A Scalable and Expedient Route to 1‑Aza[6]helicene Derivatives and Its Subsequent Application to a Chiral-Relay Asymmetric Strategy

A rapid route to diversely functionalized 1-aza[6]helicenes has been achieved via the development of a copper-mediated cross-coupling reaction, followed by PtCl₄-catalyzed cycloisomerization. Not only does this method allow access to these functionally important molecules on gram scale, but this strategy is also suitable for relaying the axial chirality of a key intermediate to the helicity of the product

Dramatic solid-state humidity-induced modification of the magnetic coupling in a dimeric fluorous copper(II)−carboxylate complex

International audienceThe very fast and efficient water vapor absorption of the dimeric fluorous copper(II)-carboxylate complex [Cu(2)(C(8)F(17)CO(2))(4)(acetone)(2)] (1) leads, in the solid state, to a dramatic decrease of the exchange magnetic coupling between the copper(II) ions and to a drastic change of its powder EPR spectrum

A fluorous copper(II)-carboxylate complex which magnetically and reversibly responds to humidity in the solid state

Reacting the fluorous carboxylate C8F17CO2*HNEt3 with Cu(OTf)2 in acetone leads, depending on the carboxylate/copper ratio, either to the neutral dimeric complex [Cu2(C8F17CO2)4(acetone)2] (2), or to the dianionic monomeric complex [Cu(C8F17CO2)4](HNEt3)2 (4). Both complexes were characterized by IR, UV-vis and EPR spectroscopies, magnetic susceptibility measurements and X-ray diffraction analysis. The complex 2 displays the classical dimeric "paddlewheel" structure with four carboxylates bridging the two copper(II) ions while the complex 4 displays an unusual monomeric structure with four monodentate carboxylates bounded to the copper(II) in a square planar geometry. In contrast to 4, the complex 2 shows a very high affinity for water vapors in the solid state and behaves as a magnetic sponge. When exposed to air, ground crystals of 2 rapidly trap up to 6 water molecules per copper ion leading to a decrease of the magnetic coupling from 2J = −480 cm−1 to 2J = −7 cm−1. The hydration process is accompanied by drastic changes of the EPR and FTIR spectra, revealing that the binding of the water molecules to the copper atoms leads to the release of the acetone ligands from the solid and to the shifting of two carboxylates from bridging bidentate to non-bridging monodentate coordination mode. When hydrated 2 is exposed to high vacuum, the water molecules are removed from the solid. The dehydration process leads to the partial recovery of the antiferromagnetic coupling between the copper(II) ions as revealed by EPR and magnetic susceptibility measurements. These data show that the hydration/dehydration process is not accompanied by a fully reversible structural rearrangement in the solid state. The unusual affinity for water and consequently, the modification of the magnetic properties, is observed neither with the fluorous dimeric complex 1, nor with the dianionic monomeric complex 4

Transformation of a norbornadiene unit to ethylenylcyclopentene requiring cooperation between boron and rhodium centers

The synthesis of the lithium salt of a bis-substituted borohydride anion containing a phenyl substituent and a 2-mercaptopyridyl unit (mp) is reported herein. This salt has been used as a pro-ligand for the synthesis of rhodium(I) complex, [Rh{κ3-H,H,S-H2B(Ph)(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene). The new boron-based ligand coordinates to the rhodium center via the thione donor and the two B–H bonds of the BPhH2 unit, with a dihydroborate motif. Reaction of complex 1 with two equivalents of triphenylphosphine leads to an unprecedented rearrangement and transfer of the former norbornadiene ligand to the boron center. The transformation occurs via an initial hydride migration from boron to rhodium center followed by a hydroboration of one of the double bonds. Finally, a ring-opening process occurs involving both boron and rhodium centers leading to an unusual boron-bound ethylenylcyclopentene unit. The product of this reaction was confirmed as [R⌈{η1-S,η2-B,C-B(Ph)(⌉CHCH2(C5H7)(mp)}(PPh3)2] (2). The net result of these transformations is the incorporation of the two hydrogen atoms from the secondary borohydride ligand [BPhH2(mp)]− into the former norbornadiene unit. The end point positions of these hydrogen atoms were confirmed by deuterium labeling experiments. Complex 2 was further reacted with carbon monoxide to generate [⌈Rh{η1-S,η2-B,C-B(Ph)(⌉CHCH2(C5H7)(mp)}(CO)(PPh3)] (3) via ligand substitution. The new ligand and the three complexes, 1, 2, and 3, have been characterized by spectroscopic techniques as well as by X-ray crystallography. Detailed characterization of 2 and 3 revealed an unusual η2-B,C coordination mode within these complexes. Further studies have demonstrated that complexes 2 and 3 react with hydrogen gas (or dimethylamine borane as a source of H2) to generate the hydrogen addition products involving the unprecedented activation of the Rh−η2-B,C motif. Complexes 2 and 3 were further found to be active catalysts for the hydrogenation of olefins and the dehydrogenation of dimethylamine borane

