Phenolate-Pyrazole Ligands in Oxidorhenium(V) Complexes: Catalyst Activation by Chlorido Abstraction

The importance of stereoisomers for catalytic perchlorate reduction was first realized with previously published phenol-oxazoline complex ReOCl(L1a)2, where both isomers N,N-cis (1a′) and N,N-trans (1a) could be separated, proving that the N,N-trans isomer exhibited higher catalytically activity. Similar phenol-pyrazole complexes ReOCl(L2a-e)2 (2a′-e′) were found to only form the unfavorable N,N-cis isomer, and hence, no catalytic activity in perchlorate reduction was observed. Here, we show that by chlorido abstraction from 2a′-e′ with AgOTf, unexpected isomerization to the N,N-trans cationic complexes ReO(L2a-e)2(OTf) (4a-e) occurred, resulting in active perchlorate reduction catalysts. Experiments to influence the stereoselectivity by the addition of different pyridine bases were also undertaken. However, no changes in stereoselectivity could be observed. Instead, in one case, a side reaction was observed, giving pyridine adduct ReOCl2(L2e)(py) (5e). The combination of the protic solvent MeOH and electron-withdrawing ligands HL2d and HL2e also gave side reactions, resulting in the isolation of complexes ReO(OMe)(L1d)2 (6d′) and ReO(OMe)(L1e)2 (6e′). Interestingly, even though these complexes adopt the N,N-cis form in the solid state, they now show some catalytic activity in perchlorate reduction. All novel complexes were fully characterized, for complexes 2b′, 4b, 4c, 5e, and 6d′, the solid-state structures were determined by single-crystal X-ray crystallography.