Agustín Galindo

Direct Bonds Between Metal Atoms: Zn, Cd, and Hg Compounds with Metal–Metal Bonds

The synthesis and characterization of [Zn(2)(eta(5)-C(5)Me(5))(2)], a stable molecular compound with a Zn-Zn bond and the first example of a dimetallocene structure, has opened a new chapter in the organometallic chemistry of zinc and in metallocene chemistry. The existence of two directly bonded zinc atoms demonstrates that the [Zn-Zn](2+) unit, the lightest Group 12 homologue of the well-known [Hg-Hg](2+) ion, can be stabilized by appropriate ligands. Activity in this area has increased enormously in the few years since the determination of the structure of this molecule. Numerous theoretical studies have been devoted to the investigation of the electronic, structural, and spectroscopic properties of this and related compounds, and new metal-metal coordination and organometallic compounds of zinc, cadmium, and mercury have been synthesized and structurally characterized. This Minireview gives an overview of activity in this field during the past three to four years.

Olefin epoxidation by hydrogen peroxide catalysed by molybdenum complexes in ionic liquids and structural characterisation of the proposed intermediate dioxoperoxomolybdenum species

The complex [Mo(4)O(16)(dmpz)(6)] (1) has been isolated as part of a study of oxodiperoxomolybdenum catalysed epoxidation of olefin substrates with hydrogen peroxide in ionic liquids. Notably, 1 is the first dioxoperoxomolybdenum species to be structurally characterised.

Synthesis, antiapoptotic biological activity and structure of an oxo-vanadium(IV) complex with an OOO ligand donor set

Abstract The oxo–vanadium(IV) complex VO(oda)(H2O)2 (1) (oda=oxydiacetate, O(CH2COO−)2) was obtained by reaction of aerobic aqueous solutions of VO(acac)2 with oxydiacetic acid, O(CH2COOH)2. The antiapoptotic biological activity of 1 was studied in insulin-producing cells. Chemically generated nitric oxide (NO) triggers apoptotic events, such as the appearance of oligonucleosomes in cytosol, and this response was prevented by the presence of 1 in the culture medium. The molecular structure of 1 has been determined by X-ray diffraction analysis.

Synthesis and molecular structure of oxydiacetate complexes of nickel(II) and cobalt(II). Theoretical analysis of the planar and non-planar conformations of oxydiacetate ligand and oxydiacetic acid

A novel synthetic route to [Ni(oda)(H2O)3]·1.5H2O 1 and [Co(oda)(H2O)2]·H2O 2 [oda = oxydiacetate, O(CH2CO2−)2] is presented. These complexes react with bidentate N-donor ligands to yield compounds of general formula [M(oda)(N–N)(H2O)] (M = Ni, N–N = bipy, 3; phen, 5; tmeda, 7. M = Co, N–N = bipy, 4; phen, 6; tmeda, 8), which can be isolated as crystalline solids with different numbers of hydration water molecules. The molecular structures of the compounds 3 and 4 have been shown by X-ray analysis to crystallize with 2.5H2O. In these compounds, the oda ligand adopts a planar, tridentate conformation (mer) in binding the octahedral metal centre. By contrast, the recent X-ray characterization of 1 has shown that the oxydiacetate ligand completes the octahedral coordination of the metal in the fac arrangement. The DFT method has been adopted to investigate theoretically the flexibility of this ligand. First, calculations have been carried out on the free oxydiacetic acid (H2oda) and its anion. Then, a model of 1 has been studied in order to find the geometric and energetic implications for the alternative oda conformations (mer and fac) in reaching the octahedral environment about the metal. In general, the geometric parameters are in good agreement with those available from crystal structures. From the energetic viewpoint, the mer conformation is favoured by a small energy difference (2.0 kcal mol−1). On the other hand, the fac conformation in 1 may be justified with the presence of the water hydration molecules (not considered by our model) which may deeply affect the energetics of the overall system by forming strong hydrogen bonds with the oxydiacetate ligand.

