Synthesis of Known and Previously Inaccessible Poly(pyrazolyl)Borates under Mild Conditions.

Poly(pyrazolyl)borate ligands have been obtained through the reaction of highly reactive haloboranes with in situ formed pyrazolides under very mild conditions. This versatile synthetic method allows the selective synthesis of bis-, tris-, or tetrakis(pyrazolyl)borates. Furthermore, the method is compatible with the use of functional groups on the heterocyclic moieties of the poly(pyrazolyl)borates that were not accessible to date. Strongly encumbered sodium and thallium(I) poly(pyrazolyl)borates with a reduced donating ability have been obtained for the first time.

Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties.

The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe2S2] complex containing a 5,5'-diisocyanide-2,2'-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe2S2] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe2S2] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe2S2] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe2S2] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF4 salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental-computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

Straightforward Synthesis of α-Chloromethylketimines Catalyzed by Gold(I). A Clean Way to Building Blocks.

α-Chloromethylketimines have been obtained through a gold-catalyzed hydroamination of aromatic and aliphatic 1-chloroalkynes with aromatic amines by using equimolar amounts of both reagents. This procedure has allowed the preparation and spectroscopic characterization of α-chloromethylketimines for the first time with a high degree of purity, complete conversion, and atom economy. The synthetic usefulness of the methodology has been demonstrated with the preparation of ß-chloroamines and indoles.

Intramolecular rhodium-catalysed [2 + 2 + 2] cycloaddition of linear chiral N-bridged triynes: straightforward access to fused tetrahydroisoquinoline core.

A route for the preparation of merged symmetrical tetrahydroisoquinolines with central chirality through a rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition involving enantiopure triynes as substrates is described. The results show that linear triynes lacking a 3-atom tether can undergo efficient cyclisation. The N-tethered 1,7,13-triynes used in our approach were easily prepared from readily accessible chiral homopropargyl amides, the basic building blocks in our approach, which were efficiently obtained by diastereoselective addition of propargyl magnesium bromide to Ellman imines. Additional substitution at the benzene rings could be attained when substituted triynes at the terminal triple bonds were employed, giving access to more complex tetrahydroisoquinolines after the rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition. Among the different transition-metal catalysts, the Wilkinson complex (RhCl(PPh3)3) afforded higher yields in the cyclisation of linear triynes; however, triynes bearing a Br substituent at the terminal positions underwent the cyclisation more efficiently in the presence of [RhCl(CO)2]2.

Triazole-Containing [FeFe] Hydrogenase Mimics: Synthesis and Electrocatalytic Behavior.

Through a Cu-catalyzed Huisgen cycloaddition between terminal alkynes and azides (CuAAC) reaction, azide [(µ-SCH2)2N(4-N3C6H4)Fe2(CO)6] has demonstrated to be a robust and versatile reagent able to incorporate the [(µ-SR)2Fe2(CO)6] fragment on a wide range of substrates, ranging from aromatic compounds to nucleosides, metallocenes, or redox and luminescent markers. The [FeIFeI]/[Fe0FeI] and [Fe0FeI]/[Fe0Fe0] reduction potentials of the triazole derivatives prepared are comparable to those of other aminodithiolate (adt) Fe-Fe hydrogenase mimics. The presence of the triazole linker influences the electrochemical behavior of these complexes depending on the strength of the acid employed.

Synthesis of substituted piperidines by enantioselective desymmetrizing intramolecular aza-Michael reactions.

An organocatalytic enantioselective intramolecular aza-Michael reaction has been described for the first time in a desymmetrization process employing substrates different from cyclohexadienones. By using 9-amino-9-deoxy-epi-hydroquinine as the catalyst and trifluoroacetic acid as a co-catalyst, a series of enantiomerically enriched 2,5-and 2,6-disubstituted piperidines have been obtained in good yields and with moderate diastereoselectivity. Depending on the catalyst/co-catalyst loading ratio, either the major or the minor diastereoisomer of the final piperidine products was achieved with high levels of enantioselectivity. Finally, some mechanistic insights have been considered by means of theoretical calculations which were in agreement with the experimental results obtained in the desymmetrization reaction.

Sequential Reactions of Alkynes on an Iridium(III) Single Site.

Sequential insertion of terminal alkynes on IrIII cyclometalated complexes allow the formation of novel metallapolycycles in a controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, C(sp3 )-H activation, and reductive coupling) on a single IrIII center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure. The sequence is a multicomponent reaction in which the metal not only promotes the different steps but also determines their stereoselectivity. This is an elegant example of the synergy between a metal-promoting reaction and a symmetry-defined stereochemistry.

Effect of a κ1-Bonded-M-1,2,3-triazole (M = Co, Ru) on the Structure and Reactivity of Group 6 Alkoxy (Fischer) Carbenes.

