Oxidation and Oxygenation of Iron Complexes of 2-Aza-21-carbaporphyrin

Abstract

Oxidation and oxygenation of (HCTPPH)Fe(II)Br an iron(II) complex of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH)H2 have been followed by 1H and 2H NMR spectroscopy. Addition of I2 or Br2 to the solution of (HCTPPH)Fe(II)Br in the absence of dioxygen results in one-electron oxidation yielding [(HCTPPH)Fe(III)Br]+. One electron oxidation with dioxygen, accompanied by deprotonation of a C(21)H fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (HCTPP)Fe(III)Br. In the subsequent step an insertion of the oxygen atom into the preformed Fe(III)-C(21) bond has been detected to produce [(CTPPO)Fe(III)Br]-. Protonation at the N2 atom affords (HCTPPO)Fe(III)Br. The considered mechanism of (HCTPPH)Fe(II)Br oxygenation involves the insertion of dioxygen into the Fe-C bond. The 1H NMR and 2H NMR spectra of paramagnetic iron(III) complexes were examined. Functional group assignments have been made with use of selective deuteration. The characteristic patterns of pyrrole and 2-NH resonances have been found diagnostic of the ground electronic state of iron and the donor nature localized at C(21) center as exemplified by the 1H NMR spectrum of intermediate-spin (HCTPP)Fe(III)Br: beta-H 7.2, -10.6, -19.2, -20.6, -23.2, -24.9, -43.2; 2-NH -76.6 (ppm, 298 K). The structures of two compounds (HCTPP)Fe(III)Br and (HCTPPO)Fe(III)Br, were determined by X-ray diffraction studies. In the first case, the iron(III) is five-coordinate with bonds to three pyrrole nitrogen atoms (Fe-N distances: 1.985(8), 2.045(7), 2.023(8) A), and the pyrrolic trigonal carbon (Fe-C: 1.981(8) A). The iron(III) of (HCTPPO)Fe(III)Br forms bonds to three pyrrole nitrogen atoms (Fe-N distances 2.104(5), 2.046(5), 2.102(5) A). The Fe-O 2.041(5) A and Fe-C(21) 2.192(5) A distances suggests a direct interaction between the iron center and the pi electron density on the carbonyl group in a eta2 fashion.

O-confusion approach in construction of carbaporphyrinoids

“…In general, carbaporphyrinoids provide a unique molecular platform that is suitable for exploring organometallic chemistry in a peculiar macrocyclic environment, forcing unusual coordination geometry and/or oxidation states of metal ions 6–8,11,12. The entrapment of metal ions in a coordination core of carbaporphyrinoids creates very efficient protection of the metal–carbon bond, and enables stabilization of extremely rare or virtually unknown oxidation/electronic states of metal ions in organometallic environments such as paramagnetic nickel(II),11,28 copper(II),29–31 silver(III),32,33 or iron(III) 34…”

“…Such couplings are usually classified as “through‐space” or “nonbonding,” because their magnitude cannot be rationalized in terms of the network of regular bonds in the molecule 42,45,46. In complementary investigations, the metal–arene interactions of Ni(II), Fe(II), and Fe(III) have been probed using their paramagnetically shifted 1 H NMR resonances 28,34,44,45,47. In our opinion, apart from pure structural considerations, the understanding of such a “borderline” MCH interaction is fundamental for exploration of the transient states active in organometallic catalysis.…”

“…[6][7][8]11,12 The entrapment of metal ions in a coordination core of carbaporphyrinoids creates very efficient protection of the metal-carbon bond, and enables stabilization of extremely rare or virtually unknown oxidation/electronic states of metal ions in organometallic environments such as paramagnetic nickel(II), 11,28 copper(II), [29][30][31] silver(III), 32,33 or iron(III). 34 Carbaporphyrinoids bear a functional resemblance to certain polydentate ligands, which, by virtue of a favorably oriented carbon donor, afford rare organometallic compounds of transition elements, often stabilizing atypical oxidation states or unusual coordination geometries. [35][36][37] Thus, the potentially terdentate monoanionic "pincer" ligands of a general formula [(2,6-(ECH 2 ) 2 -C 6 H 3 ] − , where E is a neutral, two-electron ᭤ Professor Lechos aw Latos-Gra y ski is a corresponding member of the Polish Academy of Sciences.…”

C−H and C−M Activation, Aromaticity Tuning, and Co⋅⋅⋅Ru Interactions Confined in the Azuliporphyrin Framework

“…The second scenario, resembling oxygenation of iron(III) Nconfused porphyrin, [26] implies the formation of 1-Co-O 2 as well but, in the following stage, the dioxygen insertion into CoÀC21h as been considered, yieldinga zuliperoxy-type intermediate (1-Co)-OO.T he subsequenti ntramolecular cleavage yields oxocobalt(IV) 21-oxyazuliporphyrin 2-Co-O.T he oxygen atom transfer from 2-Co-O to the suitable oxidants (1-Co or solventm olecules)p roduces the final 2-Co.F ormally, 1-Co-O and 2-Co can be recognized as structurali somers demonstrating the alternative localizationso ft he activated oxygen atom in highly oxidized species.…”

A dozen years of N-confusion: From synthesis to supramolecular chemistry

“…In the absence of covalent metal-carbon bond in these metal complexes, the center cation was coordinated by three inner nitrogen atoms and one axial ligand, and a side-on η 1 -or an agostic η 2 -interaction of inner C−H was suggested in the solid state. Oxidation of Fe II complex gave a pentacoordinated Fe III complex with a covalent Fe-C bond [39]. Moreover, several face-to-face sandwich-type dimers [(1-M II ) 2 ], wherein each metal cation is connected to one outer nitrogen and three inner nitrogen atoms as well as the inner carbon or C−H bond, were synthesized along with the monomeric complexes of Zn II [36], Fe II [37a], and Mn II [38a,b] metals (Scheme 5d).…”