A Mesoionic Diselenolene Anion and the Corresponding Radical Dianion

Dichalcogenolenes are archetypal redox non-innocent ligands with numerous applications. Herein, we describe a diselenolene ligand with fundamentally different electronic properties. By selenation of a C2-protected imidazolium salt, we have prepared a mesoionic diselenolene. This ligand is diamagnetic, which is in contrast to the paramagnetic nature of standard dichalcogenolene monoanions. The new ligand is also redox-active, as demonstrated by isolation of a stable diselenolene radical dianion. The unique electronic properties of the new ligand give rise to unusual coordination chemistry. Thus, we were able to prepare a hexa-coordinate aluminum tris(diselenolene) complex, and a Lewis acidic aluminate complex with two ligand-centered unpaired electrons.

Mécanismes d'isomérisation et d'échange de ligand de complexes octaédriques labiles de cobalt (II)

Fabrice Riblet

There are only few structural thermodynamic and kinetic studies on labile octahedral complexes available today. Octahedral paramagnetic compounds of cobalt(II) have an optimum electronic relaxation leading to NMR spectra with good resolution. Therefore, they are excellent candidates to follow substitution and isomerisation processes by multinuclear NMR at variable pressure and temperature. This work groups the study of nine labile complexes (three families of compounds with three examples each) which can exist in different isomeric forms. The first part of the work concerns octahedral complexes with bis-chelating ligands of the general formula [Co(LA)2Rpy2] with LA being a bidentate acetylacetonate ligand and Rpy a monodentate ligand of the γ-substituted pyridine type. The second part of the work concerns octahedral complexes with tris-chelating ligands of the general [Co(LA)2(LB)] with LA being a bidentate acetylacetonate ligand and LB a bidentate ligand of the γ-substituted bipyridine type. The third part of the work concerns trinuclear heterometallic complexes with the general formula [Fe2M(μ3-O)(μ2-Ac)6(Rpy)3] with M = CoII, NiII and Rpy a monodentate ligand of the γ-substituted pyridine type. Each type of the metal centres has an octahedral geometry. For each example chosen, the different species present in equilibrium in solution have been identified and the thermodynamic and kinetic parameters measured are discussed. Mechanisms for isomerisation and ligand exchange are proposed on the basis of variable temperature and pressure kinetics data. It has been shown that the bis-chelate compounds isomerise via a dissociative mechanism (with an intermediate of bipyramid geometry). The monodentate ligand participates more or less depending on its donor character. In the case of complexes with γ-methylpyridine, the mechanism consists essentially of an opening of one arm of the bidentate ligand (with little exchange of the pyridine). However, in the case of complexes with γ-trifluoromethylpyridine, a more frequent exchange of the monodentate ligand is observed competing with the opening of the arm of the bidentate ligand. In the case of tris-chelate compounds the isomerisation mechanism proposed on the bases of the exchange matrix used for spectral simulations is a twist with an intermediate of prismatic geometry. This observed rates (faster) and the obtained activation parameters (smaller) are very different of those found for complexes with bis-chelating ligands. The trinuclear heterometallic complexes maintain their structure in dichloromethane solution. Variable pressure experiments allowed assigning a dissociative mechanism (D) for the exchange of pyridine on both the divalent and the trivalent centres. Experiments at variable temperature indicated an increase of lability for the trivalent metals and a decrease in lability for the divalent metals compared to the homoleptic octahedral mononuclear complexes. The activation parameters for the trinuclear heterometallic complexes are bigger than those found for the bis- and tris-chelate compounds.

EPFL2007

Synthesis, complexation and NMR relaxation properties of Gd3+ complexes of Mes(DO3A)3

Lothar Helm, Hugues Jaccard, Pascal Miéville

Medium sized molecules endowed with multiple Gd3+ complexes are efficient high magnetic field MRI contrast agents. The novel ligand Mes(DO3A)3, presenting three DO3A (1,4,7,10-tetraazacyclododecane-N,N′,N′′-triacetatic acid) units grafted on the methyl positions of a central mesitylene (1,3,5-trimethylbenzene), has been synthesized. Designed as an MRI contrast agent, this ligand is complexed with Gd3+ and its efficiency is characterized by variable field 1H-NMR and variable temperature 17O-NMR. The evaluation of the relaxation and paramagnetic chemical shift data allowed the identification of an undesired binuclear complex which is obtained by using the classical procedure for complexation as described in the literature. An intramolecular capping mechanism appears to be responsible for the failure to introduce a third Gd3+ ion into the ligand. A new alternative method, based on pre-complexation with Mg2+ followed by transmetallation is described here and leads to the expected trinuclear Gd3+ complex [Mes{Gd(DO3A)(H2O)2}3]. The rate constants for the water exchange (kex298 = 32 × 106 s−1) for the bi- and trinuclear complex appeared to be the same, which is surprising in relation to the difference in the charge of the complex and to the difference in the number of coordinated water molecules, one and two per Gd3+ for the binuclear and trinuclear complex, respectively.

Royal Society of Chemistry2011

Mécanismes d'isomérisation et d'échange de ligand de complexes octaédriques labiles de cobalt (II)

Fabrice Riblet

EPFL2007