1,4,7-Triazacyclononane (L) and its methylated analog (L') form complexes with zeolite-exchanged Mn2+ Co2+, or Cu2+. Through comparison with diluted solutions of the complexes, the effect of the zeolite environment on the complex formation was evaluated. In the case of Mn2+ the complexation was monitored by observing the zero-field splitting in the X-and Q-band ESR spectra of the Mn2+ ion, Adsorption oft into metal Y zeolites yields bis L complexes; the L' mono complexes are formed. The latter complexes are not coordinated by the zeolite surface, as deduced from the spectral parameters of the [Mn(L')](2+) and [Cu(L')](2+) complexes. Mono [Co(L')](2+) in the zeolite is a reversible dioxygen binder, a property not previously documented for these complexes in solution. Exposure of [Mn(L)(2)](2+) or [Mn(L')](2+) containing Y zeolites to H2O2 results in intraporous formation of oxidized Mn dimers. The constrained zeolite environment seems to stabilize the oxidized dimers of Mn and L by preventing further cluster expansion.