CuH-Catalyzed Reductive Coupling of Nitroarenes with Phosphine Oxides for the Direct Synthesis of Phosphamides

A CuH-catalyzed reductive coupling of nitroarenes with phosphine oxides is developed, which produces a series of phosphamides in moderate to excellent yields with good functional group tolerance. Gram-scale synthesis and late-stage modification of nitro-aromatic functional molecule niclosamide are also successfully conducted. The mechanism study shows that the nitro group is transformed after being reduced to nitroso and a nucleophilic addition procedure is involved during the reaction.

Cobalt-catalyzed double hydroboration of pyridines

Cobalt(ii) complexes were prepared from a modular phosphinopyridonate platform and applied to the hydroboration of pyridines. The synthetically useful, yet challenging, double hydroboration toward tetrahydropyridine derivatives was successfully performed with high activity and regiocontrol. This new method enabled the direct synthesis of N-heterocyclic allylic boronates from commercial pyridines and pinacolborane (HBpin). One-pot acetylation afforded the bench-stable borylated N-acetyl tetrahydropyridines in good yields. The synthetic utility of this procedure was demonstrated by a gram-scale double hydroboration-acetylation sequence followed by chemical diversification. Mechanistic experiments indicated metal-ligand cooperativity involving ligand-centered C-H activation and the intermediacy of a cobalt(iii) hydride species.

Synthesis and Chemistry of Dihydridoborate Group 7 Metal Complexes with Varied N,E-Chelated Ligands (E = O, NH, or S)

Several dihydridoborate group 7 metal complexes have been synthesized and their structural aspects have been described from various N,S-, N,N-, and N,O-chelated borate species, such as Na[(H₃B)mp] (mp = 2-mercaptopyridyl), Na[(H₃B)amt] (amt = 2-amino-5-mercapto-1,3,4-thiadiazolyl), Na[(H₃B)hp] (hp = 2-hydroxypyridyl), Na[(H₂B)bap] (bap = bis(2-aminopyridyl)), and Na[(H₂B)bdap] (bdap = bis(2,6-diaminopyridyl)). Room temperature photolysis of [M₂(CO)₁₀] (M = Mn or Re) with these borate species afforded dihydridoborate complexes [(CO)₃M(μ-H)₂BHL] 1-6 (1, M = Mn, L = mp; 2, M = Re, L = mp; 3, M = Mn, L = amt; 4, M = Mn, L = hp; 5, M = Mn, L = ap; 6, M = Mn, L = dap, ap = 2-aminopyridyl, dap = 2,6-diaminopyridyl). In complexes 1-3, the corresponding (H₂BHL) units are coordinated to the metal centers through the (κ³-H,H,S) mode. However, in complexes 4 and 5 (or 6), the connection is via (κ³-H,H,O) and (κ³-H,H,N) modes of coordination, respectively. Complexes 1 and 5 underwent hydroboration reactions with terminal alkynes that yielded trans-hydroborated species [Mn(CO)₃(μ-H)₂(NC₅H₄E)B(PhC═CH₂)] (7, E = S; 8, E = NH). Density functional theory (DFT) calculations have been carried out to investigate the electronic structures of these dihydridoborate species as well as the nature of bonding in them.

Vanadium pyridonates: dimerization, redox behaviour, and metal-ligand cooperativity

The synthesis, structure, and reactivity of vanadium pyridonate complexes are described. Vanadium(III) pyridonate complexes were accessed through protonolysis and reduction of a tetrakis(amido)vanadium(IV) starting material. Bis(pyridonate) vanadium(IV) precursors could be isolated depending on the amount of proteoligand added. The targeted vanadium(III) species tend to form dimers, but monomeric complexes can be achieved in the presence of neutral donors such as amines or pyridine derivatives or through the use of sterically demanding proligands. The reduction process is proposed to involve dimeric intermediates and be mediated by the amine released from protonolysis, thereby forming the corresponding imine as a byproduct. Isolated amine complexes of vanadium(III) are presented. In contrast, bis(amidate)vanadium(IV) complexes were not found to undergo a similar reduction. This work informs on design principles for the synthesis and application of new vanadium pyridonate catalysts for transformations involving dimerization and PCET for changes in oxidation state.

