Dinitrogen Cleavage by a Heterometallic Cluster Featuring Multiple Uranium–Rhodium Bonds

Isolable zero-valent Ditin(0) and Diplumbum(0) complexes

Although complexes with monatomic zero-valent main group centers have been reported, diatomic zero-valent complexes are extremely rare and all previously reported examples were stabilized by either carbene or silylene ligands. Here, we present the isolation of diatomic E(0)-E(0) (E = Sn, Pb) species supported by two [N{CH₂CH₂NPiPr₂}₃Sn] fragments. The reaction of trilithium salt N{CH2CH2NLiPiPr2}3 with SnCl2 yields complex [N{CH2CH2NPiPr2}3]2Sn3 (1) with a Sn3 chain. The reduction of the mixture of 1 and SnCl2 with KC8 produces the catenated Sn4 chain [N{CH2CH2NPiPr2}3Sn2]2 (2), featuring a diatomic Sn(0)-Sn(0) unit. Further reduction of 2 with KC8 yields the alkali metal ion-bridged complex [N{CH2CH2NPiPr2}3SnK]2 (3). Moreover, the reaction of 3 with PbI2 and KC8 affords [N{CH2CH2NPiPr2}3SnPb]2 (4), which can also be generated by the reaction of KC8 with PbI2 and [N{CH2CH2NPiPr2}3SnLi]2 (5). Complex 4 features a diatomic Pb(0)-Pb(0) unit, representing a heavy diatomic zero-valent main group complex. The presence of diatomic E(0)-E(0) (E = Sn, Pb) units in complexes 2 and 4, respectively, is further confirmed by computational studies.

Synthesis and Catalytic Activity of Thorium Nitride Complex from Dinitrogen Reduction

Metal nitride species are recognized as key intermediates in the conversion of dinitrogen (N2) to ammonia (NH3). In this work, we report the isolation of a multimetallic nitride-bridged thorium complex (2) by completely cleaving the N≡N triple bond of N2. The complex was synthesized through the reduction of a thorium precursor, {N[CH2CH2N-PiPr2]3ThCl}2 (1) and chromium dichloride (CrCl2) using potassium graphite (KC8) under an N2 atmosphere. Isotopic labeling with 15N2 confirms that the nitride in complex 2 originates from N2. Under ambient conditions, complex 2 exhibits remarkable catalytic activity, converting N2 to silylamine with yields of up to 9.9 equiv per thorium molecular catalyst. This work not only represents the first isolation of a thorium nitride complex from N2 reduction but also provides a rare example of N2 functionalization promoted by an actinide catalyst.

Synthesis and Structure of Uranium Disilyl-Substituted Alkylidene Complexes

Understanding the participation of f-orbitals of actinide elements in covalent bond formations is less explored, compared to the well-studied d-orbitals of transition metals, leading to the significant interest in actinide-carbon multiple bonds. Uranium alkylidene complex, containing an alkylidene linkage of the form U═CR2 (R = H, alkyl, silyl), represents a key milestone in actinide-ligand multiple bonding, but their isolation and characterization have remained elusive. Herein, we present the synthesis of an unprecedented uranium disilyl-substituted alkylidene complex, achieved through sequential dehydrogenation reactions of a methyl group under mild conditions. Single-crystal X-ray diffraction reveals the U═C double bond length of 2.332(4) Å. Quantum chemical calculations suggest that both 5f and 6d orbitals of uranium play a key role in the U═C double bond formation.

Dipnictogen Radical Chemistry: A Dithorium-Supported Distibene Radical Trianion

Although two examples of σ-bonded trans-bent [RSbSbR]•- (R = bulky organo- or Ga-groups) that formally contain the Sb2•3- radical trianion moiety are known in p-block chemistry, d- or f-element Sb2•3- radical trianion complexes, with or without R-substituents, have remained elusive. Here, we report that reduction of a 77:23 mix of [{Th(TrenTIPS)}2(μ-η2:η2-Sb2)] (3a, TrenTIPS = {N(CH2CH2NSiPri3)3}3-):[{Th(TrenTIPS)}2(μ-SbH)] (3b) with 1.5 equiv of KC8 in the presence of 1.1 equiv of 2.2.2-cryptand yields the emerald green Sb2•3- radical complex [K(2.2.2-cryptand)][{Th(TrenTIPS)}2(μ-η2:η2-Sb2)] (4), providing an f-block Sb2•3- radical trianion complex, and the heaviest actinide-N2 radical analogue. When the recrystallization conditions are modified, a small crop of red crystals determined to be [K(2.2.2-cryptand)]3[{Th(TrenTIPS)(μ-η3:η3-Sb3)}2(μ-K)] (5) were also isolated, highlighting the complexity of heavy group 15 homodiatomic reduction chemistry. SQUID magnetometry and EPR spectroscopy suggest that the Sb2•3- radical trianion in 4 is fairly well isolated, due to electrostatic binding to Th, with pseudoaxial g-values reflecting the distinctive Sb2•3- radical trianion side-on bridging π-bonded coordination mode. Spectroscopically validated computational analysis of 3a and 4 confirms the stronger donating capability, and weaker Sb-Sb bond, of Sb2•3- radical trianion compared to the Sb22- dianion form.

