Claudio Pellecchia

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.

Syndiotactic-specific polymerization of styrene: catalyst structure and polymerization mechanism

The mechanistic investigation of syndiotactic-specific polymerization of styrene is reviewed, mainly with reference to the research work carried out at the University of Salerno. NMR analysis of the stereochemical structure of s-PS macromolecules, in some cases suitably labelled, provided valuable information concerning the regiochemistry of polyinsertion, the type of addition to the monomer double bond, and the model of stereospecific propagation. Kinetic studies and spectroscopic investigation of the catalytic systems suggested a cationic Ti(III) complex with a strongly π-co-ordinated benzyl-type growing chain as the true active species.

A FRET Enzyme-Based Probe for Monitoring Hydrogen Sulfide

Fluorescently labeled cobalt peptide deformylase (Co-PDF) can be efficiently used as a fluorescence-resonance-energy-transfer-based sensing device for hydrogen sulfide (H(2)S). The proof of concept of our sensor system is substantiated by spectroscopic, structural, and theoretical results. Monohydrogen sulfide coordination to Co-PDF and Ni-PDF was verified by X-ray crystallography. Density functional theory calculations were performed to gain insight into the characteristics of the coordination adduct between H(2)S and the cobalt cofactor in Co-PDF.

Selective detection of ATP and ADP in aqueous solution by using a fluorescent zinc receptor

We report on the successful use of a new zinc complex for the selective fluorescent detection of ADP and ATP in water. This is achieved by the complementary coordination of the phosphate groups to the metal centre and hydrogen bonding of the adenosine with the coordinated ligand.

Syndiotactic-specific polymerization of propene with Nickel-based catalysts. 3. Polymer end-groups and regiochemistry of propagation

Abstract Low molecular weight prevailingly syndiotactic polypropylene samples have been synthesized under proper conditions with homogeneous catalytic systems based on Ni(II) diimine derivatives and methylaluminoxane. 13 C NMR analysis of the natural abundance end groups formed in both the initiation and termination steps suggests that the regiospecificity of propene insertion during the propagation is prevailingly 1,2. The effects of ligand variation at the Ni catalytic center on the regiospecificity of the polymerization is in agreement with 1,2 insertion as the main mode of propagation.

New Neutral and Cationic Dialkylaluminium Complexes Bearing Imino‐Amide or Imino‐Phenoxide Ligands: Synthesis, Characterization and Reactivity With Olefins

The synthesis and the characterization of some new aluminium complexes carrying bidentate monoanionic imino-amide or imino-phenoxide ligands are described. Reaction of 1-(o,o′-diisopropylphenylamino)-6-(o,o′-diisopropylphenylimino)cyclohexene with AlMe3 proceeded by methane elimination to produce the yellow compound dimethylaluminium N-(o,o′-diisopropylphenyl)-6-[N-(o,o′-diisopropylphenyl)imido]-1-cyclohexen-1-amide (1). In contrast, reaction of the same ligand with AlEt3 under the same experimental conditions involved the 1,4-addition of AlEt3 to the α,β-unsaturated imine, and led to the highly crystalline diethylaluminium N(o,o′-diisopropylphenyl)-2-[(o,o′-diisopropylphenyl)amino]3-ethyl-1-cyclohexen-1-amide (2). The structures of compounds 1 and 2 were determined by single-crystal X-ray diffraction. Thermolysis of 2 gave rise to ethane elimination, and led to the oily bis(amido) monoethyl derivative. Treatment of the salicylaldimine ligands 3-tBu-2-(OH)C6H3CH=N−R with AlMe3 yielded the dimethylaluminium compounds {3-tBu-2-(O)C6H3CH=N−R}AlMe2 [R = C6H5 (3); 2,6-iPr2C6H3 (4); and C6F5 (5)]. Compounds 1, 3, and 5 underwent methyl abstraction reactions with B(C6F5)3; trapping of the cationic species was accomplished in the presence of THF in dichloromethane solution. Preliminary polymerization tests were carried out for the synthesized Al complexes. Toluene solutions of 3, 4, and 5, when activated with 1 equiv. of B(C6F5)3, polymerised ethylene (1 atm) to solid polyethylene with low activity.

