Ketiminate Ligands Research Articles

Zn (II) octahedral complex with new ketimine ligand: design, synthesis and single-crystal studies

Zn (II) octahedral complex with new ketimine ligand: design, synthesis and single-crystal studies

Iridium-Catalyzed Acid-Assisted Hydrogenation of Oximes to Hydroxylamines.

We found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N-chelation is crucial, with alkoxy-substituted aryl ketimine ligands providing the best catalytic performance. Several Ir-complexes were mapped by X-ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N-O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates.

Iridium‐Catalyzed Acid‐Assisted Hydrogenation of Oximes to Hydroxylamines

AbstractWe found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N‐chelation is crucial, with alkoxy‐substituted aryl ketimine ligands providing the best catalytic performance. Several Ir‐complexes were mapped by X‐ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N−O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates.

Synthesis and Coordination Behaviour of a Novel Tridentate P,N,O‐Ketiminate Ligand

AbstractTreatment of o‐(diphenylphosphino)aniline with 3‐formylacetylacetone led to the formation of the ketimine type Schiff base 3‐((o‐diphenylphosphino‐phenylamino)‐methylene)‐pentane‐2,4‐dione (o−PPh2−C6H4−NH−CH=C(COCH3)2, HL). Complexes [ML2] (M=Fe, Co, Ni, Cu) were obtained from the reaction of HL with the corresponding metal acetate or acetylacetonate derivatives. The [ML2] complexes (M=Fe, Co, Ni) exhibit a distorted octahedral coordination from two tridentate P,N,O ketiminate ligands L−. The arrangement of the ligands is meridional in the case of [FeL2] and [NiL2]. The cobalt complex [CoL2] displays a facial coordination. In the case of the nearly square planar copper complex [CuL2], only the ketiminate functionalities of the ligands are involved in coordination and the participation of the PPh2 groups is neglectable.

Synthesis and molecular geometry of unique lithium isopropoxide assisted tantalum isopropoxide cluster containing bidentate N,O-ketiminate ligands

A unique cluster of lithium isopropoxide assisted tantalum isopropoxide derivative incorporating bidentate ketiminate ligand is reported and structurally characterized. Reaction between Li(OCMeCHCMeNAr) and TaCl5 in toluene at room temperature generates compound 1, (OCMeCHCMeNAr)TaCl4, in relatively high yield. Further reacting of 1 with excess amount of LiiOPr in diethylether affords the tantalum-lithium bimetallic cluster {[Li(OCMeCHCMeNAr)]Ta(OiPr)5(LiOiPr)(LiCl)}2 (2). The molecular geometry of 2 reveals as an edge-sharing bis(open-cube) that it consists two Li-Cl edge-sharing bis(open-cube)TaLi3O5Cl units in its core structure.

Synthesis, structural and DFT interpretation of titanium bis-isopropoxide derivatives incorporating bidentate ketiminate ligands

A series of titanium compounds containing bidentate ketiminate ligands are synthesized and structurally characterized. Reaction between one or two equivalents of LDIP-H with Ti(OiPr)4 in diethyl ether results the formation of titanium bis-isopropoxide compound, Ti(LDIP)2(OiPr)2 (1). Similarly, the combination of Ti(OiPr)4 and bidentate ketiminate precursor, LMOP-H and LMOMP-H affords the respective compounds, Ti(LMOP)2(OiPr)2 (2) and Ti(LMOMP)2(OiPr)2 (3), respectively, in moderate yield. Additionally, we discuss the synthetic routes for the formation of 1 using different substrates, TiPh(OiPr)3 or TiCl(OiPr)3 with LDIP-H or lithiated ligand LiLDIPvia ligand re-distribution. We, later on, describe the concurrence of electronic disposition and steric restraint with Ti-O bond length and TiOC bond angles.

Novel aluminum complexes bearing 2-(aminomethylene)malonate ligands with high efficiency and controllability in ring-opening polymerization of ε-caprolactone

A series of Al complexes bearing diethyl 2-((arylamino)methylene)malonate and diethyl 2-((alkylamino)methylene)malonate ligands (AMM ligands) were synthesized and their application in the ring-opening polymerization of ε-caprolactone was studied. All the Al complexes bearing AMM ligands exhibited higher catalytic activity (approximately 2.8–16.5 fold) than those Al complexes bearing ketiminate ligands did, except OiPrAlMe2 (2 fold lower catalytic activity). In addition, Al complexes with steric hindrance or electron-withdrawing groups in the ligands revealed the greater catalytic activity, and the chelating effect reduced the reactivity of ring-opening polymerization.

