ABSTRACT
The structures and energetics of small tin cluster Sn(n)(-) anions up to n=15 were determined by a combination of density-functional theory and three different experimental methods: Ion mobility spectrometry, trapped ion electron diffraction, and collision induced dissociation. We find compact, quasispherical structures up to n=12. Sn(12)(-) is a slightly distorted hollow icosahedron while Sn(13)(-) to Sn(15)(-) have prolate structures, consisting of merged, hollow, in part incomplete, deltahedral subunits: Sn(13)(-) consists of a face-sharing pentagonal bipyramid and tricapped trigonal bipyramid, Sn(14)(-) comprises a face-sharing dicapped trigonal prism and capped square-antiprism, and Sn(15)(-) consists of two face-sharing tricapped trigonal prisms.
ABSTRACT
Disorder of silver atoms and high cation mobility are commonly observed and closely coupled features in silver chalcogenides. The ligand-stabilized cluster [Ag28(micro6-S)2{ArP(O)S2}12(PPh3)12] (1) (Ar=4-anisyl), with a total of 666 atoms, displays in its X-ray structure highly localized disorder at two core silver atoms. To explore the nature of this disorder, we have applied density functional methods to its internal structure and flexibility. The pseudo-S6 symmetry of the cluster provides six equivalent pockets to place the pair of silver atoms, and with the exception of populating neighboring sites, all permutations relax to structures with similar cores. The barrier to concerted motion of the central silver atoms from one set of pockets to the next of the Ci-symmetric conformer is estimated to be less than about 26 kJ mol(-1). Cluster 1 can be considered a model for bulk phase cation mobility.
ABSTRACT
Use of 1,4,7,10-tetraazacyclododecane (cyclen) as a capping ligand and 4,4'-bipyridine (4,4'-bpy) as a bridging ligand enables assembly of redox-active Ru3 triangle and Ru4 square complexes. The former is produced by reacting [(cyclen)Ru(DMSO)Cl]Cl with 4,4'-bpy in a 3:1 ethanol:water mixture to precipitate [(cyclen)3Ru3(4,4'-bpy)3]Cl6.18H2O.THF (4), whereas the latter is generated as [(cyclen)4Ru4(4,4'-bpy)4](CF3SO3)8.2CF3SO3H.5MeOH (7) by reacting (cyclen)Ru(CF3SO3)3 with 4,4'-bpy in methanol. The crystal structure of 4.11H2O reveals an equilateral triangle in which the 4,4'-bpy bridges are bowed outward, such that the pyridine rings are all forced to be perpendicular to the Ru3 triangle. Consequently, adjacent pyridine rings are essentially coplanar, and the cyclic voltammogram of [(cyclen)3Ru3(4,4'-bpy)3]6+ in acetonitrile displays three distinct one-electron oxidation events. Cyclic voltammetry measurements reveal redox processes centered at E(1/2) = 0.207, 0.342, and 0.434 V versus Cp2Fe(0/+) that are assigned to 6+/7+, 7+/8+, and 8+/9+ couples of the [(cyclen)3Ru3(4,4'-bpy)3]n+ triangle, respectively. In contrast, the structure of [(cyclen)4Ru4(4,4'-bpy)4]8+ features a regular square geometry wherein the rings of the bridging 4,4'-bpy ligands are free to rotate, leading to just one four-electron oxidation couple centered at 0.430 V. Density functional theory calculations performed on [(cyclen)3Ru3(4,4'-bpy)(3)]6+ reveal metal-based orbitals with contributions from the pi system of the bridging 4,4'-bpy ligands, providing a likely pathway for electron transfer.
ABSTRACT
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.
ABSTRACT
Simultaneous reduction of WCl6 and CCl4 with bismuth metal at 400 degrees C affords a black solid, from which the new cluster [W6CCl18]2- is extracted into aqueous HCl. The cluster is isolated in 34% yield as (Bu4N)2[W6CCl18] and features a metal-metal bonded W6 trigonal prism centered by a carbon atom and surrounded by 12 edge-bridging and 6 terminal chloride ligands. A cyclic voltammogram of [W6CCl18]2- in DMF shows the cluster undergoes two reduction and two oxidation processes, suggesting five chemically accessible redox states. Consistent with this extensive electrochemistry, DFT calculations on the diamagnetic [W6CCl18]2- species reveal closely spaced frontier orbitals, with an a2' ' HOMO situated 0.61 and 0.71 eV below unoccupied e' ' and e' orbitals, respectively. Oxidation of the cluster by a single equivalent of NO+ gives [W6CCl18]1-, which, as expected on the basis of the [W6CCl18]2- HOMO character, possesses a less elongated W6 trigonal prism. Reduction of [W6CCl18]2- with a single equivalent of cobaltocene affords [W6CCl18]3-, wherein population of a low-lying e' orbital leads to a significant Jahn-Teller distortion.
ABSTRACT
A new and potentially highly generalizable technique for synthesizing molecular fragments of binary solids is demonstrated through application to selected transition metal chalcogenides. Employing a metal atom reactor, the solids are evaporated with a tungsten heating boat, and the resulting vapor is co-condensed with triethylphosphine. Major cluster products identified from a survey of first-row transition metal sulfides include the known species Cr6S8(PEt3)6, Co6S8(PEt3)6, and Cu12S6(PEt3)8, as well as the unprecedented species Fe4S4(PBun3)4, Ni4S4(PEt3)8, and Cu6S4(PEt3)4. Reactions utilizing Cu2Se resulted in the much larger clusters Cu26Se13(PEt3)14 and Cu70Se35(PEt3)21. The core of the former has a Th-symmetry structure featuring a body-centered icosahedron of Se2- anions, while the latter adopts a triangular structure based on three hexagonal closest packed layers of Se2- anions. In both cases, the Cu+ cations occupy distorted tetrahedral or trigonal planar sites similar to those encountered in Cu2Se; however, emergence of the face-centered cubic anion lattice of the bulk solid is not yet apparent at these cluster sizes.
