Group 1 and 2 and Early Transition Metal Complexes Bearing N‑Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications

Group 1 and 2 and Early Transition Metal Complexes Bearing N‑Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Application

Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation

Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)^2–) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand <i>N</i>,<i>N′</i>-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>) and the metal alkoxide precursors M­(O^<i>i</i>Pr)₄(HO^<i>i</i>Pr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)­MCl₂(THF) (<b>2a-THF</b>, M = Zr; <b>2b-THF</b>, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative <b>2b-THF</b> confirmed the proposed formulation and the effective formation of a (OCO)­Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand <b>1</b> and Zr­(NMe₂)₄ yielded the corresponding Zr amido THF adduct (OCO)­Zr­(NMe₂)­(Cl)­(THF) (<b>3a-THF</b>) when carried in THF as a solvent, while the Zr–NHMe₂ adduct (OCO)­Zr­(NMe₂)­(NHMe₂)­(THF) (<b>3a-NHMe</b>_<b>2</b>) was isolated using CH₂Cl₂ as the reaction solvent. <b>3a-THF</b> may be readily and quantitatively converted to the dichloro derivative <b>2a-THF</b> upon addition of TMSCl. The toluene elimination reaction of protio ligand <b>1</b> and M­(CH₂Ph)₄ (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH₂MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)­M­(CH₂Ph)₂ (<b>4a</b>, M = Zr; <b>4b</b>, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. <b>4a</b> and <b>4b</b> each feature a five-coordinate metal center with both benzyl moieties binding in a η² fashion. The protonolysis reaction between species <b>4a</b> (or <b>4b</b>) and [HNMe₂Ph]­[B­(C₆F₅)₄] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation <b>5a</b>⁺ (or <b>5b</b>⁺). Remarkably, the cation <b>5a</b>⁺ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation

Neutral and Cationic N‑Heterocyclic Carbene Zirconium and Hafnium Benzyl Complexes: Highly Regioselective Oligomerization of 1‑Hexene with a Preference for Trimer Formation

Various zirconium and hafnium amido, chloro, and benzyl complexes supported by a tridentate N-heterocyclic carbene bis-phenolate dianionic ligand ((OCO)^2–) have been synthesized and structurally characterized. The alcohol elimination reaction of the protio ligand <i>N</i>,<i>N′</i>-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>) and the metal alkoxide precursors M­(O^<i>i</i>Pr)₄(HO^<i>i</i>Pr) (M = Zr, Hf) and a subsequent alkoxide/chloride exchange reaction (upon addition of trimethylsilyl chloride, TMSCl) afforded the corresponding Zr and Hf carbene dichloro complexes as THF adducts: (OCO)­MCl₂(THF) (<b>2a-THF</b>, M = Zr; <b>2b-THF</b>, M = Hf). As determined by single-crystal X-ray crystallographic studies, the molecular structure of the Hf derivative <b>2b-THF</b> confirmed the proposed formulation and the effective formation of a (OCO)­Hf chelate. In the case of Zr, an amine elimination reaction between protio ligand <b>1</b> and Zr­(NMe₂)₄ yielded the corresponding Zr amido THF adduct (OCO)­Zr­(NMe₂)­(Cl)­(THF) (<b>3a-THF</b>) when carried in THF as a solvent, while the Zr–NHMe₂ adduct (OCO)­Zr­(NMe₂)­(NHMe₂)­(THF) (<b>3a-NHMe</b>_<b>2</b>) was isolated using CH₂Cl₂ as the reaction solvent. <b>3a-THF</b> may be readily and quantitatively converted to the dichloro derivative <b>2a-THF</b> upon addition of TMSCl. The toluene elimination reaction of protio ligand <b>1</b> and M­(CH₂Ph)₄ (M = Zr, Hf) followed by a salt metathesis with 1 equiv of PhCH₂MgCl afforded the corresponding Zr and Hf carbene dibenzyl complexes (OCO)­M­(CH₂Ph)₂ (<b>4a</b>, M = Zr; <b>4b</b>, M = Hf), whose solid-state structures were confirmed by X-ray crystallography. <b>4a</b> and <b>4b</b> each feature a five-coordinate metal center with both benzyl moieties binding in a η² fashion. The protonolysis reaction between species <b>4a</b> (or <b>4b</b>) and [HNMe₂Ph]­[B­(C₆F₅)₄] afforded the clean and quantitative formation of the corresponding Zr (or Hf) anilinium benzyl cation <b>5a</b>⁺ (or <b>5b</b>⁺). Remarkably, the cation <b>5a</b>⁺ catalyzes the highly regioselective oligomerization of 1-hexene with a marked preference for trimer formation

