Density functional analysis of the trigonal uranyl equatorial coordination in hexahomotrioxacalix[3]arene-based macrocyclic complexes.

Representative models of the different geometries observed for uranyl complexes with fully deprotonated p-R-hexahomotrioxacalix[3]arene (R = tert-Bu, Me) ligands, among which is the rare trigonal geometry, have been investigated using all-electron scalar relativistic density functional theory (DFT). Optimized structures of complexes incorporating triethylammonium (HNEt3+) and 4-methylpiperidinium (HMePi+) cations are in close agreement with experimental crystal diffraction data. Possible explanations for the structural differences between these uranyl complexes are discussed in terms of varying degrees of bonding between uranium and oxygen atoms from the ether and phenoxide groups. In particular, molecular orbital analysis highlights the central role of 5f-2p hybridization in the U-O bonding.

Supramolecular control of a fast and reversible Diels-Alder reaction.

A new cyclophane featuring two opposite anthracene units linked in 9,10-positions has been synthesized thanks to the template effect of the Me4N(+) ion. It forms pseudorotaxane complexes with alkylviologen ions and undergoes a fast and reversible reaction with tetracyanoethylene. A quantitative analysis has been carried out of the formation of Diels-Alder adducts, whose distribution can be controlled by host-guest complexation. These findings open interesting perspectives in the field of Dynamic Covalent Chemistry.

Complexation of quaternary ammonium ions by tetraester derivatives of [3.1.3.1]homooxacalixarene in mobile and in fixed conformation.

In the tetraalkylation of p-tert-butyl[3.1.3.1]homooxacalixarene with BrCH2CO2R and K2CO3 in acetone, the initially formed cone conformer is converted into the more stable 1,4-alternate conformer when R = Me or Et, but not when R = i-Pr or t-Bu. In the case of R = i-Pr, derivatives in fixed 1,4-alternate conformation and in partial cone conformation were also isolated. Compounds in fixed cone conformation are good ligands for tetramethylammonium, acetylcholine, and N-methylpyridinium salts in CDCl3, but the partial cone isomer proved to be somewhat better and even the 1,4-alternate conformer turned out to be active. The possible involvement of the ester functions as additional binding sites is discussed; moreover, an insight into the energetics of the complexation and conformational isomerization processes is given.

CsF-promoted acetyl dance at the narrow rim of p-tert-butyl[3.1.3.1]homooxacalixarene.

In the acetylation of an oxygenated calix[4]arene homologue in the presence of CsF as a base, relaxation of the reaction system to the equilibrium composition takes place through several intra- and intermolecular steps that can be easily controlled to obtain the various acetyl derivatives. The effect of different bases is also discussed.

Methyl ether derivatives of p-tert-Butyl[3.1.3.1]homooxacalixarene. Formation, structure, and complexes with quaternary ammonium ions.

[structure: see text] The whole set (five compounds) of partially O-methylated products of p-tert-butyl[3.1.3.1]homooxacalixarene, currently named p-tert-butyltetrahomodioxacalix[4]arene, have been prepared. Their structure has been investigated in solution through NMR techniques and in the solid state by single-crystal X-ray diffraction. A systematic investigation, extended to the parent tetraphenol and to the tetramethyl ether derivative, has been carried out on the complexation of tetramethylammonium, acetylcholine, N-methylpyridinium, and tetraethylammonium picrate in CDCl3. The observed trends in the binding and in the selectivity of the strictly related hosts could be analyzed on the basis of the varying importance of intramolecular hydrogen bonding and its effects on the conformation of the free and of the complexed ligands. On increasing the number of methyl ether functions, the cone conformation appears to be relatively less stable but deeper, so small organic cations can be more effectively encircled.

Inclusion of diprotonated [2.2.2]cryptand in the cavity of uranyl-complexed p-phenyltetrahomodioxacalix[4]arene.

