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(rr) 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 [Cu-4(L-1)(6)(solv)(2)][PF6](2) (solv = dmf or MeOH) and [Cu-4(L-2)(4)(dmf)(4)][PF6](4) were isolated and crystallographically characterised: in all cases containing four tetragonally elongated square-pyramidal copper(rr) ions which are linked by pyrazolate bridges from the now deprotonated ligands L-1 and L-2. The approximate orthogonality of the different ligands within each complex and the approximately square array or metal ions result in a grid-like structure. In [Cu-4(L-1)(6)(solv)(2)][PF6](2) there are successively two, one, two and one pyrazolate bridges between adjacent copper(rr) ions around the Cu, square resulting in two clearly different magnetic coupling pathways; in [Cu-4(L-2)(4)(dmf)(4)][PF6](4) however, which has approximate S-4 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 [Cu-4(L-1)(6)][PF6](2) and [Cu-4(L-2)(4)][PF6](4). Magnetic susceptibility studies on these showed the presence of two antiferromagnetic exchange pathways for [Cu-4(L-1)(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 [Cu-4(L-2)(4)][PF6](4) with J = 63.5 cm(-1), consistent with the crystal structures of the dmf adducts. The EPR spectra of [Cu-4(L-1)(6)][PF6](2) and [Cu-4(L-2)(4)][PF6](4) at a variety of frequencies and temperatures can be well simulated as arising from triplet species; however the spectrum of [Cu-4(L-1)(6)][PF6](2) also contains a feature which may be ascribed to the expected thermally populated quintet state.