Dinitrogen Complexes Research Articles

Isolation and characterization of a high-spin mixed-valent iron dinitrogen complex.

We report a rare example of a mixed-valence iron compound with an FeNNFe core, which gives insight into the structural, spectroscopic, and magnetic influences of single-electron reductions and oxidations. In the new compound, the odd electron is localized as judged from Mössbauer spectra at 80 K and infrared spectra at room temperature, and the backbonding into the N2 unit is intermediate between diiron(i) and diiron(0) congeners. Magnetic susceptibility and relaxation studies on the series of FeNNFe compounds show significant magnetic anisotropy, but through-barrier pathways enable fairly rapid magnetic relaxation.

An arene-tethered silylene ligand enabling reversible dinitrogen binding to iron and catalytic silylation.

An arene-tethered silylene ligand, L (L = PhC(tBuN)2SiCH2C(tBu)NAr, Ar = 2,6-iPr2C6H3), allowed the synthesis of three-coordinate Fe(ii) silylamido and piano-stool Fe(0) dinitrogen complexes LFe[N(SiMe3)2]2 (3) and LFe-N2 (4), which not only exhibit interesting bonding but also enabled the catalytic silylation of N2 to yield N(SiMe3)3 under 1 atm of N2 at room temperature with high TONs.

Iron Dinitrogen Complexes Supported by Tris(NHC)borate Ligand: Synthesis, Characterization, and Reactivity Study

Iron Dinitrogen Complexes Supported by Tris(NHC)borate Ligand: Synthesis, Characterization, and Reactivity Study

Fundamental FH-stretching transition frequencies and oscillator strengths in hydrogen bonded FH complexes

Vibrational transition frequencies and oscillator strengths of the FH-stretching transitions in the FH⋯CO, FH⋯FH, FH⋯CO2 and FH⋯N2 complexes were calculated with different vibrational models. The calculated vibrational frequencies were found to agree well with experimental values. The experimental oscillator strengths of the FH-stretches in the FH monomer and FH dimer were also predicted well by the different vibrational models. However, with our best theoretical methods there is still a factor of 2–4 between the experimental and calculated oscillator strengths for the FH-stretches in the FH⋯CO, FH⋯CO2 and FH⋯N2 complexes.

3-Methylindole-Based Tripodal Tetraphosphine Ruthenium Complexes in N2 Coordination and Reduction and Formic Acid Dehydrogenation

The ruthenium(II) complexes RuCl2L1H, RuCl2L1CF3, RuCl2L1OMe and RuCl2L2H were synthesized from [Ru(η6-benzene)Cl(μ-Cl)]2 and the corresponding tripodal tris-3-methylindolephosphine-based ligands L1H, L1CF3, L1OMe, and L2H. Stoichiometric reduction of these complexes with KC8 yielded the corresponding ruthenium(0) dinitrogen complexes. The latter complexes were studied in the N2 reduction with chlorosilanes and KC8, yielding stoichiometric amounts of the silylamines. The synthesized ruthenium(II) complexes are also active catalysts for the formic acid dehydrogenation reaction.

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex.

One‐electron oxidation of the rhodium(I) azido complex [Rh(N3)(PNP)] (5), bearing the neutral, pyridine‐based PNP ligand 2,6‐bis(di‐tert‐butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N2)(PNP)]+ (9 +). Interestingly, complex 5 also acts as a catalyst for electrochemical N3 − oxidation (E p≈−0.23 V vs. Fc+/0) in the presence of excess azide. This is of potential relevance for the design of azide‐based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas.

End-On Bridging Dinitrogen Complex of Scandium.

The first (N═N)2- complex of a rare-earth metal with an end-on dinitrogen bridge, {K(crypt)}2{[(R2N)3Sc]2[μ-η1:η1-N2]} (crypt = 2.2.2-cryptand, R = SiMe3), has been isolated from the reduction of Sc(NR2)3 under dinitrogen at -35 °C and characterized by X-ray crystallography. The structure differs from the characteristic side-on structures previously observed for over 40 crystallographically characterized rare-earth metal (N═N)2- complexes of formula [A2Ln(THF)x]2[μ-η2:η2-N2] (Ln = Sc, Y, and lanthanides; x = 0, 1; A = anionic ligand such as amide, cyclopentadienide, and aryloxide). The 1.221(3) Å N-N distance and the 1644 cm-1 Raman stretch are consistent with the presence of an (N═N)2- bridge. The observed paramagnetism of the complex by Evans method measurements is consistent with DFT calculations that suggest a triplet (3A2) ground state in D3 symmetry involving two degenerate Sc-N2-Sc bonding orbitals. Upon brief exposure of the orange Sc3+ bridging dinitrogen complex to UV-light, photolysis to form the monomeric Sc2+ complex, [K(crypt)][Sc(NR2)3], was observed. Conversion of the Sc2+ complex to the Sc3+ dinitrogen complex was not observed with this crypt system, but it did occur with the 18-crown-6 (crown) analog which formed {K(crown)}2{[(R2N)3Sc]2[μ-η1:η1-N2]}. This suggests the importance of the alkali metal chelating agent in the reversibility of dinitrogen binding in this scandium system.

