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

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 CuII 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.