The role of diisopropanolamine (dipaH3) in cluster dimerisation and polymerisation: from spin frustrated S = 5 FeIII6 clusters to the novel 1-D covalent polymer of mixed valence [CoII3CoIII] tetramers

Abstract

The synthesis, crystal structures and magnetic properties of two hexanuclear Fe₆ clusters of general formula [Fe₆(O)₂(OH)₂(O₂CR)₁₀(dipaH₂)₂]·xMeCN.yH₂O (R = Ph, x = 5.5, y = 1 (1), R = C(Me)₃, x = 2, y = 3 (2)) are reported. The presence of the flexible amino-alcohol ligand diisopropanolamine (dipaH₃) induces the dimerisation of two trinuclear Fe^III complexes, [Fe₃O(O₂CPh)₆(H₂O)₃](NO₃) and [Fe₃O(O₂CC(Me)₃)₆(H₂O)₃](O₂CC(Me)₃), to form the hexanuclear clusters 1 and 2. DC magnetic susceptibility measurements on 1 and 2 assign ground spin states of S = 5, with zero-field splitting parameters (D) of ca. 0.25 cm⁻¹ obtained from magnetisation isotherms. AC susceptibilities showed no maxima as a function of frequency, at low temperatures, and this confirmed the lack of single-molecule magnetic behaviour. Clusters 1 and 2 are isostructural, consisting of two fused {Fe₃O} trinuclear units, bridged in two positions by one μ₂-OH⁻ unit and two μ₂-O₂CR⁻ bridging carboxylates (R = Ph (1), C(Me)₃(2)). The two singly deprotonated dipaH₂⁻ bridging ligands span the Fe1–Fe2 edges in 1 and 2via one μ₂-bridging alcohol arm and one terminal nitrogen atom while the second alcohol arm remains free. The ground spin state of S = 5 in 1 and 2 can be attributed to the presence of spin frustration within the system. 1 and 2 join a small family of spin frustrated S = 5 Fe₆ systems the magnetism of which give weight to a recent report that it is the trans position of the two shortest Fe₂ pair frustrated exchange pathways in these Fe₆ clusters that gives rise to a ground spin state of S = 5 (trans) and not a ground spin state of S = 0 (cis). The Mössbauer spectra of 1 and 2 show two quadrupole doublets, as expected, at 295 K, but a broad asymmetric lineshape at 77 K. The synthesis and magnetic properties of {[Co^II₃Co^III(OH)(O₂CC(Me)₃)₄(HO₂CC(Me)₃)₂(dipaH)₂]·2MeCN}_n (3) are reported. 3 is a covalently bonded 1D polymer of tetrameric cobalt clusters. The presence of the dipaH₃ ligand here not only dimerises the [Co^II₂(μ-H₂O)(O₂CC(Me)₃)₄(HO₂CC(Me)₃)₄] starting complex into the tetranuclear species but also polymerises the [Co^II₃Co^III] clusters in 3 by acting as the propagating ligand in the 1D chain. Magnetic susceptibility measurements on 3 show each [Co₄] moiety exhibits weak antiferromagnetic coupling between the three Co^IIS = 3/2 metal centres and fitted J values are given. The ambiguity in assignment of the spin ground state of S = 1/2 or 3/2 is discussed.