Mono(imidazolin-2-iminato) Titanium Complexes for Ethylene Polymerization at Low Amounts of Methylaluminoxane

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl₂Ti(μ-NIm^Dipp)]₂ (1) (NIm^Dipp = [1,3-bis(Dipp)imidazolin-2-iminato]^-, Dipp = C₆H₃-2,6-ⁱPr₂) with 4 and 6 equiv of KC₈ generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η⁶-Im^Dipp)Ti]₂ (2) and {[(Et₂O)₂K](μ-N-μ-η⁶:η⁶-Im^Dipp)Ti}₂ (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η⁶-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C₈H₈ (COT) forms the novel COT-bridged complex [(Im^DippN)(η⁸-COT)Ti](μ-η²:η³-COT)[Ti(η⁴-COT)(NIm^Dipp)] (4) that dissociates in THF solutions to give mononuclear (Im^DippN)Ti(η⁸-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(Im^DippN)(η⁴-COT)Ti(μ-η⁴:η⁵-COT)K(THF)(μ-η⁶:η⁴-COT)Ti(NIm^Dipp)(μ-η⁴:η⁴-COT)K(THF)₂]_n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN₃ gives the terminal and bridged imido complex [(Im^DippN)Ti(═NAd)](μ-NAd)₂[Ti(NIm^Dipp)(N₃Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(Im^DippN)Ti(NHAd)](μ-NAd)₂[Ti(C₆H₄Me)(NIm^Dipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN₃. Furthermore, treatment of 3 with 4 equiv of AdN₃ produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)₂]{[(Im^DippN)Ti(NAd)](μ-NAd)₂K[Ti(NIm^Dipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Titanium complexes with unsymmetrically substituted imidazolin-2-iminato ligands

The reaction of the unsymmetrical N-heterocyclic carbenes 1-(2,4,6-trimethylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdMes, 1a) and 1-(2,6-diisopropylphenyl)-3-(adamantyl)imidazolin-2-ylidene (IAdDipp, 1b) with trimethylsilyl azide furnished the 2-(trimethylsilylimino)imidazolines 2a (Im^AdMesNSiMe₃) and 2b (Im^AdDippNSiMe₃). Desilylation by stirring in methanol gave the corresponding imidazolin-2-imines 3a (Im^AdMesNH) and 3b (Im^AdDippNH). 2a and 2b were treated with [TiCl₄(THF)₂] (THF = tetrahydrofuran) and [CpTiCl₃] (Cp = η⁵-C₅H₅) to form the mono- and bis(imidazolin-2-iminato) titanium(IV) complexes [(Im^AdRN)TiCl₃] (4, R = Mes, Dipp), [Cp(Im^AdRN)TiCl₂] (5, R = Mes, Dipp), and [(Im^AdRN)₂TiCl₂] (6, R = Mes, Dipp). The crystal structures of all compounds except 2b were determined by X-ray diffraction analysis.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Superseding β-Diketiminato Ligands: An Amido Imidazoline-2-Imine Ligand Stabilizes the Exhaustive Series of B=X Boranes (X=O, S, Se, Te)

Boron reluctantly forms B=X (X=O, S, Se, Te) moieties, which has stimulated the quest for such species in the past few years. Based on the N,N'-chelating β-diketiminato ligand (HNacNac), a new amido imidazoline-2-imine ligand system (HAmIm) is presented, giving rise to the isolation of an exhaustive series of Lewis acid free, monomeric chalcogen B=X boranes with documented π-bond character between boron and the chalcogen. The chalcogenoboranes are isoelectronic and isolobal to the respective ketones. The chemical behavior of the oxoborane (B=O) strongly resembles the classical carbonyl reactivity in C=O bonds. The improved stability provided by HAmIm arises from the formation of more-stable five-membered boron chelates versus the six-membered NacNac analogues and from the imidazoline-2-imine moiety providing enhanced σ- and π-donation. The HAmIm ligand class may supersede the widely employed NacNac system in certain applications.

1,2,4,5-Tetrakis(tetramethylguanidino)-3,6-diethynyl-benzenes: Fluorescent Probes, Redox-Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para-ethynyl substituents (CCSi(iPr)3 or CCH) to the organic electron-donor 1,2,4,5-tetrakis(tetramethylguanidino)-benzene is evaluated. The redox-properties and redox-state dependent fluorescence are evaluated, and dinuclear CuI and CuII complexes synthesized. The Lewis-acidic B(C6 F5 )3 substitutes the proton of the ethynyl -CCH groups to give new anionic -CCB(C6 F5 )3 - substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two-electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox-active guanidine that is neutral (instead of cationic) in its oxidized form.

