Synthesis of Photochromic Phosphines by Pd-Catalyzed Annulation Reaction of Alkynes Bearing Phosphinyl Substituent with a Silacyclopropene.

The synthesis of phosphines with light controlled basicity is presented in this study. A methodological approach for the preparation of these unconventional photochromic phosphines based on a dithienylethene organic moiety is reported. It relies on the palladium-catalyzed annulation of alkynyl phosphines in the presence of a 2,3-Dithienylsilacyclopropene. Accordingly, a diphenyphosphino moiety is connected to the organic photochrome thanks to different linkers. Their influence on the photochromism and on the phosphinyl group basicity is studied and evaluated based on experimental an NMR descriptor as well as DFT calculations.

1,2-Palladasilacyclobutene: The Missing Link in the Pd-Catalyzed Annulation of Alkynes for the Silirene-to-Silole Transformation.

The palladium-catalyzed annulation reaction of alkynes enables an attractive approach to siloles. Their access from silirenes and terminal alkynes proved rather general, involving reactive intermediates that have remained elusive to date. Starting from 1,2-bis(3-thienyl)silirene as a source of photochromic siloles, the mechanism of the annulation reaction has been revisited, and palladasilacyclobutenes resulting from the activation of the silirene could be isolated and thoroughly characterized (NMR, X-ray, and DFT). Their role as reactive intermediates and their fate in the course of the reaction were also studied in situ. In combination with in-depth DFT calculations, a clearer picture of the mechanism and the reactive key species is disclosed.

Dithienylethene-Based Photochromic Siloles: A Straightforward and Divergent Synthetic Strategy.

A straightforward synthetic methodology for the preparation of photochromic siloles based on the dithienylethene motif is developed. It relies upon an efficient palladium-catalyzed annulation reaction of a 2,3-bis(3-thienyl)-silirene with terminal alkynes in mild conditions. The reaction is functional group-tolerant and can be performed in high yields with a variety of functional terminal alkynes. It can even be extended to a polymeric polypropargylmethacrylamide (PPMA) substrate affording the corresponding photochromic polymer with different degree of photochromic unit incorporation by simply adjusting the polymer/ silirene stoichiometric ratio.

Solventless and Metal-Free Synthesis of High-Molecular-Mass Polyaminoboranes from Diisopropylaminoborane and Primary Amines.

The solventless reaction of diisopropylaminoborane with n-butylamine, at room temperature, leads to a mixture of B(sp2) H-, B(sp3) H2 -, and B(sp3) H3 -containing species. At low temperature, the reaction outcome is completely modified, thus leading selectively to the formation of high-mass polybutylaminoborane. When extended to a variety of primary amines, under solventless conditions and at low temperature, this reaction provides a new, efficient, and direct metal-free access to high-molecular-mass polyaminoboranes in good to high yields under mild reaction conditions.

A Highly Effective Ruthenium System for the Catalyzed Dehydrogenative Cyclization of Amine-Boranes to Cyclic Boranes under Mild Conditions.

We recently disclosed a new ruthenium-catalyzed dehydrogenative cyclization process (CDC) of diamine-monoboranes leading to cyclic diaminoboranes. In the present study, the CDC reaction has been successfully extended to a larger number of diamine-monoboranes (4-7) and to one amine-borane alcohol precursor (8). The corresponding NB(H)N- and NB(H)O-containing cyclic diaminoboranes (12-15) and oxazaborolidine (16) were obtained in good to high yields. Multiple substitution patterns on the starting amine-borane substrates were evaluated and the reaction was also performed with chiral substrates. Efforts have been spent to understand the mechanism of the ruthenium CDC process. In addition to a computational approach, a strategy enabling the kinetic discrimination on successive events of the catalytic process leading to the formation of the NB(H)N linkage was performed on the six-carbon chain diamine-monoborane 21 and completed with a (15) N NMR study. The long-life bis-σ-borane ruthenium intermediate 23 possessing a reactive NHMe ending was characterized in situ and proved to catalyze the dehydrogenative cyclization of 1, ascertaining that bis σ-borane ruthenium complexes are key intermediates in the CDC process.

Chemical behaviour of a prototype boryl(phosphino)carbene.

