Carbon-Nitrogen Bond Formation by the Reaction of 1,2-Cumulenes with a Ditantalum Complex Containing Side-On- and End-On-Bound Dinitrogen

Hydrazido complexes prepared by methylation of an anionic end-on bridging dinitrogen dititanium complex

Here we report stepwise methylation of end-on bridging dinitrogen to a hydrazido ligand to a pentamethylhydrazinium salt, which is mediated by a titanium system with a tripodal aryloxide supporting ligand. The results constitute a synthetic cycle for pentamethylhydrazinium formation from dinitrogen and methyl iodide. We also describe silylation of the dinitrogen complex and carboxylation of the hydrazido complex.

Dinitrogen Cleavage and Multicoupling with Isocyanides in a Dititanium Dihydride Framework

Dinitrogen (N2) activation and functionalization through N-N bond cleavage and N-C bond formation are of great interest and importance but remain highly challenging. We report here for the first time N2 cleavage and selective multicoupling with isocyanides in a dititanium dihydride framework. The reaction of a dinitrogen dititanium dihydride complex [{(acriPNP)Ti}2(μ-η1:η2-N2)(μ-H)2] (1) with an excess (four or more equivalents) of p-methoxyphenyl isocyanide at room temperature gave a novel amidoamidinatoguanidinate complex [(acriPNP)Ti{NC(═NR)NC(═NR)CH2NR}Ti(acriPNP)(CNR)] (2, acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide; R = p-MeOC6H4) through N2 splitting and coupling with three isocyanide molecules. When 1 equiv of p-methoxyphenyl isocyanide was used to react with 1 at -30 °C, the hydrogenation of the isocyanide unit by the two hydride ligands in 1 took place, affording an amidomethylene-bridged dititanium dinitrogen complex [{(acriPNP)Ti}2(μ-η1:η2-N2){μ-η1:η2-CH2N(p-MeOC6H4)}] (3), which upon reaction with another equivalent of p-methoxyphenyl isocyanide at room temperature gave an amidomethylene/nitrido/carbodiimido complex [(acriPNP)Ti(N═C═NR)(μ-N)(μ-η1:η2-CH2NR)Ti(acriPNP)] (4) through N2 cleavage and N═C bond formation. Further reaction of 4 with 1 equiv of p-methoxyphenyl isocyanide led to an unprecedented four-component (carbodiimido, nitrido, isocyanide, and amidomethylene) coupling, yielding an amidoamidinatoguanidinate complex [{(acriPNP)Ti}2{NC(═NR)NC(═NR)CH2NR}] (5), which on reaction with another equivalent of p-methoxyphenyl isocyanide afforded the isocyanide-coordinated analogue 2. The reaction of 1 with 2-naphthyl isocyanide also took place in a similar multicoupling fashion. Moreover, the cross-coupling reactions of the p-methoxyphenyl isocyanide-derived amidomethylene/nitrido/carbodiimido complex 4 with 2-naphthyl isocyanide, cyclohexyl isocyanide, and tert-butyl isocyanide were also achieved, which afforded the corresponding amidoamidinatoguanidinate products consisting of two different isocyanides. Density functional theory (DFT) calculations further elucidated the mechanistic details.

Aza-Michael Addition of Dinitrogen to α,β-Unsaturated Carbonyl Compounds in a Dititanium Framework

The direct use of dinitrogen (N2) as a building block for the synthesis of NN-containing organic compounds is of fundamental interest and practical importance but has remained a formidable challenge to date. Here, we report an unprecedented 1,4-conjugate (aza-Michael) addition of N2 to α,β-unsaturated carbonyl compounds in a dititanium framework. The resulting hydrazinopropenolate products could be easily converted to diverse NN-containing organic compounds such as β-hydrazine-functionalized esters and amides, pyrazolidinones, and pyrazolines depending on the types of Michael acceptors through protonation with MeOH. Further transformations of a hydrazinopropenolate titanium complex through C-C and N-C bond formations with electrophiles such as CO2 and benzaldehyde have also been achieved. The mechanistic details of the N2 addition reaction have been elucidated by computational studies, revealing the importance of redox-active metal centers in this event. This work showcases the potential of using N2 as a building block for the synthesis of NN-containing organic compounds through activation and functionalization in a molecular metal framework.

