Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes.

ConspectusThe activation and functionalization of N2 to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N2 fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N2 to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N2 complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N2 complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N2 functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N2 unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N2 complex with Me3SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the Nß atom twice, yielding an end-on η1-hydrazido complex, Cr(III)═N-N(SiMe3)2. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N2 complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me3SiCl proceeds via the alternating pathway; the silylation occurs at both Nα and Nß atoms and generates a side-on η2-hydrazido complex, Cr(III)(η2-Me3SiN-NSiMe3). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N2 complexes with 4 equiv of Me3SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR2]+ hydrazido as the final products. More importantly, this type of Cr(0)-N2 complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N2 systems provide effective approaches for the activation and functionalization of N2, deepening the understanding of N2 electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N2 to high-value-added nitrogen-containing organic compounds.

Dinitrogen Functionalization Affording Chromium Diazenido and Side-on η2-Hydrazido Complexes.

Isolation of key intermediate complexes in dinitrogen functionalization is crucial for elucidating the mechanistic details and further investigation. Herein, the synthesis and characterization of (µ-η1:η1-N2)(η1-N2)-Cr(I) 3 and (η1-N2)2-Cr(0) complexes 4 supported by Cp* (Cp* = C5Me5) and NHC ligands were reported. Further functionalization of Cr(0)-N2 complex 4 with silyl halides delivered the key intermediates in the alternating pathway, the chromium diazenido complex 5 and the chromium side-on η2-hydrazido complex 6. Protonation of 6 led to the quantitative formation of N2H4. Moreover, the [η2-Me3SiNNSiMe3]2- unit in 6 enabled N-C bond formation reactions with CO2 and tBuNCO, giving the corresponding N,O-chelating hydrazidochromium complexes 7 and 8, respectively.

Cobalt Cyclopentadienyl-Phosphine Dinitrogen Complexes.

By applying the potassium salts of cyclopentadienyl-phosphine ligands LK to CoCl2 , the corresponding cobalt chlorides (1, LCoII Cl) were prepared. By reducing complexes 1 with KHBEt3 under a N2 atmosphere, bridging end-on complexes, LCoI -N2 -CoI L (2 a and 2 b), were successfully obtained. 15 N2 -labeled [15 N2 ]-2 a was prepared under 15 N2 /14 N2 exchange in THF solution. LCoI -N2 -CoI L complex 2 a could react with P4 molecules to release N2 and generate a Co-P4 -Co moiety 4. Further reduction of complex 2 b led to cleavage of a P-C bond in the cyclopentadienyl-phosphine ligand to provide novel µ-PCy2 -bridged Co0 -N2 complex 5. DFT calculations confirmed the experimental observations.

Dinitrogen Functionalization Affording Chromium Hydrazido Complex.

A series of trinuclear and dinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were synthesized and characterized. Further reduction of the Cr(I)-N2 complexes generated anionic Cr(0)-N2 complexes, which could react with Me3SiCl to afford the first chromium hydrazido complex from N2 functionalization. These complexes were found to be effective catalysts for the transformation of N2 into N(SiMe3)3.

Visible-Light-Driven Iron-Promoted Thiocarboxylation of Styrenes and Acrylates with CO2.

The first thiocarboxylation of styrenes and acrylates with CO2 was realized by using visible light as a driving force and catalytic iron salts as promoters. A variety of important ß-thioacids were obtained in high yields. This multicomponent reaction proceeds in an atom- and redox-economical manner with broad substrate scope under mild reaction conditions. Notably, high regio-, chemo-, and diasteroselectivity are observed. Mechanistic studies indicate that a radical pathway can account for the unusual regioselectivity.

Selective Oxytrifluoromethylation of Allylamines with CO2.

Reported is the first oxy-trifluoromethylation of allylamines with carbon dioxide (CO2 ) using copper catalysis, thus leading to important CF3 -containing 2-oxazolidones. It is also the first time CO2 , a nontoxic and easily available greenhouse gas, has been used to tune the difunctionalization of alkenes from amino- to oxy-trifluoromethylation. Of particular note, this multicomponent reaction is highly chemo-, regio-, and diastereoselective under redox-neutral and mild reaction conditions. Moreover, these reactions feature good functional-group tolerance, broad substrate scope, easy scalability and facile product diversification. The important products could also be formed with either spirocycles or two adjacent tetrasubstituted carbon centers.

Trisyl-based multidentate ligands: synthesis and their transition-metal complexes.

Herein we report the synthesis and applications of unusual trisyl-based multidentate ligands [trisyl = tris(trimethylsilyl)methyl, -C(SiMe3)3]. First, by applying a new trisyl synthon (Me3Si)2CH(SiMe2CH2Cl) 1, trisyl-based S- or N-containing compounds 2 were efficiently obtained. On treatment of these compounds 2 with MeLi, their corresponding S- or N-coordinated pincer-like trisyl-based lithium salts 3, including the S-bridged ditrisyl compound 3a [Li{C(SiMe3)2SiMe2CH2SCH2SiMe2C(SiMe3)2}Li(DME)3] and the N-coordinated monotrisyl compounds 3b [(NacNacDippLiCH2SiMe2C(SiMe3)2Li(THF)], 3c [Li(THF){C(SiMe3)2SiMe2CH2N(Me)CH2C5H4N-2}], and 3d [Li{C(SiMe3)2SiMe2CH2N(Me)CH2CH2N(iPr)2}] were synthesized and structurally characterized by single-crystal X-ray structural analysis. Second, to test these novel lithium salts as straightforward precursors for the synthesis of transition-metal complexes with unique structures, these lithium salts were applied to react with MCl2 (M = Cr, Mn, Fe, Co). Their corresponding transition-metal complexes 4-8 were obtained in high yields and structurally characterized by single-crystal X-ray structural analysis. Third, the preliminary reactivities of compound 4 were explored.

Synthesis and reactivity of asymmetric Cr(i) dinitrogen complexes supported by cyclopentadienyl-phosphine ligands.

Asymmetric trinuclear and dinuclear Cr(i) dinitrogen complexes bearing cyclopentadienyl-phosphine ligands were synthesized via reduction of their corresponding Cr(ii) chloride complexes in the presence of N2. Oxidative addition reactions of single, double and triple bonds are found to take place on the low-valent Cr(i) centre.

Oxy-Difluoroalkylation of Allylamines with CO2 via Visible-Light Photoredox Catalysis.

A selective oxy-difluoroalkylation of allylamines with carbon dioxide (CO2) via visible-light photoredox catalysis is reported. These multicomponent reactions are efficient and environmentally friendly to generate a series of important 2-oxazolidinones with functionalized difluoroalkyl groups. The good functional group tolerance, broad substrate scope, easy scalability, mild reaction conditions, and facile functionalization of products provide great potential for application in organic synthesis and pharmaceutical chemistry.

Catalytic Lactonization of Unactivated Aryl C-H Bonds with CO2: Experimental and Computational Investigation.

The first catalytic lactonization of unactivated aryl C-H bonds with CO2 to afford important phthalides is reported. Notably, this method features high selectivity, excellent functional group tolerance, smooth scalability, and facile product diversification. DFT calculations reveal that a novel insertion of two CO2 into the O-Pd bond of a palladacycle might be the key step, providing great potential and a different perspective for carbonylation with CO2.