Electronic structure predictions of the properties of non-innocent P-ligands in group 6B transition metal complexes

Abstract

Neutral group 6B (Cr, Mo, W) pentacarbonyl complexes M(CO)5-L possessing various P-ligands such as phosphines, phosphaalkenes, and phospha-quinomethanes can form radical cations and anions under redox conditions. There is significant interest in whether the radical site is localized on the metal or on a "non-innocent" ligand. Density functional theory was used to predict whether the radicals of the complexes behave as metal or ligand-centered radicals and whether these compounds could form in solution or as an ion pair with various oxidizing and reducing agents. The quinone-like ligands are predicted to be ligand centered radicals when they are anions and metal centered radicals when they are cations. The predicted reaction energies for single electron transfer (SET) reactions involving the quinone like ligands are negative or near thermoneutral for both radicals in polar solutions and as solid state ion pairs. The energetics of the SET reactions can be controlled by the nature of L, the nature of the oxidizing/reducing agent, and the solvent polarity. Such complexes could be used as flexible catalysts for single electron transfer reactions.

Analysis of Non‐innocence of Phosphaquinodimethane Ligands when Charge and Aromaticity Come into Play

“…Among these systems, the phosphaquino-dimethane ligand remained the least investigated -theoretically and experimentally -despite being the most promising non-innocent ligand. [12] Such complexes also inherently have the possibility to form distonic radical ions, i. e., having a separation of charge and radical sites. [13] Recently, we unveiled the radical formation pathway of P-CPh 3 substituted phosphaquinodimethane complexes and demonstrated a reversible one-electron reduction via cyclovoltammetry.…”

“…Electronic structures of group 6 metal complexes ( III , IV ) were investigated theoretically for the first time, focusing on bond dissociation energies (BDEs), frontier molecular orbitals (FMOs) and charge distributions [12] . One important result was that radical cations are most often metal‐centred, whereas radical anions are ligand‐centred.…”

“…One important result was that radical cations are most often metal‐centred, whereas radical anions are ligand‐centred. Among these systems, the phosphaquino‐dimethane ligand remained the least investigated – theoretically and experimentally – despite being the most promising non‐innocent ligand [12] . Such complexes also inherently have the possibility to form distonic radical ions, i. e ., having a separation of charge and radical sites [13] .…”

“…Electronic structures of group 6 metal complexes ( III , IV ) were investigated theoretically for the first time, focusing on bond dissociation energies (BDEs), frontier molecular orbitals (FMOs) and charge distributions. [12] One important result was that radical cations are most often metal‐centred, whereas radical anions are ligand‐centred. Among these systems, the phosphaquino‐dimethane ligand remained the least investigated – theoretically and experimentally – despite being the most promising non‐innocent ligand.…”

Starker Hinweis auf einen Phosphanoxylkomplex: Bildung, Bindung und Reaktivität komplexgebundener P‐Analoga von Nitroxiden

“…Diese Daten sind vergleichbar mit denen des P-OTEMP-substituierten Phosphan-Au I -Komplexes [9] (6), C10-P 1.831(2), C16-P 1.830(2), O1-P 1.639 (2), N-O1 1.482-(3);O1-P-W 121.66(6), C10-P-W 112.58 (8), C16-P-W 119.40 (8), N-O1-P 118.9(1), O1-P-C109 9.6(2), O1-P-C161 01.4(1), C16-P-C109 8.2 (2). [25] Darüber hinaus kann das W(CO) 5 -Fragment innerhalb des Komplexes 6 leichter vom P-zum O-Zentrum wandern, was schwach endergonisch ist (1.6 kcal mol À1 )u nd nur eine niedrige Barriere (3.5 kcal mol À1 ) überwinden muss.D er hieraus resultierende kO-Phosphanoyl-(auch Phosphinoyl- [11c] )Komplex 8 zeigt eine noch grçßere Delokalisation der Spindichte über das P-und O-Zentrum und das W(CO) 5 -Fragment, wobei das P-Zentrum einen hçheren Anteil als im Radikal 6 aufweist. Der metallfreie Gegenpart, das Phosphinoyl 9,wird eindeutig durch den P-zentrierten Radikalcharakter dominiert, was auch in zahlreichen Reaktionen demonstriert wurde.…”