Magnesium and zinc complexes bearing NNO-tridentate ketiminate ligands: synthesis, structures and catalysis in the ring-opening polymerization of lactides.

A series of magnesium and zinc complexes, [(L1-3)2M2(μ-OBn)2] (M = Mg (1-3), Zn (4-6)), have been synthesized by the reaction of NNO-tridentate ketiminate ligands (L1-3-H) and a stoichiometric amount of MgnBu2 (or ZnEt2) and BnOH. In addition, the reaction of these ketiminate ligands (L1-4-H) with a half equivalent of MgnBu2 (or ZnEt2) in toluene provides [M(L1-4)2] (M = Mg (7-10), Zn (11-14)) in good yields. All of these complexes have been fully characterized by NMR spectroscopy and elemental analysis, and the molecular structures of [(L1)2Mg2(μ-OBn)2] (1), [(L3)2Mg2(μ-OBn)2] (3), [(L1)2Zn2(μ-OBn)2] (4), [Mg(L1)2] (7), [Zn(L1)2] (11) and [Zn(L4)2] (14) have been further confirmed by X-ray crystallographic studies. X-ray diffraction studies revealed that complexes 1, 3 and 4 were dimeric in the solid state, bridging through the benzyloxy groups, while the solid-state structures of 7, 11 and 14 revealed a mononuclear species, six-coordinated by N, N, and O atoms of two ketiminate ligands, forming a distorted octahedron around the metal centre. Complexes 1-6 acted as efficient initiators for the ring-opening polymerization of lactides, producing PLAs with controlled molecular weights and narrow molecular weight distributions. Complex 6 [(L3)2Zn2(μ-OBn)2] exhibited the highest activity towards the ROP of lactides. And complexes 1-6 initiated rac-lactide polymerization to afford heterotactic enriched polymers (Pr up to 0.82). In addition, complexes 7-14 were also shown to efficiently catalyze the ring-opening polymerization of lactides in the presence of BnOH.

Crystal structure of the η4-ketimine titanium complex (di-phenyl-amido-κN){3-methyl-6-[(4-methyl-phen-yl)(phenyl-aza-nid-yl)methyl-idene]cyclo-hexa-2,4-dien-1-yl-κ2N,C1}(η5-penta-methyl-cyclo-penta-dien-yl)titanium(IV).

The mol-ecular structure of the title titanium(IV) half-sandwich complex, [Ti(η5-C10H15)(η4-C21H19N)(C12H10N)], shows a three-legged piano-stool geometry at the central TiIV atom, comprising of one penta-methyl-cyclo-penta-dienyl ligand, one bidentate ketimine ligand in an η4-coordination mode and one monodentate di-phenyl-amide ligand. Except for van der Waals forces, there are no significant inter-molecular inter-actions in the crystal.

Synthesis, Structure and Chemistry of Mono‐ and Digallane Complexes Supported by N,O‐Ketimine Ligand

AbstractA series of N,O‐ketimine supported gallium and digallane complexes have been synthesized and structurally characterized. The mono(ketiminate) gallium dichloride [OCMeCHCMeN(Dipp)]GaCl2, 2 (Dipp=2,6‐iPr2C6H3) and bis(ketiminate) gallium chloride [OCMeCHCMeN(Dipp)]2GaCl, 3 were isolated from the reaction of GaCl3 with 1 and 2 equivalents of lithiated ketimine, [OCMeCHCMeN(Dipp)]Li, 1 respectively. Treatment of 1 with ‘‘GaI’’ yielded diiodobis(ketiminate) digallane complex [OCMeCHCMeN‐(Dipp)]2Ga2I2, 4. To the best of our knowledge, compound 4 is the first report on N,O‐ketiminate ligand supported stable heteroleptic digallane(II) complex. The frontier molecular orbital analysis and a high Wiberg Bond Index (WBI) values support the strong interaction between the gallium centers in 4. In an attempt to synthesize alkyl substituted Ga complexes, we carried out the reaction of ketimine ligand with GatBu2Cl (1:1 ratio) that generated the adduct [OCMeCHCMeN(H)(Dipp)]GatBu2Cl, 5. In a similar fashion, dialkyl gallium complex [OCMeCHCMeN(Dipp)]GatBu2, 6 was obtained from the reaction of 1 with GatBu2Cl. All the compounds have been charecterized by 1H, 13C NMR and mass spectroscopy. The molecular structures of complexes 2–5 have also been determined by single crystal X‐ray diffraction analysis.

