Diketiminate Ligand Research Articles

Synthesis and Structure of Thiolate‐Bridged Diiron and Dicobalt Complexes Supported by Modified β‐Diketiminate Ligands

AbstractMultinuclear metal thiolate complexes represent an important class of organometallic compound owing to their capacity that they can not only serve as bio‐mimicking functional platform but can also play as excellent cooperative catalysts. In this context, it is of long‐standing interest to construct new‐type transition metal complexes featuring sulfur donors. Herein we describe a synthetic method toward a novel unsymmetric β‐diketiminate ligand containing a thioether tether in a gram‐scale. This sulfur functionalized β‐diketiminate ligand was proved to be an ideal structural skeleton for coordination and stabilization of relatively unstable FeII and CoII ions to generate new thiolate‐bridged bimetallic complexes. These new complexes have been well‐defined by various spectroscopic characterizations and single‐crystal X‐ray diffraction analysis.

Synthesis of Homo‐ and Heteroleptic Al, Ga and Zn Complexes Containing (Per)fluorinated β‐Diketiminate Ligands

Alkane elimination reactions of equimolar amounts of 17Fnac2H (17Fnac2H = FC[C(CF3)NC6F5]2H) with ZnEt2 and AlMe3 as well as 16Fnac2H (16Fnac2H = HC[C(CF3)NC6F5]2H) with GaMe3 yielded the corresponding heteroleptic complexes of 17Fnac2ZnEt (1), 17Fnac2AlMe2 (2), and 16Fnac2GaMe2 (3), whereas reactions of two equivalents of 16Fnac2H and 17Fnac2H with either ZnEt2 or Cp*2Zn gave homoleptic ZnII complexes (16Fnac2)2Zn (4) and (17Fnac2)2Zn (5), respectively. 1–5 were characterized by IR and NMR spectroscopy (1H, 13C, 19F), elemental analysis and single‐crystal X‐ray diffraction.

Selective and Nonselective Aza-Michael Additions Catalyzed by a Chiral Zirconium Bis-Diketiminate Complex

Reaction of the chiral bis-diketiminate complex rac- or (R,R)-C6H10(nacnacXyl)2ZrCl2 with AgOTf yielded the corresponding bis-triflate complex. The complex geometry changes from distorted octahedral in the dichloride complex to a pseudotetrahedral coordination involving π coordination of the diketiminate ligands. The bis-triflate complex is highly active for aza-Michael additions with turnover frequencies of 20000/h for the addition of morpholine to acrylonitrile and 1000/h for the addition of morpholine to methacrylonitrile. The enantioselectivities of the latter reaction in various solvents were low, never surpassing 19% ee. The reaction is first-order in olefin concentration and second order in amine concentration, which is explained by its participation as a base in the reaction mechanism. The presence of catalytic amounts of triethylamine slightly increases the observed rate constants and reduces the reaction order in amine to first order. Other activated alkenes such as methacrylonitrile, crotonitri...

Exceptionally high lactide polymerization activity of zirconium complexes with bridged diketiminate ligands

A cyclohexanediyl-bridged, bis(N-xylyl) diketiminate ligand, (±)-C6H10(nacnac(Xyl)H)2, LH2 (Xyl = 2,6-dimethylphenyl), was obtained from the reaction of [(2,6-dimethylphenyl)amino]-pent-3-en-2-one first with Meerwein's salt, then with (±)-cyclohexanediamine. The reaction of the ligand with Zr(NMe2)4 yielded LZr(NMe2)2. Protonation of the remaining diamide ligands with EtOH or [H2NMe2]Cl yielded LZr(OEt)2 and LZrCl2, respectively. The latter complex was also obtained by the reaction of LH2 first with nBuLi and then with ZrCl4(THF)2. The dichloride complex yielded LZr(OEt)2 and LZrMe2 upon reaction with NaOEt or MeLi/AlMe3, respectively. X-ray diffraction studies showed a trans-configuration of the ancillary ligands in LZrCl2 and LZrMe2, and a cis-configuration in LZr(NMe2)2 and LZr(OEt)2. LZr(OEt)2 was tested as a catalyst for the polymerization of rac-lactide. Kinetic investigations yielded a rate law first order in catalyst and monomer and a rate constant k = 14(1) L mol(-1) s(-1), the latter being orders of magnitude higher than typical activities for group 4 complexes in lactide polymerization. Analyses of the obtained polymer revealed an atactic polymer and broad polymer molecular weight distributions with sizeable fractions of cyclic oligomers. The influence of contaminants on the polymerization activity was examined: while lactic acid deactivates the catalyst, addition of up to 1 equiv. of water or para-toluenesulfonic acid revitalized catalysts not showing maximum activity.

