New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes.

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Stabilization of Luminescent Mononuclear Three-Coordinate CuI Complexes by Two Distinct Cavity-Shaped Diphosphanes Obtained from a Single α-Cyclodextrin Precursor.

Slightly different reaction conditions afforded two distinct cavity-shaped cis-chelating diphosphanes from the same starting materials, namely diphenyl(2-phosphanylphenyl)phosphane and an α-cyclodextrin-derived dimesylate. Thanks to their metal-confining properties, the two diphosphanes form only mononuclear [CuX(PP)] complexes (X=Cl, Br, or I) with the tricoordinated metal ion located just above the center of the cavity. The two series of CuI complexes display markedly different luminescence properties that are both influenced by the electronic properties of the ligand and the unique steric environment provided by the cyclodextrin (CD) cavity. The excited state lifetimes of all complexes are significantly longer than those of the cavity-free analogues suggesting peculiar electronic effects that affect radiative deactivation constants. The overall picture stemming from absorption and emission data suggests close-lying charge-transfer (MLCT, XLCT) and triplet ligand-centered (LC) excited states.

Enabling Stereochemical Communication and Stimuli-Responsive Chiroptical Properties in Biphenyl-Capped Cyclodextrins.

Chiroptical materials are gaining increasing interest due to their innovative character and their applications in optoelectronics and data encryption technologies. Fully harnessing the potential of building blocks from the "chiral pool", such as native cyclodextrins (CDs), as they often lack chromophores suitable for the construction of materials with significant chiroptical properties. Here, we present the synthesis and characterization of a two-level molecular stack consisting of a point-chiral element (CD) and an axially chiral element (biphenyl), capable of effectively translating the overall stereochemical information contained in CDs into stimuli-responsive chiroptical properties. α- and ß-permethylated CDs were efficiently capped with two different 2,2'-difunctionalized 1,1'-biphenyl units. In CD derivatives containing the rigid 2,2'-dihydroxy-1,1'-biphenyl cap, two intramolecular hydrogen bonds act synergistically as stereoselective actuators, enabling effective communication between the two levels and the transfer of nonchromophoric stereochemical information from the cyclic-oligosaccharide to the atropoisomeric cap. The chiroptical properties can be finely tuned by external stimuli such as temperature and solvent. The way chirality is transferred from the CD platform to the biphenyl cap was revealed thanks to crystallographic and computational analyses, together with electronic circular dichroism (ECD) studies.

Stable Luminescent [Cu(NN)(PP)]+ Complexes Incorporating a ß-Cyclodextrin-Based Diphosphane Ligand with Metal-Confining Properties.

A ß-cyclodextrin-based diphosphane with metal-confining properties was efficiently synthesized thanks to an unprecedented Smiles-like rearrangement of diphenyl-(2-phosphanylphenyl)phosphane in the presence of excess n-BuLi. The cis-chelating bidentate ligand is capable of forming very stable heteroleptic [Cu(NN)(PP)]+ complexes in which a metal-bound diimine ligand (bpy, phen, or mmp) is located within the cyclodextrin cavity. As a result of ligand encapsulation, flattening of the metal tetrahedral geometry in the excited state is disfavored, thereby resulting in enhanced luminescent properties.

A cavity-shaped cis-chelating P,N ligand for highly selective nickel-catalysed ethylene dimerisation.

The presence of a permethylated α-cyclodextrin (α-CD) cavity in a chelating P,N ligand promotes exclusive formation of 1 : 1 ligand/metal complexes. In MX2 complexes, one of the two halido ligands is forced to reside inside the CD hollow while the second one is pointing outside. Unlike its cavity-free analogue, a Ni(II) complex of the CD ligand is a highly selective precatalyst for ethylene dimerisation (96% C4 selectivity with up to 95% of 1-butene within the C4 fraction).

Benzimidazolium- and Benzimidazolilydene-Capped Cyclodextrins: New Perspectives in Anion Encapsulation and Gold-Catalyzed Cycloisomerization of 1,6-Enynes.

A new way of introducing a N-heterocyclic carbene cap onto cyclodextrins has been devised. The benzimidazolium intermediates were found to behave as receptors towards cavity matching anions. The corresponding C1 - and C2 -symmetrical regioisomeric carbene gold(I) complexes have been tested in a benchmark asymmetric cycloisomerization of 1,6-enynes. Up to 50 % ee was achieved for the enantioselective cycloisomerization of N-allyl-4-methyl-N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide.

