Synthesis and Reactions of a Redox-Active α-Diimine Aluminum Complex

Novel bidentate N-coordinated alkylaluminum complexes: synthesis, characterization, and efficient catalysis for hydrophosphonylation

Five new alkylaluminum complexes with different pyridinyl-substituted imines or cyclohexyl-substituted imines were synthesized and characterized successfully. The aluminum complex [FlCHNCH(CH3)Py]AlMe2(Py = 2-pyridyl) (1) was obtained by reacting 9-[2-pyridyl-CH(CH3)-NCH]Fl (Fl = fluorenyl) (L1) and equimolar AlMe3. The reactions of 9-(2-pyridyl-NCH)Fl (L2) and 9-[2-N(CH3)2-cyclohexyl-NCH]Fl (L3) with equimolar AlMe3 or AlEt3 afforded other alkylaluminum complexes [FlCHNPy]AlMe2(Py = 2-pyridyl) (2), [FlCHNPy]AlEt2 (Py = 2-pyridyl) (3), [FlCHNCyN(CH3)2]AlMe2 (Cy = 2-cyclohexyl) (4) and [FlCHNCyN(CH3)2]AlEt2 (Cy = 2-cyclohexyl) (5). All these complexes (1-5) were characterized using NMR spectroscopy, elemental analysis, and X-ray crystal structure analysis. The catalytic properties of these new alkylaluminum complexes for the hydrophosphonylation of aldimines were examined. Complex 5 showed the best catalytic performance under mild reaction conditions with a low catalyst loading (1 mol%), and 20 different substituents of aldimines were isolated with more than 90% yields.

Ultrahigh activity and broad temperature resistance of amine-imine cobalt precatalysts for butadiene polymerization

A series of amine-imine cobalt complexes (Co1-Co7) has been prepared and characterized. The complexes Co3, Co4, and Co6 have a distorted tetrahedral geometry, as determined by single crystal X-ray diffraction. In the presence of ethylaluminum sesquichloride (EASC), Co3 exhibited ultra-high activity toward butadiene (Bd) polymerization (up to 7813 kgpolymer mol-1 h-1). The activity is higher than any yet recorded for which yield high cis-1,4 polybutadiene by the well-defined late-transition metal catalytic system. The catalyst also exhibited excellent tolerance towards the ratio of Co/Bd and broad temperature stability. At a ratio of Bd/Co3 = 50 000, the complexes Co1-3 can afford polybutadiene with yields higher than 96% within 2 hours. At -20 °C to 100 °C, the complex Co3 afforded relatively high polymer yields at low catalyst concentrations (Bd/Co3 = 25 000). In addition, all polymers showed a relatively high molecular weight (up to 1.06 × 106 g mol-1).

Unveiling Complexity of the Oxygenation of Aluminum Alkyls by the Isolation of Unique Alkylperoxide and Oxoalkoxide Compounds

While aluminum alkyls are often considered to be exemplary compounds of main-group organometallics and an in-depth understanding of their multifaceted chemistry is continually vital, the controlled oxygenation of organoaluminum complexes still remains a largely undeveloped area. In the course of our systematic studies on the relationship between the Lewis acidity of metal centers and noncovalent interactions in the secondary coordination sphere, we report the oxygenation of dialkylaluminum complexes incorporating a pyrrole–ester ligand, as purposefully selected dormant Lewis acidic octet-compliant model compounds, and the isolation and characterization of a new, dimeric aluminum tert-butylperoxide and an unique example of an aluminum oxoethoxide cluster. Our studies provide a more in-depth look at the diversity and complexity of the oxygenation chemistry of aluminum alkyls.

We report the reactions of dialkylaluminum complexes incorporating a pyrrole−ester ligand with dioxygen as well as the isolation of a novel aluminum tert-butylperoxide and an unique example of aluminum oxoethoxide. Our studies provide a more in-depth look at the diversity and complexity of the oxygenation chemistry of aluminum alkyls.

Syntheses of Dianionic α-Iminopyridine Rare-Earth Metal Complexes and Their Catalytic Acitivities toward Dehydrogenative Coupling of Amines with Hydrosilanes

