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.     

A single-bonded cationic terminal borylene complex

The cationic terminal borylene complex [(eta5-C5H5)(CO)2FeB(eta5-C5Me5)][AlCl4] has been isolated from the reaction of [(eta5-C5H5)(CO)2FeB(Cl)(eta1-C5Me5)] with AlCl3 and on the basis of X-ray crystallographic data, spectroscopic data and a DFT calculation it is concluded that the B-->Fe bond order is one.     

Revisiting mechanistic studies on dinitrogen reduction to ammonia by an iron dinitrogen complex as nitrogenase mimic

Conversion of free nitrogen to ammonia is a required chemical reaction for both biologically and industrially but their mechanism, specifically the attachment of electron and proton transfer during the cycle, is still doubtful. In this view, a thorough knowledge of the mechanism is crucial. In this article, we employ a density functional method on [(TPB)FeN₂]⁻, the iron-dinitrogen complex carrying the tris(phosphine)borone (TPB) ligand, for the ammonia production with the inclusion of electrons and protons. The electronic structures, reactivity, and mechanistic possibilities have been extensively explored using the B3LYP functional. Both asymmetric and symmetric pathways in addition to the possible intermediates species and transition states are considered here. Our results conclude tremendously small energy barrier of 3.5 kJ/mol for the first protonation (S = 1/2) for the N─H bond activation by the [(TPB)FeN₂]⁻ species. However, high activation barrier for the third protonation was estimated to be 78.5 kJ/mol, which is explained by the high energy of the unoccupied δ_x²_-y² orbital in ¹ts4 species. The computed spectroscopic parameters such as absorption, electron paramagnetic resonance, and Mössbauer also established the electronic structure details of the species. The calculated parameters are compatible with the experimental results.     

A new tantalum dinitrogen complex and a parahydrogen-induced polarization study of its reaction with hydrogen

Reduction of Cp*(2)TaCl(2) with sodium amalgam in THF under a nitrogen atmosphere results in the formation of the novel complex (Cp*(2)TaCl)(2)(micro-N(2)). This dinuclear complex containing a micro-eta(1):eta(1) dinitrogen bridge has been characterized by NMR and X-ray crystallography. The complex possesses a C(2)-symmetric structure with each Ta bound to diastereotopic Cp* rings and chloride in addition to the micro-N(2) bridge. The Ta-N and N-N distances of 1.885(10) and 1.23(1) A, respectively, suggest modest reduction of the dinitrogen moiety. The two Cp* resonances on each Ta center remain inequivalent in solution, even up to 80 degrees C. Addition of hydrogen results in the formation of two isomers of the dihydride complex Cp*(2)TaH(2)Cl. Under parahydrogen, polarized resonances are observed for the unsymmetrical isomer with adjacent hydrides as the product of H(2) oxidative addition. The symmetric isomer of Cp*(2)TaH(2)Cl also forms, most likely by isomerization of the unsymmetrical kinetic isomer. The reactivity of (Cp*(2)TaCl)(2)(micro-N(2)) was compared to that of the related monomer, Cp*(2)TaCl(THF). The THF adduct yields the same hydrogen addition products, but the reaction is much more facile than for the nitrogen dimer, indicative of the structural integrity of the micro-N(2) complex.     

Nitride formation by thermolysis of a kinetically stable niobium dinitrogen complex

The reduction of [P(2)N(2)]NbCl (where [P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) with KC(8) under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P(2)N(2)]Nb)(2)(mu-N(2)) (2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d(1)) centers. A density functional theory calculation on the model complex [(PH(3))(2)(NH(2))(2)Nb](2)(mu-N(2)) was performed. Thermolysis of ([P(2)N(2)]Nb)(2)(mu-N(2)) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P(2)N(2)]Nb(mu-N)Nb[PN(3)] (3) (where [PN(3)] = PhPMe(CHSiMe(2)NSiMe(2)CH(2)P(Ph)CH(2)SiMe(2)NSiMe(2)N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P(2)N(2)]Nb)(2)(mu-N(2)) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic eta(2)-alkyne complex, [P(2)N(2)]Nb(eta(2)-HCCPh) (4).     

