Synthesis of [(DPPNCH2CH2)3N]3- molybdenum complexes (DPP = 3,5-(2,5-Diisopropylpyrrolyl)2C6H3) and studies relevant to catalytic reduction of dinitrogen

Dinitrogen Functionalization Affording Chromium Diazenido and Side-on η2-Hydrazido Complexes

Isolation of key intermediate complexes in dinitrogen functionalization is crucial for elucidating the mechanistic details and further investigation. Herein, the synthesis and characterization of (μ-η¹:η¹-N₂)(η¹-N₂)-Cr(I) 3 and (η¹-N₂)₂-Cr(0) complexes 4 supported by Cp* (Cp* = C₅Me₅) and NHC ligands were reported. Further functionalization of Cr(0)-N₂ complex 4 with silyl halides delivered the key intermediates in the alternating pathway, the chromium diazenido complex 5 and the chromium side-on η²-hydrazido complex 6. Protonation of 6 led to the quantitative formation of N₂H₄. Moreover, the [η²-Me₃SiNNSiMe₃]^2- unit in 6 enabled N-C bond formation reactions with CO₂ and ^tBuNCO, giving the corresponding N,O-chelating hydrazidochromium complexes 7 and 8, respectively.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Dehydrogenative Coupling of Terminal Alkynes with O/N-Based Monohydrosilanes Catalyzed by Rare-Earth Metal Complexes

Newly synthesized rare-earth metal alkyl complexes bearing a tripyrrolyl ligand act as excellent precatalysts for the cross-dehydrogenative coupling between various terminal alkynes and O/N-based monohydrosilanes of HSi(OEt)₃/HSi(NMe₂)₃, leading to the formation of a variety of alkoxysilylalkyne and aminosilylalkyne derivatives in good to high yields. The precatalysts LRE(CH₂SiMe₃)(thf)₂ (RE = Y(1a), Er(1b), Yb(1c), L = 2,5-[(2-C₄H₃N)CPh₂]₂(C₄H₂NMe), thf = tetrahydrofuran) were easily prepared in high yields via the reactions of RE(CH₂SiMe₃)₃(thf)₂ with the proligand H₂L in a single step. Mechanistic studies reveal that treatment of 1 with phenylacetylene could generate the active catalytic species: dinuclear rare-earth metal alkynides (L(thf)_n[RE(μ-C≡CPh)]₂L) (RE = Y(5a), n = 1; Yb(5c), n = 0), which could react with HSi(OEt)₃ to produce the coupling product 4aa and the dinuclear rare-earth metal hydrides (L (thf)[RE(μ-H)]₂L) (RE = Y(6a); Yb(6c)). By contrast, prior treatment of 1c with HSi(OEt)₃ proceeds via cleavage of the Si-O bond to produce the dinuclear ytterbium alkoxide (LYb(μ-OEt))₂ 7c, which is inert in the dehydrogenative coupling reaction. The results of the mechanistic studies are consistent with the observation that the reaction is greatly influenced by the addition sequence of precatalyst/alkynes/silanes.

Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes

Nitrogen fixation, the six-electron/six-proton reduction of N₂, to give NH₃, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N₂ into its bioavailable form, NH₃. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N₂ functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Crystal structure of nitridobis(tri-methyl-silanolato)[1,1,1-trimethyl-N-(tri-methyl-sil-yl)silanaminato]molybdenum(VI)

In the title compound, [Mo(C6H18NSi2)(C3H9OSi)2N], the Mo(VI) cation is located on a mirror plane and is coordinated by a nitride anion, a 1,1,1-trimethyl-N-(tri-methyl-sil-yl)silanaminate anion and two tri-methyl-silanolate anions in a distorted tetra-hedral geometry; the N atom and two Si atoms of the 1,1,1-trimethyl-N-(tri-methyl-sil-yl)silanaminato anionic ligand are also located on the mirror plane. The Mo N bond length of 1.633 (6) Å is much shorter than the Mo-N single-bond length of 1.934 (7) Å. No hydrogen bonding is observed in the crystal structure.

