Effects of the dendrimer cage on O2 binding of dendritic cobalt(II) porphyrins.
Two types of dendritically functionalized cobalt(II) porphyrins were prepared and investigated in the presence of 1,2-dimethylimidazole, pyridine, and 1-methylimidazole. Continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques revealed specific information on the oxygenated forms of these porphyrins. The ENDOR and hyperfine sublevel correlation (HYSCORE) spectra showed that in [1.CoII.1,2.DiMelm]-O2, with secondary amide moieties in the dendritic branching, no hydrogen bond forms between the bound O2 and a dendritic amide NH moiety. This hydrogen bond had earlier been proposed on basis of the large dioxygen affinity of the corresponding FeII complex. For both [1.CoII.1,2-DiMelm]-O2 and the ester derivative [2.CoII.1,2-DiMelm]-O2, which lacks H-donor centers in the periphery of the porphyrin, ENDOR experiments clearly showed that the dendritic branches are closely packed in toluene. The analysis of the g values, the cobalt hyperfine interactions, and the hyperfine and nuclear quadrupole couplings of the directly coordinated nitrogen of the axial base showed an increased ionicity in the cobalt-dioxygen bond for [1.CoII.1,2-DiMelm]-O2. This observation is linked to the packing and the polarity of the dendritic branches and can be related to the O2 and CO affinity of the corresponding FeII complexes.
Van Doorslaer
Sabine
S
Physical Chemistry Laboratory, ETH-Zürich, 8093 Zurich, Switzerland.
Zingg
Adrien
A
Schweiger
Arthur
A
Diederich
François
F