Pierre Braunstein

Hemilability of Hybrid Ligands and the Coordination Chemistry of Oxazoline-Based Systems.

Ligand design is becoming an increasingly important part of the synthetic activity in chemistry. This is of course because of the subtle control that ligands exert on the metal center to which they are coordinated. Ligands which contain significantly different chemical functionalities, such as hard and soft donors, are often called hybrid ligands and find increasing use in molecular chemistry. Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions remain very difficult, not only because of the considerable diversity encountered within the Periodic Table-different metal centers will behave differently towards the same ligand and different ligands can completely modify the chemistry of a given metal-but also because of the small energy differences involved. New systems may-even through serendipity-allow the emergence of useful concepts that can gain general acceptance and help design molecular structures orientated towards a given property. The concept of ligand hemilability, which finds numerous illustrations with hybrid ligands, has gained increased acceptance and been found to be very useful in explaining the properties of metal complexes and in designing new systems for molecular activation, homogeneous catalysis, functional materials, or small-molecule sensing. In the field of homogeneous enantioselective catalysis, in which steric and/or electronic control of a metal-mediated process must occur in such a way that one stereoisomer is preferentially formed, ligands containing one or more chiral oxazoline units have been found to be very valuable for a wide range of metal-catalyzed reactions. The incorporation of oxazoline moieties in multifunctional ligands of increasing complexity makes such ligands good candidates to display hemilabile properties, which until recently, had not been documented in oxazoline chemistry. Herein, we briefly recall the definition and scope of hemilabile ligands, present the main classes of ligands containing one or more oxazoline moieties, with an emphasis on hybrid ligands, and finally explain why the combination of these two facets of ligand design appears particularly promising.

Hemilabilität von Hybridliganden und die Koordinationschemie von Oxazolinliganden

Das Ligandendesign gewinnt auf dem Gebiet der chemischen Synthese immer mehr an Bedeutung, denn mit Hilfe von speziellen Liganden lassen sich die Eigenschaften von Komplexen in vielfaltiger Weise beeinflussen. Liganden mit verschiedenen Funktionalitaten, z. B. solche mit harten und weichen Donoren, werden oft als Hybridliganden bezeichnet und zunehmend in der Molekulchemie verwendet. Zwar ist schon lange bekannt, dass die elektronischen und sterischen Eigenschaften der Koordinationssphare eines Komplexes sehr eng mit dessen chemischen und physikalischen Eigenschaften zusammenhangen, doch ist eine Voraussage des Verhaltens von Liganden schwierig, nicht nur wegen der bei diesen Veranderungen auftretenden geringen Energiedifferenzen, sondern auch wegen der betrachtlichen Diversitat innerhalb des Periodensystems – ein bestimmter Ligand kann sich gegenuber verschiedenen Metallen ganz unterschiedlich verhalten, und der Austausch eines Liganden gegen einen anderen in einem gegebenen Metallkomplex kann dessen Chemie vollkommen verandern. Neue Systeme konnten, auch rein zufallig, zu nutzlichen, allgemein gultigen Konzepten fuhren, mit deren Hilfe Molekulstrukturen mit bestimmten Eigenschaften masgeschneidert werden konnen. Einige Eigenschaften von zahlreichen Metallkomplexen mit Hybridliganden lassen sich mit der „Hemilabilitat“ dieser Liganden erklaren – einem sehr nutzlichen Konzept, um die Eigenschaften der Metallkomplexe zu deuten und um neue Systeme fur die molekulare Aktivierung und die homogene Katalyse sowie funktionelle Materialien und Sensoren fur niedermolekulare Verbindungen zu entwerfen. In der homogenen enantioselektiven Katalyse wird in einem metallvermittelten Prozess durch sterische und/oder elektronische Kontrolle ein Stereoisomer bevorzugt gebildet. Die Verwendung von Liganden mit einem oder mehreren chiralen Oxazolinresten in diesen metallkatalysierten Reaktionen fuhrte in vielen Fallen zu guten Ergebnissen. Die Einfuhrung von Oxazolinresten in multifunktionelle Liganden erhoht deren Komplexitat und die Wahrscheinlichkeit, dass hemilabile Eigenschaften auftreten, die bis vor kurzem in der Oxazolinchemie noch nicht beobachtet worden sind. In diesem Ubersichtsartikel werden wir zunachst kurz auf die Definitionen und die Anwendungsmoglichkeiten hemilabiler Liganden eingehen, dann die Hauptklassen der Liganden mit einem oder mehreren Oxazolinresten vorstellen, wobei der Schwerpunkt auf den Hybridliganden liegt, und abschliesend erlautern, warum die Kombination dieser beiden Aspekte des Ligandendesigns besonders viel versprechend erscheint.

