Synthesis and Characterization of Iron N-Confused Porphyrins:  Structural Evidences of Agostic Interaction

Abstract

Agostic bonds are of interest in organometallic chemistry since they often lead to C-H activation. 1 The η 2 C-H interaction increases the acidity of the agostically bound proton so that deprotonation with a base frequently results. 2 Interestingly, the agostic bond was not widely considered as a possible intermediate in enzymatic hydrocarbon oxygenations until Collman and coworkers 3 recently demonstrated the inhibition of cyclohexane hydroxylation by H 2 or CH 4 in P-450 model complexes, and they suggested an agostic alternative to the P-450 rebound mechanism. Inspired by recent reports, we are aware of the intriguing tetraphenyl N-confused porphyrin (NCTPP, 1), which has an inner core C-H bond from the inverted pyrrole ring. To understand the bonding interaction between a metal center and an inner core C-H, we have prepared and structurally characterized novel iron NCTPP complexes.

[40]Nonaphyrin(1.1.1.1.1.1.1.1.1) and Its Heterometallic Complexes with Palladium–Carbon Bonds

“…In the latter, the Cu ion resides in the familiar porphyrin-like core consisting of pyrroles A, B, H, and I, the Pd1 ion is bound by C23, N6, N7, and C37 in an NNCC fashion analogous to doubly N-confused porphyrin-like coordination, [15] and the Pd2 ion is bound by N3, N4, and C22. Interestingly, the distance between Pd2 and C38 is 2.23 , which is distinctly longer than a normal C À Pd bond length, but is exactly what one would expect for an agostic interaction, thus suggesting an agostic interaction between Pd2 and the CÀH bond of pyrrole H. [16] As a consequence of the coordination of the two palladium ions, the tetrapyrrolic segment consisting of pyrroles D, E, F, and G becomes more coplanar in 13, with a mean plane deviation of 0.12 , while pyrrole H is tilted at a dihedral angle of 33.68. The absorption spectra of 11, 12, and 13 are similar to one another, and are considerably broader than those of 6 and 7 ( Figure 5).…”

Metal‐Mediated CH Bond Activation in a Carbon‐Substituted Hemiporphyrazine

“…[5] The chemistry of the N-confused porphyrins, [6] benziporphyrins, [7] and azuliporphyrins [8] all involve macrocycles in which an aromatic C À H bond is in close proximity to the metal binding site. The interior C À H bond can remain intact upon metal binding, as observed in the Mn, [9] Fe, [10] and Zn [11] complexes of N-confused porphyrins. [6] The exact nature of this CÀH metal "agostic" interaction is still under investigation, but the proximity of a metal center to this carbon atom activates it toward nucleophilic and electrophilic attack.…”

A Perspective of One‐Pot Pyrrole–Aldehyde Condensations as Versatile Self‐Assembly Processes

“…Thus, Ni II , Ni III , [59] Pd II , [52] Ag III , [60] and Mn III [61] complexes with approximately planar CTPP/CTTP ligands have been reported (Scheme 3). However, the central carbon atom tends to remain protonated in a number of transitionmetal-CTPP derivatives, in particular in Cu II , [62,63] Zn II , [64] high-spin Mn II , [65] and high-spin Fe II complexes [66] (Scheme 5). It appears that occupancy of the metal d s -type d x 2 -y 2 orbital discourages deprotonation of the central methine carbon atom.…”

“…The possibility of an agostic interaction between the metal center and the central CH unit has been suggested for these complexes. [65,66] Finally, the outer nitrogen atom can also participate in metal-ion coordination to give dimeric and other complex structures (Scheme 5). [52,[64][65][66][67][68] It is interesting to note that the availability of N-confused porphyrins through one-pot pyrrole-aldehyde condensations is largely limited to (CTPP)H 3 and (CTTP)H 3 at present.…”

“…As for other previously reported sapphyrins, NMR spectroscopy indicated (TPS)H 3 to be aromatic, but with a twist: The pyrrole ring opposite the bipyrrole unit was found to be inverted, with its NH group Scheme 5. Preparation and chemical reactivity of some Cu, [62,63] Zn, [64] Mn, [61,65] and Fe [66,69] N-confused porphyrin derivatives.…”