Synthesis and characterization of redox-active ruthenium complexes with pendant alkyl-thiol ligands designed to form self-assembled monolayers on gold

The synthesis and characterization of ruthenium(II) complexes designed to give redox-active self-assembled monolayers (SAMs) via the thiol-on-gold route are presented here. The first of these compounds is the synthetically versatile [Ru(Me4bpp)(R2mal)(L)]+ complex, where Me4bpp is 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine, R2mal is a beta-diketonate, and in this study, L is a long-chain nitrile-thiol ligand. This work represents the first reported use nitriles as ligands to form metal complexes appropriate for SAMs. A hybrid synthetic approach, which proceeded by synthesis of the free ligand to a precursor nitrile-thioacetate followed by coordination to the metal center and conversion to the desired thiol, minimized the occurrence of undesired oxidation of the terminal thiol to the disulfide and eliminated the existence of a persistent impurity created during ligand synthesis. 1H- and 13C-NMR techniques proved essential to following ligand transformations remote from the metal center after coordination and were useful in identifying the presence or lack of the undesired disulfide.;A second approach to redox-active complexes based on bis(bipyridine) ruthenium(II) complexes utilized beta-ketonato ligands substituted with long alkyl chains containing terminal functional groups. The synthesis of these complexes required modification of the hybrid synthesis utilized for the nitrile-thiol complexes resulting in coordination of a diketonate containing a terminal bromide rather than the terminal thioacetate utilized for the nitriles. A synthetic scheme was devised and implemented to achieve conversion of the terminal bromide of these ligands to the thioacetate. A short chain model of these complexes based upon 3-ethyl-2,4-pentanedionate displayed interesting NMR characteristics attributed to the chiral Ru(bpy)2 center.;The ability to coordinate the alkyl substituted beta-diketonato ligands to ruthenium(II) centers containing the tridentate Me4bpp ligand was also explored. While coordination of the diketonates proved successful, the products contained impurities resulting from changes in the inner-coordination sphere of the metal complex, as deduced from the high redox potentials observed by solution cyclic voltammetry. NMR confirmed successful synthesis of complexes of this type with acetonitrile and pyridine substituted at the sixth coordination site; however, the low yields of these complexes hampered full characterization and suggest that this might not be an ideal route to metal complexes for the formation of SAMs.