Rene S. Rojas

Nickel α‐Keto‐β‐Diimine Initiators for Olefin Polymerization

Late transition metal initiators for olefin polymerization and oligomerization are of interest owing to their low oxophilicity and high functional group tolerance. Nickeland palladiumbased initiators provide several benefits. These include control over molecular weight characterisitics, backbone stereochemistry, and comonomer incorporation. 4] “Chain walking” reactions, in which the metal center migrates along the growing polymer chain through a series of b-hydride eliminations and reinsertions, allow for the generation of a wide variety of novel materials. New ligand structures continue to appear and offer further optimization of reactivity and polymer properties. 4b, 5c,6] Scheme 1 shows representative cationic and zwitterionic nickel active sites. Example 1a (charge compensating anion is not shown) corresponds to the broadly studied class of adiimine complexes, for which the resting state is a cationic alkyl olefin species. These species can be generated from a dihalide precursor in the presence of olefin and an appropriate activator, such as methylaluminoxane (MAO). Also shown are two prototypical zwitterionic initiators (1b and 1c ; important resonance contributions are not shown). A relevant perspective for understanding the reactivity of zwitterionic species is that Lewis acid complexation onto a basic ligand site reduces the electron density of the metal center in a neutral complex. This concept is more easily appreciated by examination of 1c. Bulky substituents that occupy axial sites are important structural components for minimizing chain-growth termination in both cationic and zwitterionic systems: for example, 1b produces high-molecular-weight polyethylene (PE), whereas 1c yields a distribution of 1-alkenes. Herein, we report on the synthesis, characterization, and reactivity of a ligand–metal combination that was designed to be cationic and also to benefit from removal of electron density from the metal center by the action of a Lewis acid on the ligand framework. We targeted a nickel complex containing a bulky a-keto-b-diimine ligand with 2,6-diisopropylphenyl substituents (2, Scheme 2). The exocyclic carbonyl functionality was anticipated to provide an electronically delocalized conduit extending from the potentially cationic metal center to a site of Lewis acid interaction through the oxygen lone pairs. The previously reported b-diimine complex Scheme 1. Examples of cationic and zwitterionic polymerization/oligomerization active sites. R = alkyl.

Synthesis of new phosphorescent imidoyl-indazol and phosphine mixed ligand Cu(I) complexes – structural characterization and photophysical properties

Four mononuclear Cu(I) complexes were prepared, described as [Cu(N,N)2]PF6 (1) and [Cu(N,N)(P,P)]PF6 (2–4), where N,N is N-(1-(2H-indazol-2-yl)ethylidene)-2,6-diisopropylaniline and P,P are phosphine derived ancillary ligands (bis[2-(diphenylphosphino)phenyl]ether (POP), bis(diphenylphosphino)ethane (dppe) or 2 PPh3). These new species were characterized by NMR, FT-IR, elemental analyses, cyclic voltammetry, UV-Vis – emission spectroscopy, transient absorption spectroscopy and DFT calculations. In addition, complexes 1 and 2 were characterized by X-ray diffraction. The four complexes showed an MLCT absorption band between 400 and 450 nm, in addition to a weakly structured phosphorescence in a 4:1 ethanol:methanol glassy matrix at 77 K. Complexes 2–4 have emission profiles that resemble the phosphorescence of the protonated N,N ligand, suggesting a triplet LC character of the lowest lying excited state at 77 K. By contrast, a mixed MLCT/LC triplet emission is most likely responsible for the phosphorescence in complex 1. Weak ligand-centered emission is also detected in the solid state at room temperature but only in the case of complexes 2 and 4, suggesting thermally activated deactivation processes in the case of 1 and 3. Notably, the transient absorption spectroscopy of complexes 2–4 in CH2Cl2 solution confirms a strong contribution from a ligand-centered (LC) triplet excited state, pointing towards a mixed 3MLCT/3LC character of the transient species in solution at room temperature, undergoing a non-radiative deactivation in the μs time-scale. This behavior markedly differs from that observed for complex 1, whose short-lived 3MLCT excited state is followed by ultrafast transient absorption spectroscopy.

