Iron-Promoted Nucleophilic Additions to Diimine-Type Ligands: A Synthetic and Structural Study

We report here three examples of the reactivity of protic nucleophiles with diimine-type ligands in the presence of
FeII salts. In the first case, the iron-promoted alcoholysis reaction of one nitrile group of the ligand 2,3-dicyano-5,6-bis(2-pyridyl)-pyrazine (L1) permitted the isolation of an stable E-imido−ester, [Fe(L1‘)2](CF3SO3)2 (1), which
has been characterized by spectroscopic studies (IR, ES-MS, Mössbauer), elemental analysis, and crystallographically.
Compound 1 consists of mononuclear octahedrally coordinated FeII complexes where the FeII ion is in its low-spin
state. The iron-mediated nucleophilic attack of water to the asymmetric ligand 2,3-bis(2-pyridyl)pyrido[3,4-b]pyrazine
(L2) has also been studied. In this context, the crystal structures of two hydration−oxidation FeIII products,
[Fe(L2‘)2](ClO4)3·3CH3CN (2) and trans-[FeL2‘‘Cl2] (3), are described. Compounds 2 and 3 are both mononuclear
FeIII complexes where the metals occupy octahedral positions. In principle, L2 is expected to coordinate to metal
ions through its bipyridine-type units to form a five-membered ring; however, this is not the case in compounds 2
and 3. In 2, the ligand coordinates through its pyridines and through the hydroxyl group attached to the pyrazine
imino carbon after hydration, that is, in an N,O,N tridentate manner. In compound 3, the ligand has suffered further
transformations leading to a very stable diamido complex. In this case, the metal ion achieves its octahedral
geometry by means of two pyridines, two amido N atoms, and two axial chlorine atoms. Magnetic susceptibility
measurements confirmed the spin state of these two FeIII species:  compounds 2 and 3 are low-spin and high-spin,
respectively.