N-O Ligand Supported Stannylenes: Preparation, Crystal, and Molecular Structures

A new series of tin(II) complexes (1, 2, 4, and 5) were successfully synthesized by employing hydroxy functionalized pyridine ligands, specifically 2-hydroxypyridine (hpH), 8-hydroxyquinoline (hqH), and 10-hydroxybenzo[h]quinoline (hbqH) as stabilizing ligands. Complexes [Sn(μ-κ2ON-OC5H4N)(N{SiMe3}2)]2 (1) and [Sn4(μ-κ2ON-OC5H4N)6(κ1O-OC5H4N)2] (2) are the first structurally characterized examples of tin(II) oxypyridinato complexes exhibiting {Sn2(OCN)2} heterocyclic cores. As part of our study, 1H DOSY NMR experiments were undertaken using an external calibration curve (ECC) approach, with temperature-independent normalized diffusion coefficients, to determine the nature of oligomerisation of 2 in solution. An experimentally determined diffusion coefficient (298 K) of 6.87 × 10−10 m2 s−1 corresponds to a hydrodynamic radius of Ca. 4.95 Å. This is consistent with the observation of an averaged hydrodynamic radii and equilibria between dimeric [Sn{hp}2]2 and tetrameric [Sn{hp}2]4 species at 298 K. Testing this hypothesis, 1H DOSY NMR experiments were undertaken at regular intervals between 298 K–348 K and show a clear change in the calculated hydrodynamic radii form 4.95 Å (298 K) to 4.35 Å (348 K) consistent with a tetramer ⇄ dimer equilibria which lies towards the dimeric species at higher temperatures. Using these data, thermodynamic parameters for the equilibrium (ΔH° = 70.4 (±9.22) kJ mol−1, ΔS° = 259 (±29.5) J K−1 mol−1 and ΔG°298 = −6.97 (±12.7) kJ mol−1) were calculated. In the course of our studies, the Sn(II) oxo cluster, [Sn6(m3-O)6(OR)4:{Sn(II)(OR)2}2] (3) (R = C5H4N) was serendipitously isolated, and its molecular structure was determined by single-crystal X-ray diffraction analysis. However, attempts to characterise the complex by multinuclear NMR spectroscopy were thwarted by solubility issues, and attempts to synthesise 3 on a larger scale were unsuccessful. In contrast to the oligomeric structures observed for 1 and 2, single-crystal X-ray diffraction studies unambiguously establish the monomeric 4-coordinate solid-state structures of [Sn(κ2ON-OC9H6N)2)] (4) and [Sn(κ2ON-OC13H8N)2)] (5).

Polynuclear Iron(II) Pyridonates: Synthesis and Reactivity of Fe4 and Fe5 Clusters

The combination of pyridonate ligands with transition metal ions enables the synthesis of an especially rich set of diverse coordination compounds involving various κ- and μ-bonding modes and higher nuclearities. With iron(II) ions, this chemical space is rather poorly explored beyond some biomimetic models of the pyridone iron-containing hydrogenase. Here, the topologically new Fe₅ and Fe₄ clusters, Fe₅(L^H)₆[N(SiMe₃)₂]₄ (1) and Fe₄(L^Me)₆[N(SiMe₃)₂]₂ (2), were synthesized (L^H = 2-pyridonate; L^Me = 6-methyl-2-pyridonate). Complex 1 contained an unprecedented diamondoid Fe@Fe₄ tetrahedron with a central-to-peripheral Fe-Fe distance of ∼3.1 Å. The crystal structure of complex 2 displayed an Fe₄O₆ butterfly motif containing a planar Fe₄ arrangement. Mössbauer spectroscopy confirmed the high-spin ferrous character of all iron ions. SQUID magnetometry reveals that the Fe(II) ions are involved in weak magnetic exchange coupling across the pyridonate bridges that results in antiferromagnetic interactions. The Fe₄ cluster exhibits slow relaxation of magnetization under an applied magnetic field with an effective energy barrier of 38.5 K, rarely observed among the very rare examples of Fe(II) cluster-based single-molecule magnets. Studies of protolytic substitution of the amido ligands demonstrated the lability of the diamondoid Fe₅ core in 1 and the stability of the Fe₄ rhomboid in 2.

A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)₃ (2) with KC₈ yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)₂] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f⁴ U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I₂ to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)₂)₂(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η⁷-C₇H₇)(TDA)₂(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)₄] (1-crypt) and U(COT)₂, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)₃]₂(μ-η⁸:η³-C₈H₈) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)₄ (4) with KC₈ gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)₃(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

Functionalisation of esters via 1,3-chelation using NaOtBu: mechanistic investigations and synthetic applications

Unusual nucleophilic behavior of a metal t-butoxide in a transesterification reaction was demonstrated by NMR and real-time IR spectroscopies and deuterium-labeling experiments.

