Sujit Baran Kumar

Chemistry of molybdenum. Part 7. Reactivity of terminal oxo-ligands in cis-[MoO2(acda)2](Hacda = 2-aminocyclopent-1-ene-1-carbodithioic acid) toward proton- and electron-donor reagents. Synthesis, redox chemistry and spectroscopic characterisation of neutral seven-co-ordinate catecholato and aryldiazenido compounds of molybdenum

The reactivity of co-ordinated oxo-ligands in cis-[MoO2(acda)2](Hacda = 2-aminocyclopent-1-ene-1-carbodithioic acid) toward proton- and electron-donor reagents has been investigated. Molybdenum(VI) compounds [MoO(L)(acda)2](L = catecholate, 2a; 4-tert-butylcatecholate, 2b; 3,5-di-tert-butylcatecholate, 2c; naphthalene-2,3-diolate, 2d; or tetrachlorocatecholate, 2e are obtained when catechols are used for oxo abstraction. Substituted phenylhydrazines on the other hand form non-oxodiazenido complexes [Mo(N2C6H4X-p)(acda)3](X = H, 3a; Me, 3b; or Cl, 3c). The compounds have been characterised by electronic, IR and ESR spectroscopy and electrochemistry. In the visible region the electronic spectra are dominated by a strong ligand-to-metal charge-transfer (l.m.c.t.) band for both series of compounds. Their electrochemistry has been studied in dimethylformamide at a platinum electrode. For the catecholato compounds 2a–2e a metal-based reduction (MoVI–MoV, Epc) and a ligand-based oxidation (Epa) each involving a single electron are observed. Substituents in the catecholato ring have a significant influence on the energy of the l.m.c.t. transitions (Eop) as well as on Epc and Epa. A linear relationship exists between Eop and ΔE(redox) where ΔE(redox)=Epc–Epa. The oxidised species derived from the 0/1 + redox couple of the diazenido series 3a–3c are ESR active with hyperfine spectra due to 95,97Mo and 14N couplings. The electron-transfer orbital in this case is believed to be metal based with sufficient mixing from the diazenido nitrogen orbital.

A novel tridentate co-ordination mode for the carbonatonickel system

The trinuclear compound [Ni 2 (µ-CO 3 )(dmpd) 4 (H 2 O)][Ni(dmpd) 2 (H 2 O) 2 ][ClO 4 ] 4 ·H 2 O was obtained from the reaction of basic solutions of nickel(II) perchlorate and 2,2-dimethylpropane-1,3-diamine (dmpd) with atmospheric CO 2. It crystallises in the orthorhombic system, space group Pcab, with a = 18.634(10), b = 25.447(8), c = 25.598(4) A, Z = 8, R = 0.0437. The three nickel atoms show octahedral co-ordination with three different environments, Ni(CO 3 -O,O′)(dmpd) 2 , Ni(CO 3 -O″)(dmpd) 2 (H 2 O) and Ni(dmpd) 2 (H 2 O) 2 . The carbonate anion acts as a bridge between two nickel ions whereas the [Ni(dmpd) 2 (H 2 O) 2 ] 2+ subunit is linked by hydrogen bonds to the dinuclear group. The dinuclear [Ni 2 (µ-CO 3 )(dmpd) 4 (H 2 O)] 2+ subunit shows a moderately weak antiferromagnetic coupling with a J value of -7.8 cm -1 . The tridentate co-ordination mode found in the carbonate ligand has not previously been reported for nickel. Its magnetic behaviour is discussed.

Chemistry of molybdenum. Part 6. Synthesis, spectroscopic and redox properties of some eight-co-ordinate sulphur chelates, [MoIV(S2CNR2)n(acda)4–n] and [MoV(S2CNR2)n(acda)4–n]Br (n= 2 or 3), containing MoS8 chromophores (acda = 2-aminocyclopent-1-ene-1-carbodithioate)

Eight-co-ordinated mixed-ligand complexes [MoIV(S2CNR2)n(acda)4–n]1a–1d and [MoV(S2CNR2)n(acda)4–n]Br 2a–2d(n= 2 or 3) containing predominantly σ-donor S2CNR2(R = Et or Pr) and π-acceptor acda (2-aminocyclopent-1-ene-1-carbodithioate) ligands have been synthesised and characterised by UV/VIS, magnetic, ESR and electrochemical studies. For the lower-energy ligand-to-metal charge-transfer bands of compounds 1a–1d the observed spectral intensity showed an almost linear increment with the number of co-ordinated acda ligands. The molybdenum(V) compounds 2a–2d are ESR active, and their frozen solutions (140 K) give rise to axial spectra with grossly identical features (gII≈ 1.983, AII≈ 52.5 × 10–4 cm–1: g⊥≈ 1.984, A⊥≈ 21.5 × 10–4 cm–1) consistent with the electron being localised in a metal-centred orbital of nearly constant composition for the entire series of compounds. Cyclic voltammetry indicates the existence of three molybdenum oxidation states [MonS8] with n=III, IV and V. The E½ values (vs. saturated calomel electrode) are –0.40 and –0.18 V for the MoIII→ MoIV and MoIV→ MoV couples respectively and independent of whether the starting compound is a molybdenum(IV), 1a, 1c, or a molybdenum(V) species 2a–2d. Of the two types of ligand systems present in [Mo(S2CNR2)n(acda)4–n]m(m= 0 or +1), acda has been found to have a profound influence in controlling the redox and charge-transfer properties.

