3.2 Results and Discussion
3.2.2 Reactivity of Copper(I) Complexes of Bridged Unspenp Derivatives Towards
At −90 °C the decay of the peroxido species is very slow. After 450 s the intensities of the assigned bands are only very slightly lowered. At −35 °C still weak peroxido bands are visible, that are overlaid during 450 s by a broad starting around 600 nm, typical for copper(II) dd transfers. An indefinite decay of the peroxido species is therefore the only assumption that is possible from these initial results of the study.
3.2.2 Reactivity of Copper(I) Complexes of Bridged Unspenp Derivatives Towards
Unfortunately, the complexes were badly soluble after the oxidation reaction. Thus, a kinetic analysis of the gained UV/Vis spectra was not possible and activation parameters could not be determined. Nevertheless, a comparison of the observed bands during the reaction with dioxygen with the ones characteristic for specific oxygen intermediates provided information on the possible reaction pathway. As expected, the μ-peroxido complex is formed very rapidly even at very low temperatures. This is most likely due to the second intramolecular copper(I) atom that leads to a fast monomolecular reaction once a superoxido complex is formed. In consequence, neither the formation nor the decay of a possible end-on superoxido complex is observable.
A closer look at the band at 378 nm that most likely derives from ligand-centered imine transitions, reveals a rapid decay followed by a period of approximately one second where an increasing absorption is observable. After a reaction time of 1.5 to 2 seconds the absorption is again decreasing slowly. Unfortunately, no explanation for this phenomenon can doubtlessly be provided since there is more than one rational possibility. One of which is a rapid decay of the imine followed by a formation of a side-on bis-μ-peroxido deriving from the initially formed end-side-on-μ-peroxido complex. These two species are known to exist in an equilibrium. Both are decaying slowly to so far unknown products. Ligand hydroxylation at the Xylyl-bridge or the cleavage of the aliphatic amine bond are the most likely pathways for oxidation reactions. A similar hydroxylation is known from an 1,3-tpbd derivative.68
An underlying, intensive charge transfer band of a small amount of formed superoxido complex is also not completely deniable, but compared to the spectra of other superoxido species of similar coordination compounds or the ones of the copper(I) complex Hxyl-unspenp the superoxido band should be redshifted (compare Figure 36).
Here, only educated guesses can be made concerning the detailed reaction pathway, due to the missing kinetic analysis. The rapid formation of an end-on-μ-peroxido complex stable for more than 250 seconds at −90 °C in propionitrile is the only conclusion of the stopped flow spectra that can be proved. Until today neither oxidized ligand nor other possible oxidation products of the reaction could be isolated and characterized.
In contrast to these findings the reaction of the copper(I) complex of Hxyl-unspenp as ligand with oxygen is comparable to the properties of the related amines of the tmpa family. The depicted spectra in Figure 36 reveal charge transfer bands of both end-on superoxido and μ-peroxido species as reactive intermediates. A kinetic analysis was again impossible due to the bad solubility of the oxidation products leading to a cloudy solution, making an observation over a longer time period or in a broad temperature range impossible. From a comparison of the timetraces of the complexes bearing only one copper(I) atom and that of the copper(I) atom of Hxyl-unspenp it is obvious that the reaction rates for the formation and the decay of both superoxido and peroxido species are faster for the complex bearing two metal atoms. This is again most likely due to the acceleration of the reaction by the change between bi- and monomolecular reaction type minimizing the effects of solvent molecules.
Fig. 36 UV/Vis spectra and timetraces of the reaction of [Cu2Hxyl-unspenp]2+ with oxygen in propionitrile at T = −93 °C ( t = 0.9 s; c(O2) = saturated solution; c ([Cu2Hxyl-unspenp]SbF6) = 5×10-4
mol/L).
The decreasing band at 422 most likely derives from an intensive charge transfer transition of an end-on superoxido complex already known from the tmpa derivatives.22 The formation is too fast, to observe even at −93 °C, while the formation of the μ-peroxido species is detectable at least in the very first spectra of the stopped-flow experiment forming strong charge transfer bands at 517 and 602 nm, respectively. Both species are very reactive and decay rapidly, over a period of only some seconds. The comparison of these spectra with the related data for the related monomolecular species leads to the postulation of a possible reaction pathway shown in Figure 37. It follows the well known mechanism for the mononuclear complexes.134-135 Due to the excess amount of solved oxygen in the saturated propionitrile, a pseudo first order reaction leads to the formation of the very reactive superoxido species. The initially formed intermediate reacts intramolecular with the second copper(I) atom most likely in a first order reaction leading to the formation of a more stable end-on μ-peroxido species. Their slow decay leads to so far unknown products. Side reactions, involving the direct decay of the superoxido complex are plausible.18
A concerted reaction of both copper(I) atoms with the oxygen molecule leading directly to the end-on μ-peroxido species is not impossible but improbable due to the result already published for the comparable monomolecular copper(I) complexes.22, 131 To the best of our knowledge, the formation of the end-on superoxido complex is the initial step during the reaction of comparable copper(I) complexes with oxygen. Using these bridged ligands most likely only accelerates the formation of the μ-peroxido complex, making lower temperatures or faster spectroscopy necessary to observe the superoxido complex.
Fig. 37 Proposed mechanism for the reaction of Hxyl-unspenp with oxygen following the reaction pathway published earlier.131
From the comparison of the different timetraces(see insets of Figures 35 and 36) it is obvious that the peroxido adduct of Imxyl-unspenp is more stable than the Hxyl-unspenp adduct, possibly due to the existence of an equilibrium between end-on and side-on bound peroxido species.
3.2.2.1 Synthesis and Characterization of a Carbonate Bridged Copper(II) Dimer The copper(II) complex shown in Figure 38 derived from a solution containing copper(I) and hexafluoroantimony ions. The solution was oxidized with dioxygen and allowed to stand several days. Unexpectedly a carbonato bridged copper(II) complex formed, that could be determined by single crystal X-ray crystallography. Unfortunately the single crystals were of minor quality resulting in high R-values. Thus, the structure could not be solved properly. Nevertheless, the structure motive was determinable and is obvious from Figure 38.
Although the position of some of the carbon atoms of the ligand was not refinable, the carbonato bridges between the copper atoms are clearly recognizable. During the synthesis of the complex no carbonate was added. Therefore, the coordinated carbonate is most likely the reaction product of an reaction of carbon dioxide from air activated by the copper(I) complex and dioxygen or vice versa. Copper and zinc complexes are known for stoichiometric transition of carbon dioxide into carbonate.136-137 There are many examples for coordinated bridging carbonates, but the conversion of carbon dioxide from air into carbonate is rather rare.138 Due to the great interest of utilizing carbon dioxide from air as carbon source for chemical processes, knowledge about fixation and oxidation of carbon dioxide is essential. Copper complexes that are capable of catalyzing such reactions are therefore promising study objects.139
Fig. 38 Fragments of the molecular structure of the cation [Cu4(CO3)2(Hxyl-unspenp)2]4+. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids set to 50 %.
Only recently Angamuthu et al. reported the activation of carbon dioxide from air by a similar copper(I) complex containing two bridged dipicolyl subunits as ligand.17 Even a catalytic cycle recovering the active copper(I) species electrochemically could be demonstrated (see chapter 1.2.2).
Although the presented structure is not more than a first hint for the carbon dioxide activating capability of [Cu2(Hxyl-unspenp)](SbF6)2, it should serve as initiator for further studies concerning this topic.