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )]

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Reductive Hydrogenation of Sulfido-Bridged Tantalum Alkyl Complexes: A Mechanistic Insight

Hydrogenolysis of a series of alkyl sulfido-bridged tantalum(IV) dinuclear complexes [Ta(η⁵-C₅Me₅)R(μ-S)]₂ [R = Me, nBu (1), Et, CH₂SiMe₃, C₃H₅, Ph, CH₂Ph (2), p-MeC₆H₄CH₂ (3)] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η⁵-C₅Me₅)(μ₃-S)]₄ (4) along with the corresponding alkane. Mechanistic information for the formation of the unique low-valent tetrametallic compound 4 was gathered by hydrogenation of the phenyl-substituted precursor [Ta(η⁵-C₅Me₅)Ph(μ-S)]₂, which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta₂(η⁵-C₅Me₅)₂(H)Ph(μ-S)(μ₃-S)]₂ (5). Extending our studies toward tantalum alkyl precursors containing functional groups susceptible to hydrogenation, such as the allyl-and benzyl-substituted compounds [Ta(η⁵-C₅Me₅)(η³-C₃H₅)(μ-S)]₂ and [Ta(η⁵-C₅Me₅)(CH₂Ph)(μ-S)]₂ (2), enables alternative reaction pathways en route to the formation of 4. In the former case, the dimetallic system undergoes selective hydrogenation of the unsaturated allyl moiety, forming the asymmetric complex [{Ta(η⁵-C₅Me₅)(η³-C₃H₅)}(μ-S)₂{Ta(η⁵-C₅Me₅)(C₃H₇)}] (6) with only one propyl fragment. Species 2, in addition to the hydrogenation of one benzyl fragment and concomitant toluene release, also undergoes partial hydrogenation and dearomatization of the phenyl ring on the vicinal benzyl unity to give a η⁵-cyclohexadienyl complex [Ta₂(η⁵-C₅Me₅)₂(μ-CH₂C₆H₆)(μ-S)₂] (7). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.

Dinitrogen Activation and Addition to Unsaturated C-E (E=C, N, O, S) Bonds Mediated by Transition Metal Complexes

Dinitrogen (N₂ ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N₂ activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field.

Dinitrogen Cleavage and Functionalization with Carbon Dioxide in a Dititanium Dihydride Framework

The activation and functionalization of dinitrogen (N₂) with carbon dioxide (CO₂) are of great interest and importance but highly challenging. We report here for the first time the reaction of N₂ with CO₂ in a dititanium dihydride framework, which leads to N-C bond formation and N-N and C-O bond cleavage. Exposure of a dinitrogen dititanium hydride complex {[(^acriPNP)Ti]₂(μ₂-η¹:η²-N₂)(μ₂-H)₂} (1) (^acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) to a CO₂ atmosphere at room temperature rapidly yielded a nitrido/N,N-dicarboxylamido complex {[(^acriPNP)Ti]₂(μ₂-N)[μ₂-N(CO₂)₂]} (2, 28%) and a diisocyanato/dioxo complex {[(^acriPNP)Ti]₂(NCO)₂(μ₂-O)₂} (3, 52%) with release of H₂. When the reaction of 1 with CO₂ (1 atm) was carried out at -50 °C, complex 2 was selectively formed in 82% yield within 5 min. Heating 2 at 80 °C under 1 atm CO₂ for 30 min afforded 3 in 67% yield. When 1 was allowed to react with 1.5 equiv of CO₂ at room temperature, an isocyanato/nitrido/oxo complex {[(^acriPNP)Ti]₂(NCO)(μ₂-N)(μ₂-O)} (4) was exclusively formed in 89% yield within 5 min. The reaction of 4 with CO₂ at room temperature almost quantitatively yielded the dioxo/diisocyanato complex 3 within 5 min. The mechanistic details were clarified by the ¹⁵N- and ¹³C-labeled experiments and density functional theory (DFT) calculations, providing unprecedented insights into the reaction of N₂ with CO₂. A titanium-mediated cycle for the synthesis of trimethylsilyl isocyanate Me₃SiNCO from N₂, CO₂, and Me₃SiCl using H₂ as a reducing agent was also established.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Direct transformation of dinitrogen: synthesis of N-containing organic compounds via N−C bond formation

