Abstract
Macrocyclic host mols. bound to electrode surfaces enable the complexation of catalytically active guests for mol. heterogeneous catalysis. The authors present a surface-anchored host-guest complex with the ability to electrochem. oxidize NH3 in both organic and aqueous solutions With an adamantyl motif as the binding group on the backbone of the mol. catalyst [Ru(bpy-NMe2)(tpada)(Cl)](PF6) (1) (bpy-NMe2 is 4,4′-bis(dimethylamino)-2,2′-bipyridyl, tpada is 4′-(adamantan-1-yl)-2,2′:6′,2′′-terpyridine), high binding constants with β-cyclodextrin were observed in solution (in DMSO-d6: D2O (7:3), K11 = 492 ± 21 M-1). The strong binding affinities were also transferred to a mesoporous ITO (mITO) surface functionalized with a phosphonated derivative of β-cyclodextrin. The newly designed catalyst (1) was compared to the previously reported naphthyl-substituted catalyst [Ru(bpy-NMe2)(tpnp)(Cl)](PF6) (2) (tpnp is 4′-(naphthalene-2-yl)-2,2′:6′,2′′-terpyridine) for its stability during catalysis. Despite the insulating nature of the adamantyl substituent serving as the binding group, the stronger binding of this unit to the host functionalized electrode and the resulting shorter distance between the catalytic active center and the surface led to better performance and higher stability. Both guests are able to oxidize NH3 in both organic and aqueous solutions and the host-anchored electrode can be refunctionalized multiple times (>3) following loss of the catalytic activity, without a reduction in performance. Guest 1 exhibits significantly higher stability in comparison to guest 2 toward basic conditions, which often constitutes a challenge for anchored mol. systems. NH3 oxidation in H2O led to the selective formation of NO3- with faradaic efficiencies of up to 100%.
Roithmeyer, Helena