The chemistry of the fac-[Re(CO)2(NO)]2+ fragment in aqueous solution

Abstract

The anions [ReX₃(CO)₂(NO)]⁻ (with X = Cl, 1; X = Br, 2) have been prepared with different counterions. Complex 1 was found to lose its chloride ligands in water within 24 h. The [Re(H₂O)₃(CO)₂(NO)]²⁺ cation obtained after hydrolysis is a strong acid, which consequently undergoes a slow condensation reaction in water to form the very stable [Re(µ₃-O)(CO)₂(NO)]₄ cluster 4 at pH > 2, that precipitates from the aqueous solution and is insoluble also in organic solvents. Fast deprotonation of [Re(H₂O)₃(CO)₂(NO)]²⁺ did not lead to 4 but rather to the mononuclear species [Re(OH)(H₂O)₂(CO)₂(NO)]⁺. Subsequent attack of OH⁻ at a CO group resulted in the formation of a rhenacarboxylic acid and its carboxylate anion. For solutions of even higher pH, IR spectroscopy provided evidence for the formation of a Re(C(O)ON(O)) species. These processes were found to be reversible on lowering the pH. Starting from cluster 4 it was possible to obtain complexes of the types [ReX(CO)₂(NO)L₂] or [Re(CO)₂(NO)L₃] (L₂ = 2-picolinate, 2,2′-bipyridine, L-phenylalanate; L₃ = tris(pyrazolyl)methane, 1,4,7-trithiacyclononane) in the presence of an acid in protic solvents, but only in low yields. In further synthetic studies, complexes 1 and 2 were found to be superior starting materials for substitution reactions to form [ReX(CO)₂(NO)L₂] or [Re(CO)₂(NO)L₃] complexes.