A Novel Series of CoIII(salen-type) Complexes Containing a Seven-Membered Metallacycle: Synthesis, Structural Characterization and Factors Affecting the Metallacyclization Rate

A series of electronically tuned trans-[CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2,2′-ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis β organometallic derivative containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the −C═N– groups of the chelate. The X-ray structures of cyclized complexes, [CoIII(chelCH2)(py)(H2O)]+, show a dependence of the conformation of the seven-membered metallacycle on the different positions of substituents in the chelate. In fact, in the complex having methyls on the −C═N– groups, the conformation is characterized by having the methylene carbon atom significantly displaced (ca. 1.26 Å) from the aromatic ring plane, whereas in the complex lacking methyl groups in those positions, the atoms of the Ph–O–CH2 fragment are coplanar. The standard Gibbs energies obtained by quantum chemical calculation reveal that the different conformations observed in the solid state are mainly the result of the energetically unfavorable setup of the methyls on the −C═N– groups and of the energetically favorable displacement of the CH2 group out of plane of the aromatic ring. 1H NMR data suggest that the different conformations of the metallacycle are, at least partially, retained in solution.