Three-Coordinate Beryllium β-Diketiminates: Synthesis and Reduction Chemistry

A series of mononuclear, heteroleptic beryllium complexes supported by the monoanionic β-diketiminate ligand [HC{CMeNDipp}(2)](-) (L; Dipp = 2,6-diisopropylphenyl) have been synthesized. Halide complexes of the form [LBeX] (X = Cl, I) and a bis(trimethylsilyl)amide complex were produced via salt metathesis routes. Alkylberyllium β-diketiminate complexes of the form [LBeR] (R = Me, (n)Bu) were obtained by salt metathesis from the chloride precursor [LBeCl]. Controlled hydrolysis of [LBeMe] afforded an air-stable, monomeric β-diketiminatoberyllium hydroxide complex. [LBeMe] also underwent facile protonolysis with alcohols to form the corresponding β-diketiminatoberyllium alkoxides [LBeOR] (R = Me, (t)Bu, Ph). High temperatures and prolonged reaction times were required for protonolysis of [LBeMe] with primary amines to yield the β-diketiminatoberyllium amide complexes [LBeNHR] (R = (n)Bu, CH(2)Ph, Ph). No reactions were observed between [LBeMe] and silanes, terminal acetylenes, or secondary amines. All compounds were characterized by (1)H, (13)C, and (9)Be NMR spectroscopy and, in most cases, by X-ray crystallography. Reduction of the beryllium chloride complex with potassium metal resulted in apparent hydrogen-atom transfer between two β-diketiminate backbones, yielding two dimeric, potassium chloride bridged diamidoberyllium species. X-ray analysis of a cocrystallized mixture of the 18-crown-6 adducts of these species allowed unambiguous identification of the two reduced diketiminate ligands, one of which had been deprotonated at a backbone methyl substituent and the other reduced by hydride addition to the β-imine position. It is proposed that this process occurs by the formation of an unobserved radical anion species and intermolecular hydrogen-atom transfer by a radical-based hydrogen abstraction mechanism.

Dimethylaluminum complexes bearing a chiral diketiminate ligand: Synthesis, characterization and ring-opening polymerization of ε-caprolactone

Abstract Reaction of the chiral aliphatic N-substituted diketiminate ligand, N,N′-Di(R-phenylethyl)-2-amino-4-iminopent-2-ene (R,R-nacnacCH(Me)PhH), with AlMe3 affords the monomeric dimethylaluminum complex R,R-nacnacCH(Me)PhAlMe2 (R,R-1). Complex S,S-nacnacCH(i-Pr)PhAlMe2 (S,S-2) is prepared by S,S-nacnacCH(i-Pr)PhH and AlMe3. Complexes R,R-1 and S,S-2 are the first examples of chiral β-diketiminate ligands with main group metals. All these two complexes exhibit activity for the ring-opening polymerization of e-caprolactone in the absence of alcohol, and complex R,R-1 shows a higher activity with a narrow molecular weight distribution (PDI = 1.17) in toluene.

Highly symmetrical chromium(II) bisdiketiminate complexes

Reaction of the lithium salts of N, N′-dialkyl-2-amino-4-imino-pent-2-enes, nacnac RLi(THF) (R = CH 2Ph, Cy, nPr, iBu or S–CH(Me)Ph), with half an equivalent of CrCl 2(THF) x yielded the homoleptic complexes ( nacnac R) 2Cr. All complexes were characterized by X-ray diffraction studies and displayed a highly symmetric, square-planar coordination around the chromium center with strong boat-like distortions of the diketiminate ligands. Reaction of nacnac RLi(THF) (R = CH 2Ph, Cy) with one equivalent of CrCl 2(THF) x afforded the dimeric complexes { nacnac RCr(μ-Cl)} 2.

Zirconium Complexes of Symmetric and of Chiral Diketiminate Ligands: Synthesis, Crystal Structures, and Reactivities

Reaction of N,N-dialkyl-2-amino-4-iminopent-2-enes (nacnacRH; R = Bn (1a), Cy (1b), R-CH(Me)Ph (1c)) with nBuLi afforded the lithium salts nacnacRLi(THF), 2a−c. Further reaction with ZrCl4 or ZrCl4(THF)2 yielded the cis-dichloride complexes nacnacR2ZrCl2, 3a−c. As a side reaction in the synthesis of 3c, CH activation of CH(Me)Ph under formal HCl elimination was observed. Complex 3a could also be obtained by reaction of nacnacBnH with nBu2ZrCl2. nacnacBn2ZrMe2, 5, was formed in reactions of 3a with MeLi in the presence of excess AlMe3 and was isolated from the reaction of Me2ZrCl2 with 2a. Complex 5 reacts with aluminum chloride compounds, but not with lithium chloride, to form the dichloride complex 3a and with ethanol to form nacnacBn2Zr(OEt)2, 6. X-ray diffraction studies have been performed for 3a−c, 5, and 6.

