Anionic Oxygen Donor Research Articles

Aldimine‐Thioether‐Phenolate Based Mono‐ and Bimetallic Zinc Complexes as Catalysts for the Reaction of CO2 with Cyclohexene Oxide

A synthetic strategy for the preparation of a new class of ligands is introduced. The bis(aldimine‐thioether‐phenolate) ligands bear two anionic oxygen donors from phenolate moieties, two neutral sulfur donors and two neutral nitrogen donors belonging, respectively, to thioether and aldimine functionalities. The so designed OSNNSO ligands show two coordinative pockets and thus should be able to host two metallic centers. Instead, the aldimine‐thioether‐phenolate OSN ligands are tridentate ligands which may form monometallic complexes. Two OSNNSO and one OSN ligands of this class, have been synthesized. Satisfyingly, by direct reaction of the ligands with one or two equivalents of the zinc precursor, the corresponding bimetallic Zn(II) complexes were prepared from the bis(aldimine‐thioether‐phenolate) ligands, while the monometallic zinc complex was prepared from the tridentate ligand. These zinc amido complexes act as single component catalysts in the reaction of CO2 with cyclohexene oxide by furnishing polycyclohexene carbonate, while adding PPNCl as cocatalyst to the reaction medium, cyclohexene carbonates were obtained as the main products.

An unusual example of halogen bonding to potassium t-butoxide

1,4-diiodooctafluorobutane was found to react with potassium tert-butoxide in dioxane to afford a halogen-bonded adduct as a white precipitate. Single crystal X-ray analysis of the precipitate reveals an extended network held together by strong iodine-oxygen interactions. The oxygen-iodine bond length in the tert-butoxide complex was found to be 2.391(6) Å, much shorter than a previously published and similar complex involving dioxane (2.814(12) Å). To our knowledge, this work represents the first example of a structurally characterized halogen bonding network involving anionic oxygen donor atoms with perfluoroalkyl iodides. The tert-butoxide adduct displays the shortest iodine-oxygen distance and the longest iodine-carbon distance thus far reported for any halogen bonding involving a perfluoroalkyl iodide substrate.

ChemInform Abstract: Synthesis, Structure and Applications of [Cis‐dioxomolybdenum(VI)‐(ONO)] Type Complexes

AbstractReview: 51 refs.

Synthesis, structure and applications of [cis-dioxomolybdenum(VI)-(ONO)] type complexes

Oxo-molybdenum chemistry is of great interest since such units are found in the active sites of a majority of molybdo-enzymes. In order to mimic the biological systems, a number of oxo-molybdenum complexes have been synthesised and studied. This review describes synthesis, structure and applications of oxo-molybdenum complexes particularly cis-MoO2(L)(D) where L stands for a dianionic tridentate ONO ligand and D for a donor solvent molecule/monodentate ligand. The ligand moieties are derived from Schiff base, hydrazide Schiff base and other related tridentate ligands L(H)2. The coordination geometry around the Mo center in these complexes can be best described as a distorted octahedron in which the ONO-tridentate ligand occupies meridional position with two anionic oxygen donors mutually trans and are cis to the oxygen centers of the cis-dioxo group. Mostly the applications of cis-MoO2-(ONO) type complexes seen in literature are oxo transfer reactions like epoxidation, sulfoxidation and phosphine oxidation reactions.

Phosphinoureas: cooperative ligands in rhodium-catalyzed hydroformylation? On the possibility of a ligand-assisted reductive elimination of the aldehyde

We report the synthesis of a novel type of phosphinourea ligand, its coordination chemistry with rhodium, its use in the asymmetric hydroformylation of styrene, and investigations on the hydroformylation reaction mechanism. Complex studies on the 2:1 complex of phosphinourea to [Rh(acac)(CO)(2)] showed that a neutral trans-coordinating complex [Rh(HL-kappa P)(L-kappa O-2,P)(CO)] was formed. An anionic O,P-chelating ligand has displaced the anionic acac(-) ligand via an acid-base reaction involving the deprotonation of an acidic urea proton, giving Hacac. A second phosphinourea is coordinated as a neutral monodentate ligand and is linked to the chelating anionic ligand via an intramolecular hydrogen bond. The behavior of these supramolecular complexes in the hydroformylation reaction and the possible cooperative role of the ligands in the catalytic cycle were studied both experimentally and by computational methods. High-pressure NMR spectroscopy revealed that the catalytically active rhodium hydride species further consists of two neutral phosphinourea ligands and is in equilibrium with the neutral species [Rh(HL-kappa P)(L-kappa O-2,P)(CO)]. This equilibrium is likely an integrated part of a productive hydroformylation cycle involving a ligand-assisted reductive elimination of the aldehyde. DFT calculations revealed that the ligand-assisted mechanism could well be the preferred lower energetic pathway; however, the orientation of the anionic oxygen donor atom in [Rh(HL-kappa P)(L-K kappa O-2,P)(CO)] prevented us from finding a direct (nonsolvent assisted) transition state to connect the intermediates. We therefore cannot exclude a mechanism where [Rh(HL-kappa P)(L-kappa O-2,P)(CO)] is a dormant species outside the productive hydroformylation cycle, although the intermediate associated with this mechanism is higher in energy. Finally, the synthesis of heteroligated complexes was investigated, consisting of two electronically different phosphinoureas, which sets the stage for combinatorial supramolecular ligand approaches in catalysis. Simply mixing two electronically different phosphinoureas with metal precursor [Rh(acac)(CO)(2)] resulted in the formation of a heterobidentate ligand. A set of six new phosphinoureas was used to prepare such rhodium complexes in a combinatorial fashion for the asymmetric hydroformylation of styrene, resulting in high conversions and selectivities for the branched product and moderate enantioselectivities.

