Electronic characteristics of an extensive series of ruthenium complexes with the non-innocent o-benzoquinonediimine ligand: A pedagogical approach

Predictable electronic tuning of FeII and RuII complexes via choice of azine: correlation of ligand pKa with Epa(MIII/II) of complex

Five new mononuclear ruthenium(II) tris-ligated complexes have been synthesised, varying through the choice of azine in the family of 3-azinyl-4-(4-methylphenyl)-5-phenyl-4H-1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF6)2 (1), [Ru(Lpyridazine)](PF6)2 (2), [Ru(L4-pyrimidine)](PF6)2 (3), [Ru(Lpyrazine)](PF6)2 (4), [Ru(L2-pyrimidine)](PF6)2 (5). Three of them, 1·2MeCN·Et2O, 3·2MeCN·Et2O and 4·2MeCN, have been structurally characterised, confirming the presence of the meridional isomer, as was previously reported for the FeII analogues. Cyclic voltammetry studies, in dry CH3CN vs. Ag/0.01 M AgNO3, show that all five RuII complexes undergo a reversible RuIII/RuII process, with the midpoint potential (Em) increasing from 0.87 to 1.18 V as the azine is changed: pyridine < pyridazine < 2-pyrimidine < 4-pyrimidine < pyrazine. A strong inverse linear correlation (R2 = 0.98) is found between the RuIII/RuII redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower Em) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous FeII complexes, albeit at lower values of Em in all cases. In addition, the ionisation potentials of the RuII complexes, as well as those of the other group 8 analogues (FeII and OsII), showed a linear relationship with Epa. As the MIII/II redox potentials of a family of complexes has been previously reported to correlate with ligand pKa values, a computational protocol to calculate, in silico, the pKa of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated pKa of the Lazine ligand and the Epa of the MII complex, for all three families of complexes (R2 = 0.98).

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Ligand Electrochemical Parameter Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes

Using the density functional theory, [(N₂)Ru^IIL₅]ⁿ⁺ species are studied in silico. The properties of the Ru-N₂ bond are derived, including σ-donation, π-back donation, Ru-N and N-N bond lengths and bond orders, net charges and NN stretching frequencies, and so forth. These data are correlated using the ligand electrochemical parameter (E_L) theory, whereby the availability of electrons in the [RuL₅]ⁿ⁺ fragment is defined by its electron richness, which is the sum of the E_L parameters, ΣE_L(L₅). The objective is to better understand the binding of the N₂ ligand, leading to a molecular design whereby a specific species is constructed to have a desired property, for example, a particular bond length or charge. We supply cubic expressions linking ΣE_L(L₅) with these many metrics, allowing researchers to predict metric values of their own systems. The extended charge decomposition analysis is used. For the given target, N₂, σ-bonding does not vary greatly with the nature of ligand L, and π-back donation is the dominant property deciding the magnitudes of the various metrics. The E_L parameter provides the path to design the desired species. This contribution is devoted to dinitrogen, but the method is expected to be general for any ligand, including polydentate ligands.

Mixed-valence di-ruthenium(II,III) complexes of redox non-innocent N-aryl-o-phenylenediamine derivatives

A mononuclear ruthenium(ii), [(L1IQ)Ru2+(PPh3)2Cl2]·CHCl3 (1·CHCl3), a di-ruthenium(ii,ii), [(L2IQ)2Ru24+Cl4(PPh3)2] (2) and a mixed-valence di-ruthenium(ii,iii), [(L3IQ)Ru25+Cl5(PPh3)2]·MeOH (3·MeOH) complex, where L1IQ, L2IQ and L3IQ are, respectively, o-diiminobenzoquinone forms of redox non-innocent N-(5-nitropyridyl)-o-phenylenediamine (L1H2), N-(2,4-dinitrophenyl)-o-phenylenediamine (L2H2) and N-(3-nitropyridyl)-o-phenylenediamine (L3H2) derivatives, were successfully isolated. The molecular and electronic structures of 1·CHCl3, 2 and 3·MeOH were confirmed by single-crystal X-ray crystallography, EPR, UV-Vis-NIR spectroscopies and density functional theory (DFT) calculations. Both 1·CHCl3 and 2 exhibited reversible anodic waves due to the Ru(iii)/Ru(ii) redox couple, while the cyclic voltammogram of 3·MeOH displayed two successive cathodic waves due to ruthenium(iii)/ruthenium(ii) and (L3IQ/L3ISQ) redox couples. EPR spectroscopy and DFT calculations inferred that 1+ is a ruthenium(iii) complex of L1IQ, [(L1IQ)Ru3+(PPh3)2Cl2]+, 2+ is a mixed-valence di-ruthenium(ii,iii) complex of L2IQ, [(L2IQ)2Ru25+Cl4(PPh3)2]+ and 3- is a di-ruthenium(ii,ii) complex, [(L3IQ)Ru24+Cl5(PPh3)2]-, while 32- is a di-ruthenium(ii,ii) complex of L3ISQ, [(L3ISQ)Ru24+Cl5(PPh3)2]2-, where L3ISQ is the o-diiminobenzosemiquinonate anion radical form of the L3H2 with a significant contribution of the mixed-valence di-ruthenium(iii,ii) form, [(L3AM)Ru25+Cl5(PPh3)2]2- (L3AM is the di-amido form of the L3H2 ligand). The spin density obtained from the Mulliken spin population analyses of the broken-symmetry (BS) DFT calculations was localized on Ru(2) in 3, while it was delocalized over both Ru(1) and the o-phenylenediamine fragment in 32-. The inter-valence charge-transfer (IVCT) transitions of 2+ and 3 were relatively weaker and occurred at 1680 and 1685 nm, respectively, while 32- exhibited a broader NIR absorption band at 1500-2000 nm. The calculated electronic-coupling parameters (Hab) using the Mulliken-Hush equation for 3 and 2+ ion, respectively, were 62 and 103 cm-1, defining these as Robin-Day class II mixed-valence systems.