Coordination modes in zirconium β-diketiminate complexes

Abstract Reaction of β-diketiminate lithium salts, nacnacRLi(THF) (R = 2,6-xylyl (1a), R-CH(Me)Ph (1b)), with ZrCl4 or ZrCl4·THF yielded the mono-diketiminate complexes nacnacRZrCl3(THF), 2a and 2b. Complex 2a did not react further with a second equivalent of 1a, but with CpLi or IndLi to form pseudo-tetrahedral complexes 3 and 4. No product could be isolated from reaction of 2b with CpLi. Coordination of the diketiminate ligand in 2a is best described as a distorted κ2-, in-plane coordination. Complexes 3 and 4, however, display a haptotropic shift of the diketiminate ligand and its coordination is best described as κ2,η2-coordination. Although not previously described, analyses of available structures in the literature reveal that non-symmetric κ2,η2-coordination is an independent coordination mode, which exists next to the previously described “η5-like” coordination.

Synthesis of β-diketiminato calcium silylamides and their reactions with triethylaluminium

Reaction of the beta-diketiminato calcium amide [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe3)(2)}(OEt2)] (Ar = 2,6-di-iso-propylphenyl) with triethylaluminium yields a novel calcium aluminate complex, [kappa(3)-{ArNC(Me)C(AlEt3)HC(Me)NAr}Ca{Et4Al}], in which coordination at calcium is provided by not only bridging interactions with the ethyl groups of the aluminate anion but also a tripodal kappa(3)-N,N,C-coordinated ligand derived from further reaction of the 'spectator' beta-diketiminate ligand with triethylaluminium. In the solid state, this compound exists as a 1 : 1 mixture of coordination isomers in which the Et4Al anion binds to calcium via either mu(2)- or mu(3)-coordination modes. Although persistent, and rapidly interconverting, at room temperature in solution, variable temperature NMR studies suggest that these two isomers undergo dissociative loss of triethylaluminium from the diketiminate ligand at low temperature.

Bis(2-phenylethyl)-nacnac: A Chiral Diketiminate Ligand and Its Copper Complexes

The chiral diketiminate ligand bis-N,N′-(2-phenylethyl)-2,4-diiminopentane, 1H, was synthesized in good yields in a one-step reaction from chiral amine and acetylacetone. Reaction of 1Li(THF) with N-bromosuccinimide yielded the succinimide-substituted ligand 2H. Copper complexes were obtained by reaction of the ligand with a basic copper source in the presence of coordinating Lewis bases, and 1Cu(NCMe), 1Cu(DMAP), 1Cu(PPh3), 1Cu(2,6-xylyl isonitrile), 2Cu(PPh3), and 2Cu(2,6-xylyl isonitrile) have been prepared and, for the most part, characterized by X-ray diffraction studies. Compared to their more common analogues with aromatic substituents on N, 1 and 2 seem to be more basic (1 > 2) and sterically more demanding (2 > 1). Their copper complexes are less stable than those of aryl-substituted diketiminates and tend to decompose by disproportionation, most probably after dissociation of the coordinated Lewis base. Despite the rotational freedom around the N−R* bond, the complexes are sterically rigid, a necessary requirement for potential applications in enantioselective catalysis.

Cobalt−Dinitrogen Complexes with Weakened N−N Bonds

Reported N(2) complexes of cobalt do not have substantial weakening of the N-N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N-N bond, but in formally zerovalent complexes, cobalt and iron give similar N-N weakening. The weakening is due to cobalt-to-N(2) pi-backbonding, and potassium cations pull more electron density into N(2). These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N-N bond of N(2).

β-Diketiminate aluminium complexes: synthesis, characterization and ring-opening polymerization of cyclic esters

A series of aluminium alkyl complexes (BDI)AlEt(2) (3a-m) bearing symmetrical or unsymmetrical beta-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding beta-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize beta-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex "(BDI-2a)AlMeCl" (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe](2)(micro-O) (7) as characterized by X-ray diffraction methods. Complexes 3a-m and [(2,6-(i)Pr(2)C(6)H(3)NCMe)(2)HC]AlEt(2) (8) were found to catalyze the ring-opening polymerization (ROP) of epsilon-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (M(eta)) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of epsilon-caprolactone initiated by complexes 3a-m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66-3.74, M(w)/M(n)) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.

