Pradyut Kumar Das

Synthesis and characterization of bis(2,2′-bipyridine)(camphorquinone monoximato)ruthenium(II) perchlorate monohydrate

Abstract The electrophilic addition of cis -[RuCl(NO)(bipy) 2 ] 2+ (bipy = 2,2′-bipyridine) to camphor in boiling methanol containing sodium methoxide affords the new complex [Ru(A)(bipy) 2 ] + ( 1 ) (A = anion of camphorquinone monoxime). An alternative synthesis of 1 is provided by the reaction of cis -RuCl 2 (bipy) 2 with HA. The cation is isolated as crystalline perchlorate monohydrate. The diamagnetic complex displays an intense metal-to-ligand charge-transfer transition at ca 440 nm. A reversible rutheniuim(III)-ruthenium(II) couple near 0.8 V and two diimine reductions on the negative side of SCE are observed.

Mixed-ligand complexes of ruthenium(II)- and osmium(II)-2-(arylazo)pyridines with one 2,2′-bipyrazine/2,2′-bipyrimidine: synthesis, spectra and electrochemistry

Abstract A series of mixed-tris complexes of general formula [ML′L2](ClO4)2·H2O [M = Ru or Os; L′ = 2,2′-bipyrazine (bpyz) or 2,2′-bipyrimidine (bpym) and L = 2-(phenylazo)pyridine (papy) or 2-(m-tolylazo)pyridine (tapy)] has been prepared and characterized by physicochemical and spectroscopic methods. Using 1H NMR results (δMe of tapy), stereochemistry of ML2 fragment in these tris chelates has been determined. The complexes show a number of MLCT transitions in the visible region; the low intensity spin-forbidden transitions are observable in osmium complexes. In MeCN solution, the OsN6 unit displays one metal-centred oxidation and multiple ligand-based reductions. Relative π-acceptor ability of the ligands is clearly reflected in the electrochemical results.

Heteroleptic tris-complexes of osmium(III/IV)8-quinolinolates: synthesis, characterization and link-up of oxidation levels

SummaryA series of mixed-tris chelates of OsIII and OsIV of the general formula [OsDQ2]0/1+ [D =β-ketoenolates (D1-D3) or tropolonate (D4); Q = deprotonated 8-quinolinol] has been prepared and characterized by physico-chemical and spectroscopic methods. In MeCN solution the OsN2O4 unit displays reversible OsIV-OsIII and OsIII-OsII couples in the ca. - 1.2 to + 0.3 V range versus s.c.e. An irreversible oxidative response corresponding to a higher degree of oxidation is also observed at ca. 1.6 V. The chemistry in different oxidation states is discussed in terms of the electrochemical results.

Mixed-ligand complexes of osmium(III/IV)8-quinolinolates with one ketoximate ligand

SummaryA group of mixed-tris chelates of type [OsAQ2] [A = isonitrosoacetophenonate (A1) and isonitroso-propiophenonate (A2); Q = deprotonated 8-quinolinol (Q1) and 2-methyl-8-quinolinol (Q2)] have been prepared by two distinct synthetic approaches. [OsAQ2]+, obtained by CeIV oxidation of [OsAQ2], can be regenerated by hydrazine hydrate reduction of the former. The complexes, characterized by physico-chemical, magnetic and spectroscopic methods, exhibit several spin-allowed and spin-forbidden charge transfer transitions in their electronic spectra. In MeCN solution the OsN3O3 unit displays a nearly-reversible OsIV-OsIII change and an OsIII-OsII couple in the ca. -1.0– + 0.3V range versus s.c.e. The stability of the metal oxidation levels is discussed in terms of the chemical and electrochemical results.

Synthesis, characterization and reactivity of electrophilic mono- and binuclear nitrosyls of osmium(II)ketoximates

The reaction of [OsX2(HL)(L)] (1) {X = Cl or Br; HL = PhC(O)C(=NOH)Ph (HL1) or PhC(O)C(=NOH)Me (HL2)] with n-BuONO yields mononuclear [OsX(NO)(L1)2] (2) or binuclear [OsX2(NO)(L2)]22 (3) nitrosyls depending on L. The complexes are also obtained by reacting (1) with NaNO2 plus HCl. Molecular weight determinations are in agreement with mono- and binuclear formulations. The diamagnetic orange-red nitrosyls exhibit ν(NO) at ca. 1890 cm−1 indicating NO+ character of the bound nitrogen monoxide. In 1H-n.m.r. a single sharp L2 methyl signal is in line with the centrosymmetric geometry (4) of the binuclear nitrosyls. The complexes display both spin-allowed and -forbidden charge transfer transitions in the 1000–200 nm range. Both (2) and (3) are electroactive and reductions characteristic of mono- and binuclear compositions are observable on the negative side of s.c.e. They react smoothly with acetylacetone (acacH) in the presence of K2CO3 yielding K[Os(acamo)(L1)2] (5) and K[Os(acac)(acamo)(L2)] (6) [acamo = deprotonated MeC(O)C(=NOH)C(O)Me] respectively.

