Synthesis and Structural Diversity of Group 4 Metal Complexes with Multidentate Tethered Phenoxy-Amidine and Phenoxy-Amidinate Ligands

The coordination chemistry of the multidentate tethered amidine-phenol {4,6-<i>t</i>Bu₂C₆H₂O­(2-C­(NR)NR}­H₂ ({LON^R}­H₂, R = <i>i</i>Pr, 2,6-<i>i</i>Pr₂C₆H₃ (Ar)) and new guanidine-phenol {4,6-<i>t</i>Bu₂C₆H₂ON­(C₆H₅)­(2-C­(NR)NR}­H₂ ({LON­(Ph)­N^<i>i</i>Pr}­H₂) pro-ligands with group 4 metals has been studied. σ-Bond and salt metathesis reactions were explored to coordinate these (pro)­ligands onto zirconium and hafnium. Alkane elimination reactions between {LON^R}­H₂ and Zr­(CH₂Ph)₄ afforded mixed-ligand monobenzyl {LO^HN^R}­{LON^R}­Zr­(CH₂Ph) (R = <i>i</i>Pr; <b>1</b>) and monoligand tribenzyl {LO^HN^Ar}­Zr­(CH₂Ph)₃ (R = Ar; <b>2</b>) complexes, respectively. Alkane and amine elimination reactions between {LON­(Ph)­N^<i>i</i>Pr}­H₂ and Zr­(CH₂Ph)₄ or Hf­(NMe₂)₄ unexpectedly resulted in cleavage of the ligand backbone and eventual isolation of {(Ph)­NC₆H₂(<i>t</i>Bu)₂O}­Zr­{(<i>i</i>PrN)₂CCH₂Ph}₂ (<b>3</b>) and {(Ph)­NC₆H₂(<i>t</i>Bu)₂O}­Hf­{(<i>i</i>PrN)₂CNMe₂}₂ (<b>4</b>), respectively. Salt metathesis reactions between {LON^R}­Li₂ and ZrCl₄(THF)_<i>n</i> (<i>n</i> = 0, 2), conducted in 1:1 ratios, led upon crystallization to diverse chloro complexes: [{LON^<i>i</i>Pr}­ZrCl]₃(μ₃-O)­(μ₃-Cl) (<b>5</b>), [{LON^Ar}₂ZrCl­(μ₂-Cl)]₂[{L^HON^Ar}­ZrCl­(μ₂-Cl)]­(μ₃–OH) (<b>6</b>), and {LO^HN^Ar}­ZrCl₃(THF) (<b>7</b>). Similar salt metathesis reactions between the monolithium salts {L^HON^R}Li and ZrCl₄, conducted in 2:1 ratios, allowed the selective preparation of bis­(phenoxy-amidine) complexes with pendant amino groups {LO^HN^R}₂ZrCl₂ (R = <i>i</i>Pr, <b>8</b>; R = Ar, <b>9</b>). All complexes were authenticated by elemental analysis, X-ray crystallography, and NMR spectroscopy. Complexes <b>5</b>, <b>6</b>, <b>8</b>, and <b>9</b>, upon activation with MAO, showed poor to moderate productivities (4–172 (kg of PE) mol^–1 h^–1) in the polymerization of ethylene, giving linear polymers with large polydispersities

Aluminum Complexes of Bidentate Fluorinated Alkoxy-Imino Ligands: Syntheses, Structures, and Use in Ring-Opening Polymerization of Cyclic Esters

