Β-diketiminato Ligand Research Articles

14-Membered Macrocyclic β-Diiminato Gold(II) - A New Member for the Gold(II) Complex Family?

The chemistry of molecular gold compounds is dominated by the oxidation states +I and +III. For the intermediate oxidation state +II with 5d9 electron configuration, dimerization or disproportionation of the gold(II) radicals is favored, so that only a few mononuclear gold(II) complexes have been isolated to date. The present study addresses the one-electron reduction of the macrocyclic gold(III) complex [AuIIIL]+ of the innocent β-diiminato ligand L2- with a 14-membered macrocycle (L2-=5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-5,7,12,14-tetraenato). Electrochemistry, spectroelectrochemistry and chemical reduction of [AuIIIL]+ monitored by UV/Vis, NMR and EPR spectroscopy together with density functional theory calculations reveal disproportionation of the initially generated but elusive gold(II) complex AuIIL and provide guidelines for prospective stable mononuclear tetraazamacrocyclic gold(II) complexes.

Trapping an Elusive Phosphanyl-Phosphaalumene.

We describe our efforts to access a compound with an Al=P double bond by reaction of Al(Nacnac) towards [H2CN(Dipp)]2P(PCO) (Nacnac=HC[C(Me)N(Dipp)]2; Dipp=2,6-iPr2C6H3). Our observations are consistent with the formation of a transient phosphanyl-phosphaalumene at low temperatures (-70 °C), however this species was found to readily undergo intramolecular C-H activation of the β-diketiminato ligand upon warming to room temperature. The reactivity of the transient complex toward small molecules including dihydrogen, carbon dioxide, phosphaketenes, amines and silanes could be explored at low temperatures, showcasing that the target compound can react as both a frustrated Lewis pair (via the pendant phosphanyl moiety) or in hydroelementation reactions of the Al=P bond. The elusive target molecule could be trapped by addition of a Lewis base (tetrahydrofuran) affording an isolable molecular species that reacts in an analogous fashion to the base-free compound.

Tridentate NacNac Tames T-Shaped Nickel(I) Radical.

The reaction of a nickel(II) chloride complex containing a tridentate β-diketiminato ligand with a picolyl group [2,6-iPr2 -C6 H3 NC(Me)CHC(Me)NH(CH2 py)]Ni(II)Cl (1)] with KSi(SiMe3 )3 conveniently afforded a nickel(I) radical with a T-shaped geometry (2). The compound's metalloradical nature was confirmed through electron paramagnetic resonance (EPR) studies and its reaction with TEMPO, resulting in the formation of a highly unusual three-membered nickeloxaziridine complex (3). When reacted with disulfide and diselenide, the S-S and Se-Se bonds were cleaved, and a coupled product was formed through carbon atom of the pyridine-imine group. The nickel(I) radical activates dihydrogen at room temperature and atmospheric pressure to give the monomeric nickel hydride.

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction.

Two Ni complexes are reported with κ4-P2N2 β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF4)2, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF4)2 and X = Ph for 2(BF4)2. Electrochemical analysis together with UV–vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 12+ and 22+ to the negatively charged β-diketiminato (BDK) analogues (1-H)+ and (2-H)+ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1+ and 2+ can also rapidly evolve to the BDK (1-H)+ and (2-H)+, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at Eapp = −1.8 V vs Fc+/0, being the active species the one-electron-reduced derivative 1-H and 2-H.

Synthesis of rare-earth metal complexes with a morpholine-functionalized β-diketiminato ligand and their catalytic activities towards C-O and C-N bond formation.

Unusual tridentate β-diketiminato rare-earth metal chlorides LRECl(μ-Cl)2Li(THF)2 (RE = Y (1a), Yb (1b), and Lu (1c); L = MeC(NDipp)CHC(Me)N(CH2)2NC4H8O; Dipp = 2,6-iPr2C6H3) and the corresponding dialkyl complexes LRE(CH2SiMe3)2 (RE = Y (2a), Yb (2b), and Lu (2c)) were prepared by reaction of a morpholine-functionalized β-diketiminato proligand HL with anhydrous RECl3 and rare-earth metal trialkyl complexes RE(CH2SiMe3)3(THF)2, respectively. In 1 and 2, the morpholine moiety displayed a stable chair conformation and the resulting ligand coordinated to the rare-earth metal ion in a [NNN]-tridentate chelating fashion. Further studies demonstrated that these dialkyl complexes showed high activity towards the catalytic formation of C-O and C-N bonds in the alcoholysis of isothiocyanates and aminolysis of epoxides. A series of O-thiocarbamate derivatives were achieved in good to excellent yields by reaction of various alcohols with aromatic and aliphatic-substituted isothiocyanates at room temperature. In addition, under solvent-free conditions, the ring-opening reaction of terminal and internal epoxides with arylamines yielded the corresponding β-amino alcohols with excellent regioselectivity. More importantly, a novel β-diketiminato/anilido yttrium alkyl complex LY(NHDipp)(CH2SiMe3) (5a) was isolated and characterized by the stoichiometric reaction of 2a with diisopropylaniline.

