Azo Anion Research Articles

Performance and mechanism of chitosan-modified La-based MOF nanocomposites for high-efficiency adsorption of azo anion dye Congo red

The impact of organic dye wastewater discharge on the environment cannot be ignored following the development of modern industry. The adsorption method is widely used in wastewater treatment owing to its low cost and environmental friendliness. Improving the application performance of adsorption materials is the key to technical optimization. Herein, a lanthanum-based metal organic framework (La-BDC) and its derivative loaded with chitosan (La-MOF@CS) were synthesized with the solvothermal method, and the adsorption behavior and mechanism of anionic dye Congo red (CR) were studied. The experimental adsorption capacities of La-BDC and La-BDC@CS for 100 mg·L-1 CR at 25 ℃ were 260.46 and 227.82 mg·g−1, respectively, and maintained stable in a wide pH range and under different water quality conditions. The results of adsorption kinetics and adsorption isotherm showed that the adsorption process conformed to the pseudo-second-order kinetics and Langmuir model. The saturated adsorption capacities of the two materials were as high as 2249.56 and 2041.07 mg·g−1, respectively. La-BDC@CS maintained more than 95 % of the original adsorption capacity after ten cycles, which has practical application potential. Mechanism experiments show the adsorption of CR by La-BDC@CS is synergistically caused by various chemical mechanisms, such as hydrogen bonding, electrostatic attraction, and π-π coordination.

Isostructural behaviour in ammonium and potassium salt forms of sulfonated azo dyes.

The structures of five ammonium salt forms of monosulfonated azo dyes, derivatives of 4-(2-phenyldiazen-1-yl)benzenesulfonate, with the general formula [NH4][O3S(C6H4)NN(C6H3)RR']·XH2O [R = OH, NH2 or N(C2H4OH)2; R' = H or OH] are presented. All form simple layered structures with alternating hydrophobic (organic) and hydrophilic (cation, solvent and polar groups) layers. To assess for isostructural behaviour of the ammonium cation with M+ ions, the packing of these structures is compared with literature examples. To aid this comparison, the corresponding structures of four potassium salt forms of the monosulfonated azo dyes are also presented herein. Of the five ammonium salts it is found that three have isostructural equivalents. In two cases this equivalent is a potassium salt form and in one case it is a rubidium salt form. The isostructurality of ion packing and of unit-cell symmetry and dimensions tolerates cases where the ammonium ions form somewhat different interaction types with coformer species than do the potassium or rubidium ions. No sodium salt forms are found to be isostructural with any ammonium equivalent. However, similarities in the anion packing within a single hydrophobic layer are found for a group that consists of the ammonium and rubidium salt forms of one azo anion species and the sodium and silver salt forms of a different azo species.

Superparamagnetic Fe3O4@Al-based metal-organic framework nanocomposites with high-performance removal of Congo red

Azo dyes are one of the most widely used synthetic dyes, not only for printing and dyeing textiles, but also for dyeing leather, paper, and food. The removal of azo dyes is a major challenge to the environment. Under certain conditions, azo dyes can generate aromatic amines, which can covalently bind to human DNA bases through activation, causing human lesions and inducing cancer. Among them, Congo red (CR) is the most representative azo anion dyes. In this study, a hydrothermal method assisted by ultrasound was used to create superparamagnetic Fe3O4 @Al-based metal-organic framework (Fe3O4 @Al-MOF) nanocomposite for the removal of CR from water. According to characterization, Fe3O4 @Al-MOF was a stable nanocomposite with high specific surface area (526.73 m2·g−1) and superparamagnetic. The experimental results showed that Fe3O4 @Al-MOF could remove CR efficiently within 10 min and reach equilibrium within 50 min, and the adsorption capacity was as high as 2543.6 mg·g−1 under acidic and 313 K conditions, which was higher and faster than previously reported literature. The adsorption process was consistent with the pseudo-second-order model and the adsorption data were consistent with the Langmuir monolayer adsorption isotherm. After five cycles, the removal efficiency was still up to 83.7%, indicating the commercial potential of Fe3O4 @Al-MOF. The adsorption mechanism was investigated using FTIR and zeta potential, which revealed that the synergistic interplay of electrostatic interaction, π-π interaction, hydrogen bonding, and high porosity (56.7%) is the adsorption mechanism for CR removal by Fe3O4 @Al-MOF. Thus, Fe3O4 @Al-MOF nanocomposite is a promising material for removing CR.

