Dinitrogen Unit Research Articles

Coordination and Activation of N2 at Low-Valent Magnesium using a Heterobimetallic Approach: Synthesis and Reactivity of a Masked Dimagnesium Diradical.

The activation of dinitrogen (N2) by transition metals is central to the highly energy intensive, heterogeneous Haber-Bosch process. Considerable progress has been made towards more sustainable homogeneous activations of N2 with d- and f-block metals, though little success has been had with main group metals. Here we report that the reduction of a bulky magnesium(II) amide [(TCHPNON)Mg] (TCHPNON = 4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5% w/w K/KI yields the magnesium-N2 complex [{K(TCHPNON)Mg}2(μ-N2)]. DFT calculations and experimental data show that the dinitrogen unit in the complex has been reduced to the N22- dianion, via a transient anionic magnesium(I) radical. The compound readily reductively activates CO, H2 and C2H4, in reactions in which it acts as a masked dimagnesium(I) diradical.

High-Pressure Synthesis of Metal-Inorganic Frameworks Hf4 N20 ⋅N2 , WN8 ⋅N2 , and Os5 N28 ⋅3 N2 with Polymeric Nitrogen Linkers.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN8, and Os5N28) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N2 also leads to a non‐centrosymmetric polynitride Hf2N11 that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]∞.

High‐Pressure Synthesis of Metal–Inorganic Frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 with Polymeric Nitrogen Linkers

AbstractPolynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN8, and Os5N28) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N2 also leads to a non‐centrosymmetric polynitride Hf2N11 that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]∞.

Fe-N system at high pressure reveals a compound featuring polymeric nitrogen chains

Poly-nitrogen compounds have been considered as potential high energy density materials for a long time due to the large number of energetic N–N or N=N bonds. In most cases high nitrogen content and stability at ambient conditions are mutually exclusive, thereby making the synthesis of such materials challenging. One way to stabilize such compounds is the application of high pressure. Here, through a direct reaction between Fe and N2 in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds Fe3N2, FeN2 and FeN4. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. Fe3N2, synthesized at 50 GPa, is isostructural to chromium carbide Cr3C2. FeN2 has a marcasite structure type and features covalently bonded dinitrogen units in its crystal structure. FeN4, synthesized at 106 GPa, features polymeric nitrogen chains of [N42−]n units. Based on results of structural studies and theoretical analysis, [N42−]n units in this compound reveal catena-poly[tetraz-1-ene-1,4-diyl] anions.

Structure, mechanical, electronic, and thermodynamic properties of marcasite‐structure TMN2 (TM = Ru, Rh, Os, and Ir) under high pressure: A first‐principles study

The structural, mechanical, and electronic properties of four orthorhombic noble‐metal nitrides TMN2 (TM = Ru, Rh, Os, and Ir) (space group of Pnnm, No: 58) under 100 GPa were investigated through the first‐principles calculation using the generalized gradient approximation within the plane‐wave pseudopotential density‐functional theory. The obtained equilibrium structures are in excellent agreement with other experimental and theoretical results. The calculated formation energy indicates that the nitrides are thermodynamically metastable but mechanically stable at zero pressure. The calculated B/G ratio indicated RhN2 possesses a ductile nature at 0 GPa, while RuN2, OsN2, and IrN2 are prone to brittleness under pressures of around 20.8, 20.0, and 4.3 GPa, respectively. Then, we calculated the partial and total densities of states. The results show that these four noble‐metal nitrides are metallic and the order of metallicity from high to low is: RhN2 > IrN2 > RuN2 > OsN2. Through the charge‐density distributions, we find that strong covalency is present in the interstitial dinitrogen units for the four orthorhombic compounds. The calculated hardness of orthorhombic RuN2, RhN2, OsN2, and IrN2 are 13.66, 11.54, 16.33, and 17.92 GPa, respectively. Through the quasiharmonic Debye model, we also investigated the thermodynamic properties of these four compounds.

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](-) .

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&lt;−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]−.

High Pressure Synthesis of Marcasite-Type Rhodium Pernitride

Marcasite-type rhodium nitride was successfully synthesized in a direct chemical reaction between a rhodium metal and molecular nitrogen at 43.2 GPa using a laser-heated diamond-anvil cell. This material shows a low zero-pressure bulk modulus of K0 = 235(13) GPa, which is much lower than those of other platinum group nitrides. This finding is due to the weaker bonding interaction between metal atoms and quasi-molecular dinitrogen units in the marcasite-type structure, as proposed by theoretical studies.