Highly covalent metal-ligand π bonding in chelated bis- and tris(iminoxolene) complexes of osmium and ruthenium

The bis(aminophenol) 2,2'-biphenylbis(3,5-di-tert-butyl-2-hydroxyphenylamine) (ClipH₄) forms trans-(Clip)Os(py)₂ upon aerobic reaction of the ligand with {(p-cymene)OsCl₂}₂ in the presence of pyridine and triethylamine. A more oxidized species, cis-β-(Clip)Os(OCH₂CH₂O), is formed from reaction of the ligand with the osmium(vi) complex OsO(OCH₂CH₂O)₂, and reacts with Me₃SiCl to give the chloro complex cis-β-(Clip)OsCl₂. Octahedral osmium and ruthenium tris-iminoxolene complexes are formed from the chelating ligand tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-4-methylphenyl)amine (MeClampH₆) on aerobic reaction with divalent metal precursors. The complexes' structural and electronic features are well described using a simple bonding model that emphasizes the covalency of the π bonding between the metal and iminoxolene ligands rather than attempting to dissect the parts into discrete oxidation states. Emphasizing the continuity of bonding between disparate complexes, the structural data from a variety of Os and Ru complexes show good correlations to π bond order, and the response of the intraligand bond distances to the bond order can be analyzed to illuminate the polarity of the bonding between metal and the redox-active orbital on the iminoxolenes. The osmium compounds'π bonding orbitals are about 40% metal-centered and 60% ligand-centered, with the ruthenium compounds' orbitals about 65% metal-centered and 35% ligand-centered.

A facile non-solvent induced phase separation process for preparation of highly porous polybenzimidazole separator for lithium metal battery application

The drawbacks of low porosity, inferior electrolyte wettability, low thermal dimensional stability and permissive lithium dendrite growth of the conventional microporous polyolefin-based separators hinder their widely application in the high power density and safe Lithium ion batteries. Herein, highly porous polybenzimidazole-based separator is prepared by a facile non-solvent induced phase separation process (NIPS) using water, ethanol, chloroform and ethyl acetate as the coagulation bath solvent, respectively. It was found that the ethanol is suitable to fabricate uniform morphology macroporous separator with the porosity of 92%, electrolyte uptake of 594 wt.%, and strong mechanical strength of 15.9 MPa. In addition, the experimental tests (electrochemical analysis and XPS test) and density functional theory calculation suggest that the electron-rich imidazole ring of polybenzimidazle can enhance Li⁺ mobility electrostatic attraction interaction while the block the PF₆⁻ mobility via electrostatic repulsion interaction. Therefore, high Li⁺ transference number of 0.76 was obtained for the neat polybenzimidazole-based polymer electrolyte. As a proof of concept, the Li/LiFePO₄ cell with the polybenzimidazole-based polymer electrolyte/1.0 M LiPF₆⁻ ethylene carbonate/dimethyl carbonate (v:v = 1:1) electrolyte exhibits excellent rate capability of >100 mAh g⁻¹ at 6 C (1 C = 170 mA g⁻¹) and superior cycle stability of 1000 cycles.

