A Solid-State Route to Molecular Clusters:  Access to the Solution Chemistry of [Re₆Q₈]²⁺ (Q = S, Se) Core-Containing Clusters via Dimensional Reduction

A general method for accessing the solution chemistry of cluster constituents of solid phases exhibiting extended cluster frameworks is demonstrated. The approach is described in terms of simple metal−anion (M−X) frameworks and involves the formal incorporation of AX into a parent structure, resulting in termination of the X bridges between M centers while balancing the charge of the resulting framework with external cations A. The new structures obtained display frameworks of reduced connectedness and dimensionality. By replacing single metal centers with multinuclear cluster cores, this dimensional reduction approach is extended to cluster-containing frameworks. Its utility is demonstrated via application to the phases Re 6 Q 8 Cl 2 (Q = S, Se), exhibiting three- and two-dimensional arrays of face-capped octahedral [Re 6 (μ 3 -Q) 8 ] 2+ cluster cores covalently linked through extremely tight Re 2 Q 2 rhombic interactions of the type common to many intractable cluster frameworks (including the Chevrel phases). Stoichiometric solid-state reactions incorporating TlCl supplement the cores with additional terminal ligands, producing less connected frameworks: two-dimensional [Re 6 Se 8 Cl 3 ] 1- sheets, one-dimensional [Re 6 Q 8 Cl 4 ] 2- chains, and ultimately, isolated [Re 6 Q 8 Cl 6 ] 4- clusters. The connectivities for such [M 6 Q 8 ] frameworks are enumerated; of the 28 possibilities, three previously unknown frameworks are achieved in the structures of TlRe 6 Se 8 Cl 3, CsRe 6 Se 8 I 3, and Cs 2 Re 6 Se 8 Br 4, described herein. Alternatively, employing cesium halide as a dimensional reduction agent directly provides the unprecedented molecular clusters in water-soluble form as the phases Cs 5 Re 6 S 8 X 7 (X = Cl, Br), Cs 6 Re 6 S 8 I 8, and Cs 4 Re 6 Se 8 I 6 . The species [Re 6 S 8 X 6 ] 4- (X = Cl, Br, I) are precipitated from aqueous base upon addition of (Bu 4 N)X to give the soluble molecular products (Bu 4 N) 4 [Re 6 S 8 Cl 6 ], (Bu 4 N) 4 [Re 6 S 8 Br 6 ]·H 2 O, and (Bu 4 N) 4 [Re 6 S 8 I 6 ]·H 2 O. Treatment of yellow acetonitrile solutions of these compounds with anhydrous acid induces an immediate color change to red owing to the formation of the protonated clusters [Re 6 S 7 (SH)X 6 ] 3- . Reversible uptake of a single proton is confirmed by the single-crystal X-ray structure determinations of (Bu 4 N) 3 [Re 6 S 7 (SH)Cl 6 ], (Bu 4 N) 3 [Re 6 S 7 (SH)Br 6 ]·2Me 2 CO, and (Bu 4 N) 3 [Re 6 S 7 (SH)I 6 ]·2Me 2 CO, as well as spectrophotometric titrations and elemental analyses. The p K a of [Re 6 S 7 (SH)Br 6 ] 3- in acetonitrile is estimated at 20. An analogous workup of red Cs 4 Re 6 Se 8 I 6 affords (Bu 4 N) 3 [Re 6 Se 7 (SeH)I 6 ]·2Me 2 CO.