Catalytic aspects of metallophthalocyanines adsorbed on gold-electrode. Theoretical exploration of the binding nature role

Abstract

The need of deeper insights regarding the inner working of catalysts represents a current challenge in the search of new ways to tune their activities towards new chemical transformations. Within this field, metallophthalocyanines-based (MPc) electrocatalysis has gained tremendous attention due to their versatility, low cost, great stability and excellent turn-over properties. In this concern, here we present a quantum chemical study of the formation of supramolecular complexes based on the adsorption of MPcs on gold substrates, and the effect of the substrate on their electrocatalytic properties. For this purpose, we used iron- (FePc), cobalt- (CoPc) and copper-phthalocyanines (CuPc). To model the gold surface we used two gold clusters of different sizes, given by Au and Au accounting for gold electrode Au(111) surface. Thus, both electronic and binding strength features of the adsorption process between the complexes were analyzed in detail in order to gain a deeper description of the nature of the MPc-Au(111) formation, by using Density Functional Theory (DFT) calculations, at the PBE and TPSS levels including the dispersive contribution according to the Grimme approach (D3). Our results show that dispersion forces rule the MPc-gold interaction, with binding strengths ranging between 61 and 153 kcal mol, in agreement to the reported experimental data. To provide a detailed picture of our findings we used the non-covalent interactions index (NCIs) analysis, which offers additional chemical insights regarding the forces that control their interaction strength. Finally, our calculations revealed that among the three MPcs, CuPc required less energy for its oxidation. However, the removal of the electron involves a tremendous decrease of the MPc-gold surface interaction strength thus suggesting its desorption, which would prevent the required reversibility of the redox reaction, explaining its low performance observed experimentally.

Theoretical exploration of the forces governing the interaction between gold–phthalocyanine and gold surface clusters

“…Interestingly, this only occurred when the Au was incorporated at the center of the Pc scaffold. These results reveal the role of Au as the metal center in the binding nature as an indispensable chemical block able to assist the charge transfer process with direction from the molecule to the gold substrate and are in agreement with experimental evidence of CuPc 37 and previous theoretical results 21 regarding the direction of the process. For other metal centers such as Fe or Ni, the charge transfer process goes from the gold substrate towards the molecule, showing that AuPc with its metal center with d 9 electronic conguration follows the trend of CuPc in the charge transfer process.…”

“…The meta-GGA TPSS (Tao, Perdew, Staroverov, Scuseria) 53 functional was also used to obtain a comparison in the description of surface chemistry with the more broadly used and more validated PBE functional. 21,56 The comparison is provided in the ESI. † Since we are interested in the supramolecular assemblies that this molecules form over a surface, the inclusion of dispersion interactions is fundamental to accurately describe the non-covalent interaction between the macrocycle and the Au(111) surface.…”

“…58 Two f-type polarization functions were added Au (a f ¼ 0.20, 1.19). 21 All atoms were treated with Ahlrichs type def2-TZVP basis set, 59 adding one d-type polarization function for the optimizations, and subsequently the energies were corrected by single point calculations using the def2-TZVP basis set.…”

“…9,10 One of the most studied systems is the modication of gold surface with various types of MPcs through direct adsorption on the surface 9,[11][12][13][14] explored by a variety of surface techniques, mainly scanning tunnelling microscopy (STM). 4,[14][15][16] Also, there are extensive experimental and theoretical studies regarding rst row transition metals atoms modifying Pcs, such as Fe, Co, Cu, 10,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] but little work has been done using an Au atom coordinating the Pc. [27][28][29] Also, there are theoretical works about the AuPc using DFT in which it has been shown that the spin-density pattern is fully delocalized with an oscillating behaviour.…”

Noncovalent interactions in inorganic supramolecular chemistry based in heavy metals. Quantum chemistry point of view

“…Adsorption Energies (ΔE int ) in kJ/mol between MPc and Au n (M = Fe, Co; n = 26, 58) with counterpoise correction (CP)[140] …”