One-Electron Reduction of 2-Mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian)

The electrochemical characteristics of 2-mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian) have been investigated. One-electron reduction of dpp-mian involves the iminoketone fragment, which is revealed by the EPR spectrum obtained after the electrolysis of the dpp-mian solution in tetrahydrofuran (THF). The reduction of dpp-mian with one equivalent of metallic potassium leads to a similar EPR spectrum. The sodium complex [(dpp-mian)Na(dme)]₂ (1) produces an EPR signal with hyperfine coupling on the nitrogen atom of the iminoketone fragment of the dpp-mian ligand. Dpp-mian can also be reduced in a one-electron process by SnCl₂ ×(dioxane). In this case, complex (dpp-mian)₂ SnCl₂ (2) is formed, with the tin atom displaying an oxidation state of +4. Tin(II) chloride dihydrate, SnCl₂ ×2(H₂ O), also reduces dpp-mian, but the two ligands bound to tin in the product form a new carbon-carbon bond between the ketone moieties of the dpp-mian monoanions to form complex (bis-dpp-mian)HSnCl₃ (3). Metallic tin reduces dpp-mian to form the (bis-dpp-mian)₂ Sn (4) species. Compounds 1-4 were characterized by X-ray diffraction.

The coordination behavior of 2,3-bis(diphenylphosphino)maleic-N-phenylimide towards copper, silver, gold and palladium

In the coordination sphere of copper, silver and palladium 2,3-bis(diphenylphosphino)maleic-N-phenylimide can act as a radical, non-innocent ligand.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Radical non-radical states of the [Ru(PIQ)] core in complexes (PIQ = 9,10-phenanthreneiminoquinone)

9,10-Phenanthreneiminosemiquinonate anion radical (PIQ˙(-)) complexes of ruthenium of types trans-[Ru(II)(PIQ˙(-))(PPh3)2(CO)Cl] () and trans-[Ru(III)(PIQ˙(-))(PPh3)2Cl2] () are reported. Reactions of and with I2 afford trans-[Ru(III)(PIQ˙(-))(PPh3)2(CO)Cl](+)I3(-)·½CH2Cl2 ((+)I3(-)·½CH2Cl2) and trans-[Ru(PIQ˙(-))2(PPh3)2(μ-Cl)3](+)I3(-)·¼I2·¼toluene) ((+)I3(-)·¼I2·¼toluene), while the reaction of with Br2 yields a 9,10-phenanthreneiminoquinone (PIQ) complex of the type mer-[Ru(III)(PIQ)(PPh3)Br3]·½CH2Cl2 (·½CH2Cl2). In comparison, the reaction of trans-[Ru(III)(PQ˙(-))(PPh3)2Cl2] (PQ), a 9,10-phenanthrenequinone (PQ) analogue of affords only trans-[Ru(III)(PQ)(PPh3)2Cl2](+)Br3(-) ((+)Br3(-)). Considering the X-ray bond parameters, EPR spectra and the atomic spin densities obtained from the density functional theory (DFT) calculations, is defined as a PIQ˙(-) (average C-O/N and C-C lengths, 1.280(2) and 1.453(3) Å) complex of ruthenium(ii) while is a neutral PIQ (average C-O, C-N, C-C and C-O/N lengths, 1.248(7), 1.284(7), 1.485(8) and 1.266(7) Å) complex of the ruthenium(iii) ion. The single crystal X-ray bond parameters proposed that (+)I3(-) (average C-O/N and C-C lengths, 1.294(8) and 1.449(9) Å) and (average C-O/N and C-C lengths, 1.289(2) and 1.447(4) Å) are PIQ˙(-) complexes of ruthenium(iii), while the (+) ion (average C-O/N and C-C lengths, 1.288 ± 0.004 and 1.450 ± 0.017 Å) is a co-facial bi-octahedral complex of ruthenium(iii). In contrast, the (+) ion is a PQ complex of the ruthenium(iii) ion. EPR spectra and the calculated atomic spin densities authenticated that the (+) ion obtained after constant potential coulometric oxidation of is a PIQ complex of ruthenium(iii), while the (-) ion is a hybrid state of [Ru(II)(PIQ˙(-))] and [Ru(III)(PIQ(2-))] states. It is observed that the PIQ˙(-) state in which spin is more localized on the nitrogen (∼38% in and ∼35% in (-) ion) is stable and the coordination of the PIQ(2-) state is not observed in this study. Redox activities, UV-vis/NIR absorption spectra and their origins and the spectroelectrochemical measurements for → (+), → (-) and (+) → (2+) conversions are analyzed.