N-containing organic compounds are of vital importance to lives. Practical synthesis of valuable N-containing organic compounds directly from dinitrogen (N₂), not through ammonia (NH₃), is a holy-grail in chemistry and chemical industry. An essential step for this transformation is the functionalization of the activated N₂ units/ligands to generate N−C bonds. Pioneering works of transition metal-mediated direct conversion of N₂ into organic compounds via N−C bond formation at metal-dinitrogen [N₂-M] complexes have generated diversified coordination modes and laid the foundation of understanding for the N−C bond formation mechanism. This review summarizes those major achievements and is organized by the coordination modes of the [N₂-M] complexes (end-on, side-on, end-on-side-on, etc.) that are involved in the N−C bond formation steps, and each part is arranged in terms of reaction types (N-alkylation, N-acylation, cycloaddition, insertion, etc.) between [N₂-M] complexes and carbon-based substrates. Additionally, earlier works on one-pot synthesis of organic compounds from N₂ via ill-defined intermediates are also briefed. Although almost all of the syntheses of N-containing organic compounds via direct transformation of N₂ so far in the literature are realized in homogeneous stoichiometric thermochemical reaction systems and are discussed here in detail, the sporadically reported syntheses involving photochemical, electrochemical, heterogeneous thermo-catalytic reactions, if any, are also mentioned. This review aims to provide readers with an in-depth understanding of the state-of-the-art and perspectives of future research particularly in direct catalytic and efficient conversion of N₂ into N-containing organic compounds under mild conditions, and to stimulate more research efforts to tackle this long-standing and grand scientific challenge.

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5 N5 ]. Chemistry 2019;25:16523-16527. [PMID: 31637740 DOI: 10.1002/chem.201904362]

The side-on-end-on coordination of N₂ can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V₅ N₅ ]^- ) that can capture N₂ efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N₂ has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d-d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N₂ are the driving forces to adsorb N₂ with a high binding energy (about 2.0 eV).

Dinitrogen functionalization at a ditantalum center. Balancing N2 displacement and N2 functionalization in the reaction of coordinated N2 with CS2

The reaction of carbon disulfide (CS2) with the side-on end-on dinitrogen complex ([NPNSi]Ta)2(μ-η1:η2-N2)(μ-H)2 (1) (where [NPNSi] = [PhP(CH2SiMe2NPh)2]) has been studied and shown to generate two products, the ratio of which depends on the concentration of added carbon disulfide. At high concentrations of CS2, N-N bond cleavage and functionalization occur to generate a ditantalum complex with an isothiocyanato ligand N-bound to Ta along with bridging sulfido and nitrido moieties. At lower concentrations of CS2, less dinitrogen functionalization is observed; instead, N2 is displaced and the CS2 molecule is completely disassembled to generate a ditantalum derivative with bridging methylene and two sulfide moieties. Kinetic and DFT studies have been performed and provide clues about the product ratio and mechanistic information and shed light on why N2 functionalization is challenging.

Diamidophosphines with six-membered chelates and their coordination chemistry with group 4 metals: development of a trimethylene-methane-tethered [PN2]-type "molecular claw"

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH2CH2CH2NHPh)2 (pr[NPN]H2) and PhP(1,2-CH2-C6H4-NHSiMe3)2 (bn[NPN]H2) featuring six-membered N–C3–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN2] scaffold. In the case of the propylene-linked system pr[NPN]H2, access to the sparingly soluble dibenzyl derivative pr[NPN]ZrBn2 (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes bn[NPN]MI2 (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative bn[NPN]H2. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C1-symmetric (bn[NPN]MX2) or averaged CS-symmetric (pr[NPN]MX2) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN2]H2 protioligand was designed. This tripodal ligand system was prepared on a gram scale and its CS-symmetric dichloro complexes [PN2]MCl2 (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN2]ZrBn2 (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN2] tripod.

