Ruthenium complexes with chiral bis-pinene ligands: an array of subtle structural diversity.

A new chiral derivative of the N,N-bis(2-pyridylmethyl)ethylamine (bpea) ligand, Me-pinene[5,6]bpea [(-)-L1], has been prepared from a new aldehyde building block [Me-pinene-aldehyde, (-)-4] arising from the monoterpene chiral pool. The tridentate (-)-L1 ligand has been employed to prepare a new set of Ru-Cl complexes in combination with didentate 2,2'-bipyridine (bpy) with the general formula [RuCl((-)-L1)(bpy)](+). These complexes have been characterized in solution by cyclic voltammetry, UV-vis, and 1D and 2D NMR spectroscopy. Isomeric mixtures of trans,fac-C1a and anti,mer-C1c compounds are formed when (-)-L1 is reacted with a [Ru(bpy)(MeOH)Cl3] precursor. Density functional theory calculations of all of the potential isomers of this reaction have been performed in order to interpret the experimental results in terms of electronic and steric effects and also to unravel the observed isomerization pathway between anti,mer-C1c and trans,fac-C1a.

New dinuclear ruthenium complexes: structure and oxidative catalysis.

The synthesis of new dinuclear complexes of the general formula {[Ru(II)(trpy)]2(µ-pdz-dc)(µ-(L)}(+) [pdz-dc is the pyridazine-3,6-dicarboxylate dianion; trpy is 2,2':6',2″-terpyridine; L = Cl (1(+)) or OH (2(+))] is described. These complexes are characterized by the usual analytical and spectroscopic techniques and by X-ray diffraction analysis. Their redox properties are characterized by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex 2(+) is used as the starting material to prepare the corresponding Ru-aqua complex {[Ru(II)(trpy)(H2O)]2(µ-pdz-dc)}(2+) (3(2+)), whose electrochemistry is also investigated by means of CV and DPV. Complex 3(2+) is able to catalytically and electrocatalytically oxidize water to dioxygen with moderate efficiencies. In sharp contrast, 3(2+) is a superb catalyst for the epoxidation of alkenes. For the particular case of cis-ß-methylstyrene, the catalyst is capable of carrying out 1320 turnovers with a turnover frequency of 11.0 cycles min(-1), generating cis-ß-methylstyrene oxide stereospecifically.

Carbon dioxide reduction by mononuclear ruthenium polypyridyl complexes.

New mononuclear ruthenium complexes with general formula [Ru(bid)(B)(Cl)] (bid is (1Z,3Z)-1,3-bis(pyridin-2-ylmethylene)isoindolin-2-ide; B = bidentate ligand 2,2'-bipyridine or R(2)-bpy, where R = COOEt or OMe) were synthesized and tested as precatalysts for the hydrogenative reduction of CO(2) in 2,2,2-trifluoroethanol (TFE) as solvent with added NEt(3). Significant amounts of formic acid were produced by these catalysts and a kinetic analysis based on initial rate constants was carried out. The potential mechanisms including intermediate species for these catalytic systems were investigated by means of quantum chemical calculations to gain deeper insight into the processes. The effect of electron-donating and electron-withdrawing groups on catalyst performance was studied both experimentally and theoretically.

Mn(II) complexes containing the polypyridylic chiral ligand (-)-pinene[5,6]bipyridine. Catalysts for oxidation reactions.

A series of mononuclear and dinuclear chiral manganese(II) complexes containing the neutral bidentate chiral nitrogen ligand (-)-pinene[5,6]bipyridine, (-)-L, were prepared from different manganese salts. The chirality in these complexes arises from the pinene ring that has been fused to the 5,6 positions of one pyridine group of the bipyridine ligand. These complexes have been characterized through analytical, spectroscopic (IR, UV/Vis, ESI-MS) and electrochemical techniques (cyclic voltammetry). Single X-ray structure analysis revealed a five-coordinated Mn(II) ion in [{MnCl((-)-L)}2(mu-Cl)2] (2), [{Mn((-)-L)}2(mu-OAc)3](PF6) (3) and [MnCl2(H2O)((-)-L)] (4) and a six-coordinated one in [MnCl2((-)-L)2] (5), [Mn(CF3SO3)2((-)-L)2] (6) and [Mn(NO3)(H2O)((-)-L)2)](NO3) (7). The magnetic properties of the binuclear compounds 2 and 3 have been studied. Both compounds show a weak antiferromagnetic coupling (2, J = -0.22 cm(-1); 3, J = -0.85 cm(-1)). The catalytic activity of the whole set of complexes has been tested with regard to the epoxidation of aromatic alkenes with peracetic acid. In the particular case of styrene, good selectivities and moderate enantioselectivities were obtained. Furthermore, total retention of the initial cis configuration was achieved when epoxidizing cis-beta-methylstyrene with the chloride complexes. In general, the epoxidation activity of these manganese complexes is strongly dependent on the steric encumbrance of the substrates employed.