Synthesis and Structural Investigations of manganese carbene complexes

Abstract

The study involves the synthesis and structural characterization of manganese carbene complexes. The synthesis of dimanganese monocarbene complexes [Mn2(CO)9{C(OEt)(heteroaryl)}] was done via the classical Fischer method, and a range of complexes containing heteroaromatic substituents, e.g. 2,2'-bithiophene, thiophene, furan and N-methyl pyrrole, was isolated. These complexes displayed a novel configuration with the carbene ligand in the axial position, in contrast to the equatorial position found for the analogous rhenium compound and other dimanganese complexes known from the literature. The possibility of manipulating the position of the carbene ligand in the binuclear complexes was investigated by a nucleophilic substitution of the ethoxy substituent with an amine substituent. Only aminolysis with small, primary amines such as ammonia and propylamine, proved successful. The propylaminocarbene ligands retained their axial configuration, but a conversion to the more thermodynamically stable equatorially substituted carbene ligands was observed for the complexes [Mn2(CO)9{C(NH2)(heteroaryl)}], while mixtures of the equatorial and axial isomers were observed in solution. Structural X-ray analysis proved that although the equatorial position is more electronically favourable, steric hindrance by the second manganese pentacarbonyl moiety prevented ethoxy- and propylamino-substituted carbene ligands to adopt this configuration. A kinetic study of the aminolysis reaction was done in an effort to elucidate the reaction mechanism and to explain the axial-equatorial conversion. Due to the competing decomposition reaction of the product and reagent complexes with that of the substitution reaction, no information about reaction intermediates could be obtained. The target mononuclear complexes [Mn(CO)4{C(OEt)(heteroaryl)}X] (X = Br, I) was obtained by cleavage of the metal-metal bond of the binuclear precursor complexes. Cleaving of the Mn-Mn bond was done oxidatively by halogens, without affecting the carbene ligand. The product complexes have an assembly resembling that of the Grubbs ruthenium metathesis catalyst. Interestingly, the cleaved complexes were found to have a cis configuration of the carbene and halide ligand. Copyright 2006, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Bezuidenhout, DI 2006, Synthesis and Structural Investigations of manganese carbene complexes, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-02122007-120832 / >