Room-temperature activation of aryl chlorides in Suzuki–Miyaura coupling using a [Pd(μ-Cl)Cl(NHC)]2 complex (NHC = N-heterocyclic carbene)

A straightforwardly synthesised complex, [Pd(micro-Cl)Cl(NHC)](2) (NHC = bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr), has been employed to mediate Suzuki-Miyaura reactions involving aryl chlorides at very low catalyst loadings and at room temperature.

Reactivity of the metal-silicon bond in organometallic chemistry☆

Abstract The reactivity of molecules containing a metal-silicon bond is currently attracting considerable attention owing to its relevance to a number of important stoichiometric and catalytic transformations. The focus of this review concerns those stoichiometric reactions involving the metal-silicon bond which result in isolable Si-containing metal complex(es). Catalytic reactions are therefore only considered when strong evidence for MSi intermediate complexes is provided. Reactions in which the silicon atom leaves the metal complex are not examined. For convenience, the reactions will be classified into insertion and migration reactions, although this terminology does not necessarily have a mechanistic implication and is somewhat arbitrary since many reaction products could belong to one or the other section.

The preparation, properties, and vibrational spectra of complexes containing the AuCl2–, AuBr2–, and AuI2– ions

The preparation and isolation of the complexes [Et4N][AuX2] and [Bun4N][AuX2], where X = Cl, Br, or I, are described. The i.r. and Raman spectra of the salts have been interpreted in terms of linear anions in each case. The fundamental frequencies of each anion are compared with those of the structurally similar HgX2 molecules, and appropriate force constants are given.

Strategies for the Anchoring of Metal Complexes, Clusters, and Colloids Inside Nanoporous Alumina Membranes

Two complementary strategies are presented for the anchoring of molecular palladium complexes, of cobalt or platinum clusters or of gold colloids inside the nanopores of alumina membranes. The first consists in the one step condensation of an alkoxysilyl functional group carried by the metal complex with the hydroxy groups covering the surface of the membrane pores. Thus, using the short-bite alkoxysilyl-functionalized diphosphane ligands (Ph2P)2N(CH2)3Si(OMe)3 (1) and (Ph2P)2N(CH2)4SiMe2(OMe)] (2) derived from (Ph2P)2NH (dppa) (dppa bis(diphenylphosphanyl)amine), the palladium complexes [Pd(dmba)(kappa2-P,P-(Ph2P)2N(CH2)3Si(OMe)3)] Cl (3) and [Pd(dmba)[kappa2-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]]Cl (4) (dmba-H = dimethylbenzylamine). respectively, were tethered to the pore walls. After controlled thermal treatment. confined and highly dispersed palladium nanoparticles were formed and characterized by transmission electron microscopy (TEM). This method could not be applied to the cobalt cluster [Co4(CO)8(mu-dppa)[mu-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]] (7) owing to its too limited solubility. However, its anchoring was achieved by using the second method which consisted of first derivatizing the pore walls with 1 or 2. The covalent attachment of the diphosphane ligands provides a molecular anchor that allows subsequent reaction with the cluster [Co4(CO)10(mu-dppa)] 6 to generate anchored 7 and this step was monitored by UV/Vis spectroscopy. In addition, the presence of carbonyl ligands in the cluster provides for the first time a very sensitive spectroscopic probe in the IR region which confirms both cluster incorporation and the retaining of its molecular nature inside the membrane. The presence of the bridging dppa ligand in 6 provides additional stabilization and accounts for the selectivity of the procedure. Using this method, platinum clusters (diameter ca. 2 nm) and gold colloids (diameter ca. 13 nm) were immobilized after passing their solution through the functionalized membrane pores. The resulting membranes were characterized by TEM which demonstrated the efficiency of the complexation and showed the high dispersion of the metal loading. The successful application of these methods has demonstrated that nanoporous alumina membranes are not only unique supports to incorporate metal complexes, clusters, or colloids but can also be regarded as functional matrices or microreactors, thus opening new fields for applications.

