The chemistry of the fac-[Re(CO)(2)(NO)](2+) fragment in aqueous solution
Kurz, P × Rattat, Dirk Angst, D Schmalle, H Spingler, B Alberto, R Berke, H Beck, W #
Royal soc chemistry
Dalton transactions issue:4 pages:804-810
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(mu(3)-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) L-2] or [Re(CO)(2)(NO) L-3] (L-2 = 2-picolinate, 2,2'-bipyridine, L-phenylalanate; L-3 = 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) L-2] or [Re(CO)(2)(NO) L-3] complexes.