The oxorhenium(V) complex, Re(O)(hoz)2Cl [hoz = 2-(2'-hydroxyphenyl)-2-oxazoline] exhibits high reactivity with perchlorate (ClO4-) via an oxygen atom transfer (OAT) mechanism. The N,N-trans Re(O)(hoz)2Cl and the corresponding [Re(O)(hoz)2]+ exhibit much higher rates of monodentate dissociation and perchlorate reduction than the N,N-cis isomer. This presentation reports on a series of novel approaches and interesting findings during ongoing efforts aimed at controlled synthesis of N,N-trans Re(O)(L)2Cl complexes. First, formation mechanisms of Re(O)(hoz)2Cl isomers were identified. The two isomers initially formed with a 1:1 ratio from the anionic [Re(O)(hoz)Cl3]- intermediate, and then the conversion of N,N-cis isomer to the N,N-trans isomer was accelerated by addition of pyridine bases with moderate steric hindrance. Second, we sought to minimize N,N-cis isomer formation. Introduction of bulky isopropyl (iPr) to the oxazoline moiety of the hoz ligand successfully prevented formation of the N,N-cis isomer; however two diastereomeric isomers of N,N-trans Re(O)(iPr-hoz)2Cl (designated as A and B) were obtained instead due to the iPr-hoz ligand chirality. We established a facile and effective synthetic methodology that uses different quantities of a pyridine base to selectively obtain A and B in quantitative yield, and explored the thermodynamic and kinetic aspects for the complete interconversion between A and B. Third, when we tried to synthesize Re(O)(L)2Cl complexes with other hoz-derived ligands, we noticed the unique effect of the oxazoline moiety in hoz favoring the formation of N,N-trans structure; other derivative ligands such as oxazine-phenolate (hox) and thiazoline-phenolate (htz) favor the formation of the N,N-cis structure. Hybridization of hoz with these ligands (LN-O’) successfully yielded Re(O)(hoz)(LN-O’)Cl complexes in N,N-trans configuration, suggesting the dominating role of oxazoline in the “ligand competition” to determine the product coordination structure. Fourth, to overcome the low solubility issue generated from hybridization of hoz with other ligands in Re(O)(hoz)(LN-O’)Cl complex and from EDG/EWG modification to the hoz ligand, a series of new ligands with iPr substitution were synthesized to enable easy purification and crystallization of corresponding oxorhenium complexes, thus facilitating the study of structure-activity relationships with respect to perchlorate reduction and other green chemistry applications.

Rational design of ligands and synthetic methodologies toward novel oxorhenium(V) complexes with controlled structures and tunable catalytic properties for perchlorate reduction

249th National Meeting (2015)

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