Ruthenium-hydride-mediated stabilisation of the azo anion radical of azobis(benzothiazole) and its reversible electron-reservoir feature

Homolytic cleavage of the RII–H bond of A in contact with L facilitates the formation of H2 and L˙− derived 1–3, which undergo reversible electron exchange between 1–3 and 1+–3+.

Boehmite aerogel with ultrahigh adsorption capacity for Congo Red removal: Preparation and adsorption mechanism

As a toxic azo anion dye, Congo Red (CR) in waste-water poses a potential threat to the environment and human health. An absorbent is often used for its removal, during which, however, the role played by the functional groups is still not fully understood. In this work, a boehmite aerogel with a specific surface area as high as 305 m2/g was successfully developed by a combined sol–gel and freeze-drying method, and its CR adsorption capacity reached as high as 1962 mg·g−1. The adsorption process was dominated by a chemisorption mechanism and could be well described by the Langmuir isothermal model. The combination of the density functional theory (DFT) calculations and experimental results suggested that the amino protonation was responsible for the additional enhancement of CR adsorption in an acidic medium. The adsorption process occurred via interactions between boehmite aerogels and the SO3 functional groups of CR. Additionally, the NH3 group resultant from the protonation of NH2 functional groups of CR in an acidic medium significantly enhanced the CR adsorption on the boehmite aerogels. This work not only developed an ultrahigh efficiency anion adsorbent for CR removal, but also provided a new strategy for the exploration of new adsorbents with further improved performance.

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes.

The redox non-innocent behavior of the diaryl-azo-oxime ligand LNOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(LNO-)Cl(CO)(PPh3)2]+2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(LNO-)Cl(CO)(PPh3)2]+3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(LNO-)Cl2(PPh3)2] 4 and [Ir(LNO-)2Cl(PPh3)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(LNO-)Cl(CO2H)(PPh3)2] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO2H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(LNO˙-)Cl(CO)(PPh3)2] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Cis-Bis(2,2′-Azopyridinido)dicarbonylruthenium(II)

An [Ru(apy)2Cl2] precursor (apy = 2,2′-azopyridine) in 2-methoxyethanol was heated under a pressurized CO atmosphere to afford a diradical complex, [Ru(apy·−)2(CO)2], containing one-electron-reduced azo anion radical ligands. The electronic states of the complex were characterized by spectroscopic techniques and computational studies. Magnetic measurements revealed the existence of antiferromagnetic interactions in the diradical complex.

Barbituric acid-based mono/bi-heterocyclic dyes showing distinct spectral behaviors induced by solvents and pH

A series of barbituric acid-based heterocyclic dyes were prepared and fully characterized by ESI-MS, NMR and UV–Vis absorption spectroscopy. UV–Vis spectral comparisons reveal that in contrast to mono-heterocyclic dyes the absorption maxima of barbituric acid-based bi-heterocyclic dyes display significantly bathochromic shifts in wavelength due to the increasing π-conjugated system within the whole molecules. The effects of varying solvent media on the absorption spectra of this group of dyes were investigated, in which bi-heterocyclic dyes are liable to be affected by nature of solvents and present distinct solvatochromism in four kinds of organic solvents. Our results demonstrate that though the equilibrium between the hydrazone and deprotonated azo anion can be driven by pH values for this group of barbituric acid-based heterocyclic dyes, the acid-base effect on their absorption spectra behave differently. In order to further elucidate their different structural and spectral behavior, time-dependent density functional theory (TD-DFT) calculations with the implicit solvent model (SMD) were performed.

Monosulfonated Azo Dyes: A Crystallographic Study of the Molecular Structures of the Free Acid, Anionic and Dianionic Forms

Crystallographic studies of monosulfonated azo dyes have concentrated on the salt forms that contain the azo anion. Here we present a study that compares the structures of these anions with protonated free acid forms and with doubly deprotonated dianion forms. To this end, the new single crystal diffraction structures of 13 systematically related free acid forms of monosulfonated azo dyes are presented, together with three new structures of doubly deprotonated forms and two new structures of Na salt forms of azo anions. No structures of dideprotonated monosulfonated azo dyes have previously been reported and this paper also reports the first crystal structure of an azo dye with a hydronium cation. The geometries of the free acid, anion and dianion forms are compared to literature equivalents. It is shown that protonation of the azo bond gives predictable bond lengthening and shortening, which is of a greater magnitude than similar effects caused by azo-hydrazone tautomerisation, or the smaller again effects caused by the resonance electron donation from O or N based substituents. The dianion containing structures have twisted dianion geometries that can be understood based on the resonance effects of the phenoxide groups and upon the needs to bond to a relatively high number of metal cations.

