Celedonio M. Álvarez

Stereoselective Aldol Addition to Rhenium(I) Complexes and Reversible Dimerization with Epimerization of the Metal Center

Herein, we report several examples of stereoselective aldol additions of aldehydes or ketones to ReI tricarbonyl complexes to form monomeric derivatives in good yields. The metal-centered chirality defines the final stereochemistry of the carbon atom of the monomeric ReI complex after the addition. However, it cannot control the resulting stereochemistry of the enolate part, and thus, if the α-carbon atom of the reagent is prochiral, a mixture of diastereoisomers is obtained. On the other hand, all of the monomeric complexes can be reversibly dimerized in basic media to form cis dimers, for which an epimerization of the metal-centered chirality is required in order to avoid steric congestion. All of these results are supported by exhaustive crystallographic analysis.

η6-Hexahelicene complexes of iridium and ruthenium: running along the helix.

The first η(6)-complexes of iridium and ruthenium coordinated to helicenes have been obtained. Hexahelicene (1), 2,15-dimethylhexahelicene (2), and 2,15-dibromohexahelicene (3) react with [Cp*IrCl(2)](2) and AgBF(4) in CD(3)NO(2) to afford quantitatively the complexes [Cp*Ir(η(6)-1)][BF(4)](2) (4A), [Cp*Ir(η(6)-2)][BF(4)](2) (5A), and [Cp*Ir(η(6)-3)][BF(4)](2) (6A), respectively. In all cases, the final thermodynamic products are similar, and they exhibit coordination between the 12 e(-) metal fragment [IrCp*](2+) and the terminal ring of the helicene. Monitoring the reaction by NMR shows formation of intermediates, some of which have been fully characterized in solution. These intermediates exhibit the metal fragment coordinated to the internal rings. We have also synthesized the bimetallic complex [(Cp*Ir)(2)(μ(2)-η(6):η(6)-2)][BF(4)](4) (7), achieving coordination between two units [IrCp*](2+) and the helicene 2. Following an analogous methodology, we have prepared the complex [(η(6)-cymene)Ru(η(6)-2)][BF(4)](2) (8), which has been studied by X-ray diffraction, confirming the preferential binding to the terminal aromatic ring.

Coordination versus coupling of dicyanamide in molybdenum and manganese pyrazole complexes.

The reactions of cis-[MoCl(η(3)-methallyl)(CO)(2)(NCMe)(2)] (methallyl = CH(2)C(CH(3))CH(2)) with Na(NCNCN) and pz*H (pzH, pyrazole, or dmpzH, 3,5-dimethylpyrazole) lead to cis-[Mo(η(3)-methallyl)(CO)(2)(pz*H)(μ-NCNCN-κ(2)N,N)](2) (pzH, 1a; dmpzH, 1b), where dicyanamide is coordinated as bridging ligand. Similar reactions with fac-[MnBr(CO)(3)(NCMe)(2)] lead to the pyrazolylamidino complexes fac-[Mn(pz*H)(CO)(3)(NH═C(pz*)NCN-κ(2)N,N)] (pzH, 2a; dmpzH, 2b), resulting from the coupling of pyrazol with one of the CN bonds of dicyanamide. The second CN bond of dicyanamide in 2a undergoes a second coupling with pyrazole after addition of 1 equiv of fac-[MnBr(CO)(3)(pzH)(2)], yielding the dinuclear doubly coupled complex [{fac-Mn(pzH)(CO)(3)}(2)(μ-NH═C(pz)NC(pz)=NH-κ(4)N,N,N,N)]Br (3). The crystal structure of 3 reveals the presence of two isomers, cis or trans, depending on whether the terminal pyrazoles are coordinated at the same or at different sides of the approximate plane defined by the bridging bis-amidine ligand. Only the cis isomer is detected in the crystal structure of the perchlorate salt of the same bimetallic cation (4), obtained by metathesis with AgClO(4). All the N-bound hydrogen atoms of the cations in 3 or 4 are involved in hydrogen bonds. Some of the C-N bonds of the pyrazolylamidino ligand have a character intermediate between single and double, and theoretical studies were carried out on 2a and 3 to confirm its electronic origin and discard packing effects. Calculations also show the essential role of bromide in the planarity of the tetradentate ligand in the bimetallic complex 3.

Synthesis and decarbonylation reactions of the triiron phosphinidene complex [Fe3Cp3(μ-H)(μ3-PPh)(CO)4]: easy cleavage and formation of P-H and Fe-Fe bonds.

The binuclear phosphine complex [Fe(2)Cp(2)(μ-CO)(2)(CO)(PH(2)Ph)] (Cp = η(5)-C(5)H(5)) reacted with the acetonitrile adduct [Fe(2)Cp(2)(μ-CO)(2)(CO)(NCMe)] in dichloromethane at 293 K to give the trinuclear hydride-phosphinidene derivative [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(CO)(4)] as a mixture of cis,anti and trans isomers (Fe-Fe = 2.7217(6) Å for the cis,anti isomer). In contrast, photochemical treatment of the phosphine complex with [Fe(2)Cp(2)(CO)(4)] gave the phosphide-bridged complex trans-[Fe(3)Cp(3)(μ-PHPh)(μ-CO)(2)(CO)(3)] as the major product, along with small amounts of the binuclear hydride-phosphide complexes [Fe(2)Cp(2)(μ-H)(μ-PHPh)(CO)(2)] (cis and trans isomers), which are more selectively prepared from [Fe(2)Cp(2)(CO)(4)] and PH(2)Ph at 388 K. The photochemical decarbonylation of either of the mentioned triiron compounds led reversibly to three different products depending on the reaction conditions: (a) the 48-electron phosphinidene cluster [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(μ-CO)(2)] (Fe-Fe = 2.592(2)-2.718(2) Å); (b) the 50-electron complex [Fe(3)Cp(3)(μ-H)(μ(3)-PPh)(μ-CO)(CO)(2)], also having carbonyl- and hydride-bridged metal-metal bonds (Fe-Fe = 2.6177(3) and 2.7611(4) Å, respectively); and (c) the 48-electron phosphide cluster [Fe(3)Cp(3)(μ-PHPh)(μ(3)-CO)(μ-CO)(2)], an isomer of the latter phosphinidene complex now having three intermetallic bonds (Fe-Fe = 2.5332(8)-2.6158(8) Å).