Dinitrogen Activation and Conversion by Actinide Complexes

The efficient activation and conversion of dinitrogen (N2) represent a significant challenge in sustainable chemistry, offering potential pathways for synthesizing valuable nitrogen-containing compounds while reducing the environmental impact of traditional nitrogen fixation processes. While transition metal catalysts have been extensively studied for this purpose, actinide complexes have been less explored but have recently emerged as promising candidates due to their unique electronic properties and reactivity. This Perspective systematically examines the recent advances in N2 activation and conversion mediated by actinide complexes, with a particular focus on their synthesis, mechanistic insights, and catalytic capabilities.

Synthesis and characterization of homometallic cobalt complexes with metal-metal interactions

Complexes featuring metal-metal bonds play crucial roles in catalysis and small molecule activation due to the synergistic effects between the metals. Here, we report a series of homometallic cobalt complexes with metal-metal interactions that have been successfully stabilized by a multidentate ligand platform. Theoretical studies on metal-metal interactions in these cobalt complexes are discussed.

Oxidative Addition of E-H (E=C, N) Bonds to Transient Uranium(II) Centers

Two-electron oxidative addition is one of the most important elementary reactions for d-block transition metals but it is uncommon for f-block elements. Here, we report the first examples of intermolecular oxidative addition of E-H (E=C, N) bonds to uranium(II) centers. The transient U(II) species was formed in-situ by reducing a heterometallic cluster featuring U(IV)-Pd(0) bonds with potassium-graphite (KC8). Oxidative addition of C-H or N-H bonds to the U(II) centers was observed when this transient U(II) species was treated with benzene, carbazole or 1-adamantylamine, respectively. The U(II) centers could also react with tetracene, biphenylene or N2O, leading to the formation of arene reduced U(IV) products and uranyl(VI) species via two- or four-electron processes. This study demonstrates that the intermolecular two-electron oxidative addition reactions are viable for actinide elements.

Electronic Delocalization and σ-Aromaticity in Heterometallic Cluster with Multiple Thorium-Palladium Bonds

Research on metal-metal bonds involving f-block actinides, such as thorium, lags far behind the well-studied metal-metal bonds of d-block transition metals. The complexes with Th-TM bonds are extremely rare; all previously identified examples have only a single Th-TM bond with the Th center at an invariably +IV oxidation state. Herein, we report a series of Th2Pdn (n = 2, 3, and 6) clusters (complexes 3, 4, and 7) with multiple Th(III)-Pd bonds. Theoretical studies reveal that the Th2Pdn unit allows electronic delocalization and σ aromaticity, leading to unexpected closed-shell singlet structures for these Th(III) species. This electronic delocalization is evident in the highest occupied molecular orbital of Th(III) complexes and facilitates a 2e reduction of alkyne by complex 7, resulting in the formation of 8. Complexes 7 and 8 are distinctive in featuring a Th2Pd6 core with six and eight Th-Pd bonds, respectively, making them the largest known d-f heterometallic clusters exhibiting metal-metal bonding.

Iron promoted end-on dinitrogen-bridging in heterobimetallic complexes of uranium and lanthanides

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N2 bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)2(N2)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [MIII{N(SiMe3)2}3] (M = U, Ce, Sm, Dy, Tm) and [LnII{N(SiMe3)2}2], (Ln = Sm, Yb). Despite the lack of reactivity of the U(iii), Ln(iii) and Ln(ii) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)2}(μ-η1:η1-N2)(M{N(SiMe3)2}3)], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe)2}(μ-η1:η1-N2)(Ln{N(SiMe3)2}2)], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe)2(μ-η1:η1-N2)}2{SmII{N(SiMe3)2}2}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N2 complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(ii) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N2 with both Ln(iii) and Ln(ii) complexes with the degree of N2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)-N2Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N2 compared to free N2 due to the higher N2 polarization.