Mild sp2Carbon-Oxygen Bond Activation by an Isolable Ruthenium(II) Bis(dinitrogen) Complex: Experiment and Theory.

The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)2(N2)2(PCy3)2] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp2C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp2C–H bond activation is shown to be more facile than sp2C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.

Group 6 Transition‐Metal/Boron Frustrated Lewis Pair Templates Activate N2and Allow its Facile Borylation and Silylation

AbstractThe reaction oftrans‐[M(N2)2(dppe)2] (M=Mo,1Mo, M=W,1W) with B(C6F5)3(2) provides the adducts [(dppe)2M=N=N‐B(C6F5)3] (3) which can be regarded as M/B transition‐metal frustrated Lewis pair (TMFLP) templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes3with a hydroborane and hydrosilane occur by splitting of the B−H and Si−H bonds between the N2moiety and the perfluoroaryl borane. This reactivity of3is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.

Group 6 Transition-Metal/Boron Frustrated Lewis Pair Templates Activate N2 and Allow its Facile Borylation and Silylation.

The reaction of trans-[M(N2 )2 (dppe)2 ] (M=Mo, 1Mo , M=W, 1W ) with B(C6 F5 )3 (2) provides the adducts [(dppe)2 M=N=N-B(C6 F5 )3 ] (3) which can be regarded as M/B transition-metal frustrated Lewis pair (TMFLP) templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B-H and Si-H bonds between the N2 moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.

Ir-Catalyzed Enantioselective, Intramolecular Silylation of Methyl C-H Bonds.

We report highly enantioselective intramolecular, silylations of unactivated, primary C(sp3)-H bonds. The reactions form dihydrobenzosiloles in high yields with excellent enantioselectivities by functionalization of enantiotopic methyl groups under mild conditions. The reaction is catalyzed by an iridium complex generated from [Ir(COD)OMe]2 and chiral dinitrogen ligands that we recently disclosed. The C-Si bonds in the enantioenriched dihydrobenzosiloles were further transformed to C-Cl, C-Br, C-I, and C-O bonds in final products. The potential of this reaction was illustrated by sequential C(sp3)-H and C(sp2)-H silylations and functionalizations, as well as diastereoselective C-H silylations of a chiral, natural-product derivative containing multiple types of C-H bonds. Preliminary mechanistic studies suggest that C-H cleavage is the rate-determining step.

Catalytic Nitrogen Fixation via Direct Cleavage of Nitrogen–Nitrogen Triple Bond of Molecular Dinitrogen under Ambient Reaction Conditions

Abstract We have now found that molybdenum–iodide complexes bearing a PNP-pincer ligand have a higher catalytic activity than the so far reported molybdenum–dinitrogen complexes for ammonia formation from nitrogen gas under ambient reaction conditions, up to 830 equiv being produced based on a dinitrogen-bridged dimolybdenum complex (415 equiv of ammonia based on the molybdenum atom). This remarkable catalytic activity is induced by a novel reaction pathway, where the generation of a dinitrogen-bridged dimolybdenum–iodide complex is a key point to promote direct cleavage of the nitrogen–nitrogen triple bond of the bridging dinitrogen ligand in the Mo–N≡N–Mo core.

Synthesis and Reactivity of Iron– and Cobalt–Dinitrogen Complexes Bearing PSiP‐Type Pincer Ligands toward Nitrogen Fixation

AbstractInvited for the cover of this issue is the Nishibayashi group, University of Tokyo, collaborating with the Iwasawa group, Tokyo Institute of Technology, Japan. The cover image shows the transformation of molecular dinitrogen into silylamine as ammonia equivalent, catalyzed by Fe– and Co–dinitrogen complexes bearing PSiP‐type pincer ligands.

Synthesis of Iron Hydride Complexes Relevant to Hydrogen Isotope Exchange in Pharmaceuticals

Addition of H2 gas to the bis(arylimidazolin-2-ylidene)pyridine iron bis(dinitrogen) complex (H4-iPrCNC)Fe(N2)2 resulted in facile oxidative addition of the H–H bond to yield a mixture of (H4-iPrCNC)FeH4 and trans-(H4-iPrCNC)FeH2(N2), depending on the partial pressures of H2 and N2. Both iron hydride complexes were characterized by X-ray diffraction and proved relevant to the catalytic hydrogen isotope exchange of arene C(sp2)–H bonds. Activation of the benzene-d6 solvent at ambient temperature produced deuterated isotologues of both compounds that exhibit large isotopic perturbation of resonances in the hydride signals.

