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Replacement of a Nitrogen by a Phosphorus Donor in Biomimetic Copper Complexes: a Surprising and Informative Case Study with Calix[6]arene-Based Cryptands

The aim of the paper is to characterize Cu complexes in the P(Ar)N(3) environment provided by ligands derived from triphenylphosphine P(C(6)H(4)CH(2)NHR)(3) and compare their coordination behavior and reactivity with those obtained with all-nitrogen ligands such as tren. It is shown that coordination of the PN(3) ligand (R = iPr) to Cu(I) and Cu(II) leads to complexes whose coordination sphere is hardly controlled as they readily undergo decoordination of either one N or the P donor together with oxidation of the latter. In strong contrast, when grafted on the small rim of a calix[6]arene, the P(Ar)N(3) is geometrically constrained into a tripod that enforces the metal center to remain in the same environment with a P-Cu bond for both oxidation states. These calix[6]PN(3)-based Cu(I) and Cu(II) complexes react readily with exogenous ligands, making a comparison with calix[6]tren-based copper complexes possible. Indeed, reactivity studies in solution highlight very different behaviors. The complex [Cu(calix[6]PN(3))](2+) shows an unusual affinity for weak sigma-donors (e.g., MeCN > EtOH), while the analogous cuprous complex, [Cu(calix[6]PN(3))](+), displays a surprising affinity for hard O-donor ligands (EtOH, DMF), which has never been observed for the tren analogues. Even more surprising is the lack of reactivity of [Cu(calix[6]PN(3))](+) toward dioxygen, which contrasts strongly with the high reactivity of the [Cu(calix[6]tren)](+) complex. In an attempt to explain the observed differences in binding properties and reactivity, Density Functional Theory calculations and electronic spectra simulations were undertaken. They suggest that coordination of the soft P(Ar)(3) center allows to tune the metal ion properties, either by absorbing excess electron density from Cu(I), or by increasing the electronic density of Cu(II). This is due to the simultaneous presence of the phosphorus atom (sigma-donor) in apical position and the aromatic groups (pi-acceptors) bound to the P-atom.