Β-diketiminate Ligand Research Articles

Redox behavior of novel nickel and palladium complexes supported by trianionic non-innocent ligand containing β-diketiminate and phenol groups

A new type of nickel and palladium complexes with non-innocent β-diketiminate ligand having redox active phenol groups, 2,4-di-tert-butyl-6-(((1E,2E)-3-((3,5-di-tert-butyl-2-hydroxyphenyl)amino)-2-nitroallylidene)amino)phenol ( L H 3, fully protonated form) have been developed, and the structure, physical properties, and reactivity of their one-electron and two-electron oxidized complexes, [MII(L•2-)] and [MII(L-)]+ ( M = Ni II or Pd II ) have been examined in detail. The two-electron oxidized forms of both complexes, [MII(L-)]+, exhibited hydrogen atom abstraction ability from 1,4-cyclohexadiene (CHD) comparable to its copper analog [ Cu II ( L -)]+ (Dalton Trans. 2013; 42: 2438-2444). The one-electron oxidized form of palladium complex, [ Pd II ( L •2-)], was also found to oxidize CHD, whereas the nickel analog, [ Ni II ( L •2-)], exhibited photo-induced oxidation ability of CHD.

Synthesis and characterization of new unsymmetrical β-diketiminate tris(dimethylamido)hafnium(IV) complexes as potential precursors for the MOCVD of HfO2

The synthesis and characterization of four new unsymmetrical β-diketiminate tris(dimethylamido)hafnium(IV) complexes, [2-(2,6-diisopropylphenyl)amino-4-(phenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5a), [2-(2,6-diisopropylphenyl)amino-4-(4-methylphenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5b), [2-(2,6-diisopropyl-phenyl)amino-4-(4-methoxyphenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5c), and [2-(2,6-diisopropylphenyl)amino-4-(4-chlorophenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5d), are described. Amine elimination reactions work well for introducing unsymmetrical β-diketiminates2-(2,6-diisopropylphenyl)amino-4-(phenyl)imino-2-pentene (4a), 2-(2,6-diisopropylphenyl)amino-4-(4-methylphenyl)imino-2-pentene (4b), 2-(2,6-diisopropylphenyl)amino-4-(4-methoxyphenyl)imino-2-pentene (4c), and 2-(2,6-diisopropylphenyl)amino-4-(4-chlorophenyl)imino-2-pentene (4d) to the tetrakis(dimethylamino)hafnium centre. We discuss the synthetic procedures and characterization using 1H NMR, 13C NMR, IR, mass spectroscopy, and elemental analysis. According to the IR and NMR spectra, unsymmetrical β-diketiminate ligands are bidentate, coordinating through two nitrogens to hafnium.

Hafnium complexes with versatile β-diketiminate ligands: Synthesis, spectroscopic investigation and volatility study

Synthesis and spectral investigation of four novel β-diketiminate hafnium(IV) chloride complexes cis-dichloro-bis(2,4-bis(phenylimino)pent-3-ene)-hafnium (4a), cis-dichloro-bis(2,4-bis(4-methylphenylimino)pent-3-ene)-hafnium (4b), cis-dichloro-bis(2,4-bis(4-methoxyphenylimino)pent-3-ene)-hafnium (4c) and cis-dichloro-bis(2,4-bis(4-chlorophenylimino)pent-3-ene)-hafnium (4d) are reported. All complexes (4a–d) were characterized by spectroscopic methods (1H NMR, 13C NMR and mass) and elemental analysis. The solid-state structure of compound 4a was further examined by single crystal X-ray diffraction. The X-ray structure of 4a reveals that the complex is monomeric with the hafnium center in a distorted octahedral geometry, and the chloride ligands are in a cis arrangement. The thermal stability and volatility of the compounds were probed by TGA/DSC tests demonstrating the potential utility of compounds (4a–d) as precursors for CVD/ALD thin film growth below 200°C.

Mononuclear three-coordinate magnesium complexes of a highly sterically encumbered β-diketiminate ligand.

The highly sterically encumbered chelating β-diketiminate ligand, [HC{C(Me)N(2,6-CHPh2-4-MeC6H2)}2](-), (Ar)L(-), has been used to prepare a series of heteroleptic three-coordinate magnesium complexes. Both the bis(imine) and imine-enamine tautomers of the ligand precursor, (Ar)LH, as well as the diethyl ether adduct of the bromide complex [(Ar)LMgBr(OEt2)], the monomeric methyl complex [(Ar)LMgMe], the THF-solvated and unsolvated n-butylmagnesium complexes [(Ar)LMg(n)Bu(THF)] and [(Ar)LMg(n)Bu], and the 1-hexynyl analogue [(Ar)LMgC≡C(n)Bu] have been crystallographically characterized. Both n-butylmagnesium complexes showed remarkable stability in air, both in the solid state and in solution. Single crystals of the highly sensitive magnesium hydride, [(Ar)LMgH], underwent partial hydrolysis by solid-state water diffusion to the isostructural hydroxide compound [(Ar)LMgOH].