The coordination chemistry of bidentate fluorinated alkoxy-imino ligands onto Al­(III) centers has been studied. The proligands (CF₃)₂C­(OH)­CH₂C­(R¹)N–R² ({ON^R1,R2}­H; R¹ = Me, Ph; R² = Ph, CH₂Ph, cyclohexyl; <b>1a</b>–<b>d</b>) react selectively with AlMe₃ (0.5 or 1.0 equiv) and AlMe₂(O<i>i</i>Pr) or Al­(O<i>i</i>Pr)₃ (0.5 equiv) to give the corresponding monoligand compounds {ON^R1,R2}­AlMe₂ (<b>2a</b>–<b>d</b>) and the bis-ligand compounds {ON^R1,R2}₂AlMe (<b>3a</b>–<b>d</b>) and {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4a</b>–<b>c</b>). X-ray diffraction studies revealed that {ON^Ph,Bn}­AlMe₂ (<b>2a</b>), {ON^Me,Bn}­AlMe₂ (<b>2b</b>), {ON^Me,Bn}₂AlMe (<b>3b</b>), {ON^Ph,Ph}₂AlMe (<b>3c</b>), {ON^Me,Bn}₂Al­(O<i>i</i>Pr) (<b>4b</b>), and {ON^Ph,Ph}₂Al­(O<i>i</i>Pr) (<b>4c</b>) all adopt a mononuclear structure in the solid state; four-coordinate {ON^R1,R2}­AlMe₂ and five-coordinate {ON^Me,Bn}₂Al­(O<i>i</i>Pr) feature respectively distorted-tetrahedral and trigonal-bipyramidal geometries. The ¹H, ¹³C­{¹H}, and ¹⁹F­{¹H} NMR data indicate that the structures observed in the solid state are retained in CD₂Cl₂ or C₆D₆ solution at room temperature. The binary systems {ON^R1,R2}­AlMe₂ (<b>2</b>)/BnOH and discrete {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4</b>) are effective catalysts for the controlled ROP of ε-caprolactone and <i>rac</i>-lactide, both in bulk molten monomer and in toluene solution/slurry. In contrast to the case for {ON^RNO}­Al­(O<i>i</i>Pr), having a bridged tetradentate fluorinated dialkoxy-diimino ligand that provides isotactic-enriched polylactides, the unbridged compounds {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4</b>) produce atactic PLAs. The key element which appears to be at the origin of the absence of stereocontrol is the lack of bridge between the two imino-alkoxy moieties, possibly via a decrease in the rigidity of the compounds and/or a different positioning of N,O vs N,N heteroatoms in axial sites

Synthesis and Structural Diversity of Group 4 Metal Complexes with Multidentate Tethered Phenoxy-Amidine and Phenoxy-Amidinate Ligands

The coordination chemistry of the multidentate tethered amidine-phenol {4,6-<i>t</i>Bu₂C₆H₂O­(2-C­(NR)NR}­H₂ ({LON^R}­H₂, R = <i>i</i>Pr, 2,6-<i>i</i>Pr₂C₆H₃ (Ar)) and new guanidine-phenol {4,6-<i>t</i>Bu₂C₆H₂ON­(C₆H₅)­(2-C­(NR)NR}­H₂ ({LON­(Ph)­N^<i>i</i>Pr}­H₂) pro-ligands with group 4 metals has been studied. σ-Bond and salt metathesis reactions were explored to coordinate these (pro)­ligands onto zirconium and hafnium. Alkane elimination reactions between {LON^R}­H₂ and Zr­(CH₂Ph)₄ afforded mixed-ligand monobenzyl {LO^HN^R}­{LON^R}­Zr­(CH₂Ph) (R = <i>i</i>Pr; <b>1</b>) and monoligand tribenzyl {LO^HN^Ar}­Zr­(CH₂Ph)₃ (R = Ar; <b>2</b>) complexes, respectively. Alkane and amine elimination reactions between {LON­(Ph)­N^<i>i</i>Pr}­H₂ and Zr­(CH₂Ph)₄ or Hf­(NMe₂)₄ unexpectedly resulted in cleavage of the ligand backbone and eventual isolation of {(Ph)­NC₆H₂(<i>t</i>Bu)₂O}­Zr­{(<i>i</i>PrN)₂CCH₂Ph}₂ (<b>3</b>) and {(Ph)­NC₆H₂(<i>t</i>Bu)₂O}­Hf­{(<i>i</i>PrN)₂CNMe₂}₂ (<b>4</b>), respectively. Salt metathesis reactions between {LON^R}­Li₂ and ZrCl₄(THF)_<i>n</i> (<i>n</i> = 0, 2), conducted in 1:1 ratios, led upon crystallization to diverse chloro complexes: [{LON^<i>i</i>Pr}­ZrCl]₃(μ₃-O)­(μ₃-Cl) (<b>5</b>), [{LON^Ar}₂ZrCl­(μ₂-Cl)]₂[{L^HON^Ar}­ZrCl­(μ₂-Cl)]­(μ₃–OH) (<b>6</b>), and {LO^HN^Ar}­ZrCl₃(THF) (<b>7</b>). Similar salt metathesis reactions between the monolithium salts {L^HON^R}Li and ZrCl₄, conducted in 2:1 ratios, allowed the selective preparation of bis­(phenoxy-amidine) complexes with pendant amino groups {LO^HN^R}₂ZrCl₂ (R = <i>i</i>Pr, <b>8</b>; R = Ar, <b>9</b>). All complexes were authenticated by elemental analysis, X-ray crystallography, and NMR spectroscopy. Complexes <b>5</b>, <b>6</b>, <b>8</b>, and <b>9</b>, upon activation with MAO, showed poor to moderate productivities (4–172 (kg of PE) mol^–1 h^–1) in the polymerization of ethylene, giving linear polymers with large polydispersities