Mechanistic Inquisition on the Reduction of C17Si(NH2)2 to NH3: A DFT Study

Activation of molecular nitrogen by silicon-substituted cyclo[18]carbon and its ability to produce the C17Si-(NH2)2 derivative, as the precursor of NH3, has been recently reported. This specific acquisition has piqued an interest to investigate the possibility of NH3 formation with further addition of H2 molecules in the gaseous reaction media. The current investigations reported in this article show that two moles of molecular H2 generate two molecules of NH3 and a C17Si-H2 byproduct from its precursor. The catalyst gets restored by an in situ reaction between some unreacted C17Si-N2 and the byproduct in the media. This reaction also produces the next C17Si-(NH)2 adduct, which restarts the catalytic cycle for NH3 production again.

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

Temperature induced single-crystal to single-crystal transformation of uranium azide complexes

The monomeric and dimeric uranium azide complexes {[(CH3)2NCH2CH2NPiPr2]2U(N3)2} (2) and {[(CH3)2NCH2CH2NPiPr2]2U(N3)2}2 (3) were synthesized by treating complex 1 with NaN3 at 60 and -20 °C, respectively. A temperature-induced single-crystal to single-crystal transformation of 3 to 2 was observed. The reduction of either 2 or 3 with KC8 yields a uranium nitride complex {[(CH3)2NCH2CH2NPiPr2]4U2(μ-N)2} (4).

Nitrogen adsorption on Nb2C6H4+ cations: the important role of benzyne (ortho-C6H4)

N2 adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb2C6H4+ cation, resulting from the gas-phase reaction of Nb2+ with C6H6, is more favorable for N2 adsorption than Nb+ and Nb2+ cations. Density functional theory calculations reveal the effect of the ortho-C6H4 ligand on N2 adsorption. In Nb2C6H4+, interactions between the Nb-4d and C-2p orbitals enable the Nb2+ cation to form coordination bonds with the ortho-C6H4 ligand. Although the ortho-C6H4 ligand in Nb2C6H4+ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N2, thereby increasing the N2 adsorption energy, which effectively facilitates N2 adsorption and activation. This study provides fundamental insights into the mechanisms of N2 adsorption in "transition metal-organic ligand" systems.

A trinuclear metallasilsesquioxane of uranium(III)

The silsesquioxane ligand (iBu)7Si7O9(OH)3 (iBuPOSSH3) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U3(iBuPOSS)3]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(μ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((μ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

Dinitrogen cleavage by a dinuclear uranium(iii) complex

Understanding the role of multimetallic cooperativity and of alkali ion-binding in the second coordination sphere is important for the design of complexes that can promote dinitrogen (N2) cleavage and functionalization. Herein, we compare the reaction products and mechanism of N2 reduction of the previously reported K2-bound dinuclear uranium(iii) complex, [K2{[UIII(OSi(OtBu)3)3]2(μ-O)}], B, with those of the analogous dinuclear uranium(iii) complexes, [K(2.2.2-cryptand)][K{UIII(OSi(OtBu)3)3}2(μ-O)], 1, and [K(2.2.2-cryptand)]2[{UIII(OSi(OtBu)3)3}2(μ-O)], 2, where one or two K+ ions have been removed from the second coordination sphere by addition of 2.2.2-cryptand. In this study, we found that the complete removal of the K+ ions from the inner coordination sphere leads to an enhanced reducing ability, as confirmed by cyclic voltammetry studies, of the resulting complex 2, and yields two new species upon N2 addition, namely the U(iii)/U(iv) complex, [K(2.2.2-cryptand)][{UIII(OSi(OtBu)3)3}(μ-O){UIV(OSi(OtBu)3)3}], 3, and the N2 cleavage product, the bis-nitride, terminal-oxo complex, [K(2.2.2-cryptand)]2[{UV(OSi(OtBu)3)3}(μ-N)2{UVI(OSi(OtBu)3)2(κ-O)}], 4. We propose that the formation of these two products involves a tetranuclear uranium-N2 intermediate that can only form in the absence of coordinated alkali ions, resulting in a six-electron transfer and cleavage of N2, demonstrating the possibility of a three-electron transfer from U(iii) to N2. These results give an insight into the relationship between alkali ion binding modes, multimetallic cooperativity and reactivity, and demonstrate how these parameters can be tuned to cleave and functionalize N2.

Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(iv) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(iii)-aluminum(i) and -gallium(i) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)2(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H2 with the U(iii)-Al(i) complex affords a trihydroaluminate complex, [(C5Me5)2(MesO)U(μ2-(H)3)-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C5Me5)2(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C5Me5)Al]4 are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)2(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)2(MesO)U(CH2CH3)] or treatment of [(C5Me5)2U(i)(Me)] with KOMes forms [(C5Me5)2(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)2(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.