Crystal structure of di-μ-hydroxido-bis{[N,N'-bis-(2,6-di-methyl-phen-yl)pentane-2,4-diiminato(1-)]zinc}.

The title compound, [Zn2(C21H25N2)2(OH)2], is a binuclear zinc complex formed by two bidentate β-diketiminate (nacnac) ligands and two μ-hydroxide O atoms, bridging two mononuclear units into a centrosymmetric dimeric unit. Each Zn2+ cation is coordinated by two N-donor atoms from the nacnac ligand and two O-donor atoms of hydroxide anions to give a distorted tetra­hedral coordination environment. The Zn—O bond lengths are 1.9643 (13) and 2.0022 (14) Å, and the two Zn—N bond lengths are 1.9696 (14) and 1.9823 (14) Å. The distance between the two Zn2+ cations in the dimer is 2.9420 (4) Å. Although hydroxide groups are present in the complex, no classical hydrogen-bonding inter­ations are observed because of the bulky β-diketiminate ligands.

ChemInform Abstract: Z‐Selective Alkene Isomerization by High‐Spin Cobalt(II) Complexes.

The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky β-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.

Synthesis and characterization of group 5 imido complexes supported by the 2,6-dichloroaryl β-diketiminato ligand

The synthesis of niobium and tantalum imido complexes supported by the bis-N,N-(2,6-dichlorophenyl)-β-diketiminate ligand (BDICl) is reported. Metal chloride complexes (BDICl)M(NtBu)Cl2py (1: Nb, 2: Ta) were obtained in good yield following the treatment of M(NtBu)Cl3py2 with Li(BDICl). Complexes 1–2 were used as precursors for the formation of the dimethylated complexes (BDICl)M(NtBu)Me2 (3: M=Nb, 4: M=Ta). The new complexes 1–4 were characterized by X-ray crystallography and variable temperature NMR spectroscopy. In addition, the tantalum dialkyl imido complexes (BDICl)Ta(NtBu)Bn2 (5) and (BDICl)Ta(NtBu)(CH2SiMe3)2 (6) were prepared and characterized by NMR spectroscopy.

Facile synthesis of phosphaamidines and phosphaamidinates using nitrilium ions as an imine synthon.

Readily accessible nitrilium triflates are convenient imine building blocks for the expedient synthesis of a novel class of 1,3-P,N ligands as demonstrated for the reaction with primary phosphanes. This procedure allows variation of all substituents. X-ray crystal structures are reported for nitrilium ions, phosphaamidines, and three phosphaamidinate complexes. The lithium phosphaamidinate is N coordinated and its reaction with [AuCl(tht)] (tht = tetrahydrothiophene) gives a unique P-bridged gold trimer, while a P,N-bidentate complex results from [{RhCl(cod)}2]. The nitrilium ion methodology allows extension of the 1,3-P,N motive to bis(imino)phosphanes, which are the neutral phosphorus analogues of the valuable β-diketiminate ligand.

Cu(II) complexes of sterically and electronically divergent β-diketiminate ligands: Structural and electrochemical aspects

Cu(II) complexes of β-diketiminate ligands with divergent N-aryl substitutions are reported. Exclusive formation of mononuclear [LCu(OAc)] complexes in the case of –CN substituted ligands and hydroxy bridged dinuclear [LCu(OH)]2 complexes in the case of –NO2 substituted ligands is observed. Most of the complexes were characterized by single crystal X-ray diffraction studies. Cyclic-voltammetric studies carried out on these complexes show that the reduction potentials correlate well with the expected electronic effects.

Three-coordinate nickel(II) and nickel(I) thiolate complexes based on the β-diketiminate ligand system.