NHC Bis-Phenolate Aluminum Chelates: Synthesis, Structure, and Use in Lactide and Trimethylene Carbonate Polymerization

A novel family of Al­(III) complexes supported by a tridentate, dianionic N-heterocyclic carbene bis-phenolate ligand ((OCO)^2–) was prepared via various synthetic routes, and the derived compounds were all structurally characterized. The methane elimination reaction of the protio ligand <i><i>N,N</i></i>′-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>·H₃Cl) with AlMe₃ quantitatively led to the formation of the bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(Me)­(Cl) (<b>2</b>), whose formulation was established by X-ray diffraction studies. The deprotonation of species <b>2</b> with 1 equiv of lithium diisopropylamide (LDA) proceeded with the elimination of LiCl to afford the Al-NHC methyl derivative [(OCO)­AlMe]₂ (<b>3</b>), which was isolated as a dimer, as confirmed by X-ray diffraction studies. Alternatively, compound <b>3</b> may be accessed via a salt metathesis route involving the reaction of the NHC bis-phenolate Li salt <b>1</b>·Li₂, generated in situ via reaction of <b>1</b>·H₃Cl with 3 equiv of ^<i>n</i>BuLi (−40 °C, THF), with 1 equiv of MeAlCl₂. The serendipitous hydrolysis of compound <b>3</b> allowed the X-ray characterization of the Al-oxo dinuclear species [(OCO)­Al-O-Al-(OCO)] (<b>3</b>′), in which both Al­(III) centers adopt a distorted-trigonal-monopyramidal geometry. The reaction of the salt <b>1</b>·H₃Cl with Al­(O<i>i</i>Pr)₃ afforded the corresponding bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Cl) (<b>4</b>), which incorporates a four-coordinate tetrahedral Al center effectively κ²<i>O,O</i>-chelated by the two phenolate moieties of the OCO^2– ligand. Compound <b>4</b> may be readily converted to the Al-NHC alkoxide derivative [(OCO)­AlO<i>i</i>Pr]₂ (<b>5</b>) upon reaction with 1 equiv of LDA. Alternatively, the alcoholysis of the Al-NHC methyl species <b>3</b> with <i>i</i>PrOH also permitted access to the derived Al alkoxide <b>5</b> and proceeds via the formation of the kinetic product (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Me) (<b>6</b>) that may readily eliminate methane upon heating to produce species <b>5</b>. The Al alkoxide species <b>5</b> was shown to efficiently polymerize <i>rac</i>-lactide and trimethylene carbonate in a highly controlled manner for the production of narrow disperse materials. The observed catalytic performances are in the range of the majority of those for group 13 metal based ROP catalysts developed thus far, and all data support the noninvolvement of the NHC moiety in these polymerization reactions

Derivatization of Preformed Platinum N‑Heterocyclic Carbene Complexes with Amino Acid and Peptide Ligands and Cytotoxic Activities toward Human Cancer Cells

A simple procedure for the preparation of N-heterocyclic carbene platinum complexes with a nitrogen-based neutral ligand in <i>trans</i> geometry is presented. The lability of a <i>trans</i> pyridine ligand in an N-heterocyclic carbene–Pt­(II)–pyridine complex, namely, (3-benzyl-1-methylimidazolylidene)­PtI₂(pyridine), <b>2</b>, was probed by a displacement reaction with various nitrogen-based ligands (e.g., amines, hydrazine, amino esters, and peptides) to yield the corresponding complexes, which could be easily isolated by column chromatography. Two representative complexes could be characterized by X-ray crystallographic studies. This strategy allows generating diversity in metallodrug candidates. Preliminary results of the biological effects on various human cancer cells and comparison wih cisplatin are reported

