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.

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.

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.

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.

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.

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.

A heterotrimetallic Ir(III), Au(III) and Pt(II) complex incorporating cyclometallating bi- and tridentate ligands: simultaneous emission from different luminescent metal centres leads to broad-band light emission.

Di- and tri-nuclear metal complexes incorporating gold(III), iridium(III) and platinum(II) units linked via a 1,3,5-triethynylbenzene core are reported, together with the corresponding mononuclear complexes as models. The gold(III) and platinum(II) units comprise tridentate, cyclometallating, C^N^C and N^N^C-coordinating ligands, respectively, with the Ar-C≡C- directly bound to the metal at the fourth coordination site. The iridium moiety is an Ir(ppy)2(acac) unit bound to the triethynylbenzene through a phenyl substituent at the 3-position of the acac ligand. The multinuclear compounds are prepared, using a modular synthetic strategy, from the monometallic complexes. All of the compounds are luminescent in solution at room temperature, and their photophysical properties were studied. The triplet excited state energies of the mononuclear complexes lie in the order Au > Ir > Pt. Consistent with this order, energy transfer from Au to Ir and from Au to Pt is observed, leading to quenching of the Au emission in the gold-containing multinuclear complexes. Energy transfer from Ir to Pt occurs at a rate that only partially quenches the Ir-based emission. As a result, the dinuclear Ir-Pt and trinuclear Au-Ir-Pt complexes display broad emission across most of the visible region of the spectrum.

Ligand-controlled divergent formation of alkenyl- or allylboronates catalyzed by Pd, and synthetic applications.

The use of different ligands allows the preparation of either allyl- or alkenylboronates by Pd-catalyzed borylation of allylic carbonates containing alkyne groups. Unprecedented borylative cyclisation to alkenylboronates takes place with PCy3. The difficult dissociation of NHC ligands allows borylation of carbonates in the presence of alkynes. Oxidation, regioselective Suzuki coupling, as well as Au-catalyzed cycloisomerisation of boronates illustrate the potential synthetic applications of these reactions.

The role of water-based hydrogen atom wires in long-range electron-transfer reactions in aqueous media for the FeII-FeIII self-exchange and related systems.

Extended reach: A calculated mechanism for long-range proton-coupled electron transfer (PCET, see picture) through an array of structured water molecules between Fe(III)-Fe(III) complexes accounts for the reaction enthalpy and kinetic isotope effect previously measured for this reaction. This mechanism may be general and occur for other hydroxo-metal complexes.

Preparation of allylboronates by Pd-catalysed borylative cyclisation of dienynes.

Reaction of a variety of dienynes with bis(pinacolato)diboron catalysed by Pd bis(trifluoroacetate) affords allylic boronates containing five or six membered carbo- or heterocycles, by concomitant formation of one C-C and one C-B bonds. Resulting allylboronates can be employed for a variety of subsequent transformations.

Ni-catalyzed cascade cyclization-Kumada alkyl-alkyl cross-coupling.

Suggesting novel disconnections: A powerful Ni-catalyzed cascade reaction involving cyclization followed by cross-coupling allows the formation of up to three alkyl-alkyl bonds in a single operation by using alkene-containing alkyl iodides and Grignard reagents (see scheme; acac = acetylacetonate; TMEDA = N,N',N'-tetramethyl ethylenediamine). Mechanistic experimental and computational studies suggest a Ni(I)-Ni(II)-Ni(III) catalytic cycle and the intermediacy of radicals.

Pd-catalyzed borylative cyclisation of 1,7-enynes.

Reaction of a variety of 1,7-enynes with bis(pinacolato) diboron catalysed by Pd bis(trifluoroacetate) affords homoallylic and allylic boronates containing a six membered carbo- or heterocycle, by formation of C-C and C-B bonds.

Divergent luminescence behaviour from differential interactions in dinuclear Pt, Au, and mixed Pt-Au complexes built on a xanthene scaffold.

A diethynylxanthene unit is used to tether cyclometallated Pt(ii) and Au(iii) complexes in a geometry that favours interfacial interactions. This leads to protection from non-radiative decay pathways in the Au2 dimer and to low-energy 'aggregate-like' emission in a closely related Pt2 dimer, whilst rapid Au-to-Pt energy transfer occurs in the heterodimer.

Influence of the number of anchoring groups on the electronic and mechanical properties of benzene-, anthracene- and pentacene-based molecular devices.

One of the central issues of molecular electronics (ME) is the study of the molecule-metal electrode contacts, and their implications for the conductivity, charge-transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single-molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one-atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal-molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.

Pd-catalyzed borylative polycyclization of enediynes to alkylboronates.

Pd-catalyzed bicyclization of 1-ene-6,11-diynes with bis(pinacolato)diboron smoothly affords synthetically useful homoallylic alkylboronates under mild conditions, avoiding the use of highly nucleophilic or basic reagents. One C-B bond and two new C-C bonds are created, and two new stereogenic centers are stereospecifically formed in a single tandem operation. The obtained products suggest a cascade reaction in which the last step is the transmetalation of bis(pinacolato)diboron, this being faster than a possible ß-hydride elimination process. The reaction is general and has been applied to enediynes with different substituents on the alkene unit, the bridging groups, and the alkyne moiety. The bicyclized alkylboronates thus obtained can be functionalized by oxidation to alcohols, formation of trifluoroborate salts, or by Suzuki coupling reactions.

On/off electrochemical switches based on quinone-bisketals.

The synthesis and anodic oxidation of a variety of 2,5-diaryl or dialkynylaryl substituted 1,4-dialkoxybenzenes to quinone bisketals is described. The study of the X-ray structures and electrochemical and spectroscopic properties evidenced that these pairs constitute a first approach to the concept of a molecular nanofuse.