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.

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.

Metal Confinement through N-(9-Alkyl)fluorenyl-Substituted N-Heterocyclic Carbenes and Its Consequences in Gold-Catalysed Reactions Involving Enynes.

A series of gold(I) and gold(III) complexes containing bulky bis-N,N'-(9-alkylfluorenyl) heterocyclic carbene (R F-NHC) ligands have been prepared in high yields from appropriate imidazolinium, imidazolium and benzimidazolium salts. In all complexes, the carbene ligand provides high steric protection of the Au-X bond trans to the carbenic C atom. Irrespective of the metal oxidation state, the complexes showed high efficiency in a tandem 3,3-rearrangement/Nazarov reaction of an enynyl acetate. One of the AuIII complexes, [AuCl3 (R F-NHC)], was further found to be suitable for the efficient cyclisation of a propargylcarboxamide. Furthermore, unlike related NHC-gold(I) complexes based on conventional bulky N-heterocyclic carbenes (notably, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-(bis-tert-butyl)imidazol-2-ylidene (ItBu)), the studied [AuI Cl(R F-NHC)] complexes catalysed the conversion of a 1,6-enyne in the presence of indole into a single product; this arises from the embracing character of the ligand, which prevents indole addition on one of the catalytic intermediates. A structure/selectivity relationship was established for all carbenes tested that took into account percent buried volumes and topographic steric maps. The results illustrate the high potential of confining NHCs in organic synthesis.

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.

"Hummingbird" behaviour of N-heterocyclic carbenes stabilises out-of-plane bonding of AuCl and CuCl units.

An N-heterocyclic carbene substituted by two expanded 9-ethyl-9-fluorenyl groups was shown to bind an AuCl unit in an unusual manner, namely with the AuX rod sitting out of the plane defined by the heterocyclic carbene unit. As shown by X-ray studies and DFT calculations, the observed large pitch angle (21°) arises from an easy displacement of the gold(I) atom away from the carbene lone-pair axis, combined with the stabilisation provided by weak CH⋅⋅⋅Au interactions involving aliphatic and aromatic H atoms of the NHC wingtips. Weak, intermolecular Cl⋅⋅⋅H bonds are likely to cooperate with the H⋅⋅⋅Au interactions to stabilise the out-of-plane conformation. A general belief until now was that tilt angles in NHC complexes arise mainly from steric effects within the first coordination sphere.

Cracking cavitands: metal-directed scission of phosphinyl-substituted resorcinarenes.

Resorcinarene-derived tetramethylene cavitands bearing a diphenylphosphino group grafted to their wider rim undergo facile, directed C-O bond breaking upon reaction with transition-metal ions in the presence of nucleophiles. One possible reaction mechanism involves formation of a P,O-chelate complex, which weakens the adjacent O-CH2 bond, leading to the formation of an oxacarbenium intermediate.

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.

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.

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.

A novel ratiometric fluorescence probe based on calix[4]arene functionalized polymer dots for bisphenol A detection in real water samples.

Bisphenol A (BPA) is one of key raw materials used in the production of epoxy resins and plastics, which has toxicological effects on humans by disrupting cell functions through a variety of cell signaling pathways. Therefore, it is of great significance to develop a simple, rapid, and accurate BPA detection method in real water samples. In this study, a ratiometric fluorescence method based on yellow-emitting surface-functionalized polymer dots (PFBT@L Pdots) and blue-emitting carbon dots (Cdots) was described for the detection of BPA. Pdots as the detecting part were synthesized by using highly fluorescent hydrophobic Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-(2,1',3)-thiadiazole)] (PFBT) polymer and (R)-5,11,17,23-Tetra-tert-butyl-25,27-bis[(diphenylphosphinoyl)methoxy]-26-(3-oxabutyloxy)-28-[(1-phenylethyl)- carbamoylmethoxy]calix [4]arene (L) functionalizing ligand, and Cdots as internal reference were prepared by hydrothermal treatment of citric acid and urea. In the presence of BPA, chemical binding of the phosphorus atoms of nearby PFBT@L Pdots with BPA hydroxyl functional groups led to the aggregation of the PFBT@L Pdots aggregation and quenching their yellow emission, but the blue emission of Cdots, on the other hand, remained stable. The proposed PFBT@L Pdots probe was successfully applied for the detection of BPA in real water samples, and the results were in good agreement with those obtained by HPLC-FLD. To the best of our knowledge, this is the first report that the calixarene has been utilized to modify Pdots.