Synthesis of Optically Active Polystyrene Catalyzed by Monophosphine Pd Complexes.

Cationic Pd(II) monophosphine complexes derived from α- and ß-cyclodextrins (CDs) promote the homopolymerization of styrene under carbon monoxide pressure. Although reversible CO coordination takes place under catalytic conditions according to (13) C NMR studies with (13) C-enriched CO, both complexes catalyze the formation of CO-free styrene polymers. These macromolecules display optical activity as a result of the presence of stereoregular sequences within the overall atactic polymer.

Cyclodextrin and phosphorus(III): a versatile combination for coordination chemistry and catalysis.

With the advent of efficient methods for functionalising cyclodextrins, the synthesis of a variety of cyclodextrin-based P(iii) ligands has been made possible. Capable of acting both as first and second coordination sphere ligands towards various transition metals, these compounds have found many applications in homogeneous catalysis. This perspective article describes the different approaches that have been used to covalently associate the ubiquitous P(iii) donor atom with a cyclodextrin cavity. In addition, special emphasis is placed on the influence the cyclodextrin receptor has on the coordination and catalytic properties of these cavity-shaped ligands.

Phosphinocyclodextrins as confining units for catalytic metal centres. Applications to carbon-carbon bond forming reactions.

The capacity of two cavity-shaped ligands, HUGPHOS-1 and HUGPHOS-2, to generate exclusively singly phosphorus-ligated complexes, in which the cyclodextrin cavity tightly wraps around the metal centre, was explored with a number of late transition metal cations. Both cyclodextrin-derived ligands were assessed in palladium-catalysed Mizoroki-Heck coupling reactions between aryl bromides and styrene on one hand, and the rhodium-catalysed asymmetric hydroformylation of styrene on the other hand. The inability of both chiral ligands to form standard bis(phosphine) complexes under catalytic conditions was established by high-pressure NMR studies and shown to have a deep impact on the two carbon-carbon bond forming reactions both in terms of activity and selectivity. For example, when used as ligands in the rhodium-catalysed hydroformylation of styrene, they lead to both high isoselectivity and high enantioselectivity. In the study dealing with the Mizoroki-Heck reactions, comparative tests were carried out with WIDEPHOS, a diphosphine analogue of HUGPHOS-2.

Confining phosphanes derived from cyclodextrins for efficient regio- and enantioselective hydroformylation.

Two confining phosphane ligands derived from either α- or ß-cyclodextrin produce singly P(III) -ligated metal complexes with unusual coordination spheres. High-pressure NMR studies have revealed that rhodium hydride complexes of the same type are also formed under hydroformylation conditions. This unique feature strongly favors the formation of the branched aldehyde at the expense of the linear one with high enantioselectivity in the rhodium-catalyzed hydroformylation of styrene.

Capping methodology in cyclodextrin chemistry: use of a symmetrical diketone reagent for regiospecific installation of unsymmetrical imine-enamine and imidazole caps.

Methylated α- and ß-cyclodextrin skeletons were both equipped with an unsymmetrical N-(2-N-alkylaminoacenaphthenyl)alkylimine rigid handle. The capping reaction, which consists of condensing a diaminocyclodextrin with highly symmetrical acenaphthenequinone, was found to be regiospecific when starting from cyclodextrin-diamines without any symmetry element. All modified cyclooligosaccharides have intra-annular nitrogen donor atoms. They undergo further cyclization on oxidation, whether chemically with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone or electrochemically, to give highly strained cyclodextrins capped with an unsymmetrical 1,2-disubstituted 1H-imidazole unit.

Regioselective di- and tetra-functionalisation of γ-cyclodextrin using capping methodology.

Alkylation of γ-cyclodextrin with 3 equiv. of 1,3-bis[bis(4-tert-butylphenyl)chloromethyl]benzene, followed by permethylation afforded selectively a singly capped (A,B), as well as two doubly capped (A,B:D,E and A,B:E,F) methylated γ-CDs. Deprotection with HBF4 gave quantitatively the corresponding diols and tetrols, which constitute valuable starting compounds for further functionalisation.

TRANSDIP: a trans-chelating ligand tailor-made for probing unusual Pd0 and PdII intermediates.

