Synthesis and application of Fischer carbene complexes of group 6, 8 and 9 transition metals

Abstract

A range of novel Fischer carbene complexes (FCCs) of group 6 (Cr and W), 8 (Fe) and 9 (Co) metals was prepared using the classical Fischer route, and their electronic and steric properties evaluated. For W(0) and Cr(0), 2,3´-bithiophene (2,3ʹ-BT) was employed as carbene ligand substituent. A range of mono- and biscarbene complexes 1-13 (a = W, b = Cr) was obtained by exploiting the α-activated sites in the 2,3ʹ-BT substrates. A mutual relationship was observed between the site of activation and the ratio of products (yield) isolated. The isolation of the unprecedented 2,2´-bithiophene carbene (2,2ʹ-BT) products 6a/b and the monocarbene complex 9a during the reaction, presented evidence of unexpected C-C bond cleavage/formation during the syntheses. Comparative studies of the 2,3ʹ-BT FCCs with that of similar complexes of 2,2ʹ-BT and 3,3´-bithiophene (3,3ʹ-BT) were conducted to determine the contribution of the individual thienyl rings of the substrate bithiophenes, to the electronic properties of the FCCs. Scarcer examples of first-row groups 8 (Fe(0)) and 9 (Co(I)) FCCs were prepared with systematic variation of the aryl substituents (thiophene, 2,3´-bithiophene, N,N-dimethylaniline and ferrocene) to modify the reactivity of these complexes. Synthetic challenges included metal acylate O-alkylation to yield the desired FCCs, in contrast to the commonly found metal alkylation that leads to related ketone formation after reductive elimination from the metal complex. Only monocarbene FCCs were prepared for Fe (14-22) and Co (24-29). Similar bond breaking and bond rearrangement as found for the complexes of W(0) and Cr(0), were observed in the synthesis of the complexes of Fe(0) to lead to the isolation of the Fe(0) monocarbene complex of 2,2ʹ-BT. Modifications of the FCCs were attempted by carbene ligand variation and co-ligand substitution. Selected complexes of groups 6 (W), 8 (Fe) and 9 (Co) were successfully aminolysed in order to access the more electron donating aminocarbene analogues. Changes in carbene carbon stabilization patterns relative to the parent ethoxycarbene complex subsequent to the aminolysis processes were studied using various spectroscopic analysis approaches. Molecular structures of these complexes were confirmed by single-crystal X-ray diffraction. Complexes of Fe(0) and Co(I) adopted a trigonal-bipyramidal molecular geometry with the carbonyl ligands lying in the equatorial plane. Attempts to substitute a carbonyl ligand of the ethoxy-FCCs of Fe(0) and Co(I) by aminolysis with 1,1-dimethylethylenediamine (with available pendant amine) was unsuccessful (complex 23). Also, irradiation to facilitate the removal of the third carbonyl resulted in decomposition. Although no empirical studies were done, complexes of Fe(0) and Co(I) were found to be fairly stable under atmospheric conditions, and could be further studied for potential application in homogeneous catalysis. Electrochemistry studies of selected complexes of Fe(0) and Co(I) showed one electron (1e-) reduction of the carbene moiety and the influence of various substituents (thiophene, N,N-dimethylaniline, ferrocene, M(CO)3L (M = Fe/Co, L = PPh3, PCy3 and SnPh3), ethoxy- and amino-group) was evaluated. Chemical oxidation of complex 16 by means of a silver(I) salt as oxidant, was successful and the in situ formation of the cationic complex 16-1, [Fe(CO)3PPh3{C(OEt)Fc}]NO3 was confirmed by infrared spectroscopic studies. Selected complexes of Fe(0) and Co(I) showed catalytic activity in hydroformylation studies of 1-octene with good conversion. The complexes of iron were found to show higher yields for the isomerization/hydrogenation to alcohols, instead of the aldehydes at the same conditions used for the cobalt complexes.