Aluminum Alkyl Complexes Supported by Bidentate N,N Ligands: Synthesis, Structure, and Catalytic Activity for Guanylation of Amines

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.

Guanidinates as Alternative Ligands for Organometallic Complexes

For decades, ligands such as phosphanes or cyclopentadienyl ring derivatives have dominated Coordination and Organometallic Chemistry. At the same time, alternative compounds have emerged that could compete either for a more practical and accessible synthesis or for greater control of steric and electronic properties. Guanidines, nitrogen-rich compounds, appear as one such potential alternatives as ligands or proligands. In addition to occurring in a plethora of natural compounds, and thus in compounds of pharmacological use, guanidines allow a wide variety of coordination modes to different metal centers along the periodic table, with their monoanionic chelate derivatives being the most common. In this review, we focused on the organometallic chemistry of guanidinato compounds, discussing selected examples of coordination modes, reactivity and uses in catalysis or materials science. We believe that these amazing ligands offer a new promise in Organometallic Chemistry.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ²-{NHI^RP(Ph)NDipp}AlMe₂] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHI^RP(Ph)NDipp]⁻ [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Three aluminium alkyl complexes, [κ²-{Im^RNP(Ph)NDipp}AlMe₂] (2a–2c), supported by unsymmetrical imino-phosphanamide were synthesised and utilised as competent precatalysts for the hydroamination of carbodiimides under ambient conditions.

Synthesis and characterization of group 13 dichloride (M = Ga, In), dimethyl (M = Al) and cationic methyl aluminum complexes supported by monoanionic NNN-pincer ligands

A series of monoanionic NNN-pincer ligands effectively stabilize five-coordinate gallium and indium dichloride complexes, as well as neutral dimethyl aluminum species, and organometallic cations thereof.

Unique reactivity of an α-ketiminopyridine ligand with metal–alkyls: Synthesis and ROP of ε-caprolactone

The reaction of an α-ketimininopyridine ligand 2-((Me)CN(C₆H₃-2,6-iPr₂))-6-(OMe)C₅H₃N (L¹) with metal–alkyls, such as MeLi, Et₂Zn, Me₃Al and Me₂AlCl, was studied.

Ligands with Two Monoanionic N,N-Binding Sites: Synthesis and Coordination Chemistry

Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono- and polynuclear complexes of transition metals as well as rare-earth and main-group elements. Nitrogen-based ditopic ligands, in which two monoanionic N,N-binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β-diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N-chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).

Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac-lactide

A series of Schiff base bridged bis(indolyl) ligands were developed for aluminum chemistry. The reactions of AlEt3 or AlMe3 with the Schiff base bridged bis(indolyl) proligands R1(-N[double bond, length as m-dash]CHC8H5NH)2 (R1 = -CH2CH2- (H2L1); -CH2CH2CH2- (H2L2); -CH2CMe2CH2- (H2L3); rac-Cy (H2L4); and R,R-Cy (H2L5)) were studied leading to the synthesis of a series of aluminum alkyl complexes L1AlEt (1)-L5AlEt (5) and L3AlMe (3b) in good yields, while the reaction of H2L3 with Al(OiPr)3 gave the aluminum alkoxide complex L3AlOiPr (3a). These aluminum complexes were characterized by spectroscopic methods and elemental analyses. The solid state structures of the aluminum complexes 1-5 and 3a were confirmed by the X-ray diffraction study. X-ray analyses revealed that the aluminum centre in these complexes is five-coordinated. The coordination geometry is between square pyramidal and trigonal bipyramidal. In the presence of 1 equiv. of isopropanol, the aluminum alkyl complexes exhibited notable activity towards the ring-opening polymerization of rac-lactide at 70 °C in toluene, with good control over molecular weights and dispersities. The substituents and the length of the bridging part between the two Schiff base nitrogen atoms have an influence on either the tacticity of isolated polymers or the rate of polymerization. The kinetics of complex L3AlOiPr (3a) in C6D6 was also investigated, and the experimental results revealed that the rate of polymerization was first-order with respect to rac-lactide.

Organoaluminum Compounds as Catalysts for Monohydroboration of Carbodiimides

The effective catalytic activity of organoaluminum compounds for the monohydroboration of carbodiimides has been demonstrated. Two aluminum complexes, 2 and 3, were synthesized and characterized. The efficient catalytic performances of four aluminum hydride complexes L1 AlH2 (L1 =HC(CMeNAr)2 , Ar=2,6-Et2 C6 H3 ; 1), L2 AlH2 (NMe3 ) (L2 =o-C6 H4 F(CH=N-Ar), Ar=2,6-Et2 C6 H3 ; 2), L3 AlH (L3 =2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine; 3), and L4 AlH(NMe3 ) (L4 =o-C6 H4 (N-Dipp)(CH=N-Dipp), Dipp=2,6-iPr2 C6 H3 ; 4), and an aluminum alkyl complex L1 AlMe2 (5) were used for the monohydroboration of carbodiimides investigated under solvent-free and mild conditions. Compounds 1-3 and 5 can produce monohydroborated N-borylformamidine, whereas 4 can afford the C-borylformamidine product. A suggested mechanism of this reaction was explored, and the aluminum formamidinate compound 6 was characterized by single-crystal X-ray, also a stoichiometric reaction was investigated.