Aluminum Complexes of Bidentate Fluorinated Alkoxy-Imino Ligands: Syntheses, Structures, and Use in Ring-Opening Polymerization of Cyclic Esters

The coordination chemistry of bidentate fluorinated alkoxy-imino ligands onto Al­(III) centers has been studied. The proligands (CF₃)₂C­(OH)­CH₂C­(R¹)N–R² ({ON^R1,R2}­H; R¹ = Me, Ph; R² = Ph, CH₂Ph, cyclohexyl; <b>1a</b>–<b>d</b>) react selectively with AlMe₃ (0.5 or 1.0 equiv) and AlMe₂(O<i>i</i>Pr) or Al­(O<i>i</i>Pr)₃ (0.5 equiv) to give the corresponding monoligand compounds {ON^R1,R2}­AlMe₂ (<b>2a</b>–<b>d</b>) and the bis-ligand compounds {ON^R1,R2}₂AlMe (<b>3a</b>–<b>d</b>) and {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4a</b>–<b>c</b>). X-ray diffraction studies revealed that {ON^Ph,Bn}­AlMe₂ (<b>2a</b>), {ON^Me,Bn}­AlMe₂ (<b>2b</b>), {ON^Me,Bn}₂AlMe (<b>3b</b>), {ON^Ph,Ph}₂AlMe (<b>3c</b>), {ON^Me,Bn}₂Al­(O<i>i</i>Pr) (<b>4b</b>), and {ON^Ph,Ph}₂Al­(O<i>i</i>Pr) (<b>4c</b>) all adopt a mononuclear structure in the solid state; four-coordinate {ON^R1,R2}­AlMe₂ and five-coordinate {ON^Me,Bn}₂Al­(O<i>i</i>Pr) feature respectively distorted-tetrahedral and trigonal-bipyramidal geometries. The ¹H, ¹³C­{¹H}, and ¹⁹F­{¹H} NMR data indicate that the structures observed in the solid state are retained in CD₂Cl₂ or C₆D₆ solution at room temperature. The binary systems {ON^R1,R2}­AlMe₂ (<b>2</b>)/BnOH and discrete {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4</b>) are effective catalysts for the controlled ROP of ε-caprolactone and <i>rac</i>-lactide, both in bulk molten monomer and in toluene solution/slurry. In contrast to the case for {ON^RNO}­Al­(O<i>i</i>Pr), having a bridged tetradentate fluorinated dialkoxy-diimino ligand that provides isotactic-enriched polylactides, the unbridged compounds {ON^R1,R2}₂Al­(O<i>i</i>Pr) (<b>4</b>) produce atactic PLAs. The key element which appears to be at the origin of the absence of stereocontrol is the lack of bridge between the two imino-alkoxy moieties, possibly via a decrease in the rigidity of the compounds and/or a different positioning of N,O vs N,N heteroatoms in axial sites

Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of <i>rac</i>-Lactide: How Does Indium Compare to Aluminum?

