Reactivity of Bulky Formamidinatosamarium(II or III) Complexes with C═O and C═S Bonds

Multi-electron redox reactivity of a samarium(ii) hydrido complex

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb2+ and Eu2+ centers. Here, we report the first example of the divalent samarium(ii) hydrido complex [(CpAr5)SmII(μ-H)(DABCO)]2 (4) (CpAr5 = C5Ar5, Ar = 3,5-iPr2-C6H3; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(ii) alkyl complex [(CpAr5)SmII{CH(SiMe3)2}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO2 results in the formation of the trivalent samarium(iii) mixed-bis-formate/carbonate complex [(CpAr5)SmIII(μ-η2:η1-O2CH)(μ-η2:η2-CO3)(μ-η1:η1-O2CH)SmIII(CpAr5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS2 to afford the trivalent samarium(iii) bis-trithiocarbonate complex [(CpAr5)SmIII(μ-η2:η2-CS3)(DABCO)]2 (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.

Formation of a cyclooctatetraenylsamarium(III) inverse sandwich that ring-opens tetrahydrofuran

Reductive trapping of the cyclooctatetraenyl dianion (COT^2-) by treatment of [Sm(DippForm)₂(thf)₂] (DippFormH = N,N'-bis(2,6-diisopropylphenyl)formamidine; thf = tetrahydrofuran) with 1,3,5,7-cyclooctatetraene (C₈H₈) in toluene yields an inverse sandwich dinuclear complex [Sm₂(DippForm)₄(COT)] (1), but [Sm(DippForm)(COT)(thf)₂] (2) and [Sm(DippForm)₂(O-C₄H₈-DippForm)(thf)] (3) in thf, and 1 yields 2 and 3 on treatment with thf.

CO, CO2 and CS2 activation by divalent ytterbium hydrido complexes

Treatment of a divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (1) with CO, CO₂ and CS₂ resulted in the formation of a divalent ytterbium ethenediolate complex [(Tp^Ad,iPr)Yb]₂(cis-OCHCHO) (2), a formate complex [(Tp^Ad,iPr)Yb(κ²-O₂CH)(THF)] (3), and a trivalent ytterbium ethenetetrathiolate complex [(Tp^Ad,iPr)Yb^III]₂(C₂S₄) (4), respectively. DFT calculations were carried out to elucidate the reaction profiles of complexes 3 and 4.

Cobalt and Iron Stabilized Ketyl, Ketiminyl and Aldiminyl Radical Anions

Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.

Reductive Reactivity of the 4f75d1 Gd(II) Ion in {GdII[N(SiMe3)2]3}-: Structural Characterization of Products of Coupling, Bond Cleavage, Insertion, and Radical Reactions

The reductive reactivity of a Ln(II) ion with a nontraditional 4fⁿ5d¹ electron configuration has been investigated by studying reactions of the {Gd^II(N(SiMe₃)₂)₃]}^- anion with a variety of reagents that survey the many reaction pathways available to this ion. The chemistry of both [K(18-c-6)₂]⁺ and [K(crypt)]⁺ salts (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand) was examined to study the effect of the countercation. CS₂ reacts with the crown salt [K(18-c-6)₂][Gd(NR₂)₃] (1) to generate the bimetallic (CS₃)^2- complex {[K(18-c-6)](μ₃-CS₃-κS,κ²S',S'')Gd(NR₂)₂]}₂, which contains two trithiocarbonate dianions that bridge Gd(III) centers and a potassium ion coordinated by 18-c-6. In contrast, the only crystalline product isolated from the reaction of CS₂ with the crypt salt [K(crypt)][Gd(NR₂)₃] (2) is [K(crypt)]{(R₂N)₂Gd[SCS(CH₂)Si(Me₂)N(SiMe₃)-κN,κS]}, which has a CS₂ unit inserted into a cyclometalated amide ligand. Complexes 1 and 2 reductively couple pyridine to form bridging dipyridyl moieties, (NC₅H₄-C₅H₄N)^2-, that generate bimetallic complexes differing only in the countercation, {[K(18-c-6)(C₅H₅N)₂]}₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]} and [K(crypt)]₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]}. Complexes 1 and 2 also show similar reactivity with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the (TEMPO)^- complexes [K(18-c-6)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)] and [K(crypt)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)], respectively. The first example of a bimetallic coordination complex containing a Bi-Gd bond, [K(crypt)][(R₂N)₃Gd(BiPh₂)], was obtained by treating 2 with BiPh₃.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

The reduction of Ln(iii)-trensal complexes allows to store electrons, that become available for CO₂ reduction, trough the formation of new C–C bonds.