Magnesium complexes supported by a dianionic double layer nitrogen-phosphorus ligand: a synthesis and reactivity study

A heterobimetallic complex [MeN(CH2CH2NPiPr)2MgLiCl(THF)]2 (1) supported by a dianionic double layer nitrogen-phosphorus ligand was synthesized by the reaction of H2L1 (H2L1 = MeN(CH2CH2NHPiPr)2) with MgCl2 in the presence of n-BuLi. Reactions of complex 1 with 2 equivalents of CuI, AgI and AuCl·SMe2 led to the formation of heterobimetallic clusters [MeN(CH2CH2NPiPr)2MgCuI]2 (2), [MeN(CH2CH2NPiPr)2MgAgI]2 (3) and [MeN(CH2CH2NPiPr)2MgAuCl]2 (4), respectively. X-ray single-crystal diffraction analysis revealed that these complexes are dimers, which are composed of two [MeN(CH2CH2NPiPr)2Mg] units connected by coinage metals (i.e., Cu, Ag, and Au). The reactivity of 1 was further investigated and it was found that complex 1 could react with 4 equivalents of MeI, giving a complex [CH3N(CH2CH2NPiPr2Me)2MgI]+[I]- (5), which can be viewed as a magnesium complex supported by a neutral double layer nitrogen-phosphorus ligand (CH3N(CH2CH2NPiPr2Me)2). Complex 1 could also react with 2 equivalents of NaNH2, leading to the isolation of an amido anion bridged magnesium-sodium heterobimetallic cluster [MeN(CH2CH2NPiPr)2MgNaNH2]2 (6).

Heterometallic Clusters with Cerium-Transition-Metal Bonding Supported by Nitrogen-Phosphorus Ligands

Ligands are known to play a crucial role in the construction of complexes with metal-metal bonds. Compared with metal-metal bonds involving d-block transition metals, knowledge of the metal-metal bonds involving f-block rare-earth metals still lags far behind. Herein, we report a series of complexes with cerium-transition-metal bonds, which are supported by two kinds of nitrogen-phosphorus ligands N[CH2CH2NHPiPr2]3 (VI) and PyNHCH2PPh2 (VII). The reactions of zerovalent group 10 metal precursors, Pd(PPh3)4 and Pt(PPh3)4, with the cerium complex supported by VI generate heterometallic clusters [N{CH2CH2NPiPr2}3Ce(μ-M)]2 (M = Pd, 2 and M = Pt, 3) featuring four Ce-M bonds; meanwhile, the bimetallic species [(PyNCH2PPh2)3Ce-M] (M = Ni, 5; M = Pd, 6; and M = Pt, 7) with a single Ce-M bond were isolated from the reactions of the cerium precursor 4 supported by VII with Ni(COD)2, Pd(PPh3)4, or Pt(PPh3)4, respectively. These complexes represent the first example of species with an RE-M bond between Ce and group 10 metals, and 2 and 3 contain the largest number of RE-M donor/acceptor interactions ever to have been observed in a molecule.

Octalithium, Tetrasodium, and Decalithium Compounds Based on Pyrrolyl Ligands: Synthesis, Structures, and Activation of the C-H Bonds of Pyrrolyl Rings and C═N Bonds of a Series of Ligands by Organolithium Reagents