Iron-Catalyzed Regiodivergent Hydrostannation of Alkynes: Intermediacy of Fe(IV)-H versus Fe(II)-Vinylidene

We report an iron system, Cp*Fe(1,2-R₂PC₆H₄X), which controls the Markovnikov and anti-Markovnikov hydrostannation of alkynes by tuning the ionic metal-heteroatom bonds (Fe-X) reactivity. The sequential addition of nBu₃SnH to the iron-amido catalyst (1, X = HN^-, R = Ph) affords a distannyl Fe(IV)-H species responsible for syn-addition of the Sn-H bond across the C≡C bond to produce branched α-vinylstannanes. Activation of the C(sp)-H bond of alkynes by an iron-aryloxide catalyst (2, X = O^-, R = Cy) affords an iron(II) vinylidene intermediate, allowing for gem-addition of the Sn-H to the terminal-carbon producing β-vinylstannanes. These catalytic reactions exhibit excellent regioselectivity and broad functional group compatibility and enable the large-scale synthesis of diverse vinylstannanes. Many new reactions have been established based on such a synthetic Fe-X platform to demonstrate that the initial step of the catalysis is conveniently controlled by the activation of either the tin hydride or the alkyne substrate.

Regiocontrol in the oxidative Heck reaction of indole by ligand-enabled switch of the regioselectivity-determining step

Efficient control of regioselectivity is a key concern in transition-metal-catalyzed direct C–H functionalization reactions. Various strategies for regiocontrol have been established by tuning the selectivity of the C–H activation step as a common mode. Herein, we present our study on an alternative mode of regiocontrol, in which the selectivity of the C–H activation step is no longer a key concern. We found that, in a reaction where the C–H activation step exhibits a different regio-preference from the subsequent functionalization step, a ligand-enabled switch of the regioselectivity-determining step could provide efficient regiocontrol. This mode has been exemplified by the Pd(ii)-catalyzed aerobic oxidative Heck reaction of indoles, in which a ligand-controlled C3-/C2-selectivity was achieved for the first time by the development of sulfoxide-2-hydroxypyridine (SOHP) ligands.

Ligand-enabled switch of the regioselectivity-determining step allowed for efficient regiocontrol in the aerobic oxidative Heck reaction of indole.

Opening up the Toolbox: Synthesis and Mechanisms of Phosphoramidates

This review covers the main synthetic routes to and the corresponding mechanisms of phosphoramidate formation. The synthetic routes can be separated into six categories: salt elimination, oxidative cross-coupling, azide, reduction, hydrophosphinylation, and phosphoramidate-aldehyde-dienophile (PAD). Examples of some important compounds synthesized through these routes are provided. As an important class of organophosphorus compounds, the applications of phosphoramidate compounds, are also briefly introduced.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization

A series of Rh(κ² -BHetA)(η² -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(μ-Cl)(IPr)(η² -coe)]₂ with the corresponding anionic BHetA species. The Rh^I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h^-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ¹ -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh^III H(C≡CPh)₂ (IPr)(py)₂ intermediate gives support for an alternative oxidative route.

Intermediate snapshot on the insertion reaction of isocyanates into the Zn-Cp* bond of dizincocene Cp*2Zn2

Heteroleptic Zn(i) complexes Cp*Zn-Zn(N(R)C(Cp*)O) (R = Dipp = 2,6-i-Pr2-C6H32, t-Bu 3) with unsymmetrically η4-coordinated Cp* substituents represent snapshots of the insertion reaction of RNCO into the Zn-Cp* bond of Cp*2Zn21. The bonding situation in 2 and 3, which represent the first Zn(i) olefin complexes, was evaluated by computational calculation and further compared to other Zn(i) complexes.

Selective Room-Temperature Hydrogenation of Amides to Amines and Alcohols Catalyzed by a Ruthenium Pincer Complex and Mechanistic Insight

We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols. Compared with most previous reports for this transformation, which use high temperatures (typically, 100–200 °C) and H₂ pressures (10–100 bar), this system proceeds under extremely mild conditions (RT, 5–10 bar of H₂). The hydrogenation is catalyzed by well-defined ruthenium-PNNH pincer complexes (0.5 mol %) with potential dual modes of metal–ligand cooperation. An unusual Ru-amidate complex was formed and crystallographically characterized. Mechanistic investigations indicate that the room-temperature hydrogenation proceeds predominantly via the Ru–N amido/amine metal–ligand cooperation.

Controlled partial transfer hydrogenation of quinolines by cobalt-amido cooperative catalysis

Catalytic hydrogenation or transfer hydrogenation of quinolines was thought to be a direct strategy to access dihydroquinolines. However, the challenge is to control the chemoselectivity and regioselectivity. Here we report an efficient partial transfer hydrogenation system operated by a cobalt-amido cooperative catalyst, which converts quinolines to 1,2-dihydroquinolines by the reaction with H₃N·BH₃ at room temperature. This methodology enables the large scale synthesis of many 1,2-dihydroquinolines with a broad range of functional groups. Mechanistic studies demonstrate that the reduction of quinoline is controlled precisely by cobalt-amido cooperation to operate dihydrogen transfer from H₃N·BH₃ to the N=C bond of the substrates.

