Electrochemistry of multiply metal-metal and metal-ligand bonded building blocks for conjugated materials

The study of electronically conjugated compounds containing metal-metal and/or metal-ligand multiple bonds is currently of great interest due to the potential of these compounds to function as molecular wires and as models to study electron transfer. Underrepresented are detailed electrochemical studies to understand the effects of ancillary ligands on the redox potentials of these complexes.; We have focused our attention on two classes of compounds containing $M-4M$ and M≡C bonds that can be used as building blocks for novel transition metal containing conjugated systems, the monometallic tungsten-alkylidynes of the type W(≡CR)L_4−nL_n ′X (R = H, But, Ph, C₆H₄-4-R′; L, L′ = PMe₃, 1/2 dmpe, 1/2 depe, 1/2 dppe, P(OMe)₃, CO, N≡CBut; X = F, Cl, Br, I, Bu n, OSiMe₃, OTf, C≡CR′) and the dimetallotetraynes of the type M₂(C≡CR)₄(PMe ₃)₄ (M = Mo, W; R = H, Me, Pri, But, Ph, SiMe₃). The tungsten-alkylidyne complexes exhibit a (quasi)reversible one-electron oxidation process that is highly ligand dependent, spanning a range of ca. 1.5 V. Bimetallic tungsten-alkylidyne complexes can be assembled from the monometallic tungsten-alkylidynes in either a head-to-head or head-to-tail fashion with respect to the W≡C bond, being oxidized at potentials similar to their monometallic analogs, thus demonstrating designed redox tunability. Electrochemical studies on the dimetallotetraynes have provided strong evidence for the alkynyl ligand behaving as a π-accepting ligand in these complexes due to the strong coupling between the δ/δ* and π/π* orbitals. Additionally, the reduction and oxidation potentials of these complexes do not correlate with 1(δ → δ*) transition energies as has been observed for Mo₂X₄(PR ₃)₄ complexes. However, there is a correlation observed between 1(δ → δ*) transition energies and ΔE_1/2 +/−. Spectroelectrochemical studies on M₂ X₄(PR₃)₄ (M = Mo, W; X = halogen; R = alkyl) complexes were also conducted to provide estimates of K, the two-electron exchange integral, as a function of halogen, alkyl group on the phosphine, and metal, as the energies of the 1(δ → δ*) transition occur at roughly twice the energy obtained from electronic-absorption measurements and predicted from theoretical calculations.