Ligand pontant | EPFL Graph Search

Un ligand pontant est un ligand qui se connecte à deux atomes ou plus, généralement des ions métalliques, ce ligand pouvant être atomique ou polyatomique. Virtuellement, tous les complexes organiques sont des ligands pontants, ce terme est donc réservé à des petits ligands tels que les pseudohalogénures ou des ligands spécifiquement conçus pour se lier à deux atomes métalliques. En nomenclature des complexes, lorsqu'un seul atome se ponte à deux atomes métalliques, on précède le nom du ligand de la lettre grecque μ (mu), avec un numéro en exposant décrivant le nombre d'atomes métalliques pontés au ligand. Dans le cas des ligands pontés à deux atomes, la notation μ est souvent plus simplement notées μ. Exemples de ligands pontants Virtuellement tous les ligands sont pontants, à l'exception des amines et de l'ammoniac. On compte parmi les ligands pontants inorganiques particulièrement courants :

OH−,
O2−,
S2−,
SH−,
NH2−
NH2− (imido)
N3− (nitrido)
CO

Self-assembly of organometallic macrocycles and cages

Thomas Brasey

Neutral metallamacrocycles with four or ten metal centers were obtained by reaction of [(R3P)PdCl2]2 with 2,3-dihydroxypyridine and derivatives or 2-hydroxynicotinic acid in the presence of base. In all these assemblies, the two O-donor atoms of the ligand bind to one metal center to form a chelate ring, while the N-donor atom binds to another metal center, linking the subunits together. The tetrameric complexes showed a low affinity for small aromatic diols. The procedure was adapted for the synthesis of water-soluble macrocycles: ligands which contain both the 2,3-dihydroxypyridine motive and a tertiary amine function that can be protonated were reacted in water with the acetato-bridged complex [(Et3P)Pd(CH3COO)2]2 to give tetrameric macrocycles in over 90% yields. Scrambling experiments showed that the Pd(II) macrocycles are highly inert. The reaction of ReBr3(CO)32 with 3-hydroxy-1,2,3-benzotriazine-4(3H)-one in the presence of base gave the ionic rhenium complex [ReBr(C7H4N3O2)(CO)3][NEt4], from which a neutral metallamacrocycle was obtained upon abstraction of (NEt4)Br. Its connectivity is comparable to that of the Pd(II) macrocycles. In coordinating solvents, this macrocycle disrupts and the monomeric solvent adducts are formed. In the presence of BF4-, it undergoes an anion-templated rearrangement to give a new, trimeric macrocycle which cocrystallizes with an unusual C3-symmetric [Ag(η2-benzene)3(OH2)][BF4] complex, the tetrafluoroborate anion being encapsulated in the cavity of the macrocycle. By reacting [(cymene)RuCl2]2 with 3,5-pyridinedicarboxylic acid, a trifunctional ligand in which the three donor atoms are unable to coordinate to the same metal center, an hexameric coordination cage was obtained. This complex has a trigonal antiprism geometry. Half of the carbonyl O-atoms of the bridging ligands are positioned in close proximity to each other, forming two binding sites on the surface. The cage can thus act as an exo-receptor for alkali metal cations (Na+, K+, Cs+). 1H and 13C NMR investigations as well as a single crystal X-ray analysis revealed that upon addition of alkali metal salts, the hexanuclear assembly partly rearranges to form a dodecameric coordination cage with icosahedral geometry. Although entropically disfavored, this process takes place because in this new complex all carbonyl O-atoms are part of a binding site (eight metal cations can be accommodated on the surface of the cage). Furthermore, due to the reduced surface curvature of the dodecanuclear assembly, these O-atoms are closer together, forming more suited binding sites for the alkali guest cations. To best of our knowledge, a coordination cage with an icosahedral geometry has not been reported so far.

EPFL2006

Small Molecule Activation by Multimetallic Uranium Complexes

Marta Falcone

The development of new processes for the selective and sustainable transformation of abundant small molecules constitutes one of the major research areas in inorganic and organometallic chemistry. These molecules as CO2, CO, N2 and H2 are abundant reservoirs of chemical energy, readily available sources of carbon, nitrogen and hydrogen. In a patent from 1909, Haber reported that uranium and uranium nitride were very efficient catalysts for ammonia synthesis. In order to perform multi-electron transfer, multimetallic complexes need to be designed. Reactivity of uranium bridging nitride remained unexplored since we investigated the nitride based reactivity of a dinuclear uranium(IV) bridging nitride complex towards small molecules such as CO2, yielding the first example of a uranium complex containing a bridging dicarbamate ligand, or CO, resulting in the complete cleavage in mild conditions of one of the strongest bonds in nature, or H2, yielding one of the few examples in f-elements of metal hydride, which is readily transferred to substrates such as CO2. Beyond the ligand based reactivity, we were interested in reducing the two metal centers to access a diuranium(III) bridging nitride, to pursue nitrogen activation. This complex was shown to have high flexibility, provided by the siloxide ligands, high reducing power and a bridging atom, the nitride, that holds together the uranium centers, with the possibility to bend upon nitrogen activation. This approach led us to the isolation of a hydrazido complex, resulting from the four-electron reduction of dinitrogen by the two metal centers. Moreover, complete cleavage of the NâN single bond was achieved by addition of CO to give two cyanate ligands or by addition of H2 to give ammonia. The nitride atom linker is nonetheless, too reactive to act as a spectator and it is involved in reactivity. Hence, we target the isolation of the analogue dinuclear U(III) complex with an oxide ligand as linker. This complex showed the same reactivity towards nitrogen but DFT revealed a more ionic character of the UâO bond nature with respect to the UâN bond, leading to different reactivity of the hydrazido fragment upon addition of CO and upon addition of H2, showing a smaller extent of activation in the oxo complex with respect to the nitride. The diuranium(III) bridging oxide complex showed interesting reactivity with H2, forming a bis-hydride complex capable of reducing CO2 beyond the formate level, to methanol.

EPFL2019

Neutral metallomacrocycles with four or ten (PEt3)Pd(II) centers

Thomas Brasey, Kay Severin

The reaction of (Et3P)PdCl2 with 2,3-dihydroxypyridine, 5-chloro-2,3,-dihydroxypyridine, or 2-hydroxynicotinic acid in the presence of base gives neutral, metallomacrocyclic compounds in which the heterocycles act as dianionic, bridging ligands. The macrocycles contain four or ten palladium atoms as evidenced by single crystal X-ray analyses.

2005