New insights into the mechanism of oxodiperoxomolybdenum catalysed olefin epoxidation and the crystal structures of several oxo–peroxo molybdenum complexes

[Mo(O)(O(2))(2)(L)(2)] compounds (L = pz, pyrazole; dmpz, 3,5-dimethylpyrazole) were reacted stoichiometrically, in the absence of an oxidant, with cis-cyclooctene in an ionic liquid medium where selective formation of the corresponding epoxide was observed. However, this oxo-transfer reaction was not observed for some other olefins, suggesting that alternative reaction pathways exist for these epoxidation processes. Subsequently, DFT studies investigating the oxodiperoxomolybdenum catalysed epoxidation model reaction for ethylene with hydrogen peroxide oxidant were performed. The well known Sharpless mechanism was first analysed for the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst and a low energy reaction pathway was found, which fits well with the observed experimental results for cis-cyclooctene. The structural parameters of the computed dioxoperoxo intermediate [Mo(O)(2)(O(2))(dmpz)(2)] in the Sharpless mechanism compare well with those found for the same moiety within the [Mo(4)O(16)(dmpz)(6)] complex, for which the full X-ray report is presented here. A second mechanism for the model epoxidation reaction was theoretically investigated in order to clarify why some olefins, which do not react stoichiometrically in the absence of an oxidant, showed low level conversions in catalytic conditions. A Thiel-type mechanism, in which the oxidant activation occurs prior to the oxo-transfer step, was considered. The olefin attack of the hydroperoxide ligand formed upon activation of hydrogen peroxide with the [Mo(O)(O(2))(2)(dmpz)(2)] model catalyst was not possible to model. The presence of two dmpz ligands coordinated to the molybdenum centre prevented the olefin attack for steric reasons. However, a low energy reaction pathway was identified for the [Mo(O)(O(2))(2)(dmpz)] catalyst, which can be formed from [Mo(O)(2)(O(2))(dmpz)(2)] by ligand dissociation. Both mechanisms, Sharpless- and Thiel-type, were found to display comparable energy barriers and both are accessible alternative pathways in the oxodiperoxomolybdenum catalysed olefin epoxidation. Additionally, the molecular structures of [Mo(O)(O(2))(2)(H(2)O)(pz)] and [Hdmpz](4)[Mo(8)O(22)(O(2))(4)(dmpz)(2)]·2H(2)O and the full X-ray report of [Mo(O)(O(2))(2)(pz)(2)] are also presented.

Mononuclear and dinuclear tertiary phosphine molybdenum complexes. Oxo-molybdenum(IV), dinuclear Mo2Cl4L4 and related derivatives

Abstract Green and blue isomers of the oxo derivative MoOCl2(PMe3)3 have been obtained by an oxygen-atom abstraction reaction between MoCl4(thf)2 and equimolar amounts of water in the presence of PMe3. Methatesis with KX(X = NCO, NCS) yields MoOX2(PMe3)3 and with NaS2CNEt2, MoO(S2CNEt2)2(PMe3). The latter complex readily loses PMe3 to give MoO(S2CNEt2)2 from which it can be prepared by addition of the phosphine ligand. Reaction of the blue purple complex, MoCl3(thf)3 (I), with excess PMe3 gives mer-MoCl3(PMe3)3 which loses PMe3 on heating in toluene to afford [MoCl3(PMe3)2]2. Reduction of (I) with phosphines and zinc in tetrahydrofuran gives the dinuclear molybdenum(II) halide complexes Mo2Cl4L4 (L = PMe3, PEt3, PhMe2Ph, PEt2Ph; L2 = dppm), while Zn-acetic acid reduction yields Mo2(CO2Me)4. Interaction of the chlorocarbonyl species MoCl2(CO)2(PMe3)3 with Tl(acac) affords Mo(acac)Cl(CO)(PMe3)3 which has an unusually low CO stretching frequency for a terminal carbonyl group (1755 cm−1).

Phosphinecarbon disulfide adducts, S2CPR3: versatile ligands in coordination chemistry ☆

Abstract Phosphinecarbon disulfide adducts, S 2 CPR 3 , are very versatile ligands that exhibit a variety of coordination modes to transition metal centers. These compounds are able to formally donate from 2 to 8e − to a metal (or metals). A resume of the synthetic methodology to prepare them, a rationalization of the structural diversity found in this type of complexes, and their main spectroscopic and chemical properties will be the scope of this review.

