Reactivity of Organothorium Complexes with TEMPO

Uncovering Factors Controlling Reactivity of Metal-TEMPO Reaction Systems in the Solid State and Solution

Nitroxides find application in various areas of chemistry, and a more in-depth understanding of factors controlling their reactivity with metal complexes is warranted to promote further developments. Here, we report on the effect of the metal centre Lewis acidity on both the distribution of the O- and N-centered spin density in 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and turning TEMPO from the O- to N-radical mode scavenger in metal-TEMPO systems. We use Et(Cl)Zn/TEMPO model reaction system with tuneable reactivity in the solid state and solution. Among various products, a unique Lewis acid-base adduct of Cl2Zn with the N-ethylated TEMPO was isolated and structurally characterised, and the so-called solid-state 'slow chemistry' reaction led to a higher yield of the N-alkylated product. The revealed structure-activity/selectivity correlations are exceptional yet are entirely rationalised by the mechanistic underpinning supported by theoretical calculations of studied model systems. This work lays a foundation and mechanistic blueprint for future metal/nitroxide systems exploration.

Reduction of Thorium Tris(amido)arene Complexes: Reversible Double and Single C-C Couplings

The reduction chemistry of thorium complexes is less explored compared to that of their uranium counterparts. Here, we report the synthesis, characterization, and reduction chemistry of two thorium(IV) complexes, (AdTPBN3)ThCl (1) and (DtbpTPBN3)ThCl(THF) (4) [RTPBN3 = 1,3,5-[2-(RN)C6H4]3C6H3; R = 1-adamantyl (Ad) or 3,5-di-tert-butylphenyl (Dtbp); THF = tetrahydrofuran], supported by tripodal tris(amido)arene ligands with different N-substituents. Reduction of 1 with excessive potassium in n-pentane yielded a double C-C coupling product, [(AdTPBN3)ThK(Et2O)2]2 (3), featuring a unique tetraanionic tricyclic core. On the other hand, reduction of 4 with 1 equiv of KC8 in hexanes/1,2-dimethoxyethane (DME) afforded a single C-C coupling product, [(DtbpTPBN3)Th(DME)]2 (5), with a dianionic bis(cyclohexadienyl) core. The solid- and solution-state structures of dinuclear thorium(IV) complexes 3 and 5 were established by X-ray crystallography and NMR spectroscopy. In addition, reactivity studies show that 3 and 5 can behave as thorium(II) and thorium(III) synthons to reduce organic halides. For instance, 3 and 5 are able to reduce 4 and 2 equiv of benzyl chloride, respectively, to regenerate 1 and 4 with concomitant formation of dibenzyl. Reversible C-C couplings under redox conditions provide an alternative approach to exploiting the potential of thorium arene complexes in redox chemistry.

TEMPO coordination and reactivity in group 6; pseudo-pentagonal planar (η2-TEMPO)2CrX (X = Cl, TEMPO)

The exposure of CrCl₂(THF)₂ to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η²-O,N-TEMPO)₂CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η²-O,N-TEMPO)₂Cr(TEMPO) (2). Both 1 and 2 exhibit pseudo-pentagonal planar (PPP) geometry, instead of myriad alternatives. Calculations and spectral studies suggest the solid-state geometry persists in solution.

Gas-phase synthesis and structure of thorium benzyne complexes

The thorium benzyne complex (η²-C₆H₄)ThCl₃^- was synthesized in the gas phase through consecutive decarboxylation and dehydrochlorination from the (C₆H₅CO₂)ThCl₄^- precursor upon collision-induced dissociation. Theoretical calculations suggest that (η²-C₆H₄)ThCl₃^- exhibits a metallacyclopropene structure with two polarized Th-C_benzyne σ bonds. This procedure can be generally extended to the synthesis of a wide range of gas-phase thorium benzyne complexes.

Discrimination and quantitation of halobenzoic acid positional isomers upon Th(IV) coordination by mass spectrometry

A fast and reliable mass spectrometry-based method has been developed to discriminate the positional isomers of o-, m- and p-C₆H₄XCO₂H (X = F, Cl and Br). This is based on the distinct fragmentation patterns of isomeric ThCl₄(C₆H₄XCO₂)^- ions generated by electrospray ionization of the solutions with C₆H₄XCO₂H isomers and ThCl₄. Moreover, the composition of these positional isomers can be conveniently quantified without any pre-treatment according to the proportion of gas-phase fragmentation products.