Electron-transfer initiated nucleophilic substitution of thiophenolate anion by 1-chloro-substituted 4-(thiazol-2-ylazo)naphthalenes

In this work, the electrochemical transformation of 5-chloro-2-[(4-chloronaphthalen-1-yl)azo]thiazoles (A) into the corresponding radical anion A·− and its subsequent reaction with diphenyldisulfide (PhSSPh) was studied. It was found that the primarily generated azo anion radical A·− is able to initiate an electron transfer process which converts the disulfide into its thiolate anion PhS−. This anion was subsequently able to substitute the Cl- and H-groups by phenylmercapto moieties in the starting azo compound A. The structures of the phenylmercapto-substituted azo compounds thus generated were confirmed by thin-layer chromatography and mass spectrometry using independently prepared compounds as references.

Disulfonated azo dyes: metal coordination and ion-pair separation in twelve MII compounds of Ponceau Xylidine and Crystal Scarlet.

The structures of seven divalent metal cation compounds of Ponceau Xylidine {PX; systematic name of dication: 4-[2-(3,4-dimethylphenyl)hydrazin-1-ylidene]-3-oxo-3,4-dihydronaphthalene-2,7-disulfonate}, also known as Acid Red 26, CI 16150, and of five divalent metal cation compounds of Crystal Scarlet {CS; systematic name of dication: 8-[2-(naphthalen-1-yl)hydrazin-1-ylidene]-7-oxo-7,8-dihydronaphthalene-1,3-disulfonate}, also known as Acid Red 44, CI 16250, are presented. These are hexaaquamagnesium(II) PX dimethylformamide (DMF) monosolvate, [Mg(H2O)6](C18H14N2O7S2)·C3H7NO, (I); heptaaquacalcium(II) PX 2.5-hydrate, [Ca(H2O)7](C18H14N2O7S2)·2.5H2O, (II); catena-poly[aqua(μ-DMF)tris(DMF)bis(μ3-PX)distrontium(II)], [Sr(C18H14N2O7S2)(C3H7NO)2(H2O)0.5]n, (III); the transition-metal series hexaaquametal(II) PX DMF monosolvate, [M(H2O)6](C18H14N2O7S2)·C3H7NO, where M (metal) = Co, (IV), Ni, (V), Cu, (VI), and Zn, (VII); heptaaquacalcium(II) CS monohydrate, [Ca(H2O)7](C20H13N2O7S2)·H2O, (VIII); octaaquastrontium(II) CS monohydrate, [Sr(H2O)8](C20H13N2O7S2)·H2O, (IX); catena-poly[[triaqua(DMF)barium(II)]-μ-CS], [Ba(C20H13N2O7S2)(C3H7NO)(H2O)3]n, (X); tetrakis(DMF)(CS)copper(II) monohydrate, [Cu(C20H13N2O7S2)(C3H7NO)4]·H2O, (XI); and catena-poly[[[aquatris(DMF)zinc(III)]-μ-CS] diethyl ether hemisolvate], {[Zn(C20H13N2O7S2)(C3H7NO)3(H2O)]·0.5C4H10O}n, (XII). In all cases, the structures obtained were solvates with dimethylformamide (DMF) and/or water present. The disulfonated naphthalene-based azo anions adopt hydrazone tautomeric forms. The structures of the Mg salt and of four transition-metal forms (M = Co, Ni, Cu and Zn) of PX are found to form an isostructural series. All have solvent-separated ion-pair (SSIP) type structures and the formula [M(H2O)6][PX]·DMF. The Ca salt of PX also has an SSIP structure, but has a higher hydration state, [Ca(H2O)7][PX]·2.5H2O. In contrast, the Sr salt of PX, [Sr(PX)(DMF)2(H2O)0.5]n forms a one-dimensional coordination polymer. Both the Ca and the Sr salt of CS have an SSIP structure, namely [Ca(H2O)7][CS]·H2O and [Sr(H2O)8][CS]·H2O, whilst the heavier Ba analogue, [Ba(CS)(DMF)(H2O)3]n, forms a one-dimensional coordination polymer. Unlike PX, two CS structures containing transition metals are found to be coordination complexes, [Cu(CS)(DMF)4]·H2O and {[Zn(CS)(DMF)3(H2O)]·0.5Et2O}n. This suggests that CS is a better ligand than PX for transition metals. The Cu complex forms discrete molecules with Cu in a square-pyramidal environment, whilst the Zn species is a one-dimensional coordination polymer based on octahedral Zn centres.

