Ni(2,2':6',2''-terpyridine)2: a high-spin octahedral formal Ni(0) complex.

We have synthesised and characterised the complex Ni(tpy)2 (tpy = 2,2':6',2''-terpyridine). This formally Ni(0) complex is paramagnetic both in the solid state and in solution (S = 2). The crystal structure shows an octahedral geometry, with molecules arranged in independent dimers involving π-stacking between pairs of complexes. Magnetic measurementes and DFT calculations suggest the existence of temperature-dependent intermolecular antiferromagnetic coupling in the solid state.

2-Oxopurine Riboside: A Dual Fluorescent Analog and Photosensitizer for RNA/DNA Research.

There is significant interest in developing suitable nucleoside analogs exhibiting high fluorescence and triplet yields to investigate the structure, dynamics, and binding properties of nucleic acids and promote selective photosensitized damage to DNA/RNA, respectively. In this study, steady-state, laser flash photolysis, time-resolved IR luminescence, and femtosecond broad-band transient absorption spectroscopies are combined with quantum chemical calculations to elucidate the excited-state dynamics of 2-oxopurine riboside in aqueous solution and to investigate its prospective use as a fluorescent or photosensitizer analog. The Franck-Condon population in the S1 (ππ*) state decays through a combination of solvent and conformational relaxation to its minimum in 1.9 ps. The population trapped in the 1ππ* minimum bifurcates to either fluoresce or intersystem cross to the triplet manifold within ca. 5 ns, while another fraction of the population decays nonradiatively to the ground state. It is demonstrated that 2-oxopurine riboside exhibits both high fluorescent (48%) and significant triplet (between 10% and 52%) yields, leading to a yield of singlet oxygen generation of 10%, making this nucleoside analog a dual fluorescent and photosensitizer analog for DNA and RNA research.

Pd-catalyzed C(sp3)-C(sp) bond formation in iodocyclobutenes.

A synthesis of skipped 1,4-enynes through functionalization of the cyclobutene core with alkynes has been achieved, suggesting an unusual pathway of oxidative addition in tertiary iodoalkanes.

On the mechanism of the formation of alkyl-Ni(I) catalysts.

The formation of catalytically active alkyl-Ni(i) complexes by comproportionation of diorgano-Ni(ii) precursors and Ni(0) species proceeds easily through triplet states by alkyl ligand exchange. The process involves inversion of the configuration at the carbon that is transferred.

Triflyl-assisted reductive Pd-catalyzed Tsuji-Trost type reaction.

New (triflyl)cyclobutenes have been prepared by palladium-catalyzed hydrodetriflylation reaction using water and deuterium oxide as convenient hydrogen and deuterium sources. In addition, an investigation of the possible mechanism for this Tsuji-Trost type reaction of bis(triflyl)cyclobutenes has been facilitated by labelling studies and density functional theory (DFT) calculations.

Ni-Catalyzed Cyclization of Enynes and Alkynylboronates: Atom-Economical Synthesis of Boryl-1,4-dienes.

We report a novel atom-economical Ni-catalyzed cyclization reaction of enynes with alkynylboronates. The reaction employs a non-expensive Ni salt, a phosphine-based ligand and easy-handling alkynylboronates as boron-carbon source. The reaction provides complex fused-bicyclic compounds containing borylated 1,4-cyclohexadienes in high yields in short reaction times, involving the formation of two C-C bonds in one step. A reasonable reaction mechanism is proposed based on mechanistic experimental results.

Ni-Catalyzed Reductive Dicarbofunctionalization of Nonactivated Alkenes: Scope and Mechanistic Insights.

Olefins devoid of directing or activating groups have been dicarbofunctionalized here with two electrophilic carbon sources under reductive conditions. Simultaneous formation of one C(sp3)-C(sp3) and one C(sp3)-C(sp2) bond across a variety of unbiased π-systems proceeds with exquisite selectivity by the combination of a Ni catalyst with TDAE as sacrificial reductant. Control experiments and computational studies revealed the feasibility of a radical-based mechanism involving, formally, two interconnected Ni(I)/Ni(III) processes and demonstrated the different ability of Ni(I) species (Ni(I)I vs PhNi(I)) to reduce the C(sp3)-I bond. The role of the reductant was also investigated in depth, suggesting that a one-electron reduction of Ni(II) species to Ni(I) is thermodynamically favored. Further, the preferential activation of alkyl vs aryl halides by ArNi(I) complexes as well as the high affinity of ArNi(II) for secondary over tertiary C-centered radicals explains the lack of undesired homo- and direct coupling products (Ar-Ar, Ar-Alk) in these transformations.

Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes: Preparation of Unsymmetrical Diboronates.

We report a Ni-catalyzed diborylative cyclization of enynes that affords carbo- and heterocycles containing both alkyl- and alkenylboronates. The reaction is fully atom-economical, shows a broad scope, and employs a powerful and inexpensive catalytic Ni-based system. The reaction mechanism seems to involve activation of the enyne by Ni(0) through oxidative cyclometalation of the enyne prior to diboron reagent activation. An unprecedented dinuclear bis(organometallic) Ni(I) intermediate complex was isolated.

O-H and (CO)N-H bond weakening by coordination to Fe(ii).

New N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-ethane-1,2-diamine derivatives bearing covalently linked OH and (CO)NH groups have been synthesized. The coordination of those pendant hydroxyl/amide groups to a Fe(ii) metal center is demonstrated both in solution, even in the presence of chloride as the counterion, and in solid state, by means of X-ray diffraction crystal structures. As a result of this coordination, the experimental bond dissociation free energies (BDFE) of O-H and (CO)N-H bonds are remarkably diminished down to 76.0 and 80.5 kcal mol-1 respectively, which is also in agreement with DFT-based theoretical calculations. These BDFE values are in the range of commonly used hydrogen-atom donor reagents. The strategy presented here allows an unequivocal evaluation of the influence of metal coordination in X-H bond weakening in organic solvents which could be easily extended to other metal centers.

Copper-catalysed cross-coupling of alkyl Grignard reagents and propargylic ammonium salts: stereospecific synthesis of allenes.

Herein we describe a robust and practical method to prepare enantiomerically enriched trisubstituted allenes using alkyl Grignard reagents and bench stable propargylic ammonium salts. Excellent yields as well as regio- and stereoselectivities are observed. Our conditions provide a solution to the allene racemization, which has been a long-standing problem when using Grignard reagents.

Towards Useful Boronates through Atom-Economical Catalyzed Cascade Reactions.

Metal-catalyzed borylative cyclization reactions of polyunsaturated compounds allow concomitant formation of C-C and C-B bonds to provide cyclic boronates, which constitute synthetically useful intermediates. Recent developments using first-row transition metals as catalysts provide fully atom-economical reactions that follow mechanisms involving σ-bond metathesis.

Ligand-Controlled Electron Structure of Catalytically Active Ni Complexes.

We have performed a systematic study of the electron structure of a series of Ni(I) and Ni(II) iodo and methyl complexes with a variety of di- and tridentate nitrogen ligands to study the influence of these ligands in the structure of catalytically active complexes in cross-coupling reactions. Ni(II) compounds show the expected square-planar configuration typical of complexes of d8 metals, regardless of the kind of coordinating nitrogen atom (sp2 or sp3) found in ligands derived from either trialkylamines or pyridines. In contrast, Ni(I) complexes show different structures. Thus, the absence of orbitals capable of delocalizing the unpaired electron (such as in TMEDA and PMDTA derivatives) leads to nonplanar iodo or methyl tetracoordinate complexes. In contrast, the presence of ligands derived from pyridine allows delocalization of the unpaired electron on the ligand. This delocalization is especially effective for terpyridine species.

Sulfoxide-Induced Homochiral Folding of ortho-Phenylene Ethynylenes (o-OPEs) by Silver(I) Templating: Structure and Chiroptical Properties.

A new family of homochiral silver complexes based on carbophilic interactions with ortho-phenylene ethynylene (o-OPE) scaffolds containing up to two silver atoms are described. These compounds represent a unique class of complexes with chirality at the metal. Chiral induction is based on the inclusion of chiral sulfoxides, which allow efficient transfer of chirality to the helically folded o-OPE, leading to circularly polarized luminescence (CPL)- and vibrational circular dichroism (VCD)-active compounds. In the presence of silver(I) cations, carbophilic interactions dominate, which promote helical structures with a defined helicity. This is one of the very scarce examples of the use of such interactions in the attractive field of abiotic foldamers. The switching event has been extensively studied by using different chiroptical techniques, including circular dichroism, CPL, and VCD, and represents one of the few CPL switches described in the literature.

Iron-Catalyzed Hydroborylative Cyclization of 1,6-Enynes.

