Iron complexes of C- and N-methylated 2-aza-21-carbaporphyrin: NMR studies

Insertion of iron(II) into methylated derivatives of N-confused porphyrins 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (MeCTPPH)H, 2-aza-5, 10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (CTPPMe)H-2, and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (MeCTPPMe)H yielded N- or C-methylated high-spin iron(II) complexes (MeCTPPH) (FeBr)-Br-II, (HCTPPMe) (FeBr)-Br-II, and (MeCTPPMe) (FeBr)-Br-II. One electron oxidation of (Me-CTPP H) FeIIBr using Br-2, accompanied by deprotonation of a C(21)-H(21) fragment and formation of an Fe-C( 21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (MeCTPP) (FeBr)-Br-III. Simultaneously, a high-spin complex [(MeCTPPH)(FeBr)-Br-III](+) was formed which preserved the side-on interaction between the metal ion and the inverted pyrrole ring. [(MeCTPPH)(FeBr)-Br-III](+) was also obtained by titration of (MeCTPP)(FeBr)-Br-III with TFA due to the C(21) protonation. A titration of (HCTPPMe)(FeBr)-Br-II and (MeCTPPMe)(FeBr)-Br-II with Br-2 yielded solely corresponding high-spin iron(III) species [(HCTPPMe) (FeBr)-Br-III] + and [(MeCTPPMe) (FeBr)-Br-III](+). Dioxygen reacts cleanly with (MeCTPPH)-(FeBr)-Br-II carbaporphyrin to form solely (MeCTPP) (FeBr)-Br-III. The H-1 NMR spectra of paramagnetic iron(II) and iron(III) complexes were examined. The characteristic patterns of pyrrole, C-methyl, and N- methyl resonances were found diagnostic of the ground electronic state of iron and the coordinating nature of the N- confused pyrrole. The characteristic C-Me resonances occur in a unique window (520-420 ppm) for iron(III) C-methylated N-confused porphyrins which remains in contrast with relatively small values found for iron(II) C-methylated derivatives (50 - 80 ppm).