Conductive Self-Assembled Monolayers of Paramagnetic {Co^II } and { } Coordination Clusters on Gold Surfaces

Two polynuclear cobalt(II,III) complexes, [Co₅(N₃)₄(N-n-bda)₄(bza·SMe)₂] (1) and [Co₆(N₃)₄(N-n-bda)₂(bza·SMe)₅(MeOH)₄]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H₂bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C ₂-symmetric {Co^II $$ } core of two isosceles Co₃ triangles with perpendicularly oriented planes, sharing a central, high-spin Co^II ion residing in a distorted tetrahedral coordination environment. This central Co^II ion is connected to four outer, octahedrally coordinated low-spin Co^III ions via oxo bridges. Compound 2 comprises a semi-circular { $$ $$ } motif of four non-interacting high-spin Co^II and two low-spin Co^III centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {Cu^IILn^III} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co₅}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral Co^II centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co₁₀(N₃)₂(N-n-bda)₆(bza·SMe)₆] (3), containing 10 virtually non-interacting high-spin Co^II centers.