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

Facile and Room-Temperature Activation of C_sp3–Cl Bonds by Cheap and Air-Stable Nickel(II) Complexes of (<i>N</i>‑Thioether) DPPA-Type Ligands

Reaction of the diphosphine (<i>P</i>,<i>P</i>) ligand N­(PPh₂)₂(<i>n-</i>PrSMe) (<b>1</b>) or N­(PPh₂)₂(<i>p</i>-(SMe)­C₆H₄) (<b>2</b>) with [Ni­(NCMe)₆]­[BF₄]₂ in a 2:1 molar ratio afforded the bis-chelated, dicationic Ni­(II) complexes [Ni­(<b>1</b>)₂]­[BF₄]₂ (<b>3</b>) and [Ni­(<b>2</b>)₂]­[BF₄]₂ (<b>4</b>), respectively. Both complexes were characterized in solution by various spectroscopic techniques and in the solid state by X-ray diffraction studies. In the presence of Zn metal used as cheap reductant, complexes <b>3</b> and <b>4</b> activate the inert C–Cl bonds of dichloromethane at room temperature to afford in high yield the phosphonium ylide derivatives [Ni­((Ph₂P)­N­{P­(CH₂)­Ph₂}­(<i>n-</i>PrSMe)-<i>P</i>,<i>C</i>)₂]­[BF₄]₂ (<b>5</b>) and [Ni­((Ph₂P)­N­{P­(CH₂)­Ph₂}­(<i>p-</i>(SMe)­C₆H₄)-<i>P</i>,<i>C</i>)₂]­[BF₄]₂ (<b>6</b>), respectively. The formation of [Ni­((Ph₂P)­N­{P­(CH₂)­Ph₂}­(<i>n-</i>PrSMe)-<i>P</i>,<i>C</i>)₂]­Cl₂ (<b>5</b>′), an analogue of complex <b>5</b>, from a Ni(0) precursor supports the reduction of the Ni­(II) precursors by the Zn reagent prior to C–Cl bond activation. The structures of <b>5</b> and <b>5</b>′ were unambiguously established by X-ray diffraction analysis

Combined Experimental and Theoretical Study of Bis(diphenylphosphino)(<i>N</i>‑thioether)amine-Type Ligands in Nickel(II) Complexes for Catalytic Ethylene Oligomerization