Mononuclear nickel(II) thiolate complexes [L(tBu)Ni(SEt)] (1) and [L(tBu)Ni(aet)] (2, aet = (-)S(CH2)2NH2) (L(tBu) = [HC(C((t)Bu)NC6H3((i)Pr)2)2](-)), supported by a bulky nacnac ligand, were synthesized by treatment of the nickel(II) bromide precursor [L(tBu)Ni(Br)] (I) with the potassium salts of ethanethiol and cysteamine, respectively. The nickel atom in 1 features a planar T-shaped environment, while the Ni ion within 2 shows a distorted square planar coordination geometry, as the aminoethanethiolate (aet) is coordinated as a chelating ligand. In 2 the β-diketiminate ligand binds in a rarely observed κ(2)C,N coordination mode. Reduction of complex 1 or its benzenethiolate analogue [L(tBu)Ni(SPh)] (II) by KC8 resulted in the formation of dinuclear Ni(I) thiolates (K·OEt2)(K)[L(tBu)Ni(SEt)]2 (3) and (K·OEt2)2[L(tBu)Ni(SPh)]2 (4), respectively. In these compounds [L(tBu)Ni(SR)](-) units are held together by potassium cations produced in the reduction process. All compounds mentioned were structurally characterized by single-crystal X-ray crystallography.

Redox chemistry of nickel(II) complexes supported by a series of noninnocent β-diketiminate ligands.

Nickel complexes of a series of β-diketiminate ligands ((R)L(-), deprotonated form of 2-substituted N-[3-(phenylamino)allylidene]aniline derivatives (R)LH, R = Me, H, Br, CN, and NO2) have been synthesized and structurally characterized. One-electron oxidation of the neutral complexes [Ni(II)((R)L(-))2] by AgSbF6 or [Ru(III)(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) gave the corresponding metastable cationic complexes, which exhibit an EPR spectrum due to a doublet species (S = 1/2) and a characteristic absorption band in near IR region ascribable to a ligand-to-ligand intervalence charge-transfer (LLIVCT) transition. DFT calculations have indicated that the divalent oxidation state of nickel ion (Ni(II)) is retained, whereas one of the β-diketiminate ligands is oxidized to give formally a mixed-valence complex, [Ni(II)((R)L(-))((R)L(•))](+). Thus, the doublet spin state of the oxidized cationic complex can be explained by taking account of the antiferromagnetic interaction between the high-spin nickel(II) ion (S = 1) and the organic radical (S = 1/2) of supporting ligand. A single-crystal structure of one of the cationic complexes (R = H) has been successfully determined to show that both ligands in the cationic complex are structurally equivalent. On the basis of theoretical analysis of the LLIVCT band and DFT calculations as well as the crystal structure, the mixed-valence complexes have been assigned to Robin-Day class III species, where the radical spin is equally delocalized between the two ligands to give the cationic complex, which is best described as [Ni(II)((R)L(0.5•-))2](+). One-electron reduction of the neutral complexes with decamethylcobaltocene gave the anionic complexes when the ligand has the electron-withdrawing substituent (R = CN, NO2, Br). The generated anionic complexes exhibited EPR spectra due to a doublet species (S = 1/2) but showed no LLIVCT band in the near-IR region. Thus, the reduced complexes are best described as the d(9) nickel(I) complexes supported by two anionic β-diketiminate ligands, [Ni(I)((R)L(-))2](-). This conclusion was also supported by DFT calculations. Substituent effects on the electronic structures of the three oxidation states (neutral, cationic, and anionic) of the complexes are systematically evaluated on the basis of DFT calculations.

Structure and potential applications of amido lanthanide complexes chelated by bifunctional β-diketiminate ligand

The synthesis and structure of lanthanide(III) amides containing one bifunctional β-diketiminate ligand is reported. Compounds were obtained via a protolytic reaction of the free β-diketimine {N-[(2-MeO)C6H5]}NC(Me)CHC(Me)N(H){N′-[(2-MeO)C6H5]} (LCOH) or {N-[(2-MeO)C6H5]}NCHCHCHN(H){N′-[(2-MeO)C6H5]} (LHOH) with corresponding tris(amide) of the Ln[N(SiMe3)2]3 (Ln = Y, La, Nd, Sm, Eu, Gd and Yb) type in equimolar ratio at ambient temperature. Products were characterized by IR and/or multinuclear NMR spectroscopy techniques and structures of four of them have been determined by X-ray diffraction techniques. Complexes LCOY[N(SiMe3)2]2, LCOLa[N(SiMe3)2]2, LCONd[N(SiMe3)2]2 and LHOYb[N(SiMe3)2]2 crystalize as monomers with the six-coordinated metal centre by two amido and one tetradentate β-diketiminato ligands. Minor hydrolytic products of composition [LCOSmN(SiMe3)2]2O and (LCO)4O2(OH)4La4 were obtained and their structures contain two or four lanthanide atoms linked through the oxo- or hydroxo bridges. Diamagnetic complexes were tested for its potential applications in hydroamination and polymerization reactions, respectively, giving low conversion in all reactions.