Me CAAC=N- : A Cyclic (Alkyl)(Amino)Carbene Imino Ligand

A cyclic (alkyl)(amino)carbene (CAAC) has been shown to react with a covalent azide similar to the Staudinger reaction. The reaction of Me CAAC with trimethylsilyl azide afforded the N-silylated 2-iminopyrrolidine (Me CAAC=NSiMe3 ), which was fully characterized. This compound undergoes hydrolysis to afford the 2-iminopyrrolidine and trimethylsiloxane which co-crystallize as a hydrogen-bonded adduct. The N-silylated 2-iminopyrrolidine was used to transfer the novel pyrrolidine-2-iminato ligand onto both main-group and transition-metal centers. The reaction of the tetrabromodiborane bis(dimethyl sulfide) adduct with two equivalents of Me CAAC=NSiMe3 afforded the disubstituted diborane. The reaction of Me CAAC=NSiMe3 with TiCl4 and CpTiCl3 afforded Me CAAC=NTiCl3 and Me CAAC=NTiCl2 Cp, respectively.

Trapping an unprecedented Ti3C3 unit inside the icosahedral C80 fullerene: a crystallographic survey

The sub-nanometer cavity of fullerene cages is an ideal platform to accommodate otherwise unstable species for accurate structural characterization with, for example, rather accurate single crystal X-ray diffraction (XRD) crystallography. Herein, we report the successful entrapment of an isolated Ti3C3 moiety inside the icosahedral-C80 cage to form Ti3C3@Ih-C80 via an arc-evaporation process in the gas phase. The single crystal XRD crystallographic results unambiguously reveal that the C3-unit adopts an unprecedented cyclopropane-like structure which coordinates with the three titanium atoms in an unexpected fashion where the triangular C3-unit is nearly perpendicular to the Ti3-plane. The intercalation of a cyclopropanated C3-unit into the titanium layer is thus unambiguously confirmed. The theoretical results reveal that the Ti3C3 cluster transfers six electrons to the Ih-C80 cage so that each titanium atom has a positive charge slightly above +2 and the C3-unit is negatively charged with about -1. It is noteworthy that this is the first observation of the cyclopropane-coordination fashion in any reported organometallic complex, providing new insights into coordination chemistry.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Pentamethylcyclopentadienyl ruthenium “pogo stick” complexes with nitrogen donor ligands

The preparation and reactivity of an imidazolin-2-iminato ruthenium complex with a rare one-legged piano-stool (“pogo stick”) geometry is reported.

Synthesis, characterization and olefin polymerization behaviors of phenylene-bridged bis-β-carbonylenamine binuclear titanium complexes

Binuclear and multinuclear complexes have attracted much attention due to their unique catalytic performances for olefin polymerization compared with their mononuclear counterparts. In this work, a series of phenyl-bridged bis-β-carbonylenamine [O⁻NS^R] (R = alkyl or phenyl) tridentate ligands and their binuclear titanium complexes (Ti²L₁–Ti²L₅) were synthesized and characterized by ¹H NMR, ¹³C NMR, FTIR and elemental analysis. The molecular structure of ligand L₂ (R = ⁿPr) and its corresponding Ti complex Ti²L₂ were further investigated by single-crystal X-ray diffraction, which showed that each titanium coordinated with six atoms to form a distorted octahedral configuration along with the conversion of the ligand from β-carbonylenamine to β-imino enol form. Under the activation of MMAO, these complexes catalyzed ethylene polymerization and ethylene/α-olefin copolymerization with extremely high activity (over 10⁶ g mol (Ti)⁻¹ h⁻¹ atm⁻¹) to produce high molecular weight polyethylene. At the same time, wider polydispersity as compared with the mononuclear counterpart TiL₆ was observed, indicating that two active catalytic centers may be present, consistent with the asymmetrical crystal structure of the binuclear titanium complex. Furthermore, these complexes possessed better thermal stability than their mononuclear analogues. Compared with the complexes bearing alkylthio sidearms, the complex Ti²L₅ bearing a phenylthio sidearm exhibited higher catalytic activity towards ethylene polymerization and produced polyethylene with much higher molecular weight, but with an appreciably lower 1-hexene incorporation ratio. Nevertheless, these bis-β-carbonylenamine-derived binuclear titanium complexes showed much higher ethylene/1-hexene copolymerization activity and 1-hexene incorporation ratios as compared with the methylene-bridged bis-salicylaldiminato binuclear titanium complexes, and the molecular weight and 1-hexene incorporation ratio could be flexibly tuned by the initial feed of α-olefin commoners and catalyst structures.

Phenyl-bridged bis-β-carbonylenamine binuclear titanium complexes were synthesized, characterized and used to catalyze ethylene (co)polymerization with extremely high activity.

Organoactinides in catalytic transformations: scope, mechanisms and Quo Vadis

The last decade has witnessed brilliant and remarkable advances in the chemistry of the early actinides in stoichiometric and in challenging catalytic processes. This canvas of knowledge allows the design of chemical reactivities reaching a high level of sophistication. This review highlights the latest results obtained since 2008 on those catalytic processes.