Myoglobin as a New Fluorescence Probe to Sense H2S

A new, fast, simple and cost-effective sensing device for monitoring H(2)S has been developed. Proof-of-principle results showing that a commercial and cheap Myoglobin (Mb) can be successfully used as a biological probe for a fluorescence biosensor for H(2)S detection are reported. The two different commercial labels Cy3 and Atto620 were selected for this study. A high selectivity for detecting H(2)S against other thiols was found. The applicability of the proposed sensing system was successfully explored not only in solution but also when applied in the form of a solid state device.

Therapeutic potential of a pyridoxal‐based vanadium(IV) complex showing selective cytotoxicity for cancer versus healthy cells

Vanadium compounds can exert anticancer effects, partly due to inhibition of tyrosine phosphatases. Here, we report the effect of N,N′‐ethylenebis (pyridoxylideneiminato) vanadium (IV) complex (Pyr2enV(IV)), that induced 93% and 57% of cell mortality in A375 (human melanoma) and A549 (human lung carcinoma) cells, respectively; the mortality was <24% in other cancer cell lines and in human normal epidermal keratinocytes, lung cells and peripheral blood mononuclear cells. The mechanism of Pyr2enV(IV) effect relied on apoptosis induction; this was triggered by ROS increase, followed by mitochondrial membrane depolarization. Indeed, the addition of N‐acetyl cysteine to cell cultures abated Pyr2enV(IV)‐induced apoptosis. These results disclose the pro‐apoptotic activity of Pyr2enV(IV) and its mechanism, relying on intracellular ROS increase. J. Cell. Physiol. 228: 2202–2209, 2013.

Non-metallocene group 4 organometallic complexes as catalysts for olefin polymerization : synthesis and catalytic activity of the cationic complex [Zr(CH2Ph)3]+ [B(CH2Ph)(C6F5)3]-

Abstract The title cationic Cp-free zirconium hydrocarbyl has been synthesized by ligand abstraction from tetrabenzylzirconium with the strong Lewis acid tris (pentafluorophenyl)boron. NMR analysis indicates that the [Zr(CH 2 Ph) 3 ] + cation is stabilized by coordination of the phenyl ring of the anion [B(CH 2 Ph) (C 6 F 5 ) 3 ] − , and that this structure is maintained in aromatic solvents up to 100°C. The highly electrophilic zirconium cation reacts rapidly with Lewis bases such as THF, affording the 6-coordinate [Zr(CH 2 Ph) 3 (THF) 3 ] + adduct. The title compound is the first non-metallocene group 4 cationic complex which functions as a single-component olefin polymerization catalyst.

Heteroscorpionate-based Co2+, Zn2+, and Cu2+ complexes: coordination behavior, aerobic oxidation, and hydrogen sulfide detection.

The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-(t)Bu(2)-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me(2)-pyrazol-1-yl)methane (L2-H) have been investigated in the metal complexes (L1-H)CoCl(2) (1), (L1-H)ZnCl(2) (2), (L3)CuCl(2) (3), (L2)(2)Co(2)Cl(2) (4) (L2-H)ZnCl(2) (5), and (L2-H)CuCl(2)·H(2)O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by X-ray single crystal diffraction and (1)H NMR spectroscopy and relaxometry. The neutral L1-H is κ(2)-coordinated to the metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl(2) to yield the para-quinone derivative L3 (L3 = 2-(t)Bu-6-(bis(3,5-Me(2)-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3 resulted κ(2)-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl(2)·6H(2)O produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one cobalt is in an octahedral environment featuring two κ(3)-coordinated deprotonated ligands whereas the second cobalt center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging moieties. Interestingly L2-H, (3-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L4-H), or (5-(t)Bu-2-phenol)bis(3,5-Me(2)-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl(2). The reaction of the ligand L2-H with ZnCl(2) and CuCl(2)·2H(2)O yielded the κ(2)-coordinated tetrahedral complex 5 and the square planar complex 6, respectively. The application of the cobalt complex 1 as molecular dosimeter for H(2)S was explored and compared to that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible mechanisms of H(2)S detection by both 1 and 2 are also described.