Nitric oxide generation study of unsymmetrical β-diketiminato copper(II) nitrite complexes.

β-Diketiminato copper(II) L1CuCl-L4CuCl and their nitrite complexes L1Cu(O2N) and L2Cu(O2N) have been synthesized and characterized. X-ray analysis of the L1CuCl-L4CuCl complexes clearly reveals their mononuclear structure with a four-coordinated Cu(II) center bound by one chloride and three nitrogen atoms of unsymmetrical β-diketiminato ligands. Cyclic voltametric analysis of the Cu(II) complexes shows that the length of the pyridyl arm controls the Cu(II)/Cu(I) redox process. DFT and EPR results confirm that the geometry of the Cu(II) complexes is also controlled by the length of the chelating pyridyl arm. The oxygen atom transfer nitrite reduction of the Cu(II) nitrite complexes leads to the formation of copper(I)-PPh3 and OPPh3 which were confirmed by 1H and 31P NMR. The length of the pyridyl arm of the copper(II) nitrite complexes governs the NO-releasing ability. These findings illustrate the important bioinspired behaviour and NO generation from nitrite via oxygen atom transfer of the unsymmetrical β-diketiminato copper(II) complexes as compared to symmetrical β-diketiminato copper(II) complexes.

Theoretical insight into dihydrogen activation with β-diketiminato ligand supported Group 13 and 14 elements: mechanism and activity difference

Mechanisms and activity difference for dihydrogen activation with the BDI-supported Group 13 and 14 elements are comparatively studied.

Synthesis and structural characterization of lanthanide monoborohydride complexes supported by 2-tertbutylphenyl substituted β-diketiminate, and their application in the ring-opening polymerization of lactide

A series of lanthanide monoborohydride complexes stabilized by a bulky β-diketiminato ligand were synthesized, and their catalytic behaviors for the ring-opening polymerization of lactide were explored. Reaction of LnCl3 with two equivalents of NaL2-tBu (L2-tBu = {(2-tBuC6H4)NC(Me)}2CH−) afforded the bis(β-diketiminato) lanthanide monochlorides (L2-tBu)2LnCl (Ln = Y (1), Sm (2), Nd (3), La (4)), which can be used as starting materials to prepare bis(β-diketiminato) lanthanide derivatives. Treatment of the chlorides with NaBH4 produced the monoborohydride complexes (L2-tBu)2LnBH4 (Ln = Y (5), Sm (6), Nd (7), La (8)). The solid-state structures of lanthanide complexes 1–8 were determined by single-crystal X-ray diffraction. Complexes 5–8 were found efficient in initiating the ring-opening of l-lactide and rac-lactide to afford α,ω-telechelic polylactide diols.

Nickel(0)-Induced β-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides.

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate β-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)2 (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. β-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C3-symmetric complexes with three heteroleptic magnesium mixed β-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

A "Push-Pull" Stabilized Phosphinidene Supported by a Phosphine-Functionalized β-Diketiminato Ligand.

The use of a bis(diphenyl)phosphine functionalized β‐diketiminato ligand, [HC{(CH3)C}2{(ortho‐[P(C6H5)2]2C6H4)N}2]− (PNac), as a support for germanium(II) and tin(II) chloride and phosphaketene compounds, is described. The conformational flexibility and hemilability of this unique ligand provide a versatile coordination environment that can accommodate the electronic needs of the ligated elements. For example, chloride abstraction from [(PNac)ECl] (E=Ge, Sn) affords the cationic germyliumylidene and stannyliumylidene species [(PNac)E]+ in which the pendant phosphine arms associate more strongly with the Lewis acidic main group element centers, providing further electronic stabilization. In a similar fashion, chemical decarbonylation of the germanium phosphaketene [(PNac)Ge(PCO)] with tris(pentafluorophenyl)borane affords a “push–pull” stabilized phosphinidene in which one of the phosphine groups of the ligand backbone associates with the low valent phosphinidene center.

