a one-pot reaction from standard thorium(iv) and uranium(iv) starting materials, Li2TMTAA and KCp*. Complexes 1-6 function as valuable starting materials for salt metathesis chemistry. Treatment of precursors 4 or 5 with trimethylsilylmethyllithium (LiCH2TMS) results in the new actinide TMTAA alkyl complexes ThCp*(TMTAA)(CH2TMS) (7) and UCp*(TMTAA)(CH2TMTS) (8), respectively. The TMTAA-derived alkyl complexes (7 and 8) show unexpected stability and are stable for several weeks at room temperature in solution and in the solid-state. Additionally, double substitution of the halide ligands in 1-3 shows a strong dependence on the nucleophile used. While weaker nucleophiles, such as amides, and more sterically demanding nucleophiles, such as Cp (Cp = cyclopenadienide), favour the formation of bis-TMTAA "sandwich" complexes [An(TMTAA)2] (An = Th (9) and An = U (10)), the use of oxygen-functionalized ligands like the ODipp anion (Dipp = diisopropylphenyl) results in the formation of the doubly substituted species Th(ODipp)2TMTAA (11) and U(ODipp)2TMTAA (12). We also describe the divergent reactivity of the TMTAA ligand towards uranium(iii). Unlike the syntheses of actinide(iv) TMTAA complexes, the synthesis of a uranium(iii) TMTAA was not successful and only uranium(iv) species could be obtained.

Thorium Metallacycle Facilitates Catalytic Alkyne Hydrophosphination.

The bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex (1) undergoes reversible intramolecular C-H bond activation enabling the catalytic hydrophosphination of unactivated internal alkynes. Catalytic and stoichiometric experiments support a mechanism involving reactive Th-NHC metallacycle intermediates (Int and 2).

Kinetics of complexation of V(v), U(vi), and Fe(iii) with glutaroimide-dioxime: studies by stopped-flow and conventional absorption spectroscopy.

Uranium extracted from seawater is a promising source of uranium for nuclear energy, and the extraction technology using polymer sorbents has been shown to be feasible. However, improving selectivity for uranium over other metals, notably vanadium and iron, is essential to increase efficiency and reduce costs. In the present work, the kinetics of the binding of these three metals with glutaroimide-dioxime as a molecular analogue of polymer sorbents has been studied using stopped-flow and conventional UV Visible absorption spectroscopy to monitor the reactions over a range of time scales. Qualitatively, vanadium reacts the slowest of the three metals despite being able to form a very strong complex, with the 1 : 2 vanadium/ligand complex forming over weeks, likely due to the slow hydrolysis of the strong oxido ligands, while iron reacts fast and uranyl faster still, despite the presence of carbonate in the uranyl species. Conditional rate constants were determined for the formation of 1 : 1 glutaroimide-dioxime complexes with the three metal ions. Besides, in a narrow and near neutral pH region, a rate equation for the formation of the 1 : 1 vanadium/glutaroimide-dioxime complex was developed, showing the reaction is the first order with respect to [V], [ligand], and [H+]. These observations, some being qualitative and others quantitative, are consistent with previous marine tests of polymer adsorbents, and give mechanistic insight into how glutaroimide-dioxime forms complexes with uranium, iron, and vanadium.

Benzoquinonoid-bridged dinuclear actinide complexes.

We report the coordination chemistry of the tripodal tris[2-amido(2-pyridyl)ethyl]amine ligand, L, with thorium(iv) and uranium(iv). Using a salt-metathesis strategy from the potassium salt of this ligand, K3L, new actinide complexes were isolated, namely the dimeric thorium complex [ThCl(L)]2 (1) and the monomeric uranium complex UI(THF)(L) (2); under different crystallisation conditions, the dimeric uranium complex is also isolated, [UI(L)]2 (2-dimer). With the aim of studying electronic phenomena such as magnetic exchange between two actinide ions, we have synthesised the first examples of dinuclear, quinoid-bridged actinide complexes from dianionic 2,5-bis[2,6-(diisopropyl)anilide]-1,4-benzoquinone (QDipp) and 2,5-bis[2-(methoxy)anilide]-1,4-benzoquinone (QOMe) ligands. The resulting complexes are [Th(L)]2QDipp (3), [Th(THF)(L)]2QOMe (5) and [U(L)]2QOMe (6). The targeted [U(L)]2QDipp complex (4) could not be isolated. All isolated complexes have been characterised by spectroscopic methods and X-ray crystallography. The uranium(iv) complexes 2-dimer and 6 have been studied by SQUID magnetometry but indicate that there is negligible magnetic exchange between the two uranium(iv) ions. The reduced form of 6, [K(18-c-6)][6-] is unstable and highly sensitive, but X-ray crystallography indicates that it is a novel UIVUIV complex bridged by a quinoid-radical.

A Homoleptic Uranium(III) Tris(aryl) Complex.

The reaction of 3 equiv of Li-C6H3-2,6-(C6H4-4-tBu)2 (Terph-Li) with UI3(1,4-dioxane)1.5 led to the formation of the homoleptic uranium(III) tris(aryl) complex (Terph)3U (1). The U-C bonds are reactive: treatment with excess iPrN═C═NiPr yielded the double-insertion product [TerphC(NiPr)2]2U(Terph) (2). Complexes 1 and 2 were characterized by X-ray crystallography, which showed that the U-C bond length in 2 (2.624(4) Å) is ∼0.1 Å longer than the average U-C bond length in 1 (2.522(2) Å). Thermal decomposition of 1 yielded Terph-H as the only identifiable product; the process is unimolecular with activation parameters ΔH⧧ = 21.5 ± 0.3 kcal/mol and ΔS⧧ = -7.5 ± 0.8 cal·mol-1 K-1, consistent with intramolecular proton abstraction. The protonolysis chemistry of 1 was also explored, which led to the uranium(IV) alkoxide complex U(OCPh3)4(DME) (3·DME).