Solvent-Induced Redox Isomerism of Cobalt Complexes with Redox-Active Bisguanidine Ligands

Redox-isomeric coordination compounds, in which the magnetic and optical properties could be varied by a stimulated intramolecular electron transfer between the metal and a redox-active ligand, are of interest for several applications in catalysis and materials science. In this work, the redox chemistry of cobalt complexes with redox-active bisguanidine ligands is studied; systematic modifications at the redox-active bisguanidine and the co-ligand units allow for fine-tuning of the electronic structure, which eventually leads to the first observation of redox isomerism for cobalt complexes with redox-active guanidine ligands. Redox isomerism is triggered by a change in the solvent properties.

Switching from Metal- to Ligand-Based Oxidation in Cobalt Complexes with Redox-Active Bisguanidine Ligands

The control of the redox reactivity, magnetic and optical properties of the different redox states of complexes with redox‐active ligands permits their rational use in catalysis and materials science. The redox‐chemistry of octahedrally coordinated high‐spin Co^II complexes (three unpaired electrons) with one redox‐active bisguanidine ligand and two acetylacetonato (acac) co‐ligands is completely changed by replacing the acac by hexafluoro‐acetylacetonato (hfacac) co‐ligands. The first one‐electron oxidation is metal‐centered in the case of the complexes with acac co‐ligands, giving diamagnetic Co^III complexes. By contrast, in the case of the less Lewis‐basic hfacac co‐ligands, the first one‐electron oxidation becomes ligand‐centered, leading to high‐spin Co^II complexes with a radical monocationic guanidine ligand unit (four unpaired electrons). Ferromagnetic coupling between the spins on the metal and the organic radical in solution is evidenced by temperature‐dependent paramagnetic NMR studies, allowing to estimate the isotropic exchange coupling constant in solution. Second one‐electron oxidation leads to high‐spin Co^II complexes with dicationic guanidine ligand units (three unpaired electrons) in the presence of hfacac co‐ligands, but to low‐spin Co^III complexes with radical monocationic, peralkylated guanidine ligand (one unpaired electron) in the presence of acac co‐ligands. The analysis of the electronic structures is complemented by quantum‐chemical calculations on the spin density distributions and relative energies of the possible redox isomers.

Stimulation of Redox-Induced Electron Transfer by Interligand Hydrogen Bonding in a Cobalt Complex with Redox-Active Guanidine Ligand

Octahedrally coordinated cobalt(II) complexes with a redox‐active bisguanidine ligand and acac co‐ligands were synthesized and their redox chemistry analysed in detail. The N−H functions in a bisguanidine ligand with partially alkylated guanidino groups form N−H⋅⋅⋅O hydrogen bonds with the acac co‐ligands, thereby massively influencing the redox chemistry. For all complexes, the first one‐electron oxidation is metal‐centred, leading to Co^III complexes with neutral bisguanidine ligand units. Further one‐electron oxidation is ligand‐centred in the case of Co–bisguanidine complexes with fully alkylated guanidino groups, giving Co^III complexes with radical monocationic bisguanidine ligands. On the other hand, the hydrogen‐bond strengthening upon oxidation of the Co–bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (Co^III→Co^II) and two‐electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox‐induced electron transfer by interligand hydrogen bonding.

1,2,5,6-Tetrakis(guanidino)-Naphthalenes: Electron Donors, Fluorescent Probes and Redox-Active Ligands

New redox-active 1,2,5,6-tetrakis(guanidino)-naphthalene compounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable further in two one-electron steps to the tetracations, are reported. Protonation switches on blue fluorescence, with the fluorescence intensity (quantum yield) increasing with the degree of protonation. Reactions with N-halogenosuccinimides or N-halogenophthalimides led to a series of new redox-active halogeno- and succinimido-/phthalimido-substituted derivatives. These highly selective reactions are proposed to proceed via the tri- or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications and further to tri- and tetracations. The integration of redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values and also allows reversible oxidation in two potentially separated one-electron steps.

Intramolecular metal–ligand electron transfer triggered by co-ligand substitution

Labile co-ligands are attached to a dinuclear copper(i) complex with a redox-active bridging guanidine ligand. Their substitution triggers electron-transfer from the copper atoms to the guanidine.