Synthesis and characterization of new mononitrosyl complexes of osmium-containing quinoline-8-olates

SummaryA high yield synthetic route to osmium mononitrosyls of type [OsX(NO)Q2] (2) [X = Cl or Br, Q = anions of quinolin-8-ol (HQ1) and its substituted species (HQ2)] from [OsX2Q2] (1) is described. The orange-red complexes are diamagnetic and exhibitν(NO) at ca. 1910 cm-1 in their i.r. spectra, indicating a high degree of nitrosonium ion (NO+) character. A single sharp Q2 methyl signal is in line with the geometry of (2). The complexes display both spin-allowed and spin-forbidden charge transfer transitions in the 800–200 nm range. Chemical reactions toward nucleophiles, namely acetylacetone and propiophenone, clearly establish the electrophilic nature of the bound NO.

Synthesis and characterization oftrans-dihalogeno(isonitrosoketonato) (isonitrosoketone)osmium(III)

SummaryReaction of [OsX6]2− (X = Cl or Br) with HK [HK = PhC(=O)C(=NOH)R; R=H, Me or Ph] yielded osmium(III) complexes of the type [OsX2(K)(HK)]. The OsX2 group wastrans and the hydrogen-bonded (K)(HK) moiety behaved as a planar tetradentate N2O2 chelator. The complexes were one-electron paramagnets and exhibited characteristic osmium(III) e.p.r. spectra. Several spin-allowed and spin-forbidden charge-transfer transitions were observed in the 200–1300 nm region.

Trans-dihalo(arylazo oxime) (arylazo oximato)ruthenate(II) anion and its reaction with tertiary phosphine bases: Spectroscopic and electrochemical characterization of the stereoretentive products

Abstract Reduction (chemical or electrochemical) of red-violet trans-[Ru111X2(HA)(A)] (3) [X = Cl or Br; HA = RC(NOH)NNAr (1)] affords stereoretentive green trans-[RuIIX2(HA)(A)]−, isolated as tetraphenylarsonim salt (4). Aqueous cerium(IV) converts 4 to 3. The reaction 4 with tertiary phosphine (PR3) bases furnishes stereoretentive (IR and 1H NMR results) products [RuX(PR3(HA)(A)] (5) and [Ru(PR3)2(HA)(A)]+, the latter being isolated as its perchlorate salt (6). All the diamagnetic complexes 4–6 display intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. The metal-centred oxidation and ligand-based reduction behaviours of the complexes are studied electrochemically in acetonitrile. Conjunctive electrochemical and spectroscopic (EPR) study is helpful in ascertaining the site of electron transfer. An attempt to correlate spectroscopic and redox parameters has been made.

Reaction of 2-(arylazo)pyridine complexes of type [OsX2L2] (X = Cl or Br) with heterocyclic bases (B): spectra and electrochemistry of [OsX(B)L2]ClO4H2O

SummaryThe synthesis and characterization of a series of complexes of the type [OsX(B)L2] + (X = Cl or Br, B = pyridine (py) and pyrazine (pyz), L = 2-(phenylazo)pyridine (L1) or 2-(m-tolylazo)pyridine (L2)) is described. The cations have been isolated as crystalline perchlorate monohydrates after purification by column chromatography. The diamagnetic compounds display several spin-allowed and spin-forbidden m.l.c.t. transitions in the visible region. The metal-centred oxidations and ligand-based reductions of the complexes are studied electrochemically in MeCN. Two consecutive one-electron oxidations corresponding to OsIII/OsII and OsIV/OsIII couples occur at ca. 1.3 and ca. 2.3 V versus s.c.e. respectively. Four successive azo-reductions are observable in the potential range -0.4 to -2.4V.

High potential isomeric dihalobis[2-(arylazo)pyridine] osmium(III) perchlorate monohydrate

SummaryThe preparation of isomeric complexes [OsIIIX2L2]ClO4· H2O [{(4)} and (5): X = Cl or Br, L(1) = 2-(phenylazo)-pyridine (L1) or 2-(m-tolylazo)pyridine(L2)] via stereo-retentive oxidation of the corresponding osmium(II) precursors [(2) and (3), respectively] is described. The complexes were characterized using spectroscopic and electrochemical methods. The low-spin (idealized t2g5; S = 1/2) paramagnetic complex ions display characteristic osmium(III) e.p.r. spectra in frozen (-196° C) MeCNPhMe. In dry MeCN solution, the OsX2N4 unit exhibits irreversible [OsX2L2]2+/[OsX2L2]+ and reversible [OsX2L2]+/[OsX2L2] couples at ca. 1.8 and 1.0 V versus saturated calomel electrode (s.c.e.), respectively. The use of (4)/(5) as an oxidant is noted.