Reactivity studies, transition metal nickel

The Brookhart laboratory has contributed much of the knowledge of the polymerization mechanism for the late transition metal a-diimine catalysts. The review by Ittel provides a concise summary of the mechanistic understanding as of the year 2000 [26]. Some of the early findings will be reviewed here and additional insights reported afterward will be presented. In addition to the experimental work, many theoretical and computational studies worthy of discussion have also been carried out. These efforts have been most important in providing insight into the mechanistic details of the highly reactive nickel system, which is often difficult to study experimentally. 

Ishikawa and coworkers have studied the unique reactivity of strained cyclic disilanes (Equation 9.11) [35]. Transition metals, especially those of Group 10, readily insert into the Si—Si bond of disilacyclobutene 118 and can catalyze the addition of that bond across a variety of unsaturated acceptors. In the case of Ni(0)-catalyzed reactions of 118 with trimethylsilyl alkynes, insertion was found to occur both in a 1,2-and in a 1,1-fashion. The latter of these pathways implies a 1,2-silyl-migration, presumably occurring at the metal center. A nickel vinylidene intermediate was therefore proposed, though efforts to prove its existence were inconclusive. Similar vinylidene intermediates have been proposed by Ishikawa and coworkers to account for migrations observed in related palladium- and platinum-catalyzed reactions [36]. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

A few years ago Smalley and coworkers were able to obtain detailed experimental information about the reactivity of specific transition metal clusters with hydrogen molecules (1). The results for copper and nickel clusters were essentially as expected from the known results for surface and metal complex activities. For copper no clusters were able to dissociate whereas for nickel all clusters were active with a slow, steady increase of activity with cluster size. For the other transition metals studied, cobalt, iron and niobium, a completely different picture emerged. For these metals a dramatic sensitivity of the reactivity to cluster size was detected. No convincing explanation for these surprising results has hitherto been suggested. It should be added that there are no dramatic differences in the activity towards Hg for the metal surfaces (or the metal complexes) of nickel on the one hand and iron, cobalt and niobium on the other. 

The 3d transition metals are widely employed as catalyst and catalyst support materials in industry [3]. To gain insight into how these support materials and the catalyst support interaction influence catalytic activity, GIB-MS experiments were undertaken in our laboratory to determine the structural characteristics of cobalt oxide and nickel oxide clusters as well as their reactivity with CO. CID experiments were conducted employing Xe gas to elucidate the structural building blocks of the larger clusters. These studies provided insight into how additional (i-electrons impact the dissociation pathways and bonding motifs of 3d transition metal oxide clusters. Reactivity studies with CO were carried out, which revealed that oxide clusters composed of different 3d metals have specific stoichiometries which are most active for CO oxidation. 

In 1948, Reppe reported the first example of the transition metal-catalyzed [2+2+2] cycloaddition of alkynes by using a nickel complex [2]. Vollhardt extensively studied utilization of the cobalt-mediated [2+2+2] cycloaddition in organic synthesis [3a, b], and Yamazaki studied general reactivity of... 

Reactivity of Nickel Pincer Complexes with CO2. There is great interest involving the activation of CO2 and its conversion into value-added chemicals. One strategy utilizes late transition-metal complexes where the key step involves metal insertion to form metal carboxylates. Nickel pincer complexes were prepared and the subsequent reactivities of these complexes with CO2 were studied. The complex [(PCP)Ni(Cl)] was refluxed in THF with excess NaNH2 for 24 h to form [(PCP)Ni(NH2)] (eq 38). Further refluxing with 1 equiv of oxazole for 3 day via protonolysis, liberated ammonia to form [(PCP)Ni(oxazole)]. It is worth noting that the protonolysis is air and moisture sensitive and that if rigorously dry conditions are not employed, [(PCP)Ni(OH)] will form instead. 

The reactivity of phosphasilenes with organic compounds, for example, [2 + 2]-cycloaddition reaction (8) has been intensively investigated. The reactivity of phosphasilenes with transition metal complexes, however, is comparatively unexplored (9,10). The described phosphasilene 1 can imdergo unprecedented EtZ isomerization of the Si=P bond upon coordination to tungsten (11). Another remarkable reactivity study describes the coordination of group 10 transition metals to phosphasilene 1 generating dinuclear platinum and palladium complexes with Si—P bond cleavage and a bissilylene nickel complex from a Ni(0) precursor (12). 

In eomparative in vitro and in vivo studies, oil ash is typically more toxic than other ashes, and its toxieity is largely related to its high content of transition metals, particularly iron, nickel, and vanadium (63,67). Hauser et al. (68) reviewed the literature on short-term effects of exposure to oil ash among boiler workers and reported a new prospective study of the effects of working on oil boilers on respiratory function. Exposure to oil ash causes reduced function measured by spirometry, but no ehanges were observed in airway reactivity. There is little epidemiological information on the long-term effects of occupational exposures to inhaled oil ash. 

Particle exposure has been specifically linked to induction of pulmonary inflammation. Amdur and Chen (25) reported increased neutrophil numbers and increased bronchoalveolar lavage (BAL) protein following exposure to acid-coated particles. Exposure to fly ash, a model of outdoor air pollution, induces severe pulmonary inflammation (26) marked by increased numbers of neutrophils (27) and increased lavageable protein (26). Interestingly, the observed pulmonary inflammation has been specifically related to the metal content of the fly ash particle, particularly iron, nickel, and vanadium (26). Other studies have pointed to the effectiveness of these transition metals in inducing reactive oxygen species (ROS) in lung cells (28,29). 

Biophysical studies of the urease metal centre in the presence and absence of inhibitors, in conjunction with kinetic data provide the model of the bi-Ni site shown in 1. Certain inhibitors are thought to bridge the two nickel atoms consistent with a bridged transition state during urea hydrolysis. The ligands for nickel are believed not to contain sulphur, however, an essential cysteine is proximal to the active site. Comparisons of diethylpyrocarbonate reactivity for apo- and halo-enzyme are consistent with His as a ligand to nickel (Lee et al., 1990). 

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