Mononuclear manganese(III) complexes of a heterodonor (N2OS) ligand containing thiolate-type sulfur: synthesis, structure, redox and spectroscopic properties

Two series of five-co-ordinate high-spin (S= 2) mononuclear manganese(III) complexes have been prepared of general formulae [MnL(X)](X = Cl 1a, Br 1b, NCS 1c or N31d) and [MnL(B)]CIO4[B = pyridine (py)2a, 3-methylpyridine (3Me-py)2b or 4-methylpyridine (4Me-py)2c] using methyl 2-[2-(salicylideneamino)ethylamino]cyclopent-1-ene-1-carbodithioate (H2L) as tetradentate ligand with a (N2OS)2– donor set. The complexes have been characterised by a combination of IR and UV/VIS spectroscopy, magnetic measurements and electrochemical studies. Their UV/VIS spectra show two ligand-field transitions at ca. 685 (5B1→5E) and 570 nm (5B1→5B2) along with a strong charge-transfer band in the near-UV region (ca. 440 nm), assigned as a phenolate O→Mn(dπ·) ligand-to-metal charge-transfer transition. Cyclic voltammograms of the complexes (except 1d) exhibit a one-electron quasi-reversible MnIII–MnII reduction with E½ close, to –0.12 V (vs. saturated calomel electrode). For 1a–1c irreversible MnIII–MnIV oxidation is also observed. The compounds have an axially elongated square-pyramidal structure as dictated by the Jahn–Teller effect with ligands X or B occupying the axial position. Their electronic spectra and electrochemical properties show invariance with group X (or B) in the axial position. The crystal structure of complex 1b was solved by Patterson and Fourier methods followed by a least-squares refinement to a conventional R value of 0.037. The basal plane of the five-co-ordinated manganese centre is defined by N (two), S and O atoms, derived from the ligand L. The co-ordinated S atom behaves like a thiolate anion as reflected by the C–S bond distance 1.717(9)A.

Chemistry of molybdenum. Part 9. Non-oxomolybdenum(IV) and mononuclear oxomolybdenum(V) complexes with dithioacid ligands

The syntheses and characterisation of a series of non-oxomolybdenum(IV) and mononuclear oxomolybdenum(V) complexes are described with 2-aminocyclopent-1 -ene-1 -carbodithioate (acda) and its N-alkyl derivatives [aacda; alkyl = ethyl (eacda), propyl (pacda) or butyl (bacda)] as the bidentate (S–S)– donor ligands. The molybdenum(IV) complexes [Mo(ox)(aacda)(acda)](ox = oxalate) are feebly paramagnetic (ca. 0.9 µB) with a spin-paired d2 ground state. They exhibit rich electrochemistry consisting of two reversible one-electron couples: MoIV–MoIII(E1/2≈–0.45 V) and MoIV–MoV(E½+0.54 V), indicating conservation of the six-co-ordinated structure in [Mo(ox)(aacda)(acda)]Z(z=–1, 0 or +1). The molybdenum(V) complexes [MoOCl(L)2](L = acda 2a, eacda 2b, pacda 2c or bacda 2d) are paramagnetic (1.56 – 1.68 µB) with one unpaired electron. Their electronic spectra in the visible region are dominated by high intensity multiple bands arising from S(π)→ Mo(dπ) charge transfers. The cyclic voltammograms of the complexes show a reversible, one-electron MoV–MoIV reduction and an irreversible MoV–MoIV oxidation. In frozen solution (80 K), 2a has an axially anisotropic ESR spectrum indicating axial co-ordination of Cl. For the remaining compounds 2b–2d the features are much more complicated due to the hyperfine interactions from the chloro ligand providing evidence for possible equatorial co-ordination of Cl.

