Studies on the metalamide bond. VII. Metal complexes of the flexible N₄ ligand N,N′-bis(2′-pyridinecarboxamide)1,2-ethane

The isolation and characterization of a series of transition metal complexes is described with the ligand bpenH₂ in both neutral and deprotonated forms. The ligand acts as a planar N₄ tetradentate in the monomeric deprotonated complexes of general forms M(bpen)nH₂O (M = Cu^II, Ni^II, Pd^II or Pt^II. Metal chloride and perchlorate salts of the neutral ligand, of general formulae M(bpenH₂)Cl₂nH₂O and M₂(bpenH₂)₃(ClO₄)₄nH₂O, are reported (M = Mn^II, Fe^II, Co^II, Ni^II, Cu^II or Zn¹¹) with the ligand bonding through pyridine-N and amine-O atoms in a bridging bis-bidentate function. In the chloride complexes high-spin six-coordinate geometries are indicated for the polymeric solid products. The perchlorate complexes are dimeric and high-spin, with each metal atom exhibiting N₃O₃ six-coordination. Proton NMR studies on the ligand show coupling of the amide protons with those of the central ethylene group. Deprotonation is confirmed by loss of this coupling for the Ni¹¹, Pd¹¹ and Pt¹¹ chelates, ¹H¹⁹⁵Pt coupling being observed in the latter compound The neutral and deprotonated chelate forms also are readily differentiated by thier IR absorption. Deuterium isotopic substitution of amide protons has been used to assist IR assignments.