Dipolar Molecular Motor Candidates with Planar Chirality

Two molecular motor candidates built around ruthenium(II) centers featuring both planar chirality and a permanent dipole have been synthesized in seven steps. These motors incorporate desymmetrized cyclopentadienyl ligands bearing electron-donating (4-methoxyphenyl) and electron-withdrawing (3,5-bis(trifluoromethyl)phenyl) groups in 1,2 or 1,3 relative positions, to induce strong dipolar moments and asymmetry helpful for unidirectional rotation. Two dipolar pentaarylcyclopentadienyl ligands were synthesized and coordinated to a Ru(II) trisindazolylborate complex functionalized with thioether moieties for surface anchoring. Structural analysis and DFT calculations revealed that dipole moments increase upon complexation, reaching 5.60 D and 4.58 D for the 1,2-Cp^Ar5[Ru]Tp and 1,3-Cp^Ar5[Ru]Tp complexes, respectively. Key synthetic challenges, such as low yields in Knoevenagel condensations and side reactions, were addressed through careful modulation of precursors, reaction conditions and cation selection. This study demonstrates the feasibility of integrating permanent dipoles and chirality into molecular motor design, which could offer an enhanced control over motion under STM-induced electric fields.