Metrical Oxidation States of 2-Amidophenoxide and Catecholate Ligands: Structural Signatures of Metal–Ligand π Bonding in Potentially Noninnocent Ligands

Abstract

Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Qx)Pt(papy)] (Q = Substitutedo-Quinone oro-Iminoquinone; pap = Phenylazopyridine)

“…9 This concept, which takes all of the relevant C–C, C–O, and C–N bond lengths of such ligands into consideration for calculating MOS, is a convenient way of quantifying the structural results and correlating them to “formal” ligand oxidation states. Furthermore, such an analysis also provides valuable insight into the actual bonding situation in metal complexes of such noninnocent ligands.…”

Heptacoordinated Molybdenum(VI) Complexes of Phenylenediamine Bis(phenolate): A Stable Molybdenum Amidophenoxide Radical

“…The diamagnetism in 1'-DR is then due to the relatively strong antiferromagnetic coupling between these electrons, which results in an S = 0 state. (4) The concept of metrical oxidation state (MOS) was introduced recently by Brown 23 and it has been used to quantify the formal oxidation state of non-innocent (oxidized) amidophenoxide or catecholate ligands coordinated to metal ions by examination of their geometrical parameters.…”

A Synthetic Model of the Putative Fe(II)-Iminobenzosemiquinonate Intermediate in the Catalytic Cycle of o-Aminophenol Dioxygenases

“…Using the experimentally-derived correlations of bond distances and ligand oxidation states recently published by Brown, the O,N -donor ligand of [ 3DFT ] + has an oxidation state of −0.35(5) (i.e., partway between ISQ 1− and IBQ o ). 19 Moreoever, the highest-occupied spin-down MO (β-HOMO) of [ 3 ] + contains roughly equal Fe and ligand character (47 and 42%, respectively), and the β-LUMO is evenly delocalized over the two units (Figure S4). Thus, the DFT results suggest that the electronic structure of [ 3 ] + lies between the Fe 3+ – tBu ISQ and Fe 2+ – tBu IBQ limits.…”