Synthesis and characterisation of sulfur-rich manganese(III) and vanadium(IV) complexes containing dithioacid ligands

The chemistry of the bidentate (S,S′)– donor ligand 2-aminocyclopent-1-ene-1-carbodithioic acid (Hacda) and its N-alkylated derivatives [R = Et (Heacda), Prn(Hpacda), or Bun(Hbacda)] with vanadium(IV) and manganese(III) has been studied. The vanadium(IV) complexes [V(aacda)4]1a–1d are all eight-co-ordinated non-oxo species containing a VS8 chromophore. Their ESR spectra in frozen solution (140 K) exhibit axial anisotropy with a 16-line 51V hyperfine pattern. The observed order in spin-Hamiltonian parameters (g‖≈ 1.932, g⊥≈ 1.985, A‖≈ 175 × 10–4 cm–1 and A⊥≈ 74 × 10–4 cm–1) indicates a dx2–y2 ground state with a dodecahedral (D2d) structure. Electrochemical studies of 1a–1d show a quasi-reversible one-electron VIV–VIII reduction at ≈–0.7 V vs. saturated calomel electrode (SCE). The manganese(III) complexes [Mn(aacda)3]2a–2c are all high-spin species (µeff 5.01–5.10) and exhibit two ligand-to-metal charge-transfer bands at ≈ 610 and 530 nm. Their cyclic voltammograms reveal the presence of two quasi-Nernstian couples: E½(ox)= 0.1 V and E½(red)=–0.33 V vs. SCE due to MnIV–MnIII and MnIII–MnII electron transfers respectively.

Chemistry of molybdenum. Part 8. Syntheses, spectroscopy and electrochemical properties of molybdenum complexes containing dithioacid ligands

Complexes cis-[MoVIO2(aacda)2][aacda = 2-(alkylamino)cyclopent-1-ene-1-carbodithioate; alkyl = propyl (pacda) or butyl (bacda)] react with acids HX (X = BF4 or PF6) to form species [MoVIO(aacda)3]X 1a–1d, which contain an MoVIO core. Reactivity of the terminal oxo ligand of this core towards electron-donor reagents has been investigated. Non-oxo monomeric molybdenum(V) compounds [MoV(acda)(aacda)3]BF42a, 2b and [MoV(abt)(aacda)3]BF42c, 2d can be obtained when 2-aminocyclopent-1-ene-1-carbodithioic acid (Hacda) and o-aminobenzenethiol (Habt) are used for oxygen abstraction. With triphenylphosphine, however, the products are non-oxo molybdenum(IV) compounds [MoIV(PPh3)(aacda)3]BF43a, 3b. In frozen solution [dimethylformamide–MeCN (1 : 10 v/v), 140 K], compounds 2a–2d show ESR spectra with axial symmetry. The g and A tensor parameters (g∥≈ 1.977, g⊥≈ 1.975, A∥≈ 51.5 × 10–4 cm–1 and A⊥≈ 19.5 × 10–4 cm–1) for this series of molybdenum(V) compounds are very similar to each other indicating that the unpaired electron is located in a metal-centred orbital of essentially identical composition. Complexes 2a and 2b display two consecutive reversible one-electron waves in their cyclic voltammograms at E½=–0.24 and –0.45 V vs. saturated calomel electrode (SCE), which correspond to MoV–MoIV and MoIV–MoIII redox couples respectively. The molybdenum(IV) compounds 3a and 3b are seven-co-ordinate with a spin-triplet ground state (µeff, 2.48 and 2.53) and show two distinct redox waves due to the interconversion of the three [Mo(PPh3)(aacda)3]z(z=+1, 0 or –1) species. Half-wave potentials, E1/2, for these processes are at –0.36 (MoIV–MoIII) and –0.90 V (MoIII–MoII)vs. SCE. The presence of the strongly π-acceptor phosphine ligand facilitates the electrochemical generation of relatively scarce molybdenum(II) species.

Chemistry of molybdenum. Part 5. Synthesis, spectroscopic and electrochemical properties of bis(2-aminocyclopent-1 -ene-1- dithiocarboxylato)(arylimido)dihalogenomolybdenum(VI) complexes

The preparation of a series of arylimido complexes [Mo(NC6H4X-p)Y2(acda)2](Hacda = 2 aminocyclopent-1-ene-1 -dithiocarboxylic acid; X = H, Y = F, Cl or Br; X = Cl or NO2, Y = Cl) by O/NPh exchange reaction is reported. A novel synthetic route for the generation of arylimido-molybdenum compounds via an oxaziridine precursor has been explored. In [Mo( NPh)Y2(acda)2] complexes, a systematic shift of the ligand-to-metal charge-transfer band [S(π)→Mo(dπ)] to higher energy occurs as the ligand field strength of Y increases. Electrochemical studies (cyclic voltammetry) of these compounds in dimethylformamide reveal the presence of a quasi-Nernstian couple (E½ca.–0.12 to –0.21 V) due to MoVI–MoV electron transfer followed by a MoV–MoIV irreversible reduction process (Ep,cca.–0.4 V). It is concluded that the imido group is a poor σ-donor ligand compared to the isoelectronic oxo group.