STRUCTURE AND MAGNETISM OF A BIMETALLIC PENTANUCLEAR CLUSTER [(NI(BPM)(2))(3)(FE(CN)(6))(2)]CENTER-DOT-7H(2)O (BPM = BIS(1-PYRAZOLYL)METHANE) - THE ROLE OF THE HYDROGEN-BONDED 7H(2)O CLUSTER IN LONG-RANGE MAGNETIC-ORDERING

The synthesis and structural and magnetic properties of the pentanucle ar complex [(Ni(bpm)(2))(3)(Fe(CN)(6))(2)].7H(2)O (1) are reported. Co mplex 1 crystallizes in the tetragonal space group P4(3)2(1)2 (No. 96) with the following cell parameters at 23(1) degrees C: a = b = 12.888 (2) Angstrom, c = 42.927(2) Angstrom, V = 7130(3) Angstrom(3), and Z = 4. The structure was refined using 3197 unique reflections with I > 3 sigma(I) to give R = 0.047 and R-w = 0.038. The neutral [(Ni(bpm)(2)) (3)(Fe(CN)(6))(2)] clusters contain two unconnected Fe-III(CN)(6)(3-) groups each bridged, via three facial CN groups, to three cis-Ni(bpm)( 2) moieties, thus making the cis-(CN)(2)Ni-II(bpm)(2) centers six-coor dinate. Metal ... metal distances within the cluster are Fe ... Ni = 5 .042(3) and 5.058(2) Angstrom, Fe ... Fe = 6.438(4) Angstrom, and Ni . .. Ni = 6.654(3) and 7.028(5) Angstrom. The seven water molecules form an intriguing hydrogen-bonded ''cluster'' which connects, via hydroge n-bonding, to the other facial CN ligands on each Fe. A wide array of magnetic susceptibility and magnetization measurements has been used t o show that the Ni3Fe2 clusters display intramolecular ferromagnetic c oupling. Most importantly, the crystalline samples of 1 and most, but not all, of rapidly precipitated powder samples display long-range mag netic ordering with T-c = 23 K. The powder samples which do not displa y magnetic order yield X-ray diffraction patterns, IR spectra, and the rmogravimetric behavior identical to those of samples that display mag netic order. Experimental attempts to identify the nature of the magne tic phase transition in the ordered samples of 1 lead to ferromagnetic ordering being favored over spin-glass formation. The 7H(2)O hydrogen -bonded moiety plays a key role in intercluster exchange interactions. Dehydrated samples of 1 are not ordered. Intracluster exchange intera ctions are deduced for the dehydrated form.