Molybdenum Oxo and Imido Complexes of β-Diketiminate Ligands: Synthesis and Structural Aspects

Treatment of [MoO2(eta2-Pz)2] (Pz = 3,5-di-tert-butylpyrazolate) with the diketiminate ligand NacNacH (NacNac = CH[C(Me)NAr]2-, Ar = 2,6-Me2C6H3) at 55 degrees C leads under reduction of the metal to the formation of the dimeric molybdenum(V) compound [{MoO2(NacNac)}2] (1). The compound was characterized by spectroscopic means and by X-ray crystal structure analysis. The dimer consists of a [Mo2O4]2+ core with a short Mo-Mo bond (2.5591(5) A) and one coordinated diketiminate ligand on each metal atom. The reaction of [MoO2(eta2-Pz)2] with NacNacH in benzene at room temperature leads to a mixture of 1 and the monomeric molybdenum(VI) compound [MoO2(NacNac)(eta2-Pz)] (2). From such solutions, yellow crystals of 2 suitable for X-ray structural analysis were obtained revealing the coordination of one bidentate NacNac and one eta2-coordinate Pz ligand. This renders the two oxo groups inequivalent. Further high oxidation state molybdenum compounds containing the NacNac ligand were obtained by the reaction of [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H3) and [Mo(N-t-Bu)2Cl2(dme)] (dme = dimethoxyethane) with 1 equiv of the potassium salt NacNacK forming [Mo(NAr)2Cl(NacNac)] (3) and [Mo(N-t-Bu)2Cl(NacNac)] (4), respectively, in good yields. The X-ray structure analysis of 3 revealed a penta-coordinate compound where the geometry is best described as trigonal-bipyramidal.

Bis{N-[(1Z,3Z)-1,3-bis(4-fluorophenyl)-3-(phenylimino)prop-1-enyl]aniline(1–)}zinc(II)

In the title complex, [Zn(C27H19F2N2)2], a twofold rotation axis passes through the Zn atom, which is tetrahedrally coordinated by four N atoms from two diketiminate ligands. There are no inter­molecular inter­actions of note in the crystal structure.

Intramolecular C−H Activation Reactions Derived from a Terminal Titanium Neopentylidene Functionality. Redox-Controlled 1,2-Addition and α-Hydrogen Abstraction Reactions

Alkylation of the terminal neopentylidene titanium(IV) complex (L1)TiCHtBu(OTf) (L1- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe2)2C6H3) with LiCH2SiMe3 resulted in formation of the alkylidene−alkyl species (L1)TiCHtBu(CH2SiMe3) (1) in 82% yield. Compound 1 was fully characterized, and the molecular structure disclosed a four-coordinate titanium complex having significant α-hydrogen agostic interaction and possessing terminal alkylidene and alkyl functionalities. Attempts to alkylate (L1)TiCHtBu(OTf) with KCH2Ph in THF resulted in clean deprotonation of the methyl group attached to the β-carbon of the diketiminate ligand to form the four-coordinate titanium(IV) neopentylidene−tetrahydrofuran complex (L2)TiCHtBu(THF) (2; L22- = [Ar]NC(Me)CH(CH2)N[Ar], 64% isolated yield). Complex 2 was fully characterized and revealed a low-coordinate titanium(IV) in a C1 environment, which is supported by a chelating bis-anilide ligand. Alkylation of the alkylidene derivative (L3)TiCHtBu(OTf) (L3- = [Ar]NC(tBu)CHC(tBu)N[Ar], Ar = 2,6-(CHMe2)2C6H3) with LiCH2SiMe3 or KCH2Ph resulted in clean formation of (L3)TiCHtBu(R) (R = CH2SiMe3 (3), CH2Ph (4)). Complexes 3 and 4 were fully characterized, and the structure of 4 was determined by single-crystal X-ray diffraction studies. Complex 1 was found to decompose rapidly to several products, of which the titanacycle Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](CH2Si(Me)3) (5) and dimer [TiNAr([Ar]NC(Me)CHC(μ-CH2)CHtBu)]2 (6) were formed. Complex 5 was prepared in better yield through an independent synthesis involving Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](OTf) and LiCH2SiMe3. In THF complex 6 dissociated into the corresponding monomer (tBuHCC(CH2)CHC(CH3)N[Ar])TiNAr(THF) (8), quantitatively. Unlike complex 1, complexes 3 and 4 are kinetically more stable to intramolecular Wittig-like and C−H abstraction reactions. It was also found that one-electron reduction of the four-coordinate titanium alkylidene complexes (L1)TiCHtBu(OTf) and (L3)TiCHtBu(OTf) afforded the Ti(III) metallacycles ([Ar]NC(R)CHC(R)N[2,6-(CHMe2)(CH(CH2)(Me))C6H3])TiCH2tBu (L42-, R = Me (9); L52-, R = tBu (10)), both resulting from 1,2-addition of the proximal isopropyl CH3 group across the TiCHtBu bond. One-electron oxidation of 10 with AgOTf promotes α-abstraction to generate back the alkylidene precursor (L3)TiCHtBu(OTf). The redox-controlled 1,2-addition and α-abstraction reactions are specific only to the isopropyl methyl attached to the aryl group of the β-diketiminate ligand.

