Dinitrogen Complexes Research Articles

Diiron bridged-thiolate complexes that bind N2 at the Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I) redox states.

All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N2. Nonetheless, synthetic examples of transition metal model complexes that bind N2 and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N2-Fe(μ-SAr)Fe-N2) across at least three oxidation states (Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I)). The (N2-Fe(μ-SAr)Fe-N2) system undergoes reduction of the bound N2 to produce NH3 (∼50% yield) and can efficiently catalyze the disproportionation of N2H4 to NH3 and N2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems.

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.

Cobalt-catalyzed transformation of molecular dinitrogen into silylamine under ambient reaction conditions.

The first successful example of cobalt-catalyzed reduction of N2 with Me3 SiCl and Na as a reductant, under ambient reaction conditions, gives N(SiMe3 )3 , which can be readily converted into NH3 . In this reaction system, 2,2'-bipyridine (bpy) is found to work as an effective additive to improve substantially the catalytic activity. CoN2 complexes bearing three Me3 Si groups as ancillary ligands are considered to work as key reactive species based on DFT calculations. The DFT results also allow the proposal of a detailed reaction pathway for the transformation of N2 into N(SiMe3 )3 .

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.

Cu (II)-catalyzed asymmetric henry reaction with a novel C1-symmetric aminopinane-derived ligand.

A novel C1-symmetric dinitrogen ligand was synthesized in high yield from commercially available (1R,2R,3R,5S)-(−)-isopinocampheylamine and 1-methyl-2-imidazolecarboxaldehyde. In combination with Cu(OAc)2∙H2O, this new ligand promote the reaction between nitromethane and aliphatic aldehydes with high yields (up to 97%) and moderate enantioselectivities (up to 67% ee). The reactions with benzaldehyde required prolonged reaction time that resulted in diminished yields, but accompanied with ee-values in the 55%–76% range.

Synthesis, characterization and molecular modeling of new ruthenium(II) complexes with nitrogen and nitrogen/oxygen donor ligands

Ru(II) complexes with some dinitrogen ligands; 3,4-diamino benzoic acid (DABA), 2-hydazinopyridine (hzpy), 2,2′-bipyridyl (bipy) and anthranilic acid (anth) have been synthesized and characterized by using IR, mass, and UV–Vis spectrometry and thermal analysis. The thermodynamic parameters (ΔE, ΔH, ΔS and ΔG) have been calculated by using Coats–Redfern and Horowitz–Metzger methods. The electrochemical properties of these complexes have been studied by using cyclic voltammetry. The evaluated energies of the HOMO and LUMO are in the range of −4.94 to −4.85eV and −2.86 to −2.68eV, respectively. The complexes have been proven to have an octahedral geometry with DABA, hzpy and bipy as N2 donor ligands and NSC as monodentate ligand. The structure of the Ru(II) complexes has been geometrically optimized by using parameterized PM3 semiempirical method.

Homolytic or Heterolytic Dihydrogen Splitting with Ditantalum/Dizirconium Dinitrogen Complexes? A Computational Study

Transition metal complexes play a key role in creating efficient molecular catalysis processes leading to the ammonia production from earth-abundant dinitrogen. It is indispensable to examine the mechanism and influence of transition metal complexes on dinitrogen hydrogenation. In this paper, the mechanism of the dinitrogen hydrogenation triggered by bimetallic complexes [L2M]2(μ-η2:η2-N2) (M = Ta and Zr, L = Sita-type or Chirik-type ligand) is investigated by density functional theory calculation. Our results show that the side-on ditantalum dinitrogen complex with Sita-type ligands favors the pathway of homolytic dihydrogen splitting when hydrogenation products are generated. However, the dihydrogen splitting switches to the heterolytic pathway as the dominant mechanism when Zr is the metal center. The ditantalum dinitrogen complex undergoes hydrogenation much easier from the side-on coordination mode than the side-on-end-on mode with Sita-type or Chirik-type ligands. With these findings from the computational study, this work identifies that different metal centers and coligands (Sita-type or Chirik-type) in different binding modes (side-on-end-on or side-on bridged) dictate the pathway of dihydrogen cleavage triggered by the bimetallic complexes. This work should provide insights on factors impacting the dinitrogen hydrogenation process and shed light on designing new transition metals and ligands for dinitrogen hydrogenation catalysts in the future.