Using N-Heterocyclic Vinyl Ligands to Access Stable Divinylgermylenes and a Germylium Cation

Two efficient methods are presented to install σ- and π-electron-donating N-heterocyclic vinyl groups onto main-group elements (E): halosilane elimination and base-induced E-C bond formation. Placement of two NHC=CH^- ligands (NHC=N-heterocyclic carbene) onto a Ge^II center affords a two-coordinate germylene, a heavy congener of the elusive divinyl carbenes. The π-donating ability of this vinylic ligand scaffold was further demonstrated by the synthesis of a three-coordinate germylium cation R₃ Ge⁺ .

Advances in Guanidine Ligand Design: Synthesis of a Strongly Electron-Donating, Imidazolin-2-iminato Functionalized Guanidinate and its Properties on Iron

Imidazolin-2-imines (ImRN-), derived from N-heterocylic carbenes, have been shown to be strong electron donors when directly coordinated to metals or when used as a substituent in larger ligand frameworks. In an attempt to enhance the electron-donating properties of the popular guanidine ligand class, the effect of appending an ImRN- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(Et2O)][Im tBuN] to the aryl carbodiimide (dippN)2C (dipp = 2,6-diisopropylphenyl) cleanly affords the lithium Im tBuN-functionalized guanidinate [Li(THF)2][(Im tBuN)C(Ndipp)2] (1). Subsequent metalation of the ligand with FeBr2 gives the yellow Fe(II) complex {[(Im tBuN)C(Ndipp)2]FeBr}2 (4) in good yield. Solid-state structural analyses of both 1 and 4 shows the Im tBuN- group acts as a non-coordinating backbone substituent. Direct structural comparison of 4 to the closely related guanidinate and ketimine-guanidinate complexes {[(X)C(Ndipp)2]FeBr}2 (X = t Bu2C=N (5); N( i Pr)2 (6)), differing only in their backbone, reveals a detectable resonance contribution of the Im tBuN- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of 4 (E1/2 = -0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of 5 (E1/2 = -0.42 V) and 6 (E1/2 = -0.45 V), demonstrating the [(Im tBuN)C(Ndipp)2]- system to be a quantifiably superior electron donor.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Imidazolin-2-iminato Ligand-Supported Titanium Complexes as Catalysts for the Synthesis of Urea Derivatives

The reactions of tetrakis(dimethylamido)titanium(IV) [Ti(NMe2)4] with three different imidazolin-2-imines (Im(R)NH; R = tert-butyl (tBu), mesityl (Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the corresponding titanium imidazolin-2-iminato complexes [(Im(R)N)Ti(NMe2)3] (R = tBu, 1a; R = Mes, 1b; R = Dipp, 1c). Treatment of complex 1a with two different carbodiimides [R'N═C═NR'; R' = cyclohexyl (Cy) and isopropyl (iPr)] resulted in the formation of imidazolin-2-iminato titanium mono(guanidinate) complex of the type [(Im(R)N)Ti(R'NC(NMe2)NR') (NMe2)2 (R' = iPr; R = tBu (2a), R = Dipp (2c); R' = Cy, R = tBu (3a)], as yellow solid in 94% yield. However, a similar reaction of 1b and 1c with 2 equiv of phenyl isocyanates at ambient temperature resulted in the formation of corresponding titanium bis(ureate) complexes [(Im(R)N)Ti{κ(2)-OC(NMe2)NPh}2(NMe2)] (R = Mes, 4b and R = Dipp, 4c). Three equivalents of phenyl isothiocyanate reacted with complex 1c to afford respective titanium tris(thioureate) complex [(Im(Dipp)N)Ti{κ(2)-SC(NMe2)NPh}2{κ(1)-SC(NMe2)NPh}] (6c). The molecular structures of 1a-c, 2a, 2c, 3a, 4c, and 6c were established by X-ray diffraction analyses, and, from the solid-state structures of 1a-c, 2a, 2c, 3a, 4c, and 6c, it was confirmed that the imidazolin-2-iminato titanium bond in each case is very short and possesses a multiple-bonding character. The imidazolin-2-iminato titanium complex 1c was utilized as a precatalyst for the addition of amine N-H bond to phenyl isocyanate. High yields of the corresponding urea derivatives were achieved under mild conditions. The mechanistic study of the aforementioned catalytic reaction was performed, and the active catalyst complex 7b was isolated using 2 equiv of iminopyrrole [2-(2,6-iPr2C6H3N═CH)C4H3NH] and the complex 4b. The molecular structure of 7b was thereafter established.

Organoaluminum(iii) complexes of the bis-N,N′-(2,6-diisopropylphenyl)imidazolin-2-imine ligand

Trimethylaluminum reacts with bis-N,N′-(2,6-diisopropylphenyl)imidazolin-2-imine to form a number of new organoaluminum complexes.

