Thermodynamic stability and structure in aqueous solution of the [Cu(PTA)4]+ complex (PTA = aminophosphine‑1,3,5‑triaza‑7‑phosphaadamantane)

The chemistry of copper(I) with water-soluble phosphines is an emergent area of study which has the objective of finding ligands that stabilize copper in its lower oxidation state. Cu(I) has been found relevant in the mechanism of copper transports into cells, and the accessibility of this oxidation state has implications in oxidative stress processes. For these reasons the possibility to deal with stable, water soluble copper(I) is an attractive approach for devising new biologically relevant metal-based drugs and chelating agents. Here we present the X-ray absorption spectroscopy (XAS) and UV-visible spectrophotometric study of the [Cu(PTA)4]BF4 complex (PTA = aminophosphine‑1,3,5‑triaza‑7‑phosphaadamantane). In particular, we have studied the stability of the [Cu(PTA)n]+ species (n = 2-4) in aqueous medium, and their speciation as a function of the total [Cu(PTA)4]BF4 concentration by means of competitive UV-visible spectrophotometric titrations using metallochromic indicators. Also, the structure in solution of the Cu(I)/PTA species and the nature of the first coordination sphere of the metal were studied by transformed XAS. Both techniques allowed to study samples with total [Cu(PTA)4]BF4 concentration down to 68-74 μM, possibly relevant for biological applications. Overall, our data suggest that the [Cu(PTA)n]+ species are stable in solution, among which [Cu(PTA)2]+ has a remarkable thermodynamic stability. The tendency of this last complex to form adducts with N-donor ligands is demonstrated by the spectrophotometric data. The biological relevance of PTA towards Cu(I), especially in terms of chemotreatments and chelation therapy, is discussed on the basis of the speciation model the Cu(I)/PTA system.