Correction: Comparing l-lactide and ε-caprolactone polymerization by using aluminum complexes bearing ketiminate ligands: steric, electronic, and chelating effects

Correction for ‘Comparing l-lactide and ε-caprolactone polymerization by using aluminum complexes bearing ketiminate ligands: steric, electronic, and chelating effects’ by Yu-Hsieh Chen et al., RSC Adv., 2015, 5, 100272–100280.

Synthesis, characterization, and catalytic activity of sodium ketminiate complexes toward the ring-opening polymerization of l-lactide

Na complexes bearing ketiminate ligands revealed the greater catalytic activity and polymer controllability than that of Na complexes bearing Schiff base ligands.

Zinc complexes chelated by bifunctional ketiminate ligands: Structure, reactivity and possible applications in initiation of ROP and copolymerization of epoxides with carbon dioxide

Bifunctional ketiminate zinc complexes were synthesized via alkyl or LiCl elimination route from appropriate zinc species (ZnCl2 or ZnEt2) and methoxyaryl substituted monoanionic ketimine {[2- or 3- or 4-(MeO)C6H4]}N(H)C(Me)=CHC(Me)=O. Homo- or heteroleptic complexes were prepared and characterized by the help of multinuclear NMR spectroscopy techniques. One of the heteroleptic ethyl substituted zinc complexes - {κ2-N,O-[(2-MeO)C6H4NC(Me)–CHC(Me)–O]ZnEt}2 was selected for further reactivity experiments with alcohols and amines. Molecular structures of some products were determined by X-Ray diffraction methods showing di- or trinuclear arrangement in solid state caused by the presence of intramolecular zinc–oxygen or zinc–nitrogen bridges. {κ2-N,O-[(2-MeO)C6H4NC(Me)–CHC(Me)–O]ZnEt}2 was also screened for of its catalytic activity in co/polymerization reactions (ROP: trimethylene carbonate, ɛ-caprolactone, l-lactide; copolymerization of epoxides with CO2).

Methoxyaryl substituted aluminum ketiminate complexes and its activity in ring opening polymerization processes

The synthesis of dimethyl aluminum (LoAlMe2) and chloromethyl aluminum (LoAlMeCl) complexes containing ketiminate ligand [(2-MeO)C6H4]N(H)C(Me)=CHC(Me)=O (LoH) is reported along with the preparation of Lo3Al from LoH and LiAlH4, and ionic [Lo′Al(thf)3]I from LoAlMe2 and I2. In the later complex the pseudooctahedral geometry of the aluminum ion by terdentate ligand {[(2-O)C6H4]NC(Me)=CHC(Me)=O}2− (Lo′), originated from Lo ligand by the methyl group abstraction, and three THF molecules are observed. Structures of these complexes were determined by X-ray techniques. The activity of LoAlMe2 and LoAlMeCl as initiators of ring opening polymerization of ε-caprolactone, trimethylene carbonate and l-lactide, and (co)polymerization of cyclohexene oxide and carbon dioxide was studied.

A closer look at ε-caprolactone polymerization catalyzed by alkyl aluminum complexes: the effect of induction period on overall catalytic activity.

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time. We propose that an induction period for Al complexes for conversion to real catalytic species, Al alkoxide, should be considered because the total conversion time consists of both an induction period and polymer propagation time. Herein, the polymerization rate of a series of Al complexes bearing ketimine ligands was investigated. The catalytic results indicated complexes with more steric hindrance with an electron-withdrawing group on the ligands, or the fact that less chelating ligands demonstrated greater propagation activity. The opposite trend for these structural effects was observed on the measurement of induction periods. These features on ligands of aluminum complexes are responsible for facilitating the conversion process to Al alkoxides. The overall catalytic performance should consider both the induction period and the propagation time.

Comparing l-lactide and ε-caprolactone polymerization by using aluminum complexes bearing ketiminate ligands: steric, electronic, and chelating effects

Herein, the l-lactide polymerization rate of a series of Al complexes bearing ketimine ligands was investigated, and the polymerization characteristics between l-lactide and ε-caprolactone were compared.

