Synthesis, Characterization, and Reactivity of Aluminum Alkyl/Amide Complexes Supported by Guanidinate and Monoanionic OCO-Pincer Ligands

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.

Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide

The reaction of 9-diazo-9H-fluorene (fluN2 ) with the potassium aluminyl K[Al(NON)] ([NON]2- =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) affords K[Al(NON)(κN1 ,N3 -{(fluN2 )2 })] (1). Structural analysis shows a near planar 1,4-di(9H-fluoren-9-ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2-di(9H-fluoren-9-yl)diazene dianion as the potassium salt, [K2 (THF)3 ][fluN=Nflu] (3). The reaction of 2 with N,N'-diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2 )N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2 )2 ]2- ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.

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.

Reactivity of N-Phosphinoguanidines of the Formula (HNR)(Ph2PNR)C(NAr) toward Main Group Metal Alkyls: Facile Ligand Rearrangement from N-Phosphinoguanidinates to Phosphinimine-Amidinates

We report the reactivity of N-phosphinoguanidines of the formula (HNR)(Ph₂PNR)C(NAr) (R = ⁱPr and Ar = 2,6-ⁱPr₂C₆H₃ [Dipp] for 1a, R = ⁱPr and Ar = 2,4,6-Me₃C₆H₂ [Mes] for 1b, and R = Cy and Ar = Dipp for 1c), prepared in high yields from the corresponding trisubstituted guanidines, toward main group metal alkyls AlMe₃, ZnEt₂, MgⁿBu₂, and ⁿBuLi to obtain novel phosphinoguanidinato and phosphinimine-amidinato compounds. Reactions of 1a-c with AlMe₃ at room temperature led to the kinetic phosphinoguanidinato products [Al{κ²-N,N'-(NR)C(NAr)(NRPPh₂)}Me₂] (2a-c), whereas the mild heating (60-80 °C) of solutions of 2a-c give the thermodynamic phosphinimine-amidinato products [Al{κ²-N,N'-(NR)C(NAr)(PPh₂NR)}Me₂] (3a-c) after ligand rearrangement. The reactions of equimolar amounts of 1a-c and ZnEt₂ initially give solutions containing unstable phosphinoguanidinato compounds [Zn{κ²-N,P-(NR)C(NAr)(NRPPh₂)}Et] (4a-c), which rearrange upon mild heating to the phosphinimine-amidinato derivatives [Zn{κ²-N,N'-(NR)C(NAr)(PPh₂NR)}Et] (6a-c). Bis(phosphinoguanidinato) compounds [Zn{κ²-N,P-(NR)C(NAr)(NRPPh₂)}₂] (5a-c) can be obtained under mild conditions (<45 °C) in THF, whereas bis(phosphinimine-amidinato) compounds [Zn{κ²-N,N'-(NR)C(NAr)(PPh₂NR)}₂] (7a-c) are also accessible under more forcing conditions (55-100 °C) from (i) ZnEt₂ and 1b,c (2 equiv), (ii) 6a and 1a, or (iii) 5b,c. Equimolar mixtures of MgⁿBu₂ and 1a-c in THF at room temperature give unstable phosphinimine-amidinato monoalkyl products [Mg{κ²-N,N'-(NR)C(NAr)(PPh₂NR)}ⁿBu(THF)₂] (8a-c), whereas 2 equiv of 1a,b are required to reach the bischelate compounds [Mg{κ²-N,N'-(NⁱPr)C(NAr)(PPh₂NⁱPr)}₂] (9a,b). Finally, phosphinoguanidinato compounds [Li{κ²-N,P-(NR)C(NDipp)(NRPPh₂)}(THF)₂] (10a,c) were obtained in the reactions of 1a,c with ⁿBuLi in THF under ambient conditions. The removal of the solvent from solutions of 10a,c under partial vacuum leads to the dinuclear compounds [Li₂{μ-κ²-N,N':κ¹-N-(NR)C(NDipp)(NRPPh₂)}₂(THF)₂] (11a,c) after the decoordination of one of the THF molecules in 10a,c and dimerization. Heating solutions of 10a,c at 60 °C triggers ligand rearrangement to give phosphinimine-amidinato compounds [Li{κ²-N,N'-(NR)C(NDipp)(PPh₂NR)}(THF)₂] (12a,c). We also propose a mechanism for the ligand rearrangement reaction from 10a to give 12a, supported by DFT calculations, which fits nicely with our experimental results. It essentially involves a carbodiimide deinsertion reaction followed by a [3 + 2] cycloaddition between the resulting lithium phosphino-amide and the carbodiimide.

