d-d Dative Bonding Between Iron and the Alkaline-Earth Metals Calcium, Strontium, and Barium

Abstract

Double deprotonation of the diamine 1,1 '-(tBuCH(2)NH)-ferrocene (1-H-2) by alkaline-earth (Ae) or Eu(II)metal reagents gave the complexes1-Ae (Ae=Mg, Ca, Sr, Ba) and1-Eu.1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe...Mg distance in1-Mg is too long for a bonding interaction, but short Fe...Ae distances in1-Ca,1-Sr, and1-Ba clearly support intramolecular Fe...Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related(1)H NMR chemical-shift difference between the Cp alpha and beta protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe -> Ae dative bonding. A comprehensive bonding analysis of all1-Ae complexes shows that the heavier species1-Ca,1-Sr, and1-Ba possess genuine Fe -> Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In1-Mg, a weak Fe -> Mg donation into vacant p-orbitals of the Mg atom is observed.