Electronic interactions in (eta(6)-arene) ferracarboranes

The electronic structures of a series of di- and trinuclear ferracarboranes that incorporate FeC2B4 cluster units connected by aryl, heterocyclic, or alkynyl bridges or via direct intercluster B−B bonding have been explored by cyclic voltammetry, controlled-potential coulometry, and DFT and PM3 molecular orbital calculations. The data reflect the strong electron-donating properties of the Et2C2B4H42- carborane ligand; thus, in the ferrocenyl derivative (η6-C6H6)Fe(Et2C2B4H3-5-Fc) there is a substantial flow of electron density from the ferracarborane cluster to the ferrocenyl moiety. In all complexes examined, one or more reversible oxidations of FeII to FeIII centers were observed, with varying degrees of separation of successive E° values, indicating a range of electron-delocalization behavior. In many of the observed mixed-valent FeII−FeIII species the evidence suggests little or no metal−metal communication, paralleling earlier studies of small arene-ferracarboranes; however, significant delocalization is evidently present in the thiophene-bridged cation 2,5-[(η6-C6H6)Fe(Et2C2B4H3-5-)]2SC4H2+ (10+). Although the asymmetric dimer (η6-C6H6)Fe(Et2C2B4H3-5-)(η6-C6H5)Fe(Et2C2B4H4)+ (4+) exhibits a large (0.56 V) separation between oxidation processes, implying that 4+ is a fully delocalized Robin−Day class III system, detailed MO calculations on the latter species show that this ΔE° value results from inequivalence in the electronic environments of the two iron centers, rather than from electron delocalization between them.