Tropane and related alkaloid skeletons via a radical [3+3]-annulation process.

Tropanes and related bicyclic alkaloids are highly attractive compounds possessing a broad biological activity. Here we report a mild and simple protocol for the synthesis of N-arylated 8-azabicyclo[3.2.1]octane and 9-azabicyclo[3.3.1]nonane derivatives. It provides these valuable bicyclic alkaloid skeletons in good yields and high levels of diastereoselectivity from simple and readily available starting materials using visible-light photoredox catalysis. These bicyclic aniline derivatives are hardly accessible via the classical Robinson tropane synthesis and represent a particularly attractive scaffold for medicinal chemistry. This unprecedented annulation process takes advantage of the unique reactivity of ethyl 2-(acetoxymethyl)acrylate as a 1,3-bis radical acceptor and of cyclic N,N-dialkylanilines as radical 1,3-bis radical donors. The success of this process relies on efficient electron transfer processes and highly selective deprotonation of aminium radical cations leading to the key α-amino radical intermediates.

CS2 Reductive Coupling to Acetylenedithiolate by a Dinuclear Ytterbium(II) Complex.

The activation of CS2 is of interest in a broad range of fields and, more particularly, in the context of creating new C-C bonds. The reaction of the dinuclear ytterbium(II) complex [Yb2 L4 ], 1, [L=(OtBu)3 SiO- ] with carbon disulfide led to the isolation of unprecedented reduction products. In particular, the crystallographic characterization of complex [Yb2 L4 (µ-C2 S2 )], 2, provided the first example of an acetylenedithiolate ligand formed from metal reduction of CS2 . Computational studies indicated that this unprecedented reactivity can be ascribed to the unusual binding mode of CS2 2- in the isolated "key intermediate" [Yb2 L4 (µ-CS2 )], 3, which results from the dinuclear nature of 1.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes.

Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber-Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure-activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(III) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo-(N2) complex differs significantly from that of the nitride-(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not.

Ligand and Metal Based Multielectron Redox Chemistry of Cobalt Supported by Tetradentate Schiff Bases.

We have investigated the influence of bound cations on the reduction of cobalt complexes of redox active ligands and explored the reactivity of reduced species with CO2. The one electron reduction of [CoII(Rsalophen)] with alkali metals (M = Li, Na, K) leads to either ligand-centered or metal-centered reduction depending on the alkali ion. It affords either the [CoI(Rsalophen)K] complexes or the [CoII2(bis-salophen)M2] (M = Li, Na) dimers that are present in solution in equilibrium with the respective [CoI(salophen)M] complexes. The two electron reduction of [CoII(OMesalophen)] results in both ligand centered and metal centered reduction affording the Co(I)-Co(II)-Co(I) [Co3(tris-OMesalophen)Na6(THF)6], 6 complex supported by a bridging deca-anionic tris-OMesalophen10- ligand where three OMesalophen units are connected by two C-C bonds. Removal of the Na ion from 6 leads to a redistribution of the electrons affording the complex [(Co(OMesalophen))2Na][Na(cryptand)]3, 7. The EPR spectrum of 7 suggests the presence of a Co(I) bound to a radical anionic ligand. Dissolution of 7 in pyridine leads to the isolation of [CoI2(bis-OMesalophen)Na2Py4][Na(cryptand)]2, 8. Complex 6 reacts with ambient CO2 leading to multiple CO2 reduction products. The product of CO2 addition to the OMesalophen ligand, [Co(OMesalophen-CO2)Na]2[Na(cryptand)]2, 9, was isolated but CO32- formation in 53% yield was also detected. Thus, the electrons stored in the reversible C-C bonds may be used for the transformation of carbon dioxide.

A versatile route to homo- and hetero-bimetallic 5f-5f and 3d-5f complexes supported by a redox active ligand framework.

The salt-elimination reaction of the complex [Na2U(bis-salophen)] with metal halides provides an entry to the synthesis of well-defined homobimetallic uranium-uranium and rare heterobimetallic uranium-cobalt and uranium-nickel complexes supported by a redox-active dinucleating ligand.

