Diruthenium–Polyyn-diyl–Diruthenium Wires: Electronic Coupling in the Long Distance Regime

Reported
herein is a series of Ru2(Xap)4 capped
polyyn-diyl compounds, where Xap is either 2-anilinopyridinate (ap)
or its aniline substituted derivatives. Symmetric [Ru2(Xap)4]­(μ-C4k)­[Ru2(Xap)4] (compounds 4ka (X = 3-isobutoxy) and 4kc (X = 3,5-dimethoxy) with k = 2, 3, 4,
and 5) was obtained from the Glaser coupling reaction of Ru2(Xap)4(C2kH). Unsymmetric
[Ru2(Xap)4]­(μ-C4k+2)­[Ru2(ap)4] (compounds 4k+2b with k = 2, 3, and 4) were prepared from the Glaser
coupling reaction between Ru2(Xap)4(C2k+2H) and Ru2(ap)4(C2kH). X-ray diffraction study of compound 12c revealed both the sigmoidal topology of the polyyn-diyl bridge and
the fine structural detail about the Ru2 cores. Cyclic
and differential pulse voltammetric (CV and DPV) measurements and
spectroelectrochemical studies revealed that (i) the reduced monoanions
[Ru2–C2m–Ru2]−1 (m = 4–8) belong
to the Robin–Day class II mixed valent ions and (ii) the electronic
coupling between Ru2 termini depends on the length of the
polyyn-diyl bridge with an attenuation constant (γ) between
0.12 and 0.15 Å–1. In addition, spin-unrestricted
DFT calculations provide insight about the nature of orbitals that
mediate the long distance electronic coupling.