A computational and conceptual DFT study on the mechanism of hydrogen activation by novel frustrated Lewis pairs

A computational and conceptual density functional theory (DFT) study on the mechanism of molecular hydrogen activation by a set of three frustrated Lewis pairs (FLPs) was performed at the ωB97X-D/6-311G(d,p) level of theory. A reduced model and other two prototypes derived from experimental data, based on the donor nitrogen and acceptor boron atoms, were used. Analysis based on the energy results, geometries and the global electron density transfer at the TSs made it possible to obtain some interesting conclusions: (i) despite the well-known very low reactivity of molecular hydrogen, the catalytic effectiveness of the three FLPs produces reactions with almost unappreciable activation energies; (ii) the reactions, being exothermic, follow a one-step mechanism via polarised TSs; (iii) there are neither substituent effects on the kinetics nor on the thermodynamics of these reactions; (iv) the activation of molecular hydrogen seems to be attained when the N-B distance in the FLP derivatives is around 2.74 Å; and (v) the proposed FLP model is consistent with the behaviour of the experimental prototypes. Finally, the ability of the three FLPs as efficient catalysts was evaluated studying the hydrogenation of acetylene to yield ethylene.

Synthesis, characterization, and reactivity studies in ethylene polymerization of cyclometalated palladium(II) complexes containing terdentate ligands with N,C,N-donors

The reaction of [Na2PdCl4] with 3,5-bis(2-pyridoxy)toluene (LpyH) in acetic acid yields the cyclometalated complex [PdCl(Lpy-N, C, N)] (1). Complex 1 can be further converted into neutral species by metathesis reaction exchange of chloride by either iodide or thiocyanate to yield [PdX(Lpy-N, C, N)] (X = I (2), SCN (3)). The chloride can be replaced by neutral ligands like pyridine or acetonitrile in the presence of silver tetrafluoroborate to give the corresponding cationic compounds [PdL(Lpy-N, C, N)]BF4 (L = Py (4), MeCN (5)). In contrast, the reaction of [Na2PdCl4] with 3,5-bis(3, 5-dimethylpyrazol-1-ylmethyl)toluene (LpzH) under analogous conditions yields the neutral complex [PdCl2(LpzH-N, N)](6) with the ligand bidentate N,N-donor. The cyclometalated palladium complex [PdCl(Lpz-N, C, N)] (7) was prepared by the reaction of Pd(OAc)2 with LpzH in acetic acid followed by a metathetic reaction with lithium chloride in acetone/water. Complexes 1, 6, and 7 in the presence of methylaluminoxane (MAO) lead to an active catalyst for the polymerization of ethylene.

Synthesis and structures of N-arylcyano-β-diketiminate zinc complexes and adducts and their application in ring‐opening polymerization of L-lactide

Zinc amide complexes ZnL1N(SiMe3)2, ZnL2N(SiMe3)2 (1 and 2), their tris(pentafluorophenyl)borane adducts ZnL1N(SiMe3)2·B(C6F5)3 (3), ZnL2N(SiMe3)2·2B(C6F5)3 (4), pentafluorophenyl zinc complex ZnL1C6F5 (5) and its adduct ZnL1C6F5·B(C6F5) (6) supported by N-arylcyano-β-diketiminate ligands, as well as bis-ligated Zn(L2)2 (7) were synthesized and characterized by NMR, IR, elemental analysis and X-ray diffraction. Zinc crystal structures of 1, 4, and 7 showed mononuclear complexes, while 2 and 5 were dimmers. ROP of L-lactide with zinc complexes and their B(C6F5)3 adducts leads to generation of poly(L-LA) with high molecular weight and relatively narrow molecular weight distribution. The monomer conversion reached completion in 40 min only for zinc amide complex 1, while for other compounds it was necessary to use at least 5 hours to achieve significant polymerization yields. Coordination of the B(C6F5)3 molecule close to the metal center blocks L-lactide insertion and thus decreases the activity of respective adducts in comparison with borane-free zinc complexes.

Organonickel(II) complexes with anionic tridentate 1, 3-bis(azolylmethyl)phenyl ligands. synthesis, structural characterization and catalytic behavior

A reacao de 2-bromo-1, 3-bis(bromometil)benzeno com 3, 5-dimetilpirazol e 1H-indazol produz os ligantes tridentados 2-bromo-1, 3-bis(3, 5-dimetilpirazol-1-ilmetil)benzeno (1) e 2-bromo-1, 3-bis(indazol-2-ilmetil)benzeno (2). Estes compostos reagem com [Ni(cod)2] em tetraidrofurano (thf) para formar os complexos de adicao oxidativa [NiBr{1, 3-bis(azolilmetil)fenil-N, C, N}], azol = 3, 5-dimetilpirazol (3) e indazol (4), os quais foram isolados em bons rendimentos como solidos amarelos estaveis e caracterizados por meio de analise elementar, espectroscopia de absorcao no infravermelho com transformada de Fourier (FTIR), espectrometria de massa e ressonância magnetica nuclear (NMR). Adicionalmente, as estruturas moleculares de 2 e 4 foram determinadas por difratometria de raios X de monocristal. O complexo 4 foi testado como catalisador na reacao de polimerizacao de etileno

