Speciation Behavior of Copper( II) Acetate in Simple Organic Solvents - Revealing the Effect of Trace Water

Copper(II) acetate is frequently used as a catalyst for bond-forming reactions in organic synthesis. Unlike aqueous solutions, in which complete heterolysis to CuOAc+(aq) and AcO-(aq) prevails at low concentrations, it is clear that copper(II) acetate shows a large degree of aggregation in typical organic solvents. Here, the speciation behavior of Cu(OAc)(2) in organic solvents is probed by electrospray ionization mass spectrometry (ESI-MS), which reveals an extensive clustering of copper acetate species to form ions of the general composition [Cu-n(X)(2n-1)](+) and [Cu-n(X)(2n+1)](-) (X = OAc and OCH3), along with (solvated) monomeric species such as [Cu(OAc)(CH3OH)(n)](+). The ESI-MS measurements are complemented by solution-phase studies of Cu(OAc)(2) by electron paramagnetic resonance spectroscopy, which support an extensive aggregation of Cu(OAc)(2) in organic solvents. Collisional experiments reveal that either simple degradation or redox processes can be observed, depending on the coordinative saturation of the copper clusters. The formation of the copper clusters can be efficiently suppressed by contamination of the organic solvents by a small amount of water.