Measurement and correlation of the solubility of tin compounds and β-diketimine in selected solvents

The solubility of organotin compounds under inert atmosphere plays a significant role in homogeneous catalysis process; however, no such study has been reported. The solubilities of dimethyltin dichloride (1), stannous chloride (2), β-diketiminato ligand LH (3) (L = HC(CMeNAr)2, Ar = 2,6-Et2C6H3), and tin (II) compound [LSnCl] (4) in six organic solvents were measured using a static analytic method from 268.15 K to 313.15 K under a purified nitrogen atmosphere to select the optimum solvent for a homogeneous reaction and recrystallization. The difference in solubility can be attributed to molecular structure, coordination bonding, and molecular polarity. Six commonly used thermodynamic models were used to correlate the experimental solubility data. Some models show a great correlation with a low value of average absolute deviation (AAD). In addition, the solubility parameter of solute, partial molar excess enthalpies at infinite dilution, and thermodynamic properties were calculated to study the dissolution behavior in more depth.

Neutral and Anionic Monomeric Zirconium Imides Prepared via Selective C=N Bond Cleavage of a Multidentate and Sterically Demanding β-Diketiminato Ligand.

A sterically encumbering multidentate β-diketiminato ligand, tBu L2 (tBu L2=[ArNC(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- , Ar=2,6-iPr2 C6 H3 ), is reported in this study along with its coordination chemistry to zirconium(IV). Using the lithio salt of this ligand, Li(tBu L2) (4), the zirconium(IV) precursor (tBu L2)ZrCl3 (6) could be readily prepared in 85 % yield and structurally characterized. Reduction of 6 with 2 equiv of KC8 resulted in formation of the terminal and mononuclear zirconium imide-chloride [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(Cl) (7) as the result of reductive C=N cleavage of the imino fragment in the multidentate ligand tBu L2 by an elusive ZrII species (tBu L2)ZrCl (A). The azabutadienyl ligand in 7 can be further reduced by 2 e- with KC8 to afford the anionic imide [K(THF)2 ]{[CH(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 N(Me)CH2 ]Zr=NAr} (8-2THF) in 42 % isolated yield. Complex 8-2THF results from the oxidative addition of an amine C-H bond followed by migration to the vinylic group of the formal [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- ligand in 7. All halides in 6 can be replaced with azides to afford (tBu L2)Zr(N3 )3 (9) which was structurally characterized, and reduction with two equiv of KC8 also results in C=N bond cleavage of tBu L2 to form [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(N3 ) (10), instead of the expected azide disproportionation to N3- and N2 . Solid-state single crystal structural studies confirm the formation of mononuclear and terminal zirconium imido groups in 7, 8-Et2 O, and 10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and 1.881(3) Å, respectively.

Polar-Group Activated Isospecific Coordination Polymerization of ortho-Methoxystyrene: Effects of Central Metals and Ligands.

Stereoselective polymerization of polar vinyl monomers has been a long-standing challenge because the employed transition-metal catalysts are easily poisoned by polar groups of monomers. In this contribution, a series of β-diketiminato rare-earth metal complexes 1-5 (L1-5 Ln(CH2 SiMe3 )2 (THF)n , Ln=Gd-Lu, Y, and Sc) were successfully synthesized. In combination with AliBu3 and [Ph3 C][B(C6 F5 )4 ], complexes 1 c(Tb)-1 g(Tm) exhibited high activities and excellent isoselectivities for the polymerization of ortho-methoxystyrene (oMOS), in which, the polar methoxy group of oMOS did not poison but activated the polymerization through σ-π chelation to the active species together with the vinyl group. Moreover, the large Gd-attached precursor 1 b showed a higher activity, albeit with a slightly decreased isoselectivity. The small Sc-attached precursor 1 i was completely inert. Meanwhile, the spatial steric arrangement and the coordination mode of the β-diketiminato ligand could clearly affect and even block oMOS polymerization. This work sheds new light on the coordination polymerization of polar monomers.

Scandium complexes containing β-diketiminato ligands with pendant phosphanyl groups: competition between Sc/γ-C [4 + 2] cycloaddition and Sc/P frustrated Lewis pair reactions.

