Synthesis and characterization of oligometallic and polymetallic transition metal azolates
Creators: name: Summers, David Alexander
Publisher: name: University of British Columbia
Year: 2009

description: Several divalent transition metal azolate compounds have been prepared and characterized. Amongst these is a series of (substituted)pyrazolate transition metal polymers of general formulation [M(pz*)₂][sub x] (M = Ni (seven compounds), M = Mn (four compounds); and pz* = pyrazolate and C-substituted pyrazolates). Two manganese(II) polymeric materials with slightly different formulation, [Mn(4-Xpz)₂(4-XpzH)][sub x] (X = CI, Br), were also prepared. Indirect evidence suggests that these materials consist of linear chains of metal(II) ions doubly bridged by pyrazolate ligands. Four of the nickel(II) complexes and all of the manganese(II) compounds are paramagnetic and their magnetic susceptibilities were measured over the 2 to 300 K range. In each case the materials exhibit antiferromagnetic exchange coupling with J values ranging from -14 to -16 cm⁻¹ for the nickel compounds, -1.2 to -2.1 cm⁻¹ for the [Mn(pz*)₂][sub x] materials, and a/value of -0.4 cm⁻¹ for the two [Mn(4-Xpz)₂(4-XpzH)][sub x] complexes. In addition to the polymeric complexes, a number of oligometallic transition metal (substituted)pyrazolate compounds were prepared and characterized. Six dimetallic nickel(II) complexes of the general formula [CpNi(pz*)]₂(where Cp is the cyclopentadienyl ion and pz* = C-substituted pyrazolates), and four trimetallic nickel(II) materials of formulation [CpNi(4-X-3,5-diMepz)₂]₂Ni (X = H, CH₃, CI, and Br), were prepared and examined. Three of the dimetallic and two of the trimetallic compounds have been characterized by single crystal X-ray diffraction which revealed them to consist of nickel(II) ions doubly bridged by pyrazolate ligands and end-capped with Cp groups. A green, mixed valence (Cu(II)/Cu(I)), trimetallic copper compound of formulation [Cu₃(3,5-F₆diMepz)₅] (3.5-F₆diMepz = bis(trifluoromethyl)pyrazolate) was also prepared and examined. A single crystal X-ray diffraction study revealed that this complex consists of a triangular ring of copper ions bridged by the 3,5-F₆diMepz groups. The two copper(II) ions in this complex are connected by three pyrazolate bridges and one pyrazolate-Cu(I)-pyrazolate bridge. Variable temperature magnetic measurements (2 to 300 K) revealed strong antiferromagnetic coupling between the two Cu(II) ions ( J =-235 cm⁻¹). A three dimensional iron(II) imidazole/imidazolate polymer was prepared and characterized. A single crystal X-ray diffraction study revealed that this material consists of linear chains of tetrahedral Fe(II) ions singly-bridged by imidazolate groups. The individual chains of tetrahedral iron centers are cross-linked to six parallel chains via octahedral iron(II) centers. Neutral imidazole groups occupy two (trans) coordination sites on the octahedral centers. Variable temperature magnetic measurements revealed antiferromagnetic exchange along the tetrahedral chains (J = -2.4 cm⁻¹). A canted spin structure leads to weak ferromagnetism at temperatures below a magnetic phase transition temperature of 17 K. Two transition metal complexes containing triazolate, [M(trz)₂][sub x] (M = Cu(II), Mn(II)), were also prepared. Variable temperature magnetic studies revealed that both of these materials exhibit antiferromagnetic exchange. The copper complex exhibits longrange magnetic ordering and becomes a weak ferromagnet below a magnetic phase transition temperature of 30 K. This magnetic ordering is thought to be a result of a canted spin structure.