Redox-induced umpolung of transition metal carbenes

Abstract

An unprecedented umpolung of a nucleophilic palladium carbene complex was realized by successive one-electron oxidations to generate a cationic carbene complex, which shows electrophilic behavior toward nucleophiles resulting from a polarity inversion of the Pd–Ccarbene bond.

Synthesis and Structures of Stable PtII and PtIV Alkylidenes: Evidence for π‐Bonding and Relativistic Stabilization

“…Cyclic voltammetry experiments in o ‐difluorobenzene show reduction events at −0.56 and −1.94 V vs. Fc/Fc + , potentials virtually unchanged from the nickel complex in the same solvent (Figure ). These events have been clearly demonstrated to belong to one electron, ligand‐centered reduction in both 2 Ni and related palladium analogues …”

“…Despite the change in oxidation state and geometry, the DFT‐calculated LUMO is similar to the ligand‐centered LUMO of 2M (Figure S4, Supporting Information). However, the cyclic voltammograms of 3 are markedly different than related carbenes in the +2 oxidation state . Two irreversible reduction events are observed at −1.11 and −1.49 V vs. Fc/Fc + (Figure S6), with all subsequent features matching the cyclic voltammogram for 2 Pt .…”

“…Such an effect would lead to a better orbital energy match and orbital overlap with the vacant ligand π orbital, resulting in a stronger π‐bonding interaction, and therefore a higher energy π* orbital (LUMO). Given that the ligand‐centered π*‐type LUMO is known to be the locus of reduction in these species, information on the π*‐orbital energy levels, and therefore the bonding, can be gathered from the cyclic voltammograms of the complexes (Figure ).…”

Synthesis and Structures of Stable Pt(II) and (IV) Alkylidenes: Evidence for π-Bonding and Relativistic Stabilization

“…These events have been clearly demonstrated to belong to one electron, ligand--centred reduction in both 2 Ni 12 and related palladium analogues. 16 The CV of 2 Pt displays a lack of oxidation features, but despite this, reacts with the chlorine surrogate PhICl 2 (Scheme 2). Monitoring the reaction by 31 P{ 1 H} NMR spectrometry revealed the slow consumption 2 Pt and the clean formation of one new platinum containing product 3 at (30.6 ppm) with a concomitant decrease in the 1 J P--Pt coupling constant from 2588 Hz to 1574 Hz, consistent with oxidation of Pt (II) to Pt (IV).…”

“…39,40 Such an effect would lead to a better orbital energy match and orbital overlap with the vacant ligand π--orbital, resulting in a stronger π--bonding interaction, and therefore a higher energy π*--orbital (LUMO). As the ligand centred π*--type LUMO is known to be the locus of reduction in these species, 12,16 information on the π*--orbital energy levels, and therefore the bonding, can be gleaned from the cyclic voltammograms of the complexes (Figure 2). While the first and second reduction potentials of complex 2 Ni and 2 Pt are identical within error, the reduction potentials of 2 Pd are both shifted to more positive potentials by ca.…”