Piano-stool iron(II) complexes as probes for the bonding of N-heterocyclic carbenes:: Indications for π-acceptor ability

A series of new piano-stool iron(II) complexes comprising mono- and bidentate chelating N-heterocyclic carbene ligands [Fe(cp)(CO)(NHC)(L)]X have been prepared and analyzed by spectroscopic, electrochemical, crystallographic, and theoretical methods. Selectively substituting the L site with a series of ligands going from carbene to pyridine to CO suggests that CO is the strongest pi acceptor, while the behavior of pyridine and carbene is nearly identical. This suggests that in these complexes comprising an electron-rich iron(cp)( carbene) fragment, N-heterocyclic carbenes are not pure sigma donors but also moderate pi acceptors. Theoretical calculations support this bonding model and indicate charge saturation at the metal as key for pi back-bonding to N-heterocyclic carbenes. On the basis of voltammetric measurements, the Lever electrochemical parameter of these carbenes has been determined: E-L = + 0.29. Systematic substitution of the wingtip groups of the carbene revealed only subtle changes in the electronic properties of the iron center, thus providing a suitable methodology for ligand-induced fine-tuning of the coordinated metal.