Hydrazido Complexes Research Articles

Dinitrogen Reduction and Functionalization by a Siloxide Supported Thulium‐Potassium Complex for the Formation of Ammonia or Hydrazine Derivatives

AbstractThe dinitrogen (N2) chemistry of lanthanides remains less developed compared to the d‐block metals and lanthanide‐promoted N2 functionalization chemistry in well‐defined lanthanide complexes remains elusive. Here we report the synthesis and characterization (SQUID, EPR, DFT, X‐Ray) of the siloxide supported heterobimetallic (Tm/K) complexes [{KTm(OSi(OtBu)3)3}2(μ‐η2 : η2‐N2)] (1) and [K3{Tm(OSi(OtBu)3)3}2(μ‐η2 : η2‐N2)] (2). Complex 2 provides a rare example of a metal complex of the triply reduced N23− radical. The structure of 2 differs from the few previously reported N23− complexes as it presents two Tm and three K cations binding the N23− radical, facilitating N2 functionalization. Notably, the K3Tm2‐bound N23− moiety reacts with excess H+ to form NH4Cl in 18 % yield, and with MeOTf at room temperature to yield the dimethyl hydrazido complex [K2{Tm(OSi(OtBu)3)3}2(μ‐(CH3)NN(CH3))] (3). Protonolysis of 3 yields MeHN−NMeH ⋅ 2HCl in 18 % yield.

Dinitrogen Reduction and Functionalization by a Siloxide Supported Thulium-Potassium Complex for the Formation of Ammonia or Hydrazine Derivatives.

The dinitrogen (N2) chemistry of lanthanides remains less developed compared to the d-block metals and lanthanide-promoted N2 functionalization chemistry in well-defined lanthanide complexes remains elusive. Here we report the synthesis and characterization (SQUID, EPR, DFT, X-Ray) of the siloxide supported heterobimetallic (Tm/K) complexes [{KTm(OSi(OtBu)3)3}2(μ-η2 : η2-N2)] (1) and [K3{Tm(OSi(OtBu)3)3}2(μ-η2 : η2-N2)] (2). Complex 2 provides a rare example of a metal complex of the triply reduced N2 3- radical. The structure of 2 differs from the few previously reported N2 3- complexes as it presents two Tm and three K cations binding the N2 3- radical, facilitating N2 functionalization. Notably, the K3Tm2-bound N2 3- moiety reacts with excess H+ to form NH4Cl in 18 % yield, and with MeOTf at room temperature to yield the dimethyl hydrazido complex [K2{Tm(OSi(OtBu)3)3}2(μ-(CH3)NN(CH3))] (3). Protonolysis of 3 yields MeHN-NMeH ⋅ 2HCl in 18 % yield.

Syntheses, Characterization, and Multifaceted Coordination Chemistry of Hydrazonido Titanium Complexes.

The reaction of hydrazones with bis(π-η5:σ-η1-pentafulvene)titanium complexes leads to both hydrazonido and hydrazido complexes depending on the interaction of the hydrazone with the fulvene ligand of the metal complex. The molecular structures mostly reveal κ2N,N side-on coordination of the hydrazonido ligand due to the deprotonation of the N-H bond by one of the fulvene moieties. Instead of deprotonation, the reaction of the bis(adamantylidene fulvene)titanium complexes with cinnamon aldehyde phenylhydrazone leads to κ1N coordination. By using donating groups in the backbone of the hydrazone ligands, there are exceptions to this coordination mode due to the insertion of the C═N double bond into the Ti-Cexo bond of the pentafulvene moiety. Using 2-pyridinecarboxaldehyde phenylhydrazone, a formal κ3N,N,N ligand system is formed by the coordination of the pyridine nitrogen atom to the metal center via consecutive N-H deprotonation and insertion. Finally, the use of salicylaldehyde phenylhydrazone ultimately produces a complex with the κ3N,N,O coordination mode by double deprotonation of the hydrazone N-H and O-H functions. Because of its slow conversion to the final product, the intermediate was isolated as an insertion product with consecutive O-H deprotonation, showing a κ2N,O coordination mode of the hydrazido ligand.

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.

