Electronic Structure of Transition Metal−Isocorrole Complexes: A First Quantum Chemical Study

Abstract

Keywords: Calculations / Corrole / Isocorrole / Non-innocent ligands / Radicals / Transition metals DFT calculations indicate that the broad electronic-structural features of metalloisocorroles are rather similar to those of analogous metallocorroles. Thus, like their corrole analogues, many metalloisocorroles feature substantially non-innocent ligands. Another key point is that both corroles and isocorroles can exhibit at least two kinds of radical character, a 2 and b 1 . However, corrole and isocorrole derivatives also differ significantly in a few ways: for example, the S = 1/2 CoPh complexes of corrole and isocorrole exhibit ground states of different symmetries ( 2 AЈЈ and 2 AЈ, respectively, inCompared with the current explosion of research on corrole derivatives, [1] the chemistry of isocorrole derivatives has remained essentially unexplored since their initial synthesis and characterization by Vogel and co-workers in 1997.[2] In this context, we chose to map out the broad electronicstructural features of prototype isocorrole first-row transition-metal complexes, using density functional theory (PW91/TZP) calculations as the key tool.[3Ϫ7] While chiefly of interest to porphyrin chemists, we were gratified to find that the results impinge on a range of issues of broader interest. For example, we believe that the findings deepen our understanding of the electron distributions of high-valent transition-metal complexes and of the phenomenon of ligand non-innocence. Indirectly, the results also help place the electronic structures of high-valent heme protein intermediates in a broader chemical context. [8Ϫ12] Our results on Sc(IsoC), Ga(IsoC), Ni(IsoC), and Cu-(IsoC) (IsoC ϭ isocorrolato) as well as analogous results on the corresponding corrole [13] complexes provide a good starting point for our discussion. Table 1 presents selected adiabatic [14] ionization potentials and excitation energies for these complexes. The results show that the two lowest ionized states of Sc(IsoC) and Ga(IsoC) are almost degenerate, as they are for the analogous corrole derivatives, [13] which is clearly reminiscent of the behavior of porphyrin A 1u and A 2u radicals.[15] Figure 1 depicts the open-shell orbitals of [a]

Ligand Noninnocence in Iron Corroles: Insights from Optical and X‐ray Absorption Spectroscopies and Electrochemical Redox Potentials

“…[1,5] In af irst approximation,n o substantial spin populations were found to reside on the corrole ligand. [1,5,23] The 1 HNMR spectra (Table 2) also evince no indication of an "a 2u -type"c orrole radical, which would have led to significant paramagnetic shifts for meso-aryl protons. [24] Carefule xamination of the DFT spin density profile does, however,r eveal small excess spin populations at certain carbon atoms, including the b-carbons C 3/17 and C 8/12 and the ortho and para carbons of the axial phenyl group.…”

“…DFT calculations have long indicated an essentially innocent corrole ligand for Fe[TPC]Ph . In a first approximation, no substantial spin populations were found to reside on the corrole ligand . The 1 H NMR spectra (Table ) also evince no indication of an “a 2u ‐type” corrole radical, which would have led to significant paramagnetic shifts for meso ‐aryl protons .…”

Metal complexes with non-innocent ligands

“…163 Isocorroles, as a whole, are also similar to corroles, but their ground states differ in symmetry ( 2 A HH for corroles and 2 A H for isocorroles), resulting in different d p ± p p metal ± ligand interactions. 163,164 Porphyrins, corroles and isocorroles can exist in two oxidised radical-cationic forms, which are very similar in energy. For copper corrole and isocorrole complexes, the Cu II p-radical cation has a slightly higher energy than the diamagnetic Cu III d 8 complex.…”

A DFT overview of high-valent iron, cobalt and nickel tetraamidomacrocyclic ligand (TAML) complexes: The end of innocence?

“…6. Note that the spin density is almost entirely concentrated on the Co, which is very different from the state of affairs in cobalt corroles and related complexes [36][37][38][39][40][41]. Thus, in this case, MAC * 4À ligand may be viewed as largely innocent.…”