Cluster-to-metal magnetic coupling: synthesis and characterization of 25-electron [Re6-nOsnSe8(CN)6](5-n)-(n = 1, 2) clusters and (Re6-nOsnSe8[CNCu(Me6tren)]6)9+(n = 0, 1, 2) assemblies

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 Re(5)OsSe(8)Cl(6)](3-) and Re(4)Os(2)Se(8)Cl(6)](2-) with NaCN in a melt of NaNO(3) or KCF(3)SO(3) afford the 24-electron clusters Re(5)OsSe(8)(CN)(6)](3-) and Re(4)Os(2)Se(8)(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 Re(5)OsSe(8)(CN)(6)](3-/4-), cis-Re(4)Os(2)Se(8)(CN)(6)](2-/3-), and trans-Re(4)Os(2)Se(8)(CN)(6)](2-/3-) couples at E(1/2) = -1.843, -0.760, and -1.031 V vs FeCp(2)(0/+), respectively, in addition to other redox processes. Accordingly, reduction of Re(5)OsSe(8)(CN)(6)](3-) with sodium amalgam and Re(4)Os(2)Se(8)(CN)(6)](2-) with cobaltocene produces the 25-electron clusters Re(5)OsSe(8)(CN)(6)](4-) and Re(4)Os(2)Se(8)(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 pi* orbitals of the cyanide ligands, suggesting the possibility of magnetic superexchange. Reaction of Re(5)OsSe(8)(CN)(6)](3-) with Ni(H(2)O)(6)](2+) in aqueous solution generates the porous Prussian blue analogue Ni(3)Re(5)OsSe(8)(CN)(6)](2).32H(2)O; 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, Re(6-n)Os(n)Se(8)CNCu(Me(6)tren)](6)](9+) (n = 0, 1, 2; Me(6)tren = 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 Re(4)Os(2)Se(8)CNCu(Me(6)tren)](6)](9+). Assuming all cluster-Cu(II) 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.