The Redox Properties of Germylenes Stabilized by N‐Donor Ligands

Abstract

Germylenes, organic compounds of divalent germanium, are often considered as promising catalytic systems efficient in reactions of oxidative addition and reductive elimination, including SET reactions. However, the systematic studies of their redox properties, allowing for the quantitative evaluation of the levels of frontier molecular orbitals and the stability of single electron transfer products, are extremely limited in number. In this work, a series of germylenes with various types of stabilization: N−Ge−N, (N→)C−Ge−C(←N), N−Ge−N(←N), O−Ge−O(←N) и (N→)O−Ge−O(←N) (“−“ ‐a covalent bond, stabilization is achieved by p‐donation of a lone pair of electrons of N or O, “→” ‐donor‐acceptor bond) were characterized by cyclic voltammetry. The results were compared with UV/Vis spectroscopy data and DFT calculations. All germylenes studied in MeCN and THF with Bu4NPF6 as a supporting electrolyte are chemically irreversibly oxidized in the potential range of 0.5–1.4 V vs. Ag/AgCl. In all cases, as estimated by the quantum chemical calculations, the HOMO remains at the covalently unsaturated germanium center although being significantly redistributed over the molecular system. In turn, electroreduction of germylenes is possible only in the presence of a strong electron‐withdrawing substituent or a pyridine ring depleted in electron density donated to germanium. In both cases, the LUMO is localized exactly on these moieties and is absent on the germanium. Thus, the use of stabilizing groups makes it possible both to improve germylene stability and to fine‐tune their HOMO level that determines their ability to participate in oxidative addition reactions.

The Role of Ligands in Oxidative Addition Chemistry of Low-Valent Main Group Derivatives: Not Only Stabilization but Also Activation

“…An increase in the reactivity of germylene in the oxidative addition reaction in the presence of electron-pair donors can be associated with an increase in its relative HOMO level during complexation. This parameter can be estimated experimentally from the potentials of electrochemical oxidation, 30 which should decrease during the formation of an associate, reflecting the facilitation of electron transfer from the substrate molecule. As an example, Figure 4 shows the current−voltage curves of 1 in acetonitrile in the absence and presence of bipyridine, as well as the bipyridine oxidation curve without 1 for comparison.…”

Derivatives of penta-, hexa-, and hepta-coordinated tin with Schiff bases and 1,10-phenanthroline: structure, redox and optoelectronic properties

“…The examples include the dramatic decrease in the reduction potential and the increase in the availability of LUMO of germanium derivatives in the presence of N-donors, 2–4 the preparation of D⋯A-layered structures based on germanium complexes with 2,3-dihydroxynaphthalene and N , N ′-bidentate ligands with a narrow HOMO/LUMO gap (∼2 eV), 5 the fine tuning of the germylene HOMO level in the presence of N, S, P, and O-donor ligands, which determines the ability of germylenes to participate in oxidative addition reactions. 6,7 The key methods in the listed studies of germanium coordination compounds are voltammetry and UV-vis spectroscopy, which, when used in combination, make it possible to experimentally quantify the position of the frontier orbitals of the substrate.…”