Three cationic scandium aryloxide complexes, i.e., [LScOAr]+[B(C6F5)4]- (Ar = 2,6-tBu2C6H3; L = CH3C(2,6-iPr2C6H3N)C(R)C(CH3)(N(CH2)nCH2PPh2); 6a: n = 1, R = H; 6b: n = 1, R = CH3; 6c: n = 2, R = C6H5CH2), were prepared by a three-step synthetic route. The reactivities of tridentate NNP ligand supported scandium complexes 6 toward various unsaturated substrates were investigated. Treatment of 6a with non-conjugated small molecules such as phenyl isocyanates, phenyl isothiocyanate, and benzaldehyde resulted in formation of Sc/γ-C [4 + 2] cycloaddition products 7-9 because of nucleophilic reactivity at the β-diketiminato γ-carbon, with retention of ScP interactions. Analogous Sc/γ-C [4 + 2] cycloaddition reactions occurred for complex 6b with phenyl isocyanate and phenyl isothiocyanate, affording complexes 10 and 11, respectively. Complex 6c, with a bulky benzyl substituent at the γ position of the β-diketiminato ligand, showed typical Sc/P frustrated Lewis pair (FLP) reactivity toward phenyl isocyanate to give the 1,2-addition product 12. Complex 6c was efficient in the polymerization of γ-methyl-α-methylene-γ-butyrolactone (MMBL). The corresponding initiating species 13, which was formed by Sc/P FLP-type 1,4-addition, was isolated.

Alkaline Earth Metal Compounds of Methylpyridinato β-Diketiminate Ligands and Their Catalytic Application in Hydroboration of Aldehydes and Ketones

Ever increasing demand for green and sustainable chemical processes has set up a drive to replace transition metals with earth-abundant, nontoxic, and environmentally benign alternatives. In this regard, the alkaline earth metal complexes have attracted significant attention. Herein, we have used a β-diketiminato ligand with methyl-pyridine side arm to synthesize magnesium (1) and calcium (2) compounds. The constitutions of 1 and 2 have been confirmed by single crystal X-ray studies, which show that the magnesium and calcium atom in 1 and 2 possesses octahedral geometry. Subsequently, we have used them as catalysts (1 mol %) for hydroboration of a wide range of aldehydes using pinacolborane (HBpin) at room temperature. The strategy has further been extended to ketones with 2 mol % catalyst loading. DFT calculations have been performed to understand the mechanism.

Synthesis and characterization of three-coordinated Tin(II) chalcogenide compounds from chlorostannylene supported by β-diketiminato ligand

The stable chlorostannylene(II) LSnCl 1 reacted with bases or salts (KOH, t-BuOK, Ph3CSLi or Li2S), respectively, which led to the formation of a variety three-coordinated tin(II) chalcogens (O or S) (compounds 2–5). Compounds 2 and 5 are bridged bimetallic organotins(II) supported by a bidentate β-diketiminato ligand. Furthermore, the catalytic activity of compounds 2–5 for hydroboration with benzaldehyde and acetophenone were investigated, respectively. In the X-ray analyses of all stannylene chalcogenide compounds, the pyramidalization on the tin center are useful to enhanced p-character of lone pair. All compounds were characterized by 1H NMR and 13C NMR spectroscopy, single crystal X-ray structural analysis and elemental analysis.

Synthesis, characterization, and catalytic performance of Aluminum and Tin Compounds with β-diketiminato ligand

Two new tin (II) and tin (IV) compounds [LSnCl] (1) and [LSnCl3] (2), bearing the β-diketiminato ligand LH (L = HC(CMeNAr)2, Ar = 2,6-Et2C6H3) were synthesized by the reactions of LH with n-BuLi, then respectively with SnCl2 and SnCl4. Two new aluminum alkyl compounds, [LAlMe2] (3) and [LAlEt2] (4), were synthesized in high yield from the reactions of LH with AlMe3 and AlEt2, respectively. Compound [LAlI2] (5) was synthesized from the reaction of 3 with I2 in good yield. All these main group metal compounds bearing β-diketiminato ligand LH were fully characterized by 1H NMR and 13C NMR spectroscopy, and elemental analysis. Meanwhile, compounds 1, 2, 3, and 5 were characterized by single crystal X-ray structural analysis. Moreover, the well-defined compounds 1–4 were investigated to show good catalytic activity in hydroboration of terminal alkynes.

Side Arm Twist on Zn-Catalyzed Hydrosilylative Reduction of CO2 to Formate and Methanol Equivalents with High Selectivity and Activity

Reductive hydrosilylation of CO2 by catalysts based on environmentally benign metals such as zinc still presents an unmet challenge in terms of catalyst selectivity and activity. Here we show that tetra-coordinated neutral zinc siloxide complexes supported by the β-diketiminato ligand bearing a pendant amine arm exhibit unprecedentedly high catalytic activity toward the selective reduction of CO2 by a hydrosilane to silyl formate. More significantly, this zinc catalyst system can also effectively catalyze the subsequent reduction of silyl formate into methoxysilane in quantitative yield. To understand the mechanism of the CO2 hydrosilylation and the influence of the side arm, a series of catalytic active species (or intermediates), zinc hydrides, and formates, has been synthesized and structurally characterized; the relevant stoichiometric reactions and kinetic study were performed. DFT calculations were also carried out to determine the reaction profiles for the CO2 hydrosilylation and to explain the rel...

