Showing papers in "Journal of The Chemical Society-dalton Transactions in 1996"

Cationic Group 4 metallocene complexes and their role in polymerisation catalysis: the chemistry of well defined Ziegler catalysts

Abstract: Cationic alkyl complexes of Group 4 metallocenes of the type [MCp2R]+(M = Ti, Zr or Hf, Cp = C5H5) have been recognised as the catalytically active species in metallocene-based olefin polymerisation catalysts. These highly electrophilic 14-electron species possess a very complex chemistry in which the formation of temporarily dormant stabilised adducts plays a dominant role. Cationic metal alkyls of this kind are found to be extremely active polymerisation catalysts, with high stereoselectivities and the potential to produce numerous previously inaccessible polymeric materials. A detailed understanding of the chemistry of these species promises to lead to a new generation of well defined polymerisation catalysts. Metallocene-based catalysts already play an increasing role in major industrial polymerisation processes.

Getting excited about lanthanide complexation chemistry

Abstract: Lanthanide complexation chemistry in solution is an active research theme because of the importance of gadolinium complexes as contrast agents in magnetic resonance imaging, and of luminescent europium and terbium complexes as probes in biochemistry. The physicochemical principles and theory underpinning this work are being refined to account for recent developments such as the reappraisal of complex hydration state.

Methanol carbonylation revisited: thirty years on.

Abstract: Monsanto initiated development of its rhodium- and iodide-catalysed process for the carbonylation of methanol to acetic acid in 1966. Ownership of the technology was acquired in 1986 by BP Chemicals who have further extended it. The work of the Sheffield group, in developing a deeper understanding of the mechanism of the process, is reviewed. The rate-determining step in the rhodium–iodide catalysed reaction is the oxidative addition of methyl iodide to [Rh(CO)2I2]–1a: the product from this reaction, the reactive intermediate [MeRh(CO)2I3]–2a has been detected and fully characterised spectroscopically. The rates of the reversible reactions linking 1a, 2a and the acetyl complex [(MeCO)Rh(CO)I3]–3a, as well as activation parameters for several of the processes involved, have been measured. The efficiency of methanol carbonylation arises primarily from rapid conversion of 2a into 3a, leading to a low standing concentration of 2a, and minimising side reactions such as methane formation. By contrast, in the iridium-catalysed carbonylation, for which similar cycles can be written, the reaction of [MeIr(CO)2I3]–2b with CO to give [(MeCO)Ir(CO)2I3]–4b is rate determining. Model studies show that while kRh/kIr is ca. 1 : 150 for the oxidative addition, it is ca. 105–106 : 1 for migratory CO insertion. The migratory insertion for iridium can be substantially accelerated by adding either methanol or a Lewis acid (SnI2); both appear to facilitate substitution of an iodide ligand by CO, resulting in easier methyl migration. The carbonylation of higher alcohols (ROH) has also been successfully modelled; the corresponding alkyl iodides (RI) react much more slowly than MeI with [M(CO)2I2]–, but again give the acyls [(RCO)M(CO)I3]– for M = Rh and the alkyls [RM(CO)2I3]– for M = Ir. The greater stability of [MeIr(CO)2I3]– compared with [MeRh(CO)2I3]– accounts for the very different characters of the reactions catalysed by the two metals. It is suggested that the broad features of the Rh/Ir reactivities can be rationalised since the M–C bond to a 5d metal is generally stronger than that to the corresponding 4d metal; thus if metal–ligand bond making plays a key role in a step, then the 5d metal is more likely to react faster (e.g. in the oxidative addition), but if a metal–ligand bond-weakening or -breaking step plays a key role in a process (e.g. in the migration), it is likely that the 4d metal will be faster.