The reaction between InCl₃ and {ON^EtNO}­K₂ (<b>1a</b>), prepared from (CF₃)₂(OH)­CCH₂C­(CH₃)N–R–NC­(CH₃)­CH₂C­(OH)­(CF₃)₂ ({ON^EtNO}­H₂, R = C₂H₄ (<b>a</b>); {ON^CyNO}­H₂, R = <i>rac</i>-1,2-cyclohexylene (<b>b</b>)) and PhCH₂K, gave {ON^EtNO}­InCl (<b>2a</b>). The reaction between InCl₃ and 3 equiv of MeLi led to a mixture of [InMe₃] and Li­[InMe₄], which upon further treatment with {ON^RNO}­H₂ proligands gave the ate complexes [{ON^RNO}­Li]­InMe₂ (<b>4a</b>,<b>b</b>); the methyl complex {ON^EtNO}­InMe (<b>3a</b>) was also isolated from this reaction. Hydrocarbyl complexes {ON^RNO}­In­(CH₂SiMe₃) (<b>5a</b>,<b>b</b>) were prepared cleanly from the 1:1 reactions between In­(CH₂SiMe₃)₃ and {ON^RNO}­H₂. The solid-state molecular structures of mononuclear <b>2a</b>, <b>3a, 4b</b>, and <b>5b</b> and of [(<b>1a</b>)₂-(μ-H₂O)]_<i>n</i>, a distorted cubane-like core made up of four potassium atoms and two ligands, were determined. Compounds <b>5a</b>,<b>b</b> are moderately active initiators/catalysts for the ring-opening polymerization of <i>rac</i>-lactide, giving polymers with controlled molecular weights and narrow polydispersities, especially in the presence of added isopropyl alcohol (1–10 equiv) as exogenous initiator. Isotactic-enriched (<i>P</i>_m = 0.62–0.69) PLAs were obtained from <b>5a</b>, while <b>5b</b> gave atactic materials. The heterobimetallic compounds <b>4a</b>,<b>b</b> are also active and afforded slightly heterotactic-enriched PLAs (<i>P</i>_r = 0.57–0.62), but with broader polydispersities. Those results allowed us to discuss the initiation mechanisms according to the constitution of the systems (alkyl vs alkyl/<i>i</i>PrOH) and also stereoselective abilities as a function of the nature (In vs Al) and coordination environment (κ⁴-ONNO vs. κ²-NN) of the metal center

Hydrogen Bond Templated 1:1 Macrocyclization through an Olefin Metathesis/Hydrogenation Sequence

The construction of pyridine-containing macrocyclic architectures using a nonmetallic template is described. 4,6-Dichlororesorcinol was used as an exotemplate to self-organize two aza-heterocyclic units by OH···N hydrogen bonds. Subsequent sequential double olefin metathesis/hydrogenation reactions employing a single ruthenium–alkylidene precatalyst open access to macrocyclic molecules

Meerwein–Ponndorf–Verley-Type Reduction Processes in Aluminum and Indium Isopropoxide Complexes of Imino-Phenolate Ligands