Synthesis of Alkaline Earth Diazenides MAEN2(MAE= Ca, Sr, Ba) by Controlled Thermal Decomposition of Azides under High Pressure

The alkaline earth diazenides M(AE)N(2) with M(AE) = Ca, Sr and Ba were synthesized by a novel synthetic approach, namely, a controlled decomposition of the corresponding azides in a multianvil press at high-pressure/high-temperature conditions. The crystal structure of hitherto unknown calcium diazenide (space group I4/mmm (no. 139), a = 3.5747(6) Å, c = 5.9844(9) Å, Z = 2, wR(p) = 0.078) was solved and refined on the basis of powder X-ray diffraction data as well as that of SrN(2) and BaN(2). Accordingly, CaN(2) is isotypic with SrN(2) (space group I4/mmm (no. 139), a = 3.8054(2) Å, c = 6.8961(4) Å, Z = 2, wR(p) = 0.057) and the corresponding alkaline earth acetylenides (M(AE)C(2)) crystallizing in a tetragonally distorted NaCl structure type. In accordance with literature data, BaN(2) adopts a more distorted structure in space group C2/c (no. 15) with a = 7.1608(4) Å, b = 4.3776(3) Å, c = 7.2188(4) Å, β = 104.9679(33)°, Z = 4 and wR(p) = 0.049). The N-N bond lengths of 1.202(4) Å in CaN(2) (SrN(2) 1.239(4) Å, BaN(2) 1.23(2) Å) correspond well with a double-bonded dinitrogen unit confirming a diazenide ion [N(2)](2-). Temperature-dependent in situ powder X-ray diffractometry of the three alkaline earth diazenides resulted in formation of the corresponding subnitrides M(AE(2))N (M(AE) = Ca, Sr, Ba) at higher temperatures. FTIR spectroscopy revealed a band at about 1380 cm(-1) assigned to the N-N stretching vibration of the diazenide unit. Electronic structure calculations support the metallic character of alkaline earth diazenides.

Half-Sandwich (η6-Arene)iron(II) Dinitrogen Complexes Bearing a Disilaferracycle Skeleton as a Precursor for Double Silylation of Ethylene and Alkynes

Reaction of 2 equiv of 1,2-bis(dimethylsilyl)benzene with [Fe(mesityl)2]2 in aromatic solvent under a nitrogen atmosphere afforded the dinuclear bis(silyl)iron complexes [(η6-arene)Fe(Me2SiC6H4SiMe2)]2(μ-η1:η1-N2) (1), which have an end-on-bound bridging dinitrogen ligand. The dinitrogen unit easily dissociated from the Fe center to generate the mononuclear (η6-arene)FeII(Si)2L or (η6-arene)FeIV(Si)2H2 complexes by the reactions with CO, PPh3, and H2.

Activation and cleavage of the N–N bond in side-on bound [L2M-NN-ML2] (L = NH2, NMe2, NiPr2, C5H5, C5Me4H) dinitrogen complexes of transition metals from groups 4 through 9

The activation and cleavage of the N-N bond in side-on bound [L₂M-NN-ML₂] (L = NH₂, NMe₂, N(i)Pr₂, C₅H₅, C₅Me₄H) dinitrogen complexes of transition metals in groups 4 through 9 have been investigated using density functional theory. Emphasis has been placed on Ti, Zr, and Hf (group 4) complexes due to their experimental relevance. Calculations on these species have shown that for cases when the structural configuration corresponds to the terminal [ML₂] fragments adopting a perpendicular orientation with respect to the central [N-N] unit, a considerably higher degree of N-N activation is predicted relative to that observed in the experimentally characterized cyclopentadienyl analogues and in related systems involving end-on dinitrogen coordination. An examination of the orbital interactions between the metal-based fragments and the dinitrogen unit shows that both σ and π bonding are important in the side-on binding mode, in contrast to the end-on mode where metal-nitrogen π interactions are dominant. This analysis also reveals that the model amide systems possess the orbital properties identified as necessary for successful N-N hydrogenation. A significant result obtained for the amide complexes containing metals from groups 5 (V, Nb, Ta), 6 (Cr, Mo, W), and 7 (Mn, Tc, Re), is the presence of metal-metal bonding in configurations that are considerably distorted from planarity. As a consequence, these complexes exhibit strongly enhanced stability relative to species where metal-metal bonding is absent. In contrast, the d² metal-based configurations in the group 4 complexes of Ti, Zr, and Hf are unable to provide the six electrons required for complete reductive cleavage of the dinitrogen unit which is necessary to allow the metal centres to approach one another sufficiently for metal-metal bond formation.