The [3 + 2] cycloaddition of two different metal-bound azides, [(Me4cyclam)CoII(N3)]ClO4 and (η5-C5H5)(dppe)RuII(N3), (dppe = Ph2PCH2CH2PPh2) with Cr(0) and W(0) (ethoxy)(alkynyl) Fischer carbenes has been efficiently used for the preparation of polymetallic metal-carbene complexes. The presence of the κ1-bonded metal triazole causes a significant influence on the electronic properties, structure, and reactivity of this new class of Fischer alkoxycarbenes. For the Ru(II) derivatives, their chemical behavior is considerably influenced by the interaction of the (η5-C5H5)(dppe)RuII-triazole moiety with the empty p-carbene orbital that provokes a noticeable decrease in the electrophilicity of the M═C carbon (manifested by the shielding of the 13C NMR chemical shifts). In turn, in the Co(II) derivatives, the incorporation of the (Me4cyclam)CoII moiety diminishes the aromaticity of the triazole ring and has a marked effect on the energy and distribution of the LUSO orbital, mostly resident on the Co(II) fragment. The almost negligible participation of the carbene moiety in the LUSO makes this position unable to react with nucleophiles. The reactions reported in this work constitute the first examples of [3 + 2] cycloaddition of azides and alkynyl Fischer carbene complexes in solution.

Host-Guest Chemistry: Oxoanion Recognition Based on Combined Charge-Assisted C-H or Halogen-Bonding Interactions and Anion⋠⋠⋠Anion Interactions Mediated by Hydrogen Bonds.

Several bis-triazolium-based receptors have been synthesized and their anion-recognition capabilities have been studied. The central chiral 1,1'-bi-2-naphthol (BINOL) core features either two aryl or ferrocenyl end-capped side arms with central halogen- or hydrogen-bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge-assisted aliphatic and heteroaromatic C-H noncovalent interactions and combinations of C-H hydrogen and halogen bonding. The receptors are able to selectively interact with HP2 O7 (3-) , H2 PO4 (-) , and SO4 (2-) anions, and the value of the association constant follows the sequence: HP2 O7 (3-) >SO4 (2-) >H2 PO4 (-) . The ferrocenyl end-capped 7(2+) ⋅2 BF4 (-) receptor allows recognition and differentiation of H2 PO4 (-) and HP2 O7 (3-) anions by using different channels: H2 PO4 (-) is selectively detected through absorption and emission methods and HP2 O7 (3-) by using electrochemical techniques. Significant structural results are the observation of an anion⋅⋅⋅anion interaction in the solid state (2:2 complex, 6(2+) ⋅[H2 P2 O7 ](2-) ), and a short C-I⋅⋅⋅O contact is observed in the structure of the complex [8(2+) ][SO4 ]0.5 [BF4 ].

An Entry to Mixed NHC-Fischer Carbene Complexes and Zwitterionic Group 6 Metal Alkenyls.

The addition of NHCs to α,ß-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel group 6 (Fischer) carbene complexes.

2,4,5-Trimethylimidazolium Scaffold for Anion Recognition Receptors Acting Through Charge-Assisted Aliphatic and Aromatic C-H Interactions.

A series of two-armed 2,4,5-trimethylimidazolium-based oxoanion receptors, which incorporate two end-capped photoactive anthracene rings, being the central core an aromatic or heteroaromatic ring, has been designed. In the presence of HP2O7(3-), H2PO4(-), and SO4(2-) anions, (1)H- and (31)P NMR spectroscopical data clearly indicate the simultaneous occurrence of several charge-assisted aliphatic and aromatic C-H noncovalent interactions, i.e., significant downfield shifts were observed for the imidazolium C(2)-CH3 protons, the methylene N-CH2 protons, and the inner aromatic proton or the outer heteroaromatic protons. Density functional theory calculations confirm the occurrence of these noncovalent interaction and suggest that the interaction between the anions and the receptors is mainly electrostatic in nature.

Single Heteroatom Fine-Tuning of the Emissive Properties in Organoboron Complexes with 7-(Azaheteroaryl)indole Systems.

The application of organoboron compounds as light-absorbing or light-emitting species in areas as relevant as organic electronics or biomedicine has motivated the search for new materials which contribute to the progress of those applications. This article reports the synthesis of four-coordinate boron complexes based on the unexplored 7-(azaheteroaryl)indole ligands. An easy synthetic approach has enabled the fine-tuning of the electronic structure of the organoboron species by modifying a heteroaromatic component in the conjugated system. Furthermore, a comprehensive characterization by X-ray diffraction, absorption and emission spectroscopy, both in solution and in the solid state, cyclic voltammetry, and computational methods has evidenced the utility of this simple strategy. Large Stokes shifts have been achieved in solid thin-films which show a range of emitted light from blue to orange. The synthesized compounds have been used as biocompatible fluorophores in cell bioimaging.

Remote Control by π-Conjugation of the Emissive Properties of Fischer Carbene-BODIPY Dyads.