Photoluminescent cyclometallated diplatinum(II,II) complexes: photophysical properties and crystal structures of [PtL(PPh3)]ClO4 and [Pt2L2(µ-dppm)][ClO4]2(HL = 6-phenyl-2,2′-bipyridine, dppm = Ph2PCH2PPh2)

Abstract: The complexes [PtL(Cl)]1, [PtL(PPh3)]ClO42 and [Pt2L2(µ-dppm)][ClO4]23(HL = 6-phenyl-2,2′-bipyridine, dppm = Ph2PCH2PPh2) have been prepared and their spectroscopic and emission properties studied. Complex 3 shows a broad and intense absorption at 420–510 nm which is tentatively assigned to a metal–metal to ligand charge-transfer transition 1m.m.l.c.t. 1[dσ*→σ(π*)], where dσ* arises from the antisymmetric combination of the two platinum dz2 orbitals and σ(π*) from the symmetric combination of π* orbitals of the two L ligands. All the complexes show luminescence in both the solid state and in solution. Both 1 and 2 display 3m.l.c.t. emission in solution at room temperature. The solid-state emission of 1 and both the solid-state and fluid-state emission of 3 are assigned to the 3m.m.l.c.t. 3[(dσ*){σ(π*)}] state. The crystal structure of 2 shows an intermolecular π–π interaction between two L ligands as evidenced by the intermolecular ligand plane separation of 3.35 A. The solid-state emission of 2 is suggested to arise from a π–π excimeric interaction of the L ligands. The crystal structure of 3 shows discrete [Pt2L2(µ-dppm)]2+ units with an intramolecular Pt–Pt separation of 3.2703 A.

Ligand-stabilized metal clusters and colloids: properties and applications

Abstract: In this perspective some general phenomena of ligand-stabilized transition-metal clusters and colloids are introduced before some recent results on the special behaviour of such particles in catalysis are given.

Electronic and steric effects in manganese Schiff-base complexes as models for the water oxidation complex in photosystem II. The isolation of manganese-(II) and -(III) complexes of 3- and 3,5-substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands

Abstract: Manganese-(II) and -(III) complexes of substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands H2L (substituents are in the 3, 5 or 3,5 positions of the phenyl rings of the salen moiety) have been prepared and thoroughly characterised. The reaction of Mn(ClO4)2·6H2O with H2L in ethanol in air normally leads to manganese(III) complexes ligated by both the N2O2 ligand and water molecule(s). However, by employing electron-withdrawing substituents on the ligand, e.g. 3-Br,5-NO2, a manganese(II) complex can be obtained. A ‘borderline’ ligand is represented by the 5-NO2 derivative (nsalen), which produces a manganese(II) complex contaminated with a small amount of a manganese(III) species. Using a more rigorous oxidising agent in the synthesis, [Fe(η-C5H5)2][FeCl4], drives the reaction totally to a manganese(III) complex [Mn(nsalen)Cl(H2O)]. In addition to magnetic susceptibility studies, cyclic voltammetry has been employed. All the complexes exhibit an oxidation and reduction peak, the reversible character being confirmed by pulse voltammetry. Pulse voltammetry also confirmed the nature of the manganese(II) species [Mn(bnsalen)(H2O)2·2H2O [H2bnsalen =N,N′-bis(3-bromo-5-nitrosalicylidene)ethane-1,2-diamine] and that a slight amount of a manganese(III) species is present in [Mn(nsalen)(H2O)2]·2H2O. Six complexes have been crystallographically characterised. Despite the retention of an octahedral manganese environment in all of them, the supramolecular structures exhibit a wide diversity. The 3,5-dichloro and 5-bromo salen complexes containing co-ordinated water display combined π and hydrogen bonding, as well as dimerisation. The complex [{Mn(µ-dbsalen)(µ-O)}2](dbsalen = 3,5-dibromo derivative) offers an alternative bridging arrangement, and [Mn(bsalen)(MeOH)(OClO3)]·H2O (bsalen = 5-bromo derivative) highlights the versatility of the manganese centre in these systems where, unexpectedly, perchlorate is co-ordinated in place of a lattice water. A more subtle rearrangement of supramolecular structure is obtained in [Mn(nsalen)Cl(H2O)] where the usual combination of π- or hydrogen-bonding interaction is modified by the corresponding ability of the 5-NO2 substituent.