Shining light on dinitrogen cleavage: structural features, redox chemistry, and photochemistry of the key intermediate bridging dinitrogen complex

The key intermediate in dinitrogen cleavage by Mo(N[t-Bu]Ar)3, 1 (Ar = 3,5-C6H3Me2), has been characterized by a pair of single crystal X-ray structures. For the first time, the X-ray crystal structure of (mu-N2)[Mo(N[t-Bu]Ar)3]2, 2, and the product of homolytic fragmentation of the NN bond, NMo(N[t-Bu]Ar)3, are reported. The structural features of 2 are compared with previously reported EXAFS data. Moreover, contrasts are drawn between theoretical predictions concerning the structural and magnetic properties of 2 and those reported herein. In particular, it is shown that 2 exists as a triplet (S = 1) at 20 degrees C. Further insight into the bonding across the MoNNMo core of the molecule is obtained by the synthesis and structural characterization of the one- and two-electron oxidized congeners, (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4], 2[B(Ar(F))4] (Ar(F) = 3,5-C6H3(CF3)2) and (mu-N2)[Mo(N[t-Bu]Ar)3]2[B(Ar(F))4]2, 2[B(Ar(F))4]2, respectively. Bonding in these three molecules is discussed in view of X-ray crystallography, Raman spectroscopy, electronic absorption spectroscopy, and density functional theory. Combining X-ray crystallography data with Raman spectroscopy studies allows the NN bond polarization energy and NN internuclear distance to be correlated in three states of charge across the MoNNMo core. For 2[B(Ar(F))4], bonding is symmetric about the mu-N2 ligand and the NN polarization is Raman active; therefore, 2[B(Ar(F))4] meets the criteria of a Robin-Day class III mixed-valent compound. The redox couples that interrelate 2, 2(+), and 2(2+) are studied by cyclic voltammetry and spectroelectrochemistry. Insights into the electronic structure of 2 led to the discovery of a photochemical reaction that forms NMo(N[t-Bu]Ar)3 and Mo(N[t-Bu]Ar)3 through competing NN bond cleavage and N2 extrusion reaction pathways. The primary quantum yield was determined to be Phi(p) = 0.05, and transient absorption experiments show that the photochemical reaction is complete in less than 10 ns.     

Eta2-organoazide complexes of nickel and their conversion to terminal imido complexes via dinitrogen extrusion

1-Adamantyl- and mesitylazide react with [(dtbpe)Ni]2(eta2-mu-C6H6) to give the eta2 organic azide adducts (dtbpe)Ni(eta2-N3R) (R = Ad, 3a; Mes, 3b) that have been isolated in good yields and crystallographically characterized. These azide adducts are intermediates in the formation of the corresponding terminal imido complexes (dtbpe)NiNR (R = Ad, 4a; Mes, 4b), undergoing intramolecular loss of dinitrogen upon mild thermolysis.     

The complex cyanides of chromium(II) and chromium(0)

Summary Chromium(II) cyanide dihydrate has been isolated from the reaction between aqueous chromium(II) and KCN; it can be dehydrated at 100°in vacuo. From the red solutions containing an excess of cyanide, crystalline hexacyanochromates(II) M₄[Cr(CN)₆] (M=Na or K) have been isolated: these are dihydrates at room temperature, but become anhydrous when dried at 100°in vacuo. In liquid ammonia, the hexacyanochromates(II) are reduced to the chromium(O) compounds M₆Cr(CN)₆ by the respective alkali metals. All of these compounds are extremely air-sensitive and are low spin; i.r. and reflectance spectra are reported as well as x-ray powder data for Na₄[Cr(CN)₆], K₄[Cr(CN)₆] and K₆Cr(CN)₆.     

Reversible binding and reduction of dinitrogen by a uranium(III) pentalene complex

The U(III) mixed-sandwich compound [U(eta-Cp)(eta-C(8)H(4)[Si(i)Pr(3)-1,4](2))] 1 may be prepared by sequential reaction of UI(3) with KCp followed by K(2)[C(8)H(4)[Si(i)Pr(3)-1,4](2)], and has been crystallographically characterized. 1 reacts reversibly with dinitrogen to afford dimeric [[U(eta-Cp)(eta-C(8)H(4)[Si(i)Pr(3)-1,4](2))](2)(mu-eta(2):eta(2)-N(2))] 2, whose X-ray crystal structure reveals a sideways-bound, bridging diazenido (N(2)(2-)) ligand.     