The hydride route to the preparation of dinitrogen complexes

That the inert dinitrogen molecule can act as a ligand to a transition metal complex is one of the key discoveries in inorganic chemistry of the past century. This feature article summarises a body of work up to 2010 that describes a particularly attractive route to dinitrogen complexes that involves the direct reaction of N(2) with metal hydride derivatives. This process is shown to be general across the transition series and, depending on the metal, different levels of activation of the coordinated dinitrogen unit are observed.

First-principles investigation of the electron-phonon interaction inOsN2: Theoretical prediction of superconductivity mediated by N-N covalent bonds

A first-principles investigation of the electron-phonon interaction in the recently synthesized osmium dinitride $(\mathrm{Os}{\mathrm{N}}_{2})$ compound predicts that the material is a superconductor. Superconductivity in $\mathrm{Os}{\mathrm{N}}_{2}$ would originate from the stretching of covalently bonded dinitrogen units embedded in the transition-metal matrix, thus adding dinitrides to the class of superconductors containing covalently bonded light elements. The dinitrogen vibrations are strongly coupled to the electronic states at the Fermi level and generate narrow peaks in the Eliashberg spectral function ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$. The total electron-phonon coupling of $\mathrm{Os}{\mathrm{N}}_{2}$ is $\ensuremath{\lambda}=0.37$ and the estimated superconducting temperature ${T}_{c}\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We suggest that the superconducting temperature can be substantially increased by hole doping of the pristine compound and show that ${T}_{c}$ increases to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a doping concentration of 0.25 holes/$\mathrm{Os}{\mathrm{N}}_{2}$ unit.

Marcasite vs. arsenopyrite structural choice in MN2 (M = Ir, Os and Rh) transition metal nitrides

A first-principles density functional theory study of the recently reported IrN2 and OsN2 as well as the as yet not synthesized RhN2 transition metal nitrides is presented. It is found that although IrN2 has a slight preference for the arsenopyrite structure, OsN2 prefers the marcasite structure and RhN2 has no clear preference between these two structures. The electronic structure of these nitrides is discussed on the basis of a simple model for the octahedral MN4 chains present in both the marcasite and arsenopyrite structures. It is shown that the distortion from the marcasite to arsenopyrite structure in these nitrides is not a Peierls distortion, contrary to what is often claimed. Both metal–metal interactions along the MN4 chains and interchain interactions through the dinitrogen units compete in these compounds. IrN2 is found to be a semiconductor whereas OsN2 and RuN2 should be three-dimensional metals. It is predicted that RhN2 can exist in two metallic forms with the marcasite and arsenopyrite structures, which should exhibit different anisotropies.

Dinitrogen Activation, Partial Reduction, and Formation of Coordinated Imide Promoted by a Chromium Diiminepyridine Complex

Reduction of {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrCl (3) with NaH afforded the dinuclear dinitrogen complex {[{2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Cr(THF)]2(mu-N2)}.THF (5). Reaction carried in exclusion of dinitrogen afforded instead deprotonation of the ligand with the formation of {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(THF) (4). Further reduction of 5 with NaH yielded a curious dinuclear compound formulated as [{2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Cr(THF)][{2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(THF)](mu-N2 H)(mu-Na)2 (6) containing two sodium atoms only bound to the dinitrogen unit and the pi systems of the two diiminepyridine ligands. Subsequent reduction with NaH triggered a complex series of events, leading to the formation of a species formulated as {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(mu-NH)][Na(THF)] (7) on the basis of crystallographic, spectroscopic, isotopic labeling, and chemical degradation experiments.

Carbon-nitrogen bond formation via the reaction of terminal alkynes with a dinuclear side-on dinitrogen complex.

The dinuclear dinitrogen complex ([P2N2]Zr)2(mu-eta2:eta2-N2) reacts with terminal aryl alkynes to generate a new species in which the dinitrogen unit has been functionalized. The products formed have the general formula ([P2N2]Zr)2(mu-eta2:eta2-N2CCAr)(mu-CCAr) and display a styryl-hydrazido unit bridging the two Zr centers along with a bridging arylalkynide. The crystal structures of three of these products are reported. A mechanism is proposed for this process that involves cycloaddition of the alkyne to the side-on dinitrogen unit followed by protonation of the Zr-C bond by a second equivalent of terminal alkyne. A fluxional process is operative in solution that equilibrates the phosphorus nuclei at high temperature; in the slow exchange limit, the two [P2N2]Zr ends of complex are inequivalent as evidenced by four resonances in the 31P NMR spectrum for the inequivalent phosphorus donors. This C-N bond-forming reaction is unique in that an activated dinitrogen fragment undergoes a reaction with an alkyne.

