Chromium−Manganese Selenide Carbonyl Complexes: Paramagnetic Clusters and Relevance to CO Activation of Acetone

The paramagnetic even-electron cluster, [Et4N]2[Se2Cr3(CO)10], was found to react readily with Mn(CO)5Br in acetone to produce two unprecedented mixed chromium−manganese selenide carbonyl complexes, [Et4N][Me2CSe2{Mn(CO)4}{Cr(CO)5}2] ([Et4N][1]) and [Et4N]2[Se2Mn3(CO)10{Cr(CO)5}2] ([Et4N]2[2]). X-ray crystallographic analysis showed that anion 1 consisted of two Se−Cr(CO)5 moieties, which were further bridged by one isopropylene group and one Mn(CO)4 moiety. The dianionic cluster 2 was shown to display a Se2Mn3 square-pyramidal core with each Se atom externally coordinated by one Cr(CO)5 group. The formation of complex 1, presumably via CO activation of acetone, was further facilitated by acidification of the reaction of [Et4N]2[Se2Cr3(CO)10] with Mn(CO)5Br in acetone. Complex 1 readily transformed into 2 upon treatment with Mn2(CO)10 in a KOH/MeOH/MeCN solution. Cluster 2 was a 51-electron species, which readily converted to the known 49-electron cluster [Se2Mn3(CO)9]2− upon heating and bubbling with CO. Magnetic studies of the even-electron cluster, [Et4N]2[Se2Cr3(CO)10], and the odd-electron species, [Et4N]2[2] and [PPN]2[Se2Mn3(CO)9], were determined by the SQUID measurement to have 2, 3, and 1 unpaired electrons, respectively. In addition, the nature and formation of complexes 1 and 2 are discussed, and the magnetic properties and electrochemistry of [Se2Cr3(CO)10]2−, 2, and [Se2Mn3(CO)9]2− were further studied and elucidated by molecular orbital calculations at the PW91 level of density functional theory.