稀土和锕系配合物促进的氮气活化与转化研究

氮气分子具有高的化学惰性,氮气的活化与转化充满挑战.含氮有机物在国民经济发展中具有广泛且重要的价值,实现温和条件下由氮气直接转化为含氮有机物在科学和经济上均具有重要意义.目前对氮气的活化与转化的研究主要集中在主族与过渡金属配合物,稀土和锕系元素由于具有特殊的电子结构,在氮气的活化与转化领域展现出了区别于主族和过渡金属的特殊反应活性.我国作为稀土和钍资源大国,开展稀土及锕系元素的固氮转化研究具有重要的战略意义.本综述归纳和总结了过去五年内稀土和锕系金属氮气配合物的合成,以及由稀土和锕系配合物促进的以氮气为原料生成含氮有机物的研究.     

Molecular Epoxidation Reactions Catalyzed by Rhenium,Molybdenum, and Iron Complexes

Epoxidations are of high relevance in many organic syntheses, both in industry and academia. In this personal account, the development of rhenium, molybdenum, and iron complexes in molecular epoxidation catalysis is presented. Methyltrioxorhenium (MTO) is the benchmark catalyst for these reactions, with a thoroughly investigated mechanism and reactivity profile. More recently, highly active molecular molybdenum and iron catalysts have emerged, challenging the extraordinary role of MTO in epoxidation catalysis with high turnover frequencies (TOFs). This development is highlighted in its use of cheaper, more readily available metals, and the challenges of using base metals in catalysis are discussed. These results show the promise that relatively cheap and abundant metals, such as molybdenum and iron, hold for the future of epoxidation catalysis.     

Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP‐type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).     

Cycling between Molybdenum-Dinitrogen and -Nitride Complexes to Support the Reaction Pathway for Catalytic Formation of Ammonia from Dinitrogen

Cycling between molybdenum(I)-dinitrogen and molybdenum(IV)-nitride complexes was investigated under ambient reaction conditions. A kinetic study of the second-order reaction rate for the conversion of the molybdenum-dinitrogen complex into the molybdenum-nitride complex indicates that the formation of the dinitrogen-bridged dimolybdenum complex is involved in the rate-determining step. DFT calculations indicate that the molybdenum-dinitrogen complex transforms into the molybdenum-nitride complex via direct cleavage of the nitrogen-nitrogen triple bond of the bridging dinitrogen ligand of the dinitrogen-bridged dimolybdenum complex. The corresponding reaction of the molybdenum-nitride complex transforming into the molybdenum-dinitrogen complex proceeds via the ligand exchange of ammonia for dinitrogen at the dinitrogen-bridged dimolybdenum complexes. A new modified reaction pathway has been proposed based on the findings of our experimental and theoretical results.     

Catalytic Reduction of Molecular Dinitrogen to Ammonia and Hydrazine Using Vanadium Complexes

Newly designed and prepared vanadium complexes bearing anionic pyrrole‐based PNP‐type pincer and aryloxy ligands were found to work as effective catalysts for the direct conversion of molecular dinitrogen into ammonia and hydrazine under mild reaction conditions. This is the first successful example of vanadium‐catalyzed dinitrogen reduction under mild reaction conditions.     

Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine by Using Chromium Complexes Bearing PCP-Type Pincer Ligands**

A series of chromium-halide, -nitride, and -dinitrogen complexes bearing carbene- and phosphine-based PCP-type pincer ligands has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen under atmospheric pressure, whereby up to 11.60 equiv. of ammonia and 2.52 equiv. of hydrazine (16.6 equiv. of fixed N atom) are produced based on the chromium atom. To the best of our knowledge, this is the first successful example of chromium-catalyzed conversion of dinitrogen to ammonia and hydrazine under mild reaction conditions.     