Crystal structure and magnetic properties of the layer ferrimagnet N(n-C5H11)4MnIIFeIII(C2O4)3

Abstract: The crystal structure and magnetic properties of the molecular-based ferrimagnet N(n-C5H11)4MnIIFeIII(C2O4)3 have been determined. The compound is orthorhombic, space group C2221, a= 9.707(3), b= 16.140(3), c= 19.883(7)A(120 K), Z= 4[R 0.047 for I > 2σ(I)]. The structure consists of hexagonal layers of alternating MnII and FeIII bridged by C2O42–, separated by layers containing only N(n-C5H11)4+ with the alkyl chains extended, though with the terminal bonds twisted towards the gauche conformation. The terminal CH3 are embedded in the hexagonal pockets formed by three C2O42–. Since both metal ions have 3d5 configuration with 6A1 ground states the magnetic properties in the paramagnetic region mimic those of a two-dimensional antiferromagnet. Below TN= 27 K an uncompensated moment estimated as 8.78 × 10–5µB atom–1 arises, the direction of which was identified as parallel to the c axis by single-crystal magnetization measurements.

Outer-sphere redox reactions of [CoIII(NH3)5(HxPyOz)](m– 3)– complexes. A temperature- and pressure-dependence kinetic study on the influence of the phosphorus oxoanions

Abstract: Outer-sphere redox reactions between [CoIII(NH3)5(HxPyOz)](m– 3)–(HxPyOzm–= H2PO2–, H2PO3–, HPO32–, HP2O73–, P2O74–, γ-H2P3O103–, -HP3O104–, -P3O105–, β-H3P3O102–, -H2P3O103–, -HP3O104– or -P3O105–) and [Fe(CN)6]4– have been studied as a function of pH, HxPyOzm– oxoanion, temperature and pressure. The effect of the oxidation state, size, geometry and extent of protonation of the HxPyOzm– oxoanions on the precursor-complex formation constant, electron-transfer rate constant, and thermal and pressure activation parameters has been investigated. The values obtained indicate that all the precursor-complex formation equilibrium constants, KOS, are the same except for the non-linear β-H3P3O102–, -H2P3O103–, -HP3O104– and -P3O105– oxoanions, where the values are consistently larger, indicating that hydrogen bonding plays a very important role. The electron-transfer rate constant for a series of [Co(NH3)5(HxPyOz)](m– 3)–, with linear oxoanions, increases on decreasing the negative charge on the complex {k308= 0.73 × 10–3 and (8.5–11)× 10–3 s–1 for the γ-[Co(NH3)5(P3O10)]2– and γ-[Co(NH3)5(H2P3O10)], respectively}. For the non-linear β-P3O105– oxoanions a threshold is observed when the external oxo groups are protonated {k308= 20 × 10–3 for β-[Co(NH3)5(H3P3O10)]+ species and 0.84 × 10–3 s–1 for β-[Co(NH3)5(H2P3O10)], -[Co(NH3)5(HP3O10)]– or -[Co(NH3)5(P3O10)]2–}. The ΔH‡ values are within the range expected, while those of ΔS‡ and ΔV‡ vary considerably with the extent of protonation of the phosphorus oxoanionic ligands, being 13 J K–1 mol–1 and + 36 cm3 mol–1 and 69 J K–1 mol–1 and + 13 cm3 mol–1, respectively for the [Co(NH3)5(HP2O7)]–[Co(NH3)5(P2O7)]– couple. The ΔV‡ values depend strongly on the oxo group distribution of the oxophosphorus ligand {+ 13 and + 32 cm3 mol–1 for β- and γ-[Co(NH3)5(P3O10)]2–, respectively}. Hydrogen bonding and solvent reorganization play a key role in the interpretation of the activation parameters.

Gold – a flexible friend in cluster chemistry

Abstract: The role of metal–metal bonding interactions is reviewed in the context not only of cluster chemistry, but also in linear gold(I) compounds. The latter, although much weaker than the former, can lead to interesting infinite and catenane structures which arise from supramolecular interactions.

Hydrogenation and hydrogenolysis of thiophenic molecules catalysed by soluble metal complexes

Abstract: In this review are discussed the known hydrogenation and hydrogenolysis reactions of thiophenic molecules catalysed by soluble metal complexes. Homogeneous desulfurization reactions are also briefly summarized. The mechanistic information provided by the homogeneous modelling studies is critically evaluated through comparisons with the surface reactions that have been proposed to be implicated in the heterogeneous hydrodesulfurization catalysis.