Bis(pyridylimino)isoindolato (BPI) Osmium Complexes: Structural Chemistry and Reactivity

A series of osmium complexes with monoanionic, meridionally coordinating 1,3-bis(2-pyridylimino)isoindolates (BPI) as spectator ligands has been synthesized. Reaction of the dichlorido metal precursor [OsCl2(PPh3)3] with the lithiated BPI ligand transfer reagent gave the chlorido complex [(tBu-BPIMe)Os(PPh3)2Cl] (2) which, in turn, was reacted with lithium triethylborohydride to yield the hydrido complex [(tBu-BPIMe)Os(PPh3)2H] (3). Treatment of complex 2 with thallium hexafluorophosphate under a nitrogen pressure afforded the cationic dinitrogen complex [(tBu-BPIMe)Os(PPh3)2(N2)]PF6 (4), which contains an end-on coordinated dinitrogen molecule trans to the isoindolato nitrogen atom. To synthesize the alkynyl complex [(tBu-BPIMe)Os(PPh3)2(C≡CPh)] (5), the chlorido complex 2 was treated with lithium phenylacetylide. Complex 5 was subsequently converted quantitatively by addition of one or two equivalents of HBF4 to the vinylidene complex [(tBu-BPIMe)Os(PPh3)2(═C═CHPh)]BF4 (6), and the cationic species [(H-tBu-BPIMe)Os(PPh3)2(═C═CHPh)](BF4)2 (7), respectively; the latter being formed via protonation of one imine nitrogen atom of the BPI ligand. Upon stirring, a toluene solution of the chlorido complex 2 with benzyl potassium, the four-membered metallacycle [(tBu-BPIMe)Os(PPh3)(o-C6H4PPh2)] (8) was obtained, which reacted with molecular hydrogen and phenyl acetylene to give the hydrido complex 3 and the acetylide complex 5, respectively. Stirring of 8 with methyl acetylenedicarboxylate (DMAD) yielded [(tBu-BPIMe)Os(DMAD)(o-C6H4PPh2)] (9), while treatment with carbon monoxide gave the acyl complex [(tBu-BPIMe)Os(o–C(O)-C6H4PPh2)(CO)] (10) by an insertion of CO into the osmium carbon bond.

Well-defined copper(i) fluoroalkoxide complexes for trifluoroethoxylation of aryl and heteroaryl bromides.

Copper(I) fluoroalkoxide complexes bearing dinitrogen ligands were synthesized and the structure and reactivity of the complexes toward trifluoroethoxylation, pentafluoropropoxylation, and tetrafluoropropoxylation of aryl and heteroaryl bromides were investigated.

Well‐Defined Copper(I) Fluoroalkoxide Complexes for Trifluoroethoxylation of Aryl and Heteroaryl Bromides

AbstractCopper(I) fluoroalkoxide complexes bearing dinitrogen ligands were synthesized and the structure and reactivity of the complexes toward trifluoroethoxylation, pentafluoropropoxylation, and tetrafluoropropoxylation of aryl and heteroaryl bromides were investigated.

Electrophysiological evidence of separate pathways for the perception of depth and 3D objects

Previous studies have investigated the neural mechanism of 3D perception, but the neural distinction between 3D-objects and depth processing remains unclear. In the present study, participants viewed three types of graphics (planar graphics, perspective drawings, and 3D objects) while event-related potentials (ERP) were recorded. The ERP results revealed the following: (1) 3D objects elicited a larger and delayed N1 component than the other two types of stimuli; (2) during the P2 time window, significant differences between 3D objects and the perspective drawings were found mainly over a group of electrode sites in the left lateral occipital region; and (3) during the N2 complex, differences between planar graphics and perspective drawings were found over a group of electrode sites in the right hemisphere, whereas differences between perspective drawings and 3D objects were observed at another group of electrode sites in the left hemisphere. These findings support the claim that depth processing and object identification might be processed by separate pathways and at different latencies.