Di(tert-butyl)aluminum, -gallium and -indium β-diketonates and β-diketiminates, reactions with oxygen and formation of an unprecedented peroxo-rich hexaperoxotriindium compound

Reactions of 1,3-di(tert-butyl)acetylacetone (2,2,6,6-tetramethyl-3,5-heptanedione), (tBu2acac)H, with tri(tert-butyl)aluminum, -gallium and -indium afforded selectively the corresponding di(tert-butyl)metal derivatives (tBu2acac)MtBu2 [M = Al (1), Ga (2), In (3)] by release of isobutane. Likewise, diphenyl-β-diketamine [Ph–NC(Me)]2CH2, (Ph2nacnac)H, reacted with MtBu3 to generate the compounds (Ph2nacnac)MtBu2 [M = Ga (4), In (5)], which represent the first di(tert-butyl) complexes of gallium and indium with a β-diketiminato ligand. The reaction of 1 with molecular oxygen afforded an aluminum bisalkoxide (6) that is a dimer in the solid state. An indium peroxide (7) was reproducibly obtained in small quantities by the reaction of 3 with excess oxygen at 0 °C. Structure determination revealed a fascinating peroxo-rich triindium compound with four bridging and two terminal tert-butylperoxo moieties. The molecular core consists of an In3O7 skeleton. In contrast, THF/hexane solutions of 4 and 5 were stable towards oxygen, but slowly hydrolyzed on contact with water.

Versatile reactivities of rare-earth metal dialkyl complexes supported by a neutral pyrrolyl-functionalized β-diketiminato ligand.

Herein, rare-earth metal dialkyl complexes supported by a neutral pyrrolyl-functionalized β-diketiminato ligand with the formula LRE(CH2SiMe3)2(thf) (RE = Y (1a), Dy (1b), Er (1c), Yb (1d); L = MeC(NDipp)CHC(Me)NCH2CH2NC4H2-2,5-Me2, Dipp = 2,6-iPr2C6H3) were synthesized via the reactions of the β-diketimine HL with the rare-earth metal trialkyl complexes RE(CH2SiMe3)3(thf)2 in high yields. The reactivities of 1 with pyridine derivatives, unsaturated substrates, and elemental sulfur were investigated, and some interesting chemical transformations were observed. Ligand exchange and activation of sp2 and sp3 C-H bonds occurred during the reactions with pyridine derivatives to afford different types of mononuclear rare-earth metal pyridyl complexes, namely, LEr(CH2SiMe3)2(η1-NC5H4) (2c), LRE(η3-CH2-2-NC5H2-4,6-Me2)2 (RE = Y (3a), Er (3c)), and LRE(CH2SiMe3)(η2-(C,N)-2-(2-C6H4NC5H4)) (RE = Er (4c), Yb = (4d)). Similarly, activation of the sp C-H bond occurred during the reaction of phenylacetylene with 1c to produce the dinuclear erbium alkynyl complex [LEr(CH2SiMe3)(μ-C[triple bond, length as m-dash]CPh)]2 (5c). The mixed amidinate-β-diketiminato ytterbium complex LYb[(Dipp)NC(CH2SiMe3)N(Dipp)](CH2SiMe3) (6d) was obtained by the insertion of bis(2,6-diisopropylphenyl)carbodiimide into a Yb-alkyl bond, as well as via the direct alkane elimination of a CH2SiMe3 moiety with bis(2,6-diisopropylphenyl)formamidine to afford the erbium complex LEr(DippNCHNDipp)(CH2SiMe3) (7c). A rare sp2 C-H bond oxidation of the β-diketiminato backbone with elemental sulfur insertion was detected to provide the unprecedented dinuclear rare-earth metal thiolate complexes (LRE)2(μ-SCH2SiMe3)2(μ-SCC(Me)(NDipp)C(Me)NCH2CH2NC4H2Me2-2,5) (RE = Y (8a), Er (8c)) in the reactions of S8 with 1a and 1c, respectively. The molecular structures of the complexes 1-8 were determined by single-crystal X-ray diffraction analyses.