Hugh D. Selby

Bridged multiclusters derived from the face-capped octahedral [Re6 III(μ3-Se)8]2+ cluster core

Abstract The face-capped octahedral cluster cis-[Re6(μ3-Se)8(PEt3)4(MeCN)2](SbF6)2, site-differentiated with protecting PEt3 and substitutionally labile MeCN ligands, reacts with bidentate ligands (L) 4,4′-bipyridyl, (E)-1,2-bis(4-pyridyl)ethene, and 1,2-bis(4-pyridyl)ethane to give the first examples of cluster-based molecular squares of the general formula cyclo-[Re6(μ3-Se)8(PEt3)4L]4(SbF6)8. The complexes were characterized by microanalysis, 1H and 31P NMR, and time-of-flight electrospray ionization mass spectrometry. Cyclic voltammetry reveals in each case one quasi-reversible oxidation and, for two compounds, one reversible and one irreversible reduction process.

NEW DIRECTIONS OF CLUSTER CHEMISTRY – THE STORY OF THE [Re6(μ3-Se)8]2+ CLUSTERS

ABSTRACT Transition metal clusters are a unique class of chemical substances. Not only do they have well-defined molecular structures, they also exhibit interesting and potentially useful properties that are inherent to metal-metal bonded species. In this Comment our recent results in developing synthetic methodologies necessary to bring a cluster system out of the limited sphere of fundamental cluster chemistry and into general synthetic applicability are summarized. Specifically, we have explored the uses of site-differentiated cluster complexes, featuring the [Re6(μ3-Se)8]2+ core, as stereospecific building blocks for supramolecular construction. A great variety of cluster-supported molecular and supramolecular architectures have been realized, some of which display rather interesting electronic and spectroscopic properties. Following a distinctly different line of research, we have also pursued the activation of nitriles toward nucleophilic attack, as a result of their prior coordination to the Lewis acidic rhenium sites of the cluster. The isolation and characterization of imino ester complexes proved that the cluster core acts as a powerful Lewis acid catalyst. Subsequent photocleavage in the presence of nitriles, regenerating the procatalytic solvate, marks the completion of one cycle of a potentially photocatalytic process. It is not unreasonable to imagine applying cluster-based Lewis acid activation to a range of substrates beyond nitriles and alcohols. The work therefore marks the beginning of what promises to be an exciting new chapter in cluster chemistry.

Hydrogen-bonded supramolecular arrays of the [Re6(μ3-Se)8]2+ core-containing clusters

Site-differentiated clusters of the general formula [Re6(μ3-Se)8(PEt3)nL6−n](SbF6)2 [L = nicotinamide: 1 (n = 5), 2 (n = 4, trans-), and 3 (n = 4, cis-)] have been made by ligand substitution reactions of the corresponding acetonitrile solvates [Re6(μ3-Se)8(PEt3)n(MeCN)6−n](SbF6)2 (n = 5; n = 4, cis- and trans-) with nicotinamide. De-iodination of [Re6(μ3-Se)8(PEt3)nI6−n]In−4 [n = 4 (cis- and trans-), 5] with AgSbF6 in the presence of 3,5-pyridinedicarboxylic acid (PDCA) produced a related series of cluster complexes [Re6(μ3-Se)8(PEt3)nL6−n](SbF6)2 [L = PDCA: 4 (n = 5), 5 (n = 4, trans-), and 6 (n = 4, cis-)]. Retention of stereochemistry in each case was confirmed by 1H and 31P NMR and these new cluster derivatives were further characterized by satisfactory microanalyses (CHN). In addition, the solid-state structure of trans-[Re6(μ3-Se)8(PEt3)4(PDCA)2](SbF6)2 (5) was established crystallographically, which revealed zigzag arrays of clusters mediated by complementary hydrogen-bonding interactions involving only one of the acid groups per PDCA ligand; the second acid group extends into a small space between the chains and appears to be in close contact with a Se atom on a neighboring cluster, as well as a hydrogen atom of that clusters triethylphosphine ligands. Each polymer chain is skewed with respect to its neighbors, forming a pronounced lamellar structure.