Pentadentate and Hexadentate Pyridinophane Ligands Support Reversible Cu(II)/Cu(I) Redox Couples

Abstract

Two new ligands were synthesized with the goal of copper stabilization, N,N'-(2-methylpyridine)-2,11-diaza[3,3](2,6)pyridinophane (^PicN4) and N-(methyl),N'-(2-methylpyridine)-2,11-diaza[3,3](2,6)pyridinophane (^PicMeN4), by selective functionalization of ^HN4 and ^TsHN4. These two ligands, when reacted with various copper salts, generated both Cu(II) and Cu(I) complexes. These ligands and Cu complexes were characterized by various methods, such as NMR, UV-Vis, MS, and EA. Each compound was also examined electrochemically, and each revealed reversible Cu(II)/Cu(I) redox couples. Additionally, stability constants were determined via spectrophotometric titrations, and radiolabeling and cytotoxicity experiments were performed to assess the chelators relevance to their potential use in vivo as ⁶⁴Cu PET imaging agents.

Keywords: 64Cu PET imaging agents; bioinorganic chemistry; copper(I) complexes; copper(II) complexes; cyclic voltammetry; pyridinophane ligands; radiolabeling; reversibility.

Figure 1.

ORTEP plots (50% probability ellipsoids) of cations 1²⁺, 2⁺, 3²⁺, and 4⁺. Counterions and H atoms are omitted for clarity. The crystallographic datasets for 1·(OTf)₂, 2·OTf, 3·(OTf)₂, and 3·OTf have been deposited at CCDC under the record numbers 2049802, 2049803, 2049804, and 2049805.

Figure 2.

EPR spectrum (black) and simulation (red) of 1·(OTf)₂ (left) and 3·(OTf)₂ (right) in MeCN:PrCN (1:3) at 77K.

Figure 3.

Cyclic voltammetry of the copper complexes 1·(Otf)₂ (a), 2·Otf (b), 3·(Otf)₂ (c), and 4·Otf (d) (0.1 M Bu₄NclO₄/CH₃CN; arrow indicates the initial scan direction). The asterisk (*) corresponds to a trace amount of ^PicMeN4Cu^II(H₂O) complex.

Figure 4.

Variable pH (2.28–11.03) UV-Vis spectra of ^PicN4 in 0.1 M KCl at 25 °C (left) and its species distribution plot (right). [^PicN4]_tot = 60 μM.

Figure 5.

Variable pH (2.29–11.03) UV-Vis spectra of the ^PicN4 + Cu²⁺ system in 0.1 M KCl at 25 °C (left) and its species distribution plot (right). [Cu²⁺]_tot = [^PicN4]_tot = 50 μM.

Figure 6.

Radio-HPLC chromatograms for the ⁶⁴Cu labeled complexes of ^PicN4 and ^PicMeN4.

Figure 7.

Cell viability (% of control) of Neuro2A cells upon incubation with ^PicMeN4, ^PicN4, and their Cu^II complexes at 2, 5, 10, and 20 μM concentrations.

Scheme 1.

Synthesis of ^PicN4 and ^PicMeN4. (i) 90% H₂SO₄, reflux 2.5 h; 88% (ii) 2-(methylchloro)pyridine HCl, iPr₂EtN, MeCN, 48 h; 81% (iii) TsCl, DCM, 0 C, 3 h; 44% (iv) 2-(methylchloro)pyridine HCl, iPr₂EtN, MeCN, 48 h; 82% (v) 90% H₂SO₄, reflux, overnight; 89% (vi) formic acid, formaldehyde, reflux, overnight; 82%.

Scheme 2.

Preparation of Copper Complexes.