Derivatization of Preformed Platinum N‑Heterocyclic Carbene Complexes with Amino Acid and Peptide Ligands and Cytotoxic Activities toward Human Cancer Cells

A simple procedure for the preparation of N-heterocyclic carbene platinum complexes with a nitrogen-based neutral ligand in <i>trans</i> geometry is presented. The lability of a <i>trans</i> pyridine ligand in an N-heterocyclic carbene–Pt­(II)–pyridine complex, namely, (3-benzyl-1-methylimidazolylidene)­PtI₂(pyridine), <b>2</b>, was probed by a displacement reaction with various nitrogen-based ligands (e.g., amines, hydrazine, amino esters, and peptides) to yield the corresponding complexes, which could be easily isolated by column chromatography. Two representative complexes could be characterized by X-ray crystallographic studies. This strategy allows generating diversity in metallodrug candidates. Preliminary results of the biological effects on various human cancer cells and comparison wih cisplatin are reported

Tridentate Complexes of Group 10 Bearing Bis-Aryloxide N‑Heterocyclic Carbene Ligands: Synthesis, Structural, Spectroscopic, and Computational Characterization

A series of group 10 complexes featuring chelating tridentate bis-aryloxide N-heterocyclic carbenes were synthesized and characterized by using different techniques. Ni­(II), Pd­(II), and Pt­(II) complexes were isolated in good yields by straightforward direct metalation of the corresponding benzimidazolium or imidazolium precursors in a one-pot procedure. All of the compounds were fully characterized, including single-crystal X-ray diffractometric determination for three of the derivatives. In the solid state, the complexes adopt a typical square-planar coordination geometry around the platinum atom, sizably distorted in order to comply with the geometrical constraints imposed by the bis-aryloxide N-heterocyclic carbene ligand. For platinum and palladium derivatives, a joint experimental and theoretical characterization was performed in order to study the optical properties of the newly prepared complexes by means of electronic absorption and steady-state and time-resolved photophysical techniques as well as density functional theory (DFT) and time-dependent DFT in both vacuum and solvent. When the temperature was lowered to 77 K in frozen glassy matrix, three platinum complexes showed broad and featureless, yet weak, photoluminescence in the green region of the visible spectrum with excited-state lifetimes on the order of a few microseconds. On the basis of joint experimental and computational findings and literature on platinum complexes, such emission was assigned to a triplet-manifold metal−ligand-to-ligand charge transfer (³MLLCT) transition

Redox and Luminescent Properties of Robust and Air-Stable N‑Heterocyclic Carbene Group 4 Metal Complexes

Robust and air-stable homoleptic group 4 complexes of the type M­(L)₂ [<b>1</b>–<b>3</b>; M = Ti, Zr, Hf; L = dianionic bis­(aryloxide) N-heterocyclic carbene (NHC) ligand] were readily synthesized from the NHC proligand 1,3-bis­(3,5-di-<i>tert</i>-butyl-2-hydroxyphenyl)­imidazolinium chloride (<b>H</b>_<b>3</b><b>L,Cl</b>) and appropriate group 4 precursors. As deduced from cyclic voltammetry studies, the homoleptic bis-adduct zirconium and hafnium complexes <b>2</b> and <b>3</b> can also be oxidized, with up to four one-electron-oxidation signals for the zirconium derivative <b>2</b> (three reversible signals). Electron paramagnetic resonance data for the one-electron oxidation of complexes <b>1</b>–<b>3</b> agree with the formation of ligand-centered species. Compounds <b>2</b> and <b>3</b> are luminescent upon excitation in the absorption band at 362 nm with emissions at 485 and 534 nm with good quantum yields (ϕ = 0.08 and 0.12) for <b>2</b> and <b>3</b>, respectively. In contrast, the titanium complex <b>1</b> does not exhibit luminescent properties upon excitation in the absorption band at 310 and 395 nm. Complexes <b>2</b> and <b>3</b> constitute the first examples of emissive nonmetallocene group 4 metal complexes