Serendipitous Isolation of the First Example of a Mixed-Valence Samarium Tripyrrole Complex

The transamination reaction of Sm{N(SiMe3)2}2(THF)2 with two dipyrrole ligands of formula RR‘C(α-C4H3NH)2 (R = R‘ = Et; R = Me, R‘ = Ph) has been examined. The reactions in THF and under N2 gave two similar tetranuclear dinitrogen complexes, {[Et2C(α-C4H3N)2Sm}4(THF)2](μ-N2)·2THF and [{[(C6H5)(CH3)C(α-C4H3N)2]Sm}4(DME)2](μ-N2), where the dinitrogen unit has undergone a four-electron reduction via cooperative attack of four divalent samarium atoms and remained coordinated both side-on and end-on between the four coplanar metal centers. The same reaction carried out with diethyldipyrrolylmethane under an argon atmosphere afforded the divalent samarium macrocyclic cluster {[Et2C(α-C4H3N)2]Sm}8(THF)4·4THF. In the case of the reaction with the methylphenyldipyrrolylmethane ligand under N2, the unprecedented hexanuclear mixed-valence SmII/SmIII cluster [([(C6H5)(CH3)C(α-C4H3N)2][(C6H5)(CH3)C(α-C4H3N)(β-C4H3N)]{[(C6H5)(CH3)C]2(α-C4H3N)2(α,α‘-C4H2N)}Sm3)(THF)3]2, containing tripyrrolide, “N-confused”, and regular dipyrrolide ligands was serendipitously formed by reaction with the impurities contained in the nonpurified ligand.

Characterization of the Reaction Products of Laser-Ablated Late Lanthanide Metal Atoms with Dinitrogen. Matrix IR Spectra of LnN, (LnN)2, and Ln(NN)x Molecules

This paper comprises the final section of a two-part study of the small nitride molecules and simple dinitrogen complexes of the lanthanide metals, except promethium. Simple nitrides of the general formulas LnN and (LnN)2 have been identified for Ln = Tb, Dy, Ho, Er, Tm, Yb, and Lu. Further elucidation of the Ln(N2) and Ln(NN)2 complexes is presented. The evidence supports the anticipated conclusion that the later lanthanide metals are less prone to dinitrogen complexation and that the nitrides of the later lanthanide metals require fewer dinitrogen units to achieve saturation.

Synthesis and Structure of a Zirconium Dinitrogen Complex with a Side-On Bridging N(2) Unit.

Reduction of Zr(O-2,6-Me(2)C(6)H(3))Cl(2)[N(SiMe(2)CH(2)PPr(i)(2))(2)] with sodium amalgam under dinitrogen yields the dinuclear zirconium dinitrogen complex {[(Pr(i)(2)PCH(2)SiMe(2))(2)N]Zr(O-2,6-Me(2)C(6)H(3))}(2)(&mgr;-eta(2):eta(2)-N(2)). Solid state structural analysis shows that the dinitrogen unit is bound in a side-on mode of coordination with the N-N bond distance at 1.528(7) Å; resonance Raman spectra show a band at 751 cm(-)(1) for nu(N-N), which is consistent with this very long bond. In addition, the N(2) ligand is hinged slightly so that the Zr(2)(&mgr;-eta(2):eta(2)-N(2)) core adopts a flattened butterfly shape rather than a completely planar core as found in other related systems. Other Zr(IV) precursors of the general formula ZrCl(2)X[N(SiMe(2)CH(2)PPr(i)(2))(2)] (X = OBu(t), OCHPh(2), NPh(2)) either decompose upon reduction under N(2) or produce mixtures of products.

Dinitrogen Bridged Gold Clusters

A family of dinitrogen complexes, [(LAu) 6 (N 2 )] 2+ (L = a phosphine), with the dinitrogen unit bridging two clusters of three gold atoms have been prepared from hydrazine and [(LAu) 3 (O)] + . Structural characterization of the PPh 2 i Pr (Ph, phenyl; i Pr, isopropyl) derivative shows a nitrogen-nitrogen single-bond distance indicative of hydrazido (N 2 4− ) character for the dinitrogen unit. The complexes can be reduced and protonated to give low [13% when L = PPh 2 Me (Me, methyl)] to quantitative [100% when L = P( p -MeOC 6 H 5 ) 3 ] yields of ammonia, establishing that bonding of dinitrogen to six metal atoms can lead to facile cleavage of the nitrogen-nitrogen bond.