Mono-, di-, and Ttianionic beta-diketiminato ligands: a computational study and the synthesis and structure of [(YbL)(3)(THF)], L = [[N(SiMe(3))C(Ph)](2)CH]

A π-extended β-diketiminate ligand via a templated Scholl approach

We report a templated Scholl oxidation strategy for the preparation of the first β-diketiminate (BDI) ligands embedded within a 24-electron π-system backbone. The resulting benzo[f,g]tetracene BDI ligand was coordinated to a zinc centre and electrochemical studies showed the redox active nature of the ligand.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

A facile approach to C-functionalized β-ketoimine compounds via terminal alkylation of a tetralithiated intermediate

A series of C-functionalized β-ketoimine compounds at the terminal methyl groups of the β-ketoimine precursor L^phH₂ (L^ph = C₆H₄[NC(Me)CHC(Me)O]₂) were prepared. This convenient transformation was realized via straightforward double alkylation on the terminal C_α of a novel bis-dianionic β-ketoiminate lithium complex [L^ph'Li₄(THF)₄]₂ (L^ph' = C₆H₄[NC(Me)CHC(O)CH₂]₂) followed by hydrolysis.

Reversible Ligand-Centered Reduction in Low-Coordinate Iron Formazanate Complexes

Coordination of redox-active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox-active formazanate ligands in low-coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non-redox-active β-diketiminate complexes, and the reduced three-coordinate formazanate-iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand-centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low-coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N-I bond that is not seen with the β-diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity.

Versatile reactivities of rare-earth metal dialkyl complexes supported by a neutral pyrrolyl-functionalized β-diketiminato ligand

The first unprecedented examples of S-β-diketiminato species were disclosed in the reactions of rare-earth dialkyl complexes with S₈.

Stable, crystalline boron complexes with mono-, di- and trianionic formazanate ligands

Boron complexes are isolated with formazanate ligands in mono-, di- and trianionic form, showing their distinctive ability to function as electron-reservoir.

Synthesis, characterization and catalytic activity of rare-earth metal amides incorporating cyclohexyl bridged bis(β-diketiminato) ligands

The bridged bis(β-diketiminato) ligands supported rare-earth amides exhibited high catalytic activity towards the hydrophosphination of β-nitroalkene and α,β-unsaturated carbonyl derivatives with an excellent regioselectivity.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.

Intramolecular C-C Bond Coupling of Nitriles to a Diimine Ligand in Group 7 Metal Tricarbonyl Complexes

Dissolution of M(CO)3(Br)(L(Ar)) [L(Ar) = (2,6-Cl2-C6H3-NCMe)2CH2] in either acetonitrile [M = Mn, Re] or benzonitrile (M = Re) results in C-C coupling of the nitrile to the diimine ligand. When reacted with acetonitrile, the intermediate adduct [M(CO)3(NCCH3)(L(Ar))]Br forms and undergoes an intramolecular C-C coupling reaction between the nitrile carbon and the methylene carbon of the β-diimine ligand.

Crystalline di- or trianionic metal (Al, Sm) beta-diketiminates

The new [Al(L(Tol))Br(2)] (1) and the known [Al(L(Ph))Me(2)] (2) were prepared as potential precursors to more novel aluminium compounds. In the event, only the latter was effective. Thus 2 and two equivalents of potassium was the source of [{Al(L(Ph))Me(2)K(2)(OEt(2))}(2)] (3). The complexes [{KSm(L(Ph))(2)}(2)] (4), [Sm(2)(L(Dph))(3)] (5) and [Sm(L(Tol,Ad))(L(Tol,Ad)-H)] (6) were obtained from SmI(2) and two equivalents of the appropriate potassium beta-diketiminate [L(Ph) or L(Tol) or L(Dph) = {N(SiMe(3))C(Ar)}(2)CH, Ar = Ph or C(6)H(4)Me-4 or C(6)H(4)Ph-4; L(Tol,Ad) = N(SiMe(3))C(C(6)H(4)Me-4)C(H)C(Ad)NSiMe(3), L(Tol,Ad)-H = N(SiMe(3))C(C(6)H(4)Me-4)C(H)C(Ad)NSiMe(2)CH(2), Ad = 1-adamantyl]. Crystalline complexes 3-6 were isolated in very low (4, 5) or satisfactory (3, 6) yield and characterised by X-ray diffraction. From comparisons of the M-N, N-C, C-C and C-C(Ar) bond lengths with suitable standards, complexes 3-5 are assigned as containing Al(3+)/(L(Ph))(3-) for 3, Sm(3+)/(L(Ph))(-)/(L(Ph))(3-) for 4, and {Sm(3+)}(2)/(L(Dph))(-)/(L(Dph))(2-)/(L(Dph))(3-) for 5. Complex 6 is best formulated as a Sm(3+) compound with one "normal" (L(Tol,Ad))(-) and one deprotonated (L(Tol,Ad)-H)(2-) ligand.

