Cluster-to-Metal Magnetic Coupling:  Synthesis and\nCharacterization of 25-Electron [Re_6-<i>_n</i>Os<i>_n</i>Se₈(CN)₆]^(5-<i>ⁿ</i>^)-\n(<i>n</i> = 1, 2) Clusters and {Re_6-<i>_n</i>Os<i>_n</i>Se₈[CNCu(Me₆tren)]₆}⁹⁺\n(<i>n</i> = 0, 1, 2) Assemblies

Abstract

The first face-capped octahedral clusters with 25 metal-based valence electrons are shown to\nprovide versatile building units capable of engaging in magnetic exchange coupling. Reactions of\n[Re₅OsSe₈Cl₆]^3- and [Re₄Os₂Se₈Cl₆]^2- with NaCN in a melt of NaNO₃ or KCF₃SO₃ afford the 24-electron\nclusters [Re₅OsSe₈(CN)₆]^3- and [Re₄Os₂Se₈(CN)₆]^2-. The ¹³C NMR spectrum of a ¹³C-labeled version of\nthe latter species indicates a 1:2 mixture of cis and trans isomers. Cyclic voltammograms of the clusters\nin acetonitrile display reversible [Re₅OsSe₈(CN)₆]^3-/4-, <i>cis</i>-[Re₄Os₂Se₈(CN)₆]^2-/3-, and <i>trans</i>-[Re₄Os₂Se₈(CN)₆]^2-/3- couples at <i>E</i>_1/2 = −1.843, −0.760, and −1.031 V vs FeCp₂^0/+, respectively, in addition to other\nredox processes. Accordingly, reduction of [Re₅OsSe₈(CN)₆]^3- with sodium amalgam and [Re₄Os₂Se₈(CN)₆]^2-\nwith cobaltocene produces the 25-electron clusters [Re₅OsSe₈(CN)₆]^4- and [Re₄Os₂Se₈(CN)₆]^3-. EPR spectra\nof these <i>S</i> = ¹/₂ species in frozen DMF solutions exhibit isotropic signals with <i>g</i> = 1.46 for the monoosmium\ncluster and <i>g</i> = 1.74 and 1.09 for the respective cis and trans isomers of the diosmium cluster. In each\ncase, results from DFT calculations show the unpaired spin to delocalize to some extent into the π* orbitals\nof the cyanide ligands, suggesting the possibility of magnetic superexchange. Reaction of [Re₅OsSe₈(CN)₆]^3-\nwith [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\nof this type. Instead, a series of cyano-bridged assemblies, {Re₆_-<i>_n</i>Os<i>_n</i>Se₈[CNCu(Me₆tren)]₆}⁹⁺ (<i>n</i> = 0, 1,\n2; Me₆tren = tris(2-(dimethylamino)ethyl)amine), were synthesized to permit comparison of the exchange\ncoupling abilities of clusters with 23−25 electrons. As expected, the results of magnetic susceptibility\nmeasurements 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⁺.\nAssuming all cluster−Cu^II exchange interactions to be equivalent, the data were fit to give an estimated\ncoupling strength of <i>J</i> = 0.4 cm^-1. To our knowledge, the ability of such clusters to participate in magnetic\nexchange coupling has never previously been demonstrated.