Tuning intermetallic electronic coupling in polyruthenium systems via molecular architecture

A large number of polynuclear ruthenium complexes encompassing selective combinations of spacer (bridging ligand,BL) and ancillary (AL) functionalities have been designed. The extent of intermetallic electronic communication in mixed-valent states and the efficacy of the ligand frameworks towards the tuning of coupling processes have been scrutinised via structural, spectroelectrochemical, EPR, magnetic and theoretical investigations. Moreover, the sensitive oxidation state features in the complexes of non-innocent quinonoid bridging moieties have also been addressed

A water soluble heteropolyoxotungstate as a selective, efficient and environment friendly oxidation catalyst

A series of water soluble Keggin type heteropolyoxotungstates have been tested as oxidation catalysts in aqueous-biphasic media with dilute H2O2 (30%) as the oxygen atom donor, without using any phase transfer agent. The Zn substituted polyoxoanion {(NH4)7Zn0.5[α-ZnO4W11O30ZnO5(OH2)]·nH2O} has been found to be the most efficient catalyst, which oxidizes a wide range of organic functionalities with good turnovers and high selectivities. The functionalities that undergo oxidations are: organic sulfides, pyridines, anilines, benzyl alcohols and benzyl halides. The oxidations of sulfides to sulfoxides and/or sulfones have been studied in detail, and a simple kinetic model consisting of two consecutive reactions, is shown to give good fit with the experimental data. In the catalytic system described here product isolation is easy, and the aqueous catalyst solution can be re-used several times with little loss in its efficiency

Reversible single-crystal to single-crystal transformations in a Hg(II) derivative. 1D-polymeric chain ⇋ 2D-networking as a function of temperature

Reactions of HgX2 (X = Cl-, Br-, l-) with the ligand hep-H (hep-H = 2-(2-hydroxyethyl)pyridine) in methanol at 298 K result in 1D-polymeric chains of [(X)Hg(μ-X)2(hep-H)]∞, 1-3, respectively, where hep-H binds to the Hg(II) ions in a monodentate fashion exclusively with the pyridine nitrogen donor and the suitably ortho-positioned -(CH2)2OH group of hep-H remains pendant. The packing diagrams of 1-3 exhibit extensive intramolecular and intermolecular hydrogen bonding interactions leading to hydrogen bonded 2D network arrangement in each case. Though the single crystal of either 2 (X = Br) or 3 (X = I) loses crystallinity upon heating, the single crystal of 1 selectively transforms to a 2D-polymeric network, 4 on heating at 383 K for 1.5 h. The polymeric 4 consists of central dimeric [Hg(μ3-Cl)(hep-H)Cl]2 units, which are covalently linked with the upper and lower layers of [-(μ-Cl)2-Hg-(μ-Cl)2-Hg(μ-Cl)2-]n. The packing diagram of 4 reveals the presence of O-H-Cl and C-H-Cl hydrogen bonding interactions which in effect yields hydrogen bonded 3D-network. Remarkably, the single crystals of 4 convert back to the single crystals of parent 1 on standing at 298 K for three days

UV-vis-NIR and EPR characterisation of the redox series [MQ3]2+,+,0,−,2−, M = Ru or Os, and Q = o-quinone derivative

The neutral title compounds with Q = 3,5-di-tert-butyl-o-quinone or 4,6-di- tert-butyl-N-phenyl-o-iminobenzoquinone (Qx) were studied by UV-vis-NIR spectroelectrochemistry and by EPR spectroscopy in the case of the odd- electron monocation and monoanion intermediates. Supported by DFT and TD-DFT calculations, the results indicate stepwise electron removal from predominantly ligand-based delocalised MOs on oxidation whereas the stepwise electron uptake on reduction involves unoccupied MOs with considerably metal–ligand mixed character. In both cases, the strong near-infrared absorption of the neutral precursors diminishes. In comparison to the ruthenium series, the osmium analogues exhibit larger transition energies from enhanced MO splitting and a different EPR response due to the higher spin–orbit coupling. The main difference between the quinone (1nnn, 2nnn) and corresponding monoiminoquinone systems (3nnn, 4nnn) is the shift of about 0.6 V to lower potentials for the monoimino analogues. While the absorption features do not differ markedly, the EPR data reflect a higher degree of covalent bonding for the complexes with monoimino ligands