We report first Fe-catalyzed hydroborylative cyclization reaction. The process provides one C-C and one C-B bond in a single operation and shows a wide scope, allowing the formation of carbo- and heterocycles containing a homoallylic boryl unit that can be further functionalized. The reaction takes place in smooth conditions, with inexpensive catalytic system and full atom economy since HBpin is the borylation agent, in contrast to our previously reported Pd-catalyzed reaction. Both aryl and alkyl substituted alkynes are reactive, revealing a wide reaction scope. Mechanistic studies suggest the intermediacy of FeII -hydride active catalyst capable to react with the alkyne group prior to alkene insertion, and computational studies suggest the occurrence of barrierless σ-bond metathesis involving HBpin and Fe-C bonds along the catalytic cycle.

NiI Catalyzes the Regioselective Cross-Coupling of Alkylzinc Halides and Propargyl Bromides to Allenes.

We describe the unprecedented formation of allenes by Ni-catalyzed cross-coupling of propargyl bromides with alkylzinc halides. The reaction regioselectivity is complementary to the previously reported formation of propargyl-coupled compounds. Experiments support the formation of NiI complexes as the active species and the participation of radical intermediates. Kinetic studies showed that the reaction is first order with respect to the electrophile, zero-order with respect to the nucleophile (fast transmetalation), and one-half order with respect to the metal catalyst. Mechanistic studies support a bimetallic NiI -based pathway that involves fast homolytic cleavage of the C-Br bond by an alkyl-NiI complex, followed by radical coordination to NiI that determines the observed regioselectivity.

Copper-Catalyzed Borylative Aromatization of p-Quinone Methides: Enantioselective Synthesis of Dibenzylic Boronates.

In this report, we establish that DM-Segphos copper(I) complexes are efficient catalysts for the enantioselective borylation of para-quinone methides. This method provides straightforward access to chiral monobenzylic and dibenzylic boronic esters, with enantiomeric ratios up to 96:4, using a commercially available chiral phosphine. Standard manipulations of the C-B bond afford a variety of chiral diaryl derivatives.

Stapled helical o-OPE foldamers as new circularly polarized luminescence emitters based on carbophilic interactions with Ag(i)-sensitivity.

ortho-Oligo(phenylene)ethynylenes (o-OPEs) stapled with enantiopure 2,3-dihydroxybutane diethers have highly intense circular dichroism (CD) spectra and excellent circular polarized luminescence (CPL) responses (glum values up to 1.1 × 10-2), which are consistent with homochiral helically folded structures. In the presence of Ag(i), a change in the CPL emission is observed, representing the first example of CPL active small organic molecular emitters, which can be modulated by carbophilic interactions in a reversible manner.

Two-dimensional carbon-based conductive materials with dynamically controlled asymmetric Dirac cones.

The design of two dimensional graphene-type materials with an anisotropic electron flow direction in the X- and Y-axes opens the door for the development of novel electronic materials with multiple functions in nanoelectronics. In the present work, we have studied the electronic transport properties of a new family of 2D graphene-graphyne hybrids presenting conformationally free phenylethylene subunits. This system ensures two different conductive pathways that are perpendicular to each other: an acene nanoribbon subunit, in the X-axis, with graphene-type conduction, and a free to rotate phenylethylene subunit, in the Y-axis, in which the magnitude of the conduction depends dynamically on the corresponding torsion angle. Our calculations have confirmed that this system presents two different conduction pathways, which are related to the presence of asymmetric Dirac-type cones. Moreover, the Dirac cones can be dynamically modified in the presence of an external gate electrode, which is unprecedented in the literature.

Toward Multiple Conductance Pathways with Heterocycle-Based Oligo(phenyleneethynylene) Derivatives.

In this paper, we have systematically studied how the replacement of a benzene ring by a heterocyclic compound in oligo(phenyleneethynylene) (OPE) derivatives affects the conductance of a molecular wire using the scanning tunneling microscope-based break junction technique. We describe for the first time how OPE derivatives with a central pyrimidine ring can efficiently link to the gold electrode by two pathways presenting two different conductance G values. We have demonstrated that this effect is associated with the presence of two efficient conductive pathways of different length: the conventional end-to-end configuration, and another with one of the electrodes linked directly to the central ring. This represents one of the few examples in which two defined conductive states can be set up in a single molecule without the aid of an external stimulus. Moreover, we have observed that the conductance through the full length of the heterocycle-based OPEs is basically unaffected by the presence of the heterocycle. All these results and the simplicity of the proposed molecules push forward the development of compounds with multiple conductance pathways, which would be a breakthrough in the field of molecular electronics.