M.T. Duarte

Preparation and characterisation of new oxovanadium(IV) Schiff base complexes derived from amino acids and aromatic o-hydroxyaldehydes

Abstract A range of mostly new oxovanadium(IV) complexes is described. They contain coordinated Schiff bases, made from natural amino acids (glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, threonine, aspartic acid, and histidine) and salicylaldehyde or such derivatives as 3-, 4-, or 5-methoxy-salicylaldehyde. The coordination sphere is completed by simple ligands like water, 2,2′-bipyridyl or pyridine. The compounds are characterised and the nature of their coordination spheres shown by analysis, TLC, and by appropriate spectroscopy (EPR, IR, electronic and circular dichroism of solution and solids). In a few cases, magnetic properties are described to establish oxidation state. In several cases, the solubility of the compounds from racemic amino acids differs markedly from those containing the single enantiomer. The crystal and molecular structure of the related (and novel) compound with N-pyridoxylidene- d , l -isoleucinate, [VO(pyr- d , l -Ile)(bipy)]·H2O is described. It contains two diastereomers. Denoting the chiral vanadium centres as A or C, these are and [A(pyr- l -Ile)(bipy)] [C(pyr- d -Ile)(bipy)].

Gold complexes with dithiothiophene ligands: a metal based on a neutral molecule.

The gold complexes n-Bu4N[Au(alpha-tpdt)2] (5), n-Bu4N[Au(dtpdt)2] (4) and n-Bu4N[Au(tpdt)2] (6) based on new dithiothiophene ligands (alpha-tpdt= 2,3-thiophenedithiolate, dtpdt=2,3-dihydro-5,6-thiophenedithiolate and tpdt = 3,4-thiophenedithiolate) have been prepared and characterised. These gold(III) complexes are diamagnetic, but they can be oxidised with iodine to the paramagnetic compounds [Au(alpha-tpdt)2] (8), [Au(dtpdt)2] (7) and n-Bu4N[[Au(tpdt)2]n-2] (9), which were isolated as fine powders and which exhibit paramagnetic susceptibilities that are almost temperature independent with room temperature values of 2.5 x 10(-4), 2.0 x 10(-4) and 5 x 10(-4) emu x mol(-1), respectively. Interestingly, the neutral complex [Au(alpha-tpdt)2] (8) as a polycrystalline sample displays the properties of a metallic system with a room temperature electrical conductivity of 6 S x cm(-1) and a thermoelectric power of 5.5 microVK(-1); this is the first time that this metallic property has been observed in a molecular system based on a neutral species.

Electrochemical and X-ray studies of nickel(II) Schiff base complexes derived from salicylaldehyde: Structural effects of bridge substituents on the stabilisation of the +3 oxidation state

Abstract The oxidative chemistry of three Ni(II) complexes with Schiff base ligands derived from salicylaldehyde and diamines with different steric demands, N,N′-2-methylpropane-2,3-diyl-bis(salicylideneiminate)nickel(II) (1), N,N′-1,2-cyclohexyl-1,2-dyil-bis(salicylideneiminate)nickel(II) (2) and N,N′-2,3-dimethylbutane-2,3-diyl-bis(salicylideneiminate)nickel(II) (3), was studied by cyclic voltammetry and chronoamperometry in N,N′-dimethylformamide and (CH3)2SO. The electrogenerated species were characterised by EPR spectroscopy. All three complexes exhibited metal-centred oxidised processes and the oxidised products were low-spin six-coordinate Ni(III) species (dz2 ground state) with two solvent molecules axially coordinate. Addition of pyridine resulted in the replacement of solvent molecules with no changes in the ground state. The crystal structures of compounds 1 and 3 were determined from single crystal X-ray diffraction data, and the crystal packing for any of the complexes did not show any systematic parallel orientation of any part of the molecules. X-ray structural data for the Ni(II) complexes provided a rationale for the E1/2 values obtained in the oxidation processes and for the relative energy of the low-lying excited duplets of the electrogenerated Ni(III) species.

Cyanide and methylisocyanide complexes of rhenium(l) [NBu4][ReX(CN)(dppe)2] (X Cl or CN; dppe Ph2PCH2CH2PPh2) and trans-[ReX(CNMe)(dppe)2] (X H, F, Cl or CN): crystal structures of trans-[ReX(CNMe)(dppe)2] (X H or Cl)

