Second-sphere coordination of 5,5′-diamino-2,2′-bipyridine metal complexes with oxygen ligands through N-H⋯O hydrogen bonding

Tris(5,5′-diamino-2,2′-bipyridine)metal complexes, [M(DABP)₃]²⁺ with M=nickel(II), zinc(II) or iron(II) can be engaged in hydrogen-bonding interactions from the amino groups to oxygen-containing molecules as H-bond acceptors. This so-called second-sphere coordination of the metal complexes is investigated here by using 18-crown-6 (18-C-6) and the 1,3,5-benzene-tricarboxylate (BTC) anion for N-H⋯O bonding. Single-crystal X-ray structural studies show that [Ni(DABP)₃]²⁺ or [Zn(DABP)₃]²⁺ and 18-C-6 form strands of alternating [M(DABP)₃]²⁺ and 18-C-6 units with only two out of six amino groups getting involved in weak N-H⋯O bonding. The remaining NH₂ groups are hydrogen bonded to the chloride and nitrate anion, respectively, and solvent molecules of crystallization. The anionic BTC hydrogen-bond acceptor, however, is capable of capping the triangular face formed by the three amino groups in [M(DABP)₃]²⁺. In {[Fe(DABP)₃]·(BTC)₂}^4- the capping occurs by two BTC units from both sides, i.e. for both triangular faces and engages all six NH₂ groups of the metal complex. Charge compensation is provided by two hydrated {[Fe(DABP)₃]·aq}²⁺ complexes per formula unit.