Binuclear rhenium(I) complexes with bridging [2.2]paracyclophane-diimine ligands: probing electronic coupling through pi-pi interactions

Two pseudo-para substituted bis-diimino[2.2]paracyclophane ligands (4,16-bis(picolinaldimine)-[2.2]paracyclophane (BPPc) and 4,16-bis(methyl-picolinaldimine)-[2.2]paracyclophane (BmPPc)) were prepared by the condensation reaction of the appropriate picolinaldimine with 4,16-diamino-[2.2]paracyclophane (2). An improved synthesis of 2 from [2.2]paracyclophane also is reported. BPPc (3a): monoclinic, P2(1)/c, a = 8.2238(11) A, b = 15.336(2) A, c = 8.4532(11) A, beta = 98.578(3) degrees, V = 1054.2(2) A(3), Z = 2. To investigate the binding properties of the bis-diimino[2.2]paracyclophane ligands, binuclear rhenium(I) tricarbonyl chloride complexes [Re(CO)(3)Cl](2)(micro-BPPc) (5a) and [Re(CO)(3)Cl](2)(micro-BmPPc) (5b) were prepared and fully characterized by infrared spectroscopy, (1)H NMR spectroscopy, elemental analysis, UV-visible absorption spectroscopy, and cyclic voltammetry. Two model complexes, Re(tolyl-pyCa)(CO)(3)Cl (4) (tolyl-pyCa = N-(p-tolyl)-2-pyridinecarboxaldimine) and [Re(CO)(3)Cl](2)(micro-PBP) (6) (PBP = p-phenylenebis(picolinaldimine)), also are reported. The dimeric compounds 5 and 6 each undergo two one-electron, predominantly diimine-centered reduction processes. Spectroscopic data and comproportionation constants (5a, 23 +/- 9; 5b, 23 +/- 9; 6, 2750 +/- 540) are consistent with relatively weak interactions between the diimine groups mediated by the paracyclophane bridging group, and these results are consistent with steric and electronic factors.     

Mechanistic studies on the addition of dihydrogen to tantalocene complexes

The mechanism of dihydrogen addition to Cp(2)Ta(CH(2))(H) was examined using parahydrogen-induced polarization (PHIP), (13)C labeling, and comparison to the related complex Cp(2)Ta(CH(2))(CH(3)). The reaction of para-enriched hydrogen with Cp(2)Ta(CH(2))(H) leads to polarized resonances for both Cp(2)Ta(CH(3))(H)(2) and Cp(2)TaH(3), even at the earliest reaction times. Use of the labeled compound Cp(2)Ta((13)CH(2))(H) shows that the polarized resonances of Cp(2)Ta(CH(3))(H)(2) correspond to the two hydride ligands. The results thus support a mechanistic pathway of H(2) addition to an unsaturated Ta(III) intermediate, [Cp(2)Ta(CH(3))], rather than addition directly across the Ta=C bond. In a same sample comparison, the rates of initial H(2) addition and subsequent C-H reductive elimination for both Cp(2)Ta(CH(2))(H) and Cp(2)Ta(C(6)H(4))(H) were examined. The methylene complex exhibits greater reactivity than the benzyne complex, with the major difference due to the C-H coupling step, in which formation of methane is more facile than that of benzene. The reactivity of the related ethylene hydride complex, Cp(2)Ta(C(2)H(4))(H), with hydrogen was also examined. The PHIP study of this system leads to unusual and unexpected polarization, which is found to be due to a minor impurity in the sample.     

Anion-exchange separation of plutonium in hydrochloric-hydrobromic acid media

Plutonium can be rapidly and selectively separated from the elements that interfere in its radiochemical determination, by the use of hydrobromic acid in a hydrohalic acid anion-exchange separation procedure. Plutonium(IV) and (VI) are adsorbed onto the resin column from 9M hydrochloric acid, interfering elements such as americium and thorium are washed from the column with 9M hydrochloric acid, and the plutonium is reduced to plutoniurn(III) and washed from the column with 11M hydrobromic acid. Interfering elements such as uranium and neptunium, which are adsorbed onto the column from 9M hydrochloric acid, are retained there during the hydrochloric and hydrobromic acid washes. This system would also appear to provide the means for effectively separating plutonium from those elements that commonly interfere in such chemical methods of analysis as redox titration.     

Determination of caesium-137 in fast-reactor coolant systems

A method for the separation of caesium by ion-exchange involving ammonium 12-molybdophosphate has been investigated, and found to be 100% efficient for aqueous solutions with high sodium and potassium concentrations.     

Enhanced Photoactivated Luminescence of Selected Polychlorinated Biphenyl Congeners and Aroclor Mixtures

Fluorescence studies of selected polychlorinated biphenyl (PCB) congeners and Aroclor mixtures were conducted using enhanced photoactivated luminescence. Diphenylamine (DPA) was used as a photoactivator. DPA reacts with PCBs to form a fluorescent photoproduct under UV activation. The sensitivities of fluorescence detection using three different solid substrates were compared. Background studies of the substrates and DPA blanks were also conducted to determine the substrate with the best detection characteristics for enhanced photoactivated fluorescence. It was found that the Envi disk with a glass-fiber mesh and C-18-modified silica provides more analyte access for fluorescence detection, thereby providing best sensitivity. By comparison of the fluorescence from pure PCB congener–DPA complexes, the more coplanar congeners exhibited a red shift in excitation and emission as compared with theortho-substituted nonplanar congeners. These studies may prove useful in determining trends relating fluorescence intensity and toxicity factors.     

Ion/molecule reactions provide new evidence for the structure and origin of

Ion/molecule reactions leading to formation of the diagnostically useful [C(3)H(4)N](+) ion (m/z 54), from acetonitrile CI plasma, have been studied using a tandem quadrupole mass spectrometer. Evidence is presented to demonstrate that [C(3)H(4)N](+) is produced from an ion/molecule reaction between [C(2)H(2)N](+) (m/z 40) and neutral acetonitrile, via a [C(4)H(5)N(2)](+) (m/z 81) intermediate. Loss of HCN, where the H atom arises from neutral acetonitrile and the CN group from [C(2)H(2)N](+) (m/z 40), leads to the production of [C(3)H(4)N](+) (m/z 54). These results are consistent with a proposed concerted elimination of HCN, generating m/z 54 as a methylene vinylidene ammonium ion. Copyright 1999 John Wiley & Sons, Ltd.