Unexpected Meerwein–Ponndorf–Verley (MPV) reduction products have been obtained during attempts to prepare aluminum and indium isopropoxide complexes of an imino-phenolate ligand derived from 8-aminoquinoline ({ONN^qui}⁻). Reaction of the imino-phenol proligand {ONN^qui}H and Al­(O<i>i</i>Pr)₃ in toluene/THF at 90 °C affords selectively [{ONN^qui}­{ON^HN^qui}­Al] (<b>1a</b>), which contains a monoanionic­{ONN^qui}⁻ ligand and another dianionic ligand ({ON^HN^qui}^2–) that results from the reduction of the latter imino group into an amido moiety. The same and similar MPV reduction products <b>1a</b> and [{ONN^qui}­{ON^HN^qui}­In] (<b>5a</b>) were obtained from the treatment of {ONN^qui}­AlMe₂ (<b>2a</b>) and dialkylindium compounds {ONN^qui}­InMe₂ (<b>3a</b>) and {ONN^qui}­In­(CH₂SiMe₃)₂ (<b>4a</b>), respectively, with isopropyl alcohol in THF at room temperature. Related reactions of Al­(O<i>i</i>Pr)₃ with imino-phenol­(ate) (pro)­ligands bearing <i>N</i>-benzyl- and <i>N</i>-methyl­(2-pyridinyl) substituents or of <i>i</i>PrOH with {ON^Bn}­AlMe₂ (<b>2b</b>) and {ONN^CH2pyr}­AlMe₂ (<b>2c</b>) did not lead to MPV reduction products but to the expected {ON^Bn}₂Al­(O<i>i</i>Pr) (<b>6b</b>) and {ONN^CH2Pyr}₂Al­(O<i>i</i>Pr) (<b>6c</b>) compounds (and other unidentified products). The solid-state structures of <b>1a</b>, <b>3a</b>, <b>4a</b>, and <b>5a</b> were determined by X-ray diffraction studies. DFT computations performed on the putative complex {ONN^qui}₂Al­(O<i>i</i>Pr) (“<b>6a</b>” = <b>Ia</b>) and its isolated analogue {ON^Bn}₂Al­(O<i>i</i>Pr) (<b>6b</b>) indicated an intramolecular MPV pathway with direct hydrogen transfer. Formation of <b>1a</b> from the presumed alkoxide intermediate <b>Ia</b> is calculated to be a highly exergonic reaction which features a relatively low activation barrier. On the other hand, formation of the MPV reaction products from <b>6b</b> appears to be highly unfavorable on both thermodynamic and kinetic grounds

Meerwein–Ponndorf–Verley-Type Reduction Processes in Aluminum and Indium Isopropoxide Complexes of Imino-Phenolate Ligands

Unexpected Meerwein–Ponndorf–Verley (MPV) reduction products have been obtained during attempts to prepare aluminum and indium isopropoxide complexes of an imino-phenolate ligand derived from 8-aminoquinoline ({ONN^qui}⁻). Reaction of the imino-phenol proligand {ONN^qui}H and Al­(O<i>i</i>Pr)₃ in toluene/THF at 90 °C affords selectively [{ONN^qui}­{ON^HN^qui}­Al] (<b>1a</b>), which contains a monoanionic­{ONN^qui}⁻ ligand and another dianionic ligand ({ON^HN^qui}^2–) that results from the reduction of the latter imino group into an amido moiety. The same and similar MPV reduction products <b>1a</b> and [{ONN^qui}­{ON^HN^qui}­In] (<b>5a</b>) were obtained from the treatment of {ONN^qui}­AlMe₂ (<b>2a</b>) and dialkylindium compounds {ONN^qui}­InMe₂ (<b>3a</b>) and {ONN^qui}­In­(CH₂SiMe₃)₂ (<b>4a</b>), respectively, with isopropyl alcohol in THF at room temperature. Related reactions of Al­(O<i>i</i>Pr)₃ with imino-phenol­(ate) (pro)­ligands bearing <i>N</i>-benzyl- and <i>N</i>-methyl­(2-pyridinyl) substituents or of <i>i</i>PrOH with {ON^Bn}­AlMe₂ (<b>2b</b>) and {ONN^CH2pyr}­AlMe₂ (<b>2c</b>) did not lead to MPV reduction products but to the expected {ON^Bn}₂Al­(O<i>i</i>Pr) (<b>6b</b>) and {ONN^CH2Pyr}₂Al­(O<i>i</i>Pr) (<b>6c</b>) compounds (and other unidentified products). The solid-state structures of <b>1a</b>, <b>3a</b>, <b>4a</b>, and <b>5a</b> were determined by X-ray diffraction studies. DFT computations performed on the putative complex {ONN^qui}₂Al­(O<i>i</i>Pr) (“<b>6a</b>” = <b>Ia</b>) and its isolated analogue {ON^Bn}₂Al­(O<i>i</i>Pr) (<b>6b</b>) indicated an intramolecular MPV pathway with direct hydrogen transfer. Formation of <b>1a</b> from the presumed alkoxide intermediate <b>Ia</b> is calculated to be a highly exergonic reaction which features a relatively low activation barrier. On the other hand, formation of the MPV reaction products from <b>6b</b> appears to be highly unfavorable on both thermodynamic and kinetic grounds