Nitric oxide activation by distal redox modulation in tetranuclear iron nitrosyl complexes

A series of tetranuclear iron complexes displaying a site-differentiated metal center was synthesized. Three of the metal centers are coordinated to our previously reported ligand, based on a 1,3,5-triarylbenzene motif with nitrogen and oxygen donors. The fourth (apical) iron center is coordinatively unsaturated and appended to the trinuclear core through three bridging pyrazolates and an interstitial μ4-oxide moiety. Electrochemical studies of complex [LFe3(PhPz)3OFe][OTf]2 revealed three reversible redox events assigned to the Fe(II)4/Fe(II)3Fe(III) (-1.733 V), Fe(II)3Fe(III)/Fe(II)2Fe(III)2 (-0.727 V), and Fe(II)2Fe(III)2/Fe(II)Fe(III)3 (0.018 V) redox couples. Combined Mössbauer and crystallographic studies indicate that the change in oxidation state is exclusively localized at the triiron core, without changing the oxidation state of the apical metal center. This phenomenon is assigned to differences in the coordination environment of the two metal sites and provides a strategy for storing electron and hole equivalents without affecting the oxidation state of the coordinatively unsaturated metal. The presence of a ligand-binding site allowed the effect of redox modulation on nitric oxide activation by an Fe(II) metal center to be studied. Treatment of the clusters with nitric oxide resulted in binding of NO to the apical iron center, generating a {FeNO}(7) moiety. As with the NO-free precursors, the three reversible redox events are localized at the iron centers distal from the NO ligand. Altering the redox state of the triiron core resulted in significant change in the NO stretching frequency, by as much as 100 cm(-1). The increased activation of NO is attributed to structural changes within the clusters, in particular, those related to the interaction of the metal centers with the interstitial atom. The differences in NO activation were further shown to lead to differential reactivity, with NO disproportionation and N2O formation performed by the more electron-rich cluster.

Homolytic N-H activation of ammonia: hydrogen transfer of parent iridium ammine, amide, imide, and nitride species

The redox series [Ir(n)(NHx)(PNP)] (n = II-IV, x = 3-0; PNP = N(CHCHPtBu2)2) was examined with respect to electron, proton, and hydrogen atom transfer steps. The experimental and computational results suggest that the Ir(III) imido species [Ir(NH)(PNP)] is not stable but undergoes disproportionation to the respective Ir(II) amido and Ir(IV) nitrido species. N-H bond strengths are estimated upon reaction with hydrogen atom transfer reagents to rationalize this observation and are used to discuss the reactivity of these compounds toward E-H bond activation.

Uranium triamidoamine chemistry

Triamidoamine (Tren) complexes of the p- and d-block elements have been well-studied, and they display a diverse array of chemistry of academic, industrial and biological significance. Such in-depth investigations are not as widespread for Tren complexes of uranium, despite the general drive to better understand the chemical behaviour of uranium by virtue of its fundamental position within the nuclear sector. However, the chemistry of Tren-uranium complexes is characterised by the ability to stabilise otherwise reactive, multiply bonded main group donor atom ligands, construct uranium-metal bonds, promote small molecule activation, and support single molecule magnetism, all of which exploit the steric, electronic, thermodynamic and kinetic features of the Tren ligand system. This Feature Article presents a current account of the chemistry of Tren-uranium complexes.

Structural analysis of the coordination of dinitrogen to transition metal complexes

Transition-metal complexes show a wide variety of coordination modes for the nitrogen molecule. A structural database study has been undertaken for dinitrogen complexes, and geometrical parameters around the L(n)M-N2 unit are retrieved from the Cambridge Structural Database. These data were classified in families of compounds, according to metal properties, to determine the degree of lengthening for the dinitrogen bonding. The importance of the nature of the metal center, such as coordination number and electronic configuration, is reported. Our study reveals poor activation by coordination of dinitrogen in mononuclear complexes, always having end-on coordination. However, partial weakening of nitrogen-nitrogen bonding is found for end-on binuclear complexes, whereas side-on complexes can be completely activated.

Electronic and Steric Influences of Pendant Amine Groups on the Protonation of Molybdenum Bis(dinitrogen) Complexes