Comparison of Redox Activity between 2-Aminothioether and 2-Aminothiophenol: Redox-Induced Dimerization of 2-Aminothioether via C-C Coupling

Three chemical reactions of two 2-aminothioethers and 2-aminothiophenol with CpRu(II)Cl(PPh3)2 (Cp(-) = cyclopentadienyl anion), under identical reaction conditions, are reported. While 2-(methylthio)aniline, H2L(1) and an analogous substrate, 2-(phenylthio)aniline yielded dicationic dinuclear complexes [(PPh3)CpRu(II)(L(3/)L(4))Ru(II)Cp(PPh3)]Cl2 (where L(3) = (4E)-4-(4-imino-3-(methylthio)cyclohexa-2,5-dienylidene)-2-(methylthio)cyclohexa-2,5-dienimine ([1a]Cl2) and L(4) = (4E)-4-(4-imino-3-(phenylthio)cyclohexa-2,5-dienylidene)-2-(phenylthio)cyclohexa-2,5-dienimine ([1b]Cl2)), the reaction with 2-aminothiophenol (H2L(2)) produced a mononuclear complex [(PPh3)CpRu(II)(L(2))]Cl (where L(2) = 6-iminocyclohexa-2,4-dienethione) ([2]Cl). All these complexes are obtained in high yields (65%-75%). Formations of the products from the above reactions involve a similar level of oxidation of the respective substrate, although their courses are completely different. A comparison between the above two chemical transformations are scrutinized thoroughly. Characterizations of these complexes were made using a host of physical methods: X-ray crystallography, nuclear magnetic resonance (NMR), cyclic voltammetry, ultraviolet-visible (UV-vis), electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT). The complexes [1a]Cl2 and [1b]Cl2 showed intense metal-to-ligand charge transfer transition in the long wavelength region of the spectrum, at 860 and 895 nm, respectively, and displayed two reversible electron transfer (ET) processes at [1a](2+): -0.28 and -0.52 V; [1b](2+): -0.13 and -0.47 V, along with an irreversible ET process at 0.76 and 0.54 V, respectively. The ET processes at negative potentials are due to successive reductions of the bridging ligand, which are characterized by EPR and UV-vis spectroscopy. The one-electron reduced compound, [1a](+), showed a intraligand charge transfer transition (ILCT) at 1530 nm. The complex [2](+) showed a reversible ET process at -0.36 V and two irreversible ET processes at -1.04 and 1.18 V, respectively. DFT calculations were used to support the spectral and redox properties of the complexes and also to throw light on the difference of redox behavior between thioether and thiophenol substrates.

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.

Characterization of Porphyrin-Co(III)-'Nitrene Radical' Species Relevant in Catalytic Nitrene Transfer Reactions

To fully characterize the Co(III)-'nitrene radical' species that are proposed as intermediates in nitrene transfer reactions mediated by cobalt(II) porphyrins, different combinations of cobalt(II) complexes of porphyrins and nitrene transfer reagents were combined, and the generated species were studied using EPR, UV-vis, IR, VCD, UHR-ESI-MS, and XANES/XAFS measurements. Reactions of cobalt(II) porphyrins 1(P1) (P1 = meso-tetraphenylporphyrin (TPP)) and 1(P2) (P2 = 3,5-Di(t)Bu-ChenPhyrin) with organic azides 2(Ns) (NsN3), 2(Ts) (TsN3), and 2(Troc) (TrocN3) led to the formation of mono-nitrene species 3(P1)(Ns), 3(P2)(Ts), and 3(P2)(Troc), respectively, which are best described as [Co(III)(por)(NR″(•-))] nitrene radicals (imidyl radicals) resulting from single electron transfer from the cobalt(II) porphyrin to the 'nitrene' moiety (Ns: R″ = -SO2-p-C6H5NO2; Ts: R″ = -SO2C6H6; Troc: R″ = -C(O)OCH2CCl3). Remarkably, the reaction of 1(P1) with N-nosyl iminoiodane (PhI═NNs) 4(Ns) led to the formation of a bis-nitrene species 5(P1)(Ns). This species is best described as a triple-radical complex [(por(•-))Co(III)(NR″(•-))2] containing three ligand-centered unpaired electrons: two nitrene radicals (NR″(•-)) and one oxidized porphyrin radical (por(•-)). Thus, the formation of the second nitrene radical involves another intramolecular one-electron transfer to the "nitrene" moiety, but now from the porphyrin ring instead of the metal center. Interestingly, this bis-nitrene species is observed only on reacting 4(Ns) with 1(P1). Reaction of the more bulky 1(P2) with 4(Ns) results again in formation of mainly mono-nitrene species 3(P2)(Ns) according to EPR and ESI-MS spectroscopic studies. The mono- and bis-nitrene species were initially expected to be five- and six-coordinate species, respectively, but XANES data revealed that both mono- and bis-nitrene species are six-coordinate O(h) species. The nature of the sixth ligand bound to cobalt(III) in the mono-nitrene case remains elusive, but some plausible candidates are NH3, NH2(-), NsNH(-), and OH(-); NsNH(-) being the most plausible. Conversion of mono-nitrene species 3(P1)(Ns) into bis-nitrene species 5(P1)(Ns) upon reaction with 4(Ns) was demonstrated. Solutions containing 3(P1)(Ns) and 5(P1)(Ns) proved to be still active in catalytic aziridination of styrene, consistent with their proposed key involvement in nitrene transfer reactions mediated by cobalt(II) porphyrins.

