The Surface Trans Effect: Influence of Axial Ligands on the Surface Chemical Bonds of Adsorbed Metalloporphyrins

Abstract

The chemical bond between an adsorbed, laterally coordinated metal ion and a metal surface is affected by an additional axial ligand on the metal ion. This surface analogon of the trans effect was studied in detail using monolayers of various M(II)-tetraphenylporphyrins (MTTPs, M = Fe, Co, Zn) and their nitrosyl complexes on a Ag(111) surface. X-ray photoelectron spectroscopy (XPS) shows that the oxidation state of the Fe and Co (but not Zn) ions in the MTPP monolayers is reduced because of the interaction with the substrate. This partial reduction is accompanied by the appearance of new valence states in the UV photoelectron and scanning tunneling spectra (UPS and STS), revealing the covalent character of the ion-substrate bond. Subsequent coordination of nitric oxide (NO) to the metal ions (Fe, Co) reverses these surface-induced effects, resulting in an increase of the oxidation states and the disappearance of the new valence states. Removal of the NO ligands by thermal desorption restores the original spectroscopic features, indicating that the described processes are fully reversible. The NO coordination also changes the spin state and thus the magnetic properties of the metal ions. Density-functional theory (DFT) calculations on model systems provide structural and energetic data on the adsorbed molecules and the surface chemical bond. The calculations reveal that competition effects, similar to the trans effect, play a central role and lead to a mutual interference of the two axial ligands, NO and Ag, and their bonds to the metal center. These findings have important implications for sensor technology and catalysis using supported planar metal complexes, in which the activity of the metal center is sensitively influenced by the substrate.

Real-time electrical detection of nitric oxide in biological systems with sub-nanomolar sensitivity

“…In this case, graphene donates p-electrons to the d-orbital of Fe(III) through a cation-p interaction. When NO molecules come to coordinate with and donate electrons to Fe(III) centres, those p-electrons are partially transferred back to the valence band of graphene because of the trans-effect of the ligands 45 . Therefore, the binding of NO to Fe(III) centres is equivalent to partial electron-doping to hemin-functionalized graphene.…”

Adsorption of cobalt (II) 5,10,15,20-tetrakis(2-aminophenyl)-porphyrin onto copper substrates: Characterization and impedance studies for corrosion inhibition

“…The peripheral phenyl rings of the porphyrin rotate towards the molecular plane, because of steric repulsions between the phenyl rings and the Cu surface [39].…”

Emergence of On-Surface Magnetochemistry

“…Also, the peak width in XPS allows one to conclude on the presence or absence of unpaired electrons and has been used to study on-surface coordination chemistry on nonmagnetic substrates. 4,5,24,25 To unveil the origin of the molecule−surface exchange interaction in the presence or absence of axial ligands, numerical modeling based on density functional theory (DFT), supplemented with an additional Coulomb U term (DFT+U) has been performed. In DFT+U, the strong Coulomb interactions between the d electrons in the open d shell of the metal ion are captured by an additional Hubbard U and exchange constant J.…”