Synthesis of a sulfonate substituted β-diketimine ligand and its complexes of Co(II), Ni(II), Zn(II) and Pd(II)

Deprotonation of the β-diketimine ligand [CH2(C(Me)N-2,6-diisopropylphenyl)2] using n-BuLi followed by treatment with SO3NMe3 afforded the sulfonate substituted β-diketimine ligand Li(THF)2[O3SCH(C(Me)N-2,6-diisopropylphenyl)2] (1). Compound 1 is moderately soluble in water. It reacts with Ni(II) and Zn(II) chlorides to produce [LMCl] complexes (L=1 without Li(THF)2), in which the ligand coordinates to the metal in a tripodal fashion. Metal nitrate complexes [LM(O2NO)] [M=Co(II), Ni(II) and Zn(II)] were also synthesized and structurally characterized. The reaction of 1 with [PdCl2(CH3CN)2] was also explored. Depending on the reaction conditions, two dinuclear Pd complexes, [LPdCl2PdCl2]− and [LPdCl2PdL], were formed. In both the cases the sulfonate group remained uncoordinated.

Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

The isomerization of simple terminal alkenes to internal isomers with Z-stereochemistry is rare, because the more stable E-isomers are typically formed. We show here that cobalt(II) catalysts supported by bulky β-diketiminate ligands have the appropriate kinetic selectivity to catalyze the isomerization of some simple 1-alkenes specifically to the 2-alkene as the less stable Z-isomer. The catalysis proceeds via an "alkyl" mechanism, with a three-coordinate cobalt(II) alkyl complex as the resting state. β-Hydride elimination and [1,2]-insertion steps are both rapid, as shown by isotopic labeling experiments. A steric model explains the selectivity through a square-planar geometry at cobalt(II) in the transition state for β-hydride elimination. The catalyst works not only with simple alkenes, but also with homoallyl silanes, ketals, and silyl ethers. Isolation of cobalt(I) or cobalt(II) products from reactions with poor substrates suggests that the key catalyst decomposition pathways are bimolecular, and lowering the catalyst concentration often improves the selectivity. In addition to a potentially useful, selective transformation, these studies provide a mechanistic understanding for catalytic alkene isomerization by high-spin cobalt complexes, and demonstrate the effectiveness of steric bulk in controlling the stereoselectivity of alkene formation.

Hydrogen-bond-supported dimeric boron complexes of potentially tetradentate β-diketiminate ligands

Two dimeric boron complexes of potentially tetradentate and trianionic β-diketiminate ligands bearing phenol substituents were prepared and characterized. The synthetic routes employed were designed to circumvent the undesirable formation of β-ketimines and 2-methylbenzoxazoles observed when traditional synthetic routes toward the target β-diketiminate ligands were attempted. The title complexes were isolated via demethylation of β-diketimine ligands and boron difluoride complexes bearing 2-anisole N-aryl substituents using boron tribromide. The resulting complexes were found to contain a unique hydrogen-bond-supported boron-oxygen-boron bridge, as confirmed by X-ray crystallography. The stability of the resulting dimeric structures relative to the corresponding monomeric, tetradentate boron complexes was studied computationally, and theory confirmed that the dimeric structures were strongly favored. The absorption spectra of the dimers were red-shifted relative to the parent β-diketimine ligands. The complexes were irreversibly oxidized and reduced electrochemically and were weakly emissive at low concentrations (Stokes shifts between 23 and 31 nm), showing little solvent dependence.

Copolymerization of CO2and Cyclohexene Oxide: β-Diketiminate-Supported Zn(II)OMe and Zn(II)Et Complexes as Initiators

β-Diketiminate ligands with varying degrees of steric bulkiness and having −CN and −NO2 groups on N-aryl substituents were synthesized. The catalytic activity of some of their [LZnEt] and [LZnOMe]2 complexes (L = β-diketiminato) toward the copolymerization of carbon dioxide and cyclohexene oxide was explored. All the catalysts were found to be highly active and showed very good turnover frequency, and the polycarbonates produced have a very high percentage of carbonate linkages. The complex [LiPr,Me-CNZnEt] showed very high activity and selectivity on par with its zinc methoxide analogue. The polymers produced by [LZnEt] complexes displayed a bimodal molecular weight distribution.