Metal complexes of a chiral quadridentate Schiff base

Abstract: Chiral metal Schiff-base complexes [AlL(Et)], [TiLCl2], [VO(L)], [(FeL)2O], [CoL], [NiL], [CuL], [ZrLCl2], [RuL(CO)2] and trans-[RuL(NO)Cl]([H2L =(R,R)-(–)-N,N′-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] were synthesized and characterized. The structures of [CoL] and trans-[RuL(NO)Cl] have been established by X-ray crystallography. The former has a pseudo-square-planar geometry with Co–N and Co–O distances of 1.88 and 1.84 A, respectively. The geometry around Ru in trans-[RuL(NO)Cl] is octahedral with Ru–N (nitrosyl) and Ru–Cl distances of 1.72(2) and 2.354(4)A, respectively, and Ru–N–O 175(2)°. The cyclic voltammograms for the metal Schiff-base complexes show reversible MIII–MII and ligand-centred oxidation couples. Treatment of NiL with AgBF4 afforded air-stable [NiL]BF4, which is formulated as a nickel(II) complex of the Schiff-base cation radical.

Synthesis, photophysics and photochemistry of alkynylgold(I) phosphine complexes

Abstract: A series of alkynylgold(I) phosphine complexes with various bridging phosphines and acetylide ligands have been synthesized. All complexes have been shown to exhibit rich photophysical and photochemical behaviour. A systematic comparative study on their spectroscopy and photophysics has been attempted. The complexes undergo facile electron-transfer reactions with pyridinium electron acceptors. The redox nature of the quenching reactions has been established by time-resolved transient absorption spectroscopy.

Tetraborane(10), B4H10: structures in gaseous and crystalline phases

Abstract: Supplementing gas-phase electron-diffraction data with restraints derived from a graded series of ab initio calculations makes possible refinement of all geometrical parameters and amplitudes of vibration. By avoiding the need to fix some parameters, this technique yields structures which are more completely refined and thus have more reliable standard deviations than procedures used previously. It has been applied to the gas-phase structure of the arachno boron hydride tetraborane(10), B4H10. Salient structural parameters (rα0 structure) were found to be: r[B(1)–B(2)] 186.6(2), r[B(1)–B(3)] 173.7(5), r[B(1)–H(1,2)] 123.0(15), r[B(2)–H(1,2)] 141.7(8), r[B(1)–H(1)] 119.8(8), r[B(2)–H(2)]endo] 121.0(8) and r[B(2)–H(2)exo] 120.5(8) pm; butterfly angle 117.2(4)°. The crystal structure was also redetermined at 100 K. All gas-phase, crystallographic and ab initio structural parameters were found to be in good agreement.

Synthesis and structural characterisation of [Pd2(µ-Br)2(PBut3)2], an example of a palladium(I)–palladium(I) dimer

Abstract: The syntheses, spectroscopic characterisation and in one case (X = Br) the single-crystal structure of the novel PdI–PdI dimers [Pd2(µ-X)2(PBut3)2](X = Br or I) have been determined; preliminary results on their reactions with CO, H2, CNC6H3Me2 and C2H2 have also been obtained.

Concerted supramolecular motifs: linear columns and zigzag chains of multiple phenyl embraces involving Ph4P+ cations in crystals

Abstract: One-dimensionally infinite sequences of mutually attractive Ph4P+ cations occur in crystals, as independent supramolecular motifs. These motifs involve concerted, multiple phenyl embraces, based on the intermolecular edge-to-face (ef) interaction between phenyl groups. The linear infinite translational quadruple phenyl embrace (LITQPE) has four ef interactions between each pair of Ph4P+ ions, propagated in almost all cases by lattice translation. In the zigzag infinite sextuple phenyl embrace (ZZISPE), each Ph4P+ presents three phenyl groups to each of its neighbours, and there are six ef interactions between each pair of cations which are commonly related by a centre of inversion: the infinite sequence of P atoms is planar with approximately tetrahedral angles. In crystals, LITQPE columns and ZZISPE chains are usually parallel to stacks of anions, which vary in size from Br– to [(µ6-C)Os10(CO)24HgCF3]–: the anion stack can contain solvent, or the anion can be one-dimensionally non-molecular. 52 Crystals containing isolated LITQPE columns and 36 crystals containing ZZIPSE chains have been described, and crystal packing types (supramolecular crystal isomers) classified. The cation motifs are not disordered, whereas associated anions can be. Strong attractions exist between Ph4P+ cations within these two motifs. Calculated through-space energies for typical structures are ca. 13 kcal mol–1 net attraction per [Ph4P+]2 pair within a LITQPE, and ca. 16 kcal mol–1 net attraction within a ZZISPE. In contrast, the strongest interactions between these isolated motifs are ca. 1–3 kcal mol–1 attraction for the closest intermotif pair of Ph4P+ cations. The intramotif attractive energies have similar magnitude to the cation–anion attractive energies. The LITQPE and ZZISPE can be regarded as supramolecular crystal tectons, energetically comparable with hydrogen bonds.

