Magnetic properties of anhydrous and hydrated dimethylphosphinates of manganese(II). The crystal and molecular structure of poly-bis(μ-dimethylphosphinato)diaquomanganese(II)

Abstract

The dimethylphosphinate of manganese(II), Mn[(CH3)2PO2]2, and its dihydrate have been prepared and studied using magnetic susceptibility, differential scanning calorimetry, and electronic and vibrational spectroscopic methods. The dihydrate was obtained in crystalline form and a single crystal X-ray diffraction study revealed a polymeric structure.Crystals of poly-bis(μ-dimethylphosphinato)diaquomanganese(II) are monoclinic, a = 20.722(3), b = 4.8652(2), c = 11.0689(14) Å, β = 102.209(7)°, Z = 4, space group C2/c. The structure was solved by conventional heavy-atom methods and was refined by full-matrix least-squares procedures to R = 0.030 and Rw = 0.033 for 983 reflections with I ≥ 3σ(I). The structure consists of infinite centrosymmetric chains of Mn(II) atoms linked by double phosphinate bridges and extending along the crystallographic b axis. The water molecules are involved in both interchain and bifurcated intrachain hydrogen bonding [Formula: see text], 2.899(3) and 3.120(3) Å). The coordination about Mn is slightly distorted octahedral with libration-corrected bond lengths Mn—O(phosphinato) = 2.156(2) and 2.212(2), Mn—OH2 = 2.247(2) Å.Magnetic susceptibility studies on the dihydrate from 300 to 4.2 K reveal a magnetic moment of ~5.9 BM over most of the range and give no evidence for significant magnetic exchange. The anhydrous compound, which is considered on the basis of indirect evidence to retain the double phosphinate bridged structure exhibited by the dihydrate, shows relatively strong antiferromagnetic behaviour. The data have been analyzed according to two theoretical models both of which employ the isotropic Heinsenberg Hamiltonian. The scaling model of Wagner and Friedberg gives J = −2.94 cm−1 and g = 2.02 and the interpolation scheme of Weng gives J = −2.69 cm−1 and g = 2.01. The magnitude of the exchange coupling is considered in relation to that observed in related manganese compounds and possible reasons for the observed damping of the exchange on hydration are discussed.