Synthesis and structural characterization of iron(III) salen complexes possessing appended anionic oxygen donor ligands

The synthesis and solid-state characterization of iron(III) salen complexes bearing monodentate and bidentate anionic oxygen donor ligands are reported. The synthesis of Fe(salen)OPh (OPh=2,6–diphenylphenoxide) ( 1) was accomplished using Fe[N(Si(CH 3) 3) 2] 3 with one equivalent of H 2salen and the corresponding phenol. The complex was isolated as a crystalline sample by the slow diffusion of pentane into a concentrated solution of 1 at −20 °C. The solid-state structure of complex 1 reveals distorted square pyramidal geometry about the iron metal center. Alternatively the preparation of Fe(salen) complex 2 bearing the anionic, bidentate acetylacetonate ligand in the axial position was accomplished by thermally replacing two acetylacetonate ligands in Fe(acac) 3 with one equivalent of H 2salen. The complex was isolated and crystallographically characterized using X-ray quality crystals formed via the aforementioned protocol. The metal center adopts a very distorted octahedral geometry, indicating a preference for the square pyramidal species which is typical of other iron salen derivatives. Unfortunately, 1 and 2 were found to be inactive in the copolymerization of CO 2 and cyclohexene oxide to afford polycarbonate in the presence of a Lewis base (i.e., N-MeIm and PCy 3). The μ-[Fe(salen)] 2O ( 3) derivative was also isolated via the protonation of Fe(salen)(N(Si(CH 3) 3) 2). The complex is relatively air stable, forming large, dark crystals over a short period of time at −20 °C. The solid-state structure reveals a nearly linear Fe–O–Fe bond angle, which is due to the electronic repulsions associated with the sterically encumbering tert-butyl groups on the salen framework.

Gold(I) and Gold(III) Complexes with Anionic Oxygen Donor Ligands: Hydroxo, Oxo and Alkoxo Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Gold(I) and gold(III) complexes with anionic oxygen donor ligands: hydroxo, oxo and alkoxo complexes

The chemistry of gold(I) and gold(III) hydroxo, oxo and alkoxo complexes containing organic co-ligands is reviewed. Due to the high reactivity of the gold-oxygen bond most of these species can be employed as useful intermediates in many important processes. Alkoxides are by far the most numerous class among these species, and both gold(I) and gold(III) complexes are known with a variety of alkoxides. The recent discovery of the catalytic activity of some gold(I) and gold(III) fluoroalkoxides has enlivened the interest towards this class of compounds leading to new applications for previously known complexes, as is the case for a gold(III) siloxo complex which has found use as a precursor for CVD, as well as for the synthesis of new alkoxo derivatives. Another well represented and well known class of compounds are the gold(I) oxo complexes [O(AuPR3)3]+ which are the most effective aurating reagents. Gold(III) oxo complexes have been synthesized only recently and much of their chemistry is still largely to be explored. Preliminary studies suggest their potential in the transfer of oxygen atoms. Finally, some cationic gold(III) hydroxo complexes are found to display significant,and in some cases relevant, cytotoxic effects on cancer cell lines.

Proximal ligand control of heme iron coordination structure and reactivity with hydrogen peroxide: investigations of the myoglobin cavity mutant H93G with unnatural oxygen donor proximal ligands

