Electronic and Structural Effects of Inner Sphere Coordination of Chloride to a Homoleptic Copper(II) Diimine Complex

The reaction of CuCl₂ with 2,9-dimethyl-1,10-phenanthroline (dmp) does not lead to the formation of [Cu(dmp)₂](Cl)₂ but instead to [Cu(dmp)₂Cl]Cl, a 5-coordinated complex, in which one chloride is directly coordinated to the metal center. Attempts at removing the coordinated chloride by changing the counterion by metathesis were unsuccessful and resulted only in the exchange of the noncoordinated chloride, as confirmed from a crystal structure analysis. Complex [Cu(dmp)₂Cl]PF₆ exhibits a reversible cyclic voltammogram characterized by a significant peak splitting between the reductive and oxidative waves (0.85 and 0.60 V vs NHE, respectively), with a half-wave potential E_1/2 = 0.73 V vs NHE. When reduced electrochemically, the complex does not convert into [Cu(dmp)₂]⁺, as one may expect. Instead, [Cu(dmp)₂]⁺ is isolated as a product when the reduction of [Cu(dmp)₂Cl]PF₆ is performed with l-ascorbic acid, as confirmed by electrochemistry, NMR spectroscopy, and diffractometry. [Cu(dmp)₂]²⁺ complexes can be synthesized starting from Cu(II) salts with weakly and noncoordinating counterions, such as perchlorate. Growth of [Cu(dmp)₂](ClO₄)₂ crystals in acetonitrile results in a 5-coordinated complex, [Cu(dmp)₂(CH₃CN)](ClO₄)₂, in which a solvent molecule is coordinated to the metal center. However, solvent coordination is associated with a dynamic decoordination-coordination behavior upon reduction and oxidation. Hence, the cyclic voltammogram of [Cu(dmp)₂(CH₃CN)]²⁺ is identical to the one of [Cu(dmp)₂]⁺, if the measurements are performed in acetonitrile. The current results show that halide ions in precursors to Cu(II) metal-organic coordination compound synthesis, and most likely also other multivalent coordination centers, are not readily exchanged when exposed to presumed strongly binding and chelating ligand, and thus special care needs to be taken with respect to product characterization.