Synthesis and characterization of dioxocomplexes of molybdenum with (η-C5H5)Co{P(O)(OEt)2}3, C5H4(SiMe3) and 1,3-C5H3(SiMe3)2 ligands. X-ray crystal structure of [(η-C5H5)Co{P(O)(OEt)2}3]MoO2Cl

Abstract The interaction of (NH4)2Mo2O7 with Me3SiCl in 1,2-dimethoxyethane (dme) gives, in a one pot reaction, the known dioxo-compound MoCl2O2(dme) (1). Dioxo–molybdenum complexes of the type (L)MoO2Cl (L=monoanionic, formally tridentate, ancillary ligand like (η-C5H5)Co{P(O)(OEt)2}3, LOEt (2) C5H4(SiMe3), Cptms (3) and 1,3-C5H3(SiMe3)2, Cp2tms (4)) have been prepared using 1 as the starting material. Other known oxo complexes are also easily accessible from 1, for example, the reaction with CpTl, PMe3 and Li[(Me3SiN)2CPh].tmda affords CpMoO2Cl (5), MoOCl2(PMe3)3 (7) and Mo(O)2{(Me3SiN)2CPh}2 (8), respectively. Complex 4 acts as a good catalyst of the oxo-transfer model reaction from dimethylsulfoxide to PPh3 at room temperature. Complex (LOEt)MoO2Cl (2) has been structurally characterized by X-ray crystallography.

Trimethylphosphine complexes of molybdenum and tungsten. The synthesis and chemical properties of MoCl4(PMe3)3 and the crystal and molecular structures of WCl4(PMe3)3 and MoO(acac)2PMe3

Abstract The seven-coordinate compound MoCl 4 (PMe 3 ) 3 has been prepared by the room temperature reaction of MoCl 4 (THF) 2 and PMe 3 . This complex is a useful starting material for the synthesis of other trimethylphosphine derivatives of molybdenum. Reduction, under different experimental conditions, affords the known complexes MoCl 3 (PMe 3 ) 3 , MoCl 3 (PMe 3 ) 4 , cis -Mo(N 2 ) 2 (PMe 3 ) 4 and trans -Mo(C 2 H 4 ) 2 (PMe 3 ) 4 . Interaction with H 2 O, in the presence of an excess of PMe 3 produces MoOCl 2 (PMe 3 ) 3 which gives MoO(acac) 2 PMe 3 by reaction with Tl(acac). The oxoacetylacetonate complex crystallizes in the monoclinic space group P 2 1 / c with a 14.155(6), b 9.984(4), c 12.629(1) A, β 98.80(5)° and D calc 1.44 g cm −3 for Z = 4. A final R value of 0.026 based on 1332 observed reflections was obtained. The molybdenum coordination is pseudo-octahedral with the oxo and phosphine ligands in a cis configuration. The MoO(oxo) separation is 1.676(5) A. The crystal structure of the tungsten analogue of MoCl 4 (PMe 3 ) 3 has also been determined. WCl 4 (PMe 3 ) 3 crystallizes in the monoclinic space group P 2 1 / c with unit cell constants a 14.835(4), b 11.422(2), c 11.568(3) A, β 91.92(2)° and D calc 1.88 g cm −3 for Z = 4. The final R value based on 1613 independent observed reflections is 0.040. The tungsten atom is seven-coordinate, the ligands describing an approximate capped octahedron with a chlorine atom capping the face defined by the three phosphorous atoms. The unique WCl distance is 2.417(5) A, while the remaining WCl and WP bond lengths average 2.45(3) and 2.551(3) A respectively.

Influence of N-donor bases and the solvent in oxodiperoxomolybdenum catalysed olefin epoxidation with hydrogen peroxide in ionic liquids

Biphasic catalytic olefin epoxidation systems consisting of oxodiperoxomolybdenum catalysts in 1-n-alkyl-3-methylimidazolium hexafluorophosphate ionic liquid (IL) media with aqueous hydrogen peroxide oxidant were optimised by tuning the molecular structure of the IL and employing N-heterocyclic donor base additives to inhibit hydrolysis and enhance the activity of the catalyst. The latter study was only made possible by the solubilising properties of the IL media. Of the bases investigated, pyrazoles were identified as the most efficient additive species and the best results were obtained using 3,5-dimethylpyrazole. Immobilisation of the catalyst in the IL allowed for very efficient catalyst recycling. Finally, the compound [MoO(O(2))(2)(3-Mepz)(2)] (3-Mepz = 3-methylpyrazole) was characterised and its structure determined by X-ray crystallography.