Energetics and mechanism of ammonia synthesis through the Chatt Cycle: conditions for a catalytic mode and comparison with the Schrock Cycle

Through a series of DFT calculations the energy profile of the Chatt cycle is evaluated. This is the counterpiece of our earlier investigations of the Schrock cycle (Angew. Chem. 2005, 117, 5783; Angew. Chem. Int. Ed. 2005, 44, 5639), applying the same quantumchemical methodology and approximations. As for the Schrock cycle, decamethylchromocene acts as reductant. The protonation reactions are considered to be mediated by HBF4/diethyl ether or lutidinium. For all protonation and reduction steps the corresponding free reaction enthalpy changes are calculated. The derived energy profile and corresponding reaction mechanism bear strong similarities to the Schrock cycle. In particular, the most endergonic reaction is the first protonation of the N2 complex and the most exergonic reaction is the cleavage of the N--N bond. If lutidinium is employed as acid and Cp2*Cr as reductant, the reaction course involves steps that are not thermally allowed. For HBF4/diethyl ether as the acid and Cp2*Cr as reducant, however, a catalytic cycle consisting of thermally allowed reactions is principally feasible. This cycle involves a Mo I-fluoro complex as dinitrogen intermediate. It is shown that regeneration to the Mo 0-bis(dinitrogen) complex is thermally not accessible in this system. Moreover, the Mo I fluoro-dinitrogen complex is labile towards disproportionation. The implications of these results with respect to the realization of a catalytic system on the basis of Mo and W phosphine complexes are discussed.     

Activation and functionalization of molecular nitrogen by metal complexes

Molecular nitrogen is intrinsically unreactive, so much so that it has confounded chemists for decades in attempts to functionalize this abundant diatomic molecule. While biological systems and industrial processes can fix nitrogen to form ammonia, the challenge is to discover a process that involves a homogeneous catalyst that can utilize N(2) as a feedstock to generate higher value organonitrogen materials. In this review, the activation of molecular nitrogen by transition metal complexes is reviewed with the view to present new kinds of transformations for coordinated dinitrogen. Moreover, some reaction types that are as yet unknown are outlined to try and stimulate further research in this area.     

Interconversion of Molybdenum Imido and Amido Complexes by Proton‐Coupled Electron Transfer

Interconversion of the molybdenum amido [(^PhTpy)(PPh₂Me)₂Mo(NHtBuAr)][BArF²⁴] (^PhTpy=4′‐Ph‐2,2′,6′,2“‐terpyridine; tBuAr=4‐tert‐butyl‐C₆H₄; ArF²⁴=(C₆H₃‐3,5‐(CF₃)₂)₄) and imido [(^PhTpy)(PPh₂Me)₂Mo(NtBuAr)][BArF²⁴] complexes has been accomplished by proton‐coupled electron transfer. The 2,4,6‐tri‐tert‐butylphenoxyl radical was used as an oxidant and the non‐classical ammine complex [(^PhTpy)(PPh₂Me)₂Mo(NH₃)][BArF²⁴] as the reductant. The N?H bond dissociation free energy (BDFE) of the amido N?H bond formed and cleaved in the sequence was experimentally bracketed between 45.8 and 52.3 kcal mol^?1, in agreement with a DFT‐computed value of 48 kcal mol^?1. The N?H BDFE in combination with electrochemical data eliminate proton transfer as the first step in the N?H bond‐forming sequence and favor initial electron transfer or concerted pathways.     

Synthesis and Ligand Modification Chemistry of a Molybdenum Dinitrogen Complex: Redox and Chemical Activity of a Bis(imino)pyridine Ligand

The bis(imino)pyridine 2,6‐(2,6‐iPr₂‐C₆H₃N?CPh)₂‐C₅H₃N (^iPrBPDI) molybdenum dinitrogen complex, [{(^iPrBPDI)Mo(N₂)}₂(μ₂,η¹,η¹‐N₂)] has been prepared and contains both weakly (terminal) and modestly (bridging) activated N₂ ligands. Addition of ammonia resulted in sequential N? H bond activations, thus forming bridging parent imido (μ‐NH) ligands with concomitant reduction of one of the imines of the supporting chelate. Using primary and secondary amines, model intermediates have been isolated that highlight the role of metal–ligand cooperativity in NH₃ oxidation.     

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.     

Oxygenative Cleavage of Chlorocatechols with Molecular Oxygen Catalyzed by Non-Heme Iron(III) Complexes and Its Relevance to Chlorocatechol Dioxygenases

The degradation of halogenated arenes , which are considered as hazardous compounds from a toxicological and environmental point of view, can be effected by non-heme iron enzymes such as chlorocatechol dioxygenases. In principle, this process of biological oxygenation mimics the reaction of the iron complex shown below [Eq. (a); Py=2-pyridyl].     

