Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.Ministerio de Ciencia e Innovación Projects CTQ2010–15833, CTQ2013-45011 - P and Consolider - Ingenio 2010 CSD2007 - 00006Junta de Andalucía FQM - 119, Projects P09 - FQM - 5117 and FQM - 2126EU 7th Framework Program, Marie Skłodowska - Curie actions C OFUND – Agreement nº 26722

Alkoxycarbonylation of ethylene with cellulose in ionic liquids

Alkoxycarbonylation of ethylene with carbon monoxide and cellulose in 1-n-butyl-3-methylimidazolium methanesulfonate affords cellulose propionate with a degree of substitution of 1-2

Morphological changes during annealing of polyethylene nanocrystals

Polymer crystals are metastable and exhibit morphological changes when being annealed. To observe morphological changes on molecular scales we started from small nanometer-sized crystals of highly folded long-chain polymers. Micron-sized stripes consisting of monolayers or stacks of several layers of flat-on oriented polyethylene nanocrystals were generated via evaporative dewetting from an aqueous dispersion. We followed the morphological changes in time and at progressively higher annealing temperatures by determining the topography and viscoelastic properties of such assemblies of nanocrystals using atomic force microscopy. Due to smallness and high surface-to-volume ratio of the nanocrystals, already at 75 °C, i.e. about 60 degrees below the nominal melting point, the lateral size of the crystal coarsened. Intriguingly, this occurred without a noticeable reduction in the number of folds per polymer chain. Starting at around 110 °C, chain folds were progressively removed leading to crystal thickening. At higher temperatures, but still below the melting point, prolonged annealing allowed for surface diffusion of molten polymers on the initially bare substrate, leading eventually to the disappearance of crystals. We compared these results to the behavior of the same nanocrystals annealed in an aqueous dispersion and to bulk samples

Role of Electron-Withdrawing Remote Substituents in Neutral Nickel(II) Polymerization Catalysts

The novel neutral κ²-<i>N,O</i>-salicylaldiminato Ni­(II) complex, [κ²-<i>N,O</i>-{2,6-(3′,5′-R₂C₆H₃)₂C₆H₃-NC­(H)-(3,5-I₂-2-O-C₆H₂)}­NiCH₃(pyridine)] (<b>1a-pyr</b>, R = NO₂), with four nitro substituents on the N-terphenyl motif is a catalyst precursor for ethylene polymerization to yield linear higher molecular weight polyethylene (e.g., <i>M</i>_n 2.1 × 10⁵ g mol^–1 and only 2 methyl branches per 1000 carbon atoms). A comparison with other known catalyst precursors at various polymerization conditions shows that the catalytic properties in terms of linearity and molecular weight are similar to the fluorinated catalyst precursor with R = CF₃, showing that the latter is not singular, but rather suppression of chain transfer and branch formation by β-hydride elimination can also be brought about by nonfluorinated electron-withdrawing remote substituents

Role of Electron-Withdrawing Remote Substituents in Neutral Nickel(II) Polymerization Catalysts

The novel neutral κ²-<i>N,O</i>-salicylaldiminato Ni­(II) complex, [κ²-<i>N,O</i>-{2,6-(3′,5′-R₂C₆H₃)₂C₆H₃-NC­(H)-(3,5-I₂-2-O-C₆H₂)}­NiCH₃(pyridine)] (<b>1a-pyr</b>, R = NO₂), with four nitro substituents on the N-terphenyl motif is a catalyst precursor for ethylene polymerization to yield linear higher molecular weight polyethylene (e.g., <i>M</i>_n 2.1 × 10⁵ g mol^–1 and only 2 methyl branches per 1000 carbon atoms). A comparison with other known catalyst precursors at various polymerization conditions shows that the catalytic properties in terms of linearity and molecular weight are similar to the fluorinated catalyst precursor with R = CF₃, showing that the latter is not singular, but rather suppression of chain transfer and branch formation by β-hydride elimination can also be brought about by nonfluorinated electron-withdrawing remote substituents

Formation and evolution of chain-propagating species upon ethylene polymerization with neutral salicylaldiminato nickel(II) catalysts

Formation of Ni–polymeryl propagating species upon the interaction of three salicylaldiminato nickel(II) complexes of the type [(N,O)Ni(CH3)(Py)] (where (N,O)=salicylaldimine ligands, Py=pyridine) with ethylene (C2H4/Ni=10:30) has been studied by 1H and 13C NMR spectroscopy. Typically, the ethylene/catalyst mixtures in [D8]toluene were stored for short periods of time at +60 °C to generate the [(N,O)Ni(polymeryl)] species, then quickly cooled, and the NMR measurements were conducted at −20 °C. At that temperature, the [(N,O)Ni(polymeryl)] species are stable for days; diffusion 1H NMR measurements provide an estimate of the average length of polymeryl chain (polymeryl=(C2H4)nH, n=6–18). At high ethylene consumptions, the [(N,O)Ni(polymeryl)] intermediates decline, releasing free polymer chains and yielding [(N,O)Ni(Et)(Py)] species, which also further decompose to form the ultimate catalyst degradation product, a paramagnetic [(N,O)2Ni(Py)] complex. In [(N,O)2Ni(Py)], the pyridine ligand is labile (with activation energy for its dissociation of (12.3±0.5) kcal mol−1, ΔH≠298=(11.7±0.5) kcal mol−1, ΔS≠298 =(−7±1) cal K−1 mol−1). Upon the addition of nonpolar solvent (pentane), the pyridine ligand is lost completely to yield the crystals of diamagnetic [(N,O)2Ni] complex. NMR spectroscopic analysis of the polyethylenes formed suggests that the evolution of chain-propagating species ends up with formation of polyethylene with predominately internal and terminal vinylene groups rather than vinyl groups