CRYSTAL STRUCTURE OF A NONAMERIC NICKEL(II) METALLACROWN COMPLEX, C₇₀H₈₈Cl₂N₂₀Ni₉O₃₀.10H₂O

Aarne Pajunen, Ilpo Mutikainen, Marjatta Orama and Heikki Saarinen

Division of Inorganic Chemistry, Department of Chemistry, University of Helsinki, Box 55, FIN-00014 UNIVERSITY OF HELSINKI, Finland, E-mail: aarne.pajunen@helsinki.fi

In the course of our structural and equilibrium studies on transition metal complexes formed by different oxime ligands, we have shown that pyridine-2-carboxaldehyde oxime compounds of the type

readily form polynuclear complexes which are potential model compounds in the study of redox and magnetic-exchange interactions of metal ions [1]. The best known examples are the trinuclear copper(II) complexes characterized by the presence of a highly stable trinuclear Cu₃OH central core both in the solid state as well as in aqueous solution [2].

With nickel(II), on the other hand, most of the complexes formed by ligands HL =1 and HL = 2 have been found to belong to the mononuclear series Ni(HL)_n²⁺ (n = 1,2,3) or are oximato members of the series derived from the stepwise dissociation of the oxime protons from the parent bis and tris complexes [3]. The 6-methyl group on the pyridine ring, however, seems to have a crucial effect on the polymerization reactions of nickel(II) and binuclear and trinuclear complexes of the composition Ni₂(HL)₃H_-3⁺ and Ni₃(HL)₃H_-5⁺ are readily formed as major species in aqueous solution when HL = 3 [4].

To facilitate interpretation of the coordination models in these polynuclear structures we have now isolated one of the complexes formed between nickel(II) and HL = 3 under aqueous condition where polymerization is known to occur in the solid state and analyzed it by X-ray diffraction methods.

The compound crystallizes in the space group P(-1) with a = 15.920(7), b = 16.673(8), c = 20.583(10) Å, a = 77.80(6), b = 81.00(6) , g = 72.60(4)^o, Z = 2. Full-matrix least-squares refinement of 12584 reflections collected by means of a Rigaku AFC7S diffractometer yielded R = 0.1084.

The crystalline species isolated as perchlorate from solution, where the trinuclear species -5,3,3 predominates proved to have a very complicated [Ni₉L₁₀(HO)₆(H₂O)₆]²⁺ complex moiety, which in our formulation corresponds to the notation of -16,9,10. This metallacrown [5] structure contains an octahedral NiO₆ central core and four different octahedral NiO₄N₂ and NiO₂N₄ environments. All the oxime groups are deprotonated and the nine nickel atoms are linked together via several bi- or trifurcated oximato and hydroxo bridges.

1. Orama, M. and Saarinen, H. Acta Chem Scand., Ser A 50 (1996) 1087-1091.
2. Orama, M. Saarinen, H. and Korvenranta, J. J. Coord. Chem. 22 (1990) 183-190
3. Orama, M., Saarinen, H. and Korvenranta, J., Acta Chem. Scand., Ser A 43 (1989) 407-412.
4. Saarinen, H. and Orama, M. Acta Chem Scand. 52 (1998) To be published.
5. Pecoraro, V. L., Stemmler, A. J., Gibney, B. R., Bodwin, J. J., Wang, H., Kampf, J. W. and Barwinski, A. Progress in Inorganic Chemistry, Vol 45 (1997) 83-177.