The synthesis, structure, and complete characterization of mono- and bimetallic dyads joining Fischer carbene complexes and BODIPY chromophores are reported. In these organometallic species, the Fischer carbene complex is attached to the BODIPY moiety through a p-aminophenyl group linked at the C8 carbon atom of the BODIPY core. The photophysical properties, namely the corresponding UV/vis absorption and emission spectra of these new metal-carbene complexes, are analyzed and discussed. It is found that whereas the absorption of the considered dyads strongly resembles that of the parent 4-anilinyl-substituted BODIPY, the fluorescence emission is significantly reduced in these species, very likely as a result of a Förster-type energy transfer mechanism. At variance, the replacement of the pentacarbonyl-metal(0) fragment by a carbonyl group leads to high fluorescence emission intensity. In addition, the emissive properties of the BODIPY core in these organometallic dyads can be tuned by remote groups by means of π-conjugation, as supported by density functional theory calculations.

Alternating Ferro/Antiferromagnetic Copper(II) Chain Containing an Unprecedented Triple Formato/Hydroxido/Sulfato Bridge.

The first example of a triple formato/hydroxido/sulfato (FHS) bridge for any metal is reported in compound [Cu2(bpym)(OH)(HCO2)(SO4)(H2O)2]·3H2O (1). Its structure shows the presence of alternating triple FHS bridges and 2,2'-bipyrimidine (bpym) ones. Although in the initial synthesis the sulfate anions were introduced accidentally, here we report the rational synthesis and the magnetic properties of this compound. The magnetic properties show that 1 is an alternating ferro/antiferromagnetic (F/AF) chain compound with predominant antiferromagnetic interactions and were fit to an alternating F/AF S = (1)/2 chain with g = 2.103, JAF = -139 cm(-1), and JF = 116 cm(-1) (α = JF/|JAF| = 0.83). The JAF value found corresponds very well to those previously reported for Cu-bpym-Cu bridges (average value of ca. -150 cm(-1)). The JF value is also very close to the estimated one (ca. 100 cm(-1)) from magneto-structural correlations in triply Cu-Cu bridged compounds with both hydroxido and carboxylato bridges in equatorial positions.

Experimental and theoretical insights in the alkene-arene intramolecular π-stacking interaction.

Chiral acrylic esters derived from biomass were developed as models to have a better insight in the aryl-vinyl π-stacking interactions. Quantum chemical calculations, NMR studies and experimental evidences demonstrated the presence of equilibriums of at least four different conformations: π-stacked and face-to-edge, each of them in an s-cis/s-trans conformation. The results show that the stabilization produced by the π-π interaction makes the π-stacked conformation predominant in solution and this stabilization is slightly affected by the electron density of the aromatic counterpart.

Synthesis of oxaspiranic compounds through [3 + 2] annulation of cyclopropenones and donor­acceptor cyclopropanes.

The Sc(OTf)3-catalyzed [3 + 2]-annulation reaction between cyclopropenones and donor­acceptor cyclopropanes is described. The process leads directly to the formation of 4-oxaspiro[2.4]hept-1-ene derivatives in good to excellent reaction yields. Density functional theory calculations suggest that the [3 + 2]-annulation pathway is strongly preferred over the possible [3 + 3]-process.

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,ß-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

Competitive gold-activation modes in terminal alkynes: an experimental and mechanistic study.

The competition between π- and dual σ,π-gold-activation modes is revealed in the gold(I)-catalyzed heterocyclization of 1-(o-ethynylaryl)urea. A noticeable effect of various ligands in gold complexes on the choice of these activation modes is described. The cationic [Au(IPr)](+) (IPr=2,6-bis(diisopropylphenyl)imidazol-2-ylidene) complex cleanly promotes the π activation of terminal alkynes, whereas [Au(PtBu3 )](+) favors intermediate σ,π species. In this experimental and mechanistic study, which includes kinetic and cross-over experiments, several σ-gold, σ,π-gold, and other gold polynuclear reaction intermediates have been isolated and identified by NMR spectroscopy, X-ray diffraction, or MALDI spectrometry. The ligand control in the simultaneous or alternative π- and σ,π-activation modes is also supported by deuterium-labeling experiments.

On the use of metal purine derivatives (M=Ir, Rh) for the selective labeling of nucleosides and nucleotides.

The reactions of neutral or cationic IrIII and RhIII derivatives of phenyl purine nucleobases with unsymmetrical alkynes produce new metallacycles in a predictable manner, which allows for the incorporation of either photoactive (anthracene or pyrene) or electroactive (ferrocene) labels in the nucleotide or nucleoside moiety. The reported methodology (metalation of the purine derivative and subsequent marker insertion) could be used for the postfunctionalization and unambiguous labeling of oligonucleotides.

The selective synthesis of metallanucleosides and metallanucleotides: a new tool for the functionalization of nucleic acids.

Nucleobases team up: the efficient and selective preparation of purine-derived metallanucleosides, metallanucleotides, and metalladinucleotides having M-C bonds (M=Ir(III), Rh(III)) is reported for the first time. The results presented may be applied to the synthesis of functionalized nucleic acids, or DNA/RNA-modified segments.