Tetranuclear grid-like copper(II) complexes with pyrazolate bridges: syntheses, structures, magnetic and EPR spectroscopic properties

Abstract

Reaction of 3-(2-pyridyl)pyrazole (HL¹) and 6-(3-pyrazolyl)-2,2′-bipyridine (HL²) with nickel(II) and zinc(II) salts afforded the simple mononuclear pseudo-octahedral complexes [M(HL¹)₃][PF₆]₂ and [M(HL²)₂][PF₆]₂ respectively (M = Ni or Zn) in which the ligands co-ordinate as neutral mononucleating chelates in the same manner as e.g. 2,2′-bipyridine or 2,2′∶6′,2″-terpyridine respectively. However with Cu^II the complexes [Cu₄(L¹)₆(solv)₂][PF₆]₂ (solv = dmf or MeOH) and [Cu₄(L²)₄(dmf)₄][PF₆]₄ were isolated and crystallographically characterised, in all cases containing four tetragonally elongated square-pyramidal copper(II) ions which are linked by pyrazolate bridges from the now deprotonated ligands L¹ and L². The approximate orthogonality of the different ligands within each complex and the approximately square array of metal ions result in a grid-like structure. In [Cu₄(L¹)₆(solv)₂][PF₆]₂ there are successively two, one, two and one pyrazolate bridges between adjacent copper(II) ions around the Cu₄ square resulting in two clearly different magnetic coupling pathways; in [Cu₄(L²)₄(dmf)₄][PF₆]₄ however, which has approximate S₄ symmetry, each Cu· · ·Cu edge has a single pyrazolate bridge and the coupling pathways are all virtually equivalent. Prolonged drying of these compounds resulted in loss of the axial dmf ligands to give [Cu₄(L¹)₆][PF₆]₂ and [Cu₄(L²)₄][PF₆]₄. Magnetic susceptibility studies on these showed the presence of two antiferromagnetic exchange pathways for [Cu₄(L¹)₆][PF₆]₂ with J > 172 cm^–1 and J′ < 155 cm^–1 (strong correlation between the parameters precludes a more precise determination), but only one antiferromagnetic exchange pathway for [Cu₄(L²)₄][PF₆]₄ with J = 63.5 cm^–1, consistent with the crystal structures of the dmf adducts. The EPR spectra of [Cu₄(L¹)₆][PF₆]₂ and [Cu₄(L²)₄][PF₆]₄ at a variety of frequencies and temperatures can be well simulated as arising from triplet species; however the spectrum of [Cu₄(L¹)₆][PF₆]₂ also contains a feature which may be ascribed to the expected thermally populated quintet state.