[Show abstract][Hide abstract] 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.
Journal of the American Chemical Society 04/2011; 133(16):6206-22. DOI:10.1021/ja1093502 · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic interaction between a porphyrin-coordinated Co ion and a silver surface was switched by an axial nitric oxide (NO) ligand on the Co ion. This process is reversible and was monitored with X-ray and UV-photoelectron spectroscopy (XPS and UPS). The Co-Ag interaction leads to an electron transfer from the Ag surface to the Co ion and to a new electronic state close to the Fermi level. Both effects vanish if an axial NO ligand coordinates to the Co ion, indicating that the Co-Ag interaction is suppressed. As a possible explanation, we discuss a competition, similar to the trans effect, between the two axial ligands on the Co ion: the NO molecule and the Ag surface. These findings have important implications for sensor technology and for catalysis on supported metalloporphyrins, in which the catalytic activity of the metal center is controlled by the interaction with the underlying surface.
Journal of the American Chemical Society 11/2007; 129(40):12110-1. DOI:10.1021/ja0756725 · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).
Journal of the American Chemical Society 09/2007; 129(30):9476-83. DOI:10.1021/ja072360t · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the first example of a surface-confined two-step synthesis of axially coordinated metalloporphyrin complexes in an ultrahigh vacuum environment. Specifically, a monolayer of tetraphenylporphyrin on an Ag(111) surface was metalated with coadsorbed Zn atoms, and thereafter, NH3 ligands were attached to the metal centers. The surface reactions were monitored with X-ray photoelectron spectroscopy. The approach outlined in this work can be employed to produce and study adsorbates of various axially coordinated porphyrin complexes, including biologically relevant systems.
The Journal of Physical Chemistry C 04/2007; 111(16). DOI:10.1021/jp071531d · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that adsorbed meso-tetraphenylporphyrin molecules can coordinate Zn atoms that are pre-deposited on an Ag(111) surface, forming a complex that is identical to directly deposited tetraphenylporphyrinato-zinc(II); this reaction, which we studied with XPS, is the first example of an oxidative dissolution of a metal by a large organic ligand under ultrahigh vacuum conditions.
Chemical Communications 03/2007; DOI:10.1039/b614427f · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that well-defined monolayers of a metal complex on a surface can be prepared by direct vapor deposition of the metal atoms on monolayers of the ligand. In particular, ordered monolayers of adsorbed tetraphenylporphyrin (2H-TPP) on a silver surface were exposed to cobalt vapors, resulting in the complexation of the metal by the porphyrin. The formation of the metal complexes was monitored by means of X-ray photoelectron spectroscopy (XPS), which reveals that this metalation reaction leads to a chemical equivalence of all four nitrogen atoms. The described in situ metalation provides a convenient way to produce adsorbed monolayers of more reactive (e.g., air- or solvent-sensitive) or thermally unstable metalloporphyrins that are difficult to evaporate or even to obtain as pure compounds at room temperature.
Journal of the American Chemical Society 06/2006; 128(17):5644-5. DOI:10.1021/ja0610333 · 11.44 Impact Factor