Diverse solid-state clusters with strong metal–metal bonding. In praise of synthesis

Abstract: The synthesis, structure and bonding of two types of metal clusters are described. A large family of zirconium cluster halides of the general type AIxZr6(Z)X12 +n are obtained with A = Na to Cs, X = Cl, Br or I, 0 < x, n⩽ 6, and the required interstitial Z atom as variables. Extension to the rare-earth elements (Sc, Y, lanthanides) gives some new, hypostoichiometric cluster variations as well as condensed oligomeric and infinite-chain compounds. Zintl-phase concepts and the alkali-metal compounds that contain anionic ‘naked’ clusters or cluster networks of the triel elements Ga, In and Tl are also covered. The clusters include empty hypoelectronic examples as well as those that are centred by a heterometal or a triel element. The last compounds are remarkable parallels of the interstitial halides described.

Synthesis and characterization of gold(III) adducts and cyclometallated derivatives with 6-benzyl- and 6-alkyl-2,2′-bipyridines

Abstract: The reaction of a series of 6-substituted-2,2′-bipyridines HL (N2C10H7R, R = CH2Ph, CHMePh, CMe2Ph, CH2Me, CMe3 or CH2CMe3) with HAuCl4 or Na[AuCl4] has been investigated. Under different experimental conditions, salts [H2L][AuCl4], adducts [Au(HL)Cl3] or cyclometallated derivatives [Au(L)Cl][X](X = AuCl4, BF4 or PF6) have been isolated. The cyclometallated species arise from direct activation of a C–H bond either of a phenyl or a methyl substituent. The structures of an adduct [Au{N2C10H7(CHMePh)-6}Cl3] and two metallated species [Au{N2C10H7(CMe2C6H4)-6}Cl][AuCl4] and [Au{N2C10H7(CMe2CH2)-6}Cl][BF4]·0.5H2O have been determined by X-ray diffraction. In the adduct the gold atom is bonded to the nitrogen atom of the unsubstituted pyridine ring: a long-range interaction with the other nitrogen atom is observed, Au ⋯ N 2.758(4)A. In the two cyclometallated species the bipyridines act as tridentate N,N,C anions, giving a [5,6] and a [5,5]-fused ring system, respectively. In [Au{N2C10H7(CMe2C6H4)-6}Cl]+ the hexaatomic N,C ring adopts a boat-like conformation: the structure reveals a rather short interaction between one hydrogen of the Me group in axial position and the gold atom, Au ⋯ H 2.62 A. The pentaatomic N,C ring in [Au{N2C10H7(CMe2CH2)-6}Cl]+ is not planar: the co-ordination around the gold atom is essentially square planar. The new cyclometallated complexes [Au(N,N,C)Cl]+ are compared with those derived from similarly substituted pyridines described previously.

Syntheses and characterization of novel tri- and hexa-nuclear zinc complexes with biomimetic chelate ligands

Abstract: Reaction of Zn(ClO4)2·6H2O with pentadentate 1,3-bis(salicylidenamino)propan-2-ol (H2hsalpn) and the tridentate 4-methyl-2,6-bis(morpholinomethyl)phenol (Hbmmp) yielded the trinuclear zinc complex [Zn3(O2CPh)2(hsalpn)2]·2MeOH 1 and the hexanuclear zinc compound [Zn6(OH)6(bmmp)3][ClO4]32. The complexes were characterized by single-crystal X-ray diffraction analyses. Both structures were solved using direct methods and refined on F by full-matrix least squares. Complex 1 consists of one central zinc ion which is octahedrally co-ordinated and two terminal metals surrounded by five donor atoms in the form of a square pyramid. The left and right wing zinc ions are connected to the central metal atom via two phenolate oxygens of the dinucleating ligand and the oxygens of the benzoate ion. The six equivalent tetrahedrally co-ordinated zinc ions in 2 are arranged in form of a regular trigonal prism. Three dinuclear subunits are connected to each other by hydroxide groups. Both compounds display a new type of structure for homotri- and homohexanuclear zinc complexes.