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.

Structural and conformational analysis of a biaryl phosphine integrating a calix[4]arene cavity. Can the phosphorus atom behave as an introverted donor?

The conformational preference of a cavity-based biaryl phosphine, namely 5-(2-diphenylphosphinyl-phenyl)-25,26,27,28-tetrapropyloxycalix[4]arene (L) has been investigated by density functional theory calculations. The analysis showed that the barrier to rotation about the C-C axle of the biaryl unit is only 10.7 kcal mol-1, this rendering possible access to conformers of two types, those in which the P lone pair sits at the cavity entrance and points to the calixarene interior, others with a more open structure where the P atom is located outside the cavity. As revealed by a single crystal X-ray diffraction study, the biaryl phosphine appears virtually as an atropisomer in the solid state in which the phosphorus atom lies totally out of the cavity defined by the four phenoxy rings.

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.

Resorcin[4]arene-derived mono-, bis- and tetra-imidazolium salts as ligand precursors for Suzuki-Miyaura cross-coupling.

Eleven resorcinarene cavitands bearing either one, two or four (3-R-1-imidazolylium)-methyl substituents (R = (n)Bu, Ph, Mes, (i)Pr(2)C(6)H(3)) anchored at resorcinolic "ortho" positions have been synthesised from the appropriate bromomethylated precursor. Combining the imidazolium salts with palladium acetate and Cs(2)CO(3) gave active Suzuki-Miyaura cross coupling catalysts. The highest activities were observed with the doubly functionalised cavitands, which all have the imidazolylium groups attached to proximal resorcinol units.

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).

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.

Cavity-shaped ligands: calix[4]arene-based monophosphanes for fast Suzuki-Miyaura cross-coupling.

Five conical calix[4]arenes that have a PPh(2) group as the sole functional group anchored at their upper rim were assessed in palladium-catalysed cross-coupling reactions of phenylboronic acid with aryl halides (dioxane, 100 degrees C, NaH). With arylbromides, remarkably high activities were obtained with the catalytic systems remaining stable for several days. The performance of the ligands is comparable to a Buchwald-type triarylphosphane, namely, (2'-methyl[1,1'-biphenyl]-2-yl)diphenylphosphane, which in contrast to the calixarenyl phosphanes tested may display chelating behaviour in solution. With the fastest ligand, 5-diphenylphosphanyl-25,26,27,28-tetra(p-methoxy)benzyloxy-calix[4]arene (8), the reaction turnover frequency for the arylation of 4-bromotoluene was 321,000 versus 214,000 mol(ArBr).mol(Pd)(-1).h(-1) for the reference ligand. The calixarene ligands were also efficient in Suzuki cross-coupling reactions with aryl chlorides. Thus, by using 1 mol% of [Pd(OAc)(2)] associated with one of the phosphanes, full conversion of the deactivated arenes 4-chloroanisole and 4-chlorotoluene was observed after 16 h. The high performance of the calixarenyl-phosphanes in Suzuki-Miyaura coupling of aryl bromides possibly relies on their ability to stabilise a monoligand [Pd(0)L(ArBr)] species through supramolecular binding of the Pd-bound arene inside the calixarene cavity.

High-pressure investigations under CO/H2 of rhodium complexes containing hemispherical diphosphites.