What does successful speech-in-noise perception in aging depend on? Electrophysiological correlates of high and low performance in older adults

Aging usually decreases the ability to understand language under difficult listening conditions. However, aging is also associated with increased between-subject variability. Here, we studied potential sources of inter-individual differences and investigated spoken language understanding of younger and older adults (age ranges 21–35 and 57–74 years, respectively) in a simulated “cocktail-party” scenario. A naturalistic “stock-price monitoring” task was employed in which prices of listed companies were simultaneously recited by four speakers at different locations in space. The participants responded when prices of a target company exceeded specific values, while ignoring all other companies. According to their individual performance levels three subgroups of participants were composed, consisting of 12 high-performing and 12 low-performing older adults, and 12 young adults matching the high-performing older group. The analysis of the event-related brain potentials indicated that all older adults showed delayed attentional control (indicated by a later P2) and reduced speech processing (indicated by a reduced N400), relative to the younger adults. High-performing older adults differed in increased allocation of attention and inhibitory control (indicated by a stronger P2–N2 complex) from their low-performing counterparts. The results are consistent with the idea of an adjustment of mental resources that could help compensating potential deficiencies in peripheral and central auditory processing.

Nitrogen fixation catalyzed by ferrocene-substituted dinitrogen-bridged dimolybdenum-dinitrogen complexes: unique behavior of ferrocene moiety as redox active site.

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing metallocene-substituted PNP-pincer ligands is synthesized by the reduction of the corresponding monomeric molybdenum-trichloride complexes under 1 atm of molecular dinitrogen. Introduction of ferrocene as a redox-active moiety to the pyridine ring of the PNP-pincer ligand increases the catalytic activity for the formation of ammonia from molecular dinitrogen, up to 45 equiv. of ammonia being formed based on the catalyst (22 equiv. of ammonia based on each molybdenum atom of the catalyst). The time profile for the catalytic reaction reveals that the presence of the ferrocene unit in the catalyst increases the rate of ammonia formation. Electrochemical measurement and theoretical studies indicate that an interaction between the Fe atom of the ferrocene moiety and the Mo atom in the catalyst may play an important role to achieve a high catalytic activity.

Mechanism-based design of labile precursors for chromium(I) chemistry.

Dinitrogen complexes of the type TpR,RCr–N2–CrTpR,R are not the most labile precursors for Cr(I) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex – TptBu,MeCr(η2-C2(SiMe3)2) – proved to be much more reactive.

A new millimeter-wave observation of the weakly bound CO–N2 complex

New millimeter-wave transitions of the CO–N2 van der Waals complex have been observed using the intracavity OROTRON jet spectrometer in the frequency range of 103–159GHz. For the less abundant form, CO–paraN2, a total of 37 rotational transitions were assigned to three K=0–0, 0–1, 2–1 subbands connecting the (jCO, jN2)=(1,1) and (jCO, jN2)=(0,1) internal rotor states. The upper K=0 and K=2 “stacks” of rotational levels were probed for the first time here by millimeter-wave spectroscopy following a recent infrared study by Rezaei et al. (2013). The observation of new subbands fixes with higher precision not only these upper K=0 and K=2 but also lower K=1(f) levels, not linked with other stacks in earlier rotational studies. For the more abundant form, CO–orthoN2, five new P-branch rotational transitions of the K=0–0 “CO bending” subband are reported, thus extending previous measurements. Nuclear quadrupole hyperfine structure due to the presence of two equivalent 14N nuclei was partly resolved and analyzed to give additional information about the angular orientation of the N2 molecule in the complex.