Synthesis and Reactivity of Iron– and Cobalt–Dinitrogen Complexes Bearing PSiP‐Type Pincer Ligands toward Nitrogen Fixation

Iron– and cobalt–dinitrogen complexes bearing PSiP‐type pincer ligands are newly designed and prepared, on the basis of our previous proposal that iron– and cobalt–dinitrogen complexes bearing trimethylsilyl ligands are key reactive intermediates in the catalytic formation of silylamine from nitrogen gas under ambient reaction conditions. These iron– and cobalt–dinitrogen complexes are found to work as effective catalysts toward the catalytic transformation of nitrogen gas into silylamine.

Nitrogen-Carbon Bond Formation by Reactions of a Titanium-Potassium Dinitrogen Complex with Carbon Dioxide, tert-Butyl Isocyanate, and Phenylallene.

Nitrogen-carbon bond-forming reactions at coordinated dinitrogen in a bifunctional titanium-potassium system are reported. A titanium atrane complex with a tris(aryloxide)methyl ligand (1) was treated with two equivalents of potassium naphthalenide under N2 atmosphere to generate a bifunctional complex (2) in which N2 binds end-on to two titanium centers and side-on to three potassium cations. Dinitrogen complex 2 reacted with carbon dioxide, tert-butyl isocyanate, and phenylallene, forming nitrogen-carbon bonds and affording diverse N-functionalized products. The reaction of 2 with CO2 followed by addition of Me3 SiCl resulted in the formation of the starting complex 1 with concomitant release of silylated carboxyl hydrazines while the reaction with two equivalents of tert-butyl isocyanate proceeded by insertion into the Ti-N bonds. Treatment of 2 with phenylallene afforded vinyl-substituted hydrazido complexes.

Nitrogen–Carbon Bond Formation by Reactions of a Titanium–Potassium Dinitrogen Complex with Carbon Dioxide, tert‐Butyl Isocyanate, and Phenylallene

AbstractNitrogen–carbon bond‐forming reactions at coordinated dinitrogen in a bifunctional titanium–potassium system are reported. A titanium atrane complex with a tris(aryloxide)methyl ligand (1) was treated with two equivalents of potassium naphthalenide under N2 atmosphere to generate a bifunctional complex (2) in which N2 binds end‐on to two titanium centers and side‐on to three potassium cations. Dinitrogen complex 2 reacted with carbon dioxide, tert‐butyl isocyanate, and phenylallene, forming nitrogen–carbon bonds and affording diverse N‐functionalized products. The reaction of 2 with CO2 followed by addition of Me3SiCl resulted in the formation of the starting complex 1 with concomitant release of silylated carboxyl hydrazines while the reaction with two equivalents of tert‐butyl isocyanate proceeded by insertion into the Ti−N bonds. Treatment of 2 with phenylallene afforded vinyl‐substituted hydrazido complexes.

Enantioselective Borylation of Aromatic C-H Bonds with Chiral Dinitrogen Ligands.

The borylation of C-H bonds catalyzed by transition metals has been investigated extensively in the past two decades, but no iridium-catalyzed enantioselective borylation of C-H bonds has been reported. We report a set of iridium-catalyzed enantioselective borylations of aromatic C-H bonds. This reaction relies on a set of newly developed chiral quinolyl oxazoline ligands. This process proceeds under mild conditions with good to excellent enantioselectivity, and the borylated products can be converted to enantioenriched derivatives containing new C-O, C-C, C-Cl, or C-Br bonds.

Enantioselective Borylation of Aromatic C−H Bonds with Chiral Dinitrogen Ligands

AbstractThe borylation of C−H bonds catalyzed by transition metals has been investigated extensively in the past two decades, but no iridium‐catalyzed enantioselective borylation of C−H bonds has been reported. We report a set of iridium‐catalyzed enantioselective borylations of aromatic C−H bonds. This reaction relies on a set of newly developed chiral quinolyl oxazoline ligands. This process proceeds under mild conditions with good to excellent enantioselectivity, and the borylated products can be converted to enantioenriched derivatives containing new C−O, C−C, C−Cl, or C−Br bonds.

Dinitrogen Splitting Coupled to Protonation

AbstractThe coupling of electron‐ and proton‐transfer steps provides a general concept to control the driving force of redox reactions. N2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N−N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo−N2−Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N−N bond cleavage is an unusual case of a proton‐coupled metal‐to‐ligand charge transfer process, highlighting the use of proton‐responsive ligands for nitrogen fixation.