Free-radical ligands consisting of 4.4,5,5-tetramethyl-A2-imidazolin-l -0xy1 3-oxide, (LI), or 4.4.5.5-tetramethyl-Ae-irnidazdin-3 -oxyl, (I), fragments with diamagnetic 2-substituents R [R = 2-pyridyl((py), 6-bromo-2-pyridyl { 6-Brpy). . and Ph] are described and their interactions with closed-shell ions of nonzero nuclear spins (H+. b+, Ag+, Zn2+. Cde*, Hg2+, and Pb2+) are discussed. Unpaired spin densityon the ligmds i s 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 the e.s.r. time scale (T ca. 3 x 10-*), while complex lifelimes are long (T 9 lo-'). Interaction with Li+ does occur, but is too transient to be observable by e.s.r. spectroscopy. The derivatives (11) 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, Ki. For (I ; R = py), unpaired electron delocalization increases by as much as 0.5 A on co-ordination, and increases proportionately with &, 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 x 1 O6 to 1 X 109 I mol-l s-l on going from R = py to Ph, while complex Kt values decrease by one or two orders of magnitude. Electronic requirements on the imidazoiine ring in chelated complexes of (I; R = py) are much smaller than in those of the unidentate iigands where R = 6-Brpy. 3-BrPh, and Ph, as shown by h.f. splittings of the e.s.r. spectra. (esr.) is perhaps the most revealing physical technique employed in the study of paramagnetic metal complexes. Such studies have traditionally focused on the hyperfine interaction of the unpaired electron on the metal with the magnetic nuclei of diamagnetic ligands. The converse approach, in which the unpaired electron residing on the ligand is perturbed by the bonding interaction with the metal, has obvious advantages but has until recently been hampered by a paucity of suitable paramagnetic ligands of sufficient stability and chemical interest.
ELECTRON SPIN RESONANCEEaton has identified complexes of the o-semiquinone radical with bivalent Mg, Ca, Zn, and Cd from changes in proton hyperfine (h.f .) coupling constants in metal-ion solutions,l and several workers have investigated complexes of aliphatic nitroxides,2 of a-di-imine radical anions? and nitroxides in which the radical site does not participate in co-ordination.4 In this work we examine the e.s.r. spectra of several aromatic chelating or uni-