Ferrocenyl‐Functionalized η5‐Thiophene Cr(CO)3 Half‐Sandwich Compounds

Abstract

The synthesis of η5‐Cr(CO)3 complexed 2,5‐Fc2‐ (3a), 3,4‐Fc2‐ (3b), 2,3‐Fc2‐ (3c) and 2‐Fc‐3,4,5‐Me3‐thiophenes (3d) [Fc = Fe(η5‐C5H4)(η5‐C5H5)] is described. These half‐sandwich compounds are accessible by the reaction of Cr(CO)3(MeCN)3 with the appropriate substituted thiophenes. The compounds were analyzed by NMR spectroscopy, where a highfield shift for the ring‐carbons and substituted hydrogen atoms of the thiophene core occurred. The 2,5‐ and 3,4‐derivatives could be characterized structurally by single‐crystal X‐ray diffraction analysis, demonstrating that a low bending of the sulfur atom out of the thiophene plane of up to 9.95(17)° and an anti‐arrangement of the ferrocenyl and Cr(CO)3 fragments is characteristic. Electrochemical measurements showed that each ferrocenyl unit could be oxidized separately with higher redox separations for the ferrocenyls of Cr(CO)3 complexes 3b (372 mV) and 3c (334 mV), than for the non complexed species 1b (244 mV) and 1c (283 mV). The potentials of the oxidation processes are shifted anodically by up to 144 mV, indicating an electron‐withdrawing character of the Cr(CO)3 unit. The irreversible Cr/Cr+ oxidations occurred at 545–895 mV. Within the electrochemical studies it could be shown that upon oxidation of the ferrocenyls the Cr(CO)3 half‐sandwich compounds rapidly decompose giving their non complexed thiophenes 1a–d. Solely for the Cr(CO)3‐2,3,4,5‐Me4‐thiophene 3d a reversible redox behavior was observed.