ICP–AES法测定金属钼中微量Fe,Ni

采用ICP–AES法测定金属钼中Fe,Ni含量,以盐酸–硝酸–氢氟酸溶解样品,试验了基体元素和共存元素对Fe,Ni的光谱干扰,Fe,Ni的分析谱线分别为238.204 nm,341.477 nm。测定Fe,Ni的线性范围均为0.001%~0.01%,线性相关系数分别为0.999 4,0 999 8,检出限分别为0.000 01%,0.000 04%。方法的加标回收率为95.7%~115.0%,测定结果的相对标准偏差为2.36%~17.82%(n=8)。该方法快速、简便,能够满足金属钼中含量范围为0.001%~0.01%的Fe,Ni元素的检测要求。     

Interaction between Anions and Molybdenum Allyl Dicarbonyl Complexes of 1,4,7‐Trithiacyclononane

The labile complex [MoCl(η³‐methallyl)(CO)₂(NCMe)₂] reacts with the ligand 1,4,7‐trithiacyclononane ([9]aneS₃) and the salt NaBAr′₄ to afford [Mo(η³‐methallyl)(CO)₂([9]aneS₃)][BAr′₄] ( 1?BAr′₄ ). An analogous reaction of [MoBr(η³‐allyl)(CO)₂(NCMe)₂] yields [Mo(η³‐allyl)(CO)₂([9]aneS₃)][BAr′₄] ( 2?BAr′₄ ). The new compounds 1?BAr′₄ and 2?BAr′₄ were characterized by IR and NMR spectroscopic analysis and X‐ray diffraction studies. Both compounds feature the cyclic thioether [9]aneS₃ coordinated as a tridentate ligand to the molybdenum center. The allyl ligand in 2?BAr′₄ is aligned with the middle of the OC‐Mo‐CO angle, which is acute. Both of these features are typical of most pseudo‐octahedral allyl dicarbonyl molybdenum complexes. In contrast, the allyl group is rotated in 1?BAr′₄ , which is attributed to steric hindrance between the methyl substituent and the ligated thioether, and the OC‐Mo‐CO angle is obtuse. Compound 1?BAr′₄ undergoes rapid substitution of [9]aneS₃ by either chloride and fluoride ions in dichloromethane, and the products include the known species [{Mo(η³‐methallyl)(CO)₂}₂(μ‐Cl)₃]^? and a structurally similar new anionic complex with two fluoro and one hydroxo bridging ligands, respectively. Stable supramolecular adducts were formed in the reactions of 1?BAr′₄ and 2?BAr₄ with bromide, iodide, hydrogensulfate, and methanesulfonate compounds. The binding constants of these adducts in dichloromethane were calculated from ¹H NMR spectroscopic titration data, and the solid‐state structures of the 1?Br , 1?HSO₄ , 1?I , and 2?I adducts were determined by X‐ray diffraction studies. The surprising slightly higher stability of the iodide adduct relative to that of bromide was investigated theoretically, with the results pointing to an effect of the differential solvation of the halide ions.     

Luminescence Color Tuning of PtII Complexes and a Kinetic Study of Trimer Formation in the Photoexcited State

We investigated the luminescence properties and color tuning of [Pt(dpb)Cl] (dpbH=1,3‐di(2‐pyridyl)benzene) and its analogues. An almost blue emission was obtained for the complex [Pt(Fmdpb)CN] (FmdpbH=4‐fluoro‐1,3‐di(4‐methyl‐2‐pyridyl)benzene), modified by the introduction of ?F and ?CH₃ groups to the dpb ligand and the substitution of ?Cl by ?CN. As the concentration of the solution was increased, the color of the emission varied from blue to white to orange. The color change resulted from a monomer–excimer equilibrium in the excited state. A broad emission spectrum around 620 nm was clearly detected along with a structured monomer emission around 500 nm. Upon further increases in concentration, another broad peak appeared in the longer wavelength region of the spectrum. We assigned the near‐infrared band to the emission from an excited trimer generated by the reaction of the excimer with the ground‐state monomer. The emission lifetimes of the monomer, dimer, and trimer were evaluated as τ_M=12.8 μs, τ_D=2.13 μs, and τ_T=0.68 μs, respectively, which were sufficiently long to allow association with another Pt^II complex and dissociation into a lower order aggregate. Based on equilibrium constants determined from a kinetic study, the formation of the excimer and the excited trimer were concluded to be exothermic processes, with ΔG*_D=?24.5 kJ mol^?1 and ΔG*_T=?20.4 kJ mol^?1 respectively, at 300 K.