The organometallic compounds of lithium ions have garnered continuous interest as indispensable precursors for the syntheses of organometallic complexes of main-group metals, transition metals, lanthanide metals, and actinide metals. In this work, we present a strategy for the preparation of a series of polynuclear lithium complexes. This methodology features the utilization of organolithium reagents both as metal sources to coordinate with the ligands and as nucleophilic reagents to undergo nucleophilic addition to the C═N bonds of the ligands. Reaction of a ligand HL1 [HL1 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)phenol] with n-BuLi produced complex [Li8(L1a)4]·1.5Tol (1·1.5Tol) [H2L1a = 2-((1-(1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)pentyl)amino)phenol]. One prominent feature regarding the formation of 1·1.5Tol is the occurrence of nucleophilic addition of n-BuLi to the C═N bond of HL1, leading to the generation of a new [L1a]2- ligand that contains both aminophenol and 1-(2-pyrrolyl)alkylamine scaffolds. The developed protocol can be adapted to a series of organolithium reagents. Compounds [Li8(L1b)4] (2) and [Li8(L1c)4] (3) were afforded by treatment of HL1 with sec-BuLi and LiCH2SiMe3, respectively. Reaction of an analogous ligand HL2 [HL2 = 2-(((1-(2-(dimethylamino)ethyl)-1H-pyrrol-2-yl)methylene)amino)-4-methylphenol] with n-BuLi generated compound [Li8(L2a)4] (4). C═N bond activation was not observed in the reaction of HL1 with NaOtBu, and the complex [Na4(L1)4]·Tol (5·Tol) was obtained. A decanuclear complex [Li10(L3a)2(L3b)2] (6) was also prepared via the reaction of HL3 [HL3 = 2-(2-((((1H-pyrrol-2-yl)methylene)amino)methyl)-1H-pyrrol-1-yl)-N,N-dimethylethan-1-amine] with t-BuLi. A remarkable feature in terms of the synthesis of 6 is the simultaneous occurrence of hydrogen atom abstraction from the C-H bond of the pyrrolyl ring and nucleophilic addition to the C═N bond of the HL3 ligand by t-BuLi. A series of amines containing biologically and physiologically important moieties were achieved by hydrolysis of the crude products from the reactions of the HL1-HL3 ligands and organolithium reagents. This work provides an efficient approach to high-nuclearity lithium compounds as well as a series of amines.

Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N₂NN' (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-U and 1-Th are structurally very similar, they display disparate reactivities with TMS₃SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N₂NN')UCl₂ (1-U) and 1 equiv of TMS₃SiK in THF unexpectedly formed [Cl(N₂NN')U]₂O (2-U) featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (N₂NN')ThCl₂ (1-Th) and 1 equiv of TMS₃SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azide complex 3-Th by reaction with NaN₃. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.

Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

Dinitrogen cleavage and hydrogenation to ammonia with a uranium complex

The Haber-Bosch process produces ammonia (NH₃) from dinitrogen (N₂) and dihydrogen (H₂), but requires high temperature and pressure. Before iron-based catalysts were exploited in the current industrial Haber-Bosch process, uranium-based materials served as effective catalysts for production of NH₃ from N₂. Although some molecular uranium complexes are known to be capable of combining with N₂, further hydrogenation with H₂ forming NH₃ has not been reported to date. Here, we describe the first example of N₂ cleavage and hydrogenation with H₂ to NH₃ with a molecular uranium complex. The N₂ cleavage product contains three uranium centers that are bridged by three imido μ ₂-NH ligands and one nitrido μ ₃-N ligand. Labeling experiments with ¹⁵N demonstrate that the nitrido ligand in the product originates from N₂. Reaction of the N₂-cleaved complex with H₂ or H⁺ forms NH₃ under mild conditions. A synthetic cycle has been established by the reaction of the N₂-cleaved complex with trimethylsilyl chloride. The isolation of this trinuclear imido-nitrido product implies that a multi-metallic uranium assembly plays an important role in the activation of N₂.

A versatile strategy for the formation of hydride-bridged actinide-iridium multimetallics

Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H₃] with UCl₄ and ThCl₄(DME)₂ led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H)₃IrCp*}₄ (1) and Th{[(μ-H₂)(H)IrCp*]₂[(μ-H)₃IrCp*]₂} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI₃(1,4-dioxane)_1.5, resulted in the octanuclear complex {U[(μ²-H₃)IrCp*]₂[(μ³-H₂)IrCp*]}₂ (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.

Predicting Small Molecule Activation including Catalytic Hydrogenation of Dinitrogen Promoted by a Dual Lewis Acid

For decades, N₂ activation and functionalization have required the use of transition metal complexes. Thus, it is one of the most challenging projects to activate the abundant dinitrogen through metal-free systems under mild conditions. Here, we demonstrate a proof-of-concept study on the catalytic hydrogenation of dinitrogen (with activation energy as low as 15.3 kcal mol^-1 ) initiated by a dual Lewis acid (DLA) via density functional theory (DFT) calculations. In addition, such a DLA could be also used to activate a series of small molecules including carbon dioxide, formaldehyde, N-ethylenemethylamine, and acetonitrile. It is found that aromaticity plays an important role in stabilizing intermediates and products. Our findings provide an alternative approach to N₂ activation and functionalization, highlighting a great potential of DLA for small molecule activation.