Reactions of [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with aminomethylene-substituted pyridine 2-[O(CH₂CH₂)₂NCH₂CH₂NCH₂]C₅H₄N (1) gave the dianionic α-iminopyridine rare-earth metal amido complexes {μ-η²:σ¹:κ¹:κ¹-2-[O(CH₂CH₂)₂NCH₂CH₂NCH]C₅H₄N}₂RE₂[N(SiMe₃)₂]₂ (RE = Y(2a), La(2b), Pr(2c), Nd(2d), Sm(2e), Dy(2f), Er(2g), and Lu (2h)). However, reaction of [(Me₃Si)₂N]₃Y(μ-Cl)Li(THF)₃ with pyridin-2-ylmethyl-substituted amines such as 2-(RNHCH₂)C₅H₄N (R = ^tBu (3a) and 2,6-ⁱPr₂Ph (3b)) or benzyl-substituted amine O(CH₂CH₂)₂NCH₂CH₂NHCH₂C₆H₅ (5) afforded the corresponding yttrium complexes containing monoanionic ligands [2-(RNCH₂)C₅H₄N]₂YN(SiMe₃)₂ (R = ^tBu (4a) and 2,6-ⁱPr₂Ph (4b)) or [O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅][(Me₃Si)₂N)]Y(μ-Cl)(μ-η³-O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅)Li(THF) (6). Dianionic α-iminopyridine rare-earth metal amido complexes showed high catalytic activities for the dehydrogenation coupling reaction of hydrosilanes and amines providing a variety of silylamines in high yields.

Group 13-derived radicals from α-diimines via hydro- and carboalumination reactions

Mechanochemically synthesized alanes and (hydrido-)alanes were reacted with α-diimines to form aluminium derived radicals.

α-Diimine-Niobium Complex-Catalyzed Deoxychlorination of Benzyl Ethers with Silicon Tetrachloride

α-Diimine niobium complexes serve as catalysts for deoxygenation of benzyl ethers by silicon tetrachloride (SiCl₄) to cleanly give two equivalents of the corresponding benzyl chlorides, where SiCl₄ has the dual function of oxygen scavenger and chloride source with the formation of a silyl ether or silica as the only byproduct. The reaction mechanism has two successive trans-etherification steps that are mediated by the niobium catalyst, first forming one equivalent of benzyl chloride along with the corresponding silyl ether intermediate that undergoes the same reaction pathway to give the second equivalent of benzyl chloride and silyl ether.

Synthesis and Characterization of Neutral Ligand α-Diimine Complexes of Aluminum with Tunable Redox Energetics

The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al(L_Ar)₂Cl₂)][AlCl₄] [L_Ar = N, N'-bis(4-R-C₆H₄)-2,3-dimethyl-1,4-diazabutadiene] are structurally analogous, as determined by multinuclear NMR spectroscopy and solid-state X-ray diffraction, across a range of electron-donating [R = Me (2), ^tBu (3), OMe (4), and NMe₂ (5)] and electron-withdrawing [R = Cl (6), CF₃ (7), and NO₂ (8)] substituents in the aryl side arm of the ligand. UV-vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties, respectively, of the complexes. Both sets of properties are shown to be dependent on the R substituent. Density functional theory calculations performed on the [Al(L_Ph)₂Cl₂)][AlCl₄] complex (1) indicate primarily ligand-based frontier orbitals and were used to help support our discussion of both the spectral and electrochemical data. We also report the reaction of the L_Ph ligand with both AlBr₃ and AlI₃ and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.

α-Diimines as Versatile, Derivatizable Ligands in Ruthenium(II) p-Cymene Anticancer Complexes

α-Diimines are among the most robust and versatile ligands available in synthetic coordination chemistry, possessing finely tunable steric and electronic properties. A series of novel cationic ruthenium(II) p-cymene complexes bearing simple α-diimine ligands, [(η⁶- p-cymene)RuCl{κ² N-(HCNR)₂}]NO₃ (R = Cy, [1]NO₃; R = 4-C₆H₁₀OH, [2]NO₃; R = 4-C₆H₄OH, [3]NO₃), were prepared in near-quantitative yields as their nitrate salts. [2]NO₃ displays high water solubility. The potential of the α-diimine ligand in [3]NO₃ as a carrier of bioactive molecules was investigated via esterification reactions with the hydroxyl groups. Thus, the double-functionalized derivatives [(η⁶- p-cymene)RuCl{κ² N-(HCN(4-C₆H₄OCO-R))₂}]NO₃ (R = aspirinate, [5]NO₃; valproate, [6]NO₃) and also [4]Cl (R = Me) were obtained in good-to-high yields. UV-vis and multinuclear NMR spectroscopy and cyclic voltammetric studies in aqueous solution revealed only minor ruthenium chloride hydrolytic cleavage, biologically accessible reduction potentials, and pH-dependent behavior of [3]NO₃. Density functional theory analysis was performed in order to compare the Ru-Cl bond strength in [1]⁺ with the analogous ethylenediamine complex, showing that the higher stability observed in the former is related to the electron-withdrawing properties of the α-diimine ligand. In vitro cytotoxicity studies were performed against tumorigenic (A2780 and A2780cisR) and nontumorigenic (HEK-293) cell lines, with the complexes bearing simple α-diimine ligands ranging from inactive to IC₅₀ values in the low micromolar range. The complexes functionalized with bioactive components, i.e., [5]NO₃ and [6]NO₃, exhibited a marked increase in the cytotoxicity with respect to the precursor [3]NO₃.

