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

Abstract

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 NH 2 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 NH 2 groups of the metal complex. Charge compensation is provided by two hydrated {[Fe(DABP) 3]·aq} 2+ complexes per formula unit.