In the title compound, 4,7,13,16,21,24-hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane dioxo[7,13,21,27-tetraphenyl-3,17-dioxapentacyclo[23.3.1.1(5,9).1(11,15).1(19,23)]ditriaconta-1(29),5,7,9(30),11(31),12,14,19(32),20,22,25,27-dodecaene-29,30,31,32-tetraolato]uranium dimethyl sulfoxide trisolvate, (C(18)H(38)N(2)O(6))[U(C(54)H(40)O(6))O(2)].3C(2)H(6)OS, the uranyl ion is bound to the four phenoxide groups of the deprotonated p-phenyltetrahomodioxacalix[4]arene ligand in a cone conformation, resulting in a dianionic complex. The diprotonated [2.2.2]cryptand counter-ion is located in the cavity defined by the eight aromatic rings of the homooxacalixarene, where it is held by cation-anion, cation-pi and possibly C-H...pi interactions. Dimerization in the packing leads to the formation of sandwich assemblages in which two diprotonated [2.2.2]cryptands are encompassed by two uranyl complexes.

[2,6-Bis[(dimethylamino)methyl]pyridine-kappa3N](2,2-methylenediphenolato-kappa2O,O')dioxouranium(VI) acetone solvate.

In the title compound, [UO(2)(C(13)H(10)O(2))(C(11)H(19)N(3))].C(3)H(6)O, the U atom is in a pentagonal-bipyramidal environment, with the three N atoms of the 2,6-bis[(dimethylamino)methyl]pyridine ligand and the two O atoms of the dianionic 2,2'-methylenediphenolate ligand in the equatorial plane. The geometry is compared with that of previously reported 1:2 uranyl-diphenoxide complexes.

8,19-Dimethyl-8,19-dihydro-8,19-ethanoanthra[2'',3'':2,3;6'',7'':2',3']di-1,4-dioxino[5,6-b;5',6'-b']diquinoxaline.

The title compound, C(34)H(22)N(4)O(4), results from the reaction of 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene with 2,3-dichloroquinoxaline. The molecule, which contains a binary crystallographic symmetry axis, comprises two planar 'wings' around a central bicyclic unit. The non-ideal geometry of the latter evidences some strain, as in previous compounds with the same central core. Each molecule is involved in pi-pi interactions with four of its neighbours, oriented upside-down, which results in the formation of sheets of tightly packed molecules.

Uranyl-based metallamacrocycles: tri- and tetranuclear complexes with (2R,3R,4S,5S)-tetrahydrofurantetracarboxylic acid.

The trinuclear [UO2L]36- and tetranuclear [UO2L]48- metallamacrocycles, obtained by reaction of uranyl nitrate with the rigidly angular ligand (2R,3R,4S,5S)-tetrahydrofurantetracarboxylic acid (H4L) in a basic medium, are in equilibrium in methanol solution. Depending on the counterion, one or the other can be selectively isolated in crystal form. These rare examples of supramolecules incorporating actinide ions confirm the high potential and unique features of uranyl as a building block. Uranyl complexation through both the tri- and bidentate sites of the ligand is at variance with previous assumptions resulting from molecular modeling in nuclear waste reprocessing studies.

5,5'-Di-tert-butyl-2,2'-dihydroxy-3,3'-methylenedibenzaldehyde and 6,6'-di-tert-butyl-8,8'-methylenebis(spiro[4H-1,3-benzodioxin-2,1'-cyclohexane]).

Two related compounds containing p-tert-butyl-o-methylene-linked phenol or phenol-derived subunits are described, namely 5,5'-di-tert-butyl-2,2'-dihydroxy-3,3'-methylenedibenzaldehyde, C(23)H(28)O(4), (I), and 6,6'-di-tert-butyl-8,8'-methylenebis(spiro[4H-1,3-benzodioxin-2,1'-cyclohexane]), C(35)H(48)O(4), (II). Both compounds adopt a 'butterfly' shape, with the two phenol or phenol-derived O atoms in distal positions. Phenol and aldehyde groups in (I) are involved in intramolecular hydrogen bonds and the two dioxin rings in (II) are in distorted half-chair conformations.