Macrocycle formation by proton-template-induced dimerization of complexes with (alkoxoimino)pyridine.

The existence of a strong hydrogen bond between two molecules of an (alkoxoimino)pyridinemanganese(I) complex induces their dimerization and the formation of a metallamacrocycle. The expected intramolecular attack of the alkoxo moiety is disfavored.

Synergistic Effect of Tetraaryl Porphyrins Containing Corannulene and Other Polycyclic Aromatic Fragments as Hosts for Fullerenes. Impact of C60 in a Statistically Distributed Mixture of Atropisomers

Symmetric meso-tetraarylporphyrins bearing phenanthrene, pyrene, and corannulene moieties in meta positions have been synthesized in a straightforward procedure under microwave irradiation by quadruple Suzuki-Miyaura reactions. Their (1)H NMR spectra showed the typical pattern of four atropisomers distributed according to their statistical ratio not properly separable due to their fast isomerization. Their ability to bind buckminsterfullerene has been tested with the whole mixture, and different behaviors have been found, α4 isomer corannulene-substituted porphyrins being the best hosts in the family.

Affinity modulation of photoresponsive hosts for fullerenes: light-gated corannulene tweezers

Six azobenzene derivatives bearing polyaromatic fragments have been prepared and their reversible photoisomerization has been assessed. Corannulene-functionalized molecules have demonstrated excellent switchable hosting abilities towards fullerenes in which an interesting range of affinities has been found. The success of this design relies upon the reversible formation and destruction of tweezer-like structures.

Nuclear Magnetic Resonance Methodology for the Analysis of Regular and Non-Alcoholic Lager Beers

The presence of seven main agents responsible for beer aroma and taste (n-propanol, isobutanol, 3-methylbutanol, tyrosol/tyrosine, ethyl acetate, isoamyl acetate, and acetaldehyde) is determined by different nuclear magnetic resonance (NMR) techniques (1H PRESAT, total correlation spectroscopy, heteronuclear single quantum correlation, and heteronuclear multiple bond correlation) in five regular and five low-alcoholic or alcohol-free beers. The new methodology includes the identification of the 1H and 13C NMR chemical shifts of the analytes by a standard addition method and the consequent identification of the compounds studied in regular and non-alcoholic beers. The chemical composition is different depending on whether the beer is regular or non-alcoholic, therefore affecting the organoleptic characteristics of each type of beer.

Pyridine-2-carboxaldehyde as ligand: Synthesis and derivatization of carbonyl complexes

Thermally induced carbonyl substitutions on [M(CO)5X] (M=Mn, X=Cl, Br; M=Re, X=Br) or room temperature displacement of acetonitrile from [Mo(eta3-methallyl)Cl(CO)2(NCMe)2] produce stable crystalline complexes containing pyridine-2-carboxaldehyde (pyca) as chelate kappa2(N,O) ligands (). These react with ethylglycine to afford iminopyridine complexes containing an amino ester pendant arm in high-yield. Treatment with silver salts produce halide abstraction affording neutral complexes containing coordinated perchlorate or triflate which can be replaced by triphenyl phosphine to give cationic complexes . As confirmed by spectroscopy and X-ray crystallography the pyca ligand remains bonded as chelate kappa2(N,O) throughout these processes.

Copper Complexes in the Promotion of Aldol Addition to Pyridine-2-carboxaldehyde: Synthesis of Homo- and Heteroleptic Complexes and Stereoselective Double Aldol Addition

CuCl2·2H2O and Cu(ClO4)2·6H2O are able to promote aldol addition of pyridine-2-carboxaldehyde (pyca) with acetone, acetophenone, or cyclohexenone under neutral and mild conditions. The general and simple one-pot procedure for the aldol addition to Cu(II) complexes accesses novel Cu complexes with a large variety of different structural motifs, from which the aldol-addition ligand can be liberated by treatment with NH3. Neutral heteroleptic complexes in which the ligand acts as bidentate, or homoleptic cationic complexes in which the ligand acts as tridentate can be obtained depending on the copper salt used. The key step in these reactions is the coordination of pyca to copper, which increases the electrophilic character of the aldehyde, with Cu(ClO4)2 leading to a higher degree of activation than CuCl2, as predicted by DFT calculations. A regio- and stereoselective double aldol addition of pyca in the reaction of Cu(ClO4)2·6H2O with acetone leads to the formation of a dimer copper complex in which the novel double aldol addition product acts as a pentadentate ligand. A possible mechanism is discussed. The work is supported by extensive crystallographic studies.