Insertion reactions and structural studies of [(NHC)CuH]2 with nitrogen-based substrates

The stoichiometric and catalytic reactions of Cu-H dimers supported by N-heterocyclic carbenes (NHCs) have mainly focused on the insertions of aldehydes, ketones, CO2, and unsaturated hydrocarbons. In this report, we investigated the stoichiometric reactions of dimeric [(NHC)CuH]2 (NHC = IPr*, 6Dipp) with unsaturated nitrogen-based substrates of PhN=NPh, N3Ad, pyrazine, and N2O. The spectroscopic and structural chacterizations of the resulting monomeric, two-coordinate Cu(I) complexes containing diphenyl hydrazido, triazenido, pyrazinyl, and hydroxide ligands are discussed. The hydrazido complex of (IPr*)Cu(NPh-NHPh) and triazenido complex of (IPr*)Cu(HNNNAd) are resistant to NN bond cleavage and loss of N2, respectively, to form the corresponding amido complexes even when heated at 80 °C over several hours.

Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes.

ConspectusThe activation and functionalization of N2 to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N2 fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N2 to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N2 complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N2 complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N2 functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N2 unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N2 complex with Me3SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the Nβ atom twice, yielding an end-on η1-hydrazido complex, Cr(III)═N-N(SiMe3)2. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N2 complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me3SiCl proceeds via the alternating pathway; the silylation occurs at both Nα and Nβ atoms and generates a side-on η2-hydrazido complex, Cr(III)(η2-Me3SiN-NSiMe3). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N2 complexes with 4 equiv of Me3SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR2]+ hydrazido as the final products. More importantly, this type of Cr(0)-N2 complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N2 systems provide effective approaches for the activation and functionalization of N2, deepening the understanding of N2 electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N2 to high-value-added nitrogen-containing organic compounds.

Dinitrogen Functionalization with Carbon Dioxide and Carbon Disulfide Giving Symmetric and Unsymmetric Hydrazido Complexes.

Here we show that a tridentate bis(aryloxide)anilide-ligated titanium/potassium scaffold promotes functionalization of coordinated N2 with CO2 and CS2 through formation of N-C bonds. Treatment of a naphthalene complex with N2 gave an end-on bridging dinitrogen complex featuring a [Ti2 K2 N2 ] core. The dinitrogen complex underwent insertion of CO2 into each Ti-NN bond to afford an N,N'-dicarboxylated hydrazido complex. Stepwise nitrogen-carbon bond formation at coordinated N2 proceeded to afford an unsymmetric hydrazido complex upon sequentially treating the dinitrogen complex with CS2 and CO2 . Addition of Me3 SiCl to the dicarboxylated hydrazido complex resulted in partial silylation of the carboxylate groups but did not lead to removal of the functionalized N2 unit from the metal centers. However, reduction of the dicarboxylated hydrazido complex with potassium naphthalenide afforded an oxo-bridged dinuclear complex along with release of free potassium cyanate.

Dinitrogen Functionalization with Carbon Dioxide and Carbon Disulfide Giving Symmetric and Unsymmetric Hydrazido Complexes

AbstractHere we show that a tridentate bis(aryloxide)anilide‐ligated titanium/potassium scaffold promotes functionalization of coordinated N2 with CO2 and CS2 through formation of N−C bonds. Treatment of a naphthalene complex with N2 gave an end‐on bridging dinitrogen complex featuring a [Ti2K2N2] core. The dinitrogen complex underwent insertion of CO2 into each Ti−NN bond to afford an N,N′‐dicarboxylated hydrazido complex. Stepwise nitrogen‐carbon bond formation at coordinated N2 proceeded to afford an unsymmetric hydrazido complex upon sequentially treating the dinitrogen complex with CS2 and CO2. Addition of Me3SiCl to the dicarboxylated hydrazido complex resulted in partial silylation of the carboxylate groups but did not lead to removal of the functionalized N2 unit from the metal centers. However, reduction of the dicarboxylated hydrazido complex with potassium naphthalenide afforded an oxo‐bridged dinuclear complex along with release of free potassium cyanate.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate.

The catalytic transformation of N2 to NH3 by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N2 fixation, well-defined vanadium complexes that can catalyze the conversion of N2 to NH3 are scarce. In particular, a V(NxHy) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(tBu2PO)2-C6H3) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N2 to NH3 and N2H4. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH2), which mediated 15N2 conversion to 15NH3, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH2 possessed a N-H bond dissociation free energy (BDFEN-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH2) with [Ph2NH2][OTf] resulted in the release of NH3 and the formation of a vanadium triflate complex, which upon reduction under N2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH3. These findings provide unprecedented insights into the mechanism of N2 reduction related to FeV nitrogenase.

