Synthesis of small tetranuclear Cu(I) metallacycles based on bridging pseudohalogenide ions.

Three new 'short' metallacycles are selectively obtained from the reaction of a bimetallic Cu(I) 'U-shape' molecular clip with, respectively, the cyano CN(-), tricyanomethanide C(CN)(3)(-), and azido N(3)(-) pseudohalogenide ions. These supramolecular assemblies have been characterized by spectroscopic methods, single-crystal X-ray diffraction, and elemental analysis. The single-crystal X-ray structures of these complexes reveal no short contact interaction between the pseudohalogenide ions within these self-assembled structures despite the short Cu(I)···Cu(I) distances within the molecular clips. The formation of these discrete metallacycles is rationalized by analyzing the structural features of the Cu(I) 'U-shape' molecular clip.

Aurophilicity versus mercurophilicity: impact of d10-d10 metallophilic interactions on the structure of metal-rich supramolecular assemblies.

Treatment of U-shaped, binuclear Cu(I) complexes 1,1' (1, counterion: BF(4)(-); 1', counterion: PF(6)(-)) with metal cyanide linear linkers K[Au(CN)(2)] (3) and Hg(CN)(2) (4) lead to formation of new supramolecular assemblies 5,5' and 6,6', respectively, in good yield. These derivatives have been characterized by NMR spectroscopy, IR, and X-ray diffraction studies. Derivative 5,5' are supramolecular metallacycles in which intramolecular aurophilic interactions between the Au(I) metal centers of the linkers are observed. Derivative 5 crystallizes as a single solid phase, whereas derivative 5' is characterized in the solid state as four different pseudo-polymorphs (5'a-d). Notably in the case of phase 5'd, a dimer of supramolecular metallacycles bounded by intermolecular aurophilic interactions is formed. Conversely, derivatives 6,6' present large structural diversity depending on the nature of the counterion. Derivative 6 is a supramolecular rectangle in which the Hg(II)-Hg(II) metal distance suggests mercurophilic interaction, whereas 6' crystallizes as two different pseudo-polymorphs 6'a,b, that is, a one-dimensional coordination polymer and one oligomer with no short Hg(II)-Hg(II) metal contacts, respectively. In derivatives 6,6', short contacts between the Hg(II) metal centers and fluorine atoms of the counterions are also observed, which may explain the counterion structural dependence of these supramolecular assemblies based on Hg(II) metal cyanide linker. Comparison of the different solid-state structures characterized highlights the importance of weak secondary interactions between the linkers for the formation supramolecular metallacycles from molecular clips 1,1' and suggests the range of energies required for these interactions to form metallacycles and to induce self-aggregation.

Polyoxometalate-based metal organic frameworks (POMOFs): structural trends, energetics, and high electrocatalytic efficiency for hydrogen evolution reaction.

The grafting of the triangular 1,3,5-benzene tricarboxylate linkers (denoted trim) on tetrahedral ε-Keggin polyoxometalates (POMs) capped by Zn(II) ions, formed in situ under hydrothermal conditions, has generated three novel POM-based metal organic frameworks (POMOFs). (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)][C(6)H(3)(COO)(3)](4/3)·6H(2)O (ε(trim)(4/3)) is a 3D open-framework built of molecular Keggin units connected by trim linkers, with channels occupied by tetrabutylammonium (TBA) counterions. ε(trim)(4/3) is a novel (3,4)-connected net, named ofp for open-framework polyoxometalate, and computer simulations have been used to evaluate its relative stability in comparison with ctn- and bor-like polymorphs, showing the stability of this novel phase directly related to its greatest density. A computational study was also undertaken with the aim of locating TBA molecules, the positions of which could not be deduced from single crystal X-ray diffraction, and further rationalizes their structure directing role. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)] (ε(2)(trim)(2)), the building unit is not the molecular Keggin but a dimerized form of this POM. Their connection via trim linkers generates a 3D framework with channels filled by TBA cations. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)]·8H(2)O ([ε(trim)](∞)), zigzag chains are connected via the organic linkers, forming 2D grids. Modified electrodes were fabricated by direct adsorption of the POMOFs on glassy carbon or entrapment in carbon paste (CPE). A remarkable electrocatalytic hydrogen evolution reaction (HER) was detected with a yield greater than 95%, and a turnover number as high as 1.2 × 10(5) was obtained after 5 h. The reported POMOF-based electrodes are more active than platinum, with a roughly 260 mV anodic shift. Finally, the electrocatalytic activities of ε(trim)(4/3)/CPE electrodes in various XCl (X = Li, Na, K, Cs) media have been studied. This allowed us to detect a cation effect and propose an electrocatalytic mechanistic pathway for the HER.