Ultraviolet-circular dichroism spectra for structural analysis of copper(ii) complexes with aliphatic and aromatic ligands in aqueous solution

Abstract: An attempt has been made to identify the characteristic positions, and their eventual displacements with changing co-ordination, of the charge-transfer bands involving donor groups located on molecules of biological interest [L-malic acid, N-acetyl-L-aspartic acid, (1R,2R)-cyclohexane-1,2-diamine, L-alanyl-L-alanine, L-γ-glutamyl-L-Iµ-lysine, N-acetyl-L-histidine, β-alanyl-L-histidine and N-benzoylglycyl-L-histidyl-L-leucine] such as carboxylate, alcoholate, amine, deprotonated peptide and imidazole. Information about the species formed in solution was obtained by means of pH-metric readings while ultraviolet–circular dichroism spectra were recorded, at fixed pH values, 298 K and I= 0.1 mol dm–3, for the proton–ligand and proton–copper(II)–ligand systems, in order to evaluate a spectrum for each complex formed in solution. Intraligand and charge-transfer bands were assigned for each spectrum with the aim of relating spectral features to the structure of the species formed in solution.

Ab-initio X-ray powder diffraction structural characterization of co-ordination compounds: polymeric [{MX2(bipy)}n] complexes (M = Ni or Cu; X = Cl or Br; bipy = 4,4′-bipyridyl)

Abstract: In the absence of crystals of suitable quality, the crystal structures of five [MX2(bipy)](M = Ni or Cu; X = Cl or Br; bipy = 4,4′-bipyridyl) phases were determined, ab initio, from powder diffraction data. The nickel compounds contain symmetrically bridging halides and trans-D4h octahedrally co-ordinated metal atoms; on the contrary, the co-ordination about the copper atoms in [CuBr2(bipy)] is square planar, with long Cu ⋯ Br contacts in the axial direction, due to Jahn–Teller distortion. Interestingly, [CuCl2(bipy)] is dimorphic; both in the orthorhombic and in the monoclinic phases, the copper co-ordination is substantially square planar as in the bromide analogue, but none of the two phases is isomorphous to it.

Synthesis, characterisation and reactions of bis[2-(dimethylaminomethyl)-phenyl] diselenide: its structure and that of [2-(dimethylaminomethyl)phenyl]-selenium bromide

Abstract: Bis[(2-dimethylaminomethyl)phenyl] diselenide (RSe)2 was obtained by the organolithium route. It underwent facile reaction with stoichiometric amounts of bromine and iodine to give the corresponding arylselenium halides RSeBr and novel RSeI in which the selenium is covalently bonded to iodine. With an excess of bromine it gave the corresponding tribromide RSeBr3 but a similar reaction with iodine gave only RSeI. With diazomethane it gave the selenoether (RSe)2CH2 and with HCl the known bis(hydrochloride)(RSe)2·2HCl. A catalytic conversion of alkenes into allylic acetates using (RSe)2 was carried out. The compounds were characterised by elemental analyses, mass, multinuclear NMR (1H, 13C, 77Se), IR, Fourier-transform-Raman spectrometry and conductance measurements. The compound RSeBr3 shows evidence for the existence of Se ⋯ N interaction in solution and was found to equilibrate between co-ordinated and non-co-ordinated forms. The structures of the diselenide (RSe)2 and its bromo derivative RSeBr were determined by X-ray crystallography. The compounds are isostructural and exhibit Se ⋯ N intramolecular co-ordination. The Se atom has T-shaped three-co-ordination in both structures and the five-membered chelate rings formed by the Se, C and N atoms are puckered and exist in an envelope conformation. The Se ⋯ N interaction in RSeBr [Se–N 2.143(6)A] is, however, considerably stronger than that in (RSe)2[Se(1)–N(1) 2.856(3), Se(2)–N(2) 2.863(4)A].