On the Superior Activity of In(I) versus In(III) Cations Toward ortho-C-Alkylation of Anilines and Intramolecular Hydroamination of Alkenes

An efficient ortho-C-alkylation of unprotected anilines with a variety of styrenes and alkenes using a univalent cationic indium(I) catalyst is reported. Mechanistic studies revealed that the reaction likely proceeds via a tandem hydroamination/Hofmann-Martius rearrangement. The high compatibility between the cationic indium(I) complex and primary anilines led us to develop an In(I)⁺-catalyzed hydroamination of alkenes using unprotected primary and secondary alkenylamines. Computations support the catalytic activity of naked In(I)⁺ ions, with an outer sphere mechanism for the C-N bond formation and a potentially inner sphere protodemetallation.

Mono- and Bimetallic Aluminum Alkyl, Alkoxide, Halide and Hydride Complexes of a Bulky Conjugated Bis-Guanidinate(CBG) Ligand and Aluminum Alkyls as Precatalysts for Carbonyl Hydroboration

Tetra-aryl-substituted symmetrical conjugated bis-guanidine (CBG) ligands such as L^1-3 (3H) [L(3H) = {(ArHN)(ArHN)C═N-C═NAr(NHAr)}; Ar = 2,6-Me₂-C₆H₃ (L¹(3H)), 2,6-Et₂-C₆H₃ (L²(3H)), and 2,6-ⁱPr₂-C₆H₃ (L³(3H))] have been employed to synthesize a series of four- and six-membered aluminum heterocycles (1-8) for the first time. Generally, aluminum complexes bearing N,N'- chelated guanidinate and β-diketiminate/dipyrromethene ligand systems form four- and six-membered heterocycles, respectively. However, the conjugated bis-guanidine ligand has the capability of forming both four- and six-membered heterocycles possessing multimetal centers within the same molecule; this is due to the presence of three acidic protons, which can be easily deprotonated (at least two protons) upon treatment with metal reagents. Both mono- and dinuclear aluminum alkyls and mononuclear aluminum alkoxide, halide, and hydride complexes have been structurally characterized. Further, we have demonstrated the potential of mononuclear, six-membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane (HBpin).

Brønsted acid-catalysed intramolecular hydroamination of unactivated alkenes: metal triflates as an in situ source of triflic acid

Hydroamination of alkenes or alkynes is one of the most straightforward methods to form C-N bonds and nitrogen-containing heterocycles. A simple Lewis acid Al(OTf)₃ was found to be an effective precatalyst for the hydroamination of unactivated primary and secondary alkenylamines between 110 and 150 °C. Subsequent studies show that other metal triflates are also effective precatalysts for the hydroamination reactions. For metal triflate salts, mechanistic studies, including kinetics, are consistent with the in situ generation of triflic acid, which likely serves as the active catalyst.

Hydroamination of carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex

The hydroamination of heterocumulenes such as carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex as a precatalyst is reported here. The titanium(iv) complex [{Ph₂P(Se)NCH₂CH₂NPPh₂(Se)}Ti(NMe₂)₂] (1) was synthesised by the reaction of tetrakis-(dimethylamido)titanium(iv) [Ti(NMe₂)₄] with [{Ph₂P(Se)NHCH₂CH₂NHPPh₂(Se)}] in toluene at ambient temperature. Titanium complex 1 proved to be a competent pre-catalyst for the addition of an amine N-H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanates and phenylisothiocyanates in toluene solvents proceeding rapidly at room temperature with 5 mol% catalyst loadings to yield the corresponding urea and thio-urea derivatives up to 99%. However, ambient temperature was needed for hydroamination of 1,3-dicyclohexylcarbodiimide. The amine addition reactions with isocyanates showed first order kinetics with respect to catalyst 1 as well as substrates. The most plausible mechanism for the hydroamination reaction was established by isolating 1,1-dimethylphenyl urea as a side product.

Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions

Solutions of some of the title compounds reached an equilibrium with the aminoboranes and the corresponding carbodiimides at room temperature.

Guanidinatoaluminum complexes: synthesis, crystal structures and reactivities

The reactivities of the mononuclear guanidinatoaluminum complexes with O₂, carbodiimide or H₂O were studied for the first time.

Aluminium-catalysed intramolecular hydroamination of aminoalkenes: computational perusal of alternative pathways for aminoalkene activation

A comprehensive computational examination of alternatively plausible mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes utilising a recently reported novel phenylene-diamine aluminium amido compound is presented. On the one hand, a proton-assisted concerted N-C/C-H bond-forming pathway to afford the cycloamine in a single step can be invoked, and, on the other, a stepwise σ-insertive pathway that involves a relatively fast, reversible migratory olefin 1,2-insertion step linked to a less rapid, irreversible Al-C alkyl bond protonolysis. The present study, which employs a sophisticated and reliable computational methodology, supports the prevailing mechanism to be a stepwise σ-insertive pathway. The predicted effective barrier for turnover-limiting aminolysis compares favourably with reported catalytic performance data. Non-competitive kinetic demands militates against the operation of the concerted proton-assisted pathway, which describes N-C bond-forming ring closure triggered by concomitant amino proton delivery at the C[double bond, length as m-dash]C linkage evolving through a six-centre transition state structure. The valuable insights into mechanistic intricacies of aluminium-mediated intramolecular HA reported herein will help guide the rational design of group 13 metal-based HA catalysts.

