Β-diketiminate Ligand Research Articles

Three-Coordinate Copper(II) Alkynyl Complex in C-C Bond Formation: The Sesquicentennial of the Glaser Coupling.

Copper(II) alkynyl species are proposed as key intermediates in numerous Cu-catalyzed C-C coupling reactions. Supported by a β-diketiminate ligand, the three-coordinate copper(II) alkynyl [CuII]-C≡CAr (Ar = 2,6-Cl2C6H3) forms upon reaction of the alkyne H-C≡CAr with the copper(II) tert-butoxide complex [CuII]-OtBu. In solution, this [CuII]-C≡CAr species cleanly transforms to the Glaser coupling product ArC≡C-C≡CAr and [CuI](solvent). Addition of nucleophiles R'C≡C-Li (R' = aryl, silyl) and Ph-Li to [CuII]-C≡CAr affords the corresponding Csp-Csp and Csp-Csp2 coupled products RC≡C-C≡CAr and Ph-C≡CAr with concomitant generation of [CuI](solvent) and {[CuI]-C≡CAr}-, respectively. Supported by density functional theory (DFT) calculations, redox disproportionation forms [CuIII](C≡CAr)(R) species that reductively eliminate R-C≡CAr products. [CuII]-C≡CAr also captures the trityl radical Ph3C· to give Ph3C-C≡CAr. Radical capture represents the key Csp-Csp3 bond-forming step in the copper-catalyzed C-H functionalization of benzylic substrates R-H with alkynes H-C≡CR' (R' = (hetero)aryl, silyl) that provide Csp-Csp3 coupled products R-C≡CR via radical relay with tBuOOtBu as oxidant.

Cover Feature: A Phosphine Functionalized β‐Diketimine Ligand for the Synthesis of Manifold Metal Complexes (Chem. Eur. J. 58/2020)

A bis(diphenyl)-phosphine functionalized β-diketimine ligand is depicted in the cover image. This ligand coordinates in a very flexible manner to a number of transition metal ions. The newly designed ligand features a symmetrically placed PNNP-type pocket. Due to the hard and soft donor atoms (N vs. P) the ligand can stabilize various coordination polyhedra. A complete series ranging from coordination numbers 2 to 6 was realized, resulting in linear, trigonal planar, square planar, tetrahedral, square pyramidal, and octahedral coordination polyhedral. More information can be found in the Full Paper by P. W. Roesky et al. on page 13191.

A Phosphine Functionalized β-Diketimine Ligand for the Synthesis of Manifold Metal Complexes.

A bis(diphenyl)‐phosphine functionalized β‐diketimine (PNac‐H) was synthesized as a flexible ligand for transition metal complexes. The newly designed ligand features symmetrically placed phosphine moieties around a β‐diketimine unit, forming a PNNP‐type pocket. Due to the hard and soft donor atoms (N vs. P) the ligand can stabilize various coordination polyhedra. A complete series ranging from coordination numbers 2 to 6 was realized. Linear, trigonal planar, square planar, tetrahedral, square pyramidal, and octahedral coordination arrangements containing the PNac‐ligand around the metal center were observed by using suitable metal sources. Hereby, PNac‐H or its anion PNac− acts as mono‐, bi‐ and tetradendate ligand. Such a broad flexibility is unusual for a rigid tetradentate system. The structural motifs were realized by treatment of PNac‐H with a series of late transition metal precursors, for example, silver, gold, nickel, copper, platinum, and rhodium. The new complexes have been fully characterized by single crystal X‐ray diffraction, NMR, IR, UV/Vis spectroscopy, mass spectrometry as well as elemental analysis. Additionally, selected complexes were investigated regarding their photophysical properties. Thus, PNac‐H proved to be an ideal ligand platform for the selective coordination and stabilization of various metal ions in diverse polyhedra and oxidation states.