Starting from the new ligands bis­(diphenylphosphino)­(<i>N</i>-4-(methylthio)­phenyl)­amine (<b>4</b>, N­(PPh₂)₂(<i>p-</i>C₆H₄)­SMe) and its monosulfide derivative (Ph₂P)­N­{P­(S)­Ph₂}­(<i>p-</i>C₆H₄)­SMe (<b>4·S</b>), we have prepared and characterized, including by X-ray crystallographic studies, their Ni­(II) complexes [NiCl₂{(Ph₂P)₂N­(<i>p-</i>C₆H₄)­SMe-<i>P</i>,<i>P</i>}] (<b>5</b>) and [NiCl₂{(Ph₂P)­N­{P­(S)­Ph₂}­(<i>p-</i>C₆H₄)­SMe-<i>P</i>,<i>S</i>}] (<b>6</b>), respectively. The bis-sulfide compound N­{P­(S)­Ph₂}₂(<i>p-</i>C₆H₄)­SMe (<b>4·S</b>_<b>2</b>) was also prepared and structurally characterized. Computational studies showed that the combined influence of stronger P donors and a four-membered-ring <i>P</i>,<i>P</i> chelate leads to complex <b>5</b> being thermodynamically more stable than <b>6</b>, which contains one weaker PS donor group but a five-membered <i>P</i>,P<i>S</i> chelate ring. For comparison, the bis-chelate complex [Ni­{(Ph₂P)­N­{P­(S)­Ph₂}­(<i>p-</i>C₆H₄)­SMe-<i>P</i>,<i>S</i>}₂]­(BF₄)₂ (<b>7</b>), the monochelate complexes [NiBr₂{(Ph₂P)­N­{P­(S)­Ph₂}­(<i>p-</i>C₆H₄)­SMe-<i>P</i>,<i>S</i>}] (<b>8</b>) and the Pd­(II) analogue of <b>6</b>, [PdCl₂{(Ph₂P)­N­{P­(S)­Ph₂}­(<i>p-</i>C₆H₄)­SMe-<i>P</i>,<i>S</i>}] (<b>9</b>), were synthesized and structurally characterized and their solution behavior was investigated. The catalytic activity and selectivity in ethylene oligomerization of the Ni­(II) complexes <b>5</b> and <b>6</b> and their known <i>N</i>-(methylthio)­propyl analogues [NiCl₂{(Ph₂P)₂N­(CH₂)₃SMe-<i>P</i>,<i>P</i>}] (<b>2</b>) and [NiCl₂{(Ph₂P)­N­{P­(S)­Ph₂}­(CH₂)₃SMe-<i>P</i>,<i>S</i>}] (<b>3</b>), which were obtained from the bis­(diphenylphosphino)­(<i>N</i>-(methylthio)­propyl)­amine ligand N­(PPh₂)₂(CH₂)₃SMe (<b>1</b>) and its monosulfide derivative (Ph₂P)­N­{P­(S)­Ph₂}­(CH₂)₃SMe (<b>1·S</b>), respectively, revealed a significant influence of the nature of the <i>N</i>-substituent (aryl vs alkyl thioether) and of the chelate ring size (<i>P</i>,P vs <i>P</i>,P<i>S</i>). DFT calculations showed that the trend in Δ<i>E</i>_rel, [NiCl₂(<i>P</i>,<i>P</i>)] > [NiCl₂(<i>P</i>,P<i>S</i>)] > [NiCl₂(P<i>S</i>,P<i>S</i>)], results from the stronger covalent character of the Ni–P vs Ni–S bond. Using AlEtCl₂ as cocatalyst, mostly ethylene dimers were produced, with significant amounts of trimers (selectivity in the range 11–36%). Productivities up to 40400 and 48200 g of C₂H₄/((g of Ni) h), with corresponding TOF values of 84800 and 101100 mol of C₂H₄/ ((mol of Ni) h), were obtained with precatalysts <b>2</b> and <b>3</b>, respectively

Dietary intervention targeting increased fiber consumption for metabolic syndrome

Metabolic Syndrome is highly prevalanet in the United States and is a harbinger of diabetes and cardiovascular disease. With the staggering rise in diet-related chronic diseases such as diabetes and cardiovascular disease, simple and effective dietary intervention strategies are needed. National dietary recommendations are ever-changing and complex, which can be overwhelming and confusing for individuals who are trying to prevent or manage a chronic condition. Some evidence suggests that healthy changes in one area of diet are associated with healthy changes in other untargeted areas of diet. There is an opportunity to bridge a dietetics research gap by testing a simple dietary message focusing on fiber intake to improve risk factors for metabolic syndrome

Impact of Organometallic Intermediates on Copper-Catalyzed Atom Transfer Radical Polymerization

In atom transfer radical polymerization (ATRP), radicals (R^•) can react with Cu^I/L catalysts forming organometallic complexes, R–Cu^II/L (L = N-based ligand). R–Cu^II/L favors additional catalyzed radical termination (CRT) pathways, which should be understood and harnessed to tune the polymerization outcome. Therefore, the preparation of precise polymer architectures by ATRP depends on the stability and on the role of R–Cu^II/L intermediates. Herein, spectroscopic and electrochemical techniques were used to quantify the thermodynamic and kinetic parameters of the interactions between radicals and Cu catalysts. The effects of radical structure, catalyst structure and solvent nature were investigated. The stability of R–Cu^II/L depends on the radical-stabilizing group in the following order: cyano > ester > phenyl. Primary radicals form the most stable R–Cu^II/L species. Overall, the stability of R–Cu^II/L does not significantly depend on the electronic properties of the ligand, contrary to the ATRP activity. Under typical ATRP conditions, the R–Cu^II/L build-up and the CRT contribution may be suppressed by using more ATRP-active catalysts or solvents that promote a higher ATRP activity