title: Ligand Design for Isomer-Selective Oxorhenium(V) Complex Synthesis.
abstract: Recently, N,N-trans Re(O)(LN-O)2X (LN-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)(LN-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)(LN-O)2Cl complexes. Analysis of the crystal structures revealed that the dihedral angles (DAs) between the two LN-Oligands of N,N-cis Re(O)(LN-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)(LN-O)2X, Re(O)(LO-N-N-O)X, and Tc(O)(LN-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)(LN-O)2X and Tc(O)(LN-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.
authors.affiliation: Department of Chemical and Environmental Engineering, University of California , Riverside, California 92521, United States.|Department of Civil and Environmental Engineering, Colorado School of Mines , Golden, Colorado 80401, United States.
authors.forename: Jinyong
authors.identifier: http://orcid.org/0000-0003-1473-5377
authors.initials: J
authors.lastname: Liu
authors.affiliation: Department of Chemical Physics, University of Science and Technology of China , Hefei, Anhui 230000, China.
authors.forename: Xiaoge
authors.initials: X
authors.lastname: Su
authors.forename: Mengwei
authors.initials: M
authors.lastname: Han
authors.affiliation: Department of Chemistry and Biochemistry, The Ohio State University , Columbus, Ohio 43210, United States.
authors.forename: Dimao
authors.initials: D
authors.lastname: Wu
authors.forename: Danielle L
authors.identifier: http://orcid.org/0000-0003-0059-2096
authors.initials: DL
authors.lastname: Gray
authors.forename: John R
authors.initials: JR
authors.lastname: Shapley
authors.affiliation: Department of Civil, Architectural, and Environmental Engineering, University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Werth
authors.affiliation: Department of Civil and Environmental Engineering, Colorado School of Mines , Golden, Colorado 80401, United States.
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Strathmann