Crystal and molecular structure of [V2O3(sal-L-val)2(H2O)](sal-L-val =N-salicylidene-L-valinate) and spectroscopic properties of related complexes

Abstract: The dinuclear vanadium(v) complex [V2O3(sal-L-val)2(H2O)]1(sal-L-val =N-salicylidene-L-valinate) has been prepared and characterized and its crystal structure determined. The compound contains a double bridge by O (oxo) and O (carboxylate) and is one of the rare examples where the two VO groups make an angle near 90°. There are two independent molecules, A and B, in the asymmetric unit which are similar and each consists of two VO(sal-L-val) groups at nearly right angles to each other. They exhibit a distorted-octahedral geometry, involving in the equatorial plane the O,N,O atoms of the Schiff-base ligand and the bridging O (oxo) group and in the axial positions a carboxyl oxygen of the second VO(sal-L-val) group and a water molecule. The bridging V–O(oxo) bond lengths range between 1.787 and 1.823 A, and are significantly shorter than the bridging V–O(carboxylate), ranging between ≈1.93–1.94 A for equatorial and ≈2.40–2.43 A for axial. Several known [VO(sal-L-aa)(H2O)] and new [VO(sal-L-aa)(bipy)](sal-L-aa =N-salicylidene-L-amino acidate, bipy = 2,2′-bipyridine) complexes were also prepared and their spectroscopic (ESR, UV and CD) properties studied. A correlation observed between the CD intensities and the size of the amino acid side groups is discussed.

Extent of hydration of octadentate lanthanide complexes incorporating phosphinate donors: solution relaxometry and luminescence studies

Abstract: The behaviour of luminescent (Eu, Tb) and highly paramagnetic (Gd) complexes of 1,4,7,10-tetraazacyclododecane containing one carboxamide and three phosphinate substituents has been studied in solution. Analyses of variable-temperature nuclear magnetic relaxation dispersion profiles indicate that there is no water molecule directly co-ordinated to gadolinium(III) ions. It has been found that the observed relaxation enhancement of solvent protons is determined, in addition to the contribution from water molecules diffusing in close proximity to the paramagnetic complex, by a relatively distant water molecule in the ‘second co-ordination sphere’. This is possible because the amide carbonyl oxygen can participate in hydrogen bonding (as a hydrogen-bond acceptor) to a local water molecule, which brings the water molecule close to the metal ion. The luminescence spectra of the complexes of Eu and Tb in water and D2O are also consistent with such a hydration scheme and there is a good correlation between the non-integral q value (number of inner-sphere water molecules) determined by this method and the distance between the metal ion and the water proton estimated by relaxometric methods. It is proposed that the hydration states q= 0 and 1 may be considered to represent boundary conditions and a given complex in solution may possess intermediate values.

Direct formation of alcohols by hydrocarbonylation of alkenes under mild conditions using rhodium trialkylphosphine catalysts

Abstract: The complex[RhH(PEt3)3] catalysed the hydroformylation of hex-1-ene to heptanal and 2-methylhexanal in toluene, but heptanol and 2-methylhexanol were significant products in tetrahydrofuran especially over long reaction times(16h). In protic solvents only alcohols were produced even after short reaction times. The reactions are very rapid and also occur readily with alkenes such as hex-2-ene, propene, ethene, styrene and 3,3-dimethylbutene. The highest rates observed are for ethene (54 000 turnovers h–1) and the products in all cases are alcohols. Other phosphines containing primary alkyl groups also produced alcohols, but in contrast reactions in ethanol using rhodium complexes containing PPh3, PPh2Et, PPhEt2 or PPri3 produced significant amounts of aldehydes and/or acetals whilst Me2PCH2CH2PMe2 inhibited the reaction. The NMR studies showed that species present in equilibrium in ethanol solution are [RhH(CO)(PEt3)3], [RhH(CO)2(PEt3)2], [Rh2(CO)4(PEt3)4], [Rh2(CO)2(PEt3)6] and PEt3 but that [RhH(CO)(PEt3)3] predominates under the catalytic conditions. Reactions carried out under D2–CO in EtOH produced, 90% BuCHDCH2CD2OH/D and 10% BuCHDCH2CHDOH/D but hydrogenation of heptanal under the same conditions gave a mixture of C6H13CHDOH/D (39%) and C6H13CH2OH/D (61%). These results are interpreted to indicate that the alcohols produced from hex-1-ene are primary reaction products and not produced via intermediate aldehydes. A new mechanism for this direct hydrocarbonylation is proposed in which the key acyl intermediate becomes protonated by the alcoholic solvent because of the high electron density it bears as a result of the presence of the electron-donating trialkylphosphines. Oxidative addition of H2 followed by two H-atom transfers leads directly to the alcohol. High pressure NMR studies showed that [Rh{C(O ⋯ HOEt)Et}(CO)2(PEt3)2] is present during catalytic hydrocarbonylation of ethene in ethanol. Two different cycles are proposed to explain the products obtained from the catalytic reaction of heptanal with D2–CO. Again, protonation, this time of the metal, appears to be important.

