Synthesis and Catalysis of Redox-Active Bis(imino)acenaphthene (BIAN) Iron Complexes

Reactions of various substituted bis(imino)acenaphthenes (R-BIANs) with FeCl2(thf)1.5 afforded the tetrahedral complexes (R-BIAN)FeCl2 (2) from bulky α-diimines and the octahedral complexes [Fe(R-BIAN)3][FeCl4]2 (3) from less bulky ligands. The driving force for the formation of complexes 3 is the high ligand-field stabilization of the low-spin FeII center. The two sets of complexes exhibit distinct charge-transfer band intensities and redox activities. (R-BIAN)FeCl2 complexes showed reversible ligand-centered reductions at −0.9 V (vs. FcH/FcH+; FcH: ferrocene); further reduction led to decomposition. Irreversible oxidations were observed at 0.2 and 0.4 V, associated with a reduction at −0.4 V, as well as a ligand-centered redox event at 1.0 V. First applications of the Fe(BIAN) complexes to hydrogenations of alkenes indicated good catalytic activity under mild conditions.