Synthesis, Structures, Bonding, and Redox Chemistry of Ditungsten Butadiyne Complexes with W≡C—C≡W Backbones

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Complexes of the form XL(4)W[triple bond]C-C[triple bond]WL(4)X (L = 1/2 dmpe, 1/2 depe, P(OMe)(3); X = Cl, OTf) have been synthesized from (Bu(t)O)(3)WCCW(OBu(t))(3) in two steps via Cl(3)(dme)WCCW(dme)Cl(3), which undergoes facile four-electron reduction in the presence of L. The compounds possess formal d(2)-d(2) electron configurations. The molecular structures of Cl(dmpe)(2)WCCW(dmpe)(2)Cl and Cl{P(OMe)(3)}(4)WCCW{P(OMe)(3)}(4)Cl were determined by X-ray crystallography; bond distances within the backbone are consistent with a W[triple bond]C-C[triple bond]W canonical structure. Density-functional-theory calculations on Cl(dmpe)(2)WCCW(dmpe)(2)Cl and the model compound Cl(PH(3))(4)WCCW(PH(3))(4)Cl, and on their monometallic analogs W(CH)(dmpe)(2)Cl and W(CH)(PH(3))(4)Cl, indicate that the WCCW backbone is strongly pi-conjugated; this is supported by the observation of low-energy pi --> pi* transitions for the compounds. The calculations predict that delta symmetry d(xy)-derived orbitals should be (or lie near) the highest occupied molecular orbital. Consistent with this prediction, the electronic spectra of the compounds exhibit a band attributable to d(xy) --> pi* transition(s), as the lowest-energy feature and electrochemical studies demonstrate that they undergo sequential one-electron oxidations to produce (d(xy))(2)-(d(xy))(1) and (d(xy))(1)-(d(xy))(1) congeners. Due to the delta symmetry of the redox orbitals, the oxidized congeners maintain the W[triple bond]C-C[triple bond]W canonical structure of the parent d(2)-d(2) compounds. The first and second oxidation potentials of Cl(dmpe)(2)WCCW(dmpe)(2)Cl are separated by <or=0.4 V, corresponding to K(com) = 10(4)-10(6); the interaction between redox orbitals is largely electrostatic in origin and not the result of significant direct delta orbital overlap. The reaction between Cl(dmpe)(2)WCCW(dmpe)(2)Cl and HCl (2 equiv) produces the d(0)-d(0) dihydride ion [Cl(H)(dmpe)(2)WCCW(dmpe)(2)(H)Cl](2+), which is formulated as maintaining the W[triple bond]C-C[triple bond]W backbone on the basis of its X-ray crystal structure and NMR spectra. This family of WCCW derivatives expands the relatively small class of M[triple bond]C-C[triple bond]M compounds and is distinctive among them because their ancillary ligands should allow incorporation of the L(4)WCCWL(4) unit into interior positions of metalloyne oligomers and polymers.