A novel Wells–Dawson polyoxometalate-based metal–organic framework constructed from the uncommon in-situ transformed bi(triazole) ligand and azo anion

A novel Wells–Dawson polyoxometalate (POM)-based metal–organic framework (MOF) constructed from bi(triazole) ligand and azo anion, namely, [Ag13(L)6(N2)(HP2W18O62)] (1) (HL=4-(1H-1,2,4-triazol-3-yl)-2H-1,2,4-triazolyl) has been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, IR spectra, PXRD analysis. In compound 1, the rigid bi(triazole) ligands connected AgI ions to construct a 2D metal–organic layer, which was further extended by azo anions forming a 3D framework. The Wells–Dawson type [P2W18O62]6− anion as a multidentate inorganic ligand coordinated with the AgI ions, consolidating the 3D MOF. The L ligands and azo anions were in-situ transformed from a bi(triazole) derivative, which has never been observed in previous reports. In addition, the electrocatalytic and photocatalytic properties of compound 1 have been investigated.

Azo Anion Radical Complex of Rhodium as a Molecular Memory Switching Device: Isolation, Characterization, and Evaluation of Current–Voltage Characteristics

Two rare examples of azo anion diradical complexes of Rh(III) are reported. These complexes showed excellent memory switching properties with a large ON/OFF ratio and are suitable for RAM/ROM applications. Their electronic structures have been elucidated using a host of physical methods, including X-ray crystallography, variable-temperature magnetic susceptibility measurement, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory. The results indicate a predominant triplet state description of the systems with two ferromagnetically coupled radicals.

The cobalt(II) salt of the azo dye Orange G

Crystallizing the cobalt(II) salt of the azo dye Orange G from water was found to give the solvent-separated ion-pair species hexa­aqua­cobalt(II) 7-oxo-8-(2-phenyl­hydrazin-1-ylidene)-7,8-dihydro­naphthalene-1,3-disulfonate tetra­hydrate, [Co(H2O)6](C16H10N2O7S2)·4H2O. The asymmetric unit of the cobalt(II) salt contains three independent octa­hedral [Co(OH2)6]2+ cations, three azo anions, all with similar configurations, and 12 uncoordinated water mol­ecules. The structure is closely related to that of one of the known magnesium analogues. Both structures have Z′ = 3, feature nearly planar azo anions [maximum displacement of azo-N atoms from the plane of the phenyl ring = 0.058 (7) Å] in their hydrazone tautomeric form, form layer structures with hydro­philic and hydro­phobic layers alternating along the b-axis direction, and are stabilized by an extensive network of hydrogen bonds..

Anion radical of naphthyl-azo-imidazole in ruthenium and osmium carbonyls

The reaction of [M(H)(Cl)(CO)(PPh 3 ) 3 ] (M = Ru, Os) with 1-alkyl-2-(naphthyl-α/β-azo)imidazole (α-NaiR 1; β-NaiR, 2) in boiling dry heptane has afforded [M(Cl)(CO)(PPh 3 ) 2 (α/β-NaiR • )] (3/4 and 5/6) (α/β-NaiR • , azo anion radical) as the major product. The radicals are oxidized upon treatment with NH 4 PF 6 in dichloromethane-acetonitrile medium to one electron oxidized nou-radical salts (3 + /4 + and 5 + /6 + ). The complexes display redox couple in the range - 0.3 to -0.6 V vs SCE. The magnetic properties are studied by bulk (μ BM) and EPR measurements. The anion radicals are fluorescent and free ligands do not fluoresce (1, 2). DFT calculation shows that anion radical non-coordinated ligand is very much unstable while metal-carbonyls stabilize them.

Singlet Diradical Complexes of Chromium, Molybdenum, and Tungsten with Azo Anion Radical Ligands from M(CO)6Precursors

The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.