We recently disclosed the synthesis of a novel "push-pull" boryl(phosphino)carbene. To determine the influence of this substitution pattern on the chemical behaviour, a study into the reactivity of the prototype (1) of this new family of B(sp(2))-substituted phosphinocarbenes was undertaken. Carbene 1 exhibits one of the most common intramolecular rearrangements of singlet carbenes, involving a 1,2-mesityl shift, and typical [2+1] cycloaddition reactions with electron-poor acrylonitrile. A pronounced α,ß-ambiphilic character was also shown by the reaction of 1 with benzaldehyde, leading to phosphorylalkene 4. Due to its specific electronic properties, carbene 1 also exhibits unprecedented reactivity with chloroacrylonitrile, enabling the formation of bicyclo[1.1.0]phosphetanium salt 6 and borylcyclopropene 9, which have been fully characterised by NMR spectroscopy and X-ray crystallography.

B-H, C-H, and B-C bond activation: the role of two adjacent agostic interactions.

Tuning the nature of the linker in a L~BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ-C-H and ε-B(sp2)-H, agostic interactions. Such a unique coordination mode stabilizes a 14-electron "RuH2P2" fragment through connected σ-bonds of different polarity, and affords selective B-H, C-H, and B-C bond activation as illustrated by reactivity studies with H2 and boranes.

Synthesis of 2- and 2,7-functionalized pyrene derivatives: an application of selective C-H borylation.

An efficient synthetic route to 2- and 2,7-substituted pyrenes is described. The regiospecific direct C-H borylation of pyrene with an iridium-based catalyst, prepared in situ by the reaction of [{Ir(µ-OMe)cod}(2)] (cod = 1,5-cyclooctadiene) with 4,4'-di-tert-butyl-2,2'-bipyridine, gives 2,7-bis(Bpin)pyrene (1) and 2-(Bpin)pyrene (2, pin = OCMe(2)CMe(2)O). From 1, by simple derivatization strategies, we synthesized 2,7-bis(R)-pyrenes with R = BF(3)K (3), Br (4), OH (5), B(OH)(2) (6), and OTf (7). Using these nominally nucleophilic and electrophilic derivatives as coupling partners in Suzuki-Miyaura, Sonogashira, and Buchwald-Hartwig cross-coupling reactions, we obtained 2,7-bis(R)-pyrenes with R = (4-CO(2)C(8)H(17))C(6)H(4) (8), Ph (9), C≡CPh (10), C≡C[{4-B(Mes)(2)}C(6)H(4)] (11), C≡CTMS (12), C≡C[(4-NMe(2))C(6)H(4)] (14), C≡CH (15), N(Ph)[(4-OMe)C(6)H(4)] (16), and R = OTf, R' = C≡CTMS (13). Lithiation of 4, followed by reaction with CO(2), yielded pyrene-2,7-dicarboxylic acid (17), whilst borylation of 2-tBu-pyrene gave 2-tBu-7-Bpin-pyrene (18) selectively. By similar routes (including Negishi cross-coupling reactions), monosubstituted 2-R-pyrenes with R = BF(3)K (19), Br (20), OH (21), B(OH)(2) (22), [4-B(Mes)(2)]C(6)H(4) (23), B(Mes)(2) (24), OTf (25), C≡CPh (26), C≡CTMS (27), (4-CO(2)Me)C(6)H(4) (28), C≡CH (29), C(3)H(6)CO(2)Me (30), OC(3)H(6)CO(2)Me (31), C(3)H(6)CO(2)H (32), OC(3)H(6)CO(2)H (33), and O(CH(2))(12)Br (34) were obtained from 2. These derivatives are of synthetic and photophysical interest because they contain donor, acceptor, and conjugated substituents. The crystal structures of compounds 4, 5, 7, 12, 18, 19, 21, 23, 26, and 28-31 have also been obtained from single-crystal X-ray diffraction data, revealing a diversity of packing modes, which are described in the Supporting Information. A detailed discussion of the structures of 1 and 2, their polymorphs, solvates, and co-crystals is reported separately.

Hydroboration of vinyl halides with mesitylborane: a direct access to (mesityl)(alkyl)haloboranes.

The direct access to (mesityl)(alkyl)haloboranes (Mes(Alk)BX) (X = Br, Cl) from mesitylborane dimer and vinyl halides is presented. The involved hydroboration reaction results in the transfer of the halogen atom from the carbon of the starting material to the boron in the final product. The reactivity of the obtained Mes(Alk)BX has been evaluated for the synthesis of the bipyridyl boronium cations and 2-arylpyridine derived boron N^C-chelates. The formation mechanism of Mes(Alk)BX is apprended by DFT-calculations which shows that their formation involves two concomitant pathways derived from the regioslectivity of the hydroboration reaction.

