Efficient Chirality-Induced Spin Selectivity in Self-Assembled Monolayers of Ru25+ Paddlewheel Complexes

The use of coordination compounds provides a promising strategy to unravel the factors governing chirality-induced spin selectivity (CISS), particularly the contribution of spin-orbit coupling (SOC) to its magnitude. Here, we report the formation of self-assembled monolayers on gold substrates of a chiral paramagnetic Ru2 5+ paddlewheel complex, [Ru2(NCS){mu-(S)- or (R)-pycsa}2(mu-OAc)2] (pycsa = N-(pyridin-2-yl)-10-camphorsulfonamidate), denoted as (S)- or (R)-RuNCS. The incorporation of camphor groups confers point chirality to the molecule and causes a slight twisting around the Ru2 unit, introducing also a helical chirality, as evidenced by single-crystal X-ray diffraction and dichroic electronic absorption. The complex features an axial ligand bearing a terminal sulfur atom that serves as a robust anchoring site to the metallic surface. The resulting SAMs are homogeneous, stable, and exhibit very reproducible electrical characteristics. Magnetic-conductive atomic force microscopy measurements on enantiopure Ni/Au-RuNCS junctions reveal remarkable spin polarizations reaching values from 60 to 80%, which is particularly noteworthy considering the monolayer nature of the film and the modest helicity of the paddlewheel motif.