Computational Investigations of the Reactivity of Metalloporphyrins for Ammonia Oxidation

Density functional theory and molecular dynamics simulations were used to assess the ability of tetraphenylporphyrin (TPP)M complexes (where M = Cr, Mn, Fe, Co, Ni, Mo, Ru, W, and Os) to coordinate and weaken the N–H bonds of ammonia, as well as their reactivity towards N–N bond formation for N2 generation. Compared to other metalloporphyrins, bis-ammonia complexes (TPP)Mo(NH3)2 and (TPP)W(NH3)2 exhibit low and level N–H BDFEs due to a stabilized (TPP)M(NH3)(NH) intermediate by multiple metal–ligand bonding. These results resemble those previously obtained for polypyridyl metal complexes, suggesting that broad trends in reactivity towards N–H bond cleavage are more metal-dependent rather than ligand-dependent for a metal in a pseudo-octahedral environment with nitrogen-based ligands. We investigated N–N bond formation via NH3 nucleophilic attack on M–NH and M–N intermediates, compared to bimolecular coupling of M-NHx intermediates. We evaluated the reactivity of (TPP)Fe(NH3)2 towards N–N bond formation through a hydrazine pathway, and found amide-amide coupling to form a bridged hydrazido complex to be the most favorable pathway for N–N bond formation. Further investigation of potential N–N bond formation pathways by reaction with NH3 led us to identify a possible FeIII-•NH species with significant aminyl character that bypasses the nucleophilic attack of NH3 and that promotes homolytic N–H bond cleavage of ammonia. This reaction forms a Fe-NH2 moiety and a transient •NH2 radical that subsequently forms an N–N bond with the Fe-NH2 moiety to form a (TPP)Fe(NH3)(N2H4) species. These results indicate the need to evaluate the radical character of imido species and their reactivity towards N–H bond cleavage of ammonia.

Cooperative Reactions of Pentafulvene Niobium Complexes: Formation of Alkylidene, Imido, Hydrazido, and Niobaaziridine Complexes

Cooperative Reactions of Pentafulvene Niobium Complexes: Formation of Alkylidene, Imido, Hydrazido, and Niobaaziridine Complexes

Bonding and Activation of N2 in Molybdenum(0) Complexes Supported by Tripod Ligands with Phospholane End Groups

AbstractTripod ligands with a neopentyl backbone and one (trpd‐1 pln), two (trpd‐2 pln) or three (trpd‐3 pln) phospholane end groups are synthesized and used for the synthesis of MoX3 (X=Cl, Br, I) complexes supported by these ligands. NaxHg reduction of [MoBr3(trpd‐3 pln)], in the presence of N2 and the bidentate coligand dppm leads to the molybdenum(0) mono(dinitrogen) complex [Mo(N2)(trpd‐3 pln)(dppm)] with a moderately activated N2 ligand. An attempt to further increase the activation by using dmpm as coligand leads to a mixture containing desired [Mo(N2)(trpd‐3 pln)(dmpm)], but also the bis(dinitrogen) complex [Mo(N2)2(κ2‐trpd‐3 pln)2] and the homoleptic complex [Mo(trpd‐3 pln)2]. Protonation of [Mo(N2)(trpd‐3 pln)(dppm)] with [H(OEt2)2][Al{OC(CF3)3}4] generates the hydrazido(2‐) complex [Mo(NNH2)(trpd‐3 pln)(dppm)][Al{OC(CF3)3}4]2.

Cross-Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate-Determining Deprotonation of Bound Hydrazine.

Reported here is the Pd-catalyzed C-N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate-limiting deprotonation of a hydrazine-bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.

Cross‐Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate‐Determining Deprotonation of Bound Hydrazine

AbstractReported here is the Pd‐catalyzed C–N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate‐limiting deprotonation of a hydrazine‐bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.

A Chatt-Type Catalyst with One Coordination Site for Dinitrogen Reduction to Ammonia.

With [Mo(N2)(P2 MePP2 Ph)] the first Chatt‐type complex with one coordination site catalytically converting N2 to ammonia is presented. Employing SmI2 as reductant and H2O as proton source 26 equivalents of ammonia are generated. Analogous Mo0‐N2 complexes supported by a combination of bi‐ and tridentate phosphine ligands are catalytically inactive under the same conditions. These findings are interpreted by analyzing structural and spectroscopic features of the employed systems, leading to the conclusion that the catalytic activity of the title complex is due to the strong activation of N2 and the unique topology of the pentadentate tetrapodal (pentaPod) ligand P2 MePP2 Ph. The analogous hydrazido(2‐) complex [Mo(NNH2)(P2 MePP2 Ph)](BArF)2 is generated by protonation with HBArF in ether and characterized by NMR and vibrational spectroscopy. Importantly, it is shown to be catalytically active as well. Along with the fact that the structure of the title complex precludes dimerization this demonstrates that the corresponding catalytic cycle follows a mononuclear pathway. The implications of a PCET mechanism on this reactive scheme are considered.