Ruthenium agostic (phosphinoaryl)borane complexes: multinuclear solid-state and solution NMR, X-ray, and DFT studies.

The reactivity of the (o-phosphinophenyl)(amino)borane compound HB(N(i)Pr(2))C(6)H(4)(o-PPh(2)) prepared from Li(C(6)H(4))PPh(2) and HBCl(N(i)Pr(2)) toward the bis(dihydrogen) complex RuH(2)(H(2))(2)(PCy(3))(2) (1) was studied by a combination of DFT, X-ray, and multinuclear NMR techniques including solid-state NMR, a technique rarely employed in organometallic chemistry. The study showed that the complex RuH(2){HB(N(i)Pr(2))C(6)H(4)(o-PPh(2))}(PCy(3))(2) (3), isolated in excellent yield as yellow crystals and characterized by X-ray diffraction, led in solution to PCy(3) dissociation and formation of an unsaturated 16-electron complex RuH(2){HB(N(i)Pr(2))C(6)H(4)(o-PPh(2))}(PCy(3)) (4), with a hydride trans to a vacant site. In both cases, the (phosphinoaryl)(amino)borane acts as a bifunctional ligand through the phosphine moiety and a Ru-H-B interaction, thus featuring an agostic interaction.

[Ir(PCy3)2(H)2(H2B-NMe2)]+ as a latent source of aminoborane: probing the role of metal in the dehydrocoupling of H3B⋠NMe2H and retrodimerisation of [H2BNMe2]2.

The Ir(III) fragment {Ir(PCy(3))(2)(H)(2)}(+) has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H(3)B⋅NMe(2)H (A) to ultimately give dimeric aminoborane [H(2)BNMe(2)](2) (D). Addition of A to [Ir(PCy(3))(2)(H)(2)(H(2))(2)][BAr(F)(4)] (1; Ar(F) = (C(6)H(3)(CF(3))(2)), gives the amine-borane complex [Ir(PCy(3))(2)(H)(2)(H(3)B⋅NMe(2)H)][BAr(F)(4)] (2 a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy(3))(2)(H)(2)(H(2)B-NMe(2))][BAr(F)(4)] (3). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H(2) loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3. DFT calculations indicate that this involves metal trapping of the monomer-dimer equilibrium, 2 H(2)BNMe(2) ⇌ [H(2)BNMe(2)](2). Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy(3))(2)(H)(2)(NCMe)(2)][BAr(F)(4)] (6) liberating H(2)B-NMe(2) (B), which then dimerises to give D. This is shown to be a second-order process. It also allows on- and off-metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine-borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H(3)B⋅NMe(2)BH(2)⋅NMe(2)H (C), which ultimately was converted to D. These results indicate that the metal is involved in both the dehydrogenation of A, to give B, and the oligomerisation reaction to afford C. A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D, with 3 observed as the final organometallic product, suggesting a B-N bond cleavage mechanism. Complex 6 does not react with C, but in combination with B oligomer C is consumed to eventually give D, suggesting an additional role for free aminoborane in the formation of D from C.

Dimethylaminoborane (H2BNMe2) coordination to late transition metal centers: snapshots of the B-H oxidative addition process.

The reaction of cyclodiborazane [Me(2)N-BH(2)](2) with the chloro(dihydrogen) ruthenium complex RuHCl(η(2)-H(2))(P(i)Pr(3))(2) (1) led to the formation of the unsymmetricaly coordinated dimethylaminoborane complex RuHCl(H(2)BNMe(2))(P(i)Pr(3))(2) (2). The dimethylaminoborane coordination (H(2)BNMe(2)) to the ruthenium center in 2 was carefully studied by combining X-ray, multinuclear NMR, and density functional theory (DFT) techniques, and compared with the recently reported osmium analogue which was originally formulated as a σ-B-H borinium complex [OsH(2)Cl(HBNMe(2))(P(i)Pr(3))(2)] (4). All our data are in favor of a bis(σ-B-H) coordination mode at a very activated stage in the case of the ruthenium complex 2, whereas in the osmium complex 4, full oxidative addition is favored leading to a complex better formulated as an osmium(IV) boryl species with an α-agostic B-H interaction. The synthesis and characterization of the symmetrical dihydride complex RuH(2)(H(2)BNMe(2))(P(i)Pr(3))(2) (3) from addition of the lithium dimethylaminoborohydride to 1 is reported for comparison.

Borylated methylenephosphonium salts: precursors of elusive boryl(phosphino)carbenes.