Square-Planar Cu(II) Diketiminate Complexes in Lactide Polymerization

Cu(OiPr)2 was reacted with several β-diketimine ligands, nacnac(R)H. Sterically undemanding ligands with N-benzyl substituents afforded the dimeric heteroleptic complexes [nacnac(Bn)Cu(μ-OiPr)]2 and [3-Cl-nacnac(Bn)Cu(μ-OiPr)]2 (Bn = benzyl). With sterically more demanding amines, dimerization was not possible, and the putative nacnacCuOiPr intermediate underwent ligand exchange to the homoleptic bisdiketiminate complexes Cu(nacnac(ipp))2 and Cu(nacnac(Naph))2 (ipp = 2-isopropylphenyl, Naph = 1-napthyl). Homoleptic complexes were also prepared with N-benzyl ligands to yield Cu(nacnac(Bn))2 and Cu(3-succinimido-nacnac(Bn))2. All complexes were characterized by single-crystal X-ray diffraction. Even bulkier ligands with N-anthrylmethyl, N-mesitylmethyl, or N-methylbenzyl substituents failed to react with Cu(OiPr)2. In the case of nacnac(dipp)CuOiPr, putative nacnac(dipp)CuOiPr decomposed by β-hydride elimination. Heteroleptic complexes [nacnac(Bn)Cu(μ-OiPr)]2 and [3-Cl-nacnac(Bn)Cu(μ-OiPr)]2 are very highly active rac-lactide polymerization catalysts, with complete monomer conversion at ambient temperature in solution in 0.5-5 min. In the presence of free alcohol, the homoleptic complexes seem to be in equilibrium with small amounts of the respective heteroleptic complex, which are sufficient to complete polymerization in less than 60 min at room temperature. All catalysts show high control of the polymerization with polydispersities of 1.1 and below. The obtained polymers were essentially atactic, with a slight heterotactic bias at ambient temperature and at -17 °C.

Mechanism of Catalytic Nitrene Transfer Using Iron(I)–Isocyanide Complexes

Low-spin iron(I) complexes supported by bulky β-diketiminate ligands catalyze the formation of unsymmetric carbodiimides (RN═C═NR′, R ≠ R′) from isocyanides and organoazides. In situ EPR studies indicate that the catalytic resting state in these reactions is the tris(isocyanide)iron(I) complex LFe(CNR)3, and kinetic experiments show that the reaction is inverse second-order in isocyanide concentration, suggesting that two isocyanide molecules dissociate prior to the rate-determining step. An unusual iron–carbodiimide complex can be isolated independently, and facile displacement of the coordinated carbodiimide by both isocyanide and N2 demonstrate that product release is not kinetically limiting during catalysis. These experimental data fit a coherent mechanism for the catalytic C–N coupling reaction, and the implications for catalytic nitrene transfer are discussed.

Trichlorido{methyl[4-(methylimino)pent-2-en-2-yl]azanido-κ2N,N′}hafnium(IV)

The title complex, [Hf(C29H41N2)Cl3], was synthesized from HfCl4 and HC[C(Me)N(iPr2C6H3)]2Li(Et2O) in toluene. The structure is isotypic with those of TiIV and ZrIV complexes reported previously [Nikiforov et al. (2007 ▶). Dalton Trans. pp. 4149–4159; Kakaliou et al. (1999 ▶). Inorg. Chem. 38, 5964–5977]. There is a crystallographic mirror plane containing the Hf atom, one chloride ligand and the central diketiminate C atom. The HfIV ion has slightly distorted square-pyramidal geometry surrounded by two N atoms from the β-diketiminate ligand and by three Cl− anions. Coordination bond lengths are Hf—N = 2.181 (3) Å, and Hf—Cl = 2.3148 (15) and 2.3727 (12) Å.

Cobalt–Magnesium and Iron–Magnesium Complexes with Weakened Dinitrogen Bridges

The cooperative binding of N2 by late transition metals and main-group metals is a promising strategy for N-N bond weakening and activation. We report the use of activated Rieke magnesium for reduction of iron and cobalt complexes supported by bulky β-diketiminate ligands. Binding of N2 is accompanied by assembly of a linear M-NN-Mg-NN-M (M = Co, Fe) core with N-N bonds that are weakened, as judged by infrared spectroscopy. Both the cobalt and iron complexes require THF solvent, because of Mg-THF binding. The cobalt complex can be isolated as a pure solid, but the iron complex is stable only in solution. These results demonstrate the correlation between the binding mode and N-N weakening in heterobimetallic N2 complexes.