Second-order NLO properties of bis-cyclometalated iridium(III) complexes with β-diketiminate ancillary ligand: Substituent and redox effect

Density functional theory (DFT) calculations have been carried out to investigate the switching of the second-order nonlinear optical (NLO) properties of iridium(III) complexes. The aim of this paper is to calculate the substituent effect and redox effect on geometrical and electronic structures, redox center, UV–Vis absorption spectra and second-order nonlinear optical (NLO) properties. It is found that changing β-diketiminate ancillary ligand have little effect on first hyperpolarizabilities values, but the redox process can significantly enhance the second-order NLO responses. The βtot values of 1+,2+and3+oxidation states of the complexes are about 10.2 times, 19.7 times and 25.5 times larger than their corresponding neutral complexes, respectively. The βtot values of the complexes in their various reduction states 1-, 2- and 3- are about 13.2 times, 35.0 times and 8.5 times larger than neutral complexes, respectively. The complexes in their various reduction or oxidation states display larger first hyperpolarizabilities than neutral complexes due to the remarkable bathochromic shift of maximal absorption and the decrease of related transition energies. The theoretical investigation of these iridium complexes will be helpful for providing a fundamental guideline and reference for further research for novel NLO materials.

Silicon-carbonyl complex is stable at room temperature

Transition metals are attractively reactive but can be expensive and toxic. For years, chemists have been probing the reactivity of main-group elements to provide suitable alternatives, but they have not had much luck making carbonyl complexes from non-d-block elements, presumably because of these elements’ lack of π back-bonding orbitals. Now, Stephan Schulz and coworkers at the University of Duisburg-Essen and Justus Liebig University Giessen have synthesized the first known silicon-carbonyl complex that’s stable at room temperature (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0456-x). They were able to accomplish this by beefing up the silicon with two gallium β-diketiminate ligands, which are known to stabilize low-valent main-group complexes. The group then directly reacted CO with the electron-rich silylene in benzene at 60 °C to make the carbonyl complex in 30% yield. In the past, scientists have been able to isolate these compounds only under cryogenic conditions in solid argon. The new compound is

Chain Transfer to Toluene in Styrene Coordination Polymerization.

The chain-transfer reaction is rather important in coordination polymerization regarding catalytic efficiency, adjustment of molecular weight, and control of chain structure. To date, chain transfer to H2 and Al, Mg, and Zn alkyl compounds and β-H elimination are the commonly encountered modes. Now a novel chain transfer to toluene is reported. By introducing fluorine atoms into the β-diketimine ligands, an inert catalytic system for styrene (St) coordination polymerization was transferred into the highly active one. The activity increased with an increase in the number of fluorines in the ligands. Surprisingly, the molecular weights of resultant polystyrenes are very low (Mn =2000-6600 Da) despite of St loadings, corresponding up to 121 chains per active species. The mechanisms were investigated by DFT simulation, MALDI-TOF MS, isotope tracing experiment and 2D NMR spectrum analyses, which revealed that the fluorine activated the polymerization and directed chain transfer to toluene.

The unique β-diketiminate ligand in aluminum(i) and gallium(i) chemistry.

Over the past few decades, β-diketiminate ligands have been widely used in coordination chemistry and are capable of stabilizing various metal complexes in multiple oxidation states. Recently, the chemistry of aluminum and gallium in their +1 oxidation state has rapidly emerged. NacNacM(i) (M = Al, Ga; NacNac = β-diketiminate ligand) shows a two coordinate metal center comparable with singlet carbene-like species. The metal center also possesses a formally vacant p-orbital. In this article we present an overview of the last 10 years for aluminum(i) and gallium(i) stabilized by β-diketiminate ligands that have been widely explored in bond breaking and forming species.

The electronic structure of a β-diketiminate manganese hydride dimer.

The electronic structure of a dimeric manganese hydride catalyst supported by β-diketiminate ligands, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was investigated with density functional theory. A triple bond between the manganese centres was anticipated from simple electron-counting rules; however, calculations revealed Mn-Mn Mayer bond orders of 0.21 and 0.27 for the ferromagnetically-coupled and antiferromagnetically-coupled extremes, respectively. In accordance with experimentally determined Heisenberg exchange coupling constants of -15 ± 0.1 cm-1 (SQUID) and -10.2 ± 0.7 cm-1 (EPR), the calculated J0 value of -10.9 cm-1 confirmed that the ground state involves antiferromagnetic coupling between high spin Mn(ii)-d5 centres. The effect of steric bulk on the bond order was examined via a model study with the least sterically-demanding version of the β-diketiminate ligand and was found to be negligible. Mixing between metal- and β-diketiminate-based orbitals was found to be responsible for the absence of a metal-metal multiple bond. The bridging hydrides give rise to a relatively close positioning of the metal centres, while bridging atoms possessing 2p orbitals result in longer Mn-Mn distances and more stable dimers. The synthesis and characterization of the bridging hydroxide variant, [(2,6-iPr2PhBDI)Mn(μ-OH)]2, provides experimental support for these assessments.

