posted on 2003-12-17, 00:00authored byEric G. Tulsky, Nathan R. M. Crawford, Stéphane A. Baudron, Patrick Batail, Jeffrey R. Long
The first face-capped octahedral clusters with 25 metal-based valence electrons are shown to
provide versatile building units capable of engaging in magnetic exchange coupling. Reactions of
[Re5OsSe8Cl6]3- and [Re4Os2Se8Cl6]2- with NaCN in a melt of NaNO3 or KCF3SO3 afford the 24-electron
clusters [Re5OsSe8(CN)6]3- and [Re4Os2Se8(CN)6]2-. The 13C NMR spectrum of a 13C-labeled version of
the latter species indicates a 1:2 mixture of cis and trans isomers. Cyclic voltammograms of the clusters
in acetonitrile display reversible [Re5OsSe8(CN)6]3-/4-, cis-[Re4Os2Se8(CN)6]2-/3-, and trans-[Re4Os2Se8(CN)6]2-/3- couples at E1/2 = −1.843, −0.760, and −1.031 V vs FeCp20/+, respectively, in addition to other
redox processes. Accordingly, reduction of [Re5OsSe8(CN)6]3- with sodium amalgam and [Re4Os2Se8(CN)6]2-
with cobaltocene produces the 25-electron clusters [Re5OsSe8(CN)6]4- and [Re4Os2Se8(CN)6]3-. EPR spectra
of these S = 1/2 species in frozen DMF solutions exhibit isotropic signals with g = 1.46 for the monoosmium
cluster and g = 1.74 and 1.09 for the respective cis and trans isomers of the diosmium cluster. In each
case, results from DFT calculations show the unpaired spin to delocalize to some extent into the π* orbitals
of the cyanide ligands, suggesting the possibility of magnetic superexchange. Reaction of [Re5OsSe8(CN)6]3-
with [Ni(H2O)6]2+ in aqueous solution generates the porous Prussian blue analogue Ni3[Re5OsSe8(CN)6]2·32H2O; however, the tendency of the 25-electron clusters to oxidize in water prohibits their use in reactions
of this type. Instead, a series of cyano-bridged assemblies, {Re6-nOsnSe8[CNCu(Me6tren)]6}9+ (n = 0, 1,
2; Me6tren = tris(2-(dimethylamino)ethyl)amine), were synthesized to permit comparison of the exchange
coupling abilities of clusters with 23−25 electrons. As expected, the results of magnetic susceptibility
measurements show no evidence for exchange coupling in the assemblies containing the 23- and 24-electron clusters, but reveal the presence of weak ferromagnetic coupling in {Re4Os2Se8[CNCu(Me6tren)]6}9+.
Assuming all cluster−CuII exchange interactions to be equivalent, the data were fit to give an estimated
coupling strength of J = 0.4 cm-1. To our knowledge, the ability of such clusters to participate in magnetic
exchange coupling has never previously been demonstrated.