Synthesis, characterisation and substitution reactions of gold(iii) c,n-chelates

Abstract: New complexes of the type [AuCl2L] have been prepared, where L is a chelate consisting of a phenyl group bearing an N-donor substituent (oxazoline and/or dimethylaminomethyl). The structures of two of these, together with that of [AuCl{C6H3(CH2NMe2)2-2,6}]2[Hg2Cl6], have been determined by X-ray crystallography; the gold atoms exhibit strict square-planar geometry in all cases. The chloride ligands undergo ready substitution by other halides, thiocyanate, acetate or diethyldithiocarbamate. The monodithiocarbamate complexes [Au(S2CNEt2)L]+ contain chelated S2CNEt2 groups whereas in [Au(S2CNEt2)2L] the L ligand is monodentate (through C), one S2CNEt2 is monodentate, the other bidentate; in solution the two S2CNEt2 ligands appear equivalent on the NMR time-scale, indicating a rapid equilibrium between the two possible forms.

Cascade complexation of pseudo-halide by dicopper cryptates: a linear Cu–NNN–Cu unit

Abstract: An X-ray crystallographic structure determination of a µ-azido azacryptate has confirmed the existence of collinear Cu–NNN–Cu geometry, earlier cited as the origin of unusual spectroscopy in dicopper µ-azido cryptates; the isoelectronic µ-cyanato cryptate is also isostructural.

Structure, characterization and photoreactivity of monomeric dioxovanadium(v) schiff-base complexes of trigonal-bipyramidal geometry

Abstract: Five mononuclear cis-dioxovanadium(V) complexes of tridentate Schiff bases derived from salicylaldehyde and its derivatives and 8-aminoquinoline have been synthesized and characterized. Single-crystal X-ray analyses were performed with [VO2L1]1 and [VO2L2]2(L1 and L2 denote the [1 + 1] Schiff-base anions derived from salicylaldehyde or naphthaldehyde and 8-aminoquinoline). While the structure of 2 was refined to a final R= 0.074 (R′= 0.055), that of 1 was refined only to R= 0.134 (R′= 0.139) due to its poor diffraction quality. The complexes contain cis-oxo groups in the equatorial plane and a trigonal-bipyramidal geometry around the vanadium at which the Schiff base binds meridionally. Photoirradiation of these complexes in CH2Cl2 yielded chloride-bound VO3+ species, as studied using absorption and 51V NMR spectroscopy. These species are convertible back to the dioxovanadium(V) complexes upon addition of water to the photoirradiated solution.

Reversible adduct formation between phosphines and triarylboron compounds

Abstract: The acceptor strength of a number of Lewis-acidic fluorinated triarylboron compounds has been established and shown to depend on the amount and position of fluorine substitution. The donor strength of tert-butyl-phosphine has been found to be greater than phosphine towards these acceptor compounds; two series of adducts have been characterised and reversible adduct formation has been demonstrated for some adducts. Crystal structures of the phosphine adducts of tris(pentafluorophenyl)boron and tris(2,6-difluorophenyl)boron have been compared with those of the tert-butylphosphine adducts of tris(pentafluorophenyl)boron and tris(3,4,5-trifluorophenyl)boron to show a correlation between the length and the strength of the adduct bond. Mixing aryl groups in the acceptor compounds has not produced new adducts. The strong acceptor compounds have been found to form unstable adducts with water which act as drying agents ultimately producing arylboric acids; the crystal structure of (2,6-difluoropheny)dihydroxyborane has been determined.