Activation and Conversion of Molecular Nitrogen to the Precursor of Ammonia on Silicon Substituted Cyclo[18]Carbon: a DFT Design

Recent synthesis of sp-hybridized cyclo[18]carbon allotrope has attracted immense curiosity. Since then, a generous amount of theoretical studies concerning aromaticity, adsorption, and spectra of the molecule have been performed. However, very few stuides have been carried out concerning its reactivities and catalytic behaviour. In this article, a DFT-based inquisition has been reported regarding the reactivity of Si substituted cyclo[18]carbon molecule towards molecular N₂ . Results show that the Si substituted derivative is effective in producing adducts with molecular nitrogen. Charge calculations and IRC trapping methods indicate that only the Si center of C₁₇ Si and its (HOMO-1) level participate in N₂ addition. The N-adduct so formed, is then found to spontaneously react with molecular H₂ . The addition of two H₂ molecules to the activated nitrogen molecule to give respective amine derivatives have also been studied. The successful generation of the precursor of NH₃ by C₁₇ Si lays a clear emphasis on its potentiality.

Facile Synthesis of Uranium Complexes with a Pendant Borane Lewis Acid and 1,2-Insertion of CO into a U-N Bond

In this contribution, we illustrate uranium complexes bearing a pendant borate (i.e. 1 and 2) or a pendant borane (i.e. 3 and 4) moiety via reaction of the highly strained uranacycle I with various 3-coordinate boranes. Complexes 3 and 4 represent the first examples of uranium complexes with a pendant borane Lewis acid. Moreover, complex 3 was capable of activation of CO, delivering a new CO activation mode, and an abnormal CO 1,2-insertion pathway into a U-N bond. The importance of the pendant borane moiety was confirmed by the controlled experiments.

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal-metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation.

Recent advances in heterometallic clusters with f-block metal-metal bonds: synthesis, reactivity and applications

Due to the heterometallic synergistic effects from different metals, heterometallic clusters are of great importance in small-molecule activation and catalysis. For example, both biological nitrogen fixation and photosynthetic splitting of water into oxygen are thought to involve multimetallic catalytic sites with d-block transition metals. Benefitting from the larger coordination numbers of f-block metals (rare-earth metals and actinide elements), heterometallic clusters containing f-block metal-metal bonds have long attracted the interest of both experimental and theoretical chemists. Therefore, a series of effective strategies or platforms have been developed in recent years for the construction of heterometallic clusters with f-block metal-metal bonds. More importantly, synergistic effects between f-block metals and transition metals have been observed in small-molecule activation and catalysis. This tutorial review highlights the recent advances in the construction of heterometallic molecular clusters with f-block metal-metal bonds and also their reactivities and applications. It is hoped that this tutorial review will persuade chemists to develop more efficient strategies to construct clusters with f-block metal-metal bonds and also further expand their applications with heterometallic synergistic effects.

Progress in solid state and coordination chemistry of actinides in China

In the past decade, the area of solid state chemistry of actinides has witnessed a rapid development in China, based on the significantly increased proportion of the number of actinide containing crystal structures reported by Chinese researchers from only 2% in 2010 to 36% in 2021. In this review article, we comprehensively overview the synthesis, structure, and characterizations of representative actinide solid compounds including oxo-compounds, organometallic compounds, and endohedral metallofullerenes reported by Chinese researchers. In addition, Chinese researchers pioneered several potential applications of actinide solid compounds in terms of adsorption, separation, photoelectric materials, and photo-catalysis, which are also briefly discussed. It is our hope that this contribution not only calls for further development of this area in China, but also arouses new research directions and interests in actinide chemistry and material sciences.

Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N2 to nitrides (N3-). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N2 cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N2. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N2. Two heteroleptic diuranium nitride complexes, [K{UIV(OSi(O t Bu)3)(N(SiMe3)2)2}2(μ-N)] (1) and [Cs{UIV(OSi(O t Bu)3)2(N(SiMe3)2)}2(μ-N)] (3-Cs), containing different combinations of OSi(O t Bu)3 and N(SiMe3)2 ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K2{UIV/III(OSi(O t Bu)3)2(N(SiMe3)2)}2(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e- reduction of N2, yielding the N2 4-complex [K3{UV(OSi(O t Bu)3)2(N(SiMe3)2)}2(μ-N)(μ-η2:η2-N2)], 7. Parallel N2 reduction pathways were also identified, leading to the isolation of N2 cleavage products, [K3{UVI(OSi(O t Bu)3)2(N(SiMe3)2)([triple bond, length as m-dash]N)}(μ-N)2{UV(OSi(O t Bu)3)2(N(SiMe3)2)}]2, 8, and [K4{(OSi(O t Bu)3)2UV)([triple bond, length as m-dash]N)}(μ-NH)(μ-κ2:C,N-CH2SiMe2NSiMe3)-{UV(OSi(O t Bu)3)2][K(N(SiMe3)2]2, 9. These complexes provide the first example of N2 cleavage to nitride by a uranium complex in the absence of reducing alkali metals.

