The surface trans effect: influence of axial ligands on the surface chemical bonds of adsorbed metalloporphyrins.

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.

Tetraphenylporphyrin picks up zinc atoms from a silver surface.

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.

NO-Induced Reorganization of Porphyrin Arrays.

We studied the interaction of a highly ordered array of Co-tetraphenylporphyrin (CoTPP) with NO on Ag(111) by in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Upon NO exposure, the initially quadratically ordered CoTPP layer reorganizes, showing a wealth of highly ordered NO+CoTPP coadsorbate phases with increasing size of the unit cell, interpreted as due to attractive lateral dipole-dipole interactions between the two species. The findings not only suggest a novel approach to control the arrangement of adsorbed porphyrins in particular but also should generally be considered in the production of functional layers from large organic molecules under ambient conditions or after exposure to small electronegative molecules.

Understanding the contrast mechanism in scanning tunneling microscopy (STM) images of an intermixed tetraphenylporphyrin layer on Ag(111).

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.

Principle and mechanism of direct porphyrin metalation: joint experimental and theoretical investigation.

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).

Direct synthesis of a metalloporphyrin complex on a surface.

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.