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.