Complexing and protonation of free-radical imidazolin-1-oxyl and imidazolin-1-oxyl 3-oxide ligands: a magnetic-resonance investigation

Abstract

Free-radical ligands consisting of 4,4,5,5-tetramethyl-Δ²-imidazolin-1-oxyl 3-oxide, (II), or 4,4,5,5-tetramethyl-Δ²-imidazolin-1-oxyl, (I), fragments with diamagnetic 2-substituents R {R = 2-pyridyi(py), 6-bromo-2-pyridyl (6-Brpy), 3-bromophenyl (3-BrPh), and Ph are described and their interactions with dosed-shell ions of nonzero nuclear spins (H⁺, Li⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺) are discussed. Unpaired spin density on the ligands is distributed over both imidazoline nitrogen atoms, and metal or proton hyperfine (h.f.) splittings appear in the e.s.r. spectra on interactions, so that electronic effects can be monitored. Protonations are fast on trie e.s.r. time scale (τca. 3 × 10^–8), while complex lifetimes are long (τ 10^–7). Interaction with Li⁺ does occur, but is too transient to be observable by e.s.r. spectroscopy. The derivatives (II) can be protonated but do not complex, while (I) both protonate and complex at the N(3) atom. For a given ligand e.s.r. spectrum, changes in ¹⁴N h.f. splittings on complexing are closely correlated with determined complex-formation constants, K_f. For (I; R = py), unpaired electron delocalization increases by as much as 0·5 Å on co-ordination, and increases proportionately with K_f, as shown by matrix ENDOR. The diamagnetic 2-substituent R has a pronounced effect on the protonation and complexing interactions. Pseudo-first-order rate constants for protonation at N(3) in (I) increase from 3 × 10⁶ to 1 × 10⁹ l mol^–1 s^–1 on going from R = py to Ph, while complex K_f values decrease by one or two orders of magnitude. Electronic requirements on the imidazoline ring in chelated complexes of (I; R = py) are much smaller than in those of the unidentate ligands where R = 6-Brpy, 3-BrPh, and Ph, as shown by h.f. splittings of the e.s.r. spectra.