Abstract
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 E1/2 = 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.
Highlights
Transition metal complexes with chelating polyimines of the 1,10-phenanthroline family have been studied in detail since the early 1940s.1 In particular, copper(I) complexes prepared from 2,9-disubstituted-1,10-phenanthroline ligands have received particular attention due to their interesting photophysical, chemical, and structural properties.[2,3] Complexes of Cu(I) adopt a tetrahedral or pseudo-tetrahedral geometry and are often bright orange or red due to MLCT (d−π*) electronic transitions
Copper halides salts are no exception, and they have been widely used as starting materials for the synthesis of a variety of copper complexes.[17−20] Scheme 1 and Table 1 report the molecular structures and the crystallographic details, respectively, of the copper complexes discussed in this work
Copper chloride salts have been used for the synthesis of homoleptic compounds based on substituted bipyridine and phenanthroline ligands.[10,11]
Summary
Transition metal complexes with chelating polyimines of the 1,10-phenanthroline family have been studied in detail since the early 1940s.1 In particular, copper(I) complexes prepared from 2,9-disubstituted-1,10-phenanthroline ligands have received particular attention due to their interesting photophysical, chemical, and structural properties.[2,3] Complexes of Cu(I) adopt a tetrahedral or pseudo-tetrahedral geometry and are often bright orange or red due to MLCT (d−π*) electronic transitions. In the absence of restricting steric effects, these complexes may be readily oxidized to the more stable squareplanar, often green, Cu(II) species.[4] The geometrical changes associated with the copper oxidation states have a remarkable impact on the electrochemical properties of complexes derived from 1,10-phenanthroline with substituents in the 2,9-positions In the latter case, bulky substituents cause the geometry of Cu(II) to be nonplanar by inducing a strain in the molecule, which causes the oxidation of copper(I) to copper(II) to be thermodynamically less favorable.[5,6] For this reason, the redox potential for the couple [Cu(dmp)2]2+/+ (dmp = 2,9-dimethyl1,10-phenanthroline) is substantially more positive than that of [Cu(phen)2]2+/+ (phen = 1,10-phenanthroline).[7] In 2005, Hattori et al reported the use of [Cu(dmp)2]2+/+ and [Cu(phen)2]2+/+ as redox mediators in dye sensitized solar cells (DSSCs).[8] Their study showed significantly higher performances for the [Cu(dmp)2]2+/+ couple over [Cu(phen)2]2+/+. We show that the decision of starting the synthesis from copper(II) chloride leads to a different compound with different electrochemical properties, potentially generating undesired effects in the final electrolyte solution
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