Tuning the metal–metal separation in pyrazolate-based dinuclear complexes by the length of chelating side arms‡

Abstract

Six dinuclear cobalt(II) complexes of pyrazolate ligands with multidentate chelating side arms {3,5-(R₂NCH₂)₂C₃N₂H₂; R₂N = [Me₂N(CH₂)₃]₂N (HL¹) or (Et₂NCH₂CH₂)₂N (HL²)} have been prepared and characterised. The reaction of L¹ and L² with 2 equivalents of [Co(MeCN)₆][BF₄]₂ and NaBPh₄ proceeds via the isolable compounds [Co₂L¹(BF₄)][BPh₄]₂ 1a and [Co₂L²(BF₄)][BPh₄]₂ 2a finally to afford the dinuclear complexes [Co₂L¹F][BPh₄]₂ 1b and [Co₂L²(F)(H₂O)][BPh₄]₂ 2b, respectively, where a fluoride has been abstracted from the BF₄^– starting material in both cases. While the longer side arms in 1b allow for an exogenous bridging position of the fluoride, an additional water molecule is incorporated in 2b to form an FHO(H) moiety with an unusually short F–H–O bridge between the two cobalt centres. Evidence is reported for the reversible extrusion of the water molecule from 2b. Treatment of 2b with KBr or NaN₃ yielded the respective bromide- and azide-bridged complexes 3 and 4, where in the latter case the azide adopts a µ-1,3-bridging mode. The magnetic properties of 3 and 4 have been studied in the temperature range 8–200 K. Complexes 1b, 2b, 3 and 4 were also characterised by means of X-ray crystallography.