Syntheses and Structural Characterizations of Inorganic ansa-Metallocene Analogues: ansa-Ferratricarbadecaboranes

New linked cyclopentadienyl-tricarbadecaboranyl and bis-tricarbadecaboranyl dianions have been used to form the first examples of ansa-metallatricarbadecaboranyl complexes. The hybrid cyclopentadienyl-tricarbadecaboranyl dianion, Li2+[6-C5H4-(CH2)2-nido-5,6,9-C3B7H9]2– (1), was produced by an initial carbon-insertion reaction of a nitrile-substituted cyclopentadiene with the arachno-4,6-C2B7H12– anion, followed by deprotonation to the dianion with LiH. The linked-cage bis-tricarbadecaboranyl dianion, Li2+[6,6′-(CH2)2-nido-(5,6,9-C3B7H9)2]2– (2), was produced by a similar carbon-insertion route involving the reaction of two equivalents of arachno-4,6-C2B7H12– with succinonitrile. The reaction of 1 with an equivalent of FeCl2 produced the hybrid complex, ansa-(2-(CH2)2)-(1-η5-C5H4-closo-1,2,3,4-C3B7H9)­Fe (3), with a crystallographic determination confirming the formation of a sandwich structure where the ring and cage are linked by the ansa −CH2CH2– group with attachment to the cage at the C2 carbon. The reaction of 2 with FeCl2 produced three isomeric ansa-(CH2)2-ferrabistricarbadecaboranyl sandwich complexes, ansa-(CH2)2-(closo-C3B7H9)2Fe (4, 5 and 6). Crystallographic determinations showed that in 4, the two tricarbadecaboranyl ligands are linked by the ansa-CH2CH2- group at the C2 and C2′ cage carbons, whereas in 5 and 6 they are linked at their C2 and C4′ carbons, with the structures of 5 and 6 differing in the relative positions of the C4′ carbons in the two cages of each complex. The structural determinations also showed that, depending upon the linking position of the ansa-tether, constraints in cage-orientation, such as observed in 4, produce unfavorable intercage steric interactions. However, the cage fragments in these complexes can readily undergo a cage-carbon migration that moves one -carbon and its tether linkage to the more favorable 4-position. This isomerization reduces the cage steric interactions and produces configurations, such as those found for 5 and 6, where the iron cage bonding is enhanced as a result of the binding effect of the tether.