Reactivity of 1,3-Disubstituted Indoles with Lithium Compounds: Substituents and Solvents Effects on Coordination and Reactivity of Resulting 1,3-Disubstituted-2-Indolyl Lithium Complexes

Reactivity of 1,3-disubstituted indolyl compounds with lithium reagents was studied to reveal the substituents and solvent effects on coordination modes and reactivities resulting in different indolyl lithium complexes. Treatment of 1-alkyl-3-imino functionalized compounds 1-R-3-(R'N═CH)C₈H₅N [R = Bn, R' = Dipp (HL¹); R = Bn, R' = ^tBu (HL²); R = CH₃OCH₂, R' = Dipp (HL³); Dipp = ⁱPr₂C₆H₃] with Me₃SiCH₂Li or ⁿBuLi in hydrocarbon solvents (toluene or n-hexane) produced 1,3-disubstituted-2-indolyl lithium complexes [η¹:(μ₂-η¹:η¹)-1-Bn-3-(DippN═CH)C₈H₄NLi]₂ (1), {[η¹:(μ₃-η¹:η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N][η²:η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N][η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N]Li₃} (2), and [η¹:η¹:(μ₂-η¹:η¹)-1-CH₃OCH₂-3-(DippN═CH)C₈H₄NLi]₂ (3), respectively. The bonding modes of the indolyl ligand were kept in 1 by coordination with donor solvent, affording [η¹:(μ₂-η¹:η¹)-1-Bn-3-(DippN═CH)C₈H₄NLi(THF)]₂ (4). The trinuclear complex 2 was converted to dinuclear form with a change of bonding modes of the indolyl ligand by treatment of 2 with donor solvent THF, producing [η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄NLi(THF)]₂ (5). X-ray diffraction established that compounds 1, 3, 4, and 5 crystallized as dinuclear structures with the carbanionic sp² carbon atoms of the indolyl ligands coordinated to lithium ions in a μ₂-η¹:η¹ manner, while compound 2 crystallized as a trinuclear structure and the carbanionic atoms of the indolyl moieties coordinated to lithium ions in μ₂-η¹:η¹ and μ₃-η¹:η¹:η¹ manners. When the lithiation reaction of HL¹ with 1 equiv of ⁿBuLi was carried out in THF, the monomeric lithium complex {η¹:η¹-1-Bn-3-(DippN═CH)-2-[1'-Bn-3'-(DippNCH)C₈H₅N]C₈H₄NLi(THF)} (6) having coupled indolyl moieties was obtained. The compound 6 can also be prepared by the reaction of 1 with 0.5 equiv of HL¹ with a higher isolated yield. Accordingly, the lithium complexes [η¹:η⁴-1-Bn-3-^tBuN═CH-2-(1'-Bn-3'-^tBuNCHC₈H₅N)C₈H₄NLi(L)] (L = THF, 7a; L = Et₂O, 7b) with the coupled indolyl moieties in η⁴ mode were isolated by treatment of HL² with 2 in THF or Et₂O. All complexes were characterized by spectroscopic methods, and their structures were determined by X-ray diffraction study.

Aluminum complexes derived from a hexadentate salen-type Schiff base: synthesis, structure, and catalysis for cyclic carbonate synthesis

The unexpected aluminum complex was synthesized and found to be active in the cycloaddition reaction of terminal epoxides and CO₂ at atmospheric pressure.

Hydrosilylation in Aryliminopyrrolide-Substituted Silanes

A range of silanes was synthesized by the reaction of HSiCl3 with iminopyrrole derivatives in the presence of NEt3 . In certain cases, intramolecular hydrosilylation converts the imine ligand into an amino substituent. This reaction is inhibited by factors such as electron-donating substitution on Si and steric bulk. The monosubstituted ((Dipp) IMP)SiHMeCl ((Dipp) IMP=2-[N-(2,6-diisopropylphenyl)iminomethyl]pyrrolide), is stable towards hydrosilylation, but slow hydrosilylation is observed for ((Dipp) IMP)SiHCl2 . Reaction of two equivalents of (Dipp) IMPH with HSiCl3 results in the hydrosilylation product ((Dipp) AMP)((Dipp) IMP)SiCl ((Dipp) AMP=2-[N-(2,6-diisopropylphenyl)aminomethylene]pyrrolide), but the trisubsitituted ((Dipp) IMP)3 SiH is stable. Monitoring the hydrosilylation reaction of ((Dipp) IMP)SiHCl2 reveals a reactive pathway involving ligand redistribution reactions to form the disubstituted ((Dipp) AMP)((Dipp) IMP)SiCl as an intermediate. The reaction is strongly accelerated in the presence of chloride anions.

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.

Unexpected reactivity of an alkylaluminum complex of a non-innocent 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene ligand (dpp-bian)

The non-innocence of the bis-amido dpp-bian ligand has been documented by the reactions of compound 1 with acidic substrates, e.g. water.