New imidoyl-indazole platinum (II) complexes as potential anticancer agents: Synthesis, evaluation of cytotoxicity, cell death and experimental-theoretical DNA interaction studies

Four new neutral N,N imidoyl-indazole ligands (L1, L3, L6, L7) and six new Pt(II)-based complexes (C1-5 and C7) were synthesized and characterized by spectroscopic and spectrometric techniques. Additionally, compounds L6, L7, C3, C5 and C7 were analyzed using X-ray diffraction. An evaluation of cytotoxicity and cell death in vitro for both ligands and complexes was performed by colorimetric assay and flow cytometry, in four cancer cell lines and VERO cells as the control, respectively. Cytotoxicity and selectivity demonstrated by each compound were dependent on the cancer cell line assayed. IC50 values of complexes C1-5 and C7 were lower than those exhibited for the reference drug cisplatin, and selectivity of these complexes was in general terms greater than cisplatin on three cancer cell lines studied. In HL60 cells, complexes C1 and C5 exhibited the lowest values of IC50 and were almost five times more selective than cisplatin. Flow cytometry results suggest that each complex predominantly induced necrosis, and its variant necroptosis, instead of apoptosis in all cancer cell lines studied. DNA binding assays, using agarose gel electrophoresis and UV-visible spectrophotometry studies, displayed a strong interaction only between C4 and DNA. In fact, theoretical calculations showed that C4-DNA binding complex was the most thermodynamic favorable interaction among the complexes in study. Overall, induction of cell death by dependent and independent-DNA-metal compound interactions were possible using imidoyl-indazole Pt(II) complexes as anticancer agents.

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective

In this work, global and local descriptors of chemical reactivity and selectivity are used to explain the differences in reactivities toward ethylene of methallyl nickel complexes and their B(C6F5)3 and BF3 adducts. DFT calculations were used to explain why nickel complexes alone are inactive in ethylene polymerization while their boron adducts can activate it. It is shown that chemical potential, hardness, electrophilicity and molecular electrostatic potential surfaces describe fairly well the reactivity and selectivity of these organometallic systems toward ethylene. Experimental data indicates that addition of a borane molecule to nickel complexes changes dramatically their reactivity—behavior that is confirmed computationally. Our results show that bare complexes are unable to activate ethylene—a Lewis base—because they also behave as Lewis bases. The addition of the co-catalyst—a Lewis acid—turns the adducts into Lewis acids, making them active towards ethylene.

Development of Imine Derivative Ligands for the Exocyclic Activation of Late Transition Metal Polymerization Catalysts

Transition metal complexes bearing imine and imine derivative ligands represent a growing number of polymerization catalysts in development. The ease of synthesis and large number of structural variations which are readily accessible make these systems of great interest both academically and industrially. One subset of imine-based complexes are those which bear exocyclic functionality which can interact with Lewis acids. These systems are particularly interesting as the activation of the complex occurs remotely, away from the active center, and that the activation can proceed using stoichiometric concentrations of activators. In addition, the presence of the exocyclic functionality may present an effective method to heterogenize polymerization catalysts. In this chapter, the development of such systems and in particular ?-iminocarboxamide nickel catalysts and derivative species are discussed.

Chromium(III) complexes bearing bis(benzotriazolyl)pyridine ligands: synthesis, characterization and ethylene polymerization behavior

Abstract Reaction of benzotriazole with 2,6-bis(bromomethyl)pyridine and 2,6-pyridinedicarbonyl dichloride yields the tridentate ligands 2,6-bis(benzotriazol-1-ylmethyl)pyridine (1) and 2,6-bis(benzotriazol-1-ylcarbonyl) pyridine (2). The molecular structures of the ligands were determined by single-crystal X-ray diffraction. These ligands react with CrCl3(THF)3 in THF to form neutral complexes, [CrCl3{2,6-bis(benzotriazolyl)pyridine-N,N,N}] (3, 4), which are isolated in high yields as air stable green solids and characterized by mass spectra (ESI), FTIR spectroscopy, UV–Visible, thermogravimetric analysis (TGA), and magnetic measurements. After reaction with methylaluminoxane (MAO), the chromium(III) complexes are active in the polymerization of ethylene showing a bimodal molecular weight distribution. A DFT computational investigation of the polymerization reaction mechanism shows that the most likely reaction pathway originates from the mer configuration when the spacer is CH2 (complex 3) and from the fac configuration when the spacer is CO (complex 4).