Low-coordinate iron complexes as synthetic models of nitrogenase

Three-coordinate iron sites are potentially important intermediates in the reduction of dinitrogen by the enzyme nitrogenase, but synthetic work to outline the behavior of three-coordinate iron complexes is in its infancy. Recent work shows that bulky diketiminate ligands give a general route to three-coordinate iron complexes with a variety of third ligands. The low coordination number leads to exciting new reactions in which N—N bonds are weakened or broken. Such complexes show great promise for evaluating the ability of low-coordinate iron to perform bond cleavage reactions akin to the individual steps of the nitrogenase system.Key words: nitrogenase, three-coordinate iron, diketiminate, N–N cleavage.

A Cationic Three-Coordinate Iron(II) Complex and the Reaction of β-Diketiminate with Ethyl Diazoacetate

We report the synthesis of a cationic iron(II) complex in which the iron atom is 3-coordinate in solution and in the solid state. The complex undergoes an unusual addition of ethyl diazoacetate, in which the diketiminate forms a bond to the terminal nitrogen of the diazo group. Coordination of the ester group to iron is proposed to activate the diazo group so that it is attacked by the nucleophilic diketiminate ligand.

Synthesis and Structure of Allyl and Alkynyl Complexes of Manganese(II) Supported by a Bulky β-Diketiminate Ligand

Reaction of LK (L = HC(CMeNAr)2, Ar = 2,6-iPr2C6H3) and MnCl2(THF)1.5 in THF unexpectedly resulted in the formation of the trinuclear complex LMn(μ-Cl)2Mn(THF)2(μ-Cl)2MnL (1) in good yield. The substitution reactions of 1 with C3H5MgCl and PhCCLi, respectively, provided the monomeric LMnC3H5(THF) (2) and the dimeric [LMn(μ-CCPh)]2 (3) in moderate yield. Complexes 1−3 were characterized by single-crystal X-ray structural analysis. The structures show that compound 1 contains an ideal planar Mn3Cl4 core and a linear Mn3 trimer, the allyl ligand in 2 is in an η1 mode, and there is π-interaction between the triple bond and the metal center in 3.

Synthesis and Structural Characterization of Divalent Ytterbium Complexes Supported byβ-Diketiminate Ligands and Their Catalytic Activity for the Polymerization of Methyl Methacrylate

Three divalent ytterbium complexes supported by a β-diketiminate ligand were synthesized for the first time. The mixed-ligand ytterbium chlorides (CH3C5H4)[(DIPPh)2nacnac]YbCl (1), (C9H7)[(DIPPh)2nacnac]YbCl (2), and (ArO)[(DIPPh)2nacnac]YbCl(THF) (3) ((DIPPh)2nacnac = N,N-diisopropylphenyl-2, 4-pentanediimine anion, ArO = 2,6-di-tert-butyl-4-methylphenoxo), which were obtained in high yield by using [(DIPPh)2nacnac]YbCl2(THF)2 as precursor, were reduced by Na/K alloy in THF to afford the corresponding divalent species (CH3C5H4)[(DIPPh)2nacnac]Yb(THF) (4), (C9H7)[(DIPPh)2nacnac]Yb(THF) (5), and {(ArO)[(DIPPh)2nacnac]Yb(THF)}(THF) (6), respectively. Complexes 2−6 were well characterized, and crystal structures of complexes 4−6 were determined. These divalent complexes exhibit good catalytic activity for the polymerization of methyl methacrylate.