Out of the 14 lanthanide (Ln) ions, molecular complexes of Ln(IV) were known only for cerium and more recently terbium. Here we demonstrate that the +IV oxidation state is also accessible for the large praseodymium (Pr) cation. The oxidation of the tetrakis(triphenysiloxide) Pr(III) ate complex, [KPr(OSiPh 3 ) 4 (THF) 3 ], 1-Pr Ph , with [N(C 6 H 4 Br) 3 ][SbCl 6 ], affords the Pr(IV) complex [Pr(OSiPh 3 ) 4 (MeCN) 2 ], 2-Pr Ph , which is stable once isolated. The solid state structure, UV−visible spectroscopy, magnetometry, and cyclic voltammetry data along with the DFT computations of the 2-Pr Ph complex unambiguously confirm the presence of Pr(IV).

This study details syntheses of unsymmetrical magnesium(I)−adduct complexes, [( Ar Nacnac)-(D)Mg−Mg( Ar Nacnac)] ( Ar Nacnac = [(ArNC-Me) 2 CH] − ), Ar = xylyl (Xyl), mesityl (Mes), 2,6diethylphenyl (Dep), or 2,6-diisopropylphenyl (Dip); D = N-heterocyclic carbene or 4-dimethylaminopyridine, DMAP), which X-ray crystallographic studies show to have markedly elongated Mg−Mg bonds. Two of these highly reactive species are shown to reductively trimerize CO to yield rare crystallographically characterized examples of the planar, aromatic deltate dianion, incorporated in the complexes [{( Dip Nacnac)(D)Mg(μ-C 3 O 3 )Mg( Dip Nacnac)} 2 ] (D = DMAP or :C{N(Me)C-(Me)} 2 ). DFT calculations suggest that these complexes form via stepwise two-electron reductions of three CO molecules, resulting in the formation of three C−C bonds within the cyclic deltate unit. This work highlights the utility of activated magnesium(I) adduct complexes as soluble organometallic models for the study of reductive C−C bond-forming events in, for example, the heterogeneously catalyzed Fischer−Tropsch process.

Reduction of dinitrogen (N 2 ) is a major challenge for chemists. Cooperation of multiple metal centers to break the strong N 2 triple bond has been identified as a crucial step in both the industrial and the natural ammonia syntheses. However, reports of the cleavage of N 2 by a multimetallic uranium complex remain extremely rare, although uranium species were used as catalyst in the early Harber−Bosch process. Here we report the cleavage of N 2 to two nitrides by a multimetallic uranium−rhodium cluster at ambient temperature and pressure. The nitride product further reacts with acid to give substantial yields of ammonium. The presence of uranium−rhodium bond in this multimetallic cluster was revealed by X-ray crystallographic and computational studies. This study demonstrates that the multimetallic clusters containing uranium and transition metals are promising materials for N 2 fixation and reduction.

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA) 3 (2) with KC 8 yields the anionic bis(areneEPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f 4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a twoelectron reductant toward I 2 to form the dinuclear U(III)−U(III) triiodide species 6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η 7 -C 7 H 7 )(TDA) 2 (THF)] ( 7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA) 4 ] (1-crypt) and U(COT) 2 , while the addition of COT to complex 2 instead yields the dinuclear U(IV)−U(IV) inverse sandwich complex [U(TDA) 3 ] 2 (μ-η 8 :η 3 -C 8 H 8 ) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA) 4 (4) with KC 8 gives the mono(arene) complex [K[2.2.2]cryptand][Th-(TDA) 3 (THF)] (5). The C−C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

Subtle changes to the bulk of 1 : 1 adducts of DMAP with magnesium(i) complexes leads to steric control over the products arising from their reductive oligomerisations of carbon monoxide.

The synthesis of lanthanides other than cerium in the oxidation state +IV has remained a desirable but unmet target until recently, when two examples of TbIV with saturated coordination spheres were isolated. Here we report the third example of an isolated molecular complex of terbium(IV), where the supporting siloxide ligands do not saturate the coordination sphere. The fully characterized six‐coordinate complex [TbIV(OSiPh3)4(MeCN)2], 2‐TbPh, shows high stability and the labile MeCN ligands can be replaced by phosphinoxide ligands. Computational studies suggest that the stability is due to a strong π(O−Tb) interaction which is stronger than in the previously reported TbIV complexes. Cyclic‐voltammetry experiments demonstrate that non‐binding counterions contribute to the stability of TbIV in solution by destabilizing the +III oxidation state, while alkali ions promote TbIV/TbIII electron transfer.

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium (III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium (VI)-nitride is inert to hydrogenolysis, suggesting the 5f 1 electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.

The first examples of magnesium benzenehexolate complexes are prepared by reductive hexamerizations of CO, in reactions that are initiated (or catalyzed) by [Mo(CO) 6 ]. These reactions are closely related to Justus von Liebig's classical 1834 study on the reduction of CO gas with molten potassium, which yields K 6 C 6 O 6 (amongst other products), by an as yet unknown mechanism. File list (2) download file view on ChemRxiv C6O6 manuscript.pdf (428.67 KiB) download file view on ChemRxiv C6O6 SI.pdf (1.99 MiB)