Tuning the potential of redox-active diphosphine ligands based on the alkyne complexes [Tp*W(CO)L{η2-C2(PPh2)2}]

Eleven complexes with the general formula [Tp*W(CO)L{η²-C₂(PPh₂)₂}]ⁿ⁺ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)-borate, L = F^-, Cl^-, Br^-, I^-, MeS^-, PhS^-, pyCH₂S^-, CN^- and TfO^-; n = 0 and L = CH₃CN and pyridine; n = 1} have been synthesized and fully characterized. Depending on L, the oxidation process from W(II) to W(III) is detected between -0.28 and +0.55 V vs. Fc/Fc⁺ and the spectroscopic properties (X-ray, IR, and NMR) are influenced according to the electron-rich or electron-poor character of the central metal. The basicity of the alkyne complex-based phosphine groups was estimated by the ³¹P/⁷⁷Se coupling method of the corresponding diselenides. Selected examples of the dppa-complex ligands were converted into the corresponding κ²-PdCl₂ chelate complexes and employed in a Sonogashira reaction in order to estimate the effect of L on the catalytic behaviour of the dinuclear complexes. While the spectroscopic properties show a good correlation with the redox potential in a mostly linear fashion, catalytic activity is influenced only slightly. The effect of PdCl₂ coordination on the alkyne complex is evident from the W(II)/W(III) redox potentials measured by cyclic voltammetry supported by a change of the CO stretching frequency in IR. A comparison of the molecular structures of the alkyne complexes with terminal phosphine groups and the PdCl₂ chelate complexes all determined by XRD shows the essential flexibility of the bend-back angles in the alkyne complex moiety.

Migratory insertion of isocyanide into a ketenyl-tungsten bond as key step in cyclization reactions

Treatment of the side-on tungsten alkyne complex of ethinylethyl ether [Tp*W(CO)2(η2-C,C'-HCCOCH2CH3)]+ {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} (2a) with n-Bu4NI afforded the end-on ketenyl complex [Tp*W(CO)2(κ1-HCCO)] (4a). This formal 16 ve complex bearing the prototype of a ketenyl ligand is surprisingly stable and converts only under activation by UV light or heat to form a dinuclear complex [Tp*2W2(CO)4(μ-CCH2)] (6). The ketenyl ligand in complex 4a underwent a metal template controlled cyclization reaction upon addition of isocyanides. The oxametallacycles [Tp*W(CO)2{κ2-C,O-C(NHXy)C(H)C(Nu)O}] {Nu = OMe (7), OEt (8), N(i-Pr)2 (9), OH (10), O1/2 (11)} were formed by coordination of Xy-NC (Xy = 2,6-dimethylphenyl) at 4a and subsequent migratory insertion (MI) into the W-ketenyl bond. The resulting intermediate is susceptible to addition reactions with protic nucleophiles. Compounds 2a-PF6, 4a/b, and 7-11 were fully characterized including XRD analysis. The cyclization mechanism has been confirmed both experimentally and by DFT calculations. In cyclic voltammetry, complexes 7-9 are characterized by a reversible W(ii)/W(iii) redox process. The dinuclear complex 11 however shows two separated redox events. Based on cyclic voltammetry measurements with different conducting electrolytes and IR spectroelectrochemical (SEC) measurements the W(ii)/W(iii) mixed valent complex 11+ is assigned to class II in terms of the Robin-Day classification.

Facile Synthesis of a Stable Side-on Phosphinyne Complex by Redox Driven Intramolecular Cyclisation

Alkyne complexes with vicinal substitution by a Lewis acid and a Lewis base at the coordinated alkyne are prospective frustrated Lewis pairs exhibiting a particular mutual distance and, hence, a specific activation potential. In this contribution, investigations on the generation of a WII alkyne complex bearing a phosphine as Lewis base and a carbenium group as Lewis acid are presented. Independently on potential substrates added, an intramolecular cyclisation product was always isolated. A subsequent deprotonation step led to an unprecedented side-on λ5 -phosphinyne complex, which is interpreted as highly zwitterionic according to visible absorption spectroscopy supported by TD-DFT. Low-temperature 31 P NMR and EPR spectroscopic measurements combined with time-dependent IR-spectroscopic monitoring provided insights in the mechanism of the cyclisation reaction. Decomposition of the multicomponent IR spectra by multivariate curve resolution and a kinetic hard-modelling approach allowed the derivation of kinetic parameters. Assignment of the individual IR spectra to potential intermediates was provided by DFT calculations.

Phosphaisonitrile umpolung – synthesis and reactivity of chloro aminophosphino carbynes

The synthesis of the first P-halo-aminophosphinocarbyne complex is described in addition to exploration of its ligand-based reactivity towards nucleophilic substitution and P-halide abstraction processes as a possible route to cationic aminophosphaisonitrile derivatives.

Acetylene in Organic Synthesis: Recent Progress and New Uses

Recent progress in the leading synthetic applications of acetylene is discussed from the prospect of rapid development and novel opportunities. A diversity of reactions involving the acetylene molecule to carry out vinylation processes, cross-coupling reactions, synthesis of substituted alkynes, preparation of heterocycles and the construction of a number of functionalized molecules with different levels of molecular complexity were recently studied. Of particular importance is the utilization of acetylene in the synthesis of pharmaceutical substances and drugs. The increasing interest in acetylene and its involvement in organic transformations highlights a fascinating renaissance of this simplest alkyne molecule.