Group 4 alkoxide complexes containing [NNO]-type scaffold: synthesis, structural characterization and polymerization studies

A series of new dinuclear group 4 complexes containing [NNO]-type ligands are utilized as catalysts for polymerization reactions.

Synthesis and spectroscopic characterization of group 4 post-metallocenes bearing (σ-aryl)-2-phenolate-6-pyridyl and -isoquinolinyl auxiliaries

A new series of group 4 bis(benzyl) complexes supported by (σ-aryl)-2-phenolate-6-pyridyl [O,C,N] ligands have been prepared, and all derivatives have been characterized by multinuclear NMR spectroscopy. In the (1)H NMR spectrum of the Ti derivative where [N] = (ortho-F)-substituted isoquinolinyl, one of the two CH2 resonances is observed as a doublet of doublets (collapsing to a normal d upon (19)F-decoupling), whereas the [(1)H,(19)F]-HMQC correlation spectrum reveals a strong crosspeak for this dd resonance only, thus indicating the presence of intramolecular C-HF-C interactions. [(1)H,(19)F]-HMBC experiments have been performed which reveal a significant scalar component for this coupling and confirm that the interactions are genuine. The contrasting NMR spectral patterns for the (ortho-F)-pyridyl Hf analogue, which exhibits two sets of non-identical doublet of doublets for the methylene resonances, have been rationalized. The activities of the isoquinolinyl-based Ti-[O,C,N] catalysts for ethylene polymerization are superior to those of pyridyl-based congeners.

Preparation of octahydro- and tetrahydro-[1,10]phenanthroline zirconium and hafnium complexes for olefin polymerization

Post-metallocenes were constructed for olefin polymerization using 1,2,3,4,7,8,9,10-octahydro[1,10]phenanthroline and 1,2,3,4-tetrahydro[1,10]phenanthroline derivatives. A series of zirconium complexes - LZrCl2(NHMe2)2 [L = 2,9-H2-C12H12N2 (4), 2,9-Me2-C12H12N2 (5), 2,9-nBu2-C12H12N2 (6), and 2,9-iPr2-C12H12N2 (7)] - and hafnium complexes - LHfCl2(NHMe2)2 [L = 2,9-H2-C12H12N2 (8), 2,9-Me2-C12H12N2 (9), 2,9-nBu2-C12H12N2 (10), and 2,9-iPr2-C12H12N2 (11)] - were synthesized via the reaction of octahydro[1,10]phenanthrolines (2,9-R2-C12H12(NH)2) with (Me2N)2MCl2 (DME). The reaction of 2,9-R2-C12H12(NH)2 with (PhCH2)2ZrCl2 in the presence of a small amount of THF afforded a series of THF adduct analogs, i.e., LZrCl2(THF)2 [L = 2,9-H2-C12H12N2 (12), 2,9-Me2-C12H12N2 (13), 2,9-nBu2-C12H12N2 (14), and 2,9-iPr2-C12H12N2 (15)]. The treatment of 12 and 13 with excess Me3Al resulted in the formation of unexpected complexes, i.e., (η(4)-LAlMe2)ZrCl2(Me) [L = 2,9-H2-C12H12N2 (16) and 2,9-Me2-C12H12N2 (17)], in which the Me2Al unit forms a five-membered ring through binding with the two nitrogen donors and the MeCl2Zr unit slips to an η(4)-binding mode containing the N-C-C-N fragment. The treatment of tetrahydro[1,10]phenanthrolines [2,9-R2-C12NH9(NH)] with M(CH2Ph)4 afforded tribenzyl zirconium complexes LZr(CH2Ph)3 - [L = 2,9-Me2-C12NH9N (18) and 2,9-nBu2-C12NH9N (19)] - and hafnium complexes - LHf(CH2Ph)3 [L = 2,9-Me2-C12NH9N (20), 2,9-nBu2-C12NH9N (21), and 2,9-iPr2-C12NH9N (22)]. The structures of 4, 5, 12, 17, and 22 were elucidated by X-ray crystallography. The newly prepared complexes were screened for ethylene/1-octene copolymerization activity: 12 and 16 were potent catalysts (activities of 74 × 10(6) g mol-Zr h(-1) at ∼120 °C under 30 bar ethylene) for the production of wax-like low-molecular weight polyethylene (Mn: ∼5000), which is widely used in industry.

Imidazol-2-ylidene-N′-phenylureate ligands in alkali and alkaline earth metal coordination spheres – heterocubane core to polymeric structural motif formation

Synthetic and structural details of imidazol-2-ylidene-N′-phenylureate ligand supported two potassium complexes with a hetero-cubane K₄O₄ core and a polymeric structure, along with a mixed metal Ca–K complex are presented.