The synthesis of a new family of boryl-substituted methylenephosphonium derivatives, the phosphorus analogues of iminium salts, has been developed. They were used in the preparation of the first stable boryl(phosphino)carbene, which has been fully characterized by NMR spectroscopy and X-ray crystallography. Density functional theory calculations indicate that these carbenes can be classified as push-pull carbenes with a relatively small singlet-triplet energy gap.

Bis sigma-bond dihydrogen and borane ruthenium complexes: bonding nature, catalytic applications, and reversible hydrogen release.

Hydrogen, the simplest element in the periodic table, plays a tremendous role in organic and inorganic chemistry. For years, it was inconceivable that dihydrogen could be bound to a metal center without breaking the H-H bond. Thus, oxidative addition of H(2) was universally recognized as a key elementary step in hydrogenation processes. In 1984, Kubas and co-workers reported the first example of a complex in which dihydrogen was coordinated to a metal center without breaking of the H-H bond. This opened a new area in coordination chemistry: sigma-complexes were born, complementing the well-known Werner-type family of complexes. Since then, hundreds of stable dihydrogen complexes have been isolated, and their properties have been investigated in detail. By comparison, very little information is available for the analogous class of sigma-borane complexes, in which sigma-H-B bonds are complexed to a metal (in the manner of H-H bonds in sigma-dihydrogen complexes). Since the first example published in 1996 by Hartwig and co-workers, very few sigma-borane complexes have been isolated. Scientists have maintained a continuous interest in catalytic hydrogenation reactions. Almost a century ago, in 1912, Paul Sabatier, the father of the hydrogenation process, received the Nobel prize, and the selection of Noyori and Knowles in 2001 for their studies on enantioselective catalyzed hydrogenations amply demonstrates the ongoing importance of the field. Moreover, during the past decade, dihydrogen has attracted considerable attention as a possible "fuel of the future". This endeavor has furthered interest in sigma-borane complexes, as more and more evidence links their chemistry to that of amine-borane derivatives. Indeed, ammonia-borane (NH(3)BH(3)) is attracting significant interest for hydrogen storage applications. One of the main limitations is the lack of reversibility associated with the production of dehydrogenated (BNH)(x) materials. Of major importance will be a better understanding of the coordination of H(2) to a metal center, and more generally of the coordination of H-E bonds (E = B, C), which are likely to play a critical role in the reversible dehydrogenation process. In this Account, we review our recent results in the field of dihydrogen and borane activation, with a specific focus on the problem of reversible dehydrogenation pathways. We concentrate on the chemistry of ruthenium complexes incorporating two sigma-ligands: either two dihydrogen or two sigma-B-H bonds. We describe our synthetic strategies to prepare such unusual structures. Their characterization is discussed in detail, highlighting the importance of an experimental and theoretical approach (NMR, structural, and theoretical studies). Some catalytic applications are discussed and put into context, and their reactivity toward reversible hydrogen release is detailed.

Coordination and dehydrogenation of amine-boranes at metal centers.

There have been a number of approaches developed for the catalyzed dehydrogenation of amine-boranes as potential dihydrogen sources for hydrogen storage applications in recent years. Key advances in this area have been recently made thanks to catalytic and stoichiometric studies. In this Minireview, the fate of amine-boranes upon coordination to a metal center is discussed with a particular emphasis on B-H activation pathways. We focus on the few cases in which coordination of the resulting dehydrogenated product could be achieved, which includes the coordination of aminoborane, the simplest unit resulting from dihydrogen release of ammonia-borane.

Phosphinoborane and sulfidoborohydride as chelating ligands in polyhydride ruthenium complexes: agostic sigma-borane versus dihydroborate coordination.

A question of coordination mode: Two new borane compounds are prepared. They act as bifunctional ligands as illustrated by their reaction with ruthenium polyhydrides which leads to the formation of two complexes (see scheme) displaying either a delta-agostic interaction of a eta(2)-B-H bond involving a trivalent boron atom or a dihydroborate ligation.

A terminal borylene ruthenium complex: from B-H activation to reversible hydrogen release.

Starting from RuHCl(H2)(PCy3)2, a terminal ruthenium mesitylborylene complex was obtained via double B-H bond activation of mesitylborane and concomitant release of dihydrogen, such a process being remarkably reversible.

Functionalization of silicon surfaces with Si-C linked beta-cyclodextrin monolayers.

Vinyl-terminated heptapodyl beta-cyclodextrins react with hydrogenated silicon surfaces to generate covalently-bound molecular recognition devices.