Steric and electronic selectivity in the synthesis of Fe-1,2,4,5-tetracyanobenzene (TCNB) complexes on Au(111): From topological confinement to bond formation:

A study of the surface assisted self-assembly of 1,2,4,5-tetracyanobenzene (TCNB) acceptor molecules and Fe atoms on an Au(111) surface is presented. While conditions to get the two-dimensional arrays of stable Fe(TCNB)(4) complexes are clearly identified, ultrahigh vacuum scanning tunneling microscopy and spectroscopy (STM/STS) coupled with first-principles calculations reveals that situations may occur where Fe and TCNB survive on the surface (as Fe-4TCNB entities) at a higher density than the original molecular monolayer without forming coordination bonds with each other. It is found that the square planar coordination of the Fe(TCNB)(4) monomer complexes cannot fully develop in the presence of lateral strain due to growth-induced confinement. A phenomenon similar to steric hindrance involving a strongly modified chirality with a Fe-N-C bond angle of 120 degrees compared to the 180 degrees for the stable complex may then explain why the Fe atom keeps its metallic bond with the surface. The competition between steric and electronic effects, not reported before, may arise elsewhere in surface chemistry involved in the synthesis of new and potentially useful organic nanomaterials.This work was supported by the International Center for Frontier Research in Chemistry (Grant No. FRC-2010-JBu-0001) and the Institut Universitaire de France (IUF)