Abstract Reaction of trans-[ReCI(N2)(dppe)2] (dppe  Ph2PCH2CH2PPh2) with [NBu4]CN gives [NBu4][ReCI(CN)(dppe)2] (1) which, upon further reaction with [NBu4]CN, forms [NBu4][trans-Re(CN)2(dppe)2](2) which is readily oxidized to [Re(CN)2(dppe)2] (3). Treatment of 1 with Me3SiCF3SO3 affords [ReH(CNMe)(dppe)2] (4) which is also obtained by reaction of trans-[ReCI(CNMe)(dppe)2] (5) with Li[BEt3H]. Compound 5 reacts with [NBu4]CN (in the presence of Tl[BF4) or [NBu4]F to give trans-[Re(CN)(CNMe)(dppe)2] (6) or trans-[ReF(CNMe)(dppe)2] (7) respectively. The crystal structures of 4 and 5 have been determined by X-ray diffraction analyses which indicate very bent isocyanide ligands (CNC angles of 147.7(7)° and 139.4(10)° respectively) and rather short ReC bond lengths (1.947(6) A and 1.861(12) A respectively). Cyclic voltammetry shows that in aprotic solvent these complexes undergo two successive single-electron reversible oxidations. The oxidation potential of the first oxidation, for 7, allows the estimation of the electrochemical parameter PL of fluoride as −1.3 V.

Nitrile complexes of dicyclopentadienyl-molybdenum and -tungsten: preparation and reactivity. The structure of di-η5-cyclopentadienyliodoacetonitrile-molybdenum(IV) hexafluorophosphate, [Mo(η5-C5H5)2I(NCCH3)][PF6]

Abstract New complexes of the type [MCp2X(NCR)][PF6] (M = Mo, W; X = SR, halides; R = Me, Et, Ph) have been prepared from [MCp2X2] and TlPF6, NOPF6 or [FeCp2][PF6], in nitrile solvents. Some reactions of these cations with nucleophiles [L] have been studied. With L = PR3 or CO, the complexes [MCp2(SR)L]+ (R = Me, Ph) are formed. [MoCp2(SPh)(NCMe)]+ reacts with NaBH4 to give [MoCp2(SPh)H] and with acetone to give [MoCp2{S(Ph)C(Me)2O}]+. The W complexes [WCp2X(NCR)]+ add NHR′2 to give amidine complexes [WCp2X{HNC(R)Nr′2}]+ (X = SPh, Br). The mode of coordination of the nitriles to the [MoCp2Cl]+ fragment has been studied by EHMO calculations. The molecular structure of [MoCp2I(NCMe)][PF6] has been determined. The crystals are triclinic, space group P 1 , a 7.7989(6), b 10.3044(8), c 10.5565(5) A, α 96.218(4), β 94.466(4), γ 102.697(5)°, V 818.21 A3, Z = 2. The cation has the usual bent bis-metallocene structure, and within this family of complexes is the first reported example of a complex containing a MoI bond.

New forms of old drugs: improving without changing

In a short approach, we want to present the improvements that have recently been done in the world of new solid forms of known active pharmaceutical ingredients (APIs). The different strategies will be addressed, and successful examples will be given.

Molecular structure of two dicyclopentadienylmolybdenum derivatives containing a four-membered ring [Mo(η5-C5H5)2(2-NHNC5H4)][PF6] and [Mo(η5C5H5)2(2-ONC5H4)][PF6]

Abstract The structure of the new compound [Mo(η 5 -C 5 H 5 ) 2 (2-NHNC 5 H 4 )][PF 6 ] ( 1 ) has been determined. The crystals are orthorhombic, space group Pca 2 1 with a 20.807(1), b 8.0030(8), c 10.056(3) A, V 1674.5 A 3 , Z = 4. The structure of [Mo(η 5 -C 5 H 5 ) 2 (2-ONC 5 H 4 )][PF 6 ] ( 2 ) has also been determined. The crystals are orthorhombic, space group Pnma with a 12.727(3), b 10.174(2), c 12.918(1) A, V 1672.8 A 3 , Z = 4. The structures were solved by Patterson and difference electron density syntheses and refined by least-squares to R of 0.028 for 1287 reflections for 1 and 0.059 for 1178 reflections for 2 . Although not isostructural the two cationic complexes have equivalent geometries with the normal bent bismetallocene structure. For 1 the MoN bond lengths are 2.160(8) and 2.142(9) A, with a NMoN bond angle of 59.8(3)°, whereas for 2 MoO is 2.142(10), MoN is 2.138(11) A, the NMoO angle is 61.2(4)°. These parameters are discussed and compared with the corresponding data for similar biscyclopentadienyl complexes of molybdenum(IV). Extended Huckel molecular orbital calculations have been carried out to throw light on the nature of the bonding between the metal and the bidentate ligand.