Photochemical Synthesis of Transition Metal-Stabilized Uranium(VI) Nitride Complexes

Uranium nitrides play important roles in dinitrogen activation and functionalization and in chemistry for nuclear fuels, but the synthesis and isolation of the highly reactive uranium(VI) nitrides remains challenging. Here, we report an example of transition metal (TM) stabilized U(VI) nitride complexes, which are generated by the photolysis of azide-bridged U(IV)-TM (TM = Rh, Ir) precursors. The U(V) nitride intermediates with bridged azide ligands are isolated successfully by careful control of the irradiation time, suggesting that the photolysis of azide-bridged U(IV)-TM precursors is a stepwise process. The presence of two U(VI) nitrides stabilized by three TMs is clearly demonstrated by an X-ray crystallographic study. These TM stabilized U(V) nitride intermediates and U(VI) nitride products exhibit excellent stability both in the solid-state and in THF solution under ambient light. Density functional theory calculations show that the photolysis necessary to break the N-N bond of the azide ligands implies excitation from uranium f-orbital to the lowest unoccupied molecular orbital (LUMO), as suggested by the strong antibonding N-(N₂) character present in the latter.

Uranium-nitride chemistry: uranium-uranium electronic communication mediated by nitride bridges

Treatment of [U^IV(N₃)(Tren^TIPS)] (1, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-) with excess Li resulted in the isolation of [{U^IV(μ-NLi₂)(Tren^TIPS)}₂] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [U^III(Tren^TIPS)] (3), and together with known [{U^V(μ-NLi)(Tren^TIPS)}₂] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{U^IV(Tren^TIPS)}₂(μ-NH)(μ-NLi₂)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(IV). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of -4.1 and -3.4 cm^-1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the 'diamond-core' dinitride of 2 and matrix-isolated binary U₂N₂.

Heterometallic Clusters with Uranium-Metal Bonds Supported by Double-Layer Nitrogen-Phosphorus Ligands

ConspectusHeterometallic clusters with M-M bonds have significantly interested chemists because of their attractive structures and synergistic effects in small-molecule activation and catalysis. However, reports of the isolation of heterometallic clusters with uranium-transition metal (U-TM) bonds remain very limited. In this Account, we describe our research in the construction of heterometallic molecular clusters with multiple U-TM single or multiple bonds supported by novel double-layer N-P ligands. Multimetallic synergistic catalysis and small-molecule activation with these species are also summarized.First, according to the hard-soft acid-base theory, we employed a three-armed N-P ligand, which can be used to construct heterometallic clusters with four or six U-Ni bonds. This strategy was also effective in the construction of complexes with direct rare earth metal-TM bonding. The similar two-armed N-P ligands also are effective platforms for the synthesis of heterometallic complexes with U-Ni, U-Pd, and U-Pt bonds.Second, a set of heterometallic clusters featuring U≡Rh, U≡Co, and U≡Fe triple bonds were constructed under routine experimental conditions. X-ray diffraction analysis of these clusters exhibits the shortest U-TM bond distance (1.9693(4) Å for the U≡Fe triple bond) in these complexes. Theoretical studies reveal that the nature of the triple bond is one covalent σ bond and two TM → U dative π bonds. A large Wiberg bond index (WBI) of 2.93 and a significant degree of covalency for the U≡TM triple bonds were also found in these complexes.Third, these uranium complexes supported by the double-layer N-P ligands exhibit great potential in small-molecule activation. For instance, N₂ cleavage without an external reducing agent was achieved by a U(III)-P(III) synergistic six-electron reduction. The synergism between U(III) and P(III) enables the activation of other small molecules, such as O₂, P₄, and As⁰_(nano), and highlights the importance of the P atom in the double-layer N-P ligand for the activation of small molecules. A heterometallic cluster with U-Rh bonds can break the strong N≡N triple bond in N₂ in the presence of potassium graphite, suggesting a synergistic effect between U and Rh. This multimetallic synergistic effect was also observed in catalytic processes. A heterometallic cluster with U≡Co triple bonds shows excellent selectivity and activity in the hydroboration of a series of alkynes under mild conditions. These results lead to effective methods for the construction of heterometallic molecular clusters with U-TM single or multiple bonds and could promote the application of heterometallic clusters with U-TM bonds in catalysis and the activation of small molecules.

