Ligand Design for Isomer-Selective Oxorhenium(V) Complex Synthesis

Recently, N,N-trans Re(O)(L_N-O)₂X (L_N-O = monoanionic N-O chelates; X = Cl or Br prior to being replaced by solvents or alkoxides) complexes have been found to be superior to the corresponding N,N-cis isomers in the catalytic reduction of perchlorate via oxygen atom transfer. However, reported methods for Re(O)(L_N-O)₂X synthesis often yield only the N,N-cis complex or a mixture of trans and cis isomers. This study reports a geometry-inspired ligand design rationale that selectively yields N,N-trans Re(O)(L_N-O)₂Cl complexes. Analysis of the crystal structures revealed that the dihedral angles (DAs) between the two L_N-O ligands of N,N-cis Re(O)(L_N-O)₂Cl complexes are less than 90°, whereas the DAs in most N,N-trans complexes are greater than 90°. Variably sized alkyl groups (-Me, -CH₂Ph, and -CH₂Cy) were then introduced to the 2-(2'-hydroxyphenyl)-2-oxazoline (Hhoz) ligand to increase steric hindrance in the N,N-cis structure, and it was found that substituents as small as -Me completely eliminate the formation of N,N-cis isomers. The generality of the relationship between N,N-trans/cis isomerism and DAs is further established from a literature survey of 56 crystal structures of Re(O)(L_N-O)₂X, Re(O)(L_O-N-N-O)X, and Tc(O)(L_N-O)₂X congeners. Density functional theory calculations support the general strategy of introducing ligand steric hindrance to favor synthesis of N,N-trans Re(O)(L_N-O)₂X and Tc(O)(L_N-O)₂X complexes. This study demonstrates the promise of applying rational ligand design for isomeric control of metal complex structures, providing a path forward for innovations in a number of catalytic, environmental, and biomedical applications.