Efficient near-UV emitters based on cationic bis-pincer iridium(III) carbene complexes.

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(C(NHC)(Me)CC(NHC))2]X, where Ir(nBu)(C(NHC)(Me)CC(NHC)) is (4,6-dimethyl-1,3-phenylene-κC(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(C(NHC)(Me)CC(NHC))2]I and Ir(nBu)(C(NHC)(Me)CC(NHC))2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(C(NHC)(Me)CC(NHC))2](+). In the solid state, an emission at low energy is observed (λ(max) = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion.

An unprecedented, figure-of-eight, dinuclear iridium(I) dicarbene and new iridium(III) ‘pincer’ complexes

An unusual dinuclear Ir(I) complex bridged by two N-heterocyclic biscarbene ligands, forming a 20-membered, figure-of-eight dimetallacycle, and new CNHCCCNHC pincer complexes of Ir(III) have been obtained directly from the bis(imidazolium) precursors and [Ir(micro-Cl)(cod)]2.

Bimetallic silicon chemistry: New opportunities in coordination and organometallic chemistry

Abstract Bimetallic complexes containing silyl or siloxy ligands may display unique structures and reactivity patterns that are directly related to a subtle interplay between the metals and the ligands. Access to this class of compounds will be discussed. The recently discovered hemilabile behaviour of the bridging –Si(OR)3 ligand in heterobimetallic complexes has led to a number of developments in our group and others that are reviewed here. This will include the chemistry of Ph2PCH2PPh2 (dppm)-stabilized Fe–Pd and Fe–Pt alkyl complexes which allow, under mild conditions, controlled insertion reactions of isonitriles, alkynes or CO/olefins into the metal–carbon bond. In some cases, silicon migration reactions leading to new μ-siloxycarbene complexes have been observed. Other reactions that will be presented are the alkyne insertion into metal–hydride bonds, fluorination of the Si(OR)3 ligand and the catalytic dehydrogenative coupling of stannanes HSnR3. By altering the nature of the assembling ligand (μ-PR2 vs. μ-dppm) but keeping the metals and the silyl ligand unchanged, the first examples of intramolecular silyl migration from one metal to another were discovered. Finally, the use of aminosilyl in place of alkoxysilyl ligands led to the formation of new silylene complexes, to unprecedented examples of metal-mediated substituent exchange reactions between phosphorus and silicon, and to the characterization of the first complexes containing a bridging aminosilyl ligand. Many of these reactions involve steps that are directly relevant to the mechanisms of currently investigated catalytic systems.

The Reactivity of HRu3(CO)9C2BUt. The Synthesis and crystal structure of (PPH3)AuRu3(CO)9C2BUt, a new ruthenium-gold cluster

Abstract The (PPh 3 )AuRu 3 (CO) 9 C 3 Bu t complex has been obtained by reacting (PPh 3 )AuCl with the anionic complex [Ru 3 (CO) 9 C 2 Bu t ] − , prepared from the hydride HRu 3 (CO) 9 C 2 Bu t with a new procedure. The crystal structure of this heterometallic complex, as acetone solvate, has been determined by X-ray methods: it crystallizes in the orthorhombic space group F dd2, with 16 molecules in a unit cell of dimensions a = 33.64(2), b = 48.38(2), c = 9.23(1) A. The structure has been solved from diffractometer data by direct and Patterson methods, and refined by full-matrix least-squares to R = 0.068 for 1628 observed reflections. The complex consists of a AuRu 3 cluster in a butterfly arrangement with the Au atom on a wingtip. The Au atom of the Au(PPh 3 ) group bridges one side of the ruthenium triangle and practically substitutes the hydride ligand of the HRu 3 (CO) 9 C 2 -Bu t complex, maintaining the basic features of the Ru 3 (CO) 9 C 2 Bu t framework virtually unchange.