Carbonyl group and carbon dioxide activation by rare-earth-metal complexes

The rare-earth elements (Ln = Sc, Y, La-Lu) are widely used in stoichiometric and catalytic carbonyl group transformations. Sufficient availability, non-toxicity, high oxophilicity, tunable ion size/Lewis acidity and enhanced ligand exchangeability have been major driving factors for their successful implementation. Routinely employed reagents for stoichiometric carbonyl group transformations are divalent ytterbium and samarium compounds (e.g., ketone reduction), bimetallic CeCl3/LiR (C-C coupling), or ceric ammonium nitrate CAN (cyclic ketone oxidation). Rare-earth-metal triflates, and in particular Sc(OTf)3, are prominent examples of Lewis acid catalysts for versatile use in organic synthesis (e.g., Aldol and Michael reactions). Moreover, Ln(ii) and Ln(iii) complexes efficiently catalyze the (co)polymerization of carbonyl group-containing monomers including lactones, lactides, acrylates, and carbon dioxide. Featuring the most notorious greenhouse gas, CO2 is currently assessed as a cheap, abundant, and non-toxic C1 building block. Ln(iii) complexes are not only capable of efficient CO2 capture via reversible insertion but also of CO2 activation for catalytic conversions (copolymerization/cycloaddition with epoxides). This perspective focuses on structurally elucidated Ln complexes resulting from ketone or carbonyl derivative activation/insertion as well as carbon dioxide insertion products. The respective compounds will be sorted by structural motifs and, if applicable, details on reactivity and feasibility of catalytic reactions are presented. The article is subdivided in three parts: (i) donor and insertion products of ketones and aldehydes, (ii) redox-enhanced activation of carbonyl derivatives, and (iii) CO2 insertion/redox products and homogeneous catalytic conversion.

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

Low-coordinate dinuclear lanthanide complexes supported by siloxides effect the reduction of carbon dioxide to both carbonate and oxalate, but the cooperative binding of CO2 to the two Ln(ii) cations in the dimer favours oxalate formation.

Synthesis and reactivity of rare-earth metal phosphaethynolates

Over the course of the last six years, research on the synthesis and reactivity of molecular metal phosphaketenes (M-PCO) has gained increasing attention. However, lanthanide complexes of the heavier group 15 cyanate analogue PCO^- have not been investigated so far. Herein we present exemplar studies on the nature and reactivity of rare-earth phosphaethynolato-complexes using three characteristic representatives of the rare-earth metals: Y, Nd and Sm. Our investigations comprise both +2 and +3 redox states, and one defined amidinate-based ligand set, as well as novel reaction pathways in the presence of the sequestering agents 18-crown-6 and 2,2,2-crypt.

Sterically induced reductive linkage of iron polypnictides with bulky lanthanide complexes by ring-opening of THF

Reduction of [Cp*Fe(η⁵-E₅)] (E = P, As) with divalent lanthanide reagents usually leads to reduction of [Cp*Fe(η⁵-E₅)] followed by a Ln-E bond formation. In contrast, by using the sterically encumbered reagent [(DippForm)₂Sm(thf)₂] (DippForm = {(2,6-ⁱPr₂C₆H₃)NC(H)[double bond, length as m-dash]N(2,6-ⁱPr₂C₆H₃)}^-), ring-opening of thf and reduction of the polypnictide is observed. This leads to two new 3d/4f polyphosphide or polyarsenide complexes [(DippForm)₂Sm(Cp*Fe)E₅{(CH₂)₄O}{(DippForm)₂Sm(thf)}], in which [(DippForm)₂Sm(thf)₂] and [Cp*Fe(η⁵-E₅)] are linked by a ring-opened thf molecule and no Ln-E bond formation is observed.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion

Methyl benzoate, N,N-dimethylbenzamide, and benzophenone were reduced by U^III [N(SiMe₃ )₂ ]₃ resulting in uranium(IV) products. Reduction of benzophenone lead to U^IV [OC⋅Ph₂ )][N(SiMe₃ )₂ ]₃ , (1.1) which forms the dinuclear complex, [N(SiMe₃ )₂ ]₃ U^IV (OCPhPh-CPh₂ O)U^IV [N(SiMe₃ )₂ ]₃ (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with U^III [N(SiMe₃ )₂ ]₃ resulted in U^IV [OC⋅(Ph)(NMe₂ )][N(SiMe₃ )₂ ]₃ (2), a uranium(IV) compound and the first example of a charge-separated amide radical. In the case of methyl benzoate, the reduction resulted in U^IV (OMe)[N(SiMe₃ )₂ ]₃ (3) and benzaldehyde as the reduced organic fragment. Compound 2 showed the ability to act as a uranium(III) synthon in its reactivity with trimethylsilyl azide, a reaction that yielded U^V (=NSiMe₃ )[N(SiMe₃ )₂ ]₃ . Additionally, 2 was reduced with potassium graphite resulting in [U(μ-O)[O=C(NMe₂ )(Ph)][N(SiMe₃ )₂ ]₂ ]₂ (4), a dinuclear uranium compound bridged by oxo ligands. Reduction of 2 in the presence of 15-crown-5 afforded isolation of the mono-oxo compound, [(15-crown-5)₂ K][UO[N(SiMe₃ )₂ ]₃ ] (5). The results expand the reduction capabilities of U^III complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