Uranium: The Nuclear Fuel Cycle and Beyond

This review summarizes the recent developments regarding the use of uranium as nuclear fuel, including recycling and health aspects, elucidated from a chemical point of view, i.e., emphasizing the rich uranium coordination chemistry, which has also raised interest in using uranium compounds in synthesis and catalysis. A number of novel uranium coordination features are addressed, such the emerging number of U(II) complexes and uranium nitride complexes as a promising class of materials for more efficient and safer nuclear fuels. The current discussion about uranium triple bonds is addressed by quantum chemical investigations using local vibrational mode force constants as quantitative bond strength descriptors based on vibrational spectroscopy. The local mode analysis of selected uranium nitrides, N≡U≡N, U≡N, N≡U=NH and N≡U=O, could confirm and quantify, for the first time, that these molecules exhibit a UN triple bond as hypothesized in the literature. We hope that this review will inspire the community interested in uranium chemistry and will serve as an incubator for fruitful collaborations between theory and experimentation in exploring the wealth of uranium chemistry.

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M₃Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K-nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN^-.

Cation assisted binding and cleavage of dinitrogen by uranium complexes

N2 binding affinity decreases markedly in a series of isostructural U(iii)–alkali ions complexes with increasing cation size. N2 binding is undetectable in the Cs analogue, but the first example of cesium-assisted N2 cleavage to bis-nitride was observed at ambient condition.

Redox-induced reversible P-P coupling in a uranium complex

A synthesized redox-active multidentate N-P ligand reacted with UCl₄ in the presence of KHMDS or ⁿBuLi, where two novel U(IV) complexes with or without P-P coupling were formed, respectively. The reversible P-P coupling in these complexes was observed in redox-induced reactions.

Nitride protonation and NH3 binding versus N-H bond cleavage in uranium nitrides

The conversion of metal nitrides to NH₃ is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H⁺). Herein, we report comparative studies for the reactions of H⁺ and NH₃ with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by –N(SiMe₃)₂ ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic –N(SiMe₃)₂ ligands promote the N–H heterolytic bond cleavage of NH₃, yielding a “naked” diuranium complex containing three bridging ligands, a nitride (N³⁻) and two NH₂ ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by –OSi(O^tBu)₃ ligands, the nitride group is easily protonated to afford NH₃, which binds the U(iv) ion strongly, resulting in a mononuclear U–NH₃ complex, where NH₃ can be displaced by addition of strong acids. Furthermore, the U–OSi(O^tBu)₃ bonds were found to be stable, even in the presence of stronger acids, such as NH₄BPh₄, therefore indicating that –OSi(O^tBu)₃ supporting ligands are well suited to be used when acidic conditions are required, such as in the H⁺/e⁻ mediated catalytic conversion of N₂ to NH₃.

Ancillary ligands alter the reactivity of U-nitrides with H⁺, relevant to N₂ conversion to NH₃. The amides lead to complete ligand loss and NH₃ activation, while for siloxides, the nitride is protonated to NH₃ leaving the ancillary ligands intact.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N^3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K₂{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-η²:η²-N₂)}], B, with KC₈, yielding the tetranuclear tetranitride cluster [K₆{(OSi(O^tBu)₃)₂U^IV}₃{(OSi(O^tBu)₃)₂U^VI}(μ₄-N)₃(μ₃-N)(μ₃-O)₂], 1, a novel example of N₂ cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N═U^VI═N core analogue of UO₂²⁺. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N₂^4- ligand in complex B, resulting in N₂ cleavage and the formation of the putative diuranium(V) bis-nitride [K₄{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-N)₂}], X. Additionally, cooperative potassium binding to the U-bound N₂^4- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess ¹³CO to yield K¹³CN and KN¹³CO. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Predicting Dinitrogen Coupling with a Series of Small Molecules Catalyzed by a Pincer Complex

Due to consumption of more than 2% of the world's annual energy supply by Haber-Bosch process and the strongest triple bond (N≡N) in nature, directly coupling N₂ with small molecules is particularly important and challenging, let alone in a catalytic fashion. Here we first demonstrate that a NNN-type pincer phosphorus complex could act as a catalyst to couple dinitrogen with a series of small molecules including carbon dioxide, formaldehyde, N-ethylidenemethylamine, and acetonitrile in the presence of diborane(4) under a mild condition by theoretical calculations. N₂ fixation proceeds via a stepwise mechanism involving initial N₂ activation by diborane(4), followed by intramolecular isomerization to a key intermediate (zwitterion). Such a zwitterion can be used to couple a series of small molecules with activation barriers of 23.5-25.2 kcal mol^-1 . All these findings could be particularly useful for main group chemistry aimed at N₂ activation.