Forty years after Venanzi's seminal studies on trans-spanning diphosphines, a cavity-shaped trans chelator (TRANSDIP) has been used to monitor the stepwise formation of a carbon-carbon bond on a palladium centre. Furthermore, the ligand, which incorporates a cyclodextrin subunit, enabled the synthesis of the first η(2)-dioxygen complex in which the M-O(2) unit is entrapped within a cavity.

Non-conventional coordination of cavity-confined metal centres.

WIDEPHOS, a ß-cyclodextrin cavitand bearing two introverted P(III) donor atoms, readily accommodates two converging Au-X fragments (X = Cl, Br, I). In the corresponding [(WIDEPHOS)(AuX)(2)] complexes, severe steric crowding within the cavity results in one of the P-Au-X rods deviating from linearity, as revealed by an X-ray diffraction study and unusual (31)P magnetic shielding of one of the two phosphorus signals. The cavitand ligand is also suited for hosting strained dipalladium moieties with a single chlorido bridge, thereby forming dynamic complexes in which ligand coordination to one of the metal centres occurs via an oschelating P,O-subunit.

Regioselective opening of proximally sulfato-capped cyclodextrins.

Sulfato groups capping one or two pairs of adjacent glucose units in methylated cyclodextrin (CD) derivatives have been found to undergo regioselective opening with various nucleophiles; the reported methodology opens the way to the efficient synthesis of tridifferentiated α- and ß-cyclodextrins that constitute key starting materials for the preparation of heteropolydentate cavitands.

Regioselective double capping of cyclodextrin scaffolds.

Four different regioselective double capping reactions were applied either to α- or ß-cyclodextrin (CD) scaffolds. The first, which relied on the use of a rigid, bulky dialkylating reagent containing two trityl-like subunits, gave access to an A,B,D,E-tetrafunctionalised ß-CD regioisomer in large scale reactions. Two further capping reactions, involving the dianions PhP(2-) and S(2- , led to the synthesis of new C(1)-symmetrical ß-cyclodextrins in which pairs of neighbouring glucose units are linked by very short spacers. The last double capping reaction described allowed the high-yield preparation of unprecedented α- and ß-cyclodextrins containing two sulfate handles. Proximal capping turned out to be favoured for each of the above difunctional reagents. The structural characterisation of the capped species was achieved by thorough NMR investigations as well as by single-crystal X-ray diffraction studies.

Self-mediated stereoselective oxidation of thia-capped cyclodextrins.

Self-control: endo-Sulfoxide products can be synthesized selectively by the oxidation of thia-capped cyclodextrins using m-chloroperoxybenzoic acid (m-CPBA) in water. The reaction occurs by the formation of an oxidant-cyclodextrin inclusion complex. Operating in organic media preferentially leads to exo-sulfoxide products.

Self-assembled monolayers of alpha-cyclodextrin derivatives on gold and their host-guest behavior.

Various sulfur-modified alpha-cyclodextrin (alpha-CD) derivatives formed ordered monolayers on gold surfaces as confirmed by water contact angle goniometry, electrochemistry, X-ray photoelectron spectroscopy, and atomic force microscopy measurements. Self-assembled monolayers (SAMs) of the adsorbates showed high polarity, uniform monolayer arrangement, and low charge transfer resistance. Electrochemical capacitance measurements were used to determine the binding affinity of aliphatic carboxylic acid salts with four, six, and eight carbon atoms. The nonmethylated cyclodextrin host-guest pairs showed 1-2 orders of magnitude higher binding constants on surfaces than in solution.

BINOL-derived phosphoramidites in asymmetric hydrogenation: can the presence of a functionality in the amino group influence the catalytic outcome?

In discovering the remarkable catalytic properties of BINOL-derived phosphoramidites (binoP-NR(2)), Dutch researchers recently achieved a long-awaited breakthrough in asymmetric catalysis. For the first time, easily accessible monodentate chiral P(III) ligands turned out to provide high enantioselectivities when used in rhodium-catalysed olefin hydrogenation. The simplest ligand representative of this family is MonoPhos, which can be made straightforwardly from BINOL and hexamethylphosphorous triamide. Since the first publication dealing with such catalysts (J. Am. Chem. Soc., 2000), a variety of binoP-NRR' ligands have been reported in which the amino group bears a functional substituent or a stereogenic centre. This critical review examines the impact of the presence of such a functionality in the amino group on catalytic olefin hydrogenation reactions.