Structure and Magnetism of a Bimetallic Pentanuclear Cluster [(Ni(bpm)₂)₃(Fe(CN)₆)₂]·7H₂O (bpm = Bis(1-pyrazolyl)methane)). The Role of the Hydrogen-Bonded 7H₂O “Cluster” in Long-Range Magnetic Ordering

Abstract

The synthesis and structural and magnetic properties of the pentanuclear complex [(Ni(bpm)2)3(Fe(CN)6)2]·7H2O (1) are reported. Complex 1 crystallizes in the tetragonal space group P43212 (No. 96) with the following cell parameters at 23(1) °C: a = b = 12.888(2) Å, c = 42.927(2) Å, V = 7130(3) Å3, and Z = 4. The structure was refined using 3197 unique reflections with I > 3σ(I) to give R = 0.047 and Rw = 0.038. The neutral [(Ni(bpm)2)3(Fe(CN)6)2] clusters contain two unconnected FeIII(CN)63- groups each bridged, via three facial CN groups, to three cis-Ni(bpm)2 moieties, thus making the cis-(CN)2NiII(bpm)2 centers six-coordinate. Metal···metal distances within the cluster are Fe···Ni = 5.042(3) and 5.058(2) Å, Fe···Fe = 6.438(4) Å, and Ni···Ni = 6.654(3) and 7.028(5) Å. The seven water molecules form an intriguing hydrogen-bonded “cluster” which connects, via hydrogen-bonding, to the other facial CN ligands on each Fe. A wide array of magnetic susceptibility and magnetization measurements has been used to show that the Ni3Fe2 clusters display intramolecular ferromagnetic coupling. Most importantly, the crystalline samples of 1 and most, but not all, of rapidly precipitated powder samples display long-range magnetic ordering with Tc = 23 K. The powder samples which do not display magnetic order yield X-ray diffraction patterns, IR spectra, and thermogravimetric behavior identical to those of samples that display magnetic order. Experimental attempts to identify the nature of the magnetic phase transition in the ordered samples of 1 lead to ferromagnetic ordering being favored over spin-glass formation. The 7H2O hydrogen-bonded moiety plays a key role in intercluster exchange interactions. Dehydrated samples of 1 are not ordered. Intracluster exchange interactions are deduced for the dehydrated form.