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New β Cis Folded Organocobalt Derivatives with a Salen-Type Ligand

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posted on 2003-09-17, 00:00 authored by Renata Dreos, Giorgio Nardin, Lucio Randaccio, Patrizia Siega, Giovanni Tauzher, Višnja Vrdoljak
The reduction of [CoIII(tmsalen)py2]+ClO4-, where tmsalen = 4,4‘,7,7‘-tetramethylsalen, with NaBH4/PdCl2 in alkaline methanolic solution, followed by the oxidative addition of CH2ClI, leads to the expected trans organometallic dimeric species 1, [CH2ClCo(tmsalen)]2, provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the β cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy−iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi.

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