Metal-ion controlled reactions of 2,6-diacetylpyridine with 1,2-di-aminoethane and 2,6-diformylpyridine with o-phenylenediamine and the crystal and molecular structure of a pentagonal pyramidal cadmium(II) complex containing unidentate o-phenylenediamine

Abstract

Reaction of 2,6-diacetylpyridine (dap) with an excess of 1,2-diaminoethane (en) in the presence of Ba[ClO₄]₂ in MeOH or EtOH at room temperature yields the complex [BaL²(ClO₄)₂]·H₂O where the ligand L² is the open-chain condensate of two molecules of dap with three molecules of en. The i.r. spectrum of the solid complex indicates that L² exists in the di-imine form containing two imidazolidine rings derived from the nucleophilic addition of the two primary amine groups of the tetra-imine form across adjacent CN linkages. The use of higher reaction temperatures affords the complex [BaL¹(ClO₄)₂] of the 18-membered macrocycle L¹ derived from the [2 + 2] condensation of two molecules of dap with two molecules of en. The intermediacy of the co-ordinated open-chain ligand L² in the L¹ macrocycle formation is evidenced by the subsequent ring closure of [BaL²(ClO₄)₂]·H₂O in the absence of added diketone; a transamination mechanism is proposed. Corresponding reactions of 2,6-diformylpyridine (dfp) with o-phenylenediamine (opd) in the presence of Ba²⁺ give complexes of the 18-membered macrocycle with the isolation of any intermediate. The single Ba²⁺ in [BaL²(ClO₄)₂]·H₂O may be replaced by two Cu²⁺ with isomerization of L² to the tetra-imine form, L³. The isomerization is reversed on treatment of Cu₂L³(ClO₄)₄·2H₂O with NCS^–. The Ba²⁺in the macrocyclic complex [BaL¹(ClO₄)₂] may also be replaced by two Cu²⁺ to yield Cu₂L¹(OH)(ClO₄)₃·H₂O in which the two Cu²⁺ ions are intramolecularly bridged, and antiferromagnetically coupled, via the hydroxide ion. The use of Cd[ClO₄]₂·6H₂O as template affords the complexes [CdL⁵(en)][ClO₄]₂ and [CdL⁶(opd)][ClO₄]₂ where L⁵ is the open-chain ligand derived from the condensation of one molecule of dap with two molecules of en, and L⁶ is the corresponding ligand derived from dfp and opd. Crystals of [CdL⁶(opd)][ClO₄]₂ are triclinic with a= 9.117(9), b= 10.797(8), c= 16.084(11)Å, α= 97.89(8), β= 81.43(9), γ= 70.70(10)°, Z= 2, and space group P. 2 560 Reflections above background were measured by a diffractometer and refined by full-matrix least-squares to R 0.065. The co-ordination geometry of the Cd atom is approximately pentagonal pyramidal, being bonded to the five nitrogen atoms (Cd–N 2.35–2.46 Å) of L⁶ in the pentagonal plane and to one NH₂ group [Cd–N 2.341(9)Å] of the opd molecule in one axial position, the metal lying 0.42 Å above the equatorial N₅ plane. The other NH₂ group of opd is not co-ordinated. An incomplete X-ray analysis of [CdL⁵(en)][ClO₄]₂ indicates a seven-co-ordinate polymeric structure in which the en molecules bridge adjacent Cd²⁺ ions. Treatment of CdL⁶(opd)(ClO₄)₂·6H₂O with Ni²⁺ in the presence of O₂ yields the complex[Ni(L⁸)₂][ClO₄]₂·H₂O the properties of which suggest that the terminal NH₂ groups of L⁶ have added across the adjacent CN bonds, this being followed by oxidative dehydrogenation to generate 2,6-di(2-benzimidazolyl) pyridine.