Mixed phosphite/N-heterocyclic carbene complexes : synthesis, characterization and catalytic studies

A series of mixed P(OR)(3)/NHC Pd complexes was synthesized and fully characterized. The steric properties of both types of ligands were computationally determined using X-ray data. These structural studies clearly show that N-heterocyclic carbenes modulate their bulkiness with respect to the steric requirements of the coligands. Catalytic studies were performed using this new class of complexes for the Suzuki-Miyaura reaction. It was found that alkoxide or hydroxide bases and/or alcohols were necessary to achieve good catalytic activity. Mechanistic studies were undertaken in order to gain insights into the role of alkoxide groups. These studies suggest that alcohols or alkoxide groups play a major role in the activation of the precatalyst to generate the catalytically active species. Catalytic studies proved these systems to be efficient using 0.1 mol % of Pd loading for the coupling of aryl, benzyl, and heterocyclic chlorides with boronic acids

Stopping hydrogen migration in its tracks: the first successful synthesis of group ten scorpionate complexes based on Azaindole scaffolds

The first successful synthesis and characterization of group ten complexes featuring flexible scorpionate ligands based on 7-azaindole heterocycles are reported herein. Addition of two equivalents of either K[HB(azaindolyl)3] or Li[HB(Me)(azaindolyl)2] to [M(μ-Cl)(η1,η2-COEOMe)]2 leads to the formation of two equivalents of the complexes [M{κ3-N,N,H-HB(azaindolyl)3}(η1,η2-COEOMe)] and [M{κ3-N,N,H-HB(Me)(azaindolyl)2}(η1,η2-COEOMe)] (where M = Pt, Pd; COEOMe = 8-methoxycyclooct-4-en-1-ide), respectively. In these reactions, the borohydride group is directed towards the metal center forming square based pyramidal complexes. In contrast to analogous complexes featuring other flexible scorpionate ligands, no hydrogen migration from boron is observed in the complexes studied. The fortuitous linewidths observed in some of the 11B NMR spectra allow for a closer inspection of the B–H•••metal unit in scorpionate complexes than has previously been possible

Preparation and reactivity of rhodium and iridium complexes containing a methylborohydride based unit supported by two 7-azaindolyl heterocycles

The synthesis and characterisation of a new anionic flexible scorpionate ligand, methyl(bis-7-azaindolyl)borohydride [MeBai]- is reported herein. The ligand was coordinated to a series of group nine transition metal centres forming the complexes, [Ir(MeBai)(COD)] (1), [Rh(MeBai)(COD)] (2), [Rh(MeBai)(CODMe)] (2-Me) and [Rh(MeBai)(NBD)] (3), where COD = 1,5-cyclooctadiene, CODMe = 3-methyl-1,5-cyclooctadiene and NBD = 2,5-norbornadiene. In all cases, the boron based ligand was found to bind to the metal centres via a κ3-N,N,H coordination mode. The ligand and complexes were fully characterised by spectroscopic and analytical methods. The structures of the ligand and three of the complexes were confirmed by X-ray crystallography. The potential for migration of the "hydride" or "methyl" units from boron to the metal centre was also explored. During these studies an unusual transformation, involving the oxidation of the rhodium centre, was observed in complex 2. In this case, the η4-COD unit transformed into a η1,η3-C8H12 unit where the ring was bound via one sigma bond and one allyl unit. This is the first time such a transformation has been observed at a rhodium centre

Emergent properties of an organic semiconductor driven by its molecular chirality

Chiral molecules exist as pairs of nonsuperimposable mirror images; a fundamental symmetry property vastly underexplored in organic electronic devices. Here, we show that organic field-effect transistors (OFETs) made from the helically chiral molecule 1-aza[6]helicene can display up to an 80-fold difference in hole mobility, together with differences in thin-film photophysics and morphology, solely depending on whether a single handedness or a 1:1 mixture of left- and right- handed molecules is employed under analogous fabrication conditions. As the molecular properties of either mirror image isomer are identical, these changes must be a result of the different bulk packing induced by chiral composition. Such underlying structures are investigated using crystal structure prediction, a computational methodology rarely applied to molecular materials, and linked to the difference in charge transport. These results illustrate that chirality may be used as a key tuning parameter in future device applications.</p