Activation of CN bonds by high-valent group 6 metal chlorides, including the conversion of an α-diimine into a functionalized imidazolium

The direct interactions of WCl₆and MoCl₅with α-diimines (and also a carbodiimide in the case of MoCl₅) have been explored, leading to unusual reaction pathways.

Allowing the direct interaction of N-aryl α-diimines with a high valent metal chloride: one-pot WCl6-promoted formation of quinoxalinium salts

The full potential of a high valent metal chloride as both a chlorinating and an oxidative agent was explored by allowing WCl₆ to react with N-(2,6-diisopropylphenyl) α-diimines, in CH₂Cl₂ at room temperature. These α-diimines underwent unprecedented conversion to quinoxalinium cations via intramolecular C-N coupling.

Aluminum Complexes of N2O23- Formazanate Ligands Supported by Phosphine Oxide Donors

The synthesis and characterization of a new family of phosphine oxide supported aluminum formazanate complexes (7a,b, 8a, 9a) are reported. X-ray diffraction studies showed that the aluminum atoms in the complexes adopt an octahedral geometry in the solid state. The equatorial positions are occupied by an N₂O₂^3- formazanate ligand, and the axial positions are occupied by L-type phosphine oxide donors. UV-vis absorption spectroscopy revealed that the complexes were strongly absorbing (ε ≈ 30000 M^-1 cm^-1) between 500 and 700 nm. The absorption maxima in this region were simulated using time-dependent density functional theory. With the exception of 3-cyano-substituted complex 7b, which showed maximum luminescence intensity in the presence of excess phosphine oxide, the title complexes are nonemissive in solution and the solid state. The electrochemical properties of the complexes were probed using cyclic voltammetry. Each complex underwent sequential one-electron oxidations in potential ranges of -0.12 to 0.29 V and 0.62 to 0.97 V, relative to the ferrocene/ferrocenium redox couple. Electrochemical reduction events were observed at potentials between -1.34 and -1.75 V. In combination with tri-n-propylamine as a coreactant, complex 7b acted as an electrochemiluminescence emitter with a maximum electrochemiluminescence intensity at a wavelength of 735 nm, red-shifted relative to the photoluminescence maximum of the same compound.

Structural and Electronic Noninnocence of α-Diimine Ligands on Niobium for Reductive C-Cl Bond Activation and Catalytic Radical Addition Reactions

A d⁰ niobium(V) complex, NbCl₃(α-diimine) (1a), supported by a dianionic redox-active N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (α-diimine) ligand (ene-diamido ligand) served as a catalyst for radical addition reactions of CCl₄ to α-olefins and cyclic alkenes, selectively affording 1:1 radical addition products in a regioselective manner. During the catalytic reaction, the α-diimine ligand smoothly released and stored an electron to control the oxidation state of the niobium center by changing between an η⁴-(σ²,π) coordination mode with a folded MN₂C₂ metallacycle and a κ²-(N,N') coordination mode with a planar MN₂C₂ metallacycle. Kinetic studies of the catalytic reaction elucidated the reaction order in the catalytic cycle: the radical addition reaction rate obeyed first-order kinetics that were dependent on the concentrations of the catalyst, styrene, and CCl₄, while a saturation effect was observed at a high CCl₄ concentration. In the presence of excess amounts of styrene, styrene coordinated in an η²-olefinic manner to the niobium center to decrease the reaction rate. No observation of oligomers or polymers of styrene and high stereoselectivity for the radical addition reaction of CCl₄ to cyclopentene suggested that the C-C bond formation proceeded inside the coordination sphere of niobium, which was in good accordance with the negative entropy value of the radical addition reaction. Furthermore, reaction of 1a with (bromomethyl)cyclopropane confirmed that both the C-Br bond activation and formation proceeded on the α-diimine-coordinated niobium center during transformation of the cyclopropylmethyl radical to a homoallyl radical. With regard to the reaction mechanism, we detected and isolated NbCl₄(α-diimine) (6a) as a transient one-electron oxidized species of 1a during reductive cleavage of the C-X bonds; in addition, the monoanionic α-diimine ligand of 6a adopted a monoanionic canonical form with selective one-electron oxidation of the dianionic ene-diamido form of the ligand in 1a.