Synthesis and reactivity of a terminal uranium(iv) sulfide supported by siloxide ligands.

The reactions of the tetrasiloxide U(iii) complexes [U(OSi(O t Bu)3)4K] and [U(OSi(O t Bu)3)4][K18c6] with 0.5 equiv. of triphenylphosphine sulfide led to reductive S-transfer reactions, affording the U(iv) sulfide complexes [SU(OSi(O t Bu)3)4K2]2, 1, and [{SU(OSi(O t Bu)3)4K2}2(µ-18c6)], 2, with concomitant formation of the U(iv) complex [U(OSi(O t Bu)3)4]. Addition of 1 equiv. of 2.2.2-cryptand to complex 1 resulted in the isolation of a terminal sulfide complex, [SU(OSi(O t Bu)3)4K][Kcryptand], 3. The crucial role of the K+ Lewis acid in these reductive sulfur transfer reactions was confirmed, since the formation of complex 3 from the reaction of the U(iii) complex [U(OSi(O t Bu)3)4][Kcryptand] and 0.5 equiv. of PPh3S was not possible. Reactivity studies of the U(iv) sulfide complexes showed that the sulfide is easily transferred to CO2 and CS2 to afford S-functionalized products. Moreover, we have found that the sulfide provides a convenient precursor for the synthesis of the corresponding U(iv) hydrosulfide, {[(SH)U(OSi(O t Bu)3)4][K18c6]}, 5, after protonation with PyHCl. Finally, DFT calculations were performed to investigate the nature of the U-S bond in complexes 1, 3 and 5. Based on various analyses, triple-bond character was suggested for the U-S bond in complexes 1 and 3, while double-bond character was determined for the U-SH bond in complex 5.

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond.

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (Ad,MeArO)3tacn3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((Ad,MeArO)3tacn)U-SH] (2) is obtained from the reaction of H2S with the uranium(iii) starting material [((Ad,MeArO)3tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.

Lanthanide(II) Complexes Supported by N,O-Donor Tripodal Ligands: Synthesis, Structure, and Ligand-Dependent Redox Behavior.

The preparation and characterization of a series of complexes of the Yb and Eu cations in the oxidation state II and III with the tetradentate N,O-donor tripodal ligands (tris(2-pyridylmethyl)amine (TPA), BPA(-) (HBPA=bis(2-pyridylmethyl)(2-hydroxybenzyl)amine), BPPA(-) (HBPPA=bis(2-pyridylmethyl)(3.5-di-tert-butyl-2-hydroxybenzyl)amine), and MPA(2-) (H2 MPA=(2-pyridylmethyl)bis(3.5-di-tert-butyl-2-hydroxybenzyl)amine) is reported. The X-ray crystal structures of the heteroleptic Ln(2+) complexes [Ln(TPA)I2 ] (Ln=Eu, Yb) and [Yb(BPA)I(CH3 CN)]2 , of the Ln(2+) homoleptic [Ln(TPA)2 ]I2 (Ln=Sm, Eu, Yb) and [Eu(BPA)2 ] complexes, and of the Ln(3+) [Eu(BPPA)2 ]OTf and [Yb(MPA)2 K(dme)2 ] (dme=dimethoxyethane) complexes have been determined. Cyclic voltammetry studies carried out on the bis-ligand complexes of Eu(3+) and Yb(3+) show that the metal center reduction occurs at significantly lower potentials for the BPA(-) ligand as compared with the TPA ligand. This suggests that the more electron-rich character of the BPA(-) ligand results in a higher reducing character of the lanthanide complexes of BPA(-) compared with those of TPA. The important differences in the stability and reactivity of the investigated complexes are probably due to the observed difference in redox potential. Preliminary reactivity studies show that whereas the bis-TPA complexes of Eu(2+) and Yb(2+) do not show any reactivity with heteroallenes, the [Eu(BPA)2 ] complex reduces CS2 to afford the first example of a lanthanide trithiocarbonate complex.