Reactivity of functionalized indoles with rare-earth metal amides. Synthesis, characterization and catalytic activity of rare-earth metal complexes incorporating indolyl ligands

The synthesized rare-earth metal amido complexes displayed different catalytic activity for intramolecular hydroamination of aminoalkenes.

Synthesis, Characterization, and Catalytic Activity of a Series of Aluminium–Amidate Complexes

The aluminium complexes {[κ2-N,O-(t-BuNCOPh)]AlMe2}2 (2), [κ2-N,O-(t-BuNCOPh)]2AlMe (3), and [κ2-N,O-(t-BuNCOPh)]3Al (4) were prepared through the protonolysis reaction between trimethylaluminium and one, two, or three equivalents, respectively, of N-tert-butylbenzamide. Complex 2 was also prepared via a salt metathesis reaction between K(t-BuNCOPh) and dimethylaluminium chloride. Complexes 2–4 were characterized using 1H and 13C NMR spectroscopy. Single-crystal X-ray diffraction analysis of the complexes corroborated ligand : metal stoichiometries and revealed that all the amidate ligands coordinate to the aluminium ion in a κ2 fashion. The Al–amidate complexes 2–4 were viable catalyst precursors for the Meerwein–Ponndorf–Verley–Oppenauer reduction–oxidation manifold, successfully interconverting several classes of carbonyl and alcohol substrates.

The non-innocent phenalenyl unit: an electronic nest to modulate the catalytic activity in hydroamination reaction

The phenalenyl unit has played intriguing role in different fields of research spanning from chemistry, material chemistry to device physics acting as key electronic reservoir which has not only led to the best organic single component conductor but also created the spin memory device of next generation. Now we show the non-innocent behaviour of phenalenyl unit in modulating the catalytic behaviour in a homogeneous organic transformation. The present study establishes that the cationic state of phenalenyl unit can act as an organic Lewis acceptor unit to influence the catalytic outcome of intermolecular hydroamination reaction of carbodiimides. For the present study, we utilized organoaluminum complexes of phenalenyl ligands in which the phenalenyl unit maintains the closed shell electronic state. The DFT calculation reveals that the energy of LUMO of the catalyst is mainly controlled by phenalenyl ligands which in turn determines the outcome of the catalysis.

Phenalenyl-based organozinc catalysts for intramolecular hydroamination reactions: a combined catalytic, kinetic, and mechanistic investigation of the catalytic cycle

Herein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)-based N,N-ligands. The reactions of phenalenyl-based ligands with ZnMe(2) led to the formation of organozinc complexes [N(Me),N(Me)-PLY]ZnMe (1) and [N(iPr),N(iPr)-PLY]ZnMe (2) under the evolution of methane. Both complexes (1 and 2) were characterized by NMR spectroscopy and elemental analysis. The solid-state structures of complexes 1 and 2 were determined by single-crystal X-ray crystallography. Complexes 1 and 2 were used as catalysts for the intramolecular hydroamination of unactivated primary and secondary aminoalkenes. A combined approach of NMR spectroscopy and DFT calculations was utilized to obtain better insight into the mechanistic features of the zinc-catalyzed hydroamination reactions. The progress of the catalysis for primary and secondary aminoalkene substrates with catalyst 2 was investigated by detailed kinetic studies, including kinetic isotope effect measurements. These results suggested pseudo-first-order kinetics for both primary and secondary aminoalkene activation processes. Eyring and Arrhenius analyses for the cyclization of a model secondary aminoalkene substrate afforded ΔH(≠) =11.3 kcal mol(-1) , ΔS(≠) =-35.75 cal K(-1)  mol(-1) , and E(a) =11.68 kcal mol(-1) . Complex 2 exhibited much-higher catalytic activity than complex 1 under identical reaction conditions. The in situ NMR experiments supported the formation of a catalytically active zinc cation and the DFT calculations showed that more active catalyst 2 generated a more stable cation. The stability of the catalytically active zinc cation was further supported by an in situ recycling procedure, thereby confirming the retention of catalytic activity of compound 2 for successive catalytic cycles. The DFT calculations showed that the preferred pathway for the zinc-catalyzed hydroamination reactions is alkene activation rather than the alternative amine-activation pathway. A detailed investigation with DFT methods emphasized that the remarkably higher catalytic efficiency of catalyst 2 originated from its superior stability and the facile formation of its cation compared to that derived from catalyst 1.