In-Plane Thorium(IV), Uranium(IV), and Neptunium(IV) Expanded Porphyrin Complexes.

Here we report the first series of in-plane thorium(IV), uranium(IV), and neptunium(IV) expanded porphyrin complexes. These actinide (An) complexes were synthesized using a hexa-aza porphyrin analogue, termed dipyriamethyrin, and the nonaqueous An(IV) precursors, ThCl4(DME)2, UCl4, and NpCl4(DME)2. The molecular and electronic structures of the ligand, each An(IV) complex, and a corresponding uranyl(VI) complex were characterized using nuclear magnetic resonance (NMR) and UV-vis spectroscopies as well as single-crystal X-ray diffraction analysis. Computational analyses of these complexes, coupled to their structural features, provide support for the conclusion that a greater degree of covalency in the ligand-cation orbital interactions arises as the early actinide series is traversed from Th(IV) to U(IV) and Np(IV). The axial ligands in the present An(IV) complexes proved labile, allowing for the electronic features of these complexes to be further modified.

Isolation of a TMTAA-Based Radical in Uranium bis-TMTAA Complexes.

We report the synthesis, characterization, and electronic structure studies of a series of thorium(IV) and uranium(IV) bis-tetramethyltetraazaannulene complexes. These sandwich complexes show remarkable stability towards air and moisture, even at elevated temperatures. Electrochemical studies show the uranium complex to be stable in three different oxidation states; isolation of the oxidized species reveals a rare case of a non-innocent tetramethyltetraazaannulene (TMTAA) ligand.

A Thorium Chalcogenolate Series Generated by Atom Insertion into Thorium-Carbon Bonds.

A new thorium monoalkyl complex, Th(CH2SiMe3)(L3) (L = MeC(NiPr)2) (2), undergoes insertion of chalcogen atoms resulting in a series of thorium chalcogenolate complexes, Th(ECH2SiMe3)(L3) (E = S, SS, Se, Te; 5-8). Complex 6 represents the first alkyl disulfide thorium species and illustrates the ability of 2 to undergo controllable, stoichiometric atom insertion. All complexes have been characterized by 1H and 13C NMR spectroscopy, FTIR, EA, and melting point, and in the case of 1, 2, and 4-8, X-ray crystallography. Insertion was achieved by balancing the thermodynamic driving force of chalcogenolate formation versus the BDE of the pnictogen-chalcogen bond in the transfer reagent. Utilizing Me3NO as an oxygen atom transfer reagent led to C-H activation and SiMe4 extrusion rather than oxygen atom insertion, resulting in the alkoxide complex Th(OCH2NMe2)(L3) (4).

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).

Carbon-Nitrogen Bond Cleavage by a Thorium-NHC-bpy Complex.

Actinide complexes demonstrate unparalleled reactivity towards small molecules. However, utilizing these powerful transformations in a predictable and deliberate manner remains challenging. Therefore, developing actinide systems that not only perform noteworthy chemistry but also demonstrate controllable reactivity is a key goal. We describe a bis(NHC)borate thorium-bpy complex (1) that is capable of reductively cleaving the R-NC bond in a series of organic isocyanides. In contrast to most actinide-mediated bond activations, the dealkylation event mediated by 1 is remarkably general and yields very well-defined products that assist in mechanistic elucidation. Synthesis of the rearranged but-3-enyl product from the reaction of 1 and cyclopropylmethyl isocyanide supports the notion of a radical-based mechanism.

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.

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.

N-heterocyclic carbene gold(I) and silver(I) complexes bearing functional groups for bio-conjugation.

This work describes several synthetic approaches to append organic functional groups to gold and silver N-heterocyclic carbene (NHC) complexes suitable for applications in biomolecule conjugation. Carboxylate appended NHC ligands (3) lead to unstable Au(I) complexes that convert into bis-NHC species (4). A benzyl protected carboxylate NHC-Au(I) complex 2 was synthesized but deprotection to produce the carboxylic acid functionality could not be achieved. A small library of new alkyne functionalized NHC proligands were synthesized and used for subsequent silver and gold metalation reactions. The alkyne appended NHC gold complex 13 readily reacts with benzyl azide in a copper catalyzed azide-alkyne cycloaddition reaction to form the triazole appended NHC gold complex 14. Cell cytotoxicity studies were performed on DLD-1 (colorectal adenocarcinoma), Hep-G2 (hepatocellular carcinoma), MCF-7 (breast adenocarcinoma), CCRF-CEM (human T-Cell leukemia), and HEK (human embryonic kidney). Complete spectroscopic characterization of the ligands and complexes was achieved using (1)H and (13)C NMR, gHMBC, ESI-MS, and combustion analysis.