Lanthanide polyoxocationic complexes: experimental and theoretical stability studies and Lewis acid catalysis.

The [ε-PMo(V)(8)Mo(VI)(4)O(36)(OH)(4){Ln(III)(H(2)O)}(4)](5+) (Ln=La, Ce, Nd, Sm) polyoxocations, called εLn(4), have been synthesized at room temperature as chloride salts soluble in water, MeOH, EtOH, and DMF. Rare-earth metals can be exchanged, and (31)P NMR spectroscopic studies have allowed a comparison of the affinity of the reduced {ε-PMo(12)} core, thus showing that the La(III) ions have the highest affinity and that rare earths heavier than Eu(III) do not react with the ε-Keggin polyoxometalate. DFT calculations provide a deeper insight into the geometries of the systems studied, thereby giving more accurate information on those compounds that suffer from disorder in crystalline form. It has also been confirmed by the hypothetical La→Gd substitution reaction energy that Ln ions beyond Eu cannot compete with La in coordinating the surface of the ε-Keggin molybdate. Two of these clusters (Ln=La, Ce) have been tested to evidence that such systems are representative of a new efficient Lewis acid catalyst family. This is the first time that the catalytic activity of polyoxocations has been evaluated.

Coordination-driven hierarchical organization of pi-conjugated systems: from molecular to supramolecular pi-stacked assemblies.

The reaction of U-shaped, bimetallic, Cu(I) complexes, assembled from a heteroditopic pincer, with cyano-capped pi-conjugated linkers gives a straightforward access to pi-stacked metallocyclophanes in good yields. In these assemblies, the pi-walls have an almost face-to-face arrangement. The versatility of this rational supramolecular synthesis is demonstrated with the use of linkers that have nanoscale lengths (up to 27.7 A), different chemical compositions (oligo(para-phenylenevinylene)s OPVs, oligo(phenylene)s, oligo(phenylethynylene)s), and alternative geometries (linear, angular). Linkers that incorporate an internal pyridyne moiety can also be employed. X-ray diffraction studies revealed that the metallocyclophanes based on linear linkers self-organize into infinite pi-stacked columns in the solid state with intermolecular distances of about 3.6 A. This approach, based on coordination-driven self-assembly, provides a novel and rational strategy for the stacking of extended pi-systems in the solid state.

Zeolitic polyoxometalate-based metal-organic frameworks (Z-POMOFs): computational evaluation of hypothetical polymorphs and the successful targeted synthesis of the redox-active Z-POMOF1.

The targeted design and simulation of a new family of zeolitic metal-organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and epsilon-type Keggin polyoxometalates (POMs) as building units, named here Z-POMOFs, have been performed. A key feature is the use of the analogy between the connectivity of silicon in dense minerals and zeolites with that of the epsilon-type Keggin POMs capped with Zn(II) ions. Handling the epsilon-Keggin as a building block, a selection of 21 zeotype structures, together with a series of dense minerals were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like structure was predicted to be the most stable structure. This prediction has been experimentally validated by the targeted synthesis of the first experimental Z-POMOF structure, which was strikingly found to possess the cristobalite topology, with three interpenetrated networks. Crystals of [NBu(4)](3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)(BDC)(2)].2H(2)O (Z-POMOF1) have been isolated under hydrothermal conditions from the reduction of ammonium heptamolybdate in the presence of phosphorous acid and Zn(II) ions. Tetrabutylammonium cations play the role of counterions and space-filling agents in this tridimensional interpenetrated framework. Moreover, the electrochemistry of the epsilon-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.

Chemistry of bridging phosphanes: a comparative study within Cu(I)-Ag(I)-Au(I) triad-based homonuclear dimers.

