Isomerization dynamics and control of the eta2/N equilibrium for pyridine complexes

A series of pyridine complexes are prepared of the general form TpW(NO)(PMe3)(pyr) where pyr is either pyridine or a substituted pyridine. Depending on substitution pattern, the pyridine can be either N- or eta2-coordinated, and the role of the pyridine substituents and metal oxidation state in determining this equilibrium is explored. For eta2-pyridine complexes, the substituent pattern and solubility characteristics also determine the ratio of coordination diastereomers. Rates of both intra- and interfacial linkage isomerizations are explored along with the pyridine rotational barrier. This study is supported by DFT calculations and X-ray data and includes characterization of both eta2-pyridine and eta2-pyridinium complexes.     

Charge donation to and dearomatization of benzene attending complexation: DFT estimates of binding energies of TpMXO(L) with benzene, for Tp = hydridotris(pyrazolyl) borate, MXO = MoNO, ReCO, and WNO, and L = ammonia, N-methylimidazole, pyridine, phosphine, methyl isocyanide, and carbon monoxide

By density functional methods we characterize the bonding and charge distribution in complexes of benzene with dearomatizing agents tpReCO(L), tpMoNO(L), and tpWNO(L), where tp = hydrido Tris (pyrazolyl)borate), for a range of ligands L. Our LSDA and B3LYP density functional calculations use the Spartan LACVP+ basis and pseudopotential on Re, Mo, and W and 6-31G* on light atoms. The binding energy is strongly dependent on the nature of the ligand L, being greatest for L = ammonia and N-methylimidazole and weakest for CH3NC and CO. We find a correlation between strength of binding and electron transfer from the dearomatizing agents toward benzene. For the most strongly bound systems we find substantial (up to 500 millielectrons) charge transfer towards benzene, while for the most weakly bound systems charge is withdrawn from benzene. Structural details illustrate the ability of Re, Mo, and W species to dearomatize complexed benzene, which is extensive for all but the most weakly bound species with L = MeNC and CO. Re and W dearomatizing agents, which are computed and observed to form stable complexes with benzene, may be economic alternatives to osmium dearomatizing agents.     

Human Group IIA Phospholipase A2—Three Decades on from Its Discovery

Phospholipase A₂ (PLA₂) enzymes were first recognized as an enzyme activity class in 1961. The secreted (sPLA₂) enzymes were the first of the five major classes of human PLA₂s to be identified and now number nine catalytically-active structurally homologous proteins. The best-studied of these, group IIA sPLA₂, has a clear role in the physiological response to infection and minor injury and acts as an amplifier of pathological inflammation. The enzyme has been a target for anti-inflammatory drug development in multiple disorders where chronic inflammation is a driver of pathology since its cloning in 1989. Despite intensive effort, no clinically approved medicines targeting the enzyme activity have yet been developed. This review catalogues the major discoveries in the human group IIA sPLA₂ field, focusing on features of enzyme function that may explain this lack of success and discusses future research that may assist in realizing the potential benefit of targeting this enzyme. Functionally-selective inhibitors together with isoform-selective inhibitors are necessary to limit the apparent toxicity of previous drugs. There is also a need to define the relevance of the catalytic function of hGIIA to human inflammatory pathology relative to its recently-discovered catalysis-independent function.     

On exponential sums studied by Indlekofer and Kátai

We continue the study of sums of the form

Calculus demonstrations using MATLAB

The note discusses ways in which technology can be used in the calculus learning process. In particular, five MATLAB programs are detailed for use by instructors or students that demonstrate important concepts in introductory calculus: Newton's method, differentiation and integration. Two of the programs are animated. The programs and the graphical user interface have been specifically designed to help the student understand the processes behind these important introductory concepts. Each program has a series of demonstrations that show unusual, difficult or important cases.     

Transition metal-stabilized arenium cations: protonation of arenes dihapto-coordinated to pi-basic metal fragments

A series of metal complexes was synthesized in which arenes were dihapto-coordinated to pi-basic metal fragments having the general form [TpM(pi-acid)(L)], where Tp = hydridotris(pyrazolyl)borate, M = rhenium, molybdenum, or tungsten, pi-acid = CO or NO(+), and L = 1-methylimidazole, 1-butylimidazole, pyridine, or trimethylphosphine. The arene complexes were shown to be significantly more basic than the analogous pentaammineosmium(II) arene complexes and were protonated by moderate acids to give remarkably stable eta(2) and eta(3) arenium cation complexes. A crystal structure of [TpRe(CO)(MeIm)(5,6-eta(2)-2H-anisolium)](OTf) confirmed the eta(2) coordination of the anisolium ligand, but suggests a weak long-range interaction between the metal and C1 of the anisolium.     

Ligand-modulated stereo- and regioselective tandem addition reactions of rhenium-bound naphthalene

A series of complexes of the form TpRe(CO)(L)(eta(2)-naphthalene) (Tp = hydridotris(pyrazolyl)borate) undergoes tandem electrophile/nucleophile addition reactions with a high degree of regiocontrol depending on the auxiliary ligand, L. When L = PMe(3), the reaction of the eta(2)-naphthalene complex with triflic acid followed by a silyl ketene acetal favors the 1,4-addition product, whereas when L = pyridine, N,N-dimethylaminopyridine, N-methylimidazole, or NH(3) the 1,2-addition product is favored. These reactions proceed with excellent stereocontrol: both electrophile (H(+), D(+)) and nucleophile (silyl ketene acetal) add anti to the face of metal coordination, and a single coordination diastereomer can be isolated for each reaction. One-electron oxidation of the Re complex affords the corresponding free dihydronaphthalene in good yield.     

Estimation of the electron backscattering factor in AES

To obtain reliable results in quantitative Auger analysis one must take into account the factors dependent on the matrix of the analysed material. One of the most important among them is the so-called backscattering factor which expresses the enhancement of the Auger yield as a consequence of the backscattered primary electrons. Until now, several different approaches to its determination have been published, the obtained results, however, more or less differ one from an other. By comparing the results of various theoretical and empirical approaches with experimental data we try to choose the most appropriate method for calculation of the backscattering factor. This method should provide sufficiently accurate values. At the same time we try to avoid using such methods which are laborious, numericaly complicated or require various experimental procedures.     

On sums of squares of primes II

In this paper we continue our study, begun in G. Harman and A.V. Kumchev (2006) [10], of the exceptional set of integers, not restricted by elementary congruence conditions, which cannot be represented as sums of three or four squares of primes. We correct a serious oversight in our first paper, but make further progress on the exponential sums estimates needed, together with an embellishment of the previous sieve technique employed. This leads to an improvement in our bounds for the maximal size of the exceptional sets.