On the non-innocence and reactive: Versus non-reactive nature of α-diketones in a set of diruthenium frameworks

α-Diketones are an important class of building blocks employed in many organic synthetic reactions. However, their coordination chemistry has rarely been explored. In light of this, our earlier report on [(acac)2RuII(μ-2,2′-pyridil)RuII(acac)2] (acac = acetylacetonate) showcased the sensitivity of a diketone fragment towards oxidative C-C cleavage. Following the lead, the synthesis of similar but stable diketo fragments containing diruthenium compounds was attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione (L) were prepared: diastereomeric [(acac)2RuIII(μ-L2-)RuIII(acac)2], 1a(rac)/1b(meso), [(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2, [2](ClO4)2 and [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2, [3](ClO4)2 with ancillary ligands of different donating/accepting characteristics. The metal is stabilised in different oxidation states in these complexes: Ru(iii) is preferred in 1a/1b when σ-donating acac is used as the co-ligand whereas electron rich Ru(ii) is preferred in [2](ClO4)2 and [3](ClO4)2 when co-ligands of moderate to strong π-Accepting properties are employed. The oxidative chemistry of these systems is of particular interest with respect to the participation of varying bridging-ligands which contain phenoxide groups. On the other hand, the reduction processes primarily resulting from the metal or the ancillary ligands are noteworthy as the normally reducible 1,2-diketo-group remains unreduced. These results have been rationalised and outlined from thorough experimental and theoretical investigations. The results presented here shed light on the stability of metal coordinated α-diketones as a function of their substituents.Fil: Khan, Farheen Fatima. Indian Institute Of Technology, Bombay; IndiaFil: Mondal, Saikat. Indian Institute Of Technology, Bombay; IndiaFil: Chandra, Shubhadeep. Universität Stuttgart; AlemaniaFil: Neuman, Nicolás Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Sarkar, Biprajit. Universität Stuttgart; AlemaniaFil: Lahiri, Goutam Kumar. Indian Institute Of Technology, Bombay; Indi

Synthesis, structure, spectral and electron-transfer properties of octahedral-[Co<SUP>III</SUP>(L)<SUB>2</SUB>]<SUP>+</SUP>/[Zn<SUP>II</SUP>(L)<SUB>2</SUB>] and square planar-[Cu<SUP>II</SUP>(L){OC(=O)CH<SUB>3</SUB>}] complexes incorporating anionic form of tridentate bis(8-quinolinyl)amine [N<SUP>1</SUP>C<SUB>9</SUB>H<SUB>6</SUB>-N<SUP>2</SUP>-C<SUB>9</SUB>H<SUB>6</SUB>N<SUP>3</SUP>, L<SUP>-</SUP>] ligand

The reaction of bis(8-quinolinyl)amine [N1C9H6-N2H-C9H6N3, LH] with CoII(ClO4)2 . 6H2O in methanol under aerobic conditions results in a new class of [CoIIIN6]+ (1+) chromophore incorporating an sp2-amido nitrogen center (N2) in the ligand frame. During the course of the reaction, the cobalt ion has been oxidized from its starting +2 oxidation state to +3 state in 1. The reaction of LH with the Cu-acetate yields monomeric square planar complex, [CuII(L){OC(=O)CH3}] (2). The same copper complex 2 is also obtained from Cu(ClO4) . 6H2O in presence of CH3COONa as base. On the other hand, the reaction of Zn(ClO4) . 6H2O with LH results in octahedral complex ZnII(L)2 (3). The Cu(II) complex 2 displays a four-line EPR spectrum at room temperature. Crystal structure of the free ligand (LH) shows that the amine proton [N(2)H] is hydrogen-bonded with the terminal quinoline nitrogen centers [N(1) and N(3)]. The crystal structure of 1 confirms the meridional geometry of the complex cation. The square planar geometry of copper complex 2 is confirmed by its crystal structure where the acetate function behaves as a monodentate ligand. The free ligand, LH, is found to be highly acidic in acetonitrile-water (1:1) medium and correspondingly the amine proton (NH) readily dissociates leading to its L- form even in absence of any external base. The pKb value of L- is determined to be 2.6. Both cobalt and copper complexes do not show any expected spin-allowed d-d transitions, possibly have masked by the intense charge-transfer transitions. However, in case of cobalt complex 1, one very weak unusual spin-forbidden 1A1g &#8594; 3T1g transition has been observed at 935 nm. The quasi-reversible cobalt (III)&#8596; cobalt(II) reduction of 1 is observed at E0, -1.0 V versus SCE. The reactions of bis(8-quinolinyl)amine [N1C9H6-N2H-C9H6N3, LH] with CoII(ClO4)2 . 6H2O, ZnII(ClO4)2 . 6H2O and CuII-acetate result in octahedral-[CoIII(L-)2]+ and [ZnII(L-)2] and square planar-[CuII(L-){-OC(=O)CH3}] complexes, respectively, incorporating an sp2-amido nitrogen center (N2) in the coordinated ligand frame of L. The structural, spectral and electrochemical aspects of the complexes have been described