Indium Complexes of Fluorinated Dialkoxy-Diimino Salen-like Ligands for Ring-Opening Polymerization of <i>rac</i>-Lactide: How Does Indium Compare to Aluminum?

The reaction between InCl₃ and {ON^EtNO}­K₂ (<b>1a</b>), prepared from (CF₃)₂(OH)­CCH₂C­(CH₃)N–R–NC­(CH₃)­CH₂C­(OH)­(CF₃)₂ ({ON^EtNO}­H₂, R = C₂H₄ (<b>a</b>); {ON^CyNO}­H₂, R = <i>rac</i>-1,2-cyclohexylene (<b>b</b>)) and PhCH₂K, gave {ON^EtNO}­InCl (<b>2a</b>). The reaction between InCl₃ and 3 equiv of MeLi led to a mixture of [InMe₃] and Li­[InMe₄], which upon further treatment with {ON^RNO}­H₂ proligands gave the ate complexes [{ON^RNO}­Li]­InMe₂ (<b>4a</b>,<b>b</b>); the methyl complex {ON^EtNO}­InMe (<b>3a</b>) was also isolated from this reaction. Hydrocarbyl complexes {ON^RNO}­In­(CH₂SiMe₃) (<b>5a</b>,<b>b</b>) were prepared cleanly from the 1:1 reactions between In­(CH₂SiMe₃)₃ and {ON^RNO}­H₂. The solid-state molecular structures of mononuclear <b>2a</b>, <b>3a, 4b</b>, and <b>5b</b> and of [(<b>1a</b>)₂-(μ-H₂O)]_<i>n</i>, a distorted cubane-like core made up of four potassium atoms and two ligands, were determined. Compounds <b>5a</b>,<b>b</b> are moderately active initiators/catalysts for the ring-opening polymerization of <i>rac</i>-lactide, giving polymers with controlled molecular weights and narrow polydispersities, especially in the presence of added isopropyl alcohol (1–10 equiv) as exogenous initiator. Isotactic-enriched (<i>P</i>_m = 0.62–0.69) PLAs were obtained from <b>5a</b>, while <b>5b</b> gave atactic materials. The heterobimetallic compounds <b>4a</b>,<b>b</b> are also active and afforded slightly heterotactic-enriched PLAs (<i>P</i>_r = 0.57–0.62), but with broader polydispersities. Those results allowed us to discuss the initiation mechanisms according to the constitution of the systems (alkyl vs alkyl/<i>i</i>PrOH) and also stereoselective abilities as a function of the nature (In vs Al) and coordination environment (κ⁴-ONNO vs. κ²-NN) of the metal center

Synergistic Effect of the TiCl₄/<i>p</i>‑TsOH Promoter System on the Aza-Prins Cyclization

A novel aza-Prins cyclization promoted by a synergistic combination between a Lewis acid and a Brønsted acid to efficiently afford piperidines is described. Contrary to what has been previously reported in the literature, the generality of the reaction employing <i>N</i>-alkyl, <i>N</i>-aryl, and nonprotected homoallylamines has been demonstrated. The reaction is highly diastereoselective depending on the homoallylic amine used, <i>N</i>-PMP homoallyl amine leading preferentially to the <i>trans</i> diastereomer, and free homoallylamine affording the deprotected piperidine as single <i>cis</i> diastereomer