Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes

Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium­(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium­(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through ¹H and ¹³C NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV–vis–near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their <i>fac</i>-Re­(CO)₃-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where <i>fac</i>-Re­(CO)₃ complexes of triazoles/triazolylidenes are likely to find use

(a) Regional genetic differentiation and (b) genetic differentiation according to taxonomy, based on AFLP and chloroplast DNA sequence data (<i>trn</i>L/F suprahaplotypes).

<p>Sample size (<i>n</i>), Nei's gene diversity (<i>H_E</i>), proportion of variable markers (FP), and nucleotide diversity (<i>π</i>) with standard deviation are provided. For <i>trn</i>L/F suprahaplotypes effective genetic diversity according to Gregorius (<i>V_a</i>) is additionally displayed. The following seven geographic regions were considered: (1) Balkan Peninsula (Balk), (2) Carpathians (Carp), (3) unglaciated Eastern and Southeastern Alps (UnglaESEAlps), (4) glaciated Eastern Alps (GlaEAlps), (5) glaciated Western Alps (GlaWAlps), (6) unglaciated Central Europe (UnglaCentrEur), and (7) glaciated northern Europe (GlaNEur). <i>Arabidopsis arenosa</i> var. <i>intermedia</i> is integrated within <i>A. arenosa</i> subsp. <i>arenosa</i>. <i>Arabidopsis nitida</i> was omitted from the analyses, as it was represented by one (AFLPs) and three (<i>trn</i>L/F suprahaplotypes) accession(s) only.</p

Chloroplast DNA <i>trn</i>L/F suprahaplotype networks of the <i>Arabidopsis arenosa</i> species complex.

<p>The sizes of the circles indicate the relative frequency of a suprahaplotype. Geographic regions, taxonomic entities, and cytotypes are indicated with the same colours as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone-0042691-g002" target="_blank">Figure 2</a>. A: Visualization according to geographic regions. B: Visualization according to taxonomy. <i>Arabidopsis arenosa</i> var. <i>intermedia</i> is marked with an asterisk. C: Visualization according to ploidal level.</p

Taxonomy, ploidal level, and geographic distribution of the various taxa of the <i>Arabidopsis arenosa</i> species complex (for details refer to the text).

<p>Several taxa are awaiting taxonomic recognition (indicated with nom. prov.).</p

Heterobimetallic Cu–dppf (dppf = 1,1′-Bis(diphenylphosphino)ferrocene) Complexes with “Click” Derived Ligands: A Combined Structural, Electrochemical, Spectroelectrochemical, and Theoretical Study

Heterodinuclear complexes of the form [(dppf)­Cu­(L)]­(BF₄) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene), where L are the chelating, substituted 4,4′-bis­(1,2,3-triazole) or 4-pyridyl­(1,2,3-triazole) ligands, were synthesized by reacting [Cu­(dppf)­(CH₃CN)₂]­(BF₄) with the corresponding “click” derived ligands. Structural characterization of representative complexes revealed a distorted-tetrahedral coordination geometry around the Cu­(I) centers, with the donor atoms being the P donors of dppf and the N donors of the substituted triazole ligands. The “local-pseudo” symmetry around the iron center in all the investigated complexes of dppf is between that of the idealized <i>D</i>_5<i>h</i> and <i>D</i>_5<i>d</i>. Furthermore, for the complex with the mixed pyridine and triazole donors, the Cu–N bond distances were found to be shorter for the triazole N donors in comparison to those for the pyridine N donors. Electrochemical studies on the complexes revealed the presence of one oxidation and one reduction step for each. These studies were combined with UV–vis–near-IR and EPR spectroelectrochemical studies to deduce the locus of the oxidation process (Cu vs Fe) and to see the influence of changing the chelating “click” derived ligand on both the oxidation and reduction processes and their spectroscopic signatures. Structure-based DFT studies were performed to get insights into the experimental spectroscopic results. The results obtained here are compared with those of the complex [(dppf)­Cu­(bpy)]­(BF₄) (bpy = 2,2′-bipyridine). A comparison is made among bpy, pyridyl-triazole, and bis-triazole ligands, and the effect of systematically replacing these ligands on the electrochemical and spectroscopic properties of the corresponding heterodinuclear complexes is investigated

Heterobimetallic Cu–dppf (dppf = 1,1′-Bis(diphenylphosphino)ferrocene) Complexes with “Click” Derived Ligands: A Combined Structural, Electrochemical, Spectroelectrochemical, and Theoretical Study

Heterodinuclear complexes of the form [(dppf)­Cu­(L)]­(BF₄) (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene), where L are the chelating, substituted 4,4′-bis­(1,2,3-triazole) or 4-pyridyl­(1,2,3-triazole) ligands, were synthesized by reacting [Cu­(dppf)­(CH₃CN)₂]­(BF₄) with the corresponding “click” derived ligands. Structural characterization of representative complexes revealed a distorted-tetrahedral coordination geometry around the Cu­(I) centers, with the donor atoms being the P donors of dppf and the N donors of the substituted triazole ligands. The “local-pseudo” symmetry around the iron center in all the investigated complexes of dppf is between that of the idealized <i>D</i>_5<i>h</i> and <i>D</i>_5<i>d</i>. Furthermore, for the complex with the mixed pyridine and triazole donors, the Cu–N bond distances were found to be shorter for the triazole N donors in comparison to those for the pyridine N donors. Electrochemical studies on the complexes revealed the presence of one oxidation and one reduction step for each. These studies were combined with UV–vis–near-IR and EPR spectroelectrochemical studies to deduce the locus of the oxidation process (Cu vs Fe) and to see the influence of changing the chelating “click” derived ligand on both the oxidation and reduction processes and their spectroscopic signatures. Structure-based DFT studies were performed to get insights into the experimental spectroscopic results. The results obtained here are compared with those of the complex [(dppf)­Cu­(bpy)]­(BF₄) (bpy = 2,2′-bipyridine). A comparison is made among bpy, pyridyl-triazole, and bis-triazole ligands, and the effect of systematically replacing these ligands on the electrochemical and spectroscopic properties of the corresponding heterodinuclear complexes is investigated

Principal Component Analysis of AFLP data from the <i>Arabidopsis arenosa</i> species complex.