The two rhodium complexes [Rh(acac)(L(R))] (L(R)=(S,S)-5,11,17,23-tetra-tert-butyl-25,27-di(OR)-26,28-bis(1,1'-binaphthyl-2,2'-dioxyphosphanyloxy)calix[4]arene; 6: R=benzyl, 7: R=fluorenyl), each based on a hemispherical chelator forming a pocket about the metal centre upon chelation, are active in the hydroformylation of 1-octene and styrene. As expected for this family of diphosphanes, both complexes resulted in remarkably high selectivity towards the linear aldehyde in the hydroformylation of 1-octene (l/b≈15 for both complexes). Linear aldehyde selectivity was also observed when using styrene, but surprisingly only 6 displayed a marked preference for the linear product (l/b=12.4 (6) vs. 1.9 (7)). A detailed study of the structure of the complexes under CO or CO/H(2) in toluene was performed by high-pressure NMR (HP NMR) and FT-IR (HP-IR) spectroscopies. The spectroscopic data revealed that treatment of 6 with CO gave [Rh(acac)(CO)(η(1)-L(benzyl))] (8), in which the diphosphite behaves as a unidentate ligand. Subsequent addition of H(2) to the solution resulted in a well-defined chelate complex with the formula [RhH(CO)(2)(L(benzyl))] (9). Unlike 6, treatment of complex 7 with CO only led to ligand dissociation and concomitant formation of [Rh(acac)(CO)(2)], but upon addition of H(2) a chelate complex analogous to 9 was formed quantitatively. In both [RhH(CO)(2)(L(R))] complexes the diphosphite adopts the bis-equatorial coordination mode, a structural feature known to favour the formation of linear aldehydes. As revealed by variable-temperature NMR spectroscopy, these complexes show the typical fluxionality of trigonal bipyramidal [RhH(CO)(2)(diphosphane)] complexes. The lower linear selectivity of 7 versus 6 in the hydroformylation of styrene was assigned to steric effects, due to the pocket in which the catalysis takes place being less adapted to accommodate CO or larger olefins and, therefore, possibly leading to facile ligand decoordination. This interpretation was corroborated by an X-ray structure determination carried out for 7.

Calix[4]arene-phosphine dimers: precursors of flexible metallo-capsules and self-compacting molecules.

The first diphosphines based on a double calixarene, namely 1,4 (or 1,3)-bis-(5-diphenylphosphino-25,26,27,28-tetrapropoxycalix[4]aren-17-yl)benzene (L(2), L(3)) were each prepared in four steps starting from 5,17-dibromo-25,26,27,28-tetrapropoxycalix[4]arene. Upon reaction of L(2) with [Au(tht)(thf)]BF(4), (tht = C(4)H(8)S) a rigid metallo-capsule was quantitatively formed, which adopts an oblique form owing to the distinct nature of the spacers linking the two calixarene half-spheres. In the solid state, the 1,4-substituted phenylene linker is turned towards the gold ion, suggesting the existence of weak bonding interactions between two aromatic CH protons of this ring and the metal centre (AuH =2.67 A). In contrast to this gold complex, the related silver complex shows dynamic behaviour in solution, the exchange between two enantiomeric oblique forms being facilitated by the greater stereochemical flexibility of Ag(I) vs. Au(I). A heteronuclear (109)Ag{(1)H} HMQC experiment established strong correlations between the CH protons of the phenylene linker and the (109)Ag ion. Dynamic behaviour similar to that observed for the silver complex was further observed in trans-[PtCl(2)L(2)], a chelate complex that could be obtained quantitatively from L(2) and [PtCl(2)(PhCN)(2)]. The intended formation of a chelate complex leading to a capsule with an endo-oriented metal centre was achieved by reacting L(3) with [Pd(allyl)(thf)(2)]BF(4). The complex thus formed constitutes the first organometallic transition metal complex embedded in a cavity with large portals. Binding of [RuCl(2)(p-cymene)] to L(2) and L(3) resulted in self-compacting bimetallic complexes in which each calixarene basket entraps a Ru(p-cymene) unit, thereby forming molecules occupying a minimal volume.

Synthesis of a resorcinarene-based tetraphosphine-cavitand and its use in Heck reactions.

A resorcinarene cavitand substituted by four -CH(2)PPh(2) pendant arms was synthesised starting from a generic C(5)-resorcinarene. Combining this tetraphosphine with palladium acetate and Cs(2)CO(3) gave an active Heck catalyst. The highest activities were observed by using a tetraphosphine/Pd ratio of ca. 1:1.