Synthesis and structural characterization of iron complexes bearing N-aryl-phenanthren-o-iminoquinone ligands

Treatments of N-aryl-phenanthren-o-iminoquinone (aryl = 2,6-Me₂C₆H₃ (^MeL); 2,6-ⁱPr₂C₆H₃ (^iPrL)) with iron powder in THF at 75 °C generate complexes [η²L]₂Fe[η¹LH] (1a, L = ^MeL; 1b, L = ^iPrL) in moderate yields. The X-ray crystallography analysis reveals that the molecule of 1b consists of a Fe(iii) center coordinated by three phenanthren-o-iminosemiquinone ligands, two of which are in an η² fashion while the remaining one is in an η¹ fashion. The analysis of the bond parameters of ligands indicates that the η²-fashioned ligands are radical anions and the η¹-fashioned one is in an aminephenolato form. Reactions of ^MeL and ^iPrL with FeCl₂ in THF produce Fe(iii) complexes [L]₂FeCl (2a, L = ^MeL; 2b, L = ^iPrL) with the two ligands in the radical anionic form. However, similar reactions of PIQ ligands with FeCl₂ in CH₂Cl₂ yield ion-pair complexes {[L]₂FeCl}⁺[FeCl₄]^- (3a, L = ^MeL; 3b, L = ^iPrL), in which the iron center chelated by two neutral ligands can be formulated as Fe(ii). Reduction of 2b with sodium provides a salt-type complex [^iPrL^2-]₂Fe(ii)Na₂ (4), in which a high spin Fe(ii) atom is ligated by two amidophenolate ligands, and the sodium atoms attached to the oxygen atoms of ligands are η³-coordinated by the aryl ring in amido moieties.

Aluminum alkyl complexes: synthesis, structure, and application in ROP of cyclic esters

Aluminum alkyl complexes have very useful applications as catalysts or reagents in small molecule transformations and as cocatalysts in olefin polymerization. This short review focuses on some recent developments in the design, synthesis and structure of aluminum(iii) alkyl complexes supported by various ligands bearing nitrogen, oxygen, sulfur or phosphorus atoms, and their catalytic applications in the ring-opening polymerization (ROP) of cyclic esters. The coordination chemistry of the Al metal centre and the catalytic activity changes of the complexes caused by ligand modifications are also discussed.

Reactions of α-diimine-aluminum complexes with sodium alkynides: versatile structures of aluminum σ-alkynide complexes

Reaction of AlCl(3) with the monoanionic α-diimine ligand [NaL] yielded the complex [L˙(-)Al(III)Cl(2)(-)] (1, L = [(2,6-iPr(2)C(6)H(3))NC(Me)](2)), and subsequent reduction of by sodium metal afforded the mononuclear [L(2-)Al(III)Cl(-)(THF)] (2) and binuclear [L(2-)(THF)Al(II)-Al(II)(THF)L(2-)] (3). Compounds 2 and 3 exhibit interesting reactivities to sodium alkynides at room temperature. Treatment of dialumane 3 with 1 equiv. of 4-methylphenylacetylene in the presence of sodium metal yielded the asymmetric Al-Al-bonded compound [Na(Et(2)O)][LAl-Al(C[triple bond, length as m-dash]C(C(6)H(4)-Me))L] (4) containing an alkynyl group attached to one of the Al atoms. The reaction of 2 with 4-methylphenylacetylene and Na (or sodium 4-methylphenylacetylide) resulted in the mononuclear product [L(THF)Al(C[triple bond, length as m-dash]C-(C(6)H(4)-Me))] (5) containing a single terminal acetylide ligand. Precursor 2 reacted with 2 equiv. of phenylacetylene (or 4-methylphenylacetylene, trimethylsilylacetylene) and Na to give the tweezer "ate" complexes, [Na(THF)(DME)][LAl(C[triple bond, length as m-dash]CR)(2)] (R = C(6)H(5), ; C(6)H(4)-Me, ; Si(Me)(3), 6c), [Na(THF)](2)[LAl(C[triple bond, length as m-dash]CPh)(2)](2)(μ-C(7)H(8)) (7), [Na(C(7)H(8))][(μ-Na)][LAl(C[triple bond, length as m-dash]CSi(Me)(3))(2)](2) (8), as well as the polymeric [LAl(C[triple bond, length as m-dash]CPh)(2)Na](n) (9). In the products, two alkynyl groups coordinate terminally to one Al center and a sodium ion is embedded between these two alkynyls. Interestingly, both cycloaddition and terminal acetylide coordination of three equiv. of alkyne occurred in the reaction of with 1-hexyne, resulting in the unique dialuminum complex [Na(Et(2)O)](2)[{L(C(C(4)H(9))[double bond, length as m-dash]CH)}Al(C[triple bond, length as m-dash]C(C(4)H(9)))(2)](2) (10). Complexes 1-10 have been characterized by NMR ((1)H, (13)C) and IR spectroscopy, elemental analysis, and X-ray diffraction, and their electronic structures were studied by DFT calculations.

The ligand redox behavior and role in 1,2-bis[(2,6-diisopropylphenyl)imino]-acenaphthene nickel–TMA(MAO) systems for ethylene polymerization

The BIAN ligand in the BIAN nickel–TMA(MAO) system is redox-active and plays a more important role than just acting as a neutral ligand.