A New Generation of Alkoxyl Radical Precursors – Preparation and Properties of N-(Alkoxy)-4-arylthiazole-2(3H)-thiones

N-(Hydroxy)thiazole-2(3H)-thiones 6–10 have been prepared in a short and efficient synthesis from p-substituted acetophenones. Alkylation of heterocycles 6–10 in the form of their potassium or tetraalkylammonium salts 11–15 affords N-alkoxy-4-arylthiazole-2(3H)-thiones 16–20 in good to satisfactory yields. The hitherto unknown thiones 16–20 have been subjected to a detailed structural investigation (NMR spectroscopy and X-ray crystallography) and furthermore to a mechanistic study in order to explore their utility as sources of oxygen-centered radicals in solution. From the results of these studies, the following conclusions can be drawn: (i) X-ray analyses of the p-chlorophenyl-substituted acid 9, of the O-alkyl derivatives 19c, 19f, and of the O-mixed anhydride 19k indicate short C–S bonds [C2–S2 = 1.637(5)–1.684(2) A] and long N–O connectivities [N3–O1 = 1.369(3)–1.379(2) A] in the thiohydroxamate functionalities. Furthermore, O-alkyl- or O-acyl substituents at O1 are twisted out of the thiazolethione plane by ca. 90°, which points to lone-pair repulsion between nitrogen and oxygen atom as the underlying structural motif of the cyclic thiohydroxamate derivatives. (ii) Alkylation of ambidentate thiohydroxamate anions (salts 11–15; oxygen and sulfur nucleophiles) affords almost exclusively O-esters 16–20 (alkylation at the oxygen atom). (iii) Based on the results of X-ray diffraction studies and on the 1H- and 13C-NMR spectra, guidelines for the characterization of N-(alkoxy)thiazolethiones 16–20 and 2-(alkylsulfanyl)thiazole N-oxides 21–25, i.e. the products of S-alkylation of thiohydroxamate salts 11–15, could be derived. (iv) Photolyses of substituted N-(4-pentenoxy)-4-arylthiazolethiones 16–20 in general and in particular of p-chloro derivatives 19 were carried out in the presence of the hydrogen donor Bu3SnH, and afforded substituted tetrahydrofurans 31 or tetrahydropyrans 32 as major products in good yields. The observed stereo- and regioselectivities of ethers 31 and 32 point to alkoxyl radicals 30 as reactive intermediates, which add intramolecularly by selective 5-exo-trig or 6-endo-trig pathways to the olefinic double bonds. In terms of synthetic access and ease of handling of the radical precursors, the p-chlorophenyl-substituted thiazolethiones 9 and 19 exhibit significant advantages over all the other thiones used in this study and are considered as excellent substitutes for the pyridinethiones as efficient sources of free alkoxyl radicals. Consequently, the present compounds may be of use in both mechanistic and synthetic studies.

Structure and physical properties of (n-Bu4N)2 [Au(dcbdt)2]5

[(n-Bu) 4 N] 2 [Au(dcbdt) 2 ] 5 was obtained as brown thin plate shaped crystals by electrochemical oxidation of (n-Bu) 4 N [Au(dcbdt) 2 ], a Au complex with 4,5-dicyanobenzene-1,2-dithiolate (dcbdt) recently prepared. Cyclic voltammetry and optical spectroscopy studies indicate a mixed valence state. The crystal structure is triclinic and presents slightly pentamerised segregated stacks of the complex along (1,2,0) Single crystal electrical conductivity (σ RT ∼10 S/cm, Ea=13.5 meV) and thermopower measurements (S RT =6μ V/K) suggest a degenerate semiconducting behaviour. The static susceptibility measurements give an almost temperature independent paramagnetic value of 3 5×10 4 emu/mol. These and EPR results are discussed in relation to the crystal structure.

Tetraaza macrocycles containing pyridine and their copper(II) and nickel(II) complexes: X-ray, spectroscopic, molecular mechanics and molecular orbital studies

The behaviour in solution of nickel(II) complexes of the macrocycles L1{3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} and L2(7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} containing pyridine rings was investigated spectroscopically (UV/VIS/near IR and 1H NMR) and several species were found. In order to understand these and previous experimental data concerning stability constants, some crystals were prepared. The crystal structures of L1 and the complexes [CuL1][PF6]21 and [NiL2(ClO4)][ClO4]2 were determined, the copper complex adopting a square-planar geometry and the nickel one a square-pyramidal geometry, where the four nitrogen atoms are planar and one perchlorate occupies the axial co-ordination position. Molecular mechanics calculations were performed in order to find the preferred conformations for square-planar and square-pyramidal complexes of both L1 and L2, and the related macrocycles which do not contain a pyridine ring. The electronic preferences were analysed with extended-Huckel calculations. The introduction of pyridine into macrocycle does not significantly affect its behaviour upon co-ordination in square-planar complexes. For square-pyramidal complexes, the structures where a fifth donor occupies the axial position are usually more stable, though the introduction of methyl groups as substituents on the nitrogen may favour the presence of this donor in an equatorial site and lead to folding of the macrocycle.