Controlling the partial reduction of quinolines is challenging, given the competing overreduction to tetrahydroquinolines. Here, the authors report a cobalt-amido cooperative catalyst for the selective, partial transfer hydrogenation of quinolines to 1,2-dihydroquinolines with H₃N·BH₃ as reductant.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Synthesis, characterization, and reactivity of dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands: substituent effects on coordination and reactivity

Two series of new dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands with different haptic modes were synthesized and characterized. The treatment of [RE(CH2SiMe3)3(THF)2] with 1 equiv. of 2-(2,6-iPr2C6H3NHC[double bond, length as m-dash]O)C8H5NH (H2L1) and 2-(2-tBuC6H4NHC[double bond, length as m-dash]O)C8H5NH (H2L2) in toluene yielded the dinuclear organo-rare-earth-metal alkyl complexes {[η1:(μ2-η1:η1)-L1]RE(CH2SiMe3)(THF)2}2 [RE = Gd (1a), Dy (1b), Y (1c), Er (1d), and Yb (1e)] and {[η1:(μ2-η1:η1):η1-L2]RE(CH2SiMe3)(THF)2}2 [RE = Gd (2a), Dy (2b), Y (2c), Er (2d), and Yb (2e)] in good yields. When [RE(CH2SiMe3)3(THF)2] were treated with 2 equiv. of H2L1 or H2L2 in THF, the dinuclear organo-rare-earth-metal complexes {(η1:η1-HL)[η1:(μ2-η1:η1):η1-L]RE(THF)}2 (1ca: RE = Y, L = L1; 2ea: RE = Yb, L = L2) were obtained. The complexes could react with small organic molecules such as N,N'-diisopropylcarbodiimide (DIC), phenyl isocyanate, N-methylallylamine, phenylacetylene, pyridine, N-phenylimidazole, or 4-dimethylaminopyridine (DMAP) to yield a series of new complexes with different reactivity patterns along with the reported rare-earth-metal alkyl complexes. In the presence of cocatalysts, these dinuclear organo-rare-earth-metal alkyl complexes could initiate isoprene polymerization with high activity (100% conversion of 2000 equiv. of isoprene in 12 h), yielding polymers with high regioselectivity (1,4 polymers up to 96.1%).

Synthesis, structures and properties of six lanthanide complexes based on a 2-(2-carboxyphenyl)imidazo(4,5-f)-(1,10)phenanthroline ligand

Six lanthanide complexes [Tb(2-NCP)₂(NO₃)]_n (1), [Eu(2-NCP)₂(3-PYC)]_n (2), [Sm(2-NCP)₂(3-PYC)]_n (3), [Eu(2-NCP)(SA)]_n (4), [Tb(2-NCP)(SA)]_n (5) and [Tb(2-NCP)(AA)]_n (6) (2-HNCP = 2-(2-carboxyphenyl)imidazo(4,5-f)-(1,10)phenanthroline, 3-HPYC = pyridine-3-carboxylic acid, H₂SA = succinic acid, H₂AA = adipic acid) have been synthesized by hydrothermal route, and the crystal structures were analyzed through elemental analysis, infrared spectroscopy, single crystal X-ray diffraction. Complexes 1 and 6 present two-dimensional (2D) layers, which are further connected to three-dimensional (3D) supramolecular architectures through C–H⋯π interactions. Complexes 2 and 3 exhibit infinite one-dimensional chains. Finally, the neighboring chains are packed by C–H⋯π interactions, giving rise to 3D supramolecular network. Complexes 4 and 5 display 2D layers, which are further extend to 3D supramolecular structures via C–H⋯O intermolecular hydrogen bonding. Six complexes possess good thermal stabilities, characteristic photoluminescence properties, and photocatalytic activities for the degradation of organic dyes under visible light irradiation. In addition, the complex 6 exhibits significantly enhanced photocatalytic activity for methylene blue, and the degradation rate could reach 81.2% in 370 min. Meanwhile trapping experiments indicated that the hole, ·O₂⁻ and ·OH are the main activated species. Furthermore, by comparing the photoluminescent and photocatalytic mutation results of same metal complexes induced by interconversion of coligands, we confirm that the properties mutation induced by coligands is much obvious and controllable.

Six lanthanide complexes based on 2-HNCP possess good thermal stabilities, characteristic photoluminescence properties, and photocatalytic activities for organic dyes degradation under visible light irradiation.

Homoleptic U(iii) and U(iv) amidate complexes

Homoleptic U(iv) and U(iii) amidate complexes have been isolated and characterized; these species undergo an unusual and reversible change in coordination number upon reduction/oxidation.

Dehydrogenation of cyclic amines by a coordinatively unsaturated Cp*Ir(iii) phosphoramidate complex

The reaction of a Cp*Ir(iii) phosphoramidate complex with secondary amines gives amine, imine-bound Cp*Ir(iii) hydride complexes resulting from amine dehydrogenation.