Mg-Mg bond polarization induced by a superbulky β-diketiminate ligand.

The bulk of a recently reported superbulky β-diketiminate ligand was further increased by introducing tBu substituents in the ligand backbone. Attempts to isolate free Mg radicals with this extremely bulky ligand failed. Instead, a dinuclear Mg(i) complex with one chelating and one monodentate β-diketiminate ligand was isolated. Asymmetry in metal coordination results in a polarized Mg-Mg bond.

Electronic effect of a perfluorinated β-diketiminate ligand on the bonding nature of copper carbonyl complexes

Two copper complexes 17Fnac2Cu(C6H6) (3) and 17Fnac2CuCO (4) containing the monoanionic, perfluorinated β-diketiminate 17Fnac2- ligand (1) (17Fnac2 = FC[C(CF3)N(C6F5)]2) were synthesized and characterized by IR and NMR spectroscopy (1H, 13C, 19F), cyclovaltammometry (CV), elemental analysis and single crystal X-ray diffraction. The perfluorinated 17Fnac2- ligand marginally reduces the π-back-bonding capacity of the copper centre to the carbonyl group in 4 when compared with the corresponding 16Fnac2- substituted complexes but substantially when compared with the fluorine free substituted derivatives. Quantum chemical calculations gave deeper insight into the bonding situation of this carbonyl complex, while CV studies were performed to determine the oxidation potential of 3 in solution. Based on these data, the influence of the degree of fluorination in different β-diketimine ligands on the electronic nature of the corresponding copper complexes is discussed.

Syntheses of asymmetrical magnesium(i) complexes and their catalytic application in epoxide hydroboration

A series of novel structural magnesium(i) complexes stabilized by cyclopentyl and cyclohexyl substituted β-diketiminate ligands have been synthesized and used as highly active and regioselective pre-catalysts for various epoxides hydroboration.

Synthesis and luminescence studies of lanthanide complexes (Gd, Tb, Dy) with phenyl- and 2-pyridylthiolates supported by a bulky β-diketiminate ligand. Impact of the ligand environment on terbium(iii) emission

Structure-dependent photoluminescence was studied for lanthanide complexes with widespread Nacnac ligands, supplemented by thiolates; notable antenna properties of the latter are revealed.

Probing the non-innocent nature of an amino-functionalised β-diketiminate ligand in silylene/iminosilane systems

Electron-rich β-diketiminate ligands, featuring amino groups at the backbone β positions ("N-nacnac" ligands) have been employed in the synthesis of a range of silylene (SiII) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe3)2, P(SiMe3)2 and Si(SiMe3)3). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl SiIV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X, such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe3)2, but not for X = H, N(SiMe3)2 and Si(SiMe3)3; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups, since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the SiII centre at the β-carbon position of the N-nacnac ligand backbone.

Dinuclear Aluminum Halide Complexes Based on Bis(β-diketiminate) Ligands: Synthesis, Structures, and Electrochemical Characterization.

Twenty-three dinuclear aluminum halide complexes based on bis(β-diketiminate) ligands with various flexible and rigid bridging groups have been synthesized and fully characterized by NMR and IR spectroscopy, cyclic voltammetry, and elemental analysis. In addition, nine complexes were structurally characterized by means of single-crystal X-ray diffraction. Attempts to reduce the dinuclear species using C8K remained unsuccessful, but the reduction potentials investigated by cyclic voltammetry indicate the principle feasibility.

