New cyanometalate building units: synthesis and characterization of [Re(CN)(7)]3- and [Re(CN)(8)]3-

Two new cyanorhenate complexes of potential utility in constructing magnetic and photomagnetic materials are reported. Reaction of (Bu4N)CN with ReCl6]2- in acetonitrile affords yellow (Bu4N)3Re(CN)7] (1), featuring the pentagonal bipyramidal complex Re(CN)7]3-. The spectral and magnetic properties of 1 indicate that the complex has an S = 1/2 ground state with considerable anisotropy in the g tensor. In aqueous solution, 1 reacts with Mn2+ ions to generate the three-dimensional cyano-bridged solid fac-Mn(H2O)3]cis-Mn(H2O)2]Re(CN)7].3H2O (2) containing diamagnetic Re(CN)7]4-. Addition of KIO4 to the reaction solution, originally intended to prevent reduction of the rhenium during solid formation, instead yields white (Bu4N)3Re(CN)8] (3). As crystallized in K3Re(CN)8].2MeCN (4.2MeCN), the diamagnetic Re(CN)8]3- complex adopts a nearly perfect square antiprismatic coordination geometry. In solution, this species behaves analogously to the isoelectronic M(CN)8]4- (M = Mo, W) complexes, apparently converting to a dodecahedral geometry and photooxidizing under UV radiation to give paramagnetic Re(CN)8]2-.