Effect of metal ions and length of alkylene bridge on the strength of hydrogen bonds in diiminedioxime cobalt(III) and nickel(II) complexes

Both octahedral dinitro and dichloro cobalt(III) complexes of 3,3′-(1,4 butanediyldinitrilo)bis(2-butanone)dioxime, complexes 1 and 2, respectively, crystallized in the orthorhombic system with space group P212121. Their average OO distances are 2.434(5) and 2.419(4) Å, respectively. The trans-dichloro[3,3′-(1,2-ethanediyldinitrilo)bis(2-butanonedioximato)cobalt(III)] complex 3 crystallized in the monoclinic system with space group P21/c. Its long OO distance of 2.708(2) Å indicates that no hydrogen bond between the two cis oximes occured. It is because the ethylene bridge between the two imine nitrogens imposes more constraints on the ligand than the butylene bridge does. The square planar nickel(II) complex of 3,3′-(1,4-butanediyldinitrilo)bis(2-butanone)dioximato, complex 4, crystallized in the triclinic system with space group P1. The average OO distance is 2.432(9) Å, which is comparable to that of the corresponding cobalt complexes. The νOHO vibration occurs in the region 1700–1800 cm−1 and varies inversely as the OO distance. The hydrogen bridge proton resonances for complexes 1, 2 and 4 appear downfield (18–20 ppm). For complex 3 no such resonance was observed.

Comparison of crystal structures for diiminedioxime (BnH2 and EnH2) complexes of Co(III) and Ni(II) indicates that introduction of a propylene bridge opposite the hydrogen bond imposes no significant constraint but the ethylene bridge impose significant constraint. Hence the OO distance of [Co(EnH)Cl2] is much longer than those for [Co(BnH)(NO3)2] and [Co(BnH)Cl2].Figure optionsDownload as PowerPoint slide