Bis[bis[5-tert-butyl-2-oxido-3-(1-pyridiniomethyl)phenyl]methane]dioxouranium bis(trifluoromethanesulfonate) pyridine disolvate: a uranyl bis(diphenoxide) complex resulting from homooxacalixarenecleavage.

The title compound, [UO(2)(C(33)H(38)N(2)O(2))(2)](CF(3)SO(3))(2).2C(5)H(5)N, has been obtained by reaction of U(IV) trifluoromethanesulfonate with p-tert-butyltetrahomodioxacalix[4]arene in pyridine. The uranyl ion lies on an inversion centre and is bound to two O atoms from each diphenoxide ligand, which gives the usual square-planar equatorial environment. The zwitterionic diphenoxide species results from nucleophilic attack by pyridine on the benzylic ether C atoms of the homooxacalixarene, assisted by initial U coordination to the ether groups, with subsequent metal oxidation giving the uranyl moiety.

Dihydro[2.2.2]cryptand di-mu-hydroxo-bis[bis(nitrato-kappa 2O,O')dioxouranium(VI)] monohydrate.

In the title dinuclear uranyl complex, (C(18)H(38)N(2)O(6))[(UO(2))(2)(NO(3))(4)(OH)(2)].H(2)O, each pair of uranyl ions in the two independent centrosymmetric dianionic dimers is bridged by the two hydroxide ions, with the nitrate ions ensuring equatorial hexagonal coordination. The dihydro[2.2.2]cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) dication presents an 'in-in' conformation (endo protonation) and it is hydrogen bonded to the hydroxide ions, either directly or via a water molecule, resulting in the formation of linear hydrogen-bonded polymers.

(+/-)-6-tert-butyl-8-hydroxymethyl-2-phenyl-4H-1,3-benzodioxin and 2,2,2',2',6,6'-hexamethyl-8,8'-methylenebis(4H-1,3-benzodioxin).

Two compounds containing 1,3-benzodioxin groups are reported, namely (+/-)-6-tert-butyl-8-hydroxymethyl-2-phenyl-4H-1,3-benzodioxin, C(19)H(22)O(3), (I), and 2,2,2',2',6,6'-hexamethyl-8,8'-methylenebis(4H-1,3-benzodioxin), C(23)H(28)O(4), (II). The hydroxy groups of neighbouring molecules in (I) are hydrogen bonded to each other, giving rise to double-row chains. The molecule in (II) adopts a 'butterfly' conformation, with the O atoms in distal positions. In both compounds, the dioxin rings are in distorted half-chair conformations.

Oxidation of phenols by laccase and laccase-mediator systems.

To investigate how solubility and steric issues affect the laccase-catalysed oxidation of phenols, a series of oligomeric polyphenol compounds, having increasing size and decreasing solubility in water, was incubated with laccase. The extent of substrate conversion, and the nature of the products formed in buffered aqueous solutions, were compared to those obtained in the presence of an organic cosolvent, and also in the presence of two mediating species, i.e. N-hydroxyphthalimide (HPI) and 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO). This approach showed not only an obvious role of solubility, but also a significant role of the dimension of the substrate upon the enzymatic reactivity. In fact, reactivity decreases as substrate size increases even when solubility is enhanced by a cosolvent. This effect may be ascribed to limited accessibility of encumbered substrates to the enzyme active site, and can be compensated through the use of the appropriate mediator. While TEMPO was highly efficient at enhancing the reactivity of large, less soluble substrates, HPI proved less effective. In addition, whereas the laccase/HPI system afforded the same products as laccase alone, the use of TEMPO provided a different product with high specificity. These results offer the first evidence of the role of 'oxidation shuttles' that the mediators of laccase may have, but also suggest two promising routes towards an environmentally friendly process for kraft pulp bleaching: (a) the identification of mediators which, once oxidized by laccase, are able to target strategic functional groups present in lignin, and (b) the introduction of those strategic functional groups in an appropriate pretreatment.