Computational Analysis of Two-State Reactivity in β-Hydride Elimination Mechanisms of Fe(II)– and Co(II)–Alkyl Complexes Supported by β-Diketiminate Ligand

β-Hydride elimination in Fe(II)– and Co(II)–alkyl complexes is known to occur through two different spin multiplicity states and is thus a good example of two-state reactivity. In this study, the automated reaction path search method combined with the mixed-spin effective Hamiltonian approach has been applied to understand the detailed reaction mechanisms including the characterization of the spin-inversion points between the high-spin and low-spin potential energy surfaces for the Fe(II)–C2H5 and Co(II)–C2H5 complexes supported by a β-diketiminate ligand. Density functional theory (DFT) with different exchange-correlation functionals has been used in the reaction path search calculations. We found that the β-hydride elimination process for these complexes consists of multiple steps including two spin-inversion points. We have also investigated the substituent effect in the β-diketiminate ligand to understand the steric and electronic effects on the spin-inversion process. The efficiency of the spin-inver...

Characterization and Photophysical Properties of a Luminescent Aluminum Hydride Complex Supported by a β-Diketiminate Ligand

Aluminum hydrides are versatile compounds utilized as reducing agents, precursors of aluminum complexes, and as catalysts for polymerization reactions. However, their photophysical properties have been overlooked, although several luminescent aluminum complexes have been utilized conventionally for emitting layers in modern light-emitting devices. Herein, we report the synthesis and photophysical properties of a luminescent β-diketiminate dihydride complex through the reaction between lithium aluminum hydride and the corresponding ligand. The obtained compound exhibits crystallization-induced emission (CIE) properties at room temperature and long-lifetime phosphorescence at 80 K. Our experimental and theoretical investigations suggest that low-energy molecular vibration could play an important role in the realization of the CIE property.

A [2Fe-1S] Complex That Affords Access to Bimetallic and Higher-Nuclearity Iron-Sulfur Clusters.

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes of iron-sulfur clusters in biological and synthetic chemistry. Here we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered β-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe3 on each iron(II) ion has been crystallographically characterized. The PMe3 ligands may be removed with B(C6F5)3 to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

Chiral β-diketiminate-supported magnesium alkyl complexes: Synthesis and their application in the polymerization of rac-lactide

A series of novel magnesium alkyl complexes bearing chiral β-diketiminate ligands [CH3C[NCH(CH3)Ph]CHC(NAr)CH3]Mg(nBu)(THF) (Ar = 2,6-iPr2C6H3 (1)), [CH3C[NCH(CH3)Naph]CHC(NAr)CH3]Mg(nBu)(THF) (Naph = 1-naphthyl; Ar = 2,6-iPr2C6H3 (2), 2,6-Et2C6H3 (3), 2,6-Me2C6H3 (4)) and [CH{C[NCH(CH3)Naph]CH3}2]Mg(nBu)(THF) (5) have been prepared by reaction of the corresponding chiral ligand precursors (HL1-HL5) with MgnBu2. These complexes were characterized by various spectroscopic methods, and the solid-state structures of compounds 1, 3 and 4 were further confirmed by single-crystal X-ray diffraction analyses. Complexes 1–5 are active catalysts for the ring-opening polymerization (ROP) of rac-lactide, leading to heterotactic-rich polylactides within 5 min under mild conditions. These complexes show medium to high heteroselectivity (Pr = 0.71–0.92).

Heteroleptic Heavier Alkaline Earth Metal Amide Complexes Stabilized by a Superbulky β-Diketiminate Ligand

Heteroleptic alkaline earth metal (Ae = Ca, Sr, Ba) amide complexes with the superbulky β-diketiminate ligand DIPePBDI (CH[C(Me)N-DIPeP]2, DIPeP = 2,6-di-iso-pentylphenyl) have been prepared by dir...

Novel chalcogenide vanadium complexes with β-diimine ligand: synthesis and structural studies

Three new chalcogenide vanadium complexes with a β-diketiminate ligand have been synthesized by reduction of the elemental chalcogens (S, Se, Te) with [(µ-η6:η6-toluene){V(nacnac)}2] (1) (nacnac = HC(C(Me)NC6H3-iPr2)2). The structures of the sulfide and selenide complexes [((nacnac)V = E)2(μ,ƞ2-E2)] (E = S (2), Se (3)) are similar; the chalcogenides act both as bridging (E22-) and terminal (E2-) ligands. In compound [V2(µ-Te)2(nacnac)2] (4), all tellurium atoms are present only as bridging Te2- ligands. The compounds have been characterized by IR-spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The possibility of the formation of additional V = Te bonds in 4 has been studied using Hirshfeld analysis. It has been found that the difference in the reactivity of 1 towards Te is not due to the steric hindrance.