<p>Each symbol represents an individual. A: Visualization according to geographic regions. The following seven geographic regions were considered: (1) Balkan Peninsula (Balk), (2) Carpathians (Carp), (3) unglaciated Eastern and Southeastern Alps (UnglaESEAlps), (4) glaciated Eastern Alps (GlaEAlps), (5) glaciated Western Alps (GlaWAlps), (6) unglaciated Central Europe (UnglaCentrEur), and (7) glaciated northern Europe (GlaNEur). These regions are illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone-0042691-g001" target="_blank">Figure 1</a>. B: Visualization according to taxonomy. <i>Arabidopsis arenosa</i> var. <i>intermedia</i> is marked with an asterisk. C: Visualization according to ploidal level. Data lacking ploidal level estimates are marked in grey.</p

Distribution of accessions from the <i>Arabidopsis arenosa</i> species complex investigated.

<p>Maximal glaciation and mountain glaciers of the LGM are drawn according to Ehlers and Gibbard <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone.0042691-Ehlers1" target="_blank">[32]</a>. The borders of the seven geographic regions are indicated (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone.0042691.s001" target="_blank">Table S1</a>, where the affiliation of each accession to one of these regions is listed). A: Visualization according to taxonomy. Seven entities are distinguished: <i>A. arenosa</i> subsp. <i>arenosa</i>, <i>A. arenosa</i> subsp. <i>borbasii</i>, <i>A. carpatica</i>, <i>A. neglecta</i>, <i>A. nitida</i>, and <i>A. petrogena</i>, following Měsíček <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone.0042691-Msek1" target="_blank">[14]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone.0042691-Msek2" target="_blank">[18]</a> and Kolník <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042691#pone.0042691-Kolnk1" target="_blank">[19]</a>, and <i>Arabidopsis arenosa</i> var. <i>intermedia</i> from the Alps. B: Visualization according to ploidal level (diploids and tetraploids). Ploidal level estimates were only available for a subset of accessions. C: Visualization according to chloroplast DNA suprahaplotypes.</p

Redox Behavior of a Dinuclear Ruthenium(II) Complex Bearing an Uncommon Bridging Ligand: Insights from High-Pressure Electrochemistry

A dinuclear ruthenium complex bridged by 2,3,5,6-pyrazinetetracarboxylic acid (μ-LH₂^2–) was synthesized and characterized by X-ray crystallography, cyclic voltammetry under ambient and elevated pressures, electron paramagnetic resonance (EPR) and UV/vis-NIR (NIR = near-infrared) spectroelectrochemistry, pulse radiolysis, and computational methods. We probed for the first time in the field of mixed-valency the use of high-pressure electrochemical methods. The investigations were directed toward the influence of the protonation state of the bridging ligand on the electronic communication between the ruthenium ions, since such behavior is interesting in terms of modulating redox chemistry by pH. Starting from the [Ru^II(μ-LH₂^2–)­Ru^II]⁰ configuration, which shows an intense metal-to-ligand charge transfer absorption band at 600 nm, cyclic voltammetry revealed a pH-independent, reversible one-electron reduction and a protonation-state-dependent (proton coupled electron transfer, PCET) reversible oxidation. Deeper insight into the electrode reactions was provided by pressure-dependent cyclic voltammetry up to 150 MPa, providing insight into the conformational changes, the protonation state, and the environment of the molecule during the redox processes. Spectroelectrochemical investigations (EPR, UV/vis-NIR) of the respective redox reactions suggest a ligand-centered radical anion [Ru^II(μ-LH₂^•3–)­Ru^II]⁻ upon reduction (EPR Δ<i>g</i> = 0.042) and an ambiguous, EPR-silent one-electron oxidized state. In both cases, the absence of the otherwise typical broad intervalence charge transfer bands in the NIR region for mixed-valent complexes support the formulation as radical anionic bridged compound. However, on the basis of high-pressure electrochemical data and density functional theory calculations the one-electron oxidized form could be assigned as a charge-delocalized [Ru^II.5(μ-LH₂^2–)­Ru^II.5]⁺ valence tautomer rather than [Ru^III(μ-LH₂^•3–)­Ru^III]⁺. Deprotonation of the bridging ligand causes a severe shift of the redox potential for the metal-based oxidation toward lower potentials, yielding the charge-localized [Ru^III(μ-LH^3–)­Ru^II]⁰ complex. This PCET process is accompanied by large intrinsic volume changes. All findings are supported by computational methods (geometry optimization, spin population analysis). For all redox processes, valence alternatives are discussed

RP-HPLC-MS (Reverse phase high-performance liquid chromatography-mass spectrometry) of abdominal and gluteal fat samples.

<p>Red colors represent abdominal depots, blue colors represent gluteal depots. A,B,C Boxplots show fatty acid saturation of abdominal and gluteal adipose tissue. D,E,F Boxplots show triglyceride saturation of abdominal and gluteal adipose tissue. First numbers indicate the number of atomic carbons, second numbers indicate the number of double bonds (e.g. 50-1; 50 atomic carbons (50 C), one double bond).</p