[Cu6 (NGuaS)6 ]2+ and its oxidized and reduced derivatives: Confining electrons on a torus

The hexanuclear thioguanidine mixed-valent copper complex cation [Cu₆ (NGuaS)₆ ]⁺² (NGuaS = o-SC₆ H₄ NC(NMe₂ )₂ ) and its oxidized/reduced states are theoretically analyzed by means of density functional theory (DFT) (TPSSh + D3BJ/def2-TZV (p)). A detailed bonding analysis using overlap populations is performed. We find that a delocalized Cu-based ring orbital serves as an acceptor for donated S p electrons. The formed fully delocalized orbitals give rise to a confined electron cloud within the Cu₆ S₆ cage which becomes larger on reduction. The resulting strong electrostatic repulsion might prevent the fully reduced state. Experimental UV/Vis spectra are explained using time-dependent density functional theory (TD-DFT) and analyzed with a natural transition orbital analysis. The spectra are dominated by MLCTs within the Cu₆ S₆ core over a wide range but LMCTs are also found. The experimental redshift of the reduced low energy absorption band can be explained by the clustering of the frontier orbitals. © 2017 Wiley Periodicals, Inc.

Dual role of polyphosphoric acid-activated nitroalkanes in oxidative peri-annulations: efficient synthesis of 1,3,6,8-tetraazapyrenes

Electrophilically activated nitroalkanes play a dual role in the novel reaction with diaminoperimidines, promoting their oxidative peri-annulation to access tetraazapyrenes.

A Comprehensive Study of Copper Guanidine Quinoline Complexes: Predicting the Activity of Catalysts in ATRP with DFT

Copper complexes of the hybrid guanidine ligands 1,3-dimethyl-N-(quinolin-8-yl)-imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)-guanidine (TMGqu) have been studied comprehensively with regard to their structural and electrochemical properties and their activity in atom transfer radical polymerization (ATRP). A simple analysis of the molecular structures of the complexes gives no indication about their activity in ATRP; however, with the help of DFT and NBO analysis the influence of particular coordinating donors on the electrochemical properties could be fully elucidated. With an adequate DFT methodology and newly applied theoretical isodesmic reactions it was possible to predict the relative position of the redox potentials of copper complexes containing DMEGqu and TMGqu ligands. In addition, predictions could be made as to whether the complexes of DMEGqu or TMGqu are more active in ATRP. Four new Cu(I) complexes were tested in standard ATRP reactions and kinetically investigated both in bulk and in solution. It could be proven that complexes featuring DMEGqu possess a lower redox potential and are more active in ATRP, although the tetramethylguanidine moiety represents the stronger donor.

Optical response of the Cu2 S2 diamond core in Cu2II(NGuaS)2 Cl2

Density functional theory (DFT) and time-dependent DFT calculations are presented for the dicopper thiolate complex Cu2 (NGuaS)2 Cl2 [NGuaS=2-(1,1,3,3-tetramethylguanidino) benzenethiolate] with a special focus on the bonding mechanism of the Cu2 S2 Cl2 core and the spectroscopic response. This complex is relevant for the understanding of dicopper redox centers, for example, the CuA center. Its UV/Vis absorption is theoretically studied and found to be similar to other structural CuA models. The spectrum can be roughly divided in the known regions of metal d-d absorptions and metal to ligand charge transfer regions. Nevertheless the chloride ions play an important role as electron donors, with the thiolate groups as electron acceptors. The bonding mechanism is dissected by means of charge decomposition analysis which reveals the large covalency of the Cu2 S2 diamond core mediated between Cu dz2 and S-S π and π* orbitals forming Cu-S σ bonds. Measured resonant Raman spectra are shown for 360- and 720-nm excitation wavelength and interpreted using the calculated vibrational eigenmodes and frequencies. The calculations help to rationalize the varying resonant behavior at different optical excitations. Especially the phenylene rings are only resonant for 720 nm. © 2016 Wiley Periodicals, Inc.

Bent and twisted: the electronic structure of 2-azapropenylium ions obtained by guanidine oxidation

New bis-2-azapropenylium ions are obtained by oxidation of guanidino-substituted aromatic compounds.

The control of the electronic structure of dinuclear copper complexes of redox-active tetrakisguanidine ligands by the environment

The electronic structures of dinuclear copper complexes of the general formula [GFA(CuX₂)₂], where X = Br or Cl and GFA denotes a redox-active bridging Guanidino-Functionalized Aromatic ligand, were analysed and compared.