Coordination modes of 3-hydroxypicolinic acid: synthesis and crystal structures of palladium(II), platinum(II) and rhenium(V) complexes

Abstract

The new palladium and platinum complexes with 3-hydroxypicolinic acid (HpicOH) [M(PPh₃)₂Cl(picOH)]·CHCl₃, [M(bipy)(picOH)]Cl [M=Pd(II) or Pt(II)], K[PdCl(picOH)₂] and [Pt(picOH)₂] and the new rhenium complexes [ReOI₂(PPh₃)(picOH)] and [ReO(PPh₃)(picOH)₂]I have been prepared. The crystal structures of [M(PPh₃)₂Cl(picOH)]·CHCl₃ [M=Pd(II) 1 or Pt(II) 2] and [ReOI₂(PPh₃)(picOH)] 3 were determined by X-ray diffraction. Complex 3 exhibits a distorted octahedral geometry with the picOH⁻ ligand showing N,O-chelation with a small bite angle O–Re–N of 74.8(3)°. In complexes 1 and 2 the metal centre is surrounded by a NP₂Cl donor set in a distorted square planar arrangement. Therefore, the picOH⁻ ligand is bound through the nitrogen atom, but the distances found between the metal and the carboxylate oxygen [Pd···O(71) 2.773(5) Å or Pt···O(71) 2.734(4) Å] suggest a [4+1] coordination consistent with N,O-chelation for palladium and platinum centres. Infrared, Raman, ¹H and ¹³C-{¹H} NMR spectroscopic data for the complexes are consistent with the crystallographic results. In the solid state the complex units of 1 and 3 are aggregated in centrosymmetric dimers based on C–H···Cl or C–H···O hydrogen bonding interactions between the chlorine of the Pd–Cl bond (in 1) or the phenolic oxygen of the picOH⁻ anion (in 3) and a hydrogen atom of a phenyl group of a PPh₃ ligand.