Diazene dehydrogenation follows H2 addition to coordinated dinitrogen in an ansa-zirconocene complex

An activated side-on-bound ansa-zirconocene dinitrogen complex, [Me2Si(eta5-C5Me4)(eta5-C5H3-3-tBu)Zr]2(mu2,eta2,eta2-N2), has been prepared by sodium amalgam reduction of the corresponding dichloride precursor under an atmosphere of N2. Both solution spectroscopic and X-ray diffraction data establish diastereoselective formation of the syn homochiral dizirconium dimer. Addition of 1 atm of H2 resulted in rapid hydrogenation of the N2 ligand to yield one diastereomer of the hydrido zirconocene diazenido complex. Kinetic measurements have yielded the barrier for H2 addition and in combination with isotopic labeling studies are consistent with a 1,2-addition pathway. In the absence of H2, the hydrido zirconocene diazenido product undergoes swift diazene dehydrogenation to yield an unusual hydrido zirconocene dinitrogen complex. The N=N bond length of 1.253(5) A determined by X-ray crystallography indicates that the side-on-bound N2 ligand is best described as a two-electron reduced [N2]2- fragment. Comparing the barrier for deuterium exchange with [Me2Si(eta5-C5Me4)(eta5-C5H3-3-tBu)ZrH]2(mu2,eta2,eta2-N2H2) to diazene dehydrogenation is consistent with rapid 1,2-elimination of dihydrogen followed by rate-determining hydride migration to the zirconium. This mechanistic proposal is also corroborated by H2 inhibition and the observation of a normal, primary kinetic isotope effect for dehydrogenation.     

Catalytic oxidative cleavage of terminal olefins by chromium(III) stearate

A new synthetic methodology for the preparation of carbonyl compounds from the oxidative cleavage of terminal olefins has been developed. With the use of TBHP in combination with chromium(III) stearate, selective oxidation of double bonds conjugated with aromatic ring or carbonyl group could be achieved at ambient temperature in moderate to excellent yield. The oxidative cleavage of electron rich -methylstyrene derivatives proceeded in good to excellent yield whereas lower yields were observed in -methylstyrene derivatives containing an electron withdrawing group. This developed oxidation reaction was believed to undergo via free radical process and high valent chromium oxo species.     

A terminal palladium fluoride complex supported by an anionic PNP pincer ligand

A terminal palladium (II) fluoride complex (^FPNP)PdF (where ^FPNP is a an anionic fluoro-substituted diarylamido/bis(phosphine) pincer ligand) has been prepared and characterized spectroscopically and structurally. An X-ray diffraction study revealed an approximately square-planar environment about Pd and a short Pd-F bond distance. (^FPNP)PdF reacted with silanes containing electron-withdrawing groups on Si by exchange of fluoride with one of the substituents on Si. An analysis of the ¹⁹F chemical shifts of both the Pd-bound fluoride and of the fluorines on the backbone of the ^FPNP ligand is provided.     

Synthesis and crystal structure of a dinitrogen ruthenium(II) macrocyclic tertiary amine complex

Treatment of trans-[Ru^III(16-TMC)Cl₂]Cl (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) with zinc metal in aqueous solution under nitrogen resulted in the formation of trans-[Ru^II(16-TMC)(N₂)Cl]PF₆ (1) which has been characterized by X-ray crystal analysis. The Ru---N₂ and N---N bond distances in 1 are 1.921(6) and 1.005(10) Å, respectively.     

A possibility of functionalizing the dinitrogen in a Chatt complex by H2: Density functional studies

DFT calculations have been performed to explore the possibility of functionalizing the coordinated N₂ in a Chatt type complex by H₂, using some suitable organic co-catalysts in a homogeneous fashion. The calculated thermodynamic barriers further revealed that there is the possibility to produce ammonia by the reaction of a Chatt type complex with H₂ in different organic solvents. The electronic features of the various intermediates have also been analyzed.