Electronic, redox, and photophysical consequences of metal-for-carbon substitution in oligo-phenylene-ethynylenes.

The electronic structures, redox chemistry, and excited-state properties of tungsten-containing oligo-phenylene-ethynylenes (OPEs) of the form W[C(p-C6H4CC)n-1Ph](dppe)2Cl (n = 1-5; dppe =1,2-bis(diphenylphosphino)ethane) are reported and compared with those of organic analogues in order to elucidate the effects of metal-for-carbon substitution on OPE bonding and electronic properties. Key similarities between the metallo- and organic OPEs that bear on materials-related functions include their nearly identical effective conjugation lengths, reduction potentials, and π* orbital energies and delocalization. In addition to these conserved properties, the tungsten centers endow OPEs with reversible one-electron oxidation chemistry and long-lived emissive triplet excited states that are not accessible to organic OPEs. The electronic similarities and differences between metallo- and organic OPEs can be understood largely on the basis of π/π* orbital energy matching between tungsten and organic PE fragments and the introduction of an orthogonal mid-π/π*-gap d orbital in metallo-OPEs. These orbital energies can be tuned by varying the supporting ligands; this provides a means to rationally implement and control the emergent properties of metallo-OPE materials.

Oxidation-potential tuning of tungsten-alkylidyne complexes over a 2 V range.

The electrochemistry and electronic structures of over 30 tungsten-alkylidyne compounds of the form W(CR)L(n)L'(4-n)X (R = H, Bu(t), Ph, p-C6H4CCH, p-C6H4CCSiPr(i)3; X = F, Cl, Br, I, OTf, Bu(n), CN, OSiMe3, OPh; L/L' = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)3, CO, CNBu(t), py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal-ligand interactions. The first oxidation potentials are found to span a range of ∼2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal d(xy) orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the d(xy) orbital, being much more sensitive to the nature of the equatorial L ligands (π symmetry, with respect to d(xy), ΔE1/2 ≅ 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to d(xy), ΔE(1/2) < 0.3 V/L). The oxidation potential is linearly correlated with the calculated d(xy) orbital energy (slope ≅ 1, R(2) = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.

Synthesis, structures, bonding, and redox chemistry of ditungsten butadiyne complexes with W[triple bond]C-C[triple bond]W backbones.

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