Selective binding of ReO₄^– And PtCl₄^2– By a Pd₂L₄ cage in water

Anions such as pertechnetate (^99mTcO₄^–) and tetrachloroplatinate (^195mPtCl₄^2–) are important precursors for radioactive substances for medical imaging. Molecules that can sense and bind these small anions in water can be exploited to circumvent lengthy synthesis on radioactive material and improve these applications. The most important property of a host is its selectivity for non-biological anions over common biological anions and/or molecules. Here, we report that a previous published [Pd₂L₄]¹⁶⁺ guanidinium cage has higher selectivity for ReO₄^– and particularly for PtCl₄^2– (nonradioactive substitutes for ^99mTcO₄^– and ^195mPtCl₄^2–) over common biological molecules. This selectivity was shown by means of ¹H NMR titrations with the most noticeable affinity for PtCl₄^2– in the order of K_a = 10⁴ M⁻¹. A molecular model of PtCl₄^2– or ReO₄^– bound inside the cage revealed the best interaction complementary fit with the PtCl₄^2– anion, which is held in place by equally short N–H···Cl and C–H···Cl hydrogen bonds. This research shows that a simple coordination cage can already have selective binding for non-biological anions such as ReO₄^– and particularly for PtCl₄^2–, paving the way to improve coordination cages for use in medical imaging research.