The synthesis of a series of P(Et)P(NRR(')) (P(Et)P(NRR(')) = Et2PCH2CH2P(CH2NRR')2, R = H, R' = Ph or 2,4-difluorophenyl; R = R' = Ph or (i)Pr) diphosphine ligands containing mono- and disubstituted pendant amine groups and the preparation of their corresponding molybdenum bis(dinitrogen) complexes trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of triflic acid (HOTf) to trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes in tetrahydrofuran at -40 °C show that the electronic and steric properties of the R and R' groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N2 ligand, or a combination of these sites. For example, complexes containing monoaryl-substituted pendant amines are protonated at Mo and the pendant amine site to generate mono- and dicationic Mo-H species. Protonation of the complex containing less basic diphenyl-substituted pendant amines exclusively generates a monocationic hydrazido (Mo(NNH2)) product, indicating preferential protonation of an N2 ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo(NNH2) product that is additionally protonated at a pendant amine site. In addition, trans-Mo(N2)2(PMePh2)2(depe) (depe = Et2PCH2CH2PEt2) was synthesized to serve as a counterpart lacking pendant amines. Treatment of this complex with HOTf generated a monocationic Mo(NNH2) product. Protonolysis experiments conducted on several complexes in this study afforded trace amounts of NH4(+). Computational analysis of trans-Mo(N2)2(PMePh2)2(P(Et)P(NRR('))) complexes provides further insight into the proton affinity values of the metal center, N2 ligand, and pendant amine sites to rationalize differences in their reactivity profiles.

A tripodal benzylene-linked trisamidophosphine ligand scaffold: synthesis and coordination chemistry with group(IV) metals

A new tripodal trisamidophosphine ligand (1) based on the trisbenzylphosphine backbone has been synthesized in three steps starting from NaPH2 and phthaloyl-protected 2-aminobenzyl bromide. At elevated temperatures, 1 reacts directly with M(NMe2)4 (M = Zr, Hf) to afford the dimethylamido complexes [PN3]M(NMe2) (M = Zr, Hf) (2), which are easily converted into the corresponding triflates [PN3]MOTf (M = Zr, Hf) (3) via reaction with triethylsilyl trifluoromethanesulfonate. The related titanium chloro complex [PN3]TiCl (4-Ti) is obtained from 1 and Bn3TiCl via protonolysis. Triple deprotonation of 1 with n-butyllithium affords the tris-lithium salt Li3[PN3] (1-Li), which serves as a common starting material for the preparation of all the group(IV) chlorides [PN3]MCl (M = Ti, Zr, Hf) (4). Upon treatment of 4-Ti with Bn2Mg(thf)2, formation of a benzyltitanium species is observed, which is converted cleanly into a ligand-CH-activated species (5-Ti).

An electrochemical investigation of intermediates and processes involved in the catalytic reduction of dinitrogen by [HIPTN3N]Mo (HIPTN3N = (3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2)3N)

The redox properties of [HIPTN(3)N]Mo complexes (where HIPTN(3)N = (3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)NCH(2)CH(2))(3)N) involved in the catalytic dinitrogen reduction cycle were studied using cyclic voltammetry in fluorobenzene with Bu(4)NPF(6) as the electrolyte. MoN(2) (Mo = [HIPTN(3)N]Mo, E(1/2) = -1.96 V vs. Fc(+)/Fc at a Pt electrode), Mo≡N (E(1/2) = -2.68 V vs. Fc(+)/Fc (Pt)), and [Mo(NH(3))]BAr'(4) (Ar' = 3,5-(CF(3))(2)C(6)H(3), E(1/2) = -1.53 V vs. Fc(+)/Fc (Pt)) each undergo a chemically reversible one-electron reduction, while [Mo=NNH(2)]BAr'(4) (E(1/2) = -1.50 V vs. Fc(+)/Fc (Pt)) and [Mo=NH]BAr'(4) (E(1/2) = -1.26 V vs. Fc(+)/Fc (Pt)) each undergo a one-electron reduction with partial chemical reversibility. The acid employed in the catalytic reduction, [2,4,6-collidinium]BAr'(4), reduces irreversibly at -1.11 V vs. Fc(+)/Fc at Pt and at -2.10 V vs. Fc(+)/Fc at a glassy carbon electrode. The reduction peak potentials of the Mo complexes shift in the presence of acids. For example, the reduction peak for MoN(2) in the presence of [2,4,6-collidinium]BAr'(4) at a glassy carbon electrode shifts positively by 130 mV. The shift in reduction potential is explained in terms of reversible hydrogen bonding and/or protonation at a nitrogen site in Mo complexes. The significance of productive and unproductive proton-coupled electron transfer reactions in the catalytic dinitrogen reduction cycle is discussed.

Tris{2-[(2,6-dimethyl-phen-yl)amino]-eth-yl}amine

The title compound, C₃₀H₄₂N₄, is an aryl­ated tris­(amino­eth­yl)amine derivative which was obtained by reducing the corresponding tris-amide with AlH₃. The asymmetric unit consists of one third of a C_3v-symmetric mol­ecule with the tertiary N atom lying on a crystallographic threefold axis.