Synthesis and reactivity of Li and TaMe3 complexes supported by N,N′-bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands

Redox-active diamido ligands sponsor heterodinuclear TaLi complexes and a TaMe₃ species which undergoes photoreduction to yield a Ta(iv) dimer.

Reactions of heteroallenes with cyclam-based Zr(IV) complexes

This work describes reactions of heteroallenes with diamido-diamine cyclam-based Zr(iv) complexes of the general formula (Bn2Cyclam)ZrX2 (X = O(t)Bu, , O(i)Pr, , SPh, , NH(t)Bu, ) as well as the di-orthometallated species ((C6H4CH2)2Cyclam)Zr, . The reactions of isocyanates or isothiocyanates with , or resulted in the formation of N-bonded ureate or thioureate cyclam complexes upon [2 + 2] cycloaddition of the Zr-Namido bonds of the cyclam to the heteroallene (). DFT calculations showed that κ(2)-N,N'-ureate bonding is favoured over κ(2)-N,O-ureates, which in turn may be formed in reactions with bulky isocyanates as 1-naphthyl isocyanate (NpN[double bond, length as m-dash]C[double bond, length as m-dash]O). The reactions of with N,N'-cyclohexylcarbodiimide (CyN[double bond, length as m-dash]C[double bond, length as m-dash]NCy) and carbon disulfide afforded guanidinate and dithiocarbamate fragments, respectively, appended to one of the nitrogen atoms of the cyclam ligand. These reactions represent a reliable method for the synthesis of asymmetrically N-functionalized cyclams giving rise to C1 symmetry Zr(iv) species by addition of one equivalent of heteroallenes. The reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with one equivalent of mesityl isocyanate (MesN[double bond, length as m-dash]C[double bond, length as m-dash]O) also proceeds through insertion, involving one Zr-NH(t)Bu bond. However, it was observed that the reaction of (Bn2Cyclam)Zr(NH(t)Bu)2, , with MesN[double bond, length as m-dash]C[double bond, length as m-dash]O follows a different path if the reaction is carried out at 60 °C. In this case the reaction leads to [2 + 2] addition of the Zr-Ncyclam bond to the isocyanate, with a concomitant occurrence of orthometallation of the one benzyl pending group of the cyclam ring. The reaction of (t)BuN[double bond, length as m-dash]C[double bond, length as m-dash]O with the di-orthometallated complex ((C6H4CH2)2Cyclam)Zr, , also gave a κ(2)-N,N'-ureate fragment, by isocyanate addition to the macrocycle. DFT calculations on these systems were conducted in an attempt to rationalise the reactivity patterns observed.

Retrosynthetic approach to the design of molybdenum-magnesium oxoalkoxides

The reaction of MoCl5 methanolysis in the presence of magnesium ions was shown to produce an extensive row of heterobimetallic Mg-Mo(V, VI) oxomethoxides of different nuclearity ranging from 4 for [Mg2(CH3OH)4Mo2O2(OCH3)10] (1) to 26 for [Mg(DMF)3(CH3OH)3]2[Mo22Mg4O48(OCH3)28(DMF)6] (2) with the latter possessing a ring morphology. Examination of [Mo6O12(OCH3)16Mg4(CH3OH)6] (3), [Mo6O12(OCH3)12Mg2(DMF)4] (4a), and [Mo6O16(OCH3)4Mg2(DMF)8] (5a) X-ray structures revealed the presence of the well known tetranuclear core {Mo4O8(OCH3)2}(2+) thus similar reactivity patterns leading to their formation were assumed. For convenient synthesis of such heterobimetallic oxoalkoxides, the retrosynthetic approach based on speculative deconstruction of a target molecule onto simpler fragments was suggested and successfully employed. Namely, the reaction of the stoichiometric amounts of appropriately chosen Mo(V), Mo(VI) and Mg(2+) synthons led to their assembling resulting in the formation of heterometallic clusters 3, 5a and [Mo6O12(OCH3)12Mg2(CH3OH)4]·2CH3OH (4b) characterized by means of elemental analysis, UV-Vis, IR spectroscopy, and X-ray crystallography.