Variable‐Valent Rhenium Chemistry of Conjugated Nitrogenous Ligands: Structure and Reactivity

AbstractThe valence states of particular concern in this review are VI–I and the ligand types are diimine, azoimine, imineamide and azooximeall bidentate and N,N‐coordinating. Metal reduction potentials and stability of oxidation states are subject to control via variation of the nature and extent of N,N‐chelation and coligation. Back‐bonding plays a major role in determining stability and isomer specificity, particularly in the valence domain III–I. Three types of mediated oxygen atom transfer processes have been scrutinized and modeled: from ReVO to tertiary phosphanes and variable‐spacer diphosphanes, from water to diimine and from oxime to metal/coligand. Other reactivity topics include isomerizations associated with ligand substitution, stable azo anion radical generation and azo cleavage. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Unusual Reduction of Ammonium Heptamolybdate to Novel Molybdenum(IV)-Stabilized Azo Anion Radical Complexes

In an unusual reaction of the polyacid, ammoniumheptamolybdate tetrahydrate ((NH(4))(6)[Mo(7)O(24)].4H(2)O), and the ligand, 2-[(arylamino)phenylazo]pyridine (general abbreviation HL), in the presence of PPh(3) afforded the brown oxo free molybdenum complexes of type [Mo(L)(2)] in high yields (ca. 80%). The reaction occurs smoothly in ethanol. It is slow on a steam bath (25 h) but is complete in about an hour in a microwave oven. X-ray structures of two representatives are reported. In these complexes the ligand acted as a tridentate ligand using its pyridyl(N), azo(N), and the deprotonated amine(N), respectively. The geometry is meridional, and the relative orientations within the coordinated pairs of nitrogens are cis, trans, and cis, respectively. Bond length data of the coordinated ligands are consistent with a Mo(IV)[L.](2) ([L.](2-) = azo dianion radical formed by one electron reduction of the deprotonated anionic ligand, [L](-)) description. For example, the N-N lengths (1.349(5)-1.357(2) A) in these complexes are appreciably longer than that (1.246(3) A) in the uncoordinated and protonated salt of a representative ligand, [H(2)L(d)]ClO(4). The N-N lengths, however, correspond well with metal complexes of the ligand containing azo ion radical. The complexes are diamagnetic and showed highly resolved (1)H NMR and (13)C NMR spectra. The two coordinated ligands in these are magnetically equivalent, and resonances for only one ligand were observed in their spectra. Diamagnetism in the present molybdenum complexes is attributed to strong antiferromagnetic coupling between Mo(IV)(4d(2)) and the two planar radical [L.](2-) ligands. The complexes display multiple redox responses. The ESR spectrum of electrogenerated [1a](-) showed a characteristic spectrum for Mo(III) with weak hyperfine lines due to the presence of molybdenum isotopes having nonzero nuclear spin. Visible range multiple charge transfer transitions in these complexes are ascribed to ligand-to-metal transitions.

Monocarbaborane anion chemistry: use of an anilinyl residue as an anchor group for the construction of rod-like monocarbaboranyl diazo and imino compounds

Reaction of the [1-(4-H 2N–C 6H 4)- closo-1-CB 9H 9] − anion 1 with Me 2CHCH 2CH 2ONO and dilute HCl gives neutral [1-(4-N 2–C 6H 4)- closo-1-CB 9H 9] 3, which reacts with PhNH 2 to form the 19.5 Å [1-{4-(4-H 2N–C 6H 4–NN)–C 6H 4}- closo-1-CB 9H 9] − azo anion 2. Anion 1 with ketones and aldehydes gives imino compounds: with (CH 3) 2CO it gives neutral [1-{4-(Me 2CNH)–C 6H 4}- closo-1-CB 9H 9] 6 and with the para-dialdehyde C 6H 4-1,4-(CHO) 2 it gives the 30.5 Å extended rod-like neutral species [C 6H 4-1,4-{1-(CHNH–C 6H 4)- closo-1-CB 9H 9} 2] 8.

Azo anion radical complexes of osmium and related nonradical species.

The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap.-)(Br)(CO)(PPh3)2], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)2]Br, 7, as a minor byproduct. Upon replacing pap by the better pi-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp.-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7.CH2Cl2 have been determined. In 6b, the N-N length is 1.35(2) A, consistent with the anion radical description; in 7.CH2Cl2 the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be approximately 70% azo-pi* in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO --> LUMO transition, the LUMO being 95% pyridine-pi* in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features near g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)2]+, 6a+, which has been isolated as the diamagnetic PF6- salt; 6b+PF6- has been similarly prepared. The voltammetric reduction potentials of the 6+/6 couple follow the order 6a+/6a < 6b+/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a+ < 6b+. These trends are consistent with the pi-acidity order pap < abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P21/c (no. 14), a = 10.215(4) A, b = 17.634(7) A, c = 22.473(8) A, beta = 97.67(3) degrees , Z = 4; (7.CH2Cl2, C49H42BrCl2N3OOsP2) monoclinic, space group P2(1/n) (no. 14), a = 15.323(7) A, b = 15.201(6) A, c = 19.542(7) A, beta = 92.51(3) degrees, Z = 4.