The role of the proximal heme iron ligand in activation of hydrogen peroxide and control of spin state and coordination number in heme proteins is not yet well understood. Although there are several examples of amino acid sidechains with oxygen atoms which can act as potential heme iron ligands, the occurrence of protein-derived oxygen donor ligation in natural protein systems is quite rare. The sperm whale myoglobin cavity mutant H93G Mb (D. Barrick, Biochemistry 33 (1994) 6546) has its proximal histidine ligand replaced by glycine, a mutation which leaves an open cavity capable of accommodation of a variety of unnatural potential proximal ligands. This provides a convenient system for studying ligand–protein interactions. Molecular modeling of the proximal cavity in the active site of H93G Mb indicates that the cavity is of sufficient size to accommodate benzoate and phenolate in conformations that allow their oxygen atoms to come within binding distance of the heme iron. In addition, benzoate may occupy the cavity in an orientation which allows one carboxylate oxygen atom to ligate to the heme iron while the other carboxylate oxygen is within hydrogen bonding distance of serine 92. The ferric phenolate and benzoate complexes have been prepared and characterized by UV-visible and MCD spectroscopies. The benzoate adduct shows characteristics of a six-coordinate high-spin complex. To our knowledge, this is the first known example of a six-coordinate high-spin heme complex with an anionic oxygen donor proximal ligand. The benzoate ligand is displaced at alkaline pH and upon reaction with hydrogen peroxide. The phenolate adduct of H93G Mb is a five-coordinate high-spin complex whose UV-visible and MCD spectra are distinct from those of the histidine 93 to tyrosine (H93Y Mb) mutant of sperm whale myoglobin. The phenolate adduct is stable at alkaline pH and exhibits a reduced reactivity with hydrogen peroxide relative to that of both native ferric myoglobin, and the exogenous ligand-free derivative of ferric H93G Mb. These observations indicate that the identity of the proximal oxygen donor ligand has an important influence on both the heme iron coordination number and the reactivity of the complex with hydrogen peroxide.

Reaction of anionic oxygen donors with the antitumour copper(II)–pyridine-2-carbaldehyde thiosemicarbazone (HL) system and the crystal structure of [{Cu(HL)(H2PO4)}2][H2 PO4]2·2H3PO4 ·2H2O

The complexes [(CuL) 2 (XO 4 )]·nH 2 O (HL = pyridine-2-carbaldehyde thiosemicarbazone, X = HP, n = 6; X = Mo, n = 1), [{CuL(ptp)} 2 ]·4H 2 O (Hptp = p-tritylphenol) and [{CuL(tip)} 2 ] (Htip = 2,4,6-triiodophenol) have been prepared from the reaction of aqueous solutions of [{CuL(CH 3 CO 2 )} 2 ] with salts of the appropriate oxygen donor. Also prepared were [{Cu(HL)(H 2 PO 4 )} 2 ][H 2 PO 4 ] 2 ·xH 3 PO 4 ·nH 2 O (x = 0, n = 3; x = 2, n = 2) from the reaction of [{CuL(CH 3 CO 2 )} 2 ] with 2 mol dm -3 H 3 PO 4 and phosphate species (arising from the hydrolysis of 2 mol dm -3 HPF 6 ) respectively. The complexes have been characterized by a range of physicochemical techniques and the crystal and molecular structure of [{Cu(HL)(H 2 PO 4 )} 2 ][H 2 PO 4 ] 2 ·2H 3 PO 4 ·2H 2 O determined by single-crystal X-ray diffraction. The complex cation is a centrosymmetric dimer, the monomeric units having one-atom bridges from two H 2 PO 4 - anions. The copper atom has a distorted square-pyramidal geometry with three donor atoms (NNS) from the neutral, tridentate HL ligand and the fourth in-plane donor (an oxygen) from the bridging anion. The fifth position is occupied by a less strongly bound oxygen from the second bridging anion. An extensive hydrogen-bonding scheme involving the bridging H 2 PO 4 - anions, non-co-ordinated H 2 PO 4 - anions, H 3 PO 4 moieties, water molecules and ligand exists with one very strong contact of 2.436 A. These complexes may serve as simple models for the interaction of this antitumour system with oxygen donors.

Cisplatin analogues with sulfur donor ligands. (Ethylenediamine)platinum(II) complexes with ligands possessing a sulfinyl or sulfanyl group linked to an anionic oxygen donor atom. Reactivity and cytotoxicity

Complexes of formula [Pt(en)L]NO3, where en = ethylenediamine and L is a ligand in which a methylsulfinyl or methylsulfanyl group is linked to a carboxylate or phenolate moiety, have been synthesized and characterized by spectroscopic techniques. The ligand L is chelated to Pt via the sulfur and anionic oxygen atoms. The reactivities of these complexes towards nucleophiles [Nu = Cl– or guanosine 5′-monophosphate(2–), GMP] have been investigated by 1H NMR spectroscopy; GMP is more reactive than Cl–. The Pt–O bonds react more readily than the Pt–S bonds and, depending on the nature of L and Nu, the reactions either stop at the monosubstituted complex [Pt(en)L(Nu)](with L monodentate and co-ordinated to Pt via the sulfur atom), or yield [Pt(en)(Nu)2]. The complex with L = 2-(methylsulfanyl)phenolate is unreactive. The stable monosubstituted products have been characterized. There is little correlation between the rates and the modes of reactivity of complexes [Pt(en)L]NO3 towards GMP and their cytotoxicities against the human ovarian carcinoma A2780 cell line.