Facet-Dependent Reduction Reaction of Diruthenium Metal-String Complexes by Face-to-Face Linked Gold Nanocrystals

The facet-dependent redox reactions of diruthenium metal-string complexes by gold nanoparticles (AuNPs) are explored by using the surface-enhanced Raman scattering (SERS) technique. Gold nano-rhombic dodecahedrons (AuRDs), gold nanocubes (AuNCs), and gold octahedrons (AuOhs) with exclusive facets {110}, {100}, and {111}, respectively, were synthesized. These AuNPs linked face-to-face by metal-string complexes Ru₂M(dpa)₄Cl₂ (dpa = dipyridyl amino, M = Ni, Cu) with chloride axial ligands serve as both SERS substrates and reducing agents in the reactions. We employ the diruthenium core in these complexes with multiple redox states to study the reduction ability of varied AuNP facets upon plasmonic excitation. In Ru₂Ni(dpa)₄Cl₂, the Ru-Ru stretching mode ν_Ru-Ru str. lies at 327 cm^-1 on the SERS substrate AuOh, but this band shifts to 313 cm^-1 on the AuRD and AuNC. The diruthenium moiety was reduced to [Ru₂]⁴⁺ by the AuRD facet {110} and the AuNC {100}. The gold nanorods in the solution prepared with metal-string complexes bridging head-to-head on {111} facets were used for the SERS substrate. The SERS curves of the complexes in these self-assembled head-to-head rods display ν_Ru-Ru str. at 327 cm^-1, which is assigned to having an [Ru₂]⁵⁺ core. Hence, facets {110} and {100} have a reduction reactivity greater than that of {111}. In Ru₂Cu(dpa)₄Cl₂, the ν_Ru-Ru str. is observed to lie at 312 cm^-1 on AuRD, but shifts to 320 cm^-1 on the AuNC and AuOh. In the latter cases, the diruthenium moiety was reduced to having a charge of 4+ with electronic configuration π*²δ*², whereas the former case band at 312 cm^-1 with a weaker Ru-Ru bonding is also attributed to [Ru₂]⁴⁺ but with electron configuration π*⁴. π*⁴ lies at an energy greater than π*²δ*². The electrochemical SERS spectra of diruthenium complexes were recorded to verify their oxidation states. Conclusively, these results yield the reduction reactivity of the following facet: {110} > {100} > {111}. According to the results of the redox reactions, the valence states of the diruthenium metal-string complexes are verified. In the [Ru₂] core, n = 4 π*⁴, 4 π*²δ*², 5 π*²δ*, and 6 π*δ*, and the ν_Ru-Ru str. is 312, 320, 327, and 337 cm^-1, respectively.