Variations in the molecularity of bis(formylpyrrolyl) cobalt(II) complexes

Abstract This work aims to extend the coordination chemistry of Co(II) with bis(2-formylpyrrolyl) ligands to different donor molecules. Treatment of a mixture of anhydrous CoCl 2 and excess of pyridine in THF with two equivalents of sodium 2-formylpyrrolyl gives rise to complex [Co{κ 2 N , O -NC 4 H 3 -C(H) O} 2 (Py) 2 ] ( 1 ) in good yields, after standard work-up procedures. By contrast, the reaction of two equivalents of sodium 2-formylpyrrolyl with anhydrous CoCl 2 in neat THF or MeCN solutions systematically led, after standard work-up, to the identification of the tetrameric complex [Co{κ 2 N , O -NC 4 H 3 -C(H) O} 2 ] 4 ( 2 ) and other unknown contaminations. Analytically pure samples of complex 2 were only prepared when the synthesis reaction was performed in toluene at 90 °C. Complexes 1 and 2 are paramagnetic and were characterized by elemental analysis, magnetic susceptibility measurements (Evans method) and X-ray diffraction. Complex 1 is a hexacoordinated octahedral complex with a low-spin ( S  = 1/2) electronic configuration. On the other hand, the tetrameric complex 2 results from the self-assembling of the elusive and highly unsaturated complex “[Co{κ 2 N , O -NC 4 H 3 -C(H) O} 2 ]” giving rise to a tetranuclear distorted cubane structure, comprising two hexacoordinated octahedral and two pentacoordinated trigonal bipyramidal Co(II) centers. Complex 2 revealed an overall high-spin ( S  = 3/2 per Co center) nature, as suggested in solution by the Evans method and in the solid state by SQUID magnetometry.