Bridging ligands in Ag(I) and Au(I) bimetallic complexes: The P moiety of bis(2-pyridyl)phosphole acts as a symmetrically, semi-, or non-bridging donor in Ag(I) dimers (see figure). In related Au(I) complexes, only the non-bridging mode is observed. An unsaturated Ag(I) dimer is used as an adaptive molecular clip for the synthesis of pi-stacked metallocyclophanes.Bis(2-pyridyl)phosphole C reacted with AgPF(6) and dppm giving rise to dicationic dimer 1 in which C acts as a 1kappaN:1,2kappaP:2kappaN-donor with a symmetrically bridging P-centre. This complex gives the first example of dimers based on trigonal planar metal centres bearing a bridging sigma(3),lambda(3)-phosphane ligand. The reaction of two equivalents of C with AgPF(6) and Ag{Al[OC(CF(3))(3)](4)} afforded dicationic dimers 2 and 3 of general formula Ag(2)(C)(2) (2+), respectively. X-ray diffraction studies showed that one ligand C acts as a 1kappaN:1,2kappaP:2kappaN-donor with a symmetrically bridging P-centre and one behaves as a 1kappaN:2kappaP-chelate. In complex 3, one toluene molecule interacts with an Ag centre, giving the first example of a mixed coordination dimer featuring a bridging P-donor. Ag(I)-dimers 4 a-c of the general formula Ag(2)(C)(3) (2+), 2 X(-) (X=PF(6), BF(4), Al[OC(CF(3))(3)](4)) have been prepared according to different routes and characterised by X-ray diffraction study. In these derivatives, C exhibits a variety of coordination modes depending on the nature of the counteranion and of the solvent molecules included in the asymmetric units. These complexes can feature 0, 1, or 2 bridging P-donors. These results definitively demonstrate that the P-centre of phosphane ligands can easily switch from a bridging to a semi-bridging and a non-bridging coordination mode. Dimer 2 is used as an "adaptive" molecular clip for the synthesis of pi-stacked metallocyclophanes 7 a, b and 8 upon reaction with ditopic linkers. X-ray diffraction studies revealed that these supramolecular species behave as receptors for the counteranions (PF(6), BF(4)) and that the nature of the anion impacts the stereochemistry of the supramolecular metallocyclophanes. Au(I)-Dimers 10 a, b bearing two ligands C acting as 1kappaN:2kappaP-chelates are prepared and characterised in the solid-state.

Heterometallic 3d-4f cubane clusters inserted in polyoxometalate matrices.

Unprecedented molecular and bidimensional compounds based on monovacant polyoxometalates capped by heterometallic 3d-4f {LnCu(3)(OH)(3)O} (Ln = La, Gd, Eu) cubane fragments have been characterized and their magnetic properties investigated.

Chemistry of bridging phosphanes: CuI dimers bearing 2,5-bis(2-pyridyl)phosphole ligands.

Bis(2-pyridyl)phosphole 1 reacted with Cu(I) sources giving rise to dicationic or neutral dimers 2,3. In these derivatives, 1 acts as a 1kappaN:1,2kappaP:2kappaN donor with a symmetrically bridging P centre. X-ray diffraction studies of these species revealed no constraint due to the unusual coordination mode of the P donor. A comparative study with a monometallic Cu(I) complex in which 1 acts as a P,N chelate is presented. The acetonitrile ligands of the dicationic complex 2 can be displaced by a variety of donors. Bipyridine (bipy) acts as a chelating donor, while 1,1'-bis(diphenylphosphino)methane (dppm) and bis(2-pyridyl)phosphole 1 behave as bridging ligands. By using dppm and 1, the complexes arising from the stepwise displacement of the acetonitrile ligands of complex 2 can be isolated. X-ray diffraction studies performed on these novel complexes revealed that the P centre can easily switch from a bridging to a semibridging coordination mode. Of particular interest, within the same unit cell, complexes with P centres exhibiting bridging and semibridging coordination modes are observed. This switching can be induced by weak effects such as a different conformation of the incoming ligand. Cu(I) dimers assembled by 1 are air-stable derivatives that are not water sensitive. Hydrolysis of the PF(6) (-) counterion occurs under drastic conditions and results in the formation of a PO(2)F(2) fragment coordinated to a Cu(I)-1 fragment.

Mimicking [2,2]paracyclophane topology: molecular clips for the coordination-driven cofacial assembly of pi-conjugated systems.

Self-assembly of CuI dimers, featuring a bridging phosphole ligand, and ditopic cyano-substituted chromophores affords nanosize supramolecular cationic rectangles. Due to the short Cu-Cu distance in the bimetallic clips, the coordinated chromophores are forced to participate in cofacial pi-pi interactions as evidenced by X-ray data. In addition, intermolecular pi-pi interactions in the solid state are observed leading to infinite columns of pi-stacked (para-phenylenevinylene)-based chromophores.