Control of the axial coordination of a surface-confined manganese(III) porphyrin complex

Abstract

The organization and thermal lability of chloro(5,10,15,20-tetraphenyl porphyrinato)manganese(III) (Cl-MnTPP) molecules on the Ag(111) surface have been investigated under ultra-high vacuum conditions, using scanning tunnelling microscopy, low energy electron diffraction and x-ray photoelectron spectroscopy. The findings reveal the epitaxial nature of the molecule-substrate interface, and moreover, offer a valuable insight into the latent coordination properties of surface-confined metalloporphyrins. The Cl-MnTPP molecules are found to self-assemble on the Ag(111) surface at room temperature, forming an ordered molecular overlayer described by a square unit cell. In accordance with the threefold symmetry of the Ag(111) surface, three rotationally equivalent domains of the molecular overlayer are observed. The primitive lattice vectors of the Cl-MnTPP overlayer show an azimuthal rotation of ±15° relative to those of the Ag(111) surface, while the principal molecular axes of the individual molecules are found to be aligned with the substrate (0(-)11) and ((-)211) crystallographic directions. The axial chloride (Cl) ligand is found to be orientated away from the Ag(111) surface, whereby the average plane of the porphyrin macrocycle lies parallel to that of the substrate. When adsorbed on the Ag(111) surface, the Cl-MnTPP molecules display a latent thermal lability resulting in the dissociation of the axial Cl ligand at ~423 K. The thermally induced dissociation of the Cl ligand leaves the porphyrin complex otherwise intact, giving rise to the coordinatively unsaturated Mn(III) derivative. Consistent with the surface conformation of the Cl-MnTPP precursor, the resulting (5,10,15,20-tetraphenyl porphyrinato)manganese(III) (MnTPP) molecules display the same lattice structure and registry with the Ag(111) surface.

Zinc(II) Tetraphenylporphyrin on Ag(100) and Ag(111): Multilayer Desorption and Dehydrogenation

“…Similar ordering has been found for TPPs on metal surfaces, and is primarily driven by the intermolecular phenylphenyl interactions. 18,19,22,24,25,[27][28][29][30]32,34,35,39,52,64 NEXAFS measurements performed by other groups indicate that the phenyl groups are typically oriented approximately 60 • with respect to the porphyrin macrocycle when bound to the substrate in a densely-packed monolayer, rather than nearly perpendicular as in the gas phase. 26,32 The phenyl groups of neighboring 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 molecules may take the form of so-called T-stacking, i.e., perpendicular to one another, or π − π stacking, i.e., parallel to one another.…”

“…A variety of techniques were used such as scanning tunneling microscopy (STM) to gain insight into adsorption geometries, near edge X-ray absorption fine structure (NEXAFS) to elucidate intramolecular features such as the relative orientation of the phenyl groups with respect to the macrocycle, 25,26,32,[40][41][42][43][44][45] Xray, 17,21,33,34,[37][38][39][41][42][43][44][45][46][47] ultraviolet, 17,28,34,40,46,47 and inverse 17,28,40 photoemission spectroscopy (XPS, UPS, and IPS) to probe directly the electronic structure and chemistry, and temperature programmed desorption (TPD) to track molecular desorption and hydrogen evolution with temperature. 27,33,48 On metal surfaces, due a strong porphyrin core iminic nitrogenssubstrate interaction, the mesophenyls are typically rotated and can even become coplanar with the porphyrin core.…”

Surface chemistry of porphyrins and phthalocyanines

“…Since the resulting positive excess charge at the Mn(III) ion cannot be fully compensated by the porphyrin ligand with its formal -2 charge, it was suggested that the complex is neutralized by electron transfer from the substrate [776]. A later study of the same (Cl)MnTPP/Ag(1 1 1) system arrived at rather different results: Dissociation of the Cl-Mn bond required temperature above 500 K and the remaining complex was a Mn(II) porphyrin.…”

Detection of different oxidation states of individual manganese porphyrins during their reaction with oxygen at a solid/liquid interface

“…The majority of these studies use ensemble techniques in solution, such as electrochemistry and absorption, electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The scanning tunneling microscope (STM) has proven to be a promising tool to study reactions at the molecular level [8][9][10][11][12] , and initial experiments have been reported in which the reactive properties of metal porphyrins were studied on a surface at the single molecule level by STM in ultrahigh vacuum (UHV) [13][14][15][16][17] and under ambient conditions 18,19 .Here we report detailed STM studies of the complex redox chemistry of reactions between 10,15,R,R,porphyrin manganese(III) chloride (Mn1Cl, Fig. 2a) and different oxygen donors at a solid/liquid interface.…”