Improved syntheses of [] and []: Structural characterization of Na[Cp∗MoO3]·5H2O and []

syn and anti conformations in 2-hydroxy-5-[(E)-(4-nitrophenyl)diazenyl]benzoic acid and two related salts

The crystal structures of 2-hydroxy-5-[(E)-(4-nitrophenyl)diazenyl]benzoic acid, C13H9N3O5, (I), ammonium 2-hydroxy-5-[(E)-phenyldiazenyl]benzoate, NH4(+)·C13H9N2O3(-), (II), and sodium 2-hydroxy-5-[(E)-(4-nitrophenyl)diazenyl]benzoate trihydrate, Na(+)·C13H8N3O5(-)·3H2O, (III), have been determined using single-crystal X-ray diffraction. In (I) and (III), the phenyldiazenyl and carboxylic acid/carboxylate groups are in an anti orientation with respect to each other, which is in accord with the results of density functional theory (DFT) calculations, whereas in (II), the anion adopts a syn conformation. In (I), molecules form slanted stacks along the [100] direction. In (II), anions form bilayers parallel to (010), the inner part of the bilayers being formed by the benzene rings, with the -OH and -COO(-) substituents on the bilayer surface. The NH4(+) cations in (II) are located between the bilayers and are engaged in numerous N-H···O hydrogen bonds. In (III), anions form layers parallel to (001). Both Na(+) cations have a distorted octahedral environment, with four octahedra edge-shared by bridging water O atoms, forming [Na4(H2O)12](4+) units.

Iron-molybdenum-oxo complexes as initiators for olefin autoxidation with O2

The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular [Mo-O-Fe-O-]2 core containing high-spin iron(ii) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(ii) and cobalt(ii) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)](2+). The cobalt analogue 4 reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O8(4-) structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*.

Speciation of [Cp*(2)M(2)O(5)] in polar and donor solvents

The speciation of compounds [Cp*2 M2 O5 ] (M=Mo, W; Cp*=pentamethylcyclopentadienyl) in different protic and aprotic polar solvents (methanol, dimethyl sulfoxide, acetone, acetonitrile), in the presence of variable amounts of water or acid/base, has been investigated by (1) H NMR spectrometry and electrical conductivity. Specific hypotheses suggested by the experimental results have been further probed by DFT calculations. The solvent (S)-assisted ionic dissociation to generate [Cp*MO2 (S)](+) and [Cp*MO3 ](-) takes place extensively for both metals only in water/methanol mixtures. Equilibrium amounts of the neutral hydroxido species [Cp*MO2 (OH)] are generated in the presence of water, with the relative amount increasing in the order MeCN≈acetone<MeOH<DMSO. Addition of a base (Et3 N) converts [Cp*2 M2 O5 ] into [Et3 NH](+) [Cp*MO3 ](-) , for which the presence of a NH⋅⋅⋅OM interaction is revealed by (1) H NMR spectroscopy in comparison with the sodium salts, Na(+) [Cp*MO3 ](-) . These are fully dissociated in DMSO and MeOH, but display a slow equilibrium between free ions and the ion pair in MeCN and acetone. Only one resonance is observed for mixtures of [Cp*MO3 ](-) and [Cp*MO2 (OH)] because of a rapid self-exchange. In the presence of extensive ionic dissociation, only one resonance is observed for mixtures of the cationic [Cp*MO2 (S)](+) product and the residual undissociated [Cp*2 M2 O5 ] because of a rapid associative exchange via the trinuclear [Cp*3 M3 O7 ](+) intermediate. In neat methanol, complex [Cp*2 W2 O5 ] reacts to yield extensive amounts of a new species, formulated as the mononuclear methoxido complex [Cp*WO2 (OMe)] on the basis of the DFT study. An equivalent product is not observed for the Mo system. The addition of increasing amounts of water results in the rapid decrease of this product in favor of [Cp*2 W2 O5 ] and [Cp*WO2 (OH)].

Rational synthesis and characterization of the mixed-metal organometallic polyoxometalates [Cp*Mo(x)W(6-x)O18]- (x = 0, 1, 5, 6)

The reaction between the oxometallic complexes Cp*(2)M(2)O(5) and Na(2)M'O(4) (M, M' = Mo, W) in a 1:10 molar ratio in an acidic aqueous medium constitutes a mild and selective entry into the anionic Lindqvist-type hexametallic organometallic mixed oxides [Cp*Mo(x)W(6-x)O(18)](-) [x = 6 (1), 5 (2), 1 (3), 0 (4)]. All of these compounds have been isolated as salts of nBu(4)N(+) (a), nBu(4)P(+) (b), and Ph(4)P(+) (c) cations and two of them (1 and 3) also with the n-butylpyridinium (nBuPyr(+), d) cation. The compounds have been characterized by elemental analyses, thermogravimetric analyses, electrospray mass spectrometry, and IR spectroscopy. The molecular identity and geometry of compounds 1c, 2a, and 2c have been confirmed by single-crystal X-ray diffraction. Density functional theory calculations on models obtained by replacing Cp* with Cp (I-IV) have provided information on the assignment of the terminal M═O and bridging M-O-M vibrations.