CS2 activation at uranium(III) siloxide ate complexes: the effect of a Lewis acidic site.

Multimetallic cooperative binding of heteroallenes provides an attractive route to their activation, but the reduction of CS(2) at heterobimetallic sites, associating an electron-rich metal with a main group Lewis acid has not been explored. Here we show that the presence of a heterometallic U, K site plays an important role in the CS(2) reduction by uranium(iii) complexes of the electron-rich and the sterically demanding tris(tert-butoxy)siloxide ligand. Specifically, the ion-pair complex [K(18c6)][U(OSi(O(t)Bu)(3))(4)], 1, leads preferentially to the reductive disproportionation of CS(2) to K(2)CS(3) and CS. The crystal structure of the thiocarbonate intermediate complex [U(OSi(O(t)Bu(3)(4) (µ(3)-κ(2):κ(2):κ(2-)CS(3))K(2)(18c6)(2)], 2, isolated from the toluene reaction mixture has been determined. In contrast, the heterobimetallic complex [U(OSi(O(t)Bu(3)(4)K], 3, promotes preferentially the reductive dimerization of CS(2) to K(2)C(2)S(4) and K(2)C(3)S(5). The [K(2)C(2)S(4)(DMSO)(3)](n), 5, and [U(OSi(O(t)Bu)(3))(4)K(2)(C(3)S(5))](n), 6, polymeric compounds were isolated from this reaction and structurally characterized.

Tuning lanthanide reactivity towards small molecules with electron-rich siloxide ligands.

The synthesis, structure, and reactivity of stable homoleptic heterometallic LnL4K2 complexes of divalent lanthanide ions with electron-rich tris(tert-butoxy)siloxide ligands are reported. The [Ln(OSi(OtBu)3)4K2] complexes (Ln=Eu, Yb) are stable at room temperature, but they promote the reduction of azobenzene to yield the KPhNNPh radical anion as well as the reductive cleavage of CS2 to yield CS3(2-) as the major product. The Eu(III) complex of the radical anion PhNNPh is structurally characterized. Moreover, [Yb(OSi(OtBu)3)4K2] can reduce CO2 at room temperature. Release of the reduction products in D2O shows the quantitative formation of both oxalate and carbonate in a 1:2.2 ratio. The bulky siloxide ligands enforce the labile binding of the reduction products providing the opportunity to establish a closed synthetic cycle for the Yb(II)-mediated CO2 reduction. These studies show that the presence of four electron-rich siloxide ligands renders their Eu(II) and Yb(II) complexes highly reactive.

Synthesis of electron-rich uranium(IV) complexes supported by tridentate Schiff base ligands and their multi-electron redox chemistry.

The synthesis, structure, and reactivity of a new complex of U(IV) with the tridentate Schiff base ligand Menaphtquinolen are reported. The reduction of the bis-ligand complexes [UX2((Me)naphtquinolen)2] (X = Cl, (1-Cl) ; I (1-I)) with potassium metal affords the U(IV) complex of the new tetranionic hexadentate ligand µ-bis-(Me)naphtquinolen formed through the intramolecular reductive coupling of the imino groups of each (Me)naphtquinolen unit. The solid state structure of the [U(µ-bis-(Me)naphtquinolen)]2 dimer 2 isolated from toluene confirms the presence of a U(IV) complex of the reduced ligand. Reactivity studies with molecular oxygen and 9,10-phenanthrenequinone show that complex 2 can act as a multielectron reducing agent releasing two electrons through the cleavage of the C-C bond to restore the original imino function of the ligand. In the resulting U(IV) and U(VI) complexes [U(9,10-phenanthrenediol)((Me)naphtquinolen)2], 3, and [UO2((Me)naphtquinolen)2], 4, the restored tridentate Schiff base allows for the coordination of the reduced substrate to the metal. Electrochemical studies of complex 2 show the presence of irreversible ligand centered reduction processes and of a reversible U(IV)/U(III) couple.