Formation and structure of the first metal complexes comprising amidino-guanidinate ligands

The first metal complexes comprising amidino-guanidinate ligands have been prepared and structurally characterized, namely bis-[μ-N,N',N'',N'''-tetraisopropyl-1-(1-butyl-amidinato)guanidinato-κ3N1,N2:N2]bis-[(tetra-hydro-furan)lithium], [Li2(C18H37N4)2(C4H8O)2], (2), and [bis-(tetra-hydro-furan)-lithium]-di-μ-chlorido-{(N,N'-di-cyclo-hexyl-1-butyl-amidinato-κ2N1,N2)[N,N',N'',N'''-tetra-cyclo-hexyl-1-(1-butyl-amidinato)guanidinato-κ2N1,N2]holmate(III)}, [HoLiCl2(C4H8O)2(C17H31N2)(C30H53N4)], (3). The novel lithium amidino-guanidinate precursors Li[ n BuC(=NR)(NR)C(NR)2] [1: R = Cy (cyclo-hex-yl), 2: R = i Pr) were obtained by treatment of N,N'-diorganocarbodi-imides, R-N=C=N-R (R = i Pr, Cy), with 0.5 equivalents of n-butyl-lithium under well-defined reaction conditions. An X-ray diffraction study of 2 revealed a ladder-type dimeric structure in the solid state. Reaction of anhydrous holmium(III) chloride with in situ-prepared 2 afforded the unexpected holmium 'ate' complex [ n BuC(=NCy)(NCy)C(NCy)2]Ho[ n BuC(NCy)2](μ-Cl)2Li(THF)2 (3) in 71% yield. An X-ray crystal structure determination of 3 showed that this complex contains both an amidinate ligand and the new amidino-guanidinate ligand.

Crystal structures of two ytterbium(III) complexes comprising alkynylamidinate ligands

Two ytterbium(III) complexes comprising alkynylamidinate ligands, namely bis-(η5-cyclo-penta-dien-yl)(3-cyclo-propyl-N,N'-diiso-propyl-propynamidinato-κ2N,N')ytterbium(III), [Yb(C5H5)2(C12H19N2)] or Cp2Yb[( i Pr2N)2C-C≡C-c-C3H5] (1) and tris-(3-phenyl-N,N'-di-cyclo-hexyl-propynamidinato-κ2N,N')ytterbium(III), [Yb(C21H27N2)3] or Yb[(CyN)2C-C≡C-Ph]3 (Cy = cyclo-hex-yl) (2) have been synthesized and structurally characterized. Both complexes are monomers; for complex 2, the contribution to the scattering from highly disordered toluene solvent molecules in these voids was removed with the SQUEEZE routine [Spek (2015). Acta Cryst. C71, 9-18] in PLATON. The stated crystal data for Mr, μ etc. do not take these into account.

1,2,4-Diazaphospholide complexes of lanthanum(iii), cerium(iii), neodymium(iii), praseodymium(iii), and samarium(iii): synthesis, X-ray structural characterization, and magnetic susceptibility studies

A few heteroleptic, charge-separated heterobimetallic, and polymeric alkali metalate complexes of 1,2,4-diazaphospholide lanthanum(iii), cerium(iii), neodymium(iii), praseodymium(iii), and samarium(iii) were simply prepared via the metathesis reaction of MCl3 (THF)m (m = 1-2) and K[3,5-R2dp] ([3,5-R2dp](-) = 3,5-di-substituent-1,2,4-diazaphospholide; R = tBu, Ph) in a varied ratio (1 : 3, 1 : 4, and 1 : 5, respectively) at room temperature in tetrahydrofuran. All the complexes were fully characterized by (1)H, (13)C{(1)H}, (31)P{(1)H}, IR, and X-ray single crystal diffraction analysis despite their paramagnetism (excluding La(iii) complexes). The structures of the complexes were found to feature varied coordination modes. The magnetic properties of several compounds were studied by magnetic susceptibility, and the complexes presented the magnetic moments close to or lower than the theoretical values for the free ions in the trivalent oxidation states (Pr(3+), Nd(3+)).

Exploring the effect of the LnIII/LnIIredox potential on C–F activation and on oxidation of some lanthanoid organoamides

The role of the Ln^IIoxidation state (Ln = Eu, Yb, Sm) in C–F activation reactions has been explored.

Metal-free access of bulky N,N′-diarylcarbodiimides and their reduction: bulky N,N′-diarylformamidines

A facile synthetic protocol for the preparation of N,N′-diarylcarbodiimides and their reduction with sodium borohydride in ethanol to obtain N,N′-diarylformamidines has been demonstrated.

Heterometallic rare-earth metal complexes with imino-functionalized 8-hydroxyquinolyl ligands: synthesis, characterization and catalytic activity towards hydrophosphinylation of trans-β-nitroalkene

The heterobimetallic complexes exhibited high catalytic activities on the hydrophosphinylation of β-nitroalkenes.