Hydrogen-bonded assemblages in 2,6-bis(hydroxymethyl)-4-R-phenol, with R = methyl, methoxy, phenoxy and 1-(4-methoxyphenyl)-1-methylethyl.

Four derivatives of 2,6-bis(hydroxymethyl)phenol, with various para substituents, have been investigated; these are 2,6-bis(hydroxymethyl)-4-methylphenol, C(9)H(12)O(3), (I), 2,6-bis(hydroxymethyl)-4-methoxyphenol, C(9)H(12)O(4), (II), 2,6-bis(hydroxymethyl)-4-phenoxyphenol, C(14)H(14)O(4), (III), and 2,6-bis(hydroxymethyl)-4-[1-(4-methoxyphenyl)-1-methylethyl]phenol, C(18)H(22)O(4), (IV). All four structures display hydrogen-bonding networks resulting in sheets, with possible weak inter-sheet pi-pi interactions in one case. In all the structures but one, the molecules form centrosymmetric dimeric subunits held together by two hydrogen bonds between the hydroxymethyl groups and, in two cases, by probable pi-pi interactions.

1,3-Bridged p-methyloctahomotetraoxacalix[4]arene-bis-crown-3.

The title compound, 13,21,35,43-tetramethyl-3,6,9,17,25,28,31,39,46,49-decaoxaheptacyclo[21.21.3.3(11,33).0(2,41).0(10,15).0(19,24).0(32,37)]pentaconta-1,10,12,14,19,21,23,32,34,36,41,43-dodecaene, C(44)H(52)O(10), differs from previously reported 1,3-bridged calix[4]arene[bond]bis-crown compounds in having an enlarged calixarene ring and shorter polyoxyethylene bridges. The cavity is partly filled by the bridges.

First- and second- sphere coordination of the uranyl ion by bis[2-(2-hydroxyphenoxy)ethoxy]ethane.

Uranyl nitrate hexahydrate reacts with bis[2-(2-hydroxyphenoxy)ethoxy]ethane (C(18)H(22)O(6)), denoted LH(2) hereafter, in the presence of triethylamine to give triethylammonium aqua[2,2'-(3,6-dioxaoctane-1,8-diyldioxy)diphenolato-kappa(2)O,O'](nitrato-kappa(2)O,O')dioxouranium(VI), (Et(3)NH)[UO(2)(H(2)O)L(NO(3))], which possesses a symmetry plane. The uranyl ion is coordinated to the two phenoxide O atoms, a nitrate ion and a water molecule (first sphere); the water molecule is itself held in the crown ether chain by hydrogen-bonding interactions, thus ensuring second-sphere coordination by the ligand L.

2,3,6,7-Tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene bis(1,4-dioxane) solvate.

2,3,6,7-Tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene crystallizes with 1,4-dioxane to give a bis-solvate, C(18)H(18)O(4).2C(4)H(8)O(2). The bis(catechol) molecule is located on a twofold axis and the two aromatic rings form a dihedral angle of 130.61(4) degree. Hydrogen bonds are formed between the hydroxyl groups and either a neighbouring bis(catechol) molecule or the ether-O atom of a dioxane molecule.

Bis[2-(2-hydroxyphenoxy)ethyl] ether methanol solvate.

The title compound, 2,2'-(3-oxapentane-1,5-diyldioxy)diphenol, crystallizes as a methanol solvate, C16H18O5*CH4O. The methanol molecule, roughly perpendicular to the mean plane of the polyether molecule, is hydrogen bonded to the two polyether OH groups and to the central ether O atom. Possible C-H...pi intermolecular interactions are present.