Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy

Abstract

Tetrahedral amorphous carbon (a-C) films have been grown by pulsed laser deposition to investigate a liquid phase process for surface immobilization of electroactive [Mo6Ii8]4+ transition metal cluster cores using a complexation reaction with a pyridine-terminated alkyl monolayer covalently bonded to the a-C surface (PyS–alkyl/a-C). These films are stable against thermally-assisted grafting of alkene molecules and the covalent CC interface provides a robust monolayer/a-C assembly. Octahedral [Mo6Ii8]4+ cluster cores with iodine inner ligands and labile triflate apical ligands [Mo6Ii8(CF3SO3)a6]2− have been immobilized through partial complexation in apical positions by surface pyridine groups (PyS). The remaining CF3SO3− apical ligands of [Mo6Ii8 (PyS)ay(CF3SO3)a6−y] cluster units were further substituted with bromopyridine (Py-Br) to obtain air stable surface with expected final composition [Mo6Ii8 (PyS)ay(Py-Br)a6−y]. The yield of the different reaction steps is followed by X-ray photoelectron spectroscopy, providing cluster coverage ΣMo6I8=9×1012cm−2. Each [Mo6I8]4+ cluster is bound to the carbon surface through multiple anchoring metal sites (NPYR=3 or 4), indicating that pyridine-terminated alkyl chains are flexible enough to accommodate four bonds. Electrical transport through Hg//Mo6I8–PyS–alkyl/a-C/p-Si(111) junctions shows rectifying current–voltage characteristics but does not reveal any signature of cluster immobilization.