Chapter 5
MnClTPP on Ag(111): Axial Ligand Transformations

3d transition metal porphyrins are ideal candidates for a large number of applications due to their rich coordination chemistry [206]. In particular, manganese-based porphyrin complexes have been shown to selectively catalyse the halogenation of C–H bonds [207] and are often used as catalysts for the chemical transformation of alkenes into epoxides [208, 209]. By using electronegative ligands, the metal atom can be oxidized from the +2 to the +3 oxidation state. Given the capabilities of porphyrins to bind and release gases and to act as an active center in catalytic reactions in biological systems, porphyrin-based films on surfaces are extremely appealing as chemical and gas sensors as well as nanoporous catalytic materials [210, 211].

In this chapter, STM, DFT calculations, XAS and XPS are used to study the reaction of molecular oxygen with the surface supported monolayer (ML) of (5,10,15,20-tetraphenylporphyrinato)Mn(III)Cl (MnClTPP). When deposited onto the Ag(111) surface, MnClTPP form a close-packed square monolayer. The molecules adopt a saddle conformation, with the axial Cl-ligand pointing out of the plane into the vacuum.

Upon annealing at 510K, the Cl ligand desorbs from the MnClTPP molecule, however the (III) oxidation state of the central Mn ion is unchanged. This oxidation state is stabilised through ligation by the substrate. Exposure of the Mn(III)TPP layer to molecular oxygen oxidises the metal centre to the Mn(IV) state, and the molecule binds both oxygen atoms as a peroxide, to form Mn(IV)O2TPP. Finally, this bidentate peroxide ligand can be removed by further annealing to 445K, refreshing the monolayer to the Mn(III)TPP/Ag(111) state.

 5.1 Experimental details
 5.2 MnClTPP
 5.3 MnClTPP self-assembly
  5.3.1 Bias dependence of axial ligand resolution
 5.4 Axial ligand transformation
  5.4.1 Removing the Cl-ligand
  5.4.2 Reaction with O2
 5.5 Conclusions