Tuning the gas phase redox properties of copper(ii) ternary complexes of terpyridines to control the formation of nucleobase radical cations

Abstract

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) of ternary copper(II) complexes of [Cu(terpyX)(M)]²⁺ (where terpyX = is a substituted 2,2′:6′,2″-terpyridine ligand; M = the nucleobases: adenine, guanine, thymine and cytosine) was examined as a means of forming radical cations of nucleobases in the gas phase. The following substituents were examined: 4′-NMe₂-2,2′:6′,6″-terpyridine; 4′-OH-2,2′:6′,6″-terpyridine; 4′-F-2,2′:6′,6″-terpyridine; 2,2′:6′,6″-terpyridine; 4′-Cl-2,2′:6′,6″-terpyridine; 4′-Br-2,2′:6′,6″-terpyridine; 4′-CO₂H-2,2′:6′,6″-terpyridine; 4′-NO₂-2,2′:6′,6″-terpyridine and 6,6″-dibromo-2′,2:6′,2″-terpyridine. Each of the ternary complexes [Cu(terpyX)(M)]²⁺ was mass selected and subjected to collision induced dissociation (CID) in a quadrupole ion trap. The types of fragmentation reactions observed for these complexes depend on the nature of the substituent on the terpyridine ligand, while the yields of the radical cations of the nucleobases follow the order of their ionization energies (IEs): G (lowest IE) > A > C > T (highest IE). In general, radical cation formation is favoured for electron withdrawing substituents (e.g. NO₂) while loss of the neutral nucleobase is favoured for electron donating substituents (e.g. NMe₂). Loss of the protonated nucleobase is a major fragmentation pathway for the OH substituted terpyridine system, consistent with its ability to bind to a metal centre as a deprotonated ligand. Crystal structure determinations of (6,6″-dibromo-2′,2:6′,2″-terpyridine)bis(nitrato)copper(II) and diaqua(4′-oxo-2,2′:6′,6″-terpyridine)copper(II) nitrate monohydrate were performed and correlated with the ESI results.