Crystal structure of a flavin–metal complex, bis(10-methylisoalloxazine)lead(II) perchlorate tetrahydrate

Abstract

The title complex has been prepared and characterized by X-ray techniques. The yellow crystals are monoclinic, space group P2₁/c, with lattice constants a= 6·987(2), b= 13·806(7), c= 17·012(5)Å, β= 105·69(3)°, and Z= 4 for 1/2[Pb(ClO₄)₂]C₁₁H₈N₄O₂,2H₂O. The structure was solved by the heavy-atom method and refined by least-squares to R 8·9%, for 878 counter-measured reflections. The lead ions are disordered in two sites 0·75 Å above and below a plane of four oxygens, two of which are the centrosymmetrically related carbonyl oxygens O(2) of the ligand, [Pb–O 2·46(2) and 2·63(2)Å], the other two being water oxygens [Pb–O 2·38(2) and 2·69(2)Å]. The next nearest atoms to Pb are N(1) of the same flavin at 2·99(2)Å, and another water molecule at 3·12(2)Å. This is the first structure with a metal in the secondary chelate site of the flavin molecule but none in the primary site. Co-ordination of Pb²⁺ to oxygen rather than nitrogen is consistent with its intermediate character on the Pearson hard–soft scale.