Variable noninnocence of substituted azobis(phenylcyanamido)diruthenium complexes

The synthetic chemistry of substituted 4,4'-azobis(phenylcyanamide) ligands was investigated, and the complexes [{Ru(tpy)(bpy)}2(μ-L)][PF6]2, where L = 2,2':5,5'-tetramethyl-4,4'-azobis(phenylcyanamido) (Me4adpc(2-)), 2,2'-dimethyl-4,4'-azobis(phenylcyanamido) (Me2adpc(2-)), unsubstituted (adpc(2-)), 3,3'-dichloro-4,4'-azobis(phenylcyanamido) (Cl2adpc(2-)), and 2,2':5,5'-tetrachloro-4,4'-azobis(phenylcyanamido) (Cl4adpc(2-)), were prepared and characterized by cyclic voltammetry and vis-near-IR (NIR) and IR spectroelectrochemistry. The room temperature electron paramagnetic resonance spectrum of [{Ru(tpy)(bpy)}2(μ-Me4adpc)](3+) showed an organic radical signal and is consistent with an oxidation-state description [Ru(II), Me4adpc(•-), Ru(II)](3+), while that of [{Ru(tpy)(bpy)}2(μ-Cl2adpc)](3+) at 10 K showed a low-symmetry Ru(III) signal, which is consistent with the description [Ru(III), Cl2adpc(2-), Ru(II)](3+). IR spectroelectrochemistry data suggest that [{Ru(tpy)(bpy)}2(μ-adpc)](3+) is delocalized and [{Ru(tpy)(bpy)}2(μ-Cl2adpc)](3+) and [{Ru(tpy)(bpy)}2(μ-Cl4adpc)](3+) are valence-trapped mixed-valence systems. A NIR absorption band that is unique to all [{Ru(tpy)(bpy)}2(μ-L)](3+) complexes is observed; however, its energy and intensity vary depending on the nature of the bridging ligand and, hence, the complexes' oxidation-state description.

Ester-substituted cyclometallated rhodium and iridium coordination assemblies with π-bonded dioxolene ligand: synthesis, structures and luminescent properties

A series of functionalized cyclometallated rhodium and iridium coordination assemblies2–7displaying a π-bonded dioxolene ligand is described.

Metrical oxidation states of 2-amidophenoxide and catecholate ligands: structural signatures of metal-ligand π bonding in potentially noninnocent ligands

Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.

Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives

Recent advancement on the redox properties of a selection of transition metal complexes of the azoaromatic ligands: bidentate L(1) [2-(arylazo)pyridine] and tridentate HL(2) [2-(aminoarylphenylazo)pyridine] are described and compared. Due to the presence of a low lying azo-centered π*-orbital, these azoaromatic ligands may exist in multiple valent states. The coordination chemistry of the L(1) ligands was thoroughly studied during the 1980s. These complexes undergo facile reduction in solution at low accessible potentials. One electron reduced azo-complexes, though known for a long time to occur in solution, have only recently been isolated in a crystalline state. New synthetic protocols for the synthesis of metal-bound azo-radical complexes have been developed. Low-valent metal complexes such as metal carbonyls have been found to be excellent starting materials for this purpose. In a few selected cases, syntheses of these complexes were also achieved from very high valent metal oxides using triphenylphosphine as both a reducing and oxo-abstracting agent. Issues related to the ambiguities of the electronic structures in the azo-metal complexes have been discussed considering bond parameters, redox and spectral properties. Unusual redox events such as RIET (Redox-Induced Electron Transfer) phenomena in a few systems have been elaborated and compared with the known example. Novel examples of N=N bond cleavage reactions via four-electron reduction and subsequent C-N bond formation in metal-bound coordinated ligands have been noted.