Molybdenum Bis(Dinitrogen) Complexes Supported by NP3 Ligands containing Propylene Linkages: Symmetry Breaking and Geometrical Isomerism

AbstractThe syntheses of two new NP3 ligands containing propylene linkages, prNP3 (1) = N(CH2CH2CH2PPh2)3 and prPNetP2 (2) = (Ph2PCH2CH2CH2)N(CH2CH2PPh2)2 are reported. Using these ligands, bis(dinitrogen) complexes of molybdenum are synthesized and characterized by IR and NMR spectroscopy. The new systems are compared with the literature‐known complex [Mo(N2)2(NP3)] (3) supported by the ligand NP3 [NP3 = N(CH2CH2PPh2)3], which only contains ethylene linkages. Application of the new ligand reduces the symmetry of the complex and leads to the occurrence of different isomers.

Molybdenum complexes supported by mixed NHC/phosphine ligands: activation of N2 and reaction with P(OMe)3 to the first meta-phosphite complex.

Molybdenum(0) dinitrogen complexes, supported by the mixed NHC/phosphine pincer ligand PCP, exhibit an extreme activation of the N2 ligand due to a very π-electron-rich metal center. The low thermal stability of these compounds can be increased using phosphites instead of phosphines as coligands. Through an amalgam reduction of [MoCl3(PCP)] in the presence of trimethyl phosphite and N2 the highly activated and room-temperature stable dinitrogen complex [Mo(N2)(PCP)(P(OMe)3)2] is obtained. As a second product, the first transition metal complex containing the meta-phosphite ligand P(O)(OMe) originates from this reaction.

Low‐Temperature N2 Binding to Two‐Coordinate L2Fe0 Enables Reductive Trapping of L2FeN2− and NH3 Generation

AbstractThe two‐coordinate [(CAAC)2Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<−80 °C). The resulting putative three‐coordinate N2 complex, [(CAAC)2Fe(N2)], was trapped by one‐electron reduction to its corresponding anion [(CAAC)2FeN2]− at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β‐nitrogen atom. The redox‐linked complexes [(CAAC)2FeI][BArF4], [(CAAC)2Fe0], and [(CAAC)2Fe−IN2]− were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBArF4⋅2 Et2O at −95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below −78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron‐transfer step to generate reactive [(CAAC)2FeN2]−.

Low-temperature N2 binding to two-coordinate L2Fe(0) enables reductive trapping of L2FeN2(-) and NH3 generation.

The two-coordinate [(CAAC)2Fe] complex [CAAC = cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T<-80 °C). The resulting putative three-coordinate N2 complex, [(CAAC)2Fe(N2)], was trapped by one-electron reduction to its corresponding anion [(CAAC)2FeN2](-) at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β-nitrogen atom. The redox-linked complexes [(CAAC)2Fe(I)][BAr(F)4], [(CAAC)2Fe(0)], and [(CAAC)2Fe(-I)N2](-) were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC8 and HBAr(F)4⋅2 Et2O at -95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N2 reduction activity is highly temperature dependent, with significant N2 reduction to NH3 only occurring below -78 °C. This reactivity profile tracks with the low temperatures needed for N2 binding and an otherwise unavailable electron-transfer step to generate reactive [(CAAC)2FeN2](-) .

Mononuclear iridium dinitrogen complexes bonded to zeolite HY.

The adsorption of N2 on structurally well-defined dealuminated HY zeolite-supported iridium diethylene complexes was investigated. Iridium dinitrogen complexes formed when the sample was exposed to N2 in H2 at 298 K, as shown by infrared spectra recorded with isotopically labeled N2 . Four supported species formed in various flowing gases: Ir(N2 ), Ir(N2 )(N2 ), Ir(C2 H5 )(N2 ), and Ir(H)(N2 ). Their interconversions are summarized in a reaction network, showing, for example, that, in the presence of N2 , Ir(N2 ) was the predominant dinitrogen species at temperatures of 273-373 K. Ir(CO)(N2 ) formed transiently in flowing CO, and in the presence of H2 , rather stable iridium hydride complexes formed. Four structural models of each iridium complex bonded at the acidic sites of the zeolite were employed in a computational investigation, showing that the calculated vibrational frequencies agree well with experiment when full calculations are done at the level of density functional theory, independent of the size of the model of the zeolite.