Bis(imidazolin-2-iminato) Rare Earth Metal Complexes: Synthesis, Structural Characterization, and Catalytic Application

Imidazolin-2-iminato Cerium(IV) Chlorides: Synthesis, Structure and Electrochemical Properties

The ligand field greatly affects the redox properties of cerium. Herein, cerium(III) and cerium(IV) complexes supported by imidazolin-2-iminato ligands were synthesized and structurally characterized, and their electrochemical properties were investigated. Silylamine elimination of cerium(III) amide Ce{N(SiMe₃ )₂ }₃ with imidazolin-2-imine Im^R NH (R=Mes, tBu, iPr) provided imidazolin-2-iminato cerium(III) complexes, [(Im^Mes N)(Im^Mes NH)Ce{N(SiMe₃ )₂ }₂ ] (1), [(Im^tBu N)₂ (Im^tBu NH)Ce{N(SiMe₃ )₂ }] (2) and [(Im^iPr N)₂ (Im^iPr NH)Ce(μ-Im^iPr N)]₂ (4) in 71-85% yields. These cerium(III) complexes were successfully oxidized by Ph₃ CCl or C₂ Cl₆ to afford imidazolin-2-iminato cerium(IV) chlorides, [(Im^Mes N)₂ Ce{N(SiMe₃ )₂ }Cl] (5), [(Im^tBu N)₃ CeCl] (6) and [(Im^iPr N)₂ Ce(μ-Im^iPr N)Cl]₂ (7) in 70%-76% yields. All complexes were characterized by the single-crystal X-ray diffraction, which showed that 1, 2, 5 and 6 are monomers while 4 and 7 are dimers. The electrochemical studies indicated that the Ce(III/IV) couples for 5 and 6 are more negative than those of silylamido cerium(IV) complexes, and the Ce(III/IV) couples for 1 and 2 have a similar trend.

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.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ2-{NHIRP(Ph)NDipp}AlMe2] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHIRP(Ph)NDipp]- [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

An imido ligand significantly enhances the effective energy barrier of dysprosium(iii) single-molecule magnets

We report herein an imido ligand 1,3-bis(2,6-diisopropylphenyl) imidazolin-2-imine (Im^DippNH) that can form a very short Dy-N bond (2.12 Å) with the dysprosium(iii) ion, which leads to a much larger effective energy barrier for magnetisation reversal (803 K) compared to the analogous alkoxide ligand (53 K). Moreover, we predict that a linear two-coordinate [Dy(Im^DippN)₂]⁺ complex may have an effective energy barrier larger than 4000 K.

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.

An imidazolin-2-iminato ligand organozinc complex as a catalyst for hydroboration of organic nitriles

Catalytic hydroboration of nitriles with pinacolborane (HBpin) using the imidazolin-2-iminato zinc alkyl complex [{(Im^tBuN)Zn(CH₂CH₃)}₂] (1c) under mild and solvent-free conditions to afford corresponding diboronate esters in high yield is reported.

Rigid NON-donor pincer ligand complexes of lutetium and lanthanum: synthesis and hydroamination catalysis

A rigid NON-donor pincer ligand was employed for the synthesis of neutral lutetium and anionic lanthanum alkyl complexes; the former is highly active for both intra- and inter-molecular hydroamination.

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.

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.

Platinum(ii) complexes with hybrid amine-imidazolin-2-imine ligands and their reactivity toward bio-molecules

Two new Pt(ii) complexes with imidazolin-2-imines as ancillary ligands were synthesized and characterized and their reactivity toward bio-molecules was tested.

Palladium(ii) complexes with highly basic imidazolin-2-imines and their reactivity toward small bio-molecules

Pd(ii) complexes with chelating imidazolin-2-imine ligands were synthesized, and the pK_a values and reactivity of these complexes were investigated.

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.

Uranium-mediated ring-opening polymerization of ε-caprolactone: a comparative study

This study shows and compares the catalytic activity and the mechanism of the uranium complexes [(Im^DippN)₂U(NMeEt)₂] (3), [(C₅Me₅)₂U(NMe₂)₂] (4) and [(C₅Me₅)₂U(NCMePh)₂] (5) in the ring-opening polymerization of ε-caprolactone, among which the bis(imidazolin-2-iminato) uranium complex3displayed the highest catalytic activity.

Heteroleptic alkyl and amide iminoanilide alkaline earth and divalent rare earth complexes for the catalysis of hydrophosphination and (cyclo)hydroamination reactions

[{N^N}M(X)(thf)n] alkyl (X=CH(SiMe3)2) and amide (X=N(SiMe3)2) complexes of alkaline earths (M=Ca, Sr, Ba) and divalent rare earths (Yb(II) and Eu(II) ) bearing an iminoanilide ligand ({N^N}(-)) are presented. Remarkably, these complexes proved to be kinetically stable in solution. X-ray diffraction studies allowed us to establish size-structure trends. Except for one case of oxidation with [{N^N}Yb(II){N(SiMe3)2}(thf)], all these complexes are stable under the catalytic conditions and constitute effective precatalysts for the cyclohydroamination of terminal aminoalkenes and the intermolecular hydroamination and intermolecular hydrophosphination of activated alkenes. Metals with equal sizes across alkaline earth and rare earth families display almost identical apparent catalytic activity and selectivity. Hydrocarbyl complexes are much better catalyst precursors than their amido analogues. In the case of cyclohydroamination, the apparent activity decreases with metal size: Ca>Sr>Ba, and the kinetic rate law agrees with R(CHA) =k[precatalyst](1)[aminoalkene](1). The intermolecular hydroamination and hydrophosphination of styrene are anti-Markovnikov regiospecific. In both cases, the apparent activity increases with the ionic radius (Ca<Sr<Ba) but the rate laws are different, and obey R(HA) =k[styrene](1)[amine](1)[precatalyst](1) and R(HP) =k[styrene](1)[HPPh2 ](0)[precatalyst](1), respectively. Mechanisms compatible with the rate laws and kinetic isotopic effects are proposed. [{N^N}Ba{N(SiMe3)2}(thf)2] (3) and [{N^N}Ba{CH(SiMe3)2}(thf)2] (10) are the first efficient Ba-based precatalysts for intermolecular hydroamination and hydrophosphination, and display activity values that are above those reported so far. The potential of the precatalysts for C-N and C-P bond formation is detailed and a rare cyclohydroamination-intermolecular hydroamination "domino" sequence is presented.