Aluminum methyl and isopropoxide complexes with ketiminate ligands: Synthesis, structural characterization and ring-opening polymerization of cyclic esters

Mono(ketiminate) aluminum dimethyl complexes 1a–5a and bis(ketiminate) aluminum methyl complex 2b were synthesized via the reactions of AlMe3 with various N-aryl substituted ketimine compounds in a 1:1 or 1:2 molar ratio, respectively. The reaction of Al(OiPr)3 with the ketimine compound L2H resulted in a bis(ketiminate) aluminum isopropoxide complex 2c. All the complexes were characterized by 1H and 13C{1H} NMR spectroscopy, either EA or HRMS analysis. X-ray diffraction measurement of complexes 2a and 3a confirmed that in the solid state both 2a and 3a exist as a tetrahedron structure with the aluminum atom center surrounded by the oxygen and nitrogen donors of the bis-chelating ketiminate ligand as well as two methyl groups. Besides, a triligated aluminum complex 3 (Al(L3)3) with a six-coordinated metal core and a dinuclear aluminum isopropoxide complex 6d [(L6)2Al(μ-OiPr)2Al(OiPr)2] with C2-symmetry were isolated and characterized by X-ray diffraction studies. All aluminum methyl complexes behaved as active initiators in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), affording isotactic-enriched polylactides (PLAs) with high molecular weights and broad molecular weight distributions at 70°C. The steric and electronic characters of the ancillary ligands show significant effects both on the activity and the stereoselectivity. The introduction of an electron-withdrawing group on the para-position of N-aryl ring of the ligand resulted in an increase of the catalytic activity. However, the electronic character of ancillary ligands showed a neglectable effect on the ROP of ε-CL initiated by these aluminum complexes. For both monomers, bis(ketiminate) aluminum complex 2b was found to be more active than the corresponding mono(ketiminate) complex 2a of the same ligand. Aluminum isopropoxide complex 2c exerted better control for the polymerizations of both monomers, providing polymers with narrower molecular weight distributions and smaller molecular weights which are comparable to the theoretical values

Lewis Base Assisted Magnesium Complexes Incorporating Pyrrolyl and Ketiminate Ligands: Synthesis, Structural Diversity and Characterization

AbstractA series of four, five and six‐coordinated magnesium derivatives integrating with substituted pyrrole and ketimine ligands are conveniently synthesized. Reaction of two equiv of 2‐dimethylaminomethyl pyrrole with Mg[N(SiMe3)2]2 in THF affords the monomeric magnesium complex Mg[C4H3N(2‐CH2NMe2)]2 (THF)2 (1) in high yield along with elimination of two equiv of HN(SiMe3)2. Similarly, the reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe3)2]2 in THF renders the magnesium derivative, Mg[C4H3N(2‐CH2NHtBu)]2(THF)22(2) in good yield. Interestingly, reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe3)2]2 in toluene, instead of THF, generates Mg[C4H3N(2‐CH2NHtBu)]2 (3), also in high yield. Furthermore, the assembly of two equiv of ketimine ligand, HOCMeCHCMeNAr (Ar = C6H3‐2,6‐iPr2) and Mg[N(SiMe3)2]2, yields five‐coordinated magnesium derivatives, Mg(OCMeCHCMeNAr)2(THF) (4) and Mg(OCMeCHCMeNAr)2(OEt2) (5), using THF and diethyl ether, respectively. All the aforementioned derivatives are characterized by 1H and 13C NMR spectroscopy as well as 1, 3, 4 and 5 are subjected to X‐ray diffraction analysis in solid state.

Synthesis and characterization of tungsten bis-imido complexes containing bi-dentate pyrrole-amine, pyrrole-imine or ketiminate ligands

Three new imido tungsten derivatives containing bi-dentate pyrrole-amine, pyrrole-imine or ketiminate precursor were synthesized and characterized. Reacting W(NtBu)2(NHtBu)2 with two equiv of C4H3NH(2-CH2NHtBu) and C4H3NH(2-CHNCH2Py) in diethyl ether and toluene generates [W(NtBu)2{C4H3N(2-CH2NHtBu)}2] (1) and [W(NtBu)2{C4H3N(2-CHNCH2Py)}2] (2) in good yield. Similarly, while subjecting two equiv of (ArNCMeCHCMeOH) (Ar=2-MeOC6H4) with W(NtBu)2(NHtBu)2 in diethyl ether, affords [W(NtBu)2{OCMeCHCMeNAr}2] (3) in high yield. Single crystal X-ray structure reveals that in all three complexes the 6-coordinate W atoms belong to a distorted octahedral environment.

Ring-opening polymerization ofl-lactide catalyzed by calcium complexes

A series of calcium complexes supported by NNO-tridentate ketiminate ligands have been synthesized and characterized. X-ray structural studies indicate that these complexes bear a mononuclear feature with hexa-coordinated calcium centers bonding to two ketiminate ligands. Polymerization of L-lactide catalyzed by these complexes proceeds rapidly and well controlled in the presence of a variety of alcohols. In addition, these complexes reveal good catalytic activity even with the addition of up to 80 equiv. of benzyl alcohol as a transfer agent. Furthermore, kinetic studies show ROP of L-lactide is a first-order dependency on [LA] and a second-order dependency on [BnOH].