Synthesis, structure and magnetic behavior of five-coordinate bis(iminopyrrolyl) complexes of cobalt(II) containing PMe3 and THF ligands

The new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(kappa N-2,N'-NC4H3C(R)=N-2,6-(Pr2C6H3)-Pr-i)(2)(PMe3)] (R = H 3a; Me 3b) were synthesized in high yields (ca. 80-90%), using THF and diethyl ether as solvents, respectively, by (a) treatment of CoCl2(PMe3)(2) with the corresponding iminopyrrolyl Na salts (Ie or If) or (b) reaction of anhydrous CoCl2 and PMe3 with Ie or If. A third route was tested, involving the addition of excesses of PMe3 to the complexes [Co(kappa N-2,N'-NC4H3C(R)=N-2,6-(Pr2C6H3)-Pr-i)(2)] (R = H 1e; Me 1f), which was only successful for the synthesis of 3a, in lower yields (ca. 30%). The synthesis of 3b in THF was unfruitful because of the kinetic competition of the solvent, giving rise to mixtures of 1f and its coordinated THF adduct 4b. The synthesis of the new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(kappa N-2,N'-NC4H3C(R)=N-2,6-(Pr2C6H3)-Pr-i)(2)(THF)] (R = H 4a; Me 4b) were carried out in high yields (ca. 80-90%) by the reaction of COCl2(THF)(1.5) with the corresponding iminopyrrolyl Na salt. All the compounds have been characterized by X-ray diffraction, with 3a and 3b showing axially compressed trigonal bipyramidal geometry (with the PMe3 ligand lying on the equatorial plane), whereas complexes 4a and 4b exhibit distorted square pyramidal geometries with the THF molecule occupying the axial position. Complex 4a shows clearly a compressed geometry, but for complex 4b, two polymorphs were characterized, exhibiting molecules with different Co-O (THF) bond lengths, one of them being compatible with an elongated form. Magnetic measurements either in the solid or in the liquid phases indicate that complexes 3a and 3b have low-spin ground states (S = 1/2). In toluene solution, the geometry is fully confirmed by EPR data, which further indicates a d(x)(-y)(/)(2)(2)d(xy), ground state. However, compounds 4a and 4b behave unusually because they show magnetic moments that are compatible with high-spin ground states (S = 3/2) in the solid state, but conform to low-spin ground states (S = 1/2) when both complexes are dissolved in toluene solutions. The low-spin ground states in toluene solution are confirmed by EPR spectroscopy, which further supports, for complexes 4a and 4b, an axially elongated square pyramidal geometry and a d(z)(2) ground state. Thus the change in the ground-state and, consequently, in the geometry of complexes 4a and 4b from solid state to toluene solution might be a consequence of the elongation of the Co-O(THF) bond length. DFT studies performed on complexes 3 and 4 corroborate their different structure and magnetic behaviors and verify, for the latter complexes, the structural differences observed in the solid state and in toluene solution.