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1–5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis- NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(RuII) → π*(pyrazine, tppz) back- bonding. The widely separated one-electron oxidation steps, RuIIRuII/RuIIRuIII and RuIIRuIII/RuIIIRuIII, result in large comproportionation constants (Kc) of ≥1010 for the mixed-valent intermediates. The syn-configurated 3n exhibits a particularly high Kc of 1012 for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru–N bond lengths for this RuIIRuIII intermediate. The electrogenerated mixed-valent states 1+–5+ exhibit anisotropic EPR spectra at 110 K with average values of 2.304–2.234 and g anisotropies Δg = g1–g3 of 0.82–0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1+–5+ in the NIR region at 1660 nm–1750 nm (ε ≈ 2700 dm3 mol−1 cm−1, Δν1/2 ≈ 1800 cm−1). DFT calculations of 1+ and 3+ yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru2.5)2. Rather large spin densities of about −0.4 were calculated for the tppz bridges in 1+ and 3+. The calculated electronic interaction values (VAB) for 1+–5+ are about 3000 cm−1, comparable to that for the Creutz–Taube ion at 3185 cm−1. The DFT calculations predict that the RuIIIRuIII forms in 12+–52+ prefer a triplet (S = 1) ground state with ΔE (S = 0 − S = 1) [similar]5000 cm−1. One-electron reduction takes place at the tppz bridge which results in species [Cl(L)RuII(μ-tppz˙−)RuII(L)Cl]− (1˙−–3˙−, 5˙−) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4n is distinguished by lability of the Ru–Cl bonds

Valence and spin situations in isomeric [(bpy)Ru(Q′)2]n (Q′ = 3,5-di-tert- butyl-N-aryl-1,2-benzoquinonemonoimine). An experimental and DFT analysis

The article deals with the ruthenium complexes, [(bpy)Ru(Q′)2] (1–3) incorporating two unsymmetrical redox-noninnocent iminoquinone moieties [bpy = 2,2′-bipyridine; Q′ = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, aryl = C6H5 (Q′1), 1; m-Cl2C6H3 (Q′2), 2; m-(OCH3)2C6H3 (Q′3), 3]. 1 and 3 have been preferentially stabilised in the cc-isomeric form while both the ct- and cc-isomeric forms of 2 are isolated [ct: cis and trans and cc: cis and cis with respect to the mutual orientations of O and N donors of two Q′]. The isomeric identities of 1–3 have been authenticated by their single-crystal X-ray structures. The collective consideration of crystallographic and DFT data along with other analytical events reveals that 1–3 exhibit the valence configuration of [(bpy)RuII(Q′Sq)2]. The magnetization studies reveal a ferromagnetic response at 300 K and virtual diamagnetic behaviour at 2 K. DFT calculations on representative 2a and 2b predict that the excited triplet (S = 1) state is lying close to the singlet (S = 0) ground state with singlet–triplet separation of 0.038 eV and 0.075 eV, respectively. In corroboration with the paramagnetic features the complexes exhibit free radical EPR signals with g [similar]2 and 1HNMR spectra with broad aromatic proton signals associated with the Q′ at 300 K. Experimental results in conjunction with the DFT (for representative 2a and 2b) reveal iminoquinone based preferential electron-transfer processes leaving the ruthenium(II) ion mostly as a redox insensitive entity: [(bpy)RuII(Q′Q)2]2+ (12+–32+) [leftrightharpoons] [(bpy)RuII(Q′Sq)(Q′Q)]+ (1+–3+) [leftrightharpoons] [(bpy)RuII(Q′Sq)2] (1–3) [leftrightharpoons] [(bpy)RuII(Q′Sq)(Q′Cat)]−/[(bpy)RuIII(Q′Cat)2]− (1−–3−). The diamagnetic doubly oxidised state, [(bpy)RuII(Q′Q)2]2+ in 12+–32+ has been authenticated further by the crystal structure determination of the representative [(bpy)RuII(Q′3)2](ClO4)2 [3](ClO4)2 as well as by its sharp 1H NMR spectrum. The key electronic transitions in each redox state of 1n–3n have been assigned by TD–DFT calculations on representative 2a and 2b

