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

Abstract

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.

Synthesis and Characterisation of Novel (Guanidine)manganese Complexes and Their Application in the Epoxidation of 1‐Octene

“…[17,18] There are only few further examples of fivecoordinate Mn complexes with a Mn 2 (μ-Cl) 2 Cl 2 core. [6,19] In agreement with the reported five coordinated complexes, the terminal Mn-Cl bonds are with 2.366 (1) [6,19] With regard to the Mn···Mn vector, the Mn 2 (μ-Cl) 2 Cl 2 core in C2 is more contracted than in C1. The bite angles defined by the quinoline ligands are with 75.34(6) and 75.73(5)°n early identical in both complexes.…”

“…[14] geometry; τ = 1 perfect trigonal-bipyramidal geometry). [15] The planar Mn 2 [6,[16][17][18][19] In the literature a series of dinuclear Mn II complexes with this chloridobridging motif is described, but the majority of these complexes are six-coordinate [16] and also polymeric structures are …”

N‐Tetradentate SPANamine Derivatives and Their MnII‐Complexes as Catalysts for Epoxidation of Alkenes

“…The improvement of catalytic performance with the use of additives has already been stated for analogous catalytic Mnbased systems. This positive effect is, in some cases (when using peracids as oxidant), related to an increase in the pH value [32,39] that may lead to longer half-life of the intermediate species formed, [40] although coordination of additives to Mn prior to the oxygen transfer step (leading to alternative, more effective intermediate species) has also been postulated. [41,42] In our case, the significant increase in the selectivity when using either imidazole or sodium hydrogen carbonate indicates that the mechanistic pathway leading to the epoxide should be favored with respect to alternative undesired routes and thus the additives might have an active role in determining which mechanistic path is preferred.…”

“…A similar behavior was observed when related Mn catalysts also bearing pinene type ligands were used. [32,39] In most cases the use of ten equivalents of an additive such as imidazole or NaHCO 3 remarkably increased the conversion, especially when sodium hydrogen carbonate was used. The selectivity for the epoxide product was also increased to a similar extent for both types of additive.…”

“…[9,[27][28][29] Nitrogen-based ligands have well-known advantages such as chemical robustness, wide availability in enantiomerically pure form (from the chiral pool or through well-established racemate resolutions), rich coordination chemistry in combination with inexpensive first-row transition metals, and stabilization of high-oxidation-state transition-metal species (useful for oxidation catalysis). Among this type of ligands, those containing sp 2 -hybridized nitrogen-donor atoms such as chiral oxazolines [6,7] or pyridines, [30][31][32] porphyrines [33] and Salen [17][18][19] have received much attention during the last decades.…”