Metal/Metal Redox Isomerism Governed by Configuration

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.

Thiocarbonylphosphorane and arsorane ligands

Independently unstable thiocarbonylphosphorane or arsorane ligands SCAR₃ (A = P, As) are observed in the salts [W(η²-C,S-SCAR₃)(CO)₂(Tp*)]PF₆ [AR₃ = PCy₃, PMe_nPh_3-n: n = 0, 1, 2, AsMePh₂; Tp* = tris(dimethylpyrazolyl)borate] which arise from the reactions of phosphonio- or arsoniocarbynes [W(≡CAR₃)(CO)₂(Tp*)]⁺ with sulfur.

Development and application of redox-active cyclometallating ligands based on W(II) alkyne complexes

The assembly of dinuclear complexes with the ultimate smallest ditopic ligands based on side-on complexes of methyl substituted alkynes is presented. In fact, the coordination of Ru(II) and Ir(III) by a W alkyne complex based cyclometallating metalla-ligand causes close intermetallic electronic cooperation, which substantially changes the electrochemical and photophysical properties of the single isolated moieties.

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.

Metal/Metal Redox Isomerism Governed by Configuration

A pair of diastereomeric dinuclear complexes, [Tp′(CO)BrW{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)(PPh₃)], in which W and Ru are bridged by a phosphinyl(thiolato)alkyne in a side‐on carbon P,S‐chelate coordination mode, were synthesized, separated and fully characterized. Even though the isomers are similar in their spectroscopic properties and redox potentials, the like‐isomer is oxidized at W while the unlike‐isomer is oxidized at Ru, which is proven by IR, NIR and EPR‐spectroscopy supported by spectro‐electrochemistry and computational methods. The second oxidation of the complexes was shown to take place at the metal left unaffected in the first redox step. Finally, the tipping point could be realized in the unlike isomer of the electronically tuned thiophenolate congener [Tp′(CO)(PhS)W{μ‐η²‐C,C′‐κ²‐S,P‐C₂(PPh₂)S}Ru(η⁵‐C₅H₅)‐(PPh₃)], in which valence trapped W^III/Ru^II and W^II/Ru^III cationic species are at equilibrium.