Oligodentate Phosphine Ligands with Phospholane End Groups: New Synthetic Access and Application to Molybdenum‐Based Synthetic Nitrogen Fixation

A new synthetic access to oligodentate phosphine ligands with phospholane end groups, starting from lithium phospholanide, is established. Based on this building block, the tridentate ligand prPP(Ph)P‐pln was synthesized and used for the synthesis of [MoX3{prPP(Ph)P‐pln}] (X = Cl, Br, I) precursors. Sodium amalgam reduction in the presence of N2 and either mono‐ or bidentate ligands leads to several molybdenum(0) mono‐ and bis(dinitrogen) complexes, respectively. With the diphosphine dppm a mixture of facial and meridional isomers of [Mo(N2){prPP(Ph)P‐pln}(dppm)] is formed. Using the monophosphines PMePh2 and PMe2Ph mer‐[Mo(N2){prPP(Ph)P‐pln}(PMe2Ph)2] and trans‐[Mo(N2)2{prPP(Ph)P‐pln}(PMePh2)] could be obtained. The spectroscopic properties and reactivity of the latter towards protonation was investigated, and a hydrazido complex could be obtained and characterized.

Dinitrogen Functionalization Affording Chromium Hydrazido Complex.

A series of trinuclear and dinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were synthesized and characterized. Further reduction of the Cr(I)-N2 complexes generated anionic Cr(0)-N2 complexes, which could react with Me3SiCl to afford the first chromium hydrazido complex from N2 functionalization. These complexes were found to be effective catalysts for the transformation of N2 into N(SiMe3)3.

Direct Access to Terminal Titanocene Hydrazides via Bis(η5:η1-pentafulvene)titanium Complexes and 1,1-Diphenylhydrazine

The formation of the titanocene hydrazido complex 2 via double N–H bond activations at ambient temperature of 1,1-diphenylhydrazine by the use of bis(η5:η1-adamantylidenepentafulvene)titanium complex 1 have been investigated. Starting from the titanium hydrazido complex 2, [2 + 2] cycloaddition and bond activation reactions are realized. In reaction with carbodiimides, the asymmetric N,N-bound ureato(2−) product 3 is observed. By use of nitriles, in addition to the [2 + 2] cycloaddition, the insertion of a second equivalent of the nitrile into the Ti–N bond is found (4). By reaction of 2 with carbon disulfide the symmetric S,S-bound thioureato(2−) complex 5 is isolated, whereas in reaction with carbon monoxide the cleavage of the N–N bond of the hydrazide unit is observed, resulting in the formation of the titanocene monocyanate 6. In contrast to the [2 + 2] cycloaddition reactions, for sterically demanding boranes such as 9-borabicyclo[3.3.1]nonane and catecholborane the hydroboration of the Ti═N double ...

Effects of N2 Binding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex.

Distinguishing the reactivity differences between N2 complexes having different binding modes is crucial for the design of effective N2-functionalizing reactions. Here, we compare the reactions of a K-bridged, dinuclear FeNNFe complex with a monomeric Fe(N2) complex where the bimetallic core is broken up by the addition of chelating agents. The new anionic iron(0) dinitrogen complex has enhanced electron density at the distal N atoms of coordinated N2, and though the N2 is not as weakened in this monomeric compound, it is much more reactive toward silylation by (CH3)3SiI (TMSI). Double silylation of N2 gives a three-coordinate iron(III) hydrazido(2-) complex, which is finely balanced between coexisting S = 1/2 and S = 3/2 states that are characterized by crystallography, spectroscopy, and computations. These results give insight into the interdependence between binding modes, alkali dependence, reactivity, and magnetic properties within an iron system that functionalizes N2.

Four-electron Reduction and Functionalization of N₂ by a Uranium(III) Bridging Nitride.

N2 is a cheap and widely available but very unreactive molecule. Notably, the only industrial process for the conversion of dinitrogen is the Haber-Bosch process to form ammonia, but under very harsh conditions (high temperature and pressure). Here, we review our recent research leading to the reduction of dinitrogen by four electrons, under ambient conditions by a uranium(III) bridging nitride, K3 U-N-U, where two uranium(III) cations are linked by a nitride group and a flexible metal-ligand scaffold. We also show that the bound dinitrogen can be further functionalized under mild conditions: the addition of acid, hydrogen and protons or carbon monoxide to the uranium hydrazido complex yields to the cleavage of the N-N single bond and to the formation of new N-H and N-C bonds.