Ligand exchange of metal carbonyls by chain mechanisms. Electrochemical kinetics of electron transfer catalysis

The metal carbonyl derivatives LM(CO)_n of manganese, rhenium, molybdenum and tungsten undergo facile ligand substitution by an electrode-mediated process. Phosphine, pyridine and isocyanaide substitutions are shown to be chain reactions by coulometric analysis and by cyclic voltammetry of the metal carbonyl solutions containing the added nucleophiles L. Reversible electrochemical parameters can be obtained for both LM(CO)_n and the substitution product LM(CO)_n. These allow the digital simulation of the cyclic voltammograms by Feldberg's method, which provides a detailed analysis of the kinetics of the electrocatalytic mechanism for ligand substitution. Generally, the radical cation LM(CO)_n⁺ produced initially at the anode undergoes rapid exchange with L to afford the cationic substituted species LM(CO)_n⁺. This step is followed by electron transfer with the reactant LM(CO)_n to yield the substitution product LM(CO)_n and regenerate the radical cation LM(CO)_n⁺, which completes the chain propagation sequence. Multiple repetition of this cycle is indicated by the high current efficiencies which are obtainable. The second order rate constants for the ligand exchange of the 17-electron radical cations LM(CO)_n⁺ with added L are evaluated, and found to be more than 10⁶ times larger than that for the neutral, diamagnetic precursor LM(CO)_n. The radical cations LM(CO)_n⁺ also react readily with phosphine and alkyl isocyanide nucleophiles, leading to a characteristic distortion of the CV waves. Digital simulation of such cyclic voltammograms allows the kinetics of these processes, particularly the redox catalysis in the oxidation of phosphines by LM(CO)_n⁺, to be evaluated quantitatively. The enhanced reactivity of the 17-electron radical cations LM(CO)_n⁺ is discussed in relationship to recent reports of the substitution lability in other 17- and 19-electron metal carbonyls.     

The charge-transfer motif in crystal engineering. Self-assembly of acentric (diamondoid) networks from halide salts and carbon tetrabromide as electron-donor/acceptor synthons

Unusual strength and directionality for the charge-transfer motif (established in solution) are shown to carry over into the solid state by the facile synthesis of a series of robust crystals of the [1:1] donor/acceptor complexes of carbon tetrabromide with the electron-rich halide anions (chloride, bromide, and iodide). X-ray crystallographic analyses identify the consistent formation of diamondoid networks, the dimensionality of which is dictated by the size of the tetraalkylammonium counterion. For the tetraethylammonium bromide/carbon tetrabromide dyad, the three-dimensional (diamondoid) network consists of donor (bromide) and acceptor (CBr(4)) nodes alternately populated to result in the effective annihilation of centers of symmetry in agreement with the sphaleroid structural subclass. Such inherently acentric networks exhibit intensive nonlinear optical properties in which the second harmonics generation in the extended charge-transfer system is augmented by the effective electronic (HOMO-LUMO) coupling between contiguous CBr(4)/halide centers.     

Isolation and spectral characteristics of Ferric Iodide

Ferric iodide is isolated pure for the first time as a dark purple solid when a hexane solution of diiodotetracarbonyliron(II) and diiodine is exposed to actinic radiation. Its identity is confirmed by elemental analysis and quantitative conversion to the known tetraiodoferrate(III) by treatment with tetra-n-butylammonium iodide. Although persistent in the solid state, ferric iodide is readily decomposed in solution to ferrous iodide and 0.5 mol diiodine quantitatively.     

Electron transfer activation of the Diels-Alder reaction: Quantitative relationship to charge transfer excited states

The Diels-Alder cycloaddition of anthracene to tetracyanoethylene (TCNE) is quantitatively compared to alkylmetal insertion under the same reaction conditions. In both systems, the observation of transient charge transfer (CT) absorption bands is related to the presence of 1:1 electron donor-acceptor complexes of anthracene (Ar) and alkylmetal (RM) donors with the TCNE acceptor. The activation free energies ΔG3 for anthracene cycloaddition and alkylmetal insertion are found to be equal to the energies of ion-pair formation, i.e. [Ar⁺TCNE^?] and RM⁺TCNE^?], which are evaluated from the CT transition energies hν_CT. Indeed, the differences in the rates of alkylmetal insertion and anthracene cycloaddition by a factor of more than 10⁹, are shown quantitatively to arise from the differences in ion-pair solvation ΔG^s. The same differences in ΔG^s also apply quantitatively to the free ions, [Ar⁺] and [RM⁺], independently derived from the electrochemical and iron(III) oxidations of alkylmetals and aromatic compounds, respectively, by outer- sphere electron transfer. The charge transfer formulation of the activation process but provides a unifying basis for comparing such diverse processes as Diels-Alder cycloadditions and organometal cleavages, when a common electron-deficient acceptor is employed. The relationship to the concerted mechanisms of the Diels-Alder reaction is discussed.     