Reductive Reactivity of the 4f75d1 Gd(II) Ion in {GdII[N(SiMe3)2]3}-: Structural Characterization of Products of Coupling, Bond Cleavage, Insertion, and Radical Reactions

The reductive reactivity of a Ln(II) ion with a nontraditional 4fⁿ5d¹ electron configuration has been investigated by studying reactions of the {Gd^II(N(SiMe₃)₂)₃]}^- anion with a variety of reagents that survey the many reaction pathways available to this ion. The chemistry of both [K(18-c-6)₂]⁺ and [K(crypt)]⁺ salts (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand) was examined to study the effect of the countercation. CS₂ reacts with the crown salt [K(18-c-6)₂][Gd(NR₂)₃] (1) to generate the bimetallic (CS₃)^2- complex {[K(18-c-6)](μ₃-CS₃-κS,κ²S',S'')Gd(NR₂)₂]}₂, which contains two trithiocarbonate dianions that bridge Gd(III) centers and a potassium ion coordinated by 18-c-6. In contrast, the only crystalline product isolated from the reaction of CS₂ with the crypt salt [K(crypt)][Gd(NR₂)₃] (2) is [K(crypt)]{(R₂N)₂Gd[SCS(CH₂)Si(Me₂)N(SiMe₃)-κN,κS]}, which has a CS₂ unit inserted into a cyclometalated amide ligand. Complexes 1 and 2 reductively couple pyridine to form bridging dipyridyl moieties, (NC₅H₄-C₅H₄N)^2-, that generate bimetallic complexes differing only in the countercation, {[K(18-c-6)(C₅H₅N)₂]}₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]} and [K(crypt)]₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]}. Complexes 1 and 2 also show similar reactivity with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the (TEMPO)^- complexes [K(18-c-6)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)] and [K(crypt)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)], respectively. The first example of a bimetallic coordination complex containing a Bi-Gd bond, [K(crypt)][(R₂N)₃Gd(BiPh₂)], was obtained by treating 2 with BiPh₃.

Ligand Conjugation Directs the Formation of a 1,3-Dihydropyridinate Regioisomer

The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry. The reaction of (I₂P^2-)Al(THF)Cl [1; I₂P = bis(imino)pyridine; THF = tetrahydrofuran] with the one-electron oxidant (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) afforded (I₂P^-)Al(TEMPO)Cl (2), which can be reduced with sodium to the twice-reduced ligand complex (I₂P^2-)Al(TEMPO) (3). Compounds 2 and 3 serve as precursors for high-yielding and selective routes to an aluminum-supported 1,3-dihydropyridinate complex via the reaction of 2 with 3 equiv of potassium metal or the reaction of 3 with KH.

Thorium(iv) alkyl and allyl complexes of a rigid NON-donor pincer ligand with flanking 1-adamantyl substituents

A rigid new pincer pro-ligand, 4,5-bis(1-adamantylamino)-2,7-di-tert-butyl-9,9-dimethylxanthene, is described, with deprotonation and complexation to afford thorium(iv) chloro, alkyl, and allyl derivatives.

New vistas in the molecular chemistry of thorium: low oxidation state complexes

Although the molecular chemistry of thorium is dominated by the +4 oxidation state accounts of Th(iii) complexes have continued to increase in frequency since the first structurally characterised example was reported thirty years ago. The isolation of the first Th(ii) complexes in 2015 and exciting recent Th(iii) and Th(ii) reactivity studies both indicate that this long-neglected area is set to undergo a rapid expansion in research activity over the next decade, as previously seen since the turn of the millennium for analogous U(iii) small molecule activation chemistry. In this perspective article, we review synthetic routes to Th(iii) and Th(ii) complexes and summarise their distinctive physical properties. We provide a near-chronological discussion of these systems, focusing on structurally characterised examples, and cover complementary theoretical studies that rationalise electronic structures. All reactivity studies of Th(iii) and Th(ii) complexes that have been reported to date are described in detail.

Synthesis of Coordinatively Unsaturated Tetravalent Actinide Complexes with η(5) Coordination of Pyrrole

The synthesis of new actinide complexes utilizing bridged α-alkyl-pyrrolyl ligands is presented. Lithiation of the ligands followed by treatment with 1 equiv of actinide tetrachloride (uranium or thorium) produces the desired complex in good yield. X-ray diffraction studies reveal unique η(5):η(5) coordination of the pyrrolyl moieties; when the nonsterically demanding methylated ligand is used, rapid addition of the lithiated ligand solution to the metal precursor forms a bis-ligated complex that reveals η(5):η(1) coordination as determined by crystallographic analysis.

White phosphorus activation by a Th(iii) complex

Lappert's original Th(iii) complex, [Th{C₅H₃(SiMe₃)₂-1,3}₃], reduces white phosphorus to give a cyclo-P₄ dianion, which exhibits an unprecedented μ–η¹:η¹-binding mode in the dinuclear Th(iv) product.