Metal ion-mediated selective activations of C-H and C-Cl bonds. Direct aromatic thiolation reactions via C-S bond cleavage of dithioacids

The reactions of potassium salt of dithiocarbonate, R'OCS2K,4 (R'=Me, Et,nPr,nBu,iPr,iBu, -CH2Ph) with the low-spinctc-RuII(L)2Cl2 1,ctc-OsII(L)2Br2 2 andmer-[CoII(L)3](ClO4)2.H2O3 [L=2-(arylazo)pyridine, NC5H4-N=N-C6H4(R), R=H,o-Me/Cl,m-Me/Cl,p-Me/Cl;ctc: cis-trans-cis with respect to halides, pyridine and azo nitrogens respectively) in boiling dimethylformamide solvent resulted in low-spin diamagnetic RuII(L')2,5, OsII(L')2 6 and [CoIII(L')2]ClO4 7 respectively (L'=o-S-C6H3(R)N=NC5H4N). In the complexes5, 6 and7 ortho carbon-hydrogen bond of the pendant phenyl ring of the ligands (L') has been selectively and directly thiolated via the carbon-sulphur bond cleavage of4. The newly formed tridenate thiolated ligands (L') are bound to the metal ion in a meridional fashion. In the case of cobalt complex (7), during the activation process the bivalent cobalt ion in the starting complex3 has been oxidised to the trivalent CoIII state. The reactions are highly sensitive to the nature and the location of the substituents present in the active phenyl ring. The presence of electron donating Me group at the ortho and para positions of the pendant phenyl ring with respect to the activation points can only facilitate the thiolation process. The complexes1c, 2c and3c) having chloride group at the ortho position of the active phenyl ring underwent the thiolation reaction selectively via the carbon-chloride bond activation process. The rate of carbon-chloride activation process has been found to be much faster compared to the C-H bond activation. The reactions are sensitive to the nature of the solvent used, taking place only in those having high boiling and polar solvents. The rate of the reactions is also dependent on the nature of the R' group present in4, following the order: Me~Et&gt;nPr&gt;nBu&gt;iPr>iBu&#187;-CH2Ph. The molecular geometry of the complexes in solution has been established by 1H and 13C NMR spectroscopy. The thiolated complexes (5, 6, 7) exhibit metal to ligand charge-transfer transitions in the visible region and intraligand &#960;-&#960;&#8727; and n-&#960;&#8727; transitions in the UV region. In acetonitrile solution the complexes display reversible MIII&#8596; MII reductions at 0.43 V for Ru (5a), 0.36 V for Os (6a) and -0.13 V for Co (7a) vs saturated calomel electrode (SCE)

Unconventional mixed-valent complexes of ruthenium and osmium

Recent developments have helped to extend the repertoire of mixed-valent ruthenium and osmium complexes beyond conventional systems. This extension has been achieved by using sophisticated ligands and by creating more variegated coordination patterns. The strategies employed include the use of multidentate ligands (which give rise to multinuclear and chelate complexes) and the use several redox active components (non-innocent ligands and oxidation-state ambivalence). The results offer enhanced chemical insight into metal–ligand electron-transfer situations and suggest that mixed-valent materials may eventually be exploited in molecular electronics and molecular computing