N,N′‐Dimethylpyrazinediium bis(tetrafluoroborate) and N,N′‐diethylpyrazinediium bis(tetrafluoroborate): new examples of anion–π triads

Crystallization of N,N′‐dimethylpyrazinediium bis(tetrafluoroborate), C₆H₁₀N₂²⁺·2BF₄⁻, (I), and N,N′‐diethylpyrazinediium bis(tetrafluoroborate), C₈H₁₄N₂²⁺·2BF₄⁻, (II), from dried acetonitrile under argon protection has permitted their single‐crystal studies. In both crystal structures, the pyrazinediium dications are located about an inversion center (located at the ring center) and each pyrazinediium aromatic ring is π‐bonded to two centrosymmetrically related BF₄⁻ anions. Strong anion–π interactions, as well as weak C—H...F hydrogen bonds, between BF₄⁻ and pyrazinediium ions are present in both salts.     

Synthesis of anionic methylpalladium complexes with phosphine-sulfonate ligands and their activities for olefin polymerization

Reaction of diarylphosphinobenzene-2-sulfonic acids with tertially amines, followed by addition of [PdMeCl(cod)], provided anionic methylpalladium(II) complexes with bidentate phosphine-sulfonate ligands, which show high activity for copolymerization of ethylene with methyl acrylate.     

A Kinetic Investigation of Thermal Shrinkage of Aromatic Polymers by Thermomechanical Analysis

The kinetics of thermal shrinkage of the stretched polypyromellitimide (PI) films, Du Pont Kapton H, were investigated by thermomechanical analysis (TMA) at a constant rate of heating. The two-stage model of extended polymers has been applied to analyze the TMA data. The activation energy of the contraction reaction could be obtained from TMA curves at various heating rates. The one-step shrinkage of the 12.5% stretched PI film gives an activation energy of 10 kcal/mole for the first shrinking. From the 30% stretched sample, the second activation of contraction is 25 kcal/mole, and from 40% sample the third one is 33 kcal/mole. These three contraction reactions are observed successively for the TMA curves for highly stretched samples, and correspond to the various kinds of molecular motion of this special rigid polymer structure, which are also observed in the dynamic mechanical and dielectric properties of the same polymer.

Polypyromellitimide (PI) is a typical thermally stable polymer due to the rigid aromatic and heterocyclic ring structures in its backbone chain. This polymer is able to be cold-drawn in the glassy state, and it shrinks markedly on heating. The thermal shrinkage was investigated thermoanalytically by thermomechanical analysis (TMA) at a uniform heating rate. The activation energies of thermal shrinkage in three stages are estimated by this method.     

Molecular and electronic structures of the long-bonded pi-dimers of tetrathiafulvalene cation-radical in intermolecular electron transfer and in (solid-state) conductivity

Tetrathiafulvalene (TTF) as the prototypical electron donor for solid-state (electronics) applications is converted to the unusual cation-radical salt, TTF+* CB- (where CB- is the non-coordinating closo-dodecamethylcarboranate), for crystallographic and spectral analyses. Near-IR studies establish the spontaneous self-association of TTF+* to form the diamagnetic [TTF+,TTF+] dication and to also undergo the equally rapid cross-association with its parent donor to form the mixed-valence [TTF+*,TTF] cation-radical. The latter, most importantly, represents the first (dyad) member of a series of p-doped tetrathiafulvalene (stacked) arrays, and the thorough scrutiny of its electronic structure with the aid of Mulliken-Hush (two-state) analysis of the diagnostic (intervalence) NIR band reveals Robin-Day Class II behavior. The theoretical consequences of the unique structure of the mixed-valence [TTF+*,TTF] dyad on (a) the electron-transfer mechanism for self-exchange, (b) the molecular-orbital analysis of the Marcus reorganization energy, and (c) the ab initio computation of the coupling element or transfer integral in p-doped (solid-state) arrays are discussed.