Synthesis and structural characterization of novel mixed-valent samarium and divalent ytterbium and europium complexes supported by amine bis(phenolate) ligands

The amine-elimination reactions of Ln[N(SiMe3)2]2(THF)2(Ln=Sm, Yb and Eu) with amine bis(phenol)s (L1H2=[BunN(CH(2)-2-OC6H(2)-3,5-But2)2]H2; L2H2=[Me2NCH2CH2N(CH(2)-2-OC6H(2)-3,5-But2)2]H2) were investigated. It was found that the number of heteroatom(s) in the ligands has a profound effect on the reaction outcome for the samarium systems. Reaction of the tetradentate diamino-bis(phenol)s L2H2 with Sm[N(SiMe3)2]2(THF)2 afforded a yellow solution, which indicated the complete oxidation of the SmII species, yellow being the characteristic color of SmIII species, while the same reaction with Eu[N(SiMe3)2]2(THF)2 gave a divalent complex with a dimeric structure (EuL2)2. Using the tridentate amine bis(phenol)s L1H2 as the reagent, the novel mixed-valent samarium complex SmIII2SmIIL1(4) was prepared by the same reaction. Both reactions of L1H2 with Yb[N(SiMe3)2]2(THF)2 and Eu[N(SiMe3)2]2(THF)2 yielded the normal divalent lanthanide complexes: monomeric complex for YbII, YbL1(THF)3 and dimeric complex for EuII, (EuL1)2. All of the complexes are well characterized with elemental analyses, IR and 1H NMR spectra for , and , as well as X-ray crystal structure determination in the cases of complexes , , and .

Synthesis and Structural Characterization of a Series of Lanthanide(II) Amides: Steric Effect of Amido Ligand

The synthesis and structures of a series of lanthanide(II) and lanthanide(III) complexes supported by the amido ligand N(SiMe3)Ar were described. Several lanthanide(III) amide chlorides were synthesized by a metathesis reaction of LnCl3 with lithium amide, including {[(C6H5)(Me3Si)N]2YbCl(THF)}2.PhCH3 (1), [(C6H3-iPr2-2,6)(SiMe3)N]2YbCl(mu-Cl)Li(THF)3.PhCH3 (4), [(C6H3-iPr2-2,6)(SiMe3)N]YbCl2(THF)3 (6), and [(C6H3-iPr2-2,6)(SiMe3)N]2SmCl3Li2(THF)4 (7). The reduction reaction of 1 with Na-K alloy afforded bisamide ytterbium(II) complex [(C6H5)(Me3Si)N]2Yb(DME)2 (2). The same reaction for Sm gave an insoluble black powder. An analogous samarium(II) complex [(C6H5)(Me3Si)N]2Sm(DME)2 (3) was prepared by the metathesis reaction of SmI2 with NaN(C6H5)(SiMe3). The reduction reaction of ytterbium chloride 4 with Na-K alloy afforded monoamide chloride {[(C6H3-iPr2-2,6)(SiMe3)N]Yb(mu-Cl)(THF)2}2 (5), which is the first example of ytterbium(II) amide chloride, formed via the cleavage of the Yb-N bond. The same reduction reaction of 7 gave a normal bisamide complex [(C6H3-iPr2-2,6)(SiMe3)N]2Sm(THF)2 (8) via Sm-Cl bond cleavage. This is the first example for the steric effect on the outcome of the reduction reaction in lanthanide(II) chemistry. 5 can also be synthesized by the Na/K alloy reduction reaction of 6. All of the complexes were fully characterized including X-ray diffraction for 1-7.

Ytterbium(II) Complex Bearing a Diaminobis(phenolate) Ligand: Synthesis, Structure, and One-Electron-Transfer and ε-Caprolactone Polymerization Reactions

The first divalent ytterbium complex supported by a diaminobis(phenolate) ligand, YbL(THF)2.0.5C7H8 (1; THF = tetrahydrofuran), was synthesized in good yield by the amine elimination reaction of Yb[N(SiMe3)2]2(THF)2 with H2L (L = [Me2NCH2CH2N(CH2-2-OC6H2-3,5-But2)2]) in a 1:1 molar ratio. X-ray structural determination shows complex 1 to be a THF-solvated monomer, which adopts a distorted octahedral coordination geometry around the Yb atom. Complex 1 can react with PhNCO and PhCCH, as a single electron-transfer reagent, to give the corresponding reduction coupling product [(YbLOCNPh)(THF)]2.4THF (2) and the alkynide complex YbLCCPh(DME) (3; DME = 1,2-dimethoxyethane). Complexes 2 and 3 have been characterized by X-ray crystal structural analysis. In complex 2, the dianionic oxamide ligand resulting from the reductive coupling of two phenyl isocyanate molecules coordinates to two Yb atoms in a mu,eta4 fashion. Complex 3 has a monomeric structure with a Yb-C(terminal phenylacetynide) bond length of 2.374(3) A. Complex 1 is also a highly efficient catalyst for ring-opening polymerization of epsilon-caprolactone.

Organolanthanide mediated catalytic cycles: a computational perspective

In this perspective the contribution of recent theoretical studies to our understanding of lanthanide (Ln) catalysis is explored. In general, the results of computational studies have proven consistent with available experimental evidence. Considerable success has been obtained in elucidating the mechanisms for C-H bond activation (sigma-bond metathesis in particular) and the addition of C-X bonds across an unsaturated functionality (and the hydroamination of alkenes in particular). Ln catalysts are computationally challenging because relativistic effects are important, and large ligands are required to restrict high coordination numbers, in addition to limiting facile redistribution processes. Thus, key technical issues relating to the computational investigation of organolanthanide complexes are discussed. Increasing computational resources have seen studies expand from the optimisation of simple molecules to the study of catalytic cycles where the Ln is coordinated by larger and more complex ligands. The ability of theoretical studies to complement experimental developments by supplying a deeper understanding of the mechanistic process is reviewed with emphasis on the elucidation of transition state structures, intermediates, spectator ligand coordination, and negative entropy steps. Recent computational investigations of the catalytic cycle for Ln mediated hydroamination are a focus, as these have provided substantial and detailed rationalisations for the regio- and stereo-selectivity of inter- and intra-molecular hydroamination. Examination of transition state geometries and electronic structure appears to offer insights that could be used to facilitate the rational design of new Ln-based catalysts.